Abstract:
An image forming apparatus includes at least two image bearing members, a transferring unit that makes a physical contact with both the image bearing members, and a lubricant applying unit that applies lubricant on the surface of the image bearing members. Images can be selectively formed on any one of the image bearing member. Although an image is being formed on one image bearing member, the transferring is made to come in physical contact with the other image bearing member.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present document incorporates by reference the entire contents of Japanese priority documents, 2003-298916 filed in Japan on Aug. 22, 2003 and 2004-200964 filed in Japan on Jul. 7, 2004 
   BACKGROUND OF THE INVENTION 
   1) Field of the Invention 
   The present invention relates to an image forming apparatus, such as a copier, a printer, and a facsimile of electro photography-type or electrostatic-recording type, which can form images with two or more colors. 
   2) Description of the Related Art 
   In color image forming apparatuses, whether an image is to be formed in a monochrome mode or a color mode can be selected. Such an image forming apparatus includes an image formation unit corresponding to each color. Moreover, each image formation unit includes a photosensitive body. 
   When an image is to be formed in the color mode, for instance, the photosensitive body corresponding to black is not used. In a conventional image forming apparatus, when the image is to be formed in the color mode, for instance, the photosensitive body corresponding to black is physically separated from a transfer body to reduce friction between them and thus lengthen the life of the photosensitive body. However, there is a drawback in this technique, that the productivity lowers because there is a need to physically separate the photosensitive body from the transfer body. 
   Japanese Patent Application-Laid-Open No. 2000-293003 discloses an image forming apparatus that can surely, and at low power, perform switching between the color mode and the monochrome mode. Moreover, Japanese Patent Application-Laid-Open No. 2002-139887 discloses an image forming apparatus that decreases the running cost and the manufacturing cost by using a belt transfer device having a simpler structure. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide an image forming apparatus in which there is no need to physically separate the photosensitive body from the transfer body. 
   An image forming apparatus according to an aspect of the present invention includes a first image bearing member and a second image bearing member; a first developing unit and a second developing unit; a selection unit that selects any one developing unit to be used, from among the first developing unit to thereby form an image on the first image bearing member, and the second developing unit to thereby form an image on the second image bearing member; a transferring unit that makes physical contact with both the first image bearing member and the second image bearing member, and onto which images from both the first image bearing member and the second image bearing member are transferred; a first lubricant applying unit that applies a lubricant to the first image bearing member; and a second lubricant applying unit that applies a lubricant to the second image bearing member, wherein if the selection unit selects the first developing unit, the transferring unit is made to come in physical contact with the second developing unit and if the selection unit selects the second developing unit, the transferring unit is made to come in physical contact with the first developing unit. 
   An image forming apparatus according to another aspect of the present invention includes a first image bearing member and a second image bearing member, wherein an image can be selectively formed on any one of the first image bearing member and the second image bearing member, and a lubricant applying unit that applies a lubricant to the first image bearing member and the second image bearing member. 
   An image forming apparatus according to still another aspect of the present invention includes a first image bearing member and a second image bearing member, wherein an image can be selectively formed on any one of the first image bearing member and the second image bearing member, and a coefficient of friction of the first image bearing member and a coefficient of friction of the second image bearing member are both at the most 0.3. 
   The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of relevant portions of an image forming apparatus according to an embodiment of the present invention; 
       FIG. 2  is a side view of a photosensitive unit of the image forming apparatus; and 
       FIG. 3  is a side view of the entire image forming apparatus. 
   

   DETAILED DESCRIPTION 
   Exemplary embodiments of an image forming apparatus according to the present invention are explained below while referring to the accompanying drawings. 
     FIG. 1  is a side view of relevant portions of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus includes a photosensitive drum  5  and an electric charger  4  that charges the photosensitive drum  5 . The photosensitive drum  5  rotates in a direction represented by arrows E and C. The photosensitive drum  5  is an image bearing member that carries an image. The electric charger  4  includes an electrification roller  14  as a charger. 
   The image forming apparatus includes a writing unit  6  that forms an electrostatic latent image on the surface of the photosensitive drum  5  by exposing the surface of the photosensitive drum  5 . The image forming apparatus further includes a developing device  10  and a transfer device  70 . The developing device  10  forms a visible image (that is, a toner image) by developing the electrostatic latent image with a developer  60  on the surface of photosensitive drum  5 . 
   The developing device  10  includes a roller  11  that stirs as well as transports a toner  60  to the surface of photosensitive drum  5 . A clutch mechanism  12  drive controls this roller  11 . 
   The transfer device  70  transfers the visible image that is present on the surface of the photosensitive drum  5  to a transfer body P such as a paper. After the transfer, traces of toner sometimes remain on the surface of the photosensitive drum  5 . Therefore, the image forming apparatus is provided with a cleaning device  71  that cleans and removes such toner. 
   The cleaning device  71  includes a cleaning blade  72 . The tip of the cleaning blade  72  is configured in such a manner that it touches the surface of photosensitive drum  5  at an optimal angle and an optimal contact pressure so that the toner can be easily and completely removed from the surface of the photosensitive drum  5 . 
     FIG. 2  is a side view of a photosensitive unit of the image forming apparatus. The photosensitive unit can be detachably fit into a main body  1  of the image forming apparatus shown in  FIG. 3 . The photosensitive unit includes a main fixing part  51  that is used as a reference when fitting the photosensitive unit in the main body  1 . The photosensitive unit also has a bracket  50  that has parts  52  and  53 . The part  53  is used for positioning in the front side and the part  52  is used for positioning in the interior side. As a result, the photosensitive unit can surely and firmly be attached to a predetermined position inside the main unit  1 . 
