Research On Loose Mechanism And Vibration Test Method Of Subway Bolts

Bolts are widely used in the connection of subway vehicle parts due to their advantages of simple structure,convenient disassembly and low price.The connection performance of bolts in important parts will have an important impact on the safety and reliability of the overall structure of subway vehicles.Because it is very important to study the mechanism of bolt loosening,experts and scholars at home and abroad have conducted extensive research on it through a large number of experiments and theoretical analysis.At present,most of the special bolt coupling structural loosening testing machine is used to conduct transverse vibration test to study its loosening mechanism,but most of the subway bolts are subjected to axial load and random vibration,so it is difficult to study them directly by transverse vibration test.Therefore,this paper takes subway center pin bolt as the object,determines its main failure mode through FMECA analysis,reveals its main failure mechanism,and studies its vibration test method based on the vibration test system,providing important theoretical guidance for subway bolt loosening prevention.The main research work of the thesis is as follows:Firstly,the background and significance of this paper are briefly introduced,the mechanism of bolt looseness and the research status of bolt looseness test at home and abroad are summarized,and the main factors affecting bolt looseness and the measures to prevent bolt looseness commonly used in engineering are summarized.On this basis,the research content of this paper is put forward.Secondly,the installation position and working condition of subway center pin bolt are analyzed,and its load characteristics are determined.Based on the theory of bolt connection,the load on each ring thread of the bolt in the center of subway is analyzed under the given preload moment.The stress and strain nephogram of the engagement of screw teeth is obtained by using finite element software.Thirdly,according to the fault report of the maintenance personnel and the failure condition of the same type of bolt under the similar use environment,the FMECA analysis of the subway bolt is completed in combination with the probability level and the damage degree level of various failure modes.It is determined that the main failure mode of the central pin bolt of the subway is loose.The main failure mechanism is that the fretting wear between the contact surfaces caused by vibration causes the pre-tightening force to decrease,thereby causing looseness.Fourthly,based on the vibration test system,the vibration test fixture of subway center pin bolt is designed.The resistance strain gauge is used to monitor the axial strain of the pad to characterize the change of bolt connection pretension,so as to monitor its loosening process.Axial vibration tests were carried out under different random vibration orders and different initial preload,and the relationship between vibration orders and initial preload degradation of bolts was analyzed.Based on the test data,the degradation model and acceleration model of preload under random vibration are established,and the acceleration test method which can accelerate bolt looseness in subway is put forward.The vibration test after applying letai 542 thread fastening adhesive is further carried out,which provides a way to improve the reliability of subway bolts.At last,the research work is summarized and the research direction of this topic is given.