Abstract:
Plural conveying paths including guide plates are arranged on both sides of a sheet in its thickness direction, being linked from an upstream side to a downstream side in a conveying direction of the sheet. A stopper is arranged in at least one of the conveying paths to make contact with a leading edge of the sheet conveyed through the conveying paths thereby stopping the sheet and forming a bending portion at the sheet. A pair of folding rollers is arranged between the conveying paths to nip the bending portion of the sheet to form a folding line at the sheet. An adjusting member is provided on the guide plates to adjust a space between the guide plates.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese priority document 2007-229313 filed in Japan on Sep. 4, 2007 and Japanese priority document 2008-160602 filed in Japan on Jun. 19, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a technology for performing a sheet folding operation by conveying a recording medium through a plurality of conveying paths. 
     2. Description of the Related Art 
     Japanese Utility Model Application Laid-open No. S62-68973 and Japanese Patent Application Laid-open No. H04-64577 disclose a sheet folding device that performs a folding operation such as an operation of folding a recording medium (hereinafter, “sheet”) in a Z fold. In such a sheet folding device, when a sheet is conveyed in a first conveying path toward a pair of folding rollers, the leading end of the sheet is brought into contact with a first stopper in the first conveying path, so that a portion of the sheet is bent by the first stopper. The bending portion is then conveyed between the folding rollers whereby a first folding operation is performed on the sheet to fold the sheet. The folded sheet is then conveyed toward a second stopper in a second conveying path, and the same process as in the first conveying path is performed in the second conveying path, so that a second folding operation is performed on the sheet to fold the sheet again. Each of the first conveying path and the second conveying path is formed by guide plates arranged on opposite sides of the sheet in a thickness direction. 
     However, in the sheet folding device described above, a space between the guide plates in the first conveying path is the same as that in the second conveying path, and the size of the space is not particularly specified for each of the first conveying path and the second conveying path. When the above folding operation is performed on a sheet, the sheet passing through each of the conveying paths has a different thickness, and every time the folding operation is performed on the sheet, the thickness of the sheet is increased. Specifically, the sheet passing through the first conveying path has the thickness of the original sheet because the folding operation has not been performed on the sheet yet. The sheet passing through the second conveying path has the thickness of double the original sheet because the sheet has been folded. Thus, because the thickness of the sheet is increased after the folding operation, a problem can occur if the space between the guide plates is the same in the first conveying path and the second conveying path. If the space between the guide plates is larger than the thickness of the sheet, it is difficult to convey the sheet in a stable manner or to fold the sheet at an appropriate position. On the other hand, if the space between the guide plates is smaller than the thickness of the sheet, friction between the sheet and the guide plates is increased, and therefore the sheet can be easily jammed between the guide plates. 
     Japanese Patent Application Laid-open No. 2002-332159 discloses a sheet folding device in which a sheet can be folded at an appropriate position in a desired manner, and can be conveyed in a conveying path without causing a jam, by setting a space between guide plates arranged downstream of folding rollers larger than that between guide plates arranged upstream of the folding rollers. 
     However, some sheet folding devices include a plurality of conveying paths and a plurality of pairs of folding rollers, and perform different folding operations (single fold, Z fold, outside triple fold, inside triple fold, simple quadruple fold, and gate fold) based on sheet folding modes by using different combinations of the conveying paths. In some sheet folding modes (folding operations), the thickness of a sheet conveyed through a conveying path arranged upstream of the folding rollers is the same as that in a conveying path arranged downstream of the folding rollers. In other sheet folding modes, however, the thickness of a sheet is changed (increased) when the sheet is conveyed from the conveying path arranged upstream of the folding rollers to the conveying path arranged downstream of the folding rollers. Therefore, even if the space between the guide plates arranged downstream of the folding rollers is larger than that between the guide plates arranged upstream of the folding rollers as disclosed in Japanese Patent Application Laid-open No. 2002-332159, it is difficult to convey a sheet in a smooth manner. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to an aspect of the present invention, there is provided a sheet folding device including a plurality of conveying paths for conveying a sheet, each including guide plates arranged on both sides of the sheet in its thickness direction, the conveying paths being linked from an upstream side to a downstream side in a conveying direction of the sheet; a stopper that is arranged in at least one of the conveying paths and is configured to make contact with a leading edge of the sheet conveyed through the conveying paths thereby stopping the sheet and forming a bending portion at the sheet; a pair of folding rollers that is arranged between the conveying paths, and nips the bending portion of the sheet to form a folding line at the sheet; and an adjusting member that is provided on the guide plates to adjust a space between the guide plates. 
