Abstract:
A device for conveying a sheet through a printing machine includes a cylinder, formed with a channel wherein grippers are disposed for holding the sheet at a leading edge, a chain conveyor with grippers fixed to endless chains for holding and picking up the sheet at its leading edge from the cylinder and for holding the sheet at its trailing edge, and rotating sprockets carrying the chains. An actuating device is provided for opening and closing the grippers, and a drive is provided for the cylinder and the sprockets. Gripper pads and gripper tips for the trailing-edge grippers, in opened condition of the grippers, are able to run past the cylinder. The gripper pads and tips lie underneath a path described by the sheet. The trailing-edge grippers are closable in a rotational angle region wherein the chains run off the sprockets for the cylinder.

Description:
BACKGROUND OF THE INVENTION 
     FIELD OF THE INVENTION 
     The invention relates to a device for conveying a sheet through a printing machine, including a cylinder, grippers disposed in a channel formed in the cylinder for holding the sheet at a leading edge thereof, a chain conveyor with grippers fixed to endless chains for holding and picking up the sheet from the cylinder at the leading edge of the sheet and for holding it at a trailing edge of the sheet, and rotating sprockets over which the chains are disposed. The conveying device further includes coaxially revolving supporting elements for the sheet, the supporting elements being disposed between the rotating sprockets and being in rolling contact with the circumferential surface of the cylinder, and coaxially revolving suction grippers provided between the sprockets for holding the sheet at the trailing edge thereof, an actuating device for opening and closing the grippers, and a drive for the cylinder and the rotating sprockets. 
     German Published, Non-Prosecuted Patent Application DE 100 14 417 A1, corresponding to U.S. Pat. No. 6,578,846 B2 and to U.S. Patent Application Publication No. US 2002/0135123 A1, describes a device for transporting a sheet for a rotary printing press, wherein a sheet is transferred from a cylinder to a chain gripper system. In the chain gripper system, the sheet is held at the leading edge and at the trailing edge thereof by grippers. The trailing edge grippers pick up the sheet in an exit pocket or outlet wedge between supporting surfaces that revolve with sprockets and the circumferential surface of the cylinder. Until the sheet is picked up by the trailing edge grippers, the sheet is held by suction grippers, which likewise revolve with the sprockets. The gripper tips of the trailing edge grippers lie above a path described by the sheet held on the circumferential surface of the cylinder. In order for the trailing edge grippers to pass the circumferential surface of the cylinder without collision, appropriate recesses must be provided in the circumferential surface, which calls for additional manufacturing effort. 
     In the sheet delivery for a printing press according to German Published, Non-Prosecuted Patent Application DE 101 57 117 A1, the sheets are picked up by a gripper system only at the leading edge of the sheets. In order to avoid the formation of waves in the sheets in the outlet region of the chains from sprockets, the radii of the pitch circles of the sprockets are formed larger than the radius of the circular path of gripper pads about the axis of the sprocket shaft. The cylinder located opposite the sprockets is recessed in the regions opposite the sprockets. One embodiment provides for the sprockets to be disposed outside the end surfaces of the impression cylinder from which a sheet is picked up. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a device for conveying a sheet through a printing machine, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which affords improved sheet guidance with reduced effort. 
     With the foregoing and other objects in view, there is provided, in accordance with the invention, a device for conveying a sheet through a printing machine. The device comprises a cylinder, grippers disposed in a channel formed in the cylinder for holding the sheet at a leading edge thereof, a chain conveyor with grippers fixed to endless chains for holding and picking up the sheet from the cylinder at the leading edge of the sheet and for holding the sheet at a trailing edge thereof, rotating sprockets over which the chains are disposed and coaxially revolving supporting elements for the sheet. The supporting elements are disposed between the rotating sprockets and are in rolling contact with the circumferential surface of the cylinder. Coaxially revolving suction grippers are provided between the sprockets for holding the sheet at the trailing edge thereof. An actuating device is provided for opening and closing the grippers. A drive is provided for the cylinder and the rotating sprockets. Gripper pads and gripper tips are provided for the grippers for the trailing edge of the sheet. The grippers for the trailing edge, in the opened condition thereof, are able to run past the cylinder. The gripper pads and the gripper tips of the grippers for the trailing edge lie underneath a path described by the sheet held on the circumferential surface. The grippers for the trailing edge are closable in a rotational angle region wherein the chains run off the sprockets assigned to the cylinder. 
     In accordance with another feature of the invention, the sheet-conveying device further includes radially movable suction grippers disposed in circumferential direction in the vicinity of the gripper pads for the trailing edge and, outside the contact region with the cylinder, being radially higher than the gripper pads for the trailing edge. 
     In accordance with a further feature of the invention, the chains unwound at the sprockets have a length less than the sheet length maintained between the grippers for the leading edge and the radially movable suction grippers. 
