Abstract:
A conveyor including a conveyance element configured to rotate in a first rotary direction for sending in a first direction a sheet conveyed along a first path and in an opposite second rotary direction for sending the sheet to a second path including: a guide member for rotating between first and second positions; a first transmission element for transmitting a drive force to the conveyance element; a lever for rotating the guide member to at least one of the first and second positions; a second transmission element connected with the first transmission element to transmit the drive force to the lever; and a transmission controller for controlling transmission of the drive force from the second transmission element to the lever, wherein the transmission controller transmits the drive force from the second transmission element to the lever while the lever rotates the guide member.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a conveyor for conveying a sheet and an image forming apparatus including the conveyor. 
     2. Description of the Related Art 
     An apparatus configured to perform various processes on a sheet are widely used. Such an apparatus may comprise a mechanism configured to switch a conveyance path and/or a conveyance direction of a sheet according to a type of process performed on a sheet. 
     For example, a certain image forming apparatus comprises a conveyance mechanism configured to switch a conveyance path of a sheet using an electromagnetic solenoid. The electromagnetic solenoid is a relatively expensive component and does not meet demands on cost reduction in production. Furthermore, operating sound of the electromagnetic solenoid is inappropriate for an apparatus used in an area where quiet environment is required. 
     An improved image forming apparatus comprises a roller and a claw. This image forming apparatus switches a conveyance direction of a sheet while nipping the sheet between the roller and the claw. According to the principle of this image forming apparatus, the above mentioned drawbacks of the electromagnetic solenoid are solved. 
     The nip between the roller and the claw, however, may damage the sheet (particularly damage to a formed image) although the improved image forming apparatus has advantages of reduction in production cost and silence. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an inexpensively manufactured and silent conveyor configured to switch a conveyance direction of a sheet with giving a sheet less load and an image forming apparatus including the conveyor. 
     One aspect of the present invention provides a conveyor including a conveyance element configured to rotate in a first rotary direction for sending in a first direction a sheet conveyed along a first path and in a second rotary direction opposite to the first rotary direction for sending the sheet to a second path including: a guide member configured to rotate between a first position where the first path opens and a second position where the first path closes and the second path opens; a first transmission element configured to transmit a drive force to the conveyance element; a lever configured to rotate the guide member to at least one of the first position and the second position; a second transmission element connected with the first transmission element to transmit the drive force to the lever; and a transmission controller configured to control transmission of the drive force from the second transmission element to the lever, wherein the transmission controller transmits the drive force from the second transmission element to the lever while the lever rotates the guide member. 
     Another aspect of the present invention provides an image forming apparatus for forming an image on a sheet including: an image forming unit configured to form the image on the sheet; a housing configured to define a first path for guiding the sheet fed from the image forming unit and a second path for guiding the sheet on which the image is formed back to the image forming unit; and a conveyor including a conveyance element configured to rotate in a first rotary direction to send the sheet transferred along the first path in a direction to discharge the sheet outside the housing and in a second rotary direction to pull the sheet back into the housing to send the sheet to the second path, wherein the conveyor includes: a first transmission element configured to transmit drive force to the conveyance element; a lever configured to rotate a guide member to at least one of a first position where the first path opens and a second position where the first path closes and the second path opens; a second transmission element connected with the first transmission element to transmit the drive force to the lever; and a transmission controller configured to control transmission of the drive force from the second transmission element to the lever, and the transmission controller transmits the drive force from the second transmission element to the lever while the lever rotates the guide member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram depicting an internal structure of an image forming apparatus according to one embodiment. 
         FIG. 2  is a schematic diagram depicting the internal structure of the image forming apparatus according to one embodiment. 
         FIG. 3  is a perspective view depicting an interlocking mechanism of the image forming apparatus shown in  FIGS. 1 and 2 . 
         FIG. 4A  is a schematic perspective view depicting an exploded torque limiter used for the interlocking mechanism shown in  FIG. 3 . 
         FIG. 4B  is a schematic perspective view depicting the assembled torque limiter used for the interlocking mechanism shown in  FIG. 3 . 
         FIG. 5  is a perspective view depicting the interlocking mechanism of the image forming apparatus shown in  FIGS. 1 and 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A conveyor and an image forming apparatus according to one embodiment are now described with reference to the accompanying drawings. Directional terms such as “up”, “down”, “left” and “right” to be used hereinafter are merely for clarifying the description, and shall not in any way limit principles of the conveyor and the image forming apparatus. A term “sheet” to be used in the following description refers to copy paper, tracing paper, thick paper, an OHP sheet and other sheets on which images may be formed. 
