Patent Publication Number: US-8540229-B2

Title: Sheet processing apparatus and image forming apparatus for stably aligning sheets having a long length

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet processing apparatus for processing sheets. 
     2. Description of Related Art 
     There are conventional image forming apparatuses such as copying machines including a sheet processing apparatus. For example, after images are formed on sheets, such sheet processing apparatus aligns or binds the sheets. 
     One such sheet processing apparatus executes an alignment process while supporting a sheet by an intermediate stacking tray and a pair of supporting portions. These supporting portions face each other and are arranged downstream of the intermediate stacking tray in a sheet conveyance direction. In addition, the supporting portions support side edges of the sheet in a width direction perpendicular to the sheet conveyance direction (see Japanese Patent Application Laid-Open No. 2006-306518 and Japanese Patent Application Laid-Open No. 2003-335450). However, according to the conventional techniques, it is difficult to stably align sheets having a long length (particularly, sheets having twice or more than twice the length of predetermined-size sheets) in the sheet conveyance direction perpendicular to the sheet width direction. Therefore, an apparatus capable of suitably aligning large-size sheets is demanded. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a sheet processing apparatus and an image forming apparatus capable of stably aligning sheets having a long length. 
     In one aspect of the present invention, there is provided a sheet processing apparatus, including: a conveyance portion configured to convey a sheet in a sheet conveyance direction; a sheet stacking portion configured to stack a sheet conveyed by the conveyance portion; a sheet supporting portion arranged downstream of the sheet stacking portion in the sheet conveyance direction and configured to support side edges of the sheet in a width direction perpendicular to the sheet conveyance direction; and an alignment unit arranged on one side of the sheet supporting portion in the width direction and configured to align the position of the sheet supported by the sheet supporting portion in the width direction by moving toward a center side in the width direction. The alignment unit includes: an upstream-side pressing portion configured to press a side edge of the sheet supported by the sheet supporting portion in the width direction; and a downstream-side pressing portion arranged downstream of the upstream-side pressing portion in the sheet conveyance direction and configured to press the side edge of the sheet supported by the sheet supporting portion. The downstream-side pressing portion includes: a first pressing portion arranged downstream of the center of gravity of a predetermined-size sheet supported by the sheet supporting portion in the sheet conveyance direction and upstream of a downstream edge of the predetermined-size sheet in the sheet conveyance direction and configured to press the predetermined-size sheet; and a second pressing portion arranged downstream of the downstream edge of the predetermined-size sheet supported by the sheet supporting portion in the sheet conveyance direction and downstream of the center of gravity of a large-size sheet having twice or more than twice the length of the predetermined-size sheet and configured to press the large-size sheet. The second pressing portion is formed to protrude further toward the center side in the width direction than the first pressing portion. 
     According to the present invention, predetermined-size sheets and large-size sheets can be stably aligned. 
     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 processing apparatus according to a first exemplary embodiment of the present invention. 
         FIG. 2  is a perspective view of the sheet processing apparatus. 
         FIGS. 3A and 3B  illustrate configurations of front- and rear-side alignment portions of the sheet processing apparatus, respectively. 
         FIGS. 4A and 4B  illustrate the sheet processing apparatus that has received an A4-size sheet. 
         FIG. 5  illustrates the sheet processing apparatus that has aligned the A4-size sheet. 
         FIGS. 6A to 6C  illustrate the sheet processing apparatus that has aligned an A3-size sheet. 
         FIGS. 7A and 7B  illustrate other shapes of cut portions formed on supporting portions of the front- and rear-side alignment portions. 
         FIGS. 8A and 8B  illustrate configurations of front- and rear-side alignment portions of a sheet processing apparatus according to a second exemplary embodiment of the present invention, respectively. 
         FIG. 9  illustrates a configuration of a rear-side alignment portion of a sheet processing apparatus according to a third exemplary embodiment of the present invention. 
         FIGS. 10A and 10B  illustrate a configuration of a rear-side alignment portion of a sheet processing apparatus according to a fourth exemplary embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings. 
     Next, exemplary embodiments of the present invention will be described in detail with reference to the drawings.  FIG. 1  illustrates a configuration of an image forming apparatus  10  including a sheet processing apparatus  200  according to a first exemplary embodiment of the present invention. In  FIG. 1 , the image forming apparatus  10  includes an image forming apparatus body  100  (which will hereinafter be referred to as the apparatus body  100 ). This apparatus body  100  includes: an image forming unit  102 ; a sheet feeding unit  101  for feeding a sheet to the image forming unit  102 ; a fixing unit  103 ; and a sheet discharging unit  104 . 
     In addition, the sheet processing apparatus  200  is arranged on the top of the apparatus body  100 . After the image forming unit  102  forms images on sheets, the sheet processing apparatus  200  executes a process such as a stapling process on selectively-conveyed sheets. In addition, an image reading apparatus  300  reading a document image is arranged on the top of the sheet processing apparatus  200 . 
     The image forming unit  102  includes: a photosensitive drum  111  rotating in an arrow direction (clockwise); an exposure unit  113 ; a charging roller  112  generally sequentially arranged along the rotation direction of the photosensitive drum  111 ; a development unit  114 ; and a transfer roller  115 . The image forming unit  102  forms a toner image on a sheet S, by causing these process units to execute an image formation process. 
