Pitch diameter is used to measure the diameter of gears and screws. Pitch measures the distance between teeth on a gear and the lead on a screw. The pitch diameter of a screw falls between the major and minor diameter of the thread. Gears have both a pitch and an outside diameter, with the pitch diameter measured as if a circle ran around the gear and passed through the center of each tooth.
Pitch diameter is a term used to measure the diameter of a gear or screw using pitch points. Lean points on a gear could be found if a straight line was drawn from two of the gear teeth located next to each other and the two lines were connected by a curve. In the case of a gear or sprocket, pitch basically measures the distance between the teeth. In the case of a screw, the lead is shown by a straight line through the lead of the screw. Tread is the term for the spirals along the end of a screw that allow it to drive into a piece of wood and hold the screw in place.
In a screw, this measurement, often called the pitch diameter of the thread, falls between the measurement of the major and minor diameter of the tread. The major diameter is the measurement of the tread at its largest point: if a circle ran around the outside of the screw and grazed the outer edges of the tread, this would mark the major diameter. The minor diameter is also the measurement of an imaginary circle, but this circle goes around the inside of the tread where it stops turning and becomes the full cylinder of the screw.
Gears, on the other hand, do not measure pitch diameter in the same way, although the concepts are similar. A gear (and pinion) has both a pitch and an outside diameter. The outside diameter is the distance across an imaginary circle that goes around the outside of the gear. If a piece of string was placed around the gear along the top of the teeth and didn’t dip into the spaces between the teeth, this would be the outside diameter.
The pitch diameter, sometimes called the pitch circle diameter, for both gears and sprockets is measured as if a circle ran around the gear and passed approximately through the center of each tooth rather than across the top. To find the fit more precisely, someone can imagine a circle between each gear tooth that fills the gap and is roughly in line with the top of the gear tooth. Measuring from the center of one of these circles to the center of the circle directly across from it is the diameter of the pitch circle.
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