Abstract:
A cartridge is detachably mountable on a main casing. The main casing is provided with a coupling member providing a driving force. The cartridge comprises: a first roller extending in a first direction and configured to rotate about a first rotational axis; a driving force receiving portion configured to receive the driving force from the coupling member to drive the first roller and rotate about a second rotational axis, the driving force receiving portion comprising a contact portion configured to be in contact with the coupling member; and a first gear comprising a gear tooth, the gear being configured to transmit the driving force received at the driving force receiving portion to the first roller. The contact portion is overlapped with the gear tooth when projected in the first direction.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. application Ser. No. 12/632,914, filed Dec. 8, 2009, which claims priority from Japanese Patent Application No. 2008-312011 filed Dec. 8, 2008. The entire content of the priority applications is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a process cartridge and a developing cartridge mounted in an electrophotographic image forming device. 
       BACKGROUND 
       [0003]    A conventional developing cartridge mounted in an image forming device includes a toner hopper, a supply roller and a developing roller. Toner accommodated in the toner hopper is supplied to the supply roller and then to the developing roller. The developing roller and the supply roller are respectively provided with a drive gear, while the developing cartridge itself is provided with an input gear to which driving force from a motor of the image forming device is transmitted via a coupling member. The input gear is meshingly engaged with each of the drive gears. With this configuration, the driving force from the motor is transmitted to both drive gears simultaneously via the coupling member and the input gear, thereby rotating the supply roller and the developing roller. 
       SUMMARY 
       [0004]    There is a recent demand that the developing cartridge be made smaller. Simply making each component of the developing cartridge compact inevitably leads to smaller gears to be provided in the developing cartridge. 
         [0005]    However, as a consequence of each gear becoming smaller in size, each bearing that rotatably supports the corresponding gear also has to become smaller. Hence, such a smaller bearing cannot withstand driving force transmitted from the image forming device. Especially, the input gear is required to have an improved strength to stably transmit the driving force. 
         [0006]    In view of the foregoing, it is an object of the present invention to provide a compact-sized process cartridge and a developing cartridge capable of stably transmitting driving force from an image forming device to a developing roller and a supply roller. 
         [0007]    In order to attain the above and other objects, there is provided a cartridge that is detachably mountable on a main casing, the main casing being provided with a coupling member providing a driving force, the cartridge comprising: a first roller extending in a first direction and configured to rotate about a first rotational axis; a driving force receiving portion configured to receive the driving force from the coupling member to drive the first roller and rotate about a second rotational axis, the driving force receiving portion comprising a contact portion configured to be in contact with the coupling member; and a first gear comprising a gear tooth, the gear being configured to transmit the driving force received at the driving force receiving portion to the first roller. The contact portion is overlapped with the gear tooth when projected in the first direction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    In the drawings: 
           [0009]      FIG. 1  is a cross-sectional view of a printer, with a developing cartridge mounted therein, according to a first embodiment of the present invention; 
           [0010]      FIG. 2A  is a left-side view of the developing cartridge according to a first embodiment; 
           [0011]      FIG. 2B  is a left-side view of the developing cartridge shown in  FIG. 2A  with a gear cover thereof taken off; 
           [0012]      FIG. 2C  is a left-side view showing the developing cartridge of  FIG. 2B  with some gears removed therefrom; 
           [0013]      FIG. 3  is an enlarged view illustrating gears (an input gear, a supply roller gear, a first idle gear and a developing roller gear) and surroundings thereof provided in the developing cartridge shown in  FIG. 2B ; 
           [0014]      FIG. 4  is a left-side view conceptually illustrating a state where the input gear is meshingly engaged with the supply roller gear; 
           [0015]      FIG. 5  is a perspective view of the input gear when viewed from downward left; 
           [0016]      FIG. 6  is a perspective view of the input gear when viewed from frontward left; 
           [0017]      FIG. 7  is a left-side view of the input gear; 
           [0018]      FIG. 8  is a cross-sectional view of the input gear taken along a line VIII-VIII shown in  FIG. 7 ; 
           [0019]      FIG. 9  is a plan view illustrating a state where the input gear is connected to a coupling member provided in a main casing of the printer; 
           [0020]      FIG. 10  is a cross-sectional view of the input gear taken along a line X-X shown in  FIG. 9  in which the input gear is coupled to the coupling member; and 
           [0021]      FIG. 11  is a cross-sectional view of the developing cartridge taken along a line XI-XI shown in  FIG. 2A . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    A color printer  1  according to a first embodiment of the present invention will first be described with reference to  FIGS. 1 through 11 . In the following description, orientations will be referred to based on arrows shown in respective drawings. 
         [0023]    Also note that a left-to-right direction is identical to a widthwise direction. 
         [0024]    1. Printer 
         [0025]    As shown in  FIG. 1 , the printer  1  includes a main casing  2  within which four process cartridges  13 , a sheet cassette  7  that accommodates sheets P, a sheet feeding unit  8 , a conveyor belt  9 , four transfer rollers  10  and a fixing unit  11  are provided. A discharge tray  12  is formed on an upper surface of the main casing  2 . 
