High carbon chromium bearing steel (SUJ2) is generally used for the rolling element of ball bearing as its material. Normal form of the bearing steel is wire rod. Then rough spheres are made by forging and they are developed gradually into high accurate rolling elements by heat treatment and polishing. However, it is known heretofore that there is inhomogeneous stress distribution caused by forging and heat treatment inside the rolling element. Recently, usage environment of rolling bearings tightens, in other words, high-speed and high temperature conditions are increasingly applied. Though increasing the environmental temperature is well-known as a stress releasing technique for metal stock, moderate deformation arises in the material. When such stress releasing inside the rolling element is enforced at work with being heated to a high temperature, the geometrical form error of rolling element enlarge and the bearing performance will decay. Therefore in this research, relating to the bearing steel ball and silicon nitride ball, the relationship between the non-repetitive run-out (NRRO) of the bearing and the changes in dimensional and geometrical form (roundness) of the rolling element which is exposed to high temperature for one year, assuming its use in a hot environment, are described. As a result, the followings were clarified. (1) For SUJ2 ball, high temperature environment enforced the change of not only geometrical form error but also diameter insomuch as differing accuracy grade. This causes the deterioration of bearing performance. (2) For silicon nitride (Si
3N
4) ball, under the same condition, the changes of geometrical form error and diameter are very few. Then, the authors confirmed that the silicon nitride ball has a beneficial effect on the usage in a hot environment.
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