Patent Publication Number: US-7584954-B2

Title: Sheet feeder device, image reading apparatus and image forming apparatus

Description:
CROSS REFERENCE 
   This Non provisional application claims priority under 35 U.S.C. §119(a) on patent application Ser. No. 2003-171137 filed in Japan on Jun. 16, 2003, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a sheet feeder device for feeding document sheets, an image reading apparatus and an image forming apparatus. 
   Recently, efficient image reading or image forming has become possible by setting, on an image forming apparatus for example, a document reading apparatus configured to automatically feed documents in the form of sheets for successive reading. Further, with increasing speed in reading an image of a document, converting the image into electronic data and forming an image based on the electronic data by virtue of the development of digital technology, it has become possible to process a larger number of document sheets at high speed. Such a recent document reading apparatus allows a very large number of document sheets, for example about 100 to 200 sheets, to be set thereon at a time. Further, the improvement of the document feeder section of the document reading apparatus has made it possible to feed various types of document sheets. 
   The above-described image reading apparatus, which is configured to feed a number of sheets such as document sheets set thereon and read images formed on the sheets, picks up the document sheets stacked on a document tray one by one for image reading. To feed only a single sheet of the document stack to a reading section, use is made of a separating mechanism comprising a combination of a feed roller and a reverse roller. Specifically, when plural sheets are picked up at a time, the lower sheet(s) contacting the reverse roller is returned to the document tray so that only the sheet contacting the feed roller can be fed to a feed path leading to the reading section. 
   In such an arrangement, the reverse roller of the separating mechanism is connected, via a torque limiter, to a driving section for rotating the reverse roller in a direction opposite to the rotating direction of the feed roller. At a load torque working when only a single document sheet is pinched by the separating mechanism, the reverse roller slips relative to the driving section and hence rotates in the direction of movement of the document sheet, thereby allowing the sheet to pass through the separating mechanism. On the other hand, when two or more sheets are to be pinched, the load torque increases and the torque limiter prevents the reverse roller from slipping, so that the reverse roller rotates in the direction opposite to the sheet feed direction. As a result, only a single sheet is allowed to pass through the separating mechanism reliably. 
   Generally, the feed roller confronting the reverse roller and the pick-up roller for paying out sheets are each supported on the apparatus body in a cantilevered fashion so that they can be replaced easily at the time of maintenance or in the case where the function for preventing sheets from being fed as superposed on each other is deteriorated due to friction or the like or any other trouble occurs. (See paragraphs 0023-0025 and FIGS. 1 and 2 of Japanese Patent Laid-Open Publication No. 2002-60066, and paragraphs 0008-0013 and FIG. 4 of Japanese Patent Laid-Open Publication No. HEI 6-144607, for example.) 
   However, for the reasons that the reverse roller contacting the feed roller is generally disposed under the feed roller and the feed roller is supported in a cantilevered fashion, when the reverse roller confronting the feed roller is also supported in a cantilevered fashion as in the aforementioned Japanese Patent Laid-Open Publications No. 2002-60066 and No. HEI 6-144607, the two rollers cannot be held in contact with each other stably, which likely results in lowered sheet separating performance. 
   When the sheet separating performance is lowered, problems such as feeding of sheets as superposed on each other or a jam are likely to occur. For this reason, it is a general practice to support the reverse roller at both ends thereof. However, such a both ends support structure makes attachment or detachment of the roller difficult, thus rendering the maintenance troublesome. 
   SUMMARY OF THE INVENTION 
   A feature of the present invention is to provide a sheet feeder device including a separating section having a reverse roller which is supported at both ends thereof and is easily attachable/detachable for replacement. Another feature of the invention is to provide an image reading apparatus and an image forming apparatus each provided with such a sheet feeder device. 