     In accordance with an added feature of the invention, the sheet-conveying device further includes revolving support elements for the sheet and, for chains having polygon-type links, the pitch circle diameter of the sprockets is at most equal to the diameter of the revolving supporting elements. 
     In accordance with an additional feature of the invention, the sheet-conveying device further includes actuating elements for circumferentially adjusting the radially movable suction grippers. 
     In accordance with yet another feature of the invention, the actuating elements are piezoelectric actuators for effecting the circumferential adjustment. 
     In accordance with yet a further feature of the invention, the sheet-conveying device further includes a suction gripper bar whereon the radially movable suction grippers are disposed. The actuating elements are independently controllable for acting upon both sides of the suction gripper bar. 
     In accordance with a concomitant feature of the invention, the sheet-conveying device further includes radially movable suction grippers disposed in circumferential direction in the vicinity of the gripper pads for the trailing edge. The grippers for the trailing edge are closed in a rotational angle of the sprockets wherein the circular path of the surface of the suction grippers intersects the path of the gripper pads for the trailing edge. 
     Thus, according to the invention, trailing edge grippers which are fixed to a chain conveyor are led in opened condition past a cylinder from which a sheet is to be picked up. In this regard, the gripper pads and gripper tips for the trailing edge lie outside a path described by the sheet held on the circumferential surface of the cylinder. This ensures that the trailing edge grippers can pass the closed circumferential surface of the adjacent cylinder without collision. The trailing edge grippers are closed only in a rotational angle region of sprockets wherein the revolving path of the gripper pad intersects the radius of the crown line of the suckers. 
     In order to remove a sheet from the cylinder, suction grippers are provided which revolve with the sprockets of the chain conveyor and which pick up the sheet at the trailing edge thereof in the vicinity of the transfer center line. In order for the sheet to be picked up reliably and reproducibly by the suction grippers, supporting elements having supporting surfaces are provided, which press the sheet onto the circumferential surface of the cylinder. This means that a sheet is still held on the circumferential surface of the cylinder by the contact surface of the supporting surface when the suction grippers are just touching the sheet. In order to be able to pick up a sheet reliably from a cylinder, the suction grippers are forcibly moved against the trailing edge of the sheet, by making the radius of the suction grippers larger than the radius of the supporting surfaces. The suction grippers are disposed in such a way that they can move radially, with the suction grippers moving out to the original height thereof again after leaving the region of contact with the adjacent circumferential surface of the cylinder. 
     If a sheet is conveyed at the leading edge and at the trailing edge thereof, a minimum dimension for a sheet sag is required, for example because of one-sided action using a dryer. The device offers the possibility of adjusting the suction grippers in the circumferential direction by actuators, in order to set the sheet sag to a desired dimension. If two actuators acting upon a suction gripper bar are differently driven, the suction gripper bar can be set obliquely. Therefore, any desired positional corrections of a sheet can be performed as it is conveyed through a printing press, in order for example to effect compensation for a skewed position on front guides or for the diagonal register. Rotary transformers are provided for the power supply to the actuators. The position of the suction gripper bar can be regulated, for which purpose sensors can be disposed that supply signals relating to the current position of the sheet in the transfer center line on the cylinder and of the suction gripper bar. 
     A further possibility of obtaining a desired sheet sag is provided by the configuration of the pitch circle diameter of the sprockets and the chain pitch in relation to the diameter of the circumferential surface of the adjacent cylinder. In this regard, it is important that the length of the chains unwound from the sprockets be smaller than the sheet length in the same rotational angle region. 
     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 device for conveying a sheet through a printing 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. 
     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 fragmentary, diagrammatic, side-elevational view of a delivery of a printing press having a chain gripper system; 
         FIG. 2  is an enlarged, fragmentary view of  FIG. 1  diagrammatically illustrating the sheet transfer between an impression cylinder of the printing press and the chain gripper system; 
         FIG. 3  is an enlarged, fragmentary view of  FIG. 2  in a phase fifteen degrees before a sheet trailing edge is picked up; 
         FIG. 4  is an enlarged, fragmentary view of  FIG. 3  showing a dot-dash encircled region relating to the radial position of suction grippers; 
         FIG. 5  is an enlarged, fragmentary view of  FIG. 2  in a phase wherein a sheet trailing edge is being picked up by suction grippers; 
         FIG. 6  is an enlarged, fragmentary view of  FIG. 5  showing a dot-dash encircled region wherein the suction grippers are in contact engagement with the trailing edge of a sheet; 
         FIG. 7  is an enlarged, fragmentary view of  FIG. 2  in a phase 195 degrees after a sheet trailing edge has been picked up; and 
         FIG. 8  is an enlarged, fragmentary view of  FIG. 7  showing a dot-dash circle wherein the sheet trailing edge is picked up by trailing edge grippers of the chain gripper system. 
     
