High-performance steel doubles life of axlebox bearings

Precision bearing manufacturer Schaeffler’s Mancrodur rolling bearing steel, in combination with carbonitriding (a case hardening process that uses carbon and nitrogen), is already proving successful in wind turbines that are subjected to high loads, as well as in steel mills. With its new high-capacity Tarol Class K (HCT-K) tapered roller bearings that are used in heavy freight rail traffic, Schaeffler has expanded its range of applications for this high-performance steel.

Since April 2016, Mancrodur bearings have been undergoing rigorous testing under actual operating conditions at an Australian customer, and the results have been very promising.

According to Alexander Käbe, project manager high-capacity Tarol at Schaeffler: “Even when subjected to extreme loads and shocks, this bearing consistently demonstrates its high performance capabilities. Compared to the case hardening steel that is typically used in these applications, we are seeing considerable performance gains with bearings made from Mancrodur.”

To help rail operators immediately achieve gains in the operating life of their fleets and to substantially lower their total cost of ownership (TCO), existing AAR Class K steering knuckles can easily be fitted with Schaeffler’s new HCT-K axlebox bearings.

Greater surface hardness, but improved ductility

Compared with standard materials, Mancrodur’s load-carrying capacity is approximately 30% higher. As a result, the nominal rating life of Mancrodur is more than twice as long. Combining a high-performance steel with carbonitriding results in the highest load-carrying capacity for a given design envelope. From a materials-engineering perspective, this means this development represents the current state-of-the-art with respect to bearing performance and efficiency. Carbonitriding generates fine carbides and carbonitrides that are evenly distributed throughout the material. This results in a raceway surface that is harder and more resistant to wear. In addition, the heat treatment process increases the ductility of the raceway surface. Consequently, the material is less sensitive to contamination-induced overrolling.

In summary, compared to conventional case hardening, carbonitriding produces a material with high surface hardness and increased ductility. But only the combination of these two characteristics results in a considerable increase in fatigue life.

In addition to the new material, Schaeffler development engineers have added a number of other features to ensure that the HCT-K bearings can reliably handle millions of kilometres of use in heavy goods traffic. These innovations include an inner ring that is heat stabilised up to 200°C; a cage made from glass fibre-reinforced polyamide; and a bearing assembly that is fitted with extremely low-friction cartridge seals on both sides. To ensure a reliable automated mounting process, the bearing also features a support ring.