Patent Publication Number: US-9405271-B2

Title: Cleaning device and image forming apparatus incorporating same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation application of U.S. patent application Ser. No. 14/602,462, filed Jan. 22, 2015, which is based on and claims priority pursuant to 35 U.S.C. §119(a) to Japanese Patent Application No. 2014-010086, filed on Jan. 23, 2014, and Japanese Patent Application No. 2014-133543, filed on Jun. 30, 2014, in the Japan Patent Office. The entire disclosures of each of the above are incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Technical Field 
     Embodiments of this disclosure relate to a cleaning device including multiple cleaning members to scrape off adhered substance on a surface of a cleaning target and an image forming apparatus employing the cleaning device to remove toner on a surface of an image bearer. 
     2. Description of the Related Art 
     Image forming apparatuses are used as, for example, copiers, printers, facsimile machines, and multi-functional devices having at least one of the foregoing capabilities. Such an image forming apparatus may include an image bearer to bear a toner image on a surface thereof and a cleaning device to remove adhered substance, e.g., toner particles from the surface of the image bearer. 
     For example, as illustrated in  FIG. 20 , an image forming apparatus includes a cleaning device  900  and an intermediate transfer belt  908  serving as an image bearer. In  FIG. 20 , the cleaning device  900  includes a first sub unit  901 , a second sub unit  911 , a third sub unit  921 , and a sub-unit holder  950  to hold the first sub unit  901 , the second sub unit  911 , and the third sub unit  921 , respectively. The first sub unit  901 , the second sub unit  911 , and the third sub unit  921  hold multiple members including, e.g., a cleaning brush roller  902 , a cleaning brush roller  912 , and a cleaning brush roller  922  with a sub-unit casing  906 , a sub-unit casing  916 , and a sub-unit casing  926 , respectively. 
     The sub-unit holder  950  has a first support  951  to support the first sub unit  901  in such a manner that the first sub unit  901  is slidable in a longitudinal direction thereof (a direction perpendicular to a sheet surface on which  FIG. 20  is printed). The sub-unit holder  950  also has a second support  952  to support the second sub unit  911  in such a manner that the second sub unit  911  is slidable in a 
     longitudinal direction thereof and a third support  953  to support the third sub unit  921  in such a manner that the third sub unit  921  is slidable in a longitudinal direction thereof. With such sliding movement, the first sub unit  901 , the second sub unit  911 , and the third sub unit  921  are removably mountable relative to the sub-unit holder  950 . 
     For such a configuration, when one of the cleaning brush roller  902 , the cleaning brush roller  912 , and the cleaning brush roller  922  comes to the end of product life, a user can replace only one of the multiple sub cleaning units including the cleaning brush roller having come to the end of product life with a new one and continue to use the other sub cleaning units remaining in the cleaning device  900 . Such a configuration can reduce servicing cost of the cleaning device as compared with a configuration in which a single holder holds multiples components. 
     SUMMARY 
     In at least one aspect of this disclosure, there is provided an improved cleaning device including a cleaner, a plurality of cleaning sub-units, a sub-unit holder, a detachable transmission assembly, and a plurality of joint couplers. The cleaner scrapes off adhered material from a surface of a cleaning target while contacting the surface of the cleaning target. The plurality of cleaning sub-units each includes a holder to hold the cleaner and a drive-receive rotator to receive a driving force. The sub-unit holder holds the plurality of cleaning sub-units. The detachable transmission assembly includes a driving-force receive rotator to receive a driving force from an external unit and a plurality of drive transmission rotators to transmit the driving force received by the driving-force receive rotator to the drive-receive rotators of the plurality of cleaning sub-units. The plurality of joint couplers mounted on the drive-receive rotators and the plurality of drive transmission rotators couple the drive-receive rotators with the plurality of drive transmission rotators in a rotation axial direction. 
     In at least one aspect of this disclosure, there is provided an improved image forming apparatus including an image bearer, an image forming device, and a cleaning device. The cleaning device includes a cleaner, a plurality of cleaning sub-units, a sub-unit holder, a detachable transmission assembly, and a plurality of joint couplers. The cleaner scrapes off toner as adhered substance from the surface of the image bearer. The plurality of cleaning sub-units each includes a holder to hold the cleaner and a drive-receive rotator to receive a driving force. The sub-unit holder holds the plurality of cleaning sub-units. The detachable transmission assembly includes a driving-force receive rotator to receive a driving force from an external unit and a plurality of drive transmission rotators to transmit the driving force received by the driving-force receive rotator to the drive-receive rotators of the plurality of cleaning sub-units. The plurality of joint couplers mounted on the drive-receive rotators and the plurality of drive transmission rotators couples the drive-receive rotators with the plurality of drive transmission rotators in a rotation axial direction. 
     In at least one aspect of this disclosure, there is provided an improved unit device detachably attachable relative to a machine body. The device includes a plurality of sub units, a sub-unit holder, a plurality of cleaning sub-units, a sub-unit holder, a detachable transmission assembly, and a plurality of joint couplers. The plurality of sub units each includes a driver to be driven by a driving force. The sub-unit holder holds the plurality of sub-units. The plurality of cleaning sub-units each includes a holder to hold the cleaner and a drive-receive rotator to receive a driving force. The sub-unit holder holds the plurality of cleaning sub-units. The detachable transmission assembly includes a driving-force receive rotator to receive a driving force from an external unit and a plurality of drive transmission rotators to transmit the driving force received by the driving-force receive rotator to the drive-receive rotators of the plurality of cleaning sub-units. The plurality of joint couplers mounted on the drive-receive rotators and the plurality of drive transmission rotators couple the drive-receive rotators with the plurality of drive transmission rotators in a rotation axial direction. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The aforementioned and other aspects, features, and advantages of the present disclosure would be better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic view of a configuration of a part of an image forming apparatus according to at least one embodiment of this disclosure; 
         FIG. 2  is an enlarged view of a configuration of a belt cleaning device and a periphery thereof in the image forming apparatus; 
         FIG. 3  is an enlarged schematic view of a chevron patch formed on an intermediate transfer belt of the image forming apparatus; 
         FIG. 4  is a schematic view of the intermediate transfer belt and a toner consumption pattern formed on a surface of the intermediate transfer belt; 
         FIG. 5  is an enlarged view of a configuration of the belt cleaning device of the image forming apparatus; 
         FIG. 6  is an enlarged view of a configuration of a first cleaning sub-unit of the belt cleaning device and its circumference; 
         FIG. 7  is an enlarged view of a configuration of a second cleaning sub-unit of the belt cleaning device and its circumference; 
         FIG. 8  is an enlarged view of a configuration of a third cleaning sub-unit of the belt cleaning device and its circumference; 
         FIG. 9  is a back view of a back side plate of a sub-unit holder casing of the belt cleaning device; 
         FIG. 10  is a back view of an upper portion of the back side plate and the third sub cleaning unit; 
         FIG. 11  is a partial, enlarged perspective view of the third cleaning sub-unit and joint caps; 
         FIG. 12  is a partial, enlarged perspective view of a third back main-positioning pin of the third cleaning sub-unit and the back side plate of the sub-unit holder casing; 
         FIG. 13  is a partial, enlarged perspective view of a third back sub-positioning pin of the third cleaning sub-unit and the back side plate of the sub-unit holder casing; 
         FIG. 14  is a perspective view of a belt drive assembly of a transfer unit of the image forming apparatus and a portion of the third sub cleaning unit according to an embodiment of this disclosure; 
         FIG. 15  is a partial, enlarged perspective view of a front end of the belt cleaning device; 
         FIG. 16  is a perspective view of a part of the belt cleaning device with a transmission assembly mounted on the front side plate, seen from a front side of the belt cleaning device; 
         FIG. 17  is a perspective view of a part of the belt cleaning device with the transmission assembly mounted on the front side plate, seen from the front side of the belt cleaning device and a different angle from that of  FIG. 16 ; 
         FIG. 18  is a perspective view of the transmission assembly seen from a back face side thereof; 
         FIG. 19  is a plan view of a front edge of the belt cleaning device with the transmission assembly mounted on the front side plate, seen from a front side of the belt cleaning device; 
         FIG. 20  is an enlarged view of a comparative example of a cleaning device of an image forming apparatus with an intermediate transfer belt; 
         FIG. 21  is a partial perspective view of a part of a belt cleaning device in a state in which a transmission assembly is removed in an image forming apparatus according to an embodiment of this disclosure, seen from a side at which a front side plate is disposed; and 
         FIG. 22  is a perspective view of the transmission assembly in a state of being mounted on a body of the belt cleaning device. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION 
     In describing 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 similar results. 
     Although the embodiments are described with technical limitations with reference to the attached drawings, such description is not intended to limit the scope of the disclosure and all of the components or elements described in the embodiments of this disclosure are not necessarily indispensable. 
     Referring now to the drawings, embodiments of the present disclosure are described below. In the drawings for explaining the following embodiments, the same reference codes are allocated to elements (members or components) having the same function or shape and redundant descriptions thereof are omitted below. 
     As an image forming apparatus according to an embodiment of this disclosure, a tandem-type printer of an intermediate transfer system (to be simply referred to as a printer hereinafter) is described below. However, the image forming apparatus is not limited to the tandem-type printer of an intermediate transfer system and may be any other suitable type of image forming apparatus. First, a basic configuration of the image forming apparatus according to this embodiment is described below.  FIG. 1  is a schematic view of a configuration of a part of the image forming apparatus according to this embodiment. The printer includes four process units  6 Y,  6 M,  6 C, and  6 K to form toner images in yellow, magenta, cyan, and black (hereinafter referred to as Y, M, C, and K). The four process units  6 Y,  6 M,  6 C, and  6 K have drum-shaped photoconductors  1 Y,  1 M,  1 C, and  1 K, respectively. The photoconductors  1 Y,  1 M,  1 C, and  1 K have charging devices  2 Y,  2 M,  2 C, and  2 K, developing devices  5 Y,  5 M,  5 C, and  5 K, drum cleaning devices  4 Y,  4 M,  4 C, and  4 K, neutralizing devices, and the like therearound, respectively. The process units  6 Y,  6 M,  6 C, and  6 K use Y, M, C, and K toners having colors different from each other, but have the same configurations, respectively. Above the process units  6 Y,  6 M,  6 C, and  6 K is disposed an optical writing unit to irradiate a laser beam L on surfaces of the photoconductors  1 Y,  1 M,  1 C, and  1 K to write electrostatic latent images. 
     Below the process units  6 Y,  6 M,  6 C, and  6 K is disposed a transfer unit  7  including an endless intermediate transfer belt  8  serving as an image bearer. In addition to the intermediate transfer belt  8 , the transfer unit  7  includes, e.g., a plurality of extension rollers disposed inside a loop of the intermediate transfer belt  8 , a secondary transfer roller  18  disposed outside the loop, a tension roller  16 , a belt cleaning device  100 , and a lubricant application device  200 . 
     Inside the loop of the intermediate transfer belt  8 , four primary transfer rollers  9 Y,  9 M,  9 C, and  9 K, a driven roller  10 , a driving roller  11 , a secondary-transfer opposed roller  12 , three cleaning opposed rollers  13 ,  14 , and  15 , and an applying brush opposed roller  17  are disposed. Each of all the rollers functions as an extension roller having a peripheral surface on which the intermediate transfer belt  8  is partially hung to extend the belt. As a necessary condition of the cleaning opposed rollers  13 ,  14 , and  15 , the cleaning opposed rollers  13 ,  14 , and  15  need not necessarily give a predetermined tension to the belt. The cleaning opposed rollers  13 ,  14 , and  15  may be driven and rotated with the rotation of the intermediate transfer belt  8 . The intermediate transfer belt  8  is endlessly moved in the counterclockwise direction in  FIG. 1  with the rotation of the driving roller  11  rotationally driven in the counterclockwise direction in  FIG. 1  by a driving unit. 
