Patent Publication Number: US-6334563-B1

Title: Retensioning device for strapping machines

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a retensioning device for strapping machines. 
     2. Background Art 
     It should be mentioned as the background of the invention that strapping machines normally have a tape guide frame which is constructed vertically on a work table and in which a stack of products, for example a stack of periodicals, to be strapped with a tape is introduced. A strapping tape is then pulled off a supply roll by a tape drive device and shot into the tape guide frame until the leading end of the tape enters a welding arrangement beneath the stack of products in the work table. The leading end is held and the strapping tape retracted by a retractor. It issues from the tape guide frame and wraps itself round the stack of products. 
     In this state, the strapping tape has to be tightened round the stack of products with a defined retensioning force. This retensioning force is controlled for various reasons and is difficult in many respects. It can basically be assumed that retensioning takes place with a pair of tape tensioning rollers through whose nip the strapping tape passes. The contact between strapping tape and pair of rollers accordingly has to be controlled in such a way that the force exerted on the tape and causing the retensioning can be defined. This is due to the technical requirement of strapping, for example, a stack of periodicals with a few copies only with a slight retensioning force owing to its low flexibility, whereas a high stack of periodicals is to be strapped with a far greater retensioning force owing to the compressibility of the periodicals and the much greater stability of the stack to kinking of the individual volumes. It must be possible to adjust the retensioning force individually from stack to stack as stacks which follow one another directly and have markedly differing heights have to be processed in the strapping machine in modern packaging lines of, for example, printing plant. 
     The retensioning force which can be produced in the strapping tape by the pair of tape tensioning rollers is accordingly dependent on very different parameters, for example the coefficient of friction between the strapping belt and the rollers, the force with which the two rollers are pressed toward one another and the angle of wrap with which the tape is generally guided round one of the two rollers. Furthermore, the torque provided by the rollers obviously has a significant influence on the retensioning force. 
     DE 32 49 559 C2 shows a feed and tensioning device for a strapping tape which is to be tensioned round a package. A revolving pressure roller is provided as counter roller which cooperates with a tensioning wheel, arranged on a pivoting lever, as pivot roller. Tape guide and angle of wrap of the retensioning device shown therein are designed in such a way that the pivoting lever is loaded by the retensioning force building up during the retensioning process via the pivoting roller in such a way that the pivoting roller is pressed against the counter roller. The feed force which can be exerted on the tape is therefore increased. The completion of the retensioning process and, in particular, the drive of the pivoting roller are controlled by the excursion of the pivoting lever. On attainment of a specific force, the pivoting lever is deflected so far from its spring-loaded basic position that a contact switch for the drive motor of the pivoting roller is actuated and the motor is switched off. The maximum retensioning force can be adjusted by an energy store in the form of a helical compression spring which loads the pivoting lever against its excursion direction. The bias of the helical compression spring and therefore the loading of the pivoting lever can be adjusted by means of a hand wheel. Therefore, the maximum retensioning force cannot be adjusted in an economical and automatable manner. 
     It can be seen that the above-described retensioning device according to the prior art has a very complex construction and can only be controlled with difficulty with respect to the adjustment and control of the retensioning process. This is all the more problematic since modem strapping machines operate with extremely short cycle times of less than one second. This means that the actual retensioning process lasts only one to two tenths of a second and therefore does not allow a time frame for complicated control measures. 
     SUMMARY OF THE INVENTION 
     Starting from the aforementioned problems, the object of the invention is to provide a retensioning device which, despite its simplicity of construction, makes possible proper control of the retensioning force, in particular also with respect to extremely quick retensioning processes. 
     This object is achieved by the loading device for the defined loading of the lever being accordingly coupled to the pivoting lever so that the pivoting roller can be adjusted toward the counter roller with a defined loading force. This loading force opposes the retensioning force building up in the tape during the retensioning process in such a way that the pivoting roller momentarily lifts from the counter roller when the loading force is exceeded and therefore interrupts the retensioning process. Owing to the lifting of the pivoting roller from the counter roller, the strapping tape becomes virtually free so the retensioning force collapses and the pivoting roller can be pressed back against the counter roller by means of the loading device. The tape is therefore retensioned again and the foregoing sequence of operation is repeated within the retensioning device. An equilibrium of forces is adjusted overall owing to the opposing forces on the lever, and the retensioning force which can be produced in the strapping tape can be controlled as a function of the loading force of the pivoting lever and by means of it. On attainment of the maximum retensioning force, a mechanical control circuit comes into effect in practice owing to the excursion of the pivoting lever and keeps the retensioning force almost constant during rotation of the roller. 
     Further features, details and advantages of the invention can be inferred from the following description in which an embodiment of the subject of the invention is described in more detail with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a highly schematic side view of a retensioning device in the inactive state, and 
     FIG. 2 is a side view similar to FIG. 1 in the active state of the retensioning device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The retensioning device shown in the two figures is arranged in the base of a strapping machine below its work table. At its heart, it has a pair of tape tensioning rollers  1  formed, on the one hand, by a stationary, rotatably mounted counter roller  2  and a pivoting roller  3 . The pivoting roller  3  is mounted rotatably on a one-armed pivoting lever  4  of which the pivot axis  5  extends parallel to the two axes of rotation  6 ,  7  of the counter and pivoting roller  2 ,  3 . The piston rod  9  of a pneumatic cylinder  10  acts on the free end  8  of the pivoting lever  4 , loads the pivoting lever  4  in a direction at right angles to itself with a defined force F cyl  and therefore presses the pivoting roller  3  toward the counter roller  2  of the pair of tape tensioning rollers  1 . 
