Abstract:
An image formation device includes an image-bearing body that rotates, a charging roller that contacts the image-bearing body and is rotated by the rotation of the image-bearing body, and charges the image-bearing body, a cleaning roller that contacts the charging roller and is rotated by the rotation of the charging roller, and cleans the charging roller, and a first auxiliary roller that is provided coaxially with the charging roller, contacts the image-bearing body and is rotated by the rotation of the image-bearing body. The image formation device satisfies the following relation: 
       F1&gt;F2&gt;F3 
     where F 1  represents a friction force between the first auxiliary roller and the image-bearing body, F 2  represents a friction force between the image-bearing body and the charging roller and F 3  represents a friction force between the charging roller and the cleaning roller.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to an image formation device such as a photocopier, a printer or the like which employs an electrophotographic system, and more particularly relates to an image formation device which includes a contact charging-type charging roller and a cleaning member of the charging roller, the charging roller rotating and contacting an image-bearing body which is driven to rotate and electrostatically charging a surface of the image-bearing body. 
         [0003]    2. Related Art 
         [0004]    In recent years, as charging apparatuses for image formation devices such as photocopiers, printers and the like which employ electrophotographic systems, in order to facilitate reductions in ozone emissions, reductions in device sizes, reductions of costs of high-power supplies and so forth, bias charge rollers (BCR) which contact or are disposed close to image-bearing bodies have been employed instead of conventional non-contact-type chargers such as scorotrons and the like. 
         [0005]    In a charging apparatus with such a non-contact type charging system, because the charging roller constantly contacts the image-bearing body, there is a problem in that soiling tends to occur because of adherence of extraneous matter to the surface of the charging roller. The image-bearing body repeatedly performs an image formation operation. At a downstream side from a transfer stage, the surface of the image-bearing body passes through a cleaning stage, which removes extraneous matter such as residual toner after transfer and the like, and then advances into a region of a charging stage. However, even though the surface has passed through the cleaning stage, particles which are finer than the toner, such as bits of the toner, external additives of the toner and suchlike, remain on the image-bearing body rather than being cleaned, and adhere to the surface of the charging roller. The extraneous matter that adheres to the surface of the charging roller causes variations in surface resistance values of the charging roller. Thus, unusual charging, unstable charging and the like occur, and charging uniformity deteriorates. 
       SUMMARY 
       [0006]    According to an aspect of the present invention, an image formation device includes an image-bearing body that rotates, a charging roller that contacts the image-bearing body and is rotated by the rotation of the image-bearing body, and charges the image-bearing body, a cleaning roller that contacts the charging roller and is rotated by the rotation of the charging roller, and cleans the charging roller, and a first auxiliary roller that is provided coaxially with the charging roller, contacts the image-bearing body and is rotated by the rotation of the image-bearing body. The image formation device satisfies the following relation: 
         [0000]      F1&gt;F2&gt;F3 
         [0007]    where F 1  represents a friction force between the first auxiliary roller and the image-bearing body, F 2  represents a friction force between the image-bearing body and the charging roller and F 3  represents a friction force between the charging roller and the cleaning roller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Exemplary embodiments of the present invention will be described in detail based on the following figures, in which: 
           [0009]      FIG. 1  is a structural view showing general structure of an image formation device according to an embodiment of the present invention; 
           [0010]      FIG. 2  is an enlarged view showing structure of a photosensitive drum, a charging roller and a cleaning roller which are installed in the image formation device of  FIG. 1 ; 
           [0011]      FIG. 3  is a partial sectional side view showing structure of the photosensitive drum, the charging roller, the cleaning roller, auxiliary rollers and a holder according to the embodiment of the present invention; 
           [0012]      FIG. 4  is a perspective view showing a state in which the charging roller and the cleaning roller according to the embodiment of the present invention are axially supported at a bearing member; 
           [0013]      FIG. 5  is a partial sectional side view showing the state in which the charging roller and the cleaning roller according to the embodiment of the present invention are axially supported at the bearing member; and 
           [0014]      FIG. 6  is an enlarged view, corresponding with  FIG. 3 , showing mounting positions of the auxiliary rollers according to the embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Hereinbelow, an image formation device relating to an exemplary embodiment of the present invention will be described with reference to the drawings. 
         [0016]    An image formation device  10  of the present embodiment, which is shown in  FIG. 1 , is a four cycle-type full-color laser printer. As shown in  FIG. 1 , a photosensitive drum  12  is provided inside the device, slightly upward and to the right of the middle of the device, to be rotatable. As this photosensitive drum  12 , for example, a conductive cylindrical body whose surface is covered with a photosensitive layer formed of OPC or the like is employed, and the photosensitive drum  12  is driven to rotate in the direction of the arrow at a predetermined processing speed by an unillustrated motor. 
