Abstract:
A method and an apparatus for feeding printing plates to a plate cylinder of a printing press, load the printing plate in the vicinity of its front edge with a curvature which extends convexly with regard to the plate cylinder axis.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority, under 35 U.S.C. §119, of German Patent Application DE 10 2009 013 172.8, filed Mar. 13, 2009; the prior application is herewith incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The invention relates to a method and an apparatus for feeding printing plates to a cylinder of a printing press, in particular to a plate cylinder. 
         [0003]    During changing of a printing plate, in particular during feeding of a new or used printing plate to a plate cylinder of a printing press, the contact of the printing plate on the periphery of the plate cylinder is of great significance for the print quality. 
         [0004]    There is the problem that deformations, inhomogeneous stress distributions and unevennesses, which cannot be avoided completely in the feeding and clamping process, can be produced by the clamping operation of the printing plate onto the plate cylinder surface. As a result, the register qualities which can be achieved, in particular the reproducibility of the plate clamping operation and the quality of the multiple color overprinting, can be reduced. 
         [0005]    European Patent EP 0 570 702 B2, corresponding to U.S. Pat. No. 5,331,892, has disclosed an apparatus for feeding printing plates to a plate cylinder of a printing press having a holding apparatus for a printing plate, which is disposed pivotably on a printing unit. The plate cylinder is mounted in the printing unit and has respective clamping rails for receiving a front edge of the printing plate and a rear edge of the printing plate. 
         [0006]    As a result of a pivoting movement of a lower part of the printing plate holding apparatus, the printing plate is given a curvature which extends transversely with respect to the format width of the printing plate and is directed with its concave side toward the plate cylinder. In that case, the front edge of the printing plate is oriented parallel to the rotational axis of the plate cylinder. 
       SUMMARY OF THE INVENTION 
       [0007]    It is accordingly an object of the invention to provide a method and an apparatus for feeding a printing plate to a plate cylinder, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and apparatuses of this general type in such a way that the printing plate can be attached onto the plate cylinder without stress. 
         [0008]    With the foregoing and other objects in view there is provided, in accordance with the invention, a method for feeding printing plates to a plate cylinder of a printing press. The method comprises feeding a front edge of a printing plate to a clamping device of the plate cylinder for the front edge of the printing plate, and applying a curvature extending convexly relative to an axis of the plate cylinder to the printing plate in vicinity of the front edge of the printing plate before closing the clamping device for the front edge of the printing plate. 
         [0009]    With the objects of the invention in view, there is also provided an apparatus for feeding a printing plate to a plate cylinder of a printing press. The apparatus comprises a pivotably disposed guiding device having a first guiding region shaped convexly relative to an axis of the plate cylinder. 
         [0010]    It is particularly advantageous if the front edge of the printing plate is fed to the plate cylinder with a convex curvature which extends transversely with respect to the feeding direction of the printing plate, in which the side edges of the printing plate are close to the plate cylinder while the center of the printing plate is spaced apart somewhat more from the plate cylinder. As a result of this measure, the printing plate can be relieved or stress during closing of the clamping device at the front edge of the printing plate and likewise during further attachment of the printing plate on the plate cylinder. 
         [0011]    The measure of giving the printing plate a concave curvature which is opposed to the convex curvature of the front edge in a region which is spaced apart from the front edge is particularly advantageous, that is to say the printing plate has a curvature in this region, which curvature is deflected concavely transversely with respect to the sheet transport direction and is thus opposed to the convex curvature at the front edge of the sheet. 
         [0012]    In one advantageous configuration, the feed device has guiding elements, for example rotatably mounted guiding rollers, which are spaced apart next to one another transversely with respect to the printing plate feeding direction and are disposed at different height levels with regard to a feed plane, with the result that the printing plate can be given the desired concave and convex curvatures. 
         [0013]    The extent of the curvature can be set by an adjustable height of the guiding rollers in relation to the guiding plane. 
         [0014]    Other features which are considered as characteristic for the invention are set forth in the appended claims. 
         [0015]    Although the invention is illustrated and described herein as embodied in a method and an apparatus for feeding printing plates, 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. 
         [0016]    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 
         [0017]      FIG. 1  is a diagrammatic, longitudinal-sectional view of a sheet-fed rotary printing press; 
           [0018]      FIG. 2  is an enlarged, fragmentary, sectional view of a printing plate changing device; 
           [0019]      FIG. 3  is a longitudinal-sectional view of a convexly-shaped feed plane; 
           [0020]      FIG. 4  is a longitudinal-sectional view of the convexly-shaped feed plane with guiding rollers which are disposed in such a way that their height can be adjusted; 
           [0021]      FIG. 5  is a longitudinal-sectional view of a concavely-shaped feed plane with guiding rollers of different diameters; 
           [0022]      FIG. 6  is a longitudinal-sectional view of the concavely-shaped feed plane; 
           [0023]      FIG. 7  is an enlarged, fragmentary, sectional view of an alternative printing plate changing device; and 
           [0024]      FIG. 8  is an enlarged, fragmentary, sectional view of a further exemplary embodiment of a printing plate changing device which is configured in one piece. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Referring now to the figures of the drawings in detail and first, particularly, to  FIG. 1  thereof, there is seen a machine, for example a printing press  1 , which processes sheets  7  and has a feeder  2 , at least one printing unit  3 ,  4  and a delivery  6 . The sheets  7  are removed from a sheet stack  8  and are fed, individually or overlapping, over a feed table  9  to the printing units  3 ,  4 . The printing units  3 ,  4  each contain a respective plate cylinder  11 , in a known manner. The plate cylinders  11  have respective apparatuses  13 ,  14  for fastening flexible printing plates. Moreover, each plate cylinder  11  is assigned an apparatus  16  for semi-automated or fully automated printing plate changing. 
