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

In the present study, ball-bearing-like specimens are prepared with three groove factors (GF≡r/D; r: radius of groove; D: diameter of ball). Based on the three GF values, the specimens are designed with the number of balls and the raceway dimensions, except for ball diameter, to be slightly different. Analyses of operating conditions are conducted for the thrust loads applied to the dry-contact specimen running at a rotational speed such that the maximum contact stress created in every specimen has been obtained 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 the inner raceway, and the average friction coefficient (μ) of ball-bearing-like 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 affected by the average value of SR. Based on a theoretical model, the fluctuations of frictional torque arising at the time behind the running-in instability are confirmed to be due to the instability arising in the base material. The theoretical prediction for T _BS is determined when μ ̅ reaches the threshold value, (μ ̅) _threshold . In the specimens with the same GF, increasing (Ra) _inner can elevate the average slip ratio ((SR)). In the specimens with a relatively smaller (Ra) _inner , specimen's GF is the dominant factor for (SR); however, (Ra) _inner becomes the dominant factor for (SR) if the inner raceways are prepared with a relatively larger (Ra) _inner . Increasing specimen's GF can result in the μ reduction, irrespective of (Ra) _inner value. For the specimens with the same GF, a decrease in (Ra) _inner is advantageous for the reduction of μ. Both T _BS and T _BR are reduced by increasing the (SR) value. The wear losses of ball and the two raceways however increase with increasing the (SR) value. Periodic grove-like vibration signals in the running-in process have been proved to be caused by the surface wavinesses of the as-received raceways. 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.