Patent Publication Number: US-9835975-B2

Title: Developing device, process cartridge, and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a developing device used in an image forming apparatus, and a process cartridge attachable to and detachable from the image forming apparatus. 
     The image forming apparatus is an apparatus configured to form an image on a recording medium using an electrophotographic image forming method. Examples of the electrophotographic image forming apparatus include an electrophotographic copier, an electrophotographic printer (e.g., laser beam printer, light-emitting diode (LED) printer), a facsimile machine, a word processor, and a multifunction peripheral (multifunction printer) which combines the devices described above. 
     The developing device is a device configured to visualize an electrostatic latent image on an image bearing member, such as an electrophotographic photosensitive drum, using a developer. The process cartridge is formed by combining a charging unit, a developing unit or cleaning unit, and an electrophotographic photosensitive drum serving as an image bearing member, so as to be attachable to and detachable from the main body of the electrophotographic image forming apparatus. The process cartridge may be formed by combining at least one of a charging unit, a developing unit, and a cleaning unit with an electrophotographic photosensitive drum, so as to be attachable to and detachable from the main body of the electrophotographic image forming apparatus. The process cartridge may be formed by combining at least a developing unit and an electrophotographic photosensitive drum, so as to be attachable to and detachable from the main body of the electrophotographic image forming apparatus. 
     Description of the Related Art 
     An image forming apparatus using an electrophotographic process, such as a printer, uniformly charges a photosensitive drum serving as an image bearing member, forms a latent image by selectively exposing the photosensitive drum to light, and visualizes the latent image with toner serving as a developer to form a toner image on the photosensitive drum. Then, the image forming apparatus performs image recording by transferring the toner image onto a recording medium, and applying heat and pressure to the transferred toner image to fix the toner image to the recording medium. 
     The image forming apparatus described above requires toner supply and maintenance on various processing units. To facilitate the toner supply and maintenance operation, a cartridge formed by putting together the photosensitive drum, charging unit, developing unit, and cleaning unit into a frame has been put to practical use. 
     In recent years, image forming apparatuses configured to form a color image by multicolor printing have been proposed. To meet demands for faster printing speeds and application to multifunction printers, an in-line type image forming apparatus has been proposed as a color image forming apparatus. The in-line type image forming apparatus includes a plurality of photosensitive drums which are designed for multicolor printing and arranged side by side in line in a horizontal direction. 
     Examples of the in-line type image forming apparatus including a plurality of photosensitive drums horizontally arranged include an image forming apparatus disclosed in Japanese Patent Laid-Open No. 2008-170951. This image forming apparatus has a configuration in which a plurality of photosensitive drums are arranged side by side below a transfer belt for transferring toner images, or below a recording-medium conveying belt for conveying a recording medium to which a toner image is transferred. This configuration is advantageous in that developing devices are less affected by heat from a fixing unit because, even in the main body of the image forming apparatus, the fixing unit can be placed away from the developing devices, with the transfer belt or the like interposed therebetween. 
     A developing device in each process cartridge is formed by combining a developing container and a toner container for containing toner. The developing container includes a developing roller for supplying a developer (toner) to an electrostatic latent image formed on a photosensitive drum, a toner supply roller for supplying toner to the developing roller, and a developer regulating member for regulating the amount of toner on the developing roller. The toner container has a blade-like stirring member and a toner opening. The toner in the toner container is conveyed by the stirring member through the toner opening to a developing chamber. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved developing device. Specifically, the present invention provides a developing device and a process cartridge capable of stably supplying a developer to a developing chamber. 
     A developing device according to an embodiment of the present invention includes a developing chamber having a developer bearing member for bearing a developer; and a container disposed below the developing chamber in a vertical direction, configured to contain the developer, and having an endless developer conveying belt with at least one fin portion for conveying the developer to the developing chamber. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a cartridge according to a first embodiment. 
         FIG. 2  is a cross-sectional view of a color electrophotographic image forming apparatus according to the first embodiment. 
         FIG. 3  is a perspective view of a toner conveying belt according to the first embodiment. 
         FIG. 4  is another perspective view of the toner conveying belt according to the first embodiment. 
         FIG. 5  is a cross-sectional view of a cartridge according to a second embodiment. 
         FIG. 6  is a cross-sectional view of a color electrophotographic image forming apparatus according to the second embodiment. 
         FIG. 7  illustrates assembly of a toner conveying sheet member and a mandible-like member according to the second embodiment. 
         FIG. 8  illustrates assembly of a toner drop sheet member according to the second embodiment. 
         FIGS. 9A and 9B  illustrate assembly of a driving roller, a driven roller, and a developer conveying belt according to an embodiment. 
         FIGS. 10A and 10B  illustrate assembly of a toner conveying unit according to the second embodiment. 
         FIGS. 11A and 11B  are cross-sectional views illustrating a loosened state and a stretched state, respectively, of a developer conveying belt according to the second embodiment. 
         FIGS. 12A and 12B  are perspective views illustrating a securing unit for securing the driven roller according to the second embodiment. 
         FIG. 13  illustrates a drive transmission unit for the developer conveying belt according to the second embodiment. 
         FIG. 14  illustrates a state of the developer conveying belt, toner conveying sheet member, and toner drop sheet member according to the second embodiment. 
         FIG. 15  illustrates another state of the developer conveying belt, toner conveying sheet member, and toner drop sheet member according to the second embodiment. 
         FIG. 16  illustrates another state of the developer conveying belt, toner conveying sheet member, and toner drop sheet member according to the second embodiment. 
         FIG. 17  is a cross-sectional view of a cartridge according to a third embodiment. 
         FIG. 18  is a cross-sectional view of a color electrophotographic image forming apparatus according to the third embodiment. 
         FIGS. 19A and 19B  are cross-sectional views of the cartridge according to the third embodiment. 
         FIGS. 20A and 20B  are exploded perspective views illustrating a configuration of a developing device according to the third embodiment. 
         FIGS. 21A and 21B  are cross-sectional views of a toner chamber illustrating a configuration of a developer conveying belt according to the third embodiment. 
         FIG. 22  is a perspective view of the developing device according to the third embodiment. 
         FIGS. 23A and 23B  are perspective views of the developer conveying belt according to the third embodiment. 
         FIGS. 24A and 24B  are cross-sectional views of the developing device according to the third embodiment. 
         FIG. 25  is a principal cross-sectional view of a cartridge according to a fourth embodiment. 
         FIG. 26  is a principal cross-sectional view of an image forming apparatus according to the fourth embodiment. 
         FIGS. 27A and 27B  are principal cross-sectional views illustrating an area around a developing device of the cartridge according to the fourth embodiment. 
         FIGS. 28A and 28B  are exploded perspective views illustrating a configuration of a developer conveying belt according to the fourth embodiment. 
         FIGS. 29A and 29B  are exploded perspective views illustrating a configuration of the developing device according to the fourth embodiment. 
         FIG. 30  is an exploded perspective view illustrating a configuration of the developing device according to the fourth embodiment. 
         FIGS. 31A and 31B  are exploded perspective views illustrating a configuration of the developing device according to the fourth embodiment. 
         FIGS. 32A and 32B  are cross-sectional views illustrating a configuration of the developing device according to the fourth embodiment. 
         FIGS. 33A and 33B  are perspective views illustrating a securing unit for securing the driven roller according to the fourth embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present invention will now be described in detail with reference to the drawings. The dimensions, materials, shapes and relative arrangement of components described in the following embodiments should be changed appropriately depending on the configuration and various conditions of devices and apparatuses to which the present invention is applied. Therefore, the scope of the present invention is not intended to be limited to the following embodiments unless otherwise specified. 
     First Embodiment 
     A color electrophotographic image forming apparatus (hereinafter referred to as an image forming apparatus) and a process cartridge (hereinafter referred to as a cartridge) according to a first embodiment will be described. 
     (General Configuration of Image Forming Apparatus) 
     A general configuration of the image forming apparatus will be described with reference to  FIG. 2 .  FIG. 2  is a principal cross-sectional view of an image forming apparatus  100  according to the first embodiment. Also, a cartridge  7  will be described with reference to  FIG. 1  while the details will be described later on.  FIG. 1  is a principal cross-sectional view of the cartridge  7  containing toner T according to the first embodiment. 
     The image forming apparatus  100  includes electrophotographic photosensitive drums  1  (hereinafter referred to as photosensitive drums) which are image bearing members arranged side by side in a horizontal direction. The photosensitive drums  1  are each driven by a driving member (not shown) to rotate in the direction of arrow Q (see  FIG. 1 ). Each photosensitive drum  1  is surrounded by the following processing units, which are arranged sequentially in the direction of rotation of the photosensitive drum  1  and configured to act on the photosensitive drum  1 . The processing units surrounding the photosensitive drum  1  include a cleaning member  6  configured to remove a developer (hereinafter referred to as toner) remaining on the surface of the photosensitive drum  1  after transfer, a charging roller  2  configured to uniformly charge the surface of the photosensitive drum  1 , and a developing device  4  configured to develop an electrostatic latent image using toner. As illustrated in  FIG. 2 , the image forming apparatus  100  also includes a scanner unit  3  and an intermediate transfer belt  5 . The scanner unit  3  is configured to emit laser beams on the basis of image information to form electrostatic latent images on the photosensitive drums  1 . Toner images of four colors on the photosensitive drums  1  are transferred onto the intermediate transfer belt  5 . Each photosensitive drum  1  and the cleaning member  6 , the charging roller  2 , and the developing device  4  arranged therearound are combined to form the cartridge  7 . The cartridge  7  is detachably attached to the image forming apparatus  100  by the user. 
     The intermediate transfer belt  5  is stretched over a driving roller  10  and a tension roller  11 . Primary transfer rollers  12  ( 12   a  to  12   d ) are arranged on the inner side of the intermediate transfer belt  5  to face the respective photosensitive drums  1  ( 1   a  to  1   d ). A bias applying unit (not shown) applies a transfer bias to the intermediate transfer belt  5 . 
     As the photosensitive drums  1  rotate in the direction of arrow Q (see  FIG. 1 ) and the intermediate transfer belt  5  runs in the direction of arrow R (see  FIG. 2 ), the toner images (developer images) formed on the photosensitive drums  1  are sequentially primary-transferred onto the intermediate transfer belt  5  by applying a positive bias to the primary transfer rollers  12 . The toner images of four colors superimposed on the intermediate transfer belt  5  are conveyed to a secondary transfer unit  15 . 
