PitchDiameter

Pitch diameter is the diameter of the pitch circle, an imaginary cylinder on which gear teeth are designed to mesh or, in threading, the cylinder on which mating threads engage. It is a key reference dimension that determines how parts fit and interact.

In gears, the pitch diameter is central to geometry and mating compatibility. If N is the number of teeth and P is the diametral pitch (or, in metric systems, the module m), the pitch diameter d can be computed as d = N / P (or d = mN for metric gears). Two gears that mesh must have the same module or diametral pitch and hence compatible pitch diameters, ensuring proper tooth engagement. The center distance between two gears is (d1 + d2)/2, and the circular pitch is related to the pitch diameter by p = πd/N.

In screw threads, the pitch diameter (often called the mean diameter) is the Effective diameter where the thread thickness equals the space thickness. It lies between the major (outer) diameter and the minor (root) diameter and is used to analyze thread engagement and fit. The exact pitch diameter depends on the thread form and tolerance standards (for example, unified or metric threads) and is specified by governing standards.

Overall, the pitch diameter serves as the reference dimension for determining mating geometry, center distances, and fit in both gear trains and threaded connections.