Patent Publication Number: US-8991808-B2

Title: Sheet processing apparatus, image forming system, and sheet binding method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application Nos. 2012-108787, filed on May 10, 2012, and 2013-030113, filed on Feb. 19, 2013, in the Japan Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a sheet processing apparatus to bind together a bundle of sheets; an image forming system including the sheet processing apparatus and an image forming apparatus, such as a copier, a facsimile machine, a printer, or multifunction machine capable of at least two of these functions; and a sheet binding method used in a sheet processing apparatus. 
     2. Description of the Background Art 
     There are sheet processing apparatuses, so-called finishers or post-processing apparatuses, that align a bundle of sheets (hereinafter “a sheet bundle”) output from an image forming apparatus and bind the sheet bundle with metal staples. Such sheet processing apparatuses can automatically staple a number of sheet bundles on which images are formed and are widely used for convenience and efficiency thereof. 
     Additionally, there are hand-held staplers, so-called staple guns or powered staplers, capable of binding sheets without metal staples. A tooth form may be used to press multiple sheets so that fibers of the sheets tangle with each other and thereby tie the sheets together, or bind the sheets together using other types of processing such as half blanking, lancing, bending, and inserting. For example, JP-S36-13206-Y discloses a hand-held stapler capable of clamp binding, and JP-S37-7208-Y discloses a hand-held stapler that makes cut holes in sheets, bends cut portions, and inserts the cut portions into the cut holes. 
     Sheets bundles free of staples can be directly put through a shredder. Thus, such binding tools can reduce consumption of consumables, make recycling easier, and be effective to save resources. It is to be noted that, hereinafter clamp binding refer to a binding method that involves pressing multiple sheets with a tooth form to tie the sheets, thereby causing fibers of the sheets to tangle with each other. Use of clamp binding in sheet processing apparatuses is expected to increase owing to the above-described advantages. 
     In conventional approaches, a pressure lever that does not include a driving source is moved by a one-rotation cam to bind or bond sheets together. 
     For example, JP-2010-189101-A proposes a sheet binding device to bind a bundle of sheets by forming projections and recesses in the direction of the thickness of the sheet bundle, according to the thickness of the sheet bundle. Specifically, the sheet binding device includes a pair of tooth forms movable in the thickness direction of the sheet bundle, to squeeze the sheet bundle to form the projections and the recesses in the thickness direction, and a pressure applying member to apply pressure to the pair of tooth forms. The pressure is increased as the thickness of the sheet bundle increases. 
     Additionally, the pressure applied to the tooth forms may be increased as the thickness of the sheet bundle increases by a configuration that includes a rotary member, a driving source to rotate the rotary member, and a flexible member to apply pressure to the tooth form that is movable. The rotary member includes a contact portion that slidingly contacts the flexible member. As the rotary member rotates in one direction and the opposite direction, the amount by which the rotary member shifts to the flexible member increases and decreases, respectively. The rotational position of the rotary member can be changed to increase the shift amount of the flexible member as the thickness of the sheet bundle increases. 
     To bind the sheet bundle, the pressure lever is shifted in the former approach, and the pressure applying member including the rotary member applies pressure in the latter approach. In such configurations, typically the sheet bundle is pressed with a pressure of 1000 N or greater to cause the sheet fibers to tangle with each other. A motor may be used to generate the pressure. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention provides a sheet processing apparatus that includes a pair of squeezing members and a pressure applying unit to apply pressing force to the pair of squeezing members. The squeezing members have a projection and a recess to engage each other. A sheet bundle is inserted therebetween, squeezed in a direction of thickness of the sheet bundle, and thus bound. The pressing force generated between the squeezing members by the pressure applying unit increases in strength as a relative distance between the squeezing members decreases. 
     In another embodiment, an image forming system includes an image forming apparatus to form images on recording media sheets and the above-described sheet processing apparatuses. 
     Yet another embodiment provides a method of binding multiple sheets. The method includes a step of inserting a sheet bundle between a pair of squeezing members shaped to have a projection and a recess to engage each other, a step of primarily squeezing the sheet bundle with a first pressing force from when a relative distance between the squeezing members reaches a predetermined distance set according to the thickness of the sheet bundle to when the relative distance equals to the thickness of the sheet bundle, and a step of secondarily squeezing the sheet bundle with a second pressing force stronger than the first pressing force, from when the relative distance between the squeezing members equals to the thickness of the sheet bundle to completion of sheet binding. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIGS. 1A and 1B  are schematic diagrams illustrating two states of an image forming system according to an embodiment of the present invention; 
         FIG. 2  is a plan view of a sheet processing apparatus shown in  FIGS. 1A and 2B ; 
         FIG. 3  is a front view of the sheet processing apparatus shown in  FIGS. 1A and 1B ; 
         FIG. 4  is a schematic diagram illustrating a main portion of the sheet processing apparatus when a branch pawl is at a position for transporting sheets; 
         FIG. 5  is a schematic diagram illustrating the main portion of the sheet processing apparatus when the branch pawl is at a position for switchback operation; 
         FIGS. 6A and 6B  illustrate the sheet processing apparatus being in an initial stage of online binding; 
         FIGS. 7A and 7B  illustrate a state immediately after a first sheet output from an image forming apparatus is received in the sheet processing apparatus; 
         FIGS. 8A and 8B  illustrate a state in which the trailing end of the sheet released from a nip between a pair of entrance rollers is beyond a bifurcation channel; 
         FIGS. 9A and 9B  illustrate the switchback operation for changing a conveyance route in which the sheet is transported; 
         FIGS. 10A and 10B  illustrate a state in which the first sheet is retained in the bifurcation channel, and a second sheet is received in the sheet processing apparatus; 
         FIGS. 11A and 11B  illustrate a state in which the second sheet is received in the sheet processing apparatus; 
         FIGS. 12A and 12B  illustrate a state in which a last sheet is aligned with the preceding sheets, forming a sheet bundle; 
         FIGS. 13A and 13B  illustrate binding operation subsequent to the state shown in  FIGS. 12A and 12B ; 
         FIGS. 14A and 14B  illustrate a state in which the sheet bundle is discharged; 
         FIG. 15  is a schematic view of a binding device according to a comparative example, being at a position for receiving sheets; 
         FIG. 16  is a schematic view of the comparative binding device shown in  FIG. 15 , being at a position for binding sheets; 
         FIG. 17  illustrates a pressure applying unit according to another comparative example; 
         FIG. 18  illustrates a pressure applying unit according to an embodiment; 
         FIG. 19  is a graph illustrating relations between the size of clearance between tooth and force applied to the tooth according to an embodiment and a comparative example; and 
         FIG. 20  is a block diagram that schematically illustrates a control configuration of the system including the sheet processing apparatus and the image forming apparatus shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. 
