Abstract:
A method for automatically providing a printing plate to a plate cylinder includes the steps of aligning a first edge of a printing plate against a cylinder adjacent to a plate cylinder, moving the first edge of the printing plate into a gap in the plate cylinder, and wrapping the printing plate about the plate cylinder. The present invention also includes a device for attaching a printing plate to a plate cylinder comprising a first set of grippers for gripping a printing plate and aligning the printing plate against a cylinder adjacent to the plate cylinder, and a second set of grippers for moving the printing plate toward a gap in the plate cylinder. An offset printing press having the device is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to printing presses and more particularly to a method and device for providing a printing plate to a plate cylinder during an automatic plate change operation. 
     2. Background Information 
     U.S. Pat. No. 5,390,603 discloses a method for automatic changing of printing plates on a plate cylinder of a printing press. The plate cylinder has a locking or clamping device for a lead edge and trail edge of the printing plate. To change the printing plates, the plate cylinders are driven to a release position and the trail edge of the plates is released. The plate cylinders are then turned in the opposite direction to unwind the printing plates. A new plate for each cylinder is then fed in by suction arms which hold the plate so that the lead edge of the new plate can be attached to the lead edge locking device. 
     U.S. Pat. Nos. 5,495,805 and 5,443,006 disclose devices for carrying away and supplying printing plates to a plate cylinder. In these patents, a magazine has two holding shafts into which cassettes are insertable from above. One cassette is originally empty and can receive used printing plates, while the other holds new printing plates to be attached to the plate cylinder. A plate supplying apparatus takes plates from the new plate cassette and a plate removing apparatus removes the plates from the plate cylinder and places them in the old plate cassette. The plate removing apparatus includes suction elements disposed in a first carriage movable by a driving apparatus along a first guide. To remove the plate, the rear-edge clamping device of the plate cylinder is released, so that the inherent elasticity of the printing plate causes the rear edge of the printing plate to bump against a guide roller. The plate cylinder is then rotated backwardly so that the plate is inserted into the magazine, where the suction elements are activated and, simultaneously with a further backward rotation of the plate cylinder, the first carriage with the suction elements moves the old printing plate into an upper position, the lead edge having also been released. The old plate is then placed in the old plate cassette. To supply a new plate, the plate supplying apparatus has suction elements on a second carriage. The lead edge of the new printing plate is carried to the front-edge clamping device of the plate cylinder. The plate is aligned and clamped. The plate cylinder is rotated in the forward direction, pulling the printing plate onto its outer cylindrical surface. Due to a weakened vacuum of the suction elements, the printing plate still can be held by the suction elements while sliding over the suction elements sue to the plate cylinder rotation. Before the printing plate trail edge passes the last two suction elements, the suction elements are deactivated and a pressing-in roller presses the trail edge radially into the trail-edge clamping device. The trail edge is clamped and the plate tensioned. 
     U.S. Pat. No. 5,701,822 discloses a device for changing printing plates on an offset printing press. The plate cylinder has an automatic clamping device in a gap or channel of the plate cylinder. A holding element to aid in position of the printing plate on the plate cylinder is provided, as is a printing plate loading unit, which holds a printing plate to be loaded. The holding element grips a plate to be inserted and positions the lead edge against the plate cylinder. As the plate cylinder rotates, the lead edge slips into the gap or channel and is clamped by the automatic clamping device. The printing form then can be tautly wrapped around the circumference of the plate cylinder and the trail edge fastened therein, aided by a roller. 
     A problem with the devices and methods described above is that the lead edge of the plate may skew or become misaligned with the plate gap before insertion. This skew or misalignment can cause malfunction of an automatic plate device. Especially when used with very small gap plate cylinders, the above methods and devices are prone to malfunction or cannot be used at all. 
