GCSE-readyLinkages and pulleys
Linkages send movement round corners and reverse it. Pulleys pass turning movement from one shaft to another.
Linkages
A fixed pivot is fixed to a base and cannot move. A moving pivot joins links and moves with them.
| Linkage | What it does | Example |
|---|---|---|
| Reverse motion | Output moves in the opposite direction to the input | Some pop-up books and toys |
| Parallel motion | Output moves in the same direction as the input | Cantilever toolbox trays |
| Bell crank | Changes the direction of motion through 90° | Bicycle brake mechanisms |
| Crank and slider | Changes rotary motion to reciprocating motion (or back) | Car engine pistons, pumps |
Belts and pulleys
The driver pulley is turned by the motor or handle; the driven pulley is turned by the belt.
If the driven pulley is bigger, it turns slower but with more turning force.
A belt can slip if overloaded, which can protect the motor. Crossing the belt makes the driven pulley turn the opposite way.
Velocity ratio
Output speed = input speed ÷ velocity ratio.
A simple single fixed pulley used to lift a load changes the direction of the force: you pull down to lift up.
A motor turns a 30 mm driver pulley at 600 rpm. The driven pulley is 90 mm. Find the velocity ratio and the speed of the driven pulley.
- Velocity ratio = driven diameter ÷ driver diameter.
- 90 ÷ 30 = 3, so the ratio is 3:1.
- Output speed = 600 ÷ 3 = 200 rpm.
Answer: Velocity ratio 3:1; the driven pulley turns at 200 rpm.
Linkages change the direction or size of movement: reverse motion, parallel motion, bell crank (through 90°) and crank and slider (rotary to reciprocating). For pulleys, velocity ratio = driven diameter ÷ driver diameter.
- Bell crank
- A linkage that changes the direction of motion through 90°.
- Velocity ratio
- For pulleys, driven diameter ÷ driver diameter.
- Linkage
- Levers joined by pivots to change the direction or size of movement.
- Rotary motion
- Turning round in a circle, like a wheel.
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2 more questions
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In a parallel motion linkage, how does the output move?
- In the opposite direction
- Through 90°
- In a circle
- In the same direction as the input
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Answer: In the same direction as the input
Parallel motion keeps the same direction.
Through what angle does a bell crank turn motion?
- 180°
- 45°
- 360°
- 90°
Show the answer
Answer: 90°
Bell cranks turn motion round a corner.
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Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.