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The approximate torque response model for the engagement of a wet friction clutch, developed by Berger [1, 2], was modified and enhanced. The modified Reynolds equation for the film thickness and force balance for the wet clutch pack are solved numerically. The Reynolds equation relates the film hydrodynamic pressure to the film thickness, physical properties of friction materials, and operating parameters. The torque is calculated from the film and asperity pressure distribution at the friction interface. The applied pressure and the interface temperature as a function of time during engagement are considered in the model. The correct average flow factors of Patir and Cheng [3, 4] for ATF flow between rough surfaces are incorporated into the model. The permeability of friction materials and the temperature gradient inside the friction material are considered.

A theoretical model for predicting the life cycle of a friction material used in wet friction clutches has been developed and verified. For a given friction material, the degradation mechanism can be identified by performing a Thermal Gravimetric Analysis (TGA) on the samples of worn friction materials. The samples are taken from the friction plates after they undertake various periods of the continuous slip experiments on the full-pack test machine. The degradation rate constants are obtained by performing the TGA experiments on the samples from the continuous slip experiments with different input powers and interface temperatures. The degradation for a dynamic engagement cycle is calculated by integrating the degradation rate with the temperature history near the friction interface as a function of time. The temperature history is predicted by the Borg-Warner computer model for the engagement of a wet clutch [1], which has been verified experimentally.

A Mathematical model describing the heat transfer and fluid hydrodynamics during the engagement cycles of “wet” clutches have been developed. The model equations of heat balance for the separator, friction materials, core plate, and automatic transmission oil (ATF) or air in the clearance between separators and friction materials were formulated. The momentum balance for ATF flow in the friction lining (porous medium) during the engagement was considered. Proper boundary conditions at the interface and boundary conditions at the inner and outer radius of the clutches were accounted for. The model includes the simulation of engagement cycles for both the traditional (two-sided) wet clutches and the single-sided wet clutches.