The Influence of Contact Pressure on the Dynamic Friction Coefficient in Cylindrical Rubber-Metal Contact Geometries

As it is commonly know, classic Coulomb’s and Amonton’s friction laws, which mainly establish that the friction coefficient is independent of the area of contact, are proven to be not valid in the case of rubber-like materials. In this particular case, and due to their specific mechanical properties, the friction coefficient should be expressed as a function of contact pressure, sliding speed, temperature and lubrication regime, if the latter were the case.

The dependence with the contact pressure is associated to the varying ratio of real (microscopic level) to apparent (macroscopic level) area of contact when the vertical load (contact pressure) is rising. The problem increases in complexity when neither the contact pressure distribution nor the ratio of real to apparent area of contact are uniform along the apparent area of contact, being the cylindrical contact geometry a typical example of this situation. In the present paper, the dependence of the dynamic friction coefficient value with the contact pressure in cylindrical rubber-aluminium contact geometries is analysed in detail.

As commented before, contact pressure distribution is not constant along the whole cylindrical area of contact and, thus, friction coefficient value has to be calculated indirectly from friction force results obtained in tribotesting by means of a mathematical method. In addition, the apparent area of contact in the cylindrical case is also not constant when the vertical load increases. Thus, the robustness of the method has to be improved by comparison of experimental measurements of the apparent area of contact with FEM results of the tribotesting, combining them with the adjustment of the rubber material model. As it will be explained along the paper, the method consists on combining FEM simulations of the tribotesting to obtain contact pressure distributions along the cylindrical area of contact for different vertical loadings, and then on develo** a mathematical procedure for obtaining a final analytical expression for the dynamic friction coefficient vs. the contact pressure. Finally, it is checked that this method provides good correlations with already existing friction models which can be found in the literature [2, 3].

Key words: Tribology, polymers, dynamic friction coefficient, contact pressure, finite element method, cylindrical contact geometry.