Abstract:
An image heating apparatus includes: a rotating mechanism for rotating a belt unit in a direction for returning a belt into a predetermined zone; a displacing mechanism for permitting a first supporting member to be displaced in a direction for substantially equalizing forces, from the first supporting member, urging the belt toward a rotatable heating member at opposite end portions of the first supporting member with respect to a belt widthwise direction and to permit a second supporting member to be displaced in a direction for substantially equalizing forces, from the second supporting member, urging the belt toward the rotatable heating member at opposite end portions of the second supporting member; and a limiting mechanism for limiting the amount of the displacement permitted by the displacing mechanism within a predetermined amount.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image heating apparatus which heats a toner image on a sheet of recording medium. 
     In the field of an electrophotographic image forming apparatus, it has been a common practice to fix a toner image formed on a sheet of recording medium with the use of an electrophotographic process, to the sheet of recording medium, by applying heat and pressure to the sheet and the toner image thereon, with the use of a fixing apparatus (device) which is an example of an image heating apparatus (device). 
     In the recent years, an electrophotographic image forming apparatus has been increased in speed. Thus, it has become a common practice to equip a fixing apparatus (device) with an external means for externally heating the fixation roller (rotational heating member) of the fixing device. One of such external heating means which employ a heat belt has been proposed in Japanese Laid-open Patent Applications 2004-198658, and 2007-212896. 
     More concretely, in the case of the apparatus disclosed in Japanese Laid-open Patent Application 2004-198659, an external heat belt is supported and kept stretched by three belt supporting rollers, and is placed in contact with the peripheral surface of the fixation roller. In the case of Japanese Laid-open Patent Application 2007-212896, an external heat belt is suspended and kept stretched by two belt supporting rollers, and is placed in contact with the peripheral surface of the fixation roller. 
     Realistically speaking, it is rather difficult to assembly a fixing device so that its rollers for supporting, and keeping stretched, its external heat belt, become, and remain, virtually perfectly parallel to each other. However, unless the two rollers remain perfectly parallel to each other, the external heat belt deviates in its widthwise direction, and therefore, it is likely for the external heat belt to become unstable in its rotational movement. Thus, there have been devised various methods for controlling the external heat belt in its deviation in its widthwise deviation. One of such methods is to slant one of the two belt supporting rollers relative to the other. However, in a case where the external heat belt is employed to heat a fixation roller, it is difficult to satisfactorily employ this method, for the following reason. 
     That is, in the case of this method, the external heating unit is structured so that one of the lengthwise ends of one of the belt supporting rollers is displaced relative to the other lengthwise end. Thus, it is possible that a part of the external heat belt, which is to remain in contact with the fixation roller, is separated from the fixation roller by the displacement of the belt supporting roller. Therefore, it is possible that the external heat belt will fail to satisfactorily heat the fixation roller. Therefore, it is possible that unsatisfactory fixation will occur. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided an image heating apparatus includes a rotatable heating member configured to heat a toner image on a recording material; a belt unit including an endless belt configured and positioned to contact with said rotatable heating member to heat it, and first and second supporting members rotatably supporting an inner surface of said belt and configured to urge said belt to said rotatable heating member; a detector configured and positioned to detect that said belt is deviated from a predetermined zone with respect to a widthwise direction said belt; a rotating mechanism configured to rotate said belt unit in a direction for returning said belt into the predetermined zone; a displacing mechanism configured to permit said first supporting member to displace, with rotation of said belt unit by said rotating mechanism, in a direction for substantially equalizing forces, from said first supporting member, urging said belt toward said rotatable heating member at opposite end portions of said first supporting member with respect to the widthwise direction and to permit said second supporting member to displace, with the rotation of said belt unit by said rotating mechanism, in a direction for substantially equalizing forces, from said second supporting member, urging said belt toward said rotatable heating member at opposite end portions of said second supporting member with respect to the widthwise direction; and a limiting mechanism configured and positioned to limit an amount of the displacement permitted by said displacing mechanism within a predetermined amount. 
     According to another aspect of the present invention, there is provided an image heating apparatus comprising a rotatable heating member configured to heat a toner image on a recording material; a belt unit including an endless belt configured and positioned to contact with said rotatable heating member to heat it, and first and second supporting rollers rotatably supporting an inner surface of said belt and configured to urge said belt to said rotatable heating member; a detector configured and positioned to detect that said belt is deviated from a predetermined zone with respect to a widthwise direction said belt; a rotating mechanism configured to rotate said belt unit in a direction for returning said belt into the predetermined zone; a displacing mechanism configured to permit said first and second rollers to displace, with rotation of said belt unit by said rotating mechanism, into a positional relation in which axes of said first and second supporting rollers are skewed relative to each other; and a limiting mechanism configured and positioned to limit an amount of the displacement permitted by said displacing mechanism within a predetermined amount. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing for describing the structure of a typical image forming apparatus to which the present invention is applicable. 
         FIG. 2  is a schematic drawing for describing the structure of the fixing device in the first embodiment of the present invention. 
         FIG. 3  is a schematic drawing for describing the engaging-disengaging mechanism, in the first embodiment, for placing the external heating belt in contact with, or separating the external heating belt from, the fixing roller. 
         FIG. 4  is a schematic drawing for describing the mechanism for rotationally moving the holding frames. 
         FIG. 5  is a schematic drawing for describing the angle between the generatrix of the fixation roller and that of the external heat belt. 
         FIG. 6  is a schematic drawing for describing how the rotational movement of the external heating unit can cancel the effect of the controlling of the lateral deviation of the external heat belt (how rotational movement of the external heating unit can prevent nip between external heat belt and fixation roller from becoming nonuniform in internal pressure). 
         FIG. 7  is a schematic drawing for describing the steering mechanism for steering the external heat belt. 
         FIG. 8  is a schematic drawing for describing the driving portion section for driving the steering mechanism. 
         FIG. 9  is an enlarged schematic view of the driving portion of the steering mechanism. 
         FIG. 10  is a schematic drawing for describing the positioning of the sensor for detecting the amount of lateral deviation of the external heat belt. 
         FIG. 11  is a schematic drawing for describing the relationship between the direction of the external heat belt deviation and the direction of the rotational movement of the sensor flag. 
         FIG. 12  is a schematic drawing for describing the comparative external heating unit. 
         FIG. 13  is a schematic drawing for describing the positioning of the regulating portion in the first embodiment. 
         FIG. 14  is a schematic drawing for describing the movement of the holding frames. 
         FIG. 15  is a perspective view of the regulating portion. 
         FIG. 16  is a schematic drawing for describing the operation of the regulating portion. 
         FIG. 17  a schematic drawing for describing the rotational angle of the holding frames relative to each other. 
         FIG. 18  is a schematic drawing for describing the range to which the angle by which the holding frames are allowed to rotationally move relative to each other is limited. 
         FIG. 19  is a schematic drawing for describing the effects of the first embodiment. 
         FIG. 20  is a schematic drawing for describing the positioning of the regulating portion in the second embodiment. 
         FIG. 21  is a schematic drawing for describing the positioning of the thermistors in the second embodiment. 
         FIG. 22  is a schematic drawing for describing the structure of the mechanism for preventing the sensor supporting shaft from rotating relative to the holding frame. 
         FIG. 23  is a schematic drawing for describing the rotational movement of the holding frames relative to each other. 
         FIG. 24  is a schematic drawing for describing the operation of the regulating portion in the second embodiment. 
         FIG. 25  is a schematic drawing of the regulating portion in the third embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the embodiments of the present invention are described in detail with reference to appended drawings. 
     &lt;Image Forming Apparatus&gt; 
       FIG. 1  is a schematic drawing for describing the structure of a typical image forming apparatus to which the present invention is applicable. Referring to  FIG. 1 , an image forming apparatus  100  is a full-color printer of the tandem-type, and also, of the intermediary transfer type. It has image formation stations Pa, Pb, Pc and Pd for forming yellow, magenta, cyan and black toner images, respectively, and an intermediary transfer belt  130 . The four image formation stations are aligned in parallel (tandem) along the intermediary transfer belt  130 . 
