Keywords: ride comfort, bending vibration, vibration isolation, imbalance of wheelset, rubber bush, traction link.

Even a small mass imbalance in wheelsets frequently induces vertical bending vibration of railway vehicle carbodies which worsens the ride comfort for passengers. Since such carbody vibration is excited via the longitudinal bogie vibration caused by the rotation of the wheelsets with the imbalance, vibration isolation between the carbody and bogies ina longitudinal direction is expected to be an effective countermeasure against the vibration.

The authors therefore focus on rubber bushes for traction links, which connect the bogies to the carbody and transmit the longitudinal force between them, and have developed the "displacement-dependent rubber bush" with a small gap between the inner fixture and the buffer rubbers constituting the link. The developed rubber bushes installed in the traction links have low longitudinal stiffness when the relative displacement between the bogie and the carbody is small, and vice versa.

Since a bogie motion caused by driving, breaking and yawing is quasi-static or has a low frequency component and the longitudinal force applied to the traction link is large, the relative displacement between the carbody and the bogie becomes resultantly large. In such a displacement range, the developed rubber bush has high stiffness, i.e. the almost identical stiffness with the original one. On the other hand, the excitation force induced by the rotation of the wheelsets with mass imbalance has a high frequency component and the displacement between the carbody and the bogie is relatively small. In the small displacement range, the stiffness of the rubber bush is much lower than that of the original one and the vibration isolation between the carbody and bogies is realized. Thus, the vertical bending vibration induced by the imbalance in the wheelsets is reduced without affecting the fundamental function of the traction links.

Running tests on a commercial line have been conducted to verify the effectiveness of the vibration reduction performance by the displacement-dependent rubber bushes under the actual running conditions. A commuter type test vehicle with the traction links respectively applying the original or the developed rubber bushes has run through the test section (length: 25km), and the vertical accelerations of some position on the carbody are measured. The running speed has been determined as 83km/h; the rotating frequency of the wheelsets for this speed corresponds to the natural frequency of one of the bending modes of the carbody. Bending vibration induced by the rotation of the wheelsets, has been drastically reduced when the developed rubber bushes are applied. The ride quality level which is a common index for evaluating ride comfort of railway vehicles in Japan, has been reduced effectively at the floor centre, i.e. the proposed rubber bushes have improved the ride comfort, throughout the running section.

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