Patent Publication Number: US-2015076759-A1

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

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2013-190376 filed in Japan on Sep. 13, 2013. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet binding device, a sheet processing apparatus, and an image forming system. 
     2. Description of the Related Art 
     Image forming systems of a conventionally known type include a sheet processing apparatus configured to bind a sheaf of sheets, on which an image is formed by an image forming apparatus, using a binding tool which is a binding unit. 
     International publication No. WO 2009/110298 discloses a sheet processing apparatus including a crimping-binding-type sheet binding device that binds a sheaf of sheets without using a metal staple, but by crimping the sheets by strongly engage a pair of crimping toothed jaws, which are crimping members each having a shape with protrusions and recesses, with the sheet sheaf therebetween, thereby entangling fibers of the sheets. Binding a sheet sheaf by crimping binding without using a metal staple allows saving trouble of removing the metal staple from the sheet sheaf when discarding the sheet sheaf or putting the sheet sheaf into a shredder. 
       FIG. 24  is a diagram illustrating a pair of crimping toothed jaws according to international publication No. WO 2009/110298.  FIG. 24  is a view of the crimping toothed jaws as viewed from a direction parallel to a sheet surface of a sheet sheaf and orthogonal to a direction along which protrusions and recesses of the crimping toothed jaws are arranged. 
     As illustrated in  FIG. 24 , top portions of protrusions  172   a  of a lower crimping toothed jaw  261   b  are faces parallel to the sheet surface of the sheet sheaf. Recesses  172   b  of the lower crimping toothed jaw are substantially V-grooves in shape. Protrusions  171   a  and recesses  171   b  of an upper crimping toothed jaw  261   a  are substantially V-grooves and inverted-V-grooves, respectively, in shape. 
     As illustrated in  FIG. 24 , when the pair of crimping toothed jaws is engaged, the protrusions  171   a  of the upper crimping toothed jaw  261   a  are in contact with the recesses  172   b  of the lower crimping toothed jaw  261   b  with a clearance S left between the recesses  171   b  of the upper crimping toothed jaw  261   a  and the protrusions  172   a  of the lower crimping toothed jaw  261   b.    
     According to international publication No. WO 2009/110298, when a pressure is applied to paper from above and below, the clearance S allows a load applied to fibers of the paper to be relieved as a whole and, in some parts, prevents the fibers from being extended beyond their limit, thereby allowing the fibers to be entangled without causing paper breakage. Thus, according to the disclosure, binding strength is maintained without breakage of the entire sheet. 
     However, in the configuration described in international publication No. WO 2009/110298, the protrusions  171   a  of the upper crimping toothed jaw  261   a  are V-shaped and have pointed top portions. This leads to a disadvantage that a pressure concentrates onto portions where the sheet sheaf contacts the top portions of the protrusions  171   a  of the upper crimping toothed jaw  261   a  and breaks sheet(s) of the sheaf. 
     The configuration described in international publication No. WO 2009/110298 is also disadvantageous in that breakage of sheet(s) of the sheaf can occur. This is because the clearance is formed only between the recesses  171   b  of the upper crimping toothed jaw  261   a  and the protrusions  172   a  of the lower crimping toothed jaw  261   b , fibers are not sufficiently prevented from being extended beyond their limit. 
     In light of the foregoing, there is a need for a sheet binding device, a sheet processing apparatus, and an image forming system capable of reducing damage to a sheet(s). 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     A crimping-binding-type sheet binding device includes: a pair of crimping members each including alternately-arranged multiple recesses and multiple protrusions, the protrusions and the recesses of one of the pair of crimping members being arranged in a direction parallel to a direction in which the protrusions and the recesses of other one of the pair of crimping members are arranged, to bind a sheaf of sheets by fitting the recesses and the protrusions with the sheet sheaf interposed therebetween. The pair of crimping members are configured in such a manner that top portions of the protrusions of each of the pair of crimping members are faces parallel to a sheet surface of the sheet sheaf, side faces of the protrusions of each of the pair of crimping members are slanted faces slanted relative to the sheet surface, and when the recesses and the protrusions of the pair of crimping members are fitted, the slanted faces of the one of the pair of crimping members and the slanted faces of the other one of the pair of crimping members are in contact with each other with a clearance left both between the top portions of the protrusions of the one of the crimping members and bottom portions of the recesses of the other one of the crimping members and between the top portions of the protrusions of the other one of the crimping members and bottom portions of the recesses of the one of the crimping members. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are schematic configuration diagrams each illustrating an example of an overall configuration of an image forming system according to an embodiment of the present invention; 
         FIG. 2  is a schematic configuration diagram illustrating an example configuration of an image forming apparatus of the image forming system according to the embodiment; 
         FIG. 3  is a plan view illustrating an example configuration of a sheet processing apparatus of the image forming system according to the embodiment; 
         FIG. 4  is a front view of the sheet processing apparatus illustrated in  FIG. 3 ; 
         FIG. 5  is an explanatory diagram illustrating a bifurcating claw, which selectively directs a sheet conveyed into the sheet processing apparatus, at a home position; 
         FIG. 6  is an explanatory diagram illustrating the bifurcating claw at a position for directing a sheet conveyed into the sheet processing apparatus to a branch path; 
         FIG. 7  is an explanatory diagram illustrating an example of a binding tool and a drive mechanism of the binding tool in a state where toothed jaws are open; 
         FIG. 8  is an explanatory diagram illustrating the example of the binding tool and the drive mechanism of the binding tool in a state where the toothed jaws are closed; 
         FIGS. 9A and 9B  are a plan view and a front view, respectively, showing the internal of the sheet processing apparatus at completion of initialization; 
         FIGS. 10A and 10B  are a plan view and a front view, respectively, showing the internal of the sheet processing apparatus receiving a sheet thereinto; 
         FIGS. 11A and 11B  are a plan view and a front view, respectively, showing the internal of the sheet processing apparatus positioning the sheet in a sheet width direction; 
         FIGS. 12A and 12B  are a plan view and a front view, respectively, showing the internal of the sheet processing apparatus positioning a trailing end of the sheet; 
         FIGS. 13A and 13B  are a plan view and a front view, respectively, showing the internal of the sheet processing apparatus receiving a subsequent sheet thereinto; 
         FIGS. 14A and 14B  are a plan view and a front view, respectively, showing the internal of the sheet processing apparatus receiving a next subsequent sheet thereinto; 
         FIGS. 15A and 15B  are a plan view and a front view, respectively, showing the internal of the sheet processing apparatus in a state where a sheet sheaf has been aligned but a binding process is not started yet; 
         FIGS. 16A and 16B  are a plan view and a front view, respectively, showing the internal of the sheet processing apparatus at start of discharging the sheet sheaf having undergone the binding process; 
         FIGS. 17A and 17B  are a plan view and a front view, respectively, showing the internal of the sheet processing apparatus discharging the sheet sheaf having undergone the binding process; 
         FIGS. 18A to 18D  are explanatory diagrams of a crimping binding method; 
         FIG. 19  is a plan view of a lower crimping toothed jaw; 
         FIG. 20  is a diagram of the lower crimping toothed jaw as viewed in a direction A of  FIG. 19 ; 
         FIG. 21  is a diagram of the lower crimping toothed jaw as viewed in a direction B of  FIG. 19 ; 
         FIGS. 22A and 22B  are diagrams describing behavior of the sheet sheaf during crimping binding; 
         FIG. 23  is a graph illustrating a result of a verification test; and 
         FIG. 24  is a diagram illustrating a conventional pair of crimping toothed jaws. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention are described below with reference to the accompanying drawings. 