         [0026]    The four process cartridges  13  are detachably mounted in the main casing  2  and juxtaposed in a front-to-rear direction. The four process cartridges  13  respectively correspond to four colors of black, cyan, magenta and yellow. In accordance with four colors, the four process cartridges  13  will be referred to as process cartridges  13 K,  13  C,  13 M, and  13 Y respectively. 
         [0027]    Each process cartridge  13  includes a process casing  14  within which a photosensitive drum  3 , a Scorotron charger  4 , an LED unit  5 , a developing roller  6 , a supply roller  15  and a toner hopper  16  are provided. Each of the Scorotron charger  4 , the LED unit  5  and the developing roller  6  is disposed in opposition to the photosensitive drum  3 . Just like the process cartridges  13 , the photosensitive drums  3 , which are also juxtaposed in the front-to-rear direction, will be referred to individually as a photosensitive drums  3 K,  3 C,  3 M and  3 Y in accordance with four colors of toner images formed on thereon. 
         [0028]    In each process cartridge  13 , the photosensitive drum  3 , the developing roller  6  and the supply roller  15  are rotatably supported to the process casing  14 . Each of the photosensitive drum  3 , the developing roller  6  and the supply roller  15  has a rotational shaft extending in a widthwise direction. Toner accommodated in the toner hopper  16  is supplied to the developing roller  6  by the supply roller  15  and carried on the surface of the developing roller  6 . 
         [0029]    Each of the four transfer rollers  10  is disposed at a position opposing to each of the photosensitive drums  3  via the conveyor belt  9 . That is, the conveyor belt  9  is disposed between each photosensitive drum  3  and each transfer roller  10  corresponding thereto. 
         [0030]    Each surface of the photosensitive drums  3  is uniformly charged by corresponding Scorotron charger  4 , and then exposed to light by LEDs (not shown) of the corresponding LED unit  5 . In this way, an electrostatic latent image is formed on each surface of the photosensitive drums  3  according to image data. Subsequently, toner carried on respective developing rollers  6  is supplied to each electrostatic latent image, thereby forming a visible toner image on the surfaces of the respective photosensitive drums  3 . 
         [0031]    The sheet P is conveyed from the sheet cassette  7  to the conveyor belt  9  via the sheet feeding unit  8  including a plurality of rollers. In the process, the sheet P is guided in a direction first frontward but then rearward. The toner image formed on each photosensitive drum  3  is sequentially superimposed onto the sheet P with transfer bias applied to each of the transfer rollers  10  while the sheet P is conveyed on the conveyor belt  9 . The sheet P is then conveyed to the fixing unit  11  whereby the toner image transferred on the sheet P is thermally fixed thereon. The sheet P is then conveyed while turning the direction thereof from rearward to frontward via a variety of rollers, and finally discharged onto the discharge tray  12 . 
         [0032]    2. Developing Cartridge 
         [0033]    The developing roller  6 , the supply roller  15  and the toner hopper  16  constitute a developing cartridge  17  as a unit. The developing cartridge  17  is detachably mounted on the process casing  14  as a photosensitive cartridge. 
         [0034]    Hereinafter a detailed configuration of the developing cartridge  17  will be described with reference to  FIGS. 1 to 11 . 
         [0035]    As shown in  FIG. 1 , the developing cartridge  17  includes a developing casing  30  as a frame main body. The developing casing  30  is formed in a box shape elongated with respect to the widthwise direction. When the developing cartridge  17  is mounted on the process casing  14 , the developing casing  30  slopes diagonally upward and forward in a right side view. 
         [0036]    A partitioning wall  31  is provided on the middle portion of the developing casing  30  with respect to the vertical direction. The partitioning wall  31  extends in the widthwise direction for partitioning the interior of the developing casing  30  into a first chamber  32  and a second chamber  33 . The first chamber  32  is located above the second chamber  33 . A through-hole  34  is formed in the partitioning wall  31  to allow communication between the first chamber  32  and the second chamber  33 . 
         [0037]    The first chamber  32  corresponds to the interior of the toner hopper  16  and accommodates toner therein. An agitator  35  is provided within the first chamber  32  for agitating the toner within the first chamber  32 . The agitator  35  includes a rotational shaft  36  extending in the widthwise direction and a blade  37  provided on the rotational shaft  36 . As the blade  37  pivotally moves about the rotational shaft  36 , the toner accommodated in the first chamber  32  is agitated, thereby discharging the toner to the second chamber  33  through the through-hole  34 . 
         [0038]    The second chamber  33  accommodates the developing roller  6  and the supply roller  15 . The supply roller  15  is disposed below and adjacent to the through-hole  34 . The developing roller  6  is disposed rearward (more precisely, diagonally rearward and downward) of the supply roller  15 . The developing roller  6  is in pressure contact with the supply roller  15  at a position diagonally upward and forward of the developing roller  6 . A nip  39  is formed between the developing roller  6  and the supply roller  15 . The developing roller  6  is in contact with the photosensitive drum  3  at a position diagonally downward and rearward of the developing roller  6  while the contact position is exposed from the developing casing  30 . The toner discharged out of the first chamber  32  via the through-hole  34  is supplied from the supply roller  15  to the developing roller  6  via the nip  39 , thereby visualizing the electrostatic latent image formed on the photosensitive drum  3 . 
