Abstract:
A sheet stacking device for forming a stack of subsequent sheets includes a rotatably arranged flipping element and a closed-loop shaped friction element. The flipping element includes a slot at its circumferential edge for accepting at least a portion of a sheet. In a first rotation zone, the flipping element is able to accept the sheet into the slot and, in a second rotation zone, the sheet is conveyed out of the slot onto the top of the stack of subsequent sheets. The friction element is moveably arranged on the flipping element, and is controllable to move into a first radial position and a second radial position. In the first radial position, the friction element does not apply a frictional force to a sheet in the transport path and in the second radial position, the friction element does apply a frictional force to a sheet in the transport path.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of International Application No. PCT/EP2012/071899, filed on Nov. 6, 2012, and for which priority is claimed under 35 U.S.C. §120. PCT/EP2012/071899 claims priority under 35 U.S.C. §119(a) to Application No. 11188411.0, filed in Europe on Nov. 9, 2011. The entire contents of each of the above-identified applications are hereby incorporated by reference into the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to a sheet stacking device for forming a stack of subsequent sheets and to a printing system comprising such a sheet stacking device. 
     2. Description of Background Art 
     A typical example of such a sheet stacking device is known from U.S. Pat. No. 5,065,997. This known device comprises a pair of disks, which have been mounted onto a driven rotation shaft. Each one of the disks comprises a pair of acceptance slots, as well as a pair of friction elements. During the course of a cycle, incoming sheets may be accepted into the slots that have been made in the disk. If applicable, the incoming sheets have already sustained an initial lateral registration course and are conveyed into a slot by means of an input clamping arrangement. An accepted sheet is conveyed onto a receiving plane during part of the revolution, after which the friction elements will convey the sheet against a stop during part of the revolution. 
     A disadvantage of this known device is that, in use, it is not sufficiently precise in producing accurately formed stacks. In use, it regularly happens that sheets stacked by the known device fail to end up in an orderly manner on the stack formed on the receiving plane. Deviations in the orientation of sheets relative to one another and relative to the reference planes are highly undesirable, particularly if the stacks are to be further post-processed, such as in binding applications. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a stacking device for incoming sheets, where the sheets are accurately stacked in a reliable manner. To this end, a device has been invented according to the present invention, wherein in operation a sheet follows a transport path from a sheet supply to a receiving plane on which the stack of subsequent sheets is formed. The sheet stacking device comprises a rotatably arranged flipping element and closed-loop shaped friction element. The rotatably arranged flipping element includes a slot at a circumferential edge thereof for accepting at least a portion of a sheet to be stacked. The rotatably arranged flipping element is configured such that, in a first rotation zone, the rotatably arranged flipping element is able to accept the sheet from the sheet supply into the slot and, in a second rotation zone, the sheet is conveyed out of the slot onto the top of the stack of subsequent sheets on the receiving plane. The closed-loop shaped friction element is moveably arranged on the rotatably arranged flipping element, such that the closed-loop shaped friction element is controllable to move into one of a first radial position and a second radial position. In the first radial position, the closed-loop shaped friction element does not apply a frictional force to a sheet in the transport path, and in the second radial position, the closed-loop shaped friction element does apply a frictional force to a sheet in the transport path. 
     In the device according to the present invention, the registration of incoming sheets already performed is retained more effectively and there is no additional obstruction to the incoming movement of sheets. Due to the positioning of the friction elements according to the present invention, a significant improvement of the alignment of the stacks formed on the receiving plane is achieved. Any further binding processes may therefore form more accurate documents and the user will experience, also in cases where no further post-processing is performed, a higher quality of stacks and documents delivered. It shall be clear for the skilled person that the closed-loop shaped friction element does not necessarily consist of a closed-loop element. The function of the element is based upon the function of the friction element, such as the curved friction element as described and depicted herein. It is not necessary that the loop is closed. 
