Influence of axle load on wheel/rail adhesion under wet conditions in consideration of running speed and surface roughness

This paper describes the influence of axle load (contact pressure) on the adhesion coefficient of wheel and rail in accordance with the running speed of a vehicle, the surface roughness of wheel/rail and wet conditions created by rain, snow, etc. The adhesion characteristics were investigated by theoretical analysis with a numerical model adopting the Elasto-Hydrodynamic Lubrication (EHL) theory and the Greenwood-Williamson׳s (G-W) stochastic model in which the surface asperity heights of contact surfaces vary according to the Gaussian distribution, as well as laboratory experiments by means of a twin-disc rolling contact test machine which simulates the contact situation of wheel and rail. The numerical calculation and experimental results indicate that the relationship between the adhesion coefficient and the axle load (contact pressure) is affected by the running speed of a vehicle and the surface roughness of wheel/rail. The adhesion coefficient increases with an increase of the contact pressure in case of smaller surface roughness and higher running speed. In contrast, the adhesion coefficient decreases with an increase of the contact pressure in case of larger surface roughness at any running speed. In the past studies concerning the adhesion of wheel and rail in the railway system by other researchers, the experimental results also indicated a similar tendency of the adhesion coefficient in accordance with the contact pressure. Consequently, it can be stated with confidence that the influence of the axle load on the adhesion of wheel and rail is not proportional, but depends on the surface roughness and the running speed of a vehicle; it means that there is a critical value of the surface roughness at which the relationship between the adhesion coefficient and the contact pressure will change in its characteristics. Furthermore, a higher running speed creates a larger critical value of the surface roughness than a lower running speed.