Patent Publication Number: US-8523178-B2

Title: Discharge mechanism and image forming device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2009-061186 filed on Mar. 13, 2009. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a discharge mechanism and an image forming device. 
     SUMMARY 
     A first aspect of the present invention is a discharge mechanism including: a rotating shaft that is rotatable; plural roll portions provided around the rotating shaft; an opposing member provided above the roll portions so as to oppose the roll portions, the opposing member holding and conveying a material to be conveyed, in cooperation with the roll portions so that the material to be conveyed is discharged onto a discharge section; and a protrusion formed on an outer periphery of the rotating shaft, the protrusion not protruding beyond the outer diameter of the roll portions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic view showing the structure of an image forming device relating to the present exemplary embodiment; 
         FIG. 2  is a schematic view, partially in cross section, of the structure of a discharge mechanism relating to the present exemplary embodiment; 
         FIG. 3  is a schematic perspective view showing the structure of a discharge roller relating to the present exemplary embodiment; 
         FIGS. 4A ,  4 B and  4 C are schematic views showing modifications of the shape of protrusions formed on the discharge roller relating to the present exemplary embodiment; 
         FIGS. 5A and 5B  are schematic views each showing discharge operation of a recording medium in the discharge mechanism relating to the present exemplary embodiment; 
         FIG. 6  is a schematic plan view showing a state in which a trailing end portion of the recording medium rests on a rotating shaft of the discharge roller relating to the present exemplary embodiment; 
         FIG. 7  is a schematic view showing a way in which a leading end portion of the recording medium meets resistance at a recording medium discharge section, the recording medium having been discharged by the discharge mechanism relating to the present exemplary embodiment; 
         FIG. 8  is a schematic view showing a comparative example in which depressions are formed in the rotating shaft of the discharge roller relating to the present exemplary embodiment; 
         FIG. 9  is a schematic view showing a state in which the discharge roller relating to the present exemplary embodiment is reversed; 
         FIGS. 10A and 10B  are schematic perspective views showing a modification in which four protrusions are formed on the discharge roller relating to the present exemplary embodiment; 
         FIGS. 11A and 11B  are schematic perspective views showing a modification in which six protrusions are formed on the discharge roller relating to the present exemplary embodiment; 
         FIG. 12  is a schematic view, partially in cross section, of the structure of a discharge mechanism relating to a modified embodiment; 
         FIG. 13  is a schematic perspective view showing the structure of a discharge roller of the discharge mechanism relating to the modified embodiment; 
         FIG. 14  is a schematic view showing the size relationship between the inner diameter of a roll portion and the outer diameter of a rotating shaft including protrusions in the discharge mechanism relating to the modified embodiment; and 
         FIG. 15  is a schematic view showing the discharge mechanism relating to the modified embodiment, which is structured such that the rotating shaft is formed in a flattened shape. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment relating to the invention will be described on the basis of the drawings. 
     Structure of Image Forming Device Relating to the Present Exemplary Embodiment 
     First, the structure of an image forming device relating to the present exemplary embodiment will be described.  FIG. 1  is a schematic view showing the structure of the image forming device relating to the present exemplary embodiment. 
     As shown in  FIG. 1 , an image forming device  10  includes a recording medium accommodating section  12 , an image forming section  14 , a conveying section  16  and a recording medium discharge section  20 . A recording medium P, which is shown as an example of materials to be conveyed, is accommodated in the recording medium accommodating section  12 . In the image forming section  14 , an image is formed on a recording surface of the recording medium P. The conveying section  16  conveys the recording medium P from the recording medium accommodating section  12  to the image forming section  14 . The recording medium P having the image formed thereon by the image forming section  14  is discharged from the recording medium discharge section  20 . Examples of the recording medium P include paper, films formed of plastics and metals, and the like. 
     The image forming section  14  includes: image forming units  22 Y,  22 M,  22 C and  22 K that form toner images of yellow (Y), magenta (M), cyan (C) and black (K), respectively; transfer members  28  that transfer the toner images formed in the image forming units  22 Y,  22 M,  22 C and  22 K to the recording surface of the recording medium P; and a fixing device  18  that fixes on the recording medium P the toner images that have been transferred by the transfer members  28 . 
     The image forming units  22 Y,  22 M,  22 C and  22 K respectively have a photoreceptor drum  30  that rotates in a certain direction (a counterclockwise direction in  FIG. 1 ). The photoreceptor drum  30  is an example of an image holding body holding the toner image that has been formed on the surface of the recording medium P. 
     A charging device  32  that charges a surface of the photoreceptor drum  30 , an exposure device  34  that exposes the surface of the photoreceptor drum  30  to form an electrostatic latent image thereon, and a developing device  36  that develops the electrostatic latent image formed on the surface of the photoreceptor drum  30  to form a toner image are provided around each of the photoreceptor drums  30  in this order from an upstream side in a direction of rotation of the photoreceptor drum  30 . 
