METHOD AND APPARATUS FOR PRODUCING HELICALLY WOUND LOCK-SEAM TUBING

Abstract: In a method for producing helically wound lock-seam tubing (33), a metal strip is fed to a forming head in which the tubing is formed. In order to prevent lock-seam slip, a clinching roller device (30-40) forms interlocking spaced deformations (41) on the lock-seam (37). An apparatus for such production comprises a clinching roller (30) provided with a recess (40) for forming the deformations (41) against a counter-roller (34).

Description: METHOD AND APPARATUS FOR PRODUCING HELICALLY WOUND LOCK-SEAM TUBING FIELD OF THE INVENTION The present invention relates to the production of helically wound tubing, and more specifically to a method and an apparatus for producing helically wound lock-seam tubing from a strip of sheet metal. The tubing can, for example, be used in ventilation duct systems. BACKGROUND ART A known machine for producing helically wound lock- seam tubing is disclosed in British patent publication GB 2,213,748, which is incorporated herein by reference. In this machine, a metal strip is fed to a forming head by means of drive rollers, and a helically wound lock-seam tube is formed by clinching rollers associated with the forming head. Another known machine for producing helically wound tubing is disclosed in British patent specification GB 1,168,178, wherein Figs 3 and 4 show different seams. The technology related to such machines is well-known to persons skilled in the art. At present, several further developed machines of a similar type are available on the market. However, these machines still suffer from some specific drawbacks. In the production of helically wound lock-seam tubes of the present type, the lock-seam tends to slip, which results in an undesirable change in tube diameter. The general solution to this problem is to increase the pressure of the clinching rollers acting on the lock-seam in the forming head. However, such an increased clinching pressure can cause damage to and deficiencies in the lock- seam as well as in the adjacent portions of the metal strip forming the tube. Thus, the adjustment of the clinching pressure is crucial to the quality of the lock- seam and the resulting tube. In addition to the clinching pressure, the operator running the machine must consider several other para¬ meters, such as the thickness of the metal strip, the lubrication of the same, and possible wear of different machine components, for example the rollers. Thus, great demands are made on the skill and experience of the ope¬ rator. In spite of the operator's skill, it is hard to avoid slip or sliding in the lock-seam. An undesirable increase of the tube diameter can occur either in production, imme¬ diately after the forming head, or during storing, han¬ dling and transportation of the tubes. It should be noted that the tube diameter may also be undesirably reduced due to lock-seam slip, for example when tubes are subject to pressure forces from the outside. The change in tube diameter is always a problem, since the tubes delivered do not fulfil the customers' specifications and requirements. Specific problems are encountered when using the tubes in ventilation duct systems mounted on different premises. For example, vibrations caused by fans and similar equipment can initiate lock-seam slip which, in turn, leads to undesirable leakage of air and pressure drops. In severe cases, lock-seam slip of the ventilation ducts can jeopardize safe function of the whole ventila¬ tion system. In ventilation duct systems, different fittings are connected to the helically wound lock-seam tubes forming the major part of the system. Such fittings are bends, T-pieces, dampers, sound attenuators, etc. The fittings are normally inserted in the end of the tubes, and a sealing ring on the inserted portion of the fitting en¬ sures safe sealing. However, if the tube has an unde¬ sirably increased diameter, there is a risk that suffi- cient sealing cannot be obtained in the joint between the fitting and the tube. This may lead to the leakage and pressure drop problems discussed above, resulting in ener¬ gy losses and increased running costs. In other cases, the fitting is not provided with a pre-mounted seal, but the joint between the outside of the fitting and the inside of the tube is filled with a mastic for sealing purposes. If the gap is too large due to an undesirably increased tube diameter, sufficient sealing may not be obtained by means of the mastic. Even if the mastic is safely fastened, problems may arise later due to increased tube diameter caused by vibrations, air pressure peaks, etc. Further, change in tube diameter is disadvantageous when prefabricated annular fastening devices and the like are to be applied on the outside of the tube. Such devices do not fit if the tube diameter has changed. SUMMARY OF THE INVENTION One object of the present invention is to remedy the above-mentioned drawbacks by providing a method and an apparatus for producing