セラミックス軸受用窒化硅素球の圧砕強度の球径依存性

抄録

Crushing load is used as a measure of static load capacity of bearing balls. The purposes of this paper are to make clear experimentally how crushing load of Si₃N₄ balls for ceramic bearings varies with the ball size, and to interpret the experimental result theoretically. Crushing tests were carried out by employing JIS B 1501 test method, i.e., by compressing two balls of the same size against each other. The ball diameter was changed in the range of 1/8-3/4 inch (3.175-19.05mm). The experiment showed that the relation between crushing load P_c and the ball radius R was expressed as P_c∝ R^λwith λ≅1.8. This implies that the maximum contact pressure at final fracture is not constant but decreases with increase of the ball size. In order to interpret this experimental result, a probabilistic theory of crushing strength of ceramic balls was proposed assuming that the fracture strength of a volume element of a ball follows a two parameter Weibull distribution. The theory leads to the relation of P_c∝ R^λ with λ being smaller than 2, and thus agrees with the experimental result. It was also shown that the crushing strength of the surface-finished ball is almost equal to that of the unfinished ball. This implies that the former can be known from crushing tests of unfinished balls.