Patent Publication Number: US-10768548-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application Nos. 2018-246386 filed on Dec. 28, 2018, and 2019-143891 filed on Aug. 5, 2019, the content of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     Aspects of the disclosure relates to an image forming apparatus. 
     BACKGROUND 
     A known image forming apparatus includes an image forming unit, a pinch roller for transporting a sheet from a sheet tray to the image forming unit, and a paper powder removal roller facing the pinch roller. 
     The known image forming apparatus further includes a drawer supporting a plurality of photosensitive drums. The drawer is movable between an inside position at which the drawer is housed in a housing, and an outside position at which the drawer is shifted from the inside position toward an exterior of the housing. 
     The pinch roller is rotatably supported by the housing. The paper powder removal roller is rotatably supported by the drawer and moves as the drawer moves. When the drawer moves from the outside position to the inside position, the paper powder removal roller, which is urged by a coil spring toward the pinch roller, moves toward and is positioned relative to the pinch roller. 
     SUMMARY 
     In the known image forming apparatus, due to dimensional or assembly errors of components, the paper powder removal roller may be positioned skewly relative to the pinch roller. This may cause skew of a sheet transported relative to the photosensitive drums and skew of an image formed relative to the sheet, resulting in hindrance to image forming with an improved accuracy. 
     Aspects of the disclosure provide an image forming apparatus configured to form an image with an improved accuracy. 
     According to one or more aspects of the disclosure, an image forming apparatus includes a housing, a drawer, a photosensitive member rotatably supported by the drawer, a first roller rotatably supported by the drawer, a second roller, an urging member urging the second roller toward the first roller, and a positioner. The drawer is movable between a housed position at which the drawer is housed in the housing, and a shifted position at which the drawer is shifted from the housed position toward an exterior of the housing. The second roller is rotatably supported by the housing, disposed to face the first roller, and movable relative to the first roller. The second roller and the first roller are configured to pinch a sheet therebetween and transport the sheet toward the photosensitive member. The positioner is disposed at the drawer and configured to position the second roller relative to the first roller when the drawer is at the housed position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an image forming apparatus according to a first embodiment of the disclosure. 
         FIG. 2  is a partial perspective view of a drawer, a first holder, a first roller, a frame, a second holder, and a second roller of the image forming apparatus. 
         FIG. 3  is an exploded perspective view of a structure for holding the second roller, showing the frame, the second holder, the second roller, a second gear, a third gear, compressed coil springs. 
         FIG. 4  is a partial side view showing a second rotation shaft of the second roller positioned by a positioner of the first holder in a state where the drawer is at a housed position. 
         FIG. 5  is a partial side view of the second holder moving linearly relative to a housing. 
         FIG. 6  is a partial side view of the second holder pivoting rearward relative to the housing. 
         FIG. 7  is a partial side view of the second holder pivoting frontward relative to the housing. 
         FIG. 8  is a partial side view of the second rotation shaft of the second roller guided by a second guide of the first holder when the drawer moves. 
         FIG. 9  is a partial side view of the second rotation shaft of the second roller guided by a first guide of the first holder when the drawer moves. 
         FIG. 10  is an exploded perspective view of a structure for holding a second roller, showing a frame, a gear cover with a sleeve, a left second holder to which the gear cover is assembled, a right second holder with a sleeve, and a second gear according to an image forming apparatus of a second embodiment. 
         FIG. 11  is a partial side view showing the sleeve of the gear cover pinched between a second rotation shaft of the second roller and a positioner of a first holder, and the second rotation shaft positioned by the positioner in a state where the drawer is at the housed position according to the image forming apparatus of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Illustrative embodiments of the disclosure will now be described with reference to the drawings. 
       FIG. 1  shows an image forming apparatus  1  according to a first embodiment of the disclosure. The image forming apparatus  1  is a laser printer for electrophotographically forming an image on a sheet. 
     A front-rear direction and an up-down direction are shown in  FIG. 1  by defining right and upper sides of the page of  FIG. 1  as front and upper sides of the image forming apparatus  1 , respectively. A front-rear direction and an up-down direction shown in  FIG. 2  and subsequent drawings correspond to the directions shown in  FIG. 1 . A left-right direction is shown in  FIGS. 2 and 3  by defining a side facing out of the page of  FIG. 1  as a left side of the apparatus. Elements of the image forming apparatus  1  will now be described with reference to  FIG. 1  and other drawings. 
     Overall Structure of Image Forming Apparatus 
     As shown in  FIG. 1 , the image forming apparatus  1  includes a housing  9 , a sheet tray  4 C, a feeder  4 , an image forming unit  5 , and a discharge unit  6 . 
     The housing  9  includes a plurality of inner frames (not shown). A frame  90  shown in  FIGS. 2 and 3  is one of the inner frames. 
     As shown in  FIG. 1 , the sheet tray  4 C is disposed at the bottom of the housing  9 . The sheet tray  4 C stores therein a stack of sheets SH. A discharge tray  9 T is disposed at the top of the housing  9 . A sheet SH having an image formed thereon is discharged onto the discharge tray  9 T. 
