Study On Relaxation Of Initial Preload Loss Of Single Bolted Joint

With the continuous development of modern industry,the assembly process has more and more important influence on the service performance of large construction machinery.Bolt connection,as a common assembly method,is widely used in all kinds of construction machinery and has an important influence on the working performance of all kinds of construction machinery.At present,the main problem restricting the continuous development of bolt connection technology is the pre-tightening precision of bolts,in which the pre-tightening precision of single bolted joints is mainly affected by the short-term relaxation effect,but the research on the formation mechanism of short-term relaxation effect and its influence on the technological performance of bolted joints has not been fully carried out.The control of tightening precision of single bolted joint is the basic part of the whole process analysis,and the initial input condition of subsequent group bolts tightening process design and anti-loosening performance analysis.Compared to the regularity of group bolts elastic interaction,the discreteness of single bolted joint preload force is the key reason to the uncertainty of the bolt tightening process.Therefore this article will focus on short-term relaxation effect which affects the accuracy of single bolted joint pre-tightening precision,aiming to carry out meaningful measures to improve bolt connection technology.The specific research content is as follows:Firstly,according to the pre-tightening principle and stress situation of single bolted joints,the theoretical calculation is carried out to determine the internal stress distribution of single bolted joints.Through the analysis and comparison of various theoretical material creep models,the theoretical material creep model adopted in this paper is selected.Secondly,short-term relaxation effect experiments were designed and carried out.The mechanical properties parameters of the test pieces were determined by material tensile test.The correctness of the testing system is verified by the experiment of ultimate contact stress measurement.By testing the short-term relaxation effect under different loading modes,the existence of short-term relaxation effect and its important influence on the pre-tightening precision of single bolt were verified,and the average experimental data were obtained for the creep characteristic fitting of the material model.Thirdly,isight software was used for data fitting to endow the material with creep characteristics.A real thread finite element model is established,and the validity of the finite element model is verified by comparing the calculated results with the experimental data.Last,the finite element analysis of single bolted joint is carried out to analyze elements which affect short-term relaxation,including geometric parameters such as non-parallelism of end face,bolt hole,thread angle,thread diameter,thread pitch and tightening process parameters such as initial tightening load and repeated loading times.Aiming to analyze the causes of short-term relaxation effect and put forward effective measures to improve the pre-tightening precision of single bolted joint.