Research On Influence Of Surface Defects On Fatigue Lifetime Of Casehardened Gears

Gear transmission is widely used in mechanical transmission.With the continuous development of gear transmission system,tooth surface pitting damage and fatigue crack propagation have become the most common failure model of case hardened gears.Pitting corrosion will damage the integrity of the hardened layer on the gear tooth surface,resulting in stress concentration during gear operation,if not found in time,it will cause the large area spalling of the gear tooth surface and even fracture failure,thus affecting the service life of the gear.In practical engineering application,the tooth root of gear is prone to crack under high stress,and the crack expands rapidly,which leads to the fracture of gear teeth.Therefore,it is particularly important to perform failure analysis on damaged gears,especially gears with early damage,to correctly determine the gear damage type and calculate the remaining fatigue lifetime.In this paper,the development of gear defects in practical work and the influence of tooth surface pitt ing and tooth root crack on the fatigue lifetime of casehardened gears are analyzed and studied.The main researches and conclusions are the following:(1)Combining geometric physical characteristics and micro-effects,a three-dimensional EHL model of gear is established,and the influence law of various influencing factors(such as speed,load,surface morphology,etc.)on the lubrication state of gear meshing surface is studied.Based on the predicted minimum film thickness ratio,the effect of surface roughness on the pitting damage of tooth surface is evaluated,and the pitting initiation and propagation process are compared through gear contact fatigue test.(2)The common pitting pit shapes on the gear tooth surface is compared and analyzed in detail,and the influence of pitting pits of various shapes such as ellipsoid pits,cone pits,pentagonal pits,and triangular pits on the stress concentration factor under the same basic size is studied.The influence of the size change of ellipsoid pit and cone pit on the stress concentration factor is studied in detail,and the approximate calculation model of the stress concentration factor of ellipsoid pit and cone pit with three-dimensional size is established.According to the contact fatigue test data of a gea r,the established calculation model is analyzed and verified,which provides a reference for the subsequent research on the development mechanism and defect evaluation of gear surface defects.(3)The gear contact fatigue lifetime prediction model of stress concentration factor method and the nominal stress method prediction fatigue lifetime model of dynamic simulation are established,and the influence law of three-dimensional morphology of pitting pit on gear contact fatigue lifetime is analyzed.Finally,the two kinds of the contact fatigue lifetime prediction results of gear pitting defects are compared and analyzed.The variation law of the residual lifetime after the gear pitting with the length width ratio and depth width ratio of pitting is analyzed respectively when the ship lift of the Three Gorges is under the misloaded water depth of 5cm and 10 cm.(4)Based on the fracture mechanics method and fatigue crack propagation theory,the fatigue crack propagation law of gear tooth root is analyzed.For t he semi-elliptical cracks that often occur in practical work,the crack propagation law is analyzed,and the influence law of load,initial crack size,location and other factors on the stress intensity factor and crack propagation lifetime is studied respectively.The influence of the initial size and location of the root crack on the residual lifetime is analyzed when the ship lift of the Three Gorges is under the misloaded water depth of 10 cm.(5)The contact fatigue test of 18 Cr Ni Mo7-6 gear is carried out,and the change trend of contact fatigue lifetime caused by tooth surface pitting and the pitting failure situation of gear tooth surface are obtained.The contact fatigue test results are compared with the results of two kinds of the fatigue lifetime estimation models of gear pitting defects,which verifies the accuracy of the fatigue lifetime estimation models of gear pitting defects.(6)The bending fatigue test of 18 Cr Ni Mo7-6 gear is carried out,and the data points of gear bending fatigue test and the fracture failure situation of gear root are obtained.The propagation lifetime of different initial crack sizes under the same test load is calculated and compared with the test lifetime,which verifies the accuracy of the simulation analysis results.