         [0039]    As shown in  FIG. 2A , the developing casing  30  has a left side surface which is covered by a gear cover  47 . A window  38  is provided on the left side surface of the developing casing  30  at a position corresponding to the first chamber  32 . On the gear cover  47  as well, a hole is formed at a position corresponding to the window  38 . The window  38  enables a user to detect how much toner is left in the first chamber  32 . 
         [0040]    Under the gear cover  47 , as shown in  FIG. 2B , an input gear  40 , a supply roller gear  41  (as a transmission gear), a first idle gear  42 , a developing roller gear  43 , a second idle gear  44 , an agitator gear  45  and a detection gear  46  (as intermediary gears) are provided. Each of these gears is formed in a columnar shape having a rotational axis extending in the widthwise direction. Note that in  FIGS. 2A to 2C , the developing cartridge  17  (the developing casing  30 ) is shown upright for the sake of explanatory purpose. 
         [0041]    The input gear  40  is disposed at a position substantially center of the developing casing  30  with respect to the vertical direction. When the input gear  40  is projected onto the developing cartridge  17  in the widthwise direction, the input gear  40  is arranged to overlap with the partitioning wall  31  as shown in a dotted circle in  FIG. 1 . That is, the input gear  40  is linearly aligned with the portioning wall  31  in the widthwise direction. An input gear shaft  49  is provided on the left side surface of the developing casing  30  at a position coinciding with a center of the input gear  40 . The input gear shaft  49  is a cylindrical boss protruding leftward from the left side surface of the developing casing  30 . The input gear shaft  49  penetrates the center of the input gear  40 , thereby rotatably supporting the input gear  40 . That is, the input gear  40  can rotate about the input gear shaft  49  (See  FIG. 11 ). 
         [0042]    The input gear  40  has a connecting portion  77  and a gear portion  79  formed adjacent to the connecting portion  77  and a flange portion  78  partitioning the connecting portion  77  and the gear portion  79  (See  FIG. 5 ). The input gear  40  has a left end surface formed with a depressed portion  48 . The depressed portion  48  is exposed leftward from the gear cover  47 , as shown in  FIG. 2A . Detailed configuration of the input gear  40  will be described later. 
         [0043]    The supply roller gear  41  includes a gear main body  41 A formed in a disk shape whose circumferential surface is formed with gear teeth. The supply roller gear  41  is rotatable about a rotational axis that is a center of the gear main body  41 A. The gear main body  41 A has an outer surface  41 B facing leftward. The supply roller gear  41  is disposed downward (more precisely, diagonally downward and forward) of the input gear  40 . The supply roller gear  41  has a front end portion exposed from the gear cover  47  but the supply roller gear  41  is almost covered with the gear cover  47  from leftward except the front end portion, as shown in  FIG. 2A . The gear teeth of the supply roller gear  41  are meshingly engaged with the gear portion  79  of the input gear  40  at a position upward of the supply roller gear  41  (i.e., downward of the input gear  40 ). 
         [0044]    The left end of the rotational shaft of the supply roller  15  protrudes from the left side surface of the developing casing  30  at a position coinciding with the center of the supply roller gear  41 , as shown in  FIG. 2C . The rotational shaft of the supply roller  15  penetrates the center of the supply roller gear  41 , thereby supporting the supply roller gear  41 . The left end of the rotational shaft of the supply roller  15  has a substantially D-shaped cross section, while the center of the supply roller gear  41  through which the left end of the rotational shaft of the supply roller  15  penetrates is formed with a substantially D-shaped through-hole. In this way, the supply roller gear  41  and the supply roller  15  integrally rotate about the axis of the rotational shaft of the supply roller  15 . 
         [0045]    The first idle gear  42  has gear teeth on the circumferential surface thereof. The first idle gear  42  is disposed rearward of the supply roller gear  41 . The first idle gear  42  exposes a bottom end portion thereof from the gear cover  47 , but the first idle gear  42  as a whole is almost covered with the gear cover  47  as shown in  FIG. 2A . The gear teeth of the first idle gear  42  are meshingly engaged with the gear teeth of the supply roller gear  41  at a position forward of the first idle gear  42  (i.e., rearward of the supply roller gear  41 ). 
         [0046]    A first idle gear shaft  50  protrudes from the left side surface of the developing casing  30  at a position coinciding with a center of the first idle gear  42 , as shown in  FIG. 2C . The first idle gear shaft  50  is a cylindrical boss protruding leftward from the left side surface of the developing casing  30 . The first idle gear shaft  50  penetrates the center of the first idle gear  42  and thus rotatably supports the first idle gear  42 . That is, the first idle gear  42  can rotate about the first idle gear shaft  50 . 