     It is to be noted that the term “sheet” used in this specification includes recording media sheets. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views thereof, and particularly to  FIG. 1 , a system including an image forming apparatus and a sheet processing apparatus according to an embodiment of the present invention is described. 
       FIGS. 1A and 1B  are schematic diagrams illustrating two states of an image forming system according to an embodiment of the present invention. An image forming system  100  according to the present embodiment includes an image forming apparatus  101  and a sheet processing apparatus (i.e., a finisher or post-processing apparatus)  201 . The sheet processing apparatus  201  includes a sheet binding mechanism and disposed inside a conveyance channel through which sheets are output from the image forming apparatus  101 . Thus, the sheet processing apparatus  201  is a channel-internal binding apparatus. The sheet processing apparatus  201  is disposed inside the conveyance channel of the image forming apparatus  101  in  FIG. 1A  and outside the conveyance channel in  FIG. 1B . 
     The sheet processing apparatus  201  has two capabilities, aligning sheets stacked inside the conveyance channel and stapling the sheets inside the conveyance channel. In  FIG. 1A , the sheet processing apparatus  201  processes sheets inside the housing of the image forming apparatus  101  and thus is also called a housing-internal processing device. Thus, the sheet processing apparatus  201  according to the present embodiment is compact and can be mounted inside the housing or to a side of the image forming apparatus  101  in accordance with the configuration thereof. 
     The image forming apparatus  101  includes an image forming engine  105 , an image reader  103  to read and convert images into image data, and an automatic document feeder (ADF)  104 . The image forming engine  102  includes an image processing unit and a sheet feeder. In the state shown in  FIG. 1A , a discharge tray to which sheets on which images are formed are output is formed inside the housing of the image forming apparatus  101 . In the state shown in  FIG. 1B , the discharge tray is positioned outside the image forming apparatus  101 . 
       FIGS. 2 and 3  are respectively a plan view and a front view of the sheet processing apparatus  201  shown in  FIGS. 1A and 2B . In the configuration shown in  FIGS. 2 and 3 , the sheet processing apparatus  201  includes an entry detector  202 , a pair of entrance rollers  203 , a branch pawl  204 , a binding device  210 , and a pair of discharge rollers  205 , and these components are arranged in that order from an entrance side along a conveyance channel  240 . The entry detector  202  detects the presence of a sheet received in the sheet processing apparatus  201  after discharged from the image forming apparatus  101 . Specifically, the entry detector  202  detects the leading end and the trailing end of the sheet. For example, the entry detector  202  can be a reflection type photosensor. Alternatively, a transmission-type photosensor may be used. The entrance rollers  203  are positioned at the entrance of the sheet processing apparatus  201  to receive sheets discharged by discharge rollers  102  of the image forming apparatus  101  and forward the sheets to the binding device  210 . Additionally, a drive source, such as a drive motor, is provided for the entrance rollers  203 . A central processing unit (CPU)  201 - 1  shown in  FIG. 20 , serving as a controller, controls the stop, rotation, and a conveyance amount of the drive source. The entrance rollers  203  correct skew of the sheet with the leading end of the sheet stuck in a nip between the entrance rollers  203 . 
     The branch pawl  204  is disposed downstream from the entrance rollers  203  in the direction in which the sheet is transported (hereinafter “sheet conveyance direction”). The branch pawl  204  guides the trailing end of the sheet to a bifurcation channel  241 . In this case, after the trailing end of the sheet passes by the branch pawl  204 , the branch pawl  204  pivots clockwise in  FIG. 3 , thereby transporting the sheet in reverse. Thus, the trailing end of the sheet is led to the bifurcation channel  241 . The branch pawl  204  can pivot driven by a solenoid  250  shown in  FIG. 4 , which is described in further detail later. Instead of the solenoid  250 , a motor may be used. When the branch pawl  204  pivots counterclockwise in  FIG. 3 , the branch pawl  204  can press a single sheet or multiple sheets against a conveyance face of the bifurcation channel  241 . Thus, the branch pawl  204  can retain the single or multiple sheets not to move in the bifurcation channel  241 . 
     The discharge rollers  205  are disposed immediately upstream from the exit of the conveyance channel  240  of the sheet processing apparatus  201 . The discharge rollers  205  transport, shift, and discharge the sheets. A drive source for the discharge rollers  205  is provided similarly to the entrance rollers  203 , and the controller controls the stop, rotation, and a conveyance amount thereof. A shift mechanism  205 M (shown in  FIG. 2 ) shifts the discharge rollers  205 . The shift mechanism  205 M includes a shift link  206 , a shift cam  207 , a cam stud  208 , and a home position (HP) detector  209 . 
     The entrance rollers  203  and the discharge rollers  205  together form a conveyance unit to transport the sheet bundle  272 . 
