White structure flaking (WSF) in wind turbine gearbox bearings: Effects of ‘butterflies’ and white etching cracks (WECs)

Abstract

The actual service life of wind turbine gearboxes is often well below the desired 20 years. One of the prevalent failure modes in gearbox bearing raceways is white structure flaking (WSF) by the formation of butterflies and white etching cracks with associated microstructural change called white etching areas. Despite these failures having been observed for two decades in various industries, the detailed reasons and mechanisms for their formation are not fully understood. In this review, white etching area formation mechanisms are discussed, specifically grain refinement, and effects of carbon/carbide in a range of bearing steels of widely differing carbon content. The review also highlights the severe transient, cyclic loading and tribochemical operating conditions of gearbox bearings and explains how these may act as drivers to produce WSF. Much previous research has focused on the detrimental effects of hydrogen, but other work suggests that hydrogen is not the only cause for WSF. Possible methods for preventing WSF are discussed, with attention paid to special steels such as high chromium steels, low carbon stainless nitrogen alloy steels and carbonitrided steels. Beneficial compressive residual stresses, surface coatings and enhanced lubrication and additive packages are shown to offer degrees of prevention, although the mechanisms leading to improvements are not fully understood.

Mapping of axial plastic zone for roller bearing overloads using neutron transmission imaging

“…This suggests that subsurface fatigue crack growth became sufficiently high enough that the bearing lost its load-bearing capacity. It has been documented that higher than average carbon content in roller bearings, may give rise to WEA, increasing material hardness and providing initiation sites for fatigue cracking [12,46]. This experiment confirmed the significant effect that an overload of 34 kN has on the reduction in life of the selected bearing.…”

“…It has been observed that subsurface plastic deformation damage in bearings are often associated with butterfly cracking at the white etch area (WEA) [11]. WEA is a region associated with nano-recrystallised carbide-free ferrite [12]. EBSD has been used to measure strain associated with fatigue cracks [13].…”

Analysis of Vibration Influencing Factors of High Precision Cylindrical Roller Bearing by Poor Information (Part I: Theory)

“…It has features of big load capacity, strong radial load carrying ability, small friction between roller and ring race, and can be used for high speed rotation. Because of its special structure, it plays an irreplaceable role in the motor, generator, gas turbine, machine tool spindle and so on [3,4]. With the development of technology and the increasing demand of the main engine performance, the quality of cylindrical roller bearing has been put forward more and more high demand, and its vibration control has become one of the main problems which restrict the quality of this kind of bearing.…”

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

“…Figure 1 shows an optical image of a subsurface-initiated WEC in a bearing inner ring, taken during a test in this study. More information about the characteristics of WECs can be found in the literature [8][9][10] .…”