Download Unit 03 - Delivery guide

Cambridge
TECHNICALS
CAMBRIDGE TECHNICALS
IN ENGINEERING
LEVEL 3 UNIT 3 – PRINCIPLES OF MECHANICAL
ENGINEERING
DELIVERY GUIDE
Version 1
OCR LEVEL 3 CAMBRIDGE TECHNICALS IN ENGINEERING
CONTENTS
Introduction3
Related Activities
4
Key Terms
5
Misconceptions7
Suggested Activities:
Learning Outcome (LO1)
8
Learning Outcome (LO2)
10
Learning Outcome (LO3)
11
Learning Outcome (LO4)
12
Learning Outcome (LO5)
13
PRINCIPLES OF MECHANICAL ENGINEERING
2
This Delivery Guide has been developed to provide practitioners with a variety of
creative and practical ideas to support the delivery of this qualification. The Guide
is a collection of lesson ideas with associated activities, which you may find helpful
as you plan your lessons.
Unit 3 Principles of mechanical engineering
LO1
Understand systems of forces and types of loading on mechanical
components
OCR has collaborated with current practitioners to ensure that the ideas put forward in
this Delivery Guide are practical, realistic and dynamic. The Guide is structured by learning
outcome so you can see how each activity helps you cover the requirements of this unit.
LO2
Understand fundamental geometric properties
LO3
Understand levers, pulleys and gearing
We appreciate that practitioners are knowledgeable in relation to what works for them
and their learners. Therefore, the resources we have produced should not restrict or
impact on practitioners’ creativity to deliver excellent learning opportunities.
LO4
Understand properties of beams
LO5
Understand principles of dynamic systems
PRINCIPLES OF MECHANICAL ENGINEERING
INTRODUCTION
Whether you are an experienced practitioner or new to the sector, we hope you find
something in this guide which will help you to deliver excellent learning opportunities.
Unit aim
All machines and structures are constructed using the principles of mechanical
engineering. Machines are made up of components and mechanisms working in
combination. Engineers need to understand the principles that govern the behaviour of
these components and mechanisms. This unit explores these principles and how they are
applied.
By completing this unit learners will develop an understanding of:
•
•
•
•
•
systems of forces and types of loading on mechanical components
the fundamental geometric properties relevant to mechanical engineering
levers, pulleys and gearing
the properties of beams
the principles of dynamic systems
Opportunities for English and maths
skills development
We believe that being able to make good progress in English and maths is essential to
learners in both of these contexts and on a range of learning programmes. To help you
enable your learners to progress in these subjects, we have signposted opportunities
for English and maths skills practice within this resource. These suggestions are for
guidance only. They are not designed to replace your own subject knowledge and
expertise in deciding what is most appropriate for your learners.
EnglishMaths
Please note
The timings for the suggested activities in this Delivery Guide DO NOT relate
to the Guided Learning Hours (GLHs) for each unit.
Assessment guidance can be found within the Unit document available from
www.ocr.org.uk.
The latest version of this Delivery Guide can be downloaded from the OCR website.
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OCR LEVEL 3 CAMBRIDGE TECHNICALS IN ENGINEERING
If you have any feedback on this Delivery Guide or suggestions for other resources you
would like OCR to develop, please email [email protected].
3
OCR LEVEL 3 CAMBRIDGE TECHNICALS IN ENGINEERING
RELATED ACTIVITIES
The Suggested Activities in this Delivery Guide listed below have also been related to other Cambridge Technicals in Engineering units/Learning Outcomes (LOs). This could help with
delivery planning and enable learners to cover multiple parts of units.
This unit (Unit 3)
Title of suggested activity
Other units/LOs
LO1
Resolving forces
Unit 1 Mathematics for engineering
LO4 Be able to use trigonometry in the context of engineering problems
LO1
Resultant forces
Unit 1 Mathematics for engineering
LO1 Understand the application of algebra relevant to engineering problems
LO1
Tension and compression
Unit 2 Science for engineering
LO4 Understand properties of materials
LO5
Newton’s Laws of Motion
Unit 2 Science for engineering
LO2 Understand fundamental scientific principles of mechanical engineering
Unit 1 Mathematics for engineering
LO1 Understand the application of algebra relevant to engineering problems
Unit 2 Science for engineering
LO2 Understand fundamental scientific principles of mechanical engineering
Unit 1 Mathematics for engineering
LO1 Understand the application of algebra relevant to engineering problems
LO5
Constant acceleration formulae
LO5
Conservation of energy
Unit 2 Science for engineering
LO2 Understand fundamental scientific principles of mechanical engineering
LO5
Work done and energy
Unit 1 Mathematics for engineering
LO1 Understand the application of algebra relevant to engineering problems
LO5
Power
Unit 1 Mathematics for engineering
LO1 Understand the application of algebra relevant to engineering problems
LO5
Conservation of momentum
Unit 1 Mathematics for engineering
LO1 Understand the application of algebra relevant to engineering problems
PRINCIPLES OF MECHANICAL ENGINEERING
4
UNIT 3 – PRINCIPLES OF MECHANICAL ENGINEERING
Explanations of the key terms used within this unit, in the context of this unit
Key term
Explanation
Bending moment
Bending moments result from loading on a beam. Bending moments result from equal and opposite tensile and compressive stresses that act on opposite edges
of the beam and maintain the internal equilibrium of the cross section of the beam.
