| With the advantage of high transmission efficiency,big contact ratio,smooth operation and high bearing capacity spiral bevel gears are widely used in the transmission of vehicles,such as tanks,special vehicles and so on.They are the key parts in transmission system of high loaded vehicle,and can effectively change the torque transmission.The service life of transmission system is affected by the manufacturing accuracy and mechanical properties of spiral bevel gear that determines the reliability of the high load vehicle.The spiral bevel gears are manufactured with low carbon alloy steel,and the main manufacturing processes are consisted of tooth cutting,carburizing-die quenching tempering and so on.It is very important to control the manufacturing deformation of spiral bevel gear,due to the complexity of geometry.The deformation is mainly existed in the quenching process,and the oversize deformation can cause a decline in the gear precision and meshing stationarity,leading to the increasing noise in transmission system.A partial load takes place on tooth surface for high-duty gears,leading to the premature failure of gear and the decline of service life for the transmission system.So the quenching deformation control of spiral bevel gear is always a difficult problem in manufacturing process.Although the deformation can be reduced by using die quenching process,the physical process is more complex for the gear quenching with fast cooling rate,due to the introduction of the mold constraints: the deformation is affected by various factors,such as temperature history,temperature gradient,phase transformation sequence,final phase,constraint of die,springback after removing load and so on;in previous study,the variation of material composition(hardenability)has a significant influence on gear quenching process,leading to the different deformation under the same process conditions.It is difficult to be known well for the deformation mechanism and law of large spiral bevel gear,because of the mentioned factors.Lacking of quantitative forecasting model,the adjustments of process parameters almost completely depend on personnel experience of workers in the gear production,therefore,the manufacturing quality and efficiency cannot be guaranteed;more seriously,the assembly interchangeability is greatly reduced without the understanding of deformation law and control.Spiral bevel gears must be paired before they are assembled into actuating mechanism,causing a series of subsequent difficulties,such as installation,maintenance and so on.For the bottleneck problem of quenching deformation control for large spiral bevel gear in manufacturing process,multi field coupling simulation and process experiment are used in this thesis.The complete mechanism research is conducted by studying the material model,carburizing-die quenching process and the influence of the deformation on gear meshing.The deformation law is revealed,and the process parameters are optimized.Hence,the main researching of the following thesis can be presented as follows:(1)The hardenability distribution of 22 CrMo H steel for spiral bevel gear manufacturing is tested by jominy test and testing of alloy compositions.The concrete influence of hardenability on material properties are determined by the test of kinetics of phase transformation,static mechanical properties,transformation plasticity and thermal physical parameters for high and low hardenability material.According to the testing data,the material models are built based on the high and low hardenability.(2)In order to verify material model and ensure the boundary conditions during quenching process,a quenching experiment is designed and conducted to extract the temperature history for spiral bevel gear,meanwhile,the microstructures are observed after quenching.A corresponding temerature-transformation couling model is built based on the experimental parameters.The accuracy of material model and boundary conditions is verified by comparing the simulation and testing results of temperature history and microstructures(3)The complete carburizing-quenching models are built based on the material models with high and low hardenability according to the heat treatment process of spiral bevel gears.The effects of hardenability on microstructures and deformations of spiral bevel gear are studied under assigned process parameters.A reasonable optimization way is suggested according to influence of process parameters on quenching deformation.The reliability of simulation and analysis results is verified by the testing data of manufacturing process.Hardenability affects the kinetics of phase transformation during quenching process,which is mainly controlled by the alloy elements.After complete carburizingdie quenching process,martensite and a small amount of residual austenite formed on gear surface and bainite formed inside gear tooth for both high and low hardenability gear.The microstructures inside root base are mainly influenced by the hardenability.It is the mixture of bainite and ferrite/pearlite for low hardenability gear and main bainitefor high hardenability gear.The big gap of quenching deformation is caused by the different phase transformation and the transformation sequence for high and low hardenability gear.The buckling deformation of high hardenability gear occurred at both ends with distortion of 0.091 mm at inner circle and 0.079 mm at outer circle.The main buckling deformation of low hardenability gear is 0.375 mm at outer circle.The buckling deformation has direct effects on the accuracy of the tooth surface.The width of tooth deformation range is smaller for high hardenability gear and the width of deformation range is 0.044 mm on concave and 0.059 mm on convex.While the width of deformation range is 0.138 mm on concave and 0.151 mm on convex for low hardenability gear.The relationship between process parameters and buckling deformation is studied for the gear with big deformation.A definition of deformation parameter is put forward to measure the whole deformation on bottom surface and the effects of die load and inclination angle of bottom surface in quenching machine on deformation parameter is analyzed.Considering the non-uniform deformation in quenching process,the center of the parameter range that has little effects on deformation variation is selected.The inclination angle is determined in order to get the smallest buckling deformation under the loading conditions.Finally the die quenching process parameters are optimized to 250 psi for inner die and 350 psi for outer die,and the inclination angle tuned to 0.2°?Compared with the testing data in manufacturing process,the simulation results are well matched.The quenching deformation and the width of deformation are controlled well by the die quenching process with the optimized parameters.Finally,the coupling model with die quenching and meshing for spiral bevel gear is built to study difference of the meshing properties between ideal gear model and the gear with quenching deformation based on the die quenching model.Quenching deformation do not only affect the accuracy of tooth surface,but also the meshing properties.Under the same load,the contact ratio is smaller and there is the partial load on tooth surface for gear with quenching deformation.If the buckling deformation is big,the contact area moved to the end of tooth surface,leading to the spalling of material on the surface of tip and root.The bending fatigue strength of the tooth is reduced,leading to the eventual tooth fracture. |
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