TPT November 2009

During forming, edge of sheet will form at the first bend, so remedy the error of 115°-180° wall thickness at the first pass; and distribute the error of 0°-115° wall thickness to all main roll passes before fin pass. In this way, after the roll shape contour with error compensation is formed, error of wall thickness may be easily reduced to meet the requirement of high accuracy welded tubes with uniform wall thickness. 4 Conclusions 1) With the same outside diameter, the roundness error clearly increases along with wall thickness. By analysing the fitting results of the curve, the polynomial equation for wall thickness and roundness error is δ t =0.493-0.585 t +0.191 t 2 ,. 2) When wall thickness is constant, roundness error reduces and outside diameter increases. By analysing fitting results of the curve, the polynomial equation for outside diameter and roundness error is δ D =1.484-0.139 D+0.004 D 2 . 3) When the curve of wall thickness changes, total trend is degressive. There are three larger values of sheet thickness at the half circle, which have appeared near the axis of symmetry (0°), positions of sheet edge (180°) and close to 90°. From the result, the three larger values are all produced at the final pass, which shows that very large compressive stress occurs on sheet edge. The various simulations result in a larger changing range at the end of sheet, which are caused by distortion at the end elements. 4) By conducting the above mentioned roundness error analysis and the changing rule of wall thickness, it is necessary to modify roll gap according to wall thickness errors distribution during design in order to reach uniform wall thickness and realise high accuracy thick-wall tube forming. The rolls contour is not only a simple arc, but a spline curve fitted according to wall thickness changing, thereby determining the actual rolls contour curve. Acknowledgements Thanks to Mr Zhao, Yun, GM of Huzhou Weiyun Moulds Co Ltd and Mr Zhou, Shijia, GM of Ningbo Huaguan Machine Manufacturing Co Ltd for support and help during the investigation of the topic. References Albert Sedlmaier. Predicting The Properties of Welded Roll Formed Tubes for Subsequent Processes Using the Finite Element Method[C]. Valley. Germany. George T Halmos, Roll Forming Handbook, Taylor & Francis, 2005. H Ona, Jiying Liu, Roll Forming Technology, Chemical Industry Press, Beijing 2007.

 Figure 10 : Errors of welded tubes with various ODs and same wall thickness

The fitted equation of the curve is δ D

=1.484-0.139 D +0.004 D 2 ,

where δ D is the dependent variable roundness error, and D is the independent variable outside diameter, the curve of δ D along with D is the left half of a second-degree parabola. 3.4 Analysis of wall thickness changing To investigate the symmetry of welded tubes, divide a half circle of the welded tube into ten equal parts (Fig. 7), analyse thickness changes, Fig. 11 is the changing curve. changing

 Figure 11 : Wall thickness changes on a half circle of welded tubes in all specifications

3.5 Comparing with other research

 Figure 12 : Relation between strip material thickness and perimeter position German dataM company and Darmstadt University performed simulation and experiments for 60mmx1.5mm ERW tube, and obtained results as Fig. 12. The extreme values appear near the axis of symmetry (0°), positions of sheet edge (180°) and close to 90°. The trend of the curve is similar with the result of simulation here, which proves that the result of this simulation is authentic. 3.6 Methods for reducing wall thickness error According to the wall thickness changing after forming (Fig. 11), it is hard to meet the requirement of accurate forming if you only depend on the roll flower to design the outer contour line of the rolls. In order to reduce wall thickness error, it is necessary to modify roll gap based on the former roll flower design, and decide the corrected value according to wall thickness error in the diagram.

North China University of Technology – China Fax: +86 10888 03581 Email: fao@ncut.edu.cn Website: www.ncut.edu.cn

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