Thermomechanics of tread braking: Braking capacity of railway wheels

Doktorsavhandling, 2024

Stopping trains remains as vital a task as running them, and the former of these is the purpose of the braking system. Although most modern passenger trains one might have come across primarily rely on regenerative braking, more or less every train is also equipped with mechanical friction brakes. For freight trains, most of the braking effort may come from a system which has barely been changed since its inception in the latter half of the 19th century: the tread brake. This is where modern train braking began, and for the topic of this work, also partially where it continues. It is a simple, inexpensive and maintenance-efficient system capable of assisting more modern regenerative alternatives or carrying the full brake load by itself. The drawback is that the wheel is used as a friction-heated component, transforming the kinetic energy of the train to heat, which can cause damage at high braking loads. Future requirements of increased speeds, higher axle loads, lower noise emissions and the ever-present desire to minimise costs have however made the following question relevant:

What are the actual limits of the tread braking system?

The primary aim of this thesis is to determine safe and efficient usage limits of the tread braking system via a combination of numerical simulations, laboratory experiments and field tests. Temperature behaviour, thermomechanical effects, material damage and wheel-rail rolling contact as well as the combinations of them are all considered. A custom-made brake roller rig developed during the course of this project is used in combination with modern measurement techniques such as high-speed radiometry to study the behaviour and thermomechanical effects of elevated and non-uniform temperatures. Modelling and simulation of the experimental and other cases are then investigated to both improve current finite element models and to attempt to accurately provide computational results that can gauge the limits of the tread braking system and its components.