Patent Publication Number: US-9885986-B2

Title: Fixing device and image forming apparatus comprising a thermal fuse including a fuse element supported by an elastic member

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2016-111177 filed Jun. 2, 2016. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a fixing device and an image forming apparatus. 
     SUMMARY 
     According to an aspect of the invention, a fixing device includes a planar heating element and a thermal fuse. The planar heating element is configured to heat a fixing member that fixes a toner image to a recording medium. The thermal fuse includes a fuse element configured to be in contact with the heating element, and an elastic member configured to support the fuse element on a support body by a tension at which a fusing temperature of the fuse element is lower than a rated fusing temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a view illustrating a configuration example of an image forming apparatus; 
         FIG. 2  is a view illustrating an exemplary configuration of a fixing device when viewed along a rotation axis; 
         FIG. 3  is a view illustrating an exemplary cross-sectional configuration of a fixing belt; 
         FIG. 4  is a view illustrating an exemplary cross-sectional configuration of a heater; 
         FIG. 5  is a schematic view illustrating an exemplary configuration of a thermal fuse when viewed along a transport direction of a paper; 
         FIG. 6  is a graph representing an exemplary relationship between a tension and a fusing temperature of a fuse element; 
         FIG. 7  is a view illustrating an exemplary configuration for changing a tension of a fuse element; 
         FIG. 8  is a view illustrating an exemplary evaluation circuit for a thermal fuse; and 
         FIG. 9  is a graph representing exemplary variations in temperature of respective parts of a fixing device in the evaluation circuit. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, yellow will be represented by Y, magenta will be represented by M, cyan will be represented by C, and black will be represented by K. When it is necessary to distinguish respective constituent elements and toner images (images) from color to color, a color sign Y, M, C, or K corresponding to each color will be added to the end of a reference number and descriptions will be made with reference thereto. In addition, hereinafter, when collectively referring to the respective constituent elements and toner images without distinguishing the constituent elements and toner images from color to color, a color sign will be omitted at the end of a reference numeral and descriptions will be made thereon. 
     (Overall Configuration) 
     As illustrated in  FIG. 1 , inside an apparatus body  10 A of an image forming apparatus  10 , an image processing unit  12  is provided to perform an image processing of converting input image data into gradation data of four colors of Y, M, C, and K. 
     In addition, at the center side of the apparatus body  10 A, image forming units  16  of respective images, which respectively form color toner images, are arranged to be spaced apart from each other in an inclined direction relative to the horizontal direction. In addition, a primary transfer unit  18  is provided at the vertically upper side of the image forming units  16  of the respective colors. Toner images formed by the image forming units  16  of the respective colors are transferred to the primary transfer unit  18  in a superimposed manner. 
     In addition, a secondary transfer roller  22  is provided at a lateral side (the left side of  FIG. 1 ) of the primary transfer unit  18 . The secondary transfer roller  22  transfers the toner images, which are transferred to the primary transfer unit  18  in the superimposed manner, to paper P which is an exemplary recording medium transported along a transport path  60  by a supply transport unit  30  which will be described later. 
     A fixing device  24  is provided downstream of the secondary transfer roller  22  in the transport direction of the paper P (hereinafter, referred to as a “paper transport direction”). The fixing device  24  fixes the toner images, which are transferred to the paper P, on the paper P by heat and pressure. 
     In addition, a discharge roller  28  is provided downstream of the fixing device  24  in the paper transport direction. The discharge roller  28  discharges the paper P having the toner images fixed thereto to a discharge unit  26  provided in the upper portion of the apparatus body  10 A of the image forming apparatus  10 . 
     Meanwhile, the supply transport unit  30  is provided vertically below and lateral to the image forming units  16 . The supply transport unit  30  supplies and transports the paper P. In addition, above the primary transfer unit  18  in the vertical direction, four toner cartridges  14 K to  14 Y by colors are arranged side by side in an apparatus width direction. The toner cartridges  14 K to  14 Y are attachable to/detachable from the apparatus body  10 A from the front side of the apparatus body  10 A and are charged with a toner to be replenished to a developing device  38 . The toner cartridge  14  of each color has, for example, a cylindrical shape extending in an apparatus depth direction. Each toner cartridge  14  is connected to one of the developing devices  38  of the respective colors through a supply pipe (not illustrated). 
     (Image Forming Unit) 
     As illustrated in  FIG. 1 , all of the image forming units  16  of the respective colors are configured to be substantially the same as each other. In addition, each image forming unit  16  includes a rotating cylindrical image carrier  34  and a charging unit  36  that charges the surface of the image carrier  34 . 
     In addition, the image forming unit  16  includes a light emitting diode (LED) head  32  that irradiates the surface of the charged image carrier  34  with exposure light. In addition, the image forming unit  16  includes a developing device  38  that develops an electrostatic latent image, which is formed via the irradiation of exposure light by the LED head  32 , using a developer (in the present exemplary embodiment, a negatively charged toner) so as to visualize the electrostatic latent image as a toner image. In addition, the image forming unit  16  includes a cleaning blade (not illustrated) that cleans the surface of the image carrier  34 . 
