Patent Publication Number: US-2011052259-A1

Title: Pressure adjusting mechanism for a fixing device and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 11/352,214, filed Feb. 13, 2006, which claims priority under 35 USC 119 from Japanese Patent Application No. 2005-211468, the disclosures of which are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fixing device that fixes a toner image on a recording medium between a heating member and a pressuring member, and to an image forming apparatus, such as a copier or a laser printer, disposed with this fixing device. 
     2. Description of the Related Art 
     In image forming apparatus such as copiers and printers, an image carrier such as a photoconductor is charged and irradiated with laser light such that an electrostatic latent image is formed on the surface of the image carrier. The electrostatic latent image is made visible by a developing device, and a toner image comprising powder toner is formed. Then, the toner image is transferred to a recording medium such as paper, and thereafter the toner image on the recording medium is fixed onto the recording medium by a fixing device, whereby an image is formed. 
     The fixing device includes a heating roller, which comprises a rotor disposed with an internal heater, and a pressuring roller, which presses the recording medium against the heating roller. The recording medium on which the toner image has been formed is passed between the heating roller and the pressuring roller, and the toner image is fused and fixed onto the recording medium as a result of being heated and pressured. Sometimes, in consideration of maintenance and the like, the fixing device is configured as a unit that is attachable to and detachable from the image forming apparatus. 
     In such a fixing device, a recording medium such as an envelope sometimes becomes wrinkled when a heavy load is applied thereto when the toner image on the recording medium is fixed at the portion where the heating roller and the pressuring roller press the recording medium. For this reason, fixing devices have conventionally been proposed where the load of the pressuring roller can be varied manually. However, when the load of the pressuring roller is manually varied by the user, the recording medium cannot be completely prevented from becoming wrinkled, and the recording medium can end up becoming wrinkled due to mistakes or the like. 
     In Japanese Patent Application Publications (JP-A) Nos. 8-69202 and 2001-249569, fixing devices have been proposed which are configured to automatically adjust the load of the pressuring roller pressing against the heating roller by causing a swinging arm that presses a support shaft of the pressuring roller to swing. However, these fixing devices have many parts and expensive configurations because they include various kinds of parts such as a one-way clutch, a motor and a cam floor. 
     In Japanese Patent Application Publication No. 2004-109521, a fixing device has also proposed where the number of parts is reduced by disposing a reference position changing cam. With this configuration, the cost can be reduced, but the ease with which the unit can be loaded into and unloaded from the image forming apparatus and the stability in the precision of the unit and cam position are not taken into consideration. 
     That is, among conventional fixing devices, there have been none that can satisfy the stability in the precision of the unit and cam position while maintaining the ease with which the unit can be loaded into and unloaded from the image forming apparatus body. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of these circumstances and provides a fixing device and an image forming apparatus that can suppress the occurrence of wrinkles in the recording medium by automatically varying the load of the pressuring roller on the heating roller, and which have excellent stability in precision as a unit. 
     A first aspect of the invention provides a fixing device for an image forming apparatus, the device including: a heating member disposed with an internal heating source and a pressuring member that presses against the heating member; brackets that retain the heating member; pressuring levers that retain the pressuring member and are movably supported in the brackets; urging means that push the pressuring levers in a direction where the pressuring member presses against the heating member; and tension adjusting means that are rotatably supported in the brackets, contact the pressuring levers, and adjust the pressuring force of the pressuring member, wherein the fixing device is attachable to and detachable from the image forming apparatus body. 
     According to the first aspect, the pressuring levers support the pressuring member and are movably supported in the brackets retaining the heating member. The pressuring levers are pushed by the urging means, and the pressuring member presses against the heating member. When the tension adjusting means rotatably supported in the brackets are rotated, the tension adjusting means contact the pressuring levers and the pressuring force of the pressuring member is adjusted. Thus, the load of the pressuring member on the heating member can be appropriately adjusted, and the occurrence of wrinkles in the recording medium can be suppressed. 
     Further, because the drive source that causes the tension adjusting means to rotate is disposed in the image forming apparatus side to which the fixing device is attached, the power source can be configured to be in the “enclosed” state among the brackets, the pressuring levers, the urging means and the tension adjusting means, which is advantageous in terms of preventing deformation and ensuring precision stability of the constituent parts. The assembly is also easy in this structure. 
     In a second aspect of the invention, the fixing device includes a first mode where the tension adjusting means do not contact the pressuring levers and a second mode where the tension adjusting means contact the pressuring levers and the pressuring levers move to a predetermined pressuring position. 
     According to the second aspect, it is possible to switch between the first mode and the second mode depending on the type of recording medium. For example, in the first mode (for plain paper or the like), the tension adjusting means do not contact the pressuring levers and the pressuring member has a fixed load. In the second mode (for envelopes or the like), the pressuring levers can be fixedly displaced (interaxially fixed) to move to a predetermined pressuring position. Thus, the stroke of the pressuring levers can be reduced, which is advantageous in terms of the design of the tension adjusting means. 
