Patent Publication Number: US-8534666-B2

Title: Sheet stacking apparatus and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/849,964 filed Sep. 4, 2007, which claims priority from Japanese Patent Application Nos. 2006-242077 filed Sep. 6, 2006 and 2007-214887 filed Aug. 21, 2007, all of which are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet stacking apparatus and an image forming apparatus. 
     2. Description of the Related Art 
     In recent years, thanks to technological advances, an image forming apparatus has become capable of forming images at high speed. Together with the increase in image forming speed, sheet discharging speed from the image forming apparatus has also increased. As a result, demand for a high-volume sheet stacking apparatus capable of precisely aligning the sheets is increasing. 
     Japanese Patent Application Laid-Open No. 2006-124052, for example, discusses a sheet stacking apparatus which includes a pressing member that presses a sheet to a sheet stacking tray so that the sheet can be more speedily discharged onto the sheet stacking tray. 
       FIG. 10  illustrates a configuration of a conventional sheet stacking apparatus  100  which enables high-volume output. The sheet stacking apparatus is attached to a conveying belt  508  that rotates clockwise and includes a gripper  503 . The gripper  503  rotates together with the conveying belt  508  to convey a sheet while holding a leading edge of the sheet. Further, the sheet stacking apparatus includes a leading edge pressing member  506  and a trailing edge pressing member  507  configured to press down a leading edge and a trailing edge of a sheet. 
     In the sheet stacking apparatus having such a configuration, a sheet discharged from an image forming apparatus (not shown) is received by an inlet roller  501  and then a leading edge of the sheet is passed on to the gripper  503  by a conveyance roller  502 . Then, the conveying belt  508  rotates, and the gripper  503  moves together with the conveying belt  508  while holding the leading edge of the sheet. In this way, the sheet is conveyed along the upper portion of a sheet stacking tray  505 . 
     When the leading edge of the sheet abuts against a leading edge stopper  504 , the gripper  503  releases the sheet so that the sheet is discharged onto the sheet stacking tray  505 . In this manner, a predetermined number of sheets are stacked. Every time a sheet is stacked, an alignment member (not shown) performs a jogging process in a direction perpendicular to the sheet conveying direction (hereinafter referred to as width direction) so that an alignment of the sheets is improved. 
     When sheets are stacked at high speed, possibility of a sheet jam is increased, which occurs when a sheet interferes with a trailing edge of a preceding sheet stacked on the sheet stacking portion  505 . Therefore, during sheet stacking, the leading edge pressing member  506  and the trailing edge pressing member  507  press down a leading edge and a trailing edge of a sheet against the sheet stacking tray so that the sheet reaches the sheet stacking tray  505  more quickly. 
     In other words, when sheets are stacked at high speed, the leading edge pressing member  506  and the trailing edge pressing member  507  press a leading edge and a trailing edge of a sheet against the sheet stacking tray  505  at the time the sheet is discharged to the sheet stacking tray  505  so that the sheet is out of the way of the next sheet. 
     However, in such a conventional sheet stacking apparatus, when a sheet is pressed to the sheet stacking tray by the leading edge pressing member  506  and the trailing edge pressing member  507 , a path on which the sheet takes from a release step to landing is not fixed. Therefore, accuracy of stacking position is considerably poor. 
     Especially when priority is given to pressing by the trailing edge pressing member  507  to increase stacking speed, the sheet can be pressed in a state that the sheet leans against the stacking wall Y. In this case, not only damage is given to the sheet but also accuracy of stacking deteriorates. 
     Also, in a case where the leading edge pressing member  506  presses a leading edge portion of a sheet, the leading edge pressing member  506  can be configured to wait at a position  506 ′ and then rotate counterclockwise around a center of rotation X. In this case, a pressing force acts also in the right direction as shown in  FIG. 10 . 
     Thus, in a case when a leading edge of a sheet is pressed by the leading edge pressing member  506 , not only a force acts on the sheet stacking stray  505  but also acts in the right direction in  FIG. 10 . Accordingly, the leading edge of the sheet is curled. As a result, stacking accuracy is decreased and an undesired curl will be formed on the sheet. 
     Consequently, if a sheet discharged onto the sheet stacking tray  505  is simply pressed by the leading edge pressing member  506  and the trailing edge pressing member  507 , stable stacking with high accuracy is not achieved. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a sheet stacking apparatus capable of stacking sheets at a high speed with stability and a high degree of accuracy, and an image forming apparatus including such a sheet stacking apparatus. 
