Patent Publication Number: US-2023136526-A1

Title: Developing cartridge having electrode contactable with another electrode by pivot motion of the cartridge

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 17/117,314, filed Dec. 10, 2020, which is a continuation of U.S. patent application Ser. No. 16/290,152, filed Mar. 1, 2019, now U.S. Pat. No. 10,890,857, which claims priority from Japanese Patent Application No. 2018-067902 filed Mar. 30, 2018. The entire content of the priority applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a developing cartridge attachable to and detachable from a drum unit. 
     BACKGROUND 
     There is conventionally known an electro-photographic type image forming apparatus such as a laser printer and an LED printer. The image forming apparatus includes a drum unit and a plurality of developing cartridges. The drum unit includes a plurality of photosensitive drums. The developing cartridge includes a developing roller. The plurality of developing cartridges are attachable to and detachable from the drum unit. Upon attachment of the developing cartridge to the drum unit, the photosensitive drum of the drum unit and the developing roller of the developing cartridge are in contact with each other. 
     SUMMARY 
     According to the above described image forming apparatus, a bias voltage is applied to a shaft of the developing roller in order to carry developing agent on the surface of the developing roller. To this effect, an electrode is provided at an end portion of the developing roller. Upon attachment of the developing cartridge to the drum unit, the electrode positioned at the end portion of the developing cartridge contacts an electrode provided at the drum unit. Hence, bias voltage is supplied to the shaft of the developing roller through the electrodes. According to the conventional structure, a spring is provided between a casing of the developing cartridge and the electrode for ensuring contact between electrodes. 
     In view of the foregoing, it is an object of the disclosure to provide a developing cartridge capable of making contact between an electrode of a developing cartridge and an electrode of a drum unit without employment of a spring between a casing of the developing cartridge and the electrode at a time of attachment of the developing cartridge to the drum unit. 
     In order to attain the above and other objects, according to one aspect, the disclosure provides a developing cartridge attachable to and detachable from a drum unit. The developing cartridge includes a casing having an outer surface, a developing roller, a first electrode, and a boss. Developing agent is accommodatable in the casing. The developing roller is movable together with the casing and includes a developing roller shaft extending along a first axis extending in a first direction. The developing roller is rotatable about the first axis. The first electrode is positioned at one end portion in the first direction of the developing roller shaft, and electrically connected to the developing roller shaft. The boss is positioned at one end portion in the first direction of the outer surface. The boss extends in the first direction. A part of the first electrode is configured to contact with a second electrode of a drum unit as a result of pivotal movement of the developing cartridge about the boss relative to the drum unit in a case where the developing cartridge is attached to the drum unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the embodiment(s) as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG.  1    is a conceptual diagram of an image forming apparatus; 
         FIG.  2    is a perspective view of a drum unit to which four developing cartridges according to one embodiment are attached; 
         FIG.  3    is a perspective view of the drum unit; 
         FIG.  4    is a perspective view of the developing cartridge according to the embodiment; 
         FIG.  5    is a plan view of the developing cartridge according to the embodiment as viewed in a first direction; 
         FIG.  6    is a cross-sectional view of the developing cartridge according to the embodiment taken along a plane perpendicular to the first direction; 
         FIG.  7    is a perspective view illustrating one end portion in the first direction of a developing roller and an electrode in the developing cartridge according to the embodiment; and 
         FIG.  8    is a plan view of the developing cartridge according to the embodiment as viewed in the first direction, and particularly illustrating positional relationship between the developing cartridge and components of the drum unit. 
     
    
    
     DETAILED DESCRIPTION 
     A developing cartridge according to one embodiment will be described with reference to the accompanying drawings. Firstly, an image forming apparatus  100  will be described with reference to  FIG.  1   . 
     1. Image Forming Apparatus 
     The image forming apparatus  100  illustrated in  FIG.  1    is an electro-photographic type printer, such as a laser printer and an LED printer. The image forming apparatus  100  includes a main casing  101 , a controller  102 , a drum unit  1 , and four developing cartridges  2 . Each of the developing cartridges  2  is attachable to and detachable from the drum unit  1  independent of each other. Further, the drum unit  1  to which four developing cartridges  2  are attached is attachable to and detachable from the main casing  101 . 
