Abstract:
A device for transporting sheets in recto or verso printing operation has tongs grippers each with upper and lower grippers. The upper gripper, or rigid gripper, that forms the gripper pad can be locked directly or indirectly in recto printing operation.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention lies in the printing technology field. More specifically, the invention relates to a device for transporting sheets by way of tongs grippers that can be changed from straight printing (recto printing) to perfecting (recto/verso printing). 
     A device of that type is known, for example from German patent DE 24 14 998 and its counterpart U.S. Pat. No. 4,101,122. There, a generic device for transporting sheets wherein an upper and lower gripper of a tongs gripper system is in each case controlled by a control cam. The tongs gripper can carry out both a pivoting movement and an opening movement in turning operation. In recto printing operation, the tongs gripper system is controlled by recto printing control cams which are arranged parallel to the turner control cams and which ensure that the upper gripper forms the gripper pad as what is known as a rigid gripper. This measure has the advantage that an adjustment of the gripper pad to different sheet thicknesses can be dispensed with. However, a series of forces distributed over the axial length of the gripper system act on the individual gripper pads arranged beside one another. In particular, mention should also be made here of the torsional forces which, overall, can lead to faulty tolerances with respect to the gripper pad height. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a device for transporting sheets in recto or recto/verso printing by way of grippers which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows minimizing the gripper pad height tolerance, in particular in recto printing operation. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a device for transporting sheets, comprising: 
     a tongs gripper system configured to selectively transport sheets in recto printing operation or in turning operation, the gripper system having a plurality of grippers each with an upper gripper and a lower gripper; 
     wherein the upper gripper is configured to form a gripper pad in recto printing operation, and the rigid upper gripper is disposed to be locked, directly or indirectly, in recto printing operation. 
     It is a particular advantage of the invention that the upper grippers functioning as a gripper pad in recto printing operation can be locked and supported, which results in an exactly defined gripper pad height. This permits satisfactory gripping and holding of the sheets and thus ensures fault-free sheet transport. 
     A preferred embodiment of the locking, which can be implemented very simply, is a supporting bearing arranged between the grippers and having a latching mechanism formed in such a way that it can be brought into operative contact with the gripper shaft. By means of this measure, indirect locking of the gripper system is made possible. 
     In a second advantageous embodiment, a holding clamp is provided which permits the simultaneous holding of the lower and upper gripper during sheet transport. This measure permits direct locking of the gripper system. 
     In a third device, a rack system is provided for the indirect locking of the upper gripper and/or of the gripper pad. 
     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 device for transporting sheets in recto or verso printing operation by means of tongs grippers provided, 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 three specific exemplary embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a rotary press in section in a schematic illustration; 
         FIG. 2  is a schematic sectional view of a first exemplary embodiment of a locking device, taken along the section line II-II in  FIG. 3 ; 
         FIG. 3  is a plan view showing the first exemplary embodiment; 
         FIG. 4  is a schematic sectional view of a second exemplary embodiment of a locking device, taken along the section line IV-IV in  FIG. 5 ; 
         FIG. 5  is a plan view showing the second exemplary embodiment; 
         FIG. 6  is a schematic sectional view of a third exemplary embodiment of a locking device, taken along the section line VI-VI in  FIG. 8 ; 
         FIG. 7  is a plan view onto a section, taken along the section line VII-VII in  FIG. 6 , through the third exemplary embodiment; and 
         FIG. 8  is a plan view onto the device according to the invention shown in  FIG. 6 . 
     
    
    
     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 processing sheets  7 , for example a press  1 . The machine has a feeder  2 , at least one printing unit  3  and  4  and a delivery  6 . The sheets  7  are removed from a sheet stack  8  and, separated or overlapping in imbricated relationship, are fed over a feed table  9  to the printing units  3  and  4 . The latter each contain a plate cylinder  11 ,  12 , in a known manner. The plate cylinders  11  and  12  each have an apparatus  13 ,  14  for fixing flexible printing plates. Furthermore, an apparatus  16 ,  17  for the semiautomatic or fully automatic printing plate change is assigned to each plate cylinder  11 ,  12 . 
     The sheet stack  8  lies on a stack board  10  that can be raised under control. The sheets  7  are removed from the top side of the sheet stack  8  by way of a suction head  18 , as it is known, which inter alia has a number of lifting and dragging suckers  19 ,  21  for the separation of the sheets  7 . Furthermore, the blowing devices  22  for loosening the upper sheet layers and sensing elements  21  for tracking the stack 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. 