     The transfer member  28  faces the photoreceptor drum  30  across a conveying belt  54 , which will be described below. A transfer position, at which the toner image on the surface of the photoreceptor surface  30  is transferred onto the recording medium P, is formed between the transfer member  28  and the photoreceptor drum  30 . At this transfer position, the transfer member  28  transfers the toner image on the surface of the photoreceptor drum  30  onto the recording medium P by contact pressure and electrostatic force. 
     The conveying section  16  includes a feed roller  50  that feeds the recording medium P accommodated in the recording medium accommodating section  12 , pairs of conveying rollers  52  that hold therebetween and convey the recording medium P that has been fed by the feed roller  50 , and the conveying belt  54  that is an example of a conveying body and conveys to the transfer position the recording medium P that has been conveyed by the pairs of conveying rollers  52 . 
     The feed roller  50 , the pairs of conveying rollers  52  and the conveying belt  54  are disposed in this order along a first conveying path  17  extending from the recording medium accommodating section  12  toward the recording medium discharge section  20 . 
     The conveying belt  54  is formed in a loop around entrainment rollers  56  and  58 . The entrainment roller  56  is circular and disposed at a downstream side in a direction in which the recording medium P is conveyed (hereinafter referred to as “the conveying direction of the recording medium P” or simply “the conveying direction”). The entrainment roller  58  is disposed at an upstream side in the conveying direction of the recording medium P. The conveying belt  54  is structured so as to be rotated (driven in cycles) in a certain direction (a clockwise direction in  FIG. 1 ) by either one of the entrainment rollers  56  and  58  being driven to rotate in a certain direction (the clockwise direction in  FIG. 1 ). 
     A charging roller  57  is provided adjacent to the conveying belt  54  and at the upstream side thereof in the conveying direction. The charging roller  57  charges the surface of the conveying belt  54  and presses, against the conveying belt  54 , the recording medium P that is electrostatically attracted to the conveying belt  54 . 
     The conveying belt  54  is not limited to that structured so as to electrostatically attract and hold the recording medium P, and may be structured so as to hold the recording medium P by non-electrostatic devices such as suction or adhesion. 
     The fixing device  18  is disposed downstream of the transfer positions in the conveying direction and fixes, on the recording medium P, the toner images that have been transferred onto the recording medium P at the transfer positions. 
     Downstream of the fixing device  18  in the conveying direction is disposed a discharge mechanism  60  that discharges, to the recording medium discharge section  20 , the recording medium P having the toner images fixed thereon by the fixing device  18 . The discharge mechanism  60  is formed so as to discharge the recording medium P to the recording medium discharge section  20  with the recording surface facing down. The structure of the discharge mechanism  60  will be described later. 
     Further, a second conveying path  19 , which inverts the recording medium P having an image formed on one side thereof back into the first conveying path  17 , is disposed so as to face the first conveying path  17  across the conveying belt  54 . 
     Plural pairs of conveying rollers  23  for holding therebetween and conveying the recording medium P are disposed at the second conveying path  19 . When images are formed on both sides of the recording medium P, the recording medium P with an image formed on one side thereof is redirected to the second conveying path  19  by the discharge mechanism  60  and conveyed downstream and back to the first conveying path  17  by the plural pairs of conveying rollers  23 . 
     In the image forming device  10  relating to the present exemplary embodiment having the above-described structure, first, the recording medium P that has been fed from the recording medium accommodating section  12  is sequentially fed into the transfer positions corresponding to the respective colors of yellow, magenta, cyan and black by the conveying belt  54 . 
     The toner images of the respective colors formed on the photoreceptor drums  30  are transferred by the transfer members  28  onto the recording medium P that is fed to the transfer positions, and are superposed to form a color image on the recording medium P. The recording medium P is further conveyed to the fixing device  18  where the transferred toner images are fixed. When an image is formed on only one side of the recording medium P, the recording medium P is discharged to the recording medium discharge section  20  by the discharge mechanism  60  after the toner images are fixed. At this time, the recording medium P is discharged out to the recording medium discharge section  20  with the recording surface having the image formed thereon facing down. 
     When images are formed on both sides of the recording medium P, after an image is formed on one side thereof, the recording medium P is inverted and redirected to the second conveying path  19  by the discharge mechanism  60 . The recording medium P is further fed from the second conveying path  19  back into the first conveying path  17 , where an image is formed on the opposite side in the same way as described above, whereby the images are formed on both sides of the recording medium P. The series of image forming processes are carried out as described above. 
