PMMI Mechanical Drives Practice Test

The key idea is that the belt’s motion is governed by the pitch circles of the pulleys. The pitch diameter is the effective diameter of the pulley where the belt pitch runs, so the belt’s linear travel per revolution depends on that circle’s circumference. Because of this, the rotational speed relationship between two pulleys (the speed ratio) is determined by the ratio of their pitch diameters: the speed ratio equals the ratio of the opposite pulley’s pitch diameter. That’s why the statement saying the pitch diameter is used to calculate pulley speed ratio is the correct one.

Outer diameter is just the physical size of the pulley and is typically larger than the pitch diameter due to belt thickness and the rim. It doesn’t directly determine the speed ratio. While belt length calculations involve geometries that include pitch diameters and center distance, saying the pitch diameter is used to determine belt length is not the primary relation described here.