Patent Publication Number: US-9405245-B2

Title: Fixing device comprising deformation preventing member for preventing deformation of fixing belt and image forming apparatus including same

Description:
INCORPORATION BY REFERENCE 
     This application is based on and claims the benefit of priority from Japanese Patent application No. 2014-227117 filed on Nov. 7, 2014, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a fixing device configured to fix a toner image onto a recording medium and an image forming apparatus including the fixing device. 
     Conventionally, an electrographic image forming apparatus, such as a copying machine or a printer, includes a fixing device configured to fix a toner image onto a recording medium, such as a sheet. 
     For example, there is a fixing device including a fixing belt, a pressuring member configured to come into pressure contact with the fixing belt so as to form a fixing nip, a heat source configured to heat the fixing belt, and a heating stop device configured to face an outer circumferential face of the fixing belt. In such a fixing device, upon an excessive rise in temperature of the fixing belt, the heating stop device operates so as to stop the fixing belt from heating by the heat source. 
     In the fixing device configured as described above, there is a concern that, when a facing interval between the fixing belt and the heating stop device is too narrow, the heating stop device operates even though the temperature of the fixing belt does not excessively rise. On the other hand, there is a concern that, when the facing interval is widened, if the fixing belt is broken in the circumferential direction, a timing at which the heating stop device operates delays. 
     SUMMARY 
     In accordance with an embodiment of the present disclosure, a fixing device includes a fixing belt, a pressuring member, a heat source, a pressing member, a heating stop device and a deformation preventing member. The fixing belt is configured to be rotatable around a rotation axis. The pressuring member is configured to be rotatable and to come into pressure contact with the fixing belt so as to form a fixing nip. The heat source is configured to heat the fixing belt. The pressing member is configured to press the fixing belt to a side of the pressuring member. The heating stop device is configured to face an outer circumferential face of the fixing belt and to operate at an operating temperature so as to stop the heat source from heating the fixing belt. The deformation preventing member is configured to face an inner circumferential face of the fixing belt. In a state where the fixing belt is not broken in a circumferential direction, the fixing belt and the deformation preventing member are arranged at an interval, and when the fixing belt is broken in the circumferential direction, the fixing belt is deformed and comes into contact with the deformation preventing member so that the fixing belt is prevented from being deformed to a side far from the heating stop device. 
     In accordance with an embodiment of the present disclosure, an image forming apparatus includes the above-mentioned fixing device. 
     The above and other objects, features, and advantages of the present disclosure will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present disclosure is shown byway of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a printer according to one embodiment of the present disclosure. 
         FIG. 2  is a sectional view showing a fixing device according to the one embodiment of the present disclosure. 
         FIG. 3  is a side view showing the fixing device according to the one embodiment of the present disclosure. 
         FIG. 4  is a block diagram showing a control system of the fixing device according to the one embodiment of the present disclosure. 
         FIG. 5  is a sectional view showing a state that the fixing belt is broken in a circumferential direction in the fixing device according to the one embodiment of the present disclosure. 
         FIG. 6  is a sectional view showing a state that the fixing belt is broken in a circumferential direction in the fixing device according to another different embodiment of the present disclosure. 
         FIG. 7  is a side view showing a fixing device according to still another different embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     First, with reference to  FIG. 1 , the entire structure of an electrographic printer  1  (an image forming apparatus) will be described. Hereinafter, it will be described so that the front side of the printer  1  is positioned at the front side of  FIG. 1 . Arrows Fr, Rr, L, R, U and Lo appropriately added to each of the drawings indicate the front side, rear side, left side, right side, upper side and lower side of the printer  1 , respectively. 
     The printer  1  includes a box-formed printer main body  2 . In a lower part of the printer main body  2 , a sheet feeding cartridge  3  configured to store sheets (recording medium) is installed and, on the top surface of the printer main body  2 , a sheet ejecting tray  4  is mounted. On the top surface of the printer main body  2 , an upper cover  5  is openably/closably attached at a left-hand side of the sheet ejecting tray  4  and, below the upper cover  5 , a toner container  6  is installed. 
     In an upper part of the printer main body  2 , an exposure device  7  composed of a laser scanning unit (LSU) is installed below the sheet ejecting tray  4 . Below the exposure device  7 , an image forming unit  8  is installed. In the image forming unit  8 , a photosensitive drum  10  as an image carrier is rotatably installed. Around the photosensitive drum  10 , a charger  11 , a development device  12 , a transfer roller  13  and a cleaning device  14  are located along a rotating direction (refer to arrow X in  FIG. 1 ) of the photosensitive drum  10 . 
