Patent Publication Number: US-11048203-B2

Title: Developing cartridge including engaging member movable with helical gear and engageable with gear cover

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/229,168, filed Dec. 21, 2018, which claims priority from Japanese Patent Application No. 2018-034778 filed Feb. 28, 2018. The entire contents of the aforementioned applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a developing cartridge including a developing roller and a gear rotatable in accordance with rotation of the developing roller. 
     BACKGROUND 
     Conventionally, there are known developing cartridges each including a developing roller and gears rotatable in accordance with rotation of the developing roller, such as a developing gear, a supply gear, and an agitator gear, for example. 
     SUMMARY 
     In a state where the developing cartridge is attached to an image-forming apparatus, one of the gears rotates in a first rotational direction upon receipt of a driving force from the image-forming apparatus, thereby rotating the developing gear in a prescribed rotational direction to enable the image-forming apparatus to perform image formation. On the other hand, in a case where the gear rotates in a second rotational direction opposite to the first rotational direction, the developing roller is caused to rotate in a direction opposite to the prescribed rotational direction, which may result in leakage of developing agent. 
     In view of the foregoing, it is an object of the present disclosure to provide a structure capable of restricting a gear rotatable in accordance with rotation of a developing roller from rotating in a direction opposite to a prescribed rotational direction. 
     In order to attain the above and other objects, according to one aspect, the present disclosure provides a developing cartridge including a housing, a developing roller, a first helical gear, a second helical gear, a cover and an engaging member. The housing is configured to accommodate toner therein and has an outer surface. The developing roller is rotatable about a first axis extending in an axial direction. The first helical gear is positioned at the outer surface and is rotatable about a second axis extending in the axial direction. The first helical gear is rotatable in accordance with rotation of the developing roller. The second helical gear is positioned at the outer surface and is rotatable in a first rotational direction and a second rotational direction opposite to the first rotational direction about a third axis extending in the axial direction. The second helical gear is movable in the axial direction between a first position and a second position positioned farther away from the outer surface than the first position is from the outer surface. The second helical gear is moved to the first position by a first thrust force generated by meshing engagement between the first helical gear and the second helical gear in a case where second helical gear rotates in the first rotational direction. The second helical gear is moved to the second position by a second thrust force generated by the meshing engagement between the first helical gear and the second helical gear in a case where the second helical gear rotates in the second rotational direction. The cover is positioned at the outer surface and covers at least part of the second helical gear. The engaging member is rotatable about the third axis together with the second helical gear and is movable in the axial direction together with the second helical gear. The second helical gear is rotatable in the first rotational direction in accordance with the rotation of the first helical gear in a case where the second helical gear is at the first position. The engaging member is engaged with a part of the cover to terminate the rotation of the second helical gear in the second rotational direction in a case where the second helical gear is at the second position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a schematic view illustrating an internal structure of a printer that can accommodate a developing cartridge according to an embodiment of the present disclosure; 
         FIG. 2  is a vertical cross-sectional view of a process cartridge including the developing cartridge according to the embodiment; 
         FIG. 3  is an exploded perspective view illustrating components constituting one end portion of the developing cartridge according to the embodiment in a first direction; 
         FIG. 4A  is a perspective view of an idle gear of the developing cartridge according to the embodiment as viewed from a point outward thereof in the first direction; 
         FIG. 4B  is a perspective view of the idle gear of the developing cartridge according to the embodiment as viewed from a point inward thereof in the first direction; 
         FIG. 5  is perspective view illustrating an inner structure of a first gear cover of the developing cartridge according to the embodiment; 
         FIG. 6  is a view illustrating gears and the first gear cover of the developing cartridge according to the embodiment as viewed from a point outward thereof in a second direction in a state where the idle gear is at a first position; and 
         FIG. 7  is a view illustrating the gears and the first gear cover of the developing cartridge according to the embodiment as viewed from a point outward thereof in the second direction in a state where the idle gear is at a second position. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, one embodiment of the disclosure will be described in detail while referring to accompanying drawings. 
     As illustrated in  FIG. 1 , a laser printer  1  of the embodiment mainly includes a main casing  2 , a sheet feeding portion  3 , an image forming portion  4 , and a controller CU. The laser printer  1  is an image-forming apparatus configured to form images onto sheets S. 
     The main casing  2  includes a front cover  2 A and a discharge tray  2 B. The discharge tray  2 B is positioned at an upper portion of the main casing  2 . The sheet feeding portion  3  and the image forming portion  4  are disposed within the main casing  2 . A developing cartridge  10  according to the embodiment can be attached to and removed from the main casing  2  while the front cover  2 A is open. 
     The sheet feeding portion  3  is configured to accommodate the sheets S therein. The sheet feeding portion  3  is configured to feed the sheets S one by one to the image forming portion  4 . 
     The image forming portion  4  includes a process cartridge  4 A, an exposure device (not illustrated), a transfer roller  4 B, and a fixing device  4 C. 
     As illustrated in  FIGS. 1 and 2 , the process cartridge  4 A includes a drum cartridge  5  and the developing cartridge  10 . The developing cartridge  10  can be attached to the drum cartridge  5 . More specifically, the developing cartridge  10  can be attached to and removed from the drum cartridge  5 . With the developing cartridge  10  attached to the drum cartridge  5 , the developing cartridge  10  and the drum cartridge  5 , as the process cartridge  4 A, can be attached to and removed from the main casing  2  of the laser printer  1 . The drum cartridge  5  includes a frame  5 A, and a photosensitive drum  5 B rotatably supported by the frame  5 A. 
     As illustrated in  FIG. 2 , the developing cartridge  10  includes a housing  11 , a developing roller  12 , a supply roller  13 , and an agitator  14 . 
