Patent Publication Number: US-10768568-B1

Title: Sheet binding apparatus and image forming system

Description:
FIELD 
     Embodiments described herein relate generally to a sheet binding apparatus, methods of binding a sheet stack, and an image forming system. 
     BACKGROUND 
     There is known a sheet binding apparatus that binds an edge portion of a sheet bundle with an adhesive tape up to now. The sheet binding apparatus includes a bundle forming section and a tape attaching section. The bundle forming section stacks a plurality of sheets to forma sheet bundle. The bundle forming section forms a side portion of the sheet bundle in a step shape in order to secure a surface area during tape attachment. The tape attaching section attaches the adhesive tape to an edge portion of the sheet bundle to bind the sheet bundle. The tape attaching section includes a tape holding section that holds the adhesive tape. The tape attaching section includes a first roller and a second roller opposed to each other in the sheet bundle thickness direction. The adhesive tape is peeled from the tape holding section by inserting the sheet bundle, which is shifted in the step shape, toward the adhesive tape held by the tape holding section. Thereafter, the sheet bundle is rushed inbetween the first roller and the second roller together with the adhesive tape to stuck the adhesive tape to the edge portion of the sheet bundle. 
     However, the following problem is likely to occur depending on the magnitude of a pressing force on the sheet bundle inbetween the first roller and the second roller (between the rollers). For example, if the pressing force on the sheet bundle between the rollers is too small, the adhesive tape cannot be sufficiently adhered to the edge portion of the sheet bundle. On the other hand, for example, if the pressing force on the sheet bundle between the rollers is too large, the sheet bundle cannot be rushed into between the first roller and the second roller. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view showing an image forming system in a first embodiment; 
         FIG. 2  is a front view showing an internal configuration of a sheet binding apparatus in the first embodiment; 
         FIGS. 3A and 3B  are side views showing operation for changing a shift amount among sheets,  FIG. 3A  being a diagram showing the operation performed if the shift amount among the sheets is relatively small and  FIG. 3B  being a diagram showing the operation performed if the shift amount among the sheets is relatively large; 
         FIG. 4  is a front view showing the operation of the sheet binding apparatus; 
         FIG. 5  is a front view showing the operation of the sheet binding apparatus following  FIG. 4 ; 
         FIG. 6  is a front view showing the operation of the sheet binding apparatus following  FIG. 5 ; 
         FIG. 7  is a front view showing the operation of the sheet binding apparatus following  FIG. 6 ; 
         FIG. 8  is a front view showing the operation of the sheet binding apparatus following  FIG. 7 ; 
         FIG. 9  is a front view showing the operation of the sheet binding apparatus following  FIG. 8 ; 
         FIG. 10  is a front view showing the operation of the sheet binding apparatus following  FIG. 9 ; 
         FIG. 11  is a front view showing the operation of a sheet binding apparatus in a comparative example; 
         FIG. 12  is a front view showing an internal configuration of a sheet binding apparatus in a first modification of the first embodiment; 
         FIG. 14  is a front view showing an internal configuration of a sheet binding apparatus in a second embodiment; 
         FIG. 15  is a front view showing an internal configuration of a sheet binding apparatus in a third embodiment; 
         FIG. 16  is a diagram showing a state in which a first roller in the third embodiment is opposed to a step-like end face of a sheet bundle; 
         FIG. 17  is a diagram showing a state in which the first roller in the third embodiment passes the step-like end face of the sheet bundle; and 
         FIG. 18  is a front view showing an internal configuration of a sheet binding apparatus in a first modification of the third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a sheet binding apparatus includes a first roller, a second roller, and a nip-pressure adjusting section. The first roller attaches a tape to an edge portion of a sheet bundle. The second roller is opposed to the first roller in the sheet bundle thickness direction. The nip-pressure adjusting section is capable of adjusting, on the basis of a position of an edge portion leading end of the sheet bundle, a pressing force of a nip formed by the first roller and the second roller. Before the sheet bundle is inserted between the first roller and the second roller, an interval between the first roller and the second roller is equal to or smaller than the thickness of the sheet bundle. 
     Sheet binding apparatuses and image forming systems in embodiments are explained below with reference to the drawings. In the figures, the same components are denoted by the same reference numerals and signs. Redundant explanation of the components is sometimes omitted. In this application, various sheet-like media including paper are referred to as “sheet”. 
     First, one embodiment is explained with reference to  FIGS. 1 to 10 . 
       FIG. 1  is a front view showing an image forming system  1  in this embodiment. The image forming system  1  in this embodiment includes a sheet binding apparatus  3  that binds an edge portion  5   a  (see  FIG. 9 ) of a sheet bundle  5  with a tape. For example, the sheet binding apparatus  3  is a post-processing apparatus that is disposed beside an image forming apparatus  2  and performs post-processing on sheets S conveyed from the image forming apparatus  2 . 
     The image forming apparatus  2  is briefly explained first. 
     As shown in  FIG. 1 , the image forming apparatus  2  includes a control panel  11 , a scanner section  12 , a printer section  13 , a paper feeding section  14 , a paper discharging section  15 , and a control section  16 . 
     The control panel  11  includes various keys. The control panel  11  receives operation of a user. 
     The scanner section  12  reads image information of a copying target object. 
     The printer section  13  forms an image on the sheet S on the basis of image information received from the scanner section  12  or an external apparatus. 
     The paper feeding section  14  supplies the sheet S to the printer section  13 . 
     The paper discharging section  15  conveys the sheet S discharged from the printer section  13  to the sheet binding apparatus  3 . 
     The control section  16  controls various operations of the control panel  11 , the scanner section  12 , the printer section  13 , the paper feeding section  14 , and the paper discharging section  15 . 
     The sheet binding apparatus  3  is explained. 
     The sheet binding apparatus  3  includes a bundle forming section  22 , a sheet shifting section  23 , a tape processing section  24 , a nip-pressure adjusting section  80  (see  FIG. 2 ), a storing section  25 , and a control section  26 . 
     The bundle forming section  22  is explained. 
       FIG. 2  is a front view showing an internal configuration of the sheet binding apparatus  3 . 
     As shown in  FIG. 2 , the bundle forming section  22  stacks a plurality of sheets S to form the sheet bundle  5 . The bundle forming section  22  includes a main guide  31 , a sub-guide  32 , a stopper  33 , and a switching member  34 . 
