Abstract:
A sheet processing apparatus is disclosed. The sheet processing apparatus has a stacking unit configured to stack a sheet discharged from an image forming unit, the stacking unit including a stacker being configured to support a lower end of the sheet and a supporter being configured to support an under side of the sheet; a driver configured to drive the stacker along the supporter; and an opening configured for allowing removal of the sheet on the stacker to an outside; and a controller configured to control the stacker so that the stacker is driven to the opening along the supporter if a process is cancelled and if the sheet on the stacker needs to be removed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent Ser. No. 12/343,076, filed Dec. 23, 2008, which is based upon and claims the benefit of priority from; U.S. provisional application 61/016,930, filed on Dec. 27, 2007; and U.S. provisional application 61/036,447, filed on Mar. 13, 2008, the entire contents of each of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a sheet finisher, an image forming apparatus using the sheet finisher, and a sheet finishing method, and particularly to a sheet finisher, an image forming apparatus, and a sheet finishing method folding a sheet. 
     BACKGROUND 
     There was a sheet finisher which perform a sheet finishing such as punching or saddle-stitching of sheets printed by an image forming system such as a copier, a printer, and a multi-functional peripheral (MFP). 
     Functions of the sheet finisher become diverse. There is suggested a sheet finisher which has a function of folding a part of sheets or a function of saddle-stitching which saddles an intermediate part of the sheets after the intermediate part of the sheets is stapled in addition to the functions of punching and saddle-stitching (see JP-A-2004-59304, JP-A-2003-182928, etc.). 
     The sheet finisher that has the saddle-stitching functions is also capable of producing (binding) plural printed sheets into a book. 
     In the known intermediate saddle-stitching functions suggested in the past, the intermediate part of sheets is stapled and the stapled part of the sheets are saddled by a pair of rollers called a pair of saddle rollers. In this case, a plate-shaped member called a folding blade is put on the stapled portion of a bundle of sheets and inserts the bundle of sheets into a nip of the pair of saddle rollers to form the folding line on the bundle of sheets. 
     There is disclosed a technique for discharging the sheets by force, if it is detected that a series of discharge operations are canceled due to jam occurrence or the like during stacking sheets (for example, see JP-A-2000-177919, etc.). 
     However, if it is difficult to discharge sheets forcibly in jam occurrence in the vicinity of a sheet discharging mechanism of the sheet finisher, the sheets stopped in an erect tray have to be removed. In this case, it is difficult for a user to remove the sheets since an opening is narrow in a state where jogger fences of the erect tray guide the sheets. 
     If the sheets are relatively small in size, it is more difficult for the user to reach and remove the sheet. That is because the sheet finisher stacks the sheets on the erect tray at a location which is away from the opening. 
     In the past, there was a method of detecting the upper portion of the books which a saddle tray stacks by using a sensor in order to detect a fullness state of the saddle tray and the like subjected to folding. Moreover, there was suggested a method of calculating and estimating the fullness state using the number of sheets and the size and weight of the sheets discharged to the saddle tray. However, it is determined that the saddle tray is full even in a case of the half of actual stacking limit, since it is necessary to operate the sheet finisher in consideration of a large margin in the actual fullness state. 
     If a job for detecting that the saddle tray is full is canceled like the above method and stacking the books up to the mechanical limit of the saddle tray is intended, resistance of the stacked books causes a torque limiter or the like to operate and thus it is difficult to push a new book, thereby resulting in a discharge error. 
     SUMMARY 
     The invention is devised in view of the above-mentioned circumstances and an object of the invention is to provide a sheet finisher, an image forming apparatus, and a sheet finishing method which performs saddle-stitching and is capable of removing sheets in an erect tray. 
     Moreover, the invention is devised in view of the above-mentioned circumstances and another object of the invention is to provide the sheet finisher, the image forming apparatus, and the sheet finishing method capable of stacking books at a location of mechanical limit and detecting that a saddle tray is full without mechanical discharge jam. 
     According to an aspect of the invention, in order to attain the objects, a sheet finisher includes: a folding mechanism configured to form a folding line by folding a sheet which an image forming unit discharges; a discharging mechanism configured to discharge the sheet including the folding line which the folding mechanism forms; a stacking mechanism configured to stack the sheet which the discharging mechanism discharges; and a slip amount calculator configured to output a fullness state of the stacking mechanism to the image forming unit, if a slip amount of the discharging mechanism exceeds a threshold. 
     According to another aspect of the invention, an image forming apparatus includes: an image forming unit configured to print an image on a sheet; a folding mechanism configured to form a folding line by folding a sheet which the image forming unit discharges; a discharging mechanism configured to discharge the sheet including the folding line which the folding mechanism forms; a stacking mechanism configured to stack the sheet which the discharging mechanism discharges; and a slip amount calculator configured to output a fullness state of the stacking mechanism to the image forming unit, if a slip amount of the discharging mechanism exceeds a threshold. 
