Patent Publication Number: US-8971752-B2

Title: Belt device and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2011-150913 filed in Japan on Jul. 7, 2011. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a belt device and an image forming apparatus. 
     2. Description of the Related Art 
     In an image forming apparatus of an intermediate transfer system among various image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction peripherals having functions of a copier, a printer, and a facsimile machine, a toner image formed on a photosensitive drum is transferred onto a transfer belt (primary transfer) and then the toner image on the transfer belt is transferred onto a recording sheet (secondary transfer). The belt device having a transfer belt of this type includes a cleaning unit on the upstream side of the photosensitive drum in order to remove non-transferred toner attached to the transfer belt. An example of the belt device having a cleaning unit of this type is disclosed in, for example, Japanese Patent Laid-open Publication No. 2008-9011 (hereinafter, described as “Patent Document 1”). 
     In the belt device disclosed in Patent Document 1, a photosensitive drum on the most upstream side is disposed such that the center shaft thereof is positioned closer to the transfer belt than the center shafts of other photosensitive drums so that the amount of contact between the transfer belt and a cleaning opposing roller can be increased to improve the cleaning performance of the cleaning unit (see, for example, a paragraph [0036] of Patent Document 1). 
     However, in the configuration disclosed in Patent Document 1, the belt is wound around the opposing roller by being stretched outward by the opposing roller. Therefore, the opposing roller needs to be disposed at an offset position outward from a belt extended line between the photosensitive drum located on the most upstream side and the driving roller. Therefore, the thickness of the belt device (the maximum size between the outer periphery on a belt forwarding side and the outer periphery on a belt returning side) increases, resulting in an increase in a space needed for the belt device in the thickness direction. In particular, when the angle of contact between the belt and the opposing roller is increased to improve the cleaning performance, the opposing roller needs to be disposed further outward, and therefore, the above-mentioned problems become more apparent. 
     Meanwhile, when the image forming apparatus switches from a color mode to a monochrome (single color) mode, there is a demand to stop and separate the photosensitive drums other than the photosensitive drum used in the monochrome mode from the transfer belt in order to reduce abrasion of the surfaces of the photosensitive drums or to save power consumption. Therefore, there is a need for the belt device that can meet the above demand. 
     Therefore, there is a need to provide a belt device and an image forming apparatus that can easily cope with image formation in the monochrome mode and that are compact in size. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to an aspect of the present invention, there is provided a belt device including: a belt that has an endless shape and sequentially passes through a plurality of image forming units, each of the image forming units including a transfer member and an image carrier as a pair, and the transfer member being movable in a direction in which the transfer member comes into contact with the belt and in a direction in which the transfer member is separated from the belt; a cleaning unit that cleans an outer periphery of the belt after the belt passes through all the image forming units; and a pressing member that is disposed between the cleaning unit and the image forming unit located on the most upstream side and that presses the outer periphery of the belt, wherein when the transfer member of at least one of the image forming units moves in the direction in which the transfer member is separated from the belt, the transfer member of rest of the image forming units and the pressing member support the belt. 
     According to another aspect of the present invention, there is provided an image forming apparatus includes: a belt device including a belt that has an endless shape and sequentially passes through a plurality of image forming units, each of the image forming units including a transfer member and an image carrier as a pair, and the transfer member being movable in a direction in which the transfer member comes into contact with the belt and in a direction in which the transfer member is separated from the belt, a cleaning unit that cleans an outer periphery of the belt after the belt passes through all the image forming units, and a pressing member that is disposed between the cleaning unit and the image forming unit located on the most upstream side and that presses the outer periphery of the belt, wherein when the transfer member of at least one of the image forming units moves in the direction in which the transfer member is separated from the belt, the transfer member of rest of the image forming units and the pressing member support the belt; a secondary image-transfer unit that transfers an unfixed image formed on the belt of the belt device onto a recording medium; and a fixing unit that fixes the image on the recording medium. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a color image forming apparatus according to an embodiment; 
         FIG. 2  is a side view illustrating a schematic configuration of a belt device according to the embodiment; 
         FIG. 3  is an enlarged view of the belt device illustrated in  FIG. 2 ; 
         FIG. 4  is a side view illustrating a schematic configuration of the belt device when a single-color image is formed; 
         FIG. 5  is a side view illustrating a schematic configuration of a belt device according to another embodiment; 
         FIG. 6  is a side view illustrating a schematic configuration of a belt device according to still another embodiment; 
         FIG. 7  is a schematic diagram for explaining an outer perimeter of a projected image of a toner; 
         FIG. 8  is a schematic diagram for explaining an outer perimeter of a perfect circle with the same projected area as that of the toner; and 
         FIG. 9  is a schematic diagram for explaining a technology related to the belt device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention will be explained in detail below with reference to the accompanying drawings. In the drawings illustrating the embodiments, the same elements such as components or component parts having the same functions or the same shapes are denoted by the same reference numerals or symbols as long as the elements are identifiable, and therefore, the same explanation will not be repeated below. 