     The main guide  31  guides the sheet S along a sheet conveying direction X 1 . The plurality of sheets S are loaded in order on the main guide  31  to form the sheet bundle  5 . The main guide  31  guides the sheet bundle  5  toward between a first roller  91  and a second roller  92 . The main guide  31  guides the sheet bundle  5  to cause an edge portion leading end of the sheet bundle  5  to face the inner side of an inter-center width D 1  between the first roller  91  and the second roller  92 . The downstream-side end portion of the main guide  31  in the sheet conveying direction X 1  is formed in a comb teeth shape to avoid a first roller  41  of the sheet shifting section  23 . 
     The sub-guide  32  is opposed to the main guide  31  in a thickness direction Z of the sheet bundle  5  (hereinafter referred to as sheet bundle thickness direction Z). A space in which the sheets are loaded is provided between the main guide  31  and the sub-guide  32 . The downstream-side end portion of the sub-guide  32  in the sheet conveying direction X 1  is formed in a comb tooth shape to avoid a second roller  42  of the sheet shifting section  23 . 
     The stopper  33  is provided at the downstream-side end portion of the main guide  31  in the sheet conveying direction X 1 . The stopper  33  is movable between a restricting position (indicated by a solid line in  FIG. 2 ) and a releasing position (indicated by an alternate long and two short dashes line in  FIG. 2 ) by a not-shown moving mechanism. In the restricting position, the stopper  33  projects further upward than the upper surface of the main guide  31 . In the restricting position, the end portions of the sheets S come into contact with the stopper  33 , whereby the stopper  33  blocks the sheets S. Therefore, the sheets S stay on the main guide  31  and the sheet bundle  5  is formed. On the other hand, in the releasing position, the stopper  33  retracts further downward than the upper surface of the main guide  31 . In the releasing position, the stopper  33  allows the sheet bundle  5  on the main guide  31  to pass toward the switching member  34 . 
     The switching member  34  switches a conveying path of the sheet bundle  5 . A direction in which the sheet bundle  5  is conveyed toward the tape processing section  24  (specifically, a tape attaching section  59 ) is referred to as “first conveying direction (inserting direction)”. On the other hand, a direction in which the sheet bundle  5  is conveyed to a position (e.g., below the bundle forming section  22 ) different from the tape attaching section  59  is referred to as “second conveying direction). The switching member  34  switches the conveying path of the sheet bundle  5  between the first conveying direction and the second conveying direction. 
     The sheet shifting section  23  is explained. 
     The sheet shifting section  23  shifts the plurality of sheets S in the sheet conveying direction X 1  in order little by little to form a state in which the plurality of sheets S forming the sheet bundle  5  are shifted from one another at the edge portion  5   a  of the sheet bundle  5 . For example, the sheet shifting section  23  forms a state in which the plurality of sheets S are shifted in a step shape at the edge portion  5   a  of the sheet bundle  5 . 
     The sheet shifting section  23  includes the first roller  41  and the second roller  42 . The first roller  41  and the second roller  42  form an example of a “bundle conveying section  40 ” in cooperation with each other. The bundle conveying section  40  conveys the sheet bundle  5  located between the main guide  31  and the sub-guide  32  toward between the first roller  91  and the second roller  92 . 
     The first roller  41  is attached to a first shaft  43 . For example, the first roller  41  is a driving roller driven by a not-shown motor via the first shaft  43 . The first roller  41  is fixed in a fixed position. The material of the first roller  41  is not particularly limited. For example, the first roller  41  is formed of ethylene propylene diene rubber (EPDM). 
     The second roller  42  is attached to a second shaft  44 . For example, the second roller  42  is a driven roller that rotates following the rotation of the first roller  41 . The second roller  42  is movable in a direction approaching the first roller  41  and a direction away from the first roller  41  by a not-shown moving mechanism. The second roller  42  is moved toward the first roller  41  to come into contact with the sheet bundle  5  from the opposite side of the first roller  41 . 
     An outer circumferential surface  42   s  of the second roller  42  is softer than an outer circumferential surface  41   s  of the first roller  41  and is deformable along the surface of the sheet bundle  5 . For example, the second roller  42  is formed of sponge, rubber having a hollow on the inside, or the like. If the second roller  42  is brought close to the first roller  41 , the outer circumferential surface  42   s  of the second roller  42  is deformed in an arcuate shape conforming to the outer circumferential surface  41   s  of the first roller  41 . 
       FIGS. 3A and 3B  are side views showing operation for changing a shift amount d among the sheets S with the sheet shifting section  23 .  FIG. 3A  shows the operation performed if the shift amount d among the sheets S is relatively small. On the other hand,  FIG. 3B  shows the operation performed if the shift amount d among the sheets S is relatively large. 
     As shown in  FIG. 3 , the sheet shifting section  23  can reduce the shift amount d among the sheets S by setting a rotation angle of the first roller  41  smaller than a reference amount set in advance. On the other hand, the sheet shifting section  23  can increase the shift amount d among the sheets S by setting the rotation angle of the first roller  41  larger than the reference amount. 
     The tape processing section  24  is explained. 
     As shown in  FIG. 2 , the tape processing section  24  includes an unwinding section  51 , a tape conveying section  52 , a separating member  53 , a winding section  54 , a guide table  55 , a cutter  56 , a cutting-length changing section  57 , a tape holding section  58 , and a tape attaching section  59 . 
     The unwinding section  51  is an example of a “tape supplying section”. For example, the unwinding section  51  holds a web roll obtained by winding a belt-like tape T (hereinafter simply referred to as “tape T”). The unwinding section  51  supplies the tape T in the length direction of the tape T. In a state in which the tape T is held by the unwinding section  51 , the tape T includes an adhesive layer  61 , a protection film (a first film)  62 , and a peeling film (a second film)  63 . The protection film  62  covers the adhesive layer  61  from one side. The protection film  62  is integral with the adhesive layer  61  during use of the tape T. On the other hand, the peeling film  63  covers the adhesive layer  61  from the opposite side of the protection film  62 . The peeling film  63  is peeled from the adhesive layer  61  before the use of the tape T. The peeling film  63  is wound by the separating member  53  and the winding section  54 . 
     The tape conveying section  52  conveys, along the length direction of the tape T, the tape T supplied from the unwinding section  51 . For example, the length direction of the tape T is a direction substantially parallel to the sheet bundle thickness direction Z. For example, the tape conveying section  52  is a conveying roller pair that conveys the tape T. 
     The guide table  55  is an example of a tape conveying guide forming a conveying path of the tape T. The guide table  55  guides the tape T from which the peeling film  63  is separated. The guide table  55  supports the tape T during holding and during cutting of the tape T. A conveying direction of the tape T (the length direction of the tape T) crosses the vertical plane. 