     A developing roller  39  is disposed in the developing device  38  to face the image carrier  34 . The developing device  38  develops an electrostatic latent image, which is formed on the image carrier  34 , with a developer using the developing roller  39  to visualize the electrostatic latent image as a toner image. 
     In addition, the charging unit  36 , the LED head  32 , the developing roller  39 , and the cleaning blade are arranged in this sequence from the upstream side to the downstream side of the image carrier  34  in the rotation direction to face the surface of the image carrier  34 . 
     (Transfer Unit (Primary Transfer Unit/Secondary Transfer Roller)) 
     The primary transfer unit  18  includes an endless intermediate transfer belt  42 , and a driving roller  46  on which the intermediate transfer belt  42  is wound. The driving roller  46  is rotationally driven by a motor (not illustrated) to circulate the intermediate transfer belt  42  in the direction indicated by the arrow A. In addition, the primary transfer unit  18  includes a tension imparting roller  48  and an assist roller  50 . The intermediate transfer belt  42  is wound on the tension imparting roller  48 . The tension imparting roller  48  imparts tension to the intermediate transfer belt  42 . The assist roller  50  is disposed vertically above the tension imparting roller  48  and driven to rotate by the intermediate transfer belt  42 . In addition, the primary transfer unit  18  includes primary transfer rollers  52 , which are respectively located opposite to image carriers  34  of the respective colors with the intermediate transfer belt  42  being interposed therebetween. 
     With this configuration, toner images of the respective colors of Y, M, C, and K, which are sequentially formed on the image carriers  34  of the image forming units  16  of the respective colors, are transferred to the intermediate transfer belt  42  in the superimposed manner by the primary transfer rollers  52  of the respective colors. 
     In addition, a cleaning blade  56  is disposed opposite to the driving roller  46  with the intermediate transfer belt  42  being interposed therebetween to come in contact with the surface of the intermediate transfer belt  42  so as to clean the surface of the intermediate transfer belt  42 . 
     In addition, the secondary transfer roller  22  is provided opposite to the assist roller  50  with the intermediate transfer belt  42  being interposed therebetween to transfer the toner images, which are transferred to the intermediate transfer belt  42 , to the paper P that is being transported. In addition, the secondary transfer roller  22  is grounded, and the assist roller  50  forms a counter electrode of the secondary transfer roller  22 . When a secondary transfer voltage is applied to the assist roller  50 , the toner images are transferred to the paper P. 
     (Supply Transport Unit) 
     The supply transport unit  30  is disposed vertically below the image forming units  16  within the apparatus body  10 A, and includes a paper feeding member  62  in which plural sheets of paper P are loaded. 
     In addition, the supply transport unit  30  includes a paper feeding roller  64 , a separation roller  66 , and a registration roller  68 . The paper feeding roller  64  delivers paper P loaded in the paper feeding member  62  to the transport path  60 . The separation roller  66  separates the paper P delivered by the paper feeding roller  64  one by one. The registration roller  68  adjusts a transport timing of the paper P. In addition, the respective rollers are arranged in this order from the upstream side to the downstream side of the paper transport direction. 
     With this configuration, the paper P supplied from the paper feeding member  62  is delivered at a predetermined timing to a contact portion (secondary transfer position) between the intermediate transfer belt  42  and the secondary transfer roller  22  by the rotating registration roller  68 . 
     (Image Forming Process) 
     First, gradation data of the respective colors are sequentially output from the image processing unit  12  to LED heads  32  of the respective colors. Then, the surfaces of the image carrier  34  charged by the charging units  36  are irradiated with exposure lights that are respectively emitted from the LED heads  32  based on the gradation data. Thus, electrostatic latent images are formed on the surfaces of the image carriers  34 . The electrostatic latent images formed on the image carriers  34  are developed by the developing devices  38  of the respective colors, respectively, and are visualized as toner images of the respective colors of Y, M, C, and K, respectively. 
     In addition, the toner images of the respective colors formed on the image carriers  34  are transferred to the circulating intermediate transfer belt  42  in the superimposed manner by the primary transfer rollers  52  of the primary transfer unit  18 . 
     The toner images of the respective colors, which are transferred to the intermediate transfer belt  42  in the superimposed manner, are secondarily transferred to paper P at a secondary transfer position by the secondary transfer roller  22  when the paper P is transported to the secondary transfer position along the transport path  60  from the paper feeding member  62  by the paper feeding roller  64 , the separation roller  66 , and the registration roller  68 . 
     In addition, the paper P, to which the toner image is transferred, is transported to the fixing device  24 , and the toner image is fixed to the paper P by the fixing device  24 . Then, the paper P, to which the toner image is fixed, is discharged to the discharge unit  26  by the discharge roller  28 . 
