| Gear is one of the most important mechanical parts in mechanical transmission.Conventional gear processing methods can be divided into cutting and plastic forming.Due to the advantages of high material utilization,high production efficiency,good mechanical performance and the ability to overcome a series of problems such as big deformation force,incomplete filling of the tooth corner,mold easily damaged,gear rolling received increasing attention.In the forming process,the forming relation between the mold and the cylindrical blank is similar to that between the cutter and the blank in gear cutting,and the cylindrical blank is gradually formed through continuous rolling by the tooth profile of the mold.Gear rolling technology is divided into two basic processes:gear rolling and rack rolling based on the mold.In the gear rolling process,the gear mold can mesh the blank repeatedly,so the accuracy of the formed gear is higher,and it overcomes the problem that the time of mesh is limited by the length of the rack mold,and it saves space,so the gear rolling with the gear roller has a better application prospect.However,in the initial phase of gear rolling,slippage occurs easily on the contact surface between the gear roller and the blank,which causes staggered teeth and other defects,influencing forming quality of the gear to be rolled seriously;In the middle stage of gear rolling,sharp protrusions appear and gradually grow larger on both sides of the tooth top,the rabbit ear forms.The existence of the rabbit ear makes metal prone to fold in the late phase of gear rolling,and the removal of the rabbit ear results in time and materials wasting,and metal fiber cut,which will decrease the mechanical properties of the gear.The existence of the slippage and the rabbit ear can seriously affect the forming quality of the gear rolling,restrict the actual application and development of gear rolling.At the same time,the forming theory and technology system of gear rolling are not perfect,the influence of various factors on the forming quality has not been proved.Therefore,it is necessary to study problems above and others encountered in the research and application of spur gear rolling process by the use of the theoretical analysis,numerical simulation,optimization and experimental research.Based on the metal plastic forming theory and the generation-motion relationship between the gear roller and the blank,this paper analyzes the relevant process problems in gear rolling.According to the requirement of the cent tooth uniformity,based on the principle of the equal chord length,a method for determining the initial diameter of the blank is proposed;For the gear rolling process with a single gear roller,or that with two gear rollers rolling the gear of singular or plural tooth number,the stress state of the blank in the initial stage of gear rolling is analyzed,and the factors influencing the natural biting are determined,the natural biting condition that the blank rotating with the gear roller without slippage in the initial stage of the gear rolling is established;Besides the initial diameter of the blank,whether there is any slippage and the size of the slippage in the course of the bite also affects the cent tooth uniformity.A forced biting method is put forward in order to avoid slipping in gear rolling,and the allowable range of the slippage which can meet the requirement of the cent tooth uniformity is proposed.In order to ensure the forming quality of the gear,aiming at the tooth pitch of the blank decreasing continuously in the gear rolling process,based on the equal linear velocity principle of the gear roller and the blank,the matching relationship between the velocity of gear roller and that of the blank is given,and in order to avoid the influence of the variable pitch on the pure-rolling relationship between the gear roller and the blank,the speed of the blank should be adjusted;In the gear rolling process,the maximum allowable value of the feed rate should ensure that the cumulative pitch error is less than the extreme of staggered teeth for the requirement of the cent tooth uniformity,base on this and the analytic formula of reasonable range of feed rate is set up;Based on the generation-motion relationship between the gear roller and the blank,the influence of the tooth structural parameters of the gear roller on the forming quality of the tooth is analyzed,the design criteria for the tooth structural parameters of the gear roller are presented;Based on the engagement theory and the tooth profile equation,the angle corresponding to the contact and detachment position of the gear roller and the blank is deduced,the contact arc length on the uniform discrete position in the range of the spatial angle is obtained by using the tooth profile equation of the gear roller,and the numerical solution of the contact arc length changing with the rotation angle of the gear roller is acquired based on the geometric parameters of the gear roller and the forming gear of the research object in the paper.In the initial phase of gear rolling,slippage occurs easily on the contact surface between the gear roller and the blank,which seriously affects the quality of the gear to be formed.In this paper,according to the generation-motion relationship between the gear roller and the blank,the stress state of the blank in the initial stage of the gear rolling is