Abstract:
Sheets are conveyed through a printing machine with improved reliability of the suction grippers. The apparatus for conveying a sheet through a printing machine has a cylinder with grippers in a channel for holding the sheet at its leading edge, circulating grippers for holding and for transferring the sheet from the cylinder at its leading edge, and circulating, radially positionable suction grippers for holding the sheet at its trailing edge. An actuating apparatus opens and closes the grippers and controls vacuum for the suction grippers. A drive for the cylinder and circulating grippers and suction grippers. A control device switches on and off the sheet feed to the cylinder. The suction grippers can be moved in the radial direction into a rest position when the sheet feed is switched off.

Description:
BACKGROUND OF THE INVENTION  
     FIELD OF THE INVENTION  
       [0001]     The invention relates to an apparatus for conveying a sheet through a printing machine. The machine has a cylinder with grippers arranged in a channel for holding the sheet at its leading edge, circulating grippers for holding and for transferring the sheet from the cylinder at its leading edge, and circulating, radially positionable suction grippers for holding the sheet at its trailing edge. There is provided an actuating apparatus for opening and closing the grippers and for controlling vacuum for the suction grippers, a drive for the cylinder and circulating grippers and suction grippers, and a control device for switching on and off the sheet feed to the cylinder.  
         [0002]     Commonly assigned U.S. Pat. No. 6,578,846 B2 and German published patent application DE 100 14 417 A1 describe an apparatus for transporting a sheet for a rotary press wherein a sheet is transferred from a cylinder to a chain gripper system. In the chain gripper system, the sheet is held by grippers at the leading edge and at the trailing edge. The trailing edge grippers pick up the sheet in the exit pocket between supporting surfaces circulating with the chain wheels and the cylinder circumferential surface. Until the transfer by the trailing edge grippers, the sheet is held by means of suction grippers, which likewise circulate with the chain wheels. The suction grippers, which are located outside the smallest format width of the sheets, can be adjusted in the direction of the axis of rotation of the suction grippers, so the adjustment of the elements containing the supporting surfaces is possible in the direction parallel to the axis of rotation, in order to be able to set the sheet support on print-free margins of the sheets. The suction grippers located in the format width of the sheets remain permanently in one radial working position. Without any sheet feed, the suction grippers come into contact with the abrasively acting cylinder circumferential surface, so that it is possible for high wear of the suction grippers containing elastomers to occur.  
       SUMMARY OF THE INVENTION  
       [0003]     It is accordingly an object of the invention to provide a device for transporting a sheet through a printing machine which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for an apparatus for conveying a sheet through a printing machine which improves the reliability of the suction grippers.  
         [0004]     With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for conveying a sheet through a printing machine, comprising:  
         [0005]     a cylinder formed with a channel and grippers disposed in the channel for holding the sheet at a leading edge thereof;  
         [0006]     circulating leading-edge grippers for holding and for transferring the sheet from the cylinder at the leading edge, and circulating, radially positionable, trailing edge suction grippers for holding the sheet at a trailing edge thereof;  
         [0007]     an actuating apparatus for opening and closing the grippers and for controlling a vacuum for the suction grippers;  
         [0008]     a drive for the cylinder and the circulating grippers and suction grippers; and  
         [0009]     a control device for selectively switching on and off a sheet feed to the cylinder;  
         [0010]     wherein the suction grippers are mounted so as to be movable in a radial direction into a rest position when the sheet feed is switched off.  
         [0011]     In other words, circulating suction grippers are moved radially inward into a rest position, or position of respite, when the sheet feed is switched off. As soon as a last sheet has been conveyed, the suction grippers are brought out of contact with the surface of the cylinder conveying the sheet. In all the suckers that are out of contact with the surface, the vacuum is switched off. Suction grippers which are outside the format are permanently set in such a way that they have no contact with the surface. The process of switching off grippers which are outside the format is carried out automatically during the normal format setting of the cylinders of the printing machine that convey the sheets.  
         [0012]     In accordance with an added feature of the invention, when the sheet feed is switched on, the suction grippers are moveable in the radial direction into a rest position in dependence on a sheet width of the sheet. That is, only those suction grippers that are used in the given print job are driven into operational status, and the other grippers are placed at rest.  
