Patent Publication Number: US-2013243473-A1

Title: Cleaning device and image forming apparatus using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2012-055900 filed Mar. 13, 2012. 
     BACKGROUND  
     (i) Technical Field 
     The present invention relates to a cleaning device and an image forming apparatus using the same. 
     (ii) Related Art 
     In general, in an image forming apparatus, such as an electrophotographic copying machine or a laser beam printer, a toner image (developer image) is formed on the surface of a photoconductor drum (an exemplary image carrier, an exemplary member to be cleaned) in accordance with image data. Then, the toner image is transferred to a recording medium (such as a sheet) using a transfer member (an exemplary transfer unit, an exemplary member to be cleaned), to fix the transferred toner to the recording medium by heating, so that a recorded image is obtained. 
     The following type of recent full-color copying machines and full-color laser beam printers is also known. That is, in this type, toner images formed on photoconductor drums are first-transferred to an intermediate transfer belt (an exemplary image carrier, an exemplary member to be cleaned) using a first transfer member (an exemplary transfer unit, an exemplary member to be cleaned), and the toner images of four colors, yellow, cyan, magenta, and black, are superimposed upon each other on the intermediate transfer belt. The composite toner image is collectively second-transferred to a recording medium using a second transfer member (an exemplary transfer unit, an exemplary member to be cleaned), to form a full-color recorded image. 
     Transfer efficiency of a toner image is influenced by variations in the resistance value of the intermediate transfer belt and the resistance value of the recording medium as the surface state of any photoconductor drum, the surface state of the intermediate transfer belt, and the temperature/humidity change. Therefore, it is difficult to always maintain the transfer efficiency of the toner image to 100%. Even after the transfer of the toner image, toner remains and is adhered to the surface of a photoconductor drum and the surface of the intermediate transfer belt, to which surfaces the toner image is transferred. 
     Therefore, hitherto, cleaning devices are provided at a downstream-side of toner-image transfer portions for the photoconductor drums, the transfer member, the first transfer member, the second transfer member, and the intermediate transfer belt. Prior to the formation of the next toner image, residual toner is removed. 
     A cleaning device having the following structure is known as one cleaning device of this type. In this structure, a cleaning brush having the form of a roller and having conductive fibers implanted therein is brought into contact with a member to be cleaned (such as, the second transfer member), and a potential difference (bias voltage) that is in accordance with a charging polarity of toner is applied between the cleaning brush and the member to be cleaned, so that residual toner and other residual particles are electrostatically attracted to the cleaning brush from the member to be cleaned. 
     In the cleaning device having such a structure, there is a limit as to the amount of toner that is capable of being carried by the cleaning brush. If the cleaning brush holding a large amount of toner contacts the member to be cleaned, the cleaning performance itself is reduced. Therefore, what is called a detoning mechanism for separating and collecting the toner from the cleaning brush is required. That is, a collecting roller, which is called a detoning roller, is provided within the range of rotation of the cleaning brush, to collect the toner of the member that is cleaned removed by the cleaning brush. Further, the detoning roller is provided with a scraper. The scraper scrapes off the toner collected by the detoning roller, so that the toner is collected in a collecting box using a transporting unit. 
     In the cleaning device having this structure, by applying bias voltage, attraction force is generated between the member to be cleaned and brush portions (exemplary contactors), themselves, of the cleaning brush. Therefore, a torque that is larger than an ordinary rotating torque is required at a motor (exemplary driving unit) that rotationally drives the cleaning brush and the member to be cleaned. This required torque is increased as the bias voltage is increased. 
     In order to maintain the cleaning performance, it is necessary to apply the required amount of bias voltage to the brush portions as the member to be cleaned changes with time. If the torque varies in accordance with this, the operation of the motor becomes unstable. Therefore, it becomes necessary to stabilize the torque. 
