Abstract:
A sheet loader for loading a folded sheet bundle provided from a sheet bundle provider with a folded edge of the folded sheet bundle in a lead, including: an undersurface support configured to support an undersurface of the folded sheet bundle provided from the sheet bundle provider; a folded edge blocker configured to take a blocking position to block the folded edge of the folded sheet bundle on the undersurface support and a releasing position to release the folded sheet bundle; and a joint configured to evacuate from an orbit of the folded edge blocker returning from the releasing position to the blocking position, the joint is configured to join the undersurface support with the folded edge blocker at the blocking position after the returning to the blocking position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This non-provisional application is based upon and claims the benefit of priority from: U.S. provisional application 60/943,597, filed on Jun. 13, 2007; U.S. provisional application 60/944,962, filed on Jun. 19, 2007; U.S. provisional application 60/968,249, filed on Aug. 27, 2007; and U.S. provisional application 60/970,139, filed on Sep. 5, 2007, the entire contents of each of which are incorporated herein by reference. 
     This application is also based upon and claims the benefit of priority from Japanese Patent Application No. 2007-262761, filed on Oct. 5, 2007, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     Exemplary embodiments described herein relate to a sheet loader, a sheet folding apparatus, and a sheet finishing system. More particularly, exemplary embodiments described herein relate to a sheet tray to load folded sheet bundles. 
     BACKGROUND 
     JP-H11-322163-A2 describes a problem in paragraph 0295 and FIG. 63 that the height of a stack of folded sheet bundles is much higher only at a side of the folded edge if the folded sheet bundles are stacked with each folded edge overlapping one another because of a spring effect each possesses even if each bundle is folded strongly. In the situation of stacked folded sheet bundles, the open sides of the stack, that is, the side opposite of the folded edges, do not have such a high height. If extra sheet bundles are continually added on the stack, the stack eventually collapses towards the side of the open ends. 
     JP-H11-322163-A2 further describes a stay  106   a  to avoid such an occasion of stack collapse. The stay  106   a  is almost the same height as the height of a stack of predetermined number of sheet bundles with their folded edges overlapping each other. The stay  106   a  is set under the open end side of the stack. However, the stay  106   a  is not sufficient enough to support various kinds of sheet bundles because the individual height of the sheet bundles changes depending on such factors as temperature and humidity. 
     JP-H11-322163-A2 yet describes a proposed solution to avoid such a voluminous stacking in paragraph 0293 and FIG. 62. The proposed solution is to stack the sheet bundles with shifting each folded edge of a bundle off from the folded edge of other bundles to an open end side of a sheet bundle below, individually. However, this proposed solution raises another problem. Specifically, an increasing number of sheet bundles undesirably increases the size of the footprint of the stack. 
     Moreover, JP-2003-261256-A2 describes controlling a moving distance of a sheet stopper mechanism moving in a horizontal direction on a basis of the height of a stack of sheet bundles on an inclined sheet stacker to increase a load capacity. 
     But the control does not work well before the stack exceeds a predetermined height. In other words, the stack of sheet bundles tends to be unstable when the stack is higher than the predetermined height. The stack of sheet bundles also tends to be unstable after stacking many sheet bundles because sheet bundles stop at the horizontal floor where the sheet stopper moves around. 
     SUMMARY 
     The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key or critical elements, nor to delineate the scope of the claimed subject matter. Rather, the sole purpose of this summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented hereinafter. 
     According to an exemplary embodiment, one aspect of the invention is a sheet loader for loading a folded sheet bundle provided from a sheet bundle provider with a folded edge of the folded sheet bundle in a lead, including: an undersurface support configured to support an undersurface of the folded sheet bundle provided from the sheet bundle provider; a folded edge blocker configured to take a blocking position to block the folded edge of the folded sheet bundle on the undersurface support and a releasing position to release the folded sheet bundle; and a joint configured to evacuate from an orbit of the folded edge blocker returning from the releasing position to the blocking position, the joint is configured to join the undersurface support with the folded edge blocker at the blocking position after the returning to the blocking position. 
     Another aspect of the invention relates to a sheet folding apparatus, including: a sheet folder configured to fold a sheet bundle; a sheet bundle discharger configured to provide a folded sheet bundle with a folded edge of the folded sheet bundle in a lead; an undersurface support configured to support an undersurface of the folded sheet bundle discharged from the sheet bundle discharger; a folded edge blocker configured to take a blocking position to block the folded edge of the folded sheet bundle on the undersurface support and a releasing position to release the folded sheet bundle; and a joint configured to evacuate from an orbit of the folded edge blocker returning from the releasing position to the blocking position, the joint is configured to join the undersurface support with the folded edge blocker at the blocking position after the returning to the blocking position. 
     Yet another aspect of the invention relates to a sheet finishing system, including: a printer configured to print images on a plurality of sheets; a sheet folder configured to fold a sheet bundle including the plurality of sheets already printed by the printer; a sheet bundle discharger configured to provide a folded sheet bundle with a folded edge of the folded sheet bundle in a lead; an undersurface support configured to support an undersurface of the folded sheet bundle discharged from the sheet bundle discharger; a folded edge blocker configured to take a blocking position to block the folded edge of the folded sheet bundle on the undersurface support and a releasing position to release the folded sheet bundle; and a joint configured to evacuate from an orbit of the folded edge blocker returning from the releasing position to the blocking position, the joint is configured to join the undersurface support with the folded edge blocker at the blocking position after the returning to the blocking position. 
     To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described. The following description and the annexed drawings set forth in detail certain illustrative aspects of the invention. However, these aspects are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following description when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention and attendant advantages therefore are best understood from the following description of the non-limiting embodiments when read in connection with the accompanying Figures, wherein: 
         FIG. 1  is a diagram illustrating examples of sheets folded at center of their longitudinal direction; 
         FIG. 2  is a diagram illustrating examples of sheet bundles folded at center of their longitudinal direction; 
         FIG. 3  is a diagram illustrating examples of stacks of sheet bundles; 
         FIG. 4  is a diagram illustrating examples of stacks of sheet bundles for an explanation of a basis of embodiments; 
         FIG. 5  is a diagram illustrating examples of sheet bundles and stacks of sheet bundles for an explanation of a basis of embodiments; 
         FIG. 6  is a diagram illustrating an exemplary perspective view of a sheet loader according to a first exemplary embodiment; 
         FIG. 7  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a first exemplary embodiment; 
         FIG. 8  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a first exemplary embodiment; 
         FIG. 9  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a first exemplary embodiment; 
         FIG. 10  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a first exemplary embodiment; 
         FIG. 11  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a first exemplary embodiment; 
         FIG. 12  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a first exemplary embodiment; 
         FIG. 13  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a first exemplary embodiment; 
         FIG. 14  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a first exemplary embodiment; 
         FIG. 15  is a flowchart illustrating an exemplary operation of a sheet loader according to a first exemplary embodiment; 
         FIG. 16  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a first exemplary embodiment; 
         FIG. 17  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a first exemplary embodiment; 
         FIG. 18  is a diagram illustrating an exemplary perspective view of a sheet loader according to a second exemplary embodiment; 
         FIG. 19  is a diagram illustrating an exemplary perspective view of a sheet loader around a guard and a base plate according to a second exemplary embodiment; 
         FIG. 20  is a diagram illustrating an exemplary perspective view of a sheet loader around a guard and a base plate according to a second exemplary embodiment; 
         FIG. 21  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a second exemplary embodiment; 
         FIG. 22  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a second exemplary embodiment; 
         FIG. 23  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a second exemplary embodiment; 
         FIG. 24  is a diagram illustrating an exemplary perspective view of a sheet loader according to a third exemplary embodiment; 
         FIG. 25  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a third exemplary embodiment; 
         FIG. 26  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a third exemplary embodiment; 
         FIG. 27  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a third exemplary embodiment; 
         FIG. 28  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a third exemplary embodiment; 
         FIG. 29  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a third exemplary embodiment; 
         FIG. 30  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a third exemplary embodiment; 
         FIG. 31  is a diagram illustrating an exemplary perspective view of a sheet loader according to a fourth exemplary embodiment; 
         FIG. 32  is a diagram illustrating an exemplary cross-sectional views of a base plate of a sheet loader according to a fourth exemplary embodiment; 
         FIG. 33  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a fourth exemplary embodiment; 
         FIG. 34  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a fourth exemplary embodiment; 
         FIG. 35  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a fourth exemplary embodiment; 
         FIG. 36  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a fourth exemplary embodiment; 
         FIG. 37  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a fourth exemplary embodiment; 
         FIG. 38  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a fourth exemplary embodiment; 
         FIG. 39  is a diagram illustrating an exemplary perspective view of a sheet loader according to a fifth exemplary embodiment; 
         FIG. 40  is a diagram illustrating an exemplary perspective view of a sheet loader according to a fifth exemplary embodiment; 
         FIG. 41  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a fifth exemplary embodiment; 
         FIG. 42  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a fifth exemplary embodiment; 
         FIG. 43  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a fifth exemplary embodiment; 
         FIG. 44  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a fifth exemplary embodiment; 
         FIG. 45  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a fifth exemplary embodiment; 
         FIG. 46  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a fifth exemplary embodiment; 
         FIG. 47  is a diagram illustrating an exemplary perspective view of a sheet loader according to a sixth exemplary embodiment; 
         FIG. 48  is a diagram illustrating an exemplary perspective view of a sheet loader according to a sixth exemplary embodiment; 
         FIG. 49  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a sixth exemplary embodiment; 
         FIG. 50  is a diagram illustrating an exemplary cross sectional view of a sheet loader according to a sixth exemplary embodiment; 
         FIG. 51  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a sixth exemplary embodiment; 
         FIG. 52  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a sixth exemplary embodiment; 
         FIG. 53  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a sixth exemplary embodiment; 
         FIG. 54  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a sixth exemplary embodiment; 
         FIG. 55  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a sixth exemplary embodiment; 
         FIG. 56  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a sixth exemplary embodiment; 
         FIG. 57  is a diagram illustrating an exemplary perspective view of a sheet loader according to a seventh exemplary embodiment; 
         FIG. 58  is a diagram illustrating an exemplary cross-sectional view around a forearm and a base plate of a sheet loader according to a seventh exemplary embodiment; 
         FIG. 59  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a seventh exemplary embodiment; 
         FIG. 60  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a seventh exemplary embodiment; 
         FIG. 61  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a seventh exemplary embodiment; 
         FIG. 62  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a seventh exemplary embodiment; 
         FIG. 63  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a seventh exemplary embodiment; 
         FIG. 64  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a seventh exemplary embodiment; 
         FIG. 65  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a seventh exemplary embodiment; 
         FIG. 66  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a seventh exemplary embodiment; 
         FIG. 67  is a diagram illustrating an exemplary cross-sectional view of a sheet loader according to a seventh exemplary embodiment; 
         FIG. 68  is a diagram illustrating an exemplary perspective view of a sheet loader according to a modification of a seventh exemplary embodiment; 
         FIG. 69  is a diagram illustrating an exemplary perspective view around a guard and a base plate of a sheet loader according to a modification of a seventh exemplary embodiment; 
         FIG. 70  is a diagram illustrating an exemplary cross-sectional view around a flap of a sheet loader according to a modification of a seventh exemplary embodiment; 
         FIG. 71  is a diagram illustrating an exemplary cross-sectional view around a flap of a sheet loader according to a modification of a seventh exemplary embodiment; 
         FIG. 72  is a diagram illustrating an exemplary perspective view of a sheet finishing system; 
         FIG. 73  is a diagram illustrating an exemplary cross-sectional view of a sheet finishing system; 
         FIG. 74  is a diagram illustrating an exemplary perspective view of a sheet folding apparatus; 
         FIG. 75  is a diagram illustrating an exemplary perspective view around a sheet sensor of a sheet folding apparatus; 
         FIG. 76  is a diagram illustrating an exemplary perspective view around a sheet sensor of a sheet folding apparatus; 
         FIG. 77  is a diagram illustrating an exemplary perspective view of a mechanical sensor unit and an electrical sensor unit of a sheet folding apparatus; 
         FIG. 78  is a diagram illustrating an exemplary perspective view of an electrical sensor unit of a sheet folding apparatus; 
         FIG. 79  is a diagram illustrating an exemplary side view of an electrical sensor unit of a sheet folding apparatus; and 
         FIG. 80  is a diagram illustrating an exemplary rear side view of a mechanical sensor unit of a sheet folding apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the Figures in which like reference numerals designate identical or corresponding parts throughout the several views. 