     In addition, the sheet feeding unit  101  includes: upper and lower sheet feeding cassettes  105  and  106 , respectively, each storing a stack of sheets S on which images are to be formed; upper sheet feeding rollers  107 ; lower sheet feeding rollers  108 ; conveyance guides  109 ; and registration rollers  110 . The fixing unit  103  includes: a fixing roller  116 ; a pressure roller  117  that is in contact with the fixing roller  116  from below; and sheet discharge rollers  118 . In addition, the sheet discharging unit  104  includes: a sheet discharge port switch member  120 ; conveyance rollers  121  capable of rotating clockwise and counterclockwise; a sheet discharge guide  122 ; sheet discharge rollers  123 ; a sheet discharge unit  124  arranged on the upper surface of the apparatus body  100 ; and a full-load detection lever  125 . 
     The position of the sheet discharge port switch member  120  can be switched between a sheet processing position (as illustrated in  FIG. 1 ) for conveying a sheet on which an image has been formed to the sheet processing apparatus  200  and a sheet discharge position for discharging the sheet to the sheet discharge unit  124 . In addition, a sheet re-feeding path  126  is arranged among the image forming unit  102 , the fixing unit  103 , and the upper sheet feeding cassette  105 . This sheet re-feeding path  126  is used when the image forming unit  102  executes two-sided printing on a sheet S. 
     Next, an image forming operation executed by the image forming apparatus  10  having the above configuration will be described. First, when the image forming unit  102  receives image information from a computer (not illustrated) or a network (not illustrated) such as a local area network (LAN) connected to the apparatus body  100  or when the image forming unit  102  receives image information read by the image reading apparatus  300 , the exposure unit  113  emits laser light L based on the image information. Next, a surface of the photosensitive drum  111  is exposed to the laser light L, the surface being evenly charged with a predetermined polarity and potential by the charging roller  112 . 
     As a result, charges are removed from the exposed portion on the surface of the photosensitive drum  111 , and an electrostatic latent image is formed on the surface of the photosensitive drum  111 . Next, the development unit  114  applies toner to this electrostatic latent image to develop the image as a toner image. The toner image formed on the photosensitive drum  111  in this way is conveyed to a transfer nip portion between the photosensitive drum  111  and the transfer roller  115 , along with the rotation of the photosensitive drum  111  in the arrow direction. 
     A sheet S on which an image is to be formed is separately fed from the upper or lower sheet feeding cassette  105  or  106  via the sheet feeding rollers  107  or  108 . Next, the sheet S is conveyed to the registration rollers  110  along the corresponding conveyance guide  109 . Since the registration rollers  110  are in a stopped state, the sheet S is temporarily stopped by the registration rollers  110 . Next, the temporarily-stopped sheet S is supplied to the transfer nip portion by the registration rollers  110 , which start to rotate in synchronization with the toner image formed by the image forming unit  102 . 
     Consequently, the toner image on the photosensitive drum  111  is transferred to the sheet S by the transfer roller  115 . Next, the sheet S having the toner image that has been transferred from the photosensitive drum  111  is conveyed to the fixing unit  103 , where the sheet S is pinched and conveyed by a fixing nip portion formed between the fixing roller  116  and the pressure roller  117 . Next, the toner image is fixed on the front side of the sheet S by being heated and pressed. If the sheet S does not need to be further processed, the discharge port switch member  120  is set at the position for conveying the sheet S to the sheet discharge rollers  123 . Thus, the sheet S, on which the toner image has been fixed, is conveyed by the conveyance rollers  121  along the discharge guide  122  and is discharged facedown onto the sheet discharge unit  124  by the sheet discharge rollers  123 . Namely, the sheet S is discharged, with the side including the toner image laid facedown. 
     The full-load detection lever  125  is arranged above the sheet discharge unit  124 . This full-load detection lever  125  detects whether the sheet discharge unit  124  is fully loaded with discharged sheets S. If the full-load detection lever  125  detects that the sheet discharge unit  124  is fully loaded with discharged sheets S, a control unit (not illustrated) does not allow the apparatus body  100  to execute further image formation until discharged sheets S are removed from the sheet discharge unit  124 . 
     If a mode for two-sided printing is set for the sheet S, after the trailing edge of the sheet S including the toner image fixed on one side (front side) passes through the conveyance rollers  121 , a sheet re-feeding path switch member  127  is switched to a sheet re-feeding position. Next, the sheet S is conveyed in the reverse direction and is guided to the paper re-feeding path  126 . Next, as the sheet S is conveyed through the image forming unit  102  and the fixing unit  103 , an image is formed on the back side of the sheet S. 
     If a process such as a stapling process is set for the sheet S after image formation or if the sheet S is set to be discharged from the sheet processing apparatus  200  without any process, the discharge port switch member  120  is switched in advance to the sheet processing position for conveying the sheet S to the sheet processing apparatus  200 , as illustrated in  FIG. 1 . In this way, the sheet S is conveyed from the apparatus body  100  to the sheet processing apparatus  200  along a conveyance path  128 . 
     As described above, the sheet processing apparatus  200  executes a process such as a binding process on a sheet S after an image is formed thereon. The sheet processing apparatus  200  includes: conveyance rollers  201   a  to  201   c  conveying a sheet S from the apparatus body  100 ; an intermediate stacking portion  203  onto which a sheet S conveyed by the conveyance rollers  201   a  to  201   c  is temporarily loaded to execute an alignment process; and a removable discharge roller pair  204  discharging a sheet S. 
