Patent Publication Number: US-7212773-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus, such as a copying machine, a facsimile machine, a printer, having a latent image carrier, such as a photosensitive drum, for carrying a latent image on the surface thereof, and a developing device for developing the latent image into a visible image, provided with a developing agent carrier, such as a developing roller, for carrying an image forming material on the surface thereof. More particularly, the present invention relates to a drive force transmission device used in an image forming apparatus of this kind for receiving the rotational drive force of a drive source side and transmit this force to the driven side. 
   2. Description of the Related Art 
   Generally, in an image forming apparatus, such as a copying machine, facsimile machine, printer, or the like, many rotating bodies, such as a drum-shaped photosensitive body, a developing roller, and the like, are used. In order to transmit rotational drive force to these rotating bodies, a method is known whereby a rotating axle of a rotating body (driven side) and a rotating axle of a drive source side are coupled by means of an axle joint, in such a manner that rotational drive force is transmitted from the rotating axle of the latter to the rotating axle of the former. However, misalignment of the central axes is liable to occur between the rotational coupling section on the driven side which causes the rotating body to rotate and the rotational coupling section on the drive source side which transmits rotational drive force to the rotational coupling section on the driven side, and hence there are problems to be resolved, such as fluctuation in the rotational speed of the rotating bodies caused by the aforementioned loss of central alignment. 
   Technologies relating the present invention are (also) disclosed in, e.g., Japanese Patent Laid-open No. 6-332285 and Japanese Patent Laid-open No. 2000-227690. 
   SUMMARY OF THE INVENTION 
   The present invention was devised with the foregoing problem in view, an object thereof being to provide a drive force transmission device and an image forming apparatus using same, which make it possible to prevent fluctuation in the rotational speed of rotating bodies caused by misalignment between the central axes of a driven side rotational coupling section for rotating a rotating body, such as a developer carrier, and a drive source side rotational coupling section for transmitting rotational drive force to the driven side rotational coupling section. 
   In accordance with the present invention, a drive force transmission device comprises a driven side rotational coupling section, coupled with a drive side rotational coupling section driven in rotation and receiving rotational drive force from same; and a transmission gear for transmitting rotational drive force by meshing with the driven side rotational coupling section. The position of the driven side rotational coupling section is corrected in a plane orthogonal to the axis of rotation, by engaging with the drive side rotational coupling section by approaching same in the direction of the axis of rotation thereof. 
   In accordance with the present invention, a developing device for an image forming apparatus comprises a developer carrier for conveying an image forming material carried on a moving surface to a position opposing a latent image carrier of the image forming apparatus, in accordance with movement of the surface, and creating a visual image of the latent image carried on the latent image carrier; and a drive force transmission device for receiving rotational drive force from a drive side rotational coupling section and transmitting same to the developer carrier, by means of a driven side rotational coupling section coupled with the drive side rotational coupling section provided inside the image forming apparatus and driven in rotation. The drive force transmission device comprises a driven side rotational coupling section, coupled with a drive side rotational coupling section driven in rotation and receiving rotational drive force from same; and a transmission gear for transmitting rotational drive force by meshing with said driven side rotational coupling section. The position of the driven side rotational coupling section is corrected in a plane orthogonal to the axis of rotation, by engaging with said drive side rotational coupling section by approaching same in the direction of the axis of rotation thereof. 
   In accordance with the present invention, there is provided a processing unit in which at least a latent image carrier for carrying a latent image and a developing device for developing a latent image on said latent carrier by means of an image forming material carried on the surface of a developer carrier are supported by a common supporting body. The processing unit is integrally detachable and attachable with respect to an image forming apparatus, as one unit. The developing device comprises a developer carrier for conveying an image format material carried on a moving surface to a position opposing a latent image carrier of the image forming apparatus, in accordance with movement of the surface, and creating a visual image of the latent image carried on the latent image carrier; and a drive force transmission device for receiving rotational drive force from a drive side rotational coupling section and transmitting same to said developer carrier, by means of a driven side rotational coupling section coupled with the drive side rotational coupling section provided inside the image forming apparatus and driven in rotation. The drive force transmission device comprises a driven side rotational coupling section, coupled with a drive side rotational coupling section driven in rotation and receiving rotational drive force from same; and a transmission gear for transmitting rotational drive force by meshing with the driven side rotational coupling section. The position of the driven side rotational coupling section is corrected in a plane orthogonal to the axis of rotation, by engaging with the drive side rotational coupling section by approaching same in the direction of the axis of rotation thereof. 
   In accordance with the present invention, a transfer device comprises a surface moving body having a moving surface, and a drive force transmission device for receiving rotational drive force from a drive side rotational coupling section disposed in an image forming apparatus and driven in rotation, and transmitting same to said surface moving body for transferring a visible image carried on a visible image carrier of the image forming apparatus onto the surface moving body or onto a recording body held on the surface of same. The drive force transmission device comprises a driven side rotational coupling section, coupled with a drive side rotational coupling section driven in rotation and receiving rotational drive force from same; and a transmission gear for transmitting rotational drive force by meshing with the driven side rotational coupling section. The position of the driven side rotational coupling section being corrected in a plane orthogonal to the axis of rotation, by engaging with the drive side rotational coupling section by approaching same in the direction of the axis of rotation thereof. 
   In accordance with the present invention, an image forming apparatus for forming images by means of a processing unit wherein at least a latent image carrier for carrying a latent image and a developing device for developing a latent image on said latent carrier by means of an image forming material carried on the surface of a developer carrier are supported by a common supporting body. The processing unit is integrally detachable and attachable with respect to an image forming apparatus, as one unit. The developing device comprises a developer carrier for conveying an image format material carried on a moving surface to a position opposing a latent image carrier of the image forming apparatus, in accordance with movement of the surface, and creating a visual image of the latent image carried on the latent image carrier; and a drive force transmission device for receiving rotational drive force from a drive side rotational coupling section and transmitting same to the developer carrier, by means of a driven side rotational coupling section coupled with the drive side rotational coupling section provided inside the image forming apparatus and driven in rotation. The drive force transmission device comprises a driven side rotational coupling section, coupled with a drive side rotational coupling section driven in rotation and receiving rotational drive force from same; and a transmission gear for transmitting rotational drive force by meshing with the driven side rotational coupling section. The position of the driven side rotational coupling section is corrected in a plane orthogonal to the axis of rotation, by engaging with the drive side rotational coupling section by approaching same in the direction of the axis of rotation thereof. 
   In accordance with the present invention, an image forming apparatus comprises visible image forming means for forming a visible image on the surface of a visible image carrier, and a transfer device for transferring a visible image on the visible image carrier onto a recording body, either directly or via an intermediate transfer body. The transfer device comprises a surface moving body having a moving surface, and a drive force transmission device for receiving rotational drive force from a drive side rotational coupling section disposed in an image forming apparatus and driven in rotation, and transmitting same to the surface moving body. The transfer device transfers a visible image carried on a visible image carrier of the image forming apparatus onto the surface moving body or onto a recording body held on the surface of same. The drive force transmission device comprises a driven side rotational coupling section, coupled with a drive side rotational coupling section driven in rotation and receiving rotational drive force from same; and a transmission gear for transmitting rotational drive force by meshing with the driven side rotational coupling section. The position of said driven side rotational coupling section is corrected in a plane orthogonal to the axis of rotation, by engaging with the drive side rotational coupling section by approaching same in the direction of the axis of rotation thereof. 
