Abstract:
A developing apparatus includes a developing container for accommodating a developer including magnetic particles; a developer feeding member, provided in the developing container and having a magnetic shaft, for feeding the developer; a bearing rotatably supporting the shaft; a bearing accommodating portion; and a magnet, provided around the shaft at a position inside the bearing in the bearing accommodating portion, for forming a magnetic seal of magnetic particles by a magnetic field formed between itself and the shaft; wherein the bearing accommodating portion includes a first space between the magnet and the bearing and a second space which is provided outside of the bearing with respect to a diametrical direction and which is in fluid communication with the first space to receive the developer from the first space.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to a developing apparatus which develops an image with the use of developer having magnetic particles. In particular, it relates to the structure for a developing apparatus. More specifically, it relates to such a developing apparatus structure that forms a magnetic seal of magnetic particles, on the developing device housing side of a bearing which supports the shaft of a developer conveying member, in order to prevent developer from leaking out of the developing device housing. 
         [0002]    An image forming apparatus, such as a copying machine, a printer, a facsimile machine, a multi functional image forming apparatus capable of performing two or more functions of the preceding image forming apparatuses, which uses an electrophotographic or electrostatic image forming method, forms an electrostatic latent image on its image bearing member, such as a photosensitive drum, and develops the electrostatic latent image into a visible image by adhering developer to the latent image. It has been known that some developing apparatuses used for developing an electrostatic latent image use two-component developer (which hereafter will be referred to simply as developer) which is made up of nonmagnetic toner particles and magnetic carrier particles. 
         [0003]    Developing apparatuses such as the above described one charge toner by stirring the toner particles while it conveys the toner particle in the housing of their developing device. More specifically, the developer in the developing device housing is stirred while being conveyed by a developer conveying member, such as a screw which has a spiral blade. The lengthwise ends of the developer conveying member are supported by a pair of bearings, one for one. Thus, as the developer conveying member is rotationally driven, the bearings are heated by the friction between the developer conveying member and bearings. Thus, it is possible that as the developer travels to the bearing, it will be melted by the frictional heat. Thus, it has been known to provide a magnetic seal between the housing and bearing of a developing device (Japanese Laid-open Patent Application 2003-215919, Japanese Laid-open Patent Application 2003-162146, Japanese Laid-open Patent Application 2003-21968, and Japanese Laid-open Patent Application H11-52731). 
         [0004]    According to the abovementioned patent applications, magnets are disposed around the peripheral surface of the magnetic shaft. Thus, the magnetic particles of developer agglomerate in the form of a brush, between the shaft and magnets, forms thereby a magnetic seal. 
         [0005]    However, the magnetic seal is not in contact with the shaft nor magnets. Therefore, it sometimes occurs that the amount by which the developer in the developing device is made to slip by the magnetic seal, by the force by which the developer is conveyed, becomes substantial. As the developer slips by the magnetic seal by a substantial amount, the developer gradually accumulates in the space between the magnet and bearing. Eventually, the space is filled up with the developer. 
         [0006]    Referring to  FIG. 7 , a bearing holding portion  303 , in which a bearing  301  and a magnetic  302  are disposed, has a space  304  between the bearing  301  and magnetic  303 . Normally, this space  304  is surrounded by a cylindrical supporting surface  305  which supports the bearing  301  and magnet  303 . 
         [0007]    In other words, the space  304  between the bearing  301  and magnetic  302  in terms of the direction parallel to the axial line of the bearing holding portion  303  is surrounded by the cylindrical supporting surface  305 . Therefore, the space  304  is rather small. Therefore, it is likely to be filled up with the developer in an early stage of developing device usage. The bearing  301  is on the outward side of the space  304 , and is heated by the heat generated by the friction between the shaft of a developer conveying member, and bearing  301 . Thus, as the developer reaches the bearing  301 , it will be melted by the frictional heat of the bearing. Thus, it is possible that the image forming apparatus will be stopped while the developer is still melted. Thus, it is possible that the developing device will cool down while the melted developer is between the shaft of the developer conveying member, and bearing. As the developing device cools down, the melted developer between the shaft of the developer conveying member, and the bearing, solidifies between the shaft and bearing, welding the shaft and bearing to each other. Thus, it will be rather difficult for the developer conveying member to be rotated when the image forming apparatus is started up next time. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is made in consideration of the above-described issue. Thus, its primary object is to prevent the problem that the space between the bearing and magnet is filled up by the developer having slipped by the magnetic seal, in the early stage of developing device usage. 
         [0009]    According to an aspect of the present invention, there is provided a developing apparatus comprising a developing container for accommodating a developer including magnetic particles; a developer feeding member, provided in said developing container and having a magnetic shaft, for feeding the developer by rotation of the shaft; a bearing rotatably supporting said shaft; a bearing accommodating portion for accommodating said bearing; and a magnet, provided around said shaft at a position inside said bearing in said bearing accommodating portion, for forming a magnetic seal of magnetic particles by a magnetic field formed between itself and said shaft; wherein said bearing accommodating portion includes a first space between said magnet and said bearing and a second space which is provided outside of said bearing with respect to a diametrical direction and which is in fluid communication with said first space to receive the developer from said first space. 
         [0010]    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 
         [0011]      FIG. 1  is a schematic drawing of an image forming apparatus equipped with the developing device in the first embodiment of the present invention, and shows the general structure of the apparatus. 
           [0012]      FIG. 2  is a schematic sectional view of the developing device in the first embodiment, and shows the general structure of the device. 
