Abstract:
A fixing device includes a heat roller rotatably supported by a bearing, a pressure roller, a heater for heating the heat roller, a thermal fuse connected to a power circuit to supply electricity to the heater and disposed adjacent to an outer circumference of the heat roller while keeping an insulation distance between the heat roller and the thermal fuse based on a safety standard, and a thermostable stopper which keeps the insulation distance between the heat roller and the fuse to a specific distance based on safety standards even if the bearing melts.

Description:
This application is a division of application Ser. No. 09/431,174 filed on Nov. 1, 1999, now U.S. Pat. No. 6,269,230. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fixing device and an image forming apparatus using the fixing device. More particularly, the present invention relates to a fixing device including a heat roller supported by bearings. 
     2. Discussion of the Background 
     In image forming apparatuses, such as copying machines, printers, and the like, there is known an image forming apparatus provided with a fixing device that conveys a transfer sheet while nipping the transfer sheet with a heat roller and a pressure roller to affix a toner image on the transfer sheet, which is transferred onto the transfer sheet from a photoconductor drum. Namely, heat and pressure are applied to the transfer sheet in the sheet conveying process by the fixing device to affix the toner image on the transfer sheet. 
     A known fixing device illustrated in FIG. 11 is an example of such a fixing device. In the fixing device illustrated in FIG. 11, an elastic piece  103  having claw-shaped hooks  102  on respective upper ends is formed in a body of a C-shaped bearing  101  by which each end of a heat roller  100  is rotatably supported. The bearing  101  is secured to a supporting member  104  such as a housing by the hooks  102  elastically hooked to the supporting member  104 . 
     When the fixing device is constructed, at first the bearing  101  is secured to the supporting member  104 , and the heat roller  100  is fit into the bearing  101 , by elastically expanding a distance between both ends of the bearing  101 . Then, the pressure roller  105  is brought into contact with the heat roller  100 . 
     The operation fitting the heat roller  100  into the bearing  101  is performed by pressing the heat roller  100  in a direction orthogonal to the axis of rotation of the heat roller  100 . Thus, the fixing device is intended to be easily and automatically constructed. 
     In a fixing device having a heat roller and a pressure roller, a heater is generally provided inside the heat roller, and the heat roller and the pressure roller are set in a housing. The heater is composed of a quartz glass tube, both ends of which are sealed, and a heat wire provided in the quartz glass tube. The heater is further composed of a lead wire, an end of which is connected to the heat wire through a metal foil and the other end of which extends from the quartz glass tube, and the like parts. A heater terminal is connected to the other end of the lead wire that extends from the quartz glass tube and is affixed to the housing by a screw. A wire harness, which supplies electric power to the heater, is connected to the heater terminal. 
     Since the fixing device emits a large amount of heat, the housing is required to have thermostability and rigidity and is made of glass-containing synthetic resin. It is a current trend that the bearing is also made of a synthetic resin for the purposes of cost-reduction and weight-reduction. However, in fact the bearing is generally made of expensive synthetic resin, such as PPS (polyphenylene sulfide). 
     In addition, the heat roller is charged with static electricity generated by friction between the heat roller and the transfer sheet. Therefore, the heat roller is made of electric conductive material, such as metal, or the like. In order to ground the static electricity stored in the heat roller through the bearing, the bearing is also required to be electroconductive. Accordingly, the material used in the bearing becomes more and more expensive. 
     In FIG. 11, when the bearing  101  is attached to or detached from the supporting member  104 , the elastic piece  103  has to be bent to be hooked to the supporting member  104 . Therefore, the length of the elastic piece  103  must be relatively long so that the elastic piece  103  becomes flexible. An amount of the material necessary to form a bearing  101  thereby increases resulting in increase manufacture costs. 
     Further, the heater generates a relatively large amount of heat when image forming operations are performed. Therefore the housing is expanded by the heat, and the heater terminals affixed to the housing tend to move in such a direction to that the heater terminals pull the lead wires. 