   According to an embodiment of the present invention, a sheet feeder device comprises: a sheet accommodating section for accommodating a stack of sheets; a sheet pick-up section operative to feed each of the sheets toward a sheet feed path when abutting against the stack; and a sheet separating section having a feed roller and a reverse roller which are located downstream of the sheet pick-up section and biased to abut against each other, wherein: the reverse roller is supported on a rotation support shaft in a manner to allow the reverse roller to be fitted around and removed from the rotation support shaft in an axial direction of the rotation support shaft; the rotation support shaft has opposite ends rotatably supported by a device body via a first support and a second support; and the first support is movable in a direction to move the reverse roller away from the feed roller, while the rotation support shaft is detachable from the first support. 
   In this construction, the reverse roller is supported by the device body at both ends thereof via the first support and the second support and hence can be stably held abutting against the feed roller of the sheet separating section even when the feed roller is supported in a cantilevered fashion. Therefore, the sheet separating function works stably to ensure a satisfactory sheet feeding condition. 
   At the occasion of replacement or checking of the reverse roller or like occasions, the first support supporting the reverse roller is allowed to pivot to separate the reverse roller away from the feed roller. By so doing, the rotation support shaft can be detached from the first support with easy operation and without being hindered by the feed roller. Thereafter, the reverse roller can be easily detached from the rotation support shaft. Thus, the maintenance of the sheet feeder device can be performed easily. 
   In another embodiment of the present invention, the rotation support shaft is supported via a universal joint for swinging movement. This arrangement allows the rotation support shaft to swing when the first support pivots in such a direction as to move the reverse roller away from the feed roller. Therefore, the separation of the reverse roller from the feed roller can be achieved smoothly. 
   In still another embodiment of the present invention, the first support is pivotable relative to the device body, the first support comprising: a first support member pivotally supported by the device body; and a second support member formed to be removably fitted on the first support member and supporting the rotation support shaft, the rotation support shaft being detachable from the second support member when the second support member is detached from the first support member. 
   In this arrangement, the first support member and the second support member of the first support are separable from each other and, hence, the second support member can be detached from the rotation support shaft by separating the second support member from the first support member while keeping the first support member as it is. Therefore, the reverse roller can be pulled out from the rotation support shaft easily, whereby the replacement of the reverse roller can be achieved easily. 
   In still another embodiment of the present invention, the sheet feeder device further comprises a resilient member bridging the first support member and the device body for biasing the reverse roller into abutment against the feed roller at a predetermined pressure. 
   In this arrangement, the resilient force of the resilient member biases the reverse roller to abut against the feed roller at a predetermined pressure. Thus, a stabilized abutting pressure can be obtained and, hence, the sheet separating function can be stabilized. 
   In still another embodiment of the present invention, the first support member includes a release lever portion for separating the reverse roller away from the feed roller, the release lever portion and an openable external cover provided on the device body being cooperatively connected to each other via a coupling lever. 
   In this arrangement, when the external cover is opened, the release lever moves following the movement of the cover via the coupling lever, thereby separating the reverse roller away from the feed roller. Therefore, any additional operation for moving the reverse roller away from the feed roller is unnecessary, so that the replacement of the reverse roller can be achieved with high operability. Therefore, the sheet separating function can easily be prevented from lowering. 
   These and other objects, features and attendant advantages of the present invention will become apparent from the following detailed description of the preferred embodiments to be read with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view showing the construction of a sheet feeder device embodying the present invention; 
       FIG. 2  is a perspective view showing a portion of the sheet feeder device; 
       FIG. 3  is an enlarged perspective view showing a portion of the sheet feeder device; 
       FIG. 4  is a perspective view showing a portion of the sheet feeder device in a state where the second support member is separated from the first support member; 
       FIG. 5  is a view illustrating the structure of a portion of the sheet feeder device; 
       FIG. 6  is a view illustrating a portion of the sheet feeder device in a state where the reverse roller is separated from the feed roller; 
       FIG. 7  is a view illustrating the disassembling of the sheet feeder device for maintenance; 
       FIG. 8  is a view illustrating the construction of an image forming apparatus provided with the sheet feeder device; and 
       FIG. 9  is a perspective view showing the image forming apparatus. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, a sheet feeder device and an image reading apparatus provided with the sheet feeder device according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
     FIG. 1  is a vertical sectional view showing the overall construction of an image reading apparatus  1  according to an embodiment of the present invention. The image reading apparatus  1 , which generally comprises an optical system  2  and an ADF (Automatic Document Feeder)  3  disposed above the optical system  2 , is for use as a scanner of a copying machine or a facsimile apparatus and capable of reading both sides of a document (a sheet in the present invention). 
