pawl

A pawl is a hinged or fixed lever that engages with the teeth of a rotating gear or a rack to allow motion in one direction while preventing motion in the opposite direction. It is a key component of a ratchet mechanism, where the combination of a toothed wheel and a pawl converts every motion in one direction into a stepped advance.

In a typical ratchet and pawl arrangement, the pawl is spring-loaded to press against the wheel’s teeth. When the wheel turns in the allowed direction, the pawl rides over the teeth, advancing the wheel step by step. If the wheel tries to rotate backward, the pawl engages a tooth and blocks the motion, providing a mechanical brake or lock.

Pawls vary in design. A spring pawl uses a biasing spring to maintain engagement. A detent pawl works with a detent to provide discrete positions. Roller pawls employ a small roller to reduce wear and noise at the contact point. Cam pawls are actuated by a cam surface to engage or disengage as part of a more complex mechanism. Materials are typically metal alloys or durable plastics, chosen for wear resistance and fatigue strength.

Common applications include hand tools such as socket wrenches, hose reels, clocks and timers, winches, and indexing mechanisms in automation. Design considerations include wear on teeth and pawl, lubrication, back-driving resistance, and ensuring reliable engagement without jamming. See also ratchet, detent, and escapement for related one-way motion mechanisms.