     The four primary transfer rollers  9 Y,  9 M,  9 C, and  9 K disposed inside the belt loop sandwich the intermediate transfer belt  8  between the primary transfer rollers  9 Y,  9 M,  9 C, and  9 K and the photoconductors  1 Y,  1 M,  1 C, and  1 K, respectively. In this manner, primary transfer nips for Y, M, C, and K on which the front surface of the intermediate transfer belt  8  is brought into contact with the photoconductors  1 Y,  1 M,  1 C, and  1 K are formed. Primary transfer biases each having a polarity opposite that of toner are applied to the primary transfer rollers  9 Y,  9 M,  9 C, and  9 K with power supplies, respectively. 
     The secondary-transfer opposed roller  12  disposed inside the belt loop sandwiches the intermediate transfer belt  8  between the secondary-transfer opposed roller  12  and a secondary transfer roller  18  disposed outside the belt loop. In this manner, a secondary transfer nip on which the front surface of the intermediate transfer belt  8  is brought into contact with the secondary transfer roller  18  is formed. A secondary transfer bias having a polarity opposite that of toner is applied to the secondary transfer roller  18  with a power supply. A sheet conveyance belt may be bridged by a secondary transfer roller, several support rollers, and a driving roller, and the intermediate transfer belt  8  and the sheet conveyance belt may be sandwiched between the secondary transfer roller  18  and the secondary-transfer opposed roller  12 . 
     The three cleaning opposed rollers  13 ,  14 , and  15  disposed inside the belt loop sandwich the intermediate transfer belt  8  between the cleaning opposed rollers  13 ,  14 , and  15  and three cleaning brush rollers of a belt cleaning device  100  disposed outside the belt loop. In this manner, a cleaning nip on which the front surface of the intermediate transfer belt  8  is brought into contact with the three cleaning brush rollers is formed. The belt cleaning device  100  is replaceable together with the transfer unit  7 . The belt cleaning device  100  is also removable from the transfer unit  7  having removed from an apparatus body of the image forming apparatus. Furthermore, with the transfer unit  7  and the belt cleaning device  100  mounted in the apparatus body, a sub cleaning unit is removably mountable relative to the belt cleaning device  100 . The belt cleaning device  100  will be described in detail below. 
     The image forming apparatus according to this embodiment includes a sheet feed section having a sheet feed tray to store a recording sheet P, a sheet feed roller that feeds the recording sheet P from the sheet feed tray to a sheet feed path. A pair of registration rollers to receive the recording sheet P sent from the sheet feed section and sends the recording sheet P toward the secondary transfer nip at a proper timing is disposed on the right side of the secondary transfer nip in  FIG. 1 . A fixing device is disposed on the left of the secondary transfer nip in  FIG. 1 . The fixing device receives the recording sheet P sent from the secondary transfer nip and performs a fixing process of a toner image on the recording sheet P. As needed, toner supply devices for Y, M, C, and K that supply Y, M, C, and K toners to the development devices  5 Y,  5 M,  5 C, and  5 K are also disposed. 
     In recent years, the frequency of use of, in addition to a plain sheet that has been conventionally and widely used as a recording sheet, a special sheet designed to have uneven surfaces or a special recording sheet used for thermal transfer such as ironing print has been increased. When the special sheet is used, in comparison with in use of a conventional regular sheet, defective transfer is likely to occur when a toner image on the intermediate transfer belt  8  obtained by overlapping color toner images is secondarily transferred onto a sheet. Hence, in the image forming apparatus, an elastic layer having low hardness is formed on the intermediate transfer belt  8 , so that the intermediate transfer belt  8  can be transformed for the toner layer or a recording sheet having poor smoothness at the transfer nipping portion. The elastic layer having low hardness is formed on the intermediate transfer belt  8  to make the intermediate transfer belt  8  elastic, so that the surface of the intermediate transfer belt  8  can be transformed in accordance with local irregularity. In this manner, good tightness can be achieved without excessively increasing a transfer pressure on the toner layer, character missing in transfer does not occur, and a uniform transferred image in which uneven transfer does not occur on a sheet or the like having poor smoothness can be obtained. 
     In the image forming apparatus, the intermediate transfer belt  8  includes at least a base layer, an elastic layer, and a coat layer serving as the uppermost layer. 
     As a material used in the elastic layer of the intermediate transfer belt  8 , an elastic member such as an elastic material rubber or an elastomer is given. More specifically, one or more selected from the group consisting of isobutylene-isoprene rubber, fluororubber, acrylic rubber, ethylene propylene diene monomer (EPDM), nitrile rubber (NBR), acrylonitrile-butadiene-styrene rubber, natural rubber, isoprene rubber, styrene-butadiene rubber, butadiene rubber, polyurethane rubber, syndiotactic 1,2-polybutadiene, epichlorohydrin rubber, polysulfide rubber, polynorbornene rubber, and thermoplastic elastomer (for example, polystyrene resin, polyolefin resin, polyvinyl chloride resin, polyurethane resin, polyamide resin, polyurea resin, polyester resin, and fluorine resin) can be used. However, the material used in the elastic layer is not limited to the above materials. 
     The thickness of the elastic layer, depending on a hardness and a layer configuration, preferably ranges from 0.07 mm to 0.5 mm. More preferably, the thickness ranges from 0.25 mm to 0.5 mm. When the thickness of the intermediate transfer belt  8  is small, i.e., 0.07 mm or less, a pressure on toner on the intermediate transfer belt  8  at the secondary transfer nip becomes high, character missing in transfer is likely to occur, and a transfer ratio of toner decreases. 
     The hardness of the elastic layer preferably falls within a range given by 10° HS 65° (JIS-A). Although an optimum hardness changes depending on the layer thickness of the intermediate transfer belt  8 , when the hardness is lower than 10° (JIS-A), character missing in transfer is likely to occur. By contrast, when the hardness is higher than 65° (JIS-A), the intermediate transfer belt  8  is difficult to stretch between the rollers, and the intermediate transfer belt  8  extends due to long-term stretching to lose the durability. As a result, the intermediate transfer belt  8  need to be early exchanged. 
     The base layer of the intermediate transfer belt  8  is made of a resin having low elongation. More specifically, as a material used in a base layer, one or more selected from a group consisting of styrene resin (single polymer including styrene or a styrene substitute or copolymer) such as polycarbonate, fluoro resin (ethylene tetrafluoroethylene (ETFE), polyvinylidene difluoride (PVDF), or the like), polystyrene, chlorophyll styrene, poly-α, methyl styrene, styrene-butadiene copolymer, styrene-vinyl chloride copolymer, styrene-vinyl acetate copolymer, styrene-maleic acid copolymer, styrene-acrylic ester copolymer (styrene-acrylic acid methyl copolymer, styrene-acrylic acid ethyl copolymer, styrene-acrylic acid butyl copolymer, styrene-acrylic acid octyl copolymer, styrene-acrylic acid phenyl copolymer, or the like), styrene-methacrylic acid ester copolymer (styrene-methyl methacrylate copolymer, styrene-methacrylic acid ethyl copolymer, styrene-methacrylic acid phenyl copolymer, or the like), styrene-α-chlor acrylic acid methyl copolymer, styrene-acrylonitrile-acrylic ester copolymer, methyl methacrylate resin, methacrylate butyl resin, acrylic acid ethyl resin, acrylic acid butyl resin, denaturing acrylic acid resin (silicone denaturation acrylic acid resin, vinyl chloride resin denaturation acrylic acid resin, acrylic urethane resin, or the like), vinyl chloride resin, styrene-vinyl acetate copolymer, vinyl chloride-vinyl acetate copolymer, rosin-modified maleic acid resin, phenolic resin, epoxy resin, polyester resin, polyester polyurethane resin, polyethylene, polypropylene, polybutadiene, polyvinylidene chloride, an ionomer resin, polyurethane resin, silicone resin, ketone resin, ethylene-ethyl acrylate copolymer, xylene resin and polyvinyl butyral resin, polyamide resin, modified polyphenylene oxide resin, and the like can be used. However, the material used in the elastic layer is not limited to the above materials. 
     In order to prevent elongation of the elastic layer made of a rubber material or the like having high elongation, a core layer made of a material such as canvas or the like may be disposed between the base layer and the elastic layer. As a material that is used in the core layer to prevent elongation of the core layer, for example, one or more selected from the group consisting of a natural fiber such cotton or silk, a synthetic fiber such as a polyester fiber, a nylon fiber, an acrylic fiber, a polyolefin fiber, a polyvinyl alcohol fiber, a polychlorinated vinyl fiber, a polyvinylidene chloride fiber, a polyurethane fiber, a polyacetal fiber, a polyphloroethylene fiber, or a phenol fiber, an inorganic fiber such as a carbon fiber or a glass fiber, and a metal fiber such as an iron fiber or a copper fiber are used, and a fiber in the form of a thread or a woven cloth can be used. However, the material to prevent the elongation is not limited to the above materials. The thread may be one filament or obtained by twisting a plurality of filaments. A thread such as a single twist yarn, a plied yarn, or a two folded yarn that is twisted by any twisting method may be used. Fibers of materials selected from the material group described above may be blended. As a matter of course, a thread applied with an appropriate process to have electrical conductivity may be used. On the other hand, as a woven fabric, a woven fabric such as a knitted fabric that is woven by any weave can be used. A woven fabric obtained by combined weaving can also be used, and can also be applied with a process to have electrical conductivity. 
     The coat layer on the surface of the intermediate transfer belt  8  coats the surface of the elastic layer, and is constituted of a smooth layer. A material used in the coat layer is not limited to a specific one. However, in general, a material that reduces adherence of toner to the surface of the intermediate transfer belt  8  to improve secondary transfer properties is used. For example, particles made of one or more of polyurethane, polyester, an epoxy resin, and the like or one or more of materials that reduce surface energy to improve lubricity, for example, a fluorocarbon resin, a fluorine compound, fluorocarbon, a titanium oxide, and a silicon carbide can be used such that the particles are dispersed while being reduced in diameter as needed. A material such as a fluorine rubber material that is applied with thermal treatment to form a fluorine layer on the surface and to decrease surface energy can also be used. 
     As needed, as a base layer, an elastic layer, or a coat layer, in order to adjust the resistance, for example, metal powder of carbon black, graphite, aluminum, or nickel or a conductive metal oxide such as tin oxide, titanium oxide, antimony oxide, indium oxide, potassium titanate, antimony oxide-tin oxide complex oxide (ATO), indium oxide-tin oxide complex oxide (ITO), or the like can be used. In this case, the conductive metal oxide may be obtained by covering insulating fine particles of barium sulfate, magnesium silicate, calcium carbonate, or the like with a conductive metal oxide. However, the material used in the elastic layer is not limited to the above materials. 
     A lubricant is applied to the surface of the intermediate transfer belt  8  by a lubricant application device  200  to protect the belt surface. The lubricant application device  200  includes a solid lubricant  202  such as zinc stearate agglomerate and an application brush roller  201  serving as an application member that is brought into contact with the solid lubricant to apply lubricant powder obtained by scraping the solid lubricant with rotation to the surface of the intermediate transfer belt  8 . The image forming apparatus includes the lubricant application device  200 . However, depending on toner to be applied and the material and the surface friction coefficient of the intermediate transfer belt  8 , the lubricant application device  200  need not be always included, and lubricant need not be necessarily applied. 