     Therefore, various problems occur. For example, the heater terminal can become disconnected from the lead wire at the connection part thereof, the metal foil provided in the quartz glass tube can become disconnected from the lead wire at the connection part thereof, and the quartz glass tube can become cracked, thereby damaging the heater. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a fixing device including a bearing to support a heat roller, which has a simple structure so as to be easily assembled, and which is made at a low cost. Another object of the present invention is to provide an image forming apparatus using the fixing device. 
     Briefly, these objects, and other objects of the present invention as hereinafter will become more readily apparent, can be attained by a novel fixing device that includes a bearing including a C-shaped part; and a projection projected upwardly from a central part of an outer circumferential surface of the C-shaped part, in which the bearing is made of a synthetic resin having elasticity and conductivity, and a distance between both ends of the C-shaped part is elastically expanded. The fixing device further including a heat roller configured to be rotatably supported by the bearing at an upper part of an outer surface of each end of the heat roller in which the outer surface of each end of the heat roller which contacts the bearing, is made of an electroconductive material; a pressure roller configured to be located downward relative to the heat roller and that is driven by being in pressure contact with the heat roller; a housing configured to cover peripheries of the heat roller and the pressure roller and having an inserting part into which the projection of the bearing is detachably inserted from a pressure roller side; and a ground plate configured to attach to the housing and including an elastic piece that detachably hooks the C-shaped part of the bearing, and a ground piece that is grounded. 
     The housing generally includes an inserting part into which the projection of the bearing is detachably inserted from a pressure roller side, and a hooking pawl that detachably hooks the C-shaped part. 
     An additional embodiment of the fixing device includes a heat roller including a heater therein; a lead wire configured to extend from each end of the heater; and a heater terminal configured to be provided at each end of the lead wire and which is fixed to the housing, wherein a flexible space in which the heater terminal is capable of bending along a longitudinal direction of the heater is formed on at least one fixing position of the heater with the housing. 
     A further embodiment of the fixing device includes a heat roller that includes a heater; a thermal fuse configured to be connected to a circuit to supply electricity to the heater and is disposed adjacent to an outer circumference of the heat roller at an opposite side to the pressure roller while keeping an insulation distance between the heat roller and the thermal fuse based on the safety standard; and a thermostable stopper configured to keep an insulation distance between the heat roller and the thermal fuse within such a distance that an insulation strength based on the safety standards is kept even when the bearing melts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic longitudinal section illustrating a structure of an embodiment of the image forming apparatus of the present invention; 
     FIG. 2 is a schematic cross-section illustrating how a heat roller is supported at a front side of the fixing device of the present invention; 
     FIG. 3 is a schematic cross-section illustrating how the heat roller is supported at a rear side of the fixing device of the present invention; 
     FIG. 4 is a diagrammatical perspective view illustrating how a bearing that supports the heat roller is supported at the rear side of the fixing device of the present invention; 
     FIG. 5 is a side view illustrating a configuration of the heat roller and the bearing; 
     FIG. 6 is a schematic side cross-section illustrating a structure of another embodiment of the image forming apparatus of the present invention; 
     FIG. 7 is a schematic elevational section illustrating a structure of the fixing device of the present invention; 
     FIG. 8 is a schematic elevational section illustrating how a heater terminal is attached to a lower part of a housing; 
     FIG. 9 is a schematic perspective view illustrating the heater terminal; 
     FIG. 10 is a schematic elevational section illustrating how a heater terminal is attached to a lower part of a housing in yet another embodiment of the fixing device of the present invention; 
     FIG. 11 is a schematic side view illustrating a heat roller is supported in background art; 
     FIG. 12 is a schematic elevational section illustrating another embodiment of the supporting mechanism of the heat roller at the front side of a fixing device in FIG. 2; and 
     FIG. 13 is a schematic elevational section illustrating another embodiment of the supporting mechanism of the heat roller at the rear side of the fixing device in FIG.  12 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention are now described in detail referring to the drawings, wherein like reference numerals indicate identical or corresponding parts throughout the several views. 
