Cams and followers for QCE Engineering Unit 4

Syllabus dot point

“Explain how a cam and follower converts rotary motion into a programmed reciprocating or oscillating motion, and interpret the displacement diagram that describes the follower's movement”

QCEEngineeringUnit 4: Machines and mechanisms6 min readUpdated 2 Jun 2026

Quick answer

A cam is a shaped rotating part that drives a follower resting against its edge, converting rotary motion into reciprocating or oscillating motion. As the cam turns, the changing radius of its profile pushes the follower out and lets it return, producing a repeating cycle. The motion has three phases: rise (follower moves out), dwell (follower stays still while the cam radius is constant) and fall (follower returns). The displacement diagram plots follower displacement against cam angle over one full revolution, and its shape is the design of the cam. Common cams include the pear, circular (eccentric) and heart-shaped cams; followers may be knife-edge, flat or roller, and are held against the cam by gravity or a spring.

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What this dot point is asking
The answer
Exam-style questions

What this dot point is asking

QCAA wants you to explain the cam-and-follower mechanism: how a specially shaped rotating cam pushes a follower through a precise, repeating pattern of movement, and how the displacement diagram captures that pattern. Cams are how machines produce a custom, timed motion from plain rotation, so understanding the rise, dwell and fall and the displacement diagram is central to the mechanisms content.

The answer

How a cam and follower works

A cam is a non-circular or off-centre rotating part. A follower rests against its profile, held in contact by a spring or by gravity. As the cam rotates at a steady speed, the distance from the cam's centre to its edge changes, so the follower is pushed outward where the profile is far from the centre and allowed back where it is close. One revolution of the cam produces one complete cycle of follower movement. The cam converts uniform rotary input into a programmed reciprocating (straight-line) or oscillating (pivoting) output.

Rise, dwell and fall

The follower's motion over one revolution has up to three phases:

Rise: the cam profile moves the follower outward (away from the cam centre).
Dwell: the profile radius is constant, so the follower stays still even though the cam keeps turning.
Fall (return): the profile lets the follower move back inward.

The order and length of these phases, set by the cam shape, define exactly when and how far the follower moves, which is why cams are used to time events such as opening engine valves.

Types of cam and follower

The cam profile is chosen for the motion required:

Pear cam: gives a long dwell with a smooth rise and fall, common for valve timing.
Circular (eccentric) cam: a circle mounted off-centre, giving smooth continuous rise and fall with no dwell, like simple harmonic motion.
Heart-shaped cam: gives a uniform-velocity rise and fall with no dwell, used to wind thread evenly.

Followers also vary: a knife-edge follower follows complex profiles but wears quickly; a flat follower spreads contact; a roller follower reduces friction and is common in engines.

The displacement diagram

The displacement diagram is the key drawing for a cam. It plots the follower's displacement (vertical axis) against the cam's angle of rotation through one full revolution, $0$ to $360^\circ$ (horizontal axis). A rising line is the rise, a horizontal line is a dwell, and a falling line is the return. The diagram is effectively the design specification of the cam: from it the cam profile can be drawn.

Reading a cam displacement diagram

A pear cam rotates at $300\,\text{rev/min}$ . Its displacement diagram shows: rise from $0\,\text{mm}$ to $24\,\text{mm}$ uniformly from $0^\circ$ to $120^\circ$ , dwell at $24\,\text{mm}$ from $120^\circ$ to $240^\circ$ , then fall back to $0\,\text{mm}$ uniformly from $240^\circ$ to $360^\circ$ . Find the follower position at a cam angle of $60^\circ$ and the time for one complete revolution.

Step 1: Locate the cam angle within the displacement diagram

At $60^\circ$ the cam is inside the rise phase, which runs from $0^\circ$ to $120^\circ$ . Because the rise is described as uniform, the follower displacement is directly proportional to the cam angle within this phase.

Step 2: Calculate the lift rate for the rise phase

The total lift during the rise is $24\,\text{mm}$ over $120^\circ$ of rotation. Dividing gives the lift per degree:

\frac{24\,\text{mm}}{120^\circ} = 0.20\,\text{mm per degree}

Step 3: Find the follower position at $60^\circ$

Multiply the lift rate by the cam angle reached within the rise phase. The rise starts at $0^\circ$ , so the follower has risen through $60^\circ$ of the $120^\circ$ rise:

\text{lift} = 0.20 \times 60 = 12\,\text{mm}

This is exactly half the maximum lift, as expected halfway through a uniform rise.

Step 4: Convert rotational speed to period

The cam completes $300$ revolutions every minute. The time per revolution is the reciprocal of this rate:

T = \frac{60\,\text{s}}{300\,\text{rev}} = 0.20\,\text{s per revolution}

Final answer: At a cam angle of $60^\circ$ the follower is at $12\,\text{mm}$ . One complete rise-dwell-fall cycle takes $0.20\,\text{s}$ .

Why this matters for machines and mechanisms

Cams give a machine a custom, repeatable motion profile that gears and simple linkages cannot. An engine camshaft opens each valve at exactly the right moment for exactly the right duration; automated machinery uses cams to sequence operations. Designing the displacement diagram first and then the cam profile is the standard workflow, and it appears in Unit 4 as a way to turn a required output motion into a physical mechanism.

Common errors

Confusing the displacement diagram with the cam shape: The diagram plots displacement against angle on straight axes; the cam itself is the curved profile derived from it. They are related but not the same drawing.
Forgetting the follower needs holding in contact: A follower stays on the cam only because a spring or gravity pushes it down. Without that, it loses contact during the fall.
Treating dwell as no rotation: During a dwell the cam keeps turning; it is the follower that stays still because the profile radius is constant.
Reading lift as proportional to angle in every phase: Displacement is proportional to angle only when the rise or fall is uniform. A curved profile gives a non-linear relationship.

Exam-style questions

Questions in the style of QCAA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

Practice6 marks

A cam rotates at a constant

90\ \text{rev/min}

. Its displacement diagram shows the follower rise of

20\ \text{mm}

over the first

120^\circ

of cam rotation, then dwell. Determine the time for one full revolution and the time taken for the rise phase.

Show worked answer →

A 6 mark determine question rewards converting rotational speed to time per revolution then the rise time.

Time for one revolution: at $90\ \text{rev/min}$ , $T = \dfrac{60}{90} = 0.667\ \text{s}$ per revolution.

The rise occupies $120^\circ$ of the full $360^\circ$ , so it takes a fraction $\dfrac{120}{360} = \dfrac{1}{3}$ of a revolution: $t_{\text{rise}} = \dfrac{1}{3} \times 0.667 = 0.222\ \text{s}$ .

Markers reward the period of $0.667\ \text{s}$ from the rotational speed and the rise time of about $0.22\ \text{s}$ from the angular fraction of the cycle.

Practice4 marks

Explain how a displacement diagram describes the motion of a follower, and what the rise, dwell and fall regions represent.

Show worked answer →

A 4 mark explain answer needs the meaning of the diagram and its regions.

A displacement diagram plots follower displacement (vertical axis) against cam angle or time (horizontal axis) for one revolution, so its shape is the program of follower motion the cam profile produces. Rise is where the follower moves outward as the cam radius increases; dwell is a flat region where the follower is stationary because the cam radius is constant; fall is where the follower returns as the radius decreases.

Markers reward describing the diagram as displacement against cam angle and correctly identifying rise (increasing), dwell (constant) and fall (decreasing).

Practise this

Sources & how we know this

Engineering General senior syllabus 2025 — QCAA