     Meanwhile, when forming images on opposite sides of paper P, the paper P, on which the toner image is fixed to one side (the front side) by the fixing device  24 , is not directly discharged to the discharge unit  26  by the discharge roller  28 , and the paper transport direction of the paper P is switched by reversely rotating the discharge roller  28 . Then, the paper P is transported along a double-sided transport path  72  by transport rollers  74  and  76 . 
     The paper P transported along the double-sided transport path  72  is reversed upside down and transported again to the registration roller  68 . Then, after a toner image is transferred and fixed to the other side (the back side) of the paper P, the paper P is discharged to the discharge unit  26  by the discharge roller  28 . 
     In addition, in the image forming apparatus  10 , the transport speed of the paper P may particularly be referred to as a “process speed,” and the process speed of the image forming apparatus  10  is predetermined. In this case, as the process speed of the image forming apparatus  10  is higher, the number of papers, on which images are formed per unit time, is increased. 
     In addition, there are various kinds of image forming apparatuses  10 , such as one corresponding to only a single process speed, and another one corresponding to plural process speeds. 
     (Fixing Device) 
     Next, the fixing device  24  of the image forming apparatus  10  will be described in detail. 
     As illustrated in  FIG. 2 , the fixing device  24  according to the present exemplary embodiment includes a pressurizing roller  241  and a fixing belt  249  which is an example of an endless belt. The pressurizing roller  241  is rotated in the direction indicated by the arrow  41  by a driving device (a motor which is not illustrated). The fixing belt  249  contacts with the pressurizing roller  241  and is thus rotated following the rotation of the pressurizing roller  241  in the direction indicated by the arrow  43 . In addition, as will be described later, the fixing belt  249  is heated to a preset temperature by a heater  245  provided therein. In addition, the temperature of the fixing belt  249  is set based on, for example, the process speed of the paper P. 
     The paper P transported in the direction indicated by the arrow  40  is pinched into a nip portion  44 , which is formed by the pressurizing roller  241  and the fixing belt  249 , while the pressurizing roller  241  and the fixing belt  249  of the fixing device  24  are rotated together. Then, the fixing device  24  fixes the toner image to the paper P by, while heating the toner image transferred to the paper P using the fixing belt  249 , pressing the toner image against the paper P using pressing force generated by the pressurizing roller  241  and the fixing belt  249  when the paper P is pinched into the nip portion  44 . Thus, the fixing belt  249  is one example of a fixing member for fixing the toner image on the paper P. 
     The fixing belt  249  is an endless belt having a cylindrical shape and includes a fixing pad  243 , an inner structure  244 , the heater  245 , and a thermal fuse  246  therein. The fixing belt  249  is disposed such that the height direction of the cylinder follows the direction orthogonal to the transport direction of the paper P which is indicated by the arrow  40 , i.e. the width direction of the paper P. Hereinafter, the direction of the fixing belt  249 , which is disposed along the width direction of the paper P, is referred to as the “width direction of the fixing belt  249 .” 
     In addition, a planar heater  245  is mounted on the fixing belt  249  to be in contact with the fixing belt  249  over a predetermined length thereof. To this end, one end of the heater  245  is sandwiched and fixed between the fixing pad  243  and the inner structure  244 , and the other end of the heater  245  is brought into contact with the fixing belt  249  as a free end rather than being fixed. 
     The heater  245  generates heat depending on, for example, the magnitude of a current supplied to the heater  245 , and heats the fixing belt  249 , which is in contact with the heater  245 . Although the planar heater  245  is rounded to contact the fixing belt  249  and is formed substantially in the cylindrical shape. At this time, the heater  245  is formed such that the diameter of the rounded heater  245  is larger than the diameter of the fixing belt  249 . When the heater  245  formed as described above is mounted inside the fixing belt  249 , restoration force, by which the heater  245  returns to an original shape thereof, acts on the fixing belt  249 , thereby causing the heater  245  to be naturally brought into close contact with the fixing belt  249 . 
     In addition, like the heater  245 , for example, a heating element having a property of being deformable depending on the shape of a member to be heated (the fixing belt  249  in the present exemplary embodiment) may be referred to as a “flexible heater.” 
     The fixing pad  243  is one example of a pressing member formed of, for example, a liquid crystal polymer, and is provided at a position facing the pressurizing roller  241 . The nip portion  44  is formed by the pressurizing roller  241  and the fixing pad  243 . The surface of the fixing pad  243  facing the nip portion  44  presses the paper P together with the pressurizing roller  241  while contacting with the rotating fixing belt  249 , thereby fixing the toner image transferred to the paper P to the paper P. 
     The inner structure  244  is provided, for example, on the top of the fixing pad  243  so that one end of the heater  245  is sandwiched together with the fixing pad  243 . In addition, the inner structure  244  includes, for example, a circuit for supplying current to the heater  245  (hereinafter, referred to as a “current circuit”). 
