Abstract:
An image forming apparatus includes a rotatable image bearing member; a plurality of rotatable developer carrying members; a plurality of developing devices in each of which a respective one of the developer carrying members is provided; and a developing device supporting member rotatable in a direction of codirectional peripheral movement with the image bearing member where the supporting member and the image bearing member are opposed to each other and integrally supporting the plurality of said developing devices. The peripheral speed of the developing device supporting member divided by a peripheral speed of the image bearing member at the time when each of the developer carrying members departs from the surface of the image bearing member and is still rubbing the surface of the image bearing member with the developer is larger than 0 and smaller than 1.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to a full-color image forming apparatus employing a developing process of the rotary type. 
     There have been known full-color image forming apparatuses which have a single image bearing member and multiple developing apparatuses. In the case of these image forming apparatuses, the multiple developing apparatuses are supported together by a rotatable supporting member. In an image forming operation carried out by this type of image forming apparatus, multiple electrostatic latent images, which correspond, one for one, to primary color components of an image to be formed, are sequentially formed on the single image bearing member, and each electrostatic latent image is developed by a corresponding developing apparatus placed in the development position by rotating the developing supporting rotary member with a preset timing. 
     An image forming apparatus, such as the one described above, which employs a developing apparatus supporting rotatable member (rotary), which supports multiple developing apparatuses together and sequentially develops multiple electrostatic latent images to form a single full-color image, has been referred to as an image forming apparatus of the rotary type. 
     Examples of the structure of a conventional image forming apparatus of the rotary type are disclosed in Japanese Laid-open Patent Application 2005-148319, and Japanese Laid-open Patent Application H11-15265. 
     Generally speaking, in the case of the development process carried out by an image forming apparatus of the rotary type, it is necessary to sequentially place each of the developing apparatuses provided, one for one, for multiple developers, different in color, in contact with the peripheral surface of an image bearing member, and to separate therefrom. As for the switching of the developing apparatus, the developing supporting member is rotated while the developing apparatus is not in contact with the peripheral surface of the image bearing member. 
     A conventional image forming apparatus of the rotary type is structured so that each developing apparatus is placed in contact with, or separated from, an image bearing apparatus, by moving a developing apparatus supporting member in the radius direction of the image bearing member, with the use of a cam, or the like, having its own driving force. 
     However, a conventional image forming apparatus of the rotary type, such as those described above, suffers from the following problems. 
     As described above, a conventional image forming apparatus of the rotary type is structured so that each developing apparatus is placed in contact with, or separated from, the image bearing member by moving the developing apparatus supporting member in the radius direction of the image bearing member. Therefore, the main assembly of the image forming apparatus had to be provided with the space for moving the developing apparatus supporting member. It had to be also provided with a driving means, such as a cam, for driving the developing supporting member to place each developing apparatus in contact with, or separate from, the image bearing member. 
     Thus, it has been difficult to reduce a conventional image forming apparatus of the rotary type in size and cost, because the conventional image forming apparatus of the rotary type requires the space, and also, driving means, for moving the entirety of the developing apparatus supporting member to place each developing apparatus in contact with, or separated from, the image bearing member. 
     SUMMARY OF THE INVENTION 
     The present invention was made in consideration of the above described problems, and its primary object is to provide an image forming apparatus of the rotary type, which is smaller in size and lower in cost than a conventional image forming apparatus of the rotary type, and yet, is excellent in image quality. 
     These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical sectional view of the image forming apparatus in the first preferred embodiment of the present invention, and shows the general structure of the image forming apparatus. 
         FIG. 2  is a vertical sectional view of the developing apparatus in the first preferred embodiment of the present invention, and shows the general structure of the developing apparatus. 
         FIG. 3  is a phantom view of the rotary, and the adjacent members to the rotary, in the first preferred embodiment of the present invention. 
         FIG. 4  is a drawing which shows the state of contact between the development roller and photosensitive drum. 
         FIGS. 5(   a ),  5 ( b ), and  5 ( c ) are drawings which show the state of contact between the development roller and photosensitive drum under three conditions which are different in the peripheral velocity ratio between the development roller and photosensitive drum. 
         FIG. 6  is a cross-sectional view of the development roller and photosensitive drum in the second preferred embodiment, and shows the state of the contact between the development roller and photosensitive drum. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, referring to the appended drawings, the most preferable embodiments of the present invention will be described in detail. However, the measurements, materials, and shapes of the structural components of the image forming apparatuses in the following embodiments of the present invention, and the positional relationship among them, are not intended to limit the present invention in scope, unless specifically noted. 
     Embodiment 1 
     First, referring to  FIGS. 1-6 , the image forming apparatus in the first preferred embodiment of the present invention will be described. 
     (General Structure of Image Forming Apparatus) 
