Theoretical And Experimental Study On Heat Transfer Of The Form Grinding Process Of Involute Gears

Precision gears are widely used in various industry sectors.The transmission performance as well as the load capacity of the gears determines,to a great extent,determine the performance and quality of the mechanical equipment.The transmission capacity and service life can be increased by several times and the sizes and weights of the actuators can be significantly reduced due to the use of the hardening technology.The hardened gears are usually obtained by heat treatment after pre-machining process.Grinding is typically used,as the main finish process after the surface hardening,to eliminate the gear tooth deflection caused by the heat treatment and to further improve the accuracy and the tooth surface roughness.Form grinding is an effective process for the finish machining of precision gears,which is mainly used for the machining of gears with large modules and dimensions after case hardening.To ensure the load capacity and service life of the heavy-duty gears,the physical and mechanical properties,i.e.,the surface integrity,of the tooth flanks must be strictly controlled.The grinding force,grinding thermal and grinding power present complex variation with the variation of the gear parameters and grinding parameters in the grinding process,in many cases,will deteriorate the gear surface quality,changing the micro-structure of the surface material,reducing the surface hardness,producing tensile residual stress on the gear tooth surfaces,significant impact on the contact fatigue strength of gearing tooth surfaces.The heat transfer problem of plane grinding and cylindrical grinding which the contact relationship between the workpiece and grinding wheel is relatively simple had been researched by the many scholars at home and abroad.The analytical model of grinding temperature and the sub heat distribution model had been established base on the moving heat source model,inclined moving heat source model and pitch arc moving heat source model.But it is difficult to study the transient heat transfer problem of gear grinding,in which process the contact relationship is more complex.There are many problems need to be solved.First,to establish cambered moving heat source model under the condition of three-dimensional contact.Second,to establish the three-dimensional non-uniform distribution of heat flux on the gearing tooth surfaces.Third,to study the variation of heat partitioning along the gear tooth profile.This thesis is focused on the study of form grinding process of involute spur gears.Through systematic analysis of the variation of the contact geometry between the grinding wheel and gear surface along the tooth profile,the models for predicting the grinding force,grinding power and grinding temperatures are established for the first time whether at home or abroad.A theoretical model to calculate the heat partition to the different heat sinks,i.e.workpiece,chips,fluid and grits,are established.Based on the theoretical analysis,the variation of heat partitioning along the gear tooth profile under different process parameters are investigated.Based on the grinding force distribution model and the heat partition model,the three-dimensional heat flux distribution on the interface between the grinding wheel and the gear flank is further determined.The burn thresholds of power in the form grinding of carburizing steel gears are determined by relating the grinding temperatures with the Barkhausen Noise and micro-hardness measurements on the ground surfaces.The models and the determined burn thresholds can be used for presetting the process parameters and conditions and monitoring the grinding process to avoid the occurrence of thermal damages to the gear surfaces.Numerical simulation of three dimension stress field in gear form grinding is carried out by using the results of the three-dimensional heat flux distribution on the interface between the form grinding wheel and the gear flank as inputs of heat load.The distributions of residual stresses in the machined gear along the tooth profile and the depth direction were calculated respectively.The effects of the gear parameters,i.e.,modulus and number of teeth,on the geometric contact relationship between grinding wheel and the gear flank,the distribution of grinding force,grinding temperature and residual stress are comparatively analyzed to support the building of gear grinders for making high precision gears with large dimensional sizes.The theoretical analysis and experimental results show that the grinding force on the tooth profile is not uniformly distributed,it increases almost linearly with the change of the rolling angle.The non-uniformly distribution of the grinding force on the tooth profile would lead to uneven wear of the grinding wheel.The grinding power under different grinding parameters and conditions are measured by a Hall-effect power measuring meter.The distribution of grinding force under different grinding parameter conditions are measured using a specially designed test rig.Both the grinding power and grinding force measured are compared with the results calculated using the analytical models.It shows that the experimental and theoretical results have a reasonally good agreement.The heat flux distributed along the wheel/gear interface has a non-uniform source with three-dimensional curved shape.The surface heat source has a triangular distribution along the contact length,whilst its distribution along the gear profile.has the same trend as the product of the grinding force and wheel speed.The model of heat partitioning to the workpiece along gear profile either under wet or dry grinding condition are also established.Based on the theoretical analysis,the variation of heat partitioning along the gear tooth profile under different process parameters has been investigated.The rolling angle has significant effects on the distribution of the temperatures along the profile of the straight spur gears studied in the present work.Along the tooth profile,the temperature increases with the rolling angle.The maximum grinding temperature appears on the tip of the tooth profile.The grinding temperatures are measured using a special test rig designed with thermocouples imbedded into the workpiece at different measurement positions.The theoretical temperature distribution pattern along the gear profile has a rather good agreement with the experimental measurement results.The theoretical and experimental studies of the distribution of residual stress in the gear tooth profile grinding are carried out,both the simulation results and measurement results of residual stress show that the direction of the principal stress is in the direction of tooth profile.The tensile residual stress generated on the area where the grinding temperature is over 300℃ of the tooth surface while no residual stress generated on the area where the grinding temperature is below 300℃.The distribution of residual stress on the tooth surface is uneven and the maximum residual stress appears on the top of the tooth.The shape of the distribution of grinding temperature along the tooth profile is more flat when grinding those gears with large module and large number of teeth.The maximum residual stress on the tooth profile reduced with the increase of the module and number of gear.These phenomena indicate that under the premise of that the grinding ability of the grinding wheel is steady the tooth profile with high surface integrity will be obtained adopts form grinding method when grinding those gears with large module and large number of teeth.