     According to still another aspect of the invention, a sheet finishing method comprising acts of: forming a folding line by folding a sheet which an image forming unit discharges; discharging the sheet to a stacking mechanism by a discharging mechanism; and outputting a fullness state of the stacking mechanism to the image forming unit, if a slip amount of the discharging mechanism exceeds a threshold. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       In accompanying drawings, 
         FIG. 1  is a perspective view illustrating a major configuration example of the image forming apparatus according to the embodiment; 
         FIG. 2  is a sectional view illustrating a detailed configuration example of the image forming apparatus; 
         FIG. 3  is a sectional view illustrating a detailed configuration example of a saddle stitcher; 
         FIG. 4  is a perspective view illustrating a whole configuration example of the saddle stitcher; 
         FIG. 5  is a perspective view illustrating a jogger fence of the saddle stitcher; 
         FIG. 6  is an upper external view illustrating the jogger fence of the saddle stitcher; 
         FIG. 7  is an expanded view illustrating a stacker of the saddle stitcher; 
         FIG. 8  is a block diagram illustrating functions of the stacker and the pair of the jogger fences; 
         FIG. 9  is a schematic view illustrating an example of a mechanism which a pair of exit rollers causes jam occurrence; 
         FIG. 10  is a flowchart illustrating an operation example of the stacker and the jogger fence; 
         FIG. 11  is a perspective view illustrating a whole configuration example of a discharging mechanism; 
         FIG. 12  is a block diagram illustrating functions of the discharging mechanism; 
         FIG. 13  is a flowchart illustrating operations of the discharging mechanism; 
         FIG. 14  is a block diagram illustrating functions of a transport mechanism; and 
         FIG. 15  is a flowchart illustrating operations of the transport mechanism. 
     
    
    
     DETAILED DESCRIPTION 
     A sheet finisher, an image forming apparatus, and a sheet finishing method will be described with reference to the accompanying drawings according to an embodiment. 
     (1) Structure of Image Forming Apparatus 
       FIG. 1  is a perspective view illustrating a basic configuration example of the image forming apparatus according to the embodiment. 
       FIG. 1  shows an image forming apparatus  10  according to this embodiment. The image forming apparatus  10  includes a reading unit  11  reading a document, an image forming system  12  printing image data of the read document on a sheet using an electrographic method, and a sheet finisher  20  performing finishing such as sorting, punching, folding, and saddle-stitching on printed sheets. The image forming system  12  includes a display panel  9   a  and an operation unit  9  used for a user to carry out various operations. 
       FIG. 2  is a sectional view illustrating a detailed configuration example of the image forming apparatus  10 . 
     The image forming system  12  of the image forming apparatus  10  includes a photoconductive drum  1  in the vicinity of the middle portion. The image forming system  12  has a charging unit  2 , an exposure unit  3 , a development unit  4 , a decal unit  5 A, an electric charge removing unit  5 B, a separation claw  5 C, and a cleaning unit  6  in the circumference of the photoconductive drum  1 . The image forming system  12  has a fixing unit  8  on a downstream side of the electric charge removing unit  5 B. 
     The charging unit  2  of the image forming system  12  charges uniformly on the surface of the photoconductive drum  1 . The image forming system  12  converts a document which the reading unit  11  reads into image data, and inputs the image data to the exposure unit  3 . The exposure unit  3  emits a laser beam onto the photoconductive drum  1  according to a level of the image data, and forms an electrostatic latent image on the photoconductive drum  1 . The photoconductive drum  1  develops the electrostatic latent image with toner supplied from the development unit  4 , and forms a toner image. 
     The image forming system  12  transports each of sheets which a sheet receiving unit  7  has to a decal position (a gap between the photoconductive drum  1  and the decal unit  5 A) through several transport rollers. At the decal position, the decal unit  5 A decals the toner image from the photoconductive drum  1  onto the sheet. The electric charge removing unit  5 B removes an electric charge on the surface of the sheet. The separation claw  5 C separates the sheet from the photoconductive drum  1 . An intermediate transport unit  7 B transports the sheet to the fixing unit  8 . The fixing unit  8  fixes the toner image on the sheet by heating and pressurizing the sheet. A discharging unit  7 C discharges the sheet fixed the toner image to the sheet finisher  20 . 