     Shown in  FIG. 1  is the general structure of the image forming apparatus in this embodiment. The image forming apparatus in this embodiment is a color laser printer of the rotary type. 
     The color laser printer is provided with a rotatable photosensitive drum  2  (image bearing member). Further, the color laser printer is provided with a charge roller  3 , an exposing device  4 , and a cleaning apparatus  6 , which are disposed in the adjacencies of the peripheral surface of the photosensitive drum  2  in a manner to surround the peripheral surface of the photosensitive drum  2 . The charge roller  3  is for uniformly charging the peripheral surface of the photosensitive drum  2 . The exposing device is for forming an electrostatic latent image on the peripheral surface of the photosensitive drum  2  by projecting a beam of laser light in a manner to scan the peripheral surface of the photosensitive drum  2 . The cleaning apparatus  6  is for cleaning the peripheral surface of the photosensitive drum  2 . 
     Further, the color laser printer is provided with developing apparatuses  18   a - 18   d , which are means for developing an electrostatic latent image on the photosensitive drum  2  by supplying the latent image with developer. The developing apparatuses  18   a - 18   d  are different in the developer color (yellow, magenta, cyan, and black, respectively). 
     The developing apparatuses  18   a - 18   d  are supported together by a roughly cylindrical rotary  102  (developing apparatus supporting member), which is rotatable in such a direction that the direction in which its peripheral surface moves in the area of development is the same as the direction in which the peripheral surface of the photosensitive drum  2  moves in the area of development. Incidentally, the color laser printer may be structured so that the developing apparatuses  18   a - 18   d  are removably mountable in the rotary  102 . Structuring the printer in such a manner makes it possible to individually supply the developing apparatuses  18   a - 18   d  with developer, and also, to individually maintain them. 
     The image forming operation of the color laser printer is as follows: First, the photosensitive drum  2  is rotated in the direction (counterclockwise direction) indicated by an arrow mark in  FIG. 1 , in synchronism with the rotation of the intermediary transfer belt  7 . 
     Then, the peripheral surface of the photosensitive drum  2  is uniformly charged by the charge roller  3 , across the area which came into contact with the charge roller  3 . Then, while the peripheral surface of the photosensitive drum  2  is charged as described, the charged area of the peripheral surface of the photosensitive drum  2  is scanned (exposed) by the beam of light, which corresponds to the yellow component, for example, of the image to be formed, projected by the exposing apparatus. As a result, an electrostatic latent image, which corresponds in color to the yellow color component of an intended image, is formed on the peripheral surface of the photosensitive drum  2 . 
     Meanwhile, the driving force transmitting mechanism, which will be described later, is driven to rotate the rotary  102  to move the yellow developing apparatus  18   a  into the development position before the formation of the electrostatic latent image. The development position is the position in which the developing apparatus  18   a  opposes the photosensitive drum  2 . 
     When the developing apparatus  18   a  is in the development position, the electrostatic latent image on the photosensitive drum  2  can be developed into a visible image formed of yellow developer, by applying a voltage, which is the same in polarity as the developer, to the rotatable development roller  182   a  (developer bearing member) of the developing apparatus  18   a . That is, the voltage causes the yellow developer to adhere to the electrostatic latent image on the photosensitive drum  2 . Hereafter, an image formed of developer will be referred to as a developer image. 
     After the development of the electrostatic latent image, the developing apparatus  18   a  is separated from the peripheral surface of the photosensitive drum  2 . Then, a voltage, which is opposite in polarity to the developer is applied to the primary transfer roller  81 , which is disposed on the inward side of the loop which the intermediary transfer belt  7  forms. As the voltage is applied, the developer image on the peripheral surface of the photosensitive drum  2  is transferred (primary transfer) onto the intermediary transfer belt  7 . 
     As soon as the above described primary transfer of the yellow developer image is completed, the rotary  102  is moved back into its separation position, and then, is rotated to move the magenta developing apparatus  18   b  into the development position where the yellow developing apparatus  18   a  was. 
     Then, the same image forming process as that used to form the yellow developer image is repeated to sequentially form and transfer (primary transfer) magenta, cyan, and black images onto the intermediary transfer belt  7 , after the sequential positioning of the developing apparatuses  18   b - 18   d  in the development position. As a result, four developer images, different in color, are transferred in layers onto the intermediary transfer belt  7 . 
     While the four developer images, different in color, are transferred (primary transfer) onto the intermediary transfer belt  7 , the secondary transfer roller  82  is kept separated from the intermediary transfer belt  7 , and so is the cleaning unit  9  for cleaning the intermediary transfer belt  7 . 
     As for the sheets S of recording medium (which hereafter will be simply referred to as sheets S), they are stored in layers in the sheet feeder cassette  51 , which is located in the bottom portion of the main assembly of the color laser printer. Each sheet S is fed into the main assembly by the sheet feeder roller  52  while being separated from the rest, and then, is conveyed to the pair of registration rollers  53 . 
     As the fed sheet S is delivered to the registration rollers  53 , the registration rollers  53  send the delivered sheet S to the nip, between the intermediary transfer belt  7  and secondary transfer roller  82 , where the secondary transfer roller  82  and intermediary transfer belt  7  are kept pressed upon each other ( FIG. 1 ). 
     The process for transferring (secondary transfer) a developer image onto the sheet S is as follows: First, the sheet S is conveyed to the nip, and then, a voltage, which is opposite in polarity to the developer charge, is applied to the secondary transfer roller  82  to transfer together (secondary transfer) all the developer images on the intermediary transfer belt  7 , onto the sheet S. 
