Patent Publication Number: US-10761465-B2

Title: Fuser device having belt supporting part and image forming apparatus having the same

Description:
TECHNICAL FIELD 
     This invention relates to a fuser device that fuses an image to a medium, and an image forming apparatus provided with the fuser device. 
     BACKGROUND 
     A fuser device used in an electrophotographic image forming apparatus may use an endless fuser belt. Disclosed in Patent Document 1 is a fuser device that rotatably holds a fuser belt by a flange-shaped holding member installed on the inner circumferential side of the fuser belt. Also, in order to regulate the width-direction displacement of the fuser belt, driven ring are disposed on both sides in the width direction of the fuser belt. The driven rings contact with the width-direction ends of the fuser belt and rotate following the fuser belt. 
     RELATED ART 
     [Patent Doc. 1] JP Laid-Open Patent Application Publication 2017-203873 
     However, because a rotational speed difference can easily occur between the fuser belt and the driven rings, a groove may be formed on the surface of the driven rings due to friction. If the groove on the driven rings becomes deep, it can lead to a damage to the fuser belt. 
     This invention has been made in order to solve the above-mentioned problem, and its objective is to prevent damages to the fuser belt and the driven rings. 
     SUMMARY 
     A fuser device, which is disclosed in the application, for fusing a developer image on a medium by applying heat includes an endless fuser belt, a belt supporting part that has a contact face shaped in an arc centering on a first central axis and contacts with the inner circumferential face of the fuser belt on the contact face, a driven ring that is disposed on at least one side of the fuser belt in the width direction of the fuser belt, and a ring supporting part that has a contact face shaped in an arc centering on a second central axis and contacts with an inner circumferential face of the driven ring on the contact face, wherein the first central axis is shifted from the second central axis. 
     An image forming apparatus, disclosed in the application, includes an image forming part that forms a developer image on a medium, and the fuser device described above that fuses the developer image formed by the image forming part to the medium. 
     According to this invention, because a first central axis and a second central axis are displaced, the fuser belt is prevented from continuing to contact with the same radial position of the driven rings, and as a result it becomes hard for a groove to be formed on the driven rings. Also, by a groove becoming difficult to be formed, a damage to the fuser belt is also prevented. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         FIG. 1  is a diagram showing the basic configuration of an image forming apparatus of an embodiment of this invention. 
         FIG. 2  is a cross-sectional view showing the configuration of a fuser device of the embodiment. 
         FIG. 3  is a perspective view showing the configuration of the fuser device of the embodiment. 
         FIG. 4  is a perspective view showing the configuration of the fuser device of the embodiment with a fuser belt removed. 
         FIG. 5  is a perspective view showing the configuration of the fuser device of the embodiment with the fuser belt and a top cover removed. 
         FIGS. 6A-6C  respectively show a perspective view, a front view, and a side view showing a flange member (supporting body) of the embodiment. 
         FIG. 7  is a schematic diagram for explaining the depth of a groove part of the flange member of the embodiment. 
         FIGS. 8A and 8B  respectively show a perspective view and a front view showing a driven ring of the embodiment. 
         FIGS. 9A and 9B  respectively show a schematic diagram (A) showing the flange member and the driven ring and a schematic diagram (B) showing a state of supporting the fuser belt of the embodiment. 
         FIG. 10  is a block diagram showing the control system of the image forming apparatus of the embodiment. 
         FIG. 11  is a perspective view of the fuser device for explaining the operation of a swing lever of the embodiment with the fuser belt and the top cover removed. 
         FIG. 12  is a perspective view of the fuser device for explaining the operation of the swing lever of the embodiment. 
         FIGS. 13A-13C  respectively show a diagram (A) showing the positional relation of a fuser belt, a driven ring, and an abutting face, a schematic diagram (B) showing the surface condition of the driven ring, and a schematic diagram (C) showing an example of the surface roughness distribution of a comparative example. 
         FIGS. 14A-14C  respectively show a diagram (A) showing the positional relation of the fuser belt, the driven ring, and an abutting face, a schematic diagram (B) showing the surface condition of the driven ring, and a schematic diagram (C) showing an example of the surface roughness distribution of the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     &lt;Configuration of Image Forming Apparatus&gt; 
     First, explained is an image forming apparatus  1  provided with a fuser device  10  in an embodiment of this invention.  FIG. 1  shows the configuration of the image forming apparatus  1  in this embodiment. The image forming apparatus  1  is a printer that forms a color image using an electrophotographic method, and is provided with a medium supply part  7  that supplies a medium P such as printer sheets, an image forming part  8  that forms a toner image (developer image) on the supplied medium P, a fuser device  10  that fuses the toner image to the medium P, and a medium ejection part  9  that ejects the medium P with the toner image fused to the outside of the image forming apparatus  1 . 
     The medium supply part  7  has a sheet feeding tray  71  as a medium accommodation part that accommodates the medium P in a stacked state, a pickup roller  72  that extracts the medium P mounted on the sheet feeding tray  71  by one piece at a time, a feed roller  73  and a retard roller  74  that forward the extracted piece of medium P to a carrying route, and a carrying roller pairs  75  and  76  that carry the medium forwarded to the carrying route to the image forming part  8 . 
     The image forming part  8  has four process units (developer image forming parts)  80 K,  80 Y,  80 M, and  80 C arranged in series in the below-mentioned X direction (from right to left in  FIG. 1 ) along the carrying route of the medium P. The process units  80 K,  80 Y,  80 M, and  80 C form toner images with black, yellow, magenta, and cyan toners (developers), respectively. Because the process units  80 K,  80 Y,  80 M, and  80 C have a common configuration except for toners used, they are explained as a “process unit  80 ”. 
