Abstract:
A belt is disposed in a main body and moves circularly to transport a recording medium placed thereon. A cleaning roller contacts an outer surface of the belt for removing extraneous matter deposited on the outer surface of the belt. A pair of backup rollers is disposed in opposition to the cleaning roller with the belt interposed between the backup rollers and the cleaning roller. The pair of backup rollers contacting the belt at first and second positions and the cleaning roller contacting the belt at a third position where the third position is interposed between the first and second positions. A holder rotatably holds the pair of backup rollers. Rotational axes of the backup rollers are kept parallel to each other. An urging unit urges the backup rollers toward the cleaning roller. A driving unit is supported on the main body and moves the holder to change a distance between the pair of backup rollers and the cleaning roller.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority from Japanese patent application No. 2007-277857 filed Oct. 25, 2007. The entire contents of the priority application are incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to an image-forming device. 
       BACKGROUND 
       [0003]    The laser printer and other image-forming devices described in Japanese Patent Application Publication No-2007-199675 are well known in the art. This type of image-forming device includes a casing accommodating an endless belt for conveying paper or another recording medium, and an image-forming unit for forming images on the recording medium. 
         [0004]    In such a device, paper dust generated from the paper, toner generated from the image-forming unit, and other extraneous matter is often deposited on the outer surface of the belt. One technique for removing this extraneous matter from the belt is as follows. First, a single cleaning roller is disposed in contact with the outer surface of the belt and is urged against the belt with pressure. In addition, a single backup roller is disposed on the inside surface of the belt at a position opposing the cleaning roller through the belt. By urging the backup roller toward the cleaning roller, the force with which the cleaning roller contacts the belt can be increased. 
         [0005]    With this construction, the extraneous matter is transferred onto the cleaning roller by driving the cleaning roller to rotate. The matter transferred to the cleaning roller is subsequently transferred to a metal roller in contact with the cleaning roller and then scraped off the metal roller by a blade that contacts the metal roller with pressure. 
       SUMMARY 
       [0006]    With the above construction, the lifespan of the backup roller is often different from the lifespan of the cleaning roller. Therefore, it is conceivable to configure the backup roller and cleaning roller to be relatively displaceable in order to replace either the backup roller or the cleaning roller when the two are separated. 
         [0007]    However, after replacing one of the rollers in the above construction, there is a danger that the backup roller and cleaning roller will have shifted relative to each other when the rollers are once again in a state pinching the belt. 
         [0008]    For example, if the relative positions of the backup roller and cleaning roller shift so that the rotational axes are no longer parallel, the belt is pinched between the two rollers only in a region contacted by both the backup roller and cleaning roller (a region intersecting the backup roller and cleaning roller). Consequently, the pressure from the cleaning roller is insufficient in regions of the belt that are not pinched between both rollers, potentially reducing the effect of the cleaning roller for removing extraneous matter from the belt. 
         [0009]    In view of the foregoing, it is an object of the present invention to provide an image-forming device for improving the effects of the cleaning roller in removing extraneous matter from the belt. 
