The Reduction Of The Shaft Cross-sectional Area Has An Effect On The Fatigue Life Of The Connecting Rod

As a high-efficiency thermal power machine,the internal combustion engine is widely used in many fields such as automobiles,ships,railways,engineering and construction.The diesel engine for vehicles develops in the direction of lightness,high speed,high pressure and high power.The strengthening index is continuously improved and the mechanical load is increasing.The requirements for engine design are also raised:improving efficiency,increasing reliability,reducing weight,reducing the rate of the consumption and emissions,etc.,the corresponding engine components also put forward higher strength requirements.The connecting rod is an important part of the crank and connecting rod mechanism.During the reciprocating working cycle,the connecting rod is subjected to periodic alternating action from the peak explosion pressure and the reciprocating inertial force of the piston assembly.It requires the connecting rod to have sufficient structural rigidity,and fatigue strength to ensure reliable workTaking the connecting rod of a non-road high-pressure common rail diesel engine as the research object,UGNX9.0 was used to establish a three-dimensional model including the connecting rod body,the connecting rod cover and the bushing Analyze the effects of yield,fatigue,buckling,lubrication of big-end bearings,and fatigue life of the connecting rod by changing the width and thickness of the cross-section geometry.The connecting rod is optimized by changing the geometrical dimensions of the connecting rod shaft.Then the static and buckling analysis of the optimized model is carried out,and then analysis the sensitivity of the yield,fatigue and buckling safety factor of the connecting rod decreases with the reduction of the cross-sectional area of the connecting rod.Based on the finite element method and multi-body dynamics method,the dynamic fluid lubrication dynamic model of the big-end bearing of the connecting rod is established.The accuracy of the finite element model of the connecting rod is verified by the modal test of the connecting rod.On the basis of dynamic stress recovery,the fatigue life analysis of the connecting rod was carried out by using fatigue analysis software(ncode DesignLife).The main findings are as follows:(1)Statics and buckling analysis of connecting rodThrough the static analysis of the connecting rod,under the maximum compressive stress condition,the maximum stress position of the shaft part is close to the point A of the big end of the connecting rod,and the stress value is 235.22 MPa.The position where the tensile stress of the connecting rod is the largest is the position of the oil hole at the top of the small-end,and the stress value is 97.49 MPa.With the gradual decrease of the section width of the connecting rod,the maximum compressive stress position is the same in the five schemes,and the maximum compressive stress value decreases first and then increases.After the cross section of the connecting rod body changes in the thickness direction,the position of the maximum compressive stress remains unchanged,and the maximum stress values are substantially the same,and the difference is substantially 0.02 MPa.It can be seen that in the five schemes of the thickness direction of the connecting rod shaft section,the change of the width of the connecting rod section has almost no influence on the maximum stress position and the stress value under the action of the pressure.The Linear buckling analysis of the connecting rod is mainly to predict the sensitivity and buckling mode of the member,and to provide initial defects for the subsequent nonlinear buckling calculation.After the initial defect is introduced,the nonlinear buckling analysis of the connecting rod of original size is carried out,and the buckling critical load of the original size of the connecting rod is 312.03 kN,and the critical buckling stress is 601.87MPa.The critical buckling load will gradually decrease as the section size decreases.(2)Sensitivity analysis of the yield,fatigue and buckling safety factor of the connecting rod decreases with the cross-sectional area of the connecting rodThe static calculation of the model after changing the section size of the shaft is used to obtain the stress values of each reference point under the peak explosion pressure condition and the maximum tension condition.The nonlinear buckling analysis results in the critical buckling stress of the connecting rod.The three-standard sensitivity analysis of the diminishing width and thickness of the integrated link section shows that the sensitivity of the fatigue safety factor is the highest in the process of width and thickness variation,followed by the buckling sensitivity.Moreover,the variation of the width and the thickness in both directions,the buckling sensitivity under the width is 3.24,and the buckling sensitivity under the thickness is 4.65,and the buckling sensitivity in the thickness direction is greater than the buckling sensitivity in the width direction(3)Elastic fluid lubrication dynamics of connecting rod big head bearingThe modality of the structure is related to the accuracy of the simulation calculation.The modal analysis of the connecting rod is carried out by finite element calculation and modal test.The relative error between the simulated modal and the experimental modal of the connecting rod is within 5%,which verifies the accuracy of the finite element model of the connecting rod.From the data analysis,it can be seen that the maximum compressive load of the big-end bearing increases first and then decreases in width and thickness schemes,and the maximum tension load decreases gradually.The maximum oil film pressure of large head bearings increases with the increase of rotational speed,and the maximum oil film pressure also increases correspondingly.From the fluctuation of data,it can be concluded that under the width scheme,the maximum oil film pressure has little effect.Under the thickness scheme,the maximum oil film pressure increases first and decreases(4)Fatigue life analysis of connecting rodOn the basis of the multi-body dynamics calculation of the connecting rod,the stress recovery of the connecting rod is obtained,and the load spectrum of the connecting rod under a complete crank angle is obtained.The software(ncode DesignLife)calculates the number of fatigue life cycles of the connecting rod.The minimum fatigue life cycle number of the connecting rod under each model is greater than 107,which meets the design requirements of infinite life.