Ratchet (device)

A ratchet consists of a round gear or a linear rack with teeth, and a pivoting, spring-loaded finger called a pawl (or click, in clocks and watches[1][2]) that engages the teeth.

When the teeth move in the opposite (backward) direction, however, the pawl will catch against the steeply sloped edge of the first tooth it encounters, thereby locking it against the tooth and preventing any further motion in that direction.

This backward motion—which is limited to a maximum distance equal to the spacing between the teeth—is called backlash.

In cases where backlash must be minimized, a smooth, toothless ratchet with a high friction surface such as rubber is sometimes used.

Since the backward travel distance is primarily a function of the compressibility of the high friction surface, this mechanism can result in significantly reduced backlash.

Animation of ratchet gear (green) and pawl (pink).

Red arrows indicate which way force is applied to the gear.
A ratchet featuring a gear (1) and pawl (2) mounted on a base (3)
Animation of ratchet gear rack (green) and pawl (pink).

Red arrows indicate which way force is applied to the gear rack. The rack and pawl are both restricted to only linear movement (not shown).