       FIGS. 1A and 1B  are schematic configuration diagrams each illustrating an example of an overall configuration of an image forming system according to an embodiment of the present invention.  FIG. 1A  illustrates an image forming system  100  having an example configuration in which a sheet processing apparatus  201  is incorporated in an image forming apparatus  101  serving as an image forming device which forms an image on a sheet of paper based on an input image.  FIG. 1B  illustrate another example configuration of the image forming system  100  in which the sheet processing apparatus  201  is connected to the image forming apparatus  101 . 
     The image forming system  100  according to the embodiment electrophotographically forms an image, which is a toner image, on a sheet. Alternatively, the image forming system  100  may form an image by other method such as an inkjet method. In the embodiment, an image forming apparatus configured as the image forming apparatus  101  combined with the sheet processing apparatus  201  is described, but not limited thereto. The present invention is applicable to an image forming apparatus configured as the image forming apparatus  101  in which the sheet processing apparatus  201  is built. 
     The present invention is also applicable to a configuration in which the sheet processing apparatus  201  is independent of the image forming apparatus  101 . When this independent design is employed, the sheet processing apparatus may include a cassette or a tray where sheets to be bound are to be placed and a tray onto which a sheet sheaf(s) is to be output. 
       FIG. 2  is a schematic configuration diagram illustrating an example configuration of the image forming system  100  according to the embodiment. 
     Referring to  FIG. 2 , the image forming system  100  is a tandem color image forming apparatus adopting an indirect transfer scheme with use of an intermediate transfer member. An image forming unit  110  serving as a toner-image forming unit is arranged at substantially center of the image forming apparatus  101 . The image forming unit  110  includes image forming stations  111 Y,  111 M,  111 C, and  111 K for four colors (Y: yellow, M: magenta, C: cyan, and K: black) arranged in a predetermined direction. Hereinafter, suffixes Y, M, C, and K are omitted as appropriate. 
     The image forming apparatus  101  further includes multiple sheet feeding trays  120  which are sheet feeding units serving as recording-medium supplying units. The sheet feeding trays  120  are arranged under the image forming unit  110 . The image forming apparatus  101  further includes a sheet-feeding conveying path (vertical conveying path)  130  for conveying a sheet, which is a recording medium, picked up from the sheet feeding tray  120  to a secondary transfer unit  140  and a fixing unit  150 . The image forming apparatus  101  further includes a branch sheet discharging path  160  for conveying a sheet, onto which an image (toner image) is fixed, to the sheet processing apparatus  201  and a duplex-printing conveying path  170  for turning a sheet, on a first side (front side) of which an image (toner image) is formed, upside down so that an image is formed on a second side (back side). 
     The image forming apparatus  101  further includes a scanner unit  180  serving as an image reading unit and an automatic document feeder (ADF)  185  serving as an original-document supplying unit. The scanner unit  180  reads an image of an original document (hereinafter, “document”), which is an image-reading subject, placed on a glass surface serving as a document table and converts the image into an electric signal. One or more sheets of the document to be read by the scanner unit  180  are placed in the ADF  185 . The ADF  185  conveys each sheet of the document to the glass surface which is at a reading position of the scanner unit  180 . 
     The image forming unit  110  includes photosensitive drums as image bearers for the respective colors (Y, M, C, and K) of the image forming stations  111 . An electrostatic charger unit, a developing unit, a primary transfer unit, a cleaning unit, and an electrostatic discharging unit are arranged around and along the outer periphery of each of the photosensitive drums. The image forming unit  110  includes an optical writing unit (not shown) serving as an exposure unit and an intermediate transfer belt  112  serving as an intermediate transfer member. The optical writing unit is arranged under the image forming stations  111  and emits light onto each of the photosensitive drums according to image data generated for each of the colors based on a reading result output from the scanner unit  180 , thereby forming electrostatic latent images. The intermediate transfer belt  112  is arranged above the image forming stations  111 . Images (toner images) formed on the photosensitive drums are transferred onto the intermediate transfer belt  112  by the primary transfer unit. 
     The intermediate transfer belt  112  is rotatably supported by multiple support rollers. A support roller  114 , which is one of the support rollers, faces a secondary transfer roller  115  in the secondary transfer unit  140  with the intermediate transfer belt  112  therebetween. The images (toner images) on the intermediate transfer belt  112  are transferred, as secondary transfer, onto a sheet in the secondary transfer unit  140 . Replaceable toner containers  116  are arranged above the intermediate transfer belt  112 . 
     Meanwhile, an image forming process performed by such an image forming apparatus configured as described above (tandem color image forming apparatus adopting the indirect transfer method) is known and does not have direct relation with the gist of the present invention. Accordingly, detailed description is omitted. 
     The sheet, onto which the image is fixed by the fixing unit  150 , is conveyed by conveying rollers  162 . A conveying direction of the sheet is switched by a conveying-path switching member  161 . Thereby, the image-fixed sheet is conveyed to one of the branch sheet discharging path  160  and the duplex-printing conveying path  170 . 
     The sheet processing apparatus  201  according to the embodiment includes a conveying-path binding mechanism as a sheet binding unit which, as postprocessing for multiple sheets including one or more image-formed sheets, binds a sheaf of the multiple sheets. The conveying-path binding mechanism includes a structure for stacking sheets and aligning the sheets on the sheet conveying path, and a binding tool serving as a binding unit which binds the stacked sheets. 
