Schaeffler’s innovative solutions help to increase the resistance of bearings to white etching cracks and prevent premature bearing failure.
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 higher and higher costs, especially in the case of offshore plants. Reliable components therefore play a key role here. A phenomenon that has a negative influence on the reliability of bearings, both in the wind power industry and elsewhere, are so-called white etching cracks (WEC). WEC are structural changes in the material that form below the surface of the bearing. 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 cause the affected bearing to prematurely fail. These cracks occur in both through hardened and case hardened rolling bearings. Schaeffler‘s innovative solutions help to increase the resistance of bearings to white etching cracks and prevent premature bearing failure.
In addition to bearings for wind turbines, WEC can also be found in bearing supports used in other industrial applications such as electric motors, paper machines, industrial gearboxes (e.g. in large hydraulic excavators), pump systems, and drives in marine applications as well as in the automotive sector. Influences that could have an effect on component fatigue, such as contact pressure, an insufficient level of steel purity, or overloading, do not cause WEC to occur.
It is therefore still not completely clear what causes WEC to occur. According to current knowledge, however, additional stresses in the form of dynamics, mixed friction, and the effects of electrical systems create the conditions needed for WEC to form. Schaeffler has comprehensive testing facilities for creating white etching cracks, analyzing the causes 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 and materials. An optimum design can reduce the stresses 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 solutions for making bearing supports more resistant to white etching cracks and protecting them against premature failure.
Through hardened rolling bearings with Durotect B
The technology that Schaeffler recommends and has statistically verified for efficiently and cost-effectively reducing WEC damage is the through hardening of the bearings in combination with black oxide coating on the inner and outer rings and the rolling elements. Through continuous optimization, the experts at Schaeffler’s Surface Technology Competence Center have developed an ideal coating system that significantly increases bearings’ resistance to WEC. The Durotect B coating system 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 protection against WEC. For example, WEC occurred in less than 100 of the 482,000 through hardened bearings coated with Durotect B that were produced and delivered by Schaeffler for use in wind turbines during the last few years. This corresponds to a failure rate of less than 0.02%.
Carbonitrided rolling bearings made from Mancrodur with Durotect B
Schaeffler offers carbonitrided rolling bearings made from Mancrodur material and coated with Durotect B for applications that require high load ratings. During carbonitriding, the bearings are subjected to a special heating process in which the bearing surface is enriched with carbon and nitrogen. This increases the bearings’ surface hardness and wear resistance.
Cronidur 30 special steel with a high chromium content
According to current knowledge, Cronidur special steel with a high chromium content can be used to completely eliminate the risk of WEC. Consequently, there has never been a known case of WEC with Cronidur 30 bearings. An inspection program conducted by Schaeffler for determining individual solutions depending on the application and additional stresses showed that no cases of WEC occurred where Cronidur 30 was used. The use of this material also allows the load rating to be increased by up to 70% and thus the rating life extended and the degree of corrosion protection improved.
As a development partner to the wind power sector, Schaeffler produces high-performance bearing solutions for rotor shafts, gearboxes, generators, and wind tracking systems. State-of-the-art calculation and simulation programs ensure that optimum designs for bearings in wind power applications are produced. The entire system is considered, starting from single rolling bearings and their components through to the adjacent construction and the entire power transmission system. The comprehensive service that Schaeffler offers for wind turbines also includes a range of remote monitoring systems and diagnostic programs.