     In synchronization with the image forming operation described above, a sheet S (recording medium) is conveyed by a conveying unit including a feeding device  13  and a registration roller pair  17 . The feeding device  13  includes a feeding cassette  24  containing sheets S, a feeding roller  8  configured to feed a sheet S, and a conveying roller pair  16  configured to convey the fed sheet S. The feeding cassette  24  can be pulled out toward the front of the main body of the image forming apparatus  100  in  FIG. 2 . The sheets S contained in the feeding cassette  24  are separated and conveyed one by one by a separating pad  9  while being pressed into contact with the feeding roller  8  (frictional piece separation technique). 
     The sheet S conveyed from the feeding device  13  is further conveyed by the registration roller pair  17  to the secondary transfer unit  15 , where a positive bias is applied to secondary transfer rollers  18 . This causes the toner image of four colors on the intermediate transfer belt  5  to be secondary-transferred onto the conveyed sheet S. 
     A fixing unit  14  is configured to fix the toner image on the sheet S by applying heat and pressure thereto. A fixing belt  14   a  has a cylindrical shape, and is guided by a belt guide member (not shown) having a heating unit, such as a heater, bonded thereto. The fixing belt  14   a  and a pressure roller  14   b  form a fixing nip with a predetermined contact pressure. 
     The sheet S conveyed from the image forming unit and having an unfixed toner image thereon is subjected to heat and pressure at the fixing nip between the fixing belt  14   a  and the pressure roller  14   b . Thus, the unfixed toner image on the sheet S is fixed to the sheet S. Then, the sheet S having the fixed toner image thereon is discharged by a discharge roller pair  19  to a discharge tray  20 . 
     Toner remaining on the surface of each photosensitive drum  1  after toner image transfer is removed by the cleaning member  6 . The removed toner is collected in a removed toner chamber  22   a  (see  FIG. 1 ) in a photosensitive unit  26 . 
     Toner remaining on the intermediate transfer belt  5  after secondary transfer onto the sheet S is removed by a transfer belt cleaning device  23  (see  FIG. 2 ). The removed toner passes through a waste toner conveying path (not shown) and is collected in a waste toner collection container (not shown) disposed at the back of the apparatus. 
     (Configuration of Cartridge) 
     The cartridge  7  to which the present invention is applied will be described with reference to  FIG. 1 .  FIG. 1  is a principal cross-sectional view of the cartridge  7  containing toner T according to the first embodiment. Note that  FIG. 2  illustrates an exemplary image forming apparatus in which four cartridges  7  containing respective toners of yellow, magenta, cyan, and black are arranged side by side. A cartridge  7   a  containing yellow toner, a cartridge  7   b  containing magenta toner, a cartridge  7   c  containing cyan toner, and a cartridge  7   d  containing black toner have the same configuration. 
     Each cartridge  7  is formed by the photosensitive unit  26  including the photosensitive drum  1 , and the developing device  4  including a developer bearing member (hereinafter referred to as a developing roller)  21 . Each component will now be described. 
     The developing device  4  includes the developing roller  21  that rotates in the direction of arrow B while being in contact with the photosensitive drum  1 , a developing frame  28 , a lid member  50 , and a mandible-like member  51 . The developing roller  21  is rotatably supported by the developing frame  28 , with developing side plates  29 R and  29 L attached to both sides of the developing frame  28  interposed therebetween. A developing chamber  28   b  includes a developer supply member (hereinafter referred to as a toner supply roller)  30  rotating in the direction of arrow C while being in contact with the developing roller  21 , and a developing blade  31  for regulating the thickness of a toner layer on the developing roller  21 . A container  28   a  disposed below the developing chamber  28   b  includes a developer conveying belt  60  configured to stir the toner T in the container  28   a  and serve as a supply unit for supplying toner to the developing chamber  28   b.    
     The photosensitive drum  1  is rotatably attached to a cleaning frame  22 , with a bearing (not shown) interposed therebetween. The charging roller  2  and the cleaning member  6  are disposed on the periphery of the photosensitive drum  1 . Residual toner removed from the surface of the photosensitive drum  1  by the cleaning member drops into the removed toner chamber  22   a . The photosensitive drum  1  is driven to rotate in accordance with the image forming operation, by the driving force of a driving motor (not shown) transmitted to the photosensitive unit  26 . 
     The developing device  4  is coupled to the photosensitive unit  26  such that it can swing about a coupling shaft  33  engaged in holes in the plates  29 R and  29 L. During image formation in the cartridge  7 , the developing device  4  is biased by a developing-device pressure spring  34  and pivots about the coupling shaft  33  in the direction of arrow D. Thus, the developing roller develops an electrostatic latent image on the photosensitive drum  1  while being in contact with the photosensitive drum  1 . During non-image formation, the developing device  4  is biased by a developing-device biasing member (not shown) in the direction opposite the direction of pressure applied by the developing-device pressure spring  34 . This causes the developing roller  21  to be separated from the photosensitive drum  1 . The developing roller  21  is thus separated from the photosensitive drum  1  during non-image formation. This is to prevent the surface of the developing roller  21  having elasticity from being deformed at the contact nip between the photosensitive drum  1  and the developing roller  21 . During the separation, the developing device  4  is not driven, and no sliding stress is applied to the developing roller  21 , the toner supply roller  30 , and the toner T in the developing device  4 . 
     (Detailed Configuration of Developing Device) 
     The developer conveying belt  60  serves as a toner conveying unit in the present embodiment. 
     As illustrated in  FIG. 1 , a driving roller  61  and driven rollers  62  and  69  are inserted into the inner space of the developer conveying belt  60  of endless type. The driving roller  61  is disposed to a side of the developing chamber  28   b  in the horizontal direction, whereas the driven rollers  62  and  69  are disposed below the driving roller  61  and in the lower part of the container  28   a  in the vertical direction. The driven rollers  62  and  69  are disposed at both ends of the container  28   a  in the horizontal direction. The driven roller  69  is disposed to coincide with the photosensitive drum  1  in the vertical direction. 
     The driving roller  61  and the driven rollers  62  and  69  have a plurality of lightening portions  61   d ,  62   d , and  69   d  to prevent an increase in torque during toner conveyance. The lightening portions  61   d ,  62   d , and  69   d  are formed in both axial and radial directions. 
     The developer conveying belt  60  is stretched over the driving roller  61  and the two driven rollers  62  and  69  in a substantially triangular shape. The developer conveying belt  60  is in contact with a contact portion  28   c  of the developing frame  28  below the driving roller  61  in the vertical direction. The contact portion  28   c  is located below a supply opening  28   e  in the vertical direction. The developer conveying belt  60  is deflected on one side of the triangle by being partly in contact with the contact portion  28   c . Although the developer conveying belt  60  is stretched over the three rollers in a substantially triangular shape in the present embodiment, the developer conveying belt  60  may be stretched in a rectangular, pentagonal, or other shape by adding one or more rollers depending on the shape of the container  28   a.    
     The developer conveying belt  60  has a plurality of fin portions  60   a  integral therewith for conveying and stirring toner. The description of the fin portions  60   a  will be omitted here, as the more detailed description will be given later on. For example, a resin sheet made of polyethylene terephthalate (PET) resin, polycarbonate (PC) resin, or polyphenylene sulfide (PPS) resin may be used to form the developer conveying belt  60 . 
       FIGS. 3 and 4  are detailed perspective views of the developer conveying belt  60 . The fin portions  60   a  are arranged in the axial direction of the driving roller  61  and driven rollers  62  and  69  and in the conveying direction of the developer conveying belt  60 . Specifically, the developer conveying belt  60  has 18 to 20 rows of fin portions  60   a  in the conveying direction, with each row having 5 or 6 fin portions  60   a  in the axial direction. The fin portions  60   a  in adjacent rows are arranged not to overlap each other. The fin portions  60   a  are rectangular in shape. This is to maximize the amount of toner accumulated on the fin portions  60   a.    
     The fin portions  60   a  each have an opening  60   b , as they are formed by pressing a resin sheet. The openings  60   b  are slightly greater in size than the fin portions  60   a . The fin portions  60   a  are bent for conveying toner. 
     With the configuration described above, the driving torque of the developer conveying belt  60  can be made smaller than that in the case where each row has about three or four fin portions  60   a  in the axial direction. If each row has ten or more fin portions  60   a  in the axial direction, the stiffness of the developer conveying belt  60  is lowered due to the resulting increase in the space of the openings  60   b . It is thus desirable that the number of fin portions  60   a  in each row be about five to ten in the axial direction. 
     In the present embodiment, the developer conveying belt  60  of endless shape is formed by pressing a single resin sheet into a predetermined shape and welding the end portions of the resin sheet to each other. Alternatively, the developer conveying belt  60  may be formed by a plurality of resin sheets in the axial direction. 
     A toner conveying unit for supplying toner from the container  28   a  to the developing chamber  28   b  will be described in detail with reference to  FIG. 1 . 
     As illustrated in  FIG. 1 , the developing device  4  has the developing chamber  28   b  and the container  28   a . The developing chamber  28   b  contains the developing roller  21 , the toner supply roller  30 , and the developing blade  31 . The container  28   a  is disposed below the developing chamber  28   b  and contains the toner T to be supplied to the developing chamber  28   b . The developing chamber  28   b  and the container  28   a  have the supply opening  28   e  therebetween for supplying toner from the container  28   a  to the developing chamber  28   b.    
     In the container  28   a , the developer conveying belt  60  is supported by the driving roller  61  and the driven rollers  62  and  69  such that it can run in the direction of arrow P. 
     As the developer conveying belt  60  runs, the fin portions  60   a  convey toner in the container  28   a  to the developing chamber  28   b  while allowing their end portions to slide and rub against the inner wall of the developing frame  28  and the lid member  50 . The toner conveyed by the fin portions  60   a  moves upward while being in contact with the lid member  50 . The fin portions  60   a  can convey a large amount of toner because they are set to be inclined toward the upstream side in the belt running direction (direction of arrow P). This state of the fin portions  60   a  is maintained while the fin portions  60   a  are in contact with the lid member  50 , that is, during movement of the fin portions  60   a  from the vicinity of the driven roller  62  to the vicinity of the driving roller  61 . Therefore, toner held between adjacent fin portions  60   a  at the lower end of the container  28   a  is conveyed to the upper end of the container  28   a  as the developer conveying belt  60  runs. 
     When the toner reaches the upper end of the container  28   a , the contact between the lid member  50  and the fin portions  60   a  is released. As the developer conveying belt  60  starts to move downward in the vertical direction, the toner retained between adjacent fin portions  60   a  is delivered to the developing chamber  28   b . When the amount of toner in the developing chamber  28   b  exceeds the capacity of the developing chamber  28   b , the toner is returned back through the supply opening  28   e  to the container  28   a . The openings  60   b  in the developer conveying belt  60  allow the toner to drop therethrough. 