           [0013]      FIG. 3  is a schematic sectional view of the developing device in the first embodiment, at a plane A-A in  FIG. 2 . 
           [0014]      FIG. 4  is an enlarged vertical sectional view of a part of the developing device in the first embodiment. 
           [0015]      FIG. 5(   a ) is a schematic sectional view of the bearing holding portion, and its adjacencies, of the developing device in the first embodiment, and  FIG. 5(   b ) is a schematic sectional view of the bearing holding portion, and its adjacencies, of the developing device in the first embodiment, at a plane B-B in  FIG. 5(   a ). 
           [0016]      FIG. 6  is a schematic cross-sectional view of the bearing holding portion, and its adjacencies, of the developing device in the second embodiment of the present invention. 
           [0017]      FIG. 7  is a schematic sectional view of the bearing holding portion of the comparative developing device. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       [0018]    Referring to  FIGS. 1-5 , the first embodiment of the present invention is described. First, referring to  FIG. 1 , the general structure of the image forming apparatus in this embodiment is described. The measurements, materials, and shapes of the structural components in the following embodiments of the present invention, and the positional relationship among the structural components, are not intended to limit the present invention in scope, unless specifically noted. Further, the image forming apparatuses in the following embodiments are described as an image forming apparatus which forms full-color images. It is needless to say, however, that the following embodiments are not intended to limit the present invention in scope in terms of the image forming apparatus with which the developing devices in the following embodiments are usable. 
       [Image Forming Apparatus] 
       [0019]    Referring to  FIG. 1 , the image forming apparatus  100  in this embodiment has image forming stations PY, PM, PC and PK which form yellow (Y), magenta (M), cyan (C) and black (K) images, respectively. These image forming stations PY, PM, PC and PK are aligned in tandem in the direction parallel to the rotational direction of an intermediary transfer belt  24 . They are roughly the same in structure. Therefore, in the following description of the image forming apparatus, the suffixes Y, M, C and K which indicate the color of images to be formed by the image forming stations PY, PM, PC and PK, respectively, are omitted. 
         [0020]    A photosensitive drum  110 , which is an image bearing member, is rotatably disposed. Its peripheral surface is uniformly charged by a primary charging device  21 . Then, an electrostatic latent image is formed on the uniformly charged portion of the peripheral surface of the photosensitive drum  110 , by the exposure of the uniformly charged portion of the peripheral surface of the photosensitive drum  110  to a beam of laser light, for example, emitted by an exposing device  22  while being modulated by information signals. Then, the electrostatic latent image is developed into a visible image (toner image) by a developing device  200 , on the peripheral surface of the photosensitive drum  110 . 
         [0021]    Next, the visible image (toner image) is transferred onto the intermediary transfer belt  24  by a primary transfer charging device  23 . During this process of transferring, toner images, different in color, are sequentially transferred in layers onto the intermediary transfer belt  24 , from the image forming stations PY, PM, PC and PK, one for one. Then, the layered toner images, different in color, on the intermediary transfer belt  24  are conveyed to a secondary transfer station  29 , in which they are transferred (secondary transfer) onto a sheet  27  of recording medium, such as ordinary paper (plain paper), OHP film delivered to the secondary transfer station  29  through a recording medium conveyance passage  28 . After the transfer of the toner images onto the sheet  27  of recording medium, the sheet  27  is sent to a fixing device  25 , by which the images are fixed to the sheet  27 . Meanwhile, the transfer residual toner, that is, the toner remaining on the peripheral surface of the photosensitive drum  110  after the primary transfer, is removed by a cleaning device  26 . As the toner in the developing device  200  is consumed for development, the developing device  200  is replenished with toner from a replenishment toner container. It is not only with toner, but also, carrier that the developing device  200  is replenished. 
       [Developing Device] 
       [0022]    Next, referring to  FIGS. 2 and 3 , the general structure of the developing device  200  in this embodiment is described. In this embodiment, two-component developer (which hereafter will be referred to simply as developer), which is a mixture of nonmagnetic toner particles and magnetic carrier particles, is used. Further, the developing device  200  in this embodiment is of the so-called vertical stir type. That is, it has two developer conveyance screws  205  and  206  as developer conveying members which convey developer while stirring the developer. The two developer conveyance screws  205  and  206  are horizontally disposed in the housing of the developing device  200 , in such a manner that when the image forming apparatus  100  is in the proper attitude for image formation, the developer conveyance screw  205  is above the developer conveyance screw  206  in terms of the gravity direction. 
         [0023]    The developing device  200  is provided with the housing  201  in which the developer is held. It is also provided with a development sleeve  208 , which faces the opening of the housing  201 , which faces the photosensitive drum  110 . Further, the developing device  200  is provided with a development chamber  213  as the first chamber, and a stirring chamber as the second chamber  214 . The developing device  200  is structured so that when the image forming apparatus  10  is in the proper attitude for image formation, the development chamber  213  and stirring chamber  214  are vertically stacked, that is, the stirring chamber  214  is on the bottom side of the development chamber  213 , in terms of the gravity direction. The two chambers  213  and  214  are on the opposite side of the development sleeve  208  from the abovementioned opening of the housing  201 , and are separated by a partition wall  217 . Further, the developing device  200  is provided with a developer circulation passages through which the developer is circulated between the development chamber  213  and stirring chamber  214 . 
