Study of the influence falling friction on the wheel/rail contact in railway dynamics

Abstract

The complexity of the railway interaction comes from the coupling between the train and the track introduced through the forces appearing in the wheel/rail contact area. These forces are governed by the friction coefficient through the Coulomb s law, characterised by the static and kinematic values, although most of the contact models in railway dynamics consider a constant friction along the simulations. Nevertheless, it is well known that the friction coefficient falls with the slip velocity [1,2] from a maximum point determined by the static value to a point of saturation corresponding to the kinematic value. The question under debate is if the slope of this fall since recent test-rig experiments seem to reduce it ostensibly compared to friction curves generally estimated in the literature [3]. Rudd [4] proposed this negative slope as mechanism responsible for the generation of an instability phenomenon called railway curve squeal, which has received special attention from researchers [5 9]. The self-excited oscillations that characterise this phenomenon occur when the train is passing along a narrow curve, generating a strong tonal noise in the high-frequency domain. Although falling friction is the most widely accepted mechanism, other possibilities have been proposed to explain, getting more credit the mode-coupling mechanism [10 11]. For this instability, the oscillation frequencies of two structural modes of an undamped system come closer and closer together until they merge and a pair of an unstable and a stable mode results [12 13]. This work proposes a model based on a mass-spring-damper oscillator to evaluate its stability when submitted to a variable friction curve. Considering a single-d.o.f. (degrees of freedom) model, the paper studies the unstable conditions of the slip-dependent friction that can make the steady-state unstable. The study is extended to a two-d.o.f. case with two different geometric configurations in order to analyse the influence of the geometric coupling between the normal and tangential directions arisen from the contact and if it may instabilise the system even considering constant friction.