Here, aiming at design of a coaxial contra-rotating main reducer gearbox of a certain type of unmanned helicopter, a 25-DOF dynamic model for multi-stage helical gear transmission system in the gearbox was established by using the lumped parameter method considering effects of time-varying meshing stiffness, bearing support stiffness, transmission shaft torsional stiffness and meshing error. Influence laws of input rotating speed, scale and position of highspeed level gear tooth surface spalling defects on dynamic characteristics of the transmission system were analyzed. The results showed that dynamic meshing force of upper and lower rotor wing shaft gear pairs fluctuates more largely, and fluctuation range of dynamic meshing force of upper rotor wing shaft gear pair is 1.7 times that of lower rotor wing shaft gear pair; with increase in rotating speed, the amplitude of double-input stage meshing frequency in dynamic responses of various gear pairs increases most significantly; when spalling defects appear on high-speed level gear tooth surface, a sideband can appear near meshing frequency, and vibration impact can increase with increase in defect size; at different defect positions, vibration impact amplitude caused due to defect is the largest at junction position of double-tooth meshing zone and three-tooth meshing zone; the study results provide a theoretical reference for vibration and noise reduction and fault diagnosis of coaxial contra-rotating main reducer gearbox of unmanned helicopter. © 2022 Chinese Vibration Engineering Society. All rights reserved.
