| Screw joint is widely used in aerospace,general machinery,construction installation and commercial manufacture.In many cases,the screw joint needs to bear cyclic load.Under the condition of vibration,the loosening of screw joint is more likely to occur.It is a huge hidden danger that the screw joint in the key position loosens during the service period.There are many factors that affect the looseness of screw joint,the mechanism of looseness is not unified,and the methods of preventing looseness have advantages and disadvantages.In this paper,the screw joint is taken as the research object,and the loosening mechanism of screw joint under cyclic load is studied and analyzed from three aspects of theoretical derivation,finite element simulation and experimental verification,and the anti loosening methods of single bolt joint and bolt group joint are preliminarily discussed.(1)Based on the Yamamoto theory,the deformation of the engaged thread of the thread joint under the axial load is analyzed,and then the load distribution of the thread joint is studied based on the Yamamoto model.The corresponding model parameters are calculated for the 8.8-grade M8 common thread(triangular thread),and the analytical solution of the load proportion of each thread is obtained.The results show that the first thread bears 25.0%of the load,the second thread bears 19.3%of the load,the third thread bears 15.1%of the load,the first three thread bears 59.4%of the total load,and the last thread bears almost no load.It is concluded that the load distribution of the common thread joint is seriously uneven,the load of the first thread of the engagement thread is the largest,decreasing in turn,and the front first three threads carry most of the load.(2)According to the characteristics of screw joint and the behavior of materials under alternating load,based on the elastic-plastic theory and the mixed hardening model obeying Ziegler's follow-up hardening criterion,the elastic-plastic constitutive model of materials is constructed in ABAQUS,and the finite element simulation model of screw joint is established.Through simulation experiment and vibration experiment,the reason of screw joint entering material loosening period is studied and analyzed,so as to explore the mechanism of screw joint loosening.According to the monitoring of the stress concentration area and the axial force in the simulation experiment,the ratchet effect of the screw joint with a pre-tightening torque of 25.5Nm is obtained,and the axial force decreases with time going.However,the axial force of the threaded joint with the pre tightening torque of 5 Nm has almost no decrease.It is concluded from the results that the plastic deformation occurs in the area with large load-bearing ratio under large pre-tightening torque,and ratchet effect occurs in the material under the action of alternating load,which results in the expansion of plastic deformation area,leading to the screw joint entering the material loosening period.According to the mechanism of research and analysis,a method of thread joint anti loosing is put forward.Through the finite element simulation,the load distribution of the screw joint with different ring groove parameters is analyzed,and a kind of evenly distributed ring groove nut is obtained.(3)Based on the theory of elastic interaction,the theoretical model of bolt group joint sequence tightening is established.Using ABAQUS,a bolt group simulation model of six M16 Grade8.8 bolts with uniform circumferential distribution is established,and 11 tightening sequences(modes)are simulated and analyzed.By statistical analysis of the final preload results,a better uniformly distributed tightening sequence is obtained,so as to avoid loosening due to uneven stress. |
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