Patent Publication Number: US-11377319-B2

Title: Sheet loading device, sheet post-processing device provided therewith, and image forming system

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2019-138019 filed on Jul. 26, 2019, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a sheet loading device on which a sheet such as a paper sheet is loaded after an image is formed thereon by an image forming apparatus such as a copier, a facsimile machine, a printer, or the like, a sheet post-processing device provided therewith, and an image forming system provided therewith. 
     Sheet post-processing devices have conventionally been used which are capable of stacking a plurality of sheets (paper sheets) on which images have been formed by an image forming apparatus such as a copier, a printer, or the like, and performing post-processing such as stapling processing of binding the stacked sheets as a sheet bundle with a staple, punching processing of forming a hole (punched hole) using a punching device, etc. 
     Such conventional sheet post-processing devices each include a sheet loading device having a discharge roller pair which discharges a sheet having undergone post-processing and a loading tray on which the sheet discharged by the discharge roller pair is loaded. A known example of a sheet loading device includes a paddle member which slaps an upstream part (a rear end part) of a sheet in a discharge direction after the sheet passes through a nip portion of a discharge roller pair, and thereby makes the sheet fall onto the loading tray. 
     For example, a sheet discharge device is known which is provided with a holding member rotatable about a rotation shaft arranged below a rotation shaft of a sheet discharge roller (a discharge roller pair). The holding member and the sheet discharge roller are respectively driven to rotate by a holding-member drive motor and a sheet-discharge drive motor, which are separate from each other. The holding member is caused to rotate by a driving force received from the holding-member drive motor, and thereby holds a rear end part of a sheet in a sheet discharge direction as if by slapping the rear end part from above. 
     Another sheet discharge device is known which is provided with a scraping member; the scraping member is arranged coaxial with a driven discharge roller of a discharge rotary body pair (a discharge roller pair), and is driven to rotate by a sheet discharged by the discharge rotary body pair. The scraping member has a plurality of flexible blade members projecting from its outer peripheral surface. The blade members scrape a rear end part of a sheet having passed through the discharge rotary body pair, and further hold down the rear end part of the sheet loaded on a loading tray. 
     SUMMARY 
     According to an aspect of the present disclosure, a sheet loading device includes a discharge roller pair, a loading tray, a paddle member, a sheet holding member, a tray lifting-lowering drive portion, a top surface detection sensor, and a control portion. The discharge roller pair includes a drive roller and a driven roller which follows the drive roller to rotate, and discharges a sheet. The loading tray is arranged on a downstream side of the discharge roller pair with respect to a discharge direction of the sheet, and the sheet discharged by the discharge roller pair is loaded on the loading tray. The paddle member is arranged coaxial with the drive roller, and rotates in a same direction as the drive roller to thereby come into contact, from above, with an upstream part, in the discharge direction, of the sheet discharged by the discharge roller pair. The sheet holding member is arranged below the discharge roller pair, and is swingable between a holding position at which the sheet holding member holds the upstream part of the sheet in the discharge direction loaded on the loading tray and a retraction position at which the sheet holding member releases holding of the sheet. The tray lifting-lowering drive portion lifts and lowers the loading tray. The top surface detection sensor switches between an on state in which the top surface detection sensor performs detection of a sheet loading surface of the loading tray or of a top surface of the sheet loaded on the sheet loading surface and an off state in which the top surface detection sensor does not perform the detection. The control portion controls the tray lifting-lowering drive portion. The control portion is capable of performing a lifting-lowering operation to arrange the loading tray at a reference position by lowering the loading tray with the sheet holding member arranged at the holding position to turn the top surface detection sensor into the off state, then lifting the loading tray to turn the top surface detection sensor into the on state, and then lowering the loading tray again to turn the top surface detection sensor into the off state and stop the loading tray. With the loading tray at the reference position, a predetermined clearance is provided between the top surface of the sheet loaded on the loading tray and a rotational orbit of the paddle member. 
     Still other objects of the present disclosure and specific advantages provided by the present disclosure will be made further apparent from the following description of an embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an image forming system including an image forming apparatus and a sheet post-processing device according to an embodiment of the present disclosure. 
         FIG. 2  is a side sectional view showing an internal structure of the sheet post-processing device of the present embodiment. 
         FIG. 3  is an enlarged sectional view of and around a processing tray shown in  FIG. 2 . 
         FIG. 4  is a perspective view showing a structure of a projection drive portion in a sheet loading device of the present embodiment, showing a state in which a projection member is located at a retraction position. 
         FIG. 5  is a perspective view showing the structure of the projection drive portion in the sheet loading device of the present embodiment, showing a state in which the projection member is located at a projection position. 
         FIG. 6  is a perspective view showing a structure of a discharge drive portion and a paddle drive portion in the sheet loading device of the present embodiment. 
         FIG. 7  is a partial perspective view of and around a discharge roller pair in the sheet loading device of the present embodiment. 
         FIG. 8  is a diagram for illustrating a sheet loading operation performed by the sheet loading device of the present embodiment, showing a state in which a paddle member is located at a retraction position. 
         FIG. 9  is a diagram for illustrating the sheet loading operation performed by the sheet loading device of the present embodiment, showing a state in which the paddle member has started to rotate from the state in  FIG. 8 . 
         FIG. 10  is a diagram for illustrating the sheet loading operation performed by the sheet loading device of the present embodiment, showing a state in which the paddle member has rotated from the state in  FIG. 9  into contact with an upstream part of a sheet in a discharge direction of the sheet. 
         FIG. 11  is a diagram for illustrating the sheet loading operation performed by the sheet loading device of the present embodiment, showing a state in which the paddle member rotates from the state in  FIG. 10  toward the retraction position. 
         FIG. 12  is a block diagram showing an example of control paths in the sheet post-processing device. 
         FIG. 13  is a flowchart showing an example of control in a lifting-lowering operation of a loading tray performed in the sheet loading device of the present embodiment. 
         FIG. 14  is a side sectional view of the sheet loading device, showing a state in which a top surface detection sensor is off. 
         FIG. 15  is a side sectional view of the sheet loading device, showing a state in which the top surface detection sensor is on. 
         FIG. 16  is a side sectional view of the sheet loading device where the top surface detection sensor is arranged at a position overlapping with a leading end of a sheet holding member arranged at a holding position, showing a state in which the upper surface detection sensor is off. 
         FIG. 17  is a side sectional view of the sheet loading device where the top surface detection sensor is arranged at the position overlapping with the leading end of the sheet holding member arranged at the holding position, showing a state in which the upper surface sensor is on. 
         FIG. 18  is a side view of and around the discharge roller pair, showing a state in which the loading tray has descended to a reference position, the loading tray having descended by a descent distance twice as long as the thickness of a sheet bundle of a maximum number of sheets. 
         FIG. 19  is a flowchart showing an example of control in a discharging operation performed when a lower limit position of the loading tray has been detected by a lower limit detection sensor. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an embodiment of the present disclosure will be described with reference to the accompanying drawings,  FIG. 1  is a schematic diagram of an image forming system S which includes an image forming apparatus  200  and a sheet post-processing device  5  according to an embodiment of the present disclosure. The image forming system S includes the image forming apparatus  200  and the sheet post-processing device  5 . 
