Research On Bending Fatigue Life Of Transmission Gear For Metro Vehicle Gearbox

In the transmission system of metro vehicles,the gear as an important part of the mechanical transmission system has the characteristics of high efficiency,long service life,accurate transmission ratio and high working reliability.The working conditions of urban subway vehicles are changeable,and the transmission gear is easily subjected to the alternation of loads such as light load and heavy load.With the increasing mileage of urban rail subway transit network,the service mileage of vehicle is getting longer and the working environment is more and more bad.It increases the probability of bending fatigue and failure of transmission gears,and seriously affects the safety of driving.In order to check and maintain the bending fatigue crack of the driving gear in the gear box,the government will spend huge manpower and material resources every year,and fatigue cracks bring huge economic losses to the domestic urban rail transit industry.Firstly,the characteristic curve of the traction motor of a metro vehicle is analyzed in this paper.The loads of different working conditions are calculated and the tooth root stress of the gear of the gearbox is analyzed.According to the establishment of the gear pair finite element model,the driving torque of the vehicle under different working conditions is used as the basis for the finite element transient dynamic analysis of the gear meshing process.The stress-time history curve of the tooth root in the gear meshing process is obtained,and the stress at each node of the tooth root is calculated.The location where bending fatigue occurs is analyzed.Through finite element analysis,it can be seen that the gear tooth roots are subjected to the pulsating cyclic load during the meshing process.The maximum tensile stress occurs when the gear meshes with the indexing circle,and the direction of the maximum principal stress at the tooth root is along the tangential direction of the tooth root.Secondly,based on a large number of parts for bending fatigue test,the life and fatigue limits of gears under various stress levels are obtained and plotted as S-N curves.After that,the position and path of the fatigue crack initiation of the tooth root are analyzed by a profiler.The test results of tooth root bending fatigue show that the crack originates in the middle of the arc of the tooth root,and the direction is the vertical direction of the root tangent.Combined with the finite element analysis results,it can be found that the crack at the root of the tooth is originated from the position of maximum tensile stress,and its expansion behavior is dominated by the maximum tensile stress.Finally,the load of the running metro is made into the load spectrum,and the low-high-cycle composite load bending fatigue test is performed on the driving gear.Based on the fault diagnosis theory,the strain values of crack initiation and expansion on the tooth side of the test gear are monitored.Through data processing,the relationship between strain and crack length,crack length and bending fatigue life are obtained,and the crack initiation life and crack propagation are judged.Through low-high-cycle composite fatigue test,the prediction model of crack initiation and extended bending fatigue life based on the load-load ratio parameters under service conditions are proposed.For the crack initiation life prediction model,the single stress amplitude or stress peak in the traditional stress-life model is replaced by the load ratio(?_H/?_L).For the crack propagation life prediction model,these two constant parameters C and m in the formula are fitted as these new parameters can be added to the model by using the primary functions C(?_H/?_L)and m(?_H/?_L)of the load ratio as independent variables,which can more directly reflect effects of load ratio and composite load on crack propagation.By changing the physical quantity and parameters,the improved method is simpler and the intention is more obvious,and a more accurate life prediction model is obtained.It is verified that the maximum error of the fatigue crack initiation life prediction model is 17.7%,and the maximum error of the fatigue crack propagation life prediction model is 14.2%.