     Referring to FIGS. 1 to  5 , a first embodiment of the present invention is explained. An internal structure of an image forming apparatus is explained hereinbelow referring to FIG.  1 . Numeral  1  denotes a main body of the image forming apparatus having functions of a copying machine, a printer, and a facsimile. An image reading device  2  that reads an image of an original document is mounted on an upper part of the main body  1  of the image forming apparatus. A process cartridge  3  is provided inside the main body  1  of the image forming apparatus. A sheet conveying path  4  that guides a transfer sheet S is formed under the process cartridge  3 . 
     The image reading device  2  includes a reading section  5  that reads an image of an original document and an original tray  6  that stacks the original documents to be fed toward the reading section  5 . The original tray  6  is supported so as to be able to rotate around a fulcrum axis  7  from a horizontal position A (current position) to a standing position B that is indicated by a double-dot and a-dash line. In addition, the original tray  6  can be stably positioned at either the horizontal position A or the standing position B by a stopping member (not shown). 
     In the process cartridge  3 , a photoconductor  9  is rotatably provided in a photoconductor case  8 , and a charging brush roller  10  to charge a surface of the photoconductor  9  and a developing section  11  are provided in the vicinity of the photoconductor  9 . A transfer section  12  is provided at the main body  1  of the image forming apparatus so as to face a lower part of the circumferential surface of the photoconductor  9 . The developing section  11  includes a developing case  13 , which is detachably provided in the photoconductor case  8 , and a rotatable agitator  14  that agitates a developer contained in the developing case  13 . The developing section  11  also includes a developing roller  15  that contacts the photoconductor  9 , a supply roller  16  that supplies the developer to the developing roller  15 , and a blade  17  that contacts the developing roller  15 . An optical writing device  18  is provided at an upper part of the process cartridge  3 . The optical writing device  18  scans the photoconductor drum  9 , which has been charged, with laser light that is modulated on the basis of an image signal to form an electrostatic latent image thereon. 
     A tray  19  that supports the original document that has been read by the image reading device  2  is provided at one side of the main body  1  of the image forming apparatus. The tray  19  also supports the transfer sheet S that is stacked in an inclined state. In addition, a sheet feed roller  20  that is rotated, and a pressure plate  21  that is biased in a direction to press the transfer sheet S on the tray  19  toward the sheet feed roller  20  are provided under the tray  19 . In addition, a separation pad  22  and a separation roller  23  that prevents the transfer sheet S from being double fed by elastically contacting the sheet feed roller  20 , respectively, is also provided under the tray  19 . 
     The sheet feeding roller  20 , the separation pad  22 , and the separation roller  23  are located at an entrance side of the sheet conveying path  4 . In addition, a bottom path  24  is connected to the sheet conveying path  4  at a downstream side from the sheet feeding roller  23 . Further, a fixing device  25  that fixes the image transferred on the transfer sheet S is provided at a downstream side of the sheet conveying path  4 . 
     Furthermore, a sheet stacker  26  is provided in the main body  1  of the image forming apparatus so as to be disposed between an upper part of the fixing device  25  and the original tray  6 . The sheet stacker  26  has a receiving plate  27  that is positioned on an extended surface of the original tray  6  and that supports the transfer sheet S when the original tray  6  is in the standing position B. In addition, a sheet discharge exit  28  that discharges the transfer sheet S, which is discharged from the fixing device  25 , in a horizontal direction is formed near a side of the main body  1  of the image forming apparatus, which is opposite to the side of which the tray  19  is provided. Also, a sheet turnover path  29  that leads the transfer sheet S discharged from the fixing device  25  to the sheet stacker  26  so that the transfer sheet S is turned over is formed near the side of the main body  1  of the image forming apparatus. 
     Pairs of sheet discharge rollers  30  are disposed at an upper and lower part of the sheet turn-over path  29 . In addition, a selecting pick  31  is rotatably mounted at a crossing point of the sheet turn-over path  29  and the sheet discharge exit  28  to switch a discharging direction of the transfer sheet S. 
