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.

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Learn It

  • We can use pulleys to increase or decrease the speed of rotary motion.
  • Look at the diagram below

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  • 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.

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  • So in this case we can see that…

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  • The output speed of a pulley system is calculated as follows.

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  • So in this case we can see that

SR_formula_in.PNG

  • 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.
    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?

Badge It - Gold

  • 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.

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?