Samuel H. Tersigni

Fundamentals of anti-shudder durability: Part I - Clutch plate study

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. The basic reason for the lack of correlation is that friction plate glazing does not affect boundary friction coefficients. In almost all the cases studied, changes in fluid properties have a greater influence on boundary friction than does surface glazing.

ATF Friction Properties and Shift Quality

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. To understand the relationship between ATF friction properties and shifting clutch vibration we present a theoretical model that identifies several possible causes: (1) self-excitation instability, (2) a reduction in friction holding torque during engagement, or (3) resonance caused by periodic pressure oscillations. The ATF and friction material properties that affect these sources of vibration include the friction level, μ, and the friction slopes with respect to sliding speed ∂μ/∂v, pressure ∂μ/∂P, and temperature ∂μ/∂T. These properties must be carefully balanced to ensure that a clutch will deliver good shift quality and high torque capacity with effective vibration suppression.

Low-Speed Carbon Fiber Torque Capacity and Frictional Properties Test for ATFs

Since the mid-1990s, 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.

Effect of Detailed Base Oil Structure on Oxidation Performance of Automatic Transmission Fluids

Automatic transmission fluids (ATF) should be oxidatively stable so that their frictional properties are maintained as the fluids are aged. To test the oxidative stability of ATFs, automobile manufacturers have created oxidation tests in which ATFs are aged in operating transmissions. In these tests, the total acid number (TAN) of the oil is measured throughout the test, and at the end of the test the TAN of the oil must be below specified limits. In general, oxidation of oils occurs by formation of free radicals that can react with the oils to form acidic species that are detected by the TAN of the used oils [1, 2]. Peroxides also form when an oil is oxidized and the peroxides can react with the oil to form acids [1,2]. Base oil structure, presence of wear metals, and the amount of oxygen dissolved in the oil can all affect the oxidative stability of oils [1,2]. Therefore, we investigated how each of these three factors affect changes in TAN as oils are aged in the GM cycling and GM oxidation tests (GMOT). Base oil structure is the major factor affecting the oxidative stability of ATFs. In particular, we have found that the cyclo-paraffin concentration in the base oils used to formulate ATFs can be related to oxidative stability. The lower the number of cycloparaffins in the base oil, the better the oxidative stability of the ATF.Copyright