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Engineering Studies Preliminary Course Stage 6 Household appliances ES/S6 – Prelim 41080 P0020451 Acknowledgments This publication is copyright Learning Materials Production, Open Training and Education Network – Distance Education, NSW Department of Education and Training, however it may contain material from other sources which is not owned by Learning Materials Production. Learning Materials Production would like to acknowledge the following people and organisations whose material has been used. Board of Studies, NSW All reasonable efforts have been made to obtain copyright permissions. All claims will be settled in good faith. Matrials development: John Burns Revised version: Brian Jobson, Jeff Appleby, Joesphine Wilms and Stephen Russell Coordination: Jeff Appleby and Nicola Pegum Illustrations: Tom Brown and David Evans DTP: Nick Loutkovsky and Carolina Barbieri Copyright in this material is reserved to the Crown in the right of the State of New South Wales. Reproduction or transmittal in whole, or in part, other than in accordance with provisions of the Copyright Act, is prohibited without the written authority of Learning Materials Production. © Learning Materials Production, Open Training and Education Network – Distance Education, NSW Department of Education and Training, 1999. 51 Wentworth Rd. Strathfield NSW 2135. Revised 2001 Module contents Subject overview ................................................................................ iii Module overview................................................................................ vii Module components ................................................................ viii Module outcomes ...................................................................... ix Indicative time ............................................................................x Resource requirements...............................................................x Icons ..................................................................................................... xi Glossary............................................................................................. xiii Directive terms.................................................................................. xix Part 1: Household appliances – development......................... 1–33 Part 2: Household appliances – materials .............................. 1–41 Part 3: Household appliances – mechanics ............................ 1–41 Part 4: Household appliances – electricity and communication ................................................................ 1–63 Part 5: Household appliances – Engineering report............... 1–20 Bibliography........................................................................................21 Module evaluation .............................................................................23 i ii Subject overview Stage 6 Engineering Studies Preliminary Course and HSC Course each have five modules. Engineering Studies Preliminary Course Household appliances examines common appliances found in the home. Simple appliances are analysed to identify materials and their applications. Electrical principles, researching methods and techniques to communicate technical information are introduced. The first student engineering report is completed undertaking an investigation of materials used in a household appliance. Landscape products investigates engineering principles by focusing on common products, such as lawnmowers and clothes hoists. The historical development of these types of products demonstrates the effect materials development and technological advancements have on the design of products. Engineering techniques of force analysis are described. Orthogonal drawing methods are explained. An engineering report is completed that analyses lawnmower components. Braking systems uses braking components and systems to describe engineering principles. The historical changes in materials and design are investigated. The relationship between internal structure of iron and steel and the resulting engineering properties of those materials is detailed. Hydraulic principles are described and examples provided in braking systems. Orthogonal drawing techniques are further developed. An engineering report is completed that requires an analysis of a braking system component. iii Bio-engineering examines both engineering principles and also the scope of the bio-engineering profession. Careers and current issues in this field are explored. Engineers as managers and ethical issues confronted by the bio engineer are considered. An engineering report is completed that investigates a current bio- engineered product and describes the related issues that the bio-engineer would need to consider before, during and after this product development. Irrigation systems is the elective topic for the preliminary modules. The historical development of irrigation systems is described and the impact of these systems on society discussed. Hydraulic analysis of irrigation systems is explained. The effect on irrigation product range that has occurred with the introduction of is detailed. An engineering report on an irrigation system is completed. iv HSC Engineering Studies modules Civil structures examines engineering principles as they relate to civil structures, such as bridges and buildings. The historical influences of engineering, the impact of engineering innovation, and environmental implications are discussed with reference to bridges. Mechanical analysis of bridges is used to introduce concepts of truss analysis and stress/strain. Material properties and application are explained with reference to a variety of civil structures. Technical communication skills described in this module include assembly drawing. The engineering report requires a comparison of two engineering solutions to solve the same engineering situation. Personal and public transport uses bicycles, motor vehicles and trains as examples to explain engineering concepts. The historical development of cars is used to demonstrate the developing material list available for the engineer. The impact on society of these developments is discussed. The mechanical analysis of mechanisms involves the effect of friction. Energy and power relationships are explained. Methods of testing materials, and modifying material properties are examined. A series of industrial manufacturing processes is described. Electrical concepts, such as power distribution, are detailed are introduced. The use of freehand technical sketches. Lifting devices investigates the social impact that devices raging from complex cranes to simple car jacks, have had on our society. The mechanical concepts are explained, including the hydraulic concepts often used in lifting apparatus. The industrial processes used to form metals and the methods used to control physical properties are explained. Electrical requirements for many devices are detailed. The technical rules for sectioned orthogonal drawings are demonstrated. The engineering report is based on a comparison of two lifting devices. v Aeronautical engineering explores the scope of the aeronautical engineering profession. Career opportunities are considered, as well as ethical issues related to the profession. Technologies unique to this engineering field are described. Mechanical analysis includes aeronautical flight principles and fluid mechanics. Materials and material processes concentrate on their application to aeronautics. The corrosion process is explained and preventative techniques listed. Communicating technical information using both freehand and computer-aided drawing is required. The engineering report is based on the aeronautical profession, current projects and issues. Telecommunications engineering examines the history and impact on society of this field. Ethical issues and current technologies are described. The materials section concentrates on specialised testing, copper and its alloys, semiconductors and fibre optics. Electronic systems such as analogue and digital are explained and an overview of a variety of other technologies in this field is presented. Analysis, related to telecommunication products, is used to reinforce mechanical concepts. Communicating technical information using both freehand and computer-aided drawing is required. The engineering report is based on the telecommunication profession, current projects and issues. Figure 0.1 Modules vi Module overview In Part 1 you will begin to investigate the historical developments of household appliances. Some useful terms will be introduced. You will explore the development of floor cleaners, electric irons and refrigerators. To help you understand the importance of material selection for household appliances you will learn about the methods of classifying materials. The atomic structure and bonding of materials will be analysed. Lastly you will learn about research methods to prepare you for writing an engineering report at the end of the module. You will build on your knowledge of materials and components by studying about the use of metals. Types of metals, such as ferrous and non-ferrous, will be explained. You will then learn about cutting and joining currently used in household appliances. Polymer materials and ceramic materials and their uses are explored. Additional researching techniques are introduced, which you will need to help you eventually complete your Engineering Report. In part 3 you will develop a basic understanding of engineering mechanical principles. Concepts such as mass and force are defined. Next scalar and vector quantities are classified and a method of determining components of a force is explained. In Part 4 of this module you will study basic forms of electricity/electronics for household appliances. Principles such as potential difference, current, and components are described. You will learn how to identify principles of electrical safety and learn about induction. Freehand orthogonal drawing concepts are detailed. By part 5 you have practiced and completed your research methods and will know most of the steps for writing your engineering report. vii It will be time to pull all those skills together and collate your information into one report. You will need to research and study one household appliance. You will develop your report using your writing skills to communicate information and drawing skills to illustrate your work. Module components Each module contains three components, the preliminary pages, the teaching/learning section and additional resources. • The preliminary pages include: – module contents – subject overview – module overview – icons – glossary – directive terms. Figure 0.2 Preliminary pages • Figure 0.3 Teaching/learning section viii The teaching/learning parts may include: – part contents – introduction – teaching/learning text and tasks – exercises – check list. • The additional information may include: – module appendix – bibliography – module evaluation. Additional resources Figure 0.4 Additional materials Support materials such as audiotapes, video cassettes and computer disks will sometimes accompany a module. Module outcomes At the end of this module, you should be working towards being able to: • describe the types of materials, components and processes used to make household appliances and explain any implications for engineering development (P1.2) • explain the relationship between properties, uses and applications of materials in engineering (P2.1) • develop written, oral and presentation skills and apply these to engineering reports (P3.2) • applied graphics as a communication tool (P3.3) • describe the developments in technology and their impact on engineering products (P4.1) describe the influence of technological change on engineering and its effect on people (P4.2) • • identify the social, environmental and cultural implications of technological change in engineering (P4.3). Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to <http://www.boardofstudies.nsw.edu.au> for original and current documents. ix Indicative time The Preliminary course is 120 hours (indicative time) and the HSC course is 120 hours (indicative time). The following table shows the approximate amount of time you should spend on this module. Preliminary modules Percentage of time Number of hours Household appliances 20% 24 hr Landscape products 20% 24 hr Braking systems 20% 24 hr Bio-engineering 20% 24 hr Elective: Irrigation systems 20% 24 hr HSC modules Percentage of time Number of hours Civil structures 20% 24 hr Personal and public transport 20% 24 hr Lifting devices 20% 24 hr Aeronautical engineering 20% 24 hr Telecommunications engineering 20% 24 hr There are five parts in Household appliances. Each part will require about four to five hours of work. You should aim to complete the module within 20 to 25 hours. Resource requirements During this module you will need to access a range of resources including: • access to an early and late model household appliance • technical drawing equipment such as - rule, pencils, eraser and compasses - protractor and set squares. For research you are encouraged to use the Internet, CD Roms and books through your local, school or TAFE library. x Icons As you work through this module you will see symbols known as icons. The purpose of these icons is to gain your attention and to indicate particular types of tasks you need to complete in this module. The list below shows the icons and outlines the types of tasks for Stage 6 Engineering studies. Computer This icon indicates tasks such as researching using an electronic database or calculating using a spreadsheet. Danger This icon indicates tasks which may present a danger and to proceed with care. Discuss This icon indicates tasks such as discussing a point or debating an issue. Examine This icon indicates tasks such as reading an article or watching a video. Hands on This icon indicates tasks such as collecting data or conducting experiments. Respond This icon indicates the need to write a response or draw an object. Think This icon indicates tasks such as reflecting on your experience or picturing yourself in a situation. xi Return This icon indicates exercises for you to return to your teacher when you have completed the part. (OTEN OLP students will need to refer to their Learner's Guide for instructions on which exercises to return). xii Glossary As you work through the module you will encounter a range of terms that have specific meanings. The list below explains the terms you will encounter in this module. These terms are bolded the first time they occur in the text. alternating current when electrons flow (electric current) first in one direction and then flows back again and continue this back and forth motion atom a component of all matter body-centred cubic A type of crystal structure abbreviated to BCC box up divide a shape into sections/areas/cubes so that it is broken down into easier to draw segments brush A connector used to maintain electric contact between stationary and moving parts in a motor. ceramics materials are versatile engineering materials, including such common items as brick, porcelain, glass, and cement commutator a device for reversing the direction of an electric current in a motor dichlorodifluoromethane a ‘non toxic’ substitute for ammonium as a coolant, now banned, abbreviated to CFC-12 coefficient of expansion a measurement that describes the change in size of material relative to the temperature compressive force describes a force applied to an object that attempts to compress the object conventions agreed upon rules or practices crystalline material the atoms form into definite repeating patterns or ‘lattice’ structures density a measurement of a materials mass per unit volume xiii xiv direct current when electrons flow (electric current) in one direction along the conductor direction a measurement of the angle (normally measured off the horizontal) measured in degrees drawing into wire pulling metal through a die to form a wire ductile able to be stretched without failure elasticity the property of a material to return its original shape after a distorting force has been removed electrical current the movements of electrons along the conductor in a particular direction which produces an electric current electrical conductivity the ability of a material to conduct electricity electrons orbit the nucleus in layers (shells) and are negative in charge electro-chemical bonds hold the atoms more rigidly together electromagnet a magnet made by passing electrical current through a coiled conductor wrapped around a ferrous core element passing an electric current through a wire, thereby creating heat elements materials composed of only one type of atom equilibrant an equal force acting as a balance to maintain, or bring about, a state of equilibrium ergonomics designing for bodily needs of a given working environment face-centred cubic A tpe of crystal structure abbreviated to FCC ferrous metals metals which contain primarily iron with small proportions of other materials force the interaction between bodies force component a component of a vector is the effect of that vector in a specific direction. The components of a vector add to equal the original vector formability a material’s ability to be deformed or shaped by bending, stretching, compressing freon a coolant used in the refrigeration process – is damaging to the ozone layer frictional properties the property of a material that describes the amount of grip a material has when sliding in contact with another surface fusible able to be joined together hardness the property of a material to resist penetration or scratching when brought into contact with another material hexagonal close packed A type of crystal structure abbreviated to HCP lattice created when the atoms form into definite repeating patterns magnetic flux density magnetic field strength; a magnetic field is represented by lines of ‘flux’ – the denser the field or lines of flux, or the closer the lines are, the stronger the magnetic filed magnetic properties the ability of a material to become magnetic magnetic induction a process enabling the spin axes of electrons to be aligned thus creating a magnet magnitude the size or how much of something mass the amount of matter that is contained within that object melting point the temperature at which the material begins to become a liquid neutrons are located in the nucleus, have no electrical charge and are a similar size to the protons newton is a unit of force represented by the symbol N nichrome a metal alloy of nickel and chromium non ferrous metals metals which contain little or no iron non-oxidising does not form an oxide, chemically stable nucleus lies at the centre of the atom, consisting of protons and neutrons opacity the property of a material that stops the passage of light, that is, you cannot see through it orthogonal drawing where an object is fully described by projecting a series of related views from various viewing positions (above, front on, side on) xv xvi porcelain a ceramic product that is stable, smooth and is an insulator primary bonds the main bonds holding material together protons are located in the nucleus, are positive in charge and are equal in number to the electrons orbiting the nucleus prototype an initial version of an item, often produced at the development stage of a product radiated heat heat that is transmitted through the air relative density a measurement for material describing how compact the mass of the object is relative to other materials resistance to corrosion describes the property of a material that allows it to not corrode quickly in a service application resistance to creep the property of a material to maintain its original length for a long period of time when a small load is applied resultant force describes effects when a system of two or more forces is analysed rotor Rotating coil used in simple electric motors scalar quantity is a quantity that requires only magnitude (size) for its complete understanding secondary bonds weak bonds normally caused by attraction between positive and negative parts of molecules that are close to one another stability describes how a material reacts to external changes stator Are the stationary magnets used in an electric motor Steel an alloy of iron and carbon stiffness the property of a material to maintain shape strength the property of a material to withstand forces. Normally strength is specified as yield strength, tensile strength, compressive strength, shear strength tensile force describes a force applied to an object that attempts to stretch the object thermal conductivity the property of a material to conduct heat thermopolymer polymers which, once set, can be melted and re-formed thermosetting polymers which, once set, cannot be remelted or reshaped tonne a tonne, represented by the symbol T, is equal to 1000 kg tough able to have force applied without failure toxicity describes how harmful a material is to the environment Uniform Resource Locater address of an Internet site abbreviated to URL vector quantity that has magnitude and direction/sense vector quantity a quantity that requires direction as well as magnitude for its complete understanding xvii xviii Directive terms The list below explains key words you will encounter in assessment tasks and examination questions. account account for: state reasons for, report on; give an account of: narrate a series of events or transactions analyse identify components and the relationship between them, draw out and relate implications apply use, utilise, employ in a particular situation appreciate make a judgement about the value of assess make a judgement of value, quality, outcomes, results or size calculate ascertain/determine from given facts, figures or information clarify make clear or plain classify arrange or include in classes/categories compare show how things are similar or different construct make, build, put together items or arguments contrast show how things are different or opposite critically (analyse/evaluate) add a degree or level of accuracy depth, knowledge and understanding, logic, questioning, reflection and quality to (analysis/evaluation) deduce draw conclusions define state meaning and identify essential qualities demonstrate show by example xix describe provide characteristics and features discuss identify issues and provide points for and/or against distinguish recognise or note/indicate as being distinct or different from; to note differences between evaluate make a judgement based on criteria; determine the value of examine inquire into explain relate cause and effect; make the relationships between things evident; provide why and/or how extract choose relevant and/or appropriate details extrapolate infer from what is known identify recognise and name interpret draw meaning from investigate plan, inquire into and draw conclusions about justify support an argument or conclusion outline sketch in general terms; indicate the main features of predict suggest what may happen based on available information propose put forward (for example a point of view, idea, argument, suggestion) for consideration or action recall present remembered ideas, facts or experiences recommend provide reasons in favour recount retell a series of events summarise express, concisely, the relevant details synthesise putting together various elements to make a whole Extract from The New Higher School Certificate Assessment Support Document, © Board of Studies, NSW, 1999. Refer to <http://www.boardofstudies.nsw.edu.au> for original and current documents. xx Household appliances Part 1: Household appliances – development Part 1 contents Introduction............................................................................................ 2 What will you learn?.................................................................... 2 Overview of early technology............................................................ 3 Household appliances ........................................................................ 6 Common household appliances ................................................. 7 Developing your research skills........................................................ 25 Exercises............................................................................................ 27 Progress check................................................................................... 31 Exercise cover sheet......................................................................... 33 Part 1: Development of household appliances 1 Introduction In this part of the module you will examine the development of different household appliances through case studies. Looking at developments in household appliances allows you to gain an appreciation for engineering innovation through past achievements and focus on the connection between the needs of society and the engineering field which drives the development of new and better products. When researching developments you will need to find out about early forms of technology, the societal effects of the scientific and industrial revolution and the effect on engineering development. As you are reading keep in mind how developments in household appliances affect different groups of people in society. For example, ask yourself would a 19th century copper kettle be safe for people to use, such as an older person with arthritis, a child under twelve, a slightly built person, easy to produce, affordable to buy and economical to repair? Keep these issues in mind in your study of the developments of household appliances. You will need to write an engineering report in the last part of this module. This will require research into the material used in a household appliance. What will you learn? You will learn about: • the historical and societal influences by studying; – the historical developments of household appliances – the effects of engineering innovation on people’s lives. You will learn to: • outline the historical development of household appliances • describe the effect of engineering innovation on people’s lives. Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to <http//www.boardofstudies.nsw.edu.au> for original and current documents. 2 Household appliances Overview of early technology The development of engineering principles and techniques can be closely linked to the general development of societies. Engineering has been a central force in all economic and social growth. In this part of the module you will examine several engineering developments and consider some effects they had on society. Before you begin … What would you consider is the most important mechanical invention of all time? Our choice will be revealed later! It is not necessary to study ancient civilisations in order to gain some understanding of the effect of engineering. However, you could research the effect the development of tools had on ‘stone age’ society. You could research the developments of agricultural implements and the agrarian revolution. You could research the earliest use of metals (and their smelting) and the dramatic effect they had on societies during the bronze and iron ages. There is also a history of engineering and science principles. These are revealed in the groundbreaking books and manuscripts of Aristotle (Greek civilisation) Michelangelo, Leonardo da Vinci and Machiavelli (renaissance period in Italy). This list could continue to fill a book! Technological advancement has tended to ebb and flow over the many centuries. There have been periods of development followed by periods of little change. This pattern ended by about 1750, the start of the ‘Industrial Revolution’. The period from 1750 to the 1900s saw change like the world had never seen previously. This revolution in technology has only been surpassed by the technology revolution you are living with today. The following list provides a few examples of the many thousands of engineering developments during this period. Part 1: Development of household appliances 3 Try to think of one additional invention or development for each of the following areas: • • • • • 4 materials – developments in the iron making industry – automation of textile manufacture – development of coal as an industrial energy source – development of building materials such as bricks and reinforced concrete – mass production of glass transport – invention of the steam engine – development of a railway system – expansion of the canal systems – development of the steam turbine (ship propulsion) – development of the internal combustion engine – development of the automobile – development of the air ship tool making – development of accurate clocks for navigation – development of accurate distance/size measuring devices – development of accurate cutting devices for machine manufacture chemical knowledge – artificial fertiliser – explosives – dyes and inks – medicines (iodine, anaesthetics, carbolic acid) – voltaic battery communication knowledge – printing press – telegraph – telephone – typewriter – photography/video Household appliances • • – computer – satellite electricity – electric lighting – electric motor – electric generator food and agriculture – mechanised farm machinery – food processing plants – food preservation, packaging and refrigeration. Back to the original question, which would you consider the most important mechanical invention of all time? If you answered the ‘wheel’ then you agree with most people. The invention of the wheel The wheel appeared as early as 3 500 BC in Mesopotamia where the first application was the potters’ wheel. This technology was quickly adopted for transportation in the four-wheeled carts and chariots. You have now had a brief overview of some technology developments. You will build on this information in the later modules. You will now read about developments in household appliances. Part 1: Development of household appliances 5 Household appliances When you look at household appliances and how they have developed over the last century you will find it is an interesting way of identifying technological and engineering change. Many current models only look vaguely similar to the products first developed. When you consider the models of today, you can see that many changes to the design have taken place. Some of the reasons for new developments include: • the development of new materials such as polymers • the invention of new power sources, for example, electricity and solar power. List two other factors that may have brought about new developments in household appliances. 1 _______________________________________________________ 2 _______________________________________________________ Did you answer? The development of control devices and the new applications of existing materials Engineers, over time, have modified the household appliances to suit today’s environment, modifying the external appearance, changing the materials used, and addressing safety issues. There are also less obvious changes and modifications to appliances to make the product more attractive and profitable in the market place. 