Effects of groove factor and surface roughness of raceway in ball-bearing-like specimens on tribological behavior and the onsets of two instabilities of dry contacts

Ball-bearing-like specimens are prepared with three groove factors (GF ≡ r/D; r: radius of groove; D: diameter of ball). Analyses of operating conditions are conducted for the thrust loads applied to the dry-contact specimens running with the maximum contact stress close to 2.0 GPa. The combined effect of surface roughness ((Ra)inner) of the inner raceway and groove factor becomes the controlling factor for the slip ratio (SR) arising at the tribocontact of ball and inner raceway, and the average friction coefficient (μ¯) of specimen. Both the inception time (TBR) of the instability in the running-in process and the inception time (TBS) of the instability arising in specimen's base material are governed by the average value (SR¯) of SR. The wavinesses of raceway are the controlling factor for the significant rise of frictional torque in the running-in regime and the TBR value. The fluctuations of frictional torque arising at the time behind TBR are confirmed to be the instability arising in the base material. The theoretical prediction for TBS is determined when μ¯ reaches the threshold value, (μ¯)threshold. For the specimens with the same GF, increasing (Ra)inner can elevate SR¯ and μ¯. In the specimens with a relatively smaller (Ra)inner, GF is the dominant factor for SR¯; however, (Ra)inner becomes the dominant factor for SR¯ if the inner raceway is prepared with a relatively larger (Ra)inner. Increasing GF can result in aμ¯ reduction, irrespective of the (Ra)inner value. Both TBS and TBR are reduced by the increasing SR¯. The wear losses of ball and the two raceways, however, increase as increasing SR¯.