転がり軸受の内輪と外輪にある程度の電位差が生じると油膜を突き破って放電が起こり，軌道面に放電痕が形成される.それが繰り返されると表面性状が悪化し，音•振動が大きくなる不具合が発生する.この不具合は"電食"と呼ばれており，鉄道車両用転がり軸受でよく見られる損傷形態として知られている.転がり軸受の電食に関する研究は，1970年以前に行われており，電食発生の目安となる電流密度も示されていた.しかし，最近ではインバータの普及にともなって家電品用小型モータの軸受でも電食が起こるようになってきた.そこで筆者らは小型玉軸受を対象にした電食の研究を行い，従来から報告されていた電食発生電流密度よりも小さな値で電食が発生すること，油膜厚さが大きいほど振動上昇が大きくなることなどを明らかにしてきた.%When a difference of electrical potentials is occurred between the inner and outer rings of rolling bearing, electrical discharge arises inside the bearing. Then, the sound and vibration, which are one of the defects of bearing, from the bearing increase in a short time. This phenomenon is called "electrical pitting". In the raceway surface, a large linear undulation, which can be visual, is formed in the high density after the occurrence of electrical pitting. This undulation is called "ridge mark". It is well known that the ridge mark is a typical damage morphology of electrical pitting. However, there is no report on how the ridge mark is formed on the raceway surface. Therefore, in this research, the author observed the formation process of ridge mark arising from electrical pitting experiment by using thrust bearing which is very easy to disassemble. Consequently, the following was confirmed.(1)The ridge mark did not grow evenly in all-round by little and little, but it was formed partially on the raceway. (2) Next, it spread into circumferential direction. (3) Finally, it enlarged and was formed in all-round the raceway surface.
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