Tore V Vernersson

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.

Thermally induced roughness of tread braked railway wheels, Part 1: Brake rig experiments

Roughness (corrugation, waviness) having circumferential wavelengths of 20 to 200 mm on the tread surface of a running railway wheel leads to vibrations in wheels, bogies and superstructure and also in rails, sleepers and roadbed. The vibrations radiate noise to the surroundings. The most efficient way to reduce the rolling noise would be to bring down the roughness on wheel tread and rail head. In the present work mechanisms causing wheel roughness are investigated. Results from full-scale tread braking experiments on an inertia dynamometer are presented. In these tests the influence of various parameters such as block material, wheel speed and braking force is investigated. The temperatures of the wheel tread and the temperatures and vibrations of the block are measured during braking. Tread surface temperatures are measured using IR techniques. The evolution of wheel tread roughness is measured after each braking cycle. Finally, the hardness of the tread is registered and the tread microstructure is determined by use of metallographic methods. In parallel to the experiments, a mathematical model of the interaction between brake block and wheel has been developed. Thermal loading, contact mechanics and surface wear are considered. A detailed understanding of the braking phenomena causing the growth of wheel roughness is aimed at. Countermeasures could then be developed.

Temperatures at railway tread braking. Part 1: Modelling:

Abstract A thermal model of railway tread braking is developed for use in routine calculations of wheel and brake block temperatures. Two-dimensional finite-element models of block(s) and wheel are coupled via a contact interface that controls the heat generation and also the heat partitioning between block and wheel through thermal contact resistances. The surface temperature variations around the wheel circumference as caused by frictional heating and intermediate cooling are accounted for in a mean sense, assuming high-speed sliding conditions. The thermal power generated at the block-wheel interface during braking is determined from train braking data. A model for heat transfer from the rolling wheel into the rail is developed where a film with thermal contact resistance is placed at the wheel-rail contact interface. The present model can be used to efficiently design tread braking systems for both freight and passenger trains. It can handle stop braking, drag braking at constant brake power, and also intermediate periods of cooling. The temperature history during a full train route can be calculated. The inclusion of heat transfer from wheel to rail means that the model is useful for comparing brake rig tests, where normally the chilling influence from the rail is not included, with in-field tests. Two companion papers with experimental results supplement the present numerical modelling. A brief numerical example demonstrates the heat partitioning and the influence of rail chill (about 30 per cent) for two braking configurations.

Temperatures at railway tread braking. Part 2: calibration and numerical examples

Abstract Temperatures in wheels and brake blocks at railway tread braking are studied under brake rig conditions. Results from rig experiments are reported for drag braking at constant brake power for brake blocks made of cast iron and of sinter and composite materials. The influence of block configuration, brake power, and brake speed is studied. A thermal model of railway tread braking, developed in a companion paper, is calibrated using the experimental data. This model analyses wheel and block temperatures and also the partitioning of heat between wheel and block in the brake tests. Numerical examples indicate the influence of brake block type on temperatures and on wheel-block heat partitioning. The influence of rail chill is studied in a complementary paper.

Thermally induced roughness of tread braked railway wheels. Part 2: Modelling and field measurements

Roughness (corrugation, waviness) on the tread of a rolling railway wheel leads to vibrations in wheels, bogies and superstructure and also in rails, sleepers and roadbed. These vibrating components radiate noise to the surroundings. A lowering of the noise levels from tread braked freight cars is of utmost importance for the future of rail traffic. The thermomechanical interaction between brake block and wheel tread during braking has been found to cause hot spots on the wheel tread. This phenomenon is believed to be a main contributor to the development of a wavy tread surface. Due to thermal expansion of the rim material, the hot spots will protrude from the wheel tread and be more exposed to wear during the wheel/block contact than the rest of the tread surface. The non-even wear results in roughness on the tread of the wheel after cooling. In the present paper, a theoretical and numerical model of the interaction between wheel rim and brake block has been developed. Simulations with this model demonstrate the principal phenomena occurring at the wheel/block contact. Furthermore, results from field measurements of wheel roughness are presented.

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.

Wheel Tread Damage: A Numerical Study of Railway Wheel Tread Plasticity Under Thermomechanical Loading

A numerical study is presented where the impact of simultaneous thermal and mechanical loading on a railway wheel tread, as imposed by braking and rolling contact, is reported. A comparison is made of two-dimensional (2D) and 3D finite-element simulations of the thermomechanical problem featuring a material model that accounts for thermal expansion and plastic deformations. It is found that 2D simulations give unrealistic predictions of plastic deformations. The 3D simulations demonstrate a significant influence of the thermal loading also in cases of rather moderate temperature increases. In particular, the combination of thermal loading and high traction is found to be very detrimental.

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.

Rolling Contact Fatigue Prediction for Rails and Comparisons With Test Rig Results

Tests in a full-scale roller rig, a full-scale linear test rig, and a twin-disc machine are numerically evaluated and compared in terms of rolling contact fatigue loading. From previously evaluated contact patch sizes, a Wöhler curve relationship is established and matched towards experimentally established fatigue lives. In addition, non-linear finite-element simulations are carried out. Key data needed for the numerical evaluations, as well as difficulties in translating experimental data to numerical models are highlighted. A key parameter here is the interfacial wheel—rail friction. Additional simulations were carried out to establish the latter. However, the conformal contact in the linear test rig makes such simulations very uncertain.

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).