Full definition
The effective working diameter of a pulley, measured at the pitch line — the circumferential line where the belt tension cord rides and where the belt effectively transmits force. For V-belts, the pitch diameter is inside the groove (smaller than the outside diameter) because the belt wedges into the groove; for timing pulleys, it is at the center of the belt tooth engagement. The pitch diameter (not the outside diameter) is the correct value for calculating drive ratios: i = Dp2/Dp1. Relationship to outside diameter varies by profile: for SPA, Dp = OD - 4 mm; for SPB, Dp = OD - 5 mm; for SPC, Dp = OD - 8 mm (approximate, per ISO 4183). Belt pitch length is calculated using pitch diameters. Per ISO 4183, DIN 2211, and ISO 5296 (timing). Catalogs from Gates, Optibelt, and Continental list both OD and pitch diameter for every standard pulley.