     Next, an image reading operation and a printing operation are explained hereinbelow referring to FIG.  1 . When the image of the original document is read by the reading section  5 , the original document is fed to the reading section  5  while the original tray  6  is kept to be in the horizontal position A. The original document, whose image has been read by the reading section  5  is stacked on the tray  19  in a standing state. The image information thus read can be transmitted to the outside (At this time, the image forming apparatus functions as a facsimile transmission mode). The read image information can also be printed on the transfer sheet S fed from the tray  19  (At this time, the image forming apparatus functions as a copying machine). In addition, the image information transmitted from the outside can also be printed on the transfer sheet S (At this time, the image forming apparatus functions as a facsimile receiving mode). 
     When the printing operation is performed, the surface of the photoconductor  9  is charged by the charging brush roller  10  while the photoconductor  9  is rotated clockwise. The optical writing device  18  irradiates the surface of the photoconductor  9  with laser light according to the read image information of the original document or the image information sent from the outside, resulting in formation of an electrostatic latent image. The electrostatic latent image is developed by the developing section  11 . The developed image is transferred onto the transfer sheet S that is fed from the tray  19  by the sheet feed roller  20 . The transferred image is fixed to the transfer sheet S when the transfer sheet S on which the image is transferred passes through the fixing device  25 . The transfer sheet S is discharged to either the sheet discharge exit  28  or the sheet stacker  26  depending on the direction of the selecting pick  31 . 
     Referring to FIGS. 2 to  5 , a structure of the fixing device  25  is explained hereinbelow. FIG. 2 is a schematic cross-section illustrating how the heat roller  36  is supported at a front side of the fixing device  25 . FIG. 3 is a schematic cross-section illustrating how the heat roller  36  is supported at a rear side of the fixing device  25 . FIG. 4 is a diagrammatical perspective view illustrating how a bearing  37  that supports the heat roller  36  is supported at the rear side of the fixing device  25 . FIG. 5 is a side view illustrating a configuration of the heat roller  36  and the bearing  37 . 
     In FIG. 2, a housing  32  of the fixing device  25  is formed by connecting an upper housing  33  and a lower housing  34  with a screw  35 . The lower housing  34  is affixed to a bottom of the main body  1  of the image forming apparatus such that a lower part of the lower housing  34  is fixed to the main body  1  of the image forming apparatus by a screw (not shown). 
     The bearing  37  by which both ends of the heat roller  36 , whose outer circumferential surface is made of metal are supported is provided in the upper housing  33 . The bearing  37  having a C-shape supports an upper half of the circumferential surface of an end of the heat roller  36  and is made of a synthetic resin such as PPS (polyphenylene sulfide). A plurality of ribs  38  that support the bearing  37  are formed at both sides of the upper housing  33 . As illustrated in FIG. 4, the ribs  38  are located so as to face each other in a conveying direction of the transfer sheet S (indicated by arrows in FIGS.  2  and  3 ). An inserting part  39  in which a projection  44  is inserted between the ribs  38 . In addition, gaps  40  are formed in the ribs  38 . At an edge of the ribs  38  that face each other in a conveying direction of the transfer sheet S, steps  41  are formed adjacent to the inserting parts  39 . 
     The bearing  37  is made of material having good resistance to heat, and elasticity to some degree, such as PPS (polyphenylene sulfide). Carbon is contained in the material to impart electroconductivity. The bearing  37  has a C-shaped part  42  having a radial angle of slightly larger than 180° by which the upper side of the circumferential surface of each end of the heat roller  36  is rotatably supported. In addition, the bearing  37  has a rib  43  formed at an outer circumferential surface of the C-shaped part  42  and the projection  44  that upwardly projects from a middle of the rib  43 . 
     The width of the rib  43  is set so as to fit into a gap  40  of the rib  38  as shown in FIG.  4 . Steps  45  and  46  are formed at both ends of the rib  43 . A cave  47  is formed at each end of the inner surface of the C-shaped part  42  such that the heat roller  36  can be easily inserted into the inner surfaces of the C-shaped part  42 . 
