BENDING OF THIN WALLED PIPES USING THE HEAT REALISED BY FRICTION

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1 56 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI BENDING OF THIN WALLED PIPES USING THE HEAT REALISED BY FRICTION Branko TADIC, Nenad MARJANOVIC, Bogdan NEDIC Faculty of Mechanical Engineering Kragujevac, Serbia ABSTRACT This paper considers appliance of heat generated by friction to relieve deformation of light metals, having as example of bending thin walled copper pipes. Based on numerous, primarily experiment analysis the conclusions have been applied for designing a machine for bending of thin walled copper pipes having small bend radius. The machine s design is relatively simple, but it provides realisation of needed temperatures within required narrow area. Large number of tests has shown the reliability of suggested methodology. KEYWORDS: Bending of thin walled pipes, heat, friction, copper arc.. INTRODUCTION Friction welding has been in use since 960 s. This technology became irreplaceable in many branches of industry thanks to its very good characteristics [3, 4, 7]. That period of time signifies treating heat generated by friction from very negative aspect. Thanks to the new point of view for heat generated by friction, researches that followed, had resulted not just by expansion in field of use of materials assembling technology, but also by a tendency for using heat effects of friction in material processing. According to the literature sources researches in this field are going in multiple directions. In late 70 s there was a method of a stitch forming that has been used for creating a shroud on ends of solid cylinder parts. The results in domain of forming front hatch on bar and board parts were received as well. Now heat generated by friction has the most practical appliance in operations on tighte-ning and widening ends of the pipes and forming (welding) ends of the pipes and solid profiles [5]. Even if this procedure has been already called thermo friction processing, it may be said that this field is practically in its starting phase. One thing that is characteristic in heat effects of friction appliance in all those applications is that there is a small number of papers dealing with theoretical review of this problem. Experiments anticipate the theoretical researches in most cases. This points out the complex process and the impossibility of placing reliable dependences between quantity of released heat and obtained temperature in function of input parameters of process itself (speed, pressure, friction modules etc. Authors research in this field is primarily oriented to review possible expansion in field of practical appliance for the heat generated by friction. Theoretical analysis was performed in a smaller extent. The interest has been given to input of the process. The amount of heat and the intensity of the heat development can be controlled by this input [, 8, 9]. A large number of experiments has been carried out in intention to review a possibility of appliance for the heat generated by friction to bending thin walled pipes. There was no research in this field according to accessible literature. A thin walled copper pipes bending machine was realised as a result of a large number of experiments. It works according to the concept of heating critical bending zone using the heat generated by friction. It has been showed that the heat and its intensity, generated by friction, can be relatively easy managed in a wide range in this type of forming. Over 00,000 high quality product units have been manufactured using the realised machine. It was practically showed that operation of forming (bending pipes) based on principle of heat generated by friction has also extremely convenience applying economic effects. The machine for straightening and enlarging pipe ends has been realized together with the machine for thin walled pipes bending. Researches associated to this problem will not be represented here in consideration that the straightening of bended pipe is, in most part, implied to the bending procedure. 2. BASIC THEORETICAL POSTULATES OF THE PROCESS Pipe bending is a particular and specific type of bending [2, 6]. During the bending process pipes are

