Abstract:
A belt cleaning device includes a cleaning part positioned to be capable of being in contact with a surface of a belt, the cleaning part having a bias voltage and including a cleaning roller, and a metal roller. A scraping member is in contact with surface of the metal roller of the cleaning part. A conductive holding member holds the scraping member. A conductive member is disposed proximate to the cleaning part. A same potential keeping part includes at least a first connection member which is configured to electrically connect the cleaning part and the conductive holding member and to electrically connect the conductive holding member and the metal roller, and keeps the conductive member and the cleaning part at approximately the same potential.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority from Japanese Patent Application No. 2005-374807 filed Dec. 27, 2005. The entire content of this priority application is incorporated herein by reference. 
   TECHNICAL FIELD 
   The present invention relates to a belt cleaning device, by applying a bias voltage thereon which electrostatically cleans a belt surface, and an image forming apparatus including the same. 
   BACKGROUND 
   Image forming apparatuses such as laser printers, may utilize endless belts for conveyance mechanisms for conveying paper and intermediate transfer mechanisms for conveying developer images transferred from photosensitive drums. In such image forming apparatuses, belt cleaning devices using rollers and brushes are generally included to remove extraneous matters (residual toner, paper powder and the like) on the belts. For example, in Japanese Patent Laid-Open No. 2002-132060 and Japanese Patent Laid-Open No. 2003-345212, rotationally driven cleaning rollers which are brought into pressure contact with the surfaces of belts and to which bias voltage is applied, and electrostatically attracts extraneous matter on the belt while physically scraping them off, and scraping away the extraneous matters transferred to the cleaning rollers by blades. 
   In the image forming apparatuses which clean the belts by applying bias voltage to the above described cleaning rollers, the conductive members disposed around the cleaning rollers (for example, metal members supporting the above described blades) cause noise if they are electrically isolated, and therefore, they are generally connected to ground. In doing so, in order to avoid occurrence of current leak between the cleaning rollers and the conductive members, they have to be disposed to be separated at a certain distance, which exerts a harmful effect on reduction in size of the cleaning devices, and ultimately on the image forming apparatuses. 
   SUMMARY 
   A belt cleaning device of the present invention includes a cleaning part which is positioned to be capable of being in contact with a surface of a belt, the cleaning part having a bias voltage, a conductive member disposed proximate to the cleaning part, and a same potential keeping part which keeps the conductive member and the cleaning part at approximately the same potential. 
   According to this aspect, the conductive member which is disposed around the cleaning part to which the bias voltage is applied is at approximately the same potential as its cleaning part. Therefore, by being at approximately the same potential, the occurrence of current leak can be suppressed, and both the members can be disposed close to each other. 
   “Belt” of the present invention can include a conveyor belt which conveys a recording medium (not limited to a paper recording medium such as paper, but may be a recording medium or the like of a plastic such as an OHP sheet), an intermediate transfer belt to which a developer image is transferred, a photosensitive belt as an image carrier and the like. 
   “An extraneous matter” can include a developer, paper powder and the like. 
   “A cleaning part” can include the construction with only a cleaning roller directly in contact with a belt, and the construction having a metal roller which further attracts the extraneous matter which is attracted by the cleaning roller in addition to the cleaning roller. 
   “Changes the pressing state” includes changing the backup roller between the position where it is pressed to the cleaning part via a belt and a separation position where it is separated from the belt, and increasing and decreasing the pressing force in the state in which the backup roller is in contact with the belt. 
   “Image forming apparatus” is not only a printing apparatus such as a printer (for example, a laser printer), but also may be a facsimile, and a multifunctional machine including a printer function and reading function (scanner function) and the like. The image forming apparatus is not limited to a tandem (single path) type including an image carrier for each development unit, but may be four cycle (single drum) type in which each development unit performs development for a common image carrier, if only it has the above described belt. Further, it may be either of a direct transfer type which directly transfers a developer image to a recording medium, or an intermediate transfer type which indirectly transfers the developer image via an intermediate transfer belt. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Illustrative aspects in accordance with the invention will be described in detail with reference to the following figures wherein: 
       FIG. 1  is a side sectional view showing a schematic construction of a laser printer according to one aspect of the invention; 
       FIG. 2  is a side sectional view showing a paper conveying part and a belt cleaning part by enlarging them; 
       FIG. 3  is a perspective view showing a belt cleaning unit and a pressing force changing mechanism; 
       FIG. 4  is a left side view showing the belt cleaning unit and the pressing force changing mechanism; 
       FIG. 5  is a schematic view (separation state) of the belt cleaning unit and the pressing force changing mechanism; 
       FIG. 6  is a schematic view (contact state) of the belt cleaning unit and the pressing force changing mechanism; 
       FIG. 7  is a perspective view showing the belt cleaning unit and a part of a bias supply part; 
       FIG. 8  is a top view showing the belt cleaning unit and a part of the bias supply part; 
       FIG. 9  is a top view of an enlarged input terminal portion of the belt cleaning unit; and 
       FIG. 10  is a perspective view of the enlarged input terminal portion of the belt cleaning unit. 
