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How do gears and pulleys change the speed and force of rotary motion, and how are the ratios calculated?

Gear types and pulley and belt systems, including simple and compound gear trains, idler gears, bevel gears and rack and pinion, with velocity ratio, gear ratio and revolutions per minute calculations.

A focused answer to Edexcel GCSE Design and Technology 1.5 on gears and pulleys, covering simple and compound gear trains, idler, bevel and rack and pinion gears, V-belt pulleys, and velocity ratio, gear ratio and RPM calculations.

Generated by Claude Opus 4.811 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
Gear types
Gear and velocity ratio calculations
Pulleys and belts

What this dot point is asking

This is Edexcel key idea 1.5, the gears and pulleys part: pulleys and belts (1.5.6) and gear types (1.5.8). Edexcel wants the performance, principles, applications and the calculations for each. Because Section A guarantees 10 marks of calculations, the gear ratio, velocity ratio and revolutions per minute (RPM) sums here are high-value and almost always tested. This page pairs with the levers, cams and linkages page.

Gear types

Simple gear train: two meshing gears. A small driver and large driven gear reduces speed and increases torque (force); the reverse increases speed and reduces torque.
Idler gear: a gear placed between driver and driven. It reverses the output direction back to the same as the driver and bridges a gap, but does not change the overall ratio.
Compound gear train: two gears fixed on the same shaft, so ratios multiply. This gives a large speed or force change in a small space.
Bevel gear: angled teeth transmit drive around a corner, typically through 90 degrees (as in a hand drill).
Rack and pinion: a round pinion gear meshes with a straight toothed rack, converting rotary motion into linear motion (as in a car steering rack).

Gear and velocity ratio calculations

\text{output speed} = \frac{\text{input speed}}{\text{gear ratio}}

A gear ratio greater than 1 means the output is slower than the input but has more turning force (torque); a ratio less than 1 speeds it up. The teeth ratio and the speed change are inversely related: more teeth means slower turning.

Pulleys and belts

A pulley and belt system transmits rotary motion between shafts, often using a V-belt that grips the pulley groove to reduce slip.

A large driven pulley turns slowly with more force; a small one turns fast with less. Belts can also cross to reverse direction, and a stepped pulley (as on a pillar drill) gives several speeds. Pulleys allow slip, which can protect a mechanism but loses some drive compared with gears.

Calculating a compound gear train output speed (model answer)

"In a compound gear train, gear A (driver, 20 teeth) drives gear B (60 teeth). Gear C (15 teeth) is fixed to the same shaft as B and drives gear D (45 teeth). Gear A turns at 900 rpm. Calculate the output speed of gear D. (4 marks)"

step 1: Find the first ratio (A to B)

$\text{ratio}_1 = \dfrac{\text{driven}}{\text{driver}} = \dfrac{60}{20} = 3$ . The B shaft turns at $\dfrac{900}{3} = 300$ rpm.

step 2: Find the second ratio (C to D)

C and B share a shaft, so C also turns at 300 rpm. $\text{ratio}_2 = \dfrac{45}{15} = 3$ .

step 3: Find the output speed of D

$\text{output} = \dfrac{300}{3} = 100\ \text{rpm}$ .

step 4: State the answer

Gear D turns at 100 rpm. The compound train gives an overall ratio of $3 \times 3 = 9$ , so a small set of gears slows 900 rpm down to 100 rpm with much more torque. Markers reward each ratio, carrying the B/C shaft speed across, and the final value with units.

Exam-style practice questions

Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Edexcel 20224 marksA driver gear with 20 teeth meshes with a driven gear of 60 teeth. The driver turns at 300 rpm. Calculate the gear ratio and the output speed of the driven gear. (4 marks)

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A calculation question (Section A guarantees 10 marks of calculations). Markers award the method and the correct values.

Gear ratio uses the teeth: $\text{gear ratio} = \dfrac{\text{driven teeth}}{\text{driver teeth}} = \dfrac{60}{20} = 3$ , written as 3:1 (2 marks).

Output speed: a larger driven gear turns more slowly, so $\text{output speed} = \dfrac{\text{input speed}}{\text{gear ratio}} = \dfrac{300}{3} = 100\ \text{rpm}$ (2 marks: method and value with units).

Markers reward driven over driver for the ratio and dividing the input speed by the ratio. A common error is inverting the ratio (giving 900 rpm), which wrongly speeds the gear up.

Edexcel 20213 marksExplain the purpose of an idler gear in a simple gear train. (3 marks)

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A 3-mark Explain rewards the function described and its effect.

An idler gear is a gear placed between the driver and the driven gear (1). Its purpose is to change the direction of rotation of the driven gear so it turns the same way as the driver, rather than the opposite way (1). It does not change the overall velocity ratio between the driver and driven gears, because its own size cancels out in the calculation; it only reverses direction and can bridge a gap between two gears (1).

Markers reward (1) it sits between driver and driven, (2) it reverses the output direction, (3) it does not change the overall ratio. A frequent error is claiming the idler changes the speed ratio.

Related dot points

Sources & how we know this

Pearson Edexcel GCSE (9-1) Design and Technology (1DT0) specification — Pearson Edexcel (2022)