Development of a Laboratory Hypoid Gear Spalling Test

The laboratory tests used to define API GL5 have been the cornerstone of gear oil development for well over thirty years. In that time they have served the market very well. Lubricants developed with these test methods have provided adequate protection of axle components from severe wear, scuffing, corrosion, and oxidation. Recently, however, there has been an increasing trend toward extended drain intervals which changes the picture. Coupled with longer oil drain intervals there is a continuing increase of power throughput in the equipment. The combination of increased power and extended service life places significant stress on the oil such that the load carrying ability and thermal and oxidative stability could be greatly diminished under these conditions.

During the past ten years the industry has been actively working toward a new gear oil specification that will address the performance needs of today's vehicles. Many new or upgraded tests are already in place and are being used by the lubricant industry to develop better fluids for these applications. One key area still undefined is surface fatigue or spalling. There is currently no defined test available to predict the performance of a lubricant with respect to the surface fatigue life of a hypoid gearset. This work describes the development of a hypoid gear spalling test that is capable of differentiating oils and correlates to observed field performance.