Abstract:
A sheet passage changing apparatus includes a plurality of swingable changing devices for changing a sheet transporting direction; common drivers for swinging the plurality of changing devices; a controller for actuating the drivers, before a sheet reaches an upstream one of the changing devices, to actuate the upstream one of the changing devices, thus changing the sheet transporting direction and for actuating the drivers, after the sheet passing through the upstream changing device and before the sheet reaches a downstream one of the changing device, to actuate the downstream changing device, thus changing the sheet transporting direction.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to a switching apparatus for switching the direction in which a sheet is conveyed, and a sheet sorting apparatus comprising such a switching apparatus. More specifically, it relates to a sheet processing apparatus (sorter) which is employed in an image forming apparatus such as a copying machine, a printer, a facsimile machine, and the like, and allows the user to optionally sort sheets and deliver them into a plurality of delivery trays. 
     Recently, a typical image forming apparatus such as a copying machine, a printer, or a facsimile machine is equipped with a sheet processing apparatus which allows the user to optionally sort printed sheets and deliver them into a plurality of delivery trays. 
     This is for the following reason. In the case of an image forming apparatus such as a network printer or the like which is used by more than one person, it is feared that when a large number of printed sheets are delivered into a single tray, it is may become impossible to identify sets of sheets outputted for each user from among a large number of sheet sets accumulated on the delivery tray. Therefore, the sorter is enabled to allow the users to optionally sort the printed sheets into a plurality of delivery trays so that each set of printed sheets remains separated from the others. 
     In the case of a conventional sorter, it receives a printed sheet delivered from the main assembly of an image forming apparatus, and sorts it into a designated tray among a plurality of delivery trays, through a common sheet path. As it is well known, this type of sorter is employed in a medium to high speed copying machine which is normally used to make more than one copy, or in a large printer which is used to produce a large number of copies. 
     In some of the sheet processing apparatuses of the above described type, the delivery trays are fixedly disposed. More specifically, those sorters comprise a plurality of delivery trays, and a plurality of discharging roller pairs for discharging a sheet into the plurality of discharge trays, wherein the delivery trays and the discharging roller pairs are fixed to the main assembly of the sheet processing apparatus. Each delivery tray is mated with a sheet conveyance guide, a flapper, and a solenoid switch. The sheet conveyance guide forms a branch path for guiding a sheet from the aforementioned common sheet path to a delivery roller pair. The flapper allows the user to optionally switch the sheet delivery direction at the branching point. The solenoid switch drives the flapper. 
     However, in the case of a conventional sorter such as the one described above, the solenoid for driving the flapper which leads a sheet into a predetermined delivery tray is provided for each delivery tray, which increases cost. This is one of the problems of a conventional sorter. 
     Further, in recent years, the sorter market has been demanding a small and inexpensive apparatus which allows the user to randomly select the delivery trays during a continuous printing operation, and has a larger number of delivery trays than conventionally. However, it has been difficult for a conventional sorter to satisfy the demand for such a sorter, in terms of cost and size, since a conventional sorter must be provided with a flapper, a solenoid, or the like, for each delivery tray. 
     Further, as the number of the delivery trays is increased, the number of the solenoids must be increased to match the number of the delivery trays, which requires an increase in the number of electrical components for driving the solenoids, adding to the cost increase. In particular, in the case of a sheet processing apparatus, the cost of the actual sorting section greatly contributes to the overall (cost of a sheet processing apparatus. Therefore, the need for providing a solenoid switch for each delivery tray gives a conventional sorter a great disadvantage in terms of apparatus cost. 
     SUMMARY OF THE INVENTION 
     Accordingly, the primary object of the present invention is to suppress the cost or size increase which results from the increase in the number of delivery trays, so that it becomes possible to provide a highly reliable sheet processing apparatus capable of stably conveying a sheet. 
     According to a representative structure of the present invention which accomplishes the above object, a sheet processing apparatus capable of allowing the user to optionally sort a sheet into a plurality of delivery trays comprises: means for conveying a sheet; a common sheet path for conveying a sheet substantially in parallel to the direction in which the delivery trays are arranged; a member selectively pivotable between a position at which it guides a sheet to one of the delivery trays from the common sheet path and a position at which it does not block the common sheet path so that a sheet is guided along the common sheet path; means for conveying a sheet to the delivery trays after the direction in which the sheet is conveyed is switched by the pivotable member; a single means for pivoting a plurality of the pivotable members; means for linking the plurality of pivotable members to the single pivoting means; and a plurality of elastic members, each of which is disposed between the linking means and each of the plurality of pivotable members, wherein the single means for pivoting the plurality of pivotable members is structured so that when a sheet is in the sweeping area of one of the pivotable members, the pivotable member is held at a position at which it does not block the common sheet path. 
     According to the above structure, a sheet which is conveyed through the common sheet path is guided toward (sorted into) an optionally selected delivery tray by one of the plurality of pivotable members which are pivoted together by the single pivotable member pivoting means, through the linking means and the elastic members. When a pivotable member that is to sort a sheet is the one on the downstream side, of the two pivotable members that pivot together, the pivotable member on the upstream side remains at a position at which it does not block the common sheet path (more specifically, it does nothing to a sheet although it comes in contact with a sheet). Therefore, it does not interfere with sheet conveyance. In other words, even though the pivotable member on the upstream side is pivoted together with the pivotable member on the downstream side, a sheet is smoothly sorted by the pivotable member on the downstream side. 
     As described above, according to the present invention, when a sheet, which is being conveyed through a common sheet path to be sorted into a predetermined sorting path by a predetermined pivotable member among a plurality of pivotable members which are pivoted together by a single driving means, is in the sweeping area of one of the plurality of pivotable members, the pivotable members are held at positions at which they do not block the common sheet path. Therefore, a plurality of pivotable members can be driven by a single driving means, making it possible to reduce the number of the pivotable member driving means, and thereby making it possible to realize an inexpensive sorting apparatus which has a larger number of delivery trays into which a sheet can be randomly sorted. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical section of the sheet processing apparatus in the first embodiment of the present invention. 
