According to Mini Science, a pulley is a simple machine consisting of a grooved wheel through which is run a chain or rope that can be attached to a heavy object that can be lifted and moved. The direction in which force is exerted to lift a heavy object is changed when a pulley is used. When lifting an object from the ground, the object is usually picked up, but when an object on the ground is lifted using a pulley, the rope is pulled down to raise the object up.
The Society of Women Engineers reports Sicilian Engineer Archimedes is the developer of the pulley system, along with many other mathematical and engineering principles. Legend tells of Archimedes boasting to King Hieron that he could move any weight with enough pulleys. Eventually Archimedes used a pulley system to prove he could move a boat in an easier way than the force of many men pulling the ship. Since its invention, the pulley has been used for a variety of reasons, which, according to the Simple Machines Learning Site, includes use on flag poles, cranes, clotheslines, and sailboats.
Different types of pulleys provide different benefits. For example, the movable pulley allows heavy objects to be moved in any direction. Science Class explains that the movable pulley uses a rope attached to an object with the pulley moved along the rope instead of being statically attached to a surface as in the fixed pulley. By using ropes, Mini Science explains that the flexibility of the rope, instead of a rigid object, allows the pulley to be routed through any pathway for lifting in any direction. A pulley system can also be used with a motor shaft, allowing the rope to wound in and out to a variety of lengths with no effect on the performance of the pulley.
The Automata and Mechanical Toys group reports that pulleys can be used in a similar way to gears in engines and movable objects. A pulley in an engine is referred to as a belt. A commonly used pulley in an automobile is a fan belt. Unlike gears in an engine, a pulley does not need to come into contact with other metal objects using grease and oil to ease movement. Another advantage of the pulley system over gears is the simple design that can be made with little cost.