Abstract:
An image forming apparatus includes a plurality of image carriers and a transfer assembly. The transfer assembly includes a plurality of transferees, a supporter, and a plurality of transferee holders. The plurality of transferers is each configured to electrostatically transfer a toner image carried on one of the image carriers onto an intermediate transfer member or a recording medium. The supporter is configured to support the transferers. The transferer holders are each configured to hold one of the transferers. Each of the transferee holders and the transferer held thereby form an integrated individual unit independently attachable to and detachable from the supporter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent specification claims priority from Japanese Patent Application Nos. JP2006-314800, filed on Nov. 21, 2006, and JP2007-177707, filed on Jul. 5, 2007 in the Japan Patent Office, the entire contents of each of which are hereby incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a transfer assembly used in an electronographic tandem image forming apparatus including a laser printer, an LED (light-emitting diode) printer, a facsimile machine, etc., and an electronographic tandem image forming apparatus including the transfer assembly. 
     2. Discussion of the Background 
     In recent years, tandem image forming apparatuses have attracted attention as color image forming apparatuses because there is a need to output full color images as fast as a speed with which monochrome images are output. 
     Tandem image forming apparatuses include a plurality of photoreceptors and a transfer assembly including a plurality of transferers and an intermediate transfer belt or a transfer and transport belt. A different color image is formed on each of the photoreceptors. Two method are used to transfer images on the photoreceptors, a direct transfer belt method and an intermediate transfer belt method. 
     In the direct transfer belt method, the images are transferred sequentially by transferers and superimposed one on another directly onto a recording medium that is transported by a transfer and transport belt. In the intermediate transfer method, the images are transferred sequentially by transferees and superimposed one on another on an intermediate transfer belt, and then the superimposed image is transferred by a secondary transferee onto the recording medium. In both methods, the transferees in the transfer assembly are facing one of the photoreceptors via the transfer and transport belt or the intermediate transfer belt. 
     Although transferees typically include a conductive rubber or a conductive sponge material, which deteriorates over time, compatibility thereof is rarely considered because useful life thereof is longer than that of the intermediate transfer belt or the transfer and transport belt included in the transfer assembly together with the transferees. 
     Further, when a transferer including a conductive rubber or a conductive sponge material is touched by hand during maintenance work, the transferer may be stained, deformed, or damaged. Although an axis of the transferee can be touched, the axis is typically unexposed. 
     Image forming apparatuses typically manage the useful life of the transfer assembly based on usage history thereof, such as the number of sheets printed, and do not manage the useful life of the transferees included therein individually. 
     In a related-art image forming apparatus, transfer rollers are held in sub-frames in a transfer belt assembly as shown in  FIG. 1 . In  FIG. 1 , a transfer belt assembly  300  includes a transfer belt  301  stretched around a driving roller  310  to rotate the transfer belt  301 , a tension roller  320  to apply tension to the transfer belt  301 , and driven rollers  330  and  340 . The transfer belt assembly  300  further includes a black sub-frame  350 , in which a transfer roller  370  for black is attached, and a full-color sub-frame  360 , in which three transfer rollers  370  are attached. In both the black sub-frame  350  and the full-color sub-frame  360 , each transfer roller  370  is located at a position to contact a photoreceptor drum, not shown, via the transfer belt  301 . Each transfer roller  370  is provided with a bearing  380  and pressed against the transfer belt  301  by a spring  390 . 
     The black sub-frame  350  and the full-color sub-frame  360  are for simplifying engagement and disengagement of the transfer rollers  370  with/from the transfer belt  300 . More specifically, in monochrome printing, the three transfer rollers  370  held in the full-color sub-frame  360  are disengaged from the transfer belt  301  altogether by an action of the full-color sub-frame  360  because these transfer rollers  370  are not used. In color printing, all three transfer rollers  370  engage the transfer belt  301  by an action of the full-color sub-frame  360 . 
     However, in the configuration described above, replacing individual transfer roller  370  is not easy. For example, when one of the three transfer rollers  370  held in the full-color sub-frame  360  is to be replaced at the end of its useful life, it is necessary to remove the bearing  380  and the spring  390 . To save trouble removing the bearing  380  and the spring  390 , the full-color sub-frame  360  should be replaced with a new one, thus wasting the other two transfer rollers  370  that are still useful. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, in one illustrative embodiment of the present invention, a transfer assembly is included in an image forming apparatus including a plurality of image carriers each configured to carry a toner image. The transfer assembly includes a plurality of transferers, a supporter, and a plurality of transferer holders. The plurality of transferers is each configured to electrostatically transfer the toner image on one of the image carriers onto one of an intermediate transfer member and a recording medium. The supporter is configured to support the plurality of transferers. The plurality of transferer holders are each configured to hold one of the plurality of transferees. Each of the plurality of transferer holders and the transferer held thereby form an integrated individual unit independently attachable to and detachable from the supporter. 
