Patent Publication Number: US-10331062-B2

Title: Image forming apparatus and image forming method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119 to Japanese Patent Application Nos. 2016-210472, filed on Oct. 27, 2016, and 2017-022061, filed on Feb. 9, 2017, in the Japanese Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Exemplary aspects of the present disclosure relate to an image forming apparatus and an image forming method, and more particularly, to an image forming apparatus for forming a toner image on a recording medium and an image forming method performed by the image forming apparatus. 
     Description of the Background 
     Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; 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 rotator, such as a fixing roller, a fixing belt, and a fixing film, heated by a heater and an opposed rotator, such as a pressure roller and a pressure belt, pressed against the fixing rotator to form a fixing nip therebetween through which a recording medium bearing a toner image is conveyed. As the recording medium bearing the toner image is conveyed through the fixing nip, the fixing rotator and the opposed rotator apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium. 
     SUMMARY 
     This specification describes below an improved image forming apparatus. In one embodiment, the image forming apparatus includes a fixing device to fix a toner image on a recording medium. The fixing device includes a fixing rotator being rotatable in a forward direction and a backward direction opposite the forward direction. A heater heats the fixing rotator. An opposed rotator presses against an outer circumferential surface of the fixing rotator to form a fixing nip between the fixing rotator and the opposed rotator, through which the recording medium bearing the toner image is conveyed. At least one rotary body conveys the recording medium to the fixing nip. A controller rotates the fixing rotator in the forward direction to fix the toner image on the recording medium. The controller stops the heater and rotates the fixing rotator in a predetermined rotation direction when a failure occurs while the fixing device is activated. The controller determines that the predetermined rotation direction of the fixing rotator is one of the forward direction and the backward direction according to the at least one rotary body that sandwiches the recording medium, if the fixing rotator and the opposed rotator at the fixing nip and the at least one rotary body sandwich the recording medium simultaneously when the failure occurs while the fixing device is activated. 
     This specification further describes an improved image forming method. In one embodiment, the image forming method includes detecting that a recording medium is jammed; turning off a heater; starting rotating a fixing rotator at a decreased linear velocity; measuring a fixing rotator driving load imposed on the fixing rotator; determining that the fixing rotator driving load is a predetermined value or greater; braking a fixing motor; interrupting driving of the fixing rotator; and rotating the fixing rotator in a backward direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the embodiments and many of the attendant advantages and features thereof can be readily obtained and understood from the following detailed description with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic vertical cross-sectional view of an image forming apparatus according to an embodiment of the present disclosure; 
         FIG. 2  is a schematic vertical cross-sectional view of a fixing device incorporated in the image forming apparatus depicted in  FIG. 1 ; 
         FIG. 3  is a vertical cross-sectional view of the fixing device depicted in  FIG. 2 , illustrating a non-shield position of a heat shield; 
         FIG. 4  is a perspective view of the fixing device, illustrating the heat shield situated at the non-shield position depicted in  FIG. 3 ; 
         FIG. 5  is a partial perspective view of the fixing device depicted in  FIG. 4 , illustrating a support mechanism that supports the heat shield; 
         FIG. 6  is a partial perspective view of the fixing device depicted in  FIG. 4 , illustrating a driving mechanism that drives the heat shield; 
         FIG. 7A  is a schematic vertical cross-sectional view of the fixing device depicted in  FIG. 3 , illustrating a switcher that places a pressure roller at a depressurization position; 
         FIG. 7B  is a schematic vertical cross-sectional view of the fixing device depicted in  FIG. 3 , illustrating the switcher that places the pressure roller at a pressurization position; 
         FIG. 8  is a timing chart illustrating a sequence of driving times of components of the fixing device depicted in  FIG. 3  after a print job finishes normally; 
         FIG. 9  is a schematic vertical cross-sectional view of the fixing device depicted in  FIG. 3 , illustrating a fixing belt that is deformed; 
         FIG. 10  is a timing chart illustrating the sequence of the driving times of the components of the fixing device depicted in  FIG. 3  in a first case in which the fixing belt rotates forward when a failure occurs; 
         FIG. 11  is a block diagram of the image forming apparatus depicted in  FIG. 1 , illustrating a configuration of the fixing device according to a first embodiment; 
         FIG. 12  is a flowchart illustrating a control for selecting a rotation direction of the fixing belt when a failure occurs; 
         FIG. 13  is a timing chart illustrating the sequence of the driving times of the components of the fixing device depicted in  FIG. 3  in a second case in which the fixing belt rotates backward when a failure occurs; 
         FIG. 14  is a partial vertical cross-sectional view of the image forming apparatus depicted in  FIG. 1 , illustrating a configuration of the fixing device according to a second embodiment; 
         FIG. 15  is a flowchart illustrating the control for selecting the rotation direction of the fixing belt incorporated in the fixing device depicted in  FIG. 14  according to the second embodiment; 
         FIG. 16  is a diagram of the fixing belt incorporated in the fixing device depicted in  FIG. 3 , illustrating a configuration of the fixing device according to a third embodiment; 
         FIG. 17  is a flowchart illustrating the control for selecting the rotation direction of the fixing belt depicted in  FIG. 16  of the fixing device according to the third embodiment; 
         FIG. 18  is a schematic vertical cross-sectional view of a fixing device installable in the image forming apparatus depicted in  FIG. 1 , which incorporates a plurality of lateral end heaters; 
         FIG. 19  is an exploded perspective view of a nip formation unit incorporated in the fixing device depicted in  FIG. 18 ; 
         FIG. 20  is a perspective view of the nip formation unit depicted in  FIG. 19 ; and 
         FIG. 21  is a diagram of a plurality of halogen heaters and the plurality of lateral end heaters incorporated in the fixing device depicted in  FIG. 18 . 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. Also, identical or similar reference numerals designate identical or similar components throughout the several views. 
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     In describing 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 have a similar function, operate in a similar manner, and achieve a similar result. 
     As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 1 , an image forming apparatus  1  according to an embodiment is explained. 
       FIG. 1  is a schematic vertical cross-sectional view of the image forming apparatus  1 . The image forming apparatus  1  may be a copier, a facsimile machine, a printer, a multifunction peripheral or a multifunction printer (MFP) having at least one of copying, printing, scanning, facsimile, and plotter functions, or the like. According to this embodiment, the image forming apparatus  1  is a color printer that forms a color toner image on a recording medium by electrophotography. Alternatively, the image forming apparatus  1  may be a monochrome printer that forms a monochrome toner image on a recording medium. 
     Referring to  FIG. 1 , a description is provided of a construction of the image forming apparatus  1 . 
     In the drawings for explaining embodiments of this disclosure, identical reference numerals are assigned as long as discrimination is possible to components such as members and component parts having an identical function or shape, thus omitting description thereof once it is provided. 
     As illustrated in  FIG. 1 , the image forming apparatus  1  is a color laser printer. Four image forming devices  4 Y,  4 M,  4 C, and  4 K are disposed in a center portion of the image forming apparatus  1 . Although the image forming devices  4 Y,  4 M,  4 C, and  4 K contain developers (e.g., yellow, magenta, cyan, and black toners) in different colors, that is, yellow, magenta, cyan, and black corresponding to color separation components of a color image, respectively, the image forming devices  4 Y,  4 M,  4 C, and  4 K have an identical structure. 
     For example, each of the image forming devices  4 Y,  4 M,  4 C, and  4 K includes a drum-shaped photoconductor  5  serving as a latent image bearer or an image bearer that bears an electrostatic latent image and a resultant toner image and a charger  6  that charges an outer circumferential surface of the photoconductor  5 . Each of the image forming devices  4 Y,  4 M,  4 C, and  4 K further includes a developing device  7  that supplies toner to the electrostatic latent image formed on the outer circumferential surface of the photoconductor  5 , thus visualizing the electrostatic latent image as a toner image and a cleaner  8  that cleans the outer circumferential surface of the photoconductor  5 .  FIG. 1  illustrates reference numerals assigned to the photoconductor  5 , the charger  6 , the developing device  7 , and the cleaner  8  of the image forming device  4 K that forms a black toner image. However, reference numerals for the image forming devices  4 Y,  4 M, and  4 C that form yellow, magenta, and cyan toner images, respectively, are omitted. 
     Below the image forming devices  4 Y,  4 M,  4 C, and  4 K is an exposure device  9  that exposes the outer circumferential surface of the respective photoconductors  5  with laser beams. For example, the exposure device  9 , constructed of a light source, a polygon mirror, an f-θ lens, reflection mirrors, and the like, emits a laser beam onto the outer circumferential surface of the respective photoconductors  5  according to image data sent from an external device such as a client computer. 
     Above the image forming devices  4 Y,  4 M,  4 C, and  4 K is a transfer device  3 . For example, the transfer device  3  includes an intermediate transfer belt  30  serving as an intermediate transferor, four primary transfer rollers  31  serving as primary transferors, and a secondary transfer roller  36  serving as a secondary transferor. The transfer device  3  further includes a secondary transfer backup roller  32 , a cleaning backup roller  33 , a tension roller  34 , and a belt cleaner  35 . 
     The intermediate transfer belt  30  is an endless belt stretched taut across the secondary transfer backup roller  32 , the cleaning backup roller  33 , and the tension roller  34 . As a driver drives and rotates the secondary transfer backup roller  32  counterclockwise in  FIG. 1 , the secondary transfer backup roller  32  rotates the intermediate transfer belt  30  counterclockwise in  FIG. 1  in a rotation direction D 30  by friction therebetween. 
     The four primary transfer rollers  31  sandwich the intermediate transfer belt  30  together with the four photoconductors  5 , forming four primary transfer nips between the intermediate transfer belt  30  and the photoconductors  5 , respectively. The primary transfer rollers  31  are coupled to a power supply. The power supply applies at least one of a predetermined direct current (DC) voltage and a predetermined alternating current (AC) voltage to each of the primary transfer rollers  31 . 
     The secondary transfer roller  36  sandwiches the intermediate transfer belt  30  together with the secondary transfer backup roller  32 , forming a secondary transfer nip between the secondary transfer roller  36  and the intermediate transfer belt  30 . Similar to the primary transfer rollers  31 , the secondary transfer roller  36  is coupled to the power supply that applies at least one of a predetermined direct current (DC) voltage and a predetermined alternating current (AC) voltage thereto. 
     The belt cleaner  35  includes a cleaning brush and a cleaning blade that contact an outer circumferential surface of the intermediate transfer belt  30 . A waste toner drain tube extending from the belt cleaner  35  to an inlet of a waste toner container conveys waste toner collected from the intermediate transfer belt  30  by the belt cleaner  35  to the waste toner container. 
     A bottle holder  2  situated in an upper portion of the image forming apparatus  1  accommodates four toner bottles  2 Y,  2 M,  2 C, and  2 K detachably attached to the bottle holder  2 . The toner bottles  2 Y,  2 M,  2 C, and  2 K contain fresh yellow, magenta, cyan, and black toners to be supplied to the developing devices  7  of the image forming devices  4 Y,  4 M,  4 C, and  4 K, respectively. For example, the fresh yellow, magenta, cyan, and black toners are supplied from the toner bottles  2 Y,  2 M,  2 C, and  2 K to the developing devices  7  through toner supply tubes interposed between the toner bottles  2 Y,  2 M,  2 C, and  2 K and the developing devices  7 , respectively. 
     In a lower portion of the image forming apparatus  1  are a paper tray  10  serving as a sheet tray that loads a plurality of recording media P (e.g., sheets) and a feed roller  11  that picks up and feeds a recording medium P from the paper tray  10  toward the secondary transfer nip formed between the secondary transfer roller  36  and the intermediate transfer belt  30 . The recording media P may be thick paper, postcards, envelopes, plain paper, thin paper, coated paper, art paper, tracing paper, overhead projector (OHP) transparencies, and the like. Optionally, a bypass tray that loads thick paper, postcards, envelopes, thin paper, coated paper, art paper, tracing paper, OHP transparencies, and the like, rather than plain paper, may be attached to the image forming apparatus  1 . 
