Mixed Lubrication in High Loaded Squeeze Contact

We analyze the influence of surfaces roughness on the strongly non-stationary squeeze process of an oil film sandwiched between the chain-pin and the pulley in continuously variable transmissions. As recently demonstrated for a Newtonian oil in the elasto-hydrodynamic conditions [G. Carbone, M. Scaraggi, L. Soria, ASME Journal of Mechanical Design, 131 (1), 2009], the spatial fluid pressure distribution is characterized by a non-central annular peak, which firstly appears in the outer region of the contact domain and moves toward the center of the pin with rapidly decreasing speed. In particular, an high-viscosity oil central dimple is generated, which is able to keep separated the solid surfaces, avoiding direct metal-metal interactions for typical pin-pulley travelling time of 0.01s. In this work we include in the lubrication model the interactions between asperities of randomly rough surfaces, adopting as solid contact model the Persson's mean field contact theory. We show that the coupling between oil rheology and surface roughness determines a wide range of lubrication conditions for high-pressure squeeze contacts. In particular we find that increasing the surface roughness anticipates the transition time at which the lubrication regime changes from a fully lubricated to a mixed lubricated regime. The knowledge of such peculiar behaviour is of fundamental importance to determine the pin-pulley frictional and wear properties.