Cam Mechanisms: Pear, Circular, Eccentric, Snail & Drop Cams

This lesson covers cam mechanisms — devices that convert rotary motion into controlled reciprocating or oscillating motion. Cams are a key topic in AQA GCSE Design and Technology (8552), Section 3.1.5.

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What Is a Cam?

A cam is a shaped disc (or other profile) mounted on a rotating shaft. As the cam rotates, its profile pushes against a follower, causing the follower to move up and down (or in and out) in a controlled pattern.

Key Components

Component	Function
Cam	The shaped rotating disc that drives the mechanism
Follower	The component that follows the cam profile and produces the output motion
Shaft (camshaft)	The rotating axle on which the cam is mounted
Spring (return spring)	Keeps the follower in contact with the cam surface (in many designs)

Input and Output

Input	Output
Rotary motion (cam rotates on the shaft)	Reciprocating motion (follower moves up and down)

The diagram below shows the cam and follower motion cycle as the cam completes one full rotation:

graph LR
    A["Cam rotates\n(rotary input)"] --> B["Cam profile\npushes follower UP\n(rise)"]
    B --> C["Follower at\nhighest point\n(dwell)"]
    C --> D["Cam profile\nallows follower DOWN\n(fall)"]
    D --> E["Spring returns\nfollower to start"]
    E --> A

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Types of Follower

The shape of the follower affects how it rides on the cam surface:

Follower Type	Description	Best Suited For
Flat follower	Flat surface rides on the cam	Simple cams where the follower moves straight up/down
Roller follower	A small wheel that rolls on the cam surface	Reduces friction; used with complex cam profiles
Knife-edge follower	A pointed tip follows the exact cam profile	Precise motion tracking but wears quickly

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Cam Profiles

The shape of the cam determines the pattern of motion of the follower. Different cam profiles produce different displacement patterns.

1. Pear-Shaped Cam (Drop Cam)

Feature	Detail
Shape	Teardrop / pear shape
Motion	Follower rises gradually as the cam rotates, then drops suddenly
Rise	Smooth and gradual over most of the rotation
Fall	Sudden drop near the narrow end of the pear
Dwell	Brief dwell at the highest point

Applications:

• Sewing machine — the needle rises gradually and drops suddenly to pierce the fabric
• Stamping machines — gradual approach, sudden stamp

2. Circular (Round) Cam / Eccentric Cam

Feature	Detail
Shape	A perfect circle, but the shaft passes through an off-centre hole
Motion	Smooth, continuous sinusoidal (wave-like) rise and fall
Rise	Gradual
Fall	Gradual (symmetrical to the rise)
Dwell	No true dwell — the follower is always moving

Applications:

• Steam engines — smooth, continuous valve operation
• Vibrating sanders — smooth oscillation of the sanding pad
• Jigsaws — converting rotary motor motion to reciprocating blade motion

AQA Exam Tip: An eccentric cam produces a smooth, symmetrical rise and fall with no dwell period. This distinguishes it from all other cam types and is a frequently tested fact.

3. Snail Cam (Spiral Cam)