Lisnagarvey High School
Technology and Design GCSE
Mechanical Control Systems Content (Unit 2)
(From 2009)
A. General Concepts
2.
2.
2.
3.
4.
3-4.
Load
Effort
Fulcrum
Mechanical Advantage
Velocity ratio
Simple calculations of Mechanical Advantage and Velocity Ratio
B. Transmission of Motion Using Gears
5
5
6
6-7
7
7,11
9
8-10
pages 11-14
Flat belts
Toothed belts
Round and Vee belts
Sprocket and Chain
Jockey Pulleys
D. Conversion of Motion
14
14-15
15
15
16
17
17
17
17
18-19
20
21
22
23
pages 5-11
Spur Gears
Bevel Gears
Worm and Wheel
Rack and Pinion
Simple Gear Train
Idler Gear
Compound Gear Train
Velocity (Gear Ratio) and transmission speeds.
C. Other Transmission Systems
11
12
12
13
13-14
pages 2-4
pages 14-23
Types of Motion and Cams
Eccentric Cam
Pear shaped Cam
Heart shaped Cam
Snail Cam
Knife Follower
Flat Follower
Roller Follower
Crank and Slider
Screw Threads
Ratchet and Pawl
Moments of Force
Bell Cranks
Parallel linkages
Appendix – All Mechanical Systems Symbols
Name:_________________________________
Mechanical Systems General Concepts
A mechanism is simply a device which takes an input motion and force, and outputs a different motion
and force. The point of a mechanism is to make the job easier to do. The mechanisms most commonly
used in mechanical systems are levers, linkages, cams, gears, and pulleys as well as others.
A lever is the simplest kind of mechanism. There are three different types or classes of lever. Common
examples of each type are the crowbar, the wheelbarrow and the pair of tweezers.
All levers are one of three types, usually called classes. The class of a lever depends on the relative
position of the load, effort and fulcrum:
Class

The load is the object you are trying to move.

The effort is the force applied to move the load.

The fulcrum (or pivot) is the point where the load is pivoted.
Description
A class 1 lever has the load and the effort on opposite sides of the
fulcrum, like a pair of kitchen scales (or see-saw).
1
Moving the load closer to the fulcrum or the effort further away
means that less effort is needed to lift the load.
Other examples of class-one levers are a crowbar, pair of pliers
and a crane ( a crane needs a counterbalance on one side of the
fulcrum to stop it toppling over when lifting a load).
A class 2 lever has the load and the effort on the same side of the
fulcrum, with the load nearer the fulcrum. An example of a classtwo lever is a wheelbarrow.
2
In the diagram, the wheel or fulcrum on the wheelbarrow is
helping to share the weight of the load. This means that it takes
less effort to move a load in a wheelbarrow than to carry it.
Nutcrackers are another example of a class 2 lever.
Examples
3
A class 3 lever does not have the mechanical advantage of classone levers and class-two levers, so examples are less common.
The effort and the load are both on the same side of the fulcrum,
but the effort is closer to the fulcrum than the load, so more force
is put in the effort than is applied to the load. These levers are
good for grabbing something small, fiddly or dirty, or picking up
something that could be squashed or broken if too much pressure
is applied. The common example of class 3 levers is a pair of
barbeque tongs or a pair of tweezers.
Mechanical advantage and velocity ratio
Class 1 and class 2 levers both provide mechanical advantage (M.A). This means that they allow you
to move a large output load with a small effort. Load and effort are forces and are measured in
Newtons (N). Remember 1kg mass = 10N. (i.e to change a mass in kg to a force in Newtons, multiply
it by 10) Mechanical advantage is calculated as follows: You must use Newtons in your calculations.