     According to one aspect of the present invention, a sheet stacking apparatus includes a sheet discharging portion configured to discharge a sheet, a sheet stacking portion configured to stack the sheet discharged from the sheet discharging portion, a shifting unit configured to shift a sheet edge to a predetermined position on the sheet stacking portion, and a pressing member configured to press the sheet discharged from the sheet discharging portion to the sheet stacking portion. The sheet is pressed to the sheet stacking portion by the pressing member while the sheet edge is kept at the predetermined position by continuing a shifting operation of the shifting unit. 
     The sheet shifting unit shifts to maintain an aligned state of the sheet discharged from the discharging portion while the pressing member presses the sheet to the sheet stacking portion so that sheets can be stacked at a high speed with stability and a high degree of accuracy. 
     Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  illustrates a configuration of an image forming apparatus including a sheet stacking apparatus according to an exemplary embodiment of the present invention. 
         FIG. 2  illustrates a block diagram of a control unit provided in the above-described image forming apparatus. 
         FIG. 3  illustrates a configuration of a stacker connected to a main body of the image forming apparatus. 
         FIG. 4  is a flowchart illustrating basic control of the stacker. 
         FIG. 5  illustrates an enlarged view of a stacking portion of the stacker. 
         FIG. 6  is an enlarged view of the stacking portion of the stacker illustrating a sheet stacking operation. 
         FIG. 7  is an enlarged view of the stacking portion of the stacker illustrating the sheet stacking operation. 
         FIG. 8  is an enlarged view of the stacking portion of the stacker illustrating the sheet stacking operation. 
         FIG. 9  is an enlarged view of the stacking portion of the stacker illustrating another configuration of the stacker. 
         FIG. 10  illustrates a configuration of a conventional high-volume sheet stacking apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings. 
       FIG. 1  illustrates a configuration of an image forming apparatus including a sheet stacking apparatus according to an exemplary embodiment of the present invention. 
       FIG. 1  illustrates an image forming apparatus  900  and an image forming apparatus main body  901 . The image forming apparatus main body  901  is provided with an image scanning apparatus  951  having a scanner unit  955  and an image sensor  954 , an image forming unit  902  configured to form an image on a sheet, a double-side printing device  953 , and a platen glass  952 . Further, a document feeding apparatus  950  configured to feed a document to the platen glass  952  is provided on the upper part of the image forming apparatus main body  901 . 
     The image forming unit  902  includes a cylindrical photosensitive drum  906 , a charging unit  907 , a developer  909 , and a cleaning apparatus  913 . Also, a fixing apparatus  912  and a discharge roller pair  914  are provided downstream of the image forming unit  902 . A stacker  100  (i.e., a sheet stacking apparatus) is connected to the image forming apparatus main body  901 . The stacker  100  is configured to stack image-formed sheets discharged from the image forming apparatus main body  901 . A control unit  960  mounted on the image forming apparatus main body  901  controls the image forming apparatus main body  901  and the stacker  100 . 
     Next, an image forming operation of the image forming apparatus main body  901  having the above configuration will be described. 
     When the control unit  960  outputs an image forming signal, the document feeding apparatus  950  places a document on the platen glass  952 . Then, the image scanning apparatus  951  scans an image of the document, and the scanned digital data is input to an exposure apparatus  908 . The exposure apparatus  908  irradiates the photosensitive drum  906  with a light corresponding to the digital data. 
     At this time, the surface of the photosensitive drum  906  is evenly charged by the charging unit  907 . When laser beams from the exposure apparatus  908  scans the photosensitive drum  906 , an electrostatic latent image is formed on the surface of the photosensitive drum  906 . The developer  909  develops the electrostatic latent image, and a toner image is formed on the surface of the photosensitive drum  906 . 
     On the other hand, when the control unit  960  outputs a sheet feed signal, a sheet S set on one of cassettes  902   a  through  902   e  is conveyed to a registration roller  910  by corresponding feeding rollers  903   a  through  903   e  and a conveyance roller pair  904 . 
