Abstract:
A device for transporting sheets to a sheet processing machine includes a feed table, and at least one suction belt endlessly revolvable over the feed table. The suction belt is subjectible to vacuum from suction regions of different pressure levels disposed behind one another in a direction of sheet transport and the suction regions are producible by a single vacuum source. The suction belt is formed with through openings, and the feed table is formed with suction openings and ventilation openings, both of which correspond with the through openings formed in the suction belt.

Description:
BACKGROUND OF THE INVENTION 
     FIELD OF THE INVENTION 
     The invention relates to a sheet-transporting device having a device for adapting or adjusting vacuum in a suction belt feed table of a sheet feeder. At least one suction box is disposed below a sheet transport plane and, as viewed in sheet transport direction, is divided into a plurality of chambers subjectible in common to vacuum. 
     Transport of an imbricated stream of sheets on the feed table of a feeder of a sheet fed rotary printing press may be divided into three phases.
         The first phase is distinguishable by the sheet being drawn from a sheet pile or stack and not lying with full imbricated length on the belt.   The second phase is distinguishable by the sheet being underlappingly transported and an imbricated length thereof lying on the belt.   The third phase is distinguishable by the sheet being transported to the front guide (the leading edge of the sheet no longer being on the belt) and being cut off by the next following sheet.       

     A greater suction force is necessary in the first and third phases than in the second phase, because the supporting surface of the sheet is smaller in the first and third phases than in the second phase. 
     It is necessary to provide different vacuum levels beneath the sheet stream in order for the different operating conditions to meet expectations. That can be achieved, on one hand, by employing a plurality of vacuum sources with different vacuum levels or, on the other hand, as is known from the prior art as exemplified in German Patent DE 44 16 286 C2 corresponding to U.S. Pat. No. 5,697,606, by a switching valve which connects the suction chamber of the middle transport region to atmospheric air. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a sheet-transporting device having a suction belt module with a blower, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which has only a single vacuum source, by which the holding force on the sheet in the middle suction region is reduced in comparison with the holding force in the other suction regions. 
     With the foregoing and other objects in view, there is provided, in accordance with the invention, a device for transporting sheets to a sheet processing machine, in particular a printing press. The sheet-transporting device comprises a feed table, and at least one suction belt endlessly revolvable over the feed table. The suction belt is subjectible to vacuum from suction regions of different pressure levels disposed behind one another in a direction of sheet transport. The suction regions are producible by a single vacuum source. The suction belt is formed with through openings. The feed table is formed with suction openings and ventilation openings, both of which correspond with the through openings formed in the suction belt. 
     In accordance with another feature of the invention, the suction openings and the ventilation openings are disposed in a second or middle suction region of the feed table. 
     In accordance with a further feature of the invention, the sheet-transporting device further includes suction boxes disposed in parallel in edge regions of the suction belt for acting upon the suction openings formed in the feed table. 
     In accordance with an added feature of the invention, the sheet-transporting device further includes suction boxes disposed behind one another in a V shape for acting with vacuum upon the suction openings formed in the feed table. 
     In accordance with an additional feature of the invention, the ventilation openings formed in the feed table are respectively disposed between the suction boxes. 
     In accordance with a concomitant feature of the invention, the sheet-transporting device further includes a rotary valve through which a respective suction box of an end suction region is connected to the single vacuum source. 
     It is of great advantage that the middle region is operated with low vacuum. A reduction in the suction of air from between the sheets and, thereby, in the adhesion of the sheets to one another results. Despite using only one blower, the vacuum in the middle region can be reduced by using simple devices for minimizing the suction area. 
     One preferred configuration provides for a specific number of openings in the suction belt to have vacuum applied thereto, while the remaining openings have atmospheric air applied thereto. Due to this measure, a holding force is exerted on the sheet only in the region of the openings which have vacuum applied thereto. 
     Placing ventilation openings in the transport belt between two rows of suction openings prevents the possibility for a vacuum to form in the entire suction belt region. 
     A second exemplary embodiment provides for the suction boxes in the middle suction belt region to be disposed in a V shape in the direction of transport at a spaced distance from one another. Due to this measure, the openings of the transport belt come alternately into operative contact with the suction holes and the ventilation holes of the feed table. The V-shaped configuration, in this regard, supports the tensioning or stretching of the transport belt transversely to the direction of transport, and produces a centering effect. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a sheet-transporting device having a suction belt module with a blower, 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. 