         [0010]    To achieve the above and other objects, one aspect of the invention provides an image-forming device including a main body, a belt, a cleaning roller, a pair of backup rollers, a holder, an urging unit, and a driving unit. The main body includes an image-forming unit for forming images on a recording medium. The belt is disposed in the main body and moves circularly to transport the recording medium placed thereon. The belt has an outer surface. The cleaning roller contacts the outer surface of the belt for removing extraneous matter deposited on the outer surface of the belt. The pair of backup rollers is disposed in opposition to the cleaning roller with the belt interposed between the backup rollers and the cleaning roller. Each of the backup rollers having a rotational axis. The pair of backup rollers contacting the belt at first and second positions and the cleaning roller contacting the belt at a third position where the third position is interposed between the first and second positions. The holder rotatably holds the pair of backup rollers. Rotational axes of the backup rollers are kept parallel to each other. The urging unit urges the backup rollers toward the cleaning roller. The driving unit is supported on the main body and moves the holder to change a distance between the pair of backup rollers and the cleaning roller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    In the drawings: 
           [0012]      FIG. 1  is a side cross-sectional view showing the overall structure of a printer according to a first embodiment of the present invention; 
           [0013]      FIG. 2  is an enlarged cross-sectional view showing a belt unit and a cleaning unit according to the first embodiment; 
           [0014]      FIG. 3  is an enlarged plan view of backup rollers according to the first embodiment; 
           [0015]      FIG. 4  is an enlarged perspective view showing a structure for mounting the backup rollers and a mounting member and a first spring according to the first embodiment; 
           [0016]      FIG. 5  is an enlarged side view showing the structure of the urging unit according to the first embodiment; 
           [0017]      FIG. 6  is an enlarged cross-sectional view showing the structure of a pivoting unit according to the first embodiment; 
           [0018]      FIG. 7A  is a cross-sectional view along the plane VII-VII in  FIG. 5 ; 
           [0019]      FIG. 7B  is a cross-sectional view along the plane VII-VII in  FIG. 5  according to a modification to the cross-sectional view of  FIG. 7A ; 
           [0020]      FIG. 8  is an enlarged plan view showing a backup roller according to a second embodiment of the present invention; 
           [0021]      FIG. 9  is an enlarged perspective view showing the structure for mounting the backup rollers, a lever and a second spring; 
           [0022]      FIG. 10  is an enlarged side view showing the structure of the lever, the second spring, the pin, and pinhole; 
           [0023]      FIG. 11  is a perspective view showing a mounting member and a lever according to the second embodiment; 
           [0024]      FIG. 12  is an enlarged side view showing the state of a cam according to a third embodiment during a cleaning operation; 
           [0025]      FIG. 13  is an enlarged side view showing the state of the cam according to the third embodiment during an image-forming operation; and 
           [0026]      FIG. 14  is an enlarged plan view showing a second base unit according to the third embodiment. 
       
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
       [0027]    Next, a first embodiment of the present invention applied to a printer  1  will be described while referring to  FIGS. 1 through 7A .  FIG. 1  is a side cross-sectional view showing the overall structure of the printer  1 . In the following description, the right side of the printer  1  in  FIG. 1  will be referred to as the “front” and the left side of the printer in  FIG. 1  will be referred to as the “rear.” 
         [0028]    The printer  1  includes a casing  2 , and a paper tray  7  disposed in a bottom section of the casing  2  for retaining stacked sheets of a paper  3 . A feeding roller  10  disposed above the front end of the paper tray  7  conveys the topmost sheet of paper  3  stacked in the paper tray  7  toward registration rollers  12 . The registration rollers  12  function to correct skew in the sheet of paper  3  received from the feeding roller  10  and subsequently to convey the sheet of paper  3  onto a belt unit  35  in an image-forming unit  20 . 
         [0029]    The image-forming unit  20  includes the belt unit  35 , a scanning unit  18 , a process unit  70 , and a fixing unit  42 . 
         [0030]    The belt unit  35  is detachably mounted in the casing  2 . As shown in  FIG. 2 , the belt unit  35  includes a pair of supporting rollers  36  and  37 , and a belt  38  formed of polycarbonate or the like that is wound around the supporting rollers  36  and  37 . By driving the supporting roller (driving roller)  36  disposed on the rear side to rotate, the belt  38  is moved circularly in the counterclockwise direction in  FIG. 1 , thereby conveying the sheet of paper  3  on the top surface of the belt  38  rearward. 
         [0031]    A tension spring  43  is attached to the supporting roller (follower roller)  37  on the front side. The tension spring  43  ( FIG. 2 ) urges the supporting roller  37  in a forward direction so that a fixed tension is applied to the belt  38  looped around the supporting rollers  36  and  37 . 
         [0032]    The scanning unit  18  includes laser light-emitting units (not shown) for emitting laser beams L that are irradiated onto the surfaces of corresponding photosensitive drums  30 . 
         [0033]    The process unit  70  includes a frame  21 , and four developer cartridges  22  removably mounted in the frame  21  and corresponding to the four colors black, yellow, magenta, and cyan. The frame  21  can be pulled out through the front of the casing  2  after opening a front cover  6  disposed on the front surface of the casing  2 . In the bottom of the frame  21 , the photosensitive drums  30  and Scorotron chargers  31  corresponding to each of the developer cartridges  22  are provided. The surfaces of the photosensitive drums  30  are coated with a positive-charging photosensitive layer. Since the developer cartridges  22 , photosensitive drums  30 , and Scorotron chargers  31  have identical structures, only those parts on the left in  FIG. 1  have been labeled. 
