Patent Publication Number: US-8528895-B2

Title: Sheet feeder and image forming apparatus with the same

Description:
This application is based on and claims the benefit of priority from Japanese Patent Application Ser. No. 2011-016264 filed with Japan Patent Office on Jan. 28, 2011, the contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The present disclosure relates to a sheet feeder for feeding a sheet such as a document to a predetermined document reading position and an image forming apparatus with the same. 
     An image forming apparatus such as a copier includes a sheet feeder for conveying a document sheet such as a document with an image to a predetermined reading position, an image reader for reading a document image of a document sheet at the reading position to obtain image data, and an image forming unit for forming an image based on image data on a recording sheet. 
     The sheet feeder includes, as basic constituent elements, a pickup roller for feeding document sheets placed on a sheet tray one by one to a conveyance path, a feed roller for further feeding a document sheet picked up by the pickup roller to the conveyance path, a plurality of conveyor roller pairs for conveying a document sheet to a document reading position and a cover unit holding these members while allowing the rotation thereof. 
     Such a sheet feeder also includes a stopper. The stopper is a member arranged between the pickup roller and the feed roller and comes into contact with the leading end in a feeding direction of a document sheet placed on the sheet tray to restrict the position of the leading end in the feeding direction, i.e. align the position of the leading end in the feeding direction before a feeding operation by the pickup roller. By aligning the position of the leading end in the feeding direction, it is suppressed that the document sheet is fed in an oblique posture to the conveyance path at the time of the feeding operation by the pickup roller. 
     In a conventional sheet feeder using such a stopper, the stopper is coupled to a drive shaft of a pickup roller via a one-way clutch. When a document sheet is placed on a sheet tray and the leading end thereof in a feeding direction comes into contact with the stopper, a load of a drive system acts on the stopper via the one-way clutch, wherefore the stopper can specify the position of the leading end in the feeding direction. On the other hand, when the drive shaft of the pickup roller rotates, the load of the drive system acting on the stopper is released by the action of the one-way clutch and the stopper is pushed up (rotated) by a pressing force given by the document sheet being fed, thereby being separated from the leading end in the feeding direction. In this way, the document sheet is fed to the conveyance path. 
     In the above sheet feeder constructed as described above, when a jam occurs due to a feeding operation by the pickup roller or the feed roller, a user can remove the jammed document sheet by opening the cover unit and exposing the conveyance path to the outside. However, if the user does not know about the procedure of such a jam process, he tries to grab a part (e.g. trailing end in the feeding direction) of a jammed document sheet and forcibly pull out the jammed sheet from the conveyance path if the document sheet is jammed and the trailing end in feeding direction remains on the sheet tray. Thus, the stopper is likely to be caught by the document sheet being pulled out. If the stopper is caught by the document sheet, the document sheet may be torn. 
     Accordingly, in view of the above situation, an object of the present disclosure is to provide a sheet feeder capable of suppressing document sheet breakage even if a jammed document sheet is forcibly pulled out and an image forming apparatus with the same. 
     SUMMARY 
     To achieve this object, a sheet feeder as one aspect of the present disclosure has a feeding mechanism, a supporting body and a stopper member. 
     The feeding mechanism feeds a sheet (S) along a sheet feeding route. The stopper member ( 532 ) is located in the sheet feeding route and is supported on the supporting body ( 531 ) rotatably about a first rotational pivot point. The stopper member ( 532 ) includes a contact end portion ( 538 ); and the stopper member ( 532 ) changes its posture among (i) a restricting posture ( FIG. 8 ) in which the contact end portion ( 538 ) comes into contact with a leading end of the sheet in a feeding direction to restrict the position of the leading end in the feeding direction before a feeding operation of feeding the sheet by the feeding mechanism; (ii) a feed allowing posture ( FIG. 9 ) which is assumed during the feeding operation and in which the contact end portion ( 538 ) is separated from the position of the leading end of the sheet in the feeding direction in the restricting posture in a first direction (F) in which the sheet (S) is fed; and (iii) a rotational posture ( FIG. 10 ) reached from the feed allowing posture ( FIG. 9 ) by the rotation of the stopper piece ( 532 ) about the first rotational pivot point so that the contact end portion ( 538 ) faces in a second direction (R) opposite to the first direction (F). 
     The second direction (R) is a direction in which the sheet is pulled out from the feeding mechanism when a jam occurs in the feeding mechanism. 
     According to the sheet feeder of the present disclosure, the posture of the stopper member is changed among the restricting posture, the feed allowing posture and the rotational posture. The stopper member assumes the restricting posture to restrict the position of the leading end of the sheet in the feeding direction by the contact end portion before the feeding operation by the feeding mechanism. Further, since the stopper member assumes the feed allowing posture, in which the contact end portion is separated from the leading end in the feeding direction, during the feeding operation, it does not interfere with the feeding operation. 
     Further, when a jam occurs in the feeding mechanism, the stopper member rotates about the first rotational pivot point from the feed allowing posture to assume the rotational posture so that the contact end portion faces in the second direction in which the sheet is pulled out. This suppresses that the sheet is caught by the stopper member. As a result, a pressure received by the stopper member from the sheet being pulled out is reduced and breakage of the document sheet is suppressed. 