       FIG. 1  is a schematic configuration diagram of a color image forming apparatus according to an embodiment of the present invention. The color image forming apparatus illustrated in  FIG. 1  includes an apparatus main body  1 , an exposing unit  2 , an image forming unit  3 , a belt device  4 , a sheet feed unit  5 , a conveying path  6 , a secondary image-transfer unit  7 , an image fixing unit  8 , a discharging unit  9 , or the like. 
     The exposing unit  2  is located in the upper side of the apparatus main body  1  and includes a light source that emits a laser beam or various optical systems. Specifically, the exposing unit  2  emits a laser beam for each of color-separated components of an image to be generated based on image data obtained by an image acquiring unit (not illustrated) toward a photosensitive member of the image forming unit  3  to be described  11  below, thereby exposing the surface of the photosensitive member. 
     The image forming unit  3  is located below the exposing unit  2  and includes a plurality of process units  31  that are detachably attachable to the apparatus main body  1 . Each of the process units  31  includes a photosensitive drum  32  that serves as an image carrier for carrying toner being a developer on the surface of the photosensitive drum  32 , a charging roller  33  that uniformly charges the surface of the photosensitive drum  32 , a developing device  34  that supplies toner to the surface of the photosensitive drum  32 , a cleaning unit  35  that cleans the surface of the photosensitive drum  32 , or the like. As the process units  31 , four process units  31  ( 31 Y,  31 C,  31 M, and  31 Bk) are provided for respective colors of yellow, cyan, magenta, and black that are the color-separated components of a color image. The process units  31  have the same configurations except for colors of toner stored therein. 
     The belt device  4  is located just below the image forming unit  3 . The belt device  4  includes an intermediate transfer belt  43  that is an endless belt wound around a driving roller  41  and a driven roller  42 , which serve as supporting members, in a rotationally movable manner, a cleaning unit  44  that cleans the surface of the intermediate transfer belt  43 , a pressing roller  45  that serves as a pressing member for pressing the outer periphery of the intermediate transfer belt  43  inward, primary transfer rollers  46  that serve as a transfer member and are located opposite the photosensitive drums  32  across the intermediate transfer belt  43 , or the like. A waste toner container  47  for housing waste toner collected by the cleaning unit  44  through cleaning is disposed below the intermediate transfer belt  43  via a waste toner transfer hose (not illustrated). 
     The cleaning unit  44  includes a cleaning blade  44   a  that slides on the outer periphery of the intermediate transfer belt  43  to clean up toner from the outer periphery of the intermediate transfer belt  43 , and a cleaning opposing roller  44   b  that is in contact with the inner periphery of the intermediate transfer belt  43  at the position opposite the cleaning blade  44   a.    
     The photosensitive drums  32  of the process units  31  and the primary transfer rollers  46  of the belt device  4  form image forming units  48 , respectively. 