   In accordance with the present invention, an image forming apparatus comprises a latent image carrier for carrying a latent image on the surface thereof, a developing device for creating a visible image of the latent image, having a driven side rotational coupling section for receiving rotational drive force from an external source, and provided with a developer carrier for carrying an image forming material on the surface thereof; and a drive side rotational coupling section for transmitting rotational drive force to the driven side rotational coupling section. The driven side rotational coupling section is fixed and the position thereof is corrected by means of the driven side rotational coupling section engaging with the drive side rotational coupling section. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: 
       FIGS. 1–3  are cross-sectional diagrams showing the composition of a drive coupling section in a conventional developing device; 
       FIG. 4  is a cross-sectional diagram showing the composition of a drive coupling section of a developing device relating to a first embodiment of the present invention; 
       FIGS. 5A and 5B  are external oblique views showing the composition of the principal part of this drive coupling section; 
       FIG. 6  is a cross-sectional diagram showing the composition of the principal part of the drive coupling section; 
       FIGS. 7 and 8  are diagrams showing the composition of a drive coupling section of a developing device relating to one modification relating to the first embodiment; 
       FIG. 9  is a diagram showing the general composition of a laser printer relating to the first embodiment; 
       FIG. 10  is a diagram showing the general composition of a yellow processing unit in the aforementioned laser printer; 
       FIG. 11  is a diagram showing the state of engagement between the coupling gear and the rotational coupling section shown in  FIG. 5 ; 
       FIG. 12  is a diagram showing the composition of a yellow processing unit of a printer relating to a second embodiment of the present invention; 
       FIG. 13  is a plan view cross-sectional diagram showing the composition of this processing unit and side plates of the main body of the printer; 
       FIG. 14  is a plan view cross-sectional diagram showing the composition of a transfer unit and side plates of the main body of the printer, relating to a third embodiment of the present invention; 
       FIG. 15  is a diagram showing a drive output shaft and coupling gear prior to engagement, and a side plate of a developing device, in a printer relating to a modification of the third embodiment; 
       FIG. 16  is a diagram showing a drive output shaft and coupling gear after engagement, and a side plate of a developing device, in this printer; and 
       FIG. 17  is a lateral cross-sectional view showing the composition of a gear carrying section in the side plate, and a gear carrying section. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Before describing the present invention, the prior art and the problems associated with same will be described. 
     FIG. 1  and  FIG. 2  show the composition of a developing device of a conventional image forming apparatus. As shown in these diagrams, the developing device  140  is constituted by a developing roller  142  forming a developer carrier disposed in such a manner that it is partially exposed from the opening of the casing  141 , and a conveyance screw and developer ductor, and the like, which are not illustrated. A developing roller axle  150  extends from the interior of the casing  141 , via the side face of the casing  141  (the right-hand side in the diagram), and a roller gear  151  is fixed to the developing roller axle  150  on the outer side of the casing  141 . A coupling gear  152  supported rotatably on the side face of the casing  141  meshes with the roller gear  151 . Two fingers  153  forming a projecting section are provided in a protruding manner on the end face of the coupling gear  152 . On the other hand, a drive output axle  160  receiving drive force from a drive motor (not illustrated) is provided on the main body side of the image forming apparatus. The drive output axle  160  forming a drive axle comprises an axle  161  and a coupling section  162  fixed to this end section. The coupling section  162  has two fingers  163  corresponding respectively to the two fingers  153  of the engaging gear  152 , on the end face thereof. 
   When the disk control section  140  is installed in a printer, the coupling gear  152  on the disk control section  140  and the coupling section  162  on the drive output axle  160  of the printer main body project towards each other, as illustrated in  FIG. 2 . The coupling section  162  on the drive output axle  160  and the coupling gear  152  are coupled together. An axle joint is constituted by the end structure of the coupling gear  152  and the coupling section  162  forming the end structure of the drive output axle  160 . By means of this axle joint, the rotational drive force of the drive output axle  160  is connected to the coupling gear  152 . The rotational drive force is transmitted to the roller gear  151  which meshes with the coupling gear  152 , thereby causing the developing roller  142  to rotate. 
   Besides this, an Oldham coupling has been used as an axle joint for coupling the two rotating bodies, and this coupling is disclosed in Japanese Patent Laid-open No. 6-332285 and Japanese Patent Laid-open No. 2000-227690 described above, and the like. However, since the Oldham coupling causes two coupling members and a slider member to rub against one another as the members rotate, it is not suitable for transmitting rotational drive force in a smooth manner. 
   Problems of the following kind arise in an image forming apparatus comprising a conventional drive transmission section as described above. 
   In other words, misalignment between the central axes of the drive output axle  160  and the axle  154  fixed to the coupling gear  152  occurs, as illustrated in  FIGS. 2 and 3 . This is because the developing roller axle  150  provided on one end of the developing roller is inserted into the same side plate of the main body of the image forming apparatus as the side plate through which the drive output axle  160  is inserted. The reason for inserting the developing roller axle  150  into the main body of the image forming apparatus is that the developing roller must be positioned with respect to the main body of the image forming apparatus. Moreover, a further problem arises in that the aforementioned misalignment of the central axes may also occur as a result of vibrations in the developing device. 
   This axial misalignment causes slight fluctuation in the rotational speed of the developing roller  142 , and for the following reasons, this gives rise to uneven developing density in the image. 
   More specifically, in the developing stage, the toner of a two-component developer provided on the developing roller  142  is removed from a magnetic carrier and transferred onto the electrostatic latent image on the photosensitive body which forms the image carrier. In this, it is desirable that the developing roller  142  rotates at a uniform speed, whilst maintaining a uniform gap between the developing roller  142  and the photosensitive body. However, if the rotational speed of the developing roller  142  fluctuates slightly due to the aforementioned axial misalignment, then the amount of toner conveyed to the developing position, namely, the position where the photosensitive body opposes the developing roller  142 , per unit time, will vary, thereby giving rise to uneven developing density. 
   In particular, if an image having a relatively large surface area ratio, such as a photograph, is printed, then the unevenness in developing density will be more pronounced than in the case of an image of relatively small surface area ratio, such as text. In a monochrome image having high surface area ratio, a uniform unevenness in the developing density will cause a striped pattern in the direction of paper conveyance, thus impairing the appearance of the image. Moreover, in a full colour image having a large surface area ratio, unevenness in the developing density for each colour will appear as a shift in the colour tone. In the general marketplace, businesses such as design companies, for example, have to handle large images of this kind, and the occurrence of stripe-shaped unevenness in the developing density diminishes the product value markedly. 
   In recent years, there have been increasing demand for improved image quality, in addition to demands for increased durability of apparatuses. In order to improve image quality, there has been a tendency for the gap between the developing roller  42  and the photosensitive body to become narrower. However, as this gap becomes narrower, the unevenness in developing density caused by rotational speed fluctuations becomes ever more noticeable, and hence the problem of axial misalignment cannot be ignored. Moreover, with moves towards more compact apparatuses in recent years, there has also been a tendency for the coupling members themselves to become smaller in size. As the coupling members decrease in size, so the ratio of the central misalignment with respect to the diameter of the coupling becomes larger. 
   Furthermore, uneven developing density may also arise as a result of axial misalignment caused by slight vibrations occurring when the gear teeth make contact, but this has been improved significantly by changing from flat-tooth gears to oblique-tooth gears. 
   As described above, if there is misalignment between the central axes of the drive output axle  160  and the axle  154  fixed to the coupling gear  152 , then the rotational speed of the rotating body, such as the developing roller  42 , will vary. Therefore, if the rotating body is a developing roller  42 , then unevenness will occur in the developing density, due to the variation in the rotational speed of the developing roller  42 . Even in the case of a rotating body other than the developing roller  42 , variation in the rotational speed is liable to cause a problem of same kind. For example, if the rotating body is a drive roller about which an intermediate transfer belt moves in an endless fashion, then central misalignment between the rotational coupling section on the drive source side, for instance, the drive output axle, and the rotational coupling section on the driven side, for instance, the coupling gear, will give rise to variation in the rotational speed of the drive roller. In turn, this will cause the speed of the intermediate transfer belt to vary, and hence uneven density will occur in the transfer image transferred from the image carrier, such as the photosensitive body, onto the intermediate transfer belt. 
   Below, a first embodiment wherein the present invention is applied to a tandem-type electrophotographic colour laser printer (simply called “printer” hereinafter), which is a image forming apparatus, will be described. 
