FIELD OF THE INVENTION RELATED ART 
     The present invention relates to a cartridge, an image forming apparatus, and a drum attaching method. In particular, it is applicable to an image forming apparatus, such as a copy machine, a printer (for example, laser printer and LED printer), which form an image on recording medium (for example, ordinary paper and OHP sheet). 
     The cartridge mentioned in this specification is such a cartridge that is removably mountable in the main assembly of an image forming apparatus. It contains at least an electrophotographic photosensitive drum. It includes also a cartridge which is removably mountable in the main assembly of an image forming apparatus and integrally contains an electrophotographic photosensitive drum, and one or more means for processing the electrophotographic photosensitive drum. The cartridge is removably mountable in the main assembly of an image forming apparatus by a user him- or herself. Therefore, it can make it easier to maintain an image forming apparatus. 
     One of the conventional methods for attaching a photosensitive drum (image bearing member) to the frame of a cartridge is disclosed in Japanese Laid-open Patent Application 2000-75733. According to this application, the drum supporting shaft with which one of the lengthwise end of the photosensitive drum is provided is inserted through the hole of the cartridge frame, which is substantially greater in diameter than the drum supporting shaft, until the other lengthwise end of the photosensitive drum is positioned at a preset point in the cartridge frame, in terms of the lengthwise direction of the cartridge frame. Then, another drum supporting shaft is inserted into the hole with which the other end of the photosensitive drum is provided. Then, the bearing into which the drum supporting shaft of the first lengthwise end of the photosensitive drum is fitted is fitted into the aforementioned hole of the cartridge frame. 
     SUMMARY OF THE INVENTION 
     The present invention is an improvement of the aforementioned conventional art. Thus, the primary object of the present invention is to provide a cartridge, an image forming apparatus, and a photosensitive drum attaching method, which are simpler and more precise in terms of how a photosensitive drum is supported by the frame of a process cartridge. 
     According to an aspect of the present invention, there is provided a cartridge detachably mountable to an image forming apparatus, said cartridge comprising a photosensitive drum having a shaft provided adjacent one axial end; a frame; said frame being provided with a first bearing portion supporting a circumference of said shaft; said frame being provided with a second bearing portion supporting a circumference of said shaft, said second bearing portion being spaced from said first bearing portion with respect to the axial direction and being disposed in a side opposite from said first bearing portion with respect to a plane including an axis of said shaft at the time when said shaft is supported by said first bearing portion; a first clearance space, provided in a position opposite said first bearing portion with respect to the plane, wherein when causing said shaft to be supported by said first bearing portion, said shaft is capable of entering said first clearance space; and a second clearance space, provided in a position opposite said second bearing portion with respect to the plane, wherein when causing said shaft to be supported by said second bearing portion, said shaft is capable of entering said second clearance space. 
     According to another aspect of the present invention, there is provided an image forming apparatus for forming an image on a recording material, said image forming apparatus comprising a photosensitive drum having a shaft provided adjacent one axial end; a frame; said frame being provided with a first bearing portion supporting a circumference of said shaft; said frame being provided with a second bearing portion supporting a circumference of said shaft, said second bearing portion being spaced from said first bearing portion with respect to the axial direction and being disposed in a side opposite from said first bearing portion with respect to a plane including an axis of said shaft at the time when said shaft is supported by said first bearing portion; a first clearance space, provided in a position opposite said first bearing portion with respect to the plane, wherein when causing said shaft to be supported by said first bearing portion, said shaft is capable of entering said first clearance space; and a second clearance space, provided in a position opposite said second bearing portion with respect to the plane, wherein when causing said shaft to be supported by said second bearing portion, said shaft is capable of entering said second clearance space. 
     According to a further aspect of the present invention, there is provided a mounting method for manufacturing a cartridge detachably mountable to an image forming apparatus, the cartridge including a photosensitive drum having a shaft provided adjacent one axial end; a frame; the frame being provided with a first bearing portion supporting a circumference of the shaft; the frame being provided with a second bearing portion supporting a circumference of the shaft, the second bearing portion being spaced from the first bearing portion with respect to the axial direction and being disposed in a side opposite from the first bearing portion with respect to a plane including an axis of the shaft at the time when the shaft is supported by the first bearing portion; a first clearance space, provided in a position opposite the first bearing portion with respect to the plane, wherein when causing the shaft to be supported by the first bearing portion, the shaft is capable of entering the first clearance space; a second clearance space, provided in a position opposite the second bearing portion with respect to the plane, wherein when causing the shaft to be supported by the second bearing portion, the shaft is capable of entering the second clearance space, the method comprising a first step of moving the photosensitive drum such that a part of the shaft enters the first clearance space in a state that the photosensitive drum extends in a direction crossing with the axis; a second step, after the first step, of moving the photosensitive drum such that a part of the shaft enters the second clearance space in a state that the photosensitive drum extends in a direction crossing with the axis; and a third step of moving such that the shaft is brought into contact to the first bearing portion and the second bearing portion from a state that the photosensitive drum extends in a direction crossing with the axis. 
     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 THE DRAWINGS 
         FIG. 1(A)  is a schematic sectional view of the photosensitive drum supporting bearing in the first preferred embodiment of the present invention, and shows the basic structure of the bearing.  FIG. 1(B)  is a schematic sectional view of the image forming apparatus in the first preferred embodiment of the present invention, and shows the general structure of the apparatus. 
         FIG. 2  is a schematic sectional view of the process cartridge in the first preferred embodiment of the present invention, and shows the general structure of the cartridge. 
         FIG. 3(A)  is an exploded perspective view of the development chamber portion of the developing apparatus in the first preferred embodiment of the present invention.  FIG. 3(B)  is an exploded perspective view of the toner storage chamber portion of the developing apparatus. 
         FIG. 4  is an exploded perspective view of the cleaning apparatus, and shows the general structure of the cleaning device. 
         FIG. 5  is a drawing for describing how the charge roller is retracted when the photosensitive drum is mounted into the main assembly of the image forming apparatus. 
