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A double pulley is a system of two pulleys used together to move a load. This term may refer to two separate pulleys joined by ropes and used together. It may also refer to a single unit of two pulleys next to one another rotating on a common axle. Multiple pulleys used together are referred to as a compound pulley system, and a double pulley is the simplest type of such a system.
Consisting of a grooved wheel supported on a central axle, a pulley is one type of simple machine. A rope, chain, or cable wrapped around the wheel, which is also called a sheave, is seated in the groove on the edge of the wheel. One end is typically attached to the load while the other end is pulled to exert a force on the load in order to move it.
Using a pulley provides a mechanical advantage that makes it easier to move a load. The term "mechanical advantage" refers to the force needed to move a load divided by the resistance force that the load exerts, such as its weight. Higher mechanical advantage means that less effort is required to move the same load.
The purpose of a double pulley is to increase the mechanical advantage of the system, thereby reducing the effort required to move a given load. As more pulleys are added to a compound pulley system, the weight of the load is divided and the load becomes easier to move. Friction between the rope, chain, or cable and the wheels of the pulleys, however, increases as more pulleys are added to the system. This makes it more difficult to move the load, so the number of pulleys a compound system may practically incorporate is limited. Two pulleys, however, typically involve sufficiently little friction that the effort that must be exerted to move the load is nearly halved.
Double pulley systems or other compound pulley systems are commonly found in environments where heavy lifting must be done manually. For example, heavy loads may need to be moved manually on boats or ships. The additional mechanical advantage of a compound pulley is helpful or even necessary in such instances. Search and rescue teams may also use double pulley systems for operations in wilderness situations where power is not available to do the heavy labor.
Where power is available, a double pulley may be used to reduce the wear on a motor or allow a smaller motor to be used. An elevator is a common example of a powered application that typically makes use of such a pulley system. One pulley is fixed at the top of the shaft while the other travels with the elevator compartment as it moves up and down. A truck may use a double pulley to reduce the load on a winch when pulling a heavy load or dislodging one that is stuck. Industrial and construction operations often use them with cranes and hoists to move large pallets of materials or to lift materials for building construction.