     Next, the sheet S is conveyed to a transfer unit including a charging unit  905  at a timing such that the leading edge of the sheet synchronizes with the toner image on the photosensitive drum  906  owing to the registration roller  910 . At the transfer unit, a transfer bias is applied to the sheet S by the charging unit  905 , and a toner image on the photosensitive drum  906  is transferred to the sheet. 
     Subsequently, the sheet S with the transferred toner image is conveyed to the fixing apparatus  912  by a conveying belt  911 . The toner image is thermally fixed while the sheet is sandwiched between and conveyed by the heating roller and the pressure roller of the fixing apparatus  912 . At this time, undesired matter such as remaining toner which was not transferred to the sheet is scraped off by a blade of the cleaning apparatus  913  from the photosensitive drum  906 . As a result, the surface of the photosensitive drum  906  is cleaned and ready for the next image forming process. 
     The image-fixed sheet is conveyed to the stacker  100  by the discharge roller pair  914  or conveyed to the double-side printing device  953  where the sheet is reversed by a flapper  915  to form an image again. 
       FIG. 2  is a block diagram illustrating a configuration of the control unit  960 . The control unit  960  has a central processing unit (CPU) circuit unit  206 . The CPU circuit unit  206  includes a CPU (not shown), a read only memory (ROM)  207 , and a random access memory (RAM)  208 . Further, a document feeder (DF) control unit  202 , an operation unit  209 , an image reader control unit  203 , an image signal control unit  204 , a printer control unit  205 , and a stacker control unit  210  are controlled overall according to a control program stored in the ROM  207 . The RAM  208  temporarily stores control data and also provides a working area for calculation processing required for the control. 
     The DF control unit  202  performs control to drive the document feeding apparatus  950  based on an instruction from the CPU circuit unit  206 . The image reader control unit  203  performs control to drive the scanner unit  955  and the image sensor  954  arranged on the image scanning apparatus  951 , and transfers an analog image signal output from the image sensor  954  to the image signal control unit  204 . 
     The image signal control unit  204  converts an analog image signal sent from the image sensor  954  to a digital signal, processes the digital signal, converts the processed digital signal to a video signal, and outputs the video signal to the printer control unit  205 . 
     The image signal control unit  204  also performs various types of processing to the digital signal input from a computer  200  or from an external apparatus through an external I/F  201 , and converts the digital image signal to a video signal which is then output to the printer control unit  205 . The CPU circuit unit  206  controls the processing operation performed by the image signal control unit  204 . 
     The printer control unit  205  drives the exposure apparatus  908  through an exposure control unit (not shown) based on the input video signal. The operation unit  209  includes a plurality of keys configured to set various types of functions for forming an image, and a display unit for displaying a setting state. Further, the operation unit  209  outputs key signals corresponding to each key operation to the CPU circuit unit  206  and also displays information corresponding to signals sent from the CPU circuit unit  206 . 
     The stacker control unit  210  is mounted on the stacker  100  and performs control to drive the entire stacker by exchanging information with the CPU circuit unit  206 . The control of the stacker control unit  210  will be described later. The stacker control unit  210  can also be integrated in the CPU circuit unit  206  of the image forming apparatus  901  so that the stacker  100  can be directly controlled from the image forming apparatus main body  901 . 
       FIG. 3  illustrates a configuration of the stacker  100 . The stacker  100  has a top tray  106  configured to stack sheets discharged from the image forming apparatus main body  901  on its top face. Further, the stacker  100  has a stacking portion  130  including a stacker tray  112 , which is a sheet stacking portion configured to stack sheets, and also a switching flapper  103  configured to guide the sheet S conveyed to the stacker  100  to the top tray  106  or to the stacking portion  130 . 
     Furthermore, a solenoid (not shown) drives an outlet switching flapper  108  illustrated in  FIG. 3  so that the flapper  108  moves to a position shown by a broken line when the destination of the sheet is a sheet processing apparatus at a downstream side (not shown). 
     Next, a basic control of the stacker  100  performed by the stacker control unit  210  will be described referring to the flowchart illustrated in  FIG. 4 . 
     The sheet S discharged from the image forming apparatus main body  901  is conveyed into the stacker  100  by an inlet roller pair  101  and then conveyed to the switching flapper  103  by conveyance roller pairs  102 . 
     Before the sheet is conveyed, the CPU circuit unit  206  of the control unit  960  in the image forming apparatus main body  901  sends in advance sheet information including sheet size, sheet type, and destination of the sheet to the stacker control unit  210 . 
