This article has considered the possible physical mechanisms underlying the characteristic friction-sliding speed behavior of wet clutches; that is, high friction that increases progressively with increasing sliding speed. Despite the importance of this friction behavior in determining wet clutch performance, its origins are still not fully clarified in the literature, despite considerable previous research. A crucial feature of wet clutch contacts is that the very rough, fiber-based morphology of the friction material results in the friction material/steel contact consisting of many tiny contact units separated by regions where the gap between the friction material valleys and steel is relatively large. This large gap, coupled with the porosity of the friction material, means that it is impossible for a significant hydrodynamic pressure to be generated within the overall contact to separate the contact units. Furthermore, the very small size of the contact units means that negligible hydrodynamic film can build up within the contact units, at least up to a sliding speed of 1 m/s. This means that the friction behavior is controlled by the shear stress of the boundary lubricating films present on the steel and friction material surfaces within the contact units. These boundary lubricating films are created by the additives present in the ATF (automatic transmission fluid), the most important of which is the organic friction modifier, which provides the important characteristics of low friction at low speed and friction that increases with sliding speed. The authors suggest that this behavior originates simply from the fact that the shear stress of most organic friction modifier boundary films increases with the logarithm of the sliding speed. This has been previously reported for Langmuir-Blodgett and self-assembled monolayers and has also been quite widely reported in molecular modeling work on monolayers and in the AFM literature. However, it does not appear to have been previously considered in the context of ATFs and wet clutches. The role of other 'friction-enhancing' additives used in ATFs, such as dispersants and detergents, is probably to partially disrupt the absorbed monolayer produced by the organic friction modifier to raise the latter's friction over the whole speed range while maintaining the required positive friction/log(sliding speed) gradient. When friction material is rubbed against steel in dry conditions, friction is found to increase with increasing sliding speed. This means that the high friction measured at low speeds with base oil and some very weak boundary lubricating liquids is not simply due to lack of an effective boundary film. Instead, it may be due either to the base oil inhibiting the formation of a resin transfer film from the friction material on to the steel surface or, more likely in the authors' opinion, to capillary forces enhancing the effective load within the friction material-steel contact at low speeds.
Frictional properties of automatic transmission fluids: Part II Origins of friction-sliding speed behavior
Tribology Transactions ; 54 , 1 ; 154-167
2011
14 Seiten, 21 Bilder, 2 Tabellen, 51 Quellen
Aufsatz (Zeitschrift)
Englisch
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