Patent Publication Number: US-8967616-B2

Title: Sheet ejection device

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a sheet ejection device that stacks sheets and divides the sheets into plural sets while stacking by setting the sheets off alternately (forward and backward) along an ejection direction of the sheets. 
     2. Background Arts 
     Recently, a great large number of print sheets are used for printing and copying in printers (such as inkjet printers, stencil printers and laser printers) and copiers. A Japanese Patent Application Laid-Open No. H10-109808 discloses a sheet ejection device used in such printers and copiers. According to the disclosed sheet ejection device, a large number of sheets can be stacked orderly and uniformly. 
     The sheet ejection device, although it is not shown by drawings, includes a bottom plate, an end plate, a pair of side fences extending along a sheet ejection direction and faced to each other, fins provided on the side fences, respectively, and guide members swingably provided on the side fences vertically, respectively, in order to stack a large number of sheets orderly. 
     SUMMARY OF THE INVENTION 
     A condition of sheets to be ejected varies due to a change of their environment, and thereby dropping behaviors of ejected sheets may become unstable. Therefore, the ejected sheers may be damaged when ejected, and stacking alignment of the ejected sheets may degrade. 
     Specifically, in a case of sheets having a light bias weight and a small size (e.g. bias weight: 60 g/m 2  or less and size: B5) under a hot and humid condition (e.g. temperature: 30° C. and humidity: 70% or less), contact frictions with the side fences become large while the sheets are dropping off and balancing between both side edges of the sheets becomes hard to be kept well. Therefore, the sheets may lean on one of the side fences, and thereby the damages of sheets and the degradation of stacking alignment may easily occur. 
     On the other hand, in a case of sheets having a light bias weight and a small size (e.g. bias weight: 60 g/m 2  or less and size: B5) under a cold and dry condition (e.g. temperature: 10° C. and humidity: 40% or less), contact frictions with the side fences become large while the sheets are dropping off and contact areas between dropping sheets and the side fences are large, so that the sheets may be drawn to one of the side fences due to electrostatic charge (e.g. almost 10 kV) of the dropping sheet. Therefore, the sheets may lean on the side fence, and thereby the damages of sheets and the degradation of stacking alignment may easily occur. 
     An object of the present invention is to provide a sheet ejection device that can provide superior sheet ejection performance without affected by its environment. 
     An aspect of the present invention provides a sheet ejection device that includes a sheet tray on which ejected sheets are stacked; a pair of side fences for restricting positions of the ejected sheets along a lateral direction perpendicular to an ejection direction; and at least one rib is formed vertically on each inner side of the side fences, wherein the ribs are configured to contact with side edges of each of the ejected sheets while the each of the ejected sheets falls down to the sheet tray to align the ejected sheet along the lateral direction. 
     According to the above aspect, both side edges of each of the ejected sheets contact with the ribs on the side fences while the each of the ejected sheets falls down to the sheet tray, and thereby the each of the ejected sheet can be aligned along the lateral direction. Therefore, an attitude of the each of the ejected sheets can be corrected adequately by the ribs, and then stacked on the sheet tray after its lateral position is aligned correctly. As a result, superior sheet ejection performance can be provided without affected its environment such as humidity. In addition, a drop speed of each of the ejected sheets contacting with the ribs becomes faster that contacting with both inner surfaces of the side fences. Therefore, the number of ejection sheets per unit time can be made larger because the next sheet ejection can be done earlier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a sheet ejection device according to an embodiment; 
         FIG. 2  is a cross-sectional side view showing the sheet ejection device; 
         FIG. 3A  is an enlarged plan view showing a state where a sheet contacts with a laterally-restricting plate in the sheet ejection device; 
         FIG. 3B  is a further enlarged plan view showing the above state; 
         FIG. 4A  is an enlarged cross-sectional view showing an offset guide plate shifting mechanism in the sheet ejection device in a state where an offset guide plate is set at its waiting position; 
         FIG. 4B  is an enlarged cross-sectional view showing the offset guide plate shifting mechanism in a state where the offset guide plate is set at its offset position; 
         FIG. 5A  is an enlarged side view showing the offset guide plate shifting mechanism in the state where the offset guide plate is set at the waiting position; 
         FIG. 5B  is an enlarged side view showing the offset guide plate shifting mechanism in the state where the offset guide plate is set at the offset position; 
         FIG. 6  is an enlarged cross-sectional view showing an end plate shifting mechanism in the sheet ejection device; 
         FIG. 7A  is a rear view showing the end plate shifting mechanism in a state where a second sheet aligning plate is weighed down; 
         FIG. 7B  is a rear view showing the end plate shifting mechanism in a state where the second sheet aligning plate is lifted up; 
         FIG. 8  is a block diagram of the sheet ejection device; and 
         FIGS. 9A to 9J  are side views schematically showing sheet dividing operations by the sheet ejection device. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, an embodiment will be explained with reference to the drawings. In the drawings, an identical or equivalent component is indicated by an identical reference number. Note that the drawings show components schematically, and it should be considered that the components in the drawings are not shown precisely as they are. In addition, actual dimensions of the components and actual dimensional proportions among the components may be shown differently in the drawings. 
     Further, the embodiment described below is explained as an example that specifically carries out the subject matter of the present invention. In addition, materials, shapes, structures, arrangements of the components are not limited to those in the embodiment. The embodiment may be modified within the scope of the claims (e.g. arrangement of the components may be changed from the embodiment). 
     Furthermore, the sheet ejection device can adapt to any size of sheets. Although sheets are printed by stencil printing or by inkjets in the following descriptions, they can be printed by other methods. A printing method is not limited. 
     In the following descriptions, “right” and “left” are used based on a viewpoint from a downstream side of a sheet ejection direction U (i.e. viewed counter to a sheet ejection flow), as indicated in  FIG. 1 . Similarly, “forward (front)” and “backward (rear)” are used in relation to forward (front side) or backward (rear side) along the sheet ejection flow, as indicated in  FIG. 1 . 
       FIGS. 1 and 2  show a main portion (a sheet set dividing unit  1 ) in a sheet ejection device  8  according to the present embodiment. The sheet ejection device  8  can be applied to a printer such as an inkjet printer, a stencil printer and a laser printer, and to an image forming apparatus that forms images on a sheet P by using a copier or the like. In the present embodiment, the sheet ejection device  8  is disposed on a sheet ejection side of an image forming apparatus (not shown). 
