Patent Publication Number: US-6905118-B2

Title: Sheet finisher and image forming system using the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet finisher mounted on or operatively connected to a copier, printer or similar image forming apparatus for folding, sorting, stacking, stapling, center-stapling and binding, folding or otherwise finishing a sheet or a sheet stack, and an image forming system consisting of the sheet finisher and image forming apparatus. 
     2. Description of the Background Art 
     A sheet finisher positioned at the downstream side of an image forming apparatus for stapling or otherwise finishing a sheet stack is well known in the art. To meet the increasing demand for multiple functions, a sheet finisher having a center-stapling capability in addition to the conventional edge-stapling capability has recently been proposed. Further, a sheet finisher with a center-folding capability in addition to the center-stapling capability has been proposed to fold a center-stapled sheet stack at the center for thereby producing a pamphlet. 
     A sheet finisher with the binding capability mentioned above uses, in many cases, one or more pairs of fold rollers to fold a sheet stack. In this type of sheet finisher, a flat fold plate is caused to contact the stapled position of a sheet stack and push it into the nip of the fold roller pair, thereby folding the sheet stack. When use is made of, e.g., a first and a second fold roller pair, after the first roller pair has folded a sheet stack, the second roller pair presses the resulting fold of the sheet stack for thereby reinforcing it. 
     A problem with the above configuration, causing a fold roller pair to fold a sheet stack, is that the pressing force of the roller pair cannot be sufficiently transferred to a sheet stack because the entire width of a sheet stack passes the nip of the roller pair in an extremely short period of time. To solve this problem, Japanese Patent Laid-Open Publication Nos. 9-183566 and 9-183567, for example, propose to control the rotation speed of a fold roller pair for thereby enhancing folding quality. However, a pressing time available with a single fold roller pair is limited because the nip width of the roller pair is extremely small. Further, the above proposal reduces productivity. In light of this, Japanese Patent Laid-Open Publication No. 2000-143088 teaches the use of two fold roller pairs, which seems to be advantageous over the use of a single fold roller pair from the folding quality standpoint. 
     In any case, however, a period of time over which a sheet stack is pressed by the nip of a fold roller pair is short because the axis of each fold roller extends perpendicularly to a direction of sheet conveyance. This, coupled with the fact that the pressure of the fold roller pair, pressing the entire portion of a sheet stack to be folded, is scattered, prevents the sheet stack from being sharply folded. 
     Usually, a person folds a sheet stack by nipping the portion of the sheet stack to be folded with fingers and can therefore fold it with a relatively weak force. This is presumably because a sheet stack is not folded over the entire width at a time, but is folded part by part, so that a force to act on each part for a unit length increases. Taking this into account, Japanese Patent Laid-Open Publication No. 62-16987 proposes to surely fold a sheet stack by causing a roller to roll on the sheet stack in the direction perpendicular to the direction of sheet conveyance, i.e., parallel to the direction of a fold. More specifically, in a folding device configured to fold a sheet stack by conveying the sheet stack via the nip of a roller pair, a reinforce roller is positioned at the downstream side of the above roller pair and movable substantially perpendicularly to the direction of sheet conveyance for again pressing the fold of the sheet stack folded by the roller pair. The reinforce roller reinforces the fold of a sheet stack by being driven by a ball screw in the direction perpendicular to the direction of sheet conveyance. 
     In the configuration taught in Laid-Open Publication No. 62-16987 mentioned above, the reinforce roller presses the fold of a sheet stack in the direction perpendicular to the direction of sheet conveyance, so that load concentrates on one portion of the fold. In addition, the reinforce roller rolls on the fold of a sheet stack while exerting pressure on the entire fold of the sheet stack. The reinforce roller can therefore easily make the fold of the sheet stack sharper. However, the reinforce roller scheme taught in the above document has the following problems (1) through (7) when the sheet stack is thick. 
     (1) When the reinforce roller rolls on the fold of the sheet stack, it is likely that the roller sinks into the sheet stack and therefore moves on the fold without rotating, so that the image surface of a sheet is rubbed and smeared. 
     (2) The reinforce roller, fully pressed the fold of the sheet stack, comes down from the fold onto a lower guide plate. At this instant, the reinforce roller is apt to produce noise due to an impact. 
     (3) If a movable support member, supporting the reinforce roller, tilts while the roller is in movement, then the roller itself tilts with the result that the pressing force of the roller expected to act on the fold escapes. This prevents the reinforce roller from neatly reinforcing the fold. 
     (4) When a belt, which transfers a driving force to the reinforce roller, twists due to the tilt of the reinforce roller, it is likely that the durability of the belt is reduced or the belt slips out. 
     (5) If a guide member, which guides the movable support member, bends due to the pressing force of the reinforce roller while the roller is in movement, then the pressing force of the roller, acting on the fold, escapes, again preventing the roller from neatly reinforcing the fold. 
     (6) If a position where the reinforce roller and lower guide plate contact each other is different in level or height from the nip of a folding device located upstream of the roller, then it is likely that the sheet stack is formed with two folds. 
     (7) If the level at which the reinforce roller and lower guide plate contact each other varies in accordance with the position of the roller being moved, then the fold of the sheet stack is apt to be oblique. 
     Further, the reinforce roller scheme of Laid-Open Publication No. 62-16987 has the following problems (8) through (10) unsolved. 
     (8) When the number of sheets stapled together is small, the interval between consecutive sheet stacks is short, making a period of time necessary for the reinforce roller to press each sheet stack unavailable. 
     (9) When the number of sheets stapled together is large, each sheet stack cannot be sufficiently folded unless the reinforce roller presses the sheet stack a larger number of times or over a longer period of time. 
     (10) It is difficult to reduce the folding time of the reinforce roller while enhancing the durability of the roller. 
     When a roller pair is used to reinforce the fold of a sheet stack while conveying it, the roller pair is generally formed of an elastic material because it must exert a conveying force. Therefore, even when the sheet stack is relatively thick, noise to be produced when the trailing edge of the sheet stack leaves the nip of the roller pair is low and unnoticeable. By contrast, the reinforce roller, movable perpendicularly to the direction of sheet conveyance while rolling on the fold of a sheet stack, does not have to exert a conveying force, so that the reinforce roller and lower guide plate both can be formed of a hard material for the reinforcing effect. However, the reinforce roller, formed of a hard material, produces high, noticeable noise when coming down from the sheet stack onto the lower guide plate. The construction of Laid-Open Publication No. 62-16987 indicates that this problem is not addressed to. 
     On the other hand, if a jam occurs when the reinforce roller is moving in the direction perpendicular to the direction of sheet conveyance, then it is difficult to deal with the jam because of a relation between the direction of the nip and the direction of sheet conveyance. In the case of a roller pair, a person may forcibly pull out the jamming sheet stack or a rotatable knob may be arranged by a relatively simple, low cost method. However, when the reinforce roller stops moving halfway on the sheet stack, forcibly pulling out the sheet stack by hand is apt to damage the machine or the rotatable knob makes the configuration sophisticated. 
     Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-2426, 2001-10759, 2001-19269 and 2002-145516. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a sheet finisher capable of neatly folding a sheet stack with a reinforce roller, and an image forming system including the same. 
     It is another object of the present invention to provide a sheet finisher capable of preventing a reinforce roller, rolling on the fold of a sheet stack, from rubbing the image surface of a sheet and smearing it, and an image forming system including the same. 
     It is another object of the present invention to provide a sheet finisher capable of preventing a reinforce roller from producing noise when coming down from the fold of a sheet stack onto a lower guide plate, and an image forming system including the same. 
     It is another object of the present invention to provide a sheet finisher capable of preventing a belt, which transfers a driving force to a reinforce roller, from twisting, and an image forming apparatus including the same. 
     It is another object of the present invention to provide a sheet finisher insuring jam processing, protecting a machine from damage and reducing the downtime of the entire system when reinforcing the fold of a sheet stack, and an image forming system including the same. 
     It is another object of the present invention to provide a sheet finisher capable of efficiently reinforcing the fold of a sheet stack without producing noise or dislocating the sheet stack, and an image forming system including the same. 
     It is another object of the present invention to provide a sheet finisher capable of sufficiently reinforcing the fold of a sheet stack without reducing productivity even when the interval between consecutive sheets is short, and an image forming system including the same. 
     It is still another object of the present invention to provide a sheet finisher capable of sufficiently folding a sheet stack without regard to the number of sheets constituting the sheet stack, and an image forming system including the same. 
     It is yet another object of the present invention to provide a sheet finisher capable of reducing the folding time and enhancing the durability of a reinforce roller, and an image forming system including the same. 
     It is a further object of the present invention to provide a sheet finisher capable of allowing a jamming sheet stack to be easily, surely removed, and an image forming system including the same. 
     A sheet finisher of the present invention is included in an image forming system and folds a stack of sheets sequentially transferred from an image forming apparatus thereto. The sheet finisher includes a fold roller pair for holding the stack of sheets being conveyed via a nip thereof. A reinforce roller reinforces the fold of the folded sheet stack in cooperation with a guide plate. A drive mechanism causes the reinforce roller to move in a direction perpendicular to a direction of sheet conveyance. A shock absorbing member is located at a position where the reinforce roller and guide plate contact each other. 
    
    
     
       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 a first embodiment of the image forming system including a sheet finisher and an image forming system in accordance with the present invention; 
         FIG. 2  is a fragmentary, enlarged isometric view showing a shifting mechanism included in the sheet finisher; 
         FIG. 3  is a fragmentary, enlarged isometric view showing a shift tray elevating mechanism included in the sheet finisher; 
         FIG. 4  is an isometric view showing part of the sheet finisher configured to discharge sheets to the shift tray; 
         FIG. 5  is a plan view showing a staple tray included in the finisher, as seen in a direction perpendicular to a sheet conveying surface; 
         FIG. 6  is an isometric view showing the staple tray and a mechanism for driving it; 
         FIG. 7  is an isometric view showing a mechanism included in the sheet finisher for discharging a sheet stack; 
         FIG. 8  is an isometric view showing an edge stapler included in the sheet finisher together with a mechanism for moving it; 
         FIG. 9  is an isometric view showing a mechanism for rotating the edge stapler; 
         FIGS. 10 through 12  are views demonstrating the consecutive operating conditions of a sheet stack steering mechanism included in the sheet finisher; 
         FIGS. 13 and 14  are views demonstrating the consecutive operating conditions of a fold plate included in the sheet finisher; 
         FIG. 15  shows the staple tray and fold tray in detail; 
         FIG. 16  is a front view showing a reinforce roller unit included in the illustrative embodiment; 
         FIG. 17  is a side elevation of the reinforce roller unit; 
         FIG. 18  shows the reinforce roller in a position where it presses a sheet stack and a position where it contacts a lower guide plate; 
         FIG. 19  is a front view of the reinforce roller unit in which a flange is formed on one side of the reinforce roller; 
         FIG. 20  is a front view showing a condition in which the reinforce roller is tilted; 
         FIGS. 21A through 21C  show the configuration of a support member supporting the shaft of the reinforce roller; 
         FIG. 22  is a front view of the reinforce roller unit in a condition in which the support member is tilted; 
         FIG. 23  is a front view of the reinforce roller unit including a member configured to prevent the support member from rotating; 
         FIG. 24  is a rear view of the reinforce roller unit shown in  FIG. 23 ; 
         FIG. 25  shows how a guide member bends when the fold of a relatively thick sheet stack is reinforced; 
         FIG. 26  is a front view of the fold roller unit including a bend-preventing member; 
         FIG. 27  is a schematic block diagram showing a control system included in the illustrative embodiment; 
         FIG. 28  is a flowchart demonstrating a non-staple mode A available with the sheet finisher; 
         FIG. 29  is a flowchart demonstrating a non-staple mode B available with the sheet finisher; 
         FIGS. 30A and 30B  are flowcharts demonstrating a sort/stack mode available with the sheet finisher; 
         FIGS. 31A through 31C  are flowcharts demonstrating a staple mode available with the sheet finisher; 
         FIG. 32  is a flowchart demonstrating part of a center staple and bind mode available with the sheet finisher; 
         FIG. 33  is a flowchart demonstrating another part of the center staple and bind mode; 
         FIG. 34  is a flowchart demonstrating still another part of the center staple and bind mode; 
         FIG. 35  shows how a sheet stack is positioned on the staple tray in the center staple and bind mode; 
         FIG. 36  shows how a sheet stack is stacked and stapled at the center on the staple tray in the center staple and bind mode; 
         FIG. 37  shows the initial condition wherein the sheet stack steering mechanism steers a sheet stack stapled at the center on the staple tray in the center staple and fold mode; 
         FIG. 38  shows a condition wherein the sheet stack steering mechanism has steered the sheet stack stapled in the center staple and bind mode toward a fold tray; 
         FIG. 39  shows a condition wherein the sheet stack is positioned at a fold position on the fold tray in the center staple and bind mode; 
         FIG. 40  shows a condition wherein a fold plate has started folding the sheet stack on the fold tray in the center staple and bind mode; 
         FIG. 41  shows a condition wherein after the fold plate has started folding the sheets stack on the fold tray in the center staple and bind mode, the reinforce roller is reinforcing the fold of the sheet stack; 
         FIG. 42  shows a condition wherein the fold of a sheet stack is creased; 
         FIG. 43  is a front view showing the reinforce roller unit in which holding members are provided for holding a sheet stack during reinforcement; 
         FIG. 44  is a front view of the reinforce roller unit in which the holding members are biased toward each other; 
         FIG. 45  shows how the reinforce roller rolls on a sheet stack; 
         FIG. 46  shows the reinforce roller and support member supported by a movable support member such that they are rotatable, but not movable in the up-and-down direction; 
         FIG. 47  shows the reinforce roller rotatably supported by the support member and the support member configured to be movable in the up-and-down direction while sliding on the movable support member; 
         FIG. 48  is a side elevation showing a specific position where a sheet stack sensor is located; 
         FIG. 49  shows the sheet stack sensor located in the pressing range of the reinforce roller; 
         FIG. 50  shows a protuberance formed in a sheet stack; 
         FIG. 51  shows the sheet stack sensor located outside of the pressing range of the reinforce roller; 
         FIG. 52  is a flowchart demonstrating a reinforce roller initializing procedure; 
         FIG. 53  is a front view showing a modification of the lower guide plate; 
         FIG. 54  is a front view showing a modification of the guide member; 
         FIG. 55  is a front view of the reinforce roller unit in which the position of the lower guide plate is determined in relation to the position of the nip of the fold roller pair; 
         FIG. 56  is a front view showing the reinforce roller unit in a condition in which the above two positions are shifted from each other; 
         FIG. 57  shows a condition in which the reinforce roller presses a sheet stack introduced into the reinforce roller unit in a bent position; 