     The photoconductive drum  1  removes developer remaining on the surface on a downstream side of the separation claw  5 C by a cleaning unit  6   
     If the image forming system  12  perform printing on both sides of a sheet, the image forming system  12  branches the sheet on which the toner image is fixed from a normal discharge passage by a transport passage switching plate  7 D, and performs switchback to reverse the both sides of the sheet on a reverse transport unit  7 E. The image forming system  12  performs the same printing as the printing on one side of the sheet on the other side of the reversed sheet, and discharges the sheet from the discharging unit  7 C to the sheet finisher  20 . 
     The sheet finisher  20  includes a saddle stitcher  30  and a saddle tray  50  in addition to a sorter sorting sheets. A controller (CPU: central processing unit) based on an instruction input from a user controls operations of constituent elements in the sheet finisher  20 . 
     The saddle stitcher  30  staples the intermediate part of the plural sheets subjected to the printing, which the image forming system  12  discharges, and saddles the sheets. The saddle stitcher  30  outputs a book which the saddle stitcher  30  produces by saddle-stitching to the saddle tray  50 . The saddle tray  50  receives the book. 
       FIG. 3  is a sectional view a detailed configuration example of the saddle stitcher  30 . 
     The saddle stitcher  30  allows a pair of entrance rollers  31  to receive the sheet which the discharging unit  7 C of the image forming system  12  discharges, and transports the sheet to a pair of intermediate rollers  32 . The pair of intermediate rollers  32  transports a pair of exit rollers  33 . The pair of exit rollers  33  transports the sheet to an erect tray  34  having a placing slope surface. Then, a front end of each sheet faces to an upper portion of the slope of the erect tray  34 . 
     The saddle stitcher  30  has a stacker  35  below the erect tray  34 . A stacker claw  35   a  formed in the lower portion of the stacker  35  receives the lower end of the sheet reversed and dropped from the upper slope portion of the erect tray  34  and aligns the rear end of the sheet. A jogger fence  36  aligns the sheets in a width direction (a direction perpendicular to a sheet transport direction) of the sheet. 
     The saddle stitcher  30  has a stapler  37  in the middle portion of the erect tray  34 . If the stapler  37  staples a bundle of sheets, the erect tray  34  adjusts a location of the stacker  35  so that the location (the intermediate portion in the sheet transport direction) where the bundle of sheets is subjected to the stapling is opposed to the stapler  37 . 
     When the stapler  37  stitches the bundle of sheets, the stacker  35  descends until a portion (which is the intermediate portion in the sheet transport direction and the location where a staple is inserted) where a folding line is to be formed in the bundle of sheets reaches the front of a folding blade  38 . 
     When the portion where the folding line is to be formed reaches the front of the folding blade  38 , a front end  38   a  of the folding blade  38  inserts a surface which becomes an inner surface after the folding blade  38  saddles the bundle of sheets. 
     The saddle stitcher  30  has a pair of folding rollers  39  at the end in a movement direction of the folding blade  38 . A nip of the pair of folding rollers  39  winds the bundle of sheets which the folding blade  38  inserts, and forms the folding line on the intermediate portion of the bundle of sheets. The folding blade  38  and the pair of folding rollers  39  form folding unit. 
     The pair of folding rollers  39  forms the folding line on the bundle of sheets, and transports the bundle of sheets to a discharging mechanism  40  disposed at a further downstream side of the pair of folding rollers  39 . The discharging mechanism  40  temporarily stops a transport of the bundle of sheets which the pair of folding rollers  39  transports. 
     The discharging mechanism  40  includes a pair of saddle reinforcing rollers  41  (an upper saddle reinforcing roller (second roller)  41   a  and a lower saddle reinforcing roller (first roller)  41   b ) as a saddle reinforcing unit and a pair of transport rollers  42 . The saddle reinforcing rollers  41  moves while pressurizing the folding line in a direction (which is a direction along the folding line) perpendicular to a transport direction of the bundle of sheets to enhance the folding line. 
     The pair of saddle reinforcing rollers  41  enhances the folding line of the bundle of sheets, and outputs the bundle of sheets to the saddle tray  50 . The saddle tray  50  places the bundle of sheets as a book. 
     A feeding sensor  322 , an entrance sensor  326  of a connection part, a first transport path sensor  327 , and a second transport path sensor  328  detect the sheet between the pair of entrance rollers  31  and the pair of intermediate rollers  32 . A transport sensor  33   a  detects the sheet between the pair of intermediate rollers  32  and the pair of exit rollers  33 . A stacker sensor  330  detects the sheet on the stacker  35 . A discharging sensor  46  detects the sheet which the pair of transport rollers  42  discharges. The whole of the feeding sensor  322 , the entrance sensor  326 , the first transport path sensor  327 , the second transport path sensor  328 , the transport sensor  33   a , stacker sensor  330 , and the discharging sensor  46  is called “transport sensor  146 ” as follows. 
