Patent Publication Number: US-7909454-B2

Title: Inkjet printing press

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to an inkjet printing press for printing sheets with inkjets. 
     2. Background Art 
     Inkjet printing presses are known which have an inkjet head provided at the outer periphery of a rotary printing drum to jet ink against a sheet held on the peripheral surface of the printing drum for printing. 
     With such an inkjet printing press, the smaller the gap between the sheet and the inkjet head having a plurality of nozzles, the less the interference to ink dots jetted from the nozzles and thus the better the printing quality. But, if the gap is too small, the sheet might contact the inkjet head, thus causing smudges or accidents. Practically, the press is used with the inkjet head in proximity to the sheet to a limit where there is no trouble such as mechanical contact while the inkjet head and the sheet are moving relative to each other for printing. Because this gap is normally extremely small, if the sheet is not held properly on the peripheral surface of the printing drum, accurate printing is impossible and the sheet can contact the inkjet head, thus causing smudges on the print surface of the sheet. 
     Therefore, with such a press, it is necessary to hold the sheet stably on the printing drum so that the entire surface of the sheet is kept in close contact with the peripheral surface of the printing drum. 
     As means for holding the sheet, the following two are known. One involves attracting the sheet directly on the peripheral surface of the printing drum by static electricity or air suction, and the other involves holding the ends of the sheet by clamp strips or divided clamps. 
     With the method of holding a sheet by surface attraction such as by static electricity as disclosed in JP Patent 2868723, the sheet holding force is strong in the transverse direction but weak in the vertical direction. Thus, in printing a firm sheet, attraction at the sheet end tends to be insufficient, and if the sheet is thick, the sheet end tends to separate from the peripheral surface of the printing drum. Also, in order to hold a sheet being fed at high speed, large static electricity is required. This may cause electrical discharge with an inkjet press in which the inkjet head has to be provided in proximity to the printing drum. 
     Also, with the method of holding the sheet by negative pressure as disclosed in JP Patent 2868723, it is possible to attract the sheet onto the printing drum properly at a closed area such as at the center of the sheet, but at the peripheral portion of the sheet, because air is sucked from sheet ends, negative pressure cannot be maintained, so that sheet ends cannot be held effectively. Further, if the sheet is absorptive such as paper, negative pressure applied to the sheet may cause the ink applied by the inkjet head to permeate into the sheet. This results in change of ink color depending on the degree of permeation and makes it difficult to provide stable printing quality. 
     On the other hand, with the method of holding the sheet mechanically by clamps, the sheet ends are prevented from separating from the drum surface because the sheet is clamped at its ends. But there are the following problems. 
     That is, because ordinary clamps are of such a structure that they open and close radially of the printing drum, they cannot impart a tensile force to the sheet. Thus, the sheet tends to be loose and lift off the drum surface, so that it is not possible to obtain a stable mounting state in which the entire sheet is in close contact with the drum surface. 
     As a device which uses clamps to impart tension to the sheet, the device disclosed in Japanese patent publication 7-195780A is known. But because it imparts tension by dividing a portion of the rotary drum corresponding to the trailing end of the sheet so as to be movable, it is not suited for use with general-purpose presses which print sheets of different sizes at high speed. 
     Also, because the clamps holds the sheet while pulling both ends of the sheet extending axially toward the center of the drum, if the sheet is thick, the clamps tend to run obliquely on the sheet ends and warp on the sheet with the central portions of the clamps protruding radially outwardly from the drum surface. Thus, the clamps have to be rigid and thick. In this arrangement, it is difficult to assure a gap of proper size between the inkjet head and the peripheral surface of the printing drum. 
