Patent Publication Number: US-8526871-B2

Title: Fixing device and image forming apparatus incorporating same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Division of and claims the benefit of priority under 35 U.S.C. §120 from U.S. Ser. No. 12/714,812, filed Mar. 1, 2010 and claims priority to Japanese Patent Application Nos. 2009-060631, filed on Mar. 13, 2009, and 2009-051583, filed on Mar. 5, 2009, in the Japan Patent Office, each of which is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device. 
     2. Description of the Related Art 
     Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium (e.g., a sheet) according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium. 
     Such fixing device may include a fixing roller, a fixing belt wound around the fixing roller, and a pressing roller pressed against the fixing roller via the fixing belt to form a nip portion between the fixing belt and the pressing roller through which a sheet bearing a toner image passes. The fixing belt and the pressing roller apply heat and pressure to the sheet bearing the toner image as the sheet passes through the nip portion to fix the toner image on the sheet. Thereafter, the sheet bearing the fixed toner image is separated from the fixing belt and the pressing roller by a separator contacting the fixing belt, and conveyed to the outside of the image forming apparatus. 
     However, the separator that contacts the fixing belt to separate the sheet bearing the fixed toner image from the fixing belt is capable of scratching and damaging the fixing belt. When the damaged fixing belt contacts the fixed toner image on the sheet, especially a glossy color toner image, scratches on the fixing belt may spoil the image. To address this problem, a non-contact separator may be provided to separate the sheet bearing the fixed toner image from the fixing belt without contacting the fixing belt. However, in this case, the fixing belt is required to have a greater-than-usual degree of curvature to facilitate separation of the sheet from the fixing belt. 
     On the other hand, increasing demand for high-speed toner image formation requires that the nip portion formed between the fixing belt and the pressing roller have a greater length in a sheet conveyance direction, so that sufficient heat and pressure can be applied to the sheet to securely fix the toner image even when the fixing belt and the pressing roller rotate at faster speeds. 
     To address such requirements, the fixing belt may be wound around a plurality of rollers to provide the greater curvature required to facilitate separation of the sheet from the fixing belt and the greater length of the nip portion. For example, a separation roller may be provided downstream from the nip portion formed between the fixing roller and the pressing roller in the sheet conveyance direction, and pressed against the pressing roller via the fixing belt to form a second nip portion between the separation roller and the pressing roller. Thus, the first nip portion formed between the fixing roller and the pressing roller and the second nip portion formed between the separation roller and the pressing roller provide the greater nip length needed to apply sufficient heat and pressure to the sheet. Also, the fixing belt wound around the fixing roller and the separation roller provides the greater curvature to facilitate separation of the sheet from the fixing belt. 
     However, there are drawbacks to the above-described configuration. For example, at an intermediate nip portion provided between the first nip portion and the second nip portion in the sheet conveyance direction, the fixing belt may not be pressed against the pressing roller properly, and may separate from the pressing roller. Further, a circumferential velocity of the fixing belt may differ from a circumferential velocity of the pressing roller due to reduced friction between the fixing belt and the pressing roller at the intermediate nip portion. Consequently, the fixing belt may become slack or slip, resulting in formation of a faulty toner image. 
     BRIEF SUMMARY OF THE INVENTION 
     This specification describes below a fixing device according to exemplary embodiments of the present invention. In one exemplary embodiment of the present invention, the fixing device fixes a toner image on a recording medium, and includes a fixing roller, a separation roller, an endless fixing belt, a pressing roller, a first driving system, and a second driving system. The separation roller is provided downstream from the fixing roller in a recording medium conveyance direction. The fixing belt is wound around at least the fixing roller and the separation roller. The pressing roller is pressed against the fixing roller and the separation roller via the fixing belt to form a first nip portion between the fixing belt and the pressing roller through which the recording medium bearing the toner image passes. The first driving system is connected to the fixing roller and the separation roller, and includes a first motor and a first transmission. The first motor generates a first driving force. The first transmission transmits the first driving force to the fixing roller and the separation roller, and includes a first one-way clutch via which the first driving force is transmitted to the fixing roller. The second driving system is connected to the pressing roller, and includes a second motor and a second transmission. The second motor generates a second driving force. The second transmission transmits the second driving force to the pressing roller. When a circumferential velocity of the pressing roller is greater than a circumferential velocity of the fixing roller, the first one-way clutch idles to allow rotation of the fixing roller in accordance with rotation of the pressing roller. 
     This specification describes below an image forming apparatus according to exemplary embodiments of the present invention. In one exemplary embodiment of the present invention, the image forming apparatus includes a fixing device for fixing a toner image on a recording medium. The fixing device includes a fixing roller, a separation roller, an endless fixing belt, a pressing roller, a first driving system, and a second driving system. The separation roller is provided downstream from the fixing roller in a recording medium conveyance direction. The fixing belt is wound around at least the fixing roller and the separation roller. The pressing roller is pressed against the fixing roller and the separation roller via the fixing belt to form a first nip portion between the fixing belt and the pressing roller through which the recording medium bearing the toner image passes. The first driving system is connected to the fixing roller and the separation roller, and includes a first motor and a first transmission. The first motor generates a first driving force. The first transmission transmits the first driving force to the fixing roller and the separation roller, and includes a first one-way clutch via which the first driving force is transmitted to the fixing roller. The second driving system is connected to the pressing roller, and includes a second motor and a second transmission. The second motor generates a second driving force. The second transmission transmits the second driving force to the pressing roller. When a circumferential velocity of the pressing roller is greater than a circumferential velocity of the fixing roller, the first one-way clutch idles to allow rotation of the fixing roller in accordance with rotation of the pressing roller. 
     This specification describes below a fixing device according to exemplary embodiments of the present invention. In one exemplary embodiment of the present invention, the fixing device fixes a toner image on a recording medium, and includes a fixing roller, a separation roller, an endless fixing belt, and a pressing roller. The fixing roller includes a first elastic layer as a surface layer and has a roller hardness A. The separation roller is provided downstream from the fixing roller in a recording medium conveyance direction, and has a diameter smaller than a diameter of the fixing roller. The separation roller includes a second elastic layer as a surface layer and has a roller hardness B smaller than the roller hardness A of the fixing roller. The fixing belt is wound around at least the fixing roller and the separation roller. The pressing roller is pressed against the fixing roller and the separation roller via the fixing belt to form a nip portion between the fixing belt and the pressing roller through which the recording medium bearing the toner image passes. The pressing roller includes a third elastic layer as a surface layer and has a roller hardness C not smaller than the roller hardness A of the fixing roller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and the many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic view of an image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 2  is a sectional view of a fixing device included in the image forming apparatus shown in  FIG. 1  seen from a front side of the image forming apparatus at which a user operates the image forming apparatus; 
         FIG. 3  is a sectional view of a pressure adjuster included in the fixing device shown in  FIG. 2 ; 
         FIG. 4  is a sectional view of a fixing device according to another exemplary embodiment of the present invention; 
         FIG. 5  is a sectional view of the fixing device shown in  FIG. 4  for explaining movement of a pressure adjuster included in the fixing device and change in diameter of a fixing roller included in the fixing device; 
         FIG. 6  is a sectional view of the fixing device shown in  FIG. 5  for showing a fixing belt included in the fixing device contacting at two positions on the fixing roller shown in  FIG. 5 ; 
         FIG. 7  is a sectional view of the fixing device shown in  FIG. 6  for showing deformation of a fixing roller, a separation roller, and a pressing roller included in the fixing device, and the fixing belt shown in  FIG. 6  wound around the fixing roller, the separation roller, and the pressing roller; 
         FIG. 8  is a sectional view of the fixing device shown in  FIG. 6  for showing deformation of a fixing roller, a separation roller, and a pressing roller included in the fixing device, and the fixing belt shown in  FIG. 6  wound around the fixing roller, the separation roller, and the pressing roller when the fixing roller, the separation roller, and the pressing roller do not have predetermined roller hardnesses, respectively; 
         FIG. 9A  is a sectional view of the fixing device shown in  FIG. 4  for explaining driving systems included in the fixing device; 
         FIG. 9B  is a block diagram of the fixing device shown in  FIG. 9A ; 
         FIG. 10  is a graph illustrating control of a number of rotations of a first motor performed by a motor controller included in the fixing device shown in  FIG. 9B ; 
         FIG. 11  is a graph illustrating another control of a number of rotations of a first motor performed by a motor controller included in the fixing device shown in  FIG. 9B ; 
         FIG. 12  is a graph illustrating yet another control of a number of rotations of a first motor performed by a motor controller included in the fixing device shown in  FIG. 9B ; 
         FIG. 13  is a graph illustrating yet another control of a number of rotations of a first motor performed by a motor controller included in the fixing device shown in  FIG. 9B ; 
         FIG. 14A  is a sectional view of the fixing device shown in  FIG. 4  for explaining correction of twisting of a fixing belt included in the fixing device; 
         FIG. 14B  is a block diagram of the fixing device shown in  FIG. 14A ; 
         FIG. 15A  is a perspective view of the fixing device shown in  FIG. 2 ; 
         FIG. 15B  is a block diagram of the fixing device shown in  FIG. 15A ; 
         FIG. 16A  is a front view of the fixing device shown in  FIG. 15A  seen from a direction S 1  in  FIG. 15A ; 
         FIG. 16B  is a partial side view of the fixing device shown in  FIG. 16A  seen from a direction S 2  in  FIG. 16A ; 
         FIG. 16C  is a partial top view of the fixing device shown in  FIG. 16A  seen from a direction S 3  in  FIG. 16A ; 
         FIG. 16D  is a sectional view of a fixing roller and a fixing gear included in the fixing device shown in  FIG. 16A ; 
         FIG. 16E  is a sectional view of a shaft driving gear, a driving transmission shaft, and a one-way gear included in the fixing device shown in  FIG. 16A ; 
         FIG. 17  is a graph illustrating a relation between a number of rotations of a first motor included in the fixing device shown in  FIG. 15A  and electric current of the first motor; 
         FIG. 18  is a schematic view of a fixing device, a gloss finisher, and a conveyance roller pair included in the image forming apparatus shown in  FIG. 1 ; 
         FIG. 19A  is a perspective view of the fixing device and the gloss finisher shown in  FIG. 18 ; 
         FIG. 19B  is a block diagram of the fixing device and the gloss finisher shown in  FIG. 19A ; 
         FIG. 20  is a graph illustrating a relation between a number of rotations of a third motor included in the gloss finisher shown in  FIG. 19A  and electric current of the third motor; 
         FIG. 21A  is a flowchart illustrating processes performed in the image forming apparatus shown in  FIG. 1  in a gloss mode; 
         FIG. 21B  is a flowchart illustrating processes performed in the image forming apparatus shown in  FIG. 1  in a non-gloss mode for a sheet having a length smaller than 210 mm; 
         FIG. 21C  is a flowchart illustrating processes performed in the image forming apparatus shown in  FIG. 1  in a non-gloss mode for a sheet having a length not smaller than 210 mm; and 
         FIG. 22  is a sectional view of a fixing device according to yet another exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In describing exemplary embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular to  FIG. 1 , an image forming apparatus  100  according to an exemplary embodiment of the present invention is explained. 
       FIG. 1  is a schematic view of the image forming apparatus  100 . As illustrated in  FIG. 1 , the image forming apparatus  100  includes an image reading portion  100 A, an image forming portion  100 B, and a sheet supply portion  100 C. 
     The image reading portion  100 A includes a scanner  1  and an auto document feeder (ADF)  4 . 
     The image forming portion  100 B includes a writer  2 , a development device  3 , a fixing device  5 , a gloss finisher  6 , a conveyance roller pair  7 , an output device  8 , an intermediate transfer member  30 , photoconductors  31 , a second transfer device  34 , a conveyance belt  35 , and a cleaner  36 . 
     The sheet supply portion  100 C includes a conveyance path  37 , a registration device  38 , and a sheet container  41 . The sheet container  41  includes trays  41   a ,  41   b ,  41   c , and  41   d.    
     As illustrated in  FIG. 1 , the image forming apparatus  100  can be a copier, a facsimile machine, a printer, a multifunction printer having at least one of copying, printing, scanning, plotter, and facsimile functions, or the like. According to this exemplary embodiment of the present invention, the image forming apparatus  100  functions as a digital color copier for forming a color image on a recording medium. 
     The image reading portion  100 A is provided in an upper portion of the image forming apparatus  100 . The image forming portion  100 B is provided in a center portion of the image forming apparatus  100 . The sheet supply portion  100 C is provided in a lower portion of the image forming apparatus  100 . 
     In the image reading portion  100 A, the ADF  4  loads a plurality of originals and feeds the originals successively toward the scanner  1 . The scanner  1  optically reads an image on the original to generate image data. 
     In the image forming portion  100 B, the intermediate transfer member  30  has a belt shape and includes a transfer surface extending in a horizontal direction. A mechanism for forming images in complementary colors for color separation is provided above the intermediate transfer member  30 . Specifically, the four photoconductors  31  serving as image carriers for carrying toner images in complementary colors (e.g., yellow, magenta, cyan, and black) are arranged along the transfer surface of the intermediate transfer member  30 . 
     The writer  2  is provided above the photoconductors  31 , and emits light beams onto surfaces of the photoconductors  31  according to the image data generated by the scanner  1  or image data sent from an external device so as to form electrostatic latent images on the surfaces of the photoconductors  31 , respectively. The photoconductors  31  include drums rotatable counterclockwise in  FIG. 1  in an identical direction, respectively. A charger, the development device  3 , a first transfer device, and the cleaner  36  surround the photoconductor  31  to form a toner image while the photoconductor  31  rotates. Specifically, the charger charges the surface of the photoconductor  31 . The writer  2  emits a light beam onto the charged surface of the photoconductor  31  to form an electrostatic latent image on the photoconductor  31 . The development device  3  develops the electrostatic latent image into a toner image. The first transfer device transfers the toner image formed on the photoconductor  31  onto the intermediate transfer member  30 . The cleaner  36  collects residual toner from the surface of the photoconductor  31  after the toner image is transferred from the photoconductor  31  onto the intermediate transfer member  30 . The four development devices  3  contain yellow, magenta, cyan, and black toners, respectively. 
     The intermediate transfer member  30  is wound around a driving roller and driven rollers, and opposes the photoconductors  31  to move in an identical direction with the photoconductors  31 . The second transfer device  34  includes a transfer roller opposing one of the driven rollers. The conveyance belt  35 , the fixing device  5 , the gloss finisher  6 , and the conveyance roller pair  7  are disposed in a sheet conveyance path extending from the second transfer device  34  in this order in a sheet conveyance direction. 
     In the sheet supply portion  100 C, the sheet container  41  includes the trays  41   a ,  41   b ,  41   c , and  41   c  for loading and containing sheets serving as recording media. A conveyance device includes the conveyance path  37  and the registration device  38 . The conveyance device feeds the sheets loaded on the tray  41   a ,  41   b ,  41   c , or  41   d  one by one through the conveyance path  37  toward the second transfer device  34  via the registration device  38  by separating an uppermost sheet from other sheets loaded on the tray  41   a ,  41   b ,  41   c , or  41   d . The registration device  38  corrects skew of the sheet sent from the tray  41   a ,  41   b ,  41   c , or  41   d , and feeds the sheet to the second transfer device  34  at a proper time at which a color toner image formed on the intermediate transfer member  30  is transferred onto the sheet. 
     The following describes an image forming operation performed in the image forming apparatus  100 . The chargers uniformly charge the surfaces of the photoconductors  31 , respectively. The writer  2  emits light beams onto the charged surfaces of the photoconductors  31  according to image data generated by the scanner  1  or image data sent from an external device to form electrostatic latent images corresponding to yellow, magenta, cyan, and black colors, respectively. The development devices  3  containing yellow, magenta, cyan, and black toners make the electrostatic latent images visible as yellow, magenta, cyan, and black toner images, respectively. The first transfer devices applied with a predetermined bias transfer the yellow, magenta, cyan, and black toner images onto the intermediate transfer member  30 , respectively. Specifically, the yellow, magenta, cyan, and black toner images are successively superimposed on a same position on the intermediate transfer member  30  by an electrostatic force to form a color toner image on the intermediate transfer member  30 . 
