A Laboratory Test to Predict Lubricant Quality as Measured by the FZG Load Stage Test

Full-scale gear tests are commonly used to evaluate the load-carrying capacity of lubricants intended for use in gearbox, axle, and hydraulic systems. The FZG load stage gear test is among the most commonly used procedures in industry, while the Ryder gear test is used to qualify certain lubricants for military needs. However, tests of this type are costly and require a relatively large lubricant sample, particularly for product development. Mathematical modeling indicates that for the great majority of lubricants, the FZG gear tooth contact produces boundary-lubricated sliding with only partial fluid film formation. Previously, an effective bench scale methodology was developed to predict the results obtained from the Ryder gear test. The procedure is referred to as the Gear Oil Scuff Test (GOST) and provides good repeatability at very low cost. Boundary lubricating quality is measured under conditions of simple sliding and the result mathematically adjusted to account for viscosity. In the present study, the GOST was used to predict data obtained using the FZG load stage test with a wide range of oil types. The laboratory procedure remains unchanged from that originally developed to predict the Ryder gear test. However, mathematical adjustment of the results was required to obtain a linear correlation, due to the different contact geometries and reporting conventions used for the FZG apparatus. Good correlation was observed with results obtained using the FZG load stage procedure, while repeatability remains unchanged from the level previously reported during development of the test.