       FIG. 1  schematically shows an internal structure of an image forming apparatus according to one embodiment. The image forming apparatus shown in  FIG. 1  is exemplified as an apparatus incorporating a conveyor constructed on the basis of principles to be described below. The principles to be described below may be applied to various apparatuses configured to perform other processes than image forming process on sheets. In the following description, a color printer is exemplified as the image forming apparatus. A monochrome printer, a copier, a facsimile apparatus or other apparatuses configured to form an image on a sheet may be used as the image forming apparatus. 
     The image forming apparatus  1  receives, for example, external signals including information on an image and forms the image on the basis of the signals. The image forming apparatus  1  comprises a substantially rectangular boxed housing  2  for accommodating various apparatuses configured to form an image on a sheet S based on the external signals (e.g. various elements constituting a later mentioned cassette  31 , image forming unit  52 , fixing unit  53  and discharging unit  54 ). 
     The image forming apparatus  1  comprises a storage unit  3  configured to store the sheet S. The storage unit  3  includes a cassette  31  to be set in the housing  2  and an insertion tray  32  which protrudes from a left surface of the housing  2 . The insertion tray  32  is configured to support the sheet S. A feed roller  41  and a separation pad  42  are disposed at a base end of the insertion tray  32 . The sheet S placed on the insertion tray  32  inclined downward toward the base end of the insertion tray  32  contacts the feed roller  41  and/or the separation pad  42 . The sheet S is fed from the insertion tray  32  into the housing  2  when the feed roller  41  rotates. When several sheets S are fed into the housing  2  by the feed roller  41 , the separation pad  42  causes a frictional force to prevent the sheets S except the top sheet S from being fed into the housing  2 . As a result, the sheets S are fed into the housing  2  from the insertion tray  32  one by one. 
     A first feeding path  43  extends to the right from the feed roller  41  and/or the separation pad  42 . The first feeding path  43  in the housing  2  is defined between an upper wall  431  and a lower wall  432 . The lower wall  432  below the upper wall  431  confronts the upper wall  431 . Several conveyance roller pairs  433  configured to convey the sheet S is disposed in position along the first feeding path  43 . The first feeding path  43  is curved and extends upward from the horizontal direction to the vertical direction. 
     The cassette  31  is disposed below the horizontally extending first feeding path  43 . The detachable cassette  31  is inserted into the housing  2 . The substantially rectangular boxed cassette  31  upwardly opens so that a user put the sheet S therein. A lift plate  311  configured to support the sheet S is disposed in the cassette  31 . The lift plate  311  is upwardly inclined toward the right. 
     A pickup roller  44  is disposed above a right edge of the lift plate  311 . The sheet S is nipped by the right edge of the lift plate  311  and the pickup roller  44 . The sheet S is fed from the cassette  31  when the pickup roller  44  rotates. A feed roller  45  and a separation roller  46  are disposed after the pickup roller  44 . The feed roller  45  rotates in the same direction as the pickup roller  44  so as to feed the sheet S downstream. The separation roller  46  underneath the feed roller  45  rotates so as to feed the sheet S, which has been sent by the pickup roller  44 , back to the cassette  31 . If the pickup roller  44  feeds several sheets S from the cassette  31 , the separation roller  46  brings the sheets S except the top sheet back to the cassette  31 . Thus the sheets S are fed downstream by the feed roller  45  one by one. 
     A guide wall  33 , which curves upward to the right, is placed after the feed roller  45 . A conveyance roller pair  48  is disposed above the guide wall  33 . The conveyance roller pair  48  conveys the sheet upward. A second feeding path  49  configured to guide the sheet S fed from the cassette  31  joins together the above mentioned first feeding path  43  after the conveyance roller pair  48  to form a confluent conveyance path  50 . In this embodiment, the confluent conveyance path  50  is exemplified as the first path along which the sheet is conveyed. 
     A resist roller pair  51 , the image forming unit  52 , the fixing unit  53  and the discharging unit  54  are disposed, respectively, in the middle of the upwardly extending confluent conveyance path  50 . The resist roller pair  51  is disposed near a lower end of the confluent conveyance path  50 . The resist roller pair feeds the sheet S to the image forming unit  52  in synchronization with the image forming process of the image forming unit  52 . The resist roller pair  51  includes a first resist roller  511  installed on an inner wall of the housing  2  and a second resist roller  512  installed in a conveyance unit  20 . The first and second resist rollers  511 ,  512  nip and convey the sheet S upward. 