     The shift link  206  is provided to a shaft end  205   a  of the discharge rollers  205  and receives a force for shifting the discharge rollers  205 . The shift cam  207  is a rotary disc-shaped member and includes the cam stud  208 . For example, the shaft of the discharge rollers  205  is movably inserted into a shift link slot  207   a  via the cam stud  208 , and the discharge rollers  205  are moved in a direction perpendicular to the sheet conveyance direction by rotation of the shift cam  207 . Thus, the discharge rollers  205  are shifted. The cam stud  208  is geared to the shift link slot  207   a  and converts the rotational motion of the shift cam  207  to linear movement in the axial direction of the discharge rollers  205 . The HP detector  209  detects a position of the shift link  206 , and the detected position is deemed a home position of the shift link  206 , used as a reference to control rotation of the shift cam  207 . The rotation of the shift is controlled by the above-described CPU  201 - 1 . 
     The binding device  210  includes a sheet end detector  220 , a binding home position (HP) detector  221 , and a guide rail  230  to guide movement of the binding device  210 . The binding device  210  is a so-called stapler to bind together multiple sheets into a sheet bundle although staples are not used in the present embodiment. In the present embodiment, the binding device  210  squeezes sheets using a pair of tooth forms  261  and a pressure applying unit  269  (shown in  FIG. 18 ), thereby deforming the sheets so that fibers thereof tangle each other. This is called clamp binding. There are hand-held staplers to binds sheets using half blanking, lancing, bending, and inserting in addition to clamp binding. Such binding methods without staples reduce consumption of consumables, make recycling easier, and enable shredding of sheet bundles as is. Therefore, with the binding device  210 , sheets can be bound together using sheets alone without staples even in sheet processing apparatuses, so-called finishers. 
     The sheet end detector  220  detects a lateral end of the sheet, and sheets are aligned with reference to the position detected by the sheet end detector  220 . The binding HP detector  221  is movable in a sheet width direction perpendicular to the sheet conveyance direction and detects a position of the binding device  210 . The home position of the binding device  210  is set to a position not to interfere with a maximum size sheet processed by the image forming system  100 . The guide rail  230  guides the binding device  210  so that the binding device  210  can move reliably in the sheet width direction. The guide rail  230  extends in a range to guide the binding device  210  moving in the direction perpendicular to the conveyance channel  240  (sheet conveyance direction) from the home position to a position to binds a smallest sheets processed by the image forming system  100 . A shift unit including a drive motor moves the binding device  210  along the guide rail  230 . 
     The conveyance channel  240  extends from the entrance of the sheet processing apparatus  201  to the exit thereof. The bifurcation channel  241  bifurcates from the conveyance channel  240 . The sheet is transported in reverse (switchback) and transported from the trailing end to the bifurcation channel  241 . The bifurcation channel  241  serves as a stacking channel in which multiple sheets are stacked and aligned. The sheets are transported so that the trailing ends thereof contact a contact face  242  provided at a downstream end of the bifurcation channel  241 . Thus, the contact face  242  serves as a reference plane to align the trailing end of the sheets. 
     In the present embodiment, the pair of tooth forms  261  includes a first tooth form  261   a  (shown in  FIG. 18 ) on the upper side and a second tooth form  261   b , on the lower side, configured as a set of squeezing members having projections and recesses mating with each other. The first and second tooth forms  261   a  and  261   b  face each other and squeeze the sheets inserted between them for clamp binding. 
       FIGS. 4 and 5  are schematic diagram illustrating a main portion around the branch pawl  204  of the sheet processing apparatus  201 .  FIG. 4  illustrates a state in which the branch pawl  204  forwards the sheet along the conveyance channel  240 , and  FIG. 5  illustrates switchback operation. The branch pawl  204  is pivotable in a predetermined angle range relative to a support shaft  204   b  to switch the sheet conveyance route between the conveyance channel  240  and the bifurcation channel  241 . The position of the branch pawl  204  shown in  FIG. 4  serves as a home position to forward the sheet received from the right in  FIG. 4  to the downstream side without interfering it. A spring  251  constantly and elastically biases the branch pawl  204  counterclockwise in  FIG. 4 . 
     The spring  251  is hooked to a lever  204   a  to which a plunger of the solenoid  250  is connected. It is to be noted that the sheet can be kept clamped inside the bifurcation channel  241  when the branch pawl  204  returns to the position shown in  FIG. 4  after the sheet is transported to the branch pawl  204  in the state shown in  FIG. 5 . The conveyance route can be switched by turning on and off the solenoid  250 . Specifically, as the solenoid  250  turns on, the branch pawl  204  rotates in the direction indicated by arrow R 1  shown in  FIG. 5 , blocking the conveyance channel  240  and opening the bifurcation channel  241 . Thus, the sheet is led to the bifurcation channel  241 . 
       FIGS. 6A through 14B  illustrate online binding operation performed by the binding device  210  of the sheet processing apparatus  201 . 
     Among  FIGS. 6A through 14B , the drawings given number with subscript “A” are plan views, and drawings given number with subscript “B” are front views. Additionally, the term “online binding” means that, after the image forming apparatus  101  forms images on the sheets, the sheets are consecutively received by the sheet processing apparatus  201  disposed at the discharge port of the image forming apparatus  101 , aligned, and bound thereby. By contrast, the term “independent binding” and “offline binding” mean that the binding device  210  of the sheet processing apparatus  201  binds sheets independently from the image forming apparatus  101 , and the sheets thus bound are not limited to those outputs from the image forming apparatus  101 . Offline binding is not consecutive with image formation by the image forming apparatus  101 . 
       FIGS. 6A and 6B  illustrate the sheet processing apparatus  201  being in an initial stage of online binding. Referring to  FIGS. 6A and 6B , when the image forming apparatus  101  starts outputting sheets, the respective components of the sheet processing apparatus  201  move to their home positions, thus completing the initial stage. 
