Abstract:
A method and an apparatus for holding down a sheet on a feed table of a sheet-processing machine include a lowerable smoothing bar which can be driven according to a speed profile in time with the sheet-processing machine, which results in a targeted deflection of the smoothing bar. The deflection is preserved for the purpose of smoothing the sheet from the middle to the side.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority, under 35 U.S.C. §119, of German Patent Application DE 10 2014 002 468.7, filed Feb. 21, 2014; the prior application is herewith incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The invention relates to a method and an apparatus for holding down a sheet on a feed table of a sheet-processing machine, in particular a punch or a printing press. 
         [0003]    The invention is based on overcoming the problem that sheets, in particular thin sheets, tend to form curves or undulations at their front edges which can lead to tensions or crease formation of the sheet at its front edge during further transport of the sheet by way of downstream gripper systems. 
         [0004]    German Patent DE 4028426 C2 discloses an apparatus for smoothing a front edge of a sheet before being transported by downstream gripper devices. In that case, the front edge of the sheet is suctioned fixedly to the feed table by using fixing nozzles and is pressed against the feed table and therefore clamped by using clamping rollers. 
       SUMMARY OF THE INVENTION 
       [0005]    It is accordingly an object of the invention to provide a method and an apparatus for holding down a sheet on a feed table of a sheet-processing machine, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and apparatuses of this general type and in which the sheet is not clamped in. 
         [0006]    With the foregoing and other objects in view there is provided, in accordance with the invention, a method for holding down a sheet on a feed table of a sheet-processing machine, which comprises placing a smoothing bar above the feed table, lowering the smoothing bar in a direction of the feed table, and carrying out the lowering movement of the smoothing bar according to a predefinable speed profile. 
         [0007]    With the objects of the invention in view, there is also provided an apparatus for holding down a sheet on a feed table of a sheet-processing machine, comprising a smoothing bar disposed above the sheet and being lowerable in a direction of the feed table, the smoothing bar being mounted in such a way that it can be moved vertically on both sides of the feed table in time with the sheet-processing machine. 
         [0008]    It is one special advantage of the invention that a smoothing bar is provided which smoothes the sheet without clamping it. This measure ensures that a surface of the sheet is not damaged. The sheet is smoothed uniformly toward the sides and fold formation is therefore avoided by using a special smoothing movement of the smoothing bar, namely from the middle to the sides of the sheet. 
         [0009]    In accordance with another feature of the invention, in order to produce the smoothing movement of the smoothing bar, a drive, in particular an electric motor, is provided which can be driven in time with the sheet-processing machine and is controlled by using characteristic curves. 
         [0010]    In accordance with a further feature of the invention, a four-bar linkage is provided which connects the drive to the smoothing bar. 
         [0011]    As a result of the smoothing bar according to the invention, all grammages can be processed reliably without leaving markings on the sheet, in order to ensure that the sheet can be gripped without folds by gripping devices. 
         [0012]    In accordance with a concomitant special feature of the invention, all grammages can be processed reliably without markings being produced on the sheet. Non-tensioned sheets secure a double-free print result and high register accuracy in printing presses. 
         [0013]    Other features which are considered as characteristic for the invention are set forth in the appended claims. 
         [0014]    Although the invention is illustrated and described herein as embodied in a method and an apparatus for holding down a sheet on a feed table of a sheet-processing machine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
         [0015]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0016]      FIG. 1  is a diagrammatic, longitudinal-sectional view of a sheet punching and embossing machine for sheet-shaped material; 
           [0017]      FIG. 2  is a side-elevational view of a sheet hold-down according to the invention; 
           [0018]      FIG. 3  is a front-elevational view of the hold-down as viewed counter to a sheet transport direction; 
           [0019]      FIGS. 4A ,  4 B and  4 C show the hold-down in various positions during a smoothing operation; and 
           [0020]      FIG. 5  is a diagram illustrating actuation of an electric motor. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Referring now to the figures of the drawings in detail and first, particularly, to  FIG. 1  thereof, there is seen a fundamental construction of a sheet punching and embossing machine  100  for punching, breaking out, separating multiple copies and depositing sheets made from paper, cardboard, plastic and the like. The punching and embossing machine  100  has a feeder  1 , a punching station  2 , a breaking-out station  3  and a delivery  4  with a station for depositing and separating multiple copies, which are supported and enclosed by a common machine housing  5  and driven by a main drive  17 . The processing stations  2 ,  3 ,  4  are accessible from one side, the so-called operator side. The drive train of the sheet punching and embossing machine  100  is situated on the opposite side, the so-called drive side. A machine controller  15  controls the sequences within the punching machine  100 . 
