Abstract:
An apparatus for turning a sheet during transport through a printing press provides better printed results as a result of improved sheet guidance. A transfer drum has gripper systems disposed in channels for holding the sheet at the leading and trailing sheet edge, and guide elements covering the channels.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority, under 35 U.S.C. §119, of German Patent Application DE 10 2006 053 131.0, filed Nov. 10, 2006; the prior application is herewith incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an apparatus for turning a sheet during transport through a printing press. The apparatus includes a first transfer drum having gripper systems disposed in channels for holding the sheet at the leading and trailing sheet edge, a second transfer drum having a gripper system for accepting the sheet from the first transfer drum at the trailing sheet edge, and at least one drive for synchronous revolution of the transfer drums. 
     In order to keep down the production costs of a multicolor sheet-fed printing press, printing units are constructed to print on only one side of a sheet and the printing units are fabricated with a high degree of part repetition. If, in the case of a sheet-fed printing press having an inline configuration of the printing units, the intention is to print on both sides of the sheet, a turning apparatus for the sheets is provided between the last printing unit for printing on the front side and a following printing unit for printing on the rear side. Conventional turning apparatuses include a transfer drum, a storage drum and a turning drum between impression cylinders of the printing units. The transfer drum has a gripper system for holding a sheet at the front edge. The storage drum is implemented with a diameter twice as large as the impression cylinders and has two gripper systems for holding the sheet at the front edge and two suction gripper systems for holding the sheet at the rear edge. The turning drum has a tongs-type gripper system for holding the sheet edge trailing on the storage drum. All of the sheet-carrying cylinders are driven so as to revolve synchronously, for example by a gear mechanism. 
     The gripper systems of the storage drum and of the turning drum each include individual grippers disposed along a straight gripper closing line. The gripper closing lines in each case lie parallel to the axis of rotation of the storage drum and the turning drum. As is seen in the axial direction of the storage drum and of the turning drum, gripper closing lines of the turning drum and of the storage drum during a sheet transfer form a common tangent line which runs through a center line that passes through the axes of rotation. 
     In order to turn a sheet, it is transferred from the gripper system of the transfer drum to a gripper system for the front sheet edge of the storage drum. During the rotation on the storage drum, the rear sheet edge is held by an associated suction gripper system of the storage drum. The sheet is led past the tangent line by the leading edge gripper system of the storage drum. Once the rear edge of the sheet reaches the tangent line, a transfer is made to the tongs-type grippers of the turning drum. During the further revolution of the drums, the sheet is peeled off the circumferential surface of the storage drum and the suction action of the suction grippers is cancelled. The vacuum of the suction grippers is led to the storage drum through a rotary leadthrough. If the leading-edge gripper system of the storage drum opens at a predefined angle of rotation, a free flight phase of the sheet begins. The sheet is then held only at one sheet edge by the tongs-type grippers of the turning drum until it is transferred to the gripper system of the impression cylinder disposed downstream. 
     During the detachment from the storage drum, at the edge on which the tongs-type grippers act, the sheet has a different speed from the freely flying new rear edge. Before leaving the storage drum, the future rear edge of the sheet is moved at the orbital velocity of the storage drum. The speed of the future rear edge subsequently decreases to zero. The direction of movement of the rear edge then changes from zero to an orbital velocity which results from the rotation of the tongs-type grippers of the turning drum. 
     Due to the speed changes, the sheet is moved out of its ideal path as a result of dynamic forces and other external interfering forces. The storage drum does not have an ideal aerodynamic external contour because of channels with the gripper components located therein. The sheet guided forcibly at the front edge by the tongs-type grippers of the turning drum and carried freely at the rear edge tends to form waves and, as a result of flapping effects, to collide with components of the storage drum and guide elements, which leads to undesired printed results. 
     In order to avoid a collision between a sheet and components of a storage drum, in a turning apparatus according to German Published, Non-Prosecuted Patent Application DE 199 49 412 A1, corresponding to U.S. Pat. Nos. 6,722,276 and 7,207,265, the storage drum is constructed with a recess that is open at the edge. Air is introduced through the recess in a vacuum region, which results in a space between the storage drum, the turning drum and the sheet during the detachment. 
     A sheet transport drum described in German Published, Non-Prosecuted Patent Application DE 101 02 226 A1, corresponding to U.S. Pat. No. 6,659,456, includes comb segments which can be rotated relative to one another in order to set them to a format length. Interspaces between prongs of the comb segments are provided with a covering for aerodynamic reasons. Channel regions of the drum are kept open for a passage of gripper systems of adjacent drums. 
     BRIEF SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide an apparatus for turning a sheet during transport through a printing press, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known apparatuses of this general type and which permits better printing results as a result of improved sheet guidance. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for turning a sheet during transport through a printing press. The apparatus comprises a first transfer drum having gripper systems disposed in channels for holding the sheet at leading and trailing sheet edges. A second transfer drum has a gripper system for accepting the sheet from the first transfer drum at the trailing sheet edge. At least one drive is provided for the synchronous revolution of the transfer drums. Guide elements cover the channels. 
