The wet clutch system (WCS) is a complex combination of friction plates, separator plates and fluid (lubricant). The basic function of the WCS is to transfer torque under various operating conditions such as slipping, shifting, start/launch and/or torque converter clutch (TCC) operation. Under these conditions the slope of the coefficient of friction (μ or COF) versus slip speed (μ-v) curve must be positive to prevent shudder of the WCS, a highly undesirable condition in the lubricated friction system. An extended durability duty cycle test procedure is required to evaluate the WCS during which the μ-v curve is monitored for a negative slope, a condition indicating the potential for shudder. The friction plates, separator plates, and lubricant must be tested together and remain together during the test to be properly evaluated as a WCS.
This paper describes a new test procedure which builds on the basics of the SAE J2964 - Low Speed Continuous Slip μPVT Procedure [1] by adding a durability duty cycle to age the WCS. The test includes measurement of the μ-v curve at several intervals during the test (before break-in, after break-in, and after each 12-hour aging segment of testing). The aging segments are run at a specific continuous slip speed and constant power condition. The lubricant temperature is also controlled and the plate interface temperatures are measured. The dμ/dv slope is calculated at different speed intervals and graphed to indicate when the slope changes from positive to negative. A negative dμ/dv slope can indicate a potential for shudder in the WCS. This new procedure can be used to compare the performance of different friction material and lubricant chemistry combinations by analyzing the resulting dμ/dv change over time. It can also assess the effect of changes in the groove pattern design, friction and separator plate surface finish and separator plate materials on the dμ/dv slope.