News / WEC Resistant Rolling Bearings for Durable Wind Turbines
Date: 2016-10-01
WEC Resistant Rolling Bearings for 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. Schaeffler has developed simulation processes to research the causes of WEC and offers specially designed bearings to prevent early failure.
A phenomenon having a negative impact on the reliability of bearings is the so called “white etching cracks” (WEC). For an efficient and economical reduction of these microstructure changes in the material, Schaeffler through hardens the bearings and coats rings and rolling elements with Durotect B. Damage caused by WEC has occurred in less than 0.01 percent of the more than 750,000 bearings that have been produced by Schaeffler since 2005 for use in bearing positions affected by WEC.
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 the fact that Schaeffler was 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 that can all be sources of WEC damage. This allows them to validate the simulation methods, analyze the root causes and develop suitable countermeasures. Measures may include an optimized 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. 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 drive train 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 statistically proven technology recommended by Schaeffler for preventing WEC damage in the long term 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 percent of the more than 750,000 black-oxided 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 provides the bearings with greater surface hardness and wear resistance, making load rating increases of up to 30 percent 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 percent and thus a longer service life as well as improved corrosion protection.
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