     When image information is sent from a personal computer or the like, the printer rotationally drives the driving roller  11  to endlessly move the intermediate transfer belt  8 . Extension rollers except for the driving roller  11  are rotated by following the belt. At the same time, the photoconductors  1 Y,  1 M,  1 C, and  1 K of the process units  6 Y,  6 M,  6 C, and  6 K are rotationally driven. While the surfaces of the photoconductors  1 Y,  1 M,  1 C, and  1 K are uniformly electrically charged with the charging devices  2 Y,  2 M,  2 C, and  2 K, electrostatic latent images are formed by irradiating the laser beam L on the electrically charged surfaces. The electrostatic latent images formed on the surfaces of the photoconductors  1 Y,  1 M,  1 C, and  1 K are developed with the development devices  5 Y,  5 M,  5 C, and  5 K to obtain Y, M, C, and K toner images on the photoconductors  1 Y,  1 M,  1 C, and  1 K. The Y, M, C, and K toner images are superposed and primarily transferred on the front surface of the intermediate transfer belt  8  at the primary transfer nips for the Y, M, C and K. In this manner, a toner image in four colors is formed on the front surface of the intermediate transfer belt  8 . 
     On the other hand, at the sheet feed section, recording sheets P are sent one by one from the sheet feed tray with a sheet feed roller  27  and transported to the pair of registration rollers. The pair of registration rollers are driven at a timing that can be synchronized with the toner image in four colors on the intermediate transfer belt  8  to send the recording sheet P to the secondary transfer nip, and the toner image in four colors on the belt is secondarily transferred on the recording sheet P in a lump. In this manner, a full-color image is formed on the surface of the recording sheet P. The recording sheet P after the full-color image is formed is transported from the secondary transfer nip to the fixing device to apply a fixing process for the toner image. 
     With respect to the photoconductors  1 Y,  1 M,  1 C, and  1 K after the Y, M, C, and K toner images are primarily transferred onto the intermediate transfer belt  8 , a cleaning process for residual toner after transfer is performed by the drum cleaning devices  4 Y,  4 M,  4 C, and  4 K. Thereafter, after neutralization is performed with a neutralization lamp, the belt is uniformly electrically charged with the charging devices  2 Y,  2 M,  2 C, and  2 K to prepare the next image formation. The intermediate transfer belt  8  after the image is primarily transferred onto the recording sheet P, a cleaning process of residual toner after transfer is performed by the belt cleaning device  100 . 
     On the right of the process unit  6 K for K in  FIG. 2 , an optical sensor unit  90  is disposed to face the front surface of the intermediate transfer belt  8  with a predetermined interval. The optical sensor unit  90 , as illustrated in  FIG. 2 , has a Y optical sensor  91 Y, a C optical sensor  91 C, an M optical sensor  91 M, and a K optical sensor  91 K arrayed in the width direction of the intermediate transfer belt  8 . Each of all the sensors is formed of a reflective photosensor in which light emitted from a light-emitting element is reflected by the front surface of the intermediate transfer belt  8  and the toner image on the belt, and the reflected light quantity is detected by a light-receiving element. A controller, on the basis of output voltages from these sensors, can detect a toner image on the intermediate transfer belt  8  or detect an image density (amount of adhesion of toner per unit area) of the toner image. 
     In the printer, in a power-on state or each time a predetermined number of prints are completed, image density control is executed to optimize the image densities of the colors. The image density control, as illustrated in  FIG. 2 , automatically forms gradation patterns Sk, Sm, Sc, and Sy in colors at positions facing the optical sensors  91 Y,  91 M,  91 C, and  91 K on the intermediate transfer belt  8 . The gradation patterns in colors constitute of ten toner patches having different image densities and areas of 2 cm×2 cm. Charging potentials of the photoconductors  1 Y,  1 M,  1 C, and  1 K obtained when the gradation patterns Sk, Sm, Sc, and Sy in colors are formed are different from a uniform drum charging potential in a print process, and have values that are gradually increased. While a plurality of patch electrostatic latent images to form gradation pattern images by scanning of a laser beam are formed on the photoconductors  1 Y,  1 M,  1 C, and  1 K, respectively, the plurality of patch electrostatic latent images are developed with the development devices  5 Y,  5 M,  5 C, and  5 K for Y, M, C, and K. In this development, the values of development biases applied to the development rollers for Y, M, C, and K are gradually increased. With the development, Y, M, C, and K gradation patterns are formed on the photoconductors  1 Y,  1 M,  1 C, and  1 K, respectively. The gradation pattern images are primarily transferred to be arrayed at predetermined intervals in a main scanning direction of the intermediate transfer belt  8  indicated by arrow D 1  in  FIG. 2 . An amount of adhesion of toner of the toner patch in each of the gradation patterns is 0.1 mg/cm 2  at the minimum and 0.55 mg/cm 2  at the maximum. When toner charge-per-diameter (Q/d) distributions are measured, the polarities of the toners are almost equalized to the regular charging polarity. 
     The toner patterns (Sk, Sm, Sc, and Sy) formed on the intermediate transfer belt  8  pass through positions facing the optical sensors  91  with the endless movement of the intermediate transfer belt  8 . At this time, the optical sensors  91  receive amounts of light depending on amounts of adhesion of toner per unit area to the toner patches of the gradation patterns, respectively. 
     On the basis of output voltages from the optical sensors  91  when the toner patches are detected and an adhesion amount conversion algorithm, amounts of adhesion on the toner patches of the toner patterns in colors are calculated. On the basis of the calculated amounts of adhesion, image forming conditions are adjusted. More specifically, on the basis of the results of detection of the amounts of adhesion of toner on the toner patches and development potentials obtained when the toner patches are formed, a function (y=ax+b) representing the linear graph is calculated by regression analysis. Target values of the image densities are assigned to the function to calculate appropriate development biases, so that development biases for Y, M, C, and K are specified. 
     In the memory, an image formation condition data table is stored in which several tens of kinds of development biases and appropriate drum charging potentials respectively corresponding to the biases are associated with each other. With respect to the process units  6 Y,  6 M,  6 C, and  6 K, development biases that are closest to the specified development biases are selected from the image formation condition table to specify drum charging potentials associated with the selected development biases. 
     The printer is configured to also perform color shift correction processing when the printer is powered on or when a predetermined number of prints are completed. In the color shift correction processing, color shift detection images called chevron patches PV as shown in  FIG. 3  and constituted of Y, M, C, and K toner images are formed at each of one end and the other end of the intermediate transfer belt  8  in the width direction, respectively. The chevron patch PV, as shown in  FIG. 3 , is a line pattern group in which the Y, M, C, and K toner images are arrayed at predetermined pitches in a belt moving direction that is a sub-scanning direction (indicated by arrow D 2 ) while the toner images are tilted at about 45° relative to the main scanning direction (indicated by arrow D 1 ). An amount of adhesion of the chevron patch PV is about 0.3 mg/cm 2 . 
     The color toner images in the chevron patches PV formed at both the ends of the intermediate transfer belt  8  in the width direction are detected. In this manner, positions in the main scanning direction D 1  (axial direction of the photoconductor) in the color toner images, positions in the sub-scanning direction (belt moving direction) D 2 , magnification errors in the main scanning direction D 1 , and skew from the main scanning direction D 1  are detected. The main scanning direction D 1  mentioned here is a direction in which a laser beam is phase-shifted on the surface of the photoconductor with reflection on a polygon mirror. 
     Detection time differences between the Y, M, and C toner images in the chevron patch PV and the K toner image are read by the optical sensors  91 . In  FIG. 3 , the vertical direction corresponds to the main scanning direction D 1 , and the Y, M, C, and K toner images are sequentially arrayed from the left. Thereafter, K, C, M, and Y toner images having postures different from those of the above Y, M, C, and K toner images by 90° are further arrayed. 
     On the basis of differences between measured values and ideal values of detection time differences tky, tkm, and tkc obtained with reference to the K color serving as a reference color, deviations of the color toner images in the sub-scanning direction D 2 , i.e., registration deviations, are calculated. On the amount of registration deviations, every other surface of the polygon mirror in the optical writing unit constituting part of a toner image forming unit, i.e., using one scanning line pitch as one unit, an optical writing start timing for a photoconductor  1  is corrected to reduce the registration deviations of the color toner images. 
     On the basis of the differences of deviations in sub-scanning direction D 2  between both the ends of the belt, tilts (skews) of the color toner images from the main scanning direction D 1  are calculated. On the basis of the result, plate tilt correction of an optical reflecting mirror is performed to reduce skew deviations of the color toner images. Processing that corrects an optical writing start timing and a plate tilt on the basis of timings at which the toner images in the chevron patch PV are detected to reduce registration deviations and skew deviations is color shift correction processing. The color shift correction processing can suppress color shift of an image caused by shifting forming positions of the color toner images on the intermediate transfer belt  8  with time due to a change in temperature or the like. 
     When the image forming operation in a small image area continues, old toner continuously remaining in the development device for a long time increases. For this reason, the toner charging characteristics are deteriorated, and use of the toner in image formation deteriorates image quality (deterioration in development capability and transfer properties). The image forming device includes a refresh mode in which old toner is ejected into a non-image region of the photoconductor  1  at a predetermined timing not to be accumulated in the development device, and, after the toner is ejected, a new toner is supplied to the development device having a decreased toner concentration to refresh the interior of the development device. 
     A controller stores consumptions of toner of the development devices  5 Y,  5 M,  5 C, and  5 K and operation times of the development devices  5 Y,  5 M,  5 C, and  5 K therein in advance. At a predetermined timing, the controller examines whether a consumption of toner for an operation time in a predetermined period of each development device is a threshold value or less in each of the developments, and executes the refresh mode to a development device having a consumption of toner that is the threshold value or less. 
     When the refresh mode is executed, a toner consumption pattern A is formed in a non-image-forming region corresponding to a region between sheets of the photoconductor  1  and transferred to the intermediate transfer belt  8  ( FIG. 4 ). An amount of adhesion to the toner consumption pattern is determined on the basis of a consumption of toner for an operation time in a predetermined period of the development device, and a maximum amount of adhesion per unit area on the intermediate transfer belt  8  may be about 1.2 mg/cm 2 . When a toner Q/d distribution of the toner consumption pattern A transferred to the intermediate transfer belt  8  is measured, the charging property is almost equalized to the regular charging polarity. In this embodiment, the size of the toner consumption pattern is set to 25 mm×250 mm. 
     The color gradation patterns, the chevron patches, and the toner consumption pattern formed on the intermediate transfer belt  8  are collected by the belt cleaning device  100 . At this time, the belt cleaning device  100  removes a large amount of toner from the intermediate transfer belt  8 . However, a conventional type of cleaning device including a polarity controller and a brush roller or a conventional type of cleaning device including a brush roller for removing toner having a positive polarity and a brush roller for removing toner having a negative polarity may not remove a large amount of toner from the intermediate transfer belt  8 . More specifically, such a conventional type of cleaning device may not remove the non-transferred toner images such as the color gradation patterns, the chevron patches, and the toner consumption pattern in a lump. In such a case, the toner on the intermediate transfer belt  8  that cannot be completely cleaned is transferred onto a recording sheet in the next print operation, and an abnormal image may be formed. 
     Thus, the belt cleaning device  100  of the printer is configured such that non-transferred toner images such as the color gradation patterns, the chevron patches, and the toner consumption pattern can be removed in a lump. 
       FIG. 5  is an enlarged view of a configuration of the belt cleaning device  100  according to this embodiment. In  FIG. 5 , the belt cleaning device  100  includes a first sub cleaning unit  101  and a second sub cleaning unit  111  adjacent to and downstream of the first sub cleaning unit  101  in a moving direction of the intermediate transfer belt  8 . The belt cleaning device  100  also includes a third sub cleaning unit  121  adjacent to and downstream of the second sub cleaning unit  111  in the moving direction of the intermediate transfer belt  8 . 