     FIG. 2 is a schematic vertical section of an image forming apparatus equipped with the sheet processing apparatus illustrated in FIG.  2 . 
     FIG. 3 is an enlarged vertical section of the flappers and their adjacencies in the sheet processing apparatus in the first embodiment of the present invention. 
     FIG. 4 is a vertical section of the essential portion of the sheet processing apparatus in the second embodiment of the present invention. 
     FIG. 5 is a vertical section of the essential portion of the sheet processing apparatus in the second embodiment of the present invention. 
     FIG. 6 is a vertical section of the essential portion of the sheet processing apparatus in the third embodiment of the present invention. 
     FIG. 7 is a graph which depicts the characteristics of the tension spring of the sheet processing apparatus in the third embodiment of the present invention. 
     FIG. 8 is a vertical section of the essential portion of the sheet processing apparatus in the fourth embodiment of the present invention. 
     FIG. 9 is a longitudinal section of the compression spring, that is, one of the link members, and its adjacencies, in the sheet processing apparatus in the fourth embodiment of the present invention. 
     FIG. 10, is a longitudinal section of the compression spring, that is, one of the link members, and its adjacencies, in the sheet processing apparatus in the fourth embodiment of the present invention. 
     FIG. 11 is a vertical section of the flapper containing portion of the sheet processing apparatus in the second embodiment of the present invention. 
     FIG. 12 is a vertical section of the essential portion of the sheet processing apparatus in the fifth embodiment of the present invention, depicting the flappers and the conveyer roller pairs. 
     FIG. 13 is a vertical section of the essential portion of the sheet processing apparatus in the fifth embodiment of the present invention, depicting the flappers and the conveyer roller pairs. 
     FIG. 14 is a vertical section of the essential portion of another embodiment of the present invention, depicting the flappers and the conveyer roller pairs. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the sheet processing apparatus in accordance with the present invention will be concretely described with reference to the drawings. In the following embodiments, the present invention will be described with reference to a sheet processing apparatus which is usable in an image forming apparatus such as a copying machine. 
     Embodiment 1 
     The first embodiment will be described with reference to FIGS. 1 and 2. FIG. 1 is a vertical section of the sheet processing apparatus in the first embodiment, and depicts the general structure thereof. FIG. 2 is a schematic vertical section of an image forming apparatus equipped with the sheet processing apparatus illustrated in FIG. 1, and depicts the general structure thereof. 
     First, referring to FIG. 2, the general structure of an image forming apparatus will be concisely described. As is illustrated in FIG. 2, on the top surface of an image forming apparatus  1 , an automatic original feeding apparatus  2 , which automatically circulates originals, is disposed. On the downstream side (left side of the drawing), a sheet processing apparatus, which comprises a face-up tray  22  and a plurality of face-down trays  50 , is disposed. 
     The image forming apparatus  1  is an image forming apparatus employing one of the well-known electrophotographic systems, and its detailed description will be omitted at this time. The image of an original positioned on a platen glass  3  is formed on a photosensitive drum  4  by an unillustrated optical system. The formed image, a latent image, is visualized (as a toner image) by a developing device (or devices)  5  disposed around the photosensitive drum  4 . The visualized image (toner image) is transferred from the photosensitive drum  4  onto a sheet of transfer material by a transferring device  6 , and is permanently fixed to the transfer material by a fixing device  7 . 
     Normally, the transfer sheets on which a permanent image was formed as described above are sequentially delivered into a face-up tray, with the printed surface facing upward, by a delivered roller pair  3 . However, when image formation is carried out in the order of page number (for example, starting from the first page when copying a set of original which consists of 10 pages), and the finished copies are sequentially accumulated, with the printed surfaces facing upward, the copies are accumulated in the order opposite to the original page order. In order to accumulate the finished copies in the original page order, a sheet processing apparatus  10  of this embodiment, which is equipped with a sheet inverting mechanism such as the one illustrated in the drawing, is disposed adjacent to a copy delivery opening  9  of the image forming apparatus  1 . With this arrangement, the finished copies can be accumulated in the same order as the original page order. More specifically, when an image forming operation is carried out in the order opposite to the original page order, the finished copies are sequentially deposited in the face-up tray  22 , with their printed surfaces facing upward, and when an image forming operation is carried out in the sama order as the page order, the finished copies are sequentially turned over and deposited into one of the face-down trays  50 , with their printed surfaces facing downward. 
     Next, referring to FIG. 1, the structure of the sheet processing apparatus in this embodiment will be described in detail. In FIG. 1, reference numerals  11 ,  12  and  13  designate conveyer rollers. In order to take in a sheet S and discharge it after turning it over, a plurality (two in this embodiment) of free-rolling rollers (rollers  12  and  13  in this embodiment) are disposed in contact with the peripheral surface of the conveyer roller  11 , with a predetermined contact pressure, wherein the conveyer roller  11  is rotatable only in the direction indicated by an arrow mark. The conveyer roller  11 , and roller  12  which is in contact with the conveyer roller  11  from above, constitute a take-in roller pair which takes in the sheet S, whereas the conveyer roller  11 , and the roller  13  which is in contact with the conveyer roller  11  from below, constitute a discharge roller pair which discharges the sheet S. In other words, the sheet S is taken in by the conveyer roller  11  and the pressing roller  12 , and is discharged by the conveyer roller  11  and the pressing roller  13 . 
     A reference numeral  14  designates a flapper, which is disposed on the downstream side of the aforementioned take-in roller pair ( 11 ,  12 ). The flapper  14  is mounted on an axis  14   a , being pivotable by an unillustrated pivoting means such as a solenoid about the axis  14   a , so that it can be selectively pivoted between a position outlined by a solid line and a position outlined by a double dot chain line. In other words, whether or not a sheet S is conveyed into a plurality of sheet paths located on the downstream side of the conveyer roller  11  is determined by the selected position of the flapper  14 . More specifically, as the position of the flapper  14  is switched to the position illustrated by the solid line in FIG. 1, the sheet S is guided into an inverting path  24 . As the position of the flapper  14  is switched to the position outlined by the double dot chain line in FIG. 1, the sheet S is guided into a face-up delivery path  20 , without being inverted, in other words, with the printed surface facing upward. Thus, the sheet S can be deposited with its printed surface facing upward or downward by selectively switching the position of the flapper  14 . 