     The stacker control unit  210  determines a destination of the sheet transferred from the control unit  960  (step S 101 ). If the destination of the sheet is the top tray  106  (step S 110 ), the stacker control unit  210  controls the switching flapper  103  driven by a solenoid (not shown) (step S 111 ) so that the flapper  103  changes its position to a position shown in a broken line in  FIG. 3 . Thus, the sheet S is guided to conveyance roller pairs  104  and discharged onto the top tray  106  by a top tray discharge roller  105  (step S 112 ) and stacked. 
     If the destination of the sheet is the stacker tray  112  (stack portion  130 )(step S 120 ), the sheet conveyed by the conveyance roller pair  102  is discharged to the stacker tray  112  by a conveyance roller pair  107  and a discharge roller  110  constituting the sheet discharging portion (step S 121 ), and stacked. 
     If the destination of the sheet is a sheet processing apparatus at a downstream side (step S 130 ), a solenoid (not shown) drives the outlet switching flapper  108  (step S 131 ) so that the flapper  108  changes its position to a position shown in a broken line in  FIG. 3 . Thus, the sheet conveyed by the conveyance roller pair  102  is conveyed by the conveyance roller pair  107 , led to a delivery roller pair  109 , and conveyed to the downstream sheet processing apparatus. 
     As shown in  FIG. 3 , the stacker tray  112  of the stack portion  130  is arranged so that it can independently move up and down in the directions shown in arrows C and D by a driving device (not shown). 
     In  FIG. 3 , a shifting unit  115  shifts a sheet into a downstream side in a sheet discharging direction. The shifting unit  115  includes a knurled belt  116 , which is rotated counterclockwise by a driving device (not shown) to shift a discharged sheet into the downstream side of the stacker tray  112  in the sheet discharging direction. Further, the shifting unit  115  includes a taper portion  115   b  configured to guide the sheet to the knurled belt  116 . The shifting unit  115  also includes a leading edge stopper  121  (i.e., abutting portion) configured to position a leading edge of the sheet at a predetermined position. 
     The sheet is drawn by the knurled belt  116  until the sheet edge on a downstream side in a sheet discharging direction abuts against the leading edge stopper  121 . The shifting unit  115  is mounted on a slide shaft  118  and is movable along the slide shaft  118  in directions shown in arrows A and B. Also, the shifting unit  115  can be moved to a position corresponding to the sheet size (i.e., sheet length in the sheet discharging direction) by a driving device (not shown). 
     A sheet surface detection sensor  117  is a sensor configured to keep a constant distance between the shifting unit  115  and the top sheet. It is to be noted that the top sheet in the stacker tray  112  is not only detected by the sheet surface detection sensor  117  but also by a sheet surface detection sensor  113  in the stacking portion  130 , which is illustrated in  FIG. 5  (i.e., an enlarged view of the stacking portion  130 ). 
     The sheet surface detection sensor  113  detects a home position of the stacker tray  112  at an initial operation but functions as a sheet surface detection sensor for the stacker tray  112  during a stacking operation. In  FIG. 5 , the stacker tray  112  is at a home position for stacking sheets according to detection of the sheet surface detection sensor  113 . 
     In  FIG. 5 , a drive belt  131  is wound around a drive roller  131   a  and a driven roller  131   b  and rotated counterclockwise by a driving device (not shown). Grippers  114   a  and  114   b  are attached to the drive belt  131  and convey a sheet by pinching (holding) a trailing edge of the sheet. The grippers  114   a  and  114   b  and the drive belt  131  constitute the sheet discharging portion  132 . The sheet discharging portion  132 , which is arranged separate from the stacker tray  112 , conveys a sheet along the stacker tray  112 , and discharges the sheet onto the stacker tray  112 . 
     The grippers  114   a  and  114   b  are attached to the drive belt  131  and urged in a clockwise direction by a torsion coil spring (not shown). A driving device (not shown) drives the grippers  114   a  and  114   b  so that the grippers  114   a  and  114   b  move to a position where they hold a sheet, and to a position where they release the sheet. 
     Further, a pressing portion  122  is located above the stacker tray  112 . The pressing portion  122  includes a plurality of pressing members  122   a  through  122   c  which move up and down to press the discharged sheet down on the stacker tray  112 . In  FIG. 5 , a timing sensor  111  is arranged upstream of the discharge roller  110 . The timing sensor  111  is configured to detect a timing at which the leading edge of a sheet passes. An alignment plate  119  (alignment member) is adapted to align the sheet at an end portion in a direction perpendicular to the sheet discharging direction. 