     Due to the nature of the motor, as the rotational speed is increased, the upper limit of the torque is reduced. Therefore, when the cleaning brush is highly dependent upon the bias voltage, it is necessary to reduce the torque of the entire cleaning brush. However, if a force at an end is reduced for reducing the torque (that is, if an interference of an end of each brush portion with respect to the member to be cleaned is reduced), the cleaning performance is sacrificed. 
     SUMMARY  
     According to an aspect of the invention, there is provided a cleaning device including a cleaning unit including a shaft portion and contactors, the shaft portion being rotatably provided, the contactors being provided along an outer periphery of the shaft portion so as to contact a member to be cleaned, the contactors being formed of conductive fiber, the cleaning unit cleaning the member to be cleaned by rotating and by electrostatically attracting dirt adhered to the member to be cleaned. In the cleaning device, the contactors satisfy the following condition: 0&lt;fiber density (kF/inch 2 )×interference (mm) with respect to the member to be cleaned/fineness (denier)&lt;48. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a conceptual view of an exemplary image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 2  is a conceptual view of an enlarged principal portion of the image forming apparatus shown in  FIG. 1 ; 
         FIG. 3  is a conceptual view of an off-line bench used in examining cleaning devices according to the exemplary embodiment of the present invention; 
         FIG. 4  is a table of patterns of bias voltages that are applied to cleaning brushes and detoning rollers of the cleaning devices according to the exemplary embodiment of the present invention; 
         FIG. 5  is a table showing the relationship between the rotational frequencies of the cleaning brushes of the cleaning devices according to the exemplary embodiment of the present invention and the bias voltages that are applied to the cleaning brushes of the cleaning devices according to the exemplary embodiment of the present invention; 
         FIG. 6  is a table showing the results of measurements obtained by examining the cleaning devices according to the exemplary embodiment of the present invention; 
         FIG. 7  is a graph showing the relationship between the torque difference and the value equal to fiber density (kF/inch 2 )×interference (mm)/fineness (denier), which are based on the results of measurements obtained in  FIG. 6 ; 
         FIG. 8  is a graph showing the relationship between the torque ratio and the value equal to fiber density (kF/inch 2 )×interference (mm)/fineness (denier), which are based on the results of measurements obtained in  FIG. 6 ; 
         FIG. 9  is a photograph of the behaviors of two types of cleaning brushes; 
         FIG. 10  shows a device used for photographing the behaviors of the cleaning brushes; and  FIG. 11  illustrates linear contact of a brush portion. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment of the present invention will hereunder be described in detail on the basis of the drawings. In the drawings for illustrating the exemplary embodiment, in general, corresponding structural elements are given the same reference numerals, and the same descriptions will not be repeated below. 
       FIG. 1  is a conceptual view of an exemplary image forming apparatus  1  according to an exemplary embodiment of the present invention. 
     The image forming apparatus  1  according to the exemplary embodiment is, for example, a tandem color printer. The image forming apparatus  1  includes image forming units  20 , an intermediate transfer belt (an exemplary image carrier, an exemplary member to be cleaned)  30 , a backup roller  41  and a second transfer device (an exemplary transfer unit, an exemplary member to be cleaned)  42  that form a pair, sheet supply trays  50   a  and  50   b , a sheet transporting system  60 , and a fixing device  70 . 
     The image forming units  20  include four color image forming units  20 Y,  20 M,  20 C, and  20 K, and image forming units  20 CL and  20 CL for transparent colors. The color image forming units  20 Y,  20 M,  20 C, and  20 K form, for example, toner images (exemplary developer images) of four colors, yellow, magenta, cyan, and black. The image forming units  20 CL and  20 CL transfer, for example, toner images of transparent colors. In order for toner images formed in accordance with pieces of image information of corresponding colors to be first-transferred to the intermediate transfer belt  30 , the six image forming units  20 CL,  20 CL,  20 Y,  20 M,  20 C, and  20 K are disposed in accordance with a transparent color, a transparent color, yellow, magenta, cyan, and black in that order along the direction of rotation of the intermediate transfer belt  30 . Instead of the image forming units for transparent colors, for example, an image forming unit for a light color, such as light yellow, light magenta, light cyan, or light black, that transfers a toner image of a light color may be provided. Alternatively, an image forming unit  20 CL for a transparent color and an image forming unit for a light color may be disposed side by side. 