     The sheet set dividing unit  1  in the sheet ejection device  8  includes an ejection unit  10 , a tray elevating mechanism  20 , a pair of laterally-restricting plates (side fences)  31  and  32  for laterally restricting the sheets P stacked on a sheet tray  21 , and a laterally-restricting plate shifting mechanism (side fence shifting mechanism)  30 . The ejection unit  10  sequentially ejects sheets P supplied from the image forming apparatus along the sheet ejection direction U. The tray elevating mechanism  20  receives the sheets P that are sequentially ejected and then dropped off onto a sheet tray  21  to stack the ejected sheets P on the sheet tray  21 , and shifts the sheet tray  21  vertically according to the number of the sheets P stacked on the sheet tray  21 . The laterally-restricting plate shifting mechanism  30  shifts the laterally-restricting plates  31  and  32  along a lateral direction perpendicular to the sheet ejection direction U according to a size (lateral width) of the ejected sheets P. The sheet set dividing unit  1  further includes an offset guide plate shifting mechanism  50  and an end plate shifting mechanism  80 . 
     The offset guide plate shifting mechanism  50  shifts an offset guide plate (guide fence)  52  and a first sheet aligning plate  53  (see  FIGS. 4A and 4B ) forward-and-lower or backward-and-upper by a predetermined offset OS (see  FIGS. 9C and 9H ) for dividing each sheet set. According to this configuration, when dividing the sheets P, trailing edges of the sheets P stacked on the sheet tray  21  can be set off by the offset guide plate  52  provided vertically on a first bracket  51  at an upstream side of the sheet tray  21  and at a height level higher than an upper surface  21   e  of the sheet tray  21 . 
     The end plate shifting mechanism  80  shifts an end plate (end fence)  82  and a second sheet aligning plate  97  (see  FIG. 6 ) forward or backward by the offset OS (see  FIGS. 9E and 9H ) for dividing each sheet set, and lifts the second sheet aligning plate  97  up by using an electromagnetic solenoid (upward shifting mechanism)  93  (see  FIG. 6 ). When the end plate  82  reaches its offset position, the second sheet aligning plate  97  is weighed down with its own weight. According to this configuration, when dividing the sheets P, leading edges of the sheets P stacked on the sheet tray  21  can be set off by the end plate  82  provided vertically on a second bracket  81  at a downstream side of the sheet tray  21  and at a height level higher than the upper surface  21   e  of the sheet tray  21 . 
     In addition, the sheet ejection device  8  further includes a controller  100  for controlling the above-explained components. An installation position of the controller  100  is not limited. The controller  100  may be provided integrally with a controller of the image forming apparatus, or may be provided integrally with a controller that controls an entire of the sheet ejection device  8 . Further, the controller  100  may be disposed at an arbitrary position in the sheet set dividing unit  1 . 
     Here, the sheet tray  21  can be shifted vertically while keeping its horizontal attitude, and the sheets P stacked on the sheet tray  21  are divided into plural sets (staggered forwardly and backwardly) by the offset guide plate  52  (that restricts the trailing edges of the sheets P), the end plate  82  (that restricts the leading edges of the sheets P) and the pair of laterally-restricting plates  31  and  32  (that restricts the sheets P laterally). Namely, the sheets P stacked on the sheet tray  21  are divided into plural sets (staggered alternately forward and backward) while they are surrounded by these four plates  31 ,  32 ,  52  and  82 . The plates  31 ,  32 ,  52  and  82  are provided separately (independently) from the sheet tray  21 . 
     (Ejection Unit  10 ) 
     The ejection unit  10  is attached to an upper portion of an uppermost rear panel  2  of the sheet set dividing unit  1 . In the ejection unit  10 , two pairs of sheet feed rollers  11  and an ejection roller  12  are provided along a sheet guide plate  13 . The two pairs of sheet feed rollers  11  sequentially feed the sheets P supplied from the image forming apparatus. The ejection roller  12  is disposed at a sheet ejection position. 
     In addition, a pair of first wing members  5 L and  5 R and a pair of second wing members  6 L and  6 R are provided on both sides of the sheet guide plate  13 , respectively. The first wing members  5 L and  5 R are made of metal, and each of their downstream end faces has an almost triangle shape. The first wing members  5 L and  5 R are disposed symmetrically with respect to a central line (along the sheet ejection direction U) of the sheet guide plate  13 . The second wing members  6 L and  6 R are also made of metal, and each of their downstream end faces has an almost trapezoidal shape. The second wing members  6 L and  6 R are also disposed symmetrically with respect to the central line of the sheet guide plate  13 . 
     An upper face of the first wing member  5 L ( 5 R) is made inclined to form a continuous surface together with an upper inclined face of the second wing member  6 L ( 6 R). The wing members  5 L and  6 L can be moved vertically by a drive mechanism (not shown) in synchronization with the wing members  5 R and  6 R, so that the height level of the wing members  5 L and  6 L ( 5 R and  6 R) can be adjusted to make the sheet P slightly curved. When the sheet P is slightly curved, the sheet P hardly sags down loosely and hardly waves during its ejection. Therefore, an adequate curvature for the sheet P can be variably formed by the wing members  5 L,  5 R,  6 L and  6 R according to a size of the sheet P. 
     In the present embodiment, the ejection unit  10  is provided on the sheet set dividing unit  1  to set the ejection position of the sheet P precisely. However, the ejection unit  10  including the same configuration as explained above may be provided on the image forming apparatus to eject sheets P sequentially from the image forming apparatus to the sheet set dividing unit  1 . 
     (Tray Elevating Mechanism  20 ) 
     In the tray elevating mechanism  20 , the sheet tray  21  is disposed horizontally between the rear panels  2  of the sheet set dividing unit  1  and a front panel  3  distanced from the rear panels  2 . Its drive mechanism (not shown) for elevating the sheet tray  21  is disposed on a right side of the sheet tray  21 . The sheet tray  21  includes a front end  21   a  on a downstream side along the sheet ejection direction U, a rear end  21   b  on an upstream side along the sheet ejection direction U, a left-side end  21   c  and a right-side end (not shown) parallel to the left-side end  21   c , and has a rectangular shape in its plan view. 
     In addition, the sheet tray  21  further includes a bottom center stem  21   p  and bottom ribs  21   q  and  21   r . The bottom ribs  21   q  are aligned on the left side, and each of them extends from the left side toward the bottom center stem  21   p . The bottom ribs  21   r  are aligned on the right side, and each of them extends from the right side toward the bottom center stem  21   p . The bottom center stem  21   p  is disposed at a center between the left-side end  21   c  and the right-side end along the sheet ejection direction U, and has a flat upper surface  21   e . Each of the bottom ribs  21   q  has an inclined upper surface  21   f  that is made gradually higher toward the left-side end  21   c . Each of the bottom ribs  21   r  has an inclined upper surface  21   g  that is made gradually higher toward the right-side end. By the inclined upper surfaces  21   f  and  21   g , the above-explained curvature is formed on the sheets P stacked on the sheet tray  21 . 