     In another illustrative embodiment of the present invention, an image forming apparatus includes a plurality of image carriers each configured to carry a toner image thereon and the transfer assembly described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic illustration of an example of a related art transfer belt assembly; 
         FIG. 2  is a schematic illustration of a tandem image forming apparatus according to an example embodiment of the present invention; 
         FIG. 3  is a schematic illustration of a transfer belt assembly; 
         FIG. 4  shows an integrated individual roller unit for the image forming apparatus shown in  FIG. 2 ; 
         FIG. 5  illustrates a state in which the individual roller unit shown in  FIG. 4  is attached to a sub-frame; 
         FIG. 6  illustrates the individual roller unit shown  FIG. 4  detached from the sub-frame; 
         FIG. 7  is a schematic illustration of an individual roller unit for one of yellow, cyan, and magenta for the image forming apparatus shown in  FIG. 2 ; 
         FIG. 8  is a schematic illustration of a full color sub-frame in which the individual roller unit shown in  FIG. 7  is attached; 
         FIG. 9  is an overhead view illustrating an inner configuration of the individual roller unit shown in  FIG. 7 ; 
         FIG. 10  illustrates the individual roller unit of  FIG. 7  detached from a sub-frame; 
         FIG. 11  is a schematic illustration of an individual roller unit for black for the image forming apparatus shown in  FIG. 2 ; 
         FIG. 12  is a schematic illustration of an individual roller unit for black according to another embodiment of the present invention; 
         FIG. 13  is a schematic illustration of an individual roller unit for black according to another embodiment of the present invention; 
         FIG. 14  is a flowchart of degradation level determination performed by a controller of the image forming apparatus according to an example embodiment; and 
         FIG. 15  is a schematic illustration of a tandem image forming apparatus according to another example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views thereof, and particularly to  FIG. 2 , a tandem image forming apparatus A according to an example embodiment of the present invention is described. 
     Referring to  FIG. 2 , the image forming apparatus A includes image forming units  2 B,  2 Y,  2 C, and  2 M having photoreceptor drums  1 B,  1 Y,  1 C, and  1 M, respectively. The image forming apparatus A further includes an intermediate transfer belt  3  located to contact the photoreceptor drums  1 B,  1 Y,  1 C, and  1 M, stretched around tension rollers  6  through  9 , and a transfer belt assembly  30  that is a transferee and moves the intermediate transfer belt  3  counterclockwise, that is, in a direction shown by arrow Y, in  FIG. 1 . The image forming units  2 B,  2 Y,  2 C, and  2 M are located sequentially from downstream in a moving direction of the intermediate transfer belt  3  (belt rotation direction), according to an order in which primary transfer is performed. 
     Suffixes B, Y, C, and M attached to reference numerals indicate that components indicated by those reference numerals are for forming black, yellow, cyan, and magenta toner images, respectively. 
     At positions facing the photoreceptor drums  1 B,  1 Y,  1 C, and M 1  via the intermediate transfer belt  3 , primary transfer rollers  4 B,  4 Y,  4 C, and  4 M are provided. A power source, not shown, applies a primary transfer bias to each of the primary transfer rollers  4 B,  4 Y,  4 C, and  4 M. The image forming apparatus A further includes a cleaner  5  facing the tension roller  7  via the intermediate transfer belt  3 , and a secondary transfer roller  15  facing the tension roller  6  via the intermediate transfer belt  3 . 
     The image forming apparatus A further includes a fixer  16  located downstream from a place where the tension roller  6  and the secondary transfer roller  15  contact each other via the intermediate transfer belt  3  in a sheet transport path, and a pair of discharge rollers  17  located downstream of the fixer  16 . 
     On the photoreceptor drums  1 B,  1 Y,  1 C, and  1 M, electrostatic latent images are formed in charge and exposure processes. The electrostatic latent images are developed with black, yellow, cyan, and magenta toners, respectively, and then transferred and superimposed one on another on the intermediate transfer belt  3  by the primary transfer rollers  4 B,  4 Y,  4 C, and  4 M, respectively. Thus, a superimposed toner image (full color image) is formed in a primary transfer process. 
     The image forming apparatus A further includes a sheet feeder  10 , located at a bottom portion thereof, and a controller  56 . The sheet feeder  10  includes a sheet cassette  11 , from witch a sheet of recording media is fed by a feed roller  12 , and transported through the sheet transport path by a pair of transport rollers  13  to a pair of registration rollers  14 . The controller  56  includes a CPU (central processing unit), a RAM (random access memory), a ROM (read only memory), etc. 
     The registration rollers  14  stop the sheet, and then forward the sheet to a secondary transfer nip, which is a place where the intermediate transfer belt  3  and the secondary transfer roller  15  contact each other, such a way that the sheet overlaps the superimposed toner image. The superimposed toner image is then transferred from the intermediate transfer belt  3  onto the sheet at the secondary transfer nip in a secondary transfer process. 
     After the fixer  16  thermally fixes the full color image on the sheet, the discharge rollers  17  discharge the sheet from the image forming apparatus A. 
     As described above, the image forming apparatus A shown in  FIG. 2  adopts an intermediate transfer belt method, and the transfer belt assembly  30  transfers an image on the intermediate transfer belt  3 , which is an intermediate transfer member, secondarily onto a sheet by the transfer belt assembly  30 . 
       FIG. 3  illustrates the transfer belt assembly  30  viewed from a side opposite the the side illustrated in  FIG. 2 .  FIG. 4  illustrates an individual roller unit  40  to be attached to the transfer belt assembly  30 .  FIG. 5  illustrates the transfer belt assembly  30  in which the individual roller units  40  are attached from a front and a side. Because the four individual roller units  40  have a similar configuration, the letters B, Y, C, and M to be added to the reference numerals thereof are omitted. Similarly, the letters B, Y, C, M in the reference numerals of the photoreceptor drum  1 B,  1 Y,  1 C, and  1 M and the primary transfer rollers  4 B,  4 Y,  4 C, and  4 M are omitted when distinction is unnecessary. 
     Referring to  FIGS. 3 through 5 , an example of a configuration of the transfer belt assembly  30  is described below. 
