Ceramic Rolling Element Damage Sensitivity and Gearbox System Response

The rotorcraft industry is constantly pushing for higher temperatures, speeds, and loads, while simultaneously minimizing weight. Hybrid bearings (steel rings with ceramic rolling elements) offer improved performance over all-steel bearings plus significant weight savings. However, there are still concerns about using them within the aerospace industry. Two of the main concerns regarding hybrid bearings are that any manufacturing defects may lead to premature failure and that failures may be catastrophic in nature. In this manuscript, recent research performed on the sensitivity of hybrid bearings to preexisting damage and the sensitivity of an engine gearbox to hybrid bearing failure is presented. First, elemental testing was performed on ceramic rolling elements during which it was found that it is difficult to damage a ceramic rolling element and that they are far more tolerant of damage than was previously believed. Next, damaged balls were run in a single ball test rig at operating stresses to determine number of cycles to failure as a function of damage severity and predict time to failure in the application. Finally, the damaged rolling elements were run in a gearbox under maximum normal operating conditions and produced a soft failure within 15% of the time predicted by single ball testing.