EP0494843A1 - Method for making a double-walled tubular piece . - Google Patents

Abstract

To make a double-walled tubular piece, a workpiece (1) comprising two tubes (3, 5) inserted one into the other at least virtually without play is bent and then preferably introduced into a cavity (57) which is bounded by a mould (51) having separable mould parts (53, 55). A fluid (79), for example water, is then forced between the two tubes (3, 5) and preferably also into the interior of the inner tube (3) and the outer tube (5) is expanded by the pressure of the fluid. At least over part of its length, there is then a free interspace (91) between the walls, formed by the two tubes (3, 5), of the tubular piece made in this way. By virtue of the fact that the workpiece (1) is bent with the tubes (3, 5) resting at least approximately with a close fit one inside the other, the bending operation and the entire manufacturing process can be carried out in a relatively rapid and simple manner. <IMAGE>

Description

The invention relates to a method for producing a dimensionally stable, double-walled, at least partially curved line piece, in which there is an intermediate space between its mutually enclosing walls at least in a region of its length. The line piece thus forms a double-walled elbow.

A double-walled pipe section of the type mentioned, for example with walls made of stainless steel, can serve in particular as part of an exhaust system or - in short - an exhaust of an internal combustion engine.

In the case of double-walled, cross-sectional pipe sections of exhaust systems, the radial distance between the outer surface of the inner wall and the inner surface of the outer wall is normally approximately 2 mm to 5 mm. In known methods for the production of such a pipe section, two straight pipes are first inserted into one another, the two pipes having the intended diameters of the inner or the outer wall of the pipe section to be produced. Before the tubes are bent, the space between the two tubes and possibly also the interior of the inner tube are filled with a solid filling material. This consists, for example, of sand, ice or an alloy melting at approximately 70 ° C., the ice or the alloy being filled in in the liquid state and then solidified in the tubes by cooling. When the pipes are nested the filler material is removed from the pipes.

In these known methods, it is relatively difficult to hold the pipes in coaxial positions with each other when filling the filling material without severely hindering the filling material. If sand is used as filling material, it also slides relatively slowly into the narrow, annular-gap-shaped space between the two pipes and possibly into the interior of the inner pipe before bending and even worse and slower out of them after the pipes have been bent . If ice is used as the filling material, the tubes must be cooled to below their freezing temperature after the water has been filled in, kept at a temperature below the melting temperature of the ice during the entire bending process and reheated after the bending. If the alloy melting at approx. 70 ° C is used as filler material, the pipes and the alloy must be heated above their melting temperature both for the introduction and the removal of the latter. The introduction and removal of the filling material is therefore cumbersome and time-consuming for all the filling materials mentioned. If the filling material does not completely fill the space between the two tubes during the bending process or cannot flow away, there is also the risk that the two tubes will not remain coaxial to one another during bending and / or their cross-sectional shapes will change differently. The result of this is that the distance between the two tubes varies undesirably from location to location and that the two tubes may even touch each other.

In a method known from JP-A-61-172 625 for the production of a curved, double-walled line piece, a straight inner tube is inserted into a straight outer tube and the space between the two tubes is filled with a filling material before bending. The filling material consists in this process of ethyl silicate and powder of Al₂O₃ and SiO₂. If the filling material has reached a hard rubber-like state after being introduced between the two pipes, the pipes which are plugged into one another are bent and then heated, the filling material being sintered when heated. This method, like the other previously described methods, in which a filler material is introduced between the pipes to be bent, has the disadvantage that the two pipes must be held in coaxial positions with respect to one another when the filler material is filled. Furthermore, the filling material is presumably still deformable during bending in such a way that the tubes which are inserted into one another change their cross-sectional shapes differently during bending and after the bending no longer have the desired distance from one another everywhere. Furthermore, in the case of a pipe section produced by this method, there is no hollow space between the two pipes, but rather sintered filler material, which undesirably increases the weight of the pipe section and is likely to reduce the thermal insulation. Since a pipe section serving as an exhaust pipe can be heated to a great extent during use, there is also the risk that the organic component of the sintered filler material burns in whole or in part when the pipe section is used, and that the unburned constituents then uncontrollably become in one place or at accumulate in several places of the space between the pipes.