     A conveyance path R extends from the feed roller  11  to an output roller pair  13  to convey the recording medium P picked up from the paper tray  10  onto an outside of the image forming apparatus  1  through the secondary transfer nip. The conveyance path R is provided with a timing roller pair  12  (e.g., a registration roller pair) disposed upstream from the secondary transfer nip formed between the secondary transfer roller  36  and the intermediate transfer belt  30  in a recording medium conveyance direction A 1 . The timing roller pair  12  conveys the recording medium P conveyed from the feed roller  11  toward the secondary transfer nip at a proper time. 
     The conveyance path R is further provided with a fixing device  20  disposed downstream from the secondary transfer nip in the recording medium conveyance direction A 1 . The fixing device  20  fixes an unfixed toner image, which is transferred from the intermediate transfer belt  30  onto the recording medium P, on the recording medium P. The conveyance path R is further provided with the output roller pair  13  disposed downstream from the fixing device  20  in the recording medium conveyance direction A 1 . The output roller pair  13  ejects the recording medium P bearing the fixed toner image onto the outside of the image forming apparatus  1 , that is, an output tray  14  disposed atop the image forming apparatus  1 . The output tray  14  stocks the recording medium P ejected by the output roller pair  13 . 
     The image forming apparatus  1  further includes a controller  90  that controls an entire operation of the image forming apparatus  1 . The controller  90  controls driving of the components of the image forming apparatus  1  according to a control program stored in the controller  90 . The controller  90  may be located inside the image forming apparatus  1  or the fixing device  20 . 
     Referring to  FIG. 1 , a description is provided of an image forming operation performed by the image forming apparatus  1  having the construction described above to form a full color toner image on a recording medium P. 
     As a print job starts, a driver drives and rotates the photoconductors  5  of the image forming devices  4 Y,  4 M,  4 C, and  4 K, respectively, clockwise in  FIG. 1  in a rotation direction D 5 . The chargers  6  uniformly charge the outer circumferential surface of the respective photoconductors  5  at a predetermined polarity. 
     The exposure device  9  emits laser beams onto the charged outer circumferential surface of the respective photoconductors  5  according to yellow, magenta, cyan, and black image data, respectively, thus forming electrostatic latent images on the outer circumferential surface of the photoconductors  5 . The image data used to expose the respective photoconductors  5  is monochrome image data produced by decomposing a desired full color image into yellow, magenta, cyan, and black image data. The developing devices  7  supply yellow, magenta, cyan, and black toners to the electrostatic latent images formed on the photoconductors  5 , visualizing the electrostatic latent images as yellow, magenta, cyan, and black toner images, respectively. 
     Simultaneously, as the print job starts, the secondary transfer backup roller  32  is driven and rotated counterclockwise in  FIG. 1 , rotating the intermediate transfer belt  30  in the rotation direction D 30  by friction therebetween. The power supply applies a constant voltage or a constant current control voltage having a polarity opposite a polarity of the charged toner to the primary transfer rollers  31 , creating a transfer electric field at the respective primary transfer nips formed between the photoconductors  5  and the primary transfer rollers  31 . 
     When the yellow, magenta, cyan, and black toner images formed on the photoconductors  5  reach the primary transfer nips, respectively, in accordance with rotation of the photoconductors  5 , the yellow, magenta, cyan, and black toner images are primarily transferred from the photoconductors  5  onto the intermediate transfer belt  30  by the transfer electric field created at the primary transfer nips such that the yellow, magenta, cyan, and black toner images are superimposed successively on a same position on the intermediate transfer belt  30 . Thus, a full color toner image is formed on the outer circumferential surface of the intermediate transfer belt  30 . 
     After the primary transfer of the yellow, magenta, cyan, and black toner images from the photoconductors  5  onto the intermediate transfer belt  30 , the cleaners  8  remove residual toner failed to be transferred onto the intermediate transfer belt  30  and therefore remaining on the photoconductors  5  therefrom, respectively. Thereafter, dischargers discharge the outer circumferential surface of the respective photoconductors  5 , initializing the surface potential thereof. 
     On the other hand, the feed roller  11  disposed in the lower portion of the image forming apparatus  1  is driven and rotated to feed a recording medium P from the paper tray  10  toward the timing roller pair  12  through the conveyance path R. The timing roller pair  12  temporarily halts the recording medium P conveyed through the conveyance path R. 
     Thereafter, the timing roller pair  12  resumes rotation at a predetermined time to convey the recording medium P to the secondary transfer nip at a time when the full color toner image formed on intermediate transfer belt  30  reaches the secondary transfer nip. The secondary transfer roller  36  is applied with a transfer voltage having a polarity opposite a polarity of the charged yellow, magenta, cyan, and black toners constructing the full color toner image formed on the intermediate transfer belt  30 , thus creating a transfer electric field at the secondary transfer nip. 
     The transfer electric field secondarily transfers the yellow, magenta, cyan, and black toner images constructing the full color toner image formed on the intermediate transfer belt  30  onto the recording medium P collectively. After the secondary transfer of the full color toner image from the intermediate transfer belt  30  onto the recording medium P, the belt cleaner  35  removes residual toner failed to be transferred onto the recording medium P and therefore remaining on the intermediate transfer belt  30  therefrom. The removed toner is conveyed and collected into the waste toner container. 
     Thereafter, the recording medium P bearing the full color toner image is conveyed to the fixing device  20  that fixes the full color toner image on the recording medium P. Thereafter, the recording medium P bearing the fixed full color toner image is ejected by the output roller pair  13  onto the outside of the image forming apparatus  1 , that is, the output tray  14  that stocks the recording medium P. 
     The above describes the image forming operation of the image forming apparatus  1  to form the full color toner image on the recording medium P. Alternatively, for example, the image forming apparatus  1  may form a monochrome toner image by using any one of the four image forming devices  4 Y,  4 M,  4 C, and  4 K or may form a bicolor toner image or a tricolor toner image by using two or three of the image forming devices  4 Y,  4 M,  4 C, and  4 K. 
     Referring to  FIGS. 2 and 3 , a description is provided of a construction of the fixing device  20  incorporated in the image forming apparatus  1  described above. 
       FIG. 2  is a vertical cross-sectional view of the fixing device  20 , illustrating a shield position.  FIG. 3  is a vertical cross-sectional view of the fixing device  20 , illustrating a non-shield position. 
     As illustrated in  FIG. 2 , the fixing device  20  (e.g., a fuser or a fusing unit) includes a fixing belt  21 , a pressure roller  22 , a halogen heater  23 , a nip formation pad  24 , a stay  25 , a reflector  26 , a heat shield  27 , a temperature sensor  28 , and a recording medium sensor  29 . 
     The fixing belt  21  serves as a fixing rotator or a fixing member that is rotatable in a rotation direction D 21  and a direction opposite the rotation direction D 21 . The pressure roller  22  serves as an opposed rotator, an opposed member, or a pressure rotator that is rotatable in a rotation direction D 22  and a direction opposite the rotation direction D 22 . The pressure roller  22  separably or inseparably contacts an outer circumferential surface of the fixing belt  21 . The halogen heater  23  serves a heater or a heat source that heats the fixing belt  21 . The nip formation pad  24  is disposed opposite an inner circumferential surface of the fixing belt  21  and presses against the pressure roller  22  via the fixing belt  21  to form a fixing nip N. The stay  25  serves as a support that supports the nip formation pad  24 . The reflector  26  reflects heat or light radiated from the halogen heater  23  to the fixing belt  21 . The temperature sensor  28  serves as a first temperature detector that detects the temperature of the outer circumferential surface of the fixing belt  21 . The recording medium sensor  29  serves as a recording medium detector that detects the recording medium P. 
     The fixing belt  21  and the components disposed inside a loop formed by the fixing belt  21 , that is, the halogen heater  23 , the nip formation pad  24 , the stay  25 , the reflector  26 , and the heat shield  27 , may construct a belt unit  21 U separably coupled with the pressure roller  22 . The heat shield  27  shields the fixing belt  21  from the halogen heater  23  in a non-conveyance span of the fixing belt  21  where the recording medium P is not conveyed. 
     A detailed description is now given of a construction of the fixing belt  21 . 
     The fixing belt  21  is a thin, flexible endless belt or film. For example, the fixing belt  21  includes a base layer constructing the inner circumferential surface of the fixing belt  21  and a release layer constructing the outer circumferential surface of the fixing belt  21 . The base layer constructing the inner circumferential surface of the fixing belt  21  is made of metal such as nickel and SUS stainless steel or resin such as polyimide (PI). The release layer constructing the outer circumferential surface of the fixing belt  21  is made of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), polytetrafluoroethylene (PTFE), or the like. Optionally, an elastic layer made of rubber such as silicone rubber, silicone rubber foam, and fluoro rubber may be interposed between the base layer and the release layer. 
     If the fixing belt  21  does not incorporate the elastic layer, the fixing belt  21  has a decreased thermal capacity that improves a fixing property of being heated quickly. 
     However, as the pressure roller  22  and the fixing belt  21  sandwich and press an unfixed toner image T on the recording medium P to fix the toner image T on the recording medium P while the recording medium P passes through the fixing nip N, slight surface asperities of the fixing belt  21  may be transferred onto the toner image T on the recording medium P, producing variation in gloss of a solid portion of the toner image T on the recording medium P. To address this circumstance, the fixing belt  21  incorporates the elastic layer having a thickness not smaller than 100 micrometers. The elastic layer having the thickness not smaller than 100 micrometers elastically deforms to absorb slight surface asperities of the fixing belt  21 , preventing variation in gloss of the toner image T on the recording medium P. 
     According to this embodiment, in order to decrease the thermal capacity of the fixing belt  21 , the fixing belt  21  is thin and has a decreased loop diameter. For example, the fixing belt  21  is constructed of the base layer having a thickness in a range of from 20 micrometers to 50 micrometers; the elastic layer having a thickness in a range of from 100 micrometers to 300 micrometers; and the release layer having a thickness in a range of from 10 micrometers to 50 micrometers. Thus, the fixing belt  21  has a total thickness not greater than 1 mm. 
     A loop diameter of the fixing belt  21  is in a range of from 20 mm to 40 mm. In order to decrease the thermal capacity of the fixing belt  21  further, the fixing belt  21  may have a total thickness not greater than 0.20 mm and preferably not greater than 0.16 mm. Additionally, the loop diameter of the fixing belt  21  may not be greater than 30 mm. 
     A detailed description is now given of a construction of the pressure roller  22 . 
     The pressure roller  22  is constructed of a core bar  22   a ; an elastic layer  22   b  coating the core bar  22   a  and made of silicone rubber foam, silicone rubber, fluoro rubber, or the like; and a release layer  22   c  coating the elastic layer  22   b  and made of PFA, PTFE, or the like. A switcher  60  described below presses the pressure roller  22  against the nip formation pad  24  via the fixing belt  21 , bringing the pressure roller  22  into contact with the fixing belt  21 . 
     The pressure roller  22  pressingly contacting the fixing belt  21  deforms the elastic layer  22   b  of the pressure roller  22  at the fixing nip N formed between the pressure roller  22  and the fixing belt  21 , thus defining the fixing nip N having a predetermined length in the recording medium conveyance direction A 1 . According to this embodiment, the pressure roller  22  is pressed against the fixing belt  21 . Alternatively, the pressure roller  22  may merely contact the fixing belt  21  with no pressure therebetween. 
