Abstract:
An image forming unit including a photosensitive member on the surface of which an electrostatic latent image is formed, a charger for charging the photosensitive member, a developing roller for developing the electrostatic latent image into a visible image, a box for supporting the photosensitive member, the charger and the developing means. A first flange that is integrally formed into one unit including a first taper hole portion that is a positioning part with respect to the apparatus main body for positioning the photosensitive member at the image forming position of the apparatus main body and a first coupling portion that is a rotation engaging portion for receiving a rotation driving force from the apparatus main body is attached to one end of the photosensitive member. Thereby, it is possible to realize an image forming unit having a simple structure, capable of being reduced in size and weight, and realizing a high quality image, and an image forming apparatus capable of improving the positioning and the rotating accuracy of the photosensitive member at the image forming position by using the image forming unit.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an image forming apparatus applicable as, for example, a color printer, a color copying machine, a color facsimile, and the like, and in particular, to a color electrophotographic apparatus forming a color image by using electrophotography and to an image forming unit used therefor. 
     BACKGROUND OF THE INVENTION 
     FIG. 7 is a cross-sectional view showing a conventional image forming unit taken along a plane through its center. The image forming unit shown in FIG. 7 includes a photosensitive member, a charger, a developing means, a cleaning means, a box, and the like. 
     In FIG. 7, numeral  100  denotes an image forming unit, which is formed integrally as one unit including a photosensitive member  110  and process elements provided therearound. Each element includes the following parts. 
     Numeral  121  denotes a charger for homogeneously charging the photosensitive member  110 , and  123  denotes a developing means including a developing roller  122 . Numeral  125  denotes a toner hopper containing toner  124 . The toner  124  is supported on the surface of the developing roller  122  of the photosensitive means  123  and develops the photosensitive member  110 . Numeral  126  denotes a cleaning means for removing toner remaining on the surface of the photosensitive member  110 , which includes a rubber cleaning blade  127  and a waste toner case  128  for containing waste toner. Numeral  129  denotes an exposure window opening for a laser beam to enter the image forming unit  100 . 
     FIG. 8 is a perspective view showing a drive side flange of the photosensitive member with which a gear for driving to rotate the photosensitive member is integrated. 
     As shown in FIG. 8, at one end of the photosensitive member  110 , the drive side flange  111  is fixed. The drive side flange  111  is formed integrally into one unit with a gear portion  111 A receiving a rotation driving force from the apparatus main body. On the other end of the photosensitive member  110  opposing to the drive side flange  111 , a non-drive side flange (not shown) is fixed. The photosensitive member  110  is held in a box  130  by a photosensitive member axis  112  supporting while rotating each flange attached to the photosensitive member  110  at the axis of the photosensitive member  110 . Furthermore, the photosensitive member  110  is positioned at the image forming position of the apparatus main body at both ends of the photosensitive member axis  112 . Moreover, numeral  131  denotes washers provided at both ends of the photosensitive member  112  so that the photosensitive member axis  112  is not prevented from dropping from the box  130 . Furthermore, at the drive side flange  111 , a metal plate  113  is provided in contact with both the photosensitive member  110  and the photosensitive member axis  112  so that the photosensitive member  110  conducts to the apparatus main body. 
     Also in a color printer, a color copying machine, a color facsimile, and the like, similar to monochrome ones, there is a demand towards a high quality image, small size and low cost by producing the image forming apparatus by forming each image forming member as one unit, thus facilitating the handling. Above all, in order to improve the image quality, it is required to improve the positioning accuracy and the rotating accuracy of the photosensitive member of each image forming unit in the photosensitive member at the image forming position. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an image forming unit having a simple structure, capable of reduction in size and weight, and realizing a high quality image, and an image forming apparatus capable of improving the positioning and the rotating accuracy of the photosensitive member at the image forming position by using the image forming unit. 
