Abstract:
An electrophotographic photosensitive drum is for a process cartridge detachably mountable to a main assembly of an electrophotographic image forming apparatus. The main assembly includes a motor, and a gear having a non-circular twisted hole. The drum includes a cylinder having a non-circular twisted projection engageable with the hole to receive a driving force from the main assembly when the cartridge is mounted to the main assembly. A recess is formed in the projection and has a bottom portion and a side surface portion. When the projection receives a pulling force toward the hole by rotation of the gear with the projection and the hole being engaged with each other, the bottom portion contacts a spherical surface portion provided at a central portion of the hole, and the side surface portion contacts the spherical surface portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The present application is a Divisional Application of U.S. application Ser. No. 10/960,054 filed Oct. 8, 2004, allowed. 
     
    
     FIELD OF THE INVENTION AND RELATED ART  
       [0002]     The present invention relates to an electrophotographic photosensitive drum (which hereinafter will be referred to simply as “photosensitive drum”), a process cartridge employing an electrophotographic photosensitive drum, and an electrophotographic image forming apparatus employing such a process cartridge.  
         [0003]     Here, an electrophotographic image forming apparatus is an apparatus for forming an image on a recording medium (for example, paper, OHP sheet, etc.) with the use of an electrophotographic image forming method. For example, an electrophotographic copying machine, an electrophotographic printer (for example, an LED printer, a laser beam printer, etc.), a facsimile machine, a word processor, etc., can be included in the category of an electrophotographic image forming apparatus.  
         [0004]     A process cartridge is a cartridge which is removably mountable in the main assembly of an electrophotographic image forming apparatus, and in which a minimum of one among a charging means as a processing means, a developing means as a processing means, a cleaning means as a processing means, and an electrophotographic photosensitive drum, are integrally placed in order to make them removably mountable in the main assembly of the image forming apparatus. It also includes a cartridge which is removably mountable in the main assembly of an electrophotographic image forming apparatus, and in which a minimum of a developing means as a processing means, and an electrophotographic photosensitive drum, are integrally placed in order to make them removably mountable in the main assembly of the image forming apparatus.  
         [0005]     A process cartridge system has long been employed in the field of an electrophotographic image forming apparatus.  
         [0006]     A process cartridge system makes it possible for a user to maintain an electrophotographic image forming apparatus by himself, without relying on a service person, improving substantially operational efficiency. Therefore, it is widely used in the field of an image forming apparatus.  
         [0007]     In recent years, demand for an electrophotographic color image forming apparatus capable of forming a color image has been increasing.  
         [0008]     An electrophotographic image forming apparatus forms four independent images different in their colors. Thus, if any of the four images deviates from a target (ideal) position on the peripheral surface of the photosensitive drum, this results in the formation of a color image with a color deviation.  
         [0009]     A countermeasure for preventing the formation of a color image with the above-described color deviation is as follows. The main assembly of an electrophotographic color image forming apparatus is provided with a gear which receives a driving force from a motor. This gear is provided with a spiral hole, which is located in the center thereof to transmit the driving force to the photosensitive drum. One of the lengthwise ends of the photosensitive drum is provided with a spiral projection, which fits into the spiral hole on the main assembly side to transmit the driving force. With the provision of this structural arrangement, the driving force from the motor on the main assembly side is transmitted to the photosensitive drum through the wall of the spiral hole on the main assembly side, and the spiral projection on the photosensitive drum side, precisely rotating the photosensitive drum (Japanese Laid-open Patent Application 2003-5475).  
         [0010]     In other words, the above-described structural arrangement raises the level of accuracy at which the photosensitive drum is rotated.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention is a further development of the above described prior art.  
         [0012]     The primary object of the present invention is to provide a combination of a photosensitive drum, a process cartridge, and an electrophotographic image forming apparatus, which is superior to that of the prior art, in terms of the level of accuracy at which a photosensitive drum is rotated.  
         [0013]     Another object of the present invention is to provide a combination of a photosensitive drum, a process cartridge, and an electrophotographic image forming apparatus, which prevents the photosensitive drum from fluctuating in rotational velocity.  
         [0014]     Another object of the present invention is to provide a combination of a photosensitive drum, a process cartridge, and an electrophotographic image forming apparatus, which precisely positions the photosensitive drum relative to the main assembly of the image forming apparatus by placing the hemispherical portion of the drive shaft on the main assembly side of the image forming apparatus, in contact with the bottom of the hole of the projection on the photosensitive drum side, in terms of the lengthwise direction of the photosensitive drum, and by placing the base portion of the hemispherical portion in contact with the lateral surface of the hole, in terms of the direction perpendicular to the lengthwise direction of the photosensitive drum.  
         [0015]     Another object of the present invention is to provide a combination of a photosensitive drum, a process cartridge, and an electrophotographic image forming apparatus, which is superior in image quality to that of the prior art.  
         [0016]     Another object of the present invention is to provide a combination of a photosensitive drum a process cartridge, and an electrophotographic image forming apparatus, which prevents the photosensitive drum from fluctuating in rotational velocity even if the axial line of the driving gear on the main assembly side of the image forming apparatus becomes tilted relative to the axial line of the photosensitive drum.  
         [0017]     Another object of the present invention is to provide a combination of a photosensitive drum, a process cartridge, and an electrophotographic image forming apparatus, which minimizes the fluctuation of the rotational velocity of the photosensitive drum in order to minimize the color deviation to form a high quality image.  
