Patent Publication Number: US-9405247-B2

Title: Fixing device and image forming apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2013-071975 filed Mar. 29, 2013. The entire content of the priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a fixing device that thermally fixes a transferred developing agent image to a sheet and an image forming apparatus including the fixing device. 
     BACKGROUND 
     Japanese Patent Application Publication No. S62-14675 discloses a fixing device including a heater, a pressure belt providing a nip region in cooperation with the heater, and two rollers supporting an inner peripheral surface of the pressure belt. The pressure belt is nipped between the heater and each of the two rollers, which are disposed at an entry position and an exit position of sheets. Therefore, an enlarged nipping region can be provided, and contacting width between the sheet and the heater can be increased, whereupon enhanced heating efficiency can be obtained. 
     SUMMARY 
     In the nip region, if the contact pressure between the pressure belt and the heater is insufficient, sheet slippage relative to the pressure belt may occur at an entry position of the nip region. Therefore, an increase in contact pressure between the pressure belt and the heater is required. 
     One possible solution is to increase the contact pressure between the heater and the pressure belt. To this effect, pressing the heater against the pressure belt is required over an entire area of the belt between the rollers. That is, load application is required on the nip region, i.e. the entire range between the two rollers. However, if a heavy load is applied on the heater in order to further increase the contact pressure against the pressure belt at the sheet entry position, almost the same amount of the load at the sheet entry may be applied on the sheet exit position, because the two rollers are disposed to support the load almost equally. As a result, the load applied on the entire fixing device may become larger, and the fixing device may require complicated structure. 
     Therefore, the object of the present invention is to provide a fixing device with a simple structure that prevents a failure of sheet conveyance at the sheet entry position, and to provide an image forming apparatus including the fixing device. 
     In order to attain the above and other objects, the present invention provides a fixing device that includes a heater, a first endless belt, a first back up member, and a second back up member. The first endless belt provides a nip region upon contacting with the heater, and is movable in a first direction at the nip region. The first back up member provides a first position where the first back up member is configured to nip the first endless belt in cooperation with the heater. The first endless belt and the heater provide a first contact pressure at the first position. The second back up member is positioned downstream of the first back up member in the first direction and spaced away therefrom. The second back up member provides a second position where the second back up member is configured to nip the first endless belt in cooperation with the heater. The first endless belt and the heater provide a second contact pressure at the second position; the first contact pressure is higher than the second contact pressure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a schematic cross-sectional view showing a structure of a color laser printer having a fixing device according to one embodiment of the present invention; 
         FIG. 2A  is a schematic cross-sectional view of the fixing device according to the embodiment; 
         FIG. 2B  is a graph indicating a distribution of a contact pressure P at a contact position C between a heating device and a fusing belt in a frontward/rearward direction; 
         FIG. 3A  is a perspective view, as viewed from above, of a regulating member in the fixing device according to the embodiment; 
         FIG. 3B  is a perspective view, as viewed from below, of the regulating member and a stay assembled therewith in the fixing device according to the embodiment; 
         FIG. 3C  is a bottom view of the regulating member and the stay assembled therewith in the fixing device according to the embodiment; 
         FIG. 4  is a right side view of the fixing device according to the embodiment; 
         FIG. 5A  is a partial cross-sectional view of the fixing device as the fusing belt is not in contact with a pressure belt according to a second embodiment; 
         FIG. 5B  is a partial cross-sectional view of the fixing device as the fusing belt is in contact with a pressure belt according to the second embodiment; 
         FIG. 6  is a schematic cross-sectional view of a fixing device according to a third embodiment; 
         FIG. 7  is a schematic cross-sectional view of a fixing device according to a fourth embodiment; and 
         FIG. 8  is a schematic cross-sectional view of a fixing device according to a fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A fixing device according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings. As shown in  FIG. 1 , the color laser printer  1  includes a main frame  2 , a sheet supplying unit  5  for supplying a sheet  51  as a recording medium, an image forming unit  6  for forming an image on the sheet  51 , and a sheet discharge unit  7  for discharging a sheet on which an image has been formed. 
     Incidentally, in the following description, unless otherwise stated, the vertical direction of  FIG. 1  is referred to as a vertical direction; the left side of  FIG. 1  is referred to as front, and the right side as rear; and the back side of the paper surface is referred to as left, and the front side of the paper surface as right. In this manner, each of the directions is indicated. In this case, the left and the right are defined based on the directions in which a person standing in front of a color laser printer  1  is viewing. 
