Abstract:
A process for producing tubes ( 1 ), in particular for use in heat exchangers, from at least one metallic strip on a rolling mill train, wherein the strip, on a rolling mill train, is provided with predetermined breaking points, deformed and brought together to form the tube and finally the tubes are torn off at the predetermined breaking points between two pairs of rolls having a speed difference. The process is improved, in terms of the tearing off of individual tubes, in that an areal compressive force (Ak) is applied to the tube at least at the moment of tearing off and a sufficient tearing force (Rk) is produced and transmitted to the tube to be torn off.

Description:
RELATED APPLICATIONS 
       [0001]    This Application claims priority to German Patent Application No. 10 2009 036 006.9, filed Aug. 4, 2009, the entire contents of which are hereby incorporated by reference herein. 
       BACKGROUND 
       [0002]    The invention relates to a process for producing tubes, for example for use in heat exchangers, from at least one metallic strip on a rolling mill train. 
         [0003]    A production process and a corresponding rolling mill train are described in DE 10 2006 033 568 A1. In said application, individual tubes are broken off or cut off at perforations or predetermined breaking points present in the strips. 
         [0004]    With respect to the tearing off of the tubes U.S. Pat. No. 5,653,022 describes a pair of pull rollers and a pair of guide rollers, between which individual tubes are torn off. In the case of pairs of rolls of this type, undesirable slippage may occur during the tearing off. In said document, the tubes themselves are of an extruded type, i.e. they have not been produced from at least one metallic strip. 
         [0005]    In EP 714 342 B1, extruded tubes are likewise torn off between a stationary clamp assembly and a movable clamp assembly. The clamp assembly moves rectilinearly in the direction of the tube. 
       SUMMARY 
       [0006]    The object of the present invention is to improve the above-defined production process with respect to the tearing off of individual tubes by means of pairs of rolls. In particular, slippage between the tube and the pairs of rolls should be prevented or the probability of slippage movements occurring should at least be reduced significantly. 
         [0007]    One aspect of the production process provides for an areal compressive force to be applied to the tube at least at the moment of tearing off and for a sufficient tearing force to be transmitted to the tube to be torn off. 
         [0008]    A rolling mill train for carrying out a process for producing tubes from at least two “endless” metallic strips, wherein the rolling mill train is equipped with pairs of rolls and has at least one station for introducing predetermined breaking points into the strips, a section for deforming the strips to form the tube, and a tearing-off device for tubes which comprises at least two pairs of rolls, is characterized in that at least the second pair of rolls, lying in the conveying direction of the tube, of the tearing-off device is formed from at least two pairs of rolls, the rolls of which are connected by means of at least two transport belts. 
         [0009]    It goes without saying that the tearing-off device can also be used, for example, for extruded tubes or tubes produced in another way. 
         [0010]    Since, according to this proposal, areal compressive forces and thus significantly greater compressive forces are transmitted to the tube by means of the transport belts, slippage can be avoided. Accordingly, it is also possible for greater tearing forces to be transmitted to the tube. Overall, therefore, this proposal also contributes to the fact that the rolling mill train can be operated reliably at a greater speed, and this is a factor which increases productivity. 
         [0011]    As a result, individual tubes which have no dents or other defects at all on the tube ends and are therefore ideal for use as tubes in a heat exchanger are produced. 
         [0012]    The invention will now be described with reference to the accompanying drawings in two exemplary embodiments. The description which follows may contain information, features and advantages that may prove to be particularly significant at a later point in time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows a rolling mill train in principle with a tearing-off device at the end. 
           [0014]      FIGS. 2 and 2   a  show a side view of the tearing-off device according to the invention with upstream pairs of rolls. 
           [0015]      FIG. 3  shows a second exemplary embodiment. 
           [0016]      FIG. 4  shows a tube in cross section. 
