Studies On Surface Strengthing System And Corresponding Properties Using Longitudinal-torsional Ultrasonic Rolling Principle

As one of the important parts in transmission fields such as motion and power,gears play a key role in some mechanical assemblies and their performances directly determine the level of one national equipment.By now,more and more gears with high accuracy are needed in fields such as high-speed transportation,aerospace,new energy needs,etc.Simultaneously,the requirements for gears’performance and fatigue life are more serious compared with those for classical ones.Although classical machining methods can make gears meet the demand of the geometry accuracy,traditional ways can not satisfy the physicochemical requirement.Actually,complex and time-varying loads in the transmission process weaken the fatigue life of gears.Thus,strengthening technology is vital to eliminate contacting fatigue failure and improve gear’s fatigue life.As one important method to control geometry and performance with high precision at the same time,longitudinal-torsional ultrasonic rolling(LTUR)is used more and more commonly and becomes a hot research topic.Therefore,embracing advantages of ultrasonic machining technologies,based on the meshing theory,this thesis proposes a new surface-modification method——LTUR for the first time.Basic theories and surface integrity are studied in this paper and main contents of this paper are listed as follows:(1)A set of ultrasonic rolling equipment for surface strengthening is developed.Combining the longitudinal-torsional ultrasonic vibration with gear meshing,rolling forces are performed using a damper,a machining device is designed.Using this device,the rolling process can be controlled with high precision,efficiency,and reliability.First,according to the demand for the stiffness of the longitudinal ultrasonic vibration equipment,the frequency equation of the vibration system with double nodes is established and positions of nodes are determined.Then,the system’s frequency,node position,and amplitude are designed and verified by the finite element method and vibration test.Finally,this designing strategy can be taken as a reference by researchers in this field.(2)The residual stress model is built based on elastoplastic mechanics,Hertz contact and Llyushin total plasticity theory considering vibration forces on the contact stress.Based on the meshing theory,residual stress is modeled and proved by experiments.Using this model,it can be seen that the effect of vibration forces plays a key role on surface strengthening.The value of residual stress increases firstly and then decreases along the involute surface.The results reveal that time-varing of the residual stress in the ultrasonic machining process and the connection between dynamic forces,damping torque,residual stress is built.(3)Considering the initial grinding surface,the rolling process is divided into three steps based on the state of triangular convex bodies.Firstly,the relationship between initial-,final-roughness and rolling pressures is deduced in the first and second steps using slip line field theory.the largest pressing depth is calculated.Then,the indentation is modeled and the time-varing surface roughness is built using kinematics.Finally,the principle of surface evaluation is studied.(4)A series of experiments using the designed platform are conducted to obtain surface integrity and the distribution is gained and analyzed.Some time-varing properties are obtained.First,the residual stress and hardness decrease with the increase of the rotational speed.Roughness incline with the decline of damping forces along the involute surface.Stress increase with the incline of amplitude.Then,nonuniform distribution can be concluded along the involute surface.Finally,longitudinal torsional composite ultrasonic vibration affects the surface integrity significantly.Using the ultrasonic vibration method,the residual stress can be improved by 425%～528%,subsurface tissue can be refined,lattice distortion and phase transformation of retained austenite can be found,XRD diffraction peaks are widened.All these factors make the growth of residual stress and microhardness.It can prove that the proposed method can ameliorate surface performance and the proposed method provides a new technology for the gear processing.(5)Based on the distribution of stress on the critical interface along depth direction,the extreme angle of stress critical interface is obtained.Using the multi-axial fatigue criterion,contact fatigue life is computed,and verified by experiments.It can also be found that rotation cycles of cracks and pitting are1.9×10~7,2.4×10~7 respectively using traditional methods and 3.0×10~7,3.8×10~7 for the method in this paper.From this,it proves the result induced by theory in this paper and the validity of the proposed method.Number of Figures:106,Number of Tables:22,Number of Literatures:180.