Sliding contact between friction surfaces occurs in numerous torque transfer elements: torque converter clutches, shifting clutches, launch or starting clutches, limited slip differential clutches, and in the meshing of gear teeth under load. The total temperature in a friction interface is the sum of the equilibrium temperature with no sliding and a transient temperature rise, the flash temperature, caused by the work done while sliding. In a wet shifting clutch the equilibrium temperature is typically the bulk oil temperature and the flash temperature is the temperature rise during clutch engagement. The flash temperature is an important factor in the performance and durability of a clutch since it affects such things as the reactivity of the sliding surfaces and lubricant constituents (e.g., oxidation) and thermal stress in the components. Knowing how high the flash temperature becomes is valuable for the formulation of ATF, gear oil, engine oil and other lubricants.
This paper presents two models for approximating flash temperature in clutch interfaces for: (1) a large thermal mass with a good conduction path to an equilibrium temperature boundary, and (2) a small thermal mass without a good conduction path to a constant temperature boundary. The explicit expressions given for flash temperature in terms of material and friction properties are useful for applications such as estimating thermal effects on the lubricant and changes in the friction properties and clutch torque. Friction heating expressions for different slipping conditions are given and are used in the models to predict surface flash temperatures for specific cases: (a) plate #1 (front steel plate) in SAE #2 rig tests such as the MERCON® V and DEXRON® III H friction tests, (b) plate #3 (interior steel plate) in SAE #2 tests, and (c) the steel plate in an LVFA test.