FR-A-2 364 710 discloses a method and a device for producing a bellows. The device has clamping rings lying on the outer surface of a cylindrical bellows to be deformed and annular support parts spaced apart from one another in order to support a deformable inner sleeve arranged in the interior of the bellows in places. During the manufacturing process, a liquid - e.g. water - is placed in the between the originally cylindrical bellows and the Inner sleeve existing annular space introduced. The bellows and the inner sleeve are then hydraulically compressed in the axial direction, so that the bellows between the clamping rings are bulged outwards and the inner sleeve between the support parts is bulged inwards and forms waves. After this deformation of the originally cylindrical bellows into a bellows, the likewise deformed inner sleeve is separated from the bellows as waste material. This known method is therefore not used to produce a curved, double-walled line piece, but rather to produce a bellows with a straight axis.

DE-B-1 068 206 relates to a method for producing a curved pipe fitting. In this process, an originally cylindrical, seamless piece of pipe is first bent, then inserted into a cavity delimited by two molded parts and expanded in places by a pressure fluid pressed into its interior. The pipe fitting produced by this method is therefore not double-walled.

The invention has for its object to provide a method for producing a double-walled pipe section, which eliminates disadvantages of the known methods and in which it is particularly not necessary to insert a filling material between them for bending the nested pipes, which then again after bending must be removed.

This object is achieved by a method which, according to the invention, has the features of claim 1.

The invention further relates to a device for performing the method, the device according to the invention having the features of claim 8.

Advantageous refinements of the method and the device emerge from the dependent claims.

The pipes used to form the pipe section are preferably made of a metallic material, for example stainless steel, which can be provided with a thin aluminum coating to beautify the surface, at least on the outside of the pipe forming the outer wall. The tubes should have uninterruptedly closed sheaths in cross section, which preferably have smooth, continuous outer and inner surfaces along their circumference. For example, each tube can have a jacket butt-welded along a weld seam, the weld seam being smoothed, for example, by post-processing. Depending on the intended use of the pipe section, the pipes may also consist of a non-metallic material, but the pipes should be bendable under plastic deformation and the outer pipe should also be expandable due to a plastic deformation.

The two pipes used to produce a line piece should preferably be able to be plugged into one another with at most little play and, if possible, almost play-free and snugly fitting. The dimensioning depends on the accuracy of the pipes used, the desired accuracy of the pipe section to be manufactured and, above all, the cross-sectional dimensions of the commercially available pipes.

The tubes used are preferably circular in cross section or - more precisely - circular. In the case of mutually coaxial tubes, the radially measured distance of the outer surface of the inner tube before the expansion of the latter can be preferably at least 0.01%, for example at least 0.03% and for example at most 3% of the outer radius of the inner tube. The outside radius The inner tube in a line piece provided for an exhaust system of a motor vehicle can be, for example, at least about 10 mm, at most about 100 mm or possibly even more and often 15 mm to 60 mm. The mentioned, radially measured distance can then be at least about 0.01 mm and at most about 1 mm and preferably at most about 0.3 mm. When the outer tube is expanded after the tubes have been bent, the said radially measured distance can then be increased by at least 1 mm and for example by 2 mm to 5 mm.

In special cases, the pipes can have an elliptical, oval or polygonal shape in cross section. In the latter case, the polygon corners and / or even certain polygon sides are preferably replaced by curved transitions, so that the tubes have a cross-sectional shape composed partly of straight lines and partly of arches. In this case, the conditions that were previously given for the radially measured distances and radii then apply to the distances measured at right angles to the surfaces of the tubes and the half outer cross-sectional dimensions of the inner tube corresponding to the outer radius of the inner tube and measured with opposing circumferential locations Rohrs.

Before bending, the two tubes can, for example, be cut from long tubes with the required lengths and then inserted into one another. Thereafter, the workpiece formed from a pair of tubes which are inserted into one another - for example with the aid of a tube bending device having a mandrel which can be inserted into the inner tube - is bent into the desired shape. If, according to the previous information, the tubes inserted into each other for bending fit into each other with a small amount of play and accordingly are at least approximately in contact with one another, they remain at least approximately and practically completely coaxial with one another during bending - that is, without any special measures. If when bending of the tubes - depending on the type of bending used - changes in the cross-sectional shapes of the tubes take place at least in places, these changes are the same for both tubes, ie in such a way that the outline of the two tubes remains parallel to one another in cross-section. The tubes which are plugged into one another can therefore be bent without a filler material having to be introduced between the two tubes and into the inner tube before the bending, and without having to be removed from the tubes again after the bending.

The fluid used to expand the outer tube is preferably a liquid, for example water or possibly a low viscosity, i.e. a low viscosity oil. However, a gas may be used instead of a liquid.