     In the face-up delivery path  20 , a delivery roller pair  21  is disposed. The delivery roller pair  21  delivers the sheet S into the face-up tray  22  after the sheet S is conveyed to the delivery roller pair  21  through the face-up delivery path  20 . The face-up tray  22  is removably attached to the main assembly of the apparatus  10 , and accumulates and holds the sheet S sequentially delivered by the delivery roller  21 . 
     A reference numeral  16  designates a reverse conveyer roller, which is continuously rotated in the direction (indicated by an arrow mark in the drawing) opposite to the rotational direction of the conveyer roller  11  to reversely convey a sheet S which is taken into the reversing path  24 . The reverse conveyer roller  16  is disposed below a line which is drawn tangent to the take-in roller pair ( 11 ,  12 ), through the nip of the take-in roller pair ( 11 ,  12 ). Also, it is disposed closer to the take-in roller pair ( 11 ,  12 ) than to the leading end of the sheet S which has been taken in by the take-in roller pair ( 11 ,  12 ). Therefore, after the sheet S is guided into the reversing path  24  by the flapper  14  which has been moved to the position outlined by the solid line in FIG. 1, it is conveyed deeper into the reversing path  24  without contacting the reverse conveyer roller 
     Further, the flapper  14  is provided with a roller  15  as a slave roller to the reverse conveyor roller  16 . The roller  15  is rotatively attached to the flapper  14 , opposing the reverse conveyer roller  16 . As the flapper  14  is moved to the position outlined by the double dot chain line in FIG. 1, the roller  15  comes in contact with the reverse conveyor roller  16 , and follow the rotation of the reverse conveyor roller  16 . As the position of the flapper  14  is switched to the position outlined by the solid line in FIG. 1, the roller  15  becomes separated from the reverse conveyor roller  16 , and remains separated. 
     After a sheet S is introduced into the reversing path  24  by the flapper  14  which has been moved to the position outlined by the solid line in FIG. 1, it is discharged onto an external reversing tray  25  from a temporary discharge opening  24   a . During this movement of the sheet  5 , the leading end of the sheet S does not touch the reverse conveyor roller  16 . After this movement, the sheet S is temporarily exposed from the apparatus. The temporary discharge opening  24   a  is located between the face-up tray  22 , and a face-down tray  51  which will be described later. With this arrangement, the sheet S temporarily exposed from the apparatus through the temporary discharge opening  24   a  is protected by both the trays  22  and  51 , being prevented from being easily touched by the user. Therefore, the sheet S is prevented from being conveyed askew, or being damaged, by coming in contact with the user; the sheet S can be smoothly conveyed through the reversing path  24 . Further, since the temporarily exposed sheet S can be hidden by the trays  22  and  51 , the apparatus becomes more desirable in terms of its appearance when in operation. 
     The tray  25  disposed below the temporary discharge opening  24   a  prevents the temporarily discharged sheet S from coming in contact with another sheet S which has been already deposited in the face-down tray  51 . With this arrangement, the sheets S which have been accumulated in the face-down tray  51  are prevented from becoming misaligned by coming in contact with the sheet S which would have come in contact with the sheets S if it were not for the face-down tray  51 . Therefore, the sheets S on the face-down tray  51  can be kept in the desirable state of accumulation. 
     On the upstream side of the take-in roller pair ( 11 ,  12 ), a sensor ( 17 ,  18 ) is disposed as means for detecting the trailing end of a sheet S. As the trailing end of a sheet S is detected by the sensor ( 17 ,  18 ) during face-down delivery, the position of the flapper  14  is switched to the position outlined by the solid line in FIG. 1 by the unillustrated moving means such as a solenoid in response to the detection signal from the sensor ( 17 ,  18 ). As a result, the tip of the flapper  14  is moved from the nip of the take-in roller pair ( 11 ,  12 ) to the nip of discharge roller pair ( 11 ,  13 ). 
     The sheet S is nudged toward the reverse conveyor roller  16  by the downwardly pivoting flapper  14 . In other words, as the position of the flapper  14  is switched to the position outlined by the double dot chain line in FIG. 1, the roller  15  attached to the flapper  14  comes in contact with the reverse conveyor roller  16 , with the sheet S being pinched between the reverse conveyor roller  16  and the roller  15 . As a result, the sheet S is conveyed toward the discharge roller pair ( 11 ,  13 ), that is, in the direction opposite to the direction in which it was taken in. 
     After being discharged from the reversing path  24  by the discharge roller pair ( 11 ,  13 ), the sheet S is guided into one of a plurality (five in this embodiment) of face-down delivery openings (openings  61 - 65 ) located on the downstream side of the discharge roller pair ( 11 ,  13 ), and then is deposited face down into one of the plurality (five in this embodiment) of face-down trays (trays  51 - 55 ), that is, the tray correspondent to the selected face-down delivery opening. For example, as a command is issued to discharge a sheet S into the uppermost face-down tray  51 , a flapper  81  is moved from a position outlined by a solid line in FIG. 1 to a position. outlined by a double dot chain line in FIG. 1, whereby the sheet S is cumulatively delivered into the face-down tray  51  by a delivery roller pair  71  disposed adjacent to the uppermost delivery opening  61 ; the sheet S is cumulatively delivered, with the printed surface facing downward (in the order in which a sheet S is produced) as shown in FIG.  1 . 
     Referring again to FIG. 1, a reference numeral  19  designates a guide member, which guides a sheet S to the nip of the take-in roller pair ( 11 ,  12 ) after the sheet S is delivered from a delivery opening  9  of the image forming apparatus  1 . The guide member  19  is pivotable about the rotational axis of the conveyer roller  11 , so that it can be aligned with the sheet delivery portion (position of discharge opening, nip of delivery roller pair, or the like) of an image forming apparatus. Therefore, the sheet processing apparatus in this embodiment can accommodate various image forming apparatuses which are different in the location from which a sheet S is delivered. 