     A transport path P 1  is defined in the housing  9 . The transport path P 1  has a substantially S shape. The transport path P 1  extends from a front end of the sheet tray  4 C upward to curve in a U shape, extends rearward substantially horizontally, and then extends, at the rear of the housing  9 , upward in a U shape to the discharge tray  9 T. 
     The feeder  4  includes, along the U-shaped curved portion of the transport path P 1 , a feed roller  45 , a separation roller  46 , a separation piece  46 A, a transport roller  47 A, a pinch roller  47 B, a sensor S 1 , a first roller  41 , a second roller  42 , and a sensor S 2 . The U-shaped curved portion extends from the front end of the sheet tray  4 C. The pinch roller  47 B is pressed against the transport roller  47 A. 
     The first roller  41  and the second roller  42 , which will be described in detail later, are disposed at an upper end NT of the U-shaped curved portion which is an upstream portion of the transport path P 1 . The second roller  42  is disposed facing, from below, the first roller  41  and is pressed against the first roller  41 . 
     A controller (not shown) causes the feeder  4  to feed from the sheet tray  4 C one sheet SH at a time, fed by the feed roller  45  and separated by the separation roller  46  and the separation piece  46 A, toward the transport roller  47 A and the pinch roller  47 B. 
     Subsequently, in the feeder  4 , the transport roller  47 A and the pinch roller  47 B pinch the sheet SH therebetween and transport the sheet SH toward the first roller  41  and the second roller  42 . In this case, the controller stops rotation of the first roller  41  and the second roller  42  until a predetermined time has elapsed after detection by the sensor S 1  of a leading edge of the sheet SH transported by the transport roller  47 A and the pinch roller  47 B. Thereafter, the controller resumes rotating the rollers  41  and  42 . The first roller  41  and the second roller  42 , against which the leading edge of the sheet SH is abutted, corrects or reduces skewing of the sheet SH. Then the first roller  41  and the second roller  42  pinch and transport the sheet SH toward the image forming unit  5 . The controller controls the image forming unit  5  based on a timing of the sensor S 2  detecting the leading edge of the sheet SH transported by the first roller  41  and the second roller  42 . 
     The first roller  41  and the second roller  42  are configured to align a leading edge of the sheet SH reaching the first roller  41  and the second roller  42  and then transport the sheet SH toward the image forming unit  5 . 
     The image forming unit  5  is disposed above the sheet tray  4 C in the housing  9 . The sheet SH fed by the feeder  4  passes through the image forming unit  5  along a substantially horizontal portion of the transport path P 1 . 
     The image forming unit  5  is of the direct tandem type capable of color printing. The image forming unit  5  includes a drawer  8 , photosensitive drums  5 D, and developer cartridges  5 C. The photosensitive drums  5 D are each an example of a photosensitive member. 
     The drawer  8  has a known structure and thus is schematically shown in the drawings. The drawer  8  is a frame including side walls opposite to each other in directions facing into and out of the page of  FIG. 1  and each extending in the front-rear direction, and a plurality of connectors each extending in the left-right direction to connect the opposite side walls. 
     Four photosensitive drums  5 D, which correspond to black, yellow, magenta, and cyan toners, are rotatably supported by the drawer  8  in tandem along the substantially horizontal portion of the transport path P 1 . 
     Four developer cartridges  5 C, which correspond to black, yellow, magenta, and cyan toners, are detachably held by the drawer  8  in tandem along the substantially horizontal portion of the transport path P 1 . Each developer cartridge  5 C includes a developing roller  5 E, a charger  5 F, and a toner storage  5 G which are disposed around a corresponding photosensitive drum  5 D. 
     In a state where a front cover  9 F is pivoted to a position shown by a two-dot chain line in  FIG. 1  to be open at the front of the housing  9 , the drawer  8  is movable in the front-rear direction between a housed position at which the drawer  8  is housed in the housing  9 , and a shifted position at which the drawer  8  is shifted from the housed position toward an exterior of the housing  9 . 
     In this example, the drawer  8 , when at the shifted position, is removed from the housing  9  and the entire drawer  8  is outside the housing  9 . When the drawer  8  moves frontward from the housed position to the shifted position, the drawer  8  traces a gently oblique upward path. When the drawer  8  moves rearward from the shifted position to the housed position, the drawer  8  traces the reverse path. 
     The drawer  8  located at the shifted position allows for removal of a sheet jammed in the transport path P 1  and maintenance, such as replacement of the developer cartridges  5 C. 
     The image forming unit  5  includes a transfer belt  5 B, a scanner  5 S, and a fixer  5 H. 
     The transfer belt  5 B is disposed below the photosensitive drums  5 D to define therebetween the substantially horizontal portion of the transfer path P 1 . The transfer belt  5 B circulates while pinching, in conjunction with the photosensitive drums  5 D, a sheet being transported. 
     The scanner  5 S includes laser emitters, a polygon mirror, lenses, and reflecting mirrors, which are known elements. The scanner  5 S downwardly emits laser beams to each photosensitive drum  5 D for a corresponding color of black, yellow, magenta, or cyan. 