     Note that the structure of the image forming device is not limited to that described above. For example, the image forming device may be of indirect transfer type having an intermediate transfer body. Various structures are possible for the image forming device. 
     Structure of Discharge Mechanism  60  Relating to Present Exemplary Embodiment 
     The structure of the discharge mechanism  60  relating to the present exemplary embodiment will be described first.  FIG. 2  is a schematic view, partially in cross section, of the structure of the discharge mechanism  60  relating to the present exemplary embodiment.  FIG. 3  is a schematic perspective view showing the structure of a discharge roller  62  relating to the present exemplary embodiment. 
     As shown in  FIG. 2 , the discharge mechanism  60  relating to the present exemplary embodiment includes the discharge roller  62  and an opposing roller  72  disposed above the discharge roller  62 . 
     As shown in  FIGS. 2 and 3 , the discharge roller  62  includes a rotating shaft  64  that is rotatable, and roll portions  66  that are formed in the shape of a ring and provided around the rotating shaft  64 . 
     Similarly to the discharge roller  62 , the opposing roller  72  includes a rotating shaft  74  that is rotatable, and roll portions  76  that are formed in the shape of a ring and provided around the rotating shaft  74 . The roll portion  76  is provided above the roll portion  66  of the discharge roller  62  and opposes the roll portion  66 . Namely, the roll portion  76  serves as an opposing member that opposes the roll portion  66 . The opposing member may be an opposing belt formed by a belt body, or may be the rotating shaft  74  itself. 
     In the present exemplary embodiment, the roll portions  66  of the discharge roller  62  are disposed at two locations of the rotating shaft  64  such that a central portion of the rotating shaft  64  in an axial direction thereof is located between the roll portions  66 . The roll portions  66  may be disposed at three or more locations of the rotating shaft  64 . 
     The roll portions  76  and  66  are formed of, for example, rubber materials, while the rotating shafts  74  and  64  are formed of, for example, resin, metal or the like. Further, the roll portion  66  and the rotating shaft  64  may be an integrally formed resin product. 
     The discharge roller  62  serves as a driving roller. A rotational force is applied via a transmission member, such as a gear, to the rotating shaft  64  from a drive  63  that drives to rotate the rotating shaft  64 . The opposing roller  72 , on the other hand, serves as a driven roller that is not driven by the drive  63  but is rotated by the discharge roller  62  serving as the driving roller. The opposing roller  72  may also be a driving roller. 
     The discharge roller  62  and the opposing roller  72  are structured so as to hold, convey and discharge the recording medium P, which is an example of the materials to be conveyed, to the recording medium discharge section  20 . Specifically, the recording medium P is held between the roll portion  66  of the discharge roller  62  and the roll portion  76  of the opposing roller  72  and conveyed thereby. In  FIG. 2 , the direction in which the discharge roller  62  is rotated is indicated by arrow A, while the conveying direction (discharging direction) of the recording medium P is indicated by arrow H. 
     Protrusions  68  are formed on an outer periphery of the rotating shaft  64  and protrude outward from an outer peripheral surface of the rotating shaft  64  in a radial direction thereof. The height of the protrusion  68  is set such that the protrusion  68  does not extend beyond the outer diameter of the roll portion  66 . The protrusion  68  also extends along the axial direction of the rotating shaft  64 . 
     In the present exemplary embodiment, the protrusions  68  are composed of protrusions  68 A and  68 B. The protrusions  68 A and  68 B are disposed at positions at which they overlap each other in the axial direction of the rotating shaft  64 . Namely, the positions through which the protrusions  68 A and  68 B pass when the rotating shaft  64  is rotated overlap. 
     Further, the protrusions  68 A and  68 B are disposed at different positions in a peripheral direction of the rotating shaft  64 , as shown in  FIG. 2 . The protrusion  68 A is disposed on the rotating shaft  64  at a position opposite to the protrusion  68 B. Specifically, the protrusions  68 A and  68 B are disposed at positions at which at least portions of the protrusions  68 A and  68 B overlap each other when the rotating shaft  64  is viewed in the radial direction. More specifically, the protrusions  68 A and  68 B are disposed in such a positional relationship that they are disposed at the same positions in the axial direction of the rotating shaft  64  and spaced 180° apart thereon. 
     Moreover, the protrusions  68 A and  68 B are disposed between the roll portions  66 , namely, at the axial direction central portion of the rotating shaft  64 . 
     As shown in  FIG. 2 , the protrusions  68 A and  68 B have a first surface  67 , which is formed along the radial direction of the rotating shaft  64  when seen from one end side of the rotating shaft  64  in the axial direction thereof, and faces the rotational direction of the rotating shaft  64  when the recording medium P is discharged. The first surface  67  is formed along the axial direction of the rotating shaft  64  when seen from an outer side of the rotating shaft  64  in the radial direction thereof. 
