A study of white etching crack bearing failure detection using electrostatic sensing in wind turbine gearboxes

White etching cracks (WECs) have been found to form in rolling element bearings as early as 6-24 months into operation, especially in large wind turbine gearboxes. Despite the number of investigations conducted in both industrial and academic research over the past two to three decades, WEC formation and initiation mechanisms are still not well understood. This is due to the complexity of the factors that influence WEC formation, such as speed, load (mechanical and electrical) and lubrication, as well as a lack of effective monitoring techniques that can detect WECs prior to severe bearing failures, such as spalls on the bearing surface. Recent research has suggested that WECs can form in rolling element bearings under the influence of electrical load (potential or current). To investigate the feasibility of WEC detection using the electrostatic (ES) sensing technique, an ES wear site sensor was installed on a bearing test-rig where WECs had been created under the influence of electrical load. The ES responses were compared with those from an acoustic emission (AE) sensor that had been shown to detect WEC failures in a previous study. The physical findings related to WEC failures in the bearings and basic analysis of the sensor signals have been reported in a parallel paper by Zuercher et al. This paper focuses on the analysis of ES and AE signals using a time-frequency method, where correlations between the electrostatic charge signals and AE measurements are found. The results suggest that electrostatic sensors have the potential to detect WEC formation in rolling element bearings.