Abstract:
A device for stacking sheets for a printing machine, including at least a sheet deflection device which can be driven in rotation, and which is provided for receiving or grabbing a front edge of a sheet and for stacking the sheet after it has moved through a rotation path, and having at least two sheet conveyance devices that can be rotated substantially independently of each other about the common axle, and thus one of these sheet conveyance devices is ready to receive or grab a next sheet, when another one of these sheet conveyance devices is still occupied with the transport and the stacking of a preceding sheet.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a device for stacking sheets for a printing machine, preferably an electrophotographic printing machine, comprising at least a drivable sheet conveyance device which can be driven in rotation, and which is provided for receiving or grabbing a front edge of a sheet and for stacking the sheet after it has moved through a rotation path.  
       BACKGROUND OF THE INVENTION  
       [0002]     A stacking device for sheets, which includes a sheet conveyance device, which can be driven in rotation, is known, for example, from U.S. Pat. No. 4,252,309. In this known stacking device, the front edge is inserted into a slit of the substantially disk-shaped sheet conveyance device. The sheet, which has thus been grabbed, is moved along by an upper grabbing place by a half rotation of the sheet conveyance device and turned, and then it is stacked on the reached lower level on a stacking place where it is stacked on other sheets. The sheet to be stacked is released automatically from the sheet because the conveyance device moves past a stop, which retains the sheet.  
       SUMMARY OF THE INVENTION  
       [0003]     The invention is based on the problem of providing a variant of a sheet stacking device, which is such that the device can be used optimally, and it is preferably also suitable for sheets having different formats and weights.  
         [0004]     This problem is solved according to the invention by the fact that at least two sheet conveyance devices are provided, in such a manner that these several sheet conveyance devices can be rotated substantially independently of each other about the common axle, and thus one of these sheet conveyance devices is ready to receive or grab a next sheet, when another one of these sheet conveyance devices is still occupied with the transport and the stacking of a preceding sheet.  
         [0005]     The sheet conveyance device, which is known from the state of the art, which was cited above, presents two diametrically opposite slits for the reception of sheet front edges, so that the sheet conveyance device in principle is already again prepared to receive a next sheet, while it deposits a preceding sheet. However, in reality this occurs only if the sheets to be taken over have a fitting format. Indeed, a sheet is preferentially transferred by transport rollers at the end of a transport path for printing materials to the sheet conveyance device. A good stacking of the sheet is achieved, in particular, if the transport rollers at that time are just transferring the back edge of the sheet, as soon as the sheet front edge reaches a stop at an output place, because the sheet is then stacked flexibly onto the stack of sheets. This means that the sheet should have a length, which is such that it just surrounds approximately half of the sheet conveyance device, and thus corresponds approximately to half the circumference of the surface of the sheet conveyance device. However, this is far from being always the case. Longer or shorter formats are also transported. Moreover, it is possible, for example, that heavier and stiffer printing material cannot be bent to a significant degree without suffering damage, so that the radius of the sheet conveyor device, in view of the use of such a printing material, is chosen to be larger, as a preventive measure, than would perhaps be appropriate for the length of the sheet format. On the other hand, to fully use the installation, the sheets should be transported in, as regular as possible a flow, and the gap between two successive sheets should not be too large. However, successive sheets also should not overlap.  
         [0006]     Thus, it can happen that the free slit, which is intended for the next reception, could be ready either too early or too late, depending on the situation. In view of the described situation, it would also not be a solution of the problem to fit the sheet conveyance device with a larger number of slits (or with other jaw-like openings).  
         [0007]     However, the solution according to the invention very definitely presents optimization possibilities, because an additional sheet conveyance device can be readied, independently and with precise timing, with adaptation to the format of the sheet.  
         [0008]     An additional special embodiment of the invention provides for the use of a sheet deflection device which works in cooperation with the sheet conveyance devices, where the sheet deflection device forms, with a surface which serves as support for the sheet, a substantially curved path for the sheet to be conveyed, and each sheet conveyance devices presents at least one overlapping element for overlapping the received sheet front edge, in such a manner that the sheet front edge can be grabbed, in a manner so it can be moved along, between one of these overlapping elements of a sheet conveyance device and the surface of the sheet deflection device.  
         [0009]     If a reception slit for the sheet front edge is used, there is a risk that the sheet front edge will be damaged in the slit and it may occur that the sheet is not held sufficiently reliably and in the correct position until the desired stacking in the slit, into which it only has been inserted.  
         [0010]     The invention is therefore based on the secondary problem of indicating a functional alternative. It is advantageous, in the mentioned solution according to the invention of these problems, to distribute functions over different elements, for the purpose of achieving a more targeted, and more reliable functionality.  
         [0011]     The sheet deflection device substantially assumes the function of a disk body and it provides a device and a support for the sheet, where this sheet deflection device also does not necessarily have to be in the shape of a disk, rather, according to an embodiment, it can also be segmented in the shape of a wheel or in any other manner.  