     An entrance motor drives the pair of entrance rollers  31 . A stacker motor drives the stacker  35  in the top and the bottom. A saddle transporting motor drives the pair of intermediate rollers  32  and the pair of exit rollers  33 , and transports the sheet from a feeding intake to the stacker  35 . A discharging motor  45  drives the pair of folding rollers  39  and the pair of transport rollers  42 . The whole of the entrance motor, the stacker motor, the saddle transporting motor, and the discharging motor  45  is called “transport motors  145 ” as follows. 
     The functions and operations of the stacker  35 , the jogger fence  36 , and the discharging mechanism  40  by the control of the sheet finisher  20  will be described in detail in sequence. 
     (2) Configuration, Function, and Operation of Stacker  35  and Jogger Fence  36   
       FIG. 4  is a perspective view illustrating a whole configuration example of the saddle stitcher  30 . 
     The saddle stitcher  30  includes the erect tray  34  and the stacker  35 . The stacker  35  is disposed below the erect tray  34 . 
       FIG. 5  is a perspective view illustrating the jogger fence  36  included in the saddle stitcher  30 .  FIG. 6  is an upper external view illustrating the jogger fence  36  included in the saddle stitcher  30 . The erect tray  34  of the saddle stitcher  30  includes the jogger fence  36  aligning the sheets in the width direction of the sheets. 
       FIG. 7  is an expanded view illustrating the stacker  35  included in the saddle stitcher  30 . The stacker claw  35   a  in the lower portion of the stacker  35  receives the lower end of the sheets reversed and dropped from an upper slope portion of the erect tray  34 . 
       FIG. 8  is a block diagram illustrating the functions of the stacker  35  and the jogger fence  36 . 
     The sheet finisher  20  functions as a size information acquiring unit  101 , a jam detector  102 , a jogger fence drive controlling unit  103 , a stacker claw drive controlling unit  104 , and a notification unit  105  by execution of a program by a controller included in the sheet finisher  20 . Note that the sheet finisher  20  may have each of units  101 - 105  as hardware. 
     The size information acquiring unit  101  has a function to acquire information on a size of the printed sheet, which the image forming system  12  discharges to the sheet finisher  20 , from the CPU of the image forming system  12 . 
     The jam detector  102  has function to recognize a location of jam occurrence during the saddle-stitching and cancels a job, if one of the transport sensors  146  detects jam occurrence of a sheet (sheet jam, hereinafter referred to as “jam”). For example, the detection of jam occurrence by the one of the transport sensors  146  is that the one continues detecting the sheet over threshold time. The jam detector  102  has function to cancel a job, too. The saddle-stitching stops due to the cancel. The saddle-stitching can restart, if a user removes the jammed sheet and all sheets stopped in the sheet transport passage. 
     The jogger fence drive controlling unit  103  has a function to drive the jogger fence  36  to be located at a home position (which is a position from which elements of the jogger fence are spaced from each other), if the jam detector  102  cancels the job. 
       FIG. 9  is a schematic view illustrating an example of a mechanism which the pair of exit rollers  33  causes jam occurrence. 
     The pair of exit rollers  33  transports the sheet that the pair of intermediate rollers  32  transports to the erect tray  34  so that the front end of each sheet faces to the upper portion of the slope of the erect tray  34 . If the pair of exit rollers  33  transports each sheet to the erect tray  34 , the transport sensor  33   a  of an upstream side of the pair of exit rollers  33  detects whether the jam occurs in the pair of intermediate rollers  32  in accordance with an timer interval from an ON state of detecting the sheet to an OFF state of not detecting the sheet. 
     The stacker claw drive controlling unit  104  has function to determine whether the stacker claw  35   a  in the lower portion of the stacker  35  can be driven, if the jam detector  102  cancels the job. The stacker claw drive controlling unit  104  has function to drive the stacker claw  35   a  in the lower portion of the stacker  35  up to an upper opening where the sheet is removed and moves the bundle of sheets to the upper opening, if the stacker claw  35   a  can drive. The stacker claw drive controlling unit  104  drives the stacker claw  35   a  to the upper opening, only if a sheet length in the sheet transport direction based on a sheet size acquired by the size information acquiring unit  101  is equal to or less than a threshold. The stacker claw drive controlling unit  104  may control a drive range of the stacker claw  35   a  so as to be variable in accordance with the sheet size which the size information acquiring unit  101  acquires. 
     The notification unit  105  has function to notify the jam occurrence location which the jam detector  102  recognizes to the image forming system  12 . The image forming system  12  displays the notified jam occurrence location which the notification unit  105  notifies on the display panel  9   a  of the image forming system  12 . 