         [0018]     According to an aspect of the present invention, there is provided an electrophotographic photosensitive drum for a process cartridge detachably mountable to a main assembly of an electrophotographic image forming apparatus for forming an image on a recording material. The main assembly includes a motor, a main assembly gear for transmitting a driving force from the motor, and a non-circular twisted hole having a section with a plurality of corner portions provided at a central portion of the main assembly gear. The non-circular twisted hole is rotatable integrally with the main assembly gear. The electrophotographic photosensitive drum comprises (a) a cylinder having a photosensitive layer on a peripheral surface thereof; and (b) a non-circular twisted projection provided on one longitudinal end of the cylinder and having a section with a plurality of corner portions. The non-circular twisted projection is engageable with the hole of the main assembly of the apparatus to receive a driving force from the main assembly of the apparatus when the process cartridge is mounted to the main assembly of the apparatus. A recess is formed in a central portion of the projection and has a bottom portion and a side surface portion. When the projection receives a pulling force toward the hole by rotation of the main assembly gear with the projection and the hole being engaged with each other, the bottom portion contacts a spherical surface portion provided at a central portion of the hole, and the side surface portion contacts the spherical surface portion. When the electrophotographic photosensitive drum is mounted to the main assembly of the apparatus, and the electrophotographic photosensitive drum receives a driving force for rotation from the main assembly of the apparatus, an end of the spherical surface portion contacts the bottom portion, and the spherical surface portion contacts the side surface portion.  
         [0019]     The present invention can improve a photosensitive drum in the level of accuracy at which it is rotated.  
         [0020]     The present invention can prevent a photosensitive drum from fluctuating in rotational velocity.  
         [0021]     According to the present invention, the photosensitive drum is precisely positioned relative to the main assembly of the image forming apparatus by placing the hemispherical portion of the drive shaft on the main assembly side of the image forming apparatus, in contact with the bottom of the hole of the projection on the photosensitive drum side, in terms of the lengthwise direction of the photosensitive drum, and by placing the base portion f the hemispherical portion in contact with the lateral surface of the hole, in terms of a direction perpendicular to the lengthwise direction of the photosensitive drum.  
         [0022]     The present invention can improve the quality of images formed by an electrophotographic image forming apparatus.  
         [0023]     The present invention can prevent a fluctuation in the rotational velocity of the photosensitive drum even if the axial line of the gear on the main assembly side of an image forming apparatus becomes tilted relative to the axial line of the photosensitive drum.  
         [0024]     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  
       [0025]      FIG. 1  is a sectional view of the color image forming apparatus in one of the preferred embodiments of the present invention.  
         [0026]      FIG. 2  is a sectional view of the process cartridge in the preferred embodiment of the present invention.  
         [0027]      FIGS. 3A and 3B  are perspective views of the process cartridge in the preferred embodiment of the present invention.  
         [0028]      FIG. 4  is a schematic perspective view of the main assembly of the image forming apparatus and process cartridge, showing how the process cartridge is mounted into the apparatus main assembly.  
         [0029]      FIG. 5  is a perspective view of the drum flange.  
         [0030]      FIG. 6  is a sectional view of the drum driving force transmission mechanism in the preferred embodiment of the present invention.  
         [0031]      FIG. 7  is a sectional view of the drum driving force transmission mechanism in the preferred embodiment of the present invention.  
         [0032]      FIG. 8  is a sectional view of the drum driving force transmission mechanism in the preferred embodiment of the present invention.  
         [0033]      FIG. 9  is a perspective view of the drum driving force transmission mechanism in the preferred embodiment of the present invention.  
         [0034]     FIGS.  10 ( a ) and  10 ( b ) are graphs that provide data obtained by measuring the rotational velocities of the photosensitive drum in the preferred embodiment and a photosensitive drum in accordance with the prior art, respectively, in order to compare the preferred embodiment of the present invention with the embodiment of the prior art, in terms of an irregularity in the rotational velocity of a photosensitive drum. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     Hereinafter, the combination of the electrophotographic photosensitive drum, the process cartridge, and the electrophotographic image forming apparatus, in one of the preferred embodiments of the present invention, will be described with reference to the appended drawings.  
         [0036]     In the following description of the preferred embodiment of the present invention, an image forming apparatus is described as an electrophotographic full-color image forming apparatus employing four process cartridges removably mountable in the main assembly of the image forming apparatus. However, this embodiment is not intended to limit the number of the process cartridges to be removably mounted in an image forming apparatus; in other words, the number of the process cartridges to be removably mounted in an image forming apparatus is optional, and may be determined according to the requirements of use. For example, in the case of an image forming apparatus for forming a monochromatic image, the number of the cartridges mounted in the main assembly of the apparatus is one. Also in the following description of the preferred embodiment of the present invention, the image forming apparatus is described as a printer, which is one form of an image forming apparatus. However, this embodiment is not intended to limit the application of the present invention to a printer. In other words, the present invention is also applicable to image forming apparatuses other than a printer, for example, a copying machine, a facsimile machine, or a machine (multifunction image forming apparatus) capable of performing one or more functions of the preceding image forming apparatuses. Moreover, the application of the present invention is not limited to an image forming apparatus employing an electrostatic transfer belt. That is, the present invention is also applicable to an image forming apparatus which employs an intermediary transferring member, and in which a plurality of developer images different in color are sequentially transferred in layers onto the intermediary transferring member, and then, are transferred all at once onto a recording medium.  
         [0037]     Further, the materials and configurations of the structural components in the following embodiment of the present invention, and their positional relationship, are optional, and are to be changed as necessary according to the structure of an apparatus to which the present invention is applied, and various other factors. In other words, the following embodiment of the present invention is not intended to limit the scope of the present invention.  
         [0000]     [General Structure of Full-color Image Forming Apparatus] 
         [0038]     First, referring to  FIG. 1 , an electrophotographic full-color image forming apparatus in accordance with the present invention will be described.  FIG. 1  is a vertical sectional view of a full-color laser beam printer, which is one form of an image forming apparatus, showing the general structure thereof.  
         [0039]     As shown in  FIG. 1 , the image forming apparatus  100  in accordance with the present invention employs four process cartridges  7 , one for each of the four color components, that is, yellow ( 7 Y), magenta ( 7 M), cyan ( 7 C), and black ( 7 K), and a conveying means for conveying a recording medium to the four process cartridges  7  and discharges the recording medium out of the main assembly of the image forming apparatus. Each of the four cartridges  7  comprises an electrophotographic photosensitive drum (which hereinafter will be referred to simply as “photosensitive drum”)  1  which is rotated at a predetermined peripheral velocity.  
