Study On The Stability Of Thin-walled Pipe During Rotary Straightening

Thin-walled metal pipes are widely used in energy,chemical industry,medical treatment,aerospace and other fields.In order to obtain high-precision thin-walled pipes,oblique roller straightener is usually used to straighten the pipes.The initial curvature of each section before straightening is different in size and direction.First,equal curvature straightener is used to make the pipes undergo large plastic deformation,so that the curvatures of the pipes tend to be consistent in all directions,and then high-precision thinwalled pipes can be obtained after undergoing small curvature reverse bending elastic recovery.Therefore,the basic method of straightening thin-walled pipes is "unified before straightening".However,thin-walled pipes are prone to overall instability in the stage of large curvature uniform deformation,resulting in surface wrinkles,cross-sectional distortion,pipe fracture and other defects.At present,the domestic straightening of high-precision thin-wall pipes often adopts the method of repeated test straightening,which has low production efficiency and precision and poor quality stability.In order to improve the qualified rate of pipe straightening and obtain high-quality thin-wall pipes,this paper analyzes the large curvature plastic deformation stage which is easy to cause pipe instability during straightening,and through the stability research of curvature unified stage,finds out the key factors influencing pipe instability,analyzes the instability phenomenon,and also provides a solution for the determination of core parameters such as the radius of curvature of straightening roller and the lead of straightening roller.This paper mainly studies the following aspects:(1)Starting from the actual straightening model of pipes,the straightening mechanism of pipes is analyzed,and the influence of the lead of straightening roller and bending curvature of pipes on the uniform speed and accuracy of curvature is analyzed by graphical and mathematical induction.The experimental verification platform is designed based on the principle of four-point bending of pipes,which is verified effectively.Finally,the selection principle of straightening roller curvature is determined.(2)The expression of cross-section bending moment in the process of pipe reciprocating bending is established,and the key factors influencing the stability of pipe rotating straightening process are found out through mathematical modeling method.The critical value related to pipe instability was obtained by finite element simulation analysis method,and the relationship between allowable curvature and lead of straightening roller was determined.Meanwhile,the influence trend of different parameters on pipe stability was analyzed.Finally,it was verified by field test method.For different types of pipes,reliable data are provided for setting the lead and curvature of straightening roller in the uniform curvature stage.(3)Through simulation and field tests,the causes of fold,fracture and distortion defects of thin-walled pipe during straightening are found out.Finally,it is concluded that the essence of defects is that the bending curvature of the pipe exceeds the critical allowable value,but the form of defects is also related to the lead of the straightening roller,the initial defects of the pipe,the specifications of the pipe and the curvature of the straightening roller.It provides a reference basis for the analysis of defects during actual straightening.The method to avoid pipe straightening instability is obtained.