Patent Publication Number: US-2012027427-A1

Title: Sheet finisher, image forming apparatus and sheet finishing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from: U.S. provisional application 61/368,625 filed on Jul. 28, 2010, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a sheet finisher, an image forming apparatus, and a sheet finishing method. 
     BACKGROUND 
     In the related art, there is known a sheet finisher which is disposed downstream of an image forming apparatus such as a copier, a printer or an MFP (Multi-Functional Peripheral), and performs a finishing process such as a punching process or a stitching process for a printed sheet. 
     Recently, as the function of this sheet finisher is diversified, a sheet finisher is proposed which has, in addition to the function of the punching process and the stitching process, the function of a folding process to fold a part of a sheet or the function of a saddle-stitching and folding process to staple the central area of a sheet and then to fold the sheet at the central area. 
     In the sheet finisher having the function of the saddle-stitching and folding process, it becomes possible to form a booklet (to bind a book) from a plurality of printed sheets. 
     In the saddle-stitching and folding process proposed in the related art, after the central area of sheets is stitched with staples or the like, a process is performed for forming a fold line on the stitched part by a pair of rollers called fold rollers and for forming folding. At this time, a plate-like member called a fold blade is brought into contact with the stitched part of the sheet bundle, and is pressed into a nip section of the fold roller pair to form the fold line on the sheet bundle. 
     However, since the time when the folded part of the sheet bundle is pressed by the nip section of the fold rollers is short, and the whole folded part is simultaneously pressed by the nip section of the fold rollers, the pressure is dispersed to the whole fold line. Thus, the fold line formed by the fold rollers becomes such a fold line that the pressure is not sufficiently applied thereto. Particularly, in a case where the number of sheets is large, or in a case where a thick sheet is contained in the sheet bundle, the fold line often becomes incomplete. 
     In order to deal with this problem, there is proposed a fold line reinforcing device in which the fold line pushed out of the fold rollers is inserted into a nip section of a pair of fold reinforcing rollers and the fold line is reinforced by moving the pair of fold reinforcing rollers along the fold line. 
     The movement direction of the fold reinforcing rollers is a direction that is orthogonal to a transport direction of the sheet bundle. Thus, if a sheet jam occurs due to any cause during movement of the fold reinforcing rollers, it is difficult to deal with this problem. Even though a user desires to pull out the sheet bundle along the transport direction, since the transport direction of the sheet bundle and the rotation direction of the fold reinforcing rollers are perpendicular each other, it is difficult to smoothly pull out the sheet bundle. 
     Upon the occurrence of the sheet jam, there is proposed such a technique that an alarm is displayed or the fold reinforcing rollers return to a home position. However, with this technique, the whole apparatus should be temporarily stopped. Thus, the booklet forming efficiency is lowered. In addition, a user is forced to extra work for removing the sheet bundle from the apparatus. 
     Accordingly, it is desirable to provide a sheet finisher, an image forming apparatus and a sheet finishing method which can effectively reduce the possibility that the apparatus is temporarily stopped due to a sheet jam. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing an outer appearance example of an image forming apparatus according to an exemplary embodiment; 
         FIG. 2  is a sectional view showing a configuration example of the image forming apparatus; 
         FIG. 3  is a sectional view showing a configuration example of a saddle stitch processing unit; 
         FIG. 4  is a perspective outer appearance view showing the whole structure of a fold reinforcing unit; 
         FIGS. 5A and 5B  are schematic sectional views for mainly illustrating a structure of a support section; 
         FIG. 6  is a perspective outer appearance view showing a structural example of a roller unit; 
         FIG. 7  is a view of the fold reinforcing unit seen from a transport destination of a sheet bundle; 
         FIG. 8  is a first view illustrating a mechanism of up-and-down driving of an upper roller; 
         FIG. 9  is a second view illustrating the mechanism; 
         FIG. 10  is a diagram showing a detailed configuration example of a drive unit; 
         FIG. 11  is a diagram illustrating a switch mechanism of torque of the drive unit; 
         FIG. 12  is a flowchart illustrating an example of an operation when a sheet jam occurs in a sheet finisher according to the exemplary embodiment; 
         FIG. 13  is a diagram illustrating a switching operation of a drive torque when the sheet jam occurs and an operation of the fold reinforcing roller thereafter; and 
         FIG. 14  is a flowchart illustrating an example of the switching operation according the number of sheets of the sheet bundle or the presence or absence of a thick sheet. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of a sheet finisher and an image forming apparatus will be described with reference to the accompanying drawings. 