         FIGS. 6(A) ,  6 (B), and  6 (C) are drawings for describing the procedure for mounting the photosensitive drum into the main assembly of the process cartridge, and show the state of the photosensitive drum and its adjacencies before, during, and after, respectively, the mounting of the drum. 
         FIGS. 7(A) ,  7 (B), and  7 (C) are drawings for describing the procedure for fitting one of the lengthwise ends of the shaft of the photosensitive drum into the corresponding bearing of the frame of the cartridge, and show the state of the end of the drum shaft, and the corresponding bearing, before, during, and after, respectively, the fitting of the shaft into the bearing. 
         FIG. 8  relates to the photosensitive drum bearing in the first preferred embodiment of the present invention, and  FIG. 8(A)  is a sectional view of the photosensitive drum bearing of the cartridge in the first preferred embodiment of the present invention, as seen from the direction of the axial line of the photosensitive drum;  FIG. 8(B) , a sectional view of the first portion of the bearing, as seen from the direction of the axial line of the photosensitive drum;  FIG. 8(C) , a sectional view of the second portion of the bearing, as seen from the direction of the axial line of the photosensitive drum;  FIG. 8(D) , a sectional view of the joint between the first and second portions of the bearing, as seen from the direction of the axial line of the photosensitive drum; and  FIG. 8(E)  is a sectional view of the joint between the first and second portions of the bearing, at a plane which coincides with the axial line of the photosensitive drum. 
         FIG. 9  relates to the photosensitive drum bearing in the second preferred embodiment of the present invention, and  FIG. 9(A)  is a sectional view of the photosensitive drum bearings of the cartridge in the second preferred embodiment of the present invention, as seen from the direction of the axial line of the photosensitive drum;  FIG. 9(B) , a sectional view of the first portion of the photosensitive drum bearing, as seen from the direction of the axial line of the photosensitive drum;  FIG. 9(C) , a sectional view of the second portion of the bearing, as seen from the direction of the axial line of the photosensitive drum;  FIG. 9(D) , a sectional view of the joint between the first and second portions of the bearing, as seen from the direction of the axial line of the photosensitive drum; and  FIG. 9(E)  is a sectional view of the joint between the first and second portion of the bearing, at a plane which coincides with the axial line of the photosensitive drum. 
         FIG. 10  relates to the photosensitive drum bearing in the third preferred embodiment of the present invention, and  FIG. 10(A)  is a sectional view of the photosensitive drum bearings of the cartridge in the third preferred embodiment of the present invention, as seen from the direction of the axial line of the photosensitive drum;  FIG. 10(B) , a sectional view of the first portion of the photosensitive drum bearing, as seen from the direction of the axial line of the photosensitive drum;  FIG. 10(C) , a sectional view of the second portion of the photosensitive drum baring, as seen from the direction of the axial line of the photosensitive drum;  FIG. 10(D) , a sectional view of the joint between the first and second portions of the bearing, as seen from the direction of the axial line of the photosensitive drum; and  FIG. 10(E)  is a sectional view of the joint between the first and second portions of the bearing, at a plane which coincides with the axial line of the photosensitive drum. 
         FIG. 11  relates to the photosensitive drum bearing in the fourth preferred embodiment of the present invention, and  FIG. 11(A)  is a sectional view of the photosensitive drum bearing of the cartridge in the fourth preferred embodiment of the present invention, as seen from the direction of the axial line of the photosensitive drum;  FIG. 11(B) , a sectional view of the first portion of the photosensitive drum bearing, as seen from the direction of the axial line of the photosensitive drum;  FIG. 11(C) , a sectional view of the second portion of the photosensitive drum baring, as seen from the direction of the axial line of the photosensitive drum;  FIG. 11(D) , a sectional view of the joint between the first and second portions of the bearing, as seen from the direction of the axial line of the photosensitive drum; and  FIG. 11(E)  is a sectional view of the joint between the first and second portions of the bearing, at a plane which coincides with the axial line of the photosensitive drum. 
         FIG. 12  relates to the bearing in the fifth preferred embodiment of the present invention, and  FIG. 12(A)  is a sectional view of the bearing at a plane perpendicular to the plane which coincides with the axial line of the bearing;  FIG. 12(B) , a sectional view of the bearing as seen from the axial line of the bearing; and  FIG. 12(C)  is a perspective view of the bearing. 
         FIG. 13  relates to one of the modifications of the bearing in the fifth preferred embodiment of the present invention, and  FIG. 13(A)  is a sectional view of the bearing at a vertical plane which coincides with the axial line of the bearing; and  FIG. 13(B)  is a sectional view of the bearing as seen from the direction of the axial line of the bearing. 
         FIG. 14  relates to the procedure for mounting the photosensitive drum into the cartridge frame, and  FIG. 14(A)  shows the state of the photosensitive drum immediately before the mounting of the photosensitive drum;  FIG. 14(B) , the state of the photosensitive drum during the mounting of the photosensitive drum; and  FIG. 14(C)  shows the state of the photosensitive drum after the mounting of the drum. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the preferred embodiments of the present invention are described with reference to the appended drawings. In the following description of the preferred embodiments of the present invention, and the related drawings, the components which are the same or correspondent in function are given the same referential codes. 
     Embodiment 1 
     General Structure 
       FIG. 1(B)  is a schematic sectional view of the electrophotographic image forming apparatus A (laser beam printer, for example), in the first preferred embodiment of the present invention, in which a process cartridge B is in the process cartridge chamber E of the main assembly of the apparatus A. It shows the general structure of the image forming apparatus. The process cartridge B is removably mountable in the main assembly of the image forming apparatus A which forms an image on recording medium. 