    
    
     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 last printing unit  1  of a sheet-fed printing press, with a delivery  2  disposed downstream therefrom. The printing unit  1  has a sheet transport drum  3 , an impression cylinder  4 , a transfer cylinder  5  and a plate cylinder  6 . Rollers of an inking unit  7  and a dampening unit  8  are associated with the plate cylinder  6 . The delivery  2  includes a chain gripper system having a multiplicity of leading edge grippers  9 . 1  to  9 . 7  and trailing edge grippers  10 . 1  to  10 . 7 , which are fixed to gripper bars. The gripper bars are connected to endless chains, which are disposed over sprockets  11  and  12  and guided in chain paths or tracks  13 . At an end of lower runs of the chain paths  13 , in a conveying direction represented by an arrow  14 , a stacking or pile table  15  is provided, whereon a sheet pile or stack  16  is disposed. The stacking table  16  is lowerable by a chain mechanism  17  in accordance with the progress or advancement of production. 
     Sheets  18  are conveyed by the sheet transport drum  3  from the preceding printing unit to the impression cylinder  4 . On the impression cylinder  4 , the sheets  18  are held in grippers  19  and are guided through a printing nip  20  between the transfer cylinder  5  and the impression cylinder  4 . A printing form  21  stretched out on the plate cylinder  6  is inked in accordance with an image with the aid of the dampening unit  8  and the inking unit  7 . The transfer cylinder  5  has a rubber-elastic or resilient covering  22 , with which the printing ink on the printing form  21  is transferred to the sheet  18 . In the delivery  2 , the sheets  18  are picked up at the leading edge thereof by the leading edge grippers  9 . 1  to  9 . 7 , with the trailing edge of the sheets  18  being held by the trailing edge grippers  10 . 1  to  10 . 7 . Shortly upstream of the pile or stack  16 , the respective grippers  9 . 1  to  9 . 7  and  10 . 1  to  10 . 7  are opened, so that a respective sheet  18  falls onto the pile or stack  16 . A leading edge stop  23  limits movement in the conveying direction represented by the arrow  14 , of the sheet  18  which has been allowed to fall. 
     In  FIG. 2 , the elements for conveying a sheet  18  from the impression cylinder  4  onto the leading edge grippers  9 . 1  to  9 . 7  and the trailing edge grippers  10 . 1  to  10 . 7  are illustrated in greater detail. The cylinder  4  which is double the size, i.e., has twice the diameter, of the printing unit cylinders, i.e formed with two channels  24  and  25 , wherein the grippers  19  shown in  FIG. 1  are arranged on gripper bare. These gripper bars extend over the width of a sheet  18  which is held at the leading edge thereof. The impression cylinder  4  has two circumferential surface segments  26  and  27 , whereon, respectively, a sheet  18  lies during conveyance. The length of the circumferential surface segments  26  and  27  in the circumferential direction corresponds to the maximum length of a sheet  18  to be conveyed. The impression cylinder  4  and the sprockets  11  are driven synchronously in the directions of the arrow  28  and  29 , respectively by a drive  51 . The pitch circle diameter  30  of the sprockets  11 , and the diameter  31  of the impression cylinder  4  are of approximately equal length. A connecting line between the respective axes  32  and  33  of the impression cylinder  4  and the sprocket,  11  forms a transfer center line  34 . In a direction perpendicular to the plane of the drawing of  FIG. 2 , an intersection region runs between the circular path of the circumferential surfaces  26  and  27  and a cylinder to which the pitch circles of the sprockets  11  belong. The intersection region and the transfer center line  34  are perpendicular to one another. The leading edge grippers  9 . 1  to  9 . 7  and the trailing edge grippers  10 . 1  to  10 . 7  are arranged at the width of the format on gripper bars  35  and  36 , which are fixed to separate synchronously revolving chains. The spaced distance between the leading edge grippers  9 . 1  to  9 . 7 , on the one hand, and the trailing edge grippers  10 . 1  to  10 . 7 , on the other hand, is adjustable by shifting or displacing the phases of the chains with respect to the sheet length. Between the leading edge gripper bar  35  and the trailing edge gripper bar  36 , supporting elements  37  for a sheet  18  are arranged on the axis  33 , revolving with the sprockets  11 . The supporting elements  37  carry the sheet  18  at the print-free marginal edges thereof. For the purpose of effecting adjustments to the sheet width, the supporting elements  37  are displaceable on the axis  33 . The number of supporting elements  37  in the circumferential direction is variable in accordance with the sheet length. Furthermore, suction grippers, which are arranged on a suction gripper bar  38 , revolve with the supporting elements  37 . The suction gripper bar  38  is assigned to the trailing edge gripper bar  36  with regard to the rotational position about the axis  33 . The suction grippers pick up or accept the sheet  18  at the trailing edge thereof. 