         [0040]     Each cartridge  7  comprises a photosensitive drum  1  ( 1 Y,  1 M,  1 C, and  1 K) which is rotationally driven, a charge roller  2  ( 2 Y,  2 M,  2 C, and  2 K) for uniformly charging the peripheral surface of the photosensitive drum  1 , a development unit  4  ( 4 Y,  4 M,  4 C, and  4 K) having a development roller  40  as a developing means for developing an electrostatic latent image formed on the photosensitive drum  1 , by adhering developer to the electrostatic latent image, a cleaner unit  50  ( 50 Y,  50 M,  50 C, and  50 K) having a cleaning blade  60  as a cleaning means for removing the residual developer, that is, the developer remaining on the peripheral surface of the photosensitive drum  1  after the image transfer. The charge roller  2 , development unit  4 , and cleaner unit  50  are placed in the adjacencies of the peripheral surface of the photosensitive drum  1 . The main assembly of the image forming apparatus is provided with a plurality of scanner units ( 3 Y,  3 M,  3 C, and  3 K) for forming an electrostatic latent image on the peripheral surface of the photosensitive drum  1 .  
         [0041]     The image formation process of this image forming apparatus is as follows: First, the peripheral surface of the photosensitive drum  1  is uniformly charged by the charge roller  2  as a charging means. Then, the uniformly charged peripheral surface of the photosensitive drum  1  is exposed to a beam of laser light projected by the scanner unit while being modulated with video signals, forming thereby an electrostatic latent image on the peripheral surface of the photosensitive drum  1 . This electrostatic latent image is developed by the developing means (development roller  40 ) of the development unit  4 , which uses developer.  
         [0042]     The conveying means for conveying a recording medium to the cartridge  7  has the following structure. In a cassette  17  located in the bottom portion of the apparatus main assembly  100 A, a plurality of recording media S are stored. The recording media S are fed out of the cassette  17  by a feed roller  18  while being separated one by one, and are sent to a pair of registration rollers  19 . Then, each recording medium S is conveyed by the pair of registration roller  19  to an electrostatic transfer belt  11  in synchronism with the progression of an image forming operation. Then, the recording medium S is conveyed by the electrostatic transfer belt  11  sequentially along the four cartridges  7 . The transfer belt  11  is stretched around, and suspended by, the rollers  13 ,  14   a ,  14   b , and  15 , and is circularly driven. The transfer belt  11  keeps the recording medium S electrostatically adhered thereto, and conveys the recording medium S through the location, in which the recording medium S is placed in contact with the peripheral surface of the photosensitive drum  1 .  
         [0043]     In this embodiment, the conveying means is made up of the feed roller  18 , the pair of registration rollers  19 , the electrostatic transfer belt  11 , and the pair of discharge rollers  23 . The discharge rollers  23  will be described later.  
         [0044]     Within the loop formed by the transfer belt  11 , four transfer rollers ( 12 Y,  12 M,  12 C, and  12 K) are placed in parallel in the positions in which they oppose the four photosensitive drums  1  ( 1 Y,  1 M,  1 C, and  1 K), respectively, and contact the transfer belt  11 . In these positions, the developer images, which are formed on the peripheral surfaces of the photosensitive drums  1 , one for one, and are different in color, are sequentially transferred in layers by the application of transfer bias to the transfer rollers  12 , onto the recording medium S, while the recording medium S is conveyed by the transfer belt  11 . As a result, a color developer image is formed on the recording medium S.  
         [0045]     After the formation of the color developer image on the recording medium S, the recording medium S is conveyed to a fixing station  20  in which the next step is carried out. In the fixing station  20 , the color developer image is fixed to the recording medium S by the application of heat and pressure. Thereafter, the recording medium S is discharged by the pair of discharge rollers  23  into a delivery station  24  located on top of the apparatus main assembly.  
         [0000]     [Structure of Process Cartridge] 
         [0046]     Next, referring to  FIGS. 3 and 4 , the general structure of the cartridge  7  will be described. In this embodiment, the cartridge  7 Y having yellow developer, the cartridge  7 M having magenta developer, the cartridge  7 C having cyan developer, and the cartridge  7 K having black developer are identical in structure. Each cartridge  7  has the photosensitive drum  1 , and one or more processing means, which are a charging means (processing means), developing means (processing means), and cleaning means (processing means). The processing means are placed in the adjacencies of the peripheral surface of the photosensitive drum  1 .  
         [0047]     The photosensitive drum  1  comprises a cylindrical substrate  63 , for example, an aluminum cylinder, and a photosensitive layer coated on the peripheral surface of the aluminum cylinder  63 . The photosensitive drum  1  is rotatably supported by the pair of shafts attached to the lengthwise ends of the aluminum cylinder  63 . To one of the lengthwise ends of the photosensitive drum  1 , the driving force from a motor is transmitted, rotating the photosensitive drum  1  (in counterclockwise direction).  
         [0048]     The charging means is for uniformly charging the peripheral surface of the photosensitive drum  1 . In this embodiment, a charge roller  2 , which is electrically conductive, is employed as the charging means. The charge roller  2  is kept in contact with the peripheral surface of the photosensitive drum  1  by a pressure generating means, such as a pair of springs. As a charge bias is applied to the charge roller  2 , the peripheral surface of the photosensitive drum  1  is uniformly charged by the charge roller  2 .  
         [0049]     The developing means, which uses developer, is for developing an electrostatic latent image formed on the peripheral surface of the photosensitive drum  1 . To describe the structure of the developing means, it comprises a developer container (developer storage portion)  41  in which developer is contained, and from which the developer is sent to a developer supply roller  43  by a developer sending mechanism  42  located within the developer container  41 . The developer supply roller  43  is rotated in the clockwise direction ( FIG. 2 ). It supplies the development roller  40  with the developer, and also, strips the developer from the development roller  40  after the development. The developer supplied to the development roller  40  is evenly coated, while being given electric charge, on the peripheral surface of the development roller  40  by a development blade  44  kept pressured against the peripheral surface of the development roller  40 . As a development bias is applied to the development roller  40  from the apparatus main assembly  100 A, the development roller  40  develops the latent image; it adheres the developer to the latent image. The development roller  40  is positioned in parallel to the photosensitive drum  1 , and is rotated in the clockwise direction indicated by an arrow mark in  FIG. 2 .  
