Example Belt Drive Calculation

A 600 watt alternator produces full output at 1000 revs. per minute and has an overall efficiency of 70%.The alternator pulley has a diameter of 40 mm, the coefficient of friction between the belt and pulley is 0.3 (assume the belt and pulleys are 'flat') and the angle of wrap of the belt around the pulley in 140o.
Determine: (i) the minimum values of the belt tension T1 and T2 that will provide the necessary output and (ii) the maximum normal pressure between the belt and the pulley if the belt is 6 mm wide.

In the solution below it is assumed that the element of belt subtends a small angle and when resolving radially, sine of half the small angle is approximately equal to the angle in radians and (resolving in a tangential direction) the cosine is approximately equal to 1.
The maximum normal pressure between the belt and the pulley will be just before the belt leaves the pulley on the tight side.


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David J Grieve, 20th November 2001.