Patent Publication Number: US-7720420-B2

Title: Fixing device for thermally fixing an image developed on a recording sheet and use with an image-forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2006-230448, filed on Aug. 28, 2006, the entire contents of which are incorporated herein by reference. 
   TECHNICAL FIELD 
   The present invention relates to a fixing device for thermally fixing an image developed on paper (a recording sheet) and an image forming apparatus including the fixing device. 
   BACKGROUND 
   An image forming apparatus, such as a laser printer, is generally includes a fixing device for thermally fixing an image transferred to the sheet from a photosensitive element. JP-A-8-48432 discloses a fixing device including a heating roller heated by a heat source, a pressing roller pressed toward the heating roller, and a bearing that rotatably supports the heating roller and that is formed of a conductive resin. According to this device, an earth plate is connected to the bearing formed of conductive resin, thereby allowing electric charges accumulated in the heating roller to be discharged. Depending on the structure of the fixing device, there is a case where a conductive member for supplying power to the heat source and a power supply member having a thermal fuse as a safety device are disposed in the vicinity of the bearing of the heating roller. 
   However, in a case where the bearing is formed of a conductive resin and the power supply member is disposed in the vicinity of the bearing, when the bearing has become fused as a result of occurrence of a failure in temperature control of the heating roller, the fused bearing may contact the power supply member. When the bearing contacts the power supply member as mentioned above, a short circuit arises between the power supply member and the heating roller by way of the conductive bearing. This may eventually result in infliction of damage on the fixing device. 
   SUMMARY 
   According to a first aspect of the invention, there is provided a fixing device for thermally fixing an image developed on a recording sheet, including: a heating roller heated by a heat source and having a first end side and a second end side with respect to an axial direction; a power supply member for supplying the heat source with power for use in heating the heating roller; a first bearing formed of a conductive material, attached to the first end side of the heating roller and connected to a ground; and an second bearing formed of an insulating material and attached to the second end side of the heating roller, wherein the power supply member is disposed at the second end side of the heating roller. 
   According to a second aspect of the invention, there is provided a fixing device for thermally fixing an image developed on a recording sheet, including: a heating roller heated by a heat source; a temperature fuse that shuts off a power supply when a predetermined temperature or more is achieved and disposed to oppose to a circumferential face of the heating roller; and a cylindrical insulating tube that sheathes the temperature fuse, wherein an axial direction of the insulating tube is aligned with the heating roller, and the insulating tube is rotatably arranged around the temperature fuse. 
   According to a third aspect of the invention, there is provided an image forming apparatus for forming an image on a recording sheet, including: an exposure device that performs a scanning operation with a laser beam in accordance with an input signal of the image; a photosensitive element subjected to the scanning operation, whereby an electrostatic latent image is formed; a developing unit that supplies the photosensitive element with developer; a transfer unit that transfers a developed image formed from the developer onto the recording sheet; and a fixing device that thermally fixes the developed image on the recording sheet, wherein the fixing device includes: a heating roller heated by a heat source and having a first end side and a second end side with respect to an axial direction; a power supply member for supplying the heat source with power for use in heating the heating roller; a first bearing that is formed of a conductive material and attached to the first end side of the heating roller and connected to a ground; and an first bearing that is formed of an insulating material and attached to the second end side of the heating roller wherein the power supply member is disposed at the second end side of the heating roller. 
   According to a fourth aspect of the invention, there is provided an image forming apparatus for forming an image on a recording sheet, including: an exposure device that performs a scanning operation with a laser beam in accordance with an input signal of the image; a photosensitive element subjected to the scanning operation, whereby an electrostatic latent image is formed; a developing unit that supplies the photosensitive element with developer; a transfer unit that transfers a developed image formed from the developer onto the recording sheet; and a fixing device that thermally fixes the developed image on the recording sheet, wherein the fixing device includes: a heating roller heated by a heat source; a temperature fuse that shuts off a power supply when a predetermined temperature or more is achieved and disposed to oppose to a circumferential face of the heating roller; and a cylindrical insulating tube which sheathes the temperature fuse, wherein an axial direction of the insulating tube is aligned with the heating roller, and the insulating tube is rotatably arranged around the temperature fuse. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side cross-sectional view showing a laser printer serving as an example image forming apparatus of a first embodiment of the present invention; 
       FIG. 2  is a schematic view showing a relationship among components of a fixing device of the laser printer shown in  FIG. 1 ; 
       FIG. 3  is a side view showing the relationship among the respective components of the fixing device of the laser printer shown in  FIG. 1 ; 
       FIG. 4  is a schematic diagram showing movement of a heating roller achieved when a bearing has become fused and flow of an electric current; and 
       FIG. 5  is a schematic diagram showing a fixing device of a second embodiment of the present invention. 
