Abstract:
An electrophotographic photosensitive drum usable with an electrophotographic image forming apparatus, includes a non-circular twisted projection provided at one longitudinal end of the drum, and engageable with a non-circular twisted hole being disposed at a central portion of a driving rotatable member of a main assembly of the image forming apparatus, and an electroconductive member provided on the projection, electrically connected to the drum, and contactable with an electroconductive member electrically connected with a main assembly of the image forming apparatus and disposed in the hole to ground the drum. The contact position between the electroconductive members moves in interrelation with the projection being moved toward the hole by rotation of the driving rotatable member when the projection is engaged with the hole.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This is a divisional application of application Ser. No. 10/960,117, filed Oct. 8, 2004 now U.S. Pat. No. 7,003,247. 

   FIELD OF THE INVENTION AND RELATED ART 
   The present invention relates to an electrophotographic photosensitive drum, a process cartridge comprising an electrophotographic photosensitive drum, and an electrophotographic image forming apparatus. 
   Here, an electrophotographic image forming apparatus means an apparatus for forming an image on recording medium with the use of one of the electrophotographic image forming methods. The examples of an electrophotographic image forming apparatus include an electrophotographic copying machine, an electrophotographic printer (for example, a laser printer, an LED printer, etc.), a facsimile apparatus, a word processor, etc. 
   A process cartridge is a cartridge which is removably mountable in the main assembly of an image forming apparatus, and in which a minimum of one among a charging means, a developing means, and a cleaning means, as a processing means, and an electrophotographic photosensitive member, are integrally placed so that they can be removably mountable in the main assembly of an image forming apparatus. 
   A photosensitive drum unit means an electrophotographic photosensitive member in the form of a drum, employed by the main assembly of an electrophotographic image forming apparatus. 
   An electrophotographic image forming apparatus such as a copying machine, a laser printer, a facsimile machine, etc., forms an electrostatic latent image by selectively exposing numerous points of the peripheral surface of the electrophotographic photosensitive drum uniformly charged by a charging apparatus. Then, it develops the electrostatic latent image into a visible image (image formed of toner) by adhering toner to the electrostatic latent image, with the use of the developing apparatus, and yields a copy of an intended image by transferring the toner image onto a recording medium such as a sheet of paper. After the transfer of the toner image, it removes the toner remaining on the peripheral surface of the electrophotographic photosensitive drum, with the use of the cleaning apparatus, preparing the photosensitive drum for the following image formation operation. 
   In the field of an electrophotographic image forming apparatus employing an electrophotographic image formation process, it has been a common practice to employ a process cartridge system, in which an electrophotographic photosensitive drum, and a single or plurality of processing means, such as a charging apparatus, a developing apparatus, a cleaning apparatus, etc., which act on the electrophotographic photosensitive drum, are integrally placed in a cartridge removably mountable in the main assembly of an image forming apparatus. According to this process cartridge system, apparatus maintenance can be carried out by a user by himself, without the need for relying on service personnel, drastically improving an image forming apparatus in operational efficiency. Thus, a process cartridge system has been widely used in the field of an image forming apparatus. 
   An electrophotographic photosensitive drum is structured to allow residual electric charge to escape from the electrophotographic photosensitive drum to the main assembly of the image forming apparatus. More specifically, the electrophotographic photosensitive drum comprises a drum cylinder formed of electrically conductive substance, and a pair of flanges having a shaft portion by which the electrophotographic photosensitive drum is supported by the frame of a process cartridge or the main assembly of the image forming apparatus. The flanges are attached to the lengthwise ends of the drum cylinder by being pressed into the openings of the lengthwise ends of the drum cylinder. One of the flanges is provided with an electrically conductive member, which is located at the center of the flange. This conductive member is provided with a projection, which contacts the internal surface of the drum cylinder, and a flat portion by which the conductive member of the flange contacts the conductive member on the main assembly side of the image forming apparatus. According to the prior art, the conductive member on the main assembly side is kept in contact with the conductive member of the flange by utilizing the resiliency of a spring or the like, in order to keep the electrophotographic sensitive drum electrically connected to the apparatus main assembly (Japanese Laid-open Patent Application 2000-112200). 
   SUMMARY OF THE INVENTION 
   The primary object of the present invention is to provide an electrophotographic photosensitive drum capable of reliably establishing electrical connection between it and the main assembly of an electrophotographic image forming apparatus, a process cartridge comprising said electrophotographic photosensitive drum, and an electrophotographic image forming apparatus compatible with said electrophotographic photosensitive drum and process cartridge. 
   Another object of the present invention is to provide an electrophotographic photosensitive drum capable of wiping its electrically conductive member in order to be reliably grounded, a process cartridge comprising said electrophotographic photosensitive drum, and an electrophotographic image forming apparatus compatible with said electrophotographic photosensitive drum and process cartridge. 
   Another object of the present invention is to provide an electrophotographic photosensitive drum capable of rotating the first and second electrically conductive members together in order to prevent the conductive members from being frictionally worn, a process cartridge comprising said electrophotographic photosensitive drum, and an electrophotographic image forming apparatus compatible with said electrophotographic photosensitive drum and process cartridge. 
   According to an aspect of the present invention, there is provided an electrophotographic photosensitive drum usable with an electrophotographic image forming apparatus, comprising a non-circular twisted projection having a cross-section with a plurality of corner portions, the non-circular twisted projection being provided at one longitudinal end of the electrophotographic photosensitive drum, and being engageable with a non-circular twisted hole having a section with a plurality of corner portions, the non-circular twisted hole being disposed at a central portion of a driving rotatable member of a main assembly of the image forming apparatus; and a second electroconductive member contactable with a first electroconductive member electrically connected with a main assembly of the image forming apparatus, the first electroconductive member being disposed in the hole to ground the electrophotographic photosensitive drum, the said second electroconductive member being provided on the projection and being electrically connected with the electrophotographic photosensitive drum, wherein a contact position between the second electroconductive member and the first electroconductive member moves in interrelation with the projection being moved toward the hole by rotation of the driving rotatable member when the projection is engaged with the non-circular twisted hole of the driving rotatable member. 
   According to another aspect of the present invention, there is provided an electrophotographic photosensitive drum usable with an electrophotographic image forming apparatus, comprising a non-circular twisted projection having a cross-section with a plurality of corner portions, the non-circular twisted projection being provided at one longitudinal end of the electrophotographic photosensitive drum, and being engageable with a non-circular twisted hole having a section with a plurality of corner portions, the non-circular twisted hole being disposed at a central portion of a driving rotatable member of a main assembly of the image forming apparatus; a second electroconductive member contactable with a first electroconductive member electrically connected with a main assembly of the image forming apparatus, the first electroconductive member being disposed in the hole to ground the electrophotographic photosensitive drum, the second electroconductive member being provided on the projection and being electrically connected with the electrophotographic photosensitive drum, wherein while the projection is in engagement with the hole and is being rotated, the second electroconductive member and the first electroconductive member are integrally rotated while being in contact with each other. 
   According to a further aspect of the present invention, there is provided a process cartridge detachably mountable to an electrophotographic image forming apparatus, the process cartridge comprising an electrophotographic photosensitive drum; a non-circular twisted projection having a cross-section with a plurality of corner portions, the non-circular twisted projection being provided at one longitudinal end of the electrophotographic photosensitive drum, and being engageable with a non-circular twisted hole having a section with a plurality of corner portions, the non-circular twisted hole being disposed at a central portion of a driving rotatable member of a main assembly of the image forming apparatus; and a second electroconductive member contactable with a first electroconductive member electrically connected with the main assembly of the image forming apparatus, the first electroconductive member being disposed in the hole to ground the electrophotographic photosensitive drum, the second electroconductive member being provided on the projection and being electrically connected with the electrophotographic photosensitive drum, wherein a contact position between the second electroconductive member and the first electroconductive member moves in interrelation with the projection being moved toward the hole by rotation of the driving rotatable member when the projection is engaged with the hole of the driving rotatable member. 
   According to a further aspect of the present invention, there is provided a process cartridge detachably mountable to an electrophotographic image forming apparatus, the process cartridge comprising an electrophotographic photosensitive drum; a non-circular twisted projection having a cross-section with a plurality of corner portions, the non-circular twisted projection being provided at one longitudinal end of the electrophotographic photosensitive drum, and being engageable with a non-circular twisted hole having a section with a plurality of corner portions, the non-circular twisted hole being disposed at a central portion of a driving rotatable member of a main assembly of the image forming apparatus; and a second electroconductive member contactable with a first electroconductive member electrically connected with the main assembly of the image forming apparatus, the first electroconductive member being disposed in the hole to ground the electrophotographic photosensitive drum, the second electroconductive member being provided on the projection and being electrically connected with the electrophotographic photosensitive drum, wherein while the projection is in engagement with the hole and is being rotated, the second electroconductive member and the first electroconductive member are integrally rotated while being in contact with each other. 