     Two points P illustrated in FIG. 5 indicate both terminals of the contact area of the inner surface of the C-shaped part  42  with the outer surface of the heat roller  36 . A distance between two points P is set to a value slightly smaller than the outer diameter of the heat roller  36  when the heat roller  36  is not inserted into the C-shaped part  42 . That is, when the heat roller  36  is fit into the bearing  37  while elastically expanding the distance between the both ends of the C-shaped part  42 , the heat roller  36  is held so as not to be separated from the bearing  37  because the bearing  37  slightly tightens the heat roller  36  due to the elasticity of the bearing  37 . 
     The projection  44  is disposed so that the angle formed by the step  45  is a (90°) and the angle formed by the step  46  is B that is slightly larger than 90°. 
     As illustrated in FIGS. 3 and 4, a ground plate  48  is provided to flow static electricity stored in the heat roller  36  through the bearing  37  that is electroconductive. The ground plate  48  has an elastic piece  49  capable of being hooked on or detached from the step part  46  of the bearing  37 . Also, the ground plate  48  has a ground piece  50  that is grounded to the ground base of the main body  1  of the image forming apparatus. In addition, as illustrated in FIG. 2, a hooking pawl  51  capable of hooked on or detached from the step part  45  of bearing  37  is united in the upper housing  33 . 
     As illustrated in FIGS. 2 and 3, each end of the pressure roller  52  of the fixing device  25  is supported by a movable bearing  54 , which is pressed upward by a spring  53  supported by the lower housing  34 , such that the pressure roller is movable in a vertical direction in the lower housing  34 . 
     In addition, the fixing device  25  in the first embodiment of the present invention includes a thermostable stopper  55  that allows displacement of the heat roller  36  in a direction toward a thermal fuse  79  (depicted in FIG. 7) (upward in FIG. 2) within a range such that an insulation distance can be maintained to obtain a predetermined insulation strength even when the bearing  37  made of a synthetic resin melts. 
     In FIG. 7, an insulation distance “a” between the thermal fuse  79  and a heat roller  61  is set so as to be equal to or more than 4 mm to satisfy European and North American safety standards. However, because the thermal fuse  79  is covered with an insulation tube  92  in this embodiment, as described later, the bearings  62  and the thermal fuse  79  are configured such that an insulating distance “b” between the top of the screw  35  that is electroconductive and is not covered and the heat roller  61  is equal to or more than 4 mm. Accordingly, even when a current flows from an AC power source to the heater  67  and the thermal fuse  79 , the heat roller  61  is surely insulated from the thermal fuse  79 . 
     The construction of the thermal fuse  79  is now explained in detail. 
     A lead wire  91  of the thermal fuse  79  is provided with a leading part  92  that is lead from each end of the thermal fuse  79  along a centerline thereof. The lead wire  91  of the thermal fuse  79  is provided with a folded part  90  that is lead from the leading part  92  and that has a crank-shape folded in a direction separating from the heat roller  61 . Further, the lead wire  91  of the thermal fuse  79  is provided with a connecting part  95  that is connected to a circuit (not shown) through a screw  82  and flat-plate shaped conductive members  80  and  81 . In addition, the lead wire  91  of the thermal fuse  79  including a folded part  90  is covered with an insulation tube  94 . 
     The stopper  55  is made of a metal plate having elasticity. As illustrated in FIG. 4, the stopper  55  is provided with a nipping part  56  to elastically nip the ribs  38  of the upper housing  33 , and a projection  57  that is inserted into the inserting part  39  located between the ribs  38 . The stopper  55  does not move toward the thermal fuse  79  even when the bearing  37  melts and the heat roller  36  contacts the stopper  55  by a pressing force of the pressure roller  52  because the end of the projection  57  contacts the steps  41  of the ribs  38 . (FIG.  7 ). Accordingly, an insulation space between the heat roller  36  and the thermal fuse  79  is maintained. 
     As can be understood from FIG. 4, when the fixing device  25  is assembled, in the above-described configuration, the stopper  55  can be attached to the upper housing  33  by inserting the projection  57  into the inserting part  39  between the ribs  38  formed in the upper housing  33 . Namely, the nipping part  56  of the stopper  55  elastically nips the ribs  38  and thereby the stopper  55  does not drop off from the upper housing  33 . 