     In this description, the folded edges side of a stack of folded sheet bundles, where folded edges of sheet bundles overlap with each other, is positioned to overlap an open end of the preceding stack of folded sheet bundles. As a consequence, a footprint of a support for all of the sheet bundles is shorter than the footprint of a conventional support for all of the sheet bundles laid their folded edge with the folded edge of each bundle overlapping on the open end of each other adjacent bundle as described in the paragraph 0293 and FIG. 62 of the JP-H11-322163-A2. Furthermore, the stacking orientation in accordance with the invention avoids the undesirable stability fluctuation of the stack caused by height differences in the stack when all folded edges are aligned. Moreover, sheet bundles are well aligned with each folded edge overlapping on each open end of the others respectively by collapsing the stack with the stack sliding. 
     (1) Definition about a Sheet 
       FIG. 1 to 3  respectively illustrate a diagram of a sheet, a sheet bundle, and a stack of sheet bundles. They are folded at centers of their longitudinal direction, respectively. However, the sheets can be folded at any position. 
     (1-1) Sheet 
     As illustrated in  FIG. 1(   a ) and  FIG. 1(   b ), center-folding makes a fold line  101  on a sheet S at the center of portrait or landscape orientation. As a result, one of faces of the sheet S turns into a couple of inner faces  103  which are face to face to each other, and the other of the faces turn into a couple of outer faces  104  which are back to back to each other (facing away from each other). One of the outer faces  104  touching the ground is an outer-undersurface. 
     A direction along the fold line  101  is a lateral direction of the sheet S, and a span of the sheet S on the lateral direction is a width of the sheet S. Further, a direction orthogonal to the fold line  101  is a longitudinal direction of the sheet S, and a span of the sheet S on the longitudinal direction is a length of the sheet S. To make a fold line at any position on a sheet S is simply called a folding. 
     A left edge of the sheet S illustrated in  FIG. 1(   b ), that is the fold line between the couple of outer faces, is a folded edge  105 . A right edge of the sheet S illustrated in  FIG. 1(   b ), that is the opposite side of the folded edge and capable to separate, is an open end  106 . A couple of ends connecting the folded edge with the open end are side ends. Assuming the folded edge as a front, a near end of the side ends is a left side end  115 , and a far end of the side ends is a right side end  116 . 
     As illustrated in  FIG. 1(   c ), leaves on the both sides of the fold line  101  are pages  111  and  112 . Four sides of the couple of pages are a superolateral page face  110 , a superomedial page face  109 , an inferomedial page face  108 , and an inferolateral page face  107 , respectively. The page  111  as a lower page has the inferolateral page face  107  and the inferomedial page face  108 . The page  112  as an upper page has the superolateral page face  110  and the superomedial page face  109 . 
       FIG. 1(   d ) illustrates a diagram of a letter “Z” shaped folded sheet (hereinafter, “Z” folded sheet). The “Z” folded sheet has an additional fold line parallel to the folded edge at the medium of the upper page  112 . 
     Although its shape is different from the center-folded sheet, a left edge of the sheet S illustrated in  FIG. 1(   d ) is a folded edge  113 . A right edge of the sheet S illustrated in  FIG. 1(   d ) also is an open end  114 . In other words, each fold edge has a corresponding open edge. 
     If the inferolateral page face  107  of the folded sheet is laid on a plane, the span from a top of the superolateral page face  110  of the folded sheet to the plane in a direction perpendicular to the plane is a height of the sheet. A region around the maximum height position in the longitudinal direction of the sheet is a bulge portion. A lap portion is a region where the pages are in touch with each other. 
     (1-2) Sheet Bundle 
     A plane sheet bundle T is a plurality of sheets, each sheet overlapping on top of an adjacent sheet as depicted by sheets S 1 , S 2  and S 3  illustrated in  FIG. 2(   a ). A sheet bundle T may be a plurality of folded sheets in which each folded edge of each folded sheet is inserted into an open end of an adjacent folded sheet so that an outer face of the folded sheet meets with an inner face of the adjacent folded sheet and covers the folded sheet. 
     A left edge of the sheet bundle T illustrated in  FIG. 2(   b ) and  FIG. 2(   c ) is a folded edge of the sheet bundle T. A right edge of the sheet bundle T illustrated in  FIG. 2(   b ) is an open end of the sheet bundle T. A couple of ends connecting the folded edge with the open end are side ends. Assuming the folded edge as a front, a near end of the side ends is a left side end, and a far end of the side ends is a right side end. 
       FIG. 2(   d ) illustrates a diagram of a letter “Z” shaped folded sheet bundle (hereinafter, “Z” folded sheet bundle). The “Z” folded sheet bundle has an additional fold line parallel to the folded edge at the medium of the upper pages. 
     (1-3) Stack of Sheet Bundles (or of Sheets) 
       FIG. 3(   a ) illustrates a folded sheet S 2  positioned so that its folded edge overlaps a folded edge of the preceding folded sheet S 1 . In  FIG. 3(   a ), a folded sheet S 3  can also be positioned with its folded edge overlapping the folded edge of the preceding folded sheet S 2 . An entire group of sheets overlapping such as the sheets S 1  and S 2  (a group of S 1 , S 2 , and S 3  as well) is a sheet stack P. 
     A sheet stack P may also be, as illustrated in  FIG. 3(   b ), a plurality of “Z” folded sheets aligned with their folded edges facing the same direction with their folded edges overlapping each other. 
     In addition, a sheet stack P may be, as illustrated in  FIG. 3(   c ) and  FIG. 3(   d ), a plurality of folded sheet bundles including sheet bundles from T 1  through T 3  aligned with their folded edges facing toward the same direction with their folded edges overlapping each other. 
     Furthermore, a folded edge of a stack is a side where each folded edge of a sheet bundle overlaps on an adjacent sheet bundle&#39;s folded edge, and an open end of the stack is the side where each open end of sheet bundles overlaps on the adjacent sheet bundle&#39;s open end. 
     (2) Explanation of a Basis of Embodiments 
       FIG. 4  illustrates diagrams of stacks of sheet bundles for an explanation of a basis for the embodiments.  FIG. 4(   a ) illustrates a stack of sheet bundles  206  including sheet bundles  202 ,  203  and  204 . Each folded edge of the stack overlaps on top of the adjacent folded edge on a platform  201  which has a horizontal surface as an undersurface support. 
     The platform  201  connects to a guard  205  which has a vertical surface (that is, the guard has a surface at least substantially perpendicular to the platform surface). The guard  205  is not necessary if the stack of the sheet bundles moves slowly enough to keep itself stable. The guard  205  is illustrated here only for ease in understanding a transition of the platform  201 . The location where the sheet bundles are fed from does not change its position. 
     The stack shifts to the direction toward its folded edge side after the stack grows to include a predetermined amount of sheet bundles, as illustrated in  FIG. 4(   b ). The predetermined amount may be measured in height, determined by number of sheets, or determined by number of sheet bundles. The stack may shift together with the platform  201  as illustrated in  FIG. 4(   b ), and also may shift relative to the platform  201  instead of the platform  201  shifting. 
     In one embodiment, the distance of the shift is shorter than a length of the stack of the sheet bundles. The length of the stack may vary according to individual posture of the sheet bundles, but does not vary so much from the length of the sheet bundle if they are aligned stable. In another embodiment, the distance of the shift may be longer than one third of the length of the stack to load a bulge portion of the following stack on a lap portion of the stack. 
     After the shift, sheet bundles of the subsequent stack are fed on the platform  201  from the same location where the preceding sheet bundles are fed from. As a result, a folded edge of a sheet bundle  207  is loaded partially covering the preceding stack on a position slightly backing off from the bulge portion of the preceding stack. 
     After the preceding stack shifts away, a new stack is formed with its sheet bundles at the same vertical position (for example, folded edges of each sheet bundle within the stack are aligned), as illustrated in  FIG. 4(   c ). As a result, the sheet bundles come into a condition in which the folded edges side of the stack in which the folded edges of the sheet bundles overlap with each other, are positioned so that there is overlap with the open ends of the preceding stack. In other words, the sheet bundles come into a condition in which the bulge portion of a stack is positioned with overlap with the lap portion of the preceding adjacent stack. 
     Although  FIG. 4  illustrates a situation where the stack does not break apart during the shift, the stack may break apart on a shift by the inertia of the stack as illustrated in  FIG. 5(   b ) if friction between the sheet bundles is not sufficiently strong. As a result, each bulge portion of the sheet bundles is respectively on a lap portion of a sheet bundle below the sheet bundle. Bulge portions of the following sheet bundles are loaded in an organized and neat manner on the lap portions of the sheet bundles included in the stack which has already broken apart. If the stack has already broken apart before the stack becomes unstable, it is unnecessary to provide any concern for the stability of the stack. 
     (2-1) Embodiment 1 
       FIG. 6  illustrates a first exemplary embodiment of a sheet loader. The sheet loader  310  includes an outlet  300  as a part of a sheet bundle provider, a wall  301 , a platform  302  as an undersurface support, a path  303 , a discharge sensor  304 , a guard  305 , a rack gear  306 , a pinion gear  307 , a motor  308 , a button  309 , a load sensor  311 , and a controller  312 . The wall  301  and the path  303  may be parts of the sheet bundle provider. 
