Hybrid bearings are used in tough application conditions, e.g., poor lubrication and/or high contamination levels , . Because of the higher stiffness of the ceramic material the Hertzian contact area is slightly smaller in a hybrid bearing, leading to higher contact pressure and subsurface stress compared with an all-steel bearing of equivalent geometry. In principle this should cause a reduction of the fatigue life of the bearing. However, it has been observed that in typical applications hybrid bearings last longer. How to explain this odd behaviour? How to model it? This article addresses these questions and also shows that the GBLM can model and explain well the observations in the field. SKF GBLM for hybrid bearings is now being released.
Hybrid bearings have rings made of bearing steel and rolling elements made of bearing grade silicon nitride (Si3N4) (fig. 1). Silicon nitride is a ceramic (i.e., non-metallic) material characterized by high hardness, high modulus of elasticity, high temperature capability and chemical resistance, low density and poor conductivity and ductility. Because the silicon nitride ceramic material is such an excellent electrical insulator, hybrid bearings can be used to effectively insulate the housing from the shaft in both AC and DC motors as well as in electrical generators. Hybrid bearings perform well under poor lubrication and solid contamination conditions compared with all-steel bearings, even though under equal load (due to higher stiffness of ceramic rolling elements) the contact stresses are higher.