Patent Publication Number: US-2022236689-A1

Title: Drum unit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 17/174,540, filed Feb. 12, 2021, which is a continuation of U.S. patent application Ser. No. 16/926,870, filed Jul. 13, 2020, which is a continuation of U.S. patent application Ser. No. 16/656,893, filed Oct. 18, 2019, which is a continuation of U.S. patent application Ser. No. 16/144,178, filed Sep. 27, 2018, which further claims priority from Japanese Patent Application No. 2018-067899 filed on Mar. 30, 2018. The contents of the aforementioned applications are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     Aspects of the disclosure relate to a drum unit. 
     BACKGROUND 
     Electrophotographic image forming apparatuses known in the art include laser printers and light-emitting diode (LED) printers. An image forming apparatus includes developer cartridges and a drum unit. The drum unit includes a plurality of photosensitive drums. Each photosensitive drum is rotatable about a drum axis extending in an axial direction. The developer cartridges are removable from the drum unit. When a developer cartridge is attached to the drum unit, a developing roller of the developer cartridge contacts the photosensitive drum of the drum unit. 
     The drum unit includes a frame for holding a plurality of developer cartridges, and lock levers for locking the developer cartridges held in the frame. Each developer cartridge is attached to a corresponding slot of the frame. Each lock lever is pivotable about a pivot axis extending in the axial direction between a lock position and a release position. When the lock lever is at the lock position, the developer cartridge is locked on the frame. When the lock lever is at the release position, the developer cartridge is removable from the frame. 
     SUMMARY 
     To allow each lock lever to pivot about the pivot axis from the lock position to the release position, this image forming apparatus provides a clearance for each lock lever between the developer cartridges held in the frame. In this structure, adjacent developer cartridges cannot be arranged close to each other. Techniques are awaited for downsizing the drum unit in a direction in which the photosensitive drums are arranged. 
     In response to the above issue, one or more aspects of the present invention are directed to a drum unit including adjacent developer cartridges arranged close to one another, and thus downsized in the direction in which the photosensitive drums are arranged. 
     A first aspect of the disclosure provides a drum unit with the structure described below. The drum unit includes a photosensitive drum, a frame, and a lock lever. The photosensitive drum is rotatable about a drum axis extending in a first direction. The frame includes a first side plate located at first end in the first direction and a second side plate spaced from the first side plate in the first direction. The frame rotatably supports the photosensitive drum between the first side plate and the second side plate in the first direction. The frame holds a developer cartridge. The lock lever is movable, and in some examples, is pivotable about a pivot axis extending in a second direction intersecting with the first direction. The lock lever is movable between a lock position at which the developer cartridge is locked on the frame and a release position at which the developer cartridge is unlocked from the frame. 
     A second aspect of disclosure provides a drum unit with the structure described below. The drum unit includes a photosensitive drum, a frame, and a lock lever. The photosensitive drum is rotatable about a drum axis extending in a first direction. The frame includes a first side plate located at a first end of the frame in the first direction and a second side plate spaced from the first side plate in the first direction. The frame rotatably supports the photosensitive drum between the first side plate and the second side plate in the first direction. The lock lever is received in an opening of the first side plate of the frame. The lock lever has a first end, and a second end. The second end of the lock lever is positioned opposite to the first end. The second end of the lock lever is movable, and some examples, is pivotable about a pivot axis extending in a second direction intersecting with the first direction. The lock lever is pivotable between a lock position at which at least a portion of the second end of the lock lever is located between the first side plate and the second side plate, and a release position at which at least a portion of the second end of the lock lever extends from the first side plate away from the second side plate in the first direction. 
     A third aspect of the disclosure provides a drum unit with the structure described below. The drum unit includes a plurality of photosensitive drums, a frame, a plurality of lock levers. Each of the plurality of photosensitive drum is rotatable about a drum axis extending in a first direction. The plurality of photosensitive drums are arranged in a second direction intersecting with the first direction. The frame includes a first side plate located at a first end of the frame in the first direction and a second side plate spaced from the first side plate in the first direction. The frame rotatably supports the plurality of photosensitive drums between the first side plate and the second side plate in the first direction. The frame holds a plurality of developer cartridges. Each of the plurality of lock levers are movably, and in some examples, is pivotable supported by the frame and spaced apart in the second direction. Each of the plurality of lock levers may move between a lock position and a release position. Each of the plurality of lock levers is positioned so that a first outer surface of the corresponding developer cartridge held by the frame completely overlaps the lock lever as viewed in the first direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual diagram of an image forming apparatus. 
         FIG. 2  is a perspective view of a drum unit and developer cartridges. 
         FIG. 3  is a perspective view of a developer cartridge. 
         FIG. 4  is a perspective view of the developer cartridge as viewed in a direction different from  FIG. 3 . 
         FIG. 5  is a side view of the developer cartridge at one end of the developer cartridge in a first direction. 
