Pipe and Tube Bending Machinery's Handbook, 31st Edition
1459
where u = percent ovality of tube D max = maximum outer tube diameter after bending (inch or mm) D min = minimum outer tube diameter after bending (inch or mm) D = initial outside tube diameter (inch or mm)
When the ratio of the tube diameter to the wall thickness is small enough, the tube can be bent on a relatively small radius. The material-specific diagram in Fig. 67 may be used for a first proposition to determine whether a tube or pipe with defined dimensions (outer diameter and wall thickness) can be bent at all.
impossible
Fig. 67. Technical Limits of the Tube/Pipe Bending Process Bending without workpiece failure is impossible below the bending limit determined by stretching. The bending limit due to wrinkling separates the range where bending with mandrel and wiper shoe is possible from the range in which the tubes/pipes can also be bent without a mandrel. Due to the elastic-plastic behavior of metallic materials, the tube/pipe will spring back by a certain angle after every bending attempt because of the phenomenon of elasticity. While in the valid range of Hooke’s law (elastic line), the shaping energy is completely given back as work of elastic strain in the form of resiliency. But after the external strain has been removed, it is partly dissipated as work of plasticity when performing the elastic-plastic shaping. In this case, the extent of springback is only caused by the elastic (reversible) part of the shaping workpiece that is stored in the tube/pipe as potential energy during the bending process. Springback is an inevitable phenomenon of bending and can only be compensated for by overbending the workpiece. Different methods can be used for bending tubes/pipes, depending on the material in use and the required finishing precision. The most common processes for bending tubes or pipes include press bending (ram tube bending), rotary draw bending (round bending), compression bending, and 3-roll bending. Press Bending.— Press bending, also called ram tube bending , was probably the first tube/pipe bending method used to cold-form materials. When press bending is applied, the bending tool with the inwrought bending radius is pressed against two counter rollers, either manually or by means of hydraulics. This motion forces the tube/pipe inserted be- tween the radius block (die) and the counter-rollers to bend around the die radius (Fig. 68). The tube/pipe cannot be supported by the mandrel. This tube-bending method creates some cross-sectional ovality; therefore, this method is suitable for thick-walled pipes and large bending radii where high levels of cross-sectional ovality are acceptable, such as in furniture tubing and handrails. Large sweeping curves can be bent in small increments, moving the tube/pipe for each bend.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online