How does a ratchet allow only one turning direction?

A ratchet permits turning in one direction while restraining turning in the other. It combines a wheel with unevenly shaped teeth and a pivoting part called a pawl. The pawl's tip rests against the teeth, so contact between the two parts controls which movement is possible.

In the allowed direction, a sloping tooth face can lift the pawl. The tooth passes underneath, and the pawl returns into the next space. In the opposite direction, a blocking face presses against the pawl rather than lifting it out of the way. That contact prevents the reverse turn in this ideal mechanism.

The initial arrangement shown permits clockwise rotation. Mirroring both the teeth and pawl reverses which turn is allowed. Clockwise motion is therefore not a universal property of ratchets; the permitted direction comes from their particular contact geometry.

Lifting the pawl clear of the teeth removes the one-way restraint in this model. The pawl does not supply the energy that drives the wheel; it constrains movement supplied by something else. The explanation assumes intact parts in the intended contact and does not calculate wear, load limits or real locking-device performance.