   According to a further aspect of the present invention, there is provided a electrophotographic image forming apparatus to which a process cartridge is detachably mountable, comprising 
   (i) rotational drive member; 
   (ii) a non-circular twisted hole having a cross-section with a plurality of corner portions, the non-circular twisted hole being disposed at a central portion of a driving rotatable member of a main assembly of the apparatus; 
   (iii) a first electrophotographic member electrically connected with the main assembly of the image forming apparatus; 
   (iv) a process cartridge demountably mounted to a cartridge mounting portion, the process cartridge including, 
   an electrophotographic photosensitive drum, 
   a non-circular twisted projection having a cross-section with a plurality of corner portions, the non-circular twisted projection being provided at one longitudinal end of the electrophotographic photosensitive drum, and being engageable with the non-circular twisted hole, and 
   a second electroconductive member contactable with a first electroconductive member electrically connected with a main assembly of the image forming apparatus to electrically ground the electrophotographic photosensitive drum, the second electroconductive member being provided on the projection and being electrically connected with the electrophotographic photosensitive drum, 
   wherein a contact position between the second electroconductive member and the first electroconductive member moves in interrelation with the projection being moved toward the hole by rotation of the driving rotatable member when the projection is engaged with the hole of the driving rotatable member. 
   According to a further aspect of the present invention, there is provided an electrophotographic image forming apparatus to which a process cartridge is detachably mountable, comprising 
   (i) rotational drive member; 
   (ii) a non-circular twisted hole having a cross-section with a plurality of corner portions, the non-circular twisted projection being disposed at a central portion of a driving rotatable member of a main assembly of the apparatus; 
   (iii) a first electroconductive member electrically connected the main assembly of said image forming apparatus, the first electroconductive member being disposed in the hole; 
   (iv) a process cartridge demountably mounted to a cartridge mounting portion, the process cartridge including, 
   an electrophotographic photosensitive drum, 
   a non-circular twisted projection having a cross-section with a plurality of corner portions, the non-circular twisted projection being provided at one longitudinal end of the electrophotographic photosensitive drum, and being engageable with the non-circular twisted hole, and 
   a second electroconductive member contactable with a first electroconductive member electrically connected with the main assembly of the image forming apparatus to electrically ground the electrophotographic photosensitive drum, the second electroconductive member being provided on the projection and being electrically connected with the electrophotographic photosensitive drum, 
   wherein while the projection is in engagement with the hole and is being rotated, the second electroconductive member and the first electroconductive member are integrally rotated while being in contact with each other. 
   These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic sectional view of the multicolor image forming apparatus in one of the preferred embodiments of the present invention. 
       FIG. 2  is a schematic sectional view of the image forming apparatus in the preferred embodiment of the present invention, the front cover of which has been opened to expose the opening through which process cartridges are inserted. 
       FIG. 3  is an enlarged cross-sectional view of the process cartridge in the preferred embodiment of the present invention. 
       FIG. 4  is a perspective view of the process cartridge in the preferred embodiment of the present invention. 
       FIG. 5  is a perspective view of the process cartridge in the preferred embodiment of the present invention. 
       FIG. 6  is a schematic sectional view of the main assembly of the image forming apparatus in the preferred embodiment of the present invention, for describing how the process cartridges are mounted into the main assembly. 
       FIG. 7  is a perspective view of the photosensitive drum unit in the preferred embodiment of the present invention. 
       FIG. 8  is a combination of a perspective view and sectional views of the photosensitive drum unit, for describing the steps followed to attach the drum grounding plate. 
       FIG. 9  is a detailed vertical sectional view of the drum grounding plate in the preferred embodiment of the present invention, describing the structure thereof. 
       FIG. 10  is a detailed vertical sectional view of the drum grounding plate in the preferred embodiment of the present invention, describing the structure thereof. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, the preferred embodiments of the present invention will be described with reference to the appended drawings. 
   Embodiment 1 
   (1) General Structure of Image Forming Apparatus 
     FIG. 1  is a vertical sectional view of the multicolor image forming apparatus in the preferred embodiment of the present invention, showing the general structure thereof. This multicolor image forming apparatus is a full-color laser beam printer which uses an electrophotographic process of a transfer type, and employs a plurality of process cartridges removably mountable in its cartridge compartment. It has a process cartridge compartment in which a plurality of process cartridges are virtually vertically stacked in parallel. 
   Designated by a referential number  100  is the main assembly of the image forming apparatus (which hereinafter will be referred to simply as apparatus main assembly), and designated by a referential number  101  is a front cover of the apparatus (which hereinafter will be referred to simply as front cover). This front cover  101  is hinged to the apparatus main assembly  100 , being enabled to be opened or closed by being rotated about the hinge shaft  101   a  located at the bottom edge of the front cover  101 . In  FIG. 1 , the front cover  101  is closed against the apparatus main assembly  100 , and in  FIG. 2 , the front cover  101  has been opened toward an operator, exposing the opening  91  through which process cartridges are inserted into the apparatus main assembly  100 . 
   Designated by referential numbers  7   a ,  7   b ,  7   c , and  7   d  are four process cartridges (which hereinafter will be referred to simply as cartridges), that is, first to fourth cartridges for forming toner images of magenta, cyan, yellow, and black colors, which correspond to the color components into which the optical image of an intended full-color image is separated. These cartridges  7  are stacked in parallel in the direction slightly tilted from the true vertical direction, in the cartridge compartment of the apparatus main assembly  100 , being stacked in the listed order, with the cartridge  7   a  positioned at the bottom. 
   Each cartridge  7 ( a - d ) has an electrophotographic photosensitive member, as an image bearing member  1 ( a - d ), in the form of a drum (which hereinafter will be referred to as photosensitive drum). It also has such electrophotographic processing devices as a charging apparatus (charging means)  2 ( a - d ) for uniformly charging the peripheral surface of the photosensitive drum  1 , a developing apparatus (developing means)  4  for developing the electrostatic latent image formed on the peripheral surface of the photosensitive drum  1  into a toner image (image formed of toner) by adhering toner to the electrostatic latent image, a cleaning apparatus (cleaning means)  6 ( a - d ) for removing the toner remaining on the peripheral surface of the photosensitive drum  1  after the transfer of the toner image onto a transfer medium (recording medium), etc. 
   The developers stored in the developing apparatuses  4 ( a - d ) of the first to fourth cartridges  7 ( a - d ) are magenta, cyan, yellow, and black toners, respectively. 
   Designated by reference characters  3   a ,  3   b ,  3   c , and  3   d  are four scanner units which correspond to the four cartridges  7  one for one. The scanner unit is an exposing means for forming an electrostatic latent image on the peripheral surface of the photosensitive drum  1 , by projecting a beam of laser light (image forming light) L onto the uniformly charged peripheral surface of the photosensitive drum  1 . More specifically, the beam of laser light L outputted from the laser diode (unshown) while being modulated with image formation data is reflected (deflected) by the polygon mirror  9 ( a - d ) being rotated at a high speed by the scanner motor (unshown). The reflected beam of laser light L is sent through the image forming lens  10 ( a - d ), being thereby focused on the uniformly charged peripheral surface of the photosensitive drum  1 . As a result, numerous points of the uniformly charged peripheral surface of the photosensitive drum  1  are selectively exposed, forming an electrostatic image on the peripheral surface of the photosensitive drum  1 . 
   Designated by a reference number  93  is a partitioning wall in the apparatus main assembly  100 . It partitions the cartridge compartment, in which the four cartridges  7 ( a - d ) are mounted, from the scanner unit compartment, in which the four scanner units  7 ( a - d ) are located. The beam of laser light L outputted from each of the scanner units  7 ( a - d ) enters the corresponding cartridge  7  through the corresponding window  95 , that is, one of the windows with which the partitioning wall  93  is provided, and scans the peripheral surface of the corresponding photosensitive drum  1 , selectively exposing the numerous points of the peripheral surface of the photosensitive drum  1 . 
   Designated by a referential number  5  is an electrostatic transferring apparatus (electrostatic transferring means), which is attached to the inward side of the front cover  101 . Thus, the front cover  101  is opened or closed, along with this electrostatic transferring apparatus  5 , against the apparatus main assembly  100  ( FIG. 2 ). The electrostatic transferring apparatus  5  is provided with an electrostatic transfer belt  11 , which is circularly driven in contact with all of the photosensitive drum  1  of the first to fourth cartridges  7  after the front cover  101  is closed against the apparatus main assembly  100 . Designated by referential numbers  12   a ,  12   b ,  12   c , and  12   d  are four transfer rollers, which are placed within the loop formed by the electrostatic transfer belt  11 , being positioned so that the electrostatic transfer belt  11  remains pinched between the photosensitive drums  1  of the first to fourth cartridges  7 ( a - d ), and electrostatic transfer belt  11 . 
   Designated by a reference number  16  is a transfer medium conveying portion located in the bottom portion of the apparatus main assembly  100 . It is a portion for causing a transfer of a medium S to the electrostatic transfer belt  11  of the electrostatic transferring apparatus  5 . Designated by a reference number  17  is a sheet feeder cassette of the transfer medium conveying portion  16 , in which a plurality of transfer media S are stored. Designated by reference numbers  18  and  19  are a sheet feeder roller (semicylindrical roller), and a pair of registration rollers, respectively. 
   Designated by a referential number  20  is a fixation station located in the top portion of the apparatus main assembly  100 . It fixes to the transfer medium S a plurality of toner images different in color having been transferred onto the transfer medium S. It has a rotational heat roller  21   a , a pressure roller  21   b  kept pressured upon the heat roller  21   a  to apply pressure to the transfer medium S, etc. Designated by referential numbers  23  and  24  are a pair of discharge rollers, and a delivery tray portion which catches the transfer medium S on which an image has just been formed. 