     In a third aspect of the invention, the pressuring levers and the tension adjusting means are disposed at both end portions of the pressuring member based on the first or second aspect, the fixing device further includes a shaft that transmits the rotation of one of the tension adjusting means to the other of the tension adjusting means and a reading plate that is disposed on an end portion of the shaft and is for reading the rotational position of the tension adjusting means, and the reading plate is disposed near the other of the tension adjusting means. 
     In the third aspect of the invention, the shaft becomes kinked because there is rotational resistance in the tension adjusting means (the other tension adjusting means) opposite from the tension adjusting means (the one tension adjusting means) disposed near the drive. The reading precision can be raised by reading, with the reading plate, the rotational position of the tension adjusting means at the kinked side (the other tension adjusting means). 
     In a fourth aspect of the invention, the reading plate of the third aspect is attached to the other of the tension adjusting means. 
     According to the fourth aspect, because the reading plate is attached to the tension adjusting means opposite from the tension adjusting means near the drive, the reading precision of the rotational direction of the tension adjusting means can be raised more than in a configuration where the reading plate is fitted together with the shaft or the like. 
     In a fifth aspect of the invention, a gear that transmits driving force to the shaft based on the fourth or fifth aspect is disposed, and a shaft bearing of the gear is disposed in the image forming apparatus body. 
     According to the fifth aspect, the gear can be firmly retained by the shaft bearing because the shaft bearing of the gear that transmits driving force to the shaft is disposed in the image forming apparatus. For this reason, the shaft and the fixing device themselves do not end up moving due to the driving of the gear, even if the torque of the tension adjusting means is large. 
     In a sixth aspect of the invention, the gear based on the fifth aspect is fixed to a D-cut surface of the shaft, and an end portion of the shaft penetrating the gear is supported in the shaft bearing. 
     According to the sixth aspect, the gear is directly fixed to the D-cut surface of the shaft, and the end portion of the shaft is guided by the shaft bearing. Thus, the positional precision can be improved. 
     In a seventh aspect of the invention, the gear based on the fifth or sixth aspect includes teeth that are high teeth. 
     According to the seventh aspect, the teeth of the gear are high teeth. Thus, when the fixing device unit is to be attached to the image forming apparatus body, it is difficult for the tooth tips of the gear and the gear of the image forming apparatus body to knock against each other, and the fixing device can be smoothly attached to the image forming apparatus body. 
     When the gear has usual teeth, the tooth tips of the gear and the gear of the image forming apparatus body knock against each other, and the fixing device cannot be smoothly attached. This problem can be solved by tapering the tooth tips of the gear, which can be accomplished in two ways: with high teeth or with a displacement gear. In the case of a displacement gear, the drive tangent becomes oblique, the fixing device easily moves, and there is skipping. If the teeth of the gear are high teeth, the drive tangent becomes perpendicular and there is no skipping. 
     In an eighth aspect of the invention, a snap-fit portion disposed in the gear based on any of the fifth to seventh aspects is snap-fitted into a groove formed in the shaft. 
     According to the eighth aspect, the gear is fixed by being snap-fitted into the groove in the shaft. Thus, it is not necessary to screw the gear onto the shaft, and the assembly becomes easy. 
     In a ninth aspect of the invention, a sensor that detects a slit in the reading plate based on any of the third to eighth aspects is disposed in the image forming apparatus body. 
     According to the ninth aspect, because it becomes difficult for the fixing device to move when the fixing device has been attached to the image forming apparatus body, precise detection becomes possible even when the sensor is disposed in the image forming apparatus body. The cost can also be reduced by disposing the sensor is disposed in the image forming apparatus body. 
     In a tenth aspect of the invention, the shaft bearing based on the fifth or sixth aspect doubles as a member that fixes and/or positions the fixing device with respect to the image forming apparatus body. 
     According to the tenth aspect, the shaft bearing fixes and/or positions the unit. And because the shaft bearing has a configuration where the positional precision of the reading plate is high, the sensor of the reading plate can be disposed in the image forming apparatus body. For this reason, a reduction in cost is possible. 
     In an eleventh aspect of the invention, the rotational direction of the tension adjusting means based on any of the first to tenth aspects is a forward direction where the pressuring levers and the tension adjusting means are not associated. 
     When the rotational direction of the tension adjusting means is a direction associated with the movement of the pressuring levers, the movement of the pressuring levers becomes jerky no longer smooth. But according to the eleventh aspect of the invention, the movement of the pressuring levers can be made smooth because the rotational direction of the tension adjusting means is the forward direction that is not associated with the pressuring levers. 
     In a twelfth aspect of the invention, the gear that transmits driving force to the shaft is disposed opposite from a drive mechanism that drives the heating member or the pressuring member based on any of the fifth to eleventh aspects. 