     In the image formation station Pa, a yellow toner image is formed on a photosensitive drum  3   a , and is transferred (primary transfer) onto the intermediary transfer belt  130 . In the image formation station Pb, a magenta toner image is formed, and is transferred (primary transfer) onto the intermediary transfer belt  130 . In the image formation stations Pc and Pd, cyan and black toner images, respectively, are formed, and are transferred (primary transfer) onto the intermediary transfer belt  130 . That is, the yellow, magenta, cyan, and black toner images are sequentially transferred (primary transfer) onto the intermediary transfer belt  130 . 
     Sheets P of recording medium in a recording medium cassette  10  are moved out of the cassette  10  one by one, and each sheet P is conveyed to a pair of registration rollers  12 , at which the sheet P is kept on standby. Then, the registration rollers  12  convey the sheet P to the secondary transfer station T2, with such a timing that the sheet P reaches the secondary transfer station T2 at the same time as the four toner images, different in color, on the intermediary transfer belt  130 . Then, while the sheet P is conveyed through the secondary transfer station T2, the toner images are transferred (secondary transfer) from the intermediary transfer belt  130  onto the sheet P. Then, the sheet P is conveyed to the fixing device  9 , in which the sheet P and the toner images thereon are subjected to heat and pressure, whereby the toner images are fixed to the sheet P. Then, the sheet P is discharged into the external delivery tray  7  of the image forming apparatus  100 . 
     The image formation stations Pa, Pb, Pc and Pd are practically the same in structure, although they are different in the color of the toner used by their developing devices  1   a ,  1   b ,  1   c  and  1   d , respectively. Thus, only the image formation Pa is described, in order not to repeat the same descriptions. 
     The image formation station Pa has the photosensitive drum  3   a , a charge roller  2   a , an exposing device  5   a , a developing device  1   a , a primary transfer roller  6   a , and a drum cleaning device  4   a . The charge roller  2   a , exposing device  5   a , developing device  1   a , primary transfer roller  6   a , and drum cleaning device  4   a  are disposed in the adjacencies of the peripheral surface of the photosensitive drum  3   a , in the listed order. The photosensitive drum  3   a  is made up of an aluminum cylinder, and a photosensitive layer formed on the peripheral surface of the aluminum cylinder. 
     The charge roller  2   a  uniformly charges the peripheral surface of the photosensitive drum  3   a  to a preset potential level. The exposing device  5   a  writes an electrostatic image on the peripheral surface of the photosensitive drum  3   a , by scanning the uniformly charge portion of the peripheral surface of the photosensitive drum  3   a , with a beam of laser light which it emits. The primary transfer roller  6   a  transfers (primary transfer) the toner images on the peripheral surface of the photosensitive drum  3   a  onto the intermediary transfer belt  130 , by being given voltage. 
     The drum cleaning device  4   a  is provided with a cleaning blade. It recovers the transfer residual toner, which is the toner having escaped from the primary transfer process, and therefore, remaining adhered to the peripheral surface of the photosensitive drum  3   a  after the primary transfer, by causing the cleaning blade to scrape the peripheral surface of the photosensitive drum  3   a . The belt cleaning device  15  recovers the transfer residual toner, which is the toner having escaped from the process carried out in the secondary transfer station T2 to transfer the toner on the intermediary transfer belt  130  onto the sheet P of recording medium, and therefore, remaining on the intermediary transfer belt  130  after the secondary transfer. 
     Embodiment 1 
     Referring to  FIG. 2 , a fixation roller  101 , which is an example of a rotational member, rotates in contact with a sheet P of recording medium. An external heat belt  105 , which is an example of member in the form of a belt is for adjusting the fixation roller  101  in thermal characteristic. It rotates in contact with the fixation roller  101 . 
     The first and second support rollers  103  and  104 , which are examples of multiple belt supporting members (rotational belt supporting members), support, and keep stretched, the external heat belt  105 . A thermistor  123  which is an example of a temperature detecting member (temperature detection element) is placed in contact with the outward surface of the external heat belt  105  to detect the temperature of the belt  105 . 
     Referring to  FIG. 3 , a holding frame  206   a  which is an example of the first holding member (which is part of displacing mechanism or skewing mechanism, as well, which is described later) rotatably holds one of the lengthwise ends of the first support roller  103 , and the corresponding lengthwise end of the second support roller  104 . A holding frame  206   b  which is an example of the second holding member (which is part of displacing mechanism, which will be described later) rotatably holds the other lengthwise end of the first support roller  103 , and the corresponding lengthwise end of the second holding roller  104 . A middle frame  208  which is an example of a displacing mechanism (rotational mechanism) rotatably supports the support frames  206   a  and  206   b  in such a manner that the first and second support rollers  103  and  104  can be slanted (angled) relative to each other. 
     Referring to  FIG. 7 , a worm wheel  118  which is an example of a rotational mechanism controls the external heat belt  105  in position in terms of the widthwise direction of the external heat belt  105 , that is, the direction parallel to the lengthwise direction of the first and second rollers  103  and  104 . The worm wheel  118  rotationally moves the middle frame  208  in such a manner that the generatrix of the external heat belt  105  and the generatrix of the fixation roller  101  are angled relative to each other. A photo-interrupters  133  and  134  detect the position of the external heat belt  105  in terms of the widthwise direction of the external heat belt  105 , that is, the direction parallel to the lengthwise direction of the first and second support rollers  103  and  104 . A control section  140  is a part of the abovementioned rotational mechanism. It controls the external heat belt  105  in position, by moving the worm wheel  118  in response to the outputs of the photo-interrupters  133  and  134 . 
     Referring to  FIG. 15 , regulating portions  300 A and  300 B, which are examples of a regulating mechanism, function as a stopper for limiting to a preset value, the maximum angle by which the holding frames  206   a  and  206   b  are rotationally movable relative to each other. Next, referring to  FIG. 18 , a preset angle (γ) is greater than the maximum angle (β max ) by which the holding frames  206   a  and  206   b  are allowed to be rotationally move relative to each other, in order to control the lateral deviation of the external heat belt  105 . 
     Referring to  FIG. 15 , bent portions  301   a  and  301   b ), which are examples of the first portion of contact, extend from the inward edge of the holding frame  206   a  toward the holding frame  20   b , along the first and second support rollers  103  and  104 . Flat portions  302   a    302   b , which are examples of the second portion of contact, extend from the inward edge of the holding frame  206   b  toward the holding frame  206   a , along the first and second support rollers  103  and  104 . In terms of the direction in which the external heat belt  105  is suspended and stretched, the bent portion  301   a  and  301   b  are positioned, on one side of the combination of the first and second holding frames  206   a  and  206   b , in such a manner that as the holding frames  206   a  and  206   b  are rotationally moved relative to each other, they overlap with each other. Further, the bent portion  302   a  and  202   b  are positioned, on the other side of the combination of the first and second holding frames  206   a  and  206   b , in such a manner that as the holding frames  206   a  and  206   b  are rotationally moved relative to each other, they overlap with each other. 
     Referring to  FIG. 14 , a thermistor  123   a  is attached to one of the lengthwise ends of a leaf spring  123   m , which is an example of a beam-like member (pressing member). The other lengthwise end of the leaf spring  123   m  is fixed to the holding frame  206   a . That is, the leaf spring  123   m  is attached to the holding frame  206   a , acting thereby like a cantilever. It keeps the thermistor  123   a  in contact with the external heat belt  105  by being elastically bent. The aforementioned preset angle (γ) is set to be smaller than the maximum angle by which the holding frames  206   a  and  206   b  are allowed to rotationally move relative to each other while ensuring that the elastic deformation of the leaf spring  123   m  can keep thermistor  123   a  in contact with the external heat belt  105 . 
     (Fixing Device) 
       FIG. 2  is a schematic drawing for describing the structure of the fixing device in the first embodiment of the present invention. Referring to  FIG. 2 , the fixing device  9  has the fixation roller  101  and pressure roller  102 . It is structured so that the pressure roller  102  is pressed upon the fixation roller  101  to form a nip N, through which a sheet P of recording medium, across which an unfixed toner image K is borne, is conveyed, remaining pinched by the fixation roller  101  and pressure roller  102 , so that the toner, of which the unfixed toner image K is formed, is melted and becomes fixed to the surface of the sheet P. 