     About four thousand of sheets P having an identical size such as A3 and A4 can be stacked on the sheet tray  21  (on the upper surfaces  21   e ,  21   f  and  21   g ). Therefore, a movable vertical range of the sheet tray  21  is set to about 400 to 500 mm. 
     In addition, a pair of inner vertical brackets  22  and  23  is attached at the right side of the sheet tray  21  to form a distance therebetween. Further, a pair of outer vertical brackets  22 B and  23 B is vertically provided outside the pair of the inner vertical brackets  22  and  23 . The pair of outer vertical brackets  22 B and  23 B is also attached to the right side of the sheet tray  21 . Two reinforcement rods  24  are provided between the pair of outer vertical brackets  22 B and  23 B (lower one is not shown in  FIG. 1 , but shown in  FIG. 2 ). The reinforcement rods  24  penetrate the inner vertical brackets  22  and  23 . Furthermore, a pair of rack members  28  is vertically provided on the right side of the pair of the outer vertical brackets  22 B and  23 B. 
     Furthermore, as shown in  FIG. 1 , a stepping motor  25  is attached to the outer vertical bracket  23 B on the rear side, and the rotational output of the stepping motor  25  is transmitted to a gear  27  fixed at one end of a shaft  26  via a gear set (not shown). The shaft  26  is rotatably supported by the inner vertical brackets  22  and  23 , and another gear  27  (see  FIG. 2 ) is also fixed at another end of the shaft  26 . The gears  27  fixed at the both ends of the shaft  26  are meshed with rack gears formed on the rack members  28 , respectively. When the shaft  26  is rotated by the stepping motor  25 , the gears  27  meshing with the rack gears on the rack members  28  vertically shift the outer vertical brackets  22 B and  23 B and the inner vertical brackets  22  and  23 . As a result, the sheet tray  21  is vertically shifted integrally with the outer vertical brackets  22 B and  23 B and the inner vertical brackets  22  and  23 . 
     As shown in  FIG. 2 , two pairs of a light emitter  29   a  and a light receiver  29   b  composing an optical sensor  29  for detecting a height level of the stacked sheets P are attached to the rear panel  2  and the front panel  3  of the sheet set dividing unit  1 , respectively. Each pair of the light emitter  29   a  and the light receiver  29   b  are faced to each other, but distanced from each other to interpose the stacked sheets P therebetween. One of the pairs detects a height level of the upper surface  21   e  of the sheet tray  21  (the bottom center stem  21   p ), and another of the pairs detects a height level of the uppermost sheet P of the stacked sheets P. 
     While the sheets P are sequentially ejected from the ejection unit  10  (or supplied from the image forming apparatus) and then drop off onto the sheet tray  21 , the controller  100  controls the stepping motor  25  to shift the height level of the sheet tray  21  based on the detection results of the optical sensors  29  so that a sheet drop height H (see  FIG. 2 ) between a sheet ejection height level and the height level of the upper surface  21   e  (when no sheets P are stacked) or the height level of the uppermost sheet P is kept constant. Since the sheet drop height H is kept constant, the sheets P ejected from the ejection unit  10  (or supplied from the image forming apparatus) can keep its adequate attitude while dropping off, and can be stacked on the sheet tray  21  with its adequate attitude. Note that an optical sensor (not shown) for detecting that the sheet tray  21  is shifted downward to its lowermost position is also disposed at a lower portion of the pair of rack members  28 . 
     (Laterally-Restricting Plates  31  and  32 , and Laterally-Restricting Plate Shifting Mechanism  30 ) 
     As shown in  FIGS. 1 to 3B , in the sheet ejection device  8 , the laterally-restricting plates  31  and  32  are vertically disposed on the left and right sides of the sheet tray  21 , and distanced from each other with interposing the sheet tray  21  therebetween. In addition, the laterally-restricting plate shifting mechanism  30  is disposed above the laterally-restricting plates  31  and  32  to shift the laterally-restricting plates  31  and  32  laterally according to the size (lateral width) of the stacked sheets P. 
     Since the laterally-restricting plates  31  and  32  have symmetrical shapes to each other, only the laterally-restricting plate  32  will be explained hereinafter as a representative of them, and explanations for the laterally-restricting plate  31  will be omitted. 
     The laterally-restricting plate  32  includes a frame  35 . In the frame  35 , plural walls  32   a  to  32   d  are extended vertically, and each of the walls  32   a  to  32   d  integrally connects an upper bar with a middle bar of the frame  35 . A window W is formed between each adjacent pair of the walls  32   a  to  32   d . In the frame  35 , feet F are integrally extended downward from the center bar, and the feet F are almost associated with the walls  32   a  to  32   d , respectively. Each width of the feet F along the sheet ejection direction U and a distance between each adjacent pair of the feet F are set so as to the feet F are located in some interspaces between the bottom ribs  21   r  on the sheet tray  21 . When the laterally-restricting plate  32  is shifted laterally by the laterally-restricting plate shifting mechanism  30 , the feet F laterally shifted in the some interspaces between the bottom ribs  21   r.    
     On inner surfaces of some of the feet F, vertical ribs La and Lb are formed. In the present embodiment, the ribs La and Lb are formed on two of the feet F disposed on a near side to the ejection unit  10  (on an upstream side along the sheet ejection direction U). The ribs La and Lb are vertically extended from the foot F to an uppermost edge of the frame  35  through the walls  32   a  and  32   b . No ribs are formed on inner surfaces of the walls  32   c  and  32   d  on a far side from the ejection unit  10  (on a downstream side along the sheet ejection direction U) and the foot F under the walls  32   c  and  32   d , but the walls  32   c  and  32   d  and the foot F under the walls  32   c  and  32   d  forms a flat surface S. The flat surface S and ridges Lt (see  FIG. 3B ) of the ribs La and Lb are on a single plane. Here, the ribs La and Lb are formed to have a predetermined distance to interpose the window W therebetween. Both of the ribs La and Lb are located on the upstream side from the center of the laterally-restricting plate  32  along the sheet ejection direction U. In addition, the ribs La and Lb are “vertically” extended as explained above, but the term “vertically” includes a minute or slight inclination that does not prevent the sheet(s) P from dropping off onto the sheet tray  21  while its lateral edge PE (see  FIG. 3B ) is being contacted with the ribs La and Lb. 