     The image forming operation of the apparatus A is as follows: The photosensitive drum  7  (image bearing member) is uniformly charged across its peripheral surface. Then, the exposing apparatus  1  scans the uniformly charged portion of the peripheral surface of the drum  7  with the beam of light which it projects while modulating the beam with the image formation signals generated in accordance with the information of the image to be formed. As a result, an electrostatic latent image is formed on the peripheral surface of the drum  7 . This electrostatic latent image is developed into a visible image formed of developer (which hereafter will be referred to as toner). Meanwhile, one of the layered sheets  2  of recording medium (recording sheet, OHP sheet, fabric, etc.) in a cassette  3   a  is fed into the main assembly of the apparatus A, while being separated from the rest, by a combination of the a pickup roller  3   b  and a sheet pressing member  3   c  (which is kept pressed on pickup roller  3   b ). Then, the sheet  2  of recording medium is conveyed further into the main assembly while being guided by a sheet conveyance guide  3   e   1 . While the sheet  2  of recording medium is being conveyed, the toner image on the drum  7  is transferred onto the sheet  2  of recording medium by the application of voltage to a transfer roller (transferring means). After the transfer, the sheet  2  of recording medium is conveyed to a fixing means  5  while being guided by the sheet guide  3   e   2 . The fixing means  5  is made up of a driver roller  3   a  and a rotational fixing member  5   d . The rotational fixing member  5   d  is a cylindrical sheet (endless sheet) and is supported by a supporting member  5   c  which holds a heater  5   b . As the sheet  2  of recording medium is conveyed through the fixing means  5 , the fixing means  5  fixes the transferred unfixed toner image on the sheet  2  to the sheet  2  by applying heat and pressure to the sheet  2  and the toner image thereon. After the sheet  2  is conveyed out of the fixing means  5 , the sheet  2  is conveyed further and is discharged into a delivery tray  6  by a pair of discharge rollers  3   d  through a recording sheet conveyance path which delivers the sheet P in such a manner that the surface of the sheet  2 , which was facing upward in the cassette  3   a , faces downward in the delivery tray  6 . In this embodiment, the pickup roller  3   b , sheet pressing member  3   b , discharge rollers  3   d , etc., make up the recording medium conveying means. 
     (Process Cartridge) 
     Referring to  FIG. 2 , the process cartridge B in this embodiment is made up of a cleaning device C by which the drum  7  is rotatably supported, and a developing device  10  which develops an electrostatic latent image on the peripheral surface of the drum  7 , into a visible image. 
     The developing device D is in connection to the cleaning device C, and is rotationally movable relative to the cleaning device C. The cassette B is structured so that the developing device D and cleaning device C are kept pressed upon each other. As to the image formation by the cartridge B, while the drum  7  having a photosensitive layer is rotated, its peripheral surface is uniformly charged by the application of a preset voltage to the charge roller  8 . Then, the uniformly charged portion of the peripheral surface of the drum  7  is exposed to the beam of light projected upon the peripheral surface of the drum  7  by the exposing apparatus  1  ( FIG. 1(B) ) through an exposure opening  9   b  while being modulated with the signals generated based on the information of the image to be formed. As a result, an electrostatic latent image is formed on the peripheral surface of the drum  7 . This electrostatic latent image is developed by the developing means  10  (developing apparatus) into a visible image formed of toner (developer). After the formation of the toner image on the peripheral surface of the drum  7 , the toner image is transferred onto the sheet  2  of recording medium by a transfer roller  4  ( FIG. 1(B) ) which faces the transfer opening  9   a.    
     (Developing Apparatus) 
     Next, referring to  FIGS. 2 and 3 , the developing apparatus D which is an integral part of the process cartridge B is described. Referring to  FIG. 2 , the frame of the developing apparatus D in this embodiment is made up of a first portion  10   f   1  and a second portion  10   f   2 . It has: a chamber  10   a  which stores toner; and a chamber  10   i  in which an electrostatic latent image is developed with the use of the toner in the chamber  10   a . The toner in the toner storage chamber  10   a  is sent to the development chamber  10   i  through a toner delivery opening  10   k  by a rotatable member  10   b , which is a toner delivering means. The rotational member  10   b  has: a toner stirring shaft  10   b   1 ; and a sheet  10   b   2  solidly attached to the shaft  10   b   1 . The developing apparatus D has also a development roller  10   d , which is a developer bearing member. There is a stationary magnet  10   c  in the hollow of the development roller  10   d . As the development roller  10   d  is rotated, a layer of frictionally charged toner is formed on the peripheral surface of the development roller  10   d  by a development blade  10   e  which is a member for regulating in thickness the toner layer formed on the peripheral surface of the development roller  10   d  as the development roller  10   d  is rotated. The toner particles in this toner layer are transferred onto the peripheral surface of the drum  7  in the pattern of the electrostatic latent image on the peripheral surface of the drum  7 , developing thereby the latent image into a visible image, that is, an image formed of toner. This toner image is transferred onto the sheet  2  of recording medium by the application of voltage to the transfer roller  4  ( FIG. 1(B) ). The voltage applied to the transfer roller  4  is opposite in polarity to the toner. 
     Next, referring to  FIG. 3(A) , the developing apparatus D has: the development roller  10   d ; development blade  10   e ; and components for supplying the development roller  10   d  with electric power. These components are attached to the frame of the cartridge B during the assembly of the cartridge B. Each of the lengthwise end portions of the development roller  10   d  is provided with: a gap maintaining member  10   m  for keeping a preset amount of gap between the development roller  10   d  and drum  7 ; a seal  10   r  for preventing toner from leaking; and an end member  10   g . The end member  10   g  has an arm  10   g   7  which has a hole  10   g   8  for rotatably attaching the developing apparatus D to the cleaning apparatus C. With the provision of this structural arrangement, the first portion  10   f   1  of the frame of the developing apparatus D is rotatably supported by the frame  11   d  of the cleaning apparatus C, in such a manner that the development roller  10   d  is kept parallel to the drum  7  with the presence of the aforementioned preset amount of gap between the development roller  10   d  and drum  7 . 