2 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI 57 exposed to the forces deforming their primarily shape (cross section), and there is a possibility that the pipes can loose their continuous shape of cross section. There is a danger of changing original shape in critical bending zone. Therefore that critical zone should be secured by adequate designs, as much as possible. The bending of thin walled pipes is considered to be a difficult type of bending, especially if the pipes are bended to a relatively small bending radius. Series of methods and devices for rectilinear and circular pipe bending has been developed in practice [2, 6, 7]. However, it is not possible to produce curvilinear bending of copper arc that has proportion of bending radius and diameter of pipe r.5 d by these classical methods (bending over cylinders, bending with slot link etc.). The bending of this arcs is considered as a special technology of bending, based on heating critical zone by special inductive heaters. Inductive heating of the critical zones requires special electro-energetic facility, and production of arcs by this method is economically justified only in terms of bulk production. Starting from the fact that a substantially amount of heat in short amount of time can be generated by friction, depending on friction terms between two parts it came an idea for using that heat for warming critical zone while bending copper arcs [8]. ϕ ω Fig.. Concept scheme of friction of contact pairs. A certain amount of heat is developed by friction between two bodies in contact, in function of contact realization terms. If the heat features in contact of two bodies in presence of normal stress F n, F N V rapidity of sliding ν are analysed (fig. ), it can be concluded that developed amount of heat is complex function of the normal load F n, the velocity of sliding ν, the angle φ and many other geometric, mathematic and tribological characteristics of both contact bodies. The heat process that occurs due to the contact between pipe and bush 2 (fig. 2) can be analysed analogically to previous review. The pipe is moving by certain auxiliary motion velocity v p, while the bush 2 is revolving by spin velocity v. There is a certain overlap δ between the pipe and the bush. In this case the developed amount of heat and generated heat intensity will be function of spin velocity v, auxiliary motion velocity v p, overlap δ, contact length L and second pipe and bush 2 tribological characteristics. Mathematically it can be expressed by the functions: Q = f (v, v p, δ, L, T K, t) () dq IQ = f2( v,v p, δ,l,t K,t) = (2) dt where: Q the developed amount of heat, I Q the intensity of generated heat, T K other, tribological and thermal characteristics of pipe and bush. Defining developed amount of heat and intensity of generated heat as complex non-steady functions of contact realization terms represents a very complex problem of theoretical aspect [4]. However, from both practical engineering aspect, the developed amount of heat and the intensity of generated heat can be controlled in certain range trough one of the parameters that define terms of contact realization. For example, functions Q and I Q can be controlled in a very wide interval for chosen value of overlap δ, velocity v and other parameters. Specific, needed amount of heat can be obtained by finding parameters v, v p or some other parameter that is adequate to that amount of heat. A very complex stress and deformation state occurs in certain bending zone during pipe bending [2]. Warming of the pipes is necessary because of great scale of deformation that occurs at proportion r.5d the heat should be brought right to the critical bending zone. δ L 2 v p v Fig. 2. Heating pipes by friction.

3 58 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI 2 K P Q N N P Fig. 3. Scheme of the pipe bending. In the first step, according to figure 3, one of the possible methods of pipe bending will be reviewed in stress and deformation state analysis in the bending zone. The pipe is, by this method, repressed by force F through inner die an outer die 2. The pipe is subjected to extension stress above neutral N axis, left to the P-P section and depression stresses under the axis and left to the P-P section. Area of bending restrained by K outline represents bending zone where complete plastic pipe deformation is performed. The bending zone is partially spreading right to the P-P section due to inevitable presence of gaps between dieses and pipes and other factors. Complex stress and deformation state that has one random dimension caused by presence of the gaps, digressions of pipe s mechanical characteristics and other disables them to be quantitatively expressed as parameter functions defining the terms of bending. In case of bending without heating the pipes, due to great scale of deformation (r.5d), the wrinkles are created on the arc in the pressured zone. The digression of mechanical pipe material characteristics and deformation scale s enhancement by reduction of certain amount of heat Q into bending zone enables a proper bending process. Basic theoretical postulates of this thin walled pipe bending process were held to presumption that acquired amount of heat Q can be provided by friction between some element (bush) and the pipe in the most depression and extension zone, and that with that amount of heat that can be manipulated. Stress condition of thin walled pipe can be analysed according to scheme in figure 4. The most stressed pipe cord from depression stress aspect and most critical by creating wrinkles is the cord that runs trough point A, B and C. If the bush made of tough material (high-speed cutting steel, hard metal etc.) is spinning by certain number