   

   DETAILED DESCRIPTION 
   An illustrative aspect of the present invention will be described with reference to  FIGS. 1 to 10 . 
     FIG. 1  is a side sectional view showing a schematic construction of a laser printer  1  as an image forming apparatus of this illustrative aspect. The laser printer  1  is a so-called direct tandem type color laser printer including four photosensitive drums  30  corresponding to respective colors of, for example, black, cyan, magenta and yellow. In a body casing  2  (one example of “an apparatus body”) of the laser printer  1 , a paper feeding part  4  for feeding paper  3  as a recording medium, a scanner part  18  which exposes the above described photosensitive drum  30 , an image forming unit  20  for forming an image on the fed paper  3 , a paper conveying part  35  which conveys the paper  3  to the image forming unit  20 , a belt cleaning unit  41  as a belt cleaning device and the like are included. In this illustrative aspect, the paper conveying part  35  is attachable and detachable from a later-described opening  2   a  of the body casing  2  as a belt unit, and the belt cleaning unit  41  is also attachable and detachable from the opening  2   a . Note that in the following description, the right side of the paper surface in  FIG. 1  is set as “front, front side” of the laser printer  1 . 
   (1) Paper Feeding Part 
   The paper feeding part  4  includes a paper feeding tray  7  as a supply tray attachably and detachably mounted on a bottom portion inside the body casing  2 , a separation roller  8  and a separation pad  9  positioned above a front end portion of the paper feeding tray  7 , a pickup roller  10  positioned behind the separation roller  8 , a pair of paper powder removing rollers  11  and  11  disposed above the front side of the separation roller  8 , and a pair of registration rollers  12 A and  12 B positioned above the paper powder removing rollers  11  and  11 . 
   The paper feeding tray  7  forms a shallow box shape with its top face opened to allow the paper  3  for forming an image to be stacked therein. A front wall  13  positioned at a front end portion of the paper feeding tray  7  is disposed at the lower side of a front cover  6  in the front surface of the body casing  2 , and by pulling the front wall  13  to the front side, the paper feeding tray  7  can be horizontally drawn out forward of the body casing  2 . A paper pressing plate  7 A on which the paper  3  is mountable in a stacked state is positioned on a bottom surface of the paper feeding tray  7 , and the paper pressing plate  7 A is rotatably supported at a rear end portion, and is biased in the upward direction at the front end portion by a spring not shown. Thereby, the paper  3  stacked in the paper feeding tray  7  is in the state in which its front end side is biased upward. 
   The paper  3  on the uppermost position of the paper feeding tray  7  is pressed toward the pickup roller  10  by the biasing force of the paper pressing plate  7 A, and is started to be conveyed toward a position between the separation roller  8  and the separation pad  9  by the rotation of the pickup roller  10 . Then, the paper  3  is handled one by one when caught between the separation roller  8  and the separation pad  9 , and is fed by the rotation of the separation roller  8 . The separated paper  3  is conveyed to the registration rollers  12 A and  12 B, after the paper powder removing roller  11  removes paper powder or dust thereon. 
   The registration rollers  12 A and  12 B are constructed by the drive roller  12 A and the follower roller  12 B, and convey the paper  3  after resisting it to turn it onto a transfer belt (paper conveying belt)  38  of the paper conveying part  35  which will be described later via a paper feed path  14  forming a U shape which is folded to the rear from the front. 
   (2) Scanner Part 
   A scanner part  18  as exposure means is positioned at the uppermost portion inside the body casing  2 . The scanner part  18  irradiates a laser light L based on a predetermined image data onto the surface of the corresponding photosensitive drum  30  with high-speed scanning. Four laser lights L corresponding to the respective colors are emitted diagonally downward from a bottom surface of the scanner part  18 . The optical paths of the respective laser lights L are disposed with constant distances spaced longitudinally from one another in parallel with one another. 
   (3) Image Forming Unit 
   Above the paper feeding tray  7  in the front of the body casing  2 , the opening  2   a  is formed so as to be openable and closable by the front cover  6  pivotally supported at the lower end portion. In the body casing  2 , a unit housing part  19 , which communicates with the opening  2   a  at the lower side of the scanner part  18 , is positioned, and the image forming unit  20  which is capable of being drawn forward and attachable and detachable is housed in the unit housing part  19 . The image forming unit  20  includes a frame  21 , and in this frame  21 , the drums  30  as image carriers, which can be photosensitive drums, chargers  31  as charging means, which can be scorotron chargers, four development cartridges  22  as developing devices, and cleaning brushes  33  are held. Since these constructions corresponding to the respective colors of black, cyan, magenta and yellow are all of the same structures, reference numerals are assigned to only the one at the left end of the paper surface, and those of the others are omitted in  FIG. 1 . 