     In order to attain the above-mentioned object, an image forming unit according to the present invention includes a photosensitive member on the surface of which an electrostatic latent image is formed; a charger for charging the photosensitive member; a developing means for developing the electrostatic latent image into a visible image; and a box for supporting the photosensitive member, the charger and the developing means; and wherein a first flange that is integrally formed into one unit including a positioning part with respect to the image forming apparatus main body for positioning the photosensitive member at the image forming position of the image forming apparatus main body and a rotation engaging portion for receiving a rotation driving force from the image forming apparatus main body is attached to at least one end of the photosensitive member. According to this configuration of the image forming unit, it is possible to lighten and simplify the photosensitive member, to support the photosensitive member axis by the image forming apparatus main body, and to rotate the photosensitive member easily by the axis itself. Therefore, positioning accuracy and rotating accuracy of the photosensitive member are improved. As a result, it is possible to align colors accurately, which is important when forming a color image. Thus, a high quality image can be obtained. 
     Furthermore, it is preferable in the image forming unit according to the present invention that the positioning part is a hole or a concave portion provided concentrically with respect to the photosensitive member axis. According to such a preferable configuration, it is possible to guide the positioning means of the image forming apparatus main body to the positioning part easily and to carry out the positioning by the photosensitive member axis. As a result, it is possible to reduce rocking in rotation of the photosensitive member at the time of rotation driving, and thus a high quality image can be obtained. 
     Furthermore, it is preferable in the image forming unit according to the present invention that a second flange that is integrally formed into one unit including a positioning part for positioning with respect to the image forming apparatus main body is attached to the other end of the photosensitive member opposing the first flange. According to such a preferable configuration, it is possible to simplify the structure of the photosensitive member. Furthermore, since the positioning parts of both of the first flange and the second flange with respect to the image forming apparatus main body are provided in the flange main body, the photosensitive member can be positioned at the image forming position of the image forming apparatus main body at high accuracy. Furthermore, in this case, it is preferable that the second flange is provided with a rotation engaging portion for conveying the rotation driving force from the image forming apparatus main body. According to such a preferable configuration, since it is possible to convey the rotation driving force of the photosensitive member to the positioning means of the image forming apparatus main body that engages the second flange, the positioning means is allowed to rotate together with the second flange so as to inhibit the sliding between the rotation engaging portion of the second flange and the positioning means. As a result, it is possible to prevent the positioning part of the second flange from being deformed and, at the same time, to prevent mislocation in positioning the photosensitive member, and a high quality image can be obtained. Furthermore, in this case, it is preferable that both ends of the photosensitive member are rotatably bearing supported by the box in the outer circumference of the first and second flanges. According to such a preferable configuration, it is possible to use the side face of the flange effectively. In particular, by positioning the photosensitive member at the axis of the photosensitive member of each flange, the rotation accuracy of the photosensitive member can be increased. Thus, the accuracy in aligning colors can be enhanced, thus to obtain a high quality image. Furthermore, it is preferable in this case that the bearing supporting portions for supporting the first and second flanges formed in the box are formed in approximately U-shaped grooves having a thickness that is larger than an outer diameter of the part of the first and second flanges that is bearing supported. According to such a preferable configuration, it is possible to attach/detach the photosensitive member to/from the box easily. 
     Furthermore, it is preferable in the image forming unit according to the present invention that at least one of the first flange and the second flange is made of a conductive material. According to such a preferable configuration, conduction to the image forming apparatus main body can be taken simply without using an additional component for conducting to the image forming apparatus main body in the photosensitive member. Furthermore, since there is no load due to the sliding friction of the component for conducting to the image forming apparatus main body, it is possible to enhance the rotation accuracy of the photosensitive member. 
     Furthermore, it is preferable in the image forming unit according to the present invention that the rotating engaging portion of the first flange is a concave and convex surface formed on one end face of the first flange and the concave and convex surface is located at the outer-most side portion. According to such a preferable configuration, the moving amount of the rotation driving means of the image forming apparatus main body can be reduced, thus to realize the miniaturization of the image forming apparatus main body. Furthermore, in this case, it is preferable that the image forming unit further includes a cover that has substantially the same height as the rotation engaging portion of the first flange and protects the first flange. According to such a preferable configuration, since it is possible to protect the entire part of the flange outer circumference of the bearing supported flange and the rotation engaging portion, it is possible to prevent the positioning part, rotation engaging portion, and bearing supported outer circumference from being damaged due to impact from the outside. 
