Tube Bending and Pipe Bending Principles

Tube bending is one of the fundamental processes in metal fabrication. Industrial pipes and tubes are a vital item in manufacturing for both construction and the transportation of other materials. The majority of bent tubes are used as structural components such as handrails, as elements of metal furniture frames and car roll cages, while piping is used for oil 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. They each apply force to bend the metal and there are common principles to all of them, including elongation and bend radius. There are also tool functions that also have specific ways of affecting the bend such as mandrels and wipers. The correct application of all the principles influence the quality of the pipe and tube manufacture.

Bending Forces

When a tube is bent, the wall of the tube at the point it is bent changes in a combination of ways. The exterior wall stretches and becomes thinner, while the inner section becomes thicker and compressed. It is an important part of the pipe bending process to control this deformation to make sure a smooth bend is achieved.

titanium tube

The larger the tube, and the wider the radius of the bend, the less likely deformation is a problem. It is the thinner tubes and tighter bends that tend to create problems. Deciding how a tube should be treated is gauged 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 centreline radius of the pipe and the external diameter to determine if the bending radius is within manageable limits. Combining the two factors allows the manufacturer to define the complexity of the bend, and therefore the correct bending technique to use.

Where the walls of the tube will not be particularly compromised, a basic die set can be used, such as bend, clamp and pressure die array. Where the bend will have a more pronounced effect on the material of the tube, a mandrel will be more appropriate.

Mandrels

Where a basic die set is not suitable, a madrel will be used. It is a device that can be inserted inside the pipe at the point of the bend to provide support for the pipe while it undergoes the bending process. Where a bend will produced significant thinning of the tube wall, and an increased risk of a flattening of the pipe at the point of the bend the mandrel will protect against this weakness.

Mandrels can be constructed in several ways, including a single plug or a series of connected balls that flex and adjust themselves while the bend is applied to the tube. As well as providing this internal support, a plug mandrel can help to apply additional bending force on thicker pipes that can be more difficult to shape.

Wiper Dies

When a tube is placed under significant stress during bending, such as when a thin pipe is bent to a tight radius, internal wall compressions can occur resulting in wrinkling of the pipe wall. A wiper die reduces the chance of wrinkling occurring. The wiper is designed to be wedged into the groove between the tube and the bending die. This makes for a tighter fit between the die and the pipe and leaves no space for the wrinkle to develop. Wiper Dies are often used along with a mandrel to give the pipe a maximum degree of support.

titanium sheet

Elongation

Elongation describes the degree to which the material a pipe is made from can be stretched before suffering structural failure. As stretching of the material will always occur to some extent during pipe bending, this is an important concern for manufacturers. The general rule is that as the bending radius grows smaller, the more the material will stretch.

In many cases the selection of the right 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 to produce a tube from, but it also has a higher maximum elongation than other grades of steel, so it can be chosen as the most appropriate material if the pipe requires bends with a tight radius.