     According to the invention, a transfer drum has gripper systems disposed in channels and guide elements thereon partly or completely covering the channels. Therefore, the aerodynamics of the transfer drum are improved in such a way that a sheet accepted from an adjacent drum does not enter into a channel of the transfer drum, so that there is no collision between the sheet and grippers on the transfer drum. 
     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 an apparatus for turning a sheet during transport through a printing press, 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 SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a diagrammatic, cross-sectional view of two printing units and a turning apparatus of a sheet-fed press; 
         FIG. 2  is an enlarged, cross-sectional view of a storage drum having guide devices fixed on a gripper side; 
         FIG. 3  is a cross-sectional view of a storage drum having guide devices fixed on a sucker side; 
         FIG. 4  is a cross-sectional view of a storage drum having guide devices fixed on the gripper and sucker sides; 
         FIG. 5  is a cross-sectional view of a storage drum having pneumatic guide devices; 
         FIG. 6  is a cross-sectional view of a storage drum having folding guide devices; 
         FIG. 7  is a cross-sectional view of a storage drum having guide elements that can be pushed into one another in circumferential direction; and 
         FIG. 8  is a cross-sectional view of a storage drum having guide devices standing centrally in channels. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures of the drawings in detail and first, particularly, to  FIG. 1  thereof, there are seen two printing units  1 ,  2  of a sheet-fed press, in each case having an inking unit  3 ,  4 , a damping unit  5 ,  6 , a plate cylinder  7 ,  8 , a transfer cylinder  9 ,  10  and an impression cylinder  11 ,  12 . A turning apparatus, which is disposed between the printing units  1 ,  2 , has a transfer drum  13 , a storage drum  14  and a turning drum  15 . All of these cylinders and drums are coupled to a gear train or drive, represented by arrows, for synchronous revolution in the direction of the arrows. The impression cylinders  11 ,  12 , the transfer drum  13 , the storage drum  14  and the turning drum  15  have gripper systems  16 - 21  for holding a sheet  22  at the leading sheet edge. The storage drum  14  additionally has suction gripper systems  23 ,  24  for holding a sheet  22  at the trailing sheet edge. Pneumatic sheet guides  25 - 27 , from which blown air  28  emerges and vacuum  29  enters, are disposed along the conveying or transport path of the sheet  22  between the storage drum  14  and the turning drum  15 . The drums  14  and  15  may be described as first and second transfer drums, with regard to the transfer of the sheets  22 . 
     As is seen in more detail in  FIG. 2 , the gripper systems  19 ,  20  and the suction gripper systems  23 ,  24  are disposed in channels  30 ,  31  in the storage drum  14 . The storage drum  14  is constructed with a diameter twice as large as the impression cylinders  11 ,  12 . The storage drum  14  has two circumferential surface segments  32 ,  33  with a circumferential length l that can be set to the length of the sheets  22  to be conveyed. During the setting of the circumferential surface segments  32 ,  33 , the gripper systems  19 ,  23  and  20 ,  24  are likewise set to the sheet length. Depending on the sheet length, the channels  30 ,  31  extend over a resultant circumferential length k. The circumferential surface segments  32 ,  33  each include two groups of curved holders  34 . 1 ,  34 . 2 ,  35 . 1 ,  35 . 2  which, as seen in axial direction of the storage drum  14 , interengage in the manner of combs and have sheet support surfaces with suction grooves. The gripper systems  19 ,  20  each include a large number of pivotable gripper fingers  38 ,  39  which are disposed in a row on a gripper shaft  36 ,  37  and which interact with gripper pads  40 ,  41 . The mountings of the gripper shafts  36 ,  37  and the gripper pads  40 ,  41  are in each case structurally connected to the holders  34 . 1 ,  35 . 1 . The suction gripper systems  23 ,  24  each include a large number of suckers disposed in a row. Suction lines  42 ,  43  lead from the suckers, through a rotary leadthrough on one journal  44  of the storage drum  14 , to a stationary vacuum source. The suction gripper systems  23 ,  24  are structurally connected to the holders  34 . 2 ,  35 . 2 . 
