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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.

This report focuses on an extended investigation of the durability of Dual Clutch Transmission (DCT) fluids. The performance requirements of DCT fluids differ from those of traditional step automatic transmission fluids. For that reason, key performance lab tests are discussed in this paper. Friction durability is measured with a modified version of the JASO M348 SAE#2 friction plate test. In addition, results from a vehicle chassis dynamometer test are discussed. This test involves running a 2008 Volkswagen GTI for 60,000 dynamometer miles (42,000 cycles) of severe acceleration and high speed conditions. Finally, a new DCT fluid, which performs well in these tests, offers friction stability and superior wear protection of transmission hardware, when compared to the commercial reference fluid.

This paper reports its findings in three separate parts. First, a comparative study is made among existing commercial dual clutch automatic transmission fluids (DCTFs). Significant differences in fluid torque capacity, friction material compatibility and copper corrosion performance were found among the fluids. Second, both a new vehicle chassis dynamometer durability test and a SAE#2 durability procedure are offered, specifically designed for DCTs. A 2008 VW GTI did well in the severe 60,000 mile chassis dynamometer procedure. Third, a new DCT fluid is discussed.

1 Fluids for new generations of step-automatic transmissions must provide durable service under severe conditions in a variety of environments. Fluid degradation under severe stress can lead to changes in frictional properties, potentially resulting in undesirable noise, vibration and harshness (NVH) events. This paper describes the development of a new transmission fluid that delivers significant improvement in squawk durability. The formulation approach resulted in optimum friction characteristics that are essential to overcome stress-induced loss of friction and to reduce NVH. A factorial design of experiments was used in the development process to relate additive effects with friction characteristics of both fresh and aged fluids. Friction durability after laboratory aging was compared with friction characteristics and durability data obtained from field-aged fluids

Multiple plate disc clutches are used extensively for shifting gears in automatic transmissions. In the active clutches that engage or disengage during a shift the automatic transmission fluid (ATF) and friction material experience large changes in pressure, P, sliding speed, v, and temperature, T. The coefficient of friction, μ, of the ATF and friction material is a function of these variables so μ = μ(P,v,T) also changes during clutch engagement. These changes in friction coefficient can lead to noise or vibration if the ATF properties and clutch friction material are improperly matched. A theoretical understanding of what causes noise, vibration and harshness (NVH) in shifting clutches is valuable for the development of an ATF suitable for a particular friction material. Here we present a theoretical model that identifies the slope, ∂μ/∂T, of the coefficient of friction with respect to temperature as a major contributor to the damping in a clutch during engagement.

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.

To develop an automatic transmission fluid (ATF) that meets DEXRON® III-H specifications, the ATF must pass two critical tests, the GM oxidation test (GMOT) and the GM cycling test (GMCT), in addition to many other performance tests. The specification on the GMOT is that delta TAN (difference in total acid number compared with the fresh oil) at the end of the test does not exceed 3.25 while the specifications on GMCT are that delta TAN cannot exceed 2.0 and the 1-2 shift time must stay between 0.30 and 0.75 seconds throughout the test. For this work, we analyze oil oxidation and changes in oils' surface tension, drum and band surface degradation and deposit formation. We have found that with respect to the delta TAN limits of the DEXRON® III-H specification, the GMCT is more severe than the GMOT. The effect of base oil chemistry on oxidation in these tests has been quantified. Oil oxidation is not responsible for the GMCT 1-2 shift time increase.

Multiple plate disk clutches are used extensively for shifting gears in automatic transmissions. In a shift from one gear to another one or more clutches is engaging or disengaging. In these active clutches the automatic transmission fluid (ATF) and friction material experience large changes in pressure P, temperature T, and sliding speed v. The coefficient of friction, μ, of the ATF and friction material depends on v, P and T, and also changes during clutch engagement. Changes in μ can lead to vibration and poor shift quality if the ATF and clutch friction material are improperly selected. An in-depth theoretical understanding of the cause of vibration in shifting clutches is crucial in the development of a suitable ATF to work with a particular friction material.

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.

Since the mid-1990's, original equipment manufacturers (OEMs) of automobiles have been implementing torque converter clutches in automatic transmissions with a continuous, controlled slip mode, in order to improve the fuel economy of their vehicles. These Continuously Slipping Torque Converter Clutches (CSTCCs) are prone to an undesirable phenomenon commonly called shudder. This phenomenon has been attributed to specific shapes or slopes in the friction coefficient versus sliding speed curve of the fluid/clutch interface. Here, a method is explained that was developed to be able to screen fluids for shudder tendency, both in fresh and used states. Also included is a description of the reason for implementing CSTCCs, some background on shudder, and supporting data showing how the test method can distinguish between fluids that have different shudder tendencies.

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.

A model of an automotive powertrain equipped with a slip-controlled torque converter clutch (TCC) is presented that incorporates the clutch control system and the friction-related properties of the automatic transmission fluid (ATF) and clutch friction material. Prior research has established that stability of a slip-controlled TCC is enhanced by maintaining a positive slope of the coefficient of friction, μ, with respect to sliding speed, v. The model presented here agrees with this result, but suggests that it is neither a necessary nor sufficient condition guaranteeing stability. The model indicates that other factors affecting stability at the equilibrium sliding speed include the magnitude of μ, the engine speed, the engine torque-speed slope, the ATF pressure, and the time constants of the clutch control system. This model will aid in the development of future wet clutch systems with improved friction stability performance.

This study focused on two areas of interest to CVT fluid developers. The primary set of experiments investigated whether continuously variable transmissions (CVTs) and their conventional step-type automatic transmission counterparts stress their fluids differently. The investigation compared the push-belt CVT (PB-CVT) in a '98 Nissan Bluebird to the General Motors (GM) Hydra-Matic 4L60 (four-speed) automatic transmission. Chemical, physical and performance comparisons of fresh and used fluids revealed the fluid stresses from the Nissan Bluebird CVT were similar to those from the 4L60 transmission. A second set of tests probed the question of how well results from two standard steel-on-steel friction bench tests correlate to CVT dynamometer (belt box) findings. The bench test models captured about half of the critical trends in the belt box data, as evidenced by prediction R2 values in the 0.5 to 0.7 range.