In an advantageous embodiment of the method, in order to expand the outer tube, not only fluid is pressed into the interior of the inner tube between the two tubes, but also fluid with the same pressure. This makes it easy to ensure that the inner tube maintains its shape when the outer tube is expanded. If the inner tube is sufficiently pressure-resistant to withstand the fluid pressure required to expand the outer tube, however, it may not be necessary to press fluid into the inner tube. Furthermore, prior to expanding the outer tube, it would be possible to temporarily insert an articulated link or flexible rod into the inner tube to stiffen it while expanding the outer tube without introducing fluid into the inner tube .

When expanding the outer tube, its outline shape is preferably said through the cavity of a molding tool, or short - a shape, with two or possibly more mold parts which can be pressed against one another and brought to bear against one another and then separated again from one another or - in short - mold parts. The mold can be closed after inserting a workpiece consisting of two tubes which are inserted into one another and bent and opened again after the outer tube has been expanded to remove the workpiece. The shape is preferably designed in such a way that in the closed state it encompasses a short end section of this or - more precisely - of its peripheral surface at the opposite ends of the outer pipe arranged in it, and thereby fits the end sections of the outer pipe against one Extension ensures. The entire remaining part of the outer tube located between these two end sections can then be expanded in shape. The length of that part of the outer tube which is widened during the production of the tube piece is expediently at least 50%, preferably at least 80% and for example even approximately or at least 90% of the total length of the outer tube.

If the two end sections of the outer tube are not expanded during the expansion process in accordance with the previously described embodiment of the method, at least one or both of the end portions of the workpiece having one non-expanded end portions of the workpiece can be subsequently separated from the central main portion of the workpiece if necessary. If both said workpiece end sections are cut off, the space between the two pipes in the finished pipe section then extends over the entire length of the two pipes or - if the pipes should be of unequal length - over the entire length of the shorter pipe.

The outer tube can, however, under certain circumstances - especially if only small demands are placed on the dimensional and dimensional accuracy of the pipe section produced - expanded without the expansion process being carried out in a hollow shape.

Since it is not necessary in the method according to the invention to introduce a filler material between the pipes that extend into one another before the bending and to remove the filler material after the bending, the workflow can be simplified and the time required for the production of a line piece compared to the known ones, the introduction of a filler material required procedures are reduced. A significant advantage of the method according to the invention is also that it is largely or even completely automatic - i.e. completely automatic - in the series production of line sections with relatively simple, apparatus-related means. without manual work - can be done.

The method according to the invention makes it possible to produce a line section whose walls formed by two pipes only touch one another at their ends. Otherwise, there is the possibility of cutting off the mutually contacting end sections of the pipes at one end or at both ends of the line section. If the latter is done, the ends of the two pipes or walls of the pipe section can be connected to one another by flanges or other connecting elements in such a way that their surfaces facing one another are spaced apart over the entire lengths of the pipes or walls.

The space formed by the expansion of the outer tube between this and the inner tube can remain free in the finished pipe section, ie only contain air or even be evacuated. The double-walled pipe section therefore enables both good thermal insulation and good sound insulation.

As described, the inner tube can rest against the outer tube when the intermeshing tubes are bent and can be supported by a fluid or possibly in some other way when the outer tube is expanded. The manufacturing process can be carried out without the jacket of the inner tube having to absorb very large compressive forces. This enables the inner tube to be made relatively thin-walled, so that it is correspondingly light and has only a small heat capacity.

A line piece produced by the method according to the invention can be used, for example, in an exhaust system to connect the exhaust outlet of a gasoline engine or possibly a diesel engine of a road motor vehicle to a catalytic converter. The achievable good thermal insulation of a line section produced by the method according to the invention and the likewise achievable, low heat capacity of the inner wall of the line section then result, inter alia, in the advantage that the catalytic converter quickly and quickly starts to run through the exhaust gas when and after the internal combustion engine is started Temperature is heated, which is necessary to trigger the chemical reactions that are to take place in the catalyst. The rapid heating of the catalytic converter to said temperature in turn gives the advantage that the exhaust gas in the catalytic converter is freed from pollutants practically immediately from the start of the engine, without an additional auxiliary catalytic converter being provided for the start and warm-up phase of the engine and the exhaust system got to.

