Ratchet Wheel

The ratchet wheel is a toothed wheel that converts an oscillating motion to an intermittent circular motion.

A is the ratchet wheel. B is an oscillating lever that moves the driving pawl, C. A supplementary pawl at D prevents backward motion of the wheel. When the arm B moves counterclockwise, pawl C will move the wheel part of a revolution. Supplementary pawl D allows such motion. When arm B moves back (clockwise), pawl C slides over the points of the teeth and the wheel remains motionless due to supplementary pawl D. The pawls are often held in contact with the wheel via springs.

The radial edge of the teeth must be shaped so that back pressure from the wheel forces the pawls inwards. This is done by insuring that the angle between the line from the pawl pivot to the contact point and the line of the radial portion of the tooth is less than 90 deg.