Abstract:
A sheet treating apparatus includes a tray liftable and lowerable while sheets are stacked on the tray. A sheet pressing member is movable between a pressing position in which the sheets on the tray are pressed by the sheet pressing member and a retracted position. A sheet pressing member driving device retracts the sheet pressing member when the tray is lowered and for moving the sheet pressing member to the pressing position when the tray is lifted. A detecting device detects a height position of the sheets on the tray to stop lifting the tray.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet treating apparatus, and more particularly it relates to a sheet treating apparatus for successively receiving sheets such as copying paper discharged from an image forming apparatus such as a copying machine, a printing device or a laser beam printer after image formations and for effecting treatment such as alignment and stitching of the sheets and for discharging and stacking the treated sheets onto a stacking portion. 
     2. Related Background Art 
     In conventional image forming apparatuses such as printing devices, copying machines, printers or the like, sheets on which images are formed in a main body of the image forming apparatus are temporarily stacked on a treating tray (first stacking means) by a pair of discharge rollers (sheet discharging means) within a sheet treating apparatus, where, sheet treatment such as alignment and stitching of a sheet bundle is effected. Thereafter, the treated sheet bundle is bundle-discharged onto a stack tray (second stacking means) outside the main body by bundle discharging means. 
     A trailing end of the sheet bundle discharged on the stack tray is detected by a sheet surface height detecting sensor, and, on the basis of detection, an uppermost level of the sheets on the second stacking means is maintained to a predetermined level. 
     However, in such a conventional apparatus, if the sheet bundle discharged on the second stacking means is curled, erroneous detection of the sheet height is effected by the sheet height detecting sensor, so that the proper sheet height cannot be detected. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a sheet treating apparatus capable of properly detecting a height of sheets on stacking means without being influenced by curl in the sheet discharged on the stacking means and capable of moving an upper level of the sheets to a proper position in an up-and-down direction. 
     A sheet treating apparatus according to the present invention comprises tray means liftable and lowerable while sheets are stacked on the tray means, a sheet pressing member movable between a pressing position in which the sheets on the tray means are pressed and a retracted position, sheet pressing member driving means for retracting the sheet pressing member when the tray means is lowered and for moving the sheet pressing means to the pressing position when the tray means is lifted, and detecting means for detecting a height position of the sheets on the tray means to stop lifting the tray means. 
     Further, the sheet pressing member may receive the discharged sheet in the pressing position. 
     Further, the sheets may be discharged in a sheet bundle and the tray means are lowered and lifted whenever the sheet bundle is discharged. 
     Furthermore, the sheet pressing member may include an engagement portion for holding a trailing end of the discharged sheet bundle, and a length of the engagement portion may be greater than a flying amount of the discharged sheet bundle. 
     In addition, the sheet surface height detecting means for detecting the trailing end of the sheet on the tray means may be disposed in the vicinity of the sheet pressing member. 
     According to the present invention, since there is provided the sheet pressing member capable of pressing the trailing end of the sheet discharged on the tray means with a simple construction and capable of being rotated to the sheet pressing position when the sheet is lifted and being retracted to the retracted position when the tray means is lowered, if curled sheets are stacked, proper sheet height can be maintained and can be detected by the sheet surface height detecting means, whereby the tray means can be maintained in a proper height. 
     Further, when the sheet is discharged on the tray means, since the sheets on the tray means are pressed, the sheet(s) on the tray means are not moved by the sheet being discharged, thereby not disordering the aligned sheets. Further, since the length of the pressing member is greater than the flying amount of the sheet bundle upon the discharge, the trailing end of the sheet bundle is prevented from being caught by the pressing member thereby not affecting a bad influence upon the alignment, thereby improving the stacking ability for stacking the sheets on the tray means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view showing the entire construction of a sheet treating apparatus according to the present invention; 
     FIG. 2 is a vertical sectional front view showing an example of an image forming apparatus to which the sheet treating apparatus can be applied; 
     FIG. 3 is a plan view showing a moving mechanism for a back roller and an aligning member provided on a treatment tray; 
     FIGS. 4A,  4 B,  4 C and  4 D are side views showing operations of the back roller and a bundle discharging belt; 
     FIG. 5 is a side view showing a sheet dropping member and a sheet trailing end dropping member; 
     FIG. 6 is an enlarged side view of the sheet dropping member; 
     FIG. 7 is a view for explaining operations of the sheet dropping member and the sheet trailing end dropping member; 
     FIG. 8 is a view for explaining operations of the sheet dropping member and the sheet trailing end dropping member; 
     FIG. 9 is a view for explaining an operation of the back roller; 
     FIG. 10 is a side view showing the treat tray on which a sheet bundle is rested and a bundle discharging belt portion; 
     FIG. 11 is a view for explaining a bundle discharging operation of the bundle discharging belt; 
     FIG. 12 is a front view of the sheet trailing end dropping member, looking in a direction indicated by the arrow B in FIG. 5; 
     FIG. 13 is a side view of an aligning means portion and the bundle discharging belt portion; 
     FIG. 14 is a vertical sectional front view showing a regulating portion formed on an aligning plate; 
     FIG. 15 is a plan view of the aligning member and a driving portion therefor; 