     A number of sheet-carrying cylinders  27  to  33  ensure the sheet transport from the feed table  9  through the printing units  3 ,  4  to the chain delivery system  34 . The sheet-transporting cylinder  32  is constructed as a turner drum having at least one tongs gripper system  36 . The gripper system  36  is distinguished by the fact that it is able to carry out both an opening and closing movement and also a pivoting movement under cam control in order to turn the sheets. The tongs gripper system  36  comprises a number of upper and lower grippers  38 ,  37  arranged beside one another and distributed over the axial length of the turner drum  32 . 
     The grippers are carried by a gripper shaft  39 , which is arranged in a gripper tube  41  enclosing the gripper shaft  39  coaxially, the upper grippers  38 , also designated rigid grippers, being fixed to the gripper shaft  39  and the lower grippers  37  being fixed to the gripper tube  41 . 
     In order that the upper grippers  38  are supported sufficiently well during the sheet transport in recto printing operation and, at the same time, are secured against torsional fluctuations, provision is made, according to  FIGS. 2 and 3 , for the gripper shaft  39  to be locked with respect to the turner drum  32 . A plurality of supports  42 , fixed to the cylinder body of the turner drum  32 , are disposed axially at a distance from one another, such that, between two pairs of grippers  37 ,  38 - 37 ,  38 , they can be brought into operative connection with a latching mechanism  43 , which is arranged on the gripper shaft  39 . The latching mechanism  43  has a spring-loaded ball  44  which corresponds with a receptacle  46  in the support  42 . This is an indirect, non-controllable locking means. 
     In a second exemplary embodiment according to  FIGS. 4 ,  5 , provision is made to lock the grippers  37 ,  38  directly during the sheet transport in recto printing operation, the locking being carried out under control. To this end, provision is made to mount locking tongs  49  having two flexible locking arms  47 ,  48  on the cylinder body of the turner drum  32  in the axial direction of the turner drum  32 . The locking arms  47 ,  48  each have at their ends a hook-like holder  50 ,  52  which, in the locked state, acts on the rigid gripper  38  so that upper and lower gripper  38 ,  37  are held together during the sheet transport in recto printing operation. For the purpose of unlocking, the lower gripper  37 —also designated a sprung gripper—is controlled downward, preferably by means of the contour of a control cam to such an extent that, via the locking arms or tongs limbs  47 ,  48  running obliquely, it pivots the latter away from each other until opening of the tongs  49  allows the rigid gripper  38  to leave the closing range of the tongs  45 . The sprung gripper  37  is of narrower design than the spacing of the hooks  52  of the locking arms  47 ,  48 , so that the latter can enter and leave the locking range of the tongs  49  without hindrance. The entry of the rigid gripper  38  on the upper side of the locking arms  47 ,  48  on the hook  50 ,  52  is also made possible by bevels  53 ,  54  made thereon. As a result of the contact with the rigid gripper  38 , the locking arms  47 ,  48  are forced apart by means of the bevels  53 ,  54 , counter to a spring force, until the rigid gripper  38  dips underneath the hook  52  into the locking range of the tongs  49 . 
     In a third exemplary embodiment according to  FIGS. 6 ,  7 , the gripper shaft  39  has a toothed segment  56 , which is in engagement with a rack  57 . The rack  57  is mounted such that it can be displaced in the circumferential direction of the turner drum  32  counter to a compression spring  58 . A wedge bar  59  mounted such that it can be displaced in the axial direction acts on the rack  57  and is used to fix the rack  57  and thus also to fix the gripper shaft  39  in engagement with the rack  57  during recto printing operation. The wedge connection can be provided on a single gripper system, on a plurality of selected gripper systems or on all the gripper systems. 
     During pivoting movement of the rigid gripper  38 , the toothed segment  56  moves into the rack  57  and, as a result of the incline of a wedge-shaped part  62 , moves it against the spring  58 . At the closing point of the rigid gripper  38 , the wedge bar  59  is moved against a spring  61  by an axial cam  64 . In the process, the wedge-shaped part  62  of the wedge bar  59  moves against the stop  63 , likewise formed in a wedge shape at this point, of the rack  57 . The rack  57  and thus also the gripper shaft  39  connected to it via the toothing are locked. Shortly before the opening of the tongs grippers  39 , the blocking of the rack  57  is canceled again by the wedge-shaped part  62  of the wedge bar  59  being forced out of the range of the rack  57  under the control of an axial cam  64 . The gripper shaft  39  is then able to execute rotational movements again. 
     This application claims the priority, under 35 U.S.C. § 119, of German patent application No. 10 2005 010 426.6, filed Mar. 8, 2005; the disclosure of the prior application is herewith incorporated by reference in its entirety.