         [0047]    As to the supply roller  15 , a bearing  55  is provided on the left side surface of the developing casing  30 , as shown in  FIG. 2C . The bearing  55  is formed in a substantially rectangular plate shape when viewed from leftward. The bearing  55  is formed with a through-hole  56  at a position substantially center of the bearing  55 . The left end of the rotational shaft of the supply roller  15  penetrates the through-hole  56 . In this way, the bearing  55  rotatably supports the supply roller  15  about the axis of the rotational shaft of the supply roller  15 . 
         [0048]    A claw  57  is formed on the left side surface of the developing casing  30 . As show in  FIG. 2C , the claw  57  engages the bearing  55  so that the bearing  55  can be fixed to the left side surface of the developing casing  30 . The bearing  55  is further formed with two recesses  58 . One of the recesses  58  is formed at a corner of the bearing  55  located diagonally upward of the through-hole  56 , whereby the input gear shaft  49  is in contact with the bearing  55  at a deepest position in the recess  58 . Another recess  58  is formed at a corner of the bearing  55  located rearward of the through-hole  56 , whereby the first idle gear shaft  50  is in contact with the bearing  55  at a deepest position in this recess  58 . Hereinafter, whenever necessary, the input gear shaft  49  and the first idle gear shaft  50  are collectively referred to as a contingence portion  59 . 
         [0049]    The developing roller gear  43  has gear teeth on the circumferential surface thereof. The developing roller gear  43  is disposed downward of the first idle gear  42  and is exposed leftward from the gear cover  47  at a bottom portion thereof, as shown in  FIG. 2A . As shown in  FIG. 2B , the gear teeth of the developing roller gear  43  are meshingly engaged with the gear teeth of the first idle gear  42  at a position upward of the developing roller gear  43  (i.e., downward of the first idle gear  42 ). 
         [0050]    The left end of the rotational shaft of the developing roller  6  is exposed from the left side surface of the developing casing  30  at a position coinciding with a center of the developing roller gear  43 . The left end of the rotational shaft of the developing roller  6  penetrates the center of the developing roller gear  43  for supporting the developing roller gear  43 . Note that, the developing roller gear  43  is configured not to rotate relative to the left end of the rotational shaft of the developing roller  6 . In other words, the developing roller  6  and the developing roller gear  43  are configured to able to rotate in conjunction with each other about the axis of the rotational shaft of the developing roller  6 . 
         [0051]    The second idle gear  44  has a right portion (closer to the left side surface of the developing casing  30 ) and a left portion with respect to the widthwise direction. The right portion has a diameter smaller than that of the left portion, but each portion is formed with gear teeth on the circumferential surface thereof. The second idle gear  44  is disposed diagonally rearward and upward of the input gear  40  and is covered with the gear cover  47  from leftward, as shown in  FIG. 2A . The gear teeth of the second idle gear  44  are meshingly engaged with the gear portion  79  of the input gear  40  at a position forward of the second idle gear  44  (i.e., rearward of the input gear  40 ). 
         [0052]    A second idle gear shaft  51  is provided on the left side surface of the developing casing  30  at a position coinciding with a center of the second idle gear  44 , as shown in  FIG. 2C . The second idle gear shaft  51  is a cylindrical boss protruding leftward from the left side surface of the developing casing  30 . The second idle gear shaft  51  penetrates the center of the second idle gear  44 , thereby rotatably supporting the second idle gear  44 . In this way, the second idle gear  44  can be made to rotate about the second idle gear shaft  51 . 
         [0053]    The agitator gear  45  is formed with gear teeth on the circumferential surface thereof. The agitator gear  45  is disposed diagonally upward and forward of the second idle gear  44  and covered with the gear cover  47  from leftward, as shown in  FIG. 2A . The gear teeth of the agitator gear  45  are meshingly engaged with the gear teeth of the second idle gear  44  at a position downward of the agitator gear  45  (i.e., upward of the second idle gear  44 ). 
         [0054]    The left end of the rotational shaft  36  of the agitator  35  is exposed from the left side surface of the developing casing  30  at a position coinciding with a center of the agitator gear  45  (see  FIG. 2C ). The left end of the rotational shaft  36  of the agitator  35  penetrates the center of the agitator gear  45 , thereby supporting the agitator gear  45 . The left end of the rotational shaft  36  of the agitator  35  has a substantially D-shaped cross section, while the center of the agitator gear  45  is also formed with a substantially D-shaped through-hole. In this way, the agitator gear  45  and the rotational shaft  36  of the agitator  35  integrally rotate about the axis of the rotational shaft  36 . Note that the left end of the rotational shaft  36  of the agitator  35  is exposed leftward from the gear cover  47 , as shown in  FIG. 2A . 
         [0055]    The detection gear  46  is partially formed with gear teeth on the right circumferential surface thereof. The detection gear  46  is disposed upward of the agitator gear  45 . A detection gear shaft  52  is provided on the left side surface of the developing casing  30  at a position coinciding with a center of the detection gear  46 , as shown in  FIG. 2C . The detection gear shaft  52  is a cylindrical boss protruding leftward from the left side surface of the developing casing  30 . The detection gear shaft  52  penetrates the center of the detection gear  46 , thereby supporting the detection gear  46 . That is, the detection gear  46  is rotatably supported to the left side surface of the developing casing  30  about the detection gear shaft  52 . 