         FIG. 58  is a front view showing a modification of the lower guide plate; 
         FIG. 59  is a side elevation showing another modification of the lower guide plate; 
         FIG. 60  is a side elevation showing another modification in which position control members are provided on the lower guide member of  FIG. 58  or  59 ; 
         FIG. 61  is a front view showing the modification of  FIG. 60 ; 
         FIG. 62  is a side elevation showing a condition in which the position of the lower guide plate is not controlled; 
         FIG. 63  is a front view showing a condition in which the position of the lower guide plate is not controlled; 
         FIG. 64  is a flowchart demonstrating part of a center staple and bind mode representative of a second embodiment of the present invention; 
         FIG. 65  is a flowchart demonstrating another part of the center staple and bind mode; 
         FIGS. 66 through 71  are views for describing speed control unique to a third embodiment of the present invention; 
         FIG. 72  is a flowchart showing a speed control procedure particular to the third embodiment; 
         FIG. 73  is a flowchart showing a speed control procedure representative of a fourth embodiment of the present invention; 
         FIG. 74  is a front view showing a reinforce roller unit representative of a fifth embodiment of the present invention; 
         FIG. 75  is a side elevation of the reinforce roller unit shown in  FIG. 74 ; 
         FIG. 76  is a flowchart showing part of a center staple and bind mode representative of a sixth embodiment of the present invention; 
         FIG. 77  is a flowchart showing another part of the center staple and bind mode; 
         FIG. 78  is a flowchart showing a reinforce roller initializing procedure available with the sixth embodiment; 
         FIGS. 79A and 79B  are flowcharts showing a decision procedure included in the sixth embodiment for dealing with an error; 
         FIG. 80  shows a relation between the position and the speed of the reinforce roller representative of a seventh embodiment of the present invention; 
         FIG. 81  is a flowchart showing part of a center staple and bind mode representative of an eighth embodiment of the present invention; 
         FIG. 82  shows another part of the center staple and bind mode; 
         FIG. 83  is a flowchart showing part of a center staple and bind mode representative of a ninth embodiment of the present invention; 
         FIG. 84  shows the movement of the reinforce roller included in the ninth embodiment from a front position sensor toward a rear position sensor; 
         FIG. 85  shows the movement of the reinforce roller included in the ninth embodiment from the rear position sensor toward the front position sensor; 
         FIG. 86  shows how the reinforce roller moves back and forth between the front and rear positions sensors; 
         FIG. 87  is a flowchart demonstrating a enter staple and bind mode representative of a tenth embodiment of the present invention; 
         FIG. 88  is a flowchart showing a modification of the tenth embodiment; 
         FIG. 89  is a flowchart showing a center staple and bind mode representative of a eleventh embodiment of the present invention; 
         FIG. 90  is a flowchart showing part of a center staple and bind mode representative of a twelfth embodiment of the present invention; 
         FIG. 91  is a flowchart showing another part of the center staple and bind mode; 
         FIG. 92  is a flowchart showing part of a center staple and bind mode representative of a thirteenth embodiment of the present invention; 
         FIG. 93  is a flowchart showing another part of the center staple and bind mode; 
         FIG. 94  is a plan view showing a reinforce roller unit representative of a fourteenth embodiment of the present invention; 
         FIG. 95  is a front view of the fourteenth embodiment; 
         FIG. 96  is a side elevation of the fourteenth embodiment as seen from the right; 
         FIG. 97  is a plan view showing a reinforce roller unit representative of a fifteenth embodiment of the present invention; 
         FIG. 98  is a front view of the fifteenth embodiment; 
         FIG. 99  is a side elevation of the fifteenth embodiment as seen from the right; 
         FIG. 100  is a plan view showing a reinforce roller unit representative of a sixteenth embodiment of the present invention; 
         FIG. 101  is a front view of the sixteenth embodiment; 
         FIG. 102  is a side elevation of the sixteenth embodiment as seen from the right; 
         FIG. 103  shows an unlocked condition particular to the sixteenth embodiment; 
         FIG. 104  shows an upper guide plate held in an open position in the sixteenth embodiment; 
         FIG. 105  is a front view showing a modification of the sixteenth embodiment; 
         FIG. 106  shows an unlocked condition in the modification of  FIG. 105 ; 
         FIG. 107  shows the upper guide plate of  FIG. 105  held in an open position; 
         FIG. 108  is a plan view showing a seventeenth embodiment of the present invention; 
         FIG. 109  is a front view of the seventeenth embodiment; 
         FIG. 110  is a side elevation of the seventeenth embodiment as seen from the right; 
         FIG. 111  shows an unlocked condition in the seventeenth embodiment; 
         FIG. 112  shows the lower guide plate held in an open position in the seventeenth embodiment; 
         FIG. 113  is a front view showing a modification of the seventeenth embodiment; 
         FIG. 114  is a side elevation of the modification of  FIG. 113  as seen from the right; 
         FIG. 115  shows an unlocked condition in the modification of  FIG. 113 ; and 
         FIG. 116  shows the lower guide position held in an open position in the modification of FIG.  113 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the sheet finisher and image forming system in accordance with the present invention will be described hereinafter. Identical structural elements are designated by identical reference numerals and will not be repeatedly described in order to avoid redundancy. 
     First Embodiment 
     Referring to  FIG. 1  of the drawings, an image forming system embodying the present invention is shown and directed mainly toward the first object. As shown, the image forming system is generally made up of an image forming apparatus PR and a sheet finisher PD operatively connected to one side of the image forming apparatus PR. A sheet or recording medium driven out of the image forming apparatus PR via an outlet  95  is introduced in the sheet finisher PD via an inlet  18 . In the sheet finisher PD, a path A extends from the inlet  18  and includes finishing means for finishing a single sheet. In the illustrative embodiment, this finishing means is implemented as a punch unit or punching means  100 . Path selectors  15  and  16  steer the sheet coming in through the path A to any one of a path B terminating at an upper tray  201 , a path C terminating at a shift tray  202 , and a processing tray F. The processing tray F is used to position, staple or otherwise process a sheet or sheets and, in this sense, will sometimes be referred to as a staple tray hereinafter. 
     Sheets sequentially brought to the staple tray F via the paths A and D are positioned one by one, stapled or otherwise processed, and then steered by a guide plate  54  and a movable guide  55  to either one of the path C and another processing tray G. The processing tray G folds or otherwise processes the sheets and, in this sense, will sometimes be referred to as a fold tray hereinafter. The sheets folded by the fold tray G are further strongly folded by a reinforce roller  400  and then guided to a lower tray  203  via a path H. The path D includes a path selector  17  constantly biased to a position shown in  FIG. 1  by a light-load spring not shown. An arrangement is made such that after the trailing edge of a sheet has moved away from the path selector  17 , among rollers  9  and  10  and a staple outlet roller  11 , at least the roller  9  and a refeed roller  8  are rotated in the reverse direction to convey the trailing edge of the sheet to a prestacking portion E and cause the sheet to stay there. In this case, the sheet can be conveyed together with the next sheet superposed thereon. Such an operation may be repeated to convey two or more sheets together. 
     On the path A merging into the paths B, C and D, there are sequentially arranged an inlet sensor  301  responsive to a sheet coming into the finisher PD, an inlet roller pair  1 , the punch unit  100 , a waste hopper  101 , roller pair  2 , and the path selectors  15  and  16 . Springs, not shown, constantly bias the path selectors  15  and  16  to the positions shown in FIG.  1 . When solenoids, not shown, are energized, the path selectors  15  and  16  rotate upward and downward, respectively, to thereby steer the sheet to desired one of the paths B, C and D. 
     More specifically, to guide a sheet to the path B, the path selector  15  is held in the position shown in  FIG. 1  while the solenoid assigned thereto is deenergized. To guide a sheet to the path C, the solenoids are energized to rotate the path selectors  15  and  16  upward and downward, respectively. Further, to guide a sheet to the path D, the path selector  16  is held in the position shown in  FIG. 1  while the solenoid assigned thereto is turned off; at the same time, the solenoid assigned to the path selector  15  is turned on to rotate it upward. 
     In the illustrative embodiment, the finisher PD is capable of selectively effecting punching (punch unit  100 ), jogging and edge stapling (jogger fence  53  and edge stapler S 1 ), jogging and center stapling (jogger fence  53  and center stapler S 2 ), sorting (shift tray  202 ) or folding (fold plate  74  and fold rollers  81  and reinforce roller  400 ), as desired. 
     A shift tray outlet section I is located at the most downstream position of the sheet finisher PD and includes a shift outlet roller pair  6 , a return roller  13 , a sheet surface sensor  330 , and the shift tray  202 . The shift tray outlet section I additionally includes a shifting mechanism J shown in  FIG. 2 and a  shift tray elevating mechanism K shown in FIG.  3 . 
     As shown in  FIGS. 1 and 3 , the return roller  13  contacts a sheet driven out by the shift outlet roller pair  6  and causes the trailing edge of the sheet to abut against an end fence  32  shown in  FIG. 2  for thereby positioning it. The return roller  13  is formed of sponge and caused to rotate by the shift outlet roller  6 . A limit switch  333  is positioned in the vicinity of the return roller  13  such that when the shift tray  202  is lifted and raises the return roller  13 , the limit switch  333  turns on, causing a tray elevation motor  168  to stop rotating. This prevents the shift tray  202  from overrunning. As shown in  FIG. 1 , the sheet surface sensor  330  senses the surface of a sheet or that of a sheet stack driven out to the shift tray  202 . 
     As shown in  FIG. 3  specifically, the sheet surface sensor  330  is made up of a lever  30 , a sensor  330   a  relating to stapling, and a sensor  330   b  relating to non-stapling  330   b . The lever  30  is angularly movable about its shaft portion and made up of a contact end  30   a  contacting the top of the trailing edge of a sheet on the shift tray  202  and a sectorial interrupter  30   b . The upper sensor  330   a  and lower sensor  330   b  are mainly used for staple discharge control and shift discharge control, respectively. 
     More specifically, in the illustrative embodiment, the sensors  330   a  and  330   b  each turn on when interrupted by the interrupter  30   b  of the lever  30 . Therefore, when the shift tray  202  is lifted with the contact end  30   a  of the lever  30  moving upward, the sensor  330   a  turns off. As the shift tray  202  is further lifted, the sensor  330   b  turns off. When the outputs of the sensors  330   a  and  330   b  indicate that sheets are stacked on the shift tray  202  to a preselected height, the tray elevation motor  168  is driven to lower the shift tray  202  by a preselected amount. The top of the sheet stack on the shift tray  202  is therefore maintained at a substantially constant height. 
     The shift tray elevating mechanism K will be described in detail with reference to FIG.  3 . As shown, the mechanism K includes a drive unit L for moving the shift tray  202  upward or downward via a drive shaft  21 . Timing belts  23  are passed over the drive shaft  22  and a driven shaft  22  under tension via timing pulleys. A side plate  24  supports the shift tray  202  and is affixed to the timing belts  23 . In this configuration, the entire unit including the shift tray  202  is supported by 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 elevation motor  168 , which is a reversible drive source. Torque output from the tray elevation motor  168  is transmitted to the last gear of a gear train mounted on the drive shaft  21  to thereby move the shift tray  202  upward or downward. The worm gear  25  included in the driveline allows the shift tray  202  to be held at a preselected position and therefore prevents it from dropping by accident. 
     An interrupter  24   a  is formed integrally with the side plate  24  of the shift tray  202 . A full sensor  334  responsive to the full condition of the shift tray  202  and a lower limit sensor  335  responsive to the lower limit position of the shift tray  202  are positioned below the interrupter  24   a . The full sensor  334  and lower limit sensor  335 , which are implemented by photosensors, each turn off when interrupted by the interrupter  24   a . In  FIG. 3 , the shift outlet roller  6  is not shown. 
     As shown in  FIG. 2 , the shifting mechanism J includes a shift motor  169  and a cam  31 . When the shift motor or drive source  169  causes the cam  31  to rotate, the cam  31  causes the shift tray  202  to move back and forth in a direction perpendicular to a direction of sheet discharge. A pin  31   a  is studded on the shift cam  31  at a position spaced from the axis of the shift cam  31  by a preselected distance. The tip of the pin  31   a  is movably received in an elongate slot  32   b  formed in an engaging member  32   a , which is affixed to the back of the end fence  32  not facing the shift tray  202 . The engaging member  32   a  moves back and forth in a direction perpendicular to the direction of sheet discharge in accordance with the angular position of the pin  31   a , entraining the shift tray  202  in the same direction. The shift tray  202  stops at a front position and a rear position in the direction perpendicular to the sheet surface of  FIG. 1  (corresponding to the positions of the shift cam  31  shown in FIG.  2 ). A shift sensor  336  is responsive to a notch formed in the shift cam  31 . To stop the shift tray at the above two positions, the shift motor  169  is selectively energized or deenergized on the basis of the output of the shift sensor  336 . 
     Guide channels  32   c  are formed in the front surface of the end fence  32 . The rear edge portions of the shift tray  202  are movably received in the guide channels  32   c . The shift tray  202  is therefore movable up and down and movable back and forth in the direction perpendicular to the direction of sheet discharged, as needed. The end fence  32  guides the trailing edges of sheets stacked on the shift tray  202  for thereby aligning them. 
       FIG. 4  shows a specific configuration of the arrangement for discharging a sheet to the shift tray  202 . As shown in  FIGS. 1 and 4 , the shift roller pair  6  has a drive roller  6   a  and a driven roller  6   b . A guide plate  33  is supported at its upstream side in the direction of sheet discharge and angularly movable in the up-and-down direction. The driven roller  6   b  is supported by the guide plate  33  and contacts the drive roller  6   a  due to its own weight or by being biased, nipping a sheet between it and the drive roller  6   a . When a stapled sheet stack is to be driven out to the shift tray  202 , the guide plate  33  is lifted and then lowered at a preselected timing, which is determined on the basis of the output of a guide plate sensor  331 . A guide plate motor  167  drives the guide plate  33  in such a manner in accordance with the ON/OFF state of a limit switch  332 . 
       FIG. 5  shows the staple tray F as seen in a direction perpendicular to the sheet conveyance plane.  FIG. 6  shows a drive mechanism assigned to the staple tray F while  FIG. 7  shows a sheet stack discharging mechanism. As shown in  FIG. 6 , sheets sequentially conveyed by the staple outlet roller pair  11  to the staple tray F are sequentially stacked on the staple tray F. At this instant, a knock roller  12  knocks every sheet for positioning it in the vertical direction (direction of sheet conveyance) while jogger fences  53  position the sheet in the horizontal direction perpendicular to the sheet conveyance (sometimes referred to as a direction of sheet width). Between consecutive jobs, i.e., during an interval between the last sheet of a sheet stack and the first sheet of the next sheet stack, a controller  350  (see  FIG. 26 ) outputs a staple signal for causing an edge stapler S 1  to perform a stapling operation. A discharge belt  52  with a hook  52   a  immediately conveys the stapled sheet stack to the shift outlet roller pair  6 , so that the shift outlet roller pair  6  conveys the sheet stack to the shift tray  202  held at a receiving position. 
     As shown in  FIG. 7 , a belt HP (Home Position) sensor  311  senses the hook  52   a  of the discharge belt  52  brought to its home position. More specifically, as shown in  FIG. 37 , two hooks  52   a  and  52   a ′ are positioned on the discharge belt  52  face-to-face at spaced locations in the circumferential direction and alternately convey sheet stacks stapled on the staple tray F one after another. The discharge belt  52  may be moved in the reverse direction such that one hook  52   a  held in a stand-by position and the back of the other hook  52   a ′ position the leading edge of the sheet stack stored in the staple tray F in the direction of sheet conveyance, as needed. The hook  52   a  therefore plays the role of positioning means at the same time. 