     Inside the printer main body  2 , a sheet conveying path  15  is arranged. At an upper stream end of the conveying path  15 , a sheet feeder  16  is positioned. At an intermediate stream part of the conveying path  15 , a transferring unit  17  constructed of the photosensitive drum  10  and transfer roller  13  is positioned. At a lower stream part of the conveying path  15 , a fixing device  18  is positioned. At a lower stream end of the conveying path  15 , a sheet ejecting unit  20  is positioned. Below the conveying path  15 , an inversion path  21  for duplex printing is arranged. 
     Next, the operation of forming an image by the printer  1  having such a configuration will be described. 
     When the power is supplied to the printer  1 , various parameters are initialized and initial determination, such as temperature determination of the fixing device  18 , is carried out. Subsequently, in the printer  1 , when image data is inputted and a printing start is directed from a computer or the like connected with the printer  1 , an image forming operation is carried out as follows. 
     First, the surface of the photosensitive drum  10  is electrically charged by the charger  11 . Then, exposure corresponding to the image data on the photosensitive drum  10  is carried out by a laser (refer to two-dot chain line P in  FIG. 1 ) from the exposure device  7 , thereby forming an electrostatic latent image on the surface of the photosensitive drum  10 . Subsequently, the electrostatic latent image is developed to a toner image with a toner (a developer) in the development device  12 . 
     On the other hand, a sheet fed from the sheet feeding cartridge  3  by the sheet feeder  16  is conveyed to the transferring unit  17  in a suitable timing for the above-mentioned image forming operation, and then, the toner image on the photosensitive drum  10  is transferred onto the sheet in the transferring unit  17 . The sheet with the transferred toner image is conveyed to a lower stream on the conveying path  15  to go forward to the fixing device  18 , and then, the toner image is fixed on the sheet in the fixing device  18 . The sheet with the fixed toner image is ejected from the sheet ejecting unit  20  to the sheet ejecting tray  4 . Toner remained on the photosensitive drum  10  is collected by the cleaning device  14 . 
     Next, the fixing device  18  will be described in detail with reference to  FIGS. 2 and 3 . Arrow Y in  FIG. 2  indicates a sheet conveying direction. Arrow I in  FIG. 3  indicates an inside in forward and backward directions, and arrow O in  FIG. 3  indicates an outside of the forward and backward directions. 
     As shown in  FIGS. 2 and 3  and other figures, the fixing device  18  includes a fixing belt  22 , a pressuring roller  23  (pressuring member) which is arranged below (outside) the fixing belt  22 , a heater  24  (heat source) which is arranged at an inner diameter side of the fixing belt  22 , a reflecting plate  25  (reflecting member) which is arranged at the inner diameter side of the fixing belt  22  and below the heater  24 , a supporting member  26  which is arranged at the inner diameter side of the fixing belt  22  and below the reflecting plate  25 , a pressing member  27  which is arranged at the inner diameter side of the fixing belt  22  and below the supporting member  26 , cover members  28  which are fixed to both front and rear end parts of the supporting member  26  at the inner diameter side of the fixing belt  22 , a thermocut  29  (heating stop device) which is arranged above (outside) the fixing belt  22 , shape restricting members  30  which are attached to the both front and rear end parts of the fixing belt  22 , and a deformation preventing member  31  which is arranged at the inner diameter side of the fixing belt  22  and above the heater  24 . In addition,  FIG. 3  is a perspective view of the inside of the fixing belt  22 . 
     The fixing belt  22  is formed in a nearly cylindrical shape elongated in the forward and backward directions. The fixing belt  22  is provided rotatably around a rotation axis A elongated in the forward and backward directions. That is, in the present embodiment, the forward and backward directions are a rotation axis direction of the fixing belt  22 . 
