Abstract:
A printing press for printing on both sides of sheets is developed with low expenditure on costs and material, makes it possible to print on both sides of sheets in one pass and which has a small overall size. The printing press for printing on both sides of sheets, has at least one printing unit, in which in each case at least one form cylinder and at least one sheet-guiding impression cylinder are provided. A feeder feeds sheets individually one after another to the printing unit. A delivery guides the sheets away from the printing unit, and grippers convey the sheets individually one after another along a path which runs back and forth, the paths crossing one another.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority, under 35 U.S.C. §119, of German application DE 10 2008 052 181.7, filed Oct. 17, 2008; the prior application is herewith incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a printing press for printing on both sides of sheets. The printing press has at least one printing unit, in which in each case at least one form cylinder and at least one sheet-guiding impression cylinder are provided. A feeder feeds sheets individually one after another to the printing unit. A delivery guides the sheets away from the printing unit, and grippers convey the sheets individually one after another along a path which runs back and forth. 
     Conventional sheet-fed printing presses contain a plurality of printing units of the same type which are arranged horizontally next to one another in a row. Here, sheets are separated from a stack in a feeder and are fed to a first recto printing unit. In the sheet run, a last recto printing unit is followed by a turner apparatus and a row of verso printing units. A delivery, in which the finished sheets are conveyed onto a stack, is arranged after a last verso printing unit. A turner apparatus contains a multiplicity of machine elements which work and tumble or turn over a sheet. The turner apparatus increases the overall size of the printing press. A printing press according to published, non-prosecuted German patent application DE 26 39 900 A1 contains a module having a turner apparatus and a dryer. The length of the machine is increased by the module length. 
     Furthermore, printing presses are known which make it possible to print sheets on both sides without special turner apparatuses. 
     In a sheet-fed printing press according to published, non-prosecuted German patent application DE 197 56 796 A1, the sheets are printed in two passes, a feeder stack and a delivery stack being arranged on one and the same side of the machine. The machine time is more than doubled by the sheets being fed through twice. 
     Published, non-prosecuted German patent application DE 197 56 990 A1 describes a double-sided printing press, in which there are two ink transport paths in each printing unit. A rubber-covered cylinder of a double printing unit is configured as a sheet-guiding cylinder. The feeder and delivery are situated on different sides of the printing press. The ink transport paths are configured differently vertically downward and upward, in each case according to the ink flow. The printing zones are formed in each case between two rubber-covered cylinders. 
     Japanese patent application JP 11-300924 A discloses a sheet-fed printing press having a feeder, a plurality of double printing units of inline construction, a turner apparatus and a delivery. The feeder and delivery lie on the same side of the printing press. The double printing units contain in each case one impression cylinder which lies in one horizontal plane with an adjacent impression cylinder. The sheets run along a first path from the feeder to a turner apparatus and along a separate second path from the turner apparatus back to the delivery. The turner apparatus works and tumbles a sheet by way of three drums. The turner apparatus increases the overall size of the machine. The inking and dampening units which are provided below the plane differ from those above the plane on account of the vertical ink flow direction. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a printing press for printing on both sides of sheets which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which has low expenditure on costs and material, makes printing possible on both sides of sheets in one pass, and has a low overall size. 
     With the foregoing and other objects in view there is provided, in accordance with the invention a printing press for printing on both sides of sheets. The printing press has at least one printing unit with in each case at least one form cylinder and at least one sheet-guiding impression cylinder. A feeder feeds the sheets individually one after another to the printing unit. A delivery guides the sheets away from the printing unit and grippers convey the sheets individually one after another along paths which run back and forth and cross one another. 
     According to the invention, the sheets run through paths which cross one another. The movement direction of the sheets is reversed at the end of an outward path, at least one sheet-guiding cylinder guiding a sheet both in the outward path and in the return path. 
