Ratchet (device)

In mechanical engineering, a ratchet is a device that allows linear or rotary motion in only one direction, while preventing motion in the opposite direction. Ratchets are used in many other mechanisms, including clocks, capstans, turnstiles, spanners, winders, cable ties, jacks, screwdrivers, and hoists. The term is also used for a socket wrench.

Ratchet and pawl

Ratchets consist of a gearwheel or linear rack with teeth, and a pivoting springloaded finger called a pawl (or in horology a "click" [cite web|title=Know your terminology - Clocks|work=Hints and Tips|publisher=British Horological Institute|url=http://www.bhi.co.uk/hintsandtips.html|accessdate=2008-05-15] ) that engages the teeth. Either the teeth, or the pawl, are slanted at an angle, so that when the teeth are moving in one direction, the pawl slides up and over each tooth in turn, with the spring forcing it back with a 'click' into the depression before the next tooth. When the teeth are moving in the other direction, the angle of the pawl causes it to catch against a tooth and stop further motion in that direction.

Because the ratchet's teeth can only stop 'backward' motion at discrete points, a ratchet does allow a limited amount of 'backward' motion, or backlash, to a maximum of the spacing between its teeth. Where it is desired to eliminate this, a smooth toothless ratchet with a high friction surface like rubber is sometimes used. The pawl bears against this at an angle. Any backward motion causes the pawl to jam against the surface by friction, preventing backward motion with very little backlash.

ee also

* Freewheel
* Sprag

References

External links

* [http://www.technologystudent.com/cams/ratch1.htm An animated display of ratchet mechanism]