     As described later in detail, each developing cartridge  2  includes a developing roller  22  ( FIG.  5   ), and the drum unit  1  includes four photosensitive drums  11  ( FIG.  3   ). In a state where the developing cartridge  2  is attached to the drum unit  1 , extending direction or an axial direction of the developing roller  22  will be referred to as “first direction”. Further, a direction of an array of the four photosensitive drums  11  will be referred to as “second direction” crossing the first direction. Further, a direction crossing the first and second directions will be referred to as “third direction”. Preferably, the third direction is perpendicular to the first and second directions, and is coincident with or approximately equal to a gravitational direction. 
     The four developing cartridges  2  store developing agent (for example, toner) of different colors such as for example, cyan, magenta, yellow, and black. The image forming apparatus  100  is configured to form an image on a recording surface of a printing sheet by toner supplied from respective four developing cartridges  2 . However, numbers of the developing cartridges may be 1 through 3 or five or more. 
     The controller  102  is positioned inside the main casing  101 . The controller  102  is provided by, for example, a circuit board including a processor such as CPU, and various memories. The controller  102  performs various processing executed in the image forming apparatus  100  by operating the processor in accordance with various programs. 
     2. Drum Unit 
     As illustrated in  FIGS.  2  and  3   , the drum unit  1  includes the four photosensitive drum  11 , and a frame  12 . 
     The photosensitive drum  11  is configured to receive toner supplied from the developing cartridge  2  to form a toner image, and the toner image is transferred to a printing sheet. Four photosensitive drums  11  are arrayed in the second direction with a space between neighboring photosensitive drums. Each photosensitive drum  11  has a cylindrical outer surface extending in the first direction. The outer surface is made from a photosensitive material. Further, each photosensitive drum  11  is rotatable about a rotation axis extending in the first direction. 
     The frame  12  is configured to hold the four photosensitive drums  11 . The frame  12  includes four holding portions  120  each for holding each developing cartridge  2 . The four holding portions  120  are arrayed in the second direction with an interval between neighboring holding portions  120 . Each photosensitive drum  11  is positioned at one end portion in the third direction of each holding portion  120 . Each developing cartridge  2  is attachable to and detachable from each holding portion  120 . Upon completion of attachment of the developing cartridge  2  to the holding portion  120 , an outer surface of the developing roller  22  is in contact with an outer surface of the photosensitive drum  11 . 
     The frame  12  includes a first side plate  121 , a second side plate  122 , a third side plat  123 , and a fourth side plate  124 . The first side plate  121  extends in the second direction and the third direction and perpendicular to the first direction. The first side plate  121  is positioned at one side of the four photosensitive drums  11  in the first direction. Each one end portion in the first direction of each photosensitive drum  11  is rotatably supported to the first side plate  121 . The second side plate  122  extends in the second direction and the third direction and perpendicular to the first direction. The second side plate  122  is positioned at another side of the four photosensitive drums  11  in the first direction. Each another end portion in the first direction of each photosensitive drum  11  is rotatably supported to the second side plate  122 . 
     The third side plate  123  extends in the first direction and the third direction. The third side plate  123  is connected between one end portion in the second direction of the first side plate  121  and one end portion in the second direction of the second side plate  122 . The third side plate  123  is positioned at one side in the second direction of the array of the four photosensitive drums  11 . The fourth side plate  124  extends in the first direction and the third direction. The fourth side plate  124  is connected between another end portion in the second direction of the first side plate  121  and another end portion in the second direction of the second side plate  122 . The fourth side plate  124  is positioned at another side in the second direction of the array of the four photosensitive drums  11 . A handle  126  is provided at the third side plate  123 , and another handle  126  is positioned at the fourth side plate  124 . A user grips the handles  126  for the attachment and detachment of the drum unit  1  to and from the main casing  101 . 
     As illustrated in  FIG.  8   , the first side plate  121  of the drum unit  1  is provided with a support portion  13 , a first pressure portion  14 , a second pressure portion  15  and the electrode  16  as an example of second electrode. 
     The support portion  13  protrudes from the first side plate  121  toward the second side plate  122 , and tubular in shape. The support portion  13  has a support surface  131  extending in a direction perpendicular to the third direction. The support surface  131  faces in a direction opposite to the photosensitive drum  11 . 