     As the primary transfer rollers  46 , four primary transfer rollers  46  ( 46 Y,  46 C,  46 M, and  46 Bk) are disposed opposite the four photosensitive drums  32  ( 32 Y,  32 C,  32 M, and  32 Bk) for the respective colors of yellow, cyan, magenta, and black as the color-separated components of a color image. Therefore, the four image forming units  48  ( 48 Y,  48 C,  48 M, and  48 Bk), each of which is formed of a corresponding pair of the photosensitive drum  32  and the primary transfer roller  46 , are formed at four positions on the intermediate transfer belt  43  in a moving direction of the intermediate transfer belt  43 . The cleaning unit  44  is located upstream of the image forming unit  48 Y on the most upstream side and is disposed linearly along the four image forming units  48 . The cleaning opposing roller  44   b  of the cleaning unit  44  and the primary transfer rollers  46  ( 46 Y,  46 C,  46 M, and  46 Bk) are disposed such that the intermediate transfer belt  43  wound around the driving roller  41  and the driven roller  42  is stretched outward. The primary transfer rollers  46  press the inner periphery of the intermediate transfer belt  43  at the respective positions, so that primary transfer nips are formed between the primary transfer rollers  46  and the photosensitive drums  32 , respectively. 
     To cope with a monochrome mode to be described later, of all the primary transfer rollers  46 , the primary transfer rollers  46 Y,  46 C, and  46 M except for the primary transfer roller  46 Bk of the image forming unit  48 Bk for black are movable by a driving mechanism (not illustrated) in directions in which the primary transfer rollers  46 Y,  46 C, and  46 M come into contact with and away from the intermediate transfer belt  43 . 
     The sheet feed unit  5  is located in the lower side of the apparatus main body  1  and includes a sheet feed tray  51  for housing a recording sheet P as a recording medium, a sheet feed roller  52  for taking out the recording sheet P from the sheet feed tray  51 , or the like. 
     The conveying path  6  is a conveying pathway for conveying the recording sheet P taken out from the sheet feed unit  5 . Conveying roller pairs (not illustrated) including a registration roller pair  61  are appropriately disposed on the conveying path  6  toward the discharging unit  9  to be described later. 
     The secondary image-transfer unit  7  is located in the middle of the conveying path  6  and includes the driving roller  41  of the intermediate transfer belt  43 , and a secondary transfer roller  71  disposed opposite the driving roller  41  across the intermediate transfer belt  43 . The secondary transfer roller  71  presses the outer periphery of the intermediate transfer belt  43 , so that a secondary transfer nip is formed between the secondary transfer roller  71  and the driving roller  41 . 
     The image fixing unit  8  is located downstream of the secondary image-transfer unit  7  on the conveying pathway and includes a fixing roller  81  heated by a heat source (not illustrated), a pressurizing roller  82  that applies pressure to the fixing roller  81 , or the like. 
     The discharging unit  9  is located on the most downstream side of the conveying path in the apparatus main body  1  and includes a sheet discharge roller pair  91  for discharging the recording sheet P to the outside and a sheet discharge tray  92  for stacking a discharged recording medium. 
     A basic operation of the color image forming apparatus described above will be explained below with reference to  FIG. 1 . 
     In the image forming apparatus, when an image formation in a color mode is started, the photosensitive drums  32  of the process units  31 Y,  31 C,  31 M, and  31 Bk are rotated clockwise in  FIG. 1  by a driving device (not illustrated), and the charging rollers  33  uniformly charge the surfaces of the photosensitive drums  32  with a predetermined polarity. The exposing unit  2  applies laser beams for respective color components of an image to be formed to the charged surfaces of the photosensitive drums  32 , so that electrostatic latent images are formed on the surfaces of the photosensitive drums  32 . At this time, image information exposed on each of the photosensitive drums  32  is information on a single-color image that is obtained by separating a desired full-color image into pieces of color information on yellow, cyan, magenta, and black. The developing devices  34  supply toner as a developer onto the electrostatic latent images formed on the photosensitive drums  32 , so that the electrostatic latent images are visualized as toner images (developed images) that are visible images. As the developer, one-component developer formed of only toner or two-component developer formed of toner and carrier may be used. 