         [0017]    A surface of the photosensitive drum  12  is electrostatically charged to a predetermined potential by a charging roller  14 , which is disposed substantially directly below the photosensitive drum  12 . Then, image exposure is implemented by a laser beam LB, from an exposure apparatus  16  which is disposed below the charging roller  14 , and an electrostatic latent image is formed in accordance with image information. 
         [0018]    The electrostatic latent image that has been formed on the photosensitive drum  12  is developed by a rotating developing unit  18 , in which developers  18 Y,  18 M,  18 C and  18 K for the colors yellow (Y), magenta (M), cyan (C) and black (K), respectively, are arranged along a circumferential direction, to form a toner image of a predetermined color. 
         [0019]    Here, the respective stages of charging, exposure and development of the surface of the photosensitive drum  12  are repeated a predetermined number of times, in accordance with colors of an image that is to be formed. For the development stage, the rotating developing unit  18  turns and the developing unit  18 Y,  18 M,  18 C or  18 K of a corresponding color is moved to a development position facing the photosensitive drum  12 . 
         [0020]    For example, in a case of forming a full-color image, the respective stages of charging, exposure and development are repeated four times on the surface of the photosensitive drum  12 , in correspondence with each of the colors yellow (Y), magenta (M), cyan (C) and black (K), and toner images corresponding to the respective colors yellow (Y), magenta (M), cyan (C) and black (K) are sequentially formed on the surface of the photosensitive drum  12 . A number of rotations through which the photosensitive drum  12  turns for the formation of the toner image differs depending on the size of the image. For example, for A4 size, a single image is formed by the photosensitive drum  12  turning through three rotations. That is, over three turns of the photosensitive drum  12 , toner images corresponding to the colors yellow (Y), magenta (M), cyan (C) and black (K) are formed at the surface of the photosensitive drum  12 . 
         [0021]    The toner images of the colors yellow (Y), magenta (M), cyan (C) and black (K) that are sequentially formed on the photosensitive drum  12  are transferred by a primary transfer roller  22  at a primary transfer position, at which an intermediate transfer belt  20  is wound round an outer periphery of the photosensitive drum  12 , with conditions such that the toner images are mutually superposed on the intermediate transfer belt  20 . 
         [0022]    The toner images of yellow (Y), magenta (M), cyan (C) and black (K) which have been multiply transferred onto the intermediate transfer belt  20  are transferred, all at once, by a secondary transfer roller  26  onto recording paper  24 , which is supplied with a predetermined timing. 
         [0023]    Meanwhile, the recording paper  24  is fed out by a pickup roller  30  from a paper supply cassette  28 , which is disposed at a lower portion of the image formation device  10 , and the recording paper  24  is supplied, by a feeding roller  32  and a retarding roller  34 , in a state in which one sheet at a time is being handled. The recording paper  24  is conveyed to a secondary transfer position at the intermediate transfer belt  20 , having been synchronized, by a registration roller  36 , with the toner image that has been transferred onto the intermediate transfer belt  20 . 
         [0024]    The intermediate transfer belt  20  spans between a wrap-in roller  38 , the primary transfer roller  22 , a wrap-out roller  40 , a backup roller  42 , a first cleaning backup roller  46  and a second cleaning backup roller  48 , with a predetermined tension. The wrap-in roller  38  defines a wrapping position of the intermediate transfer belt  20  at an upstream side, in a direction of turning, of the photosensitive drum  12 . The primary transfer roller  22  transfers a toner image formed on the photosensitive drum  12  onto the intermediate transfer belt  20 . The wrap-out roller  40  defines a wrapping position of the intermediate transfer belt  20  at a downstream side of the wrapping position of the wrap-in roller  38 . The backup roller  42  abuts against the secondary transfer roller  26  with the intermediate transfer belt  20  therebetween. The first cleaning backup roller  46  and the second cleaning backup roller  48  oppose a cleaning apparatus  44  of the intermediate transfer belt  20 . The intermediate transfer belt  20  is driven in accordance with, for example, rotation of the photosensitive drum  12  so as to circulatingingly turn at a predetermined processing speed. 
         [0025]    Herein, in order to facilitate a reduction in size of the image formation device  10 , the intermediate transfer belt  20  is structured such that a cross-sectional form in which the intermediate transfer belt  20  stretches is a flat, long, thin, substantially trapezoid shape. 
