Patent Publication Number: US-8983337-B2

Title: Image forming apparatus

Description:
INCORPORATION BY REFERENCE 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2013-068842, filed Mar. 28, 2013. The contents of this application are incorporated herein by reference in their entirety. 
     BACKGROUND 
     The present disclosure relates to image forming apparatuses provided with a mechanism for moving a developing roller closer to and away from an image bearing member. 
     Some image forming apparatuses (for example, printers and copiers) include a photosensitive drum (image bearing member) and a developing unit. In addition, a gap roller is provided near each axial edge of a developing roller. The photosensitive drum bears a toner image on its peripheral surface. The developing unit includes the developing roller. The developing roller supplies toner to the photosensitive drum. In the printing operation, the photosensitive drum and the developing roller are brought into the state where the respective peripheral surfaces are opposed to each other with a predetermined gap therebetween (the state where each gap roller abuts against a non-image forming region of the peripheral surface of the photosensitive drum). In for example maintenance or jam-handling, on the other hand, when the developing unit is pulled out of the main body of the apparatus, the photosensitive drum and the developing roller are brought into the state where the respective peripheral surfaces are away from each other so as not to damage the peripheral surfaces. 
     For example, an image forming apparatus is suggested that includes a pressure member to bring the peripheral surface of the developing roller closer to the peripheral surface of the photosensitive drum. The developing unit is disposed to be horizontally movable within the main body. The developing unit is provided with a pin member on a side wall thereof. The pin member is guided by a horizontally elongated slot. The developing unit is urged in a horizontal direction by the pressure member. The pressure member presses the peripheral surface of the developing roller toward the peripheral surface of the photosensitive drum. Upon release of the pressure exerted by the pressure member, the developing roller moves away from the photosensitive drum. 
     SUMMARY 
     An image forming apparatus according to one aspect of the present disclosure includes a main housing, a drive unit, an image bearing member, a developing unit, a support unit, an urging member, and a switching mechanism. The drive unit has a drive output shaft that generates a rotational drive force for axial rotation. The image bearing member is disposed in the main housing and has a peripheral surface for bearing a toner image. The developing unit is attached to the main housing to be detachable by pulling out. The developing unit includes: a developing roller that has a peripheral surface for carrying toner thereon and that supplies the toner to the image bearing member; and a first drive input section that supplies a rotational drive force to the developing roller. The support unit is supported by the main housing to be freely pivotable about an axis of a pivot shaft and places and supports the developing unit in position. The urging member generates an urging force for urging the support unit to pivot in one of pivoting directions about the pivot shaft. The one pivoting direction coincides with an approaching direction for moving the peripheral surface of the developing roller toward the peripheral surface of the image bearing member. The switching mechanism switches the position of the support unit supporting the developing unit. The switching is made between a first position in which the peripheral surface of the developing roller is located relatively close to the peripheral surface of the image bearing member and a second position in which the peripheral surface of the developing roller is located relatively away from the peripheral surface of the image bearing member. The support unit includes: a second drive input section that is disposed coaxially with the pivot shaft and that receives a rotational drive force from the drive output shaft; and a transmission mechanism that transmits the rotational drive force received by the second drive input section to the first drive input section. The rotational drive force generated by the drive output shaft is for rotation in a direction of pivoting the support unit in the approaching direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an appearance of an image forming apparatus according to an embodiment of the present disclosure. 
         FIG. 2  is a cross sectional view of an internal configuration of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 3  is a perspective view of an appearance of the image forming apparatus according to an embodiment of the present disclosure, in the state where a front cover is removed. 
         FIG. 4  is a perspective view showing a developing unit and its surrounding units of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 5  is a perspective view showing a support unit and a switching mechanism of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 6  is a perspective view showing the developing unit of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 7  is a cross sectional view showing the support unit in a fitting position (first position), of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 8  is a cross sectional view showing the support unit in a releasing position (second position), of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 9  is a perspective view showing the switching mechanism in the state where switching to the fitting position is effected, of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 10  is a perspective view showing the switching mechanism in the state where switching to the releasing position is effected, of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 11  is a perspective view showing a drive unit of a main housing of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 12  is a perspective view showing the drive unit in an assembled state with the developing unit and the support unit, of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 13  is a perspective view showing a rear side of the support unit of the image forming apparatus according to the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following describes an embodiment of the present disclosure with reference to the accompanying drawings. In each figure, the Z1 direction corresponds to “up”, Z2 to “down”, Y1 to “rear”, Y2 to “front”, X1 to “right”, and X2 to “left”. 
     First, with reference to  FIG. 1 , the following describes a schematic configuration of an image forming apparatus  1  according to the embodiment of the present disclosure.  FIG. 1  is a perspective view of an appearance of the image forming apparatus  1  according to the embodiment of the present disclosure. 
     The image forming apparatus  1  is a monochrome or color printer having a copier function, for example. The image forming apparatus  1  includes a main housing  10  (the housing of the main body) having a boxlike shape. The main housing  10  includes a lower housing  11 , an upper housing  12  disposed above the lower housing  11 , a connecting housing  13  disposed between the lower housing  11  and the upper housing  12 . The connecting housing  13  constitutes a right face  10 R of the main housing  10 . 
     The lower housing  11  accommodates various units used for performing image forming processing on a sheet. The upper housing  12  accommodates a scanner that optically reads an image of an original document sheet. The scanner operates when the image forming apparatus  1  functions as a copier. 
     A front cover  110  covers the front face of the lower housing  11 . A sheet cassette  14  is disposed below the lower housing  11  to be freely detachable. The sheet cassette  14  stores a stack of sheets which are to be subjected to image forming processing. 
     The main housing  10  has an in-body paper ejection space  15 . The in-body paper ejection space  15  is a space surrounded by the upper face of the lower housing  11 , the lower face of the upper housing  12 , and the left face of the connecting housing  13 . Sheets having gone through image forming processing are ejected to the in-body paper ejection space  15 . The in-body paper ejection space  15  is provided at the bottom with an in-body paper ejection tray  151  for receiving sheets. 
