Troubleshooting Noise and Vibration Issues of a Metro Braking System

Purpose: The braking system used on a recently upgraded metro vehicle showed very high low-frequency noise and vibrations levels in service affecting passengers' comfort and leading to both brake calliper lever failures and to premature wear of brake pads. The characteristics of the phenomenon were such that it was classified as a groan noise type. As brake pads with different characteristics were tested trying to attenuate the issue without success, the paper describes the approach adopted by the authors to tackle the issue. Methods: The dynamic behaviour of all the components involved during the braking phase (wheelset, callipers, brake discs and part of the bogie frame) was analysed by numerical simulations and tests during service, finding a coincidence of the natural frequencies of the wheelset and the brake support. As this resonance was believed to be the root cause of the problem, a number of structural modifications were proposed. Results and Conclusions: After a careful selection process that included robustness and sensitivity analyses, the accepted modification proved to be effective in solving the issue and was applied with satisfaction to the whole fleet.