       FIG. 3  and  FIG. 4  are a plan view and a front view, respectively, illustrating an example configuration of the sheet processing apparatus  201 , which is included in the image forming system  100 , including the conveying-path binding mechanism. 
     The sheet processing apparatus  201  includes an entry sensor  202 , entry rollers  203 , a bifurcating claw (switching claw)  204 , sheet discharging rollers  205 , a shift link  206 , a shift cam  207 , a shift cam stud  208 , a shift home position (HP) sensor  209 , and a binding tool  210 . 
     The entry sensor  202  detects a leading end, a trailing end, and presence/absence of a sheet conveyed by sheet discharging rollers  102  of the image forming apparatus  101  into the sheet processing apparatus  201 . 
     The entry rollers  203  are arranged at an entry port of the sheet processing apparatus  201  and have a function of conveying a sheet into the sheet processing apparatus  201 . Furthermore, abutment skew correction can be applied to a sheet by using a nip between the entry rollers  203 . The entry rollers  203  are driven by a controllable driving source (not shown). The driving source is controlled by a controller (not shown) which controls rotation and stop of the entry rollers  203  and a conveyance amount of a sheet by the entry rollers  203 . The controller may be provided in the image forming apparatus  101 . 
     The bifurcating claw  204  is a pivotable claw provided to switch a conveying path so that a trailing end of a sheet is guided to a branch path  241 . The bifurcating claw  204  is configured to be capable of pressing the sheet against a conveyance surface of the branch path to immobilize the sheet by pressing the sheet in this manner. 
     The sheet discharging rollers  205  are arranged at an exit port of the sheet processing apparatus  201  and have a function of conveying, shifting, and discharging a sheet. The sheet discharging rollers  205  are driven by a controllable driving source (not shown). The driving source is controlled by a controller which will be described later and which controls rotation and stop of the sheet discharging rollers  205  and a conveyance amount of a sheet by the sheet discharging rollers  205 . 
     A conveying unit for conveying a sheet in the sheet processing apparatus  201  of the embodiment is made up of, for example, the entry rollers  203 , the sheet discharging rollers  205 , and the driving sources driving the rollers  203  and  205 . 
     The shift link  206  is provided at a shaft end of the sheet discharging rollers  205  and is a part which receives a moving force for the shifting function. 
     The shift cam  207  including the shift cam stud  208  is a rotatable disc-like component. Rotation of the shift cam  207  shifts the sheet discharging rollers  205 , which are connected to an elongated hole in the shift link  206  via the shift cam stud  208 . 
     The shift cam stud  208  interlocked with the elongated hole in the shift link  206  converts a circular motion of the shift cam  207  into a linear motion in an axial direction of the sheet discharging rollers  205 . 
     The shift HP sensor  209  detects a position of the shift link  206 . The detected position is defined as a home position (standby position). 
     The binding tool  210  is a tool or a device configured to bind a sheet sheaf together by a squeezing and crimping process without using a metal staple. The embodiment employs the binding tool  210  which pinches a sheaf of sheets between a pair of toothed jaws made up of an upper toothed jaw and a lower toothed jaw each having protrusions and recesses on its surface, thereby deforming the sheets and entangling fibers of the sheets. As the binding tool  210  of this type, a known binding tool disclosed in, for example, Japanese Examined Utility Model Publication No. S36-13206 can be used. A binding tool of another type may be used which binds a sheet sheaf without using a metal staple by cutting and bending a U-shaped or tongue-shaped notch in the sheet sheaf, simultaneously cutting a slit near a basal portion of the tongue, and passes a distal end portion of the tongue through the slit in a manner not to easily come out of the slit. An example of a binding tool of this type is disclosed in Japanese Examined Utility Model Publication No. S37-007208. Note that the binding unit for binding a sheet sheaf is not limited to the binding tool of the embodiment. Any binding unit which binds a sheaf of sheets by crimping the sheet sheaf to thereby entangle fibers of the sheets can be used. 
     A sheet edge sensor  220  serving as a sheet-edge detecting unit is a sensor which detects a side edge of a sheet. Sheet alignment is performed with reference to a position detected by the sheet edge sensor  220 . 
     A binding-tool HP sensor  221  is a sensor which detects a position of the binding tool  210  that is movable in a width direction crossing the sheet conveying direction. A position where, even when a sheet of a maximum size is fed, the binding tool  210  does not interfere with the sheet is set as a home position (standby position), and this position is detected by the binding-tool HP sensor  221 . 
     A binding-tool guide rail  230  guides movement of the binding tool  210  so that the binding tool  210  can move in the sheet width direction stably. 
     A conveying path  240  is a regular path for conveying and discharging a sheet conveyed into the sheet processing apparatus  201 . The branch path  241  is a conveying path provided to stack and align sheets. A sheet is conveyed backward onto the branch path  241  in such a manner that a trailing end of the sheet enters the branch path  241  first. 
     A binding tray (staple tray)  243  is a sheet tray serving as a sheet container where sheets to be bound are housed. An abutment surface  242  is a reference surface against which trailing ends of the sheets in the binding tray  243  are brought into abutment for alignment. In the embodiment, for example, crimping toothed jaws  261  are a pair of toothed jaws having shapes with protrusions and recesses which mesh with each other. The crimping toothed jaws  261  pinch sheets therebetween, thereby deforming the sheets and entangling fibers of the sheets. 
       FIGS. 5 and 6  are explanatory diagrams illustrating an example configuration of the bifurcating claw  204 , which selectively directs a sheet conveyed into the sheet processing apparatus  201 , and elements around the bifurcating claw  204  in detail.  FIG. 5  is an explanatory diagram illustrating a home position of the bifurcating claw  204 .  FIG. 6  is an explanatory diagram illustrating the bifurcating claw at a position for directing a sheet conveyed into the sheet processing apparatus  201  to the branch path  241 . 
