Abstract:
A sheet brake for a press contains braking elements diverging in the sheet running direction in order to tauten the printing material sheets transversely. The braking elements are driven so as to circulate at a nonuniform speed. The nonuniform speed of the diverging braking elements results in more effective transverse tautening of the printing material sheets.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a-sheet brake for a press and has braking elements diverging in the sheet running direction in order to tauten the printing material sheets transversely.  
         [0003]     In published, non-prosecuted German patent application DE 39 39 212 A1, a sheet brake is described whose braking elements are formed as suction rings. The suction rings are aligned divergently, that is to say obliquely outward opposite to one other from the center of the machine, so that the printing material sheet is pulled obliquely outward on both sides by the suction rings and is tautened transversely with respect to the sheet running direction. However, the effectiveness of this transverse tautening still requires improvement.  
         [0004]     Further prior art is contained in published, non-prosecuted patent applications DE 44 35 988 A1 and DE 196 20 938 A1, corresponding to U.S. Pat. No. 6,056,287.  
       SUMMARY OF THE INVENTION  
       [0005]     It is accordingly an object of the invention to provide a sheet brake for a press which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which provides more effective transverse tautening.  
         [0006]     The sheet brake for a press according to the invention, has braking elements diverging in the sheet running direction in order to tauten the printing material sheets transversely. The braking elements are driven so as to circulate at a nonuniform speed.  
         [0007]     In the sheet brake according to the invention, the braking elements revolve at a nonuniform speed with a uniform printing or machine speed of the press. In this case, the braking elements in sheet contact with the incoming printing material sheets are periodically accelerated and retarded again. The speed profile of the speed of circulation of the sheet brake resulting from this guarantees a particularly high efficiency of the transverse tautening. This is because trials have resulted in the surprising effect that the transverse sheet tautening effected by the divergent oblique alignment of the braking elements is boosted noticeably by the dynamic cyclic operation of the sheet brake.  
         [0008]     In one development, the braking elements are mounted in two braking modules and the sheet brake contains only these two braking modules, that is to say no further braking module.  
         [0009]     According to a further development, the braking elements are constructed as endless bands or belts.  
         [0010]     In a further development, the sheet brake or its motor has a control link to a control device, which controls the nonuniform speed in accordance with a specific speed profile.  
         [0011]     Apart from the sheet brake according to the invention, the invention also includes a press equipped therewith and a control method for the speed control of the sheet brake.  
         [0012]     Other features which are considered as characteristic for the invention are set forth in the appended claims.  
         [0013]     Although the invention is illustrated and described herein as embodied in a sheet brake for a 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.  
         [0014]     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  
       [0015]      FIG. 1  is a diagrammatic, side-elevational view of a press having a sheet brake according to the invention;  
         [0016]      FIG. 2  is a diagrammatic, plan view of the sheet brake;  
         [0017]      FIG. 3  is a graph showing a speed profile of the sheet brake; and  
         [0018]      FIG. 4  is a graph showing an alternative speed profile of the sheet brake. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a press  1  for recto and verso printing. The press  1  contains a sheet deliverer  2  having a chain conveyor  3  and a sheet brake  4  disposed under the latter. The chain conveyor  3  has gripper bars  5  for holding printing material sheets  6  firmly at their leading edges. The sheet brake  4  brakes the printing material sheets  6  and, in the process, tautens them transversely with respect to a sheet running direction  7 , before the printing material sheets  6  are deposited on a delivery stack  8 .  
         [0020]      FIG. 2  shows that the sheet brake  4  contains only a first braking module  9  and a second braking module  10  and no further braking module. During braking and transverse tautening, the first braking module  9  makes contact with the one printed image-free side edge  11  of the printing material sheet  6 , and the second braking module  10  makes contact with the other. Between the two braking modules  9 ,  10 , the printing material sheet  6  is unsupported or self-supporting over its width.  
         [0021]     Each braking module  9 ,  10  has at least one braking element  14 ,  15  which circulates about two geometric axes  12 ,  13  and which, at least in some sections, is aligned obliquely with respect to the sheet running direction  7 . With respect to  FIG. 1 , in which the chain conveyor  3  with its gripper bars  5  circulates in the clockwise direction, the braking elements  14 ,  15  circulate in the counterclockwise direction. The braking elements  14 ,  15  can be in the form of bands or belts.  
         [0022]     In the example shown, each braking element  9 ,  10  has two round belts guided in parallel as the braking elements  14 ,  15 , the braking runs of the round belts that make contact with the printing material sheets  6  during braking and transverse tautening being sections of the braking elements  14 ,  15  aligned obliquely with respect to the sheet running direction  7 . In the region of its braking run, each braking element  14  of the first braking module  9  is inclined with respect to the left-hand sheet side edge at an angle of inclination to be measured relative to the sheet running direction and amounting to a few degrees. In the region of its braking run, each braking element  15  of the second braking module  10  is aligned with respect to the right-hand sheet side by an angle of inclination which is just as large but has the opposite sign. Consequently, the braking elements  14 ,  15  diverge and effect the transverse tautening of the printing material sheet  6  via a frictional connection.  
         [0023]     The braking modules  9 ,  10  in each case contain a suction opening  16  to which vacuum can be applied in order to attract the printing material sheet  6  onto the braking elements  14 ,  15  by suction.  
