Abstract:
An image forming apparatus including a plurality of image forming sections each having an electrostatic latent image bearing member to form a latent image on its surface, a charging device to uniformly charge the electrostatic latent image bearing member, a developing device to develop the electrostatic latent image formed on the electrostatic latent image bearing member into a visible image and a cleaning device to remove developer adhered to the electrostatic latent image bearing member. The image forming sections are disposed in series in close proximity to or contacting a transfer sheet conveying belt spanned rotatively and are configured to differentiate an amount of a lubricant to be supplied to the surface of each electrostatic latent image bearing member such that at least one image forming section is differentiated from other image forming sections.

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, a printer, etc., and more particularly to an image forming apparatus capable of preventing a lifetime of an electrostatic latent image bearing member from being decreased by friction with an appropriate supply of a lubricant to the electrostatic latent image bearing member such as a photoconductive element. 
     2. Discussion of the Background 
     In an electrophotographic image forming apparatus, such as a copying machine, a facsimile, a printer, etc., such an image forming apparatus is well known that includes an image forming section having an electrostatic latent image bearing member to form an electrostatic latent image on its surface, a charging device to uniformly charge the electrostatic latent image bearing member, a developing device to develop the electrostatic latent image formed on the surface of the electrostatic latent image bearing member into a visible image and a cleaning device to remove developer adhered to the electrostatic latent image bearing member. 
     FIG. 9 is a schematic drawing illustrating an exemplary construction of an image forming section  1  in an electrophotographic image forming apparatus in the art. A part having the reference numeral of  10  in FIG. 9 is a photoconductive element, for example an electrostatic latent image bearing member, and a surface of the photoconductive element  10  is uniformly charged by a charging roller  11 . A desired electrostatic latent image is formed on the surface of the charged photoconductive element  10  with an exposure light  12  irradiated from an optical writing device (not shown). The electrostatic latent image is then developed into a visible toner image with toner in a developing device  13 . The toner image formed on the surface of the photoconductive element  10  is transferred onto a transfer sheet carried on a transfer sheet conveying belt  14  with a transfer bias applied by a transfer brush  15 . The transfer sheet having the transferred toner image is conveyed to a fixing device (not shown) where the toner image is fixed onto the transfer sheet by being heated and pressed. A part having the reference numeral of  16  is a cleaning device including a cleaning blade  17 . The cleaning device  16  is provided so as to remove residual toner remaining on the surface of the photoconductive element  10  with the cleaning blade  17 . The removed toner is conveyed to a used toner container (not shown) by a used toner conveying screw  19 . Further, a cleaning brush roller  18 , which is a cleaning support member, to roil and remove toner on the surface of the photoconductive element is disposed at an upstream side of the cleaning blade  17  in the moving direction of the photoconductive element. 
     An image forming apparatus illustrated in FIG. 10 includes four image forming sections. Each of which has the same construction as that illustrated in FIG.  9  and is located in parallel to form images of different colors. The image forming apparatus also includes a transfer sheet conveying belt  14  to convey a transfer sheet to the four image forming sections  1 B,  1 C,  1 M and  1 Y. Reference numerals  12 Y,  12 C,  12 M and  12 B represent exposure light irradiated in the four image forming sections  1 Y,  1 C,  1 M and  1 B respectively. In the back surface side of the transfer sheet conveying belt  14 , transfer brushes  15 B,  15 C,  15 M and  15 Y, which transfer a toner image formed at each image forming section  1 B,  1 C,  1 M and  1 Y, are provided opposing to the respective photoconductive elements  10 B,  10 C,  10 M and  10 Y so as to transfer the toner image formed at each image forming section  1 B,  1 C,  1 M and  1 Y onto a transfer sheet carried on the transfer sheet conveying belt  14  one after another superimposing each color toner image on each other. In an image forming apparatus illustrated in FIG. 10, a desired full color image can be obtained when each color of toner; black (B), cyan (C), magenta (M) and yellow (Y) is used at respective developing devices  13 B,  13 C,  13 M and  13 Y of the image forming sections  1 B,  1 C,  1 M and  1 Y. Such an image forming apparatus as described above is commonly known as a tandem color image forming apparatus. 