     Then, the projection  44  of the bearing  37  is inserted into the inserting part  39  of the ribs  38 . As illustrated in FIG. 2, the bearing  37  of the front side is hooked on the hooking pawl  51  at the step part  45  of the bearing  37 . Accordingly, the bearing  37  of the front side is held by the upper housing  33 . At this point, rotation of the bearing  37  of the front side is prevented because the projection  44  is inserted into the inserting part  39 . Therefore, even though only one step part  45  is hooked on the hooking pawl  51 , the bearing  37  of the front side does not drop off from the hooking pawl  51 . In addition, although the bearing  37  is made of synthetic resins having elasticity, i.e. a PPS resin, PPS resin is relatively hard among synthetic resins and therefore cannot be bent largely. However, the projection  44  is separated from the hooking pawl  51 , which hooks on the end part (step part  45 ) of the C-shaped part  42 , at an angle of 90° in a circumferential direction. Therefore, the C-shaped part  42  can be hooked on or detached from the hooking pawl  51  without difficulty by bending the end part (step part  45 ) of the C-shaped part  42  inwardly. At this point, since the C-shaped part  42  is bent while the projection  44  is a base point, the C-shaped part  42  can be bent without causing relatively large stress. 
     As illustrated in FIG. 3, the bearing  37  of the rear side is attached to the upper housing  33  by hooking the elastic piece  49  of the ground plate  48 , which is elastically bent, on the step part  46 . 
     Thereby, the bearing  37  is held in the upper housing  33 . In this case, movement of the bearing  37  of the rear side in a circumferential direction is prevented by inserting the projection  44  into the inserting part  39 . As a result, the bearing  37  of the rear side is prevented from dropping from the elastic piece  49  even when only one step part  46  is in a state of being hooked on the elastic piece  49  of the ground plate  48 . 
     Then, the heat roller  37  is attached to the upper housing  33  by fitting the heat roller  36  into the bearing  37  while expanding the distance between the both ends of the C-shaped part  42  of the bearing  37  of the front and rear side by elastically bending the both ends thereof. Of course, it is possible that at first the bearing  37  and the heat roller  36  is fit and then the combination is attached to the upper housing  33 . When the heat roller  36  is fit into the bearing  37 , the relatively long arched part of the C-shaped part  42  is bent in the circumferential direction. Therefore, even when the distance between the two points P, illustrated in FIG. 5, is expanded to such an extent that dimension the distance slightly exceeds the outer diameter of the heat roller  36 , a relatively large stress does not occur and the bearing  37  is not broken. 
     Thereafter, the upper housing  33  to which the heat roller  36  is attached and the lower housing  34  to which the pressure roller  52  is attached are connected by a screw  35 . Thus, the fixing device  25  is assembled. In the thus assembled fixing device  25 , the heat roller  36  is pressed upwardly by the pressure roller  52 . The bearing  37  is secured by being brought into contact with the lower part of the ribs  38  having a semicircular shape. In this state, an inner circumferential surface of the bearing  37  and an outer circumferential surface of the heat roller  36  are kept to be in close contact with each other. The fixing device  25  can be dismantled by operations opposite to the above-described assembling operations. 
     As illustrated in FIG. 3, when the bearing  37  is attached to the rear side of the upper housing  33 , the bearing  37  can be held by bending the elastic piece  49  of the ground plate  48 . Also, when the bearing  37  is attached to the front side of the upper housing  33 , the hooking pawl  51  of the upper housing  33  is hooked on the end part (step part  45 ) of the C-shaped part  42 . The distance between the both ends of the C-shaped part  42  can be elastically expanded to support the heat roller  36 , as illustrated in FIG.  2 . Accordingly, in both of these two cases illustrated in FIGS. 2 and 3, there is no need to use a relatively long elastic piece having a hooking part to hook the bearing  37  on the upper housing  33 , which is used in background bearings. 
     As a result, the amount of the material used in the bearing  37  is decreased and thereby the cost of the materials used for the bearing  37  can be saved. In addition, the two bearings  37  of the rear side and the front side are commonly used in this embodiment. In addition, the radial angle of the contact area of the bearing  37  with the heat roller  36  is set so as to be slightly larger than 180° in this embodiment. However, the angle can be set so as to be less than 180° as illustrated in FIGS. 12 and 13. 
