Abstract:
A method and an apparatus for turning a sheet during its transport through a printing press permit improved printing results as a result of improved sheet guidance. A sheet is tautened following a transfer from a first transport device to a second transport device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the priority, under 35 U.S.C. §119, of German Patent Application DE 10 2006 052 126.9, filed Nov. 6, 2006; the prior application is herewith incorporated by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention relates to a method for turning a sheet during its transport through a printing press, in which a leading edge of the sheet is moved on a first transport device along a desired path and a trailing edge of the sheet is transferred to a second transport device. The invention also relates to an apparatus for turning a sheet during its transport through a printing press, including a first transfer drum having gripper systems for holding the sheet at the leading and trailing edges, each gripper system at the trailing sheet edge including a row of suction grippers placed at intervals and connected to a controllable vacuum source, 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 the synchronous revolution of the transfer drums. 
   In order to keep production costs of a multicolor sheet-fed printing press low, printing units thereof are constructed to print on only one side of a sheet and the printing units are fabricated with a high level of repetition. If, in a sheet-fed printing press having an inline configuration of the printing units, printing is to be carried out on both sides of the sheets, 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 wheel 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 or turning drum. As is seen in the axial direction of the storage drum and of the turning drum, the gripper closing lines of the turning drum and of the storage drum during a sheet transfer form a common tangent line, through which there runs a center line which goes 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 to the tongs-type grippers of the turning drum is carried out. During the further revolution of the drums, the sheet is peeled off the circumferential surface of the storage drum, with the suction action of the suction grippers being stopped. The vacuum of the suction grippers is led to the storage drum through a rotary leadthrough, as is described in German Published, Non-Prosecuted Patent Application DE 42 10 009 A1, corresponding to U.S. Pat. No. 5,439,029, for example. The vacuum is maintained only in a rotational angle range of the storage drum between the transfer of the rear edge from the transfer drum to the turning drum. If the leading edge gripper system of the storage drum opens at a predefined rotational angle, a free flight phase of the sheet begins. The sheet is 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. 
   Since it is separated from the storage drum, at the edge at which the tongs-type grippers act, the sheet has a different speed than that of the free-flying new rear edge. Before leaving the storage drum, the future rear edge of the sheet is moved at the path speed 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 a path speed 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 sheet guided freely at the rear edge tends to form waves and, as a result of whipping effects, to collide with components of the storage drum and guide elements, which leads to undesired printing results. 
   In order to improve separation of a sheet from the circumferential surface of a storage drum, in the case of a turning apparatus according to German Published, Non-Prosecuted Patent Application DE 199 49 412 A1, corresponding to U.S. Pat. No. 6,722,276, the storage drum is constructed with a recess that is open at one edge. Air is introduced through the recess in a vacuum region which, during the separation, forms in a space between the storage drum, the turning drum and the sheet. The control of the vacuum on the rear edge of the sheet is not affected. 
   BRIEF SUMMARY OF THE INVENTION 
   It is accordingly an object of the invention to provide a method and an apparatus for turning a sheet during its transport through a printing press, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and apparatuses of this general type and which permit improved 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, a method for turning a sheet during its transport through a printing press. The method comprises moving a leading edge of the sheet on a first transport device along a desired path, transferring a trailing edge of the sheet to a second transport device, and tautening the sheet in a predefined rotational angle range following the transfer. 
   With the objects of the invention in view, there is concomitantly provided an apparatus for turning a sheet during its transport through a printing press. The apparatus comprises a first transfer drum having gripper systems for holding the sheet at a leading and a trailing edge. Each of the gripper systems at the trailing edge includes a row of suction grippers placed at intervals and connected to a controllable vacuum source. A second transfer drum has a gripper system for accepting the sheet at the trailing edge from the first transfer drum. At least one drive is provided for synchronous revolution of the transfer drums. A braking configuration is provided for the sheet running off the first 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 a method and an apparatus for turning a sheet during its 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, sectional view of two printing units and a turning apparatus of a sheet-fed printing press; 
       FIG. 2  is a perspective view of drums of a turning apparatus; 
       FIG. 3  is a diagrammatic view illustrating vacuum control on suction grippers; 
       FIG. 4  is a diagram illustrating vacuum control on a storage drum; and 
       FIG. 5  is an elevational view illustrating tautening of a sheet with a magnetic field. 
   

   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 printing press, each having an inking unit  3 ,  4 , a dampening unit  5 ,  6 , a plate cylinder  7 ,  8 , a transfer cylinder  9 ,  10  and an impression cylinder  11 ,  12 . A turning apparatus disposed between the printing units  1 ,  2  has a transfer drum  13 , a storage drum  14  and a turning drum  15 . All of the cylinders and drums are coupled to a gear train and revolve in directions indicated by 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 a first transport device in the from of 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  exits and vacuum  29  enters, are disposed along a transport path of the sheet  22  between the storage drum  14  and the turning drum  15 . The suction gripper systems  23 ,  24  are fed through a pneumatic rotary leadthrough  30  on a journal  31  of the storage drum  14 . A suction line  32  leads from the rotary leadthrough  30  to a suction connection of a suction blower  33 . The suction connection is also connected to a secondary air valve  34 , which can be controlled by a motor  35 . The rotational position of the storage drum  14  is registered through a rotary encoder  36  on the transfer drum  13 . The rotary encoder  36 , the motor  35  and the blower  33  are connected to a control device  37 . 