Mechanical advantage = load ÷ effort
Example 1. If a wheelbarrow is used to lift a load=300N and
the necessary effort=100N, then the mechanical advantage
would be:
load ÷ effort
i.e
300N ÷ 100N = 3:1 or 3
Example2. If a crowbar is used to lift a load=500N and the
necessary effort=100N, then the mechanical advantage
would be:
load ÷ effort
i.e
500N ÷ 100N = 5:1 or 5
Questions:
1. Complete the following tables. (examples are done)
Mass
3kg
Remember 1kN = 1000N
Force
Load
Effort
M.A
30N
300N
150N
2
70N
900N
0.5kg
250
65N
300g
3
2kN
500N
500N
25N
5
Velocity ratio
The mechanical advantage gained with class-one levers and class-two levers makes it seem like you are
getting something for nothing: moving a large load with a small effort. The catch is that to make the
effort smaller, you have to move a greater distance. In the wheelbarrow diagram the trade-off is that
you need to lift the handles of the wheelbarrow further (300mm) to lift the load up a smaller distance
(100mm). This trade-off is calculated by the velocity
ratio:
Velocity ratio = distance moved by effort ÷
distance moved by load
i.e 300mm ÷ 100mm = 3:1
*Important – the units of
measurement must always be
changed to the same units*
Questions:
1. Complete the following table. (example is done)
Distance
moved by
effort
Distance
moved by
load
Velocity
Ratio
500mm
100mm
5:1
250
2:1
1000mm
10:1
2m
500mm
5m
2m
2. A crowbar is used to lift up a large rock a distance of 200mm. The end of the crowbar had to be
pushed down a distance of 800 mm. What is the velocity ratio of the lever?
V.R =
3. A mechanical system with a velocity ratio of 5:1 lifts a load up a distance of 3m. What distance
did the effort have to move to do this?
Distance moved by effort =
Transmission of Motion using Gears
Gears are used to increase or decrease the speed or power of rotary motion. Like other mechanical
systems, Gear systems also have velocity ratios, however it is commonly called gear ratio and is
normally calculated in a different way (see later) although it can be worked out using the normal way.
Spur Gear
Spur gears are toothed wheels fixed to rotating shafts. The teeth of each gear mesh together to transmit
rotary motion and torque. The gears can be different sizes in which case they will turn at different
speeds (smaller goes quicker). They are generally used in simple gear boxes where you want to change
the output speed of a parallel shaft
Bevel Gear
Bevel gears like transmit torque and rotary
motion through 90º.
Unlike spur gears, bevel gears have teeth cut
on a cone instead of a cylinder blank.
The rotary velocity of the output gear shaft
can be increased or reduced by using different sized gears. (See later) The small gearwheel will go
faster than the big one.
Hand drills make use of bevel gears to achieve a rotary velocity increase. The handle of the drill is
attached to the big gear wheel which is then turned by hand. The small gearwheel is attached to the drill
bit which then turns much faster (because it is smaller). The same type of bevel gear system is used in
food mixers (hand whisks or electric mixers)
Bevel gears are found in car differential gear boxes (see picture above). They allow torque and rotary
motion to be transmitted through 90º from the propeller shaft through to the back axles in order to drive
the car forwards.
Worm and wheel
Worm gears are used when large speed reductions are required. The worm, which is always attached to
the driver shaft, has one tooth and takes the form of a screw thread. The worm can drive the wheel
round, but the wheel cannot drive the worm. This means that worm gears are good to use in hoists, the
load will not fall back when the motor stops.
Like bevel gears a worm and wheel transmits torque and rotary motion through 90º.
The worm has one long continuous tooth therefore the velocity (gear) ratio is simply the number of
teeth in the wheel e.g if the wheel has 20 teeth then the gear ratio is 20:1. (see later)
A Worm and wheel can be used in car steering systems. The steering wheel is attached to the same
shaft as the worm (driver shaft) as shown in the photos below. The driven shaft is then used to turn the
wheels.
Rack and Pinion
A rack and pinion is used to transform rotary motion into linear motion and vice versa.
The pinion is a round spur gear fixed to a shaft. The rack is a spur gear with teeth set in a straight line.
