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 fourth of a series, research by bearing manufacturer NTN-SNR is explored. Whilst they claim that their bearings are largely unaffected by WEC/WSF, they have undertaken studies by Ruellan at INSA-Lyon concerning WEC/WSF formation under conditions of non-hydrogen charging, high slippage and moderate speeds/loadings. He concludes that WEC/WSF are formed by surface tribo-chemical factors causing lubricant degradation with the formation of hydrogen that diffuses into the surface. Lubricant additives may accelerate the degradation as well as dissolved/free water. However, Ruellan does not cover technical solutions but elsewhere suggests that surface coatings may interrupt lubricant breakdown.
Additionally, he suggests that counter-measures to WEC/WSF formation include high chromium/vanadium steels, case hardening, bainitic steels, lubricants with less detergent additives, elimination of electrical activity and lowering of dissolved water.
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