         [0024]    The first developer conveyance screw  205 , as the first developer conveying member, is in the development chamber  213 , and the second developer conveyance screw  206 , as the second developer conveying member, is in the stirring chamber  214 . Each of the first and second developer conveyance screws (which hereafter will be referred to simply as conveyance screws) has a spiral blade, and conveys developer by being rotationally driven by a motor M as a driving force source. More specifically, the first conveyance screw  205  is in the bottom portion of the development chamber  213 , and extends roughly in parallel to the axial line of the development sleeve  208 . It conveys the developer in the development chamber  213  (first chamber) in the direction parallel to its axial line (rightward in  FIG. 3 ) by being rotated. As for the second conveyance screw  206 , it is in the bottom portion of the stirring chamber  214 , and extends roughly in parallel to the first conveyance screw  205 . It conveys the developer in the stirring chamber  214  (second chamber) in the opposite direction from the developer conveyance direction of the first conveyance screw  205 , (leftward in  FIG. 3 ). 
         [0025]    Further, the second conveyance screw  206  conveys the developer in the stirring chamber  208  is such a manner that as it conveys the developer in the stirring chamber  208  in the opposite direction from the developer conveyance direction of the first conveyance screw  205 , it pushes the developer up into the development chamber  213 , through a developer passage  211  which is at the left end ( FIG. 3 ) of the developing device  200 . More specifically, is, referring to  FIG. 4  which will be described later, the left end of the second conveyance screw  206  is provided with a return blade  206   b , which is angled so that as the developer in the stirring chamber  214  is conveyed to the left end of the stirring chamber  208  by the main portion of the second conveyance screw  206 , the return blade  206   b  pushes the developer backward. Thus, as the developer is conveyed to the left end of the stirring chamber  208  by the second conveyance screw  206 , it is pushed up into the development chamber  213  through the passage  211  as indicated by an arrow mark in  FIG. 4 . Meanwhile, as the developer in the development chamber  213  is conveyed to the right end ( FIG. 3 ) of the development chamber  213 , it falls into the stirring chamber  214  through the developer passage  212  which is between the development chamber  213  and stirring chamber  214 . As described above, the first and second conveyance screws  205  and  206  convey the developer in the development chamber  213  and stirring chamber  214 , respectively, and also, the passages  211  and  212 , respectively, creating thereby a circulatory passage for the developer as indicated by an arrow mark in  FIG. 3 . 
         [0026]    Further, the stirring chamber  214  is provided with an unshown replenishment toner inlet, which is on the upstream end of the stirring chamber  214  in terms of its developer conveyance direction. Thus, as the stirring chamber  214  is replenished with a fresh supply of toner by the replenishment toner container  20 , the fresh supply of toner is conveyed while being stirred along with the developer which was in the stirring chamber  214 . Thus, the developer in the stirring chamber  214  is made uniform in toner density. After being conveyed, while being stirred, through the stirring chamber  214 , the developer is conveyed into the development chamber  213 , in which it is borne by the development sleeve  203 , and is conveyed by the development sleeve  208 . More specifically, there is disposed a stationary magnet in the hollow of the development sleeve  208 . Thus, the developer is born on the peripheral surface of the development sleeve  208  by the magnetic field generated by the magnet. Then, as the development sleeve  208  rotates about the magnet, the developer on the peripheral surface of the development sleeve  208  is conveyed to the development area, in which the development sleeve  208  opposes the photosensitive drum  110 . Thus, the electrostatic latent image on the peripheral surface of the photosensitive drum  110  is developed. 
         [0027]    As described above, the developing device  200  of the vertical stir type, shown in  FIGS. 2 and 3 , is structured so that its development chamber  213  is on the top side of the stirring chamber  214  in terms of the gravity direction. Therefore, it is advantageous in that it is relatively small in the amount of space it occupies in terms of the horizontal direction. Thus, it is possible to reduce in size even a color image forming apparatus of the so-called tandem type, for example, in which multiple developing devices are horizontally aligned in tandem. Further, this developing device  200 , which is of the vertical stir type, has also the following advantages. That is, referring to  FIGS. 2 and 3 , the developer which was conveyed to the development area by being borne by the development sleeve  208 , but was not used for development, is not recovered into the development chamber  213  by the rotation of the development sleeve  208 . Instead, it is recovered into the stirring chamber  214 . Thus, there is always present in the development chamber  213 , the developer which has been fully stirred in the stirring chamber  214 . Therefore, the development sleeve  208  is supplied with only the developer which is uniform in toner density, being thereby prevented from developing an electrostatic latent image into a visible image (toner image) which is nonuniform in density in terms of the direction parallel to the axial line of the development sleeve  208 , and the nonuniformity of which is attributable to the insufficient stirring of the developer. That is, the developing device  200  makes it possible to obtain an image which is uniform in density. 
       [Bearing Holding Portion] 
       [0028]    Next, referring to  FIGS. 4 and 5 , a bearing holding portion  210 , with which one of the lengthwise ends of the developing device  200  is provided, is described. By the way,  FIG. 4  shows in detail only one (left one in  FIG. 3 ) of the lengthwise ends of the developing device  200 , with the omission of some portions.  FIG. 5  shows top half of the bearing holding portion  210  shown in  FIG. 4 . 
         [0029]    The first conveyance screw  205  has a shaft  205   a  formed of a magnetic substance. It conveys the developer by being rotated about the axial line of the shaft  205   a . Similarly, the second conveyance screw  206  has a shaft  206   a  formed of a magnetic substance. It conveys the developer by being rotated about the axial line of the shaft  206   a . The shafts  205   a  and  206   a  are rotatably supported by a pair of bearings  207   a  and a pair of bearings  207   b , respectively, which are fitted in a pair of bearing holding portions  210 , with which the lengthwise ends of the housing  201  of the developing device  200  is provided, one for one. 