     A driver (e.g., a fixing motor  50  described below) disposed inside the image forming apparatus  1  depicted in  FIG. 1  drives and rotates the pressure roller  22 . As the driver drives and rotates the pressure roller  22 , a driving force of the driver is transmitted from the pressure roller  22  to the fixing belt  21  at the fixing nip N, thus rotating the fixing belt  21  in accordance with rotation of the pressure roller  22  by friction between the pressure roller  22  and the fixing belt  21 . Alternatively, the driver may also be connected to the fixing belt  21  to drive and rotate the fixing belt  21 . 
     According to this embodiment, the pressure roller  22  is a solid roller. Alternatively, the pressure roller  22  may be a hollow roller. In this case, a heater such as a halogen heater may be disposed inside the hollow roller. The elastic layer  22   b  may be made of solid rubber. Alternatively, if no heater is situated inside the pressure roller  22 , the elastic layer  22   b  may be made of sponge rubber. The sponge rubber is more preferable than the solid rubber because the sponge rubber has an increased insulation that draws less heat from the fixing belt  21 . 
     A detailed description is now given of a configuration of the halogen heater  23 . 
     The halogen heater  23  is disposed opposite the inner circumferential surface of the fixing belt  21  and upstream from the fixing nip N in the recording medium conveyance direction A 1 . For example, a hypothetical line L is defined by a center Q of the fixing nip N in the recording medium conveyance direction A 1  and a rotation axis O of the pressure roller  22 . The halogen heater  23  is disposed upstream from the hypothetical line L in the recording medium conveyance direction A 1 , that is, below the hypothetical line L in  FIG. 2 . 
     The power supply situated inside the image forming apparatus  1  supplies power to the halogen heater  23  so that the halogen heater  23  generates heat. The controller  90  (e.g., a processor), that is, a central processing unit (CPU) provided with a random-access memory (RAM) and a read-only memory (ROM), for example, operatively connected to the halogen heater  23  and the temperature sensor  28  controls the halogen heater  23  based on the temperature of the outer circumferential surface of the fixing belt  21  that is detected by the temperature sensor  28 . Thus, the temperature of the fixing belt  21  is adjusted to a desired fixing temperature. 
     Instead of the temperature sensor  28  that detects the temperature of the fixing belt  21 , a temperature sensor that detects the temperature of the pressure roller  22  may be disposed opposite the pressure roller  22  so that the temperature of the fixing belt  21  is estimated based on a temperature of the pressure roller  22  that is detected by the temperature sensor. 
     According to this embodiment, the fixing device  20  includes two halogen heaters  23 . Alternatively, the fixing device  20  may include one halogen heater  23  or three or more halogen heaters  23  according to the size of the recording medium P or the like available in the image forming apparatus  1 . Alternatively, instead of the halogen heater  23 , an induction heater (IH), a resistive heat generator, a carbon heater, or the like may be employed as a heater that heats the fixing belt  21 . 
     A detailed description is now given of a construction of the nip formation pad  24 . 
     The nip formation pad  24  includes a base pad  241  and a low-friction slide sheet  240  disposed on an opposed face of the base pad  241  disposed opposite the fixing belt  21 . The base pad  241  extends in a longitudinal direction thereof parallel to an axial direction of the fixing belt  21  or the pressure roller  22 . 
     As the base pad  241  is exerted with pressure from the pressure roller  22 , the base pad  241  defines the shape of the fixing nip N. According to this embodiment, the fixing nip N is planar. Alternatively, the fixing nip N may define a recess, a curve, or other shapes. 
     As the fixing belt  21  rotating in the rotation direction D 21  slides over the base pad  241 , the slide sheet  240  decreases friction between the fixing belt  21  and the base pad  241 . If the base pad  241  is made of a low-friction material, the slide sheet  240  may not be interposed between the fixing belt  21  and the base pad  241 . 
     The base pad  241  is made of a heat resistant material resistant against temperatures of about 200 degrees centigrade. Thus, the nip formation pad  24  is immune from thermal deformation at temperatures in a fixing temperature range desirable to fix the toner image T on the recording medium P, retaining the shape of the fixing nip N and quality of the toner image T formed on the recording medium P. 
     Additionally, the base pad  241  is made of a rigid material to secure the mechanical strength of the nip formation pad  24 . For example, the base pad  241  is made of resin such as polyether sulfone (PES), polyphenylene sulfide (PPS), liquid crystal polymer (LCP), polyether nitrile (PEN), polyamide imide (PAI), and polyether ether ketone (PEEK). Alternatively, the base pad  241  may be made of metal or ceramics. 
     The base pad  241  is mounted on and supported by the stay  25 . Accordingly, even if the nip formation pad  24  receives pressure from the pressure roller  22 , the nip formation pad  24  is not bent by the pressure and therefore produces a uniform nip length in the recording medium conveyance direction A 1  throughout the entire width of the pressure roller  22  in the axial direction thereof. The stay  25  is made of metal having an increased mechanical strength, such as stainless steel and iron, to prevent bending of the nip formation pad  24 . 
     A detailed description is now given of a configuration of the reflector  26 . 
     The reflector  26  is secured to and supported by the stay  25  such that the reflector  26  is disposed opposite the halogen heater  23 . The reflector  26  reflects radiant heat or light radiated from the halogen heater  23  toward the fixing belt  21 , suppressing conduction of heat from the halogen heater  23  to the stay  25  and the like and thereby heating the fixing belt  21  effectively and saving energy. The reflector  26  is made of aluminum, stainless steel, or the like. If the reflector  26  is constructed of an aluminum base treated with vapor deposition of silver having a decreased emissivity and an increased reflectance, the reflector  26  enhances heating efficiency in heating the fixing belt  21 . 
     A detailed description is now given of a configuration of the heat shield  27 . 
     The heat shield  27  is manufactured by contouring a metal plate having a thickness in a range of from 0.1 mm to 1.0 mm into an arch in cross-section along the inner circumferential surface of the fixing belt  21 . The heat shield  27  is interposed between the halogen heater  23  and the fixing belt  21  and movable in a circumferential direction of the fixing belt  21 . 
     According to this embodiment, as illustrated in  FIGS. 2 and 3 , the fixing belt  21  has a circumferential heated span α and a circumferential non-heated span β spanning in the circumferential direction thereof. The circumferential heated span α is heated directly by the halogen heater  23 . The circumferential non-heated span β is not heated by the halogen heater  23  directly. The circumferential heated span α is disposed opposite a front of the halogen heater  23  directly. The circumferential non-heated span β is disposed opposite components (e.g., the reflector  26 , the stay  25 , and the nip formation pad  24 ) interposed between the halogen heater  23  and the fixing belt  21  and secured to side plates or the like and therefore is not heated by the halogen heater  23  directly. 
     As illustrated in  FIG. 2 , when the heat shield  27  is requested to shield the fixing belt  21  from the halogen heater  23 , the heat shield  27  selectively moves to one or more shield positions where the heat shield  27  is disposed opposite the circumferential heated span α of the fixing belt  21 . Conversely, as illustrated in  FIG. 3 , when the heat shield  27  is not requested to shield the fixing belt  21  from the halogen heater  23 , the heat shield  27  moves to the non-shield position where the heat shield  27  is disposed opposite the circumferential non-heated span β of the fixing belt  21 . Thus, the entire heat shield  27  is retracted to the non-shield position where the heat shield  27  is situated behind the reflector  26  and the stay  25 . 
     As the heat shield  27  rotates, the heat shield  27  changes the area of the circumferential heated span α of the fixing belt  21 , adjusting an amount of radiant heat radiated from the halogen heater  23  to the fixing belt  21 . Since the heat shield  27  is requested to be heat resistant, the heat shield  27  is made of metal such as aluminum, iron, and stainless steel or ceramics. 
     A detailed description is now given of a configuration of the recording medium sensor  29 . 
     The recording medium sensor  29  is disposed downstream from the fixing nip N in a recording medium conveyance direction A 2 . The recording medium sensor  29  detects the recording medium P as the recording medium P passes over the recording medium sensor  29 . For example, the recording medium sensor  29  is a photointerrupter or the like. 
     A description is provided of a configuration of a plurality of flanges  40 . 
       FIG. 4  is a perspective view of the fixing device  20 , illustrating the heat shield  27  situated at the non-shield position depicted in  FIG. 3 . As illustrated in  FIG. 4 , the fixing device  20  further includes the plurality of flanges  40  serving as a plurality of belt holders disposed opposite the inner circumferential surface of the fixing belt  21  at both lateral ends of the fixing belt  21  in the axial direction thereof, respectively. The flanges  40  are inserted into both lateral ends of the fixing belt  21  in the axial direction thereof, respectively, to rotatably support the fixing belt  21 . The flanges  40 , the halogen heater  23 , and the stay  25  are secured to and supported by the pair of side plates of the fixing device  20 . 
     A description is provided of a support mechanism of the fixing device  20 , which supports the heat shield  27 . 
       FIG. 5  is a partial perspective view of the fixing device  20 , illustrating the support mechanism that supports the heat shield  27  situated at the non-shield position depicted in  FIG. 3 . As illustrated in  FIG. 5 , the heat shield  27  is supported by the flange  40  through a slider  41  that is mounted on the flange  40  and arch-shaped. 
     For example, the heat shield  27  includes a projection  27   a  disposed at a lateral end of the heat shield  27  in a longitudinal direction thereof. The slider  41  includes a slit  41   a . As the projection  27   a  is inserted into the slit  41   a , the heat shield  27  is coupled with the slider  41 . The slider  41  further includes a projection  41   b . The flange  40  includes a groove  40   a  that is arch-shaped. As the projection  41   b  is inserted into the groove  40   a , the slider  41  moves and slides along the groove  40   a.    
     Hence, the heat shield  27  is pivotally movable in a circumferential direction of the flange  40  together with the slider  41 . According to this embodiment, each of the flange  40  and the slider  41  is made of resin. 
       FIG. 5  illustrates the support mechanism disposed at one lateral end of the heat shield  27  in the longitudinal direction thereof. Similarly, at another lateral end of the heat shield  27  in the longitudinal direction thereof, the heat shield  27  is pivotally supported by the flange  40  through the slider  41 . 
     A description is provided of a driving mechanism of the fixing device  20 , which drives the heat shield  27 . 
       FIG. 6  is a partial perspective view of the fixing device  20 , illustrating the driving mechanism that drives the heat shield  27  situated at the non-shield position depicted in  FIG. 3 . As illustrated in  FIG. 6 , the driving mechanism that drives the heat shield  27  includes a motor  42  serving as a driving source and a driving force transmitter  46  including a gear train constructed of a plurality of gears  43 ,  44 , and  45 . 
     The gear  43  situated at one end of the gear train is coupled to the motor  42 . The gear  45  situated at another end of the gear train meshes with a gear portion  41   c  mounted on the slider  41  in a circumferential direction thereof. As the motor  42  is driven, a driving force generated by the motor  42  is transmitted to the slider  41  through the gear train. Accordingly, the heat shield  27  pivots in a forward direction in which the heat shield  27  moves from the circumferential non-heated span β to the circumferential heated span α and a backward direction in which the heat shield  27  moves from the circumferential heated span α to the circumferential non-heated span β. For example, the motor  42  is a stepping motor. In this case, the controller  90  controls the position of the heat shield  27  by changing the number of driving pulses. Alternatively, instead of the stepping motor, the motor  42  may be a direct current (DC) motor or the like. 