     The housing  11  includes a container  11 A and a cover  11 B. The container  11 A of the housing  11  can accommodate toner T therein. 
     The developing roller  12  includes a developing-roller shaft  12 A and a roller body  12 B. The developing-roller shaft  12 A and the roller body  12 B extend in a first direction. The first direction denotes an axial direction of the developing roller  12  and will be referred to simply as the axial direction, hereinafter, wherever necessary. The developing-roller shaft  12 A defines a first axis  12 X extending in the axial direction. The developing-roller shaft  12 A is made of metal, for example. The roller body  12 B is provided over an outer peripheral surface of the developing-roller shaft  12 A. The roller body  12 B is made of an electrically conductive rubber, for example. 
     The developing roller  12  is rotatable about the first axis  12 X of the developing-roller shaft  12 A extending in the axial direction. The developing roller  12  is rotatably supported by the housing  11  so as to be rotatable about the first axis  12 X of the developing-roller shaft  12 A. That is, the roller body  12 B is rotatable together with the developing-roller shaft  12 A. The developing roller  12  is applied with a developing bias from the controller CU. 
     The container  11 A and the cover  11 B of the housing  11  face each other in a second direction. The second direction crosses the first direction. Preferably, the second direction is orthogonal to the first direction. The developing roller  12  is positioned at one end portion of the housing  11  in a third direction. The third direction crosses the first direction and the second direction. Preferably, the third direction is orthogonal to the first direction and the second direction. 
     The supply roller  13  includes a supply-roller shaft  13 A and a roller body  13 B. The supply-roller shaft  13 A and the roller body  13 B extend in the first direction. The supply-roller shaft  13 A defines an axis  13 X extending in the axial direction. The supply-roller shaft  13 A is made of metal, for example. The roller body  13 B is provided over an outer peripheral surface of the supply-roller shaft  13 A. The roller body  13 B is made of a sponge material, for example. The supply roller  13  is rotatable about the axis  13 X of the supply-roller shaft  13 A extending in the axial direction. The roller body  13 B is rotatable together with the supply-roller shaft  13 A. 
     The agitator  14  includes an agitator shaft  14 A and a flexible sheet  14 B. The agitator shaft  14 A defines an axis  14 X extending in the axial direction. The agitator shaft  14 A is rotatable about the axis  14 X. The agitator shaft  14 A is rotatably supported by the housing  11  so as to be rotatable about the axis  14 X. The agitator  14  is rotatable in accordance with rotation of a coupling  22  described later. The flexible sheet  14 B has a base end fixed to the agitator shaft  14 A. The flexible sheet  14 B has a tip end configured to contact an inner surface of the housing  11 . In accordance with rotation of the agitator  14 , the agitator  14  is configured to agitate the toner T with the flexible sheet  14 B. 
     The drum cartridge  5  includes a pressing member  5 C and an urging member  5 D. The urging member  5 D is configured to urge the pressing member  5 C toward the photosensitive drum  5 B. The pressing member  5 C urged by the urging member  5 D is configured to contact the developing cartridge  10  attached to the drum cartridge  5  and urge the developing roller  12  against the photosensitive drum  5 B. Hence, in a state where the developing cartridge  10  is attached to the drum cartridge  5 , the developing roller  12  is pressed against the photosensitive drum  5 B by the pressing member  5 C and the urging member  5 D. 
     As illustrated in  FIG. 1 , the transfer roller  4 B faces the photosensitive drum  5 B. The transfer roller  4 B is configured to convey the sheet S with the sheet S nipped between the photosensitive drum  5 B and the transfer roller  4 B. 
     The photosensitive drum  5 B is configured to be charged by a charger (not illustrated), and then exposed to light by the exposure device. An electrostatic latent image is thus formed on a peripheral surface of the photosensitive drum  5 B. The toner T is then supplied to the electrostatic latent image to form a toner image on the photosensitive drum  5 B. The toner image on the photosensitive drum  5 B is then transferred onto the sheet S fed from the sheet feeding portion  3  while the sheet S passes between the photosensitive drum  5 B and the transfer roller  4 B. 
     After the toner image is transferred onto the sheet S, the fixing device  4 C is configured to thermally fix the toner image to the sheet S. After the toner image is thermally-fixed to the sheet S, the sheet S is discharged out of the main casing  2  onto the discharge tray  2 B. 
     The controller CU is configured to control overall operations of the laser printer  1 . 
     The laser printer  1  includes a sensor  7 . The sensor  7  is configured to detect whether the attached developing cartridge  10  is new, or a specification of the attached developing cartridge  10 . The sensor  7  includes a lever  7 A and an optical sensor  7 B. The lever  7 A is pivotably supported by the main casing  2 . The lever  7 A is disposed at such a position that the lever  7 A can come into contact with detection protrusions  33 A of a detection gear  33  (described later). The detection protrusions  33 A can rotate together with the detection gear  33 . The optical sensor  7 B is electrically connected to the controller CU so that the optical sensor  7 B can output a detection signal to the controller CU. The controller CU is configured to identify the specification of the attached developing cartridge  10 , for example, based on the signal received from the optical sensor  7 B. The optical sensor  7 B is configured to detect displacement of the lever  7 A and transmit a detection signal to the controller CU based on the detection. Specifically, the optical sensor  7 B may be a sensor unit configured of a light emitter and a light receiver, for example. 
     Next, a detailed configuration of the developing cartridge  10  according to the embodiment will be described. 
