Abstract:
In a method for producing helically wound lock-seam tubing, a metal strip is fed to a forming head in which the tubing is formed. In order to frictionally interlock this lock seam, a flexible string is inserted into and compressed in the lock seam during manufacture.

Description:
This application is a divisional of application No. 09/219,239, filed Dec. 22, 1998, now U.S. Pat. No. 6,085,801, which is a divisional of application No. 08/925,545, filed Sep. 8, 1997, now U.S. Pat. No. 6,003,220, which is a divisional of application No. 08/747,998, filed Nov. 12, 1996, now U.S. Pat. No. 5,720,095, which is a continuation of application No. 08/400,752, filed Mar. 8, 1995, now abandoned. 
    
    
     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 (see FIG. 1) 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 shows 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 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 parameters, 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 operator. 
     In spite of the operator&#39;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, immediately after the forming head, or during storing, handling 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&#39; 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 ventilation 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 ensures safe sealing. However, if the tube has an undesirably increased diameter, there is a risk that sufficient 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 energy 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 fitted with a mastic for sealing purposes. If the gap is too large due to an undesirable 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 storing, handling, transportation, delivery and mounting on the final site. 
     A specific object of the invention is to provide a method and apparatus which are easily adapted to existing machines. 
     These objects are achieved by a method for producing helically wound lock-seam tubing from metal strip having longitudinal edge portions. The method comprises the following steps: feeding the strip to a forming head; forming the strip into a helical form in the forming head, whereby the edge portions of the helically formed strip are brought into engagement with each other; clinching the engaging edge portions of the strip to form a helically wound lock-seam on a tube formed in the forming head; and feeding the helically wound lock-seam tube out of the forming head. In order to interlock the engaging edge portions of the strip in the lock-seam, the method further comprises the step of providing spaced deformations on the lock-seam. 
     The objects are also achieved by an apparatus for producing helically wound lock-seam tubing from metal strip having longitudinal edge portions. The apparatus comprises: means for feeding the strip to a forming head; means for forming the strip into helical form in the forming head and for bringing the edge portions of the helically formed strip into engagement with each other; a clinching assembly for clinching said engaging edge portions of the strip to form a helically wound lock-seam on a tube formed in the forming head; and means for feeding the helically wound lock-seam tube out of the forming head. In order to interlock the engaging edge portions of the strip in the lock-seam, the apparatus further comprises means for providing spaced deformations on the lock-seam. 
     The invention makes it possible to avoid lock-seam slip and achieve further advantages. 
     An important advantage is that the spaced deformations on the lock-seam immediately 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 the interlocking deformations can either be incorporated in new machines or easily mounted on existing machines as additional equipment. 
    
    
     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 prior art machine for producing helically wound lock-seam tubing. 
     FIG. 2 illustrates schematically a clinching device incorporated in a machine according to the present invention. 
     FIG. 3 shows a helically wound lock-seam tube produced by a method and an apparatus according to the present 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 clinching 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 invention can in principle be applied to a machine of the type as generally shown in FIG.  1 . This known machine corresponds to the machine described in the publication GB-A-2,213,748 mentioned by way of introduction. 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 production, 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 connection 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 bearing  32  (schematically shown) mounted on the forming 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 pressure is adjustable by a device  39  (schematically shown) acting on the shaft  31  in the directions of the arrows. 
     The clinching roller  30  has a peripheral groove  39  with a 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 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  14  formed by the recess  40  of the clinching roller  30 . In the first embodiment shown in FIG. 2, the spaced formations  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 constant diameter. Slip in the lock-seam  37  is prevented. 
     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 thickness 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 . In 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 clinching 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 operation, such as 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 portion  16  of the strip  12  and thereby provide the interlocking effect. As can be seen, the compressed flexible string  47  can be inserted in different places between the bent edge portions  16 . 
     Since the string  47  is wound throughout the lock-seam  37 , 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 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  an be combined with the interlocking deformation  41 ,  43  obtained by the clinching rollers  30  described. 
     ALTERNATIVE EMBODIMENTS 
     According to the clinching roller embodiments described, the clinching roller is arranged outside the tube, whereas the counter roller is arranged inside the tube. It should 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 counter 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.