     The fixation roller  101  has: a metallic core  101   a ; an elastic layer  101   b  formed across the entirety of the peripheral surface of the metallic core  101   a ; and a parting layer  101   c  formed across the entirety of the outward surface of the elastic layer  101   b . The fixation roller  101  is driven by a driving mechanism  141  which includes an unshown gear train. It is rotated in the direction indicated by an arrow mark A in  FIG. 2 , at a process speed of 300 mm/sec. 
     The pressure roller  102  has: a metallic core  102   a ; an elastic layer  102   b  formed across the entirety of the peripheral surface of the metallic core  102   a ; and a parting layer  102   c  formed across the entirety of the outward surface of the elastic layer  102   b . It is driven by the driving system  141 , and rotates in the direction indicated by an arrow mark B in  FIG. 2 . The pressure roller  102  is placed in contact with, or separated from, the fixation roller  101 , by being driven by an unshown pressure applying mechanism which employs an eccentric cam. The unshown pressure applying mechanism applies a preset amount of pressure to the pressure roller  102  to press the pressure roller  102  upon the fixation roller  101 , forming the nip N between the fixation roller  101  and pressure roller  102 . 
     The halogen heater  111  is non-rotationally disposed in the hollow of the metallic core  101   a  of the fixation roller  101 . A thermistor  121  is disposed in contact with the fixation roller  101  to detect the surface temperature of the fixation roller  101 . The control section  140  turns on or off the halogen heater  111  in response to the surface temperature of the fixation roller  101  detected by the thermistor  121 , in order to keep the surface temperature of the fixation roller  101  at a preset target level, which is set according to recoding medium type. 
     The halogen heater  112  is non-rotationally disposed in the hollow of the metallic core  102   a  of the pressure roller  102 . A thermistor  122  is placed in contact with the pressure roller  102  to detect the surface temperature of the pressure roller  102 . The control section  140  turns on or off the halogen heater  112  in response to the surface temperature of the pressure roller  102  detected by the thermistor  122 , in order to keep the surface temperature of the pressure roller  102  at a preset target level. 
     (External Heat Belt) 
     Referring to  FIG. 2 , the image forming apparatus is required to be high in productivity (print output count per unit length of time) even when such recording medium as a sheet of cardstock or the like which is large in basis weight (weight per unit area), is used for image formation. In order to keep the image forming apparatus  100  high in productivity even when the recording medium used for an image forming operation is large in basis weight, the fixing device  9  of the image forming apparatus has to be enabled to remain high in heating performance even when the recording medium used for the image forming operation is large in basis weight. The amount by which recording medium which is large in basis weight robs heat from the fixation roller  101  is larger than the amount by which ordinary paper robs heat from the fixation roller  101 . Therefore, the amount of heat which the former require for fixation is greater than that for the latter. Thus, the fixing device  9  is structured so that the external heat belt  105  can be placed in contact with, or separated from, the fixation roller  101 . The external heat belt  105  increases the first and second support rollers  103  and  104  in the efficiency with which the rollers  103  and  104  can heat the fixation roller  101 , by increasing in size the area of indirect contact between the first and second rollers  103  and  104  and the fixation roller  101 , through which heat is conducted from the two rollers  103  and  104  to the fixation roller  101 . 
     The external heat belt  105  is placed in contact with the peripheral surface of the fixation roller  101 , forming thereby a nip Ne, in which it externally heats the fixation roller  101 . The external heat belt  105  has a substrative layer formed of a metallic substance such as stainless steel and nickel, or resinous substance such as polyimide. In order to prevent toner from adhering to the substrative layer of the external heat belt  105 , the surface of the substrative layer is provided with a heat resistant slippery layer formed of fluorinated resin. The external heat belt  105  is driven by the friction which occurs between the peripheral surface of the fixation roller  101  and external heat belt  105  as the fixation roller  101  is rotated; it is rotated by the rotation of the fixation roller  101  in the direction indicated by an arrow mark C in  FIG. 2 . 
     The first support roller  103  is formed of a metallic substance, such as aluminum, iron, stainless steel, etc., which is high in thermal conductivity. There is stationarily disposed a halogen heater  113 , in the hollow of the first support roller  103 , in such a manner that the axial line of the halogen heater  113  coincides with the rotational axis of the first support roller  103 . A thermistor  123  is placed in contact with the portion of the external heat belt  105 , which is supported by the first support roller  103 , and detects the temperature of the external heat belt  105 . The control section  140  turns on or off the halogen heater  113  in response to the temperature of the external heat belt  105  detected by the thermistor  123 , in order to keep the temperature of the first support roller  103  at a preset target level. 
     The second support roller  104  is formed of a metallic substance, such as aluminum, iron, stainless steel, etc., which is high in thermal conductivity. There is stationarily disposed a halogen heater  114 , in the hollow of the second support roller  104 , in such a manner that the axial line of the halogen heater  114  coincides with the rotational axis of the second support roller  104 . A thermistor  124  is placed in contact with the portion of the external heat belt  105 , which is supported by the first support roller  104 , and detects the temperature of the external heat belt  105 . The control section  140  turns on or off the halogen heater  114  in response to the temperature of the external heat belt  105  detected by the thermistor  124 , in order to keep the temperature of the second support roller  104  at a preset target level. 
     The target levels for the temperature control of the first and second support rollers  103  and  104  are set higher than the target level for the temperature control of the fixation roller  101 . Because the surface temperature of the first support roller  103  and the surface temperature of the second support roller  104  are kept higher than the surface temperature of the fixation roller  101 , heat is efficiently supplied to the fixation roller  101 , as the fixation roller  101  reduces in surface temperature. More concretely, in an image forming operation in which sheets of cardstock or the like are continuously conveyed, the target temperature level for the fixation roller  101  is set to 165° C., whereas the target temperature level for the first support roller  103 , and that for the second support roller  104 , are set higher by 75° C. than that for the fixation roller  101 . 
     The surface layer of the external heat belt  105  is soiled by adhesive contaminants such as toner particles, paper dust, and the like which offset to the external heat belt  105  from a sheet P of recording medium. The cleaning roller  108  has a surface layer formed of silicon rubber, and adheres the toner particles, paper dust, and the like on the surface layer of the external heat belt  105 , to its surface layer. The cleaning roller  108  is kept pressed upon the external heat belt  105  by a preset amount of pressure. It cleans the surface of the external heat belt  105  by being rotated by the rotation of the external heat belt  105 . 
     When the fixing device  9  is kept on standby for the next job, its external heat belt  105  is kept separated from its fixation roller  101 . As an image formation job is sent to the image forming apparatus  100 , various preparatory operations are started by various devices in the image forming apparatus  100 . One of the preparatory operation is the warmup operation started by the fixing device  9 . As the fixation roller  101 , and pressure roller  102  reach their target temperature level in the warmup operation, the external heat belt  105  is pressed upon the fixation roller  101 . Then, the image formation job is started. As the image formation job is completed, the external heat belt  105  is separated from the fixation roller  101 , and then, it is kept separated from the fixation roller  101  until the next image formation job is started. 
     (Angle Between Two Support Rollers) 
       FIG. 3  is a schematic sectional drawing for an engaging-disengaging mechanism for placing the external heat belt  105  in contact with the fixation roller  101 , or separating the external heat belt  105  from the fixation roller  101 .  FIG. 4  is a schematic drawing for describing the mechanism for rotationally moving the holding frames.  FIG. 5  is a schematic drawing for describing the skew angle between the generatrix of the fixation roller and that of the external heat belt  105 .  FIG. 6  is a schematic drawing for describing the effects of the angle of rotational movement of the external heating unit  150  of the fixing device  9 , upon the prevention of the problem that controlling the external heat belt  150  in lateral deviation makes the nip between the external heat belt  105  and fixation roller  101 , nonuniform in internal pressure. 
     Referring to  FIG. 3 , the external heating unit  150  is structured so that the external heat belt  105  is suspended and kept stretched by the first and second support rollers  103  and  104 , in such a manner that the external heat belt  105  is rotated by the rotation of the fixation roller  101 . 
