# 11. Pulleys and Belts

## Pulleys

A simple fixed pulley mechanism is made up of a rimmed wheel and a cable, which sits inside the rimmed wheel. Although it provides no mechanical advantage, it can make items easier to lift by changing the direction of force.

When two or more pulleys are used together in a block and tackle system, they can reduce the magnitude of force required to lift a load.

## Belt and Pulley Systems

In these systems, two or more pulleys are connected by a belt to transfer force and rotary motion from the driver pulley to the driven pulley.

These systems are widely used in car engines and washing machines.

It is important that the belt remains taut, so they are normally made of rubber to prevent slippage.

## Calculating the Velocity Ratio

Using different sized wheels changes the speed and force of rotation.

When the driver pulley is larger than the driven pulley, the driven pulley rotates faster with less torque.

When the driver pulley is smaller than the driven pulley, the driven pulley rotates slower with more torque.

The velocity ratio is the velocity between the two pulleys; it is calculated by dividing the diameter of the driven pulley by the diameter of the driver pulley.