     The second embodiment of the present invention will be explained referring to FIGS. 6 to  9 . The parts substantially the same as those illustrated in FIGS. 1 to  5  are denoted by the same numerals or codes and the description thereof is omitted. A basic construction of the image forming apparatus  1  is the same as that of the first embodiment of the present invention except that the fixing device  25   a  is changed. 
     The construction of the fixing device  25   a  is described in detail referring to FIG.  7 . The fixing device  25   a  has a housing  58 , which is fixed on the main body  1  of the image forming apparatus. The housing  58  is connected to upper housing  59  and a lower housing  60  by a screw (not shown). In the lower housing  60 , a cylindrical heat roller  61  having openings at both ends is rotatably supported by the bearing  62 . Also, in the upper housing  59 , a pressure roller  63  is rotatably supported by movable bearings  64  at both ends of the pressure roller  63 . The movable bearings  64  are downwardly biased by springs  65 . An outer surface of the heat roller  61  and an outer surface of the pressure roller  63  are in pressure contact with each other by the biasing force of the springs  65 . 
     A heater  66  is disposed in the heat roller  61 . The heater  66  is provided with a quartz glass tube  67 , both ends of which are sealed and in which a heat wire  68  is provided. The heater  66  is also provided with metal foil  69  that is sealed in sealing parts at both ends of the quartz glass tube  67  and connected to the heat wire  68 . Also, the heater  66  is provided with lead wires  70   a  and  70   b , each one end of which is connected to the metal foil  69  and the other end which extends from the respective ends of the quartz glass tube. 
     At the lead wire  70   a  of the heater  66 , a flat-plate shaped heater terminal  71  is welded, and the heater terminal  71  is affixed to the lower housing  60  by a screw  72 . At the other lead wire  70   b , a heater terminal  73  that is bent into an L shape is welded. The heater terminal  73  is affixed to a sidewall part  60   a  of the lower housing  60  by a flanged nut  74  and a screw  75 . A flexible space  76  is formed by chamfering a part in the sidewall part  60   a , which is surrounded by two sides of the L-shaped bent part of the heater terminal  73 . The flexible space  76  allows the heater terminal  73  to bend in a direction indicated by an arrow A (FIG. 8) along a longitudinal direction of the heater  66 . At this point, the heater terminal  73  bends around a part of the heater terminal  73  such that the part of the heater terminal  73  fixed with the sidewall part  60   a  by the screw  75 , is a fulcrum. 
     As shown in FIG. 9, a slit  77  is formed at the L-shaped bent part of the heater terminal  73 . By forming the slit  77 , flexible parts  78  that cause the heater terminal  73  to be easily bent in the direction indicated by the arrow A are formed at both sides of the slit  77 . 
     Referring again to FIG. 7, plate-shaped conductive members  80  and  81 , to which both ends of the thermal fuse  79  are respectively connected, are attached to the lower housing  60 . The conductive member  80  is affixed to the lower housing  60  by the screw  72  and is connected to the heater terminal  71  to supply electricity to the heater  66 . The other conductive member  81  is affixed to the lower housing  60  by a screw  82 . A wire harness  83  to supply electricity is connected to the end of the conductive member  81  affixed by the screw  82  and the affixed part of the heater terminal  73 . 
     In the fixing device having such a construction, the heater  66  generates heat during an image forming operation and the housing  58  (upper housing  59  and lower housing  60 ) is expanded by heat. The housing  58  is expanded in an outer circumferential direction thereof by heat. The heater terminals  71  and  73 , which are affixed to the lower housing  60 , are moved in such a direction as separating from the heater  66  by heat expansion of the lower housing  60 . With the movement of the heater terminals  71  and  73 , a pulling force affects to the lead wires  70   a  and  70   b , which are connected to the heater terminals  71  and  73 , respectively. 
