An investigation of conventional glassware techniques for determining the thermal-oxidation stability of gear lubricants revealed a lack of correlation with field service results. Therefore, a new apparatus and procedure were developed to provide industry with a reliable method of qualifying military gear lubricants. The history of the technique development is presented in this paper as well as test data. The results show that the thermal-oxidation stability of gear lubricants may be satisfactorily predicted with good repeatability and the test data agree with field results in both order and magnitude of rating.
Following the development of the test technique, a new project was initiated to enhance existing knowledge of temperature and oxygen influences on the thermal-oxidation phenomena using the apparatus previously described. The effect of oxygen was studied by conducting tests with oxygen, air, and various inert gases while keeping the test oil temperature constant. The temperature effects were determined by conducting tests at various temperatures while metering air and other gases into the gearcase. The results of this study showed that oxygen, air, helium, and nitrogen atmospheres made little, if any, difference in the viscosity and oxygenated material content of finished oils. Oxidation of base oils, however, was accelerated by oxygen and retarded by nitrogen and helium atmospheres. The effect of gear meshing appeared to significantly influence thermal-oxidative degradation although the reaction was not catalyzed by the steel test gears or copper. Deposition was influenced by oil composition and test conditions and did not appear to correlate with used oil properties. Polymerization had a strong influence on the degradation process, and the critical temperatures of several finished lubricants were determined. The additives and a number of typical additive components were shown to inhibit the thermal-oxidation process.


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