     Each image forming unit  20  includes a photoconductor drum (an exemplary image carrier, an exemplary member to be cleaned)  21 , a charging device  22 , an exposure device  23 , a developing device  24 , a first transfer roller  25  (an exemplary transfer unit, an exemplary member to be cleaned), and a drum cleaner  26 . Each charging device  22  charges the surface of its corresponding photoconductor drum  21  to a specified potential. Each exposure device  23  irradiates the corresponding charged photoconductor drum  21  with laser light L to form an electrostatic latent image. Each developing device  24  develops the electrostatic latent image formed on the corresponding photoconductor drum  21  by the corresponding exposure device  23  to form a toner image. Each first transfer roller  25  transfers the toner image carried on its corresponding photoconductor drum  21  to the intermediate transfer belt  30  at a first transfer section. Each drum cleaner  26  removes residual toner or paper powder from the surface of its corresponding photoconductor drum  21  after the transfer of the toner image. Toner cartridges  27  that supply developer to the developing devices  24  are set at upper sides of the respective image forming units  20 . 
     The first transfer rollers  25  of the corresponding image forming units  20  are disposed so that the first transfer rollers  25  and the corresponding photoconductor drums  21  nip the intermediate transfer belt  30 . By applying a transfer bias voltage having a polarity that is opposite to that of a charging polarity of toner (an exemplary developer) to each first transfer roller  25 , electric fields are formed between the photoconductor drums  21  and the corresponding first transfer rollers  25 . Therefore, the toner images that are charged on the corresponding photoconductor drums  21  are transferred to the intermediate transfer belt  30  by coulomb forces. The photoconductor drums  21  rotate clockwise during the first transfer. 
     The intermediate transfer belt  30  is a member to which the toner images of the corresponding color components, formed by the corresponding image forming units  20 , are successively transferred (first-transferred) for carrying the toner images. The intermediate transfer belt  30  is an endless belt that is placed on supporting rollers  31   a  to  31   f  and the backup roller  41 . The toner images formed by the corresponding image forming units  20 CL,  20 CL,  20 Y,  20 M,  20 C, and  20 K are first-transferred to the intermediate transfer belt  30  while the intermediate transfer belt  30  rotates counterclockwise in a peripheral direction. 
     The backup roller  41  and the second transfer device  42  that form a pair constitute a mechanism for forming a full-color image by collectively transferring (second-transferring) the toner images transferred to and superimposed on the intermediate transfer belt  30  to a sheet (an exemplary recording medium), and are disposed so as to oppose each other with the intermediate transfer belt  30  being nipped therebetween. A portion where the backup roller  41  and the second transfer device  42  oppose each other corresponds to a second transfer section. 
     The backup roller  41  is rotatably set at the inner surface of the intermediate transfer belt  30 . The second transfer device  42  is rotatably set while opposing a toner-image transfer surface of the intermediate transfer belt  30 . The backup roller  41  and the second transfer device  42  are disposed so that their directions or rotational axes (that is, their directions perpendicular to the plane of  FIG. 1 ) are parallel to each other. 
     The second transfer device  42  includes a second transfer transport belt  42   a  disposed at the toner-image transfer surface of the intermediate transfer belt  30 . The second transfer transport belt  42   a  is placed on a drive roller  42   b  and a driven roller  42   c.  The drive roller  42   b  is formed of, for example, a metal (such as SUS) or is a rubber roller that is semiconductive. The drive roller  42   b  is driven by a motor (an exemplary driving unit) (not shown). The driven roller  42   c  is, for example, a rubber roller. The second transfer transport belt  42   a  is an endless annular belt that is formed of conductive resin, such as polyimide, and that is not scratched often. The second transfer transport belt  42   a  is rotationally driven by the drive roller  42   b , and is subjected to tension by the drive roller  42   b  and the driven roller  42   c.    