     In addition, the linear thermal fuse  246  is provided on an opposite surface of the heater  245  (hereinafter, referred to as “the inner surface of the heater  245 ”) to the surface contacting with the fixing belt  249 , so as to be in contact with the heater  245  along the width direction of the fixing belt  249 . Specifically, the thermal fuse  246  includes a fuse element  247  and a support body  248  on which the fuse element  247  is mounted. The fuse element  247  is provided to be in contact with the inner surface of the heater  245 . The fuse element  247  detects the temperature of the heater  245 . 
     The pressurizing roller  241  is a driving roller having a diameter of about 28 mm. The pressurizing roller  241  includes a metallic rotating shaft  250 , a silicone rubber layer  251 , and a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) tube  252 . The rotating shaft  250  is a cylindrical body rotated by a drive power of a motor (not illustrated) in the direction indicated by the arrow  41 . The silicone rubber layer  251  has a thickness of about 5 mm and is wound around the circumferential surface of the rotating shaft  250 . The outer surface of the silicone rubber layer  251  is covered with the PFA tube  252 . An elastic material (e.g., the silicone rubber layer  251 ) is wound around the circumferential surface of the rotating shaft  250 . Thus, when the paper P is pressed by the nip portion  44 , the pressurizing roller  241  presses the paper P while being deformed by the reaction force against the pressing force of the paper P. 
     In the fixing device  24  according to the present exemplary embodiment, for example, the length of each of the pressurizing roller  241  and the fixing belt  249  in the width direction of the paper P is about 200 mm, and the length of the heater  245  in the width direction of the paper P is set to, for example, about 220 mm to be greater than the length of the fixing belt  249  in the width direction of the paper P. This configuration is adopted to suppress the temperature of the fixing belt  249  from becoming uneven as the temperature of the end of the fixing belt  249  is lowered than the temperature of the central portion of the fixing belt  249  when the fixing belt  249  and the heater  245  have the same length in the width direction of the paper P. 
     In addition, the length of the heater  245  from the one end thereof fixed by the fixing pad  243  and the inner structure  244  to the free end is about 55 mm. The range of about 45 mm thereof (i.e., the range indicated by R 1  in  FIG. 2 ) is in contact with the fixing belt  249  along the circumferential direction of the fixing belt  249 . In the range within which the fixing belt  249  and the heater  245  are in contact with each other, the fixing belt  249  is pressed against the heater  245  by a force of about 1.2 kg which is the restoration force of the heater  245 . Thereby, the fixing belt  249  is in close contact with heater  245 . 
     In addition, when an alternating current voltage of 100 V is applied to the heater  245  according to the present exemplary embodiment, the rated power is about 700 W. 
     In addition, the length of the nip portion  44  of the fixing device  24  according to the present exemplary embodiment is about 8 mm in the transport direction of the paper P, and the pressing force of the paper P in the nip portion  44  is adjusted to about 20 kg. 
     In addition, the aforementioned specific numerical values related to the fixing device  24  are given by way of an example, and the present exemplary embodiment is of course not limited thereto. 
     Next, the details of the fixing belt  249  will be described.  FIG. 3  is a view illustrating an exemplary cross-sectional configuration of the fixing belt  249 . As illustrated in  FIG. 3 , the fixing belt  249  includes three layers, i.e. a surface release layer  100 , an elastic layer  102 , and a base member layer  104 , in this order from one surface thereof that comes into contact with the paper P to the other surface thereof that is in contact with the heater  245 . 
     The surface release layer  100  is formed of, for example, tetrafluoroethylene-perfluoroalkyl vinyl ether polymer (PFA), polytetrafluoroethylene (PTFE), a silicone copolymer, or a composite thereof, and is configured as a layer having a thickness of about 10 μm or more and less than 50 μm. 
     The elastic layer  102  is formed of, for example, an elastic material (e.g., silicone rubber) having a hardness of about 10° or more and less than 60°, and is configured as a layer having a thickness of about 100 μm or more and less than 400 μm. 
     In addition, the base member layer  104  is formed of, for example, a resin material (e.g., polyimide) having a thickness of about 50 to 100 μm. 
     In addition, although an endless belt having a diameter of about 30 mm is used as the fixing belt  249  according to the present exemplary embodiment, the present exemplary embodiment is not limited in relation to the diameter of the fixing belt  249 . 
     Next, the details of the heater  245  will be described.  FIG. 4  is a view illustrating an exemplary cross-sectional configuration of the heater  245 . 
     As illustrated in  FIG. 4 , the heater  245  has a five-layered structure including five layers, i.e. a heat conducting layer  110 , an insulating layer  112 , a heating layer  116 , an insulating layer  112 , and a support layer  114  in this order from one surface that is in contact with the fixing belt  249  to the inner surface of the heater  245  at the position indicated by the dashed line B. The heater  245  is configured as a flexible heater having a thickness of about 140 μm. 