     The protrusions  68 A and  68 B also have a second surface  69  that is formed at the opposite side of the first surface  67 . As the second surface  69  extends towards the radial direction outer side of the rotating shaft  64 , the second surface  69  becomes gradually closer to the first surface  67  when seen from the axial direction one end side of the rotating shaft  64 . Specifically, the second surface  69  gradually follows the shape of an arc as it extends toward the radial direction outer side of the rotating shaft  64 . 
     The second surface  69  is formed along the axial direction of the rotating shaft  64  when seen from the radial direction outer side thereof. A corner portion  65 , at which the first surface  67  and the second surface  69  connect at the radial direction outer side of the rotating shaft  64 , is formed at the protrusion  68 . 
     Various shapes may be used for the protrusion  68 . For example, as shown in  FIG. 4A , the second surface  69  may be a straight inclined surface when seen from the axial direction one end side of the rotating shaft  64 . In this case, the protrusion  68  has a substantially triangular configuration when seen from the axial direction one end side of the rotating shaft  64 . 
     Further, similarly to the first surface  67 , the second surface  69  may be formed along the radial direction of the rotating shaft  64  when seen from the axial direction one end side of the rotating shaft  64 , as shown in  FIG. 4B . In this case, the protrusion  68  has a substantially quadrangular configuration when seen from the axial direction one end side of the rotating shaft  64 . 
     Furthermore, as shown in  FIG. 4C , the first surface  69  and the second surface  67  may be formed in the shape of a continuous arc when seen from the axial direction one end side of the rotating shaft  64 . In this case, the protrusion  68  has a substantially semicircular configuration when seen from the axial direction one end side of the rotating shaft  64 . 
     Moreover, in the present exemplary embodiment, when the image forming section  14  sequentially form images on the recording media P, the drive  63  rotates the rotating shaft  64  at least half a turn during the time duration from when the roll portion  66  discharges the recording medium P to the recording medium discharge section  20  to when the subsequent recording medium P reaches the roll portion  60 . In this structure, the two protrusions  68  are formed along the peripheral direction of the rotating shaft  64  and at the positions at which the projections  68  overlap each other in the axial direction of the rotating shaft  64 . Thus, after the roll portion  66  discharges the recording medium P to the recording medium discharge section  20  and before the subsequent recording medium P reaches the roll portion  60 , each protrusion  68  passes at least once over the entire periphery of the rotating shaft  64 . 
     Further, the drive  63  rotates the rotating shaft  64  at least half a turn within the duration of time, which is the sum of the time from when the roll portion  66  discharges the recording medium P to the recording medium discharge section  20  to when the rotation of the rotating shaft  64  is stopped, and the time from when the rotating shaft  64 , the rotation of which has been stopped, is rotated again to when the subsequent recording medium P reaches the roll portion  66 . 
     In this case as well, the protrusion  68  passes at least once over the entire periphery of the rotating shaft  64  during the time duration from when the roll portion  66  discharges the recording medium P to the recording medium discharge section  20  to when the rotation of the rotating shaft  64  is stopped, and the time duration from when the rotating shaft  64 , the rotation of which has been stopped, is rotated again to when the subsequent recording medium P reaches the roll portion  60 . 
     Discharge of the recording medium P to the recording medium discharge section  20  by the roll portions  66  can be detected by, for example, sensing the recording medium P in the recording medium accommodating section  12  or the first conveying path  17 , and determining the conveying time taken from the sensing to the discharge of the recording medium P, on the basis of conveying time that has been measured in advance. 
     Operation of Discharge Mechanism  60  Relating to the Present Exemplary Embodiment 
     Operation of the discharge mechanism  60  relating to the present exemplary embodiment will be described next. 
     In the discharge mechanism  60  relating to the present exemplary embodiment, the recording medium P that has been fed from the fixing device  18  is held between the roll portions  66  of the discharge roller  62  and the roll portion  76  of the opposing roller  72 , and conveyed and discharged to the recording medium discharge section  20 , as shown in  FIG. 5A . 
     During this conveyance of the recording medium P, when a trailing end portion of the recording medium P has a different shape as shown in  FIG. 6 , the entire recording medium P may not be discharged to the recording medium discharge section  20 , and the trailing end portion (the upstream-side end portion in the conveying direction) of the recording medium P may rest on the rotating shaft  64  at a position between the two roll portions  66 , as shown in  FIGS. 5B and 6 . 
     The reason for this may be that, for example, a leading end portion (the downstream-side end portion in the conveying direction) of the recording medium P impinges on a bottom surface or the like of the recording medium discharge section  20  and meets resistance thereat, shown in  FIG. 7 . 