     As shown in  FIG. 3B , each of the ribs La and Lb has an almost trapezoid cross-sectional shape in its plan view. Specifically, each upstream surface Le of the ribs La and Lb is inclined by an angle θ to the lateral edge PE of the sheet P (to the sheet ejection direction U). Therefore, the upstream surface Le is formed as a gentle slope between an upstream edge Lk of the rib La (Lb) and the ridge Lt. Here, the gentle angle θ that forms the upstream surface Le as a gentle slope is an angle that can absorb an impact between the rib La (Lb) and the lateral edge PE of the ejected sheet P and can make the behavior of the sheet P stable to avoid the sheet P being damaged. 
     In addition, some of the feet F include extension members B at their lower ends, respectively (see back face of the laterally-restricting plate  31  on the left in  FIG. 1 ). Each of the extension members B can slide vertically, and can swing laterally outward. When the sheet tray  21  is shifted downward, the lower ends of the extension members B are separated away from the sheet tray  21 . However, the pair of laterally-restricting plates  31  and  32  always covers the range of the above-explained sheet drop height H (see  FIG. 2 ). 
     On the other hand, the laterally-restricting plate shifting mechanism  30  is attached to a top panel (not shown) disposed above the sheet set dividing unit  1  so as to cover the sheet set dividing unit  1 . Namely, a first motor  34  is attached to a first motor bracket  33 R, and the first motor bracket  33 R is fixed to the top panel. Note that the first motor  34  can be rotated forwardly and reversely. 
     The rotation of the first motor  34  is transmitted to a first timing pulley (not shown) fixed on an output shaft of the first motor  34 . Further, a first timing belt  38  is wound around the first timing pulley and a second timing pulley  37  rotatably attached to a bracket  33 L paired with the above-mentioned motor bracket  33 R. The bracket  33 L is also attached to the top panel. Two guide shafts  39  and  40  are disposed parallel to the extended first timing belt  38  with interposing the first timing belt  38  therebetween. 
     A first hanger  41  is fixed with the laterally-restricting plate  31  on the left, and a second hanger  42  is also fixed with the laterally-restricting plate  32  on the right. The first hanger  41  is slidably coupled with the guide shafts  39  and  40 . The second hanger  42  is also slidably coupled with the guide shafts  39  and  40 . The first hanger  41  is fixedly connected with a rear-side path of the first timing belt  38  near the laterally-restricting plate  31 . The second hanger  42  is fixedly connected with a front-side path of the first timing belt  38  near the laterally-restricting plate  32 . 
     When the first timing belt  38  is driven forwardly by driving the first motor  34  forwardly, the first and second hangers  41  and  42  are moved inward toward each other. Therefore, the laterally-restricting plates  31  and  32  are shifted toward each other to narrow the lateral distance therebetween. On the other hand, when the first timing belt  38  is driven reversely by driving the first motor  34  reversely, the first and second hangers  41  and  42  are distanced away from each other. Therefore, the laterally-restricting plates  31  and  32  are distanced from each other to widen the lateral distance therebetween. In this manner, the lateral distance between the laterally-restricting plates  31  and  32  can be adjusted according to the size (lateral width) of the stacked sheets P to restrict the stacked sheets P laterally. 
     (Offset Guide Plate Shifting Mechanism  50 ) 
     The offset guide plate shifting mechanism  50  configures one of featured portions in the present embodiment. The offset guide plate shifting mechanism  50  is unitized and attached to the uppermost rear panel  2  just beneath the ejection unit  10  (or the image forming apparatus) so that the sheets P sequentially ejected from the ejection unit  10  (or the image forming apparatus) can drop off onto the sheet tray  21 . 
     Hereinafter, the offset guide plate shifting mechanism  50  will be explained with reference to  FIGS. 4A to 5B . As shown in  FIGS. 4A and 4B , a first bracket  51  as a base member of the offset guide plate shifting mechanism  50  is made of a sheet metal to include a pair of front plates  51   a  and  51   b  laterally distanced from each other and vertically fixed to the uppermost rear panel  2  (see  FIGS. 1 and 2 ), a pair of side plates  51   c  and  51   d  extended rearward from the front plates  51   a  and  51   b , respectively, and a back plate  51   e  connecting the pair of side plates  51   c  and  51   d . In addition, the above-mentioned offset guide plate  52  is disposed in an inner space surrounded by the pair of the side plates  51   c  and  51   d  and the back plate  51   e.    
     The offset guide plate  52  includes a top plate  52   a  having a narrow width, a front guide plate  52   b  for restricting trailing edges of the stacked sheets P on the sheet tray  21  to set a set(s) of the stacked sheets P off, and a left-side plates  52   c  extended rearward from the left-side edge of the front guide plate  52   b . In addition, a lower center portion of the front guide plate  52   b  is cutout to form a cutout  52   b   1 . The above-mentioned first sheet aligning plate  53  made of a sheet metal is held in the cutout  52   b   1 . The first sheet aligning plate  53  is weighed down with its own weight to extend just behind the rear end  21   b  of the sheet tray  21 , or to contact its lower end with the uppermost sheet P of the stacked sheets P on the sheet tray  21   
     The first sheet aligning plate  53  is configured to align the trailing edges of the stacked sheets P on a front face of the front guide plate  52   b . Rounded protrusions  53   a  are formed at both lateral ends and the center of the lower edge of the first sheet aligning plate  53 , so that the first sheet aligning plate  53  can be softly contacted with the uppermost sheet P of the stacked sheets P to restrict a trailing edge of a sheet P to be stacked next together with the front guide plate  52   b . Therefore, no gap is formed between the offset guide plate  52  and the uppermost sheet P when the first sheet aligning plate  53  contacts with the upper most sheet P of the stacked sheets P to divide the stacked sheets P into plural sets as explained later in detail, so that it become possible to restrict the trailing edge of the sheet P to be stacked next, and surfaces of the sheets P never be damaged. 
     In addition, a tension spring  65  is set between a lower portion of the front late  51   a  of the first bracket  51  and an operation support unit  49  disposed beside the side plates  51   c  as explained later in detail. Further, a drive motor  56  that can be rotated forwardly and reversely is attached to a bottom of a motor bracket  55 , and the offset guide plate  52  is shifted by the drive motor  56 . Furthermore, the side plate  51   c  of the first bracket  51  and after-explained rotational shafts  66  and  67  are linked by after-explained upper and lower link members  68  and  89  and a link plate  72 . By this link mechanism, the offset guide plate  52  can be shifted, integrally with the first sheet aligning plate  53 , almost along the sheet ejection direction U (also shifted downward). 
