Tread braking – fatigue life of railway wheel webs

Tread braking is a common braking system for railway vehicles which is used mainly on freight wagons but also as a component on trains with mixed braking systems. In general, railway wheels for tread braking are designed only by considering a maximum allowed braking power according to the wheel design standard EN 13979-1. The idea is to avoid derailment risks due to change of wheelset gauge or global wheel fracture caused by excessive tensile stresses in the wheel rim. In the present paper, the fatigue life of the wheel web is studied in detail when subjected to a combination of thermomechanical loading from tread braking and mechanical loading from wheel-rail contact forces. Two generic wheel designs are assessed where one represents the classical freight wheel with a slightly S-shaped web and the other is a so-called low-stress wheel, with a more elaborate shape of the wheel web, specifically designed for high thermal loads. The wheel designs are studied both with new rims and with fully worn down rims. The results show that railway wheels generally have sufficient fatigue lives for regular service with respect to thermomechanical loading. Moreover, it is shown that the studied wheels have different sensitivities to mechanical and thermomechanical loadings at some critical positions on the wheel web. Towards the wheel rim, the damage as induced by tread braking is dominating and towards the hub the damage from mechanical loading is more important. A general conclusion is that maintenance of the wheel webs is important for preserving a high safety of the railway system.