Elastohydrodynamic Lubrication with Polyolester Lubricants and HFC Refrigerants, Final Report, Volume 2

Abstract

Lubrication properties of refrigeration lubricants were investigated in high pressure nonconforming contacts under different conditions of temperature, rolling speed, and refrigerant concentration. The program was based upon the recognition that the lubrication regime in refrigeration compressors is generally elastohydrodynamic or hydrodynamic, as determined by the operating conditions of the compressor and the properties of the lubricant. Depending on the compressor design, elastohydrodynamic lubrication conditions exist in many rolling and sliding elements of refrigeration compressors such as roller element bearings, gears, and rotors. The formation of an elastohydrodynamic film separating rubbing surfaces is important in preventing the wear and failure of compressor elements. It is, therefore, important to predict the elastohydrodynamic (EHD) performance of lubricants under realistic tribocontact renditions. This is, however, difficult as the lubricant properties that control film formation are critically dependent upon pressure and shear, and cannot be evaluated using conventional laboratory instruments. In this study, the elastohydrodynamic behavior of refrigeration lubricants with and without the presence of refrigerants was investigated using the ultrathin film EHD interferometry technique. This technique enables very thin films, down to less than 5 nm, to be measured accurately within an EHD contact under realistic conditions of temperature, shear, and pressure. The technique was adapted to the study of lubricant refrigerant mixtures. Film thickness measurements were obtained on refrigeration lubricants as a function of speed, temperature, and refrigerant concentration. The effects of lubricant viscosity, temperature, rolling speed, and refrigerant concentration on EHD film formation were investigated. From the film thickness measurements, effective pressure-viscosity coefficients were calculated. The lubricants studied in this project included two naphthenic mineral oils (NMO), four polyolesters (POE), and two polyvinyl ether (PVE) fluids. These fluids represented viscosity grades of ISO 32 and ISO 68 and are shown in a table. Refrigerants studied included R-22, R-134a, and R-410A. Film thickness measurements were conducted at 23 C, 45 C, and 65 C with refrigerant concentrations ranging from zero to 60% by weight.