     A technology is widely known which suppresses an abrasion of a surface of a photoconductive element caused by a cleaning blade, etc., by supplying the photoconductive element with a lubricant and thereby preventing a lifetime of the photoconductive element from being decreased due to the abrasion, and various systems for supplying a lubricant are proposed. Two systems are known to supply a lubricant. A first type supplies the lubricant by pressing it directly against a surface of a photoconductive element (for example, in Japanese Patent Laid-Open Publication No. 9-62163). A second type supplies the lubricant by abutting a rotatable brush roller on the photoconductive element while abutting the lubricant on the rotatable brush roller (for example, in Japanese Patent Laid-Open Publication No. 6-324603). In the latter system, since a cleaning device to remove residual toner remaining on the surface of the photoconductive element is often configured to include a cleaning brush roller, such a system has been proposed that utilizes the cleaning brush roller as the above-described brush roller. 
     A plurality of image forming sections in the above-described tandem image forming apparatus are configured to have the same structure as to each other. However, a supply of a lubricant to a photoconductive element has to be adjusted to a proper amount in each image forming section. 
     An electrical action for a lubricant in a transfer section will now be discussed. Fluorine-containing resins polytetrafluoroethylene (four fluoridation ethylene resin) and PVDF (polyvinylidene fluoride) used as a lubricant electrically have negative characteristics. In a reverse development that uses negatively charged toner, positive bias is applied to a transfer section. Consequently, a force is exerted that electrically removes the lubricant adhered to a photoconductive element. 
     FIG. 11 explains a change in an amount of a removed lubricant over a lapse of time according to a different voltage level of a transfer bias. In this explanation, a coefficient of friction on a photoconductive element is measured as substitutive characteristics of an amount of a lubricant on a surface of the photoconductive element. It is observed that the higher a voltage of the transfer bias to be supplied, the larger the amount of the lubricant is removed when the apparatus is operated by varying the voltage level of the transfer bias with using a photoconductive element possessing a nearly constant coefficient of friction. 
     In the above-described tandem color image forming apparatus having four image forming sections, a technology is commonly known in which a smooth transfer is achieved by intensifying a transfer bias to be applied to an image forming section in order of image forming section from an upstream to a downstream in a moving direction of a transfer sheet conveying direction. In the configuration that intensifies the transfer bias in order of the image forming section from the upstream to the downstream, a lubricant will be easier to be removed from a surface of a photoconductive element in order of the image forming section from the upstream to the downstream, and an application of a lubricant to increase a lifetime of a photoconductive element will become less effective. 
     A proper amount of a lubricant must be supplied so as not to be inconvenienced by a mixture of a foreign substance due to an accumulated lubricant on a surface of a photoconductive element. It is preferable that the lubricant on the surface of the photoconductive element is removed completely before the next supply of lubricant is made after the lubricant has been supplied so that a fresh lubricant layer is always formed on the surface of the photoconductive element. 
     Further, when a mechanical condition in each image forming section is to be differentiated, it is advantageous from the view point of cost effectiveness to achieve the differentiation with minimum replacement parts while maintaining a maximum commonality of parts. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-discussed and other problems, and addresses the above-discussed and other problems. 
     The present invention advantageously provides a novel image forming apparatus supplying a proper amount of a lubricant to a surface of each photoconductive element to increase a lifetime of the photoconductive element without being inconvenienced by an oversupply of lubricant. 
