Chapter 31, Problem 1RQ

How does the pitch diameter differ from the major diameter for a standard screw thread?

To determine

The difference in the major diameter and the pitch diameter of a standard screw thread.

Explanation of Solution

A screw thread is a helix of uniform and continuous ridges over cylindrical or conical surfaces. Threads could be internal or external containing continuous crests and troughs. They are most commonly employed for fastening the objects or in transmitting controlled motion applications.

Standard screw threads are defined using the major, minor and pitch diameters. Here, major diameter is the largest diameter of the thread measuring the distance between the crests at a particular cross section for external threads and troughs for internal threads. Minor diameter is the smallest diameter of the thread expressing the distance between the troughs for external threads and crests for internal threads at a particular cross section.

However, the pitch diameter is an imaginary diameter equidistant between the major and minor diameters. It is the diameter of the cylinder or cone on which the threads are manufactured. It basically signifies the point of contact between the two threaded bodies which are supposed to mating.