6 Household appliances Common household appliances A tour of the average home will reveal many household appliances. The kitchen probably has more household appliances than any other room in the home, you are likely to find a fridge, freezer, juicer, toaster, non-stick pan, microwave oven, wall oven, cook top, coffee percolator, even an electric knife. 1 Spend five minutes in the kitchen and list all the household appliances specifically designed for food preparation. _______________________________________________________ _______________________________________________________ _______________________________________________________ Did you answer? Some of the kitchen appliances you may have found in the kitchen include fridge, freezer, blender, juicer, toaster, electric fry pan, electric wok, microwave oven, wall oven, cook top, coffee percolator, food processor, electric knife, electric can opener, popcorn maker … Many kitchen appliances were invented as labour saving items. They use electric motors instead of manual labour to get jobs done. For example, there are electric food processors for chopping, electric blenders to puree, electric beaters to mix and electric brooms to sweep up the mess when you are finished cooking. In another part of a typical house you will find the entertainment area complete with television, radio, cassette recorder, compact disc player, some of which today are controlled remotely. These labour saving devices were only possible after the discovery of electricity and the invention of small electric motors. Most of them weren't invented until after homes in North America and Europe were ‘electrified’, that is connected to a source of electricity, in the 20th century. Common household appliances include: • food mixer • floor cleaners • clothes iron • bread toaster • kettle • refrigerator. Part 1: Development of household appliances 7 Before the introduction of electricity, many household appliances were manually operated. You will examine the developments of common household appliances over time and the impact of engineering innovations on individuals. Complete the following table: a describe the functional features of the early model household appliances b identify a significant change in design to this product in the late model equivalent. Appliance Early model Late model kettle In the 1800s a kettle was commonly made from iron. The water was heated by a wood fire. The product was heavy and hot to touch In the 1990s a kettle was commonly made from polymer. The water was heated by an electric element. The product is light weight and cool to touch. clothes iron __________________________ _________________________ __________________________ _________________________ __________________________ _________________________ __________________________ _________________________ __________________________ _________________________ __________________________ _________________________ __________________________ _________________________ __________________________ _________________________ __________________________ _________________________ __________________________ _________________________ __________________________ _________________________ __________________________ _________________________ stove Did you answer? clothes iron • made of cast iron • made from aluminium and polymers stove • made from cast iron • steel powered by gas and/or electricity • source of heat wood 8 Household appliances How did you go? You would have found that today people use household appliances to save time and energy. You may also have noticed `these appliances have changed many times in the last 50 years. The food mixer Many household appliances only vaguely resemble the products first developed. Examine figure 1.1 which shows an early model food mixer. Figure 1.1 The early 1920s food mixer Compare this early model food mixer to a late model food mixer by listing three design differences. 1 _______________________________________________________ 2 _______________________________________________________ 3 _______________________________________________________ Did you answer? • exposed motor housing and gearing • revolving base plate/mixing bowl • metal frame (insulated). Part 1: Development of household appliances 9 When you consider the models of today, you can see that many changes to the design have taken place. Over time engineers have modified the food mixer by: • changing the external appearance to make it more appealing • using the latest materials • taking safety issues into account for example, reduced the noise level • using the latest manufacturing techniques to increase efficiency, productivity and most importantly profitability. Floor cleaners This case study looks at the historical development of floor cleaning appliances. In this section you will examine the: • 1858 carpet sweeper • 1920s pump vacuum • 1930s electric vacuum • 2000 electric upright vacuum. The floor sweeper In 1858 H.H. Herrick designed the Brush sweeper. Figure 1.2 10 Brush sweeper Figure 1.3 Sectioned view of the brush sweeper Household appliances The functional features of this appliance included: • ability to clean carpets and smooth floor surfaces such as timber • the mechanism was contained within the strong, lightweight case • the brush consisted of helical rows of tufts • every second row of tufts were to suit flat timber floors whilst the other rows of tufts were designed for carpeted surfaces. • the brush height was adjustable • the sweeper was pushed back and forth to turn the brushes. The dust was collected in pans that could be opened and emptied • furniture was protected by a rubber guard that went round the outer case. The advantage of this type of floor cleaner was that it used no electricity, which many households did not have at that stage. The electric vacuum cleaner Hubert Cecil Booth, an Englishman, designed and patented the first practicable vacuum cleaner, but a number of patents for machines like this had already been granted to other inventors. Figure 1.4 Early electric vacuum cleaner The problem for the early electric vacuum-cleaners was the motor. Early electric motors were both large and heavy. This led to the development of ducted vacuum cleaning systems that catered for many Part 1: Development of household appliances 11 rooms, but only had one motor. Public buildings and blocks of flats were suited to this ducted vacuum system which had the motor located in a plant room or basement. This is similar to the ducted systems that some homes have today. Early in the twentieth century, Axel Wenner-Gren, was determined to develop a vacuum cleaner that was low priced and light in weight for easy handling. He developed a design that had a ‘closed fan’. The fan is housed, and air is forced through a confined intake. This development gave powerful suction from a small cleaner and is still a component in current vacuum cleaner designs. The upright vacuum cleaner The electric upright type of vacuum cleaner was developed from the earlier carpet sweeper. The tuft brushes are now motor-driven and beater bars have been developed and incorporated to move dust and dirt up enabling the brushes to be more effective. A powerful fan forces the dust and dirt into a disposable bag and the exhaust air is cleaned leaving the vacuum cleaner by passing through a disposable filter. Figure 1.5 Electric upright vacuum cleaner The cylinder style vacuum The cylinder style vacuum cleaner is designed with most components such as the inlet tube, motor, fans and dust bag to be in-line. This style of vacuum cleaner relies on suction alone and thus has neither revolving brushes nor beater bars. 12 Household appliances Figure 1.6 Sectioned view of a cylinder style vacuum cleaner Recent developments The vacuum cleaner shown below is the latest design. It has developed into a light but powerful machine, and is also stylish and suitable for mass production. stick design optional cord see-through canister specialist attachments power head Figure 1.7 Late model vacuum cleaner It is an important task to analyse the effect of any product and the effect that product has on people’s lives. While the latest machine improves the cleaning of the floors, and greatly decreases the time required to clean the floors, there are other social and environmental effects. The machine requires an initial outlay of household funds. It will require maintenance. Use of the machine will add slightly to the electricity bill. It will take up storage space. At the same time, the machine will allow a carpet floor covering to be kept low in dust, and therefore much healthier for the occupants of the house. Part 1: Development of household appliances 13 The domestic clothes iron Stones heated in the fire or in boiling water were the first tools used for smoothing out wrinkles in clothes. The flat iron By 1850 there were two iron options available: • the flat iron – was made of iron and heated on a fire. Manufactured in the eighteenth and nineteenth centuries. • the box iron – where a piece of preheated iron or a lump of coal was placed in the box to keep the iron hot. Flat irons, sometimes known as ‘sad irons’, were made from cast iron. This made them very heavy and ironing became a time consuming chore that required considerable muscular effort, both in the home and industrial situation. Figure 1.8 Early flat iron Weight, cleanliness and the hot handle were the major concerns for the user. The handle problem was partly overcome when a detachable wooden handle was patented in 1865. The weight of the hot iron was important as this helped with the flattening of fabric. The iron was made in different shapes and sizes for different purposes, such as pressing sleeves and hats. Box irons provided an option to the flat iron where a hollow section was incorporated into the design to receive either: • a piece of preheated iron • a lump of coal to keep the box iron hot. This type of iron was an improvement on the flat iron heated in the fireplace as it collected soot and had to be cleaned before use. 14 Household appliances Bellows were used to blast air into the box and keep the coals hot. An outlet was included to allow the smoke from the coals to escape from the box. Figure 1.9 Box iron and bellows What effect do you think the escaping smoke could have on the ironing environment? __________________________________________________________ __________________________________________________________ Did you answer? • the coal smoke would make the clothes smell after ironing The self-heating irons The next development in iron technology were self-heating irons. They were fuelled with natural gas, gasoline, or alcohol that often exploded. The electric iron was patented in 1882 but it didn't become popular until electricity became available in homes. Figure 1.10 Sectioned-view of a self heating iron Part 1: Development of household appliances 15 The diagram above shows a very early electric iron that has been sectioned to allow you to see the heating lamp. Heating elements were developed later. Also note that timber was still used for the handle. The electric irons Early electric irons were similar to ‘sad irons’. As time went by polymers were developed and instead of the wooden handles a material called Bakelite was used. Ironing was now starting to become faster and easier due to weight reduction and manufacturers paying more attention to the style of the products. Westinghouse, a major manufacturer, designed a streamlined iron that started a trend. The Bakelite handle was designed to fit a woman's hand. Different temperature settings were available for various fabrics. The steam iron Sunbeam produced a steam iron called the Steam-0-Matic. The use of steam, rather than the weight of the iron was the major factor in removing wrinkles from the clothes. Figure 1.11 Steam iron © Goldman Ruben, S. 1998, Toilets, Toasters & Telephones, Hardcourt Brace & company, Florida, p63 With emphasis on steam rather than weight, alterative materials were used in the manufacture of irons. Aluminium rather than iron meant that irons were becoming much lighter. Recent developments Today irons have been designed by engineers using the latest materials and manufacturing processes, incorporate a range of features to ensure: 16 • comfort • appearance • safety and ergonomics Household appliances Figure 1.12 A late model iron © Koninklijke Phillips Electronics N.V. 1 2 From the previous diagram, list two the functional features of the latest model iron. i ___________________________________________________ ii ___________________________________________________ Explain the benefits of one of these functional features. _______________________________________________________ _______________________________________________________ _______________________________________________________ 3 List new materials and explain the advantages of the use of these materials. _______________________________________________________ _______________________________________________________ _______________________________________________________ 4 List two safety features that are in the latest model of the iron. i ___________________________________________________ ii ___________________________________________________ Part 1: Development of household appliances 17 Did you answer? 1 i ii temperature control steam spray function 2 The temperature – can be change so as to not damage the materials to be ironed. 3 Plastics have decreased the weight of the iron and because they do not conduct heat well they are safe to use 4 i ii auto turn off – if the iron is accidentally left on it will turn itself off plastic body ensures the operator is safe from electric shock The refrigerator Refrigeration is the process of lowering the temperature of a substance. It is a common method of food preservation. The time-line below gives you an overview of the main historical developments of the refrigerator. Notice how the materials and shape has changed through the years. Figure 1.13 Evolution of the refrigerator Early ice boxes The icebox was the first household appliance used for refrigeration. The ice manufacturing factory produced the ice and it was delivered to businesses and houses by cart. Early ice boxes used wood for the cabinet, sawdust for insulation and tin or zinc as lining. 18 Household appliances Figure 1.14 Early ice boxes Early model refrigerators The first refrigerated storage machines were developed during the early 1900s. The machines were very large, steam driven, manually operated and sometimes leaked ammonia. The first refrigerators and freezers driven by electricity were developed in the 1920s and the 1930s. It was not until the late 1940s that the first researchers successfully scaled down the machines to a size suitable for shops. Figure 1.15 Early 1920s model refrigerator In the 1950s and 1960s a further development took place. Frost-free refrigerators became available. These were more efficient and further reduced the time required in household maintenance of the machine. Part 1: Development of household appliances 19 Over time, research and development vastly improved the machines, but by the 1960s it was evident that the Chloro Fluoro Carbons (CFCs) had a harmful effect on the earth’s ozone layer and, therefore, the environment. By the 1970s new refrigerants had been developed which were very energy efficient. All CFCs were eliminated from the machines. In 1992, researchers developed ‘Green freeze’, a hydrocarbon refrigerant. This material is now becoming the coolant most commonly used. Late model refrigerators Late model refrigerators can come equipped with a computer within the door. This computer has a large screen and key pad situated on the door front that allows Internet ordering of food and other functions. Figure 1.16 Computer refrigerator The bread toaster In the eighteenth-century English people made toast in their fireplaces with a rack called the hanging griller. Another tool was the salamander, a metal disk with a long handle or simply used long handled forks to toast their bread. Since the time of Thomas Edison, engineers have been using wire as an element. When used for lighting, the wire is sealed in a vacuum or surrounded by an inert gas. This prevents the element from oxidising or burning. The toaster element had to perform the heating task in open air. 20 Household appliances The basic principle for most toasters is cooking the bread by radiant heat. The heat is created by passing an electric current through a wire, known as an element. In 1905 an engineer called Albert Marsh applied for a patent on an alloy of Nickel and Chromium, which came to be known as Nichrome. The alloy can be described as being: • very low in electrical conductivity • very fusible • non-oxidising to a very high degree • tough and sufficiently ductile to permit drawing into wire. Consider that at this time electricity was not commonly wired into houses. The common wall power outlet was still only a dream for the future. The electric flip sided toaster General Electric, in 1909, made the first successful electric toaster called ‘D-12’. It was made from a wire rack and heating element attached to a porcelain base that toasted one side of bread at a time. There were variations of the early toasters, some were made with two doors while others had slots or perforated, decorative designs. Figure 1.17 Flip sided toaster © LMP One model from Universal had porcelain knobs that were cool to the touch. Westinghouse developed the ‘Turnover Toaster’ which turned the toast when you opened its doors. Part 1: Development of household appliances 21 Some manufacturers produced a ‘Combo Toaster’ that cooked toast on the table instead of on the stove. This could make coffee and toast at the same time. Hotpoint developed the ‘El Grillo Perc-O-Toaster’ oven which could cook eggs and fry bacon. Regulating time for cooking was a problem. People did not enjoy eating burnt toast or burning their fingers. This problem lead to an automatic one-slice pop-up toaster being invented in 1919. The automatic pop up toaster The first automatic pop up toaster for the home came in 1926. It was called the ‘Toastmaster’. The Toastmaster is considered to be the most popular appliance ever produced. Figure 1.18 Automatic up toaster © Goldman Ruben, S. 1998, Toilets, Toasters & Telephones, Hardcourt Brace & company, Florida, p40 Functional features of the automatic toaster included: 22 • The motif design on each side of the toaster served a purpose. It took attention away from any scratches or dents on the chrome surface. • A manual temperature control to make toast light, medium or dark and automatic pop up mechanism. • It was sleek, a simple shape, shiny chrome, rounded corners, and had horizontal lines. Household appliances Identify possible safety problems in the automatic Pop up toaster. __________________________________________________________ __________________________________________________________ Did you answer? Some of the safety issues you may have identified include: • potential burns risk resulting from the metal housing which conducts heat • potential fire risk resulting from an inaccurately adjusted manual temperature or malfunctioning automatic pop up. Today’s toasters are designed for many consumer benefits Feature Benefit spring-loaded tray pops toast up polymer parts safe to touch, convenient to use nichrome wire wrap good conductor heats to red hot hinged/removable crumb tray easy clean, hygienic electric-cord storage tidy, safe storage wide double/single slot accommodate variable sized slices time release lever select degrees of browning temperature sensor consistent browning electronic safety cut-out switch safe to operate automatic switch off mechanism prevent overheating Part 1: Development of household appliances 23 A late model toaster Figure 1.19 Late model toaster Improvements on today's toasters include: four or six slots, warming racks for heating croissants and slide-out trays for cleaning. The materials used in the appliance include: 24 • polypropylene for top and side housing • chrome-plated steel for the top plate • polyvinyl chloride (PVC) for the cord set • polycarbonate for the crumb tray. Household appliances Developing your research skills Research is a critical function for professional engineers. You will be refining your research skills by researching the history of household appliances in preparation for your engineering report. If an engineer is going to produce a new household appliance, most certainly a research process will be implemented. Research To do research involves a process or a series of linked activities moving from beginning to end. The research process is not absolutely rigid. However, there is a sense in which the research process will be weakened or made more difficult if the first steps are not executed carefully. Process 1 Clarifying the issue During Phase 1 the researcher clarifies the issue to be researched and selects a research method(s). This may require selecting sample materials, experimentation, working collaboratively with others. 2 Collecting data During phase 2 the researcher collects evidence about the research question. Sources such as the Internet, CD-ROM, encyclopaedia, specialist text, journals are all locations where information can be gathered. NOTE: Care must be taken when gathering information from the Internet. Check for authenticity by checking whether the source is qualified, it cannot be assumed that the person(s) who placed the information on the Internet are authorities on the subject. Check the site is fully maintained by a reliable source such as a University or large organisation. Part 1: Development of household appliances 25 3 Analysing and interpreting information During phase 3 the researcher relates the evidence collected to the research question asked, draws conclusions about the question and acknowledges the limitations of the research. Reminder In ‘Developing your research skills’ you have learnt the basic skills for researching. You should now select a household appliance and begin to investigate the development of the product from the earliest to the latest model. This will help you prepare for the first section of the engineering report - the background information on your chosen appliance. Turn to the exercise sheet and complete exercise 1.1 to 1.6. 26 Household appliances Exercises Exercise 1.1 a b List an invention or innovation that occurred in the period 1750 to 1900 in the following areas: • materials ____________________________________________ • transport ___________________________________________ • tool making __________________________________________ • chemical knowledge ___________________________________ • communications ______________________________________ • power sources _______________________________________ • food and agriculture ___________________________________ List an invention or development in the following areas that has occurred since the 1900s: • material _____________________________________________ • transport ___________________________________________ • communication _______________________________________ • power sources _______________________________________ Exercise 1.2 Choose three of the inventions or innovations listed in Exercise 1.1 and describe the effects that these inventions have had on people’s lives. a _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ Part 1: Development of household appliances 27 b _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ c _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ Exercise 1.3 Compare an early food mixer, to a modern mixer. The one you have at home will be fine for a comparison, even if it is 15 years old. In your comparison, comment on the following aspects: a materials _______________________________________________________ _______________________________________________________ _______________________________________________________ b safety _______________________________________________________ _______________________________________________________ _______________________________________________________ c easy of use _______________________________________________________ _______________________________________________________ _______________________________________________________ 28 Household appliances Exercise 1.4 Explain some of the disadvantages of the early ice box. In your answer examine four of the following areas: • ease of use • running costs • safety • operating systems • the storage capacity • the environmental and social effects. a _______________________________________________________ _______________________________________________________ _______________________________________________________ b _______________________________________________________ _______________________________________________________ _______________________________________________________ c _______________________________________________________ _______________________________________________________ _______________________________________________________ d _______________________________________________________ _______________________________________________________ _______________________________________________________ Exercise 1.5 a What recent technology has been incorporated into the latest model refrigerator to reduce the environmental effects of CFCs? _______________________________________________________ _______________________________________________________ _______________________________________________________ b Identify four safety features on the latest model fridge. You may need to research this information at an electrical store or on the Internet by typing in some of the well known brand names of refrigerators. i ___________________________________________________ ii ___________________________________________________ iii ___________________________________________________ iv ___________________________________________________ Part 1: Development of household appliances 29 Exercise 1.6 List three features of a late model electric toaster and outline the benefits of each. Feature 30 Benefit Household appliances Progress check During this part you examined the development of a range of household appliances, such as the food mixer, floor cleaner, clothes iron, refrigerator and toaster. ✓ ❏ Disagree – revise your work ✓ ❏ Uncertain – contact your teacher Uncertain Agree – well done Disagree ✓ ❏ Agree Take a few moments to reflect on your learning then tick the box that best represents your level of achievement. I have learnt about • historical and societal influences by studying; – the historical developments of household appliances – the effects of engineering innovation on people’s lives. I have learnt to • outline the historical development of household appliances • describe the effect of engineering innovation on people’s lives. Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to <http://www.boardofstudies.nsw.edu.au> for original and current documents. During the next part you will investigate a range of engineering materials commonly used in household appliances. Part 1: Development of household appliances 31 32 Household appliances Exercise cover sheet Exercises 1.1 to 1.6 Name: _______________________________ Check! Have you have completed the following exercises? ❐ Exercise 1.1 ❐ Exercise 1.2 ❐ Exercise 1.3 ❐ Exercise 1.4 ❐ Exercise 1.5 ❐ Exercise 1.6 If you study Stage 6 Engineering Studies through a Distance Education Centre/School (DEC) you will need to return the exercise pages with your responses. Return the exercise pages with the Title Page cover attached. Do not return all the notes, they should be filed for future reference. If you study Stage 6 Engineering Studies through the OTEN Open Learning Program (OLP) refer to the Learner’s Guide to determine which exercises you need to return to your teacher along with the Mark Record Slip. Part 1: Development of household appliances 33 Household appliances Part 2: Household appliances – materials Part 2 contents Introduction ..........................................................................................2 What will you learn? ...................................................................2 Appropriate selection of materials....................................................3 Material descriptions ..................................................................4 Material comparisons .................................................................5 Properties of materials................................................................7 The atomic structure of material ................................................ 10 Bonding of material .................................................................. 11 Metals ..................................................................................... 16 Polymers ................................................................................. 24 Ceramics................................................................................. 27 Developing your research skills...................................................... 31 Exercises............................................................................................ 33 Progress check.................................................................................. 39 Exercise cover sheet ........................................................................ 41 Part 2: Materials and household appliances 1 Introduction Of particular interest to the engineer is the development of materials and how this has affected the appliance design. In this part you will be examining material classification, properties and the structure to gain a good understanding about the appropriate selection of materials. What will you learn? You will learn about: • classification of materials • properties of materials – physical and mechanical • the structure and bonding of materials • the types of metals suitable for cutting and joining methods • the types of polymers including thermopolymers and thermosets • ceramics and the types used in household appliances. You will learn to: • distinguish between and explain reasons for the use of ferrous and non-ferrous metals as components of household appliances • compare the suitability of joining and cutting methods used on metals • distinguish between thermopolymers and thermosets • identify the types of ceramics used in household appliances. Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to <http://www.boardofstudies.nsw.edu.au> for original and current documents. 2 Household appliances Appropriate selection of materials Material selection is critical in household appliances. The design engineer, when selecting a material for a particular purpose, must consider many things. Some of these considerations are summarised in figure 2.1. safety durability ergonomics performance Functionality ease of manufacture short term Availability Workability ease of repair long term Selecting a material feel initial cost Cost Aesthetics look manufacture cost energy efficiency Environmental impact Special properties thermal pollution disposability sustainability Figure 2.1 radioactivity electrical chemical Materials selection chart When designing, the engineer will find it convenient to have engineering materials classified into groupings for quick reference. Part 2: Materials and household appliances 3 Material descriptions There are a number of classification methods for identifying and grouping materials. Some methods are shown below. 