How to Calculate Pulley Sizes

Antique pulley mechanism
Antique pulley mechanism (Image: polea-1 image by RAMON CAMI from Fotolia.com)

Whether you need a pulley for a household clothesline or for your sailboat, determining the necessary pulley size requires just a few simple calculations. The two basic pulley systems are "rope and pulley" and "belt and pulley" which are made of two different sized pulleys: the driver pulley, the smaller, and the driven pulley, the larger. The diameter and revolutions of the driver and driven pulleys are what determines the pulley system's "size."

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A pulley's diameter is the distance from end to end of the pulley going directly through the center point. In order to find the diameter of the driver pulley, plug the appropriate values here:

(Diameter of driven pulley) x (Revolutions of driven pulley) / (Revolutions of driven pulley) = (Diameter of driver pulley)

In order to find the diameter of driven pulley, plug in the appropriate values for this equation:

(Diameter of driver pulley) x (Revolutions of driver pulley) / (Revolutions of driven pulley) = (Diameter of driven pulley)

A pulley's revolutions is measured in rpms if the it is driven by a motor. Otherwise, revolutions describes the amount of revolutions the pulleys will make in tandem. In order to find the revolutions of the driver pulley, plug in the appropriate values:

(Diameter of driven pulley) x (Revolutions of driven pulley) / (Diameter of driver pulley) = (Revolutions of driver pulley)

To find the revolutions of driven pulley, plug in the missing values:

(Diameter of driver pulley) x (Revolutions of driver pulley) / (Diameter of driven pulley) = (Revolutions of driven pulley)

Tips & Warnings

  • You can use algebra to reorganize the equations and find the value of any of the variables.
  • When finding the diameter of the pulleys in a "rope and pulley system," be sure to consider the depth of the groove where the rope sits.

References

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