Friction in wheel - rail contacts

Abstract

In the autumn, railroad traffic is often interrupted due to the occurrence of slippery tracks. The Dutch main operator (NS) and the infrastructure manager (ProRail) struggle with repeated delays. The layer between wheel and rail causing this phenomenon has not yet been identified, but can be quantified by rheological properties determined by correlating the measured friction between wheel and rail and the developed friction model. To this end, the research described in this thesis is twofold: theoretical; in which the wheel - rail contact is modelled in terms of contact and friction, and experimental; in which the friction between wheel and rail is measured to validate the developed model. The contact model between wheel and rail is approximated to be elliptical, which is shown to be accurate for the described purpose. The low friction situation is assumed to be caused by an interfacial layer, which is acting like a lubricant. Combining these two assumptions, a mixed lubrication friction model is developed for the Hertzian elliptical contact situation in which the interfacial layer is acting as a lubricant governed by the Eyring model. The mixed lubrication friction model results in the so-called Stribeck curve and/or the traction curve, which both take frictional heating and starved conditions into consideration. In addition, the changes in attack angle are also taken into account and a general viscoelastic model is proposed for both interfacial and boundary layers. For the experiments, two devices were developed to be mounted on a train and successfully used on the real track. One is a sliding sensor which measures the coefficient of friction between a curved shaped specimen pressed against and sliding along the rail, i.e. simple sliding conditions. A second device measures traction curves between a measuring wheel and the head (top) of the rail, i.e. rolling/sliding conditions, at velocities of up to 100 km/h. Based on the results from the measuring campaign combined with the modelled wheel - rail friction model, conclusions were drawn and important recommendations made to both the Dutch train operator (NS) and the rail infra manager (ProRail) from theoretical and practical points of view.