   

   DESCRIPTION 
   First Embodiment 
   &lt;Overall Configuration of Laser Printer&gt; 
   An overall configuration of a laser printer serving as an exemplification of an image forming apparatus of the present invention will first be described briefly.  FIG. 1  is a side cross-sectional view showing the laser printer serving as an exemplification of the image forming apparatus of a first embodiment of the present invention. 
   As shown in  FIG. 1 , a laser printer  1  includes a feeder unit  4  for feeding sheets  3  into a main-unit casing  2  and an image forming unit  5  for forming an image on the supplied sheet  3 . 
   &lt;Structure of the Feeder Unit&gt; 
   The feeder unit  4  includes a sheet-feeding tray  6  removably attached to an inner bottom of the main-unit casing  2  and a sheet-pressing plate  7  provided within the sheet-feeding tray  6 . The feeder unit  4  also includes a sheet-feeding roller  8  and a sheet-feeding pad  9  which are disposed at elevated positions above an end of one end side of the sheet-feeding tray  6 ; and paper-dust rollers  10 ,  11  which are disposed at downstream positions with respect to the paper-feeding roller  8  in a direction in which the sheet  3  is conveyed (hereinafter called a “conveying direction”). The feeder unit  4  additionally includes register rollers  12  disposed downstream with respect to the paper-dust rollers  10  and  11 . In the following descriptions, there is a case where a downstream side and an upstream side in the conveying direction of the sheet  3  are called simply a downstream side and an upstream side. 
   In the feeder unit  4  constructed as mentioned above, the sheets  3  in the paper-feeding tray  6  are pulled toward the paper-feeding roller  8  by means of the sheet press plate  7 . The sheets are then fed by means of the sheet-feeding roller  8  and the sheet-feeding pad  9  and conveyed one at a time to the image forming unit  5  after having passed through the various types of rollers  10  to  12 . 
   &lt;Structure of the Image Forming Unit&gt; 
   The image forming unit  5  has a scanner unit  16 , a process cartridge  17 , a fixing device  18 , and the like. 
   &lt;Structure of the Scanner Unit&gt; 
   The scanner unit  16  is disposed at an elevated position within the main-unit casing  2 . The scanner unit  16  includes a laser-emitting unit (not shown), a polygon mirror  19  to be rotationally driven, lenses  20  and  21 , reflecting mirrors  22 ,  23 ,  24 , and others. A laser beam, which is determined by image data and which is emitted from the laser-emitting unit, sequentially passes through or undergoes reflection on the polygon mirror  19 , the lens  20 , the reflecting mirrors  22  and  23 , the lens  21 , and the reflecting mirror  24 . The laser beam is then radiated over a surface of a photosensitive drum  27  by means of a high-speed scanning. 
   &lt;Structure of the Process Cartridge&gt; 
   The process cartridge  17  is disposed below the scanner unit  16  and removably attached to the main-unit casing  2 . A developing cartridge  28 , a photosensitive drum  27 , a scorotron-type charging device  29 , and a transfer roller  30  are primarily provided within a hollow housing  51  constituting an exterior framework of the process cartridge  17 . 
   The developing cartridge  28  is removably attached to the housing  51  and includes a developing roller  31 , a layer thickness regulation blade  32 , a feed roller  33 , and a toner hopper  34 . Toner in the tonner hopper  34  is supplied to the developing roller  31  by means of rotation of the feed roller  33  in an arrowed direction (a counterclockwise direction). At this time, toner is positively electrified between the feed roller  33  and the developing roller  31 . The toner fed over the developing roller  31  enters between the layer thickness regulation blade  32  and the developing roller  31  in association with rotation of the developing roller  31  in the arrowed direction (the counterclockwise direction), and held as a thin layer of given thickness over the developing roller  31 . 
