Patent Publication Number: US-9405246-B2

Title: Fixing device comprising heating stop device to stop heat source from heating 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. 2014-227114 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, 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 heat source causes a runaway (when the heat source heats the fixing belt in a state where the rotation of the fixing belt is stopped), 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 and a first heating stop device. The fixing belt is provided 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 arranged at an inner diameter side of the fixing belt, provided at a position displaced from the rotation axis and configured to heat the fixing belt. The pressing member is configured to press the fixing belt to a side of the pressuring member. The first heating stop device is configured to face an outer circumferential face of a closest part to the heat source of the fixing belt and to operate at a first operating temperature so as to stop the heat source from heating the fixing belt. When the heat source heats the fixing belt in a state where a rotation of the fixing belt is stopped, the closest part to the heat source of the fixing belt is deformed by a thermal expansion and comes into contact with the first heating stop device and the first heating stop device operates. 
     In accordance with an embodiment of the present disclosure, an image forming apparatus includes the 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 an embodiment of the present disclosure. 
         FIG. 2  is a sectional view showing a fixing device according to the embodiment of the present disclosure. 
         FIG. 3  is a side view showing the fixing device according to the embodiment of the present disclosure. 
         FIG. 4  is a block diagram showing a control system of the fixing device according to the embodiment of the present disclosure. 
         FIG. 5  is a sectional view showing a state that a heater heats a fixing belt in a state in which rotation of the fixing belt is stopped according to the embodiment of the present disclosure. 
         FIG. 6  is a graph showing a relationship between the thickness of the elastic layer of the fixing belt and the amount of deformation of the upper end part of the fixing belt when the temperature of the fixing belt is 400° C. in the fixing device according to the embodiment of the present disclosure. 
         FIG. 7  is a sectional view showing a state in which the fixing belt is broken in the circumferential direction in the fixing device according to the 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 unit  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 unit  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 , 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 unit  18 , and then, the toner image is fixed on the sheet in the fixing unit  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 in 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 , shape restricting members  30  which are attached to both end parts of the fixing belt  22 , a first thermocut  31  (first heating stop device) which is arranged above (outside) the fixing belt  22  and a second thermocut  32  (second heating stop device) which is arranged at a left side (outside) of the fixing belt  22 . 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  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 non-sheet passing region R 2  is a region through which the sheets of the maximum size do not pass. 
     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. 
     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 . Hence, in the present embodiment, an upper end part  22   a  of the fixing belt  22  is a part of the fixing belt  22  which is the closest to the heater  24 . 
     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 . Across 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) 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 . In 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) 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) to the right side (the downstream side in the sheet conveying direction). 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) 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 attachment part  58  is attached to each one of both left and right side faces of the supporting member  26 . 
     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 ring piece  61  of each shape restricting member  30  is formed in an annular shape. The ring piece  61  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 first thermocut  31  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 first thermocut  31  is arranged directly above the upper end part  22   a  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  22   a  of the fixing belt  22 . The first thermocut  31  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 ). 
     The second thermocut  32  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 second thermocut  32  is arranged just beside (equator position) of the left end part  22   b  of the fixing belt  22  (an edge part in the sheet conveying direction) of the fixing belt  22 , and faces an outer circumferential face of the left end part  22   b  of the fixing belt  22 . The second thermocut  32 , like the first thermocut  31 , 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 ). 
     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 1  of the first thermocut  31  (a first operating temperature at which the first thermocut  31  stops heating the fixing belt  22  by the heater  24 ) and a second operating temperature T 2  (a second operating temperature at which the second thermocut  32  stops heating the fixing belt  22  by the heater  24 ). The operating temperature T 2  of the second thermocut  32  is set to be lower than the operating temperature T 1  of the first thermocut  31 . 
     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 first thermocut  31  and the second thermocut are serially provided. The first thermocut  31  is configured to operate at the operating temperature T 1 , cut a power supply from the power supply  74  to the heater  24 , and stop the heater  24  from heating the fixing belt  22 . The second thermocut  32  is configured to operate at the operating temperature T 2 , 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 upper end part  22   a  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 upper end part  22   a  of the fixing belt  22  and the first thermocut  31  is too narrow, if the upper end part  22   a  of the fixing belt  22  overshoots as described above, even though the temperature of the fixing belt  22  does not excessively rise, the first thermocut  31  operates. When the first thermocut  31  operates once, it is difficult to restore the first thermocut  31  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. However, there is a concern that, when the facing interval is widened, and when the heater  24  causes a runaway (when the heater  24  heats the fixing belt  22  in a state where the rotation of the fixing belt  22  is stopped), a timing at which the first thermocut  31  operates delays. Hence, in the present embodiment, even when the heater  24  causes a runaway, the first thermocut  31  is operated at an adequate timing as follows. 