     The temperature sensor  28  depicted in  FIG. 2  or the like detects the temperature of the fixing belt  21  at a center and a lateral end of the fixing belt  21  in the axial direction thereof. The controller  90  controls the heat shield  27  to change a shield span where the heat shield  27  shields the fixing belt  21  from the halogen heater  23  according to a detected temperature of the fixing belt  21  or a temperature differential between a temperature of the center of the fixing belt  21  and a temperature of the lateral end of the fixing belt  21 . A detailed description of motion of the heat shield  27  is omitted. 
     A description is provided of a construction of the switcher  60  that moves the pressure roller  22  between a pressurization position and a depressurization position at the fixing nip N. 
       FIG. 7A  is a schematic vertical cross-sectional view of the fixing device  20 , illustrating the switcher  60  that places the pressure roller  22  at the depressurization position.  FIG. 7B  is a schematic vertical cross-sectional view of the fixing device  20 , illustrating the switcher  60  that places the pressure roller  22  at the pressurization position. The switcher  60  switches pressure exerted by the pressure roller  22  to the fixing belt  21  at the fixing nip N. As the switcher  60  presses the pressure roller  22  against the fixing belt  21 , the pressure roller  22  is at the pressurization position where the pressure roller  22  exerts pressure to the fixing belt  21  at the fixing nip N. Conversely, as the switcher  60  separates the pressure roller  22  from the fixing belt  21 , the pressure roller  22  is at the depressurization position where the pressure roller  22  releases pressure exerted to the fixing belt  21  at the fixing nip N. 
     The switcher  60  also detects where the pressure roller  22  is at the fixing nip N, the pressurization position or the depressurization position. The switcher  60  includes a lever  61 , a cam  62 , a resilient member  63 , a feeler  64  serving as a detected member, and a sensor  65  serving as a detector as main components. 
     The lever  61  is supported by a shaft O 1  disposed at one end of the lever  61  in a longitudinal direction thereof such that the lever  61  is pivotable about the shaft O 1 . A cam face (e.g., an outer circumferential face) of the cam  62  contacts another end of the lever  61  in the longitudinal direction thereof. The core bar  22   a  exposed at each lateral end of the pressure roller  22  in the axial direction thereof contacts an intermediate portion of the lever  61  in the longitudinal direction thereof. 
     The cam  62  is supported by a shaft O 2  that is eccentrically disposed such that the cam  62  is pivotable about the shaft O 2 . A motor (e.g., a pressurization-depressurization motor) or the like that serves as a driver drives and rotates the cam  62 . The resilient member  63  (e.g., a tension spring) generates a resilient force that presses the lever  61  against the cam face of the cam  62 . 
     The pressure roller  22  is supported such that the pressure roller  22  slides horizontally in  FIGS. 7A and 7B  to come into contact with and separate from the fixing belt  21 . As illustrated in  FIG. 7A , when the shaft O 2  of the cam  62  is nearest to the lever  61  and the cam face defining a shortest diameter of the cam  62  from the shaft O 2  contacts the lever  61 , the resilient force generated by the resilient member  63  biases the lever  61  in a separation direction in which the lever  61  separates from the core bar  22   a  of the pressure roller  22 . 
     Accordingly, the pressure roller  22  separates from the fixing belt  21 , releasing pressure exerted to the fixing belt  21  at the fixing nip N. Conversely, as illustrated in  FIG. 7B , when the shaft O 2  of the cam  62  is farthest from the lever  61  and the cam face defining a greatest diameter of the cam  62  from the shaft O 2  contacts the lever  61 , the lever  61  presses the core bar  22   a  of the pressure roller  22  toward the fixing belt  21  with pressure received from the cam  62 . Thus, the pressure roller  22  exerts pressure to the fixing belt  21  at the fixing nip N. 
     Referring to  FIG. 8 , a description is provided of a sequence of driving times of the components of the fixing device  20  after a print job finishes normally. 
       FIG. 8  is a timing chart illustrating the sequence of the driving times of the components of the fixing device  20  after the print job finishes normally. 
     The controller  90  of the image forming apparatus  1  depicted in  FIG. 1  sends a stop signal to the fixing device  20  at a time when a trailing edge of a last recording medium P of a print job is ejected from the fixing nip N. After the fixing device  20  receives the stop signal, the halogen heater  23  is turned off. Subsequently, a heater relay is turned off. Thereafter, if the fixing device  20  incorporates the heat shield  27 , the heat shield  27  returns to a default position, that is, a home position (HP). 
     Although the fixing belt  21  continues rotating, when a predetermined time t 1  elapses after the heat shield  27  returns to the default position, the controller  90  stops the fixing motor  50 . After the fixing motor  50  stops, the switcher  60  depicted in  FIGS. 7A and 7B  causes the pressure roller  22  to release pressure exerted to the fixing belt  21  at the fixing nip N. 
     The fixing belt  21  continues rotating for the predetermined time t 1  after the heat shield  27  returns to the default position to prevent the fixing belt  21  from being heated locally with residual heat and therefore prevent the fixing belt  21  from suffering from temperature deviation. The time t 1  is set by estimating a time taken to even an amount of heat stored in the fixing belt  21 . For example, end of the time t 1  is determined based on the temperature of the fixing belt  21  detected by the temperature sensor  28 . Alternatively, end of the time t 1  is defined as a time when a preset time has elapsed. 
     If a user removes a recording medium P jammed at the fixing nip N while the pressure roller  22  exerts pressure to the fixing belt  21  at the fixing nip N, the user pulls the recording medium P sandwiched between the fixing belt  21  and the pressure roller  22  at the fixing nip N. Accordingly, the recording medium P may damage the fixing belt  21  and the pressure roller  22  or the recording medium P may be torn off, making it difficult for the user to remove the jammed recording medium P properly. Conversely, if the pressure roller  22  releases pressure exerted to the fixing belt  21  at the fixing nip N before the fixing belt  21  interrupts rotation, the fixing belt  21  may slip and suffer from temperature increase locally, resulting in deformation of the fixing belt  21 . To address those circumstances, after the fixing belt  21  interrupts rotation, the switcher  60  places the pressure roller  22  at the depressurization position where the pressure roller  22  releases pressure exerted to the fixing belt  21  at the fixing nip N. 
     The above describes the sequence of the driving times of the components of the fixing device  20  when the print job finishes normally. However, during image formation from input of an image signal to the image forming apparatus  1  until ejection of the recording medium P bearing the fixed toner image T onto the output tray  14 , if the recording medium P is jammed in the conveyance path R or a failure occurs in any of the components of the image forming apparatus  1 , the image forming apparatus  1  may perform emergency stop. If the fixing belt  21  interrupts rotation immediately when the image forming apparatus  1  stops emergently, the fixing belt  21  may be heated by residual heat and may suffer from temperature irregularity, resulting in deformation of the fixing belt  21  as illustrated in  FIG. 9  with a deformation region Q 1 .  FIG. 9  is a schematic vertical cross-sectional view of the fixing device  20 , illustrating the fixing belt  21  that is deformed. 
     A description is provided of a construction of a first comparative fixing device. 
     The first comparative fixing device includes a heater, a fixing rotator heated by the heater with radiant heat, and an opposed rotator that contacts the fixing rotator to form a fixing nip therebetween. As an image forming apparatus incorporating the first comparative fixing device starts a print job, a toner image is transferred onto a sheet serving as a recording medium. As the sheet passes through the fixing nip formed between the opposed rotator and the fixing rotator heated to a predetermined temperature, the fixing rotator and the opposed rotator melt and fix the toner image on the sheet under heat and pressure. 
     Compared to other components of the image forming apparatus, the first comparative fixing device consumes more power and is requested to save energy. To address this request, a second comparative fixing device includes a fixing rotator that is an endless belt (e.g., a fixing belt) that has a reduced thermal capacity and is thin like film. A heater heats the fixing belt directly not through a metal thermal conductor also serving as a support. Even if the second comparative fixing device is installed in the high speed image forming apparatus, the second comparative fixing device attains an improved fixing performance. The second comparative fixing device includes a pressure roller disposed opposite an outer circumferential surface of the fixing belt and a nip formation pad stationarily disposed inside a loop formed by the fixing belt. The pressure roller is pressed against the nip formation pad via the fixing belt to form a fixing nip between the pressure roller and the fixing belt. 
     However, in the second comparative fixing device in which the heater heats the fixing belt directly, the fixing belt is thin and has an enhanced responsiveness to radiant heat radiated from the heater. Accordingly, when the radiant heat from the heater irradiates the fixing belt while the fixing belt interrupts rotation, the temperature of the fixing belt increases excessively to a temperature higher than a heat resistant temperature of silicone rubber and fluoro rubber contained in the fixing belt and the pressure roller. 
     To address this circumstance, a controller performs a control to rotate the fixing belt forward to prevent temperature increase of the fixing belt. For example, when the print job finishes normally, the fixing belt rotates forward further for a predetermined time after the heater is powered off. Thereafter, the fixing belt interrupts rotation. Thus, the controller evens the temperature of the fixing belt in a circumferential direction of the fixing belt and prevents breakage of the fixing belt due to overheating of the fixing belt and the pressure roller. 
     An upstream recording medium detector and a downstream recording medium detector are disposed upstream and downstream from the fixing nip in a recording medium conveyance path, respectively. The controller performs a control to rotate the fixing belt backward to prevent temperature increase of the fixing belt. For example, the fixing belt rotates backward further for a predetermined time after the heater is powered off. Thereafter, the fixing belt interrupts rotation. Thus, even if a recording medium is jammed, the controller prevents breakage of the fixing belt due to overheating of the fixing belt and the pressure roller. 
     However, if no recording medium is between the upstream recording medium detector and the downstream recording medium detector, when a failure occurs, for example, when the recording medium is jammed, and the image forming apparatus stops emergently, the controller performs the control to rotate the fixing belt forward to prevent temperature increase of the fixing belt. Accordingly, even if the controller performs the control to rotate the fixing belt forward to prevent temperature increase of the fixing belt, the components of the second comparative fixing device, for example, the fixing belt, may be damaged according to a condition of the recording medium or the second comparative fixing device when the image forming apparatus stops emergently. 
     The driving times of the components of the fixing device  20  when a failure occurs differ between two cases: a first case in which the fixing belt  21  rotates forward and a second case in which the fixing belt  21  rotates backward. 
     Since the fixing belt  21  rotates forward during image formation, the fixing belt  21  is not configured to rotate backward during image formation. Hence, if the fixing belt  21  is configured to rotate forward even during emergency stop, damage to the fixing belt  21  is reduced. 
     Even if the recording medium P remains in the conveyance path R when the recording medium P is jammed, if a leading edge of the recording medium P has passed through the fixing nip N and has separated from the fixing belt  21 , even if the fixing belt  21  further rotates forward in a feeding direction in which the fixing belt  21  feeds the recording medium P downstream in the recording medium conveyance direction A 2  depicted in  FIG. 2 , the recording medium P is not wound around the fixing belt  21 . 
     Even before the recording medium P remaining in the conveyance path R passes through the fixing nip N when the recording medium P is jammed, a separator disposed downstream from an exit of the fixing nip N in the recording medium conveyance direction A 2  separates the recording medium P from the fixing belt  21 . Accordingly, even if the fixing belt  21  rotates forward further, the recording medium P is not wound around the fixing belt  21 . Hence, as the sequence of the driving times of the components of the fixing device  20  during normal emergency stop, after the halogen heater  23  is turned off, the fixing belt  21  rotates forward to attain stable conveyance of the recording medium P. 
     Referring to  FIG. 10 , a description is provided of the sequence of the driving times of the components of the fixing device  20  if the fixing belt  21  rotates forward when a failure occurs, which is called a forward rotation against failure. 
       FIG. 10  is a timing chart illustrating the sequence of the driving times of the components of the fixing device  20  in the first case in which the fixing belt  21  rotates forward when a failure occurs. 