     An example of the different shape of the trailing end portion may be a shape that becomes narrow toward the trailing end portion of the recording medium P, specifically a triangle formed at the trailing end portion. An example of the recording medium P having a trailing end portion of different shape is an envelope with a flap being open toward the trailing end portion side. 
     In the present exemplary embodiment, when the trailing end portion of the recording medium P rests on the rotating shaft  64 , the rotating shaft  64  is rotated at least half a turn by the time when the subsequent recording medium P reaches the roll portions  66 . The protrusions  68  formed on the rotating shaft  64  thrust the trailing end portion of the recording medium P, whereby the recording medium P is discharged to the recording medium discharge section  20 . 
     If, as shown in  FIG. 8 , depressions  64 A are formed in the curved outer peripheral surface of the rotating shaft  64  of the discharge roller  62  in place of the protrusions  68 , when the trailing end portion of the recording medium P rests on the rotating shaft  64 , the state in which the trailing end portion rests on the outer peripheral surface of the rotating shaft  64  continues, and it is difficult for the trailing end portion to go into the depression  64 A. Consequently, the trailing end portion of the recording medium P cannot be sufficiently thrust by an edge of the depression  64 A. 
     On the contrary, in the present exemplary embodiment, the protrusion  68  is provided on the curved outer peripheral surface of the rotating shaft  64 . Therefore, the trailing end portion of the recording medium P resting on the outer peripheral surface of the rotating shaft  64  is sufficiently thrust by the protrusion  68 . 
     Further, the recording medium P is discharged with the recording surface facing the discharge roller  62 . Since the protrusion  68  does not extend beyond the roll portion  66 , scraping, by the protrusion  68 , of the recording surface of the recording medium P is prevented. As a result, defects caused by the protrusion  68  scraping the recording surface are prevented. 
     Furthermore, as shown in  FIG. 9 , even when the discharge roller  62  is reversed in the direction of arrow B in  FIG. 9  at the time of recording images on both sides of the recording medium P, the recording medium P held in the recording medium discharge section  20  abuts the second surface  69  of the protrusion  68 . Thus, the recording medium P is hardly subjected to resistance, and pickup of the recording medium P held in the recording medium discharge section  20  is prevented. Consequently, formation of jams due to the recording medium P that has already been discharged going into the second conveying path  19  for duplex (two-sided) recording is prevented. 
     The plural protrusions  68  may be four protrusions such as protrusions  68 C,  68 D,  68 E and  68 F as shown in  FIGS. 10A and 10B . The protrusions  68 C,  68 D,  68 E and  68 F are disposed between the two roll portions  66 . 
     The protrusions  68 C and  68 D are disposed at positions that overlap in the axial direction of the rotating shaft  64 , and the same applies to the protrusions  68 E and  68 F. Namely, when the rotating shaft  64  is rotated, the positions at which the protrusions  68 C and  68 D pass overlap, and the positions at which the protrusions  68 E and  68 F pass overlap. 
     The protrusions  68 C and  68 D, and the protrusions  68 E and  68 F are formed at different positions of the rotating shaft  64  in the axial direction thereof. The protrusions  68 C and  68 D, and the protrusions  68 E and  68 F are disposed at positions closer to the roll portions  66  than the center of the rotating shaft  64 . 
     The protrusions  68 C and  68 D are disposed at different positions of the rotating shaft  64  in the peripheral direction thereof. The same applies to the protrusions  68 E and  68 F. 
     The protrusion  68 C is disposed on the rotating shaft  64  at a position opposite to the protrusion  68 D, while the protrusion  68 E is disposed on the rotating shaft  64  at a position opposite to the protrusion  68 F. Specifically, the protrusions  68 C and  68 D are disposed at positions at which at least portions of the protrusions  68 C and  68 D overlap each other, and the protrusions  68 E and  68 F are disposed at positions at which at least portions of the protrusions  68 E and  68 F overlap each other, when the rotating shaft  64  is viewed in a radial direction thereof. 
     More specifically, the protrusions  68 C and  68 D are disposed in such a positional relationship that they are disposed at the same positions in the axial direction of the rotating shaft  64  and spaced 180° apart thereon. Similarly, the protrusions  68 E and  68 F are disposed in such a positional relationship that they are disposed at the same positions in the axial direction of the rotating shaft  64  and spaced 180° apart thereon. 
     The protrusions  68 C and  68 E are disposed on the rotating shaft  64  at positions at which they overlap each other in the peripheral direction. Similarly, the protrusions  68 D and  68 F are disposed on the rotating shaft  64  at positions at which they overlap each other in the peripheral direction. Namely, when seen from the axial direction one end side of the rotating shaft  64 , the protrusions  68 C and  68 E overlap each other, and the protrusions  68 D and  68 F overlap each other. 
