Roger Lundén

TRAIN-TRACK INTERACTION AND MECHANISMS OF IRREGULAR WEAR ON WHEEL AND RAIL SURFACES.

Summary High-frequency train-track interaction and mechanisms of wheel/rail wear that is non-uniform in magnitude around/along the running surface are surveyed. Causes, consequences and suggested remedies to relieve the problems are discussed for three types of irregular wheel/rail wear: (1) short-pitch rail corrugation on tangent tracks and large radius curves, (2) wheel corrugation as caused by tread braking, and (3) wheel polygonalisation. The state-of-the-art in modelling of dynamic train-track interaction in conjunction with prediction of irregular wear is reviewed.

Influence of Short-Pitch Wheel/Rail Corrugation on Rolling Contact Fatigue of Railway Wheels

Abstract A numerical procedure to integrate simulation of high-frequency dynamic train-track interaction and prediction of rolling contact fatigue (RCF) impact is presented. Features of the included models and possibilities of applications are outlined. The influence of short-pitch rail corrugation and wheel out-of-roundness (OOR) on RCF of a high-speed passenger train is investigated. It is shown how the corrugation and the OOR will have a profound effect in that levels of wheel and rail irregularities that have been measured in the field may be sufficient to generate subsurface-initiated RCF. In particular, the high-frequency content of the contact forces is of importance. Errors induced by neglecting such high-frequency components in measurements and/or simulations are investigated by comparing RCF indices based on contact forces that have been low-pass filtered with various cut-off frequencies. To avoid cracking due to RCF, a maximum roughness level in the wavelength interval up to 10 cm is sought. To limit the effects of corrugation, grinding practices have been altered leading to a significant decrease in RCF.

Contact region fatigue of railway wheels under combined mechanical rolling pressure and thermal brake loading

Abstract The combined effect on railway wheels of a periodically varying contact pressure and an intermittent thermal brake loading is investigated in this paper. The development and redistribution of residual stresses in the wheel rim are clarified. Elasto-plastic strain cycles are calculated for some often occurring loading histories. These cycles are used as input to a damage mechanics model where the relative lifetime of the wheel rim can be estimated. Shakedown, cyclic plasticity and ratchetting are central phenomena discussed in the paper. A commercial finite element model computer programme for thermo-elasto-plastic analysis is employed. Temperature-dependent material data for a real wheel steel are used. Simulations with varying material data are carried out and measures to increase the lifetime of railway wheels are discussed.

Temperatures at railway tread braking. Part 3: wheel and block temperatures and the influence of rail chill

Abstract Tread braking generates high temperatures in railway wheels and brake blocks as the kinetic energy of the running train is transformed into heat. The temperatures induced in the components are here analysed with particular focus on the cooling influence from the rolling contact between the hot wheel and a cold rail. Controlled brake rig tests are reported, where the rolling contact is studied using a so-called rail-wheel in contact with the braked wheel, along with results from field tests. The data from these experimental studies are used for calibration of a simulation tool for calculation of wheel and block temperatures. The calibrated model analyses heat partitioning between block, wheel and rail and finds the resulting temperatures at braking. The rail chill is found to have a considerable influence on the wheel temperatures for long drag braking cycles. A successful calibration of the model using data from field tests is also reported.

Disc brakes for heavy vehicles: an experimental study of temperatures and cracks

A better understanding of the thermomechanical loading of brake discs is important for controlling material fatigue and crack propagation in the disc. In the present study, full-scale drag braking experiments were performed on brake discs made from eight different grey cast iron alloys. The well-performing materials were also tested with an alternative brake pad material. A testing procedure with repeated drag brakings was used. The disc and pad temperatures were registered by thermocouples embedded at selected locations, and the disc surface temperatures by a thermocamera. Extensive analyses of the measured temperatures were performed. The results for the thermocouples at the mid-radius of the disc and at the end of brake applications indicatd that the two sides of the disc have opposite deviations from the mean temperature. The temperature deviations are generally temporally alternating, but also stationary variations can be found. The thermocamera gives the possibility of identifying the phenomena behind the temperature variations found from the thermocouples. Banding of the disc–pads contact with alternating one band and two bands of high temperatures is observed for the studied brake discs exposed to severe braking load cases. Moreover, it was found that hot-spot patterns develop on the disc surface, which are spatially fixed during each brake application. However, they may be either slowly migrating or fixed relative to the disc during consecutive brake applications. Thermal images show that small cracks do not affect hot-spot migration as a hot spot migrates over the crack. However, at a critical length of the crack, the heat becomes localized at the crack and increases its growth, thus limiting the life of the disc. The tests indicate that a combination of hot-spot migration, alternating bands and small temperature differences over the disc are significant factors to be considered when improving the lifespan of the discs.