     These and other objects, features and advantages of the sheet feeder of the disclosure will become apparent upon reading the following detailed description along with the accompanied drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing the external appearance of an image forming apparatus according to one embodiment of the present disclosure, 
         FIG. 2  is a perspective view showing the external appearance of an automatic document feeder, 
         FIG. 3  is a sectional view showing the internal structure of the image forming apparatus, 
         FIG. 4  is a sectional view of an essential part of the automatic document feeder, 
         FIG. 5  is a perspective view enlargedly showing a part of a document feeder unit of the automatic document feeder, 
         FIG. 6  is a perspective view of the document feeder unit when viewed from below, 
         FIG. 7  is a perspective view of an upper cover unit of the automatic document feeder when viewed from below, 
         FIG. 8  is a side view diagrammatically showing a stopper mechanism in a state where a stopper member (stopper piece) of the stopper mechanism is in a restricting posture, 
         FIG. 9  is a diagram showing a state where the stopper member (stopper piece) is in a feed allowing posture, and 
         FIG. 10  is a diagram showing a state where the stopper member (stopper piece) is in a rotational posture. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an embodiment of the present disclosure is described in detail with reference to the drawings.  FIG. 1  is a perspective view showing the external appearance of an image forming apparatus  1  according to one embodiment of the present disclosure,  FIG. 2  is a perspective view showing the external appearance of an automatic document feeder  3 , and  FIG. 3  is a sectional view showing the internal structure of the image forming apparatus  1 . Although a copier of an internal discharge type is illustrated as the image forming apparatus  1  here, the image forming apparatus may be a printer, a facsimile machine, or a complex machine provided with these functions. 
     The image forming apparatus  1  includes an apparatus main body  2  having a substantially rectangular parallelepipedic housing structure and including an internal space (internal discharging portion  24 ), the automatic document feeder  3  (sheet feeder) arranged on the upper surface of the apparatus main body  2  and an extension sheet feeder unit  4  assembled at a lower side of the apparatus main body  2 . 
     The apparatus main body  2  performs an image forming process on a sheet. The apparatus main body  2  includes a substantially rectangular parallelepipedic lower housing  21 , a substantially rectangular parallelepipedic upper housing  22  arranged above the lower housing  21 , and a coupling housing  23  coupling the lower housing  21  and the upper housing  22 . Various devices for image formation are housed in the lower housing  21 , and various devices for optically reading a document image are housed in the upper housing  22 . An internal space enclosed by the lower housing  21 , the upper housing  22  and the coupling housing  23  serves as an internal discharge portion  24  capable of storing a sheet after image formation. The coupling housing  23  is arranged at a side of the right surface of the apparatus main body  2  and provided with a discharge opening  961  for discharging a sheet to the internal discharge portion  24 . 
     The internal space utilized as the internal discharge portion  24  is exposed to the outside at the front surface and the left surface of the apparatus main body  2 . A user can take out a sheet after image formation from the internal discharge portion  24  by inserting his hand through these exposed parts. A bottom surface  241  of the internal space is defined by the upper surface of the lower housing  21 , and sheets discharged from the discharge opening  961  are stacked thereon. 
     An operation panel unit  25  is provided to project from the front surface of the upper housing  22 . The operation panel unit  25  is provided with operation keys  251  including a numerical keypad and a start key, an LCD touch panel  252 , etc. and receives input of various operation instructions from the user. The user can input the number of sheets to be printed, print density, etc. by means of the operation panel unit  25 . 
     A sheet cassette  211  for storing recording sheets on which an image forming process is to be performed is mounted in the lower housing  21 . The extension sheet feeder unit  4  also includes sheet cassettes  41 ,  42  for storing recording sheets on which the image forming process is to be performed. These sheet cassettes  211 ,  41  and  42  are provided for automatic sheet feeding and a large number of recording sheets can be stored according to sizes. Further, the sheet cassettes  211 ,  41  and  42  can be withdrawn forward from the front surface of the lower housing  21  or the extension sheet feeder unit  4 . Note that only the sheet cassette  211  of the lower housing  21  is drawn in  FIG. 3 . 
     A multi-tray unit M enabling the user to manually feed a sheet is mounted on the right surface of the apparatus main body  2 . The multi-tray unit M includes a feed tray  43 , on which a recording sheet to be manually fed is to be placed, and a feeding unit  44  for feeding the recording sheet to an image forming station in the lower housing  21 . The feed tray  43  is openably and closably mounted on the lower housing  21  at a lower end portion thereof and is in a closed state when not used. The user opens the feed tray  43  and places a recording sheet thereon in the case of manually feeding the sheet. 
     The automatic document feeder  3  is rotatably mounted on the rear side of the upper surface of the apparatus main body  2 . Note that this automatic document feeder  3  is not shown in  FIG. 3 . The automatic document feeder  3  automatically feeds a document sheet to be copied toward a predetermined document reading position (position where a first contact glass  222  is mounted) in the apparatus main body  2 . On the other hand, when the user manually places a document sheet on a predetermined document reading position (arrangement position of a second contact glass  223 ), the automatic document feeder  3  is opened upward. 
     With reference to  FIG. 2 , the automatic document feeder  3  includes a main housing  30 , a document feed tray  31 , a document conveying unit  32 , a document discharge tray  33  and a document reversing tray  31 B. The main housing  30  is a housing for housing various mechanisms provided in the automatic document feeder  3  and includes a front wall portion  301  and a rear wall portion  302  raised upward at the left side where the document conveying unit  32  is housed and a substantially flat low-level part on the right side. 
     The document feed tray  31  is a tray on which a document sheet to be fed to the image reading position is to be placed, and attached to the main housing  30  in such a manner as to extend from a feed opening  30 H of the main housing  30 . The document feed tray includes a pair of cursors  311  for aligning the width of a placed document sheet. 
     The document conveying unit  32  includes a conveyance path and a conveying mechanism for conveying a document sheet on the document feed tray  31  to the document discharge tray  33  via the image reading position. The document conveying unit  32  includes an upper cover unit  32 U fitted in an opening between the front wall portion  301  and the rear wall portion  302  of the main housing  30 . The cover unit  32 U is openable and closable relative to the main housing  30 . These are described in detail later based on  FIG. 4 , etc. 
     The document discharge tray  33  is a tray to which a document sheet is discharged after a document image thereof is optically read. The upper surface of the low-level part on the right side of the main housing  30  serves as the document discharge tray  33 . The document reversing tray  31 B is a tray to which a document sheet is temporarily discharged in reading the document sheet including document images on both sides. 
