Patent Publication Number: US-9417563-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This disclosure relates to an image forming apparatus such as a copier, a printer, a facsimile, and a multifunction printer having a plurality of their functions. 
     2. Description of the Related Art 
     An intermediate transfer-type image forming apparatus which transfers a toner image formed on a photosensitive drum to an intermediate transfer belt and conveys the toner image to a transfer portion which transfers the toner image to a recording medium has been widely used. For example, JP-A-2007-57803 discloses an intermediate transfer-type image forming apparatus configured to clean an intermediate transfer belt by using an electrostatic cleaning-type belt cleaning unit. 
     By the way, the intermediate transfer belt is inclined to be attached with transfer residual toner charged with a polarity reverse to a toner charging polarity after transferring a toner image on a recording medium. Due to that, JP-A-2007-57803 collects the toner in an electrostatic manner from the intermediate transfer belt by abutting a second brush roller to which voltage of a polarity reverse to the toner charge polarity is applied with the intermediate transfer belt downstream of a first brush roller to which voltage of a same polarity with the toner charge polarity is applied. 
     A transfer belt-type image forming apparatus transferring a toner image borne on a photosensitive drum or the like to a recording medium carried on a transfer is also widely used. JP-A-2006-259367 discloses a transfer belt-type image forming apparatus configured to clean a transfer belt by using an electrostatic cleaning-type belt cleaning unit. 
     There is a case when the transfer belt is adhered with the toner charged with a polarity reverse to the charge polarity of the toner transferred from a patch toner image formed between images consecutively formed after separating the recording medium on which the toner image has been transferred. Due to that, JP-A-2006-259367 recovers the toner in an electrostatic manner from the secondary transfer belt by abutting a second brush roller to which voltage of a same polarity with the toner charge polarity is applied with the secondary transfer belt downstream of a first brush roller to which voltage of a polarity reverse to the toner charge polarity is applied. 
     It is desirable to be able to recover the toner more efficiently from the endless belt such as the intermediate transfer belt and the transfer belt with the configuration including the first and second brush rollers as described above. 
     SUMMARY OF THE INVENTION 
     According to one aspect of this disclosure, there is provided an image forming apparatus which includes a moving endless belt; a first brush roller configured to contact an outer circumferential surface of the endless belt; a first roller being in contact with an inner circumferential surface of the endless belt at a position corresponding to the first brush roller; a first power supply generating an electric field in a first direction between the first brush roller and the first roller; a second brush roller contacting the outer circumferential surface of the endless belt downstream of the first brush roller in a moving direction of the endless belt; a second roller being in contact with the inner circumferential surface of the endless belt at a position corresponding to the second brush roller; and a second power supply generating an electric field in a second direction which is a direction opposite to the first direction between the second brush roller and the second roller; in which relationships of LB1&gt;LR1 and LN1&lt;LN2 are satisfied, where LB1 is a length of a first contact area in the moving direction in which the first brush roller and the endless belt come into contact with each other, LR1 is a length of a second contact area in the moving direction in which the first roller and the endless belt come into contact with each other, a third contact area is an area in which the brush roller and the endless belt come into contact with each other, a fourth contact area is an area in which the second roller and the endless belt come into contact with each other, LN1 is a length of an area in which the first contact area overlaps with the second contact area in the moving direction, and LN2 is a length of an area in which the third contact area overlaps with the fourth contact area in the moving direction. 
     According to another aspect of this disclosure, there is provided an image forming apparatus which includes a moving endless belt; a first brush roller contacting an outer circumferential surface of the endless belt; a first roller being in contact with an inner circumferential surface of the endless belt at a position corresponding to the first brush roller; a first power supply generating an electric field in a first direction between the first brush roller and the first roller; a second brush roller contacting the outer circumferential surface of the endless belt downstream of the first brush roller in a moving direction of the endless belt; a second roller being in contact with the inner circumferential surface of the endless belt at a position corresponding to the second brush roller; and a second power supply generating an electric field in a second direction which is a direction opposite to the first direction between the second brush roller and the second roller, in which relationships of LN1/LB1&lt;LN2/LB2 and LN1&lt;LN2 are satisfied, where LB1 is a length of a first contact area in the moving direction in which the first brush roller and the endless belt come into contact with each other, a second contact area is an area in which the first roller and the endless belt come into contact with each other, LB2 is a length of a third contact area in the moving direction in which the second brush roller and the endless belt come into contact with each other, a fourth contact area is an area in which the second roller and the endless belt come into contact with each other, LN1 is a length of an area in which the first contact area overlaps with the second contact area overlap in the moving direction, and LN2 is a length of an area in which the third contact area overlaps with the fourth contact area in the moving direction. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory diagram illustrating a configuration of an image forming apparatus according to a first embodiment. 
         FIG. 2  is an explanatory diagram illustrating a configuration of a secondary transfer belt unit according to the first embodiment. 
         FIG. 3  is an explanatory diagram illustrating an experiment in which diameters of a support roller and of a drive roller are changed. 
         FIG. 4  is an explanatory diagram illustrating a contact length between a fur brush and the support roller according to the first embodiment. 
         FIG. 5  is an explanatory diagram illustrating a face of a secondary transfer belt around of which the drive roller is wound according to the first embodiment. 
         FIG. 6  is an explanatory diagram illustrating a configuration of a belt cleaning unit according to a second embodiment. 
         FIG. 7  is an explanatory diagram illustrating a contact length between a fur brush and a support roller according to the second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     A first embodiment will be described using  FIGS. 1 to 5 . 
     (Image Forming Apparatus) 
       FIG. 1  is a schematic diagram illustrating a configuration of an image forming apparatus. As illustrated in  FIG. 1 , the image forming apparatus  100  is a tandem intermediate transfer-type full color printer in which image forming portions PY, PM, PC, and PK are arranged along a rotating direction (moving direction) of an intermediate transfer belt  40 . 
     In the image forming portion PY, a yellow toner image is formed on a photosensitive drum  1 Y and is transferred to the intermediate transfer belt  40 . In the image forming portion PM, a magenta toner image is formed on a photosensitive drum  1 M and is transferred to the intermediate transfer belt  40 . In the image forming portions PC and PK, a cyan toner image and a black toner image are formed on photosensitive drums  1 C and  1 K and are transferred to the intermediate transfer belt  40 . 
     The toner images of the four colors which have been transferred to the intermediate transfer belt  40  are conveyed to a transfer portion N and are secondarily transferred to a recording medium P. The recording medium P is picked up from a recording media cassette  35 , is separated one by one by a separating roller  36 , and is sent to a registration roller  13 . The registration roller  13  sends the recording medium P to the transfer portion N in synchronism with the toner image on the intermediate transfer belt  40 . 
     The recording medium P on which the four-color toner image has been secondarily transferred is conveyed by a conveyor belt  61  and is sent to a fixing unit  60  to undergo heating and pressurizing process and to fix the image on the surface thereof. The fixing unit  60  applies a predetermined pressure and a quantity of heat at a nip formed by a fixing roller  60   a  provided with a heater  60   c  therein and a press roller  60   b  to melt and fix the toner image on the recording medium P. 
     (Image Forming Portion) 
     The image forming portions PY, PM, PC, and PK are constructed substantially in the same manner except of that the colors of toners used in developing units  5 Y,  5 M,  5 C, and  5 K are different from each other as yellow, magenta, cyan, and black. Accordingly, the image forming portion PY will be typically described, and an overlapped descriptions about the other image forming portions PM, PC, and PK will be omitted here. 
     In the image forming portion PY, a charging unit  3 Y, an exposing unit  4 Y, the developing unit  5 Y, a primary transfer roller  6 Y, and a drum cleaning unit  7 Y are arranged surrounding the photosensitive drum  1 Y. The photosensitive drum  1 Y is provided with a photosensitive layer formed on an outer circumferential surface of an aluminum cylinder and rotates in a direction of an arrow A at a predetermined process speed. 