   The optical system  2  includes a CCD (Charge Coupled Device) reading unit  11  as a first reading section. The optical system  2  performs image reading by using a light source unit  13  and a mirror unit  14  to form an image of the document, which is placed flat on a platen  12 , on the CCD reading unit  11  fixed at a predetermined position. The CCD reading unit  11  includes an imaging lens  11   a , a CCD image sensor  11   b , and the like. 
   The light source unit  13  comprises a light source  13   a , a mirror  13   b  for condensing image reading light from the light source  13   a  onto a predetermined image reading position on the platen  12 , a slit  13   c  allowing only reflected light from the document to pass therethrough, and a mirror  13   d  for changing the direction of the light path of the light having passed through the slit  13   c  by 90°. The mirror unit  14  comprises a pair of mirrors  14   a  and  14   b  for changing the direction of the light path of light from the mirror unit  13  by 180°. When the light source unit  13  moves at a velocity V, the mirror unit  14  moves at a velocity of V/2. The light source unit  13  moves in the direction of arrow  15  (secondary scanning direction) to such positions as indicated by reference characters  13   e  and  13   f  to read the document image. 
   Though not illustrated, the CCD reading unit  11  may be structured to move a unit of an optical reading system for reduced-size reading or actual-size reading comprising at least CCD image sensor  11   b , imaging lens  11   a  and light source  13   a  for scanning at a velocity V. 
   The optical system  2  further includes another platen  16  spaced apart from the glass platen  12  in the secondary scanning direction. The light source unit  13 , when moved to and held at a position below the platen  16 , can read the image on a first surface (hereinafter will be defined as an obverse surface) of a document sheet being transported over the platen  16 . Adjacent the outlet for the document sheet thus transported is provided an ejected sheet tray  17 . 
   The ADF  3  includes a CIS (Contact Image Sensor)  21  as a second reading section and feeds the document sheets stacked on a document tray (sheet accommodating section of the present invention)  22  one by one. The CIS reads the image on a second surface (hereinafter will be defined as a reverse surface) of the document sheet being transported. 
   For this purpose, the ADF  3  further comprises different types of rollers R 1  to R 10 , detectors S 1  to S 7 , curved feed path  23 , and a registration/skew-correction region  24 . The CIS  21  is a contact image sensor comprising an array of image sensors, an array of light guide portions (for example a lens array of selfoc lenses), a light source (an LED array or a fluorescent lamp), and the like. 
   When optical document detector S 1  comprising an actuator S 1   a  and a sensor body S 1   b  detects that a stack of document sheets is set on the document tray  22 , lifting of the document tray  22  is started at a predetermined timing. When a pick-up roller detector S 2  detects that the uppermost sheet of the stack presses up the pick-up roller R 1  which is supported by an arm  25  for up-and-down movement, the lifting of the document tray  22  is stopped temporarily. Thus, a standby state is provided. 
   In the case where a signal to start the image reading is not inputted for a predetermined period after the document tray  22  carrying the document sheets has been held in the standby state following lifting, the document tray  22  is preferably lowered to a predetermined level to stand by at that position for preventing the pick-up roller (sheet pick-up section of the present invention) R 1  from being deformed, although the standby state of the document tray  22  at the lifted position may be maintained. 