     After the transfer of the developer images onto the sheet S, the sheet S is sent to a fixing device  54 . In the fixing device  54 , the sheet S is subjected to heat and pressure, whereby the developer images are permanently fixed to the sheet S. Thereafter, the sheet S is discharged from the fixing device  54  into a delivery portion, which is an integral part of the top cover  55  of the main assembly of the image forming apparatus. 
     (Structure of Developing Apparatus) 
     Next, referring to  FIG. 2 , the structure of developing apparatuses  18   a - 18   d  in this embodiment will be described regarding their structure.  FIG. 2  shows the general structure of the developing apparatus  18   a  in this embodiment. Here, only the developing apparatus  18   a , which corresponds to yellow, will be described. Since the developing apparatuses  18   b ˜ 18   d  is are the same in structure as the developing apparatus  18   a , their structure will not be described. 
     The developing apparatuses employed in this embodiment employ a developing method of the contact type. The developing apparatus  18   a , which is of the contact type, is provided with a development roller  182   a  (as developer bearing member), a regulatory blade  183   a , a developer supply roller  184   a , and a developer storage chamber  185   a.    
     The development roller  182   a  is structured so that it is rotatable. It supplies the electrostatic latent image, which has just been formed on the photosensitive drum  2  before the development roller  182   a  is placed in contact with the photosensitive drum  2 , by being placed in contact with the peripheral surface of the photosensitive drum  2  while bearing developer on its peripheral surface and being rotated. 
     In this embodiment, the direction in which the peripheral surface of the development roller  182   a  is moved in the interface between the development roller  182   a  and photosensitive drum  2  is the same as the peripheral surface of the photosensitive drum  2  is moved in the interface. The peripheral velocity of the development roller  182   a  is to be set to 160% of that of the photosensitive drum  2 . 
     The development roller  182   a  used in this embodiment is made up of a metallic core, a layer of silicon rubber, and a layer of urethane resin. The metallic core is made of SUS. The silicon layer is adhered, as the base layer, to the peripheral surface of the metallic core. The urethane layer is coated, as the surface layer, on the silicon rubber layer. 
     As the regulation blade  183   a , a thin (80 μm in thickness) piece of SUS is used. The regulation blade  18   a  is positioned so that its regulating edge will be on the upstream side of its base portion in terms of the rotational direction of the development roller  182   a . Positioning the regulation blade  183   a  as described makes it possible to regulate the amount by which the developer is allowed to remain coated on the peripheral surface of the development roller  182   a  as the development roller  182   a  is rotated. 
     The developer supply roller  184   a  is made up of a metallic core, and a layer of urethane sponge wrapped around the peripheral surface of the metallic core. The developer supply roller  184   a  is temporarily impregnated with the developer, and then, the developer in the developer supply roller  184   a  is supplied to the peripheral surface of the development roller  182   a , in the interface between the developer supply roller  184   a  and development roller  182   a.    
     The development roller  182   a  and developer supply roller  184   a  are rotated in the same direction. That is, in their interface, the peripheral surface of the development roller  182   a  and the peripheral surface of the developer supply roller  184   a  move in the opposite direction relative to each other. 
     Further, the image forming apparatus is structured so that a voltage is applied to various members of the developing apparatus which is in the development position, into which each developing apparatus is moved by the operation, which will be described later; when the developing apparatus  18   a  is in the development position, the voltage is applied to the various members of the developing apparatus  18   a.    
     During the development operation of the image forming apparatus in this embodiment, the potential level of the unexposed portion of the peripheral surface of the photosensitive drum  2  is −500 V, and the potential level of the exposed portion of the peripheral surface of the photosensitive drum  2  is −150 V, whereas the potential level of the voltage applied to the development roller  182   a , regulation blade  183   a , and developer supply roller  184   a  is roughly −350 V, for example. 
     Since the potential levels of the abovementioned members, portions, etc., are set as described above, the negatively charged developer does not adhere to the unexposed portions of the peripheral surface of the photosensitive drum  2 , and is adhered to the exposed portions of the peripheral surface of the photosensitive drum  2  by electrostatic force. 
     (Structure of Developing Apparatus Supporting Member) 
     Next, referring to  FIG. 3 , the structure of the rotary  102  (developing apparatus supporting member), and the structure of the members adjacent to the rotary  102 , in this embodiment, will be described.  FIG. 3  shows the state of the rotary, and the state of the developing apparatuses in the rotary, in which the development roller  182   a , which is rotatably supported by the developing apparatus  18   a , is developing the electrostatic latent image on the peripheral surface of the photosensitive drum  2 . 
     The rotary  102  is roughly cylindrical, and is rotatably structured. Its peripheral surface is provided with gear teeth, which are in engagement with a driving gear  172 . Thus, as driving force is transmitted from an unshown driving force source to the driving gear  172 , the rotary  102  rotates. As the driving gear  172  rotates in the direction indicated by an arrow mark A in  FIG. 3 , the rotary  102  rotates in the direction indicated by an arrow mark B. As the driving gear  172  stops, the rotary  102  also stops. 
     The driving gear  172  is supported by the main assembly of the image forming apparatus; the driving gear  172  is supported by the shaft  107 , which is supported by the main assembly. As the unshown driving force source stops, the driving gear  172  stops. However, the driving force source cannot be driven by the driving gear  172 . 
