Pipe and Tube Bending Principles

Bending is an important step in the process of manufacturing industrial pipes and tubing, which serve a vital role in both construction and the transportation of materials. Most bent pipes and tubes function as structural components or as “passageway” units that facilitate the transfer of substances. Structural tubing includes items such as handrails, handlebars, furniture frames, and automobile roll cages, while passageway tubing is found in fuel and water lines, hydraulic systems, and exhaust lines.

 

Tube and pipe bending can be accomplished through a number of different processes, including draw, press, ram, and roll bending, but each method is based upon the same fundamental concepts. Bending principles such as elongation and bend radius, as well as the tool functions of mandrels and wipers, form the foundation for most tube and pipe bending operations. These principles intersect in several ways that influence the effectiveness of pipe and tube manufacturing.

 

Bending Forces

 

As a tube or pipe is being bent, the exterior wall at the point of the bend begins to stretch and thin out. Simultaneously, the corresponding interior segment of the workpiece becomes thicker and more compressed. Controlling these degrees of physical deformation is important for creating a smooth rounded bend. Thick walled tubes bent at a wide radius are likely to have a relatively low degree of deformation, but thinner tubes may not. Bending servicestypically measure the wall factor, which is the ratio of a tube’s wall thickness to its external diameter, in order to determine whether a tube should be treated as a thin or thick workpiece.

 

A similar comparison is made between the centerline radius and the tube’s external diameter to determine if a bending radius is tight or wide. The combination of the bending radius and the wall factor is used to designate the complexity of the bend. Under parameters in which the interior and exterior walls would not be seriously comprised, a standard bending procedure can be performed with a basic die set, such as a bend, clamp, and pressure die array. The clamp die holds the tube in position, while the pressure die forces it against the bend die to curve into the desired shape.

 

Mandrel Functions

 

In many cases, the tubing workpiece does not fit the ideal criteria and cannot be properly

shaped using a basic die set apparatus. As the wall factor measurement grows larger from the external wall thinning, the bend radius also grows tighter and increases the chances of producing a flattened bend. This usually occurs if the wall is too thin to maintain its integrity at the angle of the bend. A mandrel is often used to compensate for this weakness. The mandrel is a device that can be affixed to the interior of the tube at the point of the bend to provide support throughout the operation. It can be designed as a single plug or a sequence of balls that flex and adjust according to the bend. Aside from providing internal support for thin tubes, a plug mandrel can also be used to exert additional bending force on thicker tubes that are more difficult to shape.

 

The Wiper Die

 

Under more severe bending conditions, like those involving thin tubing undergoing a tight bend radius, internal wall compression may develop unevenly, resulting in a wrinkle defect. A wiper die may be necessary in order to reduce the risk of wrinkling on the workpiece. This wiper is designed to be wedged into the groove between the tube and the bending die, and it has a thin tip that reaches to the point where the tube will start to bend.  The wiper completes the gap between the bending die and the tube, leaving the tube constricted and removing any space for a wrinkle to develop. Wipers are often used in conjunction with a mandrel to further reduce the potential for deformation.

 

Elongation

 

Elongation is the degree to which a tube can stretch before undergoing structural failure or cracking. Given that material stretching occurs in essentially all tube and pipe bending procedures, elongation can be an important concern for manufacturers. In general, as a bending radius grows tighter, the material will stretch more. In some cases, material selection is dictated by the expected level of elongation. For example, stainless steel has a higher maximum elongation than other grades of steel, making it easier to bend without fracturing along a tight radius.

source : http://www.thomasnet.com/articles/custom-manufacturing-fabricating/bending-principles