The subject matter of the invention is subsequently explained using an exemplary embodiment shown in the drawing. The drawing shows:
1 shows a workpiece consisting of two straight tubes which are inserted into one another,
2 shows a schematic tube bending device when bending the workpiece, the latter being drawn on a smaller scale than in FIG. 1,
3 shows a schematic device, partly in view and partly in section, for expanding the outer tube of the workpiece, the latter being drawn on an even smaller scale than in FIG. 2,
FIG. 4 shows a section, designated IV in FIG. 3, from the device drawn in this and the workpiece, but the latter being drawn in section and on a larger scale,
5 shows a plan view of the lower mold part of the shape shown in FIGS. 3, 4 for determining the outline shape of the outer tube on the same scale as FIG. 4,
FIG. 6 shows a section corresponding to FIG. 4, but the outer tube is expanded,
7 shows a longitudinal section of the workpiece after the outer tube has been widened to approximately the same scale as FIG. 2 and
8 shows a longitudinal section through the finished pipe section.

In order to produce an elongated, dimensionally stable, double-walled, at least partially curved line piece, in which there is an intermediate space between its walls at least in a region of its length, two metallic straight, circular-cylindrical tubes 3, 5 shown in FIG. 1 are first provided, cut off from longer, commercially available pipes, for example. The outside diameter of the tube 3 is slightly smaller than the inside diameter of the tube 5. The tubes 3, 5 can be formed, for example, from commercially available tubes with an outside diameter of 44.45 mm or 47.5 mm and wall thicknesses of 1.5 mm. The inner radius of the tube 5 is then about 0.025 mm larger than the outer radius of the tube 3. Otherwise, the narrower tube 3 is preferably a little longer than the further tube 5.

It should be noted here that the walls of the two tubes in FIG. 1 and also in other figures that have been described have been drawn with disproportionate thicknesses for reasons of drawing.

The two tubes 3, 5 consist of a metallic material, namely at least essentially of stainless steel. To beautify the visible outer surface of the finished pipe section, the outer surface of the further pipe 5 and, for example - depending on its type of manufacture - also the inner surface thereof can be provided with a thin, about 0.01 mm to 0.03 mm thick aluminum coating. The narrower tube 3, which is no longer visible at least for the most part in the finished pipe section, can be made entirely of stainless steel or may also be provided at least on the outside with an aluminum coating.

The tube 3 is inserted into the tube 5 manually or mechanically by means of a suitable device, the two tubes being able to be lubricated with a lubricant, for example oil, if necessary. By inserting the tube 3 into the tube 5, the workpiece designated 1 in FIG. 1 is produced, which accordingly has two straight tubes 3, 5 coaxial with an axis 7. The inner tube 3 protrudes at least at its end on the right in FIG. 2, namely at both ends a little - for example at least about 0.5 mm and at most about 5 mm - from the outer tube 5.

The device for producing a double-walled line section has, inter alia, the bending device 11 shown schematically in FIG. This can consist, for example, of a tube bending device of conventional design and have a frame 13, only indicated schematically, on which a guide element 15 is rigidly but, for example, adjustably fastened and a bending disk 17, for example consisting of a roller, is pivotably mounted. The latter is provided with a clamping jaw 19 for releasably fastening the workpiece 1. The bending device 11 also has a rigid, but adjustable, mandrel 21 fastened to the frame 11, the free end of which fits into the inner tube 3 of the workpiece 1 with at most small, radial play. The originally straight workpiece 1 can be at least partially attached to the mandrel 21 and bent by pivoting the bending disk 17 and the clamping jaw 19. This can occur at normal room temperature - i.e. by cold working - done. The bending device 11 can be actuated manually or with muscle-free working, for example electrical and / or hydraulic and / or pneumatic drive means during bending and may also be designed for largely automatic operation. The workpiece 1 can be bent, for example, according to FIG. 2, with two longitudinal regions spaced apart from one another along a plane, so that the axis 7 of the workpiece 1 lies in one plane after the bending and for example approximately the shape of a somewhat elongated letter S or Z. Has.

The device used to manufacture the double-walled line section also includes a press 31, which is shown in a simplified and schematic manner in FIG. 3. This press has, for example, a frame 33 on which a lower support 35 and an upper support 37 are held. The lower support 35 is rigidly connected to the frame 33, for example, belongs to it and forms approximately its base. The Upper support 37 is guided vertically displaceably by columns of the frame and can be adjusted vertically with a schematically indicated adjusting device 39. The adjusting device 39 has at least one hydraulic cylinder 41 fastened to the frame 33 and a piston 43 which can be displaced therein and whose shaft is connected to the upper support 37.