   The photosensitive drums  1  in the first to fourth cartridges  7  are sequentially rotated in the counterclockwise direction indicated by an arrow mark with predetermined timings of the image formation sequence. Then, the scanner units  3 ( a - d ) are sequentially driven in synchronism with the rotations of the photosensitive drums  1  of the corresponding cartridges  7 . Further, the electrostatic transfer belt  11  of the electrostatic transferring apparatus  5  is circularly driven by a driver roller  13  in the clockwise direction indicated by an arrow mark at the peripheral velocity matching the peripheral velocities of the photosensitive drums  1 . 
   As each photosensitive drum  1  is rotated as described above, its peripheral surface is uniformly charged (primary charge) by the charging apparatus  2 ( a - d ) to predetermined polarity (negative in this embodiment) and potential level. The charged peripheral surface of the photosensitive drum  1  is exposed to the beam of laser light L outputted from the scanner unit  3  while being modulated with the image formation data. As a result, an electrostatic latent image in accordance with the image formation data is formed on the peripheral surface of the photosensitive drum  1 . 
   The electrostatic latent image is developed (in reverse with use of toner, inherent polarity of which is negative, in this embodiment) into a toner image (image formed of toner) by the developing apparatus  4 . As a result, toner images of magenta, cyan, yellow, and black colors are formed on the peripheral surfaces of the photosensitive drums  1  of the first to fourth cartridges  7 ( a - d ), respectively, with predetermined sequence control timings. 
   Meanwhile, the feed roller  18  of the transfer medium conveying portion  16  is rotationally driven with the predetermined sequence control timing, feeding the transfer mediums S into the apparatus main assembly  100  from the cassette  17 , while separating them one by one. As the leading end of each transfer medium S is conveyed, it comes into contact with the nip formed by the pair of registration rollers  19  which are not being rotated. As it comes into contact with the pair of registration rollers  19 , it is temporarily kept on standby, arcing upward. Then, the registration rollers  19  begin to be rotationally driven in synchronism with the circular movement of the electrostatic transfer belt  11  and the movement of the line of the peripheral surface of the photosensitive drum  1 , at which the toner image begins to be written. As a result, the transfer medium S is conveyed to the tension roller side of the upwardly moving side of the electrostatic transfer belt  11 , and is electrostatically adhered to the surface of the electrostatic transfer belt  11  by the static electricity naturally induced in the electrostatic transfer belt  11 , being thereby reliably held to the electrostatic transfer belt  11 . Then, the transfer medium S is conveyed to the transfer station, or the most downstream station, by the movement of the electrostatic transfer belt  11 . The transfer medium conveying portion  16  may be provided with a charging means, such as an electrostatic adhesion roller, or the like, for intentionally charging the transfer medium S and/or electrostatic transfer belt  11  in order to electrostatically adhere the recording medium S to the electrostatic transfer belt  11 . 
   While being conveyed as described above, the transfer medium S sequentially receives in layers the toner images formed on the peripheral surfaces of the photosensitive drums  1 ; the toner images formed on the peripheral surfaces of the photosensitive drums  1  of the first to fourth cartridges  7  are sequentially transferred in layers onto the recording medium S by the electric field formed between the photosensitive drums  1  and corresponding transfer rollers  12 . In this embodiment, bias with the positive polarity is applied to each transfer roller  12 , causing thereby positive electric charge to be applied to the transfer medium S through the electrostatic transfer belt  11 , generating the electric field, which transfers the toner images on the photosensitive drums  1 , which are positive in polarity, onto the transfer medium S being conveyed in contact with the photosensitive drums  1 ( a - d ), during the image transfer operation. 
   More specifically, the transfer medium S is electrostatically adhered to the surface of the electrostatic transfer belt  11 , being held thereto, and is conveyed upward by the rotation of the electrostatic transfer belt  11 . While the transfer medium S is conveyed upward by the electrostatic transfer belt  11  as described above, it sequentially receives in layers the toner images of magenta, cyan, yellow, and black colors formed on the peripheral surfaces of the photosensitive drums  1  of the first to fourth cartridges  7 ; it receives one toner image in each transfer station. As a result, an unfixed full-color toner image is synthesized on the surface of the recording medium S. 
   After the reception, in layers, of the four toner images different in color, the transfer medium S is separated from the electrostatic transfer belt  11  by the curvature of the driver roller  13  of the transfer medium conveying portion  16 , and is conveyed into the fixation station  20 . In the fixation station  20 , the transfer medium S is conveyed through the fixation nip formed by the rotating heat roller  21   a , and pressure roller  21   b  rotated while being pressed against the heat roller  21   a . As a result, the plurality of toner images different in color are fixed to the transfer medium S by the heat and pressure applied to the transfer medium S by the pair of rollers  21   a  and  21   b . After the fixation of the toner images to the transfer medium S in the fixation station  20 , the transfer medium S is discharged by the pair of discharge rollers  23  into the external delivery tray  24  of the apparatus main assembly  100 , with the image bearing surface of the transfer medium S facing downward. 
   The residues such as the toner remaining on the peripheral surface of the photosensitive drum  1  in each of the first to fourth cartridges  7  after the transfer of the toner image onto the transfer medium S are removed by the cleaning apparatus  6 , and the photosensitive drum  1  is used for the following image formation. 
   (2) Process Cartridge  7   
     FIG. 3  is an enlarged cross sectional view of the cartridge  7 , and  FIGS. 4 and 5  are schematic perspective views of the cartridge  7 . 
   In this embodiment, the photosensitive drum  1  is an integral part of the cartridge  7 , and therefore, it is removably mounted into the apparatus main assembly  100  as the cartridge  7  is removably mounted into the apparatus main assembly  100 . 
   In the following description of this embodiment, the widthwise direction of the cartridge  7  or the structural components thereof is the direction parallel to the direction in which the cartridge  7  is mounted into, or removed from, the apparatus main assembly  100 , whereas the lengthwise direction means the direction intersectional (perpendicular) to the direction in which the cartridge  7  is mounted into, or removed from, the apparatus main assembly  100 . The front side of the cartridge  7  means the side of the cartridge  7  which faces the direction from which the cartridge  7  is inserted into the apparatus main assembly  100 , or the direction toward which the cartridge  7  is removed from the apparatus main assembly  100 ; in other words, it is the side of the cartridge  7  where the hole of the cartridge  7  through which the photosensitive drum  1  is exposed is present. The rear side of the cartridge  7  means the side opposite to the front side. The left and right of the cartridge  7  means the left and right sides, as seen from the front side of the cartridge  7 . The top and bottom sides of the cartridge  7  means the sides which will be on the top and bottom sides, respectively, after the mounting of the cartridge  7  into the apparatus main assembly  100 . 
   The first to fourth cartridges  7 ( a - d ) are the same (in structure) except for the developers stored in the toner container portions (developer storage portions) of the developing apparatuses  4 ( a - d ) of the first to fourth cartridges  7 ( a - d ); the toner container portions of the first to fourth cartridges  7   a ,  7   b ,  7   c , and  7   d  contain magenta toner, cyan toner, yellow toner, and black toner, respectively. 
   The cartridge  7  is made up of the cleaner unit  50  and development unit  4 A. The cleaner unit  50  comprises the photosensitive drum  1 , charging means  2 , and cleaning means  6 , whereas the development unit  4 A comprises the developing means for developing an electrostatic latent image on the peripheral surface of the photosensitive drum  1 . 
   The cleaner unit  50  also comprises a frame  51  to which the photosensitive drum  1 , primary charging apparatus  2  for uniformly charging the photosensitive layer, or the outermost layer, of the photosensitive drum  1 , cleaning blade  60  as the cleaning means  6  for removing the developer (residual toner) remaining on the peripheral surface of the photosensitive drum  1  after the image transfer, and a flexible sheet  80 , etc., are attached. 
   The photosensitive drum  1  comprises an aluminum cylinder, and a photosensitive layer formed on the peripheral surface of the aluminum cylinder. It is provided with flanges  72  and  75 , which are attached to the lengthwise ends of the photosensitive drum  1 , one for one. The flanges  72  and  75  are rotatably supported by supporting members (bearings)  31   a  and  31   b , with which the left and right walls of the cleaner unit frame  51  are provided. Of the two flanges  72  and  75 , the flange  72  functions as a driving force transmitting member which couples with the rotational driving force transmitting member (unshown) of the apparatus main assembly  100 , and receives the driving force from the driving force transmitting member (unshown) of the apparatus main assembly  100 . The configurations of the driving force transmitting member of the apparatus main assembly  100  and the flange  72 , and the manner of their connection, will be described in Section (4). 
   As the charging apparatus  2 , a charging apparatus of a contact type may be used. The charging member is an electrically conductive roller, the peripheral surface of which is placed in contact with the peripheral surface of the photosensitive drum  1 . The roller is rotated by the rotation of the photosensitive drum  1 . The peripheral surface of the photosensitive drum  1  is uniformly charged by applying charge bias voltage to the roller while the roller is rotated by the rotation of the photosensitive drum  1 . 