     The cutter  56  cuts the belt-like tape T supplied from the unwinding section  51  to forma sheet-like tape T. For example, the cutter  56  is a rotor cutter. The cutter  56  includes a cutting edge  56   a  and a supporting shaft  56   b . The supporting shaft  56   b  is rotated by a not-shown motor, whereby the cutting edge  56   a  is driven to rotate. The configuration of the cutter  56  is not limited to the example explained above. The configuration of the cutter  56  may be any configuration if the cutter  56  can cut the tape T supplied from the unwinding section  51 . The cutter  56  is movable in a direction approaching the tape T and a direction away from the tape T by a not-shown moving mechanism. 
     The cutting-length changing section  57  changes length L (see  FIG. 7 ) of the tape T cut by the cutter  56 . “The length L of the tape” in this application is the length (the width) of the tape T in the sheet bundle thickness direction Z. In other words, “the length L of the tape” is length in a direction in which the tape T wraps the edge portion  5   a  of the sheet bundle  5  from a first surface  7   a  toward a second surface  7   b  of the sheet bundle  5 . 
     The cutting-length changing section  57  includes a moving mechanism  71  that changes a relative position of the cutter  56  with respect to a leading end Te of the tape T supplied from the unwinding section  51 . For example, the moving mechanism  71  moves the cutter  56  to change the relative position of the cutter  56  with respect to the leading end Te of the tape T. For example, the moving mechanism  71  moves the cutter  56  along the sheet bundle thickness direction Z. “The relative position of the cutter  56  with respect to the leading end Te of the tape T” is, for example, a relative position of the cutter  56  with respect to the leading end Te of the tape T when the tape T is cut by the cutter  56 . 
     In this embodiment, the moving mechanism  71  includes a supporting member  72  that supports the cutter  56  and a driving source  73  that moves the cutter  56  via the supporting member  72 . For example, the supporting member  72  is a ball screw coupled to the cutter  56 . The driving source  73  is a motor that drives the ball screw to move the cutter  56 . The configurations of the supporting member  72  and the driving source  73  are not limited to the example explained above. For example, the supporting member  72  may be a cam or the like that is in contact with the cutter  56 . The driving source  73  may be a solenoid or the like that moves the cutter  56  via the supporting member  72 . In this case, the supporting member  72  is a coupling member that couples the cutter  56  and the solenoid. 
     The configuration of the moving mechanism  71  is not limited to the example explained above. For example, the moving mechanism  71  may change the relative position of the cutter  56  with respect to the leading end Te of the tape T by, for example, changing a let-out length of the tape T with respect to the cutter  56  fixed in a fixed position. 
     In this embodiment, the cutting-length changing section  57  is controlled by the control section  26  (see  FIG. 1 ). For example, the control section  26  controls the driving source  73  of the cutting-length changing section  57  to move the cutter  56  and change the length L of the tape T cut by the cutter  56 . For example, the operation of the cutting-length changing section  57  explained below is performed by controlling the cutting-length changing section  57  with the control section  26 . 
     In this embodiment, the cutting-length changing section  57  changes, on the basis of the shift amount d among the sheets S changed by the control section  26 , the length of the tape T cut by the cutter  56 . For example, if the shift amount d among the sheets S is increased by the control section  26 , the cutting-length changing section  57  increases the length L of the tape T cut by the cutter  56 . On the other hand, if the shift amount d among the sheets S is reduced by the control section  26 , the cutting-length changing section  57  reduces the length L of the tape T cut by the cutter  56 . 
     The tape holding section  58  supports the tape T in a state in which the posture of the tape T is retained substantially flat. The tape holding section  58  is movable along the length direction of the tape T by a not-shown moving mechanism. The tape holding section  58  is movable in a direction approaching the tape T and a direction away from the tape T by the not-shown moving mechanism. 
     The tape holding section  58  includes a first tape supporting section  58   a  and a second tape supporting section  58   b  that support the tape T. The first tape supporting section  58   a  and the second tape supporting section  58   b  respectively extend along an inserting direction of the sheet bundle  5  (the sheet conveying direction X 1 ). The first tape supporting section  58   a  and the second tape supporting section  58   b  are disposed at an interval from each other in the conveying direction of the tape T. The first tape supporting section  58   a  and the second tape supporting section  58   b  respectively have sharp shapes tapered toward a bonding surface of the tape T (an adhesive surface of the adhesive layer  61 ). 
     The tape attaching section  59  (a tape wrapping section) includes the first roller  91 , the second roller  92 , a first spring  93  (a first urging member), and a second spring  94  (a second urging member). The first roller  91  and the second roller  92  are disposed side by side in the conveying direction of the tape T (the sheet bundle thickness direction Z). 
     The first roller  91  has a perfect circle shape. If the tape T is attached to the edge portion  5   a  of the sheet bundle  5 , the first roller  91  is opposed to the step-like end face of the sheet bundle  5 . 
     The second roller  92  has a shape, the radius of which before insertion of the sheet bundle  5  is smaller than a threshold and the radius of which after the insertion of the sheet bundle  5  is equal to or larger than the threshold. The radius before the insertion of the sheet bundle  5  means a distance from a rotating shaft  92   c  (an axis) before the insertion of the sheet bundle  5  to a nip  90 N. The radius after the insertion of the sheet bundle  5  means a distance from the rotating shaft  92   c  (the axis) after the insertion of the sheet bundle  5  to the nip  90 N. In this embodiment, the second roller  92  has an elliptical shape. 
     The second roller  92  includes a minimum radius section  92   a  having a minimum radius and a maximum radius section  92   b  having a maximum radius. The minimum radius section  92   a  is a radius on a minor axis side of the second roller  92 . The maximum radius section  92   b  is a radius on a major axis side of the second roller  92 . Before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 , the minimum radius section  92   a  forms the nip  90 N. If the tape T is attached to the edge portion  5   a  of the sheet bundle  5 , the second roller  92  is opposed to the surface on the opposite side of the step-like end face of the sheet bundle  5  (see  FIG. 9 ). 
     The first spring  93  urges the first roller  91  toward the second roller  92 . The second spring  94  urges the second roller  92  toward the first roller  91 . The first roller  91  and the first spring  93  form an example of a “first urging section” in cooperation with each other. The second roller  92  and the second spring  94  form an example of a “second urging section” in cooperation with each other. If the tape T is attached, the edge portion  5   a  of the sheet bundle  5  is inserted between the first roller  91  and the second roller  92  together with the tape T. Consequently, the tape T is bent to wrap the edge portion  5   a  of the sheet bundle  5  by the tape attaching section  59 . The tape T is attached to the edge portion  5   a  of the sheet bundle  5 . 