           [0017]      FIG. 5  shows, in the form of a perspective view, a sectional view and a detail from the sectional view, a pair of rolls for producing predetermined breaking points. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Since the exemplary embodiment shown refers to three-part tubes ( FIG. 4 , two wall parts a, b and an inner insert c), it can be gathered from  FIG. 1  that three strip rolls R 1 , R 2 , R 3  are present as starting material. These are sheet aluminum. The strip roll R 1  generates part a, the strip roll R 2  generates part c and, finally, the strip roll R 3  generates part b of the flat tube  1 . As shown in the illustration, in each case one very large loop is present in the strip directly behind the strip rolls R 1 , R 2 , R 3 , and these loops serve to compensate different speeds or else stoppages of the strip. Depending on requirements, a plurality of loops can also be provided, which is also the case from a practical point of view. The first predetermined breaking point station S 1  is already situated directly at the start of the rolling mill train, and said station introduces the predetermined breaking points into the strip roll R 2  (part c). Part c is then deformed by means of pairs of rolls (not shown in detail) over a corresponding distance in such a manner as to provide the configuration shown in  FIG. 4 . The first predetermined breaking point station S 1  is thus adjoined by a section of the rolling mill train in which one strip material is deformed so as to form part of the subsequent tube. The strip rolls R 1  (part a) and R 3  (part b) merely roll through said distance without being significantly deformed there. The upper strip roll R 1  then reaches the second predetermined breaking point station S 2 . This is followed at a short distance by the third predetermined breaking point station S 3 , through which the lower strip roll R 3  runs in order to be correspondingly provided with predetermined breaking points S. The edge deformations on parts a and b are then formed and part c is mounted in between parts a and b (not shown in detail). However, reference can be made in this regard to patent application DE10-2006-029 378.9. In doubt, the entire content of said application should be regarded as disclosed at this point. Approximately in said section, the predetermined breaking points  3  in the three parts a, b, c are also brought into alignment, with known open-loop and closed-loop control means (not shown) being used for this purpose. As is well known to a person skilled in the art, said section should be situated upstream from the region in which parts a, b, c have already been connected to one another and are in rigid physical contact. When parts a, b, c are subsequently joined to form the tube  1 , then an endless tube  1  is initially provided (as shown in the cross section in  FIG. 4 ), from which the individual tubes  1  are then to be separated. 
         [0019]    The three predetermined breaking point stations S 1 , S 2  and S 3  ( FIG. 5 ) can be of identical design. However, a single station which incorporates all three stations may also be present. In this case, in contrast to the description above, the stations or devices are all located at approximately the same height. The number and physical positioning of predetermined breaking point stations S is dependent on the individual application, for example on the specific design of the tube  1 . In the exemplary embodiment, the predetermined breaking point stations S 1 , S 2 , S 3  comprise a pair of rolls PP. One (the lower) roll P preferably runs freely and guides part a, b or c, which is transported between the rolls P. The other (upper) roll P is formed with a projecting predetermined breaking point punch SS. Open-loop and closed-loop control means known per se are used to hold the other roll P with the predetermined breaking point punch SS in a waiting position, in which the predetermined breaking point punch SS is not in engagement. In this position, the predetermined breaking point punch SS is located horizontally on the roll P. The means mentioned then ensure that the roll P is moved abruptly with a high rotational speed so as to come into the action position shown, in which the predetermined breaking point punch SS is in engagement and the predetermined breaking points  3  are produced. The rotational speed or the peripheral speed of the rolls P is preferably higher than the transport speed of the strip, in order to ensure that the strip does not curl up. 
         [0020]      FIGS. 2 and 2   a  show an exemplary embodiment in which each of the two pairs of rolls WP 1  and WP 2  has been formed in each case from two pairs of rolls WP 1 . 1  and WP 1 . 2  and, respectively, WP 2 . 1  and WP 2 . 2 . 
         [0021]    The upper rolls of the second pair of rolls WP 2  in the figure are connected by a first transport belt  10  and the lower rolls in the figure have a second transport belt  20 . The first pair of rolls WP 1  has been equipped accordingly, with third and fourth transport belts  30  and  40  being present there. Two eccentrics  60  have been indicated above the second pair of rolls WP 2 . Camshafts or pressure-activatable working cylinders can also be involved here, and these are used to increase the compressive force Ak, for example at the moment when the tube  1  is torn off. The camshafts  60  are activated owing to corresponding pulses. This configuration provides advantageous functions. For example, firstly greater tearing forces Rk can be transmitted because the areal contact surface on the tube  1  is considerably larger. Secondly, it is also possible to increase the areal compressive forces slightly since the at least slightly elastic upper and lower transport belts  20 ,  30  can be pressed against one another by simple means between the rolls. 