helically wound lock-seam tubing with no or very low risk of lock-seam slip or sliding in any direction. Another object of the present invention is to provide a method and an apparatus for producing helically wound lock-seam tubes which maintain their diameter during stor¬ ing, handling, transportation, delivery and mounting on the final site. A specific object of the invention is to provide a method and an apparatus which are easily adapted to exist¬ ing machines. In accordance with a first aspect of the invention, these objects are achieved by a method and an apparatus as set forth in the appended independent claims 1 and 9, preferred embodiments and variants thereof being defined in subclaims 2-8 and 10-19, respectively. The invention makes it possible to avoid lock-seam slip and achieve further advantages. An important advantage according to said first aspect is that the spaced deformations on the lock-seam imme¬ diately indicate that the tube in question has a securely interlocked lock-seam which in turn results in a constant diameter of the tube. Another advantage is that the means for providing such interlocking deformations can either be incorporated in new machines or easily mounted on existing machines as additional equipment. If the interlocking deformations on the lock seam of the tube, in accordance with a preferred embodiment, are formed as expanded portions, the wear of the means provid¬ ing the interlocking deformations can be kept very low. Most preferably, a clinching roller of the clinching assembly has a peripheral groove which is provided with at least one recess for forming the expanded portions of the lock seam. Preferably, at least one interlocking deformation is provided on each helical turn of the lock seam in order to reduce the risk of slipping in the same. In accordance with a second aspect of the invention, the objects are also achieved by a method and an apparatus as set forth in the appended independent claims 20 and 23, preferred embodiments and variants thereof being defined in the subclaims 21-22 and 24-25, respectively. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described with reference to the accompanying schematic drawings showing preferred embodiments by way of example. Fig. 1 illustrates schematically a machine for pro¬ ducing helically wound lock-seam tubing. Fig. 2 illustrates schematically a clinching assembly incorporated in a machine according to the present inven- tion. Fig. 3 shows a helically wound lock-seam tube pro¬ duced by a method and an apparatus according to the pre¬ sent invention. Fig. 4 shows, on a larger scale, a lock-seam of the tube shown in Fig. 3. Fig. 5 shows, on a larger scale, a cross-section of the lock-seam shown in Fig. 3. Fig. 6 is a partial view of an embodiment of a clin¬ ching roller. Fig. 7 shows a recess of the clinching roller of Fig. 6. Fig. 8 shows a portion of the clinching roller of Fig. 6 from the side. Fig. 9 is a sectional view of the clinching roller of Fig. 6 illustrating a lock-seam indicated by dash-dot lines. Fig. 10 is a partial view of a second embodiment of a clinching roller. Fig. 11 shows a ridge of the clinching roller of Fig. 10. Fig. 12 shows a portion of the clinching roller of Fig. 10 from the side. Fig. 13 is a sectional view of the clinching roller of Fig. 10 illustrating a lock-seam indicated by dash-dot lines. Fig. 14 shows a tube produced by use of a clinching roller of Fig. 10. Fig. 15 shows, on a larger scale, the lock-seam of the tube shown in Fig. 14. Fig. 16 shows, on a larger scale, a cross-section of the lock-seam shown in Fig. 14. Fig. 17 shows a cross-section of a lock-seam in yet another embodiment of the invention. Figs 18-22 are cross-sectional views of alternative lock-seams in accordance with the invention. DESCRIPTION OF PREFERRED EMBODIMENTS The method and the apparatus according to the inven¬ tion can in principle be applied to a machine of the type as generally shown in Fig. 1. This machine bascially cor- responds to the machine described in the publication GB-A-2, 213,748 mentioned by way of introduction, some components 47, 48 related to one embodiment of the invention, to be described later, being added in Fig. 1. A metal strip 12 is fed to a forming head 14 by drive rollers 11 (arrow A). The longitudinal edge portions 16 of the strip 12 can be preformed in a known manner by rollers 13. In the forming head 14, the strip 12 is formed into a helical shape, whereby the edge portions 16 are brought into engagement with each other. A clinching assembly 22, 23, 24 is associated with the forming head 14 and includes clinching rollers 17, 18, 19 for clinching the engaging edge portions 16 of the strip 12 to form a helically wound lock-seam 25 on a tube generally denoted 15. In produc¬ tion, the tube 15 is positively fed out of the forming head 14 in a direction shown with an arrow B by the in- feeding force of the strip 12. The tube 15 is cut to desired lengths by a cutting device (not shown). The machine