Analysis Of Transmission Mechanism Of Bolt And Coupled Dynamic Characteristics Of Hydropower Station

With the rapid development of hydropower and the increasing installed capacity,the structure being bigger and frequent changes of working conditions have become an inevitable trend.The mutual influence and stability problems between coupling structures have become increasingly prominent.Many parts of the hydropower station are combined by bolts,and the stiffness of the combined components is relatively low.It also has certain influence on the vibration characteristics of the overall coupled structure,which should be paid attention to in the design.However,due to the numerous and complex influencing factors,there has not been much research on the bolting problem of hydropower station coupling at home and abroad.It is limited to study the influence of vibration on the bolt relaxation creep.In this paper,the connection sliding law and dynamics modeling method of bolts are discussed.The finite element model is established based on the actual engineering.Different schemes are used to simulate and analyze the looseness and different preloading conditions,and the effects of bolting connection to the hydropower station coupling structure is discussed.The main conclusions are as follows:1.The finite element model displacement-load curve of single lap and single bolt is analyzed and the four dynamic modeling methods are compared.It is concluded that increasing preloading will improve the tangential bearing capacity of the bolted joint connection.The slip process can be divided into three stages: friction bonding,partial slip and full slip.Compared with rigid connection method,MPC method and spring damping method,virtual medium method has advantages of convenient modeling,high versatility and high calculation accuracy.It is more suitable for engineering with a large number of bolts,which provides an effective modeling method for studying the dynamic characteristics of coupled structures of hydropower stations.2.The virtual medium method is used to establish the finite element model of the hydropower station with the lower frame.The looseness of the bolts of the foundation plate is simulated and the modal and dynamic response analysis is carried out.Also,the horizontal forces of each arm are extracted and compared.The results show that: Loosening of the bolt will reduce the minimum static stiffness of the frame and the pier and the radial static stiffness.It will also increase the von-Mises stress at each point of the lower frame,which is easy tocause material damage,which is not conducive to stable operation of the unit;loosening also reduces the natural frequency of the coupled structure.The local arm vibration form appears,and the amplitude of each part of the frame and the plant is increased,which is unfavorable for the structure to resist vibration;the horizontal bearing capacity of the loose arm connecting surface is drastically reduced,and the force of the adjacent two arms is sharply increased.Bolts are prone to fatigue failure first at these two places.3.The modal and harmonic response analysis of the hydropower station coupling structure under five preloading schemes is carried out.The results show that the preloading force has a greater lateral impact on the plant structure than the longitudinal direction.The increase of the preloading force improves the unit support to a certain extent.The rigidity of the coupling structure of the plant greatly reduces the amplitude of the frame,which is conducive to the stable operation of the unit;when the preloading force is small,the connection is not tight and the vibration loss is severe.At this time,the actual preloading state should be fully considered in the research process.In order to ensure the accuracy and reliability of the calculation results,when the bolt preloading reaches 60-70% of the material yield strength,the rigid connection can be directly used in the modeling to improve the working efficiency,and the calculation accuracy can be ensured.