     As shown in  FIG. 5 , a discharge motor  157  causes the discharge belt  52  to move via a discharge shaft  65 . The discharge belt  52  and a drive pulley  62  therefor are positioned at the center of the discharge shaft  65  in the direction of sheet width. Discharge rollers  56  are mounted on the discharge shaft  65  in a symmetrical arrangement. The discharge rollers  56  rotate at a higher peripheral speed than the discharge belt  52 . 
     A processing mechanism will be described hereinafter. As shown in  FIG. 6 , a solenoid  170  causes the knock roller  12  to move about a fulcrum  12   a  in a pendulum fashion, so that the knock roller  12  intermittently acts on sheets sequentially driven to the staple tray F and causes their trailing edges to abut against rear fences  51 . The knock roller  12  rotates counterclockwise about its axis. A jogger motor  158  drives the jogger fences  53  via a timing belt and causes them to move back and forth in the direction of sheet width. 
     As shown in  FIG. 8 , a mechanism for moving the edge stapler S 1  includes a reversible, stapler motor  159  for driving the edge stapler S via a timing belt. The edge stapler S is movable in the direction of sheet width in order to staple a sheet stack at a desired edge position. A stapler HP sensor  312  is positioned at one end of the movable range of the edge stapler S 1  in order to sense the stapler S brought to its home position. The stapling position in the direction of sheet width is controlled in terms of the displacement of the edge stapler S 1  from the home position. 
     As shown in  FIG. 9 , the edge stapler S 1  is capable of selectively driving a staple into a sheet stack in parallel to or obliquely relative to the edge of the sheet stack. Further, at the home position, only the stapling mechanism portion of the edge stapler S 1  is rotatable by a preselected angle for the replacement of staples. For this purpose, an oblique motor  160  causes the above mechanism of the edge stapler S 1  to rotate until a sensor  313  senses the mechanism reached a preselected replacement position. After oblique stapling or the replacement of staples, the oblique motor  160  causes the stapling mechanism portion to return to its original angular position. 
     As shown in  FIGS. 1 and 5 , a pair of center staplers S 2  are affixed to a stay  63  and are located at a position where the distance between the rear fences  51  and their stapling positions is equal to or greater than one-half of the length of the maximum sheet size, as measured in the direction of conveyance, that can be stapled. The center staplers S 2  are symmetrical to each other with respect to the center in the direction of sheet width. The center staplers S 2  themselves are conventional and will not be described specifically. Briefly, after a sheet stack has been fully positioned by the jogger fences  53 , rear fences  51  and knock roller  5 , the discharge belt  52  lifts the trailing edge of the sheet stack with its hook  52  to a position where the center of the sheet stack in the direction of sheet conveyance coincides with the stapling positions of the center staplers S 2 . The center staplers S 2  are then driven to staple the sheet stack. The stapled sheet stack is conveyed to the fold tray G and folded at the center, as will be described in detail later. 
     There are also shown in  FIG. 5  a front side wall  64   a , a rear side wall  64   b , and a sensor responsive to the presence/absence of a sheet stack on the staple tray F. 
     Reference will be made to  FIG. 15  as well as to  FIG. 1  for describing a mechanism for steering a sheet stack. To allow the sheet stack stapled by the center staplers S 2  to be folded at the center on the fold tray G, sheet stack steering means is located at the most downstream side of the staple tray F in the direction of sheet conveyance in order to steer the stapled sheet stack toward the fold tray G. 
     As shown in  FIG. 15 , the steering mechanism includes the guide plate  54  and movable guide  55  mentioned earlier. As shown in  FIGS. 10 through 12 , the guide plate  54  is angularly movable about a fulcrum  54   a  in the up-and-down direction and supports the press roller  57 , which is freely rotatable, on its downstream end. A spring  58  constantly biases the guide plate  54  toward the discharge roller  56 . The guide plate  54  is held in contact with the cam surface  61   a  of a cam  61 , which is driven by a steer motor  161 . 
     The movable guide  55  is angularly movably mounted on the shaft of the discharge roller  56 . A link arm  60  is connected to one end of the movable guide  55  remote from the guide plate  54  at a joint  60   a . A pin studded on the front side wall  64   a ,  FIG. 5 , is movably received in an elongate slot  60   b  formed in the link arm  60 , limiting the movable range of the movable guide  55 . A spring  59  holds the link arm  60  in the position shown in FIG.  10 . When the steer motor  161  causes the cam  61  to rotate to a position where its cam surface  61   b  presses the link arm  60 , the movable guide  55  connected to the link arm  60  angularly moves upward along the surface of the discharge roller  56 . A guide HP sensor  315  senses the home position of the cam  61  on sensing the interrupter portion  61   c  of the cam  61 . Therefore, the stop position of the cam  61  is controlled on the basis of the number of drive pulses input to the steer motor  161  counted from the home position of the cam  61 , as will be described later in detail. 
       FIG. 10  shows a positional relation to hold between the guide plate  54  and the movable guide  55  when the cam  61  is held at its home position. As shown, the guide surface  55   a  of the movable guide  55  guides a sheet stack on the path extending to the shift outlet roller  6 . 
       FIG. 11  shows a condition wherein the guide plate  54  is moved about the fulcrum  54   a  counterclockwise (downward) by the cam  61  with the press roller  57  pressing the discharge roller  57 . 
       FIG. 12  shows a condition wherein the cam  61  has further rotated from the above position to move the movable guide  55  clockwise (upward). In this condition, the guide plate  54  and movable guide  55  form the route extending from the staple tray F toward the fold tray G.  FIG. 5  shows the same relation as seen in the direction of depth. 
     While in the illustrative embodiment the guide plate  54  and movable guide  55  share a single drive motor, each of them may be driven by a respective drive motor, so that the timing of movement and stop position can be controlled in accordance with the sheet size and the number of sheets stapled together. 
     The fold tray G will be described specifically with reference to  FIGS. 13 and 14 . As shown, the fold tray G includes a fold plate  74  for folding a sheet stack at the center. The fold plate  74  is formed with elongate slots  74   a  each being movably received in one of pins  64   c  studded on each of the front and rear side walls  64   a  and  64   b . A pin  74   b  studded on the fold plate  74  is movably received in an elongate slot  76   b  formed in a link arm  76 . The link arm  76  is angularly movable about a fulcrum  76   a , causing the fold plate  74  to move in the right-and-left direction as viewed in  FIGS. 13 and 14 . More specifically, a pin  75   b  studded on a fold plate cam  75  is movably received in an elongate slot  76   c  formed in the link arm  76 . In this condition, the link arm  76  angularly moves in accordance with the rotation of the fold plate cam  75 , causing the fold plate  74  to move back and forth perpendicularly to a lower guide plate  91  and an upper guide plate  92  (see FIG.  15 ). 
     A fold plate motor  166  causes the fold plate cam  75  to rotate in a direction indicated by an arrow in FIG.  13 . The stop position of the fold plate cam  75  is determined on the basis of the output of a fold plate HP sensor  325  responsive to the opposite ends of a semicircular interrupter portion  75   a  included in the cam  75 . 
       FIG. 13  shows the fold plate  74  in the home position where the fold plate  74  is fully retracted from the sheet stack storing range of the fold tray G. When the fold plate cam  75  is rotated in the direction indicated by the arrow, the fold plate  74  is moved in the direction indicated by an arrow and enters the sheet stack storing range of the fold tray G.  FIG. 14  shows a position where the fold plate  74  pushes the center of a sheet stack on the fold tray G into the nip between a pair of fold rollers  81 . When the fold plate cam  75  is rotated in a direction indicated by an arrow in  FIG. 14 , the fold plate  74  moves in a direction indicated by an arrow out of the sheet stack storing range. 
     While the illustrative embodiment is assumed to fold a sheet stack at the center, it is capable of folding even a single sheet at the center. In such a case, because a single sheet does not have to be stapled at the center, it is fed to the fold tray G as soon as it is driven out, folded by the fold plate  74  and fold roller pair  81 , and then delivered to the lower tray  203 , FIG.  1 . 
     The reinforce roller unit  400  will be described in detail hereinafter. As shown in  FIG. 1 , the reinforce roller unit  400  is positioned on the path H between the fold roller  81  and the outlet roller pair  83  and configured to reinforce the fold of a sheet stack folded by the fold plate  74 . 
     As shown in  FIGS. 16 and 17 , the reinforce roller unit  400  is generally made up of a reinforce roller  409 , a support mechanism supporting the reinforce roller  409 , and a drive mechanism for driving the reinforce roller  409 . The drive mechanism includes a drive pulley  402 , a driven pulley  404 , a timing belt  403  passed over the pulleys  402  and  404 , and a pulse motor  401  for causing the timing belt  403  to turn. The support mechanism includes a slider or support member  407  slidable on a guide member  405  in a preselected direction, an upper guide plate  415 , and a coil spring or biasing means  411 . The upper guide plate  415  extends to a position above the slider  407  and remote from the reinforce roller  409  and prevents the reinforce roller  409  from tilting while preventing the guide member  405  from bending. The coil spring  411  constantly biases the reinforce roller  407  toward the folding direction, i.e., downward as viewed in FIG.  17 . The support mechanism extends in the direction perpendicular to the direction of sheet conveyance. The drive mechanism causes the reinforce roller  409  to move in the direction in which the support mechanism extends. 
     The output torque of the pulse motor  401  is transferred to the slider  407 , which is connected to the timing belt  403 , via the timing belt  403  passed over the drive pulley  402  and driven pulley  404 . The slider  407  therefore slides on the guide member  405  in the direction of thrust while being guided by the guide member  405 . A bend-preventing member  406  is positioned between the slider  407  and the upper guide plate  415  and implemented as a roller rotatably supported by the slider  407 . The bend-preventing member  406  is therefore movable integrally with the slider  407  in the axial direction of the guide member  405 . The reinforce roller  409  is positioned between the slider  407  and a lower guide plate  416 . A friction member  410  is fitted on the circumference of the reinforce roller  409 . 
     The reinforce roller  409  is supported by a roller support member  408 , which is supported in such a manner as to be movable in the up-and-down direction in sliding contact with the slider  407 . The coil spring  111  constantly biases the roller support member  408  downward. In this configuration, the reinforce roller  409 , when sliding on the guide member  405  together with the slider  407 , is constantly pressed toward the lower guide plate  416  by the coil spring  411  while being movable in the up-and-down direction. Position sensors  412  and  413  are positioned at opposite sides in the direction of thrust of the guide member  405 . The position sensor  412  is responsive to the slider  407  brought to a home position while the position sensor  413  is responsive to the slider  407  brought to an end-of-reinforcement position. A sheet stack sensor  414  is located at the inlet of the reinforce roller unit  400  for sensing a sheet stack introduced into the unit  400 . 
       FIG. 18  shows the reinforce roller  409  in two different positions, i.e., one in which the roller  409  is rolling on the top of a sheet stack and the other in which it is rolling on the lower guide plate  416 . As shown, a step is formed between the top of the sheet stack and the lower guide plate  416 , depending on the thickness of the sheet stack. As a result, when the reinforce roller  409  comes down from the top of the sheet stack onto the lower guide plate  416 , the reinforce roller  409  produces noise on directly contacting the lower guide plate  416 . 
     To obviate noise mentioned above, a flange  419 , formed of an elastic material, is mounted on one side of the reinforce roller  409  that does not contact the sheet stack. The flange  419  absorbs an impact when the reinforce roller  409  rolls down from the top of the sheet stack onto the lower guide plate  416 , thereby reducing noise. 
     As shown in  FIG. 20 , when the reinforce roller  409  is pressing the top of the folded portion of a sheet stack, the reinforce roller  409  tends to tilt due to the thickness of the folded portion because the coil spring  411  constantly biases the roller support member  408  toward the lower guide plate  416 . The resulting pressure obliquely acting on the folded portion fails to neatly reinforce the fold of the sheet stack. In light of this, as shown in  FIG. 21 , (c), a flange a is formed and held in contact with the roller support member  408 . In this configuration, as shown in  FIG. 22 , when the reinforce roller  409  and roller support member  408  tend to tilt, they support each other and are therefore prevented from tilting. The reinforce roller  409  can therefore neatly reinforce of the fold of the sheet stack even when the sheet stack is relatively thick. 
     As shown in  FIG. 22 , the thicker the sheet stack, the more the reinforce roller  409 , roller support member  408  and slider  407  tend to tilt. This again causes the pressure to obliquely act on the fold of the sheet stack and thereby prevents the reinforce roller  409  from neatly reinforcing the fold. To solve this problem, as shown in  FIG. 23 , a lug  420  is provided on the slider  407  and movably received in an elongate slot  415   a  formed in the upper guide plate  415 , so that the slider  407  can move along the guide member  405  without rotating about the guide member  405 . This successfully prevents the roller support member  408  and reinforce roller  405  from tilting. If desired, the slot  415   a  may be formed in a stationary member separate from the upper guide member  405  so long as the slot  415   a  is parallel to the slider  407 . 
     As shown in  FIG. 25 , as the thickness of the sheet stack increases, a force that acts on the guide member  405  upward due to the bias of the coil spring  411  becomes strong. As a result, the guide member  405  bends in one direction and causes the pressure expected to act on the fold of the sheet stack to escape. Moreover, the guide member  405  thus bent prevents the slider  407  to smoothly slide thereon. In light of this, as shown in  FIGS. 16 ,  17  and  26 , the bend-preventing member  406  mentioned earlier is rotatably supported by the slider  407  such that when the guide member  405  bends, the bend-preventing member  406  contacts the upper guide plate  415 . The bend-preventing member  406  therefore prevents the pressure expected to act on the fold of the sheet stack from escaping even when the guide member  405  bends. Further, because the bend-preventing member  406  is rotatable, the slider  407  can smoothly move in the direction of thrust of the guide member  405  even when the member  406  contacts the upper guide plate  415 . 
     Reference will be made to  FIG. 27  for describing a control system included in the illustrative embodiment. As shown, the control system includes a control unit  350  implemented as a microcomputer including a CPU (Central Processing Unit)  360  and an I/O (Input/Output) interface  370 . The outputs of various switches arranged on a control panel, not shown, mounted on the image forming apparatus PR are input to the control unit  350  via the I/O interface  370 . Also input to the control unit  350  via the I/O interface  370  are the output of the inlet sensor  301 , the output of an upper outlet sensor  302 , the output of a shift outlet sensor  303 , the output of a prestack sensor  304 , the output of a staple discharge sensor  305 , the output of a sheet sensor  310 , the output of the belt HP sensor  311 , the output of the staple HP sensor  312 , the output of the stapler oblique HP sensor  313 , the output of a jogger fence HP sensor  314 , the output of the guide home position sensor  315 , the output of a stack arrival sensor  321 , the output of a movable rear fence HP sensor  322 , the output of a fold position pass sensor  323 , the output of a lower outlet sensor  324 , the output of a fold plate HP sensor  325 , the output of sheet surface sensors  330 ,  330   a  and  330   b , and the output of the guide plate sensor  331 . 