         [0030]    Each bearing holding portion  210  has a pair of cylindrical supporting portions  210   a  and  210   b , and a cover  209 . The cylindrical supporting portions  210   a  and  210   b  are integral parts of the wall  201   a  of the lengthwise end of developing device housing  201 . The cover  209  is on the opposite side of developing device housing  201  from the pair of cylindrical supporting portions  210   a  and  210   b . The wall portion  201   a , cylindrical supporting portion  210   a , and cover  209  make up the bearing holding first portion  240 . The wall portion  201   a , cylindrical supporting portion  210   b , and cover  209  make up the bearing holding second portion  241 . 
         [0031]    The opening of the cylindrical supporting portion  210   a  of the bearing holding first portion  240 , which is on the developing device housing  201  side, is in connection to the development chamber  213 . It is through this opening that the shaft  205   a  of the first conveyance screw  205  in the development chamber  213  extends into the cylindrical supporting portion  210   a . Similarly, the opening of the cylindrical supporting portion  210   b  of the second bearing holding portion  241 , which is on the developing device housing  201  side, is in connection to the stirring chamber  214 . It is through this opening that the shaft  206   a  of the second conveyance screw  206  in the stirring chamber  214  extends into the cylindrical supporting portion  210   b.    
         [0032]    The shafts  205   a  and  206   a  extend through not only the cylindrical supporting portions  210   a  and  210   b , but also, the through holes  209   a  and  209   b  of the cover  209 , in such manner that one of their lengthwise ends is exposed from the bearing holding portion  210 . Thus, it is possible for driving force to be transmitted to the shafts  205   a  and  206   b  from the motor M ( FIG. 3 ) through a gear train or the like, as described above. Incidentally, in the case of the lengthwise end of the shaft  205   a  or  206   a , to which driving force is not transmitted from the motor M, it does not need to be extended beyond the bearing holding portion  210 . 
         [0033]    The bearings  207   a  and  207   b  are fitted in the bearing supporting portions  210   a  and  210   b  of the bearing holding portion  210 , being supported by the inward surfaces  230  and  231  of the cylindrical supporting portions  210   a  and  210   b , respectively. The shafts  205   a  and  206   a  are rotatably supported by the bearings  207   a  and  207   b , respectively. In the case of the cylindrical supporting portions  210   a  and  210   b , shown in the drawings, their inward surfaces  230  and  231  are provided with steps  230   a  and  231   a , respectively. Thus, as the bearings  207   a  and  207   b  are inserted into the cylindrical supporting portions  210   a  and  210   b , the inward ends of the peripheral portions of the bearings  207   a  and  207   b , come into contact with the steps  230   a  and  230   b , being thereby precisely positioned relative to the cylindrical supporting portion  210   a  and  210   b , in terms of the direction (leftward-rightward direction in  FIG. 4 ) parallel to the axial lines of the cylindrical supporting portions  210   a  and  210   b , respectively. As for the cover  209 , it is fixed to the wall portion  201   a  of the housing  201  with the use of multiple small screws. The left end of the bearing  207   a  ( 207   b ) is held by the cover  209 , by its outward side in terms of its radius direction, whereby the bearings  207   a  ( 207   a ) is prevented from becoming disengaged from the cylindrical supporting portion  210   a  ( 210   b ). 
         [0034]    Further, there are cylindrical magnets  202   a  and  202   b  in the cylindrical supporting portions  210   a  and  210   b , being supported by the inward surfaces  230  and  231  of the cylindrical supporting portions  210   a  and  210   b , respectively. The cylindrical magnets  202   a  and  202   b  are on the inward side (housing side) of the bearings  207   a  and  207   b  in terms of the direction parallel to the axial lines of the bearings  207   a  and  207   b . More specifically, the magnets  202   a  and  202   b  are fitted around the magnetic shafts  205   a  and  206   a , forming thereby a magnetic seal  204  of the carrier, which is made up of magnetic particles, by the magnetic field formed between the magnet  202   a  ( 202   b ) and magnetic shaft  205   a  ( 206   a ). The magnetic seal (magnetic brush)  204  seals the developer circulation passage by filling up the gap between the inward surface of the magnets  202   a  ( 202   b ) and shafts  205   a  ( 205   b ) by capturing the carrier particles, which are magnetic particles, with its magnetism. Thus, the developer is prevented from leaking out of the development chamber  213  and stirring chamber  214  of developing device housing  201 . 
         [0035]    In order to form the magnetic seal  204  which is uniform in properties in terms of the circumferential direction of the shaft  205   a  ( 206   b ), the magnet  202   a  ( 202   b ), and bearing  207   a  ( 207   b ) are concentrically disposed. That is, the magnet  202   a  ( 202   b ), and bearings  207   a  ( 207   b ) are fitted into the cylindrical supporting portion  210   a  ( 210   b ) so that they are supported by the inward surfaces  230  ( 231 ) of the cylindrical supporting portions  210   a  ( 210   b ). The portion of the inward surface  230  ( 231 ), which supports the magnet  202   a  ( 202   b ), and bearings  207   a  ( 207   b ), is processed with the use of tools which are concentrically positioned with the cylindrical supporting portion  210   a  ( 210   b ). Therefore, it is ensured that as the magnet  202   a  ( 202   b ) and bearings  207   a  ( 207   b ) are inserted into the cylindrical supporting portion  210   a  ( 210   b ), they are concentrically positioned at a high level of accuracy, by the inward surfaces  230  and  231 . Therefore, a gap which is uniform in terms of the circumferential direction of the shaft  205   a  ( 206   b ) is formed between the shaft  205   a  ( 206   a ) and magnet  202   a  ( 202   b ). 
