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Pipe and Tube Bending

Principles

Bending is an important step in the processof manufacturing industrial pipes andtubing, which serve a vital role in bothconstruction and the transportation ofmaterials. Most bent pipes and tubesfunction as structural components or aspassageway units that facilitate thetransfer of substances. Structural tubingincludes 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 bendingcan be accomplished through a number of differentprocesses, including draw, press, ram, and roll bending, but each method isbased upon the same fundamental concepts. Bending principles such aselongation and bend radius, as well as the tool functions of mandrels andwipers, form the foundation for most tube and pipe bending operations. Theseprinciples intersect in several ways that influence the effectiveness of pipe andtube manufacturing.Bending ForcesAs a tube or pipe is being bent, the exterior wall at the point of the bend beginsto stretch and thin out. Simultaneously, the corresponding interior segment ofthe workpiece becomes thicker and more compressed. Controlling thesedegrees of physical deformation is important for creating a smooth roundedbend. Thick walled tubes bent at a wide radius are likely to have a relativelylow degree of deformation, but thinner tubes may not.Bending servicestypically measure the wall factor, which is the ratio of a tubes wall thickness toits 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 tubesexternal diameter to determine if a bending radius is tight or wide. Thecombination of the bending radius and the wall factor is used to designate thecomplexity of the bend. Under parameters in which the interior and exteriorwalls would not be seriously comprised, a standard bending procedure can be

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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 itagainst the bend die to curve into the desired shape.Mandrel FunctionsIn many cases, the tubing workpiece does not fit the ideal criteria and cannotbe properlyshaped using a basic die set apparatus. As the wall factor measurement growslarger from the external wall thinning, the bend radius also grows tighter andincreases the chances of producing a flattened bend. This usually occurs if thewall is too thin to maintain its integrity at the angle of the bend. A mandrel isoften used to compensate for this weakness. The mandrel is a device that canbe affixed to the interior of the tube at the point of the bend to provide supportthroughout 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 internalsupport for thin tubes, a plug mandrel can also be used to exert additionalbending force on thicker tubes that are more difficult to shape.The Wiper DieUnder more severe bending conditions, like those involving thin tubingundergoing a tight bend radius, internal wall compression may developunevenly, resulting in a wrinkle defect. A wiper die may be necessary in orderto reduce the risk of wrinkling on the workpiece. This wiper is designed to bewedged into the groove between the tube and the bending die, and it has a thintip that reaches to the point where the tube will start to bend. The wipercompletes the gap between the bending die and the tube, leaving the tubeconstricted and removing any space for a wrinkle to develop. Wipers are oftenused in conjunction with a mandrel to further reduce the potential fordeformation.ElongationElongation is the degree to which a tube can stretch before undergoingstructural failure or cracking. Given that material stretching occurs inessentially all tube and pipe bending procedures, elongation can be animportant concern for manufacturers. In general, as a bending radius growstighter, the material will stretch more. In some cases, material selection isdictated by the expected level of elongation. For example, stainless steel has ahigher maximum elongation than other grades of steel, making it easier tobend without fracturing along a tight radius.

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