     FIG. 16 is a perspective view of the aligning member and the driving portion therefor; 
     FIG. 17 is a vertical sectional side view showing a moving mechanism for a stack tray; 
     FIG. 18 is a back view showing the moving mechanism for the stack tray, looking in a direction indicated by the arrow A in FIG. 17; 
     FIG. 19 is a side view of a sheet surface height detecting sensor and the sheet trailing end pressing member when a lowering of the stack tray is started; 
     FIG. 20 is a side view of the sheet surface height detecting sensor and the sheet trailing end pressing member when a lifting of the stack tray is started; 
     FIG. 21 is a perspective view of the sheet surface height detecting sensor, the sheet trailing end pressing member and a driving portion therefor; 
     FIG. 22 is a vertical sectional front view showing an arrangement position of a plurality of sheet surface height detecting sensors for detecting a height level of sheets on the stack tray; 
     FIG. 23 is a perspective view showing the driving portion for the sheet surface height detecting sensor and the sheet trailing end pressing member when the stack tray is lifted; 
     FIG. 24 is a perspective view showing the driving portion for the sheet surface height detecting sensor and the sheet trailing end pressing member when the stack tray is lowered; 
     FIG. 25 is a block diagram for controlling the sheet treating apparatus; 
     FIG. 26 is a plan view showing alignment plate waiting positions in a non-sort mode and a sort mode of the sheet treating apparatus; 
     FIG. 27 is a plan view showing a first aligning position of the alignment plate in the non-sort mode and the sort mode of the sheet treating apparatus; 
     FIG. 28 is a plan view showing a second aligning position of the alignment plate in the non-sort mode and the sort mode of the sheet treating apparatus; and 
     FIG. 29 is a plan view showing an aligning operation of the alignment plate in a staple sort mode of the sheet treating apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, an embodiment of a sheet treating apparatus according to the present invention and an image forming apparatus having such a sheet treating apparatus will be explained with reference to the accompanying drawings. 
     FIG. 1 is a sectional view showing a construction of the sheet treating apparatus according to the present invention, and FIG. 2 is a vertical sectional front view showing a construction of the image forming apparatus according to the present invention. 
     In FIG. 1, a sheet treating apparatus (finisher)  1  according to the invention is connected to a main body  300  of the image forming apparatus, and an original feeder (RDF)  305  of circulating type is rested on the main body  300  of the image forming apparatus. 
     As shown in FIG. 2, in the main body  300  of the image forming apparatus, an original is automatically fed by the original feeder  305  of circulating type and an image on the original is read by an image reading portion  306 . In accordance with image information read by a controller (not shown), a signal is sent to a laser generator to emit a laser beam. 
     Then, the laser beam is reflected by a rotating polygon mirror  309  and is illuminated, through reflection mirrors  310 , onto an electrophotographic photosensitive drum (image forming means)  312  a surface of which is uniformly charged, thereby forming an electrostatic latent image. The electrostatic latent image on the photosensitive drum  312  is developed by a developing device  311  as a toner image which is in turn transferred onto a sheet S such as a paper sheet or an OHP sheet. 
     The sheets S are selectively picked up from a sheet cassette  351  or  352  by a pick-up roller (sheet supplying means)  313  and are separated one by one by separation means  307 , and the separated sheet is fed and conveyed up to a pair of registration rollers  314 ,  315  by pairs of ante-registration rollers  316 ,  317 . Skew-feed of the sheet is corrected by the pair of registration rollers  314 ,  315 , and then, the sheet is conveyed between the photosensitive drum  312  and an opposed transfer device in synchronism with rotation of the photosensitive drum (image forming means)  312 . Under the action of the transfer device, the toner image formed on the photosensitive drum  312  is transferred onto the sheet S. 
     Thereafter, the sheet S is directed to a pair of fixing rollers  301 ,  302 , and the sheet is heated and pressurized by the pair of fixing rollers  301 ,  302  to permanently fix the toner image to the sheet S. A fixing upper separation pawl  303  and a fixing lower separation pawl  304  are contacted with the pair of fixing rollers  301 ,  302 , respectively so that the sheet S can be separated from the pair of fixing rollers  301 ,  302  by such pawls. 
     The separated sheet S is conveyed out of the main body  300  of the image forming apparatus by a pair of discharge rollers  399  of the main body and is directed to the sheet treating apparatus  1  connected to the main body  300  of the image forming apparatus. 
     In FIG. 1, the connection between the sheet treating apparatus  1  and the image forming apparatus is accomplished by positioning and attaching a lock arm  2  provided on an upper part of the sheet treating apparatus  1  to a hold member  3  attached to the image forming apparatus. Further, a slide unit  4  secured to the image forming apparatus is disposed at a lower part of the sheet treating apparatus so that the sheet treating apparatus  1  can be moved in a sheet discharging direction (direction indicated by the double-headed arrow Y in FIG.  1 ). If poor sheet conveyance occurs in the image forming apparatus or the sheet treating apparatus  1 , when the sheet trapped in the main body is removed, the sheet treating apparatus  1  itself is moved away in the direction Y by rotating the lock arm  2  in a direction indicated by the arrow X, thereby detaching the sheet treating apparatus from the image forming apparatus. 
     The sheet S discharged from the pair of discharge rollers  399  of the main body is further sent to a downstream direction through a sheet path  7  defined by an upper guide  5  and a lower guide  6  within the sheet treating apparatus  1 . A sheet detecting sensor  8  provided in the sheet path  7  serves to detect the passing sheet and the trapped sheet, and a pair of discharge rollers  9  comprise a discharge roller  9   a  and a discharge sub-roller  9   b  urged against the discharge roller  9   a.    