     The drive mechanism that drives the heating member or the pressuring member and the drive mechanism of the tension adjusting means require space. However, according to the twelfth aspect of the invention, the drive mechanisms are disposed at opposite sides, which is advantageous in terms of space. 
     In a thirteenth aspect of the invention, the fixing device based on any of the first to twelfth aspects further comprises a motor that drives the tension adjusting means, is a DC motor, is slowed down by a worm gear and transmits driving force. 
     In this configuration, the required rotation of the tension adjusting means is an extremely low rotation and a high load. According to the thirteenth aspect of the invention, the combination of the DC motor and the worm gear is slowed down and transmits driving force, which is advantageous in terms of space, cost, and output. 
     In a fourteenth aspect of the invention, the motor based on the thirteenth aspect is a brush motor. 
     The time during which the tension adjusting means are caused to rotate is extremely limited. Thus, the inexpensive brush motor of the fourteenth aspect of the invention can be used, and the cost can be reduced. 
     In a fifteenth aspect of the invention the urging means based on the first or second aspect are configured by guide members disposed such that they extend from the brackets, open holes that are formed in the pressuring levers and through which the guide members are inserted, and compression springs through which the guide members are inserted, with the compression springs including one end that contacts edge portions of the open holes and another end that contacts flanges formed on end portions of the guide members. 
     According to the fifteenth aspect of the invention, the pressuring levers are pushed in the direction of the guide members of the brackets by disposing in the urging means the compression springs that include one end that contacts edge portions of the open holes and another end that contacts flanges formed on end portions of the guide members. The pressuring levers can also be easily moved to predetermined movement positions as a result of the compression springs being compressed by the rotation of the tension adjusting means. 
     In a sixteenth aspect of the invention, the urging means based on the first or second aspect are configured by first spring receivers formed in the brackets, second spring receivers that are formed in the pressuring levers and are positioned nearer to the tension adjusting means than the first spring receivers, and spring receiver-use compression springs that are disposed between the first spring receivers and the second spring receivers. 
     According to the sixteenth aspect, the spring receiver-use compression springs are disposed between the first spring receivers and the second spring receivers, whereby the pressuring levers can be pushed toward the brackets and the configuration can be simplified. 
     In a seventeenth aspect of the invention, bearings are disposed on contact surfaces of the tension adjusting means based on any of the first to sixteenth aspects where the tension adjusting means contact the pressuring levers. 
     According to the seventeenth aspect, the sliding resistance of the tension adjusting means becomes smaller as a result of disposing the bearings on the contact surfaces. 
     An eighteenth aspect of the invention provides a fixing device for an image forming apparatus, the device including: a heating member disposed with an internal heating source and a pressuring member that presses against the heating member; brackets that retain the heating member; pressuring levers that retain the pressuring member and are movably supported in the brackets; tension springs that include one end coupled to the pressuring levers and pull the pressuring levers in a direction where the pressuring member presses against the heating member; and cams that are rotatably supported in the brackets, support the other ends of the tension springs, and change the support positions of the tension springs, wherein the fixing device is attachable to and detachable from the image forming apparatus body. 
     In the eighteenth aspect of the invention, the cams include one end coupled to the pressuring levers and pull the pressuring levers in the direction where the pressuring member presses against the heating member. The tension adjustment means are rotatably supported in the brackets, support the other ends of the cams, and change the support positions of the cams. Thus, the pressuring force of the pressuring member can be adjusted, and the configuration can be further simplified. 
     In a nineteenth aspect, the tension adjusting means based on the eighteenth aspect are tension spring-use cams to which leading ends of the cams are attached. 
     According to the nineteenth aspect, the tension of the cams can be adjusted by causing the tension spring-use cams to rotate. 
     In a twentieth aspect of the invention, grooves corresponding to hooks formed on the other ends of the cams are disposed in the tension spring-use cams based on the nineteenth aspect. 
     According to the twentieth aspect, the hooks of the cams can be firmly retained in the cams by hooking the hooks into the grooves disposed in the tension spring-use cams. 
     In a twenty-first aspect of the invention, bearings are disposed on outer peripheries of the tension spring-use cams. 
     According to the twenty-first aspect, the sliding resistance of the cams, the springs and the hooks can be becomes smaller as a result of disposing the bearings on the outer peripheries of the tension spring-use cams. 
     In a twenty-second aspect of the invention, an image forming apparatus includes the fixing device based on any of the first to twenty-first aspects. 
     According to the twenty-second aspect, the image forming apparatus includes the fixing device based on any of the first to twenty-first aspects. Thus, the pressuring force of the pressuring member on the heating member can be appropriately adjusted, and the occurrence of wrinkles in the recording medium can be suppressed. The movement of the fixing device attached to the image forming apparatus can be suppressed, and the precision stability can be improved. 
