Pulley is a wheel over which a rope or belt is passed for the purpose of transmitting force and doing work. The rim of the pulley is grooved when the pulley carries either a rope or a belt that has a V-shaped cross section. However, if the pulley is to carry a belt that has a flat cross section, the rim of the pulley is barrel-shaped. The flat belt rides on the highest part of the rim.
The simplest pulley is a grooved wheel on a fixed axle. A rope passing over this wheel is tied to the load to be lifted, and a pull is applied to the other end of the rope. This type of pulley gives no mechanical advantage of lift, but it changes the direction of the force applied to the load. This is important when the space directly under the load is hard to get at, as when the load is in a boat.
When the pulley is to carry a continuous turning motion, the two ends of the rope are laced together. A belt used in this way is usually manufactured as an unbroken circular unit. A second pulley, which is connected to the source of energy, transmits a steady rotation to the first pulley. If the driver pulley and the driven pulley are of the same size, the only advantage is a choice of directions from which the force may come. If the pulleys are of different sizes, an advantage of either speed or force may be obtained. Crossing the rope or belt that runs between the two pulleys reverses the direction of turn of the driven pulley.
The second basic type of pulley is a movable pulley. The load is attached to the axle of this pulley. One end of the rope that passes through the pulley is attached to a fixed support above the load. A pull is applied to the free end of the rope in the same direction the load is to move. The mechanical advantage of a movable pulley is 2. This means that the pull applied to the free end of the rope need be only half the weight of the load. The rope attached to the fixed support also carries half the weight of the load.