     The nip-pressure adjusting section  80  is explained. 
     The nip-pressure adjusting section  80  is capable of adjusting, on the basis of the position of an edge portion leading end  5   e  (see  FIG. 4 ) of the sheet bundle  5 , a pressing force (hereinafter referred to as “nip pressure” as well) of the nip  90 N formed by the first roller  91  and the second roller  92 . The nip pressure means a pressing force against the sheet bundle  5  between the first roller  91  and the second roller  92 . In this embodiment, an outer circumferential portion (an elliptical-shape forming portion) of the second roller  92  forms an example of the nip-pressure adjusting section  80 . 
     In  FIG. 4 , reference sign W 1  indicates the thickness of the sheet bundle  5 . Before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 , an interval between the first roller  91  and the second roller  92  (hereinafter referred to as “roller interval” as well) is equal to or smaller than the thickness W 1  of the sheet bundle  5 . In this embodiment, the roller interval is zero before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 . In other words, before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 , the first roller  91  and the second roller  92  are in contact with each other. 
     The control section  26  (see  FIG. 1 ) is formed by a control circuit or the like including a CPU, a ROM, and a RAM provided in the sheet binding apparatus  3 . For example, a processor such as a CPU executes a computer program, whereby the control section  26  controls the operation of the sheet binding apparatus  3 . For example, the control section  26  controls various operations of the bundle forming section  22 , the sheet shifting section  23 , and the tape processing section  24 . 
     An operation example of the sheet binding apparatus  3  is explained.  FIGS. 4 to 10  are front views showing the operation example of the sheet binding apparatus  3 . 
     First, as shown in  FIG. 2 , the sheet binding apparatus  3  moves the stopper  33  to the restricting position to block the sheet S conveyed to the main guide  31 . Consequently, the plurality of sheets S are stacked in order and the sheet bundle  5  is formed. Subsequently, the sheet binding apparatus  3  moves the stopper  33  to the releasing position. The sheet binding apparatus  3  switches the switching member  34  toward the second conveying direction. 
     Subsequently, as shown in  FIG. 3 , the sheet binding apparatus  3  moves the second roller  42  toward the first roller  41 . Consequently, the sheet bundle  5  and the outer circumferential surface  42   s  of the second roller  42  is deformed into an arcuate shape conforming to the outer circumferential surface  41   s  of the first roller  41 . The sheet binding apparatus  3  normally rotates the first roller  41  in a state in which the sheet bundle  5  is held between the first roller  41  and the second roller  42 . 
     Consequently, the second roller  42  rotates according to the rotation of the first roller  41  while maintaining a state in which the outer circumferential surface  42   s  of the second roller  42  is recessed along the outer circumferential surface  41   s  of the first roller  41 . As a result, a state in which the plurality of sheets S are shifted in a step shape in the sheet conveying direction X 1  at the edge portion  5   a  of the sheet bundle  5  is formed. “The edge portion  5   a  of the sheet bundle  5 ” in the following explanation means the edge portion  5   a  of the sheet bundle  5  in which the plurality of sheets S are shifted in the step shape. 
     Subsequently, the sheet binding apparatus  3  moves the second roller  42  in a direction away from the first roller  41 . Consequently, the recess of the outer circumferential surface  42   s  of the second roller  42  is eliminated. Subsequently, the sheet binding apparatus  3  reversely rotates the first roller  41  and the second roller  42  to move the sheet bundle  5  toward an opposite direction X 2  of the sheet conveying direction X 1 . Subsequently, the sheet binding apparatus  3  switches the switching member  34  to switch the conveying path from the second conveying direction to the first conveying direction. The sheet binding apparatus  3  normally rotates the first roller  41  and the second roller  42  to move the sheet bundle  5  toward the tape attaching section  59 . 
     As shown in  FIG. 4 , before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 , the sheet binding apparatus  3  sets the roller interval to the thickness of the sheet bundle  5  or less. In this embodiment, before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 , the sheet binding apparatus  3  brings the first roller  91  and the second roller  92  into contact with each other to set the roller interval to zero. Before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 , the sheet binding apparatus  3  forms the nip  90 N with the first roller  91  and the minimum radius section  92   a  of the second roller  92 . 
     The sheet binding apparatus  3  in this embodiment changes, on the basis of the shift amount d among the sheets S changed by the control section  26  (see  FIG. 1 ), the length L of the tape T cut by the cutter  56 . For example, in this embodiment, the control section  26  controls the driving source  73  (see  FIG. 2 ) of the cutting-length changing section  57 , whereby the position of the cutter  56  is changed. 
     Subsequently, as shown in  FIG. 5 , the sheet binding apparatus  3  brings the tape holding section  58  into contact with the tape T to support the tape T in a state in which the posture of the tape T is retained. In this embodiment, the sheet binding apparatus  3  brings the tape holding section  58  into contact with both ends (an upstream end and a downstream end in the conveying direction of the tape T) of the guide table  55  to support the substantially flat (linear) tape T. 
     Subsequently, as shown in  FIG. 6 , the sheet binding apparatus  3  moves the tape holding section  58  to between the sheet bundle  5  and the tape attaching section  59 . For example, the tape holding section  58  disposes the tape T to extend across the first roller  91  and the second roller  92 . For example, the tape holding section  58  disposes the tape T such that the center of the linear tape T faces the nip  90 N of the first roller  91  and the second roller  92 . In other words, the tape holding section  58  causes the center between the first tape supporting section  58   a  and the second tape supporting section  58   b , which hold the tape T, to face the nip  90 N of the first roller  91  and the second roller  92 . 
     Subsequently, as shown in  FIG. 7 , the sheet binding apparatus  3  cuts the belt-like tape T with the cutter  56  to form the sheet-like tape T. Consequently, the tape T is cut into a necessary length. 
     Subsequently, as shown in  FIG. 8 , the sheet binding apparatus  3  moves the sheet bundle  5  toward the tape attaching section  59  with the sheet shifting section  23  (see  FIG. 2 ). For example, the sheet binding apparatus  3  normally rotates the first roller  41  and the second roller  42  (see  FIG. 2 ) to move (insert) the sheet bundle  5  toward the tape attaching section  59 . The sheet binding apparatus  3  conveys the sheet bundle  5  located between the main guide  31  and the sub-guide  32  toward between the first roller  91  and the second roller  92 . The sheet binding apparatus  3  causes the edge portion leading end  5   e  of the sheet bundle  5  to face the inner side of the inter-center width D 1  between the first roller  91  and the second roller  92 . The sheet binding apparatus  3  inserts the sheet bundle  5  into the tape T held by the tape holding section  58  to thereby peel the tape T from the tape holding section  58 . The sheet binding apparatus  3  inserts the edge portion  5   a  of the sheet bundle  5  between the first roller  91  and the second roller  92  together with the tape T. 
