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 antishudder 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.