Abstract:
A sheet finisher for executing preselected processing with a sheet introduced thereinto from an image forming apparatus and then discharging the sheet is disclosed. The sheet finisher includes a stacking device configured to temporarily stack sheets sequentially delivered thereto. Jogger fences jog each sheet within the stacking device. A stapler staples the sheet stack jogged in the stacking device. The stapler is supported by a guide shaft such it is movable along the guide shaft in a direction perpendicular to the direction of sheet conveyance and angularly movable in a direction perpendicular to the direction of guide.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a sheet finisher constructed integrally or separately from a copier, printer or similar image forming apparatus for executing sorting, stacking, jogging, stapling, center stapling and binding, punching or similar processing with sheets carrying images thereon and then discharging the sheets, and an image forming system made up of the sheet finisher and image forming apparatus. 
   2. Description of the Background Art 
   A sheet finisher configured to automatically execute processing of the kind described above with sheets sequentially driven out of an image forming apparatus has been proposed in various forms in the past. Particularly, various methods have been proposed for the movement of a stapler. Japanese Patent Laid-Open Publication No. 9-235070, for example, discloses a sheet finisher including a stapler mounted on a guide shaft, which extends between the front and rear side walls of a staple tray. The stapler is movable in a direction perpendicular to the direction of sheet conveyance and slidable in the direction of sheet conveyance as well. 
   More specifically, in the above conventional sheet finisher, after the trailing edge of a sheet stack has been positioned by being abutted against a reference fence located below the staple tray, a hook affixed to a timing belt or similar band-like drive transmitting means lifts the trailing edge of the sheet stack for thereby causing the sheet stack to be driven out to a tray. The stapler is allowed to slide in the direction of sheet conveyance such that it does not contact a pulley or similar rotary member, which drives the drive transmitting means, when moving in the direction perpendicular to the direction of sheet conveyance. 
   However, to allow the stapler to move in both of the direction of sheet conveyance and the direction perpendicular thereto, the conventional sheet finisher needs a number of parts and is therefore sophisticated in configuration. In addition, such a number of parts increase the cost of the sheet finisher. 
   Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2000-169028, 2001-171898 and 2002-273705. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a sheet finisher allowing a stapler to move in the direction perpendicular to the direction of sheet conveyance without contacting a pulley or similar rotary member with a simple configuration, and an image forming system including the same. 
   It is another object of the present invention to provide a sheet finisher capable of reducing drive loads necessary for a stapler to move in the direction perpendicular to the direction of sheet conveyance and angularly move about a guide shaft and desirable in durability, and an image forming system including the same. 
   A sheet finisher of the present invention, which executes preselected processing with a sheet introduced thereinto from an image forming apparatus and then discharges it, includes a stacking device configured to temporarily stack sheets sequentially delivered thereto. Jogger fences jog each sheet within the stacking device. A stapler staples the sheet stack jogged in the stacking device. The stapler is supported by a guide shaft such it is movable along the guide shaft in a direction perpendicular to the direction of sheet conveyance and angularly movable in a direction perpendicular to the direction of guide. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: 
       FIG. 1  is a view showing an image forming system embodying the present invention and made up of a sheet finisher and an image forming apparatus; 
       FIG. 2  is an isometric view showing a shifting mechanism included in the sheet finisher; 
       FIG. 3  is a fragmentary perspective view showing a shift tray elevating mechanism included in the sheet finisher; 
       FIG. 4  is an isometric view showing a outlet section included in the sheet finisher for discharging sheets to a shift tray; 
       FIG. 5  is a front view showing a staple tray included in the sheet finisher, as seen in a direction perpendicular to a sheet conveying surface thereof; 
       FIG. 6  is an isometric view showing the staple tray, a driving mechanism associated therewith, and an exclusive drive source assigned to a knock roller; 