     The image forming unit  52  forms a toner image on the sheet S conveyed along the confluent conveyance path  50 . A magenta unit  52 M configured to form a toner image using magenta toner, a cyan unit  52 C configured to form a toner image using cyan toner, a yellow unit  52 Y configured to form a toner image using yellow toner, and a black unit  52 Bk configured to form a toner image using black toner, are disposed in the housing  2 . The magenta unit  52 M, the cyan unit  52 C, the yellow unit  52 Y and the black unit  52 Bk are sequentially disposed from left to right. 
     Each of the units  52 M,  52 C,  52 Y and  52 Bk has a developing apparatus  522  configured to supply toner to a photosensitive drum  521 . After an electrostatic latent image is formed on a circumferential surface of the photosensitive drum  521 , the toner is supplied from the developing apparatus  522  to form a toner image (visible image) corresponding to the electrostatic latent image. 
     The photosensitive drum  521  in  FIG. 1  rotates clockwise. The toner image formed on the photosensitive drum  521  is transferred to an intermediate transfer belt  525  which moves above the photosensitive drum  521 . 
     Four removable toner cartridges  520  configured to contain the magenta toner, the cyan toner, the yellow toner and the black toner, respectively, are attached in the housing  2 . These toner cartridges  520  are disposed between the intermediate transfer belt  525  and an upper wall  21  of the housing  2  configured to support the sheet S discharged from the discharging unit  54 . The toner is replenished to the units  52 M,  52 C,  52 Y and  52 Bk, respectively, through toner replenishment ducts (not shown), which extend from the toner cartridges  520  to the units  52 M,  52 C,  52 Y and  52 Bk, respectively. 
     The image forming unit  52  further includes charging devices  523  disposed below the photosensitive drums  521  of the units  52 M,  52 C,  52 Y and  52 Bk respectively, and an exposure device  524  disposed below the charging devices  523 . The charging devices  523  uniformly charge the circumferential surface of the photosensitive drums  521 . The exposure device  524  irradiates a laser beam onto the circumferential surface of the charged photosensitive drum  521  based on the digital signals about the received image data. As a result, an electrostatic latent image corresponding to a color component of an original document is formed on the circumferential surface of each photosensitive drum  521  of the units  52 M,  52 C,  52 Y or  52 Bk. Then the developing apparatus  522  supplies the toner to the circumferential surface of the photosensitive drum  521 . As a result, the toner electrostatically adheres to the electrostatic latent image to form the toner image. 
     The intermediate transfer belt  525  above each of the photosensitive drums  521  extends between a right drive roller  525   a  and a left idler  525   b . The lower surface of the lower half of the intermediate transfer belt  525  (moves to the right) abuts the circumferential surfaces of the photosensitive drums  521 , respectively. The outer surface of the intermediate transfer belt  525  is configured to bear the toner. Transfer rollers  526  are disposed above the photosensitive drums  521 , respectively. The intermediate transfer belt  525  pressed against the circumferential surfaces of the photosensitive drums  521  by the transfer rollers  526  runs between the drive roller  525   a  and the idler  525   b . A tension roller  525   c  is disposed between the drive roller  525   a  and the idler  525   b . The tension roller  525   c  is biased upward by a biasing member (not shown). The tension roller  525   c  pushed upward by the biasing member creates a convex profile of the tensed intermediate transfer belt  525 , which protrudes upward near the idler  525   b.    
     While the intermediate transfer belt  525  rotates, the photosensitive drum  521  of the magenta unit  52 M transfers a toner image of the magenta toner to the intermediate transfer belt  525 . Then the cyan unit  52 C transfers a toner image of the cyan toner onto the magenta colored toner image. After that, the yellow unit  52 Y transfers a toner image of the yellow toner onto the superposed toner images of the magenta toner and the cyan toner. Finally the black unit  52 Bk transfers a toner image of the black toner onto the superposed toner images of the magenta toner, the cyan toner and the yellow toner to complete a full color toner image. The complete full color toner image on the intermediate transfer belt  525  is transferred to the sheet S conveyed from the storage unit  3 . 