     As illustrated in  FIG. 3 , the developing cartridge  10  includes the housing  11 . The housing  11  has one end portion in the first direction. A first gear cover  21 , the coupling  22 , a developing gear  23 , a supply gear  24 , a first agitator gear  25 , an idle gear  26 , a first bearing member  27 , and a cap  28  are disposed at the one end portion of the housing  11  in the first direction. 
     The first gear cover  21  includes a shaft  21 B (see  FIG. 5 ) for supporting the idle gear  26 . The first gear cover  21  also covers at least one of the gears positioned at the one end portion of the housing  11  in the first direction. Specifically, the first gear cover  21  covers a portion of the coupling  22 , the supply gear  24 , the first agitator gear  25 , and the idle gear  26 . The first gear cover  21  is fixed to an outer surface  11 C of the housing  11  with screws  29 . That is, the first gear cover  21  is positioned at the outer surface  11 C. The outer surface  11 C is an outer surface of the one end portion of the housing  11  in the first direction. 
     The coupling  22  is configured to rotate the gears including the developing roller  12 . The coupling  22  is rotatable in accordance with rotations of the developing roller  12  and other gears. The coupling  22  is rotatable about an axis  22 X thereof extending in the axial direction. The coupling  22  is positioned at the one end portion of the housing  11  in the first direction. That is, the coupling  22  is positioned at the outer surface  11 C. The coupling  22  is rotatable upon receipt of a driving force. 
     Specifically, the coupling  22  is configured to receive the driving force from the laser printer  1 . The coupling  22  is rotatable by engagement with a driving member (not illustrated) provided in the laser printer  1 . The coupling  22  has one end in the axial direction formed with a first recess  22 A. The first recess  22 A is recessed inward in the first direction. The first recess  22 A is configured to receive the driving member to engage therewith. More specifically, the first recess  22 A is configured to engage the driving member of the laser printer  1  to receive the driving force from the laser printer  1 . 
     The coupling  22  includes a first gear  22 B and a second gear  22 C. The first gear  22 B meshingly engages with the developing gear  23 . The second gear  22 C meshingly engages with the supply gear  24 . The first gear  22 B has a diameter that is different from a diameter of the second gear  22 C. Specifically, the diameter of the first gear  22 B is greater than the diameter of the second gear  22 C. 
     The developing gear  23  is mounted to the developing-roller shaft  12 A. The developing gear  23  is rotatable together with the developing roller  12  about the first axis  12 X. The developing gear  23  is positioned at the one end portion of the housing  11  in the first direction. That is, the developing gear  23  is positioned at the outer surface  11 C. The developing gear  23  includes a gear portion  23 A. The gear portion  23 A meshingly engages with the first gear  22 B of the coupling  22 . 
     The supply gear  24  is mounted to the supply-roller shaft  13 A. The supply gear  24  is rotatable together with the supply roller  13  about the axis  13 X extending in the axial direction. The supply gear  24  is positioned at the one end portion of the housing  11  in the first direction. That is, the supply gear  24  is positioned at the outer surface  11 C. The supply gear  24  includes a gear portion  24 A. The gear portion  24 A meshingly engages with the second gear  22 C of the coupling  22 . 
     The first agitator gear  25  is mounted to the agitator shaft  14 A. The first agitator gear  25  is rotatable about the axis  14 X extending in the axial direction. The first agitator gear  25  is rotatable together with the agitator  14  in accordance with rotation of the coupling  22 . The first agitator gear  25  is positioned at the one end portion of the housing  11  in the first direction. That is, the first agitator gear  25  is positioned at the outer surface  11 C. The first agitator gear  25  includes a gear portion  25 A. The gear portion  25 A meshingly engages with the idle gear  26 . 
     The idle gear  26  is positioned at the one end portion of the housing  11  in the first direction. That is, the idle gear  26  is positioned at the outer surface  11 C. The idle gear  26  meshingly engages with the coupling  22  and the first agitator gear  25 . Specifically, the idle gear  26  includes a large-diameter portion  26 A and a small-diameter portion  26 B (see  FIGS. 4A and 4B ). The large-diameter portion  26 A meshingly engages with the first gear  22 B of the coupling  22 . The small-diameter portion  26 B meshingly engages with the gear portion  25 A of the first agitator gear  25 . The idle gear  26  is rotatably supported by the shaft  21 B (see  FIG. 5 ) of the first gear cover  21 . The idle gear  26  is rotatable about an axis  26 X extending in the axial direction. The idle gear  26  functions to slow down a rotation speed of the coupling  22  and transmit the same to the first agitator gear  25 . In the first direction, the large-diameter portion  26 A is positioned farther away from the housing  11  than the small-diameter portion  26 B is from the housing  11 . 
     The idle gear  26  is rotatable in a first rotation direction D 1  upon receipt of the driving force from the coupling  22 . The idle gear  26  is also rotatable in a second rotation direction D 2  opposite to the first rotation direction D 1 . That is, the idle gear  26  is supported by the first gear cover  21  and the housing  11  such that the idle gear  26  is rotatable in the first rotation direction D 1  and the second rotation direction D 2  relative to the first gear cover  21  and the housing  11 . 
     The cap  28  covers one end portion of the developing-roller shaft  12 A in the first direction. The cap  28  may be made of resin whose type is different from a type of resin which the first gear cover  21  is made of. 
     The first bearing member  27  rotatably supports the developing-roller shaft  12 A, the supply-roller shaft  13 A, and the coupling  22 . The first bearing member  27  is secured to the one end portion of the housing  11  in the first direction. The first bearing member  27  includes a base portion  27 A and a shaft  27 B. The shaft  27 B protrudes from the base portion  27 A outward in the first direction. 
     The base portion  27 A has a first insertion hole H 1  and a second insertion hole H 2 . The developing-roller shaft  12 A of the developing roller  12  is inserted in the first insertion hole H 1 . The supply-roller shaft  13 A of the supply roller  13  is inserted in the second insertion hole H 2 . 
