Pitch - Glossary - Technical Information - Hobby and Engineering

Glossary - Pitch

The pitch circle is an imaginary circle on a gear, pulley or similar mechanical device that divides the teeth at the optimal contact point. The optimal contact point is where two gears or a belt and pully contact each other when in correct mesh. The pitch is the distance from center to center of any two adjacent teeth of a gear, toothed belt, sprocket, toothed pulley or similar mechanical device as measured along the pitch circle. Pitch diameter is the diameter of the pitch circle.

Pitch is also used to describe the distance between threads of a screw, ballscrew or acme thread. It can also describe the centre distance between holes in a predrilled item such as shafting, rails or gear racks.

Timing belt and pulley pitches are usually specified as the distance between the teeth in mm.

For gears, metric pitch uses the module designation. If you divide the pitch circle diameter of a metric spur gear by the number of teeth you arrive at the module. A spur gear of module 1.0 will have a pitch circle diameter (PD) equivalent to the number of teeth eg, module 1 with 25 teeth = 25mm pitch circle diameter. A Spur gear with a module of 0.8 will have a pitch circle diameter equivalent to 0.8 times the number of teeth eg; module 0.8 with 25 teeth = 20mm pitch circle diameter.

Imperial sized spur gears use the American Standard DP (Diametrical Pitch) numbering system eg 16DP, 24DP, 32DP. To arrive at the DP for imperial sized spur gears you divide the number of teeth by the pitch diameter in inches. A 16DP gear with 16 teeth will have a 1” (25.4mm) pitch diameter and a 32DP gear with 16 teeth will have 1/2" (12.7mm) pitch diameter.

The pitch diameter is the diameter across the gear from half way between the top and the base of the tooth. So for a module 1 metric spur gear with 50 teeth, the pitch diameter will be 50mm, but the overall diameter of the gear across the teeth will be 51mm (pitch diameter + the module) and the diameter fron the base of the teeth on one side to the base on the opposite side will be 49mm.

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