     A heat roller and a pressure roller of the fixer  5 H pinch a sheet SH having passed below the developer cartridges  5 C, and heat and press the sheet SH. 
     The image forming unit  5  thus structured forms an image on a sheet SH, as described below. In each developer cartridge  5 C, after the charger  5 F uniformly and positively charges the surface of the photosensitive drum  5 D, the scanner  5 S irradiates the surface of the photosensitive drum  5 D. An electrostatic latent image, which corresponds to an image to be formed on the sheet SH, is formed on the surface of the photosensitive drum  5 D. Subsequently, the developing roller  5 E supplies toner contained in the toner container  5 G to the surface of the photosensitive drum  5 D, in accordance with the electrostatic latent image. The toner held on the surface of the photosensitive drum  5 D is transferred onto the sheet SH. The fixer  5 H heats and presses the sheet SH with the toners transferred from the photosensitive drums  5 D, thereby fixing the toners on the sheet SH. 
     A discharge roller and a pinch roller of the discharge unit  6 , which are disposed most downstream in the transport path P 1 , pinch the sheet SH with the fixed toners and discharge the sheet SH onto the discharge tray  9 T. 
     In the image forming apparatus  1 , as shown in  FIGS. 1 and 2 , the first roller  41  is rotatably supported by the drawer  8 . In contrast, as shown in  FIG. 2 , the second roller  42  is rotatably supported by the frame  90 . The separation roller  46  and the transport roller  47 A of the feeder  4  are also supported by the frame  90 . Although omitted from the drawings, the feed roller  45  of the feeder  4  is also rotatably supported by the frame  90 . 
     As shown in  FIGS. 1 and 2 , the first roller  41  moves to an exterior of the housing  9  when the drawer  8  moves to the shifted position. As shown in  FIG. 4 , a first axis X 41  of the first roller  41  is located further to the front than a second axis X 42  of the second roller  42 . Thus, the second roller  42  is unlikely to prevent the first roller  41  from moving frontward. 
     A nip line NL 1  for the first roller  41  and the second roller  42  is defined as a straight line passing through the first axis X 41  and the second axis X 42  in a state where the drawer  8  is located at the housed position. The nip line NL 1  is defined in a state where the first roller  41  and the second roller  42  are properly positioned as designed. When the first roller  41  and the second roller  42  are deviated from the proper positions as designed due to dimensional or assembly errors of components, a nip line for the first roller  41  and the second roller  42  is deviated from the nip line NL 1 . 
     In the image forming apparatus  1 , when the drawer  8  moves from the shifted position to the housed position, the second roller  42  is positioned relative to the first roller  41  by the structure described below. 
     Structures of First Roller, First Holder, Positioner, First Guide, and Second Guide 
     As shown in  FIG. 2 , the drawer  8  includes first holders  50 L and  50 R. A left first holder  50 L is fixed at a lower-front corner of the left side wall of the drawer  8 . A right first holder SOR is fixed at a lower-front corner of the right side wall of the drawer  8 . 
     The left first holder  50 L and the right first holder  50 R are symmetrically opposite to each other and identical in shape. Hereinafter, the left first holder  50 L will be mainly described and shown in the drawings and the right first holder  50 R will be simply described and shown or omitted. 
     The first roller  41  includes a first rotation shaft  41 S and a cylindrical portion  41 A. The first rotation shaft  41 S extends in the left-right direction along the first axis X 41 . The cylindrical portion  41 A extends in an axial direction of the first rotation shaft  41 S and is fixed to the first rotation shaft  41 S to be rotatable integrally with the first rotation shaft  41 S. A first gear  71  is fixed to a left end of the first rotation shaft  41 S to be rotatable integrally with the first rotation shaft  41 S. 
     The left first holder  50 L includes a first bearing  59 . Although omitted from the drawings, the right first holder SOR also includes a first bearing  59 . The left first bearing  59  protrudes leftward from the drawer  8  and has a cylindrical shape with its end closed. 
     A left end of the first rotation shaft  41 S is inserted in and rotatably supported by the first bearing  59  of the left first holder  50 L. A right end of the first rotation shaft  41 S is inserted in and rotatably supported by the first bearing  59  of the right first holder  50 R. 
     As described above, the first roller  41  is supported by the drawer  8 , via the first holders  50 L and  50 R, rotatably about the first axis X 41 . The photosensitive drums  5 D and the first roller  4  are rotatably supported by the drawer  8 . Thus, the positioning accuracy of the first roller  41  relative to the photosensitive drums  5 D is unchanged and maintained high regardless of variations in positioning of the drawer  8  relative to the housing  9 . 
     As shown in  FIGS. 2 and 4 , the left first holder  50 L includes a positioner  53 , a first guide  51 , and a second guide  52 . Although omitted from the drawings, the right first holder SOR also includes a positioner  53 , a first guide  51 , and a second guide  52 . 