         [0024]      FIG. 3  shows a graph whose abscissa depicts time t and whose ordinate depicts speed v. In the graph, the conveying speed of the printing material sheet  6  (sheet speed v S ) and the speed of circulation of the sheet brake  4  (brake speed V B ) are illustrated as curves. The maximum sheet speed which corresponds to what is known as the machine speed and the speed of circulation of the chain conveyor  3  is designated v S/max . The minimum sheet speed v S/men  is zero and is inherent in the printing material sheet  6  when it is lying on the delivery stack  8 .  
         [0025]     At curve point A, the printing material sheet  6  is attracted by the suction openings  16  and, consequently, brought into contact with the braking elements  14 ,  15  which, at this time, are circulating at the maximum brake speed V B/max , which is lower than the maximum sheet speed v S/max  or could be equal to the maximum sheet speed v S/max . The curve point A designates what is known as the jumping time, at which the printing material sheet  6  jumps onto the sheet brake  4 .  
         [0026]     At the curve point B, the grippers of the gripper bar  5  open, so that the printing material sheet  6  is braked by friction by the sheet brake  4  between the curve point B and the curve point C. The sheet brake  4  advantageously already tautens the printing material sheet  6  in the time interval between the curve points A and B, that is to say while the printing material sheet  6  is still held firmly at its leading edge by the gripper bar  5  and is transported in the sheet running direction  7 .  
         [0027]     At the curve point C, the sheet speed v S  is exactly as high as the maximum brake speed v B/max . Between the curve points A and C there is a sliding phase  17  of the printing material sheet  6 , in which the latter slides on the braking elements  14 ,  15  with slippage in the sheet running direction  7 . From the curve point C as far as the curve point D, the braking elements  14 ,  15  and the printing material sheet  6  lying on the latter move at a uniform, common speed v S =V B , which corresponds to the maximum brake speed V B/max .  
         [0028]     From the curve point E as far as the curve point F, the braking elements  14 ,  15  together with the printing material sheet  6  move with a uniform, common speed v S =V B , which corresponds to the minimum brake speed v B/min .  
         [0029]     From the curve point D as far as the curve point E, a reduction in the brake speed v B  takes place (braking phase), so that the braking elements  14 ,  15  and the printing material sheet  6  are retarded together.  
         [0030]     Between the curve point C and the curve point F, there is what is known as an adhesion phase  18 , in which there is no speed difference in the sheet running direction  7  between the braking bands  14 ,  15  and the printing material sheet  6  and adhesive friction is virtually present. In the adhesion phase  18 , the lateral stretching (transverse tautening) of the printing material sheet  6  also takes place as a result of the braking elements  14 ,  15  pulling the printing material sheet  6  apart laterally from inside to outside in the process.  
         [0031]     At the curve point F, the printing material sheet  6  leaves the sheet brake  4  and, at the curve point G, comes to rest on the delivery stack  8 . Between the curve points F and G there is what is known as the free flight phase  19  of the printing material sheet  6 , in which the latter sinks down onto the delivery stack  8 .  
         [0032]     From the curve point E as far as the curve point L, the sheet brake  4  runs at the minimum brake speed v B/min , the curve point H designating the end of the clock cycle of the preceding printing material sheet and the start of the clock cycle of the following printing material sheet  6 .  
         [0033]     The clock cycles of the dynamically driven sheet brake  4  are synchronized with what is known as the machine angle (rotary angular position) and the machine speed of the press by an electronic control device  20  (see  FIG. 2 ).  
         [0034]     From the curve point L as far as the curve point M, which is chronologically before the curve point A and in which the sheet brake  4  has reached the maximum brake speed v B/max  again, the brake speed v B  is increased (acceleration phase).  
         [0035]     From the curve point M as far as the curve point D, the sheet brake  4  maintains its maximum brake speed v B/max .  
         [0036]     The graph illustrated in  FIG. 3  makes it clear that the braking elements  14 ,  15  are accelerated to the maximum brake speed v B/max  even before the respective gripper bar  5  has released the printing material sheet  6  to be braked and the latter has jumped onto the sheet brake  4 . In addition, it becomes clear that, after the opening of the gripper bar  5 , the sheet brake  4  conveys the printing material sheet  6  and brakes it with a defined retardation. A control device  20  ensures that the printing material sheet is braked to the minimum brake speed v B/min , that is to say to what is known as the deposition speed of the printing material sheet  6 , before the force connection between the sheet brake  4  and the printing material sheet  6  is canceled.  
         [0037]     The control device  20  drives a motor  21  driving the circulating movement of the sheet brake  4  (see  FIG. 2 ) in accordance with the speed profile and, in the event of a change in the printing or machine speed, automatically adapts the curve of the brake speed v B  to the latter and thus to the maximum sheet speed v S/max .  
         [0038]     The advantage with the speed profile in  FIG. 3  is that the printing material sheet  6  is tautened transversely in an optimum way by the sheet brake  4  at every time of its contact with the latter.  
         [0039]     In order to limit the jolt occurring within the movement of the sheet, a speed profile according to  FIG. 4  can also be used. The designations in  FIG. 4  correspond to those in  FIG. 3  with the same effect.  
         [0040]     This application claims the priority, under 35 U.S.C. §119, of German application DE 10 2005 020 251.9, filed Apr. 28, 2005; the prior application is herewith incorporated by reference in its entirety.