     The process unit  80  has a photosensitive drum  81  as an image carrier that carries a toner image. The photosensitive drum  81  is a drum-shaped member having photosensitive layers (a charge generation layer and a charge transportation layer) installed on the surface of a conductive base body, and rotates clockwise in the figure. 
     The process unit  80  has a charging roller  82  as a charging member that uniformly charges the surface of the photosensitive drum  81 , an exposure head  83  as an exposure device having an LED (Light-Emitting Diode) array for example that radiates light onto the surface of the uniformly-charged photosensitive drum  81 , thereby forming an electrostatic latent image, a development roller  84  as a developer carrier that develops the electrostatic latent image with toner, a supply roller  85  as a supply member that supplies toner to the development roller  84 , and a cleaning member  86  that scrapes off toner remaining on the surface of the photosensitive drum  81 . 
     Also, the process unit  80  is provided with a detachable toner cartridge  87  as a developer supply part that supplies toner to the development roller  84  and the supply roller  85 . 
     Opposing the photosensitive drums  81  of the process units  80 K,  80 Y,  80 M, and  80 C, four transfer rollers  88  are disposed. Applied to each of the transfer rollers  88  is a transfer voltage for transferring the toner image formed on the photosensitive drum  81  to the medium P. 
     Disposed in the downward direction (−Z direction mentioned below) of the process units  80 K,  80 Y,  80 M, and  80 C are an endless transfer belt  89  that adsorbs the medium P by an electrostatic force and carries it, a belt drive roller  90  for driving the transfer belt  89 , and a tension roller  91  that gives a tension to the transfer belt  89 . 
     In the downstream side of the image forming part  8  along the carrying route of the medium P, a fuser device  10  is installed. The fuser device  10  applies heat and a pressure to the toner image on the medium P, thereby melting and fusing the toner image to the medium P. The configuration of the fuser device  10  is mentioned below. 
     In the downstream side of the fuser device  10  along the carrying route of the medium P, a medium ejection part  9  is disposed. The medium ejection part  9  has ejection roller pairs  92 ,  93 , and  94  that eject the medium P with the toner image fused. Installed on the top of the main body of the image forming apparatus  1  is a stacker part  95  for stacking the ejected medium P. 
     In  FIG. 1 , the carrying direction (the moving direction of the medium P) when the medium P passes the fuser device  10  is denoted as X direction. Also, the width direction of the medium P carried in the X direction is denoted as Y direction. The Y direction is parallel to the rotation axis of the photosensitive drum  81 . Also the direction perpendicular to the X direction and the Y direction is denoted as Z direction. 
     As for the X direction, the carrying direction when the medium P passes the fuser device  10  is denoted as +X direction, and the opposite direction as −X direction. As for the Y direction, facing the +X direction, the left direction is denoted as +Y direction, and the right direction as −Y direction. As for the Z direction, in  FIG. 1  the upward direction is denoted as +Z direction, and the downward direction as −Z direction. 
     &lt;Configuration of Fuser Device&gt; 
     Next, explained is the configuration of the fuser device  10  in this embodiment.  FIG. 2  is a cross-sectional view showing the fuser device  10 . The fuser device  10  has a fuser belt  11  that is an endless belt, a heater  15  disposed on the inner circumferential side of the fuser belt  11 , and a pressure application roller  2  as a pressure application member disposed on the outer circumferential side of the fuser belt  11 . 
     The fuser belt  11  has a base layer of a metal (such as stainless steel), an elastic layer such as silicone rubber formed on the surface of the base layer, and a coating layer such as PFA (tetrafluoroethylene-perfluoroalkylvinylether copolymer) formed on the surface of the elastic layer. The thickness of the base layer is 30 μm for example, the thickness of the elastic layer is 300 μm for example, and the thickness of the coating layer is 20 μm for example. The inner diameter of the fuser belt  11  is 30 mm for example. The width direction of the fuser belt  11  is the Y direction. 
     The pressure application roller  2  is disposed in the downward direction (−Z direction) of the fuser belt  11 . The pressure application roller  2  has a metallic shaft  21 , an elastic layer  22  such as silicone rubber formed on the surface of the shaft  21 , and a coating layer  23  such as PFA formed on the surface of the elastic layer  22 . The axial direction of the pressure application roller  2  is the Y direction. The pressure application roller  2  forms a nip region between it and the fuser belt  11 , and rotates by a rotation being transmitted from a fuser motor  214  ( FIG. 10 ) mentioned below. At the nip region, the medium is pressed and heated so that toner disposed on the medium is fused while passing through the nip region. 
     Disposed on the inner circumferential side of the fuser belt  11  are a stay  12 , a heater supporting member  13 , a heat conducting member  14 , a heater  15 , a heat diffusing member  16 , and a temperature sensor  17 . 
     The stay  12  is a member elongated in the Y direction, and has an approximately U-shaped cross section on the plane perpendicular to the Y direction. More specifically, the stay  12  comprises two side plate parts  12   b  opposing each other in the X direction, and a top plate part  12   a  that connects the upper ends of the side plate parts  12   b . The stay  12  is a structural part that supports the heater supporting member  13 , the heat conducting member  14 , the heater  15 , the heat diffusing member  16 , and the temperature sensor  17 . The stay  12  is formed of a metal such as electrogalvanized steel sheet for example. 