     The shaft  27 B has a cylindrical shape. The shaft  27 B rotatably supports the coupling  22 . Specifically, an outer peripheral surface of the shaft  27 B rotatably supports the coupling  22 . An inner end of the shaft  27 B (another end in the first direction) is closed by the base portion  27 A. 
     As illustrated in  FIG. 1 , the developing cartridge  10  also includes a second gear cover  31 , a second agitator gear  32 , the detection gear  33 , a second bearing member  34 , a developing electrode  35 , and a supply electrode  36 , all of which are positioned at another end portion of the housing  11  in the first direction. 
     The second gear cover  31  covers at least a portion of the detection gear  33 . The second gear cover  31  is positioned at an outer surface  11 E of the housing  11 . The outer surface  11 E is an outer surface positioned at the other end portion of the housing  11  in the first direction. That is, the outer surface  11 E is opposite the outer surface  11 C in the first direction. The second gear cover  31  has an opening  31 A formed therein. The portion of the detection gear  33  is exposed through the opening  31 A. 
     The second agitator gear  32  is positioned at the other end portion of the housing  11  in the first direction. That is, the second agitator gear  32  is positioned at the outer surface  11 E. The second agitator gear  32  is mounted to the agitator shaft  14 A (see  FIG. 2 ). The second agitator gear  32  is thus rotatable together with the agitator shaft  14 A of the agitator  14  about the axis  14 X extending in the axial direction. 
     The detection gear  33  is positioned at the other end portion of the housing  11  in the first direction. The detection gear  33  is rotatable by rotation of the second agitator gear  32  when the detection gear  33  comes to meshing engagement with the second agitator gear  32 . The detection gear  33  includes a plurality of the detection protrusions  33 A each configured to come into contact with the lever  7 A of the sensor  7 . Note that the number/positions of the detection protrusions  33 A may be varied according to the specifications of the developing cartridge  10  so that the controller CU can identify the specification of the developing cartridge  10  in a state where the developing cartridge  10  is attached to the main casing  2  of the laser printer  1 . 
     The second bearing member  34  rotatably supports the developing-roller shaft  12 A and the supply-roller shaft  13 A. The second bearing member  34  is fixed to the outer surface  11 E while supporting the developing-roller shaft  12 A and the supply-roller shaft  13 A. 
     The developing electrode  35  is positioned at the other end portion of the housing  11  in the first direction. The developing electrode  35  is configured to supply electric power to the developing-roller shaft  12 A. The developing electrode  35  is made of an electrically conductive resin, for example. 
     The supply electrode  36  is positioned at the other end of the housing  11  in the first direction. The supply electrode  36  is configured to supply electric power to the supply-roller shaft  13 A. The supply electrode  36  is made of an electrically conductive resin, for example. 
     The developing electrode  35  and the supply electrode  36  are screw-fixed to the outer surface  11 E of the housing  11 , together with the second bearing member  34 , with screws  38 . 
     In the present embodiment, the coupling  22  serves as an example of a first helical gear, and the idle gear  26  serves as an example of a second helical gear. More specifically, as illustrated in  FIG. 3 , the first gear  22 B of the coupling  22  is a helical gear with each gear tooth inclined relative to the first direction and a rotation direction of the coupling  22 . The large-diameter portion  26 A of the idle gear  26  is a helical gear with each gear tooth inclined relative to the first direction and a rotation direction of the idle gear  26 . Here, the rotation direction of the idle gear  26  includes the first rotation direction D 1  and second rotation direction D 2 . 
     As illustrated in  FIGS. 4A and 4B , the idle gear  26  includes the large-diameter portion  26 A, the small-diameter portion  26 B, a disc portion  26 C, a shaft portion  26 D, and an engaging member  50 . 
     The disc portion  26 C has a disc shape centered on the axis  26 X. The disc portion  26 C has an end surface  26 E facing a portion of the first gear cover  21  in the first direction. That is, the end surface  26 E faces outward in the first direction. 
     The shaft portion  26 D extends inward in the first direction from a center portion of the disc portion  26 C. The shaft portion  26 D has a cylindrical shape centered on the axis  26 X. The shaft portion  26 D of the idle gear  26  is supported by the shaft  21 B (see  FIG. 5 ) of the first gear cover  21  so that the shaft portion  26 D is movable in the axial direction relative to the shaft  21 B. 
     The idle gear  26  is movable between a first position (illustrated in  FIG. 6 ) and a second position (illustrated in  FIG. 7 ) in the axial direction. The idle gear  26  is positioned closer to the outer surface  11 C at the first position (illustrated in  FIG. 6 ) than at the second position (illustrated in  FIG. 7 ) in the first direction. At the first position, the engaging member  50  and a part of the first gear cover  21  (first protrusions  41  described later) are disengaged from each other. That is, at the first position, the engaging member  50  and the part of the first gear cover  21  do not engage each other. 
     The idle gear  26  is positioned farther away from the outer surface  11 C at the second position (illustrated in  FIG. 7 ) than at the first position (illustrated in  FIG. 6 ) in the first direction. At the second position, the engaging member  50  and the part of the first gear cover  21  (the first protrusions  41 ) are engaged with each other. More specifically, at the second position, the engaging member  50  and the part of the first gear cover  21  (a first surface  41 A of each first protrusion  41  described later) are in engagement with each other in the second rotation direction D 2 . 