     At this point in time, a case in which a plurality of sheets S are successively fed into the sheet processing apparatus, and are sequentially accumulated in the face-down tray  51  will be described. As the trailing end of the sheet S is detected by the sensor ( 17 ,  18 ), the flapper  14  is moved downward, as described above, with such timing that the sheet S begins to be guided by the flapper  14  toward the discharge roller pair ( 11 ,  13 ) as soon as the trailing end of the sheet S comes out of the nip of the take-in roller pair ( 11 ,  12 ). At the same time, the reverse conveyer roller pair ( 15 ,  16 ) pinches the sheet S, and begins to convey it to the discharge roller pair ( 11 ,  13 ). 
     Referring again to FIG. 1, the flapper  14  is structured so that its tip overlaps with the conveyer roller  11  as seen from the axial direction of the conveyer roller  11 . Therefore, the sheet S can be efficiently conveyed by the friction which is caused between the conveyer roller  11  and the sheet S by the pressure from the flapper  14 . Further, the flapper  14  functions as a guide for smoothly guiding the sheet S into the nip of the discharge roller pair ( 11 ,  13 ). It should be noted here that when the flapper  14  is disposed in such a manner that as the flapper  14  is pivoted to the bottom side position (position outlined by double dot line), the tip of the flapper  14  goes down below the rotational axis of the conveyer roller  11 , and therefore, the sheet S can be more smoothly guided to the nip of the discharge roller pair ( 11 ,  13 ) without allowing the leading end (trailing end before inversion) of the sheet S to strike the conveyer roller  11 . 
     More specifically, after coining out of the nip of the take-in roller pair ( 11 ,  12 ), the trailing end of the sheet S is pressed against the conveyer, roller  11  by the tip of the pivotable flapper  14 , and therefore, even after it comes out of the nip of the discharge roller pair ( 11 ,  13 ), it is still conveyed in the same direction as the direction in which it came out of the nip, by the friction between the conveyer roller  11  and itself. This conveyance of the sheet S lasts until the friction between the conveyer roller  11  and the sheet S disappears, that is, until the trailing end of the sheet S passes through the contact area between the conveyer roller  11 , and the tip of the flapper  14  located at the bottom side position (position outlined by the double dot chain line). As the trailing end of the sheet S comes out of the aforementioned contact area, the flapper  14  comes down further. The trailing end of the sheet S clears the conveyer roller  11  before the sheet S begins to be pinched and conveyed in the reverse direction by the reverse conveyer roller pair ( 15 ,  16 ), and therefore, it is smoothly guided to the nip of the discharge roller pair ( 11 ,  13 ). 
     As the leading end (trailing end before reversing) of the sheet S is pinched by the discharge roller pair ( 11 ,  13 ), the flapper  14  is pivoted upward, separating the slave roller  15  from the reverse conveyor roller  16 . At this moment, the leading end of the following sheet is pinched by the take-in roller pair ( 11 ,  12 ) to be guided to the reversing path  24 . Therefore, the following sheet is guided to the temporary discharge opening  24   a  along the top surface (printed surface) of the preceding sheet S, without coming in contact with the reverse conveyor roller  16 . As the trailing end (leading end before reversing) of the preceding sheet cores out of the nip of the discharge roller pair ( 11 ,  13 ), the trailing end of the following sheet is detected by the sensor ( 17 ,  18 ). Then, the aforementioned sheet movement reversing operation is started again as soon as the trailing end of the following sheet comes out of the nip of the take-in roller pair ( 11 ,  12 ). 
     Thus, the sheets which are continuously taken in can be reliably conveyed at a high speed in the direction reverse to their incoming direction, making it possible to provide a sheet processing apparatus which is particularly suitable for a high speed image forming apparatus. Further, the conveyor roller  11  or the reverse conveyor roller  16  is continuously rotated only in one direction; in other words, the complicated driving mechanism and driving control system, which are necessary in a conventional apparatus to rotate the rollers forward or backward, are unnecessary. Therefore, the present invention can provide an inexpensive apparatus. 
     Next, referring to FIG. 1, the mechanism for sorting sheets into predetermined face-down trays will be described in detail in terms of its structure. In this embodiment, the present invention is described with reference to a sheet processing apparatus which has five face-down trays as illustrated in FIG.  1 . However, the application of the present invention is not limited to the apparatus in this embodiment; the number of the trays may be adjusted as necessary. 
     First, referring to FIG. 1, the general structure of the sheet processing apparatus  10  will be described. In FIG. 1, each of reference numerals  51 - 55  designates a delivery tray (face-down tray), in which the sheets discharged from the delivery opening are cumulatively held. Each of reference numerals  71 - 75  designates a delivery roller pair as a sheet delivery means, which is disposed for each delivery tray to deliver a sheet into the tray. A reference numeral  30  designates a common sheet path, through which the sheets taken in after a recording operation are vertically conveyed to a predetermined sorting portion (portion at which a path leading to one of the trays branches off from the common sheet path). It is a sheet conveying path which leads from the sheet entrance of the sheet processing apparatus  10  to the delivery roller pair  75  located most downstream, and is substantially parallel to the direction in which the trays are aligned. Along the common sheet path  30 , conveyer roller pairs  31 - 34  as conveying means are disposed with predetermined intervals A sheet is vertically (downward direction in FIG. 1) conveyed through the common sheet path  30  by these conveyer roller pairs  31 - 34 . Each of reference numerals  81 - 84  designates a flapper, an pivotable member, which guides a sheet to one of the delivery roller pairs  71 - 74 , which is optionally selectable by the user. The rotational centers of the flappers  81 - 84  are located on the sheet delivery opening side, relative to the common sheet path  30 , and are pivotable between a position at which they do not block the common sheet path (position outlined by the solid line in the drawing), and a position at which they block the common sheet path  30  (position outlined by the double dot chain line in the drawing). The conveyer roller pairs  31 - 34  are disposed immediately after (downstream side) the correspondent flappers  81 - 84 , relative to the sheet conveyance direction in the common sheet path  30 . 