     Furthermore, an image forming apparatus according to the present invention includes a plurality of image forming units for different colors having a photosensitive member; an image forming unit conveying means for switching the plurality of image forming units by moving them successively between an image forming position and a waiting position; a positioning means for positioning the photosensitive member at a predetermined image forming position; an exposure means for exposing the photosensitive member; a transfer means for transferring sequentially the toner images of many colors, which are formed on the photosensitive member, to form a toner image in which toner images of a plurality of colors are overlapped on the photosensitive member, and a rotation driving means for driving to rotate the photosensitive member and the transfer means; and wherein an image forming unit according to the present invention is used as the image forming unit. According to this configuration of the image forming apparatus, it is possible to realize an image forming apparatus capable of positioning the photosensitive member at the image forming position of the image forming apparatus main body at high accuracy and at the same time of driving to rotate the photosensitive member at high accuracy. As a result, a high quality image can be obtained. 
     Furthermore, it is preferable in the image forming apparatus according to the present invention that at least one of the positioning means and the rotation driving means is made of a conductive material that is in contact with and conducts to the first flange or second flange. According to such a preferable configuration, conduction to the photosensitive member can be taken simply and easily without using additional component. 
     Furthermore, it is preferable in the image forming apparatus according to the present invention that a brake means for providing the photosensitive member with a braking power via the second flange is further included. According to such a preferable configuration, it is possible to suppress the change in rotation of the photosensitive member when the photosensitive member is rotated by the rotation driving force of the developing roller that is in contact with the photosensitive member when the photosensitive member is rotating. Therefore, the rotating accuracy of the photosensitive member can be improved. As a result, a high quality image can be obtained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view showing an overall configuration of an image forming-apparatus in one embodiment according to the present invention. 
     FIG. 2 is a cross-sectional view showing an image forming unit, which is formed integrally as one unit including a photosensitive member and process elements provided therearound in one embodiment according to the present invention. 
     FIG. 3 is a perspective view showing a first flange in which a first taper hole portion, which is provided in the center of the flange, for positioning with respect to the apparatus main body, and a first coupling portion for rotating the photosensitive member by receiving the rotation driving force from the apparatus main body are formed into one unit in one embodiment according to the present invention. 
     FIG. 4 is a perspective view showing a second flange in which a second taper hole portion, which is provided in the center of the flange, for positioning with respect to the apparatus main body, and a second coupling portion conveying the rotation force of the photosensitive member to the brake means of the apparatus main body are formed into one unit in one embodiment according to the present invention. 
     FIG. 5 is a cross-sectional view showing a state in which the image forming unit and driving mechanism are cut off at the image forming position in one embodiment according to the present invention. 
     FIG. 6 is a perspective view showing a drive side flange and a photosensitive member positioning and rotation driving mechanism of the image forming unit in one embodiment according to the present invention. 
     FIG. 7 is a cross-sectional view showing a conventional image forming unit which is cut at its center. 
     FIG. 8 is a perspective view showing a conventional drive side flange in which a positioning part and a gear portion of the image forming unit are integrally formed into one unit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the present invention will be described by way of embodiments with reference to drawings. 
     First, the entire configuration and operation of the color image forming apparatus of one embodiment according to the present invention will be described with reference to FIG.  1 . 
     [Structure of the Entire Apparatus] 
     (Carriage) 
     In FIG. 1, the right-hand face is the front face of the apparatus. The apparatus main body  1  includes a carriage  2  at approximately the center thereof, a front alligator  1 A on the front face and a top door  17  on a top face. 