       FIG. 7  is a perspective view showing a mechanism included in the sheet finisher for discharging a sheet stack; 
       FIG. 8  is a front views showing a relation between the staple tray, a stapler, and a guide shaft shown in  FIG. 1 ; 
       FIG. 9  is a plan view showing a relation between the staple tray, a guide stay, and a cam groove; 
       FIG. 10  is a perspective view showing a relation between the guide shaft, the stapler, the guide stay, and the cam groove; 
       FIGS. 11 and 12  are respectively a plan view and a front view showing a relation between the guide shaft, the stapler, a bracket and a stapler rotation bracket shown in  FIG. 1 ; 
       FIG. 13  shows a relation between a cam surface and a guide roller included in the sheet finisher; 
       FIG. 14  shows a comparative relation between the cam surface and the guide roller; 
       FIG. 15  is a fragmentary front view showing a relation between the guide shaft, the stapler, the guide stay, an auxiliary plate and a compression spring shown in  FIG. 1 ; 
       FIG. 16  is a schematic block diagram showing a control system included in the illustrative embodiment, particularly a controller for controlling the sheet finisher; 
       FIG. 17  is an isometric view showing a guide shaft representative of an alternative embodiment of the present invention; and 
       FIG. 18  is a section showing a mechanism included in the alternative embodiment for causing the guide stay to slide on the guide shaft. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1  of the drawings, an image forming system embodying the present invention is shown. As shown, the image forming system is generally made up of a sheet finisher PD and an image forming apparatus PR. The sheet finisher PD is connected to one side of the image forming apparatus RP, so that a sheet or recording medium driven out of the latter is introduced into the former. The sheet introduced into the sheet finisher PD is conveyed along a path A on which finishing means for finishing a single sheet is positioned. In the illustrative embodiment, the finishing means is implemented as a punch unit or punching means  100 . 
   The path A merges into a path B terminating at an upper tray  201 , a path C terminating at a shift tray  202 , and a path D terminating at a staple tray or processing tray F, which performs positioning and stapling. Path selectors  15  and  16  each steer the sheet coming out of the path A to designated one of the paths B through D. A stack of sheets positioned and stapled on the staple tray F is guided to either one of the path C and a fold tray or processing tray G by a guide plate and a movable guide  55 , which constitute steering means. The sheet stack stapled on the fold tray G is driven out to a lower tray  203  via a path H. 
   A path selector  17  is positioned on the path D and constantly biased by a light-load spring to a position shown in  FIG. 1 . An arrangement is made such that after the trailing edge of the sheet has moved away from the path selector  17 , among rollers  9  and  10  and a stapler inlet roller  11 , at least the roller  9  can be rotated in the reverse direction to introduce the trailing edge of the sheet into a prestacking section E. This allows a plurality of sheets sequentially stacked in the prestacking section E to be conveyed together. 
   An inlet sensor  301  responsive to the sheet, an inlet roller  1 , the punch unit  100 , a hopper  101  for storing sheet scraps, a roller  2  and the path selectors  15  and  16  re sequentially positioned on the path in the direction of sheet conveyance. Springs, not shown, bias the path selectors  15  and  16  to positions shown in  FIG. 1 . When solenoids assigned to the path selectors  15  and  16 , respectively, are turned on, the path selectors  15  and  16  are angularly moved upward and downward, respectively, for thereby steering the sheet to designated one of the paths B through D. 
   More specifically, to steer the sheet to the path B, the path selector  15  is held in the position of  FIG. 1  while the solenoids are turned off. To steer the sheet to the path C, the solenoids are turned on to move the path selectors  15  and  16  upward and downward, respectively. Further, to steer the sheet to the path D, the solenoid assigned to the path selector  16  is turned off while the solenoid assigned to the path selector  15  is turned on to move the path selector  15  upward. The reference numerals  3 ,  4 ,  5 ,  7  and  8  designate rollers for conveying the sheet. 
   The sheet finisher PD is capable of selectively punching a sheet with the punch unit  100 , jogging and edge-stapling sheets with a pair of jogger fences  53  and an edge-stapler S 1 , jogging and center-stapling sheets with the jogger fences  53  and center staplers S 2 , sorting sheets with the shift tray  202  or folding sheets with a fold plate  74  and fold rollers  81  and  82 , as desired. 