     The outlet  300  opens on the wall  301 . The wall  301  can correspond to an outer wall of a sheet folding apparatus. Typically, folded sheets and folded sheet bundles are discharged from the outlet  300  to the platform  302  with the folded edges in the lead. Hereinafter, each of the folded sheets and the folded sheet bundles is simply called a sheet bundle. The outlet  300  connects to the path  303 . 
     The discharge sensor  304  is positioned inside of and close to the outlet  300 . The discharge sensor  304  senses the sheet bundles conveyed through the path  303  to count the number of sheet bundles discharged from the outlet  300 . 
     The platform  302  is positioned below the outlet  300 . The platform  302  has an upper surface as the undersurface support to support an undersurface of the sheet bundle initially discharged from the outlet  300 . The platform  302  extends from and backs off to the wall  301  horizontally. The traveling direction of the platform  302  is parallel to a projection of the discharging direction of the outlet  300  on a horizontal plane. 
     The guard  305  stops the sheet bundle discharged from the outlet  300  to avoid and prevent overrun from the platform  302 . The guard  305  has a face to contact the folded edge of the discharged sheet bundle. The guard  305  takes a minimum distance between the face and the wall  301  during waiting for the sheet bundle discharged from the outlet  300 . The minimum distance may be about the same as the length of the sheet bundle. 
     The motor  308  drives the pinion gear  307  to move the platform  302  through the rack gear  306 . 
     The button  309  extends out (typically up) from the upper surface of the platform  302 , and is depressed into the upper surface of the platform  302  by the sheet bundle initially discharged on the upper surface of the platform  302 . The load sensor  311  detects whether the button  309  is extended or depressed. The load sensor can optionally be equipped to detect the extended distance of the button  309  which can correlate to a predetermined number of sheet bundles on the button  309 . 
     The controller  312  controls driving of the motor  308  based on the detection of the discharge sensor  304  and the load sensor  311 . The controller  312  counts the times of detection for sheet bundles of the discharge sensor  304  as the number of the sheet bundles discharged from the outlet  300 . The controller  312  increments a count each time the discharge sensor  304  detects a sheet bundle while the load sensor  311  is detecting whether the button  309  is depressed. The controller  312  makes the motor  308  drive to advance the platform  302  after a predetermined moment after the count meets or exceeds a predetermined threshold. For example, the predetermined threshold is set to three in this embodiment. The predetermined moment has a sufficient enough length of time for the sheet bundle to remain stable on the platform  302 , or on the preceding sheet bundles, after the discharge sensor  304  detects the sheet bundle, and also is shorter than a discharging interval between sheet bundles. Of course, it is an acceptable configuration to increase the discharging interval between sheet bundles more than usual on making the motor  308  drive, if possible. 
     When the load sensor  311  detects the button  309  in an extended position, the controller  312  clears the count to zero and initiates the motor  308  drive to back off the platform  302 . 
     An exemplary operation of the sheet loader  310  is explained with snapshots in  FIGS. 7 to 12 , and a flowchart in  FIG. 15 . 
       FIG. 7  illustrates a cross-sectional snapshot of the sheet loader  310  before a sheet bundle T 1  in the path  303  is discharged to the platform  302 . The controller  312  starts a count procedure illustrated in  FIG. 15  on detecting a sheet bundle T 1  passing in front of the discharge sensor  304  (Act  350 ). 
       FIG. 8  illustrates a cross-sectional snapshot of the sheet loader  310  after the sheet bundle T 1  is discharged to the platform  302 . The sheet bundle T 1  lands on the platform  302  with its folded edge in the lead and depresses the button  309  to be about even with or under the upper surface of the platform  302 . 
     If a sheet bundle passes in front of the discharge sensor  304  with the button  309  extended (reference “No” of Act  351 ), the controller  312  clears the count to zero before incrementing the count (Act  352 ) and holding the count (Act  353 ) as one. Otherwise, if the sheet bundle passes in front of the discharge sensor  304  with the button  309  depressed (reference “Yes” of Act  351 ), the controller  312  increments the count and holds the count without clearing or resetting to zero (Act  353 ). So, the count is held as one after a transition from the situations illustrated in  FIG. 7  and  FIG. 8 . 
       FIG. 9  illustrates a cross-sectional snapshot of the sheet loader  310  after sheet bundles T 2  and T 3  are discharged on the sheet bundle T 1  on the platform  302 . A stack of the sheet bundles is formed with the sheet bundle T 1  and the following sheet bundles T 2  and T 3  positioned on the sheet bundle T 1 . The sheet bundles T 2  and T 3  pass in front of the discharge sensor  304  with the button  309  in a depressed state due to the sheet bundle T 1 , the controller  312  increments the count twice and holds the count as three after a transition from the situations illustrated in  FIG. 8  and  FIG. 9 . 
     After holding the count, the controller  312  determines whether the platform  302  is advanced or not (Act  354 ). If the count is not equal to the predetermined threshold (reference “No” of Act  354 ), then the controller  312  finishes the count procedure without advancing the platform  302 . If the count is equal to the predetermined threshold (reference “Yes” of Act  354 ), then the controller  312  makes the motor  308  advance the platform  302  (Act  355 ) after the predetermined moment as illustrated in  FIG. 10  and finishes the count procedure. 
     The distance to advance the platform  302  may be between one third and two thirds of the length of the sheet bundle. However, it may be shorter than one third if the bulge portions of the sheet bundles are small because of the weak strength of folding expansive force. In other words, the distance to advance the platform  302  has a sufficient enough length to avoid overlapping the bulge portion of a sheet bundle to be discharged from the outlet  300  on the bulge portion of the preceding stack of sheet bundles. It may be possible to configure the distance to advance the platform  302  shorter if the stack is soft enough for its bulge portion to be pressed as likely to turn into a lap portion by the following sheet bundle. It also may be possible to configure the distance to advance the platform  302  to change according to the type of sheets constituting the stack. The distance should be configured to be shorter if the folding expansive force of the sheets are relatively weak. 
       FIG. 11  illustrates a cross-sectional snapshot of the sheet loader  310  after sheet bundles T 4  and T 5  are discharged on the lap portion of the stack of sheet bundles (T 1 , T 2 , and T 3 ) on the platform  302 . Folded edges of the sheet bundles T 4  and T 5  overlap with the lap portion of the stack. The controller  312  holds the count as five in the time of  FIG. 11 . 
     Even if the lap portion of the stack is relatively low, it has a Slight thickness that raises the folded edge of a sheet bundle overlapping there. As a result, the stabilities of different stacks are different between of the first stack and the second stack overlapping the first stack. Consequently, it may be possible to configure a fewer number of the sheet bundles constituting the first stack than the number of sheet bundles of the second stack. 
     In addition, it may be possible to configure to form a third stack of sheet bundles overlapping on a lap portion of the second stack, and to mount a bulge portion of a stack N+1 on a lap portion of the preceding stack N (positive integer). 
       FIG. 12  illustrates a cross-sectional snapshot of the sheet loader  310  after the stacks are removed from the platform  302 . The button  309  extends from the upper surface of the platform  302 , and then, the controller  312  clears the count to zero and makes the motor  308  drive to back the platform  302  off toward the wall  301  to the position similar to that as illustrated in  FIG. 7 . 
     If a length of the following sheet bundle is longer than the sheet stack removed from the position above the button  309  on the platform  302 , a distance to back the platform  302  off may be shortened, or the platform  302  may stay unchanged, to prepare a sufficient enough distance for the following sheet bundles to be positioned between the wall  301  and the guard  305 . 
     Needless to say, the number of the sheet bundles that constitute the stack is not limited to only two or three as illustrated in the figures, but the number may be less or more. Moreover, the structure to move the platform  302  is not limited to the rack-and-pinion components shown. There are many alternative ways to configure the structure such as a rack with a worm gear. 
     Although  FIG. 10  and  FIG. 11  illustrate a situation where the stack does not break apart during movement of the platform  302 , the stack may break apart when the platform  302  shifts by the inertia of the stack as illustrated in  FIG. 13 . If friction between the sheet bundles is not sufficiently strong, each bulge portion of the sheet bundles is respectively on a lap portion of a sheet bundle below the sheet bundle. Bulge portions of the following sheet bundles are loaded in an organized and neat manner on the lap portions of sheet bundles included in the stack which has already broken apart, as illustrated in  FIG. 14 . If the stack has already broken apart before the stack becomes unstable, it is unnecessary to provide concern over the stability of the stack. 
     The platform  302  may be configured so as to decline from the outlet  303  side as illustrated in  FIG. 16  and  FIG. 17 , as well. There are sheet bundles supported on the declining upper surface of the platform  302 . The guard  305  supports a folded edge of a first sheet bundle and the bulge portion of the first sheet bundle supports a bulge portion of the following sheet bundle. Each of the sheet bundles is prevented from sliding down the slope by a bulge portion of the preceding sheet bundle. As the result, sheet bundles are loaded in an organized and neat manner on the platform  302 . 
     Moreover, sheet bundles are stabilized and compressed since the bulge portions are pressed together by the gravity force of the following sheet bundles sliding down the slope. As a result, the loading capacity on the platform  302  becomes higher than a substantially level bed. 
     Such a beneficial effect cannot be not attained by the techniques described in JP-2003-261256-A2 where the sheet stopper mechanism moves in a horizontal direction connecting at the bottom of the slope, although the sheets slide down the slope of the inclined sheet stacker. In this configuration, the first and some of the following sheets stop at the bottom of the slope and overtake the preceding sheet thereby causing the sheets to be out of order. 
     (2-2-1) Embodiment 2 
       FIG. 18  illustrates a second exemplary embodiment of a sheet loader. The sheet loader  400  includes an outlet  401  as a part of a sheet bundle provider, a wall  402 , a platform  403  as an undersurface support, a path  404 , a guard  405 , and a spring  406 . The wall  402  and the path  404  may be parts of the sheet bundle provider. 
     The outlet  401  opens on the wall  402 . The wall  402  corresponds to, for example, an outer wall of a sheet folding apparatus. Folded sheet bundles are discharged from the outlet  401  to the platform  403  with their own folded edges in the lead. The outlet  401  connects to the path  404 . 
     The platform  403  is positioned below the outlet  401 . The platform  403  is configured so as to decline from the side by the outlet  401 . 
     The guard  405  supports a folded edge of the sheet bundle so that the sheet bundle does not to slide down and off the platform  403 . The guard  405  may shift along the decline of the upper surface of the platform  403  in parallel with the platform upper surface. A width of the guard  405  is sufficient to support the folded edge of a sheet bundle, such as about as same length of the shorter side of a post card. A center of the guard  405  can correspond to a center of the sheet bundle discharged from the outlet  401 . The spring  406  biases the guard  405  toward the wall  402 . The guard  405  is pushed downward along the decline of the upper surface of the platform  403  by the gravitational weight of the sheet bundles on the platform  403 . The guard  405  goes far away from the wall according to the weight of the sheet bundles on the platform  403 . 