     The external heat belt  105  can be placed in contact with, or separated from, the fixation roller  101  by the engaging-disengaging mechanism  200 . The mechanism  200  doubles as the mechanism for pressing the first and second support rollers  103  and  104  against the fixation roller  101  with the placement of the external heat belt  105  between the two support rollers  103  and  104  and the fixation roller  101 . A pressure application frame  201  is pivotally movable relative to the frame  9   f  of the fixing device  9 , about a pivot  203 , by which the pressure application frame  201  is supported. 
     There is disposed a compression spring  204  between the lengthwise opposite end portion of the pressure application frame  201  from the pivot  203 , and the frame  9   f  of the fixing device  9 . Thus, the compression spring  204  presses downward the opposite end of the pressure application frame  201  from the pivot  203 , pressing thereby the pressure application frame  201  toward the fixation roller  101 . The middle frame  208  is supported by a pair of middle rollers  210 , disposed on the front and rear sides of the middle frame  208 , in such a manner that they can be rotationally moved relative to the pressure application frame  201 . While the first and second support rollers  103  and  104  are remaining pressed against the fixation roller  101 , with the presence of the external heat belt  105  between the two rollers  103  and  104  and the fixation roller  101 , the overall amount of pressure generated by the compression spring  204  is 392 N (roughly 40 kgf). 
     A pressure removal cam  205  is placed in contact with, or separated from, the bottom surface of the tip portion of the pressure application frame  201 . The control section  140  controls a motor  210  to rotate the pressure removal cam  205  to pivotally move the pressure application frame  201  about the axle  205   a  so that the tip portion of the pressure application frame  201  moves upward or downward. As the pressure removal cam  205  is separated from the pressure application frame  201 , the compression spring  204  is allowed to extend to move downward the tip portion of the pressure application frame  201 , and therefore, the external heat belt  105  is pressed upon the fixation roller  101 . As the pressure removal cam  205  moves the pressure application frame  201  upward while compressing the compression spring  204 , the external heat belt  105  is separated from the fixation roller  101 . 
     Referring to  FIG. 4 , the front end of the first support roller  103 , and the front end of the second support roller  104 , are supported by the front holding frame  206   a , which is supported by the axles  207   a  and  207   b  so that it is allowed to rotationally move relative to the middle frame  208 . The holding frame  206   a  rotatably holds the front end of the first support roller  103 , and the front end of the second support roller  104 , with the placement of unshown thermally insulating bushing, and bearing, between the holding frame  206   a  and first and second support rollers  103  and  104 . That is, one of the lengthwise ends of the first support roller  103 , and the corresponding end of the second support roller  104 , are rotationally supported by the unshown bearings fixed to the holding frames  206   a , which is rotatably supported by the axles  207   a  and  207   b , respectively, with which the middle frame  208  is provided. 
     The rear end of the first support roller  104 , and the rear end of the second support roller  104 , are supported by the rear holding frame  206   b , which is supported by the axles  207   c  and  207   d  so that it is allowed to rotationally move relative to the middle frame  208 . The holding frame  206   b  supports the rear end of the first support roller  103 , and the rear end of the second support roller  104 , with the placement of unshown thermally insulating bushing, and bearing, between the holding frame  206   b  and first and second support rollers  103  and  104 . That is, one of the lengthwise ends of the second support roller  103 , and the corresponding end of the second support roller  104 , are rotationally supported by unshown bearings fixed to the holding frame  206   b , which is rotatably supported by the axles  207   c  and  207   d , respectively, with which the middle frame  208  is provided. The shafts  207   a - 207   d  are coaxial. 
     Referring to  FIG. 4 , there are disposed compression springs  204   a  and  204   b  at the lengthwise ends of the pressure application frame  201 , one for one. The compression springs  204  apply a preset amount of pressure to the first and second support rollers  103  and  104  to press the external heat belt  105  upon the peripheral surface of the fixation roller  101 . The line which is perpendicular to the axle  207  ( 207   a ,  207   b ,  207   c  and  207   d ) and coincident to the center of the axle  207  and the axial line of the fixation roller  101  is perpendicular to, and bisects, the line which is coincident to the center of the first support roller  103  and the center of the second support roller  104 . 
     Referring to  FIG. 5 , in a case where the angle between the generatrix of the external heat belt  105  and that of the fixation roller  101  is θ, the rear end of the first support roller  103  or the rear end of the second support roller  104  begins to press the fixation roller  101  before the other, and at the same time, the corresponding front end of the first support roller  103  or the second support roller  104  begins to press the fixation roller  101  before the other. That is, the first support roller  103  (or second support roller  104 ) is slanted (angled) in such a manner that one of the lengthwise end of the first support roller  103  (or second support roller  104 ) digs into the fixation roller  101 , and the other lengthwise end separates from the fixation roller  101 . Therefore, unless the external heating unit  150  is not provided with a displacing mechanism (rotational mechanism), the nip between the fixation roller  101  and external heat belt  105  becomes nonuniform in internal pressure in terms of the lengthwise direction of the fixation roller  101 . 
     On the other hand, in the case of a fixing device provided with a displacing mechanism (rotational mechanism) as shown in  FIG. 6(   a ), the difference between the first and second support rollers  103  and  104  in the amount of pressure they apply to the fixation roller  101  causes the front and rear holding frames  206   a  and  206   b , respectively, to rotationally move. Thus, the first and second support rollers  103  and  104  are autonomously equalized in the amount of pressure they apply to the fixation roller  101 . More concretely, the front and rear holding frames  206   a  and  206   b  are rotationally moved relative to each other in the direction perpendicular to their lengthwise direction, being thereby positioned to accommodate the curvature of the fixation roller  101 . There is no limit to the angle by which the first and second support rollers  103  and  104  are rotationally moved relative to each other. Therefore, the first and second support rollers  103  and  104  autonomously adjust themselves in attitude, positioning themselves to accommodate the curvature of the fixation roller. Therefore, the external heat belt  105  is kept perfectly in contact with the fixation roller  101 . 
     Next, referring to  FIG. 6(   b ), on the front side of the fixing device  9 , the holding frame ( 206   a ) rotates about the axles  207   a  and  207   b  in such a manner that the difference between the amount of pressure between the first support roller  103  and fixation roller  101 , and that between the second support roller  104  and fixation roller  101  is eliminated. Next, referring to  FIG. 6(   c ), on the rear side of the fixing device  9 , holding frame ( 206   b ) rotates about the axles  207   c  and  207   d  in such a manner that the difference between the amount of pressure between the first support roller  103  and fixation roller  101 , and that between the second support roller  104  and fixation roller  101  is eliminated. Therefore, even though the angle θ between the external heat belt  105  and fixation roller  101  is changed by the controlling of the lateral deviation of the external heat belt  105 , the nip between the external heat belt  105  and fixation roller  101  remains uniform in the internal pressure. 
     That is, the first and second support rollers  103  and  104  become the same in the amount of pressure by which they are pressed against the fixation roller  101 . Therefore, the fixation roller  101  is satisfactorily heated by the first and second support rollers  103  and  104  through the external heat belt  105 , across its front side as well as the rear side. 
     (Steering Mechanism) 
       FIG. 7  is a schematic drawing for describing the mechanism (rotational mechanism) for steering the external heat belt  105 .  FIG. 8  is a schematic drawing for describing the driving portion of the steering mechanism.  FIG. 9  is an enlarged view of the driving portion of the steering mechanism. 
     Referring to  FIG. 2 , the fixing device  9  is structured so that the adverse effect of the controlling of the lateral deviation of the external heat belt  105  is cancelled by rotationally moving the external heating unit  150  while keeping the first and second support rollers  103  and  104  unchanged in positional relationship. 
     Referring to  FIG. 5 , in a case where the angle between the external heat belt  105  and fixation roller  101  is θ when the external heat belt  105  came into contact with the fixation roller  101 , the rotation of the fixation roller  101  generates a force that presses the external heat belt  105  in the direction which is parallel to the lengthwise direction of the first and second support rollers  103  and  104 . This principle (phenomenon) is used by the fixing device  9  to set the direction in which the external heat belt  105  is made to laterally shift. That is, the angle θ is intentionally changed to set the direction in which the external heat belt  105  is made to laterally shift. 
