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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.

In order to prevent shudder in automatic transmissions, friction must decrease as the sliding speed between the friction plates in clutches decreases. Theoretical studies have shown that friction in wet clutches is a combination of boundary friction and the friction due to flow of fluid through the friction materials (thin-film friction). Therefore, these physical properties of oils should control the anti-shudder performance of automatic transmission fluids. Recently, we demonstrated that boundary and thin-film friction contribute to friction measured at low speeds in JASO SAE No.2 and LVFA tests. Two different friction materials are used in these tests and the relative effect of thin-film friction on low speed friction is greater in the JASO SAE No. 2 test than in the JASO LVFA test.

Friction plate degradation and/or friction plate glazing has often been related to the loss of friction control in automatic transmissions. However, in JASO SAE No.2 and LVFA tests, friction material glazing has been found to not be a sufficient condition for the loss of anti-shudder performance or a reduction in torque capacity durability. Therefore, changes in automatic transmission fluid properties rather than changes to the friction surfaces would be expected to play a dominant role in controlling anti-shudder performance and torque capacity. Earlier theoretical studies have proposed that friction in wet clutches is a combination of boundary and hydrodynamic friction. Therefore, changes in these properties should control anti-shudder durability and torque capacity. In this paper, we confirm that boundary and thin-film friction contribute to friction measured in JASO SAE No.2 and LVFA tests.

Extended gear fatigue pitting life is an essential performance requirement for today's gear oils in automotive driveline applications. One of the important industrial standard tests used to evaluate fully formulated oil's ability to extend gear pitting fatigue life is the FZG pitting test. To understand the fatigue pitting behavior in these gears we have conducted surface analyses on the FZG gears to determine fatigue modes. We have found that micro-pitting is the major fatigue mode and pitting/spalling is mostly initiated by micro-pitting in the FZG test. To help further understand how pitting and micro-pitting relate to gear oil properties and gear surface morphology, we have also carried out a statistical analysis correlating fatigue pitting life with four major physical parameters: boundary friction coefficient, oil film thickness, oil corrosiveness, and surface roughness of the gear tooth.

In automatic transmission technology development the degradation of paper friction plates has often been considered a major failure mechanism by which transmissions lose their anti-shudder characteristics. One of the most common degradation processes for paper friction plates is known as glazing. In this study, we focus on the relationship between friction plate glazing and anti-shudder durability in the Japanese Automobile Standards Organization (JASO) low velocity friction apparatus (LVFA) rig test following the procedure M349-98. We also investigate the impact of used friction plates and used oil on torque capacity durability as measured by an SAE No. 2 machine following the JASO procedure M348-95. We find that friction plate glazing has no correlation with anti-shudder durability. A completely glazed plate can have long anti-shudder durability but a barely glazed plate can have short anti-shudder durability.

Rheological properties of automatic transmission fluids (ATFs) are typically characterized by their kinematic (ASTM D 445) and Brookfield (ASTM D 2983) viscosities. However, ATFs contain polymeric viscosity modifiers, which often result in non-Newtonian fluid behavior as the polymers align and stretch under the shear stresses experienced in automatic transmissions. Therefore, the standard rheological tests, which are normally run under low shear stresses, may not adequately characterize an ATF's flow properties under the operating conditions of the automatic transmission. This study was designed to characterize the rheological properties of ATFs containing different amounts of viscosity modifiers, different base oil types and different levels of permanent shear stability under the shear and temperature conditions which exist in automatic transmissions.