Rotary draw bending
The rotary- is a common and precise bending process for pipes with small bending ratio (bending radius / tube diameter) and a high wall thickness factor (pipe diameter / wall thickness). Mainly pipes made of steel , stainless steel , aluminum , copper , brass and other bendable alloys are bent . Since the forming in this process takes place below the recrystallization temperature of the material, it is also known as cold forming .
Process and bending tools
With rotary tension bending, the pipe is clamped between the shaping tool (bending template, bending roller, bending form) with an integrated inner clamping jaw and this outer clamping jaw with the help of an outer clamping jaw, and by rotating both tools around the so-called bending axis, it is "pulled around" the bending template. The rear, straight leg of the pipe is supported by a slide rail (slider), which acts as a counter bearing and absorbs the transverse forces resulting from the bending. The dimensions of the bending template are specially tailored to the outside diameter of the pipe and also specify the radius of the bend. It supports the interior of the arch during the forming process. Due to the elastic-plastic deformation behavior of metallic materials, the tube springs back by a certain amount of angle, the so-called springback angle, after the load is released (i.e. after the clamp has been opened) . At the same time there is a slight increase in the arc radius (springback). However, this material behavior can already be taken into account when designing the bending tools.
During the bending process, the inner workpiece layers of the pipe bend experience compressive stress combined with material compression, while the outer layers are subjected to tensile stress and stretched in the direction of the legs. The arch tends to sag or buckle inward. In order to maintain the pipe cross-section as much as possible and to prevent excessive compression of the material in the interior (creasing) during bending, two additional bending tools are used: the bending mandrel and the crease smoother. The bending mandrel is pushed inside the tube into the forming zone and prevents the tube cross-section from collapsing there. Depending on the bending ratio and wall thickness factor, different types of mandrels can be used, which extend differently into the arch area.
A basic distinction is made between the following types of mandrel:
- Plug mandrel (or finger mandrel)
- Spoon pin (with shaped end)
- Ball mandrel (has a movable ball at the end)
- Link mandrel (has several balls mounted on joints)
The crease smoother, in turn, is a molded piece that is inserted behind the forming zone between the pipe and the bending template in order to prevent creases from forming on the inner bend. Particularly with the sliding rail, mandrel and crease smoother bending tools, it is important to select a suitable material in order to minimize the frictional forces occurring during the bending process.
Rotary draw bending machines
Modern rotary draw bending machines are forming machines in which, depending on the degree of automation, it is possible to control the bending axis, the tube transport and rotation axes up to various auxiliary and secondary axes. The drive is hydraulic, electrical or hybrid. Semi-automated machines are often used in matching piping or on-site piping on construction sites, while fully automatic CNC tube bending machines are mainly used in pipe prefabrication , from flexible individual production to series production of bent pipe parts. The bending process itself can be significantly simplified by using special tube bending software , as the data required to control the bending process (bending angle, transport and rotation values) can be automatically calculated and transferred to the tube bending machine.
Limits of rotational tension bending
Rotary tension bending is very well suited for small bending ratios or bending factors. With the various mandrel types, a wide range of applications can also be realized with regard to the wall thickness factor of the pipes.
In addition to the bending tools used, the material-specific properties of the pipe material also have an influence on the bending limits. Bending is impossible below a stretch-related bending limit. Wrinkles and / or cracks occur during the bending process. The material fails.
literature
VDI Society for Production and Logistics: VDI manual for production and manufacturing processes, Volume 2: Manufacturing processes (VDI 3430, VDI 3431)