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High performance rolling

High performance rolling bearing material for particularly demanding applications MACHINE ELEMENTS When rolling bearings fail prematurely in the steel, mining, or railway industries due to contamination or inadequate lubrication, this ends up costing the operator many 10000 of € every day. Schaeffler has carbonitrided the new Mancrodur rolling bearing material as a solution for such difficult cases. This provides more than twice the operating life compared to standard rolling bearing materials. For a long time now, materials experts have been trying to make steels for rolling bearing less sensitive to contamination. This is because when a particle penetrates into the raceway of the rolling contact, the displaced material produces a bulge, which is subjected to a lot of stress every time the rolling element rolls over it. The material very quickly becomes fatigued at this spot, which leads to the formation of micro-cracks and causes the bearing to fail prematurely. Inadequate lubrication gives rise to friction Inadequate lubrication can also cause micro-cracks, although the process is a different one. The tangential (tensile) stresses arising from the rolling contact in the raceway are readily tolerated with the proper design and full lubrication. However, if the lubrication in the rolling contact is insufficient, then the rolling partners touch metalto-metal in places, with the high metallic friction between the rolling partners, it causes the temperature in the surface to go up considerably. This rise in temperature not only ages the lubricant, but also decrease the strength of the steel – even to a point where tangential decreases are able to produce micro-cracks in the bearing material. A material needs to be hard as well as soft! The effects of contamination and inadequate lubrication on the material can generally be handled well with a ductile surface. The softer and more malleable the material surface, the better and faster the bulge will smoothen out, thereby reducing the high stresses around the foreign particle. No cracks are formed, and material fatigue is greatly delayed. However, this results in a conflict of interests that needs to be resolved. It is necessary for bearing surfaces to be very hard and the material to have high fatigue strength so that the bearing will attain a load-carrying capacity as high as possible. Offering solutions to the conflict of interests, Schaeffler demonstrates its new ‘Mancrodur’ rolling bearing material combined with carbonitriding. Meanwhile tests results show that these new bearings have more than twice the operating life under extremely demanding conditions. Special heat treatment of Mancrodur During carbonitriding, carbon and added nitrogen are embedded in the surface layer. On the one hand, this increases the surface hardness, and on the other, the heat treatment specially tailored to the Mancrodur steel forms a microstructure with finely distributed spherical carbides. In comparison, conventional rolling bearing steels have carbides with an elongated needle shape. They are not only embedded in the grains, but are also at the grain boundaries, thereby increasing the risk of crack formation. In Mancrodur, the carbides are distributed more uniformly in the material and not concentrated as much at the grain boundaries, helping to make the material more ductile. (Fig. 01 and 02). Temperature-stable retained austenite at a percentage with a tight tolerance increases the ductility of the new rolling bearing steel even more. The combined effect of these individual microstructural properties decreases the risk of crack formation considerably where there is either contamination or inadequate lubrication. The raceway surface is harder, more resistant to wear, and yet more ductile. Higher load rating with full lubrication The new high-performance rolling bearing steel features two other properties as well. There is less temperature-related loss of strength due to inadequate lubrication compared to other steels, reducing the risk of micro-cracks even further. In addition, the heat treatment is set up in such a way that residual compressive stresses are concentrated on the raceway surface. These compressive stresses reduce the resulting tangential stresses (tensile stresses) caused by the rolling contact. Under normal lubrication conditions (full lubrication), carbonitrided Mancrodur offers a 30 % higher load rating due to its material composition and heat treatment, equivalent to a 240 % increase in the nominal operating life. Test results with full lubrication have shown that the operating life of Mancrodur is up to six times longer. However, in harsh environments with contamination, mixed friction, or inadequate lubrication and heavy loads – like those that can be found in the railway and steel industries and sometimes in the area of wind energy as well – it is endurance tests under difficult conditions and experi- WORLD OF INDUSTRIESMOTION, DRIVE & AUTOMATION 2/2017

ence from pilot projects that really count. The field experience so far with carbonitrided Mancrodur is very promising. Higher operating life in steel industry One of the largest hot strip mills of Thyssenkrupp Steel Europe (TKSE) is located at its Bochum site. The hot strip mill is specially designed for milling high-tensile quality steels. The rolling bearings need to be able to withstand 45 million Newtons at high temperatures and with a large amount of contamination. After downtimes occurred because of the premature failure of conventional bearings, TKSE installed four-row FAG tapered roller bearings made of carbonitrided Mancrodur. The international steel producer has confirmed that these bearings have a significantly longer operating life. 01 Grinding patterns from case-hardened steels (left) and carbonitrided Mancrodur (right) Higher load-carrying capacity in wind turbine gearboxes This is how Bernhard Köhler, Business Development Manager in the Regional Wind Business Unit at Schaeffler, describes the current situation in the development of wind turbine gearboxes: “Since design lifetimes of at least 20 years apply in the wind energy industry – corresponding to 175,000 hours – and the bearing positions more and more frequently max out the planetary bearings’ load ratings, the industry is in desperate need of bearing solutions that will provide longer bearing life even under extreme lubrication conditions.” With a load-carrying capacity that is 30% greater and an operating life that has now been all but proven on the test stand to be at least twice as long, the wind energy industry can once again tap into margins of higher power density and longer runtimes by using carbonitrided Mancrodur. “For instance, downsizing planetary wheel bearings makes it possible to increase the wall thickness of the planetary wheels or the ring gear within the same installation space, which is precisely where the limits of feasibility are often reached. Besides the lower mechanical load of the gear rings, the greater wall thickness also reduces their elastic deformation, improving meshing considerably,” explains the specialist. 02 Comparison of the effect of mixed friction on the operating life of bearings made of standard bearing material with that of bearings made of carbonitrided Mancrodur Handling overloading in freight transport “Occasionally, operators will look for various non-standard ways to increase the productivity of their machines and facilities. In rail freight transport, for example, this might be done by regularly overloading the freight cars with over 10% more payload than specified by the design,” stresses Schaeffler Senior Manager Matthias Kilian. He goes on to say, “The overloaded bearings, such as inch-based AAR class K bearings (AAR = Association of American Railroads) with about a 157 mm bore therefore need to be replaced early and often unexpectedly. The premature material fatigue is usually the result of the mechanical overload combined with mixed friction and excessively long maintenance intervals for these load conditions.” By exchanging the axlebox bearings with structurally identical bearings made of carbonitrided Mancrodur, the operator can still have the increased payloads while returning to reasonable maintenance intervals, and it will no longer be necessary to purchase new bearings as frequently. However, the greatest economic benefit for the operator comes from the fact that the cars’ existing bogies can still be used. “Otherwise, the operator would have to resort to the next larger bearing series, such as AAR class G bearings with a bore that is around 20 mm larger, which would naturally make larger wheelsets, wheelset shafts, and other bogie components necessary,” Matthias Kilian explains. Photographs: Schaeffler 03 Benefits of Mancrodur depending on application area About Schaeffler AG Schaeffler Group is a leading integrated auto motive and industrial supplier. The company based in Herzogenaurach, Germany, generated revenues of approximately 13.3 billion Euros in 2016. With around 86,600 employees, Schaeffler is one of the world’s largest family owned companies. The group has approximately 170 operating locations in more than 50 countries with a worldwide network of manufacturing plants, research and development centers, and sales subsidiaries. Schaeffler makes key contributions towards ‘mobility for tomorrow’ with precision components and systems in engines, transmission and chassis applications as well as rolling and plain bearing solutions for industrial applications. WORLD OF INDUSTRIESMOTION, DRIVE & AUTOMATION 2/2017


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