     An operation panel  16  is attached to the front face of the upper housing  12 . The operation panel  16  receives user operation information input to the image forming apparatus  1 . 
     On the right face  10 R of the main housing  10 , a manual feed tray  17  is provided for manually feeding a sheet. The manual feed tray  17  can be freely opened and closed relative to the right face  10 R. 
     Next, mainly with reference to  FIGS. 2 and 3 , the following describes an internal configuration of the image forming apparatus  1  according to the embodiment of the present disclosure.  FIG. 2  is a cross sectional view of the internal configuration of the image forming apparatus  1 .  FIG. 3  is a perspective view showing the image forming apparatus  1  in the state where the front cover  110  is removed. In each of  FIGS. 2 and 3 , the upper right half of the lower housing  11  and the connecting housing  13  are shown. The cross section shown in  FIG. 2  is taken along the right-and-left direction (X direction) of the image forming apparatus  1 . 
     The lower housing  11  accommodates units used for image forming. The units used for image forming include a drum unit  21 , an exposure unit  22 , a developing unit  23 , a toner container  24 , a fixing unit  25 , and a conveyance unit  26 . These units are disposed such that each unit can be separately pulled forward relative to the lower housing  11  (a housing frame  111 ). 
     The drum unit  21  includes a photosensitive drum  211  (image bearing member), a charging device  212 , and a cleaning device  214 . The charging device  212  is disposed around the photosensitive drum  211 . The photosensitive drum  211  rotates about its axis. The photosensitive drum  211  bears an electrostatic latent image and a toner image on its peripheral surface. The charging device  212  uniformly charges the peripheral surface of the photosensitive drum  211 . 
     The exposure unit  22  emits laser light to the peripheral surface of the photosensitive drum  211  to form an electrostatic latent image. 
     The developing unit  23  includes a developing roller  231 . The developing roller  231  is disposed in proximity to the photosensitive drum  211 . The developing roller  231  supplies toner to the peripheral surface of the photosensitive drum  211  to develop the electrostatic latent image formed on the peripheral surface of the photosensitive drum  211 . 
     The toner container  24  replenishes toner to the developing unit  23 . 
     The conveyance unit  26  includes a transfer roller  213  pressed against the photosensitive drum  211 . The transfer roller  213  forms a transfer nip portion N1 with the photosensitive drum  211 . The transfer roller  213  transfers the toner image on the photosensitive drum  211  to the sheet. 
     At a downstream location near the transfer nip portion N1, a separator section  215  is disposed. The cleaning device  214  cleans the peripheral surface of the photosensitive drum  211  after the toner image transfer. The separator section  215  separates, from the photosensitive drum  211 , the sheet onto which the toner image is transferred in the transfer nip portion N1. The sheet thus separated is forwarded to the fixing unit  25 . 
     The fixing unit  25  includes a fixing roller  251  and a pressure roller  252 . The fixing roller  251  includes a heat source therein. The fixing roller  251  and the pressure roller  252  together form a fixing nip portion N2. 
     The fixing unit  25  applies heat and pressure through the fixing nip portion N2 to the sheet onto which the toner image has been transferred in the transfer nip portion N1. The sheet subjected to the fixing process (the application of heat and pressure) by the fixing unit  25  is ejected from a sheet ejecting outlet  131  into the in-body paper ejection tray  151 . 
     As shown in  FIG. 2 , a sheet conveyance path for conveying sheets is provided inside the main housing  10 . The sheet conveyance path includes a main conveyance path P1 leading to the sheet ejecting outlet  131 . The main conveyance path P1 extends in the up-and-down direction (Z direction) from a location near the bottom of the lower housing  11  to the connecting housing  13  via the transfer nip portion N1 and the fixing nip portion N2. In addition, a reverse conveyance path P2 is provided for conveying sheets in reverse for duplex printing. The reverse conveyance path P2 extends from the downstream end of the main conveyance path P1 to a location near the upstream end of the main conveyance path P1. 
     The sheet cassette  14  (see  FIG. 1 ) is provided with a sheet accommodating section for accommodating a stack of sheets. At a location near the top right of the sheet accommodating section, a pickup roller  271  and a paper feed roller pair  272  are disposed. The pickup roller  271  picks up sheets one by one from the topmost sheet in the stack. The paper feed roller pair  272  forwards the sheet picked up by the pickup roller  271  to the upstream end of the main conveyance path P1. 
     A registration roller pair  273  is disposed upstream from the transfer nip portion N1 in the main conveyance path P1. The registration roller pair  273  forwards the sheet to the transfer nip portion N1 with predetermined timing. 
     In the image forming apparatus  1  according to the present embodiment, the inner face (left face) and the outer face (right face) of the conveyance unit  26  together form the main conveyance path P1 and the reverse conveyance path P2. For example, an immediately upstream portion of the main conveyance path P1 from the transfer nip portion N1 is defined by the inner face of the conveyance unit  26  and a pre-transfer guide  274  opposed to the conveyance unit  26 . In addition to the transfer roller  213  described above, the conveyance unit  26  includes one roller in the registration roller pair  273  and one roller in a conveyance roller pair (not shown) for conveying a sheet in the reverse conveyance path P2. 
     The following describes an image forming operation of the image forming apparatus  1  mainly with reference to  FIG. 2 . 
     First, the charging device  212  charges the peripheral surface of the photosensitive drum  211  substantially uniformly. Next, the charged peripheral surface of the photosensitive drum  211  is exposed to laser light emitted by the exposure unit  22 . As a result, an electrostatic latent image conforming to image data (image to be formed on a sheet) is formed on the peripheral surface of the photosensitive drum  211 . Next, the developing unit  23  supplies toner to the peripheral surface of the photosensitive drum  211 . As a result, the toner image is visualized on the peripheral surface of the photosensitive drum  211 . 