       FIGS. 7A and 7B  illustrates a state immediately after a first sheet P 1  output from the image forming apparatus  101  is received in the sheet processing apparatus  201 . Before the first sheet P 1  is received by the sheet processing apparatus  201 , the CPU of the sheet processing apparatus  201  obtains sheet processing data such as processing type and sheet data (sheet-related variables) and enters a standby state for receiving sheets according to the data. 
     The processing types include straight transport, shifted discharge, and binding. For the straight transport, the entrance rollers  203  and the discharge rollers  205  start rotating in the sheet conveyance direction in the standby state, and the first sheet P 1  through a last sheet Pn are transported sequentially. After the last sheet Pn is discharged, the entrance rollers  203  and the discharge rollers  205  stop. It is to be noted that “n” is an integer equal to greater than “2”. 
     For the shifted discharge, the entrance rollers  203  and the discharge rollers  205  start rotating in the sheet conveyance direction in the standby state. In the shifted discharge, after the trailing end of the first sheet P 1  exits from the entrance rollers  203 , the shift cam  207  rotates a predetermined amount, and the discharge rollers  205  move in the axial direction. At that time, the first sheet P 1  moves together with the discharge rollers  205 . After the first sheet P 1  is discharged, the shift cam  207  rotates to the home position and is prepared for the subsequent sheet. This shifting operation is repeated until the last sheet Pn of that copy (a bundle) is discharged. Thus, a bundle of sheets, to be bound into a sheet bundle  272 , is stacked, shifted to one side. When a first sheet P 1  of a subsequent copy is received, the shift cam  207  rotates in the direction reverse to the direction for the previous copy. 
     For binding, in the standby state, the entrance rollers  203  are motionless, and the discharge rollers  205  start rotating in the sheet conveyance direction. Additionally, the binding device  210  moves to a standby position withdrawn a predetermined amount from the sheet width and goes standby. In this case, the entrance rollers  203  also serve as a pair of registration rollers. Specifically, the first sheet P 1  is received in the sheet processing apparatus  201 . Then, the leading end of the sheet is detected by the entry detector  202  and gets stuck in the nip between the entrance rollers  203 . Further, with the leading end thereof stuck in the entrance rollers  203 , the first sheet P 1  is transported by the discharge rollers  102  of the image forming apparatus  101  by an amount to cause slackening. Subsequently, the entrance rollers  203  start rotating. Thus, skew of the first sheet P 1  is corrected.  FIGS. 7A and 7B  illustrate this state. 
       FIGS. 8A and 8B  illustrates a state in which the trailing end of the sheet is released from the nip between the entrance rollers  203  and gets beyond the bifurcation channel  241 . The conveyance amount of the first sheet P 1  is measured based on the detection of the trailing end of the sheet by the entry detector  202 , and thus the CPU  201 - 1  of the sheet processing apparatus  201  recognizes the position of the first sheet P 1 . 
     After the trailing end of the sheet passes by the nip between the entrance rollers  203 , the entrance rollers  203  stop rotating to receive the second sheet P 2 . Simultaneously, the shift cam  207  rotates in the direction indicated by arrow R 4  shown in  FIG. 8A  (clockwise in  FIG. 8A ). The discharge rollers  205  start moving in the axial direction with the first sheet P 1  retained in the nip thereof. Thus, the first sheet P 1  is transported while being moved obliquely as indicated by arrow D 1  in  FIG. 8A , obliquely to the sheet conveyance direction. Subsequently, when the sheet end detector  220 , disposed adjacent to or incorporated in the binding device  210 , detects the lateral end of the sheet P, the shift cam  207  stops and rotates in reverse. Then, the shift cam  207  stops in a state in which the sheet end detector  220  does not detect the presence of the sheet P. When the trailing end of the sheet P reaches a predetermined position beyond a leading end of the branch pawl  204 , the discharge rollers  205  stop. 
       FIGS. 9A and 9B  illustrate the switchback operation for changing the conveyance route in which the sheet P 1  is transported. Subsequent to the state shown in  FIGS. 8A and 8B , the branch pawl  204  is rotated in the direction indicated by arrow R 5  shown in  FIG. 9B  to switch the conveyance route to the bifurcation channel  241 , after which the discharge rollers  205  are rotated in reverse. With this operation, the first sheet P 1  is switchbacked in the direction indicated by arrow D 2  (hereinafter “direction D 2 ”), and the trailing end of the first sheet P 1  enters the bifurcation channel  241 . Further, the trailing end of the sheet contacts the contact face  242  and is aligned with reference to the contact face  242 . When the first sheet P 1  is thus aligned, the discharge rollers  205  stop. At that time, the discharge rollers  205  slip as the trailing end of the first sheet P 1  contacts the contact face  242  so as not to apply conveyance force thereto. In other words, the discharge rollers  205  no longer buckle the first sheet P 1  after the trailing end of the switchbacked first sheet P 1  is aligned by the contact face  242 . 
       FIGS. 10A and 10B  illustrate a state in which the first sheet P 1  is retained in the bifurcation channel  241 , and the second sheet P 2  is received in the sheet processing apparatus  201 . After the preceding first sheet P 1  is aligned by the contact face  242 , the branch pawl  204  rotates in the direction indicated by arrow R 6  shown in  FIG. 10B . With this operation, a lower face  204   c  (hereinafter “pressing face  204   c ”) of the branch pawl  204  presses the trailing end of the sheet, which is positioned in the bifurcation channel  241 , against a lower face of the bifurcation channel  241  to keep the first sheet P 1  from moving. When the second sheet P 2  is received from the image forming apparatus  101 , the entrance rollers  203  correct skew thereof similarly to the first sheet P 1 . Subsequently, the entrance rollers  203  and the discharge rollers  205  start rotating in the sheet conveyance direction simultaneously. 
