pulley

A pulley is a simple machine consisting of a wheel with a grooved rim that a rope, belt, or cable runs around. Its primary function is to change the direction of a force, and in many configurations it can also provide a mechanical advantage that reduces the effort required to lift or move a load. The efficiency of a pulley system depends on the fit of the rope in the groove, bearing quality, and the presence of friction.

Common forms include the fixed pulley, which is attached to a support and changes the direction of the input force but does not multiply it; and the movable pulley, which is attached to the load and doubles the effort advantage, effectively providing a 2:1 mechanical advantage in an uncomplicated setup. More complex arrangements, such as block and tackle systems, combine multiple pulleys to increase the supporting rope segments and achieve greater mechanical advantage. Pulleys used to transmit power via belts are called belt pulleys or sheaves, and they create a variable speed or torque ratio between shafts.

In ideal conditions, the mechanical advantage equals the number of rope segments supporting the load, and the velocity ratio matches the movement of the rope. Real systems experience friction and inertia, reducing efficiency. Pulleys are widely used in construction hoists, elevators, theater rigging, sailing rigs, mining, and various manufacturing processes, as well as in belt-driven machinery to transmit rotational motion. History shows their use in ancient and medieval engineering, with continued relevance in modern mechanical design.