The big idea: A cam is a shaped disc on a rotating shaft, with a follower resting on its edge under gravity or a spring.
Where the profile is far from the shaft the follower is pushed up; where it is close the follower drops. So the shape of the cam is the motion it produces.
Each cam drawn beside the follower displacement it produces over one full turn — the graph is computed from the same profile.
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| Cam | The motion it gives | Where you see it |
|---|---|---|
| Pear | A dwell for about half a turn, then one rise and fall — the follower is held still while the shaft keeps turning | An engine valve, a mechanical timer, a gas-lighter striker |
| Circular | A round disc mounted off-centre: one smooth, symmetrical rise and fall per turn, with no dwell and no shock | A sewing machine, a slow oscillating toy, a gentle pump |
| Eccentric | The same geometry under another name; the size of the OFFSET sets the stroke, not the size of the disc | Sanders, vibrating screens, small pumps, clamping levers |
| Triangular | Three rises and falls per revolution, so the follower moves three times as often as the shaft turns | Automata, indexing mechanisms, toys that flap quickly |
| Oval | Two smooth rises and falls per revolution, gentler than a pear cam | Agitators, slow display mechanisms, valve gear |
| Snail (drop) | A gradual rise all the way round, then a sudden drop off the step — and it works in ONE direction only | A striking clock, an indexing wheel, a spring-release trigger |
Dwell is the word examiners look for: A dwell is a stretch of constant radius, where the follower holds still while the shaft keeps turning.
It is the reason a pear cam is used for an engine valve: the valve must stay open for a set part of the cycle, not merely be tapped.
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From the movement you want to the shape you cut
Draw the motion first
Plot follower height against shaft angle: rise here, dwell there, fall over this much of the turn.
Wrap the graph round the shaft
Each height on the graph becomes a radius at that angle. The profile is the graph in polar form — nothing more.
Check the follower can keep up
A very steep rise throws a spring-loaded follower off the profile at speed. Smooth the corners, or run it slower.
Choose the follower type
A knife-edge follows detail but wears fast; a roller wears well; a flat follower cannot enter a concave curve at all.
Two facts a cam answer should contain: That the shape is the motion — this is the one mechanism where the profile is the specification.
And that a cam is timed: because it sits on the same shaft as everything else, its rise and fall happen at a chosen point in the cycle rather than whenever.
How this is tested — identifying cam shapes and outlining how they are used. It comes up two ways:
Paper 1 — multiple choice
- Name the cam shape shown in a drawing.
- Choose the follower motion a stated cam produces.
Paper 2 — analysing a product
- Identify the cam in a named product and explain the motion it gives.
- Explain why one cam shape was chosen over another for a product.
The trap: Saying a cam "makes something move up and down" and stopping. The marks are for HOW it moves — how many times per turn, whether there is a dwell, and whether the fall is gradual or sudden.
An automaton toy makes a wooden bird peck three times for every turn of its handle, then pause. Explain the cam arrangement that produces this.
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