Cantilever Beam
A beam that is supported at one end, preventing transverse and rotational movement. The other end is unsupported and is free to move as load is applied to the
beam.
Centroid
The point at which the weight of a component can be considered to act.
Concurrent forces
A system of forces that not only act in the same plane but all pass through a single point. This is associated with problems of particle mechanics.
Coplanar forces
A system of forces that all act in the same 2D plane.
Direct Strain
Strain (ξ) is defined as the change in length (∆l) divided by the original length (l); ξ=∆l/l
Equilibrium
5
Stress (σ) is defined as the load carried per unit area;
σ = Force/Area = F/A
A component, or body, is said to be in equilibrium if all forces and moments acting on the component, or body, are balanced so there is no net force or moment.
Mechanical
Advantage
Mechanical Advantage (MA) is the ratio of the output force to the input force of a simple machine (such as a lever or gear system).
Mechanical Energy
A body, or component, can possess mechanical energy either because it has linear or rotational velocity (kinetic energy) or because of its position (height) relative
to a defined datum position (potential energy).
Mechanical Power
Mechanical power is the rate of change of mechanical energy or the rate at which work is done.
Momentum
Momentum is a vector quantity defined as the product of its mass times its velocity. In a collision of two objects, if no external forces are involved, the total
momentum of the system will always be conserved.
Non-concurrent
forces
A system of forces that do not pass through the same point. This is associated with rigid body mechanics.
Particle mechanics
An engineering situation that can reasonably be represented by a component that is a single point. All forces acting on the particle should, therefore, be
concurrent.
Prism
Component with a constant cross section along its length.
OCR LEVEL 3 CAMBRIDGE TECHNICALS IN ENGINEERING
Direct Stress
PRINCIPLES OF MECHANICAL ENGINEERING
KEY TERMS
OCR LEVEL 3 CAMBRIDGE TECHNICALS IN ENGINEERING
Explanations of the key terms used within this unit, in the context of this unit
Key term
Explanation
Rigid body
mechanics
An engineering situation where the spatial relationship between forces must be represented.
Shear force
A transverse force acting in the same plane as the cross section of a component.
Shear stress
Shear stress (τ) is defined by as the shear force (SF) carried by each unit area of the cross section (A) carrying the shear force. τ = SF/A
Simply Supported
Beam
A beam with two supports which prevent transverse movement but allow rotational movement at the support.
Velocity Ratio
Velocity Ratio (VR) is the ratio of the output speed to input speed of a simple machine.
Work done
The work done by a force is defined as the product of the force and the distance the force moves (in its direction of action).
PRINCIPLES OF MECHANICAL ENGINEERING
6
7
What is the misconception?
How can this be overcome?
Resources which could help
Algebraic manipulation
Practice of basic techniques.
Unit 01 LO1 of this qualification
Vector and scalar quantities
Learners should be alert to the difference between scalar and vector quantities - for example the
displacement and the distance travelled by a particle in a given time are not necessarily the same.
Unit 02 LO2 of this qualification
http://www.physicsclassroom.com/
class/1DKin/Lesson-1/Scalars-andVectors
Units of measurement
Learners should always be required to state quantities using appropriate metric units.
http://www.engineeringtoolbox.
com/si-unit-system-d_30.html
PRINCIPLES OF MECHANICAL ENGINEERING
Some common misconceptions and guidance on how they could be overcome
OCR LEVEL 3 CAMBRIDGE TECHNICALS IN ENGINEERING
MISCONCEPTIONS
OCR LEVEL 3 CAMBRIDGE TECHNICALS IN ENGINEERING
SUGGESTED ACTIVITIES
LO No:
1
LO Title:
Understand systems of forces and types of loading on mechanical components
Title of suggested
activity
Suggested activities
Suggested timings
Learners must understand different types of loading that could be applied to a mechanical
component. This fundamental topic could be taught with reference to examples that are familiar to
the learner. Common examples are:
30 minutes
Types of loading
•
•
•
Also related to
Direct forces
–– lifting/pushing/pulling an object
Turning forces
–– tightening a bolt
–– drive shaft in a car
Shear forces
–– riveted joints
–– simple pinned connections (eg. clevis pins)
Learners should understand that turning forces are often produced by direct forces acting at a
distance from the component.
