A study of the lubrication of cylindrical roller bearings with oil-refrigerant mixtures using capacitive oil-film measurements

Traditional methods for life calculations for rolling element bearings have shown limitations when used for bearings in refrigeration applications. There is a need for design data that can be used in the selection of bearings and the design of lubrication systems. There is also a need for diagnostic methods that will reduce the need for long endurance tests in order to verify the integrity of the bearing arrangement and the lubrication system. A bearing test apparatus has been modified to test a SKF NU 209 ECM radial rolling element bearing under conditions similar to those found in refrigeration screw compressors. The bearing is mounted so that it can experience a pure radial load. It has been tested at loads of 1, 2, 3 and 4kN at speeds between 500 and 6000rpm. The lubricant used was a mixed-acid polyolester with a base oil viscosity of 220cSt. The lubricant was diluted with R-134a to obtain viscosities between 9 and 55cP. The lubrication of the bearing was studied by measuring the capacitance of the contact between the rollers and the races. The results from the test show a strong correlation between the measured capacitance with load, viscosity and speed. The results clearly indicate under what conditions bearing wear occurs due to aspherity contacts in the bearing. The work has proven that this method is very useful in predicting bearing wear in refrigeration applications.