     As the external heat belt  105  is rotated, it laterally deviates in the direction parallel to the lengthwise direction of the first and second support roller  103  and  104 . The cause of this lateral deviation of the external heat belt  105  is that the first and second support roller  103  and  104  are not perfectly in parallel to each other, and also, that the aforementioned angle θ is not zero. 
     Referring to  FIG. 7 , the axle  209  is positioned so that it extends in the direction which is perpendicular to the area of contact between the fixation roller  101  and external heat belt  105 . The axle  209  is a shaft about which the external heating unit  150  is rotationally moved to change the angle θ between the external heat belt  105  and fixation roller  101 . In terms of the direction perpendicular to the moving direction of the external heat belt  105 , the axle  209  is at the center of the external heating unit  150 . Therefore, the external heating unit  150  can keep the frond and rear sides of the nip, balanced in internal pressure, in terms of the lengthwise direction of the fixation roller  101 . 
     In order to control the direction in which the external heat belt  105  laterally shifts, the control section  140  changes the angle θ between the generatrix of the external heat belt  105  and that of the fixation roller  101  by rotationally moving together the first and second support rollers  103  and  014  about the axle  209 . That is, in order to keep the lateral deviation of the external heat belt  105  within a preset range, the control section  140  externally forces the angle θ between the generatrix of the external heat belt  105  and that of the fixation roller  101  to change in order to reverse the external heat belt  105  in the direction of its lateral shift. 
     An axle  203  by which the pressure application frame  201  is rotatably supported is fixed to the lateral plates  202  of the main assembly of the external heating unit  150 , by its lengthwise ends. The middle frame  208  and external heat belt  105  are rotationally movable together relative to the pressure application frame  201 , about the axle  209 . The axle  207   a  is fixed to the middle frame  208 . Further, the lateral plate  202 , which corresponds in position to the axle  207   a , is provided with such a hole that provides a certain amount of clearance between the axle  207   a  and lateral plate  202 . Thus, the axle  207   a  is allowed to be moved by the movement of the arm portion  118   a  of the worm portion  118  in the directions indicated by arrow marks H and J, within the range which the clearance affords. 
     The worm wheel  118  is shaped like a fan, and is rotationally movable about the axle  119 . It is in engagement with the worm gear  120 . As the worm wheel  118  is rotated in the direction indicated by an arrow mark G, by the rotation of a motor  125  in the normal direction, the arm portion  118   a  is moved in the direction indicated by the arrow mark H, causing thereby the axle  207   a  to move in the direction of the arrow mark H. As the worm wheel  118  is rotated in the direction indicated by the arrow mark I by the rotation of the motor  125  in the reverse direction, the arm portion  118   a  is moved in the direction indicated by the arrow mark J, causing thereby the axle  207   a  to move in the direction of the arrow mark J ( FIGS. 7 and 8 ). 
     As the middle frame  208  is moved in such a direction that its front end moves in the direction indicated by the arrow mark H or J, the first and second support rollers  103  and  104  are made to rotationally moved together about the axle  209 . Consequently, the first and second support rollers  103  and  104  become angled relative to the fixation roller  101  by the angle θ. There is a correlation between the angle θ between the fixation roller  101  and external heat belt  105 , and the speed at which the external heat belt  105  is made to laterally shift. The amount of the force generated in the direction to laterally move the external heat belt  105  is affected by the distance by which the arm portion  118   a  is moved. Thus, the direction in which the external heat belt  105  is made to laterally shift, and the speed with which the external heat belt  105  is made to laterally shift, are controlled by the direction in which the arm portion  118   a  is moved, and the distance by which the arm portion  118   a  is moved, respectively. 
     As the axle  207   a  is moved in the direction indicated by the arrow mark H from the position in which the amount of the force which works in the direction to laterally shift the external heat belt  105  is zero, the force which works in the direction to move the external heat belt  105  rearward (indicated by arrow mark M) of the fixation roller  101  increases. On the other hand, as the axle  207   a  is moved in the direction indicated by the arrow mark J from the position in which the amount of the force which works in the direction to laterally shift the external heat belt  105  is zero, the force which works in the direction to move the external heat belt  105  frontward (indicated by arrow mark L) of the fixation roller  101  increases. In other words, the direction (indicated by arrow mark M or L) in which the external heat belt  105  is made to laterally shift can be controlled by changing the direction (indicated by H or J, respectively) in which the axle  207   a  is moved. 
     In this embodiment, the home position for the axle  207   a , which is set by the worm wheel  118 , is such a position that makes the external heating unit  150  parallel to the fixation roller  101 . Whether the axle  207   a  is in its home position or not is detected by a photo-interrupter  135  attached to the worm wheel  118 . 
     As the external heat belt  105  is rotated by the rotation of the fixation roller  101 , the external heat belt  105  laterally shifts frontward or rearward of the fixation roller  101 . Thus, the control section  140  moves the axle  207   a  in the direction to move the external heat belt  105  in the opposite direction from the direction in which the external heat belt  105  has shifted. 
     (Belt Position Sensor) 
       FIG. 10  is a schematic drawing for describing the positioning of the belt position sensor.  FIG. 11  is a schematic drawing for describing the positional relationship between the direction of the belt deviation, and the direction of the rotational movement of the sensor flag. Referring to  FIG. 10(   b ), an arm  129  and a roller  128  rotate together about an axle  137 . A sensor flag  132  rotates about the axle  137 . 
     Referring to  FIG. 10(   a ), the arm and sensor flag  132  are connected to each other by an axle  138 , and transmit the rotational movement of the combination of the arm  129  and roller  128 . The roller  128  is in contact with the edge of the external heat belt  105 . A torsion spring  131  provides the arm  129  with a torsional force that keeps the roller  128  pressed in the direction indicated by an arrow mark Q. 
     Referring to  FIG. 10(   b ), as the external heat belt  105  deviates in the direction indicated by the arrow mark Q, the axle  138  is moved in the direction indicated by an arrow mark P. On the other hand, as the external heat belt  105  deviates in the direction indicated by the arrow mark R, the axle  138  is moved in the direction indicated by an arrow mark O. 
     Referring to  FIG. 11(   a ), there are disposed a pair of photo-interrupters  133  and  134  along semicircular edge of the sensor flag  132 . The sensor flag  132  is provided with a pair of slits, which provides the sensor flag  132  with four edges, which are detected by the photo-interrupters  133  and  134 , which reverse their output as they detect the edges. The four edges of the sensor flag  132  are correlated to the amount of the deviation of the external heat belt  105 . For example, the photo-interrupters  132  and  133  are positioned so that the distance by which the external heat belt  105  laterally shifts before it reverses in direction becomes 5 mm. As the external heat belt  105  deviates in the direction indicated by the arrow mark R, the arm  129  is rotationally moved in the direction indicated by the arrow mark S. Thus, the sensor flag  132  is rotationally moved in the direction indicated by the arrow mark T. Consequently, the photo-interrupter  133  is turned off, and the photo-interrupter  134  is turned on. 
     Referring to  FIG. 11(   b ), as the external heat belt  105  deviates in the direction indicated by the arrow mark Q, the arm  129  is rotationally moved in the direction indicated by the arrow mark U. Thus, the sensor flag  132  is rotationally moved in the direction indicated by an arrow mark V. Consequently, the photo-interrupter  133  is turned on, and the photo-interrupter  134  is turned off. 
     (Comparative External Heating Unit) 
       FIG. 12  is a schematic drawing for describing the comparative fixing device. 
     Referring to  FIG. 7 , the external heating unit  150  places its external heat belt  105  in contact with the fixation roller  101  to directly heat the peripheral surface of the fixation roller  101 . The external heating unit  150  rotationally moves about the axle  209  to change the angle θ between the generatrix of the external heat belt  105  and that of the fixation roller  101 , controlling thereby the external heat belt  105  in lateral shift. As the external heat belt  105  is controlled in lateral deviation, the holding frames  206   a  and  206   b  are rotationally moved relative to each other, about the combination of the axles  207   a  and  207   b , and the combination of the axles  207   c , and  207   d , respectively. Thus, the first and second support rollers  103  and  104  are angled relative to each other. Consequently, the front and rear sides of the nip between the first support roller  103  and fixation roller  101 , and the front and rear sides of the nip between the second support roller  104 , become uniform in internal pressure in terms of the lengthwise direction of the fixation roller  101 . 