     Next, the flapper movement during a sheet sorting operation will be described. Each of reference numerals  36  and  37  designates a solenoid, as means for moving the flapper, which selectively pivots the flappers  81 - 84 . The solenoid  36  pivots the flappers  81  and  83 , and the solenoid  37  pivots the flappers  82  and  84 . They are independently fixed to the frame (unillustrated) of the sheet processing apparatus  10 . Reference numerals  38  and  39  reach designates a link, as connecting means. They are connected to the moving portions of the solenoid  36  and  37 , respectively, to be moved with the moving portion of the solenoids. They are movable only in the vertical direction FIG.  1 . Reference numerals  40  and  41  each designates a tension spring, one end of which is anchored to the hook portion of the link, and the other end of which is anchored to a hook portion integrally formed with the apparatus frame. Reference numerals  42 - 45  designate a tension spring as an elastic member. The tension springs  42  and  43  connect the hook portions of the link  38  which is connected to the solenoid  36 , to the hook portions of the flappers  81  and  83 , respectively, and the tension springs  44  and  45  connect the hook portions of the link  39  which is connected to the solenoid  37 , to the hook portions of the flappers  82  and  84 , respectively. 
     FIG. 1 depicts a state in which the solenoids are off. When the solenoids  36  and  37  are off, the links  38  and  39  remain at the bottom positions due to the tensional force of the tension spring. In this state, the flappers  81 - 84  are at the positions (position outlined by the solid line in the drawing) at which they do not block the common sheet path  30 . This is because the projections  81   b ,  82   b ,  83   b  and  84   b , which are formed so as not to intrude into the sheet path, are pushed downward by the end portion of the links  38  and  39 , that is, the and portions adjacent to the hook portions, and therefore, the flappers  81 - 84  are pivoted in the counterclockwise direction about the axes  81   a ,  82   a ,  83   a  and  84   a . More specifically, as the solenoid  36  (or  37 ), for example, is turned on, the link  38  (or  39 ) is pulled up (in the direction indicated the arrow mark ),and therefore, the flappers  81  and  83  (or  82  and  84 ) are pivoted in the clockwise direction by the tensional force of the tension spring  42  and  43  (or  44  and  45 ) to be moved to the position (position outlined by the broken line) at which they block the common sheet-path  30 . 
     Next, a series of operations which are carried out by the apparatus while a sheet is guided to the sheet delivery opening  64  by the fourth flapper  84  will be described. A sheet, an which an image has been recorded by the image forming apparatus  1 , is transferred to the sheet processing apparatus  10 . After the direction in which the sheet is conveyed is reversed, the leading end (trailing end before reversing) is detected by the sensor ( 46  and  47 ) at the entrance to the common sheet path  30 . Then, the sheet is conveyed downward from the top end of the common sheet path  30  at a predetermined speeds by the conveyer roller pairs  31 ,  32  and  33 . Based on the detection signal from the sensor ( 46 ,  47 ), the solenoid  37  is turned on with such timing that allows the leading end of the sheet to reach the conveyer roller pair  33  located immediately after the third flapper  83 . As the solenoid  37  is turned on, force is applied to the link  39  in the direction to pull it up. Consequently, force is applied to the flappers  82  and  84  in the direction to pivot them in the clockwise direction. However, the tensional force of the tension spring  45 , which connects the second flapper  82  and the link  39 , is set to be sufficiently greater than the resistive force which opposes the upward pivoting of the flapper  82 , but less than the bending resistance of a sheet. Therefore, only as the tip of the flapper  82 , which is in the process of pivoting to the position at which the second flapper  82  blocks the common sheet path  30 , comes in contact with a sheet which is being conveyed through the common sheet path  30 , the tension spring  45  is stretched by the sheet whose bending resistance is greater than the tensional force of the tension spring  45 . As a result, the flapper  82  is stopped, remaining in contact with the sheet, at a position at which it has come in contact with the sheet, without blocking the common sheet path  30 . On the other hand, the fourth flapper  84  is pivoted to the position at which it blocks the common sheet path  30 . Then, as the sheet is conveyed further downward by the conveyer roller pair  33  disposed along the common sheet path  30 , it collides with the flapper  84 . The direction in which force is applied to the flapper  84  by the leading and of the sheet when the sheet collides with the flapper  84  is such that the flapper  84  is pivoted in the clockwise direction. Therefore, the sheet is reliably guided toward the fourth delivery roller pair  74 , by which the sheet is delivered into the fourth delivery tray  64 . During the above series of operations, a sheet rubs against the free end portion of the flapper  82 . However, the free end portion (portion which comes in contact with a sheet) of each flapper is rounded as illustrated in the drawing, and therefore, a sheet is smoothly conveyed downward, rubbing against the free end portion of the flapper without becoming hung up on it, by the conveyer roller pair located immediately after the flapper. 
     A detailed description will not be given here. However, as is illustrated in FIG. 3, the same description as the above can also be said about the series of operations through which a sheet is guided to the sheet delivery opening  63  by the third flapper  83 . As for the tensional forces of the tension springs  42 - 44 , they are set to be the same as the tensional force described above. As for the shapes of the free end portions of the flappers  81 ,  83  and  84 , they are also rounded as is that of the flapper  82 . 
     In the case of an apparatus structured as described above, in order to discharge a sheet into the bottom most delivery tray  55 , it is only necessary to move none of the flappers (to keep solenoid  36  and  37  in the OFF state). A sheet is guided to a sheet delivery opening  65  through the common sheet path  30 , and is cumulatively discharged into the delivery tray  55  by the delivery roller pair  75 . 
     Further, in a case in which an apparatus is structured to move two flappers by a single solenoid so that a sheet is guided toward the deliverer roller side by the third or fourth flapper as described above, the sheet rubs against the first or second flapper as it is conveyed. In order to prevent a sheet from becoming restrained as it rubs against the first or second flapper, each of the conveyer roller pairs  31 - 34  disposed along the common sheet path  30  is to be provided with sheet conveying force sufficiently greater than the aforementioned frictional resistance. 