     For example, in the single-side printing process, a sheet is fed from the sheet cassette  14  (see  FIG. 1 ) or the manual feed tray  17  to the main conveyance path P1. Next, the transfer nip portion N1 causes the toner image to be transferred to the sheet. Subsequently, the fixing nip portion N2 causes the transferred toner image to be fixed onto the sheet. The sheet onto which the toner image is fixed is ejected through the sheet ejecting outlet  131  into the in-body paper ejection tray  151 . 
     In the duplex printing process, on the other hand, the transfer and fixing processes described above are carried out on one side of the sheet. Then, when the sheet is partially ejected out of the sheet ejecting outlet  131  to the in-body paper ejection tray  151 , switch-back conveyance is executed to pull the sheet back. In detail, the sheet is brought back up to a location close to the upstream end of the main conveyance path P1 through the reverse conveyance path P2. Then, the other surface (unprinted surface) of the sheet is subjected to the transfer process and the fixing process. After the transfer and fixing processes on the both surfaces of the sheet, the sheet is ejected through the sheet ejecting outlet  131  onto the in-body paper ejection tray  151 . 
     The following describes operation of the image forming apparatus  1  when the developing unit  23  or the drum unit  21  is pulled out of the main housing  10  for maintenance or jam-handling for example, mainly with reference to  FIG. 13 .  FIG. 13  is a perspective view showing a rear side of a support unit  30 . 
     The image forming apparatus  1  according to the present embodiment further includes a mechanism for moving the peripheral surface of the photosensitive drum  211  and the peripheral surface of the developing roller  231  away from each other when the developing unit  23  or the drum unit  21  is pulled out of the main housing  10 . In detail, the developing roller  231  includes a pair of gap rollers  23 D. Each gap roller  23 D is disposed in proximity to an axial edge of the developing roller  231 . The photosensitive drum  211  has a non-image forming region  211 D on the peripheral surface along each axial edge. Such a mechanism ensures that the image forming apparatus  1  according to the present embodiment is less prone to damage on the respective peripheral surfaces of the photosensitive drum  211  and the developing roller  231 . 
     For example, at the time of image formation (hereinafter, referred to as “during normal operation”), the developing roller  231  and the photosensitive drum  211  are brought into a state where a predetermined gap (hereinafter, referred to as a first gap) between the respective peripheral surfaces. During the normal operation, each gap roller  23 D is urged against and in contact with the corresponding non-image forming region  211 D. Since the developing roller  231  is provided with the gap rollers  23 D, the first gap is secured between the peripheral surface of the developing roller  231  and the peripheral surface of the photosensitive drum  211  during the normal operation. 
     On the other hand, when the developing unit  23  or the drum unit  21  is pulled out of the main housing  10  for maintenance or jam-handling for example, the urging force against the gap rollers  23 D is released. As a consequence, the photosensitive drum  211  and the developing roller  231  are moved away from each other. As a result, a gap significantly wider than the gap during the normal operation (the first gap) is secured between the peripheral surface of the photosensitive drum  211  and the peripheral surface of the developing roller  231  (this wider gap is referred to as a second gap). 
     Mainly with reference to  FIGS. 4-8  and  13 , the following describes details of the mechanism for moving the photosensitive drum  211  and the developing roller  231  to have the respective peripheral surfaces closer to each other or away from each other.  FIG. 4  is a perspective view showing the developing unit  23  along with its surrounding units. In  FIG. 4 , the photosensitive drum  211  is illustrated alone, from among components of the drum unit  21 , in a manner to reflect its relative location.  FIG. 5  is a perspective view of the support unit  30  and a switching mechanism  40 .  FIG. 6  is a perspective view showing the developing unit  23  alone.  FIGS. 7 and 8  each show the image forming apparatus  1  with the respective units assembled in the main housing  10 . 
     As shown in  FIG. 4 , the image forming apparatus  1  includes the support unit  30 , the switching mechanism  40 , a pair of front and rear coil springs (urging members)  50 . The support unit  30  is supported by the main housing  10  to be freely pivotable about an axis of a pivot shaft A. The pivot shaft A extends in the front-and-rear direction (Y direction). The support unit  30  places and supports the developing unit  23  in position. 
     As shown in  FIG. 5 , the support unit  30  includes a base  31 , a left wall  32  (first side wall), and a rear wall  33  (second side wall). The base  31  is formed of a horizontally extending plate. In detail, the base  31  is formed of a rectangular plate elongated in the front-and-rear direction (Y direction). As shown in  FIG. 4 , the developing unit  23  is mounted on the upper face of the base  31 . The lower face of the base  31  faces toward the switching mechanism  40 . 
     A right guide section  34  is disposed to stand vertically upward from a right edge  31 R of the base  31 . The right guide section  34  has an inverted L shape in cross section taken along the right-and-left direction (X direction). In addition, left guide sections  35  are disposed near a left edge  31 L of the base  31 . Each left guide section  35  is a convex ridge extending in the front-and-rear direction (Y direction). When slid into the support unit  30  from the front (from the Y2 direction), the developing unit  23  is guided by the right guide section  34  and the left guide section  35 . 
     As shown in  FIG. 5 , the left wall  32  stands vertically upward from the left edge  31 L (one edge) of the base  31 . As shown in  FIG. 13 , the left wall  32  is a side wall parallel to the developing roller  231 . The left wall  32  is higher at its front edge portion  32 F and rear edge portion  32 R than at its central portion. 
     As shown in  FIG. 5 , a fulcrum member  36  is provided inside the front edge portion  32 F. The fulcrum member  36  is a hollow cylinder that is open toward the front (toward the Y2 direction). As shown in  FIG. 13 , a pin  74  is disposed inside the rear edge portion  32 R to project toward the rear (toward the Y1 direction). The fulcrum member  36  and the pin  74  are coaxial. The fulcrum member  36  and the pin  74  together constitute the pivot shaft A of the support unit  30 . As shown in  FIGS. 7 and 8 , the fulcrum member  36  is for receiving insertion of a pin member  112  of the main housing  10 . In addition, the pin  74  is for insertion into a pin receiver  72  (see  FIG. 11 ) of a drive unit  70 , which will be described later. By the insertion of the fulcrum member  36  and the pin  74  with their associated members, the support unit  30  is freely pivotable about the axis of the pivot shaft A. 
