Non-metallic inclusions in the material of rolling bearings are well known to induce subsurface initiated spalling. Among the inclusions, oxides in particular may often give harm to the bearing life. In a recent research, improvement in rolling contact fatigue (RCF) life of bearing steel in which oxides were bonded to matrix by hot isostatic pressing (HIP) has been confirmed. It has to be noted that RCF life was evaluated by thrust-type RCF tests. In those tests, contact load direction relative to rolling direction of the steel bar is different from the most widely used deep groove ball bearings and other radial bearings. In addition, those tests were performed under large contact load inducing plastic deformation. Such large load is not usually applied to actual bearings. In this study, RCF tests were carried out for deep groove ball bearings made from low cleanliness bearing steel to which HIP had been applied. From the experimental results, it was confirmed that the life improvement of radial bearings can be derived from bonding of the oxides with the matrix. Furthermore, void closing analyses were carried out using ideal shape void model. From the correlation between the void closing rates obtained by these analyses and the interface bonding rates measured for the inner rings, it is found that the void closing analysis is effective in predicting the state of the interface between the oxide and the matrix in the material.
In an effort to further extend bearing life, the authors have attempted to acquire greater knowledge regarding lubricating grease behavior in a bearing. While conducting experiments, some kinds of difficulties commonly arise when attempting to observe grease behavior directly from the bearing exterior without removing seals and shields. Making a breakthrough such a troubling aspect, X-ray computed tomography (CT), which is one of the non-destructive inspection techniques, was employed and resulted in visualizing remarkable details of grease distribution in a resin ball bearing. Hydrodynamic grease transition from churning to channeling state was well revealed by the mixture distribution of urea and barium-based greases which have different properties of X-ray absorption capability. Furthermore, the three dimensional unsteady liquid-gas multi-phase flows analysis was performed. Hydrodynamic feature of grease was regarded as a non-Newtonian fluid, which shows a highly non-linear flow curve, and the constitutive equation of modified Bingham plastic model proposed by Papanastasiou was applied to rheological property. Through these novel experimental and calculation approaches, several new insights about grease behavior inside a ball bearing were brought out.