     The bifurcating claw  204  is configured to be pivotable to switch between the conveying path  240  and the branch path  241 . As illustrated in  FIG. 5 , a position at which a sheet conveyed from the right of  FIG. 5  can be conveyed without resistance is set as the home position of the bifurcating claw  204 . The bifurcating claw  204  is constantly pressed by a spring  251  as illustrated in  FIG. 5 . The spring  251  is hooked onto a bifurcating-claw moving lever  204   a . A bifurcating solenoid  250  is also coupled to the bifurcating-claw moving lever  204   a  via a link. The conveying surface of the branch path  241  and the bifurcating claw  204  are configured to be capable of pinching a sheet therebetween on the conveying path. Switching of the conveying path is performed such that, when the bifurcating solenoid  250  is switched on, the bifurcating claw  204  is rotated in a direction indicated by arrow A 1  in  FIG. 6  to close the conveying path  240  and guide a sheet to the branch path  241 . 
     In the embodiment, a unit which stacks multiple sheets to be bound to form a sheet sheaf is made up of the entry rollers  203 , the sheet discharging rollers  205 , the bifurcating claw  204 , the binding tray  243  including the abutment surface  242 , and the driving sources driving these elements. 
       FIGS. 7 and 8  are explanatory diagrams illustrating an example of configuration and operations of the binding tool  210 .  FIG. 7  is an explanatory diagram illustrating an example of the binding tool  210  and a drive mechanism of the binding tool  210  in a state where the crimping toothed jaws  261  are open.  FIG. 8  is an explanatory diagram illustrating the example of the binding tool  210  and the drive mechanism of the binding tool  210  in a state where the crimping toothed jaws  261  are closed. Note that the configuration of the binding tool  210  is not limited to that illustrated in  FIGS. 7 and 8 . 
     Referring to  FIG. 7 , the crimping toothed jaws  261  includes the upper crimping toothed jaw  261   a  and the lower crimping toothed jaw  261   b  formed to mesh with each other. The upper crimping toothed jaw  261   a  is assembled onto a distal end of a movable link member  263 . The lower crimping toothed jaw  261   b  is assembled onto a stationary link member  264  in a manner to face the upper crimping toothed jaw  261   a . The movable link member  263  is configured in such a manner that pivoting motion of a pressing lever  262  moves the crimping toothed jaws  261  toward and away from each other. The pressing lever  262  is pivoted in a direction indicated by arrow A 3  in  FIG. 8  by a cam  266  which rotates in a direction indicated by arrow A 2  in  FIG. 8 . The cam  266  is controlled so as to be rotated by a driving force fed from a drive motor  265  and situated at a detection position based on detection information output of the cam HP sensor  267 . The detection position where the cam  266  is detected by the cam HP sensor  267  is defined as a home position (standby position) of the cam  266 . At this position, the crimping toothed jaws  261  are in the open state. 
     Operation is made as shown in  FIG. 8  when sheets are bound. Sheets P are interposed between the pair of crimping toothed jaws  261  in the open state. The cam  266  is rotated in the direction indicated by arrow A 2  in  FIG. 8  by rotation of the drive motor  265 . The cam surface of the cam  266  is displaced, causing the pressing lever  262  to pivot in the direction indicated by arrow A 3  in  FIG. 8 . Rotating force of the pressing lever  262  is multiplied via the movable link member  263  that utilizes leverage, and transmitted to the upper crimping toothed jaw  261   a  which is at the end of the movable link member  263 . At a point in time where the cam  266  has rotated a certain degree, the upper crimping toothed jaw  261   a  and the lower crimping toothed jaw  261   b  are engaged with each other and pinch the sheets P therebetween. By being pinched in this way, the sheets P are deformed and pressed, and fibers in adjacent sheets are entangled. The sheets P are thus bound together. Thereafter, the drive motor  265  is rotated backward, and stopped at the detection position where the cam  266  is detected by the cam HP sensor  267 . The pressing lever  262 , which has resilience, is deformed when an excessive load is placed on the pressing lever  262 , thereby relieving the excessive load. 
     In the binding tool  210  configured as illustrated in  FIGS. 7 and 8 , a binding strength, at which the sheets P are bound by entanglement of fibers of the sheets, changes with a binding force, which is an engaging force at which the pair of crimping toothed jaws  261  are engaged with each other and pinch the sheets P to thereby deform and press the sheets P. The binding force being the force by which the pair of crimping toothed jaws  261  are engaged varies with a rotating force (torque) at which the pressing lever  262  is pivoted via the cam  266  or, in other words, a torque (the moment of a force) generated by the drive motor  265 . The torque generated by the drive motor  265  varies with an electric current supplied to the drive motor  265 . Accordingly, it is possible to change the binding force of the binding tool  210  to thereby change the binding strength of the sheet sheaf according to a binding mode, such as a permanent binding mode or a temporary binding mode, by controlling the electric current supplied to the drive motor  265 . 
     An example of a binding operation performed by the sheet processing apparatus  201  is described below. 
       FIGS. 9A to 17B  are plan views and front views of the sheet processing apparatus  201  performing the example binding operation.  FIGS. 9A ,  10 A, . . . , and  17 A are plan views of the sheet processing apparatus  201 .  FIGS. 9B ,  10 B, . . . , and  17 B are front views of the sheet processing apparatus  201 . 
     Referring to  FIGS. 9A and 9B , when the image forming apparatus  101  starts discharging a sheet, the units of the sheet processing apparatus  201  move to their home positions to complete initialization. 
     Subsequently, referring to  FIGS. 10A and 10B , before the sheet P discharged from the image forming apparatus  101  is conveyed into the sheet processing apparatus  201 , the sheet processing apparatus  201  receives information about an operation mode and information about the sheet P and enters a receipt-standby state based on the received information. The operation mode in the embodiment is any one of a straight mode, a shift mode, and a binding mode; however, it is not limited thereto. 
     Operation of the sheet processing apparatus  201  in the straight mode and that in the shift mode are described below. 
     How the sheet processing apparatus  201  operates in the straight mode is described first. 
     Upon receiving information indicating the straight mode and information about the sheet P, the sheet processing apparatus  201  enters a receipt-standby state for the straight mode. More specifically, each of the pair of entry rollers  203  and sheet discharging driving rollers  205   a  starts rotating in a predetermined rotating direction so that the sheet P conveyed into the sheet processing apparatus  201  is conveyed in a predetermined conveying direction (to the left in the drawing). The sheet P is fed into the sheet processing apparatus  201  in this receipt-standby state by rotation of the sheet discharging rollers  102  of the image forming apparatus  101 . The sheet P fed into the sheet processing apparatus  201  is conveyed by the pair of entry rollers  203  and then by a pair of sheet discharging rollers, which is made up of the sheet discharging driving rollers  205   a  and sheet discharging driven rollers  205   b , and discharged. When a last sheet has been discharged, the pair of entry rollers  203  and the sheet discharging driving rollers  205   a  are stopped. 