     The first sub cleaning unit  101  includes a first cleaning brush roller  102  serving as a cleaner to scrape off post-transfer residual toner from the surface of the intermediate transfer belt  8 . The first sub cleaning unit  101  also includes a first collection roller  103  to collect post-transfer residual toner from the first cleaning brush roller  102  by rotating in contact with the first cleaning brush roller  102  and a first scraping blade  104  to scrape post-transfer residual toner from a surface of the first collection roller  103 . The first sub cleaning unit  101  further includes a first sub-unit casing  106  serving as a holder. The first sub-unit casing  106  includes a first transport screw  105  to discharge post-transfer residual toner scraped off from the first collection roller  103  to the outside of the first sub-unit casing  106 . 
     Similarly with the first sub cleaning unit  101 , the second sub cleaning unit  111  has the following configuration. For example, the second sub cleaning unit  111  includes a second cleaning brush roller  112  serving as a cleaning member, a second collection roller  113 , a second scraping blade  114 , a second transport screw  115 , and a second sub-unit casing  116 . Similarly with the first cleaning sub-unit  101 , the third cleaning sub-unit  121  has the following configuration. For example, the third cleaning sub-unit  112  includes a third cleaning brush roller  122  serving as a cleaning member, a third collection roller  123 , a third scraping blade  124 , a third transport screw  125 , and a third sub-unit casing  126 . 
     Each of the first cleaning sub-unit  101 , the second cleaning sub-unit  111 , and the third cleaning sub-unit  121  is removably mounted relative to a sub-unit holder casing  150  serving as a sub-unit holder of each belt cleaning device. 
     Most of toner particles constituting the post-transfer residual toner are charged to have a negative polarity that is a regular charging polarity. For the first cleaning sub-unit  101  at the most upstream side of the three cleaning sub-units, the first cleaning brush roller  102  is applied with a cleaning bias of a polarity (positive polarity) opposite the regular charging polarity of toner. Thus, post-transfer residual toner charged with the charging polarity (negative polarity) generated on the intermediate transfer belt  8  is electrostatically captured to a brush of the first cleaning brush roller  102 . The first collection roller  103  is applied with a collection bias of positive polarity of a value greater than the cleaning bias. For the belt cleaning device  100 , the cleaning bias is set so that about 90% of post-transfer residual toner generated on the surface of the intermediate transfer belt  8  is removed with the first cleaning brush roller  102 . 
     The post-transfer residual toner may slightly include toner charged with the polarity opposite the regular charging polarity. Hence, the second cleaning brush roller  112  of the second cleaning sub-unit  111  is applied with a cleaning bias of the same polarity as the regular charging polarity (negative polarity) of toner. Thus, of the post-transfer residual toner, toner particles charged with the opposite polarity are electrostatically captured to a brush of the second cleaning brush roller  112 . The second collection roller  113  is applied with a collection bias of negative polarity of a value greater than the cleaning bias. The second cleaning brush roller  112  acts as a polarity regulator to inject charges of negative polarity, which is the regular charging polarity of toner, to toner particles charged with the opposite polarity to return the toner particles charged with the opposite polarity to the regular polarity. 
     The third cleaning brush roller  122  of the third cleaning sub-unit  121  is applied with a cleaning bias of positive polarity opposite the regular charging polarity of toner. The third collection roller  123  is applied with a collection bias of positive polarity of a value greater than the cleaning bias. 
     The second sub-unit casing  116  of the second cleaning sub-unit  111  has a first insulation seal  171  at the vicinity of an opening for exposing the second cleaning brush roller  112  to the outside of the second sub-unit casing  116 , to prevent falling of toner particles charged with the opposite polarity. The first insulation seal  171  prevents occurrence of electric discharge between the first cleaning brush roller  102  and the second cleaning brush roller  112 . The first insulation seal  171  also prevents migration of toner particles charged with the opposite polarity from the second sub-unit casing  116  to the first sub-unit casing  106 . 
     The third sub-unit casing  126  of the third cleaning sub-unit  121  has a second insulation seal  172  at the vicinity of an opening for exposing the third cleaning brush roller  122  to the outside of the third sub-unit casing  126 , to prevent falling of post-transfer residual toner. The second insulation seal  172  prevents occurrence of electric discharge between the third cleaning brush roller  122  and the second cleaning brush roller  112 . The second insulation seal  172  also prevents migration of post-transfer residual toner (charged with the regular polarity) from the third sub-unit casing  126  to the second sub-unit casing  116 . 
     The third sub-unit casing  126  of the third cleaning sub-unit  121  has a third insulation seal  173  above an opening for exposing the third cleaning brush roller  122  to the outside of the third sub-unit casing  126 . The third insulation seal  173  prevents occurrence of electric discharge between the third cleaning brush roller  122  and the tension roller  16  (illustrated in  FIG. 1 ). 
     The cleaning brush rollers  102 ,  112 , and  122  include metal rotation shafts  102   a ,  112   a ,  122   a  and brushes  102   b ,  112   b ,  122   b , respectively. Each of the metal rotation shafts  102   a ,  112   a ,  122   a  is rotationally supported. Each of the brushes  102   b ,  112   b ,  122   b  has a plurality of raising fibers standing on the peripheral surface of the rotation shaft and has an external diameter φ of 15 mm to 16 mm. Each of the raising fibers has a two-layered core-sheath structure in which the inside of each of the raising fibers is made of a conductive material such as conductive carbon, and the surface portion of each of the rising fibers is made of an insulating material such as polyester. In this manner, the cores have substantially the same potential as that of the cleaning bias applied to the cleaning brush rollers  102 ,  112 , and  122 , and toner can be electrostatically attracted to the surfaces of the raising fibers. As a result, toner particles on the intermediate transfer belt  8  are electrostatically captured to the raising fibers of the cleaning brush rollers  102 ,  112 , and  122 . 
     In some embodiments, the raising fibers of the cleaning brush rollers  102 ,  112 , and  122  are formed of only conductive fibers, not the two-layered core-sheath structure. In some embodiments, fibers are planted to be slanted with respect to a normal direction of the rotation shaft  102   a ,  112   a ,  122   a . In some embodiments, the raising fibers of the first cleaning brush roller  102  and the third cleaning brush roller  122  applied with a cleaning bias of positive polarity have core-sheath structures, and the raising fibers of the second cleaning brush roller  112  is formed of only conductive fibers. When the raising fibers of the second cleaning brush roller  112  applied with a cleaning bias of negative polarity are formed of only conductive fibers, charge injection from the second cleaning brush roller  112  to toner is facilitated. In this manner, the second cleaning brush roller  112  can equalize the toner on the intermediate transfer belt  8  to the negative polarity. On the other hand, when the raising fibers of the first cleaning brush roller  102  and the third cleaning brush roller  122  have core-sheath structures, charge injection into the toner can be suppressed, thus suppressing positive charging of the toner on the intermediate transfer belt  8 . Such a configuration can suppress occurrence of toner not electrostatically removed with the first cleaning brush roller  102  and the third cleaning brush roller  122 . 
     The cleaning brush rollers  102 ,  112 , and  122  are arranged to dig into the intermediate transfer belt  8  by 1 mm. The cleaning brush rollers  102 ,  112 , and  122  are rotated by a driving unit such that the raising fibers move in a direction (counter direction) opposing the moving direction of the intermediate transfer belt  8  at a contact position. At the contact position, the cleaning brush rollers  102 ,  112 , and  122  are rotated to move the rising fibers in the counter direction so that a difference in linear velocity between the intermediate transfer belt  8  and each of the cleaning brush rollers  102 ,  112 , and  122  can be increased. In this manner, a probability of contact to the raising fibers increases in a period until a certain position of the intermediate transfer belt  8  passes through contact areas to the cleaning brush rollers  102 ,  112 , and  122 , and the toner can be preferably removed from the intermediate transfer belt  8 . 
     In this embodiment, each of the collection rollers  103 ,  113 , and  123  are made of stainless steel (SUS). In some embodiments, the collection rollers  103 ,  113 , and  123  are made of any other material capable of achieving the following function. The function is to transfer toner adhered to the cleaning brush rollers  102 ,  112 , and  122 , from the cleaning brush rollers  102 ,  112 , and  122  to the collection rollers  103 ,  113 , and  123  by potential gradient between the raising fibers and the collection rollers  103 ,  113 , and  123 . For example, in some embodiments, each of the collection rollers has a roller resistance log R of from 12Ω to 13Ω obtained by, e.g., covering a conductive metal core with a high-resistance elastic tube having a size of several micrometers to 100 micrometers or coating the conductive metal core with an insulator. Using the stainless steel (SUS) rollers as the collection rollers allows cost reduction or suppression of an application voltage to a low level, and electric power saving can be advantageously achieved. On the other hand, setting the roller resistance log R to be 12Ω to 13Ω allows suppression of electric charge injection into toner in collection of the toner in the collection rollers. As a result, the polarity of toner becomes the same as the polarity of the voltage applied to the collection rollers, thus suppressing a reduction in toner collection efficiency. 
     Conditions of the cleaning brush rollers  102 ,  112 , and  122  are as follows.
         Material of brush: conductive polyester (having a core-sheath structure in which the interior of fiber is made of conductive carbon and the surface of fiber is made of polyester)   Resistance of brush: 10E6Ω to 10E8Ω   Planting density of brush fibers: one hundred thousand per square inch   Diameter of brush fiber: about 25 μm to about 35 μm   Flattening of brush edges: None   Brush diameter φ: 14 mm to 22 mm   Depth of brush fibers digging into the intermediate transfer belt  8 : 1 mm       

     The cleaning bias supplied to the first cleaning brush roller  102  is set to such a value that a good cleaning performance can be obtained when a large amount of post-transfer residual toner arise on the intermediate transfer belt  8 . The cleaning bias supplied to the second cleaning brush roller  112  is set to a relatively high absolute value so as to inject charges to toner on the intermediate transfer belt  8 . Planting density of brush fibers, resistance of brush, fiber diameter, voltage applied, fiber type, and depth at which brush fibers dig into the intermediate transfer belt  8  can be optimized according to a system used, and are not limited to the above-described values. Examples of usable fiber type include nylon, acryl, polyester, and so on. 
     Conditions of the collection rollers  103 ,  113 , and  123  are as follows.
         Material of cored bar of collection roller: SUS 303   Depth of brush fibers digging into collection roller: 1.5 mm   Material of the collection rollers, the depth of brush fibers digging into each collection roller, voltage applied can be optimized for a system used, and are not limited to the above-described values.   Conditions of the scraping blades ( 104 ,  114 , and  124 ) are as follows.   Material of the scraping blades: SUS 304   Contact angle of blade: 20°   Thickness of blade: 0.1 mm   Depth of blade digging into collection roller: 1.0 mm       

     The contact angle of blade, the thickness of blade, and the depth of blade digging into collection roller can be optimized for a system used, and are not limited to the above-described values. 
     In  FIG. 5 , as the intermediate transfer belt  8  moves, post-transfer residual toner having passed a secondary transfer nip goes over a contact portion at which the intermediate transfer belt  8  contacts an entry seal  174 , and is conveyed to a position of the first cleaning brush roller  102 . The first cleaning brush roller  102  is applied with a cleaning bias of a polarity (positive polarity) opposite the regular charging polarity of toner. By action of an electric field formed by a difference in surface potential between the intermediate transfer belt  8  and the first cleaning brush roller  102 , toner charged with negative polarity on the intermediate transfer belt  8  are electrostatically adsorbed to the first cleaning brush roller  102 . Toner of negative polarity migrated to the first cleaning brush roller  102  is transported to a contact position at which the first cleaning brush roller  102  contacts the first collection roller  103  applied with a collection bias of positive polarity greater than that of the first cleaning brush roller  102 . By action of an electric field formed by a difference in surface potential between the first cleaning brush roller  102  and the first collection roller  103 , post-transfer residual toner in the brush of the first cleaning brush roller  102  is electrostatically transferred to the first collection roller  103 . Then, the transferred residual toner is scraped off from the surface of the first collection roller  103  with the first scraping blade  104  and transported to a toner storage part of the first cleaning sub-unit  101  with the first transport screw  105 . 