     As described above, the driving roller  61  and the driven rollers  62  and  69  over which the developer conveying belt  60  is stretched have the lightening portions  61   d ,  62   d , and  69   d . Thus, toner caught between the developer conveying belt  60  and the driving roller  61  and driven rollers  62  and  69  can be temporarily dropped into the lightening portions  61   d ,  62   d , and  69   d , and then dropped through the openings  60   b  into the container  28   a . This is possible because, as illustrated in  FIGS. 3 and 4 , the openings  60   b  of the developer conveying belt  60  coincide with the corresponding lightening portions  61   d ,  62   d , and  69   d  of the rollers  61 ,  62 , and  69  in the longitudinal direction (axial direction). It is thus possible to reduce an increase in frictional resistance of each roller caused by toner caught between the developer conveying belt  60  and the rollers  61 ,  62 , and  69 . 
     As described above, the developer conveying belt is in contact with the contact portion  28   c  of the developing frame  28  below the driving roller  61  in the vertical direction. Thus, toner conveyed by the fin portions  60   a  can be efficiently delivered to the developing chamber  28   b . Without the contact portion  28   c , the conveyed toner may be partly brought back to the container  28   a . On the other hand, with the contact portion  28   c , the fin portions  60   a  are folded by the contact portion  28   c  and this facilitates delivery of toner to the developing chamber  28   b.    
     As described above, the present embodiment can provide a developing device and a process cartridge capable of stably supplying toner to the developing chamber  28   b.    
     Second Embodiment 
     A color electrophotographic image forming apparatus (hereinafter referred to as an image forming apparatus) and a process cartridge (hereinafter referred to as a cartridge) according to a second embodiment will now be described. 
     A general configuration of the image forming apparatus and a configuration of the cartridge are the same as those in the first embodiment, except for the detailed configuration of the developing device. Therefore, components having the same functions as those in the first embodiment are denoted by the same reference numerals as in  FIGS. 5 and 6 , and their description will be omitted.  FIG. 5  is a principal cross-sectional view of the cartridge  7  containing toner T according to the second embodiment.  FIG. 6  is a principal cross-sectional view of the image forming apparatus  100  according to the second embodiment. 
     (Detailed Configuration of Developing Device) 
     A detailed configuration of a developing device according to the present embodiment will now be described. In the present embodiment, the developing device  4  uses the developer conveying belt  60 , a toner conveying sheet member  80  in the developing chamber  28   b , and a toner drop sheet member  90  in the container  28   a  as a toner conveying unit. The configuration of the toner conveying sheet member  80 , toner drop sheet member  90 , and developer conveying belt  60  of the developing device  4  according to the present embodiment will be described in order of assembly, with reference to  FIGS. 7, 8, and 9A and 9B .  FIGS. 7, 8, and 9A and 9B  illustrate assembly of the developing device  4 . For ease of explanation, only parts related to the present invention will be described and the description of other parts will be omitted. 
       FIG. 7  is an assembly diagram illustrating how the toner conveying sheet member  80  and the mandible-like member  51  are assembled to the developing frame  28 . As illustrated in  FIG. 7 , the toner conveying sheet member  80  has holes  80   a   1 ,  80   a   2 , and  80   a   3 , into which bosses  28   k   1 ,  28   k   2 , and  28   k   3 , respectively, of the developing frame  28  are inserted. Then, the bosses  28   k   1 ,  28   k   2 , and  28   k   3  are thermally caulked at their ends to retain the toner conveying sheet member  80  (see  FIG. 5 ). After assembly of the toner conveying sheet member  80 , faces  51   a   1  and  51   a   2  of the mandible-like member  51  are welded or bonded to faces  28 L 1  and  28 L 2 , respectively, of the developing frame  28 , so that the mandible-like member  51  combines with the developing frame  28  to form a single unit. 
       FIG. 8  is an assembly diagram illustrating how the toner drop sheet member  90  is assembled in the developing frame  28 . As illustrated in  FIG. 8 , the toner drop sheet member  90  has holes  90   a   1 ,  90   a   2 , and  90   a   3 , into which bosses  28   m   1 ,  28   m   2 , and  28   m   3 , respectively, of the developing frame  28  are inserted. Then, the bosses  28   m   1 ,  28   m   2 , and  28   m   3  are thermally caulked at their ends to retain the toner drop sheet member  90  (see  FIG. 5 ). For example, a resin sheet made of polyethylene terephthalate (PET) resin, polycarbonate (PC) resin, or polyphenylene sulfide (PPS) resin may be used to form the toner conveying sheet member  80  and the toner drop sheet member  90 . 
       FIGS. 9A and 9B  are assembly diagrams illustrating how a driving roller and a driven roller are assembled to a developer conveying belt.  FIGS. 10A and 10B  are assembly diagrams illustrating how a toner conveying unit formed, for example, by the developer conveying belt is assembled in a container.  FIG. 11A  is a cross-sectional view illustrating a loosened state of the developer conveying belt.  FIG. 11B  is a cross-sectional view illustrating a stretched state of the developer conveying belt.  FIGS. 12A and 12B  are perspective views illustrating a securing unit for securing the driven roller. 
     First, as illustrated in  FIG. 9A , the driving roller  61  and the driven roller  62  are inserted into the inner space of the developer conveying belt  60  of endless type. The developer conveying belt  60  has the fin portions  60   a  integral therewith for conveying and stirring toner (see  FIG. 5 ). The fin portions  60   a  are not shown in FIGS.  9 A and  9 B to  FIGS. 11A and 11B . The description of the fin portions  60   a  will be omitted here, as the more detailed description will be given later on. For example, a resin sheet made of polyethylene terephthalate (PET) resin, polycarbonate (PC) resin, or polyphenylene sulfide (PPS) resin may be used to form the developer conveying belt  60 . The driving roller  61  has sprockets  61   a  at both ends thereof. The driving force of the driving roller  61  is transmitted to the developer conveying belt  60  by engaging the sprockets  61   a  in holes  60   c  at both ends of the developer conveying belt  60 . As illustrated in  FIG. 9B , the driven roller  62  is rotatably supported by a support member  66 , whose shaft portion  66   a  is inserted in the driven roller  62 . By securing a shaft end portion  66   c  to a protrusion  66   a   1 , the movement of the driven roller  62  in the longitudinal direction is regulated by shaft end portions  66   b  and  66   c.    
     The assembly of the driving roller  61  will now be described. As illustrated in  FIG. 10A , one end of the driving roller  61  in the longitudinal direction is rotatably supported by a driving roller shaft  63 , which passes through a shaft hole  50   c , from outside the lid member  50 . The coupling of the driving roller  61  and the driving roller shaft  63  is made by engaging a two-way shaft  63   a  of the driving roller shaft  63  in a two-way hole  61   b  at the one end of the driving roller  61 , and engaging a retaining nail  63   b  of the driving roller shaft  63  with a securing portion (not shown) for securing the driving roller  61 . A seal member  64  is provided between the driving roller shaft  63  and the shaft hole  50   c . The seal member  64  prevents toner from leaking through the shaft hole  50   c  of the lid member  50 . 
     As illustrated in  FIG. 10B , the driving roller  61  has an end shaft  61   c  at the other end thereof in the longitudinal direction. The end shaft  61   c  is fitted into a groove portion  50   d  of the lid member  50  and retained by a retaining portion  28   n   1  of the developing frame  28 , whereby the driving roller  61  is rotatably supported. A bottom face (not shown) of the groove portion  50   d  rotatably supporting the end shaft  61   c  is in the shape of a U-shaped or V-shaped bearing. The driving roller shaft  63  has a gear portion  63   c  integral therewith. The driving roller shaft  63  transmits, to the driving roller  61 , a driving force transmitted from a driving-force transmitting unit (not shown) having a gear train. 
     The assembly of the driven roller  62  will now be described. As illustrated in  FIG. 9B , the driven roller  62  is rotatably supported by the support member  66 . As illustrated in  FIG. 10A , one end of the support member  66  in the longitudinal direction is rotatably supported by a driven roller shaft  65 , which passes through a shaft hole  50   e , from outside the lid member  50 . The coupling of the support member  66  and the driven roller shaft  65  is made by engaging a two-way shaft  65   a  of the driven roller shaft  65  in a two-way hole  66   b   1  at the one end of the support member  66 , and engaging a retaining nail (not shown) of the driven roller shaft  65  with a securing portion (not shown) for securing the support member  66 . A seal member  64  is provided between the driven roller shaft  65  and the shaft hole  50   e . The seal member  64  prevents toner from leaking through the shaft hole  50   e  of the lid member  50 . 
     As illustrated in  FIG. 10B , the other end of the support member  66  in the longitudinal direction is rotatably supported by an end shaft  66   c   1  in the shaft end portion  66   c  of the support member  66 . The end shaft  66   c   1  is fitted into a groove portion  50   f  of the lid member  50  and retained by a retaining portion  28   n   2  of the developing frame  28 , whereby the support member  66  is rotatably supported. A bottom face (not shown) of the groove portion  50   f  rotatably supporting the end shaft  66   c   1  is in the shape of a U-shaped or V-shaped bearing. 
     As illustrated in  FIG. 9A , an axial line F of the shaft portion  66   a  (see  FIG. 9B ) of the support member  66  supporting the driven roller  62  is displaced from an axial line G connecting the center of the two-way hole  66   b   1  in the shaft end portion  66   b  of the support member  66  and the center of the end shaft  66   c   1  (see  FIG. 10B ) in the shaft end portion  66   c  of the support member  66 . Thus, rotating the driven roller shaft  65  (see  FIG. 10A ) allows the driven roller  62  to swing about the axial line G. 
     A further description will be given with reference to the cross-sectional views of  FIGS. 11A and 11B .  FIG. 11A  is a cross-sectional view illustrating a loosened state of the developer conveying belt  60 .  FIG. 11B  is a cross-sectional view illustrating a stretched state of the developer conveying belt  60 . When the driven roller  62  is rotated in the direction of arrow H, the driven roller  62  swings to stretch the developer conveying belt  60  between the driven roller  62  and the driving roller  61 . The lid member  50  is provided with ribs  50   h . This is to prevent the sprockets  61   a  at both ends of the driving roller  61  from being disengaged from the holes  60   c  at both ends of the developer conveying belt  60  when the developer conveying belt  60  is loosened. The ribs  50   h  are disposed near the sprockets  61   a  at both ends of the driving roller  61 , and longitudinally positioned to avoid interference with the fin portions  60   a  and the openings  60   b  of the developer conveying belt  60 . 