     The fixing belt  22  has flexibility, and is endless in a circumferential direction. The fixing belt  22  includes a base material layer  35 , an elastic layer  36  which is provided around this base material layer  35  and a release layer  37  which covers this elastic layer  36 , for example. The base material layer  35  of the fixing belt  22  is made of a metal, such as SUS or nickel. In addition, the base material layer  35  of the fixing belt  22  may be made of a resin, such as a PI (polyimide). The elastic layer  36  of the fixing belt  22  is made of a silicon rubber, for example, and has a larger thermal expansion coefficient than a thermal expansion coefficient of the base material layer  35  of the fixing belt  22 . The thickness of the elastic layer  36  of the fixing belt  22  is 270 μm, for example. The release layer  37  of the fixing belt  22  is made of a PFA tube, for example. The thickness of the release layer  37  of the fixing belt  22  is 20 μm, for example. 
     At an upper part (a part at a far side from the pressuring roller  23 ) of the fixing belt  22 , a first part P 1  and second parts P 2  formed on both of left and right sides of the first part P 1  are formed. The first part P 1  is positioned at an upper end part of the fixing belt  22 , and the first part P 1  of the fixing belt  22  is the closest to the heater  24 . Each of the second parts P 2  is slightly farther from the heater  24  than the first part P 1 . 
     The fixing belt  22  includes a sheet passing region R 1  and non-sheet passing regions R 2  which are provided at both front and rear sides (an outside in the forward and backward directions of the sheet passing region R 1 ) of the sheet passing region R 1 . The sheet passing region R 1  is a region through which sheets of a maximum size pass. Each of the non-sheet passing regions R 2  is a region through which the sheets of the maximum size do not pass. 
     The pressuring roller  23  is formed in a nearly columnar shape elongated in the forward and backward directions. The pressuring roller  23  comes into pressure contact with the fixing belt  22  so as to form a fixing nip  39  between the fixing belt  22  and the pressuring roller  23 . The pressuring roller  23  is rotatably provided. 
     The pressuring roller  23  includes a columnar core material  40 , an elastic layer  41  which is provided around this core material  40  and a release layer  42  which covers this elastic layer  41 , for example. The core material  40  of the pressuring roller  23  is made of a metal, such as an iron. The elastic layer  41  of the pressuring roller  23  is made of a silicon rubber, for example. The release layer  42  of the pressuring roller  23  is made of a PFA tube, for example. 
     The heater  24  is configured as a halogen heater, for example. The heater  24  is arranged at an upper part (a part at a far side from the pressuring roller  23 ) in an internal space of the fixing belt  22 , and is provided at a position displaced upward (the far side from the pressuring roller  23 ) from the rotation axis A of the fixing belt  22 . 
     The reflecting plate  25  is formed in a shape elongated in the forward and backward directions. The reflecting plate  25  is made of a metal, such as an aluminum alloy for brightness. The reflecting plate  25  is arranged between the heater  24  and the supporting member  26 . A cross section of the reflecting plate  25  is formed in a U shape which protrudes upward (a far side from the pressuring roller  23 ). 
     The reflecting plate  25  includes a main body part  44  which is provided nearly horizontally, and guide parts  45  which are bent downward from both left and right end parts (end parts at an upstream side and a downstream side in the sheet conveying direction Y) of the main body part  44 . A top face of the main body part  44  is a reflection face (mirror face) which faces the heater  24 , and reflects a radiation heat radiated from the heater  24 , to an inner circumferential face of the fixing belt  22 . 
     The supporting member  26  is formed in a shape elongated in the forward and backward directions. An upper part of the supporting member  26  is inserted between the guide parts  45  of the reflecting plate  25 . The supporting member  26  supports the reflecting plate  25  via a spacer  51 , and is not in direct contact with the reflecting plate  25 . The supporting member  26  is formed by combining a pair of L-shaped sheet metals  52 , and has a nearly rectangular cross-sectional shape. At a lower right corner part of the supporting member  26 , an engaging protrusion  53  which protrudes downward is formed. The engaging protrusion  53  is formed by elongating one of the sheet metals  52  downward. 
     The pressing member  27  is formed in a long flat shape in the forward and backward directions. The pressing member  27  is made of a heat-resistant resin, such as an LCP (Liquid Crystal Polymer). At a right end part of a top face of the pressing member  27 , an engaging convex part  55  is formed. The engaging convex part  55  engages with the engaging protrusion  53  of the supporting member  26 . On the top face of the pressing member  27 , a plurality of bosses  56  are formed so as to protrude. An upper end part of each boss  56  comes into contact with a lower face of the supporting member  26 . According to the above-mentioned configuration, the supporting member  26  supports the pressing member  27 , and restricts a warp of the pressing member  27 . 