     As the idle gear  26  rotates, the large-diameter portion  26 A of the idle gear  26  and the first gear  22 B of the coupling  22  generate a thrust force. Specifically, as the idle gear  26  rotates in the first rotation direction D 1 , the large-diameter portion  26 A and the first gear  22 B generate a first thrust force F 1  that causes the idle gear  26  to move inward in the first direction. In other words, as the idle gear  26  rotates in the first rotation direction D 1 , the large-diameter portion  26 A and the first gear  22 B generate the first thrust force F 1 , with which force the idle gear  26  is caused to move toward the outer surface  11 C in the first direction. Thus, in a case where the idle gear  26  rotates in the first rotation direction D 1 , the idle gear  26  is moved to the first position by the first thrust force F 1  generated by the meshing engagement between the idle gear  26  and the coupling  22 . 
     As the idle gear  26  rotates in the second rotation direction D 2 , the large-diameter portion  26 A and the first gear  22 B generate a second thrust force F 2 . The second thrust force F 2  causes the idle gear  26  to move outward in the first direction. In other words, as the idle gear  26  rotates in the second rotation direction D 2 , the large-diameter portion  26 A and the first gear  22 B generate the second thrust force F 2 , with which force the idle gear  26  is caused to move toward the first gear cover  21  in the first direction. Thus, in a case where the idle gear  26  rotates in the second rotation direction D 2 , the idle gear  26  is moved to the second position by the second thrust force F 2  generated by the meshing engagement between the idle gear  26  and the coupling  22 . 
     Note that the coupling  22  is immovable in the axial direction relative to the housing  11 , since the coupling  22  is in contact with the first gear cover  21  or the first bearing member  27 . In the present disclosure, the description “immovable in the axial direction relative to the housing  11 ” includes both cases: the coupling  22  does not move at all relative to the housing  11 ; and the coupling  22  does move slightly relative to the housing  11  due to play or backlash therebetween. 
     The engaging member  50  illustrated in  FIG. 4A  serves to allow rotation of the idle gear  26  in the first rotation direction D 1 . The engaging member  50  further serves to restrict rotation of the idle gear  26  in the second rotation direction D 2 . 
     The engaging member  50  is rotatable about the axis  26 X together with the idle gear  26 . Specifically, the engaging member  50  is rotatable, together with the idle gear  26 , in the first rotation direction D 1  and in the second rotation direction D 2 . 
     The engaging member  50  is also movable in the axial direction together with the idle gear  26 . Specifically, the engaging member  50  is movable in the axial direction between the first position (illustrated in  FIG. 6 ) and the second position (illustrated in  FIG. 7 ) together with the idle gear  26 . While the idle gear  26  is at the first position, the engaging member  50  and the part of the first gear cover  21  (the first protrusions  41 ) are disengaged from each other. While the idle gear  26  is at the second position, the engaging member  50  and the part of the first gear cover  21  are engaged with each other. 
     While the idle gear  26  is at the first position, the engaging member  50  is also at the first position together with the idle gear  26 . At this time, the idle gear  26  is rotatable in conjunction with rotations of the coupling  22  and the developing gear  23 , for example. While the idle gear  26  is at the second position, the engaging member  50  is also at the second position together with the idle gear  26 . At this time, the engaging member  50  and the part of the first gear cover  21  are in engagement with each other. The engagement between the engaging member  50  and the part of the first gear cover  21  prevents the idle gear  26  from rotating further in the second rotation direction D 2 . 
     As illustrated in  FIG. 3 , in the present embodiment, the second gear  22 C of the coupling  22 , the gear portion  23 A of the developing gear  23 , and the gear portion  24 A of the supply gear  24  are also helical gears with each gear tooth inclined relative to the first direction and corresponding rotation direction. In the embodiment, the developing gear  23  and the supply gear  24  are also immovable in the axial direction relative to the housing  11 , similar to the coupling  22 . 
     Specifically, the developing gear  23  and the supply gear  24  are immovable in the axial direction relative to the housing  11 , since the developing gear  23  and the supply gear  24  are in contact with the first gear cover  21  or the first bearing member  27 . Alternatively, the developing gear  23  and the supply gear  24  may be fixed to the developing-roller shaft  12 A and supply-roller shaft  13 A, respectively, in order to make the developing gear  23  and the supply gear  24  immovable in the axial direction relative to the housing  11 . 
     As illustrated in  FIG. 5 , the first gear cover  21  includes a side wall  21 A, the shaft  21 B, and an opening  21 C. 
     The side wall  21 A has an opposing surface  21 D facing the idle gear  26  in the first direction. The opposing surface  21 D is part of a surface of the side wall  21 A facing inward in the first direction. The opposing surface  21 D is positioned at another end portion of the side wall  21 A in the first direction. The opposing surface  21 D has a circular shape centered on the axis  26 X. 
     The shaft  21 B protrudes from the opposing surface  21 D of the side wall  21 A inward in the first direction. The shaft  21 B has a cylindrical shape centered on the axis  26 X. The shaft  21 B rotatably supports the idle gear  26 . Specifically, an outer peripheral surface of the shaft  21 B rotatably supports the idle gear  26 . 
     The opening  21 C serves to expose a part of the coupling  22  therethrough. More specifically, the first recess  22 A is exposed through the opening  21 C. The coupling  22  is thus allowed to engage the driving member (not shown) of the laser printer  1 . 
     The first gear cover  21  includes the plurality of first protrusions  41 . Specifically, the first gear cover  21  has six of the first protrusions  41 . Each of the first protrusions  41  has an arcuate shape centered on the axis  26 X. Each first protrusion  41  protrudes from the opposing surface  21 D inward in the axial direction. The first protrusions  41  are positioned to surround the shaft  21 B. The first protrusions  41  are arranged in the rotation direction of the idle gear  26 . The first protrusions  41  are arranged to form an annular shape. Each of the first protrusions  41  has a first surface  41 A and a second surface  41 B. The first surface  41 A extends in the first direction. The second surface  41 B is inclined relative to the first direction. 