     As shown in  FIG. 4 , the positioner  53  is a recess which is recessed from a lower end of the first holder  50 L ( 50 R) upwardly into the first holder  50 L ( 50 R) along the nip line NL 1 . The positioner  53  includes the most recessed portion  53 C, a first side end  53 A, and a second side end  53 B. 
     The most recessed portion  53 C is inclined to be orthogonal to the nip line NL 1  and extends in the front-rear direction. The first side end  53 A is connected to a rear end of the most recessed portion  53 C and extends downward along the nip line NL 1 . The second side end  53 B is connected to a front end of the most recessed portion  53 C and extends downward along the nip line NL 1 . The second side end  53 B rearwardly faces the first side end  53 A and extends to a lower position than the first side end  53 A. In other words, the second side end  53 B extends downwardly beyond the first side end  53 A. 
     The first guide  51  is a portion of the lower end of the first holder  50 L ( 50 R) and is connected to a lower end of the first side end  53 A of the positioner  53 . The first guide  51  is located opposite to the second side end  53 B relative to the first side end  53 A. The first guide  51  extends rearward and relatively horizontally at a position above a lower end of the second side end  53 B of the positioner  53 . 
     The second guide  52  is a portion of the lower end of the first holder  50 L ( 50 R) and is located further to the rear than the first guide  51 . As the second guide  52  extends toward the front, the second guide  52  is inclined downwardly toward the first guide  51  and is connected to a lower end of the first guide  51 . 
     Structures of Second Roller, Second Holder, and Compressed Coil Spring 
     A description will be made on a structure for holding the second roller  42 . 
     As shown in  FIGS. 2 and 3 , the second roller  42  includes a second rotation shaft  42 S, a first cylindrical portion  42 L, and a second cylindrical portion  42 R. The second rotation shaft  42 S extends in the left-right direction along a second axis X 42 . The first cylindrical portion  42 L and the second cylindrical portion  42 R extend in an axial direction of the second rotation shaft  42 S and are fixed to the second rotation shaft  42 S to be rotatable integrally with the second rotation shaft  42 S. The first cylindrical portion  42 L and the second cylindrical portion  42 R are spaced from each other in the left-right direction by a predetermined distance. A second gear  72  is fixed to a left end of the second rotation shaft  42 S to be rotatable integrally with the second rotation shaft  42 S. 
     The housing  9  includes second holders  60 L and  60 R via which the second roller  42  is rotatably supported by the frame  90 . As shown in  FIG. 3 , a left second holder  60 L and a right second holder  60 R are symmetrically opposite to each other and similar in shape. The left second holder  60 L includes a third rotation shaft  73 S while the right second holder  60 R does not. Hereinafter, the left second holder  60 L will be mainly described and shown in the drawings and the right second holder  60 R will be simply described and shown or omitted. 
     Each of the second holders  60 L and  60 R includes a flat plate  61 , a second bearing  62 , and a guide  63 . 
     As shown in  FIGS. 3 and 4 , the flat plate  61  is substantially rectangular and extends along the nip line NL 1 . The flat plate  61  has a slot  68  formed through its lower portion in the left-right direction. The slot  68  extends along the nip line NL 1 . 
     The flat plate  61  of the left second holder  60 L includes the third rotation shaft  73 S. The third rotation shaft  73 S is a cylindrical shaft protruding leftward from a position above the slot  68  and further to the front than the slot  68 . The third rotation shaft  73 S rotatably supports a third gear  73 . A substantially cylindrical connector  73 J protrudes leftward from a left side of the third gear  73 . 
     As shown in  FIG. 3 , the second bearing  62  is connected to an upper end portion of the flat plate  61  and formed into a cylinder having a round through-hole extending in the left-right direction. 
     A left end of the second rotation shaft  42 S is inserted in the second bearing  62  of the left second holder  60 L. Specifically, the left end of the second rotation shaft  42 S is located between the second gear  72  and the first cylindrical portion  42 L. A right end of the second rotation shaft  42 S is inserted in the second bearing  62  of the right second holder  60 R. 
     The second holders  60 L and  60 R rotatably supports the second rotation shaft  42 S of the second roller  42 . 
     In this state, the left end of the second rotation shaft  42 S protrudes leftward beyond the second bearing  62  and the second gear  72 . The left end of the second rotation shaft  42 S is fittable into the positioner  53  of the left first holder  50 L. The right end of the second rotation shaft  42 S protrudes rightward beyond the second bearing  62 . A right end of the second rotation shaft  42 S is fittable into the positioner  53  of the right first holder  50 R. 
     In this state, the third gear  73  rotatably supported by the third rotation shaft  73 S of the left second holder  60 L is meshed with the second gear  72 . As shown in  FIG. 2 , a right end of a coupling  74  is connected to the connector  73 J of the third gear  73 . 
     A drive source (not shown) is connected to a left end of the coupling  74  via a plurality of gears and the like. A drive force of the drive source is transmitted, via the coupling  74 , the third gear  73 , and the second gear  72 , to the second rotation shaft  42 S. 