     The second transfer device  34  transfers the color toner image formed on the intermediate transfer member  30  onto a sheet sent from the tray  41   a ,  41   b ,  41   c , or  41   d  of the sheet container  41 . The conveyance belt  35  conveys the sheet bearing the color toner image to the fixing device  5 . The fixing device  5  fixes the color toner image on the sheet. The gloss finisher  6  adds gloss to the fixed color toner image on the sheet as needed. The conveyance roller pair  7  feeds the sheet bearing the fixed color toner image to the output device  8 . The output device  8  conveys the sheet bearing the fixed color toner image through an output path to an outside of the image forming apparatus  100 . Thus, a series of image forming processes is finished. 
       FIG. 2  is a sectional view of the fixing device  5  seen from a front side of the image forming apparatus  100  depicted in  FIG. 1  at which a user operates the image forming apparatus  100 . 
     As illustrated in  FIG. 2 , the fixing device  5  includes a fixing belt unit  11 U, a pressing roller  14 , a heater  14   h , a web cleaning unit  14   c , a separator  43 , and temperature sensors  62  and  72 . The fixing belt unit  11 U includes a fixing belt  11 , a fixing roller  12 , a separation roller  13 , a heating roller  15 , a heater  15   h , a tension roller  16 , and a driven roller  17 . The fixing roller  12  includes an elastic layer  12 E. The separation roller  13  includes an elastic layer  13 E. The pressing roller  14  includes an elastic layer  14 E. 
     The fixing roller  12  has a cylindrical shape. The fixing belt  11  is stretched over the fixing roller  12 , the separation roller  13 , the heating roller  15 , the tension roller  16 , and the driven roller  17  with a predetermined tension. The pressing roller  14  is rotatably pressed against the fixing roller  12  via the fixing belt  11  to form a nip portion N 1 . The pressing roller  14  is also rotatably pressed against the separation roller  13  via the fixing belt  11  to form the nip portion N 1 . In other words, the nip portion N 1  has a double-nip structure in which the pressing roller  14  is pressed against the fixing roller  12  and the separation roller  13  via the fixing belt  11  at two nip positions. The fixing belt  11 , the fixing roller  12 , the separation roller  13 , the heating roller  15 , the heater  15   h , the tension roller  16 , and the driven roller  17  are integrated into the fixing belt unit  11 U. The separator  43  is provided downstream from the nip portion N 1  in the sheet conveyance direction. Specifically, a front edge of the separator  43  is disposed close to the pressing roller  14  to prevent a sheet P serving as a recording medium from winding around the pressing roller  14 . 
     The fixing belt  11  serves as an endless belt for fixing a toner image T on the sheet P. The fixing belt  11  may include three layers, which are a base layer, an elastic layer provided on the base layer, and a releasing layer provided on the elastic layer. The base layer may include nickel, stainless steel, and/or polyimide. The elastic layer may include silicon rubber. 
     For example, the fixing belt  11  has an inner diameter of about 115 mm, and includes the base layer including endless polyimide resin having a high heat resistance, a low thermal expansion, and a relatively great strength. The elastic layer including silicon rubber and having a thickness of about 200 μm is provided on the base layer. The releasing layer serves as an outermost layer having a tube shape covering the elastic layer, and includes fluorocarbon resin such as PFA (tetrafluoroethylene-perfluoroalkylvinylether copolymer) which is released from toner easily. Alternatively, the fixing belt  11  may be coated with fluorocarbon resin. 
     The fixing roller  12  includes a base roller having a hollow cylindrical shape, and the heat-resistant elastic layer  12 E serving as a first elastic layer provided on an outer circumferential surface of the base roller. The heat-resistant elastic layer  12 E includes silicon rubber (e.g., solid silicon rubber) or silicon sponge (e.g., silicon rubber foam). For example, the heat-resistant elastic layer  12 E including silicon rubber foam having a thickness of about 14 mm is provided on the outer circumferential surface of the base roller so that the fixing roller  12  has an outer diameter of about 65 mm. 
     The separation roller  13  has an outer diameter smaller than the outer diameter of the fixing roller  12 , and includes a core metal and the heat-resistant elastic layer  13 E serving as a second elastic layer. A heat pipe is provided inside the core metal to reduce temperature fluctuation in an axial direction of the separation roller  13 . The heat-resistant elastic layer  13 E includes silicon rubber (e.g., solid silicon rubber) having a hardness lower than a hardness of the fixing roller  12  or silicon sponge (e.g., silicon rubber foam). For example, an aluminum roller having a thickness of about 1 mm is coated with silicon rubber or fluorocarbon resin so that the separation roller  13  has an outer diameter of about 16 mm. Alternatively, the separation roller  13  may be coated with fluorocarbon rubber or solid rubber. The separation roller  13  is rotational about a center axis of the fixing roller  12 , and the pressing roller  14  is pressed against the separation roller  13  via the fixing belt  11 . 
     The tension roller  16  applies a predetermined tension to the fixing belt  11  by using springs. For example, the springs provided in both ends of the tension roller  16  in an axial direction of the tension roller  16  apply a tension of about 9.8 N, respectively, so that the tension roller  16  applies a total tension of about 19.6 N to the fixing belt  11 . 
     The heating roller  15  serves as a hollow roller including aluminum or iron. For example, the heating roller  15  may be a hollow cylindrical aluminum roller having an outer diameter of about 35 mm and a thickness of about 0.6 mm. The heater  15   h  (e.g., a halogen heater) serving as a heat source is provided inside the heating roller  15  to heat the fixing belt  11 . Accordingly, a heat source is not provided inside a loop formed by the fixing belt  11  at a position which receives pressure applied by the pressing roller  14 , that is, at the nip portion N 1 . Alternatively, an induction heater (IH) may serve as a heat source. The temperature sensor  62  detects temperature of a region of the fixing belt  11  contacted by the heating roller  15 . 
     The pressing roller  14  serves as a cylindrical roller in which the heat-resistant elastic layer  14 E serving as a third elastic layer including silicon rubber (e.g., solid silicon rubber) or silicon sponge (e.g., silicon rubber foam) is provided on a core metal including aluminum or iron. For example, silicon rubber having a thickness of about 1.5 mm covers an outer circumferential surface of the hollow steel core metal having a thickness of about 1 mm. An outermost layer having a tube shape and including PFA covers the elastic layer  14 E, so that the pressing roller  14  has an outer diameter of about 65 mm. 
     The heater  14   h  is provided inside the pressing roller  14 , and is turned on and off according to temperature of the pressing roller  14  detected by the temperature sensor  72 . Accordingly, the pressing roller  14  does not draw heat from the sheet P when the sheet P passes through the nip portion N 1 . 
     The web cleaning unit  14   c  contacts an outer circumferential surface of the pressing roller  14  to remove offset toner and paper dust from the outer circumferential surface of the pressing roller  14 . 
       FIG. 3  is a sectional view of the fixing device  5 . As illustrated in  FIG. 3 , the fixing device  5  further includes a pressure adjuster  70 . The pressure adjuster  70  includes a pressing lever  76 , a pressing member  76   a , a support shaft  76   b , a pressing portion  76   c , a spring  77 , and a cam  78 . 
     The pressure adjuster  70  presses the pressing roller  14  against the fixing roller  12  and the separation roller  13  via the fixing belt  11  to form or release the nip portion N 1 . A nip length of the nip portion N 1  is adjustable according to type of the sheet P and finishing of the toner image T (e.g., a gloss mode or a non-gloss mode).  FIG. 3  illustrates a state in which rotation of the cam  78  illustrated in a chain double-dashed line applies great nip pressure at the nip portion N 1  and a state in which rotation of the cam  78  illustrated in a solid line applies small nip pressure at the nip portion N 1 . 
     The following describes an operation of the pressure adjuster  70  to press the pressing roller  14  against the fixing roller  12 . When an external driving force rotates the cam  78  counterclockwise in  FIG. 3  in a rotation direction D 1  by a predetermined rotation angle, the cam  78  pushes up the pressing member  76   a  in a direction D 2 . Accordingly, the spring  77  mounted on the pressing member  76   a  pushes up a swing end of the pressing lever  76  in a direction perpendicular to an axial direction of the pressing roller  14  with predetermined pressure. Consequently, the pressing lever  76  rotates about the support shaft  76   b  counterclockwise in  FIG. 3  in a rotation direction D 3 . Thereafter, the pressing portion  76   c  provided between the swing end of the pressing lever  76  and the support shaft  76   b  contacts a shaft of the pressing roller  14  to push the pressing roller  14  toward the fixing roller  12 . Finally, the pressing roller  14  is pressed against the fixing roller  12  and the separation roller  13  via the fixing belt  11 . Accordingly, the pressing roller  14  is pressed against the fixing roller  12  at a first nip region F formed between the pressing roller  14  and the fixing roller  12  with constant pressure. Similarly, the pressing roller  14  is pressed against the separation roller  13  at a second nip region S formed between the pressing roller  14  and the separation roller  13  with constant pressure. Thus, the first nip region F, the second nip region S, and an intermediate nip region I provided between the first nip region F and the second nip region S form the nip portion N 1  for fixing the toner image T on the sheet P. Alternatively, the spring  77  may be omitted. In this case, the cam  78  directly pushes up the swing end of the pressing lever  76 . 
     The pressing roller  14  sinks into the fixing roller  12  via the fixing belt  11  by a predetermined depth, for example, in a range from about 3.0 mm to about 3.5 mm. The separation roller  13  is pressed against the pressing roller  14  by predetermined pressure, for example, about 9.8 N at each of both ends of the separation roller  13  in the axial direction of the separation roller  13 . Accordingly, the nip portion N 1  has a predetermined nip length of about 35 mm, for example. The great nip length of the nip portion N 1  provides proper fixing for various types of paper, high-speed fixing, and improved productivity. 
     As illustrated in  FIG. 2 , when the fixing device  5  is driven, a driving motor provided for the fixing device  5  rotates the pressing roller  14  counterclockwise in  FIG. 2 . The fixing roller  12  and the separation roller  13  rotate clockwise in  FIG. 2  to rotate the fixing belt  11  clockwise in  FIG. 2  in a direction to convey the sheet P in a state in which the driven roller  17  and the tension roller  16  press against the fixing belt  11  and apply proper tension to the fixing belt  11 . In order to fix the toner image T on the sheet P, heat generated by the heater  15   h  provided inside the heating roller  15  heats the fixing belt  11  up to a predetermined temperature (e.g., a proper fixing temperature) based on a temperature detected by the temperature sensor  62 . 
     Thereafter, when the sheet P bearing the toner image T passes through the nip portion N 1  from right to left in  FIG. 2 , pressure and heat are applied to the sheet P at the nip portion N 1  to melt and fix the toner image T on the sheet P. 
     Specifically, as illustrated in  FIG. 3 , the toner image T is mostly fixed on the sheet P at an entrance region, that is, the first nip region F of the nip portion N 1 . Thus, the toner image T is melted sufficiently and therefore has a great viscosity. Accordingly, when the sheet P passes through the intermediate nip region I of the nip portion N 1  in a state in which the sheet P is adhered to the fixing belt  11  due to the great viscosity of the toner image T, nip pressure not smaller than about 5 N/cm 2  is needed to convey the sheet P properly. The nip pressure is suppressed to about 15 N/cm 2  or smaller, that is, below a level required for gloss finishing. The sheet P is separated from the fixing belt  11  by a great curvature of the separation roller  13  having a small diameter, and is separated from the pressing roller  14  by the separator  43 . Thus, the sheet P is discharged from the nip portion N 1 . 
     The nip portion N 1  provides a total nip time not smaller than about 60 m/s with respect to a linear velocity of the sheet P. Nip pressure in a range from about 15 N/cm 2  to about 30 N/cm 2  is applied at about 50 percent or more of the total nip length of the nip portion N 1  in the gloss mode. Thus, the fixing device  5  fixes a toner image on various types of paper including thick paper having a weight of about 300 g/m 2 . 
     Pressure applied between the pressing roller  14  and the fixing roller  12  via the fixing belt  11  is adjusted to increase and decrease the nip length of the intermediate nip region I of the nip portion N 1 . When the sheet P has a weight not greater than a weight of plain paper, pressure applied between the pressing roller  14  and the fixing roller  12  is adjusted to increase the nip length of the intermediate nip region I of the nip portion N 1  applied with nip pressure in a range from about 5 N/cm 2  to about 15 N/cm 2 . Thus, when a thin sheet such as plain paper is used and therefore the thin sheet receives excessive heat, nip pressure is decreased to suppress gloss finishing. In other words, even when a thin sheet having a weight not greater than a weight of plain paper is used in the non-gloss mode and the thin sheet receives excessive heat, the nip length of the intermediate nip region I of the nip portion N 1  is adjusted to apply a gloss equivalent to a gloss applied to a thick sheet. Further, in the gloss mode, the nip length of the intermediate nip region I of the nip portion N 1  is adjusted by considering thickness of the sheet P to adjust nip pressure so as to provide a uniform gloss to finalized toner images formed on various types of sheets. Thus, the fixing device  5  provides improved reliability by applying a desired gloss in each of the gloss mode and the non-gloss mode. 
     For example, as nip pressure distribution at the nip portion N 1 , a load in a range from about 15 N/cm 2  to about 30 N/cm 2  is applied to the entrance region of the nip portion N 1  in the sheet conveyance direction, that is, the first nip region F formed between the pressing roller  14  and the fixing roller  12 . A load in a range from about 15 N/cm 2  to about 30 N/cm 2  is applied to an exit region of the nip portion N 1  in the sheet conveyance direction, that is, the second nip region S formed between the pressing roller  14  and the separation roller  13 . A load in a range from about 5 N/cm 2  to about 15 N/cm 2  is applied to the intermediate nip region I provided between the entrance region and the exit region in the sheet conveyance direction. 
     When gloss paper is used as the sheet P in the gloss mode, the pressure adjuster  70  adjusts the nip length of the first nip region F formed between the pressing roller  14  and the fixing roller  12  to about 20 mm, the nip length of the intermediate nip region I provided between first nip region F and the second nip region S to about 13 mm, and the nip length of the second nip region S formed between the pressing roller  14  and the separation roller  13  to about 2 mm. When plain paper is used as the sheet P in the non-gloss mode, the pressure adjuster  70  adjusts the nip length of the first nip region F to about 15 mm, the nip length of the intermediate nip region I to about 13 mm, and the nip length of the second nip region S to about 1 mm. 
       FIG. 4  is a sectional view of a fixing device  5 ′ equivalent to the fixing device  5  depicted in  FIG. 3 . As illustrated in  FIG. 4 , the fixing device  5 ′ includes a spring  13   s , a support roller  16 ′, and a guide  45 ′. The support roller  16 ′ replaces the tension roller  16  and the driven roller  17  depicted in  FIG. 3 . The other elements of the fixing device  5 ′ are equivalent to the elements of the fixing device  5 . 
       FIG. 5  is a sectional view of the fixing device  5 ′ for explaining movement of the pressure adjuster  70  and change in the diameter of the fixing roller  12 . 
       FIG. 6  is a sectional view of the fixing device  5 ′ for showing the fixing belt  11  contacting the fixing roller  12  at two positions on the fixing roller  12 . 
       FIG. 7  is a sectional view of the fixing device  5 ′ for showing deformation of the fixing roller  12 , the separation roller  13 , and the pressing roller  14 , and the fixing belt  11  wound around the fixing roller  12 , the separation roller  13 , and the pressing roller  14 . 
       FIG. 8  is a sectional view of the fixing device  5 ′ for showing deformation of the fixing roller  12 , the separation roller  13 , and the pressing roller  14 , and the fixing belt  11  wound around the fixing roller  12 , the separation roller  13 , and the pressing roller  14  when the fixing roller  12 , the separation roller  13 , and the pressing roller  14  do not have predetermined roller hardnesses, respectively. 
     When the fixing device  5 ′ uses oilless toner, separation of a sheet P at an exit of the nip portion N 1  depends on property of wax included in the toner and separation curvature with respect to the sheet conveyance direction at the exit of the nip portion N 1 . According to this exemplary embodiment, the fixing belt  11  contacting a toner image T on the sheet P and being wound around the fixing roller  12  and the separation roller  13  is wound around the pressing roller  14  by the fixing roller  12  and the separation roller  13 . The sheet conveyance direction at the exit of the nip portion N 1  corresponds to a tangent direction of the pressing roller  14 , that is, a direction directed to a side opposite to the toner image T, and is extended obliquely downward in  FIG. 6 . Thus, the sheet P is separated from the fixing belt  11  easily. 
     As illustrated in  FIG. 6 , a radius r 0  of the separation roller  13  wound by the fixing belt  11  is smaller than a radius R 0  of the fixing roller  12  wound by the fixing belt  11 , and therefore the separation roller  13  has a curvature greater than a curvature of the fixing roller  12 . Accordingly, when the separation roller  13  is disposed at the exit of the nip portion N 1 , the sheet P is separated from the fixing belt  11  more easily than when the fixing roller  12  is disposed at the exit of the nip portion N 1 . Consequently, the sheet P separated from the separation roller  13  at the exit of the nip portion N 1  is guided by the guide  45 ′ (depicted in  FIG. 4 ) provided above the exit of the nip portion N 1  to a conveyance path. Thus, the sheet P is conveyed properly. When the sheet P bears a solid toner image T on a front side thereof and no toner image on a back side thereof in duplex printing, the sheet P is adhered to the pressing roller  14  easily. To address this, the separator  43  (depicted in  FIG. 4 ) provided below the exit of the nip portion N 1  separates the sheet P from the pressing roller  14  and guides the sheet P to the conveyance path. 
     In the fixing device  5 ′ according to this exemplary embodiment, the fixing roller  12 , the separation roller  13 , and the pressing roller  14  have roller hardnesses A, B, and C, respectively, which have a relation shown by a formula (1) below.
 
