Investigation of Microstructural Alterations in Low- and High-Speed Intermediate-Stage Wind Turbine Gearbox Bearings

Abstract

Wind turbine drive train system failures have been the major source of down time and costly repairs. One of the main failure modes associated with gearbox failures is the formation of white etching cracks (WECs) in bearings. In this paper, low-speed intermediate shaft (LSIS) and high-speed intermediate shaft (HSIS) bearings are collected from failed gearboxes to examine microstructural alterations and evaluate damage. Butterfly wing formations are detected in LSIS bearings around nonmetallic inclusions at depths of up to about 260 µm from the contact surface and the measured span length ranging from 5 to 70 µm. Circumferential cracks oriented parallel to the contact surface are detected at depths greater than 300 μm. Irregular white etched cracks are revealed in HSIS bearings with random crack orientations. The crack depth is measured to be 7000 μm in the cross section perpendicular to the bearing axis. The bearing characterization revealed that microstructural alterations can occur with and without the formation of cracks. Evidence found supports the existing hypotheses that (1) white etch areas precedes crack formations and (2) cracks are prerequistes for white etch formation. The microstructural alterations detected in LSIS and HSIS bearings are observed to have different crack morphology and could be initiated by different mechanisms. The results presented in this paper can be used to develop materials-based prognostic life prediction models. This can be achieved by linking WECs and butterfly wings qualitative observations to turbine real operating conditions, bearing design, and manufacturing process.