     As shown in  FIG. 3 , the transfer belt assembly  30  includes a sub-frame  35  for black and sub-frame  36  for yellow, cyan, and magenta (full color). The sub-frame  35  holds an individual roller unit  40  for black. The sub-frame  36  holds three individual roller units  40  for yellow, cyan, and magenta. More specifically, the sub-frame  35  includes a pair of side plates, facing each other, on each of which a slot  351  extending in a vertical direction in  FIG. 3  is provided, and the individual roller unit  40  for black is located between the side plate. Similarly, the sub-frame  36  includes a pair of side plates, facing each other, on each of which three slots  361  extending in the vertical direction are provided horizontally, and the individual roller units  40  for yellow, cyan, and magenta are located between the side plates. 
     The sub-frames  35  and  36  are further described below. 
     As shown in  FIG. 3 , the image forming apparatus A further includes a usage history manager  59  in addition to the controller  56  described above that serves as a usage life manager configured to manage useful life of the individual roller units  40 . The usage history manager  59  includes a newness indicator  57  and a history recorder  58 . The newness indicator  57  is provided for each individual roller unit  40 . When an integrated individual roller unit  40  is unused, the newness indicator  57  thereof indicates being unused (an unused state) to the controller  56 . The history recorder  58  records a usage history of each individual roller unit  40 . 
     Referring to  FIG. 4 , the individual roller unit  40  is described below. 
     The individual roller unit  40  includes a frame  41  as a holder to hold the primary transfer roller  4 , a bearing  43  configured to support a roller axis  42  of the primary transfer roller  4 , and a screw hole  51 . Reference numeral D 1 , indicates a distance between a center of the screw hole  51  and a center of the roller axis  42 . 
     Referring to  FIG. 5 , attachment of the transfer roller  40  is described below. Although  FIG. 5  illustrates one of the individual transfer rollers  40  for full color being attached to the sub-frame  36 , the individual roller unit  40  for black is attached to the sub-frame  35  in a similar manner. 
     As illustrated in  FIG. 5 , each individual roller unit  40  is attached to the sub-frame  36  of the transfer belt assembly  30  shown in  FIG. 3  with a step screw  48  and a plate  47 . The plate  47  includes a pin  47 A protruding from a back surface thereof, and a hole which the step screw  48  spinably engages. 
     The pin  47 A on the back surface of the plate  47  penetrates the slot  361  from outside the sub-frame  36 , with a tip thereof engaging the bearing  43  from a side opposite the side of the roller axis  42 . The step screw  48  penetrating the plate  47  further penetrates the slot  361  from outside, with a tip thereof engaging the screw hole  51  on the frame  41  of the individual roller unit  40 . With the engagement described above, the sub-frame  36  supports the individual roller units  40  for yellow, cyan, and magenta. 
     As illustrated in  FIG. 5 , springs  46  are provided on both side plates of the sub-frame  36 . An end of each spring  46  is fixed at the sub-frame  36 , and the other (unfixed) end thereof penetrates a hole provided on a top plate of the frame  41  and presses the bearing  43  against the intermediate belt  3  and the photoreceptor drum  1 . Therefore, the springs  46  press whole the individual roller unit  40  to the intermediate belt  3  and the photoreceptor drum  1 , contacting the primary transfer roller  4  with the photoreceptor drum  1  via the intermediate transfer belt  3 . 
     The bearing  43  is electrically conductive, and the power source, not shown, applies a transfer bias to the primary transfer roller  4  through the springs  46 . 
     According to the present embodiment, the primary transfer roller  4  and components to hold the primary transfer roller  4 , such as the frame  41  and the bearing  43 , form each integrated individual roller unit  40  attachable to and detachable from the transfer belt assembly  30 . Further, each individual roller unit  40  is independently attachable to and detachable from the transfer belt assembly  30 . Therefore, when the useful life of one of the primary transfer rollers  4  ends, only that primary transfer roller  4  need to be replaced and can be done so easily, without removing and reinstalling the bearings  43 , the springs  46 , etc. 
     Further, because the primary transfer roller  4  is not attached to and detached from the transfer belt assembly  30  singularly, a user need not hold the small roller axis  42  when detaching the primary transfer roller  4  from the transfer belt assembly  30  so as not to soil his/her hands. Further, protecting the primary transfer roller  4  with the frame  41  can prevent or reduce stains on and/or damage to the primary transfer roller  4  that are caused by hand contact or by contact with other components during replacement. 
     Further, cost can be reduced by integrating a replacement part and related components in a unit having a similar or identical configuration applicable to different types of image forming apparatuses. For example, in tandem type image forming apparatuses, the configuration of a unit including transfer members may differ according to type, although each transfer member and a configuration around the transfer member are similar or the same. 
     For example, assume that there are image forming apparatus models B 1  and B 2 . The model B 1  uses a transfer belt assembly similar to the transfer belt assembly  30  illustrated in  FIG. 3 , except that a primary transfer roller thereof is not attachable to or detachable from the transfer belt assembly as an integrated individual roller unit. The model B 2  uses a transfer belt assembly similar to the transfer belt assembly included in the model B 1 , except that a pitch between primary transfer rollers thereof in the belt rotation direction is different from that of the model B 1 . 
     In these cases, when one of the primary transfer rollers for yellow, cyan, and magenta is to be replaced in each of the models B 1  and B 2 , it is necessary to replace a whole full color unit including these three primary transfer rollers together with a sub-frame thereof. Moreover, because the pitch between the primary transfer rollers is different in the models B 1  and B 2 , the models B 1  and B 2  cannot use the same type of full color unit. 
     By contrast, the image forming apparatus A according to the present embodiment uses integrated individual roller units  40 , in each of which one primary transfer roller  4  and related components are configured as a unit. Therefore, the individual roller unit  40  can be used in common with another type of image forming apparatus, regardless of the pitch between the primary transfer rollers  4 . Therefore, cost can be reduced by integrating a replacement part with related components as a common unit usable in different image forming apparatus types. 