     As shown in  FIG. 2 , in normal use of the heater  24  (when the heater  24  heats the fixing belt  22  in a state where the fixing belt  22  is rotating), the upper end part  22   a  of the fixing belt  22  faces the thermocut  31  with a constant interval. 
     By contrast with this, upon a runaway of the heater  24  (when the heater  24  heats the fixing belt  22  in a state where the rotation of the fixing belt  22  is stopped), as shown in  FIG. 5 , the upper end part  22   a  of the fixing belt  22  is deformed upward (a close side to the first thermocut  31 ) by a thermal expansion, and comes into contact with the first thermocut  31 . According to this, the temperature of the first thermocut  31  reaches an operating temperature T 1 , the first thermocut  31  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 heater  24  causes a runaway, by placing the upper end part  22   a  of the fixing belt  22  in contact with the first thermocut  31 , it is possible to operate the first thermocut  31  at an adequate timing. 
     Further, the upper end part  22   a  of the fixing belt  22  is configured to automatically come into contact with the first thermocut  31  when the heater  24  causes a runaway. Hence, it is not necessary to set a narrow facing interval such that, when the heater  24  causes a runaway, a timing at which the first thermocut  31  operates does not delay. Consequently, it is possible to set a wide facing interval, and avoid a situation that the first thermocut  31  operates even though the temperature of the fixing belt  22  does not excessively rise. 
     Further, when seen from the forward and backward directions (the rotation axis direction of the fixing belt  22 ), at a forward-and-backward direction center part of the upper end part  22   a  of the fixing belt  22 , a radiation heat from the heater  24  concentrates the most in the upper end part  22   a  of the fixing belt  22 . Therefore, at the forward-and-backward direction center part of the upper end part  22   a  of the fixing belt  22 , the amount of deformation caused by a thermal expansion is the largest in the upper end part  22   a  of the fixing belt  22 . Hence, in the present embodiment, at a position meeting the forward-and-backward direction center part of the fixing belt  22 , the first thermocut  31  is provided. By applying such a configuration, when the heater  24  causes a runaway, it is possible to reliably place the upper end part  22   a  of the fixing belt  22  in contact with the first thermocut  31 . 
     Further, to the both front and rear end parts of the fixing belt  22 , the shape restricting members  30  which restrict the shape of the fixing belt  22  are attached. By applying such a configuration, it is possible to prevent a pressing force which the pressing member  27  applies to the fixing belt  22  in a state where the fixing device  18  is pressured (a state where the fixing nip  39  is formed), from being deformed in a horizontally long elliptical shape. According to this, when the heater  24  causes a runaway, it is possible to easily place the upper end part  22   a  of the fixing belt  22  in contact with the first thermocut  31 . 
     Next, the thickness of the elastic layer  36  of the fixing belt  22  and a setting range of the facing interval according to the present embodiment will be described mainly with reference to  FIG. 6 .  FIG. 6  illustrates a relationship between the thickness of the elastic layer  36  of the fixing belt  22  and the amount of deformation of the upper end part  22   a  of the fixing belt  22  when the temperature of the fixing belt  22  is 400° C. in the fixing device  18  according to the present embodiment. 
     In the fixing device  18  according to the present embodiment, even when the temperature of the fixing belt  22  does not excessively rises, and when the upper end part  22   a  of the fixing belt  22  is locally heated by a remaining heat of the heater  24  upon a stop of the fixing belt  22 , the upper end part  22   a  of the fixing belt  22  is deformed upward (the close side to the first thermocut  31 ) by about 1 mm. Hence, in the present embodiment, when the facing interval is g (mm), g≧1.5 . . . equation (1) 
     holds. When equation (1) is satisfied, it is possible to set a sufficiently wider facing interval than the amount of deformation (about 1 mm) of the upper end part  22   a  of the fixing belt  22  upon the stop of the fixing belt  22 . According to this, it is possible to reliably avoid a situation that the first thermocut  31  operates even though the temperature of the fixing belt  22  does not excessively rise. 
     Further, the elastic layer  36  thermally expands the most among each layer of the fixing belt  22 , and the amount of deformation caused by the thermal expansion of the upper end part  22   a  of the fixing belt  22  is proportional to the thickness of the elastic layer  36  of the fixing belt  22 . Furthermore, in the present embodiment, when the amount of deformation caused by the thermal expansion of the upper end part  22   a  of the fixing belt  22  is 1.5 mm, the thickness of the elastic layer  36  of the fixing belt  22  is 100 μm. Hence, when the thickness of the elastic layer  36  of the fixing belt  22  is t (μm), t≧100 . . . equation (2) holds. 
     When the above equation (2) is satisfied, it is possible to sufficiently increase the amount of deformation caused by the thermal expansion of the upper end part  22   a  of the fixing belt  22 . According to this, when the heater  24  causes a runaway, it is possible to reliably place the upper end part  22   a  of the fixing belt  22  in contact with the first thermocut  31 . 