     General Structure of Laser Printer 
     The sheet supplying unit  5  includes a sheet supply tray  50  and a sheet supplying mechanism M 1 . The sheet supply tray  50  is mounted in the main frame  2  and is detachable from the main frame  2  at a front side thereof by a sliding operation. The sheet supplying mechanism M 1  is configured for lifting the sheets  51  from a front side of the sheet supply tray  50  in a diagonally upward and frontward direction and then reversing the sheet  51  rearward. 
     The sheet supplying mechanism M 1  is disposed near the front end portion of the sheet supply tray  50 , and includes a pick-up roller  52 , a separation roller  53 , a separation pad  54 , a paper dust removing roller  55 , and a pinch roller  56 . A conveying path  57  is provided above the sheet supplying mechanism M 1 , and a conveyer belt  73  is provided above the sheet supply tray  50  and downstream of the conveying path  57 . 
     An uppermost sheet  51  of a sheet stack on the sheet supply tray  50  is separated in an upward direction through cooperative operation of the pick-up roller  52 , the separation roller  53 , and the separation pad  54 . As the sheet  51  fed in the upward direction passes between the paper dust removing roller  55  and the pinch roller  56 , paper dust is removed from the sheet  51 . Then, the sheet  51  is conveyed along the conveying path  57  while the conveying direction of the sheet  51  is changed to a rearward direction. Subsequently, the sheet  51  is conveyed onto the conveyor belt  73 . 
     The image forming unit  6  includes a scanning unit  61 , a process unit  62 , a transfer unit  63 , and a fixing device  100 . The scanning unit  61  is disposed in an upper section of the main frame  2 , and includes four sub-scanning units each corresponding to one of four colors cyan, magenta, yellow, and black. Although not shown in the drawings, each of the sub-scanning units includes a laser emitting section, a polygon mirror, a plurality of lenses, and a reflecting mirror. The laser emitting section emits a laser beam, which is scanned at a high speed by the polygon mirror in the left-to-right direction and passes through and is reflected by the plurality of lenses and the reflecting mirror so as to irradiate a surface of a corresponding photosensitive drum  31  described later. 
     The process unit  62  is disposed below the scanning unit  61  and above the sheet supplying unit  5 , and includes a drum unit  3 . The drum unit  3  has four sub-drum units  30  and four developing cartridges  40  corresponding to the sub-drum units  30 . 
     Each sub-drum unit  30  includes the photosensitive drum  31  and a scorotron charger  32 . Each developing cartridge  40  includes a toner supply roller  41 , a developing roller  42 , and a doctor blade (toner layer thickness regulation blade)  43 , and accommodates therein toner of specific color. 
     During the image forming operation, the toner in the developing cartridges  40  is supplied to the developing roller  42  via the toner supply roller  41 . In this case, the toner is charged with a positive polarity by triboelectric charging. The toner conveyed on the developing roller  42  becomes a thin layer having a uniform thickness by the doctor blade  43  in accordance with the rotation of the developing roller  42 . 
     In the sub-drum units  30 , the surface of the photosensitive drum  31  is uniformly charged with a positive polarity by the scorotron charger  32 , Then, the surface is subjected to high speed scan of the laser beam from the scanning unit  61  based on the image data. Thus, an electrostatic latent image is formed on the surface of the photosensitive drum  31 . 
     The developing roller  42  supplies the toner onto the electrostatic latent image on the rotating photosensitive drum  31 ; the latent image has been formed by the discharge of the positively charged surface as a result of the exposure to the laser beam. Thus, the reversal development process is carried out in which the photosensitive drum  31  obtains a visible toner image formed of each color of the toner; in other words, the electrostatic latent image is converted into a toner color image. 
     The transfer unit  63  includes a drive roller  71 , a driven roller  72 , the conveyor belt  73 , a plurality of transfer rollers  74 , and a cleaning unit  75 . The drive roller  71  and the driven roller  72  are disposed in parallel with and separated from each other. The conveyor belt  73  is an endless belt disposed over the drive roller  71  and the driven roller  72 . An outer surface of the conveyor belt  73  serves as a conveying surface and contacts each of the photosensitive drums  31 . The transfer rollers  74  are disposed in opposition to the corresponding photosensitive drums  31  via the conveyor belt  73 , and are applied with transfer bias from a high-voltage circuit board (not shown). During the image forming operation, the conveyor belt  73  conveys the sheet. Subsequently, the sheet  51  conveyed by the conveyor belt  73  is nipped between the photosensitive drum  31  and the transfer roller  74  via the conveyor belt  73 , whereby a toner image is transferred from the photosensitive drum  31  onto the sheet  51 . 
