Characterisation and evaluation of global uneven heating during railway tread braking – Brake rig testing and field study

Abstract

In this paper, an effort has been made to characterise and evaluate the risks caused by global uneven heating, which can occur on tread braked railway wheels during prolonged braking. Brake rig experiments and a field study were performed on tread braked railway wheels exposed to brake power levels corresponding to down-hill braking. The brake rig testing was performed at the brake test rig at Chalmers University, with controlled testing at constant power levels of three different types of railway freight wheels. The field experiments were performed on the Ofoten Line outside of Narvik, where the wheel temperatures were measured thermographically. It is found that both the rig testing and the field study may generate a substantially uneven circumferential distribution of the wheel temperatures during tread braking. For the brake rig results an initially even distribution of tread temperatures changes into a state with one concentrated hot zone covering roughly one-fourth to one-third of the wheel circumference. Temperature differences between hot and cold zones on the wheel may then be more than 200°C. Similar behaviour is seen for wheels in the field study at the Ofoten Line, with temperature differences of up to 70°C. Ultrasonic measurements on wheels tested in the brake rig show that the residual stress level in the wheel rim had shifted significantly towards a detrimental tensile state compared to what could be expected from uniform heating. The thermal behaviour is similar for the three wheels that have different web shapes and propensities for the build-up of residual stresses. The global uneven heating is most pronounced at intermediate brake power levels of 25 to 40 kW. The conclusions drawn from the present study are that there can be severe consequences of an assumption of uniform wheel heating with respect to the thermomechanics of the brake–wheel–rail system.