         [0050]     The developer container  41 , the developer supply roller  43 , the development roller  40 , and the development blade  44  belong to the development unit  4 .  
         [0051]     The cleaning means is for removing the developer remaining on the peripheral surface of the photosensitive drum  1  after the developer image is transferred from the photosensitive drum  1 . The cleaning blade  60 , as the cleaning means, belongs to the cleaner unit  50 . In other words, the cleaner unit  50  comprises the cleaning blade  60  for removing the developer remaining on the peripheral surface of the photosensitive drum  1 , a removed developer storage chamber (removed developer storing portion)  55  for storing the developer removed from the peripheral surface of the photosensitive drum  1  by the cleaning blade  60 , and a flexible sheet  80  for preventing the developer removed by the blade  60  from leaking from the cleaner unit frame  51 . The sheet  80  is positioned so that a small amount of contact pressure is generated between its tip and peripheral surface of the photosensitive drum  1 .  
         [0052]     The unit  50  also comprises the photosensitive drum  1  and the charge roller  2 , in addition to the cleaning means, and is connected to the development unit  4  so that the two units  50  and  4  are rotatable relative to each other. In other words, the cartridge  7  is made up of the cleaner unit  50  and development unit  4 .  
         [0053]     To describe the invention in detail, the photosensitive drum  1  is rotatably attached to a frame  51  of the cleaner unit  50 , with a pair of bearings  30  and  31  placed between the photosensitive drum  1  and frame  51 . Further, the charge roller  2 , the cleaning blade  60 , and the sheet  80  are placed in contact with the photosensitive drum  1 , as described before. Further, the removed developer storage chamber  55  is a part of the cleaner unit  50 .  
         [0054]     The development unit  4  has a frame  46  formed by joining two separate frames  45   a  and  45   b  by ultrasonic welding. To this development unit frame  46 , the development roller  40 , the developer container  41 , the developer supply roller  43 , and the development blade  44  are attached.  
         [0055]     The development unit  4  is connected to the cleaner unit  50  in the following manner. First, the two units  4  and  50  are positioned so that connective holes  47  and  48  of the development unit  4 , located at the lengthwise ends of the development unit frame  46 , align with supportive holes  52  and  53  of the cleaner unit  50 , located at the lengthwise ends of the cleaning unit frame  51 , respectively. Then, a pair of pins (unshown) are inserted through the holes  47  and  52 , and holes  48  and  53 , from outside the lengthwise ends of cleaner unit frame  51 . As a result, the development unit  4  is connected to the cleaner unit  50  so that it is rotatable relative to the cleaner unit  50 .  
         [0056]     The development unit  4  is kept under the pressure generated by a pair of springs (unshown), or the like elastic members, toward the cleaner unit  50 , in order to keep a pair of spacer rings (unshown) fitted around the development roller  40 , pressed upon the peripheral surface of the photosensitive drum  1 . The pair of springs (unshown) are attached to the lengthwise ends of the unit  50 , one for one.  
         [0000]     [Structure for Mounting Process Cartridge into Main Assembly of Image Forming Apparatus or Removing it therefrom] 
         [0057]     Next, referring to FIGS.  3 ( a ),  3 ( b ) and  4 , the structure of the means for removably mounting the cartridge  7  into the main assembly  100 A of the image forming apparatus  100  will be described.  
         [0058]     Referring to  FIG. 4 , the image forming apparatus main assembly  100 A is provided with a front cover  101 , which is hinged to the main assembly  100 A. The aforementioned transfer belt  11  is attached to the inward side of the front cover  101 . In other words, the front cover  101  and transfer belt  11  are attached to the apparatus main assembly  100 A so that they can be opened or closed against the apparatus main assembly  100 A. When the front cover  100  and transfer belt  11  are open, the cartridge  7  can be removably mounted into the main assembly  100 A. The aforementioned transfer belt  11  is attached to the inward side of the front cover  101 . In other words, the front cover  101  and transfer belt  11  are attached to the apparatus main assembly  100 A so that they can be opened or closed against the apparatus main assembly  100 A. When the front cover  100  and transfer belt  11  are open, the cartridge  7  can be removably mounted into the main assembly  100 A.  
         [0059]     Referring to  FIG. 3 , the cartridge  7  is provided with a pair of guiding portions  30   a  and  31   a , which are located at the lengthwise ends of the cartridge  7  to guide the cartridge  7  when mounting the cartridge  7  into the apparatus main assembly  100 A or removing it therefrom. The cartridge  7  is also provided with a pair of handles  30   b  and  31   b , which are located at the lengthwise ends of the cartridge  7 .  
         [0060]     As for the apparatus main assembly  100 A, it is provided with a pair of guide rails (unshown) for guiding the guiding portions  30   a  and  31   a  of the cartridge  7 .  
         [0061]     When mounting the cartridge  7  into the apparatus main assembly  100 A, the cartridge  7  is to be inserted into the apparatus main assembly  100 A so that the guiding portions  30   a  and  31   a  follow the guide rails of the apparatus main assembly  100 A. This makes it possible to properly mount the cartridge  7  into the apparatus main assembly  100 A. When taking the cartridge  7  out of the apparatus main assembly  100 A, the cartridge  7  is to be pulled outward so that the guiding portions  30   a  and  31   a  follow the guide rails of the apparatus main assembly  100 A. This makes it easier to remove the cartridge  7  from the apparatus main assembly  100 A. In other words, the provision of the guiding portions  30   a  and  31   a  and the guiding rails makes it easier to mount or dismount the cartridge  7 .  