     Post-transfer residual toner on the intermediate transfer belt  8 , which has not been removed with the first cleaning brush roller  102 , includes, for example, toner of negative polarity or weakly charged toner of positive polarity. Such post-transfer residual toner is transported to a contact position at which the intermediate transfer belt  8  contacts the second cleaning brush roller  112 . The second cleaning brush roller  112  is applied with a voltage of the same polarity (negative polarity) as the regular charging polarity of toner. By charge injection or electric discharge, the polarity of toner on the intermediate transfer belt  8  is unified to negative polarity. At the same time, by action of an electric field formed by a difference in surface potential between the intermediate transfer belt  8  and the second cleaning brush roller  112 , toner charged with negative polarity on the intermediate transfer belt  8  is electrostatically transferred into the brush of the second cleaning brush roller  112 . Then, after electrostatically transferred to the second collection roller  113 , the transferred residual toner is scraped off from the second collection roller  113  with the second scraping blade  114  and transported to a toner storage part of the second cleaning sub-unit  111  with the second transport screw  115 . 
     Toner of negative polarity having not been removed with the second cleaning brush roller  112  is conveyed to a contact position at which the intermediate transfer belt  8  contacts the third cleaning brush roller  122 , with movement of the intermediate transfer belt  8 . The amount of residual toner of negative polarity conveyed as described above is quite small. Such residual toner is electrostatically transferred onto the third cleaning brush roller  122  and the third collection roller  123  in turn, and is scraped off from the third collection roller  123  with the third scraping blade  124 . Then, residual toner is transported to a toner storage part of the third cleaning sub-unit  121  with the third transport screw  125 . 
       FIG. 6  is an enlarged view of a configuration of the first cleaning sub-unit  101  of the cleaning device  100  and its circumference. The sub-unit holder casing  150  of the cleaning device  100  includes two supports, in this example, a first left support  151  and a first right support  152  to support the first cleaning sub-unit  101  so as to be detachably attachable relative to the sub-unit holder casing  150 . In a state in which the first cleaning sub-unit  101  is fully set in the sub-unit holder casing  150 , as illustrated in  FIG. 6 , the first sub-unit casing  106  floats up from the first left support  151  and the first right support  152 . By contrast, during an operation of sliding the first cleaning sub-unit  101  in a longitudinal direction relative to the sub-unit holder casing  150  for detachment or attachment, the first sub-unit casing  106  is placed on the first left support  151  or the first right support  152 . 
     The first left support  151  supports a left lateral end of the first sub-unit casing  106  of the first cleaning sub-unit  101  in a short direction of the first sub-unit casing  106 , from a lower side in a vertical direction. More specifically, the first left support  151  allows movement of the left lateral end of the first sub-unit casing  106  in the short direction while supporting the left lateral end of the first sub-unit casing  106  such that the first sub-unit casing  106  can slide and move in the longitudinal direction of the first sub-unit casing  106  (which is a direction perpendicular to a face of a paper sheet on which  FIG. 6  is printed). The first left support  151  has a receive portion to receive the left lateral end in a state of extending in the longitudinal direction. The receive portion allows the left lateral end to be supported so as to slide and move in the longitudinal direction. However, the first left support  151  has no portion to stop movement of the left lateral end in the short direction. Such a configuration allows movement of the left lateral end in the short direction. 
     The first right support  152  supports a right lateral end of the first sub-unit casing  106  of the first cleaning sub-unit  101  in a short direction of the first sub-unit casing  106 , from a lower side in a vertical direction. More specifically, the first right support  152  has a receive portion to receive the right lateral end of the first sub-unit casing  106  in a state of extending in the longitudinal direction of the first sub-unit casing  106  and support the left lateral end so as to be slidable in the longitudinal direction. Besides the receive portion, the first right support  152  has a left side wall located at a left side of the right lateral end of the first sub-unit casing  106  and a right side wall located at a right side of the right lateral end of the first sub-unit casing  106 . The right lateral end of the first sub-unit casing  106  is inserted between the left side wall and the right side wall of the first right support  152 . The distance between the left side wall and the right side wall is set to be larger than a length of the right lateral end of the first sub-unit casing  106  in the short direction. Accordingly, as illustrated in FIG.  6 , gaps G are formed between the right lateral end of the first sub-unit casing  106  and the left side wall of the first right support  152  and between the right lateral end of the first sub-unit casing  106  and the right side wall of the first right support  152 . The first right support  152  allows movement of the right lateral end of the first sub-unit casing  106  in the short direction within a total size of the gaps G. 
     In such a configuration, the size of the first sub-unit casing  106  serving as a holder in the short direction might become larger than the designed size within a movable range of the first sub-unit casing  106 . In such a case, the first sub-unit casing  106  can be inserted between the first left support  151  and the first right support  152  and supported with the first left support  151  and the first right support  152 . Thus, even if the first sub-unit casing  106  has a dimensional error or extends in the short direction, the first cleaning sub-unit  101  can be installed to the sub-unit holder casing  150 . 
       FIG. 7  is an enlarged view of a configuration of the second cleaning sub-unit  111  of the cleaning device  100  and its circumference. The sub-unit holder casing  150  of the cleaning device  100  includes two supports, in this example, a second left support  153  and a second right support  154  to support the second cleaning sub-unit  111  so as to be detachably attachable relative to the sub-unit holder casing  150 . In a state in which the second cleaning sub-unit  111  is fully set in the sub-unit holder casing  150 , as illustrated in  FIG. 7 , the second sub-unit casing  116  floats up from the second left support  153  and the second right support  154 . By contrast, during an operation of sliding the second cleaning sub-unit  111  in a longitudinal direction relative to the sub-unit holder casing  150  for detachment or attachment, the second sub-unit casing  116  is placed on the second left support  153  or the second right support  154 . 
     The second left support  153  supports a left lateral end of the second sub-unit casing  116  of the second cleaning sub-unit  111  in a short direction of the second sub-unit casing  116 , from a lower side in a vertical direction. The second left support  153  allows movement of the left lateral end of the second sub-unit casing  116  in the short direction while supporting the left lateral end of the second sub-unit casing  116  such that the second sub-unit casing  116  can slide and move in the longitudinal direction of the second sub-unit casing  116 . The second right support  154  supports a right lateral end, the other end of the second sub-unit casing  116  in the short direction of the second sub-unit casing  116 , from a lower side in the vertical direction. The second right support  154  has a receive portion to receive the right lateral end of the second sub-unit casing  116  in a state of extending in the longitudinal direction of the second sub-unit casing  116  and support the left lateral end so as to be slidable in the longitudinal direction. Besides the receive portion, the second right support  154  further has a left side wall and a right side wall. The right lateral end of the second sub-unit casing  116  is inserted between the left side wall and the right side wall of the second right support  154 . The distance between the left side wall and the right side wall is set to be larger than a length of the right lateral end of the first sub-unit casing  106  in the short direction. Accordingly, as illustrated in  FIG. 7 , gaps G are formed between the right lateral end of the second sub-unit casing  116  and the left side wall of the second right support  154  and between the right lateral end of the second sub-unit casing  116  and the right side wall of the second right support  154 . The second right support  154  allows movement of the right lateral end of the second sub-unit casing  116  in the short direction within a total size of the gaps G. 
     For such a configuration, even if the second sub-unit casing  116  has a dimensional error or extends in the short direction, the second cleaning sub-unit  111  can be installed to the sub-unit holder casing  150 . 
       FIG. 8  is an enlarged view of a configuration of the third cleaning sub-unit  121  of the cleaning device  100  and its circumference. The sub-unit holder casing  150  of the cleaning device  100  includes two supports, in this example, a third left support  155  and a third right support  156  to support the third cleaning sub-unit  121  so as to be detachably attachable relative to the sub-unit holder casing  150 . In a state in which the third cleaning sub-unit  121  is fully set in the sub-unit holder casing  150 , as illustrated in  FIG. 8 , the third sub-unit casing  126  floats up from the third left support  155  and the third right support  156 . By contrast, during an operation of sliding the third cleaning sub-unit  121  in a longitudinal direction relative to the sub-unit holder casing  150  for detachment or attachment, the third sub-unit casing  126  is placed on the third left support  155  or the third right support  156 . 
     The third left support  155  supports a left lateral end, one end of the third sub-unit casing  126  of the third cleaning sub-unit  121  in a short direction of the third sub-unit casing  126 , from a lower side in a vertical direction. The third left support  155  allows movement of the left lateral end of the third sub-unit casing  126  in the short direction while supporting the left lateral end of the third sub-unit casing  126  such that the third sub-unit casing  126  can slide and move in the longitudinal direction of the third sub-unit casing  126 . The third right support  156  supports a right lateral end, the other end of the third sub-unit casing  126  in the short direction of the third sub-unit casing  126 , from a lower side in the vertical direction. The third right support  156  has a receive portion to receive the right lateral end of the third sub-unit casing  126  in a state of extending in the longitudinal direction of the third sub-unit casing  126  and support the left lateral end so as to be slidable in the longitudinal direction. Besides the receive portion, the third right support  156  further has a left side wall and a right side wall. The right lateral end of the third sub-unit casing  126  is inserted between the left side wall and the right side wall of the third right support  156 . The distance between the left side wall and the right side wall is set to be larger than a length of the right lateral end of the third sub-unit casing  126  in the short direction. Accordingly, as illustrated in  FIG. 8 , gaps G are formed between the right lateral end of the third sub-unit casing  126  and the left side wall of the third right support  156  and between the right lateral end of the third sub-unit casing  126  and the right side wall of the third right support  156 . The third right support  156  allows movement of the right lateral end of the third sub-unit casing  126  in the short direction within a total size of the gaps G. 
     For such a configuration, even if the third sub-unit casing  126  has a dimensional error or extends in the short direction, the third cleaning sub-unit  121  can be installed to the sub-unit holder casing  150 . 
     The first cleaning sub-unit  101 , the second cleaning sub-unit  111 , and the third cleaning sub-unit  121  are pushed and installed into the sub-unit holder casing  150  while being moved to slide from a front side to a rear side in a direction perpendicular to a surface of a paper sheet on which  FIG. 1  is printed. By contrast, the first cleaning sub-unit  101 , the second cleaning sub-unit  111 , and the third cleaning sub-unit  121  are removed from the sub-unit holder casing  150  while being moved to slide from the rear side to the front side in the direction perpendicular to the surface of the paper sheet on which  FIG. 1  is printed. Thus, in installing or removing the cleaning sub-units, an operator inserts or extract the cleaning sub-units in front of the image forming apparatus illustrated in  FIG. 1 . Hereinafter, the term “front (side)” represents a forward side of the image forming apparatus illustrated in  FIG. 1  in the direction perpendicular to the surface of the paper sheet on which  FIG. 1  is printed. The term “back (side)” represents a rearward side of the image forming apparatus illustrated in  FIG. 1  in the direction. 
       FIG. 9  is a back view of a back side plate  160  of the sub-unit holder casing  150 . In  FIG. 9 , the back side plate  160  is shown from the back side of the sub-unit holder casing  150 . The back side plate  160  includes a first storage receiving hole  162   a , a first main-positioning-pin receiving hole  163   a , a first sub-positioning-pin receiving hole  164   a , and a first collection-bias-pin receiving hole  166   a  which correspond to the first cleaning sub-unit. The back side plate  160  further includes a first cleaning-bias-pin receiving hole  165   a . The back side plate  160  includes a second storage receiving hole  162   b , a second main-positioning-pin receiving hole  163   b , a second sub-positioning-pin receiving hole  164   b , a second collection-bias-pin receiving hole  166   b , and a second cleaning-bias-pin receiving hole  165   b  which correspond to the second cleaning sub-unit. The back side plate  160  includes a third storage receiving hole  162   c , a third main-positioning-pin receiving hole  163   c , a third sub-positioning-pin receiving hole  164   c , a third collection-bias-pin receiving hole  166   c , and a third cleaning-bias-pin receiving hole  165   c  which correspond to the third cleaning sub-unit. The back side plate  160  further includes a drive-shaft receiving hole  161 . 