     As described above, in the transfer belt assembly  30  shown in  FIG. 3 , one end of each spring  46  is attached to one of the sub-frames  35  and  36 , and the unfixed end thereof penetrates the hole in the top plate of the frame  41  of the individual roller unit  40 . Alternatively, one end of the spring  46  may be fixed on the frame  41 , and the other end thereof may be set at a spring set position provided on the sub-frames  35  and  36 . 
     In the present embodiment, the individual roller units  40  for black, yellow, cyan, and magenta have a similar configuration. Therefore, it is not necessary for a user to stock four individual roller units  40 , one each for black, yellow, cyan, and magenta. As long as the user has one individual roller unit  40  in stock, when the useful life of one of four individual roller units  40  ends, the user can replace the individual roller unit  40  with the stock, regardless of color. 
     Because the individual roller units  40  for black, yellow, cyan, and magenta are similar to each other, the frames  41  of the individual roller units  40  also have an attachment part whose disposition pattern is similar to each other. The attachment part is a part which fasteners to fix the individual roller unit  40  to one of the sub-frame  35  and  36  engage. Although in the present embodiment the fasteners are the step screw  48  and the pin  47 A on the plate  47 , the fasteners may be any items that detachably attach the individual roller unit  40  to one of the sub-frame  35  and  36 . The attachment part includes the screw hole  51  and the bearing  43 , which the step screw  48  and the pin  47 A on the frame  47  engage. 
     The attachment part, the screw hole  51  and the bearing  43  are provided at each of the side plates of the frame  41  facing each other. As shown in  FIG. 4 , the screw hole  51  and the bearing  43 , which is located under the screw hole  51  in  FIG. 4 , are located at the distance D 1 , on each side plate of the flame  41 . The screw holes  51  on both side plates have a similar diameter and are aligned horizontally. Similarly, the bearings  43  on both side plates have a similar diameter and are aligned horizontally. That is, the disposition pattern of the attachment part is the screw holes  51  and the bearings  43  on both side plates of the frame  41  that are aligned horizontally, and the screw hole  51  and the bearing  43  on each side plate thereof are located at the distance D 1 . 
     As described above, the four individual roller units  40  have a similar arrangement pattern, and positions to attach the fasteners, the step screw  48  and the pin  47 A on the plate  47 , are the same or similar in the four individual roller units  40 . Therefore, because the four individual roller unit  40  can be attached to or detached from the transfer belt assembly  30  in a similar procedure, replacement thereof can be simplified. 
     Referring to  FIGS. 7  though  9 , an individual roller unit  40 A according to another embodiment of the present invention is described below. It is to be noted that, unless otherwise described, the individual roller unit  40 A is used in the image forming apparatus A shown in  FIG. 2  and image forming apparatuses similar thereto. 
     The individual roller unit  40 A is for one of yellow, cyan, and magenta, and holds a primary transfer roller  4 , which faces one of photoreceptor drums  1 Y,  1 C, and  1 M shown in  FIG. 2  via an intermediate transfer belt  3 , therein. As shown in  FIG. 7 , the individual roller unit  40 A includes a frame  41 A, a pair of bearings  43 A to hold a roller axis of the primary transfer roller  4 , a pair of spring  46 , two pairs of guide rails  49 A and  49 B, and screw holes  51 . The frame  41 A is a transferee holder and holds the primary transfer roller  4 , the bearings  43 A, the springs  46 , etc. A transfer bias is applied to the primary transfer roller  4  through the bearings  43 A and the springs  46 . 
       FIG. 8  illustrates a state in which the individual roller unit  40 A is attached to the sub-frame  36 A for full color. The individual roller unit  40 A is fixed between a pair of side plates of the sub-frame  36 A by screws  50 , each of which penetrates the side plate of the sub-frame  36 A from outside with an end thereof engaging the screw hole  51  provided in the frame  41 A. 
     As illustrated in  FIG. 8 , the bearings  43 A are provided at both end portions of the individual roller unit  40 A in a width direction of the intermediate transfer belt  3 . At each end portion, each bearing  43 A, guided by one guide rail  49 A and one guide rail  49 B, although the guide rail  49 B is omitted in  FIG. 8 , can move toward and away from the intermediate transfer belt  3 , in a vertical direction in  FIG. 8 , while holding the roller axis  42  of the primary transfer roller  4 . In the individual roller unit  40 A, each spring  46  biases only the bearing  43 A and the primary transfer roller  4  with one end thereof fixed inside the frame  41 A. This arrangement is different from that of the individual roller unit  40  shown in  FIG. 4  in which each spring  46  biases whole the individual roller unit  40  with one end thereof fixed on the sub-frame  36 . 
       FIG. 9  is an overhead view of the individual roller unit  40 A. As described above, each bearing  43 A engages the guide rails  49 A and  49 B so as to be able to slide vertically. In the individual roller unit  40 A, two guide rails  49 A are provided in one end portion and two guide rails  49 B are provided in the other end portion in the belt width direction. The guide rails  49 A and  49 B extend in a vertical direction, that is, a direction perpendicular to the surface of the paper on which  FIG. 9  is drawn. One guide rail  49 A and one guide rail  49 B to face each other at a given distance. 
     There are several types of image forming apparatuses including a transfer belt assembly whose configuration is similar. However, a different transfer belt assembly is necessary for each type of image forming apparatus, if specification, usage condition, etc., of a transferer included in the transfer belt assembly are different. 
     By contrast, in the present embodiment of the present invention, a unit including a replacement part can be used in different image forming apparatus types, and thus cost can be reduced. Further, providing different integrated individual roller unit types, for example, a high quality item and a low cost item, means providing different transfer belt assembly types, and thus product line can be expanded and needs of individual customers can be satisfied. 
