Abstract:
A method of retaining a sheet on at least one of a transfer cylinder and a guide surface of a sheet-guiding device in a sheet-processing machine by pneumatically activatable nozzles for producing an air cushion beneath the respective sheet, which comprises producing an air cushion by an air-pressure distribution resulting in air flows beneath the respective sheet, the air-pressure distribution being based virtually only on radially decelerated air flow, and being capable of retaining the respective sheet in position; and a retaining device for performing the method.

Description:
BACKGROUND OF THE INVENTION 
     FIELD OF THE INVENTION 
     The invention relates to a method and device for contact-free retention of sheets on the circumferential surface of a cylinder or a planar guiding surface of a sheet-processing machine, in particular, a sheet-fed rotary printing machine. 
     The published German Patent Document DE 198 15 794 A1 has already disclosed heretofore a practice of installing, on a transfer cylinder of a sheet-processing machine, blowing/suction elements which subject to suction action a sheet transported primarily by grippers, and thus retain the sheet on the circumferential surface of the cylinder. The blowing or blast air that is applied here flows out parallel to and beneath the sheet and, assisted by suitably provided suction openings, against the cylinder body. In the case of sheets which are freshly printed on the reverse side thereof, contact between the sheet and the cylinder body may result in smearing problems. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a method and a device for implementing the method, by which, on a sheet-transporting cylinder or a guiding surface in a sheet-processing machine, the sheet is subjected to retaining forces which retain the sheet in a more-or-less contact-free manner in the vicinity of the circumferential surface of the cylinder or of the sheet-guiding surface. 
     With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a method of retaining a sheet on at least one of a transfer cylinder and a guide surface of a sheet-guiding device in a sheet-processing machine by pneumatically activatable nozzles for producing an air cushion beneath the respective sheet, which comprises producing an air cushion by an air-pressure distribution resulting in air flows beneath the respective sheet, the air-pressure distribution being based virtually only on radially decelerated air flow, and being capable of retaining the respective sheet in position. 
     In accordance with another mode, the method of the invention includes producing an air pressure adapted to the weight per unit area of the sheet. 
     In accordance with a further mode, the method of the invention includes setting the air pressure at between 5,000 and 500,000 pascals. 
     In accordance with another aspect of the invention, there is provided a device for retaining sheets on at least one of a transfer cylinder and a guide surface of a sheet-guiding device in a sheet-processing machine, comprising pneumatically activatable nozzles for producing an air cushion beneath the respective sheets, the nozzles having respective central blast-air openings as a center thereof, and an outflow feature selected from the group consisting of outflow openings arranged in a circle around the center and at least one outflow channel. 
     In accordance with a further aspect of the invention, there is provided a device for retaining sheets on at least one of a transfer cylinder and a guide surface of a sheet-guiding device in a sheet-processing machine, comprising pneumatically activatable nozzles for producing an air cushion beneath the respective sheets, the nozzles constituting a swivel nozzle as a center, and an outflow feature selected from the group consisting of outflow openings arranged in a circle around the center and at least one outflow channel. 
     In accordance with another feature of the invention, the blast-air opening is a borehole, and the outflow openings are boreholes. 
     In accordance with a further feature of the invention, the swivel nozzle has at least two boreholes which are offset relative to one another and are inclined relative to an outlet surface. 
     In accordance with an added feature of the invention, an angle smaller than 60° is enclosed by the boreholes, respectively, and the outlet surface. 
     In accordance with an additional feature of the invention, the boreholes have an angular offset from one another at least approximately resulting from a division corresponding to the number of boreholes. 
     In accordance with yet another feature of the invention, the blast-air opening is connected to a pressure source, and the outflow openings are connected to atmosphere. 
     In accordance with yet a further feature of the invention, the swivel nozzle is connected to a pressure source, and the outflow openings are connected to atmosphere. 
     In accordance with yet an added feature of the invention, the blast-air opening is a ring-gap nozzle. 
     In accordance with yet an additional feature of the invention, the ring-gap nozzle is formed of a number of boreholes arranged in a circle. 
     In accordance with still another feature of the invention, the blast-air openings are able to be supplied with a pressure of from 5,000 to 500,000 pascals. 
     In accordance with still a further feature of the invention, the swivel nozzles are able to be supplied with a pressure of from 5,000 to 500,000 pascals. 
     In accordance with a concomitant feature of the invention, the sheet-processing machine is a rotary printing machine. 
     A particular advantage of the invention is that of retaining the sheet on an air cushion produced by a free blast-air jet which produces an air flow beneath the sheet and, utilizing the hydrodynamic paradox, subjects the sheet to a retaining force. In particular, when used on transfer cylinders wherein the freshly printed side of the sheet is directed towards the circumferential surface of the cylinder, the method and device according to the invention avoid smearing of the fresh ink on the cylinder. When the invention is applied to a storage drum, the sheet can be clamped easily by transfer systems acting upon the trailing edge thereof. 
     A further location for a planar sheet-guiding surface used with the invention is, for example, a feed table, in which case the sheet is aligned initially at the leading edge thereof and then laterally. So-called front blowers, which blow on the leading edge of the sheet counter to the sheet-transporting direction, aid the separation from a following sheet of the sheet which is to be aligned. In order that the sheet which is to be aligned is not blown upwardly from the feed table, however, provision is made for applying the method and device according to the invention in order, on the one hand, to retain the sheet on the feed table and, on the other hand, to facilitate the lateral alignment thereof on an air cushion. 