     Moreover, the plural protrusions  68  may be six protrusions, such as protrusions  68 A through  68 F shown in  FIGS. 11A and 11B . The protrusions  68 A through  68 F have the same structure as that of the protrusions with the same reference numerals shown in  FIGS. 2 ,  3 ,  10 A and  10 B. 
     The protrusions  68 A and  68 B, and the protrusions  68 C ( 68 E) and  68 D ( 68 F) are disposed on the rotating shaft  64  at different positions in the peripheral direction thereof. Specifically, when seen from the axial direction one end side of the rotating shaft  64 , the protrusions  68 A,  68 C ( 68 E) and  68 D ( 68 F) are disposed on the rotating shaft  64  in such a positional relationship that the protrusion  68 A is spaced 90° apart from the protrusions  68 C ( 68 E) and  68 D ( 68 F). Similarly, the protrusions  68 B,  68 C ( 68 E) and  68 D ( 68 F) are disposed on the rotating shaft  64  in such a positional relationship that the protrusion  68 B is spaced 90° apart from the protrusions  68 C ( 68 E) and  68 D ( 68 F). 
     The protrusions  68 C ( 68 D) and  68 E ( 68 F) are disposed closer to the roll portions  66  than the protrusion  68 A ( 68 B) is. Further, the protrusions  68 C,  68 D,  68 E and  68 F protrude beyond the protrusions  68 A and  68 B in the radial direction of the rotating shaft  64 . 
     Because of this structure, when the trailing end portion of the recording medium P rests on the rotating shaft  64 , the recording medium P can be thrust onto the recording medium discharge section  20  even when the recording medium P is separated from the rotating shaft  64  at a position close to the roll portion  66 . 
     On the other hand, the recording medium P held between the roll portions  66  of the discharge roller  62  and the roll portion  76  of the opposing roller  72  and conveyed is hardly deflected toward the rotating shaft  64  at positions near the roll portions  66 . Thus, the protrusion  68  hardly scrapes the recording surface of the recording medium P. As a result, defects caused by the protrusion  68  scraping the recording surface are prevented. 
     Modified Embodiment of Discharge Mechanism Relating to the Present Exemplary Embodiment 
     Next, the structure of a discharge mechanism  80  relating to a modified embodiment will be described.  FIG. 12  is a schematic view showing the structure of the discharge mechanism  80  relating to the modified embodiment. Note that the same reference numerals are used to designate parts that are the same as those in the discharge mechanism  60 , and detailed description thereof will be omitted. 
     The discharge mechanism  80  relating to the modified embodiment includes a discharge roller  82 , and the opposing roller  72  above the discharge roller  82 , as shown in  FIG. 12 . 
     Similarly to the discharge roller  62 , as shown in  FIGS. 12 and 13 , the discharge roller  82  includes the rotating shaft  64  that is rotatable, and the roll portions  66  that are provided around the rotating shaft  64 . 
     In this discharge roller  82 , as shown in  FIG. 13 , plural protrusions  88  are formed on the outer periphery of the rotating shaft  64  and protrude outward in the radial direction thereof from the outer peripheral surface of the rotating shaft  64 . The height of the protrusion  88  is set such that the protrusion  88  does not extend beyond the outer diameter of the roll portion  66 . The protrusion  88  also extends along the axial direction of the rotating shaft  64 . 
     As shown in  FIG. 12 , the protrusion  88  has the first surface  67 , which is formed along a substantially radial direction of the rotating shaft  64  when seen from the axial direction one end side of the rotating shaft  64 , and faces the rotational direction of the rotating shaft  64  when the recording medium P is discharged. The first surface  67  is formed so as to extend substantially along the axial direction of the rotating shaft  64  when seen from the radial direction outer side of the rotating shaft  64 . 
     The protrusion  88  also has the second surface  69  that is formed at the opposite side of the first surface  67 . As the second surface  69  extends in the radial direction outer side of the rotating shaft  64 , the second surface  69  is gradually closer to the first surface  67  when seen from the axial direction one end side of the rotating shaft  64 . Specifically, the second surface  69  gradually follows the shape of an arc as it extends toward the radial direction outer side of the rotating shaft  64 . 
     The second surface  69  is formed so as to extend substantially along the axial direction of the rotating shaft  64  when seen from the radial direction outer side thereof. The corner portion  65 , at which the first surface  67  and the second surface  69  connect at the radial direction outer side of the rotating shaft  64 , is formed at the protrusion  88 . 
     Various shapes may be used for the protrusion  88 . For example, the second surface  69  may be a straight inclined surface when seen from the axial direction one end side of the rotating shaft  64  (see  FIG. 4A ). In this case, the protrusion  88  has a substantially triangular configuration when seen from the axial direction one end side of the rotating shaft  64 . 