     How the sheet processing apparatus  201  operates in the shift mode is described below. 
     Upon receiving information indicating the shift mode and information about the sheet P, the sheet processing apparatus  201  enters a receipt-standby state for the shift mode. More specifically, each of the pair of entry rollers  203  and the sheet discharging driving rollers  205   a  starts rotating in the predetermined rotating direction so that the sheet P conveyed into the sheet processing apparatus  201  is conveyed in the predetermined conveying direction (to the left in the drawing) as in the straight mode. The sheet P is fed from the image forming apparatus  101  into the sheet processing apparatus  201  in this receipt-standby state. The sheet fed into the sheet processing apparatus  201  is conveyed by the pair of entry rollers  203  and the pair of sheet discharging rollers as in the straight mode. Subsequently, when a trailing end of the sheet has exited the pair of entry rollers  203 , the shift cam  207  is rotated a predetermined degree. As a result, the sheet discharging driving rollers  205   a  are moved in their axial direction. The sheet P is moved together with the sheet discharging driving rollers  205   a . When the sheet P has been discharged, the shift cam  207  rotates to return to its home position to receive a next sheet. This operation of the sheet discharging driving rollers  205   a  is repeatedly performed until all sheets belonging to the same “stack” have been discharged. When a sheet belonging to a next “stack” is conveyed into the sheet processing apparatus  201 , the shift cam  207  rotates in a direction opposite to the previous direction, and the sheet is shifted to the opposite side and discharged. 
     Upon receiving information indicating the binding mode and information about the sheet P, the sheet processing apparatus  201  enters a receipt-standby state for the binding mode. More specifically, in the receipt-standby state for the binding mode, the pair of entry rollers  203  are stopped, and the sheet discharging driving rollers  205   a  start rotating in a direction indicated by arrow A 6  in the drawing so that the sheet P fed into the sheet processing apparatus  201  is conveyed in the predetermined conveying direction (to the left in the drawing). The binding tool  210  moves to its standby position (home position) away from an end of the sheet P in the width direction a preset distance and enters a standby state. 
     Thereafter, when the sheet P is conveyed into the sheet processing apparatus  201 , a leading end of the sheet P is detected by the entry sensor  202 . The sheet P is conveyed a predetermined distance (being a distance which brings the leading end of the sheet P into abutment with the nip between the entry rollers  203  and resiliently deforms the sheet P a predetermined degree) from when the leading end is detected by the entry sensor  202 . After the sheet P is conveyed the predetermined distance, the entry rollers  203  start rotating. Skew correction of the sheet P is performed in this manner. 
     Referring to  FIGS. 11A and 11B , a conveyed distance of the sheet P is counted based on detection information output from the entry sensor  202  on detection of the trailing end of the sheet P. Positional information about the sheet P is obtained in this way. When the trailing end of the sheet P has exited the nip between the entry rollers  203 , the entry rollers  203  stop rotating to receive a next sheet. Concurrently therewith, the shift cam  207  rotates in a direction indicated by arrow A 7  (clockwise) in the drawing, causing the sheet discharging rollers  205  to start moving in their axial direction together with the sheet P. As a result, the sheet P is conveyed obliquely in a direction indicated by arrow A 8  in the drawing. Thereafter, when the sheet P is detected by the sheet edge sensor  220  attached to or built in the binding tool  210 , the shift cam  207  stops rotating, and then rotates backward. This backward rotation of the shift cam  207  stops when the sheet P becomes not detected by the sheet edge sensor  220  anymore. When the operation described above is completed and the trailing end of the sheet reaches a predetermined position where the trailing end has passed over a distal end of the bifurcating claw  204 , rotation of the sheet discharging rollers  205  in the direction indicated by arrow A 9  in the drawing is stopped. 
     Subsequently, referring to  FIGS. 12A and 12B , the bifurcating claw  204  is rotated in a direction indicated by arrow A 10  (clockwise) in the drawing to switch the conveying path. Thereafter, the sheet discharging rollers  205  rotate in a direction indicated by arrow A 11  (counterclockwise) in the drawing to convey the sheet P in a direction indicated by arrow A 12  the drawing so that the trailing end of the sheet P is conveyed onto the branch path  241 . By being conveyed in this manner, the sheet P is brought into abutment against the abutment surface  242  of the binding tray  243  and aligned. The sheet discharging rollers  205  then stop. The sheet discharging rollers  205  are configured to have a weak conveying force so as to slip after the sheet P is brought into abutment. 
     Subsequently, referring to  FIGS. 13A and 13B , the bifurcating claw  204  is rotated in a direction indicated by arrow A 13  (counterclockwise) in the drawing. The sheet P on the branch path  241  is on standby with the trailing end of the sheet P firmly pressed by the bifurcating claw  204  at a contact surface where the bifurcating claw  204  contacts the sheet P. When a following sheet P′ is conveyed out from the image forming apparatus  101 , the entry rollers  203  apply skew correction to the sheet P′ as in the case of the sheet P. Concurrently when the entry rollers  203  start rotating, the sheet discharging rollers  205  start rotating in the sheet-conveying rotating direction (the direction indicated by arrow A 6  in the drawing). 
     Referring to  FIGS. 14A and 14B , the operation illustrated in  FIGS. 11A to 12B  described above is performed on each of the second and following sheets P″, . . . , to move the sheets one sheet by one sheet to the intended position and stack the sheets, thereby stacking an aligned sheet sheaf Ps on the conveying path. 
     Referring to  FIGS. 15A and 15B , when a last sheet has been stacked on the aligned sheet sheaf Ps, the sheet discharging rollers  205  rotate in the direction indicated by arrow A 14  (clockwise) in the drawing to convey the sheet sheaf Ps a predetermined distance, and stop. This motion of the sheet discharging rollers  205  can cancel the deformation of the sheets caused when the trailing ends of the sheets are brought into abutment against the abutment surface  242 . Thereafter, the bifurcating claw  204  is rotated in the direction indicated by arrow A 15  (clockwise) in the drawing. As a result, orientation of the distal end of the bifurcating claw  204  is changed, and a pressure applied to the sheet sheaf Ps is released. 