     The shaft  107  of the driving gear  172  is connected to the rotational axle of the rotary  102  by an arm  103 , which is rotatably supported by the shaft  107 . Further, one end of the arm  103  is under the pressure from an arm spring  104  attached to the main assembly. Thus, the arm  103  remains under such a pressure that works in the direction to rotate the arm  103  about the shaft  107 . 
     The rotary  102  supports the developing apparatuses  18   a - 18   d  so that the outermost point of each of the development rollers  182   a - 182   d  of the development apparatuses  18   a - 18   d , respectively, in terms of the radius direction of the rotary  102 , roughly coincides with the peripheral surface of the rotary  102 . Incidentally, the image forming apparatus may be structured so that the developing apparatuses  18   a - 18   d  are fixed to the rotary  102 , or removably mountable in the rotary  102 . 
     Further, the rotary  102  is provided with a rotatable cam  101 , which is coaxial with the rotary  102 . The cam  101  has four recesses  101   a - 101   d , which are on the outward side of the cam  101  in terms of the radius direction of the rotary  102 . The four recesses  101   a - 101   d  are the same in shape, and are distributed with equal intervals in terms of the circumferential direction of the cam  101 . The cam  101  is connected to the rotary  102  so that its rotational axis coincides with that of the rotary  102 . 
     That is, the rotary  102  is structured so that the cam  101  always moves in synchronism with the main assembly of the rotary  102 . In this embodiment, the rotary  102  and cam  101  are independent from each other. However, the cam  101  may be formed as a part of the main assembly of the rotary  102 . 
     Further, the main assembly of the image forming apparatus is provided with a regulation roller  105 , which is positioned next to the cam  101  so that it remains in contact with the peripheral surface of the cam  101 . The regulation roller  105  is rotatably supported, while remaining in contact with the peripheral surface of the cam  101 , by a roller holder  106 , with which the main assembly is provided. 
     The surface layer of the regulation roller  105  is formed of elastic rubber. Providing the regulation roller  105  with the surface layer made of elastic rubber can significantly reduce the noises attributable to the contact between the regulation roller  105  and the peripheral surface of the cam  101 . Moreover, the coefficient of friction of the rubber layer, which is significantly higher than that of the substrate portion of the regulation roller  105 , ensures the cam  101  is rotated by the regulation roller  105 . 
     In this embodiment, the regulation roller  105  is rotatably supported by the roller holder  106 . However, in a case where a roller, the peripheral surface of which is excellent in terms of slipperiness, is used as the regulation roller  105 , it is unnecessary for the regulation roller  105  to be rotatable. Besides, it is unnecessary for the regulating member ( 105 ) to be in the form of a roller. In other words, all that is necessary is that it is ensured that as the cam  101  is rotated, the regulating member ( 105 ) remains in contact with the peripheral surface of the cam  101  without interfering with the rotation of the cam  101 . 
     Referring to  FIG. 3 , the regulation roller  105  is in the adjacencies of the recess  101   a  (- 101   d ) of the cam  101 , being positioned so that the regulation roller  105  does not contact the cam  101 . 
     The arm  103 , which is under the pressure from the arm spring  104 , presses on the rotary  102 . This pressure applied to the rotary  102  generates the contact pressure between the development roller  182   a  (- 182   d ) and photosensitive drum  2 . 
     As described above, in  FIG. 3 , the image forming apparatus is structured so that the development roller  182   a  is kept pressed upon the peripheral surface of the photosensitive drum  2 , by the pressure from the arm spring  104 , with the presence of a proper amount of contact pressure between the development roller  182   a  and photosensitive drum  2 . However, as the rotary  102  is rotated, the development roller  182   a  is kept separated from the photosensitive drum  2  while the rotary  102  is rotated. 
     That is, while a latent image is developed, the rotary  102  is kept stationary. Then, as soon as the development of the latent image is completed, the rotary  102  is rotated again, causing therefore the development roller  182   a  to be separated from the peripheral surface of the photosensitive drum  2 . 
     Then, virtually at the same time as the development roller  182   a  separates from the peripheral surface of the photosensitive drum  2 , the cam  101  comes into contact with the regulation roller  105 . 
     The peripheral surface of the cam  101  is shaped so that while the regulation roller  105  is in contact with the portions of the peripheral surface of the cam  101  other than the recess portions  101   a - 101   d , developing apparatuses  18   a - 18   d  do not contact the photosensitive drum  2 . 
     Therefore, the developing apparatuses  18   a - 18   d  can be sequentially moved into the development position, and be placed in contact with the photosensitive drum  2 , without affecting the photosensitive drum  2  at all. 
     More concretely, as the developing apparatus  18   b  (- 18   d ) is moved into the development position, an unshown controller cuts off the driving force to the driving gear  172 , and the recessed portion  101   b  (- 110   d ) of the cam  101  comes into the adjacencies of regulation roller  105 . Thus, the development roller  182   b  (- 182   d ) is placed in contact with the photosensitive drum  2  so that a preset amount of contact pressure is generated between the development roller  182   b  and photosensitive drum  2 . Thus, the electrostatic latent images sequentially formed on the peripheral surface of the photosensitive drum  2  are sequentially developed by the developing apparatuses  18   a - 18   d.    
     As described above, in this embodiment, the image forming apparatus is structured so that all that is necessary to sequentially place the development rollers  182   a - 182   d  in contact with the peripheral surface of the photosensitive drum  2 , and separate them from the peripheral surface of the photosensitive drum  2 , is to rotate the rotary  102 . 