     A right side part (a part at a downstream side in the sheet conveying direction Y) of the lower face of the pressing member  27  is inclined downward (toward the pressuring roller  23 ) from the left side (an upstream side in the sheet conveying direction Y) to the right side (the downstream side in the sheet conveying direction Y). The lower face of the pressing member  27  presses the fixing belt  22  downward (toward the pressuring roller  23 ). 
     Each cover member  28  is formed in a nearly U shape when seen from a front view. A position in the forward and backward directions of each cover member  28  meets each non-sheet passing region R 2  of the fixing belt  22  and has a function of blocking a radiation heat traveling from the heater  24  to each non-sheet passing region R 2  of the fixing belt  22 . 
     Each cover member  28  includes a curved part  57  which is curved upward in an arc shape, and attachment parts  58  which are bent downward from both left and right end parts (end parts at the upstream side and the downstream side in the sheet conveying direction Y) of the curved part  57 . The curved part  57  is arranged along the inner circumferential face of the fixing belt  22 . A lower end part of each of the attachment parts  58  is attached to each one of both left and right side faces of the supporting member  26 . 
     The thermocut  29  is a thermostat of a bimetallic type (a type which configures a contact point by using two types of metals having different thermal expansion coefficients), for example. The thermocut  29  is arranged directly above the upper end part of the fixing belt  22  (a part of the fixing belt  22  which is the closest to the heater  24 ), and faces an outer circumferential face of the upper end part of the fixing belt  22 . The thermocut  29  is provided at a position meeting a forward-and-backward direction center part Z (corresponding to a forward-and-backward direction center part of the entire fixing belt  22 , too) of the sheet passing region R 1  of the fixing belt  22 ). 
     Each shape restricting member  30  is arranged closer to the outside in the forward and backward directions than each cover member  28 . Each shape restricting member  30  includes a restricting piece  60  and a ring piece  61  which is attached to the restricting piece  60 . 
     The restricting piece  60  of each shape restricting member  30  includes a base part  62 , and a restricting part  63  which is formed in a face at an inside in the forward and backward directions of the base part  62  so as to protrude. A through-hole  64  which penetrates the base part  62  and the restricting part  63  is provided to the restricting piece  60  along the forward and backward directions, and the heater  24  penetrates this through-hole  64 . The restricting part  63  is curved in an arc shape along an outer circumference of the through-hole  64 , and is formed in a nearly downward C shape. The restricting part  63  is inserted in the both front and rear end parts of the fixing belt  22 . Consequently, the shape of the fixing belt  22  is restricted (deformation of the fixing belt  22  is prevented). The upper end part (apex) of the restricting part  63  comes into contact with the inner circumferential face of the fixing belt  22  in normal use (when the fixing belt  22  is not broken in the circumferential direction). 
     The ring piece  61  of each shape restricting member  30  is formed in an annular shape. The ring piece is attached to an outer circumference of the restricting part  63  of the restricting piece  60 . The ring piece  61  is arranged at the outside in the forward and backward directions of the both front and rear end parts of the fixing belt  22 , and restricts meandering of the fixing belt  22  (movement to the outside in the forward and backward directions). The ring piece  61  is arranged at the inside in the forward and backward directions of the base part  62  of the restricting piece  60 , and thereby restricts movement of the ring piece  61  to the outside in the forward and backward directions. 
     The deformation preventing member  31  is provided above the rotation axis A of the fixing belt  22  and at a position except for a position directly above the heater  24 . The deformation preventing member  31  is elongated in the forward and backward directions, and is provided over an entire region of the fixing belt  22  in the forward and backward directions. Both front and rear end parts of the deformation preventing member  31  are fixed to the shape restricting members  30  or are fixed to a fixing frame (not shown) which retains the shape restricting members  30 . A thermal expansion rate of the deformation preventing member  31  is equal to or less than a thermal expansion rate of the base material layer  35  of the fixing belt  22 . 