     According to an embodiment of the present invention, an image forming apparatus includes a plurality of image forming sections having an electrostatic latent image bearing member to form a latent image on its surface, a charging device to uniformly charge the electrostatic latent image bearing member, a developing device to develop the electrostatic latent image formed on the electrostatic latent image bearing member into a visible image and a cleaning device to remove developer adhered to the electrostatic latent image bearing member. The image forming sections are disposed side by side in a line in close proximity to or contacting a transfer sheet conveying belt spanned rotatively. An amount of a lubricant to be supplied to the surface of the each electrostatic latent image bearing member is set such that at least one of the plurality of the image forming sections is differentiated from other of the plurality of the image forming sections. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1A is a schematic drawing illustrating a color image forming apparatus; 
     FIG. 1B is a schematic drawing illustrating one of the image forming sections of the color image forming apparatus in FIG. 1A; 
     FIGS. 2A and 2B are schematic drawings illustrating an exemplary construction of a lubricant applying device provided in each image forming section of the image forming apparatus in FIGS. 1A and 1B; 
     FIG. 3 is a schematic drawing illustrating a structure of one image forming section according to another embodiment of the present invention; 
     FIG. 4 is a schematic perspective view illustrating the image forming section with the lubricant applying device detached; 
     FIG. 5 is a schematic drawing illustrating an example of a shape of a lubricant holder and a lubricant in an image forming section of each color; 
     FIG. 6 is a schematic drawing illustrating a length of a lubricant bitten or contacted by a cleaning brush roller in an image forming section of each color according to another embodiment of the present invention; 
     FIG. 7 is a schematic drawing illustrating an exemplary construction of a color image forming apparatus according to another embodiment of the present invention; 
     FIG. 8 is a schematic perspective view illustrating an image forming section configured to differentiate a rotating speed of a cleaning brush roller; 
     FIG. 9 is a schematic drawing illustrating an exemplary construction of an image forming section of an electrophotographic image forming apparatus in the art; 
     FIG. 10 is a schematic drawing illustrating an exemplary construction of a tandem color image forming apparatus in the art; and 
     FIG. 11 is a graph explaining a change in an amount of a removed lubricant over a lapse of time according to a different voltage level of a transfer bias. A coefficient of friction on a photoconductive element is measured as substitutive characteristics of an amount of a lubricant on a surface of the photoconductive element. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, FIGS. 1A and 1B illustrate an embodiment of the present invention. FIG. 1A is a schematic drawing illustrating a color image forming apparatus in which four image forming sections are arranged side by side in a line. FIG. 1B is a schematic drawing illustrating an exemplary construction of one of the image forming sections in the color image forming apparatus. 
     In the color image forming apparatus illustrated in FIG. 1A, four image forming sections  1 B,  1 C,  1 M and  1 Y are disposed in series and a transfer sheet conveying belt  14  which conveys a transfer sheet to the four image forming sections is arranged. FIG. 1B illustrates a structure of the black image forming section  1 B as one of the image forming sections in the color image forming apparatus. The black image forming section  1 B is identical to other image forming sections of cyan  1 C, magenta  1 M and yellow  1 Y both in structure and performance unless otherwise specified. 
     Referring to FIGS. 1A and 1B, when an image forming operation is started, a surface of a photoconductive element  10 B as an electrostatic latent image bearing member disposed in the black image forming section  1 B is uniformly charged by a charging roller  11 B. A desired electrostatic latent image is formed on the surface of the charged photoconductive element  10 B with an exposing light  12 B irradiated from an optical writing device (not shown). The electrostatic latent image is developed into a visible toner image with black toner (B) in a developing device  13 B. The same image forming operation as in the black image forming section  1 B is performed with a predetermined time lag in the cyan image forming section  1 C, the magenta imaging forming section  1 M and the yellow image forming section  1 Y, and a toner image of each color of cyan (C), magenta (M), yellow (Y) is formed on a surface of respective photoconductive elements  10 C,  10 M and  10 Y In back surface side of the transfer sheet conveying belt  14 , transfer brushes  15 B,  15 C,  15 M and  15 Y are provided opposing the respective photoconductive elements  10 B,  10 C,  10 M and  10 Y of each image forming section so as to transfer the toner image formed on each of the photoconductive elements  10 B,  10 C,  10 M and  10 Y. By applying a predetermined transfer bias to the transfer brushes  15 B,  15 C,  15 M and  15 Y, the toner image of each image forming section  1 B,  1 C,  1 M and  1 Y is transferred onto a transfer sheet carried on the transfer sheet conveying belt  14  one after another being superimposed on each other. The transfer sheet having the transferred toner image is conveyed to a fixing device (not shown) where the toner image is fixed on the transfer sheet by being heated and pressed, and the full color image is obtained. Residual toner remaining on the surface of the photoconductive elements  10 B,  10 C,  10 M and  10 Y after the toner image has been transferred is removed by a cleaning blade ( 17 B) and a cleaning brush roller ( 18 B) of cleaning devices  16 B,  16 C,  16 M and  16 Y The removed toner is conveyed to a used toner container (not shown) by a used toner conveying screw ( 19 B). 