         FIG. 6  is a side view of the developer cartridge at other end of the developer cartridge in the first direction. 
         FIG. 7  is a perspective view of the drum unit. 
         FIG. 8  is a side view of the drum unit showing inner structure of the drum unit during attachment of the developer cartridge to the drum unit. 
         FIG. 9  is a side view of the drum unit showing inner structure of the drum unit, in which the developer cartridge is attached to the drum unit. 
         FIG. 10  is a side view of the developer cartridge during attachment of the developer cartridge to the drum unit. 
         FIG. 11  is a side view of the developer cartridge that is attached to the drum unit. 
         FIG. 12  is a schematic diagram describing forces acting relative to one another during attachment of the developer cartridge to the drum unit. 
         FIG. 13  is a perspective view of the drum unit, showing the lock lever attachment of the developer cartridge to the drum unit. 
         FIG. 14  is a perspective view of the drum unit, showing the lock lever when the developer cartridge is attached to the drum unit. 
         FIG. 15  is an enlarged view of the lock lever shown in  FIG. 13  at a release position. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will now be described with reference to the drawings. 
     A first direction herein refers to the direction along a drum axis of a photosensitive drum. A second direction herein refers to a direction in which a plurality of photosensitive drums are arranged. A third direction herein refers to the longitudinal direction of each end face of a developer cartridge attached to the drum unit. The first direction, the second direction, and the third direction intersect with one another (at right angles in some embodiments). 
     1. Structure of Image Forming Apparatus 
       FIG. 1  is a conceptual diagram of an image forming apparatus  100 . The image forming apparatus  100  is an electrophotographic printer. The image forming apparatus  100  may be a laser printer or a light-emitting diode (LED) printer. As shown in  FIG. 1 , the image forming apparatus  100  includes a drum unit  3  and a plurality of developer cartridges  1 . The drum unit  3  includes a frame that be configured to hold the developer cartridges  1 . 
       FIG. 2  is a perspective view of the drum unit  3  and the developer cartridges  1 . The developer cartridges  1  are individually attachable to the drum unit  3 . The drum unit  3  holding the developer cartridges  1  is configured to be mounted onto the body casing  101  of the image forming apparatus  100  (refer to  FIG. 1 ). The developer cartridges  1  each contain toner (developer) of a different color (e.g., cyan, magenta, yellow, or black). The image forming apparatus  100  forms (outputs) an image on the recording surface of a print sheet with toner fed from the developer cartridges  1 . The drum unit  3  in the present embodiment holds four developer cartridges  1 . In some embodiments, the drum unit  3  may hold one to three, or five or more developer cartridges  1 . 
     2. Structure of Developer Cartridge 
       FIGS. 3 and 4  are perspective views of the developer cartridge  1 .  FIGS. 5 and 6  show the end faces of the developer cartridge  1  in the first direction. As shown in  FIGS. 3 to 6 , the developer cartridge  1  according to the present embodiment includes a housing  11 , a developing roller  12 , an agitator (not shown), and a coupling  24 . 
     The housing  11  is a casing configured to contain toner. The housing  11  has a chamber (not shown) configured to contain toner. The housing  11  extends in the first direction. The housing  11  has a first outer surface  111 , and a second outer surface  112  located away from the first outer surface  111  in the first direction. The first outer surface  111  is one end face (first end face) of the housing  11  in the first direction. The longitudinal direction of the first outer surface  111  corresponds to the third direction. The second outer surface  112  is the other end face (second end face) of the housing  11  in the first direction. The longitudinal direction of the second outer surface  112  corresponds to the third direction. The housing  11  has a first end  113  in the third direction, and a second end  114  located away from the first end  113  in the third direction. 
     The developing roller  12  is rotatable about a roller axis extending in the first direction. The developing roller  12  is located between the first outer surface  111  of the housing and the second outer surface  112  of the housing  11  in the first direction. The developing roller  12  is located nearer the first end  113  than the second end  114  in the third direction. 
     More specifically, the developing roller  12  includes a developing roller body and a developing roller shaft  12   c . The developing roller body is cylindrical. The developing roller shaft  12   c  is cylindrical. The developing roller shaft  12   c  extends through the center of the developing roller body. The developing roller shaft  12   c  rotates together with the developing roller body. The developing roller shaft  12   c  has two ends in the first direction supported by the two end faces of the housing  11  in a rotatable manner. 
     The developing roller  12  (developing roller body) has an uncovered surface  12   a  and a covered surface  12   b . The uncovered surface  12   a  is located at one end (first end) of the developing roller  12  in the second direction, which is exposed outside the housing  11 . The covered surface  12   b  is located at other end (second end) of the developing roller  12  in the second direction, which is located inside the housing  11 . When the developer cartridge  1  is attached to the drum unit  3 , the peripheral surface (uncovered surface  12   a ) of the developing roller  12  contacts the peripheral surface of the photosensitive drum  31 . 
