pitch diameter (D_w)

applies only for worm gears and is the diameter of a pitch circle. Equal to twice the perpendicular distance from the axis to the pitch point. The nominal gear size is usually the pitch diameter. Note that for a worm it is still measured in a plane perpendicular to the gear axis, not a tilted plane.

Subscipt " _w " denotes the worm, " _g " denotes the gear