     The guard  405  in this embodiment is connected to a base plate  407  as illustrated in  FIG. 19 . The base plate  407  has a flat plane parallel to the upper surface of the platform  403 , as its upper surface. A width of the base plate  407  can be same as the guard  405 . The base plate  407  shifts together with the guard  405 . 
     The base plate  407  has a length along the direction where the guard  405  shifts according. The base plate  407  supports rollers  408  and  409  rotatably around a horizontal axis which is perpendicular to the upper surface of the slope  412 . The rollers  408  and  409  are aligned in the direction with a distance therebetween sufficient enough to be stable. Such a structure is effective for the guard  405  to keep its shift movement smooth and its posture stable. 
     A slope  412  has a flat plane parallel to the upper surface of the platform  403 , as its upper surface. The slope  412  supports the base plate  407  through the rollers  408  and  409 . The rollers  408  and  409  roll on the region surrounded with broken lines on the upper surface of the slope  412  illustrated in  FIG. 19 . 
     A platform cover  413  is attached to the slope  412  and covers regions on the upper surface of the slope  412  other than the region where the base plate  407  is located and moves across. The upper surface of the platform cover  413  is set on the same plane as the upper surface of the base plate  407 . 
     Furthermore, the base plate  407  has other rollers  410  and  411 . Rollers  410  and  411  are supported by the base plate  407  rotatably around an axis perpendicular to the upper surface of the slope  412 . 
     The rollers  410  and  411  roll on vertical guide walls which the platform cover  413  supports inside of itself. The vertical guide walls prevent the base plate  407  and the guard  405  from moving the wrong way on the slope  412 . 
     The guard  405  has a trench  415  on the surface where there is some contact with the folded edge of the sheet bundle. The guard  405  provides support to the sheet bundle for added stability because the folded edge of the sheet bundle is supported at two points which are both edges of the trench. The trench  415  is a clearance in which to put user&#39;s fingers, allowing the user to remove the sheet bundle easily. 
     The structure concerning the guard  405  is not limited to the above. For example, the guard  405  may connect to beams  414  instead of the base plate  407  which is for supporting the rollers  408  through  411 . The beams  414  are hidden under the platform cover  413 , and are exposed after the guard  405  moves down the slope  412 . 
     An exemplary operation of the sheet loader  400  is explained with snapshots in  FIGS. 21 to 23 . 
       FIG. 21  illustrates a cross-sectional snapshot of the sheet loader  400  before a sheet bundle T 1  in the path  404  is discharged to the platform  403 . 
     The spring  406  biases the guard  405 , but there is no sheet bundle on the platform  403 , so the guard  405  is at the nearest position in a range where the guard  405  can move or position itself along the decline of the platform  403 . 
       FIG. 22  illustrates a cross-sectional snapshot of the sheet loader  400  after the sheet bundle T 1  is discharged to the platform  403 . 
     The weight of the sheet bundle T 1  extends the spring  406  by gravitational force on the guard  405 , and the guard  405  slides down the decline of the platform  403  slightly. 
       FIG. 23  illustrates a cross-sectional snapshot of the sheet loader  400  after sheet bundles T 2  through T 5  are discharged on the platform  403 . 
     The distance between the wall  402  and the guard  405  increases in accordance with a number of sheet bundles laid on the platform  403 . That is, a space for putting the sheet bundles with the bulge portion of each (except the first bundle) positioned on top of an adjacent bundle&#39;s lap portion respectively is enlarged by the increasing gravitational force of sheet bundles themselves. 
     Even if relatively large size sheet bundles are discharged on the platform  403 , the space for stacking the large size sheet bundles can be acquired by the guard  405  moving away as caused by increasing heaviness of the sheet bundles. 
     The distance between the wall  402  and the guard  405  may be longer than a length of the sheet bundle before the sheet bundle is discharged on the platform  403 . As a result, the sheet bundles slide down the decline to mount their bulge portions on top of a preceding bundle&#39;s lap portion. 
     The angle of the decline of the platform  403  slows down the sliding speed of the sheet bundle so that it does not run over the bulge portion of the preceding sheet bundle. As a result, each bulge portion of the sheet bundles is on the lap portion of an adjacent sheet bundle under the sheet bundle. 
     (2-2-2) Embodiment 3 
       FIG. 24  illustrates a third exemplary embodiment of a sheet loader. The sheet loader  500  includes an outlet  501  as a part of a sheet bundle provider, a wall  502 , a platform  503  as an undersurface support, a path  504 , a guard  505 , and a spring  506 . These features respectively correspond to the outlet  401 , the wall  402 , the platform  403 , the path  404 , the guard  405 , and the spring  406  of Embodiment 2. The wall  502  and the path  504  may be parts of the sheet bundle provider. The guard  505  may be a folded edge blocker. 
     The sheet loader  500  further includes a magnet  507  and a steel plate  508 . The magnet  507  is supported on the guard  505 , and the steel plate  508  is supported on the platform  503 . The magnet  507  has a sufficient magnetic force to attract the steel plate  508  to keep the guard  505  only, without supporting any sheet bundles, at the nearest position in a range where the guard  505  can move along the decline of the platform  503 . The magnet  507  and a steel plate  508  may be parts of a canceller. 
     The magnetic force keeps the guard  505  at position nearest magnetic  507  before a total weight of sheet bundles put on the platform  503  exceeds a threshold limit. If the total weight of the sheet bundles put on the platform  503  exceeds the threshold limit, the guard  505  starts to slide down the decline of the platform  503 . An initial sliding distance just after the guard  505  starts to slide down the decline of the platform  503  may be longer than a sliding distance of the guard  505  per a sheet bundle after then. 
     An exemplary operation of the sheet loader  500  is explained with snapshots in  FIGS. 25 to 28 . 
       FIG. 25  illustrates a cross-sectional snapshot of the sheet loader  500  before a sheet bundle T 1  in the path  504  is discharged to the platform  503 . A total force of the magnet  507  and the spring  506  bias the guard  505  including no sheet bundle on the platform  503 , so that the guard  505  is at the nearest position in the range where the guard  505  can move along the decline of the platform  503 . 
       FIG. 26  illustrates a cross-sectional snapshot of the sheet loader  500  after the sheet bundle T 1  and the following sheet bundle T 2  are discharged to the platform  503 . The guard  505  does not slide down the decline of the platform  503  at this time because the total force of the magnet  507  and the spring  506  sustains a total weight of the sheet bundles T 1  and T 2  (the combined force of the magnet and the spring is greater than the gravitational force of the weight of sheet bundles T 1  and T 2 ). A stack is formed with the sheet bundles T 1  and T 2 . 
       FIG. 27  illustrates a cross-sectional snapshot of the sheet loader  500  after a sheet bundle T 3  is discharged on the stack of the sheet bundle T 1  and the sheet bundle T 2 . The total force of the magnet  507  and the spring  506  cannot sustain the weight of a stack including the sheet bundles T 1  through T 3  (the combined force of the magnet and the spring is less than the gravitational force of the weight of sheet bundles T 1 , T 2 , and T 3 ). Due to the gravitational force, the guard  505  slides down the decline of the platform  503  with the stack. As the result, the stack is ready for being overlapped by a folded edge side of the next following sheet bundle discharged from the outlet  501 , on the open end side of the ready stack. 
     As just described, the stack is ready for being overlapped by a folded edge of the following sheet bundle on its lap portion by movement of the stack toward its folded edge side.  FIG. 28  illustrates a cross-sectional snapshot of the sheet loader  500  after sheet bundles T 4  and T 5  are discharged on the stack including the sheet bundles T 1  through T 3 , and a folded edge side of a stack of the sheet bundles T 4  and T 5  overlaps on the open end side of the stack of the sheet bundles T 1  through T 3 . 
     After the stacks are removed from the platform  503 , the guard  505  climbs back to and reassumes the position just as illustrated in  FIG. 25  by the force of the spring  506 , and the magnet  507  uses its force to securely attract the steel plate  508 . 
     The guard  505  may slide down halfway of the range at a time when the guard  505  starts to slide down as illustrated in  FIG. 27  if the force of the spring is set relatively strong. The guard  505  may slide down to the bottom of the range at a time when the force of the spring is set relatively weak, as well. 
     Although  FIGS. 27 and 28  illustrate a situation where the stack does not break apart during the shift of the platform  503 , the stack may break apart during the shift by the inertia of the stack as illustrated in  FIG. 29  if friction between the sheet bundles is relatively weak. As a result, each bulge portion of the sheet bundles is respectively on a lap portion of a sheet bundle below the sheet bundle. And then, the next following sheet bundles T 4  and T 5  are put on the platform  503  with their folded edges overlapping on an open end of their respective preceding sheet bundle, as shown in  FIG. 30 . If the stack on the guard  505  is already broken apart before the stack become unstable, it is unnecessary to be concerned with the stability of the stack. 
     In addition, the magnet  507  may be a temporary magnet including similar devices to the discharge sensor  304 , the button  309 , the load sensor  311  and the controller  312  of the sheet loader  500  in Embodiment 1, and change the Act  355  of the  FIG. 15  with to release the electromagnetic force of the magnet  507  (of course, the electromagnetic force should work before then). A lock released by a magnetic force of a temporally magnet may be employed to retain the guard  505  at the top of the range. 
     (2-2-3) Embodiment 4 
       FIG. 31  illustrates a fourth exemplary embodiment of a sheet loader. The sheet loader  600  includes an outlet  601  as a part of a sheet bundle provider, a wall  602 , a platform  603  as an undersurface support, a path  604 , a guard  605 , a spring  606 , a magnet  607 , and a steel plate  608 . They respectively correspond to the outlet  501 , the wall  502 , the platform  503 , the path  504 , the guard  505 , the spring  506 , the magnet  507 , and the steel plate  508  of Embodiment 3. The wall  602  and the path  604  may be parts of the sheet bundle provider. The magnet  607  and a steel plate  608  may be parts of a canceller. The guard  605  may be a folded edge blocker. 
     A base plate  609  connecting to the guard  605  of this embodiment has a hill or mound across its lateral direction. The hill has a peak or apex. A ridge line of the peak or apex is along a folded edge of a sheet bundle which is supported by the guard  605 , a direction along the ridge line of the peak may be perpendicular to a direction where the guard  605  moves back and forth. The peak of the hill may be rounded or cornered. The base plate  609  with the hill may also be the undersurface support.  FIG. 32(   a ) is a side view of the sheet loader  600 . 
     A distance Lp indicated in  FIG. 32(   a ) is a distance between the peak and a face of the guard  605  which contacts a folded edge of the sheet bundle. The distance Lp is along the direction where the guard  605  moves back and forth. The distance Lp may be shorter than a half of a length of the sheet bundle. If the outlet  601  discharges various sizes of sheet bundles, the distance Lp may be shorter than a half of a length of the maximum size of the various sheet bundles. 