     The agitator (not shown) is rotatable about an agitator axis extending in the first direction. The agitator includes a plurality of agitation blades. The agitator is located between the first outer surface  111  of the housing  11  and the second outer surface  112  of the housing  11  in the first direction. In the third direction, the agitator is located nearer the second end  114  of the housing  11  than the developing roller  12 . The agitator rotates to agitate the toner contained in the chamber of the housing  11 . 
     The coupling  24  shown in  FIGS. 3 and 5  receives a drive force applied from the body of the image forming apparatus  100 . The coupling  24  is rotatable about an coupling axis extending in the first direction. The coupling  24  is located nearer the second end  114  of the housing  11  than the developing roller  12  in the third direction. The coupling  24  has a recess  115  in the first direction. When the developer cartridge  1  attached to the drum unit  3  is mounted onto the body casing  101  of the image forming apparatus  100 , the drive shaft of the body casing  101  of the image forming apparatus  100  is received in the recess  115 . This connects the coupling  24  to the drive shaft in a manner non-rotatable relative to each other. The rotation of the drive shaft rotates the coupling  24 . The rotation of the coupling  24  then rotates the developing roller  12  and the agitator. 
     3. Structure of Drum Unit 
     The structure of the drum unit  3  will now be described with reference mainly to  FIG. 7 .  FIG. 7  is a perspective view of the drum unit  3 . 
     The drum unit  3  according to the present embodiment is a drum cartridge. As shown in  FIGS. 7 to 9 , the drum unit  3  includes a plurality of photosensitive drums  31 , a first side plate  33 , a second side plate  34 , and a pullout plate  35 . In the present embodiment, the drum unit  3  includes four photosensitive drums  31 . 
     The photosensitive drums  31  transfer toner fed from the developer cartridges  1  to a print sheet. The photosensitive drums  31  are arranged at intervals in the second direction. Each photosensitive drum  31  is cylindrical. Each photosensitive drum has a peripheral surface. Each photosensitive drum extends in the first direction. The peripheral surface of the photosensitive drum  31  is coated with a photosensitive material. Each photosensitive drum  31  is rotatable about a drum axis of rotation extending in the first direction. 
     The first side plate  33 , the second side plate  34 , and the pullout plate  35  together define a frame  38  for holding the photosensitive drums  31 . The frame  38  has an opening located at one end of the frame in the third direction (in the direction opposite the force of gravity in the present embodiment). 
     The first side plate  33  supports first ends of the photosensitive drums  31  in the first direction. The first side plate  33  extends perpendicular to the first direction. The first side plate  33  extends in the second direction. The first side plate  33  is a plate. The first side plate  33  includes a plurality of (four in the present embodiment) developer cartridge holders  30 . The developer cartridge holders  30  are located at the inner surface of the first side plate  33  in the first direction. The developer cartridge holders  30  are arranged at intervals in the second direction. The developer cartridge holders  30  of the first side plate  33  receive first ends of the developer cartridges  1  in the first direction (ends at the first outer surfaces  111 ). 
     The second side plate  34  supports second ends of the photosensitive drums  31  in the first direction. The second side plate  34  extends perpendicular to the first direction. The second side plate  34  extends in the second direction. The second side plate  34  is a plate. The second side plate  34  includes a plurality of (four in the present embodiment) developer cartridge holders (not shown). The developer cartridge holders are located at the inner surface of the second side plate  34  in the first direction. Number of the developer cartridge holders of the second side plate  34  is equal to number of the developer cartridge holders  30  of the first side plate  33 . The developer cartridge holders of the second side plate  34  receive second ends of the developer cartridges  1  in the first direction (ends at the second outer surfaces  112 ). 
     The pullout plate  35  connects a second end of the first side plate  33  in the second direction and a second end of the second side plate  34  in the second direction. The pullout plate  35  extends perpendicular to the second direction. The pullout plate  35  extends in the first direction. The pullout plate  35  is a plate. The pullout plate  35  has a handle  36 . The handle  36  is located at an outer side surface of the pullout plate  35  in the second direction. The handle  36  is gripped by a user to pull or push the drum unit  3  out of or into the body casing  101 . 
     When the developer cartridge  1  is attached to the drum unit  3 , the first end of the housing  11  in the first direction held by the developer cartridge holder  30  of the first side plate  33  and the second end of the housing  11  in the first direction held by the developer cartridge holder of the second side plate  34 . When the developer cartridge  1  is attached to the drum unit  3 , the peripheral surface (uncovered surface  12   a ) of the developing roller  12  contacts the peripheral surface of the photosensitive drum  31 . 