shown in Fig. 1 can be improved by means of a clinching device of the type illustrated in Fig. 2. This clinching device can be mounted either in or in con¬ nection with the forming head 14. The clinching device according to a first embodiment comprises a clinching roller 30 having a shaft 31 which is rotatably supported by bearings 32 (schematically shown) mounted on the form¬ ing head 14 (cf. Fig. 1). A counter-roller 34 having a shaft 35 which is rotatably supported by bearings 36 (schematically shown) mounted on the forming head 14 (cf. Fig. 1) is located inside a tube 33. A lock-seam 37 of the tube 33 is clinched between the peripheries of the two rollers 30, 34 at a clinching point. The clinching pres¬ sure is adjustable by a device 38 (schematically shown) acting on the shaft 31 in the directions of the arrows. The clinching roller 30 has a peripheral groove 39 with at least one transverse recess 40 (see Figs 6-9). At the clinching point between the two rollers 30, 34, this recess 40 forms interlocking deformations 41 on the lock- seam 37. The interlocking deformations 41 are spaced on the helical lock-seam 37. The device shown in Fig. 2 works as follows. A metal strip 12 with longitudinal edge portions 16, which may be preformed by rollers 13 (cf. Fig. 1), is formed in the forming head 14. In order to avoid slip in the lock-seam, the engaging edge portions 16 are deformed and interlocked by the deformations 41 formed by the recess 40 of the clinching roller 30. In the first embodiment shown in Fig. 2, the spaced deformations 41 are in the shape of expanded portions of the lock-seam 37. In this way, the engaging edge portions 16 are fixed together in the lock- seam 37, which in turn results in a tube 33 with a con¬ stant diameter. Slip in the lock-seam 37 is prevented. An important advantage relating to the feature that the interlocking deformations on the lock seam 37 are provided in the shape of expanded portions thereof is that the wear of the clinching means for forming the deforma¬ tions can be kept at a very low level in relation to known equipment for the production of helically wound lock seam tubing. A swollen or expanded deformation 41 is shown in more detail in Fig. 4, and the lock-seam 37 is shown in Fig. 5. The recess 40 of the clinching roller 30 of the first embodiment is shown in detail in Figs 6-9. The recess 40 is broader than the groove 39 with respect to the thick- ness of the clinching roller 30. The section of Fig. 9 shows the deformation 41 of the lock-seam in the recess 40. Of course, the interlocking deformations of the lock- seam 37 can be achieved in other ways. A second embodiment of the clinching roller 30 is shown in Figs 10-12. In this embodiment, there is not a recess, but a transverse ridge 42 in the groove 39. At the clinching point between the rollers 30, 34 (cf. Fig. 2), the ridge 42 is pressed into the lock-seam 37, thereby causing an indentation in the same. The ridge 42 can have the same width as the groove 39. The section shown in Fig. 13 illustrates how the interlocking deformation 43 of the lock-seam is formed in the groove 39 of the clinching roller 30. The ridge 42 forms an indentation in the lock-seam. Fig. 14 shows a tube 33 with a lock-seam 37 with spaced indentations or deformations 43 formed by a clinch¬ ing roller according to the second embodiment. Fig. 15 shows such a deformation 43 in detail. The indentation 43 must be so deep that sufficient interlocking between the edge portions in the lock-seam 37 is achieved. In the two clinching roller embodiments described, it is preferred that at least one interlocking deformation 42, 43 is provided on each helical turn of the lock-seam 37. However, if a very strong interlocking effect is desired, several deformations 42, 43 may be formed on each helical turn. As an alternative to the interlocking deformations 41, 43 described above, the surfaces of the edge portions 16 of the strip 12 can be provided with knurls 45 which are brought into frictional engagement with each other in a clinching operation forming the lock-seam 37. This is shown in Fig. 17. The knurls 45 are formed on the edge portions 16 by a device 46 schematically shown in Fig. 17. The knurling is preferably performed as a preforming ope- ration, such as by using the rollers 13 in Fig. 1. Such knurling rollers (not shown) may be driven. Still another aspect of the invention is illustrated in Figs 18-22. In this case, a flexible string 47, for example of rubber, is compressed in the lock-seam 37. By appropriate adjustment of the clinching pressure, the string 47 will act as a friction-enhancing element between the hook-shaped edge portions 16 of the strip 12 and thereby provide the interlocking effect. As can be seen, the compressed flexible string 47 can be inserted in dif- ferent places between the bent edge portions 16. Since the string 47 is wound throughout the lock-seam 37, an extremely good sealing effect can be achieved. The string 47 is inserted in the lock-seam 37 