     The sheet S conveyed from the apparatus body  100  is conveyed to the intermediate stacking portion  203  serving as a sheet stacking portion, along a conveyance path  202  by the conveyance rollers  201   a  to  201   c  serving as a conveyance portion for conveying a sheet in the sheet conveyance direction. If a process such as a binding process is not executed on the conveyed sheet S, the discharge roller pair  204  is set in a nip state. Thus, the conveyed sheet S is discharged onto a sheet discharge tray  210 , without being temporarily held at the intermediate stacking portion  203 . If a binding process is executed on the conveyed sheet S, the discharge roller pair  204  is not in a nip state. Thus, the conveyed sheet S is temporarily held at the intermediate stacking portion  203 . 
     As illustrated in  FIG. 2 , the sheet processing apparatus  200  includes: a conveyance-direction alignment portion  205  aligning the trailing edge, which is the sheet-conveyance-direction upstream edge of the sheet conveyed onto the intermediate stacking portion  203 ; and a width-direction alignment portion  200 A aligning the sheet in the width direction perpendicular to the sheet conveyance direction. In addition, the sheet processing apparatus  200  includes: a stapler  209  serving as a binding portion executing a binding process on an aligned sheet stack; the sheet discharge tray  210  onto which the processed or conveyed sheets are loaded; and a side-edge-alignment reference wall  215  that comes into contact with a side edge of the sheet during a width-direction alignment operation. 
     The conveyance-direction alignment portion  205  includes: a trailing-edge-alignment reference wall  216  that comes into contact with the trailing edge, which is the upstream edge of a sheet in the sheet conveyance direction, and that serves as an alignment reference for aligning the trailing edge of the sheet; and rubber rollers  205   a  that are friction members, for example. These rubber rollers  205   a  are moved by an actuator (not illustrated) between a retracted position that is above a sheet to be aligned and a contact position where the rubber rollers  205   a  come into contact with a sheet. When the trailing edge of the sheet is aligned, the rubber rollers  205   a  are brought into contact with the top surface of the sheet. Next, the rubber rollers  205   a  are rotated to press the trailing edge of the sheet to the trailing-edge-alignment reference wall  216 , which serves as a trailing edge contact portion that is brought into contact with the trailing edge of a sheet, so that the sheet is conveyed in the direction of the trailing-edge-alignment reference wall  216 . In this way, an alignment process in the conveyance direction is executed. 
     The width-direction alignment portion  200 A includes a front-side alignment portion  207  serving as a first width-direction alignment portion arranged downstream of the intermediate stacking portion  203 ; a rear-side alignment portion  206  serving as a second width-direction alignment portion; and an intermediate-stacking-portion side alignment portion  208  arranged at the intermediate stacking portion  203 . The rear- and front-side alignment portions  206  and  207  are arranged to face each other downstream of the intermediate stacking portion  203 . In addition, these rear- and front-side alignment portions  206  and  207  serving as a pair of alignment portions support both edges of a sheet in the width direction perpendicular to the sheet conveyance direction, including the center of gravity G of the sheet placed on the intermediate stacking portion  203 . In addition, these rear- and front-side alignment portions  206  and  207  can be reciprocally moved in the width direction by an actuator (not illustrated), to align the sheet position in the width direction. 
     The rear-side alignment portion  206  includes: a supporting portion  206   a  serving as a sheet supporting portion for supporting the sheet bottom surface; an upper restriction portion  206   b  for preventing a sheet edge from being raised by curling or the like; and a vertical portion  206   c  connecting the supporting portion  206   a  and the upper restriction portion  206   b.    
     In addition, the front-side alignment portion  207  includes: a supporting portion  207   a  serving as a sheet supporting portion for supporting the sheet bottom surface; an upper restriction portion  207   b  for preventing a sheet edge from being raised by curling or the like; and a vertical portion  207   c  connecting the supporting portion  207   a  and the upper restriction portion  207   b . The rear- and front-side alignment portions  206  and  207  and the intermediate-stacking-portion side alignment portion  208  serve as moving portions and are driven by a drive motor  477  in the width direction. 
     As illustrated in  FIG. 3A , an alignment reference wall  207   d  is arranged on the vertical portion  207   c  of the front-side alignment portion  207 . The alignment reference wall  207   d  has a protrusion toward the center of the sheet in the width direction. As will be described below, when a sheet is aligned in the width direction, this alignment reference wall  207   d  is brought into contact with a sheet edge and is used as a reference for the alignment. In addition, as illustrated in  FIG. 3B , two pressing members  211  and  212  are formed on the vertical portion  206   c  of the rear-side alignment portion  206 , which constitutes an alignment unit with the intermediate-stacking-portion side alignment portion  208 . The pressing members  211  and  212  protrude toward the center of the sheet in the width direction and can move in the width direction along a guiding portion  206   d  formed on the vertical portion  206   c.    
     When the rear-side alignment portion  206  moves in the width direction, these pressing members  211  and  212  constitute a downstream-side pressing portion that presses a side edge in the width direction at a position downstream of the center of gravity G of the sheet in the sheet conveyance direction. In addition, an elastic member  213  is arranged between the pressing member  211  and the guiding portion  206   d , and an elastic member  214  is arranged between the pressing member  212  and the guiding portion  206   d . These elastic members  213  and  214  can bring the pressing members  211  and  212  into contact with a side edge of the sheet, by causing the pressing members  211  and  212  to protrude elastically. 