         [0013]     In accordance with another feature of the invention, there is provided:  
         [0014]     a chain conveyor with endless chains having fixed thereto the leading-edge grippers and the trailing-edge grippers;  
         [0015]     rotating chain wheels carrying the endless chains;  
         [0016]     coaxially circulating supporting elements for the sheet disposed between the chain wheels assigned to the cylinder, the supporting elements being in rolling contact with a peripheral surface of the cylinder and being movably mounted in a direction of an axis of rotation in dependence on a sheet transverse format; and  
         [0017]     wherein the suction grippers are movably disposed in the radial direction into the rest position as a result of an actuation of positioning elements for the axial displacement of the supporting elements.  
         [0018]     In accordance with again another feature of the invention, at least one valve spool is connected to the supporting elements via a pull cable, for switching off respective the suction grippers disposed laterally outside the given sheet width.  
         [0019]     In accordance with a further feature of the invention, a suction gripper includes a valve nozzle that rests against a sealing surface in the rest position of the suction gripper, to thereby suppress a vacuum feed to the suction gripper. In a preferred embodiment of the invention, there is provided a bellows that carries the vacuum.  
         [0020]     In accordance with a concomitant feature of the invention, a cam shaft and plungers are connected to the suction grippers for moving the suction grippers.  
         [0021]     Other features which are considered as characteristic for the invention are set forth in the appended claims.  
         [0022]     Although the invention is illustrated and described herein as embodied in an apparatus 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.  
         [0023]     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  
       [0024]      FIG. 1  shows a schematic drawing of a delivery of a press having a chain gripper system;  
         [0025]      FIG. 2  shows a drawing relating to the sheet transfer between an impression cylinder and the chain gripper system;  
         [0026]      FIG. 3  shows an enlarged view from  FIG. 2  with a representation of an overall space for structural elements for holding and operating a suction gripper;  
         [0027]      FIG. 4  shows functional elements for setting suckers on and off;  
         [0028]      FIG. 5  shows a three-dimensional representation of functional elements according to  FIG. 4 ;  
         [0029]      FIG. 6  shows suckers in the rest position  
         [0030]      FIG. 7  shows blocking of suckers in the rest position; and  
         [0031]      FIG. 8  shows a vacuum control system of a blocking lever having valve spools. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]     Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a last printing unit  1  of a sheet-fed press having a delivery  2  disposed downstream in a transport direction.  
         [0033]     The printing unit  1  contains a sheet transport drum  3 , an impression cylinder  4 , a transfer cylinder  5  and a plate cylinder  6 . The plate cylinder  6  is assigned rolls of an inking unit  7  and of a dampening unit  8 .  
         [0034]     The delivery  2  comprises a chain gripper system having a large number of leading edge grippers  9 . 1  to  9 . 7  and trailing edge rippers  10 . 1  to  10 . 7 , which are fixed to gripper bars. The ripper bars are connected to endless chains, which travel round chain wheels  11 ,  12  and are guided in chain tracks  13 . In the conveying direction  14 , at the end of the lower runs of the chain tracks  13 , there is a stacking table  15 , and a stack  16  supported thereon. The stacking table  15  can be lowered by a chain mechanism  17  in accordance with the progress of production.  
         [0035]     Sheets  18  are conveyed from a preceding printing unit by way of the sheet transport drum  3  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 or press nip  20  between the transfer cylinder  5  and the impression cylinder  4 . By using the dampening unit  8  and the inking unit  7 , a printing form  21  clamped on the plate cylinder  6  is inked in accordance with an image. The transfer cylinder  5  has a resilient cover  22 , with which the printing ink is transferred to the sheet  18  in the press nip  21 . In the delivery  2 , the sheets  18  are picked up at the leading edge by the leading edge grippers  9 , the trailing edge of the sheets  18  being held by the trailing edge grippers  10 . Shortly before reaching the stack  16 , the grippers  9 ,  10  are opened, so that the sheet  18  is allowed to fall onto the stack  16 . A leading edge stop  23  or front lay limits the movement in the conveying direction  14  of the sheet  18  as it is allowed to fall.  