     Subsequently, referring to  FIGS. 16A and 16B , the sheet discharging rollers  205  rotate in the direction indicated by arrow A 16  in the drawing, thereby conveying the sheet sheaf Ps a distance which brings the sheet sheaf Ps to a position where the crimping toothed jaws  261  of the binding tool  210  positionally coincides with a position (binding position) where the sheet sheaf Ps is to be processed, and stop. The crimping toothed jaws  261  of the binding tool  210  are positionally coincided with the position (binding position) where the sheets are to be processed in the sheet conveying direction in this manner. The binding tool  210  is moved in a direction indicated by arrow A 17  in the drawing a distance which brings the binding tool  210  to the position where the crimping toothed jaws  261  of the binding tool  210  positionally coincides with the position where the sheets are to be processed, and stopped. The crimping toothed jaws  261  of the binding tool  210  are positionally coincided with the position (binding position) where the sheets are to be processed in the sheet width direction in this manner. At this time, the bifurcating claw  204  is rotated in the direction indicated by arrow A 18  (counterclockwise) in the drawing. As a result, the orientation of the distal end of the bifurcating claw  204  is changed, and the bifurcating claw  204  returns to a sheet receiving state. Thereafter, the drive motor  265  of the binding tool  210  is switched on, and the crimping toothed jaws  261  press and squeeze the sheet sheaf Ps, thereby entangling fibers of the sheets P with each other to join the sheets together and binding the sheet sheaf Ps. 
     Subsequently, referring to  FIGS. 17A and 17B , the sheet discharging rollers  205  further rotate in the direction indicated by arrow A 16  in the drawing to discharge the bound sheet sheaf Ps. After the sheet sheaf Ps has been discharged, the shift cam  207  rotates in a direction indicated by arrow A 19  in the drawing to return to its home position. The binding tool  210  is moved in a direction indicated by arrow A 20  to return to its home position. Hence, the operation of binding the sheet sheaf Ps is completed. 
       FIGS. 18A to 18D  are explanatory diagrams of a crimping binding method. 
     As illustrated in  FIG. 18A , the binding tool  210  is positioned in such a manner that the upper crimping toothed jaw  261   a  and the lower crimping toothed jaw  261   b , each having a shape with protrusions and recesses, face each other so as to pinch the sheet sheaf Ps therebetween. As illustrated in  FIG. 18B , at least one of the crimping toothed jaws is moved to engage the upper crimping toothed jaw  261   a  and the lower crimping toothed jaw  261   b  with the sheet sheaf Ps therebetween, thereby applying a pressure to the sheet sheaf Ps. As the pressure is increased, fibers of paper are entangled, causing the sheet sheaf Ps to be bound. In this crimping binding, the sheet sheaf Ps can be bound by fibers being entangled and adhering between the sheets by crimping and squeezing the sheet Ps by fitting protrusions and recesses, thereby f. 
     Thereafter, as illustrated in  FIG. 18C , at least one of the crimping toothed jaws is moved again to separate the upper crimping toothed jaw  261   a  and the lower crimping toothed jaw  261   b  away from each other. The shape with protrusions and recesses is imprinted into the crimping-bound sheet sheaf Ps as illustrated in  FIG. 18D . 
     In the embodiment, the shape with protrusions and recesses of each of the upper crimping toothed jaw  261   a  and that of the lower crimping toothed jaw  261   b  has slope portions slanted at arbitrary angle. The upper crimping toothed jaw  261   a  and the lower crimping toothed jaw  261   b  are configured such that when engaged, top portions of the upper crimping toothed jaw  261   a  do not contact bottom portions of the lower crimping toothed jaw  261   b . With this configuration, the sheet sheaf Ps is to be crimping-bonded by the slope portions of the shapes with protrusions and recesses of the upper crimping toothed jaw  261   a  and the lower crimping toothed jaw  261   b . As illustrated in  FIG. 18D , recesses  6   a  of the sheet sheaf Ps are portions squeezed and extended by the protrusions of the upper crimping toothed jaw  261   a . Protrusions  6   b  of the sheet sheaf Ps are portions squeezed and extended by the protrusions of the lower crimping toothed jaw  261   b . Slope portions  6   c  of the sheet sheaf Ps are portions bound by crimping. 
     The shapes with protrusions and recesses of the pair of crimping toothed jaws, which are a feature of the embodiment, are described below. The upper crimping toothed jaw  261   a  and the lower crimping toothed jaw  261   b  are identical in shape. Accordingly, only the lower crimping toothed jaw  261   b  is described below. 
       FIG. 19  is a plan view of the lower crimping toothed jaw  261   b .  FIG. 20  is a diagram of the lower crimping toothed jaw  261   b  as viewed in a direction A of  FIG. 19 .  FIG. 21  is a diagram of the lower crimping toothed jaw  261   b  as viewed in a direction B of  FIG. 19 . The dashed lines in  FIGS. 20 and 21  indicate the upper crimping toothed jaw  261   a . The long dashed short dashed lines indicate the sheet sheaf Ps. 
     As illustrated in  FIG. 19 , the lower crimping toothed jaw  261   b  has the shape with protrusions and recesses formed by arranging multiple protrusions  70   b  at predetermined intervals. 
     Each of the protrusions  70   b  has a shape obtained by cutting a quadrangular pyramid near a height center parallel to a base of the pyramid and removing the upper portion. Top portions  71   b  of the protrusions  70   b  are faces parallel to sheet surface of the sheet sheaf. Meanwhile, “face parallel to the sheet surface” encompasses faces substantially parallel to the sheet surface and slightly-protruding faces such as that illustrated in  FIG. 20 . Each of the protrusions  70   b  has four side faces, each having a base side being one of sides of the base of the protrusion  70   b  and a top side being one of sides of the top face of the protrusion  70   b . The four side faces are configured as slope portions  72   b  slanted relative to the sheet surface of the sheet sheaf Ps. 
     In  FIG. 20 , B denotes a crimping height which indicates a contact area between the slope portion  72   b  of the lower crimping toothed jaw  261   b  and slope portion  72   a  of the upper crimping toothed jaw  261   a . Arrows Z in FIG.  20  indicate a portion where sheets are extended by the protrusions  70   b . In  FIG. 21 , θ 2  denotes a slope angle which is 60 degrees in the embodiment. In  FIG. 21 , S denotes a crimping area where sheets are to be crimped. 
       FIGS. 22A and 22B  are diagrams describing behavior of the sheet sheaf Ps during crimping binding. 
