TIFF stands for tooth interior fatigue fracture, a failure of case-hardened gears in which a crack starts below the surface near the middle of the active flank, around the case-core transition, and grows through the tooth until it breaks off. The fracture runs at an angle across the tooth instead of from the root fillet. It is linked to high contact load with relatively thin cases or weak cores, and it is often missed because the flank looks healthy beforehand.
Fracture starts mid-flank under the case, not at the root, that's TIFF, so look at case depth and core hardness.
Also heard as
- tooth interior fatigue fracture
- TFF
- tooth flank fracture
Related terms
Case depth
How deep the hardened surface layer of a gear tooth goes, usually measured to a set hardness such as 50 HRC.
Carburizing
A heat treatment that diffuses carbon into the surface of low-carbon steel gears, then quenches for a hard case.
Tooth breakage
A gear tooth cracking off, most often by bending fatigue starting in the root fillet on the loaded side.
Spalling
Large, irregular flakes broken out of a tooth surface, usually where pits have grown and joined.