A surface fatigue mode in which countless pits around 10 to 20 microns deep form where the oil film is thinner than the surface roughness. The affected area looks matte gray rather than pitted, and under magnification shows tiny cracks and craters. It is common on hardened, ground gears in slow, heavily loaded drives such as wind turbines, and it can remove enough material to change the profile and start macropitting.
Micropitting band on every tooth at the dedendum, the oil's too thin for this load, talk to the lube supplier.
Viscosity and surface finish are the big levers; smoother flanks and thicker oil both raise the film-to-roughness ratio.
Also heard as
- gray staining
- grey staining
- frosting
Do not mix it up with
- Pitting: Macropitting leaves distinct craters visible to the eye; micropitting is a uniform gray texture.
Related terms
Frosting
Shop word for the dull, frosted gray patches on gear teeth that turn out to be micropitting.
Pitting
Surface fatigue of gear teeth that breaks out small craters, typically on the flank just below the pitch line.
Lambda ratio
Oil film thickness divided by combined surface roughness, showing how well the lubricant separates the teeth.
Superfinishing
Polishing ground gear teeth to a mirror-like finish, chemically assisted, to cut friction and surface fatigue.