To illustrate the method of finding the size of a pulley, suppose a shaft is to make 360 R. P. M. and that it is driven from a line-shaft making 180 R. P. M. The larger pulley on the line-shaft is already in place and is 16 in. in diameter. What diameter should we make the pulley on the shaft making 360 R. P. M?

Since we know the ratio of the speeds and diameters, we have the proportion: Speed of small pulley is to speed of large pulley as diameter of large pulley is to diameter of small pulley, or using the same figures as above, we have

360 :180 = 16 : diameter of small pulley

The rule thus deduced is as follows:

The diameter of the driving pulley multiplied by its speed equals the diameter of the driven pulley multiplied by its speed.

In practice it is found that a belt creeps or slips so that it does not usually drive a pulley quite so fast as the calculations indicate. For this reason, the relative speeds of pulleys are only approximately exact and are always subject to slight variation.