         [0034]    Each developer cartridge  22  includes a toner-accommodating chamber  23  accommodating a toner of the corresponding color. A supply roller  25  disposed in the developer cartridge  22  supplies toner accommodated in the toner-accommodating chamber  23  to a developing roller  26 . At this time, the toner is positively tribocharged between the supply roller  25  and developing roller  26 . 
         [0035]    As the photosensitive drum  30  rotates, the Scorotron charger  31  applies a uniform positive charge to the surface of the photosensitive drum  30 . Subsequently, the charged surface of the photosensitive drum  30  is exposed to the laser beam L irradiated by the scanning unit  18 , forming an electrostatic latent image on the surface of the photosensitive drum  30  corresponding to an image to be formed on the paper  3 . 
         [0036]    Next, the rotating developing roller  26  supplies toner onto the surface of the photosensitive drum  30 . The toner is deposited only on areas of the surface that were exposed by the laser beam L, developing the electrostatic latent image into a visible toner image. 
         [0037]    Transfer rollers  39  are disposed on the inside of the belt  38  at positions opposing the photosensitive drums  30 , with the belt  38  interposed therebetween. As a sheet of paper  3  passes between the photosensitive drums  30  and corresponding transfer rollers  39 , toner images carried on the surfaces of the photosensitive drums  30  are sequentially transferred onto the paper  3  as a result of a transfer bias voltage applied to the transfer rollers  39 . 
         [0038]    After toner images are transferred onto the paper  3 , the belt  38  conveys the paper  3  to the fixing unit  42 . In the fixing unit  42 , the toner images are fixed to the paper  3  by heat. Subsequently, the paper  3  is discharged onto a discharge tray  47  provided on the top surface of the casing  2 . 
         [0039]    As shown in  FIG. 2 , a cleaning unit  41  is detachably mounted in the casing  2  beneath the belt unit  35 . The cleaning unit  41  functions to remove extraneous matter  71  deposited on the outer surface of the belt  38 . Examples of the extraneous matter  71  include toner printed on the belt  38  for calibrating density or registering color images, toner deposited on the belt  38  when a paper jam occurs, and paper dust produced from the paper  3 . 
         [0040]    The cleaning unit  41  includes a case  50  provided below the belt  38 . The case  50  has a narrow shape elongated in the left-to-right (front-to-rear) direction. An opening  51  is formed in the top surface of the case  50  on the front side thereof. A cleaning roller  40  is rotatably provided inside the opening  51 . The cleaning roller  40  is configured of a metal shaft  65  covered with a roller body  72  that is formed of a conductive foam material. The cleaning roller  40  is mounted such that the roller body  72  contacts the outer bottom surface of the belt  38 . 
         [0041]    A pair of metal backup rollers  54  is provided in the belt unit  35  at a position opposing the cleaning roller  40  via the belt  38 . Since the cleaning unit  41  is detachably mounted in the casing  2 , as described above, the cleaning roller  40  provided in the cleaning unit  41  and the backup rollers  54  provided in the belt unit  35  are relatively displaceable. 
         [0042]    The cleaning roller  40  is driven by the force of a motor (not shown) provided in the casing  2  so that the outer surface of the cleaning roller  40  moves in a direction opposite the moving direction of the belt  38 . By applying a prescribed bias between the cleaning roller  40  and backup rollers  54  at this time, the cleaning roller  40  can not only physically scrape the extraneous matter  71  from the outer surface of the belt  38 , but can also electrically attract the extraneous matter  71 . At this time, the backup rollers  54  rotate in the counterclockwise direction of  FIG. 2 , following the circular movement of the belt  38 . 
         [0043]    A metal recovery roller  52  is disposed diagonally below and rearward of the cleaning roller  40 . The recovery roller  52  is capable of rotating while contacting the cleaning roller  40  with pressure. A predetermined bias is also applied between the recovery roller  52  and cleaning roller  40 , causing the extraneous matter  71  deposited on the surface of the cleaning roller  40  to be electrically attracted to the surface of the recovery roller  52 . 
