Abstract
Wind turbine gearbox (WTG) bearings can fail prematurely, significantly affecting wind turbine operational availability and the cost of energy production. The current most commonly accepted theory of failure mechanism is that the bearing subsurface is weakened by white etching crack (WEC) networks that eventually lead to the flaking away of material from the bearing surface. Subsurface damage due to rolling contact fatigue (RCF) is thought to be the main cause of premature failure, resulting from the initiation of micro-cracks, often at non-metallic inclusions or other material defects, which then propagate to the bearing surface. This study proposes a hypothesis that impact loading together with high levels of surface traction and contact pressure are important factors contributing to the initiation of micro-cracks and white etching areas (WEAs) at non-metallic inclusions which may lead to the formation of WEC networks. Both repeated impact and twin-disc RCF tests were designed to investigate inclusion-initiated micro-cracks and WEAs at subsurface in order to test this hypothesis. This led to the recreation of inclusion-initiated micro-cracks and WEAs in tested specimens, similar to the subsurface damage observed at inclusions in failed WTG bearing raceways. Tests were carried out to determine threshold levels of contact pressure, surface traction, and impact loading required for the formation of inclusion-initiated micro-cracks and WEAs.
Highlights
Operational wind turbine (WT) availability is significantly affected by downtime caused by wind turbine gearbox (WTG) failures [1, 2]
A hypothesis of the effect of impact loading and rolling contact fatigue on damage initiation at non-metallic inclusions is proposed. It focuses on the investigation of micro-crack initiation and white etching areas (WEAs) formation at non-metallic inclusions due to factors related to WTG bearing impact loading and surface sliding, and their interactions with nonmetallic inclusions
Threshold maps for subsurface inclusion-initiated microcracks and WEAs in bearing steel specimens have been investigated by rolling contact fatigue (RCF) tests at a range of surface sliding levels and contact pressures, some of which had been predamaged with 50,000 oblique impact loading cycles
Summary
Operational wind turbine (WT) availability is significantly affected by downtime caused by wind turbine gearbox (WTG) failures [1, 2]. A recent investigation focused on measuring the operational conditions in the high-speed stage of a WTG has reported that, when tested at lower torque and speed than the rated conditions, the measured speeds of the bearing cage and rolling elements are 30 ~ 40% lower than their theoretical values, indicating significant sliding of rolling elements over the raceways, even under steady-state conditions [32] These periods of heavy and dynamic loading lead to transient raceway stresses sometimes exceeding 3.1 GPa and generator engagements and disengagements can lead to stresses up to 2.5–4 times higher than those during normal operating conditions [5], well above the yield strength of bearing steels [6]. Most recent studies on failed bearings from field returned WTGs have shown WECs preferentially initiate as butterfly cracks at dual-phase inclusions of aluminium with sulphur and manganese [47], around oxide and dual-phase inclusions [48], and at Type D globular duplex inclusions especially when an inclusion has low aspect ratio (ratio of inclusion lengths along major to minor axes) of 2:1 [49]
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