# 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.

### Badge It - Silver

- Click on this link to complete the quiz based on the calculations below.
- 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.
- 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.
- 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.
- 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?
- 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?

### Badge It - Gold

- Use the Wacano sets to put together the following mechanisms
- A 1:2 pulley ratio
- A 2:3 pulley ratio
- A 1:1 pulley ratio that reverses the direction of rotation

- Photograph your work and put them all onto one document and upload this.

### Badge It - Platinum

- We can also use Pulley Systems to lift loads.
- Look at the video below and set up the same pulley system with your Wacano set.
- What is the relationship between the length of string you pull through the top pulley, and the height the second pulley moves?
- Does this ratio alter if you use different sized pulleys?