   The four development cartridges  22  are mounted to be attachable to and detachable from the frame  21 , and respectively correspond to the respective colors of black, cyan, magenta and yellow. The development cartridge  22  includes a box-shaped housing case  23  with a lower side opened, and a toner housing chamber  24  in which a toner T (polymerized toner, developer), which can include one nonmagnetic component with positive electrostatic property of each color, is formed at an upper portion inside the housing case  23 . An agitator  24 A is positioned in the toner housing chamber  24 , and the agitator  24 A is rotationally driven by input of the drive force from a motor not shown, thereby agitating the toner T therein. At a lower side of the toner housing chamber  24 , a supply roller  25 , a development roller  26  as a developer carrier and a thickness restricting blade  27  are positioned. 
   The supply roller  25  is rotatably supported at the housing case  23  of the development cartridge  22 , and can be constructed by covering a roller shaft of a metal with a roller formed by a conductive foamed material. The supply roller  25  is rotationally driven by input of the drive force from a motor not shown. 
   The development roller  26  is rotatably supported at the housing case  23  of the development cartridge  22  in the state in which it is in contact with the supply roller  25  in such a manner as to be compressed by each other, in a diagonally lower rear side of the supply roller  25 . The development roller  26  is in contact with the photosensitive drum  30  to be opposed to it in the state in which the development cartridge  22  is mounted on the frame  21 . The development roller  26  can be constructed by covering a roller shaft of a metal with a roller body formed by conductive urethane rubber or silicon rubber including fine carbon particles. A coat layer of urethane rubber or silicon rubber containing fluorine can be coated on the surface of the roller body. Developing bias is applied to the development roller  26  at the time of development. The development roller  26  is rotationally driven by input of the drive force from a motor not shown. 
   The thickness restricting blade  27  includes a pressing portion semicircular shape in section formed by insulating silicon rubber at a tip end portion of the blade body formed by a metal plate spring material. The thickness restricting blade  27  is supported at the housing case  23  above the development roller  26 , and the pressing portion is in pressure contact with the development roller  26  by an elastic force of the blade body. 
   At the time of development, the toner T released from the toner housing chamber  24  is supplied to the development roller  26  by the rotation of the supply roller  25 , and at this time, it is triboelectrically charged to positive polarity between the supply roller  25  and the development roller  26 . The toner T supplied onto the development roller  26  advances into between the thickness restricting blade  27  and the development roller  26  with the rotation of the development roller  26 , where it is triboelectrically charged sufficiently, and is carried on the development roller  26  as a thin layer of a uniform thickness. 
   The photosensitive drum  30  is formed into a cylindrical shape and includes a drum body of a metal which is grounded, and can be constructed by covering its surface layer with a photosensitive layer of a positive electrostatic property made of polycarbonate or the like. The photosensitive drum  30  is positioned rotatably around a drum shaft by the drum shaft of a metal as the shaft, which extends along a longitudinal direction of a drum body in an axial center of the drum body, being supported at the frame  21 . The photosensitive drum  30  is rotationally driven by input of the drive force from a motor not shown. 
   The scorotron type charger  31  is disposed to be opposed to the photosensitive drum  30  spaced at a predetermined distance so as not to be in contact with the photosensitive drum  30 , at a diagonally rear side above the photosensitive drum  30 . The scorotron type charger  31  uniformly charges the surface of the photosensitive drum  30  to positive polarity, by causing corona discharge from a charging wire of tungsten or the like. 
   The cleaning brush  33  is disposed at the rear side of the photosensitive drum  30  to be opposed to and in contact with the photosensitive drum  30 . 
   The surface of the photosensitive drum  30  is uniformly charged positively to, for example, +900 V by the scorotron charger  31  first at the time of its rotation. Thereafter, it is exposed by high-speed scanning of the laser light from the scanner part  18  so that the surface potential is partially made, for example, +100 V, whereby an electrostatic latent image corresponding to the image to be formed on the paper  3  is formed thereon. 
   Next, by rotation of the development roller  26 , the toner T which is carried on the development roller  26  and is positively charged to, for example, +450 V is supplied to the electrostatic latent image formed on the surface of the photosensitive drum  30  when it is opposed to and in contact with the photosensitive drum  30 . Thereby, the electrostatic latent image of the photosensitive drum  30  is converted into a visible image, and the toner image (developer image) by the reversal development is carried on the surface of the photosensitive drum  30 . 
   Thereafter, the toner image carried on the surface of the photosensitive drum  30  is transferred to the paper  3  by transfer bias (for example, −700 V) of negative polarity which is applied to the transfer roller  39  while the paper  3  (conveyed by the later-described transfer belt  38 ) passes through the transfer position between the photosensitive drum  30  and the transfer roller  39 . The paper  3 , to which the toner image is thus transferred, is conveyed to a fuser  42  next. 