         [0056]    When the developing cartridge  17  is mounted on the process casing  14  for the first time, the partial gear teeth of the detection gear  46  are meshingly engaged with the gear teeth of the agitator gear  45  at a position upward of the agitator gear  45 . The left end surface of the detection gear  46  is provided with a plurality of protrusions  53  protruding leftward, as shown in  FIG. 2B . The protrusions  53  are arranged along the periphery of the left end surface of the detection gear  46 . The locations of the developing cartridge  17  are corresponds to information on the developing cartridge  17 . More specifically, the locations of the protrusions  53  corresponds to information indicating whether or not the developing cartridge  17  is new and indicating how many more pages can be printed with the developing cartridge  17 . 
         [0057]    As also shown in  FIG. 2A , the gear cover  47  is formed with an opening  54  for exposing the protrusions  53 . When the detection gear  46  rotates, the protrusions  53  are exposed leftward from the gear cover  47  through the opening  54 . 
         [0058]    When the developing cartridge  17  is mounted in the process casing  14  as shown in  FIG. 1 , the depressed portion  48  of the input gear  40  is coupled to a coupling member  90  provided on the main casing  2  (to be described later with reference to  FIG. 9 ). 
         [0059]    The coupling member  90  is connected to an output shaft of a motor (not shown) disposed within the main casing  2 . Hence, when the motor is driven and thus the coupling member  90  starts rotating, driving force from the motor is transmitted from the coupling member  90  (i.e., outside of the developing cartridge  17 ) to the input gear  40  via the depressed portion  48  coupled to the coupling member  90 . Upon receipt of the driving force, the input gear  40  starts rotating in a direction indicated by a dotted arrow A (i.e., in a clockwise direction) in  FIGS. 2B and 3 . 
         [0060]    The driving force received at the input gear  40  is then transmitted to the supply roller gear  41  and the second idle gear  44  each of which are in engagement with the input gear  40  meshingly. Accordingly, the supply roller gear  41  starts rotating in a direction indicated by a dotted arrow B (i.e., in a counterclockwise direction) in  FIGS. 2B and 3 . In accordance with the rotation of the supply roller gear  41 , the supply roller  15  is made to rotate in the direction B the same as the supply roller gear  41 . In other words, the supply roller gear  41  drives the supply roller  15  to rotate. 
         [0061]    At this time, each gear surface of the input gear  40  presses each gear surface of the supply roller gear  41  at the position where the input gear  40  and the supply roller gear  41  are meshingly engaged with each other. This pressing force of the gear surfaces of the input gear  40  against the gear surfaces of the supply roller gear  41  will be illustrated in a heavy arrow X in  FIGS. 2B and 3 . The pressing force X works in a direction substantially parallel to the direction B as well as rearward at the engaging position of the input gear  40  and the supply roller gear  41 . 
         [0062]    The second idle gear  44 , on the other hand, is also made to rotate in a direction indicated by a dotted arrow C (i.e., in the counterclockwise direction) in  FIG. 2B , in accordance with the rotation of the input gear  40  in the direction A. 
         [0063]    As the supply roller gear  41  rotates, the driving force is further transmitted to the first idle gear  42  which is meshingly engaged with the supply roller gear  41 . Accordingly, the first idle gear  42  starts rotating in a direction indicated by a dotted arrow D (i.e., in the clockwise direction) in  FIGS. 2B and 3 . At this time, each gear surface of the supply roller gear  41  presses each gear surface of the first idle gear  42  at the position where the supply roller gear  41  and the first idle gear  42  are meshingly engaged with each other. At this engaging position, reaction force of the first idle gear  42  acts against the pressing force of the supply roller gear  41 . In other words, the gear surfaces of the first idle gear  42  presses the gear surfaces of the supply roller gear  41  at this engaged position. This pressing force of the first idle gear  42  is illustrated in a heavy arrow Y in  FIGS. 2A and 3 . The pressing force Y works in a direction substantially opposite to the direction D as well as upward at the engaged position of the first idle gear  42  and the supply roller gear  41 . 
         [0064]    As a result of combination of the pressing force X and the pressing force Y, resultant force Z is generated and acts in a direction diagonally upward and rearward between the input gear shaft  49  and the first idle gear shaft  50 , which is shown by a heavy arrow Z in  FIGS. 2B ,  2 C and  3 . This resultant force Z acts on the supply roller gear  41 , the left end of the rotational shaft of the supply roller  15  that supports the supply roller gear  41 , and the bearing  55  that supports the left end of the rotational shaft of the supply roller  15 . The contingence portion  59  (the input gear shaft  49  and the first idle gear shaft  50 ) contacts the bearing  55  from downstream in the working direction of the resultant force Z within the corresponding recesses  58 , as shown in  FIG. 2C . 