     The charging unit  3 Y charges the photosensitive drum  1 Y with a potential of a uniform negative polarity. The exposing unit  4 Y scans a laser beam which is generated based on an image signal obtained by developing image data to a scanning line by using a rotating mirror and draws an electrostatic latent image based on the image data on the surface of the photosensitive drum  1 Y. 
     The developing unit  5 Y transfers toner negatively charged to the electrostatic latent image on the photosensitive drum  1 Y and develops the electrostatic latent image as a toner image. A developer replenishing unit  51 Y replenishes developer to the developing unit  5 Y in accordance to the toner or the like consumed in the developing unit  5 Y in forming the image. 
     The primary transfer roller  6 Y as a first transfer member presses the intermediate transfer belt  40  and forms a primary transfer portion between the photosensitive drum  1 Y and the intermediate transfer belt  40 . When a voltage with a positive polarity is applied to the primary transfer roller  6 Y, a toner image with a negative polarity which is borne on the photosensitive drum  1 Y is transferred to the intermediate transfer belt  40 . 
     The drum cleaning unit  7 Y collects transfer residual toner adhering on the surface of the photosensitive drum  1 Y by rubbing the photosensitive drum  1 Y by a cleaning blade 
     (Intermediate Transfer Belt) 
     The intermediate transfer belt  40 , i.e., one exemplary image bearing member, rotates while bearing a toner image. The image forming portion PY forms a toner image, and causes the intermediate transfer belt  40  to bear the toner image. 
     The intermediate transfer belt  40  is stretched around a drive roller  43 , a tension roller  41 , and a secondary transfer inner roller  42 , and rotates at a rotation speed of 250 mm/sec to 300 mm/sec in a direction of an arrow G by being driven by the drive roller  43 . A circumferential length of the intermediate transfer belt  40  is 2000 mm, and diameters of the photosensitive drums  1 Y,  1 M,  1 C, and  1 K are 80 mm. 
     A thickness of the intermediate transfer belt  40  becomes 250 μm by arranging an elastic layer of a rubber material of 120 μm to 180 μm thick on a base layer of a resin material such as polyimide, polycarbonate, or the like, of 70 μm thick, and by arranging a surface layer of 5 μm to 10 μm thick on the elastic layer. The rubber material is urethane rubber, chloroprene rubber, or the like. The surface layer includes a fluororesin material which lessens adhesion of the toner to the surface of the intermediate transfer belt  40  and makes transferring of the toner to the recording medium P easier in the transfer portion N. Volume resistivity of the intermediate transfer belt  40 , is adjusted to 1−10 9  Ω·cm by containing an appropriate amount of carbon black i.e., an antistatic agent, in each layer. 
     The tension roller  41  applies a constant tensile force to the intermediate transfer belt  40  by pressure springs  41   s  disposed at both ends of a rotational axis and biased so as to protrude toward the intermediate transfer belt  40 . A belt cleaning unit  45  collects transfer residual toner on the surface of the intermediate transfer belt  40  by contacting the intermediate transfer belt  40  by two fur brushes to which voltages of polarities reverse to each other are applied. 
     (Secondary Transfer Belt) 
     As illustrated in  FIG. 1 , a secondary transfer belt unit  56  causes the secondary transfer belt  12  to carry the recording medium P and to pass through the transfer portion N. It is possible to easily separate the recording medium P from the intermediate transfer belt  40  after transferring the toner image in the transfer portion N by using the secondary transfer belt  12 . 
     The secondary transfer belt  12 , i.e., an exemplary endless belt, moves while carrying the recording medium on an outer circumferential surface thereof. A secondary transfer outer roller  10 , i.e., a transfer member or a second transfer member, transfers the toner image borne on the intermediate transfer belt  40  to the recording medium carried on the secondary transfer belt  12 . 
       FIG. 2  is a schematic diagram illustrating a configuration of the secondary transfer belt unit. As illustrated in  FIG. 2 , in the secondary transfer belt unit  56 , the secondary transfer belt  12  is stretched around the secondary transfer outer roller  10 , a separating roller  21 , a tension roller  22 , and a drive roller  23 . A circumferential length of the secondary transfer belt  12  is 200 mm. 
     The secondary transfer belt  12  is formed of a resin material in which volume resistivity is adjusted to 1×10 9  Ω·cm to 1×10 14  Ω·cm by containing an appropriate amount of carbon black, i.e., an antistatic agent, to a resin material such as polyimide, polycarbonate, or the like. The secondary transfer belt  12  is of a single layer structure and is 0.07 mm to 0.1 mm thick. A value of Young&#39;s modulus of the secondary transfer belt  12  measured by a tensile test (JIS K 6301) is 100 MPa or more and less than 10 GPa. 
     The secondary transfer outer roller  10  is formed so as to have an outer diameter of 24 mm by arranging an elastic layer  10   b  of ion conductive foaming rubber (NBR rubber) at the outer periphery of a cored metal  10   a , i.e., a stainless steel round bar. The ten point average surface roughness Rz of the elastic layer  10   b  is 6.0 μm to 12.0 μm, and Asker-C hardness is approximately 30 to 40. A resistance value of the secondary transfer outer roller  10  measured by applying 2 kV in a normal temperature and humidity (N/N:23° C., 50% RH) is 1×10 5  Ω·cm to 1×10 7  Ω·cm. 
     In the transfer portion N, the secondary transfer inner roller  42  supports the inner circumferential surface of the intermediate transfer belt  40 . 
     The secondary transfer outer roller  10  forms the toner image transfer portion N between the intermediate transfer belt  40  and the secondary transfer belt  12  by nipping the intermediate transfer belt  40  and the secondary transfer belt  12  between the secondary transfer inner roller  42  and the secondary transfer outer roller  10 . A secondary transfer power supply  11  whose output current is variable is connected to the secondary transfer outer roller  10 . 
     Constant current control is made on output voltage of the secondary transfer power supply  11  such that a transfer current of +40 μA to 60 μA flows. The secondary transfer power supply  11  applies a transfer voltage to the secondary transfer outer roller  10  and secondarily transfers the toner image borne on the intermediate transfer belt  40  to the recording medium P on the secondary transfer belt  12 . The recording medium P is adsorbed to the secondary transfer belt in an electrostatic manner along with the secondary transfer of the toner image. 
     The separating roller  21  separates a recording medium from the secondary transfer belt  12  on the downstream of the transfer portion N. The recording medium P on the secondary transfer belt  12  reaches the separating roller  21 , and is then curvature-separated from the secondary transfer belt  12  on a curved face of the secondary transfer belt  12  along the circumferential surface of the separating roller  21 . A separation claw  32  prevents the recording medium P separated from the secondary transfer belt  12  from electrostatically winding around the secondary transfer belt  12  again. 
     The drive roller  23  is connected with a drive motor (not illustrated) and drives the secondary transfer belt  12  to rotate in a direction of an arrow B direction. Both ends of the tension roller  22  are biased by pressure springs in a direction in which the tension roller  22  projects toward the secondary transfer belt  12  to apply predetermined tension to the secondary transfer belt  12 . 
     The recording medium P which is curvature-separated from the secondary transfer belt  12  is sent to the fixing unit  60  by being conveyed to the conveyor belt  61 . The recording medium on which the image has been fixed by the fixing unit  60  is discharged out of the image forming apparatus  100 . 
     (Patch Cleaning) 
     The image forming apparatus  100  forms a patch toner image of each color between toner images (between images) formed consecutively on the respective recording media and measures density (reflected light amount of infrared light) of the patch toner image transferred onto the intermediate transfer belt  40  using an optical sensor KS ( FIG. 1 ). Controls in which density of the patch toner image is fed back include setting of laser power, setting of developing voltage, an adjustment of a toner replenishment amount, and the like. While an output voltage of the secondary transfer power supply  11  is turned OFF when a part between images, on which the patch toner image is to be formed, passes through the transfer portion N, there is a case in which a part of toner of the patch toner image is transferred to the secondary transfer belt  12  from the intermediate transfer belt  40 , because the intermediate transfer belt  40  and the secondary transfer belt  12  being in close contact with each other are pressed. 
     There is also a case in which a toner image which has not been transferred to the recording medium remains on the intermediate transfer belt  40  when a recording medium is jammed in a middle of an image forming operation and a process of taking out the jammed recording medium is performed. In this case, there is a possibility that part of toner of the toner image moves to the secondary transfer belt  12  when the remaining toner image passes through the transfer portion N. 
