Minimal models for squealing of railway block brakes

Block brakes have been used to brake railway vehicles for approximately 200 years. During the last 40 years, disk brakes have replaced the block brakes at passenger cars but block brakes are still used for all freight wagons. One problem of block brakes is that they show an enormous tendency to squeal. Although the block brake is a very common and old technical component, there exists almost no scientific work on its noise behavior regarding squeal. On the other hand, a lot of work has been done on the problem of disk brake squeal especially concerning the modeling of the excitation mechanism. The goal of this paper is to investigate whether and how models from disk brake squeal can be modified to model block brake squeal. The starting point of these investigations is a minimal model for an automotive disk brake introduced by von Wagner et al. (J Sound Vibration, 51(1-2):223-237, 2007) that is adapted to the block brake problem. It can be shown that such a simple converted model does not show any instability at all. A deeper analysis suggests that the reasons for squeal in block brakes could originate from in-plane vibrations of the brake disk or specific geometrical properties of the railway wheel. The self-excited vibrations explaining the squeal occur at relatively low rotational speeds far below the first critical rotor speed which has rarely been observed in rotor dynamics.