     The heater supporting member  13  is fixed to the lower side (−Z direction) of the stay  12 . The heater supporting member  13  supports the heater  15  to the stay  12 . The heater supporting member  13  comprises two side plate parts  13   b  fixed to the inner sides of the two side plate parts  12   b  of the stay  12 , and a bottom plate part  13   a  that connects the lower ends of the side plate parts  13   b.    
     The lower face (−Z direction face) of the bottom plate part  13   a  of the heater supporting member  13  is in contact with the heat conducting member  14  explained below. Formed on each of both ends in the X direction of the bottom plate part  13   a  is a groove part  13   d  elongated in the Y direction. The heater supporting member  13  is formed of a resin such as PEEK (polyetheretherketone) for example. 
     The heat conducting member  14  is a plate-shaped member for example, that is disposed between the heater supporting member  13  and the heater  15  explained next. The heat conducting member  14  is formed of a material such as stainless steel (SUS) having high heat resistance and high heat conductivity. The upper face (+Z direction face) of the heat conducting member  14  is in contact with the lower face of the bottom plate part  13   a  of the heater supporting member  13 . 
     The heater  15  is a plate-shaped heat source (plate-shaped heater) that applies heat to the fuser belt  11 . The heater  15  has a resistance wire as a heat emitting body, and emits heat by supplying an electric current to the resistance wire. The upper face of the heater  15  is in contact with the lower face of the heat conducting member  14 . 
     The heat diffusing member  16  is disposed between the heater  15  and the fuser belt  11 , and transmits heat of the heater  15  to the fuser belt  11 . The heat diffusing member  16  is formed of a material having high heat resistance and high thermal conductivity, such as stainless steel with a glass coating. The upper face of the heat diffusing member  16  is in contact with the lower face of the heater  15 , and the lower face of the heat diffusing member  16  is in contact with the inner circumference face of the fuser belt  11 . 
     The heat diffusing member  16  is formed by bending upwards (to the heater  15  side) both ends of a plate-shaped member elongated in the Y direction to form a pair of bent parts  16   a . Each of the bent parts  16   a  is inserted and fixed to the groove part  13   d  of the bottom plate part  13   a  of the heater supporting member  13 . The heat conducting member  14  and the heater  15  are held in a state of being sandwiched in the Z direction by the bottom plate part  13   a  of the heater supporting member  13  and the heat diffusing member  16 . 
     Also, given to the lower face of the heat diffusing member  16  (that is, a face in contact with the fuser belt  11 ) is a sliding grease for reducing swing resistance (a frictional force) with the inner circumferential face of the fuser belt  11 . 
     Also, the temperature sensor  17  is disposed in contact with the upper face of the heat conducting member  14 . The temperature sensor  17  detects temperature of the heater  15  through the heat conducting member  14 , and is supported by the heater supporting member  13 . The output signal of the temperature sensor  17  is sent to a heater control part  211  ( FIG. 10 ) mentioned below, based on which the temperature control of the heater  15  is performed. 
       FIG. 3  is a perspective view showing the fuser device  10 .  FIG. 4  is a perspective view of the fuser device  10  shown with the fuser belt  11  removed. The fuser device  10  has a pair of side frames  51  opposing each other in the Y direction, a base part  50  that supports the side frames  51  from below (−Z direction), and a top cover  52  positioned in the upper (+Z direction) part of the side frames  51 . 
     The fuser belt  11 , the stay  12 , the heater supporting member  13 , the heat conducting member  14 , the heater  15 , the heat diffusing member  16 , the temperature sensor  17 , and the pressure application roller  2  mentioned above are disposed between the pair of side frames  51  in the Y direction. Also, the pressure application roller  2  is rotatably supported by this pair of side frames  51 . 
     As shown in  FIG. 4 , installed on the inner sides in the Y direction of the pair of side frames  51  is a pair of swing levers  55 . Each of the swing levers  55  is swingably attached to the side frame  51  by a support shaft  56  in the Y direction. The support shaft  56  is disposed on the lower end (−Z direction end) and −X direction end part of the swing lever  55 . 
     Attached to each of the swing levers  55  is a flange member  3  in an approximate cylindrical shape. The flange member  3  supports the fuser belt  11  from the inner circumferential side on both sides in the width direction of the fuser belt  11  ( FIG. 3 ). Also, driven rings  4  are installed opposing end parts in the width direction of the fuser belt  11  ( FIG. 3 ). Note that one of the driven rings  4  is hidden behind the side frame  51  in  FIGS. 3 and 4 . 
     Fixed to the swing levers  55  are Y direction end parts of the stay  12  penetrating the interior of the flange members  3  as mentioned below. That is, the pair of swing levers  55  supports the fuser belt  11  through the pair of flange members  3 , and supports the stay  12  and the components (the heater supporting member  13 , the heat conducting member  14 , the heater  15 , the heat diffusing member  16 , and the temperature sensor  17 ) supported by it. 
       FIG. 5  is a perspective view showing a state where the fuser belt  11  and the top cover  52  are removed from the fuser device  10 . Installed on the upper end (+Z direction end) and +X direction end part of the swing lever  55  is a contact plate  57 . The contact plate  57  has a plate face perpendicular to the X direction for example. 
     Attached to each of the side frames  51  on the −X side of the contact plate  57  of the swing lever  55  is a coil spring  62  as a bias member. The winding axis direction of the coil spring  62  is the X direction. The −X direction end part of the coil spring  62  is in contact with the contact plate  57 . The +X direction end part of the coil spring  62  is in contact with a wall part  51   a  of the side frame  51 . Thereby, the coil spring  62  biases the contact plate  57  of the swing lever  55  in a direction indicated with an arrow R in  FIG. 5 . 