     Specifically, as shown in  FIGS. 5A and 5B , a worm  57  is attached to an output shaft of the drive motor  56 , and meshes with a worm wheel  59  attached to a shaft (not shown) rotatably supported by a side plate  55   b  of the motor bracket  55 . The worm wheel  59  further meshes with a gear  61  whose shaft  60  is rotatably supported by the side plate  55   b . The gear  61  further meshes with a gear  63  rotatably about its shaft  62 . The gear  63  meshes with a gear  63   u  fixed with the upper rotational shaft  66 . The upper rotational shaft  66  and the lower rotational shaft  67  are provided rotatably on the first bracket  51  to have a distance vertically therebetween, and each of them penetrates the side plate  51   c  of the first bracket  51 . 
     Between the side plates  51   c  of the first bracket  51  and the left-side plates  52   c  of the offset guide plate  52 , one end of the above-mentioned link member  68  is fixed with the upper rotational shaft  66 . Another end of the link member  68  is pivotally attached to a pivot point  70  at an upper corner of the left-side plates  52   c . Similarly, one end of the above-mentioned link member  69  is fixed with the lower rotational shaft  67 . Another end of the link member  69  is pivotally attached to a pivot point  71  at a lower corner of the left-side plates  52   c . An upper short link member  68 B is fixed with the upper rotational shaft  66  outside the side plate  51   c . Similarly, a lower short link member  69 B is fixed with the lower rotational shaft  67  outside the side plates  51   c . The above-mentioned link plate  72  links ends of the short link members  68 B and  69 B at pivot points  73  and  74 , respectively, to synchronize rotational angles of the rotational shafts  66  and  67 . 
     Therefore, when the upper rotational shaft  66  is rotated by the drive motor  56  via the gears  57 ,  59 ,  61 ,  63  and  63   u , the lower rotational shaft  67  is passively rotated by the links  68 B,  69 B and  72 . In addition, when the rotational shafts  66  and  67  are synchronously rotated forwardly (clockwise in  FIGS. 5A and 5B ), the link members  68  and  69  are swung to shift the offset guide plate  52  almost along the sheet ejection direction U. On the other hand, the offset guide plate  52  is retracted when the rotational shafts  66  and  67  are synchronously rotated reversely (counter-clockwise in  FIGS. 5A and 5B ). Note that the tension spring  65  absorbs rattles of the above-explained link mechanism. Further, a shading plate  75   a  is fixed to the upper rotational shaft  66  and a pair of optical sensors  75   b  are attached to the side plate  51   c  of the first bracket  51  to detect the rotational position of the rotational shaft  66  (i.e. the shifted position of the offset guide plate  52 ). 
     According to the above-explained link mechanism, a maximum shift stroke of the offset guide plate  52  along the sheet ejection direction U is almost 30 mm, and a vertical shift stroke is almost 10 mm. Therefore, when dividing the sheets P into plural sets, the offset OS (see  FIGS. 9A to 9J ) of the trailing edges of the sheets P is set to ±30 mm that is equivalent to the shift stroke of the offset guide plate  52  along the sheet ejection direction U. In addition, a maximum vertical stroke of the first sheet aligning plate  53  is set to 10 mm (or larger) that is equivalent to the vertical shift stroke of the offset guide plate  52 . 
     Namely, in the above-explained offset guide plate shifting mechanism  50 , the offset guide plate  52  and the first sheet aligning plate  53  restrict the trailing edges of the sheets P according to the offset OS regardless of the size (length along the sheet ejection direction U) of the sheets P. Therefore, the offset guide plate shifting mechanism  50  can reduce costs by integrally shifting the offset guide plate  52  and the first sheet aligning plate  53  by the predetermined offset OS when dividing the sheets P into each set. 
     (End Plate Shifting Mechanism  80 ) 
     The end plate shifting mechanism  80  also configures one of featured portions in the present embodiment. The end plate shifting mechanism  80  is unitized and provided on a side of the front end  21   a  of the sheet tray  21 . Hereinafter, the end plate shifting mechanism  80  will be explained with reference to  FIGS. 6 to 7B . As shown in  FIG. 6 , the end plate shifting mechanism  80  includes a second bracket  81  made of a sheet metal, and the above-mentioned end plate  82  attached to the second bracket  81  vertically. The end plate  82  is faced to the offset guide plate  52  to have a distance equivalent to the length of the sheets P along the sheet ejection direction U. The second bracket  81  and the end plate  82  can be integrally shifted along the sheet ejection direction U according to the length of the sheets P. 
     As explained above, the end plate  82  can be shifted along the sheet ejection direction U, but the end plate  82  cannot be shifted vertically. Since the sheet tray  21  is elevated according to the number of the stacked sheets P when dividing the sheets P into plural sets, the end plate  82  is kept at a fixed height level higher than the upper surface  21   e  of the sheet tray  21 . The end plate shifting mechanism  80  (the shifting mechanism for shifting the second bracket  81  and the end plate  82  integrally) is attached to the top panel (not shown) disposed above the sheet set dividing unit  1  so as to cover the sheet set dividing unit  1 . Namely, a second motor  84  is attached to a second motor bracket  83 , and the second motor bracket  83  is fixed to the top panel. Note that the second motor  84  can be rotated forwardly and reversely. 
     The rotation of the second motor  84  is transmitted to a third timing pulley  86  fixed on an output shaft of the second motor  84 . Further, a second timing belt  88  is wound around the third timing pulley  86  and a fourth timing pulley  87  rotatably attached to the upper portion of the uppermost rear panel  2 . Two guide shafts  89  and  90  are disposed parallel to the extended second timing belt  88  with interposing the second timing belt  88  therebetween. 
     A third hanger  91  (see  FIG. 1 ) is fixed with the second bracket  81  (see  FIG. 6 ). The third hanger  91  is slidably coupled with the guide shafts  89  and  90 , similarly to the first hanger  41  (the second hanger  42 ) and the guide shafts  39  and  40  in the laterally-restricting plate shifting mechanism  30 . The third hanger  91  is fixedly connected with one of left-side and right-side paths of the second timing belt  88 . 
     When the second timing belt  88  is driven forwardly by driving the second motor  84  forwardly, the third hanger  91  is moved toward the offset guide plate  52 . Therefore, the second bracket  81  and the end plate  82  are integrally shifted to set the sheets P off backwardly. On the other hand, when the second timing belt  88  is driven reversely by driving the second motor  84  reversely, the third hanger  91  is distanced away from the offset guide plate  52 . Therefore, the second bracket  81  and the end plate  82  are shifted to set the sheets P off forwardly. In this manner, the offset OS for the sheets P can be set forwardly (+) or reversely (−) by restricting leading edges of the stacked sheets P. 