   The photosensitive drum  27  is supported by the housing  51  so as to be rotatable in an arrowed direction (a clockwise direction). A drum main body of this photosensitive drum  27  is connected to a ground, and a surface of the photosensitive drum  27  is formed of a photosensitive layer whose surface is formed of polycarbonate and which exhibits positive charging properties. 
   The scorotron-type charging device  29  is disposed above and opposite the photosensitive drum  27  with a predetermined space, to thus avoid contacting the photosensitive drum  27 . This scorotron-type charging device  29  is used for positive charging purpose which produces a corona discharge from a charting wire, such as a tungsten wire, and which is configured to uniformly charge the surface of the photosensitive drum  27  with positive charges. 
   The transfer roller  30  is disposed to contact the photosensitive drum  27  and supported by the housing  51  so as to be rotatable in the arrowed direction (the counterclockwise) This transfer roller  30 A is constructed by means of sheathing a metallic roller shaft with a conductive rubber material. At the time of transfer operation, a transfer bias is applied to this transfer roller  30 . 
   After having been uniformly electrified with positive electric charges by means of the scorotron-type charging device  29 , the surface of the photosensitive drum  27  is exposed by means of a high-speed scan of a laser beam from the scanner unit  16 . As a result, an electric potential of an exposed area is decreased, whereby an electrostatic latent image is formed in accordance with image data. Here, the word “electrostatic latent image” means an exposed area, of which electric potential has decreased upon exposure to the laser beam, in a uniformly, positively-charged surface of the photosensitive drum  27 . Next, when the toner held over the developing roller  31  opposes and contacts the photosensitive drum  27  by rotating the developing roller  31 , the toner is supplied to the electrostatic latent image formed on the surface of the photosensitive drum  27 . As a result of toner being selectively held over the surface of the photosensitive drum  27 , the electrostatic latent image is made visible. Thus, a toner image is formed by means of discharged-area development. 
   Subsequently, the photosensitive drum  27  and the transfer roller  30  are rotationally driven such that the sheet  3  is conveyed while being nipped between the drum  27  and the roller  30 . Thus, the toner image held over the surface of the photosensitive drum  27  is transferred onto the sheet  3 . 
   &lt;Structure of the Fixing Device&gt; 
   The fixing device  18  is disposed at a downstream position with respect to the process cartridge  17 . The fixing device  18  includes a heating roller  41 , a pressing roller  42  pressed toward the heating roller  41 , and a pair of conveyor rollers (a conveyor roller pair)  43  disposed downstream with respect to the heating roller  41  and the pressing roller  42 . The fixing device  18  thermally fixes the toner image transferred on the sheet  3  during the course of the sheet  3  passing between the heating roller  41  and the pressing roller  42 . Subsequently, the sheet  3  is conveyed to a sheet discharge path  44  by means of the conveyor roller pair  43 . The sheet  3  conveyed to the sheet discharge path  44  is discharged onto a sheet discharge tray  46  by means of sheet discharge rollers  45 . 
   &lt;Detailed Structure of the Fixing Device&gt; 
   The detailed structure of the fixing device will now be described.  FIG. 2  is a schematic view showing a relationship among elements of the fixing device of the laser printer shown in  FIG. 1 , and  FIG. 3  is a side view showing the relationship among the respective elements of the fixing device of the laser printer shown in  FIG. 1 . The previously-described structure of the fixing device shown in  FIG. 1  is omitted, as required, for the sake of description. 
   As shown in  FIG. 2 , in addition to the heating roller  41  and the pressing roller  42 , the fixing device  18  also includes a conductive resin bearing  60 A and an insulating resin bearing  60 B for the heating roller  41 , a bearing  61  for the pressing roller (a pressing roller bearing), a housing  62 , and pressure arms  63 . 
   The heating roller  41  has into a hollow, cylindrical shape and is to be heated by means of a halogen heater HH incorporated in the roller  41 . This heating roller  41  is supported rotatably by the housing  62  by way of the conductive resin bearing  60 A and the insulating resin bearing  60 B (a supported area is not illustrated). 
