A Phenomenologic Explanation of Hollow Balls in Rolling Elements Bearings

Rolling Element Bearings(REB) are "high-tech" machine elements through their complex geometry, high performances materials, fine heat and surface treatments, extremely accurate precision, and the overall detailed manufacturing structure (Casting, forging, machining, grinding, polishing, ...) that allow to reach large cost effective productions. Hyperstatic systems, they can be subjected to heavy working conditions (Combined loads, high speeds, large temperature intervalles, ...) and exhibit improved mechanical performances such as high static and dynamic load ratings, upper endurance strengths, low internal losses, high speed rates and can be subjected to complex working conditions. To achieve such services, they exploite advantageously the tribo-system at the interface between their rolling elements and the raceways on which they run and through which accurate motion guidance and external loads are transferred to the machine bed. Some of those working conditions can impose the rolling elements to extremely high (combined) stresses of diverse natures : centrifugal or inertial loads, extreme orbital speeds, multiple instantaneous center of rotation, rolling and sliding speeds, high surface temperatures, ... all of this damaging more or less the rolling-element/raceway tribo-system and leading to potential misslubrication. In, usually, high speed angular contact ball bearings, some extremely rare working parameter combinations may subject the tribosystem to a kind of metastable heat input that bring the balls to high temperature under heavy contact and inertial stresses. As for result, a reduction of the mechanical characteristics of the ball material with the heat increase, leading potentially to material flow in the direction of the heaviest stresses. The result of this phenomenon is the obtention of "hollow balls", a strange and insidious failure occurring in service. The phenomenology of this extremely rare failure pattern is not at all well understood and the present article intends to give a trial of explanation to this rare failure case.