### Wheels, pulleys and gears

```Wheels, pulleys and
gears
The Wheel
Possibly the most important mechanical
 From the tiny gears in a watch to cars and
computers, nearly every machine constructed
since the Industrial Revolution (in the early
nineteenth century) uses a wheel in its design.
 Earliest known application was a potter’s wheel,
used in Mesopotamia (modern day Iraq) around
3500 BC.
 Probably first used for transportation on chariots
in this region around 3200 BC.

Wheels can be levers
A wheel on an axle is actually a special kind of
lever.
 The wheel acts as a lever and the pivot is the
centre of the axle.
 Wheels can act as force multipliers or speed
multipliers.

◦ Steering wheels are force multipliers, the larger the
steering wheel, the easier the axle will be to turn.
◦ Car wheels operate as speed multipliers in the
opposite manner to the steering wheel. A large force
is supplied to the axle of the wheel by the motor
which makes the wheel move faster.
Pulleys
A pulley is like a wheel
with a groove around it
into which a cable or rope
can fit.
 It also acts like a lever, with
its axle as the pivot.
 Simple pulleys allow us to
lift upwards by pulling
downwards.
 They do not alter the size
of the force applied, just
the direction.

Pulleys

To increase the size of the force,
we need to use a system
involving more than one pulley.

We need to apply a force over a
greater distance, but multiple
pulleys allow us to lift large loads
with minimal effort. They act as a
force multiplier.

The multiple pulley system
shown in the picture is called a
‘block and tackle’.

The ‘block’ is the outer casing
around the pulley wheels, and the
‘tackle’ is the cable or rope
connecting them.
Using pulleys



To calculate the effort
we divide the load by the
number of ropes (do not
count the rope that goes
to the effort).
The image on the right
shows a four pulley
system.
The person lifting the
a pull equal to only 50kg
(200kg/4).
Pulley question 1

What is the maximum
with this system?

For every 2 metres the
rope is pulled through
what height does the
ground?

What is the
Pulley question 2

What is the minimum
effort that must be

For every 2 metres the
rope is pulled through
what height does the
ground?

What is the
Pulley question 3

What is the maximum
with this system?

For every 2 metres the
rope is pulled through
what height does the
ground?

What is the
Gears
Gears or cogs are like wheels with teeth
that mesh together.
 When one gear turns, the one it
interlocks with also turns, but in the
opposite direction.
 Sometimes gears are joined by a chain,
such as in a bicycle or the overhead cam
shaft in some car engines.

Driving and Driven Gears




The cog that turns first is
called the driving gear.
Gears that are made to turn
by this are called driven
gears.
If the driving gear is smaller
than the driven gear, the
system acts as a force
multiplier.
If the driving gear is larger
than the driven gear, then the
system acts as a speed
multiplier.
Idler Gears

An idler gear can be
used in a gear
combination to make
the driving and
driven gears rotate in
the same direction.
Bevelled Gears
Bevelled gear wheels
are positioned at
right angles to each
other
 This changes the
plane of rotation.
 Used in hand-drills
and in many other
machines.

Rack and pinion gears


Rack and pinion gears
are made up of a row
of teeth (called the
rack), and a gear wheel
that rolls on top of
this.
One use of this type of
system is to stop
mountain trains
slipping on steep
slopes.
Bicycle Gears

A bike with twenty-one gears gives us
a choice of twenty-one possible
gearing combinations.

The front chain wheel has three
sprockets and the rear wheel has
seven.

To select the lowest gear, we use the
largest rear sprocket and the smallest
chain wheel sprocket.

This combination of gears makes it
easier to go up hills.

To select the highest gear, we use
the largest chain wheel sprocket and
smallest rear wheel sprocket.

The combination of a large front
sprocket and a small rear sprocket
makes the bike go faster.
Bicycle Gears




The gears used in a
bicycle are called
sprockets.
The pedal and crank
are attached to a gear
sprocket called the
chain wheel.
A chain connects this
to the rear sprocket.
Pedalling turns the
chain wheel, which
turns the rear wheel,
via the chain.
Gear Ratios

Are calculated by
dividing the number
of teeth on the
driving sprocket by
the number of teeth
on the driven
sprocket.
GR = teeth on driver
teeth on driven gear
Example:
 If the driving gear has 9
teeth and the driven
gear has 3
 GR=9/3=3
Gear ratio question 1

Gear "B" has 36
teeth and gear "A"
has 12.

Calculate the gear
ratio of this gear
train if "B" is the
driver.
Gear ratio question 2

Look at the image on
the right. Gear "B"
has 12 teeth and gear
"A" has 24.

Calculate the gear
ratio of this gear
train if "B" is the
driver.
```