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 fifth of a series, research carried out by bearing manufacturer NSK is reviewed. Yamada and Uyama have investigated WSF in various test rigs. In a grease lubricated bearing used in an automotive application, WSF could be suppressed by a black oxide coating providing it remained intact. No increase in hydrogen content of the steel was observed, suggesting that breakdown of the grease had not occurred.
In another test rig, a traction oil (known to break down easily) was used with the result that WSF occurred, with the black oxide layer being removed and increased hydrogen seen in the raceways. In hydrogen-precharged samples, steels with increased manganese, silicon, chromium and molybdenum improved WSF life. Using traction oil, the composition of the steel could be optimised to maximise bearing life before WSF formation.