Ratchet, in horology , is the mechanism usually employed to make a wheel turn in one direction but not in the opposite direction. The ratchet comprises three parts (See Clockmaking Plates, Plate III, fig. 7 ): the ratchet wheel, 7; the click or pawl, c c ; and the click spring, r r . Their action is so simple that it can be readily understood from the illustration alone: click pawl cc turns on screw uu and is held against the teeth of ratchet wheel 7 by spring rr . Consequently, the ratchet wheel cannot turn from 14 to 7, but can readily turn from 7 to 14, since the click does not oppose motion in that direction. During this motion the click is raised by the teeth, and as each one escapes the click drops by the action of the spring to the bottom of the next tooth. This causes the sound heard when winding a clock or a watch. When this sound is very noticeable, some people say their watch has a strong spring, but as we see, the mainspring has no effect on it. For the click to function, it must block the motion of the ratchet wheel in the most efficient way; therefore, it must be pushed towards its center of motion, r r , [1] in a direction tangent to the ratchet wheel.

“Ratchet” also applies to any combination of wheel, click or pawl, and spring. See Click, Ratchet, and Spring.

Notes

1. In fig. 7, rr indicates the click spring; the click pivot screw is uu [Translator’s note].