       FIGS. 12A and 12B  are perspective views illustrating a configuration of a securing unit for securing the driven roller.  FIG. 12A  is a perspective view illustrating a state where the developer conveying belt  60  is biased in a loosened state.  FIG. 12B  is a perspective view illustrating a state where the developer conveying belt  60  is biased in a stretched state. The developing frame  28  and the lid member  50  illustrated in  FIGS. 12A and 12B  are joined together by welding or the like. A tension spring  67  serving as a biasing unit is stretched between a boss  65   d  of the driven roller shaft  65  and a boss  28   p  of the developing frame  28 . Thus, when the developer conveying belt  60  is in a loosened state, the driven roller  62  is biased toward an inner surface  50   j  (see  FIG. 11A ) of the lid member  50 . When the driven roller shaft  65  is rotated in the direction of arrow H, the developer conveying belt  60  is stretched by the biasing force of the tension spring  67  upon passage of the boss  65   d  of the driven roller shaft  65  over an axial line I which connects the center of the boss  28   p  of the developing frame  28  and the rotation center of the driven roller shaft  65 . This configuration allows the developer conveying belt  60  to be reliably stretched, because variation in circumference caused by the tolerance of the developer conveying belt  60  can be accommodated. 
     A drive transmission unit for the developer conveying belt  60  will now be described with reference to  FIG. 13 .  FIG. 13  is a lateral view of the developing device  4  for illustrating the drive transmission unit for the developer conveying belt  60 . A driven gear  71  at one end of the developing device  4  in the longitudinal direction engages with a driving gear (not shown) of the image forming apparatus  100 , thereby transmitting the driving force of the driving motor (not shown) in the direction of arrow J. Through an idler gear  72  engaging with the driven gear  71 , the driving force is transmitted to a developing roller gear  21   a  and a toner supply roller gear  30   a  in the directions of arrows B and C, respectively. Through idler gears  73  and  74  engaging with the driven gear  71 , the driving force is transmitted to the gear portion  63   c  of the driving roller shaft  63  in the direction of arrow O. The drive is thus transmitted to the developing roller  21 , the toner supply roller  30 , and the developer conveying belt  60 . 
     A toner conveying unit for supplying toner from the container  28   a  to the developing chamber  28   b  will now be described in detail with reference to  FIGS. 14 to 16 . 
       FIG. 14  illustrates a state immediately before the fin portions  60   a  of the developer conveying belt  60  are brought into contact with the toner conveying sheet member  80  and the toner drop sheet member  90 .  FIG. 15  illustrates a state where the fin portions  60   a  of the developer conveying belt  60  are in contact with the toner conveying sheet member  80  and the toner drop sheet member  90 .  FIG. 16  illustrates a state where the fin portions  60   a  of the developer conveying belt  60  are spaced from (or out of contact with) the toner conveying sheet member  80  and the toner drop sheet member  90 . 
     As illustrated in  FIG. 14 , the developing device  4  has the developing chamber  28   b  and the container  28   a . The developing chamber  28   b  contains the developing roller  21 , the toner supply roller  30 , and the developing blade  31 . The container  28   a  is disposed below the developing chamber  28   b  and contains the toner T to be supplied to the developing chamber  28   b . The lid member  50  attached to the developing frame  28  is provided with the developer conveying belt  60  having the fin portions  60   a . The supply opening  28   e  for supplying toner from the container  28   a  to the developing chamber  28   b  and a discharge opening  28   f  for returning toner from the developing chamber  28   b  to the container  28   a  are provided between the developing chamber  28   b  and the container  28   a . The discharge opening  28   f  is disposed below the supply opening  28   e  in the vertical direction. A supply-opening end face  28   e   1  on the lower side of the supply opening  28   e  is disposed above a nip E between the toner supply roller  30  and the toner conveying sheet member  80 . More specifically, the toner conveying sheet member  80  is disposed below the toner supply roller  30 , and the toner conveying sheet member  80  is attached to an inclined surface (inclined portion)  28   q  extending at an angle from the supply opening  28   e  (supply opening side) toward the discharge opening  28   f  (discharge opening side). An end portion  80   b , which is one end of the toner conveying sheet member  80 , is set to protrude from the supply-opening end face  28   e   1  on the lower side of the supply opening  28   e . The other end of the toner conveying sheet member  80  protrudes from an end face  28   r  of the inclined surface  28   q , extends beyond the nip E between the toner supply roller  30  and the toner conveying sheet member  80 , and has a bent portion  80   c  above the discharge opening  28   f . As described with reference to  FIG. 7 , the toner conveying sheet member is retained by inserting the bosses  28   k  of the developing frame  28  into the holes  80   a  and thermally caulking the end portions of the bosses  28   k . Thus, the toner conveying sheet member  80  is disposed to be movable between the thermally caulked portions of the bosses  28   k  and the inclined surface  28   q  of the developing frame  28 . The end portion  80   b  of the toner conveying sheet member  80  is disposed to be in contact with the fin portions  60   a  of the developer conveying belt  60 . 
     As described above, the toner drop sheet member  90  is disposed in the container  28   a . The toner drop sheet member  90  is attached to a bottom surface (inclined portion)  28   s  located at the bottom of the container  28   a  and inclined in the direction from the discharge opening  28   f  (discharge opening side) toward the supply opening  28   e  (supply opening side). As described with reference to  FIG. 8 , the toner drop sheet member  90  has the holes  90   a , into which the bosses  28   m  of the developing frame  28  are inserted. The toner drop sheet member  90  is retained by thermally caulking the end portions of the bosses  28   k . Thus, the toner drop sheet member  90  is disposed to be movable between the thermally caulked portions of the bosses  28   m  and the bottom surface  28   s  of the developing frame  28 . An end portion  90   b , which is a lower end of the toner drop sheet member  90  in the direction of inclination, protrudes from an end portion  28   s   1  of the bottom surface  28   s  of the developing frame  28 . The end portion  90   b  of the toner drop sheet member  90  is disposed to be in contact with the fin portions  60   a  of the developer conveying belt  60 . Thus, when the developer conveying belt  60  runs, the fin portions  60   a  are brought into and out of contact with the end portion  90   b  of the toner drop sheet member  90 . 
     In the container  28   a , the developer conveying belt  60  is supported by the driving roller  61  and the driven roller  62  such that it can run in the direction of arrow P. The developer conveying belt  60  has the fin portions  60   a  extending substantially horizontally in the longitudinal direction of the developing device  4 . A width u of the fin portions  60   a  on the short side is set to be longer than a width v defined by the developer conveying belt  60  and the inner surface  50   j  (see  FIG. 11A ) of the lid member  50 . 
     The fin portions  60   a  are set to be inclined with respect to the belt running direction or the direction of arrow P (as in the state of the fin portion  60   a   1 ). When the developer conveying belt  60  moves from this state, the fin portions  60   a  are brought into contact with the toner drop sheet member  90  in the vicinity of the driven roller  62  at the lower end of the container  28   a , and inclined in the direction opposite the direction of travel (as in the state of the fin portion  60   a   2 ). 
     When the developer conveying belt  60  moves further, the fin portions  60   a  make a transition from the state of being in contact with the toner drop sheet member  90  to the state of being in contact with the lid member  50  while maintaining the inclination in the direction opposite the direction of travel (as in the state of the fin portion  60   a   3 ). 
     This state of the fin portions  60   a  is maintained while the fin portions  60   a  are in contact with the lid member  50 , that is, during movement of the fin portions  60   a  from the vicinity of the driven roller  62  to the vicinity of the driving roller  61 . Thus, toner held between adjacent fin portions  60   a  at the lower end of the container  28   a  is conveyed to the upper end of the container  28   a  as the developer conveying belt  60  runs. 
     When the toner is conveyed to the upper end of the container  28   a , the contact between the fin portions  60   a  and the lid member  50  is released. Then, the inclination of the fin portions  60   a  maintained in the direction opposite the direction of travel of the developer conveying belt  60  is released and returned to the inclination in the direction of travel (as in the state of the fin portion  60   a   1 ). By the elastic force generated at the release of the inclination, toner on the fin portions  60   a  (in the state of the fin portion  60   a   4 ) is delivered onto the toner conveying sheet member  80 . 
     The angle of inclination of the fin portions  60   a  in a natural state where the fin portions  60   a  are not in contact with the developing frame  28  (i.e., inclination of the fin portion  60   a   1 ) is set to be greater than the angle of repose of the developer. Therefore, after the contact between the fin portions  60   a  and the lid member  50  is released, toner on the fin portions  60   a  is supplied to the developing chamber  28   b  without remaining on the fin portions  60   a.    
       FIG. 15  illustrates a state reached by moving the developer conveying belt  60  of  FIG. 14  in the direction of arrow P. The fin portions  60   a  of the developer conveying belt  60  (in the state of fin portion  60   a   5 ) are brought into contact with the end portion  80   b  of the toner conveying sheet member  80  to cause the toner conveying sheet member  80  to warp. In the container  28   a , the fin portions  60   a  of the developer conveying belt  60  (in the state of the fin portion  60   a   6 ) are brought into contact with the end portion  90   b  of the toner drop sheet member  90  to cause the toner drop sheet member  90  to warp. 
       FIG. 16  illustrates a state reached by moving the developer conveying belt  60  of  FIG. 15  in the direction of arrow P. The fin portions  60   a  of the developer conveying belt  60  are spaced from (or out of contact with) the end portion  80   b  of the toner conveying sheet member  80 . The toner conveying sheet member  80  has elasticity. By being released from the warped state, the toner conveying sheet member  80  vibrates by elastic force and conveys toner thereon toward the toner supply roller  30 . The toner accumulated on the upstream side of the nip E is applied to the toner supply roller  30  at the nip E and further supplied to the developing roller  21 . 
     By vibration transmitted through the toner conveying sheet member  80 , toner conveyed from the developer conveying belt  60  onto the toner conveying sheet member  80  is reliably supplied to the toner supply roller  30 . Since the angle of inclination of the inclined surface  28   q  can be set to be smaller than the angle of repose of the toner, the size of the developing chamber  28   b  in the height direction can be reduced. 
     When the toner conveying sheet member  80  vibrates, the nip E between the toner supply roller  30  and the toner conveying sheet member  80  is disengaged to create a small gap w. This allows excess toner to pass through the gap w to reach the downstream side of the disengaged nip E. Then, the toner is accumulated between the disengaged nip E and the bent portion  80   c  of the toner conveying sheet member  80 . If the amount of toner on the upstream side of the disengaged nip E is insufficient due to the accumulation at the bent portion  80   c , the toner accumulated at the bent portion  80   c  is applied to the toner supply roller  30 . If a large amount of toner is accumulated between the disengaged nip E and the bent portion  80   c  of the toner conveying sheet member  80 , the toner is returned through the discharge opening  28   f  into the container  28   a.    
     The excess toner described above is toner that remains after continuous printing of low-coverage-rate images, because only a small amount of toner is developed by the developing roller  21 . The excess toner does not circulate and accumulates on the downstream side of the disengaged nip E between the toner supply roller  30  and the toner conveying sheet member  80 . 