     Next, the internal construction of the apparatus main body  2  is described based on  FIG. 3 . Toner containers  99 Y,  99 M,  99 C and  99 K, an intermediate transfer unit  92 , an image forming station  93 , an exposure unit  94  and the above sheet cassette  211  are housed in this order from top in the lower housing  21 . 
     The image forming station  93  includes four image forming units  10 Y,  10 M,  10 C and  10 K for forming toner images of yellow (Y), magenta (M), cyan (C) and black (K) to form a full-color toner image. Each of the image forming units  10 Y,  10 M,  10 C and  10 K includes a photosensitive drum  11 , and a charger  12 , a developing device  13 , a primary transfer roller  14  and a cleaner  15  arranged around the photosensitive drum  11 . 
     The photosensitive drum  11  rotates about its shaft and has an electrostatic latent image and a toner image formed on its circumference surface. A photosensitive drum using an amorphous silicon (a-Si) containing material can be used as the photosensitive drum  11 . The charger  12  uniformly charges the circumferential surface of the photosensitive drum  11 . The circumferential surface of the photosensitive drum  11  after charging is exposed to light by the exposure unit  94  to form an electrostatic latent image. 
     The developing device  13  supplies toner to the circumferential surface of the photosensitive drum  11  to develop an electrostatic latent image formed on the photosensitive drum  11 . The developing device  13  is for a two-component developer and includes agitating rollers  16 ,  17 , a magnetic roller  18  and a developing roller  19 . The agitating rollers  16 ,  17  charge the toner by conveying the two-component developer in a circulating manner while agitating it. The two-component developer is carried on the circumferential surface of the magnetic roller  18 , and the toner is transferred to the circumferential surface of the developing roller  19  due to a potential difference between the magnetic roller  18  and the developing roller  19 , whereby a toner layer is formed and carried on the circumferential surface of the developing roller  19 . The toner on the developing roller  19  is supplied to the circumferential surface of the photosensitive drum  11 , thereby developing the electrostatic latent image. 
     The primary transfer roller  14  forms a nip portion together with the photosensitive drum  11  with an intermediate transfer belt  921  of the intermediate transfer unit  92  sandwiched therebetween, and primarily transfers the toner image on the photosensitive drum  11  to the intermediate transfer belt  921 . The cleaner  15  cleans the circumferential surface of the photosensitive drum  11  after the transfer of the toner image. 
     The yellow toner container  99 Y, the magenta toner container  99 M, the cyan toner container  99 C and the black toner container  99 K are respectively for storing toners of the respective colors, and supply the toners of the respective colors to the developing devices  13  of the image forming units  10 Y,  10 M,  10 C and  10 K corresponding to the respective YMCK colors via unillustrated supply paths. 
     The exposure unit  94  includes a light source and various optical components such as a polygon mirror, a reflecting mirror and a deflecting mirror, and irradiates the circumferential surfaces of the photosensitive drums  11  provided in the respective image forming units  10 Y,  10 M,  10 C and  10 K with beams based on image data of a document image to form electrostatic latent images. 
     The intermediate transfer unit  92  includes the intermediate transfer belt  921 , a drive roller  922  and a driven roller  923 . Toner images from a plurality of photosensitive drums  11  are superimposed on the intermediate transfer belt  921  (primary transfer). The superimposed toner images are secondarily transferred to a recording sheet supplied from the sheet cassette  211  in a secondary transfer unit  98 . The drive roller  922  and the driven roller  923  for rotationally driving the intermediate transfer belt  921  are rotatably supported on the lower housing  21 . 
     The sheet cassette  221  ( 41 ,  42 ) stores a sheet stack composed of a plurality of recording sheets stacked one over another. A pickup roller  212  is arranged above the right end of the sheet cassette  211 . By driving the pickup roller  212 , the uppermost recording sheet of the sheet stack in the sheet cassette  211  is picked up one by one and conveyed to a carry-in conveyance path  26 . On the other hand, a recording sheet manually placed on the feed tray  43  is conveyed to the carry-in conveyance path  26  by driving a feed roller  45  of the feeding unit  44 . 
     A sheet conveyance path  28  extending up to the discharge opening  961  via the secondary transfer unit  98 , a fixing unit  97  and a discharge unit  96  to be described later is provided downstream of the carry-in conveyance path  26 . An upstream part of the sheet conveyance path  28  is formed between an inner wall formed in the lower housing  21  and an inner wall forming the inner side surface of a reversing unit  29 . Note that an outer side surface of the reversing unit  29  constitutes one surface of a reversing conveyance path  291  for reversing and conveying a sheet at the time of duplex printing. A pair of registration rollers  27  are arranged at a position of the sheet conveyance path  28  upstream of the secondary transfer unit  98 . The sheet is temporarily stopped by the pair of registration rollers  27  and fed to the secondary transfer unit  98  at a predetermined timing for image transfer after a skew correction. 
     The fixing unit  97  and the discharge unit  96  are housed in the coupling housing  23 . The fixing unit  97  includes a fixing roller and a pressure roller and performs a fixing process by heating and pressing a recording sheet having a toner image secondarily transferred in the secondary transfer unit  98 . The recording sheet with the fixed color image is discharged from the discharge opening  961  toward the internal discharge portion  24  by the discharge unit  96  arranged downstream of the fixing unit  97 . 
     The first contact glass  222  and the second contact glass  223  are embedded in the upper surface of the upper housing  22 . The first contact glass  222  is provided for reading a document sheet automatically fed by the automatic document feeder  3 . The second contact glass  223  is provided for reading a manually placed document sheet. 
