Categorized as a simple machine, a pulley is used to reduce the amount of force needed to lift a load. It basically consists of a grooved wheel mounted on an axle or shaft and a rope or belt that runs within the groove. The three different pulley parts are a wheel, an axle, and a rope. The advantage of using a pulley is that it both changes the direction and point of application of force, making it much easier to lift weights.
The grooves on the pulley wheels prevent the rope or belt from slipping off. Most often, the rope is stretched tight and kept in a state of tension because a loose belt would otherwise slip, causing the pulley system to fail. Pulley wheels rotate on a central shaft and are typically attached to it with what is known as a "key." The key is mostly tapped into position in such a way that it holds both the pulley wheel and the shaft together. They are fitted tightly in order to prevent the pulley wheel from slipping off the shaft.
Pulleys vary in how the different pulley parts are organized. The fixed pulley is one where the grooved wheel has a fixed axle, and the whole machine is attached to a surface. This type of pulley provides no real mechanical advantage as it still requires a person to apply the same amount of force; it only changes the direction of the force applied. A movable pulley, on the other hand, has a free axle, allowing the axle to move and significantly reduce the amount of overall force needed. As 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 pulleys feature a mixture of both movable and fixed pulley systems and reduce the amount of force required to less than half of the weight of the object being lifted. The block and tackle pulley is a type of compound pulley where a number of pulleys are mounted on each axle. This type of pulley makes optimum use of the different pulley parts and is used for lifting extremely heavy loads, such as shifting cargo from ships to land. No one really knows when and how pulleys were first developed or who designed the different pulley parts. Archimedes is credited with building the first block and tackle pulley system, which he used along with other compound pulleys to move an entire battleship filled with 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 for most machine operations in one way or another. Various types of pulleys make the best use of the different pulley parts to do a number of tasks in many industries. They are used in the aerospace, automobile, and other industries. For instance, pulleys are critical for many climbing and rescue operations.