     The sheet finisher according to an embodiment includes: a fold roller which folds a central area of a sheet bundle to form a fold line on the sheet bundle; a fold reinforcing roller which moves along a direction of the fold line while pressing the fold line of the sheet bundle to reinforce the fold line; and a drive unit which includes a motor and transmits a drive torque of the motor to the fold reinforcing roller, so that the drive torque can be changed, to move the fold reinforcing roller along the fold line. 
     (1) Configuration 
       FIG. 1  is an outer appearance perspective view showing a basic configuration example of an image forming apparatus  10  according to an embodiment. The image forming apparatus  10  includes a read unit  11  which reads an original document, an image forming unit  12  which prints image data on the read original document to a sheet by an electro-photographic technique, and a sheet finisher  20  which performs a finishing process such as a sorting process, a punching process, a folding process or a saddle stitching process, for the printed sheet. Further, the image forming unit  12  is provided with an operation unit  9  by which a user performs various operations. 
       FIG. 2  is a sectional view showing a detailed configuration example of the image forming apparatus  10 . 
     The image forming unit  12  of the image forming apparatus  10  includes a photoconductive drum  1  in the vicinity of the central area thereof, and a charging unit  2 , an exposing unit  3 , a developing unit  4 , a transfer unit  5 A, a charge removing unit  5 B, a separating pawl  5 C, and a cleaning unit  6  are respectively disposed around the photoconductive drum  1 . Further, a fixing unit  8  is provided on a downstream side of the charge removing unit  5 B. An image forming process is performed by these units roughly in the following procedure. 
     First, a surface of the photoconductive drum  1  is uniformly charged by the charging unit  2 . On the other hand, an original document read by the read unit  11  is converted into image data, and is input to the exposing unit  3 . In the exposing unit  3 , a laser beam corresponding to the level of the image data irradiates the photoconductive drum  1 , and an electrostatic latent image is formed on the photoconductive drum  1 . The electrostatic latent image is developed with a toner supplied from the developing unit  4 , and thus a toner image is formed on the photoconductive drum  1 . 
     On the other hand, a sheet contained in a sheet containing unit  7  is transported to a transfer position (gap between the photoconductive drum  1  and the transfer unit  5 A) through some transport rollers. At the transfer position, the toner image is transferred from the photoconductive drum  1  to the sheet by the transfer unit  5 A. Electric charges on the surface of the sheet on which the toner image is transferred are erased by the charge removing unit  5 B, and are separated from the photoconductive drum  1  by the separating pawl  5 C. Thereafter, the sheet is transported by an intermediate transport section  7 B, and is heated and pressed by the fixing unit  8 , so that the toner image is fixed to the sheet. The sheet on which the fixing process is completed is discharged from a discharge section  7 C and is output to the sheet finisher  20 . 
     The developer remaining on the surface of the photoconductive drum  1  is removed by the cleaning unit  6  on the downstream side of the separating pawl  5 C, and is ready for the next image formation. 
     When duplex printing is performed, the sheet on the surface of which the toner image is fixed is branched from a normal discharge path by a transport path switching plate  7 D, is switched back in a reversal transport section  7 E to be turned upside down. A print process similar to one-side printing is performed on the back side of the reversed sheet, and then the sheet is output from the discharge section  7 C to the sheet finisher  20 . 
     The sheet finisher  20  includes a saddle stitch processing unit  30  and a sheet bundle placement section  40  in addition to a sorter section (not shown) which sorts the sheets. 
     The saddle stitch processing unit  30  performs a process (saddle stitch process) for stitching a central area of a plurality of printed sheets discharged from the image forming unit  12  with staples and performing folding, so as to form a booklet. 
     The booklet saddle-stitched by the saddle stitch processing unit  30  is output to the sheet bundle placement section  40 , and the bound booklet is finally placed thereon. 
       FIG. 3  is a sectional view showing a detailed configuration example of the saddle stitch processing unit  30 . 