PRINCIPLES OF MECHANICAL ENGINEERING
Resolving forces
This has considerable overlap with LO4 of Unit 1 dealing with trigonometric functions of sine, cosine
and tangent. In this context it would be helpful for teachers to stress the need to define the positive
axes (typically x and y) before carrying out calculations. Learners should always define the coordinate
system they are using to define orthogonal forces. Understanding could be reinforced by working
through suitable examples.
Concurrent and Nonconcurrent systems of
co-planar forces
Teachers could use real world examples to illustrate the difference between concurrent and non1 hour
concurrent force systems. For example for some calculations an aircraft in flight might be represented
as a particle acted on by four forces (lift, weight, thrust, drag) whereas in other situations the same
aircraft would be represented by a much more complex mathematical model involving many more
forces. There are many other suitable contexts such as the human body and motor vehicles that could
be used to illustrate this point.
Force diagrams
1 hour
Unit 1 LO4
In problems of particle mechanics all forces acting on an object should be shown passing through a
single point (the particle). In problems involving rigid body mechanics, it is essential that the line of
action (i.e. the exact placement) of forces is shown.
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Suggested activities
Suggested timings
Also related to
Equilibrium
It is important that learners understand that for problems of particle mechanics there are two
independent conditions of equilibrium (usually taken to be horizontal and vertical equilibrium), but
for rigid body problems a third (rotational) equilibrium condition is added.
30 minutes
Resultant forces
This builds on work already covered (above), adding horizontal components and vertical components
of several forces to find the result of all forces as orthogonal components. Learners should be able to
use trigonometry to determine the angle at which the resultant will act. Learners should understand
that the equilibrant is a force that is equal in magnitude but opposite in direction to the resultant of
the set of forces.
1 hour
Unit 1 LO1
2 hours
Unit 2 LO4
PRINCIPLES OF MECHANICAL ENGINEERING
Title of suggested activity
See Lesson Element Equilibrium For non-concurrent forces learners should be able to find the line of action of the resultant to produce
the same turning effect as the individual forces.
of a Rigid Body
Direct stress and strain and
Young’s Modulus
Stress v. strain graphs
It is important that teachers introduce learners to the concepts of Stress and Strain as a way of
understanding the effects of loads on the material rather than the effects of loads on a component
made from that material. Length, including the conditions required for the assumption of uniform
stress and strain to be valid might be a good starting point. This is well presented in
https://www.youtube.com/watch?v=ZiSwnUDnnIM
Discussion around the basic formula (Stress = Force/Area and Strain = Extension/Original) A simple
model made from a soft elastic material (e.g. a synthetic sponge) could be useful to illustrate uniform
stress and strain.
Learners could be provided with a selection of stress v strain curves representing different materials
and asked to describe how a component made from the material would behave when put into axial
tension or compression.
Learners could be set a task to research the behaviour of common engineering materials and thereby
to develop an understanding of how the properties of the material can be identified from the graph
of stress against strain.
A good overview of this topic can be seen at https://www.youtube.com/watch?v=0qGrbZPeQew
Shear stress
9
Teachers could make simple demonstration pieces to illustrate a clevis pin in single and double shear.
This should help to make clear to learners the cross sectional area carrying the shear load. Worked
examples and follow up questions could be used to reinforce understanding.
30 minutes
OCR LEVEL 3 CAMBRIDGE TECHNICALS IN ENGINEERING
Tension and compression
OCR LEVEL 3 CAMBRIDGE TECHNICALS IN ENGINEERING
SUGGESTED ACTIVITIES
LO No:
2
LO Title:
Understand fundamental geometric properties
Title of suggested activity
Suggested activities
Suggested timings
Irregular 2D shapes
Learners should practice breaking down a complex shape into a combination of simple rectangles,
right angled triangles and semi-circles using addition and subtraction of these shapes.
30 minutes
The volume of a regular
prism
These are straightforward calculations. Greater engineering context might be achieved by basing
30 minutes
calculations on real examples (e.g. structural components around the school/college) using measured
dimensions and researched material densities.
The mass of a body
The centroid of a body
Learners must understand that the centroid of a body defines the point at which the weight of that
body will act. This is important for many problems of rigid body mechanics.