   When a signal to start the feeding of the document sheets is inputted, the pick-up roller R 1  is rotated so that the sheets are picked up one by one from the uppermost one of the document stack. On the downstream side of the pick-up roller R 1  are disposed feed roller (separating roller) R 2  and reverse roller R 2   a  which serve as a sheet separating section. The pick-up roller R 1  is supported by the arm  25 , which is pivotally supported by the rotating shaft of the feed roller R 2 . 
   The pick-up roller R 1  becomes abutting against the document by its own weight or by a biasing force. A stopper, which will be described later, prevents the pick-up roller R 1  from lowering more than necessary. The arm  25  is formed with a projection, while the pick-up roller detector S 2 , which comprises a photosensor and the like, detects the height of the pick-up roller R 1  based on the pivoting angle of the arm  25 . 
   In this embodiment, the projection is provided on the arm  25  as noted above for allowing the pick-up roller detector S 2  to detect the position of the pick-up roller S 2 . However, the pick-up roller detector S 2  may be located as spaced apart from the arm  25 . In such a case, the height of the arm  25  may be detected by utilizing a coupler such as a link mechanism. It is to be noted that the document tray  22 , pick-up roller R 1 , feed roller R 2  and reverse roller R 2   a  described above constitute a sheet feeder device  51  according to the present invention. 
   The reverse roller R 2   a , which is provided with a torque limiter, is arranged to confront the feed roller R 2  so that the document sheets are reliably fed separately one by one without feeding document sheets as superposed on each other. Therefore, even when plural document sheets are collectively taken in by the pick-up roller R 1 , only the uppermost sheet intimately contacting the pick-up roller R 1  is fed toward the curved feed path  23  by the combination of the feed roller R 2  and the reverse roller R 2   a.    
   Sheet feed detector S 3 , which comprises an actuator S 3   a  and a sensor body S 3   b , detects whether or not the uppermost document sheet is separately fed properly. Thereafter, each document sheet is guided to the curved feed path  23  on the downstream side at a predetermined timing. 
   Each document sheet is then transported along the curved feed path  23  by means of feed rollers R 3  to R 7 . Sheet feed detector S 4 , which comprises an actuator S 4   a  and a sensor body S 4   b , for detecting the passage of each document sheet through the curved feed path  23 , detects whether or not each document sheet is guided smoothly. The feed path  23  has such a curvature as to allow any kind of document sheets, even the thickest or the stiffest document sheet among the readable documents, to be transported stably. 
   An openable external cover  41  provided on the apparatus body so as to cover the curved feed path  23  and the combination of feed roller R 2  and reverse roller R 2   a . The external cover  41  in an open state is depicted with broken lines. There is also provided a coupling lever  42  for causing the support mechanism of the reverse roller R 2   a  to move cooperatively with the open/close movement of the external cover  41 . Though not shown, the coupling lever  42  may be configured not to move along with the external cover  41  but to be operated from the outside of the apparatus body. 
   Each document sheet outgoing from the curved feed path  23  is transported to the registration/skew-correction region  24 . When the leading edge of each document sheet is detected by sheet feed detector S 5  disposed at a location just before reaching a pair of register rollers R 8  and R 9  located adjacent the outlet of the registration/skew-correction region  24 , the rotation of the register rollers R 8  and R 9  is interrupted. In this state, the document sheet is pressed against the rollers R 8  and R 9  by the feeding force from the upstream side for a predetermined period, whereby registration and skew-correction of the sheet are achieved. The sheet feed detector S 5  comprises an actuator S 5   a  and a sensor body S 5   b.    
   For the registration and skew-correction described above to be achieved properly, the registration/skew-correction region  24 , which extends between the most downstream feed rollers R 6 ,R 7  of the curved feed path  23  and the register rollers R 8 ,R 9 , is so designed that each document sheet S can be kept flat between the pair of rollers R 6 ,R 7  and the pair of rollers R 8 ,R 9  without the possibility of deformation caused by the guide surface of the feed path. 
