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A pulley is a simple machine that reduces the force needed to lift a load. It consists of a wheel, an axle, and a rope. Grooves prevent slipping, and keys hold the pulley and shaft together. Fixed pulleys change direction, but not force, while movable pulleys reduce force. Compound sheaves reduce force by more than half. Pulleys are used in many industries, including aerospace and automotive, and have various applications, such as in cranes and rescue operations.
Classified as a simple machine, a pulley is used to reduce the amount of force required to lift a load. It basically consists of a splined wheel mounted on an axle or shaft and a rope or webbing that runs inside the spline. The three different parts of the pulley are a wheel, an axle and a rope. The benefit of using a pulley is that it changes both the direction and point of application of force, making it much easier to lift weights.
Grooves on the pulleys prevent the rope or webbing from slipping off. Most often, the rope is taut and held in a taut state because otherwise a slack belt would slip, causing the pulley system to fail. Pulleys rotate on a central shaft and are typically attached to it with what is known as a “key”. The key is mostly tapped into place so that both the pulley and shaft are held together. They are securely fastened to prevent the pulley from slipping off the shaft.
Pulleys vary in how the different parts of the pulley are arranged. Fixed pulley is one in which the grooved wheel has a fixed axle and the whole machine is fixed to one surface. This type of pulley provides no real mechanical advantage as it still requires a person to apply the same amount of force; only the direction of the applied force changes. A movable pulley, on the other hand, has a free axis, which allows the axis to move and significantly reduce the amount of overall force needed. When one end of the rope is fixed in place, a force exerted on the other end of the rope applies a double force to the object being lifted.
Compound sheaves feature a combination of movable and stationary sheave systems and reduce the amount of force required to less than half the weight of the object being lifted. Block and tackle sheave is a type of compound sheave in which a number of sheaves are mounted on each axle. This type of pulley makes optimal use of the different parts of the pulley and is used for lifting extremely heavy loads, such as moving goods from ships to land. No one really knows when and how pulleys were first developed or who designed the different parts of the pulley. Archimedes is credited with building the first hoist and pulley system, which he used in conjunction with other compound pulleys to move an entire warship full of people.
The block and tackle pulley is also used in cranes and to raise the sails on ships. Pulleys have numerous applications and are vital to most machine operations in one way or another. Various types of pulleys make the best use of different parts of the pulley to perform a variety of tasks in many industries. They are used in aerospace, automotive and other industries. For example, pulleys are critical to many climbing and rescue operations.
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