Which statement best describes pitch diameter?

• A. The diameter of the pulley measured at the outer rim
• B. The diameter of the belt path around the pulley
• C. The diameter where belt engages the pulley teeth
• D. The diameter used to calculate pulley ratio

Answer: (D)

Pitch diameter is the imaginary circle on which the belt pitch effectively sits as it travels around the pulley. This diameter sets the belt’s pitch circumference and is the size you use to determine how fast one pulley turns relative to another—the pulley ratio. In a belt drive, the speed or rotation ratio between pulleys equals the ratio of their pitch diameters, not their outer rims or the belt’s contact path. The diameter where the belt engages teeth or the belt path around the pulley doesn’t directly define that ratio, especially for smooth or non-toothed belts. Therefore, the diameter used to calculate pulley ratio best describes pitch diameter.