     Furthermore, according to another aspect of the present invention, there is provided an image forming apparatus including a sheet folding device that includes a plurality of conveying paths for conveying a sheet, each including guide plates arranged on both sides of the sheet in its thickness direction, the conveying paths being linked from an upstream side to a downstream side in a conveying direction of the sheet; a stopper that is arranged in at least one of the conveying paths and is configured to make contact with a leading edge of the sheet conveyed through the conveying paths thereby stopping the sheet and forming a bending portion at the sheet; a pair of folding rollers that is arranged between the conveying paths, and nips the bending portion of the sheet to form a folding line at the sheet; and an adjusting member that is provided on the guide plates to adjust a space between the guide plates. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a sheet folding device according to a first embodiment of the present invention; 
         FIGS. 2A to 2F  are schematic diagrams for explaining sheet folding modes according to the first embodiment; 
         FIGS. 3A to 3E  are schematic diagrams of the sheet folding device for explaining a folding operation in a single fold mode; 
         FIGS. 4A to 4E  are schematic diagrams of the sheet folding device for explaining a folding operation in a Z fold mode; 
         FIGS. 5A to 5E  are schematic diagrams of the sheet folding device for explaining a folding operation in an outside triple fold mode; 
         FIGS. 6A to 6E  are schematic diagrams of the sheet folding device for explaining a folding operation in an inside triple fold mode; 
         FIGS. 7A to 7E  are schematic diagrams of the sheet folding device for explaining a folding operation in a simple quadruple fold mode; 
         FIGS. 8A to 8F  are schematic diagrams of the sheet folding device for explaining a folding operation in a gate fold mode; 
         FIG. 9  is an enlarged view of a relevant part of the sheet folding device for explaining arrangement of idler rollers; 
         FIG. 10  is an enlarged view of a relevant part of the sheet folding device for explaining an operation of adjusting a space between guide plates; 
         FIGS. 11 and 12  are flowcharts for explaining an operation of adjusting the space in each of conveying paths in the sheet folding device; and 
         FIG. 13  is a schematic diagram of an image forming apparatus according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention are explained in detail below with reference to the accompanying drawings. 
       FIG. 1  is a schematic diagram of a sheet folding device according to a first embodiment of the present invention. The sheet folding device receives a recording medium (hereinafter, “sheet”) on which an image is formed from an image forming apparatus (not shown), and then performs a predetermined folding operation on the sheet. Afterward, if post-processing, such as punching or stapling, is to be performed on the sheet by a post-processing device (not shown), the sheet folding device discharges the sheet toward the post-processing device. If the post-processing is not to be performed on the sheet, the sheet folding device causes the sheet to be stacked in a stacker  400  arranged in the sheet folding device. 
     The sheet folding device includes a first conveying path  101 , a second conveying path  102 , a third conveying path  103 , a fourth conveying path  104 , a fifth conveying path  105 , a sixth conveying path  106 , and a seventh conveying path  107  through which a sheet is conveyed. Each of the conveying paths  101  to  107  is formed by guide plates that are arranged on opposite sides of a sheet in its thickness direction with a predetermined space between the guide plates. The guide plates are arranged to guide a sheet conveyed through each of the conveying paths  101  to  107 . The conveying paths  101  to  107  are directly connected, or are connected through a pair of folding rollers. A first stopper  501 , a second stopper  502 , and a third stopper  503  are arranged in the third conveying path  103 , the fourth conveying path  104 , and the fifth conveying path  105 , respectively. When a sheet is conveyed to each of the conveying paths  103  to  105 , the leading end of the sheet is brought into contact with each of the stoppers  501  to  503  whereby each of the stoppers  501  to  503  stops the sheet. 
       FIGS. 2A to 2F  are schematic diagrams for explaining sheet folding modes (a single fold mode, a Z fold mode, an outside triple fold mode, an inside triple fold mode, a simple quadruple fold mode, and a gate fold mode) according to the first embodiment. With the above configuration, the sheet folding device performs a folding operation in each of the sheet folding modes. 