     The first surfaces  41 A function to restrict the idle gear  26  from rotating in the second rotation direction D 2 . The first surfaces  41 A extend to cross the rotation direction of the idle gear  26 . Preferably, the first surfaces  41 A are orthogonal to the rotation direction of the idle gear  26 . As the idle gear  26  rotates in the second rotation direction D 2 , the first surfaces  41 A face the engaging member  50  and come into contact therewith in the second rotation direction D 2 . The first surfaces  41 A thus prevent rotation of the idle gear  26  in the second rotation direction D 2 . 
     The second surfaces  41 B function to move the idle gear  26  and the engaging member  50  from the second position toward the first position in accordance with rotation of the idle gear  26  in the first rotation direction D 1 . The second surfaces  41 B are inclined relative to the rotation direction of the idle gear  26 . Specifically, each second surface  41 B is inclined such that the second surface  41 B extends inward in the first direction toward downstream in the first rotation direction D 1 . More specifically, each second surface  41 B is inclined such that the second surface  41 B separates away from the opposing surface  21 D toward downstream in the first rotation direction D 1 . Hence, as the idle gear  26  rotates in the first rotation direction D 1 , the second surfaces  41 B face and come into contact with the engaging member  50  in the first rotation direction D 1 . The idle gear  26  thus moves toward the first position as the engaging member  50  moves inward in the first direction over the second surfaces  41 B. 
     As illustrated in  FIG. 4A , the engaging member  50  is positioned on the end surface  26 E of the disc portion  26 C of the idle gear  26 . The engaging member  50  includes a plurality of second protrusions  51 . Specifically, the engaging member  50  includes six of the second protrusions  51 . Each of the second protrusions  51  has an arcuate shape centered on the axis  26 X. Each second protrusion  51  protrudes outward in the first direction from the end surface  26 E of the disc portion  26 C. The second protrusions  51  are positioned around the axis  26 X. The second protrusions  51  are aligned with one another in the rotation direction of the idle gear  26 . The second protrusions  51  are arranged to form an annular shape. 
     The second protrusions  51  are formed integrally with the disc portion  26 B. The second protrusions  51  are part of the idle gear  26 . That is, the idle gear  26  includes the plurality of second protrusions  51 . Put different way, the idle gear  26  includes the engaging member  50 . 
     Each second protrusion  51  has a third surface  51 A and a fourth surface  51 B. The third surface  51 A extends in the first direction. The fourth surface  51 B is inclined relative to the first direction. 
     The third surfaces  51 A function to restrict the idle gear  26  from rotating in the second rotation direction D 2 . The third surfaces  51 A extend to cross the rotation direction of the idle gear  26 . Preferably, the third surfaces  51 A are orthogonal to the rotation direction of the idle gear  26 . The third surfaces  51 A are configured to come into contact with the first surfaces  41 A of the first protrusions  41 , respectively. More specifically, the third surfaces  51 A are configured to make surface-contact with the corresponding first surfaces  41 A. 
     The fourth surfaces  51 B function to move the idle gear  26  from the second position to the first position while the idle gear  26  rotates in the first rotation direction D 1 . The fourth surfaces  51 B are each inclined relative to the rotation direction of the idle gear  26 . Specifically, each fourth surface  51 B is inclined such that the fourth surface  51 B extends inward in the first direction toward downstream in the first rotation direction D 1 . That is, the fourth surfaces  51 B are inclined such that the fourth surface  51 B approaches the end surface  26 E toward downstream in the first rotation direction D 1 . The fourth surfaces  51 B are configured to contact the second surfaces  41 B of the first protrusions  41 , respectively. Specifically, the fourth surfaces  51 B are respectively configured to make surface-contact with the second surfaces  41 B. 
     Note that a length of each second protrusion  51  in the first direction is substantially identical to a length of each first protrusion  41  of the first gear cover  21  in the first direction. 
     Further, a moving distance of the idle gear  26  from the second position to the first position is greater than the lengths of each first protrusion  41  and each second protrusion  51  in the first direction. With this structure, when the idle gear  26  is at the first position, the engagement between each second protrusion  51  and corresponding first protrusion  41  of the first gear cover  21  can be reliably released. Hence, when the idle gear  26  is at the first position, the second protrusions  51  can be reliably separated from the corresponding first protrusions  41  of the first gear cover  21  in the first direction. The second protrusions  51  and the first protrusions  41  can thus be prevented from interfering with each other in a case where the idle gear  26  rotates in the first rotation direction D 1  together with the engaging member  50 . 
     Next, operations of the developing cartridge  10  will be described. Specifically, operations of the developing cartridge  10  attached to the drum cartridge  5  will be described hereinafter. That is, description will be given on how the idle gear  26  operates while rotating in the first rotation direction D 1  or in the second rotation direction D 2  in a state where the developing roller  12  is pressed against the photosensitive drum  5 B by the pressing member  5 C and the urging member  5 D. 
     As illustrated in  FIG. 6 , when the idle gear  26  is at the first position, the second protrusions  51  of the engaging member  50  are separated away from the opposing surface  21 D of the first gear cover  21  in the axial direction. Hence, at this time, the second protrusions  51  are disengaged from the first protrusions  41  of the first gear cover  21 . 
     As the coupling  22  rotates upon receipt of a driving force from the laser printer  1  for printing in the state where the idle gear  26  is at the first position, the idle gear  26  is caused to rotate in the first rotation direction D 1 . In accordance with rotation of the coupling  22 , the developing gear  23 , the supply gear  24 , and the first agitator gear  25  are also caused to rotate. The developing roller  12 , the supply roller  13 , and the agitator  14  thus rotate respectively in prescribed directions, as illustrated by arrows in  FIG. 2 . 