   In the illustrative embodiment, using an electrophotographic process, the image forming apparatus PR optically scans a photoconductive drum or similar image carrier in accordance with image data to thereby form a latent image, develops the latent image with toner, transfers the resulting toner image to a sheet, fixes the toner image on the sheet, and then drives the sheet or pint out of the apparatus. Such an image forming apparatus is conventional and will not be shown or described specifically. Of course, the electrophotographic image forming apparatus may be replaced with an ink jet printer or any other image forming apparatus known in the art. 
   A shift tray outlet section I, located at the most downstream side of the sheet finisher PD, includes an outlet roller pair  6 , a return roller  13 , a sheet surface sensor  330 , the shift tray  202 , a shifting mechanism J (see  FIG. 2 ), and a shift tray elevating mechanism K (see  FIG. 3 ). As shown in  FIGS. 1 through 3 , the return roller  13  presses the trailing edge of the sheet driven out by the outlet roller pair  6  against an end fence  32 ,  FIG. 2 , for thereby positioning the sheet. The return roller  13  is driven by the shift roller pair  6 . A limit switch  333  adjoins the return roller  13  and turns on when the shift tray  202  is elevated to push the return roller  13  upward, thereby turning off a tray motor  168 . This prevents the shift tray  202  from overrunning. As shown in  FIG. 1 , the sheet surface sensor or sheet surface position sensing means  330  also adjoins the return roller  13  and senses the surface position of a sheet or a sheet stack driven out to the shift tray  202 . 
   As shown in  FIG. 3 , the sheet surface sensor  330  includes a lever  30  and sensors  330   a  and  330   b  assigned to a staple mode and a non-staple mode, respectively. The lever  30  is angularly movable about its shaft portion and includes a contact portion  30   a  contacting the top sheet stacked on the shift tray  202  and a sectorial interrupter portion  30   b . The upper sensor  330   a  and lower sensor  330   b  are mainly used for staple discharge control and non-staple discharge control, respectively. 
   More specifically, the sensors  330   a  and  330   b  each turn on when the optical path thereof is interrupted by the interrupter portion  30   b  of the lever  30 . When the shift tray  202  is elevated while causing the contact portion  30   a  of the lever  30  to move upward, the sensors  330   a  and  330   b  are sequentially turned off in this order. When the sheet stack on the shift tray  202  reaches a preselected height, as determined by the sensors  330   a  and  330   b , the tray motor  168  is driven to lower the shift tray  202  by a preselected distance. Consequently, the sheet surface on the shift tray  202  is held at substantially the same height. 
   The shift tray elevating mechanism will be described with reference to  FIG. 3 . As shown, a drive unit L causes the shift tray  202  to move upward or downward via a drive shaft  21 . Timing belts  23  are passed over the drive shaft  21  and a driven shaft  22  via timing pulleys under preselected tension. A support plate  24  supports the shift tray  202  and is affixed to the timing belts  23 . In this configuration, the unit including the shift tray  202  is suspended from the timing belts  23  in such a manner as to be movable up and down. 
   The drive unit L includes a worm gear  25  in addition to the tray motor  168 , which is a reversible motor or drive source. The output torque of the tray motor  168  is transferred to the last gear of a gear train affixed to the drive shaft  21  via the worm gear  25 , moving the shift tray  202  upward or downward. The worm gear  25  present in the driveline allows the shift tray  202  to remain at a preselected position and obviates the fall or similar accident of the shift tray  202 . 
   An interrupter  24   a  is formed integrally with the support plate  24  and turns on or turns off a full sensor  334  and a lower limit sensor  335 , which are positioned below the interrupter  24   a . The full sensor  334  and lower limit sensor  335  are responsive to the full condition and lower limit position of the shift tray  202 , respectively. The full sensor  334  and lower limit sensor  335  are implemented as photosensors, and each turns on when the optical path thereof is interrupted by the interrupter  24   a . The outlet roller pair  6  is not shown in  FIG. 3 . 
   As shown in  FIG. 2 , the shifting mechanism assigned to the shift tray  202  includes a shift motor or drive source  169  and a cam  31 . The shift motor  169  causes the shift tray  202  to move in the direction perpendicular to the direction of sheet discharge via the cam  31 . A pin  31   a  is studded on the cam  31  at a position remote from the axis of the cam  31  by a preselected distance. The fee end of the pin  31   a  is loosely fitted in an elongate slot  32   b  formed in an engaging member  32   a , which is affixed to the rear surface of the end fence  32  where the shift tray  202  is absent. In this configuration, the engaging member  32   a  and therefore shift tray  202  moves in the direction perpendicular to the direction of sheet discharge in accordance with the movement of the pin  31   a  of the cam  31 . 