       FIGS. 3A to 3E  are schematic diagrams of the sheet folding device for explaining a folding operation in the single fold mode.  FIGS. 4A to 4E  are schematic diagrams of the sheet folding device for explaining a folding operation in the Z fold mode.  FIGS. 5A to 5E  are schematic diagrams of the sheet folding device for explaining a folding operation in the outside triple fold mode.  FIGS. 6A to 6E  are schematic diagrams of the sheet folding device for explaining a folding operation in the inside triple fold mode.  FIGS. 7A to 7E  are schematic diagrams of the sheet folding device for explaining a folding operation in the simple quadruple fold mode.  FIGS. 8A to 8F  are schematic diagrams of the sheet folding device for explaining a folding operation in the gate fold mode. 
     When the sheet folding device receives a sheet from the image forming apparatus, the sheet is guided to the first conveying path  101  by a first switching claw  301  ( FIG. 3A ). The sheet is then guided to the third conveying path  103 , so that the leading end of the sheet is brought into contact with the first stopper  501  that is movable depending on a fold position of the sheet ( FIG. 3B ). When the leading end of the sheet is brought into contact with the first stopper  501 , a portion of the sheet is bent. The bending portion is then conveyed through a nip (a first nip) formed between a first folding roller  201  and a second folding roller  202  whereby a first folding operation is performed on the sheet. Thus, the single fold operation is completed ( FIG. 3C ). 
     After the single fold operation is completed, the sheet is conveyed through a nip (a second nip) formed between the second folding roller  202  and a third folding roller  203 , and a nip (a third nip) formed between the third folding roller  203  and a fourth folding roller  204  without entering the fourth conveying path  104  ( FIG. 3D ). The folding operation is not performed on the sheet at the second nip and the third nip. 
     The sheet is then guided to the seventh conveying path  107  by a third switching claw  303 , and is stacked in the stacker  400  ( FIG. 3E ). If the post-processing is to be performed on the sheet after the folding operation is performed, the sheet is conveyed through the second nip and the third nip without entering the fourth conveying path  104 , and is guided to the sixth conveying path  106  by the third switching claw  303 , so that the sheet is conveyed toward the post-processing device. Alternatively, the sheet can be conveyed toward the post-processing device through the fourth conveying path  104  after the first folding operation is performed by the first folding roller  201  and the second folding roller  202  (in such a case, the second stopper  502  is removed from the fourth conveying path  104 ). 
     When the sheet folding device receives a sheet from the image forming apparatus, the sheet is guided to the second conveying path  102  by the first switching claw  301  and a second switching claw  302  ( FIG. 4A ). The sheet is conveyed through the first nip, and is then guided to the fourth conveying path  104 , so that the leading end of the sheet is brought into contact with the second stopper  502  that is movable depending on a fold position of the sheet ( FIG. 4B ). When the leading end of the sheet is brought into contact with the second stopper  502 , a portion of the sheet is bent. The bending portion is conveyed through the second nip whereby a first folding operation is performed on the sheet ( FIG. 4C ). 
     Then, the sheet is guided to the fifth conveying path  105 , so that the leading end of the sheet is brought into contact with the third stopper  503  that is movable depending on a fold position of the sheet. When the leading end of the sheet is brought into contact with the third stopper  503 , a portion of the sheet is bent. The bending portion is conveyed through the third nip whereby a second folding operation is performed on the sheet ( FIG. 4D ). Thus, the Z fold operation is completed. 
     After the Z fold operation is completed, the sheet is guided to the seventh conveying path  107  by the third switching claw  303 , and is stacked in the stacker  400 . Alternatively, if the post-processing (punching, stapling, shifting, or mixed size stacking) is to be performed on the sheet after the folding operation is performed, the sheet is guided to the sixth conveying path  106  by the third switching claw  303 , and is conveyed toward the post-processing device ( FIG. 4E ). 
     When the sheet folding device receives a sheet from the image forming apparatus, the sheet is guided to the first conveying path  101  by the first switching claw  301  ( FIG. 5A ). The sheet is then guided to the third conveying path  103 , so that the leading end of the sheet is brought into contact with the first stopper  501  ( FIG. 5B ). When the leading end of the sheet is brought into contact with the first stopper  501 , a portion of the sheet is bent. The bending portion is then conveyed through the first nip whereby a first folding operation is performed on the sheet ( FIG. 5C ). 
     Then, the sheet is guided to the fourth conveying path  104 , so that the leading end of the sheet is brought into contact with the second stopper  502 . When the leading end of the sheet is brought into contact with the second stopper  502 , a portion of the sheet is bent. The bending portion is then conveyed through the second nip whereby a second folding operation is performed on the sheet ( FIG. 5D ). Thus, the outside triple fold operation is completed. 