     The cleaning unit  75  is disposed below the conveyor belt  73  for removing toner adhered to the conveyor belt  73 . A toner accumulation section  76  is disposed below the cleaning unit  75  for accumulating toner removed by the cleaning unit  75 . 
     The fixing device  100  is disposed rearward of the transfer unit  63 . The toner image transferred onto the sheet  51  is thermally fixed thereon as the sheet  51  passes through the fixing device  100  (described later). 
     In the sheet discharge unit  7 , a paper-discharge-side conveying path  91  is so formed as to extend upward from an outlet of the fixing device  100  and to make a turn to the front side. A plurality of conveying rollers  92  for conveying the sheet  51  is disposed on the paper-discharge-side conveying path  91 . A discharge tray  93  is provided on the upper surface of the main frame  2  for accommodating the sheet  51  discharged from the paper-discharge-side conveying path  91 . 
     Detailed Configuration of Fixing Device 
     As shown in  FIG. 2A , the fixing device  100  includes a heater  101 , a pressure belt  200 , and a fixing frame  180  that supports the above components (See  FIG. 4 ). The pressure belt  200  will be described later. 
     The heater  101  includes a fusing belt  110 , a halogen lamp  120 , a nip plate  130 , a reflection plate  140 , a stay  160 , and a regulating member  170  (See  FIG. 3A ). 
     The fusing belt  110  is an endless belt that has heat resistance and flexibility. The fusing belt  110  is so formed as to come in contact with the pressure belt  200  (described later) and to follow the motion of the pressure belt  200 . The fusing belt  110  includes a metal element tube that is made of stainless steel or any other metal. The fusing belt  110  may include a rubber layer formed over a surface of the metal element tube, and may further include a nonmetallic release layer such as fluorine coating formed over a surface of the rubber layer. Incidentally, the fusing belt  110  of the present embodiment follows only the motion of the pressure belt  200 , and is not driven by other members. 
     The halogen lamp  120  is a heating element that heats the toner on the sheet  51  by heating the nip plate  130  and the fusing belt  110 . On the internal space of the fusing belt  110 , the halogen lamp  120  is disposed away from the inner surface of the fusing belt  110  and nip plate  130  by predetermined intervals. 
     The nip plate  130  is a plate-like member that receives radiation heat from the halogen lamp  120 . The fusing belt  110  is nipped between the pressure belt  200  and the nip plate  130 . The nip plate  130  conveys the radiation heat received from the halogen lamp  120  to the toner on the sheet  51  via the fusing belt  110 . 
     The nip plate  130  has a generally U-shaped cross-section and is made from a material such as aluminum having a thermal conductivity higher than that of the stay  160  (described later) made from steel. More specifically, for fabricating the nip plate  130 , an aluminum plate is bent into substantially U-shape to provide a base section  131  and bent sections  132 . When viewed in cross-section, the base section  131  extends in the frontward/rearward direction (or direction in which the pressure belt  200  moves), and the bent sections  132  are bent upward from both ends of the base section  131 . The bottom of the base section  131  provides a base surface  131 A in contact with the pressure belt  200 , and each bent section  132  has a bent surface  132 A in contact with the pressure belt  200 . Each bent surface  132 A has a radius of curvature smaller than that of the base surface  131 A. 
     The reflection plate  140  is adapted to reflect radiant heat from the halogen lamp  120  (most of the radiant heat is emitted in the frontward/rearward direction and in an upward direction) toward the nip plate  130  (an inner surface of the base section  131 ). As shown in  FIG. 2 , the reflection plate  140  is positioned in the internal space of the fusing belt  110  and surrounds the halogen lamp  120  with a predetermined distance therefrom. 
     Thus, radiant heat from the halogen lamp  120  can be efficiently concentrated onto the nip plate  130  to promptly heat the nip plate  130  and the fusing belt  110 . 
     The reflection plate  140  has a substantially U-shaped cross-section and is made from a material such as aluminum having high reflection ratio regarding infrared rays or far infrared rays. The reflection plate  140  has a substantially U-shaped reflection portion  141  and a flange sections  142  extending outward from each end portion of the reflection portion  141  in the frontward/rearward direction. A mirror surface finishing is applicable on the surface of the reflection portion in order to enhance the heat reflection ratio of the reflection plate  140 . 
