Study On Loosening Mechanism Of Flexible Flange Bolt Connection In Steel Pipe Tower

Transmission tube towers are increasingly used in large-span and high-voltage lines with its unique advantages.Steel tube tower members are usually connected by flanged joints.It is often subjected to wind load,earthquake load and other dynamic loads.When the external load is impact load or cyclic load,the force of tower member will change.The flange forces the connecting bolts to bear axial forces and bending moments.Lead to deformation of bolts,and loose phenomenon.Therefore,the safety and stability of flanged joint are very important to the reliable operation of the tower..Based on the ultra high-voltage transmission line engineering,this paper studies the joints with flexible flanged bolts under axial load.Considering the thread flexibility of bolt connection,the mechanical model of flange bolt is established.The bearing capacity distribution and loosening conditions of bolts under axial load were deduced.A fine finite element model of thread connection was established by finite element analysis.The response of different contact threads under preload and axial load was analyzed.The stress distribution,contact stress of meshing thread and plastic deformation of screw thread were analyzed.The relationship between it and bolt loosening process was investigated.The stress,contact state,plastic deformation and bolt axial force of working thread were analyzed according to the combined conditions of different preload and axial load.Based on this,the influence of different alternating loads and initial preload on bolt loosening is given.The results are as follows: when the friction coefficient of the thread contact interface is greater than the tangent value of the thread Angle,the thread surface will not slip,so the influence of plastic deformation on bolt loosening should be considered when the bolt connection meets the requirement.With the increase of alternating load,thread stress,plastic deformation and plastic deformation accumulation increase,and axial force decrease rapidly.With the increase of the initial preload,the axial force increases first and then decreases,and the plastic accumulation of the thread decreases,indicating that the bolt connection structure is the most stable when the value of preload is in a range.Overall consideration,it is suggested that the initial preload is about 60% of the yield load of bolts.This paper lays a foundation for the study of structural stability of steel tube tower and provides a new idea for the method of preventing bolt looseness in engineering practice.