     As shown in  FIG. 9 , if the edge portion  5   a  of the sheet bundle  5  is inserted between the first roller  91  and the second roller  92  together with the tape T, the first roller  91  and the second roller  92  move along the external shape of the edge portion  5   a  of the sheet bundle  5 . Consequently, the first roller  91  and the second roller  92  press the tape T against the edge portion  5   a  of the sheet bundle  5 . As a result, the tape T sequentially follows and adheres to the step-like portion of the sheet bundle  5 . The edge portion  5   a  of the sheet bundle  5  includes the first surface  7   a , the second surface  7   b , and an end face  7   c . The first surface  7   a  and the second surface  7   b  are surfaces extending along the sheet conveying direction X 1 . The second surface  7   b  is located on the opposite side of the first surface  7   a . The end face  7   c  is located between the first surface  7   a  and the second surface  7   b . The plurality of sheets S are shifted in a step shape. The sheets S are attached over the first surface  7   a , the end face  7   c , and the second surface  7   b  at the edge portion  5   a  of the sheet bundle  5 . Consequently, all the sheets S including an intermediate page of the sheet bundle  5  are integrated by the tape T. Consequently, processing for attaching the tape T to the edge portion  5   a  of the sheet bundle  5  is completed. 
     In this embodiment, if the edge portion  5   a  of the sheet bundle  5  is inserted between the first roller  91  and the second roller  92  together with the tape T, the first roller  91  and the second roller  92  respectively rotate around rotating shafts  91   c  and  92   c . After the insertion of the sheet bundle  5 , a portion having a larger radius than the minimum radius section  92   a  of the second roller  92  (a portion closer to the maximum radius section  92   b ) forms the nip  90 N. Consequently, after the insertion of the sheet bundle  5 , a nip pressure is larger than a nip pressure before the insertion of the sheet bundle  5 . Therefore, before the insertion of the sheet bundle  5 , since the nip pressure is smaller than the nip pressure after the insertion of the sheet bundle  5 , it is easy to insert the sheet bundle  5  between the rollers. On the other hand, after the insertion of the sheet bundle  5 , since the nip pressure is larger than the nip pressure before the insertion of the sheet bundle  5 , it is possible to apply a sufficient sticking force of the tape T to the edge portion  5   a  of the sheet bundle  5 . 
     Subsequently, as shown in  FIG. 10 , the sheet binding apparatus  3  reversely rotates the first roller  41  and the second roller  42  to extract the sheet bundle  5  from between the first roller  91  and the second roller  92 . The sheet binding apparatus  3  further reversely rotates the first roller  41  and the second roller  42  to discharge the sheet bundle  5  to a discharging section of the sheet binding apparatus  3 . 
     Consequently, a series of operation by the sheet binding apparatus  3  ends. 
     The operation of a sheet binding apparatus in a comparative example is explained. 
       FIG. 11  is a front view showing the operation of the sheet binding apparatus in the comparative example. 
     As shown in  FIG. 11 , the sheet binding apparatus in the comparative example does not include the nip-pressure adjusting section  80  (see  FIG. 4 ). 
     In the comparative example, the first roller  91  and the second roller  92  respectively have perfect circle shapes. Therefore, the following problem is likely to occur depending on the magnitude of a pressing force (a nip pressure) on the sheet bundle  5  between the first roller  91  and the second roller  92  (between the rollers). For example, if the nip pressure is too small, the tape T cannot be sufficiently stuck to the edge portion  5   a  of the sheet bundle  5 . On the other hand, for example, if the nip pressure is too large, the sheet bundle  5  cannot be rushed into between the first roller  91  and the second roller  92 . 
     On the other hand, in the embodiment, as shown in  FIG. 4 , the sheet binding apparatus  3  includes the nip-pressure adjusting section  80  capable of adjusting the nip pressure on the basis of the position of the edge portion leading end  5   e  of the sheet bundle  5 . Therefore, it is less likely that the nip pressure is too small and the tape T cannot be sufficiently stuck to the edge portion  5   a  of the sheet bundle  5 . In addition, it is less likely that the nip pressure is too large and the sheet bundle  5  cannot be rushed into between the first roller  91  and the second roller  92 . 
     According to the embodiment, the sheet binding apparatus  3  includes the first roller  91 , the second roller  92 , and the nip-pressure adjusting section  80 . The first roller  91  attaches the tape T to the edge portion  5   a  of the sheet bundle  5 . The second roller  92  is opposed to the first roller  91  in the sheet bundle thickness direction Z. The nip-pressure adjusting section  80  is capable of adjusting, on the basis of the position of the edge portion leading end  5   e  of the sheet bundle  5 , a pressing force (a nip pressure) of the nip  90 N formed by the first roller  91  and the second roller  92 . Before the sheet  5  is inserted between the first roller  91  and the second roller  92 , an interval between the first roller  91  and the second roller  92  (a roller interval) is equal to or smaller than the thickness of the sheet bundle  5 . With the configuration explained above, the following effects are achieved. 
     The nip pressure can be adjusted by the nip-pressure adjusting section  80  on the basis of the position of the edge portion leading end  5   e  of the sheet bundle  5 . Compared with when the nip pressure is fixed irrespective of the position of the edge portion leading end  5   e  of the sheet bundle  5 , it is less likely that the nip pressure is too small and the tape T cannot be sufficiently stuck to the edge portion  5   a  of the sheet bundle  5 . In addition, it is less likely that the nip pressure is too large and the sheet bundle  5  cannot be rushed into between the first roller  91  and the second roller  92 . Therefore, it is easy to insert the sheet bundle  5  between the rollers before the insertion of the sheet bundle  5 . It is possible to apply a sufficient sticking force of the tape T to the edge portion  5   a  of the sheet bundle  5  after the insertion of the sheet bundle  5 . In addition, before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 , compared with when the roller interval is larger than the thickness of the sheet bundle  5 , it is easy to cause the tape T to follow the edge portion  5   a  of the sheet bundle  5 . Therefore, it is possible to more surely bind the sheet bundle  5 . 