     The heat conducting layer  110  is formed of, for example, stainless steel having a thickness of about 30 μm. The heat conducting layer  110  conducts heat of the heating layer  116  to the fixing belt  249  by contacting the fixing belt  249 , to thereby heat the fixing belt  249 . 
     In the insulating layers  112 , for example, a resin material (e.g., polyimide) having a thickness of about 25 μm is used. The heating layer  116  is sandwiched between the two insulating layers  112  so that the heating layer  116  is electrically insulated. 
     In the heating layer  116 , for example, stainless steel having a thickness of about 30 μm, is used as in the heat conducting layer. The heating layer  116  is connected to, for example, a current circuit provided in the inner structure  244 , and has a structure in which stainless steel generates heat depending on the magnitude of a supplied current when the current is supplied from the current circuit. 
     In the support layer  114 , for example, stainless steel having a thickness of about 30 μm is used as in the heat conducting layer  110  and the heating layer  116 . The support layer  114  covers the insulating layer  112 , reinforces the structural strength of the heater  245 , and supports the heat conducting layer  110 , the insulating layer  112 , and the heating layer  116 . 
     The heater  245  having the above-described configuration is formed into a cylindrical shape having a diameter of about 35 mm, and is in close contact with the fixing belt  249  to heat the fixing belt  249 . In addition, the thermal fuse  246  is provided such that the fuse element  247  is in contact with the support layer  114 . 
     Next, the details of the thermal fuse  246  will be described.  FIG. 5  is a diagram illustrating a structure of the thermal fuse  246  when viewed along the transport direction of the paper P. 
     As illustrated in  FIG. 5 , the thermal fuse  246  includes a fuse element  247  and the support body  248 . The fuse element  247  is in contact with the support layer  114  of the heater  245  and fused when the temperature of the heater  245  becomes equal to or higher than an allowable temperature. The support body  248  supports the fuse element  247 . 
     One end of a conductive elastic member  20  (e.g., a metal spring) is mounted on each of the opposite ends of the fuse element  247  in the width direction of the fixing belt  249 , and the other end of the elastic member  20  is mounted on the support body  248 . Thus, the fuse element  247  is mounted on the support body  248  in a form of being pulled from the opposite ends thereof by the elastic members  20 . In addition, pulling the fuse element  247  using the elastic members  20  to mount the fuse element  247  on the support body  248  is referred to as “stretching the fuse element  247 .” 
     The other end of each elastic member  20  mounted on the support body  248  is connected to a connection line (not illustrated). In addition, the connection line is connected to, for example, a relay coil (not illustrated) and a direct current power source (not illustrated), which are provided inside the inner structure  244 . That is, the fuse element  247 , the elastic member  20 , the connection line (not illustrated), the relay coil (not illustrated), and the direct current power source (not illustrated) are connected to one another in series to form a closed circuit. 
     Thus, when the temperature of the heater  245  reaches near the allowable temperature and the fuse element  247  is fused, the current flowing through the closed circuit formed to include the fuse element  247  is interrupted, and a contact driven by the relay coil (not illustrated) is switched off. Therefore, the fixing device  24  may detect the situation in which the temperature of the heater  245  reaches near the allowable temperature. 
     In addition, in  FIG. 5 , although the elastic members  20  are mounted on the opposite ends of the fuse element  247  so as to stretch the fuse element  247 , the form of stretching the fuse element  247  is not limited thereto. For example, one end of the fuse element  247  may be mounted on the support body  248  using the elastic member  20 , and the other end of the fuse element  247  may be directly mounted on the support body  248  using, for example, a conductive wire, rather than using the elastic member  20 . 
     In addition, in the case where it is difficult to directly mount the elastic member  20  to the fuse element  247 , the fuse element  247  and the elastic member  20  may be connected to each other via, for example, a conductive wire having a composition to be easily mounted on the fuse element. 
     Even in the above-described case, the fuse element  247  is stretched on the support body  248  by the elastic members  20 . In addition, the positions where the relay (not illustrated) and the direct current power source (not illustrated) are provided are also not limited to the inside of the inner structure  244 . 
     In addition, as illustrated in  FIG. 5 , the fuse element  247  is configured by covering a cylindrical fusible body  247 A, which has a diameter of about 0.4 mm and a length of about 200 mm in the width direction of the fixing belt  249 , with a heat resistant insulating tube  247 B, which is formed of, for example, a resin material (e.g., polyimide) and has a hollow shape, of which the inner diameter is about 0.5 mm and the outer diameter is about 0.54 mm. 
     In addition, flux may be introduced into the space formed by the heat resistant insulating tube  247 B and the fusible body  247 A. The flux suppresses the degree to which oxidation progresses when the fusible body  247 A directly contacts with air, and also suppresses re-oxidation of the fusible body  247 A due to the heat of the heater  245 . 