     In a case where an external heating unit ( 150 ) is structured so that there is no limit to the angle by which the first and second heat rollers  103  and  104  are allowed to rotationally moved relative to each other, it is possible that when the external heating unit ( 150 ) is assembled, and/or when the external heating unit ( 150 ) is lifted independently from the other portions of a fixing device, the angle between the first and second support rollers  103  and  104  will become excessive. 
     Referring to  FIG. 12(   a ), the surface temperature of the external heat belt  105  is detected by the thermistors  123  ( 123   a  and  123   b ) which are in contact with the portions of the external heat belt  105 , which are in contact with the first support roller  103 . As the angle between the first and second support rollers  103  and  104  becomes excessive, the distance between the thermistors  123   a  and external heat belt  105 , and the distance between the thermistor  123   b  and external heat belt  105 , become smaller or larger than the amount of separation which is anticipated to occur as the external heat belt  105  is controlled in its lateral deviation. 
     Referring to  FIG. 12(   b ), in particular, the thermistor  123   a , which is positioned close to the center of the external heat belt  105 , in terms of the widthwise direction of the external heat belt  105 , to detect the temperature of the center portion of the external heat belt  105 , is seriously affected by the distance. More specifically, if the leaf spring  123   m  by which the thermistor  123   a  is supported is permanently deformed by the excessive increase in the angle between the first and second support rollers  103  and  104 , it is possible that the thermistor  123   a  will fail to remain in contact with the outward surface of the external heat belt  105 . 
     In this embodiment, therefore, the comparative external heating unit  150  is provided with a limiter for limiting the angle by which the first and second support rollers  103  and  104  are allowed to be slanted (angled) relative to each other, in order to solve the above described problem which the comparative external heating unit  150  suffers. That is, in the case of the external heating unit  150  in the first embodiment, the limiter regulates the angle by which the first and second support rollers  103  and  104  are allowed to be angled relative to each other. Therefore, the amount by which the leaf spring  123   m , by which the thermistor  123   a  is kept in contact with the external heat belt  105  to detect the surface temperature of the external heat belt  105 , is deformed is reduced. 
     (Regulating Portion) 
       FIG. 13  is a schematic drawing for describing the positioning of the regulating portions (limiting mechanisms) in the first embodiment.  FIG. 14  is a schematic drawing for describing the operation of the holding frames.  FIG. 15  is a perspective view of the regulating portions.  FIG. 16  is a schematic drawing for describing the operation of the regulating portions. 
     Referring to  FIG. 13 , the external heating unit  150  is separable into the top and bottom portions  150 U and  150 D. The top portion  150 U rotationally moves the middle frame  208  about the axle  209  to move the middle frame  208  relative to the pressure application frame  201 . The bottom portion  150 D is rotatably hung by the axles  207   a ,  207   b ,  207   c  and  207   d , with which the middle frame  208  is provided. The bottom portion  150 D rotatably supports one of the lengthwise end of the first support roller  103 , and the corresponding lengthwise end of the second support roller  104 , by its holding frame  206   a . Further, it rotatably supports the other lengthwise end of the first support roller  103 , and the corresponding lengthwise end of the second support roller  104 , by its holding frame  206   b.    
     Referring to  FIG. 14 , the bottom portion  150 D rotationally moves the holding frames  206   a  and  206   b  relative to each other in such a manner that the combination of the holding frames  206   a  and  206   b  is twisted at the center of the combination in terms of the lengthwise direction of the combination. As the holding frames  206   a  and  206   b  are rotationally moved relative to each other, the first and second support rollers  103  and  104  become angled relative to each other. Consequently, the nip between the external heat belt  105  and fixation roller  101  autonomously becomes roughly uniform in its internal pressure, in terms of the lengthwise direction of the external heat belt  105 . 
     Referring to  FIG. 13 , the holding frames  206   a  and  206   b  have regulating portions  300 A and  300 B which limit the angle by which the holding frames  206   a  and  206   b  are allowed to be rotationally moved relative to each other to limit the angle (amount of displacement) by which the first and second support rollers  103  and  104  are allowed to rotationally move relative to each other. 
     Referring to  FIG. 15 , the holding frame  206   a  has two bent portions  301   a  and  301   b , each of which was formed by bending a part of the holding frame  206   a  at two positions. In terms of the lengthwise direction of the bottom portion  150 D, the bent portions  301   a  and  301   b  are at the center of the  150 D. In terms of the widthwise direction of the  150 D, the bent portions  301   a  and  301   b  are at the widthwise ends of the  150 D, one for one. Further, the bent portion  301   a  is on one side of the axle  207   b , and the bent portion  301   b  is on the other side of the axle  207   b . The holding frame  206   b  is provided with flat portions  302   a  and  302   b , which protrude toward the holding frame  206   a  from the inward edge of the main portion of the holding frames  206   b . The flat portion  302   a  is on one side of the axle  207   c , and the flat portion  302   b  is on the other wide of the axle  207   c . Regarding the regulating portion  300 A, its bent portion  301   a , which is a part of the holding frame  206   a , extends toward the holding frame  206   b , far enough to reach the area above the flat portion  302   a  of the holding frame  206   b , overlapping with the flat portion  302   a  of the holding frame  206   b  in terms of the lengthwise direction of the bottom portion  150 D. As for the regulating portion, the bent portion  301   b , which is a part of the holding frame  206   a , extends to the area above the flat portion  302   b  of the holding frame  206   b , overlapping with the flat portion  302   b  in terms of the lengthwise direction of the bottom portion  150 D. 
     Referring to  FIG. 16(   a ), as the holding frame  206   b  rotationally moves relative to the holding frame  206   a  in the direction indicated by an arrow mark A, the flat portion  302   b  of the holding frame  206   b  comes into contact with the bent portion  301   b  of the holding frame  206   a , preventing thereby holding frame  206   b  from rotationally moving further. 
     Referring to  FIG. 16(   b ), as the holding frame  206   b  rotationally moves relative to the holding frame  206   a  in the direction indicated by an arrow mark B, the flat portion  302   a  of the holding frame  206   b  comes into contact with the bent portion  301   a  of the holding frame  206   a , preventing thereby holding frame  206   b  from rotationally moving further. 
     As described above, as the holding frame  206   b  rotationally moves relative to the holding frame  206   a , the flat portions  302   a  and  302   b  of the holding frame  206   b  come into contact with the bent portions  301   a  and  301   b  of the holding frame  206   a , respectively. Therefore, it does not occur that the angle between the first and second support rollers  103  and  104  becomes excessive. 
     (Rotational Angle of Holding Frames Relative to Each Other) 
       FIG. 17  is a schematic drawing for describing the angle by which the holding frames are allowed to rotationally move relative to each other.  FIG. 18  is a schematic drawing for describing the range to which the angle by which the holding frames are allowed to be rotationally moved relative each other is limited.  FIG. 19  is a schematic drawing for describing the effects of the first embodiment. 
     Referring to  FIG. 17(   a ), the angle by which the external heating unit  150  is rotationally moved about the axle  209  to change the angle between the generatrix of the fixation roller  101  and that of the external heat belt  105 , in order to control the lateral deviation of the external heat belt  105  in the direction parallel to the lengthwise direction of the first and second support rollers  103  and  104 . The angle α between the fixation roller  101  and external heat belt  105  is controlled according to the change in the speed of the lateral deviation of the external heat belt  105 , change in the distance of the lateral deviation of the external heat belt  105 , etc. The angle α is set to be no more than a certain numerical value. In a case where the lateral deviation of the external heat belt  105  cannot be controlled even if the angle α is set to the maximum value, it is determined that something is wrong with the external heating unit  150 . Then, an error message is displayed. In the first embodiment, the maximum value α x  for the angle α is set to 2°. 