     The image forming apparatus  100  with the above structure forms an image on a print sheet in the manner described below. As the coupling  24  and also the photosensitive drum  31  rotate, the toner is fed from the chamber of the housing  11  to the peripheral surface of the photosensitive drum  31  through the developing roller  12 . The toner retained on the peripheral surface of the developing roller  12  moves from the developing roller  12  to the photosensitive drum  31  in accordance with an electrostatic latent image formed on the peripheral surface of the photosensitive drum  31 . The electrostatic latent image thus appears on the peripheral surface of the photosensitive drum  31 . The photosensitive drum  31  then transfers the toner onto the print sheet. 
     4. Detailed Structure for Attachment and Removal 
     The structure according to the present embodiment for attaching and removing the developer cartridges  1  to and from the drum unit  3  will now be described with reference to  FIGS. 3 to 15 .  FIG. 8  is a side view of the drum unit  3  showing inner structure of the drum unit  3  during attachment of one developer cartridge  1  to the drum unit  3 .  FIG. 9  is a side view of the drum unit  3  showing inner structure of the drum unit  3 , in which the developer cartridge  1  is attached to the drum unit  3 .  FIG. 10  is a side view of the developer cartridge  1  during attachment of the developer cartridge  3  to the developer cartridge holder  30  of the drum unit  3 .  FIG. 11  is a side view of the developer cartridge  1  that is attached to the developer cartridge holder  30  of the drum unit  3 .  FIG. 12  is a schematic diagram describing forces acting relative to one another during attachment of the developer cartridge  1  to the developer cartridge holder  30  of the drum unit  3 .  FIG. 13  is a perspective view of the drum unit  3 , showing the lock lever  48  during attachment of the developer cartridge  3  to the drum unit  3 .  FIG. 14  is a perspective view of the drum unit  3 , showing the lock lever  48  when the developer cartridge  1  is attached to the drum unit  3 .  FIG. 15  is an enlarged view of the lock lever  48  shown in  FIG. 13  at the release position. 
     The developer cartridge  1  according to the present embodiment includes a first boss  13 , a first collar  14 , a first pressure receiving surface  15 , a second boss  16 , a second collar  17 , a first protrusion  18 , a third boss  19 , a second pressure receiving surface  21 , a fourth boss  22 , and a second protrusion  23 . 
     The first boss  13  extends in the first direction. The first boss  13  in the present embodiment is a separate component from the housing  11 , and is fixed to the housing  11 . In some embodiments, the first boss  13  may be integral with the housing  11 . The first boss  13  is movable together with the housing  11 . The first boss  13  is located at the first outer surface  111 . More specifically, the first boss  13  protrudes from the first outer surface  111  outward in the first direction. As described in detail later, the first boss  13  bears the weight of the developer cartridge  1 . The first boss  13  is located away from the developing roller  12  in both the second and third directions. As described in detail later, the first boss  13  is supported by a support surface  41  (described later) of the drum unit  3  when the developer cartridge  1  is attached to the drum unit  3 . 
     The first collar  14  is cylindrical. The first collar  14  is rotatable about the first boss  13 . The first collar  14  has hollow. The first collar  14  receives the first boss  13 . In some embodiments, the first collar  14  may not be cylindrical. For example, the first collar  14  may be polygonal. The developer cartridge  1  may not include the first collar  14 . In this case, the peripheral surface of the first boss  13  may serve as the first collar. 
     The first pressure receiving surface  15  has an arc-shaped surface curved along an arc about a straight line in the first direction. The first pressure receiving surface  15  is located inward from the first outer surface  111  in the first direction. In the second direction, the first pressure receiving surface  15  is located farther away from the developing roller  12  than the first boss  13 . 
     The first boss  13  is located away from the developing roller  12  by a first distance D 1  in the third direction. The first pressure receiving surface  15  is located away from the developing roller  12  by the first distance D 1  or a second distance D 2  smaller than the first distance D 1  (D 2 ≤D 1 ) in the third direction. In the present embodiment, as shown in  FIGS. 5 and 6 , the distance between the first pressure receiving surface  15  and the developing roller  12  in the third direction is substantially equal to the first distance D 1 . 
     In the present embodiment, the first distance D 1  can be defined as the distance between the outer surface of the first boss  13  and the peripheral surface of the developing roller  12 , and the second distance D 2  can be defined as the distance between the outer surface of the first pressure receiving surface  15  and the peripheral surface of the developing roller  12 . In some embodiments, these distances may be defined differently. For example, the first distance D 1  may be the distance between the center of rotation of the first boss  13  and the center of rotation of the developing roller  12 , and the second distance D 2  may be the distance between the center of an arc defined by the outer surface of the first pressure receiving surface  15  and the center of rotation of the developing roller  12 . In other embodiments, the first distance D 1  may be the distance between the peripheral surface of the first boss  13  and the peripheral surface of the developing roller shaft  12   c , and the second distance D 2  may be the distance between the outer surface of the first pressure receiving surface  15  and the peripheral surface of the developing roller shaft  12   c.    