     The leading edge gripper bar  35  dips into the channel  25  of the impression cylinder  4  as the gripper bar  35  passes the transfer center line  34 . Due to the presence of the channels  24  and  25 , there is no risk of collision between the leading edge grippers  9 . 1  to  9 . 7  and the impression cylinder  4  at this point. On the other hand, for the trailing edge grippers  10 . 1  to  10 . 7 , precautions must be taken to ensure that the trailing edge grippers  10 . 1  to  10 . 7  pass the circumferential surfaces  26  and  27  without contact. For this purpose, the trailing edge grippers  10 . 1  to  10 . 7  are opened at a correct time before reaching the transfer center line  34 , to such an extent that the trailing edge gripper bar  36  can pass without collision. 
       FIG. 3  shows the sprockets  11  in a rotational position of about 15 degrees before the transfer center line  34 . Gripper fingers  41  are pivoted back to such an extent that they lie reliably under a path  42  described by the supporting surface of the supporting elements  37 , by rotating a gripper shaft  39  in a direction represented by an arrow  40 . The gripper pad  43  is also located deeper in the radial direction than the path  42 . The supporting elements  37  extend in the circumferential direction to as far as the suction gripper bar  38 , so that the sheet  18  remains held by the supporting surface when suckers  44  of the suction grippers  38  just touch the sheet  18 . 
       FIG. 4  illustrates a detail IV of  FIG. 3  in an enlarged view. In order to achieve a reliable takeover or pick-up of the sheet  18  at the trailing edge thereof, the radial height of the suckers  44  is located a distance x above the path  42  of the supporting elements  37 . The suckers  44  are displacable in the radial direction represented by a double-headed arrow  45  by actuating elements  52 . The suction openings  46  of the suckers  44  are connected to a vacuum source via a duct  47 . 
     In  FIG. 5  and in a portion VI thereof shown in an enlarged view in  FIG. 6 , the suckers  44  are illustrated exactly on the transfer center line  34 . In this position, the gripper fingers  41  remain pivoted away from the gripper pads  43 . The suckers  44  pick up the sheet  18  at the trailing edge thereof. As the sprockets  11  continue to rotate, the sheet  18  leaves the circumferential surface  27  of the impression cylinder  4  and is carried at the marginal edges thereof by the supporting elements  37 . The suckers  44  are pushed-in radially by the respective contact thereof with the sheet  18  and the circumferential surface  27  on the transfer center line  34 . After leaving the transfer center line  34 , the suckers  44  move back to the original radial height or level thereof again, which they inherently had in the region before the transfer center line  34 . Therefore, in the region after the transfer center line  34 , a difference in height exists for the trailing edge between the radial level of the suckers  44  and the gripper pad  43 . 
       FIG. 7  and a portion XIII thereof shown in an enlarged view in  FIG. 8  show the acceptance or pick-up of the trailing edge of the sheets  18  by the suction grippers  44  into the trailing edge grippers  10 . 1  to  10 . 7 . In the region wherein the chains run off the sprockets  11 , the gripper pads  43  for the trailing edge of the sheet  18  come close to the radial level of the suckers  44 . In this region, there is necessarily a point at which the path of the gripper pads  43  intersects the circular path of the surface of the suckers  44 . The connecting line between this point and the axis  3  of the sprockets  11  results in a further transfer center line  48 . On this transfer center line  48 , the trailing edge of the sheet  18  is transferred to the trailing edge grippers  10 . 1  to  10 . 7 . The respective trailing edge gripper  10 . 1  to  10 . 7  is closed on the transfer center line  48 , so that the trailing edge of the sheet  18  is held between the gripper fingers  41  and the gripper pads  43 . The vacuum to the suckers  44  is switched off, so that the suckers  44  release the sheet  18 , or the vacuum remains switched on and the sheets  18  are torn from the suckers  44 . The sheet  18  is conveyed onto the pile or stack  16  in accordance with the chain track  13 , held at the leading edge and trailing edge of the sheet  18 . 
     Assuming a revolving diameter d of the supporting elements  37 , a pitch circle diameter d 0  of the sprockets  11  and a length 1 of the sheets  18 , a sheet shortening or contraction a is given by: 
     