         [0026]    The intermediate transfer belt  20  integrally structures an image formation unit  52  with the photosensitive drum  12 , the charging roller  14 , the intermediate transfer belt  20 , the plural rollers  22 ,  38 ,  40 ,  42 ,  46  and  48  between which the intermediate transfer belt  20  spans, the cleaning apparatus  44  for the intermediate transfer belt  20 , and a cleaning apparatus  78  for the photosensitive drum  12 , which will be described later. It is possible to remove the whole image formation unit  52  from the image formation device  10 , by opening a top cover  54  of the image formation device  10  and manually lifting up a handle (not shown) which is provided at an upper portion of the image formation unit  52 . 
         [0027]    The cleaning apparatus  44  of the intermediate transfer belt  20  is provided with a scraper  58  and a cleaning brush  60 . The scraper  58  is disposed so as to abut against the surface of the intermediate transfer belt  20  that is stretched against the first cleaning backup roller  46 , and the cleaning brush  60  is disposed so as to abut against the surface of the intermediate transfer belt  20  that is stretched against the second cleaning backup roller  48 . Residual toner, paper dust and the like is removed by the scraper  58  and the cleaning brush  60 , and is recovered to an interior portion of the cleaning apparatus  44 . 
         [0028]    The cleaning apparatus  44  is a structure which is provided to be capable of swinging in the anti-clockwise direction of  FIG. 1 , about a swinging shaft  62 . Until secondary transfer of a final color toner image is complete, the cleaning apparatus  44  is withdrawn to a position which is separated from the surface of the intermediate transfer belt  20 , and when the secondary transfer of the final color toner image is complete, the cleaning apparatus  44  abuts against the surface of the intermediate transfer belt  20 . 
         [0029]    The recording paper  24  to which a toner image has been transferred from the intermediate transfer belt  20  is conveyed to a fixing apparatus  64  and is heated and pressured by this fixing apparatus  64 . Thus, the toner image is fixed onto the recording paper  24 . Thereafter, in a case of single-sided printing, the recording paper  24  to which the toner image has been fixed is simply ejected, by an ejection roller  66 , to an ejection tray  68  which is provided at an upper portion of the image formation device  10 . 
         [0030]    On the other hand, in a case of double-sided printing, the recording paper  24 , to a first face (front face) of which the toner image has been fixed by the fixing apparatus  64 , is not simply ejected to the ejection tray  68  by the ejection roller  66 . In a state in which a trailing end portion of the recording paper  24  has been nipped by the ejection roller  66 , the ejection roller  66  is rotated in reverse, and a conveyance path of the recording paper  24  is switched to a duplex paper conveyance path  70 . The recording paper  24  is inverted, front to back, by conveyance rollers  72  which are provided at this duplex paper conveyance path  70 . In this state, the recording paper  24  is again conveyed to the secondary transfer position of the intermediate transfer belt  20 , and a toner image is transferred onto a second face (rear face) of the recording paper  24 . Then, the toner image at the second face (rear face) of the recording paper  24  is fixed by the fixing apparatus  64 , and the recording paper  24  is ejected to the ejection tray  68 . 
         [0031]    Furthermore, optionally, a manual feeding tray  74  can be openably/closeably mounted at the image formation device  10 , at a side face of the image formation device  10 . The recording paper  24 , of arbitrary size and type, which is placed on the manual feeding tray  74 , is supplied by a paper supply roller  76  and is conveyed, via a conveyance roller  73  and the registration roller  36 , to the secondary transfer position of the intermediate transfer belt  20 . Thus, it is possible to form images on the recording paper  24  with arbitrary sizes and types. 
         [0032]    In each turn of the photosensitive drum  12 , after the stage of transfer of the toner image has been completed, residual toner, paper dust and the like at the surface of the photosensitive drum  12  is removed by a cleaning blade  80  of the cleaning apparatus  78 , which is disposed diagonally below the photosensitive drum  12 , and the surface is provided to the stage of formation of the next image. 
         [0033]    As shown in  FIG. 2 , the charging roller  14  is disposed so as to touch the photosensitive drum  12 , at a lower end portion of the photosensitive drum  12 . At the charging roller  14 , a charging layer  14 B is formed around a conductive shaft  14 A, and the shaft  14 A is axially supported to be rotatable. At a lower end portion of the charging roller  14 , at a side thereof which is opposite from the side thereof at which the photosensitive drum  12  is disposed, a cleaning roller  100  is provided. The cleaning roller  100  is a roller-form cleaning member which contacts the surface of the charging roller  14 . At this cleaning roller  100 , a sponge layer  100 B is formed around a shaft  100 A, and the shaft  100 A is axially supported to be rotatable. 