     The second boss  16  extends in the first direction. The second boss  16  in the present embodiment is a separate component from the housing  11 , and is fixed to the housing  11 . In some embodiments, the second boss  16  may be integral with the housing  11 . The second boss  16  is movable together with the housing  11 . The second boss  16  is located at the first outer surface  111 . More specifically, the second boss  16  protrudes from the first outer surface  111  outward in the first direction. The second boss  16  is located farther away from the developing roller  12  in both the second and third directions than the first boss  13 . In other words, the second boss  16  is opposed to the developer roller  12  toward the first boss  13  in the second direction. The second boss  16  is opposed to the developer roller  12  toward the first boss  13  in the third direction. As described in detail later, the second boss  16  receives a pressing force directed from the second end  114  toward the first end  113  in the third direction when the developer cartridge  1  is attached to the drum unit  3 . 
     The second collar  17  is cylindrical, and is rotatable about the second boss  16 . The second collar  17  has hollow. The second collar  17  receives the second boss  16 . In some embodiments, the second collar  17  may not be cylindrical. For example, the second collar  17  may be polygonal. The developer cartridge  1  may not include the second collar  17 . In this case, the peripheral surface of the second boss  16  may serve as the second collar. 
     The first protrusion  18  extends in the first direction. The first protrusion  18  is located at a first end of the developing roller  12  in the first direction. More specifically, the first protrusion  18  is located at a first end of the developing roller shaft  12   c  in the first direction. The first protrusion  18  is a separate component from the developing roller  12 , and is attached to the developing roller  12 . More specifically, the first protrusion  18  is a separate component from the developing roller shaft  12   c , and is attached to the developing roller shaft  12   c . In some embodiments, the first protrusion  18  may be integral with the developing roller  12 . More specifically, the first protrusion  18  may be integral with the developing roller shaft  12   c . As described in detail later, the first protrusion  18  contacts a guide  49  of the drum unit  3  when the developer cartridge  1  is attached to the drum unit  3 . The first protrusion  18  thus positions the developer cartridge  1  relative to the drum unit  3 . 
     The third boss  19  shown in  FIGS. 4 and 6  extends in the first direction. The third boss  19  in the present embodiment is a separate component from the housing  11 , and is fixed to the housing  11 . In some embodiments, the third boss  19  may be integral with the housing  11 . The third boss  19  is movable together with the housing  11 . The third boss  19  is located at the second outer surface  112 . More specifically, the third boss  19  protrudes from the second outer surface  112  outward in the first direction. The third boss  19  is located at an axis as the first boss  13  in the first direction. The developer cartridge  1  includes a third collar that is rotatable about the third boss  19 . As described in detail later, the third boss  19  and the first boss  13  together bear the weight of the developer cartridge  1 . 
     The second pressure receiving surface  21  has an arc-shaped surface curved along an arc about a straight line in the first direction. The second pressure receiving surface  21  is located away from the first pressure receiving surface  15  in the first direction. The second pressure receiving surface  21  is located inward from the second outer surface  112  in the first direction. The second pressure receiving surface  21  overlaps the first pressure receiving surface  15  when viewed in the first direction. In other words, the second pressure receiving surface  21  is located farther away from the developing roller  12  than the third boss  19  in the second direction. 
     The third boss  19  is located away from the developing roller  12  by a first distance D 1  in the third direction. The second pressure receiving surface  21  is located away from the developing roller  12  by the first distance D 1  or a second distance D 2  smaller than the first distance D 1  in the third direction (D 2 ≤D 1 ). 
     The fourth boss  22  extends in the first direction. The fourth boss  22  in the present embodiment is a separate component from the housing  11 , and is fixed to the housing  11 . In some embodiments, the fourth boss  22  may be integral with the housing  11 . The fourth boss  22  is movable together with the housing  11 . The fourth boss  22  is located at the second outer surface  112 . More specifically, the fourth boss  22  protrudes from the second outer surface  112  outward in the first direction. The fourth boss  22  is located at the same axis as the second boss  16  extending in the first direction. The developer cartridge  1  has a collar that is rotatable about the fourth boss  22 . 
     The second protrusion  23  extends in the first direction. The second protrusion  23  is located at a second end of the developing roller  12  in the first direction. More specifically, the second protrusion  23  is located at a second end of the developing roller shaft  12   c  in the first direction. The second protrusion  23  is a separate component from the developing roller  12 , and is attached to the developing roller  12 . More specifically, the second protrusion  23  is a separate component from the developing roller shaft  12   c , and is attached to the developing roller shaft  12   c . In some embodiments, the second protrusion  23  may be integral with the developing roller  12 . More specifically, the second protrusion  23  may be integral with the developing roller shaft  12   c . The second protrusion  23  is located at the same axis as the first protrusion  18  extending in the first direction. As described in detail later, the second protrusion  23  and the first protrusion  18  together position the developer cartridge  1  relative to the drum unit  3 . 