     Also, as disclosed in JP Patent 2559043, a sheet trailing end clamp has been put to practical use which is divided into a plurality of parts axially of the printing drum. But because it is adapted to clamp only part of the trailing end of the sheet, if the sheet supplied has a bent end, the clamp can not hold the sheet. Also, with this type of printing drum, the drum surface is divided circumferentially into rings, and the divided clamps are movably held between the rings, thereby holding sheets of different sizes. But in order to make smooth the drum surface on which divided sheets have to be held in a smooth state, a complicated mechanism requiring high accuracy is needed, and thus the device tends to be very expensive. 
     An object of the present invention is to provide an inkjet printing press which can hold sheets extremely stably on the peripheral surface of the printing drum and thus can print at high speed with high accuracy. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, according to a first aspect of the present invention, the inkjet printing press includes a rotary printing drum and an inkjet head provided at an outer periphery of the printing drum and having nozzles through which ink is jetted against a sheet held on the peripheral surface of the printing drum for printing, characterized in that the printing drum is provided with a sheet holding mechanism for holding a leading end of the sheet on the printing drum, clamps which can be opened and closed for pressing a trailing end of the sheet against the peripheral surface of the printing drum, a clamp opening/closing mechanism for opening and closing the clamps, a position changing mechanism for adjusting the positions of the clamps in cooperation with the clamp opening/closing mechanism circumferentially of the printing drum according to the position of the trailing end of sheets having different lengths, and an attracting force producing means for producing a force for attracting the clamps together with the trailing end of the sheet to the peripheral surface of the printing drum. 
     Also, according to a second aspect of the present invention, each clamp is a thin plate strip extending axially of the printing drum and the thickness of the clamps is set so that the sum of the thickness of each clamp and the thickness of the sheet is smaller than a gap defined between the outer peripheral surface of the printing drum and the inkjet head. This assures that the sheet can be held without interfering with the inkjet head. 
     Also, according to a third aspect of the present invention, after the sheet holding mechanism has held a new sheet supplied to be held on the printing drum, in the step of holding the trailing end of the sheet by a clamp, when the clamp moves from its open position where it is off the peripheral surface of the printing drum to its closed position where it is on the trailing end of the sheet and the peripheral surface of the printing drum, the clamp moves first forward of the rotating direction of the printing drum and then backward. This assures that the frictional force acting on the contact surface between the clamp and the sheet imparts tensile force to the sheet and removes looseness of the sheet, thereby holding the sheet stably. 
     Also, according to a fourth aspect of the present invention, the sheet holding force of the sheet holding mechanism is set to be larger than the tensile force in a tangential direction of the periphery of the printing drum, imparted to the sheet when the clamp holds the sheet in the third aspect of the invention, and the frictional force by contact between the clamp and the sheet is set to be larger than the frictional force by contact between the sheet and the surface of the printing drum. This assures that while the clamp imparts tension to the sheet to remove looseness, the tip of the sheet will not get off the sheet holding mechanism. 
     In the inkjet printing press according to the first to third aspects of the invention, a plurality of sheet holding mechanisms are provided circumferentially of the printing drum at equal intervals and the clamps are each provided so as to correspond to one of the sheet holding mechanisms. This makes it possible to print a plurality of sheets simultaneously and improve the printing efficiency. 
     In the inkjet printing press according to the first and second aspects of the invention, as the attracting force producing means, an attracting body having a magnetic attracting force acting on the outer periphery of the printing drum is used, and the clamps may be formed of a magnetic material so as to be attracted by the attracting body. The attracting force producing means may comprise a multiplicity of suction holes formed in the outer periphery of the printing drum, and a suction head provided in the printing drum to apply suction force to the suction holes closed by the clamp to attract the clamp to the peripheral surface of the printing drum. 
     A rotatable hold-down roller may be provided at the outer periphery of the printing drum so as to be moved into and out of contact with the outer periphery of the printing drum and to press the sheet against the peripheral surface of the printing drum while the sheet is rotating together with the printing drum with its leading end held by the sheet holding mechanism. Because the sheet can be pressed against the outer periphery of the printing drum by the hold-down roller, the sheet can be brought into close contact with the printing drum more effectively. 