         [0000]     [Structure for Precisely Positioning Photosensitive Drum relative to Main Assembly of Image Forming Apparatus] 
         [0062]     Next, referring to  FIGS. 5-8 , the structure for precisely positioning the photosensitive drum  1  relative to the main assembly  100 A of the image forming apparatus will be described.  
         [0063]     The main assembly  100 A of the image forming apparatus is provided with a driving force transmitting mechanism for transmitting a driving force to the photosensitive drum  1 .  
         [0064]     Referring to  FIGS. 5, 6 , and  9 , this diving force transmission mechanism of the apparatus main assembly  100 A comprises a motor (unshown), and a driving gear  71  with which the apparatus main assembly  100 A is provided to transmit the driving force from the aforementioned motor. The driving gear  71  has a shaft portion  71   d , which projects from the center of the driving gear  71 . This shaft portion  71   d  has a non-circular spiral hole  71   a , the cross section (perpendicular to its axial direction) of which has a plurality of apexes. The transmission mechanism also comprises a drive shaft  70 , which is put through the center portion of the driving gear  71 , and which projects from the center of the bottom wall of the spiral hole  71   a  of the shaft portion  71   d ; in other words, the drive shaft  70  is put through the shaft portion  71   d . The end of the drive shaft  70 , on the shaft portion side, is made hemispheric, forming a hemispherical portion  70   a . The hemispherical portion  70   a  is positioned so that its center coincides with the axial line of the spiral hole  71   a , and slightly protrudes outward beyond the edge (end surface) of the spiral hole  71   a.    
         [0065]     Next, the driving force receiving mechanism with which the cartridge  7  is provided will be described.  
         [0066]     Referring to  FIG. 6 , the photosensitive drum  1  is provided with a pair of flanges  61  and  62 , which are attached, one for one, to the lengthwise ends of the cylinder  63 , the peripheral surface of which is coated with the photosensitive layer. The flange  61  is provided with a shaft portion  61   a , which is located at the outward end of the flange  61 , and by which the photosensitive drum  1  is rotatably supported by a bearing  31  ( FIG. 3 ), whereas the flange  62  is provided with a shaft portion, which is located at the outward end of the flange  62 , and by which the photosensitive drum  1  is rotatably supported by a bearing  30 . Next, referring to  FIG. 5 , the shaft portion  61   a  is provided with a non-circular spiral projection  61   b , as a portion for receiving the driving force from the main assembly  100 A of the image forming apparatus, the cross section of which has a plurality of apexes. As the cartridge  7  is mounted into the apparatus main assembly  100 A, this projection  61   b  fits into the aforementioned spiral hole  71   a , and receives from the apparatus main assembly  100 A the driving force for rotating the photosensitive drum  1 . To describe this structure in more detail, the driving gear  71  is rotated, with the projection  61   b  positioned in the spiral hole  71   a . Therefore, the rotational force from the driving gear  71  is transmitted to the photosensitive drum  1  through the projection  61   b , while the projection  61   b  remains subjected to the force generated in a direction to draw the projection  61   b  into the hole  71   a , at the interface between the projection  61   b  and the wall of the spiral hole  71   a.    
         [0067]     Referring to  FIG. 7 , the projection  61   b  has a blind center hole  65 , which opens at the end surface of the projection  61   b , on the main assembly side. This hole  65  has: the bottom  65   a  (bottom surface) which comes into contact with the tip (peak)  70   a   2  of the hemispherical portion  70   a  of the of the drive shaft  70 , which protrudes beyond the edge of the spiral hole  71   a ; and the lateral surface  65   b , which comes into contact with the base portion  70   a   1  of the hemispherical portion  70   a  of the drive shaft  70 . In other words, as the projection  61   b  having entered the spiral hole  71   a  receives from the apparatus main assembly  100 A the driving force for rotating the photosensitive drum  1 , the bottom  65   a  of the hole  65  of the projection  61   b  comes into contact with the tip (peak)  70   a   2  of the hemispherical portion  70   a  of the drive shaft  70 , and the lateral surface  65   b  of the hole  65  of the projection  61   b  comes into contact with the base portion  70   a   1  of the hemispherical portion  70   a  of the drive shaft  70 . In this embodiment, the lateral wall  65   b  is made roughly perpendicular to the bottom  65   a . To describe this structure in more detail, first, the projection  61   b  enters the hole  71   a , and then, the tip (peak)  70   a   2  of the hemispherical portion  70   a  comes into contact with the bottom  65   a  (contact point  65   a   1  in  FIG. 7 ). The tip (peak)  70   a   2  comes into contact with the bottom  65   a  in such a manner that after the contact between the tip (peak)  70   a   2  and bottom  60   a , it virtually coincides with the axial line of the photosensitive drum  1 . Further, the base portion  70   a   1  of the hemispherical portion  70   a  comes into contact with the lateral surface  65   b  (contact point  61   a   2  in  FIG. 7 ) in such a manner that the rotational axis of the base portion  70   a   1  virtually coincides with the axial line of the photosensitive drum  1 . Thus, as the driving gear  71  is rotated, the rotational force from the driving gear  71  is transmitted to the photosensitive drum  1  through the interface between the wall of the spiral hole  71   a  and the projection  61   b , while the projection  61   b  remains subjected to the force (thrust) generated in a direction to draw the projection  61   b  into the spiral hole  71   a  at the interface. As a result, the hemispherical portion  70   a  is drawn deeper into the hole  65 , and the tip (peak)  70   a   2  of the hemispherical portion  70   a  is placed in contact with the bottom  65   a  by the above described projection drawing force (thrust), ensuring that the tip (peak)  70   a   2  comes into contact with the bottom  65   a , and the base portion  70   a   1  comes into contact with the lateral surface  65   b . The cross section of the hole  65  perpendicular to the axial line of the flange  61  is circular. In other words, the hole  65  is cylindrical the internal diameter of the hole  65  and the external diameter of the hemispherical portion  70   a  are made roughly the same, allowing therefore the hemispherical portion  70   a  to enter the hole  65 . The reason why the two diameters are roughly the same, that is, the external diameter of the hemispherical portion  70   a  is made slightly smaller than the internal diameter of the hole  65 , is to take the manufacturing tolerances into consideration, in order to ensure that the hemispherical portion  70   a  is allowed to enter the hole  65 . The hole  65  is positioned so that its axial line coincides with the axial line of the photosensitive drum  1 , and the base portion  70   a   1  of the hemispherical portion  70   a  contacts the lateral surface  65   b  of the hole  65  so that the axial line of the base portion  70   a   1  coincides with that of the photosensitive drum  1 . Further, the tip (peak)  70   a   2  of the hemispherical portion  70   a  contacts the flat bottom  65   a  of the hole  65 .  