       FIGS. 11A and 11B  illustrate a state in which the second sheet P 2  is received in the sheet processing apparatus  201 . After the state shown in  FIGS. 10A and 10B , as the subsequent sheets P 3  through Pn are transported from the image forming apparatus  101 , operations shown in  FIGS. 10A through 11B  are executed to sequentially transport the sheets P to a predetermined position and align the sheets P there. Thus, a sheet bundle  272  is stacked in the conveyance channel  240 . 
       FIGS. 12A and 12B  illustrate a state in which the last sheet Pn is aligned with the preceding sheets P, forming the sheet bundle  272 . After the last sheet Pn is aligned and the sheet bundle  272  is formed, the discharge rollers  205  are rotated a predetermined amount in the sheet conveyance direction. This operation can eliminate the slackening of the sheet P caused when the trailing end of the sheet P contacts the contact face  242 . Subsequently, the branch pawl  204  rotates in the direction indicated by arrow R 5  to disengage the pressing face  204   c  from the bifurcation channel  241 , thereby canceling the pressure applied to the sheet bundle  272 . Thus, the sheet bundle  272  is released from the branch pawl  204  and can be transported by the discharge rollers  205 . 
       FIGS. 13A and 13B  illustrate binding operation. After the state shown in  FIGS. 12A and 12B , the discharge rollers  205  rotate in the sheet conveyance direction and stop when a binding position in the sheet bundle  272  reaches the pair of tooth forms  261  of the binding device  210 . Thus, the binding position in the sheet bundle  272  is aligned with the position of the tooth forms  261  in the sheet conveyance direction. Additionally, the binding device  210  is moved in the direction indicated by arrow D 3  shown in  FIG. 13A  (hereinafter “direction D 3  or sheet width direction”), perpendicular to the sheet conveyance direction, until the pair of tooth forms  261  is aligned with the binding position in the sheet bundle  272  in the sheet width direction. 
     Accordingly, the binding position in the sheet bundle  272  is aligned with the tooth forms  261  in the sheet conveyance direction as well as the width direction. Then, the branch pawl  204  rotates in the direction indicated by arrow R 6  shown in  FIG. 13B  and returns to the state for receiving the subsequent sheet P. Subsequently, the drive motor  265  is turned on, and the pair of tooth forms  261  squeezes the sheet bundle  272 , thereby binding the sheet bundle  272  (i.e., clamp binding or squeezing and binding). It is to be noted that, although the description above concerns the binding device  210  employing clamp binding, other type of binding, for example, half blanking, lancing, and bending and inserting can be used instead. 
       FIGS. 14A and 14B  illustrate a state in which the sheet bundle  272  is discharged. After the sheet bundle  272  is bound together as shown in  FIGS. 13A and 13B , the discharge rollers  205  rotate to discharge the sheet bundle  272 . After the sheet bundle  272  is discharged, the shift cam  207  rotates in the direction indicated by arrow R 7  shown in  FIG. 14A  to the home position (shown in  FIG. 6A ). In parallel to this operation, the binding device  210  moves in the direction indicated by arrow D 4  shown in  FIG. 14A  to the home position shown in  FIGS. 6A and 6B . Thus, alignment and binding of a single copy of sheets (a bundle of sheets) is completed. The operations shown in  FIGS. 6A through 14B  are repeated for binding subsequent copies, if any. 
     Before a distinctive feature of the present embodiment is described, a binding device according to a comparative example is described. 
       FIGS. 15 and 16  illustrate a binding device  210 X according to a comparative example. Referring to  FIGS. 15 and 16 , a binding device  210 X includes a pair of tooth forms  261 X, a pressure lever  262 , a group of links  263 , a drive motor  265 , an eccentric cam  266 , and a cam home position (HP) detector  267 . The pair of tooth forms  261 X are disposed vertically in pair and shaped to engage each other. The pair of tooth forms  261 X is positioned at an output end of the group of links  263  combined together, and the pressure lever  262  is positioned at an input end (driving end) of the group of links  263 . The tooth forms  261 X engage and are disengaged from each other as the pressure lever  262  applies pressure to and release the pressure. 
     The pressure lever  262  is rotated by the eccentric cam  266 . The drive motor  265  drives the eccentric cam  266 , and the rotational position thereof is controlled with reference to detection by the cam HP detector  267 . The rotational position of the eccentric cam  266  defines the distance from a rotation axis  266   a  and to a cam surface thereof, based on which the pressing amount by the pressure lever  262  is determined. The home position of the eccentric cam  266  is set to a position at which a feeler  266   b  provided to the eccentric cam  266  is detected by the cam HP detector  267 . As shown in  FIG. 15 , when the eccentric cam  266  is at the home position, the tooth forms  261 X are disengaged from each other. In this state, binding is not feasible and sheets can be received in the binding device  210 . 
     For binding sheets, the sheets are inserted between the tooth forms  261 X at the position shown in  FIG. 15 , and then the drive motor  265  rotates. When the drive motor  265  starts rotating, the eccentric cam  266  rotates in the direction indicated by arrow R 2  shown in  FIG. 16 . As the eccentric cam  266  rotates, the cam surface thereof shifts, and the pressure lever  262  rotates in the direction indicated by arrow R 3  shown in  FIG. 16 . The force of rotation increases in strength through the group of links  263  using leverage and is transmitted to the pair of tooth forms  261 X at the output end. 
     When the eccentric cam  266  rotates a predetermined amount, the upper and lower tooth forms  261 X engage each other, thus squeezing the sheets interposed therebetween. The squeezed sheets deform, and fibers of adjacent sheets tangle each other. Subsequently; the drive motor  265  rotates in reverse and stops in response to a detection result generated by the cam HP detector  267 . Then, the upper and lower tooth forms  261 X return to the state shown in  FIG. 15  and become capable of transporting the sheets. The pressure lever  262  has a capability of spring and can deform to let an excessive load out when the excessive load is applied thereto. 