   A gear G is fastened to one end of the heating roller  42 . Thus, as a result of transmission of driving force from a driving unit (not shown) to the heating roller  41  by way of the gear G, the heating roller  41  is rotated. The conductive resin bearing  60 A is attached to one end side of the heating roller  41 , and the insulating resin bearing  60 B is attached to the other end side of the heating roller  41 . 
   The conductive resin bearing  60 A is formed of a conductive resin and is connected to the ground by way of a metallic wire etc. connected to an appropriate point on the bearing. Any resin may also be adopted as a conductive resin. It is better to adopt; for example, conductive PPS (polyphenylene sulfide), conductive PC (polycarbonate), conductive POM (polyacetal), and the like. 
   The insulating resin bearing  60 B is formed of an insulating resin. Any resin may also be adopted as a insulating resin. It is better to adopt; for example, insulating PPS (polyphenylene sulfide), insulating POM, and the like, 
   The pressing roller  42  has a cylindrical shape. A rotary shaft  42   a  is provided so as to protrude to the outside from the center of each end of the roller  24 . This rotary shaft  42   a  is supported rotatably by means of the pressing roller bearing  61 . The pressing roller  42  is an example of a pressing member. The pressing member is not limited to a cylindrical roller, and may be other configuration such as a belt. 
   As shown in  FIG. 3 , the pressure arms  63  has an elongated shape and presses (urges) the pressing roller  42  toward the heating roller  41  as by urging toward the heating roller  41  by means of a tension spring S. One end  63   a  of each pressure arm  63  is supported rotatably by the housing  62 . The substantially center of each pressure arm  63  supports the pressing roller bearing  61 , and the other end side of the same is urged by the tension spring S. 
   As shown in  FIG. 2 , the housing  62  mainly includes a pair of sidewalls  62   a  and  62   b,  which oppose respective end faces of the heating roller  41 , and an upper wall  62   c  disposed at a position across the heating roller  41  from the pressing roller  42 . Of the sidewalls  62   a  and  62   b , the sidewall  62   a  is provided with a first electrode  66  connected to one end of the halogen heater HH, and the other sidewall  62   b  is provided with a second electrode  67  connected to the other end of the halogen heater HH. 
   The second electrode  67  is formed to have an L-shaped in cross-section and includes a connection portion  67   a  connected to the halogen heater HH and a first conductive portion  67   b  provided along an inner surface of the sidewall  62   b.  In order to supply power to the second electrode  67 , a second conductive portion  68  for transmitting electricity from an external power supply to the second electrode  67  is formed integrally to the first conductive portion  67   b  of the second electrode  67 . This second conductive portion  68  is provided along an inner surface of the upper wall  62   c  of the housing  62 . The halogen heater HH is activated by power supplied from the external power supply to the second conductive portion  68  and the first electrode  66  by way of a channel such as an electric wire (not shown). 
   The second conductive portion  68  is provided with, as appropriate, a temperature fuse  69  and a thermostat  70 . The second conductive portion  68 , the temperature fuse  69 , and the thermostat  70  are provided at the other end side of the heating roller  41  (the side of the roller  41  where the insulating resin bearing  60 B is provided) as well as on the side across the heating roller  41  from the pressing roller  42 . 
   A fuse element which is fused as a result of an increase in ambient temperature is provided in the temperature fuse  69 . When the temperature of the heating roller  41  has reached a predetermined temperature or more, the temperature fuse is fused, thereby physically shutting off circuitry and the power supply. The temperature fuse  69  is disposed in the vicinity of the other end of the heating roller  41 , thereby detecting the temperature of the other end of the heating roller  41 . This temperature fuse  69  is sheathed with a cylindrical insulating tube  71 . 
   The insulating tube  71  is a cylindrical element formed of an insulating material, such as a resin. The axial direction of the insulating tube  71  is aligned with the longitudinal direction of the heating roller  41  so as to be rotatable with respect to the temperature fuse  69 . Specifically, this insulating tube  71  is rotatable along the circumferential direction of the outer peripheral surface of the substantially cylindrical temperature fuse  69 . Here, the insulating tube  71  may be: an element that is formed by cylindrically rolling a plastic film or an insulating tape and connecting the thus-rolled film or tape; a plastic molded article or a rubber molded article cylindrically formed by means of a molding die; or the like. 