         [0065]    Since the first idle gear  42  is meshingly engaged with each of the supply roller gear  41  and the developing roller gear  43 , the driving force from the supply roller gear  41  is transmitted to the developing roller gear  43  via the first idle gear  42 . Hence, the developing roller gear  43  is made to rotate in a direction indicated by a dotted arrow E (i.e., counterclockwise) in  FIGS. 2B and 3 . The developing roller  6  is therefore to rotate in conjunction with the rotation of the developing roller gear  43  in the direction E. That is, the developing roller gear  43  drives the developing roller  6  to rotate. 
         [0066]    In accordance with the rotation of the second idle gear  44  upon receipt of the driving force from the input gear  40 , the agitator gear  45 , which is in engagement with the second idle gear  44  meshingly, is made to rotate in response to the driving force transmitted thereto from the second idle gear  44 . The agitator gear  45  rotates in a direction indicated by a dotted arrow F (i.e., clockwise) shown in  FIG. 2B . As a result, the agitator  35  is to rotate in conjunction with the rotation of the agitator gear  45  in the direction F. 
         [0067]    In accordance with the rotation of the agitator gear  45 , the detection gear  46  is then made to rotate upon receipt of the driving force transmitted from the agitator gear  45 . The detection gear  46  rotates in a direction indicated by a dotted arrow G (i.e., counterclockwise) shown in  FIG. 2B . 
         [0068]    3. Configuration of Input Gear 
         [0069]    Next, a configuration of the input gear  40  will be described in more details with reference to  FIGS. 4 through 11 . 
         [0070]    As shown in  FIG. 5 , the input gear  40  includes the connecting portion  77  and the gear portion  79  arranged adjacent to the connecting portion  77  along the rotational axis of the input gear  40  (i.e., widthwise direction of the printer  1 ). The input gear  40  also includes the flange portion  78  that partitions the connecting portion  77  and the gear portion  79 . The connecting portion  77  has a dimension (diameter) greater than that of the gear portion  79  in a direction perpendicular to the rotational axis of the input gear  40 . The input gear  40  is formed of a resin (more precisely, a polyacetal resin). 
         [0071]    The connecting portion  77  is formed in a cylindrical shape whose center corresponds to the rotational axis of the input gear  40 . The connecting portion  77  includes an outer circumferential wall  80  and a pair of engaging sections  81  protruding inward from the outer circumferential wall  80 , as shown in  FIGS. 6 and 7 . The outer circumferential wall  80  has a cylindrical shape and includes a cylindrical-shaped inner surface whose center is the rotational axis of the input gear  40 . That is, the outer circumferential wall  80  extends along the whole circumference of the input gear  40 . The outer circumferential wall  80  is supported by an inner surface of a protrusion  100  of the gear cover  47  (described later) when the input gear  40  rotates. 
         [0072]    The engaging sections  81  are symmetrically positioned with respect to the rotational axis of the input gear  40 , as shown in  FIGS. 6 and 7 . The engaging sections  81  serves as a contact portion. Each engaging section  81  includes a first wall  82 , an inner circumferential wall  83  and a second wall  84 . The first wall  82  is formed in a linear shape extending from the outer circumferential wall  80  toward the rotational axis of the input gear  40 . The inner circumferential wall  83  is formed such that the inner circumferential wall  83  extends, from an inner end of the first wall  82 , in a circumferential direction of the input gear  4  about the rotational axis thereof. That is, the inner circumferential wall  83  is concentrically with the cylindrical-shaped inner surface of the outer circumferential wall  80 . The second wall  84  is formed in a linear shape extending from another end of the inner circumferential wall  83  toward the outer circumferential wall  80 . The second wall  84  is to be in contact with the coupling member  90  provided in the main casing  2  as will be described later (See  FIG. 10 ). The first wall  82 , the inner circumferential wall  83 , the second wall  84  and the outer circumferential wall  80  are formed integrally. The outer circumferential wall  80  serves to reinforce the engaging section  81  (more specifically, the second walls  84 ) that contacts the coupling member  90  as the contact portion (a point of action). 
         [0073]    The connecting portion  77  is further formed with a first bottom wall  87 , a platform  85  and a projection  86  as also shown in  FIGS. 6 through 8 . The first bottom wall  87  is formed as a portion of the flange portion  78  inside the input gear  40 . The platform  85  is formed in a cylindrical shape having the rotational axis of the input gear  40  as a center thereof, protruding outward from the first bottom wall  87  in the axial direction of the input gear  40 . The projection  86  is formed in a domical shape and disposed at the center of the platform  85 . The projection  86  is to contact the coupling member  90  of the main casing  2  in the axial direction of the input gear  40  when the coupling member  90  is inserted into the connecting portion  77 , thereby serving to position the input gear  40  relative to the coupling member  90  with respect to the axial direction of the input gear  40 . The outer circumferential wall  80 , the first walls  82 , the inner circumferential walls  83 , the second walls  84 , the first bottom wall  87 , the platform  85  and the projection  86  constitute the depressed portion  48  exposed leftward from the gear cover  47  in the widthwise direction. 