     In the saddle stitch processing unit  30 , the sheet discharged from the discharge section  7 C of the image forming unit  12  is received by an inlet roller pair  31  and is delivered to an intermediate roller pair  32 . The intermediate roller pair  32  delivers the sheet to an outlet roller pair  33 . The outlet roller pair  33  sends the sheet to a standing tray  34  having an inclined placement surface. The leading edge of the sheet is directed toward an upper part of the inclination of the standing tray  34 . 
     A stacker  35  is provided below the standing tray  34 , and receives the lower edge of the sheet which is switched back and falls from the upper part of the inclination of the standing tray  34 . 
     A stapler  36  is provided at the middle of the standing tray  34 . When the saddle stitch process (stapling) is performed on the sheet bundle, the position of the stacker  35  is adjusted so that the position of the sheet bundle to be stapled (a central area of the sheet bundle in the up-and-down direction) faces the stapler  36 . 
     When the sheet bundle is stapled by the stapler  36 , the stacker  35  descends until the position of the sheet bundle where a fold line is to be formed (the central area of the sheet bundle in the up-and-down direction and the position where the staples are inserted) comes to the front of a fold blade  37 . 
     When the position where the fold line is to be formed comes to the front of the fold blade  37 , a leading edge  37   a  of the fold blade  37  pushes a surface which becomes an inner surface after the sheet bundle is folded. 
     A fold roller pair  38  is provided ahead of the fold blade  37  in a traveling direction. The sheet bundle pushed by the fold blade  37  slides into a nip section of the fold roller pair  38 , and the fold line is formed at the central area of the sheet bundle. 
     The sheet bundle on which the fold line is formed by the fold roller pair  38  is transported to a fold reinforcing unit  50  provided on the downstream side thereof. The sheet bundle transported to the fold reinforcing unit  50  is temporarily stopped there. 
     The fold reinforcing unit  50  is provided with a fold reinforcing roller pair  51  which includes an upper roller  51   a  and a lower roller  51   b.  The fold reinforcing roller pair  51  moves in a direction (direction along the fold line) orthogonal to the transport direction of the sheet bundle while pressing the fold line, to reinforce the fold line. 
     The sheet bundle whose fold line is reinforced by the fold reinforcing unit  50  again starts to be transported, is pulled by an discharge roller pair  39  and is output to the sheet bundle placement section  40 , and the sheet bundle (booklet) which is saddle-stitched is placed on the sheet bundle placement section  40 . 
     (2) Structure and Operation of Fold Reinforcing Unit 
       FIG. 4  is a perspective outer appearance view showing the whole structure of the fold reinforcing unit  50 . The fold reinforcing unit  50  includes a fold reinforcing roller unit  60  (hereinafter, simply referred to as a roller unit  60 ), a support section  70  and a drive unit  80 . 
     The roller unit  60  includes the fold reinforcing roller pair  51 . The fold reinforcing roller pair  51  nips and presses the fold line of the sheet bundle pushed out of the upstream fold roller pair  38 , and moves along the fold line to reinforce the fold line. 
     The support section  70  supports the roller unit  60  so that the roller unit  60  can slide in the fold line direction, and includes a nipping member of the sheet bundle, a structural member of the whole fold reinforcing unit  50 , and the like. 
     The drive unit  80  includes a drive motor  81 , and drives the roller unit  60  along the fold line by the drive motor  81 . 
     Among the roller unit  60 , the support section  70  and the drive unit  80 , the structure of the support section  70  will be firstly described with reference to  FIG. 4  and  FIGS. 5A and 5B .  FIGS. 5A and 5B  are schematic sectional views for mainly illustrating the structure of the support section  70 .  FIG. 5A  is a sectional view when the roller unit  60  is at a home position (standby position: left end position in  FIG. 4 ), and  FIG. 5B  is a sectional view when the roller unit  60  is moving (the fold line is reinforced). 
     The support section  70  includes a frame  71 , and the frame  71  includes a top plate  711 , right and left side plates  712   a  and  712   b,  a bottom plate  713 , a back plate  714 , a sheet bundle placement table  715  (see  FIGS. 5A and 5B , etc.) and the like. 
     The top plate  711  is provided with a support hole  711   a  extending in its longitudinal direction. 