     The CPU  360  controls, based on the above various inputs, the tray motor  168  assigned to the shift tray  202 , the guide plate motor  167  assigned to the guide plate, the shift motor  169  assigned to the shift tray  202 , a knock roller motor, not shown, assigned to the knock roller  12 , various solenoids including the knock solenoid (SOL)  170 , motors for driving the conveyor rollers, outlet motors for driving the outlet rollers, the discharge motor  157  assigned to the belt  52 , the stapler motor  159  assigned to the edge stapler S 1 , the jogger motor  158  assigned to the jogger fences  53 , the steer motor  161  assigned to the guide plate  54  and movable guide  55 , a motor, not shown, assigned to rollers for conveying a sheet stack, a rear fence motor assigned to the movable rear fence  73 , a fold roller motor, not shown, assigned to the fold roller  81 , and the pulse motor  401  assigned to the reinforce roller  409 . The pulse signals of a staple conveyance motor, not shown, assigned to the staple discharge rollers are input to the CPU  360  and counted thereby. The CPU  360  controls the knock SOL  170  and jogger motor  158  in accordance with the number of pulse signals counted. The fold roller motor is implemented by a stepping motor and controlled by the CPU  360  either directly via a motor driver or indirectly via the I/O  370  and motor driver. 
     Further, the CPU  360  causes the punch unit  100  to operate by controlling a clutch or a motor. The CPU  360  controls the finisher PD in accordance with a program stored in a ROM (Read Only Memory), not shown, by using a RAM (Random Access Memory) as a work area. 
     Specific operations to be executed by the CPU  360  in various modes available with the illustrative embodiment will be described hereinafter. 
     First, in a non-staple mode A, a sheet is conveyed via the paths A and B to the upper tray  201  without being stapled. To implement this mode, the path selector  15  is moved clockwise, as viewed in  FIG. 1 , to unblock the path B. The operation of the CPU  360  in the non-staple mode A will be described with reference to FIG.  28 . 
     As shown, before a sheet driven out of the image forming apparatus PR enters the finisher PD, CPU  360  causes the inlet roller pair  1  and conveyor roller pair  2  on the path A to start rotating (step S 101 ). The CPU  360  then checks the ON/OFF state of the inlet sensor  301  (steps S 102  and S 103 ) and the ON/OFF state of the upper outlet sensor  302  (steps S 014  and S 105 ) for thereby confirming the passage of sheets. When a preselected period of time elapses since the passage of the last sheet (YES, step S 106 ), the CPU  360  causes the above rollers to stop rotating (step S 107 ). In this manner, all the sheets handed over from the image forming apparatus PR to the finisher PD are sequentially stacked on the upper tray  201  without being stapled. If desired, the punch unit  100 , which intervenes between the inlet roller pair  1  and conveyor roller pair  2 , may punch the consecutive sheets. 
     In a non-staple mode B, the sheets are routed through the paths A and C to the shift tray  202 . In this mode, the path selectors  15  and  16  are respectively moved counterclockwise and clockwise, unblocking the path C. The non-staple mode B will be described with reference to FIG.  29 . 
     As shown, before a sheet driven out of the image forming apparatus PR enters the finisher PD, CPU  360  causes the inlet roller pair  1  and conveyor roller pair  2  on the path A and the conveyor roller pair  5  and shift outlet roller pair  6  on the path C to start rotating (step S 201 ). The CPU  360  then energizes the solenoids assigned to the path selectors  15  and  16  (step S 202 ) to thereby move the path selectors  15  and  16  counterclockwise and clockwise, respectively. Subsequently, the CPU  360  checks the ON/OFF state of the inlet sensor  301  (steps S 203  and S 204 ) and the ON/OFF state of the shift outlet sensor  303  (steps S 205  and S 206 ) to thereby confirm the passage of the sheets. 
     On the elapse of a preselected period of time since the passage of the last sheet (YES, step S 207 ), the CPU  360  causes the various rollers mentioned above to stop rotating (S 208 ) and deenergizes the solenoids (steps S 209 ). In this manner, all the sheets entered the finisher PD are sequentially stacked on the shift tray  202  without being stapled. Again, the punch unit  100  intervening between the inlet roller pair  1  and conveyor roller pair  2  may punch the consecutive sheets, if desired. 
     In a sort/stack mode, the sheets are also sequentially delivered from the path A to the shift tray  202  via the path C. A difference is that the shift tray  202  is shifted perpendicularly to the direction of sheet discharge copy by copy in order to sort the sheets. The path selectors  15  and  16  are respectively rotated counterclockwise and clockwise as in the non-staple mode B, thereby unblocking the path C. The sort/stack mode will be described with reference to  FIGS. 30A and 30B . 
     As shown, before a sheet driven out of the image forming apparatus PR enters the finisher PD, CPU  360  causes the inlet roller pair  1  and conveyor roller pair  2  on the path A and the conveyor roller pair  5  and shift outlet roller pair  6  on the path C to start rotating (step S 301 ). The CPU  360  then energizes the solenoids assigned to the path selectors  15  and  16  (step S 302 ) to thereby move the path selectors  15  and  16  counterclockwise and clockwise, respectively. Subsequently, the CPU  360  checks the ON/OFF state of the inlet sensor  301  (steps S 303  and S 304 ) and the ON/OFF state of the shift outlet sensor  303  (step S 305 ) 
     If the sheet passed the shift outlet sensor  303  is the first sheet of a copy (YES, step S 306 ), then the CPU  360  turns on the shift motor  169  (step S 307 ) to thereby move the shift tray  202  perpendicularly to the direction of sheet conveyance until the shift sensor  336  senses the tray  202  (steps S 308  and S 309 ). When the sheet moves away from the shift outlet sensor  303  (YES, step S 310 ), the CPU  360  determines whether or not the sheet is the last sheet (step S 311 ). If the answer of the step S 311  is NO, meaning that the sheet is not the last sheet of a copy, and if the copy is not a single sheet, then the procedure returns to the step S 303 . If the copy is a single sheet, then the CPU  360  executes a step S 312 . 
     If the answer of the step S 306  is NO, meaning that the sheet passed the shift outlet sensor  303  is not the first sheet of a copy, then the CPU  360  discharges the sheet(step S 310 ) because the shift tray  202  has already been shifted. The CPU  360  then determines whether or not the discharged sheet is the last sheet (step S 311 ). If the answer of the step S 311  is NO, then the CPU  360  repeats the step S 303  and successive steps with the next sheet. If the answer of the step S 311  is YES, then the CPU  360  causes, on the elapse of a preselected period of time, the inlet roller pair  1 , conveyor roller pairs  2  and  5  and shift outlet roller pair  6  to stop rotating (step S 312 ) and deenergizes the solenoids assigned to the path selectors  15  and  16  (step S 313 ). In this manner, all the sheets sequentially entered the finisher PD are sorted and stacked on the shift tray  202  without being stapled. In this mode, too, the punch unit  100  may punch the consecutive sheets, if desired. 
     In a staple mode, the sheets are conveyed from the path A to the staple tray F via the path D, positioned and stapled on the staple tray F, and then discharged t the shift tray  202  via the path C. In this mode, the path selectors  15  and  16  both are rotated counterclockwise to unblock the route extending from the path A to the path D. The staple mode will be described with reference to  FIGS. 31A through 31C . 
     As shown, before a sheet driven out of the image forming apparatus PR enters the finisher PD, CPU  360  causes the inlet roller pair  1  and conveyor roller pair  2  on the path A and the conveyor roller pairs  7 ,  9  and  10  and staple outlet roller  11  on the path D and knock roller  12  to start rotating (step S 401 ). The CPU  360  then energizes the solenoid assigned to the path selector  15  (step S 402 ) to thereby cause the path selector  15  to rotate counterclockwise. 
     After the stapler HP sensor  312  has sensed the edge stapler S 1  at the home position, the CPU  360  drives the stapler motor  159  to move the edge stapler S 1  to a preselected stapling position (step S 403 ). Also, after the belt HP sensor  311  has sensed the belt  52  at the home position, the CPU  360  drives the discharge motor  157  to bring the belt  52  to a stand-by position (step S 404 ). Further, after the jogger fence motor HP sensor has sensed the jogger fences  53  at the home position, the CPU  360  moves the jogger fences  53  to a stand-by position (step S 405 ). In addition, the CPU  360  causes the guide plate  54  and movable guide  55  to move to their home positions (step S 406 ). 
     If the inlet sensor  301  has turned on (YES, step S 407 ) and then turned off (YES, step S 408 ), if the staple discharge sensor  305  has turned on (YES, step S 409 ) and if the shift outlet sensor  303  has tuned on (YES, step S 410 ), then the CPU  360  determines that a sheet is present on the staple tray F. In this case, the CPU  360  energizes the knock solenoid  170  for a preselected period of time to cause the knock roller  12  to contact the sheet and force it against the rear fences  51 , thereby positioning the rear edge of the sheet (step S 411 ). Subsequently, the CPU  360  drives the jogger motor  158  to move each jogger fence  53  inward by a preselected distance for thereby positioning the sheet in the direction of width perpendicular to the direction of sheet conveyance and then returns the jogger fence  53  to the stand-by position (step S 412 ). The CPU  360  repeats the step S 407  and successive steps with every sheet. When the last sheet of a copy arrives at the staple tray F (YES, step S 413 ), the CPU  360  moves the jogger fences  53  inward to a position where they prevent the edges of the sheets from being dislocated (step S 414 ). In this condition, the CPU  360  turns on the stapler S 1  and causes it to staple the edge of the sheet stack (step S 415 ). 
     On the other hand, the CPU  360  lowers the shift tray  202  by a preselected amount (step S 416 ) in order to produce a space for receiving the stapled sheet stack. The CPU  360  then drives the shift discharge roller pair  6  via the shift discharge motor (step S 417 ) and drives the belt  52  by a preselected amount via the discharge motor  157  (step S 418 ), so that the stapled sheet stack is raised toward the path C. As a result, the stapled sheet stack is driven out to the shift tray  202  via the shift outlet roller pair  6 . After the shift outlet sensor  303  has turned on (step S 419 ) and then turned off (step S 420 ), meaning that the sheet stack has moved away from the sensor  303 , the CPU  360  moves the belt  52  and jogger fences  53  to their stand-by positions (steps S 421  and S 422 ), causes the shift outlet roller pair  6  to stop rotating on the elapse of a preselected period of time (step S 423 ), and raises the shift tray  202  to a sheet receiving position (step S 424 ). The rise of the shift tray  202  is controlled in accordance with the output of the sheet surface sensor  330  responsive to the top of the sheet stack positioned on the shift tray  202 . 
     After the last copy or set of sheets has been driven out to the shift tray  202 , the CPU  360  returns the edge stapler S 1 , belt  52  and jogger fences  53  to their home positions (steps S 426 , S 427  and S 428 ) and causes the inlet roller pair  1 , conveyor roller pairs  2 ,  7 ,  9  and  10 , staple discharge roller pair  11  and knock roller  12  to stop rotating (step S 429 ). Further, the CPU  360  deenergizes the solenoid assigned to the path selector  15  (step S 430 . Consequently, all the structural parts are returned to their initial positions. In this case, too, the punch unit  100  may punch the consecutive sheets before stapling. 
     The operation of the staple tray F in the staple mode will be described more specifically herein after. As shown in  FIG. 6 , when the staple mode is selected, the jogger fences  53  each are moved from the home position to a stand-by position 7 mm short of one end of the width of sheets to be stacked on the staple tray F (step S 405 ). When a sheet being conveyed by the staple discharge roller pair  11  passes the staple discharge sensor  305  (step S 409 ), the jogger fence  53  is moved inward from the stand-by position by 5 mm. 
     The staple discharge sensor  305  senses the trailing edge of the sheet and sends its output to the CPU  360 . In response, the CPU  360  starts counting drive pulses input to the staple motor, not shown, driving the staple discharge roller pair  11 . On counting a preselected number of pulses, the CPU  360  energizes the knock solenoid  170  (step S 412 ). The knock solenoid  170  causes the knock roller  12  to contact the sheet and force it downward when energized, so that the sheet is positioned by the rear fences  51 . Every time a sheet to be stacked on the staple tray F 1  passes the inlet sensor  301  or the staple discharge sensor  305 , the output of the sensor  301  or  305  is sent to the CPU  360 , causing the CPU  360  to count the sheet. 
     On the elapse of a preselected period of time since the knock solenoid  170  has been turned off, the CPU  360  causes the jogger motor  158  to move each jogger fence  53  further inward by 2.6 mm and then stop it, thereby positioning the sheet in the direction of width. Subsequently, the CPU  360  moves the jogger fence  53  outward by 7.6 mm to the stand-by position and then waits for the next sheet (step S 412 ). The CPU  360  repeats such a procedure up to the last page (step S 413 ). The CPU  360  again causes the jogger fences  53  to move inward by 7 mm and then stop, thereby causing the jogger fences  53  to retain the opposite edges of the sheet stack to be stapled. Subsequently, on the elapse of a preselected period of time, the CPU  360  drives the edge stapler S 1  via the staple motor for thereby stapling the sheet stack (step S 415 ). If two or more stapling positions are designated, then the CPU  360  moves, after stapling at one position, the edge stapler S 1  to another designated position along the rear edge of the sheet stack via the stapler motor  159 . At this position, the edge stapler S 1  again staples the sheet stack. This is repeated when three or more stapling positions are designated. 
     After the stapling operation, the CPU  360  drives the belt  52  via the discharge motor  157  (step S 418 ). At the same time, the CPU  360  drives the outlet motor to cause the shift outlet roller pair  6  to start rotating in order to receive the stapled sheet stack lifted by the hook  52   a  (step S 417 ). At this instant, the CPU  360  controls the jogger fences  53  in a different manner in accordance with the sheet size and the number of sheets stapled together. For example, when the number of sheets stapled together or the sheet size is smaller than a preselected value, then the CPU  360  causes the jogger fences  53  to constantly retain the opposite edges of the sheet stack until the hook  52   a  fully lifts the rear edge of the sheet stack. When a preselected number of pulses are output since the turn-on of the sheet sensor  310  or the belt HP sensor  311 , the CPU  360  causes the jogger fences  53  to retract by 2 mm and 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 it moves away from the upper ends of the jogger fences  53 . 
     On the other hand, when the number of sheets stapled together or the sheet size is larger than the preselected value, the CPU  360  causes the jogger fences  53  to retract by 2 mm beforehand. In any case, as soon as the stapled sheet stack moves away from the jogger fences  53 , the CPU  360  moves the jogger fences  53  further outward by 5 mm to the stand-by positions (step S 422 ) for thereby preparing it for the next sheet. If desired, the restraint to act on the sheet stack may be controlled on the basis of the distance of each jogger fence from the sheet stack. 
