Failure Analysis and Toughness Improvement for a Pinion Head Tapered Roller Bearing in Differential System of a Commercial Vehicle

Tapered roller bearings are utilized in applications where operating loads are high with simultaneous radial and axial type of loads. A failure case was reported in pinion gear of commercial vehicle application where bearing cone was observed fractured at groove area in multiple locations. The subjected bearing cone was made of SAE 52100 material and surface treated with patented life improvement treatment for improved wear and fatigue resistance with longer service life. Fracture analysis through scanning electron microscopy (SEM) revealed multi-point fracture initiation from groove area causing the fracture to initiate and propagate towards larger face of bearing cone, ultimately causing flange separation and bearing failure. The fracture was observed taking place under bending stress. No material or heat treatment non-conformity was observed in metallurgical and fracture analysis. Further analysis revealed that the bearing cone which was fitted against pinion gear of differential system has discontinuous support due to roots of gear tooth causing cantilever loaded condition. The abutment available, material and surface treatment selection proved inadequate for given application conditions and more toughness was essential for bearing flange to operate in service. As a countermeasure, low carbon steel with case carburizing heat treatment was proposed for the bearing. The improvement in fracture load capacity of the low carbon steel bearing cone was compared with existing 52100 steel bearing cone with help of specially designed fixture on universal testing machine. The new bearing with low carbon steel with case carburizing treatment showed higher fracture loads for flange as compared to bearing made from SAE 52100 steel.