 C≧A&gt;B   (1)
 
     The roller hardness may be measured by a known method and may represent hardness of each roller as Japanese Industrial Standards JIS-A or C hardness. 
     When the fixing belt  11  is separated from the pressing roller  14  at the intermediate nip region I provided between the fixing roller  12  and the separation roller  13  as illustrated in  FIG. 6 , the toner image T may be shifted from a proper position on the sheet P or heat applied to the sheet P may fluctuate. Specifically, shifting of the toner image T may occur when a moving velocity of the fixing belt  11  facing the toner image T differs from a circumferential velocity of the pressing roller  14  for feeding the sheet P in a state in which the fixing belt  11  is separated from the pressing roller  14  at the intermediate nip region I. Fluctuation of heat applied to the sheet P may occur when the sheet P is partially separated from the fixing belt  11  at the intermediate nip portion I at which the fixing belt  11  is separated from the pressing roller  14 . 
     To address this, according to this exemplary embodiment, the fixing roller  12 , the separation roller  13 , and the pressing roller  14  have the roller hardnesses A, B, and C defined by the above formula (1). Accordingly, the fixing roller  12 , the separation roller  13 , and the pressing roller  14  are deformed and wound by the fixing belt  11  properly as illustrated in  FIG. 7 . Specifically, the roller hardness A of the fixing roller  12  is smaller than the roller hardness C of the pressing roller  14 , and therefore pressure applied by the pressing roller  14  dents the fixing roller  12  to have a concave shape. Thus, the fixing roller  12  presses the fixing belt  11  against the pressing roller  14 . 
     The separation roller  13  has the small diameter to provide an improved separation curvature at the exit of the nip portion N 1 . However, the roller hardness B of the separation roller  13  is smaller than the roller hardness A of the fixing roller  12 , and is even smaller than the roller hardness C of the pressing roller  14 , as shown in the above formula (1). Accordingly, regardless of the small diameter, the separation roller  13  is deformed at the nip portion N 1  easily. Specifically, the pressing roller  14  presses against and dents the separation roller  13  to have a concave shape. Thus, the separation roller  13  presses the fixing belt  11  against the pressing roller  14 . Accordingly, the fixing belt  11  is pressed against the pressing roller  14  strongly at upstream and downstream positions from the intermediate nip region I in the sheet conveyance direction. Moreover, the fixing belt  11  has a predetermined level of rigidity, and is stretched by tension. Accordingly, the fixing belt  11  is pressed against the pressing roller  14  without being separated from the pressing roller  14  even at the intermediate nip region I. Consequently, shifting of the toner image T on the sheet P and fluctuation of heat applied to the sheet P are suppressed. 
     By contrast, when the roller hardness B of the separation roller  13  does not satisfy the relation shown by the formula (1), that is, when the roller hardness B of the separation roller  13  is not smaller than the roller hardness A of the fixing roller  12 , the separation roller  13  dents the pressing roller  14  to have a concave shape at the second nip region S at which the separation roller  13  is pressed against the pressing roller  14 , as illustrated in  FIG. 8 . Accordingly, the fixing belt  11  is wound around the separation roller  13 . Further, at an entrance to the second nip region S, which is provided upstream from the separation roller  13  in the sheet conveyance direction, a force for attracting the fixing belt  11  having rigidity toward the separation roller  13  is applied to the fixing belt  11 . As a result, the fixing belt  11  may not be wound around the pressing roller  14  easily, and therefore may be separated from the pressing roller  14  at the intermediate nip region I. To address this, the roller hardness B of the separation roller  13  needs to be smaller than the roller hardness A of the fixing roller  12 . 
     The separation roller  13  may be a tension roller for applying tension to the fixing belt  11  at the intermediate nip region I provided between the fixing roller  12  and the separation roller  13  in the sheet conveyance direction. For example, as illustrated in  FIG. 4 , a mechanism including the spring  13   s  may cause the separation roller  13  to apply a predetermined tension to the fixing belt  11 . One end of the spring  13   s  is attached to the separation roller  13 , and another end of the spring  13   s  is attached to the fixing roller  12 . A tension of about 9.8 N may be applied to each of both ends of the separation roller  13  in the axial direction of the separation roller  13 . Thus, a total tension of about 19.6 N is applied to the separation roller  13 . Accordingly, in addition to effects provided by the roller hardnesses A, B, and C defined by the above formula (1), the fixing belt  11  is wound around the pressing roller  14  stably by the tension applied by the separation roller  13 . 
     Twisting of the fixing belt  11  due to the fixing roller  12  including an elastic body may occur due to difference between nip pressure at one end in an axial direction of the fixing belt  11  and nip pressure at another end in the axial direction of the fixing belt  11  at the nip portion N 1 . In a structure in which the fixing belt  11  contacts the fixing roller  12  again at a contact position downstream from the nip portion N 1  in the sheet conveyance direction like the fixing belt  11  illustrated in a chain double-dashed line in  FIG. 6 , movement of the fixing belt  11  may differ between both ends in the axial direction of the fixing belt  11  at the contact position, and therefore the fixing belt  11  may be twisted. Specifically, when the fixing roller  12  including the elastic body is deformed at the nip portion N 1 , a radius R 1  of the fixing roller  12  at positions immediately upstream and downstream from the nip portion N 1  in the sheet conveyance direction is greater than the radius R 0  of the fixing roller  12  which is not deformed. Accordingly, the fixing belt  11  for conveying the sheet P moves fast at the first nip region F of the nip portion N 1 . The fixing roller  12  serving as a driving roller receives a rotation force at a position at which the fixing roller  12  has the radius R 1 . Accordingly, the fixing roller  12  rotates slowly at a position at which the fixing roller  12  has the radius R 0 . Therefore, when the fixing belt  11  contacts the fixing roller  12  again at the contact position downstream from the nip portion N 1  in the sheet conveyance direction as illustrated in the chain double-dashed line in  FIG. 6 , the fixing belt  11  may be twisted. To address this, according to this exemplary embodiment, the fixing belt  11  is supported by the support roller  16 ′ and the heating roller  15  at positions downstream from the nip portion N 1  in the sheet conveyance direction as illustrated in  FIG. 4 . Thus, the fixing belt  11  does not contact the fixing roller  12  at a position other than the nip portion N 1 , and therefore is not twisted. 
     As illustrated in  FIG. 2 , in order to cause the fixing roller  12 , the separation roller  13 , and the pressing roller  14  to have the roller hardnesses A, B, and C satisfying the above formula (1), respectively, the elastic layer  14 E of the pressing roller  14  may include a material equivalent to an elastic material of the elastic layer  12 E of the fixing roller  12 , but may have a thickness smaller than a thickness of the elastic layer  12 E of the fixing roller  12 . The elastic layer  13 E of the separation roller  13  may include a material having a hardness smaller than a hardness of the elastic material of the elastic layer  12 E of the fixing roller  12 . For example, the fixing roller  12  and the pressing roller  14  include an identical elastic material such as silicon rubber or silicon sponge, but the elastic layer  14 E of the pressing roller  14  is thinner than the elastic layer  12 E of the fixing roller  12 . Accordingly, the fixing roller  12  is deformed easily to have a concave shape by pressure applied by the pressing roller  14 , and presses the fixing belt  11  against the pressing roller  14 . 
     The separation roller  13  is coated with an elastic material (e.g., low-hardness silicon rubber or low-hardness silicon sponge) softer than the elastic material of the fixing roller  12 . Accordingly, even with the small diameter, the separation roller  13  is deformed more easily to have a concave shape by pressure applied by the pressing roller  14  than the fixing roller  12 . Further, tension applied by the spring  13   s  (depicted in  FIG. 4 ) winds the fixing belt  11  around the pressing roller  14  stably, suppressing separation of the fixing belt  11  from the pressing roller  14  at the intermediate nip region I stably. 
     When the pressure adjuster  70  depicted in  FIG. 4  moves the pressing roller  14  to adjust the nip length of the first nip region F formed between the pressing roller  14  and the fixing roller  12 , the fixing device  5 ′ may include a biasing member (e.g., the spring  13   s  depicted in  FIG. 4 ) for moving the separation roller  13  in accordance with movement of the pressing roller  14  caused by the pressure adjuster  70 . 
     As described above, the pressure adjuster  70  moves the pressing roller  14  in such a manner that the pressing roller  14  applies pressure to the fixing belt  11  stably, so as to adjust the nip length of the nip portion N 1  formed between the pressing roller  14  and the fixing roller  12  via the fixing belt  11  or release the nip portion N 1 . Change in the nip length of the first nip region F of the nip portion N 1  formed between the pressing roller  14  and the fixing roller  12  causes a relation of the radiuses R 0 , R 1 , and R 2  of the fixing roller  12  to satisfy a following formula (2) as illustrated in  FIG. 5 .
 
 R 1 &gt;R 2 &gt;R 0  (2)
 
