Tread Braking of Railway Wheels - Noise-related Tread Roughness and Dimensioning Wheel Temperatures
Block braking is a commonly used braking system for freight wagons and other types of railway vehicles. One or several brake blocks are pressed against the tread (running surface) of the wheel, which is also in rolling contact with the rail. The kinetic energy of the running train is then transformed into heat, which is partitioned between block(s) and wheel and is conducted from block to block holder and from wheel to rail and also dissipated into the surroundings by convection and radiation.
During the last decade, one aspect of block braking has become an environmental issue: the high rolling noise levels generated by trains with block braked wheels. A roughness (corrugation, waviness) develops on the wheel tread during the sliding contact between block and wheel, and this tread roughness (out-of-roundness) induces vibrations and noise when the train rolls. The roughness level strongly depends on the properties of the brake block material. Here cast iron blocks generate higher roughness levels than do composite and sinter blocks. In Europe, cast iron is presently the most commonly used brake block material on freight wagons, but composite and sinter materials are now gradually being introduced.
In the first part of the present thesis, the thermomechanical interaction between brake block and wheel tread is studied to search for the mechanisms behind the growth of wheel roughness. The evolution of hot spots, i e areas on the wheel tread with significantly higher temperature than the rest of the tread, is believed to be a key phenomenon that can explain the differences between the different block materials. Results from full-scale tread braking experiments with forged wheels in an inertia dynamometer are reported. A numerical model for studying the thermoelastic contact between brake block and wheel tread demonstrates the principal phenomena. Also, results from field measurements of wheel roughness are presented.
The tendency of cast iron brake blocks to generate high roughness levels on wheel treads has propelled a general shift away from cast iron to other materials which do not give disturbing roughness levels. However, this change of block material will affect the heat partitioning between wheel and block. Excessive heating of the wheel may cause damage and may result in problems with axial deflection of the wheel rim (change of wheelset gauge). High tensile stresses in the wheel rim after its cooling down can lead to initiation and growth of transverse cracks in the wheel rim.
In the second part of the present thesis, a thermal model of railway tread braking is developed for use in routine calculations of wheel and block temperatures, including the cooling influence from the rail. Brake rig tests have been performed for drag braking at constant brake power for blocks made of cast iron and sinter and composite materials. Results on the influence of block configuration, brake power and brake speed on wheel and block temperatures are reported. Rolling contact heat transfer from wheel to rail is studied in the brake rig using a so-called rail-wheel in contact with the braked wheel, along with results from field tests. The model has been calibrated by using data from the experiments and can be employed to calculate temperatures and the heat partitioning between block, wheel and rail. The rail chill is found to have a considerable influence on the wheel temperature for long brake cycles. 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 thus be calculated.