     The image forming apparatus  200  is what is called a monochrome multifunction peripheral having functions of, for example, printing (print), scanning (image reading), and facsimile transmission. In the image forming apparatus  200 , as shown in  FIG. 1 , a document transport portion  203  is placed on atop surface of a main body portion  201 . At a position that is below the document transport portion  203  but is inside the main body portion  201 , an image reading portion  204  is provided. The image reading portion  204  reads an image on a document loaded on the document transport portion  203  or an image on a document placed on an unillustrated contact glass provided on a top surface of the image reading portion  204 . 
     The image forming apparatus  200  further includes a sheet feeding portion  205 , a sheet transport portion  206 , an exposure portion  207 , an image forming portion  208 , a transfer portion  209 , a fixing portion  210 , a sheet discharge portion  211 , a relay portion  212 , and a main body control portion  213 . 
     The sheet feeding portion  205  holds a plurality of sheets P, and feeds them out one by one, separately from each other, during printing. The sheet transport portion  206  transports a sheet P fed out from the sheet feeding portion  205  to the transfer portion  209  and the fixing portion  210 , and further transports the sheet P having undergone fixing to the sheet discharge portion  211  or to the relay portion  212 . The exposure portion  207  emits laser light controlled based on image data toward the image forming portion  208 . 
     The image forming portion  208  includes a photosensitive drum  2081  as an image carrier and a development device  2082 . In the image forming portion  208 , the laser light emitted from the exposure portion  207  forms, on a surface of the photosensitive drum  2081 , an electrostatic latent image based on a document image. The development device  2082  supplies toner to the electrostatic latent image to develop it into a toner image. The transfer portion  209  transfers, onto a sheet P, the toner image formed by the image forming portion  208  on the surface of the photosensitive drum  2081 . The fixing portion  210  applies heat and pressure to the sheet P having the toner image transferred thereon to fix the toner image on the sheet P. 
     After the fixing, the sheet P is transported to the sheet discharge portion  211  or to the relay portion  212 . The sheet discharge portion  211  is arranged below the image reading portion  204 . The sheet discharge portion  211  has an opening in its front face, and the sheet P after the printing (a printed sheet) is taken out from the front-face side. The relay portion  212  is arranged below the sheet discharge portion  211 . The relay portion  212  has its downstream end in a sheet transport direction connected to the sheet post-processing device  5 . The sheet P (printed matter) after the printing having been sent to the relay portion  212  passes through the relay portion  212  to be transported into the sheet post-processing device  5 . 
     The main body control portion  213  includes a CPU, an image processing portion, and a storage portion, of which none is illustrated, and other unillustrated electronic circuits and components. The CPU, based on a control program and control data stored in the storage portion, controls operations of various constituent elements provided in the image forming apparatus  200  to perform processing related to functions of the image forming apparatus  200 . The sheet feeding portion  205 , the sheet transport portion  206 , the exposure portion  207 , the image forming portion  208 , the transfer portion  209 , and the fixing portion  210  each individually receive an instruction from the main body control portion  213 , and coordinate with each other to perform printing with respect to the sheet P. The storage portion is configured as a combination of non-volatile storage devices such as a program ROM (Read Only Memory) and a data ROM and a volatile storage device such as a RAM (Random Access Memory). 
     The sheet post-processing device  5  is attachably and detachably connected to a side face of the image forming apparatus  200 . The sheet post-processing device  5  includes a post-processing housing  50 , and a post-processing mechanism  6 , a sheet transport mechanism  7 , a processing tray  8 , a sheet loading device  10 , and a post-processing control portion  100 , which are arranged inside the post-processing housing  50 . 
     In such a side face of the post-processing housing  50  as faces the image forming apparatus  200 , a sheet receiving port  41  is provided. After passing through the relay portion  212 , the sheet P passes through the sheet receiving port  41  to be transported into the sheet post-processing device  5 . 
     A sheet transport path  42  extends from the sheet receiving port  41 , in a direction away from the image forming apparatus  200  (leftward in  FIG. 1 ), to a position above the processing tray  8 . 
     The post-processing mechanism  6  performs predetermined post-processing with respect to the sheet P transported along the sheet transport path  42 . The post-processing mechanism  6  includes a punching processing portion  61  and a staple processing portion  62 . 
     The punching processing portion  61  is arranged at an intermediate position between the sheet receiving port  41 , which is an upstream end of the sheet transport path  42  in the sheet transport direction (an arrow-H 11  direction in  FIG. 2 ), and a downstream end of the sheet transport path  42 . The punching processing portion  61  performs punching processing with respect to the sheet P transported along the sheet transport path  42  to form a punched hole (a binding hole) in the sheet P. Here, the punched hole is formed along one of opposite edges of the sheet P in its width direction which is perpendicular to the sheet transport direction. 
     The staple processing portion  62  is arranged at a position that is below the sheet transport path  42  but is on an upstream side of the processing tray  8  in the sheet transport direction. The sheet post-processing device  5 , by using the staple processing portion  62 , can perform staple processing (binding processing) of binding a bundle of sheets P (hereinafter referred to simply as “sheet bundle”) placed on the processing tray  8  with a staple needle to bind the sheet bundle. Here, what is called edge binding processing is performed, in which a sheet bundle is bound with a staple needle at a corner or at an edge. 
     The sheet transport mechanism  7  transports the sheet P in the sheet transport direction which is along the sheet transport path  42 . The sheet transport mechanism  7  has a transport roller pair  71  (see  FIG. 2 ), an intermediate roller pair  72 , and a discharge roller pair  73 , which are arranged in this order from an upstream side in the sheet transport direction. 
     The processing tray  8  is arranged below a downstream part of the sheet transport path  42  in a sheet discharge direction. In other words, the processing tray  8  is located at a position that is on a downstream side of the intermediate roller pair  72  in the sheet discharge direction but is below the intermediate roller pair  72 . A plurality of sheets P having passed through the sheet transport path  42  to reach the processing tray  8  are placed on the processing tray  8 , where the staple processing is performed on them by the staple processing portion  62 . 
     The sheet loading device  10  has a loading tray  11  arranged on a downstream side of the processing tray  8  in the sheet discharge direction to be adjacent to the processing tray  8 . The sheet bundle, with respect to which the staple processing has been completed on the processing tray  8 , is discharged by the discharge roller pair  73  to be loaded on the loading tray  11 . Here, in a case where the staple processing portion  62  does not perform the staple processing, the sheets P are transported to the loading tray  11  without being loaded on the processing tray  8 . A detailed configuration of the sheet loading device  10  will be described later. 
     The post-processing control portion  100  includes a CPU and a storage portion, of which neither is illustrated, and other unillustrated electronic circuits and components. The post-processing control portion  100  is communicably connected to the main body control portion  213 . The post-processing control portion  100  receives an instruction from the main body control portion  213 , and, by using the CPU, based on a control program and control data stored in the storage portion, controls operations of various constituent elements provided in the sheet post-processing device  5  to perform processing related to functions of the sheet post-processing device  5 . The post-processing mechanism  6 , the sheet transport mechanism  7 , the processing tray  8 , and the sheet loading device  10  each individually receive an instruction from the post-processing control portion  100 , and coordinate with each other to perform the post-processing with respect to the sheet P. The storage portion is configured as a combination of storage devices such as a program ROM, a data ROM, a RAM, etc., of which none is illustrated. A detailed control path in the post-processing control portion  100  will be described later. 
