Ratchet

A Ratchet mechanism allows movement in one direction only.

In the illustration, the Ratchet Wheel lifts the Pawl as it rotates anti-clockwise but clockwise rotation is stopped by it. Such a system is used in Winches to allow loads to be lifted up without slipping down again. The Ratchet Wheel could be fixed to the Winch shaft via a Key as shown. Alternatively, use of a square or hexagonal winch shaft would ensure it rotates with the Ratchet.

A similar idea can be used to create a Linear Ratchet which allows motion in one direction but not the other. A Pawl type mechanism allows each squeeze of a trigger to tighten a Bar Clamp for example. They are found also as Quick Release Mechanisms on some Wood VIces and are the basis of the humble Cable Tie,