in connection with the forming head 14. Preferably, the string 47 to be inserted is fed from a spool 48 mounted on the machine. The spool 48 is schematically shown in the machine of Fig. 1. Thanks to the frictional effect of the string 47, slippage of the lock-seam can be avoided while securing good sealing at the same time. Preferably, the string 47 has a round cross-section before insertion and compression in the lock-seam. The insertion of the string 47 can be combined with the interlocking deformations 41, 43 obtained by the clinching rollers 30 described. ALTERNATIVE EMBODIMENTS According to the clinching roller embodiments de¬ scribed, the clinching roller is arranged outside the tube, whereas the counter-roller is arranged inside the tube. It will be appreciated, however, that the aimed-at indentations or deformations can also be achieved if the clinching roller is arranged inside the tube and the coun¬ ter-roller is arranged outside the tube. The means (for example a recess or a ridge) for providing the inter- locking deformations on the lock-seam can be provided on the periphery either of the clinching roller or of the counter-roller. Further, there can be more than one such recess or ridge on the clinching roller or the counter- roller. The important thing is that the aimed-at inter- locking effect on the lock-seam is achieved. It will also be appreciated that the two clinching roller embodiments can be combined in such a way that a clinching roller is provided with both recesses and ridges of the type shown.

Claims: CLAIMS 1. A method for producing helically wound lock-seam tubing from a metal strip having longitudinal edge por¬ tions, comprising the following steps: feeding said strip to a forming head; forming said strip into a helical form in said forming head, whereby said edge portions of said helically formed strip are brought into engagement with each other; clinching said engaging edge portions of said strip to form a helically wound lock-seam on a tube formed in said forming head; feeding said helically wound lock- seam tube out of said forming head; and providing spaced deformations on said lock-seam in order to interlock said engaging edge portions of said strip in said lock-seam. 2. The method of claim 1, wherein said interlocking deformations are provided in an operation performed in connection with said forming head. 3. The method of claim 1, wherein said interlocking deformations are formed as expanded portions of said lock- seam. 4. The method of claim 1, wherein said interlocking deformations are formed as indentations on said lock-seam from the outside of said tube. 5. The method of claim 1, wherein said interlocking deformations are formed as indentations on said lock-seam from the inside of said tube. 6. The method of claim 1, wherein said interlocking deformations are formed as knurls. 7. The method of claim 1, wherein at least one inter¬ locking deformation is provided on each helical turn of said lock-seam. 8. The method of claim 1, wherein said edge portions of said strip are preformed before said step of feeding into said forming head. 9. An apparatus for producing helically wound lock- seam tubing from a metal strip having longitudinal edge portions, comprising: means for feeding said strip to a forming head; means for forming said strip into helical form in said forming head and for bringing said edge por¬ tions of said helically formed strip into engagement with each other; a clinching assembly for clinching said engag¬ ing edge portions of said strip to form a helically wound lock-seam on a tube formed in said forming head; means for feeding said helically wound lock-seam tube out of said forming head; and means for providing spaced deformations on said lock-seam in order to interlock said engaging edge portions of said strip in said lock-seam. 10. The apparatus of claim 9, wherein said deforma- tion means comprises a rotatable clinching roller having a periphery and a rotatable counter-roller having a peri¬ phery, said lock-seam being clinched between said periphe¬ ries of said rollers at a clinching point, and either of said rollers having means on its periphery for providing said interlocking deformations on said lock-seam at said clinching point. 11. The apparatus of claim 10, wherein said clinch¬ ing roller is provided with a peripheral groove which com¬ prises at least one recess for providing said interlocking deformations in the shape of expanded portions of the lock seam. 12. The apparatus of claim 10, wherein said clinching roller is provided with a peripheral groove which compri¬ ses at least one ridge. 13. The apparatus of claim 10, wherein said clinching roller is arranged outside said tube being formed, and said counter-roller being arranged inside said tube being formed. 14. The apparatus of claim 10, wherein said clinching roller is arranged inside said tube being formed, and said counter-roller is arranged outside said tube being formed. 