   The shift tray  202  is caused to stop at the front and rear positions as seen in the direction perpendicular to the sheet surface of  FIG. 1 . To control the stop of the shift tray  202 , the shift motor  169  is selectively turned on or turned off in accordance with the output of a shift sensor  336  responsive to a notch formed in the cam  31 . 
   Ridges  32   c  are formed on the front surface of the end fence  32  while the rear end of the shift tray  202  is engaged with the ridges  32   c  to be movable up and down. The shift tray  202  is therefore supported by the end fence  32  in such a manner as to be movable up and down and in the direction perpendicular to the direction perpendicular to the direction of sheet discharge, as needed. The end fence  32  additionally serves to guide and position the rear edges of sheets stacked on the shift tray  202 . 
     FIG. 4  shows the section for discharging the sheet to the shift tray  202  more specifically. As shown in  FIGS. 1 and 4 , the outlet roller pair  6  is made up of a drive roller  6   a  and a driven roller  6   b . The driven roller  6   b  is rotatably supported by the free end of a guide plate  33 , which is angularly movable up and down about its upstream end in the direction of sheet discharge. The driven roller  6   b  is held in contact with the drive roller  6   a  due to its own weight or by a biasing force, so that a sheet or sheet stack is driven out to the shift tray  202  by the two rollers  6   a  and  6   b . When a stapled sheet stack is to be driven out, the guide plate  33  is moved upward and then lowered at preselected timing in accordance with the output of a discharge sensor  303 . The guide plate  33  is brought to a stop at a position determined by the output of a guide plate open/close sensor  331  and is driven by a guide plate motor  167 , which is, in turn, driven in accordance with the ON/OFF of a guide plate limit switch  332 . 
   The staple tray F will be described with reference to  FIGS. 5 through 7  in detail. As shown in  FIG. 6 , sheets are sequentially conveyed to and stacked on the staple tray F by the stapler inlet roller  11 . Every time a sheet is laid on the staple tray F, a knock roller  12  knocks the sheet to thereby position it in the vertical direction or direction of sheet conveyance. Subsequently, the jogger fence  53  positions the sheet in the horizontal direction or direction perpendicular to the direction of sheet conveyance. During the interval between consecutive jobs, i.e., between the last sheet of a sheet stack and the first sheet of the next sheet stack, a controller  350  (see  FIG. 16 ) sends a staple signal to the edge stapler S 1 , causing the stapler S 1  to staple a sheet stack. The stapled sheet stack is immediately conveyed to the outlet roller pair  6  by a belt or timing belt  52  and then driven out to the tray  202 , which is located at a receiving position. 
   As shown in  FIG. 7 , a belt HP (Home Position) sensor  311  senses a hook  52   a  brought to a home position. More specifically, two hooks  52   a  are position on the outer surface of the belt  52  in such a manner as to face each other, and each turns on and turns off the belt HP sensor  311 . The hooks  52   a  alternately move sheet stacks brought to the staple tray F one after another. If desired, the belt  52   a  may be moved in the reverse direction, as needed, so that the two hooks  52   a  can position the leading edge of the sheet stack laid on the staple tray F with their backs. In this sense, the hooks  52   a  play the role of positioning means for positioning a sheet stack in the direction of sheet conveyance as well. 
   As shown in  FIG. 5 , a motor  157  drives a drive shaft  65  for causing the belt  52  to move. The belt  52  and a drive pulley  62  over which the belt  52  is passed are positioned on the shaft  65  at the center in the widthwise direction of a sheet. Rollers  56  are affixed to the drive shaft  65  symmetrically with respect to the drive pulley  62 . The rollers  56  each are rotated at a higher peripheral speed than the belt  52 . 