     As illustrated in  FIG. 8 , the velocity of the fixing belt  11  at the nip portion N 1  changes to cause the velocity of the moving fixing belt  11  to be close to a circumferential velocity of the fixing roller  12  at the first nip region F of the nip portion N 1 . However, the nip length of the first nip region F decreases, and pressure applied by the fixing roller  12  to press the fixing belt  11  against the pressing roller  14  decreases. Accordingly, the fixing belt  11  is separated from the pressing roller  14  easily at the intermediate nip region I, resulting in shifting of a toner image T on a sheet P and fluctuation of heat applied to the sheet P. To address this, according to this exemplary embodiment, the separation roller  13  moves in accordance with movement of the pressing roller  14 . Therefore, even when the nip length of the first nip region F of the nip portion N 1  formed between the pressing roller  14  and the fixing roller  12  changes, the nip length of the second nip region S of the nip portion N 1  formed between the pressing roller  14  and the separation roller  13  does not change, or changes in an amount smaller than an amount of change of the nip length of the first nip region F of the nip portion N 1 . Accordingly, the separation roller  13  presses the fixing belt  11  against the pressing roller  14  with great pressure to stretch the fixing belt  11  constantly, suppressing shifting of the toner image T on the sheet P and fluctuation of heat applied to the sheet P. 
     The separation roller  13  having the diameter smaller than the diameter of the fixing roller  12  and moving in accordance with movement of the pressing roller  14  separates the sheet P from the fixing belt  11  at the exit of the nip portion N 1 . In other words, the sheet P is separated from the fixing belt  11  easily at a constant position. 
     In the fixing device  5 ′ according to this exemplary embodiment, the pressing roller  14  serves as a primary driving roller driven and rotated by a driving system such as a motor. An outer circumferential velocity of the primary driving roller is equivalent to a conveyance velocity for conveying a sheet P. The pressing roller  14 , which has the elastic layer  14 E thinner than the elastic layer  12 E of the fixing roller  12  and has surface temperature with little fluctuation, may be used as the primary driving roller. 
       FIG. 9A  is a sectional view of the fixing device  5 ′. As illustrated in  FIG. 9A , the fixing device  5 ′ further includes a first driving system  12   s  and a second driving system  14   s ′. The first driving system  12   s  includes a first motor  12   m  and gears  101 ,  102 , and  103 . The second driving system  14   s ′ includes a second motor  14   m ′ and gears  111 ,  112 ,  113 , and  114 . 
       FIG. 9B  is a block diagram of the fixing device  5 ′. As illustrated in  FIG. 9B , the fixing device  5 ′ further includes a motor controller  200 ′. The motor controller  200 ′ includes a first motor regulator  12   m A, a second motor detector  14   m ′D, and a calculator  200 C. 
     The first motor  12   m  for driving the fixing roller  12  is provided separately from the second motor  14   m ′ for driving the pressing roller  14 . The motor controller  200 ′ includes a CPU (central processing unit), a ROM (read-only memory), and a RAM (random-access memory), for example, and controls the first motor  12   m  and/or the second motor  14   m ′ so that the moving velocity of the fixing belt  11  is not greater than the circumferential velocity of a surface of the pressing roller  14 . 
     The first motor  12   m  generates a driving force to be transmitted to the fixing roller  12  via the gears  101  to  103  to rotate the fixing roller  12 . The rotating fixing roller  12  rotates the fixing belt  11 . The fixing belt  11  rotated by the rotating fixing roller  12  rotates the heating roller  15 , the separation roller  13 , and the support roller  16 ′ having a cylindrical shape. The second motor  14   m ′ generates a driving force to be transmitted to the pressing roller  14  via the gears  111  to  114  to rotate the pressing roller  14 . 
     A driver of the fixing device  5 ′ is divided into the first motor  12   m  for driving the fixing roller  12  and the second motor  14   m ′ for driving the pressing roller  14 , so as to adjust a difference between the velocity of the fixing belt  11  and the velocity of the pressing roller  14  at the intermediate nip region I generated due to a temperature difference between the fixing roller  12  and the pressing roller  14  and variation in thickness of a sheet P. Accordingly, the fixing belt  11  and the pressing roller  14  move at an identical velocity at the intermediate nip region I constantly. Consequently, tension applied by the separation roller  13  to the fixing belt  11  prevents the fixing belt  11  from separating from the pressing roller  14  at the intermediate nip region I stably, preventing shifting of a toner image T on the sheet P and fluctuation of heat applied to the sheet P with improved stability. 
     The motor controller  200 ′ adjusts a number of rotations (e.g., the RPM) of the first motor  12   m  based on torque change of the second motor  14   m ′. For example, the first motor regulator  12   m A adjusts the number of rotations of the first motor  12   m  based on a torque of the second motor  14   m ′ detected by the second motor detector  14   m ′D. 
     As described above, shifting of the toner image T on the sheet P and fluctuation of heat applied to the sheet P may occur when the fixing belt  11  and the pressing roller  14  move at different velocities, respectively, as the fixing belt  11  is separated from the pressing roller  14  at the intermediate nip region I. In this case, unlike when the fixing belt  11  and the pressing roller  14  move at the identical velocity, the fixing belt  11  and the pressing roller  14  partially scratch each other with a great force, increasing torque of the first motor  12   m  and the second motor  14   m ′. By contrast, under a lowest torque, the fixing belt  11  and the pressing roller  14  move at the identical velocity, and therefore the fixing belt  11  is not separated from the pressing roller  14 . To address this, according to this exemplary embodiment, since driving of the pressing roller  14  determines the conveyance velocity for conveying the sheet P, a number of rotations (e.g., the RPM) of the pressing roller  14  is maintained at a predetermined value to detect change in torque of the second motor  14   m ′. Based on the change in torque of the second motor  14   m ′ detected by the second motor detector  14   m ′D, the first motor regulator  12   m A changes the number of rotations of the first motor  12   m  for driving the fixing roller  12  which rotates the fixing belt  11  to prevent shifting of the toner image T on the sheet P and fluctuation of heat applied to the sheet P. 
       FIG. 10  is a graph illustrating one example of control of the number of rotations of the first motor  12   m  performed by the motor controller  200 ′ depicted in  FIG. 9B . 
     For example, when the number of rotations of the second motor  14   m ′ is changed to a predetermined value, torque (e.g., electric current) of the second motor  14   m ′ increases. To address this, the motor controller  200 ′ increases the number of rotations of the first motor  12   m . Thereafter, the motor controller  200 ′ adjusts (e.g., increases and decreases) the number of rotations of the first motor  12   m  for several times based on a detection result of torque of the second motor  14   m ′. When the second motor  14   m ′ has a lowest level of torque, the number of rotations of the first motor  12   m  is stabilized. 
     As the total nip length of the nip portion N 1  and pressure distribution at the nip portion N 1  change depending on a pressing state in which the pressing roller  14  is pressed against the fixing roller  12 , a pressing state in which the fixing belt  11  is pressed against the pressing roller  14  changes at the intermediate nip region I. To address this, the calculator  200 C of the motor controller  200 ′ calculates and stores in advance the number of rotations of the first motor  12   m  to cause the lowest torque level of the second motor  14   m ′ for various pressing conditions under which the pressing roller  14  is pressed against the fixing roller  12 . Since the number of rotations of the second motor  14   m ′ is constant, the calculator  200 C of the motor controller  200 ′ also calculates and stores a ratio between the number of rotations of the first motor  12   m  and the number of rotations of the second motor  14   m ′. Accordingly, when the fixing device  5 ′ is turned on, the first motor regulator  12   m A of the motor controller  200 ′ selects a proper ratio between the number of rotations of the first motor  12   m  and the number of rotations of the second motor  14   m ′ from the ratios stored in the calculator  200 C, which corresponds to a pressing state in which the pressing roller  14  is pressed against the fixing roller  12 . Thereafter, the motor controller  200 ′ starts driving the first motor  12   m  and the second motor  14   m ′ according to the selected ratio. Thus, operation time for adjusting the number of rotations of the first motor  12   m  is shortened to a short time period equivalent to a time period required in a normal state, that is, a normal pressing state in which the pressing roller  14  is pressed against the fixing roller  12 , so as to adjust the number of rotations of the pressing roller  14  and the fixing roller  12  to an optimum velocity. Consequently, a sheet P is conveyed at the optimum velocity when the pressing roller  14  and the fixing roller  12  start feeding the sheet P, improving reliability of moving the sheet P. 
       FIG. 11  is a graph illustrating another example of control of the number of rotations of the first motor  12   m  (depicted in  FIG. 9A ) performed by the motor controller  200 ′ (depicted in  FIG. 9B ). In  FIG. 11 , “NR 1 ” represents the number of rotations of the first motor  12   m  in the normal state. “NR 2 ” represents the number of rotations of the first motor  12   m  under a different pressing condition according to this exemplary embodiment. 
     As illustrated in  FIGS. 9A and 9B , when the stopped fixing device  5 ′ resumes driving, the motor controller  200 ′ determines the ratio between the number of rotations of the first motor  12   m  and the number of rotations of the second motor  14   m ′ depicted in  FIG. 9A  corresponding to a pressing condition of the pressing roller  14  pressed against the fixing roller  12  and a conveyance condition under which the pressing roller  14  and the fixing roller  12  convey a sheet P based on the pressing condition, and starts driving the first motor  12   m  at the number of rotations NR 2  of the first motor  12   m  calculated based on the determined ratio. Thereafter, the motor controller  200 ′ increases and decreases the number of rotations of the first motor  12   m  for several times based on a detected torque of the second motor  14   m ′ detected by the second motor detector  14   m ′D. When the second motor  14   m ′ has a lowest torque, the number of rotations of the first motor  12   m  is stabilized, and the first motor  12   m  is driven. A time period required for the number of rotations of the first motor  12   m  to be stabilized after driving of the first motor  12   m  is started is equivalent to a time period required for the number of rotations NR 1  of the first motor  12   m  to be stabilized after driving of the first motor  12   m  is started under a normal condition. 
     When a thick sheet P passes between the pressing roller  14  and the fixing roller  12 , the pressing roller  14  moves in a direction to separate from the fixing roller  12  for an amount corresponding a thickness of the thick sheet P. When a sheet P having a lower surface friction coefficient passes between the pressing roller  14  and the fixing roller  12 , the sheet P slips slightly at the nip portion N 1 . To address this, the motor controller  200 ′ needs to increase the conveyance velocity for conveying the sheet P. Accordingly, the calculator  200 C of the motor controller  200 ′ calculates and stores in advance the number of rotations of the first motor  12   m  when the second motor  14   m ′ has the lowest torque per thickness or type of a sheet P to be used. Since the number of rotations of the second motor  14   m ′ is constant, the calculator  200 C calculates and stores the ratio between the number of rotations of the first motor  12   m  and the number of rotations of the second motor  14   m ′. Accordingly, when the motor controller  200 ′ starts driving the fixing device  5 ′, the first motor regulator  12   m A of the motor controller  200 ′ selects a proper ratio between the number of rotations of the first motor  12   m  and the number of rotations of the second motor  14   m ′, which corresponds to thickness or type of the sheet P, from the ratios stored in the calculator  200 C. Thereafter, the controller  200  starts driving the first motor  12   m  and the second motor  14   m ′ based on the selected number of rotations of the first motor  12   m  and the second motor  14   m ′. Consequently, the number of rotations of the first motor  12   m  is adjusted in a shortened time period. In other words, the conveyance velocity for conveying the sheet P is adjusted to the optimum linear velocity in the shortened time period equivalent to a time period required in the normal state in which a normal sheet P is used. Thus, the fixing device  5 ′ conveys the sheet P at the optimum linear velocity when the fixing device  5 ′ starts feeding the sheet P, improving reliability of moving the sheet P. 
     The thickness or type of the sheet P may be selected by a user on a control panel provided in the image forming apparatus  100  depicted in  FIG. 1 , and is sent to the motor controller  200 ′. 
       FIG. 12  is a graph illustrating yet another example of control of the number of rotations of the first motor  12   m  (depicted in  FIG. 9A ) performed by the motor controller  200 ′ (depicted in  FIG. 9B ). In  FIG. 12 , “NR 1 ” represents the number of rotations of the first motor  12   m  in the normal state. “NR 2 ” represents the number of rotations of the first motor  12   m  when a thick sheet P or a coated sheet P having a low surface friction coefficient is used. 
     As illustrated in  FIGS. 9A and 9B , when the stopped fixing device  5 ′ resumes driving, the motor controller  200 ′ determines the ratio between the number of rotations (e.g., the RPM) of the first motor  12   m  and the number of rotations (e.g., the RPM) of the second motor  14   m ′ corresponding to a sheet condition such as thickness or type of a sheet P to be used and a conveyance condition under which the pressing roller  14  and the fixing roller  12  convey the sheet P based on the pressing condition of the pressing roller  14  pressed against the fixing roller  12 , and starts driving the first motor  12   m  at the number of rotations NR 2  of the first motor  12   m  calculated based on the determined ratio. Thereafter, the motor controller  200 ′ increases and decreases the number of rotations of the first motor  12   m  for several times based on a detected torque of the second motor  14   m ′ detected by the second motor detector  14   m ′D. When the second motor  14   m ′ has a lowest torque, the number of rotations of the first motor  12   m  is stabilized, and the first motor  12   m  is driven. A time period required for the number of rotations of the first motor  12   m  to be stabilized after driving of the first motor  12   m  is started is equivalent to a time period required for the number of rotations NR 1  of the first motor  12   m  to be stabilized after driving of the first motor  12   m  is started under the normal condition. 
     The first motor regulator  12   m A of the motor controller  200 ′ increases and decreases the number of rotations of the first motor  12   m  by a predetermined number of rotations with respect to a reference number of rotations X, and judges which of the increased number of rotations or the decreased number rotations of the first motor  12   m  provides a smaller torque value Y of the second motor  14   m ′ which is detected by the second motor detector  14   m ′D of the motor controller  200 ′. Thereafter, the first motor regulator  12   m A of the motor controller  200 ′ changes the reference number of rotations X to a number of rotations (e.g., the RPM) of the first motor  12   m  corresponding to the smaller torque value Y by a predetermined number of rotations repeatedly until the detected torque value Y becomes smallest, so as to adjust the number of rotations of the first motor  12   m  The above-described control is performed constantly while the first motor  12   m  and the second motor  14   m ′ rotate. 
       FIG. 13  is a graph illustrating yet another example of control of the number of rotations of the first motor  12   m  performed by the motor controller  200 ′ depicted in  FIG. 9B . 
     As illustrated in  FIGS. 9A and 9B , the first motor regulator  12   m A of the motor controller  200 ′ increases and decreases the number of rotations of the first motor  12   m  slightly from a reference number of rotations X, and judges which of the increased number of rotations and the decreased number of rotations of the first motor  12   m  provides a smaller torque value Y of the second motor  14   m ′ detected by the second motor detector  14   m ′D of the motor controller  200 ′. The first motor regulator  12   m A changes the reference number of rotations X of the first motor  12   m  to the number of rotations corresponding to the smaller torque value Y of the second motor  14   m ′ so as to change the number of rotations of the first motor  12   m  to a number of rotations (e.g., the RPM) corresponding to a smallest torque Y. According to this exemplary embodiment, the motor controller  200 ′ performs the above-described control constantly while the motor controller  200 ′ drives the first motor  12   m  and the second motor  14   m ′. Accordingly, an optimum number of rotations of the first motor  12   m  is obtained to correspond to change in conditions such as change in temperature of the pressing roller  14  and the fixing roller  12  when a sheet P passes between the pressing roller  14  and the fixing roller  12  and job interruption to use sheets P of various thicknesses or types, improving reliability of moving the sheet P. 
       FIG. 14A  is a sectional view of the fixing device  5 ′. As illustrated in  FIG. 14A , the fixing device  5 ′ further includes gears  16   a  and  17   a , a slit  17   b , a sensor  17   c , and a motor  17   m .  FIG. 14B  is a block diagram of the fixing device  5 ′. 
     In order to prevent the fixing belt  11  from twisting in a width direction of the fixing belt  11  perpendicular to the sheet conveyance direction at a position downstream from the separation roller  13  in the sheet conveyance direction, a position of one end of the support roller  16 ′ in a width direction, that is, in an axial direction of the support roller  16 ′, is changed within a predetermined range in a direction T 1  perpendicular to a connection line C 1  connecting a midpoint on a wound portion of the fixing belt  11  wound around the support roller  16 ′ to a roller center of the support roller  16 ′. The connection line C 1  is parallel to a direction C 2  in which the support roller  16 ′ applies tension to the fixing belt  11 . Specifically, a driving force generated by the motor  17   m  serving as a support roller motor is transmitted to the support roller  16 ′ via the gear  17   a  mounted on a driving shaft of the motor  17   m  and the gear  16   a  mounted on one end of the support roller  16 ′ in the axial direction of the support roller  16 ′. When the sensor  17   c  detects that the fixing belt  11  is twisted, the motor  17   m  moves one end of the support roller  16 ′ in the axial direction of the support roller  16 ′ along the slit  17   b  serving as a guide in a predetermined direction. 
     The following describes control operations for correcting twisting of the fixing belt  11  with the above-described structure. The sensor  17   c  serving as a fixing belt detector is provided at a position near the support roller  16 ′. For example, the sensor  17   c  may include two sensors provided near both ends of the fixing belt  11  in the width direction of the fixing belt  11 , respectively.  FIG. 14A  illustrates the sensor  17   c  provided at a position upstream from the support roller  16 ′ in a rotation direction of the fixing belt  11  rotating clockwise in  FIG. 14A . Alternatively, the sensor  17   c  may be provided at a position downstream from the support roller  16 ′ in the rotation direction of the fixing belt  11 . 
     The sensor  17   c  detects a position of the fixing belt  11  in the width direction of the fixing belt  11 . The motor controller  200 ′ identifies the position of the fixing belt  11  based on a detection result provided by the sensor  17   c , and determines a direction in which the support roller  16 ′ moves. The motor controller  200 ′ rotates the motor  17   m  according to the determined direction to move one end of the support roller  16 ′ in the axial direction of the support roller  16 ′ in the determined direction along the slit  17   b . Namely, the motor controller  200 ′ drives the motor  17   m  to tilt the support roller  16 ′. The motor controller  200 ′ stops the motor  17   m  at a position at which detection by the sensor  17   c  is switched. Thus, even when the fixing belt  11  is twisted in the width direction of the fixing belt  11 , the fixing device  5 ′ corrects twisting of the fixing belt  11 , maintaining stability of moving the fixing belt  11  and improving reliability of correcting twisting of the fixing belt  11 . 
     Referring to  FIGS. 15A and 15B , the following describes control operations for controlling rotation of the pressing roller  14  and the fixing roller  12  of the fixing device  5 .  FIG. 15A  is a perspective view of the fixing device  5 . As illustrated in  FIG. 15A , the fixing device  5  further includes a first driving system  11   s  and a second driving system  14   s . The first driving system  11   s  includes a first motor  11   m , a joint gear  11   g , and a first transmission  11   t . The second driving system  14   s  includes a second motor  14   m , a joint gear  14   g , and a second transmission  14   t.    
       FIG. 15B  is a block diagram of the fixing device  5 . As illustrated in  FIG. 15B , the fixing device  5  further includes a motor controller  200 . The motor controller  200  includes a first motor regulator  11   m A and a first motor detector  11   m D. 
     The fixing device  5  includes driving systems independently provided for the pressing roller  14  and the fixing belt unit  11 U including the fixing belt  11 , the fixing roller  12 , the separation roller  13 , the heating roller  15  (depicted in  FIG. 3 ), the tension roller  16  (depicted in  FIG. 3 ), and the driven roller  17  (depicted in  FIG. 3 ), respectively. 
     The fixing device  5  includes the first driving system  11   s  including the first motor  11   m , a one-way clutch, the joint gear  11   g , and the first transmission  11   t  including a plurality of transmission gears, which correspond to the fixing belt unit  11 U. The first driving system  11   s  transmits a driving force generated by the first motor  11   m  to the fixing roller  12  via the one-way clutch, and transmits the driving force generated by the first motor  11   m  to the separation roller  13 . The fixing device  5  further includes the second driving system  14   s  including the second motor  14   m , the joint gear  14   g , and the second transmission  14   t  including a plurality of transmission gears, which correspond to the pressing roller  14 . The second driving system  14   s  transmits a driving force generated by the second motor  14   m  to the pressing roller  14 . 
     The first motor  11   m  and the second motor  14   m  may be provided in the image forming apparatus  100  depicted in  FIG. 1 . When the fixing device  5  is attached to the image forming apparatus  100 , the first motor  11   m  is connected to the joint gear  11   g  serving as a driving force input portion of the fixing belt unit  11 U. Similarly, the second motor  14   m  is connected to the joint gear  14   g  serving as a driving force input portion of the pressing roller  14 . 
     With the above-described structure, the first transmission  11   t  transmits a driving force generated by the first motor  11   m  to the fixing belt unit  11 U via the joint gear  11   g . The second transmission  14   t  transmits a driving force generated by the second motor  14   m  to the pressing roller  14  via the joint gear  14   g . The first driving system  11   s  and the second driving system  14   s  independently control driving of the fixing belt unit  11 U and the pressing roller  14 , respectively. 
       FIGS. 16A ,  16 B,  16 C,  16 D, and  16 E illustrate a transmission channel for transmitting a driving force with the transmissions  11   t  and  14   t  of the fixing device  5 .  FIG. 16A  is a front view of the fixing device  5  seen from a direction S 1  in  FIG. 15A . 
     As illustrated in  FIG. 16A , the first transmission  11   t  includes a fixing gear  18 , a double-gear  19 , a shaft driving gear  20 , a driving transmission shaft  21 , a one-way gear  22 , and a separation roller gear  23 . The second transmission  14   t  includes a pressing idler gear  24  and a pressing gear  25 . 
       FIG. 16B  is a partial side view of the fixing device  5  seen from a direction S 2  in  FIG. 16A .  FIG. 16C  is a partial top view of the fixing device  5  seen from a direction S 3  in  FIG. 16A .  FIG. 16D  is a sectional view of the fixing roller  12  and the fixing gear  18 . As illustrated in  FIG. 16D , the fixing roller  12  includes a flange  12   a . The fixing gear  18  includes a one-way clutch  18   a.    
       FIG. 16E  is a sectional view of the shaft driving gear  20  and the one-way gear  22 . As illustrated in  FIG. 16E , the one-way gear  22  includes a one-way clutch  22   a.    
     In  FIGS. 16A ,  16 B,  16 C,  16 D, and  16 E, the first motor  11   m  and the second motor  14   m  are omitted. 
     As illustrated in  FIG. 16A , the first transmission  11   t  includes the joint gear  11   g , the fixing gear  18 , the double-gear  19 , the shaft driving gear  20 , the driving transmission shaft  21 , the one-way gear  22 , and the separation roller gear  23 . The joint gear  11   g  is connected to the first motor  11   m  depicted in  FIG. 15A . The joint gear  11   g  engages the fixing gear  18 . The double-gear  19  includes two gears. One of the two gears of the double-gear  19  engages the fixing gear  18 . Another one of the two gears of the double-gear  19  engages the shaft driving gear  20 . The driving transmission shaft  21  serves as a rotatable shaft mounted with the shaft driving gear  20  and the one-way gear  22 . The one-way gear  22  engages the separation roller gear  23 . 
     The second transmission  14   t  includes the joint gear  14   g , the pressing idler gear  24 , and the pressing gear  25 . The joint gear  14   g  is connected to the second motor  14   m  depicted in  FIG. 15A , and engages the pressing idler gear  24 . The pressing idler gear  24  engages the pressing gear  25 . 
     The following describes transmission of a driving force in the first driving system  11   s  depicted in  FIG. 15A . The first motor  11   m  generates and transmits a driving force to the joint gear  11   g  connected to the first motor  11   m . Thereafter, as illustrated in  FIG. 16A , the joint gear  11   g  transmits the driving force to the fixing gear  18  directly. As illustrated in  FIG. 16D , the one-way clutch  18   a  is provided inside the fixing gear  18 . The driving force is transmitted to the flange  12   a  of the fixing roller  12  via the one-way clutch  18   a  serving as a first one-way clutch. Thus, the fixing roller  12  receives the driving force and rotates. 
     As illustrated in  FIG. 16A , the fixing gear  18  transmits the driving force to the double-gear  19 . The double-gear  19  amplifies a number of rotations (e.g., the RPM) caused by the driving force, and transmits the driving force to the shaft driving gear  20 . The shaft driving gear  20  transmits the driving force to the driving transmission shaft  21 . The driving transmission shaft  21  transmits the driving force to the one-way gear  22  via the one-way clutch  22   a  serving as a second one-way clutch as illustrated in  FIG. 16E . Finally, the one-way gear  22  transmits the driving force to the separation roller gear  23  to rotate the separation roller  13  depicted in  FIG. 15A . 
     The one-way clutches  18   a  and  22   a  provide directional coupling. Specifically, when a gear of a driving shaft rotates faster than a gear of a driven shaft, the one-way clutches  18   a  and  22   a  clutch both gears to transmit the driving force. By contrast, when the gear of the driving shaft rotates slower than the gear of the driven shaft, the one-way clutches  18   a  and  22   a  release clutching of both gears to idle so as not to transmit the driving force. For example, when the fixing gear  18  serving as the gear of the driving shaft rotates faster than the flange  12   a  serving as the gear of the driven shaft, the one-way clutch  18   a  transmits the driving force generated by the first motor  11   m  in a state in which the one-way clutch  18   a  clutches both gears. By contrast, when the fixing gear  18  rotates slower than the flange  12   a , the one-way clutch  18   a  releases clutching of both gears to idle so as not to transmit the driving force generated by the first motor  11   m . On the other hand, when the shaft driving gear  20  serving as the gear of the driving shaft rotates faster than the one-way gear  22  serving as the gear of the driven shaft, the one-way clutch  22   a  transmits the driving force generated by the first motor  11   m  in a state in which the one-way clutch  22   a  clutches both gears. By contrast, when the shaft driving gear  20  rotates slower than the one-way gear  22 , the one-way clutch  22   a  releases clutching of both gears to idle so as not to transmit the driving force generated by the first motor  11   m.    
     When the one-way clutches  18   a  and  22   a  clutch (e.g., lock) the gears, a gear ratio of the double-gear  19  is set in such a manner that a circumferential velocity Vf of the fixing roller  12  is smaller than a circumferential velocity Vs of the separation roller  13  to satisfy a following formula (3).
 