     U.S. Pat. Nos. 5,553,544 and 5,284,093 show small gap plate cylinders, where the trail and lead edges of the plate are inserted into the same gap so that only a small space separates them. U.S. Pat. No. 5,678,487 also shows a small gap plate cylinder and discloses a tucking device for the trail edge. These three patents, as well as U.S. Pat. Nos. 5,495,805 and 5,443,006 discussed above, are hereby incorporated by reference herein. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an improved method for automatically attaching printing plates to a plate cylinder. An additional object is to provide an improved device for providing a printing plate to the plate cylinder. 
     The present invention provides a method for automatically providing a printing plate to a plate cylinder including the steps of aligning a first edge of a printing plate against a cylinder adjacent to a plate cylinder, moving the first edge of the printing plate into a gap in the plate cylinder, and wrapping the printing plate about the plate cylinder. 
     The present method provides for better alignment of the plate before insertion into a gap in the plate cylinder. The lead edge of the plate thus more reliably fits into the gap in proper alignment. Thus with even small gapped plate cylinders, the plate can be inserted more reliably. 
     The present method further may include the step of fastening the first edge in the gap. The plate cylinder may then be rotated to wrap the plate about the plate cylinder. A tucking device may then be used to aid in inserting the trail edge of the plate into the gap. 
     Preferably, the adjacent cylinder is a blanket cylinder of an offset printing press. The blanket cylinder advantageously provides a large adjacent surface on which to align the plate. 
     The present invention also provides a method for attaching a printing plate to a plate cylinder comprising the steps of holding a printing plate through a first set of grippers, moving the first set of grippers so that a lead edge of the plate aligns against a cylinder adjacent to the plate cylinder, holding the printing plate through a second set of grippers, releasing the fist set of grippers, moving the second set of grippers toward a gap in the plate cylinder, and inserting the lead edge of the printing plate into the gap. 
     The present invention also provides a device for attaching a printing plate to a plate cylinder having a first set of grippers for gripping a printing plate and aligning the printing plate against a cylinder adjacent to the plate cylinder and a second set of grippers for moving the printing plate toward a gap in the plate cylinder. 
     Advantageously, the first set of grippers includes spring-mounted grippers, so that the plate may align against the adjacent cylinder. The first set of grippers preferably moves in a direction solely radial to the adjacent cylinder. The second set of grippers preferably is movable in a direction substantially radial to the plate cylinder. 
     The device may further include a tucking device for tucking in the trail edge of the printing plate. 
     The present invention also provides an offset printing press having a plate cylinder with a plate locking mechanism and having a receiving gap on an outer surface, a blanket cylinder forming a nip with the plate cylinder; and a plate attachment device having a first set of grippers for gripping a printing plate and aligning the printing plate against the blanket cylinder and a second set of grippers for moving the printing plate toward the receiving gap. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One preferred embodiment of the present invention is described below by reference to the following drawings, in which: 
     FIG. 1 shows a side view of the device of the present invention with the plate being held by the first gripper set; 
     FIG. 2 shows a side view of the device of the present invention with the plate aligned on the blanket cylinder; 
     FIG. 3 shows a side view of the device of the present invention with the plate being held by the second gripper set and moved into the gap of the plate cylinder; FIG.  4  shows a side view of the device of the present invention with the plate inserted into the gap and the plate locking device closed for rotation; 
     FIG. 5 shows a side view of the device of the present invention with the plate being rotated through the plate-blanket nip; 
     FIG. 6 shows a side view of the device of the present invention with the trail edge of the plate being tucked into the plate cylinder gap; 
     FIG. 7 shows a cross-sectional view of a possible configuration of the first gripper set of the present invention; 
     FIG. 8 shows a section of alternate construction for supporting the first and second gripper sets of the present invention; 
     FIG. 9 shows schematically the details of the construction of FIG. 8; and 
     FIG. 10 shows a possible positioning of the first and second gripper sets with the alternate construction of FIG.  8 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a schematic side view of plate attachment device  10 , which includes a first gripper set  20 , a second gripper set  30  and a tucking device  40 . The plate attachment device is located next to an offset printing unit  1 , which includes a plate cylinder  2 , an adjacent upper blanket cylinder  3  and a lower blanket cylinder  4 . A plate locking device  5  is located in a channel at the surface of plate cylinder  5  and moves a plate clamp  6  toward or away from the surface of plate cylinder  5 . The locking mechanism may be similar, for example, to those disclosed in U.S. Pat. Nos. 284,093, 5,553,544, and 678,487. A gap  7  is formed between the channel side wall and plate clamp  6 , for accepting a lead edge  62  and a trail edge  64  of a printing plate  60 . 