     The carriage  2  contains four image forming units  3 Y,  3 M,  3 C and  3 Bk for four colors (yellow, magenta, cyan and black). The carriage  2  is supported on a cylindrical drum  21  rotatably, thereby switching the image forming units  3  by sequentially shifting a photosensitive member  30  of the image forming units  3  for each color between an image forming position  10  and other waiting positions. 
     The image forming unit  3  is attached to the apparatus main body  1  detachably. When one of the image forming units  3  needs to be replaced with a new unit, it can be done after rotating the carriage  2  so that the image forming unit  3  of the color to be replaced is located underneath the top door  17 , and opening the top door  17 . 
     Inside the carriage  2 , the image forming unit  3  can form images only at the image forming position  10  where the photosensitive member  30  is irradiated with a laser beam  8  and a transfer belt unit  5  is in contact with the photosensitive member  90 . The image forming unit  8  is connected to a driving source or a power source of the apparatus main body  1  at the image forming position  10 , thereby performing the image forming operation. The other positions are waiting positions in which the image forming units  3  are not operated. 
     (Alligator, Attachment of Transfer Belt Unit, and Treatment of Paper Jamming) 
     The front alligator  1 A is hinged to the apparatus main body  1  with a hinge axis  1 B, and can be lowered and opened toward the front. The front alligator  1 A is provided with a fixing device  15 , a secondary transfer roller  9 , an electricity-removing needle  7 , a front side of paper guide  13   a,    13   b,    13   c  and  13   d,  and a front side of a resist roller  16 . When the front alligator  1 A is lowered toward the front, these components also are lowered toward the front together. Therefore, it is possible to open the front surface of the apparatus main body  1  widely and to attach/detach the transfer belt unit  5  into/from this opened part. At the same time, it is possible to remove paper easily at the time of paper jamming. 
     The transfer belt unit  5  is positioned reliably at a predetermined position when it is attached to the apparatus main body  1 , and the portion facing the photo sensitive member  30  at the image forming position  10  of the intermediate transfer belt  50  is in contact with the photosensitive member  30 . Furthermore, at the same time, each portion of the transfer belt unit  5  is electrically connected to the side of the apparatus main body  1  and the driving pulley  55 A is connected to the driving means at the side of the apparatus main body  1 . Thereby, the intermediate transfer belt  50  can be rotated. 
     Furthermore, the electricity-removing needle  7  prevents a toner image from being disturbed when the recording paper is separated from the intermediate transfer belt  50 . 
     (Optical System) 
     Numeral  6  denotes a laser exposure device, which is placed underneath the transfer belt unit  5 . The laser exposure device  6  includes a semiconductor laser (not shown), a polygon mirror  6 A, a lens system  6 B, a first mirror  6 C, and the like. The pixel laser signal light  8  corresponding to a transient serial electrical pixel signal for image information passes through an optical window  22  formed between a waste toner container  37  of the yellow image forming unit  3 Y and the toner hopper  39  of the black image forming unit  3 Bk. The pixel laser signal light  8  passes through a window (not shown) that is opened in a part of the cylindrical drum  21 ; is incident on a mirror  19  of the cylinder drum  21  (the mirror  19  is fixed to the apparatus main body  1 ); and is reflected from the mirror  19  and incident into the yellow image forming unit  3 Y from an exposure window of the image forming unit  3 Y located at the image forming position  10 . Then, the pixel laser signal  8  is incident into an exposure portion of the left side face of the photosensitive member  30 , and the photosensitive member  30  is exposed to light by scanning in the direction of the main line. 
     (Paper Feed System) 
     Numeral  12  denotes a paper feed unit,  14  denotes a paper feed roller,  16  denotes a resist roller,  18  denotes a paper ejecting roller, and  13   a,    13   b,    13   c  and  13   d  denote a paper guide, respectively, which link between the above-mentioned rollers as well as between the contact point of the intermediate transfer belt  50  and the secondary transfer roller  9  and the fixing device  15 . 
     [Operation of the Apparatus] 
     The following is a description of the color image formation process. 