     When the fixing device  20  receives a failure detection signal from the image forming apparatus  1 , the halogen heater  23  is turned off. Subsequently, the heater relay is turned off. After the heater relay is turned off, the controller  90  slows down the linear velocity of the fixing belt  21  to a low linear velocity that is lower than an image formation linear velocity at which the fixing belt  21  conveys the recording medium P to fix the toner image T on the recording medium P. The fixing belt  21  rotates forward for a predetermined time Ta at the low linear velocity. After the time Ta elapses, the fixing belt  21  interrupts rotation. Thereafter, the switcher  60  moves the pressure roller  22  to the depressurization position where the pressure roller  22  releases pressure exerted to the fixing belt  21  at the fixing nip N. After changing the driving times of the components of the fixing device  20  described above, the controller  90  notifies the user of the failure with a notification device of the image forming apparatus  1 . For example, the notification device is a control panel that displays a message, an indicator lamp that emits light, an alarm that generates a noise, or the like, to alert the user to the failure. After notifying the user of the failure, the controller  90  prohibits the user from using the fixing device  20 . 
     While the fixing belt  21  rotates forward after the halogen heater  23  is turned off, the halogen heater  23  heats the fixing belt  21  with residual heat in an unshielded span of the circumferential heated span α depicted in  FIG. 2  of the fixing belt  21  where the heat shield  27  does not shield the fixing belt  21  from the halogen heater  23 . As the fixing belt  21  rotates forward, the recording medium P passes through the fixing nip N and draws heat from the fixing belt  21 , causing no substantial temperature deviation of the fixing belt  21 . Accordingly, the fixing device  20  prevents temperature irregularity and deformation of the fixing belt  21 . 
     The time Ta for which the fixing belt  21  rotates forward is long enough for the fixing belt  21  to conduct heat to the recording medium P as the entire circumference of the fixing belt  21  passes through the fixing nip N and to conduct heat to the pressure roller  22  after the recording medium P passes through the fixing nip N. For example, the time Ta is a time for which the fixing belt  21  performs one rotation. The controller  90  switches the linear velocity of the fixing belt  21  while the fixing belt  21  rotates forward to the low linear velocity to facilitate conduction of heat from the fixing belt  21  to the recording medium P, thus reducing temperature deviation of the fixing belt  21 . 
     A description is provided of the sequence of the driving times of the components of the fixing device  20  if the fixing belt  21  rotates backward when a failure occurs, which is called a backward rotation against failure. 
     The image forming apparatus  1  uses various types of the recording media P (e.g., sheets). For example, the image forming apparatus  1  forms a toner image on a long-length sheet used in electronics retail stores more frequently. The long-length sheet has a thickness, a length, a type, and a surface that vary depending on the usage of the long-length sheet. Hence, the long-length sheet is more susceptible to jamming during conveyance and erroneous setting by the user than plain paper. 
     When the long-length sheet is jammed during conveyance, the feed roller  11 , the timing roller pair  12 , the secondary transfer roller  36 , and the secondary transfer backup roller  32 , which are hereinafter referred to as the plurality of rotary bodies, and the fixing belt  21  and the pressure roller  22 , which are hereinafter referred to as the fixing nip N, may sandwich the recording medium P simultaneously. In this case, the recording medium P is sandwiched with a substantial force by the plurality of rotary bodies, that is, the feed roller  11 , the timing roller pair  12 , the secondary transfer roller  36 , and the secondary transfer backup roller  32 , which are disposed upstream from the fixing nip N in the recording medium conveyance direction A 1 . Accordingly, even if the controller  90  controls the fixing belt  21  to rotate forward, a load imposed on the fixing belt  21  (e.g., a fixing rotator driving load) when driving the fixing belt  21  may be too great to rotate the fixing belt  21 . Consequently, the halogen heater  23  may heat the fixing belt  21  with residual heat, generating temperature irregularity of the fixing belt  21 . 
     In addition to the long-length sheet, the load imposed on the fixing belt  21  when driving the fixing belt  21  may increase according to the type, the thickness, or the like of the recording medium P when the recording medium P is jammed. Accordingly, the fixing belt  21  may not rotate. 
     For example, the timing roller pair  12  has a great driving torque and sandwiches the recording medium P with a substantial force. If the fixing belt  21  and the pressure roller  22  at the fixing nip N and the timing roller pair  12  sandwich the recording medium P simultaneously when a failure occurs, a conveyance force of the fixing belt  21  to convey the recording medium P is smaller than a conveyance force of the timing roller pair  12  to convey the recording medium P. Accordingly, even if the controller  90  controls the fixing belt  21  to rotate in a forward direction, the controller  90  may fail to rotate the fixing belt  21  as intended. To address this circumstance, the controller  90  is requested to determine to rotate the fixing belt  21  in a backward direction opposite the forward direction. 
     Conversely, if the timing roller pair  12  does not sandwich the recording medium P, that is, if the fixing nip N formed by the fixing belt  21  and the pressure roller  22  and the secondary transfer nip defined by the secondary transfer roller  36  pressed against the secondary transfer backup roller  32  via the intermediate transfer belt  30  sandwich the recording medium P simultaneously when a failure occurs, the conveyance force of the fixing belt  21  to convey the recording medium P is greater than a conveyance force of the secondary transfer nip (e.g., the secondary transfer roller  36 ) to convey the recording medium P. Accordingly, the controller  90  controls the fixing belt  21  to rotate forward as intended. In this case, the controller  90  is requested to determine to rotate the fixing belt  21  in the forward direction. 
     To address this request, according to this embodiment, when a failure occurs, if the fixing nip N and at least one of the plurality of rotary bodies sandwich a single recording medium P, that is, an identical recording medium P, simultaneously, the controller  90  determines to rotate the fixing belt  21  forward in the predetermined forward direction or backward in the backward direction opposite the forward direction according to the at least one of the plurality of rotary bodies sandwiching the recording medium P. For example, according to this embodiment, if a conveyance nip other than the fixing nip N sandwiches the recording medium P, the controller  90  determines in which direction the fixing belt  21  rotates, forward or backward, based on at least one of a rotation load imposed on the fixing belt  21 , presence (e.g., a position) of the recording medium P detected by a recording medium detector, and a rotation speed of the fixing belt  21  detected by a speed detector when the fixing belt  21  starts rotation. 
     A description is provided of a configuration of the fixing device  20  according to a first embodiment. 
       FIG. 11  is a block diagram of the image forming apparatus  1 . As illustrated in  FIG. 11 , according to the first embodiment, in order to prevent temperature irregularity generated by the causes described above, the image forming apparatus  1  includes a rotation load detector  108  that measures the fixing rotator driving load (e.g., the rotation load) imposed on the fixing belt  21 . When a failure occurs, the rotation load detector  108  measures the fixing rotator driving load imposed on the fixing belt  21  that rotates forward. If the fixing rotator driving load imposed on the fixing belt  21  is a predetermined value or greater, the controller  90  rotates the fixing belt  21  backward, not forward. Accordingly, even if at least one of the plurality of rotary bodies disposed upstream from the fixing nip N in the recording medium conveyance direction A 1  sandwiches the recording medium P, the fixing belt  21  rotating backward loosens the recording medium P. Consequently, a small driving force generated by the fixing motor  50  rotates the fixing belt  21 , preventing temperature irregularity of the fixing belt  21 . 
     Referring to  FIG. 12 , a description is provided of a control for selecting the rotation direction of the fixing belt  21 , forward or backward, when a failure occurs. 
       FIG. 12  is a flowchart illustrating the control for selecting the rotation direction of the fixing belt  21 , forward or backward, when a failure occurs. 
     In step S 101 , the controller  90  of the image forming apparatus  1  detects that the recording medium P is jammed. In step S 102 , the controller  90  turns off the halogen heater  23 . In step S 103 , the controller  90  turns off the heater relay. After the controller  90  turns off the heater relay, the controller  90  slows down the linear velocity of the fixing belt  21  to the low linear velocity that is lower than the image formation linear velocity at which the fixing belt  21  conveys the recording medium P to fix the toner image T on the recording medium P in step S 104 . In step S 105 , while the fixing belt  21  rotates forward for the predetermined time Ta at the low linear velocity, the controller  90  causes the rotation load detector  108  to measure the fixing rotator driving load imposed on the fixing belt  21 . The rotation load detector  108  is a measurement device that measures a driving torque of the fixing belt  21 . Alternatively, the rotation load detector  108  may be a calculation device that calculates the driving torque of the fixing belt  21  by measuring an electric current value of the fixing motor  50 . In step S 106 , the controller  90  determines whether or not the fixing rotator driving load is a predetermined value X [Nm] or greater. 
     If the controller  90  determines that the fixing rotator driving load is the predetermined value or greater (YES in step S 106 ), the controller  90  determines that the fixing rotator driving load imposed on the fixing belt  21  may be too great for the fixing belt  21  to rotate. In step S 107 , the controller  90  brakes the fixing motor  50  to stop. In step S 108 , the fixing motor  50  interrupts driving of the fixing belt  21 . After the fixing belt  21  interrupts rotation, the controller  90  rotates the fixing belt  21  backward for a predetermined time Td in step S 109 . Subsequently, the controller  90  stops the fixing motor  50 . The controller  90  controls the switcher  60  to move the pressure roller  22  from the pressurization position to the depressurization position, thus finishing the backward rotation of the fixing belt  21  against failure in step S 110  and therefore finishing the control for selecting the rotation direction of the fixing belt  21 . 
     If the controller  90  determines that the fixing rotator driving load is not the predetermined value or greater (NO in step S 106 ), the controller  90  determines that the fixing belt  21  is rotatable in the forward direction. In step S 111 , the controller  90  performs the forward rotation of the fixing belt  21  against failure that rotates the fixing belt  21  forward at the low linear velocity as illustrated in  FIG. 10 . Subsequently, the controller  90  stops the fixing motor  50 . The controller  90  controls the switcher  60  to move the pressure roller  22  from the pressurization position to the depressurization position, thus finishing the forward rotation of the fixing belt  21  against failure in step S 112  and therefore finishing the control for selecting the rotation direction of the fixing belt  21 . 
     Referring to  FIG. 13 , a description is provided of the sequence of the driving times of the components of the fixing device  20  if the fixing belt  21  rotates backward when a failure occurs, which is called the backward rotation against failure. 
       FIG. 13  is a timing chart illustrating the sequence of the driving times of the components of the fixing device  20  in the second case in which the fixing belt  21  rotates backward when a failure occurs. 
     When the fixing device  20  receives a failure detection signal from the image forming apparatus  1 , the halogen heater  23  is turned off. Subsequently, the heater relay is turned off. After the heater relay is turned off, the controller  90  slows down the linear velocity of the fixing belt  21  to the low linear velocity that is lower than the image formation linear velocity at which the fixing belt  21  conveys the recording medium P to fix the toner image T on the recording medium P. While the fixing belt  21  rotates forward for the predetermined time Ta at the low linear velocity, the controller  90  causes the rotation load detector  108  to measure the fixing rotator driving load imposed on the fixing belt  21 . When the fixing rotator driving load is the predetermined value X [Nm] or greater, the controller  90  brakes the fixing motor  50  for a predetermined time Tb immediately, stopping the fixing belt  21 . 
     The controller  90  forcibly stops the fixing motor  50  for a predetermined time Tc. Thereafter, the controller  90  drives and rotates the fixing motor  50  backward for a predetermined time Td. After the predetermined time Td elapses, the controller  90  stops the fixing motor  50 . Thereafter, the switcher  60  moves the pressure roller  22  to the depressurization position where the pressure roller  22  releases pressure exerted to the fixing belt  21  at the fixing nip N. After changing the driving times of the components of the fixing device  20  described above, the controller  90  notifies the user of the failure with the notification device of the image forming apparatus  1 . After notifying the user of the failure, the controller  90  prohibits the user from using the fixing device  20 . 