   (Overall Composition) 
     FIG. 9  shows the general composition of a laser printer relating to this first embodiment. The laser printer comprises four processing units ( 1 Y,  1 M,  1 C,  1 K) for forming images of respective colours, namely, yellow (Y), magenta (M), cyan (C) and black (K). Naturally, the suffixes Y, M, C and K appended after the respective reference numbers indicate members used for yellow, magenta, cyan and black, respectively. This also applies similarly below. In addition to the processing units  1 Y,  1 M,  1 C and  1 K, the printer also comprises an optical writing unit  10 , a transfer unit  11 , a pair of resist rollers  19 , three paper supply cassettes  20 , a fixing unit  21 , and the like. 
   (Optical Writing Unit) 
   The optical writing unit  10  comprises four optical writing elements. Each of these optical writing elements comprises a light source, a polygonal mirror, an f-θ lens, a reflecting mirror, and the like, and irradiates laser light onto the surface of the photosensitive body (described hereinafter), on the basis of image data. 
   (Processing Units) 
     FIG. 10  shows the general composition of a yellow processing unit  1 Y, of the aforementioned processing units  1 Y,  1 M,  1 C and  1 K. Since the other processing units  1 M,  1 C and  1 K each have the same composition, separate description of these units is omitted here. In  FIG. 10 , the processing unit  1 Y comprises a drum-shaped photosensitive body  2 Y, a charging device  30 Y, a developing device  40 Y, a drum cleaning device  48 Y, and the like. 
   The charging device  30 Y provides the drum surface with a uniform electrical charge, by rubbing a charging roller supplied with an AC voltage against the photosensitive body  2 Y. Modulated and polarized laser light is irradiated and scanned by the optical writing unit  10  onto the surface of the charged photosensitive body  2 Y. In so doing, a latent electrostatic image is formed on the surface of the drum. The latent electrostatic image thus formed is then developed by the developing device  40 Y, thereby creating a yellow toner image. 
   The developing device  40 Y has a developing roller  42 Y disposed in such a manner that it is exposed partially via an opening in the casing of the developing device. Furthermore, it also comprises a first conveyance screw  43 Y, a second conveyance screw  44 Y, a developer ductor  45 Y, a toner density sensor (hereinafter, called T sensor)  46 Y, and the like. 
   The two-component developer comprising a magnetic carrier and a negatively charged yellow toner is accommodated inside the casing. This two-component developer is churned and conveyed by the first conveyance screw  43 Y and the second conveyance screw  44 Y, and becomes charged through friction, whereupon it is carried on the surface of the developing roller  42 Y. The thickness of the layer of developer is restricted by the developer ductor  45 Y, and it is then conveyed to the developing area opposing the photosensitive body  2 Y, where the yellow toner is caused to adhere to the latent electrostatic image on the photosensitive body  2 Y. By means of the toner adhering to the latent image in this way, a yellow toner image is formed on the photosensitive body  2 Y. The two-component developer after consumption of yellow toner in the developing process is returned to the casing by the rotation of the developing roller  42 Y. 
   A partition  47 Y is provided between the first conveyance screw  43 Y and the second conveyance screw  44 Y. By means of this partition  47 Y, the interior of the casing is divided into a first supply section for accommodating the developing roller  42 Y and the first conveyance screw  43 Y, and the like, and a second supply section for accommodating the second conveyance screw  44 Y. The first conveyance screw  43 Y is caused to rotate by drive means (not illustrated), and it conveys the two-component developer inside the first supply section from the front side in the drawing towards the rear side, thereby supplying the developer to the developing roller  42 Y. The two-component developer conveyed to the vicinity of the end section of the first supply section by the first conveyance screw  43 Y is then introduced into the second supply section via an opening (not illustrated) provided in the partition  47 Y. In the second supply section, the second conveyance screw  44 Y is caused to rotate by drive means (not illustrated), and it conveys the two-component developer moved from the first supply section, in the opposite direction to the first conveyance screw  43 Y. The two-component developer conveyed to the vicinity of the end section of the second supply section by the second conveyance screw  44 Y is returned to the first supply section via a further opening (not illustrated) provided in the partition  47 Y. 
   The T sensor  46 Y, which is a magnetic permeability sensor, is provided in the base wall of the second supply section in the vicinity of the centre thereof, and it outputs a voltage corresponding to the magnetic permeability of the two-component developer passing over it. The magnetic permeability of the two-component developer has a certain correlation to the toner density, and therefore the T sensor  66 Y outputs a voltage of a value that corresponds to the density of the yellow toner. This output voltage value is sent to a control section (not illustrated). The control section is provided with a RAM, where it stores a yellow Vtref, which is a target value for the output voltage from the T sensor  46 Y. Moreover, it also stores data for a magenta Vtref, a cyan Vtref and a black Vtref, which are target values for the output voltages from T sensors (not illustrated) provided in the other developing devices. The Y Vtref is used to control driving of a yellow toner conveyance device (not illustrated). More specifically, the control section adjusts the supply of yellow toner to the second supply section  49 Y by controlling the drive of the yellow toner conveyance device (not illustrate), in such a manner that the value of the output voltage from the T sensor  46 Y approaches the value of the yellow Vtref. By adjusting the supply in this way, the density of the yellow toner in the two-component developer held in the developing device  40 Y can be maintained within a prescribed range. A similar toner supply control process is implemented in the developing devices of the other processing units, as well. 
   The yellow toner image formed on the yellow photosensitive body  2 Y is transferred onto transfer paper conveyed by a paper conveyance belt, which is described hereinafter. The surface of the photosensitive body  2 Y after transferring the image is cleaned of residual toner by a drum cleaning device  48 Y, whereupon the charge is removed from the drum by means of a charge removing device (not illustrated). The photosensitive body  20 Y is then prepared for the next image formation operation by being charged uniformly by the charging device  30 Y. The same applies to the other processing units. Each processing unit is detachable with respect to the printer main body and is replaced when it has reached the end of its useful life. 
   (Transfer Unit) 
   In  FIG. 9  described above, the transfer unit  11  forming an image transfer device comprises a paper conveyance belt  12 , a drive roller  13 , a tension roller  14 , and four transfer bias rollers  17 Y,  17 M,  17 C and  17 K, and the like. The paper conveyance belt  12  is held in a tensed state about the drive roller  13  and the tension rollers  14  and  15 , and it is caused to move in an endless fashion in the anti-clockwise direction in the diagram, by means of a drive system (not illustrated). The four transfer bias rollers  17 Y,  17 M,  17 C and  17 K are respectively supplied with a transfer bias from a power source (not illustrated). They press the paper conveyance belt  12  from the rear surface thereof towards the photosensitive bodies  2 Y,  2 M,  2 C and  2 K, thereby forming respective transfer nips. A transfer electric field is formed between the photosensitive body and the transfer bias roller at each of these transfer nips, due to the effects of the transfer bias. The yellow toner image formed on the yellow photosensitive body  2 Y is transferred onto transfer paper P conveyed on the paper conveyance belt  12 , due to the effects of the transfer electric field and the nip pressure. Magenta, cyan and black toner images formed respectively on the photosensitive bodies  2 M,  2 C and  2 K are transferred successively and superimposed on this yellow toner image. By means of this superimposed transfer of images, a full colour toner image combined with the white colour of the paper is formed on the transfer paper P conveyed by the paper conveyance belt  12 . 
   (Paper Supply Cassettes) 
   Three paper supply cassettes  20  for accommodating a plurality of stacked sheets of transfer paper P are provided in a stepped fashion below the transfer unit  11 . In each of the cassettes, a paper supply roller presses against the uppermost sheet of transfer paper P. When the paper supply roller is caused to rotate at a prescribed timing, the uppermost sheet of transfer paper P is conveyed into the paper conveyance path. 
