Transient wheel–rail adhesion characteristics under the cleaning effect of sliding

Investigation of traction and braking phenomena in rail vehicles requires proper modelling of friction in the wheel–rail contact in the mode of full sliding. Adhesion characteristic with falling velocity-dependent friction is therefore often employed. Such a function however still describes a steady state, steady dry friction conditions. It is observed that the conditions may be variable, though; energy dissipation in the contact area brings a cleaning (conditioning) effect marked particularly at low adhesion and improving friction above the initial level. The present paper explains the role of this effect in measured wheel–rail adhesion characteristics obtained in laboratory experiments. They are understood and evaluated as transitions between steady characteristics for contaminated and cleaned surfaces. Mathematical description of the transition is proposed, suitable for use with any contact model using the coefficient of friction as an input.