         [0036]    The magnet  202   a  ( 202   b ) is sandwiched by the step portion  230   b  ( 231   b ) of the inward surface  230  ( 231 ) of the cylindrical supporting portions  210   a  ( 210   b ) and a magnet holding member  203   a  ( 203   b ), being thereby accurately positioned in terms of the direction parallel to its axial line. The magnet holding member  203   a  ( 203   b ) is provided with multiple (four, for example) springy claws  203 A, the external diameter of which is larger than that of the cylindrical inward surface  230  ( 231 ). Therefore, as the magnet holding member  203   a  ( 203   b ) is inserted into the cylindrical supporting portion  210   a  ( 210   b ), following the inward surface  230  ( 231 ) in the direction parallel to the axial line of the cylindrical supporting portion  210   a  ( 210   b ), the springy claws  203 A are elastically deformed inward of the cylindrical supporting portion  210   a  ( 210   b ) in terms of the radius direction of the cylindrical supporting portion  210   a  ( 210   b ), and then, snap into the recesses, one for one, with which inward surface  230  ( 231 ) is provided, becoming fixed to the inward surface  230  ( 231 ). 
         [0037]    As will be described later, the first and second holding portion  240  and  241  of the bearing holding portion  210  are provided with developer holding portions  220  and  221 , respectively, as the second spaces. Therefore, the cylindrical supporting portion  210   a  ( 210   b ) is provided with a gap  250  which is in connection to the developer holding portion  220  ( 221 ). By the way,  FIG. 5(   a ) shows only the gap  250  of the cylindrical supporting portion  210   a  of the first holding portion  240 . However, the cylindrical supporting portion  210   b  of the second holding portion  241  has also a gap similar to the gap  250  shown in  FIG. 5(   a ). With the provision of cylindrical supporting portion  210   a  ( 210   b ) with the gap  250 , the springy claw  203 A of the magnet holding members  203   a  ( 203   b ) does not function in the area where the gap  250  is present. 
         [0038]    Therefore, there are disposed multiple ribs  251  in the area which corresponds to the gap  250 , as shown in  FIG. 5(   a ), in order to ensure that the entirety of the force which can be generated by the springy claw  203 A is utilized. Thus, the ribs  251  are positioned so that no matter which direction the magnet holding member  203   a  ( 203   b ) is rotated, the entirety of the force which can be generated by the spring claw  203  is utilized. Further, in order to prevent the ribs  251  from interfering with the discharging of the developer through the area having the gap  250 , the ribs  251  are regulated in their intervals, and the distance between the rib  251  and the edge of the gap  250 . For example, the minimum distance is set to 2 mm, whereas the particle size of the developer is in a range of several micrometers-several tens of micrometers. 
         [0039]    The rib  251  is shaped so that it protrudes from the wall  201   a  of developing device housing  201  toward the shaft, and supports the magnet  202   a  ( 202   b ) and magnet holding member  203   a  ( 203   b ), by their peripheral surface. The height of the rib  251  (distance by which it protrudes from wall  201   a ) is such that after the magnet holding member  203   a  ( 203   b ) is pressed into the cylindrical supporting portion  210   a  ( 210   b ) so that the magnet  202   a  ( 202   b ) is pressed on the step  230   b  ( 231   b ), the rib  251  is greater than the springy claw  203 A in terms of height from the wall  201   a . For example, referring to  FIG. 5(   b ), the rib  251  protrudes by 1.5 mm (distance C in  FIG. 5(   b )) beyond the springy claw  203 A. 
       [Developer Holding Portion] 
       [0040]    The bearing holding portion  210  structured as described above has the developer holding portion  220  ( 221 ) as the second space. The developer holding portion  220  ( 221 ) is in connection to the first space  222  ( 223 ) which is between the magnet  202   a  ( 202   b ) and bearing  207   a  ( 207   b ). Referring to  FIG. 5 , the first space  222  ( 223 ) is the space formed by the extension of the peripheral surface of the magnet  202   a  ( 202   b ) in the direction parallel to the axial line of the cylindrical supporting portion  210   a  ( 210   b ), magnet  202   a  ( 202   b ), and bearing  207   a  ( 207   b ). Therefore, it is possible for the developer in the first space to be transferred into the developer holding portion  220  ( 221 ). In this embodiment, the image forming apparatus  100  is structured so that when it is in the proper attitude for image formation, the developer holding portion  220  ( 221 ) will be on the downward side of the first space in terms of the gravity direction. Therefore, the developer in the first space  222  ( 223 ) is accepted (discharged into) the developer holding portion  220  ( 221 ) by being made to fall into the developer holding portion  220  ( 221 ) by the gravity. Further, in order to allow the developer in the first space  222  ( 223 ) to be discharged into the developer holding portion  220  ( 221 ) as described above, the cylindrical supporting portion  210   a  ( 210   b ) is provided with a gap such as the above described gap  250 . 
