Measurement and modelling of friction of rubber

It can be difficult to find relevant data for the coefficient of friction for situations involving rubber as one of the contact bodies, and so rather arbitrary values are often used in FE analyses. A review of the literature reveals the reason: rubber friction depends not only strongly on factors such as surface contamination, velocity and temperature, but also the commonly assumed proportionality between frictional stress and normal stress is in fact not generally obeyed. Friction results for normal stresses ranging up to high values, typical of engineering applications, are presented for natural rubber sliding on steel tracks. Models for the dependence of friction on normal stress are compared with the results and it is concluded that the empirical equation of Thirion (1946), while imperfect, has the merits of simplicity and of providing a reasonable fit over a broad range of normal stress. This equation implies that the frictional stress asymptotes to a maximum value as the normal pressure is raised. FEA results are also presented, highlighting the complications involved in modelling rubber friction.