Application of Ultrasonic Sensing to Monitoring Lubrication Conditions in a Refrigeration Compressor

Abstract

ABSTRACTUnderstanding lubrication conditions in a refrigeration compressor is valuable for machine design and ensures the reliability of air conditioners. In this work, the lubrication conditions of a journal bearing in a scroll compressor were evaluated by applying ultrasonic sensing. An ultrasonic probe was attached to a journal bearing supporting a crank shaft in the compressor. The probe transmitted ultrasonic pulses toward the lubricant film between the shaft and the bearing and then received reflections. The lubricant film can be monitored by measuring the reflection amplitude of the ultrasound, which increases with the thickness of the thin lubricant film. With the whirling shaft in the bearing, the minimum reflection amplitude was measured as an index of the minimum lubricant film thickness in the bearing during stable and transient operations of the compressor. Then the measured reflection amplitude was compared with the Sommerfeld number, which is also known to increase with lubricant film thickness. The measured minimum reflection amplitude mostly changed in trend similar to that of the Sommerfeld number. However, the measured minimum reflection amplitude behaved differently during the first 400 s in the startup process. An observation of the lubricant reservoir showed generation of small bubbles in the lubricant in the period. Overall, test results support the usability of the ultrasonic measurement to trend monitoring of the lubricant film thickness in the journal bearing in the compressor. The existence of bubbles in the journal bearing was indicated by the results of the ultrasonic measurement and observation.