         [0022]    A sufficient areal compressive force Ak can be transmitted to the tube  1  using the means described above at least at the moment when the tube  1  is torn off, and the friction between the transport belts  10 ,  20  and the tube also makes it possible to transmit a tearing force Rk to the tube  1 . 
         [0023]    In an exemplary embodiment which is not shown but nevertheless presently preferred, the means mentioned (eccentrics  60  or the like) are not present, i.e. a sufficiently great, but substantially static areal compressive force Ak is constantly transmitted. The areal compressive force Ak can be set by means of a spindle or the like. It therefore acts constantly—in contrast to the first exemplary embodiment—and not just at the moment of tearing off. The magnitude of said force has to be finely tuned to the compressive force in the first pair of rolls WP 1 . This also means that the distance a which can be seen in  FIGS. 2 and 3  between the upper and lower transport belts  10  and  20  does not have to be present in these embodiments which are not shown. A further result of this is that the second pair of rolls WP 2  or the drive motor thereof (not shown) receives a rotational acceleration pulse, at the moment of tearing off, in order to produce the tearing force Rk and to transmit the latter by means of the friction mentioned. A sensor (not shown) is present for this purpose and detects a predetermined breaking point S situated between the pairs of rolls WP 1  and WP 2  and triggers a corresponding rotational acceleration pulse in the drive motor of the second pair of rolls WP 2 . 
         [0024]      FIG. 2   a  shows an enlargement of an excerpt of  FIG. 2 , and it should be clear from  FIG. 2   a  that it is advantageous to arrange a series of rollers  70  on the inner side of the transport belts  10 ,  20 ,  30 ,  40  that improve the uniformity of the transmission of the areal compressive force Ak to the tube  1 . Only the upper or lower peripheral sections of the rollers  70  can be seen in  FIG. 2   a . A possible tearing-off zone Rz, in which the tearing off operation according to the proposal is to take place, has also been marked in  FIG. 2   a.    
         [0025]    The rolls can also be designed in the manner of gearwheels or have a ribbed surface. The inner side of the transport belts  10 ,  20 ,  30 ,  40  may then have a structure which corresponds to the teeth or to the ribbing. This affords an improved transmission of force from the rolls to the transport belts. In addition, as mentioned, suitable drive units, servomotors or the like are also present for the rolls, but have not been depicted. 
         [0026]    The tearing force is produced by means of the second pair of rolls WP 2 , since the finished tube comes from the right-hand side in the image. As mentioned, the rotational speed V 2  of the second pair of rolls can also be regulated accordingly. In order to produce the tearing force Rk, said rotational speed has to be higher than the rotational speed V 1  in the first pair of rolls. The individual tubes  1  are torn off between the pairs of rolls WP 1  and WP 2  at the predetermined breaking point S located there at that time. In this case, the predetermined breaking points S are notches in the strips, although perforations also allow the same effects. It can be seen that the tearing force Rk acts approximately in the longitudinal direction of the tube  1 , whereas the areally applied compressive force Ak acts approximately perpendicularly thereto, for example perpendicularly on the two wall parts a and b in the tube shown in  FIG. 4 . Since the tube has an inner insert c, it is also sufficiently stable to absorb high compressive forces without being impaired. Since the compressive force is applied over a very large area, it is also possible to tear off less stable tubes cleanly in the proposed manner. 
         [0027]      FIG. 3  shows a second exemplary embodiment, in which merely the second pair of rolls WP 2  has been configured as described. The first pair of rolls WP 1  can be a simple pair of rolls. The surfaces of the rolls can be covered with rubber. In  FIG. 3 , only a few reference symbols have been used because the elements shown correspond to those in  FIGS. 2 and 2   a . The relatively large distance between the pairs of rolls WP 1  and WP 2 , which can be seen in  FIG. 3 , is actually not intended but rather arises as a result of the drawing.