     Attached to each of the side frames  51  on the +X side of the contact plate  57  of the swing lever  55  is a cam  60  as a drive mechanism. In  FIG. 5 , one of the cams  60  is hidden behind the side frame  51 . The pair of cams  60  is attached to a rotation shaft  61  extending in the Y direction. Both ends of the rotation shaft  61  are rotatably supported by supporting holes  51   b  formed on the side frames  51 . The outer circumferential face of the cam  60  is in contact with the +X side face of the contact plate  57 . That is, the contact plate  57  is pressed against the outer circumferential face of the cam  60  by the bias force of the coil spring  62 . 
     The cam  60  rotates by a rotation transmitted from a cam motor  216  ( FIG. 10 ) mentioned below. By the cam  60  rotating, the position of the contact plate  57  varies, thereby the swing lever  55  swings centering on the support shaft  56 . By this swing lever  55  swinging, the fuser belt  11  ( FIG. 3 ) moves toward or away from the pressure application roller  2 . 
     Attached on one of the side frames  51  is a terminal part  58  for supplying an electric current to the heater  15  ( FIG. 2 ) on the inner circumferential side of the fuser belt  11 . A wiring  59  is extracted from the terminal part  58  and connected to a heater control part  211  ( FIG. 10 ). The output of the temperature sensor  17  ( FIG. 2 ) is also outputted from the terminal part  58  to the heater control part  211  ( FIG. 10 ). 
       FIGS. 6A-6C  are respectively a perspective view (A), a front view (B), and a side view (C) showing the flange member  3 . Note that although only one flange member  3  is shown in  FIGS. 6A-6C , the other flange member  3  has a symmetric shape with the flange member  3  shown in  FIGS. 6A-6C  with respect to the Y direction center. 
     The flange member  3  is formed of a resin such as PPS (polyphenylene sulfide) for example. As shown in  FIGS. 6A-6C , the flange member  3  has a belt supporting part  31  in an approximate cylindrical shape that supports the fuser belt  11 , and a connecting part  32  adjacent in the Y direction to the belt supporting part  31 . 
     The belt supporting part  31  has a contact face  31   a  shaped in an arc centering on a central axis A (first central axis) extending in the Y direction. This contact face  31   a  is the face where the inner circumferential face of the fuser belt  11  slides. The radius (that is, the distance from the central axis A to the contact face  31   a ) of the belt supporting part  31  is 14.9 mm for example. 
     The belt supporting part  31  has a rectangular inner circumferential part  31   b  that fits with the stay  12  ( FIG. 2 ) mentioned above. The Y direction end part of the stay  12  shown in  FIG. 4  penetrates the inner circumferential part  31   b  of the belt supporting part  31  and is fixed to the swing lever  55 . 
     The belt supporting part  31  is not perfectly cylindrical but has an opening part  31   c  on its lower end part (−Z direction end part). Via this opening part  31   c , the heater supporting member  13  ( FIG. 2 ) attached to the say 12 opposes the fuser belt  11 . 
     The belt supporting part  31  has a tapered face  31   d  on the opposite side of the connecting part  32  in the Y direction. This tapered face  31   d  is installed for making it easy to attach the fuser belt  11  to the contact face  31   a  of the belt supporting part  31 . 
     The connecting part  32  is an approximate ring-shaped part extruding radially to the outside from the belt supporting part  31 . The face (−Y direction face) of the connecting part  32  on the side of the belt supporting part  31  is an abutting face  32   a  that contacts with the driven ring  4 . The abutting face  32   a  is a face parallel to the XZ plane. Formed on the lower end part (−Z direction end part) of the connecting part  32  is a tapered face  32   b  that is inclined relative to the XZ plane, see  FIGS. 6A and 6C . 
     Formed on the upper end part (+Z direction end part) of the connecting part  32  is a fixing hole  32   c  for a screw to penetrate. By a screw penetrating the fixing hole  32   c , the flange member  3  is fixed to the swing lever  55 . 
     As shown in  FIG. 6C , formed on the connecting part  32  side end part of the belt supporting part  31  is a ring supporting groove  33  as a ring supporting part that supports the driven ring  4 . This ring supporting groove  33  is a part that rotatably supports the driven ring  4 . The bottom part (contact face)  33   a  of this ring supporting groove  33  extends in an arc shape centering on a central axis B (second central axis) extending in the Y direction. The distance from the central axis B to the bottom face  33   a  of the ring supporting groove  33  is 14.4 mm for example. 
     When defining first angle θ 1 , which is around the central axis A, of the contact face  31   a  and second angle θ 2 , which is around the central axis B, of the contact face  33   a , it is preferred to satisfy the follow:
 
θ1=θ2
 
210°≤θ1≤225°
 
210°≤θ2≤225°.
 
     As shown in  FIGS. 6B and 6C , the central axis A of the arc that defines the contact face  31   a  of the belt supporting part  31  and the central axis B of the arc that defines the bottom face  33   a  of the ring supporting groove  33  are in mutually shifted positions. 
       FIG. 7  is a schematic diagram for explaining the groove shape of the ring supporting groove  33 . The depth D of the ring supporting groove  33  (that is, the distance from the contact face  31   a  of the belt supporting part  31  to the bottom face  33   a  of the ring supporting groove  33 ) varies along the circumferential direction of the belt supporting part  31  (that is, the extending direction of the ring supporting groove  33 ). Such a configuration as this allows realizing a positional relation that the central axis A and the central axis B are shifted from each other. 
