Wheel-rail Systems Research Articles

Apporoximation of contact geometry in the dynamical simulation of wheel-rail

Models for the geometrical contact of wheel and rail are a basic component of multibody system (MBS) models for wheel-rail systems. Approximations are used to get sufficiently differentiable contact conditions that can be evaluated efficiently. We discuss an approximation that is essentially based on polynomial 2D-tensorproduct splines and minimizes a functional that combines a weighted least squares approximation of a rigid contact model with a smoothing term. An efficient algorithm to compute the tensorproduct spline is developed, the parallelization on a cluster of workstations is discussed. We report on results of both the sequential and the parallel algorithm and give simulation results for a rigid wheelset on a straight track.

NEW ASPECTS OF CONTACT MODELLING AND VALIDATION WITHIN MULTIBODY SYSTEM SIMULATION OF RAILWAY VEHICLES

The development of advanced railway vehicles requires an efficient modelling and simulation of wheel-rail systems under complex conditions and maneuvers. For that reason, the multibody system simulation software SIMPACK was enhanced recently with a generally applicable wheel-rail contact module to be outlined in this paper. This module comprises the possibility of applying a constrained or an elastic contact model, single or multiple contact locations per wheel-rail element, analytic or measured track geometries including irregularities and rail profiles, varying in longitudinal direction (switches). A simulation study of a passenger car running through a switch demonstrates the efficiency of the constrained wheel-rail contact model extended for the simulation of multiple contacts. As a second example a freight train is investigated and its simulation results are compared to measurements in order to validate the simulation tool as well as the model.

Thermal stresses for frictional contact in wheel-rail systems

Based on a work by Knothe et al. (Wear 189 (1995) 91–99) a fully analytical solution for the temperature field, produced by sliding contact on the surface of a halfplane, is presented applying the Laplace transform technique. Further, the stress state in the halfplane under plane strain is analysed. A full solution is presented for the whole stress state as well as its extrema. All results are given in dimensionless form depending only on two parameters (dimensionless thermal penetration depth, Poisson's ratio).

Contact vibration with high damping alloy

Following the formation of corrugations on a two-disk machine in the laboratory, theoretical analysis predicted that contact damping would be an effective factor to prevent corrugations. One idea to get high contact damping is the use of high damping materials. Recently a high damping alloy was developed in Japan and there is the possibility of applying it in wheel-rail systems. The value of the logarithmic decrement of this high damping alloy is about 10 times that of stainless steels, although its strength is almost the same as usual steels. In this paper, the authors tried to examine the contact vibration characteristics using high damping alloy by basic experiments. The experimental results showed that there was no initiation of corrugations on the surface of the high damping alloy, although severe corrugations occurred on the stainless steel. The difference in contact damping on this machine between high damping alloy test wheels and wheels made from other steels was not demonstrated in tests with the micro shaker, although the special experiments with a small needle and the high damping alloy show high damping characteristics. The authors conclude, therefore, that it may be some other property of the high damping alloy which prevents the formation of corrugations.

Determination of temperatures for sliding contact with applications for wheel-rail systems

Contact temperatures and temperature fields of components in relative sliding motion can be analyzed by Laplace transforms and the method of Green's functions. For the case of constant and ellipsoidal pressure distribution the temperatures obtained agree with the results of earlier work. The solution technique can also be used to solve arbitrary pressure distributions, which, coupled with the general normal contact problem, leads to extended solutions. The case of loadings with fluctuations due to surface roughness and indents are investigated. It can be shown that each kind of fluctuation causes a rise of the maximum contact temperature. All applications are for wheel-rail systems. The results are discussed.

Friction-Induced Vibration, Chatter, Squeal, and Chaos—Part II: Dynamics and Modeling

This part provides a comprehensive account of the main theorems and mechanisms developed in the literature concerning friction-induced noise and vibration. Some of these mechanisms are based on experimental investigations for classical models. Bilinear and nonlinear dynamical models have been considered to explain such friction phenomena as stick-slip, chatter, squeal, and chaos. Nonlinear modeling includes two types of nonlinearities which differ from those encountered in structural dynamics. These nonlinearities, in addition to the observed uncertainty of friction between sliding surfaces, form a formidable difficulty in developing accurate and reliable modeling. They include the inherent nonlinearity of contact forces (eg, Hertzian contact), and the nonlinear relationship between friction and sliding relative velocity. Research activities in this area are a mixture of theoretical, numerical, and experimental investigations. Theoretical investigations are prevailed by deterministic analysis with few attempts of stochastic treatment. The models include classical and practical engineering models such as the mass-spring model sliding on a running belt or on a surface with Hertzian contact, a pin sliding on a rotating disk, beams with friction boundaries, turbine blades, water-lubricated bearings, wheel-rail systems, disc brake systems and machine cutting tools. There is a strong need for further research to promote our understanding of the various friction mechanisms and to provide designers of sliding components with better guidelines to minimize the deteriorating effects of friction.

Slip-time history influences on the interaction between friction and wear in contaminated rolling contacts of wheel-rail systems

Low friction of contaminated rails can adversely affect the braking performance of railway vehicles. In this paper we report an experimental study carried out on a down-scaled test machine to determine the longitudinal traction-slip and wear characteristics of lubricated wheel-rail contact. In most of the tests the contact was lubricated by a low viscosity reference contaminant specified by Union International des Chemin de Fer Leaflet 541−05. The brake torque (traction force) has been measured over a wide range of rolling speeds with slip (slide/roll ratio) ranging from 0% to 80%. At higher slip the traction-slip characteristics display two distinct branches depending on whether slip is increased or decreased. The branches are characterized by transitions in friction and wear which are defined by two different slip thresholds. The tribological state of the wheel surface evolves during the course of rolling and sliding in the wheel-rail interface as a result of plastic deformation, frictional heating and wear. Over a wide range of rolling speeds it is observed that the traction force and wear are interactively influenced by the wheel slip and its time history.