     As shown in  FIG. 6 , a second bracket  81  as a base member of the end plate shifting mechanism  80  is made of a sheet metal to include a top plate  81   a  fixed with the above-mentioned third hanger  91 , a front plate  81   b  extended vertically downward from the top plate  81   a , and a left-side plate  81   c  extended forward from the left side edge of the front plate  81   b . The end plate  82  is fixed onto the rear face of the front plate  81   b  vertically to face to the offset guide plate  52  with interposing the sheet tray  21 . An electromagnetic solenoid (i.e. the above-mentioned upward shifting mechanism)  93  for shifting the second sheet aligning plate  97  upward is attached to the front face of the front plate  81   b . A movable iron core  93   a  of the electromagnetic solenoid  93  is extended downward. A lower end of the movable iron core  93   a  is pivotally attached to a movable pivot point  94   a  on a lever  95 . In addition, a base end of the lever  95  is pivotally attached to a stationary pivot point  94   b  on the front plate  81   b . When the movable pivot point  94   a  is lifted up by the electromagnetic solenoid  93  via the movable iron core  93   a , the lever  95  is swung upward about the stationary pivot point  94   b.    
     In addition, the second sheet aligning plate  97  made of a sheet metal is slidably coupled with the second bracket  81  on the front side of the second bracket  81 . An upper end of the second sheet aligning plate  97  is hooked with a roller  96   a  rotatably attached to another end of the lever  95  via a rotational axis  96   b . The second sheet aligning plate  97  includes a pair of sheet aligning arms  97   p  made of resin at its lower ends, so that the second sheet aligning plate  97  can be softly contacted with the uppermost sheet P of the stacked sheets P to restrict a leading edge of a sheet P to be stacked next. Further, the second sheet aligning plate  97  includes a shading plate  97   a  at its upper end and an optical sensor  98  is attached to the front plate  81   b  of the second bracket  81  to detect the vertical position of the second sheet aligning plate  97 . 
     When the electromagnetic solenoid  93  is de-energized as shown in  FIG. 7A , the second sheet aligning plate  97  is weighed down with its own weight to contact the sheet aligning arms  97   p  with the upper surface  21   e  of the sheet tray  21  on which no sheets P are stacked or with the uppermost sheet P of the stacked sheets P on the sheet tray  21 . Here, the lever  95  is also swung downward due to its own weight and the weight of the second sheet aligning plate  97 . Therefore, no gap is formed between the second sheet aligning plate  97  (the sheet aligning arms  97   p ) and the uppermost sheet P when the second sheet aligning plate  97  contacts with the upper most sheet P of the stacked sheets P to divide the stacked sheets P into plural sets as explained later in detail, so that it become possible to restrict the leading edge of the sheet P to be stacked next, and surfaces of the sheets P never be damaged. 
     On the other hand, when the electromagnetic solenoid  93  is energized to swing the lever  95  upward by the movable iron core  93   a  as shown in  FIG. 7B , the second sheet aligning plate  97  is lifted up by the lever  95 . Therefore, the second sheet aligning plate  97  (the sheet aligning arms  97   p ) is separated away from the uppermost sheet P. A vertical shift stroke of the second sheet aligning plate  97  is almost 10 mm similarly to that of the offset guide plate  52  (the first sheet aligning plate  53 ). Note that a tension spring  99  is provided to absorb rattles of the above-explained shifting mechanism. 
     Namely, in the above explained end plate shifting mechanism  80 , the end plate  82  and the second sheet aligning plate  97  restrict the leading edges of the sheets P according to the above-explained offset OS and the size (length along the sheet ejection direction U) of the sheets P. Therefore, the end plate shifting mechanism  80  integrally shifts the end plate  82  and the second sheet aligning plate  97  by the second motor  84  by the predetermined offset OS when dividing the sheets P into each set. And the end plate shifting mechanism  80  shifts the second sheet aligning plate  97  upward by the energization of the electromagnetic solenoid  93 , and shifts the second sheet aligning plate  97  downward by the de-energization of the electromagnetic solenoid  93 . 
     In the present embodiment, the electromagnetic solenoid  93  is used as the upward shifting mechanism for shifting the second sheet aligning plate  97  upward. However, the upward shifting mechanism is not limited to the electromagnetic solenoid  93 , and may have another mechanism as long as it can softly contact the second sheet aligning plate  97  with the uppermost sheet P of the stacked sheets P. For example, a piezo-stack actuator or a motor may be used as the upward shifting mechanism. 
     (Controller  100 ) 
     As shown in  FIG. 8 , the controller  100  includes, in its inside, a CPU  100   a  for executing arithmetic processings and judgment processings, a ROM  100   b  in which an operation program of the sheet set dividing unit  1  and so on are stored, and a RAM  100   c  for temporally storing variable information for the sheet set dividing unit  1 . In addition, the controller  100  controls the ejection unit  10 , the tray elevating mechanism  20 , the laterally-restricting plate shifting mechanism  30 , the offset guide plate shifting mechanism  50 , and the end plate shifting mechanism  80 . 
     (Operations for Dividing Sheets into Sets) 
     Hereinafter, operations for dividing the sheets P sequentially ejected from the ejection unit  10  (or the image forming apparatus) into plural sets on the sheet tray  21  will be explained step by step with reference to  FIGS. 9A to 9J . Note that the optical sensors  29  for detecting a height level SL of the upper surface  21   e  of the sheet tray  21  on which no sheets P are stacked ( FIG. 9A ) or a height level SL of the uppermost sheet P of the stacked sheets P on the sheet tray  21  ( FIGS. 9B to 9J ) are shown only in  FIGS. 9A and 9F . In stead, the height level SL is indicated by dashed-dotted lines in  FIGS. 9B to 9E  and  9 G to  9 J. Hereinafter, the height level SL is referred as a sheet level SL. In the present embodiment, the size (length along the sheet ejection direction U) of the sheets P stacked on the sheet tray  21  is only a single variety. 
     In addition, when dividing the sheets P sequentially ejected from the ejection unit  10  (or the image forming apparatus) into plural sets by setting sheet sets off alternately (forwardly and backwardly), the predetermined number of the sheets P in a single set may be set to a constant value for each set, or may be set differently (variously) for each set according to print jobs or the like. Further, when setting sheet sets off alternately (forwardly and backwardly), a first stacking position (backward offset position) and a second stacking position (forward offset position) can be set, and the two positions are set alternately. 