     In one preferred variant, the printing press contains at least one printing unit having an impression cylinder, the effective diameter of which is at least twice as large as a form cylinder which is assigned to the impression cylinder. One sheet is fed in with every revolution of the impression cylinder. This produces gaps between the sheets which run through, through which gaps the returning sheets are conveyed. A reflector drum is situated at the end of the outward path, which reflector drum causes a sheet at the last impression cylinder of the outward path to be moved by the circumferential face in a first semicylindrical sector into the circumferential face of the second semicylindrical sector. Only half of the impression cylinders interacts with an adjacent transfer cylinder. The respective second half acts as a pure sheet-guiding half without printing function. This prevents printing ink being transferred onto the sheet-guiding half if no sheet is being conveyed on the half. The double-sized impression cylinders have in each case two diametric gripper systems, the grippers of the first gripper system being used only for conveying in the outward path and the grippers of the second gripper system being used only for conveying in the return path. If a plurality of printing units are arranged next to one another, the printing units for printing on the front side and the rear side of the sheets are arranged alternately next to one another. 
     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 printing press for printing on both sides of sheets, 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 
         FIGS. 1A-1D  are diagrammatic side views of a printing press in four printing phases according to the invention; 
         FIG. 2  is an illustration of an outward path and a return path; 
         FIG. 3  is a diagram of the printing press having printing units which are disposed vertically above one another and a separate reflector drum; 
         FIG. 4  is a diagram of the printing press having printing units which are disposed vertically above one another without a separate reflector drum; and 
         FIG. 5  is a diagram of the printing press with a flexible arrangement of feeder and delivery. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures of the drawing in detail and first, particularly, to  FIGS. 1A-1D  thereof, there is shown an offset printing press, by way of which it is possible to print sheets in four colors on both sides in one pass. The printing press contains four recto printing units S 1 -S 4  and four verso printing units W 1 -W 4 . The recto printing units S 1 -S 4  in each case contain a form cylinder  1 - 4 , a transfer cylinder  5 - 8  and an impression cylinder  9 - 12 . The verso printing units W 1 -W 4  in each case contain a form cylinder  13 - 16 , a transfer cylinder  17 - 20  and an impression cylinder  21 - 24 . Each printing unit S 1 -S 4 , W 1 -W 4  contains, furthermore, an inking unit and a dampening unit which are assigned in each case to the form cylinders  1 - 4 ,  13 - 16 . The form cylinders  1 - 4 ,  13 - 16  in each case carry a wet offset printing form. An elastic cover is pulled in each case onto the transfer cylinders  5 - 8 ,  17 - 20 , by way of which elastic cover printing ink is transferred from the printing forms onto a sheet which is being conveyed on the respective impression cylinder  9 - 12 ,  21 - 24 . Each impression cylinder  9 - 12 ,  21 - 24  has two semicylindrical sectors  25 - 40 . The hatched sectors  25 ,  27 ,  29 ,  31 ,  33 ,  35 ,  37 ,  39  have a circumferential face which carries a sheet. The impression cylinders  9 - 12 ,  21 - 24  do not have a surface which builds up a pressure in the sectors  26 ,  28 ,  30 ,  32 ,  34 ,  36 ,  38 ,  40  which are not hatched. Gripper systems for holding a sheet at the leading edge are situated diametrally in all the sectors  25 - 40 . The impression cylinder  9  is assigned a transfer drum  41  which likewise has diametral gripper systems. The impression cylinder  21  is assigned a reflector drum  42 , on which a gripper system is arranged. The active diameters of the impression cylinders  9 - 12 ,  21 - 24  and of the transfer drum  41  are twice as large as the diameters of the form cylinders  1 - 4 ,  13 - 16 , of the transfer cylinders  5 - 8 ,  17 - 20  and of the reflector drum  42 . All the cylinders  1 - 24  and drums  41 ,  42  are mounted rotatably in side walls of the printing press and can be driven synchronously in the direction of the arrows  43  by gearwheel mechanisms and/or individual drives. The rotational axes  44 - 53  of the impression cylinders  9 - 12 ,  21 - 24  and of the drums  41 ,  42  lie in a horizontal plane  54 . The grippers of the gripper systems of the impression cylinders  9 - 12 ,  21 - 24  and of the drums  41 ,  42  run on circular paths which are tangent in transfer lines  55 - 63 . Transfer lines  64 ,  65  are situated below and above an axis  52  of the transfer drum  41 , in which transfer lines  64 ,  65  a sheet is transferred from a feeder  66  to a gripper system of the transfer drum  41  or from a gripper system of the transfer drum  41  to a delivery  67 . 