     The first pressure portion  14  is pivotally movable about a shaft positioned at the first side plate  121  and extending in the second direction. The first pressure portion  14  has a first pressure surface  141 . The first pressure surface  141  has a width in the second direction, and is arcuate in shape. The first pressure portion  14  is pivotally movable between a lock position and a release position. When the first pressure portion  14  is positioned at the release position, the first pressure surface  141  faces the second side plate  122 . The first pressure surface at the lock position is positioned closer to the second side plate  122  than the first pressure surface at the release position is to the second side plate  122 . A resiliently deformable member such as a spring (not illustrated) is connected to the first pressure portion  14  so as to urge the first pressure portion  14  to its lock position. 
     The second pressure portion  15  is positioned adjacent to a surface of the first side plate  121 , the surface facing the second side plate  122 . The second pressure portion is movable in the second direction. The second pressure portion  15  has a second pressure surface  151  extending perpendicular to the second direction. The second pressure portion  15  is movable in the second direction between a pressure position and a retracted position. The second pressure surface  151  at its pressure position is positioned closer to the photosensitive drum  11  in the second direction than the second pressure surface  151  at its retracted position is to the photosensitive drum  11 . A resiliently deformable member such as a spring (not illustrated) is connected to the second pressure portion  15  so as to urge the second pressure portion  15  to its pressure position. 
     The electrode  16  is fixed to the surface of the first side plate  121 , the surface facing the second side plate  122 . The electrode  16  is made from electrically conductive material, for example, electrically conductive metal and electrically conductive resin. The electrode  16  has a contact surface  161  extending perpendicular to the third direction, and facing to the photosensitive drum  11 . The electrode  16  is configured to be supplied with voltage from the above-described controller  102 . 
     The second side plate  122  of the drum unit  1  is provided with a support portion, a first pressure portion, and a second pressure portion, those corresponding to the support portion  13 , the first pressure portion  14 , and the second pressure portion  15  of the support portion  13 , the first pressure portion  14 , and the second pressure portion  15 , respectively. Further, the second side plate  122  has a contact surface. The contact surface of the second side plate  122  and the contact surface  161  of the second electrode  16  of the first side plate  121  are positioned on an identical linear line extending in the first direction. Further, the contact surface of the second side plate  122  has a shape the same as that of the contact surface  161  of the electrode  16  of the first side plate  121 . However, the contact surface of the second side plate  122  is made from non-electrically conductive material. 
     3. Developing Cartridge 
     As illustrated in  FIGS.  4  through  6   , the developing cartridge  2  includes a casing  21 , the developing roller  22 , a supply roller  23 , an electrode  24  as an example of a first electrode, a first boss  25 , and a second boss  26 . 
     The casing  21  is configured to accommodate therein developing agent. The casing  21  has a first outer surface  211  and a second outer surface  212 , and extends in the first direction directing from the first outer surface  211  to the second outer surface  212  and vice versa. A storage chamber  213  is provided inside the casing  21  for accommodating the developing agent. The casing  21  has one end portion in the third direction where an opening  214  is formed. The storage chamber  213  is communicated with an outside through the opening  214 . 
     The developing roller  22  is rotatable about a first axis A 1  extending in the first direction. The developing roller  22  is positioned at the opening  214 . That is, the developing roller  22  is positioned at one end portion in the third direction of the casing  21 . The developing roller  22  is movable together with the casing  21 . The developing roller  22  includes a developing roller body  221 , and a developing roller shaft  222 . The developing roller body  221  is hollow cylindrical in shape extending in the first direction, and is made from rubber having elasticity. The developing roller shaft  222  extends through the developing roller body  221 , and is solid cylindrical in shape extending along the first axis A 1 . The developing roller shaft  222  is made from metal or electrically conductive resin. 
     The developing roller body  221  is fixed to the developing roller shaft  222  avoiding relative rotation therebetween. The developing roller shaft  222  has one end portion in the first direction fixed to a developing roller gear (not illustrated) positioned at the second outer surface  212 . Hence, rotation of the developing roller gear causes rotation of the developing roller shaft  222  and the developing roller body  221 . Upon attachment of the developing cartridge  2  to the drum unit  1 , an outer surface of the developing roller body  221  is in contact with an outer surface of the photosensitive drum  11 . 