     As shown in  FIG. 13 , a second coupling gear  73  (a second drive input section) is assembled to the rear edge portion  32 R of the left wall  32 . The second coupling gear  73  is disposed coaxially with the pin  74  (or with the pivot shaft A). The second coupling gear  73  includes, for example, gear teeth formed along an edge of a hollow cylinder that is coaxial with the pin  74 . The second coupling gear  73  receives a rotational drive force from a first coupling gear  71  (see  FIG. 11 ) of the later-described drive unit  70 . 
     As shown in  FIG. 5 , the rear wall  33  stands vertically upward from a rear edge  31   b  (another edge) of the base  31 . The rear wall  33  is a side wall perpendicular to the left wall  32 . The left edge of the rear wall  33  is continuous with the rear edge of the left wall  32 . The rear wall  33  has a hollow interior  331  in which a transmission gear  37  (transmission mechanism) is rotatably held. The transmission gear  37  receives a rotational drive force from the second coupling gear  73  (see  FIG. 13 ). The rear wall  33  is provided with a first locating hole  381  and a second locating hole  382  that are vertically aligned on the front face (in detail, at a location closer to the right edge) of the rear wall  33 . In addition, a coupling member  39  is disposed on the rear wall  33  at a location closer toward the left edge (toward the X2 direction) of the rear wall  33 . The coupling member  39  receives a rotational drive force transmitted from the second coupling gear  73  (see  FIG. 13 ). 
     The base  31  is open at its front edge  31 F (the edge toward the Y2 direction). As shown in  FIGS. 7 and 8 , a pair of hooks  311  (first engaging piece) is disposed below the base  31 . Each hook  311  is a plate that extends obliquely downwardly from the base  31  to form an angle of approximately 30° or so at a location near the right of the base  31 . The hook  311  has a sloped portion  312 . A gap is present between the sloped portion  312  and the lower face of the base  31 . 
     The following describes the configuration of the developing unit  23  mainly with reference to  FIGS. 6 and 7 . 
     As shown in  FIG. 6 , the developing unit  23  includes a developing housing  60 . The developing housing  60  accommodates the developing roller  231 , a first stirring screw  232 , a second stirring screw  233 , and a toner supplying screw  234 . 
     As shown in  FIG. 6 , the developing housing  60  is elongated in the front-and-rear direction (Y direction). The developing housing  60  is formed of a bottom plate  601 , a front plate  602 , a rear plate  603 , a left plate  604 , and a top plate  605 . As shown in  FIG. 7 , the developing housing  60  has an opening at a location near the right edge (the edge toward the X1 direction) of the developing housing  60 . The opening opens toward the Z1 direction and extends in the front-and-rear direction (Y direction). In the opening, the developing roller  231  is exposed halfway around the peripheral surface. 
     As shown in  FIG. 6 , the developing roller  231  is for bearing toner on its peripheral surface. The developing roller  231  is supported by the developing housing  60  to be freely rotatable. As shown in  FIG. 7 , the peripheral surface of the developing roller  231  is opposed to the peripheral surface of the photosensitive drum  211 . The toner carried on the developing roller  231  is supplied to the photosensitive drum  211 . With the toner supplied to the developing roller  231 , the electrostatic latent image formed on the peripheral surface of the photosensitive drum  211  is developed into a toner image. 
     Each gap roller  23 D described above (see  FIG. 13 ) is disposed at a location near the axial edge of the developing roller  231 . As shown in  FIG. 7 , the first and second stirring screws  232  and  233  convey toner while stirring the toner within the developing housing  60 . In addition, the first and second stirring screws  232  and  233  charge the toner. The toner supplying screw  234  is for uniformly supplying toner to the second stirring screw  233  (in greater detail, within the housing space accommodating the second stirring screw  233 ). 
     As shown in  FIG. 6 , the top plate  605  has a toner inlet  61  at a location near its front edge. The toner inlet  61  is for introducing toner into the developing housing  60 . At the time of toner replenishment, toner stored in the toner container  24  (see  FIG. 4 ) is supplied to an intermediate hopper  241  (see  FIG. 4 ) and then into the developing housing  60  from the toner inlet  61 . 
     The following now describes a configuration of the intermediate hopper  241  mainly with reference to  FIG. 4 . 
     The intermediate hopper  241  includes a receiving port, a discharge port, and a toner conveyance path. The receiving port receives toner supplied from the toner container  24 . The discharge port is opposed to the toner inlet  61 . The discharge port includes a shutter member. The shutter member opens and closes in response to detachment/attachment of the developing housing  60  (see  FIG. 6 ) from/to the main housing  10 . The toner conveyance path connects the receiving port and the discharge port. 
     In the image forming apparatus  1  according to the present embodiment, the intermediate hopper  241  is located between the toner container  24  and the developing unit  23 . With such a configuration, the image forming apparatus  1  allows the developing unit  23  to be detached alone from the main housing  10 , i.e., separately from the toner container  24 . 
     With reference to  FIGS. 6 and 7 , the following continues the description of the configuration of the developing unit  23 . With the developing unit  23  mounted on the base  31  of the support unit  30 , the bottom plate  601  faces toward the base  31  as shown in  FIG. 7  and the rear plate  603  (see  FIG. 6 ) faces toward the rear wall  33  as shown in  FIG. 13 . As shown in  FIG. 6 , a gear housing  62  is disposed to project at a location on the rear plate  603  near the right edge thereof. The gear housing  62  accommodates a developing-roller driving gear  63  (first drive input section). 
