Abstract:
The invention relates to a stacking device as well as to a method for depositing sheets on a stack. The task of the invention is to provide a stacking device that deposits sheets on a stacking unit quickly, cost effectively and reliably. The invention solves this task with a method for depositing sheets and a stacking device that contains two gripping devices for alternately gripping and depositing sheets.

Description:
FIELD OF THE INVENTION 
     The invention relates in general to a method and apparatus for stacking sheets in a printing press. More specifically, after the run through the printing module of a printing press, the printed sheets are stacked in a stacking unit of the printing press where various solutions are used to pick up the sheet from the sheet path and to deposit it on the stacking unit. 
     BACKGROUND OF THE INVENTION 
     One prior art mechanism for stacking sheets involves the gripping of the sheet in a transport path with a mechanical gripper, which has two gripper plates that partially grasp the sheet and carry it from the path to a stacking unit. The gripper then deposits the sheet on the stacking unit. However, the speed of this stacking solution is limited, since only one sheet per run is grasped by the gripper and carried to the stacking unit. The speed characteristic is an important criterion for modern high speed, high productivity printing machines. 
     SUMMARY OF THE INVENTION 
     This invention deposits sheets on a stacking unit quickly, cost effectively and reliably. The invention is directed to a method for depositing sheets, and a stacking device that contains two gripping devices, for alternately picking up and depositing sheets relative to a stack. A first gripping device picks up a sheet advantageously within reach of the sheet transport, while a second gripping device simultaneously holds the sheet stack, whereby the sheets to be deposited are at a lower level to prevent the sheet stack from moving during this process. The arrangement of the sheet on this sheet stack is in registered alignment, or stacking may also be offset lengthwise to the sheet transport path. Conveyor rollers are used for this purpose, which slow down the run of the transported sheet at a specific point in time and thus determine the stacking of various sheets in an offset manner in the lengthwise direction. The sheets may then be removed by the operator in stacks from the printing press at the rate of approximately one offset stack per printing job. 
     The invention and its advantages will be better understood from the ensuing detailed description of preferred embodiments, reference being made to the accompanying drawings in which like reference characters denote like parts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the detailed description of the preferred embodiment of the invention presented below, reference is made to the accompanying drawings, in which: 
     FIG. 1 is a schematic front view of a stacking device of a printing press, whereby a gripping device holds down the sheet stack and a second gripping device waits for a sheet from the sheet path; 
     FIG. 2 is a schematic top view of a stacking device according to FIG. 1; 
     FIG. 3 shows a schematic view of a crank mechanism for the stacking device of this invention; 
     FIG. 4 shows a schematic view of the crank mechanism of FIG. 3 being in a further position; 
     FIG. 5 shows a schematic view of the crank mechanism of FIG. 3 being in a still further position; and 
     FIG. 6 shows a schematic side view of a gripping device utilized with this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the accompanying drawings, FIG. 1 shows a schematic front view of a stacking device of a printing press. Following the printing process, sheets  5  are delivered in the direction of the observer (out of the plane of the drawing) to the sheet-stacking unit. The conveyance of the sheets  5  is essentially performed by the conveyor rollers  40  and the side conveyor rollers  50 , which turn in the same direction as the delivery direction of the sheet  5 . A complete transfer of the sheet  5  between the conveyor rollers  40  and the side conveyor rollers takes place at this point. There is a gap between the side conveyor rollers  50  that is smaller than the size of the smallest sheet format used with the printing press. A reliable transport of various sheet formats is ensured in this manner. 
     Grippers of a first gripper device  10  are located at approximately the height of the side conveyor rollers  50 . The first gripper device  10  is arranged approximately 50-60 mm above and beside a sheet stack  90 . A second gripping device  20  is located on the side under the first gripping device  10 , whereby in the present situation, grippers of the second gripping device  20  are positioned on the sheet stack  90  and retain and hold down sheets  5  already deposited on the sheet stack  90 . The gripper devices  10 ,  20  essentially contain at least one gripper respectively, and in this example, the gripper devices  10 ,  20  each contain five individual grippers, with an upper gripper plate  12 ,  22  and a lower gripper plate  14 ,  24 , respectively. Further, there is provided a device for moving the gripping devices  10 ,  20  between at least two positions and a control device for controlling the opening/closing of the grippers and the movement between at least of the two positions, whereby the alternating positions of the gripping devices  10 ,  20  at the sheet stack  90  and close to the sheet path  8  of the printing press are defined by two positions, as described above and which can be seen in FIG.  1 . The stacking device or stacker contains a tray  80  that is inclined and perpendicular to a wall of the stacking unit or bordering on the stacker wall, which provides a stacker ledge  70  for the sheets  5 . 