     Note that it is preferable to terminate the drive of the jogger fence  36  by the transverse alignment drive controlling unit  103  and the drive of stacker claw  35   a  by the stacker claw drive controlling unit  104 , before the jam which the jam detector  102  recognizes is displayed on the display panel  9   a.    
       FIG. 10  is a flowchart illustrating an operation example of the stacker  35  and the jogger fence  36 . 
     First, the CPU of the sheet finisher  20  acquires the information on the size of the printed sheet, which the image forming system  12  discharges to the sheet finisher  20 , from the CPU of the image forming system  12  (Act  1 ). Subsequently, the sheet finisher  20  starts the saddle-stitching (Act  2 ). 
     The sheet finisher  20  determines whether at least one of the transport sensors  146  detects the jam occurrence during the saddle-stitching (Act  3 ). If the result is Yes in Act  3 , that is, if the sheet finisher  20  determines that the jam is not detected, the sheet finisher  20  determines whether to terminate the saddle-stitching (Act  4 ). If the result is Yes in Act  4 , that is, if the sheet finisher  20  determines that the saddle-stitching is terminated, the sheet finisher  20  terminates the saddle-stitching. On the other hand, if the result is No in Act  4 , that is, if the sheet finisher  20  determines that the saddle-stitching is not terminated, the intermediate the sheet finisher  20  continues saddle-stitching, and then the sheet finisher  20  determines whether at least one of the transport sensors  146  detects the jam occurrence (Act  3 ). 
     If the result is No in Act  3 , that is, if the sheet finisher  20  determines that the jam occurs, the sheet finisher  20  cancels the job (Act  5 ). Upon canceling the job in Act  5 , the sheet finisher  20  stops the saddle-stitching. Therefore, the sheet finisher  20  does not restart the saddle-stitching, if the user does not remove the jammed sheet and all the sheets stopped in the sheet transport passage. 
     If the sheet finisher  20  cancels the job in Act  5 , the sheet finisher  20  drives the jogger fence  36  to the home position (Act  6 ). 
     Subsequently, the sheet finisher  20  determines whether the stacker claw  35   a  in the lower portion of the stacker  35  can be driven (Act  7 ). If the result is Yes in Act  7 , that is, if the sheet finisher  20  determines that the stacker claw  35   a  in the lower portion of the stacker  35  can be driven, the sheet finisher  20  determines whether the sheet length in the sheet transport direction based on the sheet size acquired in Act  1  is equal to or less than the threshold (Act  8 ). If the result is Yes in Act  8 , that is, if the sheet finisher  20  determines that the sheet length in the sheet transport direction is equal to or less than the threshold, the sheet finisher  20  drives the stacker claw  35   a  (Act  9 ). 
     In Act  8 , the sheet finisher  20  may determine whether the sheet length in the sheet transport direction is equal to or less than a second threshold larger than the first threshold. In this case, when the sheet length in the sheet transport direction is equal to or less than the first threshold, the sheet finisher  20  drives the stacker claw  35   a  by the maximum drive amount. On the other hand, when the sheet length in the sheet transport direction is larger than the first threshold and equal to or less than the second threshold, the sheet finisher  20  drives the stacker claw  35   a  by the half of the maximum drive amount. 
     Subsequently, the sheet finisher  20  displays the jam occurrence location which Act  3  detects on the display panel  9   a  (Act  10 ) and prompts the user to remove all the sheets stopped in the sheet transport passage. 
     On the other hand, if the result is No in Act  7 , that is, if the sheet finisher  20  determines that the stacker claw  35   a  in the lower portion of the stacker  35  cannot be driven, the sheet finisher  20  does not drive the stacker claw  35   a  and displays the jam occurrence location on the display panel  9   a  (Act  10 ). 
     In addition, when the result is No in Act  8 , that is, when the sheet length in the sheet transport direction is equal to or less than the threshold, the sheet finisher  20  does not drive the stacker claw  35   a  and displays the jam occurrence location on the display panel  9   a  (Act  10 ). 
     Subsequently, if the user removes all the sheets stopped in the sheet transport passage, the sheet finisher  20  restarts the saddle-stitching, and proceeds to the operation of Act  3 . 
     In the sheet finisher  20  functioning as the units  101  to  105  or operating in Acts  1  to  10 , it is easy for the user to remove the bundle of sheets stopped in the erect tray  34 , when the jam occurs in one of locations in the sheet finisher  20 . 
     (3) Structure, Function, and Operation of Discharging Mechanism  40   
       FIG. 11  is a perspective view illustrating a whole configuration example of the discharging mechanism  40 . 
     The discharging mechanism  40  includes the pair of saddle reinforcing rollers  41 , the pair of transport rollers  42  (the lower transport roller  42   a ), a torque limiter  43 , a shaft  44 , the discharging motor  45 , and the discharging sensor  46 . The pair of saddle reinforcing rollers  41  moves while pressurizing the folding line in a direction perpendicular to the transport direction of the bundle of sheets to enhance the folding line. 
