On the Design of a Yaw Colloidal Damper Used to Suppress the Hunting Motion and to Improve the Travelling Stability of a Bullet Train

Yaw damper represents a major source of excitation for flexural vibration of the railway carbody. In order to reduce transmissibility of such undesired excitation, yaw damper should allow for large force transmission at low working frequencies, but should behave as vibration isolator at high working frequencies. Unfortunately, the yaw oil damper, which is nowadays in service, has poor intrinsic elastic capabilities and provides damping forces varying as a power function versus the piston speed. Since colloidal damper has intrinsic elastic capabilities and larger damping forces at lower excitation frequencies, it occurs as an attractive alternative solution to traditional yaw dampers. In this Chapter, firstly a simple but reliable analytical expression to estimate the negative damping occurring spontaneously during the hunting motion of the railway wheelset is presented, and the working conditions to be fulfilled by the yaw damper are clarified. Concretely, technical options to diminish the effects of the wheelset unstable hunting mode are listed, and the influence of the carriage geometry, hunting wavelength, and lateral perturbation on the stroke of the yaw damper is discussed. In order to estimate the effectiveness of the yaw damper, ride comfort evaluation of the bullet train subjected to lateral excitation is discussed relative to the standard procedure, and also by taking into account some particular frequency weightings which account for the discomfort sensation during the reading and writing activities performed by passengers. Then, the dynamic characteristics of a yaw colloidal damper, destined to carbody suspension of a full-scale bullet train, are evaluated from the experimental results obtained during horizontal vibration tests, performed on a ball-screw shaker. Both the frictional and colloidal effects are discussed versus the working stroke and frequency of the yaw colloidal damper. Compared to the corresponding classical yaw oil damper, the trial yaw colloidal damper allowed for: weight reduction of 31.6%; large damping force, dissipated energy and spring constant at long piston stroke under low excitation frequency; low damping force, dissipated energy and spring constant at short piston stroke under high excitation frequency. Observed elastic features of the yaw colloidal damper are justified based on a model which includes the effect of a porous lyophobic matrix on the behavior of a classical liquid spring.