         [0044]    A rubber scraping blade  53  is provided for contacting the bottom surface of the recovery roller  52  with pressure. The extraneous matter  71  deposited on the surface of the recovery roller  52  is scraped off by the scraping blade  53  and collected in the case  50 . 
         [0045]    As shown in  FIGS. 4-6 , each backup roller  54  is provided with a shaft  75  that protrudes out from both ends of the backup roller  54  in a direction following the rotational axis of the same. 
         [0046]    As shown in  FIG. 3 , coupling members  80  are provided for coupling like ends of the backup rollers  54  so that the rotational axes of the backup rollers  54  are maintained in a parallel state. The coupling members  80  are formed of a synthetic resin and are substantially elliptical in shape when viewed from the end, as shown in  FIGS. 4-6 . A depression  81  is formed in the bottom surface of the coupling member  80  in  FIG. 5  so that the surface of the coupling member  80  recedes from the cleaning roller  40 . 
         [0047]    When the belt unit  35  and cleaning unit  41  are mounted in the casing  2  and held at their proper positions relative to the casing  2 , the cleaning roller  40  is positioned between the pair of backup rollers  54 , whose rotational axes are maintained in a parallel state. 
         [0048]    Two through-holes  82  penetrate the coupling member  80  in left and right ends of  FIG. 5  in a direction orthogonal to the surface of the drawing. The shafts  75  of the backup rollers  54  are rotatably inserted into the through-holes  82 . The gap between the through-holes  82  is set greater than the outer diameter of the backup roller  54  so that the backup rollers  54  are separated from each other. 
         [0049]    Mounting members  83  are mounted in the belt unit  35  outside of the coupling members  80  in  FIG. 3  in the left-to-right direction (direction along the rotational axes of the backup rollers  54 ). The mounting members  83  can freely slide vertically through a guide mechanism (not shown). Each mounting member  83  is formed of a synthetic resin and has a substantially rectangular parallelepiped shape. 
         [0050]    As shown in  FIG. 7A , a pin  84  substantially columnar in shape is provided on the mounting member  83  so as to protrude toward the backup roller  54 . A pinhole  85  in which the pin  84  can be inserted is formed in the coupling member  80  at a position corresponding to the pin  84  and penetrates the coupling member  80  in the left-to-right direction in  FIG. 7  (along a rotational axis of the backup roller  54 ). The pin  84  and the pinhole  85  constitute a driving unit. As shown in  FIG. 6 , the pinhole  85  is elongated in the left-to-right direction (a direction orthogonal to the rotational axis of the backup roller  54 ) so that the coupling member  80  can pivot relative to the mounting member  83  in a direction orthogonal to the rotational axis of the backup roller  54 . 
         [0051]    As shown in  FIG. 6 , a space between inner top and bottom walls of the pinhole  85  is slightly larger than the outer diameter of the pin  84 , and a space between inner left and right walls of the pinhole  85  is set sufficiently larger than the outer diameter of the pin  84 . 
         [0052]    As shown in  FIG. 5 , a recessed part  86  is formed in the upper surface of the mounting member  83 . A first spring  87  is inserted into the recessed part  86 . The mounting member  83  and the first spring  87  constitute an urging unit. The bottom end of the first spring  87  is attached to the inner bottom surface of the recessed part  86 , and the top end, though not shown in the drawing, is fixed to the belt unit  35 . The first spring  87  functions to urge the mounting member  83  downward in  FIG. 5  (toward the cleaning roller  40 ). Accordingly, the mounting member  83  is mounted on the belt unit  35  so as to be capable of being displaced vertically in  FIG. 7 . 
         [0053]    Next, the operations and effects of this embodiment will be described. As an example, when replacing the cleaning unit  41 , the operator opens the front cover  6 , removes the belt unit  35 , and subsequently removes the cleaning unit  41 . 
         [0054]    The method of installing a new cleaning unit  41  is as follows. After mounting the cleaning unit  41  in the casing  2 , the belt unit  35  is similarly mounted in the casing  2 . At this time, the belt  38  contacts the top of the cleaning roller  40  with pressure. 