         [0022]    Sheets  6  are separated from a stack  6 . 1  by the feeder  1 , are fed to a sheet transport system  7 , are gripped at their front edge by grippers which are fastened to gripper bars of a gripper carriage  8 , and are pulled intermittently through the different stations  2 ,  3  and  4  of the punching and embossing machine  100  in a sheet transport direction B. 
         [0023]    The sheet transport system  7  has a plurality of gripper carriages  8 , as result of which a plurality of sheets  6  can be processed at the same time in the different stations  2 ,  3  and  4 . The gripper carriages  8  can be driven by a chain drive or, in an alternative embodiment, by way of an electromagnetic linear drive with linear motors, as described, for example, in German Utility Model DE 20 2007 012 349 U1. 
         [0024]    The punching station  2  is formed of a lower platen, which is a so-called lower table  9 , and an upper platen, which is a so-called upper table  10 . The upper table  10  is mounted in such a way that it can be moved back and forth vertically and is provided with an upper die  22  with punching and scoring blades. The lower table  9  is mounted fixedly on the machine frame and is provided with a plate  18  corresponding to the punching and scoring blades. As an alternative, the upper table  10  can also be stationary and the lower table  9  can be movable. During embossing, embossing dies, in particular in the form of so-called embossing plates, are used instead of the punching and scoring dies. 
         [0025]    The gripper carriage  8  transports the sheet  6  from the punching and embossing station  2  into the following breaking-out station  3  which is equipped with breaking-out dies  19 ,  21 . In the breaking-out station  3 , waste pieces  11  which are not required are ejected downward from the sheet  6  with the aid of the breaking-out dies  19 ,  21 , as a result of which the waste pieces  11  fall into a carriage-like container  12  which is pushed in below the station. 
         [0026]    The sheet  6  passes from the breaking-out station  3  into the delivery  4 , where the sheet  6  is either merely simply deposited, or a separation of the individual multiple copies of a respective sheet  6  takes place at the same time. To this end, the delivery  4  has a die  19 ,  21  for separating multiple copies. The delivery  4  can also include a pallet  13 , on which the individual sheets  6  or copies are stacked in the form of a stack  14 , so that the pallet  13  with the sheet stack  14  can be moved away from the region of the punching and embossing machine  100  after a defined stack height has been reached. Auxiliary stack devices can be used, so that it is not necessary to stop the machine  100  during the stack change. 
         [0027]    A sheet hold-down  23  is disposed at one end of a feed table  16 , which is the end that faces the punching and/or embossing stations  2  to  4 . 
         [0028]    As is seen in  FIGS. 2 and 3 , the hold-down  23  substantially has a smoothing bar  24  which is disposed above the feed table  16  transversely with respect to the sheet transport direction and can be lowered in time with the sheet-processing machine, for example a sheet-fed printing press, in particular a punch, in the direction of the feed table  16  from a position H 0  above the feed table  16  into a position H 2 , as seen in  FIG. 5 . The smoothing bar  24  is fastened to couplers  26 ,  27  which are disposed on the sides of the feed table  16  and are in each case part of a four-bar linkage  28 ,  29 . Moreover, each four-bar linkage  28 ,  29  has two bearing levers  31 ,  32 ;  33 ,  34  which are disposed horizontally parallel to one another. They are mounted pivotably and have a first end disposed in a stationary manner on a side frame of the feed table  16  and a second end disposed in each case on a respective coupler  26 ;  27 . 