     The torque limiter  43  delivers the power of the discharging motor  45  to the shaft  44  and the lower transport roller  42   a  through the pair of saddle reinforcing rollers  41 . 
     The discharging sensor  46  is an optical sensor which detects the book which the pair of saddle reinforcing rollers  41  transports. 
     The pair of saddle reinforcing rollers  41  enhances the folding line of the bundle of sheets, outputs the bundle of sheets from the pair of transport rollers  42  to the saddle tray  50 . The saddle tray  50  stacks the book as the bundle of sheets. 
       FIG. 12  is a block diagram illustrating functions of the discharging mechanism  40 . 
     The sheet finisher  20  functions as a size information acquiring unit  111 , a rotation determining unit  112 , a detection determining unit  113 , a slip amount calculator  114 , an overload determining unit  115 , a notification unit  116 , and a discharging motor controlling unit  117  by execution of a program by a controller included in the sheet finisher  20 . In addition, the sheet finisher  20  may have the units  111  to  117  as functions of a CPU as hardware. 
     The size information acquiring unit  111  has a function to acquire the information on the size of the printed sheet, which the image forming system  12  discharges to the sheet finisher  20 , from the CPU of the image forming system  12 . 
     The rotation determining unit  112  has a function to determine whether rotation of the discharging motor  45  with respect to one book is equal to or less than an allowed amount of rotation on the basis of the information on the size of the sheet which the size information acquiring unit  111  acquires. Note that the allowed amount of rotation is an amount of rotation of the discharging transport motor  45  corresponding to “a sheet length in the sheet transport direction based on the size of the sheet” plus “an allowance slip length”. 
     The detection determining unit  113  has a function to determine whether the discharging sensor  46  of the upstream side of the discharging mechanism  40 , if the rotation determining unit  112  determines that the amount of rotation of the discharging motor  45  is equal to or less than the allowed amount of rotation. 
     The slip amount calculator  114  has a function to calculate an amount of slip on the basis of a distance corresponding to the amount of the rotation of the discharging motor  45 , the information on the size of the sheet which the size information acquiring unit  111  acquires, and time when the discharging sensor  46  continues detecting the book. For example, the slip amount calculator  114  calculates the amount of slip based on “the distance corresponding to a unit amount of rotation of the discharging motor  45 ” times “an amount of rotation of the discharging motor  45  per unit hour” times “the time when the discharging sensor  46  continues detecting the book” minus “the sheet length in the sheet transport direction based on the size of the sheet”. For example, “the amount of rotation of the discharging motor  45  per unit hour” is in proportion to the number of electrical pulses which the discharging motor  45  obtains. For example, the slip amount includes a slip amount to occur to release a load of the torque limiter  43 , which exceeds a threshold. For example, the slip amount includes a slip amount to occur even if the torque limiter  43  does not slip between the discharging mechanism  40  and the book. 
     The overload determining unit  115  has a function to determine that the discharging mechanism  40  is not in an overload state, if the amount of slip which the slip amount calculator  114  calculates is equal to or less than a third threshold. If the amount of slip is larger than the third threshold, the overload determining unit  115  has a function to determine that the discharging mechanism  40  is in the overload state due to a fullness state of the saddle tray  50 , that is, due to contact of the book which the saddle tray  50  loads with the book to be transported next time. The overload determining unit  115  has a function to determine that the discharging mechanism  40  is in the overload state due to transport jam in the discharging mechanism  40 , if the rotation determining unit  112  determines that the amount of rotation of the discharging motor  45  with respect to one book is larger than the allowed amount of rotation. 
     The notification unit  116  has a function to notify the overload state of the discharging mechanism  40  to the CPU of the image forming system  12 . The image forming system  12  displays the fact that the saddle tray  50  is full or the fact that a discharge jam occurs on the display panel  9   a  of the image forming system  12 . 
     The image forming system  12  displays the fact that the notified saddle tray  50  is full on the display panel  9   a  of the image forming system  12 , and warns a user that the saddle tray  50  is full. The image forming system  12  displays the fact that the notified discharge jam occurs on the display panel  9   a  of the image forming system  12 , and stops discharging the sheet to the sheet finisher  20 . 
     The discharging motor controlling unit  117  controls the discharging motor  45  to be stopped, if the detection determining unit  113  detects the book and if the notification unit  116  notifies that the discharging from the image forming system  12  stops. 
