Math CentralQuandaries & Queries


Subject: Help
Name: Chris
Who are you: Other

If I have a motor that's spinning at 950 RPM's with a pulley that's 6in diameter with a belt running to a generator, What size pulley would I need on the generator to make it spin at 3600 RPM

Hi Chris,

The circumference C of a circle is given by

C = 2 π r

Thus in 1 minute your motor has completed 950 revolutions and the belt has moved

950 x (2 π x 6) inches.


Suppose that the smaller pulley has radius r inches. Thus in 1 minute the smaller pulley has completed 3600 revolutions and the belt has moved

3600 x (2 π x r) inches.

If there is no slippage in the belt then

950 x (2 π x 6) = 3600 x (2 π x r)


6 x 950 = r x 3600 or
r = 6 x 950/3600 = 1.58 inches


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