     According to an embodiment of the present invention, lubricant applying devices  30 B,  30 C,  30 M and  30 Y are arranged at each image forming section  1 B,  1 C,  1 M and  1 Y in the image forming apparatus constructed and operating as described above. According to the exemplary construction illustrated in FIGS. 1A and 1B, the lubricant applying devices  30 B,  30 C,  30 M and  30 Y are located between the cleaning devices  16 B,  16 C,  16 M and  16 Y and the charging rollers  11 B,  11 C,  11 M and  11 Y Referring to the black image forming section  1 B, the lubricant applying device  30 B includes a lubricant  31 B, a holder  32 B and a pressurizing spring  33 B as illustrated in FIG.  1 B. The holder  32 B and the lubricant  31 B are pressed with a predetermined pressure against the rotatable photoconductive element  10 B by the pressurizing spring  33 B. A surface of the lubricant  31 B is scraped and the scraped lubricant is applied to the surface of the photoconductive element  10 B to form a layer of the lubricant on the surface of the photoconductive element  10 B. 
     As a lubricant, a solid or a sheet-formed material having lubricating properties may be used. To make a sheet-formed lubricant  34  abut on a photoconductive element, the sheet-formed lubricant  34  may be fixed to a holder  35  by an adhesive double coated tape or the like as illustrated in FIG. 2A, or an elastic element, such as a sponge  37 , may be inserted between the sheet-formed lubricant  34  and a holder  36  to uniformly apply an abuting pressure so that the lubricant uniformly abuts on a photoconductive element, as illustrated in FIG. 2B, thereby preventing an uneven abutment. 
     In an image forming apparatus in which each image forming section  1 B,  1 C,  1 M and  1 Y for black, cyan, magenta and yellow is disposed respectively side by side in a line as illustrated in FIG. 1A, the pressurizing springs  33 B,  33 C,  33 M and  33 Y used to pressurize the lubricants  31 B,  31 C,  31 M and  31 Y of the lubricant applying devices  30 B,  30 C,  30 M and  30 Y provided in each image forming section  1 B,  1 C,  1 M and  1 Y are arranged such that the pressurizing force of the image forming sections decrease in the order of the yellow, magenta, cyan and black such that the pressing force is largest in the image forming section of yellow and is smallest in that of black. Therefore, sufficient amount of the lubricant is supplied even though a force to remove the lubricant applied to each photoconductive element is increased in the order of the location of the image forming section, i.e., in the order of the image forming section located at an upstream to a downstream in the moving direction of the transfer sheet conveying belt  14  such that the force is largest at the downstream and smallest at the upstream. Inconvenience due to an excess supply of a lubricant may not be caused because appropriate amount of supply of a lubricant can be set according to an image forming condition. 