 Vs&gt;Vf   (3)
 
     Accordingly, when a circumferential velocity Vp of the pressing roller  14  is greater than the circumferential velocity Vf of the fixing roller  12 , the one-way clutch  18   a  idles, and the fixing roller  12  rotates in accordance with rotation of the pressing roller  14 . 
     When the circumferential velocity Vp of the pressing roller  14  is greater than the circumferential velocity Vs of the separation roller  13 , the one-way clutch  22   a  idles, and the separation roller  13  rotates in accordance with rotation of the pressing roller  14 . 
     On the other hand, in the second driving system  14   s  illustrated in  FIG. 15A , the second motor  14   m  transmits a driving force to the joint gear  14   g  connected to the second motor  14   m . Thereafter, as illustrated in  FIG. 16A , the joint gear  14   g  transmits the driving force to the pressing idler gear  24 . The pressing idler gear  24  transmits the driving force to the pressing gear  25 . Since no one-way clutch is provided inside the pressing idler gear  24  and the pressing gear  25 , the pressing gear  25  transmits the driving force to a flange of the pressing roller  14  straight. Thus, the pressing roller  14  receives the driving force and rotates. 
     As illustrated in  FIG. 15A , with the above-described structure, the first driving system  11   s  and the second driving system  14   s  independently drive and control the fixing belt unit  11 U and the pressing roller  14 , respectively. The fixing roller  12  and the separation roller  13  receive the driving force via the one-way clutches  18   a  and  22   a , respectively. Accordingly, even when the circumferential velocity Vp of the pressing roller  14  is set to be greater than the circumferential velocities Vf and Vs of the fixing roller  12  and the separation roller  13 , respectively, the fixing roller  12  and the separation roller  13  rotate in accordance with rotation of the pressing roller  14 , and therefore the circumferential velocities Vf and Vs of the fixing roller  12  and the separation roller  13 , respectively, are not smaller than the circumferential velocity Vp of the pressing roller  14 . In other words, a relative velocity difference between the fixing belt unit  11 U and the pressing roller  14  is decreased at the nip portion N 1 , preventing formation of a faulty image at the intermediate nip region I of the nip portion N 1  depicted in  FIG. 3 . 
     Driving of the first driving system  11   s  and/or the second driving system  14   s  may be controlled to satisfy a following formula (4) when the fixing device  5  conveys a sheet P.
 
 Vs≧Vp&gt;Vf   (4)
 
       FIG. 17  is a graph illustrating a relation between the number of rotations (e.g., the RPM) of the first motor  11   m  depicted in  FIG. 15A  and electric current of the first motor  11   m . The number of rotations of the first motor  11   m  is set by the gear ratio of the double-gear  19  depicted in  FIG. 16A  in such a manner that the circumferential velocity Vs of the separation roller  13  depicted in  FIG. 15A  is greater than the circumferential velocity Vf of the fixing roller  12  depicted in  FIG. 15A  under the constant number of rotations (e.g., the RPM) of the pressing roller  14  depicted in  FIG. 15A . The electric current of the first motor  11   m  may be electric power or torque.  FIG. 17  shows a relation between the circumferential velocities Vf and Vs of the fixing roller  12  and the separation roller  13 , respectively, and the circumferential velocity Vp of the pressing roller  14  when the one-way clutches  18   a  and  22   a  clutch (e.g., lock) the gears. 
     Basically, the relation between the number of rotations of the first motor  11   m  and the electric current of the first motor  11   m  shows a proportional relation in which the greater the number of rotations of the first motor  11   m , the greater the electric current of the first motor  11   m . However, inclination (e.g., an increase rate) of a line showing the proportional relation is divided into three regions according to the relation among the circumferential velocities Vf, Vs, and Vp of the fixing roller  12 , the separation roller  13 , and the pressing roller  14 , respectively. 
     Specifically, when the number of rotations of the first motor  11   m  is small, the relation among the circumferential velocities Vf, Vs, and Vp of the fixing roller  12 , the separation roller  13 , and the pressing roller  14 , respectively, is shown by a following formula (5).
 
 Vp&gt;Vs&gt;Vf   (5)
 
     Actions of the one-way clutches  18   a  and  22   a  depicted in  FIGS. 16D and 16E  rotate the fixing roller  12  and the separation roller  13 , respectively, in accordance with rotation of the pressing roller  14 . Therefore, the torque, that is, the electric current or the electric power, of the first motor  11   m  is small, and the torque of the first motor  11   m  is also small in a region G 1 . As long as the fixing roller  12  and the separation roller  13  rotate in accordance with rotation of the pressing roller  14 , even when the number of rotations of the first motor  11   m  increases, the electric current, the electric power, or the torque increases slightly. The torque means torque to be output by the first motor  11   m  according to an instruction issued by a driver controller of the fixing device  5  to a controller (e.g., the motor controller  200  depicted in  FIG. 15B ) connected to the first motor  11   m.    
     When the number of rotations of the first motor  11   m  is increased, the circumferential velocity Vs of the separation roller  13  is greater than the circumferential velocity Vp of the pressing roller  14  according to the above formula (3). Accordingly, the circumferential velocities Vf, Vs, and Vp of the fixing roller  12 , the separation roller  13 , and the pressing roller  14 , respectively, have a relation indicated by a following formula (6) in a region G 2 .
 
 Vs≧Vp&gt;Vf   (6)
 
     Thus, in the first driving system  11   s , the one-way clutch  18   a  idles, and the fixing roller  12  rotates in accordance with rotation of the pressing roller  14 . The one-way clutch  22   a  clutches the one-way gear  22  depicted in  FIG. 16E  to transmit the driving force generated by the first motor  11   m  to the separation roller  13 . The torque, that is, the electric current, the electric power, or the torque, of the first motor  11   m  increases by a driving amount of the separation roller  13 . Thus, the inclination of the line in the region G 2  is greater than the inclination in the region G 1 . 
     When the number of rotations of the first motor  11   m  is increased further, the circumferential velocity Vf of the fixing roller  12  is greater than the circumferential velocity Vp of the pressing roller  14 . Accordingly, the circumferential velocities Vf, Vs, and Vp of the fixing roller  12 , the separation roller  13 , and the pressing roller  14 , respectively, have a relation indicated by a following formula (7) in a region G 3 .
 
 Vs&gt;Vf&gt;Vp   (7)
 