   The output torque of the motor  157  is transferred to the belt  52  via a timing belt and timing pulleys. The drive pulley or timing pulley  62  and rollers  56  are mounted on a single shaft  65 . When the relation in speed between the rollers  56  and belt  52  should be varied, an arrangement may be made such that the rollers  56  are capable of idling on the shaft  65  while the output torque of the motor  157  is divided and transferred to the rollers  56 . This arrangement provides the setting of a speed reduction ratio with freedom. 
   The circumferential surfaces of the rollers  56  are formed of rubber or similar material having high frictional resistance. The rollers  56  exert a conveying force on a sheet or a sheet stack in cooperation with driven rollers  57 , which are pressed against the rollers  56  due to its own weight or by a biasing force. There are also shown in  FIG. 5  a front and a rear side wall  64   a  and  64   b  included in the sheet finisher PD, a stack branch motor for driving the movable guide  55 , and cams  61  included in the drive mechanism. 
   As shown in  FIG. 6 , a knock solenoid  170  causes the knock roller  12  to swing about a fulcrum  12   a  like a pendulum, thereby causing a sheet arrived at the staple tray F to abut against a rear fence  51 . In  FIG. 6 , the knock roller  12  is rotated in the counterclockwise direction. The knock roller  12  is driven by a knock motor  156 , which is driven by a CPU  360  (see  FIG. 16 ) via a motor driver independently of the other drive sources, as will be described specifically later. In the illustrative embodiment, the knock motor  156  is implemented as a stepping motor. The knock solenoid  170  is also driven by the CPU  360  via a driver. 
   The jogger fences  53  are driven back and forth by a reversible jogger motor  158  via a timing belt in the direction perpendicular to the direction of sheet conveyance. 
   As shown in  FIG. 5 , a reversible stapler shift motor  159  causes the edge stapler S 1  to move via a timing belt  46  (see  FIG. 10 ) in the widthwise direction of a sheet, thereby stapling a sheet stack at a preselected edge position. A stapler HP sensor  312 ,  FIG. 1 , responsive to the home position of the edge stapler S 1  is positioned at one end of the movable range of the edge stapler S 1 . The edge-stapling position is controlled on the basis of the displacement of the edge stapler S 1  from the home position. 
   More specifically, as shown in  FIGS. 8 through 10 , the edge stapler S 1  moves in the direction perpendicular to the direction of sheet conveyance on a guide shaft  40 , which is parallel to the rear fence  51 . The edge stapler S 1  is guided by a cam slot or stapler guide  41   a  formed in a guide stay  41 . The cam slot  41   a  is configured to cause the edge stapler S 1  to move in the following manner. The edge stapler S 1  is angularly moved about the guide shaft  40  to a position indicated by a phantom line in  FIG. 8  when moving below the lower edge of the staple tray  50 ,  FIG. 9 , and a discharge idle pulley  56   a , and then returned to a position indicated by a solid line in  FIG. 8 . 
   As shown in  FIGS. 11 and 12 , a member  45  is affixed to the timing belt  46 , nipped by a stapler shift bracket  43 , and movable on the guide shaft  40  in the widthwise direction of a sheet. In this configuration, when the member  45  is moved along the guide shaft  40 , the bracket  43 , a guide roller  42  mounted on the bracket  43 , a stapler rotation bracket  44  and the edge stapler S 1  move integrally with each other. 
   The stapler shift bracket  43 , stapler rotation bracket  44  and edge stapler S 1  angularly move along the locus of the guide roller  42 , which roll on cam surfaces  41   b ,  41   d  and  41   c  forming part of the cam slot  41   a . However, the member  45  does not angularly move because it is affixed to the timing belt  46 . 
   As shown in  FIG. 13 , the surface of the guide roller  42  contacting the cam surfaces  41   b  through  41   d  is provided with curvature, so that the contact point between the guide roller  42  and cam surfaces  41   b  through  41   d  varies when the edge stapler S 1  angularly moves. For comparison,  FIG. 14  shows a condition wherein the guide roller  42  not provided with curvature contacts the cam surfaces  41   b  through  41   d . As shown, the guide roller  42  constantly contacts the cam surfaces  41   b  through  41   d  at its edge. The guide roller  42  may, of course, be replaced with a spherical, rotary body. 