analyzed,the main factors that affect slippage are determined;A quantitative index for the degree of slippage is established and the simulation is carried out,the influence of the main factors,such as the initial bite depth,the friction coefficient,the teeth number of gear roller and the number of the gear roller,on the degree of slippage in the gear rolling process is analyzed.In the gear rolling process,the rabbit ear can cause metal folding,even if there is no folding,subsequent excision will cause waste of materials and increase of working hour,and cutting the metal fiber will lead to the reduction of mechanical properties.In this paper,through the relationship analysis between the gear roller and the blank,the finite element model of gear rolling is established,and the gear rolling process is simulated,the regularity of changes in the tooth shape of the blank during the different stages of the gear rolling is given.Based on the analysis of the stress state,deformation distribution and metal flow,the formation mechanism of the rabbit ear is revealed:the metal on the tooth profile of the blank is extruded by the tooth profile of the gear roller,and flows in the direction of smaller resistance,that is to say,towards the two sides of the tooth top,which causes the velocity difference between the metal on the two sides of the tooth top and the metal in the central,thus,the two sides of the tooth top form protrusions,namely rabbit ear.Based on the analysis of flow condition and force condition of the blank,the forming process parameters and structure parameters of the gear rollers which influence effective tooth volume in the gear rolling process are determined;The finite element model of gear rolling is established,and the gear rolling process is simulated,the proportion of the effective volume to the whole volume of a single tooth is calculated,and their mean value is taken as the quantitative evaluation index of the effective tooth.The effect of blank's initiate diameter,the feed speed of the gear roller and the rotation speed of the gear roller,the friction factor between the gear roller and the blank,the tooth number on the gear roller,material of the blank,rolling temperature and the pressure angle of the gear roller,the tip diameter of the gear roller,the root diameter of the gear roller on effective tooth is analyzed in the gear rolling process.The existence of the rabbit ear seriously affects the quality of the forming gear,in this paper,based on the forming mechanism of the rabbit ear,according to the effect of various factors on the effective tooth in the gear rolling process,the control method of the rabbit ear is analyzed and acquired in gear rolling,the proportion of the rabbit ear volume to the whole volume of a single tooth is calculated,and their mean value is taken as the quantitative evaluation index of the rabbit ear,the process parameters of gear rolling and the structure parameters of the gear roller which influence the rabbit ear are optimized by orthogonal experiment.The design scheme of trapezoidal preform and petal shaped preform is proposed,the finite element simulation results show that,the trapezoidal preform design can play a role in reducing the rabbit ear,and preformed trapezoidal blank with different size parameters is further simulated,the size parameter of preformed trapezoidal blank which can eliminate rabbit ear is acquired.The method of installing cylindrical wheel to add external force in order to prevent and restrain the radial growth of the rabbit ear is proposed,the radial return position function of the cylinder wheel is acquired,the numerical simulation result shows that the rolling device with additional cylinder wheel has obvious effect on reducing and eliminating the ear.For the reliability validation of the finite element model and finite element simulation results,a set of double gear roller experimental apparatus designed and produced by the task group is used to develop different initial bite depth and different number of teeth on the gear roller in the cent tooth phase of gear rolling,the morphology of the rabbit ear and the form law of the tooth in the forming phase of gear rolling.By imprinting method,the slip amount of the sample is calculated,the change law of the slippage with the different initial bite and the different number of teeth on the gear roller is given,the trend is consistent with the simulation results;The morphology change of the rabbit ear observed in the forming phase of gear rolling,is consistent with the simulation results;Through image grid algorithm,variation trend of the effective tooth proportion of the rolled sample with different feed speed of the gear roller is given,which is consistent with the simulation results.The teeth morphology of the rolled lead sample is perfect without rabbit ear,which indicates that when the material is soft and the carbon steel is hot rolled,the rabbit ear disappear,which is consistent with the simulation results.Based on the above research work,the basic theory of gear rolling is improved,the influence law of gear rolling process parameters on forming quality is given,the formation mechanism of the rabbit ear is revealed,some effective measures to control the forming quality of gear rolling is put forward,it will lay a foundation for promoting the development of gear rolling. |
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