     Thus, in the first driving system  11   s , the one-way clutches  18   a  and  22   a  clutch the fixing gear  18  and the one-way gear  22  (depicted in  FIG. 16A ) to transmit the driving force generated by the first motor  11   m  to the fixing roller  12  and the separation roller  13 , respectively. The torque value, that is, the electric current, the electric power, or the torque, of the first motor  11   m  increases sharply by a driving amount of the fixing roller  12  and the separation roller  13 . Accordingly, the inclination of the line in the region G 3  is greater than the inclination in the region G 2 . 
     In the region G 1 , the separation roller  13  rotates in accordance with rotation of the pressing roller  14 , and does not stretch the fixing belt  11 . Accordingly, the fixing belt  11  sags at the intermediate nip region I provided between the first nip region F and the second nip region S of the nip portion N 1 , and does not apply proper pressure to a sheet P conveyed on the fixing belt  11 . Consequently, a toner image T fixed on the sheet P may have orange peel finish, resulting in a faulty fixed toner image. Further, the fixing roller  12  and the separation roller  13  rotating in accordance with rotation of the pressing roller  14  may unfavorably increase load to the second motor  14   m  of the second driving system  14   s  depicted in  FIG. 15A  substantially. 
     The fixing roller  12  rotating faster than the pressing roller  14  in the region G 3  causes a relative velocity difference between the fixing belt  11  and the pressing roller  14 . Accordingly, the fixing belt  11  may scratch the fixed toner image T on the sheet P, resulting in formation of a faulty fixed toner image. The fixing roller  12  and the pressing roller  14  rotate in a state in which the pressing roller  14  dents the thick, heat-resistant elastic layer  12 E (depicted in  FIG. 2 ) formed of silicon rubber foam or the like of the fixing roller  12 . Accordingly, the velocity of the fixing roller  12  may fluctuate easily, resulting in unstable rotation of the fixing roller  12 . Therefore, when the fixing roller  12  and the pressing roller  14  are driven independently and separately from each other as shown in the region G 3 , substantial fluctuation in the number of rotations (e.g., the RPM) of the fixing roller  12  may unfavorably increase load to the first motor  11   m  and the second motor  14   m.    
     The above problems may be solved in the region G 2 . Since the circumferential velocity Vp of the pressing roller  14  is greater than the circumferential velocity Vf of the fixing roller  12  in the region G 2 , the fixing roller  12  rotates in accordance with rotation of the pressing roller  14 , stabilizing rotation of the fixing roller  12  and causing no relative velocity difference between the fixing roller  12  and the pressing roller  14 . Thus, a faulty fixed toner image, such as a scratched toner image, may not be formed. 
     Further, the second driving system  14   s  drives the pressing roller  14 , and the driven pressing roller  14  drives the fixing roller  12 , preventing or reducing fluctuation in the number of rotations of the fixing roller  12 . Thus, unnecessary load may not be applied to the first motor  11   m  and the second motor  14   m.    
     In the region G 2 , the circumferential velocity Vs of the separation roller  13  is greater than the circumferential velocity Vp of the pressing roller  14 . In other words, the separation roller  13  rotates faster than the pressing roller  14 . Since the diameter of the separation roller  13  is smaller than the diameter of the fixing roller  12 , and the circumferential velocity Vp of the pressing roller  14  is greater than the circumferential velocity Vf of the fixing roller  12 , the circumferential velocity Vp of the pressing roller  14  determines the conveyance velocity at the nip portion N 1  for conveying a sheet P. Further, at the nip portion N 1  at which the pressing roller  14  contacts the fixing belt  11 , the surfaces of the pressing roller  14  and the fixing belt  11  move at an identical velocity. Accordingly, the separation roller  13  rotates and slides over an inner circumferential surface of the fixing belt  11  while stretching the fixing belt  11  against the nip portion N 1 . Consequently, the rotating separation roller  13  adjusts nip pressure at the intermediate nip region I of the nip portion N 1  to predetermined pressure by stretching the fixing belt  11 , preventing formation of a faulty fixed toner image having orange peel finish, for example. In order to stretch the fixing belt  11  properly, friction coefficient of the surfaces of the separation roller  13  and the fixing belt  11  may be considered. For example, the separation roller  13  may include silicon rubber. 
     As described above, when the circumferential velocities Vf, Vs, and Vp of the fixing roller  12 , the separation roller  13 , and the pressing roller  14 , respectively, have the relation indicated by the above formula (4), that is, Vs≧Vp&gt;Vf, the fixing belt  11  may not scratch a toner image on a sheet, preventing formation of a faulty fixed toner image having orange peel finish, for example. Further, load applied to the first motor  11   m  and the second motor  14   m  may be decreased, resulting in an improved load balance. 
     For example, as illustrated in  FIGS. 15A and 15B , the motor controller  200  controls the second driving system  14   s  to drive and rotate the pressing roller  14  at the constant circumference velocity Vp. The motor controller  200  controls the first driving system  11   s  to drive the fixing roller  12  and the separation roller  13  in such a manner that the circumferential velocities Vf and Vs of the fixing roller  12  and the separation roller  13 , respectively, satisfy the above formula (4). Alternatively, the motor controller  200  may control the second driving system  14   s  to drive the pressing roller  14  to satisfy the above formula (4) because the double-gear  19  is configured to cause the circumferential velocity Vs of the separation roller  13  to be greater than the circumferential velocity Vf of the fixing roller  12 . 
     The circumferential velocities Vf, Vs, and Vp of the fixing roller  12 , the separation roller  13 , and the pressing roller  14 , respectively, may fluctuate due to difference in temperature, thermal expansion, and denting of the pressing roller  14 , the separation roller  13 , and the fixing roller  12 . Therefore, it is difficult to drive the pressing roller  14 , the separation roller  13 , and the fixing roller  12  to satisfy the above formula (4) constantly under a constant driving condition of the first motor  11   m  and the second motor  14   m.    
     To address this, the fixing device  5  includes the first motor detector  11   m D and the first motor regulator  11   m A. The first motor detector  11   m D detects one of the electric current, the electric power, and the torque of the first motor  11   m  serving as a servomotor. The first motor regulator  11   m A changes the number of rotations of the first motor  11   m.    
     Specifically, when a sheet P is conveyed, the motor controller  200  performs feed-back control to cause the first motor regulator  11   m A to change the number of rotations of the first motor  11   m  to a number of rotations (e.g., the RPM) corresponding to a predetermined range (e.g., a preset range) based on a detection result provided by the first motor detector  11   m D, so that the circumferential velocities Vf, Vs, and Vp of the fixing roller  12 , the separation roller  13 , and the pressing roller  14 , respectively, satisfy the relation indicated by the above formula (4). The predetermined range of the number of rotations of the first motor  11   m  means a range of the electric current, the electric power, or the torque of the first motor  11   m  in the region G 2  depicted in  FIG. 17 . 
     More specifically, in the fixing device  5 , the number of rotations of the first motor  11   m  and the electric current, the electric power, or the torque of the first motor  11   m  have the relation illustrated in the graph in  FIG. 17 . Accordingly, the first motor detector  11   m D detects one of the electric current, the electric power, the torque of the first motor  11   m . When a detected value provided by the first motor detector  11   m D is not in the predetermined range (e.g., the preset range), the first motor regulator  11   m A changes the number of rotations of the first motor  11   m  to a number of rotations (e.g., the RPM) corresponding to the predetermined range. 
     For example, when the detected value is in the region G 1 , the first motor regulator  11   m A increases the number of rotations of the first motor  11   m . By contrast, when the detected value is in the region G 3 , the first motor regulator  11   m A decreases the number of rotations of the first motor  11   m . Thus, the relation indicated by the above formula (4) is satisfied. 
     The above-described control may be performed continuously when sheets P are conveyed. Alternatively, the control may be performed periodically or whenever an image formation mode (e.g., the gloss mode or the non-gloss mode) is switched. 
     In the fixing device  5 , control for satisfying the relation indicated by the above formula (4) may be performed with a structure in which the one-way clutch  18   a  is provided inside the fixing gear  18  of the first driving system  11   s . However, as illustrated in  FIG. 15A , when the fixing device  5  is turned on to start driving the second driving system  14   s  for driving the pressing roller  14  and the first driving system  11   s  for driving the fixing belt unit  11 U, the first motor  11   m  may not be connected to the joint gear  11   g  precisely depending on a state in which the fixing belt unit  11 U is attached to the fixing device  5 . In this case, the first motor  11   m  may be connected to the joint gear  11   g  after the first motor  11   m  starts rotating and rotates for a certain amount. Accordingly, some time lag may generate between start of rotating the pressing roller  14  and start of rotating the fixing roller  12  and the separation roller  13  included in the fixing belt unit  11 U. During the time lag, the separation roller  13  stops while the pressing roller  14  rotates, and therefore the separation roller  13  and the fixing belt  11  may receive a shearing force generated by the pressing roller  14 . To address this, the one-way clutch  22   a  may be favorably mounted on the one-way gear  22 , as illustrated in  FIG. 16E . In other words, when the motor controller  200  starts driving the second driving system  14   s , the second driving system  14   s  rotates the pressing roller  14 , and the one-way clutches  18   a  and  22   a  cause the fixing roller  12  and the separation roller  13  to rotate in accordance with rotation of the pressing roller  14 . Thereafter, the motor controller  200  starts driving the first driving system  11   s  for driving the fixing belt unit  11 U. Thus, the above-described problems may be solved. 
       FIG. 18  is a schematic view of the fixing device  5 , the gloss finisher  6 , and the conveyance roller pair  7  included in the image forming apparatus  100  shown in  FIG. 1 . 
     As illustrated in  FIG. 18 , the image forming apparatus  100  further includes guides  45  and  95 . 
     The gloss finisher  6  includes a heating roller  80 , a temperature sensor  82 , a separator  83 , a heater  85 , a pressing roller  90 , a pressure adjuster  91 , and a cleaner  93 . The heating roller  80  includes a core metal  80   a  and an elastic layer  80   b . The pressing roller  90  includes a core metal  90   a  and an elastic layer  90   b . The pressure adjuster  91  includes a pressing lever  96 , a pressing member  96   a , a support shaft  96   b , a pressing portion  96   c , a spring  97 , and a cam  98 . 
     The conveyance roller pair  7  includes rollers  7   a  and  7   b.    
     The image forming apparatus  100  provides the gloss mode for applying gloss to a fixed toner image T on a sheet P, and the non-gloss mode for not applying gloss to the fixed toner image T. In the image forming apparatus  100 , the fixing device  5 , the gloss finisher  6 , and the conveyance roller pair  7  are provided on a path line PL for conveying the sheet P in this order in the sheet conveyance direction. Alternatively, the fixing device  5 ′ depicted in  FIG. 4  may replace the fixing device  5 . 
     The fixing device  5  includes the fixing belt  11  serving as a rotatable fixing member, and the pressing roller  14  serving as a pressing member for pressing against the fixing member to form the nip portion N 1  for fixing the toner image T on the sheet P. 
     The gloss finisher  6  includes the heating roller  80  serving as a first rotary member or a heating member, the heater  85  serving as a heater provided inside the first rotary member, and the pressing roller  90  serving as a second rotary member or a pressing member pressed against the first rotary member to form a nip portion N 2  for applying gloss to the fixed toner image T on the sheet P. 
     The conveyance roller pair  7  is provided at a position separated from a downstream end of the nip portion N 1  of the fixing device  5  in the sheet conveyance direction by a distance L 1  (e.g., 210 mm) or smaller, and feeds the sheet P sent from the gloss finisher  6 . 
     In the non-gloss mode, when a length of the sheet P is smaller than 210 mm in the sheet conveyance direction, the gloss finisher  6  decreases nip pressure applied by the pressing roller  90  to the heating roller  80  to a level lower than nip pressure applied by the pressing roller  90  to the heating roller  80  in the gloss mode, so that the pressing roller  90  and the heating roller  80  feed the sheet P. When the length of the sheet P is not smaller than 210 mm, the gloss finisher  6  releases the nip portion N 2  formed between the pressing roller  90  and the heating roller  80 , so that the conveyance roller pair  7  feeds the sheet P. 
     The following describes the fixing device  5 . The fixing device  5  has the structure shown in  FIG. 2 . When the fixing belt  11  and the pressing roller  14  rotate, the surface of the fixing belt  11  is heated up to a predetermined temperature. When a sheet P bearing a toner image T passes through the nip portion N 1  leftward in  FIG. 18 , the fixing belt  11  and the pressing roller  14  apply heat and pressure to the sheet P at the nip portion N 1  to melt and fix the toner image T on the sheet P. When the sheet P bearing the toner image T is discharged from the nip portion N 1 , the separator  43  separates the sheet P from the pressing roller  14 . 
     As described above by referring to  FIGS. 15A and 15B , the motor controller  200  controls the first driving system  11   s  and the second driving system  14   s  to satisfy the relation indicated by the above formula (4), thus preventing formation of a faulty fixed toner image such as a scratched toner image and an orange peel toner image, decreasing load applied to the first motor  11   m  and the second motor  14   m , and improving a load balance. 
     The sheet P discharged from the fixing device  5  is sent to the gloss finisher  6 . The guide  45  is provided between the fixing device  5  and the gloss finisher  6 . For example, the guide  45  includes two plate members provided above and below the path line PL, respectively, to form a gap through which the sheet P discharged from the fixing device  5  is conveyed to the gloss finisher  6 . The gap becomes narrower toward the gloss finisher  6 . The sheet P separated from the fixing belt  11  by the curvature of the separation roller  13  may curl easily. To address this, the guide  45  corrects curl of the sheet P to direct a leading edge of the sheet P toward the gloss finisher  6 . Thus, the sheet P is not creased or jammed in the gloss finisher  6 , providing stability in conveying the sheet P. According to this exemplary embodiment, the toner image T which is fixed on the sheet P properly by the fixing device  5  does not degrade even when the toner image T contacts the guide  45 . 
     The following describes the gloss finisher  6 . In the gloss finisher  6 , the heater  85  is provided inside the heating roller  80  serving as a first rotary member having a hollow cylindrical shape. The pressing roller  90  serving as a second rotary member is pressed against the first rotary member to form the nip portion N 2  at which the heating roller  80  and the pressing roller  90  apply heat and pressure to the fixed toner image T on the sheet P to apply gloss to the fixed toner image T. 
     In the heating roller  80 , the elastic layer  80   b  including silicon rubber is provided on an outer circumferential surface of the core metal  80   a  having a cylindrical shape including aluminum or iron. The heater  85  is provided inside the core metal  80   a.    
     In the pressing roller  90 , the elastic layer  90   b  including silicon rubber is provided on an outer circumferential surface of the core metal  90   a  having a round-bar shape including aluminum or iron. The elastic layer  90   b  of the pressing roller  90  is thinner than the elastic layer  80   b  of the heating roller  80 , and therefore fluctuation in surface temperature of the pressing roller  90  is smaller than fluctuation in surface temperature of the heating roller  80 . Accordingly, the pressing roller  90  serves as a primary driving roller of the gloss finisher  6 . 
     The temperature sensor  82  is provided to face the heating roller  80  at a position near an entrance to the nip portion N 2  to detect surface temperature of the heating roller  80  at a position upstream from the nip portion N 2  in a rotation direction of the heating roller  80 . The heater  85  (e.g., a halogen heater) is turned on and off based on the detected surface temperature to maintain the constant surface temperature of the heating roller  80 . 
     The surface temperature of the heating roller  80  is controlled to apply gloss to the fixed toner image T properly in the gloss mode. For example, the surface temperature of the heating roller  80  contacting the fixed toner image T on the sheet P is lower than the surface temperature of the fixing belt  11  serving as a fixing member of the fixing device  5 . Alternatively, the surface temperature of the heating roller  80  may be favorably not lower than a temperature of the sheet P entering the gloss finisher  6  and not higher than a temperature of the sheet P immediately after the sheet P is discharged from the fixing device  5 . 
     The surface temperature of the heating roller  80  may be preferably not lower than a softening temperature of toner used to form the toner image T which is measured by a flow tester and not higher than a half-flow start temperature at which half of the toner starts flowing, and more preferably not lower than the softening temperature and not higher than a flow start temperature at which the toner starts flowing. A solid-state temperature of the toner may be measured with a flow tester model CFT-500D available from Shimadzu Corporation under a load of 5 kg/cm 2 , a temperature increase velocity of 3.0 degrees centigrade per minute, a die diameter of 1.0 mm, a die length of 10.0 mm, and may be calculated based on a relation between temperature and piston stroke. The half-flow start temperature at which half of the toner starts flowing is defined as a midpoint between the flow start temperature at which the toner starts flowing and a flow finish temperature at which the toner finishes flowing. 
     For example, the surface temperature of the heating roller  80  may be preferably in a range from 60 degrees centigrade (e.g., a softening temperature in the solid-state temperature of the toner) to 137 degrees centigrade (e.g., a half-flow start temperature in the solid-state temperature of the toner), more preferably in a range from 60 degrees centigrade to 120 degrees centigrade (e.g., a flow start temperature in the solid-state temperature of the toner), and yet more preferably in a range from 80 degrees centigrade to 100 degrees centigrade. The temperature (e.g., the solid-state temperature) of toner fluctuates depending on lot and color of the toner. The above-mentioned temperatures are average temperatures. 
     According to this exemplary embodiment, in a fixing process in which a sheet P passes through the fixing device  5 , an unfixed toner image T on the sheet P receives heat and pressure at the nip portion N 1 , and therefore an entire toner layer forming the unfixed toner image T from an upper surface of the unfixed toner image T to a lower surface of the unfixed toner image T contacting the sheet P is melted and fixed. Certain leveling adheres the toner image T to the sheet P, and a substantial adhesive force generates on the upper surface of the toner image T. 
     By contrast, in a gloss application process in which the sheet P passes through the gloss finisher  6 , the toner image T has already been fixed on the sheet P, and therefore the gloss finisher  6  applies heat needed to level the surface of the toner image T. The toner image T on the sheet P receives heat and pressure at the nip portion N 2  in the gloss finisher  6 . However, the surface temperature of the heating roller  80  is not lower than the temperature of the sheet P when the sheet P enters the gloss finisher  6  and not higher than the temperature of the sheet P immediately after the sheet P is discharged from the fixing device  5 . In other words, the surface temperature of the heating roller  80  is not lower than the softening temperature of the toner forming the toner image T measured by the flow tester and not higher than the half-flow start temperature at which half of the toner starts flowing, or the surface temperature of the heating roller  80  is in a range from about 60 degrees centigrade to about 120 degrees centigrade. Accordingly, the entire toner layer is not melted, but the surface layer of the toner image T is softened. Consequently, color of the toner image T is maintained, and the smooth surface of the heating roller  80  levels the surface layer of the toner image T to improve the gloss of the toner image T. The surface of the toner image T provides an adhesive force smaller than the adhesive force provided in the fixing process. Accordingly, even when a diameter of the heating roller  80  is not smaller than about 30 mm and not greater than about 40 mm, the sheet P bearing the toner image T separates from the heating roller  80  properly. Namely, the separator  83  provided downstream from the nip portion N 2  in the sheet conveyance direction may be omitted to simplify the structure of the gloss finisher  6  and reduce manufacturing costs. Further, offset caused by melting the entire toner layer in the fixing process does not generate in the gloss application process. Accordingly, the cleaner  93  for removing toner from a surface of the pressing roller  90  may be omitted to simplify the structure of the gloss finisher  6  and reduce manufacturing costs. 
     The pressing roller  90  may be a cylindrical roller in which the elastic layer  90   b  including silicon rubber is provided on the core metal  90   a  including aluminum or iron. The pressure adjuster  91  includes the pressing lever  96 , the pressing member  96   a , the support shaft  96   b , the pressing portion  96   c , the spring  97 , and the cam  98 , and contacts the pressing roller  90  to press the pressing roller  90  against the heating roller  80  in the gloss mode. 
     The following describes operations of the pressure adjuster  91 . When a driving force generated by an external device rotates the cam  98  clockwise in  FIG. 18  in a rotation direction D 4  by a predetermined angle, the cam  98  pushes up the pressing member  96   a  in a direction D 5 . The pressing member  96   a  causes the spring  97  mounted on the pressing member  96   a  to push up one end of the pressing lever  96  in a direction perpendicular to an axial direction of the pressing roller  90  with predetermined pressure. Accordingly, the pressing lever  96  rotates about the support shaft  96   b  clockwise in  FIG. 18  in a rotation direction D 6 . Thereafter, the pressing portion  96   c  provided between one end of the pressing lever  96  mounting the spring  97  and the support shaft  96   b  contacts a shaft of the pressing roller  90  and pushes the pressing roller  90  toward the heating roller  80 . Finally, the pressing roller  90  contacts the heating roller  80  and presses against the heating roller  80  with predetermined pressure to form the nip portion N 2  for applying gloss to the toner image T on the sheet P. Alternatively, the pressure adjuster  91  may not include the spring  97  so that the cam  98  directly pushes up one end of the pressing lever  96 . 
     The rotation angle of the cam  98  is adjusted to change pressure applied by the pressure adjuster  91 . Specifically, when the cam  98  is at a predetermined rotation position, the pressing roller  90  separates from the heating roller  80  to release the nip portion N 2 . 
     In the gloss mode, the pressure adjuster  91  may adjust nip pressure applied at the nip portion N 2  to a value in a range from about 15 N/cm 2  to about 30 N/cm 2 . Accordingly, when the sheet P sent from the fixing device  5  passes through the gloss finisher  6 , heat and predetermined pressure are applied to the fixed toner image T on the sheet P at the nip portion N 2  to level the surface layer of the fixed toner image T to apply gloss to the fixed toner image T. 
     When a sheet P having a length smaller than 210 mm is used in the non-gloss mode, the pressure adjuster  91  adjusts the nip pressure applied at the nip portion N 2  to a value smaller than the nip pressure applied at the nip portion N 2  in the gloss mode, for example, preferably smaller than about 15 N/cm 2  and more preferably not greater than about 5 N/cm 2 . The nip pressure applied at the nip portion N 2  is defined as average pressure applied at a whole nip length of the nip portion N 2 . Accordingly, the heating roller  80  and the pressing roller  90  nip the sheet P with the small nip pressure to convey the sheet P without increasing gloss of the fixed toner image T on the sheet P. 
     When a sheet P having a length not smaller than 210 mm is used in the non-gloss mode, the pressure adjuster  91  may release the nip portion N 2 , that is, pressure applied between the heating roller  80  and the pressing roller  90 . 
     In the non-gloss mode, an A3 size sheet having a weight not greater than about 80 g/m 2  may be used as a thin long sheet P. However, a slight difference between the linear velocities of the fixing device  5  and the gloss finisher  6  for conveying the sheet P may bend or stretch the sheet P, creasing the sheet P slightly. To address this, the pressing roller  90  separates from the heating roller  80  in the gloss finisher  6 . In this case, the sheet P passes through the gloss finisher  6  without receiving pressure at the nip portion N 2 . Specifically, a leading edge of the sheet P having the length not smaller than 210 mm discharged from the nip portion N 1  of the fixing device  5  reaches the conveyance roller pair  7 , and the conveyance roller pair  7  nips and conveys the sheet P. Thus, chances of the toner image T on the sheet P to contact the rollers (e.g., the heating roller  80  and the pressing roller  90 ) are decreased to maintain image quality and convey the sheet P precisely. 
     A roller gap provided between the heating roller  80  and the pressing roller  90  when the nip portion N 2  is released may be not greater than about 2 mm. When the roller gap is greater than about 2 mm, the sheet P may go off the path line PL and may be jammed. 
     A surface layer of each of the heating roller  80  and the pressing roller  90  may be coated with fluorocarbon resin. Specifically, even when the pressing roller  90  separates from the heating roller  80  to provide the roller gap not greater than about 2 mm in the non-gloss mode, and the toner image T on the sheet P contacts the heating roller  80  partially while the sheet P passes through the roller gap, the surface layer coated with the fluorocarbon resin may release the sheet P from the heating roller  80  easily, and may prevent the heating roller  80  from scratching the toner image T on the sheet P. 
     As described above, the gloss finisher  6  provides desired gloss stably in the gloss mode, and improves reliability of the desired gloss in each of the gloss mode and the non-gloss mode. 
     The gloss finisher  6  (e.g., the heating roller  80  and the pressing roller  90 ) is disposed at a position at which the leading edge of the sheet P reaches the nip portion N 2  of the gloss finisher  6  before a trailing edge of the sheet P leaves the nip portion N 1  of the fixing device  5 . For example, a distance L 2  from the exit of the nip portion N 1  to the entrance to the nip portion N 2  may be preferably in a range from about 60 mm to about 182 mm, more preferably in a range from about 70 mm to about 150 mm, and yet more preferably in a range from about 80 mm to about 100 mm. When the distance L 2  is smaller than about 60 mm, the two plate members of the guide  45  may tilt sharply to provide a predetermined opening gap at an entrance to the guide  45  and another predetermined opening gap at an exit of the guide  45 . Thus, the sheet P may be jammed in the guide  45  easily. An upper limit of the distance L 2  may be a length of a smallest sheet which can be handled by the image forming apparatus  100 . For example, the distance L 2  of 182 mm is a distance corresponding to a short length of a B5 size sheet when the B5 size sheet is conveyed in a short direction of the B5 size sheet. The upper limit of the distance L 2  is 150 mm when a half letter size sheet is conveyed in a short direction of the half letter size sheet. 
       FIG. 19A  is a perspective view of the fixing device  5  including the first driving system  11   s  and the second driving system  14   s , and the gloss finisher  6  including a third driving system  80   s . As illustrated in  FIG. 19A , the third driving system  80   s  includes a joint gear  80   g , a third motor  80   m , and a third transmission  80   t.    
       FIG. 19B  is a block diagram of the fixing device  5  and the gloss finisher  6 . As illustrated in  FIG. 19B , the motor controller  200  further includes a third motor detector  80   m D and a third motor regulator  80   m A. 
     The third driving system  80   s  drives the heating roller  80 . In the third driving system  80   s , the third transmission  80   t  includes one or more transmission gears, and transmits a driving force generated by the third motor  80   m  to the heating roller  80 . 
     The third motor  80   m  is provided in the image forming apparatus  100 . When the gloss finisher  6  is attached to the image forming apparatus  100 , the third motor  80   m  is connected to the joint gear  80   g  serving as a driving force input portion of the heating roller  80 . 
     With the above-described structure, the third transmission  80   t  transmits the driving force generated by the third motor  80   m  to the heating roller  80  via the joint gear  80   g . Thus, the gloss finisher  6  is driven. According to this exemplary embodiment, the third driving system  80   s  drives the heating roller  80 . Alternatively, the third driving system  80   s  may drive the pressing roller  90 . 
     When “V 1 ” represents a conveyance velocity at which the fixing device  5  conveys a sheet P, and “V 2 ” represents a conveyance velocity at which the gloss finisher  6  conveys the sheet P, the third driving system  80   s  performs driving control satisfying a following formula (8).
 