       FIG. 10  is a back view of an upper portion of the back side plate  160  and the third cleaning sub-unit  121 . In  FIG. 10 , a drive shaft  130  of the belt cleaning device passes through the drive-shaft receiving hole  161  of the back side plate  160 . Accordingly, a rear end of the drive shaft  130  protrudes backward beyond the back side plate  160 . Such a configuration allows a coupler of the rear end to be coupled with a coupler of an end of a drive shaft disposed at the apparatus body. 
     A circular toner storage  129  of the third cleaning sub-unit  121  passes through the third storage receiving hole  162   c  of the back side plate  160 . The toner storage  129  protrudes in a cylindrical shape beyond the back side plate of the third sub-unit casing of the third cleaning sub-unit  121 . A clearance is disposed between an inner wall of the third storage receiving hole  162   c  and the toner storage  129 . Passing of the toner storage  129  through the third storage receiving hole  162   c  of the back side plate  160  allows the following operation. For example, by rotating the toner storage  129  screwed on the back side plate in a state in which the belt cleaning device  100  is removed from the apparatus body, the toner storage  129  can be removed from the back side plate of the third sub-unit casing. Then, interior waste toner can be discarded. 
     A third cleaning-bias pin  127   a  of the third cleaning sub-unit  121  passes through the third cleaning-bias-pin receiving hole  165   c  of the back side plate  160 . The third cleaning-bias pin  127   a  protrudes backward beyond the back side plate of the third sub-unit casing and conducts the third cleaning brush roller  122  via a metal bearing. A clearance is disposed between an inner wall of the third cleaning-bias-pin receiving hole  165   c  and the third cleaning-bias pin  127   a . Passing the rear end of the third cleaning-bias pin  127   a  through the third cleaning-bias-pin receiving hole  165   c  allows the following operation. For example, as illustrated in  FIG. 11 , a joint cap  191  disposed at the apparatus body is jointed to the rear end of the third cleaning-bias pin  127   a . A cleaning bias is supplied from the joint cap  191  to the third cleaning brush roller  122  via the third cleaning-bias pin  127   a  and the metal bearing. 
     In  FIG. 10 , a third cleaning-bias pin  127   b  of the third cleaning sub-unit  121  passes through the third collection-bias-pin receiving hole  166   c  of the back side plate  160 . The third cleaning-bias pin  127   b  protrudes backward beyond the back side plate of the third sub-unit casing and conducts the third collection roller  123  via a metal bearing. A clearance is disposed between an inner wall of the third collection-bias-pin receiving hole  166   c  and the third cleaning-bias pin  127   b . Passing a rear end of the third cleaning-bias pin  127   b  through the third collection-bias-pin receiving hole  166   c  allows the following operation. For example, as illustrated in  FIG. 11 , a joint cap  191  disposed at the apparatus body is jointed to the rear end of the third cleaning-bias pin  127   b . A cleaning bias is supplied from the joint cap  191  to the third the collection roller  123  via the third cleaning-bias pin  127   b  and the metal bearing. 
     In  FIG. 10 , a third back main-positioning pin  128   a  of the third cleaning sub-unit  121  is fitted in the third main-positioning-pin receiving hole  163   c  of the back side plate  160 . The third back main-positioning pin  128   a  protrudes beyond the back side plate of the third sub-unit casing of the third cleaning sub-unit  121 . No clearance is disposed between an inner wall of the third main-positioning-pin receiving hole  163   c  and the third back main-positioning pin  128   a . The inner wall of the third main-positioning-pin receiving hole  163   c  and the third back main-positioning pin  128   a  closely contact with each other. Thus, the rear end of the third cleaning sub-unit  121  is positioned. 
     A third back sub-positioning pin  128   b  of the third cleaning sub-unit  121  is fitted in the third sub-positioning-pin receiving hole  164   c  of the back side plate  160 . The third back sub-positioning pin  128   b  protrudes beyond the back side plate of the third sub-unit casing of the third cleaning sub-unit  121 . Of four inner walls, i.e., upper, lower, left, and right inner walls of the third sub-positioning-pin receiving hole  164   c , no clearance is disposed between the third back sub-positioning pin  128   b  and each of the upper and lower inner walls of the third sub-positioning-pin receiving hole  164   c . By contrast, a clearance is disposed between the third back sub-positioning pin  128   b  and each of the left and right inner walls of the third sub-positioning-pin receiving hole  164   c.    
     As illustrated in  FIG. 12 , the third back main-positioning pin  128   a  of the third cleaning sub-unit  121  is tapered. When the third cleaning sub-unit  121  is pushed into the sub-unit holder casing from the front side to the back side of the image forming apparatus, a leading edge of the third back main-positioning pin  128   a  enters the third main-positioning-pin receiving hole  163   c  even if the position of the third cleaning sub-unit  121  is slightly deviated from the sub-unit holder casing. Then, as the third cleaning sub-unit  121  is further pushed into the sub-unit holder casing, a bottom portion of the third back main-positioning pin  128   a  slides against the inner wall of the third main-positioning-pin receiving hole  163   c . The bottom portion of the third back main-positioning pin  128   a  closely contacts the entire inner wall of the third main-positioning-pin receiving hole  163   c.    
     As illustrated in  FIG. 13 , the third back sub-positioning pin  128   b  of the third cleaning sub-unit  121  is tapered. When the third cleaning sub-unit  121  is pushed into the sub-unit holder casing from the front side to the back side of the image forming apparatus, a leading edge of the third back sub-positioning pin  128   b  enters the third sub-positioning-pin receiving hole  164   c  even if the position of the third cleaning sub-unit  121  is slightly deviated from the sub-unit holder casing. Then, as the third cleaning sub-unit  121  is further pushed into the sub-unit holder casing, a bottom portion of the third back sub-positioning pin  128   b  slides against the inner wall of the third sub-positioning-pin receiving hole  164   c . Then, the bottom portion of the third back sub-positioning pin  128   b  closely contacts the entire inner wall of the third sub-positioning-pin receiving hole  164   c.    
     The positioning of a rear end of the third sub-unit casing  126  in the longitudinal direction is completed at the following timing. That is, when the bottom portion of the third back main-positioning pin  128   a  closely contacts the entire inner wall of the third main-positioning-pin receiving hole  163   c  and the bottom portion of the third back sub-positioning pin  128   b  closely contacts the entire inner wall of the third sub-positioning-pin receiving hole  164   c , the rear end of the third sub-unit casing  126  is positioned in the longitudinal direction. A bottom of the rear end of the third sub-unit casing  126  thus positioned floats up from the third left support  155  or the third right support  156 . 
     In  FIG. 10 , the third back main-positioning pin  128   a  is placed near the third cleaning-bias pin  127   a  to which a high-voltage cleaning bias is applied. Accordingly, if the third back main-positioning pin  128   a  is made of metal, electric discharge might occur between the third cleaning-bias pin  127   a  and the third back main-positioning pin  128   a . Hence, for the image forming apparatus according to this embodiment, the third back main-positioning pin  128   a  is made of insulating resin material. By contrast, the third back sub-positioning pin  128   b  is placed at a position relatively away from the third cleaning-bias pin  127   a  or the third cleaning-bias pin  127   b  to which a high-voltage collection bias is applied. Accordingly, electric discharge does not occur between the third back sub-positioning pin  128   b  and the third cleaning-bias pin  127   a  or the third cleaning-bias pin  127   b . Hence, in this embodiment, the third back sub-positioning pin  128   b  is made of metal. 
     In the above description, the relation between the components at the rear end of the third cleaning sub-unit  121  and the holes of the back side plate  160  is described. The same goes for the relation between components at the rear end of the first cleaning sub-unit  101  and corresponding holes of the back side plate  160 . The same also goes for the relation between components at the rear end of the second cleaning sub-unit  111  and corresponding holes of the back side plate  160 . 
       FIG. 14  is a perspective view of a belt drive assembly of the transfer unit  7  and a portion of the third cleaning sub-unit  121  according to an embodiment of this disclosure. In  FIG. 14 , the belt drive assembly includes an output coupling  191 , a drive output shaft  192 , a drive transmitter  193 , and a drive motor  194 . The drive transmitter  193  includes, e.g., a timing belt. The belt drive assembly is mounted on a unit back side plate of the transfer unit  7 . When the drive motor  194  starts rotating, a rotation driving force is transmitted to the drive output shaft  192  via the drive transmitter  193 , thus rotating the drive output shaft  192 . The output coupling  191  is fixed at a leading edge of the drive output shaft  192 . 
     In the belt cleaning device, as illustrated in  FIG. 10 , the drive shaft  130  passes through the drive-shaft receiving hole  161  of the back side plate  160  of the sub-unit holder casing  150 . The drive shaft  130  passes through a drive-shaft receiving hole of a front side plate of the sub-unit holder casing and the drive-shaft receiving hole  161  of the back side plate  160  via an interior of the sub-unit holder casing. The rear end of the drive shaft  130  protrudes backward beyond the back side plate  160 .  FIG. 14  shows the rear end of the drive shaft  130  thus protruding backward. As illustrated in  FIG. 14 , an input coupling (coupler)  131  is secured on the rear end, which is coupled with the output coupling  191  of the belt drive assembly. Thus, the rotation drive of the drive output shaft  192  is transmitted to the drive shaft  130  to rotate the drive shaft  130 . 
       FIG. 15  is a partial, enlarged perspective view of a front end of the belt cleaning device  100 . A front side plate  140  of the belt cleaning device  100  rotatably supports the front ends of the cleaning sub-units  191 ,  111 , and  121  and the front end of the drive shaft  130 . 
     A first sub-unit faceplate  141   a  is fixed at the front side of the front side plate  140  with a bolt. Each of the first sub-unit faceplate  141   a  and the front side plate  140  includes a first main-positioning-pin receiving hole. The first main-positioning-pin receiving hole of the first sub-unit faceplate  141   a  and the front side plate  140  are communicated with each other. Each of the first sub-unit faceplate  141   a  and the front side plate  140  includes a first sub-positioning-pin receiving hole. The first sub-positioning-pin receiving hole of the first sub-unit faceplate  141   a  and the front side plate  140  are communicated with each other. 
     A first front main-positioning pin  108   c  and a first front sub-positioning pin  108   d  protrudes from the front side plate of the sub-unit casing of the first cleaning sub-unit  101 . When the first sub-unit faceplate  141   a  is secured to the front side plate  140  of the sub-unit holder casing  150 , the front end of the first cleaning sub-unit  101  is positioned as follow. In other words, the first front sub-positioning pin  108   d  of the first cleaning sub-unit  101  fits in the first sub-positioning-pin receiving holes of the front side plate  140  and the first sub-unit faceplate  141   a . Simultaneously, the first front main-positioning pin  108   c  of the first cleaning sub-unit  101  fits in the first main-positioning-pin receiving holes of the front side plate  140  and the first sub-unit faceplate  141   a . Thus, positioning is performed. 