     First gripper set  20  includes at least a first gripper  22 , which is held in a gripper holder  24  between a first spring  26  and a second spring  27 . The first gripper set  20  is movable in a track  28  and can grip plate  60  using, for example, suction. The first gripper  22  moves in track  28  so that plate  60  moves radially with respect to blanket cylinder  3 . First gripper  22  is held by springs  26  and  27  such that first gripper  22  is slightly flexible radially in the direction of springs  26  and  27 , but are stable or fixed axially, i.e. into the page as shown in FIG. 1, and tangentially to blanket cylinder  3 . The grippers may be provided with a suction force through a vacuum device and a flexible tube connected to the grippers. The suction grippers may be similar generally to the suction pads disclosed in U.S. Pat. No. 5,443,006, for example. 
     FIG. 7 shows a cross-section view of first gripper set  20  and shows that gripper set  20  may comprise, for example, a plurality of grippers (shown by dotted lines)  22 ,  23  and  29 , held between springs. Gripper holder  24  may be an H-bar, for example, moving in track  28 . Toothed wheels run by a motor may, for example, drive gripper holder  24 . However, as defined herein a gripper set may also include a single gripper. 
     As shown in FIG. 2, once plate  60  has been gripped by first gripper set  20 , gripper holder  24  is moved radially toward blanket cylinder  3  so that the entirety of lead edge  62  contacts blanket cylinder  3 . Any skew or misalignment of lead edge  62  with respect to blanket cylinder  3  is corrected, as the springs  26  and  27  permit lead edge  62  to align with the surface of blanket cylinder  3 . Lead edge  62  thus also is aligned properly with respect to plate cylinder  2 , which is coaxial with blanket cylinder  3 . Lead edge  62 , axis  102  of plate cylinder  2  and axis  103  of blanket cylinder  3  therefore all are parallel to each other. 
     Second gripper set  30 , having a gripper  31 , a second gripper holder  32  and a track  33  is then moved to grip plate  60  in its properly aligned position. Gripper set  30  thus firmly grips, through for example suction, plate  60  in its aligned position. First gripper set  20  then is released by removing the suction, or by providing a positive air flow. 
     As shown in FIG. 3, gripper holder  32  then moves in track  33  so that lead edge  62  of plate  60  fits into gap  7  next to unlocked locking mechanism  5  of plate cylinder  2 , which is in an initial insertion position. Advantageously, track  33  may be curved to aid in providing a proper insertion angle. As an alternative to this insertion method, track  33  may be shaped so that the lead edge  62  first hits against the surface of lock-up clamp  6  so that plate  60  bends backwardly so as to create a slight spring force. Plate  60  is then moved upwardly along the surface of lock-up clamp  6  and springs into gap  7 . 
     Grippers  31  preferably are spring-supported in an axial direction, i.e. in and out of the page in FIGS. 2,  3  and  4 . The entire second gripper set  30  is movable as well in the axial direction, for example by providing a linear driver for track  33  in an axial direction. Once plate  60  is inserted in gap  7 , second gripper set  30  is moved axially so as to push the plate against an axial plate stop to provide proper axial alignment. 