     The time Td for which the fixing belt  21  rotates backward is a time for which the fixing belt  21  performs one rotation, for example. A linear velocity of the fixing belt  21  rotating backward when the failure occurs is lower than a linear velocity of the fixing belt  21  rotating forward when no failure occurs. Accordingly, the controller  90  facilitates conduction of heat from the fixing belt  21  to the recording medium P, thus reducing temperature deviation of the fixing belt  21 . 
     While the fixing motor  50  rotates the fixing belt  21  backward, the halogen heater  23  heats the fixing belt  21  with residual heat in the unshielded span of the circumferential heated span α depicted in  FIG. 2  of the fixing belt  21  where the heat shield  27  does not shield the fixing belt  21  from the halogen heater  23 . Like the first case in which the fixing belt  21  rotates forward, as the fixing belt  21  rotates backward, the recording medium P passes through the fixing nip N in a direction opposite the recording medium conveyance direction A 1 . Accordingly, the recording medium P draws heat from the fixing belt  21 . Consequently, the fixing belt  21  is immune from substantial temperature deviation, preventing temperature irregularity and deformation of the fixing belt  21 . 
     Since the controller  90  employs the control to rotate the fixing belt  21  forward or backward as described above, when the failure occurs in the image forming apparatus  1 , the controller  90  stops the fixing device  20  and the image forming apparatus  1  quickly, reducing damage to the components of the image forming apparatus  1 . Accordingly, the controller  90  prevents temperature irregularity and deformation of the fixing belt  21 . 
     As illustrated in  FIG. 13 , the controller  90  brakes and forcibly stops the fixing motor  50  before rotating the fixing motor  50  backward to prevent the fixing motor  50  from being broken when rotation of the fixing motor  50  switches from forward rotation to backward rotation. If the fixing device  20  employs the fixing motor  50  for which the controller  90  is capable of switching rotation from forward rotation to backward rotation by skipping braking and forcible stoppage, braking and forcible stoppage of the fixing motor  50  are not necessary. 
     A description is provided of a configuration of the fixing device  20  according to a second embodiment in which the controller  90  determines to rotate the fixing belt  21  forward or backward. 
       FIG. 14  is a partial vertical cross-sectional view of the image forming apparatus  1 , illustrating the configuration of the fixing device  20  according to the second embodiment in which the controller  90  determines to rotate the fixing belt  21  forward or backward. 
     According to the second embodiment, the controller  90  determines to rotate the fixing belt  21  forward or backward based on a detection result provided by a recording medium detector situated in the conveyance path R. 
     The image forming apparatus  1  includes a bypass tray  100 , a feed roller  111  that feeds a recording medium P from the bypass tray  100 , and the recording medium detector. The recording medium detector that detects presence of the recording medium P includes at least one of a paper tray downstream sensor  101  serving as a sheet tray downstream sensor, a bypass tray downstream sensor  102 , a timing roller pair upstream sensor  103 , a timing roller pair downstream sensor  104 , and a fixing device upstream sensor  105 . 
     The paper tray downstream sensor  101  detects a recording medium P remaining after the feed roller  11  feeds the recording medium P from the paper tray  10 . The bypass tray downstream sensor  102  detects a recording medium P remaining after the feed roller  111  feeds the recording medium P from the bypass tray  100 . The timing roller pair upstream sensor  103  detects the recording medium P remaining at a position disposed upstream from the timing roller pair  12  in the recording medium conveyance direction A 1 . The timing roller pair downstream sensor  104  detects the recording medium P remaining at a position disposed downstream from the timing roller pair  12  in the recording medium conveyance direction A 1 . The fixing device upstream sensor  105  detects the recording medium P remaining at a position disposed upstream from the fixing device  20  in the recording medium conveyance direction A 1 . 
     The paper tray downstream sensor  101 , the bypass tray downstream sensor  102 , the timing roller pair upstream sensor  103 , the timing roller pair downstream sensor  104 , and the fixing device upstream sensor  105  are disposed upstream from the fixing nip N in the recording medium conveyance direction A 1  to detect presence of the recording medium P at the positions disposed upstream and downstream from the plurality of rotary bodies, that is, the feed roller  11 , the timing roller pair  12 , the secondary transfer roller  36 , the secondary transfer backup roller  32 , and the fixing belt  21 . The controller  90  determines whether or not the rotary body sandwiches the recording medium P based on a detection result provided by the recording medium sensor. 
     If a long-length recording medium P as described above is jammed and all of the bypass tray downstream sensor  102 , the timing roller pair upstream sensor  103 , the timing roller pair downstream sensor  104 , and the fixing device upstream sensor  105  determine that the recording medium P remains, the fixing rotator driving load imposed on the fixing belt  21  increases. In this case, the controller  90  drives and rotates the fixing belt  21  backward. Alternatively, the recording medium detector may determine presence of the recording medium P in combination with determination based on the size of the recording medium P specified by the user. 
     Referring to  FIG. 15 , a description is provided of a control for selecting the rotation direction of the fixing belt  21 , forward or backward, when a failure occurs. 
       FIG. 15  is a flowchart illustrating the control for selecting the rotation direction of the fixing belt  21 , forward or backward, according to the second embodiment. 
     According to the second embodiment, the controller  90  performs steps S 201  to S 205  equivalent to steps S 101  to S 105  depicted in  FIG. 12 . Thereafter, the controller  90  determines whether or not all of the bypass tray downstream sensor  102 , the timing roller pair upstream sensor  103 , the timing roller pair downstream sensor  104 , and the fixing device upstream sensor  105  detect that the recording medium P remains in step S 206 . 
     If the controller  90  determines that all of the bypass tray downstream sensor  102 , the timing roller pair upstream sensor  103 , the timing roller pair downstream sensor  104 , and the fixing device upstream sensor  105  detect that the recording medium P remains (YES in step S 206 ), the controller  90  determines that the fixing rotator driving load imposed on the fixing belt  21  may be too great for the fixing belt  21  to rotate. The controller  90  performs steps S 207  to S 210  equivalent to steps S 107  to S 110  depicted in  FIG. 12  and finishes the control. 
     If the controller  90  determines that all of the bypass tray downstream sensor  102 , the timing roller pair upstream sensor  103 , the timing roller pair downstream sensor  104 , and the fixing device upstream sensor  105  detect that no recording medium P remains (NO in step S 206 ), the controller  90  determines that the fixing belt  21  is rotatable in the forward direction. The controller  90  performs steps S 211  and S 212  equivalent to steps S 111  and S 112  depicted in  FIG. 12  and finishes the control. 
     Since the controller  90  employs the control to rotate the fixing belt  21  forward or backward as described above, when the failure occurs in the image forming apparatus  1 , the controller  90  stops the fixing device  20  and the image forming apparatus  1  quickly, reducing damage to the components of the image forming apparatus  1 . Accordingly, the controller  90  prevents temperature irregularity and deformation of the fixing belt  21 . 
     The positions of the recording medium detectors and combination of the recording medium detectors used for determination of the controller  90  vary depending on the specification of the image forming apparatus  1  and therefore are set arbitrarily based on the size of the image forming apparatus  1  and the layout of the conveyance path R. 
     A description is provided of a configuration of the fixing device  20  according to a third embodiment in which the controller  90  determines to rotate the fixing belt  21  forward or backward. 
       FIG. 16  is a diagram of the fixing belt  21 , illustrating the configuration of the fixing device  20  according to the third embodiment in which the controller  90  determines to rotate the fixing belt  21  forward or backward. 
     According to the third embodiment, the controller  90  determines to rotate the fixing belt  21  forward or backward based on a detection result provided by a speed detector that detects the rotation speed of the fixing belt  21 . 
     As illustrated in  FIG. 16 , markings  106  are disposed on one lateral end of the fixing belt  21  in the axial direction thereof. A reflectance of the markings  106  is different from a reflectance of the outer circumferential surface of the fixing belt  21 . A reflection sensor  107  serving as a speed detector disposed in proximity to the fixing belt  21  detects change in the reflectance that generates in accordance with rotation of the fixing belt  21 . If a rotation cycle of the markings  106  detected by the reflection sensor  107  is longer than a predetermined rotation cycle, that is, if a rotation speed of the fixing belt  21  detected by the speed detector is a predetermined speed or lower, the fixing belt  21  does not rotate at a target rotation speed, increasing the fixing rotator driving load imposed on the fixing belt  21 . In this case, the controller  90  drives and rotates the fixing belt  21  backward. 
     Referring to  FIG. 17 , a description is provided of a control for selecting the rotation direction of the fixing belt  21 , forward or backward, when a failure occurs. 
       FIG. 17  is a flowchart illustrating the control for selecting the rotation direction of the fixing belt  21 , forward or backward, according to the third embodiment. 
     According to the third embodiment, the controller  90  performs steps S 301  to S 305  equivalent to steps S 101  to S 105  depicted in  FIG. 12 . In step S 306 , the controller  90  determines whether or not a reflectance change speed detected by the reflection sensor  107  is a predetermined reflectance change speed V [ms] or lower. 
     If the controller  90  determines that the reflectance change speed detected by the reflection sensor  107  is the predetermined reflectance change speed V or lower (YES in step S 306 ), the controller  90  determines that the fixing rotator driving load imposed on the fixing belt  21  may be too great for the fixing belt  21  to rotate. The controller  90  performs steps S 307  to S 310  equivalent to steps S 107  to S 110  depicted in  FIG. 12  and finishes the control. 
     If the controller  90  determines that the reflectance change speed detected by the reflection sensor  107  is not the predetermined reflectance change speed V or lower (NO in step S 306 ), the controller  90  determines that the fixing belt  21  is rotatable in the forward direction. The controller  90  performs steps S 311  and S 312  equivalent to steps S 111  and S 112  depicted in  FIG. 12  and finishes the control. 
     Since the controller  90  employs the control to rotate the fixing belt  21  forward or backward as described above, when the failure occurs in the image forming apparatus  1 , the controller  90  stops the fixing device  20  and the image forming apparatus  1  quickly, reducing damage to the components of the image forming apparatus  1 . Additionally, the controller  90  prevents temperature irregularity and deformation of the fixing belt  21 . 
     A description is provided of a construction of a fixing device  120  according to another embodiment. 
     Determination of the controller  90  to determine to rotate the fixing belt  21  forward or backward according to the embodiments described above is also applicable to a fixing device incorporating a lateral end heater.  FIG. 18  is a schematic vertical cross-sectional view of the fixing device  120  according to this embodiment. 
     The fixing device  120  (e.g., a fuser or a fusing unit) includes a fixing belt  121  and a pressure roller  122 . The fixing belt  121 , serving as a fixing rotator or a fixing member, is an endless belt that is thin, flexible, tubular, and rotatable in a rotation direction D 121  and a direction opposite the rotation direction D 121 . The pressure roller  122 , serving as a pressure rotator or a pressure member, contacts an outer circumferential surface of the fixing belt  121 . The pressure roller  122  is rotatable in a rotation direction D 122  and a direction opposite the rotation direction D 122 . Inside a loop formed by the fixing belt  121  is a plurality of heaters or a plurality of fixing heaters, that is, a halogen heater  123 A serving as a first halogen heater and a halogen heater  123 B serving as a second halogen heater, that heats the fixing belt  121  with radiant heat. Each of the halogen heaters  123 A and  123 B is a radiant heater serving as a main heater or a fixing heater. 