         [0074]    A gap  110  facing outward is formed within each engaging section  81 . That is, the gap  110  is bounded on the periphery by the first wall  82 , the inner circumferential wall  83 , the second wall  84  and the outer circumferential wall  80 , as shown in  FIG. 6 . Within the gap  110 , a second bottom wall  88  is formed inside the input gear  40  in the axial direction of the input gear  40  as a portion of the flange portion  78 , just like the first bottom wall  87 . In other words, the gap  110  is closed with the second bottom wall  88  at a side adjacent to the flange portion  78  with respect to the axial direction of the input gear  40 , as shown in  FIG. 8 . When the connecting portion  77  is projected toward the gear portion  79 , a pitch circle  72  of the gear portion  79  (described later) comes to a position overlapping with the second bottom wall  88 . Details of the pitch circle  72  will be described later. 
         [0075]    The input gear  40  is formed of a resin, as stated earlier. Hence, the first wall  82 , the inner circumferential wall  83 , the second wall  84  and the outer circumferential wall  80  are required to be formed in a thickness as uniform as possible. To this effect, when molding the input gear  40 , the first wall  82 , the inner circumferential wall  83 , the second wall  84  and the outer circumferential wall  80  can be made in the uniform thickness by inserting a die into the input gear  40  at a position corresponding to their center in the axial direction of the input gear  40 . Since the gear portion  79  is provided at a position coinciding with the second bottom wall  88  in the axial direction of the input gear  40 , the die cannot be removed from the gear portion  79  side. The gap  110  facing outward along the axial direction of the input gear  40  therefore serves to release the die from the input gear  40  from a side opposite to the gear portion  79  side. 
         [0076]    The gear portion  79  is formed integrally with the connecting portion  77  via the flange portion  78 . The gear portion  79  has a diameter smaller than that of the connecting portion  77 , as shown in  FIGS. 5 and 8 . The gear portion  79  includes a supported wall  89  of a cylindrical shape. The supported wall  89  is formed with gear teeth on the outer circumferential surface thereof. The supported wall  89  has an inner surface formed in a stepped manner. 
         [0077]    The flange portion  78  protrudes outward from an end of the connecting portion  77  located at the gear portion  79  side in a direction perpendicular to the axial direction of the input gear  40 , as shown in  FIG. 8 . The flange portion  78  is in contact with the gear cover  47  so as to position the input gear  40  with respect to the axial direction thereof, as will be described later. 
         [0078]    As shown in  FIG. 4 , the gear portion  79  of the input gear  40  includes an addendum circle  71 , the pitch circle  72 , and a dedendum circle  73 . The addendum circle  71  is an imaginary circle formed by connecting tops of each of the gear teeth constituting the gear portion  79 . The pitch circle  72  is an imaginary circle formed by connecting pitch points where each gear tooth of the gear portion  79  and each gear tooth of the supply roller gear  41  are in contact with each other. The dedendum circle  73  is an imaginary circle formed by connecting dedendums of each gear tooth of the gear portion  79 . 
         [0079]    Likewise, the supply roller gear  41  which is meshingly engaged with the input gear  40  includes an addendum circle  74 , a pitch circle  75 , and a dedendum circle  76 , as also shown in  FIG. 4 . The addendum circle  74  is an imaginary circle formed by connecting tops of each gear tooth of the supply roller gear  41 . The pitch circle  75  is an imaginary circle formed by connecting pitch points where each gear tooth of the supply roller gear  41  and each gear tooth of the gear portion  79  are in contact with each other. The dedendum circle  76  is an imaginary circle formed by connecting roots of each gear tooth of the supply roller gear  41 . 
         [0080]    The input gear  40  is formed such that, when the connecting portion  77  is projected onto the gear portion  79  in the axial direction of the input gear  40 , each engaging section  81  is located at a position overlapping with the pitch circle  72 . That is, the engaging sections  81  is linearly aligned with the pitch circle  72  in the axial direction of the input gear  40 . More specifically, the second wall  84  of the engaging section  81 , which contacts the coupling member  90  of the main casing  2 , is located on the pitch circle  72  of the gear portion  79  in a plane to which the connecting portion  77  is projected. With this configuration, the engaging section  81  can be located at least on the pitch circle  72  in the projected plane, thereby stably transmitting the driving force to the gear portion  79  while making the gear portion  79  compact. 
         [0081]    When the developing roller  6  is made to rotate, the coupling member  90  of the main casing  2  is inserted into the depressed portion  48  of the input gear  40 , as shown in  FIGS. 9 and 10 . The coupling member  90  is retractably provided on the main casing  2  with respect to the axial direction of the input gear  40 . Upon receipt of the driving force from the motor (not shown) disposed within the main casing  2 , the coupling member  90  rotates in a clockwise direction in  FIG. 10 . At this time, the coupling member  90  contacts each of the second wall  84  of the input gear  40 . This configuration prevents the input gear  40  from being distorted under strain, thereby further contributing to stable transmission of the driving force. 
