A belt drive system having a 12 inch driver pulley and a 6 inch driven pulley has a pulley ratio of

• A. 2
• B. 1
• C. 0.5
• D. 6

Answer: (C)

In a belt drive, the pulley ratio is determined by comparing the diameters of the driven and driving pulleys—the belt moves at the same linear speed on both pulleys, so the rotational speeds are inversely related to diameter. Here, the driven pulley is 6 inches and the driving pulley is 12 inches. Using the convention that the ratio equals driven diameter divided by driving diameter, the ratio is 6/12 = 0.5. This means the driven pulley is half the size of the driver, so it must rotate at about twice the speed of the driver to keep the belt moving at the same linear speed. (Torque on the driven is roughly half of the driver’s torque, neglecting losses.)