     Further, another aspect of the present invention may provide a fixing device for an image forming apparatus, the device including: a heating member disposed with an internal heating source and a pressuring member that presses against the heating member; brackets that retain the heating member; pressuring levers that retain the pressuring member and are movably supported in the brackets; tension springs that push the pressuring levers in a direction where the pressuring member presses against the heating member; and tension adjusting means that are rotatably supported in the brackets, contact the pressuring levers, and adjust the pressuring force of the pressuring member, wherein the fixing device is attachable to and detachable from the image forming apparatus body. 
     As described above, according to the present invention, the pressuring force of the pressuring member pressing against the heating member can be adjusted, and the occurrence of wrinkles in the recording medium can be suppressed. The precision stability as a fixing device attachable to and detachable from the image forming apparatus body can also be improved. The occurrence of curls and nip tracks in the recording medium can also be suppressed by adjusting the pressuring force of the pressuring member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will be described below with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic configural diagram showing an image forming apparatus disposed with a fixing device pertaining to a first embodiment of the invention; 
         FIG. 2  is a perspective view showing the fixing device pertaining to the first embodiment of the invention; 
         FIG. 3  is a perspective view of the fixing device shown in  FIG. 2  seen from the direction of cams; 
         FIG. 4  is a perspective view showing an attachment structure of pressuring levers that retain a pressuring roller, brackets that retain a heating roller, and the cams of the fixing device shown in  FIG. 2 ; 
         FIG. 5  is an exploded perspective view showing an attachment structure of a cam and a reading plate of the fixing device shown in  FIG. 2 ; 
         FIG. 6  is an exploded perspective view showing an attachment structure of gears and a shaft of the fixing device shown in  FIG. 2 ; 
         FIG. 7  is a side view showing the attachment structure of the pressuring levers that retain the pressuring roller, the brackets that retain the heating roller, and the cams of the fixing device shown in  FIG. 2 ; 
         FIG. 8A  is a plan view showing the attachment structure of the pressuring levers that retain the pressuring roller, the brackets that retain the heating roller, and the cams of the fixing device shown in  FIG. 2 ; 
         FIG. 8B  is a partially enlarged view of a gear that causes the cams to rotate; 
         FIG. 9  is a side view showing a state where the cams of the fixing device shown in  FIG. 2  have rotated and where the pressuring levers have pivoted; 
         FIG. 10  is a configural view showing an example of a drive mechanism of the cams of the fixing device shown in  FIG. 2 ; 
         FIG. 11  is a partial perspective view showing the relevant portions of a fixing device pertaining to a second embodiment of the invention; 
         FIG. 12  is a partial perspective view showing the relevant portions of a fixing device pertaining to a third embodiment of the invention; 
         FIGS. 13A and 13B  are a partial perspective view and an exploded perspective view showing the relevant portions of the fixing device pertaining to the third embodiment of the invention; and 
         FIG. 14  is a side view showing an attachment structure of pressuring levers, brackets, and compression springs of a fixing device pertaining to a fourth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of a fixing device and an image forming apparatus pertaining to the invention will be described below on the basis of the drawings. 
     First Embodiment 
       FIG. 1  shows an image forming apparatus  11  to which a fixing device  10  pertaining to a first embodiment of the invention has been applied. 
     The image forming apparatus  11  includes an image forming apparatus body  200 . The fixing device  10  comprises a unit that is attachable to and detachable from the image forming apparatus body  200 . An open/close cover  202  is disposed on the image forming apparatus body  200 , and the fixing device  10  is loaded into the image forming apparatus body  200  as a result of the open/close cover  202  being opened. 
     When the fixing device  10  is loaded into the image forming apparatus body  200 , a connector  252  of the fixing device  10  becomes connected to a connector  250  of the image forming apparatus body  200 , such that power can be supplied to the fixing device  10  and the completion of the loading of the fixing device  10  is detected. After the fixing device  10  has been loaded into the image forming apparatus body  200 , the image forming apparatus  11  becomes operable as a result of the open/close door  202  being closed. 
     A process cartridge  204 , in which an image forming section is integrally unitized, is disposed in the image forming apparatus body  200 . A photoconductor drum  216  that rotates in one direction is disposed inside the process cartridge  204 . Disposed around the periphery of the photoconductor drum  216  are a charge roller  218  that charges the photoconductor drum  216 , a developing roller  220  that develops an electrostatic latent image formed on the photoconductor drum  216 , and a transfer roller  222  that transfers onto paper the developed toner image on the photoconductor drum  216 . A cleaning member  224  that cleans the surface of the photoconductor drum  216  after the toner image has been transferred from the photoconductor drum  216  to the paper is disposed downstream of the transfer roller  222  in the rotational direction of the photoconductor drum  216 . 
     An exposure device  214  that irradiates the photoconductor drum  216  with image light is disposed between the charge roller  218  and the developing roller  220  in the image forming apparatus body  200 . 