     The sheet S guided and conveyed along the confluent conveyance path  50  vertically extending at the right side of the image forming unit  52  is directed to a secondary transfer nip portion defined by a second transfer roller  513  and the intermediate transfer belt  525 . The second transfer roller  513  is disposed on the confluent conveyance path  50 . The second transfer roller  513  confronting the drive roller  525   a , around which the intermediate transfer belt  525  is wound, contacts the outer surface of the intermediate transfer belt  525  to form the secondary transfer nip portion. The sheet S guided along the confluent conveyance path  50  and fed to the secondary transfer nip portion between the intermediate transfer belt  525  and the second transfer roller  513 , is nipped between the intermediate transfer belt  525  and the second transfer roller  513 . As a result, the complete full color toner image on the intermediate transfer belt  525  is transferred to the sheet S. 
     The image forming apparatus  1  further includes a cleaning apparatus  528 . The cleaning apparatus  528  removes toner remaining on the intermediate transfer belt  525  after the toner image is transferred to the sheet S (normally called the secondary transfer) to clean the intermediate transfer belt  525 . The cleaning apparatus  528  confronts the idler  525   b.    
     The fixing unit  53  configured to perform a fixing process on the toner image transferred onto the sheet S by the image forming unit  52 , includes a heating roller  531  and a pressure roller  532  confronting the heating roller  531 . The heating roller  531  encloses an electric heater as a heating source. 
     The sheet S bearing the toner image is conveyed to the fixing unit  53 . While the sheet S passes between the pressure roller  532  and the heating roller  531  at a high temperature, the toner image receiving heat from the heating roller  531  is fixed to the sheet S. 
     The fixing process by the fixing unit  53  completes and the color printing ends. The color printed sheet S is guided along the confluent conveyance path  50  extending upward from the fixing unit  53  and discharged from the discharging unit  54 . The upper surface (upper wall  21 ) of the housing  2  is used as a discharge tray configured to support the color printed sheet S. 
     A relay apparatus  60  is disposed in the housing  2  along a front wall of the housing  2 . The upper surface of the relay apparatus  60  is configured to support a front edge of each toner cartridge  520 . The various components of the image forming apparatus  1  such as the photosensitive drums  521 , the intermediate transfer belt  525 , the development apparatuses  522  and the charging apparatuses  523  are disposed behind the relay apparatus  60 . The relay apparatus  60  relays the toner supplied from the toner cartridges  520  to each of the development apparatuses  522  below the intermediate transfer belt  525 . 
     The relay apparatus  60  stores the waste toner generated after the transfer process. Cleaning apparatuses  527  are disposed on the right side of the photosensitive drums  521 , respectively. The cleaning apparatus  527  removes toner remaining on the circumferential surface of the photosensitive drum  521  after finishing the transfer of the toner image to the intermediate transfer belt  525  (called primary transfer). The removed toner is stored in the relay apparatus  60  as the waste toner. The circumferential surface of each photosensitive drum  521  cleaned by the cleaning apparatus  527  moves toward the charging apparatus  523  to be subject to the charging process again. The cleaning apparatus  528  confronts the idler  525   b  at the left side of the intermediate transfer belt  525 . The cleaning apparatus  528  removes toner remaining on the outer surface of the intermediate transfer belt  525  after finishing the transfer of the toner image to the sheet S (called secondary transfer). The removed toner is stored in the relay apparatus  60  as the waste toner. The outer surface of the intermediate transfer belt  525  cleaned by the cleaning apparatus  528  then is subjected to transfer of a new toner image from each photosensitive drum  521 . 
     As shown in  FIG. 1 , the image forming apparatus  1  may print images on both sides of the sheet S. The sheet S after the image fixing process on one side by the fixing unit  53  is sent to a discharge roller pair  541  by a conveyance roller pair  540  disposed in the discharging unit  54 . In the present embodiment, the discharging unit  54  is exemplified as the conveyor configured to convey the sheet S. 
     If printing on only one side is instructed by external signals, the sheet S is directly discharged onto the upper wall  21  of the housing  2 . If double sided printing is instructed by the external signal, the discharge roller pair  541  discharges the sheet S outside the housing  2  only by a predetermined amount, then rotates in reverse so as to draw the sheet S back into the housing  2 . Then the sheet S is fed into a return path  55  extending downward along the right surface of the housing  2 . In the present embodiment, the return path  55  is exemplified as the second path along which the sheet S is conveyed. 