     Inside the loop formed by the fixing belt  121  are a nip formation pad  124 , a stay  125 , lateral end heaters  126 , a thermal conduction aid  127 , and reflectors  128 A and  128 B. The components disposed inside the loop formed by the fixing belt  121 , that is, the halogen heaters  123 A and  123 B, the nip formation pad  124 , the stay  125 , the lateral end heaters  126 , the thermal conduction aid  127 , and the reflectors  128 A and  128 B, may construct a belt unit  121 U separably coupled with the pressure roller  122 . The nip formation pad  124  presses against the pressure roller  122  via the fixing belt  121  to form the fixing nip N between the fixing belt  121  and the pressure roller  122 . The stay  125 , serving as a support, supports the nip formation pad  124 . 
     A detailed description is now given of a configuration of the nip formation pad  124 . 
     The nip formation pad  124  extending in a longitudinal direction thereof parallel to an axial direction of the fixing belt  121  is secured to and supported by the stay  125 . Accordingly, even if the nip formation pad  124  receives pressure from the pressure roller  122 , the stay  125  prevents the nip formation pad  124  from being bent by the pressure and therefore allows the nip formation pad  124  to produce a uniform nip length in the recording medium conveyance direction A 1  throughout the entire width of the pressure roller  122  in an axial direction or a longitudinal direction thereof. The nip formation pad  124  is made of a heat resistant material being resistant against temperatures up to 200 degrees centigrade and having an enhanced mechanical strength. For example, the nip formation pad  124  is made of heat resistant resin such as PI, PEEK, and PI or PEEK reinforced with glass fiber. Thus, the nip formation pad  124  is immune from thermal deformation at temperatures in a fixing temperature range desirable to fix a toner image on a recording medium P, retaining the shape of the fixing nip N and quality of the toner image formed on the recording medium P. 
     Both lateral ends of the stay  125  and the halogen heaters  123 A and  123 B in a longitudinal direction thereof are secured to and supported by a pair of side plates of the fixing device  120  or a pair of holders, provided separately from the pair of side plates, respectively. 
     A detailed description is now given of a configuration of the lateral end heaters  126 . 
     The lateral end heaters  126  are mounted on or coupled with both lateral ends of the nip formation pad  124  in the longitudinal direction thereof, respectively. The lateral end heaters  126  serve as a sub heater provided separately from the main heater or the fixing heater (e.g., the halogen heaters  123 A and  123 B). The lateral end heaters  126  heat both lateral ends of the fixing belt  121  in the axial direction thereof, respectively. The lateral end heater  126  is a contact heater that contacts the fixing belt  121  to conduct heat to the fixing belt  121 , for example, a resistive heat generator such as a ceramic heater. 
     A detailed description is now given of a configuration of the thermal conduction aid  127 . 
     The thermal conduction aid  127  also serves as a thermal equalizer that facilitates conduction of heat in the axial direction of the fixing belt  121 . The thermal conduction aid  127  covers a nip-side face of each of the nip formation pad  124  and the lateral end heaters  126 , which is disposed opposite an inner circumferential surface of the fixing belt  121 . The thermal conduction aid  127  initiatively conducts and equalizes heat in the fixing belt  121  in a longitudinal direction of the thermal conduction aid  127  that is parallel to the axial direction of the fixing belt  121 , preventing heat from being stored at both lateral ends of the fixing belt  121  in the axial direction thereof while a plurality of small recording media P is conveyed over the fixing belt  121  or while the lateral end heaters  126  are turned on. Thus, the thermal conduction aid  127  eliminates uneven temperature of the fixing belt  121  in the axial direction thereof. Hence, the thermal conduction aid  127  is made of a material that conducts heat quickly, for example, a material having an enhanced thermal conductivity such as copper having a thermal conductivity of 398 W/mk and aluminum having a thermal conductivity of 236 W/mk. 
     The thermal conduction aid  127  includes a nip-side face  127   a  being disposed opposite and in direct contact with the inner circumferential surface of the fixing belt  121 , thus serving as a nip formation face that forms the fixing nip N. As illustrated in  FIG. 18 , the nip-side face  127   a  is planar. Alternatively, the nip-side face  127   a  may be curved or recessed or may have other shapes. If the nip-side face  127   a  is recessed with respect to the pressure roller  122 , the nip-side face  127   a  directs a leading edge of the recording medium P toward the pressure roller  122  as the recording medium P is ejected from the fixing nip N, facilitating separation of the recording medium P from the fixing belt  121  and suppressing jamming of the recording medium P between the fixing belt  121  and the pressure roller  122 . 
     A temperature sensor  129  is disposed opposite the outer circumferential surface of the fixing belt  121  at a proper position thereon, for example, a position upstream from the fixing nip N in the rotation direction D 121  of the fixing belt  121 . The temperature sensor  129  detects a temperature of the fixing belt  121 . A separator  141  is disposed downstream from the fixing nip N in the recording medium conveyance direction A 1  to separate the recording medium P from the fixing belt  121 . A pressurization assembly, that is equivalent to the switcher  60  depicted in  FIGS. 7A and 7B , presses the pressure roller  122  against the nip formation pad  124  via the fixing belt  121  and releases pressure exerted by the pressure roller  122  to the fixing belt  121 . 
     Like film, the fixing belt  121  is a thin, endless belt having a decreased loop diameter to achieve a decreased thermal capacity. Since the fixing belt  121  has a construction similar to the above-described construction of the fixing belt  21  depicted in  FIG. 2 , a description of the fixing belt  121  is omitted. 
     A detailed description is now given of a construction of the stay  125 . 
     The stay  125 , having a T-shape in cross-section, includes a base  125   b  disposed opposite the fixing nip N and an arm  125   a  projecting from the base  125   b  and being disposed opposite the nip formation pad  124  via the base  125   b . The arm  125   a  is interposed between the halogen heaters  123 A and  123 B serving as the main heater to screen the halogen heater  123 A from the halogen heater  123 B. 
     A detailed description is now given of a construction of the halogen heaters  123 A and  123 B. 
     The halogen heater  123 A includes a center heat generator disposed in a center span of the halogen heater  123 A in the longitudinal direction thereof. A small recording medium P is disposed opposite the center heat generator of the halogen heater  123 A. The halogen heater  123 B includes a lateral end heat generator disposed in each lateral end span of the halogen heater  123 B in the longitudinal direction thereof. A large recording medium P is disposed opposite the lateral end heat generator of the halogen heater  123 B. The power supply situated inside the image forming apparatus  1  supplies power to the halogen heaters  123 A and  123 B so that the halogen heaters  123 A and  123 B generate heat. The controller  90  operatively connected to the halogen heaters  123 A and  123 B and the temperature sensor  129  controls the halogen heaters  123 A and  123 B based on the temperature of the outer circumferential surface of the fixing belt  121 , which is detected by the temperature sensor  129  disposed opposite the outer circumferential surface of the fixing belt  121 . Thus, the temperature of the fixing belt  121  is adjusted to a desired fixing temperature. 
     A detailed description is now given of a configuration of the reflectors  128 A and  128 B. 
     The reflector  128 A is interposed between the halogen heater  123 A and the stay  125 . The reflector  128 B is interposed between the halogen heater  123 B and the stay  125 . The reflectors  128 A and  128 B reflect light and heat radiated from the halogen heaters  123 A and  123 B to the reflectors  128 A and  128 B, respectively, toward the fixing belt  121 , thus enhancing heating efficiency of the halogen heaters  123 A and  123 B to heat the fixing belt  121 . Additionally, the reflectors  128 A and  128 B prevent light and heat radiated from the halogen heaters  123 A and  123 B from heating the stay  125  with radiant heat, suppressing waste of energy. Alternatively, instead of the reflectors  128 A and  128 B, an opposed face of the stay  125  disposed opposite the halogen heaters  123 A and  123 B may be treated with insulation or mirror finish to reflect light and heat radiated from the halogen heaters  123 A and  123 B to the stay  125  toward the fixing belt  121 . 
     The pressure roller  122  has a construction similar to the above-described construction of the pressure roller  22  depicted in  FIG. 2 . Further, the fixing device  120  has a configuration that attains a driving force transmission method or the like in which the pressure roller  122  transmits a driving force that drives and rotates the fixing belt  121  to the fixing belt  121 , which is similar to the above-described configuration of the fixing device  20  depicted in  FIG. 2 . Hence, a description of the construction of the pressure roller  122  and the configuration of the fixing device  120  that attains the driving force transmission method is omitted. 
     Referring to  FIG. 19 , a description is provided of a construction of a nip formation unit  200  incorporated in the fixing device  120  depicted in  FIG. 18 . 
       FIG. 19  is an exploded perspective view of the nip formation unit  200 , illustrating a basic structure of the nip formation unit  200 . As illustrated in  FIG. 19 , the nip formation unit  200  includes the nip formation pad  124 , the stay  125 , the thermal conduction aid  127 , and lateral end heaters  126   a  and  126   b  illustrated as the lateral end heaters  126  in  FIG. 18 . Each of the lateral end heaters  126   a  and  126   b  includes a nip-side face  126   c  serving as an opposed face disposed opposite the fixing nip N and the inner circumferential surface of the fixing belt  121 . The nip formation pad  124  includes a nip-side face  124   c  serving as an opposed face disposed opposite the fixing nip N and the inner circumferential surface of the fixing belt  121  and a stay-side face  124   d  being opposite the nip-side face  124   c  and disposed opposite the stay  125 . The stay  125  includes a nip-side face  125   c  being planar and disposed opposite the fixing nip N and the inner circumferential surface of the fixing belt  121 . The stay-side face  124   d  of the nip formation pad  124  contacts the nip-side face  125   c  of the stay  125 . For example, the stay-side face  124   d  of the nip formation pad  124  and the nip-side face  125   c  of the stay  125  mount a recess and a projection (e.g., a boss and a pin), respectively, so that the stay-side face  124   d  engages the nip-side face  125   c  to restrict each other with the shape of the stay-side face  124   d  and the nip-side face  125   c.    
     The thermal conduction aid  127  engages the nip formation pad  124  that is substantially rectangular such that the thermal conduction aid  127  covers the nip-side face  124   c  of the nip formation pad  124  that is disposed opposite the inner circumferential surface of the fixing belt  21 . Thus, the thermal conduction aid  127  is coupled with the nip formation pad  124 . For example, the thermal conduction aid  127  is coupled with the nip formation pad  124  with a claw, an adhesive, or the like. 
     Two recesses  124   a  and  124   b , each of which defines a step or a difference in thickness of the nip formation pad  124 , are disposed at both lateral ends of the nip formation pad  124  in the longitudinal direction thereof, respectively. The lateral end heaters  126   a  and  126   b  are secured to the recesses  124   a  and  124   b , thus being accommodated by the recesses  124   a  and  124   b , respectively. A description of a positional relation between the lateral end heaters  126   a  and  126   b  and the halogen heaters  123 A and  123 B is deferred. 
     The thermal conduction aid  127  includes the nip-side face  127   a  that is disposed opposite the inner circumferential surface of the fixing belt  21 . The nip-side face  127   a  serves as a slide face over which the fixing belt  121  slides. However, since a mechanical strength of the nip-side face  124   c  of the nip formation pad  124  is greater than a mechanical strength of the nip-side face  127   a  of the thermal conduction aid  127 , the nip-side face  124   c  of the nip formation pad  124  serves as a nip formation face that is disposed opposite the pressure roller  122  and forms the fixing nip N practically. 
     According to this embodiment, the lateral end heaters  126   a  and  126   b  are coupled with the nip formation pad  124  to form the fixing nip N. Hence, the lateral end heaters  126   a  and  126   b  are situated inside a limited space inside the loop formed by the fixing belt  121 , saving space. 