     As also shown in FIGS. 1 and 2, a strapping tape  11  is guided from a tape store, not shown, on the side facing the pneumatic cylinder  10  to the pair of tape tensioning rollers  1  and into the nip  12  formed between counter and pivoting roller  2 ,  3 . The strapping tape  11  is then guided via an angle of wrap  13  round the pivoting roller  3 . The strapping tape  11  is guided away from the pivoting roller  3  after the angle of wrap  13  in such a way that it runs away in a direction exactly opposed to the piston rod  9  or the cylinder force F cyl . 
     A tape guide rail  14  which brushes the angle of wrap  13  with its tape guide face  15  describing a quadrant is indicated in hatching in FIGS. 1 and 2. The tape guide rail  14  serves to guide the strapping tape  11  while the strapping tape  11  is shot into the strapping machine. 
     In terms of control and operation, it should be added with respect to the retensioning device that the two rollers  2 ,  3  are driven synchronously and in opposite directions  20 ,  21  by a motor  16  which can be switched on and off via a controller  17 . The loading pressure of the pneumatic cylinder  10  can also be adjusted so the cylinder force F cyl  can be varied rapidly and over wide ranges. This is also effected via corresponding activation of the pneumatic cylinder  10  by means of the controller  17 . 
     The mode of operation of the retensioning device shown in FIGS. 1 and 2 can be explained as follows: 
     FIG. 1 shows the inactive position of the retensioning device in which the strapping tape  11  is shot into the strapping tape machine. The pneumatic cylinder  10  is set in such a way that the piston rod  9  is driven out to a maximum and the pair of tape tensioning rollers  1  is open. This means that no force can be exerted on the strapping tape  11  despite a rotation of the rollers  2 ,  3  counter to the shooting direction  18 . The strapping tape  11  is guided along the tape guide face  15  of the rail  14  through the machine and is handled in a known manner in the strapping machine. 
     The retensioning device comes into action after retraction of the tape  11  from the tape guide frame of the strapping machine and application of the strapping tape  11  round the stack of products to be strapped. For this purpose, the pneumatic cylinder  10  is activated and loaded with a force defined by the controller  17 . The pivoting lever  4  is thus deflected in a clockwise direction with respect to FIG.  2  and the pivoting roller  3  is adjusted toward the counter roller  2  while closing the nip  12 . The strapping tape  11  is grasped by the pair of tape tensioning rollers  1  and pulled in the retensioning direction  19 . A force F R  which is dependent on the pressing force in the nip  12 , the frictional force which can be exerted on the tape by the rollers  2 ,  3  and the angle of wrap  13  act on the tape. As the retraction of the strapping tape  11  increases, the strapping tape  11  is placed increasingly tightly round the stack of products to be strapped so that a retensioning force F re  builds up in the tape. This retensioning force F re  produces a torque D on the pivoting lever  4  which counteracts the loading force F cyl . As soon as this force is exceeded, the pivoting lever  4  is pivoted out in an anticlockwise direction with respect to FIG.  2  and the nip  12  opens somewhat. The force which is indicated as a whole by F R  in FIG.  2  and can be exerted on the tape and, with it, the retensioning force F re  therefore collapses momentarily so that the pivoting lever  4  can be pivoted back into the closing direction. The retensioning force F re  is accordingly built up again and the foregoing process is repeated cyclically, the cycle time ranging from a few thousandths to hundredths of a second. It should also be mentioned that the pivot path of the pivoting lever  4  moves in the plastic range of the tape and cannot be measured in practice. 
     The foregoing statements show that the retensioning force F re  can be varied by the adjustment of the loading force F cyl  via the pneumatic cylinder  10 . It should also be mentioned that the torque of the motors does not have to be controllable for adjusting the retensioning force F re . Rather, the invention provides a very simply operating, mechanical control circuit with which the retensioning process can be completed by pure timing. If, for example, the actual retensioning process lasts 0.08 seconds and the subsequent welding of the strapping tape 0.05 seconds, it is sufficient to activate the pair of tape tensioning rollers  1  over a period of, for example, 0.15 seconds with a defined loading force F cyl  on the pneumatic cylinder  10  in order to guarantee specific retensioning and welding of the tape while maintaining this retensioning. The next strapping process can then be effected in the described manner by activation of the pair of tape tensioning rollers  1 . 
     It is pointed out that the loading device for the pivoting lever  4  can also be produced by an electromagnetic lifting device or a spring arrangement of which the bias is mechanically adjustable. 
     To sum up, the retensioning device according to the invention has various advantages: 
     Only one time set value and one set value for the loading force of the pneumatic cylinder  10  have to be predetermined for the controller. The actual control of retensioning is then effected “in situ” by the mechanical control circuit. 
     The retensioning force is infinitely variable linearly from the smallest values close to zero to a maximum force value. 
     The mechanism is easy to construct and to service. Complex adjustment work is not required. 
     The retensioning device runs very quietly as no component has to overcome large ranges.