Research On Hot Forging Process And Accuracy Control Method Of Non-circular Spur Bevel Gear

Spur bevel gear is the key basic part of power and motion transmission.It can be divided into circular spur bevel gear(CSB-gear)with fixed transmission ratio and spur non-circular spur bevel gear(NCSB-gear)with variable transmission ratio according to the characteristics of its transmission ratio.Because of its high efficiency,high reliability and high stability,CSB-gear has been widely used in many industrial fields,such as machinery,automobile,train,ship and aerospace etc.However,with the precise and integrated development of major equipment in China,the requirements for special function and high performance of key basic parts have become increasingly stringent,and it's difficult for CSB-gear with fixed transmission ratio to meet this strict requirement for major equipment.Due to the ability to realize variable gear ratio transmission,NCSB-gear possesses many special transmission performances which CSB-gear cannot achieve,which means that it's totally appropriate to be used in designing special mechanism with specific function,such as limited slip vehicle differential mechanism,variable flow gear pump,power divider machine and so on.In order to catch up with the rapid development of mechanical equipment and national defense construction,it is urgent to accelerate the research,production and application of NCSB-gear.However,due to the strict foreign technical blockade,design and manufacturing technology of NCSB-gear has been severely restricted,as a result seriously restrains the application of NCSB-gear in China.In view of the shortcomings of the existing design methods of NCSB-gear,such as complexity of the solution process and the poor adaptability,this paper proposes a precise design method of NCSB-gear based on the coordinate transformation of curved surface vector matrix.Firstly,based on the pure rolling of pitch surface of driving and driven gears,the pitch surface equation of NCSB-gear is derived.Secondly,the normal vector of pitch surface is(further)derived,and(ultimately)the normal offset surface of NCSB-gear is established.Thirdly,the kinematics of tooth profile generation is analyzed,and the mathematical model of tooth profile generation is established.Fourth,the existence conditions of the curvature interference boundary point and meshing boundary point in design of tooth profile are discussed,and the determination method of these two boundary point is proposed.By using the design method of NCSB-gear,a NCSB-gear pair which is used in the automobile differential is designed.The distribution of contact stress on tooth surface was analyzed by finite element simulation method,and the anti-slip differential test was carried out to verify the reliability of the NCSB-gear design method proposed in this research.In order to improve the rationality of metal flow during hot forging of NCSB-gear,a preformed stage is added before final forging of NCSB-gear.The design method of preformed shape of workpiece is developed according to material distribution along circumference of NCSB-gear.By using the preformed workpiece,the metal flow,strain and temperature distribution were comprehensively investigated.The research results show that,metal flowing on different gear-tooth cavities become more synchronous by using preformed workpiece,so that the gear-teeth can be filled well,and forging flashes are more uniform on different gear-teeth regions.Additionally,the homogeneity of stress and temperature distribution can be significantly improved,which is conducive to improve the performance of forged gear.Moreover,the temperature distributions are more uniform on gear-tooth by using preformed workpiece,which is beneficial to improve the service life of die.In order to realize the mass production of NCSB-gear,the strength design method of gear-tooth die must be developed.In this paper,the distribution and evolution of the die stress in the hot forging process of NCSB-gear are revealed.According to the stress state and geometrical shape characteristics of gear-tooth die,the design method of parting face of gear-tooth die is proposed.Next,a precise design method of preload die based on uniform preload force is innovatively proposed.This method breaks through the deficiency that the existing design method of preload die which is only can be used to preload the die with thick-wall cylindrical cavity.On account of the difficulty in controlling the tooth accuracy in the hot forging of NCSB-gear,a FE analysis strategy of tooth accuracy prediction is proposed on the basis of transfer relation between the stress field and the temperature field in the hot forging process.This FE analysis strategy can significantly improve the computational efficiency of tooth deviation.By using this modeling strategy,the effects of the die shrink-fitting,die elastic deformation,die thermal expansion,gear elastic recovery,and non-uniform cooling shrinkage of gear on the tooth deviation of are revealed,and a compensation method for tooth deviation is proposed.For the sake of the realization of the precision forming of NCSB-gear,a controlling method the tooth accuracy in hot forging of NCSB-gear is developed in this paper.Firstly,in order to improve the accuracy of gear-tooth die,a revised method of accuracy of gear-tooth die is proposed based on preload deformation of gear-tooth die.In this method,a BP neural network is developed to establish the nonlinear relationship between the preload interference value and the accuracy of gear-tooth die,and genetic algorithm is used to optimize the preload interference value,so as to obtain high precision die.Secondly,three key system errors which have significant effect on the gear tooth accuracy are summarized in this research: machining error of shrink interference,coaxial error of shrink-fitting and parallelism error of hot forging die.The influences of these key system errors on hot forged NCSB-gear have been comprehensively researched by using the finite element model.On this basis,by adjusting the mutual matching relations among the three types of systematic errors,the purpose to offset the tooth form deviations caused by different systematic errors to offset each other is achieved,and the effective control of the tooth form accuracy of the straight teeth of hot forging non-conical gear is realized.According to the simulation results,a control method of gear accuracy of NCSB-gear is proposed by adjusting the matching relationship between these system errors,so that the gear accuracy of hot forged NCSB-gear can be significantly improved due to the tooth deviation caused by different system errors can be offset by each other.