     As shown in  FIG. 6 , the gear housing  62  has an opening at a location toward the left (toward the X2 direction). The gear teeth of the developing-roller driving gear  63  are partly exposed through the opening. As shown in  FIG. 13 , the developing-roller driving gear  63  is attached to the rear end of the rotary shaft of the developing roller  231 . By the rotational drive force supplied to the developing-roller driving gear  63 , the developing roller  231  rotates about the axis of the rotary shaft. 
     The rear plate  603  has a window portion at a location near its left edge. A coupling socket  65  is exposed through the window portion. The coupling socket  65  is directly connected to the rear end of the rotary shaft of the toner supplying screw  234  (see  FIG. 7 ). By the rotational drive force supplied to the coupling socket  65 , the toner supplying screw  234  (see  FIG. 7 ) rotates about the axis of the rotary shaft. 
     A first locating pin  641  and a second locating pin  642  are each disposed on the rear plate  603  to project at a location near the right edge of the rear plate  603  (the edge toward the X1 direction). The first and second locating pins  641  and  642  are at locations aligned in the up-and-down direction. The first locating pin  641  is located centrally of the gear housing  62 . The second locating pin  642  is located near the lower edge of the rear plate  603 . 
     As shown in  FIG. 7 , the bottom plate  601  is provided with a right guide rib  606  (see  FIG. 7 ) along its right edge. The right guide rib  606  projects toward the right (toward the X1 direction) and extends in the front-to-rear direction (Y direction). As shown in  FIG. 6 , the bottom plate  601  is provided with a left guide rib  607  along its left edge. The left guide rib  607  projects toward the left (toward the X2 direction) and extends in the front-to-rear direction (Y direction). A blade  66  is attached to the right edge of the top plate  605 . The blade  66  limits the thickness of the toner layer carried on the peripheral surface of the developing roller  231 . 
     The following describes the method for attaching the developing unit  23  to the main housing  10  (see  FIG. 1 ), mainly with reference to  FIGS. 5-7 . 
     The developing unit  23  is attached to the main housing  10  by a user. In detail, the developing unit  23  is mounted into the support unit  30  from the front (from the Y2 direction) by sliding the developing unit  23  on the base  31  toward the rear (toward the Y1 direction) of the support unit  30 . In this way, the developing unit  23  is attached to the main housing  10 . To attach the developing unit  23  into the main housing  10 , it is preferable to fit the right and left guide ribs  606  and  607  respectively into the right and left guide sections  34  and  35  of the support unit  30 . By doing so, the right and left guide ribs  606  and  607  are respectively guided by the right and left guide sections  34  and  35  when the developing unit  23  slides toward the rear. This enables the developing unit  23  to stably slide on the base  31 . Note that the developing unit  23  is detached from the main housing  10  through a procedure in reverse of the above-described procedure. 
     In the attachment of the developing unit  23  to the main housing  10 , when the rear plate  603  of the developing unit  23  reaches a predetermined location near the rear wall  33  of the support unit  30 , the first and second locating pins  641  and  642  come to be inserted into the first and second locating holes  381  and  382 , respectively. In addition, the coupling socket  65  is coupled to the coupling member  39 . 
     By the insertion of the first and second locating pins  641  and  642  respectively into the first and second locating holes  381  and  382 , the developing unit  23  is placed into an appropriate relative location with respect to the support unit  30 . In this positional relation, the developing-roller driving gear  63  makes mesh engagement with the transmission gear  37 . The gear teeth of the transmission gear  37  are partly exposed from the housing constituting the rear wall  33 . Part of the exposed gear teeth meshes with the exposed part of the gear teeth of the developing-roller driving gear  63  described above. In addition, the coupling socket  65  is coupled with the coupling member  39 . 
     Then, the rotational drive force of the second coupling gear  73  is transmitted to the transmission gear  37  and the developing-roller driving gear  63 . This enables the developing roller  231  to rotate. Similarly, the rotational drive force is transmitted to the coupling member  39  and the coupling socket  65  to enable the toner supplying screw  234  to rotate. 
     The pair of coil springs  50  is a member that supplies an urging force to the support unit  30 . The support unit  30  is loaded with the developing unit  23 . With the urging force supplied by the coil springs  50 , the support unit  30  can pivot about the axis of the pivot shaft A. The urging force supplied by the coil spring  50  is to pivot the support unit  30  in a direction for moving the peripheral surface of the developing roller  231  toward the peripheral surface of the photosensitive drum  211  (approaching direction). 
     The pair of coil springs  50  is oriented such that each extending direction coincides with the up-and-down direction (Z direction). The upper end portion of each coil spring  50  is in contact with the lower face of the base  31  at a location near the right edge of the base  31 . The bottom end portion of each coil spring  50  is in contact with a securing frame member (not shown) of the main housing  10 . The coil springs  50  upwardly urge the lower face of the base  31  each at a location near the right edge  31 R. 
     The switching mechanism  40  switches the position of the support unit  30  supporting the developing unit  23 . The switching is made between the fitting position (first position) in which the peripheral surface of the developing roller  231  is located relatively close to the peripheral surface of the photosensitive drum  211  and a releasing position (second position) in which the peripheral surface of the developing roller  231  is located relatively away from the peripheral surface of the photosensitive drum  211  by resisting the urging force of the coil spring  50 . 
     The following describes the method in which the switching mechanism  40  performs the position switching of the support unit  30 , mainly with reference to  FIGS. 7 ,  8 , and  13 .  FIG. 7  is a cross sectional view showing the support unit  30  in the fitting position.  FIG. 8  is a cross sectional view showing the support unit  30  in the releasing position. 