     A treatment tray (sheet stocking means)  30  is constructed as an intermediate tray for temporarily stacking the sheets and for effecting alignment and stapling of the sheets. 
     A stapler  10  serves to staple a sheet bundle S on the treatment tray  30 . In explanation of the illustrated embodiment, although a description regarding this stapler is provided, it may have a construction substantially the same as that of a commercially available automatic stapling device of an electrically driven type or a motor driven type in which staple is stuck into the sheet bundle to stitch the sheet bundle. 
     A bundle discharging belt  60  serves to bundle-convey the sheets S on the treatment tray  30  and to bundle-discharge the sheets on a stack tray  80 . 
     A treatment tray unit  20  is disposed between a conveying portion for conveying the sheet S from the main body  300  of the image forming apparatus and the stack tray  80  for receiving and containing the sheet bundle S treated on the treatment tray  30 . 
     The treatment tray unit  20  comprises the treatment tray  30 , aligning means  40 , a back roller  50  and the bundle discharging belt  60 . 
     As shown in FIG. 1, the treatment tray  30  is an inclined tray with an upper downstream end (left end in FIG. 1) and a lower upstream end (right end in FIG. 1) in a sheet conveying direction, and the lower end is provided with a trailing end stopper  31 . 
     The sheet S discharged by the pair of discharge rollers  9  is slid on the treatment tray  30  by the action of weight of the sheet itself, a sheet dropping member  102  (described later) and the back roller  50  until a trailing end of the sheet S abuts against the trailing end stopper  31 . Further, the treatment tray  30  is provided with the bundle discharging belt  60  so that the bundle discharging belt  60  is moved in the sheet discharging direction by a motor  70  to discharge the sheet bundle S rested on the treatment tray  30  onto the stack tray  80 . 
     In the above-described arrangement, although the construction in which the sheets discharged into the sheet treating apparatus  1  are staced on the treatment tray  30  and then are discharged onto the stack tray  80  is briefly described, the sheet dropping member portion, the back roller portion, the aligning means, the stack tray  80  and the sheet trailing end pressing portion which serve to stack the sheets discharged from the pair of discharge rollers  9  on the treatment tray  30  will now be explained successively. 
     &lt;Sheet dropping member and sheet trailing end dropping member&gt; 
     A sheet dropping member and a sheet trailing end dropping member will now be described with reference to FIGS. 5 to  12 . 
     In FIGS. 5 and 6, in the vicinity of a downstream side of the pair of discharge rollers  9 , there are provided a sheet dropping member  102  for guiding and dropping the discharged sheet S onto the treatment tray  30 , and a sheet trailing end dropping member  105  for dropping the trailing end of the discharged sheet S. The sheet dropping member  102  is provided at its lower end with a pressing portion  102   a  for pressing an upper surface of the sheet S and is rotatable by loosely fitting an elongated slot  102   b  formed in an upper end of the member  102  onto a support shaft  101 . 
     A stopper  107   a  having a proximal end secured to a fixed member  106  abuts against an upstream side of the sheet dropping member  102  to define an initial position of the sheet dropping member  102  which can be rotated by its own weight. A stopper  107  having a proximal end secured to the fixed member  106  is disposed above the sheet dropping member  102 , and an elastic member  109  is provided on a lower surface of a free end of the stopper  107 . 
     When the discharged sheet S is discharged forcibly in the direction indicated by the arrow and pushes the sheet dropping member  102 , the sheet dropping member  102  strikes against the stopper  107 . However, since the elastic member  109  is provided as mentioned above, the rotating force of the sheet dropping member  102  is absorbed, thereby preventing the bounding due to such collision. As a result, the sheet dropping member  102  presses the upper surface of the discharged sheet S by its own weight, so that the sheet S can be dropped onto the treatment tray  30  as will be described later. The bounding of the sheet dropping member  102  which will occur if there is no elastic member  109 , and buckling of the sheet S due to the sheet bounding can be prevented. 
     Next, operations of the sheet dropping member  102  and the sheet trailing end dropping member  105  will be explained with reference to FIGS. 7 to  11 . As will be described later in connection with FIG. 7, a plurality of sheet trailing end dropping members  105  are provided and serve to press the trailing end of the discharged sheet S and drop the sheet onto the treatment tray  30 . 
     As shown in FIG. 7, the sheet dropping member  102  drops the sheet S toward the treatment tray  30  while being rotated in the direction indicated by the arrow by the sheet S discharged by the pair of discharge rollers  9 . When the trailing end of the sheet S leaves the pair of discharge rollers  9 , as shown in FIG. 8, the sheet dropping member  102  is rotated in a direction indicated by the arrow by its own weight. When the sheet dropping member  102  is rotated, the sheet S is moved toward the trailing end stopper  31  (FIG.  7 ). As will be described later, the sheet S is further moved up to the trailing end stopper  31  by the back roller  50  and is aligned (FIG.  9 ). As shown in FIG. 10, when a predetermined number of sheets S are stacked on the treatment tray  30 , the bundle discharging belt  60  and a hook portion  60   a  integral with the belt  60  are rotated to bundle-discharge a sheet bundle S 0  onto the stack tray  80 , as shown in FIG.  11 . 
     Next, the sheet trailing end dropping member  105  will be fully described with reference to FIG.  12 . 