   (4) Paper Conveying Part 
   The paper conveying part  35  is disposed under the image forming unit  20  mounted on the unit housing part  19 . The paper conveying part  35  can include a pair of belt supporting rollers  36  and  37  positioned with spaces at the rear side and the front side to be parallel with each other, and a transfer belt  38  (one example of “a belt”) looped over both the rollers  36  and  37 . The transfer belt  38  circulates (revolves) by the belt supporting roller  36  at the rear side being rotationally driven by the drive force of the motor. The belt supporting roller (drive roller)  36  at the rear side can be a roller formed by covering a substantially cylindrical metal tube material surface of aluminum or stainless steel with a rubber layer, or applying a coating layer to it, for securing a gripping force with the belt inner surface. The belt supporting roller (tension roller)  37  at the front side is a roller that can be formed by applying plating to a substantially cylindrical metal tube material surface of aluminum or stainless steel for prevention of abrasion of the surface due to friction with the belt inner surface. The transfer belt  38  is an endless belt made of a resin material of, for example, of polycarbonate or the like, and its width dimension is not less than the width dimension of the maximum printable paper size (for example, A4 size in this illustrative aspect). 
   At the inner side of the transfer belt  38 , the four transfer rollers  39 , which are disposed to be opposed to the respective photosensitive drums  30  of the aforementioned image forming unit  20 , are aligned at constant spaces in the longitudinal direction. Transfer belt  38  is positioned or in moveable contact between the respective photosensitive drums  30  and the corresponding transfer rollers  39 . Each of the transfer rollers  39  is constructed by covering an elastic member formed by a conductive rubber material around a roller shaft of a metal, so that transfer bias of negative polarity is applied thereto at the time of transfer. A belt cleaning unit  41  having a cleaning roller  40  for removing extraneous matters such as the residual toner T and paper powder adhering onto the transfer belt  38  (explanation may be made with the residual toner T cited as a representative example in the following description), is positioned at the lower side of the transfer belt  38 . The paper  3  which is fed out from the aforementioned resist rollers  12 A and  12 B passes through the paper feed path  14 , abuts on a portion in the vicinity of the front end of the top surface of the transfer belt  38 , where it is electrostatically attracted to the top surface of the transfer belt  38 , and is conveyed rearward with circulating movement of the transfer belt  38 . 
   (5) Fuser 
   The fuser  42  is disposed behind the paper conveying part  35  in the body casing  2 . The fuser  42  is constructed by a heating roller  43 , and a pressure roller  44  or the like which are positioned opposite to each other, and fuses the toner image transferred onto the paper  3  to the paper surface by heat. The paper  3  subjected to heat fusing is conveyed to a paper discharge roller  46  disposed at an upper portion of the body casing  2  by the conveying rollers  45  disposed at a diagonally rear side above the fuser  42 . A paper discharge tray  47  with its front end side substantially horizontal and its rear end side inclined downward to the rear is positioned on a top surface of the body casing  2 , and the paper  3  after image formation which is discharged from the paper discharge roller  46  is stacked on the paper discharge tray  47 . 
   &lt;Construction of Belt Cleaning Unit&gt; 
     FIG. 2  is an enlarged side sectional view showing the paper conveying part  35  and the belt cleaning unit  41 . 
   The belt cleaning unit  41  includes a box-shaped case  50  slim and long in the longitudinal direction, and the case  50  is positioned at the lower side of the transfer belt  38 . In this case  50 , an opening  51  is formed at a front end side of a top surface, and the cleaning roller  40  as a cleaning member is rotatably positioned inside the opening  51 . The cleaning roller  40  is a silicon foamed roller which can be constructed by covering a roller shaft of a metal with a roller body formed by a conductive foamed material. 
   For example, a metal roller  52  (one example of “metal roller”) formed by a hard material such as a metal is rotatably positioned at a diagonally lower rear side of the cleaning roller  40  so as to be in pressure contact with the cleaning roller  40 . 
   Further, a scraping blade  53 , or a scraping member, can be made of rubber and be positioned at a lower side of the metal roller  52 . Scraping blade  53  further includes a rear end portion which is connected to a holder  55 , or a holding member, which can be made of metal. Further, scraping blade  53  includes a front end portion which is a free end is in pressure contact with a lower surface of the metal roller  52  by an elastic force of the blade body. In order to bring scraping blade  53  into contact with the metal roller  52  with a uniform force over the substantially entire length in the longitudinal direction, the rear end portion of the scraping blade  53  is fixed with a force which is strong. To achieve this substantially uniform force, holder  55  can be made of a metal with relatively high strength. Further, a backup roller  54  formed by a conductive member such as a metal is rotatably positioned above the cleaning roller  40  with the transfer belt  38  vertically nipped between the backup roller  54  and the cleaning roller  40 . 
   As shown in  FIG. 2 , at the time of a cleaning operation which is carried out during the period before the paper  3  is discharged by the paper discharge roller  46  (after, for example, the paper  3  with an image formed there on passes through the fuser  42 ), the cleaning roller  40  is rotationally driven. Cleaning roller  40  can be driven in a direction opposed to the transfer belt  38  circulating in the counterclockwise direction in the drawing in the contact surface with it (that is, in the counterclockwise direction in the drawing), by the drive force from a motor not shown. Also, at the same time, the metal roller  52  is rotationally driven in the clockwise direction in the drawing. Meanwhile, the backup roller  54  rotates together in the counterclockwise direction in the drawing with the circulation movement of the transfer belt  38 . 
