Tube bending is one of the fundamental processes in metal fabrication. Industrial pipes and tubes are a vital element in manufacturing for both construction and transportation of other materials. Most of the bent tubes are used as structural components such as handrails, metal furniture frame elements, and automobile roll cages, while the pipes are used for petroleum and water products, hydraulic systems, and exhaust lines.
Tube bending processes
There are several metal fabrication techniques for bending tubes and pipes, including draw, press, ram, and roll bending. Each applies force to bend metal, and they all have common principles, including elongation and radius of bend. There are also tool functions that also have specific ways to affect the curve, like chucks and windshield wipers. The correct application of all the principles influences the quality of the manufacture of tubes and pipes.
bending forces
When a tube is bent, the wall of the tube at the point of bending changes in a combination of ways. The outer wall stretches and becomes thinner, while the inner section becomes thicker and more compressed. It is an important part of the pipe bending process to control this deformation to ensure that a smooth bend is achieved.
The larger the tube and the wider the radius of the bend, the less likely warping will be a problem. It is the thinner tubes and tighter bends that tend to create problems. The decision as to how a tube should be treated is measured by determining the wall factor, which is the ratio of the tubes wall thickness to its diameter. A similar comparison is then made between the pipe centerline radius and the outside diameter to determine if the bend radius is within manageable limits. The combination of the two factors allows the manufacturer to define the complexity of the bend and therefore the correct bending technique to use.
Where the tube walls are not particularly compromised, a basic die set such as a press, clamp and bend die set can be used. Where curvature has a more pronounced effect on the tube material, a mandrel will be more appropriate.
mandrels
Where a basic die set is not suitable, a motherl will be used. It is a device that can be inserted into the pipe at the point of the bend to provide support to the pipe while it undergoes the bending process. Where a bend results in significant tube wall thinning and an increased risk of flattening of the pipe at the point of the bend, the mandrel will protect against this weakness.
Mandrels can be constructed in a variety of ways, including a single plug or a series of connected balls that bend and adjust as the bend is applied to the tube. In addition to providing this internal support, a plug mandrel can help apply additional bending force on thicker pipe that may be more difficult to form.
wiper dies
When a tube is subjected to significant stress during bending, such as when a thin tube is bent at a tight radius, internal wall compressions can occur causing wrinkling of the tube wall. A wiper blade die reduces the chance of wrinkling. The cleaner is designed to fit into the slot between the tube and the bending die. This makes the fit between the die and the pipe tighter and leaves no room for wrinkles to develop. Wiper dies are often used in conjunction with a mandrel to give the pipe the maximum degree of support.
Elongation
Elongation describes the degree to which the material a pipe is made of can be stretched before suffering structural failure. Since material stretching will always occur to some extent during pipe bending, this is a major concern for manufacturers. The general rule of thumb is that as the radius of curvature gets smaller, the more the material will stretch.
In many cases, the selection of the correct metal or material for a particular application is dictated by the amount of elongation that can be expected during bending and fabrication. For example, stainless steel can be an expensive option for producing a pipe, but it also has a higher maximum elongation than other steel grades, so it may be chosen as the most appropriate material if the pipe requires tight radius bends.