         [0055]    The pair of backup rollers  54 , disposed on the opposite side of the belt  38  and in opposition to the cleaning roller  40 , apply pressure to the top of the cleaning roller  40  through the belt  38 . The backup rollers  54  are held in the mounting members  83  through the coupling members  80 , and the mounting members  83  are attached to the belt unit  35  in a manner that allows vertical movement while being urged toward the cleaning roller  40  by the first springs  87 . Accordingly, the backup rollers  54  are also urged toward the cleaning roller  40 , pressing against the belt  38  so that the belt  38  also reliably contacts the cleaning roller  40  with pressure. 
         [0056]    By applying the prescribed bias between the cleaning roller  40  and backup rollers  54 , the extraneous matter  71  deposited on the belt  38  is electrically attracted to the cleaning roller  40  while the cleaning roller  40  also physically scrapes the extraneous matter  71  off the belt  38 . A predetermined bias is also applied between the recovery roller  52  and cleaning roller  40 , attracting the extraneous matter  71  transferred onto the surface of the cleaning roller  40  to the recovery roller  52 , while the recovery roller  52  also physically scrapes the extraneous matter  71  off the surface of the cleaning roller  40 . 
         [0057]    Additionally, the rubber scraping blade  53  contacts the bottom of the recovery roller  52  with pressure, scraping the extraneous matter  71  off the surface of the recovery roller  52  so that the extraneous matter  71  accumulates in the case  50 . 
         [0058]    When the belt unit  35  and cleaning unit  41  are mounted in the casing  2  and held at their proper positions relative to the casing  2 , the cleaning roller  40  is positioned between the pair of backup rollers  54 , which are maintained in the coupling members  80  so that their rotational axes are parallel. Accordingly, the area of contact between the cleaning roller  40  and belt  38  can be increased in the region of the belt  38  pressed against the cleaning roller  40  by the backup rollers  54 , as shown in  FIGS. 5 and 6 . This configuration enables the cleaning roller  40  to more effectively remove the extraneous matter  71  from the outer surface of the belt  38 . 
         [0059]    However, if one of the belt unit  35  and cleaning unit  41  deviates from its proper position relative to the casing  2  when mounted therein, the cleaning roller  40  may be held such that its rotational axis intersects with the rotational axes of the backup rollers  54 , for example. As in the above example, the cleaning roller  40  presses against the bottom of the backup rollers  54  through the belt  38  in this case. 
         [0060]    At this time, the backup rollers  54 , which are pivotably supported in a direction orthogonal to the rotational axes of the backup rollers  54 , pivot along the surface of the cleaning roller  40  through the engagement of the pins  84  on the mounting member  83  in the pinholes  85  formed in the coupling members  80 . Accordingly, the backup rollers  54  are displaced so that the cleaning roller  40  is positioned between the parallel backup rollers  54  and the rotational axes of the backup rollers  54  are parallel to the rotational axis of the cleaning roller  40 . Consequently, the cleaning roller  40  and the backup rollers  54  are maintained in correct positions relative to each other at all times, regardless of whether the belt unit  35  and cleaning unit  41  are mounted in the proper position relative to the casing  2 . In other words, by relatively displacing the coupling members  80  and mounting members  83  engaged through the pins  84  and pinholes  85 , it is possible to absorb positional deviation between the backup rollers  54  and the cleaning roller  40 . 
         [0061]    Hence, this embodiment ensures that the cleaning roller  40  contacts the belt  38  with sufficient pressure across the entire length in the width direction (left-to-right direction), even when the cleaning unit  41  mounted in the casing  2  deviates from its proper position. Accordingly, this embodiment improves the efficiency of the cleaning unit  41  for removing extraneous matter  71  from the belt  38 . 
         [0062]    Further, in this embodiment, the coupling members  80  hold the backup rollers  54  so that a gap is maintained therebetween. This configuration stabilizes the positional relationship of the backup rollers  54  so that the backup rollers  54  can apply a stable force to the cleaning roller  40 . 
         [0063]    With this embodiment described above, the pins  84  inserted into the pinholes  85  can be displaced in a direction orthogonal to the rotational axes of the backup rollers  54 . Hence, through the simple construction of the pins  84  and pinholes  85 , the preferred embodiment can pivotably support the backup rollers  54 . 