   A perspective illustration according to  FIG. 2  shows the impression cylinders  11 ,  12 , the transfer drum  13 , the storage drum  14  and the turning drum  15  in more detail. The effective diameter of the storage drum  14  is twice as large as that of the impression cylinders  11 ,  12 . The storage drum  14  contains concentrically disposed hoop-like holders  38 . 1 - 38 . 4  having sheet support surfaces located on the outside. In each case two groups of holders  38 . 1 ,  38 . 2 ;  38 . 3 ,  38 . 4  are used to support a sheet  22  and interengage in the manner of a comb. Suction grooves  39 . 1 - 39 . 4  in the sheet support surfaces are connected to the blower  33  through the rotary leadthrough  30  and run in the circumferential direction of the storage drum  14 . The gripper systems  19 ,  20  each include a large number of individual grippers  40  on a gripper shaft  41 . The individual grippers  40  are opened and closed through the use of a cam mechanism and a gear wheel mechanism. The suction gripper systems  23 ,  24  each contain a large number of suckers  42 , which act on the rear edge of a sheet  22 . The grippers of the second transport device or gripper system  21  are constructed as tongs-type grippers each having two interacting gripper fingers  43 ,  44 . During a rotation of the turning drum  15 , the gripper fingers  43 ,  44  complete a pivoting movement about a gripper shaft  45  under cam control. 
   The transfer of a sheet  22  from the storage drum  14  to the turning drum  15  will be described by using a diagrammatic illustration in  FIG. 3  and a diagram in  FIG. 4 .  FIG. 3  shows, in profile, circles of rotation  46 - 48  of the gripper system  18  of the transfer drum  13 , of the gripper systems  19 ,  20 ,  23 ,  24  of the storage drum  14 , and of the gripper system  21  of the turning drum  15 . Straight lines joining the center of the circles  46 ,  47  and  48  are transfer center lines  49 ,  50 . 
   At a rotational angle of α=70°, the suction grooves  39 . 1  of the holders  38 . 1  have a vacuum p 1  applied to them. The pressure variation in the suction grooves  39 . 1  is indicated by a curve  50 . At a rotational angle of α=75°, the leading sheet edge reaches the transfer center line  49 , on which the sheet  22  is transferred from the gripper system  18  of the transfer drum  13  to the gripper system  19  of the storage drum  14 . At a rotational angle of α=130°, the trailing suction grooves  39 . 2  of the holders  38 . 2  have the vacuum p 1  applied to them. The pressure variation in the suction grooves  39 . 2  is illustrated as a curve  51 . At a rotational angle of α=160°, the connection of the suction grooves  39 . 1  to the suction blower  33  is interrupted. At a rotational angle of α=195°, the trailing sheet edge of the sheet  22  reaches the transfer center line  49 , and the suckers  42  of the suction gripper system  23  have a vacuum p 2  applied to them. The pressure variation on the suckers  42  of the suction gripper system  23  is seen from a curve  52 . In the further course, the front sheet edge of the first sheet  22  is guided past the transfer center line  50 . At a rotational angle of α=350°, a vacuum p 2  on the suckers  42  of the suction gripper  23  is shut off and the suckers  42  are ventilated. At a rotational angle of α=360°, the rear edge of the first sheet  22  reaches the transfer center line  50  and is accepted by the gripper fingers  43 ,  44  of the tongs-type gripper system  21 , with the leading sheet edge being released from the gripper system  19 . Starting at a rotational angle of α=22°, the first sheet  22  begins to be peeled off by the holders  38 . 2  and  38 . 1 . Up to a rotational angle of α=35°, the first sheet  22  is guided only by the gripper system  21  of the turning drum  15 . Starting from a rotational angle of α=35°, the suckers  42  of the suction gripper system  23  again have a vacuum p 3  applied to them. As a result of the renewed application of the vacuum, the first sheet  22  is held by the suckers  42  of the suction gripper system  23  again, starting from a rotational angle of α=90°. The free trailing end of the first sheet  22  is tautened. The suckers  42  of the suction gripper system  23  carry out a relative movement in relation to the sheet  22  during this process. This braking phase for the trailing end runs as far as a rotational angle of α=130° following the transfer of the trailing sheet edge to the gripper system  21 . Depending on the sheet length, the suckers  42  of the suction gripper system  23  leave the rear sheet edge in a rotational angle range between α=190° and α=130°. 
   The sheet transport described above is repeated periodically. While one sheet  22  is being accepted by the gripper system  19  at the rotational angle of α=75°, a preceding sheet  22  is being transported in the same way on the opposite holders  38 . 3 ,  38 . 4  with the gripper system  20  and the suction gripper system  24 . Curves  53 ,  54  reproduce the variation of the vacuum p 1  in the suction grooves  39 . 3 ,  39 . 4  of the holders  38 . 3 ,  38 . 4 . A curve  55  illustrates the variation of the vacuum on the suckers  42  of the suction gripper system  24 . The braking phase of the sheet  22  preceding the first sheet  22  runs in a rotational angle range between α=215° and α=310°. 
     FIG. 5  shows an apparatus in which a braking configuration for a sheet  22  is fixed to a frame. The braking configuration includes a generator  58  for an alternating magnetic field and a pneumatic guide device  59  for the sheet  22 . As the sheet  22  runs past the generator  58 , a current is induced in the ferromagnetic material of the sheet  22  or the printing ink on the sheet  22 . A magnetic field originating from an eddy current counteracts the field from the generator  58 , so that the sheet  22  is braked. The braking effect can be adjusted to the sheet thickness and/or locally to the sheet  22  by changing the field strength of the field of the generator  58 . Therefore, a formation of waves in the sheet  22  and whipping effects at the trailing sheet edge can be avoided or reduced. The guide device  59  prevents the sheet  22  from touching the generator  58 .