     Next, referring to  FIG. 3(B) , one of the lengthwise ends of the first portion  10   f   1  of the frame of the developing apparatus D has an opening  10   u  for filling the toner chamber  10   a  with toner. The opening  10   u  is sealed with a cap  10   j  after the filling of the toner chamber with toner. The first portion  10   f   1  of the frame of the developing apparatus D, which is connected to the second portion  10   f   2  of the frame of the developing apparatus D, internally holds: the stirring shaft  10   b   1  for supplying the development roller  10   d  with toner; sheet  10   b   2  solidly attached to the stirring shaft  10   b   1 , and also, for supplying development roller  10   d  with toner; a toner seal  27 ; etc. It has also an opening  10   k  as a toner passage. The toner seal  27  is for keeping sealed the opening  10   k  of the first portion  10   f   1  of the frame of the developing apparatus D, which is for allowing the toner in the toner chamber  10   a  to be supplied to the development roller  10   d . It is thermally attached to the seal seat portion  10   h , which correspond in position to the four edges of the opening  10   k . The developing means  10  has also a seal  10   b   4  for preventing the toner leaking out of the first portion  10   f   1  of the frame of the developing apparatus D. The seal  10   b   4  is fitted around the stirring shaft  10   b   1  along with a gear  10   b   3  which is for transmitting driving force to the stirring shaft  10   b   1  and regulating the stirring shaft  10   b   1  in position in terms of the lengthwise direction of the stirring shaft  10   b   1 . 
     (Cleaning Apparatus) 
     Next, referring to  FIGS. 2 and 4 , the cleaning apparatus C which holds the drum  7  is described. Referring to  FIG. 2 , the toner image formed through the development of the electrostatic latent image by the developing apparatus D is transferred onto the sheet  2  of recording medium through the transfer opening  9   a  of the transfer portion. The toner which is remaining on the drum  7  after the transfer is removed from the drum  7  by a cleaning means  11  attached to the frame  11   d  of the cleaning means  11 , and is stored in the frame  11   d . More specifically, the toner remaining on the peripheral surface of the drum  7  is scraped away from the peripheral surface of the drum  7  by the cleaning blade  11   a , and as it is scraped away, it is scooped up and collected into the waste toner storage  11   c  sealed with the seal  11   h , by a scooping sheet  11   b . Instead of the cleaning blade  11   a , a fur brush, a magnetic brush, or the like may be used as the cleaning means  11 . Referring to  FIG. 4(A) , the drum  7 , the cleaning apparatus C holds the charge roller  8 , an electrode  8   c  for supplying the charge roller  8  with electric power, the cleaning blade  11   a , the drum shutter  12 , etc., which are attached to the frame  11   d  of the cleaning means  11 . The cleaning apparatus C is structured so that when the process cartridge B is not in use, the transfer opening  9   c , through which the drum  7  faces the transfer roller  4 , remains covered with the drum shutter  12 . 
     One end of the drum  7  is fitted with a flange  71 , whereas the other end of the drum  7  is fitted with a flange  8   g . The flange  71  has: a coupler  71   b  for driving force transmission; a drum supporting shaft  71   a ; and a drum gear  71   c . The flange  85  has a hole  85   a  (FIG.  6 (A)), through which the other drum supporting shaft is put to support the drum  7 . The drum gear  71   c  is for transmitting driving force to the development roller  10  and transfer roller  4 . Next, referring to  FIG. 4(B) , the cleaning blade  11   a  is attached to a preselected portion of the frame  11   d  of the cleaning apparatus C with the use of small screws. Further, the process cartridge B has: a seal  11   e  for preventing the waste toner in the waste toner storage chamber  11   c  from leaking out of the chamber  11   c  from the lengthwise ends of the rear side of the cleaning blade  11   a ; and a seal  11   h  for preventing the toner leak which occurs on the rear side of the cleaning blade  11   a . The two seals  11   e  and  11   h  are immovably attached to preselected portions, one for one, of the frame  11   d  of the cleaning apparatus C with the use of two-sided adhesive tape, or the like. The cartridge B has also: a seal  11   f  for preventing the toner from leaking out of the cassette B at the lengthwise ends of the rubber portion of the cleaning blade  11   a ; and the sheet  11   b  (toner scooping sheet) for scooping away the adherent substances, such as the residual toner, on the drum  7 . The seal  11   f  and scooping sheet  11   b  are immovably attached to the frame  11   d  of the cleaning apparatus C with two-sided adhesive tape of the like. 
     The charge roller  8  has: an electrode  8   c  for supplying the charge roller  8  with the electric power from the main assembly A of the image forming apparatus; a bearing  8   b  which rotatably supports the charge roller  8 ; and a bearing  8   a . The electrode  8   c  is attached to the frame  11   d  of the cleaning apparatus C by being fitted in the electrode slot of the frame  11   d . The charge roller bearings  8   b  and  8   a  are assembled as integral parts of the frame  11   d . The shaft portions of the charge roller  8  are fitted in the bearings  8   a  and  8   b  one for one. The bearings  8   a  and  8   b  are under the pressures from springs. Thus, the charge roller  8  is kept pressured toward the drum  7 . Regarding the method for charging the drum  7 , the method for charging the drum  7  may be one of the so-called contact charging methods, which uses a charge roller such as the charge roller  8  in this embodiment, or any of the conventional ones. For example, the drum  7  may be uniformly charged by using a charging means made up of a piece of tungsten wire, and a metallic shield which is made of aluminum or the like and surrounds the wire from three sides. In the case of this charging means, the peripheral surface of the drum  7  is uniformly charged by moving the positive or negative ions generated by applying high voltage to the wire, onto the peripheral surface of the drum  7 . Further, the charging means may be in the form of a roller like the one in this embodiment, or a blade (charge blade), a pad, a block, a rod, etc., which have been used in the past. 
     (Mounting of Drum) 
     Next, referring to  FIGS. 5-7 , and  14 , the method for mounting the drum  7  (image bearing member which is in the form of a rotatably drum, and on which an image is formed) is described. The drum  7  is mounted into the frame  11   d  of the cleaning apparatus C in such a manner that the drum  7  is supported by the shaft  18   d  and bearing  11   d  of the frame  11   d  of the cleaning apparatus C (which hereafter will be referred to simply as cleaning apparatus frame  11   d ). Referring to  FIG. 5 , when the drum  7  is inserted into the cleaning apparatus frame  11   d , the charge roller  8  is kept retracted in the direction indicted by arrow marks to prevent the charge roller  8  from interfering with the drum  7 . 
