Does a gear's diametral pitch have to be an integer?
Gear diametral pitch: the mating system matters more than an integer label
A gear’s diametral pitch does not have to be an integer as a mathematical value. In practice, the important requirement is that the mating gear system is defined consistently: pitch convention, tooth count, pressure angle, tooth form, helix data where applicable and center distance. Do not select or reproduce a gear from a DP number alone, especially when a legacy drawing uses mixed units or a nonstandard tooth system.
Check compatibility before treating DP as a purchase specification
- Confirm the convention: distinguish DP from module and record the units used by the drawing or mating component.
- Match the pair: mating gears must share the compatible pitch system and pressure angle; similar outside diameters do not prove that teeth will mesh correctly.
- Check the geometry: tooth count, profile shift, face width, center distance and tooth form can change the practical interpretation of an incomplete description.
- Protect the replacement decision: when the original data are unclear, measure or obtain the mating geometry rather than choosing the nearest nominal DP.
The PairGears spur-gear design resource provides context for the geometry that should travel with a pitch designation.
Example: turn the request into a checkable definition
For a legacy replacement, a note reading “about 10 DP” should trigger a drawing and mating-part review. Record the measured tooth count, pressure angle, pitch or reference diameter, center distance and tooth form before asking PairGears to assess the part. That prevents a nominal DP match from creating a non-meshing or poorly contacting pair.
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