     To switch the support unit  30  into the fitting position, the switching mechanism  40  supplies no constraining force on the support unit  30 . Consequently, the urging force of the coil springs  50  is supplied on the support unit  30 . Thus, the support unit  30  pivots about the pivot shaft A (the axis of the fulcrum member  36 ) in the counterclockwise direction indicated by the arrow R11 in  FIG. 7 . Then, as the support unit  30  pivots, the developing roller  231  is pressed in a direction toward the photosensitive drum  211  as indicated by the arrow R12 in  FIG. 7 . As a result, each gap roller  23 D is pressed against the non-image forming region  211 D of the photosensitive drum  211  to leave a predetermined gap between the peripheral surface of the developing roller  231  and the peripheral surface of the photosensitive drum  211  as shown in  FIG. 13 . This state where the peripheral surface of the developing roller  231  is located relatively close to the peripheral surface of the photosensitive drum  211  is the fitting position. The fitting position is taken when the image forming apparatus  1  performs image forming processing. 
     On the other hand, to switch the support unit  30  into the releasing position, the switching mechanism  40  supplies a constraining force on the support unit  30 . In detail, the switching mechanism  40  supplies a force pressing the hooks  311  downward against the urging force of the coil springs  50 . In response, the base  31  is pressed downward along its right edge (the edge toward the X1 direction). Consequently, the support unit  30  pivots about the pivot shaft A in the clockwise direction indicated by the arrow R21 in  FIG. 8 . Then, as the support unit  30  pivots, the developing roller  231  is pressed to move in a direction away from the photosensitive drum  211  as indicated by the arrow R12 in  FIG. 8 . As a result, a relatively large gap is secured between the peripheral surface of the developing roller  231  and the peripheral surface of the photosensitive drum  211 . This state where the peripheral surface of the developing roller  231  is located relatively away from the peripheral surface of the photosensitive drum  211  is the releasing position. The releasing position is taken to allow the developing unit  23  to be pulled forward away from the support unit  30  (or to be pressed rearward into the support unit  30 ). 
     The following describes the configuration of the switching mechanism  40  mainly with reference to  FIGS. 9 and 10 . The switching mechanism  40  performs the position switching of the support unit  30  between the fitting position and the releasing position.  FIG. 9  is a perspective view showing the switching mechanism  40  in the state where switching to the fitting position is effected.  FIG. 10  is a perspective view showing the switching mechanism  40  in the state where switching to the releasing position is effected. 
     The switching mechanism  40  includes a lever member  41  movable in the front-and-rear direction (Y direction) and two movable plates  42  each movable in the right-and-left direction (X direction) cooperatively with the front-to-rear (Y direction) movement of the lever member  41 . The movable plates  42  are disposed below the lever member  41 . In detail, the two movable plates  42  are spaced apart in the front-and-rear direction (Y direction). 
     The lever member  41  is a plate elongated in the front-and-rear direction (Y direction). The lever member  41  includes an operation portion  411  at the front (toward the Y2 direction) and a transitional portion  412  continuous from the rear portion (the portion toward the Y1 direction) of the operation portion  411 . 
     The operation portion  411  occupies ⅕ or so of the entire length of the lever member  41 , starting from the front edge  41 F in the front-and-rear direction (Y direction). The operation portion  411  is relatively thick in the up-and-down direction (Z direction). The operation portion  411  has an operation hole  44  therethrough for insertion of a user&#39;s finger when the user operates the lever member  41 . In  FIGS. 3 and 4 , the forwardly (toward the Y2 direction) protruding portion corresponds to the operation portion  411 . 
     The transitional portion  412  is relatively thin in the up-and-down direction (Z direction). The outer edges of the transitional portion  412  are surrounded by a rib having the same thickness as the operation portion  411 . Of the two movable plates  42 , one at the front (toward the Y2 direction) is disposed at a location near the front edge of the transitional portion  412 . On the other hand, the movable plate  42  at the rear (toward the Y1 direction) is disposed at a location near the rear edge of the transitional portion  412 . The transitional portion  412  has slits  43  one near the front edge and another near the rear edge thereof. Each slit  43  extends in a direction oblique to the extending direction of the lever member  41 . 
     The following describes the slit  43  located at the front (toward the Y2 direction). The slit  43  has a proximal end  43 A at a location near the front edge and near the right edge of the transitional portion  412 . The slit  43  extends from the proximal end  43 A in a direction obliquely rearward left (toward the X2-Y1 direction) to reach a distal end  43 B located at a predetermined distance rearward (toward the Y1 direction) from the front edge of the transitional portion  412 . The distal end  43 B is at a location near the left edge of the transitional portion  412 . The slit  43  at the rear (toward the Y1 direction) is identical in shape to the slit  43  at the front. The distal end of the slit  43  at the rear (the portion corresponding to the distal end  43 B) reaches a location near the rear edge  41 R of the lever member  41  (the rear edge of the transitional portion  412 ). 
     Each movable plate  42  includes a base plate  421 , a first guide groove  422 , a second guide groove  423 , a fitting pin  45 , and an engaging pin  46  (second engaging piece). The base plate  421  is a flat plate elongated in the right-and-left direction (X direction). The first and second guide grooves  422  and  423  each extend through the entire base plate  421  in the up-and-down direction. The fitting pin  45  stands vertically on the upper face of the base plate  421 . The engaging pin  46  is attached to the base plate  421  at a location near the right edge of the base plate  421 . 
     The first and second guide grooves  422  and  423  are each formed at a location centrally of the base plate  421  in the front-and-rear direction (Y direction). The first and second guide grooves  422  and  423  each extend in the right-and-left direction (X direction). The first guide groove  422  is located closer to the right (toward the X1 direction), whereas the second guide groove  423  is located closer to the left (toward the X2 direction). The first guide groove  422  has, for example, a U shape that is open at the right edge (the edge toward the X1 direction) in top view. The second guide groove  423  has, for example, an elliptical shape in top view. 