     (General Procedure for Mounting Drum) 
     Referring to  FIG. 6(A) , one of the lengthwise ends of the cleaning apparatus frame  11   d  is provided with the bearing  11   g , into which the drum supporting shaft  71   a  of the drum gear  71  is inserted. The other lengthwise end of the cleaning apparatus frame  11   d  is provided with a drum shaft insertion hole  11   i , into which the drum shaft  18   d  is pressed. Next, the method for mounting the drum  7  into the cleaning apparatus frame  11   d  is described referring to  FIG. 6(B) . First, the drum supporting shaft  71   a  of the drum gear  7  is to be inserted into the bearing  11   g  from the direction indicated by an arrow mark AR 1 . Then, the hole  85   a  of the flange  85  is aligned with the drum shaft insertion hole  11   i  of the cleaning apparatus frame  11   d  by rotationally moving the drum  7  about the portion of the drum shaft  71   g , which is in the bearing  11   g , in such a direction that the flange ( 85 ) side of the drum  7  moves in the direction indicated by an arrow mark AR 2 . Then, the drum shaft  18   d  is to be moved in the direction indicated by an arrow mark AR 3 , as shown in  FIG. 6(C) , so that the drum shaft  18   d  is put through the drum shaft insertion hole  11   i  of the cleaning apparatus frame  11   d , and then, through the hole  85   a  of the flange  85 . 
     (Insertion of Drum Supporting Shaft into Bearing) 
     Referring to  FIG. 7(A) , the drum supporting shaft  71   a  is coaxial with the coupler  71   b  to which the drum driving force is transmitted from the main assembly A of the image forming apparatus. The drum supporting shaft  71   a  of the drum gear  71  is to be inserted into the bearing  11   g  in a direction which is angled relative to the axial line of the bearing  11   g . Then, it is to be placed in contact with the first and second portions  11   g   1  and  11   g   2  of the bearing  11   g , as shown in  FIG. 7(C) . The first and second portions  11   g   1  and  11   g   2  of the bearing  11   g  are shaped so that they match the drum supporting shaft  71   a  in shape and radius. Thus, the drum  7  is precisely positioned relative to the cartridge frame (and also, charge roller  8 , development roller  10   d , etc.). 
     Referring again to  FIG. 7(A) , in order to make it possible for the drum supporting shaft  71   a  to be inserted into the bearing  11   g  in the direction angled relative to the axial line of the bearing  11   g , the first and second portions  11   g   1  and  11   g   2  of the bearing  11   g  are provided with first and second recesses (clearance recesses)  11   g   3  and  11   g   4 , respectively. Further, the joint  11   g   5  between the first and second portions  11   g   1  and  11   g   2  of the bearing  11   g  is provided with the first and second recesses  11   g   3  and  11   g   4 . This structural arrangement will be described later in detail.  FIG. 7(A)  shows an extension  11   g   6  (semicircular, and the same in thickness as the joint  11   g   5 ), which extends from the second portion  11   g   2  of the bearing  11   g  in the opposite direction from the lengthwise center of the drum  7 . However, the extension  11   g   6  is just for increasing the bearing  11   g  in overall strength, and therefore, the provision of the extension  11   g   6  is not mandatory. 
     The following is a more concrete description of the method for attaching the drum  7  to the cleaning apparatus frame  11   d . That is, first, referring to  FIG. 7(A) , the drum  7  is moved in such a manner that the axial line of the drum  7  is intersectional to the axial line Ce of the bearing  11   g , and also, that the drum supporting shaft  71   a  partially enters the first recess  11   g   3  (First Step). Then, referring to  FIGS. 7(B) and 14(A) , the drum  7  is moved further in such a manner that the axial line of the drum  7  remains intersectional to the axial line Ce of the bearing  11   g , and also, that the drum shaft  71   a  partially enters the second recess  11   g   4  (Second Step). Then, referring to  FIGS. 7(C) and 14(B) , the drum  7 , which is kept in the state in which its axial line is angled relative to the axial line Ce of the bearing  11   g , is moved in such a manner that the drum supporting shaft  71  is supported by the first and second portions  11   g   1  and  11   g   2  of the bearing  11   g  (Third Step). Thus, the axial line of the drum  7  becomes coincident with the axial line Ce of the bearing  11   g . Next, referring to  FIGS. 14(B) and 14(C) , the drum shaft  18   d  is inserted through the drum shaft insertion hole  11   i  of the cleaning apparatus frame  11   d , and then, is inserted into the drum shaft insertion hole  85   a  of the flange  85  (Fourth Step). Thus, the drum  7  becomes properly positioned relative to the cleaning apparatus frame  11   d  in terms of the direction perpendicular to the axial line of the drum  7 . Further, in terms of the direction parallel to the axial line of the drum  7 , the drum  7  is regulated in movement by the lateral portion  11   d   1 , as the first regulating portion, of the cleaning apparatus frame  11   d , and the lateral portion  11   g   7  of the bearing  11   g , as the second regulating portion. That is, the drum  7  has: the end portion  85   b , which is the first portion to be regulated, and can come into contact with the lateral portion  11   d   1 ; and the end portion  71   c   1 , which is the second portion to be regulated and can contact the lateral portion  11   g   7 . The end portion  71   c   1  is the surface of the drum gear  71   c , from which the supporting shaft  71   a  projects. Further, referring to  FIG. 14(C) , the relationship among the dimensions d 1 , d 2 , and d 3  of the drum  7  and cleaning apparatus frame  11   d  in terms of the direction parallel to the axial line of the drum  7  is as follow:
 
 d 1 &lt;d 2 &lt;d 3  (1)
 
     d 1 : distance between the lateral portions  11   d   1  and  11   g   7  of the cleaning apparatus frame  11   d  in terms of the direction parallel to the axial line of the drum  7   
     d 2 : distance between the two ends  71   c   1  and  85   b  of the drum  7  in terms of the direction parallel to the axial line of the drum  7   
     d 3 : distance between the tip of the coupler  71   b  and the lengthwise end  85   b  of the drum  7  in terms of the direction parallel to the axial line of the drum  7 . 
     That is, because the bearing  11   g  is structured as described above, the distance between the lateral portions  11   d   1  and  11   g   7  of the cleaning apparatus frame  11   d  do not need to be made greater than the distance between the tip of the coupler  71   b  and the end portion  85   b . Therefore, it is possible to reduce in size the process cartridge B in terms of the direction parallel to the axial line of the drum  7 . 
