Oxidation of Axle Lubricants - Laboratory Studies and a Model to Predict Service Life

The continuing push for improved fuel economy, reduced carbon emissions, and lower operating costs has resulted in higher operating temperatures for axle lubricants in passenger cars and commercial vehicles. These higher operating temperatures, in turn, have placed more severe demands on the thermal & oxidative performance of axle lubricants. A number of industry-standard, laboratory methods exist to evaluate this key performance parameter. This paper discusses the use of laboratory methods to evaluate oil service life. We examine the behavior of five commercially available axle lubricants in the CEC L-48 oxidation test (Apparatus A). The oils were chosen so that different additives and base oils could be compared. We evaluated both the effect of time and temperature on the oxidation behavior. In agreement with previous studies, we found that infrared (IR) spectroscopy provides a convenient and meaningful way to track the extent of oxidative degradation. We define the "breaking point" of an oil as the value of IR Peak Area Increase (PAI) at which deposits first appear. We propose a simple model for estimating the total endurance life of an axle lubricant by dividing the breaking point by the rate of oxidation as measured by PAI.