   It is only necessary for the distance between the pair of rollers R 6 ,R 7  and the pair of rollers R 8 ,R 9  to be set not smaller than the length, extending in the feeding direction, of a document sheet of the smallest size among document sheets which can be handled by the sheet feeder device. Thus, the smaller the trailing edge portion of the document sheet remaining in the curved feed path  23  is, the more smoothly the registration and skew-correction can be performed. 
   After the registration and skew-correction is performed, the feeding of the document sheet is restarted at a predetermined timing and the document sheet is guided to a first read position (traveling document read position) Pos 1  for exposure and scanning of the obverse surface of the document sheet. Subsequently, the document sheet passes through a second read position Pos 2  for exposure and scanning of the reverse surface of the document sheet. The light source unit  13  is disposed to face the first read position Pos 1 , whereas the CIS  21  disposed to face the second read position Pos 2 . 
   The document sheet of which image on the obverse surface or on both of the obverse and the reverse surfaces has been read is ejected by ejecting rollers R 10  and R 11  onto the ejected sheet tray  17  supported on a lateral side of the image reading apparatus  1  at a position lower than the document ejection point. (The ejecting roller R 11  is located on the optical system  2  side.) Sheet ejection detector S 6  comprising an actuator S 6   a  and a sensor body S 6   b  detects the ejection of each document sheet. 
   The operation described above is repeated until the document tray  22  on which document sheets have been set is emptied. All the document sheets having been finished with image reading are successively ejected onto the ejected sheet tray  17 . 
   As the document sheets are fed successively, the height of the document stack on the document tray  22  decreases. Therefore, monitoring is conducted so that, when the height decreases to a certain level, the tray  22  is lifted by an amount corresponding to the amount of a drop of the position of the pick-up roller R 1 , whereby proper positional relationship is maintained between the uppermost sheet of the document stack and the pick-up roller R 1 . For this purpose, the document tray  22  is made pivotable about a supporting point  22   a , while a rib  22   b  provided at an end opposite from the supporting point  22   a  is configured to be pressed up by a lifting plate  31  for up and down movement. 
   An end of the lifting plate  31  opposite from the end provided with the rib  22   b  is fixed to a plate support shaft  32 . The plate support shaft  32  is driven for rotation by a lift motor  61  via a lift mechanism  34  comprising a train of transmission members (gears). 
   A controller, which will be described later, controls the lift motor  61  of the lift mechanism  34  based on an actuating signal from the pick-up roller detector S 2  so that the position of the document tray  22  in the standby state is maintained. The position of the tray  22  is selectable as desired depending on the number of document sheets which is likely to be set frequently, and the selected position can be set in advance by a serviceman or a user from a control section, which will be described later. 
   Further, the document tray  22  is movable up and down within the height range necessarily defined between the inlet side and the outlet side of the curved feed path  23  provided for assuring the reliable document feed, as described above. Since the tray  22  is movable up and down, a large number of document sheets can be stacked on the tray  22 . Thus, the document tray  22  is capable of raising the level of the uppermost sheet of the document stack thereon to a position appropriate for feeding to the inlet of the curved feed path  23 , whereby the document sheets can be successively fed one by one. 
   The document tray  22  is provided with a document restricting plate  30  for aligning side edges of the document sheets. The position of the document restricting plate  30  is detected by document size detector S 0  and the position thus detected is utilized for sheet selection in image formation, for example. 
   In reading a document sheet placed on the platen  12 , the light source unit  13  moves from the position Pos 3  toward the position Pos 4  in  FIG. 1  by a predetermined distance depending on the size of the document sheet detected by the document size detector S 9  which is not shown in  FIG. 1 . On the other hand, reading of a document sheet under feeding is performed with the light source unit kept stationary at the position Pos 1 . Based on the detection result obtained by the position detector of the light source unit  13 , any of the position Pos 3 , the position Pos 4 , and an intermediate position between Pos 3  and Pos 1  can be determined as the home position. The light source unit  13  is kept stationary at the home position in the standby state where it is not used. 