     Namely, the elastic member  213  is arranged between the pressing member  211  and the vertical portion  206   c , which serves as a holding portion for movably holding the pressing member  211 . Likewise, the elastic member  214  is arranged between the vertical portion  206   c  and the pressing member  212 . Each of the pressing members  211  and  212  serving as a downstream-side pressing portion is arranged to protrude further toward the center of the sheet in the width direction than the intermediate-stacking-portion side alignment portion  208  serving as an upstream-side pressing portion. 
     In addition, as will be described below, even when the pressing members  211  and  212  are pressed by a sheet in the direction opposite to the moving direction, as soon as the pressing force by the sheet is removed, the elastic force of the elastic members  213  and  214  returns the pressing members  211  and  212  to the respective home positions as illustrated in  FIG. 3B . In the first exemplary embodiment, the second pressing member  212  arranged downstream in the sheet conveyance direction is formed to protrude slightly further toward the center of the sheet in the width direction than the first pressing member  211  serving as a first pressing portion arranged upstream in the sheet conveyance direction. 
     In addition, the second pressing member  212  is arranged downstream of the trailing edge of an A4-size sheet (predetermined-size sheet) in the sheet conveyance direction. For example, when the sheet processing apparatus  200  aligns an A4-size sheet in the width direction, the second pressing member  212  does not come into contact with the sheet. However, when the sheet processing apparatus  200  aligns an A3-size sheet, which is larger than the A4-size sheet, the second pressing member  212  comes into contact with the trailing edge of the sheet in the width direction. 
     The second pressing member  212  is arranged downstream of the center of gravity G of an A3-size sheet (large size sheet) in the sheet conveyance direction. In addition, the supporting portions  206   a  and  207   a  are arranged to support the downstream portion of an A3-size sheet (large size) in the sheet conveyance direction, including the center of gravity G of the sheet. These supporting portions  206   a  and  207   a  are sufficiently long in the sheet conveyance direction, to prevent the supported A3-size sheet from slipping. 
     In addition, the intermediate-stacking-portion side alignment portion  208  is arranged to be movable in the width direction along a guiding portion (not illustrated) arranged at the intermediate stacking portion  203 . The intermediate-stacking-portion side alignment portion  208  can reciprocally be moved by a linking portion  98  in the width direction in synchronization with the rear-side alignment portion  206 . In addition, this intermediate-stacking-portion side alignment portion  208 , which moves in the width direction in synchronization with the rear-side alignment portion  206 , constitutes the upstream-side pressing portion for pressing a side edge that is upstream of the center of gravity G of the sheet in the sheet conveyance direction. An elastic member (not illustrated) is arranged between the alignment portion  208  and the guiding portion (not illustrated). 
     In this way, as will be described below, even when pressed by a sheet, once the pressing force by the sheet is removed, the elastic force of the elastic member returns the intermediate-stacking-portion side alignment portion  208  to the home position as illustrated in  FIG. 2 . For example, the synchronization of the intermediate-stacking-portion side alignment portion  208  with the rear-side alignment portion  206  can electronically be achieved by a motor that drives each of the portions  208  and  206 . 
     Next, a sheet processing operation of the sheet processing apparatus  200  having the above configuration will be described.  FIG. 4A  illustrates a positional relationship of various components when the intermediate stacking portion  203  has received an A4-size sheet. In  FIG. 4A , to clearly indicate the sheet position, the upper restriction portions  206   b  and  207   b  of the rear- and front-side alignment portions  206  and  207  are not illustrated. 
     When an A4-size sheet S is conveyed, the front-side alignment portion  207  is moved by the drive motor  477  toward the center of the sheet from a retracted position to reach a supporting position where the supporting portion  207   a  can support the bottom surface of the sheet S. When moved to this supporting position, the alignment reference wall  207   d  of the front-side alignment portion  207  is positioned in the same plane as the side-edge-alignment reference wall  215 . Likewise, the rear-side alignment portion  206  is moved by the drive motor  477  to a supporting position where the support portion  206   a  can support the bottom surface of the sheet S and the first pressing member  211  does not hinder conveyance of the sheet S. During this operation, the discharge roller pair  204  is separated from the sheet S. 
     In this way, after the sheet S is conveyed by the conveyance roller  201   c , the conveyed sheet S is placed while the bottom surface thereof is supported by the intermediate stacking portion  203  and the supporting portions  206   a  and  207   a  of the rear- and front-side alignment portions  206  and  207 . As illustrated in  FIG. 4B , the sheet S is supported by the intermediate stacking portion  203  and the pair of rear- and front-side alignment portions  206  and  207  so that the sheet S has an approximately planar shape while the center portion thereof in the width direction is prevented from sagging downwards. 
     Next, to align the sheet S in the width direction, the rear-side alignment portion  206  and the intermediate-stacking-portion side alignment portion  208  are driven by the drive motor  477  toward the center of the sheet S in the width direction as illustrated in an arrow in  FIG. 4A . The first pressing member  211  of the rear-side alignment portion  206  and a surface of the intermediate-stacking-portion side alignment portion  208  that comes into contact with a side edge of the sheet are formed to be in the same plane. Thus, the sheet S is moved as two points of the side edge thereof are pressed by the rear-side alignment portion  206  and the intermediate-stacking-portion side alignment portion  208 . 