4 Material Description Metals Normally exhibit properties such as: good conductors of heat and electricity; high density; display some formability (ductile or malleable); normally solid at room temperature, for example, iron, copper, gold. Ceramics Inorganic, non-metallic solids processed or used at high temperature. As well as the common pottery, sanitary white ware, tiles and the like, there are ‘high tech’ applications which are used extensively in industrial applications. Polymers Generally known as plastics. They are based on the chemistry of carbon and are generally excellent insulators and easily moulded into complex shapes. Natural Materials that are found naturally in the environment. They require little modification before use, for example, stone, gold. Biological A sub-set of natural materials that only includes materials that are produced from living things, for example, wood, leather. Conductors Materials that allow relatively easy transmission of heat and/or electrical current. Semi-conductors Materials that form a special category of inorganic, non-metallic solids processed at high temperature. They are insulators, but, with minute amounts of additional elements, can be made to conduct electrons in specific circumstances. They are the building blocks of transistors, solid state electronics and computers. Insulator Materials that resist the transmission of heat and/or electrical current. Household appliances Material comparisons The following tables compare the use of material in older appliances with recent appliances. This should be a useful guide for when you investigate your own appliance later in this module. Materials used to manufacture vacuum cleaners Outlined below is a comparison of the materials used in an early model vacuum cleaner with a late model vacuum cleaner. Early model vacuum cleaner Late model vacuum cleaner Steel – Steel – handles, frame members, nuts bolts, fan blades, wheels and motor parts. Polymer – limited use – electrical insulation and some body parts. Copper – specifically for electrical conductivity for wires and motor parts. Other materials – cloth dust collection bag, brass zipper, chrome coated badges, rubber belts, cloth packing in electrical cord. Part 2: Materials and household appliances nuts and bolts, motor parts. Huge reduction from 1950 model. Polymer – extensively used for: • electrical insulation • integrated body parts • nuts and bolts • wheels and tyres • brushes and hoses. Copper – specifically for electrical conductivity for wires and motor parts. Other materials – disposable paper dust bags, semi-conductor materials in electronic components. 5 Materials used to manufacture clothes irons The following table provides a comparison of the materials used in an early electric clothes iron and a later model electric clothes iron. Early model clothes Iron Late model clothes Iron • Steel – fittings • Stainless steel – base • Cast iron – base • Polymer – PVC electrical insulation • Polymer – Bakelite electrical fittings • Copper – electrical wire • Copper – electrical wire • Ceramic – electronic components • Cloth – electrical insulation Materials used to manufacture refrigerators The following table provides a comparison of the materials used in an early model refrigerator with a late model refrigerator. Common materials Early model pressed metal fridge Common materials Late model computer fridge Steel – most panels/shelving Steel – most panels Polymer – Polymer – rare – electrical components (bakelite) /door seals common – handles/door seals/drawers/shelving electrical components Insulation for heat transfer 6 Copper – Copper – all electrical components including the cooling coils and the motor components all electrical components including the cooling coils and the motor components Other material – Other material – CFC’s/ freon blended refrigerant called R406 Household appliances Materials used to manufacture toasters The following table provide a comparison of the materials used in an early toaster and a late model toaster. Early model toaster Late model toaster Steel – Steel – most of the toaster body and frame many of the body parts and attachments Polymer – Polymer – rare – in early models the thermosetting polymer ‘Bakelite’ was used in electrical plugs and electrical insulation situations. common – mainly in the electrical components, handles and as a base for the enamel paint Copper – Copper – used for all electrical connection from the wall to the element no change as all electrical components and wiring Other material – Other material – porcelain, bakelite semi-conductor, found in electronic components Properties of materials The ability of an engineer to select appropriate material is critical. Selection needs to be based on the engineering properties of the material. The service requirements of a material may involve properties which fall in one or all of the following three categories. Mechanical Properties Physical properties Chemical properties • strength • electrical conductivity • resistance to corrosion • elasticity • thermal conductivity • stability • toughness • relative density • toxicity • resistance to creep • melting point • resistance to fatigue • coefficient of expansion • frictional properties • magnetic properties • hardness Part 2: Materials and household appliances 7 Common engineering materials In order to select the most appropriate material you must be familiar with the properties of that material. You should be able to nominate its properties, including its strengths and weakness in certain situations. Examples of common engineering materials that you will need to become knowledgeable about include: • steel • ceramic • thermosoftening polymer • thermosetting polymer • cast iron • aluminium • brass • copper • glass. Characteristics of materials The material properties that describe characteristics of interest to the engineer when selecting materials include: Density: a measurement of a materials mass per unit volume. Metals are usually dense. Non-metals are usually less dense. Formability: a description of a materials’ ability to be deformed or shaped by bending, stretching, compressing. Compressive force: describes a force applied to an object that attempts to compress the object. Tensile force: describes a force applied to an object that attempts to stretch the object. Opacity: the property of a material that stops the passage of light that is, you cannot see through it. Strength: the property of a material to withstand forces. Normally strength is specified as yield strength, tensile strength, compressive strength, shear strength and so on. 8 Household appliances Elasticity: the property of a material to return its original shape after a distorting force has been removed. Toughness: the property of a material to withstand application of impact force without failure. Stiffness: the property of a material to maintains shape. Resistance to creep: the property of a material to maintain its original length for a long period of small load application. Frictional properties: the property of a material that describes the amount of grip a material has when sliding in contact with another surface. Hardness: the property of a material to resist indentation or scratching when brought into contact with another material. Electrical conductivity: the property of a material to conduct electricity. Thermal conductivity: the property of a material to conduct heat. Relative density: a measurement for material describing how compact the mass of the object is relative to other materials. Melting point: the temperature at which the material begins to become a liquid. Coefficient of expansion: a measurement that describes the change in size of material relative to the temperature. Magnetic properties: the property of a material to become magnetic. Resistance to corrosion: describes the property of a material that allows it to not corrode quickly in a service application. Toxicity: describes how harmful a material is to the environment, due to the effect on things in that environment. Stability: describes how a material reacts to external changes. Turn to the exercise sheet and complete exercise 2.1.and 2.2 Part 2: Materials and household appliances 9 Atomic structure The properties if materials are determined by their atomic structure. An element is a pure substance and is made up of one type of particle. A list of all known elements can be found in the Periodic Table. In this table all of the elements are represented by letters, for example H for hydrogen and O for oxygen. 1 2 H He Hydrogen Helium 1 Atomic number Li Be H Symbol Lithium Beryllium Hydrogen 3 11 4 5 Name 9 10 B C N O F Ne Boron Carbon Nitrogen Oxygen Fluorine Neon 13 12 6 14 7 15 8 16 17 18 Na Mg Al Si P S Cl Ar Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon 25 26 29 30 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton 19 20 21 22 23 24 46 47 49 50 Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon 72 78 79 81 82 Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Barium LANTHANIDES Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury Thallium Lead Bismuth Polonium Astatine Radon 88 89-103 104 112 113 114 115 116 117 Rf Db Sg Bh Hs Mt Uun Uuu Uub Uuq Uuh Uuo ACTINIDES Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Ununnilium Unununium Ununbium Ununquadium Ununhexium Ununoctium 87 Fr Ra Francium Radium 57 58 105 59 106 60 107 61 108 62 77 109 63 110 64 111 65 80 66 67 68 83 69 84 70 85 86 57-71 Cs 56 76 54 Caesium 55 75 53 36 Zirconium 74 52 35 Y 73 51 34 Yttrium 41 48 33 Sr 40 45 32 Strontium 39 44 31 Rb 38 43 28 Rubidium 37 42 27 71 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium 95 96 Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium 89 90 91 92 93 94 97 98 99 100 101 102 118 103 Figure 2.2 Periodic Table The particles that make up an element are called atoms and each atom will contain a number of subatomic particles. Protons, electrons and neutrons are the main subatomic particles. The protons are positively charged and the neutrons, no electrical charge, are found in the nucleus, or the centre of the atom. The electrons, negatively charged, orbit the nucleus in an outer layers called a shells. There are an equal number of protons and electrons in an atom so that it is electrically neutral. 10 Household appliances nucleus electron neutron proton Figure 2.3 Structure of an atom Bonding of materials Atoms rarely exist by themselves; it is uncommon to come by a single atom of an element. Single atoms are usually unstable and need to combine with others to form a stable substance or molecule, similar atoms form an element or when different atoms combine they form compounds. The joining together of atoms is known as bonding. There are two main categories of bond types: • strong bonds that are a result of the transfer of valence electrons. Such bonds are referred to as intramolecular bonds or primary bonds. Examples include ionic, covalent and metallic bonding • weak intermolecular bonds are caused by the attraction between positive and negative parts of molecules that are close to one another. Such bonds are sometimes referred to as secondary bonds and can include hydrogen bonding and van der Waals forces. Part 2: Materials and household appliances 11 Table of Comparative Bond strengths Intramolecular bonds Bond Type Typical bond strength (KJ/mol) Ionic 1000 covalent 300 Metallic 300 hydrogen bonding 30 Van der Waals 10 Intermolecular bonds Intramolecular bonds or primary bonds Ionic bonding is the electro-static attraction between oppositely charged ions. For example a sodium atom (Na) has 11 electrons orbiting the nucleus in three shells. The inner most shell can only contain 2 electrons and is said to be full. The next shell contains 8 electrons and is also full. This leaves one electron in the outer most electron shell. The Na has a desire to have a completed outer shell therefore it loses the outer valence election and becomes positively charged. When an atom becomes charged it is known as an ion. + Na Na Sodium (Na) Sodium ion (Na+) + e– Electron Figure 2.4 Formation of a sodium ion Chlorine (Cl) has 17 electrons, 2 in the innermost shell, 8 in the next shell and 7 in the outermost shell. The Cl finds it easier to complete the outer shell by gaining an electron rather than casting away the 7 electrons. The Na atom can supply this electron. By adding an extra electron the Cl becomes a negatively charged ion. 12 Household appliances – e– Cl Electron Chloride ion (Cl–) + Cl Chlorine (Cl) Figure 2.5 Formation of a chloride ion With the formation of these ions there develops an electrostatic attraction between the ions. + – Na Cl Figure 2.6 Attraction between 2 oppositely charged ions In reality this process occurs many times, and many Na+ and Cl– ions are formed so the electro-static attraction is shared between all of the ions. This arrangement forces the ions into a particular ordered pattern, such that there are 6 positive ions surrounding each negative ion. This is known as a crystal. Cl Na Cl Na – + – + Na Cl Na Cl + – + – Cl Na Cl Na – + – + Na Cl Na Cl + – + – Figure 2.7 Sodium Chloride crystal Do you think that the ions can move about freely? No – the ions are firmly held in place. Ionically bonded materials are typically salts in the solid state and have high melting points. Covalent bonding – involves the sharing of the outer valence electrons. Classical examples of covalently bonded atoms are with hydrogen, oxygen and carbon. Part 2: Materials and household appliances 13 The hydrogen atom has only 1 electron in its outer shell but has a desire to have this shell filled. Remember that this shell only requires 2 electrons to be filled. The Oxygen atom (O) on the other hand has 9 electrons, 2 in the inner shell and 6 in the outer shell. H O Figure 2.8 Hydrogen and Oxygen atoms To satisfy their requirements of full outer shells the O and H share their electrons. Actually the oxygen needs 2 hydrogen atoms. O H H Figure 2.9 Molecule of H2O Covalently bonded materials therefore share their outer valence electrons. Polymers are typically covalently bonded materials and are excellent thermal and electrical insulators as there are no free electrons to conduct current. Make a sketch showing how carbon and hydrogen could be covalently bonded? Metallic bonding as the name suggests is typically found in metals and in many ways is the simplest to understand. The metallic atoms simply discard their outer valence electrons, those electrons in the outermost shell. These then form a cloud of electrons that are shared with all of the positively charged metallic ions. metallic ion sea of delocalised electrons two-dimensional representation three-dimensional representation Figure 2.10 Metallic Bonded materials 14 Household appliances The electrons are free to move about this matrix and are not held in fixed positions. These electrons are called delocalised. Metals are conductors of heat and electricity as the electron cloud is not fixed and is free to move. Intermolecular bonds or secondary bonds These are weak bonds normally caused by attraction between positive and negative parts of molecules that are close to one another. Look closely at the H2O molecule that is described in the covalent bonding section above, the O atom attracts the shared electrons more strongly than the H atom. This will mean that a slight negative charge will develop on the O atom and a positive charge on the H. This will mean that the water molecule will be polar, it has a slightly negative O end and a slightly positive H end. d– O H d+ H d+ Figure 2.11 Water molecule showing polarity The importance of this polar nature becomes apparent when the water molecule changes state from liquid to solid. With the reduction in thermal energy the polarity of the molecule forces it to arrange itself in an ordered (crystal) manner. d+ – d d+ d– d+ – d Figure 2.12 Ice – showing secondary bonds This is a dramatic and common example of secondary bonding. Because of the special characteristics of hydrogen this type of bonding is known as hydrogen bonding. This is the strongest example of secondary bonding, other types of bonds are known as van der Waals forces and are generally very weak. Part 2: Materials and household appliances 15 Particle theory When considering the structure of matter it is sometimes easier to think of it as consisting of a series of particles. These particles are usually found in one of three states of matter – gas, liquid or solid. As a solid, the particles can arrange themselves randomly or in definite repeating patterns. Materials that form repeating patterns are known as crystalline and those that do not are non-crystalline or amorphous. Metals are crystalline, as the atoms form into one of three repeating patterns, hexagonal close packed (HCP), face centred cubic (FCC) or body centred cubic (BCC). Hexagonal close packed Face centred cubic Body centred cubic Figure 2.13 Metallic crystalline structures Metals Are you familiar with the term ferrous and non-ferrous metals? Write down an example of a ferrous and a non-ferrous metal. __________________________________________________________ __________________________________________________________ Did you answer? 16 Ferrous non ferrous • steel • copper • cast iron • aluminium Household appliances Ferrous metals Ferrous metals are metals that are primarily iron, with small proportions of other materials. The words iron and steel are commonly used in everyday language but their links to each other are not always understood. Iron (Fe) by itself is an element, but when it is mixed with a small amount of carbon it becomes an iron alloy known as steel. It is not known when or where people first made iron from iron ore, but it is believed that crude weapons and ornaments were made from the iron found in meteorites about 4000 BC. The process of making iron from iron ore developed in different parts of the world and by about 1000 BC, advanced civilisations were making iron. The production of steel started in the early 1800s but it was not until the late 1800s that steel could be manufactured in large, inexpensive quantities. Steel making technology developed rapidly during the 1900s. Types of steels All steels are composed of iron (Fe) and small amounts of carbon (C) – less than 2%. If carbon is present in greater quantities the material is called cast iron. Only those steels that have another element added, to give the steel special qualities, are called alloy steels for example stainless steel. Stainless steels are special steels containing elements that promote a resistance to corrosion. For example, household appliances such as kettles, toasters and pans are made with alloy steel. Figure 2.14 Stainless steel household appliances Part 2: Materials and household appliances 17 There are a number of methods for classifying steels. They can be by the way they are used or by composition. Classification of steels by use: • Dead mild steel – 0.07%C to 0.15%C, soft steel that can be severely cold worked • Mild steel – 0.15%C to 0.25%C, relatively soft, can be welded, difficult to heat treat • Medium Carbon Steel – 0.25%C to 0.55%C, these steels can have their properties altered by various heat treatment procedures. • High Carbon steels – 0.55%C to 0.9%C, used when high strengths or good wear resistance are required. • Carbon tool steels – 0.9%C to 1.6%C, used for cutting tools Classification of steels by composition: Hypo-eutectoid steels – 0.0008%C to < 0.83%C, these steels exhibit the phases ferrite and pearlite. Ferrite is a soft material but as the quantity of carbon increases so does the amount of pearlite. • Eutectoid steels – these steels are very strong and contain 100% pearlite • Hyper-eutectoid steels – > 0.83%C but usually less than 1.6%C; the pearlite phase is present however the development of a cementite phase is becoming evident. Non-ferrous metals Non-ferrous metals are metals that contain no iron or only very small quantities Examples of non-ferrous metals include: • Copper Copper (Cu) is refined from copper sulphides (Cu2S or CuFeS2). It is a reddish brown metal, which is malleable, ductile with high electrical and heat conductivity and is highly corrosion-resistant and takes a high polish Copper is used in the form of wire and strip for electrical appliances and conductors and in the form of sheet and tube for heating appliances. Common alloys of copper include the brasses (Cu –Zn) and bronzes (Cu – Sn). Brasses can be used for tube and wire, condenser tubes, marine propellers, switch gear and brazing materials, to improve the machinability of the brasses it is usual to include a small quantity of 18 Household appliances lead (Pb). Bronzes are commonly used for coinage, water fittings and bearings. • Aluminium Aluminium is refined from bauxite (Al2O3.nH2O). It is a very light metal, low specific gravity but has very good conductive capacity. It is an extremely good conductor of electricity and is used extensively for power cables. The low strength aluminium needs to be reinforced with a steel core for this purpose. Aluminium has excellent corrosive properties as well, as it readily forms an inert oxide when exposed to the atmosphere. Alloys of aluminium are often used in the aircraft industry and the automotive industry. Lead, zinc and tin are other examples non-ferrous metals that are commonly used in industry. Joining metals There are many joining techniques. During this part of the module, several traditional joining methods are described. Bolts, rivets and screws cause little disturbance to the metallurgy of the base metal whereas soldering, brazing and welding are methods of permanently joining metals together and can have a profound effect on the metallurgy of the materials that are being joined. Nuts and bolts Bolts can be secured by the addition of a nut or they can be screwed directly into a material. They are often used as a non-permanent method of fixing two or more surfaces or parts together and can be made from a range of materials depending on their application. Solid rivets A solid rivet is a very old fashioned method of joining metal. You would find it difficult to locate one on a modern appliance. For this method you need to: • drill a hole in both pieces of metal to be joined • insert the rivet into the hole • burr the rivet • set the rivet. Part 2: Materials and household appliances 19 Figure 2.15 A solid rivet This method was replaced in most situations by the use of a Pop Rivet. For this technique you need to: • drill a hole • insert the pop rivet through the hole • set the rivet using special pliers. Figure 2.16 A pop rivet It is difficult to find a pop rivet in many modern appliances. Self-tapping screw Self-tapping screws are designed to be inserted into a drilled hole that is smaller than the screw or these screws may have a drill end that self drills the hole. As the screw enters the hole it cuts its own thread (cutting a thread in a hole is termed ‘tapping’). The thread enables the screw to be removed and reinserted. The screws are generally made from hardened steel. drill Figure 2.17 20 tap self-tapping screw Household appliances Welding Welding is described as the bonding of metals by the application of heat. Arc welding and gas welding result in the localized melting of the base metal while forge and resistance welding are a product of the localised application of pressure. This can cause significant changes to the structure of the base metal. Two methods that do not appreciably alter the structure of the base metal are soldering and brazing. Soft Soldering Soft soldering is a quick method of joining metals at relatively low temperatures (250 – 350°C). Solder is an alloy of tin and lead. The parts to be joined are firstly chemically cleaned with a flux. The components are heated, solder is introduced to the joint and is then allowed to cool and harden. The common use for solder in household appliances is for electrical joints, such as an electronic components joined to circuit boards. Brazing Brazing is known as hard soldering. It is performed at higher temperatures and produces a stronger joint. Brazing alloys commonly contain 50% to 60% copper – zinc alloys and melt in the 850 to 900oC range. Alloys of silver, copper and zinc are known as silver solder and melt in the 600–800oC range. The higher melting temperature requires the use of an oxy-propane or oxy-acetylene heat source. Other welding processes are carried out at significantly higher temperatures than brazing. These welding operations can be classified as either pressure welding or fusion welding. Pressure or Resistance welding In pressure (resistance) welding, the metals are heated but are not melted and pressure is applied to effect the weld. No additional filler metal is required. Resistance welding is a process of applying an electric current to materials that are held under spot pressure. Spot welding is an example of this technique. It is extremely quick and very suitable for sheet metal Part 2: Materials and household appliances 21 products. Electrodes clamp the two sheets and a short charge of electricity is applied. The resistance to the flow of this charge, caused by the sheet metals between the clamping electrodes, produces heat. The heat and pressure at this instant causes the materials to join. You may have seen robotic arms operating in automobile factories. They resistance weld car body parts together. In a variation to this process, the electrodes can also be in the form of rollers and a continuous seam can be made. Electric arc welding Fusion welding involves the localised melting of the base metal and the application of a filler metal, a wire or a flux-coated electrode. An electric arc is struck to generate the heat required to melt the base metal and a filler material is used to complete the weld. This type of welding forms an extremely strong bond between the metals that have to be joined but usually forms a scale on the outer surface of the weld, this scale has to be removed prior to a visual inspection of the welded zone. The formation of the scale can be minimized by the introduction of an inert gas shield at the weld point thus eliminating atmospheric oxidation at the weld surface. This method is known as metal inert gas (MIG) welding. If a tungsten electrode is used as the electrode tip with the inert gas then this is known as tungsten inert gas (TIG) welding. Can you think of any other metal joining techniques used on a household appliance? Cutting metals You may be aware of several ‘high-tech’ methods of cutting. Examples would be laser cutting and high-pressure water-jet cutting. At this stage it is important to consider the basics of cutting. Shearing Shearing cuts material, but achieves the cut without removing a waste strip. Snips are an example of a shear cut, often used to cut thin sheet metal. Thick sheet metal can be cut by machine operated blades. Normally only one blade moves. Industrial punches use the shear method of cutting. It has the advantage of a smoother cut edge, a quick cutting action and can be very efficient with less waste created. 22 Household appliances Sawing Sawing removes a waste section of the material. Hacksaws are an example. In this process, the edge being cut is not bent or distorted. Hacksaws are mainly used for cutting rods, bars and angles to required lengths and thick sheet metals to shape. Hacksaws can be hand held tools or machine operated. Blades are made from tungsten steel for the cutting the hardest material or from high-speed steel for general work. The sawing process is generally slow however, and is uncommon in the industrial situation. Flame cutting Using a gas-flame torch to cut ferrous metals is common. Oxyacetylene flames are the mostly used. Steel needs temperatures in the range of 2000°C for this process, and therefore you will not find this technique used to produce fine work in household appliances. The process is more suited to preparation of larger section objects. Drilling Drilling is the cutting of round holes in metal. This is done by rotating and feeding the required drill into the work. Drilling machines can be bench mounted (usually driven by a motor and belts) or hand held electric drilling machines. The bench drill capacity is normally limited to 13 mm diameter while the hand held drills usually have a capacity ranging from 1 mm to 13 mm. Turning The lathe is used mainly for machining circular surfaces, that is cylindrical or conical but can be used for producing flat surfaces, drill holes, machine slots, and for many more functions. Milling Milling machines have a rotating cutting wheel that spins. The work to be shaped is secured to a sliding table. The work is introduced to the cutting wheel, rather than the cutting wheel moving to the work. This system is very efficient when producing machined slots or flat surfaces. Part 2: Materials and household appliances 23 Grinding Grinding has similarities with milling, but instead of a cutting wheel, a disc of bonded ceramic is introduced to the work and the surface is ground away rather than cut. Polymers Polymers commonly known as plastics are manufactured materials. Polymers can be formed into any shape, size, colour and texture. We are surrounded with polymer products. Some polymers are woven into the clothes you wear while others line the saucepans in which you cook. Leo Baekeland developed the first artificial plastic in 1907. When working on experiments to develop a substitute for shellac he mixed phenol and formaldehyde and produced phenol-formaldehyde, later known as bakelite. The term polymer comes from the combination of the Greek words ‘poly’ and ‘menos’ meaning many parts. This is exactly how polymers are made. They are the combination of many simple hydrocarbons to form a large or long chain molecule. To fully understand the mechanics of forming polymers a basic understanding of organic chemistry is required. As previously stated carbon readily forms covalent bonds with itself and hydrogen. This can be seen in ethane as shown in figure 2.17. However, sometimes there are not enough hydrogen atoms present and the carbon forms a second bond with itself as shown in figure 2.17. These are termed double bonds and because of the stress contained in the double bond they are easier to break than a single bond, so they become the weak point for future attack. Double bond H H H C C H H H H Figure 2.18 24 C H Ethane (C2H6) H C H Ethene (C2H4) Ethane and Ethene (ethylene) Household appliances The process of joining the many ethylene molecules (mers) together is known as polymerization. H H C H H + C H Ethylene Figure 2.19 H C H C H Ethylene H H H H C C C C H H H H Polyethylene Polymerisation of ethylene There are two types of polymerisation: • addition polymerisation – this process proceeds without the production of a waste. The above example of the formation of polyethylene is an addition polymerization. • condensation polymerisation. – this process involves the production of a simple waste product, often H 2O or an alcohol is a by-product. Such an example, is the production of phenol formaldehyde, bakelite, where water is produced as the by-product. Other examples include the production of Dacron and Mylar where methyl alcohol is the by-product. Types of polymers All polymers can be placed into one of two categories: • thermosetting polymers, often called thermosets, once set cannot be reshaped • thermosoftening polymers can be re-formed. Structure of polymers The structure of a polymer will often determine its physical properties. Generally polymers will form many long chain like molecules, with strong covalent primary bonds along the chain but only weak secondary bonds between the chains. This will allow the chains to readily move, or slip over each other, thus the polymer will be exceptionally weak at holding its shape, as it will be only the entanglement of the chains that will allow them to hold their shape. This is not a desirable engineering property. In the polymerisation process there are a number of ways that the slippage of the chains can be minimised. Part 2: Materials and household appliances 25 Branching – by including branches on the primary chain, when a force is applied to the chains they are more likely to become entangled. C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C Figure 2.20 C C C C C C C C C Example of branching Cross-linking – by forming links across the chains then it is necessary to break a primary bond before deformation can occur. The more cross links that are formed then the more rigid the polymer. C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C Figure 2.21 C C Cross-linking Thermosetting polymers are sometimes considered as heavily cross linked polymers however in thermosets it is difficult to define the primary chain as a three dimensional structure, a network is formed which is extremely resistant to deformation. C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C Figure 2.22 Network structure characterized by thermosetting polymers Turn to the exercise sheet and complete exercise 2.3. 