     If the inner diameter of the fuser belt  11  is set to 30 mm, and the inner diameter of the driven ring  4  to 29.2 mm, the distance S between the central axis A and the central axis B should desirably be 0.4 mm or less. As mentioned below, it is to prevent the fuser belt  11  from extruding from the inner circumferential edge of the driven ring  4 . Although the central axis A and the central axis B are shifted in both the X direction and the Z direction here, this invention is not limited to this, but they can be shifted only in the X direction or only in the Z direction. 
     Also, as shown in  FIG. 7 , the distance L 1  from the central axis A to the contact face  31   a  is 14.9 mm for example, and the distance L 2  from the central axis B to the bottom face  33   a  of the ring supporting groove  33  is 14.4 mm for example. The distance S between the central axis A and the central axis B is 0.2 mm for example. 
       FIG. 8  shows a perspective view (A) and a front view (B) showing the shape of the driven ring  4 . The driven ring  4  has an inner circumference  43  and an outer circumference  44 , both of which are circular. Also, the driven ring  4  has a first contact face  41  contacting with the end face of the fuser belt  11 , and a second contact face  42  contacting with the abutting face  32   a  of the flange member  3 . 
     It is preferred that distance S, distance L 1  from the central axis A to the contact face  31   a  and distance L 2  from the central axis B to the contact face  33   a  satisfy the follows:
 
6.67≤ S/L 1≤26.7
 
6.65≤ S/L 2≤27.4
 
0.25≤ S /( L 1− L 2)≤1.0.
 
     The driven ring  4  is formed of a resin such as PEEK (polyetheretherketone) or PPS. The inner diameter R 1  of the driven ring  4  is 29.2 mm for example, and the outer diameter R 2  is 35 mm for example. The width H of the driven ring  4  is 2.9 mm for example, which is constant over the circumferential direction. The thickness T of the driven ring  4  is 0.3 mm for example. That is, the driven ring  4  has a thickness allowing it to bend in the thickness direction. In this embodiment, it is preferred that the width H is ranged from 2.0 mm to 6.0 mm. The width may vary around its axis (or over the circumferential direction). Further, in the present invention, the width H may be determined considering a value of distance L, see  FIG. 7 . With respect to L 1 , the width H may be ranged from 13% to 40%. 
       FIG. 9A  is a schematic diagram showing a state where the driven ring  4  is attached to the ring supporting groove  33  of the flange member  3 . The width of the ring supporting groove  33  is larger than the thickness of the driven ring  4 , therefore the driven ring  4  can move in the Y direction inside the ring supporting groove  33 . The first contact face  41  of the driven ring  4  is oriented to the belt supporting part  31  side, and the second contact face  42  of the driven ring  4  opposes the abutting face  32   a.    
       FIG. 9B  is a schematic diagram showing the positional relation of the flange member  3 , the driven ring  4 , the fuser belt  11 , and the pressure application roller  2 . Once the fuser belt  11  is attached to the belt supporting part  31  of the flange member  3 , an end face in the width direction (Y direction) of the fuser belt  11  opposes the first contact face  41  of the driven ring  4 . 
     Along with the rotation of the fuser belt  11 , if the fuser belt  11  shifts in the Y direction as indicated with an arrow, the Y direction end face of the fuser belt  11  contacts the first contact face  41  of the driven ring  4 . Thereby, the driven ring  4  moves to the abutting face  32   a  side, and the second contact face  42  contacts with the abutting face  32   a . Thereby, the Y direction position of the fuser belt  11  is regulated. 
     Also, on the lower part of the belt supporting part  31  (the part where the opening part  31   c  shown in  FIG. 6  is formed), the fuser belt  11  is not in contact with the contact face  31   a , therefore when the fuser belt  11  that passed this part contacts with the contact face  31   a  again, it may be displaced in the Y direction. In that case, the driven ring  4  can bend, and this bending of the driven ring  4  can be released on the tapered face  32   b.    
     &lt;Control System of Image Forming Apparatus&gt; 
     Next, explained is the control system of the image forming apparatus  1 .  FIG. 10  is a block diagram showing the control system of the image forming apparatus  1 . The image forming apparatus  1  is provided with a control part  200 , an I/F (interface) control part  201 , receiving memory  202 , image data editing memory  203 , an operation part  204 , a sensor group  205 , a charging roller power supply  206 , a development roller power supply  207 , a supply roller power supply  208 , a transfer roller power supply  209 , a head control part  210 , a heater control part  211 , a fuser drive control part  213 , a cam drive control part  215 , a carrying control part  217 , a drive control part  219 , and a belt drive control part  221 . 
     The control part  200  comprises a microprocessor, ROM (Read Only Memory), RAM (Random Access Memory), an input/output port, a timer, etc. The control part  200  receives print data and control commands through the I/F control part  201  from a host device, and performs the print operation of the image forming apparatus  1 . 
     The receiving memory  202  temporarily stores the print data inputted through the I/F control part  201  from the host device. The image data editing memory  203  receives the print data stored in the receiving memory  202  and also records image data formed by editing the print data. 
     The operation part  204  is provided with a display part (such as an LED) for displaying the state of the image forming apparatus  1 , and an operation part (such as switches) for an operator to input instructions. The sensor group  205  includes various sensors such as a medium position sensor, a temperature/humidity sensor, and a density sensor for monitoring the operation state of the image forming apparatus  1 . 
     By the control of the control part  200 , the charging roller power supply  206  applies a charging voltage to the charging roller  82  for uniformly charging the surface of the photosensitive drum  81 . By the control of the control part  200 , the development roller power supply  207  applies a development voltage to the development roller  84  for developing an electrostatic latent image on the surface of the photosensitive drum  81 . 