     The drive roller  42   b  is disposed so as to press-contact the backup roller  41  while the second transfer transport belt  42   a  and the intermediate transfer belt  30  are nipped between the drive roller  42   b  and the backup roller  41 . The drive roller  42   b  functions as a second transfer roller for second-transferring the toner images to a sheet that is transported along the second transfer transport belt  42   a.    
     The diameter of the driven roller  42   c  is small so as not to allow the second transfer transport belt  42   a  to wind thereupon even if, for example, a thin coated sheet is transported. 
     Cleaning devices (exemplary cleaning devices) C 1  and C 2  that contact and clean the second transfer device  42  are disposed. The cleaning devices C 1  and C 2  will be described below. 
     When transferring the toner images on the intermediate transfer belt  30 , a voltage whose polarity is the same as the toner charging polarity is applied to the backup roller  41 , or a voltage whose polarity is opposite to the toner charging polarity is applied to the second transfer device  42 . This causes a transfer electric field to be formed between the backup roller  41  and the second transfer device  42 , so that the toner images carried by the intermediate transfer belt  30  are transferred to a sheet. 
     For example, sheets of various sizes and thicknesses are held in the sheet supply trays  50   a  and  50   b . The sheets in the sheet supply trays  50   a  and  50   b  are drawn out by a pickup roller (not shown) of the sheet transporting system  60 . Then, a timing of the sheets is controlled by registration rollers  62  of the sheet transporting system  60 , and the sheets are introduced into the second transfer section, so that the toner images are transferred to the sheets. Thereafter, the sheets are transported to the fixing device  70  by transporting belts  63  and  64  of the sheet transporting system  60 . 
     The fixing device  70  fixes unfixed toner images, transferred to, for example, a sheet at the second transfer section, to the sheet by thermocompression. The fixing device  70  includes a heating roller  71 , which includes a heater (not shown), and a pressure roller  72 , provided so as to oppose the heating roller  71 . 
     After the second transfer, the sheet is transported to a fixing nip where the heating roller  71  and the pressure roller  72  oppose each other, and is discharged while being nipped between the heating roller  71  and the pressure roller  72 . At this time, for example, the sheet is heated by the heating roller  71 , and is pressed by the pressure roller  72 , so that toner images are fixed to, for example, the sheet. For example, the sheet that has passed through the fixing device  70  is sent to a discharge roller (not shown) by a transporting belt  65 , and is discharged to the outside of the image forming apparatus  1 . 
     As shown in  FIG. 2 , in the exemplary embodiment, the cleaning devices serving as exemplary cleaning devices that clean the second transfer transport belt  42   a  include the first cleaning device C 1  and the second cleaning device C 2 . The first cleaning device C 1  includes a conductive cleaning brush (an exemplary cleaning unit)  73  having the form of a roller, and a detoning roller (an exemplary collecting unit)  74 . The second cleaning device C 2  includes a conductive cleaning brush (an exemplary cleaning unit)  76  having the form of a roller, and a detoning roller (an exemplary collecting unit)  77 . Scrapers  75  and  78  are provided at the respective detoning rollers  74  and  77  so as to press-contact the respective detoning rollers  74  and  77 . 
     The conductive cleaning brushes  73  and  76  are disposed in contact with the second transfer transport belt  42   a , and remove any dirt, such as toner, adhered to the second transfer transport belt  42   a . The detoning rollers  74  and  77  are disposed within the range of rotation of the cleaning brush  73  and the range of rotation of the cleaning brush  76 , respectively, and collect the dirt removed by the respective cleaning brushes  73  and  76 . The scrapers  75  and  78  scrape off the dirt collected by the respective detoning rollers  74  and  77 . The scraped off dirt is collected in a collecting box using a transporting unit (not shown). 