     Another embodiment of the present invention will now be described. FIG. 3 is a schematic drawing illustrating a structure of one image forming section out of four image forming sections of an image forming apparatus constructed similar to that illustrated in FIG.  10 . FIG. 3 illustrates a structure of black image forming section  11 B as an example. The black image forming section  1 B is identical to other image forming sections of cyan, magenta and yellow both in structure and performance unless otherwise specified. According to the image forming section  1 B illustrated in FIG. 3, a lubricant applying device is disposed in the cleaning device  16 B. A lubricant  38 B is provided so as to abut on the cleaning brush roller  18 B. The lubricant (or lubricant applicator)  38 B is scraped by the cleaning brush roller  18 B,and the scraped lubricant adheres to a surface of the cleaning brush roller  18 B. The lubricant adhered to the cleaning brush roller  181 B is transferred to a surface of the photoconductive element  10 B when the cleaning brush roller  188 B abuts on the photoconductive element  10 B, and then a layer of the lubricant is formed on the surface of the photoconductive element  10 B. 
     FIG. 4 is a schematic perspective view illustrating the image forming section in FIG. 3 with the lubricant applying device detached. A holder  39 B holding the lubricant  38 B is constructed separately from a casing  40 B, which includes the photoconductive element  10 B, the charging roller  11 B and the cleaning device  16 B. The holder  39 B is configured to be fixed to the casing  40 B by screws, snap fits or the like. 
     FIG. 5 is a schematic drawing illustrating an example of a shape of a lubricant holder and a lubricant in an image forming section of each color when the image forming section illustrated in FIG. 3 is applied to four image forming sections of a color image forming apparatus. When the image forming sections are disposed in order of that of black, cyan, magenta and yellow from an upstream to a downstream in a moving direction of a transfer sheet conveying belt, the lubricants  38 Y,  38 M,  38 C and  38 B, which are held by holders  39 Y,  39 M,  39 C and  39 B respectively, are configured such that a width of a surface of each lubricant abutting on respective brush roller is decreased in order of the lubricant of yellow, magenta, cyan and black image forming sections such that the width is largest in the image forming section of yellow and is smallest in that of black. Therefore, an amount of supply of a lubricant to a photoconductive element can be increased in order of the image forming section from the upstream to the downstream in the moving direction of the transfer sheet conveying belt such that the amount of supply of a lubricant is largest downstream and smallest upstream. 
     FIG. 6 is a schematic drawing, according to another embodiment of the present invention, illustrating an amount of a lubricant bitten by a cleaning brush roller in an image forming section of each color when the image forming section illustrated in FIG. 3 is applied to four image forming sections of a color image forming apparatus. The term “bitten” is used to refer to the state or degree to which the lubricant contacts or extends within the cleaning brush roller. When the image forming section is disposed in the order of that of black, cyan, magenta and yellow from an upstream to a downstream in a moving direction of a transfer sheet conveying belt, a length of the lubricants  38 Y,  38 M,  38 C and  38 B bitten by the respective cleaning brush rollers  18 Y,  18 M,  18 C and  18 B is decreased in the order of the image forming section of yellow, magenta, cyan and black such that the length is largest in the image forming section of yellow and is smallest in that of black. By setting a length of a lubricant bitten by a cleaning brush roller, an amount of supply of a lubricant to a photoconductive element can also be increased in order of the image forming section from the upstream to the downstream in the moving direction of the transfer sheet conveying belt such that the amount of supply of lubricant is largest in the image forming section of the downstream and smallest in that of the upstream. 
     FIG. 7 is a schematic drawing illustrating a structure of a color image forming apparatus according to another embodiment of the present invention. Each image forming section  1 B,  1 C,  1 M and  1 Y is constructed similar to those in FIG. 3. A lubricant applying device is provided in each cleaning device  16 B,  16 C,  16 M and  16 Y. The lubricants  38 B,  38 C,  38 M and  38 Y are disposed to abut on the cleaning rollers  18 B,  18 C,  18 M and  18 Y respectively. Each image forming section  1 B,  1 C,  1 M and  1 Y is constructed almost similar to those in FIG. 3. A lubricant applying device is provided in each cleaning device  16 B,  16 C,  16 M and  16 Y and the lubricants  38 B,  38 C,  38 M and  38 Y are disposed so as to abut on the cleaning brush rollers  18 B,  18 C,  18 M and  18 Y respectively. 
