Abstract
SYNOPSIS Railwaymen commonly associate low adhesion with slightly wet rails. It is shown that solid railhead debris can cause extremely low values of wheel/rail adhesion when mixed with water or other fluids. The rheological properties of common track contaminants are examined in terms of yield stress and viscosity, concentrating on mixtures of iron oxide and water. The viscosity of these mixtures is found to increase exponentially with increasing solid content. Hydrodynamic equations are then developed to assess whether the train wheel could be lubricated by a limited quantity of viscous lubricant (i.e. railhead contamination). It is concluded that there are two explanations for low adhesion caused by debris. Solid contamination can be both sufficiently strong and pliable enough to support a wheel, yet have a low shear strength. When sufficient viscous contamination is present, however, a train wheel can be hydrodynamically lubricated with the resulting coefficient of adhesion being low.
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