     In an embodiment of the present invention, the closed-loop shaped friction element is controlled to move into the first radial position, if the closed-loop shaped friction element is in the first rotation zone. In the first rotation zone as described here above, a sheet may be fed into the slot on the rotatably arranged flipping element. Therefore, the device in accordance with the present invention moves the closed-loop shaped friction element into its first radial position, in which the closed-loop shaped friction element does not apply a frictional force to a sheet in the transport path. In this retracted position, the closed-loop shaped friction element does not disturb an incoming sheet during its reception into the slot. As a result of the non-disturbance of the sheet by the closed-loop shaped element, the sheet retains its lateral alignment during its transport over the transport path from the sheet supply via the rotatably arranged flipping element to the stack formed on the receiving plane, or if a stack is already formed, onto the top of the stack of sheets. 
     In an embodiment of the present invention, the closed-loop shaped friction element is controlled to move into the second radial position if the closed-loop shaped friction element is in the second rotation zone. In the second rotation zone as described hereabove, a sheet is expelled from the slot onto the receiving plane, or if a stack has already been formed, onto the top of the stack. Therefore, the device in accordance with the present invention moves the closed-loop shaped friction element into its second radial position, in which the closed-loop shaped friction element does apply a frictional force to a sheet in the transport path. In this extended radial position, the closed-loop shaped friction element is able to apply some friction on the top of the top sheet of the stack to gently urge this sheet to align against a stopping member, which limits the motion of the sheet in the transport direction, thereby defining an alignment edge of the stack. In its second radial position the closed-loop shaped element may extend partially beyond a radius of the flipping element or may be moved completely beyond the local edge of the flipping element. 
     In an embodiment of the present invention, the control of the movement of the closed-loop shaped friction element comprises a cam and cam follower construction. By means of a cam and cam follower combination, a simple and reliable construction assures that the movement of the closed-loop shaped element is synchronized with its position, such that the effects as described here above are rendered every revolution of the rotatably arranged flipping element. 
     In a further embodiment of the present invention, the sheet stacking device further comprises a cam element, which is rotatably arranged on the rotatably arranged flipping element, wherein the closed-loop shaped friction element is mounted on the cam element. By mounting the closed-loop shaped element on a cam element, which is rotatably mounted on the flipping element, the application of a cam and cam follower combination enables a smooth variation between the first radial position of the closed-loop shaped element and the second radial position of the closed-loop shaped element. 
     In a further embodiment of the present invention, the cam element is urged into the first radial position by means of a biasing element, such as a spring. This biasing force defines a passive home position of the closed-loop shaped element during a rotation of the flipping element. The cam, which may be arranged such that in the first rotation zone the biasing force is overcome and the closed-loop shaped element is forced towards its second radial position. It may be advantageous to configure the biasing force such that the effects of the forces imposed on the closed-loop shaped element and/or the cam element on which the closed-loop shaped element is mounted as a result of the rotation of the flipping element are suppressed. 
     In an embodiment of the present invention, the movement of the closed-loop shaped element is actuated by means of an electric motion actuator, such as a linear drive motor or the like, and such that its motion is electronically controlled. Known servo-control mechanisms may be applied to implement this motion. Sensors may be implemented to synchronize the movement with the applicable rotation zones and/or radial positions of the closed-loop shaped element. 
     In another aspect of the present invention, a printing device is disclosed, comprising a sheet supply unit, a printing engine for applying a marking substance to a sheet and the sheet stacking device according to the present invention. As printing systems reach increasingly higher printing speeds, it is very advantageous that stacking units as part of such printing systems can cope with these higher speeds while maintaining or even increasing the quality of alignment of these stacks. The printing system according to the present invention fulfils this requirement with the features disclosed herein. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a schematic perspective view showing a stacking device  10  according to the present invention; 
         FIG. 2  is a schematic perspective view showing a part of a flipping device according to the present invention; 
         FIGS. 3A-D  are schematic diagrams showing a flipping device according to the present invention during the reception and expelling of a sheet; and 
         FIG. 4  is a schematic side view showing a flipping element of a flipping device according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described with reference to the accompanying drawings, wherein the same or similar elements are identified with the same reference numerals throughout the several views. 
       FIG. 1  is a diagram showing a stacking device  10  in which a device according to the present invention has been mounted. A stacking device  10  of this kind may, for example, be mounted behind a printer. By coupling a printer&#39;s sheet outlet to the entry point of a stacking device  10  of this kind, sheets from the printer may be stacked neatly when the registration actions are performed by stacking device  10 . A stacking device  10  of this kind may be embodied with various output facilities, such as, but not limited to, a pallet output facility for feeding stacks easily from the stacking device onto a pallet, a binding facility for binding the stacks together, for example using a plastic strip or another method, in order to maintain the stack shape also during transport. 