     Referring to  FIG. 17(   b ), as the angle α between the external heat belt  105  and fixation roller  101  is changed, the first and second support rollers  103  and  104  by which the external heat belt  105  is suspended and kept stretched are rotationally moved relative to each other. That is, they are slanted (angled) relative to each other. As a result, the holding frames  206   a  and  206   b  are rotationally moved (twisted) relative to each other, and therefore, they become slanted relative each other by an angle β. In this embodiment, the maximum value β max , for the angle that is, the maximum angle by which the holding frames  206   a  and  206   b  are rotationally movable relative to each other when the angle α between the generatrix of the fixation roller  101  and that of the external heat belt  105  is the maximum value α max , is set to 6°. Referring to  FIG. 18 , the angle γ between the holding frames  206   a  and  206   b  when the bent portion  301   b  of the holding frame  206   a  is in contact with the flat portion  302   b  of the holding frame  206   b , that is, when the holding frame  206   a  and  206   b  are in their positions which do not allowed to rotationally move further relative to each other, is set so that the relation between the angle γ and maximum value β max  satisfies the following mathematical formula:
 
γ≧β max .
 
     The angle γ is affected by the component tolerance. Therefore, the external heating unit  150  is desired to be designed so that the angle γ becomes greater than the maximum value β max  (γ&gt;β max ). If the angel γ is no more than the maximum value β max  the first and second support rollers  103  and  104  are insufficiently slanted relative to each other, and therefore, the nip between the external heat belt  105  and fixation roller  101  becomes nonuniform in the internal pressure in terms of the lengthwise direction of the fixation roller  101 . 
     In the first embodiment, the angle γ was set to 8° which is greater than the maximum vale β max , which was 6°. Therefore, it does not occur that when the lateral deviation of the external heat belt  105  is controlled, the nip between the external heat belt  105  and fixation roller  101  is affected in the distribution of its internal pressure in terms of the lengthwise direction of the fixation roller  101 . Further, it does not occur that the when the external heating unit  150  is assembled, and/or when the external heating unit  150  is lifted, the first and second support rollers  103  and  104 , by which the external heat belt  105  is suspended and kept stretched, are excessively slanted relative to each other. 
     Referring to  FIG. 19(   a ), in the case of the first comparative external heating unit ( 150 ), the distance between the external heat belt  105  and thermistor  123   a  is changed by the slanting of the first support roller  103  relative to the second support roller  104  is substantial. Therefore, it is possible that the leaf spring  123   m  will be permanently deformed. Referring to  FIG. 19(   b ), in the first embodiment, however, the external heating unit ( 150 ) is regulated so that the change which occurs to the distance between the external heat belt  105  and thermistor  123   a  as the first support roller  103  is slanted relative to the second support roller  104  remains relatively small. Therefore, it does not occur that the leaf spring  123   m  is permanently deformed. The first embodiment ensures that when the angle θ between the fixation roller  101  and external heat belt  105  is changed to control the external heat belt  105  in lateral deviation, the holding frames  206   a  and  206   b  are allowed to rotationally move relative to each other by an angle which is satisfactory to keep the nip between the external heat belt  105  and fixation roller  101  uniform in internal pressure in terms of the lengthwise direction of the external heat belt  105 . That is, the nip between the external heat belt  105  and fixation roller  101  is not affected in the distribution of its internal pressure, in terms of the lengthwise direction of the external heat belt  105 , by the controlling of the lateral deviation of the external heat belt  105 . In addition, the first embodiment regulates the angle by the holding frames  206   a  and  206   b  are slanted relative to each other, in order to keep within a tolerable range, the angle by which the first and second supporting rollers  103  and  104  are slanted relative to each other to control the lateral deviation of the external heat belt  105 . Therefore, it can reduce the distance by which the leaf spring  123   m  is displaced. 
     Further, according to the first embodiment, it does not occur that when the external heat belt  105  is replaced, and/or when the external heating unit  150  is individually lifted, the first and second support rollers  103  and  104  become excessively slanted relative to each other. 
     In the first embodiment, the external heating unit  150  is provided with the axle  209 , in order to change the angle between the external heating unit  150  and fixation roller  101  to control the lateral deviation of the external heat belt  105 . Further, the external heating unit  150  is structured so that the holding frame  206   a  which holds the first support roller  103  by the lengthwise ends of the roller  103 , and the holding frame  206   b  which holds the second support roller  104  by the lengthwise ends of the roller  104 , are allowed to be slanted relative to each other. 
     However, the angle by which the holding frames  206   a  and  206   b  are allowed to be slanted relative to each other is made to be greater than the maximum angle by which the first and second support rollers  103  and  104  are allowed to be slanted relative to each other to control the lateral deviation of the external heat belt  105 . Therefore, it is possible to regulate the distance by which the thermistor  123   m , which is placed in contact with the surface of the external heat belt  105  to detect the surface temperature of the external heat belt  105 , is displaced, without causing the slanting of the holding frames  206   a  and  206   b  relative to each other to affect the controlling of the lateral deviation of the external heat belt  105 . Therefore, it does not occur that when the external heating unit  150  is assembled, the first and second support rollers  103  and  104  are excessively slanted relative to each other. Therefore, it does not occur that when the external heating unit  150  is assembled, the thermistor  123   a , which is to remain in contact with the surface of the external heat belt  105  to detect the surface temperature of the external heat belt  105  is displaced by an undesirably large distance. 
     Embodiment 2 
       FIG. 20  is a schematic drawing for describing the positioning of the regulating portion of the external heating unit in the second embodiment of the present invention.  FIG. 21  is a schematic drawing for describing the positioning of the thermistors in the second embodiment.  FIG. 22  is a schematic drawing for describing the structural arrangement, in the second embodiment, for controlling the rotational movement of the sensor supporting axle relative to the holding frame.  FIG. 23  is a schematic drawing for describing the rotationally movement of the holding frames relative to each other, which is caused to slant the frames relative to each other.  FIG. 24  is a schematic drawing for describing the operation of the regulating portion in the second embodiment. Referring to  FIG. 21 , the sensor supporting shaft  303 , which is an example of a beam supportable by both of its lengthwise ends, is disposed between the holding frames  206   a  and  206   b  in such a manner that it bridges between the holding frames  206   a  and  206   b . As the holding frames  206   a  and  206   b  are rotationally moved relative to each other in the direction parallel to the recording medium conveyance direction, the first and second support rollers  103  and  104  are slanted (angled) relative to each other in the direction parallel to the recording medium conveyance direction. 
     Referring to  FIG. 12 , in the first embodiment, as the holding frames  206   a  and  206   b  are rotationally moved relative to each other, the distance between the horizontal portion of the holding frame  206   a , and the peripheral surface of the first support roller  103  substantially changes. Therefore, it is necessary that the leaf spring  123   m  fixed to the horizontal portion of the holding frame  206   a , by its base portion, substantially deforms to tolerate the large amount of displacement of the thermistor  123   m.    
     Referring to  FIG. 20 , in the second embodiment, as the holding frames  206   a  and  206   b  are rotationally moved relative to each other, the base portion of the leaf spring  123   m  is allowed to move upward or downward to reduce the distance by which the thermistor  123   m  is moved relative to the base portion of the leaf spring  123   m . Therefore, the range in which the distance between the thermistor  123   a  and the peripheral surface of the first support roller  103 , and the distance between the thermistor  123   b  and the peripheral surface of the first support roller  103 , are allowed to change as the first and second support rollers  103  and  104  are angled relative to each other, is relatively small. 
     Referring to  FIG. 20 , in the second embodiment, the sensor supporting shaft  303  is disposed between the holding frames  206   a  and  206   b , in parallel to the first support roller  103 . Further, the sensor supporting shaft  304  is disposed between the holding frames  206   a  and  206   b , in parallel to the second support roller  104 . Further, the thermistors  123   a  and  123   b  are supported by the sensor supporting shaft  303 , and the thermistors  124   a  and  124   b  are supported by the sensor supporting shaft  304 . 