     After the outside triple fold operation (an inside triple fold operation or a simple quadruple fold operation) is completed, the sheet is conveyed through the third nip without entering the fifth conveying path  105  ( FIG. 5E ). Then, the sheet is guided to the seventh conveying path  107  by the third switching claw  303 , and is stacked in the stacker  400 . Alternatively, if the post-processing is to be performed on the sheet after the folding operation is performed, the sheet is conveyed through the third nip, and is guided to the sixth conveying path  106  by the third switching claw  303 , so that the sheet is conveyed toward the post-processing device. 
     The inside triple fold operation and the simple quadruple fold operation are performed in almost the same manner as the outside triple fold operation, and therefore detailed explanation on the inside triple fold operation and the simple quadruple fold operation is omitted. Each of the processes shown in  FIGS. 6A to 6E  and  FIGS. 7A to 7E  corresponds to that shown in  FIGS. 5A to 5E . In the inside triple fold operation and the simple quadruple fold operation, the sheet is conveyed through the same conveying paths as in the outside triple fold operation, and the folding operation is performed at the same nip as in the outside triple fold operation. The difference between the outside triple fold operation, the inside triple fold operation, and the simple quadruple fold operation is a fold position of the sheet in the first folding operation. The fold position can be adjusted by changing the position of the first stopper  501 . A fold position of the sheet in the second folding operation is determined depending on the fold position in the first folding operation, and the second folding operation is performed on a corresponding fold position of the sheet. 
     When the sheet folding device receives a sheet from the image forming apparatus, the sheet is guided to the first conveying path  101  by the first switching claw  301  ( FIG. 8A ). The sheet is then guided to the third conveying path  103 , so that the leading end of the sheet is brought into contact with the first stopper  501  ( FIG. 8B ). When the leading end of the sheet is brought into contact with the first stopper  501 , a portion of the sheet is bent. The bending portion is then conveyed through the first nip whereby a first folding operation is performed on the sheet ( FIG. 8C ). 
     Then, the sheet is guided to the fourth conveying path  104 , so that the leading end of the sheet is brought into contact with the second stopper  502 . When the leading end of the sheet is brought into contact with the second stopper  502 , a portion of the sheet is bent. The bending portion is then conveyed through the second nip whereby a second folding operation is performed on the sheet ( FIG. 8D ). 
     Then, the sheet is guided to the fifth conveying path  105 , so that the leading end of the sheet is brought into contact with the third stopper  503 . When the leading end of the sheet is brought into contact with the third stopper  503 , a portion of the sheet is bent. The bending portion is then conveyed through the third nip whereby a third folding operation is performed on the sheet. Thus, the gate fold operation is completed ( FIG. 8E ). 
     After the gate fold operation is completed, the same operation as in the Z fold operation is performed. Specifically, the sheet is guided to the seventh conveying path  107  by the third switching claw  303 , and is stacked in the stacker  400 . Alternatively, if the post-processing is to be performed on the sheet, the sheet is guided to the sixth conveying path  106  by the third switching claw  303 , and is conveyed toward the post-processing device ( FIG. 8F ). 
     A first idler roller  603 , a second idler roller  604 , a third idler roller  605 , a fourth idler roller  606 , and a fifth idler roller  607  are arranged in the third conveying path  103 , the fourth conveying path  104 , the fifth conveying path  105 , the sixth conveying path  106 , and the seventh conveying path  107 , respectively. The idler rollers  603  to  607  serve as adjusting members that adjust a space between the guide plates. The adjusting member is not limited to the idler roller, but can be a Mylar. 
       FIG. 9  is an enlarged view of a relevant part of the sheet folding device for explaining arrangement of the idler rollers  603  to  607 . The third conveying path  103  is adjusted to have a space D 3  by the first idler roller  603 , the fourth conveying path  104  is adjusted to have a space D 4  by the second idler roller  604 , the fifth conveying path  105  is adjusted to have a space D 5  by the third idler roller  605 , the sixth conveying path  106  is adjusted to have a space D 6  by the fourth idler roller  606 , and the seventh conveying path  107  is adjusted to have a space D 7  by the fifth idler roller  607 . 
     Table 1 indicates that the thickness of a sheet conveyed through each of the conveying paths  103  to  107  is different depending on the sheet folding mode. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Outside 
                 Inside 
                 Simple 
                   