     Due to that, the image forming apparatus  100  is provided with an electrostatic cleaning-type belt cleaning unit  90  on the secondary transfer belt  12  to collect the toner adhering on the secondary transfer belt  12 . Because the electrostatic cleaning-type belt cleaning unit  90  uses no cleaning blade, no trouble involved in the cleaning blade occurs. 
     (Belt Cleaning Unit) 
     As illustrated in  FIG. 2 , the belt cleaning unit  90  collects the toner positively charged by using a fur brush  92 B to which a negative voltage is applied after collecting the negatively charged toner by using a fur brush  91 B to which a positive voltage is applied. 
     The fur brushes  91 B and  92 B and metallic rollers  91 C and  92 C are connected to a drive motor M 90  via a gear mechanism, and rotate in directions of arrows, respectively, by being driven by the drive motor M 90 . 
     The fur brushes  91 B and  92 B rotate in a direction opposite from a moving direction of the secondary transfer belt  12  at a position facing the secondary transfer belt  12  and contact the secondary transfer belt  12 . The fur brush  92 B rotates in a direction also opposite to a rotation direction of the metallic roller  92 C and contacts the metallic roller  92 C. The fur brush  91 B rotates in a same direction with a rotation direction of the metallic roller  91 C and contacts the metallic roller  91 C. 
     The fur brushes  91 B and  92 B are implanted with hairs of conductive nylon fiber of which volume resistivity is 10 5  Ω·cm and a diameter thereof is 18 mm. An inroad amount of the secondary transfer belt  12  into the fur brushes  91 B and  92 B is 1.5 mm. A diameter of the metallic rollers  91 C and  92 C is 20 mm, respectively. An inroad amount of the metallic rollers  91 C and  92 C into the fur brushes  91 B and  92 B is 2 mm. A support roller  91 A is an aluminum columnar roller whose diameter is 13 mm. In the drive roller  23 , a rubber material of which a thickness is 0.5 mm and volume resistivity is 10 5  Ω·cm is covered on a circumferential surface of a stainless steel columnar roller. A diameter thereof is 22 mm. 
     The support roller  91 A is a metallic roller connected to a ground potential, rotates by being driven by the secondary transfer belt  12 , and supports the secondary transfer belt  12  which is contacted by the fur brush  91 B. A power supply  91 E applies a voltage with a positive polarity to the metallic roller  91 C. The fur brush  91 B which comes into contact with the metallic roller  91 C is charged with a positive polarity and adsorbs toner negatively charged and adhering on the secondary transfer belt  12 . The toner which is collected by the fur brush  91 B is moved to the metallic roller  91 C whose positive potential is higher and is then scraped off by a cleaning blade  91 D. 
     Still further, the toner of which the charged polarity is changed from a negative polarity to a positive polarity while rotating by adhering to the fur brush  91 B returns to the secondary transfer belt  12  from the fur brush  91 B, and is collected by the fur brush  92 B thereafter, in a process of passing through the fur brush  92 B. 
     The drive roller  23  is a metallic roller  23   a  covered with conductive rubber  23   b , rotationally drives the secondary transfer belt  12 , and supports the secondary transfer belt  12  which is contacted with the fur brush  92 B. A power supply  92 E applies a voltage with a negative polarity to the metallic roller  92 C. The fur brush  92 B which comes into contact with the metallic roller  92 C is charged with a negative polarity, and adsorbs toner positively charged and adhering on the secondary transfer belt  12 . The toner collected by the fur brush  92 B is moved to the metallic roller  92 C whose negative potential is higher and is then scraped off by a cleaning blade  92 D. 
     As described above, a cleaning portion  91  includes the fur brush  91 B as one example of the first brush roller, the support roller  91 A as one example of the first roller, and the power supply  91 E as one example of the first power supply. The fur brush  91 B contacts the outer circumferential surface of the secondary transfer belt  12  from which a recording medium on which the toner image had been transferred has been separated. The support roller  91 A comes into contact with the inner circumferential surface of the secondary transfer belt  12  at a position corresponding to the fur brush  91 B. In other words, the support roller  91 A is arranged so as to nip the secondary transfer belt  12  with the fur brush  91 B. The power supply  91 E generates an electric field between the fur brush  91 B and the support roller  91 A in a first direction. Still further, the metallic roller  91 C as the first collecting roller collects toner from the fur brush  91 B while being in contact with the fur brush  91 B. The cleaning blade  91 D as the first cleaning member removes the toner from the metallic roller  91 C while being contact with the metallic roller  91 C. 
     A cleaning portion  92  includes the fur brush  92 B as an example of the second brush roller, the drive roller  23  as an example of the second roller, and the power supply  92 E as an example of a second power supply. The fur brush  92 B contacts the outer circumferential surface of the secondary transfer belt  12  downstream of the fur brush  91 B in the moving direction of the secondary transfer belt  12 . In other words, the fur brush  92 B contacts the outer circumferential surface of the secondary transfer belt  12  which has been contacted by the fur brush  91 B. The drive roller  23  comes into contact with the inner circumferential surface of the secondary transfer belt  12  at a position corresponding to the fur brush  92 B. In other words, the drive roller  23  is arranged so as to nip the secondary transfer belt  12  with the fur brush  92 B. The power supply  92 E generates an electric field between the fur brush  92 B and the drive roller  23  in a second direction which is opposite to the first direction, i.e., in a direction opposite to the electric field generated by the power supply  91 E. Still further, the metallic roller  92 C, i.e., the second collecting roller, collects the toner from the fur brush  92 B while being in contact with the fur brush  92 B. The cleaning blade  92 D, i.e., a second cleaning member, removes the toner from the metallic roller  92 C while being in contact with the metallic roller  92 C. 
     Here, in the belt cleaning unit in which voltages whose polarities are reverse to each other are applied to the fur brush  91 B and the fur brush  92 B, it is unable to collect the toner charged with polarity to be collected by the upstream fur brush  91 B by the downstream fur brush  92 B. Due to that, it has been considered to collect the toner charged with the polarity to be collected by the fur brush  91 B is collected by the fur brush  91 B by increasing voltage to be applied to the upstream fur brush  91 B. However, it turned out that, when the voltage to be applied to the fur brush  91 B is increased, an amount of toner which cannot be collected by the downstream fur brush  92 B increases, because a charge amount increases due to charge injection caused by discharge in the fur brush  91 B. 
     For example, in the image forming apparatus  100 , if an amount of toner of a patch toner image increases, an amount of toner which can be cleaned also increases by increasing the voltage to be applied to the fur brush  91 B. However, in parallel with that, toner which returns to the secondary transfer belt  12  from the upstream fur brush  91 B also increases. 
     Then, the toner which passes through without being cleaned by the fur brush  91 B is mixed with toner charged with both polarities and cannot be completely cleaned by the downstream fur brush  92 B. Therefore, if the voltage to be applied to the fur brush  92 B is increased, the charged polarity of the toner is reversed also in the fur brush  92 B, causing passing-through. 
     Accordingly, it is desirable to provide an image forming apparatus in which, even when a voltage to be applied to the upstream fur brush  91 B is increased, it is possible to exhibit high cleaning performance with respect to the endless belt such as the secondary transfer belt  12  by efficiently collecting toner using the fur brush  91 B and the fur brush  92 B. 
     Therefore, it was verified whether or not there is a room for improving cleaning performance of the belt cleaning unit  90  by changing diameters of the drive roller  23  and the support roller  91 A to multiple stages. It was verified whether or not it is possible to secure sufficient cleaning latitude by which a cleaning failure can be prevented, even in the case when an amount of toner to be cleaned such as a patch toner image for adjusting density is increased in the configuration of an electrostatic fur brush cleaning. 
     (Comparison Experiment) 
     A comparison experiment by which the cleaning performance is studied by changing diameters of the support roller and the drive roller will be described. As illustrated in  FIG. 1 , because the secondary transfer belt unit  56  is downsized, it is preferable to downsize the belt cleaning unit  90 , as well. Still further, because a process speed of the image forming apparatus  100  is 250 mm/sec to 300 mm/sec, it is desirable for the belt cleaning unit  90  to exhibit sufficient toner cleaning performance with respect to the secondary transfer belt  12  which rotates at this peripheral velocity. 
     Therefore, as illustrated in  FIG. 3 , nine combinations were made by changing diameters of the drive roller  23  and the support roller  91 A to three types ( 23 L,  23 M,  23 S,  91 AL,  91 AM, and  91 AS) of 13 mm, 16 mm, and 22 mm, respectively. For each of the combinations, cleaning performance of the belt cleaning unit  90  was evaluated and compared by rotating the secondary transfer belt  12  at the same peripheral velocity of 300 mm/sec. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Diameter of 
                 Diameter of drive 
                 Cleaning 
               