         [0034]    The cleaning roller  100  presses against the charging roller  14  with a predetermined loading, and the sponge layer  100 B resiliently deforms along the periphery of the charging roller  14  and forms a nipping portion  101 . The photosensitive drum  12  is driven to rotate in a clockwise direction of  FIG. 2  (the direction of arrow  2 ) by an unillustrated motor, and the charging roller  14  is rotated in the direction of arrow  4  in accordance with the rotation of the photosensitive drum  12 . Further, the roller-form cleaning roller  100  is rotated in the direction of arrow  6  in accordance with the rotation of the charging roller  14 . 
         [0035]    When the cleaning roller  100  is rotated, contamination such as toner and surface additives (extraneous matter) that has adhered to the surface of the charging roller  14  is cleaned off by the cleaning roller  100 . Hence, this extraneous matter is taken into cells of a foam structure of the cleaning roller  100 . The extraneous matter accommodated in the cells agglomerates and, when the extraneous matter reaches a suitable size, is returned from the cleaning roller  100  to the photosensitive drum  12  via the charging roller  14 , and is recovered by the cleaning apparatus  78  which cleans the photosensitive drum  12 . Thus, continuous maintenance of cleaning characteristics is realized. 
         [0036]    Now, the charging roller  14  and the cleaning roller  100  of the present embodiment will be described. 
         [0037]    The charging roller  14  is disposed in contact with the surface of the photosensitive drum  12  as described above and a DC voltage, or an AC voltage on a DC voltage, is applied thereto. Thus, the surface of the photosensitive drum  12  is electrostatically charged. A form of the charging roller  14  may be a roller form in which a resistive resilient layer constituting the charging layer  14 B is formed around a core which constitutes the shaft  14 A. The resistive resilient layer may have a structure which is divided into a sequence, from an outer side, of a resistive layer and a resilient layer which supports the resistive layer. Further, in order to provide the charging roller  14  with endurance and soiling resistance, it is possible, in accordance with requirements, to provide a protective layer at the outer side of the resistive layer. 
         [0038]    Hereinbelow, a case in which a resilient layer, a resistive layer and a protective layer are provided on the core will be described in more detail. 
         [0039]    Because a material of the core is to exhibit conductivity, ordinarily, iron, copper, brass, stainless steel, aluminum, nickel or the like is employed. Of materials other than metals, a material can be employed as long as it exhibits conductivity and suitable stiffness. For example, a resin-molded product in which conductive particles or the like are dispersed, or a ceramic or the like may be employed. Furthermore, besides the roller form, a hollow pipe form is possible. 
         [0040]    As a material of the resilient layer, because the material is to exhibit conductivity or semiconductivity, the material is ordinarily a material in which conductive particles or semiconductive particles are dispersed in a resin material or a rubber material. As a resin material, a combined resin of polyester resin, acrylic resin, melamine resin, epoxy resin, urethane resin, silicon resin, urea resin, polyamide resin or the like, or the like is employed. As a rubber material, ethylene propylene rubber, polybutadiene, natural rubber, polyisobutylene, chloroprene rubber, silicon rubber, urethane rubber, epichlorhydrine rubber, fluorosilicone rubber, ethylene oxide rubber or the like, or a foam material in which such a rubber is foamed, is employed. 
         [0041]    As the conductive particles or semiconductive particles, carbon black, a metal such as zinc, aluminium, copper, iron, nickel, chromium, titanium or the like, a metal oxide such as ZnO—Al 2 O 3 , SnO 2 —Sb 2 O 3 , In 2 O 3 —SnO 2 , ZnO—TiO 2 , MgO—Al 2 O 3 , FeO—TiO 2 , TiO 2 , SnO 2 , Sb 2 O 3 , In 2 O 3 , ZnO, MgO or the like, or an ionic compound such as a quarternary ammonium salt or the like, or the like can be employed. These materials may be employed singly or in a combination of two or more thereof. Furthermore, in accordance with requirements, an inorganic packing material such as talc, alumina, silica or the like, or an organic packing material such as a fluorine resin, microparticles of silicon rubber or the like, can be used singly or in a combination of two or more. 
         [0042]    For the resistive layer and the protective layer, with a material in which conductive particles or semiconductive particles are dispersed in a settled resin to control resistance, resistivity may be set to 10 3  to 10 14  Ω·cm, preferably 10 5  to 10 12  Ω·cm, and more preferably 10 7  to 10 12  Ω·cm. A layer thickness may be 0.01 to 1000 μm, preferably 0.1 to 500 μm, and more preferably 0.5 to 100 μm. As the settled resin, an acrylic resin, cellulose resin, polyamide resin, methoxymethylated nylon, ethoxymethylated nylon, polyurethane resin, polycarbonate resin, polyester resin, polyethylene resin, polyvinyl resin, polyarylate resin, polythiophene resin, polyolefin resin such as PFA, FEP, PET or the like, styrene butadiene resin, melamine resin, epoxy resin, urethane resin, silicon resin, urea resin or the like is employed. 