       FIGS. 1A-1D  show the printing press in four phases, in which in each case the impression cylinders  9 - 12 ,  21 - 24  and the transfer drum  41  are rotated further by 90°. The numbers on both sides of sheets  68  denote in each case the number of color separations which are printed over one another on the respective side. 
     According to  FIG. 1A , an unprinted sheet  68 . 1  is separated from a stack in the feeder and fed to a gripper system of the transfer drum  41 . Further preceding sheets  68 . 2 - 68 . 5  are conveyed on the impression cylinders  24 ,  23 ,  22 ,  21  in the sectors  40 ,  38 ,  36 ,  34 . As viewed from the location of the feeder  66  and delivery  67 , the sheets  68 . 1 - 68 . 5  are conveyed in a first direction  69  to the reflector drum  42 . At the same time, sheets  68 . 6 - 68 . 10  run back to the delivery  67  in the opposite direction  70 . The front edge of the sheet  68 . 10  is situated in the transfer line  65 . 
     On the way from the feeder  66  to the reflector drum  42  and back to the delivery  67 , the sheets  68  run through two different paths  71 ,  72  which, as shown in  FIG. 2 , cross one another in the transfer lines  55 - 63 . In the transfer line  63 , in each case one sheet  68  in the outward path  71  is received by the reflector drum  42  and guided around in a circular path. Here, the sheet  68  runs through a reversing point  73 . After the reversing point  73 , the sheets  68  are conveyed in the return path  72  which is illustrated by way of a dashed line. In the outward path  71 , the sheets  68  receive prints on the front side in the printing units S 1 -S 4 . In the return path  72 , the sheets  68  are printed on the rear side in the printing units W 1 -W 4 . Before being transported back by way of the impression cylinder  21 , the prints on the front side can be dried by way of at least one dryer. In the paths  71 ,  72 , the sheets  68  are conveyed on circular tracks which result from the movement of the gripper systems. In the transfer lines  55 - 63 , sheet transfers take place in each case from an impression cylinder to an adjacent impression cylinder or to a drum. 
       FIG. 1B  shows a phase in which the sheet  68 . 1  is transferred from the transfer drum  41  to the impression cylinder  9 . In each case in the outward path  71 , the sheets  68 . 2 - 68 . 4  are transferred from the impression cylinders  24 ,  23 ,  22  to the adjacent impression cylinders  10 ,  11 ,  12 . In the transfer line  63 , the sheet  68 . 5  passes from the impression cylinder  21  onto the reflector drum  42 . In the return path  72 , the sheets  68 . 6 - 68 . 9  are transferred from the impression cylinders  21 ,  22 ,  23 ,  24  to the adjacent impression cylinders  12 ,  11 ,  10 ,  9 . The sheet  68 . 10  leaves the printing press and is conveyed onto a stack by the delivery  67 . 
     A further 90° rotation of the impression cylinders  9 - 12 ,  21 - 24  results in the phase position which is shown in  FIG. 1C . Accordingly, the sheets  68 . 1 - 68 . 4  are printed on the front side in the outward path  71  in each case in the nip between the impression cylinders  9 - 12  and the transfer cylinders  5 - 8 . The sheet  68 . 5  is guided around on the reflector drum  42 . In the return path  72 , the sheets  68 . 6 - 68 . 9  are conveyed in the non-printing sectors  32 ,  30 ,  28 ,  26  of the impression cylinders  12 ,  11 ,  10 ,  9 . 