               
               
                   
                 Single 
                   
                 triple 
                 triple 
                 quadruple 
               
               
                   
                 fold 
                 Z fold 
                 fold 
                 fold 
                 fold 
                 Gate fold 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Third 
                 Sheet 
                 Not 
                 Sheet 
                 Sheet 
                 Sheet 
                 Sheet 
               
               
                 conveying 
                 thickness × 1 
                 conveyed 
                 thickness × 1 
                 thickness × 1 
                 thickness × 1 
                 thickness × 1 
               
               
                 path 103 
               
               
                 Fourth 
                 Sheet 
                 Sheet 
                 Sheet 
                 Sheet 
                 Sheet 
                 Sheet 
               
               
                 conveying 
                 thickness × 2 
                 thickness × 1 
                 thickness × 2 
                 thickness × 2 
                 thickness × 2 
                 thickness × 2 
               
               
                 path 104 
               
               
                 Fifth 
                 Not 
                 Sheet 
                 Not 
                 Not 
                 Not 
                 Sheet 
               
               
                 conveying 
                 conveyed 
                 thickness × 2 
                 conveyed 
                 conveyed 
                 conveyed 
                 thickness × 2 
               
               
                 path 105 
               
               
                 Sixth 
                 Sheet 
                 Sheet 
                 Not 
                 Not 
                 Not 
                 Not 
               
               
                 conveying 
                 thickness × 2 
                 thickness × 3 
                 conveyed 
                 conveyed 
                 conveyed 
                 conveyed 
               
               
                 path 106 
               
               
                 Seventh 
                 Sheet 
                 Sheet 
                 Sheet 
                 Sheet 
                 Sheet 
                 Sheet 
               
               
                 conveying 
                 thickness × 2 
                 thickness × 3 
                 thickness × 3 
                 thickness × 3 
                 thickness × 4 
                 thickness × 4 
               
               
                 path 107 
               
               
                   
               
             
          
         
       
     
