Abstract:
A method and an apparatus for improving the sheet transport in the grip of grippers in a sheet-processing machine, for example a printing press. The sheet corners of the print-free gripper edge are removed in order to increase the sheet stability and in order to avoid these limp sheet corners being able to turn over.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority, under 35 U.S.C. § 119, of German application DE 10 2007 012 658.3, filed Mar. 16, 2007; 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 for improving the sheet transport in a sheet-processing machine in which the sheet is gripped by means of gripper devices at a print-free edge and is passed on to following gripper devices. 
         [0003]    In such sheet-processing machines, there is often the problem that the sheets are very limp at the corners of the leading sheet edge, in particular in the case of thin materials, and therefore tend to turn over. 
         [0004]    In order to master this problem, attempts are generally made to support the sheet corners during the sheet transfer from one gripper system to the other. 
         [0005]    German patent DE 196 05 420 C1 shows a gripper system in which blast air is blown from below against the sheet leading edge located in the grip of grippers, in order to support the unfixed sheet corners in order that the latter cannot turn over downward. The possibility, however, that the sheet corners turn over upward cannot be ruled out. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    It is accordingly an object of the invention to provide a method and an apparatus for processing a sheet prior to transport in a gripper, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which ensures that the sheet corners are prevented from turning over. 
         [0007]    With the foregoing and other objects in view there is provided, in accordance with the invention, a method for processing a sheet or sheet stack before transport in a grip of grippers through a sheet-processing machine, the method which comprises removing sheet corners at a leading edge of the sheet provided for gripping the sheet. 
         [0008]    It is a particular advantage of the invention that the limp sheet corners that tend to turn over or bend over are removed before being transported through the sheet-processing machine. As a result of this measure, trouble-free sheet transport is ensured, which manages without additional means for stabilizing the sheet corners. 
         [0009]    Preferably, in sheet-processing machines without sheet turning, only the sheet corners of the leading edge are removed. In machines with turning, for example a rotary perfecting machine, the sheet corners of the trailing edge are also removed as well. 
         [0010]    In either case, in accordance with an added feature of the invention, the sheet corners are removed only in a region of a print-free edge of the sheet. 
         [0011]    In accordance with another feature of the invention, the sheet corners are removed in a pre-stacking device or in a feeder of a sheet-processing machine. 
         [0012]    With the above and other objects in view there is also provided, in accordance with the invention, an apparatus for carrying out the above method, i.e., an apparatus for processing a sheet or sheet stack prior to transport in a grip of grippers through a sheet-processing machine. The apparatus comprises a paper-milling device mounted on a vertical guide for up and down movement thereof. 
         [0013]    In accordance with again another feature of the invention, there are provided two vertical guides assigned to a sheet leading edge and adjustably disposed for freely adjusting a format transversely with respect to a sheet transport direction. In accordance with a further feature of the invention, there are also provided vertical 
         [0014]    guides assigned to a sheet trailing edge and adjustably disposed for freely adjusting a format transversely with respect to the sheet transport direction. 
         [0015]    The removal of the sheet corners is preferably carried out in a pre-stacking station which, for example, is arranged upstream of the sheet stack of a feeder. Alternatively, however, the removal can also be carried out directly at the sheet stack of the feeder. 
         [0016]    In other words, the apparatus provided for removing the sheet corners is, for example, a paper mill, which is arranged on a vertical guide and mills off the sheet corner in the region of the print-free edge by the mill being moved along at right angles to the sheet stack edge. A mill is preferably provided for each vertical sheet stack edge. The guides together with the mills can be adjusted transversely with respect to the sheet transport direction, so that they can be set to every sheet format to be processed. The rear guides can also be adjusted in the sheet transport direction, so that these can also be set to format length. 
         [0017]    In a preferred embodiment of the invention, the milling devices each include a cylindrical milling head. The milling head is mounted such that an angle thereof can be adjusted in a horizontal plane. In a preferred embodiment, the milling head is mounted to define an angle of a milling line less than 45°. 
         [0018]    In accordance with a concomitant feature of the invention, each milling device is assigned an extraction device for removing paper scraps and sheet corners. 
         [0019]    Other features which are considered as characteristic for the invention are set forth in the appended claims. 
         [0020]    Although the invention is illustrated and described herein as embodied in method and apparatus for processing a sheet before the sheet transport in the grip of grippers, 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. 
         [0021]    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 
         [0022]      FIG. 1  is a schematic side view of a rotary press in section; 
           [0023]      FIG. 2  shows a plan view of a sheet stack; 
           [0024]      FIG. 3  shows a side view of the sheet stack; and 
           [0025]      FIG. 4  shows a plan view of a sheet with the printing area identified. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is seen a sheet-processing machine, for example a printing press ball  1 . The machine  1  has a feeder  2 , at least one printing unit  3  and  4  and a delivery  6 . Sheets  7  are removed from a sheet stack  8  and, separated or overlapping, are fed over a feed table  9  to the printing units  3  and  4 . The printing units each contain, in a known way, a plate cylinder  11 ,  12 . The plate cylinders  11 ,  12  each have a device  13 ,  14  for fixing flexible printing plates. Furthermore, each plate cylinder  11 ,  12  is assigned a device  16 ,  17  for fully automatic or semiautomatic printing plate exchange. 
         [0027]    The sheet stack  8  rests on a stack board  10  that can be raised under control. The sheets  7  are removed from the top of the sheet stack  8  by means of what is known as a suction head  18  which, inter alia, has a number of lifting and dragging suckers  19 ,  21  for separating the sheets  7 . Furthermore, blowing devices  22  for loosening the upper sheet layers and sensing elements  23  for stack tracking are provided. In order to align the sheet stack  8 , in particular the upper sheets  7  of the sheet stack  8 , a number of side and rear stops  24  are provided. 
         [0028]    As seen in the sheet transport direction, a pre-stacking station  26  is arranged upstream of the sheet stack  8 . Referring now to  FIG. 2 , the pre-stacking station  26  has milling devices  27 , which are arranged such that they can be moved on vertical guides  29 . The milling devices  27  each comprise a cylindrical milling head  32 , which has a drive motor  33 . The latter is fixed to a carriage  34 , which can be moved up and down by means of a drive device. The drive device can, for example, be an electric motor, an electromagnetic linear drive, or an operating cylinder. 
         [0029]    The vertical guides  29  are arranged such that they can be adjusted transversely with respect to the sheet transport direction, so that they can be set to each respective format to be processed. The milling heads  32  are mounted such that they can be swiveled horizontally, so that an angle of about 45° can be set, preferably an angle α&lt;45°. 
         [0030]    A sheet  7  which is to be processed and printed in the recto printing mode without turning, for example in a printing press, needs to be processed only at its leading edge  37 . In this case, the sheet corners  37   a  are cut off at an angle α=about 40°. The cut line  38  in this case reaches at most as far as a printing area  39 . As a result of this measure, a sufficiently large gripper edge  41  remains for holding the sheet  7  by means of grippers  42  provided. 
         [0031]    A sheet  7  which is processed for example in the turning operation (e.g., in recto-verso printing), in which therefore its rear edge becomes the leading edge after turning, must also be processed at its sheet trailing edge  43 , i.e. the sheet corners  44 ,  46  of the sheet trailing edge  43  are removed before the processing in the sheet-processing machine, for example a printing press. 
         [0032]    Of course, the sheet-milling devices for the rear sheet corners  44 ,  46  are also arranged such that their format can be adjusted. 
         [0033]    Each milling device  27  is additionally equipped with an extraction device  47  for paper scraps and the sheet corners.