Abstract:
In a method and an apparatus for feeding sheets to a sheet-processing machine, the leading edge of the sheet is smoothed before the grippers are closed by further transport into the sheet-processing machine. A curvature or bend in the sheet transport direction is imparted to the sheet. The feed table has, in its feeder region, a curvature in the sheet transport direction, against which curvature the sheet leading edge is laid with the aid of pneumatic or mechanical devices.

Description:
BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention lies in the mechanical sheet-processing field. More particularly, the invention pertains to an apparatus for guiding sheets to a sheet-processing machine, in particular to a printing press. The assembly has a feed table and a feeder region provided at the end of the feed table with aligning means for lateral and circumferential alignment of the sheets. 
   In order to enable a transfer of a sheet accurately to a further transport device, for example a pregripper, after alignment in the feeder region of the feed table, the respective sheet must have an unstressed and optimally smoothed sheet leading edge. 
   It is known in the art, for example, to provide so-called smoothing brushes or sheet hold-downs and to used them to act mechanically on the sheet from above and avoid the formation of corrugation waves. 
   German published patent application DE 100 11 186 A1 describes a pneumatic sheet-smoothing device. In its feeder region, a feed table has, in the feed plane, so-called blowing/suction nozzles, by means of which a flow running parallel with respect to the surface of the feed table is generated. The flow produces a vacuum on the underside of the side of the sheet facing the feeder table, in order that the side rests on the feed table. The blown air jets are aligned diagonally with cutouts in the feed table which are provided for gripper stops of the pregripper. 
   SUMMARY OF THE INVENTION 
   It is accordingly an object of the invention to provide a sheet-smoothing method and device which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which ensure that the sheet leading edge has a smoothed sheet leading edge immediately before further transport to the sheet-processing machine. 
   With the foregoing and other objects in view there is provided, in accordance with the invention, a method of feeding sheets to a sheet-processing machine. The method comprises the following steps: 
   placing a sheet with a leading edge, as defined by a sheet transport direction through the machine, against front lays on a feed table; 
   smoothing the leading edge of the sheet by imparting to the sheet a curvature in the sheet transport direction; and 
   subsequently gripping the sheet with a further transport device and feeding the sheet into the machine for processing. 
   With the above and other objects in view there is also provided, in accordance with the invention, an apparatus for feeding sheets to a sheet-processing machine, in particular an apparatus for carrying out the above-summarized method. The apparatus comprises: a feed table formed with a guide surface in a feeder region thereof, the guide surface being curved in a sheet transport direction to the sheet-processing machine, and a smoothing device configured to assist in laying the sheet against the guide surface. 
   With the above and other objects in view there is also provided, in accordance with the invention, an apparatus for smoothing a leading edge of a sheet, in particular by carrying out the above-summarized method. The assembly is an improved apparatus with a feed table having a plurality of suction openings formed in the feeder region and arranged transversely with respect to a sheet transport direction, and a vacuum device configured to sequentially apply a vacuum to the suction openings one after another, starting from a center opening and in each case laterally outward. 
   A particular advantage of a smoothed sheet leading edge lies in the fact that it can be gripped very accurately by the following further transport means. 
   According to one preferred embodiment, the sheet leading edge is smoothed by having a slight curvature in the sheet transport direction imparted to it. 
   In an advantageous mechanical solution, there is provision for the feed table to have top lays which are arranged on its feeding edge and can be pivoted onto and away from said feeding edge, each have a curvature in the sheet transport direction and thus, together with a guide surface of the feed table, form a pocket for accommodating the sheet leading edge. 
   In an advantageous pneumatic solution, there is provision for the feed table to have, at its feeding edge, a guide surface having a curvature in the sheet transport direction and to be provided with a number of suction openings transversely with respect to the sheet transport direction, said suction openings sucking the sheet to the curved guide surface of the feed table, with the result that a slight curvature is imparted to the sheet. 
   In order for it to be possible to attract the sheet by suction in an unstressed manner, there is provision for vacuum to be applied to the suction openings chronologically one after another, starting from the center and in each case toward the outside. It is possible to smooth and unstress the sheet toward the sides by means of this measure. It is also possible to perform this measure in the case of a straight feeder table without curvature. 