   The residual toner (waste toner) is removed from the peripheral surface of the photosensitive drum  1  by the cleaning blade  60 , and the removed residual toner is stored in the waste toner chamber (residual toner storage chamber)  55  located above the cleaning blade  60 . Incidentally, the toner remaining on the peripheral surface of the photosensitive drum  1  after the toner image transfer therefrom moves past the contact area between the flexible sheet  80  and the peripheral surface of the photosensitive drum  1 , and reaches the cleaning blade  60 . The flexible sheet  80  is attached to the cleaner unit frame  51  in order to prevent the residual toner from leaking out of the cleaner unit frame  51  after the residual toner is removed from the peripheral surface of the photosensitive drum  1  by the cleaning blade  60 . 
   The development unit  4 A comprises: a development sleeve  40 , which is rotated (in the direction indicated by arrow mark in  FIG. 3 ), with a minute gap maintained between the peripheral surfaces of the development sleeve  40  and photosensitive drum  1  by a pair of spacer rings  40   a ; and development means frames  45   a  and  45   b , in which the toner is stored. The developing means frames  45   a  and  45   b  are joined by ultrasonic welding or the like means, forming the container unit  46 . The development sleeve  40  is rotatably supported by the developing means container unit  46 , with a pair of bearings placed between the development sleeve  40  and the unit  46 . In the adjacencies of the peripheral surface of the development sleeve  40 , a toner supply roller  43 , which is rotated in the clockwise direction indicated by an arrow mark in contact with the peripheral surface of the development sleeve  40 , and the development blade  44 , are located. Further, in the toner container portion (developer storage portion)  41  of the developing means container unit  46 , a toner conveyance mechanism  42  for conveying the toner (unshown) stored in the toner container portion  41 , to the toner supply roller  43  while stirring it, is located. 
   The development unit  4 A is provided with a pair of connective holes  47 , which are located at the lengthwise ends of the container unit  46 , one for one, whereas the cleaner unit frame  51  of the cleaner unit  50  is provided with a pair of supportive holes  52 , which are located at the length ends of the cleaner unit frame  51 . The development unit  4 A and cleaner unit  50  are connected to each other by inserting a pair of pins  49  through the connective holes  47  and supportive holes  52  while holding the two units so that the connective holes  47  and supportive holes  52  align one for one. As a result, the entirety of the development unit  4 A becomes rotatable about the pins  49 , being thereby movable relative to the cleaner unit  50  while remaining suspended from the cleaner unit  50 . Further, the development unit  4 A is kept pressured by a pair of springs (unshown) in the direction to rotate the development unit  4 A about the pins  49  so that the spacer rings  40   a  of the development sleeve  40  are kept in contact with the photosensitive drum  1  in the cleaner unit  50 . 
   During a developing operation, the toner in the toner container  41  is conveyed by the stirring mechanism  42  to the toner supply roller  43 , which is being rotated in the clockwise direction, in contact with the development sleeve  40  which is being rotated also in the clockwise direction. As a result, the peripheral surface of the supply roller  43  is rubbed against the peripheral surface of the development sleeve  40 , causing the toner on the peripheral surface of the supply roller  43  to be transferred onto the peripheral surface of the development sleeve  40 . The toner having been borne on the peripheral surface of the development sleeve  40  is brought by the rotation of the development sleeve  40  to the development blade (toner layer regulating member)  44 . Thus, as the development sleeve  40  is further rotated, the layer of the toner on the peripheral surface of the development sleeve  40  is regulated in thickness by the development blade  44 , into a thin layer of the toner uniform in thickness, while being given a predetermined amount of electric charge. Then, the thin layer of the toner on the peripheral surface of the development sleeve  40  is brought by the further rotation of the development sleeve  40  to the development station, in which the distance between the photosensitive drum  1  and development sleeve  40  is extremely small. In the development station, the toner from the thin layer of the toner on the peripheral surface of the development sleeve  40  is adhered to the electrostatic latent image on the peripheral surface of the photosensitive drum  1 , by the development bias applied to the development sleeve  40  from the electrical power source (unshown); in other words, the latent image is developed. More specifically, the development sleeve  40  forms (develops) a toner image on the peripheral surface of the photosensitive drum  1  by transferring toner onto the numerous points of the electrostatic latent image, which are lower in potential level. 
   The toner which did not contribute to the development of the latent image, that is, the toner which remained on the development sleeve  40 , is returned by the further rotation of the development sleeve  40 , into the container unit  46 , in which it is stripped from the development sleeve  40  by the supply roller  43 , in the area in which the peripheral surfaces of the supply roller  43  and development sleeve  40  are rubbing against each other; in other words, the residual toner is recovered into the container unit  46 . The recovered toner is mixed into the toner in the container unit  46  by the stirring mechanism  42 . 
   Designated by a referential number  54  is a shutter for protecting the photosensitive drum  1 . The shutter  54  is attached to the cleaner unit frame  51 . It is movable by a shutter mechanism (unshown) between the closed position ( FIGS. 3-5 ) in which it covers the photosensitive drum exposure window on the front side of the cartridge  7 , and the open position (indicated by double dot chain line in  FIG. 3 ) into which it is moved downward from the closed position to expose the photosensitive drum exposure window. When the cartridge  7  is out of the apparatus main assembly  100 , the shutter  54  is kept in the closed position, protecting thereby the portion of the peripheral surface of the photosensitive drum  1 , which will remain exposed if there were no the shutter  54 . As the front cover  101  of the apparatus main assembly  100  is closed after the mounting of the cartridge  7  into the apparatus main assembly  100 , the shutter  54  is moved into the open position by the shutter mechanism (unshown) which is moved by the movement of the front cover  101 . As a result, the electrostatic transfer belt  11  is allowed to be placed in contact with the peripheral surface of the photosensitive drum  1  through the aforementioned exposure window of the cartridge  7  ( FIG. 1 ). 
   Designated by a referential number  53  is a cartridge insertion guide of the cleaner unit frame  51 . There are two cartridge insertion guides  53  located at the left and right lengthwise ends, one for one. Designated by a referential number  90  is a handle to be used when mounting the cartridge  7  into the apparatus main assembly  100  or removing it therefrom. There are two handles  90  which project frontward from the left and right lengthwise ends of the cartridge  7 . 
   (3) Method for Mounting or Removing Cartridge  7   
   Next, the method for mounting the cartridge  7  into the apparatus main assembly  100  or dismounting it therefrom will be described. The operation for mounting the cartridges  7  into the apparatus main assembly  100  or removing them therefrom is to be carried out after opening the front cover  101  of the apparatus main assembly  100  to fully expose the cartridge insertion opening  91  of the apparatus main assembly  100  ( FIGS. 2 and 6 ). When the front cover  101  is in the closed state ( FIG. 1 ), it remains locked to the apparatus main assembly  100  by a latching mechanism (unshown). Thus, in order to mount or remove the cartridge  7 , first, the front cover  101  must be unlocked from the apparatus main assembly  100  by unlatching the latching mechanism, so that the front cover  101  can rotated (opened), along with the electrostatic transferring apparatus  5  attached thereto, frontward of the apparatus main assembly  100 , about the hinge shaft  101   a  located at the bottom of the front cover  101 . As the front cover  101  is rotated (opened) all the way, the cartridge insertion opening  91  of the apparatus main assembly  100  becomes fully exposed. 
   Through the cartridge insertion opening  91 , the first to fourth cartridges  7  are mountable into the cartridge compartment of the apparatus main assembly  100 , in a manner to be stacked in parallel in the direction slightly tilted from the true vertical direction, in the listed order, that is, with the first cartridge mounted at the bottom. More specifically, the cartridge compartment is divided into four cartridge slots, which are the magenta, cyan, yellow, and black cartridge slots, listing from the bottom. The four cartridge slots are identical in cartridge mounting mechanism. Each cartridge slot is provided with a pair of rough guides  103 , a pair of middle guides  104 , and a pair of guiding grooves  105 , for guiding the cartridge  7  into the image forming position.  FIG. 6  shows the inward side of the right plate of the apparatus main assembly  100 . The inward side of the left plate is symmetrical with that of the right plate. 
   An operator is to hold the cartridge  7  by the left and right handles  90 , by grasping the handles  90  with both hands, and to insert the cartridge  7  into the proper cartridge slot through the cartridge insertion opening  91 , so that the rear side of the cartridge  7 , that is, the side opposite to the side where the photosensitive drum  1  is exposed, faces forward, and also, so that the left and right lengthwise end portions of the cartridge  7  will be rested on the left and right rough guides  103  of the apparatus main assembly  100 , respectively. As the cartridge  7  is inserted deeper into the apparatus main assembly  100 , the aforementioned pair of cartridge insertion guides  53  are moved onto the middle guides  104 , causing thereby the cartridge  7  to be lifted from the rough guides  103 . Thereafter, the cartridge  7  is guided by the middle guides  104 . 
   As the cartridge  7  is inserted even deeper into the apparatus main assembly  100 , the left and right supporting members  31   a  and  31   b  of the cartridge  7  are inserted into the left and right guiding grooves  105 , respectively. Then, as the cartridge  7  is further inserted, the supporting members  31   a  and  31   b  come into contact with the deepest end of the guiding grooves  105 , and prevent the further insertion of the cartridge  7  into the apparatus main assembly  100 . As a result, the cartridge  7  is precisely positioned relative to the apparatus main assembly  100  in terms of the widthwise direction. 
