EN
For any of you in the pipe and tube bending business, nothing is more irritating – or frustrating – or common, than seeing wrinkles form along the inside radius on large-radius bend. This wavy ripple pattern, usually located at the inside radius or intrados, is blemish which can impact not only on the structural integrity but the quality of the entire finish. For everything from automotive and aviation to medical devices and military hardware which requires an ever-increasing variety of profile types, bending machines are often proving too inadequate to cope with increasingly sophisticated needs. This, however, may change as, in recent years, number of modern rolling machines are starting to provide answers.
Understanding the Root Cause of Wrinkling
Large radius pipe bending wrinkles are essentially function of material flow and compressive stress. When you bend pipe or tube, the material elongates (thins) on the outside of the curve, while compression is imposed on the inside (causing thickening). As one analysis put it, “Ripples occur from high amount of compression stress on the inside of pipe or tube with thin wall section”. When compressive force on the inner wall becomes too high, the material collapses and wrinkles. That’s much of the challenge involved in forming larger radius bends where material needs to move and stretch lot further around the curve.
Differential Rolling Technology: A Game Changer
With today's rolling benders using what’s called differential rolling for wrinkle prevention, they prevent wrinkling. Different rolling techniques are being employed on today's rolling benders including the use of tapered or conical-shaped rollers to produce an offset on rolling speed across the width of the roller. The technique compensates the varying speed on the different part of the workpiece being rolled by having reduced strain one side with the ability for opposite side contraction and therefore eliminating wrinkling of the bent material. Conically rolled, the circumferential speeds approach feed rates closely enough so that the amount of stretch or contract forces will keep deformation to minimum and allow bending width and thickness to be controlled well with.
Integrated Tension Control Systems
Third benefit that has come to rolling bender designs is the inclusion of an integrated tensioning control system that pulls on the part just after it exits the primary bending rollers. According to one patent, the addition of tension on strip member “after being rolled removes some compressive stress typically imparted or caused during rolling. This tension actually prevents buckling and, consequently, wrinkling. Post-bend tension control actually pulls the material just enough to reduce residual compressive forces, without stretching it beyond its ability to retain shape. When used in combination with tight inlet and outlet rollers to maintain material stability throughout the process, the added tension creates superior quality end product.
Optimized Machine Configuration for Profile Stability
Another important aspect is the machine configuration itself, especially for large radius work. The diameter to thickness ratio of the material must be maintained, because “a large diameter pipe with thin wall does not bend well and can even be difficult to bend to large radii.” More modern roll benders are constructed to overcome these challenges by having larger opening and speed-synchronized independent motor design which enable the gradual bend of the material in stages. By sharing work over greater surface area, this design also reduces stress at any given point. With over 20 years of tube and pipe bending expertise, Baorui has actually incorporated this technology into its roll benders through heavy-duty construction and intelligent control systems. This ensures wrinkle free results, regardless of radius size, material type, or application.