     One of the lengthwise ends of the first support roller  103 , and the corresponding lengthwise end of the second support roller  104 , are rotatably supported by the holding frame  206   a , which is rotatably supported by the axle  207   a  attached to the middle frame  208 . The opposite lengthwise end of the first support roller  103  from the lengthwise end supported by the holding frame  206   a , and the corresponding lengthwise end of the second support roller  104 , are rotatably supported by the holding frame  206   b , which is rotatably supported by the axle  207   b  attached to the middle frame  208 . 
     Referring to  FIG. 21 , the sensor supporting shafts  303  and  304  are put through the holding frames  206   a  and  20   b  in such a manner that they bridge between the holding frames  206   a  and  206   b . The leaf springs  123   m  and  123   n  are fixed to the sensor supporting shaft  303 , by their base portions. The thermistors  123   a  and  123   b  are fixed to the tips of the leaf springs  123   m  and  123   n , respectively. The leaf springs  124   m  and  124   n  are fixed to the sensor supporting shaft  304 , by their base portions. The thermistors  124   a  and  124   b  are fixed to the tips of the leaf springs  124   m  and  124   n , respectively. 
     Referring to  FIG. 22 , the lengthwise end portion of the sensor supporting shaft  303 , and the lengthwise end portion of the sensor supporting shaft  304 , which are to be put through the holding frame  206   a , are shaped so that they appear like a letter D in cross-section. That is, the sensor supporting shafts  303  and  304  are prevented from rotating relative to the holding frame  206   a , while being allowed to rotate relative to the holding frame  206   b . Further, after the sensor supporting shafts  303  and  304  are put through the holding frames  206   a  and  206   b , the portions of the sensor supporting shafts  303  and  304 , which are on the outward side of the holding frames  206   a  and  206   b , are fitted with a locking ring  305  which is for preventing the shafts  303  and  304  from disengaging from the holding frames  206   a  and  206   b.    
     Referring to  FIG. 23(   a ), as the holding frame  206   a  and  206   b  are rotationally moved relative to each other by the controlling of the lateral deviation of the external heat belt  105 , the first and second support rollers  103  and  104  are slanted relative to each other, and at the same time, the sensor supporting shafts  303  and  304  are also slanted relative to each other. Therefore, the first and second support rollers  103  and  104  remain roughly parallel to the sensor supporting shafts  303  and  304 , respectively. Thus, this embodiment is smaller than the first embodiment, in the amount of the change in the distance between the thermistor  123   a  and the first support roller  103 , and the distance between the thermistor  123   b  and first support roller  103 , which is caused by the rotational movement of the holding frames  206   a  and  206   b  relative to each other, and also, in the amount of change in the distance between the thermistor  124   a  and second support roller  104 , and also, the amount of change in the distance between the thermistor  124   b  and second support roller  104 . 
     Referring to  FIGS. 23(   b ) and  23 ( c ), therefore, when the first and second support rollers  103  and  104  are slanted relative to each other, the sensor supporting shafts  303  and  304  remain in parallel to the first and second support rollers  103  and  104 , respectively. Further, the sensor supporting shafts  303  and  304  are prevented by the locking rings  305  from disengaging from the holding frames  206   a  and  206   b , because the direction in which the sensor supporting shaft  303  is slanted is the same as the direction in which first support roller  103  is slanted relative to the second support roller  104 , and the direction in which the sensor supporting shaft  304  is slanted is the same as the direction in which the second support roller  104  is slanted relative to the first support roller  103 . 
     Therefore, this embodiment is smaller than the first embodiment, in the change, in position, of the areas of contact between the thermistors  123   a  and  123   b  attached to the leaf springs  123   m  and  123   n , respectively, attached to the sensor supporting shaft  303 , and the first support roller  103 , and also, in the change, in position, of the areas of contact between the thermistors  124   a  and  124   b  attached to the leaf springs  124   m  and  124   n , respectively, attached to the sensor supporting shaft  304 , and the second support shaft  104 . In the second embodiment, therefore, the leaf springs  123   m ,  123   n ,  124   m  and  124   n  are less likely to be permanently deformed by the controlling of the lateral deviation of the external heat belt  105 , than in the first embodiment. 
     However, also in the case of the second embodiment, it is possible that when the external heating unit  150  is assembled, and/or when the external heating unit  150  is independently lifted from the other components of a fixing device, the first and second support rollers  103  and  104  will be slanted relative to each other by an angle large enough to cause the leaf spring  123   m  to be permanently deformed. 
     Referring to  FIG. 20 , in this embodiment, therefore, the front side of the middle frame  208  is provided with a pair of flat portions  305   a  and  305   b  to limit the angle by which the holding frame  206   a  is allowed to rotationally move. Further, the rear side of the middle frame  208  is provided with a pair of flat portions  306   a  and  306   b  to limit the angle by which the holding frame  206   b  is allowed to rotationally move. 
     Referring to  FIG. 24 , on the front side, as the holding frame  206   a  is rotationally moved relative to the middle frame  208 , the holding frame  206   a  comes into contact with the flat portion  305   a  or  305   b  fixed to the middle frame  208 . Therefore, the angle by which the holding frame  206   a  is allowed to rotationally move relative to the middle frame  208  is regulated to be in a preset range. On the rear side, as the holding frame  206   b  is rotationally moved relative to the middle frame  208 , the holding frame  206   b  comes into contact with the flat portion  306   a  or  306   b  fixed to the middle frame  208 . Therefore, the angle by which the holding frame  206   b  is allowed to rotationally move relative to the middle frame  208  is regulated to be in a preset range. The preset ranges for the angles by which the holding frames  206   a  and  206   b  are allowed to rotationally move relative to the middle frame  208  may be the same as those in the first embodiment. That is, they may be set to be slightly wider than the angle by which the first and second support roller  103  and  104  are allowed to be slanted relative to each other to control the external heat belt  105  in lateral deviation, in order not to interfere with the controlling of the lateral deviation of the external heat belt  105 . Although  FIG. 24  shows only the structure of the holding frame  206   a  and its adjacencies, the holding frame  206   b  and its adjacencies are similar in structure to the holding frame  206   a  and its adjacencies. 
     The second embodiment is smaller than the first embodiment, in the distance by which the thermistors  123   a ,  123   b ,  124   a  and  124   b  are displaced. Therefore, it is higher than the first embodiment, in the repeatability with which the surface temperature of the external heat belt  105  (first and second support rollers  103  and  104 ) is accurately detected. On the other hand, in the second embodiment, the rotational movement of the external heating unit  150  is significantly controlled by the presence of the sensor supporting shafts  303  and  304 . Therefore, it is significantly smaller than the first embodiment, in the amount of the temperature detection error attributable to the permanent deformation of the leaf springs  123   m ,  123   n ,  124   m  and/or  124   n.    
     Embodiment 3 
       FIG. 25  is a schematic drawing for describing the regulating portion in the third embodiment of the present invention. Referring to  FIG. 25 , the number of the areas of the external heating unit  150  to which the mechanism for limiting the angle by which the first and second holding frames are allowed to be slanted relative to each other is attached may be only one. Otherwise, the external heating unit in the third embodiment is similar in structure to the external heating unit  150  in first embodiment. 
     As will be evident from the foregoing description of the first to third embodiment, the present invention encompasses various image heating apparatuses other than those in the preceding embodiments of the present invention, as long as they are structured so that the angle by which their first and second holding portions are allowed to be slanted relative to each other is limited to be in a preset range, whether they are partially or entirely different in structure from those in the preceding embodiments. 
     That is, the means for heating a rotational heating member, an endless heating belt (belt supporting members, as well), and the like, does not need to be limited to a halogen heater. For example, a rotational heating member, an endless heating belt, and the like may be provided with a heating layer in which heat can be inductively generated by an alternating magnetic flux. Further, a rotational heating member does not need to be limited to a fixation roller. For example, it may be a pressure roller capable of heating the opposite surface of a sheet of recording medium from the surface having an image. 
     Further, an image heating apparatus to which the present invention is applicable is not limited to a fixing device such as those in the preceding embodiments. That is, the present invention is also applicable to a surface heating apparatus (device) for altering a temporarily fixed image or a permanent fixed image in surface properties such as glossiness. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims priority from Japanese Patent Application No. 025457/2013 filed Feb. 13, 2013, which is hereby incorporated by reference.