     The image forming apparatus  100  according to the present embodiment includes the developer cartridge holders  30  of the first side plate  33  and the developer cartridge holders of the second side plate  34 . More specifically, the developer cartridge holders  30  of the first side plate  33  and the developer cartridge holders of the second side plate  34  in the present embodiment each include a support  46 , a pressing member  47 , a lock lever  48 , and a guide  49  as shown in  FIGS. 8 and 9 . The support  46 , the pressing member  47 , the lock lever  48 , and the guide  49  in each developer cartridge holder  30  of the first side plate  33  are paired with these components in the corresponding developer cartridge holder of the second side plate  34 . The developer cartridge holders  30  of the first side plate  33  will be mainly described, without repeatedly describing the developer cartridge holders of the second side plate  34 . 
     The support  46  has a support surface  41  as shown in  FIG. 10 . The support  46  protrudes from the inner surface of the first side plate  33  in the first direction. The support surface  41  faces in the direction opposite the force of gravity in the present embodiment. The support surface  41  is a flat surface perpendicular to the third direction. The support surface  41  supports the first boss  13  through the first collar  14  when the developer cartridge  1  is attached to the developer cartridge holder  30 . 
     The pressing member  47  has a first pressing surface  42 . The pressing member  47  is located at the inner surface of the first side plate  33 . The first pressing surface  42  is a flat surface substantially perpendicular to the second direction. The first pressing surface  42  is slidable in the second direction. The pressing member  47  includes a spring (not shown), which is an elastic member. The spring has a second length L 2  when the first pressing surface  42  is free from a pressing force in the second direction. In other words, the spring has the second length L 2  when the developer cartridge  1  is yet to be attached to the drum unit  3 . When the developer cartridge  1  has been attached to the drum unit  3 , the first pressing surface  42  is pressed against the first pressure receiving surface  15  to slide toward the second end of the drum unit in the second direction. This shortens the spring to a first length L 1 , which is smaller than the second length L 2  (L 1 &lt;L 2 ). The spring with the first length L 1  presses the first pressing surface  42  toward a first end of the drum unit  3  in the second direction. The first pressing surface  42  contacts the first pressure receiving surface  15  to apply, to the first pressure receiving surface  15 , a pressing force directed from the second end toward the first end of the drum unit  3  in the second direction when the developer cartridge  1  is attached to the developer cartridge holder  30 . 
     The lock lever  48  is movable between the lock position and the release position. In examples shown herein, the lock lever  48  is pivotable between the lock position and the release position. The lock lever  48  has a second pressing surface  43  and a guide surface  51 . The guide surface  51  is arc-shaped. The second pressing surface  43  is a flat surface continuous with the guide surface  51 . When the lock lever  48  is at the lock position, the guide surface  51  is at least partially located between the first side plate  33  and the second side plate  34 , and the second pressing surface  43  is substantially perpendicular to the third direction and faces toward a second end of the drum unit  3  in the third direction. When the lock lever  48  is at the release position, the guide surface  51  is located farther away from the second side plate  34  than when the lock lever  48  is at the lock position. The second pressing surface  43  contacts the second boss  16  through the second collar  17  when the developer cartridge  1  is inserted in the developer cartridge holder  30  of the drum unit  3 . This applies, to the second boss  16 , a pressing force directed from a first end of the drum unit  3  in the third direction toward the second end of the drum unit  3  in the third direction. The lock lever  48  prevents the developer cartridge  1  from easily separating under impact or vibrations. 
     The guide  49  has a first guide surface  44  and a second guide surface  45 . The guide  49  is located at the inner surface of the first side plate  33 . The guide  49  protrudes from the inner side surface of the first side plate  33  in the first direction. The first guide surface  44  is a flat surface substantially perpendicular to the third direction. The guide  49  contacts the first protrusion  18  when the developer cartridge  1  rotates about the first boss  13 . The guide  49  thus positions the developer cartridge  1  relative to the drum unit  3 . 
     The second guide surface  45  has a flat surface and a slope surface continuous with the flat surface. The flat surface is substantially perpendicular to the third direction. One end of the slope surface in the second direction connects to the flat surface. The slope surface slopes from one end of the slope surface in the second direction to the other end in the third direction toward the other end of the slope in the second direction. The second guide surface  45  is spaced from the first protrusion  18  in the third direction when the developer cartridge  1  rotates about the first boss  13 . This prevents the developer cartridge  1  from being greatly misaligned relative to the drum unit  3 . 
     5. Detailed Structure of Lock Mechanism 
     The structure of the lock lever  48  according to the present embodiment will now be described in detail. A lock mechanism according to the present embodiment includes the lock lever  48 . 
     As shown in  FIGS. 8 to 10 , the lock levers  48  are arranged at positions corresponding to the developer cartridges  1  to be attached to the drum unit  3 . The lock levers  48  located at the first side plate  33  are paired with the corresponding lock levers  48  located at the second side plate  34 . The lock levers  48  located at the first side plate  33  will be mainly described without repeatedly describing the lock levers  48  located at the second side plate  34 . 