     By the control of the control part  200 , the supply roller power supply  208  applies a supply voltage to the supply roller  85  for supplying toner to the development roller  84 . By the control of the control part  200 , the transfer roller power supply  209  applies a transfer voltage to the transfer roller  88  for transferring a toner image on the photosensitive drum  81  to the medium P. 
     The head control part  210  sends the image data recorded in the image data editing memory  203  to the exposure head  83 , and controls the emission of the exposure head  83 . The heater control part (fuser control part)  211  is a temperature adjustment circuit that supplies a prescribed electric current from the heater power supply  212  to the heater  15  ( FIG. 2 ) based on the output signal of the temperature sensor  17  ( FIG. 2 ) of the fuser device  10 . 
     The fuser drive control part  213  rotates the fuser motor  214  to rotate the pressure application roller  2  ( FIG. 2 ) of the fuser device  10 . The cam drive control part  215  rotates the cam motor  216  to rotate the cam  60  ( FIGS. 3-5 ). Thereby, the cam  60  swings the swing lever  55 , and the fuser belt  11  moves toward or away from the pressure application roller  2 . 
     The carrying control part  217  controls the rotation of the carrying motor  218  to rotate the pickup roller  72 , the feed roller  73 , the carrying roller pairs  75  and  76  shown in  FIG. 1  for carrying the medium P. The drive control part  219  rotates a drive motor  220  for rotating the photosensitive drum  81 , the development roller  84 , the supply roller  85 , etc. of each process unit  80 . 
     The belt drive control part  221  controls the rotation of a belt motor  222  to rotate the belt drive roller  90  for driving the transfer belt  89 . Note that the ejection roller pairs  92 ,  93 , and  94  rotate by a rotation transmitted from the fuser motor  214 . 
     &lt;Operations of Image Forming Apparatus&gt; 
     Next, explained are the operations of the image forming apparatus  1  referring to Figs. and  10 . Upon receiving a print command and print data through the I/F control part  201  from the upper-level device, the control part  200  of the image forming apparatus  1  starts an image forming (print) operation. The control part  200  temporarily records the print data in the receiving memory  202 , generates image data by editing the recorded print data, and records the data in the image data editing memory  203 . 
     The control part  200  also drives the carrying motor  218  by the carrying control part  217 . Thereby, the pickup roller  72  and the feed roller  73  rotate to forward the medium P contained in the sheet feeding tray  71  by one piece at a time to the carrying route. Furthermore, the carrying roller pairs  75  and  76  carry the medium P along the carrying route to the image forming part  8 . 
     In the image forming part  8 , the transfer belt  89  that rotates by the belt drive roller  90  adsorbs and carries the medium P. The medium passes through the process units  80 K,  80 Y,  80 M, and  80 C sequentially in that order. 
     The control part  200  performs the formation of color toner images in the process units  80 K,  80 Y,  80 M, and  80 C. That is, the control part  200  applies the charging voltage, the development voltage, and the supply voltage from the charging roller power supply  206 , the development roller power supply  207 , and the supply roller power supply  208  to the charging roller  82 , the development roller  84 , and the supply roller  85  of each of the process units  80 , respectively. 
     The control part  200  also rotates the drive motor  220  by the drive control part  219  to rotate the photosensitive drum  81 . Along with the rotation of the photosensitive drum  81 , the charging roller  82 , the development roller  84 , and the supply roller  85  also rotate. The charging roller  82  uniformly charges the surface of the photosensitive drum  81  by its charging voltage. 
     The control part  200  further controls the emission of the head control part  210  based on the image data recorded in the image data editing memory  203 . The head control part  210  exposes the surface of the uniformly charged photosensitive drum  81  by the exposure head  83  to form an electrostatic latent image. 
     The electrostatic latent image formed on the surface of the photosensitive drum  81  is developed with toner adhering to the development roller  84 , thereby a toner image is formed on the surface of the photosensitive drum  81 . Once the toner image approaches the surface of the transfer belt  89  by the rotation of the photosensitive drum  81 , the control part  200  applies the transfer voltage to the transfer roller  88  from the transfer roller power supply  209 . Thereby, the toner image formed on the photosensitive drum  81  is transferred to the medium P on the transfer belt  89 . Toner that was not transferred to the medium P is scraped off by the cleaning member  86 . 
     In this manner, the individual color toner images formed in the process units  80 K,  80 Y,  80 M, and  80 C are sequentially transferred to the medium P and superimposed over one another. The medium P to which the individual color toner images transferred is carried further by the transfer belt  89  and reaches the fuser device  10 . 
     In the fuser device  10 , the fuser belt  11  rotates, and the heater  15  is heated by the heater control part  211  and has reached prescribed fusing temperature. To the medium P carried to the fuser device  10 , heat and a pressure are applied between the fuser belt  11  and the pressure application roller  2 , thereby the toner image is fused to the medium P. 
     The medium P to which the toner image is fused is ejected to the outside of the image forming apparatus  1  by the ejection roller pairs  92 ,  93 , and  94 , and stacked on the stacker part  95 . Thereby, a color image formation to the medium P is complete. 
     &lt;Operations of Fuser Device&gt; 
     Here, explained are the operations of the fuser device  10 . First, a contact/separation operation of the fuser belt  11  and the pressure application roller  2  is explained. At the end of the fusing operation, the fuser device  10  performs a separation operation that separates the fuser belt  11  from the pressure application roller  2 . Specifically, the control part  200  drives the cam motor  216  to rotate the cam  60 . 
