The diameter of the reference circle on which tooth spacing is defined, equal to tooth count times module, or tooth count divided by diametral pitch. Two mating gears behave like two cylinders of these diameters rolling without slip, and at standard center distance the center distance equals half the sum of the two pitch diameters. The operating pitch diameter shifts if the gears run at a different center distance.
Pitch diameter's 4.000 on the 40-tooth, so center distance with the 20-tooth pinion should be three inches flat.
Also heard as
- reference diameter
- PD
- pitch circle
Related terms
Module
The metric measure of gear tooth size: pitch diameter in millimeters divided by the number of teeth.
Diametral pitch
The inch measure of gear tooth size: number of teeth per inch of pitch diameter, larger number for finer teeth.
Center distance
The distance between the axes of two meshing gear shafts, set by the housing bores.
Base circle
The circle from which the involute tooth profile is unwound, smaller than the pitch circle.