15 minutes
Shapes with axes of
symmetry
Learners should be able to use of axes of symmetry of a uniform 2D figure to find its centroid. This can
be demonstrated by using 2D models of uniform material with two or more axes of symmetry: draw
axes of symmetry to locate their crossing point. It should be possible to suspend the 2D shape from
this point without any rotation of the body.
30 minutes
Centroids of common nonsymmetrical 2D shapes
Learners should be taught the position of the centroids of
15 minutes
Moment of area
This is a straightforward technique. Results could be verified using the same 2D model technique as
above. A clear presentation of the method can be found at
https://www.youtube.com/watch?v=nAEBQQNAmzU
Also related to
• Right angled triangles
• Semi circles
This is straightforward factual knowledge. No formal proof is required for this specification. The given
values could be reinforced and demonstrated by using 2D models, as above.
1 hour
PRINCIPLES OF MECHANICAL ENGINEERING
10
LO No:
3
LO Title:
Understanding levers, pulleys and gearing
Title of suggested activity
Suggested activities
Suggested timings
Mechanical advantage (MA)
and velocity ratio (VR)
It might be useful for teachers to emphasise that the terms MA and VR apply to many types of simple
machines. MA being the ratio of output force to input force, and VR the ratio of the velocity output of
the machine to the velocity of the input. It is useful for learners to know that VR is the inverse of MA
(and vice versa).
30 minutes
Classes of lever
Use common examples of levers to illustrate the different classes of lever. For example,
• Class One - Oars in a rowing boat
• Class Two - Wheelbarrow
• Class Three - Tweezers
30 minutes
Also related to
PRINCIPLES OF MECHANICAL ENGINEERING
SUGGESTED ACTIVITIES
See Lesson Element Case study
of levers: cycle brakes
Learners must be able to identify the fulcrum of a lever and the distances from the fulcrum to the
input and output forces, and use these to calculate the MA and VR for the system.
Gears and gear systems
Learners should be able to identify and name different types of gears and gear systems, and their
applications This could be set as a research activity, or examples given of different examples of
applications of gears, for example those used in machine tools, especially lathes. There is considerable
scope for extension beyond the immediate specification content to include involute gear profiles,
helical cut gears etc.
1 hour
MA and VR for spur gears
This is a straightforward calculation, but could be illustrated using one of the many types of
construction kits (e.g. Lego, Fischer Technic) or ‘home’ made gears (see https://woodgears.ca/gear_
cutting/template.html for suitable profiles).
1 hour
MA and VR for simple
This is a relatively simple extension of the above. Worked examples and practice exercises could be
compound spur gear systems used to reinforce understanding.
1 hour
Pulley and belt drive systems
There are many examples of belt driven systems in automotive and machine tool applications. These
could form the basis for a research activity for learners to begin to understand the advantages and
disadvantages of the different types of belts and their advantages and disadvantages compared to
other methods of transmitting rotational motion.
1 hour
MA and VR for belt drive
systems
The concepts involved are very similar to those for spur gears.
1 hour
See Lesson Case study for a belt
and pulley drive system
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OCR LEVEL 3 CAMBRIDGE TECHNICALS IN ENGINEERING
more examples at http://www.enchantedlearning.com/physics/machines/Levers.shtml
OCR LEVEL 3 CAMBRIDGE TECHNICALS IN ENGINEERING
SUGGESTED ACTIVITIES
LO No:
4
LO Title:
Understand properties of beams
Title of suggested activity
Suggested activities
Suggested timings
Types of beams
Learners should know the definition of a beam - as a 2d component subject to only transverse loads.
This gives two equilibrium conditions, and restricts the types of beams that can be analysed using
static equilibrium to simply support and cantilever beams.
30 minutes
Loading applied to beams
Loading is classified as dead loads and live loads (sometimes called self-weight and imposed loading).
Also related to
See Lesson Element
Introduction to beams
Reactions of beams
Learners need to be able to calculate the reactions for simply supported and cantilever beams. This
could be taught as an extension of the work in LO1 of this unit, treating the beam as a rigid body.
Learners should understand that vertical and rotational equilibrium can be used to find two unknown
reactions; two equilibrium equations can be solved simultaneously to find two unknown values.
30 minutes
Bending Moments
To understand bending moments, learners will need to understand the concept of ‘internal
equilibrium’. This is well explained in this video
https://www.youtube.com/watch?v=Hd8w_7s_78A
2 hours
Bending moment diagrams
In doing so they will also need to understand shear forces in the beam section, although this will not
be assessed in this qualification.