   To enable the setting of a document on the platen  12  for reading, the ADF  3  can pivot upward about a hinge (not shown) provided on the rear side of the image reading apparatus (deeper side in the figure) between the ADF  3  and the optical system  2 . With this arrangement, the upper surface of the platen  12  of the image reading apparatus  1  can be exposed so that a document that cannot be fed, such as a book, can be set on the platen  12 . A document mat  35  is provided on the ADF  3  side so as to face the platen  2 . 
   The image reading apparatus  1  thus constructed can perform image reading in three modes, i.e., a stationary document reading mode, a traveling document reading mode and a both-sides reading mode. The stationary document reading mode is a mode for reading such a document as a book placed on the platen  12  by using the CCD reading unit  11  while moving the light source unit  13  and the mirror unit  14 . 
   The traveling document reading mode and the both-sides reading mode are modes for reading document sheets set on the document tray  22  while automatically feeding the sheets one by one with the ADF  3 . The image reading in the traveling document reading mode is performed using the CCD reading unit  11 , whereas the image reading in the both-sides reading mode performed using both of the CCD reading unit  11  and the CIS  21 . The maximum number of document sheets which can be set on the document tray  22  is 200, for example. 
     FIGS. 2 to 4  each show the structure of a portion of the sheet feeder device  51  in detail. As described with reference to  FIG. 1 , the sheet feeder device  51  is made up of the document tray  22  as the sheet accommodating section, the pick-up roller R 1  as the sheet pick-up section for feeding sheets, and the combination of feed roller R 2  and reverse roller R 2   a  located downstream of the pick-up roller R 1  and serving as a sheet separating section. The reverse roller R 2   a  is removably fitted on and supported by rotation support shaft  53  (See  FIG. 4 ). 
   As shown in  FIG. 2 , the rotation support shaft  53  has opposite ends supported by the device body via first and second supports  54  and  55 . The first support (the support  54  in this embodiment) is pivotally supported by the device body. With this arrangement, the reverse roller R 2   a  can be separated away from the feed roller R 2  and easily detached from the rotation support shaft  53  with the feed roller R 2  kept as it is. 
   As shown in  FIGS. 3 and 4 , the first support  54  on the near side comprises a first support member  541  pivotally supported by the device body, and a second support member  542  removably fitted to the first support member  541  and holding the rotation support shaft  53 . By detaching the second support member  542  from the first support member  541 , the near side of the rotation support shaft  53  is released. In this state, the reverse roller R 2   a  can be pulled out from the rotation support shaft  53  for detachment. 
   As shown in  FIG. 2 , the second support  55  on the deeper side comprises a bearing attached to a support plate  60  standing on the device body. The rotation support shaft  53  projecting rearward through the second support  55  is supported by a bearing  66  attached to a frame member  65  of the device body via a universal joint  64  (the universal joint defined in the present invention) comprising two joints  62  and  63 . The rotation support shaft  53  is connected to and driven by the driving system provided outside of the frame member  65 . Therefore, the near side of the rotation support shaft  53  is allowed to swing in universal direction (in all directions), which facilitates the detachment of the reverse roller R 2   a , as described above. 
   More specifically, an end of the first support member  541  is swingably (pivotally) supported on a support shaft  58  projecting above a generally L-shaped support plate  57  fixedly standing on the device body, and an engagement portion  541   a  is provided to project below the end. A resilient member  56  is provided having an end engaging the engagement portion  541   a  and the other end engaging a lower bent end  571  of the support plate  57 . The (constantly biasing) resilient force of the resilient member  56  biases the reverse roller R 2   a  to press against the feed roller R 2  (See  FIG. 5 ). 