   A lubricant spreading mechanism is arranged in the upstream of the cleaning blade  72 . This lubricant spreading mechanism includes a brush roller  15 , a lubricant  100 , and a spring  101  that presses the lubricant  100  to the brush roller  15 . The brush roller  15  is made to rotate at a speed that is 1.1 times of the line velocity of the photosensitive drum  5  in the direction indicated by an arrow B (see  FIG. 2 ). Thus, the lubricant  100  is applied on the surface of the photosensitive drum  5 . Because of the presence of the lubricant, the cleaning blade  72  scratches the toner from the photosensitive drum  5  without damaging it. The lubricant  100  is solid zinc stearate in the form of a rod of size 8 mm×8 mm and weight 20 grams. 
   As another embodiment, instead of providing the lubricant spreading mechanism, a photosensitive drum having a low coefficient of friction can be used to obtain the same effects. The photosensitive drum having the low coefficient of friction can be manufactured by forming a coat of fluorine on the surface of an ordinary photosensitive drum. 
   A problem in typical image forming apparatuses is that, because in the monochrome mode the development units corresponding to other colors rotate unnecessarily, the quality of the toners of the other colors deteriorates. To solve this problem, in the present embodiment, an arrangement is provided so that those development units that are not used for image formation do not rotate. Consequently, deterioration of the quality of the toners is prevented. 
   Another problem in the typical image forming apparatuses is that, static electricity acts on the photosensitive drum  5  and damages the photosensitive drum  5 . To solve this problem, in the present embodiment, an arrangement is provided that can adjust a charging bias and a developing bias, which are generally applied to the photosensitive drum  5 , in such a manner that the static electricity does not act on the photosensitive drum  5 . If the bias is lowered or completely turned off, static electricity does not act on the photosensitive drum  5 . Therefore, damage of the photosensitive drum  5  is prevented. 
   Still another problem in the typical image forming apparatuses is that, the toner in the image formation unit upstream mixes with the toner in the image formation unit downstream. This causes degradation in the image quality. To solve this problem, in the present embodiment, an arrangement is provided that can adjust a transfer bias. The transfer bias is lowered or completely turned off. 
   Lubricant  100  is cut-down from the rod by the brush roller  15  little by little. Because the brush roller  15  is in physical contact with the photosensitive drum  5 , the cut-down lubricant  100  spreads on the surface of the photosensitive drum  5 . If the life of the photosensitive unit is assumed to be equivalent to printing of A4 sized, portrait-oriented, 100K papers, then the amount of the lubricant  100  required for smooth operation is 15 grams. 
   The lubricant  100  is applied to the photosensitive drum  5  to such an extent that the coefficient of friction (I) of the photosensitive drum  5  is 0.2 or less. The coefficient of friction is measured using the oiler belt method, in which the photosensitive drum  5  without an image is rotated until a saturated value is obtained. Using T6200 paper for the measurements, the inventor confirmed that the photosensitive drum  5  showed no signs of damage when the coefficient of friction of the photosensitive drum  5  was 0.3 or less. 
     FIG. 3  is a side view of the entire image forming apparatus. This image forming apparatus is a color image forming apparatus of the tandem type. That is, in the main body  1  of the image forming apparatus, two or more image forming units  2 A to  2 D are lined-up along a transfer belt  3 . The image forming units  2 A to  2 D correspond to yellow (Y), black (BK), cyan (C), and magenta (M), respectively. 
   Each image forming unit  2 A to  2 D has the configuration of the photosensitive unit shown in  FIG. 2 . In other words, each image forming unit  2 A to  2 D has a photosensitive drum, a lubricant spreading unit, a cleaning blade, and a developing device. Each developing device can be driven independently. 
   One or more of the image forming units  2 A to  2 D can be selectively operated. For instance, in the monochrome mode, only the image unit  2 A is operated. On the other hand, in the color mode, only the image forming units  2 B to  2 D are operated. 
   However, in the monochrome mode, although only the image unit  2 A is operated, even the photosensitive drums of the other image forming units  2 B to  2 D are rotated and are not physically separated from the transfer belt  3 . Similarly, in the color mode, although only the image forming units  2 B to  2 D are operated, even the photosensitive drum of the image forming units  2 A is rotated and is not physically separated from the transfer belt  3 . 
   Because the lubricant is spread on the surface of the photosensitive drum, wear-out of the photosensitive drum, the cleaning blade, and the transfer belt  3  can be suppressed. Moreover, efficiency improves, because it is not necessary to physically separate the photosensitive drum and the transfer body. 
   Although it is explained above that the development units of the image forming units  2 B to  2 D are operated in the case of the monochrome mode, it is possible to stop operation of these development units to reduce power consumption. Moreover, the charging bias of the image forming units  2 B to  2 D may be lowered or completely turned off so that static electricity does not act on the photosensitive drums of the image forming units  2 B to  2 D. Furthermore, the developing bias of image forming units  2 B to  2 D may be lowered or may be completely turned off. Opposite is the case for the color mode. 
   Still another problem in the typical image forming apparatuses is that, the image formed by the image forming unit  2 A gets reverse-transferred, although in traces, onto the images formed by the image forming units  2 B to  2 D. To overcome this problem, in the present embodiment, an arrangement is provided that can adjust a transfer bias. The transfer bias can be lowered or completely turned off. Moreover, the transfer bias in those image forming units that are not operating can be lowered or completely turned off to reduce power consumption. 
   Same effects can be obtained if the transfer belt is replaced by intermediate transfer belt. 
   Thus, according to the present invention, wear-out of the photosensitive drum, the cleaning blade, and the transfer belt can be suppressed. Moreover, efficiency can be improved because there is now no need to physically separate the photosensitive drum and the transfer body. 
   Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.