     Here, there are conventionally known image forming apparatuses that can form images on both sides of each sheet. In order to perform such both-side printing, conventional image forming apparatuses are configured to form image on one side of a sheet at an image forming portion, then reverse the sheet and convey the reversed sheet back to a position upstream of the image forming portion in a sheet-conveying direction, and subsequently form an image on the back side of the sheet. 
     In such conventional image forming apparatuses, a photosensitive drum may be caused to rotate in a predetermined direction for performing image formation on each sheet, whereas the photosensitive drum may rotate in a direction opposite to the predetermined direction in order to reverse the sheet. In this configuration, a developing roller may be caused to rotate in reverse following the reverse rotation of the photosensitive drum. However, the structure according to the embodiment can prevent the developing roller  12  from rotating in reverse following the reverse rotation of the photosensitive drum  5 B. 
     Specifically, in the present embodiment, in a case where the developing roller  12  rotates in reverse due to the reverse rotation of the photosensitive drum  5 B while the idle gear  26  is at the first position illustrated in  FIG. 6 , the idle gear  26  rotates in the second rotation direction D 2  through rotations of the developing gear  23  and the coupling  22 . As a result, as illustrated in  FIG. 7 , the idle gear  26  is caused to move toward the second position by the second thrust force F 2  generated by the meshing engagement between the coupling  22  and the idle gear  26 . The idle gear  26  is positioned closer to the opposing surface  21 D of the first gear cover  21  at the second position than at the first position. 
     When the idle gear  26  arrives at the second position, the second protrusions  51  of the engaging member  50  engages the corresponding first protrusions  41  of the first gear cover  21 . Since the idle gear  26  is rotating in the second rotation direction D 2  at this time, the engaging member  50  also rotates in the second rotation direction D 2  together with the idle gear  26 . The third surfaces  51 A of the second protrusions  51  of the engaging member  50  thus come into contact with the first surfaces  41 A of the first protrusions  41 , respectively. This contact prevents the engaging member  50  from rotating further in the second rotation direction D 2 . The idle gear  26  integral with the engaging member  50  is thus prevented from rotating further in the second rotation direction D 2 . 
     In response to halt of the rotation of the idle gear  26 , rotations of the coupling  22 , rotations of the developing gear  23 , the supply gear  24 , and the first agitator gear  25  are also terminated. The developing roller  12 , the supply roller  13 , and the agitator  14  are caused to stop rotating, accordingly. The developing roller  12 , the supply roller  13 , and the agitator  14  are therefore prevented from rotating in reverse. 
     The coupling  22  rotates upon receipt of the driving force from the laser printer  1  while the idle gear  26  is at the second position. As the coupling  22  rotates, the idle gear  26  is caused to rotate in the first rotation direction D 1 . The idle gear  26  is therefore moved from the second position toward the first position by the first thrust force F 1  generated by the meshing engagement between the coupling  22  and the idle gear  26 . 
     As the idle gear  26  rotates in the first rotation direction D 1 , the engaging member  50  also rotates in the first rotation direction D 1  together with the idle gear  26 . The fourth surfaces  51 B of the second protrusions  51  of the engaging member  50  are brought into contact with the second surfaces  41 B of the first protrusions  41 , respectively. As the engaging member  50  further rotates in the first rotation direction D 1  together with the idle gear  26 , the fourth surfaces  51 B respectively move over the corresponding second surfaces  41 B. The engaging member  50  thus moves toward the first position together with the idle gear  26 . In this way, the second surfaces  41 B and the fourth surfaces  51 B serve to assist movement of the engaging member  50  and the idle gear  26  from the second position to the first position. 
     As illustrated in  FIG. 6 , when the idle gear  26  arrives at the first position, the idle gear  26  comes into contact with the housing  11 . The idle gear  26  is thus prevented from moving further inward in the first direction. When the idle gear  26  arrives at the first position, the second protrusions  51  of the engaging member  50  are disengaged from the respective first protrusions  41  of the first gear cover  21 . The idle gear  26  can therefore continue to rotate in the first rotation direction D 1  thereafter. 
     The operations described above can also be realized even in a state where the developing cartridge  10  is removed from the drum cartridge  5 . 
     The embodiment described above can achieve technical and operational advantages described below. 
     As the idle gear  26  rotates in the first rotation direction D 1 , the engaging member  50  also rotates together with the idle gear  26 . In a case where the idle gear  26  rotates in the second rotation direction D 2 , the idle gear  26  rotates slightly in the second rotation direction D 2  but is then caused to stop rotating due to the engagement of the engaging member  50  with the first surfaces  41 A of the first protrusions  41 . The idle gear  26  is thus restricted from rotating further in the second rotation direction D 2  opposite to the first rotation direction D 1 . With this structure of the embodiment, leakage of the toner T out of the housing  11  due to the reverse rotation of the developing roller  12  can be suppressed. 
     While the idle gear  26  is at the first position, the second protrusions  51  of the engaging member  50  are disengaged from the first protrusions  41  of the first gear cover  21 . That is, the engaging member  50  is in separation from the first gear cover  21 . With this structure, while the idle gear  26  is at the first position, the idle gear  26  is reliably rotatable in the first rotation direction D 1 . 
     Further, the idle gear  26  can be restricted from rotating in the second rotation direction D 2  by the first surfaces  41 A and the third surfaces  51 A that are orthogonal to the rotation direction of the idle gear  26 . Also, movement of the idle gear  26  and the engaging member  50  from the second position to the first position can be assisted by the second surfaces  41 B and the fourth surfaces  51 B both inclined relative to the rotation direction of the idle gear  26 . 