Camera tripods use rack and pinion gears for height adjustment. They are also used in drilling machines
to allow the user to raise and lower the chuck and drill bit. The third and fourth photos show a rack and
pinion being used to open and close sluice gates built into a river dam. The sluice gate is operated by
attaching a crank to the square end of the shaft which passes through the pinion. It is raised by rotating
the crank clockwise and lowered by rotating the crank anti-clockwise.
Camera Tripod
Drilling Machine
Sluice Gate Pictures
Simple Gear Train
A simple gear train uses two gears, which may be of different sizes. It
is a simple gear train if there is only one gear wheel on each shaft. If
one of these gears is attached to a motor or a crank then it is called the
driver gear. The gear that is turned by the driver gear is called the
driven gear.
In simple gear trains the driver and driven gears will rotate in opposite
directions.
Idler Gears
An intermediate gear (or gears), called an idler gear, can be inserted between the driver gear and
driven gear to make them rotate in the same direction.
* Idler gears only change the direction of rotation of simple gear chains*.
Gear Ratio (Velocity Ratio) of a simple gear train
The measure of how much the speed or power is changed by a gear
train is called the gear ratio (velocity ratio).
This is equal to the number of teeth on the driver gear divided by the
number of teeth on the driven gear.
Remember an idler gear does not change the velocity (gear) ratio of a
simple gear train.
Gear ratio = number or teeth on driven gear ÷ number of teeth on the driver gear
So the gear ratio for the simple gear train above, if the smaller gear is the driver gear, is:
Gear ratio = 30 ÷ 20 = 3/2 or 3:2.
In other words, the driver gear revolves three times to make the driven gear revolve twice.
If you know the gear ratio, and the speed input at the driver gear, you can calculate the speed output at
the driven gear using the formula:
Output speed = input speed ÷ gear ratio
So if the gear ratio is 3/2 and the driver gear is revolving at 300 rpm then
the output speed = 300 ÷ 3/2 = 200 rpm
*Remember the small gear always goes faster in a simple gear train.
Questions:
1.
A simple gear train has two gear wheels. One is turned by a motor and it has 50 teeth. This is
connected to a second gear wheel which has 100 teeth.
a. What is the gear ratio:
GR =
b. The driver gear is turning at 100rpm, what speed is the driven gear turning at?
Driven Gear Speed =
2. A simple gear train has three spur gears A, B and C connected in a line. The first gear (A) has
40 teeth and is connected to a crank handle which is turned by hand. The second gear (B) has 10
teeth. The third gear (C) has 20 teeth and is connected to a shaft which rotates and lifts a bucket.
a. Which gear wheel is the Driver ______________, and the Driven_______________?
b. What is the name of Gear B? ___________________
c. What is the purpose of Gear B? __________________________________________
d. What is the gear ratio:
GR =
e. The bucket shaft rotates at 50rpm, what speed is the crank handle being turned at?
Crank handle speed (Gear A speed) =
Compound Gear Train
Compound gear trains involve several pairs of meshing gears. They are used where large speed changes
are required or to get different outputs moving at different speeds.
Centre lathes like the ones found in school
metalwork rooms, have compound gear trains
that transmit rotary motion from an electric
motor through to the headstock spindle.
The leadscrew, which allows the tool post to
travel on automatic feed, also operates from this
compound gear train.
Compound gear trains often have two or more
gears mounted on the same shaft.
A good example of this is a car gear box, which
has to fit into a confined space, has to allow the
driver to select various gear ratios and also has
to change from forward to reverse.
Gear Ratios for Compound Gear Trains (or velocity ratios, VR) are calculated
using the same principle as for simple gear trains, i.e. VR = number of teeth on the driver
gear divided by the number of teeth on the driven gear.
However, the velocity ratio for each pair of gears must
then be multiplied together to calculate the total
velocity ratio of the gear train.
Total VR = VR1 x VR2 x VR3 x VR4 etc.