15. The apparatus of claim 9, wherein said deforma¬ tion means is arranged in connection with said forming head. 16. The apparatus of claim 10, wherein said clinching roller and said counter-roller have shafts, respectively, rotatably supported by said forming head. 17. The apparatus of claim 9, further comprising means for providing knurls on said edge portions of said strip, said knurls forming said interlocking deformations. 18. The apparatus of claim 10, further comprising means for adjusting the clinching pressure between said rollers. 19. The apparatus of claim 9, further comprising means for preforming said edge portions of said strip before said feeding into said forming head. 20. A method for producing helically wound lock-seam tubing from metal strip having longitudinal edge portions, comprising the following steps: feeding said strip to a forming head; forming said strip into a helical form in said forming head, whereby said edge portions of said helically formed strip are brought into engagement with each other; clinching said engaging edge portions of said strip to form a helically wound lock-seam on a tube formed in said forming head; feeding said helically wound lock- seam tube out of said forming head; inserting a flexible string in said lock-seam; and compressing said string in said lock-seam to frictionally interlock said edge por¬ tions of said strip in said lock-seam. 21. The method of claim 20, wherein said strip is inserted throughout said helically wound lock-seam. 22. The method of claim 20, wherein said compression is performed as a clinching operation. 23. An apparatus for producing helically wound lock- seam tubing from metal strip having longitudinal edge portions, comprising: means for feeding said strip to a forming head; means for forming said strip into helical form in said forming head and for bringing said edge portions of said helically formed strip into engagement with each other; a clinching assembly for clinching said engaging edge portions of said strip to form a helically wound lock-seam on a tube formed in said forming head; means for feeding said helically wound lock-seam tube out of said forming head; means for inserting a flexible strip in said lock-seam; and means for compressing said strip in said lock-seam to frictionally interlock said edge por¬ tions of said strip in said lock-seam. 24. The apparatus of claim 23, further comprising a clinching device for performing said compression of said strip. 25. The apparatus of claim 23, wherein said strip has a round cross-section before insertion and consists of a rubber material. AMENDED CLAIMS [received by the International Bureau on 22 July 1996 (22.07.96); original claims 1-25 replaced by amended claims 1-25 (4 pages)] 1. A method for producing helically wound lock-seam tubing from a metal strip (12) having longitudinal edge portions (16), c h a r a c t e r i s e d by the follow¬ ing steps: feeding the strip (12) to a forming head (14); forming the strip (12) into a helical form in the forming head (14), whereby the edge portions (16) of the helically formed strip (12) are brought into engagement with each other; clinching the engaging edge portions (16) of the strip (12) to form a helically wound lock-seam (37) on a tube (33) formed in the forming head (14); feeding the helically wound lock-seam tube (33) out of the forming head (14); and providing spaced deformations (41; 43; 45) on the lock-seam (37) in order to interlock the engaging edge portions (16) of the strip (12) in the lock-seam (37). 2. A method as claimed in claim 1, wherein the inter- locking deformations are provided in an operation perfor¬ med in connection with the forming head (14). 3. A method as claimed in claim 1, wherein the inter¬ locking deformations are formed as expanded portions (41) of the lock-seam (37). 4. A method as claimed in claim 1, wherein the inter¬ locking deformations are formed as indentations (43) on the lock-seam (37) from outside of the tube (33). 5. A method as claimed in claim 1, wherein the inter¬ locking deformations are formed as indentations on the lock-seam (37) from the inside of the tube (33). 6. A method as claimed in claim 1, wherein the inter¬ locking deformations are formed as knurls (45). 7. A method as claimed in claim 1, wherein at least one interlocking deformation (41; 43) is provided on each helical turn of the lock-seam (37). 8. A method as claimed in anyone of the claims 1-7, wherein the edge portions (16) of the strip (12) are pre¬ formed before said step of feeding into the forming head (14). 9. An apparatus for producing helically wound lock- seam tubing from a metal strip (12) having longitudinal edge portions (16), c h a r a c t e r i s e d by means (11) for feeding the strip (12) to a forming head (14); means for forming the strip (12) into helical form in the forming head (14) and for bringing the edge portions (16) of the helically formed strip (12) into engagement with each other; a clinching assembly (30, 34) for clinching the engaging edge portions (16) of the strip (12) to form a helically wound lock-seam (37) on a tube (33) formed in the forming head (14); means for feeding the helically wound lock-seam tube (33) out of the forming head (14); and means (40; 42; 46) for providing spaced deformations on the lock-seam (37) in order to interlock the engaging edge portions (16) of the strip (12) in the lock-seam (37). 