     The fusible body  247 A is, for example, an alloy including lead, tin, and silver, and the melting point of the fusible body  247 A, i.e. the fusing temperature of the fusible body  247 A is set by adjusting the composition ratio of the respective elements. The melting point of the fusible body  247 A set by the composition ratio of the respective elements is referred to as a rated fusing temperature of the thermal fuse  246 , and the rated fusing temperature of the thermal fuse  246  according to the present exemplary embodiment is set to a temperature T 0 . At this time, the rated fusing temperature T 0  of the thermal fuse  246  may be set to be equal to the allowable temperature of the heater  245 . 
     In addition, the fusible body  247 A has a length of about 200 mm in the width direction of the fixing belt  249 . When the fusible body  247 A is fused, the liquefied fusible body  247 A might scatter to the surroundings, thereby being adhered to the fixing device  24 . However, the fusible body  247 A is covered with the heat resistant insulating tube  247 B. Thus, when the fusible body  247 A is fused, it is possible to prevent the liquefied fusible body  247 A from being scattered to the surroundings and from adhering to the fixing device  24 . 
     In addition, in the above description, although the length of the fuse element  247  is less than the width of the heater  245  by way of an example, the fusible body  247 A having a greater length than a width of the heater  245  may be used. 
     Here, “the width of the heater  245 ” refers to the length of the heater  245  along the width direction of the fixing belt  249 . Thus, the width direction of the heater  245  coincides with the width direction of the fixing belt  249 . In addition, the length of the fuse element  247  in the width direction of the fixing belt  249  is referred to as “the length of the fuse element  247 .” 
     Next, the action of stretching the fuse element  247  will be described. 
     Assuming that the fuse element  247  is an ordinary thermal fuse having about several millimeters to several centimeters in length. In this case, when the temperature of the fuse element  247  becomes equal to or higher than the rated fusing temperature T 0 , the ends of a fusing portion of the fusible body  247 A are changed to spherical shapes due to surface tension and separated. Thereby, the fuse element  247  is fused. 
     However, if the length of the fuse element  247  is increased and becomes, for example, 100 mm or more like the thermal fuse  246  according to the present exemplary embodiment, the fusible body  247 A of the fuse element  247  starts to be expanded and loosened due to heat of the heater  245 . In this case, the gap between the heat resistant insulating tube  247 B and the fusible body  247 A is narrowed. Thus, even if the temperature of the fusible body  247 A becomes equal to or higher than the rated fusing temperature T 0  and the fusible body  247 A starts to be fused, the ends of the fusing portion of the fusible body  247 A may hardly be changed to the spherical shapes compared to the ordinary thermal fuse  246 . That is, as the length of the fuse element  247  is increased, the fuse element  247  may hardly be fused at the preset rated fusing temperature T 0  of the thermal fuse  246 . 
     Thus, in the thermal fuse  246  according to the present exemplary embodiment, as illustrated in  FIG. 5 , the opposite ends of the fuse element  247 , more specifically, the opposite ends of the fusible body  247 A constituting the fuse element  247  are pulled using the elastic members  20  so as to stretch the fuse element  247 . In this case, even if the fusible body  247 A of the fuse element  247  is expanded and loosened by the effect of heat by the heater  245 , a tension acts on opposite ends of the fusible body  247 A to pull the fusible body  247 A in the opposite directions. 
     Thus, when the temperature of the fusible body  247 A becomes equal to or higher than the rated fusing temperature T 0  and the fusible body  247 A starts to be fused, the ends of the fusing portion tend to move away from each other by the tension acting on the opposite ends of the fusible body  247 A. Therefore, the fusible body  247 A is easily fused compared to the case where the fuse element  247  is mounted on the support body  248  without being stretched. 
     Meanwhile,  FIG. 6  is a graph illustrating an example of changing the fusing temperature of the fuse element  247  relative to a tension for stretching the fuse element  247 . The horizontal axis represents a tension for stretching the fuse element  247 , and the vertical axis represents a fusing temperature of the fuse element  247 . 
     As illustrated in  FIG. 6 , it is found that the fusing temperature of the fuse element  247  falls within an allowable range that may be regarded as the rated fusing temperature T 0  when the tension for stretching the fuse element  247  is a specific threshold value N 0  or less, and that the fusing temperature tends to be linearly reduced as the tension is increased when the tension for stretching the fuse element  247  exceeds the threshold value N 0 . 
     Thus, when the fuse element  247  is stretched by the tension exceeding the threshold value N 0 , the fusing temperature of the thermal fuse  246  may be set to a specific temperature below the rated fusing temperature T 0  of the thermal fuse  246  by the thermal fuse  246  having the rated fusing temperature T 0 . 