A mold 51 which can be seen in FIG. 3 and partly in FIGS. 4 to 6 and which is also referred to below as mold 51, has two mold parts 53, 55, which are also referred to below as mold parts 53, 55. The one, lower molded part 53 is rigidly attached to the lower support 35. The other, upper molding 55 is rigidly attached to the upper support 37. The mold 51 can accordingly be closed and opened again with the help of the press, in that the upper mold part 55 is pressed against the lower mold part 53 or moved upwards away from it. Each molded part 53, 55 is provided on its side facing the other molded part with an elongated recess 53a or 55a. Each recess 53a, 55a has a central main section 53b and 55b and a narrower and shorter end section 53c and 55c at both ends thereof. When the mold is closed, that is to say when the two molded parts 53, 55 abut one another, they together delimit an elongated cavity 57 which serves to hold the workpiece 1, each of the two recesses 53a and 55a forming at least approximately and, for example, exactly half of the cavity 57 . The two main recess sections 53b, 55b together form a main cavity section 57b with a circular cross section, the radius of the main cavity section 57b being at least 1 mm and, for example, at least 2 mm larger than the outer radius of the outer tube 5 of the workpiece 1. The two end sections 53c, 55c of the recesses 53a and 55a together form a short, straight, circular cylindrical end section 55c of the cavity 55. The radius the end sections 55c is approximately equal to the outer radius of the outer tube 5 of the workpiece 1, so that the two end sections of the outer tube 5 fit snugly, ie radially at least approximately without play into the cavity end section 57c.

The cavity 55 is somewhat longer than the workpiece 1, so that at least the end section 57c located in FIG. 3 at the right end of the cavity 57 has a free area of it after the workpiece has been inserted, namely both cavity end sections 57c after the Introducing the workpiece 1 have such a free area.

It should be noted here that the difference between the radius of the cavity main section 57b and the radius of the cavity end sections 57c in FIGS. 3 to 6 is analogous to the illustration in the same way as the wall thicknesses of the two pipes in comparison to the original diameters of the pipes of disproportionate size is drawn.

It should also be noted that the cavity 57 and the workpiece 1 arranged therein can be arranged according to FIG. 3 such that the axis 7 of the workpiece 1 and the axis of the cavity 57 coinciding therewith lie in a vertical plane. The mutually facing surfaces of the two molded parts 53 and 55 which surround the recesses 53a, 55a are then flat and horizontal in vertical sections perpendicular to the plane of the drawing in FIG. The workpiece 1 and the cavity 57 were, however, drawn in FIG. 3, especially for the sake of clarity, with the axis 7 lying in a vertical plane. In the case of a workpiece axis 7 lying in one plane, the shape delimiting the cavity for receiving the workpiece will actually be designed in such a way that the axis of the workpiece and of the said cavity lie in a horizontal plane, ie at right angles to the direction of displacement of the upper molded part. The the recesses of the two molded parts on their mutually facing sides of the surfaces can then lie in planes, namely the same as the axis of the workpiece in horizontal planes. As will be explained later, in practice, the workpieces often have spatial axes.

When the mold 51 is closed, the surfaces of the two molded parts 53 and 55 which face one another and surround the recesses 53a, 55a abut one another. The form 51 is also provided with only sealing means 61 shown in FIGS. 4 to 6, which seal off the areas of the two cavity end sections 57c which are free after the insertion of a workpiece 1 when the form 51 is closed, with respect to the environment. For each cavity end section 57c, the sealing means 61 have, for example, two semicircular seals 63 and 65, which surround the outer tube 5 and form a ring in the groove of the molded part 53 and 55, and a seal 67. This is held, for example, in a groove of the molded part 53, is approximately C-shaped in the plan view of the latter shown in FIG. 5 and extends from one end of the seal 63 around the free area of the cavity end section 57c to the other end of the Seal 63. The seals 63, 65, 67 are shown schematically in Figures 4 to 6 as one-piece, for example rubber-elastic profile pieces, but can in reality consist of two or more parts with different strengths and elastic properties, as is the case for high pressure Seals is known.

The molded part 53 is provided with a hole 53d consisting approximately of a stepped bore, which connects the free area of the one cavity end section 57c to a connection 69 fastened to the molded part 53, for example welded, schematically drawn as a sleeve.

The at least one hydraulic cylinder 41 of the actuating device 39 is connected via at least one fluid line to a fluid source, not shown, for supplying and discharging a hydraulic fluid. The adjusting device 39 and / or the fluid source can be designed to adjust the upper support 37 and the upper molded part 55 fastened to it either with a relatively small force quickly over a long distance or slowly with a large force.