     Further, an amount of deformation x of the upper end part  22   a  of the fixing belt  22  is represented as
     x=11.8×(t/1000)+0.3 in case of 100≦t≦170, and   x=7.0×(t/1000)+1.1 in case of 170&lt;t≦270.
 
In this regard,
   in case of 100≦t≦170, g≦11.8×(t/1000)+0.3 . . . equation (3) holds, and,   in case of 170&lt;t≦270, g≦7.0×(t/1000)+1.1 . . . equation (4) holds.
 
When the above equation (3) and the above equation (4) are satisfied, it is possible to set the facing interval to the amount of deformation x of the upper end part  22   a  of the fixing belt  22  or less, and it is possible to more reliably place the upper end part  22   a  of the fixing belt  22  in contact with the first thermocut  31  when the heater  24  causes a runaway.
   

     By the way, when the fixing belt  22  is broken in a circumferential direction in the fixing device  18  applying the above-mentioned configuration, as shown in  FIG. 7 , the fixing belt  22  is deformed in a horizontally long elliptical shape so as to stretch in the left and right directions. According to this, the facing interval between the upper end part  22   a  of the fixing belt  22  and the first thermocut  31  widens, and therefore it is difficult to place the upper end part  22   a  of the fixing belt  22  in contact with the first thermocut  31  and the first thermocut  31  hardly operates. 
     Hence, in the present embodiment, the second thermocut  32  is arranged so as to face the outer circumferential face of the left end part  22   b  of the fixing belt  22 . Hence, when the fixing belt  22  is broken in the circumferential direction and is deformed in the horizontally long elliptical shape, the left end part  22   b  of the fixing belt  22  is deformed to the left side (the close side to the second thermocut  32 ) and comes into contact with the second thermocut  32 . According to this, the temperature of the second thermocut  32  reaches an operating temperature T 2 , the second thermocut  32  operates and the heater  24  stops heating the fixing belt  22 . By applying such a configuration, when the fixing belt  22  is broken in the circumferential direction, it is possible to operate the second thermocut  32  at an adequate timing and stop the heater  24  from quickly stopping heating the fixing belt  22 . 
     In addition, as shown in  FIG. 2  and other figures, while the first thermocut  31  faces the outer circumferential face of the upper end part  22   a  of the fixing belt  22 , the second thermocut  32  faces the outer circumferential face of the left end part  22   b  (a farther side from the heater  24  than the upper end part  22   a ) of the fixing belt  22 . Therefore, there is a concern that, when the operating temperature T 2  of the second thermocut  32  is set to the operating temperature T 1  of the first thermocut  31  or more, and when the heater  24  causes a runaway, a timing at which the second thermocut  32  operates delays compared to a timing at which the first thermocut  31  operates in case where the fixing belt  22  is broken in the circumferential direction. 
     Hence, in the present embodiment, the operating temperature T 2  of the second thermocut  32  is set lower than the operating temperature T 1  of the first thermocut  31 . By applying such a configuration, when the heater  24  causes a runaway, it is possible to prevent the timing at which the second thermocut  32  operates from delaying compared to the timing at which the first thermocut  31  operates in case where the fixing belt  22  is broken in the circumferential direction. 
     In the present embodiment, a case where the second thermocut  32  faces the outer circumferential face of the left end part  22   b  (the end part at the upstream side in the sheet conveying direction) of the fixing belt  22  has been described. Meanwhile, in other different embodiments, the second thermocut  32  may face the outer circumferential face of the right end part (the end part at the downstream side in the sheet conveying direction) of the fixing belt  22 . 
     In the present embodiment, a case where, when the fixing belt  22  is broken in the circumferential direction, the second thermocut  32  is operated to stop the heater  24  from heating the fixing belt  22  has been described. Meanwhile, in the other different embodiments, a sensor (referred to as “the sensor” below) which detects whether or not the left end part  22   b  of the fixing belt  22  has moved to the left by a standard movement amount or more may be arranged, and, when fixing belt  22  is broken in the circumferential direction and the sensor detects that the left end part  22   b  of the fixing belt  22  has moved to the left by the standard movement amount or more, the control part  71  may stop the heater  24  from heating the fixing belt  22 . For the sensor, an optical sensor which includes a light emitting part and a light receiving part can be used, for example. When the optical sensor is used in this way, desirably, the light emitting part of the sensor is provided at a position meeting one end part (e.g. a front end part) of the fixing belt  22 , and the light receiving part of the sensor is provided at a position meeting the other end part (e.g. a rear end part) of the fixing belt  22 . By applying such a configuration, sensor light emitted from the light emitting part of the sensor travels straightforward along the forward and backward directions (the rotation axis direction of the fixing belt  22 ), passes from the one end part of the fixing belt  22  to the other end part and then reaches the light receiving part of the sensor. According to this, 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 detect that the fixing belt  22  is broken in the circumferential direction. 
     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.