of revolutions in such a way that one generating bush line runs trough A, B and C points, then it will generate heat right in the critical bending zone. Depression stresses p(β) needed for the heat generation can be provided by a small overlap between pipe () and bush (2). Depression stresses p(β) based on the process geometry are provided by the inner die (3) itself. While elaborating theoretical model the starting presumption was the necessity for minimal overlap realization between bush and the pipe that can provide bending in critical zone in exactly defined area (no gap between bush and the pipe). A B C 3 b 2 p(b) Fig. 4. Assignment of depression stress on thin walled pipe. There is a small gap between inner die and the pipe that provides radial movement of pipe cord to interior, originated as a consequence of the pipe heating. It is logical to assume that the extra depression stresses and the generation of larger concentrated heat sources will come right from the bending forces. These bending forces are representing the normal reaction of die lower zone. It is entirely logical to assume that the heat distribution will follow normal pressure distribution to the pipe walls and the bending zone will be limited by relatively small space. The mechanical characteristics of the pipe material will change in function of heat generation intensity amount. From the aspect of distribution on lengthwise cords and according to specified presumptions, the largest amount of generated heat will take the cords with the largest scale of shortening apropos starching. Pipe bending is also done by the part of external die through p stress, though the influence of this stress to the total process of pipe bending is quite smaller. It is very hard to raise precise theoretical dependencies between the amount of generated heat and the distribution of temperature fields in critical

4 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI 59 zone due to complex process and the number of influence values. Presented reviews are only presumptions that pointed out the possibility of this processing procedure realization. Based on these assumptions and approximate calculations that are not presented on this occasion, the construction of the thin walled copper pipes bending machine was successfully realized. 3. DESCRIPTION OF REALIZED SOLUTION The machine for copper arcs manufacturing was projected and realized based on liable theoretical foundation. The bending procedure will be described using the scheme in figure 5. The pipe is placed over inner die 6 whose position in relation to bush 3 is adjusted by screw nuts 5. The pipe is located on inbound cone of the bush 3 in the starting position (bending in the first arc). Bush 3 is connected to a turret 2 by a detachable joint. Turret 2 is situated in the bearings 7 which are situated in carriers 4, and is revolving by certain number of revolutions trough the electromotor and the belt drive 0. The carrier 4, the electromotor, the pneumatic cylinders 9, the restrictor 6, the assemble of cutting saw 3 and the outer die 5 are attached to the floor plate. The pipe is constrained by a special chuck 8 which is connected to the piston rod of the pneumatic cylinders 9. Pipe suppression is realized by the adjustable restrictor 6 to the bush 7. The amount of heat brought to the bending zone is regulated trough the overlap size between bush 3 and the pipe and trough pipe revolution speed. Overlap size is approximately 0.03 mm. The replacement of certain elements is held during proceeding to a second bending radius apropos second pipe diameter. The bush 3 used for heating, the outer die 5, the inner die 6 and the bandage of special chuck are spare parts. The photographic display of the machine for copper arc bending based using heat generated by friction is displayed in figure 6. Proper functioning of the machine requires a solution to a lot issued details, such as:. system for pipe restriction and reliable force transfer, 2. working with a high speeds in order to have a greater production, 3. temperature maintaining in certain limits, 4. solution for a lubrication system, 5. solution for other issued details. Restriction device needs to ensure concentric pipe restriction, concerning even pressure on certain length of the outer surface. Even pressure realizes the possibility of transferring a massive suppression force trough a large pipe surface and is preventing pipe deformation where the inner die with minimal gap is used. This was achieved by using a special restriction device with radial multilateral cut embedded bushes. In this way the force worthing 25,000 N can be transferred by friction at radial identical deformation worth maximum 0.03 m, while the pipe diameter is worth only 22 mm. The bush is replaced after manufacturing of 50,000 product units. This is accomplished by adequate solution of the bush heating (bush geometry and material) and lubrication system CEV Fig. 5. Schematic review of copper arcs bending machine.