     The main housing  10  has guide pins (not shown) secured thereto. The guide pins are respectively fitted into the guide grooves  422  and  423 . The engagement of the guide pins into the guide grooves  422  and  423  prevents each movable plate  42  from being detached upward (toward Z1 direction) from the main housing  10 . By being guided by the respective guide pins engaged with the first and second guide grooves  422  and  423 , the movable plate  42  is allowed to move in the right-and-left direction (X direction). The width of the second guide groove  423  in the right-and-left direction (X direction) determines a movement stroke of the movable plate  42  in the right-and-left direction (X direction). 
     The movable plate  42  at the front (toward the Y2 direction) has an extended portion  425  extended from a location toward the rear of the base plate  421  (toward the Y1 direction). The fitting pin  45  vertically stands on the top face of the extended portion  425 . The fitting pin  45  is fitted into the slit  43 . 
     As described above, the slit  43  extends obliquely, rather than straight in the front-and-rear direction (Y direction). The lever member  41  guided by a guide member (not shown) to move straight along the front-and-rear direction (Y direction). As the lever member  41  moves in the front-and-rear direction (Y direction), the fitting pin  45  is guided by the slit  43 . Consequently, the movable plate  42  moves in the right-and-left direction (X direction). With the fitting pin  45  engaged with the slit  43 , the movement stroke of the lever member  41  in the front-and-rear direction (Y direction) is limited to the length of the slit  43  in the front-and-rear direction (Y direction). 
     The engaging pin  46  has a cylindrical shape extending in the front-and-rear direction (Y direction). A pair of front and rear retaining portions is provided to upwardly (in the Z1 direction) stand on the right edge of the upper face of the base plate  421 . The pair of front and rear retaining portions  424  retains the engaging pin  46  by holding the respective ends of the engaging pin  46 . The engaging pins  46  are engageable with the respective hooks  311  (first engaging piece) of the support unit  30  (see  FIGS. 7 and 8 ). 
     The following describes the operation of the switching mechanism  40  mainly with reference to  FIGS. 7 to 10 . 
     When the lever member  41  is pressed to the rearmost location (toward the Y1 direction), the switching mechanism  40  is placed into the state shown in  FIGS. 7 and 9  (the state where the lever member  41  is in the second location). In this state, each fitting pin  45  is in contact with the proximal end  43 A of the corresponding slit  43 . In addition, each movable plate  42  is located the rightmost (toward the X1 direction). 
     As shown in  FIG. 7 , each hook  311  has the sloped portion  312  that slopes downwardly toward the right (toward the X1 direction). An opening gap is secured between the tip (lower end) of the hook  311  and the lower face of the base  31 . When the lever member  41  is at the rearmost location (toward the Y1 direction), each engaging pin  46  is located to the right (toward the X1 direction) of the opening gap. Note that the engaging pin  46  is located higher than the lower end of the hook  311 . In the state shown in  FIG. 7 , the engaging pin  46  is retracted to a location toward the right (toward the X1 direction), the engagement between the engaging pin  46  and the hook  311  of the support unit  30  is released. When the engagement between the engaging pin  46  and the hook  311  is released, the urging force of the coil springs  50  acts on the support unit  30  without being restricted. Therefore, the support unit  30  is urged upward by the coil springs  50 . As a consequence, the support unit  30  takes the fitting position. 
     When the lever member  41  in the state shown in  FIG. 9  is pulled forward (in the direction indicated by the arrow R3 in  FIG. 10 ) to bring the front edge  41 F to the foremost location, the switching mechanism  40  is placed into the state shown in  FIGS. 8 and 10  (the state where the lever member  41  takes the first position). In this state, the fitting pin  45  is in contact with the distal end  43 B of the slit  43 . As the fitting pin  45  moves from the proximal end  43 A toward the distal end  43 B, the movable plate  42  is guided by the slits  43  to move toward left (toward the X2 direction) as indicated by the arrow R4 in  FIG. 10 . When the fitting pin  45  reaches the distal end  43 B, the movable plate  42  comes to be located at the leftmost location (toward the X2 direction). 
     As shown in  FIG. 8 , when the lever member  41  is placed to the foremost location (toward the Y2 direction), the engaging pin  46  has moved toward the left (X2 direction) to engage with the hook  311 . In detail, when the movable plate  42  moves toward the left (toward the X2 direction), the engaging pin  46  also moves toward the left (toward the X2 direction). As a consequence, the engaging pin  46  comes to engage the hook  311  at the lower end of the sloped portion  312 . Thereafter, as the engaging pin  46  slides along the sloped portion  312  toward the left (toward the X2 direction), the support unit  30  is gradually pressed downward. In this way, the support unit  30  receives via the hooks  311  a force pressing the support unit  30  downward against the urging force of the coil springs  50 . As a consequence, the support unit  30  is placed in the releasing position. 
     The image forming apparatus  1  further includes the drive unit  70 . The drive unit  70  is assembled to the main housing  10 . The drive unit  70  generates a drive force for driving the photosensitive drum  211 , the developing roller  231 , and the like. The following describes a configuration of the developing unit  70  mainly with reference to  FIG. 11 . 
     The drive unit  70  includes a gear housing  70 H and a drive motor (not shown). The gear housing  70 H accommodates a plurality of gear trains. The drive motor supplies a rotational drive force to each gear train. The gear housing  70 H is assembled to the front face (toward the Y2 direction) of a side frame  18 , which is a component of the main housing  10 . The drive motor is attached to the rear face (toward the Y1 direction) of the side frame  18 . Note that the gear housing  70 H functions as a retaining member of a high-voltage board  19 . On the high-voltage board  19 , electric components (for example, a power semiconductor) operating at high-voltage are mounted. 
     The drive unit  70  has a drive output shaft. The drive output shaft generates a rotational drive force that rotates about its axis. According to the present embodiment, the first coupling gear  71  is an example of the drive output shaft. 
