The big idea: A cam turns on a shaft and a follower rides its edge, held down by gravity or a spring, so continuous rotary motion becomes reciprocating motion at the follower.
The profile is the specification: how far the follower rises, how many times per turn, how sharply, and whether it dwells.
Each profile beside the displacement it produces — the graph is computed from the same shape that is drawn.
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| What to read | Where you read it | What it tells a designer |
|---|---|---|
| Lift or stroke | Largest radius minus smallest radius | How far the follower moves — the stroke of the needle, the valve or the arm |
| Rises per revolution | The number of lobes on the profile | One for a pear, circular or eccentric cam; two for an oval; three for a triangular |
| Dwell | A stretch of constant radius | The follower holds still while the shaft keeps turning, which is how a valve stays open for part of a cycle |
| Rate of rise | How steeply the radius changes | A steep rise throws a spring-loaded follower off the profile at speed, so the corners must be smoothed or the shaft run slower |
| Timing | The angle at which each event happens | Because the cam shares a shaft with the rest of the machine, its rise happens at a chosen point in the cycle rather than whenever |
Follower type matters as much as the profile: A knife-edge follower traces fine detail and wears out quickly. A roller follower wears well and is the usual choice. A flat follower cannot enter a concave curve at all, so the profile has to be drawn to suit it.
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From the movement to the shape
Draw the displacement graph first
Follower height against shaft angle: rise here, dwell there, fall over this part of the turn. That graph IS the specification.
Wrap it round the shaft
Each height becomes a radius at that angle. The profile is the same graph in polar form — nothing more than that.
Smooth the transitions
Sharp corners on the graph become shocks at the follower. Rounding them keeps contact at speed and reduces noise and wear.
Check the follower can keep up
If the profile falls faster than the spring can push the follower down, contact is lost and the follower bounces — which is exactly what valve float is in an engine.
Changing the shape is free: Three pecks per turn can be had by gearing the shaft up three times — two extra gears, more friction, more cost.
Or by cutting a triangular profile instead of a pear one, which costs nothing at all. With a cam, the shape does the work that mechanism would otherwise have to.
How this is tested — analysing cam systems and interpreting cam diagrams. It comes up two ways:
Paper 1 — multiple choice
- Read the lift or the number of rises from a cam drawing.
- Identify the cam that produces a described motion.
Paper 2 — analysing a product
- Analyse a cam system in a named product.
- Design a cam profile to produce a stated movement.
The trap: Saying it "makes something move up and down". Give the lift, the rises per revolution, whether there is a dwell, and whether the fall is gradual or sudden.
A sewing machine feeds the fabric one stitch length between needle strokes, using a cam on the lower shaft. Analyse the cam system.
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