     Here, the first stacking position is a position where the offset guide plate  52  and the end plate  82  are set at their upstream (backward) positions (i.e. their waiting positions), respectively. In other words, when the offset guide plate  52  and the end plate  82  are set at their waiting position, the sheets P are stacked at the first stacking position (see  FIGS. 9B and 9J ). On the other hand, the second stacking position is a position where the offset guide plate  52  and the end plate  82  are set at their downstream (forward) positions (i.e. their offset positions), respectively. In other words, when the offset guide plate  52  and the end plate  82  are set at their offset position, the sheets P are stacked at the second stacking position (see  FIG. 9F ). The sheets P are stacked at the first stacking position during odd times of the sheet dividing operations, and the sheets P are stacked at the second stacking position during even times of the sheet dividing operations. 
     As shown in  FIG. 9A , the sheet tray  21  is set at its waiting position (the uppermost height level) for the sheet dividing operation at the first stacking position before the first sheet P is stacked on the sheet tray  21 . The height level of the upper surface  21   e  of the sheet tray  21  is detected by the optical sensors  29 , and thereby the height level of the sheet tray  21  is set by the tray elevating mechanism  20  so that the sheet drop height H (between a sheet ejection level EL and the sheet level SL) is kept constant. 
     In addition, the offset guide plate  52  provided on a side of the rear panels  2  is located at a height level higher than the upper surface  21   e  of the sheet tray  21 . The offset guide plate  52  is slightly distanced from the rear end  21   b  of the sheet tray  21  and almost flat with the uppermost rear panel  2  to restrict the trailing edges of the sheets P for the first dividing operation of the sheets P. The first sheet aligning plate  53  attached to the lower portion of the offset guide plate  52  is weighed down with its own weight to restrict the trailing edges of the sheets P together with the offset guide plate  52 . 
     Further, the end plate  82  provided on a side of the front panel is moved backward so that the distance between the end plate  82  and the rear panel  2  (i.e. the retracted offset guide plate  52 ) is made almost equivalent to (or slightly wider than) the length of the sheets P to be stacked on the sheet tray  21 . Namely, the end plate  82  is set at a position associated with the size (length along the sheet ejection direction U) of the sheets P to be stacked on the sheet tray  21  to restrict the leading edges of the sheets P for the first dividing operation of the sheets P. Furthermore, the end plate  82  is located at a height level higher than the upper surface  21   e  of the sheet tray  21 . The second sheet aligning plate  97  attached to the lower portion of the end plate  82  is weighed down with its own weight (the electromagnetic solenoid  93  is de-energized) and the sheet aligning arms  97   p  at the lower ends of the second sheet aligning plate  97  are softly contacted with the upper surface  21   e  to restrict the leading edges of the sheets P together with the end plate  82 . 
     Subsequently, as shown in  FIG. 9B , the offset guide plate  52 , the first sheet aligning plate  53 , the end plate  82  and the second sheet aligning plate  97  are set at the same positions as shown in  FIG. 9A . Therefore, in the first sheet dividing operation, a sheet stacking position is set between the uppermost rear plate  2  (the offset guide plate  52  and the first sheet aligning plate  53  made almost flat with the uppermost rear panel  2 ) and the end plate  82  (the second sheet aligning plate  97 ) located at the position associated with the size (length along the sheet ejection direction U) of the sheets P. Namely, the sheets P are stacked at the first stacking position in the first sheet dividing operation (in odd times of sheet dividing operations). 
     The sheets P sequentially ejected from the ejection unit  10  (or the image forming apparatus) fall down between the rear panel  2  (the retracted offset guide plate  52 ) and the end plate  82 , and thereby are sequentially stacked on the sheet tray  21 . Here, the leading edges of the sheets P are aligned (restricted) by the second sheet aligning plate  97 . The sheet level SL is continuously detected by the optical sensors  29 , and the sheet tray  21  is shifted downward by the tray elevating mechanism  20  to keep the sheet drop height H constant. According to this downward shifting of the sheet tray  21 , the second sheet aligning plate  97  becomes separated from the upper surface  21   e . When the predetermined number of the sheets P are stacked on the sheet tray  21  as the first divided sheet set, the downward shifting of the sheet tray  21  is stopped and the first sheet dividing operation is finished. 
     Subsequently, as shown in  FIG. 9C , the offset guide plate  52  is shifted forward and downward by the offset guide plate shifting mechanism  50  for the second sheet dividing operation while the second sheet aligning plate  97  is kept contacted with the leading edges of the sheets P stacked in the first sheet dividing operation. The offset guide plate  52  is shifted forward by the offset OS along the sheet ejection direction U. By keeping the second sheet aligning plate  97  contacted with the leading edges of the sheets P, the sheets P stacked in the first sheet dividing operation are restricted so as not to get misaligned forward. Note that the sheet tray  21  is being stopped still at the position where it has been stopped when the first sheet dividing operation is finished. 
     The offset guide plate  52  is protruded from the uppermost rear panel  2 , and stopped in a state where the lower end of the first sheet aligning plate  53  is contacted with the uppermost sheet P of the first divided sheet set. The offset OS of the offset guide plate  52  (the first sheet aligning plate  53 ) for the second sheet dividing operation is set to +30 mm. Here, the first sheet aligning plate  53  is softly contacted with the uppermost sheet P at a forward position from the trailing edges of the sheets P of the first divided sheet set, but the uppermost sheet P never got misaligned forward because of the restriction by the second sheet aligning plate  97 . 
     Subsequently, as shown in  FIG. 9D , the second sheet aligning plate  97  is temporally lifted up for the second sheet dividing operation. Here, the sheet tray  21  is being stopped still at the position where it has been stopped when the first sheet dividing operation is finished. In addition, the offset guide plate  52  and the first sheet aligning plate  53  are being stopped still at the positions shown in  FIG. 9C . Although the end plate  82  is also being stopped still at the position shown in  FIG. 9C , the second sheet aligning plate  97  is temporally lifted up by energizing the electromagnetic solenoid  93  to be separated from the sheets P of the first divided sheet set. 
     Subsequently, as shown in  FIG. 9E , the end plate  82  is shifted forward by the end plate shifting mechanism  80  for the second sheet dividing operation while the second sheet aligning plate  97  is kept lifted up. The second sheet aligning plate  97  is weighed down with its own weight by de-energizing the electromagnetic solenoid  93  after the end plate  82  has been shifted forward by the offset OS as shown by dashed-two-dotted lines. Here, the sheet tray  21  is being stopped still at the position where it has been stopped when the first sheet dividing operation is finished. In addition, the offset guide plate  52  and the first sheet aligning plate  53  are being stopped still at the positions shown in  FIGS. 9C and 9D . 
