AGMA 11FTM16 PDF

Micropitting is a type of fatigue failure occurring on hardened tooth flanks of highly loaded gears. This failure consists of very small cracks and pores on the surface of tooth flanks. Micropitting looks greyish and causes material loss and a change in the profile form of the tooth flanks, which can lead to pitting and breakdown of the gears. The formation of micropitting depends on different influences. Besides material, surface roughness, and geometry of the tooth flanks, the lubricant and the operating conditions show a main influence on micropitting formation. The micropitting load-carrying capacity of gears can be calculated according to ISO/TR 15144-1, where the influence of lubricant, operating conditions, and surface roughness is considered with the specific lubricant film thickness. For this purpose, the specific lubricant film thickness of a practical gear is compared with a minimum required specific lubricant film thickness. The latter is the specific film thickness where no micropitting risk is given for a lubricant and can be determined by performing a micropitting test according to FVA 54/7. This test procedure consists of a load stage test and an endurance test. Lubricants with a high micropitting load-carrying capacity reach the failure criterion of a profile form deviation of 7.5 ?m due to micropitting in load stage greater than or equal to LS 10 of the load stage test. In the endurance test, a stagnation of micropitting formation compared with the micropitting area at the end of the load stage test is preferred but not required. In field applications, micropitting formation is often reported even though industrial gear oils with a high micropitting load-carrying capacity are used. Such oils offer a good micropitting protection determined in the load stage test, but with a low micropitting performance in the endurance test. The aim of research is therefore the investigation whether a change from an oil with low micropitting performance in the endurance test to an oil with high micropitting performance in the endurance test can stop the micropitting formation.