         [0043]    As the conductive particles or semiconductive particles, similarly to the resilient layer, carbon black, metals, metal oxides, ionic compounds such as quarternary ammonium salts and the like which exhibit ion conductivity, and the like can be employed singly or in a combination of two or more. Furthermore, in accordance with requirements, an oxidation inhibitor such as a hindered phenol, hindered amine or the like, an inorganic packing material such as a clay, kaolin, talc, silica, alumina or the like, an organic packing material such as a fluorine resin, microparticles of silicon rubber or the like, a lubricant such as silicone oil or the like, and suchlike can be added singly or in combinations of two or more. Moreover, a surfactant, a charging control agent and suchlike are added in accordance with requirements. 
         [0044]    As a method for forming these layers, a blade coating process, a Meyer bar coating process, a spray coating process, an immersion coating process, a bead coating process, an air knife coating process, a curtain coating process or the like can be employed. 
         [0045]    The cleaning roller  100  is formed from a core, which constitutes the shaft  10 A, and a porous resilient layer, which constitutes the sponge layer  100 B and which is formed at a peripheral surface of the core, and as described above, the cleaning roller  100  is disposed to contact the surface of the charging roller  14 . 
         [0046]    As a material of the core, a material which supports the porous resilient layer and exhibits stiffness to a degree capable of maintaining the state of contact with the charging roller  14  with a suitable abutting force is employed. Ordinarily, a metal such as iron, copper, brass, stainless steel, aluminium, nickel or the like, or alternatively a resin-molded product, a ceramic or the like, or such a material in which conductive particles or the like are dispersed, or a material in which an inorganic filler is dispersed or the like may be employed. Furthermore, besides the roller form, a hollow pipe form is possible. 
         [0047]    The porous resilient layer is a roller-form sponge, which is formed with a predetermined cell density. For example, an ether-based urethane foam, polyethylene foam, polyolefin foam, melamine foam, micropolymer or the like can be employed. 
         [0048]    Taking a polyurethane foam as an example and briefly describing a fabrication method thereof, the foam is fabricated using a polyol, an isocyanate, water, a catalyst (an amine catalyst, a metallic catalyst or the like) and a foam stabilizer (a surfactant), and additives such as a pigment and the like are employed in accordance with an intended application. When these ingredients are mixed and stirred, chemical reactions occur and a foam body of urethane resin can be obtained. 
         [0049]    Next, a support structure for the charging roller  14  and cleaning roller  100  relating to the present exemplary embodiment, and auxiliary rollers which are provided at each of the rollers, will be specifically described. 
         [0050]    As shown in  FIG. 3 , in the present exemplary embodiment, the charging roller  14  and the cleaning roller  100  are mounted at a box-like holder  120 , via a pair of bearing members  110 . The charging roller  14  and cleaning roller  100  are accommodated in this holder  120  and formed as a unit with the holder  120 , and are disposed at predetermined positions relative to the photosensitive drum  12 . 
         [0051]    As shown in  FIG. 4 , one of the bearing members  110  is formed in a cuboid shape (a block shape) and has a simple structure. The bearing member  110  is formed with a synthetic resin material, such as polyacetal, polycarbonate or the like, which has high stiffness and high slidability and is excellent in abrasion resistance. In order to further raise the abrasion resistance, the bearing member  110  may include glass fibers, carbon fibers or the like in the synthetic resin material. 
         [0052]    A bearing trough  112  and a bearing hole  114 , which are arranged with a predetermined spacing along a length direction of the bearing member  110  (the vertical direction of  FIG. 4 ) are formed in the bearing member  110 . The bearing trough  112  is formed with a ‘U’-shaped cross section, which opens out at an upper end face of the bearing member  110 . An internal diameter of an inner periphery face portion of the bearing trough  112 , which has the form of a semi-circular periphery face, is substantially the same as a shaft diameter of a support portion  14   a , which is provided at an end portion of the shaft  14 A of the charging roller  14 . The support portion  14   a  of the shaft  14 A of the charging roller  14  is rotatably fitted into this bearing trough  112 . Because the photosensitive drum  12  side of the bearing trough  112 , which is the upper side thereof in the drawing, is open, when the support portion  14   a  is abuttingly supported at the inner periphery face portion of the bearing trough  112 , a shape is formed in which a degree of freedom is provided to an abutting direction of the support portion  14   a  toward the photosensitive drum  12  (the direction of arrow  8 ). Meanwhile, a support portion  100   a  which is provided at an end portion of the shaft  100 A of the cleaning roller  100  is rotatably inserted into the bearing hole  114 . 
