<p>After the failure of the lubrication system of the helicopter main drive, the spiral bevel gear transmission system will experience normal lubrication and starved-oil lubrication, and finally enter the extreme working state of dry running operation. The time-varying backlash, time-varying friction and time-varying meshing stiffness caused by thermal deformation, wear and elastic deformation between meshing tooth surfaces will directly affect the dynamic characteristics and stability of the transmission system. Therefore, it is necessary to carry out research on the full backlash dynamics of spiral bevel gears considering lubrication change. Firstly, a&#xa0;time-varying full backlash calculation model considering thermal deformation, elastic deformation and wear of spiral bevel gear pair is proposed. Based on the principle of thermoelastic mechanics, Hertz contact theory and Archard wear model, the time-varying full backlash under different lubrication conditions is acquired. Secondly, the friction coefficient between the meshing tooth surfaces of the spiral bevel gear pair under different lubrication conditions is obtained by means of the friction characteristic test, and then the time-varying friction force and friction torque at the meshing point under different lubrication conditions are obtained. Thirdly, a&#xa0;time-varying meshing stiffness model considering elastic contact stiffness, temperature stiffness and oil film stiffness is established. Then, the lumped parameter method is used to establish an 8‑degree-of-freedom nonlinear dynamic model of the spiral bevel gear bending-torsion-axis coupling with time-varying full backlash, time-varying friction and time-varying meshing stiffness. The nonlinear dynamic response of the spiral bevel gear transmission system under different lubrication conditions is solved and analyzed. Finally, the correctness of the dynamic model in this paper is indirectly verified by experiments.</p>