     Further, a support shaft  75  which supports the roller unit  60 , a transport guide  72  having an L-shaped section, a drive shaft  76  (see  FIG. 5A  and  FIG. 5B , etc.) which drives the transport guide  72  in the up-and-down direction, and the like are provided between both the side plates  712   a  and  712   b.    
     A band-like flexible member  73  formed of a film-like resin member of polyethylene terephthalate (PET) or the like is extended from a bottom plate  72   a  of the transport guide  72 . A similar flexible member  74  is extended also from the sheet placement table  715 . 
     As shown in  FIGS. 5A and 5B , a fold line  100   a  of a sheet bundle  100  is nipped between the flexible members  73  and  74 , and is pressed by the fold reinforcing roller pair  51  (the upper roller  51   a  and the lower roller  51   b ) through the flexible members  73  and  74 , and thus the fold line is reinforced. The occurrence of a scratch or a wrinkle in the fold line and in the vicinity thereof is prevented through the flexible members  73  and  74 . 
     Cut sections  73   a  and  74   b  are provided at leading edges of the flexible members  73  and  74 . These cut sections  73   a  and  74   b  are provided at positions corresponding to positions of staples of the fold line, and prevent the flexible members  73  and  74  from being damaged by the staples. 
     A through hole  61  through which the support shaft  75  passes is provided in a lower part of the roller unit  60 . Further, a support roller  62  for keeping the attitude is provided in an upper part of the roller unit  60 , and the support roller  62  is moved along the support hole  711   a  provided in the top plate  711 . 
     The position (except for a position change in the movement direction) of the roller unit  60  and the three-axial attitude are regulated by the support shaft  75  and the through hole  61 , and the support hole  711   a  and the support roller  62 , and are kept constant also during the movement of the roller unit  60 . 
     Next, the structure of the roller unit  60  will be described.  FIG. 6  is a perspective outer appearance view showing a structural example of the roller unit  60 , and is a view seen from a sending source side of the sheet bundle (direction opposite to  FIG. 4 ). 
     The roller unit  60  is a unit which is provided therein with the fold reinforcing roller pair  51 , and includes a unit support section  63  that is positioned at a lower part thereof and is provided with the through hole  61 , and a unit frame  67  fixed to an upper part of the unit support section  63 . 
     In the unit frame  67 , an upper frame  67   a  having a hollow section and a lower frame  67   b  having a similar hollow section are fixed and coupled by a frame plate  67   c.    
     Further, the roller unit  60  includes an upper link member  65  and a lower link member  66 , and both the members  65  and  66  are spring-coupled by a spring  68 . One end of the spring  68  is engaged with a hook hole  65   b  of the upper link member  65 , and the other end of the spring  68  is engaged with a cut part  66   b  of the lower link member  66 .  FIG. 6  shows the spring  68  in a free state in which the other end of the spring  68  is released from the cut part  66   b,  but in a state where the other end of the spring  68  is actually engaged with the cut part  66   b,  a tensile force of the spring  68  is applied between the upper link member  65  and the lower link member  66 . 
     The lower roller  51   b  which is one of the fold reinforcing roller pair  51  is accommodated in the hollow section of the lower frame  67   b.  The lower roller  51   b  is supported around a lower roller shaft (not shown) fixed to the lower frame  67   b  to be able to rotate. 
     Further, the lower link member  66  is coupled to the side of the lower frame  67   b  through a lower link shaft  66   a  (see  FIG. 4 ) fixed to the lower frame  67   b  to be able to rotate. 
     The upper roller  51   a  which is one of the reinforcing roller pair  51  is accommodated in the hollow section of the upper frame  67   a.  The upper roller  51   a  is supported around an upper roller shaft (not shown) fixed to the upper link member  65  (not the upper frame  67   a ) to be able to rotate. 
     The rotation shaft (lower roller shaft) of the lower roller  51   b  is fixed to the lower frame  67   b  (that is, fixed to the unit frame  67 ), and even if the roller unit  60  is moved, the position of the lower roller  51   b  is not changed in the up-and-down direction. The position of the upper end of the lower roller  51   b  is adjusted to be the same as the position of the flexible member  74 . When the roller unit  60  is moved, the lower roller  51   b  is rotated while coming in contact with the lower surface of the flexible member  74 . 