5 60 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI Fig. 6. Photographic display of copper arc bending machine based on usage of the heat generated by friction. Forming arc ends (realigning of edges, enlarging and processing front surface) is carried out by heat generated by friction on the machine, specially designed for that assignment. 4. PRESENTATION OF THE ACCOMPLISHED RESULTS After solving many problems related, at first, to manage the heat generated by friction, the following results were accomplished:. the phase of experimental research was realized; 2. pilot series of 00,000 products of different dimension has been produced 3. geometry and mechanical characteristics of produced arcs were performed in accordance to DIN standards. One characteristic detail is accentuated by this occasion. According to valid DIN standards maximal deviation of arc diameter by whole bended segment is only 0.05 mm, which is an extremely narrow tolerance for bending process. All other allowed deviations are of the same range as well. The look of arcs of different sizes produced by application of heat generated by friction is represented in figure CONCLUSIONS Based on the achieved results it can be concluded that the heat generated by friction can be successfully used in thin walled pipe bending operations. There is a large number of parameters (input data) in this process trough which the amount of heat generated by friction and intensity of that heat can be successful managed. Displayed results are not qualified by too large theoretical review of this problem. However, the facts from the practical engineer aspect are obvious: the method is verified by experiment, process has a repetitive characteristic, there is a large number of parameters that process can be managed with, and the most important, generated heat is most probably supplied to the bending zone by a distribution that is consistent to normal pressures. Fig. 7. Photographical display of arcs produced by appliance of heat generated by friction.

6 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI 6 In experimental phase it was confirmed that the amount of needed heat for warming the critical zone can be accomplished with very small overlap between bush and the pipe at a speed of m/s. This method is especially convenient for bending copper pipes. The copper pipe coefficient of heat conductivity is much higher than the bush coefficient of heat conductivity. That means that the most part of the generated heat is going right into the critical bending zone. Conducting time is short especially considering the fact that the bending speed is relatively high. It is quite possible that the bending of steel pipes can be performed by this method using the heating with a bush made of hard metal or tool ceramics. It was shown by experiment also, that the process has certain repentance for certain parameters (overlap and speed). In inductive heating of critical bending zone method (today s most extensive and most recent method) the whole critical zone is heated to the same temperature value. From the aspect of spent energy (unnecessary heating), complex procedure and cost price, this method has disadvantages comparing to the above-illustrated method. According to the achieved preliminary research, the heat generated by friction can by applied in spatial pipe bending operations. Undoubtedly, this possibility should be reviewed from both theoretical and practical aspects in futher details. REFERENCES. Buckley H.D., 98, Surface effect in adhesion, friction, wear and lubrication, Elsevier Scientific Publishing Company, Amsterdam-Oxford-New York. 2. Bogojawlenskij K.N., Neubauer A., Wladimirowitch V., 985, Technologie der Fertigung von Leichtbau-profilen, VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig (in German). 3. Đurđanović M., 980, About a Possibility of Using Heat Generated by Friction, Faculty of Mechanical Engineering of Nis Proceedings, Vol. II, (in Serbian). 4. Đurđanović, M., 990, Pipe Profile Ends Forming by Friction Process Ivestigation, PhD. thesys, Nis, (in Serbian). 5. Матвеев А.Д., 987, Ковка и штамповка, цправочник, Машиностроение, Москва, (in Russian). 6. Musafija B., 988, Plastic Deformation Forming Process of Metals, Svetlost, Sarajevo, (in Serbian). 7. Popović P., Đurđanović M., 99, Applying of Heat Generated by Friction in Plastics Technologies, Tribology in Industry, No. 4/4, Kragujevac (in Serbian). 8. Tadić B., 999, Bending of thin walled pipes using the heat realized by friction, Tribology in Industry, No. /999, Kragujevac (in Serbian). 9. Tadić B., 2006, Manufacturing Processes and Special Machines and Devices, Monography, Faculty of Mechanical Engineering, Kragujevac, (in Serbian).