Influence of dither generated by rolling contact on friction damping in freight wagons

Friction dampers of freight wagons developing the two-dimensional friction in the primary suspension are exposed to the dither generated by rolling contact of wheel and rail. The dither is the effect of the wheel-rail surface irregularities and the non-homogeneity of the track's mechanical properties in the direction of rolling. The paper is devoted to the investigation of the influence of dither on the dry friction damping in such wagons. This influence has been analysed in simple test cases, when the creep forces developed on the wheels do not interfere with the friction damping. The results show the influence of the sinusoidal dither on the vertical damping, lateral damping and the combined vertical–lateral (two-dimensional) damping of a freight wagon. The combined vertical–lateral damping has been also analysed in the presence of the real dither recorded during the track tests. The dither of sufficient intensity excites the vertical sliding in the friction dampers; it makes the damping lighter and behaves like the viscous damping, because it removes the sticking in the dampers. One may say that dither smoothes dry friction as far as damping is concerned. Smoothing the two-dimensional friction by the vertical dither is stronger in the vertical direction than in the lateral. This means that the two-dimensional friction in the presence of one-dimensional dither behaves similarly with the anisotropic friction. The influence of dither on the friction damping in a freight wagon with dampers in the primary suspension should be taken into account in the numerical simulations of the wagon motion.