     On the other hand, the upper roller shaft of the upper roller  51   a  is fixed to the upper link member  65 . When the roller unit  60  starts to move away from the home position, the upper link member  65  is pulled by the spring  68 , and starts to rotate downward around a upper link shaft  65   a.  By this rotation, the upper roller  51   a  rotatably attached to the upper link member  65  starts to descend, and is moved to a position where the upper roller  51   a  comes in contact with the lower roller  51   b.  The pressing force caused by the tensile force of the spring  68  is mutually exerted between the upper roller  51   a  and the lower roller  51   b.  Actually, since the sheet bundle is nipped between the upper roller  51   a  and the lower roller  51   b  through the flexible members  73  and  74 , the fold line of the sheet bundle is reinforced by the pressing force between the upper roller  51   a  and the lower roller  51   b.    
       FIG. 7  is a diagram illustrating a position relationship of the drive unit  80 .  FIG. 7  is a view when seen from a transport destination of the sheet bundle to a transport source thereof, and also shows the roller unit  60  at the home position, the fold roller pair  38  and the drive mechanism of the fold roller pair  38 . The illustration of the structural member of the support section  70  is partially omitted for convenience of explanation. 
     The drive unit  80  includes a drive motor  81  and a gear train which will be described in a portion surrounded by a broken line in  FIG. 7 . The drive motor  81  is a DC motor, and the rotation direction and speed thereof can be controlled from the outside. 
     The drive force of the drive motor  81  is transmitted to a driving gear pulley  86   a  through the gear train. On the other hand, a unit drive belt  87  is stretched between the driving gear pulley  86   a  and a driven pulley  86   b.  The unit drive belt  87  is moved between the driving gear pulley  86   a  and the driven pulley  86   b  by the drive force of the drive motor  81 . 
     A rack is formed on the surface of the unit drive belt  87 , and the rack is engaged with teeth of a fit section  63   a  (see  FIG. 6 ) provided at the lower part of the roller unit  60 , so that the roller unit  60  can be reliably moved without sliding in the fold line direction. The movement direction of the unit drive belt  87  can be changed by reversing the rotation direction of the drive motor  81 , and thus the roller unit  60  can be reciprocated. 
     In the fold reinforcing unit  50 , the upper roller  51   a  moves up and down inside the roller unit  60  and the transport guide  72  moves up and down, in addition to the movement of the roller unit  60  in the fold line direction. The drive source of the up and down movements of these elements is the drive motor  81 . That is, the drive operation of the fold reinforcing unit  50  is carried out by the single drive motor  81 . 
       FIG. 7  shows a home position sensor  89  which detects that the roller unit  60  moves away from the home position or the roller unit  60  returns to the home position. 
       FIGS. 8 and 9  are views illustrating the mechanism of the up-and-down drive of the upper roller  51   a.  As described above, the upper link member  65  and the lower link member  66  of the roller unit  60  are spring-coupled by the spring  68  at the positions farthest from the respective rotation shafts (the upper and lower link shafts  65   a  and  66   a ). Further, the lower link member  66  is provided with a freely rotating guide roller  66   c  (see  FIG. 4 , etc.). 
     On the other hand, as shown in  FIGS. 8 and 9 , the support section  70  includes a guide rail  77  having an L-shaped section. The guide rail  77  has a slope section  77   a  in the vicinity of the home position, and is parallel to the fold line direction of the sheet bundle, (i.e., horizontal) except for the slope section  77   a.    
     When the roller unit  60  is driven by the drive belt  87  and moves away from the home position, as shown in  FIG. 9 , the guide roller  66   c  comes in contact with the bottom of the slope section  77   a  of the guide rail  77  before long. Thereafter, the guide roller  66   c  descends along the bottom of the slope section  77   a.  As the guide roller  66   c  descends, the lower link member  66  is rotated around the lower link shaft  66   a  in the counterclockwise direction in  FIG. 9 . Further, the upper link member  65  is also pulled by the spring  68  and is rotated around the upper link shaft  65   a  in the counterclockwise direction. As a result, the upper roller  51   a  between the upper link shaft  65   a  and the hook hole  65   b  of the spring  68  gradually descends while the roller unit  60  moves on the slope section  77   a,  and the interval between the upper roller  51   a  and the lower roller  51   b  is gradually shortened. Then, the upper roller  51   a  and the lower roller  51   b  come in contact with each other in the vicinity of an area where the slope section  77   a  is terminated. At this time, a pressure (pressing force) to press each other is exerted between the upper roller  51   a  and the lower roller  51   b.  The pressing force is based on the tensile force of the spring  68 . 
