Full definition
For narrow V-belt profiles (SPZ, SPA, SPB, SPC per ISO 4184), the datum diameter (Dd) is the reference diameter replacing the older pitch diameter concept. It is measured at the datum line — the standardized position where the belt width equals its nominal top width. The datum diameter is used for calculating belt datum length (Ld), drive ratios, and belt speed. For classic profiles (A, B, C, D, E), the equivalent term is pitch diameter (Dp). Relationship to outside diameter varies by profile: SPA: Dd = OD - 4 mm; SPB: Dd = OD - 5 mm; SPC: Dd = OD - 8 mm approximately. All drive design calculations — ratio, belt length, wrap angle, peripheral speed — must use datum (or pitch) diameter, never outside diameter. Per ISO 4184 and DIN 7753. Catalogs list both OD and Dd for each standard pulley size. Confusing OD with Dd is a common engineering error that leads to incorrect speed ratios.