         [0064]    Further, this embodiment described above implements urging unit for urging the backup rollers  54  against the cleaning roller  40  through the simple construction of the mounting members  83  and the first springs  87 . 
         [0065]    According to this embodiment described above, the belt unit  35  is detachably mounted in the casing  2 , and the cleaning unit  41  is also detachably mounted in the casing  2 . Accordingly, both the belt  38  and the cleaning roller  40  can easily be replaced. 
       Second Embodiment 
       [0066]    Next, a second embodiment of the present invention will be described with reference to  FIGS. 8-11 . Components in the second embodiment having the same structure as those in the first embodiment have been designated with the same reference numerals to avoid duplicating description. In the second embodiment, metal levers  92  each have a mounting member  90 . The levers  92  are mounted in the belt unit  35  on the outside of the coupling members  80  in the left-to-right direction of  FIG. 8  (along the rotational axes of the backup rollers  54 ). As shown in  FIG. 11 , the mounting members  90  are stepped pins formed of metal and are fitted into and fixed in distal ends of the metal levers  92 . 
         [0067]    As shown in  FIG. 10 , the substantially columnar pins  84  protrude from the mounting members  90  toward the backup rollers  54 . The pinholes  85  in which the pins  84  can be inserted are formed in the coupling members  80  at positions corresponding to the pins  84  and penetrate the coupling members  80  in a direction orthogonal to the surface of the drawing in  FIG. 10  (direction along the rotational axes of the backup rollers  54 ). The pinholes  85  are elongated in the left-to-right direction in  FIG. 10  (directional orthogonal to the rotational axes of the backup rollers  54 ). 
         [0068]    As shown in  FIG. 8 , each of the levers  92  has a narrow elongated plate-shape. A through-hole  94  is formed in the top end of each lever  92  in  FIG. 8  (hereinafter referred to as a “connecting end  93 ”) and penetrates the thickness of the lever  92 . An end of the mounting member  90  is fitted into and fixed in the through-hole  94 . 
         [0069]    A substantially U-shaped angled part  95  is formed on the bottom end of each lever  92  in  FIG. 8  by twice bending the end at right angles in the same direction. As shown in  FIG. 9 , through-holes  96  are formed through two opposing sides of the angled part  95 , penetrating the sides in the thickness direction of the lever  92 . One end of a metal rotational shaft  97  is inserted through the through-holes  96 , enabling the lever  92  to pivot about the rotational shaft  97 . While not shown in detail in the drawing, the rotational shaft  97  is rotatably supported in the belt unit  35 . 
         [0070]    A base part  98  is formed on the bottom edge of the lever  92  near the left-and-right center thereof in  FIG. 10 . The base part  98  can be disposed at an arbitrary position that is different from the connecting end  93 . As shown in  FIG. 9 , the base part  98  is formed by bending the metal plate constituting the lever  92  to a direction along the rotational axes of the backup rollers  54 . The bottom end of a second spring  99  is attached to the top surface of the base part  98 , while the top end of the second spring  99  is fixed to the belt unit  35  (not shown in the drawing). The second spring  99  urges the mounting member  90  downward in  FIG. 10  (toward the cleaning roller  40 ) through the lever  92 . 
         [0071]    With the second embodiment described above, the second springs  99  urge the base parts  98  toward the cleaning roller  40 , causing the levers  92  to rotate about the rotational shafts  97  toward the cleaning roller  40 . Accordingly, the mounting members  90  and coupling members  80  attached to the connecting ends  93  of the levers  92  also rotate, and the backup rollers  54  held in the coupling members  80  are thus urged toward the cleaning roller  40  (downward in  FIG. 10 ). 
         [0072]    In the second embodiment described above, the second springs  99  are attached to the levers  92  near the left-and-right centers thereof in  FIG. 10  at positions different from the connecting ends  93 . Accordingly, the stroke of the second springs  99  is less than that required for rotating the connecting ends  93  of the levers  92 . As a result, it is possible to reduce the overall size of the levers  92  and second springs  99  in the direction that the levers  92  rotate by this amount of stroke reduction. 