     Note that the slip amount calculator  114  may calculate the amount of slip in consideration of the sheet on which the image forming system  12  already starts the printing or the sheet on which the saddle stitcher  30  already starts the saddle-stitching. In this case, the overload determining unit  115  determines that the overload does not occur in the transport of the book, even if the sheet on which the image forming system  12  already starts the printing or the sheet on which the saddle stitcher  30  already starts the saddle-stitching is transported to the saddle tray  50 . 
       FIG. 13  is a flowchart illustrating the operations of the discharging mechanism  40 . 
     First, the CPU of the sheet finisher  20  acquires the information on the size of the printed sheet, which the image forming system  12  discharges to the sheet finisher  20 , from the CPU of the image forming system  12  (Act  21 ). Subsequently, the sheet finisher  20  starts the drive of the discharging motor  45  to stitch on the basis of the sheet which the image forming system  12  discharges (Act  22 ). 
     Subsequently, the sheet finisher  20  determines whether the rotation of the discharging motor  45  with respect to one book is equal to or less than the allowed amount of rotation (Act  23 ). If the result is Yes in Act  23 , that is, if the sheet finisher  20  determines that the amount of rotation of the discharging motor  45  with respect to one book is equal to or less than the allowed amount of rotation, the sheet finisher  20  determines whether the discharging sensor  46  detects the book (Act  24 ). 
     If the result is Yes in Act  24 , that is, if the sheet finisher  20  determines that the discharging sensor  46  detects the book, the sheet finisher  20  determines that the overload does not occur in the transport of the book, and stops the discharging motor  45  (Act  25 ). 
     Subsequently, the sheet finisher  20  calculates the amount of slip on the basis of the distance corresponding to the amount of rotation of the discharging motor  45 , the information on the size of the sheet which Act  21  acquires, and time when the discharging sensor  46  continues detecting the book, if the discharging motor  45  stops (Act  26 ). 
     The sheet finisher  20  determines whether the amount of slip which Act  26  calculates is equal to or less than the third threshold (Act  27 ). If the result is Yes in Act  27 , that is, if the sheet finisher  20  determines that the amount of slip is equal to or less than the third threshold, the sheet finisher  20  determines that the discharging mechanism  40  is not in the overload state, and terminates the discharging of the book. 
     Alternatively, if the result is No in Act  27 , that is, if the sheet finisher  20  determines the amount of slip is larger than the third threshold, the sheet finisher  20  determines that the discharging mechanism  40  is in the overload state due to the fullness state of the saddle tray  50 . In addition, the sheet finisher  20  notifies the CPU of the image forming system  12  that the saddle tray  50  is full (Act  28 ). Subsequently, the sheet finisher  20  terminates the transport of the book. 
     If the result is No in Act  23 , that is, if the sheet finisher  20  determines the amount of rotation of the discharging motor  45  is larger than the allowed amount of rotation, the sheet finisher  20  determines that the discharging mechanism  40  is in the overload state due to the discharge jam occurred in the discharging mechanism  40 . In addition, the sheet finisher  20  notifies that the sheet discharge from the image forming system  12  stops (Act  29 ). Subsequently, the sheet finisher  20  stops the discharging motor  45  (Act  30 ), and terminates the transport of the book. 
     The sheet finisher  20  functioning as the units  111  to  117  and operating in Acts  21  to  30  is capable of detecting that the saddle tray  50  is full while stacking the books at a location of a mechanical limit without mechanical discharge jam. The sheet finisher  20  is capable of detecting that the sheet cannot be actually carried out. It is not necessary to provide a separate sensor in the saddle tray  50 , since detecting the fullness state of the saddle tray  50  is not necessary. 
     (4) Function and Operation of Various Transport Mechanisms Including Discharging Mechanism  40   
       FIG. 14  is a block diagram illustrating functions of a transport mechanism. 
     The sheet finisher  20  functions as the size information acquiring unit  111 , a rotation detecting unit  112 A, a detection determining unit  113 A, a slip amount calculator  114 A, an overload determining unit  115 A, the notification unit  116 , and a transport motor control unit  117 A by execution of a program by a controller included in the sheet finisher  20 . In addition, the sheet finisher  20  may have the units  111  to  117 A as functions of the CPU as hardware. The same reference numerals are given to the same functions as the functions of the discharging mechanism  40  and description is omitted. 
     The rotation determining unit  112 A has a function to determine whether the amount of rotation of each transport motors  145  with respect to one sheet (or book) is equal to or less than an allowed amount of rotation in accordance with the same determining method as that of the rotation determining unit  112 . 
     If the amount of rotation of at least one of transport motors  145  is equal to or less than the allowed amount of rotation, the detection determining unit  113 A has a function to determine whether a transport sensor, which is equal to or less than the allowed amount of rotation, of an upper stream side of the roller or the stacker detects the sheet in accordance with the same determining method as that of the detection determining unit  113 . 
