Building tougher turbines

Louise Davis

Andreas Mangold on the role of crack-resistant rolling bearings in producing durable wind turbines

Feared, but preventable: when a gearbox bearing fails at an early stage, this can often be attributed to ‘white etching cracks’ (WEC). These cracks are changes in the material microstructure that form below the bearing raceway.

To find a solution to this problem, Schaeffler has developed simulation processes to research the causes of WEC and offers specially designed bearings to prevent early failure. The bearings, coat rings and rolling elements are hardened with the company’s Durotect B coating system.

Reproduction of WEC damage on the test stand

The causes of WEC have not been fully determined. Further research requires reliable testing methods. One important milestone was Schaeffler being able to reproduce the much-feared WEC damage on test stands. The rolling bearing manufacturer operates a wide range of WEC test stands to which engineers apply additional loads from friction, dynamics and electricity – which can all be sources of WEC damage. This allows them to validate the simulation methods, analyse the root causes and develop suitable countermeasures. Measures may include an optimised bearing design, the selection of a WEC-resistant material and an improved lubrication system.

Hydrogen hypothesis as the most plausible explanation

Previous investigations have found that the so- called ‘hydrogen hypothesis’ is the most plausible explanation for the development of WEC. It describes how, under certain conditions, hydrogen can ingress into the steel. This may result in changes in the material microstructure if external loads are applied. These changes look white after etching, which is why this damage is referred to as ‘white etching cracks’. Additional external loads cause stress and ultimately cracks in the material that can extend to the surface and cause the bearing to fail. 

Reducing additional loads

There are two basic approaches to tackling this issue. One consists of reducing additional loads. For instance, friction may be reduced with the help of a precisely adjusted amount of oil and viscosity; the dynamic load on the bearings caused by vibrations and torsional vibrations may be reduced or the electric fields affecting the bearings can be shielded. This requires the cooperation of all partners that produce components for the drivetrain in wind turbines.

Coating the bearings 

The other approach consists of increasing the load capacity of the bearing. Based on the hydrogen hypothesis, this means applying a suitable coating to prevent the hydrogen from entering the bearing material. The technology recommended by Schaeffler for preventing WEC damage in the longterm consists of treating the bearings with the Durotect B coating system. This system is an enhancement of conventional black oxide layers that have much more homogenous surface characteristics, which increases performance. Damage caused by WEC has occurred in less than 0.01% of the more than 750,000 black-oxide coated bearings that have been produced by Schaeffler since 2005 for use in bearing positions affected by WEC.

Carbonitrided bearings for higher load ratings

If higher load ratings are required along with a high level of WEC robustness, Schaeffler offers carbonitrided rolling bearings made from Mancrodur material with Durotect B coating.

During carbonitriding, the bearings go through a special heat treatment process that enriches the component surface with carbon and nitrogen. This equips them with greater surface hardness and wear resistance, making load rating increases of up to 30% possible. Bearings of this type have been used in volume production for around two years.

Special steels prevent WEC damage

Based on what we know today, Cronidur 30 high-chromium special steel can fully prevent the development of WEC. So far, there have been no known cases of WEC with bearings made from Cronidur 30 that have been used in this application on the market. The use of this material also helps achieve a load ratings increase of up to 70% and thus a longer service life as well as improved corrosion protection.

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Andreas Mangold is with Schaeffler.