Pulleys represent simple machines used for many operations. Pulley systems are made from two pulley wheels on a shaft with a belt joining them. One pulley is the driver pulley, and the other is the driven pulley. Pulleys can change speed, provide torque and alter rotation direction. Changing speed with pulleys entails changing the diameter of a pulley wheel.

In a two-pulley system with wheels of varying sizes, one can calculate the difference in speed between the two pulley wheels. Velocity ratio is found by using the diameter of the driven pulley divided by the diameter of the driver pulley. For example, with a driven pulley of 150 mm and a driver pulley of 50 mm, the velocity ratio is 3. Finding the output speed entails taking the input speed and dividing it by the velocity ratio. Basically the speed of the output pulley becomes the input speed for the next stage, and you can find the multi-pulley drive. If the input speed is 75 rpm, the output speed equals 75 rpm divided by 3 or 25 rpm. In turn, one can find the output torque from the driver pulley to the driven pulley by multiplying the input torque by the velocity ratio.

Two pulleys connected by a belt that are the same size will turn at the same speed under the same shaft power. A smaller pulley turning a larger pulley results in the larger one moving more slowly but with more shaft power. An example would be a truck in low gear, with its engine turning rapidly but its wheels turning slowly, yet it has significant power at reduced speed. Alternatively, a larger pulley turning a smaller pulley results in the smaller one turning faster but with less shaft power. For example, a truck in high gear would have a more slowly turning engine but fast turning wheels, resulting in greater speed of the truck. Clutches provide control over load bearing and speed.

Pulleys are used in drive belts for various machines. The belts themselves may be made of synthetic material. Pulleys and belts do not require lubrication, although the belt can wear or slip. In a snowmachine, a clutch is comprised of two pulleys connected by a wide belt that is very strong and yet flexible. When the snowmachine encounters a load or resistance, the engine slows down, and the drive pulley decelerates and spreads apart. Then, the springs in the driven clutch push together. This results in reduced track speed but more power for the snowmachine.

To adjust fan speed, an adjustable pulley can be used. Adjustable pulleys have two tapered, connected sections with one half turned onto the other. When turned to each other their belt is forced outside the pulley and the fan speed is increased. Conversely, decreasing the fan speed would entail turning the pulley halves apart and shortening the distance for the belt to travel around it. Essentially, the pulley diameter is changed to reduce the fan speed and therefore airflow.

Automobiles often employ a synchronous belt for the engine timing driving system. As speed of the driving pulley increases, so does belt vibration frequency. Likewise, when reducing the speed of the driving pulley, belt vibration decreases. Reducing vibration improves transmission stability.

Conveyor belts used in material transport such as mining rely upon a pulley system. In order for the conveyors to work safely and efficiently, their speed must be controlled. To do this, belt tension must be maintained, so the belt will not sag and create friction. Driving forces exerted on drive pulleys must be controlled to prevent belt slippage. The conveyor can decelerate after a peak of driving force on the drive pulley. This control prevents material spillage from the conveyor.

Demonstrating a simple pulley can be done in the home with a rubber band and thread spools. The rubber band represents the drive belt, and the spools represent the pulleys. By using varying sizes of the spools, one can demonstrate the number of turns the driven spool needs to turn the drive spool. The spools demonstrate the speed difference of pulley wheels.