     The stay  160  is a member that ensures rigidity of the nip plate  130  by supporting both ends of the base section  131  of the nip plate  130  in the frontward/rearward direction through the flange sections  142  of the reflection plate  140 . The stay  160  is placed opposite to the pressure belt  200  with respect to the nip plate  130 . The stay  160  has a substantially U-shaped cross-section, including an upper wall  161 , a front wall  162 , and a rear wall  163 . The front wall  162  extends downward from the front end of the upper wall  161 , and the rear wall  163  extends downward from the rear end of the upper wall  161 . The stay  160  is so disposed as to cover the reflection plate  140 . The stay  160  is formed by bending a steel plate or any other plate having high rigidity into a U-shape. 
     The stay  160  holds the nip plate  130  and the reflection plate  140  at a lower surface  162 A of the front wall  162  and at a lower surface  163 A of the rear wall  163 . The stay  160  and the halogen lamp  120  are fixed to the left and the right regulating members  170  as shown in  FIGS. 3A, 3B, and 3C . Alternatively, the halogen lamp  120  can be fixed to the fixing frame  180 . 
     Each of the regulating members  170  is disposed at each of the widthwise end portions of the fusing belt  110  for regulating the movement of the fusing belt  110  in the leftward/rightward direction. Incidentally, in the following description, only the right regulating member  170  will be described, because the left regulating member  170  has the structure the same as the right regulation member. 
     More specifically, the regulating member  170  includes an upper wall  171 , a pair of side walls  172 , and a holding wall  173 . The side walls  172  extend downward from both the front and the rear end portions of the upper wall  171 , and the holding wall  173  extends downward from an outer end portion of the upper wall  171  in the rightward/leftward direction. The regulating member  170  holds the stay  160  so that the upper wall  171 , the pair of side walls  172 , and the holding wall  173  surround the stay  160 . 
     Moreover, the regulating member  170  includes a belt regulating section  174 , and a guide portion  175 . The belt regulating section  174  is arcuate shaped that protrudes outward in the frontward/rearward direction from inner end portions of the pair of side walls  172  in the frontward/rearward direction. The belt regulating section  174  includes a belt regulating surface  174 A at an inner side in the leftward/rightward direction for restricting movement of the fusing belt  110  in the leftward/rightward direction. 
     The guide portion  175  is a rib protruding inward from the belt regulating surface  174 A in the leftward/rightward direction. The guide portion  175  has a C-shaped section with an opening at its lower side. The guide portion  175  is adapted to extend into the fusing belt  110  to suppress radially inward deformation of the fusing belt  110 . Incidentally, the shape of the regulating member  170  is not limited to the shape described above, but the regulating member  170  can be formed into any shape. 
     As shown in  FIG. 4 , the regulating member  170  is supported by the fixing frame  180  so as to be movable in a vertical direction. The fixing frame  180  includes an upper frame  181  and a lower frame  182 . On the upper frame  181 , a coil spring S is provided so as to urge the regulating member  170  downward or against the pressure belt  200 . The coil spring S is adapted to press the upper wall  171  in the downward direction. As a result, the pressure belt  200  is pressed against the first roller  210  and the second roller  220 . Thus, the pressure belt  200  is pressed against the fusing belt  110  by the reaction force from the first and second rollers  210  and  220 . 
     Substantially U-shaped support grooves  182 A is formed on each of the left and right walls of the lower frame  182  for supporting the regulating member  170  so that the regulating member  170  is movable in the vertical direction. A bearing  190  for supporting a first shaft  211  of a first roller  210  (described later) and a bearing  191  for supporting a is second shaft  221  of a second roller  220  (described later) are provided on the front side of the bottom portion of the support groove  182 A. 
     Configuration of Pressure Belt 
     As shown in  FIG. 2A , the pressure belt  200  is an endless belt that faces the fusing belt  110  and is in contact with the fusing belt  110 , thereby forming a nip region N. A portion of the pressure belt  200  that faces the fusing belt  110  is so configured as to move rearward. 
     The pressure belt  200  is made from a resin such as polyimide resin. An inner peripheral surface  200 A of the pressure belt  200  is supported by the first roller  210  and the second roller  220 . Incidentally, all that is required for the pressure belt  200  is to contain resin. 
     The first roller  210  faces the fusing belt  110 . The pressure belt  200  is held between the first roller  210  and the fusing belt  110  at a position A. The pressure belt  200  and the fusing belt  110  are held between the first roller  210  and the front side of the base surface  131 A, wherein the front side is an upstream side of the base surface  131 A in the running direction of the pressure belt  200 . 