     The fin portions  60   a  of the developer conveying belt  60  in the container  28   a  are spaced from (or out of contact with) the end portion  90   b  of the toner drop sheet member  90 . By being released from the warped state, the toner drop sheet member  90  vibrates by elastic force and conveys toner thereon toward the developer conveying belt  60 . 
     As described above, toner in the container  28   a  is conveyed while being retained in the spaces created between the fin portions  60   a  of the developer conveying belt  60  and the developing frame  28  or lid member  50 . Thus, the toner is brought up from the container  28   a  to the developing chamber  28   b  in small amounts in the spaces described above, and is delivered to the toner conveying sheet member  80 . The toner conveying sheet member  80  vibrates by contact with the fin portions  60   a  of the developer conveying belt  60 , and conveys the toner to the nip E between the toner conveying sheet member  80  and the toner supply roller  30 . The toner accumulated on the upstream side of the nip E is applied to the toner supply roller  30  at the nip E and further supplied to the developing roller  21 . 
     Since the nip E is disengaged by vibration of the toner conveying sheet member  80 , excess toner is returned through the discharge opening  28   f  to the container  28   a . That is, only the amount required for development is supplied to the developing roller  21  and excess toner is collected in the container  28   a . In other words, toner circulates between the container  28   a  and the developing chamber  28   b.    
     In the present embodiment, as described above, when the amount of toner in the developing chamber exceeds a predetermined amount, vibration of the toner conveying sheet member creates a gap between the toner conveying sheet member and the toner supply roller, thereby allowing excess toner to be returned through the discharge opening to the container. The amount of toner in the developing chamber can thus be kept below a certain amount, and toner can be prevented from packing in the developing chamber. 
     In the present embodiment, it is possible to prevent toner packing in the developing chamber, and to stably supply toner to the toner supply roller. It is thus possible to provide a developing device and a process cartridge that can achieve more stable solid image density and can supply high-quality images. 
     Third Embodiment 
     A color electrophotographic image forming apparatus (hereinafter referred to as an image forming apparatus) and a process cartridge (hereinafter referred to as a cartridge) according to a third embodiment will be described. 
     A general configuration of the image forming apparatus and a configuration of the cartridge are the same as those in the above-described embodiments, except for the detailed configuration of the developing device. Therefore, components having the same functions as those in the above-described embodiments are denoted by the same reference numerals as in  FIGS. 17 and 18 , and their description will be omitted.  FIG. 17  is a principal cross-sectional view of the cartridge  7  containing toner T according to the third embodiment.  FIG. 18  is a principal cross-sectional view of the image forming apparatus  100  according to the third embodiment. 
     (Detailed Configuration of Developing Device) 
     A supply unit for supplying toner from the container  28   a  to the developing chamber  28   b  in the developing device  4  will now be described in detail with reference to  FIGS. 19A and 19B . 
     As described above, the developing device  4  is formed by the developing frame  28  forming the developing chamber  28   b  and the container  28   a , and the lid member  50 . The developing chamber  28   b  contains the developing roller  21 , the toner supply roller  30 , and the developing blade  31 . The container  28   a  is disposed below the developing chamber  28   b . The container  28   a  contains the toner T to be supplied to the developing chamber  28   b , and the developer conveying belt  60 . A first partition  28   h  and a second partition  28   d  are provided between the developing chamber  28   b  and the container  28   a . The first partition  28   h  is provided with the supply opening  28   e , and the second partition  28   d  is provided with the discharge opening  28   f . The supply-opening end face  28   e   1  on the lower side of the supply opening  28   e  is disposed above the lowermost point of the toner supply roller  30  in the vertical direction (i.e., above the nip E between the toner supply roller  30  and the toner conveying sheet member  80 ). The developer conveying belt  60  is provided in the container  28   a  as a supply unit that supplies toner T to the developing chamber  28   b . The developer conveying belt  60  is driven to run in the direction of arrow P by a driving unit (not shown). 
     A configuration of the developer conveying belt will now be described in order of assembly, with reference to  FIGS. 20A and 20B ,  FIGS. 21A and 21B ,  FIGS. 9A and 9B , and  FIGS. 12A and 12B , which are drawings illustrating the assembly of the developing device  4 . For ease of explanation, only parts related to the present embodiment will be described and the description of other parts will be omitted.  FIGS. 9A and 9B  are assembly diagrams illustrating how a driving roller and a driven roller are assembled to a developer conveying belt.  FIGS. 20A and 20B  are assembly diagrams illustrating how a toner conveying unit formed by the developer conveying belt and the like is assembled in a container.  FIGS. 21A and 21B  are cross-sectional views illustrating a loosened state and a stretched state, respectively, of the developer conveying belt.  FIGS. 12A and 12B  are perspective views illustrating a securing unit for securing the driven roller. 
     First, as illustrated in  FIG. 9A , the driving roller  61  and the driven roller  62  are inserted into the inner space of the developer conveying belt  60  of endless type. The developer conveying belt  60  has a plurality of openings for passage of toner and a plurality of fin portions integral therewith for conveying and stirring toner. The description of the openings and the fin portions will be omitted here, as the more detailed description will be given later on. For example, a resin sheet made of polyethylene terephthalate (PET) resin, polycarbonate (PC) resin, or polyphenylene sulfide (PPS) resin may be used to form the developer conveying belt  60 . The driving roller  61  has the sprockets  61   a  at both ends thereof. The driving force of the driving roller  61  is transmitted to the developer conveying belt  60  by engaging the sprockets  61   a  in the holes  60   c  at both ends of the developer conveying belt  60 . As illustrated in  FIG. 9B , the driven roller  62  is rotatably supported by the support member  66 , whose shaft portion  66   a  is inserted in the driven roller  62 . By securing the shaft end portion  66   c  to the protrusion  66   a   1 , the movement of the driven roller  62  in the longitudinal direction is regulated by the shaft end portions  66   b  and  66   c.    
     The assembly of the driving roller  61  will now be described. As illustrated in  FIG. 20A , one end of the driving roller  61  in the longitudinal direction is rotatably supported by the driving roller shaft  63 , which passes through the shaft hole  50   c , from outside the lid member  50 . The coupling of the driving roller  61  and the driving roller shaft  63  is made by engaging the two-way shaft  63   a  of the driving roller shaft  63  in the two-way hole  61   b  at the one end of the driving roller  61 , and engaging the retaining nail  63   b  of the driving roller shaft  63  with the securing portion (not shown) for securing the driving roller  61 . The seal member  64  is provided between the driving roller shaft  63  and the shaft hole  50   c . The seal member  64  prevents toner from leaking through the shaft hole  50   c  of the lid member  50 . 
     As illustrated in  FIG. 20B , the driving roller  61  has the end shaft  61   c  at the other end thereof in the longitudinal direction. The end shaft  61   c  is fitted into the groove portion  50   d  of the lid member  50  and retained by the retaining portion  28   n   1  of the developing frame  28 , whereby the driving roller  61  is rotatably supported. The bottom face (not shown) of the groove portion  50   d  rotatably supporting the end shaft  61   c  is in the shape of a U-shaped or V-shaped bearing. The driving roller shaft  63  has the gear portion  63   c  integral therewith. The driving roller shaft  63  transmits, to the driving roller  61 , a driving force transmitted from the driving-force transmitting unit (not shown) having a gear train. 
     The assembly of the driven roller  62  will now be described. As illustrated in  FIG. 9B , the driven roller  62  is rotatably supported by the support member  66 . As illustrated in  FIG. 20A , one end of the support member  66  in the longitudinal direction is rotatably supported by the driven roller shaft  65 , which passes through the shaft hole  50   e , from outside the lid member  50 . The coupling of the support member  66  and the driven roller shaft  65  is made by engaging the two-way shaft  65   a  of the driven roller shaft  65  in the two-way hole  66   b   1  at the one end of the support member  66 , and engaging the retaining nail (not shown) of the driven roller shaft  65  with the securing portion (not shown) for securing the support member  66 . The seal member  64  is provided between the driven roller shaft  65  and the shaft hole  50   e . The seal member  64  prevents toner from leaking through the shaft hole  50   e  of the lid member  50 . 
     As illustrated in  FIG. 20B , the other end of the support member  66  in the longitudinal direction is rotatably supported by the end shaft  66   c   1  in the shaft end portion  66   c  of the support member  66 . The end shaft  66   c   1  is fitted into the groove portion  50   f  of the lid member  50  and retained by the retaining portion  28   n   2  of the developing frame  28 , whereby the support member  66  is rotatably supported. The bottom face (not shown) of the groove portion  50   f  rotatably supporting the end shaft  66   c   1  is in the shape of a U-shaped or V-shaped bearing. 
     As illustrated in  FIG. 9A , the axial line F of the shaft portion  66   a  (see  FIG. 9B ) of the support member  66  supporting the driven roller  62  is displaced from the axial line G connecting the center of the two-way hole  66   b   1  in the shaft end portion  66   b  of the support member  66  and the center of the end shaft  66   c   1  (see  FIG. 20B ) in the shaft end portion  66   c  of the support member  66 . Thus, rotating the driven roller shaft  65  (see  FIG. 20A ) allows the driven roller  62  to swing about the axial line G. 
     A further description will be given with reference to the cross-sectional views of  FIGS. 21A and 21B .  FIG. 21A  is a cross-sectional view illustrating a loosened state of the developer conveying belt  60 .  FIG. 21B  is a cross-sectional view illustrating a stretched state of the developer conveying belt  60 . When the driven roller  62  is rotated in the direction of arrow H, the driven roller  62  swings to stretch the developer conveying belt  60  between the driven roller  62  and the driving roller  61 . The lid member  50  is internally provided with the ribs  50   h . This is to prevent the sprockets  61   a  (see  FIG. 9A ) at both ends of the driving roller  61  from being disengaged from the holes  60   c  at both ends of the developer conveying belt  60  when the developer conveying belt  60  is loosened. The ribs  50   h  are disposed near the sprockets  61   a  at both ends of the driving roller  61 . 
       FIGS. 12A and 12B  are perspective views illustrating a configuration of a securing unit for securing the driven roller.  FIG. 12A  is a perspective view illustrating a state where the developer conveying belt  60  is biased in a loosened state.  FIG. 12B  is a perspective view illustrating a state where the developer conveying belt  60  is biased in a stretched state. The developing frame  28  and the lid member  50  illustrated in  FIGS. 12A and 12B  are joined together by welding or the like. The tension spring  67  serving as a biasing unit is stretched between the boss  65   d  of the driven roller shaft  65  and the boss  28   p  of the developing frame  28 . Thus, when the developer conveying belt  60  is in a loosened state, the driven roller  62  is biased toward the inner surface  50   j  (see  FIG. 21A ) of the lid member  50 . When the driven roller shaft  65  is rotated in the direction of arrow H, the developer conveying belt  60  is stretched by the biasing force of the tension spring  67  upon passage of the boss  65   d  of the driven roller shaft  65  over the axial line I which connects the center of the boss  28   p  of the developing frame  28  and the rotation center of the driven roller shaft  65 . This configuration allows the developer conveying belt  60  to be reliably stretched, because variation in circumference caused by the tolerance of the developer conveying belt  60  can be accommodated. 