     As described above, according to this embodiment, the flappers  81 - 84  are connected by the tension springs  42 - 45 , to the links  38  and  39  which are connected to the solenoids  36  and  37 , respectively. Therefore, even though the plurality of flappers are moved by the single solenoid, a sheet is not restrained by the flappers other than the one which has been selected to sort the sheet (flapper which blocks common sheet path). Therefore, the cost and size increase of a sheet processing apparatus, which occurs as the number of delivery trays is increased, can be minimized as much as possible. As a result, it becomes possible provide a highly reliable sheet processing apparatus capable of stably, conveying a sheet. 
     In order to sort a sheet with the downstream side flapper, relative to the sheet conveyance direction, while pivoting two adjacent flappers by a single solenoid as in the second and third embodiments which will be described later, the flapper movement must be completed between the time when a sheet P leaves the conveyer roller pair located after the preceding flapper, and the time when the sheet P reaches the flapper which is to sort the sheet P. 
     In a case in which the sheet processing apparatus  1  is connected to an image forming apparatus whose sheet conveying speed is fast, the pivoting of the flapper sometimes fails to be completed within the above described period. In order to prevent the occurrence of such an incident, the two flappers which are pivoted by the same solenoid are alternately disposed. 
     According to the fourth embodiment, two flappers can be pivoted by a single solenoid even when the sheet conveying speed of an image forming apparatus is high. 
     Embodiment 2 
     Referring to FIGS. 4 and 5, the sheet processing apparatus in the second embodiment will be described. FIGS. 4 and 5 are enlarged sections of the essential portion of the sheet processing apparatus in this embodiment. Since the general structure of the apparatus is substantially the same as that in the first embodiment, the members and portions having the same functions as those in the first embodiment are designated by the same referential symbols, and their detailed descriptions will be omitted here. 
     Referring to FIG. 4, the structure of the sheet processing apparatus in this embodiment is substantially the same as the structure described in the first embodiment. They are different only in how the solenoids  36  and  37  are combined with the flappers  81 - 84  which are to be pivoted by the solenoid  36  or  37 . More specifically, the first and second flappers  81  and  82  are pivoted by the solenoid  36 , and the third and fourth flappers  83  and  84  are pivoted by the solenoid  37 . 
     In the case of a sheet processing apparatus structured as described above, in order to deliver a sheet into the fourth tray, the solenoid  37  is turned on to move the link  39 , and the flapper  83  and  84  are pivoted at the same time (to the position at which they block the common sheet path  30 ). During this operation, as described in the preceding embodiment, the fourth flapper  84  pivots to the position (outlined by a solid line in FIG. 3) at which it blocks the common sheet path  30 , whereas the third flapper  83  stops as it comes in contact with a sheet which is being conveyed, being pinched, by the conveyer roller pair  33 . Then, the sheet is conveyed further by the conveyer roller  33 , being guided by the fourth flapper  84 , and is delivered into the fourth delivery tray  54  by the fourth delivery roller  74  illustrated in FIG.  5 . 
     In other words, in the first embodiment described above, a plurality of flappers to be moved at the same time by a single solenoid are alternate ones, whereas in this embodiment, a plurality of flappers to be moved at the same time by a single solenoid are consecutive ones, which also can provide the same effects as those described in the preceding embodiment. 
     Next, referring to FIG. 5, the requirements for the flappers movable by the same solenoid will be described. The requirements will be described with reference to a case in which the third and fourth flappers  83  and  84  are moved by the same solenoid  37  (FIG.  4 ). In FIG. 5, an alphabetic reference L stands for the measurement of the smallest conveyable sheet, relative to the sheet conveyance direction, and an alphabetic reference H stands for the interval between two smallest conveyable sheets S 1  and S 2  which are being consecutively conveyed. The distance between a position A, to which the flapper  83 , that is, the one located on the upstream side, of the flappers  83  and  84  moved by the solenoid  37 , is moved to block the common sheet path  30 , and a position B, that is, the position of the delivery roller  74  at which a sheet arrives after it is guided toward the sheet delivery opening by the flapper  84  located on the downstream side, is designated by an alphabetic reference M. In this case, the positional relationship among the flappers movable by the same solenoid must be such that the sheet conveyance distance M becomes less than (L+H), that is, the sum of the length L of the smallest conveyable sheet and the minimum sheet interval H. In other words, in this embodiment, the flappers  83  and  84  movable by the solenoid  37  are disposed in a manner to satisfy the above described positional relationship. 
     Embodiment 3 
     Referring to FIGS. 6 and 7, the sheet processing apparatus in the third embodiment of the present invention will be described. FIG. 6 is an enlarged section of the essential portion of the sheet processing apparatus in this embodiment, and FIG. 7 is a graph depicting the characteristics of a tension spring. Since the general structure of the entire apparatus is substantially the same as that in the first embodiment described above, the members and portions having the same functions as those in the first embodiment are given the same referential symbols, and their detailed description will be omitted here. 
     Referring to FIG. 6, the sheet processing apparatus in this embodiment is substantially the same in structure as that in the first embodiment described above. It is different in the configurations of the links  38  and  39  moved by the solenoids  36  and  37 , in the configurations of the flappers  81 - 84 , and in the characteristic of the tension springs  42 - 45 . 
     More specifically, referring to FIG. 7, the installation of the tension springs  42 - 45  is such that their tensional forces become zero when L, a distance by which they are elongated, is La; when L is less than La, compressive force is generated, and when L is greater than La, tensile force is generated. The flappers  81 - 84  are connected to the links  38  and  39  by the tension springs  42 - 45 . Referring to the flapper  61 , when the solenoid  36  is off, this flapper  81  stops at a predetermined position outlined by a solid line, with the elongation of the tension spring  42  being La, and when the solenoid  36  is on, the flapper  81  stops at a predetermined position outlined by a broken line, with the elongation of the tension spring  42  remaining at La. The provision of stoppers  90  and  91  at the predetermined stopping positions, respectively, makes the flapper  81  stop more accurately. 
     Although this is not illustrated, when the stoppers  90  and  91  are provided in the manner described above, similar stoppers are provided at the stopping position for each of the flappers  82 - 84 . 