     Specifically, an elastic member  20  having an elastic modulus that stretches the fuse element  247  using a tension that makes the fusing temperature of the thermal fuse  246  substantially equal to the allowable temperature of the heater  245 , which is lower than the rated fusing temperature T 0  may be used as the elastic member  20 . When a coil spring is used as the elastic member  20 , among plural kinds of coil springs having different spring coefficients, for example, based on  FIG. 6 , a coil spring, which has a spring coefficient that stretches the fuse element  247  using a tension that substantially corresponds to the allowable temperature of the heater  245 , which is lower than the rated fusing temperature To, may be used. 
     That is, for plural kinds of fixing devices  24  in which the allowable temperatures of the heaters  245  are equal to or lower than the rated fusing temperature T 0  and different from one another, the same kind of thermal fuses  246 , of which the rated fusing temperature is set to T 0 , may be used. Thus, a cost reduction for the fixing device  24  and the image forming apparatus  10  including the fixing device  24  is expected by commonly using the thermal fuses  246 . 
     In addition, when the image forming apparatus  10  corresponds to plural process speeds, the set temperature of the heater  245  may be changed by a difference among the process speeds. 
     The process speeds are, for example, classified into a process speed called a “low speed” of about 160 mm/s, a process speed called a “middle speed” of about 260 mm/s, and a process speed called a “high speed” of about 365 mm/s. 
     When the process speed is the low speed, the time during which the paper P is in contact with the fixing belt  249  heated by the heater  245  is increased compared to the case where the process speed is the middle speed. Thus, when the paper P passes through the fixing device  24  at the same temperature of the heater  245  as that in the case where the process speed is the middle speed in the situation in which the process speed is the low speed, the temperature of the paper P easily becomes a high temperature compared to the case where the process speed is the middle speed. That is, in view of the fact that the quality of an image may be deteriorated when the temperature at which the toner image is fixed to the paper P becomes higher than a specific temperature, the set temperature of the heater  245  may be set to be lower as the process speed is reduced. 
     On the contrary, when the process speed is the high speed, the time during which the paper P is in contact with the fixing belt  249  heated by the heater  245  is reduced compared to the case where the process speed is the middle speed. Thus, when the paper P passes through the fixing device  24  at the same temperature of the heater  245  as that in the case where the process speed is the middle speed in the situation in which the process speed is the high speed, the temperature of the paper P easily becomes a low temperature compared to the case where the process speed is the middle speed. That is, in view of the fact that the toner image may be hardly fixed on the paper P and the quality of an image may be deteriorated when the temperature at which the toner image is fixed on the paper P becomes lower than the specific temperature, the set temperature of the heater  245  may be set to be higher as the process speed is increased. 
     Thus, in the image forming apparatus  10  corresponding to the plural process speeds, the fusing temperature of the thermal fuse  246  may be changed according to the allowable temperature that depends on the set temperature of the heater  245 , which is set for each process speed. 
     Therefore, for example, as illustrated in  FIG. 7 , on an end of the support body  248 , a traction roller  253  is provided that is rotated by a motor (not illustrated) while winding a wire connected to one end of the elastic member  20 . In addition, the winding amount of the wire connected to the elastic member  20  is adjusted by controlling the rotating direction and the rotating amount of the traction roller  253 , and the tension for stretching the fuse element  247  is set to a specific value. 
     For example, it is assumed that the fuse element  247  is stretched using the tension at which the fusing temperature of the thermal fuse  246  becomes the allowable temperature of the heater  245  at the middle process speed. 
     In the above-described situation, when the process speed of the image forming apparatus  10  is switched to the low speed, the allowable temperature of the heater  245  is set to be lower than the allowable temperature at the middle process speed according to the reduction of the process speed. Thus, the traction roller  253  is rotated in the direction where the winding amount of the wire connected to one end of the elastic member  20  is increased so as to increase the tension for stretching the fuse element  247 , thereby reducing the fusing temperature of the thermal fuse  246 . 
     Meanwhile, when the process speed of the image forming apparatus  10  is switched from the middle speed to the high speed, the allowable temperature of the heater  245  is set to be higher than the allowable temperature at the middle process speed. Thus, the traction roller  253  is rotated in the direction in which the winding amount of the wire connected to one end of the elastic member  20  is reduced so as to reduce the tension for stretching the fuse element  247 , thereby increasing the fusing temperature of the thermal fuse  246 . 
     That is, for the image forming apparatus  10  of which the process speed is switchable, it is possible to protect the fixing device  24  by detecting plural temperatures using the single thermal fuse  246 . Thus, the number of thermal fuses  246  may be reduced compared to a case in which plural thermal fuses  246 , of which the rated fusing temperatures are different, are provided in the fixing device  24  according to the allowable temperature of the heater  245 , which is changed for each process speed. Thus, the cost reduction in the fixing device  24  and the image forming apparatus  10  including the fixing device  24  is expected. In addition, the size of the fixing device  24  is reduced because the number of thermal fuses  246  is reduced. 