     The deformation preventing member  31  includes an upstream side part  67  which is arranged at an upper left side of the heater  24 , and a downstream side part  68  which is arranged at an upper right side of the heater  24  with an interval from the upstream side part  67 . The upstream side part  67  and the downstream side part  68  of the deformation preventing member  31  may be connected at a side closer to an outside in the forward and backward directions than the fixing belt  22 , or may not be connected. The upstream side part  67  of the deformation preventing member  31  does not face an inner circumferential face of the first part P 1  of the fixing belt  22 , and faces an inner circumferential face of the second part P 2  (referred to as the “left second part P 2 ” below) provided closer to a left side (an upstream side in the sheet conveying direction Y) than the first part P 1  of the fixing belt  22 . The downstream side part  68  of the deformation preventing member  31  does not face the inner circumferential face of the first part P 1  of the fixing belt  22 , and faces the inner circumferential face of the second part P 2  provided closer to the right side (a downstream side in the sheet conveying direction Y) than the first part P 1  of the fixing belt  22 . 
     Next, a control system of the fixing device  18  will be described with reference to  FIG. 4 . 
     The fixing device  18  includes a control part  71  (CPU). The control part  71  is connected to a storage part  72  which is configured as a storage device, such as a ROM or a RAM, and the control part  71  is configured to control each part of the fixing device  18  based on a control program or control data stored in the storage part  72 . The storage part  72  stores an operating temperature T of the thermocut  29 . 
     The control part  71  is connected to a drive source  73  configured as a motor or the like, and the drive source  73  is connected to the pressuring roller  23 . Further, based on a signal from the control part  71 , the drive source  73  rotates the pressuring roller  23 . 
     The control part  71  is connected to a power supply  74 , and the power supply  74  is connected to the heater  24 . Further, based on a signal from the control part  71 , power is supplied from the power supply  74  to the heater  24  so as to operate the heater  24 . On a power supply route from the power supply  74  to the heater  24 , the thermocut  29  is provided. The thermocut  29  is configured to operate at the operating temperature T, cut a power supply from the power supply  74  to the heater  24 , and stop the heater  24  from heating the fixing belt  22 . 
     To fix a toner image on a sheet in the fixing device  18  applying the above-mentioned configuration, the drive source  73  rotates the pressuring roller  23  (see arrow B in  FIG. 2 ). When the pressuring roller  23  is rotated in this way, the fixing belt  22  which comes into pressure contact with the pressuring roller  23  is driven to rotate in a direction opposite to a direction of the pressuring roller  23  (see arrow C in  FIG. 2 ). When the fixing belt  22  is rotated in this way, the fixing belt  22  slides against the pressing member  27 . 
     Further, to fix a toner image on a sheet, power is supplied from the power supply  74  to the heater  24  so as to operate the heater  24 . When the heater  24  is operated in this way, the heater  24  radiates a radiation heat. Part of the radiation heat radiated from the heater  24  is directly radiated on and is absorbed in the inner circumferential face of the fixing belt  22  as indicated by arrow D in  FIG. 2 . Further, as indicated by arrow E in  FIG. 2 , another part of the radiation heat radiated from the heater  24  is reflected toward the inner circumferential face of the fixing belt  22  on the top face of the main body part  44  of the reflecting plate  25 , and is absorbed in the inner circumferential face of the fixing belt  22 . According to the above-mentioned function, the heater  24  heats the fixing belt  22 . When the sheet passes through the fixing nip  39  in this state, the toner image is heated, is melted and is fixed to the sheet. 
     By the way, in the fixing device  18  applying the above-mentioned configuration, even when the heater  24  stops heating the fixing belt  22  in response to the stop of the fixing belt  22 , the first part P 1  of the fixing belt  22  is locally heated by a remaining heat of the heater  24  and overshoots (a rise in the temperature) in some cases. There is a concern that, when a facing interval between the first part P 1  of the fixing belt  22  and the thermocut  29  is too narrow, if the first part P 1  of the fixing belt  22  overshoots as described above, even though the temperature of the fixing belt  22  does not excessively rise, the thermocut  29  operates. When the thermocut  29  operates once, it is difficult to restore the thermocut  29  to a state before the operation, and therefore it is generally necessary to exchange the entire fixing device  18 . 
     To avoid such a situation, it is necessary to widen the facing interval between the first part P 1  of the fixing belt  22  and the thermocut  29 . However, there is a concern that, when the facing interval is widened in this way, a timing at which the thermocut  29  operates upon an excessive rise in the temperature of the fixing belt  22  delays. There is a concern that, particularly when a configuration where the pressing member  27  of a flat shape presses the fixing belt  22  downward as in the present embodiment is applied, if the fixing belt  22  is broken in the circumferential direction, the fixing belt  22  is deformed in a horizontally long elliptical shape. There is a concern that, when the fixing belt  22  is deformed in the horizontally long elliptical shape in this way, the facing interval between the first part P 1  of the fixing belt  22  and the thermocut  29  further widens, and a timing at which the thermocut  29  operates further delays. Hence, in the present embodiment, even when the fixing belt  22  is broken in the circumferential direction, the thermocut  29  is operated at an adequate timing as follows. 
