Innovative technologies for preventing white etching cracks

2014-03-10 | 000-004-881 DE-EN

SCHAEFFLER AG, BARCELONA/SCHWEINFURT

Schaeffler at the EWEA 2014 in Hall 6, Booth 6C50

Innovative technologies for preventing white etching cracks

The development of wind turbines to become multi-megawatt power plants leads to higher forces and torque, which subject the components to ever increasing loads. At the same time, any downtimes and maintenance work generate increasing costs, especially in the case of offshore plants. Reliable components play a key role here.

A phenomenon that has a negative influence on the reliability of bearings, also those outside the wind power industry, are so-called white etching cracks (WEC). WEC are structural changes in the material, which form below bearing surfaces. These changes result in the formation of cracks, which extend to the surface during stress conditions under different external loads. As a result, the inner or outer ring may eventually crack and lead to premature failure of the affected bearing. These cracks occur in both through hardened and case hardened rolling bearings. Schaefflers innovative technologies help to increase the resistance of bearings to white etching cracks and prevent premature bearing failure.

Influences, which could have an effect on component fatigue, such as contact pressure, an insufficient level of steel purity or overloading, do not cause the occurrence of WEC. The reasons for the formation of WEC are not fully understood on the basis of the current level of knowledge. However, there is scientific proof that additional stresses in the form of dynamics, mixed friction or electric currents combined with certain lubrication concepts contribute to the formation of WEC. Schaeffler has comprehensive testing facilities for assessing these influences and developing solution concepts for reducing the risk of WEC.

All in all, it can be said that the WEC risk can be limited by the design of the bearing and the selection of lubricants or materials. An optimum design can reduce the loads placed on the bearing, which can, in turn, be subjected to higher loads by matching the lubrication and preservation concepts and selecting the optimum material.

Schaeffler also offers different technologies for making bearing supports more resistant to white etching cracks and protecting them against premature failure. According to current knowledge, Cronidur special steel containing a high chromium content can be used to completely eliminated the risk of WEC. There has never been a known case of WEC with Cronidur bearings. An inspection program conducted by Schaeffler for determining individual solutions depending on the application and additional loads showed that no cases of WEC occurred where Cronidur was used.

It has been proven statistically that black oxide finishing of outer and inner rings as well as rollers significantly reduces the formation of white etching cracks. The coating system Durotect B has been consistently developed from conventional black oxide finishes to give increased performance. This coating minimizes the risk of damage caused by slippage, improves running-in behavior, provides protection against corrosion, and last but not least, increases the level of robustness to WEC. For example, WEC occurred in less than 20 of the 50,000 bearings coated with Durotect B, which were produced and delivered by Schaeffler for use in wind turbines during the last five years. This corresponds to a failure rate of less than 0.04 %.

Thanks to continuous optimization, the experts from Schaefflers Surface Technology Competence Center have developed an ideal coating system for efficiently reducing WEC damage. Schaeffler therefore always recommends its customers to use gearbox bearing supports in Durotect B.

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