Pulley – a rotating part of a belt drive or a cable transmission, made in the form of a wheel around which a flexible link passes. Drive pulleys and driven pulleys, which transmit torque, are fastened to shafts by means of, for example, key or spline joints. Pulleys that do not transmit torque, such as idler and tension pulleys, turn freely on shafts or axles. Pulleys with grooved surfaces are sometimes called sheaves.
Pulleys are available in a wide variety of designs. Pulleys may be made of cast iron, steel, a light alloy, plastic, or—in some cases—wood.