         [0082]    More specifically, the coupling member  90  has a tip portion on which a shaft  93  and a pair of protruding portions  92  are formed. The coupling member  90  rotates about the shaft  93  and the protruding portions  92  protrude from the shaft  93  in directions radially opposite to each other. The protruding portions  92  contact the second walls  84  respectively, thereby rotating the input gear  40  in the direction indicated by the dotted arrow A in  FIG. 10 . In accordance with the rotation of the input gear  40  in the direction A, the developing roller gear  43  is made to rotate via the supply roller gear  41  and the first idle gear  42 , while the agitator gear  45  is made to rotate via the second idle gear  44 . 
         [0083]    As shown in  FIG. 11 , the left end of developing casing  30  is provided with the second idle gear shaft  51 , the input gear shaft  49  and the bearing  55 . The second idle gear shaft  51  and the input gear shaft  49  protrude outward from the left end surface of the developing casing  30  in a direction parallel to the rotational shaft of the supply roller  15 . As previously described, the second idle gear shaft  51  rotatably supports the second idle gear  44 . The input gear shaft  49  as a bearing boss is inserted into the supported wall  89  of the input gear  40 , thereby rotatably supporting the input gear  40 . The rotational shaft of the supply roller  15  penetrates the bearing  55 , while the supply roller gear  41  is fixed to the rotational shaft of the supply roller  15 . 
         [0084]    The left end of the input gear shaft  49  protrudes leftward than the outer surface  41 B of the supply roller gear  41 , as shown in  FIG. 11 . As previously stated, the input gear  40  is in meshing engagement with each of the second idle gear  44  and the supply roller gear  41 . With this configuration, the rotational center of the input gear  40  can be accurately positioned at least at the position where the supply roller gear  41  and the input gear  40  are meshingly engaged with each other. The input gear  40 , the second idle gear shaft  51  and the supply roller gear  41  are covered with the gear cover  47  from leftward, i.e., from a side opposite to the developing casing  30 . 
         [0085]    The gear cover  47  is formed with protrusions  100  and  102 . The protrusion  100  protrudes outward (leftward) and the protrusion  102  protrudes inward (rightward) with respect to the direction parallel to the rotational shaft of the supply roller  15 , as shown in  FIG. 11 . The protrusion  102  has a cylindrical shape and rotatably supports the second idle gear  44  from a side opposite to the second idle gear shaft  51 . The protrusion  100  is also formed in a cylindrical shape having a free end which is open toward outward (leftward). The protrusion  100  has an inner surface which is coupled to the outer circumferential wall  80  of the input gear  40 , thereby rotatably supporting the input gear  40  from a side opposite to the input gear shaft  49 . 
         [0086]    In the present embodiment, the developing cartridge  30  is made compact in size. Therefore, the gear portion  79  of the input gear  40  is thinned down, resulting in the input gear shaft  49  being slim. Hence, in accordance with the slimmed-down input gear shaft  49 , the input gear  40  is supported with the protrusion  100  of the gear cover  47  from radially outward. In this way, the input gear  40  is accurately positioned relative to the developing casing  30  in cooperation with the input gear shaft  49  and the protrusion  100  of the gear cover  47 . 
         [0087]    That is, the input gear  40  is supported not only by the input gear shaft  49  but also by the protrusion  100  of the gear cover  47  provided from the side opposite to the developing casing  30 . Hence, even though the input gear  40  is made compact, the input gear shaft  49  can, which is also made smaller in size in accordance with the compact-sized input gear  40 , stably support the input gear  40  in collaboration with the protrusion  100  of the gear cover  47 . 
         [0088]    Further, the connecting portion  77  has a dimension greater than that of the gear portion  79  with respect to the direction orthogonal to the axial direction of the input gear  40  in the present embodiment. Hence, the overall dimension of the input gear  40  can be made smaller because of the smaller gear portion  79 , while at the same time the input gear  40  can stably receive the driving force from the motor of the image forming device. 
         [0089]    Further, the outer circumferential wall  80  is so provided as to extend along the whole circumference of the input gear  40 , thereby stably supporting the input gear  40  in conjunction with the protrusion  100  that contacts the outer circumferential wall  80 . 
         [0090]    While the invention has been described in detail with reference to the specific embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
         [0091]    For example, although the process casing  14  and the developing cartridge  17  are configured to be detachably mounted in the main casing  2  integrally as the process cartridge  13  in the above-described embodiment, the developing cartridge  17  alone, separate from the process casing  14 , may be detachably mounted in the main casing  2  on which the separated process casing  14  has already been mounted. 
         [0092]    Further, while a direct-transfer method is employed in the color printer  1  according to the above embodiment, the present invention may also be applicable to a color or monochrome printer of an intermediate-transfer type. 
         [0093]    Further, instead of exposing surfaces of the photosensitive drums  3  by LEDs, a laser may be used for exposing the same. 
         [0094]    Further, while the outer circumferential wall  80  has a cylindrical shape in the above embodiment, an outer wall may have at least a curved surface whose center of radius of a curvature is coincident with the rotational axis of the input gear  40 .