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 (T_BR) of the instability in the running-in process and the inception time (T_BS) 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 T_BR value. The fluctuations of frictional torque arising at the time behind T_BR are confirmed to be the instability arising in the base material. The theoretical prediction for T_BS 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 a μ¯ reduction, irrespective of the (Ra)_inner value. Both T_BS and T_BR are reduced by the increasing SR¯. The wear losses of ball and the two raceways, however, increase as increasing SR¯.