     Stacking unit  10  as shown in  FIG. 1  receives sheets via sheet inlet  11 . A sheet that has been fed in is then conveyed against a registration wall  13  by means of a registration ruler  12 . In the embodiment shown, the transport clamping arrangements of registration ruler  12  have been formed in such a way that a force is transferred from the clamping arrangement onto the sheet in the direction of registration wall  13 . As soon as the sheet lies against the registration wall, the force of the clamping arrangement on the sheet will predominantly be applied in the direction of transportation and to a lesser extent in the direction of registration wall  13 , as sheet deformation may occur if excessive force is applied when a sheet is conveyed against a registration wall. This may be achieved by embodying the transport clamping arrangement of the registration ruler with wheels that are more flexible in the one lateral direction than in the other lateral direction. The moment a sheet lies against the registration wall, the transport wheel will bend outwards and transfer a force that is predominantly directed in the transport direction, causing a sheet that has reached the registration wall to be pushed against the wall without excessive force, thus preventing sheet deformations. 
     Because of this first registration action, all sheets enter reversing loop  14  at the same height. Behind this reversing loop  14 , an input clamping arrangement  15  is positioned, which forms the inlet transport into the module, inside which a flipping device  20  according to the present invention is mounted. This module feeds incoming sheets into a straight stack onto a receiving plane  21 . This receiving plane  21  may be height adjustable, so that the stacking capacity of the stacking unit may be increased. The stack may be removed from the unit via an opening hatch or another type of outlet opening  16 . Stacking unit  10  may also comprise means (not shown) to assist a user in removing the stack out of the unit, for example by conveying the stack out of the stacking unit, in whole or in part. This may, for example, be embodied as a power-steered drawer, which, after opening outlet opening  16 , is conveyed out of the unit. A stacking device of this kind may be either electronically connected to a printer so that the timing of sheets may be communicated, or embodied fully autonomously, where the timing of incoming sheets is detected by the stacking unit itself 
       FIG. 2  is a diagram showing a device according to the present invention. The device according to the present invention comprises a receiving plane  21  and two rotatably arranged elements  22  and  23 , which are connected to an electrically driven motor  25  and a rotation shaft  24 . The device may comprise one or more rotatably arranged elements of this kind. Electrically driven motor  25  may, for example, be an electric servo motor or stepping motor. The rotatably arranged elements comprise slots  26  and  27  in which an incoming sheet may be accepted in whole or in part from input clamping arrangement  15 . In the position shown in  FIG. 2 , rotatable elements  22  and  23  are located in the first rotation zone, in which the leading edge of a sheet may be accepted into slots  26  and  27 . 
     Cam following pivot elements  50  are mounted pivotably on each of the rotatably arranged elements  22  and  23 . Closed-loop shaped friction elements  28  and  29  are mounted on the cam following pivot elements  50 . The cam following pivot elements  50  are biased inwards by means of a spring force, while a cam engages with these cam following pivot elements, just before the sheet is urged against stop  30 . By pivoting outward in a radial direction of the rotatably arranged elements  22  and  23 , the friction elements do not interfere with an incoming sheet during acceptance of that sheet in the slot  26 ,  27 , while an urging force is applied during the urging of the sheet against the stop  30 . Note that, for reasons of clarity of illustration, the rotatably arranged elements  22  and  23  have been illustrated partially. In practice, the rotatably arranged elements  22  and  23  are formed as a full cylinder comprising two slots and two pivotable elements. By actuating motor  25 , rotation shaft  24  is driven in the direction of arrow B, where rotatably arranged elements  22  and  23 , and as a result, also slots  26  and  27 , as well as the sheet accepted within, are conveyed in the direction of stop  30  according to arrow B. During the movement of rotation shaft  24 , closed-loop shaped friction elements  28  and  29  connected to the rotation shaft will exercise a force with a component in the direction of the stop on the upper side of the sheet that was last deposited onto receiving plane  21 , so that the sheet is conveyed against stop  30 . In the description below, the movement cycle of the device will be described in more detail. It will be understood by those skilled in the art that, when receiving plane  21  does not contain any sheets, the closed-loop shaped friction elements  28  and  29  will brush over receiving plane  21 . 