The connection 69 is connected to a fluid source 75 via a fluid line 73. The fluid line 73 is preferably as short as possible, the fluid source 75 possibly actually being attached directly to the molded part 53. The fluid source 75 is designed to press a fluid 79 into the cavity 57 in the manner described above with pressure when the mold 51 is closed, and then to relieve it of the fluid pressure again and, for example, to derive at least part of the fluid present in the cavity 57 therefrom . The fluid preferably consists of a liquid, namely, for example, water. The fluid source 75 has, for example, a reservoir 77 for storing fluid 79 and two pump devices 81, 83. Of these, the pump device 81 consists of a pump which is designed to pump a large amount of fluid per unit of time, but only to generate a relatively low pressure. The pump device 83, on the other hand, is intended to generate a high pressure, but only needs to deliver a small amount of fluid per unit time. The pump device 83 can have, for example, a multi-stage pump or several separate pumps connected in series or at least one pump and a pressure booster connected to its outlet. The pump device 81 has an inlet connected to the fluid reservoir 77 and an outlet which is connected to the connection 69 via a check valve 85. The pump device 83 has an inlet connected to the fluid reservoir 77 and one directly output connected to terminal 69. Furthermore, there is also a valve 87 which, in the open state, connects the connection 69 to the fluid reservoir 77 in order to relieve pressure.

The workpiece 1 bent with the bending device 11 can be further shaped using the press 31, the mold 51 and the fluid source 75. For this purpose, the workpiece 1 is inserted into the recess 53a of the lower molded part 53 when the mold 51 is open. The mold 51 is then closed with the aid of the adjusting device 39. Thereafter, the fluid source 75 leads the connection 69 from the fluid 79 consisting of water, as indicated by an arrow in FIG. 6. The fluid, i.e. Water first comes from the connection 69 into the cavity end section 57c directly connected to it and then flows into the interior of the inner tube 3 and through it into the free area of the end section 57c of the left end of the workpiece 1 in FIG Cavity 57. Furthermore, at both ends of the workpiece 1 fluid, ie Water penetrate between the outer surface of the inner tube 3 and the inner surface of the outer tube 5. This penetration of the fluid consisting of water between the two tubes 3, 5, which are at least approximately free of play and neatly inserted into one another, is facilitated and accelerated in that the inner tube 3 protrudes from the outer tube 5 at both ends of the workpiece 1. The air present in the free areas of the cavity end sections 57c and in the workpiece 1 before the supply of water is compressed into at least one bubble by the supplied water and possibly partially dissolved in the water.

In the initial phase of the water supply process, in which only a relatively low pressure is required in order to supply water to the workpiece 1 located in the cavity 57 and in the cavity end sections 57c and in the workpiece To compress existing air, the water is supplied by the pump device 81 having a relatively high pumping rate. When the free cavity end portions 57c and the interior of the inner tube 3 are at least approximately filled with water and the pressure generated by the pumping device 81 is no longer sufficient to supply additional water, the pumping device 81 is switched off and the pumping device 83 is switched on instead has a lower pumping rate but can generate a higher pressure than the pumping device 81.

The water is pressed by the fluid source 75 during the pumping process and in particular after the pumping device 83 has been switched on with such great a pressure, for example about 200 MPa to 300 MPa, into the inner pipe 3 and between the two pipes 3, 5 that the water is the main section 5b of the outer tube 5 located in the main cavity section 57b is radially stretched and expanded by plastic deformation - namely, for example, by cold deformation taking place at normal room temperature - until it abuts the inner surface of the mold 51 delimiting the cavity 57. Because the inner and outer surfaces of the inner tube 3 have the same fluid pressure, the inner tube 3 is not deformed. The original, i.e. Before the expansion of the tube 5 in the free area of the main cavity section 57b, air is compressed during the expansion process and / or at least for the most part is pressed out of the cavity 57 between the two molded parts 53, 55.

If the outer tube 5 has been expanded in the manner described and also bears in the region of the main section 57c of the cavity 57 on the inner surfaces of the mold 51 delimiting it, the water supply is stopped by switching off the pump device 83. Furthermore, the valve 87 is now temporarily opened and thereby the cavity 57 and that Workpiece 1 relieved of the fluid pressure, water being able to flow back from the cavity 57 and workpiece into the reservoir 77. After this pressure relief, the mold 51 is opened with the adjusting device 39 and the workpiece 1 is removed from the mold 51.

The press 31 and the fluid source 75 are also provided and / or connected with control means (not shown), with which the actuating device 39 of the press and the fluid source 75 can be controlled in order to carry out the previously described work operations. The various work operations can be controlled manually, for example, or at least partially, either manually or automatically.

After the described expansion of the main section 5b of the outer tube 5 which is located in the region of the main cavity section 57b, there is a free space 91 between the inner tube 3 and the main section 5b of the outer tube 5 which extends over most of the length of the workpiece 1 extends. The workpiece 1 then has the shape shown in FIG. 7.