     As illustrated in  FIG. 22A , one of the crimping toothed jaws is moved to engage the upper crimping toothed jaw  261   a  and the lower crimping toothed jaw  261   b  with the sheet sheaf Ps therebetween. The sheets are squeezed and extended by top portions  71   a  and the top portions  71   b  of the protrusions of the crimping toothed jaws. In the embodiment, the top portions  71   a  and  71   b  of the protrusions of the crimping toothed jaws are configured as faces parallel to the sheet surface of the sheet sheaf Ps. Accordingly, a pressure applied from the top portions  71   a  and  71   b  of the protrusions of the crimping toothed jaws to the sheet sheaf Ps can be suppressed to be low. As a result, occurrence of an undesirable situation that the top portions  71   a  and  71   b  of the protrusions break the sheet sheaf Ps can be reduced. 
     In the process in which the upper crimping toothed jaw  261   a  and the lower crimping toothed jaw  261   b  are engaged, portions (enclosed by circles in  FIG. 22A ) of the sheet sheaf Ps pinched between the slope portions  72   a , which are the side faces of protrusions  70   a  of the upper crimping toothed jaw  261   a , and the slope portions  72   b  of the lower crimping toothed jaw  261   b  are drawn and compressed, causing fibers to be entangled with each other. 
     In the embodiment, sheets are crimped at the slope portions  72   a  and  72   b  which are slanted relative to the sheet surface of the sheet sheaf Ps. When a vertically-upward pressure applied from the slope portion  72   b  of the lower crimping toothed jaw  261   b  is decomposed to a component in a direction orthogonal to the slope portion  72   b  and a component in a direction parallel to the slope portion  72   b , the component parallel to the slope portion  72   b  is directed toward the top portion  71   b  of the lower crimping toothed jaw  261   b . Similarly, when a vertically-downward pressure applied from the slope portion  72   a  of the upper crimping toothed jaw  261   a  is decomposed, a component parallel to the slope portion  72   a  is directed toward the top portion  71   a  of the upper crimping toothed jaw  261   a . Accordingly, at the crimping portions of the sheet sheaf Ps indicated by the circles in  FIG. 22A , sheets closer to the lower crimping toothed jaw move up along the slope portions  72   b , while sheets closer to the upper crimping toothed jaw move down along the slope portions  72   a . These motions cause the sheets to be grinded at the crimping portions of the sheet sheaf Ps, and the fibers of the sheets to be more entangled, thereby allowing the sheets to be bound with a greater binding force. 
     As illustrated in  FIG. 22B , the crimping toothed jaws  261  are configured so that the clearance S is left both between the top portions  71   b  of the protrusions  70   b  of the lower crimping toothed jaw  261   b  and bottom portions  73   a  of recesses of the upper crimping toothed jaw  261   a  and between the top portions  71   a  of the protrusions  70   a  of the upper crimping toothed jaw  261   a  and bottom portions  73   b  of recesses of the lower crimping toothed jaw  261   b  when the upper crimping toothed jaw  261   a  is moved to its bottom dead center. As described above, in the embodiment, at the crimping portions of the sheet sheaf Ps indicated by the circles in  FIG. 21A , the sheets closer to the lower crimping toothed jaw move up along the slope portions  72   b , while the sheets closer to the upper crimping toothed jaw move down along the slope portions  72   a . The sheets are moved while being grinded at the crimping portions in this manner. Accordingly, as illustrated in  FIG. 22B , the sheets are deflected in a corrugated fashion in the clearance S between the top portions  71   a  and  71   b  of the protrusions and the bottom portions  73   a  and  73   b  of the recesses. In a configuration where, in contrast to the embodiment, the clearance S is not left but top portions of protrusions are brought into contact with bottom portions of recesses, deflected portions of sheets produced between the top portions of the protrusions and the bottom portions of the recesses are to be eventually compressed, resulting in wrinkles or a like damage of the sheets. However, the embodiment is configured such that, when the upper crimping toothed jaw  261   a  is moved to its bottom dead center, the clearance S is left between the top portions  71   a  and  71   b  of the protrusions and the bottom portions  73   a  and  73   b  of recesses. Accordingly, corrugated portions of the sheets are not compressed. Furthermore, according to the embodiment, because the sheets are appropriately pressed by being squeezed by the protrusions  70   a  and  70   b , fibers of the sheets are entangled with each other in the clearance S between the top portions  71   a  and  71   b  of the protrusions and the bottom portions  73   a  and  73   b  of the recesses. As a result, the sheet sheaf can be bound firmly. 
     Furthermore, when a pressure is applied to the sheet sheaf Ps from the slope portions  72   a  and  72   b , the sheets can move to the clearance S while being grinded. Accordingly, the sheets can be favorably grinded at the crimping portions, and a favorable binding force can be obtained. 
     In the embodiment, crimping binding of the sheet sheaf Ps is performed by entangling fibers of paper by squeezing and extending the sheets of the sheet sheaf Ps using the protrusions  70   a  and  70   b  and grinding the sheets at the crimping portions indicated by circles in  FIG. 22A . As the crimping height B, which indicates the contact area between the slope portion  72   b  of the lower crimping toothed jaw  261   b  and the slope portion  72   a  of the upper crimping toothed jaw  261   a , illustrated in  FIG. 20  increases, the crimping area S (see  FIG. 21 ) increases, causing a sheet sheaf to be bound firmly. However, as the crimping height B increases, the length between the top portions  71   b  of the protrusions of the lower crimping toothed jaw  261   b  to the top portions  71   a  of the protrusions of the upper crimping toothed jaw  261   a  in a state where the crimping toothed jaws  261  are engaged increases. As a result, sheets are to be extended longer, and thus sheet breakage resulting from extending the sheets beyond their limit can occur. 
     To avoid such an undesirable situation, the inventors intensively conducted a verification test and found an optimum value of the crimping height B. The verification test conducted by the inventors is described below. 
     The verification test was performed by producing a binding tool with the crimping height B of 0.45 mm, a binding tool with the crimping height B of 0.6 mm, and a binding tool with the crimping height B of 0.7 mm and measuring binding forces using each of the binding tools. Meanwhile, it is typical to bind a sheet sheaf of about five sheets. Accordingly, the binding forces were measured by binding a sheet sheaf of five sheets with each of the binding tools. 
     The following are other conditions of the binding tools: 
     Applied pressure: 2,000 N, 
     Number of teeth: six (the number of the protrusions), and 
     Slope angle θ 2 : 60°. 
