...From this equation the mean friction coefficient over the clutch face can be calculated, see Fig. 2.3 c). This friction is the total friction containing both viscous and adhesive effects. T„(t) = FN(t)-u.(t)-r. According to an explanation by Andersson [17] the engagement consists of three different phases. Hydrodynamic squeeze is the first phase, where the clutch plate approaches the separator plate and forces oil to flow over and through the paper based material, thus creating a hydrodynamic squeeze pressure which supports the normal load and prevents physical contact. This means full-film lubrication. The friction coefficient versus time, |l(t), rises rapidly during the initial squeeze film phase. The oil film becomes thinner, the shear velocity increases and therefore the shear force and friction also increase. The next phase is called the squash film phase because oil is squashed out from the interior of the porous material. Here some asperities produce adhesive friction. Both the oil film pressure and the asperity contacts support the load. This is a form of mixed lubrication. At the beginning of the squash phase the sliding velocity is high. Oil is still retained in the porous material, but the film is thin. According to hydrodynamic theory, this leads to a high shear force and thus high levels of viscous friction. When the velocity decreases, the hydrodynamic forces will also decrease since the oil film is mainly constant. Increased temperature will lead to decreased viscosity and thus to decreased hydrodynamic forces. In this phase adhesive contact also takes place and must be added to the friction. While the friction is decreasing, the hydrodynamic action is dominant. Finally in the adhesive phase all the load is supported by physical contact and the hydrodynamic effects are minimal. This is a boundary lubrication situation. At the end of the adhesive phase the friction rises considerably. This is when the sliding velocity approaches zero and the friction approaches static friction. In the adhesive phase the material parameters and the additives have a dominant effect on friction, and bulk oil properties are less important. There are no distinct limits between these phases. Later other phenomena have been added to this basic explanation, such as micro-cavitation and bubbles visually seen in the contact by Sanda et al. [18]. Fluid confined in very thin films leads to a tremendous raise in viscosity, see Granick [19], which may explain the rather high viscous friction, see Anleitner [20]...