Effect Of Residual Stress On Fatigue Life And Vibration Characteristics Of Helicopter Tail Reducer

Spiral bevel gears are key components of helicopter reducers due to their high overlap,strong load-bearing capacity,and smooth transmission.The residual stress generated by the quenching process of this gear is easily cracked under the coupling effect of the complex alternating cyclic stress borne by the tooth root during service.Therefore,residual stress is a factor that cannot be ignored during the service process of spiral bevel gears.This article takes the tail reducer of a light helicopter as the research object and analyzes the evolution law of temperature and stress fields of spiral bevel gears under graded quenching state.The results show that the quenching residual compressive stress is mainly distributed on the surface of the gear teeth,and the residual tensile stress is mainly present on the tooth core and tooth end face.The existence of residual stress will affect the crack evolution and load-bearing deformation of the gear during service.By analyzing the effects of transient and static meshing on tooth root cracks,this study explores the propagation path of tooth root cracks under variable working conditions,compares the stress intensity factors at the tooth root with and without residual stress,and studies the effect of residual stress on fatigue life.The results show that the bending stress at the tooth root during transient meshing is 24% greater than that during static meshing;When residual stress exists,the crack propagation rate at the root of the tooth intensifies,and the crack propagation rate at the large end of the gear tooth is much greater than that at the small end.The crack propagation rate along the tooth surface direction is greater than that along the depth direction.Residual stress reduces the number of cycles at the same crack depth by 41%.Based on the single node meshing method,the meshing stiffness of spiral bevel gears is solved,and the effects of quenching residual stress and initiation cracks on the meshing stiffness are analyzed.The coupling relationship is established using oil film stiffness to obtain the effect of residual stress on the comprehensive meshing stiffness.The results show that the overall trend of the comprehensive meshing stiffness curve is the same as that of the meshing stiffness curve.When residual stress exists,the peak value of meshing stiffness increases by 25%,and when cracks initiate,the peak value of meshing stiffness decreases by 19.41%;And residual stress increases the peak meshing stiffness by 33%when cracks initiate.Considering the nonlinear internal excitation factors such as comprehensive meshing stiffness and backlash,a centralized parameter method is used to establish a full degree of freedom coupled dynamic model of the spiral bevel gear transmission system,and the influence of residual stress on the vibration characteristics of the spiral bevel gear dynamic system is analyzed.The results show that when residual stress exists,the amplitude attenuation speed of vibration velocity is very small in a meshing cycle,but the amplitude of dynamic meshing force increases by about 3.3%.The residual stress makes the amplitude spectral density of dynamic meshing force near the meshing frequency increase by about 30%.When residual stress exists and crack initiation occurs,the amplitude spectral density of dynamic meshing force increases by about 33%.