       FIGS. 32 through 34  demonstrate a center staple and bind mode or fold reinforcement mode. In this mode, the sheets are sequentially conveyed from the path A to the staple tray F via the path D, positioned and stapled at the center on the tray F, folded on the fold tray G, again pressed by the reinforce roller  409 , and then driven out to the lower tray  203  via the path H. In this mode, the path selectors  15  and  16  both are rotated counterclockwise to unblock the route extending from the path A to the path D. Also, the guide plate  54  and movable guide plate  55  are closed, as shown in  FIG. 36 , guiding the stapled sheet stack to the fold tray G. The center staple and bind mode will be described with reference to FIG.  32 . 
     As shown, before a sheet driven out of the image forming apparatus PR enters the finisher PD, CPU  360  causes the inlet roller pair  1  and conveyor roller pair  2  on the path A and the conveyor roller pairs  7 ,  9  and  10  and staple outlet roller  11  on the path D and knock roller  12  to start rotating (step S 401 ). The CPU  360  then energizes the solenoid assigned to the path selector  15  (step S 402 ) to thereby cause the path selector  15  to rotate counterclockwise. 
     Subsequently, after the belt HP sensor  311  has sensed the belt  52  at the home position, the CPU  360  drives to the discharge motor  157  to move the belt  52  to the stand-by position (step S 503 ). Also, after the jogger fence HP sensor has sensed each jogger fence  53  at the home position, the CPU  360  moves the jogger fence  53  to the stand-by position (step S 504 ). Further, the CPU  360  moves the guide plate  54  and movable guide  55  to their home positions (steps S 505 ). 
     If the inlet sensor  301  has turned on (YES, step S 506 ) and then turned off (YES, step S 507 ), if the staple discharge sensor  305  has turned on (YES, step S 508 ) and if the shift outlet sensor  303  has tuned on (YES, step S 509 ), then the CPU  360  determines that a sheet is present on the staple tray F. In this case, the CPU  360  energizes the knock solenoid  170  for the preselected period of time to cause the knock roller  12  to contact the sheet and force it against the rear fences  51 , thereby positioning the trailing edge of the sheet (step S 510 ). Subsequently, the CPU  360  drives the jogger motor  158  to move each jogger fence  53  inward by the preselected distance for thereby positioning the sheet in the direction of width perpendicular to the direction of sheet conveyance and then returns the jogger fence  53  to the stand-by position (step S 511 ). The CPU  360  repeats the step S 407  and successive steps with every sheet. As shown in  FIG. 33 , when the last sheet of a copy arrives at the staple tray F (YES, step S 512 ), the CPU  360  moves the jogger fences  53  inward to the position where they prevent the edges of the sheets from being dislocated (step S 513 ). 
     After the step S 513 , the CPU  360  turns on the discharge motor  157  to thereby move the belt  52  by a preselected amount (step S 514 ), so that the belt  52  lifts the sheet stack to a stapling position assigned to the center staplers S 2 . Subsequently, the CPU  360  turns on the center staplers S 2  at the intermediate portion of the sheet stack for thereby stapling the sheet stack at the center (step S 515 ). The CPU  360  then moves the guides  54  and  55  by a preselected amount each in order to form a path directed toward the fold tray G (step S 516 ) and causes the upper and lower roller pairs  71  and  72  of the fold tray G to start rotating (step S 517 ). As soon as the movable rear fence  73  of the fold tray G is sensed at the home position, the CPU  360  moves the fence  73  to a stand-by position (step S 518 ). The fold tray G is now ready to receive the stapled sheet stack. 
     After the step S 518 , the CPU  360  further moves the belt  52  by a preselected amount (step S 519 ) and causes the discharge roller  56  and press roller  57  to nip the sheet stack and convey it to the fold tray G. When the leading edge of the sheet stack arrives at the stack arrival sensor  321  (step S 520 ) and then moves a preselected distance, the CPU  360  causes the upper and lower roller pairs  71  and  72  to stop rotating (step S 521 ) and then releases the lower rollers  72  from each other (step S 522 ). Subsequently, the CPU  360  causes the fold plate  74  to start folding the sheet stack (step S 523 ) and causes the fold roller pairs  81  and  82  and lower outlet roller pair  83  to start rotating (step S 524 ). The CPU  360  causes the fold roller pairs  81  to continuously rotate until the sheet stack sensor  414  included in the reinforce roller unit  400  turns on. When the sheet stack sensor  414  turns on (YES, step S 525 ), the CPU  360  causes the fold roller  81  to rotate by a preselected amount and then stop rotating (step S 526 ). By this operation, the leading edge of the sheet stack is conveyed to a position where the reinforce roller  409  can press the fold of the sheet stack. 
     When the leading edge of the sheet stack is stopped at the above position, the CPU  360  drives the pulse motor  401  assigned to the reinforce roller  409  (step S 527 ) for thereby causing the reinforce roller  409  to roll on the leading edge or fold of the sheet stack. When the position sensor  413  senses the reinforce roller  409  reached the end-of-reinforcement position (YES, step S 528 ), the CPU  360  stops driving the pulse motor  401  (step S 529 ) to thereby complete the reinforcement of the fold. The CPU  360  then causes the fold roller pairs  81  to rotate and convey the sheet stack to the lower outlet roller pair  83  (step S 530 ). 
     In the above condition, as shown in  FIG. 34 , the CPU  360  determines whether or not the trailing edge of the folded sheet stack has moved away from the lower outlet sensor  324  (steps S 531  and S 532 ). If the answer of the step S 532  is YES, then the CPU  360  drives the step motor  401  to return the reinforce roller  409  to the home position (step S 533 ). When the position sensor  412  senses the reinforce roller  409  reached the home position (YES, step S 534 ), the CPU  360  stops driving the pulse motor  401  while causing the fold roller pairs  81  and  82  and lower outlet roller pair  83  to further rotate for a preselected period of time and then stop (step S 535 ). Subsequently, the CPU  360  causes the belt  52  and jogger fences  53  to return to the stand-by positions (steps S 536  and S 537 ). The CPU  360  then determines whether or not the above sheet stack is the last copy of a single job to perform (step S 538 ). If the answer of the step S 538  is NO, then the procedure returns to the step S 506 . If the answer of the step S 538  is YES, then the CPU  360  returns the belt  52  and jogger fences  53  to the home positions (steps S 539  and S 540 ). At the same time, the CPU  360  causes the inlet roller pair  1 , roller pairs  2 ,  7 ,  9  and  10 , staple discharge roller pair  11  and knock roller  12  to stop rotating (step S 541 ) and turns off the solenoid assigned to the path selector  15  (step S 542 ). As a result, all the structural parts are returned to their initial positions. 
     As stated above, sheets sequentially introduced from the image forming apparatus PR are stapled at the center by the staple tray F, folded at the center by the fold tray G, again pressed by the reinforce roller  409 , and then stacked on the lower tray  203 . 
     The stapling and folding operations to be performed in the center fold mode will be described in more detail hereinafter. A sheet is steered by the path selectors  15  and  16  to the path D and then conveyed by the roller pairs  7 ,  9  and  10  and staple discharge roller  11  to the staple tray F. The staple tray F operates in exactly the same manner as in the staple mode stated earlier before positioning and stapling (see FIG.  34 ). Subsequently, as shown in  FIG. 35 , the hook  52   a  conveys the sheet stack to the downstream side in the direction of conveyance by a distance matching with the sheet size. After the center staplers S 2  have stapled the center of the sheet stack, the sheet stack is conveyed by the hook  62   a  to the downstream side by a preselected distance matching with the sheet size and then brought to a stop. The distance of movement of the sheet stack is controlled on the basis of the drive pulses input to the discharge motor  157 . 
     Subsequently, as shown in  FIG. 37 , the sheet stack is nipped by the discharge roller  56  and press roller  57  and then conveyed by the hook  52   a  and discharge roller  56  to the downstream side such that it passes through the path formed between the guides  54  and  55  and extending to the fold tray G. The discharge roller  56  is mounted on a drive shaft associated with the belt  52  and therefore driven in synchronism with the belt  52 , as stated earlier. Subsequently, as shown in  FIG. 38 , the sheet stack is conveyed by the upper and lower roller pairs  71  and  72  to the movable rear fence  73 , which is moved from its home position to a position matching with the sheet size beforehand and held in a stop for guiding the lower edge of the sheet stack. At this instant, as soon as the other hook  52 ′ on the belt  52  arrives at a position close to the rear fence  51 , the hook  52   a  is brought to a stop while the guides  54  and  55  are returned to the home positions to wait for the next sheet stack. 
     As shown in  FIG. 39 , the sheet stack abutted against the movable rear fence  73  is freed from the pressure of the lower roller pair  72 . Subsequently, as shown in FIG.  40 , the fold plate  74  pushes part of the sheet stack close to a staple toward the nip of the fold roller pair  81  substantially perpendicularly to the sheet stack. The fold roller pair  81 , which is caused to rotate beforehand, conveys the sheet stack reached its nip while pressing it. As a result, the sheet stack is folded at its center. 
     As shown in  FIG. 41 , the center-folded sheet stack is conveyed to the reinforce roller unit  400  and then stopped there on the basis of the output of the sheet stack sensor  414 . Subsequently, the reinforce roller  409  is driven at a position shown in  FIG. 41  in order to reinforce the fold of the sheet stack. The sheet stack is then driven out to the lower tray  203  by the fold roller pair and lower outlet roller pair  83 . At this instant, as soon as the pass sensor  323  senses the trailing edge of the sheet stack, the fold plate  74  and movable rear fence  73  are returned to their home positions while the lower roller pair  72  is released from each other so as to wait for the next sheet stack. Alternatively, the rear fence  73  may be held at the same position without being returned to the home position if the next job deals with the same sheet size and the same number of sheets. 
     As shown in  FIG. 42 , the fold roller pair  81  continuously holds the sheet stack when the reinforce roller  409  is rolling on the fold or leading edge of the sheet stack in the direction perpendicular to the direction of sheet feed to reinforce the fold. Otherwise, as shown in  FIG. 42 , the folded portion of the sheet stack PB is, in many cases, creased without being neatly folded because the individual sheet is warped. 
     The fold roller pair  81 , however, may fail to firmly nip the sheet stack alone, e.g., when the individual sheet is relatively hard. In light of this, as shown in  FIG. 43 , a roller pair  417 , serving as a holding member, may be used to nip the upstream portion of the sheet stack from the time when the reinforce roller  409  starts pressing the fold of the sheet stack to the time when it stops pressing the fold. As shown in  FIG. 44 , a biasing member  418  constantly biases the rollers of the roller pair  417  toward each other. The roller pair  417  may be freely rotatable or rotated by a pulse motor not shown, as desired. 
     As shown in  FIG. 45 , the friction member  410  mentioned earlier is fitted on part of the reinforce roller  409  that contacts the sheet stack when pressing the fold of the sheet stack, i.e., on at least the circumference of the roller  409  that contacts the sheet stack. More specifically, when the sheet stack is relatively thick, the point where the reinforce roller  409  and sheet stack contact sinks and makes it difficult for the roller  409  to rotate. In such a condition, the friction member  410  guarantees a frictional force necessary for rotation between the sheet stack and the reinforce roller  409 , preventing the fold roller  409  from slipping on and rubbing an image, which may exist on the top of the sheet stack. The image is therefore protected from smearing. 
     As shown in  FIG. 46 , assume that the reinforce roller  409  and roller support member  408  are rotatable relative to the slider  407 , but not movable in the up-and-down direction. Then, when the sheet stack is relatively thick, the reinforce roller  409  may fail to get on the sheet stack and reinforce the fold. By contrast, in the illustrative embodiment, not only the reinforce roller  409  is rotatably supported by the roller support member  408 , but also the roller support member  408  is movable in the up-and-down direction while sliding on the slider  407 , as shown in FIG.  47 . In  FIG. 47 , if the distance h by which the roller support member  408  is movable in the up-and-down direction is selected to be greater than the maximum thickness t of a sheet stack folded by the folding device preceding the reinforce roller unit  400 , the reinforce roller  409  can easily get on fold of the sheet stack. Further, the coil spring  411 , pressing the reinforce roller  409  downward, allows the roller  409  to further neatly reinforce the fold of the sheet stack. 
     If the reinforce roller  409  is positioned on the sheet stack conveyance path when a sheet stack is transferred from the folding device to the reinforce roller unit  400 , then reinforce roller  409  will stop the sheet stack on the path and will therefore fail to press the fold of the sheet stack. The reinforce roller  409  must therefore be retracted from the above path before a sheet stack enters the reinforce roller unit  400 . For this purpose, as shown in  FIG. 48 , the illustrative embodiment locates at least one sheet stack sensor  414  below the lower guide plate  416  at the center portion of the guide member  405 . The sheet stack sensor  414  senses a sheet stack via a hole formed in the lower guide plate  416 . When the sheet stack sensor  414  senses the leading edge of a sheet stack, the reinforce roller  409  is surely retracted from the conveyance path on the basis of the output of the sensor  414 . 
     As shown in  FIG. 49 , assume that the sheet stack sensor  414  lies in a pressing range w over which the reinforce roller  409  presses a sheet stack. Then, as shown in  FIG. 50 , when the reinforce roller  409  presses the fold of a sheet stack, part PB 1  of the surface of the sheet stack protrudes in accordance with the shape of the hole formed in the lower guide plate  416  and assigned to the sheet stack sensor  414 . By contrast, as shown in  FIG. 51 , if the sheet stack sensor  414  and the hole of the lower guide plate  416  are positioned outside of the above range w and if a sheet stack is conveyed by preselected pulses into the range after it has been sensed by the sheet stack sensor  414 , then the reinforce roller  409  can press the fold of the sheet stack while obviating the protuberance PB 1 . 
     Reference will be made to  FIG. 52  for describing more specifically the return of the reinforce roller  409  to the home position effected in the step S 533  of the procedure shown in FIG.  34 . As shown, if the position sensor  412  is in an OFF state (NO, step S 451 ) and if the sheet stack sensor  414  is in an OFF state (NO, step S 452 ), then the pulse motor  401  is driven to move the reinforce roller  409  toward the home position (step S 454 ). Subsequently, when the other position sensor or home position sensor  412  turns on, the pulse motor  410  is turned off (step S 455 ). If the sheet stack sensor  414  is in an ON state, as determined in the step S 452 , meaning that the sensor  414  has sensed a sheet stack before the arrival of the reinforce roller  409  at the home position, then a jam signal is output (step S 453 ). 
       FIG. 53  shows a modification of the lower guide plate  416  effective when the flange  419 ,  FIG. 19  cannot sufficiently cope with noise alone. As shown, an elastic material  421  is positioned on part of the lower guide plate  416  which the flange  419  contacts. The elastic material  421  sufficiently reduces noise in cooperation with the flange  419 . 
       FIG. 54  shows a modification of the guide member  405 . As shown, while the guide member  405  shown in  FIG. 53  has a circular section, the modified guide member  405  shown in  FIG. 54  has a rectangular section in order to prevent the reinforce roller  409  from tilting when the sheet stack is relatively thick, as described with reference to FIG.  20 . However, the crux is that the guide member  405  includes at least one corner in a section so as to prevent the slider  407 , slidable along the guide member  405 , from tilting. This allows the reinforce roller  409  to surely press the fold of a sheet stack without causing the pressure from escaping. 
