Influence of Air Bubbles Inside Lubricants on Ultrasonic Thickness Measurement and Ultrasonic Imaging of Air Bubbles

Lubricant is an important factor in ensuring the stable operation of journal bearing under hydrodynamic pressure lubrication conditions. However, when the spindle rotates at high speed inside the journal bearing, negative pressure may be generated in the area near the minimum film thickness. When the negative pressure in the lubricant is lower than the cavitation pressure, the cavitation phenomenon occurs. The presence of cavitation not only leads to a decrease in the lubrication effect of the plain bearings, but may also cause damage to the bearing. When cavitation occurs, the bubbles in the lubricant collapse on the solid surface and produce a strong impact, this impact will cause damage to the surface of the plain bearing, which will affect the performance and service life of the bearing. In order to solve this problem, this paper proposed a method of scanning and imaging the bubbles inside the lubricant using ultrasonic technology, and investigated the film thickness measurement errors of the resonance model and the spring model when there were bubbles of different sizes in the lubricant. It was shown that the presence of air bubbles in the lubricant had a significant effect on the film thickness measurement accuracy of the spring model. Even in the case of small and medium bubbles in the lubricant, the film thickness measurement errors of the spring model were as high as 233% and 15955%, respectively, which indicated that the calculation results of the spring model deviated from the true film thickness value when there were bubbles in the lubricant. In contrast, the resonance model was affected differently by the presence of air bubbles of different sizes in the lubricant. The presence of small and medium bubbles in the lubricant had a small effect on the film thickness measurements and the maximum relative error was less than 8.5%. However, when there were large air bubbles in the lubricant, the amplitude of the resonance point in the amplitude curve of the reflection coefficient would be significantly affected, and even the phenomenon of the disappearance of the resonance point would occur, which made it impossible to utilize the resonance frequency corresponding to the resonance point to obtain the value of the film thickness. Therefore, in the case of large air bubbles in the lubricant, the use of resonance frequency to determine the film thickness value would be greatly affected. In addition, when scanning air bubbles in the lubricant using ultrasound technology, it was necessary to adopt the appropriate treatment method depending on the thickness of the lubricant film. When the lubricant film was thin, according to the reflective signal amplitude of the difference between the lubricant could be realized on a single bubble or multiple bubble scanning imaging. When the lubrication film was thick, the scanning imaging was determined based on the difference in the amplitude of the resonance point. By comparing the position of bubbles scanned by ultrasound technology with the actual captured images, it could be found that the position of bubbles obtained by ultrasound imaging was almost the same as that in the actual captured images, which verified the effectiveness and accuracy of ultrasound technology in bubble localization. In summary, the research in this paper provided theoretical and technical references for the real-time monitoring of bubbles and cavitation areas inside the journal bearing, which was of great significance for improving the operational reliability and service life of the journal bearing system, and ensuring the effectiveness and stability of the lubricant.