     The discharge roller pair  541  includes an upper roller  541   a  and a lower roller  541   b . In the present embodiment, the upper roller  541   a , to which a later mentioned drive gear (not shown in  FIG. 1 ) is connected, is exemplified as the conveyance element configured to convey the sheet S. In the present embodiment, the conveyance direction of the sheet S toward the outside of the housing  2  is exemplified as the first direction. The conveyance direction of the sheet S to draw the sheet S back into the housing  2  is exemplified as the second direction. The rotary direction of the upper roller  541   a  during the conveyance of the sheet S in the first direction (clockwise in  FIG. 1 ) is exemplified as the first rotary direction. The rotary direction of the upper roller  541   a  during the conveyance of the sheet in the second direction (counterclockwise in  FIG. 1 ) is exemplified as the second rotary direction. 
     A conveyance roller pair  551  configured to convey the sheet S is disposed in position along the return path  55 . The return path  55  curving to surround the right and lower surface of the conveyance unit  20  merges to the second feeding path  49  before the nip portion defined by the conveyance roller pair  48 . Therefore the sheet S fed into the return path  55  is conveyed into the resist roller pair  51  again by the conveyance roller pair  48 . Then the resist roller pair  51  feeds the sheet S to the image forming unit  52  in synchronization with the image formation by the image forming unit  52 . In the present embodiment, the return path  55  is exemplified as the second path configured to guide the sheet S, on which the image forming process has been performed, into the image forming unit  52  again. 
     The image forming unit  52  forms an image on a blank surface (unprinted surface) of the sheet S. Then a new toner image is fixed on the sheet S by the fixing unit  53 , and the sheet S is discharged onto the upper wall  21  of the housing  2  by the discharge roller  541 . As shown in  FIG. 1 , both sides of the sheet are printed because of the formation of the confluent conveyance path  50  configured to guide the sheet fed from the image forming unit  52  to the discharging unit  54  and the return path  50  connected with the confluent conveyance path  50  in the discharging unit  54  to guide the sheet S, on which the image forming process has been performed, to the image forming unit  52  again. 
       FIG. 2  is a schematic view depicting operation of the discharging unit  54  when the sheet S is returned toward the return path  55 . The operation of the discharging unit  54  will be generally described with reference to  FIGS. 1 and 2 . 
     As shown in  FIGS. 1 and 2 , the discharging unit  54  includes the guide member  600  near the conveyance roller pair  540 . The guiding member  600  shown in  FIG. 1  is located in the first position. The guiding member  600  shown in  FIG. 2  is located in a second position. The guiding member  600  in the first position opens the confluent conveyance path  50  used as the first path whereas the guide member  600  in the second position closes the confluent conveyance path  50  and opens the return path  55  used as the second path. The guide member  600  is installed to the inner wall of the housing  2  so as to be rotatable between the first position and the second position. The phrases “open the transporting path”, “open the path” and the like used hereinafter refer to guiding the sheet S in a predetermined direction without interrupting conveyance of the sheet S. The phrases “close the conveyance path”, “close the path” and the like used hereinafter refer to interrupting downstream conveyance of the sheet S. 
     As shown in  FIG. 1 , when the tip of the guide member  600  is located in an upper position, a gently curved lower surface of the guide member  600  guides the sheet S from the conveyance roller pair  540  to the discharge roller pair  541 . Meanwhile the guide member  600  protrudes so as to cross and close the return path  55  extending from the discharge roller pair  541 . When the tip of the guide member  600  is in a lower position, as shown in  FIG. 2 , on the other hand, the lower surface of the guide member  600  lies down so as to close the confluent conveyance path  50 , and the upper surface of the guide member  600  is used as a wall surface constituting the return path  55 . While the guide member  600  is in the first position, the upper roller  541   a  of the discharge roller pair  541  rotates in the first rotary direction to discharge the sheet S to the outside of the housing  2 . While the guide member  600  is in the second position, the upper roller  541   a  of the discharge roller pair  541  rotates in the second rotary direction to draw the sheet S back into the housing  2 . 
       FIG. 3  is an enlarged perspective view of the discharging unit  54 . In  FIG. 3 , the housing  2  is partially omitted so that the guide member  600  is clearly shown. Instead, the remaining portion of the housing  2  shown in  FIG. 3  is described as “housing wall  290 ”. The discharging unit  54  is described with reference to  FIG. 3 . 
