The Difference Between A Pulley And A Sheave

Pulleys have been used for centuries in the workplace to make lifting easier. Typically made with a rope and a wheel, a pulley allows a person to lift a heavy load without using as much force as would normally be needed. The term pulley is often used interchangeably with the word sheave, but this is not technically correct. There are some differences between a pulley and a sheave.

A pulley is one of six types of simple machines. A sheave (pronounced "shiv") is actually part of the pulley system. The sheave is the rotating, grooved wheel inside the pulley. This is the piece that the rope fits into.

A fixed pulley with no sheave changes the direction in which the force is applied to move the heavy load, but it does not change the amount of force needed. Using multiple sheaves gives you a mechanical advantage. In fact, with each additional sheave you use in a pulley, you only need half of the original required force to move the object.

Just because multiple sheaves lessen the force needed to move an object, it doesn't mean that dozens of sheaves can be used in a pulley. More sheaves will make the work easier, but it also adds friction. When adding more sheaves and ropes, each one increases friction and takes away your mechanical advantage until eventually you've made your work harder instead of easier. You can use several sheaves in one pulley system, but to increase efficiency you should arrange the sheaves above or below each other with a fixed axle between them. This is known as a compound pulley.

Often times, a single sheave within a pulley will get the job done with minimal effort. For a sheave to be effective, it must have the minimum surface area possible for the rope attached, and it needs to be resistant to abrasions and warping.