     Next, the sorting operation of a sheet processing apparatus structured as described above will be described. In this embodiment, the flappers  81 - 84  remain pivotable in both directions regardless of solenoid activation. More specifically, when the flapper  81  is pivoted while a sheet is in the common sheet path  30 , its rotation stops due to the tensional force as the tip of the flapper  81  contacts the sheet. Further, when the solenoid is turned off as soon as the leading end of the sheet begins to be guided toward the sheet discharge opening, the flapper  81  rotates in the counterclockwise direction, and stops as the tip of the flapper  81  contacts the sheet, due to the compressive force of the tension spring  42 . Here, even though the flapper movement is described with reference to the flapper  81 , the same can be said about the movements of the flappers  82 - 84 . 
     With the flappers  81 - 84  being retained at neutral positions by the correspondent tension springs  42 - 45 , the flappers  81 - 84  have to be moved only when the leading end of a sheet is immediately before the flappers. Therefore, the structural restriction, which is required in the first embodiment in terms of sheet size and the like (flapper position, conveyance path length, and the like), can be eliminated. In other words, it becomes unnecessary to elaborately change the flapper moving timing. 
     Further, since the flapper solenoid can be turned off as soon as a sheet is guided into one of the sheet delivery branches, solenoid duty may be reduced. In other words, it becomes possible to employ inexpensive solenoids. Therefore, it is possible to provide an apparatus with far smaller cost. 
     Embodiment 4 
     Referring to FIGS. 8 and 10, the sheet processing apparatus in the fourth embodiment will be described. FIG. 8 is an enlarged section of the essential portion of the sheet processing apparatus in this embodiment, and FIGS. 9 and 10 are sectional drawings which depict the conditions of a spring disposed in a link. Since the general structure of the apparatus is substantially the same as the first embodiment, the members and portions having the same functions are designated by the same referential symbols, and their detailed descriptions will be omitted here. 
     Referring to FIG. 8, the structure of the sheet processing apparatus in this embodiment is substantially the same as that in the first embodiment described above. It is different only in the configurations of links  101  and  102  which are moved by the solenoids  36  and  37 , respectively, the configurations of flappers  121  and  124 , and the characteristics of springs  111 - 114 . 
     Referring to FIG. 9, the link  101  comprises a compression spring  111 , and a cap  115  which holds one end of the compression spring  111 . One end  121   a  of the flapper  121  is fitted in an engagement hole  101   a  of the link  101  with the provision of a gap (play) t. Although this is not illustrated here, the structures for engaging the link  101  with the flapper  122 , and the link  102  with the flappers  123  and  124  are the same as the structure for engaging the link  101  with the flapper  121 . 
     The state illustrated in FIG. 9 is a state in which the solenoid  36  is off. It can also be a state in which the solenoid  36  is on; the flapper  121  is blocking the common sheet path  30 ; and no sheet is in the common sheet path  30 . The state illustrated in FIG. 10 is a state in which the solenoid  36  is on, a sheet is in the common sheet path  30 , and the flapper  121  is in contact with the sheet in the common sheet path  30 . With the provision of the above described structure, as the solenoid  36  is turned on, and the link  101  moves in the direction of an arrow mark a, the end  121   a  of the flapper  121  is lifted by the cap  115  as illustrated in FIG.  9 . As a result, the flapper  121  is pivoted clockwise to a position at which it blocks the common sheet path  30 . Regarding this movement of the flapper  121 , when a sheet is in the common sheet path  30 , the flapper  121  comes in contact with the sheet, and stops after pressing down the cap  115  as shown in FIG.  10 . 
     Also in the case of the above described structure, the end  121   a  of the flapper  121  is fitted in the engagement hole  101   a  of the link  101  with the provision of the play t. Therefore, even if the solenoid  36  is turned off after a sheet is guided into one of the sheet delivery paths by the flapper  121 , and the link  101  moves downward, the flapper  121  is allowed to remain where it is (at the position to which it has been moved by turning on the solenoid  36 ). More specifically, the end  121   a  of the flapper  121  moves to the uppermost end (position outlined by a broken line in FIG. 9) of the engagement hole  101   a  of the link  101 , allowing the flapper  121  to remain blocking the common sheet path  30 . As a result, the solenoid  36  can be turned off after a sheet begins to be guided by the flapper  121 . Therefore, solenoid duty can be reduced. In other words, it is possible to employ inexpensive solenoids, which makes it possible to provide ah apparatus with far lower cost. 
     Embodiment 5 
     FIG. 11 is a schematic section of the sheet path of the sheet sorting apparatus of the fifth embodiment of the present invention. 
     The structure illustrated in this drawing is the same as the structure described in the first embodiment, except that the configurations of the flappers  31 ,  32 ,  33  and  34  are different. As for the referential symbols, they are the same as the first embodiment except for those for the flappers. 
     The movement of the flapper  33  to discharge a sheet P into a delivery tray  4  will be described. 
     The flapper  33  comprises a guide portion  33   d , and a retaining portion  33   c . The guide portion  33   d  guides the downward facing surface of the sheet P, The retaining portion  33   c  is located at the downstream end of the guide portion  33   d , relative to the sheet conveyance direction, and is disposed so as to be on the top surface side of the sheet P. The guide portion  33   d  and the retaining portion  33   c  are connected to each other outside the sheet path. 
     The sheet conveyance speed of a delivery roller pair  13  is set to be faster by a predetermined amount than that of a conveyer roller pair  28 . 
     In order to deliver a sheet P into the third tray  4 ; the solenoid is turned on as soon as the leading end of the sheet P reaches the conveyer roller  28 , so that the flapper  33  is pivoted to guide the sheet P toward the delivery roller  13 . The sheet P is conveyed between the guide portion  33   d  and the retaining portion of the flapper  33 . 
     As soon as the leading end of the sheet P is pinched by the nip of the delivery roller pair  13 , the sheet P is pulled toward the delivery side due to the difference in sheet conveyance speed between the delivery roller pair  13  and the conveyer roller pair  28 . As a result, the retaining portion  33   c  of the flapper  33  is subjected to such force that is generated by the tension of the sheet P in the direction to pushed up the sheet P; the flapper  33  is subjected to such force that works to pivot the flapper  33  in the clockwise direction about an axis  33   a . In this condition, even if the solenoid is turned off, the flapper  33  maintains the position at which it guides the sheet P toward the delivery roller pair  13 . 