     The thickness of a sheet conveyed through the conveying path arranged downstream of the folding rollers is equal to or larger than that of the sheet conveyed through the conveying path arranged upstream of the folding rollers. If the space between the guide plates is the same in all of the conveying paths  103  to  107 , in some conveying paths, the sheet can be conveyed between the guide plates in an unstable manner because of the large space, and in other conveying paths, the sheet can be jammed between the guide plates because of the small space. Thus, it is difficult to convey the sheet in a smooth manner. To solve the above problem, the idler rollers are arranged in the conveying paths upstream and downstream of the folding rollers to adjust the space between the guide plates. With this configuration, it is possible to convey the sheet in a stable manner. 
     The idler rollers are arranged such that the space between the guide plates downstream of the folding rollers is equal to or larger than that between the guide plates upstream of the folding rollers. Thus, it is possible to convey a folded sheet in a stable manner. 
     The space defined by each of the idler rollers  603  to  607  is determined based on the thickness of a sheet conveyed through each of the conveying paths  103  to  107 . In this manner, it is possible to provide an appropriate space in each of the conveying paths  103  to  107  with respect to the thickness of the conveyed sheet. 
     Specifically, the idler rollers  603  to  607  are arranged such that the spaces defined by the idler rollers  603  to  607  are set as follows: 
     The third conveying path  103 : D 3 =sheet thickness×1 
     The fourth conveying path  104 : D 4 =sheet thickness×2 
     The fifth conveying path  105 : D 5 =sheet thickness×2 
     The sixth conveying path  106 : D 6 =sheet thickness×4 
     The seventh conveying path  107 : D 7 =sheet thickness×4 
     Thus, the space defined by each of the idler rollers  603  to  607  corresponds to a maximum thickness of a sheet conveyed through each of the conveying paths  103  to  107 , so that it is possible to prevent the possibility that the sheet hits the idler rollers  603  to  607  and gets jammed between the guide plates, and to convey the sheet in a smooth manner in all of the sheet folding modes. 
     However, the thickness of a sheet conveyed through each of conveying paths is different depending on the sheet folding mode. Specifically, in the Z fold operation, the thickness of the sheet conveyed through the fourth conveying path  104  is sheet thickness×1 (because it is an unfolded sheet), and if the space D 4  is set to sheet thickness×2, the space D 4  is larger than the thickness of the sheet. Therefore, it is difficult to convey the sheet in a stable manner. To solve such a problem, the idler rollers are movable in a direction perpendicular to a conveying direction of the sheet. With this configuration, it is possible to provide an appropriate space in each of the conveying paths depending on the thickness of a conveyed sheet. 
     The sheet having sheet thickness×1 or sheet thickness×2 is conveyed through the fourth conveying path  104  depending on the sheet folding mode. On the other hand, the sheet having sheet thickness×2, sheet thickness×3 or sheet thickness×4 is conveyed through the seventh conveying path  107  depending on the sheet folding mode. If a moving distance of each of the idler rollers is the same, the space can be too small in some conveying paths, and can be too large in other conveying paths with respect to the thickness of the sheet. Therefore, the moving distance of each of the idler rollers  603  to  607  is individually set, so that it is possible to provide an appropriate space in each of the conveying paths. 
     However, in some conveying paths, the sheet having the same thickness is conveyed in all of the sheet folding modes. If a mechanism for moving the idler roller is arranged in all of the conveying paths, the mechanisms become complicated, and the costs are increased. Therefore, the idler roller is not movable in the conveying path through which the sheet having the same thickness is conveyed in all of the sheet folding modes, and the idler roller is movable in the conveying path through which the sheet having a different thickness is conveyed depending on the sheet folding mode. Specifically, the idler rollers  603  and  605  are not movable. In this manner, it is possible to omit an unnecessary mechanism, and to reduce the costs. 
     Furthermore, because the thickness of the sheet conveyed through each of the conveying paths is different depending on the sheet folding mode, the idler rollers are individually moved in the conveying paths, and the moving distance of each of the idler rollers is determined based on the sheet folding mode. In this manner, an appropriate space can be provided in each of the conveying paths depending on the sheet folding mode, and the sheet can be conveyed in a stable manner in all of the sheet folding modes. 
       FIG. 10  is an enlarged view of a relevant part of the sheet folding device for explaining an operation of adjusting the space between the guide plates by moving the second idler roller  604 . When the Z fold operation is performed on the sheet, the second idler roller  604  is moved in a direction close to the guide plate that is opposed to the second idler roller  604 , thereby defining the space D 4 . When the single fold operation is performed on the sheet, the second-idler roller  604  is moved in a direction away from the guide plate that is opposed to the second idler roller  604 , thereby forming the space D 4 ′. 
     Specifically, the space in each of the conveying paths  103  to  107  is defined as follows: 
