ATF Friction Properties and Shift Quality 2004-01-3027
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.