TPT March 2014

Article

Tracto-Technik GmbH & Co KG

resistance against torsion. At torsion factor 1, torsion is non- existent; torsion factor 2 describes plastic torsion in one direction around the longitudinal axis of the bending part; and torsion factor 3 implies occurrence of torsion in both circumferential directions (see figure 5). The semi-finished part and its geometrical dimensions are of significant influence on selecting and determining an appropriate bending process. The main focus of the geometry is on one hand the cross-sectional shape and on the other hand the relation between wall thickness and outside diameter. These two parameters of the semi-finished part have to be considered particularly when selecting the appropriate bending tools. Constructive restraint of the bending contour must be ensured in order to avoid declining quality of the bending part. According to the required geometrical arrangement of the tooling the selection of eligible bending methods is limited due to constructive restrictions of the corresponding machines. Where tubes are concerned, the wall factor (W = H / s) describes the quotient between the outside tube diameter (H) and the wall thickness (s). Reflecting the common process limitations the wall factor is classified into four categories. Due to continuous further development of tube and profile bending processes towards open, asymmetric and alterable cross-sections [8,p.288f] , this also has to be particularly considered when selecting an eligible bending process [9, p.1ff] . For this reason the criterion cross-sectional form has been introduced, which is of decisive importance for the selection of an eligible bending method and the respective bending tools. The criterion itself is categorised into closed and open profiles and for both in symmetric and asymmetric cross-sectional forms. In order to meet the increased requirements for lightweight construction there will also be a rising demand for bending parts with an alterable wall thickness distribution. This fact is recognised by the additional introduction of the wall thickness change criterion. A distinction is made between bending parts with constant wall thickness, tube bending parts with variable wall thickness above the longitudinal axis (so called “tailored tubes”) and profile bending parts with variable wall thickness above the longitudinal axis (“tailored profiles”). The term “tailored” implies the development of parts, whose wall- thickness is especially adapted to the expected operational loading conditions to save material and weight. In the simplest case of “tailored tubes” only the inside diameter of the tube differs above the longitudinal axis, so that just the inside tooling has to be extra manufactured or rather modified.

Figure 2: Danger of collision of bending parts with tools and the bending part itself – the “overbending” effect [6]

GmbH & Co KG

imaginary Cartesian coordinate system, the risk of a looping of the bending die by the bending component becomes less, the smaller the angle between the limiting rays is. As a consequence the bending component must be destroyed to rescue the bending die. The loop is also named “overbending” (figure 3). Category 1 of the intercept factor includes bending parts with bends in multiple directions. The bending angles are small. There are neither undercuts nor returning bends. Typical examples are S-shaped bending components or fitting tubes for dashboards in automobiles. With rising angle between the limiting rays increases the risk of overbending effects, so the bending contour should be checked for the scheduled bending method. The second category includes geometries with a rearward profile. This usually includes still open complex contours. The freedom of movement of the bending machine must be tested before the start of the bending procedure. The third category of the intercept factor includes multiple crossing bending characteristics. The contours can be produced only with difficulty and always require prior investigation regarding their realisation. Furthermore category 3 is assigned when there is a rearward bending and the centre points of bending are continuously plotted over multiple bending planes. In order to counteract the torsion during the bending process, it might be necessary to change the design of the bending part or to provide a moment which compensates the torsion. In contrast to inadvertent torsion, there are bending parts, in which torsion has to be specially applied and within a certain area. For this purpose and for describing inadvertent torsion, the torsion factor is very suitable. According to figure 3, this parameter describes the existence of torsion and its direction around the longitudinal axis of the bending part. The torsion factor has to be considered particularly with open profiles, because these parts generally have a very low moment of

Figure 3: The intercept factor describes the bending direction and the positions of the bending sections towards each other [6]

Intercept factor 1

Intercept factor 2

Intercept factor 3

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March 2014