       FIG. 10  illustrates a state in which the individual roller unit  40 A is detached from the sub-frame  36 A. As described above, the end of each spring  46  is fixed inside the frame  41 A, and thus the spring  46  biases the bearing  43 A engaging the roller axis  42  and presses the primary transfer roller  4 , even when the individual roller unit  40 A is not attached to the sub-frame  36 A. 
     In the configuration described above, because adjusting a position of the spring  46  between the sub-frame  36 A and the frame  41 A, putting the spring  46  into a hole provided on the frame  41 A, etc., are unnecessary, replacement can be simplified. However, there may be a case in which a bias to a primary transfer roller (tension of a spring) differs depending on transfer belt unit type. In such a case, the same integrated individual roller unit can not be used in different transfer belt unit types. By contrast, because the spring  46  is fixed on the sub-frame  36  in the individual transfer unit  40  shown in  FIGS. 4 and 5 , the same configuration for the integrated individual roller unit  40  can be used in different transfer belt unit types. 
       FIG. 11  is a schematic illustration of an individual roller unit  40 B for black according to the present embodiment adopting an indirect bias application method. A primary transfer roller  4 B is located so as not to face a photoreceptor drum  1 B vertically via the intermediate transfer belt  3 , and a rotation axis of the primary transfer roller  4 B is fixed, that is, a bearing fitted on the rotation axis does not slide. The individual roller unit  40 B includes a frame  41 B on which screw holes  51  are provided, and is attached to a sub-frame for black by screws engaging the screw holes  51 . 
     In the individual roller unit  40 A for full color shown in  FIGS. 7 and 8 , the primary transfer roller  4  presses the intermediate transfer belt  3  from a back surface thereof against the photoreceptor drum  1  to form a transfer nip, which is a contact area between the intermediate transfer belt  3  and the photoreceptor drum  1 . The intermediate transfer belt  3  follows a track away from both the primary transfer roller  4 B and the photoreceptor drum  1  immediately after passing the transfer nip. 
     By contrast, in the individual roller unit  40 B shown in  FIG. 11 , the intermediate transfer belt  3  follows a track to wind around a circumference of the photoreceptor drum  1 B, after passing a transfer nip and leaving the primary transfer roller  4 B. In this configuration, because the primary transfer roller  4 B is not pressed against the photoreceptor drum  1 B in a normal line direction thereof a spring and a mechanism to move the bearing, which are for adjusting pressure against the intermediate transfer belt  3 , are unnecessary, thus reducing the cost of the individual roller unit  40 B. 
     In the present embodiment, as shown in  FIGS. 7 and 11 , the individual roller unit  40 A for full color and the individual roller unit  40 B for black have different configurations. That is, a common integrated individual roller unit is not used as both the individual roller unit  40 A for full color and the individual roller unit  40 B. 
     When individual roller units for full color and black have different configurations, disposition patterns of attachment parts thereof are typically different and thus prevent installation of the wrong individual roller unit. However, if the disposition patterns of attachment parts are different, then attachment and detachment procedures are also different, and can be hard to remember. 
     By contrast, in the present embodiment, the individual roller unit  40 A for full color and the individual roller unit  40 B for black have attachment parts whose disposition patterns are the same or substantially similar to each other. The disposition pattern has following features: The screw holes  51  on both side plates of both the frames  41 A and  41 B, which are aligned horizontally, have the same or substantially the same diameter. The screw holes  51  on both side plates are aligned horizontally. On each side plate of both the frames  41 A and  41 B, the two screw holes  51  are located horizontally at a distance D 1 . 
     The attachment parts of the individual roller unit  40 A for full color and the individual roller unit  40 B for black have the same or substantially the same disposition pattern as well, and thus prevent installation of the wrong individual roller unit. 
     In the present embodiment, a concavity and a convexity that engage each other are provided on the sub-frame  36 A for full color and the frame  41 A of the individual roller unit  40 A for full color as identification of full color. Similarly, a concavity and a convexity that engage each other are provided on the sub-frame for black and the frame  41 B of the individual roller unit  40 B for black as identification of black. The individual roller unit  40 B for black can not be attached to the sub-frame  36 A for full color, because the concavity and the convexity provided on the individual roller unit  40 B and the sub-frame  36 A for full color do not match. Similarly, the individual roller unit  40 A for full color can not be attached to the sub-frame for black, because the concavity and the convexity on the individual roller unit  40 A and the sub-frame for black do not match. 
     Alternatively, attachment of the wrong individual roller unit may be prevented in a control method. 
       FIG. 12  is a schematic illustration of an individual roller unit  40 B 1  for black according to another embodiment of the present invention. In the individual roller unit  40 B 1 , a configuration around a primary transfer roller  4  is similar to that of the individual roller unit  40 A shown in  FIG. 7 . However, the individual roller unit  40 B 1  further includes a roller  52  as an additional member, provided upstream of the primary transfer roller  4 B in the belt rotation direction. The roller  52  enhances winding of the intermediate transfer belt  3  around the photoreceptor drum  1 B, increasing a contact area (a transfer nip) between the photoreceptor drum  1 B and the intermediate transfer belt  3 . 
     The individual roller unit  40 B 1  is attachable to the sub-frame  36 A for full color shown in  FIG. 8  by screws engaging screw holes  51  on a frame  41 C. 
     Although in a case of the individual roller unit  40 B 1 , the roller  52  is used as a nip former, alternatively, another type of member to contact and press the intermediate transfer belt  3 , such as a conductive film (e.g., Mylar™), may be used as the nip former. Further, the roller  52  may be located downstream of the primary transfer roller  4 B. With the roller  52 , a reliable transfer nip can be formed, and thus high quality images can be attained. 