     (Detailed Description of Bearing Shape) 
     Referring to  FIG. 1(A) , the bearing  11   g  in this embodiment has: the first and second portions  11   g   1  and  11   g   2 , which correspond in position to the drum supporting shaft  71   a  which is at one of the lengthwise ends of the drum  7 . In terms of the direction parallel to the axial line Ce of the bearing  11   g , the second portion  11   g   2  of the bearing  11   g  is further from the lengthwise center of the cartridge B than the first portion  11   g   1  of the bearing  11   g . Referring to  FIG. 8 , the shape and radius of each of the first and second portions  11   g   1  and  11   g   2  of the bearing  11   d  are set so that the drum supporting shaft  71   a  is precisely supported by the bearing  11   g . The second portion  11   g   2  of the bearing  11   g  is on the opposite side of a flat plane M ( FIG. 8 ), which is coincident with the axial line Ce of the first portion  11   g   1  of the bearing  11   g , from the first portion  11   g   1  of the bearing  11   g . Further, the bearing  11   g  is structured (shaped) so that there is a gap Δ between the end surface  11   g   2   a  of the second portion  11   g   2  of the bearing  11   g , which is closer to the first portion high of the bearing  11   g  than the end surface  11   g   2   a  of the second portion  11   g   2  of the bearing  11   g , and the end surface  11   g   1   a  of the first portion  11   g   1  of the bearing g, in terms of the direction parallel to the axial line Ce of the bearing  11   g  ( FIG. 1 ). Further, the bearing  11   g  is provided with the first and second recesses  11   g   3  and  11   g   4 . The first recess  11   g   3  is on the opposite side of the flat plane M from the first portion  11   g   1  of the bearing  11   g . The second recess  11   g   4  is on the opposite side of the flat plane M from the second portion  11   g   2  of the bearing  11   g . Therefore, when attaching the drum  7  to the cleaning apparatus frame  11   d  by fitting the supporting shaft  71   a  into the bearing  11   g , the supporting shaft  71   a  can be inserted into the bearing  11   g  at an angle. That is, the drum  7  can be inserted into the cleaning apparatus frame  11   d  at an angle α relative to the first and second portions  11   g   1  and  11   g   2  of the bearing  11   g . More concretely, a straight line L 1  which coincides with the edge  11   g   1   b  of the first portion  11   g   1  of the bearing  11   g , the angle of which relative to the axial line Ce of the bearing  11   g  is α, and a straight line L 2  which coincides with the edge  11   g   2   b  of the second portion  11   g   2  of the bearing  11   g , the distance L between the two straight lines L 1  and L 2 , and the radium R of the drum supporting shaft  71   a , have to satisfy the following requirement: 2L&gt;2R.
 
2 L =(2 R +Δ tan α)×cos α=2 R  cos α+Δ sin α
 
Therefore,
 
Δ&gt;2 R (1−cos α)/sin α,Δ&gt;0
 
     That is, if Δ=0, the drum  7  cannot be angularly inserted. 
       FIGS. 8(A)-8(D)  are sectional views of the bearing  11   g  at planes A 1 -A 1 , B 1 -B 1 , C 1 -C 1 , and D 1 -D 1 , respectively, in  FIG. 8(E) , which are perpendicular to the axial line of the bearing  11   g . Referring to  FIG. 8(B) , in order to allow the supporting shaft  71   a  to be inserted at an angle into the bearing  11   g , the first portion  11   g   1  of the bearing  11   g  is on one side of the flat plane M which is coincident with the axial line Ce of the bearing  11   g  (axial line of supporting shaft  71   a  or rotational axis of drum  7 ), and the first recess  11   g   2  is on the opposite side of the flat plane M from the first portion  11   g   1 . Next, referring to  FIG. 8(C) , the second recess  11   g   4  is on the opposite side of the plane M from the second portion  11   g   2  of the bearing  11   g . In this embodiment, the first and second recesses  11   g   3  and  11   g   4  are different from each other in that the former was created by eliminating a portion of the bearing  11   g , whereas the latter was created by partially removing the inward portion of the bearing  11   g . However, they are the same in that both are recesses. 
     Next, referring to  FIG. 8(D) , like the first portion  11   g   1  of the bearing  11   g , the joint  11   g   5  of the bearing  11   g , which corresponds with the recess  11   g   3 , is on the opposite side of the plane M from the recess  11   g   3 . Next, referring to  FIG. 8(C) , on the opposite side of the plane M from the second portion  11   g   2  of the bearing g, there is the second recess  11   g   4 , which is the inward side of the extension  11   g   6 . 
     In this embodiment, the supporting shaft  71   a  is attached to the drum  7 . The bearing  11   g , and the cleaning means  11  for removing the developer remaining on the drum  7 , are attached to the cleaning apparatus frame  11   d . Further, the first portion  11   g   1  of the bearing  11   g , which is on the inward side of the cleaning apparatus frame  11   d  in terms of the direction parallel to the axial line Ce, being therefore closer to the peripheral surface  7   a  of the drum  7 , has the recess  11   g   3 , which is away from the cleaning means  11  in terms of the radium direction of the drum  7 . 
     In the case of a cartridge in accordance with the prior art, if an attempt is made to insert the supporting shaft into the bearing at an angle relative to the axial line of the bearing, the bearing interferes with the insertion of the supporting shaft. Therefore, the angle by which the supporting shaft is allowed to be tilted is limited to the amount tolerable by the clearance between the peripheral surface of the supporting shaft, and the inward surface of the bearing. In comparison, in the case of the cartridge in this embodiment, the bearing  11   g  is provided with the recess  11   g   3  and  11   g   4 . Therefore, even if the supporting shaft  71   a  is inserted into the bearing  11   g  at an angle relative to the axial line of the bearing  11   g , the bearing  11   g  does not interfere with the insertion of the supporting shaft  71   a , as long as the angle is not excessive ( FIG. 7(A) . 
