Richard J. Vickerman

A new method to evaluate the overall anti-shudder property of automatic transmission fluids – multiple parameters spider chart evaluation

Formulating automatic transmission fluids (ATFs) can pose some diverse challenges. The friction response should ensure a smooth engagement process with no shudder in the automatic transmission. Friction, as a function of velocity, must show a smooth and positive/plateau curve as velocity increases in the μ−v plots. In this study, the friction behaviour of some basic additive systems for ATFs was assessed by using a variable speed friction test (VSFT) rig. The test was used to mimic the clutch engagement process and to evaluate the friction response of different formulations. The focus was anti-shudder properties of the designated formulations. A new method of evaluating VSFT results was developed to assess the overall ATF friction behaviour in conjunction to the common friction—velocity curve analysis. X-ray photoelectron spectroscopy was used to check the different surface conditions before and after the test.

Anti-Shudder Properties of ATFs—Investigation into Tribofilm Composition on Clutch Friction Material and Steel Surfaces and the Link to Frictional Performance

The friction response of an automatic transmission fluid (ATF) should ensure a smooth engagement process with no shudder in the process of power transmission. To study the effect of ATF chemistry on anti-shudder properties, the frictional characteristics of the baseline additive system used in ATFs with various friction modifiers and detergent combinations were assessed using a variable-speed friction test (VSFT) machine. Tests were performed on steel-versus-clutch friction material tribocouples, with the aim of studying the tribofilm formation on both friction couple (steel and friction material) surfaces and to establish the link between the lubricant-surface interactions and anti-shudder performance. An SAE No. 2 test machine was also used to investigate the friction performance of the same ATFs for comparison with the VSFT results. Similar performance rankings were obtained from those two tests. This article focuses on the posttest surface analysis and surface chemistry results of the test samples. Energy dispersive X-ray spectroscopy (EDX) analysis, X-ray photoelectron spectroscopy (XPS), and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) were used to assess the different surface conditions and chemical tribofilm compositions on the friction material and steel counterpart after the test. The mechanisms of the different friction performance provided by additive formulations and the correlation with the composition of tribofilms are discussed. Some adsorbed functional groups on the posttest sample surface, such as OH-, C-N, etc., were suggested to have positive effects on the anti-shudder performance of ATFs.

Anti-Shudder Properties of ATFs—An Investigation into Friction Modifying Mechanisms Using VSFT and SAE No. 2 Tests

In recent years, continuous slip torque converter clutches (CSTCCs) have been widely used in automatic transmissions, mainly to improve fuel efficiency. With the adoption of these devices the problem of shudder vibration has arisen. To help alleviate this problem, better anti-shudder characteristics are desired for automatic transmission fluids (ATFs). Positive μ-v curves are well recognized to correlate to the avoidance of shudder. A variable-speed friction tester (VSFT), which combines a low-velocity friction apparatus (LVFA)-type test with the ability to age an oil in situ, was used to determine friction characteristics of four model ATF formulations in a wet clutch tribosystem. The linear-defined multiple parameter spider chart ATF evaluation (LSAE) method was applied to present the ATFs overall friction results including anti-shudder performance and durability (Zhao, et al. (1)). One modified SAE No. 2 test was also used to test the four oils’ friction behavior, to compare with the VSFT results. Similar performance rankings have been obtained from those two tests. Both test results and evaluation parameters will be discussed in this article and the response will be linked to the physical parameters defining the posttest surface. The oil absorption ability of post test friction material is shown to be important to the overall friction performance. The environmental scanning electron microscope (ESEM) images and 3D profiling results showed that some surface topographical parameters, such as Vvc (void volume of the core) and Vvv (void volume of valleys), can be linked with the friction modifying mechanisms in the ATF tests.

Understanding Friction Behavior in Automatic Transmission Fluid LVFA Test: A New Positive Curve Parameter to Friction Coefficient Ratio Index Evaluation

The frictional characteristics of some basic additive systems used in automatic transmission fluids (ATFs) were assessed using a variable speed friction test machine. The test evaluates the coefficient of friction in unidirectional sliding contact over a range of speeds, loads, and temperatures. Tests were performed on a steel-versus-clutch friction material tribocouple, mainly to establish the link between the lubricant/surface interactions and additive system performance. A new parameter that measures the positive μ-v curve slope speed range was applied, together with friction coefficient ratio index, to show the antishudder property. It is suggested that this is a good complimentary method to the friction coefficient ratio index evaluation. Lubricant formulation is important to the overall friction performance. Both friction modifiers and detergents are shown to be important to the ATF antishudder performance. Oil absorption ability of the post-test friction material surface is proposed to be an important parameter to indicate the anti-shudder durability in the test. The detergent and antiwear agents are also suggested to be responsible for the decreased oil absorption and pore plugging of the friction material surface.