               
                   
                 support roller 
                 roller 
                 performance 
               
               
                   
                   
               
             
            
               
                   
                 large 
                 large 
                 X 
               
               
                   
                 medium 
                 large 
                 X 
               
               
                   
                 small 
                 large 
                 ◯ 
               
               
                   
                 large 
                 medium 
                 X 
               
               
                   
                 medium 
                 medium 
                 X 
               
               
                   
                 small 
                 medium 
                 Δ 
               
               
                   
                 large 
                 small 
                 X 
               
               
                   
                 medium 
                 small 
                 X 
               
               
                   
                 small 
                 small 
                 Δ 
               
               
                   
                   
               
            
           
         
       
     
     As illustrated in Table 1, the toner cleaning performance of the belt cleaning unit  90  was improved when the diameter of 13 mm ( 91 AS) was selected for the support roller  91 A, and when the diameter of 22 mm ( 23 L) was selected for the drive roller  23 . Toner cleaning performance of the belt cleaning unit  90  was degraded when the diameter of 22 mm ( 91 AL) was selected for the support roller  91 A, and the diameter of 13 mm ( 23 S) was selected for the drive roller  23 . 
     Next, a computer simulation was performed under the conditions indicated in Table 1, and a contact length of the support roller  91 A being in contact with the secondary transfer belt  12  LR1, and a contact length of the fur brush  91 B being in contact with the secondary transfer belt  12  LB1 were obtained. Still further, a nip length of the secondary transfer belt  12  in contact with the support roller  91 A and the fur brush  91 B LN1, and a nip length of the secondary transfer belt  12  in contact with the drive roller  23  and the fur brush  92 B LN2 were similarly obtained. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Contact length on 
                   
