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It is experimentally known that the surface color of bearing balls gradually becomes brown during long term operation of the bearings under appropriate lubrication conditions. That exhibits the possibility of an estimation method for residual life of ball bearings without any abnormal wear on the surfaces by precise color measurements. Therefore, we examined what set colors on bearing balls by surface observation using scanning electron microscopy and subsurface analysis using transmission electron microscopy. Results showed that an amorphous carbon layer had gradually covered ball surfaces during operation of the bearings. The layer not only changed ball color but also made overall ball shapes closer to a complete sphere. The report also introduces a uniquely developed color analyzer which enabled color measurements on metallic surfaces, such as the above-mentioned balls.

As electric powertrain for electric vehicles (EVs) and hybrid vehicles (HVs) are becoming more efficient and smaller, rolling bearings for these vehicles should be capable of operating at higher speeds than those for internal combustion engines (ICEs). One key factor in predict fatigue endurance of such bearings is the oil film thickness on the bearing raceway grooves. Direct measurement of the oil film in operating machines is virtually unfeasible, while calculation of the oil film requires the input of precise temperature variation around the film. In this study, the oil film thickness on the bearing raceway grooves was calculated while in high-speed rotation by: (1) measuring the temperature profile of the bearing raceway outer ring; (2) calculating the temperature of the raceway groove using the basic formula for heat transfer; and (3) conducting an Elasto-Hydrodynamic Lubrication (EHL) analysis based on the temperature calculated in (2).