     Paper supply cassettes  206  and  208  that accommodate paper are disposed in the lower portion of the image forming apparatus body  200  such that they can be pulled out. Paper supply rollers  205  and  207  that remove and convey the paper one sheet at a time from the paper supply cassettes  206  and  208  are disposed at positions where they remove the paper in the paper supply cassettes  206  and  208 . Conveyance rollers  210 ,  211 ,  212  and  213  that convey the paper supplied from the paper supply rollers  205  and  207  to the position where the photoconductor drum  216  and the transfer roller  222  face each other are also disposed. 
     The fixing device  10  is loaded into the image forming apparatus body  200  downstream of the transfer roller  222  in the conveyance direction of the paper. A paper discharge tray  230 , into which the paper is discharged after the toner image has been fixed thereto, is disposed downstream of the fixing device  10 . The fixing device  10  causes the toner image on the paper to be fixed to the paper between a heating roller  14  and a pressuring roller  16 . The fixing device  10  will be described in greater detail later. 
     In the image forming apparatus  11 , the photoconductor drum  216  is charged by the charge roller  216 , and an electrostatic latent image is formed on the surface of the photoconductor drum  216  as a result of the surface of the photoconductor drum  216  being irradiated with image light from the exposure device  214 . The electrostatic latent image is developed by the developing roller  220 , and a toner image is formed on the photoconductor drum  216 . 
     The paper is supplied by the paper supply roller  205  from the paper supply cassette  206  and conveyed by the conveyance rollers  210 ,  211 ,  212  and  213  to the position where the photoconductor drum  216  and the transfer roller  222  face each other. Then, the toner image on the photoconductor drum  216  is transferred onto the paper by the transfer roller  222 , and the toner image is heated and pressured by the heating roller  14  and the pressuring roller  16  of the fixing device  10 , whereby the toner image is fused and an image is fixed onto the paper. Thereafter, the paper on which the image has been formed is discharged into the paper discharge tray  230 . 
     Next, the fixing device  10  will be described. 
     As shown in  FIG. 2  and  FIG. 4 , the fixing device  10  includes a housing  12  disposed with the heating roller  14 , which is heated to a predetermined fixing temperature, and the pressuring roller  16 , which presses the paper against the heating roller  14 . The heating roller  14  comprises a hollow cylindrical member (e.g., a steel tube) including a surface disposed with a low coefficient-of-friction release layer (not shown), and a heater is disposed inside the cylindrical member. The pressuring roller  16  comprises a steel core disposed with a heat-resistant elastomer layer (e.g., silicone sponge or silicone rubber). The elastomer layer of the pressuring roller  16  is pressed against the heating roller  14 . 
     As shown in  FIG. 4 , the heating roller  14  is rotatably supported at both end portions on support members  20 A that are disposed on brackets  20 . The pressuring roller  16  is rotatably supported at both end portions on support members  22 A that are disposed on pressuring levers  22 . The pressuring levers  22  are supported such that they are pivotable about lower pivot points  24  with respect to the brackets  20 . 
     As shown in  FIG. 7 , guide pieces  21 A are disposed above the left and right brackets  20  such that the guide pieces  21 A extend substantially parallel to the axis of the heating roller  14 , and studs (as a guide portion)  26  are fastened to the guide pieces  21 A. Moving pieces  23 A are disposed above the pressuring levers  22  at both end portions such that the moving pieces  23 A extend substantially parallel to the guide pieces  21 A, and the studs  26  are inserted through open holes  23 B formed in the moving pieces  23 A. 
     Compression springs  28  are disposed on the peripheries of the studs  26 . One end of each of the compression springs  28  contacts flanges  26 A of the studs  26 , and the other end of each of the compression springs  28  contacts the moving pieces  23 A. The compression springs  28  push the moving pieces  23 A toward the guide pieces  21 A of the brackets  20 , whereby the pressuring roller  16  retained in the pressuring levers  22  is pressed by a predetermined load (pressuring force) against the heating roller  14 . 
     As shown in  FIG. 4 , contact pieces  23 C are disposed substantially parallel to the moving pieces  23 A on extension portions  23  of the pressuring levers  22 . Cams  30  are rotatably supported on retaining members  20 B of the brackets  20  at positions where the cams  30  face the contact pieces  23 C, and the cams  30  at both end portions are coupled together by a shaft  32 . A gear  34  that transmits driving force to the cams  30  is disposed on an end portion of the shaft  32  connected with one of the cams  30 . As shown in  FIG. 7  and  FIG. 8A , the cams  30  are substantially circular and disposed such that they are offset from the center portion of the shaft  32 . 
     The fixing device  10  includes a first mode that conveys plain paper or the like and a second mode that conveys envelopes or the like, and is configured such that one of the two modes is selected by a control unit (not shown) when the paper is detected by an unillustrated sensor. 