     A scanning mechanism  224  and an image pickup device  225  for optically reading document information of a document sheet are housed in the upper housing  22 . The scanning mechanism  224  includes a light source, a moving carriage, a reflecting mirror, etc. and introduces reflected light from a document to the image pickup device  225 . The image pickup device  225  photoelectrically converts the reflected light into an analog electrical signal. The analog electrical signal is input to the exposure unit  94  after being converted into a digital electrical signal in an A/D conversion circuit (not shown). 
     Next, the internal structure of the automatic document feeder  3  is described in detail based on  FIGS. 4 to 7 .  FIG. 4  is a sectional view showing an essential part (document conveying unit  32 ) of the automatic document feeder  3 . The document conveying unit  32  includes first to fifth conveyance paths  341  to  345  constituting a conveyance route for document sheets, first to fifth conveyor roller pairs  351  to  355  arranged at suitable positions of these first to fifth conveyance paths  341  to  345 , and a document feeder unit  5  for feeding a document sheet placed on the document feed tray  31  into the document conveying unit  32 .  FIG. 5  is a perspective view enlargedly showing the document feeder unit  5 ,  FIG. 6  is a perspective view of the document feeder unit  5  when viewed from below and  FIG. 7  is a perspective view of the upper cover unit  32 U described above when viewed from below. 
     The first, second and third conveyance paths  341 ,  342  and  343  constitute a U-shaped conveyance path extending from the above feed opening  30 H to a discharge opening  30 E, through which a document sheet is discharged to the document discharge tray  33 , via a reading position X where a document image is optically read. On the other hand, the fourth and fifth conveyance paths  344 ,  345  are switchback conveyance paths used to reverse a document sheet in reading the document sheet having document images on both sides. 
     The first conveyance path  341  is a conveyance path which is continuous with the document feed tray  31  and extends leftward and slightly downward from the feed opening  30  H and in which a document sheet fed from the document feeder unit  5  first passes. An upper conveying surface of this first conveyance path  341  is defined by a guide member  321  (see  FIG. 7 ) of the upper cover unit  32 U. The second conveyance path  342  is an arcuate conveyance path extending from the downstream end of the first conveyance path  341  to the document reading position X facing the first contact glass  222 . One conveying surface of this second conveyance path  342  is also defined by the guide member  321  of the upper cover unit  32 U. The third conveyance path  343  is a conveyance path extending rightward and slightly upward from the position facing the first contact glass  222  to the discharge opening  30 E. Note that a contact guide  36  to bring a document sheet into sliding contact with the first contact glass  222  is arranged at the position facing the first contact glass  222 . 
     The fourth conveyance path  344  is a conveyance path branched off from the third conveyance path  343  and extending upward and rightward. A switching lever  37  is arranged at a position where the third and fourth conveyance paths  343 ,  344  are branched. The switching lever  37  guides a document sheet to the third conveyance path  343  in the case of normal one-side reading while guiding a document sheet to the fourth conveyance path  344  when the document sheet having one side read needs to be reversed upside down in the case of reading both sides of the document sheet. The fifth conveyance path  345  is a substantially horizontal conveyance path communicating with the fourth conveyance path  344 , the first conveyance path  341  and the document reversing tray  31 B and used to receive the document sheet to be reversed upside down from the fourth conveyance path  344  and switch back and convey it to the first conveyance path  341 . 
     Each of the first, second, third, fourth and fifth conveyor roller pairs  351 ,  352 ,  353 ,  354  and  355  is composed of a combination of a drive roller  351 A,  352 A,  353 A,  354 A or  355 A for generating a rotational driving force for conveying the document sheet and a driven roller  351 B,  352 B,  353 B,  354 B or  355 B held in contact with the drive roller to be driven and rotated. 
     The first conveyor roller pair  351  is arranged between the first and second conveyance paths  341 ,  342  and feeds a document sheet toward the largely curved second conveyance path  342 . The second conveyor roller pair  352  is arranged right upstream of the document reading position X and feeds the document sheet to this document reading position X. The third conveyor roller pair  353  is arranged right downstream of the document reading position X and feeds the document sheet after image reading to the third or fourth conveyance path  343  or  344 . The fourth conveyor roller pair  354  is arranged near the discharge opening  30 E and discharges the document sheet toward the document discharge tray  33 . The fifth conveyor roller pair  355  is composed of a pair of rollers which can rotate in forward and reverse directions, arranged in the fifth conveyance path  345 , and switches back and conveys the document sheet utilizing the document reversing tray  31 B. 
     The document feeder unit  5  includes a pickup roller  51  (roller member), a document feed roller  52  arranged downstream of the pickup roller  51  in a sheet conveying direction, stopper mechanisms  53  for restricting a document sheet placed on the document feed tray  31 , a holder  50  for holding these members, a driving mechanism  56  (pivoting unit) for pivoting the holder  50 , and a torsion coil spring  57  for applying a torque to the holder  50 . As shown in  FIG. 7 , the document feeder unit  5  is mounted in the upper cover unit  32 U. The automatic document feeder  3  further includes an unillustrated motor for applying a rotational driving force in a forward or reverse direction to the driving mechanism  56 . 
     As shown in  FIGS. 5 and 6 , the holder  50  is a box-shaped member including an upper plate  500  in the form of a flat plate, a front plate  501 , a rear plate  502  and a middle plate  503  made of rib members integral to the upper plate  500 . A front tubular portion  504  and a rear tubular portion  505 , which are coaxially arranged, project from the front plate  501  and the rear plate  502 . The holder  50  pivots about tube centers of the front and rear tubular portions  504 ,  505 . 
     The pickup roller  51  has a torque applied thereto to rotate about its axis and feeds document sheets placed on the document feed tray  31  one by one to the document conveying unit  32  (first conveyance path  341 ). A rotary shaft of the pickup roller is rotatably supported at the right sides of the rear and middle plates  502 ,  503 . The pickup roller  51  shifts its position between a feeding position where it is in contact with the upper surface of the document sheet on the document feed tray  31  and a retracted position where it is spaced upward from the upper surface of the document sheet by a pivotal movement of the holder  50  about the tube centers of the front and rear tubular portions  504 ,  505 . 
