Research About Influence Of Special Connection Form On Bolt Joints Self-loosening Under Transverse Excitation

Bolt connection is one of the most widely connected forms in mechanical,aerospace,civil engineering and other fields,and seems to be small but very important.As an important connecting part,it's reliability determines the safety of the whole structure.Especially under vibration conditions,loosening is one of the main causes of bolt failure.With the development of science,the progress of the times,the form of bolted connections and the joints are also showing a diversification trend,and the study of the classic bolts loosening hasn't fully explained the new problems.So,this paper,combined with the engineering practice,explores three special forms of bolt connection,and some significative conclusions are obtained by combining experiment data and analyzing experiment phenomena.The main contents and conclusions are as follows:(1)The development process of thread and bolt theory is introduced in detail.The principle of bolt tightening and loosening is described on the theoretical level,the bolt tightening methods and anti-loosening measures are described.The finite element simulation analysis of bolt tension state is carried out,and the real stress distribution is obtained.The finite element result shows that the first 3 turns working thread of bolt bear the most load,causing the high stress.(2)Because of the great number of tests,the complicated test conditions and the large number of test variables,the detachable fixture suitable for transverse excitation has been developed to transform the variables during the test.It is important to accelerate the test and reduce the cost.(3)In the study of the influence of thickness and hardness of the non-metallic gasket on bolt loosening,it is found that the thicker non-metallic gasket can improve the loose life of the bolted connection.In the thick non-metallic gasket hardness test,it is found that the harder gasket can improve the bolt connection life;in the thin non-metallic gasket hardness test found that the soft gasket will be compacted under the preload,and can be damaged with the vibration load,so it is recommended to use more powerful,harder and stronger gasket.(4)In the probing test of the best tightening torque of the caged nut,the bolts should be fastened with calculated torque according to the torque specification.Because the spring card of the caged nut has a lower strength,excessive torque will cause the card skewed,and too small torque may not be able to tighten the bolts normally,resulting the bolted connections loosening rapidly.(5)In the study of the impact about the joint surface coating to bolt loosening found that single-layer epoxy primer is easy to be grinded off,so the metal parts will be exposed and rusty.The coating of epoxy primer +2 layer of polyurethane topcoat is thicker and more interface,resulting a low stability under the vibration environment,which can't keep a high bolt pre-tightening.And the epoxy primer + polyurethane topcoat coating is thinner than 3 layers coating and can protect the primer from being worn off,shows the best results in the tests.