     A bulge portion of a sheet bundle initially laid on the platform  603  falls into a space between the peak and the face of the guard  605 . Although the ridge line of the peak in this embodiment continues through an entire of a width of the guard  605 , the ridge line of the peak may include a plurality of independent peaks. 
     A valley wall is a surface extending on the base plate  609  toward the guard  605  from the peak. A mountain slope is a surface extending on the base plate  609  toward a side by the wall  602  from the peak. The valley wall inclines steeper than the mountain slope. Due to the increased steepness of the valley wall, friction and other resistances in a range between the peak and the guard  605  are reduced, and the folded edge of a first sheet bundle can contact the guard  605  more easily. Additionally, the first sheet bundle can contact with the guard  605  stable. 
     Such benefit is improved by setting the landing point of the first sheet bundle farther than the peak. Conversely, if the landing point is closer to the wall  602  than the peak, it is necessary to set the discharging speed of the first sheet bundle relatively fast, and to set the decline of the platform  603  steeply, suitably enough to prevent the first sheet bundle from stopping before contacting the guard  605 . 
     The peak has a sufficient height to support the first sheet bundle so as to keep a superolateral surface of the first sheet bundle as convex or flat. The peak may be set sufficiently high enough to keep a superolateral surface of the following several sheet bundles mounting on the first sheet bundle as convex or flat. A reason to keep the superolateral surface of the top sheet bundle of the stack as convex or flat is to prevent the next following sheet bundle from stopping before contacting the guard  605 . In many instances, it is undesirable for a subsequent sheet bundle to stop before contacting the guard  605 . 
     The mountain slope of the hill may be set to cross the upper surface of the platform  603  as illustrated in  FIG. 32(   c ) to keep open ends of sheet bundles closer to the platform  603 . As a result, since both corners of open ends are supported by the platform  603 , which is broader than the base plate  609 , the sheet bundle is stabilized further. 
     An end of the mountain slope close to the wall  602  may set above the upper surface of the platform  603  as illustrated in  FIG. 32(   d ) and  FIG. 32(   e ). As a result, since the open ends of sheet bundles are prevented from contacting the platform  603 , the sheet bundles are prevented from stopping before contacting the guard  603  by friction with the platform  603 . 
     An exemplary operation of the sheet loader  600  is explained with snapshots in  FIGS. 33 to 36 . 
       FIG. 33  illustrates a cross-sectional snapshot of the sheet loader  600  before a sheet bundle T 1  in the path  604  is discharged to the platform  603 . The magnet  607  and the spring  606  bias the guard  605  with their total respective forces, but there is no sheet bundle on the platform  603 , so the guard  605  is at the nearest position in the range where the guard  605  can move along the decline of the platform  603 . 
       FIG. 34  illustrates a cross-sectional snapshot of the sheet loader  600  after the sheet bundle T 1  and the following sheet bundles T 2  and T 3  are discharged to the platform  603  in numerical order. The guard  605  does not slide down the decline of the platform  603  at this time because the total force of the magnet  607  and the spring  606  sustains a total weight of the sheet bundles T 1  through T 3 . As a result, a stack is formed with the sheet bundles T 1  through T 3 . Since a superolateral surface of the sheet bundle T 2  is slightly convex by the benefit of the hill on the base plate  609 , the sheet bundle T 3  is kept more stable on the sheet bundle T 2 , and the entire stack is held more stable. 
       FIG. 35  illustrates a cross-sectional snapshot of the sheet loader  600  after a sheet bundle T 4  is discharged on the stack of the sheet bundle T 1  through T 3 . The total force of the magnet  607  and the spring  606  cannot sustain the total weight of the sheet bundles T 1  through T 4 . Then the guard  605  slides down the decline of the platform  603  with the stack. As the result, the stack is ready for being overlapped by a folded edge side of the next following sheet bundle discharged from the outlet  601 , on its open end side. It can be understood from the smaller warp of the sheet bundle T 3  as illustrated in  FIG. 35  than as illustrated in  FIG. 27  that the stack is more stable due to the hill on the upper surface of the base plate  609 . 
     As just described, the stack readies for being overlapped by a folded edge of the following sheet bundle on its lap portion by shifting the stack toward its folded edge side.  FIG. 36  illustrates a cross-sectional snapshot of the sheet loader  600  after sheet bundles T 5  through T 7  are discharged on the stack of sheet bundles T 1  through T 4 , and a folded edge side of a stack of the sheet bundles T 5  through T 7  overlaps on the open end side of the stack of the sheet bundles T 1  through T 4 . 
     After the stacks are removed from the platform  603 , the guard  605  climbs back to the position just as illustrated in  FIG. 33  by the force of the spring  606 , and the magnet  607  uses its force to attract the steel plate  608 . 
     Although  FIGS. 35 and 36  illustrate a situation where the stack does not break apart during the shift, the stack may break apart during movement of the stack by the inertia of the stack as illustrated in  FIG. 37  if friction between the sheet bundles is not sufficiently strong. As a result, each bulge portion of the sheet bundles is respectively on a lap portion of a sheet bundle below the sheet bundle. And then, the following sheet bundles T 5  and T 6  are put on the platform  603  with their folded edges overlapping on an open end of their respective preceding sheet bundle, as shown in  FIG. 38 . If the stack on the guard  605  is already broken apart before the stack become unstable like above, it is unnecessary to be concerned with the stability of the stack. 
     (2-2-4) Embodiment 5 
       FIG. 39  illustrates a fifth exemplary embodiment of a sheet loader. The sheet loader  700  includes an outlet  701  as a part of a sheet bundle provider, a wall  702 , a platform  703  as an undersurface support, a path  704 , a spring  706 , a magnet  707 , and a steel plate  708 . These features respectively correspond to the outlet  601 , the wall  602 , the platform  603 , the path  604 , the spring  606 , the magnet  607 , and the steel plate  608  of Embodiment 4. The wall  702  and the path  704  may be parts of the sheet bundle provider. 
     The sheet loader  700  further includes a guard  705  which differs from the guard  605  in Embodiment 4, a lever  707 , a stopper  708 , and a lever arm  709  as  FIG. 40  which illustrates a perspective view of the sheet loader  700 . The guard  705  may be a folded edge blocker. The lever  707 , the stopper  708 , and the lever arm  709  may be parts of a canceller. 
     The guard  705  rotatably supports the lever  707  at its center in the width direction on an axis along a folded edge of a sheet bundle to be supported by the guard  705 . 
     The lever  707  juts out from the top of the guard  705 . The lever  707  is rotated around the axis by a spilt sheet bundle sliding off the top of a stack of sheet bundles after the stack grows higher than the guard  705 . The lever  707  has a shape crooked toward the side near the outlet  701  around its top. Such shape provides the benefit of stopping the spilt sheet bundle stable after the lever  707  is pushed into a plane to contact the folded edge of the sheet bundle. 
     The lower end of the lever  707  connects to the lever arm  709  extended above the decline of the platform  703 . The other end of the lever arm  709  engages the stopper  708  on the platform  703 . The engagement between the lever arm  709  and the stopper  708  is released if the top of the lever  707  is turned by the push or force of the spilt sheet bundle. 
     If the height of the stack exceeds a threshold limit, the guard  705  starts to slide down the decline of the platform  703 . An initial sliding distance just after then may be longer than a sliding distance of the guard  705  per a sheet bundle. 
     An exemplary operation of the sheet loader  700  is explained with snapshots in  FIGS. 41 to 44 . 
       FIG. 41  illustrates a cross-sectional snapshot of the sheet loader  700  before a sheet bundle T 1  in the path  704  is discharged to the platform  703 . At this time, the stopper  708  catches the lever arm  709 , then the guard  705  is kept at the nearest position in a range where the guard  705  can move along the decline of the platform  703 . 
       FIG. 42  illustrates a cross-sectional snapshot of the sheet loader  700  after the sheet bundle T 1  and the following sheet bundle T 2  are discharged to the platform  703 . Since the stopper  708  still catches the lever arm  709 , the guard  705  is kept at the same position. As a result, a stack is formed with the sheet bundles T 1  and T 2 , and the following sheet bundles mount on the stack. 
       FIG. 43  illustrates a cross-sectional snapshot of the sheet loader  700  after a sheet bundle T 3  is discharged on the stack of the sheet bundle T 1  and the sheet bundle T 2 . Since the stack is sufficiently high, the sheet bundle T 3  slides off the top of the stack and pushes the top of the lever  707 . As a result, the lever  707  turns with the lever arm  709  to releases the stopper  708 . Since the guard  705  is not longer coupled to the stopper  708 , the guard  705  slides down the decline of the platform  703  with the stack including the sheet bundles T 1  through T 3 . As the result, the stack is ready for being overlapped by a folded edge side of the following sheet bundle discharged from the outlet  701 , on its open end side. 
     As just described, the stack is ready for being overlapped by a folded edge of the following sheet bundle on its lap portion by shifting the stack toward its folded edge side.  FIG. 44  illustrates a cross-sectional snapshot of the sheet loader  700  after sheet bundles T 4  through T 6  are discharged on the stack of the sheet bundles T 1  through T 3 , and a folded edge side of the stack of the sheet bundles T 4  through T 6  overlaps on the open end side of the stack of the sheet bundles T 1  through T 3 . 
     After the stacks are removed from the platform  703 , the guard  705  climbs back to the position just as illustrated in  FIG. 41  by the force of the spring  706 , and the lever  707  restores its posture to engage the lever arm  709  and the stopper  710 . 
     Although  FIGS. 43 and 44  illustrates a situation where the stack does not break apart during the shift, the stack may break apart on the shift by the inertia of the stack as illustrated in  FIG. 45  if friction between the sheet bundles are not relatively strong. As a result, each bulge portion of the sheet bundles is respectively on a lap portion of a sheet bundle below and adjacent the sheet bundle. And then, the following sheet bundles T 4  through T 6  are put on the platform  703  with their folded edges overlapping on an open end of their respective preceding sheet bundle, as shown in  FIG. 46 . If the stack on the guard  705  is already broken apart before the stack becomes unstable like above, it is unnecessary to provide concern for the stability of the stack. 
     (2-2-5) Embodiment 6 
       FIG. 47  illustrates a sixth exemplary embodiment of a sheet loader. The sheet loader  800  includes an outlet  801  as a part of a sheet bundle provider, a wall  802 , a platform  803  as an undersurface support, a path  804  and a spring  806 . These features respectively correspond to the outlet  601 , the wall  602 , the platform  603 , the path  604  and the spring  606  of Embodiment 4. The wall  802  and the path  804  may be parts of the sheet bundle provider. 
     The sheet loader  800  further includes a guard  805  which differs from the guard  605  in Embodiment 4, a stopper arm  807  and a tongue  812  as a cushion member, as  FIG. 48  which illustrates a perspective view of the sheet loader  800 . The guard  805  may be a folded edge blocker. The stopper arm  807  may be a part of a canceller. 