         [0012]     The sheet conveyance device takes over above all a part of the holding function, by readying an overlapping element, which clamps the sheet front edge between itself and the surface of the adjacent sheet overlapping element, and holds it reliably as a result, without damaging the front edge. These sheet conveyance devices, however, even more must not be in the shape of a disk; rather it is preferred that, according to an embodiment of the invention, they are characterized by an arrangement, which substantially includes two armed pivoting beams, which each present, in the area of each one of their two radially outward free ends, an overlapping element. By the overlapping elements at both ends, each sheet conveyance device is already prepared to take up a next sheet, while it is depositing a preceding sheet, or at least shortly thereafter. Thus, the sheet deflection device, which is already preferably substantially symmetrical with respect to an axle of rotation, is in any case in principle ready in any rotational position and at all times.  
         [0013]     The overlapping element is preferably a tongue or lash with a surface, and also optionally with spring mounted yield, so that an additional mechanism to prevent damaging the sheet front edge is provided. Here the overlapping element is substantially a tongue or cover plate, which is approximately parallel to the curvature path of the sheet deflection element.  
         [0014]     The next embodiment of the invention provides for the multitude of sheet conveyance devices and the sheet deflection device to be each doubled and arranged on the common axle with mirror symmetry with respect to each other, in such a manner that all sheet conveyance devices are arranged between the two sheet deflection devices, so that a front sheet edge, in its orientation parallel to the common axle of the sheet conveyance devices and the sheet deflection devices, can be jointly grabbed by two of the sheet conveyance devices, of which there is a total of at least four, and the two sheet deflection devices.  
         [0015]     In this manner, the sheet is advantageously reliably grabbed over its entire width and, in particular, a transverse position or rotation due to the transport is also prevented.  
         [0016]     For a particularly reliable grabbing, it is preferred to provide for the side of the overlapping elements of the sheet conveyor devices, which is turned toward the sheet, to be at a smaller radial distance from the common axle than the overlapped external side of the sheet which applies its thickness on the radius of the surfaces of the sheet deflection devices, so that the sheet front edge in its travel is forced and bent through, in a manner generating tension, in the area of the overlapping elements, approximately in the direction of the common axle.  
         [0017]     As is already known from the state of the art, the device according to the invention as well has a stop for the sheet front edges, with which the sheet conveyance devices work in cooperation during the stacking of the sheets.  
         [0018]     In view of, in particular, sheet formats which are as short as possible, this device according to the invention is preferably characterized by at least one guiding element which blocks a grabbed sheet at least in the centrifugal direction, between a grabbing place and a delivery place of the sheet to force the sheet to maintain the radius of curvature. In this manner, the grabbed and transported sheet is still reliably stacked, in an immediate position, to the surface of the first sheet conveyance device, even if its back edge were released too soon from transport rollers.  
         [0019]     A particular result of the use according to the invention of at least two sheet conveyance devices is that there time is saved during the stacking of the sheet, if one provides, if desired, at least one shifting device which is coupled with at least one of the sheet conveyance devices, for a transverse shifting of a sheet to be stacked, in a direction which is substantially parallel to the common axle of the sheet conveyance devices. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     An embodiment example of a device according to the invention, from which other inventive characteristics can also be seen, but to which the invention is not limited in its scope, is schematically represented in the drawing, in particular to illustrate the operational course of such a device. In the drawings:  
         [0021]      FIG. 1  shows sheet conveyance devices according to the invention in a side view in a first rotational position;  
         [0022]      FIGS. 2 through 4  show the sheet conveyance devices in additional rotational positions; and  
         [0023]      FIG. 5  is a perspective view of a device according to the invention with sheet conveyance devices according to  FIGS. 1 through 4 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]      FIGS. 1 through 4  show sheet conveyance devices according to the invention in a side view in different rotational positions to illustrate the functional course of such an example of a device according to the invention.  FIG. 1  is a representation of a side view of a substantially disk shaped sheet deflection device  1 , on which a sheet to be transported is placed, which sheet is transferred by the last transport rollers  2 , which form the end of the transport path for printing material, to the sheet deflection device  1 .  
         [0025]     For the forced placement of the sheet on the sheet deflection device, the pin roller  3  is used as a guiding element which blocks in the centrifugal direction, and which is preferably adjustable along the circumference of the sheet deflection device  1 , in order to be able to guide different sheet formats reliably and for a sufficiently long time.  
         [0026]     The device according to the invention includes, in the representation of  FIGS. 1 through 4 , additionally two sheet conveyance devices  4 ,  5 , which are substantially two-armed pivoting beams, and which present, at each one of their free ends, a cover plate-shaped overlapping element  6 . A sheet to be transported can be inserted with its sheet front edge into the slit between the sheet deflection device  1  and an overlapping element  6 , and held therein, and it can be conveyed by the simultaneous rotation of the sheet deflection device  1  and of the sheet conveyance device  4  or  5 , each affected by its corresponding overlapping element  6 , to a stop  7 , and stacked there on a stack of sheets,  8 . The two sheet conveyance devices  4 ,  5  can in principle be rotated independently of each other about the common rotation axle  9 , so that they can assume the corresponding different rotational positions in order to take over, in as rapid as possible a sequence, one sheet after the other from the transport rollers  2 , and to be able to deposit them on the stack of sheets,  8 . Each respective position of the determining overlapping elements  6  is better identified in  FIGS. 1 through 4  because the positions are labeled A, B, C, and D to allow them to be distinguished.  
