Ratchet and pawl

Here is a description of the ratchet and pawl, from Cornell University's wonderful Kinematic Models for Design Digital Library:

The ratchet and pawl mechanism allows motion in one direction but locks it in the other direction. In this sense it acts in the same way as a diode in an electrical circuit or a check value in a water pipe. The ratchet and pawl was often used with a winch or windlass, a horizontal drum with a rope or cable attached, which allowed the rope to be wound onto the cylinder but prevented the cylinder from unwinding the rope. Reuleaux was fascinated with ratchet mechanisms and designed several different models to illustrate the different motions and uses of the ratchet. In his machine design book The Constructor (1893), he defined the ratchet in the similar way as Leonardo da Vinci; "The object of the ratchet is to check the action of certain portions of a machine and so modify an otherwise continuous motion into some intermittent form." (Page 150). The use of control valves in steam engines and internal combustion engines in the 19th century to regulate speed represented the beginnings of automatic control of machines. Reuleaux recognized the importance of control but did not have the mathematical concepts to describe it. He saw instead the special digital nature of ratchet mechanisms, on or off, as having special significance in machine regulation and created several models to express the role of ratchets in machine control devices.