     The slip amount calculator  114 A has a function to calculate the amounts of slip on the basis of the distances corresponding to the amounts of rotation of the transport motors  145 , the information on the size of the sheet which the size information acquiring unit  111  acquires, and time when the transport sensors  146  continue detecting the sheet. 
     The overload determining unit  115 A has a function to determine that each transport mechanism is not in the overload state by the same determining method as that of the overload determining unit  115 , when the amount of slip is equal to or less than the third threshold. 
     The notification unit  116 A has a function to notify the CPU of the image forming system  12  that each transport mechanism is in the overload state. The image forming system  12  displays the fact that the saddle tray  50  is full, the fact that a transport jam occurs, or a location where the jam occurs on the display panel  9   a  of the image forming system  12 . 
     The image forming system  12  displays the fact that the notified saddle tray  50  is full on the display unit  9   a  of the image forming system  12  to warn a user. The image forming system  12  displays the fact that the notified transport jam occurs or a location where the transport jam occurs, on the display panel  9   a  of the image forming system  12 , stops discharging the sheet to the sheet finisher  20 . 
       FIG. 15  is a flowchart illustrating operations of the transport mechanism. The same act numbers are given to the same operations of the discharging mechanism  40  shown in  FIG. 13  and description is omitted. 
     The sheet finisher  20  starts drive of the transport motors  145  to stitch as the finishing on the basis of the sheet which the image forming system  12  discharges (Act  32 ). 
     Subsequently, the sheet finisher  20  determines whether the amount of rotation of all the transport motors  145  with respect to one sheet (or book) is equal to or less than the allowed amount of rotation (Act  33 ). If the result is Yes in Act  33 , that is, if the sheet finisher  20  determines that the amount of rotation of all the transport motors  145  with respect to one sheet is equal to or less than the allowed amount of rotation, the sheet finisher  20  determines whether the transport sensor which is equal to or less than the allowed amount of rotation, of the upstream side of the roller or the stacker detects the sheet (Act  34 ). 
     If the result is Yes in Act  34 , that is, if the sheet finisher  20  determines that the transport sensor which is equal to or less than the allowed amount of rotation detects the sheet, the sheet finisher  20  determines that the overload does not occurs in the transport of the sheet, and stops the transport motors  145  (Act  35 ). 
     Subsequently, if the transport motors  145  stop, the sheet finisher  20  respectively calculates the amounts of slip on the basis of the distances corresponding to the amounts of rotation of the transport motors  145 , the information on the size of the sheet which Act  21  acquires, and time when the transport sensors  146  continue detecting the sheet (Act  36 ). 
     The sheet finisher  20  determines whether all the amount of slip which Act  36  calculates is equal to or less than the third threshold (Act  37 ). If the result is Yes in Act  37 , that is, if the sheet finisher  20  determines that all the amount of slip is equal to or less than the third threshold, the sheet finisher  20  determines that each transport mechanism is not in the overload state, and terminates the transport of the sheet. 
     Alternatively, when the result is No in Act  37 , that is, if the sheet finisher  20  determines that at least one of amount of slip is larger than the third threshold, the sheet finisher  20  determines that maintenance of the sheet finisher  20  is necessary. In addition, the sheet finisher  20  notifies the CPU of the image forming system  12  that the maintenance of the sheet finisher  20  is necessary (Act  38 ). Subsequently, the sheet finisher  20  terminates the transport of the sheet. 
     If the result is No in Act  33 , that is, if the sheet finisher  20  determines that the amount of rotation of at least one of transport motors  145  is larger than the allowed amount of rotation, the sheet finisher  20  determines that the transport jam occurs in a vicinity of the roller or the stacker in which the amount of rotation is larger than the allowed amount of rotation. In addition, the sheet finisher  20  notifies that discharging the sheet from the image forming system  12  stops (Act  39 ). Subsequently, the sheet finisher  20  stops the transport motor of which the rotation is larger than the allowed amount of rotation among the transport motors  145  (Act  40 ), and terminates the transport of the sheet. 
     The sheet finisher  20  functioning as the units  111  to  117 A or operating in Acts  21  to  40  is capable of detecting timing at which the maintenance of the sheet finisher  20  is necessary without the mechanical transport jam while stacking the books up to a mechanical limit. The sheet finisher  20  is capable of detecting that the sheet cannot be actually carried out. 
     The invention is not limited to the above-described embodiment, but may be modified in various forms without departing the gist of the invention in practice steps. Various embodiments of the invention may be put into practice by appropriate combination of the plural constituent elements described above. For example, several constituent elements may be removed from the whole constituent elements of the embodiment. In addition, the constituent elements may be appropriately combined.