   As shown in  FIG. 4  for example, the first support member  541  is bent at a right angle at the generally central portion to provide a release lever portion  541   b  extending toward the other end. The release lever portion  541   b  has an end to which an end of a coupling lever  42  is attached pivotally. The coupling lever  42  has the other end pivotally attached to the openable external cover  41  (See  FIG. 1 ). Therefore, as shown in  FIG. 6  for example, as the external cover  41  moves to open, the coupling lever  42  moves in the direction indicated by the arrow in  FIG. 6  cooperatively with the movement of the cover, thereby causing the first support member  541  to pivot counterclockwise against the resilient force of the resilient member  56 . As a result, the reverse roller R 2   a  moves away from the feed roller R 2 . Although the resilient member  56  in this embodiment comprises a tension coil spring made of stainless steel wire for springs or steel wire for springs, use may be made of a leaf spring made of stainless steel sheet for springs or phosphor bronze sheet. 
   As shown in  FIG. 4 , the second support member  542 , which is removably fitted to the first support member  541 , has an end formed with a shaft hole  542   a  fitted around the support shaft  58  for rotation and a projection  542   c  fitted in a recess  541   c  formed at an end of the first support member  541 . The other end of the second support member  542  is formed with a shaft hole  542   b  holding the rotation support shaft  53  for rotation and a come-off preventive projection  542   d  in sliding engagement with a circumferential groove  53   d  formed at an end of the rotation support shaft  53 . This end is further formed with a tab  542   e  for separating the second support member  542  from the first support member  541 . 
   In this arrangement, when the second support member  542  is in the state fitted to the first support member  541 , the two members are fixedly joined together by the fit of the projection  542   c  of the second support member  542  in the recess  541   c  of the first support member  541 . Further, the second support member  542  supports the rotation support shaft  53  for rotation while preventing the shaft  53  from coming off, and the reverse roller R 2   a  is biased to press against the feed roller R 2  by the resilient force of the resilient member  56  (See  FIG. 5 ). 
   Next, the procedure for replacement of the reverse roller R 2   a  will be described (See  FIGS. 5 to 7 ). First, as indicated by broken lines in  FIG. 1 , the external cover  41  is opened. At this time, the coupling lever  42  coupling the release lever portion  541   b  of the first support member  541  to the external cover  41  cooperates with the external cover  41 , so that the state shown in  FIG. 5  changes into the state shown in  FIG. 6  where the separating section is released (i.e., the reverse roller R 2   a  is separated away from the feed roller R 2  in the separating section). 
   In this state, the reverse roller R 2   a  does not serve as a bar to the detachment of the feed roller R 2 . After removal of a coupling support member, which supports the arm  25  supporting both of the pick-up roller R 1  and the feed roller R 2  for rotation to provide an appropriate positional relationship therebetween, the pick-up roller R 1  and the feed roller R 2  are detached from respective rotation shafts (See  FIG. 7 ). 
   Subsequently, an inner cover (not shown) covering the supports for the reverse roller R 2   a  is removed and the tab  542   e  (see  FIG. 3 ) of the second support member  542  of the reverse roller R 2   a  is gripped to separate the second support member  542  from the first support member  541 . Thus, the end of the rotation support shaft  53  on the near side is made free. In this state, the reverse roller R 2   a  is pulled out from the rotation support shaft  53  and then replaced with a new reverse roller R 2   a . Thereafter, the original state is resumed by reversing the above-described procedure. At that time, the pick-up roller R 1  or the feed roller R 2  may be replaced with a new one as required. 
     FIG. 8  schematically illustrates the construction of a digital multifunction machine  100  as another embodiment of image forming apparatus according to the present invention. In the digital multifunction machine  100 , a document image read at a reading section (document reading apparatus)  110  is transmitted as image data to a non-illustrated image data input section. After having undergone predetermined image processing, the image data is temporarily stored in memory provided in an image processing section. The image data in the memory is read out in response to an output instruction and then transferred to a laser writing unit  227  as an optical writing device provided in an image forming section  210 . As shown in  FIG. 9 , an ADF  111  is mounted on the reading section (document reading apparatus)  110 , and a control panel  112  is provided on the manipulation side. 