     Incidentally, the developing roller shaft  222  needs not extend through a length of the developing roller body  221  in the first direction. For example, a pair of developing roller shafts  222  are provided, and each roller shaft  222  extends from each end in the first direction of the developing roller body  221  in the first direction. 
     The supply roller  23  is rotatable about a second axis A 2  extending in the first direction. The supply roller  23  is positioned between the developing roller  22  and the storage chamber  213 . The supply roller  23  is movable together with the casing  21 . The supply roller includes a supply roller body  231 , and a supply roller shaft  232 . The supply roller body  231  is hollow cylindrical in shape extending in the first direction, and is made from rubber having elasticity. The supply roller shaft  232  extends through the supply roller body  231 , and is solid cylindrical in shape extending along the second axis A 2 . The supply roller shaft  232  is made from metal or electrically conductive resin. 
     The supply roller body  231  is fixed to the supply roller shaft  232  avoiding relative rotation therebetween. The supply roller shaft  232  has one end portion in the first direction fixed to a supply roller gear (not illustrated) positioned at the second outer surface  212 . Hence, rotation of the supply roller gear causes rotation of the supply roller shaft  232  and the supply roller body  231 . 
     Incidentally, the supply roller shaft  232  needs not extend through a length of the supply roller body  231  in the first direction. For example, a pair of supply roller shafts  232  are provided, and each roller shaft  232  extends from each end in the first direction of the supply roller body  231  in the first direction. 
     An outer surface of the supply roller  23  and the outer surface of the developing roller  22  are in contact with each other. By rotating the developing roller  22  and the supply roller  23 , developing agent is supplied from the storage chamber  213  to the outer surface of the developing roller  22  through the supply roller  23 . In this case, the developing agent is subjected to triboelectric charging between the supply roller  23  and the developing roller  22 . Further, as described later, bias voltage is applied to the developing roller shaft  222  and the supply roller shaft  232 . Hence, the developing agent is electro-statically attracted to and carried on the outer peripheral surfaces of the supply roller body  231  and the developing roller body  221 . 
     Thereafter, the developing agent carried on the outer peripheral surface of the developing roller body  221  is supplied to the photosensitive drum  11  of the drum unit  1 . In this case, the developing agent moves from the developing roller body  221  to the photosensitive drum  11  in accordance with an electrostatic latent image formed on the outer peripheral surface of the photosensitive drum  11 . Hence, the electrostatic latent image becomes a visible toner image on the outer peripheral surface of the photosensitive drum  11 . Then, the toner image is transferred from the photosensitive drum  11  to the printing sheet. 
     The electrode  24  is positioned at the outer surface  211  of the casing  21 , and is made from electrically conductive material. The electrode functions not only as a bearing for rotatably supporting the developing roller shaft  222  and the supply roller shaft  232 , but also as an electrode for supplying bias voltage to the developing roller shaft  222  and the supply roller shaft  232 . Electrically conductive resin is preferable as a material of the electrode  24  because of easiness of producing the electrode having complicated shape. However, metal is also available as the material of the electrode  24 . 
     As illustrated in  FIG.  7   , the electrode  24  includes a base portion  240 , a first collar  241 , and a second collar  242 . The base portion  240  has a generally plate-like shape extending perpendicular to the first direction. The base portion  240  is fixed to the first outer surface  211  of the casing  21  by a screw. 
     The first collar  241  extends from the base portion  240  in a direction away from the casing  21  in the first direction, and is hollow cylindrical. The first collar  241  rotatably supports one end portion in the first direction of the developing roller shaft  222 . Specifically, the first collar  241  has a through-hole  241   a  extending in the axial direction. An inner peripheral surface of the through-hole  241   a  serves as a hollow cylindrical bearing surface  241   b . The one end portion in the first direction of the developing roller shaft  222  is rotatably fitted with the through-hole  241   a . Thus, the electrode  24  is electrically connected to the developing roller shaft  222 . An outer surface of the one end portion of the developing roller shaft  222  is in rotational-sliding contact with the bearing surface  241   b . As described above, since electrically conductive resin is used as the material of the electrode  24 , sufficient slidability between the electrode  24  (bearing surface  241   b ) and the developing roller shaft  222  is obtained rather than a case where the electrode is made from metal. 