     Subsequently, the driving roller  41  of the belt device  4  is rotated counterclockwise in  FIG. 1 , so that the intermediate transfer belt  43  moves in an arrow A direction in  FIG. 1 . A constant voltage with a polarity opposite the polarity of charged toner or a voltage subjected to constant current control is applied to the primary transfer rollers  46 . Therefore, transfer electric fields are formed at the respective primary transfer nips between the primary transfer rollers  46  and the photosensitive drums  32 . Subsequently, the toner images for the respective colors formed on the photosensitive drums  32  of the process units  31 Y,  31 C,  31 M, and  31 Bk are transferred onto the intermediate transfer belt  43  in a superimposed manner by the transfer electric fields formed at the primary transfer nips. As a result, a full-color toner image (an unfixed image), in which the images formed by the image forming units  48  are superimposed on one another, is formed on the surface of the intermediate transfer belt  43  after the intermediate transfer belt  43  passes through the image forming units  48  ( 48 Y,  48 C,  48 M, and  48 Bk). The transfer member is not limited to the primary transfer rollers  46 . A conductive member, such as a brush, may be used as the transfer member. 
     Subsequently, the cleaning unit  35  removes non-transferred toner attached to the surfaces of the photosensitive drums  32  and a neutralizing device (not illustrated) neutralizes the surfaces of the photosensitive drums  32 , so that the surface potential is initialized in preparation for next image formation. It is possible not to provide the cleaning unit  35  but to cause the developing devices  34  to collect the non-transferred toner attached to the surfaces of the photosensitive drums  32 . Alternatively, it is possible to employ various known cleaning means. The intermediate transfer belt  43  further moves along with the rotation of the driving roller  41 , so that the toner images formed on the surface of the intermediate transfer belt  43  are conveyed toward the secondary image-transfer unit  7 . 
     Meanwhile, in the lower side of the apparatus main body  1 , a recording sheet P housed in the sheet feed tray  51  is fed toward the conveying path  6  along with rotation of the sheet feed roller  52  of the sheet feed unit  5 . The recording sheet P fed to the conveying path  6  is conveyed to the secondary transfer nip between the secondary transfer roller  71  of the secondary image-transfer unit  7  and the driving roller  41  opposite the secondary transfer roller  71  at a certain timing adjusted by the registration roller pair  61 . At the secondary transfer nip, the full-color toner image formed on the surface of the intermediate transfer belt  43  is transferred onto the recording sheet P. At this time, a transfer voltage with a polarity opposite the polarity of charged toner attached to the toner image on the intermediate transfer belt  43  is applied to the secondary transfer roller  71 , so that a transfer electric field is formed at the secondary transfer nip. The toner images formed on the intermediate transfer belt  43  are collectively transferred (transfer by attractive force) onto the recording sheet P due to the transfer electric field formed at the secondary transfer nip. 
     It may be possible to apply a transfer voltage with the same polarity as the polarity of charged toner to the driving roller  41  in the secondary image-transfer unit  7  so that the toner images can be transferred onto the recording sheet P by repulsive force against the driving roller  41  (transfer by repulsive force). 
     Thereafter, non-transferred toner attached to the intermediate transfer belt  43  is removed by the cleaning blade  44   a  of the cleaning unit  44 . The removed toner is conveyed and collected into the waste toner container  47  by a screw (not illustrated), a toner transfer hose (not illustrated), or the like. 
     Subsequently, the recording sheet P on which the toner image is transferred is conveyed to the image fixing unit  8 , where the heated fixing roller  81  and the pressurizing roller  82  apply heat and pressure to the recording sheet P to fix the toner image on the recording sheet P. The recording sheet P on which the toner image is fixed is conveyed by the conveying roller pairs (not illustrated) and discharged to the sheet discharge tray  92  by the sheet discharge roller pair  91  of the discharging unit  9 . The fixation operation may be performed by the secondary image-transfer unit  7 . 
     The belt device  4  of the image forming apparatus according to the embodiment will be explained in detail below with reference to  FIGS. 2 and 3 . 
     As illustrated in  FIG. 2 , in the belt device  4 , the pressing roller  45  is disposed at a fixed position between the image forming unit  48 Y located on the most upstream side in the moving direction of the intermediate transfer belt  43  and the cleaning unit  44  (specifically, between the primary transfer nip located on the most upstream side and a contact portion of the cleaning blade  44   a  and the cleaning opposing roller  44   b ). The pressing roller  45  presses the intermediate transfer belt  43  so that the intermediate transfer belt  43  is in a reversely-bent state, in which the intermediate transfer belt  43  is bent inward from a belt extended line between the photosensitive drum  32 Y and the cleaning opposing roller  44   b.    