     In a horizontal area (that is, the effective drive area) of the guide rail  77 , the upper roller  51   a  and the lower roller  51   b  apply the pressure to the fold line of the sheet bundle while keeping the pressing force to thereby reinforce the fold line. 
     (3) Driving of Fold Reinforcing Roller 
       FIG. 10  is a diagram showing a detailed configuration example of the drive unit  80 . As described above, the drive unit  80  includes the drive motor  81  and the gear train. The rotational drive force of the drive motor  81  is transmitted to the driving gear pulley  86   a  through the gear train. Thus, the unit drive belt  87  is moved in the horizontal direction by rotation of the driving gear pulley  86   a,  and the roller unit  60  fixed to the unit drive belt  87  through the fit section  63   a  is also moved in the horizontal direction. When the roller unit  60  starts to horizontally move away from the home position, the upper roller  51   a  descends toward the lower roller  51   b  and nips the fold line of the sheet bundle. Thereafter, the fold reinforcing roller pair  51  (upper roller  51   a  and lower roller  51   b ) moves in the horizontal direction while pressing the fold line to reinforce the fold line. 
     The gear train includes a plurality of gears which forms two paths of a transmission path A and a transmission path B. The transmission path A includes a gear train having a shaft gear  801  of the drive motor  81 , a gear  802 , a gear  803  (electromagnetic clutch A), a gear  804  and a gear  805 . On the other hand, the transmission path B includes a gear train having the shaft gear  801  of the drive motor  81 , the gear  802 , a gear  806  and a gear  807  (electromagnetic clutch B). The gear  803  and the gear  807  are formed as the electromagnetic clutches. When each electromagnetic clutch is turned on, the rotation is transmitted, and when the electromagnetic clutch is turned off, the rotation transmission is cut off. 
       FIG. 11  is a diagram schematically illustrating the relationship between ON and OFF of the electromagnetic clutches A and B and selection of the transmission paths, and the like. When the electromagnetic clutch A is turned on and the electromagnetic clutch B is turned off, the transmission path A is selected. Contrarily, when the electromagnetic clutch A is turned off and the electromagnetic clutch B is turned on, the transmission path B is selected. Here, a drive torque of the transmission path B is set to be larger than a drive torque of the transmission path A. A specific method of setting different drive torques according to the transmission paths is not limitative. For example, if gear diameters and gear pitches of the gear  802 , the gear  803 , the gear  805 , the gear  806  and the gear  807  are all the same, a torque ratio Ta/Tb becomes a ratio ra/rb of a radius ra of the driving gear pulley  86   a  to a radius rb of the gear  807  (rb&gt;ra). Here, Ta (this torque is referred to as a normal torque) is a driving torque of the driving gear pulley  86   a  when the transmission path A is selected, and Tb is to a driving torque of the driving gear pulley  86   a  when the transmission path B is selected. Accordingly, in the example shown in  FIG. 11 , the drive torque transmitted to the driving gear pulley  86   a  when the transmission path B is selected becomes larger than that when the transmission path A is selected. Meanwhile, the rotation speed of the driving gear pulley  86   a  when the transmission path B is selected becomes slower than that when the transmission path A is selected. 
     Note that unit drive belt  87  is engaged with the driving gear pulley  86   a,  and the fold reinforcing roller pair  51  moves with the unit drive belt  87 . Thus, the force by which the fold reinforcing roller pair  51  is moved is proportional to the drive torque of the driving gear pulley  86   a.  Accordingly, the force by which the fold reinforcing roller pair  51  is moved becomes relatively strong when the transmission path B is selected, although the movement speed of the fold reinforcing roller pair  51  becomes relatively slow when the transmission path B is selected. 