         [0068]     The tip (peak)  70   a   2  of the hemispherical portion  70   a  is placed in contact with the bottom  65   a  of the hole  65  in the lengthwise direction of the photosensitive drum  1 , that is, the axial direction (thrust direction). As a result, the position of the photosensitive drum  1  relative to the apparatus main assembly  100 A in terms of the lengthwise (thrust) direction of the photosensitive drum  1  is fixed. Further, the base portion  70   a   1  of the hemispherical portion  70   a  contacts the lateral surface  65   b , whereby the position of the photosensitive drum  1  relative to the apparatus main assembly  100 A in terms of a direction perpendicular to the lengthwise direction of the photosensitive drum  1  is fixed; in other words, the photosensitive drum  1  is precisely positioned relative to the apparatus main assembly  100 A in terms of the radial direction (of the photosensitive drum  1 ). Further, in terms of the lengthwise direction of the photosensitive drum  1 , the hole  65  is positioned so that it overlaps with the hemispherical portion  70   a , making it possible to reduce the dimension of the apparatus main assembly  100 A in terms of the lengthwise direction of the photosensitive drum  1 .  
         [0069]     Referring to  FIG. 7 , in this embodiment, the hole  71   a  is spiral, and its cross section is roughly triangular; it is preferred that the cross section of the hole  71   a  is roughly in the form of an equilateral triangle. The projection  61   b  is also spiral, and its cross section is also roughly triangular; it is preferred that the cross section of the projection  61   b  is roughly in the form of an equilateral triangle. It should be noted that when the cross sections of the hole  71   a  and projection  61   b  are roughly in the form of an equilateral triangle, the level of accuracy at which the axial lines of the hole  71   a  and projection  61   b  align with each other is much better than otherwise; in other words, the level of accuracy with which the axial lines of the driving gear  71  and photosensitive drum  1  align with each other is much better than otherwise.  
         [0070]     In this embodiment, the hole  71   a  is in the form of a spiral hole, the cross section of which is roughly triangular, and the projection  61   b  is in the form of a spiral pillar, the cross section of which is also triangular. However, this embodiment is not intended to limit the scope of the present invention. That is, the configuration of the hole  71   a  may be different from the one in this embodiment, as long as the hole  71   a  is in the form of a spiral hole, the cross section of which is non-circular and has a plurality of apexes. Similarly, the configuration of the projection  61   b  may be different from the one in this embodiment, as long as the projection  61   b  is in the form of a spiral projection, the cross section of which is non-circular and has a plurality of apexes. In other words, as long as the projection  61   b , which fits into the non-circular spiral hole, the cross section of which has a plurality of apexes, is such a non-circular spiral projection, the cross section of which has a plurality of apexes, the configuration of the projection  61   b  may be different from the one in this embodiment. With the hole  71   a  and projection  61   b  structured as described above, as the driving gear  71  is rotated while the projection  61   b  is in the hole  71   a , the axial lines of the projection  61   b  and hole  71   a  align with each other in practical terms. In other words, as long as the projection  61   b  and hole  71   a  are structured so that the projection  61   b  receives the driving force from the apparatus main assembly  100 A while remaining subjected to the force generated in the direction to draw the projection  61   b  into the hole  71   a  by the interface between the projection  61   b  and the lateral surface of the hole  71   a , the configurations of the projection  61   b  and hole  71   a  do not need to be limited to those described above.  
         [0071]     Referring to  FIGS. 6 and 8 , the external diameter X of the shaft portion  71   d  of the driving gear  71  is set to be smaller than the internal diameter Y of the hole  74   a  of the supporting frame (supporting member)  74  (X&lt;Y) ( FIGS. 6 and 8 ). Therefore, the shaft portion  71   d  is allowed to move in the radial direction of the shaft  71   d , within the hole  74   a . While the photosensitive drum  1  is connected to the driving gear  71 , that is, while the projection  61   b  is in the hole  71   a , the driving gear  71  does not come into contact with the supporting member  74  of the apparatus main assembly  100 A; in other words, the shaft portion  71   d  does not come into contact with the internal surface of the hole  74   a . Therefore, the driving gear  71  smoothly rotates. Incidentally, while the projection  61   b  is not in contact with the internal surface of the hole  71   a , the shaft portion  71   d  is in contact with the internal surface of the hole  74   a ; in other words, the shaft portion  71   d  is supported by the supporting frame (member)  74 . The gear positioning portion  71   c  of the driving gear  71 , that is, the portion which is positioned opposite to the photosensitive drum  1 , is supported by the supporting frame  72  of the apparatus main assembly  100 A, with a bearing  73  placed between the positioning portion  71   c  and supporting frame  72 ; in other words, it is supported by the supporting frame  72 , with the provision of a minimum amount of a gap, between the positioning portion  71   c  and supporting frame  72  (bearing  73 ), necessary to allow the driving gear  71  to slide relative to the apparatus main assembly  100 A. Therefore, while the aforementioned rotational driving force is transmitted, the position of the driving gear  71  relative to the photosensitive drum  1  remains fixed in terms of the radial direction of the photosensitive drum  1 . In other words, the position of the driving gear  71  in terms of its radial direction is fixed as the position of the photosensitive drum  1  in terms of its radial direction becomes fixed. Further, as described above, the lengthwise ends of the photosensitive drum  1  are supported by the pair of bearings  30  and  31  with which the cartridge frame is provided. Therefore, the position of the photosensitive drum  1  in terms of its radial direction becomes fixed as its lengthwise ends are supported by the pair of bearings  30  and  31 . Therefore, the axial line of the drive shaft  70  of the apparatus main assembly  100 A, and the axial line of the photosensitive drum  1  coincide with each other. Incidentally, the drive shaft  70  is integrally attached to the driving gear  71 , and the drive shaft  70  and the driving gear  71  are attached to the bearing  73  so that they do not move relative to the apparatus main assembly  100 A in their axial directions. In comparison, the photosensitive drum  1  is supported by the bearings  30  and  31  so that it is allowed to move relative to the apparatus main assembly  100 A in its axial direction.  