1.05 ×V 1 □V 2 □V 1  (8)
 
       FIG. 20  is a graph illustrating a relation between a number of rotations (e.g., the RPM) of the third motor  80   m  and electric current of the third motor  80   m  in the gloss finisher  6  which receives a sheet P sent from the fixing device  5  at the predetermined conveyance velocity V 1 . The electric current of the third motor  80   m  may be electric power or torque. The graph illustrated in  FIG. 20  also shows a relation between the conveyance velocity V 1  at which the fixing device  5  conveys the sheet P and the conveyance velocity V 2  at which the gloss finisher  6  conveys the sheet P. 
     Basically, the relation between the number of rotations of the third motor  80   m  and the electric current of the third motor  80   m  shows a proportional relation in which the greater the number of rotations of the third motor  80   m , the greater the electric current of the third motor  80   m . However, inclination (e.g., an increase rate) of a line showing the proportional relation is divided into three regions according to the relation between the conveyance velocities V 1  and V 2  for conveying the sheet P. 
     Specifically, when the number of rotations of the third motor  80   m  is small, the relation between the conveyance velocity V 1  of the fixing device  5  and the conveyance velocity V 2  of the gloss finisher  6  is shown by a following formula (9).
 
 V 1&gt; V 2  (9)
 
     The conveyance velocity V 1  of the fixing device  5  has an ascendancy over the conveyance velocity V 2  of the gloss finisher  6 . Therefore, the torque, that is, the electric current or the electric power, of the third motor  80   m  is small, and the torque value of the third motor  80   m  is also small in a region I. Even when the number of rotations of the third motor  80   m  increases, the electric current, the electric power, or the torque of the third motor  80   m  increases slightly in the region I. The torque value is defined as torque to be output by the third motor  80   m  according to an instruction issued by a driver controller of the gloss finisher  6  to a controller (e.g., the motor controller  200  depicted in  FIG. 19B ) connected to the third motor  80   m.    
     When the number of rotations of the third motor  80   m  is increased, the conveyance velocity V 2  of the gloss finisher  6  is greater than the conveyance velocity V 1  of the fixing device  5  to have a relation indicated by a following formula (10) in a region II.
 
1.05 ×V 1&gt; V 2&gt; V 1  (10)
 
     When the conveyance velocity V 2  of the gloss finisher  6  increases further, the conveyance velocities V 1  and V 2  have a relation indicated by a following formula (11) in a region III.
 
 V 2&gt;1.05 ×V 1  (11)
 