     The guard  805  has a prong  808  on its top. The prong  808  is around the center of the width of the guard  805 . An upper end of the guard  805  is on both sides of the prong  808 . The prong  808  is positioned higher than the upper ends of the guard  805 . The prong  808  may also be a part of a canceller. 
     An end of the stopper arm  807  engages the prong  808 , and connects the guard  805  to the wall  802 . The other end of the stopper arm  807  rotates around a shaft  809  supported by an arm support  810  above the outlet  801  on the wall  802 . The stopper arm  807  is formed as a bath tub shape with its opening having a downward facing concave orientation. The one end of the stopper arm  807  has a rib  811  in the concave portion. The rib  811  is around a center of a width of the stopper arm  807 . The rib  811  is formed with a hook shape. 
     Both sidewalls of the stopper arm  807  have silhouettes like the rib  811  with infilling the crena of the rib  811 . The sidewalls prevent the stopper arm  807  from losing engagement with the prong  808  by sliding in the width direction. 
     A distance between the platform  803  and the sidewalls become progressively narrower with a distance from the other end of the stopper arm  807  at the time the stopper arm  807  engages the prong  808 . Therefore, if a stack has a sufficient enough height, a bulge portion of a sheet bundle sliding off the top of the stack pushes up the stopper arm  807  to release the engagement with the prong  808 . 
     If the height of the stack exceeds a threshold limit, the guard  805  starts to slide down the decline of the platform  803 . An initial sliding distance just after exceeding the threshold limit may be longer than a sliding distance of the guard  805  per a sheet bundle after then. 
     Furthermore, the stopper arm  807  has an attack angle for the guard  805  climbing back the decline of the platform  803 . Therefore, the one end of the stopper arm  807  can hurdle the prong  808  and re-engage it easily when the guard  805  climbs back the decline of the platform  803 . 
     The tongue  812  has an attack angle for a direction where a sheet bundle discharged from the outlet  801  comes along. The tongue  812  cushions an impact of the sheet bundle on the stopper arm  807 . 
     The tongue  812  rotates around the shaft  809  which the stopper arm  807  rotates around. The tongue  812  rotates across the concave portion of the stopper arm  807 . The tongue  812  has an arc downward facing convex shape. The convex portion has an attack angle for the direction where the sheet bundle discharged from the outlet  801  comes along, in the time the convex region sticks out from the bottom of the stopper arm  807 . The spring  813  stretches between the ceiling of the stopper arm  807  and a roof of the tongue  812  and pushes the tongue  812  out from the concave portion of the stopper arm  807 . 
     An exemplary operation of the sheet loader  800  is explained with snapshots in  FIGS. 49 to 54 . 
       FIG. 49  illustrates a cross-sectional snapshot of the sheet loader  800  before a sheet bundle T 1  in the path  804  is discharged to the platform  803 . At this time, the stopper arm  807  catches the prong  808 , then the guard  805  is kept at the nearest position in a range where the guard  805  can move along the decline of the platform  803 . 
       FIG. 50  illustrates a cross-sectional snapshot of the sheet loader  800  when the sheet bundle T 1  puts out its folded edge from the outlet  801 . The sheet bundle T 1  hits the convex portion of the tongue  812 , and pushes the convex portion of the tongue  812  upwards. As a result, the shock of the sheet bundle T 1  for the stopper arm  807  is cushioned by the tongue  812 , as well as reducing the momentum of the sheet bundle T 1  to land on the platform  803  stable without serious flopping. 
       FIG. 51  illustrates a cross-sectional snapshot of the sheet loader  800  after the sheet bundles T 1  through T 3  are discharged to the platform  803  in numerical order. Since the stopper arm  807  still catches the prong  808 , the guard  805  is kept at the same position. As a result, a stack is formed with the sheet bundles T 1  through T 3 , and the following sheet bundles mount on the stack. 
       FIG. 52  illustrates a cross-sectional snapshot of the sheet loader  800  when the sheet bundle T 4  puts out its folded edge from the outlet  801 . As same as the explanation in connection with  FIG. 50 , the sheet bundle T 4  hits on the convex portion of the tongue  812 , and pushes the convex portion of the tongue  812  upwards. As a result, the shock of the sheet bundle T 4  for the stopper arm  807  is cushioned by the tongue  812 , as well as reducing the momentum of the sheet bundle T 4  so that its lands on the stack stable without serious flopping. 
       FIG. 53  illustrates a cross-sectional snapshot of the sheet loader  800  after the sheet bundle T 4  is discharged on the stack of the sheet bundles T 1  through T 3 . Since the stack is already of sufficient size, the sheet bundle T 4  slides off the top of the stack and pushes the stopper arm  807  upwards. As a result, the stopper arm  807  releases the prong  808 . Since the guard  805  loses support of the stopper arm  807 , the guard  805  slides down the decline of the platform  803  with the stack including the sheet bundles T 1  through T 4 . As the result, the stack is ready for being overlapped by a folded edge side of the following sheet bundle discharged from the outlet  801 , on its open end side. 
     As just described, the stack is ready for being overlapped by a folded edge of the following sheet bundle on its lap portion by shifting the stack toward its folded edge side.  FIG. 54  illustrates a cross-sectional snapshot of the sheet loader  800  after sheet bundles T 5  through T 7  are discharged on the stack of the sheet bundles T 1  through T 4 , and a folded edge side of the stack of the sheet bundles T 5  through T 7  overlaps on the open end side of the stack of the sheet bundles T 1  through T 4 . 
     After the stacks are removed from the platform  803 , the guard  805  climbs back to the position just as illustrated in  FIG. 49  by the force of the spring  806 , and the stopper arm  807  restores its posture to engage with the prong  808 . 
     Although  FIGS. 53 and 54  illustrate a situation where the stack does not break apart during the shift, the stack may break apart on the shift by the inertia of the stack as illustrated in  FIG. 55  if friction between the sheet bundles is not sufficiently strong. As a result, each bulge portion of the sheet bundles is respectively on a lap portion of a sheet bundle below and adjacent the sheet bundle. And then, the following sheet bundles T 5  and T 6  are put on the platform  803  with their folded edges overlapping on an open end of their respective preceding sheet bundle, as shown in  FIG. 56 . If the stack on the guard  805  is already broken apart before the stack become unstable like above, it is unnecessary to provide concern for the stability of the stack. 
     (2-2-6) Embodiment 7 
       FIG. 57  illustrates a seventh exemplary embodiment of a sheet loader. 
     The sheet loader  900  includes an outlet  901  as part of a sheet bundle provider, a wall  902 , a platform  903  as an undersurface support, a path  904 , a spring  906 , an arm support  910 , a tongue  912  as a cushion member, and a spring  913 . These features respectively correspond to the outlet  801 , the wall  802 , the platform  803 , the path  804 , the spring  806 , the arm support  810 , the tongue  812 , and the spring  813  of Embodiment 6. The wall  902  and the path  904  may be parts of the sheet bundle provider. 
     The sheet loader  900  further includes a guard  905  which differs from the guard  805  in Embodiment 6, an upper arm  907  as a canceller and a forearm  908  as a folded edge blocker. 
     The guard  905  connects to a base plate  915 . The base plate  915  corresponds to the base plate  609  of Embodiment 4. The base plate  915  has a hill similar to Embodiment 4, as well. 
     An end of the upper arm  907  rotates around a shaft  909  supported by an arm support  910  above the outlet  901  on the wall  902 . The upper arm  907  is formed as a bath tub shape with a downward facing concave opening. The tongue  912  rotates around the shaft  909  which the upper arm  907  rotates around. The tongue  912  rotates across the concave portion of the upper arm  907 . 
     The upper arm  907  supports a shaft  914  around its other end. The forearm  908  rotates around the shaft  914 . When a straight line between the shaft  914  and the shaft  909  is parallel to the decline of the platform  903  and the guard  905  is set at the nearest position in a range where the guard  905  can move along the decline, the shaft  914  is at a higher position on a direction perpendicular to the decline than an upper end of the peak and is at an upper region on a direction along the decline than the peak. 
     The upper arm  907  has a prong  917  on its outer surface rounded around the shaft  909  to avoid over rotation. The prong  917  contacts the ceiling of the arm support  910  to prevent the upper arm  907  from dropping down the shaft  914  lower than a position of the shaft  914  when the straight line between the shaft  914  and the shaft  909  is parallel to the decline. 
     The base plate  915  has a slot  918  on its mountain slope. The slot  918  is positioned at about a middle region of a width of the base plate  915  along the ridge line of the peak. 
       FIG. 58  illustrates a cross-sectional view of the sheet loader  900  around the other end of the upper arm  907 , the forearm  908 , and the guard  905  with the base plate  915 . The bottom of the slot  918  is a plane almost parallel to a direction where the guard  905  shifts along. One end of the slot  918  by the side of the peak connects to a cliff rising steeply against the direction where the guard  905  shifts along. 
     The forearm  908  hangs down from the shaft  914 . The lower end of the forearm  908  fits into the slot  918 . The forearm  908  has an obverse face which faces the guard  905 , and a reverse face which faces the outlet  801 . The forearm  908  is biased around the shaft  914  so that the lower end climbs up a valley wall to get close to the wall  902 . On the other hand, the prong  916  contacts a ceiling of the upper arm  907  to prevent the forearm  908  from rolling its lower end up over the surface of the mountain slope by the bias so as not to make a gap to let a sheet bundle through and between the lower end and the mountain slope. 
     When the platform  903  does not load a sheet bundle, the forearm  908  is positioned in a gap between the obverse face and the base plate  905  shown as a posture P 1  illustrated with solid line in  FIG. 58  to prevent itself from abrasion against the base plate  905 . Sheet bundles loaded on the mountain slope push the reverse face and the obverse face contacts the cliff on the one end of the slot  918 . 
     The forearm  908  may be designed to contact the cliff without pushing by the sheet bundle to avoid a knock sound generated between the forearm  908  and the cliff. Furthermore, the cliff may have a cushion to mitigate the knock sound. 
     The reverse face of the forearm  908  may be vertical or inclined toward the guard  905  when the obverse face contacts the cliff of the guard  905  at the nearest position in the range of motion. Furthermore, the reverse face may be vertical at the time the forearm  908  is released from the cliff of the guard  905  sliding down the decline by the push of sheet bundles on the mountain slope. Of course, the forearm  908  is not limited to the above configuration. 
     If a drop distance between an open end and a folded edge of a sheet bundle held by the reverse face is too steep for a length of the sheet bundle, an open end of the sheet bundles opens enough to take the following sheet bundle into its pages. However, the mountain slop makes the drop distance sufficiently small enough to prevent the open end from opening. 
     An exemplary operation of the sheet loader  900  is explained with snapshots in  FIGS. 59 to 65 . 
       FIG. 59  illustrates a cross-sectional snapshot of the sheet loader  900  before a sheet bundle T 1  in the path  904  is discharged to the platform  903 . At this time, the straight line between shafts  909  and  914  is almost parallel to the decline of the platform  903 , and the lower end of the forearm  908  is in the slot  918  of the hill on the base plate  915 . Furthermore, the guard  905  is kept at the nearest position in the range where the guard  905  can move along the decline by the bias of the spring  906 . 
