White Structure Flaking in the wind industry sector

On 28 and 29 June, representatives from science and industry met at the VDI conference in Aachen on the subject "Damage Mechanisms in Bearings". The content of the event organised by the Association of German Engineers was provided by the Institute for Machine Elements and System Development at the renowned #RWTH Aachen University (MSE). NSK Engineering Manager Ralf Petersen explained the advantages of black oxide coated bearings - an already well-tested product solution from #NSK - in his presentation "White-structure flaking mechanism and Black oxide coating as Countermeasure for bearings in wind industry".

Mr Petersen, the first meeting after a two-year forced break - there was certainly a great need for exchange??

R. Petersen: That's right. The response was really great for a summer conference. A total of 85 experts took part in this VDI conference. Among them were scientists from RWTH Aachen University, Fraunhofer Institute and other institutes as well as experts from the supplier and user industries.

What questions were discussed together?

R. Petersen: This year we dedicated ourselves to current trends in the analysis and prevention of damage to rolling and plain bearings. For this purpose, 22 lectures were held in six different topic blocks. New insights were provided into known as well as relatively recent phenomena in roller and plain bearings. In addition, there were interesting insights into the simulation and analysis of various failure modes.?

Wind power applications played a decisive role in the presentations - an area with specific problems and also the main topic of your presentation. Can you explain the challenges in the wind industry in more detail?

R. Petersen: A phenomenon that we have increasingly noticed in wind turbine applications is the so-called 'white structure flaking'. To date, it cannot be explained one hundred percent, but there are some well-researched theories. One of them sees the penetration of free hydrogen molecules into the base material as the main cause for the oxidation process at the bearing. This hydrogen can come from the air or from the lubricant. The consequences are fatal, especially in heavy and expensive applications such as the generator drives of wind turbines, if the bearings have to be replaced prematurely.

What is NSK's solution to this?

R. Petersen: A[RP1]? protective oxide layer on the bearing slows down the penetration of hydrogen to such an extent that premature bearing failures due to white structure flaking can be avoided. The application of this protective oxide layer is also called black oxide coating. This is a layer of iron oxide that is applied to the bearing by a special finishing process. NSK offers its bearings with the Black Oxide Coating.?

But the problem of white structure flaking does not only affect the wind industry?

R. Petersen: Correct. This effect has been known for 20 or 30 years in the automotive industry, especially with alternators and belt drives. However, the problem has become more prevalent in applications in the wind industry. And there, these premature bearing failures produce considerable consequential costs.?

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Why is the problem more prevalent in the wind industry?

R. Petersen: White structure flaking is promoted by the fact that electricity runs through the application. Of course, this cannot be eliminated in a wind turbine designed to generate electricity. There are several possibilities, such as using other base materials for the bearing or changing the bearing design. But this still needs further development work. However, for us and already many users, the use of a suitably robust bearing in combination with black oxide coating is a solution that produces the best results and has therefore proven itself.

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How did you come to this conclusion?

R. Petersen: We have been able to drastically reduce premature bearing failures in practical field tests after replacing bearings with black oxide coating. We have also simulated the phenomenon through various laboratory tests. In the tests, we changed various process parameters and analysed their effects: different chemical compositions, different base materials and process times. From this we were able to conclude that there is something like an optimal process. In this process, the black oxide coating is of crucial importance.

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And these findings are already convincing practical users.?

R. Petersen: The solution with the black oxide coating has already become so established that the industry is already approaching us as a bearing manufacturer specifically with this requirement.

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What outlook can you give us for the future? Where is research at NSK going?

R. Petersen: Current-carrying applications will increasingly occupy us in the age of electromobility, especially where frequency converters are used. There are various ways to counteract this with insulation on the bearing or ceramic rolling elements. On the other hand, the current can be conducted through the bearing in a targeted manner so that it does not cause any damage there. We also think of special seals that serve as conductor. Or lubricants could be used that favour the direct passage of current. At NSK, we will continue to research in various directions in order to be able to supply the industry with the right bearings for all future developments.

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Thank you very much for the interview!

?[RP1]Further to already developed carbo-nitried bearing steel (STF and AWS) a protective……