     The offset OS of the end plate  82  (the second sheet aligning plate  97 ) for the second sheet dividing operation is set to +30 mm, similarly to the offset OS of the offset guide plate  52  (the first sheet aligning plate  53 ). The second sheet aligning plate  97  is positioned forward from the leading edges of the sheets P of the first divided sheet set, and the lower ends of the sheet aligning arms  97   p  are positioned at a height level lower than the uppermost sheet P of the first divided sheet set but not contacted with the sheet tray  21 . 
     Subsequently, as shown in  FIG. 9F , the offset guide plate  52 , the first sheet aligning plate  53 , the end plate  82  and the second sheet aligning plate  97  are set at the same positions as shown in  FIG. 9E . Therefore, in the second sheet dividing operation, a sheet stacking position is set between the offset guide plate  52  (the first sheet aligning plate  53 ) protruded from the uppermost rear panel  2  and the end plate  82  (the second sheet aligning plate  97 ) shifted forward. Namely, the sheets P are stacked at the second stacking position (in even times of sheet dividing operations). 
     The sheets P sequentially ejected from the ejection unit  10  (or the image forming apparatus) fall down between the offset guide plate  52  shifted forward and the end plate  82  shifted forward, and thereby are sequentially stacked on the first divided sheet set. The sheet level SL is continuously detected by the optical sensors  29 , and the sheet tray  21  is shifted downward by the tray elevating mechanism  20  to keep the sheet drop height H constant. When the predetermined number of the sheets P are stacked on the first divided sheet set as the second divided sheet set, the downward shifting of the sheet tray  21  is stopped and the second sheet dividing operation is finished. 
     Subsequently, as shown in  FIGS. 9G to 9J , the third sheet dividing operation is done. Since the third sheet dividing operation is almost the same as the first sheet dividing operation, it will be briefly explained hereinafter. As shown in  FIG. 9G , the second sheet aligning plate  97  is temporally lifted up after the second sheet dividing operation. This process is different from the first sheet dividing operation. Subsequently, as shown in  FIG. 9H , the offset guide plate  52  (the first sheet aligning plate  53 ) is shifted backward by the offset OS set to −30 mm, so that the offset guide plate  52  (the first sheet aligning plate  53 ) is reverted to its waiting position (the same position in the first sheet dividing position). The end plate  82  (the second sheet aligning plate  97 ) is also shifted backward by the offset OS set to −30 mm while the second sheet aligning plate  97  is temporally lifted up, so that the end plate  82  (the second sheet aligning plate  97 ) is reverted to the same position in the first sheet dividing position. 
     Subsequently, as shown in  FIG. 9I , the second sheet aligning plate  97  is weighed down with its own weight (the electromagnetic solenoid  93  is de-energized) and the sheet aligning arms  97   p  at the lower ends of the second sheet aligning plate  97  are softly contacted with the uppermost sheet P of the second divided sheet set to restrict the leading edges of the sheets P together with the end plate  82 . Subsequently, as shown in  FIG. 9J , the third sheet dividing operation is made similarly to the first sheet dividing operation. In a case where the fourth sheet dividing operation is made, it is made similarly to the second sheet dividing operation. In these manners, by repeating the plural sheet dividing operations, the sheets P are divided into the plural set on the sheet tray  21  by shifting the offset guide plate  52  and the end plate  82  alternately (forwardly and backwardly). 
     (Advantages) 
     According to the present embodiment, a sheet(s) P ejected from the ejection unit  10  of the sheet ejection device  8  contacts with the ribs La and Lb on the laterally-restricting plate  32  ( 31 ) at its side edge(s) PE extending along the sheet ejection direction U as shown in  FIG. 3 . Therefore, an attitude of the sheet P can be corrected adequately while moving forward and dropping downward, and then stacked after its lateral position is aligned correctly. As a result, superior sheet ejection performance can be provided according to the sheet ejection device  8  without affected its environment such as humidity. 
     In addition, a drop speed of a sheet P while its side edge(s) PE is aligned by the ribs La and Lb becomes faster that that while the side edge(s) PE is aligned by the flat surface(s) S. Therefore, a drop speed of a trailing edge of the sheet P (on a side of the ribs La and Lb) becomes faster than a drop speed of a leading edge of the sheet P (on a side of the flat surface(s) S). As a result, the side edge(s) PE can be aligned stably on a side of the trailing edge (on a side of the ribs La and Lb), and the number of ejection sheets per unit time can be made larger because the next sheet ejection can be done earlier. 
     In addition, the flat surface(s) S is formed on an inner surface(s) of the laterally-restricting plate (side fence)  32  ( 31 ) so that the flat surfaces S and the ridges Lt of the ribs La and Lb are on a single plane. Therefore, the side edge(s) PE on the side of the leading edge contacts with the flat surface S, so that inclination of the leading edge of the sheet P (the orientation of the leading edge becomes unparallel to the sheet ejection direction U) can be prevented while preventing reduction of the number of ejection sheets per unit time. 
     Further, each of the ribs La and Lb has the upstream surface Le gently inclined to the side edge PE of the sheet P as shown in  FIG. 3B . Specifically, the upstream surface Le is formed as a gentle slope between the upstream edge Lk and the ridge Lt, and is inclined by the gentle angle θ. Therefore, inclination of the leading edge of the sheet P (the orientation of the leading edge becomes unparallel to the sheet ejection direction U) can be prevented when the leading edge contacts with the ribs La and Lb. Here, the gentle angle θ is an angle that enables absorption of an impact of the leading edge of the sheet P on to the ribs La and Lb and avoidance of sheet damages to make behavior of the sheet P stable. 
     Furthermore, the ejection unit  10  can eject the sheets P while making them slightly curved to prevent them from sagging down loosely and waving, and can form an adequate curvature to various types of the sheets P. Therefore, the sheets P can be ejected after getting the adequate curvature according to the shapes, the positions and the number of the ribs La and Lb, 
     Note that the ribs La and Lb are aligned on an upstream side as rib formed on the inner surfaces of the laterally-restricting plates  32  and  31  in the present embodiment. By forming at least one rib on an upstream side on each of the laterally-restricting plates (side fences)  32  and  31 , behaviors of the sheets P can be corrected by the ribs and then the sheets P can be stacked while laterally aligned precisely. 
     The present invention is not limited to the above-mentioned embodiment, and it is possible to embody the present invention by modifying its components in a range that does not depart from the scope thereof. Further, it is possible to form various kinds of inventions by appropriately combining a plurality of components disclosed in the above-mentioned embodiment. For example, it may be possible to omit several components from all of the components shown in the above-mentioned embodiment. 
     The present application claims the benefit of a priority under 35 U.S.C §119 to Japanese Patent Application No. 2012-191107, filed on Aug. 31, 2012, the entire content of which is incorporated herein by reference.