     When the transfer belt unit  5  and all image forming units  3  are installed in their predetermined locations, the power for the apparatus main body  1  is turned on, and the fixing device  15  is heated up, while the polygon mirror  6 A of the laser exposing device  6  starts to be rotated, thus completing the preparations. 
     After these preparations are completed, first, an initialization operation is performed to move the image forming unit  3  of the color to be recorded to the image forming position  10 . In this initialization operation, the carriage  2 , on which all image forming units  3  are attached, rotates, and the image forming unit  3  of the color to be recorded first (in the present embodiment, the yellow image forming unit  3 Y) is moved into the image forming position  10  in the apparatus main body  1 , and it stops there. 
     First of all, an image formation process by the yellow image forming unit  3 Y, which is positioned at the image forming position  10 , starts. The driving source at the apparatus main body  1  starts to rotate the yellow photosensitive member  30  at the image forming position  10 , and at the same time, a developing device  35 , a charger  34  and an intermediate transfer belt  50  start to operate as well. Furthermore, the driving pulley  55 A is driven by the driving source of the apparatus main body  1 , and its friction forces rotate the intermediate transfer belt  50  in the arrow direction of FIG.  1 . Herein, a peripheral speed of the photosensitive member  30  and that of the intermediate transfer belt  50  are set to be substantially the same. Furthermore, at this time, the secondary transfer roller  9  and the cleaner  51  are separated from the intermediate transfer belt  50 . 
     In accordance with the timing with which a portion that is charged homogeneously by the charger  34  on the surface of the photosensitive member  30 , a detection means (not shown), for detecting the top position of the intermediate transfer belt  50 , detects the top position of the intermediate transfer belt  50 . In synchronization with this detected signal, the homogeneously charged photosensitive member  30  is irradiated with a laser beam  8 , which corresponds to the image signal, from the laser exposing device  6 , forming the electrostatic latent image on the photosensitive member  30 . The thus formed latent images are developed sequentially by the developing unit  35  so as to form into toner images. Next, the toner images formed on the photosensitive member  30  are moved toward the primary transfer position while being in contact with the intermediate transfer belt  50 , and transcribed sequentially on the intermediate transfer belt  50 . The yellow image forming operation is completed after the end of the image is transferred onto the intermediate transfer belt  50 , and the photosensitive member  30  and the intermediate transfer belt  50  stop at the initial position. 
     Moreover, at the time of image formation, the photosensitive member  30  is charged to −450V by the charger  34 . The exposure potential of the photosensitive member  30  becomes −50V. Furthermore, a DC voltage of +100 V is applied from a high-voltage source to the developing roller  35 A (see FIG. 2) when it passes through a region of the photosensitive member  30  that is not yet charged. Furthermore, a DC voltage of −250 V is applied from the high-voltage source to the developing roller  35 A when the surface of the photosensitive member  30 , onto which the electrostatic latent image has been inscribed, passes the developing roller  35 A. Furthermore, a DC voltage of +1.0 kV is applied to the guide pulley  55 C and the tension pulley  55 D of the intermediate transfer belt  50 . 
     Yellow image forming is completed and the operation of the photosensitive member  30  and the intermediate transfer belt  50  stops, the engagement between the yellow photosensitive member  30  and the driving source of the apparatus main body  1  is released, and then the carriage  2  rotates only 90° in the arrow direction of FIG.  1 . This moves the yellow image forming unit  3 Y away from the image forming position  10 , and the next, the magenta image forming unit  3 M, is positioned and stops at the image forming position  10 . When the magenta image forming unit  3 M stops at the image forming position  10 , the driving source of the apparatus main body  1  engages the photosensitive member  30 , and the image forming unit  3 M and the transfer belt unit  5  start to operate, and an image forming operation is performed, similarly as for yellow. Consequently, a magenta toner image is formed overlapping a yellow toner image on the intermediate transfer belt  50 . 
     Thus, sequential switching operations and image forming operations are repeated for cyan and black, so that four toner images are formed on the intermediate transfer belt  50 . 