       FIG. 55  shows a specific configuration providing a particular positional relation between the lower guide member  416  and the nip of the fold roller pair  81 . As shown in  FIG. 56 , assume that the nip between the reinforce roller  409  and the lower guide plate  416  is difference in level or height from the nip, labeled N 1 , of the fold roller pair  81 . Then, as shown in  FIG. 57 , the sheet stack is bent with the result that a gap a is produced between the position of the fold provided by the fold roller pair  81  and the position where the reinforce roller  409  again presses the fold. To solve this problem, in the configuration of  FIG. 55 , when the reinforce roller  409  is held in a stand-by position before pressing the fold of a sheet stack and when the former presses the latter, the nip between the reinforce roller  409  and the lower guide plate  416  is maintained at the same level or height as the nip of the fold roller pair  81 . This prevents the fold of a sheet stack from being shifted. 
       FIGS. 58 and 59  show a modified form of the modification described with reference to FIG.  55 . As shown, to obviate the gap a,  FIG. 57 , the lower guide plate  416  is configured to be movable in the up-and-down direction perpendicularly to the axis of the reinforce roller  409 . A biasing member  422  constantly biases the lower guide plate  416  with the same force as, but in the opposite direction to, the biasing member  411  biasing the reinforce roller  409 . Even when the sheet stack is relatively thick, the biasing member  422  maintains the nip between the reinforce roller  409  and the lower guide plate  416  at the same level as the nip of the fold roller pair  81 , as shown in FIG.  58 . The fold of a sheet stack is therefore free from shift. 
       FIGS. 60 and 61  show another modified form of the modification shown in FIG.  55 . As shown in  FIG. 62  or  63 , if the position of the lower guide plate  416  is not restricted, then the lower guide plate  416  may tilt when the reinforce roller  409  is pressing the fold of a sheet stack. As a result, the nip between the reinforce roller  409  and the lower guide plate  416  is shifted or the pressure expected to act on the fold of a sheet stack escapes, preventing the reinforce roller  409  from neatly reinforcing the fold. 
     In light of the above, as shown in  FIG. 60 , position regulating members or control members  423  are provided on the lower guide plate  416  and movably received in elongate slots formed in side plates  424 , so that the lower guide plate  416  can move in the up-and-down direction without tilting. In this configuration, as shown in  FIG. 61 , when the reinforce roller  409  presses the fold of a sheet stack, the nip between the reinforce roller  409  and the lower guide plate  416  is located at the same level as the nip of the fold roller pair  81  positioned upstream of the reinforce roller unit  400 . The reinforce roller  400  can therefore neatly reinforce the fold of the sheet stack. The slots in which the position regulating members  423  are received may be formed in any other members so long as they are stationary, if desired. 
     Second Embodiment 
     Reference will be made to  FIGS. 64 and 65  for describing a second embodiment of the present invention Briefly, in the center staple and bind mode or fold reinforcement mode, the illustrative embodiment causes the reinforce roller  409  to press the fold of a sheet stack during each of forward and backward movements. A step S 513  at which a procedure shown in  FIG. 64  starts follows the step S 512  of FIG.  32 . Because the procedure of  FIG. 65  is identical with the procedure described with reference to  FIGS. 32 through 34  except for the steps S 526  through S 519 , the following description will concentrate on differences between the two procedures. 
     As shown in  FIG. 64 , after a sheet stack has been conveyed to the pressing position assigned to the reinforce roller  409  in the step S 526 , whether or not the position sensor  412  responsive to the home position of the reinforce roller  409  has turned on is determined (step S 551 ). If the answer of the step S 551  is YES, then the pulse motor  401  is energized to cause the reinforce roller  409  to move forward while pressing the fold of the sheet stack (step S 527 ). The pulse motor  401  is then turned off when the other position sensor responsive to the end-of-reinforcement turns on. 
     If the answer of the step S 551  is NO, meaning that the reinforce roller  409  is not located at the home position, then whether or not the reinforce roller  409  is located at the end-of-reinforcement position is determined (step S 552 ) on the basis of the output of the position sensor  413 . If the answer of the step S 552  is YES, then the pulse motor  401  is driven in the reverse direction to move the reinforce roller  409  toward the home position in the backward direction while again pressing the fold of the sheet stack (S 553 ). Subsequently, when the position sensor  412  at the home position side turns on (YES, step S 554 ), the pulse motor  401  is turned off (step S 529 ). This is followed by the step S 530  and successive steps. 
     As stated above, in the illustrative embodiment, the reinforce roller  409  presses the fold of a sheet stack during each of forward and backward movements for thereby reinforcing the fold of the sheet stack. In addition, the reinforce roller  409  does not have to be returned to the home position every time it reaches the end-of-reinforcement position, promoting efficient operation. 
     If desired, the reinforce roller  409  may be moved back and forth while pressing the fold of a sheet stack two times. In this case, when the position sensor  413  at the end-of-reinforcement side turns on in the step S 528 , the procedure returns to the step S 551 . At this instant, because the position sensor  412  at the home position side is in an OFF state, whether or not the position sensor  413  is in an ON state is determined in the step S 552 . At this instant, because the position sensor  413  is in an ON state, the steps S 553  and S 554  are executed until the position sensor  413  turns on. When the position sensor  413  turns on, the pulse motor  401  is turned off. In this manner, the reinforce roller  409  presses the fold of the sheet stack two times. 
     As for the rest of the configuration, the illustrative embodiment is identical with the second embodiment. 
     Third Embodiment 
     A third embodiment of the present invention will be described with reference to  FIGS. 66 through 72 . In the first embodiment, the flange  419  is formed of an elastic material while the elastic material  421  is provided on the lower guide member  416 , thereby reducing noise ascribable to the step between the sheet stack and the lower guide plate  416 . The third embodiment is configured to control the moving speed of the reinforce roller  409  for the same purpose as the first embodiment. 
     A distance from the home position (abbreviated as HP hereinafter) of the reinforce roller  409  to one edge of a sheet stack, i.e., a press start position and a distance from the other edge of the sheet stack, i.e., a press end position to the stop position of the roller  409  can be calculated on the basis of sheet size information received from the image forming apparatus PR. Every sheet stack is dislocated in the direction perpendicular to the direction of conveyance before arriving at the reinforce roller unit  400 . Taking this into account, as shown in  FIG. 66 , there can be set a zone X 1  in which the reinforce roller  409  does not get on a sheet stack, a zone X 2  in which the roller  409  may get on the sheet stack, a zone X 3  in which the roller  409  presses the sheet stack, a zone X 4  in which the roller  409  comes down from the sheet stack onto the lower guide plate  416 , and a zone X 5  terminating at the stop position of the roller  409 . 
     Assume that a usual speed necessary for the reinforce roller  409  to move is V 1 , that a speed that allows the roller  409  to get on one edge of a sheet stack without leaving a roller mark on the edge is V 2 , that a speed necessary for the roller  409  to reinforce the fold of the sheet stack is V 3 , and that a speed that allows the roller  409  to come down from the other edge of the sheet stack onto the lower guide plate  416  without producing noise is V 4 . Then, as shown in  FIGS. 67 through 70 , the roller  409  is moved from HP at the speed V 1  over the zone X 1 , moved at the speed V 2  over the zone X 2 , moved at the speed V 3  over the zone X 3 , and then moved at the speed V 4  over the zone X 4 . Finally, as shown in  FIG. 71 , the roller  409  is again moved at the speed V 1  over the zone X 5 . This allows the roller  409  to press the sheets tack without leaving a roller mark on the sheet stack or producing noise. 
     In the above description, the reinforce roller  409  is assumed to start moving at the same HP every time it presses a sheet stack. By contrast, assume that the position where the roller  409  has ended pressing the preceding sheet stack is used as a press start position (HP) for the following sheet stack. Then, a relation to be described hereinafter holds between the speeds V 1  through V 4  and the zones X 1  through X 5  when the roller  409  is moved from the HP opposite to the original HP. 
     The roller  409  is moved at the speed V 1  over the zone X 5  and then moved at the speed V 2  over the range X 4  when getting on a sheet stack. Subsequently, the roller  409  is moved at the speed V 3  over the zone X 3  while pressing the sheet stack, moved at the speed V 4  over the zone X 2  when coming down from the sheet stack onto the lower guide plate  416 , and then moved at the speed V 1  over the zone X 1 . Such a procedure will be described more specifically with reference to FIG.  72 . 
     The procedure shown in  FIG. 72  is executed between the steps S 501  through S 512  of FIG.  32  and the steps S 531  through S 542  of FIG.  65 . Steps S 561  through S 565  of  FIG. 72  are substituted for the step S 527  of FIG.  33 . Because a step S 513  of  FIG. 72  follows the step S 512  of FIG.  32  and because a step S 531  and successive steps are identical with the corresponding steps of  FIG. 65 , let the following description concentrate on differences between such procedures. 
     As shown in  FIG. 72 , when the fold roller pair  81  conveys a sheet stack until the fold or leading edge of a sheet stack arrives at the pressing position (step S 526 ), the pulse motor  401  is driven to move the reinforce roller  409  at the speed V 1  over the zone X 1  (step S 561 ), move it at the speed V 2  over the zone X 2  (step S 562 ), moves it at the speed V 3  over the zone X 3  (step S 563 ), moves it at the speed V 4  over the zone X 4  (step S 564 ), and then moves it at the speed V 1  over the zone X 5  (step S 565 ). Subsequently, when the position sensor  413  positioned at the end-of-reinforcement side, the pulse motor  401  is turned off (step S 528 ). 
     By controlling the speed of the reinforce roller  409  when the roller  409  gets on a sheet stack and when the former comes down the latter as stated above, it is possible to obviate noise and protect the surface of a sheet stack from damage or smear. 
     As for the rest of the configuration, the illustrative embodiment is identical with the first embodiment. 
     Fourth Embodiment 
     A fourth embodiment of the present invention will be described with reference to FIG.  73 . In the illustrative embodiment, the second embodiment is combined with the first embodiment. More specifically, a procedure shown in  FIG. 73  is executed between the steps S 501  through S 512  of FIG.  32  and the steps S 531  through S 542  of FIG.  65 . Steps S 561  through S 565  of  FIG. 73  are substituted for the step S 527  of FIG.  33 . Because a step S 513  of  FIG. 73  follows the step S 512  of FIG.  32  and because a step S 531  and successive steps are identical with the corresponding steps of  FIG. 65 , let the following description concentrate on differences between such procedures. 
     Briefly, in the illustrative embodiment, the reinforce roller  409  presses a sheet stack during both of forward and backward movements while being controlled in speed for obviating noise and protecting the surface of a sheet stack from damage and smear as in the third embodiment. 
     Assume that, when a sheet stack is brought to the pressing position and stopped there, the position sensor  412  at the HP side is in an ON state, i.e., the reinforce roller  409  is located at the HP. Then, the steps S 582  through S 583  shown in  FIG. 73  are sequentially executed in the same manner as the steps S 561  through S 565  of the third embodiment. When the other position sensor  413  at the end-of-reinforcement side turns on, the pulse motor  401  is turned off, stopping the reinforce roller  409  at the end-of-reinforcement side. On the other hand, if the position sensor  412  at the HP side is in an OFF state (NO, step S 581 ), then whether or not the position sensor  413  at the end-of-reinforcement side is in an ON state is determined (step S 587 ). If the answer of the step S 587  is YES, the steps S 588  through S 593  are executed, causing the reinforce roller  409  to move in the forward direction. The steps S 588  through S 593  are opposite to the steps S 582  through S 586 . 
     The procedure described above is also successful to promote the efficient movement of the reinforce roller  409  while reducing noise and protecting a sheet stack from damage and smear. 
     As for the rest of the configuration, the illustrative embodiment is identical with the first through third embodiments. 
     Fifth Embodiment 
       FIGS. 74 and 75  show a fifth embodiment of the present invention. As shown in  FIG. 73 , the illustrative embodiment includes a first and a second guide member  405   a  and  405   b  extending perpendicularly to the lower guide plate  416 . The elastic member  411  is fitted on the shaft portion of the bend-preventing member  406  intervening between the slider  407  and the upper guide plate  415 . The guide members  405   a  and  405   b  are received in a guide slot  403   a  formed in the slider  407  in the vertical direction, as viewed in FIG.  74 . Spaces exist between the top of the guide member  405   a  and bottom of the guide member  405   b  and the edge of the guide slot  403   a , so that the guide members  405   a  and  405   b  are movable vertically relative to the lower guide plate  416 . 
     In the above configuration, when the reinforce member  409  presses a sheet stack, the slider  401  is elastically biased toward or away from the upper guide plate  415  in accordance with the thickness of a sheet stack to be reinforced. In addition, the two guide members  405   a  and  405   b , supporting the slider  407 , prevent the reinforce roller  409  from tilting. 
     As for the rest of the configuration, the illustrative embodiment is identical with the first to fourth embodiments. 
     As stated above, in accordance with the first to fifth embodiments of the present invention, the reinforce roller  409  can neatly reinforce the fold of a sheet stack by pressing the fold. Further, the reinforce roller  409  does not slip on the sheet of a sheet stack while pressing its fold and therefore does not rub an image, which may be present on the surface of a sheet stack. Moreover, the reinforce roller  409  does not produce noise when coming down from a sheet stack onto the lower guide plate  416 . In addition, because the guide member  405  or guide members  405   a  and  405   b  do not bend, there can be obviated defective reinforcement ascribable to the bend of the guide member  405  and the twist of a belt, which is included in drive means for driving the reinforce roller  405 . 
     Sixth Embodiment 
       FIGS. 76 and 77  show a center staple and bind mode or fold reinforcement mode representative of a sixth embodiment of the present invention. A step S 153  shown in  FIG. 76  follows the step S 512  of FIG.  32 . Because the procedure of  FIG. 76  is identical with the procedure of FIGS.  32  through  FIG. 34  except for steps S 525  through S 536 , the following description will concentrate on differences between the two procedures. 
     As shown in  FIG. 76 , the fold roller pair  81  conveys a sheet stack until the sheet stack sensor  414  included in the reinforce roller unit  400  turns on (step S 525 ). If the answer of the step S 525  is YES, then the fold roller pair  81  is rotated by a preselected amount and then stopped (step S 526   a ), thereby conveying the sheet stack to the pressing position. Subsequently, the pulse motor  401  is driven to cause the reinforce roller  409  to move from the position of the position sensor  412  to the position of the position sensor  412  while pressing the fold of the sheet stack (step S 527   a ). Then, the fold roller pair  81  and lower outlet roller pair  32  are caused to start rotating (S 528   a ). 