     The second roller  92  has a shape, the radius of which before the insertion of the sheet bundle  5  is smaller than a threshold and the radius of which after the insertion of the sheet bundle  5  is equal to or larger than the threshold. With the configuration explained above, the following effects are achieved. 
     It is possible to adjust the nip pressure on the basis of the position of the edge portion leading end  5   e  of the sheet bundle  5  making use of the shape of the second roller  92 . Compared with a configuration including a motor, an apparatus configuration is simplified. The configuration contributes to a reduction in cost. In addition, since complicated control is unnecessary, the configuration contributes to energy saving. 
     The second roller  92  includes the minimum radius section  92   a  having the minimum radius and the maximum radius section  92   b  having the maximum radius. Before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 , the minimum radius section  92   a  forms the nip  90 N. With the configuration explained above, the following effects are achieved. 
     Before the insertion of the sheet bundle  5 , a balanced state is retained by the first roller  91  and the minimum radius section  92   a  of the second roller  92 . Therefore, after the insertion of the sheet bundle  5 , a portion having a larger radius than the minimum radius section  92   a  of the second roller  92  (a portion closer to the maximum radius section  92   b ) forms the nip  90 N. Consequently, after the insertion of the sheet bundle  5 , it is possible to set a nip pressure larger than a nip pressure before the insertion of the sheet bundle  5 . In addition, after the sheet bundle  5  is extracted from between the first roller  91  and the second roller  92 , it is possible to return the first roller  91  and the second roller  92  to the balanced state before the insertion of the sheet bundle  5 . 
     The second roller  92  has an elliptical shape. With the configuration explained above, the following effects are achieved. 
     It is possible to adjust the nip pressure with a simple configuration that makes use of the elliptical shape. In addition, compared with the configuration including the motor, since complicated control is unnecessary, the configuration contributes to energy saving. 
     The first roller  91  has a perfect circle shape. The first roller  91  is opposed to the step-like end face  7   c  of the sheet bundle  5 . With the configuration explained above, the following effects are achieved. 
     Compared with when the first roller  91  having the elliptical shape is opposed to the step-like end face  7   c  of the sheet bundle  5 , it is easy to cause the tape T to follow the step-like end face  7   c  of the sheet bundle  5 . Therefore, it is possible to smoothly attach the tape T to the edge portion  5   a  of the sheet bundle  5 . 
     The second roller  92  has an elliptical shape. The second roller  92  is opposed to a surface on the opposite side of the step-like end face  7   c  of the sheet bundle  5 . With the configuration explained above, the following effect is achieved. 
     Compared with when the second roller  92  having the perfect circle shape is opposed to the surface on the opposite side of the step-like end face  7   c  of the sheet bundle  5 , it is easy to adjust the nip pressure making use of the shape of the second roller  92 . 
     The sheet binding apparatus  3  further includes the first spring  93 , the second spring  94 , and the main guide  31 . The first spring  93  urges the first roller  91  toward the second roller  92 . The second spring  94  urges the second roller  92  toward the first roller  91 . The main guide  31  guides the sheet bundle  5  to cause the edge portion leading end  5   e  of the sheet bundle  5  to face the inner side of the inter-center width D 1  between the first roller  91  and the second roller  92 . With the configuration explained above, the following effects are achieved. 
     Since it is possible to cause the tape T to follow the edge portion  5   a  of the sheet bundle  5 , it is possible to more surely bind the sheet bundle  5 . In addition, compared with when the sheet bundle  5  is guided to cause the edge portion leading end  5   e  of the sheet bundle  5  to face the outer side of the inter-center width D 1  between the first roller  91  and the second roller  92 , it is easy to rush the sheet bundle  5  into between the first roller  91  and the second roller  92 . 
     The sheet binding apparatus  3  further includes the bundle conveying section  40  that conveys the sheet bundle  5  located between the main guide  31  and the sub-guide  32  toward between the first roller  91  and the second roller  92 . With the configuration explained above, the following effects are achieved. 
     In a state in which the posture of the sheet bundle  5  is retained, it is possible to convey the sheet bundle  5  toward between the first roller  91  and the second roller  92 . Therefore, it is possible to attach the tape T to the edge portion  5   a  of the sheet bundle  5  while preventing curl of the sheet bundle  5 . 
     A first modification of the first embodiment is explained. 
     The first roller  91  is not limited to having the perfect circle shape. 
       FIG. 12  is a front view showing a sheet binding apparatus in a first modification of the first embodiment. As shown in  FIG. 12 , a first roller  191  may have an elliptical shape. In this modification, the first roller  191  and the second roller  92  respectively have elliptical shapes. The first roller  191  and the second roller  92  respectively include minimum radius sections  191   a  and  92   a  and maximum radius sections  191   b  and  92   b . Before the sheet bundle  5  is inserted between the first roller  191  and the second roller  92 , the minimum radius sections  191   a  and  92   a  of the first roller  191  and the second roller  92  form the nip  90 N. 
     According to the first modification of the first embodiment, the first roller  191  and the second roller  92  respectively have the elliptical shapes. With the configuration explained above, the following effect is achieved. 
     Compared with when one of the first roller  191  and the second roller  92  has the elliptical shape, it is easy to adjust the nip pressure making use of the respective shapes of the first roller  191  and the second roller  92 . 
     A second modification of the first embodiment is explained. 
     The second roller  92  is not limited to having the elliptical shape. 
       FIG. 13  is a front view showing a sheet binding apparatus in a second modification of the first embodiment. As shown in  FIG. 13 , a second roller  192  may have a cam shape. A rotating shaft  192   c  of the second roller  192  deviates from the center position of the second roller  192 . The second roller  192  has a perfect circular external shape. The second roller  192  includes a minimum radius section  192   a  and a maximum radius section  192   b . Before the sheet bundle  5  is inserted between the first roller  91  and the second roller  192 , the minimum radius section  192   a  of the second roller  192  forms the nip  90 N. The second roller  192  rotates with the rotating shaft  192   c  as an axis to adjust the nip pressure. 
     According to the second modification of the first embodiment, the second roller  192  has a cam shape. With the configuration explained above, the following effects are achieved. 
     It is possible to adjust the nip pressure with a simple configuration that makes use of the cam shape. In addition, compared with the configuration including the motor, since complicated control is unnecessary, the configuration contributes to energy saving. 
     A second embodiment is explained. In the second embodiment, explanation is omitted concerning the same components as the components in the first embodiment. 