         [0029]    The coupler  26  has a third lever  36  which has a first end mounted on the coupler  26  in an articulated manner and a second end connected to an eccentric  37  which is driven rotationally by a motor  38 . The motor  38  is driven in time with the sheet-processing machine, for example the punching and embossing machine  100 , according to an adjustable speed profile  39 . 
         [0030]    The speed profile  39  is configured in such a way that immediately before the end position of the lowering movement H 2  is reached, the middle of the smoothing bar  24  overshoots just enough (over-shooter) that the middle of the smoothing bar  24  reaches its end position before the ends of the smoothing bar  24  reach their end position. The oscillation of the smoothing bar  24  decays or is stopped after one overshoot of the bar middle, with the result that the oscillation of the smoothing bar  24  is stopped virtually completely when the end position of the ends of the smoothing bar  24  is assumed. 
         [0031]    The smoothing bar  24  performs precisely one oscillation of a first resonant frequency. As an alternative, the smoothing bar  24  can perform a plurality of periods of the oscillation of the first resonant frequency. 
         [0032]    The smoothing bar  24  is preferably excited to perform the oscillation of the first resonant frequency from a rest position or during the lowering movement by harmonic excitation, which oscillation decays by way of material damping or deliberate damping through the drive, while the smoothing bar  24  is moved into its end position H 2 . 
         [0033]    A superimposition of the excitation of a plurality of resonant frequencies of the smoothing bar  24  is achieved during a first contact of the smoothing bar  24  with the sheet by way of a geometry of the smoothing bar  24  which differs from the first resonant frequency. 
         [0034]    The feed table  16  has a number of suction openings  41  below the smoothing bar  24  for fixing a sheet  6 . The suction openings  41  can be charged or loaded with vacuum from a vacuum generator  42  and can be adjusted by using a throttle  43  in time with the sheet-processing machine. 
         [0035]    A sheet  6  which is conveyed over the feed table  16  is aligned on front lays  44  and is stopped. As soon as the sheet  6  stops, the smoothing bar  24  is moved downward. The vacuum is switched on at the same time as the downward movement of the smoothing bar  24 , preferably at its low point H 2 . The smoothing bar  24  therefore ensures that an air gap under the sheet  6  is small enough that the suction openings  41  can reliably attract the sheet  6  by suction. The stroke and therefore the minimum spacing of the gap between the dead center position H 2  of the smoothing bar  24  and the feed table  16  are adjustable, so that sheets  6  of different thickness can be reliably attracted by suction. 
         [0036]    In order to ensure that the attraction of the sheets by suction takes place without folds, the smoothing bar  24  has an oscillatory structure, that is to say it deflects noticeably during the downward movement. The smoothing bar  24  is excited to produce an oscillation in a targeted manner by the drive  38 , which oscillation is superimposed on a kinematic movement in time with the sheet-processing machine and causes the middle of the bar to first reach the lowest point H 2  above the sheet  6  and shortly thereafter for the ends of the smoothing bar  26  to also likewise assume that position ( FIGS. 4A to 4   c ). 
         [0037]    The temporal sequence is coordinated in such a way that a first jump t 0  according to  FIG. 5  takes place at half a period duration of the first resonant frequency of the smoothing bar  24  before the start of a suction action t 1  of the suction openings  41 , with the result that the middle of the smoothing bar reaches the lower dead center H 2  of its oscillation at the same time as the start of the vacuum. At this time, a second jump then takes place which likewise lowers the ends of the smoothing bar  24  completely and at the same time stops the periodic resonant oscillation which results from the first jump, so that the smoothing bar  24  remains at rest in the lower position H 2  at a time t 2 , that is to say the smoothing bar reaches its dead center. 
         [0038]    In addition, it is proposed to charge or load the suction openings  41  with vacuum starting from the middle toward the sides, in order to improve a sliding sweeping effect toward the side.