     [Single Fold] 
     The third conveying path  103 : D 3 =sheet thickness×1 
     The fourth conveying path  104 : D 4 =sheet thickness×2 
     The fifth conveying path  105 : arbitrary (not conveyed) 
     The sixth conveying path  106 : D 6 =sheet thickness×2 
     The seventh conveying path  107 : D 7 =sheet thickness×2 
     [Z Fold] 
     The third conveying path  103 : arbitrary (not conveyed) 
     The fourth conveying path  104 : D 4 =sheet thickness×1 
     The fifth conveying path  105 : D 5 =sheet thickness×2 
     The sixth conveying path  106 : D 6 =sheet thickness×3 
     The seventh conveying path  107 : D 7 =sheet thickness×3 
     [Outside Triple Fold, Inside Triple Fold] 
     The third conveying path  103 : D 3 =sheet thickness×1 
     The fourth conveying path  104 : D 4 =sheet thickness×2 
     The fifth conveying path  105 : arbitrary (not conveyed) 
     The sixth conveying path  106 : arbitrary (not conveyed) 
     The seventh conveying path  107 : D 7 =sheet thickness×3 
     [Simple Quadruple Fold] 
     The third conveying path  103 : D 3 =sheet thickness×1 
     The fourth conveying path  104 : D 4 =sheet thickness×2 
     The fifth conveying path  105 : arbitrary (not conveyed) 
     The sixth conveying path  106 : arbitrary (not conveyed) 
     The seventh conveying path  107 : D 7 =sheet thickness×4 
     [Gate Fold] 
     The third conveying path  103 : D 3 =sheet thickness×1 
     The fourth conveying path  104 : D 4 =sheet thickness×2 
     The fifth conveying path  105 : D 5 =sheet thickness×2 
     The sixth conveying path  106 : arbitrary (not conveyed) 
     The seventh conveying path  107 : D 7 =sheet thickness×4 
       FIGS. 11 and 12  are flowcharts for explaining an operation of adjusting the space in each of the conveying paths  103  to  107  by changing positions of the idler rollers  603  to  607 . After a power source (not shown) is turned on, initialization is performed (Step S 1 ). The idler rollers  603  to  607  are set to default positions (Step S 2 ), and then enter a standby state. Specifically, the default positions of the idler rollers  603  to  607  are defined such that the space D 3 =sheet thickness×1, the space D 4 =sheet thickness×1, the space D 5 =sheet thickness×1, the space D 6 =sheet thickness×2, and the space D 7 =sheet thickness×2. 
     After a function of setting the sheet folding mode is activated, a set sheet folding mode and positions of the idler rollers  603  to  607  corresponding to the set sheet folding mode are determined. When the idler rollers  603  to  607  are set to these positions, the folding operation is started. For example, if the outside triple fold mode is set (No at Step S 11 , No at Step S 13 , and Yes at Step S 15 ), the idler rollers  603  to  607  are set to the corresponding positions for the outside triple fold mode (Step S 16 ). 
     Specifically, the idler rollers  603  to  607  are set to the positions such that the space D 3 =sheet thickness×1, the space D 4 =sheet thickness×2, the space D 5 =sheet thickness×1, the space D 6 =sheet thickness×2, and the space D 7 =sheet thickness×3. The positions of the idler rollers  603  to  607  as shown in  FIG. 12  are determined based on the thickness of a sheet conveyed through each of the conveying paths  103  to  107  as shown in Table 1 and the space in each of the conveying paths  103  to  107  as described above. 
     With the configuration and the operation described above, it is possible to convey the folded sheet in a stable manner. 
       FIG. 13  is a schematic diagram of an image forming apparatus  90  according to a second embodiment of the present invention. The image forming apparatus  90  includes a sheet folding device  70  having the configuration according to the first embodiment and a post-processing device  80 . The image forming apparatus  90  is a copy machine that forms a toner image by an image forming process using an electrophotographic system. The image forming apparatus  90  can be a printer, a facsimile, or a multifunction product (MFP) having functions of a copy machine, a printer, and a facsimile. The image forming apparatus  90  can be an inkjet printer. 
     The image forming apparatus  90  includes an automatic document feeder (ADF)  1 , a feed tray  2 , feeding rollers  3 , a feeding belt  4 , ejecting rollers  5 , an exposure glass  6 , an original-set detecting unit  7 , a first tray  8 , a second tray  9 , a third tray  10 , a first feeding unit  11 , a second feeding unit  12 , a third feeding unit  13 , a longitudinal conveying unit  14 , a photosensitive element  15 , a conveying belt  16 , a fixing unit  17 , a discharging unit  18 , a developing unit  27 , a scanning unit  50  including an exposure lamp  51 , a first mirror  52 , a lens  53 , a coupled charge device (CCD) image sensor  54 , a second mirror  55 , and a third mirror  56 , a writing unit  57  including a laser output unit  58 , an imaging lens  59 , and a mirror  60 , the sheet folding device  70 , and the post-processing device  80 . 
     A pile of originals are placed on the feed tray  2  such that the side of the original on which an image is formed faces upward. When a start key of an operation unit (not shown) is pressed, the uppermost original is fed from the pile by the feeding rollers  3  and the feeding belt  4  to a predetermined position on the exposure glass  6 . 
     When the original is fed to the predetermined position on the exposure glass  6 , the image on the original is scanned by the scanning unit  50 . After the scanning is completed, the original is ejected to the outside by the feeding belt  4  and the ejecting rollers  5 . When the original-set detecting unit  7  detects that the next original is placed on the feed tray  2 , the next original is fed to a predetermined position on the exposure glass  6  in the same manner as described above. The feeding rollers  3 , the feeding belt  4 , and the ejecting rollers  5  are driven by a conveying motor (not shown). 
