ratchet and pawl
ratchet and pawl, mechanical device that permits motion in one direction only. The ratchet is usually a wheel with slanting teeth. The pawl is a lever tangential to the wheel with one end resting on the teeth. When the wheel rotates one way, the pawl slides over the teeth; when the wheel rotates the other way, the pawl catches in the teeth.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.