Which statement best describes the purpose of pitch diameter in pulley systems?

• A. It defines the maximum outer width of the pulley.
• B. It indicates the material of construction.
• C. It measures the rim thickness.
• D. It is the diameter used to calculate the pulley speed ratio.

Answer: (D)

Pitch diameter is the effective diameter at which the belt engages the pulley, measured along the belt’s pitch line. Because the belt contacts the pulley along this pitch circle, the belt’s linear speed around the system is tied to this diameter. That’s why the pulley speed ratio is determined by the ratio of their pitch diameters: the belt speed around each pulley must be the same, so N1/N2 equals the ratio D2_p/D1_p. In other words, this diameter is the one used to calculate how fast one pulley turns relative to the other. The other options describe physical size, material, or rim thickness, which aren’t defined by pitch diameter.