     As described above, in the present embodiment, the sheet bundle is squeezed and clamped to bind the sheet bundle. Conventionally, a force of about 1000 N is applied to squeeze and bond together the sheets. It the configuration shown in  FIGS. 15 and 16 , such a force can be given by the drive motor  265 . However, driving the sheet binding device with a large driving force is not desirable from a viewpoint of energy saving. 
     In view of the foregoing, the sheet processing apparatus according to the present embodiment includes the sheet binding device in which a pair of squeezing members includes projections and recesses, and a pressure applying unit applies pressure to the squeezing members in a direction of thickness of a sheet bundle interposed between the squeezing members. The pressure applying unit generates a pressing force between the squeezing members such that the pressing force increases as the relative distance between the squeezing members decreases. According to the present embodiment, an energy-saving sheet binding device driven by a reduced driving force, saving energy, can be attained. It is to be noted that other aims, configurations, and effects of the present embodiment are also given in the description below. 
       FIG. 17  is a partial diagram of another comparative sheet binding device  210 X that employs a crank mechanism  268  as a pressure applying unit to press a pair of tooth forms  261 X. 
     In  FIG. 17 , the tooth forms  261 X (i.e., first and second forms  261   a  and  261   b ) are disposed facing each other, and the sheet bundle is inserted therebetween. The crank mechanism  238  includes a connecting rod  268   c , and an end of the connecting rod  268   c  is rotatably or pivotably connected via a joint  268   d  to a side of the second tooth form  261   b  opposite a teeth face  261   b   1 . The other end (i.e., a base end) of the connecting rod  268   c  is connected to a rotary shaft  268   a  of a drive motor  268   m  serving as a drive source. An end of a rotary member  268   b  that rotates together with the rotary shaft  268   a  serves as a joint  268   e  to which the connecting rod  268   c  is connected rotatably. 
     When the drive motor  268   m  rotates in the direction indicated by arrow θ shown in  FIG. 17 , the second tooth form  261   b  can be moved back and forth along a line  268   f  by the connecting rod  268   c , guided by a guide member. The sheet bundle is disposed in a clearance L between the first and second tooth forms  261   a  and  261   b  and squeezed as the second tooth form  261   b  reciprocates. The rotary shaft  268   a  of the drive motor  268   m  is positioned on an extension line of the linear movement of the straight line (extension of the line  2680 . A point F 1  on which an action of the connecting rod  268   c  is exerted is also positioned on the extension line of the line  268   f.    
     When such a pressure applying unit is used, the sheet bundle is squeezed prior to being bound, and accordingly this method can be also called “squeezing and clamp binding”. 
       FIG. 18  is partial diagram of the binding device  210  according to the present embodiment, in which a squeezing and clamping unit  269  serves as a pressure applying unit. 
     Referring to  FIG. 18 , the squeezing and clamping unit  269  includes a link unit  270  and a crank unit  271  serving as a link activation unit. The link unit  270  and the crank unit  271  are rotatably connected to each other via a first joint  269   a.    
     The link unit  270  includes first and second connecting rods  270   a  and  270   b , and a first end of each of the first and second connecting rods  270   a  and  270   b  is connected to the first joint  269   a . A second end the first connecting rod  270   a  is connected to a second joint  270   c , and that of the second connecting rod  270   b  is connected to a third joint  270   d . The second joint  270   c  is disposed on a back side of the second tooth form  261   b , and the third joint  270   d  is fixed to a stationary member  270   f  not to move. The stationary member  270   f  is on a straight line  270   e  that is similar to the line  268   f  in  FIG. 17 . The straight line  270   e  can be a locus along which the second tooth form  261   b  moves, guided by a guide. 
     The crank unit  271  includes a third connecting rod  271   a , a drive motor  271   m , and a rotary member  271   c  fixed to a rotary shaft  271   b  of the drive motor  271   m  movably together with the rotary shaft  271   b , which are respectively similar to the connecting rod  268   c , the drive motor  268   m , and the rotary member  268   b  shown in  FIG. 17 . An end of the third connecting rod  271   a  is rotatably connected to an end of the rotary member  271   c  as well as a fourth joint  271   d . The other end thereof is rotatably connected to the first joint  269   a . In other words, one end of each of the first, second, and third connecting rods  270   a ,  270   b , and  271   a  is connected to the first joint  269   a . It is to be noted that the position of the rotary shaft  271   b  of the drive motor  271   m  is fixed. 
     Additionally, the first and second connecting rods  270   a  and  270   b  are connected together such that an angle α therebetween (via the first joint  269   a ) is not 180 degrees, that is, the first and second connecting rods  270   a  and  270   b  are not aligned with each other, when the second tooth form  261   b  is shifted to the first tooth form  261   a  at a maximum. Links connected in this manner may be called “doglegged links or elbow-shaped links”. 
     In the present embodiment, the doglegged link is constructed with a link unit including the first and second connecting rods  270   a  and  270   b  and the first joint  269   a  that connects the first ends of them rotatably. In this configuration, the third connecting rod  271   a  is connected to the first joint  269   a , and the first joint  269   a  is moved in the direction indicated by arrow D 1 . (hereinafter “direction D 1 ”) and the opposite direction by the rotary member  271   c  driven by the drive motor  271   m . The respective members are disposed such that a dead point of the first joint  269   a  in the direction D 1  at that time is positioned immediate short of the straight line  270   e.    
     This configuration can prevent the first and second connecting rods  270   a  and  270   b  from being aligned with each other, and a maximum pressing force can be applied to the tooth forms  261  immediately before the first and second connecting rods  270   a  and  270   b  are aligned with each other. With this configuration, the first joint  269   a  can constantly have a vertical angle, and the link unit is doglegged, and thus called a doglegged link. 