   (Pair of Resist Rollers) 
   The transfer paper P supplied to the paper conveyance path from a paper supply cassette  20  is sandwiched between a pair of resist rollers  19 . The resist rollers  19  feed the transfer paper P sandwiched between them at a timing whereby the toner images can be superimposed mutually in the respective transfer nips. By this means, the toner images are transferred onto the transfer paper P in a superimposed fashion, at the respective transfer nips. The transfer paper P formed with the full colour image is supplied to the fixing unit  21 . 
   (Fixing Unit) 
   The fixing unit  21  forming a fixing nip by means of a heating roller  21   a  having an internal heating source, such as a halogen lamp, or the like, and a pressurizing roller  21   b  which presses against the heating roller  21   a . The transfer paper P is sandwiched in the fixing nip and the full colour image is fixed onto the surface of the paper. The transfer paper P having passed through the fixing unit  21  is then output from the machine, via a pair of output rollers (not illustrated). 
   Next, the characteristic features of the composition of this printer will be described. 
     FIG. 4  shows the composition of a drive coupling section of a developing device relating to a first embodiment of the  5  present invention. As this diagram reveals, the developing device  40  is constituted by a developing roller  42  forming a developer carrier disposed in such a manner that a portion thereof is exposed via an opening in a casing  41 , and conveyance screws and a developer ductor, and the like, which are not illustrated. The developing roller axle  50  extends from the interior of the casing  41 , via a side face of the casing  41  (the right-hand side in the diagram), and a roller gear  51  is fixed to the developing roller axle  50  outside the casing  41 , in such a manner that it rotates in unison with same. A coupling gear  52  meshes with the roller gear  51 . 
     FIG. 5A  and  FIG. 5B  show the external appearance of the coupling gear  52  and a rotational coupling section  62  (described below) which is located in the main body of the printer. As these diagrams indicate, two fingers  53  forming projecting sections are provided in a protruding manner on the end face of the coupling gear  52 . Moreover, projections  55  for engaging with recesses  65  provided on the end face of the rotational coupling section  62  on the main body of the printer (described below) are formed on the end face of the coupling gear  52 , on the same side where the finger sections are provided. Moreover, as shown in  FIG. 5A  and  FIG. 5B , an opening for inserting a shaft (holding axle)  54  is provided in the central portion of the coupling gear  52 . The diameter of this opening section is set to be larger than the diameter of the shaft  54 , and the shaft  54  is inserted into the opening section without being fixed to same, thereby supporting the coupling gear  52 . 
   In this first embodiment, the diameter of the opening section in the coupling gear  52  is set to be between 1.0 mm and 0.5 mm larger than the diameter of the shaft  54 . By setting the diameter in this manner, a gap is formed between the opening section forming a through hole in the coupling gear  52 , which is the rotational coupling section on the driven side, and the shaft  54  which is inserted into this opening. This gap allows the coupling gear  52  to swing in a direction orthogonal to the direction of the axis of rotation of the rotational coupling section  62  on the main body of the printer. Here, in conventional gears also, the diameter of the through hole has been set slightly larger than the diameter of the rotational axle member, in order to avoid situations where the rotational axle member, such as a shaft, cannot be inserted into the through hole provided in the gear member, due to dimensional error in the components. However, this difference between the diameters is generally of the order of several 10 μm, which is much smaller than the difference between the diameter of the opening section of the coupling gear  52  and the diameter of the shaft  54  in the present printer. In other words, the difference between the diameter of the opening section of the coupling gear  52  and the diameter of the shaft  54  in the present printer is much greater than the difference between the diameters of the through hole and the rotational axle member in a conventional gear. By adopting a large difference in diameters of this kind, it becomes possible for the coupling gear  52  to swing in a direction orthogonal to the direction of the axis of rotation. 
   On the other hand, a drive axle  67  for receiving drive force from a drive motor (not illustrated) is provided in the main body of the printer, as shown in  FIG. 4 . The drive axle  67 , which forms a drive source axle, comprises an axle  68  and a drive gear  69  fixed to this axle  68 . The drive gear  69  is composed so as to mesh with a drive output gear  66 . The driver output gear  66  is fixed to the drive output axle  60 , in such a manner that the drive output axle  60  is caused to rotate by the drive motor, via the drive gear  69 . This drive output axle  60  comprises an axle  61  and a rotational coupling section  62  fixed to this end section. The rotational coupling section  62  has two fingers  63  corresponding respectively to the two fingers  53  of the coupling gear  52  provided on the end face thereof. Moreover, as shown in  FIG. 5A  and  FIG. 5B , a recess  65  is provided on the circumferential face of the rotational coupling section  62 , at the end face where the fingers  63  are provided. 
   When the developing device  40  is installed in the printer, the projection  55  on the coupling gear  52  of the developing device  40  engages with the recess  65  in the rotational coupling section  62  of the drive output axle  60  on the main body of the printer, as illustrated in  FIG. 4  and  FIG. 6 . In conjunction with this, the two fingers  53  and the two fingers  63  catch on each other, thereby coupling the rotational coupling section  62  on the drive output axle  60  with the coupling gear  52 . An axle joint is formed by the end structure of the coupling gear  52  and the rotational coupling section  62  forming the end structure of the drive output axle  60 . The rotational drive force of the drive output axle  60  is conveyed to the coupling gear  52  by means of this axle joint. The rotational drive force is transmitted to the roller gear  51  that meshes with the coupling gear  52 , and hence the developing roller  42  is caused to rotate. 
   The coupling gear  52  is fixed by meshing with the rotational coupling section  62 , in such a manner that its position can be adjusted finely. In other words, rather than providing an axle that is fixed to the coupling gear  52  and rotates in unison with same, as in the prior art, here the coupling gear  52  is allowed a small margin of movement with respect to the developing device, before it engages with the rotational coupling section  62  provided in the main body of the printer. By engaging with the rotational coupling section  62 , the centre of rotation of the coupling gear  52  moves to a position on the axis of the axle  61  forming the rotating axle of the rotational coupling section  62 , thereby correcting its position. By adopting a composition of this kind, it is possible for any slight misalignment occurring between the central axes of the coupling gear  52  and the rotational coupling section  62 , due to problems with precision in the components or design, to be absorbed by the coupling gear  52 . Therefore, the coupling gear  52  on the developing device side always rotates about the centre of rotation of the rotational coupling section  62 . As a result of this, the rotational coupling section  62  on the printer main body and the coupling gear  52  rotate about the same axis. Consequently, no variation occurs in the rotational speed of the developer carrier due to misalignment of the central axes, and hence there is no variation in the amount of toner conveyed to the developing position, namely, the position where the developer carrier opposes the latent image carrier, per unit time, and unevenness in the developing density can be prevented. 
   When observed at very small dimensions, for instance, at the order of nanometres (nm), it is difficult to make the axes of the coupling gear  52  and the rotational coupling section  62  coincide completely. However, from the viewpoint of the mechanical design of a drive transmission system using gears, even if there is a misalignment of the central axes in the range of 0.01 to 0.1 mm, the axes can still be regarded to be the same, without causing any problems. In the present invention, the concept “rotating about the same axis” includes axial misalignment of this degree. 
   As shown in  FIG. 6 , in the combination of the coupling gear  52  and the rotational coupling section  62 , it is also possible to make the fingers  53  on the coupling gear  52  engages with the recess  65  in the rotational coupling section  62 , as illustrated in  FIG. 6 . However, a more reliable engaging action is obtained if the coupling gear  52  is formed such that its diameter on the front end side is smaller than its diameter on the base end side, a projection  55  being provided on the front end and this projection  55  being made to engage with a round tubular recess in the rotational coupling section  62 , as shown in  FIG. 5A  and  FIG. 5B . For reference purposes, the state of engagement between the coupling gear  52  and the rotational coupling section  62  shown in  FIG. 5A  and  FIG. 5B  is illustrated in  FIG. 11 . 