* Important – two gear wheels on the same shaft
will go at the same speed, even if they are different
sizes – they will go at the speed of the shaft because
they are connected to it and will have a gear ratio
of 1:1*
Question –
a. Work out the gear ratio of the compound gear system below. (remember work out the GR
of each pair of gears and then multiply them together)
GR =
C=30
A=30
B=10
D=10
Driver Gear
b. If Gear A is turning at 180rpm –
i. What speed is Gear B turning at?
ii. What speed is Gear C turning at?
iii. What speed is Gear D turning at?
c. If Gear D is turning at 900 rpm, calculate the speed of Gear A
*Remember the gear ratio of a worm and wheel is simply the number of teeth in the wheel*
Gear Ratio of Simple Gear train with Idler Gear
A=20
C=40
The gear train on the left is a simple gear train (only 1
gear per shaft, but gear B is an idler gear. It was stated
earlier than idler gears do not change the gear ratio,
only the direction. Below is proof.
GR(AB) = 10/20 = 1/2 = 1:2
GR (BC) = 40/10 = 4/1 = 4:1
Total GR = GR (AB) x GR (BC) = ½ x 4/1 =2/1 = 2:1
Driver Gear
B=10
Now using the formula for GR for a Simple Gear Train
GR = No. of teeth in driven ÷ No. of teeth in driver
GR = 40 ÷ 20 = 4/2 =2/1 =2:1 i.e the same as above
*An idler gear only changes the direction of a simple gear train, not the gear ratio *
Other Transmission Systems
Pulley Systems
One method of transmitting force and rotary motion from one shaft to another is by using pulleys and
belts.
Various types of belts are used to connect the pulleys together depending on the application. The main
types of belts are flat, round, vee and toothed belts.
Flat Belts
This type of belt is used to
transmit torque and motion from
engines to machines. E.g. steam
powered traction engines. The
main problem with them is that
they can slip on the pulley wheel.
Round Belts
Round belts are used where the belt has to twist or where small forces
are involved. E.g. in vacuum cleaners to connect the electric motor
(driver) pulley to the fan pulley which sucks in air and dirt. They will
fit into a circular groove in the pulley itself. They are likely to slip
slightly but this is not a problem.
Vee Belts
Vee belts are the most commonly used type of pulley belt. They fit tightly
into a similar shaped groove on the pulley wheel so that slip is reduced to a
minimum. E.g. in pillar drilling machines (see photo above) with a
stepped cone pulley to allow the drill speeds to be adjusted. There is much
more surface contact of the belt with the groove in the pulley therefore
there is less slippage, although they can still slip if the force becomes too
large.
Toothed Belts
Toothed belts are used in systems where belt
slip must be eliminated. E.g. a car engine
timing belt system which is shown in the
photo to the right. The belt has ‘teeth’ which
locate accurately into similar shaped
grooves in the pulley wheel and there is
virtually no chance of any belt slippage.
Toothed pulley
Wheels and belt
Sprockets and Chains
Sprockets are toothed wheels attached to the driver and driven shafts. Chains consist of many loosely
jointed links which engage with the sprocket teeth. It is rare that a chain will come off a sprocket unless
it is very loose or if the sprocket and chain are not well-aligned.
Sprockets and chains are used when no
slip, i.e. positive drive, is essential. E.g.
on a bicycle or motor bike. See the
photos on the right.
Sprockets and chains need to be
lubricated with oil or they will rust. They
make more noise than pulleys and belts.
It is important that the chain is tensioned correctly to prevent it becoming
disengaged from the sprockets. This can be done on a bicycle by moving the position of the back wheel
or by adding or removing links from the chain although this is not generally done on modern bikes
which have gears.