         [0027]     With reference to  FIGS. 1 through 4 , the course of the operation of the device according to the invention can thus be described as follows:  
         [0028]     The first sheet conveyance device  4  is positioned in such a manner that the overlapping element A is in the sheet receiving position. The second sheet conveyance organ  5  follows the sheet conveyance device  4  with its overlapping element D at a short interval.  
         [0029]     This is a result of the fact that the overlapping element C is placed in such a manner on the opposite side (area stop  7 ) of the second sheet conveyance device  7 , that precisely one sheet can be stacked on the stop  7 .  
         [0030]     The second sheet conveyance device  7  can now remain in the described position until the first sheet conveyance device  4  with the next sheet to be stacked runs downward and must enter into the area of the stop  7 .  
         [0031]     This intermediate stop of the overlapping elements  6  in the area of the stop  7 , which tends to deposit a sheet on the stop  7 , is advantageous if, as preferably provided for, before the stop  7  an offset shift of the sheets is to be carried out, that is if at least one shifting device, which is not shown in further detail, and which is coupled with at least one of the sheet conveyance devices  1 ,  4 ,  5 , is provided, for the transverse shift of a sheet to be stacked in a direction which is substantially parallel to the common axle  9  of the sheet conveyance devices.  
         [0032]     If the overlapping element A with the next sheet to be stacked has moved downward and assumed practically the position of the overlapping element C, then the overlapping element D has already moved into the sheet receiving position.  
         [0033]     Indeed, it is as if the two sheet conveyance devices  4 ,  5  were chasing each other, without ever colliding.  
         [0034]     As an alternative, the sheet conveyance devices  4 ,  5  can take over the guidance roller function here.  
         [0035]     In this solution it is also important that, after the start of the transport of a sheet in the direction of the stop  7 , the following sheet conveyance device  4 ,  5  can be moved with an overlapping element  6  under the sheet to be stacked into the sheet receiving position.  
         [0036]     Thus, the small gap between two sheets, which arrive at the transport rollers  2 , does not have to be used to lead the next overlapping element  6  into the sheet receiving position.  
         [0037]     This results in the following sequence of steps: 
    First step/Fourth step ( FIG. 1 ): 
        Sheet conveyance device  4  with overlapping element A is in the sheet receiving position;     sheet conveyance device  5  follows with a short separation after the sheet conveyance device  4  with the overlapping element D;     simultaneously, the sheet conveyance device  5  with the segment C could deposit a sheet on the stack edge, or cause an offset movement to be performed there.    
        Second step ( FIG. 2 ): 
        Sheet conveyance device  4  moves with the newly received sheet in the direction of the stop  7 ;     sheet conveyance device  5  still remains in the preceding position;     back-up roller  3  has taken over the sheet transport.    
        Third step ( FIG. 3 ): 
        Sheet conveyance device  4  has moved the sheet over the overlapping element A further in the direction of the stop  7 ; sheet conveyance device  5  has turned until the overlapping element D is in the sheet receiving position.    
        Fourth step/First step ( FIG. 4 ): 
        Sheet conveyance device  4  has led the sheet over the overlapping element A on the stop  7 , where it is temporarily stopped. Offset shift could now be carried out with the sheet located on the stop  7 ;     the overlapping element D which is in the sheet receiving position can receive the next sheet.    
       
 
         [0051]     The sequence of the steps  3  and  4 , which has been shown in successive representations, can also take place with temporal overlap. This is preferably the case if the sheet formats are shorter. While one sheet is still being conveyed in the direction of the stop  7 , the next sheet can already be inserted in a following overlapping element  6 . As soon as the sheet back edge is conveyed out of the intervention area of the back-up roller  3 , the next overlapping element  6  with the next sheet can move into this back-up roller area.  
         [0052]     In fact, the second sheet follows the first one with the corresponding gap. For there to be no gap in the transport area of the overlapping element  7 , after the reception of two very short sheets (in the case of a total of only four overlapping elements  6 , both sheets would be on one half of the disk shaped sheet deflection device  1 ), both sheet conveyance devices  4 ,  5  are fitted with two overlapping elements  6  and not with only one. As a result, the gap is always automatically closed again, without the need to move one overlapping element  6  over half the circumference of the deflection, in an accelerated manner, into the sheet receiving position.  
         [0053]      FIG. 5  is a perspective view of an embodiment example of a device according to the invention. The same construction elements are identified with the same reference numerals as in  FIGS. 1 through 4 .  
         [0054]     In  FIG. 5  one can see, in particular, that two sheet deflection devices  1  and four sheet conveyance devices  4 ,  5  are arranged on the axle  9  in a mirror symmetric arrangement with respect to each other, in such a manner that the two sheet deflection devices  1  are located on the outside and the sheet conveyance devices  4 ,  5  are located in between.