Multiple plate disc clutches are used extensively for shifting gears in automatic transmissions. In the active clutches that engage or disengage during a shift the automatic transmission fluid (ATF) and friction material experience large changes in pressure, P, sliding speed, v, and temperature, T. The coefficient of friction, μ, of the ATF and friction material is a function of these variables so μ = μ(P,v,T) also changes during clutch engagement. These changes in friction coefficient can lead to noise or vibration if the ATF properties and clutch friction material are improperly matched.
A theoretical understanding of what causes noise, vibration and harshness (NVH) in shifting clutches is valuable for the development of an ATF suitable for a particular friction material. Here we present a theoretical model that identifies the slope, ∂μ/∂T, of the coefficient of friction with respect to temperature as a major contributor to the damping in a clutch during engagement. The model predicts that negative ∂μ/∂T during clutch engagement increases the damping and reduces the risk of self-excited vibration or other NVH phenomena. Experimental data supporting the model are presented.
In the thin film or hydrodynamic regimes of friction, ∂μ/∂T is typically negative since fluid viscosity and shear stress generally decrease as the temperature rises. However, the final lockup of an engaging clutch is controlled by boundary friction where ∂μ/∂T is normally positive. Consequently, in the boundary friction regime the ATF additive chemistry is important for suppressing NVH during clutch engagement by achieving negative ∂μ/∂T with a particular friction material.