     As shown in  FIG. 77 , in the above condition, when the fold position pass sensor  323  turns on (YES, step S 529   a ) and then turns off (YES, step S 530   a ), the lower roller pair  72  is pressed (step S 531   a ). At the same time, the fold plate  74  is returned on to the home position (step S 532   a ) while the guide plate  54  and movable guide  55  are moved to their home positions (step S 533   a ). Subsequently, when the lower outlet sensor  324  turns on (YES, step S 534   a ) and then turns off (YES, step S 535   a ), the fold roller pair  81  and lower roller pair  83  are further rotated for a preselected period of time and then stopped. (step S 536   a ). Then, the reinforce roller  409  is moved from the position of the position sensor  412  to the position of the position sensor  413 , i.e., to the home position (step S 537   a ) while the belt  52  and jogger fence  53  are returned to their home positions (steps S 536  and S 537 ). 
     In the illustrative embodiment, the reinforce roller  409  is controlled with its home position being used as a reference, so that the return of the reinforce roller  409  to the home position, i.e., initialization is important. The return of the reinforce roller  409  to the home position in the center staple and bind mode of FIG.  77  will be described more specifically with reference to FIG.  78 . 
     As shown in  FIG. 78 , if the position sensor or home position sensor  413  is in an ON state (YES, step S 601 ), meaning that the current position of the reinforce member  409  is the home position, then the procedure simply returns. If the answer of the step S 601  is NO, the pulse motor  402  is driven to move the reinforce member  409  toward the position sensor  413  (step S 602 ). When the position sensor  413  turns on (step S 603 ), the pulse motor  402  is turned off to cause the reinforce roller  409  to stop moving (steep S 604 ). 
     Decision on an error that the illustrative embodiment makes will be described hereinafter. If the position sensor  413  does not turn on in the step S 603  after the reinforce roller  409  has been moved toward the position sensor  413  in the step S 602 , then a sheet stack is, determined to have jammed the path. More specifically, the reinforce roller  409  is moved from the position of the position sensor  413  toward the position of the position sensor  412  after a sheet stack has been stopped at the preselected position. At this instant, if the position sensor  412  does not sense the reinforce roller  409  even after a preselected number of pulses input to the pulse motor  402  have been counted, then it is determined that an error, i.e., the locking of the mechanism, the stop of the roller  409  ascribable to a short drive torque or the step-out of the motor  402  has occurred. In this case, the pulse motor  401  is driven in the reverse direction to return the reinforce roller  409  toward the position of the position sensor  413 . At this instant, if the position sensor  413  senses the reinforce roller  409  within a preselected period of time, then the reinforce roller  409  is stopped at the position of the position sensor  413  while a jam message is displayed on, e.g., the control panel of the image forming apparatus PR. Alternatively or in addition, a display for displaying such an error message may be mounted on the sheet finisher PD, if desired. 
     As stated above, if the position sensor  413  does not sense the reinforce roller  409  within a preselected period of time, the pulse motor  401  is turned off while a service call or similar message, showing that an error unable to be dealt with by the user has occurred, is displayed on, e.g., the control panel of the image forming apparatus PR. After reinforcement, the fold roller pair  81  and lower outlet roller pair  83  convey the sheet stack to the lower tray  203 . At this instant, when the fold position pass sensor  323  senses the trailing edge of the sheet stack, the movable rear fence  73  is returned to the home position while the lower roller pair  72  is released to prepare for the next sheet stack. Alternatively, the rear fence  73  may not be returned to the home position if the next job deals with a sheet stack of the same sheet size and consisting of the same number of sheets. 
       FIGS. 79A and 79B  show the above error decision procedure more specifically. As shown, whether or not a movement start flag is (logical) ZERO is determined (step S 701 ). If the answer of the step S 701  is YES, then the reinforce roller  409  is moved toward the position of the position sensor  412  (step S 702 ). At the same time, a counter, not shown, starts counting the number of pulses input to the pulse motor  402  while the movement start flag is set to (logical) ONE (step S 703 ). Subsequently, whether or not the counter has counted a preselected number of pulses is determined (step S 704 ). If the answer of the step S 704  is NO, then whether or not the position sensor  412  has sensed the reinforce roller  409  is determined (step S 705 ). If the answer of the step S 705  is NO, the procedure returns to the step S 704  because the movement start flag is ONE (YES, step S 701 ). If the answer of the step S 705  is YES, then the reinforce roller  409  is caused to stop moving (step S 706 ) while the movement start flag is cleared, i.e., set to ZERO. 
     If the answer of the step S 704  is YES, then whether or not the position sensor  412  has sensed the reinforce roller  409  is determined (step S 708 ). If the answer of the step S 708  is YES, then the procedure returns to the step S 706 . The movement of the reinforce roller  409  described so far is normal. 
     On the other hand, if the position sensor  412  is in an OFF state in the step S 708 , meaning that an error has occurred, the following job, i.e., the operation for folding a sheet stack at the center is interrupted (step S 709 ). At the same time, the pulse motor  402  is rotated in the reverse direction to move the reinforce roller  409  toward the position of the position sensor  413  (step S 710 ). Subsequently, if the position sensor  413  senses the reinforce roller  409  within a preselected period of time (YES, step S 711 ), meaning that the roller  409  has returned to the home position despite any error, it is determined that the error is a simple jam. In this case, a message, showing a jam that can be dealt with by the user, is displayed on, e.g., the control panel of the image forming apparatus PR (step S 712 ) while the movement start flag is returned to ZERO. If the answer of the step S 711  is NO, meaning that the reinforce roller  409  is unable to move, then a message, showing a jam that cannot be dealt with by the user, is displayed (step S 712 ). At the same time, the movement flag is returned to ZERO. 
     With the control described above, it is possible to prevent, when a jam that cannot be dealt with by the user occurs, the user from damaging the machine and making the error more serious by performing unexpected operation. Why the following job is interrupted in the step S 709  is that when an error occurs during reinforcement, it is determined that a sheet stack being reinforced exceeds an allowable limit. This prevents the following sheet stack from being subject to the same error. 
     As for the rest of the configuration, the illustrative embodiment is identical with the previous embodiments. 
     As stated above, the illustrative embodiment allows a jam occurred during reinforcement to be surely dealt with for thereby protecting the machine from damage and reducing the downtime of the entire system. 
     Seventh Embodiment 
       FIG. 80  shows a seventh embodiment of the present invention. As shown, the reinforce roller  409  is moved from the home position to the vicinity of one edge of a sheet stack at a speed V 1 , moved at a speed V 2  over the zone in which the roller  409  gets on the sheet stack, moved at a speed V 3  to the vicinity of the other edge of the sheet stack while pressing the fold of the sheet stack, moved at a speed V 4  over the zone in which the roller  409  comes down from the sheet stack, and then moved at the speed V 1  to the position of the position sensor  413 . Subsequently, when the sheet stack is conveyed to the outside of the reinforce roller unit  400 , the reinforce roller  409  is returned to the position of the position sensor  413  at a speed V 5 . 
     The following relations hold between the speeds V 1  through V 5 :
 
V 1 ≧V 2 
 
V 2 , V 4 &lt;V 3 
 
V 5 &gt;V 3 
 
     In the illustrative embodiment, the speed V 4  is selected to be equal to the speed V 1 . Alternatively, a speed V 6  different from the speed v 1  may be selected, in which case a relation of V 6 ≦V 4  should hold. 
     The illustrative embodiment is substantially similar to  FIG. 6  as to the center staple and bind mode operation and reinforce roller initializing operation. As for the rest of the configuration, the illustrative embodiment is identical with the previous embodiments. 
     As stated above, the illustrative embodiment can efficiently reinforce the fold of a sheet stack without producing noise or dislocating the sheet stack. 
     Eighth Embodiment 
       FIGS. 81 and 82  show an eighth embodiment of the present invention. A step S 513  shown in  FIG. 81  follows the step S 512  shown in FIG.  32 . Because a procedure of  FIGS. 81 and 82  is identical with the procedure of  FIGS. 32 through 34  except for processing between the steps S 523  and S 535 , let the following description concentrate on differences between the two procedures. 
     As shown in  FIG. 81 , the fold plate  74  starts folding a sheet stack at the center (step S 523 ). Subsequently, assuming that a period of time necessary for the reinforce roller  409  to complete reinforcement is T 1  and that a time interval between consecutive sheet stacks each having n sheets is T 2 , then the periods of time T 1  and T 2  are compared (step S 524 - 1 ). If the period of time T 1  is shorter than or equal to T 2  (YES, step S 524 - 1 ), then the fold roller pair  81  is caused to start rotating to fold the sheet stack (step S 524 - 2 ). When the sheet stack sensor  414  of the reinforce roller unit  400  turns on (YES, step S 524 - 3 ), meaning that the sheet stack thus folded has entered the reinforce roller unit  400 , then the sheet stack is conveyed by a preselected distance to the pressing position, and then the fold roller pair  81  is caused to stop rotating (step S 524 - 4 ). As a result, the sheet stack is nipped by the fold roller pair  81 . 
     Subsequently, whether or not the position sensor is turned on, i.e., whether or not the reinforce roller  409  is located at the position of the position sensor  413  is determined (step S 524 - 5 ). If the answer of the step S 524 - 5  is NO, then the reinforce roller  409  is moved from the position of the position sensor  412  to the position of the position sensor  413  (step S 524 - 7 ). If the answer of the step S 524 - 5  is YES, the reinforce roller  409  is moved from the position of the position sensor  413  to the position of the position sensor  412  (step S 524 - 6 ). Then, the fold roller pair  81  and lower roller pair  83  are caused to start rotating to convey the sheet stack (step S 514 - 8 ). On the other hand, if the answer of the step S 524 - 1  is NO, the procedure advances to the step S 524 - 8  by skipping the reinforcement. 
     Thereafter, as shown in  FIG. 82 , when the fold position pass sensor  323  turns on (YES, step S 525   b ) and then turns off (YES, step S 526   b ), the lower roller pair  72  is pressed (step S 527   b ) while the fold plate  74 , guide plate  54  and movable guide  55  are returned to their home positions to prepare for the next sheet stack (steps S 528   b  and S 529   b ). When the trailing edge of the sheet stack moves away from the lower outlet sensor  324  (YES, step S 531   b ), the fold roller pairs  81  and  82  and lower outlet roller pair  83  are further rotated by a preselected period of time and then caused to stop rotating (step S 532 ). 
     Ninth Embodiment 
       FIG. 83  shows a ninth embodiment of the present invention. Steps S 501  through S 524 - 4  and steps S 524 - 8  through S 539  shown in  FIG. 83  are identical with the corresponding steps of the eighth embodiment and will not be described specifically in order to avoid redundancy. 
     As shown in  FIG. 83 , when the sheet stack sensor  414  of the reinforce roller unit  400  turns on (YES, step S 524 - 3 ), meaning that a folded sheet stack has entered the reinforce roller unit  400 , then the sheet stack is conveyed to the pressing position by a preselected distance, and then the fold roller pair  81  is caused to stop rotating (step S 524 - 4 ) As a result, the sheet stack remains nipped by the fold roller pair  81 . Subsequently, when the sheet stack sensor  321  turns on (YES, step  524 - 9 ), meaning that the sheet stack has arrived at a position just preceding the upper guide plate  92 , then the fold roller pair  81  and lower outlet roller pair  83  are caused to start rotating because a period of time for reinforcing the fold of the sheet stack is not available (step S 524 - 8 ). 
     If the answer of the step S 524 - 9  is NO, then whether or not the position sensor  413  is in an ON state is determined because a period of time for reinforcement is available (step S 524 - 5 ). If the answer of the step S 524 - 5  is YES, meaning that the reinforce roller  409  is located at the position of the position sensor  413 , then the reinforce roller  409  is moved from the position of the position sensor  413  to the position of the position sensor  412  while pressing the fold of the sheet stack (step S 524 - 6 ). Subsequently, whether or not a sheet stack has arrived at the arrival sensor  321  is again determined (step S 524 - 9 ). The procedure then returns to the step S 524 - 8  if the answer of the step S 524 - 9  is YES or returns to the step S 524 - 5  if it is NO. 
     If the answer of the step S 524 - 5  is NO, then the reinforce roller  409  is moved from the position of the position sensor  412  to the position of the position sensor  413  (step S 524 - 7 ). The procedure then returns to the step S 524 - 9 . 
     If T 1  is longer than or equal to T 2 , as determined in the step S 524 , then the procedure jumps to the step S 524 - 8  by skipping the reinforcement. 
     As for the rest of the configuration, the illustrative embodiment is identical with the first embodiment. 
     As stated above, to press the fold of a sheet stack with the reinforce roller  409  as many times as possible, the illustrative embodiment uses the sensing means positioned upstream of the first fold roller pair  81  to repeatedly press the fold of the same sheet stack at allowable timing. 
     More specifically, as shown in  FIG. 84 , assume that the movement of the reinforce roller from the position sensor  412  to the position sensor  413 , as shown in  FIG. 84 , or from the latter to the former, as shown in  FIG. 85 , is a single pressing action. Then, as shown in  FIG. 86 , the single pressing action is repeated until the sensing means positioned upstream of the first fold roller pair  81  senses the next sheet stack. 
     In the above condition, the larger the number of sheets constituting a single sheet stack, the longer the time interval between consecutive sheet stacks and therefore the larger the number of times of pressing available with the reinforce motor  409 . Every sheet stack can therefore be sufficiently folded without regard to the number of sheets constituting it. Further, because the minimum period of time T 1  necessary for the single pressing action is known beforehand, the pressing action is not available if the time interval T 2  sensed by the sensing means is shorter than or equal to the period of time T 1 . In this case, the reinforcement is not executed. While the illustrative embodiment uses the arrival sensor  321  as sensing means stated above, extra sensing means may be positioned between the sheet sensor  310  and the fold roller pair  81  shown in  FIG. 1 , if desired. 
     Tenth Embodiment 
       FIGS. 87 and 88  show a tenth embodiment of the present invention. Steps S 501  through S 524 - 4  and steps S 524 - 8  through S 539  are identical with the corresponding steps of the eighth embodiment and will not be described specifically in order to avoid redundancy. 
     As shown in  FIG. 87 , when the sheet stack sensor  414  of the reinforce roller unit  400  turns on (YES, step S 524 - 3 ), the fold roller pair  81  conveys a folded sheet stack, entered the reinforce roller unit  400 , to the pressing position by a preselected position and is then caused to stop rotating (step S 524 - 4 ). As a result, the sheet stack remains nipped by the fold roller pair  81 . 
     Subsequently, whether or not the position sensor  413  has turned on is determined (step S 524 - 5 ). If the answer of the step S 524 - 5  is YES, then the reinforce roller  409  is moved from the position of the position sensor  413  to the position of the position sensor  412  while pressing the sheet stack (step S 524 - 10 ). If the answer of the step S 524 - 5  is NO, then the reinforce roller  409  is moved from the position of the position sensor  412  to the position of the position sensor  413  while pressing the sheet stack (step S 524 - 11 ). After the step S 524 - 10  or S 524 - 11 , the fold roller pair  81  and lower roller pair  83  are rotated to convey the sheet stack (step S 524 - 8 ). 
     As for the rest of the configuration, the illustrative embodiment is identical with the eighth embodiment. 
