Gear Systems: Simple, Compound, Worm, Rack & Pinion, Bevel

This lesson covers the main types of gear systems used in mechanical products. Gears are a core topic in AQA GCSE Design and Technology (8552), Section 3.1.5, and are essential for understanding how machines control speed, force and direction.

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What Are Gears?

Gears are toothed wheels that mesh together to transmit rotary motion and force from one shaft to another. Gears can:

• Change the speed of rotation
• Change the direction of rotation
• Multiply force (torque) at the expense of speed, or vice versa

Key Gear Terminology

Term	Definition
Driver gear	The gear connected to the input (motor/handle) — provides the effort
Driven gear	The gear connected to the output — receives the motion
Teeth	The interlocking projections on the gear circumference
Gear ratio	The ratio of teeth on the driven gear to teeth on the driver gear
Meshing	When two gears are engaged (teeth interlocked)
Idler gear	A gear placed between the driver and driven gears to change direction without affecting the gear ratio
Gear train	A system of two or more meshing gears

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Simple Gear Train

A simple gear train consists of two or more spur gears meshing together. Each gear in the train meshes with the next.

The diagram below shows how meshing gears transmit motion, with the direction of rotation reversing at each gear:

graph LR
    A["🔵 Driver Gear\n(Small — 20 teeth)\nClockwise ↻"] -- "Teeth mesh" --> B["🔴 Driven Gear\n(Large — 60 teeth)\nAnticlockwise ↺"]
    B -- "Speed decreases\nTorque increases" --> C["Output:\nSlower rotation,\nMore force"]

Key Rule: Direction of Rotation

When two spur gears mesh:

• They rotate in opposite directions.
• Adding an idler gear between them causes the output to rotate in the same direction as the input.

Configuration	Driver Direction	Output Direction
Two gears (no idler)	Clockwise	Anticlockwise
Three gears (with idler)	Clockwise	Clockwise

Speed and Force Relationship

Scenario	Effect on Speed	Effect on Torque (Force)
Small driver → Large driven	Speed decreases	Torque increases
Large driver → Small driven	Speed increases	Torque decreases
Equal-sized gears	Speed unchanged	Torque unchanged

AQA Exam Tip: Remember the trade-off: gearing down (large driven gear) gives more torque but less speed — like cycling uphill in a low gear. Gearing up (small driven gear) gives more speed but less torque — like cycling on a flat road in a high gear.

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Compound Gear Train

A compound gear train has two or more gears mounted on the same shaft. This allows much larger gear ratios in a compact space.

How It Works

1. Gear A (driver) meshes with Gear B.
2. Gear B and Gear C are on the same shaft — they rotate together at the same speed.
3. Gear C meshes with Gear D (output).

The overall gear ratio is the product of the individual ratios:

$\text{Overall GR} = \text{GR}_1 \times \text{GR}_2$Overall GR=GR1​×GR2​

Worked Example

Gear	Teeth	Role
A (driver)	20	Input
B	60	Meshes with A
C (on same shaft as B)	15	Meshes with D
D	45	Output

$GR_1 = \frac{60}{20} = 3$GR1​=2060​=3

$GR_2 = \frac{45}{15} = 3$GR2​=1545​=3

$\text{Overall GR} = 3 \times 3 = 9$Overall GR=3×3=9

If the input speed is 900 RPM:

$\text{Output speed} = \frac{900}{9} = 100 \text{ RPM}$Output speed=9900​=100 RPM

Real-World Example

A hand-cranked drill uses a compound gear train. The slow rotation of the hand crank is geared up to spin the drill bit at high speed.

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Worm and Worm Wheel