     The guide member  600  which substantially looks like a triangular prism, as shown in  FIG. 3 , is located in the first position. An opening  612  is defined on an end surface  610  formed with a substantially triangular plate of the guide member  600 . A shaft  951  protruding from the inner surface of the housing wall  290  (in  FIG. 3 , a portion of the housing wall  290  connected with the guide member  600  is omitted) is inserted through the opening  612 . The guide member  600  is supported by the shaft  951  so as to rotate upward and downward. A torsion bar spring  952  is used, in addition to the shaft  951 , to connect the housing wall  290  with the guide member  600  according to the present embodiment. The torsion bar spring  952  biases the guide member  600  toward the second position. Alternatively, the torsion bar spring  952  may bias the guide member  600  toward the first position. Further alternatively, only the shaft  951  may be used for the connection of the guiding member  600  and the housing wall  290 . In the present invention, the torsion bar spring  952  is exemplified as a biasing member. 
     The guide member  600  rotates between the first position and the second position around the shaft  951  inserted into the opening  612 . A substantially cylindrical pin  611  protrudes from the end surface  610  of the guide member  600 . A tip of the pin  611  appearing outside the housing wall  290  contacts an interlocking mechanism  650  mounted on the outer surface of the housing wall  290 . The interlocking mechanism  650  rotates the guide member  600  between the first position and the second position via the pin  611 . When the upper roller  541   a  of the discharge roller pair  541  rotates in the first rotary direction (direction to discharge the sheet S outside the housing  2 ), the interlocking mechanism  650  rotates the guide member  600  toward the first position. While the guide member  600  is rotated to the second position due to the torsion bar spring  952  used for connecting the housing wall  290  with the guide member  600 , the interlocking mechanism  650  interlocks with the rotating guide member  600  not to interrupts downward movement of the pin  611 . 
     If the torsion bar spring  952  used for connecting the housing wall  290  with the guide member  600  biases the guide member  600  toward the first position, the interlocking mechanism  650  may rotate the guide member  600  toward the second position. In this case, while the guide member  600  is rotated toward the first position by the torsion bar spring  952 , the interlocking mechanism  650  preferably interlocks with the rotating guide member  600  so as not to interrupt the pin  611  from moving up. 
     The interlocking mechanism  650  may rotate the guide member  600  toward the first position and the second position. In this case, the biasing member configured to bias the guide member  600  toward the first position or the second position is not necessary. 
     In  FIG. 3 , the upper roller  541   a  of the discharge roller pair  541  is shown whereas the lower roller  541   b  (see  FIG. 1  and  FIG. 2 ) is not shown because the lower roller  541   b  is behind the housing wall  290 . 
     The interlocking mechanism  650  includes a drive gear  651  mounted on an end of the shaft  542  of the upper roller  541   a  of the discharge roller pair  541 . In the present embodiment, the drive gear  651  is exemplified as the first transmission element configured to transmit a drive force to the upper roller  541   a  of the discharge roller pair  541 . 
     A drive force from a drive source (not shown) configured to bi-directionally rotate is input to the drive gear  651  outside the housing wall  290 . Therefore the drive gear  651  may rotate in a direction to discharge the sheet S out of the housing  2 , and in a direction to draw the sheet S back into the housing  2 . The drive gear  651  shown in  FIG. 3  rotates counterclockwise. The drive gear  651  looks like a substantially multi-stage cylinder. In the drive gear  651 , a gear portion  651   b  smaller in diameter is formed between a gear portion  651   a  larger in diameter and the housing wall  290 . The drive force from the drive source is input, for example, to the larger gear portion  651   a . As a result, the drive gear  651  rotates with the shaft  542  according to the rotary direction of the drive source. As a result, the upper roller  541   a , to which the drive force from the drive source is transmitted, selectively rotates in the first rotary direction or the second rotary direction. 
     The interlocking mechanism  650  includes a relay gear  653  configured to engages with the smaller gear portion  651   b  of the drive gear  651 , a lever  654  configured to contact with the pin  611  of the guiding element  600 , and a torque limiter  800  disposed between the relay gear  653  and the lever  654 . The relay gear  653  is exemplified as a second transmission element connected with the first transmission element to transfer drive force to the lever  654 . The first transmission element and the second transmission element are not limited to those having a gear structure. For example, such structures as pullies interconnected via a belt, or frictional disks configured to rotate by friction of their circumferential surfaces contacting each other, may be used as the first transmission element and the second transmission element. The torque limiter  800  is exemplified as the transmission controller configured to control transfer of the drive force from the relay gear  653  to the lever  654 . In the present embodiment, it is likely the torque limiter  800  prevents excessive load from working on the second transmission element (relay gear  653 ) and/or the lever  654 . 