       FIGS. 11 and 12  are diagrams showing the fuser device  10  in the separation operation.  FIG. 11  shows a state where the fuser belt  11  and the top cover  52  are removed, and  FIG. 12  shows a state where the fuser belt  11  and the top cover  52  are attached. 
     As shown in  FIG. 11 , in the separation operation, the control part  200  drives the cam motor  216  ( FIG. 10 ) to rotate the cam  60  in a prescribed direction. The cam  60  presses the contact plate  57  of the swing lever  55  in the −X direction as indicated with an arrow F. Thereby, the swing lever  55  swings in a direction indicated with an arrow C 1  centering on the support shaft  56 , resisting a bias force of the coil spring  62 . Thereby, as shown in  FIG. 12 , the fuser belt  11  separates upwards from the pressure application roller  2 . 
     On the other hand, performed in starting the fusing operation is a contact operation that has the fuser belt  11  contact with the pressure application roller  2 . In this case, the control part  200  drives the cam motor  216  to rotate the cam  60  in the opposite direction from that in the separation operation. Thereby, the pressing force to the contact plate  57  by the cam  60  weakens, therefore the swing lever  55  swings in the opposite direction from that of the arrow C 1  centering on the support shaft  56  by the bias force of the coil spring  62 . Thereby, as shown in  FIGS. 2 and 3 , the fuser belt  11  contacts with the pressure application roller  2 , forming the nip part. 
     In starting the fusing operation, the control part  200  drives the fuser motor  214  ( FIG. 10 ) to rotate the pressure application roller  2 . Once the pressure application roller  2  rotates, the fuser belt  11  in contact with the pressure application roller  2  rotates following the pressure application roller  2 . 
     Also, at about the same time as the pressure application roller  2  starts rotating, an electric current is supplied to the heater  15  from the heater power supply  212  ( FIG. 10 ). The heater  15  emits heat by the electric current supplied, and heat of the heater  15  is transmitted to the fuser belt  11  through the heat diffusing member  16 . The temperature sensor  17  detects the temperature of the heater  15  through the heat conducting member  14 , and outputs it to the heater control part  211  ( FIG. 10 ). The heater control part  211  controls the electric current supplied to the heater  15  so that the temperature of the fuser belt  11  is maintained at target temperature. 
     The medium P to which the toner image was transferred in the image forming part  8  ( FIG. 1 ) enters the nip part between the fuser belt  11  and the pressure application roller  2 . Then, by heat applied by the fuser belt  11  and a pressure of being nipped by the fuser belt  11  and the pressure application roller  2 , the toner melts and is fused to the medium P. 
     &lt;Efficacy of Embodiment&gt; 
     In the above-mentioned fusing operation, along with the rotation of the fuser belt  11 , the fuser belt  11  shifts in the +Y direction or the −Y direction. In that case, as explained referring to  FIG. 9B , an end part of the fuser belt  11  contacts with the driven ring  4 , and the driven ring  4  contacts with the abutting face  32   a , thereby the width-direction position of the fuser belt  11  is regulated. 
     This driven ring  4  rotates following the fuser belt  11  due to its contact with the end face of the fuser belt  11 . At this time, because a speed difference can easily occur between the rotation speed of the fuser belt  11  and the rotation speed of the driven ring  4 , due to friction with the end face of the fuser belt  11 , the surface of the driven ring  4  wears out. Below, this point is explained. 
       FIG. 13A  is a schematic diagram showing a contact state of a fuser belt  11 , a driven ring  4 , and an abutting face  32   a  in a comparative example. In this comparative example, the central axis A of an arc formed by a contact face  31   a  of a belt supporting part  31  and the central axis of an arc formed by a bottom face  33   a  of a ring supporting groove  33  coincide with each other. 
     In this case, the fuser belt  11  rotates centering on the central axis A, and the driven ring  4  also rotates centering on the central axis A. Therefore, a contact region with the fuser belt  11  on the surface of the driven ring  4  (that is, a first contact face  41  shown in  FIG. 8 ) becomes a narrow region shaped in a ring centering on the central axis A. Therefore, the surface of the driven ring  4  can be shaved, thereby forming a groove. 
       FIG. 13B  is a schematic diagram showing a state that a groove  4   a  has occurred on the driven ring  4  in the comparative example.  FIG. 13C  is a schematic diagram showing an example of the surface roughness distribution of a part indicated with a code E in  FIG. 13B . In  FIG. 13C , the vertical axis indicates the radial position (R), and the horizontal axis the surface roughness. As mentioned above, because the contact region with the fuser belt  11  on the surface of the driven ring  4  is a narrow ring-shaped region, the surface of the driven ring  4  is shaved off to form the deep groove  4   a . If the groove  4   a  on the surface of the driven ring  4  becomes deep, a damage may also occur to the fuser belt  11  in contact with the driven ring  4 . 
       FIG. 14A  is a schematic diagram showing a contact state of the fuser belt  11 , the driven ring  4 , and the abutting face  32   a  in this embodiment. In this embodiment, as mentioned above, the central axis A of an arc formed by the contact face  31   a  of the belt supporting part  31  and the central axis B of an arc formed by the bottom face  33   a  of the ring supporting groove  33  are shifted. 