     Incidentally, instead of the through-hole  241   a , a distal end portion of the first collar  241  may have a bottom wall to provide a recessed portion. Further, the through-hole  241   a  or the recessed portion may not necessarily cover an entire peripheral surface of the one end portion of the developing roller shaft  222 . That is, the through-hole or the recessed portion covering at least a part of the peripheral surface of the one end portion of the developing roller shaft  222  is sufficient. 
     Further, not only direct contact but also indirect contact is available between the electrode  24  and the developing roller shaft  222 . In the latter case, additional electrically conductive member may be interposed between the electrode  24  and the developing roller shaft  222 . For example, the one end portion of the developing roller shaft  222  may be capped with a cap made from electrically conductive material, and the cap may be rotatably supported by the first collar  241 . 
     As illustrated in  FIG.  7   , the first collar  241  includes a large diameter portion  241   c  and a small diameter portion  241   d . The large diameter portion  241   c  protrudes from the base portion  240  in the direction away from the casing  21  in the first direction, and is hollow cylindrical. The small diameter portion  241   d  protrudes from the large diameter portion  241   c  in the direction away from the casing  21  in the first direction, and is hollow cylindrical. The outer peripheral surfaces of the large and small diameter portions  241   c  and  241   d  are coaxial with the first axis A 1 . An outer diameter of the large diameter portion  241   c  is greater than that of the small diameter portion  241   d.    
     The second collar  242  extends from the base portion  240  in a direction away from the casing  21  in the first direction, and is hollow cylindrical. The second collar  242  rotatably supports one end portion in the first direction of the supply roller shaft  232 . Specifically, the second collar  242  has a through-hole  242   a  extending in the axial direction. An inner peripheral surface of the through-hole  242   a  serves as a hollow cylindrical bearing surface  242   b . The one end portion in the first direction of the supply roller shaft  232  is rotatably fitted with the through-hole  242   a . Thus, the electrode  24  is electrically connected to the supply roller shaft  232 . That is, in this embodiment, the electrode  24  is electrically connected to both the developing roller shaft  222  and the supply roller shaft  233  so that bias voltage can be supplied to both the developing roller shaft  222  and the supply roller shaft  232  from the electrode  16  of the drum unit  1  through the electrode  24 . An outer surface of the one end portion of the supply roller shaft  232  is in rotational-sliding contact with the bearing surface  242   b . As described above, since electrically conductive resin is used as the material of the electrode  24 , sufficient slidability between the electrode  24  and the supply roller shaft  232  is obtained rather than a case where the electrode is made from metal. 
     Incidentally, instead of the through-hole  242   a , a distal end portion of the second collar  242  may have a bottom wall to provide a recessed portion. Further, the through-hole  242   a  or the recessed portion may not necessarily cover an entire peripheral surface of the one end portion of the supply roller shaft  232 . That is, the through-hole or the recessed portion covering at least a part of the peripheral surface of the one end portion of the supply roller shaft  232  is sufficient. 
     Further, not only direct contact but also indirect contact is available between the electrode  24  and the supply roller shaft  232 . In the latter case, additional electrically conductive member may be interposed between the electrode  24  and the supply roller shaft  232 . For example, the one end portion of the supply roller shaft  232  may be capped with a cap made from electrically conductive material, and the cap may be rotatably supported by the second collar  242 . 
     As illustrated in  FIGS.  4  and  5   , the first boss  25  is positioned at the first outer surface  211  of the casing  21 . The first boss  25  extends in the first direction at a position away from the developing roller  22  in the second and third directions. Specifically, the first boss  25  protrudes outward in the first direction from the first outer surface  211  of the casing  21 . The first boss  25  has a strength sufficient for supporting a weight of the developing cartridge  2 . The first boss  25  is a segment separate from the casing  21 , and is fixed to the casing  21 . However, the first boss  25  may be integral with the outer surface  211  of the casing  21 . The first boss  25  is solid cylindrical in shape extending in the first direction. However, the first boss  25  may have other shape such as hollow cylindrical shape and rectangular column. 
