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 third of a series, solutions offered by bearing manufacturer Schaeffler are explored. They recommend through-hardened bearings with Durotect B coating (a development of black oxide) on bearing components with alternatives of carbo-nitrided Mancodur steel coated with Durotect B or a high chromium steel Cronidur 30 (that is found to be free of WEC issues). Additionally, Schaeffler conclude that WEC is unrelated to the presence of non-metallic inclusions or other impurities in the steel. It is related, however, to accumulation of structural distortion within the steel matrix. They further conclude that electrical processes can cause sub-surface initiated WEC.
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Click here to access Holweger paper