     In the first mode, the cams  30  do not contact the contact pieces  23 C, but in the second mode, the cams  30  rotate to predetermined positions and contact the contact pieces  23 C. When the cams  30  contact the contact pieces  23 C, the moving pieces  23 A compress the compression springs  28 , move to a predetermined pressuring position, and cause the pressuring levers  22  to pivot, whereby the pressure of the pressuring levers  22  is adjusted (see  FIG. 9 ). Thus, in the second mode (envelopes or the like), the load (pressuring force) of the pressuring roller  16  on the heating roller  14  becomes smaller in comparison to the load in the first mode (plain paper or the like). 
     As shown in  FIGS. 2 to 4 , a gear  256  that meshes with the gear  34  is disposed in the image forming apparatus body  200 , and a drive unit  258  including a motor that transmits driving force to the gear  256  is attached to an unillustrated frame. Thus, the drive unit  258  transmits driving force to the gear  256  to cause the gear  34  to rotate, whereby one of the cams  30  (the cam  30  at the far side in  FIG. 3 ) coupled to the shaft  32  rotates, and the other cam  30  (the cam  30  at the near side in  FIG. 3 ) rotates via the shaft  32 . 
     A drive unit (not shown) for causing the heating roller  14  to rotate is disposed on the support member  20 A of the bracket  20  at the side opposite (the near side in  FIG. 4 ) from the drive unit  258  with respect to the shaft  32 . Driving force is transmitted from the unillustrated drive unit, whereby the heating roller  14  rotates and the pressuring roller  16  rotates following the rotation of the heating roller  14 . Because the drive unit  258  that causes the cams  30  to rotate and the unillustrated drive unit that causes the heating roller  14  to rotate are disposed at opposite sides, they are efficiently disposed inside the image forming apparatus  11 , which is advantageous in terms of space. 
     As shown in  FIG. 4 , a reading plate  40  including a slit (not shown) for reading the rotational position of the cams  30  is disposed on the shaft  32  opposite from the drive unit  258 . A sensor  42  that detects the slit is disposed at a position where the sensor sandwiches and faces the reading plate  40 . The sensor plate  42  is attached to the unillustrated frame of the image forming apparatus body  200  and remains in the image forming apparatus body  200  when the fixing device  10  is removed, which makes possible a reduction in cost. 
     As shown in  FIG. 5 , an L-shaped protruding portion  41 A is disposed on a shaft portion  41  of the reading plate  40  such that the protruding portion  41 A protrudes outward from the peripheral surface of the shaft portion  41 . The protruding portion  41 A is fitted together with a concave fitting portion  31 A formed in the direction orthogonal to the rotational direction inside a shaft portion of the cam  30 . 
     An elastically deformable snap-fit portion  41 B is disposed such that it protrudes inside the shaft portion  41  of the reading plate  40 . The snap-fit portion  41 B snap-fits into a groove portion  32 A formed in the peripheral surface of the shaft  32 , whereby the reading plate  40  is fixed to the shaft  32 . 
     Thus, the precision of the rotational direction of the cam  30  and the reading plate  40  can be raised in comparison to when the cam  30  and the reading plate  40  are directly attached to the shaft  32 . Also, as shown in  FIG. 4 , although there is concern that the shaft  32  will become kinked because there is rotational resistance in the cam  30  (at the near side in  FIG. 4 ) opposite from the cam  30  disposed near the drive unit  258  of the gear  34 , the reading precision can be raised by reading, with the reading plate  40 , the rotational position of the cam  30  at the kinked side. 
     As shown in  FIG. 6 , a D-cut surface  33 A is formed on a shaft end portion  33  that the shaft  32  penetrates, and the shaft end portion  33  is inserted through a shaft portion  35 , on which a D-cut surface is formed, of the gear  34 . An elastically deformable snap-fit portion  35 A is disposed inside the shaft portion  35 . The snap-fit portion  35 A is snap-fitted into a groove portion  33 B in the shaft end portion  33 , whereby the gear  34  is fixed. Because the gear  34  is fixed to the shaft end portion  33  by the snap-fit portion  35 A, it is not necessary to screw the gear  34  onto the shaft end portion  33 , and the assembly becomes easy. 
     A circular end portion  33 C is formed on the terminal end portion of the D-cut surface  33 A of the shaft end portion  33 . The circular end portion  33 C protrudes when the gear  34  is fixed to the shaft end portion  33 . The circular end portion  33 C is configured to be insertable into a lateral U-shaped shaft bearing  260 A formed in a shaft bearing member  260 . The shaft bearing member  260  is attached to the unillustrated frame of the image forming apparatus  200 . The shaft bearing member  260  fixes and/or positions the fixing device  10  with respect to the image forming apparatus body  200  as a result of the circular end portion  33 C being inserted into the shaft bearing  260 A. 
