What Are the Differences Between a Pulley, a Wheel & an Axle?

In the simplest terms, a wheel is a circular object able to turn around an axis that passes through its center. Typically, a shaft called an axle is attached to the wheel at the point where that axis meets the wheel, and the wheel either may spin freely around the axle while the axle remains stationary, or the axle may turn along with the wheel. If the wheel is able to spin freely on the axle, it is not considered to be part of a wheel-and-axle simple machine because force cannot be transferred back and forth between the wheel and the axle.

If the axle is rigidly attached to the wheel so it turns when the wheel turns, the combination becomes a simple machine that transforms a rotational force applied to either of the two parts of the machine. If a force is applied to the wheel, that force increases while the speed of rotation decreases. If the force is applied to the axle instead, the force decreases but the speed of rotation increases.

A pulley is a wheel held in a frame but allowed to turn freely. The outside of the pulley is grooved, and a rope wrapped around the pulley causes the pulley to turn when the rope is pulled. A fixed pulley can change the direction of a force applied to it, and a pulley suspended so it can move along with a load attached to it can provide a mechanical advantage in moving that load. Systems consisting of multiple pulleys often are used to change both the direction of forces and provide mechanical advantage.

Wheel-and-axle machines sometimes are confused with pulleys. Such is the case with automobile engines, in which the wheels driven by belts often are referred to as pulleys. These wheels actually are rigidly attached to axles and used to transfer power or change rotational speed and force. These qualities make them wheel-and-axle machines and not pulley systems.