     That is, in this embodiment, the direction in which the development rollers  182   a - 182   d  are moved to be placed in contact, or separated from, the peripheral surface of the photosensitive drum  2 , is parallel to the line which is tangential to the peripheral surface of the photosensitive drum  2  and coincides with the expected point of contact between the development rollers  182   a - 182   d  and photosensitive  2 . That is, the image forming apparatus in this embodiment is quite different in structure from a conventional image forming apparatus structured so that the development roller is moved in the radius direction of the development roller to be placed in contact with, or separated from, the photosensitive drum. 
     That is, in this embodiment, the image forming apparatus does not need to be structured to enable the rotary  102 , for example, to be moved in the radius direction of the photosensitive drum  2 , making it unnecessary to provide the main assembly of the image forming apparatus with the space for placing the development rollers  182   a - 182   d  in contact with, or separated from, the photosensitive drum  2 , making it thereby possible to significantly reduce in size the main assembly of the apparatus, compared to a conventional image forming apparatus of the rotary type. 
     Further, the development roller  182   a - 182   d  are placed in contact with, or separated from, the photosensitive drum  2  simply by rotating the rotary  102  to switch the developing apparatus in the development position with the next developing apparatus. Thus, the structural arrangement, driving force source, etc., dedicated to the placing of a development roller in contact with, or the separation of the development roller from, the photosensitive drum  2 , are unnecessary. Thus, the image forming apparatus in this embodiment is significantly smaller in manufacturing cost than a conventional image forming apparatus of the rotary type; the present invention can significantly reduce an image forming apparatus of the rotary type, in manufacturing cost. 
     Further, in the case of the image forming apparatus of the rotary type in this embodiment, the operation for placing one of the developing apparatuses  18   a - 18   d  in contact with, or separating from, the peripheral surface of the photosensitive drum  2 , and the operation for replacing the developing apparatus  18  in the development position with the next developing apparatus  18 , are simultaneously carried out, and therefore, the image forming apparatus of the rotary type in this embodiment is significantly higher in the speed with which the development rollers  182   a - 182   d  are sequentially placed in contact with, or separated from, the photosensitive drum  2 . 
     (Mechanism of Formation of Developer Stripe on Peripheral Surface of Image Bearing Member Due to Rubbing) 
     However, in the case of an image forming apparatus, the development rollers  182   a - 182   d  of which are placed in contact with, or separated from, the peripheral surface of the photosensitive drum  2  in the direction parallel to the tangential line of the photosensitive drum  2 , which coincides with the expected point of contact between the development roller  182  and photosensitive drum  2 , it is possible that the following problem will occur. 
     That is, when the rotation of the rotary  102  is restarted (when development roller  182  is separated from photosensitive drum  2 ) after the development of the electrostatic latent image on the photosensitive drum  2  by the development roller  182  while the rotary  102  is kept stationary, the peripheral surface of the development roller  182  rubs against the peripheral surface of the photosensitive drum  2 . 
     In this embodiment, in order to place the development roller  182  in contact with, or separated from, the peripheral surface of the photosensitive drum  2 , the development roller  182  is moved in the direction parallel to the line which is tangential to the peripheral surface of the photosensitive drum  2  and coincides with the expected point of contact between the development roller  182  and photosensitive drum  2  as described above. Thus, the peripheral surface of the development roller  182  and the peripheral surface of the photosensitive drum  2  in this embodiment are likely to rub against each other unlike the counterparts of an image forming apparatus of the conventional rotary type, the development roller  182  of which is moved in the radius direction of the image bearing member to be placed in contact with, or separated from, the image bearing member. 
     Thus, as the two surfaces rub against each other, the developer on the peripheral surface of the development roller  182   a  is rubbed against the peripheral surface of the photosensitive drum  2 . As a result, a stripe of the developer is left on the peripheral surface of the photosensitive drum  2 . The extent of the rubbing is affected by the relationship between the peripheral velocity of the rotary  102  and that of the photosensitive drum  2 . With the presence of the stripe of developer on the peripheral surface of the photosensitive drum  2 , unsatisfactory copies, for example, a copy, the back side of which is soiled, or the like, will be yielded. In other words, it is difficult to obtain a copy of high quality. 
     Thus, in order to minimize the amount by which the developer is left, in the form of a stripe, on the peripheral surface of the photosensitive drum  2  when the development roller  182  separates from the peripheral surface of the photosensitive drum  2 , the image forming apparatus in this embodiment is structured so that the rotary  102  and photosensitive drum  2  can be controlled in peripheral velocity. This is the characteristic feature of the image forming apparatus in this embodiment. Next, this feature will be described. 
     First, referring to  FIGS. 5(   a )- 5 ( c ), the mechanism of the formation of developer stripe which occurs when the development roller  182   a  is separated from the photosensitive drum  2 , will be described. The mechanism will be described with reference to the development roller  182   a . Since the development rollers  182   b - 182   d  are the same in structure as the development roller  182   a , the formation of developer stripe by them will not be described. 
       FIGS. 5(   a )- 5 ( c ) show the photosensitive drum  2  and development roller  182   a  at the moment of separation of development roller  182   a  from the photosensitive drum  2 . In  FIGS. 5(   a )- 5 ( c ), RT and Dr stand for the peripheral velocity of the rotary  102 , and the peripheral velocity of the photosensitive drum  2 , respectively. The drawings represent the cases in which the following relationships are satisfied by RT and Dr:
 