     As shown in  FIG. 2 , the cleaning brushes  73  and  76  include respective shafts (exemplary shaft portions)  73   a  and  76   a , which are rotatably provided, and brush portions (exemplary contactors)  73   b  and  76   b , which are provided at outer peripheries of the respective shafts  73   a  and  76   a.    
     Here, the shafts  73   a  and  76   a  are formed of, for example, conductive materials such as metals (aluminum, stainless steel, etc.) or materials in which conductive particles are mixed in resin or synthetic rubber. A motor (an exemplary driving unit) (not shown) rotates the shafts  73   a  and  76   a  in a direction of rotation that is the same as a direction of rotation of the drive roller  42   b  and the driven roller  42   c  (clockwise in the drawings). The shafts  73   a  and  76   a  are electrically connected to a power supply (not shown), and are such that bias voltages are applied thereto. 
     The brush portions  73   b  and  76   b  are formed on the outer peripheral surfaces of the corresponding shafts  73   a  and  76   a  by, for example, electrostatic frocking or pile weaving. The brush portions  73   b  and  76   b  are formed of, for example, conductive fibers having lengths on the order of from 4.5 to 5.0 mm. The brush portions  73   b  and  76   b  are provided so as to extend radially from the outer peripheries of the corresponding shafts  73   a  and  76   b , and contact the second transfer device  42  (more exactly, the second transfer transport belt  42   a  of the second transfer device  42 ) serving as a member to be cleaned. The brush portions  73   b  and  76   b  are formed by mixing conductive particles (such as carbon black particles) in, for example, nylon, acryl, or polyester. 
     The cleaning brush  73  including the shaft  73   a  and the brush portions  73   b  and the cleaning brush  76  including the shaft  76   a  and the brush portions  76   b  rotate around the shaft  73   a  and the shaft  76   a , respectively, so that, in accordance with the procedure that is described later, any dirt adhered to the second transfer device  42  is electrostatically attracted to the brush portions  73   b  and  76   b  to clean the second transfer device  42 . 
     The detoning rollers  74  and  77  are each in the form of a pipe in which the outer periphery of a cored bar is covered with resin, such as phenol resin. A motor (an exemplary driving unit) (not shown) rotates the detoning rollers  74  and  77  in a direction of rotation that is the same as a direction of rotation of the shafts  73   a  and  76   a  (clockwise in the drawings). The detoning rollers  74  and  77  are also electrically connected to a power supply (not shown), and are such that bias voltages are applied thereto. 
     Bias voltages having different polarities are applied to the first cleaning device C 1 , which is positioned at an upstream side in the direction of rotation of the second transfer transport belt  42   a , and the second cleaning device C 2 , which is positioned at a downstream side in the direction of rotation of the second transfer transport belt  42   a.  That is, since there are variations in the electric charge of toner remaining on the second transfer transport belt  42   a  after the transfer, a bias voltage having a positive polarity for attracting toner that is negatively charged is applied to the first cleaning device C 1 , and a bias voltage having a negative polarity for attracting toner that is positively charged is applied to the second cleaning device C 2 . 
     In the first cleaning device C 1 , a positive polarity is applied so that the bias voltage of the cleaning brush  73  and the bias voltage of the detoning roller  74  differ from each other. That is, the bias voltage at the cleaning brush  73  is low, whereas the bias voltage at the detoning roller  74  is high. In the second cleaning device C 2 , a negative polarity is applied so that the bias voltage of the cleaning brush  76  and the bias voltage of the detoning roller  77  differ from each other. That is, the bias voltage at the cleaning brush  76  is low, whereas the bias voltage at the detoning roller  77  is high. In this way, by causing the bias voltages at the detoning rollers  74  and  77  to be greater than the bias voltages at the cleaning brushes  73  and  76 , respectively, the toner collected by the cleaning brushes  73  and  76  is smoothly transferred to the detoning rollers  74  and  77 , so that the cleaning performance of the cleaning brushes  73  and  76  is maintained. 