         [0072]     Even if the photosensitive drum  1  and driving gear  71  are positioned relative to each other so that their axial lines coincide with each other, the axial line of the drive shaft  70  sometimes becomes slightly tilted relative to the axial line of the photosensitive drum  1 , because of the tolerances of the components for supporting them. Hereafter, the angle of this tilt will be referred to as declination angle θ. Referring to  FIG. 7 , in this embodiment, the photosensitive drum  1  is precisely positioned by placing the tip (peak)  70   a   2  of the hemispherical portion  70   a  in contact with the bottom  65   a  of the hole  65 , in terms of the lengthwise direction of the photosensitive drum  1 . Therefore, as the tip (peak)  70   a   2  comes into contact with the bottom  65   a , a gap U is always left between the end surface  61   b   1  of the projection  61   b  and the bottom  70   a   1  of the hole  71   a . In this embodiment, it is ensured that even if the axial line of the drive shaft  70  slightly (at declination angle of θ) deviates from that of the photosensitive drum  1 , the gap U always remains between the end surface  61   b   1  of the projection  61   b  and the bottom  70   a   1  of the hole  71   a . Therefore, the tip (peak)  70   a   2  of the hemispherical portion  70   a  never fails to contact the bottom  65   a  of the hole  65 . Therefore, even if the axial line of the driving gear  71  (drive shaft  70 ) tilts relative to the axial line of the photosensitive drum  1 , the rotational velocity of the photosensitive drum  1  is not affected. In other words, in this embodiment, even if the axial line of the drive shaft  70  deviates by the declination angle θ, the rotational velocity of the photosensitive drum  1  does not become irregular. This is because the tip (peak)  70   a   2  of the hemispherical portion  70   a  and the bottom  65   a  of the hole  65  contact each other in such a manner that the contact point between them remains coincident with the axial line of the photosensitive drum  1 . In other words, even if the axial line of the drive shaft  70  becomes tilted by the declination angle  8 , the tip (peak)  70   a   2  contacts the bottom  65   a  so that the contact point between them coincides with the axial line of the photosensitive drum  1 . Therefore, the photosensitive drum  1  of the cartridge  7  is prevented from becoming irregular in rotational velocity. Therefore, it is possible to prevent the occurrence of color deviation attributable to the irregularity in the rotational velocity of the photosensitive drum  1 , when forming a full-color image.  
         [0073]     Incidentally, designated by reference characters  71   b  is the actual gear portion of the driving gear  71 . More specifically, the actual gear portion  71   b  is helical.  
         [0074]     As described above, in this embodiment, the position of the photosensitive drum  1  relative to the apparatus main assembly  100 A in terms of the axial direction of the photosensitive drum  1  is fixed by the force generated in the direction to draw the projection  61   b  into the hole  71   a  as the driving gear  71  is rotated after the projection  61   b  fits into the hole  71   a . Further, the tip (peak)  70   a   2  of the hemispherical portion  70   a  is placed in contact with the bottom  65   a  of the hole  65 . Therefore, even if the axial line of the driving gear  71  becomes tilted relative to the axial line of the photosensitive drum  1 , the rotational velocity of the photosensitive drum  1  does not become irregular. Therefore, the photosensitive drum  1  of one cartridge  7  does not become different in rotation from the photosensitive drums  1  in the other cartridges  7 . In other words, this embodiment makes it possible to precisely control the rotation of the photosensitive drum  1 , making it possible to minimize the color deviation attributable to the irregularity in the rotation of the photo sensitive drum  1 . Therefore, this embodiment can yield a color image of higher quality.  
         [0075]     At this time, referring to  FIG. 10 , the results of the comparison between the above-described embodiment of the present invention, and a comparative example, will be described.  FIG. 10  provides data for comparing the above-described embodiment of the present invention, and the comparative example, in terms of the irregularity in the rotation of the photosensitive drum. In the case of the comparative example, the structure for positioning the photosensitive drum relative to the apparatus main assembly in terms of the lengthwise direction of the photosensitive drum is such that the photosensitive drum is positioned relative to the apparatus main assembly by placing the flat bottom surface of the hole provided on the apparatus main assembly side, in contact with the flat end surface of the projection provided on the partridge side. The comparative example was tested, with the declination angle set to 0.3°. To compare the above-described embodiment of the present invention with the comparative example, the embodiment of the present invention was tested, with the declination angle set also to 0.3°.  FIG. 10 ( a ) shows the irregularity in the rotation of the photosensitive drum  1  in the above-described embodiment, and  FIG. 10 ( b ) shows the irregularity in the rotation of the photosensitive drum in the comparative example. In both figures, the ordinate axis represents the amount of the fluctuation in the rotational velocity of the photosensitive drum, and abscissas axis represents the elapsed time. As will be evident from these graphs, the photosensitive drum  1  in the embodiment of the present invention, the test results of which are given in  FIG. 10 ( a ), is smaller in the amount of the fluctuation in the rotational velocity per unit of time (per rotation) than the photosensitive drum in the comparative example, the test results of which are given in  FIG. 10 ( b ).  