     The sheet finisher  20  according to the present embodiment switches the transmission path of the rotation of the drive motor  81  to change the drive torque of the driving gear pulley  86   a,  and thus to change the force by which the fold reinforcing roller pair  51  is moved. 
     Specifically, the sheet finisher  20  drives the fold reinforcing roller pair  51  at a normal torque and a normal speed during a normal operation. On the other hand, when the sheet jam occurs, the sheet finisher  20  drives the driving gear pulley  86   a  with a large torque to move the fold reinforcing roller pair  51  with stronger power, while decreasing the movement speed of the fold reinforcing rollers  51  to some extent. As a result, even when a large load is applied to the fold reinforcing rollers  51  due to the sheet jam and the fold reinforcing roller pair  51  is stopped, the sheet finisher  20  changes the transmission path so that the drive torque becomes large, and thus, it is possible to climb over a portion where the sheet jam occurs to further go on. 
       FIG. 12  is a flowchart illustrating an example of an operation of the sheet finisher  20  according to the present embodiment when the sheet jam occurs. 
     In ACT  1 , the torque of the drive unit  80  is set to an initial value (normal torque). In the example shown in  FIG. 11 , the transmission path A is selected by turning on the electromagnetic clutch A and by turning off the electromagnetic clutch B, and thus the torque transmitted to the driving gear pulley  86   a  is set to the normal torque. 
     In ACT  2 , the roller unit  60  (fold reinforcing roller pair  51 ) starts to move in a going path direction. In ACT  3 , the fold reinforcing process of a going path is performed, and the presence or absence of the sheet jam is detected (ACT  4 ). 
     A specific method of detecting the sheet jam occurrence is not limitative, but for example, when time elapsed after the roller unit  60  moves away from the home position exceeds a predetermined threshold, it is determined that the roller unit  60  stops in the middle thereof, that is, that the sheet jam occurs. In the normal fold reinforcing process operation without the sheet jam occurrence, the roller unit  60  is reciprocated along the fold line. Accordingly, when the roller unit  60  moves away from the home position and does not return to the home position within a predetermined elapse time, it can be considered that the roller unit  60  is stopped in the middle due to the sheet jam occurrence. The fact that the roller unit  60  moves away from the home position or returns to the home position can be detected by the home position sensor  89  shown in  FIG. 7  or the like. 
     Instead, the sheet jam may be detected using the transport time of the sheet bundle. The sheet bundle on which the fold line is to be formed by the fold rollers  38  is temporarily stopped when the fold line reaches the position of the fold reinforcing roller pair  51 , and the fold reinforcing process is performed there. When the fold reinforcing process is completed, the sheet bundle starts to move again in the transport direction, is pulled by the discharge roller pair  39 , and then is discharged to the sheet bundle placement section  40 . Thus, for example, sensors for detecting passage of the sheet bundle may be respectively provided on the outlet side of the fold rollers  38  and on the front side of the discharge roller pair  39 , and time points when the leading edge of the sheet bundle passes therethrough may be respectively measured using the two sensors. Consequently, when a difference between the time points, that is, the transport time of the sheet bundle is longer than a predetermined time, it may be determined that the sheet jam occurs during the fold reinforcing process. 
     If it is determined that the sheet jam is detected in ACT  4 , the torque of the drive unit  80  is switched from the normal torque to a high torque, in ACT  5 . Specifically, as shown in  FIG. 13 , the electromagnetic clutch A is turned off and the electromagnetic clutch B is turned on, and thus the transport path A is switched into the transport path B. As a result, the torque transmitted to the driving gear pulley  86   a  is switched from the normal torque to the high torque. Through this switching, the roller unit  60  can move with a force stronger than that at the normal torque, and can overcome the sheet jam occurrence portion to further go on. 
     Then, as shown in an upper right section of  FIG. 13 , the fold reinforcing roller pair  51  is stopped in a position which exceeds the edge part of the sheet bundle (ACT  6 ). That is, the roller unit  60  proceeds up to a position which does not obstruct discharge of the sheet bundle, and then stops there. 
     In ACT  7 , the sheet bundle is pushed out by the fold rollers  38 , and is then discharged to the sheet bundle placement section  40  by being pulled by the discharge roller pair  39 . 
     Further, after the sheet bundle is discharged, the roller unit  60  is moved to the home position (ACT  8 ). 