       FIG. 2  is a side sectional view showing an internal structure of the sheet post-processing device  5 .  FIG. 3  is a side sectional view showing a structure of and around the processing tray  8  shown in  FIG. 2 . The transport roller pair  71  is arranged on a downstream side of the punching processing portion  61  in the sheet transport direction (the arrow-H 11  direction) to be adjacent to the punching processing portion  61 . The transport roller pair  71  transports a sheet that has undergone the punching processing or a sheet that has not undergone the punching processing to a downstream side in the sheet transport direction H 11 . 
     The intermediate roller pair  72  is arranged, on the sheet transport path  42 , at a position between an upstream-side end part and a downstream-side end part of the sheet transport path  42  in the sheet transport direction. The intermediate roller pair  72  includes a first drive roller  721  which rotates on receiving a driving force from a transport drive portion  70  (see  FIG. 12 ), and a first driven roller  722  which follows the first drive roller  721  to rotate. The first drive roller  721  and the first driven roller  722  are in contact with each other under a predetermined nip pressure therebetween to form a first nip portion  72 N which nips and transports a sheet. 
     Immediately near the intermediate roller pair  72  on its downstream side, a first sheet detection portion S 1  is arranged. The first sheet detection portion S 1  is a sensor which optically detects a sheet, and detects that a leading end of a sheet transported by the transport roller pair  71  has entered into the intermediate roller pair  72 . The first sheet detection portion S 1  also detects that the sheet transported by the intermediate roller pair  72  has passed through the intermediate roller pair  72 . 
     The discharge roller pair  73  is arranged on a downstream side of the sheet transport path  42  in the sheet transport direction. The discharge roller pair  73  includes a second drive roller  731  which rotates on receiving a driving force from a discharge drive portion  90  (see  FIG. 12 ), and a second driven roller  732  which follows the second drive roller  731  to rotate. The second drive roller  731  and the second driven roller  732  are in contact with each other under a predetermined nip pressure therebetween to form a second nip portion  73 N which nips and transports a sheet. The second nip portion  73 N is released by a nip release mechanism  74  (see  FIG. 12 ) when the staple processing portion  62  performs the staple processing. 
     Immediately near the discharge roller pair  73  on its downstream side, a second sheet detection portion S 2  is arranged. The second sheet detection portion S 2  includes an actuator and a photosensor; the actuator has a contact piece, which a sheet discharged by the discharge roller pair  73  comes into contact with, and a detection piece, and the photosensor has a light emitter and a light receiver which are arranged facing each other with the detection piece located therebetween. When the leading end of a sheet transported by the intermediate roller pair  72  comes into contact with the contact piece, the actuator rotationally moves in a clockwise direction, so that the detection piece moves out of an optical path extending from the light emitter to the light receiver. Thereby, it is detected that a leading end of the sheet has entered the discharge roller pair  73  and that the sheet is being discharged by the discharge roller pair  73 . On the other hand, when a rear end of the sheet passes by the contact piece, the actuator rotationally moves in a counterclockwise direction, so that the detection piece moves into the optical path extending from the light emitter to the light receiver. Thereby, it is detected that the rear end of the sheet has passed through the discharge roller pair  73 . 
     Below the sheet transport path  42 , the processing tray  8  is arranged. The processing tray  8 , with the second nip portion  73 N of the discharge roller pair  73  released, receives the sheet transported by the intermediate roller pair  72  to have the sheet loaded thereon. A sheet bundle loaded on the processing tray  8  is subjected to the staple processing performed by the staple processing portion  62 . The processing tray  8 , having its downstream-side end part (left end part in  FIG. 2 ) in the sheet transport direction located near the discharge roller pair  73  and its upstream-side end part (right end part in  FIG. 2 ) located below the intermediate roller pair  72 , is inclined downward from its downstream-side end part toward its upstream-side end part in the sheet transport direction. 
     The processing tray  8  is provided with a bundle discharge member  81  which supports an upstream-side end part (a rear end) of a sheet bundle. The bundle discharge member  81  is fixed to a drive belt (not shown) arranged on a rear-surface side of the processing tray  8 , and part of the bundle discharge member  81  projects from a placing surface of the processing tray  8  in an L-shape in side view. Along with the drive belt being rotationally moved by the discharge drive portion  90  (see  FIG. 12 ), the bundle discharge member  81  reciprocates along the placing surface of the processing tray  8  in the sheet transport direction. 
     A sheet bundle loaded on the processing tray  8  and having been subjected to the staple processing by the staple processing portion  62  is discharged to the sheet loading device  10  by the discharge roller pair  73  with the second nip portion  73 N recovered or by the bundle discharge member  81 . 
     The sheet loading device  10  has loaded thereon sheets having been subjected to the post-processing by the post-processing mechanism  6 . The sheet loading device  10  includes the loading tray  11 , a pair of cursor members  12 , a projection member  13 , a sheet holding member  14 , and a paddle member  15 . 
     The loading tray  11  is arranged on a downstream side of the discharge roller pair  73  with respect to the sheet transport direction (hereinafter may also be referred to as the sheet discharge direction), and is a final destination to which a sheet is discharged in the sheet post-processing device  5 . The loading tray  11  has a sheet loading surface  11   a  on which are loaded sheets discharged by the discharge roller pair  73  or by the bundle discharge member  81 , such as sheets having been subjected to the punching processing by the punching processing portion  61 , a sheet bundle having been subjected to the staple processing by the staple processing portion  62 , etc. The sheet loading surface  11   a  is highest at its downstream-end part in the sheet discharge direction, and is inclined downward toward its upstream-side end part. 
     The upstream-side end part of the sheet loading surface  11   a  is located below the discharge roller pair  73 . Immediately near the sheet loading surface  11   a  on its upstream side, a sheet receiving wall  11   b  is provided upright. The sheet receiving wall  11   b  receives the upstream-side end part (the rear end) of a sheet that comes sliding down the sheet loading surface  11   a.    
     The loading tray  11  is configured to be able to be lifted and lowered by a tray lifting-lowering drive portion  113  (see  FIG. 12 ) in accordance with an amount of sheets loaded on the sheet loading surface  11   a . At a position that is slightly downstream of the upstream-side end part of the loading tray  11 , a top surface detection sensor S 3  is arranged. The top surface detection sensor S 3  is a photosensor that detects the sheet loading surface  11   a  or a top surface of a sheet loaded on the sheet loading surface  11   a . In accordance with a detection signal of the top surface detection sensor S 3 , an operation of lifting-lowering (positioning) the loading tray  11  performed by the tray lifting-lowering drive portion  113  is controlled. The operation of lifting-lowering the loading tray  11  is performed once for every predetermined number of sheets (for example, every 10 sheets) or at predetermined time intervals (for example, every several seconds). Thereby, a position of a topmost surface of sheets on the sheet loading surface  11   a  is maintained at a constant height. 
     In a lower part of the post-processing housing  50 , a lower limit detection sensor S 4  is provided which detects a lower limit position of the loading tray  11 . The lower limit detection sensor S 4  is a photosensor similar to the top surface detection sensor S 3 , and can detect, when an optical path of a detection portion is blocked by a flag  11   e  provided on and projecting from the loading tray  11 , that the loading tray  11  has descended to the lower limit position. Here, as the top surface detection sensor S 3  and the lower limit detection sensor S 4 , other sensors may be used instead of photosensors. Details of the operation of lifting-lowering the loading tray  11  will be described later. 
     The pair of cursor members  12  are supported by a holder  121  through which a shaft  122  is inserted. The shaft  122  is supported by the post-processing housing  50  so as to extend along a sheet width direction above the discharge roller pair  73 . The holder  121  is supported by the shaft  122  so as to be movable along the sheet width direction. The holder  121  supports the pair of cursor members  12  such that leading end parts of the pair of cursor members  12  are swingable in an up-down direction. 