       FIG. 60  illustrates a cross-sectional snapshot of the sheet loader  900  after the sheet bundle T 1  and the following sheet bundles T 2  and T 3  are discharged to the platform  903  in numerical order. The sheet bundles T 1  through T 3  push the reverse face of the forearm  908 , and the obverse face of the forearm  908  contacts the cliff of the hill. The sheet bundles T 1  through T 3  are stopped by the reverse face of the forearm  908  to form a stack. 
     Since a first position on the reverse face where a folded edge of the sheet bundle T 1  contacts at is far from the shaft  914 , the guard  905  slides down the decline a relatively long distance. However, a moment caused by the sheet bundle T 2  is smaller than the one the sheet bundle T 1  causes because a second position on the reverse face where a folded edge of the sheet bundle T 2  contacts is closer to the shaft  914  than the first position. As a result, the guard  905  slides down the decline a relatively short distance. Moreover, a moment caused by the sheet bundle T 3  is smaller than the one the sheet bundle T 2  causes because a third position on the reverse face of the forearm  908  where a folded edge of the sheet bundle T 3  contacts is closer to the shaft  914  than the second position. As a result, the guard  905  slides down the decline an even shorter distance. That is, the sliding distance downward of the guard  905  per one sheet bundle becomes increasingly smaller according to a number of sheet bundles on the base plate  915 . 
     Although a stack becomes more unstable according to its height (typically the higher the stack, the more unstable the stack), making the sliding down distance of the guard  905  per one sheet bundle increasingly smaller according to the number of sheet bundles in a stack on the base plate  915  is effective for avoiding the stack breaking apart. 
     Although the forearm  908  rotates around the shaft  914  because of the weight of the sheet bundles T 1  through T 3 , the guard  905  does not slide down the decline sufficiently enough to release the forearm  908  from the cliff, yet at the time illustrated in  FIG. 60 . Therefore, the following sheet bundles mount on the stack. 
       FIG. 61  illustrates a cross-sectional snapshot of the sheet loader  800  after a sheet bundle T 4  is discharged on the stack of the sheet bundles T 1  through T 3 . Since the stack is already of sufficient enough size, the sheet bundle T 4  slides off the top of the stack and pushes the upper arm  907  upwards. As a result, the lower end of the forearm  908  is released from the cliff by the pull of the upper arm  907 . 
     Even if the stack does not have a sufficiently high enough height to push the upper arm  907  upwards, the guard  905  slides enough distance down to release the forearm  908  from the cliff when the weight of the stack is sufficient to cause release of the forearm  908 . 
       FIG. 62  illustrates a cross-sectional snapshot of the sheet loader  800  after the forearm  908  is released from the slot  918 , and  FIG. 63  illustrates a cross-sectional snapshot of the sheet loader  800  later than the time illustrated in  FIG. 62 . The stack of sheet bundles T 1  through T 4  which is previously supported by the forearm  908  slides down and contacts the guard  905 . The guard  905  receives the whole weight of the stack and slides down further. 
     As just described, the stack is ready for being overlapped by a folded edge of the following sheet bundle on its lap portion by shifting the stack toward its folded edge side.  FIG. 64  illustrates a cross-sectional snapshot of the sheet loader  900  after sheet bundles T 5  through T 7  are discharged on the stack of sheet bundles T 1  through T 4 , and a folded edge side of a stack of the sheet bundles T 5  and T 7  overlaps on the open end side of the stack of the sheet bundles T 1  through T 4 . 
     After the stacks are removed from the platform  903 , the guard  905  climbs back to the position just as illustrated in  FIG. 59  by the force of the spring  906 , and the forearm  908  is rolled upwards by the biasing force around the shaft  914  to engage the lower end with the cliff as shown in  FIG. 65 . 
     Although  FIGS. 62 through 64  illustrate a situation where the stack does not break apart during the movement of the platform  903 , the stack may break apart during movement of the platform  903  by the inertia of the stack as illustrated in  FIG. 66  if friction between the sheet bundles is not sufficiently strong. As a result, each bulge portion of the sheet bundles is respectively on a lap portion of a sheet bundle below the sheet bundle. And then, the following sheet bundles T 5  through T 7  are put on the platform  903  with their folded edges overlapping on an open end of their respective preceding sheet bundle, as shown in  FIG. 67 . Such situation is easier to conduct in this embodiment than in other embodiments because the lower end of the forearm  908  lugs against the momentum of the top of the stack. If the stack on the guard  805  is already broken apart before the stack becomes unstable like above, it is unnecessary to provide concern for stability of the stack. 
     Moreover, although the forearm  908  is biased around the shaft  914  so that the lower end climbs up the valley wall of the hill to get close to the wall  902 , the lower end can not climb up the valley wall sufficiently enough to cross over the peak to refit into the slot  918  if the biasing force is too weak. 
     To avoid such a situation, the cliff may be constructed as an end of a roof of a flap  950  as illustrated in  FIG. 68 . The flap  950  covers a hole connecting and through to the one end of the slot  918  on the valley wall, and can be pushed down under the valley wall. The flap  950  may be a joint. 
       FIG. 69  is an exploded perspective view around the platform  903  of the sheet loader  900  with the flap  950 . The guard  905  connects the base plate  907  in the same width. The guard  905  and the base plate  907  ride on a chassis  957 . The chassis  957  slides on an upper surface of a slope  952  which is parallel to the decline of the platform  903 . Rollers  958  and  959  support the chassis  957  on the slope  952 , and roll on the region surrounded with broken lines on the slope  412 . 
     A roller cover  954  covers a space above the region with its ceiling. The rollers  958  and  959  fit in the space between the slope  952  and the ceiling of the roller cover  954 . The roller cover  954  has walls on the top end and bottom end in the direction along the slope  952  of the region to limit travel of the rollers  958  and  959 . 
     Furthermore, the chassis  957  has other rollers  960  and  961 . Rollers  960  and  961  are supported by the chassis  957  rotatably around axis perpendicular to the slope  952 . The roller cover  954  additionally has a guide wall which perpendicularly stands on the slope  952  along the decline of the slope  952 . The rollers  960  and  961  roll on the guide wall. The guide wall supports the rollers  960  and  961  to prevent the chassis  957  from running off track. 
     A bedcover  953  covers the slope  952  except for regions covered with the guard  905  and the base plate  907 . On a direction perpendicular to the decline of the slope  952 , the height of a roof of the bedcover  953  from the slope  952  is lower than the height of the peak from the slope  952 . The bedcover  953  is fixed to the slope  952 . 
     The chassis  957  supports the flap  950  rotatably on a fulcrum set under the roof of the flap  950 . 
     The base plate  907  does not cover regions overlapping the roof of the flap  950  and a sheet sensor  965  as a probe. 
     A sheet sensor  965  has a fulcrum on the slope  952 . The tip of the sheet sensor  965  projects above the upper surface of the base plate  905  when no sheet is on the platform  903 . The tip of the sheet sensor  965  is depressed into the base plate  905  by rotating around the fulcrum due to the presence of the sheet on the platform  903 . 
       FIG. 70  illustrates a cross-sectional view of the sheet loader  900  with the flap  950  around the other end of the upper arm  907 , the forearm  908 , and the guard  905  with the base plate  915 . The chassis  957  is exposed through the bottom of the slot  918 . One end of the roof of the flap  950  forms the cliff at the first end of the slot  918  near the side of the peak. 
     The flap  950  rotates around the fulcrum  962  supported by a stay  963  fixed on the chassis  957 . The fulcrum  962  is set under the other end of roof of the flap  950 . 
     A circular arc  940  illustrated with a dashed line presents an orbit of the lower end of the forearm  908  when the guard  905  is set at the nearest position in the range of motion and a straight line between the shaft  914  and the shaft  909  is parallel to the decline of the platform  903 . The fulcrum  962  is set more closely to the guard  905  than a position P 2  where the circular arc  940  crosses with the surface of the valley wall of the base plate  915  in a direction along which the chassis  957  slides. The fulcrum  962  is set more closely to the slope  952  than the position P 2  in a direction perpendicular to the slope  952 , as well. 
     The roof of the flap  950  is kept in plane with, or under, the valley wall by a spring  964  stretching between the chassis  957  and the ceiling of the flap  950 . On the other hand, the flap  950  has a stopper around the fulcrum  962  to prevent the roof of the flap  950  from projecting over the valley wall. 
     As illustrated in  FIG. 71 , the flap  950  moves from the circular arc  940  by the push of the lower end of the forearm  908  passing along the circular arc  940  through a section from the position P 2  to a position where the obverse face of the forearm  908  contacts the first end of the roof of the flap  950 . 
     After the lower end of the forearm  908  passes by the position where the obverse face of the forearm  908  contacts the first end of the roof of the flap  950 , the first end of the roof of the flap  950  raises up to be in plane with, or under, the valley wall by the expansion force of the spring  964 . As a result, the forearm  908  can go back to the position illustrated in  FIG. 59  to contact the first end of the roof of the flap  950  more easily. 
     (3) Embodiments of a Sheet Folding Apparatus and a Sheet Finishing System 
       FIG. 72  illustrates a perspective view of a sheet finishing system  4000  as an exemplary embodiment. The sheet finishing system  4000  includes a scanner  3000 , a printer  2000 , and a sheet folding apparatus  1000 . Generally, a side with the operation panel  9  of the printer  2000  is a so called front side, and the opposite side is a so called rear side. 
       FIG. 73  illustrates a cross-sectional view of the sheet finishing system  4000 . The scanner  3000  above the printer  2000  scans an image of a manuscript. 
     The printer  2000  may have the operation panel  9  on its upper front side. The operation panel  9  may have a button to start the scanning, and may have buttons to select a mode for an image processing and a mode for a sheet finishing from pluralities of choices. 
     The printer  2000  has an image processing portion which includes a charger  2 , an exposure unit  3 , an image developer  4 , an image transfer unit  5 A, an electric discharger  5 B, a separator  5 C, and a cleaner  6 , with all of the components being arranged around a latent image carrier  1  which rotates around its axis. 
     After the charger  2  charges the surface of the latent image carrier  1  uniformly along the axis, the exposure unit  3  exposes a laser beam to form a latent image on the charged surface of the latent image carrier  1  based on information about the manuscript obtained by the scan of the scanner  3 . The developer  4  develops the latent image to a toner image on the latent image carrier  1 . The transfer unit  5 A transfers the toner image from the latent image carrier  1  on an obverse side of a sheet which is supplied from a sheet stacker  7 A. Thereafter, the electric discharger  5 B discharges electricity on a reverse side of the sheet, the separator  5 C separates the sheet from the latent image carrier  1 , and the cleaner  6  removes residual toner from the surface of the latent image carrier  1 . Additionally, an intermediate conveyer  7 B conveys the sheet, a fixing unit  8  fixes the toner image on the sheet, and a conveying roller pair  7 C conveys the sheet. 