     As mentioned above, in the cleaning device C 1  and the cleaning device C 2  having this structure, by applying bias voltages, attraction force is generated between the brush portions  73   b  of the cleaning brush  73 , themselves, and the second transfer transport belt  42   a  of the second transfer device  42  (serving as a member to be cleaned), and between the brush portions  76   b  of the cleaning brush  76 , themselves, and the second transfer transport belt  42   a  of the second transfer device  42 . Therefore, a torque that is larger than an ordinary rotating torque is required at a motor that rotationally drives the cleaning brushes  73  and  76  and the second transfer transport belt  42   a . This required torque is increased as the bias voltages are increased. 
     In order to maintain the cleaning performance, it is necessary to apply the required amounts of bias voltages to the brush portions  73   b  and  76   b  as the second transfer transport belt  42   a  serving as a member to be cleaned changes with time. If the torque varies in accordance with this, the operation of the motor becomes unstable. Therefore, it becomes necessary to stabilize the torque. 
     Due to the nature of the motor, as the rotational speed is increased, the upper limit of the torque is reduced. Therefore, when the cleaning brushes  73  and  76  are highly dependent upon the bias voltages, it is necessary to reduce the torques of the entire cleaning brushes  73  and  76 . However, if, for reducing the torque, the interference of the end of each of the brush portions  73   b  and  76   b  with respect to the second transfer transport belt  42   a  (a radial length of a portion of each of the rotating cleaning brushes  73   b  and  76   b  that interferes with the second transfer transport belt  42   a ), that is, a force at the end of each of the brush portions  73   b  and  76   b  is reduced, the cleaning performance is sacrificed. 
     Therefore, the present inventors have considered the suppression of variations in the rotating torque of the motor that rotationally drives the cleaning brushes  73  and  76  and the second transfer transport belt  42   a  without reducing the cleaning performance. 
     Here, a conceptual view of an offline bench used in considering the suppression of variations in the rotating torque of the motor is shown in  FIG. 3 . 
     In  FIG. 3 , the structure of the first cleaning device C 1  and the structure of the second cleaning device C 2  are as described above. The structure corresponding to the second transfer device  42  (serving as a member to be cleaned) includes a belt  142   a  (corresponding to the second transfer transport belt  42   a ), one drive roller  142   b  (corresponding to the drive roller  42   b ) upon which the belt  142   a  is placed, and driven rollers  142   c  (corresponding to the driven rollers  42   c ) upon which the belt  142   a  is placed. The belt  142   a  is rotationally driven by the drive roller  142   b , and is subjected to tension by the drive roller  142   b  and the driven rollers  142   c.  In order to measure the rotating torque at the position of the drive roller  142   b , a torque gauge  142   d  is set at the drive roller  142   b.    
     Similarly to the second transfer device  42 , the belt  142   a  is an endless annular belt that is formed of a conductive resin material, such as polyimide, and that is not scratched often. The drive roller  142   b  is formed of, for example, a metal (such as SUS) or is a rubber roller that is semiconductive. The driven roller  42   c  is, for example, a rubber roller. 
     Next, the patterns of bias voltages that are applied to the cleaning brushes  73  and  76  and the detoning rollers  74  and  77  are shown in  FIG. 4 . As shown in  FIG. 4 , voltages are applied on the basis of four patterns, patterns A to D. 
     The relationships between the rotational frequencies of the cleaning brushes  73  and  76  and the bias voltages that are applied to the cleaning brushes  73  and  76  are shown in  FIG. 5 . As shown in  FIG. 5 , the load is lightest when the rotational frequency is 1092 kHz and the bias voltage is 0 V (Condition 1), and the load is heaviest when the rotational frequency is 1692 kHz and the bias voltage is ±700 V (Condition 2). Therefore, the mode in which variations of the rotating torque of the motor are reduced corresponds to a mode in which variations in the rotating torque in Conditions 1 and 2 are reduced. 