     The projection member  13  is a rod-shaped member having a predetermined width in the sheet width direction and extending in the sheet discharge direction in an are shape, and is arranged below a sheet discharge port  2 . In detail, the projection member  13  is arranged below the processing tray  8  so as to be below a discharge path via which a sheet is discharged from the discharge roller pair  73  along the processing tray  8 . In the present embodiment, the projection member  13  is arranged, for example, at each of two positions in the sheet width direction, the two positions each being at a predetermined distance from a center part of the loading tray  11  in the sheet width direction. Here, the projection members  13  are arranged at positions different from the position of the paddle member  15  with respect to the sheet width direction. 
     The projection member  13  is supported by a projection drive portion  131  shown in  FIGS. 4 and 5 , and displaced by the projection drive portion  131  along the sheet discharge direction. The projection drive portion  131  includes a guide rail  801 , a drive transmission gear group  802 , a drive transmission shaft  803 , a drive shaft  804 , a drive transmission belt  805 , a drive belt  806 , and a drive motor  807 . 
     Two guide rails  801 , two drive transmission gear groups  802 , two drive transmission shafts  803 , and two drive transmission belts  805  are provided corresponding to the two projection members  13 . One drive shaft  804 , one drive belt  806 , and one drive motor  807  are provided. 
     The guide rail  801  is arranged on an upstream side of the discharge roller pair  73  in the sheet discharge direction. The guide rail  801  is an open-topped gutter-shaped member, extending in an arc shape in the sheet discharge direction like the projection member  13 . The guide rail  801  accommodates and supports the projection member  13  inside thereof. 
     The drive transmission gear group  802  is arranged below the guide rail  801 . The drive transmission gear group  802  is composed of a plurality of gears in mesh with each other, and includes a pinion gear  8021  at an end on a side of the guide rail  801 , and a drive transmission gear  8022  at an end on a side of the drive transmission shaft  803 . 
     The pinion gear  8021  is arranged directly under the guide rail  801 . On a lower-face side of the projection member  13 , there is formed a rack (not shown) of a rack-and-pinion gear mechanism. The rack has a plurality of teeth aligned along the sheet discharge direction. The pinion gear  8021  meshes with the rack of the projection member  13 . Here, in the guide rail  801 , at a position adjacent to the pinion gear  8021 , there is provided an unillustrated window portion via which the pinion gear  8021  and the projection member  13  mesh with each other. 
     The drive transmission shaft  803  is arranged in a lower part of the drive transmission gear group  802 . The drive transmission shaft  803  extends along the sheet width direction. The drive transmission gear  8022  of the drive transmission gear group  802  is arranged coaxially with the drive transmission shaft  803 , and rotates together with the drive transmission shaft  803 . 
     The drive shaft  804  is arranged below the drive transmission shaft  803 . The drive shaft  804  extends along the sheet width direction. 
     The drive transmission belt  805  is wound around the drive transmission shaft  803  and the drive shaft  804  via pulleys. In detail, the two drive transmission belts  805  are wound around the one drive shaft  804 , and are respectively wound around the separate drive transmission shafts  803 . The drive transmission belts  805  transmits a rotational force of the drive shaft  804  to the drive transmission shafts  803 . 
     The drive belt  806  is wound around the drive shaft  804  and a rotation shaft of the drive motor  807  via pulleys. The drive belt  806  is rotated by the drive motor  807 . 
     In the projection drive portion  131 , when the drive motor  807  rotates, a rotational force of the drive motor  807  is transmitted via the drive belt  806  to the drive shaft  804 , so that the drive shaft  804  rotates. When the drive shaft  804  rotates, the rotational force is transmitted via the drive transmission belt  805  to the drive transmission shaft  803 . When the drive transmission shaft  803  rotates, the rotational force is transmitted via the drive transmission gear group  802  to the pinion gear  8021 . Thereby, the two projection members  13  are simultaneously displaced along the sheet discharge direction. The displacement of the projection members  13 , in other words, the operation of the projection drive portion  131 , is controlled by the post-processing control portion  100 . 
     Referring back to  FIG. 3 , the sheet holding member  14  is arranged on an upstream side of the loading tray  11  in the sheet discharge direction. The sheet holding member  14  is arranged below a rotation shaft  731   a  of the second drive roller  731  of the discharge roller pair  73 . In the present embodiment, as the sheet holding member  14 , for example, two sheet holding members  14  are arranged to be spaced from each other by a predetermined distance on the loading tray  11  in the sheet width direction. Here, the sheet holding members  14  are arranged at positions different from the position of the paddle member  15  with respect to the sheet width direction. 
     The sheet holding member  14  is a rod-shaped member having a predetermined width in the sheet width direction and extending substantially in the up-down direction. The sheet holding member  14  is, at its lower end part, swingably supported about a swing shaft  14   a  which extends along the sheet width direction as a swing fulcrum. The sheet holding member  14  is caused by a sheet holding drive portion  142  (see  FIG. 12 ) to swing about the swing shaft  14   a  in the sheet discharge direction, with its upper end part as a free end. The sheet holding member  14  is displaced between a holding position (see  FIG. 14 ) for holding the upstream part of a sheet loaded on the loading tray  11  in the sheet discharge direction and a retraction position (see  FIG. 3 ) for releasing the holding of the sheet. 
     The sheet holding member  14  is, as shown in  FIG. 3 , before a sheet discharging operation is started, stationary at the retraction position where it does not project toward the loading tray  11 . In this manner, the sheet holding member  14 , when out of use, does not interfere with discharging of a sheet. 
     Subsequently, the paddle member  15  is rotated, and, before the paddle member  15  passes an upstream end of the loading tray  11  in the sheet discharge direction, swinging of the sheet holding member  14  is started. Then, the sheet holding member  14  is, as shown in  FIG. 14 , displaced to the sheet-holding position where it holds the upstream part of a sheet loaded on the loading tray  11  in the sheet discharge direction. 
     According to this configuration, it is possible to hold a rear end of a curled sheet from above by means of the sheet holding member  14 . Thereby, even a case where the discharging and the loading of sheets with respect to the loading tray  11  are performed at high speed can be handled, so that the upstream part of a sheet loaded on the loading tray  11  in the sheet discharge direction can be held from above, and sheets on the loading tray  11  can be aligned preferably. 
     The paddle member  15  is arranged coaxially with the discharge roller pair  73 . In detail, the paddle member  15  is arranged coaxially with the rotation shaft  731   a  of the second drive roller  731  extending along the sheet width direction. More in detail, in the present embodiment, two paddle members  15  are provided coaxially with the rotation shaft  731   a  of each of the two second drive rollers  731 , such that a total of four paddle members  15  are provided. 
       FIG. 6  is a perspective view showing a configuration of the discharge drive portion  90  and a paddle drive portion  161  in the sheet loading device  10 . The two second drive rollers  731  are simultaneously driven to rotate by the discharge drive portion  90 . The discharge drive portion  90 , as shown in  FIG. 6 , includes a drive transmission shaft  301 , a first drive transmission belt  302 , a drive shaft  303 , a second drive transmission belt  304 , a drive transmission gear  305 , a drive gear  306 , and a drive motor  307 . 
     Two drive transmission shafts  301 , two first drive transmission belts  302 , and two second drive transmission belts  304  are provided corresponding to the two rotation shafts  731   a  of the two second drive rollers  731 . One drive shaft  303 , one drive transmission gear  305 , one drive gear  306 , and one drive motor  307  are provided. 
