RATCHET AND PAWL is an important mechanical de vice in a great many machines and appliances, enabling a move ment to be effected in one direction, with slip or freedom in the opposite direction. The mechanism is used to lock something so that it cannot slip or reverse, to hold a load as in a winch, or to give a positive feed. Some ratchets are straight, or with a moderate curve, but the majority are circular. The teeth are usually V-shaped, and the pawl is pivoted so that it drops against any tooth and remains without danger of jumping out. Where a continuous movement is desired, the objectionable clicking of the pawl may be checked by a leather facing, or by the use of a ball or roller, or a set of these, to catch in the teeth, with gravity or spring action to ensure engagement. In the case of a silent ratchet, the wheel has no teeth but an eccentrically pivoted pawl works frictionally against the periphery.
Various drives and brake mechanisms are made safe by ratchet mechanism, the ship's capstan being one of the oldest examples.
Screwdrivers and wrenches are operated by ratchet for access in difficult situations, or to help the worker in manipulative power. Braces and metal drills are actuated by ratchet when it is not possible to make the complete revolution of a handle. So also are jack screws for lifting loads rapidly. Ratchet feed, by which an arm holding the pawl is moved over, and the ratchet-wheel and a feed screw are turned, is utilized in many types of machines and is usually automatic in action. The finest example occurs in cylindrical grinding machines, in which by means of an automatic feed the grinding-wheel travels along a shaft or spindle, the ratchet mechanism setting the wheel to cut a very little smaller before each traverse takes place, this process being repeated as often as is necessary to bring the shaft to size.