 RT/Dr= 1  5(a),
 
 RT/Dr&gt; 1  5(b),
 
 RT/Dr&lt; 1  5(c).
 
     Further, C, C′, D, D′ in the drawings are for showing the distance which the peripheral surface of the development roller  182   a  moves, and the range of the peripheral surface of the photosensitive drum  2 , across which the peripheral surface of the photosensitive drum  2  is rubbed by the development roller  182   a , when the development roller  182   a  separates from the photosensitive drum  2 . 
     That is, when separating from the photosensitive drum  2 , the development roller  182   a  moves from Point C to Point C′ while rubbing the peripheral surface of the photosensitive drum  2 , whereas the peripheral surface of the photosensitive drum  2  is covered with the developer deposited on the peripheral surface of the photosensitive drum  2  by being rubbed against the peripheral surface of the photosensitive drum  2  by the development roller  182   a , across the area from Point D to Point D′.
         &lt;RT/Dr=1 ( FIG. 5(   a ))&gt;       

     First, referring to  FIG. 5(   a ), in the case where the rotary  102  and photosensitive drum  2  are the same in peripheral velocity (RT/Dr=1), the peripheral surface of the photosensitive drum  2  and peripheral surface of the development roller  182   a  rotationally move at the same velocity. 
     Therefore, the developer on the peripheral surface of the development roller  182   a  is rubbed onto the peripheral surface of the photosensitive drum  2  by the same amount per unit area as the amount of developer on the development roller per unit area. Thus, as the development roller  182   a  separates from the peripheral surface of the photosensitive drum  2 , a stripe of developer remains on the peripheral surface of the photosensitive drum  2 .
         &lt;RT/Dr&gt;1 ( FIG. 5(   b ))&gt;       

     Next, referring to  FIG. 5(   b ), in the case where the peripheral velocity RT of the rotary  102  at the moment when the development roller  182   a  separates from the peripheral surface of the photosensitive drum  2 , is greater than the peripheral velocity Dr of the photosensitive drum  2  at the moment when the development roller  182   a  separates from the photosensitive drum  2  (RT/Dr&gt;1), the developer stripe attributable to the rubbing of the peripheral surface of the development roller  182   a  and the peripheral surface of the photosensitive drum  2  relative to each other is generated across a smaller area of the peripheral surface of the photosensitive drum  2 . 
     That is, the preset amount of developer on the development roller  182   a , which is on the development roller  182   a , is rubbed onto the smaller area of the peripheral surface of the photosensitive drum  2  than when RT/Dr=1. Therefore, a narrower and denser developer stripe is generated on the peripheral surface of the photosensitive drum  2 .
         &lt;RT/Dr&lt;1 ( FIG. 5(   c )&gt;       

     Next, referring to  FIG. 5(   c ), in the case where the peripheral velocity RT of the rotary  102  at the moment when the development roller  182   a  separates from the peripheral surface of the photosensitive drum  2 , is less than the peripheral velocity Dr of the photosensitive drum  2  at the moment when the development roller  182   a  separates from the photosensitive drum  2  (RT/Dr&lt;1), the area of the peripheral surface of the photosensitive drum  2 , across which the development roller  182   a  contacts the peripheral surface of the photosensitive drum  2  while moving from the development position to the point of its separation from the photosensitive drum  2 , is larger. 
     Therefore, the preset amount of developer on the development roller is rubbed onto a larger area of the peripheral surface of the photosensitive drum  2 . Thus, the resultant developer stripe is less conspicuous. 
     As described above, the difference in the relationship (ratio) between the distance (distance from Point C to Point C′) which the peripheral surface of the development roller  182   a  moves while the development roller  182   a  separates from the photosensitive drum  2 , and the distance (distance from Point D to Point D′) which the peripheral surface of the photosensitive drum  2  moves while the development roller  182   a  separates from the photosensitive drum  2 , affects the appearance of the developer stripe generated on the peripheral surface of the photosensitive drum  2 . 
     That is, it is evident that, in order to ensure that the developer stripe which will be formed on the photosensitive drum  2  will be inconspicuous, it is effective to make the peripheral velocity (RT) of the rotary  102  at the moment when the development roller  182   a  separates from the photosensitive drum  2 , less than the peripheral velocity (Dr) of the photosensitive drum  2  at the moment when the development roller  182   a  to separated from the photosensitive drum  2 . 
     (Results of Study of Effects) 
     The inventors of the present invention studied the relationship between the ratio of the peripheral velocity of the rotary  102  to the peripheral velocity of the photosensitive drum  2 , and the appearance of the developer stripe on the peripheral surface of the photosensitive drum  2 , when the rotational direction of the photosensitive drum  2  is opposite to the rotational direction of the rotary  102  as shown in  FIG. 4 . 
     In the study, the ratio between the peripheral velocity (RT) of the rotary  102  and peripheral velocity (Dr) of the photosensitive drum  2  was to set to several values. Further, the peripheral velocity (mm/sec) of the photosensitive drum  2  was set to two values. Then, whether the developer stripe is generated or not was examined under the various conditions created by combining the abovementioned ratios with the peripheral velocities of the photosensitive drum  2 . The results of the study are shown in Table 1, in which NG stands for the condition under which distinct developer stripe was generated; F stands for the condition under which vaguely visible is developer stripe was generated; and G stands for the condition under which no visible developer stripe was generated. That is, G stands for the condition under which a preferable level of image quality was achieved. 
     In this study, the rotational direction of the rotary  102  was defined as the positive direction. As for the rotational direction of the photosensitive drum  2 , the rotational direction of the photosensitive drum  2 , which is opposite to the rotational direction of the rotary  102  was defined as the positive/direction. That is, the rotational direction of the photosensitive drum  2  was considered positive when the direction in which the peripheral surface of the photosensitive drum  2  moves in the area in which the distance between the rotary  102  and photosensitive drum  2  is smallest is the same as the direction in which the peripheral surface of the rotary  102  moves in the same area. 
     