     In a duplex image forming mode, a path switch  7 D connects a path from the fixing unit  8  to a sheet inverter  7 E to switchback the sheet at first, and the path switch  7 D reconnects the path from the fixing unit  8  to the conveying roller pair  7 C after forming an image on the reverse side of the sheet. 
     The conveying roller pair  7 C conveys the sheet to the sheet finishing apparatus FS. 
     The sheet folding apparatus  1000  has an inlet roller pair  30  to receive the sheet, and an intermediate transfer roller pair  32  to receive the sheet from the inlet roller pair  30 . 
     The intermediate transfer roller pair  32  releases the sheet to an injection roller pair  34 . The injection roller pair  34  injects the sheet upwards along an inclined direction to position the sheet on a standing tray  36  which has a surface inclined in a substantially similar direction as the injection direction in order to support the sheet. 
     A stacker  38  is positioned below the standing tray  36  to catch a lower end of the sheet which switchbacks on and falls along the standing tray  36 . The stacker  38  remains still until a plurality of sheets makes a plane sheet bundle. 
     A stapler  40  is set above the standing tray  36 . The stapler  40  staples at two points on a middle line of the length of the plane sheet bundle. 
     In a saddle stitch finishing mode, the stacker  38  is positioned to receive the sheet bundle so as to face the middle line of the sheet bundle to the stapler  40 . The stacker  38  then descends so as to face the middle line of the sheet bundle to a blade  42  after the stapler  40  staples the sheet bundle. 
     The blade  42  has a tip line almost parallel to the lower end of the sheet bundle supported by the stacker  38 . The blade  42  rams the sheet bundle with the tip line after facing the middle line of the sheet bundle. 
     A folding roller pair  44  makes a nip between its rollers on a ramming direction of the blade  42 . The nip convolves the plane sheet bundle rammed by the blade  42  to make a folded edge on the sheet bundle. 
     The folded edge of the sheet bundle comes out from the nip and is traced by a fold enhancer  46 . 
     A discharging roller pair  48  tows the sheet bundle to discharge on a sheet loader. Although the sheet loader is described here as the same as the sheet loader  900  of Embodiment 7, the sheet loader may alternatively be any of the other sheet loaders described in other embodiments or combinations thereof. 
     An inner frame  50  may support the intermediate transfer roller pair  32 , the injection roller pair  34 , a part of the standing tray  36 , the stacker  38 , the stapler  40 , the blade  42 , the folding roller pair  44 , the fold enhancer  46  and the discharging roller pair  48 . The intermediate transfer roller pair  32 , the injection roller pair  34 , a part of the standing tray  36 , the stacker  38 , the stapler  40 , the blade  42 , the folding roller pair  44 , the fold enhancer  46  and the discharging roller pair  48  are removed together with the inner frame  50  at the same time from an inside of an outer frame  54  of the sheet folding apparatus  1000 . 
       FIG. 74  illustrates a perspective view of the sheet folding apparatus  1000  with the inner frame  50  pulled out of the outer frame  54 . The inner frame  50  moves along a rail  58  extended between the front side and the rear side. A floor plate  62  fixed at a bottom of the outer frame  54  supports the rail  58  so as to move between the front side and the rear side along a longitudinal direction of the rail  58 . 
     The sheet folding apparatus  1000  has a door  56  on the front side. The inner frame  50  linearly exits out of the outer frame  54  along the rail  58  from an opening appearing after the door  56  opens. Consequently, sheets jammed in the sheet folding apparatus  1000  can be removed easily. 
     The inner frame  50  carries a controller  60  to manage control of the whole of the sheet folding apparatus  1000 . The controller  60  is located at an easily touchable position after pulling the inner frame  50  out of the outer frame  54 . 
       FIG. 75  illustrates a perspective view of the sheet loader  900  around a sheet sensor  980  projecting up from the base plate  915 . The controller  60  determines whether a tip of the sheet sensor  980  projects up from, or is depressed into, the base plate  905 . 
     The controller  60  is mounted on the inner frame  50 , and the sheet sensor  980  is mounted on the outer frame  54 . Since the inner frame  50  and the outer frame  54  move relative to each other as described above, some intracracies described below to lay out wire harnesses for transferring the state of the sheet sensor  980  to the controller  60 . 
       FIG. 76  illustrates a close-up view of the sheet folding apparatus  1000  around a sheet sensor  980  with the inner frame  50  pulled out of the outer frame  54 . A mechanical sensor unit  64  is mounted on the floor plate  62  to rotatably support the sheet sensor  980 . An electrical sensor unit  66  is mounted on the inner frame  50  to convert the motion of the sheet sensor  980  into an electrical signal. When the inner frame  50  moves straight to the rear side from the front side along the rail  58  to fit into the outer frame  54 , the mechanical sensor unit  64  and the electrical sensor unit  66  are in such relative positions that the electrical sensor unit  66  can detect motion of the mechanical sensor unit  64 . 
       FIG. 77  illustrates a close-up view of the mechanical sensor unit  64  and the electrical sensor unit  66  when they approach each other. The mechanical sensor unit  64  has an upper registration shaft  68  and a lower registration shaft  72  along the direction of movement of the inner frame  50 . The shafts are fit respectively into an upper registration slot  70  and a lower registration slot  74  of the electrical sensor unit  66 . The electrical sensor unit  66  may have one or more registration shafts, and then the mechanical sensor unit  64  may have registration slots to fit the registration shafts. 
     The mechanical sensor unit  64  is fixed on the floor plate  62  with screws  76  and  78 . The screws  76  and  78  are put respectively through oval holes on the mechanical sensor unit  64 . Major axes of the oval holes are parallel to each other and perpendicular to the direction along which the inner frame  50  moves. After the screws  76  and  78  are loosened, the mechanical sensor unit  64  can slide along the major axes of oval holes. 
     The electrical sensor unit  66  is fixed on the inner frame  50  with a screw  82 . The electrical sensor unit  66  has three oval holes including a pair of oval holes  80  and a middle oval hole  552  between the pair of oval holes  80 . Major axes of the three oval holes are parallel to each other and perpendicular to directions along which the inner frame  50  moves and the mechanical sensor unit  64  slides. The screw  82  is put through the middle oval hole  552 . After the screw  82  is loosened, the electrical sensor unit  66  can slide along the major axis of the oval hole. 
     The inner frame  50  has two cylindrical projections  84  to fit respectively into the pair of oval holes  80  to guide the slide and to avoid rotation of the electrical sensor unit  66 . 
       FIG. 78  illustrates a rear side perspective view of the electrical sensor unit  66 . A base board  86  is fixed to the inner frame  50  with the screw  82  in  FIG. 77 . Half-screws  88  and  90  are screwed on the base board  86 . 
     A movable board  92  has four holes. Diameters of two of the holes are smaller than the heads of and are bigger than necks of the half-screws  88  and  90 . The half-screws  88  and  90  are put through the two holes, respectively. 
     The movable board  92  can slide within a distance of the length of necks of the half-screws  88  and  90  from the base board  86 . 
     The remaining holes of the four holes are on axes of, and have bigger diameters than the upper registration slot  70  and the lower registration shaft  72 , respectively. 
     The movable board  92  supports a receiver  96  and an emitter  98  on its vertical reference plane. The receiver  96  and the emitter  98  work as a photo interrupter in combination with each other. There is a sensing slot between the receiver  96  and the emitter  98 . The photo interrupter detects whether something blocks a light from the emitter  98  to the receiver  96  is present in the sensing slot or not. 
     Pillars  94  stand almost perpendicular to the reference plane with their tops from the reference plane being higher than tops of the receiver  96  and the emitter  98 . 
       FIG. 79  illustrates a left side view of the electrical sensor unit  66 . Springs  554  and  556  are put around the necks of the half-screws  88  and  90 , respectively. The springs  554  and  556  stretch between the movable board  92  and the base board  86 . 
     The movable board  92  is moved toward the base board  86  when the pillars  94  are pushed by the mechanical sensor unit  64 . On the other hand, the springs  554  and  556  expand to force the movable board  92  against the mechanical sensor unit  64  to ensure a relative position between them. 
     When the pillars  94  are in contact with the mechanical sensor unit  64 , the tops of the receiver  96  and the emitter  98  have clearances from a plane where the mechanical sensor unit  64  and the pillars  94  are in contact with each other. Furthermore, the pillars  94  are long enough for a bottom of the sensing slot not to contact a breaker plate  560  of the mechanical sensor unit  64 . 
       FIG. 80  illustrates a rear side view of the mechanical sensor unit  64 . The screws  76  and  78  screw a supporting board  558  on the floor plate  62 . The supporting board  558  has screw holes on a face where the pillars  94  of the electrical sensor unit  66  contact. The upper registration shaft  68  and the lower registration shaft  72  are screwed into the screw holes, respectively. 
     The supporting board  558  has an arc slit  576  on the face where the pillars  94  of the electrical sensor unit  66  contact. A rotation shaft  562  is put at a track center of the arc slit  576 . The arc slit  576  overlaps the sensing slot. The breaker plate  560  rotates around the rotation shaft  562 . One end of the breaker plate  560  is bent behind the plane of the paper so as to be almost parallel to the rotational axis  562  around the arc slit  576 . The one end is inserted into the arc slit  576  to move across the sensing slot. The breaker plate  560  rotates to take an active position to block the light from the emitter  98  to the receiver  96  with the one end, and a rest position not to interfere with the light. The active position is lower than the rest position. 
     The breaker plate  560  is biased by a spring  572  to a clockwise direction in  FIG. 80  to push the one end up to the rest position. One end of the spring  572  connects to a stay  574  which is a part of the supporting board  558  bent in front of the plane of the paper. The other end of the spring  572  connects to the breaker plate  560 . The other end of the breaker plate  560  supports a shaft  564 . The shaft  564  supports one end of an arm  570  rotatably. The other end of the arm  570  connects to a shaft  568  rotatably. The shaft  568  is supported by the sheet sensor  980 . The supporting board  558  supports a shaft  556  to support the sheet sensor  980  rotatably. 
     When the one end of the breaker plate  560  is out of the sensing slot, the shaft  564  pulls the arm  570  by the bias of the spring  572  applied on the breaker plate  560 , the arm  570  pulls the shaft  568  to raise the tip of the sheet sensor  980  above the upper surface of the base plate  905 . 
     On the other hand, if the tip of the sheet sensor  980  is depressed into the upper surface of the base plate  905 , the shaft  568  pulls the arm  578  to rotate the breaker plate  560  against the bias of the spring  572  through the arm  570  and the shaft  564 , and the one end of the breaker plate  560  blocks the light from the emitter  98  to the receiver  96 . 
     Although the invention is shown and described with respect to certain illustrated aspects, it will be appreciated that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components, the terms used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure, which performs the function in the herein illustrated exemplary aspects of the invention.