     In FIG. 12, the sheet dropping member  102  is disposed at a center CL of the sheet discharging portion, and a plurality of sheet trailing end dropping members  105   a ,  105   b ,  105   c ,  105   d  are symmetrically arranged on both sides of the sheet dropping member  102 . Each of the sheet trailing end dropping members  105   a  to  105   d  is rotatably supported by a support plate  104  via a support shaft  103 . 
     Each of the sheet trailing end dropping members  105   a  to  105   d  is arranged to press the trailing end of the sheet in accordance with a size of the discharged sheet S. 
     That is to say, when the discharged sheet S is a “post card”, the trailing end of the sheet is pressed by the sheet trailing end dropping members  105   a , and, when the discharged sheet has A5R or B5R size, the trailing end of the sheet is pressed by the sheet trailing end dropping members  105   a ,  105   b , and, when the discharged sheet has A4R size, the trailing end of the sheet is pressed by the sheet trailing end dropping members  105   a  to  105   c , and, when the discharged sheet has B5, LTR or A4 size, the trailing end of the sheet is pressed by the sheet trailing end dropping members  105   a  to  105   d.    
     As mentioned above, by providing the sheet trailing end dropping members divided into the plural members for pressing the trailing end of the sheet in accordance with the size of the discharged sheet S, a phenomenon that the small size sheet is pressed by a large sheet trailing end dropping member to cause the buckling of the sheet can be prevented. 
     &lt;Back roller&gt; 
     Next, the back roller  50  will be explained with reference to FIG.  3  and FIGS. 4A,  4 B,  4 C and  4 D. As shown in FIGS. 4A to  4 D, the back roller  50  has a less-half circular configuration and secured to a back roller shaft  51  for swinging movement and is biased by a spring  52  to assume an arc shape around the back roller shaft  51 . 
     In FIG. 3, the back rollers  50  are attached to the back roller shaft  51  at predetermined intervals in a sheet width direction and include back rollers  50   a  each having a friction member  53  such as silicone rubber at its arc periphery and back rollers  50   b  not having such friction members  53 . 
     The back roller shaft  51  is supported by bearings  54 ,  55  for rotational movement with respect to front and rear side plates and for movement in a thickness direction of the sheets S stacked on the treatment tray  30 , and urging springs  56 ,  57  are mounted on the bearings  54 ,  55 . 
     The back roller shift  51  is connected to a motor  70  via a pulley  58  and a timing belt  59  so that, when back rollers  50  receives a driving force from the motor  70 , the back rollers  50  are rotated together with the back roller shaft  51  around the back roller shaft  51  in an counterclockwise direction in FIGS. 4A to  4 D. 
     As shown in FIG. 4A, a home position of each back roller  50  is selected as a position where the back roller does not abut against the sheet S discharged onto the treatment tray  30  by the pair of discharge rollers  9 . 
     When the sheet S is discharged from the pair of discharge rollers  9 , as shown in FIG. 4B, the back rollers  50  are rotated through one revolution around the back roller shaft  51  in the counterclockwise direction in FIGS. 4A to  4 D by the driving force of the motor  70 , thereby drawing the sheet S until the sheet abuts against the trailing end stopper  31 . 
     If the discharged sheet S leans against the lower guide, ends of the back rollers  50  catch the trailing end of the sheet S, with the result that the back rollers are rotated through one revolution around the back roller shaft  51  in the counterclockwise direction in FIGS. 4A to  4 D while scraping the trailing end of the sheet S, thereby landing the sheet S on the treatment tray  30  surely and drawing the sheet until the sheet abuts against the trailing end stopper  31 . Thereafter, the back rollers  50  are stopped at their home position, therbey preparing for discharging a next sheet S. 
     &lt;Aligning means&gt; 
     Next, the aligning means  40  will be described with reference to FIG.  3  and FIGS. 13 to  16 . 
     The aligning means  40  has a this side (front side) aligning member  41  and a that side (rear side) aligning member  42  which can be moved independently in a direction (width direction) perpendicular to the sheet conveying direction. The aligning members  41 ,  42  are provided with aligning plates  41   a ,  42   a  for abutting against and regulating both lateral edges of the sheet in a condition that the plates stand on the treatment tray  30 , and rack gears  41   b ,  42   b  extending in a width direction of the treatment tray  30 . 
     The aligning members  41 ,  42  are assembled so that the aligning plates  41   a ,  42   a  extend above the treatment tray  30  and the rack gears  41   b ,  42   b  are positioned below the treatment tray  30 . In FIGS. 15 and 16, a pinion gear  43  comprising two-stage gear is meshed with the rack gear  41   b  of the aligning plate  41  on this side, and a guide sub-roller  111  for cooperating with the pinion gear  43  to hold the rack gear  41   b  abuts against an opposite surface of the rack gear  41   b . As shown in FIG. 16, a large gear of the pinion gear  43  is meshed with a drive gear of an aligning motor  45  on this side. 
     Further, a free end of the rack gear  41   b  is movably pinched between a pair of guide sub-rollers  113  rotatably provided on a support member  115 . The guide sub-rollers  113  can slightly be moved in a direction indicated by the arrow (sheet conveying direction) so that, when the aligning plate  41  is assembled, after the position of the guide sub-rollers  113  are adjusted, an attachment state of the aligning plate  41 a can be adjusted. 
     Similarly, the rack gear  42   b  of the aligning plate  42  on that side is also moveably pinched between a pinion gear  44  and a guide sub-roller  112 , and a free end of the rack gear  42   b  is also movably pinched between similar guide sub-rollers  113 . By adjusting the position of the guide sub-rollers  113  in the direction indicated by the arrow, an attachment state of the aligning plate  42   a  on that side can be adjusted. 