       
         
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 (Rotary Speed (RT))/(Drum Speed (Dr)) 
               
             
          
           
               
                   
                 0.3 
                 0.6 
                 0.9 
                 1.0 
                 1.2 
                 2.5 
               
               
                   
                   
               
             
          
           
               
                 Drum 
                  50 
                 G 
                 G 
                 G 
                 F 
                 NG 
                 NG 
               
               
                 Speed 
                 100 
                 G 
                 G 
                 G 
                 F 
                 NG 
                 NG 
               
               
                 (Dr mm/s) 
               
               
                   
               
             
          
         
       
     
     Referring to Table 1, in the case where the peripheral velocity of the photosensitive drum  2  was set to 100 mm/sec, and the peripheral velocity of the rotary  102  was set to 2.5 times the peripheral velocity of the photosensitive drum  2  to rotate the rotary  102  faster than the photosensitive drum  2 , a conspicuous developer stripe was generated when the development roller  182  was separated from the photosensitive drum  2 . Further, a conspicuous developer stripe was generated even when the peripheral velocity of the rotary  102  was 1.2 times the peripheral velocity of the photosensitive drum  2 . 
     On the other hand, it was confirmed that as the ratio of the peripheral velocity of the rotary  102  to the peripheral velocity of the photosensitive drum  2  was set to a value which is no more than 1.0, the generated developer stripe was significantly less conspicuous. 
     That is, the distinctive feature of this embodiment is that when the development roller  182   a  separates from the peripheral surface of the photosensitive drum  2 , the relationship between the peripheral velocity of the rotary  102  and peripheral surface of the photosensitive drum  2  is made to be such that the ratio of the peripheral velocity of the rotary  102  to the peripheral velocity of the photosensitive drum  2  is no more than 1.0. This distinctive feature is the indispensable condition for preventing the formation of an image having an unwanted developer stripe. 
     As described above, the generation of a conspicuous developer stripe can be prevented by making the peripheral velocity of the rotary  102  slower than the peripheral velocity of the photosensitive drum  2  at least when the development roller  182  separates from the peripheral surface of the photosensitive drum  2 . The prevention of the formation of a conspicuous developer stripe can achieve an excellent level of image quality. 
     Although it was stated above that the peripheral velocity of the rotary  102  is to be made slower than the peripheral velocity of the photosensitive drum  2  at least when the development roller  182   a  separates from the peripheral surface of the photosensitive drum  2 , the peripheral velocity of the rotary  102  may be switched back to the previous speed (higher speed) after the separation. 
     Increasing the peripheral velocity of the rotary  102  after the separation, and keeping the peripheral velocity of the rotary  102  at the increased velocity, reduces the length of time necessary for the development process, making it possible to provide an image forming apparatus of the rotary type, which is significantly higher in the development speed than a conventional image forming apparatus of the rotary type. 
     The peripheral velocity of the rotary  102 , peripheral velocity of the photosensitive drum  2 , and the conditions under which the components other than the rotary  102  and photosensitive drum  2 , which are related to the development process, are driven, can be controlled by the unshown control portions (CPU, etc.) with which the main assembly of the apparatus is provided. 
     As will be evident from the description of the first preferred embodiment of the present invention, not only can the present invention improve an image forming apparatus of the rotary type, in image quality, but also, can reduce in size and cost an image forming apparatus of the rotary type. 
     Embodiment 2 
     Next, referring to  FIG. 6 , the image forming apparatus in the second preferred embodiment of the present invention will be described.  FIG. 6  shows the state of contact between the development roller  182   a  and photosensitive drum  2  in this embodiment. 
     Even in the case where the direction in which the development roller  182  rotates is the same as the direction in which the rotary  102  rotates, as the peripheral velocity of the photosensitive drum  2  is increased relative to the peripheral velocity of the rotary  102  as in the first embodiment, the resultant developer stripe is less conspicuous. However, it is possible that the developer adheres to the peripheral surface of the photosensitive drum  2  due to the effect of the rotation of the development roller  182 . 
     That is, even if the development roller  182  and photosensitive drum  2  satisfy the relationship in peripheral velocity between the rotary  102  and photosensitive drum  2 , which was described in the description of the first preferred embodiment, the developer is likely to be rubbed onto a small area of the peripheral surface of the photosensitive drum  2 , making it possible that the developer will adhere to the peripheral surface of the photosensitive drum  2 , because the development roller  182  is rotating. 
     It is rare that the above described occurrence of the adhesion of the developer to the peripheral surface of the photosensitive drum  2  results in the formation of an unsatisfactory copy, such as the one soiled across the back side. However, the adhesion results in the unnecessary amount of developer consumption. Further, as the peripheral surface of the development roller  182  and the peripheral surface of the photosensitive drum  2  rub against each other, the photosensitive drum  2  is likely to be changed in peripheral surface properties; for example, the amount of torque necessary to make the peripheral surface of the development roller  182  and the peripheral surface of the photosensitive drum  2  move through the area of contact between the development roller  182  and photosensitive drum  2 , is changed, causing the electrostatic latent image to become “blurry”, and/or the portion of the intermediary transfer belt  7 , which is in the transfer portion, to move, making it possible for the image forming apparatus to yield a defective image. 
     Thus, in order to reduce the amount of developer consumption by reducing the amount by which the developer adheres to the peripheral surface of the photosensitive drum  2 , the inventors of the present invention zealously studied the relationship between the ratio of the peripheral velocity of the development roller  182  to the peripheral velocity of the rotary  102 , and the presence (absence) of the developer adhesion. Given in Table 2 is the results of this study. The peripheral velocity of the photosensitive drum  2  was 100 mm/sec, and the development roller  182  was rotated at three different peripheral velocities, whereas the rotary  102  was rotated at four different, peripheral velocities. 
     Whether the developer adhered to the peripheral surface of the photosensitive drum  2  or not was checked under the following conditions. In the table, NG stands for the developer adhesion and/or presence of developer stripe; F stands for the developer adhesion; and G stands for no developer adhesion. The numbers on the right-hand side of the G, F, and NG are the ratio of the sum of the peripheral velocity of development roller  182  and peripheral velocity of rotary  102 , to the peripheral velocity (100 mm/sec) of the photosensitive drum  2 . Here, the rotational direction of the development roller  182  is defined as the positive direction when it is the same as the rotational direction of the rotary  102 . 
     
