The Slow-Speed Wear Behavior of Case-Carburized Gears Lubricated with NLGI 00 Grease under Boundary Lubrication Conditions

NLGI 00 greases are often used to lubricate gears running at low pitch line velocities, such as, for example, in large open gear drives. At low pitch line velocities, sliding wear, which under these operating conditions is referred to as slow speed wear, is often the limiting factor to gear lifetime. A thorough knowledge of the effect of different grease components on the wear behavior is therefore important when selecting a grease to effectively reduce gear wear in a given gear drive. In order to systematically investigate and analyze the influence of different grease components on the slow-speed wear behavior of case-carburized gears, systematic gear tests using the Gear Research Center's (FZG) back-to-back gear test rig were conducted. Primarily, the focus of the experimental investigations is on the influence of the base oil viscosity and type, the additive type, and also the type of soap thickener on the gear wear behavior at low pitch line velocities. To experimentally determine the influence of these different grease components on the wear behavior of case-carburized gears, a modified, more stringent wear test, based on the standard DGMK slow-speed wear test for gear oils, was developed. Different NLGI 00 greases with base oil viscosities between ν₄₀ = 70 and 1,200 mm²/s were investigated.

Base oil type and base oil viscosity were shown to have only a minor effect on the wear behavior under boundary lubrication conditions. On the other hand, the thickener type and especially the additive type play an important role in determining the wear behavior.