What Is a Pulley Assembly?

Jean Marie Asta

In its simplest form, a pulley assembly is composed of a wheel which spins freely on its axle and a belt, rope, chain or cable running over the wheel. The purpose of pulleys is to ease the amount of force it takes to lift a load, otherwise known as mechanical advantage. Manufacturing, mechanical, industrial and automotive fields make use of the pulley assembly. Pulley parts differ depending on the context in which they are used. For example, the wheel, or wheels, used for an assembly may be grooved to accommodate a rope, or rough for a belt that will spin by friction.

A metal pulley and rope.
A metal pulley and rope.

There are three different systems of simple pulley assemblies: the fixed pulley, the movable pulley, and the compound pulley. Fixed pulleys consist of an axle that stays fixed at a certain location, while a rope, belt, chain, or cable runs over it. Movable pulleys consist of one fixed axle, one free axle and one rope, belt, chain or cable. A compound pulley assembly is a combination of the fixed and movable pulley systems. These compound assemblies can significantly increase the mechanical advantage value in a system.

Wooden rope pulley blocks on a boat.
Wooden rope pulley blocks on a boat.

In the manufacturing, mechanical, industrial, and automotive fields, pulley assemblies follow the same basic patterns as simple pulley systems. The pulley parts may differ in their construction, in how force is exerted, and in whether the directional motion is continuous or not. Depending on what a pulley is used for, every individual part and action is tailored to the type of work the pulley performs.

A winch and rope pulley assembly, for example, may consist of a belt wrapped tightly around two axles/wheels. While a motor spins one axle/wheel, the friction from the tightly bound belt makes the two axles/wheels spin simultaneously and indefinitely. The flanges of the axles/wheels may also be grooved inward specifically to make sure the belt does not come loose.

Pulley assemblies can also be found in everyday objects like bicycles, which use a chain drive pulley assembly between the back wheel and pedals. At the base of the pedals and the back wheel, there are two axles that are grooved in the shape of "teeth," around which a chain is placed to connect the two. The chain is made of interlocking links, where the "teeth" fit into the spaces between the links. When the bicycle is pedaled, the axle at the pedals pulls the links of the chain, spinning both the axle at the pedals and the axle at the center of the back wheel.

Pulleys often have a groove around their circumference that guides a chain, cable, belt or rope.
Pulleys often have a groove around their circumference that guides a chain, cable, belt or rope.

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