   The thermostat  70  has spherical bimetal. When a given temperature or higher is reached, the bimetal is inverted, to thus turn off a switch and physically shut off circuitry. This thermostat  70  is disposed at a position close to the substantial center of the heating roller  41 , thereby reacting to the temperature of the substantial center of the heating roller  41 . Anything may also be adopted as the thermostat, so long as it physically shuts off circuitry. Moreover, an element for electrically shutting off circuitry in accordance with a signal from a sensor may also be applied instead of the thermostat. 
   A thermistor  72  for detecting the temperature of the substantial center of the heating roller  41  is provided in the vicinity of the thermostat  70  (at a position opposite to the temperature fuse  69 ). A signal detected by this thermistor  72  is output to a controller (not shown), and this controller controls electric current according to the temperature for the heating roller  41 . 
   The present embodiment mentions, as examples of the power supply member, the second conductive portion  68 , the temperature fuse  69 , and the thermostat  70 . However, the present invention is not limited to this embodiment. For instance, only the second conductive portion  68  and the temperature fuse  69  may also be taken as the power supply members and disposed opposite the heating roller  41 . 
   An intake-type exhaust fan  73  for discharging an air from one end side of the heating roller  41  to the other end side thereof is placed at the other end side of the heating roller  41  (the opposite side of the gear G) so as to oppose the end face of the heating roller  41 . 
   Next, operation of the controller (the thermistor  72 ), the thermostat  70 , and the fuse  69  is described. Subsequently, operation of the heating roller  41  and other operations performed when these elements fail to operate properly will be described. 
   &lt;Operation of the Controller&gt; 
   The thermistor  72  detects the temperature of the substantial center of the heating roller  41  at all time, and the detected temperature is output as temperature information to the controller (not shown). This controller appropriately controls electric current in accordance with the temperature information from the thermistor  72 . 
   &lt;Thermostat  70 &gt; 
   When an increase in the temperature of the heating controller  41  is not prevented by current control of the controller, circuitry is physically shut off by means of switching operation of the thermostat  70 . 
   &lt;Operation of the Temperature Fuse  69 &gt; 
   When a large number of sheets  3  is printed consecutively, there sometimes arises a case where heat of the substantial center of the heating roller  41  is intensively deprived by the sheets  3  passing through the substantial centers of the respective rollers  41  and  42 . When such a phenomenon of intensive deprivation of heat from the substantial center of the heating roller  41  has arisen, an anomalous increase occurs in the temperatures of both ends of the heating roller  41  even when a normal temperature is acquired at the substantial center of the heating roller  41 . In such a case, the thermostat  70  does not shut off circuitry, but the temperature fuse  69  instead physically breaks a line, to thus shut off circuitry. 
   &lt;A Case Where the Thermostat  70  and the Temperature Fuse  69  Do Not Operate&gt; 
   For instance, in the event that the temperature fuse  69  is broken when a large number of sheets  3  is printed consecutively as mentioned previously, the temperatures of both ends of the heating roller  41  increase. At this time, the air in the fixing device  18  flows from one end side of the heating roller  41  (i.e., the side of the heating roller opposing the gear G) toward the other side of the same by means of ventilation of the exhaust fan  73 . Therefore, an increase rises particularly in the temperature of the other end side of the heating roller  41 . 
   When a temperature has increased such as that mentioned above, the insulating resin bearing  60 B disposed at the other end side of the heating roller  41  is first fused, and the conductive resin bearing  60 A is fused subsequently. Thus, the bearings  60 B and  60 A are fused one after another with a predetermined time lag, whereupon the heating roller  41  is pressed toward the second conductive portion  68  by the pressing roller  42  while one end side of the heating roller  41  undergoes resistance. 
   Therefore, as shown in  FIG. 4 , the other end side of the heating roller  41  moves to the second conductive portion  68  and becomes inclined obliquely when compared with its normal position. When the heating roller  41  becomes inclined in this way, the insulating resin bearing  60 B having already become fused at the other end side of the roller  41  contacts the second conductive portion  68 . However, since the insulating resin bearing  60 B is an insulator, the electric current does not flow from the second conductive portion  68  to the insulating resin bearing  60 B. 