     As shown in  FIG. 6 , measurement results are obtained by setting the materials, the finenesses (denier), the fiber densities (kF/inch 2 ), and the interferences (mm) (that is, the interferences into the belt  142   a ) of the brush portions  73   b  and  76   b  of the corresponding cleaning brushes  73  and  76 . The items of the measurement results are: 
     the value equal to fiber density (kF/inch 2 )×interference (mm)/fineness (denier), 
     the difference between the maximum torque and the minimum torque, 
     the ratio between the maximum torque and the minimum torque, 
     the minimum torque (CN—m) (=torque in Condition 1), and 
     the maximum torque (CN·m) (=torque in Condition 2). 
     In  FIG. 6 , “the variation width of the rotating torque is small” means that the difference between the maximum torque and the minimum torque (torque difference) is small or that the ratio between the maximum torque and the minimum torque (torque ratio) is small. 
     Here, on the basis of the measurement results obtained in  FIG. 6 , the relationship between the value equal to fiber density (kF/inch 2 )×interference (mm)/fineness (denier) and the torque difference is shown in the graph of  FIG. 7 , and the relationship between the value equal to fiber density (kF/inch 2 )×interference (mm)/fineness (denier) and the torque ratio is shown in the graph of  FIG. 8 . 
     As shown in these figures, it is understood that, when the value equal to fiber density (kF/inch 2 )×interference (mm)/fineness (denier) is less than 48, the torque difference and the torque ratio are reduced suddenly. That is, the torque variation is small. Further, it is understood that, when this value is less than 31, there is almost no torque difference, so that the torque ratio approximates to 1, that is, the torque variation is further reduced. Therefore, when such cleaning brushes are used in the image forming apparatus, the members that are cleaned are reliably cleaned. 
     In general, when bias voltages are applied to cleaning brushes, the load torque of a member that is driven (the second transfer transport belt  42   a  in the exemplary embodiment) is increased in accordance with the applied bias voltage and rotational speed. However, it is found that, when brush portions manufactured by way of trial and whose materials, finenesses, fiber densities, and interferences are changed are tested, the following is true. That is, as shown in  FIGS. 6 to 8 , regardless of the rotational speeds and the applied bias voltages of the cleaning brushes, it is possible to obtain cleaning brushes that allow the load torque of the member that is driven to be stabilized, that is, that suppress variations in the load torque of the member that is driven. 
     That is, it is ascertained that, regardless of the materials of the cleaning brushes (that is, whether or not the materials of the cleaning brushes are, for example, nylon, polyester, or acryl, which is easily processed into fibers at a low cost), it is possible to obtain cleaning brushes having considerably reduced torque variations when the finenesses (denier), the fiber densities (kF/inch 2 ), and the interferences (mm) of the cleaning brushes are maintained in the following conditions. 
     That is, the conditions are: 
       0&lt;fiber density (kF/inch 2 )×interference (mm)/fineness (denier)&lt;48,
 
       desirably, 0&lt;fiber density (kF/inch 2 )×interference (mm)/fineness (denier)&lt;31.
 
     Regarding a mechanism that causes the above to occur, the behaviors of cleaning brushes are analyzed. The results of the analysis are shown in  FIG. 9 . 