     On the other hand, if the sheet jam does not occur in the fold reinforcing process of the going path (NO in ACT  4 ), a fold reinforcing process of a returning path is performed (ACT  9 ). 
     Further, if the sheet jam occurs during the fold reinforcing process of the returning path (YES in ACT  10 ), in a similar way to ACT  5 , the normal torque is switched to the high torque to overcome the sheet jam occurrence portion (ACT  11 ). After overcoming the sheet jam occurrence portion, the roller unit  60  returns to the home position. 
     If the sheet jam does not occur in the fold reinforcing process of the returning path (NO in ACT  10 ), the roller unit  60  also returns to the home position (ACT  12 ). 
     After the roller unit  60  returns to the home position, the sheet bundle is discharged. 
     According to the sheet finisher  20  according to the above-described embodiment, when the sheet jam occurs, it is possible to automatically switch the force (drive torque), which drives the roller unit  60 , to the drive torque which is larger than the normal torque. As a result, the fold reinforcing roller pair  51  which is temporarily stopped due to the sheet jam can overcome the sheet jam occurrence portion to further go on, to thereby prevent the image forming apparatus from being temporarily stopped. Further, a user does not have to perform a repairing operation of pulling the sheet bundle which is stopped in the state of being nipped in the fold reinforcing roller pair  51  by hand. 
     (4) Other Embodiments 
       FIG. 14  is a flowchart illustrating an operation example of the sheet finisher  200  according to another embodiment. In this embodiment, torque switching of the drive unit  80  is performed according to the number of sheets which forms the sheet bundle or whether a thick sheet which is equal to or thicker than a predetermined thickness is included in the sheet bundle. 
     In ACT  21 , it is determined whether the number of sheets of the sheet bundle is equal to or more than a predetermined number. For example, it is determined whether the number of sheets which forms the sheet bundle is  15  or more. If the number of sheets is equal to more than the predetermined number, in ACT  23 , the torque of the drive unit  80  is set to a high torque. The high torque setting is the same as in the above-described method. That is, the transmission path A is switched to the transmission path B by turning off the electromagnetic clutch A and by turning on the electromagnetic clutch B, and the torque transmitted to the driving gear pulley  86   a  is switched from the normal torque to the high torque. 
     Further, in ACT  22 , it is determined whether the thick sheet is included in the sheet bundle. Even when the number of sheets is less than the predetermined number, if the thick sheet is included in the sheet bundle (YES in ACT  22 ), the torque of the drive unit  80  is set to the high torque. 
     On the other hand, if the number of sheets of the sheet bundle is less than the predetermined number and the thick sheet is not included (NO in ACT  22 ), the torque of the drive unit  80  is set to the normal torque. That is, the transmission path A is selected by turning on the electromagnetic clutch A and by turning off the electromagnetic clutch B, and the torque transmitted to the driving gear pulley  86   a  is set to the normal torque. 
     In the flowchart shown in  FIG. 14 , the sheet number determination and the thick sheet presence or absence determination are all performed, but only one determination may be performed. 
     The number of sheets or the presence or absence of the thick sheet may be determined using information set by a user through the operation unit  9  of the image forming unit  12 , or may be determined using information on the number of sheets counted inside the image forming unit  12  or information on the sheet thickness detected inside the image forming unit  12 . 
     In ACT  25 , the fold reinforcing process of the sheet bundle is performed by reciprocating the fold reinforcing roller pair  51  according to the normal operation. Then, the sheet bundle on which the fold line is reinforced is discharged in ACT  26 . 
     Generally, it may be difficult to obtain a reliable fold line for the sheet bundle having a large number of sheets or the sheet bundle including a thick sheet, even though the fold reinforcing process is performed by the fold reinforcing roller pair  51 . However, in this embodiment, since the fold reinforcing roller pair  51  can be driven with a high torque with respect to the sheet bundle having the large number of sheets or the sheet bundle including the thick sheet, it is possible to form a reliable fold line compared with that of the related art. 
     By suitably combining a plurality of components disclosed in the embodiments, a variety of embodiments can be made. For example, some components may be omitted from all the components disclosed in the embodiment. Further, components included in different embodiments may be suitably combined.