     In this positioning, a floor of the front end of the sub-unit casing of the first cleaning sub-unit  101  floats up from the first left support ( 151  in  FIG. 6 ) or the first right support ( 152  in  FIG. 6 ). Ahead of this, a floor of the rear end of the sub-unit casing of the first cleaning sub-unit  101  already floats up from the first left support ( 151  in  FIG. 6 ) or the first right support ( 152  in  FIG. 6 ) with positioning of the rear end. When the first sub-unit faceplate  141   a  is secured to the front side plate  140  to position the front end of the first cleaning sub-unit  101 , the entire floor of the sub-unit casing of the first cleaning sub-unit  101  floats up from the first left support or the first right support. As described above, by floating the entire floor of the sub-unit casing up with positioning, the sub-unit casing can be securely positioned regardless of dimensional error, extension, or deformation of the sub-unit casing. By contrast, if the sub-unit casing is positioned with the floor of the sub-unit casing attached, the orientation of the sub-unit casing might be not be secured because of dimensional error, extension, or deformation of the sub-unit casing, thus hampering positioning of the sub-unit casing. 
     The first sub-unit faceplate  141   a  supports a first drive transmitter  146   a  including, e.g., a first collection gear  142   a , a first screw gear  143   a , a first cleaning pulley gear  144   a , and a first relay pulley gear  145   a . Each of the first collection gear  142   a , the first screw gear  143   a , the first cleaning pulley gear  144   a , the first relay pulley gear  145   a  is rotatably supported with the first sub-unit faceplate  141   a.    
     The first collection gear  142   a  is coupled with a shaft of the first collection roller  103  with a coupling at a back side of the first sub-unit faceplate  141   a . The first screw gear  143   a  is coupled with a shaft of the first transport screw  105  with a coupling at the back side of the first sub-unit faceplate  141   a . The first cleaning pulley gear  144   a  is coupled with a member of the first cleaning brush roller  102  with a coupling at the back side of the first sub-unit faceplate  141   a.    
     A gear portion of the first relay pulley gear  145   a  engages the first collection gear  142   a . Thus, the rotation drive of the first relay pulley gear  145   a  is transmitted to the first collection gear  142   a  to rotate the first collection roller  103  inside the first cleaning sub-unit  101 . As the first collection gear  142   a  is rotated, the first screw gear  143   a  engaging the first collection gear  142   a  rotates, thus rotating the first transport screw  105  inside the first cleaning sub-unit  101 . 
     A timing belt is stretched over the first relay pulley gear  145   a  and the first cleaning pulley gear  144   a . Thus, the rotation drive of the first cleaning pulley gear  144   a  is transmitted to the first relay pulley gear  145   a . As the first cleaning pulley gear  144   a  rotates, the first cleaning brush roller  102  is rotated inside the first cleaning sub-unit  101 . 
     A drive receive coupling  181   a  serving as a joint coupler is secured at the front edge of the first cleaning pulley gear  144   a  so as to rotate coaxially with the first cleaning pulley gear  144   a . A drive receive coupling  181   b  serving as a joint coupler is secured at a front edge of a second cleaning pulley gear  144   b  so as to rotate coaxially with the second cleaning pulley gear  144   b . A drive receive coupling  181   c  serving as a joint coupler is secured at a front edge of a third cleaning pulley gear  144   c  so as to rotate coaxially with the third cleaning pulley gear  144   c.    
     In the above description, the positioning of the front end of the first cleaning sub-unit  101  in the longitudinal direction is described. In the same manner, the front end of the second cleaning sub-unit  111  or the third cleaning sub-unit  121  is positioned. In the above description, the first drive transmitter  146   a  of the first cleaning sub-unit  101  is described. Likewise, a second drive transmitter  146   b  of the second cleaning sub-unit  111  or a third drive transmitter  146   c  of the third cleaning sub-unit  121  has a similar configuration to that of the first drive transmitter  146   a  of the first cleaning sub-unit  101 . 
     As illustrated in  FIG. 14 , the rotation drive transmitted to the drive shaft  130  at the rear side of the belt cleaning device is transmitted to the front side of the belt cleaning device  100  (see  FIG. 15 ) via the drive shaft  130 . For example, the front end of the drive shaft  130  protrudes beyond the front side plate  140  through a through-hole of the front side plate  140  of the sub-unit holder casing. An output coupling  182  is secured to the front end of the drive shaft  130 . The output coupling  182  is coupled with a coupling of a transmission assembly. 
     In this printer, a drive transmission system of the belt cleaning device  100  is centered on the front side of the front side plate  140 . By contrast, joint caps  191  to apply cleaning bias and collection bias are centered on the rear side of the back side plate  160 . Thus, a layout of preventing joint caps or bias pins applied with high voltage from approaching to the rotation metal shaft of drive transmission system is achieved, thus preventing electric discharge between the rotation metal shaft and the joint caps or bias pins. 
     Next, a configuration of an image forming apparatus according to an embodiment of this disclosure is described below.  FIG. 16  is a perspective view of a part of the belt cleaning device  100  with a transmission assembly  185  mounted on the front side plate  140 , seen from the front side of the belt cleaning device  100 .  FIG. 17  is a perspective view of a part of the belt cleaning device  100  with the transmission assembly  185  mounted on the front side plate  140 , seen from the front side of the belt cleaning device  100  and a different angle from that of  FIG. 16 . A first positioning pin  140   a  and a second positioning ping  140   b  protrude from the front side plate  140 . By contrast, a casing  186  of the transmission assembly  185  has a first positioning hole and a second positioning hole. The first positioning pin  140   a  of the front side plate  140  fits in the first positioning hole of the casing  186  of the transmission assembly  185 . The second positioning ping  140   b  of the front side plate  140  fits in the second positioning hole of the casing  186  of the transmission assembly  185 . Thus, the transmission assembly  185  is positioned relative to a body of the belt cleaning device. The transmission assembly  185  is fixed on the front side plate  140  with three bolts  183   a ,  183   b , and  183   c  in such a positioned state. 
       FIG. 18  is a perspective view of the transmission assembly  185  seen from a back face side thereof. A back face of a casing of the transmission assembly  185  rotatably supports each of a first relay gear  187  and a second relay gear  188 . An input coupling  187   a  is molded with the first relay gear  187  as a single component. When the transmission assembly  185  is mounted on the front side plate  140 , the input coupling  187   a  is coupled with the output coupling ( 182  in  FIG. 15 ) of the edge of the drive shaft ( 130  in  FIG. 15 ) which is disposed at the body of the belt cleaning device  100 . The rotation drive of the drive output shaft ( 192  in  FIG. 14 ) of the belt drive assembly disposed at the transfer unit  7  is transmitted to the first relay gear  187  of the transmission assembly  185  via the drive shaft ( 130  in  FIG. 14 ). 
     A belt is stretched over and around the first relay gear  187  and the first relay gear  187 . The rotation drive of the first relay gear  187  is transmitted to the second relay gear  188 . A rotary shaft of the second relay gear  188  engages one gear of a gear train including multiple gears within the casing  186  of the transmission assembly  185 . The gears of the gear train include, e.g., a first output gear  189   a , a second output gear  189   b , and a third output gear  189   c . The rotation drive of the second relay gear  188  is also transmitted to the first output gear  189   a , the second output gear  189   b , and the third output gear  189   c . Each of the first output gear  189   a , the second output gear  189   b , and the third output gear  189   c  protrudes beyond the back face of the casing  186  via a through-hole of the casing  186 . A drive output coupling  190   a  serving as a joint coupler is secured to a protruding portion of the first output gear  189   a  protruding beyond the casing  186  so as to rotate coaxially with the first output gear  189   a . A drive output coupling  190   b  serving as a joint coupler is secured to a protruding portion of the second output gear  189   b  protruding beyond the casing  186  so as to rotate coaxially with the second output gear  189   b . A drive output coupling  190   c  serving as a joint coupler is secured to a protruding portion of the third output gear  189   c  protruding beyond the casing  186  so as to rotate coaxially with the third output gear  189   c.    
     When the transmission assembly  185  is mounted on the front side plate  140  of the belt cleaning device  100 , the drive output coupling  190   a  of the first output gear  189   a  of the transmission assembly  185  is coaxially coupled with the drive receive coupling ( 181   a  in  FIG. 15 ) of the first cleaning sub-unit  101 . The drive output coupling  190   b  of the second output gear  189   b  of the transmission assembly  185  is coaxially coupled with the drive receive coupling ( 181   b  in  FIG. 15 ) of the second cleaning sub-unit  111 . The drive output coupling  190   c  of the third output gear  189   c  of the transmission assembly  185  is coaxially coupled with the drive receive coupling ( 181   c  in  FIG. 15 ) of the third cleaning sub-unit  121 . Thus, the driving force is transmitted to each cleaning sub-unit. 
     In  FIG. 16 , in removing the first cleaning sub-unit  101 , the second cleaning sub-unit  111 , and the third cleaning sub-unit  121 , an operator performs the following operation. First, the operator unbolts the three bolts  183   a ,  183   b , and  183   c  to remove the transmission assembly  185  from the front side plate. Then, for example, for the first cleaning sub-unit  101 , the operator loosens the bolt of the first sub-unit faceplate  141   a  to remove the first sub-unit faceplate  141   a  from the front side plate  140 . Thus, a sub-unit removal opening covered with the first sub-unit faceplate  141   a  is widely opened. Next, the operator puts his/her hand into the sub-unit removal opening of the front side plate  140  and pulls out the first cleaning sub-unit  101  from the sub-unit holder casing  150 . In installing the first cleaning sub-unit  101  into the sub-unit holder casing, the operator performs the pull-out steps in the opposite order. In the same manner, the second cleaning sub-unit  111  or the third cleaning sub-unit  121  can be installed into and removed from the sub-unit holder casing. 
     In such install and removal operations, the operator is not forced to remove the belt stretched around and between the two cleaning sub-units. Accordingly, when a plurality of cleaning sub-units is installed to or removed from the sub-unit holder casing  150 , the operator is not forced to attach or remove multiple belts, and it is enough to attach or remove the transmission assembly  185  relative to the sub-unit holder casing  150  only once. Such a configuration enhances work performance of the operator in install and removal of the plurality of cleaning sub-units relative to the sub-unit holder casing  150 . 
     In this embodiment, each of the drive output coupling  190   a  to  190   c  serving as joint couplers and the drive receive couplings  181   a  to  181   c  serving as joint couplers are is made of insulating resin. Such a configuration prevents cleaning bias applied to the cleaning brush rollers  102 ,  112 , and  122  from leaking to the outside via the couplings. 
     For the image forming apparatus, the transmission assembly  185  is installed or removed relative to the front side plate  140  located at an edge in a direction in which the cleaning sub-units  191 ,  111 , and  121  are pulled out from the sub-unit holder casing  150 . Such a configuration allows installation and removal of the transmission assembly  185  relative to the sub-unit holder casing  150  without removing the entire belt cleaning device  100  from the apparatus body of the image forming apparatus. Accordingly, the cleaning sub-units can be installed into and removed from the sub-unit holder casing  150  without pulling out the entire belt cleaning device  100  from the apparatus body, thus enhancing the operation performance in installation and removal. 
     As described above, in  FIG. 18 , after the first relay gear  187  receives an external rotation drive, the first relay gear  187  transmits the rotation drive to the first output gear  189   a , the second output gear  189   b , and the third output gear  189   c  via the second relay gear  188 . The casing  186  houses a group of gears to transmit the rotation drive of the second relay gear  188  to each of the first output gear  189   a , the second output gear  189   b , and the third output gear  189   c . As described above, the transmission assembly  185  accommodates the group of main gears within the casing  186 . 