         [0053]    As shown in  FIG. 3 , the holder  120  is integrally provided with a pair of mounting portions  124 , at which the two bearing members  110  are mounted, at each of two end portions, along an axial direction of the charging roller  14  and cleaning roller  100  (left and right side end portions in  FIG. 3 ), of a main body portion  122  of the holder  120 . 
         [0054]    A guide channel  126  is formed in each mounting portion  124  along a direction in which the mounting portion  124  extends. The bearing members  110  are fitted into these guide channels  126 , and disposed close to distal end sides thereof. The bearing members  110  are guided in the guide channels  126  and are made capable of sliding along the direction of extension of the mounting portions  124  (i.e., a direction toward and away from the photosensitive drum  12 ). 
         [0055]    A compression coil spring  128  is disposed at a base end side within each guide channel  126 . The compression coil springs  128  urge the bearing members  110  toward the photosensitive drum  12 . By spring force of the compression coil springs  128 , the bearing members  110  are urged toward the photosensitive drum  12  (i.e., in the direction of arrow  8 ), and the charging roller  14  is abutted against the photosensitive drum  12 . 
         [0056]    Thus, at the pair of bearing members  110 , between the charging roller  14  of which the support portions  14   a  at the two ends of the shaft  14 A are coaxially supported and the cleaning roller  100  of which the support portions  100   a  at the two ends of the shaft  100 A are supported, as described above, the cleaning roller  100  is pushed against the charging roller  14  with a predetermined loading, the sponge layer  100 B resiliently deforms along the peripheral surface of the charging roller  14 , and the nipping portion  101  is formed (see  FIG. 2 ). In this state, an inter-axis separation of the charging roller  14  and the cleaning roller  100  is fixed, and a relative spacing in the direction of abutting is kept constant. Furthermore, a positional relationship in a direction intersecting the abutting direction (substantially a direction of a contacting portion (the nipping portion  101 )) is fixed, and relative positions are kept constant. Consequently, a width of nipping is constant. Further, as shown in  FIG. 5 , the photosensitive drum  12  side of the holder  120 , which covers the surroundings of the charging roller  14  and the cleaning roller  100 , is open (the upper side of  FIG. 5 ), and in the state in which the holder  120  supports the rollers, a gap is formed between an upper edge portion  127  thereof and the photosensitive drum  12 . 
         [0057]    As shown in  FIGS. 3 and 4 , at the charging roller  14  of the present exemplary embodiment, a pair of first auxiliary rollers  15  are coaxially mounted to vicinities of the two end portions of the shaft  14 A. In addition, at the cleaning roller  100 , a pair of second auxiliary rollers  102 , which correspond with the two first auxiliary rollers  15 , are coaxially mounted to vicinities of the two end portions of the shaft  100 A. 
         [0058]    Each first auxiliary roller  15  is slightly spaced apart from the charging layer  14 B at the vicinity of the end portion of the shaft  14 A, and is fixed at a position so as not to contact the bearing member  110 . Furthermore, as shown in  FIG. 6 , the first auxiliary roller  15  is located outside an image formation region of the photosensitive drum  12  (i.e., at a non-image-formation region), and is disposed well away from the image formation region. 
         [0059]    The first auxiliary roller  15  has an outer diameter the same as an outer diameter of the charging roller  14  (i.e., of the charging layer  14 B), or is set to be slightly larger. Thus, the first auxiliary roller  15  contacts the surface of the photosensitive drum  12 . Further, with the first auxiliary rollers  15  of the present embodiment, a friction force between the first auxiliary rollers  15  and the photosensitive drum  12  is specified so as to be larger than a friction force between the photosensitive drum  12  and the charging roller  14 . 
         [0060]    More specifically, a resilient force of the surfaces of the first auxiliary rollers  15  is set larger than a resilient force of the surface of the charging roller  14 , or the outer diameter of the first auxiliary rollers  15  is set larger than the outer diameter of the charging roller  14 , or the first auxiliary rollers  15  are formed of a material with a higher coefficient of friction against the photosensitive drum  12  than the charging roller  14  (i.e., the charging layer  14 B), a coating with a higher coefficient of friction against the photosensitive drum  12  than the charging roller  14  (the charging layer  14 B) is applied to the surfaces of the first auxiliary rollers  15 , or the like. Thus, the first auxiliary rollers  15  are specified such that the relationship (friction force between the first auxiliary rollers  15  and the photosensitive drum  12 )&gt;(friction force between the photosensitive drum  12  and the charging roller  14 ) is satisfied. 