     At the stacking portion  130  having such a configuration, when a sheet S is conveyed from the image forming apparatus main body  901  to the discharge roller  110  in the above-described sheet conveying control operation, the timing sensor  111  detects a leading edge of the sheet. Based on the detected timing of the sheet edge passing, either of the grippers  114   a  and  114   b , which are waiting, for example, the gripper  114   a , is driven by a driving device (not shown) and pinches (holds) the leading edge of the sheet. 
     Subsequently, the drive belt  131  rotates counterclockwise, and the gripper  114   a  moves together with the drive belt  131  while holding the leading edge of the sheet. In this way, the sheet is conveyed above and along the stacker tray  112 . 
     Then, when the gripper  114   a  passes a taper portion  115   b  formed on a gripper side of the shifting unit  115  as shown in  FIG. 6 , the gripper  114   a  is driven to release the sheet. In this way, the sheet S is conveyed while its leading edge is guided by the taper portion  115   b  toward the stacker tray  112  and conveyed to the knurled belt  116 . 
     At this time, the sheet contacts the knurled belt  116  by a inertia force generated at the time the sheet is conveyed. The sheet S is conveyed by the knurled belt  116  until its leading edge abuts against the stopper  121  as shown in  FIG. 7 . Then the sheet S is stacked on the stacker tray  112  while the sheet edge on the downstream side in the sheet discharging direction is aligned. 
     In this state, the knurled belt  116  continues rotating in a direction that shifts the sheet S. According to this rotation, a force is applied to the sheet S that continuously presses the sheet S against the stopper  121 . Although the knurled belt  116  continues a shifting operation, the knurled belt  116  is configured so that the sheet S slips over the knurled belt  116  in a state that its leading edge abuts against the stopper  121 . With this configuration, undue pressure is not applied to the sheet S. Consequently, the sheet S is not curled by the knurled belt  116  although the knurled belt  116  is operating. 
     Next, as shown in  FIG. 8 , in this state, the pressing members  122   a  through  122   c  are simultaneously moved for a time down substantially vertically toward a sheet stacking face  112   a  of the stacker tray  112  by a driving device such as a solenoid and press the sheet S to the stacked sheets. In this way, air between the sheet S and the stacked sheets in the entire stacking area is removed, the sheet S can be stacked at high speed with improved stability, and the sheets in the stacker tray  112  is stacked in good condition. Also, a curl of the sheet S can be reduced. This contributes to improving of stackability. 
     It is to be noted that when the pressing members  122   a  through  122   c  press the sheet S for a time, or when the pressing members  122   a  through  122   c  stop pressing the sheet S, due to an impact, the sheet S can move in a direction parting from the stopper  121 . Even in such a case, however, since the knurled belt  116  is rotating, the sheet S is kept in place owing to the rotation force, and a good stacking state is maintained. It is not limited a leading edge in the sheet discharging direction which abuts against an abutting portion. It is possible to keep a sheet at the predetermined position by abutting any sheet edge. 
     Even when the impact is so great that the alignment state is disturbed, the sheet S immediately returns to the original position of alignment. The rotary force (shifting force) of the knurled belt  116  is adjusted so that the sheet S is not deformed when it is held at the predetermined position. 
     After that, the pressing members  122   a  through  122   c , driven by a driving device, move upward and return to their home positions. Then, a lateral end of the stack of sheets in the width direction, which is a direction perpendicular to the sheet discharging direction, is aligned by the alignment plate  119 . The alignment plate  119  retracts in by a predetermined amount after it aligns the stack of sheets and waits until a new sheet is conveyed. Thus, the alignment in the width direction prevents a leading edge of a next sheet from colliding against the trailing edge of the preceding sheet when the next sheet is discharged onto the stacker tray  112 . 
     The stacker control unit  210  continuously monitors the top sheet in the stacker tray  112  through the sheet surface detection sensors  117  and  113 . If a distance between the shifting unit  115  and the top sheet becomes smaller than a predetermined distance, a stacker tray driving device (not shown) moves down the stacker tray  112  for a predetermined distance so that the distance between the shifting unit  115  and the top sheet remains constant. By repeating this operation, the sheets are stacked on the stacker tray  112  one after another. 