   Further, at this time, the heating roller  41  nearly comes into contact the temperature fuse  69 . However, since the temperature fuse  69  is sheathed with the insulating tube  71 , an electric current does not flow from the temperature fuse  69  to the heating roller  41 . Moreover, even when driving force is still transmitted to the gear G and rotation of the heating roller  41  is continued, the insulating tube  71  rotates with respect to the temperature fuse  69 , thereby allowing the rotational force of the heating roller  41  to be released. 
   Therefore, even when the bearings  60 A and  60 B are fused as mentioned above, electric current can flow along a normal route, such as that indicated by an arrow in the drawing, without involvement of occurrence of a short circuit between the second conductive portion  68  and the heating roller  41 . 
   According to the above descriptions, the present embodiment can yield the following advantages. 
   The insulating resin bearing  60 B prevents occurrence of a short circuit between the second conductive portion  68  and the heating roller  41 , and the insulating tube  71  prevents occurrence of a short circuit between the temperature fuse  69  and the heating roller  41 . Accordingly, infliction of damage to the device, which would otherwise be induced by a short circuit, can be prevented. 
   Since the heat is discharged toward the area for which measures against a short circuit have already been taken by means of the insulating resin bearing  60 B and the insulating tube  71 , the heating roller  41  is inclined obliquely, thereby preventing occurrence of a contact between other structures for which no measures against a short circuit are taken and the heating roller  41 . Specifically, in the present embodiment, the heating roller can be prevented from contacting the thermostat  70  for which measures against a short circuit are not taken. 
   Since the exhaust fan  73  is placed at the position opposite to the gear G, a gear train which is engaged with the gear G can be arranged compactly without taking into account the presence of the exhaust fan  73 . Hence, the structure of the heating roller facing the gear G can be embodied in the minimum size that allows incorporation of the gear train. This makes it possible to attempt to miniaturize the overall apparatus. 
   The rotation of the insulating tube  71  prevents rubbing of the heating roller  41  against the insulating tube  71  (release of rotational force). Hence, even when the insulating tube  71  is formed by wrapping; e.g., an insulating tape, the insulating tube  71  does not peel off from the temperature fuse  69 . Therefore, occurrence of a short circuit can be prevented without fail by means of the insulating tube  71 . 
   The present invention is not limited to the first embodiment and can be utilized in various forms as exemplified below. 
   In the first embodiment, both the insulating resin bearing  60 B and the insulating tube  71  are adopted. However, the present invention is not limited to this embodiment. For instance, in the case of a structure where, when the bearing has become fused, the other end of the heating roller  41  and the bearing does not contact the second conductive portion  68  and contacts only the temperature fuse  69 , the bearing to be positioned on the other end side of the heating roller  41  does not need to be embodied as an insulator. Even in this case, occurrence of a short circuit can be prevented by means of the insulating tube  71 . 
   Second Embodiment 
   Next, a second embodiment of the present invention will be described in detail, as appropriate, by reference to the drawings. Since the present embodiment relates to a modification on a part of the structure of the fixing device  18  of the first embodiment, the same reference numerals are assigned to the constituent elements analogous to those of the first embodiment, and their explanations are omitted.  FIG. 5  is a schematic diagram of the fixing device of the second embodiment. 
   As shown in  FIG. 5 , a fixing device  18 ′ of the second embodiment differs from that of the first embodiment. A cover portion  62   d  formed as a part of the housing  62  is disposed between the temperature fuse  69  and the heating roller  41 . An insulating tape  74  is affixed so as to extend from this cover portion  62   d  to a position opposing the insulating resin bearing  60 B of the second conductive portion  68 . It is better to adopt; for example, a polyester adhesive tape, a polyimide adhesive tape, or an acetate adhesive tape, as the insulating tape  74 . 
   According to the above descriptions, the second embodiment can yield the following advantages. 
   Since the insulating tape  74  is provided between the second conductive portion  68  and the insulating resin bearing  60 B, occurrence of a short circuit between the second conductive portion  68  and the heating roller  41  can be inhibited more thoroughly. 