     In addition, the device of adjusting the tension for stretching the fuse element  247  illustrated in  FIG. 7  is given by way of an example, and the present exemplary embodiment is not limited thereto. For example, the traction rollers  253  may be provided on the opposite ends of the support body  248  so that tension is adjusted by pulling the fuse element  247  from the opposite ends thereof. In addition, a mechanism for changing the length L of the support body  248  may be provided on the support body  248  illustrated in  FIG. 5  such that when the tension for stretching the fuse element  247  is increased, the length L of the support body  248  may be adjusted to be longer than the length of the current state, and when the tension for stretching the fuse element  247  is reduced, the length L of the support body  248  may be adjusted to be shorter than the length of the current state. 
     (Check Operation of Thermal Fuse) 
     An operation of the thermal fuse  246  according to the present exemplary embodiment is checked using an evaluation circuit illustrated in  FIG. 8 . As illustrated in  FIG. 8 , a direct current power supply  95  is connected in series to the thermal fuse  246  having a rated fusing temperature T 0  via a coil  94 A of a relay  94 . In addition, a commercial alternating current power supply  96  is connected in series to the heater  245  of the fixing device  24  via a contact  94 B of the relay  94  and a solid state relay  93 . In addition, a temperature sensor  92  is disposed around the fixing belt  249 , and a CPU  91  in a control circuit  90  is notified of the temperature measured by the temperature sensor  92 . The CPU  91  performs a contact control of the solid state relay  93  and controls the electrical conduction time for the heater  245  using information about the temperature measured by the temperature sensor  92  so as to control the temperature of the heater  245 . 
     In addition, in  FIG. 8 , V D  represents the driving voltage of the temperature sensor  92  and the solid state relay  93 . In addition, the temperature sensor  92  is used to measure each of the temperature of the fixing belt  249 , the temperature of the heater  245 , and the temperature of the thermal fuse  246 . 
       FIG. 9  is a graph representing variations in the respective temperatures of the fixing belt  249 , the heater  245 , and the thermal fuse  246  in the case where the temperature of the heater  245  is not controlled by the control circuit  90  and the heater  245  is operated at a rated power under the assumption that the control circuit  90  is in failure, and also representing a relationship between the fusing temperature and the fusing time of the thermal fuse  246  in the case where the fuse element  247  of the thermal fuse  246  is stretched by different tensions. 
     In  FIG. 9 , the graph  97  represents the temperature of the heater  245 , the graph  98  represents the temperature of the fixing belt  249 , and the graph  99  represents the temperature of the thermal fuse  246 . In addition, in  FIG. 9 , the horizontal axis represents the electrical conduction time of the heater  245 , and the vertical axis represents the temperature. In addition, it is assumed that the magnitude of tension has a relationship of N 0 &lt;N 1 &lt;N 2 &lt;N 3 , the temperature has a relationship of T 3 &lt;T 2 &lt;T 1 &lt;T 0 , and the time has a relationship of S 1 &lt;S 2 &lt;S 3 . In addition, it is assumed that the temperature T 1  is the temperature corresponding to the allowable temperature of the heater  245  when the process speed is the high speed, the temperature T 2  is the temperature corresponding to the allowable temperature of the heater  245  when the process speed is the middle speed, and the temperature T 3  is the temperature corresponding to the allowable temperature of the heater  245  when the process speed is the low speed. 
     In the case where the tension for stretching the fuse element  247  is N 3 , the thermal fuse  246  is fused at the temperature T 3  when electrical conduction for the heater  245  is initiated. In this case, the electrical conduction time for the heater  245  is S 1 . 
     In addition, in the case where the tension for stretching the fuse element  247  is N 2 , the thermal fuse  246  is fused at the temperature T 2  when the electrical conduction for the heater  245  is initiated. In this case, the electrical conduction time for the heater  245  is S 2 . 
     In addition, in the case where that the tension for stretching the fuse element  247  is N 1 , the thermal fuse  246  is fused at the temperature T 1  when the electrical conduction for the heater  245  is initiated. In this case, the electrical conduction time for the heater  245  is S 3 . 
     That is, it has been found that the fusing temperature of the thermal fuse  246  is reduced as the tension for stretching the fuse element  247  is increased in the range within which the tension exceeds a threshold value N 0 . 
     As described above, with the fixing device  24  according to the present exemplary embodiment, the fusing temperature of the thermal fuse  246  is adjusted using the thermal fuse  246  in which the fuse element  247  is stretched by a larger tension than the threshold value N 0 . Thus, even in the case of the thermal fuse  246  of which the rated fusing temperature is T 0 , plural temperatures can be detected because the fusing temperature of the thermal fuse  246  is changed when the tension for stretching the fuse element  247  is adjusted. 
     The exemplary embodiments are described above. It should be noted that the invention is not limited to the above described exemplary embodiments. Various modifications or improvements may be applied to the exemplary embodiment without departing from the gist of the present invention, and the modified or improved forms are also included in the technical scope of the present invention. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.