   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 sheet smoothing device, 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 DRAWINGS 
       FIG. 1  a diagrammatic longitudinal section taken through a sheet-processing machine in the form of a printing press; 
       FIG. 2  is a perspective view of a feeder region of a feed table having a curved guide surface and blowing/suction nozzles; 
       FIG. 3  is a perspective view of the feeder region of the feed table having a curved guide surface and suction openings; 
       FIG. 4  is a perspective view of the feeder region according to  FIG. 3  and having electrically or magnetically driven control valves; 
       FIG. 5  is a perspective view of the feeder region of the feed table having a curved guide surface and curved top lays; and 
       FIG. 6  is a perspective view of a further exemplary embodiment having a straight guide surface and a number of suction openings. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a machine  1  that processes sheets  7 . The machine  1 —a printing press in the exemplary embodiment—has a feeder  2 , at least one printing unit  3  or  4  and a delivery  6 . The sheets  7  are removed from a sheet stack or sheet pile  8  and fed individually or overlapped to the printing units  3  and  4  via a feed table  9 . The printing units  3  and  4  have respective plate cylinders  11 ,  12 . The plate cylinders  11  and  12  each have an apparatus  13 ,  14  for fixing flexible printing plates. Furthermore, each plate cylinder  11 ;  12  is assigned an apparatus  16 ;  17  for the semiautomatic or fully automatic changing of printing plates. 
   The sheet stack  8  rests on a stack board or pile board  10  which can be lifted in a controlled manner. The sheets  7  are removed from the upper side of the sheet stack  8  by a so-called suction head  18  which, inter alia, has a number of lifting and dragging suckers  19 ,  21  to separate the sheets  7 . Furthermore, blowing devices  22  are provided for loosening the upper sheet layers and sensing elements  23  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 . 
   The feed table  9  is configured as what is referred to as a suction belt table and has a feeder region with a feeding edge  27  at its end towards the printing press  1 . The feeding edge  27  is configured in such a way that it is possible to set pivotable front and/or top lays  26 ,  30  and a pivotable pregripper  28  against it cyclically. Side pull lay devices  25  arranged in the edge region of the feed table  9  ensure lateral sheet alignment in the feeder region. In the feeder region, the feed table  9  has a guide surface  29  having a slight curvature in the sheet transport direction, preferably in the downward direction. Pneumatic devices are provided in order that the sheet leading edge also follows the curvature reliably, the former pulling the sheet leading edge against the curved guide surface  29  of the feed table. According to  FIG. 2 , provision is made here to arrange a number of blowing/suction nozzles  31  transversely with respect to the sheet transport direction. The nozzles each emit a blown air jet which is directed in the sheet transport direction parallel with respect to the guide surface  29  and parallel with respect to the underside of the sheet. The jet produces a vacuum under the sheet and fits the latter snugly against the curvature of the guide surface  29 . 
   In a further exemplary embodiment, provision is made according to  FIG. 3  for the curved guide surface  29  to have a number of suction openings  36  transversely with respect to the sheet transport direction, which suction openings  36  suck the sheet  9 , in the region of its leading edge, against the curved guide surface  29 . In order to unstress the sheet, there is provision for vacuum to be applied to the suction openings  36  chronologically one after another, starting from the center and in each case toward the outside. The vacuum can be supplied here via a rotary valve  37  or via electrically or magnetically actuatable control valves  38  ( FIG. 4 ) which are connected to a vacuum or blown air source  40 , the activation being effected by a control computer of the sheet-processing machine. If the sheet is brought into contact with the curvature via suction openings, the lateral alignment must take place at another instant, that is to say a later instant, as it is not possible to displace the sheet laterally when it is being attracted by suction. It is also conceivable in the other refinements to perform a lateral alignment at an instant other than during contact with the front lays. 
   In a third exemplary embodiment according to  FIG. 5 , provision is made for the sheet leading edge to be adapted mechanically to the curvature of the guide surface  29 . For this purpose, there is provision for top lays  30  to be arranged on the pivotable front lays  26 , the former likewise having a curvature adapted to the curvature of the guide surface  29  and the sheet leading edge being inserted into the pocket  32  formed from the top lay  30  and guide surface  29 , for example by a transport belt  33  of the suction belt table. The sheet leading edge is in each case smoothed as a result of a curvature in the sheet transport direction being imparted to it. 
   In the exemplary embodiments shown, the curvature is made in a downward direction in each case. It is also possible to make said curvature in an upward direction, depending on the space requirement or configuration of the sheet transport means. 
   In an exemplary embodiment according to  FIG. 6 , there is provision for the feed table  9  to be configured, in the feeder region, with a straight conventional guide surface  34  which has a number of suction openings  36  transversely with respect to the sheet transport direction, said suction openings  36  being actuated with vacuum chronologically one after another, starting from the center, in order to suck the sheet against the feed table  9  and therefore achieve smoothing of the sheet leading edge. Actuation can be performed by means of the rotary valve  37  or the electrically or magnetically driven valves  38 . In this exemplary embodiment, lateral alignment is also performed at another instant, for example a later instant. 
   This application claims the priority, under 35 U.S.C. § 119, of German patent application No. 103 16 646.7, filed Apr. 11, 2003; the disclosure of the prior application is herewith incorporated by reference in its entirety.