     As shown in  FIG. 2 , in normal use of the fixing belt  22  (when the fixing belt  22  is not broken in the circumferential direction), the left second part P 2  of the fixing belt  22  and the upstream side part  67  of the deformation preventing member  31  are provided with an interval, and the right second part P 2  of the fixing belt  22  and the downstream side part  68  of the deformation preventing member  31  are provided with an interval. Consequently, it is possible to prevent a heat of the fixing belt  22  from escaping to the deformation preventing member  31 . According to this, the heater  24  can intensively heat the fixing belt  22 , and it is possible to reduce a temperature rising time of the fixing belt  22 . 
     By contrast with this, when the fixing belt  22  is broken in the circumferential direction, as shown in  FIG. 5 , the upper part of the fixing belt  22  (the far side from the pressuring roller  23 ) is deformed downward (the far side from the thermocut  29 ). This deformation places the left second part P 2  of the fixing belt  22  in contact with the upstream side part  67  of the deformation preventing member  31 , and places the right second part P 2  of the fixing belt  22  in contact with the downstream side part  68  of the deformation preventing member  31 . According to this, the upper part of the fixing belt  22  is prevented from being deformed downward (the far side from the thermocut  29 ), and the fixing belt  22  is not deformed in the horizontally long elliptical shape. 
     When the heater  24  continues heating the fixing belt  22  in this state, the temperature of the fixing belt  22  rises, the temperature of the thermocut  29  facing the outer circumferential face of the first part P 1  of the fixing belt  22  also rises. According to this, the temperature of the thermocut  29  reaches the operating temperature T, the thermocut  29  operates and power supply from the power supply  74  to the heater  24  is stopped. Hence, the heater  24  also stops heating the fixing belt  22 . 
     In the present embodiment, when the fixing belt  22  is broken in the circumferential direction, the upper part of the fixing belt  22  is prevented from being deformed downward (the far side from the thermocut  29 ). Consequently, it is possible to prevent the facing interval between the first part P 1  of the fixing belt  22  and the thermocut  29  from widening. According to this, it is possible to operate the thermocut  29  at an adequate timing. 
     Further, when the fixing belt  22  is broken in the circumferential direction, the upper part of the fixing belt  22  is prevented from being deformed downward (the far side from the thermocut  29 ), and therefore it is not necessary to narrow the facing interval between the first part P 1  of the fixing belt  22  and the thermocut  29  so as not to widen the facing interval between the first part P 1  of the fixing belt  22  and the thermocut  29  too much when the fixing belt  22  is broken in the circumferential direction. Consequently, it is possible to set a wide facing interval between the first part P 1  of the fixing belt  22  and the thermocut  29 , and avoid a situation that the thermocut  29  operates even though the temperature of the fixing belt  22  does not excessively rise. 
     Further, the upstream side part  67  and the downstream side part  68  of the deformation preventing member  31  do not face the inner circumferential face of the first part P 1  of the fixing belt  22 , and face the inner circumferential face of each of the second parts P 2  of the fixing belt  22 . The upstream side part  67  and the downstream side part  68  of the deformation preventing member  31  are arranged so as not to face the inner circumferential face of the first part P 1  of the fixing belt  22  as described above, so that it is possible to prevent a radiation heat traveling from the heater  24  toward the first part P 1  of the fixing belt  22 , from being blocked by the deformation preventing member  31  (see arrow D in  FIG. 2 ). According to this, the heater  24  can efficiently heat the fixing belt  22 . Further, the upstream side part  67  and the downstream side part  68  of the deformation preventing member  31  are arranged so as to face the inner circumferential face of each of the second parts P 2  of the fixing belt  22 , so that it is possible to enhance a function of preventing the fixing belt  22  from being deformed. 
     Further, the deformation preventing member  31  is provided over the entire region of the fixing belt  22  in the forward and backward directions. By applying such a configuration, irrespective of at which position in the forward and backward directions the fixing belt  22  is broken in the circumferential direction, it is possible to reliably prevent the upper part of the fixing belt  22  from being deformed downward (the far side from the thermocut  29 ). 