     By contrast, as illustrated in  FIG. 15 , the body of the belt cleaning device  100  exposes the first drive transmitter  146   a , the second drive transmitter  146   b , and the third drive transmitter  146   c , which are supported with the front side plate  140 , to the outside. As illustrated in  FIG. 18 , the transmission assembly  185  extends three legs  186   a  from a base of the casing  186  so as to be fixed at the front side plate  140  away from the first drive transmitter  146   a , the second drive transmitter  146   b , and the third drive transmitter  146   c . The legs  186   a  is formed by sheet metal processing. Due to a relatively small thickness, when the legs  186   a  are removed from the front side plate  140 , the legs  186   a  are likely to deform. If the legs  186   a  deform, the accuracy may decrease in positioning of the transmission assembly  185  relative to the first drive transmitter  146   a , the second drive transmitter  146   b , and the third drive transmitter  146   c  disposed at the body of the belt cleaning device  100 . Such a reduced accuracy might hamper mounting of the transmission assembly  185  to the front side plate  140 . For example, gears of the drive receive couplings ( 181   a  to  181   c  in  FIG. 15 ) of the body of the belt cleaning device  100  might hit against gears of the drive output couplings ( 190   a  to  190   c  in  FIG. 18 ) in the rotation axis direction. As a result, a gear teeth of one of the drive receive couplings and the drive output couplings cannot insert to spaces between gear teeth of the other of the drive receive couplings and the drive output couplings, thus preventing two couplings from engaging with each other in the rotation axial direction. Consequently, the transmission assembly  185  might not be mounted on the front side plate  140 . 
     Next, a configuration of an image forming apparatus according to another embodiment of this disclosure is described below. Unless specifically described, the configuration of the image forming apparatus in this embodiment is similar to, if not the same as, that of the image forming apparatus according to the above-described embodiment.  FIG. 21  is a partial perspective view of a part of a belt cleaning device  100  in a state in which a transmission assembly  185  is removed in the image forming apparatus according to this embodiment, seen from a side at which a front side plate  140  is disposed. A front face of the front side plate  140  has three studs  195  serving as supports protruding in a direction in which the cleaning sub-unit is pulled out. Each of the stud  195  includes a cylindrical, hollow stud body  195   a , a female thread, and a bolt  195   b . The female thread is disposed in a hollow space of the stud body  195   a . The bolt  195   b  is screwed together with the bolt  195   b.    
       FIG. 22  is a perspective view of the transmission assembly  185  in a state of being mounted on a body of the belt cleaning device  100 . A front face plate of the transmission assembly  185  has three holes corresponding to the three studs  195 . From an outside of the front face plate of the transmission assembly  185 , the bolts  195   b  of the three studs  195  are screwed together with the female threads of the three studs  195 , which are disposed at a back side of the front face plate in  FIG. 22 . Thus, the transmission assembly  185  is fixed at the front side plate  140  of the body of the belt cleaning device  100  with the transmission assembly  185  supported with the three studs  195 . 
     The three studs  195  have a sturdy structure and is unlikely to deform. As compared with a configuration in which the transmission assembly  185  are supported with legs (e.g., the legs  186   a ), such a configuration suppresses a reduction in positioning accuracy between the transmission assembly  185  and the three drive transmitter  146   a  to  146   c  disposed at the body of the belt cleaning device  100 . Thus, operation performance in mounting the transmission assembly  185  can be enhanced. 
     The length of the studs  195  is preferably set as follow. For example, the length of each of the studs  195  is preferably set to not less than a total of a distance from the front face (edge) of the front side plate  140  a leading edge of the drive receive coupling ( 181   a  to  181   c ) and a distance from the back face of the transmission assembly  185  to the drive output coupling ( 190   a  to  190   c ). It is to be noted that a length slightly value than the total may be set as a minimum length. For example, the minimum length is a value obtained by subtracting, from the total, a depth of a recess of one of the drive receive coupling ( 181   a  to  181   c ) and the drive output coupling ( 190   a  to  190   c ) that receives the other. Accordingly, in the image forming apparatus in this embodiment, the length of each of the studs  195  is set to be greater than the value. 
     In consideration of dimensional errors of components, couplings are preferably configured to transmit drive even if the couplings are slightly shifted in the rotation axial direction. For the image forming apparatus according to this embodiment, for example, the length of the stud  195  is set to be greater than the above-described value by 0.4 mm. 
     The studs  195  are preferably three or more. The three or more studs  195  allow the transmission assembly  185  to be supported on a plane constituted of leading edges of the three or more studs  195 . For the image forming apparatus according to this embodiment, as illustrated in  FIG. 21 , one stud  195  is disposed on a right side of the front side plate  140 , another stud  195  is disposed on an upper left side of the front side plate  140 , and another is disposed on a lower left side of the front side plate  140 . To stably support the transmission assembly  185 , at least one of the studs  195  is preferably disposed around a center of the three cleaning brush rollers arranged at the body of the belt cleaning device  100 . 
     The studs  195  preferably have a certain degree or more of strength to stably support the transmission assembly  185  against a force generated by the driving. Therefore, the studs  195  are preferably a cylindrical stud including metal, such as iron, aluminum, and/or stainless steel, having a diameter of 5 mm or more. For the image forming apparatus according to this embodiment, the studs  195  has a cylindrical shape of a diameter of 5 mm or more and is made of free-cutting steel. A deformation amount at which the stud  195  deforms when applied with a load of 6 [N] is 0.1 mm or less. 
     The image forming apparatus according to this embodiment employs the studs  195  made of conductive material and applies a high voltage of X [kv] to the cleaning brush roller. The studs  195  are disposed at positions away from the cleaning brush roller by 2×X mm to prevent occurrence of electric discharge between the cleaning brush roller and each of the studs  195 . 
     The above description is just an example, and an image forming apparatus according to embodiments of this disclosure can exert, for example, an effect in each of the following aspects. 
     [Aspect A] 
     A unit device is detachably attachable relative to a machine body. The unit device includes a plurality of sub units each including a driver to be driven by a driving force received and a sub-unit holder to hold the plurality of sub-units. The unit device further includes a detachable transmission assembly. The transmission assembly includes a driving-force receive rotator to receive a rotation driving force transmitted from an external unit and a plurality of drive transmission rotators to transmit the rotation driving force received by the driving-force receive rotator to a plurality of drive receive rotators of the plurality of sub units. Each of the plurality of the drive receive rotators and the plurality of drive transmission rotators has a joint coupler to couple each of the plurality of drive receive rotators with a corresponding one of the plurality of drive transmission rotators. 
     For such a configuration, first, an operator removes the transmission assembly from the sub-unit holder ahead of removal of the plurality of sub units from the sub-unit holder. As a result, joint couplers of the drive receive rotators of the plurality of sub units are decoupled from joint couplers of the drive transmission rotators disposed at the transmission assembly so as to correspond to the drive receive rotators. Thus, the sub units become separately removable from the sub-unit holder, and the operator can remove a desired one of the sub units from the sub-unit holder. To install the sub unit to the sub-unit holder, the operator the above-described steps in the opposite order. In such install and removal operations, the operator is not forced to attach or remove the belt stretched around and between the two sub cleaning units. Accordingly, when a plurality of cleaning sub-units is installed to or removed from the sub-unit holder, the operator is not forced to attach or remove multiple belts, and it is enough to attach or remove the transmission assembly relative to the sub-unit holder only once. Such a configuration enhances work performance of the operator in install and removal of the plurality of sub-units relative to the sub-unit holder. 
     [Aspect B] 
     According to Aspect B, a cleaning device (e.g., the belt cleaning device  100 ) includes a cleaner (e.g., cleaning brush rollers  102 ,  112 ,  122 ) to scrape off adhered material from a surface of a cleaning target (e.g., intermediate transfer belt  8 ); a plurality of cleaning sub-units (e.g., cleaning sub-units  101 ,  111 ,  121 ) each including a holder (e.g., sub-unit casings  106 ,  116 ,  126 ) to hold the cleaner and a drive-receive rotator to receive a driving force; a sub-unit holder (e.g., sub-unit holder casing  150 ) holding the plurality of cleaning sub-units; a detachable transmission assembly (e.g., transmission assembly  185 ) including a driving-force receive rotator (e.g., first relay gear  187 ) to receive a driving force from an external unit and a plurality of drive transmission rotators (e.g., output gears  189   a  to  189   c ) to transmit the driving force received by the driving-force receive rotator to the drive-receive rotators of the plurality of cleaning sub-units; and a plurality of joint couplers (e.g., drive output couplings  190   a  to  190   c  and drive receive couplings  181   a  to  181   c ) mounted on the drive-receive rotators and the plurality of drive transmission rotators to couple the drive-receive rotators with the plurality of drive transmission rotators in a rotation axial direction. 
     Such a configuration enhances work performance of an operator in install and removal of the plurality of cleaning sub-units relative to the body of the belt cleaning device  100 . 
     [Aspect C] 
     According to Aspect C, in the cleaning device according to Aspect B, the transmission assembly is detachably attachable relative to a body of the cleaning device in the rotational axial direction. Thus, when the transmission assembly is moved in the rotational axial direction and installed to or removed from the sub-unit holder, joint couplers of the drive-receive rotators of the plurality of cleaning sub-units are automatically coupled with or decoupled from joint couplers of the plurality of drive transmission rotators in the transmission assembly. 
     [Aspect D] 
     According to Aspect D, in the cleaning device according to Aspect B or C, the plurality of joint couplers is made of insulating material. Such a configuration prevents cleaning bias applied to the cleaner from leaking to the outside via the joint couplers. 
     [Aspect E] 
     According to Aspect E, in the cleaning device according to any one of Aspects B to D, the transmission assembly is detachably attachable relative to a body of the cleaning device in the rotational axial direction. For such a configuration, as described in the embodiments of this disclosure, the plurality of cleaning sub-units can be installed into and removed from the sub-unit holder without pulling out the entire belt cleaning device from a body of an image forming apparatus, thus enhancing the operation performance in installation and removal. 
     [Aspect F] 
     According to Aspect F, in the cleaning device according to Aspect E, the belt cleaning device has a support (e.g., studs  195 ) protruding from a surface of the edge of the body (e.g., the front side plate  140 ) of the belt cleaning device in the direction in which the plurality of cleaning sub-units is pulled out from the sub-unit holder, and the transmission assembly is fixed on the support. As described above, such a configuration can enhance operation performance of an operation in installing the transmission assembly as compared with a configuration in which the transmission assembly is supported with legs of sheet metal. 
     [Aspect G] 
     According to Aspect G, in the cleaning device according to Aspect F, a height of the support is greater than a value obtained by subtracting a total of a length from a depth of a recess of one of the plurality of joint couplers to receive another of the plurality of joint couplers from a total of a length from the surface of the edge to a leading edge of the one of the plurality of joint couplers mounted to the drive-receive rotators in the direction and a length from a back face of a body of the transmission assembly to a leading edge of the plurality of drive transmission rotators in the direction. For such a configuration, as described above, the drive receive rotators and the drive transmission rotators can be placed within the length of the support, thus allowing the transmission assembly to be reliably mounted to the body of the belt cleaning device. 
     [Aspect H] 
     According to Aspect H, the cleaning device according to Aspect F or G includes three or more joint couplers as the plurality of joint couplers. For such a configuration, as described above, the transmission assembly can be stably supported with protruding edges of the three or more supports. 
     [Aspect I] 
     According to Aspect I, in the cleaning device according to any one of Aspects F to H, the support has a cylindrical shape having a diameter of 5 mm or greater and is made of free-cutting steel. Such a configuration effectively suppresses deformation of the support. 
     [Aspect J] 
     According to Aspect J, an image forming apparatus includes an image bearer to bear a toner image; an image forming device to form a toner image on a surface of the image bearer; and the cleaning device according to any one of Aspects B to I to scrape off toner as adhered substance on the surface of the image bearer. 
     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 above teachings, the present disclosure may be practiced otherwise than as specifically described herein. With some embodiments having thus been described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the present disclosure and appended claims, and all such modifications are intended to be included within the scope of the present disclosure and appended claims.