Determination of wheel-roller friction coefficient on roller rigs for railway applications

Railway technology is strongly influenced by the friction coefficient between wheel and rail. Friction plays a key role in many aspects of railway dynamic such as derailment, wear, noise, stability and is the most important factor to be considered in traction and braking problems. The railway system is based on the interaction of the profiles of wheel and rail, and their relative motion is subject to the presence of friction, which produces tangential forces acting on the vehicle and determining its dynamic behaviour. In case of traction and braking, the presence of friction is adopted in order to generate the required traction and braking force, and the study of friction in this field is indicated as adhesion and it is fundamental to define the vehicle performance. Since the railway system is subjected to the environmental effects, the friction coefficient can vary due to the presence of contamination, such as water, rust or weaves on the rail surface. The determination of the friction coefficient is usually obtained through complex and expensive experimental tests performed on the track using a real vehicle. Those tests are mandatory to verify the vehicle safety especially when active system are adopted to control traction and braking (WSP). An alternative to the use of test on the track, is the adoption of test on a roller-rig, a dynamic track simulator where the track is replaced by rollers. The roller rig allows to recreate in a laboratory environment test conditions comparable with the track, but with better repeatability, lower costs and higher safety. Both full scale or scaled similitude prototypes can be used on roller rig, and this strategy was adopted since the beginning of the 19th century to study traction by Carter. In the present work, a simple and efficient laboratory methodology for the estimation of the value of static friction coefficient on roller-rig is proposed. An accurate determination of the static friction coefficient is mandatory when a roller rig is adopted to study adhesion, wear or vehicle dynamic, in order to be able to perform different tests in known initial condition.