                 Cleaning 
               
               
                 upstream side 
                 Comparison of nip length 
                 performance 
               
               
                   
               
             
            
               
                 brush contact length &lt; 
                 length on upstream side = 
                 X 
               
               
                 roller contact length 
                 length on downstream side 
               
               
                 brush contact length = 
                 length on upstream side &lt; 
                 X 
               
               
                 roller contact length 
                 length on downstream side 
               
               
                 brush contact length &gt; 
                 length on upstream side &lt; 
                 ◯ 
               
               
                 roller contact length 
                 length on downstream side 
               
               
                 brush contact length &lt; 
                 length on upstream side &gt; 
                 X 
               
               
                 roller contact length 
                 length on downstream side 
               
               
                 brush contact length = 
                 length on upstream side = 
                 X 
               
               
                 roller contact length 
                 length on downstream side 
               
               
                 brush contact length &gt; 
                 length on upstream side &lt; 
                 Δ 
               
               
                 roller contact length 
                 length on downstream side 
               
               
                 brush contact length &lt; 
                 length on upstream side &gt; 
                 X 
               
               
                 roller contact length 
                 length on downstream side 
               
               
                 brush contact length = 
                 length on upstream side &gt; 
                 X 
               
               
                 roller contact length 
                 length on downstream side 
               
               
                 brush contact length &gt; 
                 length on upstream side = 
                 Δ 
               
               
                 roller contact length 
                 length on downstream side 
               
               
                   
               
            
           
         
       
     