     By disposing the shaft bearing  260 A of the gear  34  in the image forming apparatus body  200  and retaining the circular end portion  33 C at the outer side of the gear  34 , the gear  34  can be firmly fixed. Thus, even if the torque of the cams  30  is large, the shaft  32  and the fixing device  10  themselves do not end up moving due to the rotation of the gear  34 . 
     Also, because the gear  34  is directly attached to the shaft end portion  33  of the shaft  32  and the circular end portion  33 C is guided by the shaft bearing  260 A, its position and drive transmission precision can be improved. Consequently, because it becomes difficult for the fixing device  10  to move when the fixing device  10  has been attached to the image forming apparatus body  200 , the positional precision of the reading plate  40  becomes higher, and precise detection becomes possible even when the sensor  42  is disposed in the image forming apparatus body  200 . A reduction in cost is also made possible in comparison to when the sensor  42  is disposed in the fixing device  10 . 
     As shown in  FIG. 8B , the gear  34  includes teeth  34 A that are high. For this reason, when the fixing device  10  is to be attached to the image forming apparatus body  200 , it is difficult for the tooth tips of the gear  34  and the gear  256  of the image forming apparatus body  200  to knock against each other, and the fixing device  10  can be smoothly attached to the image forming apparatus body  200 . When the gear  34  has usual teeth, the tooth tips of the gear  34  and the gear  256  of the image forming apparatus body  200  knock against each other, and the fixing device  10  cannot be smoothly attached. 
     This problem can be solved by tapering the tooth tips of the gear  34 , which can be accomplished in two ways: with high teeth or with a displacement gear. In the case of a displacement gear, the drive tangent becomes oblique, the fixing device  10  easily moves, and there is skipping. If the teeth  34 A of the gear  34  are high teeth, the drive tangent becomes perpendicular and there is no skipping. 
     Next, the action of the fixing device  10  will be described. 
     As shown in  FIG. 7 , in the first mode (plain paper or the like), the cams  30  rotate in positions (home positions) where they do not contact the contact pieces  23 C of the pressuring levers  22 . Thus, the moving pieces  23 A of the pressuring levers  22  move toward the guide pieces  21 A of the brackets  20  due to the pressuring force of the compression springs  28 . For this reason, the pressuring roller  16  retained in the pressuring levers  22  is pressed by a predetermined load against the heating roller  14  retained in the brackets  20 . 
     In the first mode, the pressuring roller  16  has a fixed load, and the elastomer layer of the pressuring roller  16  is elastically deformed as a result of the pressuring roller  16  being pressed against the heating roller  14  by a high load. Thus, the plain paper or the like is conveyed between the heating roller  14  and the pressuring roller  16 , and the toner image can be fixed onto the plain paper. 
     As shown in  FIG. 9 , in the second mode (envelopes or the like), the gear  34  meshing with the gear  256  is caused to rotate as a result of the drive unit  258  causing the gear  256  to rotate in the direction of the arrow. Thus, the cam  30  coupled by the shaft  32  to the gear  34  rotates in the direction of arrow A. Then, the cams  30  rotate and contact the contact pieces  23 C, and the moving pieces  23 A compress the compression springs  28  and move to the predetermined pressuring position because the studs  26  are inserted into the open holes  23 B of the moving pieces  23 A. Thus, the pressuring levers  22  pivot to the predetermined pressuring position in the direction of arrow B around the pivot points  24 , and the pressuring roller  16  retained in the pressuring levers  22  moves in the direction where the interaxial distance between the pressuring roller  16  and the heating roller  14  increases. At this time, the pressuring roller  16  becomes fixedly displaced (interaxially fixed). 
     Thus, in the second mode (envelopes or the like), the load of the pressuring roller  16  on the heating roller  14  becomes smaller in comparison to the load in the first mode. 
     In the fixing device  10 , the load of the pressuring roller  16  on the heating roller  14  can be appropriately adjusted depending on the type of paper (such as plain paper or envelopes), and the occurrence of wrinkles in the paper can be suppressed. Also, because the power relationship between the brackets  20 , the pressuring levers  22 , the compression springs  28  and the cams  30  is closed just among four parts, this is advantageous in terms of the deformation and precision of the constituent parts. It is also easy to assemble the constituent parts. And because the stroke of the pressuring levers  22  is small, this is advantageous in terms of the design of the cams  30 . 
     Also, as shown in  FIG. 9 , the rotational direction of the cams  30  (direction of arrow A) is set in a forward direction that does not disturb the pivoting of the pressuring levers  22  (direction of arrow B). If the rotational direction of the cams  30  is set in a direction that disturbs the pivoting of the pressuring levers  22 , the movement of the pressuring levers  22  becomes jerky and the pressuring levers  22  do not move smoothly. But by making the rotational direction of the cams  30  the forward direction that does not block the pivoting of the pressuring levers  22 , the movement of the pressuring levers  22  can be made smooth. 