     As a result, by normal and reverse rotations of the aligning motor  45 ,  46 , the pinion gears  43 ,  44  are rotated, with the result that the rack gears  41   b ,  42   b  meshed with the pinion gears  43 ,  44  are also moved, thereby moving the aligning members  41 ,  42  in the width direction of the sheet S. 
     Incidentally, the aligning members  41 ,  42  are provided with sensors  47 ,  48  for detecting home positions of the aligning members  41 ,  41 . The sensors  47 ,  48  detect flags  47   a ,  48   a  provided on the rack gears  41   b ,  42   b , respectively. Normally, the aligning members  41 ,  42  are waiting at their home positions. In the illustrated embodiment, the home position of the aligning member  41  on this side is set at a most at this side, and the home position of the aligning member  42  on that side is set at a most at that side. 
     Next, configurations and supporting structure of the aligning members  41 ,  42  will be explained. In the illustrated embodiment, only the aligning member  41  on this side will be described, and explanation of the side aligning member  42  having the similar construction will be omitted. 
     In FIG. 13, an elastic member  120  is secured to an outer side of the aligning plate  41  and a base portion of the elastic member  120  is secured to an upper part of a support member  121 . A lower part  121   a  of the support member  121  is secured to the rack gear  41   b  of the aligning plate  41   a  on this side. The elastic member  120  is provided at its central portion with an elastic portion  120   a  for permitting inclination of the aligning plate  41   a  about the base portion of the elastic member  120 . 
     As mentioned above, by supporting the aligning plate  41   a  by the elastic member  120  having the elastic portion  120   a , upon lifting the stack tray  80 , even if the lifting/lowering motor steps out for any reason so that the stack tray  80  strikes against the aligning plate  41  ( 42 ), an extended portion  41 A of the aligning plate  41  is retracted inwardly to the inside of a guide surface  99  of an outer wall  100 , the thereby preventing damage of the aligning plate  41   a.    
     Further, the rack gear  41   b  integral with the aligning plate  41  and the rack gear  42   b  integral with the aligning plate  42  extend through the inside of the bundle discharging belt  60  (between a tension side and a loose side) for discharging a sheet bundle onto the treatment tray  30 . By arranging the rack gears  41   b ,  42   b  in the inside of the bundle discharging belt  60  as mentioned above, rather than the outside of the bundle discharging belt  60 , dimensions of the aligning plates  41 ,  42  and the bundle discharging belt  60  in a height direction can be reduced, thereby making the apparatus more compact. 
     As shown in FIG. 13, a downstream end of the aligning plate  41  is protruded from the guide surface  99  of the outer wall of the main body  1   a  of the apparatus by a distance of L 1 . Thus, when the sheet bundle S 0  is aligned by the aligning plates  41 ,  42 , an aligning length along the sheet conveying direction is increased and central portion of the sheet bundle can be aligned, thereby improving the aligning ability for sheets. 
     Further, a lower end of the aligning plate  41  is protruded downwardly from the stacking surface of the treatment tray  30  by a distance of L 2 . Thus, sheets S extended from the treatment tray  30  toward the stack tray  80  can be aligned, thereby improving the aligning ability for sheets. 
     As shown in FIG. 14, a regulating portion  41 c directing inwardly (toward the center of the treatment tray  30 ) is formed on an upper part of an inner surface of the aligning plate  41 . Thus, if the sheet is curled in the width direction, as shown by the alternate long and two short dashes line in FIG. 14, the lateral edge of the sheet is regulated by the regulating portion  41   c , thereby preventing distortion of the stacked sheets. Further, if the sheet is curled along the sheet conveying direction, the lateral edge of the sheet is regulated by the regulating portion  41   c , thereby improving the sheet stacking ability. A guide portion  41   a  protruding upwardly is provided on an upper part of the upstream end of the aligning plate  41 . The guide portion  41   a  has an inner surface comprising a low friction member so that the sheet S discharged from the pair of discharge rollers  9  can smoothly be brought into the aligning plates  41 ,  42 . An upper part of the guide portion  41   a  may be inclined outwardly to guide the sheet S further smoothly. 
     Next, the bundle discharging belt  60  will be explained with reference to FIGS. 3 and 4A to  4 D. As shown in FIGS. 4A to  4 D, the bundle discharging belt  60  is provided with hook portions  60   a , and the bundle discharging belt  60  is extended around pulleys  61 ,  62 , and the pulley  61  is connected to the motor  70  via a gear  63 , a one-way gear  64  and a timing belt  59 . The one-way gear  64  can transmit a driving force to the gear  63  only when the motor  70  is rotated in a clockwise direction. 
     When the back rollers  50  draw a last sheet S until such a sheet abuts against the trailing end stopper  31 , the bundle discharging belt  60  is driven by the motor  70  to be moved in a direction indicated by the arrow A (sheet conveying direction) in FIG. 4D substantially along the inclination of the treatment tray  30 , so that the sheet bundle S aligned and stapled on the treatment tray  30  is discharged onto the stack tray  80 . 
     Next, states of the aligning plates  41 ,  42  when the sheets discharged on the treatment tray  30  are aligned and when the sheet bundle is discharged from the treatment tray  30  will be explained. 