     When the top of the black toner image, transferred by primary transfer, comes to the position of the secondary transfer roller  9 , the secondary transfer roller  9  is moved. Then, recording paper, which is fed from the paper feed unit  12 , is sandwiched and conveyed between the secondary transfer roller  9  and the intermediate transfer belt  50 , and the four-color toner image is transferred in one batch onto the recording paper. During this time, a voltage of +300V is applied to the secondary transfer roller  9 . The toner image transferred onto the recording paper is fixed on the recording paper by passing the fixing device  15 , and is ejected out of the apparatus with the paper eject rollers  18 . 
     Any toner remaining on the intermediate transfer belt  50  after secondary transfer is scraped off by the cleaning blade  53  that is brought into contact with the intermediate transfer belt  50  before the top of the image comes to the cleaning position. The scraped toner is contained in a waste toner case  57  by means of a screw  52 . 
     When the secondary transfer is finished, the intermediate transfer belt  50  and the image forming unit  3  stop again, and the carriage  2  rotates 90°. Then, the yellow image forming unit  3 Y reaches the image forming position  10 , thus completing the color image forming operation. 
     In the above-mentioned embodiment, the secondary transfer may be carried out during the recording of the last black toner and may be carried out by rotating the intermediate transfer belt  50  after recording black. 
     (Image Forming Unit) 
     The following is a description of the configuration of an image forming unit according to the present invention with reference to FIGS. 2 to  4 . 
     FIG. 2 is a cross-sectional view showing an image forming unit, which is formed integrally as one unit including a photosensitive member and process elements provided therearound. FIG. 3 is a perspective view showing a first flange in which a first taper hole portion, which is provided in the center of the flange, for positioning with respect to the apparatus main body, and a first coupling portion for rotating the photosensitive member by receiving the rotation driving force from the apparatus main body are formed into one unit. FIG. 4 is a perspective view showing a second flange in which a second taper hole portion, which is provided in the center of the flange, for positioning with respect to the apparatus main body, and a second coupling portion conveying the rotation force of the photosensitive member to the brake means of the apparatus main body are formed into one unit. 
     In FIG. 2, numeral  34  denotes a corona charger for homogeneously charging the photosensitive member  30  negatively,  35  denotes a developing device including a developing roller  35 A, and  39  denotes a toner hopper. The toner hopper  39  contains negatively charged toner  32  that is made of polyester resin in which a pigment is dispersed. 
     The developing roller  35 A supporting toner  32  on its surface is rotated in the arrow direction at the rotation speed faster than that of the photosensitive member  30  while being in contact with the photosensitive member  30 , thus developing the photosensitive member  20 . Numeral  8  denotes a cleaning means for cleaning off the toner remaining on the surface of the photosensitive member  30  after transfer. The cleaning means  38  includes a rubber cleaning blade  36  and a waste toner case  37  for containing waste toner. Numeral  33  denotes an exposure window by which a laser beam can enter the image forming unit  3 . The diameter of the photosensitive member  30  is 30 mm, and the diameter of the developing roller  35 A of the developing device  35  is about 16 mm. They are rotatably supported by the box  43  of the image forming unit  3 , respectively. 
     As shown in FIGS. 3 to  5 , the first flange  40  is fixed to the end face, which is driven to be rotated by the apparatus main body  1 , of the photosensitive member  30  of the image forming apparatus  3 . And the second flange  41  is fixed to another end face. The photosensitive member  30  of the image forming unit  3  is supported rotatably in which a bearing support surface  40 C of the first flange  40  and a bearing support surface  41 C of the second flange  41  are inserted into bearings  44  and  45  in a state in which the bearing support surfaces  40 C and  41 C are arranged in a flange bearing portions  43 A and  43 B formed in the box  43  of the image forming unit  3 . The flange bearing portions  43 A and  43 B are formed in an approximately U shape having a diameter that is larger than that of the bearing support surface  40 C of the first flange  40  and the bearing support surface  41 C of the second flange  41 . Thus, the photosensitive member  30  can be attached/detached to/from the apparatus main body easily. 
