Predicting Micro-Pitting Occurrence in Wind Turbine Gearbox Roller Bearings

The standard rolling contact fatigue life calculations currently in use by the rolling bearing industry is based on the first occurrence of subsurface-initiated spalling of a raceway or roller surface. However, wind turbine gearbox roller bearings have been suffering from another damage mode, which manifests itself as micro-pitting. The micro-pitting, which is spalling on a micro scale, by itself can be tolerated in its early stages; i.e. the roller bearing will still function properly. As the damaged bearing continues to operate, the micro-pitting propagates and at the later stages, often termed peeling, the pitting becomes deep enough to reach the appearance of traditional subsurface-initiated spalling. To better understand the phenomenon micro-pitting and its causes, this study was conducted to review published literature on the topic as it relates to bearing operation. The key findings were the need for a low specific lubricant film thickness parameter, and some component of sliding velocity in the contacting surface. With this knowledge, a proposed test scheme including these variables could be created from which a method to predict the risk of micro-pitting may be determined.