     The lever  654  includes a rotatable cylinder  655  connected to the outer surface of the housing wall  290 , and an arm  656  extending from a circumferential surface of the cylinder  655 . In the present embodiment, an upper edge  659  of the arm  656  contacts the pin  611 . If the torsion bar spring  952  used for connecting the housing wall  290  with the guide member  600  biases the guide member  600  toward the first position, the arm  656  may be disposed so that a lower edge  901  of the arm  656  contacts the pin  611 . If the interlocking mechanism  650  guides the guiding member  600  to the first position and the second position, the arm  656  may be configured to interpose the pin  611  (e.g. a slit into which the pin  611  is inserted may be defined in the arm  656 ). Thus the guide member  600  is rotated to at least one of the first position and the second position by the lever  654 . 
     The relay gear  653 , the torque limiter  800  and the lever  654  rotate around the same rotational axis. The drive force from the drive gear  651  is transmitted to the relay gear  653  via the smaller gear portion  651   b . While the guide member  600  moves from the second position to the first position, the drive force from the relay gear  653  is transmitted to the lever  654  via the torque limiter  800 . As a consequence, the arm  656  of the lever  654  rotates upward around the cylinder  655 . 
       FIGS. 4A and 4B  show the torque limiter  800 .  FIG. 4A  is a perspective view of the exploded torque limiter  800 .  FIG. 4B  is a cross-sectional view of the assembled torque limiter  800 . A commercially available and general torque limiter may be used for the transmission controller or the torque limiter. The interlocking mechanism  650  is further described with reference to  FIGS. 3 to 4B . 
     The torque limiter  800  comprises a substantially cylindrical housing  810 . The housing  810  includes a bottom and an opening confronting the bottom. The torque limiter  800  further comprises a shield ring  820  configured to close the opening of the housing  810 , three plate spring rings  830  accommodated in the housing  810 , and a shaft  840  inserted in the housing  810 . The housing  810  further comprises a few bumps  811 . Concavities  812  are defined among the bumps  811  intermittently formed along the inner surface of the housing  810 , respectively. External teeth  831  are formed on a circumferential surface of the plate spring ring  830 . The external teeth  831  protruding outward are placed in the concavities  812  defined inside the housing  810 . As a consequence, the housing  810  and the plate spring ring  830  integrally rotate together. 
     The plate spring ring  830  includes an internal tooth  832 . The internal tooth  832  protruding from an inner circumferential edge of the plate spring ring  830  is pressed to the shaft  840  inserted in the plate spring ring  830 . If a torque (torsional moment) less than a predetermined value is applied to the torque limiter  800 , the shaft  840  and the plate spring ring  830  integrally rotate. If a torque (torsional moment) more than the predetermined value is applied to the torque limiter  800 , the shaft  840  runs idle with respect to the plate spring ring  830 . 
     A rotatable base end  841  of the shaft  840  is held by a concave in the bottom of the housing  810 . A tip  842  of the shaft  840  protrudes from the housing  810  through an opening  821  defined in the shield ring  820 . The tip  842  of the shaft  840  protruding from the housing  810  supports the relay gear  653 . The housing  810  is connected to the cylinder  655  of the lever  654 . In the present embodiment, the housing  810  and/or the plate spring ring  830  are/is exemplified as the first element connected to the lever  654 . The shaft  840  is exemplified as the second element connected to the relay gear  653 . 
       FIG. 5  is an enlarged perspective view of the discharging unit  54 . Like  FIG. 3 , in  FIG. 5 , the housing  2  is partially omitted to clearly show the guide member  600 . Like  FIG. 3 , the remaining portion of the housing  2  is described as “housing wall  290 ”. The guide member  600  shown in  FIG. 5  is located in the second position. The interlocking mechanism  650  is further described with reference to  FIGS. 3 to 5 . 
     As shown in  FIGS. 3 and 5 , the interlocking mechanism  650  comprises first and second pins  710 ,  720  protruding from the outer surface of the housing wall  290 . The first pin  710  is disposed above the second pin  720 . The first pin  710  and the second pin  720  collaboratively define a rotary range of the lever  654 . 
