The purpose of the present work is to study the correlation between the main geometrical tube parameters of the rotary draw bending process and the resulting quality of the cross-section of small hydraulic pipings. The method of choice, named free rotary draw bending, is a rotary draw bending process where no internal support from an internal mandrel or plug is\ud possible, because hydraulic tubes cannot be lubricated on their internal diameter. Furthermore, no axial boosting of the tube or of the pressure die is applied. Free bending is mainly used when\ud small-diameter tubes are involved, with high length and relatively thin wall. \ud A unique quality estimator QCS of the cross-section geometry of bent tubes has been developed, which is able to synthetically represent the main functional and aesthetical properties of tubes:\ud the maximum ovalization, the minimum hydraulic diameter, and so on. In the case of copper tubes for hydraulic applications a strongly deformed cross-section after bending means a poor\ud quality of the product, since it delivers an increase of head losses and of pressure drops. The deformation occurring in the cross-section of the tube has been studied through a set of FEM simulations, run with an explicit code with shell elements, and the model has been validated by comparing the experimental and numerical results of specific cases, in terms of geometry of\ud the cross-section in a 90 bend. The plan of simulations has been designed using the typical copper tube dimensions for hydro-thermo-sanitary applications and a few runs have been added with modified parameters with the aim of completing the plan for evaluating the sensitivity to a change either in geometrical variables (thickness t, outer diameter OD, mean bending radius RM) or in the tube material. An indicator (bending factor) is proposed, able to reliably estimate the difficulty of a bending process with respect to tendency to ovalization and collapse of the crosssection.\ud Some design guidelines of the bending tools are provided

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Rotary draw bending is the most spread among the industrial bending processes of profiles, due to its great versatility and reliability. When bending non-round profiles, the process often shows a behavior which is more difficult to predict than round tubes. This is due to several reasons: lack of knowledge and experience about less common geometries, the presence of corners and straight portions of the cross section, which make the tube stiffer, the inhomogeneous mechanical properties (thickness and strain hardening) of tubes around the cross section caused by the tube making process (roll forming and subsequent welding), etc. In this paper, an FEM model, which is able to take in account the differences in thickness distribution and in mechanical properties along the profile section, has been built and validated with the commercial code PAM-Tube. Then, a quantitative indicator of the process difficulty has been developed, which is able to represent, from a geometrical point of view, the degree of severity of process in terms of potential risk of shape degradation of the bent cross sections. Finally, a quality index, able to synthetically represent both the aesthetical and functional needs of the resulting cross section has been introduced and correlated to the process difficulty factor.