   The roller shaft of the backup roller  54  is grounded, and at the time of a cleaning operation, negative polarity bias of (one example of “bias voltage”), for example, −3 kV is applied to the cleaning roller  40 . Further, a negative polarity bias (one example of “bias voltage”) of, for example, −3.5 kV is applied to the metal roller  52 . Thereby, the residual toner T adhering to the transfer belt  38  moves to the cleaning roller  40  by a bias attraction force and a contact force of the cleaning roller  40  at a region in the vicinity of the opposing position of the cleaning roller  40  and the backup roller  54 . The residual toner T carried by the cleaning roller  40  moves to the hard metal roller  52  by the bias attraction force, and the residual toner T carried by the metal roller  52  is scraped by the scraping blade  53 , and finally collected into the case  50 . 
   &lt;Pressing Force Changing Mechanism of Backup Roller to Cleaning Roller&gt; 
   In the laser printer  1  of this illustrative aspect, a pressing force changing mechanism  60  is included, which causes the pressing force of the backup roller  54  to the cleaning roller  40  to differ at the time of the above described cleaning operation and at the time of the non-cleaning operation. More specifically, the pressing force changing mechanism  60  positions the backup roller  54  at a separation position separated from the transfer belt  38  during the image formation time (non-cleaning operation time) in which, for example, a start command for image formation is initiated. As the paper  3  is moved onto the transfer belt  38  from the paper feeding tray  7 , a toner image is transferred onto the paper  3 , and the toner image is fused by heat with the fuser  42 . On the other hand, during a cleaning operation, force charging mechanism  60  positions the backup roller  54  into contact with the transfer belt  38 , so that the transfer belt  38  is also positioned into contact with the cleaning roller  40 . Namely, this illustrative aspect has the construction in which the backup roller  54  is pressed against the cleaning roller  40  to ensure the pressure required for cleaning only during a cleaning operation. Thus, even in a configuration which is always rotationally driven during a cleaning operation and also during a non-cleaning operation, there is no fear of applying travel load to the transfer belt  38 . As a matter of course, the backup roller  54  does not always have to be moved to the position completely separated from the transfer belt  38  during a non-cleaning operation, and it may be in light contact with the transfer belt  38  within the range in which travel of the transfer belt  38  is not hindered. In short, if a travel load to the transfer belt  38  can be reduced, as compared to the travel load during cleaning operation, contact is suitable during a non-cleaning operation. 
     FIG. 3  is a perspective view showing the belt cleaning unit  41  and the pressing force changing mechanism  60  (the front side of the laser printer  1 , or the opening  2   a  side of the body casing  2 , is in the lower right direction of the paper surface of the drawing).  FIG. 4  is a left side view showing the belt cleaning unit  41  and the pressing force changing mechanism  60 , and the front side of the laser printer  1  (the opening  2   a  side of the body casing  2 ) is in the right direction of the paper surface of the drawing. The hollow arrow in the drawing indicates the rotational direction of each gear. 
   As shown in  FIG. 3 , the backup roller  54  is rotatably held by a pair of moveable holding arms  61  and  61  which are respectively disposed at both of its left and right ends. As shown in  FIG. 4  each of the moveable holding arms  61  has a front end portion made moveable up and down around a rear end portion pivotally supported at a support shaft body  61   a  which is parallel with the backup roller  54  and positioned at the body casing  2  side. Each of the moveable holding arms  61  has its rocking end portion (front end portion) pressed downward (the belt cleaning unit  41  side) by a pressing spring  62  as a biasing mechanism. In  FIG. 3 , the backup roller  54 , a pair of moveable holding arms  61  and pressing springs  62  are mounted on the paper conveying part  35  constructed as a belt unit (but the transfer belt  38  is omitted in the drawing for convenience to facilitate understanding). 
   The cleaning roller  40  has both end portions of its roller shaft bore positioned to protrude from a left and a right wall of the case  50 , and a rotary gear  41   a  is integrally positioned at one of the end portions (for example, the left end portion). The metal roller  52  has both end portions of its roller shaft bore positioned to protrude from the left and right wall of the case  50 , and a rotary gear  52   a  is integrally positioned at one of the end portions (for example, the left end portion) and is meshed with the above described rotary gear  41   a  to be gear-connected thereto. An input gear  63  is positioned behind the rotary gear  52   a , the input gear  63  is meshed with the rotary gear  52   a  to be gear-connected thereto, and is meshed with an output gear  64  at the body casing side in the state in which the belt cleaning unit  41  is mounted in the body casing  2 . The output gear  64  is disposed at a diagonally lower rear side of the input gear  63 , and is rotationally driven by receiving the drive force from a motor not shown. The motor rotates by a start command for image formation, for example, and the drive force is transmitted to the rotary gear  41   a  and the rotary gear  52   a  via the output gear  64  and the input gear  63 , thereby rotationally driving the cleaning roller  40  and the metal roller  52 . 