     The connector  73 J is configured to transmit the drive force of the drive source from the coupling  74  to the third gear  73  even when the coupling  74  is tilted relative to an axial direction of the third gear  73 . 
     As shown in  FIG. 3 , each guide  63  is connected to a front side of a corresponding second bearing  62 . Each guide  63  is C-shaped in section and has, on its inside, front-rear restriction surfaces  63 A and  63 B and left-right restriction surfaces  63 C and  63 D. The front-rear restriction surfaces  63 A and  63 B define an inside width W 1  therebetween. The front-rear restriction surface  63 A and  63 B and the left-right restriction surface  63 C and  63 D extend along the nip line NL 1 . 
     As shown in  FIG. 3 , the frame  90  has, at its left and right end portions, rails  93 . The left rail  93  and the right rail  93  are symmetrically opposite to each other and identical in shape. 
     Each rail  93  is L-shaped in section and extends along the nip line NL 1 . As shown in  FIG. 3 , the left rail  93  includes a rib protruding leftward and having a plate thickness T 1 . The inside width W 1  between the front-rear restriction surfaces  63 A and  63 B is set to be greater, to some extent, than the plate thickness T 1 . 
     The frame  90  has, at each of its left and right end portions, a shaft  98  and a spring holder  96 . The shaft  98  and the spring holder  96  on the left end portion, and the shaft  98  and the spring holder  96  on the right end portion are symmetrically opposite to each other and identical in shape. Thus, the shaft  98  and the spring holder  96  on the left end portion of the frame  90  are shown in the drawings and those on the right end portion of the frame  90  are omitted from the drawings. 
     The shaft  98  is cylindrical and protrudes leftward from a position lower and further to the rear than the rail  93 . As shown in  FIG. 4 , the shaft  98  has an axis serving as the pivot axis X 60 . The nip line NL 1  passes through the pivot axis X 60 . The pivot axis X 60  is located opposite to the first axis X 41  of the first roller  41  relative to the second axis X 42  of the second roller  42  and extends in the left-right direction. 
     As shown in  FIG. 3 , the spring holder  96  includes an inclined surface formed above the shaft  98  and below the rail  93 , and a protrusion protruding upward from the inclined surface. 
     As shown in  FIG. 2 , the rail  93  at the left end portion of the frame  90  is inserted in the guide  63  of the left second holder  60 L. The rail  93  at the right end portion of the frame  90  is inserted in the guide  63  of the right second holder  60 R. 
     As shown in  FIG. 3 , the inside width W 1  between the front-rear restriction surfaces  63 A and  63 B is set to be greater, to some extent, than the plate thickness T 1  of the rib of the rail  93 . This allows the guide  63  to move in the front-rear direction relative to the rail  93 . 
     A clearance between the left restriction surface  63 C and the rail  93 , and a clearance between the right restriction surface  63 D and the rail  93  are set to be small, thereby restricting rattling of the guide  63  in the left-right direction relative to the rail  93 . 
     As shown in  FIG. 4 , the shaft  98  at the left end portion of the frame  90  is inserted and located in the slot  68  of the left second holder  60 L. Although omitted from the drawings, the shaft  98  at the right end portion of the frame  90  is inserted in the slot  68  of the right second holder  60 R. 
     Thus, the second holders  60 L and  60 R are supported by the housing  9  to be movable linearly along the nip line NL 1 , as shown in  FIGS. 4 and 5 , and to be pivotable about the pivot axis X 60 , as shown in  FIGS. 6 and 7 . 
     The second roller  42  is supported by the housing  9 , via the second holders  60 L and  60 R thus structured, rotatably and movably relative to the first roller  41 . 
     As shown in  FIG. 3 , the image forming apparatus  1  includes compressed coil springs  69 L and  69 R. The compressed coil springs  69 L and  69 R are each an example of an urging member. 
     A lower end of a left compressed coil spring  69 L is held by the spring holder  96  at the left end portion of the frame  90 . An upper end of the left compressed coil spring  69 L contacts, from below, the second bearing  62  of the left second holder  60 L. A lower end of a right compressed coil spring  69 R is held by the spring holder  96  (not shown) at the right end portion of the frame  90 . An upper end of the right compressed coil spring  69 R contacts, from below, the second bearing  62  of the right second holder  60 R. The compressed coil springs  69 L and  69 R apply to the second rotation shaft  42 S of the second roller  42  an urging force acting upward along the nip line NL 1  shown in  FIG. 4 . 
     The second roller  42  is urged by the compressed coil springs  69 L and  69 R thus structured, upward and frontward, toward the first roller  41 . 
     Action of Positioner, First Guide, and Second Guide 
     As shown in  FIG. 4 , when the drawer  8  is located at the housed position, opposite ends of the second rotation shaft  42 S of the second roller  42  are fitted and located in the positioners  53  of the first holders  50 L and  50 R. Thus, the positioners  53  position the second roller  42  relative to the first roller  41 . 
