Abstract: A power coupling system with two-stage planetary gear trains is considered, which is a part of a novel hybrid power train system based on a double-rotor motor. Many fundamental factors are taken into account, such as mesh stiffness of gear pairs of the two-stage planetary gears, torsional stiffness of the central parts, torsional coupling stiffness of the connecting section, inertia of the system, etc. Purely torsional dynamic model of the coupling system is developed. The reduced-order eigenvalue problems are derived by using the related parameters of the system, and the natural characteristics of the system is shown. The vibration modes of the system are classified into three categories: overall rotational mode, front row planet mode, rear row planet mode. In overall mode, the natural frequency is a simple root and each part of the system has torsional vibration to some extent. However, the natural frequency is a double root in the other two modes and no part of system vibrates except the planet gears. It’s found that the vibration mode characteristics agree with those of the literatures. The impact of coupling stiffness to natural characteristics of the system is point out, and preliminary analysis is proposed.

Key words: Coupling stiffness, Dynamic modeling, Natural characteristics, Two stage planetary gear train