     At another end portion opposite to the photosensitive member  30  of the first flange  40 , a first taper hole portion  40 A, which is a positioning part for positioning the photosensitive member  30  at the image forming position  10  of the apparatus main body  1 , is formed. Around the first taper hole portion  40 A, the first coupling portion  40 B including twelve concave and convex surfaces is formed. The first coupling portion  40 B is formed as one unit with the first taper hole portion  40 A. Consequently, when the first coupling portion  40 B is rotated, the photosensitive member  30  is also rotated. 
     At another end portion opposite to the photosensitive member  30  of the second flange  41 , the second taper hole portion  41 A, which is a positioning part for positioning the photosensitive member  30  at the image forming position  10  of the apparatus main body  1 , is formed. Around the second taper hole portion  41 A, the second coupling portion  41 B including ten concave and convex surfaces is formed. The second coupling portion  41 B is formed as one unit with the second taper hole portion. Consequently, when the photosensitive member  30  is rotated, the second flange  41  is rotated, thus rotating the brake means of the apparatus main body  1 . 
     Furthermore, at least one of the first flange  40  and the second flange  41  is made of a conductive material. Furthermore, at the side face of the first flange  40  side, a side cover  46  covering the entire surface of the first flange  40  is provided. 
     (Photosensitive Member Positioning and Rotation Driving Mechanism) 
     The following is a detailed description of a positioning mechanism and a rotation driving mechanism for performing precise color alignment of all colors at the image forming position, with reference to FIGS. 5 and 6. FIG. 5 is a cross-sectional view showing an image forming unit taken on line passing the image forming position. FIG. 6 is a perspective view showing a first flange having an output axis and driving pin for conveying the rotation driving force from the apparatus main body to the photosensitive member and a first coupling portion. 
     First, rotation driving mechanisms  60  and  80  for precisely positioning the photosensitive member  30  at the image forming position  10  will be described. 
     Photosensitive member positioning and rotation diving mechanism  60  is attached to the right side wall  1 R of the apparatus main body and includes an output axis  70 , a driving pin R 61  that is rotated integrally with the output axis  70 , an output axis driving gear  71  and a driving mechanism for rotating these elements mentioned above. The output axis  70  is supported, movably in the thrust direction and rotatably, by the bearings  77  fixed respectively to the right side wall  1 R of the apparatus main body and a base plate  67  fixed to the right side wall  1 R. 
     One end of the output axis  70  has a tip-tapered portion  75 , which has a convex tapered surface corresponding to the first tapered hole portion  40 A of the first flange  40 . The other end of the output axis  70  has a spherical shape so as to be in contact with a thrust bearing  69  with small area. The output axis driving gear  71 , which is fixed to the output axis  70 , is a left-handed helical gear, having the same direction as the rotation direction. This output axis driving gear  71  meshes with a motor-side gear  72 . Numeral  74  denotes a compression spring, which is inserted between the bearing  77  and the output axis driving gear  71 . This compression spring  74  constantly urges the output axis  70  and the driving pin  61  toward the position that is separated from the first taper hole portion  40 A of the first flange  40 . The output axis  70  can move against the spring force by a driving means for moving the thrust bearing  69 , between a position separated from the first flange  40  and a position where the first taper hole portion  40 A of the first flange  40  engages the tip-tapered portion  75 . The motor-side gear  72  has a sufficient tooth width so that the output axis driving gear  71  meshes with the motor-side gear  72  in any position. When the output axis  70  moves in the thrust direction, the output axis driving gear  71  and the motor-side gear  72  slide against each other on the tooth surfaces. 
     Numeral  61  is a driving pin, which meshes with the first coupling portion  40 B so as to transmit power, is fixed to the output axis  70  and is rotated integrally with the output axis  70 . 
     The following is a description of the photosensitive member positioning and rotation driving mechanism  80 , which is attached to the left wall  1 L of the main body. 