     A toner conveying configuration of the developing device  4  will now be described in detail with reference to  FIGS. 19A and 19B . 
     As illustrated in  FIG. 19A , in the container  28   a , the developer conveying belt  60  is supported by the driving roller  61  and the driven roller  62  such that it can run in the direction of arrow P. The developer conveying belt  60  has the fin portions  60   a  extending substantially horizontally in the longitudinal direction of the developing device  4 . The width u of the fin portions  60   a  on the short side is set to be longer than the width v defined by the developer conveying belt  60  and the inner surface  50   j  of the lid member  50 . 
     The fin portions  60   a  are set to be inclined with respect to the belt running direction or the direction of arrow P (as in the state of the fin portion  60   a   1 ). When the developer conveying belt  60  moves from this state, the fin portions  60   a  are brought into contact with the toner drop sheet member  90  in the vicinity of the driven roller  62  at the lower end of the container  28   a , and inclined in the direction opposite the direction of travel (as in the state of the fin portion  60   a   2 ). 
     When the developer conveying belt  60  moves further, the fin portions  60   a  make a transition from the state of being in contact with the toner drop sheet member  90  to the state of being in contact with the lid member  50  while maintaining the inclination in the direction opposite the direction of travel (as in the state of the fin portion  60   a   3 ). 
     This state of the fin portions  60   a  is maintained while the fin portions  60   a  are in contact with the lid member  50 , that is, during movement of the fin portions  60   a  from the vicinity of the driven roller  62  to the vicinity of the driving roller  61 . Thus, toner held between adjacent fin portions  60   a  at the lower end of the container  28   a  is conveyed to the upper end of the container  28   a  as the developer conveying belt  60  runs. 
     When the contact between the fin portions  60   a  and the lid member  50  is released, the toner at the upper end of the container  28   a  is further conveyed by its own weight through the supply opening  28   e  to the developing chamber  28   b . Then, the inclination of the fin portions  60   a  maintained in the direction opposite the direction of travel of the developer conveying belt  60  is released and returned to the inclination in the direction of travel (as in the state of the fin portion  60   a   1 ). By the elastic force generated at the release of the inclination, toner on the fin portions  60   a  (in the state of the fin portion  60   a   4 ) is delivered onto the toner conveying sheet member  80 , and this facilitates movement of toner toward the developing chamber  28   b . The toner conveyed to the developing chamber  28   b  is supplied to the toner supply roller  30  and then sequentially supplied to the developing roller  21 . 
     The angle of inclination of the fin portions  60   a  in a natural state where the fin portions  60   a  are not in contact with the developing frame  28  (i.e., inclination of the fin portion  60   a   1 ) is set to be greater than the angle of repose of the developer. Therefore, after the contact between the fin portions  60   a  and the lid member  50  is released, toner on the fin portions  60   a  is supplied to the developing chamber  28   b  without remaining on the fin portions  60   a.    
     The toner conveying sheet member  80  is attached to the inclined surface  28   q  forming the developing chamber  28   b . The toner conveying sheet member  80  is formed by an elastic sheet and retained by inserting the bosses  28   k  of the inclined surface  28   q  into the holes on the upper side of the sheet. The toner conveying sheet member  80  interferes with the fin portions  60   a  as the developer conveying belt  60  runs, and has an elastic force smaller than that of the developer conveying belt  60 . Therefore, when the developer conveying belt  60  interferes with the toner conveying sheet member  80 , the toner conveying sheet member  80  is warped by its own elasticity as illustrated in  FIG. 19B . When the developer conveying belt  60  further runs, the interference between the toner conveying sheet member  80  and the developer conveying belt  60  is released, and the toner conveying sheet member  80  is returned to the state of  FIG. 19A  by its own elastic force. By vibration transmitted through the toner conveying sheet member  80 , toner conveyed from the developer conveying belt  60  onto the toner conveying sheet member  80  is reliably supplied to the toner supply roller  30 . Since the angle of inclination of the inclined surface  28   q  can be set to be smaller than the angle of repose of the toner, the size of the developing chamber  28   b  in the height direction can be reduced. Then, the toner is sequentially supplied from the toner supply roller  30  to the developing roller  21 . An amount of toner greater than that required for the toner supply roller  30  and the developing roller  21  is collected through the discharge opening  28   f  in the container  28   a.    
     As described above, toner in the container  28   a  is conveyed by the developer conveying belt  60  while being retained in small amounts between adjacent fin portions  60   a , and is then supplied to the developing chamber  28   b . After only the amount required for development is supplied to the developing roller  21 , excess toner is collected in the container  28   a . In other words, toner circulates between the container  28   a  and the developing chamber  28   b.    
     (Residual Quantity Detecting Configuration) 
     A detecting unit for detecting the amount of toner in the container  28   a  will now be described with reference to  FIG. 17 ,  FIG. 22 ,  FIGS. 23A and 23B , and  FIGS. 24A and 24B .  FIG. 22  is a perspective view of the developing device  4  as viewed from the lid member  50 .  FIGS. 23A and 23B  are perspective views of a configuration of the developer conveying belt  60  and a residual quantity detector, as viewed from the same side as  FIG. 22 . For convenience of illustration, other components are not shown in  FIGS. 23A and 23B .  FIGS. 24A and 24B  are cross-sectional views of the developing device  4  taken along line XXIV-XXIV in  FIG. 22 . 
     First, the residual quantity detector will be described. As illustrated in  FIG. 22 , a light emitting element  82  and a light receiving element  81  are attached to the lid member  50  of the developing device  4 . As illustrated in  FIGS. 24A and 24B , the light emitting element  82  passes through the lid member  50 , and is disposed with a light emitting portion  82   a  thereof exposed to the interior of a recessed portion  50   k  of the lid member  50 . The light receiving element  81  is similarly disposed on the lid member  50 . As illustrated in  FIGS. 23A and 23B , the light receiving element  81  is configured such that a light receiving portion  81   a  thereof faces the light emitting portion  82   a . A series of detecting operations first involves receiving light from a detecting unit (not shown) of the image forming apparatus  100  at the light emitting element  82 , receiving the light at the light receiving element  81  through the recessed portion  50   k  of the lid member  50 , and then receiving the light at the detecting unit of the image forming apparatus  100 . 
     As illustrated in  FIG. 23A , in the present embodiment, the developer conveying belt  60  has at least one V-shaped fin portion  60   d  different from the fin portions  60   a . The V-shaped fin portion  60   d  has a hole  60   e  in the center thereof in the longitudinal direction (axial direction). During movement of the developer conveying belt  60 , toner in the container  28   a  is retained in the state indicated by oblique lines in  FIGS. 23A and 24A . In this state, for example, if the toner surface is at T 2  in  FIG. 24A , the toner is discharged through the hole  60   e  into an inner space  68  of the developer conveying belt  60  as the developer conveying belt  60  moves (see  FIG. 24B ). Thus, the amount of toner retained by the V-shaped fin portion  60   d  (i.e., the amount of toner indicated by oblique lines in  FIG. 23A ) is reduced. Therefore, as illustrated in  FIG. 23B , until the hole  60   e  reaches the vicinity of the light emitting portion  82   a  and the light receiving portion  81   a  serving as the residual quantity detector, toner is absent and light can be received by the light receiving portion  81   a.    
     On the other hand, when the toner surface in the container  28   a  is at T 3  in  FIG. 24A , no toner is discharged through the hole  60   e  until the hole  60   e  reaches the vicinity of the light emitting portion  82   a  and the light receiving portion  81   a  serving as the residual quantity detector. Therefore, the amount of toner indicated by oblique lines in  FIGS. 23A and 24A  is kept unchanged, and the recessed portion  50   k  of the lid member  50  having the residual quantity detector therein is filled with toner. This means that the light receiving portion  81   a  cannot receive light from the light emitting portion  82   a.    
     As described above, in accordance with the amount of toner in the container  28   a , the amount of toner retained by the V-shaped fin portion  60   d  varies, and the amount of empty space having no toner therein when passing through the residual quantity detector also varies. This varies the length of time during which the recessed portion  50   k  of the lid member  50  having the residual quantity detector therein is not filled with toner. Therefore, by detecting a change in the duration of passage of light, the residual quantity detector can detect a change in the amount of residual toner in the container  28   a.    
     The developer conveying belt  60  has at least one V-shaped fin portion  60   d , which has surfaces inclined with respect to the developer conveying direction (i.e., direction of travel P of the developer conveying belt  60 ) and the longitudinal direction of the developer conveying belt  60 . Therefore, toner retained by the V-shaped fin portion  60   d  is reliably collected in the hole  60   e  in the center, with the help of its own weight. In the present embodiment, the hole  60   e  is located in the center of the V-shaped fin portion  60   d  in the longitudinal direction. However, as long as the V-shaped fin portion  60   d  has surfaces inclined toward the hole  60   e , the same effect as above can be achieved regardless of the location of the hole  60   e  in the longitudinal direction. 
     The developing device  4  is configured such that when the hole  60   e  passes the residual quantity detector, that is, after the timing of residual quantity detection, the openings  60   b  (see  FIG. 17 ) appropriately provided in the developer conveying belt  60  are during passage through the container  28   a . Thus, with the openings  60   b , the toner surface in the container  28   a  becomes level with the toner surface in the inner space  68  of the developer conveying belt  60 . 
     The V-shaped fin portion  60   d  has a plurality of slits  60   f  (see  FIG. 23B ). This allows the V-shaped fin portion  60   d  to smoothly operate even when passing through the vicinities of the driving roller  61  and the driven roller  62  at the top and bottom ends, respectively, of the developer conveying belt  60 . Although the V-shaped fin portion  60   d  has a plurality of slits  60   f  in the present embodiment, the configuration is not limited to this, as long as at least one slit is provided. 
     As described above, the present embodiment uses the developer conveying belt  60  in the developing device  4  in which the container  28   a  is disposed below the developing chamber  28   b . This allows supply of toner in small amounts and reduces scattering of toner. Also, by detecting toner retained in the space between the developer conveying belt  60  and the inner wall of the lid member  50 , it is possible to reduce entry of scattered toner into the space between the light emitting portion  82   a  and the light receiving portion  81   a , and detect the amount of residual toner with high accuracy. 