   As  FIGS. 9 and 10  indicate, the guide roller  42  contacts and rolls on the cam surface  41   b  (first cam surface  41   b  hereinafter), so that the edge stapler S 1  moves in the direction perpendicular to the direction of sheet conveyance for stapling the edge of a sheet stack. At this instant, as shown in  FIG. 8 , the edge stapler S 1  slidably hangs down from the guide shaft  40  and causes the guide roller  42  to contact the first cam surface  41   b  due to gravity and roll thereon while sandwiching the edge portion of the sheet stack to be stapled. In this condition, the position of the stapler S 1  is determined by the position of the guide shaft  40  and the position of the guide roller  42  contacting the first cam surface  41   b.    
   In the illustrative embodiment, in the position indicated by the solid line in  FIG. 8 , the guide roller  42  rolls on the first cam surface  41   b  with the bracket  43  being inclined (see line L 2 ,  FIG. 15 , as also shown in  FIG. 9 . On the other hand, in the position indicated by the phantom line in  FIG. 8 , the guide roller  42  rolls on the cam surface  41   c  (second cam surface  41   c  hereinafter) without the bracket  43  being inclined (line L 1 ,  FIG. 15 ; perpendicular direction or direction of gravity). When the guide roller  42  rolls on the first cam surface  41   b , the edge stapler S 1  moves while sandwiching the sheet stack and can therefore staple the sheet stack at a preselected position. When the guide roller  42  rolls on the second cam surface  41   c , the edge stapler S 1  is retracted from the discharge idler pulley  56   a.    
   As stated above, the guide roller  42  rolls on the cam surfaces  41   b  and  41   c  under the action of gravity, causing the edge stapler S 1  to angularly move over an angle α between the lines L 1  and L 2 ,  FIG. 15 . However, the edge stapler S 1  has a large mass. Consequently, when the guide roller  42  rolled on the first cam surface  41   b  rolls on the inclined cam surface  41   d  (third cam surface  41   d  hereinafter) preceding the second cam surface  41   c , acceleration ascribable to the weight of the edge stapler S 1  increases and is apt to exert a heavy shock on the second cam surface  41   c . This shock causes the guide roller  42  to hit against the surface of the guide slot  41   a  opposite to the second cam surface  41   c . As a result, the guide roller  42  moves along the guide slot  41   a  while repeatedly hitting against the opposite surfaces of the cam slot  41   a . The above shock not only produces noise, but also causes the structural elements to vibrate and thereby lowers reliability of operation. 
   Further, when the guide roller  42  rolls from the second cam surface  41   c  to the other third cam surface  41   d  preceding the other first cam surface  41   b  located at the stapling side, the guide roller  41  hits against a corner  41   e  between the cam surfaces  41   c  and  41   d , also resulting in a heavy shock. Moreover, a great force is necessary for moving the stapler S 1  having a large mass along the third cam surface  41   d , so that the stapler motor  159  must output a great torque and therefore needs a great drive current. 
   In light of the above, as shown in  FIG. 15 , a compression spring  41   g  and an auxiliary plate  41   h  are provided on the vertical edge  41   f  of the guide stay  41  while a roller  41   i  coaxial with the guide roller  42  is provided that rolls on the auxiliary plate  41   h . The auxiliary plate  41  is angularly movable about a shaft  41   j  while the compression spring  42   g  damps the angular movement. Further, when the guide roller  42  moves from the second cam surface  41   c  to the third cam surface  41   d , the impact to act on the third cam surface  41   e  is absorbed by the compression spring  42   g . Therefore, a small driving force suffices for causing the guide roller  42  to easily move from the third cam surface  41   d  to the first cam surface  41   b . This successfully reduces the output torque and therefore drive current required of the stapler motor  159 , contributing to energy saving. 
   The compression spring  41   g  may be replaced any other suitable mechanism so long as it can damps the angular movement of the auxiliary plate  41   h  and reduce the motor output torque necessary for causing the guide roller  42  to roll on the third cam surface  41   d.    
