Effect of Compression, Impact and Slipping on Rolling Contact Fatigue and Subsurface Microstructural Damage

In Wind Turbine Gearboxes (WTGs), which are designed to be in service for 20 to 25 years, the service life of their bearings is often below their design life despite depending on advanced technologies and standards in the gearbox design. There are a considerable number of factors affecting premature bearing failure. Three parameters which are widely reported as the main causes were investigated in this study, which are contact pressure, slipping and impact loading. Two failed planetary bearings from a multi-megawatt wind turbine gearbox were investigated first to evaluate their surface and subsurface damage features and to estimate the parameter testing levels. A new test rig that can apply impact loading in combination with compression and slipping was designed. Two contact discs made from bearing material (AISI 52100 steel) were used in this test rig. Sixteen tests are designed using Minitab software for design of experiments using a line contact with flat test discs with stress concentration at the contact edges because stress concentration cannot be avoided in roller bearings, which is the type usually used in the turbine gearboxes. The effect of the key parameters on the test disc life in terms of the number of cycles to failure and subsurface damage features distribution at the microscale level were analysed. These distributions were correlated with the subsurface stress distribution within the contact region to predict which stress type was responsible for each damage type. Further tests were also designed and conducted without stress concentration using fully crowned disc profiles. The investigation led to suggest a new damage estimation model depending on the contact stress levels and the number of cycles under each level. The suggested fatigue life prediction model has been tested using real operating data from SCADA and the results were quite close to the real planetary bearing life compared with the bearing selection standard.