Modelling of temperatures during railway tread braking: Influence of contact conditions and rail cooling effect

The temperature rise of wheels and blocks due to frictional heating during railway tread braking along with the transfer of heat through the wheel–rail contact is studied in this paper. In particular, heat partitioning between block, wheel and rail for stop braking cycles is considered. The wheels are of interest because they are a limiting factor for railway tread braking systems. Two types of thermal models are employed to investigate the maximum temperatures over the wheel tread. In a circumferential (plane) model of wheel, block and rail, the heat transfer problem is studied by use of a finite element formulation of the two-dimensional time-dependent convection–diffusion equation. The hot spot phenomenon is simulated by introducing a prescribed wheel-fixed contact pressure distribution between wheel and block. In an axisymmetric (axial) model of wheel, block and rail, the lateral movements of the wheel–rail contact are studied. A general result is that the cooling effect provided by the rail is important when local temperatures on the tread are considered, but not when studying bulk temperatures created in a single stop braking event. Furthermore, it is found from the lateral movements of the wheel–rail contact that slow oscillations result in maximum temperatures over the wheel tread that are somewhat lower than for travelling on straight track (rolling at the rolling circle).

Identifying the root causes of damage on the wheels of heavy haul locomotives and its mitigation

The paper illustrates how damage patterns in the form of rolling contact fatigue (RCF) on wheels, can be employed to identify and improve underlying operational conditions. The focus is on RCF of locomotive wheels operating on the Iron Ore Line in northern Sweden and Norway. Seasonal changes and damage patterns are charted. Potential root causes for observed damage patterns are identified and investigated. Mitigating actions are proposed and the efficiency of implemented actions is quantified.

Railway Axle Design: To be Based on Fatigue Initiation or Crack Propagation?

Design and standardization of railway axles from the mechanical-strength point of view is discussed and a brief state-of-the-art survey is given. Traditional fatigue design with the ‘safe-life’ approach and a design with the more recent crack-propagation/defect-tolerance approach are described and compared. Axles of a standard freight wheelset (22.5 tonne axle load) and of two heavy haul wheelsets (25 and 31 tonne axle loads) are used for illustration. From the numerical results it is concluded that a crack-propagation/defect-tolerance approach could be a useful complement to the traditional fatigue design method. A result of particular interest is the fairly strong influence of stresses from the press-/shrink-fitting on the residual life of the axles.

Tread braking of railway wheels – temperatures generated by a metro train

Tread braking of railway wheels results in the kinetic energy of the train being dissipated into the wheel and blocks in the form of heat. This heat is further conducted into adjacent structures, notably the cold rail, and also transferred into the surroundings by convection and radiation. Heat partitioning between wheel and block is, for short time periods, controlled by local thermal interactions at the contact point and by the conductive properties of the bodies. However, for a metro train that performs longer periods of intermittent braking (or for drag braking) convective and radiation cooling properties of the components come into play. In the present study, results from brake rig tests and from in-field testing of a metro train are presented and used to calibrate a simulation model. It is found that the cooling level of the wheels of the metro train is substantially lower than for the wheels of a freight wagon. Moreover, it is found that the first axle on the metro train is exposed to higher cooling levels than the remaining axles. In a numerical example, temperatures of tread braked wheels are calculated using the new findings for a metro train, and the results obtained are compared with wheel temperatures as calculated assuming freight wagon conditions.

Introduction to wheel-rail interface research

History and present situation. Phenomena in the wheel-rail interface. Research fields. Applications. Ongoing research, development and standardization efforts. System aspects and optimization. Future trends. Sources of further information and advice. Acknowledgements. References.