     The first gear cover  21  includes the plurality of first protrusions  41 . The engaging member  50  includes the plurality of second protrusions  51 . As the idle gear  26  rotates in the second rotation direction D 2 , the plurality of the third surfaces  51 A of the second protrusions  51  respectively come into contact with the plurality of the first surfaces  41 A of the first protrusions  41 . With this structure, the idle gear  26  can be reliably restricted from rotating in the second rotation direction D 2 . 
     Even in a state where the developing roller  12  is pressed onto the photosensitive drum  5 B by the pressing member  5 C and the urging member  5 D of the drum cartridge  5 , the idle gear  26  is rotatable in the first rotation direction D 1  but substantially impossible to rotate in the second rotation direction D 2 . 
     It would be apparent to those skilled in the art that the embodiment described above is merely an example of the present disclosure and modifications and variations may be made therein without departing from the spirit of the disclosure. 
     For example, while the coupling  22  serves as the first helical gear and the idle gear  26  serves as the second helical gear in the depicted embodiment, the present disclosure is not limited to this configuration. For example, a first agitator gear may serve as the second helical gear. In this case, the first agitator gear may include a gear portion which is a helical gear, and an engaging portion having the same configuration as the engaging member  50  of the embodiment. Alternatively, a coupling, or a supply gear or a developing gear may serve as the second helical gear. 
     The first helical gear may be any gear, provided that the first helical gear meshes with the second helical gear. For example, in a case where a first agitator gear serves as the second helical gear, an idle gear may serve as the first helical gear. This idle gear may include a small-diameter portion which is a helical gear. Alternatively, in a case where a coupling serves as the second helical gear, a developing gear may serve as the first helical gear. 
     In the depicted embodiment, the plurality of first protrusions  41  are provided at the first gear cover  21 . However, the first gear cover  21  may include a single first protrusion  41 . Likewise, the engaging member  50  may include a single second protrusion  51 , instead of the plurality of second protrusions  51  of the embodiment. 
     In the embodiment described above, both of the second surfaces  41 B of the first protrusions  41  and the fourth surfaces  51 B of the second protrusions  51  are inclined surfaces that are inclined relative to the rotation direction of the idle gear  26 . However, only one of the second surfaces and fourth surfaces may be inclined surfaces. 
     In the embodiment described above, the engaging member  50  is integrally formed with the idle gear  26  serving as the second helical gear. However, the engaging member and the second helical gear may be separate components. 
     In the embodiment described above, the first gear cover  21  includes the first protrusions  41  as a portion engageable with the second protrusions  51 . However, instead of protrusions, a cover may have holes each engageable with second protrusions. Specifically, the cover may have a portion formed with a first hole having a first surface and a second surface. The first hole may be a single hole or a plurality of holes. Further, the first hole may be a through-hole or a bottomed hole. 
     In the embodiment described above, the engaging member  50  includes the second protrusions  51  as a portion engageable with the first protrusions  41 . However, instead of protrusions, an engaging member may have holes each engageable with first protrusions. Specifically, the engaging member may have a portion formed with a second hole having a third surface and a fourth surface. The second hole may be a single hole or a plurality of holes. Further, the second hole may be a through-hole or a bottomed hole. 
     In the embodiment described above, the first gear cover  21  covers a part of the coupling  22 , the supply gear  24 , the first agitator gear  25 , and the idle gear  26 . However, a cover positioned at an outer surface of a housing may only cover part of the second helical gear. Specifically, the cover may only cover a part of the second helical gear at which the engaging member is provided so that the cover can engage the engaging member. 
     In the embodiment described above, the developing cartridge  10  and the drum cartridge  5  are separate components. However, the developing cartridge  10  and the drum cartridge  5  may be integrally formed as a single component. 
     The monochrome laser printer  1  is described as an example of an image forming apparatus of the disclosure. However, the image forming apparatus of the disclosure may be a color image forming apparatus, an image forming apparatus configured to perform exposure with LEDs, a copying machine, or a multifunction device. 
     It should be apparent to those who skilled in the art that the embodiment and variations described above may be combined with one another as appropriate. 
     &lt;Remarks&gt; 
     The developing cartridge  10  is an example of a developing cartridge. The housing  11  is an example of a housing. The outer surface  11 C is an example of an outer surface. The developing roller  12  is an example of a developing roller. The first axis  12 X is an example of a first axis. The coupling  22  is an example of a first helical gear and an example of a coupling. The axis  22 X is an example of a second axis. The idle gear  26  is an example of a second helical gear. The end surface  26 E is an example of an end surface. The first rotation direction D 1  is an example of a first rotational direction. The second rotation direction D 2  is an example of a second rotational direction. The axis  26 X is an example of a third axis. The first thrust force F 1  is an example of a first thrust force. The second thrust force F 2  is an example of a second thrust force. The first gear cover  21  is an example of a cover. The engaging member  50  is an example of an engaging member. The first surface  41 A and second surface  41 B are examples of a first surface and a second surface, respectively. The third surface  51 A and fourth surface  51 B are examples of a third surface and a fourth surface, respectively. The first protrusions  41  are an example of a first protrusion. The second protrusions  51  are an example of a second protrusion. The agitator  14  is an example of an agitator. The first agitator gear  25  is an example of an agitator gear. The developing gear  23  is an example of a developing gear. The first recess  22 A is an example of a recess. The drum cartridge  5  is an example of a drum cartridge. The photosensitive drum  5 B is an example of a photosensitive drum. The pressing member  5 C is an example of a pressing member.