         [0026]    The sheet stack  8  rests on a stack board  10  which can be raised in a controlled manner. The sheets  7  are removed from the upper side of the sheet stack  8  through the use of a so-called suction head  18  which has, inter alia, a number of lifting and dragging suckers  19 ,  21  to separate the sheets  7 . Moreover, blowing devices  22  are provided for loosening the upper sheet layers and sensing elements  23  are provided for stack tracking. A number of lateral and rear stops  24  are provided for aligning the sheet stack  8 , in particular the upper sheets  7  of the sheet stack  8 . 
         [0027]    In the exemplary embodiment according to  FIG. 2 , the printing plate changing apparatus  16  is disposed in front of the printing unit  3  and has an upper part  25  with guiding elements  27 ,  28  which are disposed in a stationary manner on a machine frame  26  of the printing press  1  or on a non-illustrated protective device and are preferably configured as guiding rollers. The guiding elements  27 ,  28  support the rear region of a printing plate  30  during preliminary positioning outside the plate cylinder  11 . A lower part  29  of the printing plate changing apparatus  16  is mounted in such a way that it can be pivoted in the direction of the plate cylinder  11  out of a waiting position into a printing plate feeding position. The lower part  29  has at least two guiding elements  31 ,  32  for the printing plate. The guiding element  31  is disposed on a carrier  33  which is connected in an articulated manner to a carrier  34  which carries the guiding element  32 . 
         [0028]    The guiding elements  27 ,  28 ;  31 ,  32  each include, for example, a respective number of rotatably mounted guiding rollers which are spaced apart next to one another transversely with respect to a printing plate feeding direction. 
         [0029]    The guiding rollers  27 ,  28  of the upper part  25  of the printing plate changing apparatus  16  define a straight feed plane for the printing plate  30 , in particular for the rear region of the printing plate. In this case, the guiding rollers  27 ,  28  have, for example, the same diameter D and are each disposed on a respective rotational axis  36 ;  37 . 
         [0030]    The guiding rollers  32 , which are disposed on the carrier  34  and face the plate cylinder  11 , define a concavely curved guiding region K in relation to the guiding device ( 29 ,  34 ) for the front edge of the printing plate. This is due to the guiding rollers  32  according to  FIG. 3  being disposed on a concavely bent rotational axis  38 . As a result of this measure, the front edge of the printing plate has a convex deflection in relation to a rotational axis  39  of the plate cylinder  11 . 
         [0031]    In one exemplary embodiment according to  FIG. 4 , the guiding rollers  32  have identical diameters D, but are disposed on different rotational axes  35 ,  40 . In this case, the guiding roller  32 , which is disposed in a central region, is disposed on the rotational axis  35  which is at a greater spacing from the plate cylinder  11  than the rotational axis  40 , on which guiding rollers  32 . 1  in a lateral region are disposed. 
         [0032]    The guiding rollers  31 , which are disposed on the carrier  33 , define a convexly curved guiding region L in relation to the guiding device ( 29 ,  33 ) for those regions of the printing plate  30  which come into contact with the guiding rollers  31 . 
         [0033]    As a result of this measure, the printing plate is deformed concavely with regard to the cylinder axis  39 . According to  FIG. 5 , the guiding rollers  31  are disposed on a common rotational axis  41 , but have different diameters. A diameter D 1  of a centrally disposed guiding roller  31 . 2  is thus greater than a diameter D of the guiding rollers  31  which are disposed in an edge region. 
         [0034]    In one exemplary embodiment according to  FIG. 6 , the guiding rollers  31  have identical diameters D, but are disposed on different rotational axes  42 ,  43 . In this case, the axis  42  for a centrally disposed guiding roller  31  is disposed at a level which lies above the level of the axis  43  for laterally disposed guiding rollers  31 . 1 . 
         [0035]    In an exemplary embodiment according to  FIG. 7 , the guiding rollers  31  are disposed in a stationary manner on the machine frame  26  of the printing press  1  in order to produce a concave deformation of the printing plate  30  with regard to the cylinder axis  39  of the plate cylinder. The guiding rollers  32 , which are provided for the convex deformation of the printing plate with regard to the plate cylinder axis  39 , are disposed at one end, facing the plate cylinder  11 , of a single piece, pivotably mounted carrier  46 . 
         [0036]    In one exemplary embodiment according to  FIG. 8 , the entire printing plate changing apparatus  16  has a single piece carrier  47  which is disposed pivotably on the machine frame  26 . The guiding rollers  31  for producing a convex curvature of the printing plate  30  with regard to the guiding device and a concave curvature of the printing plate with regard to the plate cylinder axis  39 , are disposed at one end of the carrier  47 , which end faces away from the plate cylinder  11 . 
         [0037]    The guiding rollers  32  for producing a convex curvature of the printing plate with regard to the cylinder axis  39  are disposed at an end of the carrier  47  which faces the plate cylinder  11 . 
         [0038]    All or individual guiding rollers  31 ,  31 . 1 ,  32  and/or  32 . 1  can be assigned setting apparatuses  48 , as seen in  FIG. 4 , for setting a vertical position with regard to the guiding regions K, L, in order for it to be possible to adjust the degree of curvature.