     The drive transmission shaft  301  is arranged below the rotation shaft  731   a  of the second drive roller  731 . The drive transmission shaft  301  extends along the sheet width direction. 
     The first drive transmission belt  302  is wound around the rotation shaft  731   a  of the second drive roller  731  and the drive transmission shaft  301  via pulleys. The first drive transmission belt  302  transmits a rotational force of the drive transmission shaft  301  to the rotation shaft  731   a.    
     The drive shaft  303  is arranged below the drive transmission shaft  301 . The drive shaft  303  extends along the sheet width direction. 
     The second drive transmission belt  304  is wound around the drive transmission shaft  301  and the drive shaft  303  via pulleys. In detail, the two second drive transmission belts  304  are wound around the one drive shaft  303 , and are respectively wound around the separate drive transmission shafts  301 . The second drive transmission belts  304  transmit a rotational force of the drive shaft  303  to the drive transmission shafts  301 . 
     The drive transmission gear  305  is provided on the drive shaft  303 . The drive transmission gear  305  is arranged coaxially with the drive shaft  303 , and rotates together with the drive shaft  303 . 
     The drive gear  306  is provided on a rotation shaft of the drive motor  307 . The drive gear  306  is rotated by the drive motor  307 . The drive gear  306  meshes with the drive transmission gear  305 . 
     In the discharge drive portion  90 , when the drive motor  307  rotates, a rotational force of the drive motor  307  is transmitted via the drive gear  306  and the drive transmission gear  305  to the drive shaft  303 , so that the drive shaft  303  rotates. When the drive shaft  303  rotates, the rotational force is transmitted via the second drive transmission belt  304  to the drive transmission shaft  301 . When the drive transmission shaft  301  rotates, the rotational force is transmitted via the first drive transmission belt  302  to the rotation shaft  731   a  of the second drive roller  731 . Thereby, the two second drive rollers  731  are simultaneously driven to rotate. The rotation of the second drive rollers  731 , in other words, the operation of the discharge drive portion  90 , is controlled by the post-processing control portion  100 . 
     The four paddle members  15  are simultaneously driven to rotate by the paddle drive portion  161 . The paddle drive portion  161 , as shown in  FIG. 6 , includes a first drive transmission shaft  501 , a first drive transmission belt  502 , a second drive transmission shaft  503 , a second drive transmission belt  504 , a drive shaft  505 , a third drive transmission belt  506 , a drive transmission gear  507 , a drive gear  508 , and a drive motor  509 . 
     Four first drive transmission belts  502  are provided corresponding to the four paddle members  15 . Two first drive transmission shafts  501 , two second drive transmission shafts  503 , two second drive transmission belts  504 , and two third drive transmission belts  506  are provided corresponding to the two rotation shafts  731   a  of the two second drive rollers  731 . One drive shaft  505 , one drive transmission gear  507 , one drive gear  508 , and one drive motor  509  are provided. 
     The paddle members  15 , as shown in  FIG. 7 , each include a paddle main body portion  51  and a shaft portion  52 . The shaft portion  52  is fixed to a side of the paddle main body portion  51  in the sheet width direction. The paddle main body portion  51  and the shaft portion  52  are configured in cylindrical shapes of which central axes extend in the sheet width direction and arranged coaxially with an axis of the rotation shaft  731   a . The paddle main body portion  51  has a smaller diameter than the second drive roller  731 . The shaft portion  52  has a smaller diameter than the paddle main body portion  51 . The rotation shaft  731   a  penetrates, in the sheet width direction, center parts of the paddle main body portion  51  and the shaft portion  52  in a diameter direction. The paddle main body portion  51  and the shaft portion  52  are rotatable independently of the rotation shaft  731   a.    
     The first drive transmission shaft  501  is arranged below the rotation shaft  731   a  of the second drive roller  731 . The first drive transmission shaft  501  extends along the sheet width direction. 
     The first drive transmission belt  502  is wound around the shaft portion  52  of the paddle member  15  and the first drive transmission shaft  501  via pulleys. In detail, the two first drive transmission belts  502  are wound around the one first drive transmission shaft  501 , and are respectively wound around the shaft portions  52  of the separate paddle members  15 . The first drive transmission belts  502  transmit a rotational force of the first drive transmission shaft  501  to the shaft portions  52  of the paddle members  15 . 
     The second drive transmission shaft  503  is arranged below the first drive transmission shaft  501 . The second drive transmission shaft  503  extends along the sheet width direction. 
     The second drive transmission belt  504  is wound around the first drive transmission shaft  501  and the second drive transmission shaft  503  via pulleys. The second drive transmission belt  504  transmits a rotational force of the second drive transmission shaft  503  to the first drive transmission shaft  501 . 
     The drive shaft  505  is arranged below the second drive transmission shaft  503 . The drive shaft  505  extends along the sheet width direction. 
     The third drive transmission belt  506  is wound around the second drive transmission shaft  503  and the drive shaft  505  via pulleys. In detail, the two third drive transmission belts  506  are wound around the one drive shaft  505 , and are respectively wound around the separate second drive transmission shafts  503 . The third drive transmission belts  506  transmit a rotational force of the drive shaft  505  to the second drive transmission shafts  503 . 
     The drive transmission gear  507  is provided on the drive shaft  505 . The drive transmission gear  507  is arranged coaxially with the drive shaft  505 , and rotates together with the drive shaft  505 . 
     The drive gear  508  is provided on a rotation shaft of the drive motor  509 . The drive gear  508  is rotated by the drive motor  509 . The drive gear  508  meshes with the drive transmission gear  507 . 
     In the paddle drive portion  161 , when the drive motor  509  rotates, a rotational force of the drive motor  509  is transmitted via the drive gear  508  and the drive transmission gear  507  to the drive shaft  505 , so that the drive shaft  505  rotates. When the drive shaft  505  rotates, the rotational force is transmitted via the third drive transmission belt  506  to the second drive transmission shaft  503 . When the second drive transmission shaft  503  rotates, the rotational force is transmitted via the second drive transmission belt  504  to the first drive transmission shaft  501 . When the first drive transmission shaft  501  rotates, the rotational force is transmitted via the first drive transmission belt  502  to the shaft portion  52  of the paddle member  15 . With this arrangement, the four paddle members  15  are driven to rotate simultaneously and are rotatable about the rotation shaft  731   a  of the second drive roller  731  independently of the second drive roller  731 . The rotation of the paddle members  15 , in other words, the operation of the paddle drive portion  161 , is controlled by the post-processing control portion  100 . 
     The paddle member  15 , as shown in  FIG. 7 , includes the paddle main body portion  51  and a paddle elastic portion  53 . The paddle main body portion  51  includes a base portion  511  which has formed therein a shaft hole and through which the rotation shaft  731   a  is inserted, and an arm portion  512  which is provided on an outer peripheral surface of the base portion  511 . 
     The arm portion  512  projects in a direction that crosses the axis of the rotation shaft  731   a  of the base portion  511  and that is away from an axial center. In detail, the arm portion  512  projects from the outer peripheral surface of the base portion  511  outward substantially in a tangent direction of the outer peripheral surface. The arm portion  512  is integrally formed with the base portion  511 . The arm portion  512  is made of a material that has a higher modulus of rigidity than a material of the paddle elastic portion  53 . 
