What is 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^ (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.

What is follower position at 60^?

The rise from 0^ to 120^ is uniform over 24\,mm, so the lift per degree is:

QLDEngineeringSyllabus dot point

How does a cam turn steady rotation into a controlled pattern of back-and-forth movement?

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

A QCE Engineering Unit 4 answer on cams and followers. Covers how a rotating cam drives a follower, the rise-dwell-fall pattern, types of cam and follower, the displacement diagram, and a worked reading of follower lift versus cam angle.

Generated by Claude Opus 4.76 min answerUpdated 2026-05-29

Reviewed by: AI editorial process; not yet individually human-reviewed

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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

Find the follower position over one revolution

A pear cam is described by this displacement diagram for one revolution. From $0^\circ$ to $120^\circ$ the follower rises uniformly from $0$ to a maximum lift of $24\,\text{mm}$ . From $120^\circ$ to $240^\circ$ it dwells at $24\,\text{mm}$ . From $240^\circ$ to $360^\circ$ it falls uniformly back to $0$ . The cam turns at $300\,\text{rev/min}$ . Find the follower position at a cam angle of $60^\circ$ and the time the cam takes to complete one revolution.

Follower position at $60^\circ$ . The rise from $0^\circ$ to $120^\circ$ is uniform over $24\,\text{mm}$ , so the lift per degree is:

\frac{24}{120} = 0.20\,\text{mm/degree}

At $60^\circ$ (halfway through the rise):

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

Time for one revolution. At $300\,\text{rev/min}$ :

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

So one complete rise-dwell-fall cycle takes $0.20\,\text{s}$ , and at the $60^\circ$ point the follower sits at $12\,\text{mm}$ , exactly half its $24\,\text{mm}$ maximum lift. The uniform rise means displacement is directly proportional to cam angle within that phase.

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.