         [0061]    Meanwhile, each second auxiliary roller  102  is slightly spaced apart from the sponge layer  100 B at the vicinity of the end portion of the shaft  100 A, and is fixed at a position so as not to contact the bearing member  110 . Furthermore, as shown in  FIG. 6 , the second auxiliary roller  102  is located to correspond with the first auxiliary roller  15  outside the image formation region of the photosensitive drum  12  (at the non-image-formation region) and, similarly to the first auxiliary roller  15 , is disposed well apart from the image formation region. 
         [0062]    The second auxiliary roller  102  has an outer diameter the same as an outer diameter of the cleaning roller  100  (i.e., the sponge layer  100 B), or is set slightly larger. Thus, the second auxiliary roller  102  contacts the surface of the first auxiliary roller  15 , and a width dimension (i.e., axial direction dimension) of the second auxiliary roller  102  is set substantially the same as a width dimension of the first auxiliary roller  15 . 
         [0063]    Further, with the second auxiliary rollers  102  of the present embodiment, a friction force between the first auxiliary rollers  15  and the second auxiliary rollers  102  is specified so as to be smaller than the friction force between the first auxiliary rollers  15  and the photosensitive drum  12 , equal to or greater than the friction force between the photosensitive drum  12  and the charging roller  14 , and greater than a friction force between the charging roller  14  and the cleaning roller  100 . 
         [0064]    More specifically, the outer diameter of the second auxiliary rollers  102  is set larger than the outer diameter of the cleaning roller  100 , or the second auxiliary rollers  102  are formed of a material with which a coefficient of friction of the second auxiliary rollers  102  against the first auxiliary rollers  15  is smaller than the coefficient of friction between the first auxiliary rollers  15  and the photosensitive drum  12 , is equal to or greater than the coefficient of friction between the photosensitive drum  12  and the charging roller  14  and is greater than a coefficient of friction between the charging roller  14  and the cleaning roller  100 , or a coating which achieves the above coefficient of friction relationships is applied to the surfaces of the second auxiliary rollers  102 , or the like. Thus, the second auxiliary rollers  102  are specified such that the relationships (friction force between the first auxiliary rollers  15  and the photosensitive drum  12 )&gt;(friction force between the first auxiliary rollers  15  and the second auxiliary rollers  102 )≧(friction force between the photosensitive drum  12  and the charging roller  14 )&gt;(friction force between the charging roller  14  and the cleaning roller  100 ) are satisfied. 
         [0065]    Next, operations of the present embodiment will be described. 
         [0066]    In the image formation device  10  of the present embodiment, when the photosensitive drum  12  rotates during an image formation operation, the charging roller  14  is rotated in accordance with the rotation of the photosensitive drum  12 , and electrostatically charges the photosensitive drum  12 . The cleaning roller  100  is also rotated in accordance with the rotation of the charging roller  14 , and cleans the charging roller  14 . Thus, the charging roller  14  which charges up the photosensitive drum  12  for image formation is cleaned of extraneous matter that has adhered to the roller surface thereof by the cleaning roller  100 , and decreases in charging capabilities are consequently restrained. Further, because the surroundings of the charging roller  14 , along with the cleaning roller  100 , are covered by the holder  120 , except at the side at which the photosensitive drum  12  is disposed, the charging roller  14  is protected from toner, dust and the like that flies from the developers  18 Y,  18 M,  18 C and  18 K and floats around in the device, and adherence of such extraneous matter is prevented. 
         [0067]    The rotation of the charging roller  14  which contacts the photosensitive drum  12  and is rotated is assisted by the pair of first auxiliary rollers  15 , which are provided coaxially with respect to the charging roller  14 , contacting the photosensitive drum  12  and being rotated together with the charging roller  14 . Further, because the friction force between the first auxiliary rollers  15  and the photosensitive drum  12  is made larger than the friction force between the photosensitive drum  12  and the charging roller  14  and the friction force between the charging roller  14  and the cleaning roller  100  is made smaller than the friction force between the photosensitive drum  12  and the charging roller  14 , even when deterioration with time of the surface of the charging roller  14  progresses over a long period of use, a decrease in drivability of the charging roller  14  with respect to the photosensitive drum  12  is suppressed. Therefore, stable charging characteristics can be maintained over long periods. Moreover, because the first auxiliary rollers  15  are disposed outside the image formation region of the photosensitive drum  12 , adverse effects on image formation are avoided. 