     The first coupling gear  71  includes a hollow cylinder and gear teeth formed along an edge of the hollow cylinder. The first coupling gear  71  is coupled with the second coupling gear  73  (see  FIG. 13 ). The first coupling gear  71  is provided with the pin receiver that is disposed on its axis. The pin receiver  72  is for insertion of the pin  74  (see  FIG. 13 ). The pin  74  is coaxial with the second coupling gear  73 . The first coupling gear  71  generates a rotational drive force for counterclockwise rotation as indicated by the arrow R5 in  FIG. 11 . 
     The following describes the assembled state of the drive unit  70  with the developing unit  23  and the support unit  30 , mainly with reference to  FIGS. 11-13 .  FIG. 12  is a perspective view showing the assembled state of the drive unit  70  with the developing unit  23  and the support unit  30 . 
     For example, the support unit  30  on which the developing unit  23  is mounted is assembled to the drive unit  70 . In the state where the support unit  30  is assembled to the drive unit  70  (the state shown in  FIG. 12 ), the rear wall  33  is in contact with the gear housing  70 H (see  FIG. 11 ). The second coupling gear  73  is exposed from the rear wall  33 . This allows the first coupling gear  71  to be coupled with the second coupling gear  73 . In addition, the pin  74  is inserted into the pin receiver  72 . In the state shown in  FIG. 12 , the rotational drive force of the first coupling gear  71  can be transmitted to the second coupling gear  73 . In the state shown in  FIG. 12 , in addition, the support unit  30  is supported axially by the pin receiver  72  (and by the pin member  112  inserted into the fulcrum member  36 ). 
     When the drive motor described above is driven in the state shown in  FIG. 12 , the first coupling gear  71  rotates in the direction indicated by the arrow R5 shown in  FIG. 11 . In addition, the second coupling gear  73  rotates in the direction indicated by the arrow R6 shown in  FIG. 13 . In addition, as the second coupling gear  73  rotates, the transmission gear  37  and the developing roller  231  rotate. 
     In the image forming apparatus  1  according to the present embodiment, the rotation direction R5 of the first coupling gear  71  and the rotation direction R6 of the second coupling gear  73  are both in the direction for moving the developing roller  231  toward the photosensitive drum  211 . The rotation directions R5 and R6 each coincide with the pivoting direction for moving the peripheral surface of the developing roller  231  toward the peripheral surface of the photosensitive drum (approaching direction), out of the two possible pivot directions about the axis of the pivot shaft A. The fulcrum member  36  and the pin  74  of the support unit  30  together constituting the pivot shaft A are both coaxial with the first and second coupling gears  71  and  73 . When each of the first and second coupling gears  71  and  73  rotates, a driving force acts on the support unit  30  for pivoting the support unit  30  about the axis of the pivot shaft A in the corresponding rotation direction. 
     In the image forming apparatus  1  according to the present embodiment, the developing unit  23  is supported by the support unit  30  to be freely pivotable about the pivot shaft A. The support unit  30  is urged by the coil springs  50  in the approaching direction for moving the peripheral surface of the developing roller  231  toward the peripheral surface of the photosensitive drum  211 . The coil springs  50  generate an urging force urging the support unit  30  to pivot. In detail, in the state mounted on the top face of the horizontal base  31 , the developing unit  23  is urged to pivot about the axis of the pivot shaft A by the urging force of the coil springs  50 . The pivot shaft A is disposed on the left wall  32 . 
     In addition, the first coupling gear  71  (the drive output shaft of the drive unit  70 ) is set to generate a rotational drive force in the direction for pivoting the support unit  30  into the approaching direction. That is, the support unit  30  is pivoted by the urging force of the coil springs  50  and also by the rotational drive force of the first coupling gear  71  (the drive output shaft of the drive unit  70 ). In the image forming apparatus  1  according to the present embodiment, the force required to pivot the support unit  30  about the axis of the pivot shaft A is supplied partly by the urging force of the coil springs  50  and partly by the driving force generated by the rotation of the first coupling gear  71 . 
     Since the rotational drive force generated by the first coupling gear  71  (the output shaft of the drive unit  70 ) contributes the pivoting of the support unit  30 , the urging force of the coil springs  50  is allowed to be smaller. In detail, when the image forming apparatus  1  is in the operating state (the state where the developing roller  231  is rotated), the driving force generated by rotation of the first coupling gear  71  serves as an additional urging force. Therefore, the force pressing the gap rollers  23 D against the non-image forming regions  211 D of the photosensitive drum  211  is unlikely to fall short even if the urging force of the coil springs  50  is small. 
     Reducing the urging force of the coil springs  50  improves the operability of the switching mechanism  40 . In detail, the switching mechanism  40  switches the support unit  30  from the fitting position to the releasing position by resisting the urging force of the coil springs  50 . Therefore, the force required for the user to apply when making an operation for effecting the switching can be made smaller. 
     As described above, the switching mechanism  40  according to the present embodiment is simple in configuration. The switching mechanism  40  according to the present embodiment can perform the position switching of the support unit  30  between the fitting position and the releasing position, simply by switching the location of the lever member  41  between the front and the rear. The switching mechanism  40  according to the present embodiment can reduce the urging force of the coil springs  50  acting on the support unit  30  when the lever member  41  is pulled toward the front (moving the lever member  41  from the second location to the first location). As a result, the operability of the lever member  41  improves. 
     As has been described above, the image forming apparatus  1  according to the present embodiment is provided with a mechanism for moving the developing roller  231  closer to and away from the photosensitive drum  211 . This can improve the user operability for causing the distal and proximal movements. 
     The embodiment described above is one example and may be modified. For example, the embodiment described above employs the first coupling gear  71  as the drive output shaft and the second coupling gear  73  as the second drive input section. In addition, the first coupling gear  71  is provided with the pin receiver  72  that is disposed on its axis, and the second coupling gear  73  is provided with the pin  74  that is disposed on its axis. Such configuration is simple and still allows the drive force to be transmitted from the drive output shaft to the second drive input section and also allows the pivot shaft and the second drive input section to be coaxial. However, the above configuration is without limitation and may be replaced by another configuration. For example, spur gears may be used instead.