The different mechanisms of fretting wear in oil and grease lubrication and methods to reduce fretting wear were examined by means of thrust ball bearings in this study.Tests of fretting wear under oil lubrication were conducted. It was confirmed that highviscosity oil can reduce fretting wear at high velocity (i.e., high frequency) through the oil film formation.In the case of grease lubrication, the influence of velocity on fretting wear was significantly different for low and high viscosity greases. Grease with low-viscosity base oils could reduce fretting wear at high velocity. In contrast, grease with high-viscosity base oils could reduce fretting wear at low velocity. Grease thickeners were found to be effective in forming a layer that could prevent fretting wear. These results highlight the large differences of effective fretting wear reduction mechanisms between oil and grease lubrication.
A measurement method using electrical impedance has been developed for simultaneous quantifications of the thickness (h) and breakdown ratio (a) of oil films in elastohydrodynamic (EHD) contacts. First, based on simplified geometrical and electrical models of EHD contacts, theoretical expressions of h and a were derived as explicit functions of the measured electrical impedance by using the Lambert function. Then, to verify the proposed measurement principle, oil film thickness measurements were conducted by using the electrical method together with the optical interferometry method in a ball-on-disc-type apparatus, which utilized the lubricated contact between a steel ball and a glass disc with a transparent conductive layer (i.e., an indium tin oxide layer). As a result, it was confirmed that the measured h-values obtained by the electrical method agreed well with those obtained by the optical method, under various test conditions with changing the entrainment speed, slide-to-roll ratio, normal load, and viscosity. Besides, it was also confirmed that the measured a-values obtained by the electrical method showed consistent correlations with the film parameter and the friction coefficient. It is hoped that the developed electrical method will be applied to practical metal-to-metal contacts (e.g., the contacts in practical ball bearings) to understand invisible behaviors of oil films in EHD contacts.
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 lowspeed 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.
In this study, we developed the electrical impedance method which simultaneously measures the thickness and breakdown ratio of oil films in elastohydrodynamic (EHD) line contacts within thrust needle roller bearings. Initially, we theoretically demonstrated that the oil film thickness and breakdown ratio can be simultaneously measured using the complex impedance that is produced when an AC voltage is applied to EHD line contacts. To verify the measurement accuracy of the electrical method, we monitored the oil film thickness of a thrust needle roller bearing and compared it with the theoretical value. The results revealed that the oil film thickness was thinner than the theoretical value immediately after starting the test, with the breakdown ratio being greater than 0 (indicating mixed lubrication); however, the breakdown ratio decreased over time, and the oil film thickness nearly matched the theoretical value one hour after starting the test, when it is believed that running-in wear is complete (i.e., breakdown ratio ≈ 0). Furthermore, following the test, after examining the race surface, we confirmed that running-in wear had indeed occurred. These results suggest that the developed method can monitor the lubrication conditions in EHD line contacts, such as those in thrust needle roller bearings, in detail.
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