       FIG. 13  is a schematic illustration of an individual roller unit  40 B 2  for black according to another embodiment of the present invention. The individual roller unit  40 B 2  includes a frame  41 D in which screw holes  51  are provided, a link  53  to hold a primary transfer roller  4 B, a tension spring  54  to press the primary transfer roller  4 B against a photoreceptor drum  1 B, and a conductive film  55  as a discharger. The conductive film  55  is provided downstream of the primary transfer roller  4 B in the belt rotation direction shown by an arrow Y and with an end thereof fixed in the individual roller unit  40 B 2 . The other end of the conductive film  55  is not fixed and pressed against the intermediate transfer belt  3  from an inner surface (back surface) thereof. The individual roller unit  40 B 2  is attachable to a sub-frame for black by screws engaging the screw holes  51 . 
     Referring to  FIG. 13 , in an area where a gap between the intermediate transfer belt  3  and the photoreceptor drum  1 B is small, a discharge phenomenon may occur due to potential differences therebetween, which causes image failure, for example, partly absent toner, scattering of letters, etc., on an output image. For example, a discharge phenomenon called separating discharge occurs in a small gap between the intermediate transfer belt  3  and the photoreceptor drum  1 B formed immediately downstream of an exit portion of the transfer nip, where the intermediate transfer belt  3  starts to leave the photoreceptor drum  1 B, in the belt rotation direction. 
     Therefore, in the individual roller unit  40 B 2 , the conductive film  55  discharges the intermediate transfer belt  3  in the small gap area, with the unfixed end thereof being in contact with the back surface of the intermediate transfer belt  3  and the fixed end thereof grounded. Thus, an occurrence of separation discharge can be prevented or reduced by preventing a transfer current from flowing to the small gap area. 
     It is to be noted that in an area immediately upstream from an entry portion of the transfer nip in the belt rotation direction, where the intermediate transfer belt  3  and the photoreceptor drum  1 B start to contact each other, another small gap is formed (upstream gap) between the intermediate transfer belt  3  and the photoreceptor drum  1 B. Accordingly, because a discharge phenomenon may also occur in the upstream gap area as well, another discharger may be provided therein. 
     Although the conductive film  55  is used as the discharger in the individual roller unit  40 B 2 , the discharger is not limited thereto. Therefore, another type of discharger, such as a roller, may be provided in the individual roller unit  40 B 2 . Further, the individual roller unit  40 B 2  may include a member to apply a bias to the intermediate transfer belt  3  depending on transfer conditions, not only to discharge the intermediate transfer belt  3 . With the conductive film pressed against the back surface of the intermediate transfer belt  3 , scattering of toner, honeycomb-shaped toner absence, etc., which occur after transferring, can be prevent or reduced, and thus high quality images can be attained. 
     As described above, the transfer belt assembly  30 , shown in  FIG. 3 , according to an example embodiment of the present invention includes the individual roller unit  40  shown  FIGS. 4 and 5 . The individual roller unit  40  includes the primary transfer roller  4  and the frame  41  that is the transferer holder. The primary transfer rollers  4 , each of which transfers an image formed on one of the photoreceptor drums  1  located along an outer circumference of the intermediate transfer belt  3 , are attachable to and detachable from the transfer belt assembly  30  together with the frame  41 . With this configuration, the primary transfer roller  4  can be replaced more easily, and cost can be reduced by using a common primary roller unit for any of black, yellow, cyan and magenta. 
     Management of the useful life of the individual roller unit  40  performed by the controller  56  is described below. 
     As described above, the image forming apparatus A shown in  FIG. 2  includes the usage history manager  59  including the newness indicator  57  and the history recorder  58  as illustrated in  FIG. 3 . The controller  56  manages the useful life of the individual roller unit  40  by making the end of the useful life as a time to replace the individual roller unit  40  for each color, and it does so by tracing the usage history of that individual roller unit  40 . 
     Specially, the controller  56  stores usage history information about each of the individual roller unit  40  in the history recorder  59 , which is included in the RAM of the controller  56  in an example embodiment. The usage history information includes operating time of the individual roller unit  40  while being attached to the transfer belt assembly  30 , the number of sheets printed, etc. When the operating time, the number of sheets, etc., reach a given value, the controller  56  issues a message to advise a user to replace the individual roller unit  40 , determining that the useful life of the individual roller unit  40  is at an end. 
     When the individual roller unit  40  at the end of its useful life is replaced with another one, the usage history information in the history recorder  58  (RAM) should be reset. However, the newly attached individual roller unit  40  is not necessarily unused. Therefore, when the newly attached individual roller unit  40  is unused, the newness indicator  57 , which is provided for each individual roller unit  40 , indicates its being unused (unused state) to the controller  56 . The controller  56  determines whether or not the newly attached individual roller unit  40  is unused by referring to the newness indicator  57  when detecting an attachment of the individual roller unit  40  to the transfer belt assembly  30 . The controller  56  then resets the usage history information corresponding to the individual roller unit  40  for that color when determining that the newly attached individual roller unit  40  is unused. 
     By contrast, when the controller  56  determines that the newly attached individual roller unit  40  is not unused, the controller  56  informs the user of that fact, for example, by displaying a message on a display of the image forming apparatus A. The message may be a warning that the end of the useful life of the individual roller unit  40  cannot be properly predicted, as well as a query as whether or not to continue to attempt to manage the newly attached individual roller unit  40 . When the user elects to manage the useful life end timing, for example, by pushing a button, the controller  56  resets the usage history information. When the user selects not to manage the useful life end timing, the controller  56  suspends managing the useful life end timing of that individual roller unit  40  until that individual roller unit  40  is replaced with another one. 