The Velocity Ratio of a sprocket and chain system is determined by comparing the number of teeth on
each sprocket. (similar to gear wheels)
Most bicycles nowadays use Derailleur gears to easily adjust velocity ratios. A low velocity ratio
allows the rider to climb hills slowly, but with the minimum of effort. High velocity ratios mean that
more effort is required but greater speeds can be achieved without having to pedal very fast. Derailleur
gear systems use spring loaded jockey wheels to correctly tension the chain. (See below)
Jockey Pulleys/wheels
Belts in pulley systems must be tensioned correctly. If they are too slack they may slip or come off the
pulleys. If they are too tight, they will apply bending forces to the pulley shafts which could cause
damage to the system. Two methods are commonly used to tension belts:
1. Jockey Wheels.
A jockey wheel (or pulley) is an additional
pulley which keeps the maximum amount of
belt in contact with the driver and driven
pulleys. Jockey wheels are often spring
loaded and can also be found in chain and
sprocket systems.
The photo shows a jockey wheel used in a bicycle derailleur gear system.
2. By having an adjustable pulley in the system.
Once adjusted this pulley is locked in place. This
is seen in the two photos on the left. The not and
bolt are loosened and the pulley held firmly in
place until the bet is at the required tension and
then the nut is tightened up again
Conversion of Motion
Motion in a circle is called rotary motion. E.g a wheel going round and round
Linear motion is motion in a straight line. E.g a train going along a straight track.
Reciprocating motion is linear motion backwards and forwards in a straight line. Sewing machines
make use of this type of motion as the needle goes up and down
Oscillating motion is motion backwards and forwards in a circular arc. E.g. playground swings and
clock pendulums.
Many mechanisms can change one type of motion to another. Examples of these are cams, crank and
sliders, screw threads and ratchet and pawls.
Cams
A cam is a specially shaped piece of metal or hard wearing plastic, which is usually fixed to a rotating
shaft.
A follower is held against the cam, either by its own weight or by a spring. As the cam rotates the
follower moves up and down in a reciprocating motion.
The distance and speed at which the follower moves depends on the shape of the cam.
Therefore cams generally change rotary motion into reciprocating motion.
Eccentric Cam
A circular cam is also known as an 'eccentric' cam. The centre of
rotation of the cam is offset from the geometric centre of the circle.
The cam rotates but this type of cam produces a smooth form of
motion called simple harmonic motion.
It is like a smooth wave effect. The further the cam is offset from
the centre the higher the wave and the movement of the cam (known
as the stroke)
One half of the cam makes the follower rise up (Rise) and the other
side makes it drop (Fall). As the cam is symmetrical the rise and fall motions are the same.
The diagrams (1 to 7) seen below show the cam rotating in an anticlockwise direction. As it rotates it
pushes the flat follower upwards and then allows it to drop downwards. The movement is smooth and
at a constant speed.
Pear-shaped Cam
Pear shaped cams are often used for controlling valves such as those
found in car engines.
This type of cam has
a long dwell period
during which time
the follower does not
move. This is
because at least half
of the cam is circular
in shape. (Remember the shaft is going through the centre of the circle, unlike the eccentric cam)
When the follower is moving the rise and fall times are equal because of the symmetrical shape of the
cam.
Heart-shaped Cam
This cam causes the follower to move with a uniform velocity.
Heart-shaped cams are essential when the follower motion needs to be
uniform or steady as, for example, in the mechanism that winds thread
evenly on the bobbin of a sewing machine.
A heart-shaped cam can be used for winding wire evenly on the former of
a solenoid.
They are often used with knife or point followers so that the cam can get
right down into the groove of the heart.
Snail Cam
A snail cam is used where the fall of the follower must be sudden.
The example snail cam shown below rotates in an anticlockwise direction. Rotating in a
clockwise direction would lead to the entire mechanism jamming. This highlights one
possible disadvantage of using this type of cam profile. Also, to ensure the rotation is
smooth, the vertical centre line of the snail/drop cam is positioned slightly to the left of
the slide (see diagram).
The diagrams below show the rotation of the snail cam. When rotating for one complete
revolution the follower stays level for approximately the first 120 degrees (diagrams 1 to 4). The
follower then rises slowly (diagrams 5 to 6) and then suddenly drops when it reaches and passes the
peak (diagram 7).