     As shown in  FIG. 5 , a separation pad  313  is arranged at a position facing the pickup roller  51  at a downstream end  312  of the document feed tray  31 . When the pickup roller  51  is at the feeding position, a nip portion is formed between the pickup roller  51  and the separation pad  313 . 
     The document feed roller  52  conveys one document sheet fed from the pickup roller  51  further to the first conveyance path  341 . The rotary shaft  521  of the document feed roller  52  is rotatably supported by the front and rear plates  501 ,  502  of the holder  50 . In feeding a document sheet, a rotational driving force is applied to the rotary shaft  521  and the document feed roller  52  rotates. Note that, as shown in  FIG. 4 , a driven roller  350  is arranged to face this document feed roller  52  in the main housing. The front and rear tubular portions  504 ,  505  of the holder  50  described above are mounted rotatably about the axis of this rotary shaft  521 . That is, the axial center of the rotary shaft  521  and the tube centers of the front and rear tubular portions  504 ,  505  are coaxial, wherefore the document feed roller is not vertically moved even if the holder  50  pivots, and constantly forms the sheet feeding nip portion together with the driven roller  350 . 
     The stopper mechanisms  53  are located between the pickup roller  51  and the document feed roller  52  in a lateral direction. The stopper mechanisms  53  are for aligning the leading end of a document sheet in a feeding direction by restricting the leading end in the feeding direction before the pickup roller  51  starts a feeding operation. By aligning the leading end in the feeding direction, it is suppressed that the document sheet is fed in an oblique posture to the first conveyance path  341 . The construction of the stopper mechanisms  53  is described in detail later. 
     A first wheel  541  having a multitude of grooves formed in the outer circumferential surface is fixed to the rotary shaft of the pickup roller  51 . Further, a second wheel  542  including similar grooves is fixed to the rotary shaft  521  of the document feed roller  52  (see  FIG. 6 ). These first and second wheels  541 ,  542  are respectively arranged at positions behind the pickup roller  51  and the document feed roller  52 . An endless belt  55  (transmission mechanism) for power transmission is mounted between the first and second wheels  541 ,  542 . A multitude of projections engageable with the grooves of the first and second wheels  541 ,  542  are formed on the inner circumferential surface of the endless belt  55 . When a rotational driving force in a direction to feed the document sheet (rotational driving force in a forward direction; rotational driving force in a clockwise direction when viewed from front) is applied to the rotary shaft  521  of the document feed roller  52 , this rotational driving force is transmitted to the rotary shaft of the pickup roller  51  via the endless belt  55 . As a result, the pickup roller  51  and the document feed roller  52  are both rotated in synchronization. 
     The driving mechanism  56  is a mechanism for transmitting the rotational driving force of the motor in the forward or reverse direction to the rotary shaft  521  of the document feed roller  52 . The driving mechanism  56  includes a coupling portion  561 , a shaft  562 , a drive input portion  563  and a pressure spring  564 . 
     The coupling portion  561  is a part to be engaged with the rotary shaft  521  and has a tubular shape for receiving the rotary shaft  521 . A groove portion  561 A extending in an axial direction of the rotary shaft  521  is formed in the tubular wall of the coupling portion  561 . On the other hand, a pin  522  projects from the peripheral wall of the rotary shaft  521 , and the above engagement is achieved by fitting this pin  522  into the groove portion  561 A. The pressure spring  564  biases the coupling portion  561  forward to make the engagement of the groove portion  561 A with the pin  522  reliable. 
     A rotational driving force is applied to the drive input portion  563  from the motor via an unillustrated gear mechanism. The coupling portion  561 , the shaft  562  and the drive input portion  563  are united, so that the coupling portion  561  rotates when the drive input portion  563  is rotated and a rotational driving force thereof is transmitted to the rotary shaft  521 . This causes the pickup roller  51  and the document feed roller  52  to rotate. 
     Since the front and rear tubular portions  504 ,  505  of the holder  50  is mounted on the rotary shaft  50  as described above, the holder  50  can rotate about the axial center of the rotary shaft  521 . Thus, when a rotational driving force in the forward direction is applied to the rotary shaft  521 , a moment in the clockwise direction acts on the holder  50 . As a result, the holder  50  rotates about the axial center of the rotary shaft  521  in the clockwise direction and the pickup roller  51  moves to the feeding position to come into contact with the upper surface of a document sheet placed on the document feed tray  31 . On the contrary, when a rotational driving force in the reverse direction (rotational driving force in a counterclockwise direction when viewed from front) is applied to the rotary shaft  521 , a moment in the counterclockwise direction acts on the holder  50 . As a result, the holder  50  moves to the retracted position to be spaced upward from the upper surface of the document sheet. Note that  FIGS. 4 and 5  show a state where the pickup roller  51  is at the retracted position. 
     A coiled part of the torsion coil spring  57  is mounted on the rotary shaft  521 , and the torsion coil spring  57  biases the holder  50  to maintain the pickup roller  51  at the retracted position. A biasing force of the torsion coil spring  57  is set to be smaller than a moment in the clockwise direction acting on the holder  50  when a rotational driving force in the forward direction is applied to the rotary shaft  521 . Accordingly, when the rotary shaft  521  rotates in the forward direction when a document sheet is fed, the holder  50  rotates in the clockwise direction about the axial center of the rotary shaft  521  against the biasing force of the torsion coil spring  57 . On the other hand, when a rotational driving force in the reverse direction is applied to the rotary shaft  521  and the pickup roller  51  moves to the retracted position, the posture of the holder  50  at that time is maintained by the biasing force of the torsion coil spring  57 . At the retracted position, the holder  50  rests on a ceiling plate  320  of the upper cover unit  32 U. 