     In the regions II and III, the torque value of the third motor  80   m , that is, the electric current, the electric power, or the torque of the third motor  80   m , has an identical increase rate (e.g., inclination of the line) and is greater than the torque of the third motor  80   m  in the region I. 
     In the region I, the conveyance velocity V 1  of the fixing device  5  for feeding the sheet P into the gloss finisher  6  is greater than the conveyance velocity V 2  of the gloss finisher  6  for discharging the sheet P from the gloss finisher  6 . Accordingly, the sheet P waves and slacks between the fixing device  5  and the gloss finisher  6 . Consequently, the sheet P or the toner image T on the sheet P may have slack creases. On the other hand, in the region III, the conveyance velocity V 2  of the gloss finisher  6  for discharging the sheet P from the gloss finisher  6  is excessively greater than the conveyance velocity V 1  of the fixing device  5  for feeding the sheet P into the gloss finisher  6 . Accordingly, the gloss finisher  6  pulls the sheet P with a substantial force. Consequently, the sheet P or the toner image T on the sheet P may have rib creases, that is, creases extending obliquely from edges toward a center of the sheet P. 
     In the region II, the gloss finisher  6  pulls the sheet P properly to prevent or reduce slack creases and rib creases. Namely, the conveyance velocity V 2  of the gloss finisher  6  is greater than the conveyance velocity V 1  of the fixing device  5  within 5 percent as shown by the above formula (8) to prevent or reduce visual faults such as slack creases and rib creases. 
     The conveyance velocities V 1  and V 2  of the fixing device  5  and the gloss finisher  6 , respectively, may fluctuate due to difference in temperature, thermal expansion, denting of the pressing roller  14 , the separation roller  13 , the fixing roller  12 , the heating roller  80 , and the pressing roller  90  depicted in  FIG. 18 , and type of a sheet P. Therefore, it is difficult to drive the pressing roller  14 , the separation roller  13 , the fixing roller  12 , the heating roller  80 , and the pressing roller  90  to satisfy the above formula (8) under a constant driving condition of the first motor  11   m , the second motor  14   m , and the third motor  80   m  depicted in  FIG. 19A . 
     To address this, as illustrated in  FIG. 19B , the gloss finisher  6  includes the third motor detector  80   m D and the third motor regulator  80   m A. The third motor detector  80   m D detects one of the electric current, the electric power, and the torque of the third motor  80   m . The third motor regulator  80   m A changes the number of rotations (e.g., the RPM) of the third motor  80   m.    
     Specifically, when a sheet P is conveyed, the motor controller  200  performs feed-back control to cause the third motor regulator  80   m A to change the number of rotations of the third motor  80   m  to a number of rotations (e.g., the RPM) corresponding to a predetermined range based on a detection result provided by the third motor detector  80   m D, so that the conveyance velocities V 1  and V 2  of the fixing device  5  and the gloss finisher  6 , respectively, satisfy the relation indicated by the above formula (8). The predetermined range of the number of rotations of the third motor  80   m  is defined as a range (e.g., a preset range) of the electric current, the electric power, or the torque of the third motor  80   m  in the region II depicted in  FIG. 20 . 
     More specifically, in the gloss finisher  6 , the number of rotations of the third motor  80   m  and the electric current, the electric power, or the torque of the third motor  80   m  have the relation illustrated in the graph in  FIG. 20 . Accordingly, the third motor detector  80   m D detects one of the electric current, the electric power, the torque of the third motor  80   m . When a detected value provided by the third motor detector  80   m D is not in the predetermined range (e.g., the preset range), the third motor regulator  80   m A changes the number of rotations of the third motor  80   m  to a number of rotations (e.g., the RPM) corresponding to the predetermined range. 
     For example, when the detected value provided by the third motor detector  80   m D is in the region I, the third motor regulator  80   m A increases the number of rotations of the third motor  80   m . By contrast, when the detected value provided by the third motor detector  80   m D is in the region III, the third motor regulator  80   m A decreases the number of rotations of the third motor  80   m . Thus, the relation indicated by the above formula (8) is satisfied. 
     The above-described control may be performed continuously when sheets P are conveyed as long as the pressing roller  90  is pressed against the heating roller  80  in the gloss finisher  6 . Alternatively, the control may be performed periodically or whenever the image formation mode (e.g., the gloss mode or the non-gloss mode) is switched. 
     As illustrated in  FIG. 18 , the sheet P discharged from or passed through the gloss finisher  6  is sent to the conveyance roller pair  7 . The guide  95  is provided between the gloss finisher  6  and the conveyance roller pair  7 . For example, the guide  95  includes two plate members provided above and below the path line PL, respectively, to form a gap through which the sheet P is conveyed. The gap becomes narrower toward the conveyance roller pair  7 . The guide  95  corrects curl of the sheet P to direct the leading edge of the sheet P toward the conveyance roller pair  7 . Thus, the sheet P is not creased or jammed in the conveyance roller pair  7 , providing stability in conveying the sheet P. 
     Referring to  FIG. 18 , the following describes the conveyance roller pair  7 . The conveyance roller pair  7  includes the cylindrical roller  7   a  including chloroprene rubber and/or silicon rubber and the cylindrical roller  7   b  including resin and contacting the roller  7   a . One or both of the rollers  7   a  and  7   b  is driven to sandwich the sheet P sent from the gloss finisher  6  and feed the sheet P toward the output path. The conveyance roller pair  7  is disposed at a position downstream within 210 mm from the exit of the nip portion N 1  of the fixing device  5  in the sheet conveyance direction. Accordingly, when the length of the sheet P in the sheet conveyance direction in the non-gloss mode is not smaller than 210 mm, that is, the length of an A4 size sheet in a short direction of the A4 size sheet, the nip portion N 2  formed between the heating roller  80  and the pressing roller  90  of the gloss finisher  6  is released. However, the leading edge of the sheet P discharged from the nip portion N 1  of the fixing device  5  reaches the conveyance roller pair  7  before the trailing edge of the sheet P leaves the nip portion N 1 . Thus, the sheet P is conveyed properly. 
     The surface temperature of the heating roller  80  is low, that is, not lower than a sheet temperature of the sheet P entering the gloss finisher  6  and not higher than a sheet temperature of the sheet P immediately after the sheet P is discharged from the fixing device  5 , not lower than the softening temperature of toner used to form the toner image T which is measured by the flow tester and not higher than the half-flow start temperature at which half of the toner starts flowing, or not lower than about 60 degrees centigrade and not higher than about 120 degrees centigrade. Accordingly, in the gloss mode, the sheet temperature of the sheet P reaching the conveyance roller pair  7  is equivalent to or lower than the sheet temperature of the sheet P immediately after the sheet P is discharged from the fixing device  5 . Consequently, the toner does not adhere to the conveyance roller pair  7 . Further, due to the similar reason, the toner does not adhere to the guide  95 . 
     Referring to  FIG. 18 , the following describes the gloss mode and the non-gloss mode of the image forming apparatus  100 . The image forming apparatus  100  may selectively provide the gloss mode to apply gloss to a fixed toner image T on a sheet P and the non-gloss mode not to apply gloss to a fixed toner image T on another sheet P when both the sheets P have a same basis weight or a same basic weight. For example, the image forming apparatus  100  includes a control panel displaying the gloss mode and the non-gloss mode so that a user can select the gloss mode or the non-gloss mode. The gloss mode uses a sheet having a high gloss in a range from about 30 percent to about 50 percent such as coated paper to form a fixed toner image on the sheet and apply gloss equivalent to gloss of the sheet P serving as a background of the fixed toner image to the fixed toner image. The gloss mode is preferably used for photogravure printing. The non-gloss mode uses a sheet having a low gloss such as plain paper to form a fixed toner image on the sheet and not apply gloss to the fixed toner image. The gloss is measured by a 60-degree glossmeter and denoted in percent. 
     Referring to  FIGS. 18 and 21A , the following describes processes performed when the gloss mode is selected, and a sheet such as coated paper having the gloss in the range from about 30 percent to about 50 percent is used.  FIG. 21A  is a flowchart illustrating processes performed in the image forming apparatus  100  in the gloss mode. 
     In step S 11 , a sheet P bearing an unfixed toner image T is sent to the fixing device  5 , and the fixing device  5  fixes the toner image T on the sheet P. Specifically, the fixing belt  11  is heated up to a proper fixing temperature by heat generated by the heater  15   h  provided inside the heating roller  15 . The cam  78  of the pressure adjuster  70  is moved to adjust nip pressure applied at the nip portion N 1  so that a region in which nip pressure in a range from about 15 N/cm 2  to about 30 N/cm 2  is applied occupies 50 percent or more of the whole nip portion N 1 . Accordingly, the toner image T on the sheet P passing through the fixing device  5  is fixed on the sheet P properly, and a gloss of 25 percent or more is applied to the fixed toner image T. 
     In step S 12 , the guide  45  corrects curl of the sheet P discharged from the fixing device  5 , and guides the sheet P so that a leading edge of the sheet P enters the gloss finisher  6  properly. 
     In step S 13 , the gloss finisher  6  applies further gloss to the fixed toner image T on the sheet P. Specifically, the surface temperature of the heating roller  80  is in a range from about 80 degrees centigrade to about 100 degrees centigrade. The pressure adjuster  91  adjusts nip pressure applied at the nip portion N 2  to a range from about 15 N/cm 2  to about 30 N/cm 2 . Accordingly, when the sheet P passes through the gloss finisher  6 , heat and predetermined pressure are applied to the fixed toner image T on the sheet P at the nip portion N 2  to level a surface layer of the fixed toner image T. A gloss within plus and minus 15 percent with respect to the gloss of the sheet P, preferably a gloss within plus and minus 10 percent, is applied to the fixed toner image T on the sheet P. 
     In step S 14 , the sheet P bearing the fixed toner image T is discharged from the gloss finisher  6 , and passes through the guide  95  and the conveyance roller pair  7  provided on the conveyance path. 
     Referring to  FIGS. 18 and 21B , the following describes processes performed when the non-gloss mode is selected.  FIG. 21B  is a flowchart illustrating processes performed in the image forming apparatus  100  in the non-gloss mode. Size of a sheet P is checked and identified as a sheet having the length smaller than 210 mm or a sheet having the length not smaller than 210 mm in the sheet conveyance direction. 
     The following describes processes performed when the sheet P is identified as a sheet having the length smaller than 210 mm in the sheet conveyance direction. 
     In step S 21 , a sheet P bearing an unfixed toner image T is sent to the fixing device  5 , and the fixing device  5  fixes the toner image T on the sheet P. Specifically, the fixing belt  11  is heated up to a proper fixing temperature by heat generated by the heater  15   h  provided inside the heating roller  15 . The cam  78  of the pressure adjuster  70  is moved to adjust nip pressure applied at the nip portion N 1  so that a region in which nip pressure in a range from about 15 N/cm 2  to about 30 N/cm 2  is applied occupies less than 50 percent of the whole nip portion N 1 . Accordingly, the toner image T on the sheet P passing through the fixing device  5  is fixed on the sheet P properly in a state in which the gloss of the fixed toner image T on the sheet P is hardly increased. Alternatively, conditions of the fixing device  5  in the non-gloss mode may be equivalent to conditions of the fixing device  5  in the gloss mode according to sheet type. 
     In step S 22 , the guide  45  corrects curl of the sheet P discharged from the fixing device  5 , and guides the sheet P so that a leading edge of the sheet P enters the gloss finisher  6  properly. 
     In step S 23 , the gloss finisher  6  sandwiches the sheet P at the nip portion N 2  and feeds the sheet P. Specifically, the surface temperature of the heating roller  80  is in a range from about 80 degrees centigrade to about 100 degrees centigrade. The pressure adjuster  91  adjusts nip pressure applied at the nip portion N 2  to a level lower than the nip pressure applied at the nip portion N 2  in the gloss mode, for example, to a value not greater than 5 N/cm 2 . Under such low nip pressure, when the sheet P passes through the gloss finisher  6 , heat and pressure are hardly applied to the fixed toner image T on the sheet P at the nip portion N 2 , not increasing the gloss of the fixed toner image T. 
     In step S 24 , the sheet P bearing the fixed toner image T is discharged from the gloss finisher  6 , and passes through the guide  95  and the conveyance roller pair  7  provided on the conveyance path. 
     Referring to  FIGS. 18 and 21C , the following describes processes performed when the non-gloss mode is selected.  FIG. 21C  is a flowchart illustrating processes performed in the image forming apparatus  100  in the non-gloss mode. The following describes processes performed when the sheet P is identified as a sheet having the length not smaller than 210 mm in the sheet conveyance direction. 
     In step S 31 , a sheet P bearing an unfixed toner image T is sent to the fixing device  5 , and the fixing device  5  fixes the toner image T on the sheet P. Specifically, the fixing belt  11  is heated up to a proper fixing temperature by heat generated by the heater  15   h  provided inside the heating roller  15 . The cam  78  of the pressure adjuster  70  is moved to adjust nip pressure applied at the nip portion N 1  so that a region in which nip pressure in a range from about 15 N/cm 2  to about 30 N/cm 2  is applied occupies less than 50 percent of the whole nip portion N 1 . Accordingly, the toner image T on the sheet P passing through the fixing device  5  is fixed on the sheet P properly in a state in which the gloss of the fixed toner image T on the sheet P is hardly increased. 
     In step S 32 , the guide  45  corrects curl of the sheet P discharged from the fixing device  5 , and guides the sheet P so that a leading edge of the sheet P enters the gloss finisher  6  properly. 
     In step S 33 , the sheet P passes through a roller gap not greater than 2 mm, which is formed between the heating roller  80  and the pressing roller  90  by separating the pressing roller  90  from the heating roller  80 . 
     In step S 34 , the sheet P bearing the fixed toner image T is discharged from the gloss finisher  6  and passes through the guide  95 , and reaches the conveyance roller pair  7 . The conveyance roller pair  7  is disposed within 210 mm from the exit of the nip portion N 1  of the fixing device  5 . Therefore, the leading edge of the sheet P reaches the conveyance roller pair  7  before a trailing edge of the sheet P leaves the nip portion N 1 . Thus, the conveyance roller pair  7  sandwiches and feeds the sheet P properly. The sheet P discharged from the conveyance roller pair  7  is sent to the output device  8  (depicted in  FIG. 1 ) through the conveyance path. 
     As described above, when the length of the sheet P in the sheet conveyance direction is smaller or not smaller than 210 mm in the non-gloss mode (e.g., a normal print mode), the fixing device  5  and the gloss finisher  6  convey the sheet P stably without increasing the gloss of the toner image T. Accordingly, the toner image T having a desired gloss can be formed without changing the path line PL in either the gloss mode or the non-gloss mode, resulting in the compact image forming apparatus  100 . 
     In the gloss mode, a nip time of the fixing device  5  for nipping the sheet P may be set to not smaller than 30 msec or preferably not smaller than 60 msec. A nip time of the gloss finisher  6  may be set to not smaller than 15 msec. Accordingly, even in the gloss mode, productivity equivalent to productivity provided in the non-gloss mode can be provided. In other words, the image forming apparatus  100  can provide high productivity in either the gloss mode or the non-gloss mode. 
     Referring to  FIG. 22 , the following describes a fixing device  5 ″ as a modified example of the fixing device  5  depicted in  FIG. 2  or the fixing device  5 ′ depicted in  FIG. 4 .  FIG. 22  is a sectional view of the fixing device  5 ″. The fixing device  5 ″ does not include the tension roller  16  and the driven roller  17  depicted in  FIG. 2  or the support roller  16 ′ depicted in  FIG. 4 . The other elements of the fixing device  5 ″ are equivalent to the elements of the fixing device  5  or  5 ′. 
     As another modified example, the above-described exemplary embodiments may be applied to an image forming apparatus not including the gloss finisher  6  depicted in  FIG. 18 . 
     As yet another modified example, the above-described exemplary embodiments may be applied to an image forming apparatus including a second fixing device replacing the gloss finisher  6  depicted in  FIG. 18 . Specifically, the second fixing device includes a second nip portion at which heat and pressure are applied to a toner image on a sheet. Namely, two fixing devices, which are the fixing device  5  depicted in  FIG. 18  and the second fixing device, fix the toner image on the sheet. 
     The fixing device  5  depicted in  FIG. 3  may include the support roller  16 ′ depicted in  FIG. 9A  instead of the tension roller  16  and the driven roller  17  to provide the above-described effects provided by the fixing device  5 ′ depicted in  FIG. 9A . Similarly, the fixing device  5 ′ may include the tension roller  16  and the driven roller  17  instead of the support roller  16 ′ to provide the above-described effects provided by the fixing device  5 . 
     As described above, the image forming apparatus  100  depicted in  FIG. 1  provides improved fixing and gloss application functions. In other words, the image forming apparatus  100  can form a toner image with gloss or without gloss on various types of sheets (e.g., a thin sheet or a thick sheet, and plain paper or coated paper) without degrading productivity. 
     According to the above-described exemplary embodiments, in the fixing device  5 ,  5 ′, or  5 ″ depicted in  FIG. 3 ,  4 , or  22 , respectively, roller hardnesses of the fixing roller  12 , the separation roller  13 , and the pressing roller  14  have a proper relation. Accordingly, the fixing roller  12  is deformed to have a concave shape by pressure applied by the pressing roller  14 , and presses the fixing belt  11  against the pressing roller  14 . Similarly, the separation roller  13  is deformed to have a concave shape by pressure applied by the pressing roller  14 , and presses the fixing belt  11  against the pressing roller  14 . Consequently, the fixing belt  11  is pressed against the pressing roller  14  with a substantial force at positions upstream and downstream from the intermediate nip region I of the nip portion N 1 . Accordingly, the fixing belt  11  is pressed against the pressing roller  14  in such a manner that the fixing belt  11  is not separated from the pressing roller  14  at the intermediate nip region I. As a result, the fixing device  5 ,  5 ′, or  5 ″ can prevent or reduce shifting of a toner image formed on a sheet and fluctuation in heat applied to the sheet while maintaining proper releasing property for releasing the sheet from the fixing belt  11  and the pressing roller  14 . Further, the fixing device  5 ,  5 ′, or  5 ″ can prevent or reduce twisting of the fixing belt  11 , providing proper movement of the fixing belt  11 . 
     In the fixing device  5  depicted in  FIG. 15A , the fixing belt unit  11 U including the fixing belt  11 , the fixing roller  12 , and the separation roller  13 , and the pressing roller  14  are driven by the first motor  11   m  and the second motor  14   m  serving as separate drivers, respectively. Accordingly, the first motor  11   m  and the second motor  14   m  adjust the circumferential velocities of the fixing belt unit  11 U and the pressing roller  14 , respectively. 
     As illustrated in  FIGS. 16D and 16E , the one-way clutches  18   a  and  22   a  are provided in the first transmission  11   t  of the first driving system  11   s . Accordingly, even when the circumferential velocity of the pressing roller  14  is greater than the circumferential velocities of the fixing roller  12  and the separation roller  13 , respectively, the fixing roller  12  and the separation roller  13  rotate in accordance with rotation of the pressing roller  14 . Consequently, a relative difference between the circumferential velocities of the fixing belt unit  11 U and the pressing roller  14  at the nip portion N 1  decreases to prevent formation of a faulty fixed toner image such as a scratched toner image or an orange peel toner image. 
     As illustrated in  FIG. 15A , when the circumferential velocity of the fixing belt unit  11 U is smaller than the circumferential velocity of the pressing roller  14 , the motor controller  200  depicted in  FIG. 15B  adjusts the number of rotations of the first motor  11   m  to cause the circumferential velocity of the fixing belt unit  11 U to be equivalent to the circumferential velocity of the pressing roller  14 . Accordingly, a decreased load is applied to the pressing roller  14 . Thus, the relative difference between the circumferential velocities of the fixing belt unit  11 U and the pressing roller  14  may not generate easily. 
     Similarly, in the fixing device  5 ′ or  5 ″ depicted in  FIG. 9A  or  22 , respectively, the fixing roller  12  and the pressing roller  14  are driven by the first motor  12   m  and the second motor  14   m ′ serving as separate drivers, respectively. Accordingly, the first motor  12   m  and the second motor  14   m ′ adjust the circumferential velocities of the fixing roller  12  and the pressing roller  14 , respectively, providing effects equivalent to the above-described effects provided by the fixing device  5 . 
     The image forming apparatus  100  includes the fixing device  5 ,  5 ′, or  5 ″ depicted in  FIG. 3 ,  4 , or  22 , respectively. The fixing device  5 ,  5 ′, or  5 ″ can prevent formation of a faulty toner image due to deformation of the intermediate nip region I of the nip portion N 1  even when various types of sheets are used or sheets are conveyed in various states. Thus, the image forming apparatus  100  can form a toner image stably. As illustrated in  FIG. 18 , the gloss finisher  6  is provided downstream from the fixing device  5  in the sheet conveyance direction to adjust the gloss of the toner image. In the gloss finisher  6  also, the driving controls according to the above-described exemplary embodiments may be performed to prevent or reduce formation of a faulty toner image and creases of the sheet. 
     The present invention has been described above with reference to specific exemplary embodiments. Note that the present invention is not limited to the details of the embodiments described above, but various modifications and enhancements are possible without departing from the spirit and scope of the invention. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative exemplary embodiments may be combined with each other and/or substituted for each other within the scope of the present invention.