     According to the embodiment of the present invention, conditions (a width of a lubricant abuts on a cleaning brush roller and a length of a lubricant bitten by a cleaning brush roller) of abutment of the lubricants  38 B,  38 C,  38 M and  38 Y on the cleaning brush rollers  18 B,  18 C,  18 M and  18 Y are set equal in every image forming section. The cleaning brush rollers  18 B,  18 C,  18 M and  18 Y is rotated by a driving source in a body of the image forming apparatus (not shown) and are configured to rotate at a speed such that the speed decreases in order of the brush roller of yellow, magenta, cyan and black such that the brush roller of yellow rotates at the highest speed and that of black rotates at the lowest speed. Therefore, an amount of supply of a lubricant to a photoconductive element can be increased in order of the image forming section from the upstream to the downstream in the moving direction of the transfer belt  14  such that the amount of supply of a lubricant is the largest at the downstream and is smallest at the upstream because the cleaning brush roller of the image forming section which is disposed in the downstream rotates at a higher speed. 
     FIG. 8 is a schematic perspective view illustrating an exemplary construction of an image forming section that differentiates a rotating speed of a cleaning brush roller of each image forming section in the color image forming apparatus in FIG.  7 . According to the embodiment of the present invention, transfer devices  50 B ( 50 C,  50 M and  50 Y) are separately constructed that include a plurality of gears  51 B ( 51 C,  51 M and  51 Y) and  52 B ( 52 C,  52 M and  52 Y) to transfer a driving force received from a driving source in a body of an image forming apparatus (not shown) to cleaning brush rollers of the cleaning devices  16 B ( 16 C,  16 M and  16 Y) in the image forming sections  1 B ( 1 C,  1 M and  1 Y). The transfer devices  50 B ( 50 C,  50 M and  50 Y) are provided to the image forming sections  1 B ( 1 C, 1 M and  1 Y). The driving force transferred from drive intake gear  51 B ( 51 C,  51 M and  51 Y) is transferred via the driving gears  52 B ( 52 C,  52 M and  52 Y) to brush roller gears  53 B ( 53 C,  53 M and  53 Y) that are provided integrally with the cleaning brush rollers in the cleaning devices  16 B ( 16 C,  16 M and  16 Y). 
     When the construction illustrated in FIG. 8 is applied to each image forming section illustrated in FIG. 7, each transfer device  50 Y,  50 M,  50 C and SOB is configured such that the rotating speed of the cleaning brush rollers  18 Y,  18 M,  18 C and  18 B decreases in the order of these of yellow, magenta, cyan and black such that the brush roller of yellow rotates at the highest speed and that of black rotates at the lowest speed. When a driving force is transferred to the cleaning brush rollers  18 B,  18 C,  18 M and  18 Y of the cleaning devices  16 B,  16 C,  16 M and  16 Y in each of the image forming sections  1 B,  1 C,  1 M and  1 Y, a driving force from a driving source in a body of the apparatus can be transferred to each of the transfer devices  50 B,  50 C,  50 M and  50 Y at the same speed. Therefore, a construction of a driving device in the body of the apparatus does not need to be modified for each image forming section. A required rotating speed may be obtained when a driving force is transferred to each transfer devices  50 B,  50 C,  50 M and  50 Y from the developing devices  13 B,  13 C,  13 M and  13 Y of the respective image forming sections, which reduces the number of transferring points for transferring the driving force from the body of the image forming apparatus to the image forming sections  1 B,  1 C,  1 M and  1 Y 
     Obviously, numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein. 
     This document claims priority and contains subject matter related to Japanese Patent Application No. 11-204512, filed on Jul. 19, 1999, and the entire contents thereof are herein incorporated by reference.