     The photosensitive member positioning and rotation driving mechanism  80  includes an input axis  81 , a driving pin  82  rotating integrally with the input axis  81 , a brake means  90 , and a mechanism moving the input axis  81  in a thrust direction. The input axis  81  is supported, movably in the thrust direction and rotatably, by the bearings  84  fixed respectively to the left side wall  1 L of the apparatus main body and a base plate  83  fixed to the left side wall  1 L. 
     One end of the input axis  81  has a tip-tapered portion  85 , which has a convex tapered surface corresponding to the second tapered hole portion  41 A of the second flange  41 . The other end of the input axis  81  has a spherical shape so as to be in contact with a thrust bearing  86  with a small area. A brake plate  91  is fixed to the input axis  81 . Numeral  87  denotes a compression spring, which is inserted between the bearing  84  and a thrust plate  93 . This compression spring  87  constantly urges the input axis  81  and the driving pin  82  toward the position that is separated from the second coupling portion  41 B of the second flange  41 . The input axis  81  can move against the spring force by a driving means for moving the thrust bearing  86 , between a position separated from the second flange  41  and a position where the second taper hole portion  41 A of the second flange  41  engages the tip-tapered portion  85 . The brake plate  91  is rotated integrally with the input axis  81  and rotates a brake pad  92  at the same cycle. The brake pad  92  generates the braking power by being brought into contact with the brake base plate  95  fixed to the left side wall  1 L of the apparatus main body when the tip-taper portion  85  of the input axis  81  engages the second taper hole portion  41 A of the second flange  41 . On the other hand, when the tip taper portion  85  of the input axis  81  does not engage the second taper hole portion  41 A of the second flange  41 , the brake pad  92  is not in contact with the brake base plate  95 , and thus the braking power is not generated. Between the brake pad  92  and the brake plate  91 , the compression spring  94  is inserted, thus generating an appropriate brake torque when the brake pad  92  is in contact with the brake base plate  95 . 
     Numeral  82  denotes a driving pin, which meshes with the second coupling portion  41 B of the second flange  41 , and conveys the rotation power of the photosensitive member  30  to the brake means  90 . The driving pin  82  is fixed to the input axis  81  and is rotated integrally with the input axis  81 . 
     [Operation of Photosensitive Member Positioning and Rotation Driving Mechanism] 
     Next, the following is a description of the operation of the photosensitive member positioning and rotation driving mechanism. 
     First, the positioning operation will be described. When the image forming unit  3  has been shifted to the image forming position  10  in the apparatus main body  1 , the driving pin  61  of the photosensitive member positioning and rotation driving mechanism  60  and the output axis  70  having the output driving gear  71  moves in the thrust direction by the thrust bearing  69 . At the engaging position, the tip-taper portion  75  of the output axis  70  is positioned corresponding to a first hole portion  40 A of the first flange  40 . At the same time, the first coupling portion  40 B of the first flange  40  engages the driving pin  61 . Input axis  81  having a driving pin  82  of the rotation driving mechanism  80  provided on the left side wall  1 L of the apparatus main body moves in the thrust direction by the thrust bearing  86  in synchronization with the motion of the output axis  70 , and tip taper portion  85  of the input axis  80  is positioned corresponding to the second taper hole portion  41 A of the second flange  41  at the engaging position and at the same time, the second coupling portion  41 B of the second flange  41  engages the driving pin  82 . Thus, the positioning operation is finished. 
     Next, the rotation driving operation of the image forming unit located at the image forming position will be described. When the motor-side gear  72  is rotated by the driving source of the apparatus main body  1 , the rotation force is conveyed to the first coupling portion  40 B of the first flange  40  via the output driving gear  71 , the output axis  70  and the driving pin  61 , so as to drive to rotate the photosensitive member  30 . On the other hand, the brake plate  91  that is integrally formed with an input axis  80 , a spring  94  and a brake pad  92  are rotated via the second coupling portion  41 B of the second flange  41 , the driving pin  82  and the input axis  80 . Thus, the brake power is generated between the brake pad  92  and the brake base plate  95 . As a result, the photosensitive member  30  rotates stably regardless of the rotation of the developing roller  35 A. 
     The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.