# 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.

• For each of the problems below - draw a diagram of the pulleys described and then calculate the answer.
1. A driver pulley has a diameter of 200mm and rotates with a speed of 60rpm. Calculate the speed of a driven pulley with a diameter of 50mm.
2. A driver pulley has a diameter of 80mm and rotates with a speed of 250rpm. Calculate the speed of a driven pulley with a diameter of 400mm.
3. A driver pulley has a diameter of 100mm and rotates with a speed of 50rpm. Calculate the required diameter of a driven pulley if we want it to rotate at 125rpm.
4. We have a driver pulley with a diameter of 60mm. And a driven pulley with a diameter of 20mm. 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 100rpm. What diameter will the driver pulley need if it can only rotate at 25rpm?

### Design It

• The mechanism you are going to build can either be cranked by hand, or run from a motor using a pulley system.
• In this next activity you are going to build the pulleys for your mechanism.
• Here are the parts for your pulley mechanism.

• Use SolidWorks to draw sketches of the pulley parts above and then extrude them by 3mm. They need to be separate parts i.e. 4 files.
• Take a screenshot of your hexagon cam with the inner cross and ensure you are including the part that proves your sketch is fully defined.
• upload the hexagon shape to bournetolearn.com

### Learn It

• You can create pulley mechanisms in SolidWorks that actually work.
• Follow the guide below and see if you can create a simple pulley mechanism.
• Firstly you'll need to create a couple of pulley wheels, as separate parts, and place them into an assembly as shown below. These pulleys have extra holes, just so that you can see them spinning.
• Now we'll need to make sure they are lined up with each other, by mating two of the faces.
• Next we add a belt. Belts can be found in the `Assembly Features` menu.
• Select the curved faces of both your pulleys.
• Lastly we'll need to float the first pulley you placed in the Assembly. At the moment it is fixed and so can't move. Right click on the pulley and select `float` from the context menu
• If you want you can now add a motor to the driver pulley, like you did in the cams lesson.