     Next, as the side edge of the sheet S is pressed, the sheet S is moved and pressed against the alignment reference wall  207   d  of the front-side alignment portion  207  and the side-edge-alignment reference wall  215 . As a result, the position of the sheet S in the width direction is aligned. The first pressing member  211  is arranged downstream of the gravity center G of the A4-size sheet in the sheet conveyance direction. The trailing edge of the A4-size sheet is in contact with the trailing-edge-alignment reference wall  216 . The intermediate-stacking-portion side alignment portion  208  is arranged near the upstream edge of the A4-size sheet S in the sheet conveyance direction, and the first pressing member  211  is arranged near the downstream edge of the A4-size sheet S in the sheet conveyance direction. In this way, the sheet S can be moved without being rotated. 
       FIG. 5  illustrates a positional relationship of various components when the A4-size sheet S has been aligned in the width direction. During the alignment operation, the rear-side alignment portion  206  is moved until the distance between the pressing plane formed by the first pressing member  211  and the intermediate-stacking-portion side alignment portion  208 , and the plane formed by the alignment reference wall  207   d  of the front-side alignment portion  207  and the side-edge-alignment reference wall  215  becomes shorter than the width of the sheet S. Namely, when a sheet is aligned in the width direction, the alignment position of the rear-side alignment portion  206  is within the width of the sheet S. 
     By setting the alignment position of the rear-side alignment portion  206  in this way, even when the apparent sheet width is decreased by tolerance of a part or the sheet width or curling, the side edge of the sheet S can be surely brought into contact with the front-side alignment portion  207  and the side-edge-alignment reference wall  215 . As a result, a stack of sheets can be aligned highly accurately. 
     The elastic force of the elastic member  213  is set so that, when the first pressing member  211  is pressed by the sheet S after the sheet S comes into contact with the front-side alignment portion  207  and the side-edge-alignment reference wall  215 , the first pressing member  211  can move in the direction opposite to the center of the sheet in the width direction. 
     In addition, as will be described below, the elastic force of the elastic member  214  is set so that, when an A3-size sheet is aligned and the second pressing member  212  is pressed by the sheet, the second pressing member  212  can move in the direction opposite to the center of the sheet in the width direction. Thus, by setting the alignment position of the rear-side alignment portion  206  in this way, the sheet S can be aligned in the width direction, without being damaged. 
     Next, after the sheet S is aligned in the width direction in this way, the rubber rollers  205   a  of the conveyance-direction alignment portion  205  are moved from the above retracted position to the contact position and are rotated (clockwise direction in  FIG. 1 ) to move the sheet S toward the trailing-edge-alignment reference wall  216 . In this way, the trailing edge of the sheet S is aligned. Next, after the trailing edge of the sheet S is aligned in this way and before the next sheet reaches the sheet processing apparatus  200 , the rubber rollers  205   a  are moved to the retracted position where the rubber rollers  205   a  do not hinder the sheet conveyance. 
     Before the next sheet reaches the sheet processing apparatus  200 , the rear-side alignment portion  206  is also retracted from the alignment position illustrated in  FIG. 5  to the sheet reception position illustrated in  FIG. 4A . As in the first exemplary embodiment, when the trailing edge of the sheet S is aligned, if the sheet S is supported by raising the sheet-conveyance-direction downstream side of the sheet S higher than the sheet-conveyance-direction upstream side of the sheet S, the sheet S is affected in the alignment direction by the self-weight. Thus, in the first exemplary embodiment, for example, as illustrated in  FIG. 4B , the rear- and front-side alignment portions  206  and  207  are inclined so that the sheet-conveyance-direction downstream side of each of the portions  206  and  207  is raised higher than the sheet-conveyance-direction upstream side. 
     In addition, in the first exemplary embodiment, a holding member (not illustrated) for pressing aligned sheets from above is arranged near the side-edge-alignment reference wall  215  to move vertically Since this holding member presses the aligned sheets, even when the next sheet is conveyed, the aligned sheets are not misaligned by the next sheet. When the next sheet is aligned, the holding member is retracted to a position where the holding member does not hinder the next sheet alignment operation. When the next sheet alignment operation is completed, the holding member is lowered to press the next sheet. Each time a sheet is discharged from the conveyance roller  201   c  onto the sheets supported by the intermediate stacking portion  203  and the supporting portions  206   a  and  207   a  of the rear- and front-side alignment portions  206  and  207 , the above alignment process in the width and conveyance directions is executed. The above alignment process in the width and conveyance directions is repeatedly executed for each discharged sheet, until the sheet processing apparatus  200  completes processing a job of a predetermined number of sheets. 
     Next, when the alignment process is completed for the predetermined number of sheets, the stapler  209  serving as a binding processing portion staples the sheet stack, and the discharge roller pair  204 , which has been separated from the sheet stack nips the sheet stack and rotates. Accordingly, the sheet stack is conveyed in the sheet conveyance direction. Next, by causing the rear- and front-side alignment portions  206  and  207  to move in the direction opposite to the center of the sheet in the width direction, the sheet stack is dropped and loaded onto the sheet discharge tray  210 . 
