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Approximation formulas are presented for the time response of the film thickness and torque in a wet clutch. The approximation formulas show the effects of various clutch parameters on the film thickness, the hydrodynamic torque and the asperity torque. Clutch parameters affecting the film thickness and torque include friction material characteristics, lubricant properties, the geometry of the clutch plates and the time-dependent apply pressure. The approximation formulas are obtained from heuristic curve fits of previously published and validated models. It is also shown that a positive gradient (dTf/dωslip > 0) of the friction torque, Tf, with respect to slip speed, ωslip, promotes friction stability. This stability gradient is obtained analytically using the approximation formulas so that the effects of the clutch parameters on friction stability are also shown.

The paper highlights the use of a light duty axle efficiency test for evaluating the fuel economy performance of automotive gear lubricants. Both final peak axle temperatures and torque efficiencies are recorded for several multigrade automotive gear lubricants. The dependence of temperature on torque efficiencies for the gear lubricants tested are discussed for a variety of driving conditions: city, highway and severe service. Temperature and torque efficiency data show strong dependence on additive system and viscosity- temperature characteristics of the gear lubricants under different driving conditions. A discussion of lubricant rheology and its importance to maintaining film strength for adequate bearing and gear lubrication as related to optimum torque efficiency and axle temperature under varying loads and pinion speeds is also provided.