An example of a snail cam is seen below. As the cam profile rotates the foot of the model rises slowly
and then suddenly drops.
Cam and Follower Symbols
Cam followers:
There are three types of cam followers, and since the type of follower influences the profile of the cam
it is worthwhile considering the advantages and disadvantages of each type. The three types are the
knife follower, the roller follower and the flat follower.
Knife follower
Roller follower
Flat follower
This is the simplest type and is
not often used due to the rapid
rate of wear. When it is adopted,
it is usually for reciprocating
motion, running in slides and
there is a considerable side force.
It is often used with a heart
shaped cam to get down into the
groove
This eliminates the problem of
rapid wear since the sliding
effect is largely replaced by a
roller action. Again, with the
roller follower, considerable side
forces are present, a
disadvantage when dealing with
reciprocating motions. These
side forces will be increased
when using small rollers.
This has the advantage that the
only side force present is that
due to the friction between the
follower and the cam. The
problem of wear is not so great
as with the knife-edge
follower, since the point of
contact between the cam and
follower will move across the
face of the follower.
Crank and Slider
A crank and slider mechanism consists of a rotating crank connected to a slider by a
connecting rod.
It can either convert rotary motion into reciprocating motion or vice versa.
In the photo, a rotating crank is
connected to a reciprocating slider
which forms part of a water pump, i.e
rotary motion is converted into
reciprocating motion.
The photo shows 4 engine pistons
operating as sliders to drive a
crankshaft. In this example
reciprocating motion is converted
into rotary motion.
Screw Thread Systems
Screw threads are used in many different ways:

to provide powerful movements (e.g. car jacks)

to position things accurately (e.g. binoculars, seats,
machine heights etc )

to hold things in place (e.g. bolts and screws, clamps)
A screw thread system requires two parts which mate
together.

A male part (often a bolt) is a cylindrical shaft which
has an external thread cut onto it.

A female part (often a nut) which is a drilled out part
and has an internal thread cut into it.
The threads on the male and female parts must be identical
if they are to screw together.
It should be noted that in all the uses of screw thread systems there is a conversion of motion –
generally rotary motion to linear motion as one of the two parts is rotated the other is pulled or
pushed along in a straight line.
Examples of products which use screw threads to transmit motion are:
Photo
Type of Thread
Uses/Applications
Ideal for holding
things in place. E.g.
set screws, nuts and
bolts.
The most common type of thread is the V
thread. Its shape causes a lot of friction.
Woodworking vices
with quick release
mechanisms.
Buttress threads are used where a force
needs to be applied in one direction only.
The leadscrew of a
centre lathe to
provide a smooth
automatic feed.
ACME SCREW THREADS
Acme threads are used for transmitting
motion in conjunction with a disengaging
nut.
As the corkscrew is rotated it pulls itself
into the cork in linear motion. Once it is in
far enough to give a good grip it can then
be pulled with enough force to pull the
cork out.
Corkscrew
Scissor Screw Car
jacks, G cramps,
vices, sash clamps
and other forms of
clamp
SQUARE SCREW THREADS
Square threads are used for the moving
parts of things. Square threads do not
produce as much friction as V threads but
do allow large forces to be applied.
In the car jack – as the handle is turned the
linkage pushes apart in a linear motion and
provides enough force to lift a car.
In a clamp or vice – as the handle is turned
it pulls the two parts of the clamp together
(linear motion) and squeezes anything
together which is between them.
Rachet and Pawl Mechanism
A ratchet is a wheel with saw-shaped teeth around its rim. The ratchet
engages with a tooth shaped lever called a pawl.
The purpose of a ratchet and pawl is to allow a shaft to rotate in one
direction only and prevent rotation in the opposite direction.
Ratchet and pawls are found on winches, car hand-brake levers
(photo), socket sets, fishing reels and screwdrivers, hoists, cranes etc.