     Next, the force to which the drum  7  is subjected is described. Referring to  FIG. 2 , the cartridge has the cleaning blade  11   a , charge roller  8 , development roller  10   d , transfer roller  4 , etc., which are drum processing means, in the adjacencies of the peripheral surface of the drum  7 . Thus, the drum  7  is subjected to the forces from these processing means. Further, during an image forming operation, the drum  7  is subjected to the friction, which occurs as the cleaning blade  11   a  rubs the peripheral surface of the drum  7 , the forces generated by the meshing of the drum gears  71  with the gear for driving the transfer roller  4  and the gear for driving the development roller  10   d . Therefore, during an image forming operation, the drum supporting shaft  71   a  continuously remains under the combination of these forces while remaining in the bearing  11   g . More concretely, referring to  FIG. 2 , the drum  7  remains pressured toward the cleaning blade  11   a . This is why the first portion  11   g   1  of the bearing  11   g , which bears the force which presses the drum  7  toward the cleaning blade  11   a , is placed close to the peripheral surface  7   a  of the drum  7  in terms of the direction parallel to the axial line of the drum  7 , as shown in  FIG. 7 . Further, in terms of the direction parallel to the axial line of the drum  7 , the dimension of the first portion  11   g   1  of the bearing  11   g  is made greater than that of the second portion  11   g   2  of the bearing  11   g . Further, when the supporting shaft  71   a  is inserted into the bearing  11   g , the first portion high of the bearing  11   g  is more likely to be impacted by the drum supporting shaft  71   a  than the second portion  11   g   2  of the bearing  11   g . Therefore, it is desired that the first portion high of the bearing  11   g  is made longer, being therefore stronger, than the second portion  11   g   2  of the bearing  11   g.    
     Embodiment 2 
       FIG. 9  shows the bearing  111   g  in the second preferred embodiment of the present invention. The bearing  111   g  is different from the bearing  11   g  ( FIG. 8 ) in the first preferred embodiment of the present invention only in that in terms of cross section, its internal surface is not entirely circular. That is, the opposing two portions of the internal surface of the bearing  111   g  are flat.  FIGS. 9(A)-9(D)  are sectional views of the bearings  111   g  at planes A 2 -A 2 , B 2 -B 2 , C 2 -C 2 , and D 2 -D 2  in  FIG. 9(E) , which are perpendicular to the axial line of the bearing  111   g . The pair of flat portions S are parallel to each other, and are perpendicular to the aforementioned flat plane M. The distance between the two flat portions S is equal to the external diameter of the supporting shaft  71   a  so that the shaft  71   a  perfectly fits between the two flat portions S. The two flat portions S regulate the supporting shaft  71   a  in the movement perpendicular to the flat portions S, making it thereby easier to insert the supporting shaft  71   a  into the bearing  111   g  at an angle relative to the axial line of the bearing  111   g.    
     Next, referring to  FIG. 9(B) , in this embodiment, the drum supporting shaft  71   a  is borne by a part of the semi-cylindrical inward surface of the bottom portion of the first portion  111   g   1  of the bearing  111   g , and the two portions of the inward surface of the bearing  111   g , which correspond in position to the intersection of the flat plane M, and the two flat portions S of the bearings  111   g . Next, referring to  FIG. 9(C) , the drum supporting shaft  71   a  is also borne by a part of the semi-cylindrical inward surface of the top portion of the second portion  111   g   2  of the bearing  111   g , and the two portions of the inward surface of the bearing  111   g , which correspond in position to the intersection of the flat plane M, and the two flat portions S of the bearing  111   g.    
     Embodiment 3 
       FIG. 10  shows the bearing  211   g  in the third preferred embodiment of the present invention. The bearing  211   g  is different from the bearing  111   g  ( FIG. 9 ) in the second embodiment only in that the portions Q ( 10 (B)) of the first portion  211   g   1  of the bearing  211   g , which has the two flat portions S 2  of the shaft bearing surface of the bearing  211   g , is in the space in which the recess  211   g   3  is present. Also in this embodiment, however, the first recess  211   g   3  (which is on the top side of flat plane M) is on the opposite side of the flat plane M from the first portion  211   g   1  of the bearing  211   g , because the portion of the first portion  211   g   1  of the bearing  211   g , which actually bears the supporting shaft  71   a , is on the bottom side of the flat plane M. That is, referring to  FIG. 10(B) , the drum supporting shaft  71   a  is borne by a part of the semi-cylindrical portion of the inward surface of the first portion  211   g   1  of the bearing  211   g , and the portions of the inward surface of the first portion  211   g   1  of the bearing  211   g , which correspond in position to the intersection of the flat plane M and the flat portions S 2  of the bearings  211   g . Also in this embodiment, the second recess  211   g   4  (which is below the flat plane M) is on the opposite side of the flat plane M from the second portion  211   g   2  of the bearing  211   g.    
       FIGS. 10(A)-10(D)  are sectional views of the bearings  211   g  at planes A 3 -A 3 , B 3 -B 3 , C 3 -C 3 , and D 3 -D 3  in  FIG. 10(E) , which are perpendicular to the axial line of the bearing  211   g . Referring to  FIG. 10(B) , not only are the portions of the first portion  211   g   1  of the bearing  211   g , which have the flat portions S 2 , in the space below the flat plane M, but also, in the space above the flat plane M, that is, the space in which the recess  211   g   3  is present. However, this setup does not interfere with the insertion of the drum supporting shaft  71   a  into the bearing  211   g  at an angle relative to the axial line of the bearing  211   g , because the portions of the supporting shaft supporting surface of the bearing  211   g , which are on the top side of the flat plane M, are not semi-cylindrical, and flat. In this embodiment, the positional relationship between the first and second portions  211   g   1  and  211   g   2  of the bearing  211   g  relative to the flat plane M is not such that the entirety of the first portion  211   g  is on one side of the flat plane M, and the entirety of the second portion  211   g   2  is on the other side of the flat plane M. However, the positional relationship between the first and second recess  211   g   3  and  211   g   4  is such that the first recess  211   g   3  is on the opposite side of the flat plane M from the second recess  211   g   4 . That is, the first recess  211   g   3  is on one side of the flat plane M, and the second recess  211   g   4  is on the other side. 