26 Household appliances Ceramics Ceramics are one of the most versatile and oldest engineering materials. Stone was possibly the first engineering material used by people. Ceramics include such common items as brick, porcelain and glass. They can be used in computer chips, high voltage insulators, grinding wheels, cements and synthetic diamonds. Look around the house and note the ceramic objects that you find. Because of the diversity of the items that you identified the method of classifying ceramics is not as straight forward as with metals and polymers. It is common to think of ceramic items as being those based on clay, however these do not include glass, alumina and cement. Ceramics are complex compounds of metals and non-metals and sometimes classification is best achieved by looking at their purpose. Clay bodies ceramics Clay results from the breakdown of rocks. Have you ever picked up a piece of clay soil? How would you describe the physical nature of the clay? Although there can be a great difference in the chemical composition of clay they all possess the following properties: • When clays are moist they are easily deformed (plastic) and if the moisture content is increased they can appear to be suspended in the solution. This is easily seen in ‘dirty water’ at flood times, the clay is suspended in the fluid. • As clays dry out they become rigid but will regain their plasticity if they are rewetted. • In order to fix their shape they need to be fired , that is heated to very high temperatures. This produces a strong, hard and permanently non-plastic material. The two most common clay minerals are kaolinite (Al2O3.2SiO2.2H2O) and montmorillonite (Al2O3.4SiO2.nH2O). You can see from their structure they are basically aluminium silicates with water attached and are extremely small. Part 2: Materials and household appliances 27 Classification of clay-bodied ceramics Clay bodied ceramics can be broadly classified as earthenware, stoneware or porcelain. Earthenwares are invariably a red-brown in colour and are fired at a relatively low temperature between 800 oC–900oC, they have high porosity usually 6–10% but may exceed 15% and are used for bricks, drainage pipes, pavers and ceramic filters. Stoneware bodies are lighter in colour, sometimes white, fired at higher temperatures 1100oC to 1200oC. Lower porosity between 1–2% makes them suitable for tableware and applications that preclude water absorption. Porcelains are usually white in colour and are fired at higher temperatures between1300–1450oC. This makes them less porous (<1%) therefore making them suited to use in scientific equipment but the fine texture of the clay body makes them suitable for the use in fine tableware. Can you tell the difference between a white-bodied stoneware and a porcelain? Porcelain is translucent. If you hold a porcelain plate up to the light with your other hand between the light source and the plate you should be able to see the outline of your hand. Glasses Glass rarely occurs naturally, although it can occasionally appear near volcanoes given the right conditions. The earliest examples of man-made glass were beads found in Mesopotamia and date back to 4 500 BC. Other early examples of glass have been found in Egypt (1 400 BC) and in Babylon (200 BC) but it was not until the 1500s that the glass making process became manageable. The structure of glass is amorphous (no crystalline pattern). It is this property that makes glass see through, you cannot see through crystalline objects. Silica is the most common glass former as it is easily cooled without crystallizing and forms the basis of all commercial glasses. Other materials are often added to the silica to enhance the network forming properties of the mix. These are known as intermediates and modifiers and act as fluxes or stabelisers. A common example is soda–lime–silicate glass, where the silica is the network former the soda 28 Household appliances (Na2) and the potash (K2O) act as fluxes and the lime (CaO), magnesia (MgO) and alumina (AlO) act as the stabelisers. Such glass is used for windows and bottles. It is not uncommon in the manufacture of window glass for impurities to be present in the mix, this can cause small regions to crystallize thus rendering them non transparent. These impurities can be quite small and are known as stones. Have a look at the windows about your house and see if you can identify any stones in the glass. Typically a stone can be smaller than 1 mm. Properties of ceramics Ceramic materials can display a wide range of properties but generally they have: • high hardness • the ability to withstand heat • the ability to resist chemical attack • no ability to conduct electricity. This makes them especially suitable for certain products, such as: • tableware • glass • electrical equipment • construction materials • abrasives. Tableware Due to a ceramics ability to resist chemical attack they can be used as containers for food and drinks. Some can withstand changes in temperature, making them suitable for refrigerator to oven use. Most ceramic tableware is made from a mixture of clay, feldspar and quartz. Glass Due to its transparency, glass is one of the most important materials. Food containers, light bulbs, windows and lenses are obvious examples but glass can transmit telephone calls and other information via optical fibres. Part 2: Materials and household appliances 29 Look in your cupboards, pantry or refrigerator and see how many containers for storage or cooking food are made from glass or ceramic material. Compare this to other types of containers. What are the other types of food storage and cooking containers generally made from? __________________________________________________________ __________________________________________________________ __________________________________________________________ Did you answer? • plastic • metal Electrical equipment Ceramics that do not conduct electricity are used as insulators in computers to electric power lines. These ceramics include alumina and porcelain. Another ceramic material, barium titanite, is used in making capacitors, which store electric charges in electronic equipment. Magnetic ceramics are used in electronic circuits and in electric motors. Construction materials Items such as cement, brick and earthenware drainpipes are all used in the construction of homes and building exteriors, while tiles, plaster for the surfaces of walls and ceilings, bathtubs, sinks, and toilets are used for the interior fittings. Abrasives Due to the hardness of ceramic materials they are used for cutting metals and for grinding, polishing and sanding. Examples include alumina and silicon carbide. Refractories The heat-resistant property makes ceramics suitable for refractories in heating furnaces. Refractory tiles even cover the surface of space vehicles, which must withstand the intense heat when exiting and reentering the earth’s atmosphere. 30 Household appliances Developing your research skills In Part 1 of this module you read about the research process. In Part 2 you will further develop your skills by researching information about the materials in the products that you will analyse for your engineering report. You will be doing this preliminary research in preparation for your final engineering report, so it will be a progressive learning task. You will then collate your research findings together for Part 5. As you should have completed the research of the historical development of your chosen household appliance or system it is now time to collect data on materials by examining the: • type of material in the product (for example, metals, ceramics and polymers) • classification of the materials • properties of the materials • structure of the materials • suitability of joining methods and cutting methods. You will also need to decide which experiments you are going to conduct and the type of data you are going to collect on your product. Turn to the exercise sheet and complete exercise 2.4 to 2.7. Part 2: Materials and household appliances 31 32 Household appliances Exercises Exercise 2.1 Rank in order of the selection criteria in developing washing machine components from 1 (highest) to 6 (lowest). Component Aesthetic control panel knob 3 Cost Electrical properties 1 body 1 motor bearings 1 6 Environmental Manufacturing Durability impact properties 5 2 4 electrical cord Exercise 2.2 a Select a household appliance. _______________________________________________________ b List some materials used to make the parts of the household appliance. i _____________________ v _______________________ ii _____________________ vi _______________________ iii _____________________ vii _______________________ iv _____________________ viii _______________________ Part 2: Materials and household appliances 33 Exercise 2.3 Describe with the aid of a sketch the structure and bonding of the following polymers. a Thermosetting polymer i Structure ___________________________________________________ ___________________________________________________ ii Bonding ___________________________________________________ ___________________________________________________ iii Sketch b Thermosoftening polymer i Structure ___________________________________________________ ___________________________________________________ ii Bonding ___________________________________________________ ___________________________________________________ iii Sketch 34 Household appliances Exercise 2.4 Complete the following table. Metal Melting Point Alloys with Uses • Steel • Structural steel • Copper • Coins • Silver • Cutlery • Gold • White Gold Aluminium °C in its pure form Copper °C in its pure form Lead 327 °C in its pure form Nickel 1455 °C in its pure form Tin °C in its pure form Zinc °C in its pure form State the references you used to complete the previous table. Internet sites: • _______________________________________________________ CD-ROM: • _______________________________________________________ Books: • _______________________________________________________ Part 2: Materials and household appliances 35 Exercise 2.5 List the three main engineering properties of each of the following engineering materials. a b c d e Steel i ___________________________________________________ ii ___________________________________________________ iii ___________________________________________________ Aluminium i ___________________________________________________ ii ___________________________________________________ iii ___________________________________________________ Thermosoftening polymer i ___________________________________________________ ii ___________________________________________________ iii ___________________________________________________ Thermosetting polymer i ___________________________________________________ ii ___________________________________________________ iii ___________________________________________________ Copper i ___________________________________________________ ii ___________________________________________________ iii ___________________________________________________ Exercise 2.6 a What is a ferrous metal? _______________________________________________________ _______________________________________________________ _______________________________________________________ b What is a non-ferrous metal? _______________________________________________________ _______________________________________________________ _______________________________________________________ 36 Household appliances c d Outline two advantages of ferrous metals when compared to nonferrous metals. i ___________________________________________________ ii ___________________________________________________ Outline two advantages of non-ferrous metals when compared to ferrous metals. i ___________________________________________________ ii ___________________________________________________ Exercise 2.7 a List two shear cutting processes or machines. _______________________________________________________ _______________________________________________________ b Describe the difference between sawing and shear cutting. _______________________________________________________ _______________________________________________________ _______________________________________________________ c List three machine-cutting operations. _______________________________________________________ _______________________________________________________ _______________________________________________________ d Name the non-ferrous material commonly used to manufacture pop rivets. _______________________________________________________ _______________________________________________________ e State the advantage, when in service, that a non-ferrous pop rivet has compared to a ferrous rivet. _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________ f Explain why self-tapping screws are commonly made from ferrous metal. _______________________________________________________ _______________________________________________________ Part 2: Materials and household appliances 37 Exercise 2.8 a State three major properties that are often displayed by ceramics. i ___________________________________________________ ___________________________________________________ ii ___________________________________________________ ___________________________________________________ iii ___________________________________________________ ___________________________________________________ b c Identify two ceramic materials used in a household situation. i ___________________________________________________ ii ___________________________________________________ State the name of two ceramic products that have been in production for more than one hundred years. _______________________________________________________ _______________________________________________________ d State the name of one ceramic product that has only been in production within the past ten years . _______________________________________________________ 38 Household appliances Progress check During this part you examined a range of materials – properties and structure. ✓ ❏ Disagree – revise your work ✓ ❏ Uncertain – contact your teacher Uncertain Agree – well done Disagree ✓ ❏ Agree Take a few moments to reflect on your learning then tick the box that best represents your level of achievement. I have learnt about • classifying materials • physical and mechanical properties of materials • the structure and bonding of materials • the types of metals and suitable joining and cutting methods • the types of polymers including thermopolymers and thermosets • ceramics and the types used in household appliances. I have learnt to • distinguish between and explain reasons for the use of ferrous and non-ferrous metals as components of household appliances • compare the suitability of joining and cutting methods used on metals • distinguish between thermopolymers and thermosets • identify the types of ceramics used in household appliances. Extract from Stage 6 Engineering StudiesSyllabus, © Board of Studies, NSW, 1999. Part 2: Materials and household appliances 39 Refer to <http://www.boardofstudies.nsw.edu.au> for original and current documents. During the next part you will explore methods of analysing mechanical situations. 40 Household appliances Exercise cover sheet Exercises 2.1 to 2.8 Name: _______________________________ Check! Have you have completed the following exercises? ❐ Exercise 2.1 ❐ Exercise 2.2 ❐ Exercise 2.3 ❐ Exercise 2.4 ❐ Exercise 2.5 ❐ Exercise 2.6 ❐ Exercise 2.7 ❐ Exercise 2.8 Locate and complete any outstanding exercises then attach your responses to this sheet. If you study Stage 6 Engineering Studies through a Distance Education School/Centre (DEC) you will need to return the exercise sheet and your responses as you complete each part of the module. If you study Stage 6 Engineering Studies through the OTEN Open Learning Program (OLP) refer to the Learner’s Guide to determine which exercises you need to return to your teacher along with the Mark Record Slip. Part 2: Materials and household appliances 41 Household appliances Part 3: Household appliances – mechanics Part 3 contents Introduction .......................................................................................... 2 What will you learn?................................................................... 2 Principles of mechanics..................................................................... 3 Mass......................................................................................... 3 Force ........................................................................................ 4 Units ......................................................................................... 5 Scalar and vector quantities........................................................ 6 Equilibrants and resultants ....................................................... 16 Developing your research skills...................................................... 27 Exercises............................................................................................ 29 Progress check ................................................................................. 39 Exercise cover sheet........................................................................ 41 Part 3: Mechanics and household appliances 1 Introduction This part explains basic mechanical engineering principles and how they relate to household appliances. Concepts such as mass and force are defined. Quantities are defined as scalar or vector and two methods of determining the components of a force are explained. Further research methods are covered so that you can build up your skills for using technology to research the latest information on engineering innovation. What will you learn? You will learn about: • the fundamentals of engineering mechanics – by studying mass and force and how they relate to household appliances – how scalar and vector qualities are classified – methods of determining components of a force forces – nature and types of forces You will learn to: • use mathematical and/or graphical methods to solve engineering problems in household appliances • conduct research using computer technologies and other resources. – applies mathematical and graphical methods to solve problems Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to <http//ww.boardofstudies.nsw.edu.au> for original and current documents. 2 Household appliances Principles of mechanics Mechanics is the study and analysis of the effect of mass and force on objects. Engineering mechanics is the practical application of those principles. Mass Objects may differ widely in size and shape from one another, but they have two things in common, they all contain matter and occupy space. The mass of an object is the amount of matter that is contained within that object. The unit for measuring mass is the kilogram and is abbreviated to kg. The term tonne is used to indicate 1000 kg. The mass of an object should not be confused with weight. Weight is a term commonly used and is often stated in kilograms. However, this can be misleading in engineering terms. A mass of 100 kg is the same as if it is on the earth, on the moon or floating in space. However, if we used scales to determine mass, then the scales would give different readings on the earth, the moon and in space. This is because the scales are not measuring mass but a weight force, which is a force that is related to gravity. On the earth the acceleration due to gravity is 9.8 m/s/s, on the moon gravity is less therefore the acceleration due to gravity is less and in space the gravity is zero therefore the acceleration due to gravity is zero too. This does not mean that there is less mass just that the weight force is less. Part 3: Mechanics and household appliances 3 Force A force is defined as the interaction between bodies or a ‘push or pull’ in a given direction. All bodies are subjected to forces or systems of forces. Such forces will either keep the body from moving or cause it to accelerate. Newton’s Law states: 1 a body will remain at rest or continue in uniform motion unless acted upon by an external force 2 a body acted upon by an external force will accelerate in proportion to the magnitude of this force and in the direction of the force 3 for every action (force) there is an equal and opposite reaction (force). The basic formulae for determining values of force are: F = ma or F = mg F = force m = mass a = acceleration g = acceleration due to gravity Note the value for Earths gravity varies depending on the position on the Earth. It is generally measured as 9.8 m/s2, but often in calculations is rounded to 10 m/s2. Force is measured in Newtons (N). Basic forces There are four basic forces: 4 1 weight force – the attraction between the mass and the Earth (F = mg) 2 action force – the force applied to an object from outside the object (F = ma) 3 reaction force – the force applied to a body to balance an action force or weight force 4 friction force – the force set up between objects that tends to prevent or reduce motion. Friction is a special reaction force. Household appliances Units The International System of Units will be used in all aspects of this course. This system provides a coherent set of units that standardise all measures and defines a set of base measures that are all multiples of the power of ten. It is important that when using any formulae the base units of this system are used exclusively. The base units are shown in the following table. Quantity Unit Definition Time second s A second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. Mass kilogram kg A kilogram is equal to the mass of the international prototype of the kilogram. Length metre m The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. Thermodynamic temperature Kelvin K A Kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. Electric Current ampere A An ampere is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 Newton per metre of length. Luminous intensity candela cd The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. Amount of a substance mole mol 1 A mole is the amount of substance, which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. 2 When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. Part 3: Mechanics and household appliances 5 When using these base units it is recommended that you use only the prefixes listed below when dealing with multiples. Prefix Multiplication factor Symbol tera 1012 (1 000 000 000 000) T gega 109 (1 000 000 000) G mega 106 (1 000 000) M kilo 103 (1 000) k milli 10-3 (0.001) m micro 10-6 (0.000 001) m nano 10-9 (0.000 000 001) n From the above: 10MN = 10 million Newtons = 10 x 106 N 65km = 65 x 103 m Scalar and vector quantities Quantities in engineering mechanics are divided into two groups, scalars and vectors. You will need to be able to differentiate between these two types of quantities. Scalar quantity A scalar quantity is one that requires only magnitude (size) amount for its complete definition, such quantities as time, distance, mass, length, area, volume and number are all scalars. Vector quantity A vector quantity is one that requires direction (sense) as well as magnitude for its complete definition. 6 Household appliances Both the direction and magnitude of these quantities must be used in the solution of an engineering mechanics problem. The following quantities are examples of vectors: • force • displacement • velocity • acceleration • momentum. Representation of vectors Vectors may be represented graphically by a straight line with its length scaled to represent the magnitude of the vector and its direction at the correct angle to a chosen reference or a compass direction. Figure 3.1 can represent a vector displacement of 10 metres north. 10 m N Scale W E 0 10 20 Metres S 10 m North Figure 3.1 Northerly vector Figure 3.2 represents a 5 N force to the right. 5N Figure 3.2 5 N force to the right Figure 3.3 represents a 7 Newton force to the right. 7N Figure 3.3 7 N force to the right Part 3: Mechanics and household appliances 7 Figure 3.4 represents a 20 N force that is inclined upward to the right at 30 o. N 20 30∞ Figure 3.4 20 N force inclined upward to the right at 30o Weight-force As mentioned earlier mass and weight are often confused. Weight is the force that a body exerts (downwards) due to gravity. To calculate the weight force we use: F = ma since a = g F = mg For example, calculate the weight force produced by a 1.2 kg kettle F = mg = 1.2 x 10 = 12 N Ø Would there be any change to the weigh-force if we added 2 litres of water to the kettle? (Note: 1 litre of water has a mass of 1kg) Yes! There is now a mass of 3.2 kgs so it will produce an equivalent weight–force of: F = mg = 3.2 x 10 = 32 N Ø Turn to the exercise sheet and complete exercise 3.1 and 3.2. 8 Household appliances Principle of transmissibility The principle of transmissibility states that a force can be applied anywhere along its line of action. The force has the same effect wherever its point of application is along its line of action. A pull of 20 N will have the same effect as a push of 20 N along the same line of action. Centre of gravity In order to simplify calculations it is possible to consider that the mass of an object be concentrated at a single point. The centre of mass of an object is a point where the mass is concentrated and will not alter the objects affect of inertia or the weight–force of the object. The centre of mass is usually considered to be the centre of the object. This is true with spheres, cubes and other simple prisms. Figure 3.5 Centre of gravity for simple shapes When considering more complex shapes the position of the centre of gravity is usually specified. Centre of gravity (C of g) C of g Figure 3.6 The centre of gravity for more complex shapes Part 3: Mechanics and household appliances 9 Adding and subtracting of vectors A person is attempting to slide a clothes iron using a force of 10 N 10 N Figure 3.7 One force pushing clothes iron on horizontal surface A second force provides an additional 7.5 N force in the same direction, the iron still does not move. 10 N 7.5 N Figure 3.8 Two forces pushing clothes iron on a horizontal surface The total force provided by the two forces can be calculated using the following analytical method: Total force calculated mathematically F (total) = F1 + F2 = 10 + 7.5 = 17.5 N Æ Therefore, the total of the two forces is 17.5 N, acting in horizontal direction towards the right. This is a resultant force, or one force that will replace the two forces. Why doesn’t the iron move? Is there another force operating? The conventions You may have noticed that both the forces in the previous example were in the same direction (horizontal) and both acted to the right. The total force was calculated by adding them together. 10 Household appliances What if one force was to the left and one force was to the right? Obviously, the final result of the forces would be different. But how do I show that mathematically? The answer is ‘by deciding on a convention’. A convention in this case is a decision regarding the definitions of a positive (+) and negative (–) force. It is a necessary tool used to describe a situation. The convention used in the previous example is that the forces, if acting toward the right, are considered positive. The convention would be indicated by: Horizontal So the calculation would be: S F = F 1 + F2 = (+10) + (+7.5) = 17.5 N Æ The resultant force is 17.5 N horizontally to the right. A force acting to the left, would be considered negative. Consider the application of a different set of forces on the iron. Determine the resultant. 15 N Figure 3.9 6N Opposing forces acting on an iron SF = F 1 + F2 = (15) – (6) = 9NÆ The resultant force is 9 N horizontally to the right. Part 3: Mechanics and household appliances 11 The graphical method – Vector diagrams Addition and subtraction of vectors can be accomplished by an entirely different technique. The forces are drawn to scale and the problems are solved using graphics. In the same situation as previously described a clothes iron is being acted upon by a force of 10 N. 10 N Illustration 10 N Vector diagram scale 10 mm = 2.5 N Figure 3.10 One force acting on a clothes iron, represented by a scaled horizontal line with an arrowhead showing sense Next, a second force provides an additional 7.5 N force in the same direction as the 10 N force. 10 N Illustration 7.5 N 10 N vectors added tip to tail 7.5 N Vector diagram scale 10 mm = 2.5 N Figure 3.11 Two forces acting on the clothes iron, shown graphically by two scaled horizontal lines, each with an arrow head indicating sense – the vectors are drawn tip to tail The total force would be measured. For example, 70 mm = 17.5 N. The same graphical technique can be used regardless of the direction of the forces (vectors). In the case above, all the vectors are horizontal. The senses of the forces (the arrow heads) were ‘both to the right’. 