     As will be described later in connection with “sheet flow”, in the aligning plates  41 ,  42 , although one of the aligning plates urges the sheets S against the other stationary aligning plate to align the sheets, when the aligned sheet bundle is discharged, since the sheet bundle is frictionally slid on the aligning surfaces of the aligning plates  41 ,  42  so that a great bundle discharging force is required, the motor  70  for the bundle discharging belt  60  for discharging the sheet bundle may step out. 
     In the illustrated embodiment, although a distance between the aligning plates  41  and  42  is set so that the aligning plates abut against the lateral edges of the sheet bundle when the sheets are aligned, when the sheet bundle is discharged, the distance between the aligning plates  41  and  42  is slightly widened (for example, by 1 mm) by controlling the rotation of the motor  70  comprising a pulse motor. The control for widening the distance between the aligning plates  41  and  42  in this way is effected by a control device (control means)  150  shown in FIG.  25 . 
     In this way, the aligned sheet bundle can smoothly be discharged onto the stack tray  80  by the bundle discharging belt  60 , and, upon bundle discharge, load on the motor  70  can be reduced, endurance of the aligning plates  41 ,  42  can be improved and poor sheet bundle discharging can be prevented. 
     &lt;Stack tray and sheet trailing end pressing member&gt; 
     Next, a construction of the stack tray  80  will be described with reference to FIGS. 17 and 18. The stack tray  80  has a sub-tray  79  therein, so that, by extending the sub-tray  79 , sheets having large size such as A3 or B4 size can be stacked. 
     Further, both sides of the stack tray  80  are supported by tray support plates  81 ,  82  which are secured to timing belts  83 ,  84 , respectively. 
     The timing belt  83  is extended around pulleys  85 ,  86  and the timing belt  84  is extended around pulleys  87 ,  88 , and the pulleys  86 ,  88  are secured to a drive shaft  90  to which a drive gear  89  is secured. The drive gear  89  is connected to a drive motor (driving means)  92  through a gear train  91 . 
     Sub-rollers  93 ,  94  are attached to the tray support plates  81 ,  82  for rotation with respect to sub-roller guides  95 ,  96 , so that, when a driving force from the drive motor  92  is transmitted, the tray support plates are moved in an up-and-down direction (directions indicated by the double-headed arrow Z in FIG.  17 ). 
     A sheet surface height detecting sensor (first sheet surface height detecting means)  97  serves to detect a height of the stacked sheet bundle, thereby adjusting a height of the stacking surface of the stack tray  80  with respect to the treatment tray  30  to a predetermined value. 
     A sheet trailing end pressing member  98  can be extended from and retracted into the guide surface  99  in response to the lifting/lowering movement of the stack tray  80  to press the trailing end of the sheet bundle S rested on the stack tray  80 , thereby preventing the sheets from being deviated in the sheet discharging direction after the sheet bundle is discharged. The sheet surface height detecting sensor  97  is disposed in the vicinity of the sheet trailing end pressing member  98 . By arranging the sheet surface height detecting sensor  97  as mentioned above, a height level of the sheets stacked on the stack tray  80  can be detected accurately without being influenced by curl in the sheet(s) S. 
     FIGS. 19 and 20 are side views showing detection of the sheet surface height of the sheet bundle on the stack tray  80 . FIG. 21 is a perspective view showing the detection. 
     In FIGS. 21 to  24 , a gear  139  is secured to the drive shaft  90  which is forwardly and reversely rotated by the drive motor  92 , and a rotational driving force of this gear  139  is transmitted to a pinion of a drive gear  141  through a timing belt and then is transmitted to a sector gear  143  through an intermediate gear  142 . The sheet trailing end pressing member  98  is attached to a support shaft  130  integral with the sector gear  143 , so that, by the forward and reverse rotations of the drive motor  92 , the sheet trailing end pressing member  98  is moved between a sheet pressing position and a retracted position in the main body la of the apparatus. 
     A sheet surface height detecting sensor (second sheet surface height detecting means)  133  for detecting a sheet surface height at a stapler  10  side is rotatably provided on the support shaft  130 , as well as the sheet surface height detecting sensor  97  and is biased to be protruded toward the stack tray  80 . The sheet surface height detecting sensors  97 ,  133  are constructed integrally with each other so that, when the sensors are urged against the sheet S on the stack tray  80 , they are moved into the main body  1   a  of the apparatus thereby to detect the sheet surface height by a flag  97   a  of the sheet surface height detecting sensor  97  and a photo-interrupter  132 . 
     Incidentally, since the sheet bundle is stitched at a left upper corner of an imaged surface, in a relation or condition of the stapler  10  shown in FIG. 3, the sheet is discharged onto the stack tray  80  with an upper edge of the imaged surface directing toward that side and with the imaged surface facing downwardly. 
     The stopping operation of the sheet trailing end pressing member  98  after the sheet trailing end pressing member  98  is moved to the pressing position on the stack tray  80  and to the retracted position in the main body la of the apparatus on forward and reverse rotations of the drive motor  92  is effected while being regulated by a rotational amount regulating mechanism shown in FIGS. 23 and 24. 
     When the drive motor  92  is rotated in the forward direction to rotate the drive gear  141  in the clockwise direction and the intermediate gear  142  in the counterclockwise direction, the sector gear  142  is rotated in the clockwise direction (in the forward direction) in FIG. 23 through a predetermined amount and then is stopped because of disengagement between the sector gear  143  and the intermediate gear  142 . As a result, the sheet trailing end pressing member  98  is stopped at a position in which the sheet trailing end pressing member  98  is protruded into the sheet pressing position (FIGS.  20  and  23 ). 