     Next, the stopper mechanism  53  is described with reference to  FIG. 8  as well as  FIGS. 4 and 5 .  FIG. 8  is a side view diagrammatically showing the stopper mechanism  53  in a state where the stopper mechanism  53  (stopper piece) is in a restricting posture to be described later. The stopper mechanisms  53  are respectively mounted on the outer surfaces of the front and rear plates  501 ,  502  of the holder  50  to be located between the pickup roller  51  and the document feed roller  52 . Since the respective stopper mechanisms  53  have the same construction, the stopper mechanism  53  mounted on the front plate  501  is described here. 
     The stopper mechanism  53  includes a supporting body  531  and a stopper piece  532 . The supporting body  531  is set to have a substantially triangular shape in a front view, and one side thereof includes a semicylindrical rotational pivot portion (hereinafter, referred to as a second rotational pivot portion  533 ) projecting rightward. The second rotational pivot portion  533  includes a rotational pivot point (second rotation pivot point) of the supporting body  531 . A shaft portion  508  projects from the front plate  501  of the holder  50 . The shaft portion  508  is located between the pickup roller  51  and the document feed roller  52  when viewed from front. The supporting body  531  is rotatable about the second rotation pivot point by being supported on the shaft portion  508  via the second rotational pivot portion  533 . A part of the one side of the supporting body  531  above the second rotational pivot portion  533  serves as a contact portion  535 . 
     The supporting body  531  further includes a hanging piece  536  substantially perpendicularly hanging down from the bottom side. The hanging piece  536  has a pair of front and rear surfaces facing in forward and backward directions, wherein a rotation support shaft  536 A projecting forward is formed on the front surface and a rotation support shaft  536 A (see  FIG. 7 ) projecting backward is also formed on the rear surface. 
     A rotation preventing piece  537  is integrally formed to the hanging piece  536 . The rotation preventing piece  537  has front and rear surfaces facing in forward and backward directions. The rotation preventing piece  537  is set to be wider than the hanging piece  536  when viewed from right or left. The position of the rotation preventing piece  537  is set to be above the rotation support shaft  536 A of the hanging piece  536 . Although described later, the rotation preventing piece  537  comes into contact with the stopper piece  532  to prevent the clockwise rotation of the stopper piece  532  when viewed from front. 
     The stopper piece  532  is a long and narrow member arranged in the sheet conveyance route. The sheet conveyance route is a route along which a document sheet is conveyed successively by the pickup roller  51 , the document feed roller  52  and the first to third conveyor roller pairs  351  to  353  as is clear from the description given using  FIG. 4 . The stopper piece  532  includes a rotation pivot portion (hereinafter, referred to as a first rotation pivot portion  534 ) and a contact end portion  538 . Note that since it is not necessary to limit the stopper piece  532  to a long and narrow member in carrying out the present disclosure, the stopper piece  532  is called a “stopper member” in claims of the present application, which means that the shape of the stopper piece  532  is not limited to a long and narrow shape. 
     The first rotation pivot portion  534  includes a rotation pivot point (first rotation pivot point) of the stopper piece  532 . The first rotation pivot portion  534  is set at a position of the stopper piece  532  near one end portion and includes a pair of front and rear walls  539 ,  540  (see  FIG. 5 ) spaced apart in forward and backward directions. A distance between the front and rear walls  539 ,  540  is set to be larger than a distance between the front and rear surfaces of the hanging piece  536 , and the hanging piece  536  is accommodated in a space between the front and rear walls  539 ,  540 . The front and rear walls  539 ,  540  are respectively formed with coaxial through holes  534 H, and a pair of rotation support shafts  536 A of the hanging piece  536  are inserted into the corresponding through holes  534 H. In this way, the stopper piece  532  is free to rotate about the first rotation pivot point relative to the supporting body  531 . 
     Further, the distance between the front and rear walls  539 ,  540  is set to be smaller than a distance between the front and rear surfaces of the rotation preventing piece  537  so that the rotation preventing piece  537  cannot be accommodated in the space between the front and rear walls  539 ,  540 . Thus, rightward facing surfaces of the respective front and rear walls  539 ,  540  serve as contact-stop surfaces  539 A,  540 S to be brought into contact with the rotation preventing piece  537 . When it is attempted to rotate the stopper piece  532  clockwise when viewed from front, the contact-stop surface  539 S is stopped by the contact with the rotation preventing piece  537 . In this way, the clockwise rotation of the stopper piece  532  is restricted. 
     As just described, the stopper piece  532  is allowed to rotate counterclockwise when viewed from front, whereas the clockwise rotation thereof is restricted by the rotation preventing piece  537 . 
     The other end portion of the stopper piece  532  serves as the contact end portion  538 . This contact end portion  538  comes into contact with the leading end in the feeding direction of a document sheet placed on the document feed tray  31  to restrict the position of the leading end in the feeding direction before the pickup roller  51  starts the feeing operation of feeding the document sheet. The contact end portion  538  includes a restricting surface  538 S which comes into contact with the leading end in the feeding direction. 
     The stopper piece  532  of the stopper mechanism  53  constructed as described above can change its posture among a restricting posture, a feed allowing posture and a rotational posture.  FIG. 8  described above is a diagram showing a state where the stopper piece  532  is in the restricting posture,  FIG. 9  is a diagram showing a state where the stopper piece  532  is in the feed allowing posture, and  FIG. 10  is a diagram showing a state where the stopper piece  532  is in the rotational posture. The respective postures of the stopper piece  532  are described below with reference to  FIGS. 8 to 10 . 