   The laser writing unit  227  generally comprises a semiconductor laser light source for emitting laser light in accordance with image data read out of the memory or image data transferred from an external device, a polygon mirror for deflecting laser light by rotation at a constant angular velocity, and an f-? lens for performing correction so that the laser light deflected at a constant angular velocity is deflected at a constant angular velocity on a photosensitive drum  222 . Although the laser writing unit is used as the writing device in this embodiment, use may be made of an optical writing head unit of a solid-state scanning type which comprises an array of light emitting elements such as LEDs or EL devices. 
   The image forming section  210  further includes, around the photosensitive drum  222 , an electrostatic charger  223  for charging the photosensitive drum  222  to a predetermined potential, a developer  224  for supplying toner onto an electrostatic latent image formed on the photosensitive drum  222  to make the image tangible, a transfer device (e.g. transfer charger)  225  for transferring the toner image formed on the surface of the photosensitive drum  222  to a sheet, a cleaner  226  for recovering excess toner, and a static eliminator (e.g. charger for static elimination)  229 . The sheet onto which the image is transferred at the image forming section  210  is then fed to a fixing unit  217 , where the image is fixed to the sheet. 
   On the ejection side of the image forming section  210  are provided, besides the fixing unit  217 , a switchback path  221  for inverting the sheet front side back for formation of an image on the reverse surface of the sheet, and a post-processing device  260  which is provided with an up-and-down tray  261  and performs stapling or other processing for sheets bearing respective images formed thereon. The sheet on which a toner image has been fixed at the fixing unit  217  is guided to the post-processing device  260  by a sheet ejecting roller  219  optionally through the switchback path  221 , subjected to appropriate post-processing, and then ejected. 
   Below the image forming section  210  is provided a sheet feeder section which comprises a manual feed tray  254 , a reversing unit  255  and a sheet tray  251  which are provided on or in the apparatus body, and sheet trays  252  and  253  provided in a multi-tier sheet feeder section  270 . The sheet feeder section further includes a transport section  250  for transporting a sheet fed from any one of the trays  251 ,  252 ,  253  and  254  to a transfer position in the image forming section  210  where the transfer device is located. 
   The reversing unit  255 , which communicates with the switchback path  221  for inverting each sheet, is used in forming images on both sides of the sheet. The reversing unit  255  is so configured such that it can be replaced with an ordinary sheet cassette. Therefore, an arrangement is possible where an ordinary sheet cassette is disposed instead of the reversing unit  255 . 
   The digital multifunction machine  100  thus constructed may be provided with the sheet feeder device  51  shown in  FIGS. 2 to 7  in the transport unit  250 , i.e.  250   a ,  250   b ,  250   c  and  250   d.    
   The image reading apparatus according to the present invention is not limited to the construction shown in  FIG. 1 . The present invention is applicable to any image reading apparatus irrespective of the construction and type thereof as far as the image reading apparatus can include a sheet feeder device comprising, at least, a sheet accommodating section for accommodating a stack of sheets; a sheet pick-up section operative to feed each of the sheets toward a sheet feed path when abutting against the stack; and a sheet separating section having a feed roller and a reverse roller which are located downstream of the sheet pick-up section and abut against each other. 
   Moreover, the image forming apparatus according to the present invention is not limited to the construction shown in  FIGS. 8 and 9 . The present invention is applicable to any image forming apparatus irrespective of the construction and type thereof as far as the image forming apparatus includes a sheet feeder device comprising, at least, a sheet accommodating section for accommodating a stack of sheets; a sheet pick-up section operative to feed each of the sheets toward a sheet feed path when abutting against the stack; and a sheet separating section having a feed roller and a reverse roller which are located downstream of the sheet pick-up section and abut against each other. 
   While only certain presently preferred embodiments of the present invention have been described in detail, as will be apparent for those skilled in the art, certain changes and modifications may be made in embodiments without departing from the spirit and scope of the present invention as defined by the following claims.