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Several new generation friction materials with high energy, high coefficient of friction; good μ-v characteristics and durability were presented. These new generation materials were developed based on fundamental tribological friction interface phenomena understanding through various predictive models as well as chemical and physical characterization methods for friction materials / fluid interface interactions. Such new generation materials include desired characteristics for various wet clutch applications, e.g., wet start clutches, shifting clutches, and continuous slip clutches.

A new successful friction material was developed to operate in the severe high-energy environment of the slipping clutch, including wet start clutch, continuous slip and limited slip applications. The new slipping material was based on the understanding of the interfacial phenomena occurring during the slipping operations. The resulting friction interface (friction material and lubricant combination) possesses the necessary heat dissipation, good shudder resistance and good launch feel. Bench, dynamometer and vehicle test results are presented and the underlying interfacial phenomena are discussed.

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.

The physical and chemical properties of a mineral-based (Fluid-M) and a partial-synthetic-based (Fluid-PS) automatic transmission fluid were compared by the analyses of Gel Permeation Chromatography (GPC), Thermogravimetry (TG), Fourier Transform Infrared Spectroscopy (FTIR), Nuclear Magnetic Resonance (NMR), Gas Chromatography (GC), viscometry, thermo-oxidative stability, torque response curve shape and metal-to-metal wear preventive characteristics. The effects of various properties of Fluid-M and Fluid-PS on wet friction material performance were investigated from the viewpoints of compressibility, durability, tensile strength, surface interactions and friction-pressure-speed-temperature characteristics. Friction material specifications for partial-synthetic fluid applications will be different from those for mineral-based fluid applications. GPC showed that Fluid-PS has a higher concentration and a lower molecular weight of VI Improver than Fluid-M.

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.

The basic friction material design considerations for continuous slip torque converter clutches are discussed. A bench test method was developed to predict the performance of continuous slip clutches at different temperatures, pressures, and velocities. A test machine which can analyze a full-sized friction plate was also designed. Two types of shudder exist in continuous slip clutches: “initial shudder” and “long-term shudder”. These shudder phenomena were investigated. The results indicated that the following factors influence the initial shudder: adsorption capability, elasticity, and “damping” properties of the friction materials, as well as the oil temperature and the slipping velocity. The long term durability and shudder resistance were found to be affected by the heat transfer capability and the surface treatment of friction material.

The techniques for characterization of the shear strength of friction materials were reviewed and a new double-lap-shear test method was developed. Various types of friction materials were tested for shear strength by this new technique. The effects of different material properties and processing factors such as resin type, resin content, resin cure rate, fiber geometry, fiber content, and test environment on the shear strength of the friction materials were also examined. Positive correlations between the shear strength and the mechanical life of wet friction materials were established. Accurate determination of shear strength in wet friction materials is vital in predicting the performance and durability of friction materials for friction plates, transmission bands, and torque converter lockup clutches.

Separator plates are utilized in clutch or brake packs for transmissions in vehicles. These plates are internally or externally splined to be operatively connected to the rotatable and/or stationary members in a torsional coupling. When pressure is applied during an engagement, separator plates mate against friction assembly plates and generate torque. Improvements can be made in the separator plate processing and finishing which will improve friction characteristics. The friction coefficients which result when clutch or brake packs are brought into contact are influenced by factors such as type of lining material, type of oil, temperature, velocity, and loading. Traditionally, varying the composition of the friction material on an assembly plate was a means to obtain different friction coefficients and in turn torque capacity. Additionally, there are other factors such as the separator plate surface finish that may affect the friction characteristics.

A number of papers published in the past several years (1, 2, 3) have sought to quantify transmission band performance parameters. Most recently researchers have explored the effects of temperature and lubrication levels, and correlation of these factors with vehicle testing (4). For the most part, these papers have focused on optimization of existing band and drum systems, and have assumed that the contribution of friction material to overall system performance remains constant. Historically, most research concerning friction material performance has focused strictly on energy level per unit of surface area. One recent research effort (5) has found that the speed and inertia components of shift energy are at least as important as the energy itself. In either case, friction coefficients have been assumed constant for a given energy level, while variations in the speed and inertia components of that energy level have not been emphasized.

Two important phenomena which govern the performance of wet friction composite materials operating in transmission oils have been examined. Both the elastic deformation behavior, and the liquid permeability of fiber reinforced polymeric wet friction composite materials are important input parameters to any accurate oil flow model which predicts the interface temperature of wet friction clutch facing polymeric materials. Liquid permeability, pore size, and pore size distribution of three different classes of wet friction materials were measured by a recently developed automated pore permeameter. Darcy's Law for laminar flow through homogenous porous media can be used to describe the anisotropic liquid permeability of wet friction materials. The elastic deformation of the three classes of wet friction materials was also determined.