     The paddle elastic portion  53  projects longer than the arm portion  512  in a direction that crosses the axis of the rotation shaft  731   a  of the paddle main body portion  51  and that is away from the axial center. In detail, the paddle elastic portion  53  is attached to the arm portion  512 , and projects longer than the arm portion  512  in the same direction as the arm portion  512 . The paddle elastic portion  53  is configured of a material having a higher elasticity modulus than the arm portion  512  (the paddle main body portion  51 ), such as a rubber. 
       FIGS. 8 to 11  are diagrams for illustrating a sheet loading operation performed by the sheet loading device  10 . A description will be given of an operation of the paddle member  15  in the sheet loading device  10 , with reference to  FIGS. 8 to 11 . 
     As shown in  FIG. 8 , the paddle member  15 , before a start of its operation, is made to stop its rotation in a state of being arranged at a retraction position where the arm portion  512  and the paddle elastic portion  53  project neither toward the processing tray  8  nor toward the loading tray  11 . That is, the paddle member  15  stands by at a predetermined position. With this arrangement, when out of use, the paddle member  15  does not interfere with discharging of a sheet P. A rotation speed of the discharge roller pair  73  is reduced by the time when the upstream end of the sheet P in the sheet discharge direction passes through the second nip portion  73 N of the discharge roller pair  73 . That is, a discharge speed of the sheet P to be discharged by the discharge roller pair  73  is reduced to a predetermined discharge speed by the time the upstream end of the sheet P in the sheet discharge direction passes through the second nip portion  73 N of the discharge roller pair  73 . 
     Next, as shown in  FIG. 9 , before the upstream end (rear end) of the sheet P in the sheet discharge direction passes through the second nip portion  73 N of the discharge roller pair  73  to be loaded on the sheet loading surface  11   a  of the loading tray  11 , the paddle drive portion  161  makes the paddle member  15  start rotating. A rotation speed of the paddle member  15  is equal to the rotation speed of the discharge roller pair  73  at which the discharge roller pair  73  rotates when the upstream end of the sheet P in the sheet discharge direction passes through its second nip portion  73 N. 
     The paddle member  15  comes into contact with the upstream part (the rear end) of the sheet P in the discharge direction in which the sheet P is discharged by the discharge roller pair  73 . Thereby, the paddle member  15  pushes down, toward the sheet loading surface  11   a , the upstream part of the sheet P in the discharge direction, in which the sheet P has been discharged from the discharge roller pair  73 , as if by slapping the upstream part from above. 
     When the paddle member  15  further rotates from the state shown in  FIG. 9 , the paddle elastic portion  53 , as shown in  FIG. 10 , comes into contact with the upstream part of the discharged sheet P in the sheet discharge direction. Thereby, the paddle member  15  pulls the sheet P along the loading tray  11  toward the upstream side of the sheet P in the sheet discharge direction. Further, the paddle member  15  holds down the upstream part of the sheet P in the sheet discharge direction toward the sheet receiving wall  11   b  of the loading tray  11 . 
     Then, when the sheet loading operation by the sheet loading device  10  is completed, as shown in  FIG. 11 , the upstream part of the sheet P in the sheet discharge direction comes in contact with the sheet receiving wall  11   b  provided on the upstream side of the loading tray  11  in the sheet discharge direction. Thereby, the sheet P is aligned at a predetermined position on the loading tray  11 . Here, the sheet receiving wall  11   b  has, on a rotational orbit of the paddle member  15 , an unillustrated slit portion through which the paddle member  15  can pass. In this manner, the arm portion  512  and the paddle elastic portion  53  reach the retraction position at which they do no project toward the loading tray  11 . 
       FIG. 12  is a block diagram showing an example of a control path for the sheet post-processing device  5 . The post-processing control portion  100  (hereinafter referred to simply as the control portion  100 ) is constituted by a CPU (Central Processing Unit) which controls operations of various portions of the sheet post-processing device  5  including the sheet loading device  10 , a ROM (Read Only Memory) which stores a control program therein, a RAM (Random Access Memory) which is used as an operation area for the CPU, etc. The control portion  100  controls the operations of the various portions of the sheet post-processing device  5  including the sheet loading device  10  by the CPU executing the control program stored in the ROM. 
     The control portion  100  controls a punching processing operation performed by the punching processing portion  61  of the post-processing mechanism  6  and a staple processing operation performed by the staple processing portion  62  of the post-processing mechanism  6 . The control portion  100  controls driving of the transport drive portion  70 , and thereby controls rotating and stopping of the transport roller pair  71  and the intermediate roller pair  72 . The control portion  100  controls driving of the discharge drive portion  90 , and thereby controls rotating and stopping of the discharge roller pair  73  or reciprocating movement of the bundle discharge member  81 . 
     The control portion  100  controls driving of a nip release drive portion  91 , and thereby controls operations of releasing and recovering the second nip portion  73 N of the discharge roller pair  73  performed by the nip release mechanism  74 . For example, in a case where the staple processing is performed by the staple processing portion  62  with respect to a sheet bundle of a predetermined number of sheets, the control portion  100 , after a first sheet is pulled into the processing tray  8 , makes the nip release drive portion  91  drive the nip release mechanism  74  to release the second nip portion  73 N. Then, after a second and subsequent sheets are pulled into the processing tray  8  and the staple processing is performed, the second nip portion  73 N is recovered to discharge the sheet bundle onto the loading tray  11 . 
     Here, in a case of discharging a sheet bundle onto the loading tray  11  by means of the bundle discharge member  81 , with the second nip portion  73 N released, the bundle discharge member  81  is moved to the downstream side in the sheet discharge direction, and the sheet bundle is pushed out and discharged onto the loading tray  11 . 
     The control portion  100  controls driving of the tray lifting-lowering drive portion  113 , and thereby controls the operation of lifting-lowering the loading tray  11 . The control portion  100  controls driving of the projection drive portion  131 , and thereby controls movement of the projection member  13  between a projection position and a retraction position along the guide rail  801 . The control portion  100  controls driving of the sheet holding drive portion  142 , and thereby controls a swinging operation which the sheet holding member  14  performs, by rotating about the swing shaft  14   a , to swing between the sheet-holding position and the retraction position. 
     The control portion  100  controls driving of the paddle drive portion  161 , and thereby controls a slapping operation which the paddle member  15  performs, by rotating about the rotation shaft  731   a , to slap, toward the loading tray  11 , a rear end of a sheet having passed through the discharge roller pair  73 , and a holding operation which the paddle member  15  performs subsequently to the slapping operation to come into contact, from above, with the rear end part of the sheet having fallen into the loading tray  11  to hold the sheet down while pulling the sheet toward the upstream side. 
     Here, in a case of performing the above-described operation of lifting-lowering (positioning) the loading tray  11  in a mode in which sheets are loaded continuously one by one on the loading tray  11 , in order to prevent the top surface detection sensor S 3  from erroneously detecting a falling sheet or a curled rear end part of a sheet, the loading tray  11  is lifted and lowered with the sheet holding member  14  holding the sheets. At this time, depending on whether the lifting-lowering operation is finished with the top surface detection sensor S 3  in an on state or the lifting-lowering operation is finished with the top surface detection sensor S 3  in an off state, the following inconvenience may occur. 
     For example, control may be performed in such a manner that the top surface detection sensor S 3  is confirmed to be on and then a top surface of sheets is detected, the loading tray  11  is once lowered and then the top surface detection sensor S 3  is confirmed to be off, and then the loading tray  11  is lifted again and the lifting-lowering operation is finished with the top surface detection sensor S 3  in the on state. In this case, the top surface of the sheets is located nearest the rotational orbit of the paddle member  15 . 