     There is a stepping motor  16  for moving the first gripper device  10  and a stepping motor  26  for moving the second gripper device  20 . Gripping device  10  is driven by a crank mechanism comprising a stepping motor  16 , a crank  13 , and a connecting rod  11 . Gripping device  20  is driven by a crank mechanism comprising a stepping motor  26 , a crank  23 , and a connecting rod  21 . The stepping motor  16  is connected to the first gripping device  10  via a crank  13  and a connecting rod  11 . The stepping motor  26  is connected to the second gripping device  20  via a crank  23  and a connecting rod  21 . Detailed description of crank mechanisms is given in FIG. 3,  4  and  5 . Driving the stepping motor  16  and the stepping motor  26  the gripping device  10  and the gripping device  20  is moved, respectively. The guiding plate  30  is driven by a stepping motor  17 . Guiding plate  30  is connected to stepping motor  17  and is moved around the axis of the stepping motor  17 . As described FIG. 1 shows two positions of the guiding plate  30 , one position shown with continuous lines and the other position shown with dashed lines. 
     Thus there is a sheet stack  90  that lies on the tray  80 , which is inclined in the direction of the stacker ledge  70 . The sheet path  8  is further bordered by at least one side guide  30 , which is located next to the sheet path and which ensures reliable delivery of the sheet  5 . The side guide  30  is indicated with solid lines in a position next to the sheet path and with dotted lines in a second position, in which the side guide is swiveled outwards. The side guide  30  contains another conveyor roller  32 , which is positioned from below on the sheets  5  in the present state. In this first operating state, according to FIG. 1, the first gripping device  10  is located at approximately the height of the sheet path close to its end and the second gripping device  20  is located below the first gripping device  10  with the sheet stack  90 . The lower gripper plates  24  of the gripper of the second gripping device  20  touch the sheet stack  90  from above and exert a force on it. For this reason, the sheets  5  of the sheet stack  90  maintain their position and are protected from slipping. 
     In FIG. 1, a sheet  5  is delivered by the conveyor rollers  40  and the side transport rollers  50  in the direction of the arrow. Next, the gripper plates  12 ,  14  of the first gripper device  10  open and allow the sheet  5  to be delivered between the gripper plates  12 ,  14 . As soon as the sheet  5  is located at a specific section between the gripping plates  12 ,  14 , the gripping plates  12 ,  14  of the gripping device  10  close and grasp the sheet  5 . The gripping device  10  is then moved from its first position in FIG. 1 to a second position in which the second gripping device  20  in FIG. 1 is located. At the same time, the second gripping device  20  is moved, controlled by the control device of the gripping devices  10 ,  20 , from the second position to the first position near the sheet stack  90 . The first gripping device  10  approaches the sheet stack  90  and the stacking wall with the stacker ledge  70 . The lower gripping plate  14  of the gripping device  10  moves past the stacker ledge  70  and touches the stacker ledge  70  in such a way that the sheet  5  is released from the grip of the lower and upper gripping plates  12  and  14  and is ejected. As a result, the sheet  5  is deposited on the sheet stack  90 , whereby the edge of the sheet borders the stacker ledge  70 , due to the ejection of the sheet  4  at the stacker ledge  70  and the inclined tray  80 . 
     When the gripping devices  10 ,  20  have reached their end position, they have switched their positions; the first gripping device  10  is located at the second position in which the second gripping device  20  is located in FIG.  1  and vice versa. The first gripping device  10  now holds down the sheet stack  90  and the second gripping device  10  opens its gripping plates  22 ,  24  to pick up another sheet. The directions of movement of the two gripping devices  10 ,  20  are both horizontal and vertical and are controlled by the control device. It is obvious from the description to date that the gripping devices  10 ,  20  have switched their positions and their respective functions at the positions; the gripping devices  10 ,  20  are used alternately to hold down the sheet stack  90  and to grasp and carry sheets  5  to the sheet stack  90  of the tray  80 . In their holding-down function, the lower gripping plates  14 ,  24 , press the sheets  5  down on the sheet stack  90 , as can be seen in FIG.  1 . 
     FIG. 2 illustrates a schematic top view of the stacking device according to FIG.  1 . It can be seen that the side conveyors  50  are arranged approximately between the grippers of a second gripping device  20  at the second position. Furthermore, the side conveyor rollers  50  are arranged above the second gripping device  20 , as can be seen in FIG.  1 . In this illustration according to FIG. 2, the first gripping device  10  is located to the left next to the second gripping device  20 ; indeed, the first gripping device  10  is located above and laterally offset to the second gripping device  20 . The upper gripping plates  12 ,  22  of the gripper taper horizontally in the direction of the sheet  5  and are shown coated on their insides that are gripping the sheets  5 , so that the sheet  5  is not damaged by the gripping or transportation by the gripping devices  10 ,  20 . The lower gripping plates  14 ,  24 , have a rectangular shape. If further developed, the stacking device can be configured with an offset of the sheet position. This means that the sheets  5  are arranged with the stacks offset lengthwise, i.e., in the sheet running direction or “in track”. This leads to a stack of sheets  5  that can be removed by the operator of the printing press in individual stacks, corresponding to a single printing job by the printing press. The offset of sheets  5  is achieved, in which the sheets  5  on their way over the sheet path are slowed down at various positions by the side conveyor rollers  50 . Accordingly, the sheets  5  are transported more or less far in the direction of the direction arrow and reach various positions of length. 