     In  FIG. 9 , each brush portion is formed of nylon, and has a length of 4.875 mm. The two upper and low photographs on the left in  FIG. 9  show the behavior of the brush portions having a fineness of 2 denier, a fiber density of 120 kF/inch 2 , and an interference of 1.0 mm (fiber density×interference/fineness=60; torque variation is increased). The upper photograph shows the behavior when a bias voltage is not applied and the cleaning brush is rotated, whereas the lower photograph shows the behavior when a bias voltage is applied and the cleaning brush is rotated. The two upper and lower photographs on the right in  FIG. 9  show the behavior of the brush portions having a fineness of  6  denier, a fiber density of 100 kF/inch 2 , and an interference of 1.0 mm (fiber density×interference/fineness=16.7; torque variation is reduced). The upper photograph shows the behavior when a bias voltage is not applied and the cleaning brush is rotated, whereas the lower photograph shows the behavior when a bias voltage is applied and the cleaning brush is rotated. 
     These photographs are taken by a device such as that shown in  FIG. 10 . That is, a thin-film layer  81  formed by evaporating a metal is formed on a transparent glass  80 , and an insulating transparent tape  82  is bonded to the thin-film layer  81 , so that a substrate  83  is formed. Cleaning brushes  73  and  76  are rotated so as to contact the transparent tape  82 , and these are photographed with an imaging device  84 , such as a charge coupled device (CCD) camera, from the opposite side. 
     In these photographs, as shown in  FIG. 11 , portions that are shining white are where ends of the brush portions  73   b  and  76   b  linearly contact the substrate  83 . 
     In the two upper and lower photographs shown on the left in  FIG. 9 , when a bias voltage is not applied (upper photograph), the fibers of the brush portions are spread, whereas, when a bias voltage is applied (lower photograph), the brush portions are attracted to the substrate  83 , and the fibers of the brush portions are aligned upward because the direction of rotation in  FIG. 9  is downward. 
     As a result, the area of contact of the brush portions due to linear contact with the substrate  83  is increased (torque variation is a huge factor), a bunch of fibers of the brush portions is loosened and contacts the substrate  83  so as to fill a gap (torque variation is an intermediate factor), and an effective nip width is increased by the attraction (torque variation is a small factor), so that the torque variation is increased. 
     In contrast, in the two upper and lower photographs shown on the right in  FIG. 9 , when a bias voltage is not applied (upper photograph) and when a bias voltage is applied (lower photograph), the brush portions are not attracted to the substrate  83 , so that the brush portions do not linearly contact the substrate  83 . 
     This is believed to be because the fibers of the brush portions are strong and spreading of the bunch of fibers is small, as a result of which the torque variation is reduced. 
     Although the invention carried out by the inventors is described in detail on the basis of an exemplary embodiment, the exemplary embodiment disclosed in the specification is an exemplification on all points, and should not to be thought of as limiting the disclosed technology. That is, the technical scope of the present invention is not to be construed in a limited sense on the basis of the explanation in the exemplary embodiment. The technical scope of the present invention should be strictly construed in accordance with the scope of the claims. Accordingly, technologies that are equivalent to the technology that is set forth in the scope of the claims and all modifications that do not depart from the gist of the scope of the claims are included. 
     For example, although, in the exemplary embodiment, two cleaning devices C 1  and C 2  are used, only one of the cleaning devices that apply a voltage having a polarity opposite to that of the toner may be used. 
     The brush portions  73   b  and  76   b  of the corresponding cleaning brushes  73  and  76  according to the exemplary embodiment may be formed of materials other than the aforementioned nylon, polyester, and acryl. 
     Further, in the exemplary embodiment, the cleaning devices C 1  and C 2 , serving as exemplary cleaning devices, are described as cleaning the second transfer device  42  serving as a transfer unit that is an exemplary member to be cleaned. As the member to be cleaned, for example, the photoconductor drum  21  (serving as an image carrier), the intermediate transfer belt  30  (serving as an image carrier), or the first transfer roller  25  (serving as a transfer unit) may also be used. 
     Although, in the foregoing description, the present invention is described as being applied to a second-transfer image forming apparatus, the present invention may also be applied to a direct-transfer image forming apparatus that directly transfers to a recording medium a developer image developed on an image carrier such as a photoconductor drum. 
     The foregoing description of the exemplary embodiment of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiment was chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.