     In this case, contact between the cylindrical portion  41 A of the first roller  41  and the first and second cylindrical portions  42 L and  42 R of the second roller  42 , restricts a fitting depth to which the second rotation shaft  42 S fits into the positioners  53 . While the most recessed portion  53 C of each positioner  53  faces an upper end of the second rotation shaft  42 S, the first side end  53 A and the second side end  53 B of each positioner  53  sandwich the second rotation shaft  42 S to thereby reduce rattling of the second rotation shaft  42 S in the front-rear direction. 
     In this case, the first guide  51  of each positioner  53  is located below an upper end of the second rotation shaft  42 S fitted and located in the positioner  53 . 
     In a state where the drawer  8  is located at the housed position, the first gear  71  and the second gear  72  are meshed with each other. Thus, a drive force of the drive source (not shown) is transmitted, via the coupling  74 , the third gear  73 , and the second gear  72 , to the first gear  71 . Accordingly, the first roller  41  and the second roller  42  are driven to rotate. 
     When the first roller  41  moves toward an exterior of the housing  9  in response to movement of the drawer  8  from the housed position toward the shifted position, the second holder  60 L ( 60 R) is urged by the compressed coil spring  69 L ( 69 R) to move up along the nip line NL 1 , as shown by way of example in  FIGS. 6 and 7 . In this case, the shaft  98  is located near a lower end of the slot  68 . 
     As shown in  FIG. 8 , when the first roller  41  approaches the second roller  42  in response to movement of the drawer  8  from the shifted position toward the housed position, the second guide  52  contacts from above the second rotation shaft  42 S before the first guide  51  does. When the drawer  8  moves further toward the housed position, the second guide  52  slidably contacts the second rotation shaft  42 . Thus, the second holder  60 L ( 60 R) and the second roller  42  gradually move down along the nip line NL 1  against the urging force of the compressed coil spring  69 L ( 69 R). 
     As shown in  FIG. 9 , when the drawer  8  moves further toward the housed position, the second guide  52  moves rearward away from the second rotation shaft  42 S, and then the first guide  51  slidably contacts from above the second rotation shaft  42 S. The first guide  51  slidably contacts from above the second rotation shaft  42 S before the positioner  53  positions the second rotation shaft  42 S. 
     As shown in  FIG. 4 , when the drawer  8  reaches the housed position, the first guide  51  moves rearward away from the second rotation shaft  42 S, and the second rotation shaft  42 S is urged by the compressed coil spring  69 L ( 69 R) to fit into the positioner  53 . The second side end  53 B located further to the front receives and stops the second rotation shaft  42 S. The second roller  42  disposed at the housing  9  is positioned relative to the first roller  41  disposed at the drawer  8  regardless of variations in positioning of the drawer  8  relative to the housing  9 . 
     When the drawer  8  moves from the housed position to the shifted position, the first guide  51  and the second guide  52  act on the second rotation shaft  42 S in a manner reverse to the above, and thus a description will be omitted. 
     Effects 
     In the image forming apparatus  1  according to aspects of the disclosure, as shown in  FIG. 1 , the drawer  8  rotatably supports the four photosensitive drums  5 D and the first roller  41 . The positioning accuracy of the first roller  41  relative to the photosensitive drums  5 D is unchanged and maintained high regardless of variations in positioning of the drawer  8  relative to the housing  9 . As shown in  FIGS. 4 to 9 , the second roller  42  is movable by the second holder  60 L ( 60 R) and the compressed coil spring  69 L ( 69 R) relative to the first roller  41 . As shown in  FIG. 4 , when the drawer  8  moves from the shifted position to the housed position, the second rotation shaft  42 S of the second roller  42  is positioned by the positioner  53  of the first holder  50 L ( 50 R) relative to the first roller  41 . 
     In short, in the image forming apparatus  1 , the second roller  42  disposed at the housing  9  moves toward the first roller  41  disposed at the drawer  8 , regardless of variations in positioning of the drawer  8  relative to the housing  9 . This may improve the accuracy in positioning the second roller  42  relative to the photosensitive drums  5 D, and prevent the second roller  42  from being positioned skewly relative to the photosensitive drums  5 D. Accordingly, sheet skew may be prevented or reduced when the first roller  41  and the second roller  42  pinch a sheet SH therebetween and transport the sheet SH toward the photosensitive drums  5 D. As a result, image skew relative to the sheet SH may be prevented or reduced in the image forming apparatus  1 . 
     Thus, the image forming apparatus  1  according to aspects of the disclosure may achieve image forming with an improved accuracy. 
     In the image forming apparatus  1 , as shown in  FIG. 4 , the second rotation shaft  42 S of the second roller  42  fits directly, not via a bearing or the like, in the positioner  53  which is a recess formed in the first holder  50 L ( 50 R). This may allow for accurate positioning of the second rotation shaft  42 S of the second roller  42  relative to the first rotation shaft  41 S of the first roller  41 , leading to the improvement of the positioning accuracy of the second roller  42  relative to the photosensitive drum  5 D. 
