GCSE-readyGears and cams
Gears change speed and turning force. Cams turn rotary motion into up-and-down movement. Together they power everything from clocks to toys.
Gears
Two meshing spur gears turn in opposite directions. Adding an idler gear between them makes the driver and driven turn the same way, without changing the gear ratio.
Gear ratio = teeth on driven gear ÷ teeth on driver gear.
A small driver turning a large driven gear makes the output slower but with more torque (turning force).
| Gear system | What it does | Example |
|---|---|---|
| Spur gears | Change speed and direction of rotation | Clocks, hand whisks |
| Bevel gears | Turn rotary motion through 90° | Hand drills |
| Rack and pinion | Change rotary motion to linear motion | Pillar drill lever, car steering |
| Worm and wormwheel | Give a very large reduction in speed | Guitar tuning pegs |
Cams and followers
So a cam changes rotary motion into reciprocating motion.
When the follower stays still for part of the turn, this is called dwell.
| Cam | Follower movement |
|---|---|
| Circular (eccentric) | Rises and falls smoothly |
| Pear | Stays still (dwells) for about half a turn, then rises and falls |
| Snail (drop) | Rises slowly, then drops suddenly; it can only turn one way |
| Heart | Rises and falls at a steady speed |
Followers
A roller follower has less friction, so it wears less.
Cams are used in toy automata, engines (to open valves) and some sewing machines.
A driver gear with 15 teeth turns a driven gear with 45 teeth at 300 rpm. What is the gear ratio and the output speed?
- Gear ratio = driven teeth ÷ driver teeth.
- 45 ÷ 15 = 3, so the ratio is 3:1.
- Output speed = 300 ÷ 3 = 100 rpm.
Answer: Gear ratio 3:1; output speed 100 rpm.
Gear ratio = driven teeth ÷ driver teeth. Meshing spur gears turn opposite ways; an idler gear makes them turn the same way. Bevel gears turn motion through 90°; rack and pinion gives linear motion. Cams change rotary motion into reciprocating motion.
- Rotary motion
- Turning round in a circle, like a wheel.
- Gear ratio
- Gear ratio = teeth on driven gear ÷ teeth on driver gear. Output speed = input speed ÷ gear ratio.
- Idler gear
- A gear placed between the driver and driven gears so they turn the same way.
- Bevel gears
- Gears with angled teeth that turn rotary motion through 90°.
- Rack and pinion
- A gear system that changes rotary motion into linear motion.
- Reciprocating motion
- Moving backwards and forwards in a straight line.
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2 more questions
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A 15-tooth driver gear meshes with a 45-tooth driven gear. What is the gear ratio (x in x:1)?
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Answer: 3
Gear ratio = driven ÷ driver = 45 ÷ 15 = 3, so 3:1.
A gear system has a ratio of 3:1. The driver turns at 270 rpm. What is the output speed?
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Answer: 90 rpm
270 ÷ 3 = 90 rpm.
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Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.