DIN ISO 20056-2:2018-11

Hybrid bearings are rolling bearings with raceways consisting of commonly used rolling bearing steel and rolling elements made from silicon nitride (for definitions, see ISO 5593). Due to the higher modulus of elasticity of the ceramic rolling elements, hybrid bearings have a noticeably smaller contact ellipse at the same load than rolling bearings with rolling elements made of rolling bearing steel. This will lead to higher contact stresses at the same load. Since the second edition of ISO 76 in 1987, the static capacity of rolling bearings is defined by a permissible Hertzian contact stress in the highest loaded contact. In rolling bearings made of steel, this contact stress will lead to a permanent plastic deformation at raceway and rolling element of approximately 1/10 000th of the rolling element diameter. In hybrid bearings, no significant plastic deformation will occur at the rolling element, resulting in a smaller total plastic deformation in the rolling contact. Therefore, in accordance with industrial practice, slightly higher permissible Hertzian stresses are defined for hybrid bearings. The guideline values of the static safety factor, S0, have been increased accordingly to maintain the same overall safety for the raceway. This document specifies methods of calculating the static load ratings for hybrid bearings with bearing rings made of contemporary, commonly used, high quality hardened bearing steel, in accordance with good manufacturing practice, and a full set of rolling elements made of silicon nitride (Si3N4) in contemporary, commonly used material and manufacturing quality.