   After the cartridges  7  are inserted into the proper cartridge slots as described above, the front cover  101 , which was kept open, is to be closed, and to be locked to the apparatus main assembly  100  by latching the latching mechanism (unshown). The following are accomplished by the means (unshown) which is moved by the closing movement of the front cover  101 : 
   1) Precise positioning of each cartridge  7  relative to the apparatus main assembly  100  in terms of the widthwise direction of each cartridge  7 , and keeping the cartridge  7  pressured in the direction in which the cartridge  7  is inserted; 
   2) Moving the shutter  54  of each cartridge  7  into the open position; and 
   3) Starting the multiple pre-rotations of the image forming apparatus in order to couple the flange  72 , on the driven side, of the photosensitive drum  1  of each cartridge  7  with the driving force transmitting member  73  on the main apparatus side. 
   The function of precisely positioning each cartridge  7  relative to the apparatus main assembly  100  in terms of the widthwise direction of the cartridge  7  is carried out by a pair of pressing members (unshown), which are moved by the movement of the front cover  101 . When the front cover  101  is open, that is, when the front cover  101  is in the state in which the cartridges  7  can be mounted into, or removed from, the apparatus main assembly  100 , the pressing members are in the positions into which they have been retracted from the guiding grooves  105 . Thus, when the cartridge  7  is inserted, the pressing members do not interfere with the supporting members  31   a  and  31   b  of the cartridge  7 . However, as the front cover  101  is closed after the insertion of the cartridges  7 , the pressing members are moved into the positions in which they press the supporting members  31   a  and  31   b  against the deepest end of the guiding grooves  105 , precisely positioning thereby the cartridge  7  relative to the apparatus main assembly  100  in terms of the widthwise direction. 
   With each cartridge  7  precisely positioned relative to the apparatus main assembly  100 , the flange  72 , on the driven side, of the cartridge  7 , and the driving force transmitting rotational member  73  of the apparatus main assembly  100  become coupled with each other, making it possible for the diving force from the motor (unshown) of the apparatus main assembly  100  to be transmitted to the flange  72 , on the driven side, of the cartridge  7 , through the driving force transmitting rotational member  73 ; in other words, it becomes possible for the photosensitive drum  1  of each cartridge  7  to be rotationally driven. It should be noted here that the development sleeve  40 , toner conveyance mechanism  42 , and toner supply roller  43  are driven by the rotation of the photosensitive drum  1  through the gear train (unshown). 
   Also, electrical connection is established between the electrical contacts (unshown) on the cartridge side and the electrical contacts (unshown) on the main assembly side, making it possible for charge bias and development bias to be applied to the cartridge  7  from the power source (unshown) of the apparatus main assembly  100 , and also, for information or the like to be exchanged between the memory element on the cartridge side and the control circuit on the apparatus main assembly side. The structure for grounding the photosensitive drum  1  will be described later in Section (4). 
   All that is necessary to remove the cartridge  7  from the apparatus main assembly  100  is to carry out in reverse the above described steps for mounting the cartridge into the apparatus main assembly  100 . That is, first, the aforementioned latching mechanism is to be unlatched to unlock the front cover  101  from the apparatus main assembly  100 . Then the front cover  101  is rotated downward about the hinge shaft  101   a  located at the bottom of the front cover  101  in order to open the front cover  101  frontward. As the front cover  101  is rotated downward, the cartridge pressing members are retracted by the means which moved by the opening movement of the front cover  101 , being stopped from pressing the cartridge  7 . Also as the front cover  101  is opened, the driving force transmitting rotational member  73  becomes disengaged from the flange  72  of the driven side, and the shutter  54  is moved into the open position. Then, an operator is to grasp the handles  90  of the cartridge  7  with both hands, and to pull the cartridge  7  in the direction opposite to the direction in which the cartridge  7  is pushed when it is mounted into the apparatus main assembly  100 . This will remove the cartridge  7  from the apparatus main assembly  100 . 
   (4) Method for Connecting Drum Grounding Plate  71 ) 
   Next, referring to  FIGS. 7-10 , the method for connecting the drum grounding plate  71  attached to the flange  72  on the driven side will be described in more detail. 
     FIG. 7  is a perspective view of the photosensitive drum (which hereinafter will be referred to as photosensitive drum unit)  1 , and the driving force transmitting rotational member  73  on the apparatus main assembly side.  FIG. 8  is a combination of a perspective view and vertical sectional views of the photosensitive drum unit  1 , showing the steps followed to assemble the photosensitive drum unit  1 .  FIGS. 9 and 10  are enlarged views of the E portion in  FIG. 8 , showing the state of connection between the drum grounding plate  71  and drive shaft  70 .  FIG. 9  shows the positional relationship between the drum grounding plate  71  and drive shaft  70  at the moment the two have just come into contact with each other, or immediately before the two become separated from each other.  FIG. 10  shows the positional relationship between the drum grounding plate  71  and drive shaft  70  when the cartridge  7  has been successfully mounted, being therefore ready for image formation. 
   First, referring to  FIG. 7 , the structure of the photosensitive drum unit  1  will be described. 
   The photosensitive drum unit  1  has a drum cylinder  74 , which comprises, as described before, an aluminum cylinder and a layer of photosensitive substance coated on the peripheral surface of the aluminum cylinder. The photosensitive drum unit  1  also has a pair of flanges  72  and  75 , which are pressed into the openings of the lengthwise ends of the drum cylinder  74 , being virtually integrated with the drum cylinder  74 . The flanges  72  and  75  are rotatably supported by the bearing members  31   a  and  31   b  ( FIGS. 4 and 5 ) with which the cartridge  7  is provided. To the flange  72  located at one of the lengthwise ends of the photosensitive drum unit  1 , the driving force is transmitted from the motor (unshown) within the apparatus main assembly  100  through the driving force transmitting rotational member  73 , rotating thereby the photosensitive drum unit  1 . In this embodiment, hereinafter, the flange  72  to which the driving force is transmitted from the apparatus main assembly  100  will be referred to as the driver flange  72 , whereas the other flange will be referred to as the non-driver flange  75 . 
     FIG. 9  is an enlarged view of the joint between the driver flange  72  and driving force transmitting rotational member  73 . The driver flange  72  is provided with a spiral projection  72   a  (coupling projection) with a non-circular cross section; the cross section of the spiral projection has a plurality of apexes. The apparatus main assembly  100  is provided with the driving force transmitting rotational member  73  for transmitting the driving force from the motor (unshown) within the apparatus main assembly  100 , to the photosensitive drum unit  1 . The driving force transmitting rotational member  73  has a spiral hole  73   a  (coupling hole) with a non-circular cross section; the cross section of the spiral hole has a plurality of apexes. The axial line of the spiral hole  73   a  coincides with that of the driving force transmitting rotational member  73 . As the cartridge  7  is mounted into the apparatus main assembly  100 , the spiral projection  72   a  enters the spiral hole  73   a , enabling the rotational driving force to be transmitted to the photosensitive drum unit  1 . As the driving force transmitting rotational member  73  is rotated by the motor on the apparatus main assembly side, a force A′ which acts in a direction to pull the spiral projection  72   a  into the spiral hole  73  is generated, causing the driving force transmitting rotational member  73  and the photosensitive drum unit  1  to be drawn toward each other. 
   The spiral projection  72   a  is provided with an electrically conductive second member  71  (which hereinafter will be referred to as a drum grounding plate) for establishing an electrical connection between the photosensitive drum unit  1  and apparatus main assembly  100 . The drum grounding plate  71  is located at the center of the spiral projection  72   a , in terms of the radius direction of the projection  72   a , and is placed in contact with the internal surface of the drum cylinder  74 . The driving force transmitting rotational member  73  is provided with an electrically conductive first member  70  (which hereinafter will be referred to as a drive shaft) for establishing our electrical connection between the photosensitive drum unit  1  and apparatus main assembly  100 . The electrically conductive first member, or the drive shaft  70 , projects a predetermined distance from the center of the bottom of the spiral hole  73   a . The first and second electrically conductive members  70  and  71  allow the residual electric charge of the photosensitive drum unit  1  to escape to the apparatus main assembly  100 . 
   Referring to  FIG. 8 , the drum grounding plate  71  is a single-piece member, and flat contact portion  71   a , and a contact portion  71   b  in the form of an arm. The flat contact portion  71   a , as one of the contacting means, is supported by the flat portion  72   g  provided within the driver flange  72 . The arm-shaped contact portion  71   b  as the other contacting means is formed by bending or the like method. In terms of the lengthwise direction, the arm-like contact portion  71   b  extends outward beyond the flat contact portion  71   a.    