12 Household appliances When solving a problem graphically, simply draw each vector in its true direction and add the arrow head to show its sense. Add vector after vector, joining them end to end (more accurately, the vector origin end is joined to the arrow head end of the last vector). This may sound difficult, but it will become clearer as you complete exercises. Vector diagrams can only be constructed using vectors! When analysing an engineering system you will need to convert scalar quantities to vector quantities. For instance, all mass (scalar quantity) must be converted into force (vector quantity). That is, a 6.4 kg toaster will exert a force of F = mg downwards when acted upon by gravity. The force would calculated as: F = mxg Data F = 6.4 x 10 mass F = 64 N gravity = = 6.4 kg 10 m/s2 Force components In the last section, it was explained that vector forces could be added and/or subtracted, in either the graphical and analytical form. The examples only contained horizontal vectors (forces). When forces are applied to an appliance they can be horizontal, vertical or in fact at any angle. To solve problems where a number of forces are being applied, it may be convenient to determine the components of the force. A component of a vector is the effect of that vector in a specific direction. A vector can have any number of components in any number of directions. Very often we use components in directions at right-angles to one another to solve problems related to a specific situation. The directions most often used are the x horizontal and y vertical axes. For example, find the horizontal and vertical components of the 100 N force acting upwards to the right at 30°. Part 3: Mechanics and household appliances 13 Y 0N 10 30∞ O X Figure 3.12 100 N force at 30° – the X and Y axis are indicated OX and OY are the directions of the required components. Their magnitudes may be found either graphically if the diagram is drawn to scale or analytically using trigonometry. OX = 100 cos 30 = 86.6 N OY = 100 sin 30° = 50 N Worked example 1 Determine the horizontal and vertical components of a 100N force acting upwards to the right at 30° to the horizontal using a graphical technique. You can use the following approach. 14 Step 1 Select a suitable scale ( 1 N = 1 mm in this case) Step 2 Select a suitable starting point, labelled 0 for origin Step 3 Draw the vector (force) to scale in the position described. In this case that would be a 100 mm line, starting at the origin point and aimed upwards to the right at 30° to the horizontal. The vector must have sense and direction, so the arrow head must be placed on the upper end of the 100 mm line. Be accurate! Step 4 Draw a horizontal line going through the origin point and going out to the right. Step 5 Draw a vertical line from the arrow end of the original 100 N force vector until it crosses the horizontal line. You now should Household appliances see a triangle, formed between the original force, the horizontal line and the vertical line. Step 6 Indicate the horizontal and vertical components on the diagram by: a placing an arrow head on the horizontal line at the point it meets the vertical line to indicate the length (magnitude) of the horizontal component. b drawing an arrow head at the top end of the vertical line, as it meets the arrow end of the original force, to show the sense of the vertical component. The two components, when combined, start at the same origin point as the original 100 N force, and end at the same point as the original 100 N force. They achieve the same result. In this case, two forces could replace one given force, and the same result is achieved. 0N 10 O V H Figure 3.13 Graphical solution – H and V components Space diagrams and free-body diagrams Space diagrams can be an artist’s impression showing all of the details of a force system acting on a particular structure. Such a diagram will often contain much more information than is needed to solve the problem, this can lead to some confusion when trying to analyse the task. Free-body diagrams use the essential information contained in the space diagram but without the excessive detail. Generally they show the positions and the directions of all of the forces acting on the body but not necessarily to scale. The body or object that the forces are acting on can be shown as a point. Part 3: Mechanics and household appliances 15 15 N 6N 15 N Space diagram 6N Free body diagram Figure 3.14 Space and free–body diagram of the forces acting on the iron Equilibrants and resultants When a system of two or more forces is analysed, the effect of all the forces is described as the resultant force. All the forces are combined to determine the net effect. A resultant force can replace the original forces. When a force system has been analysed a resultant force is determined. To maintain, or bring about, a state of equilibrium, the system would require an equal force acting as a balance. The force that can achieve this balance is called the equilibrant. It is equal in all respects to the resultant with the exception of its sense. If a resultant is 10 N to the left, the equilibrant is 10 N to the right. If a resultant is 50 N upward at 30° to the right, the equilibrant is 50 N downward at 30° to the left. Solving force vector situations (using both mathematical and graphical techniques) is a fundamental and essential skill for engineers and will feature in subsequent modules. Worked example 2 Using mathematics to find the resultant of two (2) forces. F2 10 N 10 N F1 Figure 3.15 Addition of two forces F1 and F2 16 Household appliances Step 1 is to analyse the horizontal components ( forces in the –x direction) and the vertical components (forces in the –y direction) in each of the two forces, F 1 and F2. Step 2 F1x = 10 N F2 x = 0 F1 y = 0 F2 y = 10 N is to add the horizontal (–x direction) and vertical (–y direction) forces. SFx = F1x + F2x SFy = F1y + F2y = 10 + 0 = 0 + 10 = 10 N = 10 N This tells us that the resultant of the two forces has a horizontal component of 10 N in the positive direction and a vertical component of 10 N in the positive direction. Rather obvious in this case but in latter examples this may not be so. To determine the size and the direction of the resultant. From step 2 it is possible to construct the following force polygon. R es ul ta nt Step 3 SFy = 10 N Figure 3.16 Force polygon used to determine the resultant From Pythagoras: R2 = 102 + 102 R = 14.14 N Part 3: Mechanics and household appliances 17 Because the resultant is a vector it is necessary to include a direction. sin q = = opp hyp 10 14.14 = 0.7071 q = sin –1 0.7071 q = 45o The resultant is a force of 14.14 N upwards to the right at 45 o. This has been a simple example A graphical method can be used to solve this question and verify your answer. Did you answer? Resultant force 14N upward to the right at 45° Turn to the exercise sheet and complete exercise 3.3. Worked example 3 Using mathematics to find the resultant of two (2) forces. F1 = 100 N 45∞ F2 = 60 N Figure 3.17 Two force system Step 1 is to analyse the horizontal components (forces in the –x direction) and the vertical components (forces in the –y direction) in each of the two forces, F 1 and F2. cos 45 = 18 F1x 100 F1x = 100cos 45 F1X = 70.71 Household appliances sin 45 = Step 2 F1y/100 F1y = 100sin 45 F1y = 70.71 F2x = 60 F2y = 0 is to add the horizontal (– x direction) and vertical (–y direction) forces SFx = SFy = F1y + F2y F1x + F2x = 70.71 + 60 = 70.71 + 0 = 130.71 = 70.71 This tells us that the resultant of the two forces has a horizontal component of 130.71 N in the positive direction and a vertical component of 70.71 N in the positive direction. Step 3 To determine the size and the direction of the resultant. From step 2 construct the following force polygon. R u es lta nt 70.71 N 130.71 N Figure 3.18 Force polygon used to determine the resultant From Pythagoras: R2 = 130.712 + 70.712 = 17085.1 + 4999.9 = 22085 R = 148.6 N Part 3: Mechanics and household appliances 19 Because the resultant is a vector it is necessary to include a direction. sin q = = opp hyp 10.71 148.6 = 0.4758 q = sin-1 0.4758 = 28.4o The resultant is a force of 148.6 N upwards to the right at 28.4 o. A graphical method can be used to solve this question and verify your answer. Did you answer? Resultant is 150 N upward to the right at 30° Turn to the exercise sheet and complete exercise 3.4. Worked example 4 Using mathematics to find the resultant of two (2) forces. F1 = 2 kN 60∞ F2 = 500 N 30∞ Figure 3.19 Two force system Step 1 20 is to analyse the horizontal components (forces in the –x direction) and the vertical components (forces in the –y direction) in each of the two forces, F 1 and F2. cos 30 = F1x/2000 F1x = 2000cos F1x = 732.1 N cos60 = –F2x/500 30 F2x = –500cos 60 F2x = – 250 N Household appliances sin 30 = Step 2 F1y/2000 sin 60 = F2y/500 F1y = 2000 sin 30 F2y = –500 sin 60 F1y = 1000 N F2y = –433 N is to add the horizontal (– x direction) and vertical (–y direction) forces SFx = SFy = F1y + F2y F1x + F2x = 1732.1 + –250 = 1000 + –433 = 1482.1 N = 567 N This tells us that the resultant of the two forces has a horizontal component of 1482 N in the positive direction and a vertical component of 567 N in the positive direction. Step 3 To determine the size and the direction of the resultant. From step 2 it s possible to construct the following force polygon. u lt Res ant 567 N 1482.1 N Figure 3.20 Force polygon used to determine the resultant From Pythagoras: R2 = 1482.12 + 5672 = 2196620.41 + 321489 = 2518109.41 R = 1586.9 N Because the resultant is a vector it is necessary to include a direction. (In previous examples the sin function was used to determine the angle of inclination but any trig function can be used.) tan q = = opp adj 567 1482.1 = 0.3826 q = 20.9o Part 3: Mechanics and household appliances 21 The resultant is a force of 1586.9 N upwards to the right at 20.9 o. A graphical method can be used to solve this question and verify your answer. Did you answer? The resultant is 1590 N upward to the right at 21° Turn to the exercise sheet and complete exercise 3.5. Worked example 5 50 N 60 N 30∞ 30∞ Figure 3.21 Two (2) force system What is the total resulting force created by the two forces shown? Analytical solution Step 1 Find the horizontal and vertical components of the 50 N force. 50 N 60∞ Vertical 22 Sin 60° = V 60 V = Sin 60° ¥ 50 = 0.866 ¥ 50 = 43.3 N ≠ Household appliances Horizontal Cos 60° = H 50 H = Cos 60° ¥ 50 = 0.5 ¥ 50 = 25 N Æ and find the vertical and horizontal components of the 60 N force. 60 N 30∞ Vertical Sin 30° = V 60 V = Sin 30° ¥ 60 = 0.5 ¥ 60 = 30 N ≠ Cos 30° = H 60 H = Cos 30° ¥ 60 = 0.866 ¥ 60 = 51.96 NÆ Horizontal Step 2 Add the horizontal (– x direction) and vertical (–y direction) forces. SFx = F1x + F2x S Fy = F1y + F2y = 25 + 51.96 = 43.3 + 30 = 76.9 6 N = 73.3 N This tells us that the resultant of the two forces has a horizontal component of 76.96 N in the positive direction and a vertical component of 73.3 N in the positive direction. Step 3 To determine the size and the direction of the resultant. From step2 it s possible to construct the following force polygon. Part 3: Mechanics and household appliances 23 R 73.3 N 76.9 N Figure3.22 Two (2) components of the resultant From Pythagoras: R2 = 76.92 + 73.32 = 5929 + 5329 = 11258 R = 106.1 N To find the resultant direction: Tan q = 73.3 77 = 0.9519 \ q = 43.6° Therefore the resultant is 106.1 N at 43.6°. Having determined that the resultant is 106.1 N 43.6° you know the one force that could replace the original two forces. If you had been asked to determine the one force that could ‘balance’ the system then you would be able to say 106.1 N 43.6°. This force is known as the ‘equilibrant’ it would balance the other two forces and bring the system into equilibrium. Note that it has the same magnitude (106.1 N) and the same direction ). (43.6°) but has the opposite sense ( not 24 Household appliances An ‘equilibrant’ creates equilibrium. and A ‘resultant’ is the total effect of the other forces. Now for a simpler way! Graphical solution Given the same problem, graphics can be used. 50 N 60 N 30∞ 30∞ Figure 3.23 Two (2) force system What is the total resulting force created by the two forces shown? Select a suitable drawing scale. In this case: 1 N = 1 mm 50 N = 50 mm 60 N = 60 mm Part 3: Mechanics and household appliances 25 scale: 1 mm = 1 N 60 N 30∞ R=? 50 N 60∞ 0 Figure 3.24 Vector diagram Select a starting point, call it ‘0’. Then: • draw a line through ‘0’ at 60° • measure 50 mm along this line • mark the position with an arrow head, this now represents the 50 N force • label the force 50 N • draw a line at 30° from the arrow head • measure a distance of 60 mm from the arrow head • place an arrow head at the 60 mm mark, this now represents the 60 N force • label the force 60 N • draw a line from the original ‘0’ position to the last arrow head to determine the resultant • measure the length of the line (106 mm), this determines that the magnitude of the resultant force is 106 N • measure the angle the resultant force is to the horizontal (44°), this determines the direction of the resultant. The final resultant force is 106 N 44°. Note that this is the same as in the analytical solution. This verifies that result. Note also that the two forces could have been drawn in any order. Either the 60 N or the 50 N force could start at the original position, the result would be the same. What do you think the magnitude, direction and sense of the equilibrant is? Remember it is the one force that could be added to create balance to the system. 26 Household appliances Developing your research skills The Internet is commonly used on a research tool however, care must be taken when the information you have gathered is from the Internet as this source has to be qualified. Using the Internet to research To help clarify the reliability of an Internet site, the Internet address or the Uniform Resource Locator (URL) can provide some clues. Take the following Internet address: http://www.det.nsw.edu.au The document with this address is: • in hypertext:// • on the World Wide Web • on the NSW Department of Education computer • an educational organisation • in Australia. With this type of information you can make a decision on the quality of the information found at this or any other Internet address. When you access information that is contained on a Compact Disc for example, World Book Encyclopaedia, you are aware that the source of this information is reputable. This is due to the efforts made by writers, editors and the like who are given the responsibility to ensure that all the information contained on the CD is accurate and correct. Log on to the Internet and try and locate some more information about household appliances. Part 3: Mechanics and household appliances 27 Turn to the exercise sheet and complete exercise 3.6 to 3.10. 28 Household appliances Exercises Exercise 3.1 Complete the following table by stating whether the listed quantity is a vector or a scalar. Quality Scalar or Vector 1 hour 35 mins 80 kg 10 m/s North 4 m2 9.8 m/s vertical down 25 m/s South East 4.5 m 50 N vertically up 1 m3 10 s 25 km in a North East direction 1 x 109 Part 3: Mechanics and household appliances 29 Exercise 3.2 Determine the force created in a vertical down direction by a stack of six (6) boxes each with a mass of 6.4 kg using the analytical (mathematical) method. Start with a formula and show all workings. 30 Household appliances Exercise 3.3 Determine the resultant of the three forces shown below, using the analytical (mathematical) method and verifying your result with a graphical solution. F3 15 N 20 N 10 N F1 F2 Figure 3.25 Three force system Part 3: Mechanics and household appliances 31 Exercise 3.4 Determine the resultant of the forces shown below using the analytical (mathematical) method and verifying your result with a graphical solution. F1 = 200 N 30∞ F2 = 60 N Figure 3.26 The force system 32 Household appliances Exercise 3.5 Determine the resultant of the two forces shown below using the analytical (mathematical) method and verifying your result with a graphical solution. 500 N 30∞ 20∞ 800 N Figure 3.27 Two force system Part 3: Mechanics and household appliances 33 Exercise 3.6 Using graphical methods: determine the force resultant when two forces, 70 N 45° and 80 N 30° act on the corner of a toaster. Nominate the scale you use for the vector diagram. Remember to state your resultant magnitude and its direction. 30∞ N Toaster N 80 70 a 45∞ Figure 3.28 Two forces acting on a toaster b state the equilibrant force required to balance to 70 N 80 N 30°. 45° and _______________________________________________________ 34 Household appliances Exercise 3.7 Determine using a graphical method, the total force created by two people, both pushing a refrigerator to the left along a flat surface, when the first person pushes with a 100 N effort and the second person with a 125 N effort. 125 N 100 N Figure 3.29 Two people pushing a refrigerator Part 3: Mechanics and household appliances 35 Exercise 3.8 A refrigerator is being pushed by a person. The force applied is 140 N upwards to the right at 30° (140 N 30°). Determine, using graphical method the horizontal effect on a refrigerator by the 140 N force. 0N 14 30∞ Figure 3.30 One person pushing a refrigerator 36 Household appliances Exercise 3.9 A force has a 125 N magnitude. The direction is 45° off the horizontal upward to the right. Determine, using mathematical methods, the horizontal and the vertical components of the force. That is, what effect does this 125 N force have in the horizontal direction and what effect does this 125 N force have in the vertical direction? Part 3: Mechanics and household appliances 37 Exercise 3.10 a What are the two (2) force conventions you apply when describing a force sense? i ______________________________________________________ _______________________________________________________ ii b 38 ___________________________________________________ What are three (3) other conventions used in Australian society, for example, driving a car on the left side of the road? i ___________________________________________________ ii ___________________________________________________ iii ___________________________________________________ Household appliances Progress check During this part you examined how to solve engineering problems using mathematical and graphical methods. ✓ ❏ Disagree – revise your work ✓ ❏ Uncertain – contact your teacher Uncertain Agree – well done Disagree ✓ ❏ Agree Take a few moments to reflect on your learning then tick the box that best represents your level of achievement. I have learnt about • the fundamentals of engineering mechanics – by studying mass and force and how they relate to household appliances – how scalar and vector qualities are classified – methods of determining components of a force. I have learnt to • use mathematical and/or graphical methods to solve engineering problems in household appliances • conduct research using computer technologies and other resources. Extract from Stage 6 Engineering StudiesSyllabus, © Board of Studies, NSW, 1999. Refer to <http://www.boardofstudies.nsw.edu.au> for original and current documents. During the next part you will explore the basic principles of electricity and communicating information using drawing. Part 3: Mechanics and household appliances 39 40 Household appliances Exercise cover sheet Exercises 3.1 to 3.10 Name: ______________________ Check! Have you have completed the following exercises? ❐ Exercise 3.1 ❐ Exercise 3.2 ❐ Exercise 3.3 ❐ Exercise 3.4 ❐ Exercise 3.5 ❐ Exercise 3.6 ❐ Exercise 3.7 ❐ Exercise 3.8 ❐ Exercise 3.9 ❐ Exercise 3.10 If you study Stage 6 Engineering Studies through a Distance Education Centre/School (DEC) you will need to return the exercise pages with your responses. Return the exercise pages with the Title Page cover attached. Do not return all the notes, they should be filed for future reference. If you study Stage 6 Engineering Studies through the OTEN Open Learning Program (OLP) refer to the Learner’s Guide to determine which exercises you need to return to your teacher along with the Mark Record Slip. Part 3: Mechanics and household appliances 41 Household appliances Part 4: Household appliances – electricity and communication Part 4 contents Introduction............................................................................................ 2 What will you learn?.................................................................... 2 Electricity in society .............................................................................. 3 Electrical distribution network ..................................................... 4 Changing power sources over time............................................. 5 Basic principles of electricity................................................................ 6 Conducting materials ................................................................. 7 Insulating materials..................................................................... 7 Electrical currents ...................................................................... 8 Household appliances ............................................................. 11 Magnetic induction .................................................................. 17 Induction motor ....................................................................... 25 Electrical safety................................................................................... 29 What electrical potentials (voltages) are safe?............................. 30 Electric shock.......................................................................... 34 Australian standards................................................................. 35 Forms of electrical safety .......................................................... 36 Engineering communication skills...................................................... 49 Freehand drawing.................................................................... 49 Technical drawing.................................................................... 52 Exercises............................................................................................ 53 Progress check................................................................................... 61 Exercise cover sheet......................................................................... 63 Part 4: Electricity and engineering drawing 1 Introduction In this part you will explore the basic principles of electricity. The concepts of potential difference and current are explained and basic electrical components are described. You will examine principles of electrical safety and induction. In the engineering communication skills section you will develop the skill to draw freehand orthogonal sketches. Internet research skills are discussed further. What will you learn? You will learn about: • basic principles [of electricity] … – • potential difference, current, simple circuits and components electric safety – related to Australian electrical standards • magnetic induction • the fundamentals of AC and DC circuits • electric motors. You will learn to: • explain the basic electrical principles of operation appropriate to household appliances • appreciate the importance of electrical safety when using electrical household appliances • explain the working of an induction motor • distinguish between AC and DC circuits • draw freehand, three-dimensional objects • conduct research using computer technologies and other resources. Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to <http//ww.boardofstudies.nsw.edu.au> for original and current documents. 2 Household appliances Electricity in society Electricity is probably the dominant source of power in your life. It has become an integral part of the modern world. It is hard to believe that less than 130 years ago there was no electricity in any home and electricity was still in the realm of something worked on by the occasional experimental scientist. In this part you will look at some of the history in taking electricity from a scientific oddity to an essential technology. The electrical age began in North America with the birth of the electrical companies. Electricity was seen as an alternative method of lighting. This concept in itself was revolutionary. Light produced from electricity did not require burning of anything to produce the chemical reaction from which all previous light in the home and factory had come. Factories could now be run 24 hours a day. No more ceasing work after sundown. Shift work had begun. In addition to light, the machines in the factories could now be run 24 hours a day using the same power source. Electricity was embraced by society. It was clean (because the power stations were built away from the home or factory). Electricity was adaptable. It could light up a room, send a message by telecommunications equipment and run a motor to power any machine. Today you use the electric light every evening. Devices powered by electricity entertain you. Messages are transferred across the world using communication technologies powered by electricity. So how and when did this revolution of the electrification of our lives take place? Probably the answer lies in the approach taken by a single inventor, Thomas Edison. In 1868 Edison, while working for Western Union Telegraph, read a book that was to send the inventor on a journey of discovery in the field of electricity. That book was Experimental Researches in Electricity by Michael Faraday. This inspired Edison to experiment with electricity and begin a lifelong search to develop machines that could use electricity to communicate, calculate, entertain and illuminate your life. Part 4: Electricity and engineering drawing 3 Electrical distribution network Perhaps the greatest invention of Edison was the incandescent light bulb. This was the first successful device to produce light without a flame and the constant need for a supply of fuel. When the light bulb was invented in 1879, Edison saw its potential to illuminate the world. He and his team of researchers set about developing the technology to make an electrical society. To do this they had to devise an electrical distribution network and the necessary multitude of devices such as generators, transmission wires, motors, junction boxes, internal electrical circuitry for buildings, and fuses for safety. Because the devices and systems for an electrical network did not exist in 1879, you might wonder how long it took for the Edison led team to get the system right. The answer is an amazing three years! Edison’s company opened the first commercial power station within three years in Manhattan, New York. This power station wasn’t exactly like the ones today. For a start it made direct current (DC) electricity. It served around 85 customers and powered 400 lamps. It was more a curiosity than anything else, but it was the beginning of the electrical energy dependent society in which you live. Almost any energy source can be converted to electricity. Australian power plants mostly burn fossil fuels, such as coal or gas, though some use the energy of falling water to generate electricity. Australian engineers lead the world in the development of many aspects of the solar electric cell for generating electricity. Electricity is easy to distribute and once the networks for distribution are established they require only minimal maintenance. To increase the efficiency of distribution, electricity from power plants is distributed via high-voltage transmission lines (up to 500 000 volts). Transformers reduce the voltage and increase the amperage of this electricity when it is delivered to homes and industries. The electricity you use is reliable and safely delivered at around 240 V. As a result the equipment bought to use that electricity is designed to operate at around 240 V. Together, the equipment and the supply make up a system. Because the demand for electricity is variable throughout the day, its generation is uneven. This produces problems and comes at a financial and environmental cost that must be factored in when considering the impact of supplying electrical power. 4 Household appliances Changing power sources over time The use of electricity has changed society. To understand how profound a change the use of electricity has made to society, consider a comparison between now and earlier times. Imagine what life was like. Think about what the main source of power was. Consider what equipment you would have used to perform tasks. Part 4: Electricity and engineering drawing 5 Basic principles of electricity To understand electricity, you need to investigate the structure of materials. Part 2 of this module stated that all matter is composed of atoms. The atoms of one element are different from the atoms of all other elements but they are composed of the same basic components. In the centre of the atom is the nucleus, consisting of positively charged particles called protons, and neutrons that have no charge. In an orbit around the nucleus are electrons that are negatively charged. Each atom has the same number of protons as electrons therefore the charges cancel out, which is the atoms have no overall charge. nucleus electron neutron proton Figure 4.1 Diagrammatic representation of an atom showing protons, neutrons and electrons The electrons generally stay with the atom they belong to, but sometimes they can be pulled away. If this happens, the atom that has lost on electron will be positively charged and the atom gaining the electron negatively charged. They are now called ions because they have a charge. 6 Household appliances Conducting materials Some types of atoms lose electrons more readily than others. Metals, such as iron, aluminium and copper, have some electrons that easily ‘escape’ from their atom. These materials are called conductors. If you were to put a positive charge near a metal, the electrons, which are negatively charged, would move towards the positive charge. This happens because positive and negative attract each other (‘opposites attract’). A battery has a positive terminal, so if you connected a copper wire to the positive terminal of a battery, electrons would move from the copper atoms to the positive terminal. If you made the connection complete, from negative to positive terminal, electrons would move around the circuit in a continuous flow. negatively charged electrons drift towards positive terminal electrons electrons metal ion DC source Figure 4.2 Movement of electrons between atoms Insulating materials In some materials, the electrons are held very tightly by the atoms. These materials are called insulators. The following tables list common conductors, semi-conductors and insulators. Note that of the metals, silver is the best conductor and iron the poorest conductor. Conductors Semi-conductors Insulators silver silicon polymers copper germanium gold Part 4: Electricity and engineering drawing carbon rubber pure water 7 aluminium selenium glass tungsten porcelain iron dry timber Electrical currents The movement of electrons along a conductor, in a particular direction, produces an electric current. The more electrons that move, the greater the current. Direct and alternating currents When electrons flow (electric current) in one direction along a conductor, the current is direct current (DC). This is the type of current that is produced from a battery. For example, in a torch, there is a direct current from a battery to the globe and back to the battery when the switch is on. Switch Globe DC battery Figure 4.3 Circuit in a torch Take a look at a torch and see if you can follow the wires in a complete circuit (note: sometimes wires are replaced by metal strips). You will probably have to take out the batteries and possibly the bulb to identify the connections. Don’t forget to put it back together again when you’ve finished! Current doesn’t always have to flow in one direction, and the electrons don’t have to go from one point to another point. If an electron moves just one millimetre, then there is a current while it is moving. If it moves one millimetre in one direction, and then one millimetre in the other direction, it hasn’t made much progress, but it has produced current! 