Pipe bending
As a collective term, pipe bending describes various forming processes ( bending processes ), the aim of which is to bring about a plastic deformation of the pipe through the action of force .
If the shape of the pipe changes below the recrystallization temperature of the material, it is called cold bending ( cold forming ). In the case of hot bending, on the other hand, the deformation zone is inductively heated before bending in order to enable or promote the deformation of the material .
Bending process: geometry and special features
With regard to the complexity of the bending geometry, a basic distinction can be made between the production of individual pipe bends, the production of pipe figures in one bending plane (e.g. pipe coils) and the production of complex three-dimensional workpieces with several bending planes.
Due to the elastic-plastic behavior of metallic materials, the tube springs back after every bending process by a certain angle - the elastic component. This so-called springback angle can, however, be estimated as a function of the material and dimensions of the starting pipe and the respective bending angle, or it can be determined by test bending and stored in a database. The springback angle must be taken into account accordingly before bending and the pipe must be bent over by this amount so that the desired bending angle is retained after bending.
When pipes are bent, wall thickness changes in the pipe cross-section always occur. The inner workpiece layers experience a compressive load combined with a material compression, while the outer layers are subjected to tension and stretched in the direction of the leg.
This tensile stress or expansion results in an ovalization of the originally circular pipe cross-section. The outside of the arch tends to pull against the center line, thereby flattening the pipe. The ovalization is stronger, the smaller the wall thickness of the workpiece and the bending radius are chosen. The change in the cross-sectional shape has an impact on the free flow cross-section and the strength behavior of the pipes under internal pressure.
Tube bending process
For the bending of tubes (and profiles) of the starting material, the geometry of the workpiece, and the required machining accuracy, various methods may be applied depending on the material and dimensions.
Analogous to the bending processes according to DIN 8586, a distinction can also be made between processes with rotating tool movement and processes with straight tool movement when bending pipes. The bending contour serves as a further classification criterion, which can be generated kinematically via a free-form bending process or also form-bound.
The most common shape-related pipe bending processes include press bending , compression bending and rotary tension bending . With them, the pipe is shaped into a bending tool that contains the "negative shape". In the kinematic process, simple 3-roller bending and 3-roller push bending are widely used. During press bending , the bending tool with the incorporated bending radius is pressed manually or hydraulically against two counter rollers. This movement forces the tube inserted between the bending radius and the counter rollers to bend around the radius. Since the pipes cannot be supported from the inside, this method is only suitable for thick-walled pipes and large bending radii.
In compression bending, the pipe is clamped between a sliding carriage and a stationary bending roller. By rotating the sliding carriage around the bending roller, the tube is bent to the radius of the bending roller.
Three-roll bending is used to produce workpieces with large bending radii. The process is similar to press bending, but the work roll and the two stationary counter rollers rotate and thereby form the sheet.
Rotary draw bending is much more versatile and precise than the processes mentioned above . The workpiece is fixed between the bending roller and the clamping piece and reshaped by rotating both tools around the bending axis.
application areas
Pipe bending is the essential production step in the adaptation piping and pipe prefabrication. Often bent steel pipes z. B. also used as a basic semi-finished product for downstream hydroforming .
In general, all cold-formable materials are suitable for tube bending, such as B. steel, stainless steel , copper , titanium , aluminum or alloys made of these metals . In addition to circular pipe cross-sections, square and oval pipes, flat and solid material as well as profiles with different cross-sectional shapes can also be formed by bending.
In everyday life, bent tubes are required to transport a wide variety of liquids and gases and as construction elements and structural components in almost all industrial areas. In the automotive , aerospace and shipbuilding are as widespread as in the chemical industry , the refrigeration and air conditioning, the furniture industry, the steel , the machine and plant construction or installation work.
advantages
Especially in the automotive industry and the aviation and space technology tubular design and structural components due to the demand for lightweight construction are indispensable. By bending pipes, very complex construction and structural components with high stability and low weight can be manufactured efficiently and precisely.
But also in industrial pipeline construction and plant construction, pipe bending has become increasingly important compared to conventional connection techniques (e.g. welding ) in recent years. The reason for this are economic and manufacturing advantages, such as B. the saving of fittings , whereby the material, procurement and storage costs can be reduced. In addition, the production times and costs for bending are significantly lower than for producing welded connections. By eliminating connection points that represent critical components, potential danger points in the pipeline system can be reduced to a minimum, which ultimately results in a longer service life of the pipeline system.
literature
- Franz, Wolf-Dietrich: The cold bending of pipes. Methods and machines, ISBN 3-662-11427-5 , 9783662114278
- DIN 8586: Manufacturing process for bending - Classification, subdivision, terms
- VDI 3430: Rotary tension bending of profiles, Appendix: Classification of the bending processes.