     In the image forming apparatus  1 , as shown in  FIG. 4 , the first guide  51  is located below the upper end of the second rotation shaft  42 S fitted and located in the positioner  53 . As shown in  FIG. 9 , when the drawer  8  moves from the shifted position toward the housed position, the second rotation shaft  42 S of the second roller  42  is pushed down by the first guide  51  and then is urged by the compressed coil spring  69 L ( 69 R) to fit into the positioner  53 . This may prevent frictional contact between the first roller  41  and the second roller  42  during movement of the drawer  8 , thereby reducing local abrasion of the first roller  41  and the second roller  42 . 
     In the image forming apparatus  1 , as shown in  FIG. 4 , in a state where the drawer  8  is located at the housed position, the first gear  71  fixed to the first rotation shaft  41 S of the first roller  41  is meshed with the second gear  72  fixed to the second rotation shaft  42 S of the second roller  42 . As described above, when the drawer  8  moves from the shifted position toward the housed position, the second rotation shaft  42 S of the second roller  42  is pushed down by the first guide  51  and then is urged by the compressed coil spring  69 L ( 69 R) to fit into the positioner  53 . This may prevent frictional contact between the first gear  71  and the second gear  72  during movement of the drawer  8 , thereby reducing local abrasion of the first gear  71  and the second gear  72 . 
     In the image forming apparatus  1 , as shown in  FIG. 8 , when the drawer  8  moves from the shifted position toward the housed position, the second rotation shaft  42 S of the second roller  42  is gradually pushed down by the second guide  52 , which is inclined downwardly toward the first guide  51 , and then is guided by the first guide  51 . This may reduce an abrupt increase in resistance when the drawer  8  moves from the shifted position to the housed position. Thus, the drawer  8  moves smoothly to the housed position in the image forming apparatus  1 . 
     In the image forming apparatus  1 , as shown in  FIG. 4 , when the drawer  8  reaches the housed position, the second rotation shaft  42 S fitting into the positioner  53  is reliably stopped by the second side end  53 B which extends downwardly beyond the first side end  53 A. This may further improve the positioning accuracy of the second roller  42  relative to the photosensitive drums  5 D in the image forming apparatus  1 . 
     In the image forming apparatus  1 , as shown in  FIGS. 5 to 7 , the second holder  60 L ( 60 R), which is supported movably relative to the housing  9 , rotatably supports the second rotation shaft  42 S of the second roller  42 . The second gear  72  is fixed to the second rotation shaft  42 S to be rotatable integrally with the second rotation shaft  42 S. As shown in  FIG. 3 , the left second holder  60 L rotatably supports the third gear  73  which meshes with the second gear  72  to transmit a drive force to the second rotation shaft  42 S. The second holder  60 L ( 60 R) thus structured allows the second roller  42  to properly move toward the first roller  41 . Movement of the second roller  42  toward the first roller  41  does not change the positional relation between the second gear  72  and the third gear  73 . Thus, a drive force is properly transmitted, via the third gear  73  and the second gear  72 , to the second rotation shaft  42 S of the second roller  42 . Accordingly, the first roller  41  and the second roller  42  pinch a sheet SH therebetween and transport the sheet SH smoothly toward the photosensitive drums  5 D. 
     In the image forming apparatus  1 , as shown in  FIG. 2 , the coupling  74  is connected to the connector  73 J of the third gear  73 . When the third gear  73  moves together with the left second holder  60 L as the second roller  42  moves toward the first roller  41 , the coupling  74  properly transmits a drive force, via the third gear  73  and the second gear  72 , to the second rotation shaft  42 S of the second roller  42 . 
     In the image forming apparatus  1 , as shown in  FIGS. 5 to 7 , the second holder  60 L ( 60 R) is supported by the housing  9  to be movable linearly along the nip line NL 1  and pivotable about the pivot axis X 60 . Thus, the second holder  60 L ( 60 R) allows the second roller  42  to properly move toward the first roller  41 . 
     In the image forming apparatus  1 , as shown in  FIGS. 3 and 4 , linear movement and pivoting of the second holder  60 L ( 60 R) are readily achieved by the shaft  98  of the frame  90  inserted in the slot  68  in the second holder  60 L ( 60 R). The nip line NL passes through the pivot axis X 60 . This may prevent the second roller  42  from deviating unevenly from the nip line NL when the second holder  60 L ( 60 R) pivots about the pivot axis X 60  rearward (shown in  FIG. 6 ) and frontward (shown in  FIG. 7 ). 
     In the image forming apparatus  1 , as shown in  FIG. 1 , the first roller  41  and the second roller  42  are disposed at the upper end PIT of the curved portion of the transport path P 1 . The first roller  41  is immovable when the first roller  41  receives a pressing force from a sheet SH passing the upper end NT of the curved portion. The second roller  42  moves toward the first roller  41 , thereby properly transporting the sheet SH toward the photosensitive drums  5 D. 
     In the image forming apparatus  1 , as shown in  FIG. 1 , the first roller  41  and the second roller  42 , which serve as the registration rollers, advantageously meets the requirement for high accuracy in positioning the registration rollers relative to the photosensitive drums  5 D. 