     The first side plate  33  has a plurality of openings  33   a  located at a first end of the first side plate  33  in the third direction (end in the direction opposite the force of gravity). The plurality of openings  33   a  are spaced from one another in the second direction. Each opening  33   a  is rectangular, and extends through the first side plate  33  in the first direction. In the present embodiment, the first side plate  33  has four openings  33   a  to receive four developer cartridges  1  to be attached to the drum unit  3 . The lock lever  48  is exposed through each opening  33   a.    
     In detail, each opening  33   a  is defined by a first inner wall of the first side plate  33  located at a first end of the first side plate  33  in the second direction and a second inner wall of the first side plate  33  located at a second end of the first side plate  33  in the second direction. The lock lever  48  includes a pivot shaft (not shown) between the first inner wall and the second inner wall in the second direction. The lock lever  48  is pivotable about the pivot shaft. 
     The lock lever  48  includes an arc-shaped guide surface  51 . The lock lever  48  is pivotable about a pivot axis between the lock position and the release position. When the lock lever  48  is at the lock position, the guide surface  51  is at least partially located between the first side plate  33  and the second side plate  34  in the first direction. When the lock lever  48  is at the release position, the guide surface  51  is located farther away from the first side plate  33  in the first direction than when the lock lever  48  is at the lock position. 
     The lock lever  48  at the lock position applies, to the developer cartridge  1  attached to the drum unit  3 , a pressing force directed from the first end of the drum unit  3  in the third direction toward the second end of the drum unit  3  in the third direction. This prevents the developer cartridge  1  from easily separating from the drum unit  3  under impact or vibrations. 
     The lock lever  48  at the release position allows the developer cartridge  1  to be inserted into the developer cartridge holder  30  of the drum unit  3  or to be removed from the developer cartridge holder  30  of the drum unit  3 . 
     The guide surface  51  of the lock lever  48  guides the developer cartridge  1  (a first outer surface  111  and a second outer surface  112 ) during attachment of the developer cartridge  1  to the developer cartridge holder  30  of the drum unit  3 . 
     The lock lever  48  includes an elastic spring (not shown) that applies a pressing force directed from the first end of the drum unit  3  in the third direction to the second end of the drum unit  3  in the third direction when the developer cartridge  1  is attached to the developer cartridge holder  30  of the drum unit  3 . In other words, the lock lever  48  is constantly pressed toward the lock position by the spring force. 
     6. Forces Acting Relative to One Another in Attaching Developer Cartridge 
     The forces acting relative to one another during attachment of the developer cartridge  1  to the developer cartridge holder  30  of the drum unit  3  will now be described with reference mainly to  FIGS. 8 to 15 . 
     To attach the developer cartridge  1  to the drum unit  3 , the user first holds the developer cartridge  1  to have the first end  113  of the housing  11  facing the second end of the drum unit  3  in the third direction and the second end  114  of the housing  11  facing the first end of the drum unit  3  in the third direction. The user then inserts the developer cartridge  1  into the developer cartridge holders (the developer cartridge holder  30  of the first side plate  33  and the developer cartridge holder of the second side plate  34 ) from the first end of the drum unit  3  in the third direction toward the second end of the drum unit  3  in the third direction (refer to  FIG. 8 ). As a result, the guide surface  51  of the lock lever  48  contact with the second boss  16  (second collar  17 ), causing the guide surface  51  of the lock lever  48  pivots outward in the drum unit  3 . In this manner, the lock lever  48  pivots toward the release position and the end faces (the first outer surface  111  and the second outer surface  112 ) of the developer cartridge  1  is guided along the arc-shaped guide surface  51 . As a result, the user smoothly pushes the developer cartridge  1  toward the second end of the drum unit  3  in the third direction. 
     Then, the first boss  13  is supported by the support surface  41  through the first collar  14 . The support surface  41  bears the weight of the developer cartridge  1 . 
     As the user pushes the developer cartridge  1  toward the second end of the drum unit  3  in the third direction during movement of the lock lever  48  from the lock position to the release position against the pressing force applied from the spring of the lock lever  48 , the second boss  16  moves toward the second end of the drum unit  3  in the third direction farther than the lock lever  48 . The lock lever  48  then pivots to the lock position under the pressing force applied from the spring of the lock lever  48 . The lock lever  48  then moves to the lock position under the pressing force applied from the spring of the lock lever  48 , thus causing the second pressing surface  43  to apply, to the second boss  16  (second collar  17 ), a pressing force directed from the first end of the drum unit  3  in the third direction toward the second end of the drum unit  3  in the third direction. 
