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Mechanisms and Movement
Mechanisms are used to convert between one type of motion and another. Any
machine can be looked on as a group of interconnected mechanisms which
convert one type of motion to a variety of other motions.
These changes may be to convert rotary motion to straight line motion or to
convert reciprocal (back and forth) motion to intermittent motion. They may also
transform a fixed type of motion, for example by magnifying a linear motion or by
slowing down a rotary motion.
Motion.
A comparison of different
types of movement and
how to convert between
them .
Mechanisms.
Explanations of basic
mechanical movements
Compound Mechanisms.
How basic mechanical
units fit together.
Linear motion is the most basic of all motions. Uninterrupted objects will
continue to move in a straight line indefinitely. Under every day circumstances
gravity and friction conspire to bring objects to rest.
Linear motion is measured in two parts. Speed, and direction. Together these
make up the velocity.
Reciprocating Motion
The rack and pinion is used to convert between rotary and linear motion. The
rack is the flat, toothed part, the pinion is the gear. Rack and pinion can convert
from rotary to linear of from linear to rotary.
The diameter of the gear determines the speed that the rack moves as the pinion
turns. Rack and pinions are commonly used in the steering system of cars to
convert the rotary motion of the steering wheel to the side to side motion in the
wheels.
Rack and pinion gears give a positive motion especially compared to the friction
drive of a wheel in tarmac. In the rack and pinion railway a central rack between
the two rails engages with a pinion on the engine allowing the train to be pulled
up very steep slopes.
The bell crank is used to convert the direction of reciprocating movement. By
varying the angle of the crank piece it can be used to change the angle of
movement from 1 degree to 180 degrees.
The bell crank was originally used in large house to operate the servant’s bell,
hence the name.
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“Jeeves, where’s my tea?!”
http://www.flying-pig.co.uk/mechanisms/pages/bellcrank.html
Chains are used to connect gears. They work in a similar way to pulleys but with
a positive drive rather than a reliance on friction. Gears which are connected by
chain turn in the same direction unlike gears which mesh against each other.
The crank is used to convert rotary motion to reciprocating or oscillating motion.
With careful timing it can also be used to convert motion the other way... from
reciprocating to rotary, (see the piston)
The throw of the reciprocating motion is determined by the offest of the crank.
The crank slider mechanism is the basis of the models Flying Pig and Surfin' the
Web. As the crank (yellow) turns the pushrod and slider is moved up and down,
the end point tracing out a curved, but not necessarily circular motion.
By altering the various dimensions, different motions can obtained, the key
distances are
Gears are used to change speed in rotational movement. In the example above
the blue gear has eleven teeth and the orange gear has twenty five. To turn the
orange gear one full turn the blue gear must turn 25/11 or 2.2727r turns.
Notice that as the blue gear turns clockwise the orange gear turns anti-clockwise.
In the above example the number of teeth on the orange gear is not divisible by
the number of teeth on the blue gear. This is deliberate. If the orange gear had
thirty three teeth then every three turns of the blue gear the same teeth would
mesh together which could cause excessive wear. By using none divisible
numbers the same teeth mesh only every seventeen turns of the blue gear.
The Geneva stop is named after the Geneva cross, a similar shape to the main
part of the mechanism.
The Geneva stop is used to provide intermittent motion, the orange wheel turns
continuously, the dark blue pin then turns the blue cross quarter of a turn for
each revolution of the drive wheel.
The crescent shaped cut out in dark orange section lets the points of the cross
past, then locks the wheel in place when it is stationary.
The Geneva stop mechanism is used commonly in film projectors to move the
film on one frame at a time.
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Levers are an essential part of many mechanisms. They can be used to change
the amount, the strength and the direction of movement.
The position of the force and the load are interchangeable and by moving them
to different points on the lever, different effects can be produced.
The fixed point of the lever about which it moves is known as the fulcrum.
In this example the force and the load move in opposite directions.
With the force three times closer to the fulcrum them the load lifted is only one
third of the force but it move three times as far.
(Next)
© Rob Ives / Flying Pig 2000-2004
Linkages are an essential part of many mechanisms. They can be used to
change direction, alter speed and change the timing of moving parts.
In this example two linked linkages are used to convert the small linear
movement of the drive shaft (bottom left) into first a rotational body movement
and secondly a fast hammer movement. Compare the speed of the hammer with
the speed of the drive shaft!
(next)
Pulleys
On the left is a simple pulley. As the rope
is pulled down the weight moves up by the
same distance.
In the compound pulley on the right the
rope is wrapped around two pulleys. As
the rope is pulled the weight, this time
attached to the lower pulley rather than
direct to the rope, moves up slower than
the speed that the rope is pulled.
Corresponding to this reduction in speed is
an increase in the force on the weight.
The amount of increase in the force
depends on how many times the rope
wraps round the pulleys. By wrapping the
rope several times around the pulleys it is
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easily possible to lift your own weight off
the ground!
Belt Drives
Belt drives are used transfer
rotational motion from one place to
another.
On the left, both pulleys are the
same size. Drive can be transfered
by friction of the belt on the pulley or,
if required, buy using a toothed belt.
Chain drives work in a similar way.
By crossing the belt the direction of
drive can be changed.
On the right two sizes of pulley are
used to show how speed of rotation
can be changed.
A worm is used to reduce speed. For each complete turn of the worm shaft the
gear shaft advances only one tooth of the gear.
In this case, with a twelve tooth gear, the speed is reduced by a factor of twelve.
Also, the axis of rotation is turned by 90 degrees.
Unlike ordinary gears, the motion is not reversible, a worm can drive a gear to
reduce speed but a gear cannot drive a worm to increase it.
As the speed is reduced the power to the drive increases correspondingly. Worm
gears are a compact, efficient means of substantially decreasing speed and
increasing power. Ideal for use with small electric motors.
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