     The paddle member  15  has a role of pushing down a sheet toward the sheet loading surface  11   a  of the loading tray  11  as if by slapping a rear end of the sheet from above, pulling the sheet along the loading tray  11  toward the upstream side in the sheet discharge direction, and holding down an upstream part of the sheet in the sheet discharge direction toward the sheet receiving wall  11   b  of the loading tray  11 , and thus it is preferable for the paddle member  15  to have a possible maximum paddle length. 
     Thus, if the lifting-lowering operation is finished with the top surface detection sensor S 3  in the on state, the top surface of the sheets loaded by a next lifting-lowering operation may overlap with the orbit of the paddle member  15  and make the paddle member  15  unrotatable (locked). 
     On the other hand, in a case where the lifting-lowering operation is finished with the top surface detection sensor S 3  in the off state in order to avoid the risk of the paddle member  15  coming near the top surface of the sheets, if the sheets having been loaded during continuous sheet discharge are removed, the loading tray  11  does not rise to a predetermined position in the next lifting-lowering operation. As a result, a height difference between the discharge roller pair  73  and the top surface of the sheets is increased, and this may invite an unstable sheet discharge state in which, for example, a leading end of a sheet discharged by the discharge roller pair  73  is curled or a sheet is reversed, and cause poor alignment of sheets loaded on the loading tray  11 . 
     To prevent this, in the present embodiment, first the loading tray  11  is lowered and the top surface detection sensor S 3  is confirmed to be off, then the loading tray  11  is once lifted and the top surface detection sensor S 3  is confirmed to be on, and after the top surface of the sheets is detected, the loading tray  11  is lowered again and the lifting-lowering operation is finished with the top surface detection sensor S 3  in the off state. 
       FIG. 13  is a flowchart showing an example of control of the lifting-lowering operation of lifting and lowering the loading tray  11  performed in the sheet loading device  10  of the present embodiment. With reference to  FIGS. 1 to 12  and later-described  FIGS. 14 and 15 , as necessary, along the steps shown in  FIG. 13 , a description will be given of a method of positioning the loading tray  11  of the sheet loading device  10  at a reference position (home position). 
     First, the sheet post-processing device  5  is in a mode (one-by-one mode) in which it processes sheets continuously one by one, and the loading tray  11  of the sheet loading device  10  is arranged at the reference position. 
     When discharging of sheets onto the loading tray  11  is started from this state (step S 1 ), the control portion  100  determines whether or not a predetermined number (here, ten) sheets have been discharged (step S 2 ). If the predetermined number has not been reached (No in step S 2 ), the control portion  100  determines whether or not the discharging of sheets has been finished (step S 3 ). If the discharging of sheets has not been finished (No in step S 3 ), the flow returns to step S 2 , and the discharging of sheets is continued. When the discharging of sheets has been finished (Yes in step S 3 ), the processing is finished. 
     In a case where the predetermined number of sheets have been discharged (Yes in step S 2 ), the sheet holding member  14  is moved from the retraction position to the sheet-holding position (step S 4 ). Then, the loading tray  11  is lowered (step S 5 ). 
     Next, the control portion  100  determines whether or not the top surface detection sensor S 3  has been turned off (step S 6 ). In a case where the top surface detection sensor S 3  is on (No in step S 6 ), the loading tray  11  continues to be lowered. 
       FIG. 14  is a side sectional view of the sheet loading device  10 , showing a state in which the top surface detection sensor S 3  is off. In  FIG. 14 , the loading tray  11  has descended to a position at which a topmost sheet P 1 U of sheets P 1  loaded on the sheet loading surface  11   a  does not overlap with a detection portion S 3   a  of the top surface detection sensor S 3 . In a case where the top surface detection sensor S 3  has been turned off as shown in  FIG. 14  (Yes in step S 6 ), the loading tray  11  is lifted (step S 7 ). 
     Next, the control portion  100  determines whether or not the top surface detection sensor S 3  has been turned on (step S 8 ). In a case where the top surface detection sensor S 3  is off (No in step S 8 ), the loading tray  11  continues to be lifted. 
       FIG. 15  is a side sectional view of the sheet loading device  10 , showing a state in which the top surface detection sensor S 3  is on. In  FIG. 15 , the loading tray  11  has ascended to a position at which the topmost sheet P 1 U of the sheets P 1  loaded on the sheet loading surface  11   a  overlaps with the detection portion S 3   a  of the top surface detection sensor S 3 . In a case where the top surface detection sensor S 3  has been turned on as shown in  FIG. 15  (Yes in step S 8 ), the loading tray  11  is lowered again (step S 9 ). 
     Next, the control portion  100  determines whether or not the top surface detection sensor S 3  has been turned off (step S 10 ) In a case where the top surface detection sensor S 3  is on (No in step S 10 ), the loading tray  11  continues to be lowered. In a case where the top surface detection sensor S 3  has been turned off as shown in  FIG. 14  (Yes in step S 10 ), the loading tray  11  is stopped (step S 11 ). This position is the reference position of the loading tray  11 . Thereafter, the sheet holding member  14  is moved to the retraction position (step S 12 ) and the flow returns to step S 2 , and the similar processing is repeated. 
     According to the above-described control example, after the state of the top surface detection sensor S 3  changes from off to on to off, the operation of lifting-lowering the loading tray  11  is finished and the loading tray  11  is positioned at the reference position. Thus, as compared with a case where the lifting-lowering operation is finished with the top surface detection sensor S 3  on, the clearance between the rotational orbit of the paddle member  15  and the top surface of the sheets can constantly be widened by a certain distance. Accordingly, it is possible to achieve as long a paddle length of the paddle member  15  as possible while avoiding inconveniences such as the paddle member  15  becoming unable to rotate (locked state) and unstable sheet discharge state occurring when loaded sheets are removed. 
     Further, since the topmost sheet surface is detected after the sheet holding member  14  is moved to the sheet-holding position, it is possible to prevent erroneous detection by the top surface detection sensor S 3  due to a lifted or curled rear end of a sheet. 
     Furthermore, as shown in  FIGS. 16 and 17 , by arranging the top surface detection sensor S 3  at a position at which the top surface detection sensor S 3  overlaps with the leading end of the sheet holding member  14  arranged at the sheet-holding position, the sheet holding member  14  can be detected by the top surface detection sensor S 3  when detecting a position of the top surface of the sheets. 
     According to this configuration, whether the top surface detection sensor S 3  is in the off state as shown in  FIG. 16  or in the on state as shown in  FIG. 17 , an actual position of the top surface is lower, by a thickness of the sheet holding member  14 , than in a case where the topmost sheet P 1 U is detected as shown in  FIGS. 14 and 15 . As a result, the clearance between the rotational orbit of the paddle member  15  and the top surface of the sheets can be widened to provide a sufficient margin for the paddle member  15  to rotate. Accordingly, it is possible to dealt also with cases where discharged sheets have different thicknesses and where a rear end of a sheet is curled. 
     Here, in a case where the flag  11   e  (see  FIG. 2 ) stops slightly before the lower limit detection sensor S 4  is turned on when the operation of lifting-lowering the loading tray  11  is finished, in a next lifting-lowering operation, it is detected that the loading tray  11  has reached the lower limit position. In this case, the loading tray  11  can hardly descend in the next lifting-lowering operation, and thus, after the next lifting-lowering operation is finished, the position of the top surface of sheets loaded after the next lifting-lowering operation is finished comes above a detection position (upper limit position) of the top surface detection sensor S 3 , and thus it becomes impossible for the paddle member  15  to rotate (locked state). 
