Pulley ratio is the pitch diameter of the __ pulley divided by the pitch diameter of the __ pulley.

• A. Driven, Driver
• B. Driver, Driven
• C. Master, Slave
• D. Leading, Trailing

Answer: (A)

Pulley ratio is based on how belt drives transfer motion between two pulleys. In a belt drive, the belt constrains the circumferential speeds so that the driver and driven rotate with a relationship tied to their pitch diameters: the driver’s pitch diameter times its angular speed equals the driven’s pitch diameter times its angular speed. This means the speed relationship is driven speed over driver speed equals the driver’s pitch diameter divided by the driven’s pitch diameter. When we express the pulley ratio as a simple quotient of the two pitch diameters, using the driven pulley over the driver pulley, we capture how much larger (or smaller) the driven is relative to the driver. That’s why the pulley ratio is the pitch diameter of the driven pulley divided by the pitch diameter of the driver pulley. Other pairings aren’t standard terminology for pulleys in belt drives.