         [0041]    The gap  250  makes the width of the cylindrical supporting portion  210   a  ( 210   b ), in terms of its circumferential direction, less than the external diameter of the bearing  207   a  ( 207   b ). This means that the bearing  207   a  ( 207   b ), magnet holding member  203   a  ( 203   b ), and magnet  202   a  ( 202   b ) are not supported by the cylindrical supporting portion  210   a  ( 210   b ), by the entirety of their peripheral surface. That is, they are supported by the 180°+a (for example, 240° which is dimension D in  FIG. 5(   a )). The rest corresponds to the area through which the developer falls. 
         [0042]    Further, the developer holding portion  220  ( 221 ) is a space with no outlet. It stores the developer as the developer falls into the developer holding portion  220  ( 221 ) from the first space  222  ( 223 ). Therefore, the cover  209  is positioned to cover the bearing  207   a  ( 207   b ) which is on the most outward side of the cylindrical supporting portion  210   a  ( 210   b ) in terms of the direction parallel to the axial line of the cylindrical supporting portion  210   a  ( 210   b ), as described above. Then, the cover  209  is fixed to the developing device housing  201 , with the use of small screws. In other words, the cover  209  is made to double as a lid for forming the developer holding portion  220  ( 221 ). Further, referring to  FIG. 5(   a ), the developer holding portion  220  ( 221 ) is surrounded by the wall  252 , which is an integral part of the cylindrical supporting portion  210   a  ( 210   b ). In other words, the developer holding portion  220  ( 221 ) is a space (with no outlet) surrounded by the wall portion  201   a , cover  209 , and wall  252  of developing device housing  201 . Providing the bearing holding portion  210  with the developer holding portion  220  ( 221 ) as described above makes it possible to increase the cylindrical supporting portion  210   a  in developer capacity to 3.3 times the first space  222 , and the cylindrical supporting portion  210   b  to 2.5 times the second holding portion  241 . 
         [0043]    Further, unlike developing device housing  201 , the developer holding portion  220  ( 221 ) is not subjected to the developer conveyance pressure (developer circulation pressure). Therefore, it does not require a seal. That is, no seal is placed between the adjacent components, parts thereof, etc., of which the developer holding portion  220  ( 221 ) is made up. All that was done to ensure that the developer holding portion  220  ( 221 ) remains free of developer leakage, was to make flat enough the end surface of the cylindrical supporting portion  210   a  ( 210   b ), and the surface E of the cover  209 , by which the cover  209  is placed in contact with developing device housing  201 , to allow no gap to be created in the joint between them. Thus, the mold for the cylindrical supporting portion  210   a  ( 210   b ) and the mode for the cover  209  can be simplified to improve the components in tolerance to make it unlikely for gaps to occur at the interface between the surface E of the cover  209  and developing device housing  201 . Further, in order to ensure that the surface-to-surface contact among the above described components occurs with no gap, a minute gap (0.2 mm, for example) is provided between the cover  209  and bearing  207   a  ( 207   b ). 
         [0044]    In terms of the direction in which the developing device  200  is inserted into the main assembly of the image forming apparatus, the developer holding portion  220  ( 221 ) is on the downstream side (inward side of image forming apparatus  100 ). The developer holding portion  220  is a part of the development chamber  213 , and the developer holding portion  221  is a part of the stirring chamber  214 . Referring to  FIG. 3 , in the case of the developing device  200  in this embodiment, the developer circulation pressure in the development chamber  213  is low on the downstream side (right side in  FIG. 3 ), and the more upstream (left side in  FIG. 3 ), the greater the amount of developer, being therefore higher in the developer circulation pressure, because of the manner in which the developer is circulated in the developing device  200 . In this embodiment, the upstream side of the developing device  200  in terms of the developer conveyance direction corresponds to the rear side of the developing device  200 . Therefore, the amount by which the developer slips by the magnetic seal  204  on the rear side of the developing device  200  is greater than on the front side. Therefore, it is desired that the developer holding portion  220  ( 221 ) is positioned on the rear side of the developing device  200 . 
         [0045]    Further, in terms of the projection drawing of developing device  200  in the direction parallel to the axial line of the cylindrical supporting portion  210   a  ( 210   b ), the developer holding portion  220  ( 221 ) falls within the projection of the developing device housing  201 . In other words, the developing device  200  is structured so that the projection of the developer holding portion  220  ( 221 ) falls within the projection of developing device housing  201 , contributing to the ease with which the developing device  200  can be inserted into, or extracted from, the main assembly of the image forming apparatus  100 . 
         [0046]    In the case of the developing device  200  in this embodiment, which is structured as described above, as the developer slips by the magnetic seal  204 , it can be discharged into (accepted by) the developer holding portion  220  ( 221 ) to be accumulated in the developer holding portion  220  ( 221 ). Therefore, it is possible to prevent the problem that the space between the bearing  207   a  ( 207   b ) and magnet  202   a  ( 202   b ) is filled up with the developer having slipped by the magnetic seal  204  in the early stage of developing device usage. In other words, the presence of the developer holding portion  220  ( 221 ) makes it possible to increase the amount by which the developer is allowed to slip by the magnetic seal  204 . 