It is now possible, for example, to cut off one end section of the workpiece 1, which has the non-widened end section of the outer tube 5 and the end section of the inner tube 3 which is located therein, at the separating surface designated 93 in FIG. This can be done, for example, by cutting with a saw or with a side milling cutter or with any other cutting device.

The interior of the inner tube 3 and the intermediate space 91 between the two tubes 3, 5 can still contain water after the expansion of the outer tube and the described pressure relief. The water contained in the inner tube normally flows when the mold 51 is opened and when the workpiece 1 is subsequently removed therefrom without any particular one Measures at least for the most part out of the inner tube 3. The water remaining in the intermediate space 91 after the expansion and the pressure relief can, at least in part, still remain in the intermediate space 91 when the mold 51 is opened and when the workpiece is removed therefrom. After the one end section of the workpiece 1 has been cut off, any water still present in the intermediate space 91 can then be drained out of the latter, so that the intermediate space 91 now only contains air.

The ends of the two tubes 3, 5 located at the left end of the workpiece 1 can now still be firmly and tightly connected to one another and connected to one another, that is to say welded, by a connecting element 95 consisting of a flange, as shown in FIG. The other ends of the two pipes 3, 5 can be welded tightly and tightly to a connecting element 97, which also consists of a flange, and can thus also be indirectly connected to one another. The result is the dimensionally stable line piece, designated as a whole by 99, which has two walls or jackets, which are formed by the remaining parts of the tubes 3, 5 and are at least approximately coaxial to a common axis 7 and are separated by the free space 91 over their entire lengths are.

The method and equipment for carrying it out can be changed in a number of ways.

For example, the outline shape created when the workpiece is bent can be varied within wide limits. For example, the workpiece can be bent such that it has only a single bent area extending over part of its length or more than two such areas separated from one another by straight areas or is bent over its entire length. Furthermore, in practice, the workpiece has to be bent for many uses in such a way that its Axis does not lie in one plane, but forms at least partly a more or less pronounced spatially curved line or curve, which is partly curved and partly straight or curved everywhere. Furthermore, the bending device can be equipped with heating means in order to heat the workpiece before and during the bending. Furthermore, the bending device can, for example, instead of the guide element 15, the bending disk 17 and the clamping jaw 19, a control element defining a curve and a feed slide with a displaceable support, at least two rollers rotatably mounted about fixed axes and at least one pivot arm mounted pivotably on the support exhibit. On the latter, a feeler roller rolling on the control element and scanning its curve and a bending roller can then be rotatably mounted, which acts on the workpiece and bends it in cooperation with the rollers mounted on the support.

If, after bending, the axis of the workpiece is still in one plane, this can be vertical or horizontal, as already mentioned, when expanding the outer tube, ie parallel or perpendicular to the direction of displacement of the displaceable molded part. If, on the other hand, the axis of the workpiece is spatially curved to a greater or lesser extent when it is bent, the axis of the cavity which defines the outline shape of the outer tube when the tube is expanded must of course also be correspondingly spatially curved. The two mutually adjustable molded parts should also be designed with a spatially curved axis of the workpiece such that this axis runs between the mutually facing and abutting surfaces of the two molded parts when the mold is closed and the recesses of the two molded parts are left out in thought or lies in these areas. The molded parts can then be designed, for example, in such a way that the axis of the workpiece and of the cavity both in a projection onto an axis analogous to the drawing plane in FIG Vertical plane as well as being curved at least in places in a projection onto a horizontal plane.

If a workpiece is produced with a relatively strongly spatially curved axis, the shape which delimits the cavity for receiving the workpiece when the outer tube is expanded can possibly have three or even more separable molded parts.

The press and / or the shape serving to expand the outer tube of a workpiece for receiving it can also have locking means which have at least one adjustable element, engage mechanically with one another when the shape is closed and thereby secure the molded parts against separation until the locking means be disengaged again.

Furthermore, the shape receiving the workpiece when the outer tube is expanded and / or the fluid source used for supplying a fluid to this and to the workpiece can be provided with heating means, and the shape and / or the fluid and thus the workpiece before and / or during that Extend the outer tube to heat. Such thermal deformation can reduce the fluid pressure required to expand the outer tube - given the same dimensions and with the same material of the outer tube - compared to the fluid pressure required for cold deformation.