     In the first embodiment, while a sheet P is guided toward the delivery roller side by a flapper, the solenoids  17  and  18  must be kept in the ON state. However, according to this embodiment, the solenoid may be turned off as soon as the leading end of the guided sheet reaches a delivery roller pair. 
     Therefore, the power application time for a solenoid can be shortened. 
     Embodiment 6 
     FIGS. 12 and 13 are schematic sections of the sheet path portion of the sheet sorting apparatus in the sixth embodiment of the present invention. 
     In the case of the sheet sorting apparatus  1  illustrated in FIG. 12, two flappers which are moved by the same solenoid are flappers  81  and  82 , which are adjacent to each other, and flappers  83  and  84 , which also are adjacent to each other, as illustrated by a broken line. In the case of the sheet sorting apparatus  1  illustrated in FIG. 13, two flappers which are moved by the same solenoid are flappers  81  and  83 , which are alternately positioned, and flappers  82  and  84 , which also are alternately positioned, as indicated by a broken line. Also, at least one conveyer roller pair is necessary between the flappers moved by the same solenoid, as described in the first embodiment. 
     First, referring to FIG. 12, the positioning of the conveyer roller pair in an apparatus in which consecutively positioned flappers are moved by the same solenoid will be described. 
     A conveyer roller pair  31  is disposed along the common sheet path  30 , between the first and second flappers  81  and  82  which are moved by the first solenoid. In order to guide a sheet toward the sheet delivery opening by the second flapper  82 , the pivoting of the flappers  81  and  82  must be completed between the time when a sheet leaves the conveyer roller pair  31 , and the time when the sheet reaches the flapper  82 . Therefore, the conveyer roller pair  31  is disposed as close as possible to the first flapper  81 . Similarly, the conveyer roller pair  32  is disposed between the third and fourth flappers  82  and  84 , as close as possible to the third flapper  83 . 
     Next, referring to FIG. 13, the positioning of the conveyer roller pair in an apparatus in which alternately positioned flappers are moved by the same solenoid will be described. 
     The conveyer roller pair  31  is disposed along the common sheet path  30 , between the first and third flappers  81  and  83  which are moved by the first solenoid. In order to guide a sheet toward the sheet delivery opening by the third flapper  83 , the pivoting of the third flapper  83  must be completed between the time when the sheet leaves the conveyer roller pair  31  and when it reaches the third flapper  83 . Therefore, the conveyer roller pair  31  is disposed as close as possible to the first flapper  81 . 
     When the conveyer roller pair  31  is on the upstream side of the second flapper, the distance from the conveyer roller pair  31  to the third flapper  83 , that is, the distance through which a sheet is conveyed when the sheet is sorted by the third flapper, is long. Therefore, even when sheet conveyance speed is fast, the pivoting of the third flapper  83  can be reliably completed while the sheet is conveyed to the sorting portion of the third flapper after it reaches the conveyer roller pair  31 . 
     Similarly, the conveyer roller pair  32  is disposed between the second and fourth flappers  82  and  84  which are moved by the second solenoid, immediately after the second flapper  82 . 
     With the provision of the above structure, even when the measurement of the smallest conveyable sheet in the sheet conveyance direction is greater than the maximum distance between the nips of adjacent two conveyer roller pairs or adjacent two delivery roller pairs, the number of the conveyer roller pairs can be rendered smaller than that of the flappers. 
     Miscellaneous embodiments 
     In the preceding embodiments, the present invention was exemplified with reference to a sheet processing apparatus which moves two flappers at the same time with the use of a single solenoid, but the present invention is not to be limited by those embodiments. For example, three or more flappers (FIG. 14) may be moved at the same time using a single solenoid. Further, a unit comprising two flappers and a single solenoid, and a unit comprising three flappers and a single solenoid, may be mixedly employed. 
     Further, in the preceding embodiments, the present invention was exemplified by a sheet processing apparatus in which a spring such as a tension spring or a compression spring was employed as an elastic member. However, the present invention is not to be limited by those embodiments. For example, other elastic members such as rubber may be employed. 
     Further, the present invention is not to be limited to the embodiments described above. Links as connecting means, springs as elastic members, flappers as a pivotable member, may be modified in configuration, and also, their combination may be modified, to obtain the same effects. 
     Also in the embodiments described above, the present invention was exemplified by a sheet processing apparatus capable of sorting sheets into a plurality of optionally selected delivery trays. However, the present invention is not to be limited by those embodiments; the present invention is effectively applicable to a sheet processing apparatus provided with additional processing means such as a stapler capable of performing a stapling operation or the like on a set of sheets sorted into the delivery trays. 
     Further, in the embodiments described above, the present invention was exemplified by a sheet processing apparatus employed by an image forming apparatus in which an image is formed on the top surface of a sheet. However, the present invention is also effectively applicable to a sheet processing apparatus employed by an image forming apparatus in which an image is formed on the bottom surface of a sheet. In the latter case, a delivery tray into which a sheet is delivered without being turned over constitutes a face-down tray, and a tray into which a sheet is delivered after being turned over constitutes a face-up tray. 
     Further, in the embodiments described above, an image forming apparatus to which the present invention is applicable was exemplified by a copying machine combined with a reader or the like. However, the present invention is not to be limited by those embodiments. For example, an image forming apparatus may be in the form of a facsimile apparatus having a function to transmit or receive data, or an image outputting peripheral apparatus for an information processing device such as a computer. The same effects as those in the preceding embodiments can be also obtained by applying the present invention to a sheet processing apparatus employed in these image forming apparatuses. 
     Further, in the embodiments described above, a recording system employed by an image forming means was exemplified by an electrophotographic system. However, the present invention is not to be limited by those embodiments. For example, a recording system may be an ink jet system, a thermal transfer system, a thermal system, a wire dot system, or any other recording system. The same effects as those in the preceding embodiments can be obtained by applying the present invention to a sheet processing apparatus employed in an image forming apparatus which adopts one of these recording systems. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.