     Then, a detection device (not shown) configured to detect a number of sheets discharged from the discharge roller  110  or to detect a height of the sheets stacked on the stacker tray  112 , detects that the stacker tray  112  is fully loaded. 
     Even when the stacker tray  112  is detected as fully loaded by counting a number of discharged sheets, the height of the stacked sheets is reduced by removing air between the sheet S and the stacked sheets or by correcting a curl of the sheets, which prevents the next sheet from colliding with the sheets already stacked. Alternatively, the stacker tray  112  is moved upward until the top sheet, whose height is lowered by removing the air or by correcting the curl, is detected by the detection device. In this way, a number of sheets that can be stacked on a tray is prevented from decreasing. 
     When the stacker tray  112  is fully loaded, the stacker control unit  210  controls the stacker tray  112  to move down and fixes the stacker tray  112  on a dolly  120 . Then, an operator removes the stack of image-formed sheets from the stacker  100 . 
     If the operator sets the dolly  120  and the stacker tray  112  in the stacker  100  after removing the sheets on the stacker tray  112 , the stacker tray  112  moves upward and returns to the position shown in  FIG. 3  to receive the next sheet. 
     The shifting unit  115  continues a shifting operation in a state that a leading edge of a sheet abuts against the stopper  121 , and the pressing members  122   a  through  122   c  press the sheet to the sheet stacking face  112   a  while the sheet abuts against the stopper  121 . Thus, sheets can be stacked at a high speed with stability and a high degree of accuracy by the shifting unit  115 . 
     According to the present exemplary embodiment, the sheet S is thrust against the stopper  121  by the knurled belt  116 , pressed by the pressing members  122   a  through  122   c , and aligned by the alignment plate  119 . The present invention, however, is not limited to such a sequence. For example, the sheet S can be thrust against the stopper  121  by the knurled belt  116 , aligned by the alignment plate  119  in a width direction, which is perpendicular to a sheet discharging direction, and then pressed by the pressing members  122   a  through  122   c . Even in this order, a similar effect can be achieved. 
     Further, a similar effect can also be achieved by pressing the sheet S with the pressing members  122   a  through  122   c  for a time while the sheet is drawn to the stopper  121  by the knurled belt  116 . 
     Furthermore, after the leading edge of the sheet S abuts against the stopper  121 , the pressing members  122   a  through  122   c  can be successively moved down to the stacker tray side starting from the pressing member  122   c  on the stopper side as shown in  FIG. 9 . In other words, a driving device such as a solenoid (not shown) can cause the pressing members  122   a  through  122   c  to move down in an order of the pressing member  122   c , the pressing member  122   b , and the pressing member  122   a  with a delayed timing and to press the sheet S to the stacked sheets. This is because if a plurality of pressing members  122   a  through  122   c  press the sheet at a time, it can become difficult to remove the air between the sheet S and the stacked sheets since the space for the air to pass is narrowed and the air can not easily escape. 
     If the pressing members  122   a  through  122   c  press the sheet S with a delayed timing, the air between the sheet S and the stacked sheets can be successively discharged from a stopper side to an opposite end while an alignment of the sheet S is maintained. Therefore, the sheets on the stacker tray  112   a  will be stacked in good condition. After that, the pressing members  122   a  through  122   c  retract upward to their home positions, and alignment of the sheet edge in a width direction, which is perpendicular to the sheet charging direction, will be performed by the alignment plate  119 . Accordingly, the next sheet can be discharged onto the stacker tray  112  while its leading edge does not collide with the trailing edge of the preceding sheet. 
     Although the exemplary embodiment of the present invention uses the grippers  114   a  and  114   b  in the sheet discharging portion  132  as a device configured to convey the sheet, the present invention is not limited to such a device. For example, an air attracting unit (by suction) or an electrostatic attracting unit can also be used to obtain a similar effect so long as a sheet is conveyed and discharged by holding the leading edge of the sheet. 
     While the above exemplary embodiment has been described referring to a case where one stacker tray  112  is used, the present invention is not limited to such a case. For example, a similar effect can be achieved by a plurality of stacker trays arranged side-by-side in a sheet discharging direction and each stacker tray includes a shifting unit and a pressing member of the above-described configuration. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures, and functions.