     Further, the configuration described above can relieve the user of renewing the usage history information manually. More specifically, when the individual roller unit  40  in the transfer belt  30  is replaced, the usage history information stored in the history recorder  58  should be renewed. If a user renews the usage history information for himself/herself, it may take some time and the user may forget to rewrite some of the information. 
     By contrast, in the image forming apparatus A, the individual roller unit  40  includes the newness indicator  57 , and the controller  56  detects the newness indicator  57  and renews the usage history information automatically more quickly and without omission. 
     In an example embodiment, the newness indicator  57  is a mechanical structure. For example, the newness indicator  57  includes a specular reflector and a shutter both provided on the frame  41 , and each of the sub-frames  35  and  36  includes a claw and a reflective photosensor. The shutter is slidable to an open position to expose the specular reflector and a closed position to cover the specular reflector. The shutter is at the closed position when an individual roller unit  40  is shipped to a user. 
     When the user removes an individual roller unit  40  at the end of its useful life from the sub-frame  35  or  36 , the claw thereon opens the shutter. With the opened shutter, the individual roller unit  40  is recognizable as being used. More specifically, when the used individual roller unit  40  is attached to one of the sub-frame  35  and  36 , the reflective photosensor thereon receives light reflected by the specular reflector, and thus the individual roller unit  40  is detected as being used. The controller  56  detects that an individual roller unit  40  as being unused when the reflective photosensor does not receive such reflected light. 
     Another management method of useful life of the individual roller unit  40  is described below. 
     Although life end timing of individual roller units  40  is managed for each color similarly in this method, instead of a mechanical structure, the usage history manager  59  shown in  FIG. 3  is a circuit board including an integrated circuit (IC) chip and various electronic components, fixed on the frame  41 . 
     When the transfer belt assembly  30  shown in  FIG. 3  is detached from the image forming apparatus A and then reinstalled therein, the usage history manager  59  (circuit board) of the individual roller unit  40  and the controller  56  in the image forming apparatus A are electrically continuous through a contact point therebetween. In this state, the controller  56  is able to read and write information to and from the IC chip in the history manager  59 . 
     In the usage history manager  59  as a the circuit board, both a newness indicator  57  and a history recorder  58  are included in the same IC chip. The newness indicator  57  includes newness determination information, which indicates being unused (unused state) as factory default. The newness indicator  57  further includes color identification indicating one of black, yellow, cyan, and magenta, which the individual roller unit  40  is for. When a user starts using the individual roller unit  40 , the controller  56  renews the newness determination information to a value showing being used. Therefore, the controller  56  determines whether or not the individual roller unit  40  is unused based on the newness determination information stored in the newness indicator  57  in the usage history manager  59  (circuit board). Further, the controller  56  detects an error when the wrong individual roller unit  40  is attached to the image forming apparatus A based on the color identification. 
     As described above, in the image forming apparatus A according to the present embodiment, the individual roller unit  40  is provided with the newness indicator  57 . Therefore, whether or not an individual roller unit  40  is unused and an error message indicating that the individual roller unit  40  is for the wrong color are detected through coordination between functions of the controller  56  and the newness indicator  57 . 
     Further, in the present embodiment, the usage history manager  59  (circuit board) of the individual roller unit  40  includes the history recorder  58 , and usage history information is managed through coordination between the functions of the controller  56  and the history recorder  58 . 
     By contrast, when the history recorder  58  is included in the controller  56  with the usage history information managed only by the controller  56 , the remaining life of a used individual roller unit  40  is not recognizable even if the individual roller unit  40  is detached from the image forming apparatus A before the end of its useful life. However, if the detached individual roller unit  40  is not used until its useful life ends, the individual roller unit  40  can be reused. 
     Therefore, in the present embodiment, by providing the individual roller unit  40  with the history recorder  58  is the form of an IC chip, etc., usage history information can be included in the individual roller unit  40 , facilitating reuse or recycling of used individual roller units  40 . 
     The usage history information stored in the history recorder  58  includes the operating time of the primary transfer roller  4  held in that individual roller unit. In this case, the useful life of the individual roller unit  40  is managed based on useful life of the primary transfer roller  4 . However, in addition to the useful life of the primary transfer roller  4 , the operating time of the bearings  43 , the springs  46 , and other components in the individual roller unit  40  may be stored so that individual component can be reused. In this case, the individual roller unit  40  is replaced with another one when one of the components reaches the end of its useful life, and other components whose operating time is relatively short can be prepared for reuse at a recycling factory. 
     In the example embodiments shown in  FIGS. 7  though  13 , the individual roller units  40 B,  40 B 1 , and  40 B 2  for black and the individual roller unit  40 A for full color have different configuration. Alternatively, however, one of the individual roller units for full color may have a different configuration from those of the individual roller units for other colors. Or, characteristics such as electrical resistance of the primary roller unit  4 , may differ among individual roller units even though appearance and/or physical structure is the same or substantially the same among the individual roller units for respective colors. 
     A method of determining the end of the useful life of a unit or a component is described below. 
     Although the end of the useful life of a component can be determined with a certain degree of accuracy by storing operating time as usage history information as in the embodiments described above, the relation between deterioration and operating time is typically different for each component. As a result, a predicted end of useful life might be slightly different from the actual end of useful life. 
     Therefore, in an example embodiment of the present invention, the controller  56  measures a degradation level of the primary transfer roller  4  so as to determine the end of its useful life with a higher degree of accuracy. The degradation level of the primary transfer roller  4  means the level of degradation of the conductive rubber layer included therein. The degradation level of the primary transfer roller  4  is determined based on relations among electric resistance of the conductive rubber layer, a transfer bias, and temperature, which are further described below. 