     When the guide member  600  is rotated from the second position shown in  FIG. 5  to the first position shown in  FIG. 3 , the drive gear  651  changes the rotary direction from a direction indicated by an arrow in  FIG. 5  to a direction indicated by an arrow in  FIG. 3 , so that the drive gear  651  changes the rotary direction of the upper roller  541   a  of the discharge roller pair  541  from the second rotary direction to the first rotary direction. As a result, the drive force of the relay gear  653  received from the drive gear  651  is transmitted to the lever  654  via the torque limiter  800 . As a consequence, the arm  656  of the lever  654  rotates upward. The guide member  600  rotates toward the first direction via the pin  611  contacting the upper edge  659  of the arm  656 . 
     As shown in  FIG. 3 , when the guide member  600  reaches the first position, the arm  656  of the lever  654  contacts the first pin  710 . Even after the guide member  600  reaches the first position, the upper roller  541   a  of the discharge roller pair  541  keeps rotating in the first rotary direction as the sheet S is discharged. As a result, the torsional moment applied to the torque limiter  800  increases. 
     As described in the context of  FIGS. 4A and 4B , the torsional moment applied to the torque limiter  800  more than the predetermined value causes idle running between the shaft  840  of the torque limiter  800  and the plate spring ring  830 . Consequently, the torque limiter  800  limits the transmission of the drive force from the relay gear  653  to the lever  640  during a period from arrival of the guide member  600  at the first position to initiation of the rotation of the guide member  600  toward the second position. As a result, it is less likely that excessive load is applied to the second transmission element (relay gear  653 ) and/or the lever  654 . 
     When the guide member  600  is rotated from the first position shown in  FIG. 3  to the second position shown in  FIG. 5 , the drive gear  651  changes the rotary direction from the direction indicated by the arrow in  FIG. 3  to the direction indicated by the arrow in  FIG. 5 , so that the drive gear  651  changes the rotary direction of the upper roller  541   a  of the discharge roller pair  541  from the first rotary direction to the second rotary direction. As a result, the drive force of the relay gear  653  received from the drive gear  651  is transmitted to the lever  654  via the torque limiter  800 . Consequently, the arm  656  of the lever  654  rotates downward. The guide member  600  biased toward the second position by the torsion bar spring  952  used for connection between the housing wall  290  and the guide member  600  rotates toward the second position, following up the arm  656  of the lever  654  rotating downward. 
     As shown in  FIG. 5 , when the guide member  600  reaches the second position, the arm  656  of the lever  654  contacts the second pin  720 . The upper roller  541   a  of the discharge roller pair  541  keeps rotating in the second rotary direction in order to perform the double sided printing on the sheet S even after the guide element  600  reaches the second position. As a result, the torsional moment applied to the torque limiter  800  increases. 
     As mentioned above, the torsional moment applied to the torque limiter  800  more than the predetermined value causes idle running between the shaft  840  of the torque limiter  800  and the plate spring ring  830 . Consequently, the torque limiter  800  limits the transmission of the drive force from the relay gear  653  to the lever  654  during a period from arrival of the guide member  600  at the second position to initiation of rotation of the guide member  600  toward the first position. 
     The above mentioned principle of limiting the drive force by the torque limiter  800  may be appropriately applied to the case of the lever  654  actively rotating the guide member  600  toward the second position and/or to the case of the torsion bar spring  952  used for the connection between the housing wall  290  and the guide member  600  to bias the guide member  600  toward the first position. 
     According to the present embodiment, the above mentioned principle of the conveyor is applied to the discharging unit  54  of the image forming apparatus  1 . The above mentioned principle of the conveyor may be, however, applied to any apparatuses configured to convey the sheet S. The above mentioned principle of the conveyor may be applied to any other apparatuses configured to perform other arbitrary processes on the sheet S than the image forming apparatus  1 . 
     According to the present embodiment, the upper roller  541   a  of the discharge roller pair  541  is used as the conveyance element configured to rotate in the first rotary direction and the second rotary direction. Alternatively, an arbitrary conveyance element (e.g. vacuum belt) configured to convey the sheet S may be used as the conveyance element. 
     This application is based on Japanese Patent application No. 2010-119668 filed in Japan Patent Office on May 25, 2010, the contents of which are hereby incorporated by reference. 
     Although the present invention 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.