Mechanical devices - EdexcelGear types and trains

Gears are wheels with teeth around the outside. When several wheels are interlocked, they can transfer motion from one place to another, eg in some hand whisks or on bikes.

Gears can change the direction or the speed of movement. As there are teeth around the edge of the gears they grip together and so can withstand a greater force, enabling them to move large items such as cars or bicycles.

Gear trains

Gear trains are when two or more gears are joined together. In a simple gear train, the drive gear causes the driven gear to turn in the opposite direction.

Smaller gears with fewer teeth turn faster than larger gears with more teeth. This difference in speed is called the gear ratio.

Example - Gearing down

The drive gear has 15 teeth and the driven gear has 60 teeth.

The gear ratio is calculated by

Teeth on driven gear ÷ Teeth on drive gear

= 60 ÷ 15

= 4

This means that every time the drive gear turns four times, the driven gear would rotate once.

Gear ratio = 4:1

This is known as gearing down because the driven gear rotates fewer times than the drive gear

Example - Gearing up

The reverse of gearing down is called gearing up. Here, the driven gear rotates more than the drive gear.

The drive gear has 60 teeth and the driven gear has 15 teeth.

Gear ratio = Teeth on driven gear ÷ teeth on drive gear

= 15 ÷ 60

= 0.25

This means that every time the drive gear turns a quarter (0.25), the driven gear turns once.

The gear ratio is then expressed as:

Rotations of a drive gear : Rotations of a driven gear

0.25:1

Multiply both sides of the ratio by four to avoid having a decimal in our ratio. The ratio is then 1:4.

Gear types

If the drive gear and the driven gear are separated by another gear, called the idler, they will move in the same direction.

Bevel gears

A bevel gear is a special gear that can transfer rotary through 90 degrees. The diagram below shows two gears of the same size - the name given to this arrangement is a mitre-gear.

However, the two gears can vary in size to achieve a different gear ratio. An example of this is in a hand drill, where the drive gear is larger than the driven gear.

Rack and pinion

A rack and pinion is an arrangement of a gear wheel and a rack which allows the rotary motion to be converted to linear motion. An example this is in a pillar drill, where the table bed is moved up and down.

The speed at which a gear or wheel rotates is measured in revolutions per minute (rpm). Many cars have a rev counter that tells the driver how fast the engine is spinning.

Example

A driver gear rotating at 60 rpm is connected to a gear arrangement with a ratio of 60:20.

Calculate the output speed.

Gear ratio = number of teeth on driven gear ÷ number of teeth on the drive gear

= 60 ÷ 20 = 3

Output speed = input speed ÷ gear ratio

= 60 (rpm) ÷ 3 = 20 rpm

In this arrangement the output speed is three times slower compared to the input speed.