     According to the present invention, the clamps in their closed positions are adjusted according to the size of the sheet and hold the sheet with the trailing end of the sheet attracted to the surface of the printing drum by the attracting mechanism. Therefore, it is possible to hold the trailing end of the sheet securely independently of the size of the sheet and to adopt thin metal sheets having a low rigidity as the clamps. This makes it possible to set the gap between the inkjet head and the sheet to a very small value and thus makes high quality printing possible. Also, because the sheet holding force can be produced reliably even if the area for holding the trailing end of the sheet by a clamp is small, a wider area of the sheet to near its trailing end is usable for printing as the effective printing area. 
     Also, the attracting force at the peripheral surface of the printing drum acts on the clamp portion only and has little effect on the sheet itself. Therefore, good printing quality is assured independently of the size and material of the sheet. 
     Further, the clamp for holding the trailing end of the sheet is moved backward with respect to the rotating direction of the printing drum when it is moved from their open position to closed position, so that tension is imparted to the trailing end of the sheet by frictional force produced by contact with the clamp. Therefore the sheet can be held in a stable state with the entire sheet in close contact with the peripheral surface of the printing drum and thus extremely high quality printing is possible by inkjet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially cut away front view showing one embodiment of the printing press according to this invention; 
         FIG. 2  is a vertical sectional side view of the printing drum shown in  FIG. 1 ; 
         FIG. 3  is an enlarged sectional view of a portion of the printing drum; 
         FIG. 4  is an enlarged vertical sectional front view of a portion of the printing drum; 
         FIG. 5  is a sectional view of a clamp; 
         FIG. 6  is a vertical sectional front view of another embodiment of an attracting means; 
         FIG. 7  is a vertical sectional side view of  FIG. 6 ; and 
         FIG. 8  is a plan view of the embodiment of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now an embodiment of this invention is described with reference to the drawings. As shown in  FIG. 1 , the printing press according to the present invention includes a rotary printing drum  1 , a sheet feeder  2  for feeding sheets S to the printing drum  1 , an inkjet head  3  facing the outer periphery of the printing drum  1  to jet ink to sheets S held on the periphery of the printing drum  1  and fed circumferentially for printing, and a sheet discharge unit  5  for receiving the printed sheets S from the printing drum  1  and feeding them to a sheet discharge station  4 . 
     The sheet feeder  2  feeds sheets S one after another in one direction on a sheet conveyor  2   a , grips the tip of the sheet S fed to the discharge end of the sheet conveyor  2   a  by a pivotal end of a swing gripper  2   b , and pivots the swing gripper  2   b  toward a transfer cylinder  2   c  which is rotating in one direction to deliver the sheets S from the swing gripper  2   b  to holding claws provided on the transfer cylinder  2   c  and then deliver the sheets S from the transfer cylinder  2   c  to the printing drum  1 . 
     The inkjet head  3  has a nozzle surface  3   a . A gap G about 1 mm is present between the nozzle surface  3   a  and the outer periphery of the printing drum  1 . The inkjet head  3  has a multiplicity of nozzles (not shown) in the nozzle surface  3   a  through which ink is jetted onto the periphery of the printing drum  1  to print the sheets S. 
     The sheet discharge unit  5  is a chain delivery comprising a sheet take-up cylinder  5   a , a chain  5   b  trained around the cylinder  5   a , and a plurality of gripper bars  5   c  mounted on the chain  5   b  at equal intervals. The printed sheets S are transferred from the printing drum  1  to the gripper bars  5   c . By moving the chain  5   b , the sheets S held by the gripper bars  5   c  are fed to the sheet discharge station  4 , where the sheets are released. 
       FIGS. 2 to 4  show the printing drum  1 . The drum  1  has drum shafts  10  at both ends which are rotatably supported by a pair of frames  11  of the printing press. 