     As illustrated in  FIGS.  4  and  5   , the second boss  26  is positioned at the first outer surface  211  of the casing  21 . The second boss  26  extends in the first direction at a position further away from the developing roller  22  in the second and third directions than the first boss  25  is from the developing roller  22 . Specifically, the second boss  26  protrudes outward in the first direction from the first outer surface  211  of the casing  21 . The second boss  26  is a segment separate from the casing  21 , and is fixed to the casing  21 . However, the second boss  26  may be integral with the outer surface  211  of the casing  21 . The second boss  26  is solid cylindrical in shape extending in the first direction. However, the second boss  26  may have other shape such as hollow cylindrical shape and rectangular column. 
     As illustrated in  FIG.  4   , the developing cartridge  2  further includes a positioning protrusion  27 , a third boss  28 , and a fourth boss  29  those positioned at the second outer surface  212 . The first collar  241 , the first boss  25 , and the second boss  26  at the first outer surface  211  are positioned symmetrical with respect to a center of the casing  21  in the first direction with the positioning protrusion  27 , the third boss  28 , and the fourth boss  29  at the second outer surface  212 , respectively. That is, the first collar  241  and the positioning protrusion  27  are on a linear line extending in the first direction. Further, the positioning protrusion  27  has a shape the same as that of the first collar  241 , and protrudes in the first direction opposite to the protruding direction of the first collar  241 . The first boss  25  and the third boss  28  are on a linear line extending in the first direction. Further, the third boss  28  has a shape the same as that of the first boss  25 , and protrudes in the first direction opposite to the protruding direction of the first boss  25 . The second boss  26  and the fourth boss  29  are on a linear line extending in the first direction. Further, the fourth boss  29  has a shape the same as that of the second boss  26 , and protrudes in the first direction opposite to the protruding direction of the second boss  26 . 
     4. Attaching Operation 
     Next, operation for attaching the developing cartridge  2  to the drum unit  1  will be described with reference to  FIG.  8    in which several parts of the drum unit  1  such as the photosensitive drum  11 , the support portion  13 , the first pressure portion  14 , the second pressure portion  15 , and the electrode  16  are illustrated by two dotted chain line. 
     By inserting the developing cartridge  2  into the holding portion  120 , the first boss  25  is brought into contact with the support surface  131  of the support portion  13  of the first side plate  121 . Similarly, the third boss  28  is brought into contact with a support surface of the support portion of the second side plate  122 . Hence, a weight of the developing cartridge  2  is supported by the support surface  131  of the support portion  13  of the first side plate  121  and the support surface of the support portion of the second side plate  122 . 
     Further, in the process of inserting the developing cartridge  2  into the holding portion  120 , the second boss  26  moves in the third direction while being in sliding contact with the first pressure portion  14  of the first side plate  121 . Similarly, the fourth boss  29  moves in the third direction while being in sliding contact with the first pressure portion of the second side plate  122 . In this case, the first pressure portion  14  of the first side plate  121  and the first pressure portion of the second side plate  122  are temporarily moved to their release positions. Then, the first pressure portion  14  moves from the release position to the lock position by the urging force of the spring after the second boss  26  moves past the first pressure portion  14 . Hence, the first pressure portion  14  urges the second boss  26  in the third direction toward the photosensitive drum  11 . Similarly, the first pressure portion of the second side plate  122  urges the fourth boss  29  in the third direction toward the photosensitive drum  11 . 
     As a result, the casing  21  of the developing cartridge  2  is pivotally moved about the first boss  25  and the third boss  28  as indicated by a broken line arrow with respect to the drum unit  1 . Thus, the outer peripheral surface of the first collar  241  is brought into contact with the contact surface  161  of the electrode  16 , and further, the outer peripheral surface of the positioning protrusion  27  positioned at the second outer surface  212  of the developing cartridge  2  is brought into contact with the contact surface of the second side plate  122 . Accordingly, pivotal movement of the casing  21  is stopped. Consequently, pivot posture of the developing cartridge  2  about the axis extending in the first direction is fixed relative to the drum unit  1 . 
