Rockett, Frank H. Engineering Consultant, Charlottesville, Virginia.
Last reviewed:January 2020
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A wheel, usually toothed, operating with a catch or a pawl so as to rotate in a single direction (see illustration). A ratchet and pawl mechanism locks a machine such as a hoisting winch so that it does not slip. The locking action may serve to produce rotation in a desired direction and to disengage in the undesired direction as in a drill brace. A further adaptation is to drive the catch in a to-and-fro motion against the ratchet to produce intermittent circular motion. The catch or pawl may be of various shapes such as an eccentrically mounted disk or ball bearing. Gravity, a spring, or centrifugal force (with the catch mounted internal to the ratchet) are commonly used to hold the pawl against the ratchet. A ratchet and pawl provides an arresting action, whereas an escapement provides an arresting action followed by a self-initiated momentary release. In high-speed machines, the abrupt action of a toothed ratchet produces severe shock. In such situations, a continuously variable yet directionally sensitive action is achieved by wedging rollers or specially shaped sprags between the input and output members. See also: Brake; Escapement; Pawl
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