     Fourth Embodiment 
     A color electrophotographic image forming apparatus (hereinafter referred to as an image forming apparatus) and a process cartridge (hereinafter referred to as a cartridge) according to a fourth embodiment will be described. 
     A general configuration of the image forming apparatus and a configuration of the cartridge are the same as those in the above-described embodiments, except for the detailed configuration of the developing device. Therefore, components having the same functions as those in the above-described embodiments are denoted by the same reference numerals as in  FIGS. 25 and 26 , and their description will be omitted.  FIG. 25  is a principal cross-sectional view of the cartridge  7  containing toner T according to the fourth embodiment.  FIG. 26  is a principal cross-sectional view of the image forming apparatus  100  according to the fourth embodiment.  FIGS. 27A and 27B  are principal cross-sectional views illustrating an area around the developing device  4  of the cartridge  7  according to the fourth embodiment. 
     As illustrated in  FIG. 27A , in the developing device  4 , the toner supply roller  30  is disposed to form a nip N, which is a predetermined contact portion, at the location where the toner supply roller  30  and the developing roller  21  face each other. As illustrated, the toner supply roller  30  rotates in the direction of arrow C. Here, the toner supply roller  30  is an elastic sponge roller produced by forming a foam layer on a conductive cored bar. 
     On the other hand, the developing roller  21  is a rubber roller produced by forming a rubber layer harder than the sponge layer of the toner supply roller  30  on a conductive cored bar. Thus, as illustrated in  FIG. 27A , the toner supply roller  30  and the developing roller  21  are in contact with each other, with the toner supply roller  30  being depressed by the developing roller  21  by a certain amount ΔE. At the nip N, the developing roller  21  and the toner supply roller  30  rotate in relatively opposite directions (i.e., in the directions of arrows B and C in  FIGS. 27A and 27B ). Thus, the toner supply roller  30  not only supplies toner to the developing roller  21 , but also strips off the toner remaining on the developing roller  21  after development. The toner supply roller  30  is disposed with respect to the developing roller  21  such that the downstream end of the nip (contact portion) N in the rotation direction of the toner supply roller  30  is located above the upstream end of the nip N. 
     (Detailed Configuration of Developing Device) 
     A supply unit for supplying toner from the container  28   a  to the developing chamber  28   b  will now be described in detail with reference to  FIG. 25  and  FIGS. 27A and 27B . 
     Toner can be supplied to the toner supply roller  30  most efficiently by conveying it to a location (toner accumulated portion  95 ) immediately after the nip N between the developing roller  21  and the toner supply roller  30  on the downstream side in the rotation direction of the toner supply roller  30 . In the toner supply roller  30  including an elastic layer having a plurality of cells on the outer periphery thereof, the elastic layer coming out of the nip N is released from pressure and this allows the cells to be opened immediately after the nip N. Then, because of the resulting suction of air, toner is absorbed into the elastic portion of the toner supply roller  30 . Therefore, to allow toner to be efficiently retained by the toner supply roller  30 , it is desirable to convey the toner directly to the absorbing portion. Note that the elastic portion of the toner supply roller  30  is the sponge layer (surface layer) of the toner supply roller  30 , and the plurality of cells are bubbles in the sponge layer. 
     Efficient supply of toner to the nip N between the toner supply roller  30  and the developing roller  21  will now be described in detail. The present embodiment uses the developer conveying belt  60  to supply toner in small amounts for prevention of excessive toner supply, and also to bring up toner from the container  28   a  in the lower part of the developing device  4  to the developing chamber  28   b  in the upper part of the developing device  4 . 
     As described above, the developing device  4  has the developing chamber  28   b  on the upper side and the container  28   a  on the lower side. The developing chamber  28   b  contains the developing roller  21 , the toner supply roller  30 , and the developing blade  31 . The container  28   a  contains the toner T to be supplied to the developing chamber  28   b , and the developer conveying belt  60 . The developing chamber  28   b  and the container  28   a  are formed by the developing frame  28 , the mandible-like member (second developing frame)  51 , and the lid member  50 . A partition  28   j  and the discharge opening  28   f  are provided between the developing chamber  28   b  and the container  28   a . The container  28   a  contains the developer conveying belt  60  serving as a supply unit that supplies the toner T to the developing chamber  28   b . The developer conveying belt  60  is driven to move in the direction of arrow P by a driving unit (not shown) in the image forming apparatus  100 . 
     A configuration of the developer conveying belt  60  will be described in detail in order of assembly of the developing device  4 , with reference to  FIGS. 28A and 28B  to  FIGS. 33A and 33B , which are drawings illustrating the assembly of the developing device  4 . For ease of explanation, only parts related to the present embodiment will be described and the description of other parts will be omitted.  FIGS. 28A and 28B  are assembly diagrams illustrating how a driving roller and a driven roller are assembled to a developer conveying belt.  FIGS. 29A and 29B  are assembly diagrams illustrating how a toner conveying unit formed by the developer conveying belt and the like is assembled in a container.  FIG. 30  is an assembly diagram illustrating how a driven roller is assembled to a developing frame.  FIGS. 31A and 31B  are assembly diagrams illustrating how the developer conveying belt is assembled to a developing device.  FIGS. 32A and 32B  are cross-sectional views illustrating a loosened state and a stretched state, respectively, of the developer conveying belt.  FIGS. 33A and 33B  are perspective views illustrating a securing unit for securing the driven roller. 
     First, as illustrated in  FIG. 28A , the driving roller  61  and the driven roller  62  are inserted into the inner space of the developer conveying belt  60  of endless type. The developer conveying belt  60  has fin portions (protruding portions) integral therewith for conveying and stirring toner. The description and illustration of the fin portions will be omitted here, as the more detailed description will be given later on. For example, a resin sheet made of polyethylene terephthalate (PET) resin, polycarbonate (PC) resin, or polyphenylene sulfide (PPS) resin may be used to form the developer conveying belt  60 . The driving roller  61  has the sprockets  61   a  at both ends thereof. The driving force of the driving roller  61  is transmitted to the developer conveying belt  60  by engaging the sprockets  61   a  in the holes  60   c  at both ends of the developer conveying belt  60 . 
     As illustrated in  FIG. 28B , the driven roller  62  is rotatably supported by a support member  66 , whose shaft portion  66   a  is inserted in the driven roller  62 . By securing the shaft end portion  66   c  to the protrusion  66   a   1 , the movement of the driven roller  62  in the longitudinal direction is regulated by shaft end portions  66   b  and  66   c.    
     The assembly of the driving roller  61  will now be described. As illustrated in  FIG. 29A , one end of the driving roller  61  in the longitudinal direction is rotatably supported by the driving roller shaft  63 , which passes through the shaft hole  50   c , from outside the lid member  50 . The coupling of the driving roller  61  and the driving roller shaft  63  is made by engaging the two-way shaft  63   a  of the driving roller shaft  63  in the two-way hole  61   b  at the one end of the driving roller  61 , and engaging the retaining nail  63   b  of the driving roller shaft  63  with the securing portion (not shown) for securing the driving roller  61 . The seal member  64  is provided between the driving roller shaft  63  and the shaft hole  50   c . The seal member  64  prevents toner from leaking through the shaft hole  50   c  of the lid member  50 . 
     As illustrated in  FIG. 29B , the driving roller  61  has the end shaft  61   c  at the other end thereof in the longitudinal direction. The end shaft  61   c  is fitted into the groove portion  50   d  of the lid member  50  and retained by the retaining portion  28   n   1  of the developing frame  28 , whereby the driving roller  61  is rotatably supported. The bottom face (not shown) of the groove portion  50   d  rotatably supporting the end shaft  61   c  is in the shape of a U-shaped or V-shaped bearing. The driving roller shaft  63  has the gear portion  63   c  (see  FIG. 29A ) integral therewith. The driving roller shaft  63  transmits, to the driving roller  61 , a driving force transmitted from the driving-force transmitting unit (not shown) having a gear train. 
     The assembly of the driven roller  62  will now be described. As illustrated in  FIG. 28B , the driven roller  62  is rotatably supported by the support member  66 . As illustrated in  FIG. 29A , one end of the support member  66  in the longitudinal direction is rotatably supported by the driven roller shaft  65 , which passes through the shaft hole  50   e , from outside the lid member  50 . The coupling of the support member  66  and the driven roller shaft  65  is made by engaging the two-way shaft  65   a  of the driven roller shaft  65  in the two-way hole  66   b   1  at the one end of the support member  66 , and engaging the retaining nail (not shown) of the driven roller shaft  65  with the securing portion (not shown) for securing the support member  66 . The seal member  64  is provided between the driven roller shaft  65  and the shaft hole  50   e . The seal member  64  prevents toner from leaking through the shaft hole  50   e  of the lid member  50 . 
     As illustrated in  FIG. 29B , the other end of the support member  66  in the longitudinal direction is rotatably supported by the end shaft  66   c   1  in the shaft end portion  66   c  of the support member  66 . The end shaft  66   c   1  is fitted into the groove portion  50   f  of the lid member  50  and retained by the retaining portion  28   n   2  of the developing frame  28 , whereby the support member  66  is rotatably supported. The bottom face of the groove portion  50   f  rotatably supporting the end shaft  66   c   1  is in the shape of a U-shaped or V-shaped bearing. 
     As illustrated in  FIGS. 28A and 28B , the axial line F of the shaft portion  66   a  of the support member  66  supporting the driven roller  62  is displaced from the axial line G connecting the center of the two-way hole  66   b   1  in the shaft end portion  66   b  of the support member  66  and the center of the end shaft  66   c   1  (see  FIG. 29B ) in the shaft end portion  66   c  of the support member  66 . Thus, rotating the driven roller shaft  65  (see  FIG. 29A ) allows the driven roller  62  to swing about the axial line G. 
     A second driven roller  75  for stretching the developer conveying belt  60  is assembled to the developing frame  28  illustrated in  FIGS. 29A and 29B . This will be described with reference to  FIG. 30 . The developing frame  28  has recessed portions  28   t  and  28   u  at both ends thereof in the longitudinal direction. The second driven roller  75  has shaft portions  75   a  and  75   b  at both ends thereof. The second driven roller  75  is rotatably supported by the developing frame  28  by inserting the shaft portions  75   a  and  75   b  into the recessed portions  28   t  and  28   u , respectively. 
     The developing frame  28  and the lid member  50  assembled as described with reference to  FIGS. 29A and 29B  are joined together, for example, by welding (see  FIG. 31A ). In this state, a third driven roller  76  is inserted into the inside of one end portion  60   g  of the developer conveying belt  60 . Additionally, shaft portions  76   a  and  76   b  at both ends of the third driven roller  76  are inserted into recessed portions  28   v  and  28   w , respectively, of the developing frame  28 . Then, as illustrated in  FIG. 31B , the mandible-like member  51  is attached to the developing frame  28  in the direction of arrow to prevent the third driven roller  76  from falling out of the recessed portions  28   v  and  28   w.    