     In the squeezing and clamping unit  269  configured as described above, as the drive motor  271   m  rotates in the direction θ, the third connecting rod  271   a  pushes the first joint  269   a  in the direction D 1 . As the first joint  269   a  moves in the direction D 1 , the angle α between the first and second connecting rods  270   a  and  270   b  increases. By contrast, the second tooth form  261   b  moves in the direction indicated by arrow D 2  (hereinafter “direction D 2 ”) since the third joint  270   d  is stationary. Then, while moving to the first tooth form  261   a  with the sheet bundle interposed in the clearance L therebetween, the second tooth form  261   b  applies pressing force to the sheet bundle. Thus, clamp binding is executed. It is to be noted that reference character F 2  represents a point of action exerted on the second tooth form  261   b  by the first connecting rod  270   a , and the point F 2  is positioned on the extension line of the straight line  270   e.    
     In the present embodiment, the link unit  270  is configured to move the second tooth form  261   b  with the doglegged link, and the crank unit  271  transmits a driving force to the link unit  270 . As described above, when or almost when the first and second connecting rods  270   a  and  270   b  are fully stretched, the doglegged link can generate a strong force. Therefore, doglegged links are often used in jacks for vehicles. Therefore, the relation between the link unit  270  and the crank unit  271  is set such that, in driving the link unit  270 , maximum force can be generated at a preferred timing using the crank unit  271 . 
       FIG. 19  is a graph illustrating relations between the size of the clearance L between the tooth forms  261   a  and  261   b  and force applied thereto. In  FIG. 19 , the comparative configuration shown in  FIG. 17 , using the crank mechanism alone, is compared with the present embodiment shown in  FIG. 18 , using the link unit and the crank mechanism in combination. 
     In  FIG. 19 , the abscissa represents the size of the clearance L in millimeters (mm) between the tooth forms  261  (also “distance L”), and the ordinate represents the pressing force F in newton (N) applied to the tooth forms  261  when a bundle of five sheets is squeezed and bound. Additionally, at a point B, the distance between the tooth forms  261  equals to the thickness of the sheet bundle, and a clamp biding is started (also “clamp biding start point B”). Further, reference numeral  300  in  FIG. 19  represents a clamp binding range, starting from the point B, during which clamp binding is executed. In  FIG. 19 , the clamp binding range  300  of the distance L is from zero to about 0.5 mm (0 mm&lt;L≦0.5 mm), and a force of about 1000 N is applied to the tooth forms  261 . Reference numeral  400  represents a squeezing range (0.5 mm&lt;L≦1.5 mm), and a force of about 100 N is applied to the tooth forms  261 . At a point A, squeezing is started (also “squeezing start point A), and the squeezing start point A (1.5 mm in  FIG. 19 ) can be determined according to the height of the projections of the tooth forms  261 . An optimum size can be selected experimentally, and the squeezing start point A can be preset according to the thickness of the sheet bundle. 
     It can be known from  FIG. 19  that the configuration shown in  FIG. 18  can generate a necessary amount of force efficiently and more timely. More specifically, from the force output properties shown in  FIG. 19 , the configuration shown in  FIG. 18  in which the link unit  270  is combined with the crank unit  271  can squeeze, clamp, and bind the sheet bundle more efficiently with a smaller driving force. Additionally, at the clamp biding start point B, the configuration shown in  FIG. 18  can generate a greater force than the configuration shown in  FIG. 17  using the crank mechanism  268  alone, and the force pressing the sheet bundle can increase more gradually. Thus, the configuration shown in  FIG. 18  is advantageous in that damage to the sheet bundle is smaller in addition to the increased binding capability. 
     As described above, when the mechanism to move the second tooth form  261   b  is constructed of two or more of links or cams (a link unit and a crank mechanism in the configuration shown in  FIG. 18 ), the amount of force for clamp binding can be generated timely, and the necessary driving force can be reduced. Thus, energy and resource for sheet binding can be saved. 
     Additionally, it can be also known from  FIG. 19  that, in the configuration shown in  FIG. 18 , the pressing force can be applied earlier, that is, during the squeezing range  400  prior to clamp binding, and increased gradually. Since the pressing force gradually increases during transition from the squeezing range  400  to the clamp binding range  300  and further during the clamp binding range  300 , the risk of damage to binding portions of the sheet bundle can be reduced. Thus, quality of sheet binding can improve. 
     It is to be noted that, although the link unit  270  is driven by the crank unit  271  in the configuration shown in  FIG. 18 , alternatively, the link unit  270  may be driven by a cam mechanism. 
       FIG. 20  is a block diagram that schematically illustrates a control configuration of the system including the image forming apparatus  101  and the sheet processing apparatus  201 . The control circuit of the sheet processing apparatus  201  includes, for example, a micro computer including the CPU  201 - 1  and an input/output (I/O) interface  201 - 2 . The CPU  201 - 1  performs various types of control according to signals input from either a CPU of the image forming apparatus  101  or a control panel  101 - 1 , or signals received via the I/O interface  101 - 2  from respective switches as well as sensor groups  113 D and  130  including various sensors and detectors. The control circuit further includes a pulse width module (PWM) generator  112 C. Additionally, the CPU  201 - 1  controls a solenoid  113 A, a direct current (DC) motor  113 B, and stepping motors  112 B and  113 C via a driver  111 A and motor drivers  111 B,  111 C, and  112 A. The CPU  201 - 1  acquires data from the detectors in the apparatus via the interface  201 - 2 . Further, according to what is controlled or sensors, the CPU  201 - 1  controls the motors  112 B,  113 B, and  113 C and acquires data from the sensors via the I/O interface  101 - 2 . The CPU  201 - 1  reads out program codes stored in a read only memory (ROM), and performs various types of control based on the programs defined by the program codes using a random access memory (RAM) as a work area and data buffer. The control circuit can further include a nonvolatile storage device for storing data used for control operations. 
