A scaled roller-rig for investigation on adhesion recovery

Contaminants lying on the track (water, oil, leaves, etc.) can reduce the wheel-rail adhesion level to extremely low values, with serious drawbacks, namely reduction of traction\braking performances, high creep values and severe wear of wheel profiles. However, in degraded adhesion conditions, the work of the friction forces removes the contaminant from the wheels (wheel adhesion recovery) and from the rails (rail adhesion recovery), thus partially restoring dry adhesion levels on the wheelsets. The term adhesion recovery can be used to refer to the effects of both phenomena, since they are not easy to distinguish in normal train operations. A deep knowledge of adhesion recovery in full slip conditions would be fundamental to improve contact models as well as WSP and antiskid algorithms. The investigation of the wheel adhesion recovery phenomenon was performed by many researchers with full-scale roller-rigs. The typical configuration of roller-rigs is not suitable for rail adhesion recovery studies, which require a configuration with many wheelsets running over the same surface. Therefore, the authors developed a new 1:5 scaled multi-axle roller-rig with four wheelsets running over the same pair of rollers, thus representing the four wheelsets of a two-bogie vehicle running on the track. A braking system allows to control independently the braking torque on each wheelset, while the rollers are motorized by a 6 poles brushless motor controlled in servo mode. This configuration allows to reproduce different creep levels to study the whole adhesion characteristic. The paper describes the mechanical design of the bench with great attention to the control logic and the acquisition of data measured by several sensors. The new roller-rig configuration allows to perform three kinds of experimental tests, namely simple adhesion curve tests, wheel adhesion recovery tests and rail adhesion recovery tests. Then, attention is given to presentation of the experimental results obtained in the three types of tests. Simple adhesion curves are presented for both speed and torque control modes, showing a good repeatability and a trend just as witnessed in the literature. Wheel adhesion recovery tests in wet conditions showed an adhesion recovery in the backwards (i.e., decreasing creep) cycle, but this phenomenon proved to be significantly affected by the roller speed. Finally, data from railway adhesion recovery tests demonstrated that the cleaning effect is mostly performed by the leading wheelset, so that the rear ones have a higher adhesion level.