Damage at joints meant to be stationary, such as bearing seats, splines, keyways and bolted flanges, caused by vibration or flexing that produces slips of a few microns to a few tens of microns. The debris oxidizes immediately, so steel parts show a red-brown iron oxide powder that mechanics call cocoa, while aluminum shows black. Fretting loosens fits and starts fatigue cracks. Tighter fits, better clamping and anti-fretting pastes prevent it.
Bearing OD is covered in fretting and the housing bore's oversize, it's been creeping in there for months.
Red cocoa powder at a bearing seat means the fit is wrong, not the lubricant.
Also heard as
- fretting corrosion
- fretting wear
- cocoa
Related terms
False brinelling
Dents in bearing races caused by vibration while the machine sits still, not by impact.
Adhesive wear
Wear where touching asperities weld together and tear, moving metal from one surface to the other.
Spalling
Flaking of large chunks from a bearing race or gear tooth after fatigue cracks below the surface link up.