     The embodiment shown in  FIG. 2  furthermore comprises a tool to restrict the freedom of movement of the edge of any sheets that have already been deposited onto receiving plane  21 . This will reduce any curling or other form of deformation of the sheet edge, which will have a beneficial effect on the registration behavior of sheets conveyed onto receiving plane  21 . In this example of an embodiment, retention hooks  31  and  32  have been mounted, which have been rotatably attached to the frame end and are bent at the other side, in such a way that any incoming sheets conveyed from slots  26  and  27  at the level of stop  30  onto the receiving plane may easily be conveyed under here, whilst the freedom of movement of the sheet edge is restricted. The retention force of the hooks onto the sheet edge is predominantly directed downwards and may, for example, be delivered exclusively by the hooks&#39; own weight in a gravity field and/or by means of a torsion spring at the position of attachment to the frame or by means of magnetic pull. 
       FIGS. 3A-D  are schematic diagrams showing a flipping device according to the present invention during the reception and expelling of a sheet.  FIG. 3A  illustrates the device is in its receiving position, in the first rotation zone. Slot  26  is positioned relative to input clamping arrangement  15  in such a way that a sheet  40  may be accepted into slot  26 . This first rotation zone may be very small, if the rotatably arranged element is halted whilst a sheet  40  is accepted into slot  26 . Cam following pivot element  50 , on which the friction element  28  has been mounted, is in its retracted inward orientation, such that the incoming sheet is not disturbed. If the rotatably arranged element is not halted, the relative speed of accepting sheet  40  relative to the rotation speed of the rotatably arranged element will need to be high enough to at least partially accept sheet  40 . Subsequently, sheet  40  is conveyed through input clamping arrangement  15  in slot  26 . This is illustrated in  FIG. 3B . If a sheet  40  is next accepted into slot  26 , the rotatably arranged element may be conveyed further so that the leading edge of sheet  40  will be conveyed against stop  30  during this revolution. As rotatably arranged element  22  continues to rotate from this position, whilst the movement of sheet  40  is halted by stop  30 , sheet  40  will be conveyed onto receiving plane  21  as illustrated in  FIG. 3C . Here, sheet  40  has been entirely deposited onto receiving plane  21 . Rotatably arranged element  22  may continue to rotate to the first rotation zone in which a new sheet may be accepted into slot  26 , and the cycle can start all over again. A sheet  40  that is deposited onto receiving plane  21 , does not yet have the right registration relative to stop  30  in this case. A sheet  40  may, for example, be left at some distance from stop  30 . Cam  60  is positioned such that cam following pivot element  50  is forced radially outward such that the friction element  28  is engaged with the top of the deposited sheet. As a closed-loop shaped friction element  28  is now forced beyond the radial circumferential edge of rotatable element  22 , a force with component F in the direction of stop  30  may be exercised on the upper side of a sheet  40 , which has been deposited onto receiving plane  21 , so that the sheet may still adopt the right registration relative to stop  30 . This is shown in  FIG. 3D . Closed-loop shaped friction element  28  must not obstruct the movement of sheet  40  from input clamping arrangement  15 . Therefore, the cam following pivot element  50  is urged radially inward by a spring (not shown), such that after passing the cam  60  the pivot element is urged inside the circumferential edge of the rotatably arranged element. 
       FIG. 4  is a schematic side view showing a flipping element of a flipping device according to the present invention.  FIG. 4  shows in some detail the rotatably arranged element  22  comprising slot  26  for enclosing a portion of a sheet to be stacked on receiving plane  21  on which a stack of subsequently fed sheets is formed on a stack, registered against stop  30 . Cam following pivot element  50 ,  51  comprising friction element  28  is forced radially outward by cam  60  and urged radially inward by spring  65 ,  66 . 
     Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the present invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. In particular, features presented and described in separate dependent claims and/or embodiments may be applied in combination and any combination of such claims and/or embodiments are herewith disclosed. 
     Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention. The terms “a” or “an”, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.