     As illustrated in Table 2, the cleaning performance was remarkably enhanced when a contact length of the fur brush  91 B in contact with the secondary transfer belt  12  (brush contact length LB1) is longer than that of the support roller  91 A in contact with the secondary transfer belt  12  (roller contact length LR1), and when a nip length of the secondary transfer belt  12  of the fur brush  92 B (length on a downstream side LN2) is longer than that of the fur brush  91 B (length on an upstream side LN1). 
     (Examination on Experiment Result) 
     As illustrated in  FIG. 2 , toner on the secondary transfer belt  12  which is charged with a negative polarity is moved to the fur brush  91 B to which a potential with a positive polarity is applied, in an electrostatic manner via the metallic roller  91 C. Still further, the toner which is charged with a negative polarity is moved to the metallic roller  91 C from the fur brush  91 B, and is cleaned using the cleaning blade  91 D thereafter. At this time, toner on the secondary transfer belt  12  which is charged with a positive polarity passes through the fur brush  91 B; however, the toner is collected by the fur brush  92 B which is disposed downstream of the fur brush  91 B, and to which a potential with a negative polarity is applied. 
     Still further, toner of which a charge with a negative polarity is lost by being in contact with the fur brush  91 B, or toner which is originally not charged is hardly moved to the metallic roller  91 C from the fur brush  91 B even when being transferred to the fur brush  91 B. Due to that, there is a case in which the toner of which a charge is lost, or the toner which is originally not charged returns to the secondary transfer belt  12  from the fur brush  91 B when being in contact with the secondary transfer belt  12  by passing through the metallic roller  91 C while adhering to the fur brush  91 B. 
     In many cases, in the toner which returned to the secondary transfer belt  12  when being in contact with the secondary transfer belt  12 , by passing through the metallic roller  91 C while adhering to the fur brush  91 B, a charged amount of the toner is changed due to discharging at the periphery of the fur brush  91 B. There also is a case in which a charged polarity of the toner is reversed. 
     Therefore, an experiment in which toner of which a charge is lost, or toner which is originally not charged is collected in the metallic roller  91 C was performed, by increasing a voltage with a positive polarity which will be applied to the metallic roller  91 C. However, when the voltage with a positive polarity which will be applied to the metallic roller  91 C is increased, toner particles which are charged with a high positive polarity are generated on the secondary transfer belt  12  due to discharging, and it was not possible to sufficiently collect toner in the fur brush  91 B to which a voltage with a negative polarity is applied. 
     Still further, in order to increase productivity of the image forming apparatus  100 , it is required to increase a rotation speed of the secondary transfer belt  12  by increasing a so-called process speed. In the belt cleaning unit  90 , when the rotation speed of the secondary transfer belt  12  is increased, it is required to also increase an applied voltage by increasing a rotation speed of the fur brush  91 B, since a time for the fur brush  91 B to contact the secondary transfer belt  12  becomes short. However, both of increasing the rotation speed of the fur brush  91 B and increasing the applied voltage easily cause discharging in the fur brush  91 B, which leads to an increase in toner particles of which a charged amount is large on the secondary transfer belt  12 . 
     Still further, when toner which is charged with a positive polarity and toner which is charged with a negative polarity reach the downstream fur brush  91 B together, the fur brush  92 B to which a voltage with a negative polarity is applied is incapable of completely collecting toner which is charged with a negative polarity. However, when it is set so as to collect toner which is charged with a negative polarity, by applying a voltage with a high positive polarity to the upstream fur brush  91 B, toner which returns to the secondary transfer belt  12  by being injected with a charge from the fur brush  91 B is charged with a high positive polarity. Due to that, the downstream fur brush  91 B is incapable of sufficiently collecting toner on the secondary transfer belt  12 . In an area in which the secondary transfer belt  12  passes through the fur brush  91 B with a rotation of the secondary transfer belt  12 , discharging is generated between the fur brush  91 B and toner particles on the secondary transfer belt  12 , and the toner particles on the secondary transfer belt  12  are charged with a positive polarity. Still further, the toner particles on the secondary transfer belt  12  which are charged with a high positive polarity due to discharging passes through the downstream fur brush  92 B which is applied with a potential with a negative polarity. 
     However, as illustrated in  FIG. 4 , when a diameter of the support roller  91 A is set to be small, for example, 13 mm, toner particles which are charged with a positive polarity are rarely generated on the secondary transfer belt  12 , even when a voltage with a positive polarity which will be applied to the metallic roller  91 C is increased. The reason for this is that, when a diameter of the support roller  91 A is set to be small, because the fur brush  91 B of which a diameter is relatively large starts to be separated from the secondary transfer belt  12  in an area which is a small electric field and is far from the support roller  91 A, discharging is hardly generated. It is considered that, in this manner, since the fur brush  91 B starts to be separated from the secondary transfer belt  12  at a position in which an electric field is weak by setting the diameter of the support roller  91 A small, toner is hardly injected with a charge, and toner which returns to the secondary transfer belt  12  is not charged with a high positive polarity. 
     Meanwhile, in the downstream fur brush  91 B, in an area in which the secondary transfer belt  12  is separated from the fur brush  92 B with a rotation of the secondary transfer belt  12 , there is almost no residual toner on the secondary transfer belt  12 . Due to that, there is no problem even when discharging is generated between the fur brush  92 B and the secondary transfer belt  12 . More than that, it is efficient to collect toner from the secondary transfer belt  12  by causing the fur brush  92 B to come into contact with an area of the secondary transfer belt  12  of which the inner circumferential surface is supported by the drive roller  23 , and in which a high electric field is generated as long as possible. 
     Due to that, as described above, it is considered that toner cleaning performance of the belt cleaning unit  90  is enhanced when setting a diameter of the drive roller  23  to 22 mm, compared to a case in which the diameter is set to 13 mm and 16 mm. It is considered that, in the downstream fur brush  91 B, it is possible to collect toner which is charged with a high positive polarity to some extent, by causing the fur brush  92 B contact the secondary transfer belt  12  as long as possible on the drive roller  23 . 
     (Examination on Discharging) 
     As illustrated in  FIG. 4 , a length of the first contact area in which the fur brush  91 B as an example of the first brush roller and the secondary transfer belt  12  which is an example of an endless belt come into contact with each other is set to LB1. A length of the second contact area in which the support roller  91 A as an example of the first roller and the secondary transfer belt  12  come into contact with each other is set to LR1. A length of a third contact area in which the fur brush  92 B as an example of the second brush roller and the secondary transfer belt  12  come into contact with each other is set to LB2. A length of a fourth contact area in which the drive roller  23  as an example of the second roller and the secondary transfer belt  12  come into contact with each other is set to LR2. A length of an area in which the first contact area and the second contact area overlap with each other in the moving direction of the secondary transfer belt  12  is set to LN1, and a length of an area in which the third contact area and the fourth contact area overlap with each other in the moving direction of the secondary transfer belt  12  is set to LN2. 
     As illustrated in  FIG. 4 , there is a possibility that discharging is generated in a vacant space R in which the secondary transfer belt  12  passes through the fur brush  91 B with a movement of the secondary transfer belt  12 . When a voltage is applied between the support roller  91 A and the fur brush  91 B, an electric field is strongest on a line L which is formed by connecting a center of the support roller  91 A and a center of the fur brush  91 B, and becomes gradually weak with the distance from the line L toward the upstream side and the downstream side. Due to that, discharging which is generated in the vacant space R becomes strong as being close to the line L, and becomes weak with the distance from the line L. 
     In this manner, since an electric field which causes discharging becomes strong when a distance between the vacant space R and the support roller  91 A is shorter, discharging which is generated in the vacant space R becomes strong when the distance between the vacant space R and the support roller  91 A is shorter. When such discharging is generated around toner, a charged polarity and a charged amount of toner are changed since a discharged charge jumps into toner. Accordingly, since it is possible to make the distance between the vacant space R and the support roller  91 A long by setting LB1&gt;LR1, discharging which is generated in the vacant space R is remarkably suppressed, and an amount of toner which returns to the secondary transfer belt  12  from the fur brush  91 B is reduced. 
