Bearing lubrication of refrigeration screw compressors

University dissertation from Luleå : Luleå tekniska universitet

Author: Roger Tuomas; [2001]

Keywords: Machine Elements; Maskinelement;

Abstract: The bearings in a modern refrigeration screw compressor are lubricated with a mixture of oil and refrigerant. However, little or no published bearing life data is available for the new generation non-chlorinated refrigerants. The work presented in this thesis concerns the development of a measuring technique and experimental equipment for bearing life studies. The equipment is intended to provide data about bearings lubricated with mixtures of oil/refrigerant for use by compressor designers. Bearing life is affected by the working lubricant's ability to form a film to separate the contact surfaces. To provide a sufficient film thickness in an elastohydrodynamic lubricated (EHL) contact, the viscosity, h, and pressure-viscosity coefficient, a, of the lubricant both play an important role. The film thickness in an EHL contact lubricated with an oil-refrigerant mixture with increasing amounts of refrigerant and different load ratios have been measured experimentally. The lubricant mixture tested consisted of a VG68 polyolester refrigeration oil, Solest 68, with R-134a as refrigerant. To measure the film thickness, an on-line capacitance method, Lubcheck, developed by SKF was used. The refrigerant dilution was increased until the lubricating film broke down and asperity contact occurred. The refrigerant's influence on the rheological properties of the oil was measured in a high pressure Höppler viscometer. R-134a, R-32, R-410a and R-22 refrigerant were mixed with a VG68 polyolester oil, Mobil 68 Arctic. Measurements showed that the viscosity and pressure-viscosity coefficient decrease with increasing dilution by the refrigerant. The work showed that film formation in contacts lubricated with oil/refrigerant mixtures is more sensitive to load than other investigations have indicated. A test procedure has been developed to determine the refrigerant concentration at which metal-to-metal contact occurs. The results also showed that run-in behaviour also appears in bearings used in refrigeration applications.

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