     Concurrently with or after the pivotal movement of the casing  21 , the developing cartridge  2  is pressed in the second direction by the second pressure portion  15  of the first side plate  121  and by the second pressure portion of the second side plate  122 . Specifically, a part of the outer surface of the casing  21  is pressed toward the photosensitive drum  11  by the second pressure portion  15  of the first side plate  121 , and another part of the outer surface of the casing  21  is pressed toward the photosensitive drum  11  by the second pressure portion of the first side plate  122 . Accordingly, the developing cartridge  2  moves in the second direction while its pivotal posture about the axis extending in the first direction is maintained. As a result, the outer peripheral surface of the developing roller  22  is brought into contact with the outer peripheral surface of the photosensitive drum  11 . In the present embodiment, the outer peripheral surface of the photosensitive drum  11  and the outer peripheral surface of the developing roller  22  is brought into contact with each other as a result of the casing  21  being pressed in the second direction by the second pressing portion  15 . However, the outer peripheral surface of the photosensitive drum  11  and the outer peripheral surface of the developing roller  22  may be brought into contact with each other only by pivoting movement of the casing  21  without the pressure by the first pressure portion  14  and the second pressure portion  15 . 
     In this state, the weight of the developing cartridge  2  is supported by the support surface  131  of the support portion  13  of the first side plate  121  and the support surface of the support portion of the second side plate  22 , as described above. Therefore, contacting pressure between the outer peripheral surface of the photosensitive drum  11  and the outer peripheral surface of the developing roller  22  is less susceptible to the weight of the developing cartridge  2 . Consequently, change in contacting pressure between the outer peripheral surface of the photosensitive drum  11  and the outer peripheral surface of the developing roller  22  dependent on residual amount of the developing agent accommodated in the developing cartridge  2  can be restrained. 
     Further, in a state of completion of attachment of the developing cartridge  2  to the drum unit  1 , the outer surface of the first collar  124  of the electrode  24  is in contact with the contact surface  161  of the electrode  16 . Hence, the electrode  24  and the electrode  16  are electrically connected to each other. Particularly, in the state of completion of attachment of the developing cartridge  2  to the drum unit  1 , the outer surface of the small diameter portion  241   d  of the first collar  241  and an end surface in the first direction of the large diameter portion  241   c  are in contact with the electrode  16 . Therefore, the reliability of the electrical connection between the electrode  16  and the electrode  24  can be enhanced. 
     Thereafter, the drum unit  1  to which the developing cartridge  2  is attached is attached to the main casing  101  of the image forming apparatus  100 , whereupon the main electrode positioned at the main casing  101  is electrically connected to the electrode  16  positioned in the drum unit  1 . Thus, bias voltage can be supplied from the controller  102  to the developing roller shaft  222  and the supply roller shaft  232  through the main electrode, the electrode  16 , and the electrode  24 . 
     As described above, according to the above-described embodiment, the electrode  24  of the developing cartridge  2  is brought into contact with the electrode  16  of the drum unit  1  by making use of pivotal movement of the developing cartridge  2  about the axis of the first boss  25 . Hence, the electrical contact between the first and electrodes  24  and  16  can be attained without employment of a spring between the electrode  24  and the casing of the developing cartridge  2 . 
     5. Modifications 
     According to the above-described embodiment, the electrode  24  including the base portion  240 , the first collar  241 , and the second collar  242  is a single or integral component. However, the electrode  24  may be provided by a plurality of components. For example, the base portion  240  and the first collar  241  are different components from each other. Further, the first collar  241  may be rotatable about the first axis A 1  with respect to the casing  21  in the state of insertion of the one end portion of the developing roller shaft  222  in the first collar  241 . With this structure, frictional resistance between the electrode  16  and the first collar  241  can be lowered during pivotal motion of the developing cartridge  2  with respect to the drum unit  1 . 
     Further, in the above-described embodiment, the electrode  24  is positioned at the first outer surface  211  of the casing  21 . However, the electrode  24  may be positioned at the second outer surface  212  of the casing  21 . 
     Further, in the above-described embodiment, the electrode  24  is in electrical contact with both the developing roller shaft  222  and the supply roller shaft  232 . However, the electrode  24  may be in electrical contact with the developing roller shaft  222  only. 
     Further, details on shape of the developing cartridge may not be limited to the above-described shape, and various parts and components those appearing in the above-described embodiment and modifications may be combined together as long as technical conflict is avoidable. 
     While the description has been made in detail with reference to the embodiments thereof, it would be apparent to those skilled in the art that many modifications and variations may be made therein without departing from the spirit of the disclosure.