         [0068]    Because, as mentioned above, the resilient force of the surfaces of the first auxiliary rollers  15  is made smaller than the resilient force of the surface of the charging roller  14  or the outer diameter of the first auxiliary rollers  15  is made to be equal to or greater than the outer diameter of the charging roller  14 , the relationship (friction force between the first auxiliary rollers  15  and the photosensitive drum  12 )&gt;(friction force between the photosensitive drum  12  and the charging roller  14 ) can be realized with a simple structure. 
         [0069]    With a structure in which, as in the present embodiment, the cleaning roller  100  is in contact with the charging roller  14  and follows rotation thereof, there is concern that the friction force between the cleaning roller  100  and the charging roller  14  will fall, because of a deterioration over time of the surface of the cleaning roller  100  due to long-term usage of the image formation device  10  or the like, and that drivability of the cleaning roller  100  by the charging roller  14  will fall, causing cleaning failures. 
         [0070]    However, the rotation of the cleaning roller  100  of the present embodiment to follow the charging roller  14  is assisted by the pair of second auxiliary rollers  102 , which are coaxially provided, contacting the first auxiliary rollers  15  provided at the charging roller  14  and being rotated together with the cleaning roller  100 . Further, because the friction force between the first auxiliary rollers  15  and the second auxiliary rollers  102  is made smaller than the friction force between the first auxiliary rollers  15  and the photosensitive drum  12 , a fall in drivability of the charging roller  14  with respect to the photosensitive drum  12  can be avoided, and because the friction force between the first auxiliary rollers  15  and the second auxiliary rollers  102  is made larger than the friction force between the charging roller  14  and the cleaning roller  100  (i.e., is set to at least the friction force between the photosensitive drum  12  and the charging roller  14 ), the rotation of the cleaning roller  100  caused by the rotation of the charging roller  14  can be excellently assisted. Therefore, even when deterioration with time of the surface of the cleaning roller  100  progresses over a long period of use, a decrease in drivability of the cleaning roller  100  by the charging roller  14  is suppressed, and stable charging characteristics can be maintained over long periods. 
         [0071]    Moreover, because, as mentioned above, the outer diameter of the second auxiliary rollers  102  is set to at least the outer diameter of the cleaning roller  100 , the relationship (friction force between the first auxiliary rollers  15  and the second auxiliary rollers  102 ) (≧(friction force between the photosensitive drum  12  and the charging roller  14 ))&gt;(friction force between the charging roller  14  and the cleaning roller  100 ) can be realized with a simple structure. 
         [0072]    Thus, with the image formation device  10  of the present embodiment, image defects due to failures in charging of the photosensitive drum  12  and failures in cleaning of the charging roller  14  are suppressed, and high quality images can be formed over long periods. 
         [0073]    Hereinabove, the present invention has been described in detail in accordance with the particular exemplary embodiment described above. However, the present invention is not limited to this exemplary embodiment, and it is possible to embody various modes within the scope of the present invention. 
         [0074]    For example, the exemplary embodiment described above has a structure in which the charging roller  14  and the cleaning roller  100  are both supported by the bearing members  110 , and the charging roller  14  is abutted against the photosensitive drum  12  and the cleaning roller  100  is abutted against the charging roller  14  by the urging force of the compression coil springs  128 . However, support structures, abutting structures and the like of the respective rollers are not limited thereto; the charging roller  14  and the cleaning roller  100  could be supported by separate bearing members, and could be urged for abutting by separate urging means. 
         [0075]    Further, although the charging roller  14  contacts a lower side portion of the photosensitive drum  12  and the cleaning roller  100  contacts a lower side portion of the charging roller  14  in the above structure, positional relationships of the photosensitive drum  12 , the charging roller  14  and the cleaning roller  100  are not limited thereto. For example, the present invention can be applied to a structure in which a charging roller is caused to contact an upper side portion of a photosensitive drum and a cleaning roller is caused to contact an upper side portion of a charging roller, or the like. 
         [0076]    Further again, an image formation device to which the present invention is applied is not limited to a four cycle-type structure which repeats formation of toner images onto the photosensitive drum  12  four times using the rotating developing unit  18 , as in the present embodiment. For example, with a full-color tandem structure in which image formation units for yellow, magenta, cyan and black are arranged in a row along a direction of movement of an intermediate conveyance belt, the present invention can be applied to a photosensitive drum and a holder of a charging roller and a cleaning roller at each image formation unit. 
         [0077]    While the present invention has been illustrated and described with respect to some specific exemplary embodiments thereof, it is to be understood that the present invention is by no means limited thereto and encompasses all changes and modifications which will become possible without departing from the spirit and scope of the present invention.