     Embodiment 4 
       FIG. 11  shows the bearings  311   g  of the fourth preferred embodiment of the present invention. The fourth embodiment is different from the first to third embodiments only in that in the four embodiments, the drum supporting shaft bearing surface of the bearing  311   g  does not have any semi-cylindrical portion. More concretely, in this embodiment, the shaft bearing surface of the bearing  311   g  is hexagonal in cross section. That is, unlike in the second and third embodiments in which the bearing has a pair of flat portions, the bearing  311   g  in this embodiment has three pairs of flat portions.  FIGS. 11(A)-11(D)  are sectional views of the bearings  311   g  at planes A 4 -A 4 , B 4 -B 4 , C 4 -C 4 , and D 4 -D 4  in  FIG. 11(E) , which are perpendicular to the axial line of the bearing  311   g.    
     Embodiment 5 
     This preferred embodiment is different in the joint of the bearing from the preceding ones. In the case of the bearing  411   g  shown in  FIG. 12 , the joint  411   g   5  is not perpendicular to the axial line of the first and second portions  411   g   1  and  411   g   2  of the bearing  411   g . That is, parts of the joint  411   g   5  are angled relative to the axial line of the first and second portions  411   g   1  and  411   g   2  of the bearing  411   g . In the cases of the first to fourth embodiments, when the supporting shaft  71   a  is inserted into the bearing, the joint was angled relative to the supporting shaft  71   a . In comparison, in this embodiment, when the supporting shaft  71   a  is inserted into the bearing  411   g , the supporting shaft  71   a  is perpendicular to parts of the joint  411   g   5 . 
     In the case of the bearing  511   g  shown in  FIG. 13 , the first and second portions  511   g   1  and  511   g   2  are in connection to each other only across the joint  511   g   5  of the bearing  511   g , which is in the adjacencies of the flat plane M. Further, the bearing  511   g  does not have such a portion that is equivalent to the extension  11   g   6  of the bearing  11   g  in the first embodiment, which is on the opposite side of the plane M from the first portion  11   g   1  of the bearing  11   g . Therefore, the bearings  511   g  is significantly smaller in size than those in the preceding embodiments. 
     In the preceding preferred embodiments of the present invention, the process cartridges were structured so that the drum supporting shaft was attached to the drum, and the bearing was attached to the frame of the cartridge. However, a process cartridge may be reversed in where the drum supporting shaft and bearing are attached. In other words, a process cartridge may be structured so that the drum supporting shaft is attached to the process cartridge frame, and the bearing is attached to the drum. In such a case, the drum is mounted into the cartridge frame at an angle relative to the axial line of the drum supporting shaft on the cartridge frame, in such a manner that the bearing on the drum is fitted around the drum supporting shaft on the cartridge frame. Further, the first and second portions of the bearing on the drum are placed opposite in position from those in the preceding embodiments. That is, the portion of the bearing, which first comes into contact with the tip of the drum supporting shaft on the cartridge frame, is the first portion of the bearing, and the portion of the bearing which next comes into contact with the drum supporting shaft is the second portion of the bearing. It is true also in this case that from the standpoint of strength, it is desired that the portion of the bearing, which first comes into contact with the tip of the drum supporting shaft, is made greater in dimension in terms of the axial line of the bearing than the portion of the bearing, which next comes into contact with the drum supporting shaft, as in the preceding embodiments. 
     Further, the process cartridges in the preceding embodiments described above were for an image forming apparatus for forming monochromatic images. However, the present invention is also compatible with process cartridges for a full-color image forming apparatus which has multiple developing means and is capable of forming multicolor images (two color images, three color images, or full-color images, for example). 
     In the cases of process cartridges in accordance with the prior art, the drum bearing or drum supporting shaft has to be attached to both lengthwise ends of a photosensitive drum, being therefore greater in the component count and assembly step count than process cartridge in accordance with the present invention. That is, in the cases of the preceding embodiments of the present invention, it is only to one of the lengthwise ends of the photosensitive drums that the bearing has to be attached. Further, in comparison to a process cartridge structured so that the drum supporting shaft can be inserted into the bearing at an angle relative to the axial line of the bearing, based on the clearance between the peripheral surface of the drum supporting shaft and the shaft supporting surface of the bearing, the process cartridges in the preceding embodiments of the present invention are smaller in the clearance between the drum supporting shaft and bearing, being therefore higher in the level of precision with which the drum is positioned relative to the cartridge frame, and also, are smaller in the size of the cartridge frame in terms of the direction parallel to the axial line of the drum. In other words, the present invention is suitable for reducing a process cartridge in size. 
     In the above, the present invention was described with reference to an electrophotographic image forming apparatus. However, the preceding preferred embodiments of the present invention are not intended to limit the present invention in scope. That is, the present invention is also applicable to image forming apparatuses which form latent images with the use of magnetism or electricity, instead of light. 
     The present invention makes it possible to provide a process cartridge which is significantly simpler in structure and is more precise in terms of how a photosensitive drum is supported by the cartridge frame than any of process cartridges in accordance with the prior art. 
     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 Applications Nos. 105348/2010 and 090966/2011 filed Apr. 30, 2010 and Apr. 15, 2011, respectively, which are hereby incorporated by reference.
A cartridge detachably mountable to an image forming apparatus, the cartridge includes a photosensitive drum having a shaft provided adjacent one axial end; a frame; the frame being provided with a first bearing portion supporting a circumference of the shaft; the frame being provided with a second bearing portion supporting a circumference of the shaft, the second bearing portion being spaced from the first bearing portion with respect to the axial direction and being disposed in a side opposite from the first bearing portion with respect to a plane including an axis of the shaft at the time when the shaft is supported by the first bearing portion; a first clearance space, provided in a position opposite the first bearing portion with respect to the plane, wherein when causing the shaft to be supported by the first bearing portion, the shaft is capable of entering the first clearance space; and a second clearance space, provided in a position opposite the second bearing portion with respect to the plane, wherein when causing the shaft to be supported by the second bearing portion, the shaft is capable of entering the second clearance space.