     Next, a sheet processing operation of the sheet processing apparatus  200  executed to align an A3-size sheet S 2  will be described. In this operation, the alignment process in the width direction and the trailing-edge alignment process for an A3-size sheet S 2  are the same as those for an A4-size sheet S 2 . Thus, redundant description thereof will be avoided. After the alignment process in the width direction and the trailing-edge alignment process are completed for an A3-size sheet and before the next sheet is conveyed, the rear-side alignment portion  206  is retracted to the sheet reception position from the alignment position, as illustrated in  FIG. 6A . 
     After the rear-side alignment portion  206  is retracted to the sheet reception position in this way, as illustrated in  FIGS. 6B and 6C , the supporting portions  206   a  and  207   a  support both edges of the conveyed A3-size sheet S 2  in the sheet conveyance direction. The downstream portion includes the gravity center G of the sheet S 2 . As a result, the sheet S 2  is held in such a shape that the center portion in the width direction sags downwards and both the edges are raised (hereinafter, this shape will be referred to as a gutter shape). In the case of an A4-size sheet, since the center of gravity G of the sheet S is positioned on the intermediate stacking portion  203  as illustrated in  FIG. 4 , the center portion of the sheet S in the width direction rarely sags downwards. 
     If the sheet S 2  is held in the gutter shape, since rigidity (hereinafter referred to as stiffness) in the sheet conveyance direction is increased, the sheet-conveyance-direction downstream portion of the sheet S 2  does not sag downwards by the stiffness of the sheet S 2 . As described above, the supporting portions  206   a  and  207   a  of the rear- and front-side alignment portions  206  and  207  support the downstream portion of the sheet S 2  in the sheet conveyance direction, including the center of gravity G of the sheet S 2 . Since the sheet S 2  is held in the gutter shape, even if the supporting portions  206   a  and  207   a  have a smaller area for supporting both edges of the sheet, the posture of the sheet S 2  can be maintained while the downstream portion thereof is raised. However, on the sheet-conveyance-direction upstream side where the trailing-edge-alignment reference wall  216  and the stapler  209  are arranged, it is desirable that the sheet S 2  be held in a planar shape. 
     If the sheet-conveyance-direction downstream portion of the sheet S 2  is held in the gutter shape as described above, the apparent sheet width of the downstream portion of the sheet S 2  is decreased to be less than the prescribed value of the sheet S 2 . In addition, as described above, when the sheet S 2  is aligned in the width direction, to prevent rotation of the sheet S 2 , it is desirable that side edges of the sheet S 2  be pressed upstream and downstream of the center of gravity G of the sheet S 2  in the sheet conveyance direction. 
     Thus, in the first exemplary embodiment, as illustrated in  FIG. 3B , the rear-side alignment portion  206  includes the first and second pressing members  211  and  212  for A4- and A3-size sheets, respectively, and the second pressing member  212  is formed to protrude further toward the sheet center in the width direction than the first pressing member  211 . Namely, the second pressing member  212  is arranged to jut out from the vertical portion  206   c  more than the first pressing member  211 . 
     It is desirable that the protrusion amount of the second pressing member  212  be set larger than the reduction amount of the apparent sheet width caused by holding the sheet in the gutter shape. In the first exemplary embodiment, the protrusion amount of the second pressing member  212  is set larger than the reduction amount of the apparent sheet width caused by holding the sheet in the gutter shape. Thus, by forming the second pressing member  212  to protrude in such a protrusion amount, the second pressing member  212  can be brought into contact with a sheet side edge earlier than the first pressing member  211 . Namely, the sheet side edge can be pressed upstream and downstream of the center of gravity G of the sheet S 2  in the sheet conveyance direction. 
     As a result, when the sheet S 2  is aligned in the width direction, rotation of the sheet S 2  can be prevented. Even if the sheet S 2  is rotated by the second pressing member  212 , the trailing edge of the sheet is moved toward the trailing-edge-alignment reference wall  216  as the sheet S 2  is rotated, that is, in the clockwise direction in  FIG. 6A . Thus, alignment of the sheet stack is little affected. 
     In the first exemplary embodiment, the sheet S 2  is held by causing the downstream center portion thereof to sag in the gutter shape. Thus, as illustrated in  FIG. 3A , a cut portion  207   e  is formed at a downstream end corner of the supporting portion  207   a  of the front-side alignment portion  207  on the sheet center side in the width direction. In addition, as illustrated in  FIG. 3B , a cut portion  206   e  is formed at a downstream end corner of the support portion  206   a  of the rear-side alignment portion  206  on the sheet center side in the width direction. The cut portion  206   e  has a linearly-cut chamfered shape. 
     By forming these cut portions  206   e  and  207   e  at the downstream ends of the supporting portions  206   a  and  207   a  of the rear- and front-side alignment portions  206  and  207  on the sheet center side in the width direction, the center portion of the sheet S 2  in the width direction can easily sag between the cut portions  206   e  and  207   e . In addition, in the first exemplary embodiment, these cut portions  206   e  and  207   e  are shaped so that the supporting portions  206   a  and  207   a  taper to the respective narrower downward ends. Namely, the supporting portions  206   a  and  207   a  of the rear- and front-side alignment portions  206  and  207  have a reduced sheet supporting area at the respective downstream ends, and accordingly, as moving to the downstream side, the sheet S 2  gradually changes the shape from the approximately planar shape to the gutter shape. 
