Can lubricants help mitigate the role of white etching cracks?

SPONSORED CONTENT: White etching cracking (WEC) is a type of bearing failure commonly observed in wind turbine gearboxes.

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This condition, which gets its name from the white appearance of the fissures in the microstructure of steel, can result in costly parts failure.

The actual scale of the issue is considerable: 80% of wind turbine high speed bearing failures are caused by WEC.

Worryingly, this happens without warning; as the phenomenon occurs below the surface of a bearing it is not possible to detect, even with a visual inspection.

And once WEC damage reaches the surface of a bearing it is too late — the resulting component failure can require extensive and costly maintenance to put right, particularly offshore.

ExxonMobil and Schaeffler Technologies investigated the role of lubricants in triggering WEC, and how choosing specific formulations of oils can help reduce the phenomenon, using two tests.

The FE8 test was used to examine oils and greases with regard to their wear and friction behaviour under lubricant and bearing-specific influences.

The companies used the MTM-SLIM test to investigate sub-micron tribofilms.

The scale of the tribochemical reaction — the chemical changes that occur to a lubricant and a lubricated surface when separated by a thin tribofilm — is not well understood.

Speculation has centred on the activity of hydrogen in the subsurface ‘WEC zone’, which can have a role in the bearings becoming brittle.

Lubricant insights

Bearings that failed within the first 200 hours of FE8 testing clearly exhibited WEC. No such signs were observed in specimens of bearings that lasted longer than 200 hours of testing.

In these cases, cracks propagated in the lateral as well as outward radial direction reaching to the surface, suggest non-WEC related failures as a result of surface fatigue.

Based on this observation, the FE8 test results were grouped into two categories: average hours to failure below and above 200 hours triggered by oils that cause WEC and those that do not contribute to WEC formation (referred to as WEC oils and non-WEC oils) respectively.

Tests that reached 500 hours were stopped manually.

WEC oils showed consistent bearing failure after less than 200 hours of testing, while non-WEC oils did not result in WEC failure in FE8 tests.

Also, oils formulated with metal-containing additives showed strong tendencies toward WEC failure. This is linked to the growth of relatively thick (~120 nanometres) tribofilms, and high frictional forces, during and after its formation.

Base-stock type did not show any impact on WEC occurrence and although higher viscosity base-stocks delayed WEC formation, they did not prevent it.

The MTM-SLIM results indicated that once WEC has started, the switch to a non-WEC oil delayed the WEC events but could avoid the failure.

These results have major implications in industry applications, for example to assess the turbine bearings’ susceptibility to WEC failure, operators need to assess the oil history of the gearbox and not only the oil currently in use.

WEC is a complex engineering and scientific challenge. However, as this research shows, it is possible for the wind turbine industry to offset potential issues and maintenance costs through the use of high performance non-WEC lubricants.

These findings are being used to develop a range of next generation oils and greases to help ensure that wind turbine owners have the lubricants they need to enhance their operations and reduce preventable downtime.

ExxonMobil continues to work closely with Schaeffler to ensure it offers its customers the expertise and products they need for safe and efficient turbine operation.

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This article is sponsored by Exxon

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