The photo on the left is a model of a handbrake in a car. The
handbrake lever is pulled up (often clicking can be heard) to
engage the brake. When the lever is released it stays in position
because the pawl does not allow the rachet to turn backwards as
it is ‘stuck’ in a tooth. To let the handbrake off there is a button
at the end which, when pressed, lifts the pawl out of the ratchet
tooth and allows the ratchet to reverse.
The same principle is used in the other products which use a
ratchet and pawl.
Moments of Force (Applies to levers, linkages etc)
Moment is the turning effect of a force. It is found by
multiplying the force (Newtons) by the distance
(metres) from the pivot point, or fulcrum.
i.e. Moment (Nm) = force (N) x distance (m)
A mass in kg can be changed to a force of Newtons
by multiplying it by 10
e.g 1kg = 10N, 7 kg = 70N, 0.5kg = 5N etc.
The moment of a force is also called torque.
By extending the length of the vice handle a smaller
input force can be used to apply the same force to the
work.
Equilibrium
A body is in equilibrium when the sum of the clockwise moments equals the sum of the anticlockwise
moments.
The diagram shows a lever with an effort on one side
of the fulcrum and a load on the other.
The load force tends to turn the lever in a clockwise
direction. The effort force tends to turn the lever in an
anticlockwise direction.
If these turning effects balance one another then the
state of equilibrium is not disturbed.
The product of force and distance from the fulcrum is
called the moment of a force.
For a lever to be in equilibrium the clockwise moments (CW Mo) about a fulcrum must equal the
anticlockwise moments.(ACW Mo)
i.e L (N) x D1 (m) = E (N) x D2 (m)
Complete the table below assuming the lever is in
equilibrium.
L
E
D1
ACW Mo
L
10N
50N
25N
D2
F
CW Mo
2N
10N
D1
2m
5m
4m
3m
500mm
4m
E
20N
100N
50N
3N
5N
50N
D2
1m
2.5m
Moment
20Nm
100Nm
7m
10Nm
4m
32Nm
Bell Crank Levers
Bell Cranks are useful for changing the direction of motion or
transmitting it round a corner.
Below is a picture of a bell
crank although they do vary in
shape
F
A bell crank is often shaped in a triangular shape as above. As one of the levers is pulled or pushed, the
bell crank rotates about the fulcrum point (F) and the other lever moves backwards and forwards at
approx 90 degrees.
An example of a use of a bell crank is in mountain bike brakes.
When the brake is pulled the cable is pulled upwards, however
the bell crank lever converts the movement around a corner by
almost 90 degrees so that the brake blocks pull inwards against
the wheel rim.
Bellcranks are often used in aircraft contol systems to connect the pilot's controls to the control
surfaces. For example: on light aircraft, the rudder often has a bellcrank whose pivot point in the rudder
hinge. A cable connects the pilot's rudder pedal to one side of the bellcrank. When the pilot pushes on
the rudder pedal, the rudder rotates on its hinge. The opposite rudder pedal is connected to the other
end of the bellcrank to rotate the rudder in the opposite direction.
Bellcranks are also seen in automotive applications, as part
of the linkage connecting the throttle pedal to the
carburetor, and connecting the brake pedal to the master
brake cylinder.
On the left is a photo of a bellcrank in a motorbike
suspension system.
Parallel Linkages
Parallel linkages are used to make two or more parts of a mechanism move together and stay parallel to
each other as the linkage moves.
The photo below shows a petrol engine cross cut
wood saw. Rotary motion from the engine output
shaft is tranformed into reciprocating motion
through a crank and slider. This reciprocating
motion is then transmitted to the saw blade via a
parallel motion linkage.
Parallel linkages are used in many different products such as folding tables, cantilever toolboxes,
shop window and lift security shutters as shown in the photos below.
Folding Table
Shop window/door security shutters
Cantilever Toolbox
Lift security shutters
Appendix - All Mechanical Systems Symbols