     (Restricting Posture) 
     The stopper piece  532  assumes the restricting posture to restrict the position of a leading end SU of a document sheet S in the feeding direction. As shown in  FIG. 8 , the stopper piece  532  assumes the restricting posture when the pickup roller  51  is in a retracted posture in which it is spaced from the document sheet S by a pivoting movement of the holder  50 . When the stopper piece  532  is in the restricting posture, the stopper piece  532  projects toward the downstream end  314  of the document feed tray  31  and the restricting surface  538 S of the contact end portion  538  serves as a wall surface extending perpendicularly to the document sheet S placed on the document feed tray  31 . This enables the restricting surface  538 S of the contact end portion  538  to come into contact with the leading end SU of the document sheet S in the feeding direction. Thus, the position of the leading end SU of the document sheet S in the feeding direction is restricted and the leading end SU in the feeding direction is aligned. As a result, it is suppressed that the document sheet S is fed in an oblique posture by the pickup roller  51 . 
     The restricting posture of the stopper piece  532  is maintained by a first contact piece  322  (first contact portion: see  FIG. 4 ). The first contact piece  322  is formed on the ceiling plate  320  of the upper cover unit  32 U. The holder  50  rests on the ceiling plate  320  when the pickup roller  51  is in the retracted posture. The position of the first contact piece  322  is so set that the first contact piece  322  can come into contact with the contact portion  535  of the supporting body  531  of the stopper mechanism  53  with the holder  50  resting on the ceiling plate  320 . 
     If a user places a document sheet S on the document feed tray  31  to bring a leading end SU in the feeding direction into contact with the contact end portion  538  when the stopper piece  532  is in the restricting posture, a pressing force of the document sheet S acts on the contact end portion  538 . The pressing force of the document sheet S acts to rotate the stopper piece  532  and consequently the supporting body  531  in the clockwise direction (first rotational direction) when viewed from front. However, since the first contact piece  322  comes into contact with the contact portion  535  of the supporting body  531  at this time, the clockwise rotation of the supporting body  531  is prevented. In this way, the restricting posture of the stopper piece  532  is maintained. 
     (Feed Allowing Posture) 
     The stopper piece  532  changes its posture from the restricting posture shown in  FIG. 8  to the feed allowing posture when the pickup roller  51  assumes a feeding posture in which it is in contact with a document sheet S by a pivoting movement of the holder  50  as shown in  FIG. 9 . When the holder  50  pivots so that the pickup roller  51  assumes the feeding posture, i.e. when the right end of the holder  50  is lowered, the contact portion  535  of the supporting body  531  of the stopper mechanism  53  is separated from the first contact piece  322  of the ceiling plate  320 . In this way, the clockwise rotation of the supporting body  531  is allowed. When the feed of the document sheet S is started by the rotation of the pickup roller  51 , the stopper piece  532  is pushed up by the leading end SU of the document sheet S to be fed in the feeding direction to rotate clockwise and the contact end portion  538  is separated from the leading end SU in the feeding direction. In this way, restriction of the leading end SU in the feeding direction by the contact end portion  538  is released. The clockwise rotation of the stopper piece  532  is made possible by the supporting body  531  allowed to rotate clockwise. At this time, the contact end portion  538  is facing in a direction (first direction F) to convey the document sheet S from the pickup roller to the document feed roller  52  to such a degree as not to interfere with the feed of the document sheet S. When the document sheet S is conveyed, the contact end portion  538  is located in the sheet conveyance route and the document sheet S is conveyed while sliding in contact with the contact end portion  538 . 
     The front plate  501  of the holder  50  is formed with a second contact piece  506  (second contact portion: see  FIG. 5 ). The position of the second contact piece  506  on the front plate  501  is set to be near the supporting body  531  and closer to the pickup roller  51  than the supporting body  531  when viewed from front. Further, the second contact piece  506  extends in a vertical direction of the front plate  501 . The height of the second contact piece  506  in the vertical direction is so set that the upper edge of the second contact piece  506  comes into contact with the contact portion  535  of the supporting body  531  if the supporting body  531  rotates clockwise beyond a predetermined rotation range when the stopper piece  532  is in the feed allowing posture. 
     Accordingly, when the stopper piece  532  is pushed up by the document sheet S and the supporting body  531  tries to rotate clockwise beyond the predetermined rotation range, the second contact piece  506  comes into contact with the contact portion  535  of the supporting body  531 . Since this prevents the supporting body  531  from rotating more than necessary, the feed allowing posture of the stopper piece  532  is stabilized. 
     (Rotational Posture) 
     In this embodiment, the stopper piece  532  can further assume the rotational posture shown in  FIG. 10  from the feed allowing posture shown in  FIG. 9 . The stopper piece  532  can assume the rotational posture when a document sheet S is jammed in the document conveying unit  32  (sheet conveyance route), while being in the feed allowing posture shown in  FIG. 9 . 
     In the automatic document feeder  3  of this embodiment, when the document sheet S is jammed, the jammed document sheet S can be removed by setting the upper cover unit  32 U (see  FIG. 4 ) in the open state and exposing the first and second conveyance paths  341 ,  342  to the outside. 
     However, if the user does not know about the procedure of the above jam process, he tries to grab a part (e.g. trailing end in the feeding direction) of the jammed document sheet and forcibly pull out the jammed sheet from the first conveyance path  341  or the second conveyance path  342  if the trailing end in the feeding direction remains on the document feed tray  31 . By the user forcibly pulling out the document sheet S, the stopper piece  532  in the feed allowing posture may be caught by the document sheet S to tear the document sheet S. 
     The posture change of the stopper piece  532  from the feed allowing posture to the rotational posture in this embodiment solves the above problem. That is, if the document sheet S is jammed in a state where the pickup roller  51  is in the feeding posture and the stopper piece  532  is in the feed allowing posture as shown in  FIG. 9 , the user who does not know about the procedure of the above jam process tries to grab a part (trailing end part in the feeding direction) of the document sheet S remaining at the outer side of the pickup roller  51  and pull out the document sheet S in a direction (second direction R) opposite to the first direction F. The pickup roller  51  is subjected to an upward acting pressing force from the document sheet S due to the operation of pulling out the document sheet S in the second direction R. If this pressing force is transmitted to the holder  50  via the pickup roller  51 , the holder  50  starts pivoting and the pickup roller  51  starts moving upward. That is, the pickup roller  51  starts assuming the retracted posture. 