       FIG. 18  is a side view of and around the discharge roller pair  73 , showing a state in which the loading tray  11  has descended to the reference position in the sheet loading device  10  of the present embodiment. As shown in  FIG. 18 , when the lifting-lowering operation is finished and the loading tray  11  is positioned at the reference position after the state of the top surface detection sensor S 3  changes from off to on to off, when a descending amount (descending distance) of atop surface of sheets from the detection position (indicated by a broken line L in  FIG. 18 ) of the top surface detection sensor S 3  is represented by “d”, and a thickness of a sheet bundle of a maximum number of sheets (for example, 100 sheets) dischargeable from the processing tray  8  onto the loading tray  11  at one time is represented by “t”, formula (1) below is satisfied:
 
 d&gt; 2× t    (1)
 
     According to this configuration, a secured amount of sheets loadable on the sheet loading surface  11   a  in one event of the operation of lifting-lowering the loading tray  11  exceeds twice the maximum number of sheets in one sheet bundle. Thus, even when the loading tray  11  reaches the lower limit position in the next lifting-lowering operation and the loading tray  11  hardly descends, a sheet bundle of the maximum number of sheets can be loaded on the sheet loading surface  11   a  of the loading tray  11 . Accordingly, even when the loading tray  11  reaches the lower limit position, the position of the top surface of the sheets does not exceed the detection position (upper limit position) of the top surface detection sensor S 3 , and thus it is possible to prevent the inconvenience of the paddle member  15  becoming unable to rotate (locked state). 
     In the configuration shown in  FIG. 18 , when the lower limit detection sensor S 4  detects that the loading tray  11  has reached the lower limit position, the discharging operation thereafter is stopped. Thus, in a case where a sheet bundle discharged when the loading tray reaches the lower limit position includes only a small number of sheets, the discharging operation is stopped with a margin left in the number of sheets loadable on the sheet loading surface  11   a  of the loading tray  11 , and this may result in degraded loading efficiency (post-processing efficiency). 
     To prevent this, when the lower limit detection sensor S 4  is turned into the on state, a judgment is made whether or not to stop the sheet discharging operation in accordance with the number of sheets in a next sheet bundle to be discharged. This helps prevent the inconvenience of the paddle member  15  becoming unable to rotate (locked state) with as little degradation of the loading efficiency as possible. 
       FIG. 19  is a flowchart showing an example of control in the discharging operation performed when the lower limit position of the loading tray  11  has been detected by the lower limit detection sensor S 4 . By referring to  FIGS. 1 to 18  as necessary, along the steps shown in  FIG. 19 , a description will be given of the discharging operation performed when the lower limit position of the loading tray  11  has been detected. 
     First, the sheet post-processing device  5  is set in a mode (bundle processing mode) in which the staple processing is performed with respect to a sheet bundle loaded on the processing tray  8 , and the loading tray  11  of the sheet loading device  10  is arranged at the reference position. 
     When, from this state, discharging of a sheet bundle onto the loading tray  11  is started (step S 1 ), the control portion  100  detects a number A of sheets in the sheet bundle (step S 2 ). The number of sheets in the sheet bundle can be detected by, for example, counting the number of sheets that pass the first sheet detection portion S 1  (or the second sheet detection portion S 2 ). 
     Next, the control portion  100  determines whether or not the lower limit detection sensor S 4  is on (step S 3 ). In a case where the lower limit detection sensor S 4  is not in the on state (No in step S 3 ), the loading tray  11  has not reached the lower limit position, and thus the flow proceeds to a loading-tray lifting-lowering routine shown in  FIG. 13  (step S 4 ). Specifically, each time a sheet bundle of a predetermined number of sheets is discharged, the loading tray  11  is lifted and lowered, and after the state of the top surface detection sensor S 3  changes from off to on to off, the lifting-lowering operation is finished, and the loading tray  11  is positioned at the reference position. The reference position is below the detection position (upper limit position) of the top surface detection sensor S 3  by a distance equal to twice the thickness of the maximum number of sheets (for example, 100 sheets) discharged onto the loading tray  11 . 
     In a case where the lower limit detection sensor S 4  is in the on state (Yes in step S 3 ), it is determined whether or not the number A of sheets included in the discharged sheet bundle is equal to or larger than a predetermined number A 1  (for example, 50) (step S 5 ). In a case where A≤A 1  (Yes in step S 5 ), the sheet bundle is discharged (step S 6 ). Also, a cumulative number B of sheets having been discharged after the lower limit detection sensor S 4  is turned into the on state is detected (step S 7 ). 
     Next, the control portion  100  determines whether or not the cumulative number B of the discharged sheets is equal to or larger than a predetermined number B 1  (step S 8 ). The predetermined number B 1  is set to be smaller than the maximum number of sheets loadable when the loading tray  11  is at the reference position (twice as large as the maximum number of sheets in a sheet bundle). For example, in a case where the maximum number of sheets in a sheet bundle is 100, the predetermined number B 1  is set to a number (for example, B 1 =150) that is smaller than 200 (=100×2). 
     In a case where B&lt;B 1  (No in step S 8 ), there is a margin in the number of sheets loadable on the loading tray  11 , the flow returns to step S 2 , where discharging of sheets onto the loading tray  11  is continued (steps S 2  to S 8 ). In a case where B≥B 1  (Yes in step S 8 ), there is no margin left in the number of sheets loadable on the loading tray  11 , and thus the discharging of sheets onto the loading tray  11  is stopped (step S 9 ). 
     On the other hand, in a case where A&gt;A 1  in step S 5  (No in step S 5 ), it is determined that the discharging of a sheet bundle this time will leave no margin in the number of loadable sheets, and after the sheet bundle is discharged (step S 10 ), the discharging operation is stopped (step S 9 ). 
     According to the control described above, in a case where it is detected that the loading tray  11  is at the lower limit position, when the number of sheets in a discharged sheet bundle is small, the discharging of a sheet bundle is continued. Thus, the discharging operation is never stopped with a margin left in the number of loadable sheets, and this helps reduce degradation of processing efficiency. 
     On the other hand, when the discharged sheet bundle is of a large number of sheets, discharging of a sheet bundle thereafter is stopped, and this helps prevent inconvenience such that the paddle member  15  becomes unable to rotate because the loading tray  11  does not descend. The threshold values A 1  and B 1  can be appropriately set in accordance with the maximum number of sheets in a sheet bundle dischargeable in one event of sheet discharge. 
     The present disclosure is not limited to the embodiments described above and various modifications thereto can be made without departing from the spirit and scope of the present disclosure. For example, in the embodiments described above, the projection member  13  is displaced between the projection position at which a sheet discharged by the discharge roller pair  73  comes into contact with the top surface of the projection member  13  and the retraction position at which the projection member  13  is retracted to the upstream side in the sheet discharge direction, but a configuration is possible without the projection member  13 . 
     Further, in the embodiments described above, the image forming apparatus  200  in the image forming system S is a multifunction peripheral for monochrome printing, but this is not meant to limit the present disclosure. The image forming apparatus  200  may instead be, for example, a monochrome copier, a monochrome printer, or the like, or may instead be an image forming apparatus for color printing, such as a color copier, a color printer, or the like.