Lubrication Condition Monitoring of Practical Ball Bearings by Electrical Impedance Method

In this study, the electrical impedance method was developed to monitor the thickness and breakdown ratio of oil films in elliptical elastohydrodynamic (EHD) contacts of practical ball bearings. First, it is theoretically shown that the oil film thickness and breakdown ratio can be simultaneously measured from the complex impedance generated when a sinusoidal voltage is applied to elliptical contacts. Subsequently, lubrication conditions of practical ball bearings were monitored at an ambient temperature to verify the measurement accuracy of the developed method. The oil film thickness in the low-speed range was consistent with the theoretical value calculated by Hamrock–Dowson equation. However, in the high-speed range, the oil film was thinner than the theoretical value considered an ambient temperature. In this high-speed range, the results of both the outer ring temperature and bearing torque revealed that the viscous shear heating and starvation were occurring simultaneously, thus supporting that the measured thickness would be thinner than the theoretical value. Besides, the developed method can also measure the breakdown ratio, confirming that it increases in the low-speed range where the bearing torque increases. That is, it indicates that not only the oil film thickness but also the breakdown ratio can be evaluated quantitatively.