10. An apparatus as claimed in claim 9, wherein the deformation means comprises a rotatable clinching roller (30) having a periphery and a rotatable counter-roller (34) having a periphery, the lock-seam (37) being clinch- ed between the peripheries of the rollers (30, 34) at a clinching point, and either of the rollers (30) having means (40; 42) on its periphery for providing the inter¬ locking deformations (41; 43) on the lock-seam (37) at said clinching point. 11. An apparatus as claimed in claim 10, wherein the clinching roller (30) is provided with a peripheral groove (39) which comprises at least one recess (40) for provid¬ ing the interlocking deformations in the shape of expanded portions (41) of the lock seam (37). 12. An apparatus as claimed in claim 10, wherein the clinching roller (30) is provided with a peripheral groove (39) which comprises at least one ridge (42). 13. An apparatus as claimed in anyone of the claims 10-12, wherein the clinching roller (30) is arranged out¬ side the tube (33) being formed, and the counter-roller (34) being arranged inside the tube (33) being formed. 14. An apparatus as claimed in anyone of the claims 10-12, wherein the clinching roller is arranged inside the tube (33) being formed, and the counter-roller is arranged outside the tube (33) being formed. 15. An apparatus as claimed in anyone of the claims 9-14, wherein the deformation means is arranged in con¬ nection with the forming head (14). 16. An apparatus as claimed in anyone of the claims 10-15, wherein the clinching roller (30) and the counter- roller (34) have shafts (31, 35), respectively, rotatably supported by the forming head (14). 17. An apparatus as claimed in claim 9, further com¬ prising means (46) for providing knurls (45) on the edge portions (16) of the strip (12), the knurls (45) forming said interlocking deformations. 18. An apparatus as claimed in anyone of the claims 10-17, further comprising means (38) for adjusting the clinching pressure between the rollers (30, 34). 19. An apparatus as claimed in anyone of the claims 9-18, further comprising means (13) for preforming the edge portions (16) of the strip (12) before said feeding into the forming head (14). 20. A method for producing helically wound lock-seam tubing from metal strip (12) having longitudinal edge por¬ tions (16), c h a r a c t e r i s e d by the following steps: feeding the strip (12) to a forming head (14); forming the strip (12) into a helical form in the forming head (14), whereby the edge portions (16) of the helically formed strip (12) are brought into engagement with each other; clinching the engaging edge portions (16) of the strip (12) to form a helically wound lock-seam (37) on a tube (33) formed in the forming head (14); feeding the helically wound lock-seam tube (33) out of the forming head (14); inserting a flexible string (47) in the lock- seam (37); and compressing the string (47) in the lock- seam (37) to frictionally interlock the edge portions (16) of the strip (12) in the lock-seam (37). 21. A method as claimed in claim 20, wherein the string (47) is inserted throughout the helically wound lock-seam (37). 22. A method as claimed in claim 20 or 21, wherein said compression is performed as a clinching operation. 23. An apparatus for producing helically wound lock- seam tubing from metal strip (12) having longitudinal edge portions (16), c h a r a c t e r i s e d by means (11) for feeding the strip (12) to a forming head (14); means for forming the strip ( 12 ) into helical form in the form- ing head (14) and for bringing the edge portions (16) of the helically formed strip (12) into engagement with each other; a clinching assembly (30, 34) for clinching the engaging edge portions (16) of the strip (12) to form a helically wound lock-seam (37) on a tube (33) formed in the forming head (14); means for feeding the helically wound lock-seam tube (33) out of the forming head (14); means for inserting a flexible string (47) in the lock- seam (37); and means for compressing the string (47) in the lock-seam (37) to frictionally interlock the edge por- tions (16) of the strip (12) in the lock-seam (37). 24. An apparatus as claimed in claim 23, further com¬ prising a clinching device for performing the compression of the string (47). 25. An apparatus as claimed in claim 23 or 24, where- in the string (47) has a round cross-section before inser¬ tion and consists of a rubber material.

Inventors: LENNARTSSON KENNETH, LENNARTSSON, KENNETH
Applicants: LENNARTSSON KENNETH, PROTOL AG, LENNARTSSON, KENNETH, PROTOL A.G.
Application year: 1996
Main classification: B21C37
IPC codes: B21C37, B21D39