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Peripheral Speed of Rotary (mm/sec) 
               
             
          
           
               
                   
                 30 
                 60 
                 90 
                 130 
               
               
                   
                   
               
             
          
           
               
                 Peripheral 
                 130 
                 G/1.6 
                 G/1.9 
                 G/2.2 
                 NG/2.6 
               
               
                 Speed of 
                 170 
                 G/2.0 
                 G/2.3 
                 G/2.6 
                 NG/3.0 
               
               
                 Developing 
                 220 
                 G/2.5 
                 G/2.8 
                 G/3.1 
                 NG/3.5 
               
               
                 Roller 
               
               
                 (mm/sec) 
               
               
                   
               
             
          
         
       
     
     It is evident from the results of this study that the sum of the peripheral velocity of the rotary  102  and the peripheral velocity of the development roller  182   a  is desired to be no more than 2.5 times the peripheral velocity of the photosensitive drum  2 . 
     In this embodiment, as the sum of the peripheral velocity of the rotary  102  and development roller  182   a  was greater than the peripheral velocity of the photosensitive drum  2 , the developer on the development roller  182   a  was adhered to the portion of the peripheral surface of the photosensitive drum  2 , which happened to be in contact with the development roller  182   a , by being rubbed onto this portion of the peripheral surface of the photosensitive drum  2 . 
     Thus, the amount by which developer is inadvertently adhered to the peripheral surface of the photosensitive drum  2  can be reduced by reducing the ratio of the sum of the peripheral velocity of the development roller  182   a  and the peripheral velocity of the rotary  102 , to the peripheral velocity of the photosensitive drum  2 , to a specific value (2.5) or less. 
     Prevention of the untimely adhesion of the developer onto the peripheral surface of the photosensitive drum  2  can prevent the wasteful developer consumption. In the case of the above described study, the peripheral velocity of the photosensitive drum  2  was set to 100 mm/sec. However, it has been confirmed that the above described relationship holds true regardless of the peripheral velocity of the photosensitive drum  2 . 
     As described above, in this embodiment, the unwanted adhesion of the developer to the photosensitive drum  2  was prevented by adjusting the peripheral velocity of the rotary  102 , peripheral velocity of the photosensitive drum  2 , and development roller  182   a . As a result, it was possible to achieve a satisfactory level of image quality. 
     Incidentally, it is possible that the rubbing of the peripheral surface of the development roller  182  and the peripheral surface of the photosensitive drum  2  against each other will change the peripheral surface of the photosensitive drum  2  in properties, which in turn creates problems; For example, the change in the surface properties of the photosensitive drum  2  may change the torque in the area of contact between the development roller  182  and photosensitive drum  2 , causing thereby an electrostatic latent image to be “blurred”. Further, the transfer portion, that is, the area of contact between the intermediary transfer belt  7  (sheet S) and the peripheral surface of the photosensitive drum  2 , is also affected by the change in the amount of torque, which in turn may resulting in the formation of a streaky image. 
     However, the image forming apparatus in this embodiment was designed so that the peripheral velocity of the development roller  182  at the moment when the development roller  182  separates from the peripheral surface of the photosensitive drum  2  is less than a preset value. Thus, the image forming apparatus in this embodiment is significantly less likely to suffer from the above described problems. 
     As will be evident from the description of the first and second preferred embodiments of the present invention, not only does the present invention make it possible to achieve a satisfactory level of image quality with the use of an image forming apparatus of the rotary type, but also, to reduce an image forming apparatus of the rotary type, in size and cost. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 220476/2008 filed Aug. 28, 2008 which is hereby incorporated by reference.