   In a conventional Oldham coupling, there are two reasons why misalignment of the central axes causes variation in the rotational speed of the rotating body, such as the developing roller. The first reason is that the axial misalignment causes variation in the amount of drive force transmitted from the rotational coupling section on the drive source side to the rotational coupling section on the driven side. More specifically, if there is axial misalignment, then during one rotation of the rotational coupling section on the drive side, the amount of drive force transmitted to the rotational coupling section on the driven side, per unit of rotational travel, will vary depending on the rotational position. In general, when the central axes are misaligned, the amount of drive force transmitted to the rotational coupling section on the driven side per unit of rotational travel increases gradually as the amount of rotation increases, in the region from the prescribed rotational reference position of the rotational coupling section on the drive side until a position reached by a 180° rotation from that reference position. On the other hand, in the region from the 180° position until the 360° position, the amount of drive force transmitted to the rotational coupling section on the driven side per unit of rotational travel decreases gradually as the amount of rotation increases. In this way, the amount of drive force transmitted to the rotational coupling section on the driven side varies with the rotational position of the rotational coupling section on the drive side, thereby giving rise to variation in the speed of the rotational body. 
   However, by means of the combination of the coupling gear  52  and the rotational coupling section  62  in the present printer, misalignment of the central axes is eliminated. Therefore, it is possible to avoid situations where the amount of drive force transmitted to the coupling gear  52  on the driven side per unit of rotational travel varies with the rotational position of the rotational coupling section  62  on the drive side, and hence the developing roller  42  can be caused to rotate at a stable speed. 
   The second reason why misalignment of the central axes causes variation in the rotational speed of a rotating body, such as a developing roller, in a conventional Oldham coupling is because the load on the rotational drive source, such as a motor, varies with the rotational position of the rotational coupling section on the drive side. More specifically, in a conventional Oldham coupling, a groove-shaped recess is formed on one end section of the respective rotational coupling sections, and a projection for engaging with this groove-shaped recess, slidably in the longitudinal direction of the groove, is provided on the other end section. By means of the projection slidably inside the recess in the longitudinal direction of the groove, in accordance with the rotation of the rotational coupling section on the drive side, a state is produced where no misalignment of the central axes occurs. By this means, even if there is axial misalignment between the respective rotational coupling sections, rotation via coupling of the respective rotational coupling sections can still be achieved. In an Oldham coupling having a composition of this kind, during one rotation of the rotational coupling section on the drive side, there arise rotational positions at which the frictional force between the projection and the groove-shaped recess is large, and rotational positions at which this frictional force is small. At a rotational position where the frictional force is large, a strong force acts seeking to bend the rotating axle of the rotational coupling section on the drive side in a direction orthogonal to the axial direction thereof, but since this force is used to cause the projection to make a sliding movement, the two rotational coupling sections are caused to rotate in whatever way possible. As a result of this, the load on the rotational drive source, such as the motor, varies with the rotational position of the rotational coupling section on the drive side, and hence the amount of drive force transmitted from the drive side to the driven side changes. In this way, in a conventional Oldham coupling, two rotational coupling sections whose central axes are misaligned are caused to rotate forcibly, by means of the sliding movement of the projection, rather than achieving rotation of the two rotational coupling sections by eliminating axial misalignment. 
   By contrast, by means of the combination of the coupling gear  52  and the rotational coupling section  62  in the present printer, the two elements are made to rotate whilst eliminating axial misalignment between same. Therefore, it is possible to avoid situations where the load on the rotating drive source, such as the motor, changes with the rotational position of the rotational coupling section  62  on the drive side, and hence variation in the rotational speed of the developing roller  42  caused by variation in the load can be eliminated. 
   The drive output axle  60 Y forming a drive axle is made from an electrically conductive material, and it supplies a developing bias from the printer main body, via an electrode. Since the developing bias is supplied from the drive output axle  60 Y which rotates in unison with the developing roller  42 , it is possible to prevent connection failures (contact failures). 
   The photosensitive body  2  is composed in such a manner that it is positioned with respect to the side plates  70  of the printer main body by means of a method similar to that described above, or by means of a known method. Thereby, the developing roller  42  and the surface of the photosensitive body are made to oppose each other via a prescribed gap. 
   In each of the processing units, desirably, the gap between the photosensitive body  2  and the developing roller  42  is 0.4 mm or less. Thereby, it is possible to improve the graininess of the developed toner image significantly, thus yielding an image of higher quality, compared to cases where a larger gap is allowed. If the gap is narrowed in this way, then unevenness in the developing density due to variation in the rotational speed of the developing roller is more liable to occur, but since rotational speed variation of this kind is suppressed in the present printer, it is possible effectively to suppress image deterioration caused by uneven developing density. 
   Moreover, desirably, the amount of two-component developer picked up by the roller is equalized by forming V-shaped or other shaped grooves  91  in the surface of the developing roller  42 , or by roughening the surface of the developing roller  42  by sandblasting. In so doing, it is possible to suppress not only unevenness in the developing density caused by rotational speed variation in the roller, but also unevenness in the developing density caused by variation in the amount of toner carried by the roller. In general, it is desirable to provide grooves, such as V grooves, in the developing roller, rather than subjecting the roller to sandblasting, since this option allows stable conveyance of the developer without causing abrasion, even in the case of prolonged use. 
   Furthermore, the carrier forming magnetic particles is created by providing a resin coating on magnetic core particles, and for this resin coating, it is desirable to use a resin component obtained by cross-linking a thermoplastic resin, such as acrylic, with a melamine plastic, and incorporating a charge adjusting agent. A conventional carrier is composed in order to obtain long life with the hard coating film gradually being worn away, but in the case of the present carrier, impacts are absorbed by the elastic properties of the coating layer and hence loss of the film is restricted. Consequently, even longer life than a conventional carrier can be achieved. By this means, it is possible to ensure that the amount of toner carried by the roller remains stable over a long period of time. In other words, stable quality can be expected. 
   Furthermore, in the first embodiment, a carrier having an average particle size of between 20 (μm) and 60 (μm) was used. If a magnetic carrier having a small particle size of 60 (μm) or less is used, then variation in the trace of the magnetic carrier and the half-tone images, in other words, deterioration of the graininess, can be prevented, and hence dot reproducibility can be improved and high image quality can be achieved. Moreover, using a magnetic carrier having a particle size of 20 μm or above prevents excessive decline in the fluid properties and stress in the developer. 
   Furthermore, desirably, an oil-based toner containing an oil component is used. The reasons for this are as follows. Namely, in a fixing system using an oil-free polymer toner, it is not possible to obtain glossy images, and therefore oil must be coated onto the fixing rollers in order to achieve a glossy effect. In this case, unevenness in the gloss on the image is liable to occur due to unevenness in the oil coating applied to the fixing rollers. On the other hand, if a toner containing oil is used, then it is possible to achieve a glossy appearance naturally, by means of the oil being released from the toner particles upon fixing. Therefore, unevenness in the gloss effect caused by unevenness in the oil coating can be avoided. One example of a toner containing oil is the following toner. More specifically, firstly, a pre-polymer consisting of a resin in an organic solvent is taken, and a compound which extends or cross-links with this pre-polymer, and a toner material, are dissolved or dispersed in the pre-polymer. Then, a cross-linking or extension reaction is induced in the medium, and a toner is obtained by removing the solvent from the dispersed liquid. 
   Furthermore, desirably, a low-melting-point toner having a melting point of around 140° C. is used. Since this type of toner can be fixed at a lower temperature than a conventional toner, the fixing means can be starting up quickly and the hence energy savings can be made. 
   In  FIG. 4  described previously, when the developing device  40  is installed in the main body of the printer, the developing roller axle  50  which extends significantly from the interior of the casing  41  is inserted into a positioning bearing provided in the side plate  70  of the printer main body. Thereby, the developing device  40  is positioned with respect to the printer main body, and hence with respect to the photosensitive body, by taking the developing roller  42  as a reference. In other words, by engaging the developing roller axle  50  provided on one end of the roller gear  51  forming a transmission gear with a positioning bearing forming a positioning section, the developing roller  42  is positioned with respect to the printer main body. By means of positional registration of this kind, the developing gap between the developing roller  42  and the photosensitive body is set accurately, and therefore it is possible to suppress variation in the developing performance between different products due to assembly errors. 