     More specifically, the first roller  210  is disposed at a position where the lower surface  162 A supports the base section  131 , or is aligned with the front wall  162  in the frontward/rearward direction. The nip plate  130 , the pressure belt  200 , and the fusing belt  110  are held between the first roller  210  and the lower surface  162 A of the stay  160 . Moreover, the lower surface  162 A and the first position A at which the first roller  210  is in contact with the pressure belt  200  are aligned with each other in the vertical direction. 
     The first roller  210  is coupled to a first gear  213  as shown in  FIG. 4 , which is driven by a motor  400  disposed outside. The first roller  210  includes a first shaft  211  made from metal and a first elastic layer  212  made from rubber and formed over an outer peripheral surface of the first shaft  211 . The first elastic layer  212  has a thickness smaller than that of a second elastic layer  222  (described later). Therefore, when the pressure belt  200  is nipped between the first roller  210  and the nip plate  130 , the reaction force of the first elastic layer  212  is greater than that of the second elastic layer  222 . Thus, the pressing force of the first roller  210  is larger than the pressing force of the second roller  220 . This means that the contact pressure between the pressure belt  200  and the fusing belt  110  at the first position A is larger than the contact pressure between the pressure belt  200  and the fusing belt  110  at a second position B (described later). Consequently, slippage of the sheet  51  relative to the pressure belt  200  at the first position A can be more reliably prevented. 
     Incidentally, the thickness of the first elastic layer  212  may be in the range of 0.01 to 10.00 mm, or in the range of 0.1 to 5.00 mm, or in the range of 0.15 to 3.00 mm. The thickness of the second elastic layer  222  may be in the range of 0.10 to 40.00 mm, or in the range of 2.0 to 20.00 mm, or in the range of 5.00 to 15.00 mm. 
     As shown in  FIG. 2A , the second roller  220  is positioned rearward (downstream in the running direction of the pressure belt  200 ) of the first roller  210 . The pressure belt  200  is nipped between the second roller  220  and the fusing belt  110  at the second position B remote from the first position A. More specifically, the second roller  220  is disposed at the position where the lower surface  163 A supports the base section  131 , i.e. at the same position as the rear wall  163  in the frontward/rearward direction. The second roller  220  nips the nip plate  130 , the pressure belt  200  and the fusing belt  110  in cooperation with the lower surface  163 A. When viewed in the upward/downward direction, the lower surface  163 A and the contacting portion at which the second roller  220  is in contact with the pressure belt  200 , i.e. second position B, are aligned with each other. 
     Since the second roller  220  is disposed away from the first roller  210  by predetermined interval, the pressure belt  200  is in contact with the fusing belt  110  in the frontward/rearward range extending from the front wall  162  to the rear wall  163 . Hence, a nip region N can be formed in that range, and can be extended wider in the frontward/rearward direction. According to the present embodiment, when the fusing belt  110  is not in contact with the pressure belt  200 , e.g. when the fixing device  100  is not yet assembled or when the heater  101  is separated from the pressure belt  200  due to sheet-jam processing, an upper end of the first roller  210  is at the same height as an upper end of the second roller  220  in the upward/downward direction. 
     The second roller  220  includes a second shaft  221  made from metal and the second elastic layer  222  made from a thermal insulation material such as foamable sponge and formed over an outer peripheral surface of the second shaft  221 . Because the second elastic layer  222  is a foamable elastic layer whose reaction force is smaller than that of the rubber layer of the first roller  210 , the contact pressure at the second position B is smaller than that at the first position A. Consequently, the contact pressure at the first position A is larger than that at the second position B. 
     As shown in  FIG. 4 , the second roller  220  is coupled to a second gear  223 , which is driven by the motor  400 . The circumferential velocity of the second roller  220  is set greater than that of the first roller  210  in operation. Incidentally, such velocity difference can be set by setting a first reduction ratio of a gear train for driving the first gear  213  greater than a second reduction ratio of a gear train for driving the second gear  223  such that the circumferential velocity of the second gear  223  can be greater than that of the first gear  213 . Alternatively, such velocity difference can be provided by connecting each dedicated motor to each of the gears  213 ,  223 . In the latter case, the circumferential velocity of the motor connected to the second gear  223  is greater than that of the other motor connected to the first gear  213 . 