     The individual roller unit  40  further includes a conductive feedback current roller in addition to the primary transfer roller  4 . The conductive feedback current roller is configured to contact the intermediate transfer belt  3  at a portion downstream of the primary transfer roller  4  in the belt rotation direction. The transfer current applied to the back surface of the intermediate transfer belt  3  by the primary transfer roller  4  partly flows in a thickness direction of the intermediate transfer belt  3  to the photoreceptor drum  1 . This current is an actual transfer current and affects a toner image on the photoreceptor drum  1 . In the transfer nip and an area around the transfer nip, the transfer current applied by the primary transfer roller  4  partly flows in a circumferential direction of the intermediate transfer belt  3 , this current is hereinafter referred to as the circumferential current. A ratio of the circumferential current to the actual transfer current depends on the electrical resistance of the intermediate transfer belt  3 , which changes as the temperature changes. If this ratio fluctuates, reliable transfer characteristics can not be attained. 
     Therefore, in the image forming apparatus A according to the present embodiment, a bias output from the power source is controlled so as to maintain the actual transfer current constant. More specifically, most of the circumferential current flows to the feedback current roller as a feedback current. The power source for supplying the transfer bias calculates the actual transfer current by deducting the feedback current from an output current, and controls the output bias to maintain the actual transfer current constant. 
     The image forming apparatus A further includes a temperature sensor, not shown, to detect temperature therein and output a detection result as a digital temperature signal to the controller  56 . The power source to supply the transfer bias outputs a value of the transfer bias applied to the primary transfer roller  4  to the controller  56  as a digital bias signal. When the temperature signal from the temperature detector indicates a predetermined or given temperature (e.g., 25° C.), in other words, when an electric resistance of the intermediate transfer belt  3  corresponds to the predetermined or given temperature, the controller  56  reads the bias signal. The bias signal correlates with electric resistance of the conductive rubber layer in the primary transfer roller  4 . That is, a transfer bias value under a given temperature increases as the electric resistance of the conductive rubber layer increases in accordance with the degradation of the conductive rubber layer. Therefore, the controller  56  determines a degradation level of the primary transfer roller  4  based on a bias signal under a given temperature, and stores the determined degradation level in the history recorder  58 . 
       FIG. 14  illustrates a process of degradation level determination performed by the controller  56 . Referring to  FIG. 14 , when the controller  56  determines that the temperature inside the image forming apparatus A is a given temperature at S 1  (YES at S 1 ), the controller  56  reads a bias signal generated by the power source for supplying a transfer bias at S 1 . The controller  56  includes a degradation index table storing degradation indexes, which are established through tests beforehand, which are correlated with the bias signals. The controller  56  selects one of the degradation indexes from the degradation index table corresponding to the read bias signal at S 3 , and renews degradation index information stored in the history recorder  58  as usage history at S 4 . 
     It is to be noted that the transfer belt assembly  30  can be adopted in a tandem image forming apparatus employing a direct transfer belt method, although the description above concerns a tandem image forming apparatus employing an intermediate transfer method. 
     Referring to  FIG. 15 , a tandem image forming apparatus Al employing an intermediate transfer method is described below. Each component of the image forming apparatus A 1  that is similar to a corresponding component of the image forming apparatus A is given the same reference numeral, and a description thereof thus omitted. 
     As illustrated in  FIG. 15 , the image forming apparatus A 1  includes image forming units  2 B,  2 Y,  2 C, and  2 M having a photoreceptor drums  1 B,  1 Y,  1 C, and  1 M, respectively. The image forming apparatus A 1  further includes a transfer belt assembly  30 A located to contact the photoreceptor drums  1 B,  1 Y,  1 C, and  1 M. The transfer belt assembly  30 A moves a transfer belt  3 A stretched around tension rollers  19  through  24  counterclockwise in  FIG. 15 . The image forming units  2 B,  2 Y,  2 C, and  2 M are located sequentially from downstream in a moving direction of the transfer belt  3 A (belt rotation direction), according to an order in which transfer is performed. 
     At positions facing the photoreceptor drums  1 B,  1 Y,  1 C, and  1 M via the transfer belt  3 A, transfer rollers  4 AB,  4 AY,  4 AC, and  4 AM are provided. The image forming apparatus A 1  further includes a cleaner  5  facing the tension roller  21  via the transfer belt  3 A. 
     The image forming apparatus A 1  further includes a fixer  16  having a heating belt located downstream of the image forming unit  2 B in a sheet transport path, and a pair of discharge rollers  17  located downstream of the fixer  16 . On the photoreceptor drums  1 B,  1 Y,  1 C, and  1 M, latent images are formed and developed with black, yellow, cyan, and magenta toners. 
     The image forming apparatus A further includes a sheet feeder  10  having a sheet cassette  11  and a feed roller  12 , located at a bottom portion thereof, transport rollers  13 , a pair of registration rollers  14 , and a controller  56 . 
     The toner images on the photoreceptor drums  1 B,  1 Y,  1 C, and  1 M are transferred and superimposed as a full color image on a sheet transported on the transfer and transport belt  3 A. The full color image is fixed on the sheet by the fixer  16  and discharged from the image forming apparatus A 1  with a pair of discharge rollers  17 . Thus, the image forming apparatus A 1  applies a direct transfer belt method. 
     As can be appreciated by those skilled in the art, although the transferer is a transfer roller in the description above, alternatively, other types of transferees, such as a transfer brush, may be used. Further, the present invention may be applied to a tandem image forming apparatus without an intermediate transfer belt or a transfer and transport belt. 
     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.