     Referring to FIG. 8, when the heater terminal  73  is moved in such a direction as separating from the heater  66  by heat expansion of the lower housing  60 , the heater terminal  73  is bent in a direction indicated by the arrow A. This is because a flexible space  76  is formed on the sidewall part  60   a  where the heater terminal  73  is attached. Therefore, the amount of movement of the heater terminal  73  of one end of the heater terminal  73  affixed to the lead wire  70   b  becomes relatively small compared with that of the other end of the heater terminal  73  affixed to the sidewall part  60   a . Thereby, tension that affects the lead wire  70   b  becomes small. 
     Consequently, the following problems: 
     1. The heater terminal  73  is disconnected from the lead wire  70   b  at the connection part thereof; 
     2. The metal foil  69  provided in the quartz glass tube  67  is disconnected from the lead wire  70   b  at the connection part thereof; and 
     3. The quartz glass tube  67  is cracked thereby damaging the heater  66 , can be prevented. 
     In addition, the heater terminal  73  is configured to bend more easily in the direction indicated by the arrow A because the flexible part  78  is formed in the heater terminal  73 . 
     The flat-plate shaped heater terminal  71  connected to the lead wire  70   a  cannot bend, which is different from the heater terminal  73 . However, when the heater terminal  71  moves in such a direction separating from the heater  66  due to the heat expansion of the lower housing  60 , the entire heater  66  moves to a heater terminal  71  side because the heater terminal  73  is bent. 
     Consequently, the following problems: 
     1. The heater terminal  71  is disconnected from the lead wire  70   a  at the connection part thereof; 
     2. The metal foil  69  provided in the quartz glass tube  67  is disconnected from the lead wire  70   a  at the connection part thereof; and 
     3. The quartz glass tube  67  is cracked thereby damaging the heater  66 , can be prevented. 
     In the second embodiment, a flexible part  78  is formed at both sides of the slit  77  that is formed on the heater terminal  73 . However, notches can also be formed at both sides of the heater terminal  73  in a width direction thereof to form a flexible part having a relatively narrow width between the notches. 
     Next, a third embodiment of the present invention is described referring to FIG.  10 . The parts substantially the same as those illustrated in FIGS. 6 to  9  are denoted by the same numerals or codes, and the description thereof is omitted. 
     In this embodiment, when the heater terminal  73  is affixed to the sidewall part  60   a , the side of the heater terminal  73  folded into an L-shape is separated from an upper end surface of the sidewall part  60   a . Further, a flexible space  84  is formed between the upper end surface of the sidewall part  60   a  and the heater terminal  73 . The flexible space  84  allows the heater terminal  73  to bend in a direction indicated by an arrow A along a longitudinal direction of the heater  66 . At this point, the heater terminal  73  bends around a part of the heater terminal  73  such that the part of the heater terminal  73  fixed with the sidewall part  60   a  by the screw  75 , is a fulcrum. 
     Also, in this embodiment, when the heater terminal  73  moves to the direction to separate from the heater  66  by the heat expansion of the lower housing  60 , the heater terminal  73  is bent in the direction indicated by the arrow A. This is because a flexible space  84  is formed at the upper part of the sidewall part  60   a  where the heater terminal  73  is affixed. Accordingly, the amount of movement in such a direction as separating from the heater  66  of one end of the heater terminal  73  affixed to the lead wire  70   b  becomes relatively small compared with that of the other end of the heater terminal  73  affixed to the sidewall part  60   a . Thereby, the tension that affects the lead wire  70   b  becomes small. 
     Consequently, the following problems: 
     1. The heater terminal  73  is disconnected from the lead wire  70   b  at the connection part thereof; 
     2. The metal foil  69  provided in the quartz glass tube  67  is disconnected from the lead wire  70   b  at the connection part thereof; and 
     3. The quartz glass tube  67  is cracked thereby damaging the heater  66 , can be prevented. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. 
     This document claims priority and contains subject matter related to Japanese Patent Applications Nos. 10-310414, 10-310415, 11-176255, filed on Oct. 30, 1998, Oct. 30, 1998, and Jun. 23, 1999, respectively, and the entire contents thereof are herein incorporated by reference.