poleas

Poleas, or pulleys, are wheels with a grooved rim that guide a rope, belt, or cable in mechanical systems. They serve to change the direction of a force and, in many configurations, to increase the usable effort by sharing the load across multiple supporting segments.

Pulley systems are categorized by their arrangement. A fixed pulley is mounted on a stationary support; it changes the direction of the applied force but does not reduce the required effort. A movable pulley is attached to the load; it provides a mechanical advantage by having multiple rope segments support the load, typically halving the effort for a single movable pulley. Block and tackle, or compound pulley systems, combine fixed and movable pulleys to increase the mechanical advantage, at the cost of rope length and complexity. In belt-driven machinery, a pair of pulleys with a belt transfers rotational motion between shafts; changing pulley sizes alters speed and torque.

Pulleas are made from metal, composite, or plastic materials, with the choice depending on load, speed, and environment. The groove size matches the rope or belt width, and the sheave diameter influences wear and belt life. Bearings or bushings reduce friction, and lubrication minimizes energy loss.

Applications span construction, shipping, and industry, including cranes, elevators, sailing rigging, gym machines, and manufacturing lines. The performance of a pulley system is often described by the ideal mechanical advantage (the number of rope segments supporting the load). The actual advantage equals the ideal advantage multiplied by the system efficiency, which is reduced by friction, misalignment, and wear.