     With the structure thus constructed, as the coil spring S presses the heater  101  downward, the fusing belt  110  is in contact with the pressure belt  200 , forming the nip region N. Further, the pressure belt  200  is pressed against the fusing belt  110  by the first and second rollers  210 ,  220 . Therefore, as shown in  FIG. 2B , appropriate contact pressure (shown as P) can be maintained across the entire nip region N at a contact position (shown as C). 
     Moreover, the first roller  210  and the second roller  220  are pressed against the pressure belt  200  at the first position A and at the second position B. Therefore, the contact pressure at the positions A and B is higher than at a remaining position in the nip region N. Here, assuming that the contact pressure between the pressure belt  200  and fusing belt  110  at the first position A is the same as the contact pressure between the pressure belt  200  and fusing belt  110  at the second position B. In the latter case, in an attempt to increase the contact pressure at the sheet entry position (first position A) and the sheet exit position (second position B), a total load applied to the entire nip region N must be increased. As a result, the load applied to the entire fixing device  100  becomes larger, which makes the structure of the fixing device more complex. 
     According to the present embodiment, the contact pressure of the second roller  220  is lower than the contact pressure of the first roller  210 . Therefore, in comparison with the structure in which the contact pressure at the first position is the same as the second position, the contact pressure to be applied to the entire nip region N does not have to be increased. Consequently, the simplified structure of the entire fixing device  100  can be provided, because, for example, the urging force of the coil spring S can be smaller. 
     Further, since the circumferential velocity of the second roller  220  is set greater than that of the first roller  210 , the rotation of the second roller  220  pulls an upper portion of the pressure belt  200  rearward. As a result, sufficient tension can be applied to an upper portion of the pressure belt  200 , or a portion of the pressure belt  200  in contact with the fusing belt  110  to avoid deflection of the upper portion. 
     Further, the nip plate  130 , the pressure belt  200 , and the fusing belt  110  are nipped not only between the first roller  210  and the lower surface  162 A, but also between the second roller  220  and the lower surface  163 A. Therefore, the contact pressure between the pressure belt  200  and the fusing belt  110  can be increased at the first and second positions A and B. 
     Further, the pressure belt  200  contains resin, thereby providing lower thermal conductivity of the pressure belt  200 , in comparison with a pressure belt made exclusively from metal. Consequently, the pressure belt  200  made from resin can restrain heat removal from the heater  101  while maintaining durability. 
     Further, the area of the nip region N can be increased, resulting in an improvement in heating efficiency in the nip region N. 
     Second Embodiment 
     A fixing device  100 A according to a second embodiment of the present invention will be described with reference to  FIGS. 5A and 5B , wherein like parts and components are designated by the same reference numerals and characters as those shown in the first embodiment. 
     According to the first embodiment, the first elastic layer  211  has a thickness smaller than that of the second elastic layer  222 , and the first elastic layer  212  is a rubber layer whereas the second elastic layer  222  is a foamable elastic layer. Thus, the contact pressure at the first position A is higher than that at the second position B. In contrast, according to the second embodiment as shown in  FIG. 5A , when a fusing belt  110  is not in contact with a pressure belt  200 , or for example when the fixing device  100 A is not yet assembled or when a heater  101 A is separated from the pressure belt  200  due to sheet jam processing, a first roller  210 A is configured higher than a second roller  220 A. Incidentally, the direction from the first roller  210 A to the fusing belt  110 , or upward direction, is one example of a second direction. 
     The first roller  210 A has a first shaft  211 A made from metal, and a first elastic layer  212 A made from rubber. The second roller  220 A has a second shaft  221 A made from metal, and a second elastic layer  222 A made from rubber. The first shaft  211 A has a diameter larger than that of the second shaft  221 A; the first elastic layer  212 A has a thickness equal to that of the second elastic layer  222 A. Incidentally, the first elastic layer  212 A and the second elastic layer  222 A can be foamable elastic layers. Alternatively, the thickness of the first elastic layer  212 A can be equal to that of the second elastic layer  222 A, and the diameter of the first roller  210 A can be equal to that of the second roller  220 A. In the latter case, the first roller  210 A is positioned higher than the second roller  220 A in the vertical direction. 
     With this structure, as a coil spring S (not shown) presses the heater  101 A toward the pressure belt  200 , as shown in  FIG. 5B , the reaction force of the first roller  210 A can be greater than that from the second roller  220 A because the first roller  210 A is positioned higher than the second roller  220 A. As a result, the pressing force of the first roller  210 A can be greater than that of the second roller  220 A. 