     More specifically, the second boss  16  (second collar  17 ) contacts the second pressing surface  43  of the lock lever  48  at the lock position to receive, from the second pressing surface  43 , a pressing force directed from the second end  114  of the housing  11  toward the first end  113  of the housing  11  in the third direction. At this position, the first boss  13  is supported by the support surface  41  located at a second end of the drum unit  3  in the third direction. These opposite forces cause a rotation moment in the developer cartridge  1  about the first boss  13 . This slightly rotates the developer cartridge  1  about the first boss  13  in the drum unit  3 , and moves the developing roller  12  toward the photosensitive drum  31 . As the developing roller  12  moves toward the photosensitive drum  31 , the first protrusion  18  contacts the first guide surface  44  to stop the rotation of the developer cartridge  1  about the first boss  13 . This determines the angle of contact between the developing roller  12  and the photosensitive drum  31 , and appropriately positions the developing roller  12  relative to the photosensitive drum  31 . In other words, the developer cartridge  1  is positioned in the drum unit  3 . 
     As the user inserts the developer cartridge  1  into the drum unit  3  from the first end of the drum unit  3  in the third direction toward the second end of the drum unit  3  in the third direction, the first pressing surface  42  is pushed by the first pressure receiving surface  15  to move toward the second end of the drum unit  3  in the second direction against the pressing force of the spring of the pressing member  47 . The first pressure receiving surface  15  thus receives, from the first pressing surface  42 , a pressing force directed from the covered surface  12   b  toward the uncovered surface  12   a  in the second direction at the same time as or subsequently to slight rotation of the developer cartridge  1  about the first boss  13 . At this position, the support surface  41  receives the weight of the developer cartridge  1  applied through the first boss  13 . The first boss  13  (first collar  14 ) thus slides toward the first end of the drum unit  3  in the second direction when the first boss  13  is supported by the support surface  41 . This moves the developing roller  12  further toward the photosensitive drum  31 , and allows the peripheral surface of the developing roller  12  (developing roller body) to contact the peripheral surface of the photosensitive drum  31  under an appropriate contact pressure. At this position, the support surface  41  bears the weight of the developer cartridge  1 . The weight of the developer cartridge  1  is less likely to affect the contact pressure between the developing roller  12  and the photosensitive drum  31 . In other words, the contact pressure between the developing roller  12  and the photosensitive drum  31  is determined solely by the pressing force applied from the spring of the pressing member  47  and the pressing force applied from the spring of the lock lever  48 . This prevents the contact pressure between the developing roller  12  and the photosensitive drum  31  from varying greatly depending on the amount of toner remaining in the developer cartridge  1 . 
     Immediately before the peripheral surface of the developing roller  12  (developing roller body) contacts the peripheral surface of the photosensitive drum  31 , the first protrusion  18  is held between the first guide surface  44  and the second guide surface  45  in the third direction (refer to  FIG. 10 ). This structure appropriately positions the developing roller  12  relative to the photosensitive drum  31 . The slope surface included in the second guide surface  45  smoothly guides the developing roller  12  between the first guide surface  44  and the second guide surface  45 . 
     In the manner described above, the developer cartridge  1  is appropriately positioned relative to the drum unit  3  when the developer cartridge  1  is attached to the drum unit  3 . When the developer cartridge  1  is attached to the drum unit  3 , the lock lever  48  is at the lock position, restricting the movement of the developer cartridge  1  toward the first end of the drum unit  3  in the third direction (in the direction opposite the force of gravity). The second pressing surface  43  thus locks the developer cartridge  1 . The developer cartridge  1  thus does not easily separate from the drum unit  3  under vibrations or impact applied to the image forming apparatus  100 . The structure according to the present embodiment enables reliable attachment and removal of the developer cartridge  1  to and from the drum unit  3  as well as reliable locking. 
     To remove the developer cartridge  1  from the drum unit  3 , the lock lever  48  is temporarily pivoted toward the release position to move the developer cartridge  1  toward the first end of the drum unit  3  in the third direction during movement of the first outer surface  111  and the second outer surface  112  along the curve of the arc-shaped guide surface  51 . 
     In the present embodiment, the lock lever  48  is pivotable about the pivot axis extending in the second direction. The lock lever  48  is switched between the lock position and the release position for attaching or removing the developer cartridge  1  to and from the drum unit  3 . This structure allows each of the lock levers  48  to be positioned so that the first surface  111  or the second outer surface  112  of a corresponding developer cartridge  1  held by the frame  38  completely overlaps the lock lever  48  as viewed in the first direction. In other words, each of the lock levers  48  is positioned entirely between a third outer surface and a fourth outer surface of the corresponding developer cartridge  1 . The third outer surface extends between the first outer surface and the second outer surface in the first direction. The fourth outer surface extends between the first outer surface and the second outer surface in the first direction. The fourth outer surface is spaced from the third outer surface in the second direction. This eliminates the clearance required for the lock lever  48  to pivot between the developer cartridges  1  adjacent in the second direction, and thus allows the adjacent developer cartridges  1  to be closer to one another. The drum unit  3  is downsized in the direction in which the photosensitive drums  31  are arranged.