Spherical debris particles in spalled areas of bearing raceways have been attributed to motions of the crack faces during spall generation. This note suggests an alternative generation mechanism for the particles.

Figure 1, from Failure Atlas for Hertz Contact Machine Elements, shows a spherical debris particle lying on the spalled surface of a bearing raceway. The atlas characterizes the spheroids as characteristic of spalling fatigue, and attributes the spheroids to alternating micro-motions between crack faces, but qualifies by saying that the specific mechanism of their generation is not understood. The spheroid measures about 50 microns and consists of bearing steel, oxidized at the surface.