     FIG. 3 shows a schematic view of the crank mechanism comprising stepping motor  16  connected to crank  13 , which is connected to connecting rod  11  via a link  19 . FIG. 3 makes clear the way the gripping device  10  is moved. In FIG. 3 crank mechanism is shown in a position at which crank  13  and connecting rod  11  are aligned and have their maximum length seen in the direction to plate  18 . At one end connecting rod  11  comprises a pin  17  reaching through an opening  31  in the plate  18 . As shown, opening  31  has a round form and the pin  17  is moved in the opening  31 . At the other side of the plate  18  in FIG. 3 the pin  17  is connected to the gripping device  10 . By driving the stepping motor  16 , the crank mechanism is moved, with the crank mechanism moving the gripping device  10  as described above. In FIG. 3 crank mechanism comprising stepping motor  16 , crank  13 , connecting rod  11  and plate  18  is shown exemplary. A crank mechanism comprising stepping motor  26 , crank  23 , connecting rod  21 , and plate  28  respectively is driven in the same way. 
     FIG. 4 shows another position of the crank mechanism with the stepping motor  16  continued to move and the crank  13  and the connecting rod  11  being in different alignments. Stepping motor  16  has moved the crank  13  and the connecting rod  11  and consequently moved pin  17  at the end of the connecting rod  11  in the opening  31  and moved the gripping device  10  connected to the pin  17 . The gripping device  10  relating to the position of the crank mechanism shown in FIG. 4 has another position comparing to FIG.  3  and is moved away from the sheet stack  90 , as shown in FIG.  2 . 
     FIG. 5 shows another position of the crank mechanism with the stepping motor  16  continued to move and the crank  13  and the connecting rod  11  being in different alignments with the crank mechanism and the pin  17  being in a nearly extreme position and moving back to the position shown in FIG. 1 as indicated by the arrow. Pin  17  is moving further through opening  31  and reaches the position of FIG. 1 next. 
     FIG. 6 illustrates a schematic side view of the gripping device  10 ,  20  in more detail driven by said stepping motor  16 ,  26  and crank mechanism. The upper gripping plate  12  and the lower gripping plate  14  include a clamping mouth  110  and a clamping mouth  120  (coated tabs) respectively for gripping a sheet between them. Upper gripping plate  12  is connected via a tension spring  60  to an arm  34  that initiates opening the clamping mouth  110  and the clamping mouth  120  by an arm  40  reaching from said arm  34  to the upper gripping plate  12 . Upper gripping plate  12  has a lever  50 , which touches a stop  130  in a position when the gripping device  10 ,  20  is moved in a direction towards the right side of FIG.  6 . Movement in a horizontal direction is achieved by a slider  165  sliding on a guiding device  160 , movement in a vertical direction is achieved by a slider  85  sliding on a guiding device  82 , as shown in FIG.  6 . Slider  85  is fixed to an arm  87  bent from said lower gripping plate  14 . Sliding in vertical direction and in horizontal direction causes the gripping device  10 ,  20  to slide in vertical and in horizontal direction respectively. Upper gripping plate  12  and lower gripping plate  14  are connected by a tension spring  60 . At one end upper gripping plate  12  is connected to a pivot  95  that is mounted at said lower gripping plate  14 . The arm  34  is connected to a pivot  100  that is mounted at the rear side of the lower gripping plate  14 . Lower gripping plate  14  is connected to arm  34  by a pressure spring  70 . When lever  50  is moved towards stop  130  lever  50  presses against stop  130  causing upper gripping plate  12  to open clamping mouth  110 . This is the open position for receiving a sheet between clamping mouth  110  and clamping mouth  120 . 
     Next, with slider  85  moving along the guiding device  82  and slider  165  moving along guiding device  160 , arm  34  is moved towards sheet stock  90  and is pressed against sheet stock  90 . Thereby arm  40  presses against upper gripping plate  12  and moves upper gripping plate  12  so that clamping mouth  110  moves away from clamping mouth  120 . Because of this the sheet between the clamping mouth  110  of the upper gripping plate  12  and the clamping mouth  120  of the lower gripping plate  14  let loose and falls onto the sheet stock  90 . After the arm  34  leaving paper staple  90 , force of the pressure spring  70  causes to close clamping mouth  110  and clamping mouth  120  by pressing arm  34  away from lower gripping plate  14  and consequently arm  40  releases force from upper gripping plate  12 . Then the first gripping device  10  is moved away from the paper staple  90  and in the direction to stop  130  to take over a further sheet while the second gripping device  20  is moved towards the sheet stock  90 . First gripping device  10  and second gripping device  20  take alternating positions as already described. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.