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?

• Use the Wacano sets to put together the following mechanisms:
1. A 1:2 pulley ratio
2. A 2:3 pulley ratio
3. 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.