     Moreover, the sheet processing apparatus  201  may be controlled according to instructions or data transmitted from the CPU of the image forming apparatus  101 . Users can input instructions via the control panel  101 - 1  of the image forming apparatus  101 . Then, the image forming apparatus  101  can transmit operation signals input via the control panel  101 - 1  to the sheet processing apparatus  201 , and the state or functions of the sheet processing apparatus  201  can be reported to the user or operator on the control panel  101 - 1 . 
     As described above, the present embodiment can attain the following effects. 
     1) The sheet binding device  210  according to the above-described embodiment includes first and second tooth forms  261   a  and  261   b , having projections and recesses engaging each other, and the squeezing and clamping unit  269  to apply pressing force to the first and second tooth forms  261   a  and  261   b  in the thickness direction of the sheet bundle sandwiched between the first and second tooth forms  261   a  and  261   b , thereby squeezing and binding the sheet bundle. The squeezing and clamping unit  269  is configured to apply a greater force to the first and second tooth forms  261   a  and  261   b  as the relative distance therebetween decreases. With this configuration, without increasing the strength of the driving force, the sheet bundle can be squeezed and bonded or bound together reliably. Consequently, energy required for sheet binding can be reduced. 
     2) The squeezing and clamping unit  269  is constructed with at least two displacement units such as the link unit  270  and the crank unit  271  serving as a link activation unit to activate the link unit  270 . Even if the driving force of the crank unit  271  is constant, the pressing force generated between the first and second tooth forms  261   a  and  261   b  can be increased using the link unit  270  as the distance therebetween decreases. With this configuration, the sheet bundle can be squeezed and bonded or bound together reliably with a reduced driving force. 
     3) Since the squeezing and clamping unit  269  can set the strength of pressing force in accordance squeezing steps of the sheet bundle  272  by the first and second tooth forms  261   a  and  261   b , driving force is not wasted. It is to be noted that squeezing operation (i.e., primary squeezing) of the squeezing and clamping unit  269  is executed from the point A in  FIG. 19 , at which the distance between the first and second tooth forms  261   a  and  261   b  reaches the thickness of the sheet bundle  272 , to the point B at which clamp binding is started, and the clamp binding operation is executed from the point B to the point C at which sheet binding operation completes (hereinafter “binding completion point C”). 
     4) According to the above-described embodiment, pressing force is applied to the tooth forms  261  in multiple steps. That is, in primary squeezing (i.e., the squeezing range  400 ) during which the distance L between the first and second tooth forms  261   a  and  261 . b  is reduced from the squeezing start point A, determined according to the thickness of the sheet bundle, to the clamp biding start point B, a first pressing force is applied to the tooth forms  261  for squeezing the sheet bundle. Then, a second pressing force greater than the first pressing force is applied to the tooth forms  261  in secondary squeezing (i.e., clamp binding range  300 ) narrower than the first range. With the two-step squeezing, clamp binding can be efficient. 
     5) A small driving force exerted by the drive motor  271   m  that drives the third connecting rod  271   a  can be converted into a greater force, namely, the second pressing force and transmitted to the link unit  270  including the first and second connecting rods  270   a  and  270   b . With this configuration, the second pressing force at the binding completion point C can be increased to about 1000 N or greater, in the configuration shown in  FIG. 19 . 
     6) The squeezing and clamping unit  269  includes the link unit  270  and the crank unit  271  serving as the link activation. The link unit  270  includes the first and second connecting rods  270   a  and  270   b  and the first joint  269   a  capable of rotatably connecting together the first ends of the first and second connecting rods  270   a  and  270   b . The crank unit  271  includes the third connecting rod  271   a  driven by the drive motor  271   m . The second end of the first connecting rod  270   a  is connected to the movable second tooth form  261   b , and the second end of the second connecting rod  270   b  is rotatably connected to the third joint  270   d  provided to the stationary member  270   f . The first, second, and third connecting rods  270   a ,  270   b , and  271   a  are arranged such that, when the third connecting rod  271   a  pushes the first joint  269   a  in the direction to stretch the first and second connecting rods  270   a  and  270   b , the first joint  269   a  reaches the dead point immediately before the first and second connecting rods  270   a  and  270   b  are aligned with each other into single straight line. Accordingly, even if the driving force of the drive motor  271   m  is small, greater driving force can be attained. 
     7) The above-described embodiment concerns the sheet binding method in which the sheet bundle is inserted between the first and second tooth forms  261   a  and  261 . b  having projections and recesses shaped to engage each other, the squeezing and clamping unit  269  presses the first and second tooth forms  261   a  and  261   b  to squeeze the sheet bundle therebetween in the thickness direction of the sheet bundle, and thereby the sheet bundle is clamped and bound. Specifically, the method includes the step of squeezing the sheet bundle (i.e., the squeezing range  400 ) starting from the predetermined squeezing start point A to the clamp biding start point B at which the sheets contact closely with each other and the step of clamping the sheet bundle (i.e., the clamp binding range  300 ) starting from the clamp biding start point B to the binding completion point C. The step of squeezing corresponds to the first range of the distance L from the squeezing start point A to the clamp biding start point B, during which the first pressing force is applied to the tooth forms  261 . The step of clamping corresponds to the second range of the distance L, narrower than the first range, from the point B to the point C, during which the second pressing force greater than the first pressing force is applied to the tooth forms  261 . Thus, sheet binding is executed in two steps efficiently, saving energy. 
     It is to be noted that the present invention is not limited to the specific embodiments described above, and numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that, the disclosure of this patent specification may be practiced otherwise than as specifically described herein, and such variations, modifications, alternatives are within the technical scope of the appended claims.