   A metal shaft body  65  as a rotary shaft body which is parallel with the cleaning roller  40  and has both end portions positioned to protrude from the left and right wall of the case  50  is positioned in front of the cleaning roller  40 . The metal shaft body  65  includes a gear  65   a  having a pair of tooth portions symmetrically disposed is integrally positioned at one of the end portions (for example, the left end portion) of the metal shaft body  65 . A pair of protruded portions  66  and  66  are disposed symmetrically about the shaft center at a position near to a center in the metal shaft body  65 . A rotary shaft body  67 A parallel with the metal shaft body  65  is positioned in front of the metal shaft body  65 , and an engaging arm  67  is integrally positioned at the rotary shaft body  67 A. When the tooth portions of the gear  65   a  are at the rotation position opposed to the rotary gear  41   a , a claw at a tip end of one end portion (end portion facing the rear side in  FIG. 3 ) of the engaging arm  67  is engaged with one of a pair of protruded portions  66  and  66 . The engaging arm  67  is caused to abut on a solenoid switch (not shown) at the other end portion (end portion facing to the front side in  FIG. 3 ), so that when the solenoid switch receives a start command signal for an image forming operation or a start command signal for the cleaning operation, the solenoid switch performs an ON operation to release engagement of the engaging arm  67  and the protruded portion  66 . When the engagement is released, the metal shaft body  65  is forcefully rotated to the position where the tooth portion of the gear  65   a  is meshed with the rotary gear  41   a  by a coil spring  68  as a biasing spring. 
   Further, the metal shaft body  65  is integrally provided with a pair of cams  69  and  69  having larger end portions (the left side is at the inner side of the gear  65   a ) respectively. In the state in which the belt cleaning unit  41  is mounted in the body casing  2  and the above described paper conveying part (belt unit)  35  is further mounted thereon, the end portions of the pair of moveable holding arms  61  and  61  are positioned on the peripheral surfaces of the pair of cams  69  and  69 . 
   Next, the operation of the pressing force changing mechanism  60  will be described with reference to schematic views shown in  FIGS. 5 and 6  in addition to  FIGS. 3 and 4 . 
     FIG. 3  shows the state in which each of the cams  69  has its large diameter portion faced upward, the tooth portions of the gear  65   a  are in the rotational position where they are opposed to the rotary gear  41   a  and are not meshed with it, and the metal shaft body  65  is held by the engaging arm  67 . In this state, as shown in  FIG. 5 , the rocking end portions of the moveable holding arms  61  and  61  located at both left and right sides of the transfer belt  38  and placed on the large diameter portions of the respective cams  69  are pushed upward against the biasing force of the pressing springs  62 , and thereby, the backup roller  54  is placed at the above described separation position. At this time, the transfer belt  38  is in the separation state such that the cleaning roller  40  and the backup roller  54  separated from each other, when such cleaning pressure is not applied. At this time, even if the transfer belt  38  is in contact with the cleaning roller  40  which rotates in the counterclockwise direction in the drawing, the cleaning pressure is not applied thereto, and therefore, a cleaning operation is not performed. 
   When a start command signal for a cleaning operation is sent to the solenoid switch in the separation state shown in  FIGS. 3 and 5 , engagement by the engaging arm  67  is released, and the gear  65   a  is meshed with the rotary gear  41   a  and is rotationally driven. Thereby, as shown in  FIG. 6 , each of the cams  69  has the large diameter portion faced downward, the tooth portions of the gear  65   a  are in the rotational position where they are opposed to the rotary gear  41   a  and are not meshed with it, and the metal shaft body  65  is brought into the state where it is held by the engaging arm  67  again. In this state, the end portions of the moveable holding arms  61  and  61  are pressed downward by the biasing force of the pressing spring  62 , whereby the backup roller  54  is displaced to the contact position, and is brought into the contact state with the cleaning roller  40  and the transfer belt  38  therebetween. Thereafter, a start command signal for an image formation operation is sent to the solenoid switch again, and thereby, they are returned to the separation state in  FIGS. 3 and 5 . The metal shaft body  65  can be made of a metal in order to provide rigidity corresponding to the forces which are loaded respectively to move the cams  69  and  69  in synchronism with it in the operation of the above described pressing force changing mechanism  60 . 
   As described above, at the time of a cleaning operation, the transfer belt  38  is in positioned between or in moveable contact with the backup roller  54  and the cleaning roller  40 , and during an image forming operation such as transfer and fusing to the paper  3 , the backup roller  54  and the cleaning roller  40  are separated from the transfer belt  38 . Accordingly, the turning load of the transfer belt  38  during an image forming operation is reduced to make stable movement of the paper  3  possible, and deterioration of the cleaning roller  40  by contact with the transfer belt  38  in the state where contact pressure occurs can be reduced. 
