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# 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.

### Learn It - Pulley Systems

- We can also use Pulley Systems to lift loads.
- A pulley system makes it easier to lift an object than lifting the weight by hand. A single pulley essentially changes the direction of the pull or force applied.
- When a person uses two or more pulleys in a system, then the system also multiplies the force applied in addition to changing direction.
- With one fixed and one movable pulley in a system, it essentially doubles the weight of the load you could lift without help from another person based upon which heavy objects you can lift.
- A pulley is an example of a simple machines work to reduce the force required to move an object, but the amount of work done will always remain the same. Work is a value that describes how a force (F) is exerted over a certain distance (d): Work = Force X Distance.
- As less force is needed to move a heavy weight, the distance required to move it will actually increase based on the simple machine.
- So a simple pulley system that cuts the force in half, will double the distance of rope that will be used, resulting in equivalent work.

Here is an example of 6 pulley system - note the size of the pulley does not matter.

### Badge It - Platinum

- Look at the photos 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?

### Extension Task

- Build a 6:1 pulley on Solidworks.Stuck? Look here