Pawl

A pawl is a small, spring-loaded finger or lever that engages with the teeth of a gear, ratchet wheel, or linear rack to allow motion in one direction while preventing it in the other. It is the critical component that makes ratcheting mechanisms work.

How it works

The pawl pivots on a pin and is pressed against the gear by a spring. When the gear rotates in the free direction, the pawl rides up the gently sloped edge of each tooth and drops into the next valley — usually with an audible click. When the gear tries to rotate in the opposite direction, the pawl jams against the steep face of the tooth, locking the mechanism.

Where pawls appear

• Socket wrenches: The pawl inside the ratchet head engages the gear to allow free rotation in one direction.
• Clocks and watches: The escapement mechanism uses a pallet (a type of pawl) to regulate the release of energy from the mainspring.
• Bicycle freewheels: Pawls inside the rear hub allow the wheel to spin freely when you stop pedalling.
• Cable ties: The locking ladder inside a cable tie acts as a series of pawls, engaging the teeth on the strap.
• Handcuffs: Pawls lock the ratcheting arm in place, preventing the cuffs from opening without a key.
• Roller coasters: Anti-rollback devices use pawls that click over a chain lift hill and prevent the train from rolling backward.

Design considerations

The pawl must be strong enough to withstand the forces in the locked direction, springy enough to return to position in the free direction, and hard enough to resist wear from repeated engagement. Most pawls are made from hardened tool steel and are heat-treated for durability.