     Sheets stacked on the first tray  8 , the second tray  9 , and the third tray  10  are fed by the first feeding unit  11 , the second feeding unit  12 , and the third feeding unit  13 , respectively, and are conveyed by the longitudinal conveying unit  14  to a position at which the sheet is in contact with the photosensitive element  15 . 
     The photosensitive element  15  is irradiated with a laser beam emitted from the writing unit  57  based on image data obtained by the scanning unit  50  whereby an electrostatic latent image is formed on the photosensitive element  15 . The electrostatic latent image on the photosensitive element  15  is developed by the developing unit  27 , so that a toner image is formed on the photosensitive element  15 . 
     After the sheet is conveyed to the conveying belt  16  by the longitudinal conveying unit  14 , the sheet is conveyed by the conveying belt  16  that is moved at the same speed as that at which the photosensitive element  15  is rotated, so that the toner image on the photosensitive element  15  is transferred onto the sheet. 
     The sheet having the toner image transferred thereon is then conveyed to the fixing unit  17  where the toner image is fixed to the sheet with heat. 
     The photosensitive element  15 , the conveying belt  16 , the fixing unit  17 , the discharging unit  18 , and the developing unit  27  are driven by a main motor (not shown). Each of the feeding units  11  to  13  is driven by a driving force transmitted from the main motor via a feeding clutch (not shown). The longitudinal conveying unit  14  is driven by a driving force transmitted from the main motor via an intermediate clutch (not shown). 
     The discharging unit  18  discharges the sheet having the image formed thereon to the sheet folding device  70 . The sheet folding device  70  performs the folding operation as described in the first embodiment. After the sheet folding device  70  completes the folding operation, the sheet folding device  70  discharges the sheet to the post-processing device  80 . The post-processing device  80  performs post-processing operation, such as sorting of sheets for each original or each copy of originals that is sorted by an image memory, punching, or stapling. 
     The above embodiments are described as preferred embodiments of the present invention. The present invention is not limited to the embodiments, but modifications can be made as appropriate within a scope of technical ideas of the present invention. 
     According to the embodiments, an adjusting member is arranged between guide plates of each of conveying paths arranged upstream and downstream of folding rollers to adjust a space between the guide plates, so that it is possible to convey a sheet in a stable manner. 
     Furthermore, according to the embodiments, the space between the guide plates arranged downstream of the folding rollers is equal to or larger than the space between the guide plates arranged upstream of the folding rollers, so that it is possible to convey a folded sheet in a stable manner. 
     Moreover, according to the embodiments, the space is determined depending on the thickness of a sheet conveyed through each of the conveying paths, so that it is possible to provide an appropriate space in each of the conveying paths with respect to the thickness of the sheet. 
     Furthermore, according to the embodiments, the space corresponds to a maximum thickness of a sheet conveyed through each of the conveying paths, so that it is possible to convey the sheet in a smooth manner without being hit by the adjusting member. 
     Moreover, according to the embodiments, the adjusting member is movable in a direction perpendicular to a conveying direction of a sheet by a driving source, so that the adjusting member can be moved in accordance with change of the thickness of the sheet depending on the sheet folding mode, and an appropriate space can be provided in the conveying path. 
     Furthermore, according to the embodiments, a moving distance of the adjusting member in the direction perpendicular to the conveying direction is individually set for each of the conveying paths, so that an appropriate space can be provided in each of the conveying paths. 
     Moreover, according to the embodiments, the adjusting member only in the conveying path through which a sheet having a different thickness is conveyed is movable in the direction perpendicular to the conveying direction, so that it is possible to omit an unnecessary mechanism for moving the adjusting member, and to reduce the costs. 
     Furthermore, according to the embodiments, a moving distance of the adjusting member in the direction perpendicular to the conveying direction is determined depending on the sheet folding mode, so that an appropriate space can be provided in each of the conveying paths depending on each of the sheet folding modes, and to convey the sheet in a stable manner in all of the sheet folding modes. 
     Moreover, according to the embodiments, an idler roller or a Mylar can be used as the adjusting member, so that it is possible to convey a sheet without causing damage on the sheet. 
     According to an aspect of the present invention, it is possible to provide a sheet folding device and an image forming apparatus in which a sheet can be conveyed through each of the conveying paths in a stable manner. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.