         [0047]    Thus, it is possible to make it unlikely for the shafts  205   a  ( 206   a ) and bearing  207   a  ( 207   b ) to be welded to each other by the developer (melted developer). For example, in the case of a combination of the cylindrical supporting portion  303 , bearing  301 , and magnet  302  structured as shown in  FIG. 7 , the space between the bearing  301  and magnet  302  is relatively small. Thus, it will be filled up with the developer in the early stage of developing device usage. As the space is filled up with the developer, the developer is melted by the heat generated by the friction between the bearing  301  and shaft. Then, as the melted developer cools down, it solidly welds the bearing and shaft to each other, making sometimes it necessary for the developing device to be replaced, which in turn increases the image forming apparatus in maintenance cost. In comparison, in the case of the developing device  200  in this embodiment, it is possible to prevent the shaft and bearing from being welded to each other in the early stage of developing device usage. In other words, this embodiment can extend a developing device in the length of service life, and also, can reduce a developing device in maintenance cost. 
         [0048]    Further, in the case of this embodiment, the developing device  200  is structured so that when the image forming apparatus  100  is in the proper attitude for image formation, the developer holding portion  220  ( 221 ) is on the bottom side of the first space  222  ( 223 ) in terms of the gravity direction, making it possible for the developer having slipped by the magnetic seal  204 , to free fall into the developer holding portion  220  ( 221 ) and accumulate in the developer holding portion  220  ( 221 ). That is, the developing device  200  in this embodiment does not requires a structural means dedicated to the conveyance of the developer from the first space  222  ( 223 ) to the developer holding portion  220  ( 221 ). In other words, this embodiment can reduce a developing device (hence, image forming apparatus) in size. 
         [0049]    By the way, it does not need to be on the under side of the first space  222  ( 223 ) in terms of the gravity direction that the developer holding portion  220  ( 221 ) is to be disposed. For example, it may be disposed at the same level as the first space  222  ( 223 ). In such a case, the developer having slipped by the magnetic seal  204  gradually spreads in the same direction as in the developer holding portion  220  ( 221 ). Therefore, it takes substantial length of time for the developer to reach the shaft. Unless the developer reaches the shaft, it does not occur that the melted developer solidifies between the bearing and shaft and interferes with the rotation of the shaft (melted developer solidifies below shaft), even if the developer is melted by the heat generated by the friction between the bearing and shaft. Further, the developing device  200  may be provided with a suctioning means dedicated to the conveyance of the developer having slipped by the magnetic seal  204 . In such a case, more latitude is afforded in terms of the positioning of the developer holding portion  220  ( 221 ). For example, it may be disposed so that it is above the first space  222  ( 223 ) in terms of the gravity direction. 
         [0050]    Further, all that is necessary is that at least the first holding portion  240  is provided with the developer holding portion  220  ( 221 ) as described above. That is, referring to  FIG. 1 , in the stirring chamber  214 , the developer is pushed up into the development chamber  213  by the return blade  206   b . Therefore, the second holding portion  241  is less in the amount of developer circulation pressure than the first holding portion  240 . On the other hand, the development chamber  213  is subjected to the developer which is being pushed up into the development chamber  213  from the stirring chamber  214 . Therefore, it becomes higher in developer circulation pressure than the stirring chamber  214 . Thus, the amount by which the developer slips by the magnetic seal  204  in the second storing portion  241  is greater than that in the first storing portion  240 . Therefore, it is desired that at least the first storing portion  240  is provided with the developer holding portion  220 . 
       Embodiment 2 
       [0051]    Next, referring to  FIG. 6 , the second embodiment of the present invention is described. In the first embodiment described above, the developer holding portion  220  ( 221 ), as the second space, was structured as a space with no outlet. In comparison, in this embodiment, the second space  260  ( 261 ), which is between the bearings  207   a  ( 207   b ) and magnet  202   a  ( 202   b ) ( FIG. 4 ) and is in connection to the first space  222  ( 223 ), is not a space with no outlet. That is, the developing device  200  is provided with a developer recovery system  270 , as a developer recovering means, which recovers the developer as the developer is received (discharged) from the first space  222  ( 223 ). The developer recovery mechanism is in connection to the second space  260  ( 261 ). Otherwise, the developing device  200  in the second embodiment is similar in structure and function to the developing device  200  in the first embodiment. Thus, its structural components, portions thereof, etc., which are the same in structure as the counterparts in the first embodiment, are given the same referential codes as those given to the counterparts, in order not to describe them, or simplify their description. That is, the description of the second embodiment will be concentrated to the difference of the developing device  200  in the second embodiment from the developing device  200  in the first embodiment. 
         [0052]    The developing device  200  in this embodiment is structured so that when the image forming apparatus  100  is in the proper attitude for image formation, the second space  260  ( 261 ) is on the under side of the first space  222  ( 223 ) in terms of the gravity direction. The second space  260  of the first storing portion  240 A, which is in connection to the development chamber  213  ( FIG. 4 ) is between the cylindrical supporting portion  210   a  ( 210   b ), and the walls  253  and  254  which are next to the gap  250 . In other words, the second space  260  is a space surrounded by the wall  201   a  of developing device housing  201 , cover  206  ( FIG. 4 ), and walls  253  and  254 . The second space  261  of the second storing portion  241 A, which is in connection to the stirring chamber  214  ( FIG. 4 ), is formed between the bottom portion of the cylindrical supporting portion  210   b , and the wall  255  which is an integral part of one side of the gap  250  of the cylindrical supporting portion  210   b . That is, the second space  261  is formed by the wall  201   a , cover  209 , and the bottom portion of the first storing portion  241 A. In this embodiment, the bearing holding portion  210 A is made up of the first and second storing portions  240 A and  241 A. 
