What is the belt drive ratio?

• A. Driven pitch diameter divided by drive pitch diameter
• B. Sum of pitch diameters
• C. Drive pitch diameter divided by driven pitch diameter
• D. Belt length divided by center distance

Answer: (A)

The belt drive ratio describes how the input (drive) speed relates to the output (driven) speed when two pulleys are linked by a belt that doesn’t slip. Since the belt moves at the same linear speed on both pulleys, the relationship is π times the drive pitch diameter times the drive RPM equals π times the driven pitch diameter times the driven RPM. Canceling π gives D_drive × N_drive = D_driven × N_driven, so the speed ratio N_drive / N_driven equals D_driven / D_drive. In other words, the belt drive ratio is the driven pitch diameter divided by the drive pitch diameter. This means if the drive pulley is larger, the driven pulley turns more slowly, and the ratio is less than one. The other options describe different quantities (sum of diameters or belt length over center distance) that aren’t about the speed relationship.