8 Household appliances When electrons flow (electric current) first in one direction and then flow back again, and continue this back and forth motion, the current is called alternating current (AC). This type of current is used to power home electrical appliances. +I 0 Time -I Alternating current (AC) +I 0 Time Direct current (DC) Figure 4.4 AC, DC Current doesn’t have to flow only in one direction to be useful! Turn to the exercise section and complete exercise 4.1. Simple electric circuits For electrons to produce current there must be a completed electric circuit. Figure 4.5 shows a circuit comprising: • a supply of electrons (the battery) • a control device to enable the circuit to be completed or broken (the switch) • a load (the resistor) • an ammeter to read the current flowing in the circuit. Part 4: Electricity and engineering drawing 9 Battery Switch Resistor Ammeter wires Figure 4.5 Components of a basic circuit When electric circuits are drawn they represent a real circuit and the components that make up that circuit. Because the components are difficult to draw and vary in appearance a standard set of symbols has been developed to enable circuits to be drawn and understood by all people. Circuit symbols Resistor: Power pack/battery: Va r i a b l e r e s i s t a n c e : Battery AC 6V DC voltmeter: voltage V Open Single DC ammeter: pole A Closed Earth connection: (or 0 volts) Lamp Figure 4.6 shows the circuit illustrated in figure 4.5 drawn using standard symbols. + I R - Figure 4.6 10 A + - Circuit from figure 4.5 drawn using standard symbols Household appliances Household appliances Let’s examine some simple household electrical appliances and investigate how they work. Electric kettle 240 V AC Insulated power cord carrying electric current Switch Earth wire Heating element Figure 4.7 Simplified electrical circuit of a kettle How it works When the power cord is connected to a power point, and both the power point and the kettle are switched to the ON position, an electrical circuit is created and electrical current flows. The current passes through a heating element that is a resistor. The resistance to the flow of current in the heating element causes it to get hot, and this heat causes the water to get hot. Safety devices A range of safety features are incorporated to prevent injury including: • polymer handle (and sometimes body) to prevent burns as polymers are good heat insulators • automatic switch off when the water reaches boiling point to prevent the water boiling away and the element overheating • earth wire in the power cord (see ‘earthing’ later in this part) • circuit breakers or fuses in the house wiring power point circuit. Part 4: Electricity and engineering drawing 11 Pop up toaster Inside the toaster On/off lever Nichrome wire on heating element inside toaster On/off switch (with timer mechanism) Power cord MEDIUM LIGHT Darkness adjustment knob Figure 4.8 240 V AC DARK Variable resistor to adjust current supplied to heating element Earth wire Simplified electrical circuit of a toaster How it works A toaster uses radiant heat to brown a slice of bread. When the power cord is connected to a power point, and the switch on the power point and the lever on the pop up toaster are in the ON position, a closed electrical circuit is created and electrical current flows. The circuit supplies current to a length of nichrome wire (an alloy of nickel and chromium) wrapped around insulating mica boards on both sides of the bread slot. Nichrome wire is a poor conductor of electricity compared to copper, and gets red hot when current flows through it. The radiant heat produced slowly browns the bread. A timer releases the lever, breaking the circuit and turning the toaster off. At the same time it releases the spring-loaded tray, popping the toast up. The timer can be a simple mechanism such as a bimetallic strip or an electronic circuit. Safety devices A range of safety features are incorporated to prevent injury including: 12 • polymer parts which don’t get hot and therefore prevent burns • an automatic switch off mechanism to prevent overheating • an earth wire in the power cord. Household appliances Hair dryer Heating element Motor Fan Coiled nichrome wire On Switch Off Insulated power cord carrying electric current Figure 4.9 Simplified electrical circuit of a hair dryer How it works A hair dryer uses a heating coil and a motor-driven fan to transform electrical energy into convective heat and generate a blast of hot air. When the power cord is connected to a power point, and both the power point switch and the switch on the hair dryer are in the ON position, a closed electrical circuit is created and electrical current flows. The circuit supplies current to the bare, coiled nichrome wire wrapped around insulating mica boards. This is the heating element. As in the toaster, when current flows through this nichrome wire, it gets very hot. The current also makes the electric motor located inside the fan spin, which turns the blades. The airflow generated by the fan is directed down the barrel and through the heating element producing the blast of hot air. The speed of the airflow can be adjusted by modifying the amount of current going through the motor. This is achieved by a switch, which changes the current in the motor circuit. Part 4: Electricity and engineering drawing 13 Safety devices A range of safety features are incorporated to prevent injury including: • insulation in the form of a heat shield that lines the barrel and a polymer housing to prevent burns as polymers are poor heat conductors • a hair dryer is a double insulated device to protect the user from electrocution (see double insulation later in this part) • a cut out switch, often a heat sensitive bimetallic strip, which trips (opens) the circuit thereby shutting off the motor and preventing overheating • a thermal fuse in the heating element circuit, which will blow and break the circuit if the current is excessively high • protective screens over the intake vent to prevent objects being drawn in • front grill over the outlet vent to prevent objects being placed down the barrel. Some basic electrical units Before going too much further, you need to revise some basic terms used in electricity that you would have encountered in Science. Voltage: the ‘pressure’ at which electrons are forced around a closed circuit; measured in units of ‘Volts’ and given the symbol ‘V’. How many Volts are required to make a torch work? Look at the battery of the torch you looked at earlier to find out and multiply the figure by the number of batteries. __________________________________________________________ __________________________________________________________ Did you answer? For example, if there are two batteries, each 1.5 V, then the torch uses 2 x 1.5 = 3.0 V Current: the quantity of electrons flowing around a closed circuit; measured in ‘Amperes’ (or often abbreviated to ‘Amps’) and given the symbol ‘I’. Look at the bulb in the torch. On the side there will be a voltage and current rating. For example, 2.5 V, 0.5 A. 14 Household appliances Resistance: the property of a circuit or circuit element that restricts the flow of electrons, or equivalently, the ratio of voltage to current in a circuit element measured in; measured in units of ‘Ohm s’ and given the symbol ‘R’. Voltage, current and resistance are related by Ohm's Law that states that: V = I¥R This can also be written as: I = V R or R = V I The relationship described above was identified by the German physicist Georg Ohm. Ohm’s Law states that the current flowing between any two points in any circuit is: • directly proportional to the potential difference (voltage) between them. That is, doubling the voltage doubles the current, halving the voltage halves the current and so on. • inversely proportional to the resistance of the circuit between the two points. That is, doubling the resistance halves the current, halving the resistance doubles the current and so on. a What is the resistance of the light bulb in the previous example? _______________________________________________________ _______________________________________________________ _______________________________________________________ b If you connect only 2.0 V to the bulb, how much current would flow, and what would happen to the brightness of the bulb? _______________________________________________________ _______________________________________________________ Did you answer? a b For example, 2.5 V, 0.5 A R = 2.5 = 5.0 Ohms 0.5 I V 2.0 = = 0.4 A R 5.0 = This is less current than before. The bulb will not glow as brightly. Part 4: Electricity and engineering drawing 15 Note: the resistance of the light bulb filament depends on what it’s made of and how long and thick the wire is. Therefore, it’s resistance doesn’t change, but the current through it can be changed by changing the voltage applied to it. If a 12 V electrical circuit is to be limited to 3 Amps, what is the required resistance in the circuit? There are two other units that will be particularly useful: Power: the rate at which work is done by an electrical circuit; measured in units of ‘Watts’and given the symbol ‘P’ Energy: the total amount of work done in an electrical circuit; measured units of ‘Joules’ and given the symbol ‘W’. Power is the (time) rate at which energy is delivered, and conversely, energy is the product of power and time (t): P = W t and W = P ¥ t Power is related to voltage, current and resistance via the relationships: P = V¥I Substituting for V we can obtain: P = (I ¥ R) ¥ I = I2R or substituting for I we can obtain: P = V ¥ V V2 = R R Note: Volts, Amperes, Ohms, Watts and Joules are all named after famous people who have contributed to the field of electrical engineering, and therefore we use capital letters for each of these units. The terms and formula above will be used later in our calculations. 16 Household appliances A toaster is rated at 600 Watts and operates at 240 V. How much current does it draw? How much resistance is in the circuit? If the toaster is used for 6 minutes, how much energy does it use? __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ Did you answer? Given that we have P = 600 Watts and V = 240 Volts, we can write: P = VxI or I = P 600 = = 2.5 Amps V 240 We can obtain the resistance in several ways: V2 R From P = From P = I2R we can write R = V2 ( 240) 2 = = 96 Ohms P 600 we can write R = P P I 2 = 600 ( 2.5) 2 = 96 Ohms Energy consumption is measured in Joules, which is equivalent to Watt seconds. 6 minutes is equivalent to 360 seconds. The energy consumed is thus: W = P ¥ t = 600 ¥ 360 = 216,000 Joules Turn to the exercise section and complete exercises 4.1 c and 4.2. Magnetic induction Electricity and magnetism are very closely linked. In 1820 Hans Christian Oersted, first demonstrated this by observing and later demonstrating that a current can produce a magnetic field. It was later that the opposite effect was discovered – that a magnetic field can produce electricity. Part 4: Electricity and engineering drawing 17 Electricity and magnetism are related because they both involve electrons. However, in magnetism, the relationship with electrons is not as simple as moving electrons creating electricity. The electrons in an atom can be considered to spin around in a particular axis. These ‘spin axes’ are usually randomly arranged throughout the material. However, in a magnet, lots of the axes of spin are aligned, that is, they are all lined up in the one direction. This is what gives a magnet a north and south pole. You can make a magnet out of a piece of iron, such as nail and a permanent magnet. If you stroke the iron, in one direction with the magnet, you will be able to pick up iron filings or a paper clip with the nail. By stroking the iron in one direction with the magnet, you are aligning the axes of spin of the electrons. This is called magnetic induction. You can only do this with materials that are made primarily of iron. If you have a magnet, see if you can magnetise a nail. Try to pick up a paper clip with the nail. The invisible ‘field’ around a magnet that causes it to attract a paper clip or to repel another magnet’s like pole, is called a magnetic field. We can illustrate how a magnetic field ‘looks’ with magnetic field lines as shown in figure 4.10. N S N S Unlike poles attract S N N S Like poles repel Figure 4.10 Magnetic field lines around bar magnets If you add arrows to the field lines, going from north to south, you can see why like poles repel and opposite poles attract. You may be surprised to learn that a wire that has an electric current flowing through it, also has a magnetic field around it! 18 Household appliances Does that mean that it will attract a paper clip? Well yes, if you can arrange it the right way. Let me explain … The magnetic field around a current-carrying wire looks like a set of concentric rings as shown in figure 4.11. Wire carrying a current Magnetic field lines Figure 4.11 Magnetic filed lines around a current-carrying conductor It is easier to visualise if you imagine that you are looking along the wire. Say the wire is piercing the paper of this page and going directly into the desk underneath. If the current is going into the page (designated by a cross) then the magnetic field direction is clockwise. If the current is coming out of the page (designated by a dot) then the magnetic field direction is anticlockwise. Wire with current coming up out of the page Figure 4.12 Wire with current going into the page Magnetic field direction around a current-carrying conductor There doesn’t appear to be any north or south pole here. Well, think about this. If we had a current-carrying wire in a coil, and we threaded it through the page, it would have a field that looks something like a magnetic field around a magnet. Part 4: Electricity and engineering drawing 19 Current out Current in wire Current in Page Figure 4.13 Magnetic field created by coiling a current-carrying conductor The concentric circles around the wire, as it comes out and goes into the page, have become distorted. This is because the fields affect each other. The lines of the magnetic field never cross over each other, so they tend to get ‘squashed’ together. If we coiled the wire several times, then it would look like this … Figure 4.14 Magnetic field around several coils The fields around each coil of the wire tend to combine with the adjacent ones to make one larger, combined magnetic field that looks like the field around a bar magnet if you imagined that the field lines also go through the magnet as shown in figure 4.15. 20 Household appliances N Figure 4.15 S Magnetic field lines around and ‘through’ a bar magnet So it is actually possible to make a coil of wire carrying a current behave like a bar magnet. Can you see now how closely related electricity and magnetism are? If we were to place an iron core inside the wire coil, the magnetic field would be enhanced (strengthened) by the presence of the iron. Wrapping a current-carrying coil of wire around a ferrous (iron) core produces an electromagnet. The strength of the magnetic field, or the magnetic flux density, is proportional to the number of turns in the coil and the magnitude of the current. This is a convenient way to produce a strong magnet. n turns Current Figure 4.16 Configuration of an electromagnet Generating torque using electromagnets Figure 4.17 shows three magnets arranged in a line but with the outer magnets fixed in place and the central magnet able to spin about a point. (Pinned) magnet Fixed (stator) magnet Fixed (stator) magnet Resultant torque Figure 4.17 Configuration of permanent bar magnets to induce torque Part 4: Electricity and engineering drawing 21 With the magnets arranged the way they are, the central magnet will tend to rotate until its north and south poles are in a straight line with the other two magnets. The magnetic force between the magnets creates a force that causes the central magnet to rotate. Such a force is called torque. The torque of the central magnet is the biggest when it is at q = 90° to the line of the fixed magnets. In fact, the size of this torque (T) is related to the angle q by the following formula: T µ Sinq This simple principle underlies all electrical motors. A simple motor The problem with our three magnets is that the central magnet will move around until it lines up with the fixed magnets, N to S and S to N (q = 0°) and then it will stop. A motor needs to produce continuous motion! Let’s change things around a bit using our knowledge. We can replace one, or all, of the permanent magnets with current-carrying wire coils or even electromagnets. To keep it simple, consider replacing the central magnet with a single wire coil, as shown in figure 4.18. We shall call the rotating coil the rotor and the stationary magnets the stator. axis of rotation Figure 4.18 Current-carrying coil in a magnetic field When the coil is in the position shown, it will experience a torque that will tend to rotate it until its magnetic field ‘lines up’ with those of the permanent magnets. (Think about the coil as though it has a north and south pole. The north pole, in the figure, will be below the coil and the south pole will be above the coil. Magnetic field lines point from north to south). So the coil will tend to rotate anticlockwise, or, the left side of the coil will experience a force downwards, and the right side of the coil will experience a force upwards. (The forces are opposite in direction because the current is going in opposite directions in the two sides of the coil). 22 Household appliances But we haven’t solved the original problem, that is, the coil may spin around but only until its magnetic field aligns with the magnetic field created by the fixed magnets, then it will stop. In 1833, a man called Thomas Davenport devised a way in which the direction of the current in a coil could be reversed every time the coil rotated by half a revolution. In this way each time the coil rotated to the position where its poles lined up, the current would switch direction, and therefore, the magnetic field would reverse polarity. Consequently the coil would suddenly experience a force to turn it another 180°. As long as the current continued to flow in the coil, the coil would continue to rotate. It is called the brush and commutator. Figure 4.19 shows how it works. The coil ends are attached to two half rings, or a split ring. The ring is in contact with, but not permanently attached to, some brushes made of conducting material. The brushes are fixed in position and are connected to the current supply. So as the coil rotates, the half ring that is touching the brush on the positive side rotates. When the ‘gap’ in the ring reaches the brush, the half ring that was contacting the positive brush now moves around to contact the negative brush, so the current in the coil changes direction. Very clever don’t you think? N S Figure 4.19 The brush and commutator device There is an easy way to determine what direction the coil will rotate in a simple motor such as this. It is found using what is called the right hand palm rule. Part 4: Electricity and engineering drawing 23 Hold your right hand flat as shown in figure 4.20. F B Figure 4.20 The right hand palm rule Your fingers represent the magnetic field lines (in the direction from north to south). Your thumb represents the current direction (from positive to negative). Now pretend you are pushing with your hand. The force experienced by the wire carrying the current will be in the direction of the push. Try this rule on each side of the coil shown in figure 4.18. In real motors of course, there is more than one coil and they are usually arranged in three windings to get the best possible efficiency. Figures 4.21 and 4.22 show a small motor from a remote controlled toy. You will notice that the rotor is attached to a shaft or axle. This is what allows the motor to do work. For example, the shaft could be connected to wheels to cause a toy car to move or to fan blades to move air for cooling. rotor coil contact is connected to one segment of the splitring commutator stator Figure 4.21 The rotor removed from the stator of the small motor. Note the insulated copper wire making up the coil. This wire although it appears to be bare copper wire is actually insulated with either a clear polymer lacquer such as polyurethane or a thin coating of transparent plastic. If the insulation fails the effectiveness of the motor is diminished because there are effectively less coils. 24 Household appliances insulated copper coil pole 2 wear lines from the brushes contacting the commutator pole 1 split ring commutator coil contacts laminated iron electromagnet Figure 4.22 pole 3 The armature or rotor of the motor. Note the three coils. The motor has three magnetic poles to increase its efficiency. All motors work on the basic principles discussed here. However, there are several variations of motor, each working in a slightly different way. For now you will just look at one, the induction motor. Induction motor To understand how an induction motor works, you must first review a discovery by Michael Faraday called magneto-electric induction. You have already seen that if you place a current-carrying conductor into a magnetic field (as long as the conductor and the magnetic field lines are not parallel), the conductor will experience a force. If the conductor is not fixed in position, it will move. Similarly, if you move a conductor in a magnetic field (as long as the conductor crosses field lines), then a current is induced in the conductor. Faraday discovered this phenomenon in 1831. It is the principle behind the induction motor. Of course, the same result is obtained if the conductor is stationary but the magnetic field is moving. The principle is illustrated in figures 4.23 and 4.24. Magnetic field Conductor carrying current into page Figure 4.23 Force experienced by conductor Force on a current-carrying conductor in a magnetic field Part 4: Electricity and engineering drawing 25 Length of wire moved through the magnetic field (into the page) Magnetic field Current induced in wire (no external power source) Figure 4.24 Current induced in a conductor moving through a magnetic field Most electric motors are AC induction motors. Perhaps the simplest and most reliable are the so-called three-phase squirrel cage motors. In squirrel cage motors, the rotor ‘winding’ consists of solid bars, not coils of wire, that are joined at either end by a metal ring. The term squirrel cage came about because the cage of the rotor resembles the rotating cylinder that squirrels play with when in captivity. The bars that make up the cage are generally aluminium but they can be copper or any other highly conductive material. The use of aluminium is a trade off between conductivity and lightness. Figure 4.25 The rotor cage of an induction motor (‘squirrel cage’) Squirrel cage motors are often used in heavy industrial applications such as trains, cranes and large air conditioning units. In the squirrel cage induction motor, the rotor turns because of rotation of the magnetic field. A strong magnetic field is created using electromagnets on three sides of the stator in a triangular relationship as shown in figure 4.26. 26 Household appliances A B C C Solid bars on rotor A B Figure 4.26 A schematic of a squirrel cage motor. Note how the poles of opposite magnets in the stator are different. Although presented here in this schematic figure as bar magnets, the magnets are actually electromagnets with an AC current passing through them. Opposite poles are connected in the one circuit receiving one phase of a three-phase current. That is, A and A are connected in the same circuit, B and B in a circuit and C and C in a circuit, and each lags slightly behind the other. The AC current constantly changes the polarity of the electromagnets but opposite magnets still have opposite poles facing each other. This creates an apparent rotating magnetic field. The rotating magnetic field induces a current in the bars of the squirrel cage rotor. The rotor is then literally dragged along chasing the rotating magnetic field in the stator, due to the torque. A three-phase current is used in the induction motor. This consists of three alternating currents slightly out of phase with each other. In other words, the changing direction of the currents is not in time. Each lags behind the next one by a third of the time for one complete cycle of direction change. Because the direction of the current in each of the three coils is constantly changing, and the current on each phase is sequenced to follow on from the previous coil, the direction of the electric current induced in the rotor is constantly changing. The different current phases function in tandem. It is similar to pedalling on a bicycle with your feet strapped to the pedals so you get the pull and the push on opposite sides of the rotor. The time lag between the different phases of the AC current act to create rotating magnetic fields. This ‘moving’ magnetic field induces currents in the bars of the squirrel cage rotor. Each bar experiences a torque because it is carrying a current and is in a magnetic field. So the squirrel cage rotor is caused to turn. This machine is known as an ‘induction’ motor, because the currents in the rotor are induced (not physically connected). Part 4: Electricity and engineering drawing 27 Characteristics of the induction motor The induction motor relies on rods in the rotor intersecting lines of magnetic flux, known as field lines, in order for the induction process to work. Controlling the operating speed of an induction motor can be quite difficult. Modern power electronics have been developed that largely overcome the problem, but such circuits are often complicated, and more often than not, expensive. In many applications, the motor speed is dictated by the load on the machine, and no effort is made to operate the motor at any other speed. By far the biggest advantage of induction motors is their simple and robust construction. No slip rings or commutator are required. The rotor is robust, consisting only of the squirrel cage and iron core. Figure 4.27 shows the rotor from an induction motor. The rods are clearly visible protruding from the end rings. Figure 4.27 Rotor of an induction motor Single-phase induction motors do exist, and are very common in a myriad of applications. However, their construction and operating principles are significantly different (and more complicated) than that of the three-phase version. Applications of induction motors Induction motors are used extensively where larger motors are needed and AC power is available. Refrigerators, washing machines, air-conditioners, irrigation pumps, lathes, and fans, to name but a few, all use induction motors to supply rotational energy. These motors need virtually zero maintenance, especially compared with other motor types, and are very robust. The application of induction motors is likely to increase as the cost of suitable power electronic controllers decreases. It is envisaged that induction motors will replace DC motors in variable speed drive applications in future years. Turn to the exercise section and complete exercise 4.3. 28 Household appliances Electrical safety The vast majority of Australians make use of electrical energy every day. We are so used to the presence of electrical appliances in our lives that we often take them for granted. Given that we use electrical appliances so regularly, it is easy to forget that electrical energy is dangerous. Electricity is odourless, colourless and invisible. It is almost impossible to determine if a circuit is energised by just looking at it. There are a large number of electrical standards and guidelines that equipment, appliance manufacturers and electrical workers must adhere to. These ‘electrical rules’ ensure that the electrical accident rate is minimised and that a reasonable level of protection is afforded to everyone, not just those with formal electrical training. In Australia alone there are tens of millions of electrical appliances in use each day. Estimate of how many electrical appliances your family owns, and compare that with the number of motor vehicles your family owns. __________________________________________________________ __________________________________________________________ Did you answer? The number of electrical appliance owned is generally many times the number of motor vehicles. Given this obvious disparity in number of appliances versus number of vehicles, why do you think motor vehicle fatalities are relatively common, but domestic electrical fatalities are not? Perhaps this is no accident … In this section we will look at a number of electrical safety issues, and what can be done to ensure the safety of individuals operating electrical appliances. Part 4: Electricity and engineering drawing 29 Who is responsible for electrical safety? Electrical appliance and equipment manufacturers are required by law to adhere to a minimum set of standards if they are to provide equipment for sale or use in Australia. Domestic electrical equipment must be adequately protected so that the user needs no specialist electrical knowledge to use the appliance safely. If the equipment meets the relevant standards the manufacturer is deemed to have met his or her legal requirement for safety. Having said that, in the end it is the user that has the ultimate responsibility for his or her own safety. It is unreasonable to expect equipment manufacturers or government legislation to completely protect us from our own actions. Some electrical accidents are truely unfortunate accidents that few people could have foreseen or prevented. Unfortunately, however, most electrical accidents are preventable and result from inappropriate use of electrical equipment, ‘tinkering’ with a broken electrical appliance, or work undertaken by unqualified individuals. In New South Wales it is possible to purchase a large range of specialised electrical components without the need for formal electrical qualifications. Hobbyist electronics outlets, even general hardware stores commonly sell 240V general purpose outlets (power points), electrical switches, mains rated wiring, terminals, and insulators. While it is tempting to try and repair a broken electrical appliance, unless you have specific formal training, it is not only potentially dangerous, it is also illegal. As a general rule, we should carefully follow the manufacturers instructions to ensure we use an appliance appropriately, and under no circumstances should we attempt to repair, service or modify an electrical appliance or installation. What electrical potentials (voltages) are safe? The electrical power outlets in Australian homes generally provide an electrical potential of 240V AC. Would you describe this potential as ‘low voltage’ or ‘high voltage’? What electrical potential do you think would be classed as ‘safe’? 