Influence of train axle load on wheel-rail interface friction

Abstract

The implementation of high speed railways in India are in their early phases and vast studies are being conducted for the effective implementation of the system. Maintaining the traction required for efficient operation of trains at high speeds requires the net minimum frictional coefficient to be maintained at the wheel-rail interface, across the desired range of axle loads and target speeds of train operation. With the increase in speed, wheels of the train tend to lift-off, effectively reducing the interface friction at the wheel-rail contact. In this study, the effect of increasing the axle load on the effective frictional coefficient available at wheel-rail interfaces during high speed railway operations were quantified by numerical simulations, and the results reveal that the net frictional coefficient available at the wheel-rail contacts have a trend to increase with an increase in the applied axle loads. Conversely, this instead points out that decreasing the axle loads for achieving higher speeds of train operation will result in reduction of traction, and there will be chances of wheel-slips on the rails during rapid acceleration and emergency braking situations. Modifying the rail surface friction using top-of-rail friction modifiers are a much easier alternative to making amendments in the wheel-rail profiles, for maintaining sufficient traction on rails. In the wheel-rail interfaces with high friction, lubrications in the boundary regime, mixed lubrication or hydrodynamic lubrication are applied on the basis of Stribeck curve, adopting the optimum frictional coefficients for avoiding rolling contact fatigue and high rates of wear due to plastic deformations. When there is less frictional grip, a coating of top-of-rail positive friction enhancers becomes necessary, for increasing the adhesion characteristics and achieving positive creep. This study emphasizes the importance of optimizing the contact at wheel-rail interfaces, to maintain the frictional coefficients to neither cause loss of traction due to lower grip, nor be on the higher side to cause rolling contact fatigue and high rates of material wear.