Abstract:
The holding-down arrangement for holding down a sheet ( 24 ) in a sheet-bearing region of a sheet-processing machine, in particular of a folding machine, includes a plurality of blowing devices ( 19, 21 ) which, during use, are directed toward a sheet ( 24 ) and direct an air jet ( 20, 22 ) onto the sheet ( 24 ). The holding-down arrangement may be fitted in a sheet-aligning device which is arranged downstream of a sheet infeed and includes a sheet-bearing device ( 12 ), on which an incoming sheet ( 24 ) comes to rest.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a holding-down arrangement for holding down a sheet in a sheet-bearing region of a sheet-processing machine, and to a sheet-aligning device which comprises such a holding-down arrangement.  
         DESCRIPTION OF THE BACKGROUND ART  
         [0002]    DE A 1 611 362 discloses a sheet-aligning device which comprises a sheet-bearing device which is formed by corrugated resilient strips. On one side, the sheet-bearing device is bounded by an aligning rail which can be adjusted along an axis, transversely to the sheet-infeed direction, for adjustment to the sheet width. Provided on the other side of the sheet-bearing device are a bottom guide and a top guide, between which the sheet is introduced. The top guide and the bottom guide can likewise be adjusted along the axis, transversely to the sheet-infeed direction, for adaptation to the sheet width. Above the center of the sheet-bearing device, a holding-down bar is arranged just above the sheet-bearing device, this bar preventing an incoming sheet from rising up. In the case of wide sheet-bearing devices, it is necessary to have a plurality of holding-down bars spaced apart one beside the other in order to prevent the sheet from rising up over the entire width of the sheet-bearing device. The arrangement of a plurality of holding-down bars is presented, for example, in German Utility Model 84 06 391. The plurality of holding-down bars are fixed above the sheet-bearing device. If the sheet-bearing device is to be adjusted to a smaller format, the aligning rail can be displaced in the direction of the center of the sheet-bearing device, counter to the force of the resilient elements. Since the holding-down bars are located level with the aligning rail, it is necessary to remove holding-down bars which are fitted within the adjusting path. At the same time, it is also necessary to adjust the bottom rail and the top rail, for which reason it may be necessary to remove further holding-down bars. Installation or removal of holding-down bars, however, involves very high outlay.  
         SUMMARY OF THE INVENTION  
         [0003]    The object of the invention is to provide, using means of straightforward design, a holding-down arrangement and a sheet-aligning device, provided with the holding-down arrangement, which make it possible for the sheet-aligning device to be adjusted to different sheet widths without it been necessary for holding-down elements to be installed or removed.  
           [0004]    This object is achieved by a holding-down arrangement and by a sheet-aligning device as disclosed herein.  
           [0005]    Advantageous developments of the holding-down arrangement according to the invention are also disclosed herein. Preferred embodiments of the sheet-aligning device are also disclosed herein.  
           [0006]    Since the holding-down arrangement according to the invention holds down the sheets by the air jet of the blowing devices, it is possible for the holding-down arrangement to be arranged further above the sheet-bearing device than is the case with the known holding-down bars, which can be arranged just above the sheet-bearing device. The holding-down arrangement may be arranged at a higher level, in the adjusting region of an aligning rail, than the aligning rail, with the result that there is no chance of collision with one or more holding-down arrangements during the adjustment of the aligning rail. There is thus no longer any need for the holding-down arrangement to be installed or removed during adaptation of the sheet-bearing device to a sheet width.  
           [0007]    The blowing devices are preferably arranged such that the air jet, during use, is inclined in the running direction of a sheet. It is thus possible to assist the movement of the sheet in the sheet-running direction.  
           [0008]    Outside the adjusting region of the aligning rail, the previously known holding-down profiles may be arranged just above the sheet-bearing device since there is no risk of collision with the aligning rail here.  
           [0009]    In the case of a preferred embodiment, the sheet-bearing device has, on the sides opposite the aligning rail, a sheet-bearing plate which has a plurality of blowing devices which are directed toward the underside of the sheet and are designed such that they direct onto the underside of the sheet an air jet which is inclined in the sheet-running direction. The air jet forms a kind of air cushion, which prevents static attraction of the sheet on the paper rest. Since the air jets are inclined in the sheet-running direction, this additionally assists the movement of the sheet in the sheet-running direction.  
           [0010]    The blowing devices of the sheet-bearing plate and of the holding-down arrangements are connected to a pressure-generating device, it being possible for the quantity and the pressure of the air to be regulated for optimum adjustment. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Exemplary embodiments of the invention are explained in more detail hereinbelow with reference to drawings, in which:  
         [0012]    [0012]FIG. 1 shows, schematically, a side view of a sheet-aligning device for conveying and uniformly aligning sheets,  
         [0013]    [0013]FIG. 2 shows a schematic plan view of the sheet-aligning device from FIG. 1,  
         [0014]    [0014]FIG. 3 shows the section A-A from FIG. 2,  
         [0015]    [0015]FIG. 4 shows a longitudinal section through a holding-down bar of the sheet-aligning device from FIG. 1, and  
         [0016]    [0016]FIG. 5 shows a schematic plan view of a second embodiment of a sheet-aligning device. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    The sheet-aligning device  10  which is shown in FIG. 1 serves for conveying and aligning a sheet  24  from a first folding unit FW 1  to a second folding unit FW 2 . The sheet-aligning device  10  comprises an angled-roller table  12  with a plurality of driven rollers  13  arranged one after the other in the longitudinal direction. An aligning rail  16  is arranged on that longitudinal side of the angled-roller table  12  which is at the top in FIG. 2. The rollers  13  extend transversely to the longitudinal direction of the angled-roller table  12 . The axis of the rollers  13  is inclined slightly counter to the sheet-running direction BL, in the direction of the aligning rail  16 . The first folding unit FW 1  is arranged on that side of the angled-roller table  12  which is located opposite the aligning rail  16 , at that end of the angled-roller table  12  which is on the right in FIG. 2. The second folding unit FW 2  is located on that end side of the angled-roller table  12  which is located downstream in the sheet-running direction BL.  
