Research On Key Technology Of Large Module Formed By Near-net Rolling

Gear is one of the important parts of machine. The large module gear is widely used in heavy industry. At present, cutting processing is mostly used for gear manufacturing, while the processing with multi-process and low efficiency. Gear plastic forming process can improve productivity. maintain the continuity of the metal flow lines of the teeth, and improve gear strength and service life. The precision die forging bevel gears with small module and small diameter has been taken in industrial applications, and also the small module splines has been manufactured by cold roll forming. But the forming load is very large and the demand of dies is very harsh for forging large module and big size gear. Therefore, the key technologies for near-net forming large module gear by hot roll forming process were put forward to study in this thesis.The deformation behavior of SAE8620H gear steel at high temperature was studied by using the Gleeble-1500D thermal simulation machine. The stress-strain curves were obtained. According to the experiment results, the material constitutive model was modeled. The model used hyperbolic sine functions and the effect of strains on the constitutive equation parameters was taken into account. The relationships between strains and flow stress of gear steel can be well described used the model, which provided material data for simulating the hot roll forming process of large module gear. Experimental study of the influence of temperature and holding time on SAE8620H gear austenite grain growth was done. And the influence of deformation temperature、strain rate and strain on SAE8620H gear steel austenite grain refinement was studied by unidirectional compression experiments. The microstructure evolution models were established based on these data.A hot roll forming process of large module gear was proposed. The influence of engagement backlash variation of driving gears on the relative transfer movement of the roller and the workpiece was analyzed. The parameters relationship between the roller teeth and the rolling teeth was established. The dimensions of the roller teeth and the blank were defined considering the thermal expansion. The finite element simulation model of hot roll forming large module gear was established by using the DEFORM software. The forming process was analyzed, and large module gears were obtained through experiments. The rolling forces during the rolling process were comparative analyzed between simulation and experiment.Finite simulation method was used to study the metal flow laws during the growing of the teeth. The insufficient filling reasons on the ends of the workpiece were analyzed. And also the influence of process parameters on the tooth end filling was studied. A concave blank design method was proposed to compensate the end fill. The simulation and experiments results verified the effectiveness of this method. The influence of relative sliding between roller teeth and rolling teeth on the tooth tip pulling was analyzed. The effect of process parameters on the tooth tip pulling was studied by simulation. Tooth tip pulling was eliminated by increasing the forming temperature, reducing the contact friction coefficient, and also with the press of the roller teeth root during the rolling process.The accuracy influence factors of rolled large module gear were studied. The mathematical models was established to describe the hole diameter deviation how to produce tooth pitch error and tooth profile shape deviation, and the influence laws of the hole size variation on pitch deviation and the maximum deviation of the tooth profile were analyzed. The mathematical models was established to describe the bending deformation of the shaft how to produce tooth profile error and tooth helix deviation, and the influence laws of the rolling force and shaft diameter on the maximum deviation of the tooth profile and helix deviation were analyzed. By measuring the rolled gear, the accuracies were obtained:single pitch error, helix error, and radial runout error all reach grade 12; the tooth profile error exceeds grade 12 precision 55.9μm; the cumulative pitch error tolerance is bias largely comparing with grade 12. The calculation deviation of total cumulative pitch is well corresponding to the measurement value. The total calculation deviation of tooth profile is 67.4μm. the rolled tooth profile can reach grade 12 excluding calculation deviations.SAE8620H gear steel microstructure evolution model was called in to write a subroutine. The grain size distribution law of hot roll formed large module gear was analyzed with finite element simulation. The effects of rolling temperature, feeding rate and roller rotation speed on the microstructure evolution during hot roll forming large module gear were studied. The average grain size error is 11.7% between the experiment and simulation results, which proved the reliability of the established microstructure model. The results can guide the technological practice.