     A further description will be given with reference to the cross-sectional views of  FIGS. 32A and 32B .  FIG. 32A  is a cross-sectional view illustrating a loosened state of the developer conveying belt  60 .  FIG. 32B  is a cross-sectional view illustrating a stretched state of the developer conveying belt  60 . When the driven roller  62  is rotated in the direction of arrow H, the driven roller  62  swings to stretch the developer conveying belt  60  between the driven roller  62  and the driving roller  61 . The lid member  50  is provided with the ribs  50   h  at its bottom. This is to prevent the sprockets  61   a  (see  FIG. 28A ) at both ends of the driving roller  61  from being disengaged from the holes  60   c  at both ends of the developer conveying belt  60  when the developer conveying belt  60  is loosened. The ribs  50   h  are disposed near the sprockets  61   a  at both ends of the driving roller  61 . 
       FIGS. 33A and 33B  are perspective views illustrating a configuration of a securing unit that secures the driven roller  62  for keeping the developer conveying belt  60  in a loosened state (see  FIG. 32A ) and in a stretched state (see  FIG. 32B ).  FIG. 33A  is a perspective view illustrating a state where the developer conveying belt  60  is biased in a loosened state.  FIG. 33B  is a perspective view illustrating a state where the developer conveying belt  60  is biased in a stretched state. The tension spring  67  serving as a biasing unit is stretched between the boss  65   d  of the driven roller shaft  65  and the boss  28   p  of the developing frame  28 . Thus, when the developer conveying belt  60  is in a loosened state, the driven roller  62  is biased toward the inner surface  50   j  (see  FIG. 32A ) of the lid member  50 . When the driven roller shaft  65  is rotated in the direction of arrow H, the developer conveying belt  60  is stretched by the biasing force of the tension spring  67  upon passage of the boss  65   d  of the driven roller shaft  65  over the axial line I which connects the center of the boss  28   p  of the developing frame  28  and the rotation center of the driven roller shaft  65 . This configuration allows the developer conveying belt  60  to be reliably stretched, because variation in circumference caused by the tolerance of the developer conveying belt  60  can be accommodated. 
     A toner conveying configuration of the developing device  4  will now be described in detail with reference to  FIG. 25  and  FIGS. 27A and 27B . As illustrated in  FIG. 25 , the developer conveying belt  60  is disposed in the container  28   a  and supported by the driving roller  61 , the driven roller  62 , the second driven roller  75 , and the third driven roller  76  such that it can run in the direction of arrow P. The developer conveying belt  60  extends upward from the lower end of the developing device  4  (at which the developer conveying belt  60  is supported by the driven roller  62 ) to the upper end of the developing device  4  (at which the developer conveying belt  60  is supported by the driving roller  61 ) in a substantially vertical direction. At the same time, the developer conveying belt  60  is bent, at the upper end of the developing device  4 , at a substantially right angle toward the developing chamber  28   b.    
     One end portion of the developer conveying belt  60  is located near the lower end of the container  28   a , and the other end portion of the developer conveying belt  60  is located in the upper part of the developing chamber  28   b  and above the nip (contact portion) N described above (see  FIGS. 27A and 27B ). 
     The developer conveying belt  60  has the fin portions  60   a  extending substantially horizontally in the longitudinal direction of the developing device  4 . The fin portions  60   a  are formed to be substantially orthogonal to the belt running direction (direction of arrow P). When the developer conveying belt  60  moves, the fin portions  60   a  are brought close to the developing frame  28  in the vicinity of the driven roller  62 , which is near the lower end of the container  28   a . Then, toner is held in the space defined by the developer conveying belt  60 , adjacent fin portions  60   a , and the lid member  50 . This state is maintained while the fin portions  60   a  are being close to the lid member  50 , that is, during movement of the fin portions  60   a  from the vicinity of the driven roller  62  to the vicinity of the driving roller  61  along the inner surface  50   j  of the lid member  50 . Thus, the toner held between the adjacent fin portions  60   a  at the lower end of the container  28   a  is conveyed to the upper end of the container  28   a  as the developer conveying belt  60  moves. At the location of the driving roller  61 , the developer conveying belt  60  turns to the horizontal direction to convey the toner to the developing chamber  28   b.    
     As illustrated in  FIG. 25 , the toner conveyed to the vicinity of the developing chamber  28   b  is supplied to the developing chamber  28   b  at the third driven roller  76 . That is, since the fin portions  60   a  are moved clockwise in  FIG. 25  as the developer conveying belt  60  runs, the toner retained between adjacent fin portions  60   a  drops into the developing chamber  28   b  by its own weight. The developer conveying belt  60  is configured to make a 180-degree turn at the location of the third driven roller  76 . Therefore, the fin portions  60   a  each make a half turn from upward to downward in the vertical direction, so that toner between adjacent fin portions  60   a  can be reliably supplied to the nip N between the toner supply roller  30  and the developing roller  21 . 
     The third driven roller  76  is disposed above the nip N between the toner supply roller  30  and the developing roller  21 . The position of the third driven roller  76  is set such that each fin portion  60   a  is turned toward the nip N at the location where the angle of inclination of the fin portion  60   a  exceeds the angle of repose of the developer. That is, the third driven roller  76  is configured such that toner is reliably supplied to the nip N by being dropped by its own weight. As described above, since the fin portion  60   a  is eventually turned downward in the vertical direction, toner on the developer conveying belt  60  is supplied to the developing chamber  28   b  without remaining on the developer conveying belt  60 . 
     A third sheet member (separating member)  77  is attached to the partition  28   j  forming the developing chamber  28   b . The third sheet member  77  is formed by an elastic sheet. One end of the third sheet member  77  is secured to the partition  28   j , and the other end of the third sheet member  77  is in contact with the toner supply roller  30 . The third sheet member  77  is in contact with the toner supply roller  30  on the downstream side of the contact portion (nip N) between the toner supply roller  30  and the developing roller  21  in the rotation direction of the toner supply roller  30  (i.e., in the direction of arrow C). In the present embodiment, using the developer conveying belt  60  allows toner retained between adjacent fin portions  60   a  to be sequentially supplied in small amounts. 
     If low-coverage-rate images are continuously printed by the user, an excessive amount of toner may be accumulated at the contact portion (nip N) between the toner supply roller  30  and the developing roller  21 . When the amount of toner supply appropriate in the present configuration is the amount of toner (developer) indicated by shaded areas in  FIGS. 27A and 27B , toner exceeding this amount may be supplied, for example, as indicated by a two-dot chain line (excess toner portion  96 ) in  FIG. 27B . If, as in this case, the amount of toner supplied toward the nip N by the developer conveying belt  60  exceeds a predetermined amount, the third sheet member  77  separates the excess toner from the toner supply roller  30 . As the developer conveying belt  60  runs, the excess toner separated by and accumulated on the third sheet member  77  is scratched off, collected, and conveyed by the fin portions  60   a  after supply of toner, and then collected in the container  28   a . With this configuration, no toner exceeding a required amount is retained at the nip N between the toner supply roller  30  and the developing roller  21 . 
     After collecting the excess toner from the excess toner portion  96  (see  FIG. 27B ) and conveying the collected toner to the container  28   a , each fin portion  60   a  is turned to move downward in the vertical direction at the location of the second driven roller  75  (see  FIG. 25 ). The fin portion  60   a  overrides the second driven roller  75  as it passes through the second driven roller  75 . As described above, a resin sheet made of polyethylene terephthalate (PET) resin, polycarbonate (PC) resin, polyphenylene sulfide (PPS) resin, or the like is used to form the developer conveying belt  60 . Also, the developer conveying belt  60  is in a stretched state. Therefore, when the fin portion  60   a  comes into contact with and overrides the second driven roller  75 , the fin portion  60   a  falls backward in the direction of travel of the developer conveying belt  60  by the elasticity of the resin sheet, and passes over the second driven roller  75  as the developer conveying belt runs. In the present embodiment, to stabilize the stretched state maintained by the driven roller  62 , the second driven roller  75  has a cylindrical shape with a uniform diameter in the longitudinal direction (see  FIG. 30 ). Alternatively, the driven roller  62  may have a smaller diameter at portions overlapping the fin portions  60   a  in the longitudinal direction (axial direction) than at end portions thereof. This reduces warpage of the fin portions  60   a  that occurs when the developer conveying belt  60  passes over the second driven roller  75 , and thus allows smooth running of the developer conveying belt  60 . 
     Toner is sequentially supplied to the developing roller  21  and the photosensitive drum  1 . Toner dropped from the toner supply roller  30  or developing roller  21  into a second developing chamber  28   b   2  is collected through the discharge opening  28   f  in the container  28   a . It is thus possible to prevent occurrence of toner packing in the developing chamber. 
     In the present embodiment, an amount of toner required for development is supplied above the toner supply roller  30 . Therefore, unlike the case of fixing toner to the toner supply roller by pressure below the developing roller, the occurrence of toner packing can be prevented. 
     The fin portions  60   a  of the developer conveying belt  60  are arranged at intervals that can maintain a sufficient amount of toner supply during image formation. 
     As described above, toner in the container  28   a  is conveyed by the developer conveying belt  60  while being retained in small amounts between adjacent fin portions  60   a , and then is supplied to the developing chamber  28   b . After only the amount required for development is supplied to the developing roller  21 , excess toner is collected in the container  28   a . In other words, toner circulates between the container  28   a  and the developing chamber  28   b.    
     As described above, the present embodiment uses the developer conveying belt  60  in the developing device  4  in which the container  28   a  is disposed below the developing chamber  28   b . This allows toner to be supplied, in small amounts, above the toner supply roller  30  and the developing roller  21 . Also, an amount of toner sufficient for development can be retained at the nip N between the toner supply roller  30  and the developing roller  21 . 
     After conveying toner, the developer conveying belt collects excess toner at the nip and conveys it. It is thus possible to always supply only as much toner as required for development, and to prevent image quality from being negatively affected by toner packing in the developing chamber. Additionally, since excess toner is collected in the container by the developer conveying belt, toner can circulate between the developing chamber and the container. This can prevent an imbalance in the distribution of toner degradation in the developing device. 
     The present embodiment provides a developing device, a process cartridge, and an image forming apparatus capable of maintaining good toner circulation between a developing chamber and a container while supplying a sufficient amount of toner to the developing chamber, and thereby producing high-quality images. 
     The present invention can ensure stable supply of a developer to the developing chamber. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2015-151531 filed Jul. 31, 2015, which is hereby incorporated by reference herein in its entirety.