     In this way, by disposing the pressing roller  45  between the image forming unit  48 Y and the cleaning unit  44 , it is possible to press the pressing roller  45  against the surface of the intermediate transfer belt  43  that has been cleaned. Therefore, it is possible to prevent non-transferred toner remaining on the intermediate transfer belt  43  from being attached to the surface of the pressing roller  45 . Furthermore, it is possible to prevent non-transferred toner attached to the pressing roller  45  from being attached to the intermediate transfer belt  43  again. Therefore, it is possible to prevent the intermediate transfer belt  43  from getting dirty, enabling to prevent adverse effects on image quality. As a result, it is possible to lengthen the lifetime of the intermediate transfer belt  43 , enabling to provide the color image forming apparatus that can form an image of stable quality. 
     In contrast with the present embodiment, when the pressing roller  45  is disposed upstream of the cleaning unit  44  and it is desired to prevent the above disadvantage, for example, it is necessary to apply a bias with the same polarity as the polarity of the toner to the pressing roller  45  or to coat the surface of the pressing roller  45  with a low friction material, such as fluorine resin, in order to prevent non-transferred toner from being attached to the pressing roller  45 . However, this may increase costs. By contrast, by disposing the pressing roller  45  between the image forming unit  48 Y located on the most upstream side and the cleaning unit  44  as described above, it is possible to obtain a high-quality image at low costs. 
       FIG. 3  is an enlarged view of the belt device  4 . As illustrated in  FIG. 3 , the intermediate transfer belt  43  is reversely bent by the pressing roller  45 , so that an angle of contact α between the intermediate transfer belt  43  and the outer periphery of the cleaning opposing roller  44   b  increases. The cleaning blade  44   a  slides on the intermediate transfer belt  43  at a linear pressure F to be described later within the range of the angle of contact α. In this way, by securing the adequate angle of contact α, it is possible to stably move the intermediate transfer belt  43  and cause the cleaning blade  44   a  to stably perform cleaning. 
     When the pressing roller  45  is not used, as illustrated in  FIG. 9  for example, the angle of contact α may be increased by increasing the amount of offset of the cleaning opposing roller  44   b  toward the outside (the amount of offset in a direction away from a line connecting the center of the driving roller  41  and the center of the driven roller  42 ). However, in this case, the size of the belt device  4  in the thickness direction increases and the flexibility in arrangement of the cleaning opposing roller  44   b  and the driving roller  41  in the vertical direction is limited, resulting in reduced flexibility in design or increased size of the image forming apparatus. By contrast, by reversely bending the intermediate transfer belt  43  by using the pressing roller  45  as described above, it is possible to increase the amount of pressing the pressing roller  45  inward, enabling to increase the angle of contact α. Therefore, it is not necessary to increase the amount of offset of the cleaning opposing roller  44   b  toward the outside, enabling to prevent an increase in the size of the belt device  4  in the thickness direction and reduce the entire size of the belt device. 
     When the image forming apparatus described above switches from the color mode to the monochrome mode, as illustrated in  FIG. 4 , some of the image forming units, that is, the image forming units for colors other than a color used in the monochrome mode (for example, the image forming units  48 Y,  48 C, and  48 M for colors), activate the driving mechanisms of the primary transfer rollers  46 Y,  46 C, and  46 M in order to move the primary transfer rollers  46 Y,  46 C, and  46 M in a direction in which the primary transfer rollers  46 Y,  46 C, and  46 M are separated from the intermediate transfer belt  43 . On the other hand, other image forming unit, that is, the image forming unit corresponding to the color to be used (for example, the image forming unit  48 Bk for black) does not move the primary transfer roller  46 Bk, so that the primary transfer nip is maintained. 
