The big idea: Gears transmit rotary motion from one shaft to another through meshing teeth, so there is no slip and the two shafts stay exactly in step.
The tooth counts set everything: Ratio = teeth on the driven gear ÷ teeth on the driver.
All seven gear systems, each drawn with its shafts — then the ratio rule.
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| System | What it does | Where you see it |
|---|---|---|
| Spur | Straight-cut teeth on parallel shafts; the output turns the OPPOSITE way to the input | A hand drill, a clock, a food mixer, a wind-up toy |
| Bevel | Cone-shaped teeth that carry drive round a corner, usually through 90° | A hand drill head, a car differential, a rotary egg whisk |
| Rack and pinion | A round pinion runs along a straight toothed rack: rotary becomes LINEAR, or the reverse | Car steering, a pillar drill feed, a rack railway, a sliding gate |
| Worm and wheel | A threaded shaft drives a wheel at 90°, giving a huge reduction in one step — and it cannot be back-driven | A guitar tuning peg, a wiper motor, a conveyor drive, a hoist |
| Ratchet and pawl | A toothed wheel with a sprung pawl allowing rotation one way and locking the other | A socket wrench, a winch, a cable tie, a seat-belt reel |
| Idler | A third gear between two others; it does NOT change the ratio, it restores the direction and bridges a gap | Anywhere the output must turn the same way as the input |
| Compound | Two gears fixed on one shaft, so two ratios multiply | A clock train, a gearbox, a winch, a mechanical speedometer |
The two that are always misread: An idler changes the direction of the output, not the ratio. Students routinely claim it gears the system down; it does not.
A worm cannot be driven backwards. That is a safety feature, not a limitation — it is why a hoist holds its load when you let go.
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Ratio = driven teeth ÷ driver teeth: A 10-tooth driver into a 40-tooth driven gear gives 4:1 — the output turns a quarter as fast, with about four times the torque.
Compound trains multiply: A compound train multiplies: 4:1 followed by 4:1 is 16:1, achieved with four small gears instead of one the size of a dinner plate.
Choosing a gear system
Parallel shafts, moderate ratio
Spur gears. Cheap, efficient and easy to make — the default unless something else is needed.
Drive must turn a corner
Bevel gears for 90° with a moderate ratio; a worm and wheel when the reduction must also be very large.
Rotary must become linear
Rack and pinion. It is the only gear system in the list that changes the TYPE of motion.
The load must not run back
A ratchet and pawl if it must be released deliberately; a worm if it must simply hold wherever it stops.
How this is tested — identifying gear systems and outlining how they are used. It comes up two ways:
Paper 1 — multiple choice
- Name the gear system shown in a drawing.
- Calculate a gear ratio from two tooth counts.
Paper 2 — analysing a product
- Identify the gear system in a named product and explain its purpose.
- Explain why a compound train was used instead of a single pair.
The trap: Claiming an idler changes the ratio. It restores the direction and bridges a gap; the ratio is set by the first and last gears only.
A cordless electric screwdriver uses a compound gear train between its motor and its chuck. Explain why.
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