     Meanwhile, when toner passes through at the downstream fur brush  91 B, back surface staining occurs in a recording medium such as a sheet. Toner reaching the fur brush  92 B includes not only toner which is charged with a reverse polarity (positive) but also uncharged toner with a charged amount of approximately zero which cannot be collected by the upstream fur brush  91 B. The uncharged toner adheres to the secondary transfer belt  12  due to a physical adhesive force, not an electrostatic adhesive force. In order to collect the uncharged toner, it is effective to make a contact area between the secondary transfer belt  12  which is supported by the drive roller  23  and the fur brush  92 B long by making the drive roller  23  large, compared to a case in which discharging in the vicinity of toner is reduced by the drive roller  23  being set to be small. That is, when LN1&lt;LN2, it is possible to collect much more toner from the secondary transfer belt  12  in the downstream fur brush  91 B. 
     Still further, LN1/LB1 and LN2/LB2 are ratios of an effective cleaning area (range in which cleaning electric field is strong) to a contact length of a fur brush. The larger the value, the fur brush can adsorb toner for a long time in a process of being in contact with the secondary transfer belt  12 . Due to that, when the relationships of LN1/LB1&lt;LN2/LB2, and LN1&lt;LN2 are satisfied, role sharing in which discharging is suppressed in the upstream fur brush  91 B, while enhancing toner collecting performance in the downstream fur brush  91 B becomes more clear. 
     As illustrated in  FIG. 5 , according to the first embodiment, the length LN2 in a range in which a cleaning electric field is strong is secured by causing the fur brush  92 B to come into contact with a curved face on which the secondary transfer belt  12  is wound around the drive roller  23 . 
     That is, the drive roller  23  which faces the fur brush  92 B by interposing the secondary transfer belt  12  therebetween is a roller which is wrapped by and stretches the secondary transfer belt  12 . Due to that, it is possible to make LN2 long. In contrast to this, when a secondary transfer belt  12 ′ is not stretched by the drive roller  23 , and a face of the secondary transfer belt  12 ′ which comes into contact with the fur brush  92 B is planar, a length of a range in which a cleaning electric field is strong becomes LN2′ which is shorter than LN2. Accordingly, it is possible to enhance cleaning performance of the belt cleaning unit  90  by causing the fur brush  92 B to come into contact with the curved face of the secondary transfer belt  12  which is wound around the drive roller  23 . 
     It is noted that discharging is also generated at a portion in which the secondary transfer belt  12  rushes into the fur brush  92 B with a movement of the secondary transfer belt  12 . However, a phenomenon in which toner returns to the secondary transfer belt  12  from the fur brush  92 B, easily occurs at a portion in which the secondary transfer belt  12  passes through the fur brush  92 B, not a portion in which the secondary transfer belt  12  rushes into the fur brush  92 B, and has a large influence on cleaning performance. 
     As described above, a length of the first contact area in which the fur brush  91 B comes into contact with the secondary transfer belt  12  is set to LB1. A length of the second contact area in which the support roller  92 A comes into contact with the secondary transfer belt  12  is set to LR1. 
     A length of an area in which LB1 and LR1 overlap with each other in the moving direction is set to LN1. 
     Still further, a length of a third contact area in which the fur brush  92 B comes into contact with the secondary transfer belt  12  is set to LB2. A length of a fourth contact area in which the drive roller  23  comes into contact with the secondary transfer belt  12  is set to LR2. A length of an area in which LB2 and LR2 overlap with each other in the moving direction is set to LN2. 
     According to the first embodiment, at this time, the relationships of LB1&gt;LR1 and LN1&lt;LN2 hold. At the same time, the relationships of LN1/LB1&lt;LN2/LB2 and LN1&lt;LN2 hold. 
     According to the first embodiment, due to a relationship of LB1&gt;LR1, injection of a charge into toner on the secondary transfer belt  12  due to discharging is reduced even when a voltage which is applied to the fur brush  91 B is increased. Still further, due to a relationship of LN1&lt;LN2, collecting efficiency with respect to toner with a large charged amount is improved. Accordingly, it is possible to exhibit high cleaning performance with respect to the secondary transfer belt  12  by efficiently collecting toner using the fur brush  91 B and the fur brush  92 B, even when a voltage which is applied to the fur brush  91 B is increased. 
     According to the first embodiment, since a relationship in each of the above described expressions is satisfied, a charged state of toner which passes through the fur brush  91 B, or toner which returns to the secondary transfer belt  12  enter a state in which the toner is easily collected by the fur brush  92 B, a cleaning failure hardly occurs. 
     According to the first embodiment, when setting a diameter of the support roller  91 A to DR1, and a diameter of the drive roller  23  to DR2, a relationship of DR1&lt;DR2 holds. Due to that, relationships of LB1&gt;LR1 and LN1&lt;LN2, or relationships of LN1/LB1&lt;LN2/LB2 and LN1&lt;LN2, and relationships of LB1&gt;LR1 and LN1&lt;LN2 can be easily satisfied. It is easy to design the belt cleaning unit  90  by assorting diameters of the fur brush  91 B and the fur brush  92 B, and an inroad amount of the secondary transfer belt  12  into the fur brushes  91 B and  92 B. 
     According to the first embodiment, the drive roller is a roller wrapped around and stretching the secondary transfer belt  12  after separating a recording medium therefrom. Due to that, it is not required to provide an exclusive support roller, and it is possible to downsize the secondary transfer belt unit  56 . 
     According to the first embodiment, the drive roller  23  is configured by covering the circumferential surface of a metallic roller  23   a  with conductive rubber  23   b . Due to that, a resistance between the metallic roller  23   a  and the fur brush  92 B increases compared to a case in which the metallic roller  23   a  is in direct contact with the secondary transfer belt  12 , and discharging between the secondary transfer belt  12  and the fur brush  92 B is hardly generated. 
     According to the first embodiment, a voltage of V 1  of which a polarity is reverse to a charged polarity of toner is applied to the fur brush  91 B, and a voltage of V 2  of which a polarity is the same as the charged polarity of toner is applied to the fur brush  92 B. At this time, a relationship of |V 1 |&gt;|V 2 | holds. Therefore, it is possible to collect transfer residual toner efficiently and in large quantities by the upstream fur brush  91 B while satisfying the relationship of LN1&lt;LN2, or the relationships of LN1/LB1&lt;LN2/LB2 and LN1&lt;LN2, and the relationships of LB1&gt;LR1 and LN1&lt;LN2. 
     Still further, in order to increase productivity of the image forming apparatus, it is required to increase a rotation speed of the secondary transfer belt  12  by increasing a so-called process speed. When the rotation speed of the secondary transfer belt  12  is increased, in the belt cleaning unit  90 , it is required to increase an applied voltage, as well, by increasing a rotation speed of the fur brush  91 B, since a time for the fur brush  91 B to contact a belt becomes short. 
     According to the first embodiment, it is possible to secure toner cleaning performance by making discharging is hardly generated in the fur brush  91 B, also in a case in which the rotation speed of the fur brush  91 B is increased, and also in a case in which the applied voltage is increased. Due to that, it is easy to cause a process to correspond to a high-speed process. 
     Second Embodiment 
     A second embodiment will be described using  FIGS. 6 and 7  while referring to  FIG. 1 . It is noted that in the first embodiment, the belt cleaning unit  90  of the secondary transfer belt  12  has been described. In contrast to this, in the second embodiment, the belt cleaning unit  45  of the intermediate transfer belt  40  will be described. Since configurations and effects other than that are the same as those in the first embodiment, descriptions and illustrations thereof are omitted or simplified by giving the same reference numerals to the same configurations, and hereinafter, portions which are different from those in the first embodiment will be mainly described. 
     (Belt Cleaning Unit) 
     As illustrated in  FIG. 1 , the exposing unit  4 Y and the developing unit  5 Y form a toner image, and cause the photosensitive drum  1 Y as the photosensitive member to bear the image. The primary transfer roller  6 Y as an example of the first transfer member transfers the toner image which is borne in the photosensitive drum  1 Y to the intermediate transfer belt  40  as an example of a moving endless belt. The secondary transfer outer roller  10  which is an example of the second transfer member transfers the toner image which is borne on the intermediate transfer belt  40  to a recording medium. 
     As illustrated in  FIG. 6 , the belt cleaning unit  45  collects toner which is charged with a negative polarity using a fur brush  192 B to which a voltage with a negative polarity is applied, after collecting toner which is charged with a positive polarity using a fur brush  191 B to which a voltage with a negative polarity is applied. 