         [0036]      FIG. 2  illustrates, in more detail, the elements for conveying the sheet  18  from the impression cylinder  4  onto the leading edge grippers  9  and the trailing edge grippers  10 . The impression cylinder  4  is a double-size cylinder formed with two channels  24 ,  25 , wherein the grippers  19  are disposed on gripper bars. The gripper bars reach over the width of a sheet  18 , which is held at its leading edge in the grippers  19 . The impression cylinder  4  has two circumferential surface segments  26 ,  27 , on which in each case a sheet  18  rests while it is being transported. The length of the circumferential surface segments  26 ,  27  in the circumferential direction corresponds to the maximum length of a sheet  18  to be conveyed. The impression cylinder  4  and the chain wheels  11  are driven synchronously in the directions of the arrows  28 ,  29 . The pitch circle diameter  30  of the chain wheels  11  and the diameter  31  of the impression cylinder  4  are approximately equal. A connecting line between the axes  32 ,  33  of the impression cylinder  4  and of the chain wheels  11  forms a transfer center line  34 . In the direction at right angles to the plane of the drawing there runs an intersection region between the circular path of the circumferential surfaces  26 ,  27  and a cylinder to which the pitch circles of the chain wheels  11  belong. The intersection region and the transfer center line  34  are perpendicular to each other. The leading edge grippers  9  and the trailing edge grippers  10  are all arranged at the format width of the gripper bars  35 ,  36 , which are fixed to separate, synchronously circulating chains. The distance between leading edge grippers  9  and trailing edge grippers  10  can be adjusted to the sheet length by shifting the phase of the chains. Between a leading edge gripper bar  35  and the trailing edge gripper bar  36 , on the axis  33  there are arranged supporting elements  37  for a sheet  18 , which circulate with the chain wheels  11 . The supporting elements  37  carry the sheet  18  at the print-free margins. For the purpose of adjustments to the sheet width, the supporting elements  37  can be displaced on the axis  33 . The number of supporting elements  37  in the circumferential direction can be varied in accordance with the sheet length. Furthermore, suction grippers which are arranged on a suction gripper bar  38  circulate with the supporting elements  37 . In relation to the rotational position, the suction gripper bar  38  is assigned around the axis  33  of the trailing edge gripper bar  36 . The suction grippers pick up the sheet  18  at the trailing edge.  
         [0037]     The leading edge gripper bar  35  dips into the channel  25  of the impression cylinder  4  as it passes the transfer center line  34 . Because of the channel  24 / 25 , there is at this point no risk of collision between the leading edge grippers  9  and the impression cylinder  4 . On the other hand, precautions have to be taken for the trailing edge grippers  10  that the trailing edge grippers  10  can pass the circumferential surfaces  26 ,  27  without contact. For this purpose, before they reach the transfer center line  34 , the trailing edge grippers  10  are opened in good time to such an extent that the trailing edge gripper bars  36  can pass without collision.  
         [0038]      FIG. 3  shows the chain wheels  11  in a rotational position before the transfer center line  34 . As a result of rotation of a gripper shaft  39  in the direction  40 , the gripper fingers  41  are pivoted back to such an extent that they lie securely under a path  42  described by the supporting surface of the supporting elements  37 . The gripper pad  43  is likewise lower in the radial direction than the path  42 . In the circumferential direction, the supporting elements  37  reach as far as a suction gripper bar  38 , so that a sheet  18  is still held by the supporting surface of the suckers  44  if the suction grippers  38  are just touching the sheet  18 .  
         [0039]     The overall space  45  assumed by the structural elements for holding and operating the suction grippers  44  is illustrated in dash-dotted form. The fact that the supporting elements  37  reach as far as the suction gripper bar  38  in the circumferential direction means that the overall space  45  is restricted.  
         [0040]     With reference to  FIGS. 4 and 5 , the following elements are contained in the overall space  45 : A rectangular, hollow crossmember  46  circulating about the axis  33  with the chain wheel  11  is covered by a holder  48 , as seen in the radial direction  47 . The holder  48  is fixed to the crossmember  46  by screws  49 . In the three-dimensional illustration according to  FIG. 5 , the crossmember  46 , the holder  48  and the screws  49  have been masked out. On the holder  48 , angled-over levers  51  are mounted in pins  50  such that they can be pivoted. The pivot axes of the levers  51  are parallel to the axis  33 . In each case at an end of the levers  51 , an angled-over sucker rod  52  is mounted in pins  53  such that it can be pivoted. The pivot axis of the sucker rod  52  is parallel to the axis  33 . The sucker rod  52  carries an elongated sucker  54 . The suckers  54  are aligned on a straight line which lies parallel to the axis  33 . In each case at the other end of the sucker rods  52  there is fixed a supporting roller  55 , whose axis of rotation is parallel to the axis  33  and which bears on an inner surface  56  of the crossmember  46 . The movement of the supporting rollers  55  and therefore the movement of the sucker rods  52  and the suckers  54  in the radial direction  47  is limited by stops  57  in the interior of the crossmember  46 . The suckers  54  are connected via vacuum holes  58  to a common vacuum channel  59 . For each sucker  54 , a bellows  60  and a compression spring  61  are provided. The bellows  60  encloses vacuum openings to the vacuum channel  59  for carrying a vacuum.  