       FIG. 10  shows an example of a drive unit  270  that transfers driving force to the gear  256 . The drive unit  270  includes a motor  271  and a worm  272  that is coupled to the motor  271 . A worm gear  274  meshes with the worm  272 , and a transmission gear  276  is disposed coaxially with the worm gear  274 . 
     A transmission gear  278  meshes with the transmission gear  276 , and the gear  256  meshes with the transmission gear  278 . The motor  271  is a DC motor, causes the worm gear  274  to rotate in the direction of the arrow due to the worm  272 , and causes the transmission gears  276  and  278  and the gear  256  to rotate in the directions of the arrows. 
     By disposing the worm  272  and the worm gear  274 , the high-revolution motor  271  can be slowed down and the driving force can be transmitted. If the cams  30  are required to rotate at an extremely low rotation and a high load, the combination of the DC motor, the worm  272  and the worm gear  274  is advantageous in terms of space, cost, and output. 
     Also, because the motor  271  only revolves for an extremely limited amount of time when the motor  271  causes the cams  30  to rotate, an inexpensive brush motor can be used. 
     In the fixing device  10 , bearings (not shown) may be disposed on the contact surfaces of the cams  30 . By disposing bearings, the sliding resistance between the cams  30  and the contact pieces  23 C can be lowered. 
     Second Embodiment 
     Next, a fixing device of a second embodiment of the invention will be described. 
     The same reference numerals will be given to members that are the same as those in the first embodiment, and redundant description of those same members will be omitted. 
     As shown in  FIG. 11 , the fixing device includes pressuring levers  62  that retain the pressuring roller  16  and are pivotably supported on brackets  60  that retain the heating roller  14 . End portions  66 A of tension springs  66  are attached to guide members  62 A of the pressuring levers  62 . Groove portions  64 A are formed in the peripheral surfaces of cams  64  disposed on retaining members  60 B of the brackets  60 , and hooks  66 B formed on the other end portions of the tension springs  66  are hooked in the groove portions  64 A. The pressuring levers  62  are pulled toward the brackets  60  by the tension springs  66 , whereby the pressuring roller  16  is pushed against the heating roller  14 . 
     The load of the pressurizing roller  16  is adjusted because the rotational axes of the cams  64  are eccentric, the pressuring levers  62  pivot due to the rotation of the cams  64 , and the interaxial distance between the heating roller  14  and the pressuring roller  16  is variable. Thus, the configuration can be simplified in a fixing device of the type where the interaxial distance is variable. Also, because the hooks  66 B of the tension springs  66  are hooked in the groove portions  64 A, the hooks  66 B can be firmly retained. 
     Third Embodiment 
     Next, a fixing device of a third embodiment of the invention will be described. 
     The same reference numerals will be given to members that are the same as those in the first and second embodiments, and redundant description of those same members will be omitted. 
     As shown in  FIG. 12  and  FIGS. 13A and 13B , in this fixing device, groove portions  74 A are formed in the outer peripheries of cams  74  supported in retaining members  60 B, and bearings  76  are disposed in the groove portions  74 A. The hooks  66 B of the tension springs  66  are hooked onto the bearings  76 . 
     In this fixing device, because the hooks  66 B of the tension springs  66  are hooked onto the bearings  76 , the sliding resistance between the cams  74  and the hooks  66 B becomes smaller when the cams  74  rotate. 
     Fourth Embodiment 
     Next, a fixing device of a fourth embodiment of the invention will be described. 
     The same reference numerals will be given to members that are the same as those in the first embodiment, and redundant description of those same members will be omitted. 
     As shown in  FIG. 14 , in this fixing device, leading end portions  81  of brackets  80  that retain the heating roller  14  and leading end portions  83  of pressuring levers  82  that retain the pressuring roller  16  cross at the pivoting end portion side of the pressuring levers  82 . 
     Spring receiving portions  81 A are formed on the leading end portions  81  substantially parallel to the axial direction of the heating roller  14 , and spring receiving portions  83 A are disposed on the leading end portions  83  a predetermined distance away from the spring receiving portions  81 A. Bosses  86  are attached to the spring receiving portions  81 A, and bosses  87  are attached to the spring receiving portions  83 A. Both end portions of compression springs  88  are retained on these bosses  86  and  87 . Contact pieces  23 C that can be contacted by the cams  30  are formed on the leading end portions  83 . 
     In this fixing device, when the cams  30  rotate in the direction of arrow A and contact the contact pieces  23 C, the compression springs  88  are compressed and the spring receiving portions  83 A move in the direction of arrow C. Thus, the pressuring levers  82  pivot in the direction of arrow B, and the load of the pressuring rollers  16  retained in the pressuring levers  82  is adjusted. 
     In this fixing device, because the compression springs  88  are disposed between the spring receiving portions  81 A of the brackets  80  and the spring receiving portions  83 A of the pressuring levers  80 , the studs  26  shown in  FIG. 7  can be omitted and the number of parts can be reduced.