Ratchet and Pawl mechanism is a type of intermittent motion mechanism. It allows the rotation of the shaft in one direction only and prevents its motion in the other direction.
It consists of an arm that pivots about the center of the toothed ratchet wheel. The oscillatory motion of the arm is translated into intermittent rotary motion.
There are two pawls: one driving and the other one, locking. Both the pawls are usually spring-loaded against the ratchet.
As the arm moves backward, the locking pawl moves along with it and drops into the next step on the ratchet wheel. During this period, the ratchet wheel is stationary. Now as the arm moves forward, the driving pawl pushes the ratchet wheel to rotate in one direction. The locking pawl prevents the ratchet from reversing direction while the driving pawl returns as the arm move in the backward direction again. So, the rotation is allowed in one direction only.