     In the present embodiment, the pressing roller  45  presses the intermediate transfer belt  43  from the outside to the inside to reversely bend the intermediate transfer belt  43 . Therefore, when the primary transfer rollers  46 Y,  46 C, and  46 M move, the intermediate transfer belt  43  is changed to a non-contact state, in which the intermediate transfer belt  43  is separated from the photosensitive drums  32 Y,  32 C, and  32 M of the image forming units  48 Y,  48 C, and  48 M for colors. Accordingly, the intermediate transfer belt  43  is supported by the primary transfer roller  46 Bk of the image forming unit  48 Bk for black located on the most downstream side and the pressing roller  45 . Therefore, it is possible to stop the photosensitive drums  32 Y,  32 C, and  32 M for colors during printing, enabling to reduce abrasion of the surfaces of the photosensitive drums or save power consumption. At this time, it is desirable to move the primary transfer rollers  46 Y,  46 C, and  46 M for colors to positions where the primary transfer rollers  46 Y,  46 C, and  46 M are not in contact with the intermediate transfer belt  43 . 
     To cope with the monochrome mode, as illustrated in  FIGS. 2 and 4 , it is desirable to align the rotation centers of the photosensitive drums  32 Y,  32 C, and  32 M for colors on a line and set the line to be parallel to a belt extended line between the pressing roller  45  and the image forming unit  48 Bk for black. With this configuration, a gap d between each of the photosensitive drums  32 Y,  32 C, and  32 M of the image forming units  48  for colors and the intermediate transfer belt  43  can be uniform, so that it is possible to assuredly prevent each of the photosensitive drums  32 Y,  32 C, and  32 M for colors from coming into contact with the intermediate transfer belt  43 . In this case, it is desirable to arrange the rotation center of the photosensitive drum  32 Bk for black at a position closer to the intermediate transfer belt  43  than the rotation centers of the photosensitive drums  32 Y,  32 C, and  32 M for colors. 
     As described in the present embodiment, when the pressing roller  45  is disposed between the image forming unit  48 Y located on the most upstream side and the cleaning unit  44 , the angle of contact α of the cleaning opposing roller  44   b  does not change before and after the primary transfer rollers  46 Y,  46 C, and  46 M move in the direction in which the primary transfer rollers  46 Y,  46 C, and  46 M are separated from the intermediate transfer belt  43  or in the direction in which the primary transfer rollers  46 Y,  46 C, and  46 M come into contact with the intermediate transfer belt  43 . Therefore, the cleaning unit  44  can achieve the same cleaning performance both in the color mode and in the monochrome mode. 
     In the above example, it is explained that the image forming units  48  for respective colors are arranged in the order of the image forming units  48  for yellow, cyan, magenta, and black toward the downstream side in the conveying direction of the intermediate transfer belt  43 ; however, the order of arranging the image forming units  48  is not limited to this example. Even when a single or a plurality of the image forming units  48  are used for image formation without using the rest of the image forming units  48 , similarly to the above, it is possible to move the unused image forming units  48  to the positions at which the primary transfer rollers  46  do not come into contact with the intermediate transfer belt  43 , support the intermediate transfer belt  43  by the primary transfer rollers  46  of the image forming units  48  corresponding to colors to be used and the pressing roller  45 , and separate the photosensitive drums  32  of the unused image forming units  48  from the intermediate transfer belt  43 . Furthermore, while it is explained that a part of the primary transfer rollers (the primary transfer roller  46 Bk for black) is disposed at the fixed position, it is possible to configure the image forming unit  48 Bk so that the image forming unit  48 Bk can move in directions in which the image forming unit  48 Bk comes in contact with and away from the intermediate transfer belt  43 , similarly to the primary transfer rollers  46 Y,  46 C, and  46 M. Namely, it is possible to configure all the primary transfer rollers  46 Y,  46 C,  46 M, and  46 Bk such that the primary transfer rollers  46 Y,  46 C,  46 M, and  46 Bk can move in directions approaching and away from the intermediate transfer belt  43 . 
     In the color image forming apparatus of the present embodiment, when, for example, spherical toner with average circularity of 0.98 or greater is used, the liner pressure F of the cleaning blade  44   a  against the intermediate transfer belt  43  needs to be set to at least 40 N/m or greater in order to secure adequate cleaning performance of the cleaning blade  44   a . The linear pressure F of the cleaning blade  44   a  against the intermediate transfer belt  43  is a value obtained by dividing the total weight applied to the cleaning blade  44   a  by a length of an edge line of a tip portion of the cleaning blade  44   a  pressed against the intermediate transfer belt  43 . 