     In addition to the bearings for the gripper shafts  36 ,  37  and the gripper pads  40 ,  41 , guide plates  45 ,  46  are fixed to the holders  34 . 1 ,  35 . 1 . The guide plates  45 ,  46  form guide surfaces  47 ,  48 , which are inclined relative to a contour of revolution  49  of the circumferential surface segments  32 ,  33 . The guide surfaces  47 ,  48  rise gently from the interior of the channels  33 ,  31 , in each case in the direction of the gripper systems  19 ,  20 . The radially outermost ends of the guide surfaces  47 ,  48  are located at the level of the contour of revolution  49 . The guide plates  45 ,  46  are connected to the holder  34 . 1 ,  35 . 1  through webs  50 ,  51  and bars  52 ,  53 . As a sheet  22  is pulled off the sheet supporting surfaces of the holders  34  or  35  by the tongs-type gripper system  21  of the turning drum  15 , the respective guide surface  47 ,  48  prevents the end of the sheet from entering into the channel  30  or  31 . 
       FIG. 3  shows guide plates  54 ,  55  which are each fixed to a bar  56 ,  57 , connected to the holders  34 . 2 ,  35 . 2 . The guide surfaces  47 ,  48  formed with the guide plates  54 ,  55  likewise rise from the interior of the channels  30 ,  31  to the level of the contour of revolution  49 . 
     In a variant according to  FIG. 4 , guide devices each including a pair of interacting guide plates  56 . 1 ,  56 . 2  and  57 . 1 ,  57 . 2  are provided. The guide plates  56 . 1 ,  56 . 2  and  57 . 1 ,  57 . 2  are fixed to the holders  35 . 2 ,  34 . 1  and  34 . 2 ,  35 . 1  by respective bars  58 - 61 . If, during a format setting, the holders  34 . 1 ,  34 . 2  and  35 . 1 ,  35 . 2  are rotated toward each other in the circumferential direction of the storage drum  14 , then the guide plates  56 . 1 ,  56 . 2  and  57 . 1 ,  57 . 2  are pushed toward one another, so that a channel area covered by the guide plates  56 . 1 ,  56 . 2 ;  57 . 1 ,  57 . 2  is adapted in a manner corresponding to the format of the sheet  22 . The surfaces of the guide plates  56 . 1 ,  56 . 2 ;  57 . 1 ,  57 . 2  form the guide surfaces  47 ,  48  for a sheet  22  moving away from the storage drum  14 . 
       FIG. 5  shows a variant having guide devices which in each case have a guide plate  62 ,  63  with air passage openings  64 . An air chamber  65 ,  66  is disposed underneath the guide plates  62 ,  63  in each case. The guide devices are fixed to the holders  34 . 1 ,  35 . 1  by webs  67 ,  68  and bars  69 ,  70 . Pneumatic lines  71 ,  72  lead from the air chambers  65 ,  66  to a stationary vacuum and/or blown air source. The air passage openings  64  in each case open in a guide surface  47 ,  48  predefined by the guide plates  62 ,  63  which, as described in relation to  FIGS. 2-4 , lies at an angle with respect to the contour of revolution  49 . 
       FIG. 6  shows guide devices which each include two guide plates  73 . 1 ,  73 . 2 ;  74 . 1 ,  74 . 2  connected to each other in an articulated manner. During the transfer of the sheet  22  to the turning drum  15 , the tongs-type grippers of the tongs-type gripper system  21  in each case move into an interspace between the guide plate  73 . 1 ,  74 . 1  and the suction gripper system  24  or  23 . If the turning apparatus is to be changed over in order to print on only one side, a clearance between the guide plate  73 . 1 ,  74 . 1  and the gripper system  19  or  20  must be kept free. Therefore, in recto printing operation, the guide plates  73 . 2  and  74 . 2  are folded away about joints  75 ,  76  into the interior of the respective channel  30 ,  31 . 
     In a variant according to  FIG. 7 , guide devices each having two guide plates  77 . 1 ,  77 . 2  and  78 . 1 ,  78 . 2  that can be pushed together telescopically are provided. In a way similar to the embodiment according to  FIG. 6 , clearances are created, in which the guide plate  77 . 1 ,  78 . 1  is pushed under the guide plate  77 . 2 ,  78 . 2  during recto printing operation. 
       FIG. 8  illustrates a variant in which guide plates  79 ,  80  are kept approximately centrally in the respective channel  30 ,  31  irrespective of the format setting of the holders  34 . 1 ,  34 . 2 ,  35 . 1 ,  35 . 2  in the direction of revolution of the storage drum  14 . The guide plates  79 ,  80  have webs  81 ,  82  each having a slot  83 ,  84 . Joint pins  85 - 88  of a jointed quadrilateral linkage  89 ,  90  are seated in the slots  83 ,  84 . Joint pins  91 - 94  disposed outside the slots  83 ,  84  are connected in an articulated manner to bars  95 - 98 , which are fixed to the holders  34 . 1 ,  34 . 2 ,  35 . 1 ,  35 . 2 . If the holders  34 . 1 ,  34 . 2 ,  35 . 1 ,  35 . 2  are adjusted in the circumferential direction for the purpose of format setting, the joint pins  85 - 88  move into the slots  83 ,  84  and the guide plates  79 ,  80  maintain their circumferential position in the channels  30 ,  31 .