     Suction is then removed, or a positive air flow provided, to release plate  60  from second gripper set  30 . Lock-up mechanism  5  is then locked. Plate  60  is thus fixed in gap  7  in proper alignment. Plate  2  may then be rotated in the direction of the arrow in FIG.  4 . 
     As shown by FIG. 5, plate  60  is thus pulled through the plate cylinder-blanket cylinder nip and may additionally be supported by deactivated gripper  31 . Alternately, gripper  31  may be withdrawn if the stiffness in the plate permits that plate  60  will not be damaged by any components of attachment device  10 . 
     When plate cylinder  2  returns to its original insertion position, trail edge  64  is ready to be tucked into gap  7 . Locking mechanism  6  is activated so that clamp  6  moves away from the surface of plate cylinder  2 . Tucking device  40 , which may be, for example, similar to the tucking device disclosed in U.S. Pat. No. 5,678,487, can then be activated to tuck trail edge  64  into gap  7 . Clamp  6  is retracted and tucking mechanism  40  deactivated. Plate  60  is thus firmly attached to plate cylinder  2 . 
     Removal of the plate after use can be effected in a variety of ways be releasing the trail edge, and using an automatic deplating device, for example on the other side of plate cylinder  2 , or by using plate attachment device  10  to guide the plate off the plate cylinder. 
     As an alternative to the two separate gripper movement mechanisms shown in FIG. 1, it is also possible to provide the second gripper set on gripper holder  24  shown in FIG.  7 . The gripper holder  24  as shown in FIG. 1 could then be rotated upwardly to provide insertion of the plate, after the alignment with the blanket has occurred. 
     Another alternative to the gripper movement mechanisms shown in FIG. 1 is a bar mechanism as shown in FIGS. 8 and 9. The two gripper sets  20  and  30  may be supported on a gripper support  80 , the second gripper set  30  for example being located below first gripper set  20 , as shown in FIG.  10 . 
     Support  80  is connected at its bottom end to an air cylinder support  84  and a first arm  90  of the bar mechanism by a pivot  82 . Support arm  80  is connected at its top end by a pivot  88  (FIG. 9) to one side of a lever  94  of the bar mechanism. The other side of lever  94  is attached to an end  93  of a second arm  92  by a pivot  95 . Pivot  95  in turn is connected to an air cylinder  96  supported on air cylinder support  84 . 
     Arms  90  and  92  are rotatably supported in, for example, a frame at ends  98  and  99 , respectively. An activating arm  100  for arm  90  is connected at one end to end  98  and an air cylinder  102  is connected to the other end of activating arm  100 . 
     A similar bar mechanism is located on the other side of support  80 . 
     The bar mechanism operates as follows. First gripper set  20  grips a plate from a plate supply. Air cylinder  102  is actuated, which rotates arm  90  about end  98  as shown by arrow  108 . Thus gripper support  80  is rotated downwardly in the direction of arrow  110  so that the plate bumps and aligns against a blanket. The plate is then gripped by the second gripper set  30 , and released by first gripper set  20 . Air cylinder  96  then is actuated, causing gripper support  80  to rotate in the direction of arrow  112 , thus moving the plate edge against the locking bar of the plate cylinder. Upon a further movement of the gripper support  80 , the plate springs into the gap in the plate cylinder and is aligned axially, for example through moving the entire bar mechanism axially. The lock-up mechanism in the plate cylinder is then activated and the plate released from second gripper set  30 . 
     In addition to being used to move the plate into the plate cylinder gap, the second air cylinder may also be activated to aid in gripping new plates from a new plate supply. 
     Plate cylinder  2  may be a small-gapped plate cylinder, which is defined herein to mean that the receiving gap is less than 0.25 inches. While the present invention has been described using the blanket cylinder for alignment purposes, it may also be possible to use another cylinder adjacent to the plate cylinder, for example an ink roller, to align the printing plate before insertion. The blanket cylinder preferably includes a gapes blanket. A gripper set as defined herein means at least one gripper capable of supporting the printing plate when activated.