   If the developing device  40  is positioned with respect to the printer main body in this way, by taking the developing roller  42  as a reference, then there will be slight misalignment between the central axes of the coupling gear  52  and the rotational coupling section  62 , due to dimensional errors in the various components, errors in the hole positions, or the like. If, conversely, a composition is adopted wherein the developing device  40  is positioned by taking the coupling gear  52  as a reference, with the objective of eliminating this axial misalignment, then it becomes impossible to set the developing gap accurately. However, in the present printer, it is possible to resolve both axial misalignment and variation in the developing gap, which are linked by trade off relationship in this way. The variation in the developing gap is eliminated by positioning the developing roller  42  by taking the developing roller  42  as a reference, and the axial misalignment is eliminated by adjusting the position of the coupling gear  52  as described above. 
   In the present printer, even if the position of the coupling gear  52  is corrected when it engages with the rotational coupling section  62  on the drive side, the gap between the opening section of the coupling gear  52  and the shaft  54  inserted therein is maintained. By fixing the coupling gear  52  after its position has been corrected, whilst it maintains an engaged state with respect to the rotational coupling section  62 , the non-contact state between the coupling gear  52  and the shaft  54  is preserved. In this composition, it is possible to avoid situations where a large load is applied to the rotational drive source, such as the motor, due to contact between the shaft  54  and the inner walls of the opening section of the coupling gear  52  after its position has been corrected. Moreover, it is also possible to avoid unevenness in the developing density due to vibrations being generated by friction between the shaft  54 , held in a non-rotatable state, and the inner walls of the opening in the coupling gear  52 , which is driven so as to rotate, and these vibrations being transmitted to the developing roller  42  via the casing of the developing device  40  and the developing roller axle  50 . 
   Next, a modification of the positioning structure for the developing roller with respect to the printer main body will be described. 
     FIG. 7  and  FIG. 8  show the general composition of a drive coupling section of a developing device relating to this modification. The basic composition and operation of this drive coupling section is similar to that in the first embodiment described above, and before the rotational coupling section on the developing device side is engaged with the rotational coupling section on the printer main body, the rotational coupling section on the developing device side is able to move within a small range with respect to the developing device. Below, the points of difference with respect to the first embodiment will be described. 
   In the first embodiment described above, a shaft  54  fixed to the developing device is inserted into an opening section provided in the central portion of the coupling gear  52 , but in the present modification, this shaft  54  is not provided. Instead of this, an opening section  255  for engaging with a drive output shaft section  261  in the printer main body (described hereinafter), is provided in the central region of the coupling gear  252 . Moreover, in the first embodiment described above, the coupling gear  52  was supported by the shaft  54 , but in the present modification, no shaft is provided for supporting the coupling gear  252 , but rather, it is supported by a frame composed in such a manner that it is movable within a small range. 
   Furthermore, in the first embodiment described above, the drive output axle  60  formed by the axle  61  and the rotational coupling section  62  engaged with a coupling gear  52  on the developing device side, but in the present modification, the drive output shaft  261  is composed in such a manner that it engages with the coupling gear  252  on the developing device side. The drive output shaft  261  has a taper on the front end side where it engages with the coupling gear  252 , as illustrated in  FIG. 7 , in order that it may engage smoothly with same. Moreover, in the first embodiment described above, two fingers  63  corresponding respectively to two fingers  53  on the coupling gear  52  were provided on the end face of the rotational coupling section  62  on the printer main body, but in the present modification, fixed pins  263  passing through the drive output shaft  263  are provided instead of the fingers  63 . 
   When the developing device is installed in the printer, the opening section  255  of the coupling gear  252  on the developing device side engages with the front end section of the drive output shaft section  261  in the printer main body, as illustrated in  FIG. 8 . In conjunction with this, the two fingers  253  engage with the fixed pins passing through the drive output shaft  261 , thereby coupling the drive output shaft  261  with the coupling gear  252 . An axle joint is formed by the end structure of the coupling gear  252  and the drive output shaft  261 . This axle joint transmits the rotational drive force of the drive output shaft  261  to the coupling gear  252 . The rotational drive force is then transmitted to a roller gear  51  (not illustrated) that meshes with the coupling gear  252 , thereby causing the developing roller  42  to rotate. 
     FIG. 15  shows the drive output shaft  261  and the coupling gear  252  in the printer according to the present modification, before they are mutually engaged, as well as the side plate  249  of the developing device. Moreover,  FIG. 16  shows the drive output shaft  261  and the coupling gear  252  in the printer according to the present modification, after they have been mutually engaged, as well as the side plate  249  of the developing device.  FIG. 17  shows a gear holding section  250  of the side plate  249  and the coupling gear  252 . The coupling gear  252  is fixed by engaging with the drive output shaft  261 , in such a manner that slight positional adjustment can be made. 
   More specifically, in the printer according to the present modification, a gear holding section  270  is formed projecting from the side of the side plate  249  of the developing device. This gear holding section  270  has a circular tubular structure and has a cylindrical internal cavity. The diameter of this cylindrical cavity is greater than the diameter of the outer circumferential surface of rotation of the coupling gear  252  which forms the rotational coupling section on the driven side. The coupling gear  252  is held in the cylindrical cavity of the of the gear holding section  270 , and it engages with the drive output shaft  261  forming the rotational coupling section on the drive side, which is inserted into an opening section forming a through hole passing through the coupling gear  252  in the direction of rotation. A projecting section  271  projecting in a ring shape towards the inside of the cylinder is provided in the gear holding section  270 , at the end thereof in the direction of the axis of the cylinder. This projecting section  271  has a smaller internal diameter than the diameter of the outer circumferential surface of the coupling gear  252 , and it serves to prevent the coupling gear  252  held in the cylindrical cavity from becoming detached from the cavity. Even if the coupling gear  252  swings in the direction of the cylinder axis inside the cylindrical cavity of the gear holding section  270 , the coupling gear  252  is prevented from becoming detached from the cylindrical cavity by abutting against the projecting section  271  of the gear holding section  270 . 
   As shown in  FIG. 17 , this coupling gear  252  is held movably in at least a plane orthogonal to the direction of the axis of rotation, by means of the gap between the outer rotational circumferential surface and the cylindrical cavity. Therefore, the position of the coupling gear  252  can be corrected by moving it along the axis of rotation of the drive output shaft  261 , as it engages with the drive output shaft  261 . In other words, the coupling gear  252  is not provided with an axle fixed to the coupling gear  252  so as to rotate in unison with same, as in the prior art, but rather, before it is engaged with the drive output shaft  261  on the printer main body, it is disposed movably within a small range with respect to the developing device. By engaging with the drive output shaft  261 , the coupling gear  252  is positioned. 
   By adopting a composition of this kind, it is possible for any slight misalignment occurring between the central axes of the coupling gear  252  and the drive output shaft  261  due to problems with precision in the components or design, to be absorbed by the coupling gear  252 . Therefore, the coupling gear  252  on the developing device side always rotates about the centre of rotation of the drive output shaft  261 . As a result of this, no variation occurs in the rotational speed of the developer carrier due to misalignment of the central axes, and hence there is no variation in the amount of toner conveyed to the developing position, namely, the position where the developer carrier opposes the latent image carrier, per unit time, and unevenness in the developing density can be prevented. 
   The diameter of the cylindrical cavity in the gear holding section  260  is set to 0.5 to 1.0 mm greater than the diameter of the outer rotational circumferential surface of the coupling gear  252 . Therefore, a gap is formed between the inner wall of the cylindrical cavity of the gear holding section  260  and the coupling gear  252  held in same, and this means that the coupling gear  252  is able to swing in a direction orthogonal to the axis of rotation of the coupling gear  252 , as it engages with the drive output shaft  261 . 