     In accordance with the phase position according to  FIG. 1D , the sheets  68 . 1 - 68 . 4  in the outward path  71  are transferred in the transfer lines  56 ,  58 ,  60 ,  62  from the impression cylinders  9 - 12  to the impression cylinders  24 ,  23 ,  22 ,  21 . The sheet  68 . 5  is guided in the return path  72  by the reflector drum  42 . Furthermore, in the return path  72 , the sheets  68 . 6 - 68 . 9  are transferred from the impression cylinders  12 ,  11 ,  10 ,  9  to the respectively adjacent impression cylinder  22 - 25  or to the transfer drum  41 . 
     The invention is not restricted to the cylinder configuration which is described in  FIGS. 1A-1D . For example, quadruple-sized impression cylinders can be provided instead of the double-sized impression cylinders  9 - 12 ,  21 - 24 . Furthermore, the transfer drums can be provided behind every second impression cylinder, in order to obtain greater clearance for service purposes between the printing units. 
       FIG. 3  shows one exemplary embodiment having the recto printing units S 1 -S 4  and the verso printing units W 1 -W 4  which are arranged vertically above one another. Each printing unit S 1 -S 4 , W 1 -W 4  contains, inter alia, a form cylinder  74 - 81  and a transfer cylinder  82 - 89 . The printing units S 1 , S 4 ; W 1 , W 4 ; S 2 , S 3  and W 2 , W 3  have common impression cylinders  90 - 93 . The impression cylinder  90  is assigned a transfer drum  94 . The impression cylinder  93  is assigned a reflector drum  95 . The form cylinders  74 - 81 , the transfer cylinders  82 - 89 , the impression cylinders  90 - 93  and the transfer drum  94  are configured with twice as large a diameter as the reflector drum  95 . The form cylinders  74 - 81  carry a printing form of a color separation. The transfer cylinders  82 - 89  are provided with elastic covers. In each case one gripper system for holding a sheet at the leading edge is arranged on the impression cylinders  90 - 93 . All the cylinders  74 - 93  and drums  94 ,  95  are mounted in side walls of the printing press and can be driven in the direction of the arrows  96  by gearwheel mechanisms and/or individual drives. The rotational axes  97 - 102  of the transfer drum  94 , the impression cylinders  90 - 93  and the reflector drum  95  lie in a vertical plane  103 . The cylinders  74 - 89  of the printing units S 1 -S 4 , W 1 -W 4  lie symmetrically with respect to the plane  103 . A feeder  104  and a delivery  105  are arranged below the transfer drum  94 . The transfer drum  94 , the impression cylinders  90 - 93  and the reflector drum  95  are equipped with gripper systems which make it possible to transfer sheets in each case in a transfer line  106 - 111 . 
     During printing, sheets  112  are separated in the feeder  104  and are fed one after another to the transfer drum  94 . In the transfer line  106 , in each case one sheet  112  is received by a gripper system of the transfer drum  94 . Starting from the transfer line  106 , the sheets  112  are conveyed along a first path in the direction of the reflector drum  95 . The sheets  112  are transferred one after another from the transfer drum  94  to the impression cylinders  90 - 93 . The first path is illustrated symbolically by solid arrows  113  in the transfer drum  94 , the impression cylinders  90 - 93  and the reflector drum  95 . At the end of the first path, a sheet  112  reaches a reversing point  114 . The sheets  112  are conveyed back in the direction of the transfer line  106  on a second path by the reflector drum  94 . The second path is illustrated symbolically by dashed arrows  115 . Both paths contain semicircles which are strung together and lie symmetrically with respect to the plane  103 . The paths cross one another in the transfer lines  106 - 111 . At the end of the return path, the sheets  112  are transferred to a gripper system of the delivery  105  in the transfer line  106 . In the delivery  105 , the sheets  112  are deposited onto a stack  116 . 