     A gear  12  is mounted on one end of the printing drum  1 . Driving torque is transmitted to the gear  12  to rotate the printing drum  1  in the direction of the arrows in  FIGS. 1 and 4 . 
     The printing drum  1  is formed in its outer periphery with a plurality of recesses  13  in the form of axial grooves arranged at equal circumferential intervals. A claw shaft  14  is received in each recess  13 . 
     The claw shafts  14  are rotatably supported on end walls of the recesses  13 . A plurality of holding claws  15  are mounted on the portion of each claw shaft  14  that is located in the recess  13 . Although the holding claws are used to hold sheets, any other means using negative pressure, static electricity, etc. may be used instead, provided it can hold the tips of the sheets properly. 
     As shown in  FIG. 2 , the holding claws  15  are opened and closed by a cam mechanism  16  as an opening/closing mechanism. The cam mechanism  16  has roller arms  17  each mounted at one end thereof on one end of each claw shaft  14  and rotatably carrying a roller  18  at the other end. The rollers  18  are kept in elastic contact with the outer surfaces of claw opening/closing cams  19  each mounted on the inner wall of one of the frames  11  through springs (not shown). As the printing drum  1  rotates, the rollers  18  roll along the outer surfaces of the claw opening/closing cams  19  to open and close the holding claws  15 . The claw opening/closing cams  19  on both sides work as conjugate cams. Although a cam mechanism is used as the clamp opening/closing mechanism, any other mechanism using a motor or solenoids may be used instead. 
     The holding claws  15  are normally held in closed positions. When it is necessary to feed a sheet S while the printing drum  1  is rotating, the holding claws  15  are opened and closed by the claw opening/closing cams  19  at a position where they are opposite to the transfer cylinder  2   c , so that the sheet S is transferred from the transfer cylinder  2   c  when the holding claws  15  turn from their open states to their closed states. Also, when it is necessary to discharge a sheet S held on the printing drum  1 , the holding claws  15  are opened by the claw opening/closing cams  19  at a position where they are opposed to the sheet take-up cylinder  5   a . When the claws  15  turn from their closed states to their open states, the sheet S is transferred to one of the gripper bars  5   c  of the sheet discharge unit  5 . 
     As shown in  FIGS. 1 and 4 , the peripheral surfaces of the printing drum  1  between the adjacent recesses  13  are formed by attracting bodies  20  which are sheets of magnetic rubber (rubber magnet) having a thickness of 3.2 mm and having a magnetic attracting force. The leading edge of each attracting body  20  with respect to the rotating direction of the printing drum  1  is fixed to an anvil  20   a  and its trailing edge is fixed to a bar  20   b , both by an adhesive, and the anvil  20   a  and the bar  20   b  are removably fixed to the printing drum  1  so that the attracting bodies  20  can be replaced. The attracting bodies  20  have their surfaces finished by grinding into flat surfaces and are held in close contact with the printing drum  1 , which is finished by grinding to a cylindrical shape, so that the gap between the printing drum  1  and the inkjet head  3  can be kept accurately. The accuracy of the gap can also be assured by finishing the printing drum by grinding after mounting the attracting bodies  20 , which are sheets of magnetic rubber, on the surface of the printing drum  1  into close contact therewith. Further, instead of using attracting bodies as separate parts, the printing drum  1  itself may be formed of a magnetic material. 
     As the magnetic rubber, it is necessary to use a rubber substrate having corrosion resistance to the ink used or to subject the surface of the rubber to coating to impart corrosion resistance. 
     Over each attracting body  20 , a clamp  21  is provided to press the trailing end of a sheet S against the outer periphery of the attracting body  20  with the tip of the sheet S held by the holding claws  15 . 
