Potential secondary mode I fatigue fracture at remnant defect sites of ejected spalls in planetary gear bearings causing critical uncontained failure

Planetary gear bearings being thin-rimmed are susceptible to ovalization stress. Also, due to rolling contact fatigue, spalling is a wear phenomenon. These ejected spalls leave behind crater- deep sharp-edged bottoms. This research paper investigates potential crack growth from these remnant sites of having a tendency to propagate into the core of the material leading to critical uncontained failure. The methodology presented here is coined as the RANK methodology and uses the principles of fracture mechanics coupled with a traditional analytical weight function. Also utilizing reference material data and inherent first principles, an evaluation technique is discussed. Different orientations of the rollers of the planetary gear bearing are also considered to be exhaustive in the extraction of the exact service-load stress state. Our approach robustly predicts this secondary mode I fatigue fracture failure. Through this methodology, the critical depth/thickness for the largest spall and the entire behaviour of such remnant defect sites that are sharp-edged bottoms left behind by ejected spalls can be concisely understood. The testing method to prove this hypothesis was then verified by full-scale service-load fatigue testing on a modified test rig. Here, a planetary gear bearing with manufactured defects similar in proportions to a remnant defect site left behind by an ejected spall was tested and evaluated to prove our hypothesis. Both novel, the prediction model and its experimental campaign concur well with and provide new insight onto this type of failure mechanism and is part of the ‘Innovative DEsign for Reliable PLANEt bearings’ (IDERPLANE) research project.