   Referring to  FIG. 9 , the driver shaft  70 , which is on the apparatus main assembly side, rotates with the driving force transmitting rotational member  73 . The driver flange  72  is provided with a hole  72   j  into which the drive shaft  70  fits, and which is located roughly at the center of the driver flange  72  in terms of the radius direction. When the image forming apparatus is actually ready for image formation, the tip  70   a  of the drive shaft  70  is in contact with the flat contact portion  71   a . The flat contact portion  71   a  is supported (backed) by the flat portion  72   g  provided within the driver flange  72 . As the drive force transmitting rotational member  73  is rotated, with the spiral projection  72   a  being in the spiral hole  73   a , the force A′ which acts in the direction to pull the projection  72   a  into the hole  73   a  is generated. The contact pressure generated by the force A′ applies to the tip  70   a  of the drive shaft  70  and the flat contact portion  71   a . This concludes the coupling between the driving force transmitting rotational member  73  and driver flange  72  (photosensitive drum unit  1 ). The driving force transmitting rotational member  73  is provided with a spring (unshown) for keeping the connected photosensitive drum unit  1  pressured in the lengthwise direction. Thus, the non-driver flange  75  is kept pressured upon the cleaner unit  50  (cartridge  7 ) through the photosensitive drum unit  1 . Therefore, the cleaner unit  50  is kept pressed upon-the side plate  102  of the apparatus main assembly  100 . As a result, the cartridge  7  is precisely positioned-relative to the apparatus main assembly  100  in terms of the lengthwise direction. 
   Next, the process through which the drum grounding plate  71  becomes connected to its counterpart will be described in detail. Referring to  FIG. 9  which shows the positional relationship between the drum grounding plate  71  and drive shaft  70  at the moment the two have just come into contact with each other, or immediately before the two become separated from each other, prior to the insertion of the cartridge  7  into the apparatus main assembly  100 , the driving force transmitting rotational member  73  of the apparatus main assembly  100  remains retracted, being thereby prevented from interfering with the insertion of the cartridge  7 . As the cartridge  7  is mounted into the apparatus main assembly  100 , and the front cover  101  of the apparatus main assembly  100  is closed ( FIG. 2 ), the driving force transmitting rotational member  73  is moved, while being rotated, toward the driver flange  72  in the lengthwise direction. As a result, the cartridge  7  (driver flange  72 ) and the apparatus main assembly  100  (driving force transmitting rotational member  73 ) become partially connected; the spiral projection  72   a  partially enters the spiral hole  73   a.    
   In this state, the pre-rotation step (in which the apparatus is driven for a predetermined length of time between when a print start signal is inputted and when an actual image forming operation starts) is carried out. During this pre-rotation step, a rotational driving force is inputted from the driving force transmitting rotational member  73  into the drive flange  72  integral with the photosensitive drum unit  1 . There occurs sometimes that the spiral projection  72   a  fails to enter the spiral hole  73   a  (the driver flange  72  fails to mesh with the driving force transmitting rotational member  73 ) because they fail to synchronize in rotational phase during the preceding step, that is, the closing of the front cover  101  of the apparatus main assembly  100 . However, the driving force transmitting rotational member  73  is kept pressed upon  20  the photosensitive drum unit  1  (driver flange  72 ) in the lengthwise direction, as described above. Therefore, the driver flange  72  and the driving force transmitting rotational member  73  are eventually made to synchronize in rotational phase, meshing therefore with each other, during this pre-rotation period. 
   As the rotational driving force is inputted, with the driving force transmitting rotational member  73  and the driver flange  72  partially meshed (the spiral projection  72   a  being partially placed in the spiral hole  73   a ), the force A′, which acts in the direction to draw the driving force transmitting rotational member  73  and driver flange  72  (photosensitive drum unit  1 ) toward each other (draw spiral projection  72   a  into spiral hole  73   a ), is generated. As a result, the spiral projection  72   a  is drawn into the spiral hole  73   a . Therefore, in the driver flange  72 , the drive shaft  70  and drum grounding plate  71  are drawn toward each other, causing the tip of the arm-shaped contact portion  71   b  of the drum grounding plate  71  to come into contact with the tip of the drive shaft  70 , at a point b 1  ( FIG. 9 ). 
   As the input of the rotational driving force is continued, the generation of the force A′ also continues, causing the tip of the arm-shaped contact portion  71   b  of the drum grounding plate  71  to follow the surface of the tip  70   b  of the drive shaft  70  in the radius direction of the drive shaft  70  as indicated by a referential letter c. In other words, the contact point between the tip of the arm-shaped portion  71   b  of the drum grounding plate  71  and drive shaft  70  shifts as the spiral projection  72   a  is drawn into the spiral hole  73   a . Since the drum grounding plate  71  is formed of an electrically conductive elastic substance, it elastically deforms. Therefore, the tip in the arm-shaped portion  71   b  of the drum grounding plate  71  slides on the surface of the tip  70   b  of the drive shaft  70 , while applying a slight pressure upon the surface of the tip  70   b  of the drive shaft  70 , while the spiral projection  72   a  is drawn into the spiral hole  73   a.    
   In other words, as the input of the rotational driving force continues, the spiral projection  72   a  is drawn into the spiral hole  73   a  so that the drive shaft  70  and drum grounding plate  71  are drawn to each other. As a result, the tip  70   a  of the drive shaft  70  comes into contact with the flat portion  71   a  of the drum grounding plate  71  as shown in  FIG. 10 . In other words, at the same time as the cartridge  7  is precisely positioned relative to the apparatus main assembly  100 , electrical connection is established between the apparatus main assembly  100  and present invention drum unit  1  by the contact pressure A′, which is the combination of the force A and the pressure generated by the resiliency of the springs (unshown). Until the tip  70   a  of the drive shaft  70 , and the flat portion  71   a  of the drum grounding plate  71  come into contact with each other, the drive shaft  70  is continuously rotated relative to the drum grounding plate  71 . Thus, during this period, the tip of the arm-shaped portion  71   b  keeps on sliding on the surface of the tip  70   b  of the drive shaft  70 , not only in the lengthwise direction of the drive shaft  70 , but also, in the radially outward direction of the drive shaft  70 . In other words, the tip of the arm-shaped portion  71   b  of the drum grounding plate  71  slides on the surface of the tip of the shaft  70 , from the contact point b 1  ( FIG. 9 ) to the contact point b 2  ( FIG. 10 ) as indicated by the referential letter c in  FIGS. 9 and 10 . Thereafter, the electrical connection between the apparatus main assembly  100  and photosensitive drum unit  1  is maintained at the contact point b 2  between the tip of the arm-shaped portion  71   b  of the drum grounding plate  71 , and the surface of the tip  70   a  of the shaft  70 , by the contact pressure B generated by the arm-shaped portion  71   b  of the drum grounding plate  71 . 
   With the employment of the above-described method for electrically connecting the drum grounding plate  71  to the drive shaft  70 , the contact point between the photosensitive drum unit  1  and the drive shaft  70  does not shift (slide) in the lengthwise direction during the actual image forming operation. That is, while the driver flange  72  and the driving force transmitting rotational member  73  are rotating together, with the spiral projection  72   a  engaged in the spiral hole  73   a , the drum grounding plate  71  (flat portion  71   a  and arm-shaped portion  71   b ) and the drive shaft  70  (tip  70   a  of drive shaft) rotate together while remaining in contact with each other. Therefore, the cartridge  7  is reliably grounded to the apparatus main assembly  100 . The drive shaft  70 , which rotates with the driving force transmitting rotational member  73 , is connected to the power supplying member (unshown) of the apparatus main assembly  100 . The portion of the drive shaft  70  (portion of the power supplying member), which rubs against the power supplying member is coated with electrically conductive grease to assure that the cartridge  7  is grounded to the apparatus main assembly  100 . It should be noted here that the contact point between the photosensitive drum unit  1  and drive shaft  70 , which a user can touch, is not coated with electrically conductive grease. With the provision of the above-described structural arrangement in this embodiment of the present invention in which the contact point between the apparatus main assembly  100  and the cartridge  7  does not shift in the lengthwise direction during a period in which an image is actually formed, the cartridge  7  remains better grounded than with the provision of the structural arrangement in accordance with the prior art. Further, while the driver flange  72  (drum grounding plate  71 ) and the drive shaft  70  rotate together, with the spiral projection  72   a  remaining in the spiral hole  73   a , the wall of the hole  72   j  into which the drive shaft  70  fits, and drum grounding plate  71 , are prevented from being worn by the friction between the two, minimizing therefore their frictional wear. 
   The contact pressure between the flat portion  71   a  of the drum grounding plate  71  and the tip  70   a  of the drive shaft  70  is generated by the force A′ generated as the drive shaft  70  is rotated by the rotational driving force transmitted thereto. In other words, the contact pressure is generated only while the driving force is inputted. Therefore, the members pertinent to the electrical connection between the apparatus main assembly  100  and cartridge  7  are less likely to suffer from fatigue. In addition, at the beginning of the transmission of the driving force, the contact point between the tip  70   a  of the drive shaft  70  and the flat portion  71   a  of the drum grounding plate  71  slightly shifts, creating the wiping effect. 
   As for the grounding of the photosensitive drum unit  1  through the arm-shaped portion  71   b , or the other contact point, of the drum grounding plate  71 , as the spiral projection  72   a  is drawn into the spiral hole  73   a , the tip of the arm-shaped portion  71   b  slides on the surface of the tip  70   b  of the drive shaft  70  while being kept in contact with the surface by its own resiliency. In other words, it wipes itself while wiping the surface of the tip  70   b  of the shaft  70 , better grounding therefore the photosensitive drum unit  1 . 
   The configuration of the tip (inclusive of points  70   a  and  70   b ) of the drive shaft  70  does not need to be limited to the one in this embodiment. It has only to be such that allows the tip of the arm-shaped portion  71   b  of the drum grounding member  71  to be slid on the surface of the tip (inclusive of points  70   a  and  70   b ) of the shaft  70  as the spiral projection  72   a  is drawn into the spiral hole  73   a.    