     Third Embodiment 
     A fixing device  100 B according to a third embodiment will next be described with reference to  FIG. 6 . According to the first embodiment, the second roller  220  is coupled to the second gear  223  that is driven by the motor  400  disposed outside. However, according to the third embodiment shown in  FIG. 6 , the second gear for coupling to the second roller  220  is not provided, that is, the second roller  220  do not receive a drive force from a motor disposed outside. Instead, the first roller  210  receives a drive force from a motor disposed outside. The second roller  220  includes a second shaft  221  and a second elastic layer  222  these being the same as the first embodiment. A coil spring  230  is provided at a bearing  191  rotatably supporting the second shaft  221 . 
     A coil spring  230 A has a front end portion  231  hooked on the bearing  191  and a rear end portion  232  hooked on an appropriate location of a fixing frame  180  (See  FIG. 4 ) for urging the second roller  220  in the rearward direction. Therefore, the pressure belt  200  is pulled in the rearward direction, and the pressure belt  200  therefore is constrained from flexing. 
     Fourth Embodiment 
       FIG. 7  shows a fixing device  100 C according to a fourth embodiment of the present invention. In the first embodiment, the coil spring S presses the heater  101  in the downward direction for providing the contact pressure in the nip region N. In contrast, according to the fourth embodiment shown in  FIG. 7 , a first compression spring  240 A and a second compression spring  240 B are provided instead of the coil spring S. 
     In a first roller  210 B and a second roller  220 B, a first elastic layer  212 B has a thickness equal to that of a second elastic layer  222 B. A first shaft  211 B and a second shaft  221 B are rotatably supported by bearings  190 B and  191 B. 
     A fixing frame  180 B is formed with substantially U-shaped support grooves  183 A,  183 B elongated in the upward direction, and the bearings  190 B and  191 B are movable with respect to the corresponding support grooves  183 A,  183 B. The first and second compression springs  240 A,  240 B are positioned in the U-shaped support grooves  183 A,  183 B, respectively. More specifically, the first compression spring  240 A is interposed between the bearing  190 B and the support groove  183 A such that an upper end portion  241 A of the first compression spring  240 A is in contact with the bearing  190 B, and a lower end portion  242 A of the first compression spring  240 A is in contact with a bottom of the support groove  183 A for urging the bearing  190 B upward. Similarly, the second compression spring  240 B is interposed between the bearing  191 B and the support groove  183 B such that an upper end portion  241 B of the second compression spring  240 B is in contact with the bearing  191 B, and a lower end portion  242 B of the second compression spring  240 B is in contact with a bottom of the support groove  183 B for urging the bearing  191 B upward. Thus, the first and second compression springs  240 A,  240 B are so formed as to urge the bearings  190 B,  191 B, or the first and second rollers  210 B,  220 B, toward the heater  101 . The urging force of the first compression spring  240 A is greater than that of the second compression spring  240 B. 
     With this structure, since the urging force of the first compression spring  240 A is greater than that of the second compression spring  240 B, the pressing force of the first roller  210 B is greater than that of the second roller  220 B. 
     Fifth Embodiment 
       FIG. 8  shows a fixing device  100 D according to a fifth embodiment of the present invention. According to the above described embodiments, the first roller  210  and the second roller  220  are illustrated as a first back up member and a second back up member that press the pressure belt  200  at the positions A and B. In contrast, according to the fifth embodiment shown in  FIG. 8 , a first pad  250  and a second pad  260  are provided as first and second back up members. 
     A first roller  210 C and a second roller  220 C are provided for supporting an inner peripheral surface  200 A of the pressure belt  200  similar to the foregoing embodiment. The first and second rollers  210 C,  220 C have the configuration the same as that in the fourth embodiment. A first shaft  211 C and a second shaft  221 C are rotatably supported by bearings  190 C and  191 C, respectively. 
     A fixing frame  180 C is formed with a substantially U-shaped support groove  184  having an upper open end, and the bearings  190 C and  191 C are fixed to the front and rear end portions of the support groove  184 , respectively. First and second compression springs  270 A,  270 B are provided on the support groove  184  and between the bearings  190 C and  191 C in the frontward/rearward direction. More specifically, the first compression spring  270 A is positioned rearward of the bearings  190 C, and the second compression spring  270 B is positioned frontward of the bearing  191 C. 
