Wind Turbine / Lubricant Research Update
Lubricants research update: White etching cracking wind turbine bearing failure
Any wind turbine operator will be familiar with the challenges posed by white etching cracking (WEC), a contact fatigue bearing failure that is reported to cause a significant portion of wind turbine high speed bearing failure.
Part of the reason that WEC is such a challenge is because it’s hard to detect when WEC formation begins. WEC eventually appears in the form of visible cracks, but the initial fatigue damage actually occurs well before those cracks appear. And, once damage erupts to the surface, it’s too late – it typically causes complete component failure, requiring extensive repair, unscheduled downtime and increased costs.
But, mitigating the potential for WEC is not easy, as the definitive root cause is unknown. There are various mechanical and chemical factors that have been shown to directly or indirectly contribute to WEC failure, and the correlations between these factors have yet to be fully understood.
That’s why ExxonMobil partnered with Schaeffler, a major bearing manufacturer, to conduct a multi-year research project to better understand the root causes of WEC. The research, which was conducted under diverse operating conditions and contact configurations, also aimed to understand lubrication’s role in WEC formation.
The experimental results of this collaboration were published this year in the peer-reviewed journal Tribology Transactions, and we wanted to share some of the lubrication-specific insights with all of you.
Certain lubricant formulations can contribute to WEC
The results clearly showed that lubricant formulations containing certain metal containing lubricant components can cause WEC formation. When testing various lubricant formulations, controlling for base oil type, viscosity and additive packages, the only variable that had a measurable influence on WEC formation was the additives – and metal additives in particular.
More precisely, zinc phosphate and/or the combination of overbased calcium and sodium alkyl sulfonates were demonstrated to cause WEC formation.
These additives can cause higher boundary/mix friction and increase sub-surface stress. In addition, due to hydroscopic nature of these additives, they may deliver water at the contact zone and enhance hydrogen diffusion. The combined influence of hydrogen and subsurface stress below the contact zone may lead to WEC formation.
Converting to a non-WEC oil cannot solve the damage caused by a WEC oil
For those operators hoping that a quick switch can fix any damage already caused by using the wrong oil, there’s some bad news. The research demonstrated that an oil formulated without any of the damaging metal components cannot remove the WEC-forming tribofilm or mitigate any sub-surface damage that has already occurred.
Thus, the best way to ensure that the lubricant is not contributing to WEC is to use well formulated oils in the first place. For example, Mobil SHC Gear 320 WT synthetic gear oil uses a metal-free formulation that was one of the lubricants that performed well in the testing.
Most importantly, lubricants cannot prevent WEC formation
As mentioned previously, WEC develops for a number of reasons, and lubricants represent only one of the possible causes. So, while an operator’s choice of lubricant can help minimize the risk of WEC, it cannot eliminate the possibility of WEC formation.
Hopefully this has been an informative look at the relationship between lubricants and WEC formation.