     At this time, since the contact end portion  538  of the stopper piece  532  held in contact with the jammed document sheet S is facing in the first direction F, a tensile force to move the contact end portion  538  in the second direction R acts on the contact end portion  538  of the stopper piece  532  by the operation of pulling out the document sheet S in the second direction R. By this tensile force, a force to rotate the stopper piece  532  counterclockwise acts on the stopper piece  532 . The stopper piece  532  is free to rotate about the first rotational pivot point as described above. Thus, if a tensile force acts on the contact end portion  538  of the stopper piece  532 , the stopper piece  532  rotates counterclockwise about the first rotational pivot point and the contact end portion  538  faces in the second direction R as shown in  FIG. 10 . In this way, the stopper piece  532  assumes the rotational posture. The rotation range of the contact end portion  538  is so set that the contact end portion  538  faces in the second direction R. With the contact end portion  538  facing in the second direction R, the stopper piece  532  extends substantially in parallel with the document sheet S and the surface of the stopper piece  532  opposite to the restricting surface  538 S is in contact with one side of the document sheet S. Therefore, a pressure (tensile force) received by the stopper piece  532  from the document sheet S being pulled out is reduced. In this way, breakage of the document sheet S is suppressed. 
     Note that the supporting body  531  rotatably supporting the stopper piece  532  about the first rotational pivot point also rotates counterclockwise (second rotational direction opposite to the first rotational direction) as the stopper piece  532  rotates (as the posture is changed to the rotational posture). Specifically, when a tensile force acts on the contact end portion  538  of the stopper piece  532 , this tensile force is transmitted to the supporting body  531  via the stopper piece  532 . The supporting body  531  is free to rotate about the second rotational pivot point as described above. Thus, when the tensile force is transmitted to the supporting body  531 , the supporting body  531  rotates counterclockwise as shown in  FIG. 10  from the state of  FIG. 9 . By the counterclockwise rotation of the supporting body  531 , the pressure of the document sheet S being pulled out is let to successively escape to the stopper piece  532  and the supporting body  531 . In this way, the posture of the stopper piece  532  is smoothly changed to the rotational posture. 
     The rotational posture of the stopper piece  532  is maintained by a third contact piece  507  (third contact portion: see  FIG. 5 ). The third contact piece  507  is formed on the front plate  501  of the holder  50 . The third contact piece  507  is formed at a position immediately below the bottom side of the supporting body  531  and immediately to the right of the rotation preventing piece  537  of the supporting body  531 . That is, the third contact portion  507  is formed at the position to be able to come into contact with the rotation preventing piece  537  when the stopper piece  532  is in the restricting posture. When the supporting body  531  rotates counterclockwise as the posture of the stopper piece  532  is changed to the rotational posture, the third contact piece  507  comes into contact with the rotation preventing piece  537 . This prevents the rotation of the supporting body  531  and maintains the rotational posture of the stopper piece  532 . In this way, the sheet is smoothly pulled out from the feeding mechanism. 
     According to the automatic document feeder  3  of this embodiment described above, the stopper piece  532  is supported on the supporting body  531  rotatably about the first rotational pivot point and the supporting body  531  is supported on the holder  50  rotatably about the second rotational pivot point. That is, in this embodiment, the stopper for restricting the position of the leading end SU of the document sheet S in the feeding direction has a divided construction. Since the stopper piece  532  rotates about the first rotational pivot point to assume the rotational posture when a jam occurs and a document sheet S is pulled out in the second direction R, a pressure (tensile force) received by the stopper piece  532  from the document sheet S being pulled out is reduced. As a result, breakage of the document sheet S is suppressed. In addition, since the supporting body  531  rotates counterclockwise (second rotational direction) as the posture of the stopper piece  532  is changed to the rotational posture, the pressure of the document sheet S being pulled out is let to successively escape to the stopper piece  532  and the supporting body  531 . In this way, the posture of the stopper piece  532  is smoothly changed to the rotational posture. 
     Further, according to the automatic document feeder  3  of this embodiment, the pickup roller  51  assumes the retracted posture to be separated from the document sheet S at a timing when the supporting body  531  rotates counterclockwise and the stopper piece  532  assumes the rotational posture. Thus, the jammed document sheet S is smoothly pulled out from the sheet conveyance route. In this way, it is further suppressed that the document sheet S is caught by the stopper piece  532 . 
     Furthermore, according to the automatic document feeder  3  of this embodiment, the posture change of the stopper piece  532  and that of the pickup roller  51  can be easily synchronized since the supporting body  531  is so supported on the shaft portion  508  of the holder  50  as to be rotatable about the second rotational pivot point. 
     Further, according to the automatic document feeder  3  of this embodiment, the first to third contact pieces  322  to  507  are provided to maintain the postures of the stopper piece  532 . Since the first contact piece  322  maintains the restricting posture of the stopper piece  532 , the position of the leading end SU of the document sheet S in the feeding direction is appropriately restricted before the feeding operation by the pickup roller  51 . Further, since the second contact piece  506  prevents the supporting body  531  from rotating in the first rotational direction beyond the predetermined rotation range, the feed allowing posture of the stopper piece  532  is stabilized. Further, since the third contact piece  507  maintains the rotational posture of the stopper piece  532 , the document sheet S is smoothly pulled out from the sheet conveyance route. 
     Since the image forming apparatus according to the present disclosure uses the sheet feeder capable of suppressing breakage of sheets, the image forming operation can be smoothly performed. 
     Although the sheet feeder as one aspect of the present disclosure has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention hereinafter defined, they should be construed as being included therein.