| As an important source of damage in the mechanical structure,threaded connection,improper design will cause mechanical connection failure.The large-end threaded connection of the connecting rod is a key technology in the design of the crank connecting rod mechanism.Its structure and stress form are complex and fail There are various forms,and the failure of the connection structure will cause the functions of the related components of the connecting rod to be affected or even completely lost.Aiming at the fatigue failure of the connecting rod big-end threaded connection structure,through the combination of theoretical analysis and numerical simulation,the fatigue reliability and structural optimization design are studied to improve the reliability of the connecting rod big-end bolt and ensure the s Tab operation of the crank connecting rod structure It has important engineering application and theoretical research value.In this paper,the H16 steel forged connecting rod big-end screw connection structure is taken as the research object,a mesh size that takes into account the convergence and accuracy is proposed,and a reasonable finite element model is established to affect the stress distribution law and different effects of the standard bigend screw connection structure of the connecting rod.Study on the influence of factors;evaluate the strength of connecting rod big-end screw connection structure by fatigue safety factor method;predict the fatigue life of connecting rod big-end screw connection structure based on Miner damage criterion;Combining with the fatigue life design method,an Optimized thread optimization coupling structure is proposed based on the optimized structural parameters,and compared with the conventional thread coupling structure from the perspective of fatigue reliability.Research indicates:(1)The axial load of the connecting rod big head bolts shows a gradually decreasing trend from the head to the bottom of the nut.The stress concentration at the meshed first thread is more serious.For a single thread,the stress concentration at the root of the thread is more serious.The stress failure point at the first thread is mainly concentrated in the 45°～90° and 270°～315° rotation angle of the thread.(2)When the connecting length of the big-end thread of the connecting rod is constant,the smaller the pitch,the smaller the difference in the load-bearing ratio of each twisted thread.After the thread section is more than 6mm from the top surface of the nut,the load-bearing ratio of each twisted thread tends to be uniform,And with the increase of the loading ratio,the trend is more significant;when the pitch is unchanged,the first thread is most affected by the preload force.Under the action of the bolt preload,the load ratio of the first thread will decrease by about 10%;If the length of the threaded connection is too large or too small,the meshing effect will be reduced.With the increase of the working load,the load ratio of each thread tends to be uniform.The first thread shows a decreasing trend.The shorter the threading length,the more obvious the decreasing effect.(3)For connecting rod bolts with different structural parameters,the safety factor of the first thread engagement is the smallest,followed by the connection interface between the connecting rod body and the big head cover,the safety factor of the joint between the bolt head and the screw is the largest,and the fatigue safety factor is the smallest At the meshing position of the first thread,all three areas are vulnerable areas.(4)For bolts with the same nominal diameter,as the pitch increases,the average fatigue life shows a downward trend.For bolts with a nominal diameter of 8 mm,the maximum life appears when the pitch is 1,and the size is 3.35×108 cycles;As the thread length increases,the average fatigue life increases first and then decreases.The maximum fatigue life occurs when the thread length coefficient is 0.76 and the size is 3.30×108 cycles.The non-conventional thread optimization structure can generate a larger normal contact force than the conventional thread connection pair,the friction force is also greatly increased,the anti-loosening performance is improved,and the reliability is also greatly improved. |