30 Household appliances Low voltage If you read through the Australian standards or guidelines for electrical safety you will notice many references to equipment operating at or below ‘low voltage’. You may be surprised to learn that in the standards, the term ‘low voltage’ actually refers to voltage potentials up to 1000V AC or 1 500V DC, and in no way implies safety. ‘Low voltage’ is simply a relative term to describe the precautions required for a particular group of electrical appliances or installations. In comparison to large overhead transmission lines with potentials of 330 000V AC and above, something operating at 1 000V AC can be considered ‘low voltage’. Extra low voltage The standards also describe a category of electrical equipment termed ‘extra low voltage’. This equipment operates at less than 50V AC or 120V DC (AS/NZS 3000:2000). Again, the term extra low voltage does not imply safety. Rather it indicates a particular set of precautions, level of insulation, protective barriers and other precautions that should be applied to this type of equipment. The risk of electric shock from accidental contact with extra low voltage equipment is sufficiently low (but not zero!) that live components in general need not be insulated. High voltage Any appliance or installation that does not fit the ‘low voltage’ category is deemed to be ‘high voltage’. This applies to appliances and installations that operate at potential above 1 000V AC or 1 500V DC . List two electrical devices around your home that you think might fit each equipment category (extra low voltage and low voltage). __________________________________________________________ __________________________________________________________ Did you answer? Extra low voltage Low voltage • • 12 V downlights any 240 V appliances ex: garden lighting Part 4: Electricity and engineering drawing 31 Battery powered devices Most commonly available batteries are classed as extra low voltage. An AAA alkaline dry cell (a pen battery you might find on a video remote control, or a toy) has a terminal voltage of 1.5V DC. Lead acid batteries that run a car or motor bike are commonly 6V DC or 12V DC. Some trucks and tractors use two batteries to give 24V DC. A lithium button cell (a watch or calculator battery) is commonly 3V DC, a nickel cadmium rechargeable battery (AAA NiCd cell) is 1.2V DC, Nickel metal hydride (NiMH) or NiCd battery packs for mobile phones and cordless drills, commonly range from 7.2V DC to 18V DC. Figure 4.28 Various types of batteries including alkaline, nickel cadmium and lithium We have all handled batteries at some stage, and most of us have touched the terminals of a battery on our car, or in a transistor radio without getting electrocuted. Does that mean all battery powered equipment can be considered safe? The lead acid battery in your car is more than likely 12V DC. Does the whole car electrical system run at 12V DC? Look under the bonnet of a car. Some connections are exposed meaning that they are (presumably) not dangerous. However, some other connections, particularly around the ignition system, have significant insulation. What might this indicate? 32 Household appliances Figure 4.29 An ignition coil on a car Even though the car runs on a 12V DC electrical system, the ignition coil can produce up to 50 000V. Can you tell which is the extra low voltage connection and which is the high voltage connection? Just because an electrical system is powered by a battery, it does not mean the system can be classed as extra low voltage. While the battery terminals themselves may be at low potential other parts of the circuit may not be. Classify each of the following devices on the basis of the electrical potentials within the device as being extra low voltage, low voltage or high voltage. Device Electrical potentials battery powered electric stock fence battery powered fluorescent light car ignition system hi-fi stereo system magnetron (micro wave source) in a microwave oven a neon sign a television set Did you answer? In case you didn't guess, all of the above devices, with the exception of the hi-fi system, have voltages of greater than 1000 Volts in them. The devices may appear safe, but are in fact potentially hazardous if treated incorrectly. Part 4: Electricity and engineering drawing 33 Turn to the exercise section and complete exercises 4.4 and 4.5 questions 1 to 3. Electric shock One of the most complicated objects in the known universe is the human brain. On average we each have around one hundred billion (100 000 000 000) neurons (nerve cells), with each neuron is connected to around 10 000 of its neighbouring neurons. This incredible network of connections runs on electricity. Our thoughts, senses, and muscles in our body are all controlled by electrical impulses, generated by a delicate balance of electro-chemistry in our brain. The electrical signals that control our bodies are of very low power. Since they are so small, it is relatively easy for large external electrical signals to interfere with them. An electric shock is the result of an electrical signal from an external source running through a part or parts of our bodies. If the external signal is of sufficient magnitude, it can disrupt our body's normal operation. In some cases the external signals can affect the heart's operation, and stop blood being pumped around the body. The amount of electrical energy required to start or stop our heart is surprisingly small. A defibrillation machine, used to start or stop a patient’s heart, provides electrical impulses between 200 and 400 Joules. The amount of electrical energy we have available in our homes is many thousands of times higher than this. A standard 9V alkaline battery commonly used in a transistor radio is rated at 9V, 500 milli-Ampere hours. How much electrical energy is stored in this type of battery? You can calculate how much power the battery can deliver for one hour, remembering that one Watt is equivalent to one Joule per second, and there are 3600 seconds in an hour. No electrical potential, even extra-low voltage, can be considered completely safe. 34 Household appliances Australian standards The Australian Standards are a large collection of documents that cover an enormous variety of situations. They prescribe what precautions and measures are considered appropriate for anything from paint and building materials through to electrical appliances. Almost any device, product or service you can think of is likely to have some components subject to an Australian Standard. There are a large number of Standards documents that relate to electrical equipment and installations. Some of these include : • AS/NZS 3350 Safety of household and similar electrical appliances • AS/NZS 3112 – 1993, Approval and test specification – plugs and socket outlets • AS/NZS 3194 – 1993, Approval and test specification – electric shaver supply units • AS 1430 – 1986, Household refrigerators and freezers • AS 1907 – 1990 Performance of household electrical appliances – toasters • AS 2040 – 1998, Performance of household electrical appliances – clothes washing machines • AS 3540 – 1998, Performance of motor operated food preparation appliances • AS 3106 – 1993, Approval and test specifications – electric jugs (with non-metallic bodies) • AS/NZS 3128 – 1998, Approval and test specifications – portable lamp standards and brackets • AS3000 – 1994, Electrical installation – buildings, structures and premises (SAA Wiring Rules). These documents prescribe anything from the distance a power point must be located away from a source of water, to how deep an electrical cable must be buried in the ground. The standards change regularly as new equipment and methods evolve. Safety labeling on Australian appliances Electric household appliances are increasing in use and in number. Therefore, it is essential that this equipment be used carefully. Careless use of electrical power can kill you. Part 4: Electricity and engineering drawing 35 When you purchase a household appliance or power tool, you will find that they have the seal of approval from a safety authority. Such authorities test electric products and approve only those that meet the Australian standards of safety. Look for the seal of approval from a safety authority on an electrical household appliance. Additionally, all household appliances are sold with instructions for their use. When using electric products, follow carefully the manufacturer's instructions. Forms of electrical safety Electrical appliances need protection devices incorporated into them and the circuitry to which they are connected. An electric appliance or device has three essential parts: 1 an internal piece of wire that carries current that is at a lethal voltage 2 insulation around or supporting the internal wire which prevents the frame or casing becoming live with electricity and being capable of electrocuting you 3 a frame, casing or cover which may be metallic, resin or polymer. Two faults that can cause damage and/or danger in an appliance, device or wiring installation are: • a short circuit – if insulation breaks down or if a wire becomes loose in a device and a live wire touches a metal frame, the casing, or another conductor, a short circuit occurs • overload – if a device, such as a drill, is pushed too hard into material being drilled, the speed drops and the device heats up the motor is being overloaded, excessive heat in the machine may cause the insulation to break down and this may create a short circuit. Electrical safety devices and practices can be classed as one of two types, passive (preventative) methods/devices and active (reactive) methods/devices. 36 Household appliances Passive (Preventative) electrical safety elements The most successful electrical safety element is to completely eliminate the electrical hazard. This may be done in several ways, ranging from something as simple as switching off the circuit (which is not always practical) to ensuring there are no electrically live exposed components by providing protective barriers or insulation. Preventative safety systems also include systems that prevent a hazardous situation from occurring at some stage in the future. Perhaps the most effective precaution against accidents involving electrical circuits is common sense. (Unfortunately, common sense is often less common that we might expect!) While legislators make every effort to prevent dangerous situations from occurring, no amount of legislation can hope to address each and every situation involving electricity and people. In the absence of a specific directive, we must rely on our own judgement to decide what is safe and what is not. It is simply not worth gambling with electricity! Earthing One of the most common passive electrical safety systems is the practice of providing safety earths. The electrical installation in your home is required to provide a number of safety earths, some of which you should be able to observe. When we stand either on the ground or on an object that is in contact with the ground, our bodies are normally considered to be at mains earth potential. If everything we can possibly touch or come in contact with is also at mains earth potential there is little risk of us receiving an electric shock. Manufacturers and electrical trades people go to significant lengths to ensure that as much as possible in our daily environment is kept at earth potential. Electrical service technicians often add additional earths to an electrical system to ensure their safety while they work. There should be a number of earth points on the electrical distribution system in your home. Near the distribution box you should be able to locate a wire that runs from the box down to a metal spike that is driven into the ground. The wire is commonly green, or green with a yellow stripe, or in older installations it may just be bare metal conductor. Part 4: Electricity and engineering drawing 37 Can you locate the main earth for your home? Figure 4.30 Earth stake in a domestic installation A green and yellow cable is connected to the network of safety earth wires in the power points of the house. Figure 4.30 shows the safety earth connected to a stake that is driven into the ground (‘earth’). The connecting clip is painted (white) to prevent corrosion. Many homes have an electric hot water service. Water is a conductor, and water pipes are often copper or galvanised metal, both of which are also good conductors. If there was a fault in an electric hot water service, it is possible the water, and so the pipes in your entire house may become live. For this reason the water pipes and gas pipes to your house are normally earthed. That way, if there is a fault, the pipes will remain safe to touch. Can you locate a water pipe (possibly under your house) that runs close to the distribution board and see a safety earth wire connected to it? 38 Household appliances Strain relief Electrical cables and plugs can become dangerous due to mechanical damage caused by repeated bending, or pulling on the cable sheath in the process of unplugging. Nearly all electrical connectors or wiring require some form of strain relief. This may be as simple as a clamping arrangement that prevents the cable and insulation from separating if pulled on with moderate force. Other forms of strain relief are common near a plug or where a flexible electrical cable enters an appliance. Often a polymer jacket is provided to limit the radius of curvature the cable can go through, to prevent breaking the internal metal conductors. Figure 4.31 Strain relief in a handheld hairdryer Notice that where the cable enters the appliance, it is both supported externally by a flexible polymer collar, and internally by wrapping it around a convoluted path so that the whole cable takes the strain, and not just the internal conductors. Cable clips that hold electrical wiring up out of the way are a form of strain relief that prevent the cable from being accidentally snagged and pulled on. Can you see electrical wiring under your house held by cable clips? Part 4: Electricity and engineering drawing 39 Figure 4.32 Cable clips supporting 240VAC wiring in a domestic installation Domestic electrical insulators tend to be soft polymers (some are hard polymers, others are ceramics). Where a soft coated electrical cable enters a chassis, it is usually through a grommet, or 'O' ring, to protect the insulation from damage. Remember that it is only the integrity of the insulation that makes an electrical cable, such as an extension cord, safe to touch and use. Once the insulation is damaged, it is likely that the cable is unsafe. Look at the power pack in figure 4.33. The cable is light and thin, and is thus susceptible to damage where it enters the hard polymer casing. Strain relief is offered by a soft polymer collar. The tag on the cable shows that the appliance has been tested for safety. The reverse side of the tag shows the date of next required test. Figure 4.33 40 Strain relief on a power pack Household appliances Because the outer insulation on modern electrical cables is soft, cables are often placed in a hard polymer or metallic conduit for additional protection. Figure 4.34 shows a polymer conduit carrying power cables from a main distribution board. The conduit is flexible, and being polymer offers mechanical protection and electrical insulation. Figure 4.34 Flexible polymer conduit carrying power cables in a domestic installation You should be able to identify a number of different electrical insulation materials on appliances in your home. List the insulation materials you can see used on: • a TV • an electric iron, • a toaster. Don't just look at the cable: what holds the live toasting element in place away from the chassis? Safety – do not open or dismantle any electrical appliances. Did you answer. • TV – polymer charge • electric iron – polymer body • toaster – polymer insulated components – ceramic body Part 4: Electricity and engineering drawing 41 Double insulation Another method of passive electrical safety is to use ‘double insulation’. With a single insulated device, it is common that some part or possibly all of its external casing is metallic. If the appliance develops a fault, it is possible that the outer protective casing could become live. To protect people from harm in an appliance failure, the Australian standards require that any exposed metal components be earthed, that is, connected to the bottom pin of the three-pin supply plug. With this connection, even in the case of a fault, the device casing will still be at a safe potential if someone touches it. Appliances constructed with metal panels such as refrigerators and washing machines are normally single insulated devices. You may note that for a single insulated device to remain safe, it is essential that the electrical connection to all exposed metal components is continuous all the way back to the earth connection for your home. If the earth connection is dirty, or broken, the appliance will still operate, but the user may be exposed to a hazardous potential if the appliance develops a fault, since the exposed metalwork is no longer connected to earth. There is a way to ensure the user is safe without relying on the continuity of an earth connection. This approach is called double insulation. In a double insulated device the user is protected by a number of layers of insulation. The outer casing for the device that the user may touch is normally made from an insulator (polymer and ceramics are common). Even if internal elements of the device become faulty, there is no possibility that the outer case may become live or pose a danger to the user. Double insulation is safer than single insulation since we are not relying on the continuity of an external earth connection for our continued safety. A double insulated electrical appliance typically has only two pins on its supply plug, while a single insulated device will typically have three pins on its supply plug. The doubly insulated appliance does not have an earth pin. Both the hair dryer in figure 4.31 and the power pack in figure 4.33 are doubly insulated appliances. The two pin plug on the power pack is clearly evident in the figure. Check the supply plugs for a number of electrical appliances in your home – some are likely to be double insulated, some are not and observe distinguishing the feature between a single insulated appliance, and a double insulated appliance (it may help to note what is the appliance casing made of). Turn to the exercise section and complete exercise 4.5 questions 4 to 6. 42 Household appliances Active (Reactive) electrical safety systems An active, or reactive, safety system works to prevent damage or injury after a fault occurs. Passive or preventative systems try to stop a fault from occurring in the first place. The most simply responsive action is to disconnect the faulty appliance or installation from the electrical supply. There are a number of devices designed to disconnect faulty appliances, these include fuses and circuit breakers. Fuses One of the simplest reactive protection devices is a fuse. All fuses act as a switch to disconnect an appliance from the electrical supply. The trigger for the disconnection is a higher than expected current consumption by the appliance that is protected by the fuse. A conventional fuse is remarkably simple. It is simply a fine filament of wire that is designed to melt when a current above a preset threshold passes through it. All electrical conductors have a finite current limit. Beyond this limit, the excessive current will heat the conductor, melting the insulation (in itself a dangerous factor), and eventually melting the conductor itself (a significant fire hazard). A fuse is simply a wire that is designed to melt and break before the main conductor gets too hot. Figure 4.35 shows a conventional mains voltage wire fuse holder (on the left), together with a low voltage fuse (on the right). The mains voltage fuse assembly is rated for 240VAC and 32 A. The fuse wire can be seen connected to one end of the holder (where it is held by a screw) and passing through the body of the holder to the other end. The fuse wire is special wire designed to melt at a specific level current. Blown fuses should never be repaired with ordinary wire since it will not melt to protect against over currents. The low voltage fuse is enclosed in a glass tube to protect the fine conducting filament. This fuse is rated (designed to melt) at 5 A. This type of fuse cannot be repaired, and must be replaced when damaged. Part 4: Electricity and engineering drawing 43 Figure 4.35 Mains rated and low voltage fuses Conventional fuses have a fundamental limitation. They rely on the buildup of heat to melt the conductor, and this takes a finite amount of time. It is possible to pass many times the rated current through a fuse, as long as the heat buildup in the fuse is not sufficient to melt the conductor. A standard 5 A mains fuse, will actually pass thousands of amps for a few microseconds. The short duration (microseconds) is not sufficient to heat and melt the fuse wire, but could be sufficient to damage the appliance that the fuse is (supposed to be) protecting. As with any protection device, it is important to understand its limitations, and choose a device appropriate for the protection conditions. Conventional thermal fuses are available in ratings from tens of milliAmps to tens of thousands of amps, and are common as protection devices in both domestic and industrial applications. In terms of protection, a fuse does little more than remove power from a faulty appliance, and prevent the excessive current in the appliance from causing an electrical fire. There are a large number of modern variants of a fuse. Some are capable of automatically resetting a certain time after the fault was removed, others specifically designed to rupture slowly. Some are designed for inductive low frequency loads, some are ceramic, and some are constructed from semiconducting materials. 44 Household appliances Figure 4.36 Examples of specialised fuses The fuses in figure 4.36 are rated at 500V, and 100A and 50A respectively. The bodies of the fuses are porcelain, with large electrically conducting surfaces at either end to carry the high currents. Once these fuses have melted (‘ruptured’) they have to be replaced. Remember fuses: • protect devices against short circuit • do not protect devices against light overload • do not directly protect humans. If you touch a live wire in a device, or in an extension lead in either of the following situations a fuse on its own will not protect you against electrocution – it may not even blow if you are also: • touching earthed metal (sink, water pipe or the like) • standing on damp concrete floor. Circuit breakers A fuse, while simple and effective, destroys itself in the process of protecting electrical equipment. The fuse needs to be replaced each time it operates. There are also certain electrical loads (such as highly inductive loads fed by direct current) that a fuse will not adequately protect. Circuit breakers perform the same general function as a fuse in that they remove power from an appliance if it starts to consume an excessive current. However, circuit breakers do not (normally) destroy themselves in the event of an over current condition, and can be reset by a switch that forms part of the breaker itself. Part 4: Electricity and engineering drawing 45 A circuit breaker tends to be a more reliable protection device than a fuse. furthermore, the time taken for a circuit breaker to react is more accurately specified, thus affording a greater degree of protection. As with fuses, breakers are commonly available from fractions of an amp to thousands of amp trip currents. In a modern domestic distribution board, such as is in our homes, the over current protection devices for each circuit are normally circuit breakers. Figure 4.36 shows a typical domestic installation of circuit breakers. Older installations tend to use fuse wire and ceramic holders. Figure 4.37 Circuit breakers replacing conventional fuses on a domestic distribution board In figure 4.37 the left most breaker is rated at 80A, and switches all circuits. The middle three breakers are for individual circuits and are rated at 20A and 10A. The right hand breaker is an earth leakage circuit breaker. Circuit breakers are also commonly found in many power boards. A power board is the modern equivalent of a double adapter, with the added safety feature that piggy backing too many power boards with a circuit breaker will automatically disconnect power – the conventional double adapter does not do this. Figure 4.37 shows a power board with an integral circuit breaker. If the board is subject to excessive currents, the breaker will trip, disconnecting power between the board and the mains supply. When this happens, all loads should be disconnected before resetting the breaker by pressing the red button. 46 Household appliances Figure 4.38 A power board with integral circuit breaker Residual current devices Another relatively common active protection element is a residual current device (RCD), previously an earth leakage circuit breaker (ELCB). The normal current return path for an electrical appliance is from the active supply, through the device, and return via the neutral connection. In normal operation the current in the active and neutral conductors will be exactly the same. However, because the neutral and the earth (or ground we stand on) are at the same potential, it is possible for the return current to use either the neutral and/or the earth connection. The presence of current in the earth connection, or alternatively, an imbalance in the active and neutral connections, normally indicates a fault condition. The fact that not all of the current is returning to the supply via the neutral conductor, means that a person may be exposed to an electrical shock hazard. The RCD is designed to trip when the earth current, or imbalance between active and neutral currents, exceeds some preset threshold. Most RCD’s look at the supply current balance, that is, the difference between the supply current in the active conductor and the neutral conductor. If the two currents are not the same, leaked current makes an electromagnet open a magnetic switch. This breaks the circuit. Part 4: Electricity and engineering drawing 47 In practice, there is rarely an exact balance of current between the active and neutral conductors. A variety of RCD’s are available with allowable current imbalances ranging from 10 milliamps to 30 milliamps. An RCD with an allowable current imbalance of 30 millamps or less is generally referred to as a ‘personnel grade’ RCD. These devices offer good (but not perfect) protection for people. Personnel grade RCD’s are the type commonly installed in domestic applications, and because of the relatively low allowable current imbalance these RCD’s significantly reduce the risk of receiving a significant electric shock. It should be noted that an RCD does not reduce the risk of electrocution to zero. Because an RCD is a reactive device, it will only trip after a fault has occurred. Even the relatively sensitive RCD’s that trip at a leakage current of 30 milliamps cannot guarantee to completely protect you in the case of an electrical fault. Also an RCD can not help to prevent shock if you are using an appliance that has no earth connection. All appliances that have only two pin power point connection plugs are in this category. However, the presence of an RCD or ELCB in the distribution board of your home significantly reduces the risk of electrocution. Turn to the exercise section and complete exercise 4.5 questions 7 to 9. Isolating transformers Isolating transformers are 240V to 240V transformers: • primary isolating transformers are connected to 240V mains supply • secondary isolating transformers supply 240 V to an appliance. A short circuit or an overload in an appliance connected to an isolating transformer will probably harm the appliance but not a person in contact with earth and live metal using the appliance. To enhance the effectiveness of an isolating transformer if fitted: 48 • never put a double adaptor or power board in the output of an isolating transformer • use only one appliance to one transformer. Household appliances Engineering communication skills The engineer’s most useful tool for communication is drawing. Drawing can be used to solve many of the problems that arise during the construction, from a simple one piece item to a more complicated object such as a building or machine. The household appliances you have examined would have had freehand sketches made during the development stage. These sketches might not be of the entire object, they may be of the mechanisms in the object. Freehand drawings Freehand drawings help turn thoughts into plans. They are drawn using a pencil. The use of freehand sketches is generally restricted to informal presentations and recording of ideas. Freehand drawing may require additional notes to specify the nature of changes and other forms of information that cannot be readily observed from the drawing. Freehand drawing are not always drawn to scale so the sizes of components and the processes to be used must be noted on the sketch. Different styles of drawing are made to help the person who is to use the drawing. When drawing freehand sketches a commonly used method is to make a three-dimensional pictorial sketch, known as a 3D sketch. However, the orthogonal drawing technique, using two-dimensional views, is very useful and also commonly used. It is important to note that each of the following style of freehand drawing has its own set of rules. • isometric • oblique • perspective • orthogonal Part 4: Electricity and engineering drawing 49 Isometric projection The angle used in isometric projection is 30°. This angle is used to draw both the left and right hand sides of the diagram. Figure 4.39 Isometric view of a toaster Oblique projection The angle used in oblique projection is 45°. Additionally, measurements along the 45° line in the oblique are halved in length. Figure 4.40 Oblique view of an object Perspective projection In the perspective style, each side of the diagram diminishes as it approaches a vanishing point. In figure 4.41 each side of the diagram has a vanishing point. In a variation to this style there can be a single vanishing point. 50 Household appliances Figure 4.41 Perspective view of on object Orthogonal freehand When 3D freehand sketches are complete, the sizes can be applied to the drawing. However, when the object is complicated, it is difficult to indicate the required sizes to a pictorial sketch. To show all the measurements it is more appropriate to use another style of drawing known as orthogonal drawing. Figure 4.42 shows an orthogonal drawing of a toaster. Figure 4.42 Three orthogonal views of toaster The pictorials give a quick overview of what the toaster might look like, the orthogonal gives the detail that would be needed to construct the toaster. The orthogonal views have specific locations, each view must be projected from the other views. Sizes are to a scale, and actual sizes can be added by the use of dimensioning. Part 4: Electricity and engineering drawing 51 Figure 4.43 Freehand pictorial sketch Technical drawing Technical drawing has a set of conventions and rules that can be used when making drawings. These conventions and rules are used so that people anywhere can understand the drawings. You may not know how to speak a foreign language but it is quite easy to follow the information given in a technical drawing from any country, provided you understand the rules and conventions. Turn to the exercise section and complete exercises 4.6 to 4.10. 