A metal pulley and a rope.
In its simplest form, a pulley assembly is made up of a wheel that rotates freely on its axis and a belt, rope, chain, or cable that runs over the wheel. The purpose of pulleys is to relieve the amount of force needed to lift a load, also known as mechanical advantage. The manufacturing, mechanical, industrial and automotive areas make use of the pulley assembly. Pulley parts differ depending on the context in which they are used. For example, the wheel(s) used for an assembly may be grooved to accommodate a rope or roughened for a belt that will rotate by friction.
Wooden rope pulley blocks on a ship.
There are three different systems of simple pulley assemblies: the fixed pulley, the moving pulley, and the compound pulley. Fixed pulleys consist of a shaft that remains fixed in a certain place while a rope, belt, chain or cable passes through it. Movable pulleys consist of a fixed shaft, a free shaft, and a rope, belt, chain, or cable. A compound pulley set is a combination of the fixed and moving pulley systems. These composite assemblies can significantly increase the value of mechanical advantage in a system.
Pulleys often have a groove around their circumference that guides a chain, cable, belt, or rope.
In the manufacturing, mechanical, industrial, and automotive fields, pulley assemblies follow the same basic standards as individual pulley systems. Pulley parts can differ in their construction, how the force is exerted, and whether or not the directional motion is continuous. Depending on the purpose of the pulley, each individual part and action is tailored to the type of work the pulley does.
A winch cable and pulley assembly, for example, may consist of a belt wrapped tightly around two axles/wheels. When a motor turns an axle/wheel, the friction of the tightly bound belt causes both axles/wheels to rotate simultaneously and indefinitely. The axle/wheel tabs can also be slotted inwards specifically to ensure that the strap does not come loose.
Pulley assemblies can also be found on everyday objects such as bicycles, which use a chain drive pulley assembly between the rear wheel and the pedals. At the base of the pedals and the rear wheel there are two splined axles in the shape of “teeth”, around which a chain is placed to link them together. The chain is made of interlocking links, where the “teeth” fit into the spaces between the links. When the bike is pedaled, the pedal spindle pulls on the chain links, turning both the pedal spindle and the spindle in the center of the rear wheel.