     The clamps  21  are stainless steel sheets SUS304CSP made of a magnetic material and having a thickness of 0.2 mm and a width of 20 mm so as to be attracted by the attracting bodies  20 . The thinner the clamps  21 , the more easily they can be bent along the periphery of the printing drum  1 , so that sheets S can be pressed more reliably. On the other hand, the thicker the clamps  21 , the larger the attracting force by magnetic force. Therefore, stainless steel sheets having a thickness of 0.05 to 0.5 mm, preferably 0.1 to 0.3 mm are used as the clamps  21 . As the material for the clamps, stainless steel alloy is excellent in view of magnetic force, strength and corrosion resistance, but spring steel having strong magnetism may be used according to the intended use. 
     As shown in  FIG. 3 , the clamps  21  extend axially of the printing drum  1  as elongated one-piece parts and have their ends bent at a right angle inwardly into a □-shape. Their bent portions are inserted into slots  23  formed at both ends of the outer periphery of the printing drum  1  and tips of clamp arms  24  are coupled to the bent portions. 
     As shown in  FIGS. 3 and 4 , one end of a short link  25  is coupled to each clamp arm  24  at a position near its trailing end and one end of a long link  26 , too, is coupled to the clamp arm  24  at a position nearer to its trailing end than the one end of the short link  25 . The other end of the short link  25  is supported on a first shaft  27 , which is rotatably supported by two annular plates  28  provided in the printing drum  1 , so that the short link  25  can pivot about the first shaft  27 . 
     On the other hand, the other end of the long link  26  is supported by a second shaft  29 , which is rotatably supported on the annular plates  28 , so that the long link  26  can pivot about the second shaft  29 . 
     The two annular plates  28  are coupled together by a plurality of stays  30  extending between the annular plates  28  and are supported so as to be slidable and rotatable about a control shaft  44  coaxial with the axis of the printing drum  1 . Thus, by turning the annular plates  28 , the positions of the plurality of clamps  21  are adjusted simultaneously in the circumferential direction. 
     Because the positions of the clamps  21  are adjustable in the circumferential direction of the printing drum  1  as described above, it is possible to clamp the trailing ends of sheets S of different sizes. 
     The clamps  21  are opened and closed by a cam mechanism  31  shown in  FIG. 2 . The cam mechanism  31  includes roller arms  32  each having one end thereof mounted on one of the ends of the second shaft  29 , which protrude from the sides of the printing drum  1 , and rotatable rollers  33  carried on the other ends of the roller arms  32  and kept in elastic contact with the outer surfaces of clamp opening/closing cams  34  each mounted on the inner wall of one of the frames  11  through springs (not shown). Thus, as the printing drum  1  rotates, the rollers  33  roll along the outer peripheries of the clamp opening/closing cams  34  to open and close the clamps  21 . The clamp opening/closing cams  34 , which are provided on both sides, work as conjugate cams. 
     As described above, the short link  25  coupled with each clamp arm  24  is pivotable about the first shaft  27  whereas the long link  26  is pivotable about the second shaft  29 . Therefore, when the clamp  21  in its open position moves to its closed position, where it abuts against the peripheral surface of the magnetic rubber sheet  20 , it moves first forward in the rotating direction of the printing drum  1  and then backward into its closed position as shown in  FIG. 5  by an arrow. 
     The position of each clamp  21  is adjusted beforehand according to the size of the sheet S so that it rests on the trailing end of the sheet S and the surface of the attracting body  20  in its closed position. 
     The annular plates  28 , which serve to adjust the circumferential positions of the clamps  21 , are locked relative to the printing drum  1  by a lock mechanism  35  shown in  FIG. 3 . 
     The lock mechanism  35  includes a braking plate  36  provided outside of one of the annular plates  28  so as to rotate together with the printing drum  1  and a pad  37  provided on the other side of the annular plate  28  so as to move toward the annular plate  28  to press the annular plate  28  against the braking plate  36 . 
