Methods for detection of lubrication failure applied to a swashplate compressor

J. c. Drozdek, J. D. Chappell, C. Cusano, P. S. Hrnjak, N. R. Miller, and T. A. Newell Understanding lubrication failures at the shoe/swash plate contact of automotive swash plate compressors will greatly enhance the reliability of the air conditioning system. Maintainin~ proper lubrication is not always possible during transient conditions. Therefore, a method for detection of lubricant loss is of great interest to the automotive industry. Three methods for detecting lubrication loss were examined: contact resistance, acoustic emission, and dynamiC pressure oscillations. A mobile air conditioning test stand capable of recording many system parameters was used. Oil return to the compressor was monitored using an oil separator and a refrigerant/oil concentration sensor. Data were taken during steady oil return rates and after oil shut off. The electrical contact resistance between the shoe and swashplate was used to indicate changes in the lubrication conditions at this critical interface. Measurements were taken at two oil return rates, during steady oil return tests. Preliminary results show that the minimum contact resistance is independent of the steady oil flow rates tested. In addition to the contact resistance, an acoustic emission (AE) sensor and a dynamic pressure transducer (DPX) were used to monitor oil film breakdown after oil shut off. Preliminary data indicate that the DPX may be an early indicator of low oil circulation. Even after several hours of operation after oil shut off, the magnitude of the contact resistance indicated no oil film breakdown had occurred. It was determined that the AE sensor is probably not an adequate indicator of lubrication breakdown. INTRODUCTION Failure of swashplate compressors in automotive air conditioning systems due to inadequate oil circulation is not well understood. The primary failure mode of swash plate compressors is scuffing at the shoe and swashplate interface. In order to better understand the scuffing event, there is a need to study the operating conditions during failure. Several methods for possible early detection of failure have been developed. These methods can be used during normal operation, start up and quaSi-steady clutch cycling. The primary method for early failure detection is a contact resistance measurement between the shoe and swashplate. Previous work by Yoon et al. [1999] has shown that a substantial reduction in the resistance across this contact indicates a destruction of the protective surface layer, ultimately resulting in a scuffing failure. In conjunction with the compressor instrumentation, suction line oil circulation rates are controlled and monitored to see the effects of varying oil circulation rates on compressor lubrication. Acoustic emission and sound pressure data are also taken as non-intrusive methods of early failure detection. Data from these measurements are correlated with the results from contact resistance and oil circulation measurements. Although these results are only preliminary, they are encouraging. Work on these techniques is still ongoing. Future work will include measurements that incorporate all standard components for an automotive air conditioning loop. Tests will include conditions where low oil return is likely. Transient conditions such as slugging during start up and clutch cycling conditions will be examined. Finally, data using these techniques may lead to a practical method for detection of lubrication failures.