     Further, the heater  24  is arranged at the inner diameter side of the fixing belt  22  and is provided at a position displaced upward (the far side from the pressuring roller  23 ) from the rotation axis A of the fixing belt  22 , and the thermocut  29  faces the outer circumferential face of the first part P 1  of the fixing belt  22  (the part of the fixing belt  22  which is the closest to the heater  24 ). The first part P 1  of the fixing belt  22  is a part of the fixing belt  22  whose temperature is the most likely to excessively rise and therefore, by arranging the thermocut  29  so as to face the outer circumferential face of the first part P 1  of the fixing belt  22  as described above, it is possible to reliably prevent an excessive rise in the temperature of the fixing belt  22 . 
     In the present embodiment, a case where the thermal expansion rate of the deformation preventing member  31  is equal to or less than the thermal expansion rate of the base material layer  35  of the fixing belt  22  has been described. Meanwhile, in other different embodiments, the thermal expansion rate of the deformation preventing member  31  may be larger than the thermal expansion rate of the base material layer  35  of the fixing belt  22 . By applying such a configuration, when the fixing belt  22  is broken in the circumferential direction, as shown in  FIG. 6 , the upstream side part  67  and the downstream side part  68  of the deformation preventing member  31  are deformed by a thermal expansion, and press each of the second parts P 2  of the fixing belt  22  upward (toward the thermocut  29 ). According to this, it is possible to place the first part P 1  of the fixing belt  22  in contact with the thermocut  29 , reliably operate the thermocut  29  and reliably stop the heater  24  from heating the fixing belt  22 . In addition, to increase the thermal expansion rate of the deformation preventing member  31  compared to the thermal expansion rate of the base material layer  35  of the fixing belt  22 , the deformation preventing member  31  is made of an aluminum and the base material layer  35  of the fixing belt  22  is made of a SUS, for example. 
     Further, in the other different embodiments, as shown in  FIG. 7 , the deformation preventing member  31  may be made of a material (e.g. silica glass) which has a higher rigidity than the rigidity of the fixing belt  22  and allows transmission of a radiation heat (infrared ray) radiated from the heater  24 , and the inner circumferential faces of the first part P 1  and each of the second parts P 2  of the fixing belt  22  and the deformation preventing member  31  may face each other. By making the deformation preventing member  31  of a material which allows transmission of the radiation heat radiated from the heater  24  as described above, even when the deformation preventing member  31  is arranged so as to face the inner circumferential face of the first part P 1  of the fixing belt  22 , the radiation heat (see arrow Fin  FIG. 7 ) traveling from the heater  24  toward the first part P 1  of the fixing belt  22  is not blocked by the deformation preventing member  31 . Similarly, also when the deformation preventing member  31  is arranged so as to face the inner circumferential face of each of the second parts P 2  of the fixing belt  22 , the radiation heat (see arrow G in  FIG. 7 ) traveling from the heater  24  toward each of the second parts P 2  of the fixing belt  22  is not blocked by the deformation preventing member  31 . According to this, the heater  24  can efficiently heat the fixing belt  22 . Further, the deformation preventing member  31  is arranged so as to face the inner circumferential faces of the first part P 1  and each of the second parts P 2  of the fixing belt  22 , so that it is possible to enhance the function of preventing the fixing belt  22  from being deformed. 
     In addition, when the deformation preventing member  31  is made of a material which allows transmission of a radiation heat radiated from the heater  24  as described above, as shown in  FIG. 7 , the inner circumferential faces of the first part P 1  and each of the second parts P 2  of the fixing belt  22  and the deformation preventing member  31  may face each other, or the deformation preventing member  31  may face only the first part P 1  of the fixing belt  22 . That is, as long as the deformation preventing member  31  faces at least the first part P 1  of the fixing belt  22 , it is possible to enhance the function of preventing the fixing belt  22  from being deformed. 
     In the present embodiment, a case where the heater  24  composed of the halogen heater is used as a heat source has been described. Meanwhile, in the other different embodiments, a ceramic heater or the like may be used as the heat source. 
     In the present embodiment, a case where the configuration of the present disclosure is applied to the printer  1  has been described. Meanwhile, in the other different embodiments, the configuration of the disclosure may be applied to another image forming apparatus, such as a copying machine, a facsimile or a multifunction peripheral. 
     While the present disclosure has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present disclosure.