     When the drive motor  92  is rotated in the reverse direction to rotate the drive gear  141  in the counterclockwise direction and the intermediate gear  142  in the clockwise direction, the sector hear  143  is rotated in the counterclockwise direction (in the reverse direction) in FIG. 23 through a predetermined amount and then is stopped because of disengagement between the sector gear  143  and the intermediate gear  142 . As a result, the sheet trailing end pressing member  98  is stopped at a position in which the sheet trailing end pressing member  98  is retracted in the retracted position. In this way, after the sheet trailing end pressing member  98  is rotated to the sheet pressing position and the retracted position through the predetermined amount, the rotation of the sheet trailing end pressing member  98  is regulated by the rotational amount regulating mechanism, and the drive gear  141  and the intermediate gear  142  are merelt rotated idly. 
     In FIG. 19, the trailing end of the sheet bundle discharged on the stack tray  80  by the bundle discharging belt  60  is engaged by and supported on the sheet trailing end pressing member  98 . In this case, since the sheet bundle on the stack tray  80  is pressed by the sheet trailing end pressing member  98 , the sheet bundle on the stack tray  80  cannot be moved by the sheet bundle being discharged. Further, a length L of an engagement portion at a lower part of the sheet trailing end pressing member  98  is set to be greater than a flying amount of the sheet bundle S 0  being bundle-discharged, so that the trailing end of the sheet bundle S 0  is surely held by the sheet trailing end pressing member  98  in the bundle discharging and the trailing end of the discharged sheet bundle is prevented from being caught by the sheet trailing end pressing member  98  to distort the alignment. 
     After the sheet bundle is discharged onto the stack tray  80 , when the stack tray  80  is lowered, the sheet trailing end pressing member  98  is retracted from the sheet pressing position shown in FIG. 19 to the retracted position  98 A by the action of the rotational amount regulating mechanism shown in FIG. 24, thereby releasing the trailing end of the sheet bundle S 0 . 
     After the sheet bundle is stacked on the stack tray  80 , the stack tray  80  is lowered by a predetermined amount and then lifted again. As a result, the trailing end of the sheet bundle is frictionally slid against the guide surface  99 , thereby eliminating any curl in the sheet bundle and aligning the trailing ends of the sheet bundles. 
     When the stack tray  80  is lifted, the sheet trailing end pressing member  98  is rotated to the sheet pressing position by the action of the rotational amount regulating mechanism shown in FIG. 23 so that the sheet trailing end pressing member  98  abuts against the upper surface of the lifted sheet bundle to press the sheet bundle. With this pressing action, even if there is any curled sheet, the proper sheet surface height is maintained. 
     When the sheet surface height detecting sensor  97  shown in FIG. 19 leaves the sheet bundle in response to the lowering movement of the stack tray  80 , the photo-interrupter  132  is turned OFF. However, when the sheet surface height detecting sensor  97  is pushed again by the upper surface of the sheet bundle in response to the lifting movement of the stack tray  80 , as shown in FIG. 20, the sheet surface height detecting sensor  97  turns the photo-interrupter  132  ON, thereby detecting the sheet surface height. After the detection of the sheet surface height, the stack tray  80  is lifted by a predetermined amount (for example, about 12 mm) and then is stopped. In this case, the sheet trailing end pressing member  98  presses the upper surface of the sheet bundle with proper pressure. 
     Control of the lifting/lowering movement of the stack tray  80  is effected by a control device  150  including a CPU  151  (FIG.  25 ). 
     In this way, by arranging the sheet surface height detecting sensor  97  in the vicinity of the sheet trailing end pressing member  98 , proper detection of the sheet surface height and the pressing operation of the sheet trailing end pressing member  98  with proper pressure can be realized. Further, since the trailing end of the sheet bundle bundle-discharged by the bundle discharging belt  60  is held by the sheet trailing end pressing member  98 , the sheet bundle can be bundle-discharged without pushing out the already stacked sheet bundle(s). 
     When the sheet bundle discharged onto the stack tray  80  has already stapled by the stapler  10  on the treatment tray  30 , as shown in FIG. 22, one corner of the sheet bundles is swollen by the presence of staples, with the result that the sheet surface height is locally increased. 
     A recess  80   a  is formed in the stack tray  80  at a position in which the staples are located, thereby reducing the swelling of the stapled portions of the sheet bundles more or less. 
     The detection of the sheet surface height of the stitched side of the sheet bundle is effected by a sheet surface height detecting sensor  133 , and this sheet surface height detecting sensor  133  is disposed at a spaced position from a central position in which the sheet trailing end pressing member  98  is disposed toward the stapler  10 . 
     Thus, if the stitched portions of the sheet bundles are swollen, the sheet surface height at a highest position can be detected properly, and a state that the stitched portions of the sheet bundles are protruded upwardly from the stack tray  80  and inconvenience caused thereby can be prevented. Further, by utilizing the photo-interrupter  132  of the sheet surface height detecting sensor  97  also as a photo-interrupter of the sheet surface height detecting sensor  133 , the number of parts can be reduced and the control can be simplified. 
     &lt;Sheet flow&gt; 
     Next, a flow of the sheet S in the sheet treating apparatus  1  will be explained with reference to FIG.  1  and FIGS. 26 to  29 . First of all, an operation when a non-sort mode is selected in an operating portion (not shown) of the main body  300  of the image forming apparatus will be described. 
