Abstract: The analytic eigenvalue formula of the system characteristic equation is derived based on the hunting model of a single wheelset. The critical velocity for unstable wheelset hunting as well as the corresponding frequency are deduced according to the Routh-Hurwitz stability criteria. The phase synchronization modal analysis method is adopted to eliminate the phase difference caused by the system damping, and the real modal decoupling of the non-conservative system equations of wheelset hunting motion is realized. The research indicates that reducing the mass and moment of inertia of wheelset as well as the equivalent conicity, increasing the stiffness and the damping of primary suspension in lateral and longitudinal directions, increasing wheelset diameter and distance between two contact points can increase the critical hunting velocity. Reduction of creep coefficient will not reduce the critical velocity. The frequency corresponding to the critical hunting motion is increasing with the decreasement of the mass and moment of inertia of wheelset, the increasement of the stiffness of the primary suspension in lateral and longitudinal directions, and with the reduction of the distance between contact points. The influence of the suspension damping on the corresponding hunting frequency is not obvious. The equivalent conicity, wheel diameter and creep coefficient have no influence on the frequency for critical hunting. A large enough critical velocity can be achieved in theory by the optimizing the primary suspension stiffness. There is a hunting resonance response existed at any running speed. The resonance amplitude and frequency increase with the increasement of speed. The high-damping mode of wheelset hunting motion is actually an over-damped response, while the wheelset hunting motion is dominated by the low-damping vibration mode.

Key words: wheelset, hunting motion, critical velocity, phase synchronization, modal analysis