Analysis of the premature failure of wind turbine gearbox bearings

Wind turbine gearbox bearings are the component that leads to the most downtime of operating wind turbines due to their high failure rates. Failures occur within 10 % of bearing design life, despite the fact that they are designed to the same bearing standards that satisfactorily predict bearing lifetime in many other industrial applications. No theory has yet been widely accepted to explain the reasons for this premature failure, despite intensive research effort and many theories have been suggested both from industrial and academic researchers alike. The most widely accepted theory at the current time is that the bearing subsurface is weakened by what have been termed as white etching cracks that eventually lead to material removal from the bearing contact surfaces.
Extreme loading conditions caused by a number of possible sources, which expose bearings to higher than designed contact pressures and surface traction in wind turbine operation, are investigated throughout this project. A dynamic model of a wind turbine gearbox was developed in order to calculate bearing contact stresses during transient operating conditions, which found that bearings were loading to above recommended values, even during normal operating conditions. A failed bearing from a wind turbine gearbox was then destructively investigated, leading to the conclusion that manganese sulphide inclusions were the primary cause of white etching crack initiation. These inclusions were investigated in greater detail to determine the geometry and depth of the most damaging inclusions, both in the failed bearing and on bench top test rigs. A series of hammering impact test and rolling contact fatigue tests were designed and led to the successful recreation of white etching cracks in test specimens. It was found that white etching cracks certainly initiate at MnS inclusions. These microcracks initiate due to a tensile load across inclusion tips, which are thought to be further propagated by shear loading along the cracks. Inclusion initiated microcracks have been found to develop into white etching cracks, which may link up and weaken the subsurface of bearing raceways sufficiently to cause eventual failure. Testing is carried out to find thresholds in terms of contact pressure, surface traction, impact and fatigue loading cycles, required for the formation of white etching cracks.
The key contributions of this study are identification and recreation of four different types of subsurface damage at MnS inclusions by examining a failed WTGB and carrying out testing using a reciprocating hammering impact rig and a rolling contact fatigue twin disc machine. A hypothesis of the order and mechanism of these damage events is proposed in this study, as well as the development of testing methods to investigate the damage in order to support the hypotheses. Test methods are also developed to investigate the effects of some key bearing loading parameters, including impact loading, levels of contact pressure, surface traction and number of load cycles.