   As shown in  FIG. 15 , assume that the vertical line L 1 , extending from the axis of the guide shaft  40 , is one axis while a line extending from the above axis perpendicular to the vertical line L 1  (horizontal line) is another axis. Then, the angle α between the lines L 1  and L 2  lies between the above two axes, i.e., in the fourth quadrant, obviating wasteful angular movement. 
   Five different sheet discharge modes are available with the illustrative embodiment in accordance with the finishing mode, as will be described hereinafter. In a non-staple mode a, sheets are sequentially discharged to the upper tray  201  via the paths A and B. In a non-staple mode b, sheets are sequentially delivered to the shift tray  202  via the paths A and C. In a sort/stack mode, sheets are sequentially delivered to the shift tray  202  via the paths A and C; the shift tray  202  is repeatedly shifted in the direction perpendicular to the direction of sheet discharge to thereby sort the sheets. In a staple mode, sheets are delivered to the staple tray F via the paths A and D, positioned and stapled on the tray F, and then discharged to the shift tray  202  via the path C. Further, in a center staple, bind mode, sheets are delivered to the staple tray F via the paths A and D, positioned and stapled at the center on the tray F, folded at the center on the fold tray G, and then driven out to the lower tray  203  via the path H. The staple mode will be described in detail hereinafter. The other modes will not be described specifically. 
   In the staple mode, a sheet sheered from the path A to the path D by the path selectors  15  and  16  is conveyed to the staple tray F by the rollers  7 ,  9  and  10  and stapler inlet roller  11 . When a preselected number of sheets are stacked on the staple tray F, the edge stapler S 1  staples the sheet stack. Subsequently, the hook  52   a  lifts the stapled sheet stack to the downstream side in the direction of sheet conveyance, and then the shift outlet roller  6  conveys it to the tray  202 . 
   More specifically, as shown in  FIG. 6 , the jogger fences  53  each move from its home position to a stand-by position 7 mm remote from the width of a sheet. When the stapler inlet roller  11  conveys a sheet until the trailing edge of the sheet moves away from the staple discharge sensor  305 , each jogger fence  53  is further moved by 5 mm inward of the stand-by position. The staple discharge sensor  305 , sensed the tailing edge of the sheet, sends its output to the CPU  360 . In response, the CPU  360  starts counting pulses output from a conveyance motor, not shown, which drives the stapler inlet roller  11 . On counting a preselected number of pulses, the CPU  360  turns on the knock solenoid  170  for thereby causing the knock roller  12  to knock the sheet, as stated earlier. The sheet is therefore abutted against the rear fence  51  and positioned thereby. Every time a sheet moves away from the inlet sensor  101  or the staple discharge sensor  305 , the CPU  360  increments the count of sheets. 
   On the elapse of a preselected period of time since the turn-off of the knock solenoid  170 , the jogger motor  158  moves each jogger fence  53  further inward by 2.6 mm, thereby positioning the sheet in the horizontal direction. Subsequently, the jogger motor  158  moves each jogger fence  53  outward by 7.6 mm to the stand-by position and causes it to wait for the next sheet. This operation is repeated up to the last sheet of a job. Thereafter, the jogger motor  158  again moves each jogger fence  53  inward by 7 mm to thereby nip the opposite edges of the sheet stack. On the elapse of a preselected period of time since the above step, the stapler motor drives the edge stapler S 1  for thereby stapling the edge of the sheet stack. If the sheet stack should be stapled at two or more positions, then the staple motor  159  further moves the edge stapler S 1  to an adequate position along the lower edge of the sheet stack. 
   After the stapling operation, the discharge motor  157  is driven to move the belt  52  with the result that the hook  52   a  lifts the stapled sheet stack. At the same time, the discharge motor is driven to rotate the shift discharge roller  6 , so that the sheet stack lifted by the hook  52   a  is conveyed by the roller  6 . At this instant, the jogger fences  53  are controlled in a different manner in accordance with the number or the size of sheets stapled together. For example, if the number or the size of sheets is smaller than a preselected value, then the jogger fences  53  continuously nip the sheet stack therebetween when the sheet stack is being lifted by the hook  52   a.    