         [0041]     The guidance of the reciprocating movement of the suckers  54  is in each case ensured via the sucker rod  52 , since the sucker rod  52  is attached in an articulated manner to the lever  51 . The fact that the sucker rod  52  is supported on the inner surface  56  by the supporting roller  55  means that tilting of the rod  52  and therefore a movement in the circumferential direction  62  is prevented. In  FIG. 4 , the suckers  54  are illustrated in the spring-extended state. As a result of the suckers  54  running against the surface of a sheet  18 , which is supported on a circumferential surface of segments  26 ,  27  of the impression cylinder  4 , the suckers  54  spring in in the radial direction  47 , counter to the force of the bellows  60  and of the compression springs  61 . As they spring in, the suckers  54  are displaced by a small amount in the circumferential direction  62 , since they also have one degree of freedom in the circumferential direction  62  as a result of the guide mechanism and the compression springs  61 .  
         [0042]     In  FIGS. 4 and 6 , the suckers  54  are respectively illustrated in a position during printing and with the sheet feed switched off. The bellows  60  permit the supply of the vacuum to the movable suckers  54 . By means of the bellows  60 , the vacuum is sealed off with respect to the surroundings. The bellows  60  remain dimensionally stable when vacuum is applied and withstand the movement cycles permanently. In order to move the suckers  54  out of the position during printing into a rest position or position of respite when the sheet feed is switched off, a cam shaft  63  is provided whose switching cams  64  interact with hammers  65 . The hammers  65  are formed as one-armed levers on the lever  51 . In the event-of rotation of the cam shaft  63  by a pneumatic cylinder  66  and a lever  67 , the hammers  65  slide on the switching cams  64 . Via the guide mechanism, the suckers  54  execute a radial reciprocating movement in order to reach the rest position. In the interior of the bellows  60  there are in each case valve nozzles  68  with a vacuum hole  69 . In the rest position, the end of the valve nozzles  66  is pressed in a sealing manner against the base  70  of a vacuum channel  59 , so that the vacuum feed to the respective sucker  54  is switched off.  
         [0043]     Suckers  54  which are outside the format must remain in the rest position both during printing and when the sheet feed is switched off. For the purpose of permanent setting of suckers  54  that are outside the format to the rest position, blocking levers  72  which project into the hollow crossmember  46  are mounted on the holder  48  on an axis  71 . In each case at the lower end of a blocking lever  72 , one side of a bellows  73  is fixed in a sealing manner. The other side of the bellows  73  is closed by a plate  74 . The plates  74  have an air passage opening  75 , which is connected to the interior of a pipe  76 . In the pipe  76  there are valve spools  77  which are symmetrical with respect to the center  2  of a sheet and are in each case connected to a pull cable  78 . Via the pull cables  78 , the synchronous and symmetrical adjustment of the valve spools  77  is coupled to the axial adjustment of the supporting elements  37  to the format width of the sheets  18 .  
         [0044]     In  FIG. 8 , the pipe  76  is illustrated as a detail with one of the valve spools  77 . The pipe  76  is closed at the side by covers  79  and seals  80 . In the covers  79  there is a passage for the pull cable  78 . The ends of the cables of the pull cables  78  are fixed to the valve spool  77  by screws  81 ,  82 . There are sealing rings  83 ,  84  on the valve spool  77 . In the pipe  76 , the valve spools  77  delimit the space where is no vacuum applied. Depending on the position of the supporting elements  37  in the direction of the axis  33 , the result is a position of the valve spools  77  in the pipe  76  such that the suckers  54  located between the valve spools  77  are in operation and all the others are switched off. If a sucker  54  is located in the region of the pipe  76  where vacuum prevails because of the position of the valve spool  77  then, as illustrated in  FIG. 7 , the bellows  73  is contracted by the vacuum. The blocking lever  72  completes a pivoting movement  85  about the axis  71 . On the blocking lever  72  there is a stop lug  86  which, when the bellows  73  is contracted, prevents the end  87  of the lever  51 , as illustrated in  FIG. 4 , being able to pivot in the radial direction  47  about the pin  50 .  
         [0045]     Therefore, the relevant suckers  54  remain in the rest position. If there is no vacuum on the bellows  73 , then in each case the blocking lever  72  is located in the position shown in  FIG. 4  as a result of the spring action of the bellows  73  and remains ineffective.  
         [0046]     This application claims the priority, under 35 U.S.C. §  119 , of German patent application No. 10 2004 042 257.5, filed Aug. 30, 2004; the entire disclosure of the prior application is herewith incorporated by reference.