     A printing press according to  FIG. 4  contains four recto printing units S 1 -S 4  and four verso printing units W 1 -W 4 . Each printing unit S 1 -S 4 , W 1 -W 4  contains an engraved roller  117  of a short inking unit, an ink applicator roller  118 , a form cylinder  119  and a transfer cylinder  120 . The transfer cylinders  120  of the printing units S 1  and S 4 , W 1  and W 4 , S 2  and S 3 , and W 2  and W 3  interact with common impression cylinders  121 - 124 . The rotational axes  125 - 128  of the impression cylinders  121 - 124  lie in a vertical plane  129 . The diameters of the impression cylinders  121 - 123  are twice as large as the diameters of the impression cylinder  124 , the form cylinder  119  and the transfer cylinder  120 . The impression cylinder  121  is assigned a feeder  130  and a delivery  131 . There are transfer lines  132 - 135  below the impression cylinder  121  and between the impression cylinders  121 ,  122 ;  122 ,  123 ;  123 ,  124 . 
     During printing, the sheets  136  are separated from a stack  137  and are transferred to the impression cylinder  121  in the transfer line  132 . Starting from the transfer line  132 , the sheets  136  run through an outward path and a return path in the manner of a slalom. The outward path is illustrated symbolically by way of solid arrows  138 . The outward path ends at a reversing point  139  in the track course on the impression cylinder  124 . The impression cylinder  124  acts as a reflector drum. The sheets  136  are guided around on the impression cylinder  124  from the outward path into the return path. The return path is illustrated by way of dashed arrows  140 . The outward path and return path cross one another in the transfer lines  132 - 135  and lie symmetrically with respect to the plane  129 . At the end of the return path, the sheets  136  are transferred to a gripper system of the delivery  131  in the transfer line  132  and are deposited onto a stack  141 . 
       FIG. 5  shows a further variant having recto printing units S 1 -S 4  and verso printing units W 1 -W 4  which are arranged vertically above one another. The printing units S 1 -S 4 , W 1 -W 4  contain in each case, inter alia, a form cylinder  142  and a transfer cylinder  143 . The printing units  51 , S 4 ; W 1 , W 2 ; S 2 , S 3  and W 3 , W 4  interact in each case with common impression cylinders  144 - 147 . The rotational axes  148 - 151  of the impression cylinders  144 - 147  lie in a vertical plane  148 . There are transfer lines  152 - 154  between the impression cylinders  144 ,  145 ;  145 ,  146  and  144 ,  147 . During printing, the sheets are fed individually one after another from a feeder to the impression cylinder  144 . The sheets run through a first path with the transfer lines  154 ,  152 ,  153  and receive prints one after another in the printing units S 1 , W 1 , S 2 . The impression cylinder  146  acts as a reflector drum. From a reversing point  155 , the sheets run back to the transfer line  154  in a second path. The first and second paths are illustrated symbolically by solid and dashed arrows  156 ,  157 . In the second path, the sheets receive prints one after another in the printing units S 3 , W 2  and S 4 . The zigzag-shaped paths cross one another in the transfer lines  152 - 154 . In the transfer line  154 , the sheets are transferred further to the impression cylinder  147 . During the conveying on the impression cylinder  147 , the sheets receive prints one after another in the printing units W 3  and W 4 . After the application of the final print on the rear side of the sheets in the printing unit W 4 , the sheets pass into the transfer line  154  again and are transferred to a gripper system of a delivery. The variant according to  FIG. 5  shows that, in the context of the invention, the sheets can be fed in and guided away at any desired location of the printing press. In every case, the sheets run through at least one outward and one return slaloming path, which paths cross one another.