         [0018]    A sheet-bearing plane is formed by the rollers  13  arranged in one plane. Above the sheet-bearing plane, three tubular holding-down bars  18  are arranged one beside the other in one plane, and extend in the longitudinal direction of the angled-roller table  12 , i.e. in the sheet-running direction BL. Provided in the underside of the holding-down bars  18 , in the region in which the first folding unit FW 1  is arranged, are a plurality of through-passage openings  19  (FIG. 4), which are inclined downward in the direction of the aligning rail  16  in order for an air jet  22  which passes out through the through-passage openings  19  to be directed in the direction of the aligning rail  16 . A sheet  24  passing out of the first folding unit FW 1  is conveyed onto the bearing plane of the angled-roller table  12 , perpendicularly to the longitudinal direction of the angled-roller table  12 , until it comes into abutment against the aligning rail  16 . It is retained on the bearing plane here by the air jet  22  passing out of the holding-down means  18 , the movement of the sheet  24  in the direction of the aligning rail  16  being assisted by the air jet  22 . The sheet  24  is then conveyed by the rollers  13  in the direction of the second folding unit FW 2 , abutment of the sheet edge against the aligning rail  16  being ensured on account of the rollers  13  being arranged in an angled manner in relation to the aligning rail  16 . In the region in which the sheet  24  is conveyed in the longitudinal direction of the angled-roller table  12 , a plurality of through-passage openings  21  (FIG. 4) are spaced apart from one another in the longitudinal direction in the holding-down means  18 . These through-passage openings  21  are designed such that an air jet  20  which passes out through the through-passage openings  21  is directed in the sheet-running direction BL, as a result of which the movement of the sheet  24  in the direction of the second folding unit FW 2  is assisted. At the same time, the sheet  24  is held down by the air jets  20 .  
         [0019]    As has been mentioned above, the holding-down bars  18  are of tubular design. They have a closed interior  25  (FIG. 4) which is enclosed by a tube wall  23  (FIG. 4). The interior  25  is connected to a regulatable pressure-generating device (not shown) by means of which it is possible to introduce air at a regulatable pressure into the holding-down bars  18 , this air passing out through the through-passage openings  19 ,  21  in the tube wall  23  in order to form the air jets  20 ,  22 .  
         [0020]    The second embodiment of the sheet-aligning device  30 , which is shown in FIG. 5, has two spaced-apart side-frame elements  36 ,  38  extending in the sheet-running direction BL. An aligning rail  32 , likewise extending in the sheet-running direction BL, forms, together with an angled conveying belt  34 , a unit which can be adjusted transversely to the sheet-running direction for adaptation to the sheet width. Above the aligning rail  32 , two transverse struts  40 ,  42 , which extend transversely to the sheet-running direction, are fastened on the side frames  36 ,  38 . The aligning rail  32  can be adjusted in an adjusting region VB together with the angled belt  34 . Within the adjusting region VB, three tubular holding-down bars  44 ,  46 ,  48  are fastened on the transverse struts  40 ,  42  such that they are located at a higher level than the aligning rail  32 . The holding-down bars  44 ,  46 ,  48  are designed like the holding-down bars  18  of the first embodiment, the air jet form the through-passage openings being directed in the sheet-running direction BL. The adjusting region VB is followed by a bearing plate  56  extending in the sheet-running direction BL. The surface of the bearing plate  56  is located in the bearing plane of the sheet-aligning device  30 . The bearing plate  56  has a closed interior which is connected to a regulatable pressure-generating device (not shown). Provided in the top side of the bearing plate  56  are three rows of through-passage openings  58 , these extending in the sheet-running direction BL and being designed such that an air jet from the through-passage openings  58  is inclined in the sheet-running direction BL. The left-hand row cannot be seen in FIG. 5 since it is arranged beneath a holding-down means  50 . The quantity and the pressure of the air which passes out can be regulated by regulation of the pressure-generating device. Above the bearing plate  56 , four holding-down means  50 ,  52 ,  54 ,  55  extending in the sheet-running direction are fixed on the transverse struts  40 ,  42  such that they are arranged just above the bearing plane, as is the case with known holding-down means.  
         [0021]    Since the holding-down bars  44 ,  46 ,  48  are arranged at a higher level than the aligning rail  32 , it is possible for the aligning rail  32 , together with the angled belt  34 , to be adjusted within the adjusting region without colliding with the holding-down bars  44 ,  46 ,  48 . The air which passes out of the through-passage openings  58  forms a kind of air cushion for an incoming sheet, this air cushion preventing static attraction between the sheet and the bearing plate  56 . This makes it possible for the bearing plate  56  to be of relatively wide configuration, with the result that there is no need for elements to be adjusted to the sheet width on that side of the sheet-aligning device  30  which is opposite the aligning guide  32 .