     In this embodiment, as a means for moving the pad  37  toward the annular plate  28 , a lever  40  is pivotally supported on a support  38  fixed to the braking plate  36  through a pin  39 ; the pad  37  is mounted on one end of the lever  40 ; the other end of the lever  40  is coupled with the braking plate  36  through a two-joint link  41 ; one end of a link  42  is coupled to the bent portion of the two-joint link  41 ; and an L-shaped arm  43  coupled to the other end of the link  42  is mounted on the control shaft  44 , which is arranged on the axis of the printing drum  1 . By axially moving the control shaft  44 , the pad  37  is moved relative to the annular plate  28 . 
     As shown in  FIG. 1 , a hold-down roller  45  is provided over the transfer cylinder  2   c  of the sheet feeder  2 . The hold-down roller  45  is movable into and out of contact with the outer periphery of the printing drum  1 . When in contact, it serves to press the sheet S against the peripheral surface of the printing drum  1  while the sheet is being fed circumferentially with its leading end held by holding claws  15 . 
     The printing press of this embodiment is of the structure described above. When a sheet S is fed onto the printing drum  1  by the sheet feeder  2 , it is fed circumferentially by the rotation of the printing drum  1  with its leading end clamped by holding claws  15 . 
     At this time, the hold-down roller  45  is arranged in a position where it is put into contact with the printing drum  1 . Thus, the sheet S fed in a circumferential direction of the printing drum  1  is pressed by the hold-down roller  45  against the peripheral surface of the printing drum  1  into close contact therewith. 
     When the entire area of the sheet S is on the periphery of the printing drum  1 , the cam mechanism  31  for opening and closing the clamps  21  moves one of the clamps to closed position, so that the clamp abuts the peripheral surface of the printing drum  1 . 
     At this time, because the short link  25  and the long link  26  pivot to move the clamp  21  first forward in the rotating direction of the printing drum  1  and then backward into its closed position, a pulling force is imparted to the trailing end of the sheet S by contact with the clamp  21 . 
     Now the relative position between the sheet S and the clamp  21  when feeding the sheet is described. Just before the clamp  21  begins to hold the sheet S, the clamp  21  is in the position  21   a  in  FIG. 5 , where it does not interfere with the trailing end of the sheet S. Next, in the position  21   b , the clamp  21  moves radially inwardly toward the sheet S to hold down its trailing end. In the position  21   c , the central portion of the clamp  21  is attracted by the magnetic rubber  20  and bent until a portion of the clamp  21  contacts the magnetic rubber  20  with the sheet S between them. While pressing the trailing end of the sheet S partially, the clamp  21  moves backward with respect to the rotating direction of the printing drum  1  and radially inwardly. In the position  21   d , the entire surface of the clamp  21  is attracted by the magnetic rubber  20  with the sheet S between them. In this state, the relation K&gt;M&gt;D is met, where K is the sheet holding force by the holding claws  15 ; M is the sliding frictional force between the clamp  21  and the sheet S; D is the sliding frictional force between the sheet S and the surface of the printing drum  1  (attracting body  20 ). When the clamp  21  moves backward while maintaining contact with the sheet S, tensile force is applied to the sheet S backward with respect to the rotating direction of the printing drum  1 . With this series of actions, it is possible to reliably bring the sheet S into close contact with the surface of the printing drum without looseness to the trailing end while holding the sheet S stably by the holding claws  15 . Also, such a clamp mechanism is effective to prevent the sheet from getting marred or smeared even if used with a transfer cylinder of an offset press or more complicated apparatus using a rotary drum. 
     Because by keeping the clamp  21  closed, the sheet S can be securely held in a stable state with its entire surface in close contact with the printing drum  1 , the gap between the inkjet head  3  and the sheet S can be set properly, thereby assuring highly accurate printing. Also, because the sheet holding size necessary for holding the sheet S by the clamp  21  can be shortened, it is possible to use a wider area for printing nearer to the trailing end of the sheet than before. Particularly, if the sheet holding size is three to 200 times the thickness of the sheet, preferably 20 to 100 times, it is possible to hold the sheet stably and provide a wide effective printing range. 