Learners will need to be able to calculate the bending moment at any point of a simply supported or
cantilever beam. This will repeat work from LO1 of this unit, i.e. equilibrium of a rigid body.
PRINCIPLES OF MECHANICAL ENGINEERING
The bending moment diagram can be presented as drawing a graph of the bending moment along
the length of the beam.
12
LO No:
5
LO Title:
Understand principles of dynamic systems
Title of suggested activity
Suggested activities
Suggested timings
Also related to
Newton’s Laws of Motion
Newton’s laws of motion underpin much of the content of this unit. Learners should understand that
a state of equilibrium does not necessarily imply that the body in question is stationary, although in
many practical engineering situations this may be true.
30 minutes
Unit 2 LO2
Unit 1 LO1
PRINCIPLES OF MECHANICAL ENGINEERING
SUGGESTED ACTIVITIES
In particular Newton’s second law (represented by F = ma) is fundamental to this LO, so simple worked
examples to show its application are worthwhile.
See https://www.youtube.com/watch?v=QffUhiX2uSg
Constant acceleration
formulae
The constant acceleration formula should initially be applied to problems of linear motion. There
3 hours
is considerable scope for worked examples and questions to be based in real world contexts such
as falling objects, vehicles (or objects) travelling in a straight line, objects projected vertically. It is
important that learners know that velocity, acceleration and displacement are vector quantities and so
it is vital that they define the positive axis for all calculations.
Unit 2 LO2
Unit 1 LO1
Learners should also be introduced to problems involving objects moving in two dimensions.
Typically these will be problems of projectiles moving under gravity.
Teachers could begin by introducing the concepts of Gravitational Potential Energy (GPE) and Kinetic 3 hours
Energy (KE) and the way in which one quantity is ‘converted’ to the other - for example when an object
falls. Learners will need to be familiar with the range of questions that could arise and this is probably
best accomplished through worked examples. Many examples of these questions will be found in any
mechanics, physics or engineering textbook of the appropriate level. Many typical problems could be
solved either using the principle of conservation of energy or using the constant acceleration formula.
Where applicable it would be useful for learners to verify results obtained using both methods.
Work done and energy
Teachers could show the equivalence of work done and energy gained by using the Newton’s second
law and the constant acceleration formula.
Learners need to know that the work done by forces acting on a body will cause an equivalent
increase in the energy of the body and how this can be used to solve problems involving bodies
moved by external forces. See https://www.youtube.com/watch?v=2WS1sG9fhOk
for an introduction to this topic.
See Lesson Element Work done
and Energy
This topic could be taught using worked examples of progressive complexity. Many suitable questions 3 hours
will be found in standard texts (for example motion on inclined planes) and will enable learners to
develop understanding and familiarity with the format of assessment.
Unit 2 LO2
1 hour
Unit 1 LO1
OCR LEVEL 3 CAMBRIDGE TECHNICALS IN ENGINEERING
13
Conservation of energy
OCR LEVEL 3 CAMBRIDGE TECHNICALS IN ENGINEERING
SUGGESTED ACTIVITIES
Title of suggested activity
Suggested activities
Suggested timings
Also related to
Power
Learners should understand that Power is the rate of work, or the rate of change of energy.
Power problems will almost certainly include other topics within this LO and should bring many
opportunities to use engineering contexts as their basis.
3 hours
Unit 1 LO1
LE Coefficient of Friction suggests an experimental introduction to the topic of friction between two
surfaces. Learners should be familiar with Newton’s third law and so understand the nature of the
normal contact force acting between when two bodies are touching. Learners should know that the
value of the coefficient of friction will lie between 0 and 1, and that the direction of the friction force
will always oppose the motion of the body or bodies.
2 hours
Learners should understand that when two bodies collide momentum is always conserved but
energy is only conserved if the collision is elastic. This is well presented in video:
https://www.youtube.com/watch?v=V4vzNk4qppw
2 hours
See Lesson Element
Understanding work, energy
and power
Friction
See Lesson Element Coefficient
of friction/limiting friction
Conservation of momentum
Unit 1 LO1
There is a wealth of information available to help learners understand this topic, for example;
https://www.youtube.com/watch?v=rX9GBvCSLo4
https://www.youtube.com/watch?v=FeqUREuTnJw
Learners will need to practice solving problems involving elastic collision (conservation of momentum
and conservation energy) and collisions where the two bodies coalesce (i.e. become linked together)
so that they have a common final velocity. Many such examples will be found in standard textbooks.
PRINCIPLES OF MECHANICAL ENGINEERING
14
PRINCIPLES OF MECHANICAL ENGINEERING
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