In Situ Observation of a Grease Lubricant Film on a Radial Seal by Fluorescence Induced Microscopy

Industrial robots are operated by servomotors and reduction gears in their joints. Grease lubrication of the reduction gears is important for the delicate movements of the robot, and radial lip seals prevent lubrication leakage. Their operational behavior and sealing mechanism were examined by various authors and therefore is comparatively well understood [1]. Y. Sato observed the lubrication film on the radial lip seal in oil using LIF. However, the lubrication film on the radial lip seal in grease has not been observed [2]. It has been known that microscopic infrared spectroscopy and fluorescence induced microscopy are effective in observing grease behavior in situ. These methods were used due to their special feasibility of observing film thickness and thickener separately [3,4]. Previous verification results suggest that thickener particles are present in the sealing gap and are influencing the performance of the sealing system [5]. In this study, the sliding surfaces of radial lip seals in grease lubrication were observed using a fluorescence induced microscope to verify the grease lubrication and sealing condition. A glass shaft was used to observe the lubrication film on the radial lip seal and the sliding surface was observed from the shaft side. Lithium soap grease was used as the lubricant. Two dyes were used in this grease to observe film thickness and thickener separately. We observed changes in oil film thickness and amount of thickener on the sliding surfaces and discussed the seal lubrication mechanism of the grease in radial lip seals. [1]E.T.Jagger, Proceedings of the Institution of Mechanical Engineers, Vol171, Issue1, 597-616, 5.1957 [2] S.Yuki (2001) , Kyushu University [3] Y. Hoshi et al., J. Japanese Soc. Tribol. (Toraiborojisuto), 4, 15, 201, 2020 [4] D. Kostal et al., Faculty of Mechanical Engineering, Brno University of Technology, 17, 8, 2022 [5] Sommer M. et al., Tribology International, 103, 574-583, 2016