A model for hybrid bearing life with surface and subsurface survival

Hybrid bearings have rings made of steel and rolling elements made of bearings grade silicon nitride ceramic. Due to the higher modulus of elasticity of ceramic compared to steel, the ellipse of the Hertzian contact is slightly smaller than that in a conventional bearing. This results in about a 12% increase in the contact pressure between the ceramic ball and the raceway. Following the standard Rolling Contact Fatigue theory, a higher contact pressure leads to a reduction of the dynamic load-carrying capacity and service life of the bearing. However, long experience in the use of hybrid bearings shows that, in most applications, hybrid bearings clearly outperform traditional bearings made entirely out of steel. This paper addresses this issue by applying a novel approach to Rolling Contact Fatigue. Central to the new method is the survival probability of the raceway surface which is explicitly formulated into the basic life equations of the rolling contact. This allows tailoring the contribution of the stress system close to the rolling surface to better represent the ceramic-steel interaction, which has been proven to be substantially more favourable in the case of a hybrid ceramic-steel contact. The comparison between experimentally obtained hybrid bearings fatigue lives and lives predicted using the new calculation model indicates good agreement. It is found that an increase of the fatigue resistance of the raceway surface of hybrid bearings can, in most cases, compensate for the additional stress present in the subsurface region of the rolling contact. The new methodology, by providing a direct account of the survival probability of the ceramic-steel interface gives a better representation of the expected performance of hybrid bearings, opening new opportunities for the reliable use of this type of bearing in modern machinery.