         [0076]     In other words, in the case of the embodiment of the present invention, even when the axial line of the driving gear  71  became tilted relative to the axial line of the photosensitive drum  1 , the rotation of the photosensitive drum  1  did not become irregular. This is because the aforementioned tip (peak) contacted the aforementioned bottom in such a manner that the contact point between them was always coincident with the axial line of the photosensitive drum. In other words, even when the axial line of the driving gear  71  became tilted relative to the axial line of the photosensitive drum  1 , the tip (peak) of the hemispherical portion contacted the aforementioned bottom at the axial line of the photosensitive drum  1 .  
         [0077]     Incidentally, when measuring the amount of the irregularity in the rotational velocity of the photosensitive drum, the amount of the torque applied to rotate the photosensitive drum  1  was set to 3 kgf cm, in order to match the amount of the torque applied for the test to the amount of the torque applied to rotate the photosensitive drum in an actual cartridge.  
         [0078]     Next, referring to  FIG. 8 , the structural arrangement for grounding the photosensitive drum  1  to the apparatus main assembly  100 A through the hemispherical portion  70   a  will be described. The other structural arrangements are the same as those described above, and therefore, their description given above is to be referred to for the descriptions of the structural arrangements other than that for the grounding structure.  
         [0079]     The drive shaft  70  and hemispherical portion  70   a  are made electrically conductive, and one end of a contacting member  77  is placed in contact with the rear end of the driving shaft  70 , as shown in  FIG. 8 , and the other end of the contacting member  77  is attached to the supporting frame  72 . The contacting member  77  and supporting frame  72  are formed of metallic plates and are electrically conductive. Therefore, the hemispherical portion  70   a , the drive shaft  70 , the contacting member  77 , and the supporting frame  72  are electrically connected. Further, in this embodiment, the bottom  65   a  of the hole  65  ( FIG. 8 ) is covered with one end of a piece of an electrically conductive metallic plate (photosensitive drum grounding contact)  76 , the other end of the which is electrically connected to the electrically conductive cylinder  63  of the photosensitive drum  1 . In other words, within the hole  65 , one end of the metallic plate  76 , as a grounding contact, electrically connected to the cylinder  63 , is positioned in a manner to cover the bottom  65   a  of the hole  65 , so that as the projection  61   b  fits into the hole  71   a  and receives from the apparatus main assembly  100 A the force for rotationally driving the photosensitive drum  1 , the hemispherical portion  70   a  is pressed on the metallic grounding plate  76  by the aforementioned force generated in the direction to draw the projection  61   b  into the hole  71   a . In other words, this metallic grounding plate  76  comes into contact with the tip (peak)  70   a   2  of the hemispherical portion  70   a , when the cartridge  7  is in the apparatus main assembly  100 A. Further, the one end of the metallic grounding plate  76 , which covers the bottom  65   a  of the hole  65 , and with which the tip (peak)  70   a   2  comes into contact, is flat. Therefore, while the rotational force is transmitted to the photosensitive drum  1 , with the projection  61   b  being in the hole  71   a , the photosensitive drum  1  remains electrically connected to the apparatus main assembly  100 A through the grounding metallic plate  76 , the hemispherical portion  70   a , the drive shaft  70 , the contacting member  77 , and the supporting frame  72 , because while the rotational force is transmitted to the photosensitive drum  1  from the apparatus main assembly  100 A, the aforementioned force generated in the direction to draw the projection  61   b  into the hole  71   a  keeps the hemispherical portion  70   a  pressed upon the metallic grounding plate  76 , ensuring that the hemispherical portion  70   a  and metallic grounding plate  76  remain electrically connected. In addition, the resiliency of the hemispherical portion  70   a  keeps the metallic grounding plate  76  resiliently connected to the hemispherical portion  70   a , also ensuring that the two remain electrically connected. Therefore, the photosensitive drum  1  is reliably grounded and remains grounded.  
         [0080]     With the provision of the above-described structural arrangement, the point of the hemispherical portion  70   a , by which the hemispherical portion  70   a  contacts the metallic grounding plate  76  (and vice versa), that is, by which the hemispherical portion  70   a  keeps the photosensitive drum  1  electrically connected to the drive shaft  70 , does not slide on the metallic grounding plate  76 ; in other words, it remains in contact with only a single spot of the metallic grounding plate  76 . This is because the photosensitive drum  1  and the drive shaft  70  rotate together. Therefore, it is ensured that the cartridge  7  removably mountable in the apparatus main assembly  100 A is electrically connected, and remains connected, to the apparatus main assembly  100 A.  
         [0081]     Further, the force generated in the direction to draw the projection  61   b  into the hole  71   a  during the transmission of the rotational driving force is utilized as the pressure to keep the hemispherical portion  70   a  pressed on the metallic grounding plate  76 . Therefore, a user does not need to be concerned about the fatigue of the components through which the photosensitive drum  1  is grounded. Further, the contact pressure is generated only during the transmission of the rotational driving force, and the force which works in the direction to draw the projection  61   b  into the hole  71   a  is not generated (projection drawing force is turned off) while the driving force is not transmitted; it is generated (projection drawing force is turned on) only during the transmission of the driving force. Thus, as the projection drawing force is turned on or off, the slight misalignment occurs between the axial lines of the photosensitive drum  1  and drive shaft  70 , and this misalignment has the effect of causing the hemispherical portion  70   a  and metallic grounding plate  76  to wipe each other.  
         [0082]     Further, referring to  FIG. 5 , the hole  65  is shaped so that its cross section perpendicular to the axial line of the hole  65  becomes circular, minimizing the amount of the space necessary for the hole  65 . Therefore, it is possible to reduce the size of the projection  61   b  without reducing the projection  61   b  in strength. Accordingly, it is possible to reduce size the hole  71   a  into which the projection  61   b  fit, making it therefore possible to reduce in size the main assembly  100 A of the image forming apparatus.  
         [0083]     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.  
         [0084]     This application claims priority from Japanese Patent Applications Nos. 140695/2004 and 283318/2004 filed May 11, 2004 and Sep. 29, 2004, respectively, which is hereby incorporated by reference.