   &lt;Construction for Eliminating Backlash and Preventing Current Leak of Belt Cleaning Unit&gt; 
   As shown in  FIGS. 3 and 4 , in the belt cleaning unit  41 , a pair of columnar front side support protruded parts  70  and  70  are positioned on a left and a right side surfaces of the front end side of the case  50 , and a pair of columnar rear side support protruded parts  71  and  71  are positioned on a left and a right side surfaces at the rear end side of the case  50 . Meanwhile, front side support members  72  and  72  which receive the respective front side support protruded parts  70  and  70 , and rear side support members  73  and  73  which receive the rear side support protruded parts  71  and  71  are fixed and positioned at left and right opposing walls of the unit housing part  19  of the body casing  2 . Each of the front side support members  72  is formed into a U-shape opened upward in section, and each of the front side support protruded parts  70  is housed therein. Each of the rear side support members  73  is formed into an L-shape opened upward and forward in section, and the rear side support protruded part  71  is mounted on its bottom surface. 
     FIG. 7  is a perspective view showing the belt cleaning unit  41  and a part of a bias supply part  75  (the front side of the laser printer  1  is in the upper right direction of the paper surface in the drawing).  FIG. 8  is a top view showing the belt cleaning unit  41  and a part of the bias supply part  75  (the front side of the laser printer  1  is in the upper direction of the paper surface in the drawing). In  FIGS. 7 and 8 , illustration of the transfer belt  38  is omitted as in  FIG. 3 . 
   As shown in  FIGS. 7 and 8 , the bias supply part  75  is fixed and disposed behind the belt cleaning unit  41  mounted inside the body casing  2 . In the bias supply part  75 , a pair of output terminals  76   a  and  76   b  are positioned laterally side by side at its front surface (for example, at the position to the left in this illustrative aspect) and can be formed into rod-like shapes. Biasing springs  77  and  77 , which function as biasing mechanisms which bias the output terminals  76   a  and  76   b  forward, are respectively positioned at their base end sides. The output terminal  76   a  outputs the above described negative polarity bias to the cleaning roller  40 , and the output terminal  76   b  outputs the above described negative polarity bias to the metal roller  52 . 
   Meanwhile, in the belt cleaning unit  41 , a pair of input terminals  78   a  and  78   b  are positioned laterally side by side at the rear surface of the case  50  (the position to the left in this illustrative aspect). Each of the input terminals  78   a  and  78   b  can be configured as a metal member in a long plate shape folded in an L-shape to turn onto the top surface from the rear surface of the case  50 . Both of the input terminals  78   a  and  78   b  can be positioned inside the groove formed on the case  50 , and in the position recessed lower than the outer surface of the case  50  around it. In the state in which the belt cleaning unit  41  is mounted inside the body casing  2 , the input terminal  78   a  is in contact with the above described output terminal  76   a , and has aids in the transfer of the negative polarity bias to the cleaning roller  40 . Further, the input terminal  78   b  is in contact with the above described output terminal  76   b , and has aids in the transfer of the negative polarity bias to the metal roller  52 . 
     FIG. 9  is a top view of the enlarged input terminal portion of the belt cleaning unit  41  (the front side of the laser printer  1  is in the upper direction of the paper surface of the drawing).  FIG. 10  is a perspective view of the enlarged input terminal portion of the belt cleaning unit  41  (the front side of the laser printer  1  is in the diagonally lower right direction of the paper surface of the drawing). 
   In the input terminal  78   a , its front end portion is fastened to the case  50  with a screw  80  with one end portion of a lead wire  79  positioned therebetween. As shown in  FIG. 10 , the rotary shaft of the cleaning roller  40  and the metal shaft body  65  are received by a common shaft-receiving member  81  formed by a conductive plastic (synthetic resin or the like). The shaft-receiving member  81  is fastened to the case  50  via screw  82  with the other end portion of the above described lead wire  79  positioned therebetween. By such a construction, the negative polarity bias from the input terminal  78   a  is transferred to the roller shaft of the cleaning roller  40  via the lead wire  79  and the shaft-receiving member  81 , and the metal shaft body  65  is made at the same potential as the roller shaft of the cleaning roller  40  by the shaft-receiving member  81 . Accordingly, the shaft-receiving member  81  is one example of “the second connection member”. The case  50  of the belt cleaning unit  41  is constructed by a plastic (synthetic resin or the like) having insulating properties as a matter of course. 
   The front end portion of input terminal  78   b  is in contact with the holder  55 , and is fastened to the case  50  with a screw  84  with one end portion of a lead wire  83  positioned therebetween. The roller shaft of the metal roller  52  is received by a shaft-receiving member  85  formed by a conductive plastic (synthetic resin or the like), and the shaft-receiving member  85  is fastened to the case  50  via screw  86  with the other end portion of the above described lead wire  83  positioned therebetween. By such a construction, the negative polarity bias from the input terminal  78   b  is transferred to the roller shaft of the metal roller  52  via the leadwire  83  and the shaft-receiving member  85 , and the negative polarity bias from the input terminal  78   b  is directly applied to the holder  55 , thus making the roller shaft of the metal roller  52  and the holder  55  at the same potential. Accordingly, the lead wire  83  and the shaft-receiving member  85  are one example of “the first connection member”. 