     A sheet binding apparatus is not limited to including the second spring  94  (see  FIG. 4 ) that urges the second roller  92  toward the first roller  91 . The second embodiment is different from the first embodiment in that the sheet binding apparatus does not include the second spring  94  (an urging member). In other words, in the second embodiment, the first roller  91  among the first roller  91  and the second roller  92  includes the urging member. 
       FIG. 14  is a front view showing a sheet binding apparatus  203  in the second embodiment. 
     As shown in  FIG. 14 , the sheet binding apparatus  203  includes a supporting member  96  that rotatably supports the second roller  92 . The supporting member  96  supports the second roller  92  in a fixed position. 
     The main guide  31  guides the sheet bundle  5  to cause the edge portion leading end  5   e  of the sheet bundle  5  to face a nip forming end of the second roller  92 . The nip forming end of the second roller  92  means, on the outer circumferential surface of the second roller  92 , a portion that forms the nip  90 N in cooperation with the first roller  91 . The nip forming end of the second roller  92  is equivalent to an end edge closest to the first roller  91  in the sheet bundle thickness direction Z in the second roller  92 . Before insertion of the sheet bundle  5 , the nip forming end of the second roller  92  is an end edge of the minimum radius section  92   a . In the figure, reference sign K 1  indicates an imaginary straight line that passes the edge portion leading end  5   e  of the sheet bundle  5  and the nip forming end of the second roller  92 . 
     According to the second embodiment, the sheet binding apparatus  203  includes the urging member  93 , the supporting member  96 , and the main guide  31 . The urging member  93  urges the first roller  91  toward the second roller  92 . The supporting member  96  rotatably supports the second roller  92 . The main guide  31  guides the sheet bundle  5  to cause the edge portion leading end  5   e  of the sheet bundle  5  to face the nip forming end of the second roller  92 . With the configuration explained above, the following effects are achieved. 
     Since it is possible to cause, with the urging member  93  (the first spring), the tape T to follow the edge portion  5   a  of the sheet bundle  5 , it is possible to more surely bind the sheet bundle  5 . In addition, compared with when the edge portion leading end  5   e  of the sheet bundle  5  is guided to a position deviating from the nip forming end of the second roller  92 , it is easy to rush the sheet bundle  5  into between the first roller  91  and the second roller  92 . 
     A third embodiment is explained. In the third embodiment, explanation is omitted concerning the same components as the components in the first embodiment. 
     A sheet binding apparatus is not limited to adjusting a nip pressure with a shape of a roller. The third embodiment is different from the first embodiment in that the sheet binding apparatus includes a control section that controls the nip pressure. 
       FIG. 15  is a front view showing a sheet binding apparatus  303  in the third embodiment. 
     As shown in  FIG. 15 , the sheet binding apparatus  303  includes a nip-pressure adjusting section  380  capable of adjusting the nip pressure on the basis of the position of the edge portion leading end  5   e  of the sheet bundle  5 . The nip-pressure adjusting section  380  may include a sensor  310  that detects the edge portion leading end  5   e  of the sheet bundle  5  and a control section  320  (hereinafter referred to as “nip-pressure control section  320 ” as well) that controls the nip pressure on the basis of a detection result of the sensor  310 . 
     For example, the sensor  310  is a non-contact displacement sensor such as a laser-type displacement sensor. The sensor  310  is located between the main guide  31  and the first roller  91  in an inserting direction of the sheet bundle  5 . The sensor  310  is located between the tape holding section  58  and the tape attaching section  59  in a state in which the tape holding section  58  disposes the tape T to extend across the first roller  91  and the second roller  92 . In this embodiment, the first roller  91  and the second roller  92  respectively have perfect circle shapes. 
     In  FIG. 15 , reference numeral  321  indicates a supporting plate that supports the proximal end of the second spring  94 . Reference numeral  322  indicates an elliptical cam having an elliptical shape. Reference numeral  323  indicates a cam driving source for rotating the elliptical cam  322 . The elliptical cam  322  includes a minimum radius section  322   a  and a maximum radius section  322   b . Before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 , the minimum radius section  322   a  of the elliptical cam  322  is in contact with the supporting plate  321 . The elliptical cam  322  rotates with a rotating shaft  322   c  as an axis to adjust the nip pressure. For example, the cam driving source  323  is a motor. 
     The nip-pressure control section  320  controls the cam driving source  323  on the basis of a detection result of the sensor  310 . The nip-pressure control section  320  controls the cam driving source  323  to thereby adjust the nip pressure on the basis of the position of the edge portion leading end  5   e  of the sheet bundle  5 . 
     In this embodiment, after the sensor  310  detects the edge portion leading end  5   e  of the sheet bundle  5 , the nip-pressure control section  320  sets the nip pressure (a pressing force) to be larger than a threshold (a pressing threshold) after a set time elapses. For example, the set time is set to 1 ms or more and 100 ms or less. For example, the set time is set to a time until the sensor  310  detects a second sheet S from the edge portion leading end  5   e  of the sheet bundle  5  after detecting the edge portion leading end  5   e  of the sheet bundle  5 . 
     The nip-pressure control section  320  sets the nip pressure to be larger than the threshold after insertion of a second or subsequent sheet S from the edge portion leading end  5   e  of the sheet bundle  5  in the sheet bundle  5 . The nip-pressure control section  320  sets the nip pressure on the second or subsequent sheet S to be larger than the nip pressure on the first sheet S. 
     The nip-pressure control section  320  gradually increases the nip pressure according to an increase in the number of sheets after the insertion of the second or subsequent sheet S. While the first roller  91  is opposed to the step-like end face  7   c  of the sheet bundle  5 , the nip-pressure control section  320  gradually increases the nip pressure according to an increase in the number of sheets (see  FIG. 16 ). 
     The nip-pressure control section  320  may release the application of the nip pressure if the first roller  91  passes the step-like end face  7   c  of the sheet bundle  5  according to the insertion of the sheet bundle  5  (see  FIG. 17 ). In other words, the nip-pressure control section  320  may release the application of the nip pressure while the first roller  91  is opposed to the surface of a last sheet included in the sheet bundle  5  (see  FIG. 17 ). 
     According to the third embodiment, the nip-pressure adjusting section  380  includes the sensor  310  that detects the edge portion leading end  5   e  of the sheet bundle  5  and the nip-pressure control section  320  that controls the nip pressure on the basis of a detection result of the sensor  310 . With the configuration explained above, the following effects are achieved. 