         [0053]    The developer recovery system  270  has a recovery-conveyance portion  271 , and passages  262  and  263 . The developing device  200  is structured so that when the image forming apparatus is in the proper attitude for image formation, the recovery-conveyance portion  271  will be below the second space  260  ( 261 ), and conveys the developer to a recovery container  272 , which is in the main assembly of the image forming apparatus  100  ( FIG. 1 ). It is made up of a cylindrical conveyance tube  272 , and a conveyance screw  273  disposed in the conveyance tube  272 . The conveyance screw  273  is rotationally driven by an unshown motor, and conveys the developer in the conveyance tube  272  toward the recovery container  280 . 
         [0054]    In this embodiment, the recovery-conveyance portion  271  recovers also the developer overflow from the developing device housing  201 , and conveys the developer to the recovery container  280 . That is, the developing device  200  ( FIGS. 2 ,  3 , etc.) is structured so that as the developer is circulated in developing device housing  201 , it is allowed to partially overflow from developing device housing  201  through the unshown opening. This structural arrangement is for discharging the old toner particles and carrier particles in developing device housing  201 , that is, those which have not been used for development for a substantial length of time. As the toner in developing device housing  201  is consumed for development, the developing device housing  201  is replenished with toner from the replenishment toner container  20  ( FIG. 2 ) as described above. That is, the developing device  200  is structured so that as the developer in the developing device housing  201  is discharged by a preset amount as described above, the developer in the developing device housing  201  is partially replaced. As the developer is discharged from the developing device housing  201 , it is sent to the conveyance tube  272 , and is conveyed to the recovery container  280  by the conveyance screw  273 . 
         [0055]    The developer passage  262  ( 263 ) connects the second space  260  ( 261 ) and the conveyance tube  272  of the recovery-conveyance portion  271 . That is, the second space  260  is open on its downward side in terms of the gravity direction, and is in connection to the conveyance tube  272  through the passage  262 , whereas the second space  261  is open on its lateral side, and is in connection to the conveyance tube  272  through the passage  263 . Thus, the developer in the second space  260  of the first storing portion  240 A is conveyed into the conveyance tube  272  through the passage  262 , as indicated by an arrow mark in  FIG. 6 , whereas the developer in the second space  261  of the second storing portion  241 A is conveyed to the conveyance tube  272  through the passage  263 , as indicated by another arrow mark in  FIG. 6 . 
         [0056]    The developer in the second space  260  conveys itself to the conveyance tube  272  by falling out of the second space  260 . As for the developer in the second space  261 , it is pushed into the conveyance tube  272  by the developer which is continuously discharged from the first space  223 , because the conveyance tube  272  is at the same level as the second space  261 . In this embodiment, the inward surface of the wall  255 , which is at the bottom end of the developer passage between the second space  261  and conveyance tube  272 , is tilted in such a manner that the closer it is to the conveyance tube  272 , the lower in terms of the gravity direction, to ensure that the developer is smoothly conveyed from the second space  261  to the conveyance tube  272 . 
         [0057]    As described above, as the developer is conveyed from the second space  260  ( 261 ) to the conveyance tube  272 , it is conveyed, along with the developer having overflowed from the developing device housing  201 , to the recovery container  280  by the conveyance screw  273 . In the case of this embodiment, therefore, as the developer slips by the magnetic seal  204  ( FIG. 5 ), it can be discharged into the second space  260  ( 261 ), and then, conveyed to the recovery container  280  by the recovery-conveyance portion  271 . Therefore, it is possible to prevent the problem that the space between the bearing  207   a  ( 207   b ) and magnet  202   a  ( 202   b ) is filled up with the developer having slipped by the magnetic seal  204 . 
         [0058]    In particular, in this embodiment, the developer having slipped by the magnetic seal  204  is conveyed to the recovery container  280  without being held in the developer holding portion  220  ( 221 ) as in the first embodiment. Therefore, it is possible to substantially increase the amount by which the developer is allowed to slip by the magnetic seal  204 . Therefore, it is possible to prevent for a long period of time the problem that the space between the bearing  207   a  ( 207   b ) and magnet  202   a  ( 202   b ) is filled up by the developer having slipped by the magnetic seal  204 . 
         [0059]    Also in the case of this embodiment, it is not mandatory that the second space  260  ( 261 ) is positioned directly below the first space  222  ( 223 ) in terms of the gravity direction. Further, it is not mandatory that the recovery-conveyance portion  271  is positioned directly below the first space  222  ( 223 ) in terms of the gravity direction. For example, it may be positioned at a higher level than the first space  222  ( 223 ) in terms of the gravity direction. Moreover, the recovery-conveyance portion  271  may be different from the means which recovers the developer having overflowed from the developing device housing  201 . 
       [Miscellanies] 
       [0060]    In each of the preceding embodiments of the present invention described above, the developing device was of the so-called vertical stir type. However, the present invention is also applicable to developing devices different in structure from those in the preceding embodiments. For example, it is applicable to developing devices structured so that when the image forming apparatus is in the proper attitude for image formation, the development chamber and stirring chamber are at the same level. Further, the present invention is also applicable to developing devices which use single-component developer made up of magnetic toner. 
         [0061]    According to the present invention, as developer slips by a magnetic seal, it is allowed to enter a second space. Therefore, it is possible to prevent the problem that the space between a bearing and a magnet is filled up by the developer having slipped by a magnetic seal in the early stage of developing device usage. Thus, it is possible to make it unlikely for the shaft and bearing are welded to each other by the developer. 
         [0062]    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. 
         [0063]    This application claims the benefit of Japanese Patent Application No. 2013-144163, filed Jul. 10, 2013 which is hereby incorporated by reference herein in its entirety.