     The fur brushes  191 B and  192 B, and metallic rollers  191 C and  192 C are connected to a drive motor M 190  via a gear mechanism, and rotate in the arrow direction, respectively, by being driven by the drive motor M 190 . The fur brushes  191 B and  192 B rotate in the opposite direction from the rotation direction of the intermediate transfer belt  40  at the position facing to the intermediate transfer belt  40  and contact the intermediate transfer belt  40 . The fur brushes  191 B and  192 B rotate in the same direction with the rotation direction of the metallic roller  191 C and  192 C and contact the metallic roller  191 C and  192 C. 
     The fur brushes  191 B and  192 B are implanted hairs of conductive nylon fiber of which volume resistivity is 10 5  Ω·cm, and a diameter thereof is 18 mm. An inroad amount of the intermediate transfer belt  40  into the fur brushes  191 B and  192 B is 1.5 mm. A diameter of the metallic rollers  191 C and  192 C is 20 mm. An inroad amount of the metallic rollers  191 C and  192 C into the fur brushes  191 B and  192 B is 2 mm. A support roller  191 A is an aluminum columnar roller, and a diameter thereof is 13 mm. In the drive roller  43 , a rubber material of which a thickness is 0.5 mm, and volume resistivity is 10 5  Ω·cm is covered on the circumferential surface of a stainless steel columnar roller, and a diameter thereof is 24 mm. 
     The support roller  191 A is connected to a ground potential, rotates by being driven by the intermediate transfer belt  40 , and supports the intermediate transfer belt  40  which is contacted with the fur brush  191 B. A power supply  191 E applies a voltage with a negative polarity to the metallic roller  191 C. The fur brush  191 B which comes into contact with the metallic roller  191 C is charged with a negative polarity, and adsorbs toner which is charged with a positive polarity and adheres to the intermediate transfer belt  40 . The toner which is collected by the fur brush  191 B is moved to the metallic roller  191 C of which a potential with a positive polarity is high, and is scratched off using a cleaning blade  191 D thereafter. 
     Still further, toner of which a charged polarity is changed from a negative polarity to a positive polarity while rotating by adhering to the fur brush  191 B returns to the intermediate transfer belt  40  from the fur brush  191 B, and is collected by the fur brush  192 B thereafter, in a process of passing through the fur brush  192 B. 
     The drive roller  43  is a metallic roller  43   a  which is covered with conductive rubber  43   b , rotates the intermediate transfer belt  40 , and supports the intermediate transfer belt  40  which is contacted with the fur brush  192 B. A power supply  192 E applies a voltage with a positive polarity to the metallic roller  192 C. The fur brush  192 B which comes into contact with the metallic roller  192 C is charged with a positive polarity, and adsorbs toner which is charged with a negative polarity and adheres to the intermediate transfer belt  40 . The toner which is collected by the fur brush  192 B is moved to the metallic roller  192 C of which a potential with a negative polarity is high, and is scraped off using a cleaning blade  192 D thereafter. 
     As described above, a cleaning unit  191  includes the fur brush  191 B as an example of the first brush roller, the support roller  191 A as an example of the first roller, and the power supply  191 E as an example of the first power supply. The fur brush  191 B contacts the outer circumferential surface of the intermediate transfer belt  40  after transferring a toner image to a recording medium. The support roller  191 A comes into contact with the inner circumferential surface of the intermediate transfer belt  40  at a position corresponding to the fur brush  191 B. In other words, the support roller  191 A is arranged so as to nip the intermediate transfer belt between the fur brush  191 B and the support roller. The power supply  191 E generates an electric field between the fur brush  191 B and the support roller  191 A in the first direction. Still further, the metallic roller  191 C as a first collecting roller collects toner from the fur brush  191 B while being in contact with the fur brush  191 B. The cleaning blade  191 D as the first cleaning member removes toner from the metallic roller  191 C while being in contact with the metallic roller  191 C. 
     The cleaning unit  192  includes the fur brush  192 B as an example of the second brush roller, the drive roller  43  as an example of the second roller, and the power supply  192 E as an example of a second power supply. The fur brush  192 B contacts the outer circumferential surface of the intermediate transfer belt  40  on the downstream of the fur brush  191 B in the moving direction of the intermediate transfer belt  40 . In other words, the fur brush  192 B contacts the outer circumferential surface of the intermediate transfer belt  40  in a sliding manner after being contacted with the fur brush  191 B in a sliding manner. The drive roller  43  comes into contact with the inner circumferential surface of the intermediate transfer belt  40  at a position corresponding to the fur brush  192 B. In other words, the drive roller  43  is arranged so as to nip the intermediate transfer belt  40  between the fur brush  192 B and the drive roller. The power supply  192 E generates an electric field in the second direction which is opposite to the first direction, that is, in a direction opposite to an electric field which is generated by the power supply  191 E between the fur brush  192 B and the drive roller  43 . Still further, the metallic roller  192 C as a second collection roller collects toner from the fur brush  192 B while being in contact with the fur brush  192 B. The cleaning blade  192 D as a second cleaning member removes toner from the metallic roller  192 C while being in contact with the metallic roller  192 C. 
     (Examination on Discharging) 
     As illustrated in  FIG. 7 , a length of a first contact area in which the fur brush  191 B and the intermediate transfer belt  40  come into contact with each other is set to LB1. A length in a second contact area in which the support roller  191 A and the intermediate transfer belt  40  come into contact with each other is set to LR1. A length of an area in which LB1 and LR1 overlap with each other in the moving direction is set to LN1. 
     A length of a third contact area in which the fur brush  192 B comes into contact with the intermediate transfer belt  40  is set to LB2. A length of a fourth contact area in which the drive roller  43  comes into contact with the intermediate transfer belt  40  is set to LR2. A length of an area in which LB2 and LR2 overlap with each other in the moving direction is set to LN2. 
     At this time, the relationships of LB1&gt;LR1 and LN1&lt;LN2 hold, similarly to the first embodiment also in the second embodiment. At the same time, the relationships of LN1/LB1&lt;LN2/LB2 and LN1&lt;LN2 hold. 
     Due to that, also in the second embodiment, in the cleaning unit  191 , discharging is remarkably suppressed, and an amount of toner which returns to the intermediate transfer belt  40  from the fur brush  191 B is reduced. In the cleaning unit  192 , it is possible to collect much more toner from the intermediate transfer belt  40 . Role sharing in which toner collecting performance is enhanced in the fur brush  192 B on the downstream side, while reducing discharging in the fur brush  191 B on the upstream side become clear. 
     According to the second embodiment, a relationship of DR1&lt;DR2 holds when a diameter of the support roller  191 A is set to DR1, and a diameter of the drive roller  43  is set to DR2. Due to that, it is possible to easily establish a relationship in the above described each expression. 
     According to the second embodiment, the drive roller  43  is a roller wrapped around and stretching the intermediate transfer belt  40  after transferring a toner image to a recording medium. Due to that, it is not required to provide an exclusive counter roller which faces the fur brush  192 B. 
     According to the second embodiment, the drive roller is configured by covering the circumferential surface of the metallic roller  43   a  with conductive rubber  43   b . Due to that, discharging is hardly generated between the fur brush  192 B and the intermediate transfer belt  40 . 
     According to the second embodiment, the voltage V 1  with the same polarity with a charged polarity of toner is applied to the fur brush  191 B, and the voltage V 2  with a polarity reverse to a charged polarity of toner is applied to the fur brush  192 B. At this time, the relationship of |V 1 |&gt;|V 2 | holds. Due to that, in the fur brush  191 B on the upstream side, it is possible to efficiently collect toner which is charged with a positive polarity, and is a major component in transfer residual toner while satisfying the relationship of LN1&lt;LN2. Due to that, the intermediate transfer belt  40  can easily cope with high speed. 
     OTHER EMBODIMENTS 
     The above described first and second embodiments are merely examples of the embodiments of this disclosure, and this disclosure is not limited to specific configurations of the above described first and second embodiments. 
     In the first embodiment, a belt cleaning unit of a belt which conveys a recording medium while bearing the medium has been described; however, there is no limitation to this, and it is also possible to execute this disclosure in a belt cleaning unit of a photosensitive member belt. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2014-196385, filed Sep. 26, 2014, which is hereby incorporated by reference herein in its entirety.