If, when the mold is closed, the elongated cavity present in the mold, as in the mold 51 shown in FIG. 3, has two ends located at different heights, the connection for supplying and discharging the fluid used to expand the outer tube can take place instead of as in the figure 3 at the higher end of the cavity at the lower end. Thereby it can possibly be achieved that a larger part of the water or other liquid fluid present in the cavity can flow out during the pressure relief before the mold is opened. Furthermore, suction and / or blowing agents can be provided, only after the outer tube has been widened and after the pressure has been released, but before the mold containing the workpiece is opened, as much of the liquid fluid as possible contained in the workpiece and in the cavity of the mold is drawn out suck and / or blow out.

Instead of separating the end section of the inner and outer tube located at one end of the workpiece, as shown in FIGS. 7 and 8, either the non-expanded end sections of the outer tube and the end sections of the inner tube enclosed by these can be separated at both ends of the workpiece or at leave both ends of the workpiece on it. In the latter case, the workpiece can be heated to such an extent before the pipes are welded with connecting elements that the water still present in the intermediate space between the two tubes evaporates and flows out of the intermediate space as steam.

The connecting elements can be formed from sleeves or other suitable components instead of flanges. If the line piece is used to form an exhaust system having a catalytic converter, the two pipes may perhaps be welded directly to the housing of the catalytic converter and / or another part of the exhaust system.

Furthermore, the space between the tubes can be evacuated during or after connecting the tubes with connecting elements.

Claims (10)

Method for producing a dimensionally stable, double-walled, at least partially curved line piece (99) in which there is an intermediate space (91) between its walls at least in a region of its length, a tube (3) serving to form the inner wall being inserted into a Formation of the outer wall-serving tube (5) is inserted and the workpiece (1) thus formed is at least partially bent, characterized in that after the workpiece (1) has been bent, a fluid (79) between the two tubes (3, 5) pressed in and the outer tube (5) is thereby extended in the area of its length. Method according to claim 1, characterized in that fluid (79) is pressed into the interior of the inner tube (3) with the same pressure as between the two tubes (3, 5). Method according to claim 1 or 2, characterized in that tubes (3, 5) are used in which the distance between the inner surface of the outer tube (5) and the outer surface of the inner tube (3) in the case of coaxially intermeshing tubes (3, 5) before expanding the outer tube (5) amount to at least 0.01% and at most 3% of half the outer cross-sectional dimension of the inner tube (3), preferably circular tubes (3, 5) are used in cross-section, in which the conditions mentioned for the radially measured distance with respect to the outer radius of the inner tube (3, 5) are met. Method according to one of claims 1 to 3, characterized in that tubes (3, 5) are used in which the distance of the inner surface of the outer tube (5) from the outer surface of the inner tube (3) with coaxially inserted into each other Pipes (3, 5) before expanding the outer pipe (5) is at least 0.01 mm and at most 1 mm, preferably circular pipes (3, 5) are used in cross-section, in which the radially measured distance meets the conditions mentioned and wherein the distance of the inner surface of the outer tube (5) from the outer surface of the inner tube (3) is preferably increased by at least 1 mm when the outer tube (5) is expanded. Method according to one of Claims 1 to 4, characterized in that the workpiece (1) after the bending and before the expansion of its outer tube (5) into a cavity (52) of a mold (51, 55) which can be separated from one another ) is introduced and that the outer tube (5) is then expanded by the fluid (79) until it lies in the expanded area on the inner surface of the mold (51) delimiting the cavity (57). Method according to Claim 5, characterized in that the two end sections of the outer tube (5) which face away from one another are secured against expansion after the workpiece (1) has been introduced into the mold (51). Method according to one of claims 1 to 6, characterized in that a liquid, for example water, is used as a fluid (79) to expand the outer tube (5). Device for carrying out the method according to one of claims 1 to 7, characterized in that a fluid source (75) is present to press fluid (79) between the two tubes (3, 5) and thereby the outer tube (5) expand at least in the mentioned area. Device according to claim 8, characterized in that there is a mold (51) with mold parts (53, 55) which abut against one another in the closed state and are separable to open the mold (51), and in that the mold parts (53, 55) are provided with closed form (51) together delimit an elongated cavity (58) which is designed to receive a bent workpiece (1) and, when the outer tube (5) is expanded, to define its outline shape. Device according to claim 9, characterized in that an end section (57c) of the cavity (57) is connected to the fluid source (75) in such a way that the fluid (79) supplied by it into the interior of the inner tube (3) and between the two pipes (3, 5) can flow into it and that sealing means (61) are present to close the two end sections (57c) of the cavity, each containing an end section of the workpiece (1) when the mold (51) is closed (57) against the outer tube (5) and against the environment.

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