   Referring to  FIGS. 9 and 10 , in this embodiment, the arm-shaped portion  71   b  of the drum grounding plate  71  is shaped so that while its tip slides on the surface of the tip  70   a  of the drive shaft  70 , the angle of the surface of the tip of the arm-shaped portion  71   b  relative to the line tangential to the surface of the tip  70   b  of the drive shaft  70  remains as small as possible, making it easier for the tip of the arm-shaped portion  71   a  to slide on the surface of the tip  70   a  of the drive shaft  70 . Obviously, as long as the arm-shaped portion  71   b  is shaped so that as the spiral projection  72   a  is drawn into the spiral hole  73   a , the tip of the arm-shape portion  71   b  is allowed to slide on (move in contact with) the surface of the tip  70   a  of the drive shaft  70 , the same wiping effect as that realized by this embodiment can be realized. In other words, the arm-shaped portion  71   b  of the drum grounding plate  71  may be differently bent from the shape in which it is bent in this embodiment. 
   Further, in this embodiment, the arm-shaped portion  71   b  and flat portion plate  71   b  of the drum grounding plate  71  are formed by cutting and bending a single piece of an electrically conductive springy plate. However, the same effects as those described above can be obtained even if two or more electrically conductive members are combined to form the drum grounding plate  71  having the flat and arm-shaped portions  71   a  and  71   b . Further, even if the spiral hole  73   a  with a non-circular cross section and spiral projection  72   a  with a non-circular cross section are switched in position, the same effects as those obtained by this embodiment can be obtained. In other words, even if the driver flange  72  is provided with a non-circular spiral hole  73   a  which has a cross section having a plurality of apexes, and the rotational axis of which coincides with that of the flange  72 , and the driving force transmitting rotational member  73  of the apparatus main assembly  100  is provided with a spiral projection ( 72   a ) which has a cross section having a plurality of apexes, and the rotational axis of which coincides with that of the driving force transmitting rotational member  73 , the same effects as those obtained by this embodiment can be obtained. 
   Further, the drive shaft  70  and drum grounding plate  71  may be switched in position. In other words, even if the drum grounding plate  71  is positioned at the center of the spiral hole  73   a  with a non-circular cross section, and the spiral projection  72   a  with a non-circular cross section is provided with the drive shaft  70 , the rotational axis of which coincides with that of the spiral projection  72   a , the same effects as those obtained by this embodiment can be obtained. However, placing the drum grounding plate  71  at the center of the spiral projection  72   a  with a non-circular cross section makes the hole  72   j  of the spiral projection  72   a , through which the drive shaft  70  is put to be connected to the drum grounding plate  71 , identical in diameter as the drive shaft  70 , and therefore, making it possible to reduce the spiral hole  73   a  in cross section. Therefore, placing the drum grounding plate  71  at the center of the spiral projection  72   a  is superior for the purpose of preventing foreign objects from coming into contact with the springy drum grounding plate  71  from outside. 
   (5) Attachment of Drum Grounding Plate  71   
   Next, referring to  FIG. 8 , the steps to be followed in order to attach the drum grounding plate  71  to the driver flange  72  will be described in detail. 
     FIG. 8  is a combination of a perspective view and vertical sectional views of the driver flange  72  and drum grounding plate  71 , showing the steps to be followed to attach the drum grounding plate  71 . 
   The drum grounding plate  71  is attached to the driver flange  72  following steps  1 )- 8 ) in the numerical order.  FIG. 8  (step  1 ) shows the drum grounding plate  71  and driver flange  72  prior to the attachment of the drum grounding plate  71  to the driver flange  72 . 
   The drum grounding plate  71  in this embodiment is bent roughly in the shape of a letter L. In the sectional view of the drum grounding plate  71 , at a plane perpendicular to the vertical portion of the letter “L” (direction perpendicular to lengthwise direction described before), the drum grounding plate  71  has a pair of projections  71   c  (by which drum grounding plate  71  is guided), which perpendicularly project from the longest edges of the drum grounding plate  71 , one for one. The drum grounding plate  71  also has: a positioning hole  71   d  for precisely positioning the drum grounding plate  71  within the driver flange  72 ; a projection  71   e  by which the drum grounding plate  71  is placed in contact with the internal surface of the drum cylinder to establish electrical connection between the drum grounding plate  71  and the drum cylinder  74 ; and aforementioned flat and arm-shaped portion  71   a  and  71   b.    
   The driver flange  72  is provided with a positioning pin  72   b  which fits into the positioning hole  71   d  of the drum grounding plate  71 , and the spiral projection  72   a  (coupling portion) with a non-circular cross section, which has a plurality of apexes and projects from the end of the driver flange  72 . Referring to  FIG. 8  (step  2 ), the driver flange  72  is provided with an internal space  72   c  into which the drum grounding plate  71  is inserted. When attaching the drum grounding plate  71  to the driver flange  72 , the drum grounding plate  71  is to be inserted, from its contact point side, into the driver flange  72  from the rear end of the driver flange  72  toward the front end, provided that the portion of the driver flange  72 , which has the coupling portion, is the front end portion. 
   As the drum grounding plate  71  is inserted further into the driver flange  72  as shown in  FIG. 8  (step  3 ), the pair of projections  71   c , which perpendicularly project from the longest edges of the drum grounding plate  71 , come into contact with a pair guide portions  72   d  of the driver flange  72 , which are located within the internal space  72   c . Each guide portion  72   d  is tilted backward in terms of the direction in which the drum grounding plate  71  is inserted into the drum grounding plate  71 ; in other words, the bottom end of the guide portion  72   d  is located more inward of the internal space  72   c  than the top end of the guide portion  72   d . Therefore, as the drum grounding plate  71  is further inserted, it is guided downward in the drawing by the pair of guiding portions  72   d . As a result, the tip portion of the drum grounding plate  71 , which essentially is the contact point, is elastically deformed into the space  72   c , and the positioning pin  72   b  of the driver flange  72 , which projects from the rear surface of the driver flange  72  fits into the positioning hole  71   d  of the drum grounding plate  71 . 
   The bottom end of each of the slanted guide portion  72   d  is connected to the slit (groove)  72   e , the dimension of which in terms of the vertical direction in  FIG. 8  is equal to the thickness of the drum grounding plate  71 . Thus, as the drum grounding plate  71  is further insert after its contact with the pair of guiding portions  72   d  in  FIG. 8  (step  4 ), the pair of projections  71   c  of the drum grounding plate  71  slide into the pair of slits (grooves)  72   e , one for one. Even after the sliding of the pair of projections  71   c  of the drum grounding plate  71  into the pair of slits (grooves)  72   e , the tip portion of the drum grounding plate  71  is kept within the space  72   c  by the resiliency of the drum grounding plate  71 . 
   The leading end portion of the drum grounding plate  71 , in terms of the plate insertion direction, is provided with a catch portion  71   f  formed by bending the very tip of the leading end portion of the drum grounding plate  71 . Thus, as the drum grounding plate  71  is further inserted into the driver flange  72  while elastically bending the trailing end portion of the drum grounding plate  71  in the direction indicated by an arrow mark in  FIG. 8  (step  4 ), the catch portion  71   f  is moved beyond the flat end portion  72   g  of the driver flange  72 , and is moved downward, in the drawing, sliding on the flat end portion  72   g , by the resiliency of the drum grounding plate  71  itself. In other words, the tip of the drum grounding plate  71  is moved downward in the space  72   c , as shown in  FIG. 8  (step  5 ). 
   Further, the driver flange  72  is provided with a recess  72   f  into which the catch portion  71   f  of the drum grounding plate  71  latches. As the pressure applied to the rear end of the drum grounding plate  71  is removed, the resiliency of the drum grounding plate  71  generates such force that acts in the direction to move the tip of the drum grounding plate  71  in the direction indicated by an arrow mark in  FIG. 8  (step  6 ). As a result, the back side of the flat portion  71   a  of the drum grounding plate  71  is placed flatly in contact with the flat portion  72   g  of the driver flange  72 . It should be noted here that the dimension m (distance between the rear end and flat end portion  72   g  of the driver flange  72 ) and dimension n (distance between the rear end and flat portion  71   a  of the drum ground plate  71 ) are set so that even if the combination of the tolerances of the drum grounding plate  71  and river flange  72  is substantial, the equation n=m is satisfied, ensuring that the flat portion  71   a  of the drum grounding plate  71  is placed flatly in contact with the flat end portion  72   g  of the driver flange  72 . 
   The catch portion  71   f  located at the very tip of the drum grounding plate  71  latches into the recess  72   f  of the driver flange  72 , preventing thereby the drum grounding plate  71  from floating upward, since the catch portion  71   f  is formed by bending rearward the very tip of the drum grounding plate  71 . Therefore, it entirely fits into the recess  72   f  of the driver flange  72 , and therefore, does not interfere with the electrical connection between the drum grounding plate  71  and drive shaft  70 . 
   In the step  7 ), the positioning pin  72   b  is thermally deformed to form a retainer portion  72   h  in order to prevent the drum grounding plate  71  from moving relative to the driver flange  72 . 