   Subsequently, when the CPU  360  counts a preselected number of pulses after a sheet presence/absence sensor  310  or the belt HP sensor  311  has outputs a sense signal, the jogger fences  53  are moved outward by 2 mm to release the sheet stack. The preselected number of pulses corresponds to an interval between the time when the hook  52   a  contacts the trailing edge of the sheet stack and the time when the hook  52   a  moves away from the ends of the jogger fences  53 . 
   If the number or the size of the sheets stapled together is larger than the preselected value, then the jogger fences  53  are moved outward by 2 mm before the discharge of the stapled sheet. In any case, as soon as the sheet stack moves away from the jogger fences  53 , the jogger fences  53  are further moved outward by 5 mm to the stand-by positions to prepare for the next sheet stack. Restraint to act on the sheet stack may be adjusted on the basis of the distance between the sheet stack and the jogger fences  53 . 
   As shown in  FIG. 16 , the controller  350  is implemented as a microcomputer including an I/O (Input/Output) interface in addition to the CPU  360 . The outputs of switches arranged on a control panel, which is mounted on the body of the image forming apparatus PR, and the outputs of the inlet sensor  301 , upper sheet outlet sensor, shift discharge sensor  303 , prestack sensor, stapler inlet sensor  305 , sheet presence/absence sensor  301 , belt HP sensor  311 , staple HP sensor  312 , jogger fence HP sensor, stack arrival sensor  321 , movable rear fence HP sensor, fold sensor, lower outlet sensor, sheet surface sensor  330  and so forth are input to the CPU  360  via the I/O interface  370 . 
   The CPU  360  controls, in accordance with the above inputs, the tray motor  168 , guide plate open/close motor shift motor  169 , knock motor  156 , solenoids including the knock solenoid  170 , motor for driving the rollers, outlet motor for controlling outlet motors, belt motor  157 , stapler shift motor  159 , jogger motor  158 , stack branch motor  161  and so forth. The CPU  360  counts the output pulses of the staple conveyance motor assigned to the stapler outlet roller  11  for controlling the knock solenoid  170  and jogger motor  158 . 
   An alternative embodiment of the present invention will be described with reference to  FIGS. 17 and 18 . In the previous embodiment, the edge stapler S 1  is moved along the guide slot or stapler guide  41   a  and shifted between the stapling position and the retracted position thereby. In the alternative embodiment, the guide shaft  40  is configured to serve as a stapler guide shaft. 
   As shown in  FIGS. 17 and 18 , the guide shaft, labeled  40 ′, is formed with a guide groove or cam groove  40   a  corresponding to the cam slot  41   a  of the previous embodiment. The guide groove  40   a  is made up of first guide grooves  40   b  corresponding to the first cam surfaces  41   b , second guide grooves  40   c  corresponding to the second cam surface  41   c , and third cam grooves  40   d  corresponding to the third cam surfaces  41   d . The guide grooves  40   b  through  40   d  are contiguous with each other. 
   As shown in  FIG. 18 , a guide member (bearing) is provided with a ball  41   k . When the guide stay  41  moves along the guide groove  40   a  together with the ball  41   k , the edge stapler S 1  is shifted between the position at which it moves while sandwiching a sheet stack and the position retracted from the idler pulley  56   a , as stated earlier. In the illustrative embodiment, the edge stapler S 1  moves back and forth in the direction perpendicular to the direction of sheet conveyance while being retracted from the idle pulley  56   a  as in the previous embodiment. Again, the guide shaft  40 ′ supports the stapler S 1  alone, so that the damping means included in the previous embodiment should preferably be used. As for the rest of the configuration, the illustrative embodiment is identical with the previous embodiment. 
   The illustrative embodiment makes it needless to position a cam below the stapler S 1  for thereby saving space in the up-and-down direction. 
   In summary, in accordance with the present invention, stapling means can move in the direction perpendicular to the direction of sheet conveyance while being retracted from a pulley or similar rotary member. A cam surface and a member contacting it are prevented from wearing due to friction and noticeably reducing the life of the stapling means. In addition, a load to act on the stapling means during movement is reduced. 
   Further, a single guide shaft can guide both of the above movement and angular movement of the stapling means, so that the number of parts is reduced. Moreover, the configuration of the present invention is simple and therefore low cost. 
   Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.