A mechanism for overcoming the effects of the internal resonances of coil springs on vibration transmissibility

Abstract

Internal resonances in suspension springs can result in significantly increased vibration transmission at the corresponding natural frequencies. Particularly for the case of metal coil springs used in vehicle suspensions, these internal resonances can be as low as 50 Hz and can lead to increased structure-borne noise in certain frequency bands. Although, in practice, this can be mitigated to some extent by using rubber pads in series with the coil springs, high vibration transmission in the vicinity of the internal resonance frequencies remains an important issue. In this paper, a mechanism is identified that can overcome the increased vibration transmission due to the internal resonances. This involves the use of a pivoted arm with a pivot bushing that is relatively stiff for translational motion. The quasi-static load is primarily carried by the suspension spring but the dynamic load at higher frequencies is also transmitted through the pivot bushing. By appropriate selection of parameters, in particular the moment of inertia of the pivoted arm and the stiffness of the pivot bushing, it can be arranged that the dynamic loads acting through the spring and the bushing largely cancel each other out at the spring natural frequency. It is shown that this mechanism is contained within common designs of railway vehicle primary suspension. Nevertheless, their design is largely based on their quasi-static behaviour and this principle of dynamic load cancellation has not previously been explained. The dynamic behaviour of different suspension arrangements is compared and the selection of suitable parameter values that can achieve this dynamic load cancellation is explained. Field measurements are also presented which confirm this behaviour.