Mechanism And Experimental Study Of Looseness Of Fasteners In Vibration And Shock Environment

In aerospace vehicles,bolts are used as special fasteners to connect a large number of internal structures as a whole.During operation,these aerospace fasteners not only withstand the high-frequency,random and complex vibration and shock loads generated by the engine,but also withstand the low-frequency vibration loads generated during the transportation and lifting of the assembly process,and then make them in service is in a variety of extreme environments.In recent years,related problems caused by fasteners have occurred frequently,and have become a weak link that seriously affects the stability of aerospace connection structures.Therefore,research on the bolt loosening mechanism is of great significance to the development of aerospace engineering.This article aims to explore the bolt loosening mechanism and improve the bolt anti-loosening effect.Under the vibration and shock load environment,the theory,simulation and test of bolt loosening are studied.The specific contents are as follows:1)A mathematical model of loosening and slippage of bolts with thread angles is established,including slippage analysis models for three cases of no slippage on the contact surface,slippage on the thread surface and simultaneous slippage on the bolt head surface and the thread surface.And the mathematical model of slip was used to simulate the entire loosening process with Matlab program,thereby predicting the decline of bolt preload.2)Using finite element technology,a parameterized finite element analysis model of bolt loosening was established.In the analysis of bolt loosening simulation results,the macro and micro characteristics of the bolts were analyzed comprehensively.The macro analysis reveals the variation of the force and preload distribution of the screw,the change of the rotation angle of the nut,the bearing contact force and the bearing contact friction torque.In micro-analysis,the slip factor is proposed as the main evaluation parameter of micro-slip analysis.The characteristics of the sliding distance and sliding state of each sliding surface were studied,and the corresponding bolt loosening laws were obtained.3)The mathematical model of the bolt anti-loosening of the geometric non-parallelism of the bearing face is established.For the non-parallel bearing face,a bending moment will be generated on the bolt node.The bending moment will cause the lateral force of the thread,and the nut will move laterally.Research on generating additional contact force and contact friction torque,and play a role in preventing loosening of non-parallel bearing bolts.Then,the simulation model is used to analyze the anti-loosening process,and the anti-loosening process is evaluated through the micro-slip factor,macro-preload and nut angle.Finally,the effects of middle diameter,joint thickness and thread friction coefficient on the anti-loosening of the geometric non-parallelism of the bearing face are analyzed,and the influence rules of various factors are revealed.4)In order to verify the mathematical model of bolt loosening and the finite element simulation model,a single bolt loosening test plan was designed.The two models were verified through loosening tests.A single-bolt loose test bed is designed,and the accuracy of the theoretical model and simulation model is verified by physical experiment methods.The effects of thread friction coefficient and bearing surface friction coefficient,thread angle and pitch,and lateral vibration load on bolt looseness are analyzed.The verification of the theory of geometrical unevenness prevention of bearing faces proves and explains the mechanism of geometrical unevenness prevention of bearing faces.Finally,the influence of the middle diameter,the thickness of the connecting piece and the friction coefficient of the thread on the geometrical unevenness of the bearing face is analyzed,and the corresponding laws are obtained.