White etching cracks and white structure flaking failures have afflicted bearings in wind turbine gearboxes. Research has not led to a definitive root cause, but lubricant additives, roller skidding, lubricant breakdown, impact/impulsive loadings, hydrogen formation, water in oil, hydrogen content of bearing steel and electrical currents are thought to be contributors. It is also not known whether WEC/WSF formation is surface or sub-surface initiated. The role of non-metallic inclusions is highlighted when WEC/WSF crack formations are not surface-breaking.
In the second of a series, solutions offered by bearing manufacturer Timken are explored. They recommend steel of higher cleanliness by reduction of size and number of non-metallic inclusions that act as stress raisers near the surface and a carburising process that induces residual compressive stress in the near surface. These solutions recognise that WEC may be sub-surface initiated under the effect of transient loading conditions, as opposed to surface-initiated where super-finishing, black oxide coating and/or roller coatings (Timken ES) improve micro-pitting and smearing behaviour.