       FIG. 23  is a graph illustrating a result of the verification test.  FIG. 23  illustrates a result of binding force applied to the first sheet (uppermost sheet) of a sheet sheaf and that applied to a center sheet (the third-from-top sheet) of a sheet sheaf. In  FIG. 23 , minimum values of the binding force obtained by performing measurement multiple times are plotted. 
     Referring to  FIG. 23 , when the crimping height B is equal to or longer than 0.45 mm, a binding force equal to or higher than a target level can be achieved. The binding force peaks at the crimping height B of 0.6 mm. The binding tool with the crimping height B of 0.7 mm is lower in binding force than the binding tool with the crimping height B of 0.6 mm. Because sheet breakage decreases the binding force, the reason why even though the crimping height B is higher than 0.6 mm, the binding force of the binding tool with the crimping height B of 0.7 mm is lower is presumably that one or more of sheets bound with the binding tool with the crimping height B of 0.7 mm were broken. 
     From the verification test, it is found that at least setting the crimping height B to be equal to or longer than 0.45 mm and equal to or shorter than 0.6 mm allows a favorable binding force to be obtained without causing sheet breakage. 
     Even when the crimping height is shorter than 0.45 mm, a binding force equal to or higher than the target level can be achieved by increasing the number of teeth (the number of the protrusions) or increasing the applied pressure. However, increasing the number of teeth increases the size of the binding tool, which leads to an increase in cost of materials and, eventually, to an increase in cost of the apparatus. Furthermore, consuming more materials can be waste of resources. Increasing the pressure requires that a drive force supplied from a driving source to apply the pressure be increased. This disadvantageously increases electric power consumption of the apparatus. However, by setting the crimping height B to be equal to or longer than 0.45, increases in the number of teeth and the cost of materials can be reduced, thereby reducing an increase in cost of the apparatus. Moreover, resources saving can be achieved. Still furthermore, because a lower pressure is required, energy saving can be achieved. 
     The embodiment described above is an example. Advantages each specific to the following aspects are provided. 
     First Aspect 
     According to a first aspect of the present invention, a crimping-binding-type sheet binding device, an example of which is the binding tool  210 , includes a pair of crimping members each including alternately-arranged multiple recesses and multiple protrusions, the protrusions and the recesses of one the pair of crimping members being arranged in the direction along which the protrusions and the recesses of the other one of the pair of crimping members are arranged, to bind a sheaf of sheets by fitting the recesses and the protrusions with the sheet sheaf interposed therebetween. The pair of crimping members is configured as follows. The top portions  71   a  and  71   b  of the protrusions of each of the pair of crimping members are faces parallel to a sheet surface of the sheet sheaf. Side faces of the protrusions are slanted faces, an example of which is the slope portions  72   a  and  72   b , slanted relative to the sheet surface. When the recesses and the protrusions of the pair of crimping members are fitted, the slanted faces of one of the pair of crimping members and the slanted faces of other one of the pair of crimping members are in contact with each other with the clearance S left both between the top portions  71   a  of the protrusions of the one of the crimping members and the bottom portions  73   b  of the recesses of the other one of the crimping members and between the top portions  71   b  of the protrusions of the other one of the crimping members and the bottom portions  73   a  of the recesses of the one of the crimping members. 
     According to the first aspect, when the pair of crimping members is engaged, the slanted faces, which are the side faces of the protrusions of the pair of crimping members, are brought into contact with each other. Accordingly, the sheet sheaf is bound by a pressure applied from the slope portions of the pair of crimping members. In the engaged state, the clearance is left both between the top portions of the protrusions of the one of the crimping members and the bottom portions of the recesses of the other one of the crimping members and between the top portions of the protrusions of the other one of the crimping members and the bottom portions of the recesses of the one of the crimping members. Accordingly, further reduction in load placed on fibers of paper can be achieved than that of the configuration disclosed in international publication No. WO 2009/110298, in which a clearance is left only between top portions of protrusions of one of crimping members and bottom portions of recesses of other one of the crimping members. Thus, according to the first aspect, fibers are more likely prevented from being extended beyond their limit than the configuration of international publication No. WO 2009/110298, entanglement between fibers can be achieved with less sheet breakage than with the configuration of international publication No. WO 2009/110298. 
     Furthermore, according to the first aspect, the top portions of the protrusions of both of the crimping members are faces parallel to the sheet surface of the sheet sheaf. Accordingly, both of contact between the sheet sheaf and the top portions of the protrusions of the one of the crimping-binding toothed jaws and contact between the sheet sheaf and the top portions of the protrusions of the other one of the crimping-binding toothed jaws are made by area contact, and therefore pressure concentration is prevented. As a result, occurrence of an undesirable situation that a sheet sheaf is broken during crimping binding can be reduced. 
     Second Aspect 
     According to a second aspect of the present invention, in the sheet binding device according to the first aspect, a length (the crimping height B) of a contact portion where the slanted faces are in contact with each other as viewed from a direction parallel to the sheet surface and orthogonal to the direction along which the recesses and the protrusions are arranged is equal to or longer than 0.45 mm and equal to or shorter than 0.6 mm. 
     According to the second aspect, as described in the foregoing verification test, setting the length (the crimping height B) of the contact portion where the slanted faces are in contact with each other to be equal to or longer than 0.45 mm and equal to or shorter than 0.6 mm allows a favorable binding force to be obtained while reducing sheet breakage. 
     Third Aspect 
     According to a third aspect of the present invention, the sheet processing apparatus  201  including at least a sheet binding device, such as the binding tool  210 , configured to perform a binding process on the sheet sheaf Ps includes, as the sheet binding device, the sheet binding device according to the first aspect or the second aspect. 
     According to the third aspect, as described above in the embodiment, favorable binding force can be obtained while reducing sheet breakage. 
     Fourth Aspect 
     According to a fourth aspect of the present invention, the image forming system  100  including the image forming apparatus  101  configured to form an image on a sheet, and a sheet binding device, an example of which is the binding tool  210 , configured to perform a binding process on a sheaf of sheets on which an image is formed by the image forming apparatus  101  includes the sheet binding device according to the first aspect or the second aspect as the sheet binding device. 
     According to the fourth aspect, binding a sheaf of sheets on which an image is formed can be achieved with favorable binding force while reducing sheet breakage. 
     According to an embodiment, damage to a sheet(s) can be reduced. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.