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Automatic AAR coupling mechanisms are used worldwide for the inter-connection of rail vehicles. Coupling failures, running instability, longitudinal jerks and other issues limit the usage of these coupling mechanisms. Couplers of these mechanisms require certain slack to facilitate appropriate coupling. The presence of slack even during coupled state allows the possibility of both translational and rotational motions of couplers inside the slack regime. Resulting interaction between contours of interacting coupler heads give rise to impacts and frictional rubbing actions. This study examines the implications of coupler slack on the longitudinal dynamics of rail vehicles. A simplified two degree of freedom model of such a coupling mechanism is developed, incorporating impacts and frictional actions. The study is limited to longitudinal motion, permitting only translational movement between couplers. Intermittent interactions between profiles contours at various contact points due to possible rotation between couplers are not considered. Instead, it is approximated by defining a single contact location manually. This contact location is utilised to estimate resultant impact and friction forces along the longitudinal direction. The coupler system is excited with a harmonic force and simulations are carried out for two explicit values of excitation frequency. It is observed that intermittent impacts and stick-slips resulting from slack are significant in the longitudinal force transfer between two vehicles and cause steep variations in acceleration, which in turn become the cause for jerks of considerable magnitude. Considerably higher magnitude jerks are observed at higher excitation frequencies than those at lower excitation frequencies. It is concluded that a smaller averaging sample time for acceleration is required for realistic jerk computations.

Influence of slack of automatic AAR couplers on longitudinal dynamics and jerk behaviour of rail vehicles