Seeded Fault Testing of Hybrid Bearings in a Rotorcraft Transmission

Use of hybrid bearings (bearings with ceramic rolling elements and steel raceways) in rotorcraft transmissions has become more prevalent, providing weight reduction and operational benefits, however experience and knowledge of failure characteristics are not commonly accessible. What happens when a ceramic rolling element fails? Are the benefits of hybrid bearings overshadowed if it introduced new risk? Can impending bearing failures be detected before they become catastrophic? Current rotorcraft transmission diagnostic monitoring systems rely on metallic and/or magnetic properties of bearing assemblies in order to detect debris from a bearing failure. With the use of Silicon Nitride (ceramic) rolling elements, a potential problem arises in that the ceramic material is neither metallic nor magnetic. This means that current diagnostic systems would not indicate an impending failure of a ceramic rolling element. Previous research and development rig testing performed by several bearing suppliers on hybrid bearings indicates that seeded fault ceramic rolling elements fail locally and then induce a failure in the corresponding steel raceway, which spalls in a conventional manner, however this has not been demonstrated with aircraft hardware in a rotorcraft transmission. A descriptive account of recent testing is presented in this paper, which is focused on operational and seeded fault testing of hybrid bearings. Two bearings were identified for testing, a ball bearing and a cylindrical roller bearing were selected in a CH47D Chinook Engine Transmission. SKF provided all bearings for this test, one set contained seeded faults and two sets of bearings were made without seeded faults. SKF developed the appropriate cylindrical roller profile for the rollers to allow proper contact stress distribution between the silicon nitride rolling elements and the steel raceways which is markedly different than the contact stresses experienced with steel rolling elements on steel raceways. The results of this test are consistent with previous testing done at the bearing suppliers. The seeded fault rolling elements typically fail to a minor degree and then the mating steel raceway spalled and their chips initiated an indication within its transmission chip detector. For this reason, the test was deemed successful as a validation of previous rig testing performed at bearing suppliers, demonstrating that flaws in a ceramic rolling element would providing an indication with legacy metallic/magnetic indication systems prior to catastrophic failure.