Publication

In order to prevent global warming, low torque and long life of rolling bearings are required. To improve those bearing performances, understandings of practical bearing lubrication conditions (e.g., thickness and breakdown ratio of oil films) are very important. Usually, optical interferometry methods have been utilized as accurate methods for measuring the oil film thickness in EHD (elastohydrodynamic) contacts. However, it is difficult to apply them to measurements for practical bearings because visible lights cannot be transmitted through metal bodies. In this research, the electrical impedance method, which can apply to metal bodies, has been developed for simultaneous quantifications of the oil film thickness (h) and the breakdown ratio (α) in EHD contacts. First, based on simplified geometrical and electrical models of EHD contacts, theoretical expressions of h and α were derived as explicit functions of the measured electrical impedance. Subsequently, 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, as shown in Fig. 1. Note that as the spacer layer, silica films are often used, but in this study, the ITO (indium tin oxide) film is used, not only as a “transparent film” in optical measurements, but also as a “conductive film” in electrical measurements. For electrical measurements, AC voltage (amplitude: 1 V and frequency: 1 MHz) is applied between the contact, AC current passing through the contact is measured, and the complex impedance is obtained, which is automatically performed with a commercially available impedance meter. Figure 2 shows the experimental results when the entrainment speed U was varied for the polyalphaolefin oil (the viscosity ν = 30 mm2/s at 40ºC) under the normal load W = 10 N and the slide-to-roll ratio Σ = 2. The condition Σ = 2 means the pure sliding contact with keeping the disc stationary. From the top graph of Fig. 2, it was confirmed that the measured h-values obtained by the electrical method (represented by red open circles in top graph) agreed well with those obtained by the optical method (represented by blue crosses in top graph). Besides, we find that all the measured h-values are located around the dashed line predicted by Hamrock-Dowson equation. On the other hand, it was also confirmed that the measured α-values obtained by the electrical method (represented by red open circles in middle graph) showed consistent correlations with the friction coefficient μ (represented by black open circles in bottom graph). Therefore, it was suggested that the lubrication conditions of practical bearings could be monitored quantitatively by using the electrical impedance method. It means that the developed method can contribute to improve bearing performances.