     The fuser belt  11  rotates centering on the central axis A. On the other hand, if there is a speed difference between the rotation speed of the fuser belt  11  and the rotation speed of the driven ring  4 , the driven ring  4  rotates centering on the central axis B. That is, the center of rotation of the fuser belt  11  and the center of rotation of the driven ring  4  differ. Therefore, the fuser belt  11  cannot continue to contact with the surface of the driven ring  4  in the same radial position. That is, the contact region with the fuser belt  11  on the surface of the driven ring  4  (that is, the first contact face  41  shown in  FIG. 8 ) becomes a wide region ranging from the inner circumferential vicinity to the outer circumferential vicinity of the driven ring  4 . 
       FIG. 14B  is a schematic diagram showing a state that a groove  4   b  has occurred on the driven ring  4  in this embodiment.  FIG. 14C  is a schematic diagram showing an example of the surface roughness distribution of a part indicated with a code E in  FIG. 14B . In  FIG. 14C , the vertical axis indicates the radial position (R), and the horizontal axis the surface roughness. As mentioned above, because the contact region with the fuser belt  11  on the surface of the driven ring  4  becomes a wide region ranging from the inner circumferential vicinity to the outer circumferential vicinity of the driven ring  4 , the range where the surface of the driven ring  4  is shaved off becomes large. Even if a groove  4   b  is formed on the surface of the driven ring  4 , its depth becomes small. Therefore, a damage to the driven ring  4  can be suppressed, and a damage to the fuser belt  11  in contact with this driven ring  4  can also be suppressed. 
     Also, as mentioned above, if the inner diameter of the fuser belt  11  is set to 30 mm and the inner diameter of the driven ring  4  to 29.2 mm, the distance S between the central axis A and the central axis B should desirably be 0.4 mm or less. If the distance S between the central axis A and the central axis B is one half or less of the inner diameter of the fuser belt  11  minus the inner diameter of the driven ring  4  (0.8 mm here), the fuser belt  11  would never be dislocated from the inner circumferential side of the driven ring  4 . 
     &lt;Efficacy of Embodiment&gt; 
     As explained above, in this embodiment, the belt supporting part  31  is in contact with the fuser belt  11  on the contact face  31   a  shaped in an arc centering on the central axis A (first central axis), the ring supporting groove  33  is in contact with the driven ring  4  on the bottom face  33   a  shaped in an arc centering on the central axis B (second central axis), and these central axes A and B are shifted from each other. Therefore, the center of rotation of the fuser belt  11  and the center of rotation of the driven ring  4  can be made different. As a result, the contact region with the fuser belt  11  on the surface of the driven ring  4  expands, thereby the formation of a deep groove on the surface of the driven ring  4  can be prevented. Thereby, a damage to the driven ring  4  can be prevented, and a damage to the fuser belt  11  in contact with the driven ring  4  can also be prevented. 
     Also, because the radial width H of the driven ring  4  is larger than the distance S between the central axis A and the central axis B, even if the fuser belt  11  rotates centering on the central axis A and the driven ring  4  rotates centering on the central axis B, the contact state between the fuser belt  11  and the driven ring  4  can be secured. 
     Also, because the belt supporting part  31  and the ring supporting groove  33  are formed on the common flange member  3  (supporting body), the fuser belt  11  and the driven ring  4  can be supported by the contact face  31   a  and the bottom face  33   a  in a simple configuration. 
     Also, because the ring supporting groove  33  is formed adjacent to the belt supporting part  31  in the width direction (Y direction) of the fuser belt  11 , the driven ring  4  can be supported so as to contact with an end face of the fuser belt  11 . 
     Also, because the depth D of the ring supporting groove  33  varies along the circumferential direction, a configuration that the central axis A and the central axis B are shifted can be realized in a simple configuration. 
     Also, because the flange member  3  (supporting body) has the abutting face  32   a  that can contact with the driven ring  4  on the opposite side of the belt supporting part  31  with respect to the ring supporting groove  33 , it can contact with the driven ring  4  displaced by contacting with the fuser belt  11  and regulate the width-direction position of the fuser belt  11 . 
     Also, having the heat diffusing member  16  between the heater  15  and the fuser belt  11  and holding the heater  15  between the heater supporting member  13  and the heat diffusing member  16 , heat of the heater  15  can be efficiently transmitted to the fuser belt  11 . 
     Also, because the tapered face  32   b  is formed adjacent to the lower part of the abutting face  32   a , if the fuser belt  11  is displaced in the Y direction when it passed through the lower part of the belt supporting part  31  (the part where the opening part  31   c  is formed) and contacted with the contact face  31   a , the driven ring  4  is allowed to bend, and the bending of the driven ring  4  can be released by the tapered face  32   b.    
     Note that although the belt supporting part  31 , the ring supporting groove  33 , and the connecting part  32  were formed on the common flange member  3  (supporting body) in the above-mentioned embodiment, these can be formed as separate bodies. 
     Also, although the pressure application roller  2  as a pressure application member was installed on the outer circumferential side of the fuser belt  11  in the above-mentioned embodiment, instead of the pressure application roller  2 , a pressure application pad may be installed for example. 
     Although the stay  12 , the heater supporting member  13 , the heat conducting member  14 , the heater  15 , the heat diffusing member  16 , and the temperature sensor  17  were installed on the inner circumferential side of the fuser belt  11  in the above-mentioned embodiment, this invention is not limited to such a configuration as this, but its configuration only needs to allow heating the fuser belt  11  by the heater  15  from the inner circumferential side. 
     Although an image forming apparatus that forms a color image was explained in the above-mentioned embodiment, this invention can also be applied to an image forming apparatus that forms a monochromatic image. Also, this invention can be utilized, for example, by image forming apparatuses (such as copiers, facsimile machines, printers, and multifunction peripherals) that form an image on a medium using an electrophotographic system and their fuser devices.