     Further, similarly to the first surface  67 , the second surface  69  may be formed along the radial direction of the rotating shaft  64  when seen from the axial direction one end side of the rotating shaft  64 . In this case, the protrusion  88  has a substantially quadrangular configuration when seen from the axial direction one end side of the rotating shaft  64  (see  FIG. 4B ). 
     The plural protrusions  88  are formed by eight protrusions, namely, protrusions  88 A,  88 B,  88 C,  88 D,  88 E,  88 F,  88 G and  88 H. 
     The protrusions  88 A through  88 H are disposed at positions further outward in the axial direction of the rotating shaft  64  than the roll portions  66 . 
     The protrusions  88 A through  88 D are disposed at the axial direction one end side of the rotating shaft  64 , while the protrusions  88 E through  88 H are disposed at the other end side of the rotating shaft  64  in the axial direction. 
     The protrusions  88 A through  88 H face outward with respect to the rotational direction of the rotating shaft  64 . Specifically, when seen from the radial direction outer side of the rotating shaft  64 , the protrusions  88 A through  88 H are disposed at an angle with respect to the axial direction of the rotating shaft  64 . Further, an end portion  89 A at a center side of the rotating shaft  64  in the axial direction thereof (i.e., the side close to the roll portion  66 ) is disposed at a downstream side of the rotating shaft  64  in the rotational direction. Furthermore, an end portion  89  at an outer side of the rotating shaft  64  in the axial direction thereof (i.e., the side far from the roll portion  66 ) is disposed at an upstream side of the rotating shaft  64  in the rotational direction. 
     The protrusions  88 A and  88 B are disposed at positions that overlap in the axial direction of the rotating shaft  64 , and the same applies to the protrusions  88 C and  88 D. Namely, when the rotating shaft  64  is rotated, the positions at which the protrusions  88 A and  88 B pass overlap, and the positions at which the protrusions  88 C and  88 D pass overlap. 
     The protrusions  88 A and  88 B, and the protrusions  88 C and  88 D are formed at different positions of the rotating shaft  64  in the axial direction thereof. The protrusions  88 C and  88 D are disposed at positions closer to the roll portion  66  than the protrusions  88 A and  88 B are. 
     The protrusions  88 A and  88 B are disposed at different positions of the rotating shaft  64  in the peripheral direction thereof. The same applies to the protrusions  88 C and  88 D. 
     The protrusion  88 A is disposed on the rotating shaft  64  at a position opposite to the protrusion  88 B, while the protrusion  88 C is disposed on the rotating shaft  64  at a position opposite to the protrusion  88 D. Specifically, the protrusions  88 A and  88 B are disposed at positions at which at least portions of the protrusions  88 A and  88 B overlap each other, and the protrusions  88 C and  88 D are disposed at positions at which at least portions of the protrusions  88 C and  88 D overlap each other, when the rotating shaft  64  is viewed in the radial direction thereof. 
     More specifically, the protrusions  88 A and  88 B are disposed in such a positional relationship that they are disposed at the same positions in the axial direction of the rotating shaft  64  and spaced 180° apart thereon. Similarly, the protrusions  88 C and  88 D are disposed in such a positional relationship that they are disposed at the same positions in the axial direction of the rotating shaft  64  and spaced 180° apart thereon. 
     The protrusions  88 A and  88 C are disposed on the rotating shaft  64  at positions at which they overlap each other in the peripheral direction. Similarly, the protrusions  88 B and  88 D are disposed on the rotating shaft  64  at positions at which they overlap each other in the peripheral direction. Namely, when seen from the axial direction one end side of the rotating shaft  64 , the protrusions  88 A and  88 C overlap each other, and the protrusions  88 B and  88 D overlap each other. 
     Further, the protrusions  88 E and  88 F are disposed at positions that overlap in the axial direction of the rotating shaft  64 , and the same applies to the protrusions  88 G and  88 H. Namely, when the rotating shaft  64  is rotated, the positions at which the protrusions  88 E and  88 F pass overlap, and the positions at which the protrusions  88 G and  88 H pass overlap. 
     The protrusions  88 E and  88 F, and the protrusions  88 G and  88 H are formed at different positions of the rotating shaft  64  in the axial direction thereof. The protrusions  88 E and  88 F are disposed at positions closer to the roll portion  66  than the protrusions  88 G and  88 H are. 
     The protrusions  88 E and  88 F are disposed at different positions of the rotating shaft  64  in the peripheral direction thereof. The same applies to the protrusions  88 G and  88 H. 