     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 DRAWINGS 
         FIG. 1  is a diagrammatic, side-elevational view of a sheet processing printing press incorporating the invention of the instant application; 
         FIG. 2  is a fragmentary, side-elevational view of  FIG. 1  showing an exemplary embodiment of a feeding table for sheets in a sheet feeder of the printing press; 
         FIG. 3  is a plan view of the feeding table of  FIG. 1 ; 
         FIG. 4  is an enlarged, cross-sectional view of  FIG. 3  taken along the line IV-IV through the feed table according to  FIG. 2 ; and 
         FIG. 5  is a plan view similar to  FIG. 3  of a different exemplary embodiment of the feeding table. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawings in detail and first, particularly, to  FIG. 1  thereof, there is seen a rotary printing press  1 , e.g. a printing press for processing sheets  7 , having a feeder  2 , at least one respective printing unit  3  and  4  and a delivery  6 . The sheets  7  are removed from a sheet pile or stack  8  and, singly separated or overlapping, i.e., imbricated, are fed over a feed table  9  to the printing units  3  and  4 . Each of the printing units  3 ,  4  is provided in a conventional manner with a respective plate cylinder  11 ,  12 . The plate cylinders  11  and  12  each have a respective device  13  and  14  for fastening flexible printing plates thereon. Furthermore, each plate cylinder  11 ,  12  has a respective device  16 ,  17  assigned thereto for effecting a semiautomatic or fully automatic printing-plate change. 
     The sheet pile or stack  8  lies on a controllably liftable pile or stack plate  10 . The removal of the sheets  7  takes place from the top of the sheet pile  8  by a so-called suction head  18  which, among others, has a number of lifting and dragging suckers  19 ,  21  for separating the sheets  7 . Furthermore, blast or blowing devices  22  for loosening up the top sheet layers, and sensing elements  23  for following up or tracking the pile or stack, are provided. A number of side and rear stops  24  are provided in order to align the sheet pile or stack  8 , in particular the top sheets  7  of the sheet pile or stack  8 . 
     The sheet arriving in a forward or front region of the feed table  9  is aligned in the direction of sheet transport by front guides  26  and, transversely to the direction of sheet transport, by lateral aligning elements  27 . The feed table  9  is configured as a so-called suction belt table which, as viewed in the direction of sheet transport, has at least three suction regions I, II, III shown in  FIG. 2 . The first suction region I is distinguished by a high vacuum level, in order to be able to take over a sheet reliably from the separating device. The vacuum in the suction region I is produced by a suction box  31  which is disposed below a feed plane. 
     The second or middle suction region II is distinguished by a low vacuum level which exerts a relatively small holding force on the sheet in order to hold the latter sufficiently reliably on a transport belt  29 . The vacuum in the second or middle suction region II is produced by two suction boxes  32  and  33 , seen in  FIG. 3 . The suction boxes  32  and  33  are disposed in parallel at a distance from one another transversely with respect to the direction of sheet transport and, in fact, preferably in the edge region, i.e., near the respective side edges of the transport or suction belt  29 . 
     The third or end suction region III is distinguished by a cyclic vacuum level. The vacuum in the suction region III is produced by a suction box  34  which is disposed below the feeding plane. All of the suction boxes  31 ,  32 ,  33  and  34  are connected to a single common suction source  36 . 
     The transport belt  29  is driven by a drive roller  37  and deflected around a deflection roller  38 . Tensioning devices provided for the transport belt  29  are not shown in the figures. 
     A rotary valve  39 , providing a device for cycling the vacuum in the operating or work cycle of the sheet processing printing press, is connected in a connecting line to the suction box  34 . The rotary valve  39  is additionally provided with a bypass to atmospheric pressure in order to adapt or adjust the vacuum of the suction box  34 . 
     As is further shown in  FIGS. 3 and 4 , the transport belt  29  is formed with a multiplicity of through openings  41  which, in the suction region I, are connected to the suction box  31  by suction openings  42  formed in the feed table  9 . In the suction region II, the through openings  41  in the edge regions of the transport belt  29  are connected to the respective suction boxes  32  and  33  by respective suction openings  42  and  43  formed in the feed table  9 . Those through openings  41  formed in the middle region lying between the edge regions of the transport belt  29  are connected to atmospheric air by ventilation openings  46  formed in the feed table  9 . In the suction region III, the through openings  41  are connected to the suction box  34  by suction openings  44  formed in the feed table  9 . 
     In a second embodiment of the invention according to  FIG. 5 , provision is made for suction boxes  51  to  55  which are disposed in the middle suction region II to have a V-shaped configuration. The point of the V points counter to the direction of sheet transport. Through the use of this measure, a stretching of the transport belt  29  in the transverse direction and, additionally, a centering of the transport belt  29  are achieved. 
     The suction boxes  51  to  55  are respectively connected to the common suction source  36  and disposed at a regular distance from one another. It is also possible, in this way, for the suction box  31  to have suction box attachments  56 ,  57  which are adapted to or match the V shape. 
     Suction openings  58  which are appropriately disposed in a V shape and correspond with the through openings  41  formed in the transport belt  29  are provided in the feed table  9  in the region of the suction boxes  51  to  55 . Ventilation openings  59  are provided in the feed table  9  between the suction openings  58  of the respective suction boxes  51  to  55  and  56 ,  57 . Through the use of this measure, vacuum or atmospheric air is applied alternately to one and the same through hole  41  during transport, and reduces the holding force on the sheet or the stream of sheets with respect to the holding force thereon in the suction region I.