Pulley ratio is defined as the ratio of pitch diameter of the driven pulley to the pitch diameter of the driver pulley.

• A. The ratio of the pitch diameter of the driver pulley to the driven pulley
• B. The ratio of the pitch diameter of the driven pulley to the pitch diameter of the driver pulley
• C. The ratio of belt length to pulley circumference
• D. The sum of the pitch diameters of both pulleys

Answer: (B)

In a belt-driven pulley system, the ratio that matters for speed and torque is based on the pitch diameters of the two pulleys. The pulley ratio is defined as the pitch diameter of the driven pulley divided by the pitch diameter of the driver pulley. This directly ties the sizes of the pulleys to how fast each will rotate at the same belt speed: the smaller the driver relative to the driven, the faster the driven turns, and vice versa.

If the driven pulley is larger, the ratio is greater than one, meaning the driven rotates more slowly and provides more torque; if it’s smaller, the ratio is less than one, giving a speed increase. The other options mix in belt length or circumference, or use a sum of diameters, none of which define the pulley ratio.