     The first pad  250  is positioned in alignment with the lower surface  162 A of the front wall  162 , and the second pad  260  is positioned in alignment with the lower surface  163 A of the rear wall  163 . Further, a pressure belt  200  is nipped between the first pad  250  and the fusing belt  110  at a position A. More specifically, the nip plate  130 , the pressure belt  200 , and the fusing belt  110  are nipped between the first pad  250  and the lower surface  162 A. 
     The second pad  260  is positioned rearward of the first pad  250  (downstream of the running direction of the pressure belt  200 ). The pressure belt  200  is nipped between the second pad  260  and the fusing belt  110  at a second position B away from the first position A. More specifically, the nip plate  130 , the pressure belt  200 , and the fusing belt  110  are nipped between the second pad  260  and the lower surface  163 A. 
     The first compression spring  270 A is disposed between the first pad  250  and the support groove  184  such that an upper end portion  271 A of the first compression spring  270 A is in contact with the first pad  250  and a lower end portion  272 A thereof is in contact with the bottom of the support groove  184 . The second compression spring  270 B is disposed between the second pad  260  and the support groove  184  such that an upper end portion  271 B is in contact with the second pad  260 , and a lower end portion  272 B is in contact with the bottom of the support groove  184 . The first compression spring  270 A and the second compression spring  270 B are so configured to urge the first pad  250  and the second pad  260  toward the heater  101 . Here, the urging force of the first compression spring  270 A is greater than that of the second compression spring  270 B. Thus, the pressing force of the first pad  250  is greater than that of the second pad  260 . Incidentally, two first compression springs  270 A are provided such that one of the first compression springs  270 A is positioned at left side of the pressure belt  200 , and another first compression springs  270 A is positioned at right side of the pressure belt  200 . The same is true with respect to the second compression springs  270 B. 
     Various modifications are conceivable. For example, according to the foregoing embodiments, when viewed in the vertical direction, the first position A is aligned with the lower surface  162 A, and the second position B is aligned with the lower surface  163 A. However, the first and second positions A and B may not be at the same positions as the lower surfaces  162 A and  163 A. 
     In the first embodiment shown in  FIG. 2A , the first elastic layer  212  and the second elastic layer  222  are made from materials different from each other, and the second elastic layer  222  has a thickness larger than that of the first elastic layer  212 . However, the present invention is not limited to that configuration. For example, the first elastic layer and the second elastic layer can be made from the same material, and the second elastic layer can be thicker than the first elastic layer. Alternatively, the first elastic layer can be equal in thickness to the second elastic layer, and the first elastic layer and the second elastic layer may be a rubber layer and a foamable elastic layer, respectively. 
     Further, in the second embodiment shown in  FIG. 5A , the diameter of the first roller  210 A is greater than the diameter of the second roller  220 A. However, the present invention is not limited to that configuration. For example, the diameter of the first roller can be equal to the diameter of the second roller, and the top of the first roller can be a higher than the top of the second roller. 
     Further, according to the foregoing embodiments, the first and second rollers support the inner peripheral surface  200 A of the pressure belt  200 . However, the present invention is not limited to this configuration. For example, three or more rollers can be used for supporting the pressure belt  200 . 
     Further, according to the foregoing embodiments, the heater includes the fusing belt and the nip plate. However, a heat roller is also available as the heater. 
     Further, according to the foregoing embodiments, the first roller  210  is driven by the motor  400  disposed outside of the fixing device  100 . However, the motor for driving the first roller  210  is not required, if the heater is a heating roller and if the rotation force is imparted on heating roller. In addition, the first roller may not have to be driven by a motor, if driving force is directly imparted on the second roller and provided that slippage of the pressure belt relative to the first roller does not occur. 
     Further, according to the foregoing embodiments, the first elastic layer  212  is formed over the metallic first shaft  211  in the first roller  210 . However, the elastic layer can be dispensed with. Alternatively, a metallic layer and an elastic layer can be formed over a non-metallic shaft member to provide the first roller. 
     Further, according to the foregoing embodiments, the second roller  220  includes as an outer layer the second elastic layer  222  made from thermally insulating material. However, an elastic layer with no heat insulating characteristic is available as the outer layer. 
     Further, according to the foregoing embodiments, the pressure belt  200  is made from resin. Instead, the pressure belt  200  can be made from metal. 
     Further, the foregoing embodiments are applied to the color laser printer  1 . However, the present invention is also available to an image forming apparatus other than color laser printer, such as a monochromatic printer, a copying machine and a multifunction device. 
     While the invention has been described in detail and with reference to specific embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.