52 Household appliances Exercises Exercise 4.1 a Explain the basic principle of electricity. _______________________________________________________ _______________________________________________________ _______________________________________________________ b Explain the following types of electrical supply: i direct current (DC) ___________________________________________________ ___________________________________________________ ___________________________________________________ ii alternating current (AC) ___________________________________________________ ___________________________________________________ ___________________________________________________ c Calculate: i the current that would flow in a circuit with a resistance of 3.3 Ohms that has 9V applied to it ___________________________________________________ ___________________________________________________ ___________________________________________________ ii the resistance needed in a 9V circuit if a current of 2 A is required ___________________________________________________ ___________________________________________________ ___________________________________________________ Part 4: Electricity and engineering drawing 53 Exercise 4.2 a i Find out the voltage of an appliance in your home. ___________________________________________________ ___________________________________________________ ii Calculate the amount of energy (in Joules) used in the appliance was switched on for 10 min. ___________________________________________________ ___________________________________________________ b Calculate the amount of energy used in kilowatt hours. (1 kWhr = 3 6000 000 Joules). _______________________________________________________ _______________________________________________________ _______________________________________________________ c The electricity used in your home is charged per kWhr. Find out how much this appliance has cost to run for this time. (Take a look at the electricity bill to find out the cost of a kWhr of electricity). _______________________________________________________ _______________________________________________________ _______________________________________________________ Exercise 4.3 Give a brief description of the principle of an electric motor. __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ Exercise 4.4 Explain what you understand by the term ‘safe’ when referring to electrical energy. __________________________________________________________ __________________________________________________________ __________________________________________________________ 54 Household appliances Exercise 4.5 Select the alternative A, B, C or D that best answers the question. Circle the letter. 1 2 3 4 5 It is against the law to: a report accidents that occurred more than two months after they happen b inspect appliances for obvious signs of damage c purchase electrical components from hardware stores d to repair a damaged electrical appliance without formal training. A voltage that is classified by Australian Standards as ‘low voltage’ is: a one supplied from a battery b dangerous only if you are not careful c generally safe to handle when live d potentially lethal. Voltages inside battery-powered appliances: a are always classified as ‘extra low voltage’ b rarely exceed 12–18 Volts c can reach ‘high voltage’ levels d are always the same as the battery voltage. Passive safety elements: a prevent accidents from happening b reduce the risk of accidents occurring c reduce the risk of injury in the case of an accident d all of the above. Strain relief on electrical appliances is designed to: a prevent the power cord from being damaged where it enters the appliance b disconnect the appliance from the electrical supply in case of an accident c limit the current flow in the event of an accident d all of the above. Part 4: Electricity and engineering drawing 55 6 7 8 9 56 A doubly insulated appliance: a relies on a sound earth connection to protect the user of an appliance b uses additional insulation to reduce the reliance on an external earth connection c is cheaper because it only needs a two pin plug d has two earth connections in parallel in case one of them fails. A blown fuse indicates: a a faulty earth connection b the presence of strong winds c excessive current has flowed in the fused circuit d normal operation. Circuit breakers are preferable to fuses because: a they are generally more reliable b they can be easily reset after operation c they can react to over currents faster d all of the above. Residual current devices: a disconnect the earth lead to prevent injury in the case of a fault b disconnect the neutral lead so that all of the current flows to earth c disconnects both active and neutral leads if there is a mismatch between currents in those two leads d connects the neutral wire to earth when a fault is detected. Household appliances Exercise 4.6 Draw a freehand orthogonal sketch of the ‘plug in’ end of an electrical cord. Show three views, top, front and side views. The top view is on top, the front view is below the top view, and the end view is beside the front view. No sizes are to be shown, and the freehand views should show the plug full size. Part 4: Electricity and engineering drawing 57 Exercise 4.7 Draw a freehand orthogonal sketch of a refrigerator. Show three views top, front, and side views. Remember that all views must be projected from each other. The top view is on top, the front view is below the top view, and the end view is beside the front view. Three major sizes are to be indicated, and the freehand views should show the refrigerator at an approximate scale of 1:20. 58 Household appliances Exercise 4.8 Draw a freehand pictorial sketch, using oblique principles, of a refrigerator and indicate the approximate scale used. Part 4: Electricity and engineering drawing 59 Exercise 4.9 Draw a freehand pictorial sketch, using perspective principles, of an electric toaster. Exercise 4.10 Draw a freehand pictorial sketch, using isometric principles, of a washing machine. 60 Household appliances Progress check During this part you investigated energy and engineering drawing. ✓ ❏ Disagree – revise your work ✓ ❏ Uncertain – contact your teacher Uncertain Agree – well done Disagree ✓ ❏ Agree Take a few moments to reflect on your learning then tick the box that best represents your level of achievement. I have learnt about • the basic principles [of electricity ]… – potential difference, current, simple circuits and components • electrical safety – related to Australian electrical standards • magnetic induction • fundamentals of AC and DC circuits • electric motors. I have learnt to • explain the basic electrical principles of operation appropriate to household appliances • appreciate the importance of electrical safety when using electrical household appliances • explain the working of an induction motor • distinguish between AC and DC circuits • draw freehand, three-dimensional objects • conduct research using computer technologies and other resources. Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to <http://www.boardofstudies.nsw.edu.au> for original and current documents. During the next part you will investigate a household appliance and produce an engineering report on the product. Part 4: Electricity and engineering drawing 61 62 Household appliances Exercise cover sheet Exercises 4.1 to 4.10 Name: _______________________________ Check! Have you have completed the following exercises? ❐ Exercise 4.1 ❐ Exercise 4.2 ❐ Exercise 4.3 ❐ Exercise 4.4 ❐ Exercise 4.5 ❐ Exercise 4.6 ❐ Exercise 4.7 ❐ Exercise 4.8 ❐ Exercise 4.9 ❐ Exercise 4.10 If you study Stage 6 Engineering Studies through a Distance Education Centre/School (DEC) you will need to return the exercise pages with your responses. Return the exercise pages with the Title Page cover attached. Do not return all the notes, they should be filed for future reference. If you study Stage 6 Engineering Studies through the OTEN Open Learning Program (OLP) refer to the Learner’s Guide to determine which exercises you need to return to your teacher along with the Mark Record Slip. Part 4: Electricity and engineering drawing 63 Household appliances Part 5: Household appliances – engineering report Part 5 contents Introduction.......................................................................................... 2 What will you learn?................................................................... 2 An engineering report ........................................................................ 3 Research skills.......................................................................... 3 Aims of an engineering report..................................................... 5 Structure of an engineering report............................................... 5 Developing an engineering report ............................................... 7 Sample engineering report ......................................................... 8 Exercise ..............................................................................................15 Progress check ..................................................................................17 Exercise cover sheet.........................................................................19 Bibliography........................................................................................21 Module evaluation .............................................................................23 Part 5: Household appliances – engineering report 1 Introduction During the Engineering Studies course you will complete an engineering report as part of each module. Each engineering report may vary depending on the purpose. An engineering report must contain technical information which should be communicated in a variety of formats – words are essential, but so are freehand and technical drawings. A wide range of sources of information need to be consulted and the material used documented. There must be evidence of analysis which supports conclusions or recommendations. As this is the last part of the module you should demonstrate the knowledge and skills you have gained to produce the best possible engineering report. Your engineering report should provide clear evidence of the level of achievement in this module. In this part you will investigate the materials used in a household appliance of your choice and explain manufacturing or inservice properties of the materials. What will you learn? You will learn about: • engineering report writing • communication – research methods including the Internet, CD-ROM and libraries – collaborative work practices. You will learn to: • complete an engineering report based on the analysis of one or more household appliances, integrating the use of computer software • conduct research using appropriate computer technologies and other resources. Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999. Refer to <http//ww.boardofstudies.nsw.edu.au> for original and current documents. 2 Household appliances An engineering report An engineering report is a formal, considered document which draws together information gained about a product or field, through research and analysis, to arrive at a conclusion or present recommendations based on investigation. Engineers do not communicate with words alone. In an engineering report, technical information is presented using a combination of text, tables, graphs and diagrams. An engineering report for an application module involves: • outlining the area under investigation • collecting and analysing available data • drawing conclusions and/or proposing recommendations • acknowledging contributions form individuals or groups • recording sources of information • including any relevant additional support material. An engineering report for a focus module involves covering additional aspects such as: • examining the nature of the work done by the profession • discussing issues related to the field. Research skills One of the main challenges of using the Internet to search for information is being able to refine your search to find what you are looking for. When looking for information it is important that you know exactly what terms you are looking for and/or alternative terms that will help locate the information. Part 5: Household appliances – engineering report 3 For example, when locating information on the topic ‘Magnetic Induction’ via the CD-ROM of an encyclopaedia, I found my search unsuccessful. However, using a bit of lateral thinking I was able to develop a number of alternative words and phrases that eventually proved more successful, including • induction • magnetic • electric • electric induction. The process of developing alternative terms is essential for successful research. Time spent developing these alternatives will in the long term save you time and when on the Internet, money. Preparing for your engineering report This is the last part before commencing your first engineering report. The sample engineering report will feature toasters. The engineering report is more detailed than the brief case studies you have read. Your report requires you to complete research. You will need to begin (continue?) your research this week. You may base your report on any type of household appliances you have access to. Do not dismantle any working appliance! It is most important that no electrical appliance has its electrical compartment opened. The 240 volt power will kill. Any appliance tampered with will be a danger to all future users. Do not take the risk with your life and never take the risk with other peoples lives. The report has many sections, but a critical section that requires speciality research involves material used. Try to find information about specific materials that you have identified in the product. For instance, you can see a metal part that looks like chrome. Research Chromium plated steel and prepare information for your report. It will be more difficult to specifically identify polymers. You may be able to contact the manufactures directly. In the case of a particular part of appliances, such as the electrical cord, you may research electrical cord directly rather than through the appliance manufacturer. 4 Household appliances Begin your research on the materials in a household appliance before you start Part 5. This will make it easier for you to analyse your product and to draw conclusion about the materials used. Aims of an engineering report A well structured engineering report aims to: • demonstrate effective management, research, analysis and communication skills related to the content • include data relevant to the area under investigation • present information clearly and concisely so that it is easily understood by the reader through the use of tables, graphs and diagrams to illustrate mathematical and scientific facts • justify the purpose using observations, calculations, or other evidence, to support a conclusion or recommendations. • document contributions and sources of information. Structure of an engineering report An engineering report generally includes the following sections: • title page • abstract • introduction • analysis • result summary • conclusions/recommendations • acknowledgments • bibliography • appendices. Title page The title page gives the title of the engineering report, identifies the author and gives the date when the report was completed. Part 5: Household appliances – engineering report 5 Abstract The abstract is a concise statement that describes the content of the engineering report. It covers the scope of the report (what it is about) and the approaches used to complete the analysis (how the information was assembled). The purpose of the abstract is to allow a reader to decide if the engineering report contains relevant information. The abstract should be no more than two or three paragraphs – shorter if possible. Introduction The introduction provides an overview of the subject, purpose and scope of the engineering report. It may contain background information regarding the topic. It also outlines the sections of the engineering report including why the investigation was undertaken, what research occurred, how data was collected and what anaylsis was conducted. Analysis The analysis is the body of the engineering report and should show evidence of research and experimentation. Information about materials and the mechanics of products should be collected or calculated for all engineering reports. This section must contain information required to satisfy the aim and purpose of the report. Tables and graphs, used to summarise detailed data in a concise form, are common features of an engineering report. Result summary The result summary should present the results concisely and note any limitations on the investigation. The results inform and support the conclusions and recommendations. Conclusions/recommendations The conclusions/recommendations summarises major points or issues in earlier sections of the engineering report. This section requires the author to draw conclusions or make recommendations based on data collected. If the purpose of the engineering report was to ‘select the best…..’, then the selection should be stated and the reason for the choice explained. 6 Household appliances Acknowledgments The acknowledgment section provides the opportunity to credit other people’s work that has contributed to the report. Bibliography The bibliography demonstrates that the report is well researched – all references need to be included. Bibliographic entries should follow established guidelines. A standard approach for referencing bibliographic entries includes identifying the name of the author, the year of publication, the title of the work, the name of the publisher and the place of publication. For example: Ritchie. J, and Simpson. G, 1998 Engineering Application – A projectbased approach, Butterworth-Heineman, United Kingdom. This information allows the reader to source the information for confirmation of the details or conduct further research. Appendices The appendices should contain detail that has been separated from the main body of the engineering report. The information in this section is not essential but enhances the other data. Examples could be engineering drawings of products being compared, where the overall dimensions of the product may not have been part of the report, but may be relevant to some readers. During the engineering course this section may contain a technical drawing and could include information collected from organisations. Developing an engineering report Research and collaboration are the keys to developing an accurate and informative engineering report. Research methods Research is a critical function for professional engineers. The process involves: 1 Clarifying the issue The first step involves clarifying the issue under investigation and selecting an approach. This may require selecting sample materials, experimentation, working collaboratively with others. Part 5: Household appliances – engineering report 7 2 Collecting data The second step involves collecting data. Sources such as the Internet, CD-ROM, encyclopedia, texts and journals are all locations where information can be gathered. NOTE: Take care when gathering information from the Internet. Verify the accuracy and reliability of the information by checking the qualifications of the source, it cannot be assumed that the person(s) placing the information on the Internet is an expert on the subject. 3 Analysing and interpreting information The third step involves relating the evidence collected to support conclusions drawn or recommendations made. Collaborative work practices Collaboration involves working with others. It is an effective and efficient means of obtaining information and support during a project. The degree of collaboration can range from including the contribution of others through discussion to the involvement of a team depending on the project. Sample engineering report The following section contains a sample engineering report on a household appliance – the electric toaster. The sample engineering report compares an early model to a late model appliance. You can use the sample engineering report as a guide when presenting your work. Your engineering report will investigate the materials used in a household appliance of your choice and explain manufacturing or inservice properties of the materials. 8 Household appliances Household appliances Title: Comparison of the materials used in an early model electric toaster to a late model electrical toaster. Author: B. Zarzoff Date: January 2000 Abstract The report makes a comparison of the materials used in two electric toasters – the Johnson 21B (1930) and the ‘Ubeaut’ (2000). Introduction The report examines two electric toasters, investigating the materials used. The aim of this report is to identify and distinguish various materials. The analysis section concerns identification of materials used in various parts of the toasters. The results section presents the data in a table format. The conclusion explains the differences in the materials used and suggests reasons for the change. The acknowledgement section and the bibliography section lists resources consulted. The appendices contains related information and technical drawings. Analysis The main components of the electric toaster include the: • electrical cord - wire plus insulation • electrical plug - screws, fittings and body • base • electrical element • sliding bread holder • springs • control switch • crumb catching drawer. The three main materials used in the electric toaster and the engineering properties of each are: 1 2 3 Steel – rigid and strong in service – heat resistant – wear resistant – particularly necessary at the pivot points. Copper – flexible (the electrical cord can be bent without fracturing the wire) – heat resistant – conduct electricity with little resistance. Thermosetting polymer – smooth surface finish making it easy to clean – insulator remains cool to the touch stable under variations in heat. This section should continue to identify materials used in the toaster component by component. Results summary The following table compares the materials used in the early model toaster to the materials used in a late model toaster. Part Early model toaster material Late model toaster material electrical cord copper wire, woven cloth coated with rubber copper, multistrand wire, thermosoftening polymer insulation electrical plug – screws and fittings moulded thermosetting polymer plug, steel screws thermosoftening polymer plug, no metal fasteners in plug Base/body chrome plated steel polymer coated steel heating element nichrome nichrome bread holder chrome plated steel chrome plated steel springs medium carbon steel medium carbon steel, heavily work hardened control switch/knob bakelite – thermosetting polymer thermosetting polymer crumb catching drawer mild steel polymer tray A results summary should be concise overview of the information gathered. Table format is typical, but graphs and text are also acceptable and appropriate in particular circumstances. Conclusions Based on the inspection of the two toasters, a noticeable change has occurred in the use of materials. The main trend has involved the increasing use of polymers, particularly thermosoftening types. These changes are likely to have been made by the manufacturer because: • new materials have become available that fulfil the needs of the component but can be purchased at a lower cost than the original material • new manufacturing techniques are available that are suited to particular materials, notably polymers • there appears to be an increase in the number of safety features present in the new toaster, again linked to polymers and their insulating properties. Some materials have maintained their useage. For instance, steel remains integral to both toasters. The conclusion needs to directly relate to the abstract and introduction of the report. All statements should be supported by evidence, that is data. That data should be included in the report. Acknowledgments Use this section as evidence that extensive contacts have been made. Bibliography Bolton, W, 1998, Engineering materials Technology, Newness Butterworth Heinemann Ltd, Oxford. Brown, D, 1981, Basic Albany New York. Metallurgy, Delmar Pub. Inc, Sheedy, P. A, 1989, Materials – properties, R.Brown & Associates, Bathurst NSW. <www.csiro.au> Data Use this section as evidence that extensive research has been undertaken and the report is a well researched document. Appendices Technical Drawings Isometric sketch of the toaster Figure 5.1 Isometric sketch Orthogonal drawing of the toaster Figure 5.2 Left side view Orthogonal drawing Front view Complete a technical drawing by hand, by instrument or by CAD. This technical drawing should provide evidence that the report writer is able to use technical drawing as a communication tool. Safety issues • Electrical appliances are potentially fatal. They should always be kept at a safe distance from water. • Toasters should always be unplugged before attempting to remove stuck toast. Contact with the electrical wires inside can cause death. • Any electrical appliance can cause fires. Smoke detectors should be installed in all homes. • Young children should not operate electrical equipment. • Only profession electricians should repair damaged electrical leads. The early model toaster appears to have less electrical insulation. The insulation materials used appear to have worn, as the material was cloth based. There also appears to be some mica used as insulation in various insulation positions. Heat insulation is not good on the early model toaster. The outside surfaces heat to a potentially dangerous temperature. This information, while not directly related to the topic, it is vitally important. Exercises Exercise 5.1 Report on materials used in a household appliance. • Investigate the materials used in the product, component by component. • Explain what manufacturing or service properties are required for the material in each component and why the material is suitable. The conclusion for this report should describe any trends you noticed in the application of materials for particular components or functions in household appliances. Remember, it is important to use graphics, tables and technical drawings to support your engineering report and integrate the use of computing software as appropriate. You should have completed a lot of the background information and research during each part of the module, now it is time to pull it all together into a well presented engineering report. Contact your teacher with any queries and to discuss your engineering report. Part 5: Engineering report 15 Try to: • avoid euphemisms, for example, ‘previous owned’ should be ‘second hand’ • be brief and strive for clarity in your statements • base your conclusions on the data you collect. You may choose any household product you have available. A rejected broken down appliance is probably a good choice. Read through the format and make yourself familiar with what you need to answer. If possible, telephone your teacher to discuss your choice of household appliance. Do not dismantle any electrical appliance that will be used again. Locate an old appliance that can be/has been discarded. Do not reassemble an appliance you have dismantled – electricity kills! 16 Household appliances Progress check During this part of the module you produced an engineering report on a household appliance. ✓ ❏ Disagree – revise your work ✓ ❏ Uncertain – contact your teacher Uncertain Agree – well done Disagree ✓ ❏ Agree Take a few moments to reflect on your learning then tick the box that best represents your level of achievement. I have learnt about • engineering report writing. I have learnt to • complete an engineering report based on the analysis of one or more household appliances, integrating the use of computer software. Extract from Stage 6 Engineering StudiesSyllabus, © Board of Studies, NSW, 1999. Refer to <http://www.boardofstudies.nsw.edu.au> for original and current documents. Congratulations! You have now completed Household appliances. Part 5: Engineering report 17 18 Household appliances Exercise cover sheet Exercise 5.1 Name: _______________________________ Check! Have you have completed the following exercise and included all the sections? ❐ Exercise 5.1 • title page • abstract • introduction • analysis • result summary • conclusions • acknowledgments • bibliography • appendices. If you study Stage 6 Engineering Studies through a Distance Education Centre/School (DEC) you will need to return the exercise pages with your responses. Return the exercise pages with the Title Page cover attached. Do not return all the notes, they should be filed for future reference. If you study Stage 6 Engineering Studies through the OTEN Open Learning Program (OLP) refer to the Learner’s Guide to determine which exercises you need to return to your teacher along with the Mark Record Slip. Please complete and return the module evaluation that follows. Part 5: Engineering report 19 20 Household appliances Bibliography Board of Studies, 1999, The New Higher School Certificate Assessment Support Document , Board of Studies NSW, Sydney. Board of Studies, 1999, Stage 6 Engineering Stuidies Examination, Assessment and Reporting, Board of Studies NSW, Sydney. Board of Studies, 1999, Stage 6 Engineering Stuidies Support Document , Board of Studies NSW, Sydney. Board of Studies, 1999, Stage 6 Engineering Stuidies Syllabus, Board of Studies NSW, Sydney. Eide, A. Jenison, R. and Northup, L. 1998, Introduction to Engineering Design and Problem Solving, McGaw Hill, United States. Goldman Ruben, S. 1998, Toilets, Toasters & Telephones, Hardcourt Brace & company, Florida. Johnston, Gostelow & Jones, 1999, Engineering and Society – An Australian Perspective, Longman, Melbourne. Ritchie. J, and Simpson. G, 1998 Engineering Application – A projectbased approach, Butterworth-Heineman, United Kingdom. Faraday. M, 1844–1855, Experimental Researches in Electricity, Vol 1-3, Taylor & Francis, London. 21 22 Module evaluation To help us make improvements to future learning materials we would like your comments on this material. 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Which were the most challenging parts of the material? __________________________________________________________ __________________________________________________________ Please return this form to your teacher to forward on to OTEN – LMP. Thank you for this valuable information. 24 Learning Materials Production Training and Education Network – Distance Education NSW Department of Education and Training