   In the present modification, even if the position of the coupling gear  252  is corrected as it is engaged with the drive output shaft  261  forming the rotational coupling section on the drive side, a gap is still maintained between the cylindrical inner wall of the gear holding section  270  and the coupling gear  252 . By fixing the coupling gear  252  after its position has been corrected, whilst maintaining it in an engaged state with respect to the drive output shaft  261 , the two elements are kept in a non-contact state. In this composition, it is also possible to avoid unevenness in the developing density due to vibrations being generated by friction between the gear holding section  270 , fixed in a non-rotatable state, and the coupling gear  252 , which is driven so as to rotate, and these vibrations being transmitted to the developing roller via the casing of the developing device and the developing roller axle. 
   The coupling gear  252  shown in  FIG. 15  to  FIG. 17  has a cylindrical opening section which does not change diameter from the shaft input side to the shaft output side. Instead of this opening section, it is also possible to adopt an opening section having a taper on the input side whereby the diameter becomes smaller towards the output side. In an opening section of this kind, even if there is a large misalignment between the centre of the opening section of the coupling gear  252  before engagement and the centre of the drive output shaft  261 , then the front end of the drive output shaft  261  abuts against the broad input side of the opening and the drive output shaft  261  can then be inserted smoothly into the opening section. 
   Next, a printer relating to a second embodiment where the present invention is applied will be described. Except where stated otherwise below, the composition of the printer is similar to the printer relating to the first embodiment. 
     FIG. 12  shows the composition of a yellow processing unit  301 Y of this printer. As this diagram illustrates, the processing unit  301 Y comprises a developing device  340 Y, a charging device  330 Y, a drum cleaning device  348 Y, and the like, disposed about a drum-shaped photosensitive body  302 Y. These elements are held in a casing which forms a common supporting body, and are detachable from the printer main body, in an integrated fashion as a single unit. 
     FIG. 13  shows a yellow processing unit  301 Y and a printer main body side plate  370 . A shaft hole for inserting a drum drive shaft  371 Y supported rotatably by the printer main body side plate  370  is provided in the centre of the drum of the photosensitive body  302 Y of the processing unit  301 Y. The drum drive shaft  371 Y has a pin  371   a  projecting beyond the rotating surface of photosensitive body  302 Y at a position on the base end side which is not inserted into the shaft hole, and by means of this pin catching on a projection  302 Y a  of the photosensitive body  302 Y and causing it to rotate, rotational drive force is transmitted to the photosensitive body  302 Y. 
   A drum drive gear  372 Y is fixed to the rear end side of the drum drive shaft  371 Y. An intermediate gear  373 Y and a second drive output gear  374 Y mesh with this drum drive gear  372 Y. The intermediate gear  373 Y also meshes with a drive gear  369 Y, in addition to the drum drive gear  372 Y, and the drive gear  369 Y meshes further with a first drive output gear  366 Y. 
   The drive gear  369 Y is the uppermost gear which is driven in rotation by a motor (not illustrated), and it transmits rotational drive force to the respective gears. The first drive output gear  366 Y which meshes with the drive gear  369 Y is fixed to the rear end of the first drive output axle  60 , and a first rotational coupling section  362 Y on the drive side is fixed to the front end of the first drive output axle  360 Y. 
   The first rotational coupling section  362 Y transmits rotational drive force to two screws  343 Y and  344 Y in the developing device  340 Y, and to a developing roller  342 Y. The developing device  340 Y has a first coupling gear  352 Y forming a driven side rotational coupling section, which meshes with the first rotational coupling section  362 Y. The first rotational coupling section  362 Y and the first coupling gear  352 Y respectively have the same composition as the rotational coupling section ( 62 ) and the coupling gear ( 52 ) of the printer relating to the first embodiment. The position of the first coupling gear  352 Y is corrected as it engages with the first rotational coupling section  362 Y, thereby forming a mechanism whereby any axial misalignment between the two elements is eliminated. 
   A first screw gear  375 Y and a second screw gear  376 Y mesh with the first coupling gear  352 Y. The first screw gear  375 Y and the second screw gear  376 Y are fixed to the ends of the first conveyance screw  343 Y and the second conveyance screw  344 Y. The rotational drive force from the first rotational coupling section  323 Y is transmitted via the first coupling gear  352 Y to the first screw gear  375 Y and the second screw gear  376 Y, thereby causing the first conveyance screw  343 Y and the second conveyance screw  344 Y to rotate. A roller gear  351 Y also meshes with the first screw gear  375 Y, and by transmitting rotational drive force to this gear, the developing roller  342 Y is caused to rotate. 
   The rotational drive force of the uppermost drive gear  396 Y is transmitted successively to the intermediate gear  373 Y, the drum drive gear  372 Y, and the second drive output gear  374 Y. The second drive output gear  374 Y is fixed to the rear end of the second drive output axle  377 Y, and a second drive side rotational coupling section  378 Y is fixed to the front end of the second drive output axle  377 Y. 
   The second rotational coupling section  378 Y transmits the rotational drive force a recovery screw  378 Y of the drum cleaning device  348 Y. The developing device  340 Y has a second coupling gear  380 Y forming a driven side rotational coupling section which meshes with the second rotational coupling section  378 Y. The second rotational coupling section  378 Y and the second coupling gear  380 Y respectively have a similar composition to the rotational coupling section ( 62 ) and the coupling gear ( 52 ) of the printer relating to the first embodiment. The position of the second coupling gear  380 Y is corrected as it engages with the second rotational coupling section  378 Y, thereby forming a mechanism whereby axial misalignment between the two elements is eliminated. 
   In the printer having the foregoing composition, it is also possible to eliminate the following type of speed variation, in addition to the rotational speed variation in the developing roller  342 Y caused by misalignment between the central axes of the first rotational coupling section  362 Y and the first coupling gear  352 Y. More specifically, this type of speed variation is variation in the rotational speed of the first conveyance screw  343 Y and the second conveyance screw  344 Y, caused by axial misalignment. Furthermore, variation in the rotational speed of the recovery screw  379 Y due to misalignment between the central axes of the second rotational coupling section  378 Y and the second coupling gear  380 Y can also be eliminated. 
   In the present printer, even if the position of the first coupling gear  352 Y is corrected as it engages with the first rotational coupling section  362 Y on the drive side, the gap between the opening section of the first coupling gear  252 Y and the shaft inserted into same is still maintained. By fixing the first coupling gear  252 Y after its position has been corrected, whilst maintaining it in an engaged state with the first rotational coupling section  362 Y, the two elements are kept in a non-contact state. 
   Moreover, similarly, the second coupling gear  380 Y is kept in a non-contact state with respect to the shaft inserted into the opening section therein. Therefore, it is also possible to avoid unevenness in the developing density due to vibrations being generated by friction between the shaft, held in a non-rotatable state, and the first coupling gear  252 Y, which is driven so as to rotate, and these vibrations being transmitted to the photosensitive body  302 Y via the casing of the processing unit  301 Y and the drum drive shaft  371 Y. 
   Moreover, it is also possible to avoid unevenness in the developing density due to vibrations being generated by friction between the shaft, held in a non-rotatable state, and the second coupling gear  280 Y, which is driven so as to rotate, and these vibrations being transmitted to the photosensitive body  302 Y via the casing of the processing unit  301 Y and the drum drive shaft  371 Y. 
   In this printer, the photosensitive body  302 Y is positioned inside the main body of the printer by means of the drum drive shaft  371  in the printer main body being inserting into a shaft hole of the photosensitive body  302 Y. Moreover, the processing unit  301 Y, and hence the developing roller  342 Y, is positioned inside the main body of the printer by means of a positioning pin  381 Y projecting from the side plate of the processing unit  301 Y engaging with a positioning hole provided in the side plate  370  of the printer main body. 
   Next, a printer relating to a third embodiment to which the present invention is applied will be described. Except where specified otherwise below, the composition of this printer is similar to the printer relating to the first embodiment. 