     The protrusion  88 E is disposed on the rotating shaft  64  at a position opposite to the protrusion  88 F, while the protrusion  88 G is disposed on the rotating shaft  64  at a position opposite to the protrusion  88 H. Specifically, the protrusions  88 E and  88 F are disposed at positions at which at least portions of the protrusions  88 E and  88 F overlap each other, and the protrusions  88 G and  88 H are disposed at positions at which at least portions of the protrusions  88 G and  88 H overlap each other, when the rotating shaft  64  is viewed in the radial direction thereof. 
     More specifically, the protrusions  88 E and  88 F are disposed in such a positional relationship that they are disposed at the same positions in the axial direction of the rotating shaft  64  and spaced 180° apart thereon. Similarly, the protrusions  88 G and  88 H are disposed in such a positional relationship that they are disposed at the same positions in the axial direction of the rotating shaft  64  and spaced 180° apart thereon. 
     The protrusions  88 E and  88 G are disposed on the rotating shaft  64  at positions at which they overlap each other in the peripheral direction. Similarly, the protrusions  88 F and  88 H are disposed on the rotating shaft  64  at positions at which they overlap each other in the peripheral direction. Namely, when seen from the axial direction one end side of the rotating shaft  64 , the protrusions  88 E and  88 G overlap each other, and the protrusions  88 F and  88 H overlap each other. 
     Further, the portion of the rotating shaft  64  to which the roll portion  66  is mounted is thicker than the protrusion  88 . Thus, the inner diameter of the roll portion  66  is larger than the outer diameter of the rotating shaft  64  including the protrusions  88 . In this way, the protrusions  88  are less likely to be obstacles when the roll portion  66  is mounted to the rotating shaft  64  from the axial direction outer side thereof. 
     Operation of Discharge Mechanism  80  Relating to Modified Embodiment 
     Operation of the discharge mechanism  80  relating to the modified embodiment will be described next. 
     In the discharge mechanism  80  relating to the present modified embodiment, the recording medium P that has been fed from the fixing device  18  is held between the roll portions  66  of the discharge roller  82  and the roll portion  76  of the opposing roller  72 , and conveyed and discharged to the recording medium discharge section  20 . 
     During this conveyance of the recording medium P, the entire recording medium P may not be discharged to the recording medium discharge section  20 , and the trailing end portion (the upstream-side end portion in the conveying direction) of the recording medium P may rest on the rotating shaft  64  at outer sides of the two roll portions  66 , as shown in  FIG. 13 . 
     The reason for this may be that, for example, side end portions of the recording medium P curl and lift upward. 
     Particularly in the case that a large number of the recording media P are accommodated in the recording medium discharge section  20 , when the central portion of the recording media P between the side end portions is placed on the recording medium P accommodated in the recording medium discharge section  20 , the recording medium P curls and the side end portions thereof lift upward, resulting in that the trailing end portion of the recording medium P near the sides rests on the rotating shaft  64 . 
     In the present modified embodiment, when the trailing end portion of the recording medium P rests on the rotating shaft  64 , the protrusions  88  formed on the rotating shaft  64  thrust (propel) the trailing end portion of the recording medium P, whereby the recording medium P is discharged to the recording medium discharge section  20 . 
     The protrusion  88  faces outward with respect to the rotating direction of the rotating shaft  64  and is positioned along the trailing end portion of the recording medium P having the curled side end portions. Therefore, the recording medium P is easily thrust by the protrusion  88 . 
     Further, the recording medium P is discharged with the recording surface facing the discharge roller  82 . Since the protrusion  88  does not extend beyond the roll portion  66 , scraping, by the protrusion  88 , of the recording surface of the recording medium P is prevented. 
     Furthermore, even when the discharge roller  82  is reversed at the time of recording images on both sides of the recording medium P, the recording medium P held in the recording medium discharge section  20  abuts the second surface  69  of the protrusion  88 . Thus, the recording medium P is hardly subjected to resistance, and pickup of the recording medium P accommodated in the recording medium discharge section  20  is prevented (see  FIG. 9 ). 
     The rotating shaft  64  may be formed such that the portion at which the protrusion  88  is formed has a flattened shape, as shown in  FIG. 15 . Specifically, the rotating shaft  64  has an oval cross section. It is sufficient if the flattened shape is formed by pressing and deforming the rotating shaft  64  that has a circular cross section. Further, in the structure shown in  FIG. 15 , the portion of the rotating shaft  64  at which the protrusion  88  is not formed may have a circular cross section as indicated by a dotted line in  FIG. 15 . The entire rotating shaft  64  need not be formed in the flattened shape. 
     In this structure, the roll portion  66  need not be stretched, but is pressed and made flattened when mounted to the rotating shaft  64  from the axial direction outer side thereof. In this way, damage to the roll portion  66  is prevented. 
     The present invention is not limited to the embodiments described above, and may include various variations, modifications and improvements. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.