     When the operator sets the originals on the original feeder  305  of circulating type and turns ON a start key (not shown), an image is formed on the sheet S in the main body  300  of the image forming apparatus and the pair of discharge rollers  9  of the sheet treating apparatus  1  are rotated. The sheet S discharged from the pair of discharge rollers  399  of the main body  300  of the image forming apparatus is directed into the sheet treating apparatus  1  and is conveyed therein. 
     First of all, as shown in FIG. 26, when there is no sheet S on the treatment tray  30 , i.e., when a first sheet S of the job is discharged, the aligning members  41 ,  42  which are waiting at the home positions on this side and that side are previously moved to have a distance therebetween slightly greater than the width of the sheet S. 
     The discharged sheet S starts to move toward the trailing end stopper  31  by its own weight, and the back rollers  50  which are waiting at their home positions are rotated in the counterclockwise direction in FIG. 1 by the motor  70  to help in moving the sheet S toward the trailing end stopper  31 . 
     When the trailing end of the sheet S surely abuts against the trailing end stopper  31  and is stopped there, as shown in FIG. 27, the aligning member  42  on that side continues to be stopped at a position PS 21  to act as a reference plate. The aligning member  41  on this side is moved to a position PS 12  to align the sheet S at a first aligning position. 
     Next, an operation when the operator designates a sort mode will be described. 
     When the operator sets the originals on the original feeder  305  of circulating type, designates the sort mode in the operating portion (not shown) and turns ON the start key (not shown), an image is formed on the sheet S in the main body  300  of the image forming apparatus and the pair of discharge rollers  9  of the sheet treating apparatus  1  are rotated. The sheet S discharged from the pair of discharge rollers  399  of the main body  300  of the image forming apparatus is directed into the sheet treating apparatus  1  and is conveyed therein and then is discharged onto the treatment tray  30  by the pair of discharge rollers  9 . 
     First of all, as shown in FIG. 26, when there is no sheet S on the treatment tray  30 , i.e, when a first sheet S of the job is discharged, the aligning members  41 ,  42  which are waiting at the home positions on this side and that side are previously moved to have a distance therebetween slightly greater than the width of the sheet S. 
     The discharged sheet S starts to move toward the trailing end stopper  31  by its own weight, and the back rollers  50  which are waiting at their home positions are rotated in the counterclockwise direction FIG. 1 by the motor  70  to help in moving the sheet S toward the trailing end stopper  31 . 
     When the trailing end of the sheet S surely abuts against the trailing end stopper  31  and is stopped there, as shown in FIG. 27, the aligning member  42  on that side is stopped and the aligning member  41  on this side is moved to move the sheet S to the first aligning position, thereby aligning the sheet S. 
     Thereafter, the aligning member  41  on this side is moved to a position PS 11  and is waiting at the position PS 11  for preparation for a next discharged sheet S. After the discharging of the sheet S is completed, the aligning member  41  on this side is moved to the position PS 12  again, thereby aligning the sheet S at the first aligning position. In this case, the aligning member  42  on that side continues to be stopped at the position PS 21  to act as the reference plate. The above-mentioned operations are repeated until a last sheet of that sheet bundle is treated. 
     The aligned sheet bundle S of the first copy is bundle-discharged as mentioned above to be moved onto the stack tray  80 . 
     Then, a sheet bundle S of the second copy is discharged onto the treatment tray  30 . In this case, as is in the first copy, although the aligning members  41 ,  42  are waiting at the positions PS 11 , PS 21  as shown in FIG. 26, the aligning position is moved to a second aligning position shown in FIG.  28 . The second aligning position is deviated from the first aligning position toward this side by a predetermined amount of L c . 
     Thereafter, for every sheet bundle, the sheet bundle S are successively stacked on the stack tray  80  while changing the aligning positions alternately, so that the sorted stacking can be permitted with the deviation amount of L c . 
     Next, an operation when the operator designates a staple sort mode will be described. 
     When the operator sets the originals on the original feeder  305  of circulating type and turns ON the start key (not shown), an image is formed on the sheet S in the main body  300  of the image forming apparatus and the pair of discharge rollers  9  of the sheet treating apparatus  1  are rotated. The sheet S discharged from the pair of discharge rollers  399  of the main body  300  of the image forming apparatus is directed into the sheet treating apparatus  1  and is conveyed therein and then is discharged onto the treatment tray  30  by the pair of discharge rollers  9 . 
     The discharged sheet S starts to move toward the trailing end stopper  31  by its own weight, and the back rollers  50  which are waiting at their home positions are rotated in the anti-clockwise direction in FIG. 1 by the motor  70  to help in moving the sheet S toward the trailing end stopper  31 . 
     When the trailing end of the sheet S surely abuts against the trailing end stopper  31  and is stopped there, the aligning plate  41  is stopped at the home position and the aligning plate  42  conveys the sheet S discharged on the trailing end stopper  31  up to a staple position and aligns the sheet there (FIG.  29 ). 
     After all of the sheets S of a sheet bundle S of the first copy are discharged onto the treatment tray  30  and are aligned, the stapler  10  staples the sheet bundle S. The sheet bundle S on the treatment tray  30  is bundle-discharged onto the stack tray  80  by the bundle discharging belt  60 . 
     Upon bundle discharging in the above-mentioned modes, the aligning plates  41 ,  42  are slightly widened, thereby reducing the load acting on the motor  70  during the bundle discharging, improving the endurance of the aligning plates and preventing poor bundle discharging.