Bookmark and Share
The pitch diameter of a worm is the diameter of the pitch circle (the “imaginary” circle on which the worm and worm gear mesh). There is no fixed method for determining the pitch diameter of a worm. (See Figure below)

Important: Pitch diameters can vary, but sound engineering practice dictates that they be as small as possible for the most efficient performance. Why? A small pitch diameter reduces the sliding velocity and, therefore, the efficiency of the worm.

The following formulas will be useful as you needs with regard to the selection of the correct worms.
• Diametral pitch – 3.1416 ÷ circular (linear) pitch
• Circular (linear) pitch = 3.1416 ÷ diametral pitch
• Pitch diameter = outside diameter – 2 (addendum)
• Bottom diameter = outside diameter – 2 (whole depth)
• Outside diameter = pitch diameter + 2 (addendum)