Contact fatigue caused by repeated Hertzian stress, in which cracks start at or just below the surface and release small pieces of metal, leaving craters. It usually appears first in the dedendum near the pitch line, where rolling combines with negative sliding and a single tooth pair carries the whole load. Initial pitting may stop once high spots wear in, while progressive pitting spreads and leads to spalling, noise and tooth fracture.
Pitting is just a band of little pits below the pitch line, it's not spreading yet, re-inspect next outage.
Initial pitting that stops after the gears bed in is often left alone; pitting that keeps spreading is not.
Also heard as
- macropitting
- pitting fatigue
Do not mix it up with
- Micropitting: Micropits are far smaller and show up as a gray, frosted area rather than individual craters you can see with the naked eye.
Related terms
Micropitting
Very fine surface fatigue pits, too small to see one by one, that give tooth flanks a dull gray look.
Spalling
Large, irregular flakes broken out of a tooth surface, usually where pits have grown and joined.
Contact stress
The compressive stress where two tooth flanks press together, which governs pitting life.
Dedendum
The depth of a tooth space below the pitch circle, slightly larger than the addendum to give tip clearance.