   &lt;Effect of the Illustrative Aspect&gt; 
   (1) Since the rotary shaft of the cleaning roller  40  and the metal shaft body  65  are made at the same potential, occurrence of current leak is prevented, and both of them can be positioned close to each other. Here, as the rotary shaft of the cleaning roller  40  and the metal shaft body  65  are farther away from each other, the arm length of the moveable holding arm  61  from the support shaft body  61   a  becomes larger, and the cam which abuts on its end portion cannot displace the backup roller  54  between the above described separation position and the contact position unless the diameter of the cam is made larger. On the other hand, in this illustrative aspect, the rotary shaft of the cleaning roller  40  and the metal shaft body  65  can be positioned close to each other, the backup roller  54  can be displaced between the separation position and the contact position with the cam  69  having the relatively small diameter. Since the roller shaft of the metal roller  52  and the holder  55  are made at the same potential, occurrence of current leak is prevented, and they can be positioned close to each other. Thus, a smaller protrusion length of the scraping blade  53  from the holder  55  can be realized, and the residual toner T adhering to the metal roller  52  can be scraped off by the stable pressure contact. From the above, reduction in size of the belt cleaning unit  41 , and reduction in size of the entire laser printer  1  can be achieved. 
   (2) The belt cleaning unit  41  is supported by the front side support member  72  and the rear side support member  73  as described above. However, the front side support member  72  requires a certain degree of clearance with respect to the front side support protruded portion  70  for making the belt cleaning unit  41  attachable and detachable. Because of this, there is the fear that the belt cleaning unit  41  cannot clean the transfer belt  38  with high backlash precision only utilizing the front side support members  72  and the rear side support members  73 . This is especially true when the roller shaft of the cleaning roller  40  inclines relative to the traveling direction of the transfer belt  38 , thus arising the possibility of applying a skew force to the transfer belt  38  to make it meander. In this case, a certain amount of pressure is needed to provide reliable electrical connection of the output terminals  76   a  and  76   b  and the input terminals  78   a  and  78   b . Thus, in this illustrative aspect, the biasing force (the dotted line arrow X 1  in  FIG. 4 ) of the biasing springs  77 , which bias the output terminals  76   a  and  76   b , is used for securing the contact pressure. Thus, elimination of backlash of the belt cleaning unit  41  with respect to the body casing  2  and the transfer belt  38  is realized. 
   The belt cleaning unit  41  receives a rotational force of the output gear  64 , which is connected to the input gear  63  and receives a force in the diagonally lower direction to the front side, namely, a force in the direction (the dotted line arrow X 2  in  FIG. 4 ) with the pressure angle considered with respect to the tangential line at the meshing position of the input gear  63  and the output gear  64 . Further, the belt cleaning unit  41  receives the reaction force (the dotted line arrow X 3  in  FIG. 4 ) from the cleaning roller  40  rotationally driven in contact with and opposed to the transfer belt  38  at the time of a cleaning operation. Accordingly, by the resultant force of these three forces X 1 , X 2  and X 3 , the belt cleaning unit  41  is strongly pressed against a front wall  72   a  (one example of “an opposing portion”) of the front side support member  72 , and thereby, backlash of the belt cleaning unit  41  can be reliably suppressed. The belt cleaning unit  41  is pressed against the bottom surfaces of the front side support members  72  and the rear side support members  73  by the component force in the lower direction of the force X 2  and the biasing force of the pressing spring  62 , and thereby, the belt cleaning unit  41  is positioned in the vertical direction. 
   &lt;Other Illustrative Aspects&gt; 
   The present invention is not limited to the illustrative aspect described in accordance with the above description and the drawings, but, for example, the following illustrative aspects are also included in the technical range of the present invention. 
   (1) In the above described illustrative aspect, the belt cleaning device is attachable to and detachable from the body casing  2  as a unit, but the belt cleaning device is not limited to this, and may be configured to be incapable of being attached to and detached from the body casing  2 . 
   (2) In the above described illustrative aspect, the construction including the cleaning roller  40  and the metal roller  52  is adopted as the cleaning part, but the cleaning part is not limited to this. A construction in which only the cleaning roller  40  is included and the scraping blade  53  is in contact with the cleaning roller  40  may be adopted. In this case, the holder  55  is desired to be at the same potential as the cleaning roller  40 . At this time, the construction which performs short-circuiting connection of the cleaning roller  40  and the scraping blade  53  and short-circuiting connection of the cleaning roller  40  and the holder  55  respectively by separate connection members may be adopted, or the construction which performs them by an integrally formed common connection member may be adopted. 
   (3) In the above described illustrative aspect, the construction which performs short-circuiting connection of the cleaning roller  40  and the metal shaft  65 , and the holder  55  and the metal roller  52  respectively by using the lead wire  83  and the shaft-receiving members  81  and  85  as the connection member is adopted, but the present invention is not limited to this. The construction which makes the cleaning roller  40  and the metal shaft  65  at the same potential by applying negative polarity bias at the same potential separately to them from the bias supply part  75  via the separate input terminals may be adopted.