   The above described steps  1 )- 7 ) ensure that the drum grounding plate  71  is precisely positioned in the driver flange  72 . In particular, they ensure that the aforementioned two contact points between the drive shaft  70  and drum grounding plate  71  remain stable in position. 
   In the step  8 ), or the last step, the rear end portion  72   i  of the driver flange  72  is pressed into the drum cylinder  74 . During this step, the projection  71   e  of the drum grounding plate  71  slides on the internal surface of the drum cylinder  74 . After the rear portion  72   i  of the drum grounding plate  71  is pressed into the drum cylinder  74 , the projection  71   e  is kept pressed upon the internal surface of the drum cylinder  74  by the resiliency of the drum grounding plate  71 , ensuring the electrical connection between the drum grounding plate  71  and drum cylinder  74 . 
   The above described steps for assembling the photosensitive drum unit  1  makes it possible to work on the driver flange  72 , from one end of the driver flange  72 , that is, the drum grounding plate  71  can be simply inserted into the driver flange  72 , from the rear end of the driver flange  72 , improving the efficiency with which the photosensitive drum unit  1  is assembled. Further, since the drum grounding plate  71  is formed as a single-piece component, and the interior of the driver flange  72  is structured as described above, not only is it ensured that the photosensitive drum unit  1  is properly grounded, but also, the photosensitive drum unit  1  is improved in assembly efficiency. In other words, according to this embodiment of the present invention, the drum unit  1  can be reliably grounded to the apparatus main assembly  100  with the use of the simpler structural arrangement, improving thereby the image forming apparatus in cost performance. 
   As described above, in this embodiment: 
   1) The photosensitive drum unit  1  is grounded through the electrical connection between the drive shaft  70  (first conductive member) on the main assembly side of the image forming apparatus, and the drum grounding plate  71  (second conductive member) placed within the drum cylinder  74  of the photosensitive drum unit  1 . In the case of this structural arrangement, the drum grounding plate  71  is provided with the flat contact portion  71   a  and arm-shaped contact portion  71   b . and as the rotational driving force is transmitted to the spiral projection  72   a  having entered the spiral hole  73   a , the force A′ which draws the spiral projection  72   a  into the spiral hole  73   a  (driving force transmitting rotational member  73 ) is generated, causing the photosensitive drum unit  1  to be moved toward the driving force transmitting rotational member  73 . Then, as the photosensitive drum unit  1  is moved toward the driving force transmitting rotational member  73 , the flat contact portion  71   a  of the drum grounding plate  71  comes into contact with the tip of the drive shaft  70 , and the tip of the arm-shaped portion  71   b  slides on the surface of the tip  70   a  of the drive shaft  70 . In other words, the structural arrangement in this embodiment provides two means for electrically connecting the photosensitive drum unit  1  and apparatus main assembly  100 , making it possible to better grounding the photosensitive drum unit  1 . 
   Further, the contact point of the photosensitive drum unit  1  and the contact point of the drive shaft  70  are prevented from keeping on rubbing each other after they are connected, making it possible for the cartridge  7  to be more reliably grounded to the apparatus main assembly  100 . Further, the drive flange  72  (drum grounding plate  71 ) is solidly located with the drive shaft  70 . Therefore, the wall of the hole  72   j  of the driver flange  72 , into which the drive shaft  70  fits, and the drum grounding plate  71 , are presented from being frictionally worn. 
   As for the first means for electrically connecting the photosensitive drum unit  1  to the apparatus main assembly  100 , the flat contact portion  71   a  of the drum grounding plate  71  is placed, and kept, in contact with the tip  70   a  of the drive shaft  70  by the contact pressure, or the force A′ generated in the direction to draw the spiral projection  72   a  of the driver flange  72  into the spiral hole  73   a  of the driving force transmitting rotational member  73  as the rotational driving force is transmitted to the spiral projection  72   a  placed in the spiral hole  73   a . Therefore, the contact pressure applies only while the driving force is inputted, preventing the components pertinent to the electrical connection between the photosensitive drum unit  1  and apparatus main assembly  100  from being fatigued by the contact pressure. Further, the point of contact between the contact points on the photosensitive drum unit side and the contact point on the apparatus main assembly side are structured so that they slightly shift in position, and once the connection is fully established, they do not shift in position while the driving force is transmitted. Therefore, the tip  70   a  of the drive shaft  70  and the flat contact portion  71   a  of the drum grounding plate  71  wipe each other at the beginning of the transmission of the driving force, and yet, the components pertinent to the electrical connection between the photosensitive drum unit  1  and apparatus main assembly  100  are not frictionally worn during the rest of the transmission of the driving force. 
   As for the second means for electrically connecting the photosensitive drum unit  1  to the apparatus main assembly  100 , the arm-shaped portion  71   b  of the drum grounding plate  71  slides on the surface of the tip  70   b  of the drive-shaft  70 , while being kept pressured upon the surface of the end portion  70   b  of the drive shaft  70  by the resiliency of the drum grounding plate  71 . Therefore, the point of contact between the arm-shaped portion  71   b  and tip  70   b  shifts in position only at the beginning and end of the transmission of the driving force; it does not shift in position during the rest of the driving force transmission. Therefore, the arm-shaped portion  71   b  and the tip  70   b  of the drive shaft  70  wipe each other as they become fully connected, ensuring the electrical connection between the photosensitive drum unit  1  and apparatus main assembly  100 . 
   2) The drum grounding plate  71  having the flat contact portion  71   a  and the arm-shaped contact portion  71   b  is formed as a single-piece component, reducing thereby the component count. 
   3) As the means for accurately attaching the drum grounding plate  71  to the driver flange  72 , the driver flange  72  is provided with the pair of guide portions  72   d  for guiding the drum grounding plate  71  to the predetermined position in the internal space  72   c , whereas the drum grounding plate  71  is provided with the pair of projections  71   c  by which the drum grounding plate  71  is guided by the pair of guide portions  72   d . Therefore, the drum grounding plate  71  can be attached to the driver flange  72  by simply pushing the drum grounding plate  71  into the driver flange  72  from the rear end of the driver flange  72 , improving the photosensitive drum unit  1  in assembly efficiency. 
   4) For the purpose of preventing the drum grounding plate  71  from moving after its attachment to the driver flange  72 , the drum grounding plate  71  is provided with the catch portion  71   f  bent toward the rear of the drum grounding plate  71 , whereas the driver flange  72  is provided with the recess  72   f  into which the catch portion  71   f  latches. In addition, the drum grounding plate  71  is shaped so that as the drum grounding plate  71  is attached to the driver flange  72 , the catch portion  71   f  latches into the recess  72   f  and remains latched therein, by the resiliency of the drum grounding plate  71  itself. Therefore, the actual contact portion, or the tip, of the drum grounding plate  71  is prevented from floating (becoming separated) from the driver flange  72 . Therefore, it is assured that the point of contact is precisely position. Consequently, the photosensitive drum unit  1  is better grounded. 
   5) The photosensitive drum unit  1  having the above described drum grounding features 1)-4) may be structured so that it can be directly mounted into, or removed from, the apparatus main assembly  100 , without being placed in the process cartridge. Such an arrangement also produces the same effects as those described above. 
   [Miscellanies] 
   1) As for the developing method, one of the widely known developing methods, for example, two component magnetic brush based developing method, cascade developing method, touch-down developing method, cloud developing method, etc., may be employed, instead of the developing method employed in the above described embodiment. 
   2) In the above described embodiment, the so-called contact type charging method is employed as the charging means. Obviously, the charging method does not need to be limited to the above described one. For example, one of the charging methods, which has been widely used in the past, may be employed. Namely, a piece of tungsten wire is surrounded on three sides by a shield formed of metallic substance such as aluminum, so that the positive or negative ions generated by applying high voltage to the tungsten wire can be transferred onto the peripheral surface of the photosensitive drum in order to uniformly charge the peripheral surface of the photosensitive drum. 
   As for the charging means, a blade (charge blade), a pad, a block, a rod, a wire, or the like, may be employed in stead of the aforementioned roller. 
   3) As for the method for removing the toner remaining on the photosensitive drum, a cleaning means in the form of a blade, a fur brush, a magnetic brush, or the like, may be employed instead of the above described one. 
   According to the present invention, it is possible to provide an electrophotographic photosensitive drum capable of reliably establishing electrical connection between it and the main assembly of an electrophotographic image forming apparatus, a process cartridge comprising said electrophotographic photosensitive drum, and an electrophotographic image forming apparatus compatible with said electrophotographic photosensitive drum and process cartridge. 
   Also according to the present invention, it is possible to provide an electrophotographic photosensitive drum capable of wiping its electrically conductive member in order to be reliably grounded, a process cartridge comprising the electrophotographic photosensitive drum, and an electrophotographic image forming apparatus compatible with the electrophotographic photosensitive drum and process cartridge. 
   Also according to the present invention, it is possible to provide an electrophotographic photosensitive drum capable of rotating the first and second electrically conductive members together in order to prevent the conductive members from being frictionally worn, a process cartridge comprising the electrophotographic photosensitive drum, and an electrophotographic image forming apparatus compatible with the electrophotographic photosensitive drum and the process cartridge. 
   While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
   This application claims priority from Japanese Patent Application No. 099503/2004 filed Mar. 30, 2004, which is hereby incorporated by reference.