     The average circularity of toner can be measured by using flow particle image analyzer FPIA-2000 (which is the name of a product manufactured by Sysmex Corporation). Specifically, a surfactant, or more preferably, alkylbenzene sulfonate of 0.1 milliliter to 0.5 milliliter is added as a dispersant to water of 100 milliliters to 150 milliliters in a container from which impure solids are removed in advance, and a measurement sample (toner) of approximately 0.1 gram to 0.5 gram is further added to the water. Thereafter, the suspended solution, in which the toner is dispersed, is subjected to dispersion treatment by an ultrasonic disperser for about 1 minute to 3 minutes to obtain the dispersion liquid at the concentration of 30 millions per microliter to 10 thousands per microliter, and the dispersion liquid is set in the analyzer to measure the size and the distribution of the toner. By using the measurement result, Lc/Lt is obtained, where Lc is the outer perimeter of the projected shape of the toner ( FIG. 7 ) and Lt is the outer perimeter Lc of a perfect circle having the same area as a projected area S of the toner ( FIG. 8 ), and thereafter, an average of obtained values Lc/Lt is obtained as the circularity. 
     In the present embodiment, when spherical toner with the average circularity of 0.98 or greater is used, linear pressure F 1  of the cleaning blade  44   a  is set to 45 N/m. In this way, when the linear pressure F 1  of the cleaning blade  44   a  is set to 40 N/m or greater to secure adequate cleaning performance, resistance of the cleaning blade  44   a  due to the movement of the intermediate transfer belt  43  in the moving direction increases. Therefore, the intermediate transfer belt  43  is deflected at a position upstream of the contact position of the cleaning blade  44   a  and the intermediate transfer belt  43  in the moving direction of the intermediate transfer belt  43 , so that the oscillation (behavior) of the intermediate transfer belt  43  easily becomes unstable. By contrast, in the present embodiment, because the pressing roller  45  presses the intermediate transfer belt  43 , a tensile force is applied to the intermediate transfer belt  43 . Therefore, it is possible to prevent deflection of the intermediate transfer belt  43 . 
     In the above example, it is explained that the pressing roller  45  is disposed at the fixed position. However, the pressing roller  45  may be configured such that the pressing roller  45  is movable in directions in which the pressing roller  45  comes in contact with and away from the intermediate transfer belt  43  as illustrated in  FIG. 5 . With this configuration, for example, the pressing roller  45  can be moved to and maintained at the position separated from the intermediate transfer belt  43  while the belt device  4  is deactivated or not in use. Therefore, it is possible to prevent the intermediate transfer belt  43  from being permanently deformed or being permanently curled due to the press against the pressing roller  45  for a long time. 
     Even when the pressing roller  45  is separated from the intermediate transfer belt  43  as described above, as indicated by a chain double-dashed line in  FIG. 5 , it is desirable to set the position of the cleaning opposing roller  44   b  such that the intermediate transfer belt  43  does not come into contact with the photosensitive drums  32  (in particular, the intermediate transfer belt  43  does not come into contact with the photosensitive drums  32 Y,  32 C, and  32 M during printing in the monochrome mode). When both of the photosensitive drums  32  and the intermediate transfer belt  43  are stopped, it is allowable that the photosensitive drums  32  and the intermediate transfer belt  43  come into contact with each other. 
     The secondary transfer roller  71  of the secondary image-transfer unit  7  may be configured such that the secondary transfer roller  71  comes into contact with or comes away from the intermediate transfer belt  43  by a contact-separate mechanism (not illustrated). Therefore, by separating the secondary transfer roller  71  from the intermediate transfer belt  43  when an image forming operation is not performed, it is possible to prevent plastic deformation (creep) that occurs on the secondary transfer roller  71  or the surface of the intermediate transfer belt  43  when the secondary transfer roller  71  and the intermediate transfer belt  43  are in contact with each other while being stopped for a long time. 