     Each of the lateral end heaters  126   a  and  126   b  includes a nip-side face  126   c  disposed opposite the inner circumferential surface of the fixing belt  121 . The nip-side face  126   c  of each of the lateral end heaters  126   a  and  126   b  is leveled with the nip-side face  124   c  of the nip formation pad  124  that is disposed opposite the inner circumferential surface of the fixing belt  121  in a pressurization direction in which the pressure roller  122  presses against the nip formation pad  124  so that the nip-side faces  126   c  and the nip-side face  124   c  define an identical plane. Accordingly, the pressure roller  122  is pressed against the lateral end heaters  126   a  and  126   b  via the fixing belt  121  and the thermal conduction aid  127  sufficiently. 
     Consequently, the fixing belt  121  rotates stably in a state in which the fixing belt  121  is pressed against the lateral end heaters  126   a  and  126   b  or adhered to the lateral end heaters  126   a  and  126   b  indirectly via the thermal conduction aid  127 . The fixing belt  121  is pressed against the lateral end heaters  126   a  and  126   b  with sufficient pressure, retaining improved heating efficiency of the lateral end heaters  126   a  and  126   b . Hence, the fixing device  120  enhances reliability. 
     The lateral end heaters  126   a  and  126   b  are disposed opposite the fixing nip N. Accordingly, the lateral end heaters  126   a  and  126   b  heat the fixing belt  121  in a nip span of the fixing nip N in the rotation direction D 121  of the fixing belt  121 . That is, the lateral end heaters  126   a  and  126   b  do not heat the fixing belt  121  in a circumferential span outboard from the nip span in the rotation direction D 121  of the fixing belt  121 . Hence, the lateral end heaters  126   a  and  126   b  prevent residual toner failed to be fixed on a previous recording medium P and therefore adhering to the fixing belt  121  from being melted again and degrading a toner image on a subsequent recording medium P. 
       FIG. 20  is a perspective view of the nip formation unit  200  and the halogen heaters  123 A and  123 B. As illustrated in  FIG. 20 , the stay  125  includes a first portion  125 A and a second portion  125 B, each of which is substantially L-shaped in cross-section. Thus, the stay  125  is substantially T-shaped in cross-section. Accordingly, the stay  125  attains an enhanced rigidity that prevents the nip formation pad  124  from being bent by pressure from the pressure roller  122 . The stay  125  constructed of the first portion  125 A and the second portion  125 B extends linearly in the longitudinal direction of the nip formation pad  124 . The stay  125  is secured to the nip formation pad  124 . Accordingly, the stay  125  allows the nip-side face  124   c  depicted in  FIG. 19  of the nip formation pad  124  to retain the fixing nip N precisely throughout the entire width of the fixing nip N in the longitudinal direction of the nip formation pad  124 . 
     As illustrated in  FIG. 20 , the halogen heater  123 A is disposed opposite the halogen heater  123 B via the arm  125   a  of the stay  125  in a short direction perpendicular to the longitudinal direction of the stay  125 . The arm  125   a  is interposed between the halogen heaters  123 A and  123 B to screen the halogen heater  123 A from the halogen heater  123 B. Accordingly, while the halogen heaters  123 A and  123 B are powered on, glass tubes of the halogen heaters  123 A and  123 B, respectively, do not heat each other, preventing degradation in heating efficiency of the halogen heaters  123 A and  123 B. 
     As illustrated in  FIG. 18 , each of the halogen heaters  123 A and  123 B is not surrounded by the stay  125 . For example, a center of each of the halogen heaters  123 A and  123 B in cross-section is outside a space defined or enclosed by the stay  125 . Accordingly, the halogen heaters  123 A and  123 B attain obtuse irradiation angles α and β, respectively, of light that irradiates the fixing belt  121 , thus improving heating efficiency. 
     Alternatively, the stay  125  may have shapes other than the substantially T-shape in cross-section. The first portion  125 A and the second portion  125 B depicted in  FIG. 20  may curve and extend in the longitudinal direction of the halogen heaters  123 A and  123 B as long as the arm  125   a  interposed between the halogen heaters  123 A and  123 B screens the halogen heater  123 A from the halogen heater  123 B. The arm  125   a  of each of the first portion  125 A and the second portion  125 B may be oblique relative to the nip-side face  124   c  of the nip formation pad  124 . 
     A description is provided of arrangement of the lateral end heaters  126   a  and  126   b  to correspond to recording media P of special sizes such as an A3 extension size sheet. 
       FIG. 21  is a diagram of the halogen heaters  123 A and  123 B and the lateral end heaters  126   a  and  126   b , illustrating arrangement thereof. As illustrated in  FIG. 21 , the halogen heater  123 A includes a heat generator  140 A serving as a center heat generator having a dense light distribution in the center span of the halogen heater  123 A, which is disposed opposite a center span of the fixing belt  121  in the axial direction thereof. The halogen heater  123 B includes a heat generator  140 B serving as a lateral end heat generator having a dense light distribution in each lateral end span of the halogen heater  123 B, which is disposed opposite each lateral end span of the fixing belt  121  in the axial direction thereof. The heat generator  140 B is disposed outboard from the heat generator  140 A in the axial direction of the fixing belt  121 . The halogen heater  123 A heats the center span of the fixing belt  121  in the axial direction thereof. The halogen heater  123 B heats each lateral end span of the fixing belt  121  in the axial direction thereof. 
     The heat generator  140 A of the halogen heater  123 A corresponds to small recording media P of small sizes such as an A4 size sheet in portrait orientation. The heat generator  140 B of the halogen heater  123 B corresponds to large recording media P of large sizes such as an A3 size sheet in portrait orientation. The heat generator  140 B is disposed outboard from the heat generator  140 A in the longitudinal direction of the halogen heater  123 A so that the heat generator  140 B heats a lateral end of the large recording medium P that is outboard from the heat generator  140 A in the longitudinal direction of the halogen heater  123 B. The large recording media P include a maximum standard size sheet available in the fixing device  120 . A heat generator  140 , that is, a first combined heat generator constructed of or defined by the heat generators  140 A and  140 B, corresponds to a width of the maximum standard size sheet (e.g., the A3 size sheet in portrait orientation) and does not encompass a width of an extra-large recording medium P of an extension size, which is greater than the width of the maximum standard size sheet. 
     The lateral end heaters  126   a  and  126   b  are disposed opposite both lateral ends of the halogen heater  123 B in the longitudinal direction thereof, respectively. The lateral end heaters  126   a  and  126   b  include heat generators  142   a  and  142   b  that heat both lateral ends of the extra-large recording medium P greater than the maximum standard size sheet in the longitudinal direction of the halogen heater  123 B, respectively. Thus, a heat generator  142 , that is, a second combined heat generator constructed of or defined by the heat generators  140 A,  140 B,  142   a , and  142   b , corresponds to the width of the extra-large recording medium P of the extension size (e.g., an A3 extension size sheet and a 13-inch sheet). A part of each of the heat generators  142   a  and  142   b  overlaps the heat generator  140 B in the longitudinal direction of the halogen heater  123 B. Accordingly, the fixing belt  121  of the fixing device  120  heats both lateral ends of the extra-large recording medium P greater than the maximum standard size sheet in the longitudinal direction of the halogen heater  123 B. 
     As illustrated in  FIG. 19 , the thermal conduction aid  127  covers the nip-side face  124   c  of the nip formation pad  124  and the nip-side face  126   c  of each of the lateral end heaters  126   a  and  126   b , which are disposed opposite the inner circumferential surface of the fixing belt  121  via the thermal conduction aid  127 . The thermal conduction aid  127  is made of a material having an increased thermal conductivity, such as copper and aluminum. Accordingly, even if the lateral end heaters  126   a  and  126   b  are turned off immediately when a failure occurs, the thermal conduction aid  127  may retain a high temperature for a predetermined time due to overshooting or the like. Consequently, when the fixing belt  121  interrupts rotation, the fixing belt  121  may suffer from temperature irregularity and deformation. 
     Even if the image forming apparatus  1  employs the fixing device  120  configured to save energy substantially, since the controller  90  employs the control to rotate the fixing belt  121  forward or backward as described above, when the failure occurs in the image forming apparatus  1 , the controller  90  stops the fixing device  120  and the image forming apparatus  1  quickly, reducing damage to the components of the image forming apparatus  1 . Additionally, the controller  90  prevents temperature irregularity and deformation of the fixing belt  121 . 
     The present disclosure is not limited to the details of the embodiments described above and various modifications and improvements are possible. An aspect that the controller  90  determines that the fixing belt  21  or  121  and the pressure roller  22  or  122  at the fixing nip N and at least one of the plurality of rotary bodies sandwich a single recording medium P, that is, an identical recording medium P, simultaneously when a failure occurs is not limited to the embodiments described above as long as the controller  90  determines that the fixing belt  21  or  121  and the pressure roller  22  or  122  at the fixing nip N and at least one of the plurality of rotary bodies sandwich the single recording medium P simultaneously. 
     A description is provided of advantages of the fixing devices  20  and  120 . 
     As illustrated in  FIGS. 1, 2, and 18 , a fixing device (e.g., the fixing devices  20  and  120 ) includes a fixing rotator (e.g., the fixing belts  21  and  121 ), a heater (e.g., the halogen heaters  23 ,  123 A, and  123 B), an opposed rotator (e.g., the pressure rollers  22  and  122 ), at least one rotary body (e.g., the feed roller  11 , the timing roller pair  12 , the secondary transfer roller  36 , and the secondary transfer backup roller  32 ), and a controller (e.g., the controller  90 ). 
     The fixing rotator is rotatable in a forward direction and a backward direction opposite the forward direction. The heater heats the fixing rotator. The opposed rotator presses against or contacts an outer circumferential surface of the fixing rotator to form a fixing nip (e.g., the fixing nip N) therebetween, through which a recording medium (e.g., a recording medium P) bearing a toner image (e.g., a toner image T) is conveyed. The at least one rotary body conveys the recording medium to the fixing nip. The controller rotates the fixing rotator in the forward direction to fix the toner image on the recording medium. The controller stops the heater and then rotates the fixing rotator in a predetermined rotation direction when a failure occurs while the fixing device is activated. If the fixing rotator and the opposed rotator at the fixing nip and the at least one rotary body sandwich the identical recording medium simultaneously when the failure occurs while the fixing device is activated, the controller determines that the predetermined rotation direction of the fixing rotator is one of the forward direction and the backward direction according to the at least one rotary body that sandwiches the recording medium. 
     Accordingly, the fixing device prevents overheating of the fixing rotator while the fixing rotator does not rotate with no damage to the components of the fixing device. 
     As illustrated in  FIG. 21 , the fixing device  120  employs a center conveyance system in which the recording medium P is centered on the fixing belt  121  in the axial direction thereof. Alternatively, the fixing device  120  may employ a lateral end conveyance system in which the recording medium P is conveyed in the recording medium conveyance direction D 121  along one lateral end of the fixing belt  121  in the axial direction thereof. In this case, one of the lateral end heaters  126   a  and  126   b  is eliminated. Another one of the lateral end heaters  126   a  and  126   b  is distal from the one lateral end of the fixing belt  121  in the axial direction thereof. Similarly, the fixing device  20  may employ the lateral end conveyance system. 
     According to the embodiments described above, each of the fixing belts  21  and  121  serves as a fixing rotator. Alternatively, a fixing film or the like may be used as a fixing rotator. Further, each of the pressure rollers  22  and  122  serves as an opposed rotator. Alternatively, a pressure belt or the like may be used as an opposed rotator. 
     The above-described embodiments are illustrative and do not limit the present disclosure. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements and features of different illustrative embodiments may be combined with each other and substituted for each other within the scope of the present invention. 
     Any one of the above-described operations may be performed in various other ways, for example, in an order different from the one described above.