To address the vibration problems caused by dual-input power confluence and planetary-stage power splitting in helicopter main reducers, a dual-input parallel-shaft planetary gear transmission system is investigated. A bending–torsional coupled dynamic model is established by considering gear-pair and spline-pair mesh stiffness, equivalent error, backlash, support stiffness, and spline shaft torsional stiffness. The time-varying mesh stiffness of gear pairs is calculated using the potential energy method and verified by finite element analysis. The effects of key excitation parameters, input speeds, and load conditions on the system vibration responses are analyzed. The results show that increasing gear-pair mesh stiffness generally reduces the torsional vibration responses of rotating components, while equivalent errors significantly amplify system torsional vibration. The spline shaft torsional stiffness affects vibration transmission between the parallel-shaft and planetary gear stages. The results provide a theoretical basis for error control, spline connection stiffness matching, and low-vibration optimization design of helicopter gear transmission systems.
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