# An Introduction to Mechanisms

## 1 Pulleys

### Learn It

• A Pulley is a wheel on a shaft that can move a belt along it's circumference.
• Pulleys can be used to lift loads and transmit motion.

### Learn It

• We can use pulleys to increase or decrease the speed of rotary motion.
• Look at the diagram below
• The `driver` pulley has a diameter of 40mm and revolves at 100 revolutions per minute (`rpm`).
• The `driven` pulley has a diameter of 120mm
• If we want to calculate the rpm of the driven pulley, we first need to find out the `velocity ratio`.
• So in this case we can see that…
• The output speed of a pulley system is calculated as follows.
• So in this case we can see that
• So the driven pulley will rotate at 33.3rpm.

• Click on this link to complete the quiz based on the calculations below.
1. A driver pulley has a diameter of 400mm and rotates with a speed of 60rpm. Calculate the speed of a driven pulley with a diameter of 100mm.
2. A driver pulley has a diameter of 40mm and rotates with a speed of 250rpm. Calculate the speed of a driven pulley with a diameter of 200mm.
3. A driver pulley has a diameter of 100mm and rotates with a speed of 25rpm. Calculate the required diameter of a driven pulley if we want it to rotate at 75rpm.
4. We have a driver pulley with a diameter of 120mm. And a driven pulley with a diameter of 40mm. What speed will the driver pulley need if the driven pulley must rotate at a speed of 90rpm?
5. A driven pulley has a diameter of 10mm and must rotate at a speed of 200rpm. What diameter will the driver pulley need if it can only rotate at 50rpm?