A cam’s profile controls the displacement of its follower. Pear, eccentric and snail cams convert rotary motion into distinct patterns of reciprocating motion, while a centred circular cam produces no follower displacement.
A cam is a rotating component whose edge stays in contact with a follower. As the distance from the axis to the edge changes, the follower rises or falls. Where that distance remains constant, the follower is stationary during a dwell.
| Cam shape | Motion created at the follower |
|---|---|
| Pear cam | Produces a substantial dwell, followed by a smooth rise and fall; it suits mechanisms that must pause between movements. |
| Circular cam | If centred on its geometric axis, the constant radius keeps the follower stationary, creating continuous dwell. |
| Eccentric cam | A circular disc rotates about an off-centre axis, producing one smooth rise and fall per revolution, usually without dwell. Motion is symmetrical and approximately simple harmonic. |
| Snail cam | Produces a gradual rise and sudden fall when the follower drops over the sharp edge. It provides a timed impact or release and normally rotates in one direction to prevent jamming. |
The mechanism is the changing effective radius. A pear cam has a constant-radius section for dwell; an eccentric cam changes radius smoothly; a snail cam combines gradual radial increase with an abrupt step.
A common misconception is that every circular cam creates reciprocating motion. A centred circular cam does not; the commonly illustrated “circular cam” is often an eccentric cam because its axis is off-centre.
For IB Design Technology HL A3.3, an outline response should identify the cam and state its sequence of rise, dwell and fall. In a diagram, check the axis position, rotation direction and radius changes rather than relying only on overall shape.