     FIG. 14  shows the composition of a transfer unit  411  and a printer main body side plate  470  in the present printer. The transfer unit  411  is supported rotatably by means of a supporting body  418 , and is detachable from the printer main body. A drive roller  413  which is supported rotatably on the supporting body  418  and which holds a paper conveyance belt  412  in conjunction with two tensioning rollers (not illustrated) is caused to rotate by means of a drive transmission system (described below), thereby causing the paper conveyance belt  412 , which forms a moving surface, to move in an endless fashion. 
   One end of an axle member  413   a  of a drive roller  413  projects significantly from the supporting body  418 , and a drive roller gear is fixed to the projecting portion of the axle member  413   a . Moreover, the front end of the projecting portion is inserted into a positioning bearing provided in the side plate  470  of the printer main body. Thereby, the transfer unit  411  is positioned with respect to the printer main body, and hence the respective photosensitive bodies, by taking the drive roller  413  as a reference. 
   The drive roller gear  481  meshes with a transfer coupling gear  482  forming a driven side rotational coupling section, which is supported by the supporting body  418 . This transfer coupling gear  482  meshes with a transfer rotational coupling section  483  on the drive side, which projects from the side plate  470  of the printer main body. The transfer rotational coupling section  483  and the transfer coupling gear  482  respectively have a similar composition to the rotational coupling section ( 62 ) and the coupling gear ( 52 ) of the printer relating to the first embodiment. The position of the transfer coupling gear  482 Y is corrected as it engages with the transfer rotational coupling section  483 Y, thereby forming a mechanism for eliminating any axial misalignment between these elements. 
   The transfer coupling gear  482 Y is fixed to one end of the drive output shaft  485 , and the drive output gear  484  is fixed to the other end of the drive output shaft  485 . By means of the drive output gear  484  meshing with the uppermost drive gear  486 , the rotational drive force of the drive gear  486  is transmitted successively to the drive output gear  484 , the transfer rotational coupling section  483 , the transfer coupling gear  482 , the drive roller gear  481 , and the drive roller  4134 . 
   In the present printer having this composition, the position of the transfer coupling gear  484 Y is corrected by means of it moving in a direction orthogonal to the axis of rotation, as it engages with the transfer rotational coupling section  483 Y, and hence rotational speed variation in the drive roller  413  caused by axial misalignment between the two elements can be eliminated. By this means, it is possible to suppress displacement in the transfer position of the toner image caused by variation in the endless movement speed of the paper conveyance belt  412  due to variation in the rotational speed of the drive roller  413 . 
   In this printer, even if the position of the transfer coupling gear  448 Y is corrected as it engages with the transfer rotational coupling section  483 Y on the drive side, the gap between the opening section of the transfer coupling gear  484 Y and the shaft inserted into same is still maintained. By fixing the transfer coupling gear  484 Y after its position has been corrected, whilst maintaining it in an engaged state with respect to the transfer rotational coupling section  483 Y, the two elements are kept in a non-contact state. By means of this composition, it is possible to avoid unevenness in the transfer density due to vibrations being generated by friction between the shaft, held in a non-rotatable state, and the inner walls of the opening section of the transfer coupling gear  484 Y, which is driven so as to rotate, and these vibrations being transmitted to the paper conveyance belt  412  via the roller axle support plate of the transfer unit  411  and the drive roller  413 . 
   The printer used in the respective embodiments and modifications above is one example of an apparatus to which the present invention can be applied, and it is not limited to such an apparatus. The invention was also described with respect to an example where a two-component developer is used as the developer, but it may of course also be applied to an apparatus using a single-component developer. 
   In the printer relating the respective embodiments above, an opening section forming a through hole is provided in a coupling gear ( 52 ,  352 Y,  380 Y,  482 ) forming a rotational coupling section on the driven side, said opening passing through in the direction of the axis of rotation of the gear, and a shaft, which is a body inserted into the opening section, being supported by the apparatus. The position of the coupling gear is corrected by means of it moving in a direction orthogonal to the axis of rotation, due to a gap between the opening section and the shaft inserted into same, as it engages with a drive side rotational coupling section ( 62 ,  362 Y,  378 Y and  483 ). By means of this composition, it is possible to correct the position of the coupling gear readily, by means of it moving as it engages with the rotational coupling section. 
   Moreover, in the printer relating to the modification example, the coupling gear  252  forming the rotational coupling section on the driven side is held inside a cylindrical cavity having a larger diameter than the diameter of the outer rotational circumferential surface (the gear diameter) of the coupling gear, and it engages with a drive output shaft section  261  forming is a rotational coupling section on the drive side, which is inserted into the opening section forming a through hole provided in the direction of rotation of the coupling gear. In this composition also, the position of the coupling gear  252  is corrected readily by means of it moving as it engages with the drive output shaft section  261  forming a rotational coupling section. 
   Furthermore, in the printer relating to a modification example, the coupling gear  252  forming a rotational coupling section on the driven side is provided with a taper on the side of the opening section where the rotational coupling section is inserted, in such a manner that the diameter becomes smaller towards the output side. This provides the following beneficial effects. More specifically, even if there is a large misalignment between the central axes of the opening section of the coupling gear  252  and the drive output shaft  261 , before they become engaged, it is still possible to introduce the drive output shaft  261  into the opening section smoothly, by means of the front end of the drive output shaft  261  abutting against the broad input side of the opening section. 
   Moreover, in the printer relating to the first embodiment, by engaging the developing roller axle  50  provided on one end of the roller gear  51  forming a transmission gear with a positioning bearing forming a position member, the developing roller  42  can be positioned inside the main body of the printer. In this composition, the developing gap between the developing roller  42  and the photosensitive body is set accurately, and hence any variations in developing performance between products caused by assembly errors can be suppressed. 
   Furthermore, in the printer relating to the present embodiment, the opening section of the coupling gear ( 52 ,  352 Y,  380 Y,  482 ) whose position is corrected by means of it engaging with the rotational coupling section ( 62 ,  362 Y,  380 Y,  483 ), and the shaft inserted into the opening section are maintained in a non-contact state, and therefore it is possible to avoid situations where a large load is applied to the rotational drive source, such as the motor, due to the inner walls of the opening section of the coupling gear making contact with the shaft after the position of the gear has been corrected. Furthermore, it is also possible to avoid unevenness in the developing density or in the transfer density, due to vibrations being generated by friction between the coupling gear and the shaft, and these vibrations being transmitted to the developing roller, photosensitive body or belt drive roller. 
   According to the present invention as described above, a merit is obtained in that it is possible to prevent variation in the rotational speed of a rotating body, such as a developer carrier, caused by misalignment between the central axes of the rotational coupling section on the driven side for causing the rotating body to rotate, and the rotational coupling section on the drive side for transmitting rotational drive force to the aforementioned rotational coupling section. 
   This beneficial effect is obtained for the following reasons. 
   According to the present invention as described above, the rotational coupling section on the driven side is fixed by engaging with the rotational coupling section on the drive side, in such a manner that its position can be corrected slightly. In other words, rather then providing an axle in the rotational coupling section on the driven side which is composed in such a manner that it is fixed to the rotational coupling section and rotates in unison with same, as in the prior art, the rotational coupling section on the driven side is disposed in such a manner that it can be moved within a small range with respect to the drive force transmission device and the developing device, before it is engaged with the rotational coupling section on eh drive side. By engaging it with the rotational coupling section on the drive side, the rotational coupling section on the driven side is positioned. By adopting a composition of this kind, it is possible for slight misalignment occurring between the central axes of the driven side and drive side coupling sections, due to problems with the precision of the components or design, or the like, to be absorbed by the driven side rotational coupling section. Therefore, the driven side rotational coupling section always rotates about the centre of rotation of the drive side rotational coupling section. As a result, it is possible to prevent variation in the rotational speed of the rotating body due to axial misalignment. If the rotational drive force received by the rotational coupling section on the driven side is transmitted to a developer carrier, then it is possible to eliminate any variation in the amount of toner conveyed per unit time to the developing position, namely, the position where the developer carrier opposes the latent image carrier, and hence unevenness in the developing density can be prevented. 
   Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.