How do cams, cranks, screws and rack-and-pinion convert one type of motion into another?
Motion converters: cams and followers, crank and slider, rack and pinion, and screw threads, and the motion change each produces.
A CCEA GCSE Technology and Design answer on motion converters: cams and followers, the crank and slider, rack and pinion, and screw threads, explaining the input and output motion each one produces and where they are used.
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What this dot point is asking
CCEA wants you to know the main motion converters - the cam and follower, the crank and slider, the rack and pinion, and the screw thread - and to state the input and output motion for each. These mechanisms change one type of motion into another, which the systems often need.
The answer
Cam and follower
Different cam shapes give different movements: a pear cam gives a smooth rise and fall with a rest (dwell); an eccentric (circular) cam gives smooth continuous reciprocation. Cams are used in engines (to open valves) and in automata.
Crank and slider
Rack and pinion
Screw thread
Summary of motion conversions
A quick reference for the exam:
- Cam and follower: rotary to reciprocating.
- Crank and slider: rotary to reciprocating (and back).
- Rack and pinion: rotary to linear (and back).
- Screw thread: rotary to linear, with a force advantage.
Worked example: choosing a motion converter
Examples in context
- Example 1. A car engine
- The crank and slider converts the reciprocating motion of the pistons into the rotary motion of the crankshaft, while cams open and close the valves - two converters in one machine.
- Example 2. Car steering
- Turning the steering wheel (rotary) moves a rack left and right (linear) through a pinion, steering the wheels - a rack and pinion.
- Example 3. A workshop vice
- Turning the handle (rotary) drives a screw thread that moves the jaw in a straight line (linear) with great force, clamping the work tightly.
Being able to name each converter and state its input and output motion lets you answer both the "describe how it works" and "name the input and output" questions.
Try this
Q1. What motion change does a cam and follower produce? [1 mark]
- Cue. Rotary input to reciprocating (up-and-down) output.
Q2. Name a mechanism that converts rotary motion into linear motion. [1 mark]
- Cue. A rack and pinion (or a screw thread).
Q3. Give one product that uses a crank and slider. [1 mark]
- Cue. A car engine (piston and crankshaft) or a sewing machine.
Q4. Besides changing motion, what extra benefit does a screw thread give? [2 marks]
- Cue. A large mechanical (force) advantage, so a small effort produces a large linear force.
Q5. What controls how the follower rises and falls on a cam? [1 mark]
- Cue. The shape (profile) of the cam.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksDescribe how a cam and follower works and state the motion change it produces.Show worked answer →
A cam is a specially shaped rotating piece, and a follower rests on its edge (1). As the cam rotates, its changing radius (profile) pushes the follower up and lets it fall (1).
So the cam converts the rotary motion of the cam shaft into the reciprocating (up and down) motion of the follower (1). The shape of the cam controls how the follower rises and falls (1).
CCEA style4 marksName the input and output motion for a rack and pinion and for a crank and slider.Show worked answer →
Rack and pinion: input is rotary (the pinion gear turns) and output is linear (the rack moves in a straight line) (1, 1).
Crank and slider: input is rotary (the crank turns) and output is reciprocating (the slider moves backwards and forwards), or the reverse (1, 1).
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Sources & how we know this
- CCEA GCSE Technology and Design specification — CCEA (2017)