     If such cut portions  206   e  and  207   e  are formed, the sheet S 2  is held while being into contact with a ridge line between corner portions  207   f  and  207   g  of the cut portion  207   e  and a ridge line between corner portions  206   f  and  206   g  of the cut portion  206   e , as illustrated in  FIG. 6C . As a result, the gutter shape is formed. In addition, there are cases where the sheet processing apparatus  200  processes a long sheet such as an A3 sheet, whose sheet-conveyance-direction downstream edge is located downstream of the downstream ends of the rear- and front-side alignment portions  206  and  207  in the sheet conveyance direction. In such cases, it is desirable that the corner portions  206   g  and  207   g  always be located inside both edges of the sheet in the width direction. 
     In other words, it is desirable that the intersection between the sheet-conveyance-direction downstream end of the support portion  206   a  of the rear-side alignment portion  206  and the ridge line of the cut portion always be located closer to the sheet center in the width direction than the corresponding edge of the supported sheet. The same applies to the other side. By forming the corner portions  206   g  and  207   g  at such positions as the intersections, it becomes possible to prevent the sheet placement position from changing when a sheet side edge moves over the corner portion  206   g  or  207   g . Namely, the sheet shape can be stably held. 
     However, it is not desirable that the second pressing member  212  be formed excessively downstream in the sheet conveyance direction. This is because, since the sheet has the gutter shape, when the second pressing member  212  presses the side edge of the sheet during an alignment process, the pressing force may not be sufficiently applied to the sheet. As a result, the sheet may not be moved. Thus, in the first exemplary embodiment, the second pressing member  212  is formed between a position slightly downstream of the leading edge of an A4-size sheet and a position near the cut portion  206   e  of the support portion  206   a  at which the sheet starts forming the gutter shape. 
     As described above, in the first exemplary embodiment, the cut portions  206   e  and  207   e  are formed on the supporting portions  206   a  and  207   a  of the rear- and front-side alignment portions  206  and  207 , respectively. In this way, the center of the sheet-conveyance-direction downstream portion of the sheet sags downwards. In addition, the second pressing member  212  is formed to protrude toward the center of the sheet in the width direction than the first pressing member  211 . In this way, the second pressing member  212  can come into contact with the side edge of the sheet-conveyance-direction downstream portion of the sheet whose center portion sags downwards in the width direction. 
     In this way, the sheet can be stably aligned and processed. In addition, according to the first exemplary embodiment, even when a large-size sheet is aligned, since there is no need to support the entire bottom surface of the sheet, the size of each of the alignment portions  206  and  207  does not need to be increased. Thus, a sheet stack can be aligned highly accurately at low cost. In addition, since the size of each of the alignment portions  206  and  207  can be reduced, more space for a user to reach for sheets loaded on the sheet discharge tray  210  can be created. In this way, for example, sheets can be extracted more easily, and loaded sheets can be viewed more clearly. Namely, usability is improved. 
     Other than the linearly-cut chamfered shape as illustrated in  FIG. 3A , the cut portion  207   e  may have a curved shape as illustrated in  FIG. 7A . In this way, since the cut portion  207   e  is depressed toward the alignment reference wall  207   d , the support surface of the supporting portion  207   a  is decreased. By shaping the cut portion  207   e  in this way, points of contact  207   f ′ and  207   g ′ between the sheet and the supporting portion  207   a  of the front-side alignment portion  207  are less affected by stiffness of the sheet. Thus, the sheet can stably be formed to have the gutter shape. In contrast, if the cut portion  207   e  is jutted as illustrated in  FIG. 7B  instead of having the linearly-cut shape, a point of contact  207   h  is more affected by stiffness of the sheet. More specifically, since the sheet and the supporting portion  207   a  are in contact with each other only at one contact point  207   h , the sheet and the supporting portion  207   a  are not stabilized. In addition, the sheet supporting posture is not stabilized, either. 
     While the sheet processing apparatus for processing sheets of various sizes such as A4- and A3-size sheets has thus been described, the present invention is not limited to such sheet processing apparatus. The present invention is applicable to a sheet processing apparatus for processing sheets of other sizes such as LTR and LDR-size sheets. In this case, a plurality of second pressing members  212  is arranged in the sheet conveyance direction, and when a sheet of a different size is processed, a second pressing member  212  arranged at a position corresponding to the sheet size presses a side of the sheet-conveyance-direction downstream portion of the sheet. 
     Next, a second exemplary embodiment of the present invention will be described.  FIG. 8  illustrates configurations of the rear- and front-side alignment portions  206  and  207  of a sheet processing apparatus according to the second exemplary embodiment. In  FIG. 8 , reference characters identical to those in  FIG. 2  denote identical or corresponding portions. 
     In the second exemplary embodiment, the supporting portions  206   a  and  207   a  of the rear- and front-side alignment portions  206  and  207  are not formed horizontally. Each of the supporting portions  206   a  and  207   a  is inclined downwards from the width-direction outer end toward the center of the sheet. By inclining these supporting portions  206   a  and  207   a  in this way, the downstream portion of the sheet can be more easily held in the gutter shape. 
     In the second exemplary embodiment, the width of the sheet supporting space at both the width-direction ends of each of the rear- and front-side alignment portions  206  and  207  is not changed. Namely, the distance between the support portion  206   a  and the upper restriction portion  206   b  and the distance between the supporting portion  207   a  and the upper restriction portion  207   b  are not changed. In this way, when a curled sheet is aligned at both of the ends, the upper restriction portions  206   b  and  207   b  can prevent curling at both of the ends of the sheet. 