   The temperature fuse  69  is protected by the cover portion  62   d  and the insulating tape  74  as well as by the insulating tube  71 . Accordingly, occurrence of a short circuit between the temperature fuse  69  and the heating roller  41  can be prevented more thoroughly. 
   The present invention is not limited to the second embodiment and can be utilized in various forms, such as those exemplified below. 
   In the second embodiment, the insulating tape  74  is adopted as an example insulator to be interposed between the second conductive portion  68  and the insulating resin bearing  60 B. However, the present invention is not limited to this. For instance, a rubber-molded article or a plastic-molded article which is fixed to the housing  62 , a silicon-based insulating film applied over the surface of the second conductive portion  68 , and the like, may also be adopted. 
   The present invention is not limited to the previously-described embodiments and can be utilized in various forms as exemplified below. 
   In the respective embodiments, the laser printer  1  is exemplified. However, the present invention is not limited to this printer and may also be applied to another image forming apparatus; for example, a copier, a multifunction machine, or the like. 
   In the respective embodiments, the transfer roller  30  is adopted as an example of a transfer unit. However, the present invention is not limited to this type of transfer unit, and; for example, a transfer roller of noncontact type may also be adopted. 
   In the respective embodiments, the sheet  3 , such as cardboard, a postcard, or thin paper, is adopted as an example of a recording sheet. However, the present invention is not limited to these types of paper. For example, an OHP sheet may also be adopted. 
   In the respective embodiments, the tension spring S is adopted as an example of an urging unit. However, the present invention is not limited to the spring. For instance, a press spring, a torsion spring, and the like, may also be adopted. 
   In the respective embodiments, the halogen heater HH is adopted as an example of a heat source. However, the present invention is not limited to this heater. For instance, a heater of induction heating (IH) type, a heating resistor, and the like, may also be adopted. 
   In the respective embodiments, toner is adopted as example developer; the developing cartridge  28  is adopted as example developing unit; the scanner unit  16  is adopted as an example of an exposure device; and the photosensitive drum  27  is adopted as an example of an photosensitive element. However, the present invention is not limited to them. Needless to say, a material and a structure can be modified, as appropriate, so long as the modifications are consistent with the gist of the present invention. 
   In the respective embodiments, the power supply member, such as the second conductive portion  68  and the temperature fuse  69 , is disposed across the heating roller  41  from the pressing roller  42 . However, the present invention is not limited to this layout, but the power supply member may be disposed in the vicinity of the insulating resin bearing  60 B. For instance, the power supply member may also be disposed forward or backward of the heating roller  41  shown in  FIG. 2  (at one end side or the other end side of the pressure arm  63 ; see  FIG. 3 ). Even in this case, the fused bearing may contact the power supply member and hence the structure described the embodiment is effective. 
   In the respective embodiments, the intake-type exhaust fan  73  is provided at the other end side of the heating roller  41 . However, the present invention is not limited to the exhaust fan of this type and this layout. An exhaust fan of emission type may also be disposed at one end side of the heating roller  41 . Even in this case, heat can be discharged toward the other end side of the heating roller  41  for which measures against a short circuit have already been taken. Hence, infliction of damage to the apparatus can be prevented. 
   In the respective embodiments, the intake-type exhaust fan  73  is mentioned as an example of an exhaust unit. However, the present invention is not limited to this unit. For example, an exhaust nozzle connected to a main unit of an air blower by way of a tube, or the like, may also be adopted. 
   In the respective embodiments, the bearing disposed at the other end side of the heating roller  41  is embodied as an insulating bearing. However, the present invention is not limited to the bearing of this type. For instance, when there is a concern about only a contact between the temperature fuse  69  and the heating roller  41 , such as in a case where the second conductive portion  68  is embedded in the housing  62  in connection with the configuration shown in  FIG. 2 , the bearing disposed on the other end side of the heating roller  41  may also be embodied as a conductive bearing. Even in such a case, since the temperature fuse  69  is provided with the insulating tube  71  in a rotatable manner, occurrence of a short circuit between the heating roller  41  and the temperature fuse  69  is prevented.