Mandrel-Free Pipe Bending on Small Radii – Theoretical Research and Experimental Tests

This paper presents the theoretical and experimental results of a study investigating a new method for tube bending. The method involves three-point bending of a tube without using a mandrel. The bending process was conducted with a small bending radius of Rg = 1.5Dz (where Dz is the outside diameter of the bent tube) and a large bending angle of 1800. The novelty of the proposed solution is the use of new shapes of bending roll impression. Instead of the standard circular-shaped impression, an elliptical-shaped impression was used. The aim of the study on the proposed small radius tube bending technique was to optimize the shape of roll impression in terms of minimizing ovalization and flattening of the tube cross section in the bending zone. Previous studies only showed that circular impressions were inefficient. The tube bending process conducted with a circular impression roll, without the use of a mandrel or other type of filling to achieve an angle ranging 900 ÷ 1800 , led to the flattening of the cross section. The tube wall in the upper zone would crack or its cross section would become deformed and oval. This theoretical and experimental study was conducted on tubes with an outside diameter of Dz=20 mm and a wall thickness of g=2 mm, made of 16Mo3 boiler steel and EN-AW 6060 aluminum alloy. Obtained results were then used to determine the ranges of bending roll impression parameters that ensured that the product would meet the standardized conditions of cross-sectional ovalization in the bending zone. The tool developed for this study can be applied in industrial practice.