     Next, a third exemplary embodiment of the present invention will be described.  FIG. 9  illustrates a configuration of the rear-side alignment portion  206  of a sheet processing apparatus according to the third exemplary embodiment. In  FIG. 9 , reference characters identical to those in  FIG. 3  denote identical or corresponding portions. 
     In  FIG. 9 , the rear-side alignment portion  206  includes a pressing member holder  231  for holding the second pressing member  212  and the elastic member  214 , which are attached movably in the width direction along a guiding portion  231   d  provided with the pressing member holder  231 . In addition, this pressing member holder  231  is attached so that the pressing member holder  231  can be moved by an actuator  250  in the width direction along the guiding portion  206   d  formed on the rear-side alignment portion  206 . 
     In addition, the third exemplary embodiment includes an input unit  251  for inputting information about sheet rigidity, such as sheet thickness or grammage, to the apparatus body  100  or the sheet processing apparatus  200 . In addition, the third exemplary embodiment includes a control unit  252  for controlling the actuator  250  based on the information about sheet rigidity supplied from the input unit  251 . The control unit  252  controls the position of the pressing member holder  231 , in other words, the protrusion amount of the pressing member holder  231  in the width direction. Namely, in the third exemplary embodiment, by moving the pressing member holder  231  in the width direction based on the sheet information obtained by the input unit  251 , the protrusion amount of the second pressing member  212  is changed. 
     For example, if the control unit  252  determines that a conveyed sheet is a thin sheet having low stiffness based on the sheet information supplied from the input unit  251  such as a sensor or an operation unit, the control unit  252  predicts that the downstream portion of the sheet will have a larger gutter shape and the apparent sheet width will be shorter. In such case, the control unit  252  moves the second pressing member  212  toward the sheet center in the width direction, to increase the protrusion amount of the second pressing member  212  to be greater than that of the first pressing member  211 . 
     In contrast, if a thick sheet is conveyed, the protrusion amount can be decreased. Since the second pressing member  212  is moved based on the sheet thickness/grammage so that the protrusion amount of the second pressing member  212  becomes a predetermined amount with respect to the first pressing member  211 , a sheet of an arbitrary thickness/grammage can be accurately aligned. Namely, by changing the protrusion amount of the first pressing member  211  based on the sheet thickness/grammage, a sheet of an arbitrary thickness/grammage can be accurately aligned. 
     Next, a fourth exemplary embodiment of the present invention will be described.  FIG. 10  illustrates a configuration of the rear-side alignment portion  206  of a sheet processing apparatus according to the fourth exemplary embodiment. In  FIG. 10 , reference characters identical to those in  FIG. 3  denote identical or corresponding portions. 
     In  FIG. 10 , a rotatable support portion  243  is rotatably arranged at the support portion  206   a  of the rear-side alignment portion  206 . The rotatable support portion  243  is rotated around a point of rotation  242  by an actuator  253 . The rear-side alignment portion  206  uses the support portion  206   a  and the rotatable support portion  243  to support the sheet-conveyance-direction downstream edge of the sheet. In addition, the fourth exemplary embodiment includes: an input unit  254  for inputting information about sheet rigidity; and a control unit  255  controlling the actuator  253  and rotating the rotatable support portion  243  based on the information about sheet rigidity supplied from the input unit  254 . While not illustrated, the front-side alignment portion  207  has a similar configuration. 
     In the fourth exemplary embodiment, the rotatable support portion  243  has a cut portion  243   a  at the sheet-center-side corner of the sheet-conveyance-direction downstream end. If the rotatable support portion  243  having such cut portion is rotated, since the position of the cut portion of the rear-side alignment portion  206  is changed, the sheet supporting area of the rear-side alignment portion  206  is accordingly changed. 
     Thus, in the fourth exemplary embodiment, before a sheet is conveyed, the rotatable support portion  243  is rotated based on the sheet information obtained by the input unit  254 , and the sheet supporting area of the rear-side alignment portion  206  is changed. Namely, in the fourth exemplary embodiment, the rotatable support portion  243  is rotated in an optimum rotation amount based on the sheet thickness/grammage, the position of cut portion is changed, and the sheet supporting area of the rear-side alignment portion  206  is changed. For example, when a thick sheet is aligned, the position of the cut portion is changed so that the supporting area is decreased. When a thin sheet is aligned, the position of cut portion is changed so that the supporting area is increased. In this way, the sheet can always have an optimum gutter shape at the sheet-conveyance-direction downstream portion thereof, and a sheet of an arbitrary thickness/grammage can be aligned highly accurately. 
     The present invention has been illustrated based on a sheet processing apparatus capable of executing a sheet alignment process and a binding process as sheet processing. However, the present invention is also applicable to an apparatus capable of only a sheet alignment process as sheet processing. In addition, the width-direction alignment unit  200 A using the intermediate-stacking-portion side alignment portion  208  and the rear- and front-side alignment portions  206  and  207  that move in the width direction has been described. However, the present invention is not limited to such example. For example, the present invention may be configured so that, when a sheet is aligned, the front-side alignment portion  207  is fixed and the rear-side alignment portion  206  is moved. 
     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. 
     This application claims priority from Japanese Patent Application No. 2011-214011 filed Sep. 29, 2011, which is hereby incorporated by reference herein in its entirety.