     It is possible to adjust the nip pressure with the nip-pressure control section  320  on the basis of the position of the edge portion leading end  5   e  of the sheet bundle  5 . Compared with when the nip pressure is fixed irrespective of the position of the edge portion leading end  5   e  of the sheet bundle  5 , it is less likely that the nip pressure is too small and the tape T cannot be sufficiently stuck to the edge portion  5   a  of the sheet bundle  5 . In addition, it is less likely that the nip pressure is too large and the sheet bundle  5  cannot be rushed into between the first roller  91  and the second roller  92 . Therefore, it is easy to insert the sheet bundle  5  between the rollers before the insertion of the sheet bundle  5 . It is possible to apply a sufficient sticking force of the tape T to the edge portion  5   a  of the sheet bundle  5  after the insertion of the sheet bundle  5 . Accordingly, it is possible to automatically bind the sheet bundle  5 . 
     After the sensor  310  detects the edge portion leading end  5   e  of the sheet bundle  5 , the nip-pressure control section  320  sets the nip pressure to be larger than the threshold after the set time elapses. With the configuration explained above, the following effects are achieved. 
     It is possible to optimize control timing of the nip pressure. For example, the set time is set to a time until the sensor  310  detects a second sheet S from the edge portion leading end  5   e  of the sheet bundle  5  after detecting the edge portion leading end  5   e  of the sheet bundle  5 . Consequently, it is possible to apply a sufficient sticking force of the tape T to the step-like end face  7   c  of the sheet bundle  5 . 
     The nip-pressure control section  320  sets the nip pressure to be larger than the threshold after the insertion of the second or subsequent sheet S from the edge portion leading end  5   e  of the sheet bundle  5  in the sheet bundle  5 . With the configuration explained above, the following effect is achieved. 
     It is possible to apply a sufficient sticking force of the tape T to the second or subsequent sheet S from the edge portion leading end  5   e  of the sheet bundle  5 . 
     The nip-pressure control section  320  gradually increases the nip pressure according to an increase in the number of sheets after the insertion of the second or subsequent sheet S. With the configuration explained above, the following effects is achieved. 
     It is possible to adjust the nip pressure with respect to the number of sheets stepwise. Therefore, compared with when adjustment of the nip pressure with respect to the number of sheets is performed in only one stage, it is easy to cause the tape T to follow the edge portion  5   a  of the sheet bundle  5 . Therefore, it is possible to more effectively apply a sufficient sticking force of the tape T to the second or subsequent sheet S from the edge portion leading end  5   e  of the sheet bundle  5 . 
     The first roller  91  and the second roller  92  respectively have perfect circle shapes. With the configuration explained above, the following effect is achieved. 
     It is possible to automatically bind the sheet bundle  5  using the simple first and second rollers  91  and  92 . 
     A first modification of the third embodiment is explained. 
     A nip-pressure adjusting section is not limited to including the elliptical cam  322  that adjusts the nip pressure by rotating with the rotating shaft  322   c  as an axis. 
       FIG. 18  is a front view showing a sheet binding apparatus in the first modification of the third embodiment. As shown in  FIG. 18 , a nip-pressure adjusting section  380 A may not include the elliptical cam  322  (see  FIG. 15 ). In  FIG. 18 , reference numeral  395  indicates a first supporting member that rotatably supports the first roller  91 . Reference numeral  396  indicates a second supporting member that rotatably supports the second roller  92 . Reference numeral  397  indicates a roller driving source that brings the second roller  92  close to or separates the second roller  92  from the first roller  91 . For example, the roller driving source  397  includes a piston and crank mechanism. The nip-pressure control section  320  controls the roller driving source  397  to thereby adjust the nip pressure on the basis of the position of the edge portion leading end  5   e  of the sheet bundle  5 . 
     According to the first modification of the third embodiment, the nip-pressure adjusting section  380 A does not include the elliptical cam  322 . With the configuration explained above, the following effects are achieved. 
     Compared with when the nip-pressure adjusting section includes the elliptical cam  322 , the number of components is reduced. The configuration contributes to a reduction in cost. 
     Other modifications of the embodiments are explained below. 
     Before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 , the roller interval is not limited to zero. For example, before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 , the roller interval may be the same as the thickness of the sheet bundle  5 . That is, before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 , the roller interval may be larger than zero and equal to or smaller than the thickness of the sheet bundle  5 . 
     A sheet binding apparatus is not limited to including the main guide  31  that is provided between the first roller  91  and the second roller  92  and guides the sheet bundle  5  and the sub-guide  32  opposed to the main guide  31  in the sheet bundle thickness direction Z. For example, the sheet binding apparatus may not include the sub-guide  32 . For example, the sheet binding apparatus may include the main guide  31 . 
     According to at least one embodiment explained above, the sheet binding apparatus  3  includes the first roller  91 , the second roller  92 , and the nip-pressure adjusting section  80 . The first roller  91  attaches the tape T to the edge portion  5   a  of the sheet bundle  5 . The second roller  92  is opposed to the first roller  91  in the sheet bundle thickness direction Z. The nip-pressure adjusting section  80  is capable of adjusting, on the basis of the position of the edge portion leading end  5   e  of the sheet bundle  5 , the pressing force (the nip pressure) of the nip  90 N formed by the first roller  91  and the second roller  92 . Before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 , the interval between the first roller  91  and the second roller  92  (the roller interval) is equal to or smaller than the thickness of the sheet bundle  5 . With the configuration explained above, the following effects are achieved. 
     It is possible to adjust the nip pressure with the nip-pressure adjusting section  80  on the basis of the position of the edge portion leading end  5   e  of the sheet bundle  5 . Compared with when the nip pressure is fixed irrespective of the position of the edge portion leading end  5   e  of the sheet bundle  5 , it is less likely that the nip pressure is too small and the tape T cannot be sufficiently stuck to the edge portion  5   a  of the sheet bundle  5 . In addition, it is less likely that the nip pressure is too large and the sheet bundle  5  cannot be rushed into between the first roller  91  and the second roller  92 . Therefore, it is easy to insert the sheet bundle  5  between the rollers before the insertion of the sheet bundle  5 . It is possible to apply a sufficient sticking force of the tape T to the edge portion  5   a  of the sheet bundle  5  after the insertion of the sheet bundle  5 . In addition, before the sheet bundle  5  is inserted between the first roller  91  and the second roller  92 , compared with when the roller interval is larger than the thickness of the sheet bundle  5 , it is easy to cause the tape T to follow the edge portion  5   a  of the sheet bundle  5 . Therefore, it is possible to more surely bind the sheet bundle  5 . 
     While certain embodiments have been described these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms: furthermore various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and there equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.