     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 method and device for a contact-free retention of sheets, 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, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic side elvational view of a rotary printing machine incorporating the device according to the invention; 
     FIG. 2 is a diagrammatic plan view of a transfer cylinder incorporating the device according to the invention; 
     FIG. 3 is a diagrammatic plan view of the feeding region of a sheet-feeding table incorporating the device according to the invention; 
     FIGS. 4 a  and  4   b  are sectional and plan view, respectively, of one embodiment of a retaining nozzle according to the invention; 
     FIGS. 5 a  and  5   b  are sectional and plan views, respectively, of another embodiment of the retaining nozzle; 
     FIGS. 6 and 7 are plan views of additional embodiments, respectively, of the retaining nozzle; and 
     FIGS. 8 a  and  8   b  are sectional and plan views, respectively, of yet another embodiment of the retaining nozzle. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings and, first, particularly to FIG. 1 thereof, there is shown therein a rotary printing machine, e.g., a printing machine  1  for processing sheets  7 , having a feeder  2 , at least one printing unit  3 ,  4  and a delivery  6 . The sheets  7  are removed from a sheet pile  8  and fed separately or in imbricated form to the printing units  3  and  4  via a feed table  9 . The printing units, respectively, include, in a conventional manner, a plate cylinder  11 ,  12 . The plate cylinders  11  and  12 , respectively, have a device  13 ,  14  for fastening flexible printing plates thereon. Furthermore, each plate cylinder  11 ,  12  has assigned thereto a respective device  16 ,  17  for semi-automatically or fully automatically changing printing plates. 
     The sheet pile  8  lies on a pile or stack support plate  10  which can be raised controllably. The sheets  7  are removed from the upper side of the sheet pile  8  by a so-called suction head  18  which, amongst others, has a number of lifting and pull suckers  19 ,  21  for singling or separating the sheets  7 . Also provided are blowing or blast devices  22  for loosening the top sheet layers, and feeler or sensing elements  23  for pile adjustment. In order to align the sheet pile  8 , particularly the top sheets  7  of the sheet pile  8 , a number of lateral and rear stops are provided. Furthermore, a turning or reversing device  28  is provided between the first printing unit  3  and a second printing unit  4 , the turning device  28  being selectively or optionally activatable, so that it is possible to operate both in recto printing mode and in recto/verso printing mode. 
     A transfer cylinder  26  is located between an impression cylinder  27  of the first printing unit  3  and a storage drum  24  of the turning device  28 . As shown in greater detail in FIG. 2, the transfer cylinder  26  has distributed over the circumferential jacket surface thereof a number of retaining nozzles  25  provided with a blast or blowing-air opening  29  which is enclosed, respectively, and, as shown in FIG. 5 b , by a number of outflow openings  31 , a distance a between a respective blast-air opening  29  and a respective outflow opening  31  being constant, resulting in a circular arrangement of the outflow openings  31 , with the blast-air opening  29  as a center point. The blast-air opening  29  is arranged so that the free blast-air jet from the mouth of the blast-air opening passes out approximately perpendicularly to the sheet transporting plane or radially from the cylindrical jacket surface until it comes into contact with the sheet  7  which is to be retained. 
     The feed table  9  is proposed as an example for arranging the retaining nozzles  25  according to the invention on a planar transporting surface. Provision is thereby made, as shown in FIG. 3, for arranging the retaining nozzles  25  according to the invention in the front region of the feed table  9  in the vicinity of front guides  32  and pull-type side guides  33 , i.e., in the non-imbricated region of the sheet  7 . 
     According to the embodiment of FIGS. 4 a  and  4   b , the retaining nozzles  25  are formed from a circular element  34  which is raised in relation to the guide surface of the feed table  9  or of the jacket surface of the transfer cylinder  26 , and has a free-jet blowing opening  35  in the center. In this embodiment, the necessary outflow channel  36  is formed by the space which is formed by the guide plane, an edge of the circular element  34  and the sheet  7 . 
     In another embodiment, provision is made for using an annular or ring gap  37  as the blast-air opening, the gap  37  having one or more crosspieces  38  (FIG. 7) or, in a further embodiment, the blast-air opening is produced by a number of boreholes  39  (FIG. 6) arranged closely together. 
     In an additional embodiment, as shown in FIGS. 8 a  and  8   b , the blast-air opening that is provided is a swivel nozzle which comprises at least two boreholes offset ideally 180° from one another. The boreholes are at an angle α (α&lt;60°), ideally α is approximately 30°, to the outlet surface  43 . The exemplary embodiment according to FIG. 8 thus has a swivel nozzle  42  formed with three boreholes offset 120° from one another. It is also possible for the number of boreholes of this swivel nozzle to be greater, although the angular offset between the boreholes should be a divisor of 360° which is determined by the number of boreholes, e.g., 5 boreholes result in an angular offset of 360°/5=72°. 
     The blast-air opening  29  or swivel nozzle  42  is activated by a compressed-air source  41  (note FIG. 5 a ) with a low air pressure p=approximately 5000 pascals to p=approximately 500,000 pascals. The free air jet produced here passes centrally perpendicularly out of the blast-air opening  29 , or at an angle in the case of the swivel nozzle  42 , comes into contact with the sheet  7  and spreads out approximately parallel to the sheet  7  and the guide surface until the free air jet reaches the outflow openings  31 . This produces a radially decelerated air flow, a hydrodynamic paradox, which results in the sheet  7  being retained in a contact-free manner. 
     The instant that the distance between the sheet and the retaining nozzle  25  drops below a maximum, the sheet is attached by suction by the retaining nozzle  25 . If this distance decreases to a minimum distance, the pressure beneath the sheet increases so that the sheet  7  is forced away from the retaining nozzle  25 . This measure establishes a floating state of the sheet.