     A description will be made on an image forming apparatus, especially on a structure for holding a second roller  42 , according to a second embodiment. In the second embodiment, elements shown and described in the first embodiment are designated by the same reference numerals, and thus the description thereof will be omitted. 
     As shown in  FIGS. 10 and 11 , the image forming apparatus of the second embodiment uses second holders  600 L,  600 R instead of the second holders  60 L,  60 R of the first embodiment, and a gear cover  800 L to be assembled to a left second holder  600 L for covering the second gear  72 . 
     The second holder  600 L includes a rib  600 L 1  and a protrusion  600 L 2 , in addition to the elements of the second holder  60 L of the first embodiment. Other elements of the second holder  600 L are the same as those of the second holder  60 L. 
     The gear cover  800 L includes a sleeve  1100 L, a hook  800 L 1 , and a recess  800 L 2 . The sleeve  1100 L covers a left end of the second rotation shaft  42 S. The hook  800 L 1  engages with the rib  600 L 1  of the second holder  600 L to fix the gear cover  800 L to the second holder  600 L. The sleeve  1100 L is an example of a covering member. The recess  800 L 2  engages with the protrusion  600 L 2  of the second holder  600 L to lock the gear cover  800 L together with the hook  800 L 1 . 
     A right second holder  600 R includes a sleeve  1100 R, in addition to the elements of the second holder  60 R of the first embodiment. The sleeve  1100 R is integral with the second holder  600 R and covers a right end of the second rotation shaft  42 S. Other elements of the second holder  600 R are the same as those of the second holder  60 R. 
     As shown in  FIG. 11 , a positioner  53  of a left first holder  50 L is a recess which is recessed from a lower end of the first holder  50 L upwardly into the first holder  50 L along the nip line NL 1 . The recess is sized to receive the sleeve  1100 L covering the end of the second rotation shaft  42 S. In a state where the drawer  8  is located at the housed position, the sleeve  1100 L covering the left end of the second rotation shaft  42 S is fitted and located in the positioner  53  of the first holder  50 L. Thus, the positioner  53  positions the second rotation shaft  42 S relative to the first roller  41 . 
     While the second rotation shaft  42 S rotates, the sleeve  1100 L is unable to rotate by the hook  800 L 1  and the recess  800 L 2  engaging with the second holder  600 L, and thus remains stationary relative to the positioner  53 . 
     Although omitted from the drawings, a positioner  53  of a right first holder SOR is also a recess sized to receive the sleeve  1100 R. In a state where the drawer  8  is located at the housed position, the sleeve  1100 R covering a right end of the second rotation shaft  42 S is fitted and located in the positioner  53  of the first holder  50 R. Thus, the positioner  53  positions the second rotation shaft  42 S relative to the first roller  41 . 
     While the second rotation shaft  42 S rotates, the sleeve  1100 R, which is integral with the second holder  600 R, is unable to rotate, and thus remains stationary relative to the positioner  53 . 
     This may improve the positioning accuracy of the second roller  42  relative to the photosensitive drums  5 D in the image forming apparatus of the second embodiment, as is the case with the image forming apparatus of the first embodiment. Thus, the image forming apparatus of the second embodiment may also achieve image forming with an improved accuracy. 
     In the second embodiment, while the second rotation shaft  42 S rotates, the sleeve  1100 L remains stationary relative to the positioner  53 . Thus, friction is unlikely to act between the second rotation shaft  42 S and the positioner  53  in a direction against the urging force of the compressed coil spring  69 L ( 69 R) or in a direction in which the second rotation shaft  42 S comes off the positioner  53 . This stabilizes the position of the second rotation shaft  42 S. 
     While the disclosure has been described with reference to particular examples, various changes, arrangements and modifications may be applied therein without departing from the spirit and scope of the disclosure. 
     In the above-described embodiments, the photosensitive drum  5 D is rotatably supported directly by the drawer  8  but is not limited to this structure. For example, the photosensitive drum  5 D may be rotatably supported by the drawer  8  via an additional element. Specifically, the photosensitive drum  5 D may be rotatably supported by a process cartridge which may be supported by the drawer  8 . 
     In the above-described embodiments, the first roller  41  is rotatably supported by the first holder  50 L ( 50 R) and the first holder  50 L ( 50 R) is supported by the drawer  8  but is not limited to this structure. For example, the drawer  8  may include a bearing integrally formed with the drawer  8  and the first roller  41  may be rotatably supported by the bearing such that the first roller  41  is supported directly by the drawer  8 . 
     In the above-described embodiments, the second roller  42  is movable linearly along the nip line NL 1  and pivotable about the pivot axis X 60  but is not limited to this structure. For example, the second roller  42  may be movable linearly or pivotable, relative to the first roller  41 . 
     In the above-described embodiments, the drawer  8  when at the shifted position, is removed from the housing  9  and the entire drawer  8  is outside the housing  9  but is not limited to this structure. For example, the drawer  8  may not be removed from the housing  9  and a portion of the drawer  8  may be exposed from the housing  9 .