     The longer the pressing time of the reinforce roller  409 , the sharper the fold of a sheet stack. In light of this, when a plurality of sheet stacks should be sequentially reinforced, the illustrative embodiment increases the pressing time. More specifically, the time interval T 2  between consecutive sheet stacks is calculated on the basis of information representative of the number of sheets constituting each sheet stack. It is therefore possible to calculate the speed V 1  necessary for the reinforce roller  409  to press a sheet stack by moving from the position sensor  412  to the position sensor  413 , as shown in  FIG. 84 , or from the latter to the former, as shown in  FIG. 85 , within the time interval T 2 . Therefore, if the reinforce roller  409  is moved in the direction of  FIG. 84  or  85  at the speed V 1  thus calculated while pressing a sheet stack, then the roller  409  can press the sheet stack by taking a sufficient period of time in accordance with the number of sheets of the sheet stack. The sheet stack can therefore be sufficiently pressed without regard to the number of sheets constituting it. 
     Further, as shown in  FIG. 88 , when the press roller  409  is caused to press a sheet stack a preselected number of times as in the procedure of  FIG. 86 , the speed V 2  that implements the above number of times within the time interval T 2  can be calculated. Therefore, by driving the reinforce roller  409  at the speed V 2  thus calculated (step S 524 - 10 ′ or S 524 - 11 ′), it is possible to press the fold of a sheet stack the preselected number of times without regard to the number of sheets constituting it (step S 524 - 12 ). 
     Moreover, the minimum period of time T 1  necessary for the single pressing action of the reinforce roller  409  is also known beforehand. If the time interval T 2  is shorter than or equal to the period of time T 1 , i.e., if the pressing action is not available, then the pressing operation is not executed, i.e., the step S 524 - 1  jumps to the step S 524 - 8 . It is to be noted that information representative of the number of sheets of a single sheet stack can be obtained from the image forming apparatus and the number of times of operation of jogging means. 
     Eleventh Embodiment 
       FIG. 89  shows an eleventh embodiment of the present invention. Steps S 501  through S 523 , steps S 524 - 1  through S 524 - 4  and steps S 525  through S 539  are identical with the corresponding steps of the eighth embodiment and will not be described specifically in order to avoid redundancy. 
     As shown in  FIG. 89 , whether or not the reinforce roller  409  has pressed the same sheet stack m times calculated, as stated earlier, is determined (step S 524 - 13 ). If the answer of the step S 524 - 13  is NO, then whether or not the position sensor  413  is in an ON state is determined (step S 524 - 5 ). If the answer of the step S 524 - 5  is YES, then the reinforce roller  409  is moved from the position of the position sensor  413  to the position of the position sensor  412  (step S 524 - 6 ); if otherwise (NO, step S 524 - 5 ), the roller  409  is moved from the position of the position sensor  412  to the position of the position sensor  413 . Then, whether or not the reinforce roller  409  has pressed the sheet stack m times is again determined (step  524 - 13 ). If the answer of the step S 524 - 13  is YES, then the fold roller pair  81  and lower outlet roller pair  38  are rotated (step S 524 - 8 ). This is followed by the step S 525  and successive steps. 
     On the other hand, if the time interval T 2  is shorter than or equal to the period of time T 1  (NO, step S 524 - 1 ), the procedure jumps to the step S 524 - 8  by skipping the reinforcement. 
     As for the rest of the configuration, the illustrative embodiment is identical with the eighth embodiment. 
     The larger the number of times of pressing, the sharper the fold of a sheet stack. In light of this, the illustrative embodiment calculates, when pressing a plurality of consecutive sheet stacks, the time interval V 2  between the sheet stacks on the basis of the number of sheets constituting each sheet stack and then causes the reinforce roller  409  to repeatedly press the same sheet stack a preselected number of times within the time interval T 2 . More specifically, the illustrative embodiment calculates how many times m the reinforce roller  409  can press a sheet stack while moving from the position sensor  412  to the position sensor  413  or from the latter to the former, as shown in  FIG. 86 , and causes the roller  409  to press the sheet stack. 
     Further, the minimum period of time T 1  necessary for the single pressing action of the reinforce roller  409  is also known beforehand. If the time interval T 2  is shorter than or equal to the period of time T 1 , i.e., if the pressing action is not available, then the pressing operation is not executed. Again, information representative of the number of sheets of a single sheet stack can be obtained from the image forming apparatus and the number of times of operation of jogging means. 
     As stated above, the eighth to eleventh embodiments allow the reinforce roller  409  to surely reinforce the folds of consecutive sheet stacks without reducing productivity even when the interval between the sheet stacks is short. This can be done without regard to the number of sheets constituting each sheet stack. 
     Twelfth Embodiment 
       FIGS. 90 and 91  show a twelfth embodiment of the present invention. The steps S 501  through S 505 , steps S 506  through S 512  and steps S 513  through S 525  of the first embodiment shown in  FIG. 32 , the steps S 526   a  through S 535   a  of the sixth embodiment shown in  FIGS. 76 and 77  and the steps S 536  through S 542  shown in  FIG. 77  also apply to the illustrative embodiment. The following description will therefore concentrate on differences between the illustrative embodiment and the previous embodiments. 
     As shown in  FIG. 90 , on receiving size information from the image forming apparatus PR, the CPU  360  calculates a stand-by position of the reinforce roller  409  and a distance X by which the roller  409  should move for reinforcement (step S 543   c ). Subsequently, after the steps S 501  through S 505 , the CPU  360  moves the reinforce roller  409  to the stand-by position on the basis of the size information obtained in the step S 6543   c  (step S 544   c ). The CPU  360  then repeats the steps S 506  through S 512  with every sheet. When the last sheet of a single sheet stack arrives (YES, step S 512 ), the CPU  360  determines, based on the number of sheets of the sheet stack known then, determines the number of times A the reinforce roller  409  should press the sheet stack in accordance with the number of sheets (step S 545   c ). The number of times A may be one for one to five sheets, two for five to ten sheets and so forth or may be incremented by one for every five sheets. 
     Subsequently, after the steps S 513  through S 535   a , the CPU  360  drives the fold roller pair  81  and lower outlet roller pair  83  for a preselected additional period of time and then stops driving them (step S 546   c ). The CPU  360  then causes the reinforce roller  409  to start moving the distance X (step S 547   c ) and then stop moving (steps S 548   c  and S 549   c ). Thereafter, when the reinforce roller  409  has moved A consecutive times (YES, step S 550   c ), the CPU  360  causes the belt  52  and jogger fence  53  to the stand-by positions. Thereafter, the CPU  360  executes the steps S 536  through S 542  to thereby initialize the entire mechanism. 
     Thirteenth Embodiment 
       FIGS. 92 and 93  show a thirteenth embodiment of the present invention. As shown in  FIG. 92 , the illustrative embodiment includes a step S 551   d  between the steps S 544   c  and  506  of the twelfth embodiment shown in FIG.  92 . Also, as shown in  FIG. 93 , the illustrative embodiment substitutes steps S 552   d  through S 558   d , which take account of the direction of movement of the reinforce roller  409 , for the steps S 547   c  through S 549   c . As for the rest of the configuration, the illustrative embodiment is identical with the twelfth embodiment. 
     As shown, after the step S 544   c , the CPU  360  resets a flag indicative of the direction of movement of the reinforce roller  709  (step S 551   d ) and then causes the fold roller pair  81  and lower outlet roller  83  to stop rotating (step S 546   c ). Subsequently, the CPU  360  causes the reinforce roller  409  to move, if the flag reset is ZERO, the distance X derived from the size information in the forward direction or to move, if the flag is ONE, the distance X in the reverse direction (step S 552   d ). When the reinforce roller  409  has moved the distance X, the CPU  360  causes the reinforce roller  409  to stop moving (steps S 553   d  and S 554   d ), again checks the flag (step S 555   d ), and sets, if the flag is ZERO, the flag to ONE (step S 556   d ) or sets, if the flag is ONE, the flag to ZERO (step S 558   d ). After repeating the above procedure A times (YES, step S 557   d ), the CPU  360  executes the steps S 537  through S 542 . 
     As stated above, in the twelfth and thirteenth embodiments, the reinforce roller  409  is moved to the stand-by position before pressing a sheet stack and then moved for pressing the sheet stack only by a distance two times as long as the distance between the stand-by position and the widthwise center of the sheet stack. This allows the reinforce roller  409  to start pressing the sheet stack at the earliest possible timing and move the minimum necessary distance during pressing, thereby reducing the pressing time and enhancing the durability of the roller  409 . 
     Fourteenth Embodiment 
       FIGS. 94 through 94  show a fourteenth embodiment of the present invention. The center staple and bind mode operation of the eighth embodiment shown in  FIGS. 81 and 82  also apply to the illustrative embodiment. 
     When the reinforce roller  409  stops moving halfway on a sheet stack due to a jam, the sheet stack sometimes cannot be removed from the reinforce roller unit  400 , as stated earlier. In light of this, as shown in  FIGS. 94 through 96 , a lever  431  is directly connected to the shaft of the pulley  404 , so that the pulley  404  and lever  431  can transfer rotation to each other. The lever  431  is implemented as a disk in consideration of movement to occur during usual pressing operation. 
     In the above configuration, when the operator rotates the lever  431  by hand, the rotation of the lever  431  is transferred to the slider  407  and reinforce roller  409  via the timing belt  403 . This allows, when a sheet stack jams the reinforce roller unit  400 , the operator to move the reinforce roller  409  to the outside of the pressing range and easily remove the sheet stack. 
     Fifteenth Embodiment 
     Reference will be made to  FIGS. 97 through 99  for describing a fifteenth embodiment of the present invention. Because the illustrative embodiment is identical with the fourteenth embodiment except for the configuration of the reinforce roller unit  400 , identical structural elements are designated by identical reference numerals and will not be described specifically. 
     As shown, the lever  431  and a first bevel gear  432  are respectively mounted on opposite ends of the guide member  405 . Because the guide member  405  does not transfer the driving force, a second bevel gear  433  is mounted on a shaft  403   a  and held in mesh with the first bevel gear  432 . A timing belt  434  is driven by a pulley  402  mounted on the output shaft of the pulse motor  401 . The timing belt  434  and a timing pulley  403   a  over which the timing belt  434  is passed is provided on the shaft  403   a . To cause the reinforce roller  409  to press a sheet stack, the output torque of the pulse motor  401  is transferred to the timing belt  403  via the timing belt  434 . When the reinforce roller  409  stops moving halfway due to a jam, the operator moves the guide member  405  via the lever  431  by hand. As a result, a driving force is transferred from the first bevel gear  432  to the second bevel gear  433 , so that the timing belt  403  is caused to turn while moving the reinforce roller  409 . 
     While the illustrative embodiment mounts the lever  431  on the guide member  405 , the movement of the lever  431  may alternatively be transferred to the guide member  405  via a pulley, timing belt and a gear by way of example. 
     Sixteenth Embodiment 
     A sixteenth embodiment of the present invention will be described with reference to  FIGS. 100 through 107 . Briefly, the illustrative embodiment allows the operator to remove a sheet stack by opening the upper guide plate  415  while allowing, as in the fourteenth and fifteenth embodiments, the operator to move the reinforce roller  409  via the lever  431 . 
     As shown in  FIGS. 100 through 102 , the output torque of the pulse motor  434  is transferred to the pulley  435  via the timing belt  434  to thereby drive the timing belt  403  passed over the pulleys  435  and  404 , so that the reinforce roller  409  is moved to press a sheet stack. The upper guide plate  405 , supporting the guide member  405 , is angularly movable, or openable, about the axis of the pulley  435 . Further, a locking mechanism LK is arranged on the upper guide plate  415  and made up of a lever  436 , a link  437 , a stop  438 , and a shaft  439 . 
     When part of a sheet stack, jamming the reinforce roller unit  400 , obstructs the movement of the reinforce roller  409 , the operator unlocks the locking mechanism LK, as shown in  FIG. 103 , and then opens the upper guide plate  415  loaded with the drive system for the reinforce roller  409 , as shown in FIG.  104 . In this condition, the operator can easily remove the sheet stack even when the sheet stack cannot be removed by operating the lever  431 . 
     In an alternative configuration, the output torque of the pulse motor  401  is transferred to the pulley  435  via a gear, which is a substitute for the timing belt  434 , while the upper guide plate  415  is openable about the axis of either one of the gear and pulley  435 . 
       FIGS. 105 and 106  shows a modification of the illustrative embodiment. As shown, the upper guide plate  415  supports the pulse motor  401 . A shaft  440 , supporting the upper guide plate  415  such that the plate  415  is angularly movable, is mounted on the plate  415 . In this configuration, by unlocking the locking mechanism LK, as shown in  FIG. 106 , the operator can lift the drive system assigned to the reinforce roller  409  together with the upper guide plate  415  in order to remove a jamming sheet stack. 
     As for the rest of the construction, the illustrative embodiment is identical with the fourteenth embodiment. 
     Seventeenth Embodiment 
       FIGS. 108 through 116  show a seventeenth embodiment of the present invention. The illustrative embodiment differs from the fourteenth embodiment in that it allows the lower guide plate  416  to be retracted. More specifically, as shown in  FIGS. 108 through 110 , the locking mechanism, made up of the lever  436 , link  437 , stop  438  and shaft  439 , is mounted on the lower guide plate  416 . The lower guide plate  416  is supported by a shaft  440 , which is located at the opposite side to the locking mechanism LK and extends in the direction parallel to the direction of sheet conveyance, in such a manner as to be angularly movable. 
     As shown in  FIGS. 111 and 112 , when a sheet stack jams the reinforce roller unit  400 , the operator can remove the sheet stack by unlocking the locking mechanism LK to thereby cause the lower guide plate  416  to bodily retract. 
       FIGS. 113 through 116  show a modification of the illustrative embodiment. As shown, the shaft  440  extends perpendicularly to the direction of sheet conveyance. In this case, the locking mechanism LK includes a shaft  441  and a roller  442  in addition to the lever  436 , link  437 , stop  438  and shaft  439 . More specifically, as shown in  FIG. 114 , the link  437  is generally L-shaped, as seen in a side elevation, and angularly movable about the shaft  439 . The lever  436  is mounted on one end of the link  437  while the roller  442  is mounted on the other end of the link  437  via the shaft  441 . As shown in  FIGS. 115 and 116 , by turning the lower guide plate  416  via the lever  436  and thereby uncovering the sheet path, the operator can easily remove a sheet path jamming the sheet path. 
     As for the rest of the construction, the illustrative embodiment is identical with the fourteenth embodiment. 
     In the above modification, when the operator turns the lever  431  by hand, the pulley  404  rotates with the result that the slider  407  and reinforce roller  409  are moved via the timing belt  403 . Therefore, when a sheet stack jams the reinforce roller unit  400 , the operator can remove the sheet stack by moving the reinforce roller  409  to the outside of the pressing range. Even when part of such a sheet stack blocks the movement of the reinforce roller  409 , the operator can remove the sheet stack by unlocking the locking mechanism LK and moving the lower guide member  416 . 
     As stated above, the fourteenth to seventeenth embodiments allow the operator to surely, easily remove a sheet stack jamming the reinforce roller unit  400  even when the reinforce roller  409  stops moving halfway on the sheet stack. This is true even when part of the sheet stack is spread and caught by the fold roller  409  or any one of the drive members. 
     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.