       Third Embodiment 
       [0073]    Next, a third embodiment of the present invention will be described with reference to  FIGS. 12-14 . As shown in  FIG. 14 , in the third embodiment, a second base part  198  protrudes farther outward than the belt  38  from the base part  98  of the lever  92 . A cam  100  is disposed below the second base part  198  and contacts the second base part  198  from the bottom thereof. As shown in  FIG. 13 , the cam  100  has a nose part  101 . When the nose part  101  of the cam  100  contacts the second base part  198 , the lever  92  is pushed upward. In this state, the backup rollers  54  are separated from the belt  38  and, hence, do not apply pressure to the belt  38 . 
         [0074]    However, when a surface of the cam  100  different from the nose part  101  contacts the second base part  198 , as shown in  FIG. 12 , the cam  100  no longer applies a force for pushing the lever  92  upward. Consequently, the backup rollers  54  apply a force to the belt  38 . 
         [0075]    Since the remaining structure in the third embodiment is identical to that in the second embodiment, like parts and components are designated with the same reference numerals to avoid duplicating description. 
         [0076]    During a cleaning operation with the device according to the third embodiment, a surface of the cam  100  different from the nose part  101  is placed in contact with the second base part  198 , as shown in  FIG. 12 , so that the belt  38  is pinched between the backup rollers  54  and the cleaning roller  40 . Consequently, the cleaning roller  40  can reliably remove the extraneous matter  71  deposited on the belt  38 . 
         [0077]    On the other hand, during image-forming operations, including transferring and fixing images on the paper  3 , the nose part  101  of the cam  100  is placed in contact with the second base part  198 , as shown in  FIG. 13 , separating the backup rollers  54  from the belt  38 . This configuration reduces the load placed on the belt  38  during image-forming operations, enabling the belt  38  to convey the paper  3  with stability. 
         [0078]    Further, since the cleaning roller  40  only receives pressure from the backup rollers  54  during a cleaning operation, the structure of the third embodiment can reduce degradation of the cleaning roller  40  caused by sliding against the belt  38 . 
       Variations of the Embodiments 
       [0079]    While the invention has been described in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that many modifications and variations may be made therein without departing from the spirit of the invention, the scope of which is defined by the attached claims. 
         [0080]    (1) In those embodiments described above, the pivoting unit is configured of the pins  84  and pinholes  85 , but is not limited to this construction. For example, the backup rollers  54  can be held through a ball joint construction including engaging parts having spherical surfaces and engaging part receiving parts provided with walls capable of gripping these spherical surfaces, thereby enabling the backup rollers  54  to pivot relative to the cleaning roller  40 . 
         [0081]    (2) In those embodiments described above, the cleaning unit  41  is detachably mounted in the casing  2  but is not limited to this construction. For example, the cleaning unit  41  may have any arbitrary construction, provided that the cleaning unit  41  can be pulled from the casing  2  at least to a position for replacing the cleaning roller  40  and that the cleaning roller  40  and the backup rollers  54  can be relatively displaced. 
         [0082]    (3) In those embodiments described above, the pins  84  are provided on the mounting members  83  or the mounting members  90 , and the pinholes  85  are provided on the coupling members  80 , but it is also possible to provide the pins  84  on the coupling members  80  and the pinholes  85  on the mounting members  83  or mounting members  90  as shown in  FIG. 7B . 
         [0083]    (4) Those embodiments described above, the first springs  87  and second springs  99  are configured of coil springs, but these components may be configured of any type of spring capable of urging the backup rollers  54  toward the cleaning roller  40 , such as volute springs or leaf springs. 
         [0084]    (5) In those embodiments described above, the backup rollers  54  are held in coupling members  80  so that a gap is maintained between the backup rollers  54 . However, a link mechanism well known in the art may be used to hold the backup rollers  54  so that their rotational axes are parallel while allowing the gap between the backup rollers  54  to be adjusted. 
         [0085]    (6) In those embodiments described above, both the belt unit  35  and the cleaning unit  41  are detachably mounted in the casing  2 , but it is possible to configure the printer  1  so that only one of the belt unit  35  and cleaning unit  41  is detachably mounted in the casing  2 . Alternatively, both the belt unit  35  and the cleaning unit  41  may be fixed in the casing  2 . In the latter case, the pivoting unit of the present invention would have the effect of absorbing manufacturing errors during assembly.