     When printing of the sheet S is complete, the sheet S is transferred from the printing drum  1  to the sheet discharge unit  5 . In this state, the holding claws  15  of the printing drum  1  open and one of the gripper bars  5   c  holds the tip of the sheet S. When the printing drum  1  turns to a position where the trailing end of the sheet S leaves the periphery of the printing drum  1 , the cams  34  of the cam mechanism  31  set beforehand in a position for opening the clamp  21  operate to open the clamp and release the trailing end of the sheet S. At this time, because the clamp releasing force of the cam mechanism  31  is set to be larger than the attracting force of the attracting body  20  acting on the clamp  21 , the clamp can be released extremely easily without the need of adopting a complicated arrangement using an electromagnet as an attracting body to turn the attracting force on and off. 
     The printed sheet S is fed by the sheet discharge unit  5  to the sheet discharge station  4  and discharged. 
     When the size of the sheet S to be printed changes, the positions of the clamps  21  have to be adjusted according to the size of the sheet S. 
     In adjusting the positions of the clamps  21 , all the clamps are moved to their open positions by means of the cams (not shown) provided for opening and closing the clamps, and pins (not shown) are pushed into the frames  11  to fix the phase of the annular plates  28  relative to the frames  11 . Then, the lock mechanism  35  shown in  FIG. 3  is operated to unlock the annular plates  28 . By imparting a driving force to the gear  12 , the printing drum  1  is turned by a required amount according to the size of the sheet S to be printed. Since the plurality of clamps  21  are supported by the annular plates  28 , the position of the plurality of clamps can be adjusted simultaneously. 
     After adjusting the positions of the clamps  21 , the lock mechanism  35  is operated to lock the annular plates  28 , and the pins are removed from the frames  11  to unlock the annular plates  28 . 
     In the embodiment shown in  FIG. 5 , magnetic attracting bodies are mounted on the periphery of the printing drum  1  to attract the clamps  21 . But, means for attracting the clamps  21  is not limited thereto. 
     For example, as shown in  FIGS. 6 and 7 , a plurality of suction holes  50  arranged axially at equal intervals may be formed in the peripheral wall of the printing drum  1  in rows and suction heads  51  may be provided in the printing drum  1  to apply a suction force to the suction holes  50  adapted to be closed by the clamps  21  when the clamps  21  are closed. 
     With such a suction means, the suction heads  51  and the clamps  21  should be arranged to allow circumferential position adjustment at the same phase to adapt to sheets S of different sizes. 
       FIG. 8  shows one example of the positional relationship among the clamp  21  and the suction holes  50  and suckers  52  as viewed from over the printing drum  1 . The printing drum  1  is formed with the suction holes  50  of 3 mm diameter arranged at a circumferential pitch P of 10 mm in 11 rows and the suction holes  50  in adjacent rows are offset circumferentially by a distance 0 of 1 mm. The suction heads  51  are arranged in the printing drum  1  just under the respective clamps  21 , which have a width of 20 mm, so as to be movable at the same phase as the clamps  21  and the suckers  52  have an effective suction diameter of 15 mm and are arranged at the same axial intervals as the suction holes  50 . With this arrangement, in whatever phase the clamps  21  may be, suction force can be applied to at least one row of the suction holes  50  under the clamps  21 , so that the clamps can be attracted reliably. Thus, it is possible to adapt to sheets S of different sizes. 
     Also, although in the embodiment shown in  FIGS. 1 to 5 , the clamps  21  are moved backward with respect to the rotating direction of the printing drum  1  to pull the trailing end of the sheet S with its leading end held by holding claws  15 , the clamps  21  may be arranged to be opened and closed radially of the printing drum  1  because the sheet S can be put into close contact with the periphery of the printing drum  1  by means of the hold-down roller  45 .