In mechanical engineering a ratchet is a device used to control rotation of an axle in one direction only. This sort of behaviour is useful in many areas, such as in turnstile, spanners (where rotation in confined surfaces can be achieved easily), and others.

Ratchets work generally by having a form of gearwheel that, instead of having regular teeth, have teeth set off at an angle.

Many designs include a stiff, thin protrusion that rests against the gearwheel. When the gearwheel is rotated in one direction, the protrusion raises and clicks back in place. Because of the shape of the gearwheel it is difficult to push the protrusion back and over the tooth of the gear.

Alternatively, this gearwheel can be set in a molding shaped to admit rotation of this gearwheel in one direction only. If the gearwheel is rotated backwards the molding and the teeth will clash.

In this image, the inner wheel and outer moulding are made of a relatively stiff rubber. As the inner wheel turns clockwise, the outer fins can bend and spring back with a relatively small force. However it takes much greater force to jam the inner wheel past the edge of the fin against the outer edge of the molding, causing a restriction of motion.

See also: freewheel