Research On Three-point Bending Straightening Method Of Stepped Pipe Fittings

Stepped pipe fittings are widely used in aerospace,military,and vehicle fields.Their straightness grade has great influence on performance and application,and is one of the important indicators to measure product performance and quality.If the pipe is bent and deformed,straightening correction must be performed.At present,the straightening theory for simple shafts such as equal-diameter shafts and tubes is relatively mature,and there are few researches on the straightening theory of parts with complex cross-sections such as variable-section pipe fittings,and it is difficult to accurately set the straightening process parameters,and the straightening accuracy cannot be guaranteed..In addition,at present,the straightness detection of the inner hole of the pipe before and after straightening is mostly off-line detection,the measurement process is complicated,the efficiency is low,and the influence of human factors is large,which is difficult to meet the large-scale workpiece inspection.In this thesis,the single-curved curved pipe fittings are studied.From the theoretical derivation,numerical simulation and experimental verification,the three-point bending straightening method of the stepped pipe fittings is studied.At the same time,a machine vision based pipe deflection detection is proposed.Methods:The straightness of the inner hole of the pipe before and after straightening was tested,and the applicability of the method was discussed.(1)According to the principle of simply supported beam of material mechanics,the fulcrum and pressure point combination of stepped pipe fittings are studied.Based on the linear elastoplastic theory,the stress and strain changes of pipe fittings during straightening are analyzed,and the bending moment of pipe fittings is derived.Calculation formula and minimum fulcrum distance formula;According to the requirements of the reverse bending straightening process,considering the influence of stress concentration on the straightening process,a three-point bending straightening calculation method based on stroke control is proposed.(2)Based on the finite element method,the numerical simulation of the three-point bending pressure straightening process of the stepped pipe fittings is carried out,and the correctness of the theoretical model is verified,and the straightening margin in the simulation results is explained accordingly;by simulating different sizes The parameters and the step straightening process of the stepped pipe fittings with different fulcrum distances lead to the straightening stroke and maximum stress,maximum residual stress and straightening deformation of the stepped pipe fittings,which have certain reference significance for the actual straightening.(3)Aiming at the deflection measurement of single-arc elbow,a machine vision-based pipe deflection detection method is proposed.Combined with image processing technology,a mathematical model between the spot information and the bending deflection of the elbow projection is established,and the data measured by the laser displacement sensor is used.The results show that the proposed method has better adaptability to the straightness detection of large deflection pipe fittings and the measurement accuracy is higher.(4)The experimental platform was built and the three-point bending pressure straightening experiment of the stepped pipe fittings was completed.The experiment shows that the single straightening rate of the elbow has reached more than 80%.After straightening,the straightness of the pipe fitting can meet the actual engineering requirements,and the overall straightening effect is good.The correctness of the theoretical model of the stepped pipe straightening is verified.The experimental results are limited.The comparison of the meta-results has good consistency,which indicates that it is feasible to use the finite element method instead of the experiment to study the straightening process of the stepped pipe.