       
         
           
             a 
             = 
             
               1 
               ⁢ 
               
                 
                   ( 
                   
                     1 
                     - 
                     
                       
                         d 
                         0 
                       
                       d 
                     
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
     From this sheet shortening, for a sheet with low stiffness and low sag, a maximum sheet sag h max  and a small sag result: 
     
       
         
           
             
               h 
               max 
             
             = 
             
               
                 
                   
                     3 
                     8 
                   
                   ⁢ 
                   
                     ( 
                     
                       
                         a 
                         · 
                         1 
                       
                       - 
                       
                         a 
                         2 
                       
                     
                     ) 
                   
                 
               
               . 
             
           
         
       
     
     A sheet sag is required in order to be able to hold a sheet  18  with the given gripper holding forces, for example if one side is subjected to air by a dryer. The root function results in a sensitive relationship between the shortening or contraction a and the sheet sag h max . 
     The shortening or contraction a and, therefore, the sheet sag h max  can be greatly affected due to inaccuracies in the sheet position on a circumferential surface segment  26 ,  27 . Therefore, in one alternate or varying embodiment of the invention, provision is made for actively influencing the sheet sag h max . It is possible to exert an influence in the rotational angle region of the sprockets  11  where a sheet  18  is held only by the suckers  44 , i.e., after the takeover or pick-up from the impression cylinder  4  until the instant of time of the transfer to the trailing edge gripper bar  36 . One possible way of changing the position of the trailing edge of the sheets  18  in relation to the leading edge is by the use of actuators, in particular piezoelectric actuators, which are disposed on both sides between the suction gripper bar  38  and the holders of the suction gripper bar  38  on the axis  33 . if the actuators are driven simultaneously, the suction gripper bar  38  experiences a pure circumferential adjustment. If the actuators are driven differently, a skewed position of the suction gripper bar  38  results. Therefore, any desired positional corrections of the sheet  18 , in particular compensation for a skewed position of front guides in the feeder of the printing press, and corrections of settings of the diagonal register, can be made. The position of the suction gripper bar  38  can be regulated if position sensors are provided, which are directed towards the sheet  18  on both sides of the sheet path on the transfer center line  34 . In addition, the current position of the suction gripper bar  38  in the circumferential direction of the sprockets  11  can be determined by distance sensors. The power for actuating the actuators is made available via rotary transformers on the axis  33 . 
     The foregoing considerations relating to the sheet sag h max  assume that the pitch of the sprockets  11 ,  12  is so small that the chains have no disruptive polygon effect. A further variable having an influencing effect upon the sheet sag results if the polygon effect is taken into account in the case of large-link chains in the delivery  2 . In the case of a chain which has links disposed around a sprocket  11  forming a polygon, a sag will also be established if the pitch circle diameter of the sprockets  11  has the same diameter as the circumferential surface segments  26 ,  27  of the impression cylinder  4 . The resultant sheet sag depends in this case upon the chain pitch. If a sprocket  11  has a pitch circle diameter d 0 , a number of teeth z and a chain pitch t, then: 
     
       
         
           
             t 
             = 
             
               
                 d 
                 0 
               
               · 
               
                 
                   sin 
                   ( 
                   
                     
                       180 
                       ⁢ 
                       ° 
                     
                     z 
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
     The sheet shortening or contraction a p  with the polygon effect taken into account is calculated as: 
                 a   p     =     l   ·     (     l   -     z   ·     (     t     π   ·   d       )         )         ,         
from which a corresponding sheet sag h max,p  results from the hereinafore-mentioned relationship:
 
     
       
         
           
             
               h 
               
                 max 
                 , 
                 p 
               
             
             = 
             
               
                 
                   
                     3 
                     8 
                   
                   ⁢ 
                   
                     ( 
                     
                       
                         
                           a 
                           p 
                         
                         · 
                         l 
                       
                       - 
                       
                         a 
                         p 
                         2 
                       
                     
                     ) 
                   
                 
               
               . 
             
           
         
       
     
     This consideration assumes that the sheet  18  lies at the radial height of the supporting elements  37 . It is possible to disregard the radial heights of the suckers  44  and of the gripper pad for the leading edge and the trailing edge  9 . 1  to  9 . 7  and  10 . 1  to  10 . 7 . 
     This application claims the priority, under 35 U.S.C. § 119, of German Patent Application 103 58 171.5, filed Dec. 12, 2003; the entire disclosure of the prior application is herewith incorporated by reference.