     Meanwhile, when the image forming unit  48  ( 48 Y,  48 C,  48 M, or  48 Bk) primary transfers a toner image onto the intermediate transfer belt  43  and when the secondary image-transfer unit  7  transfers the toner image onto the recording sheet P, the primary transfer roller  46  or the secondary transfer roller  71  apply a transfer bias to the intermediate transfer belt  43 . Therefore, in some cases, residual potential remains on the surface of the intermediate transfer belt  43  after the intermediate transfer belt  43  has passed through the secondary image-transfer unit  7 . In this case, to prevent charge up due to the residual potential on the surface of the intermediate transfer belt  43 , it is desirable to apply conductivity to the pressing roller  45  and to ground (earth) the pressing roller  45 . For example, if the roller is made with metal (conductive metal), a conductive roller as described above can be obtained at low costs. 
     Furthermore, if the surface of the pressing roller  45  is made with an elastic material, such as rubber or sponge, the degree of attack to the surface of the intermediate transfer belt  43  is weakened. Therefore, it is possible to lengthen the lifetime of the intermediate transfer belt  43 . At this time, if a conductive material is used as the elastic material of the surface of the pressing roller  45 , it is also possible to prevent charge up due to the residual potential. 
     Furthermore, in the present embodiment, the example is explained in which the rotatable pressing roller  45  is used as the pressing member; however, it is not limited thereto. For example, a non-rotatable member, such as a pad or a pressing bar, may be used as the pressing member. When such a non-rotatable member is used, it is desirable to form a low friction film made with fluorine resin or the like on the sliding portion of the belt. 
     The number of the photosensitive drums  32  or the primary transfer rollers  46  to be arranged is not limited to four but may be three or smaller or five or greater. 
     Furthermore, in the belt device  4  of the present embodiment, the primary transfer rollers  46  are disposed just below the photosensitive drums  32  across the intermediate transfer belt  43 ; however, it is not limited thereto. For example, as illustrated in  FIG. 6 , the primary transfer rollers  46  may be disposed at a position (an offset position) deviated from the perpendicular line extending from the respective centers of the photosensitive drums  32  toward the intermediate transfer belt  43 . Even in this case, the angle of contact α between the intermediate transfer belt  43  and the cleaning opposing roller  44   b  can be adequately secured, so that it is possible to stably move the cleaning opposing roller  44   b  and cause the cleaning blade  44   a  to stably perform cleaning. 
     While the embodiments of the present invention are described above, the present invention is not limited to the above embodiments. The present invention can be modified in various forms within the scope of the technical idea of the present invention. In the embodiments described above, the configuration of the present invention is applied to the belt device. However, the configuration of the present invention may be applied to a belt device that drives a photoreceptor belt, which is an electrostatic latent image carrier for carrying an electrostatic latent image on the surface thereof. Furthermore, the image forming apparatus according to the present invention is not limited to the color image forming apparatus illustrated in  FIG. 1  but may be applied to a monochrome image forming apparatus, a copier, a printer, a facsimile machine, or a multifunction peripheral having functions of a copier, printer, and a facsimile machine. 
     According to one embodiment of the present invention, it is possible to provide a belt device that is compact in size and can easily cope with image formation in the monochrome mode. 
     With this configuration, the belt is pressed inward by the pressing member, so that the belt is reversely bent near the cleaning unit. With the reversely-bent state of the belt, the belt can be wound around the cleaning unit. Because the belt is wound by using the inward pressing force, it is possible to reduce an increase in size of the belt device in the thickness direction and reduce the entire size of the belt device. 
     Furthermore, when some of the transfer members of the image forming units are moved in a direction away from the belt, the belt is supported by the transfer member of other image forming unit and the pressing member and the transfer nips formed between the transfer members of the other image forming units and the photosensitive drums are maintained. Therefore, it is possible to transfer images onto the belt by the other image forming units in the monochrome mode. At the same time, the transfer members of the some of the image forming units are moved in a direction away from the belt such that the photosensitive drums of the some of the image forming units are separated from the belt. Therefore, rotation of the photosensitive drums of the image forming units that are not used in the monochrome mode can be stopped. As a result, it is possible to prevent abrasion of the photosensitive drums or save power consumption. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.