Patent Publication Number: US-9851664-B2

Title: Fixing device

Description:
INCORPORATION BY REFERENCE 
     This application claims priority to Japanese Patent Application No. 2015-119348 filed on Jun. 12, 2015, the entire contents of which are incorporated by reference herein. 
     BACKGROUND 
     The present disclosure relates to a fixing device configured to apply heat and pressure to an image formed on a recording paper sheet to fix the image thereon and an image forming apparatus. 
     Fixing devices are configured to press a heat roller and a pressure roller against each other to form a nip area between them and then nip a recording paper sheet in the nip area with the heat roller heated so that an image (unfixed toner image) on the recording paper sheet is fixed by the application of heat and pressure. 
     In order to reduce the heating time of the heat roller and save energy in such a fixing device, there is a tendency to reduce the thickness of the peripheral wall of the heat roller to reduce the heat capacity of the heat roller. 
     However, if the thickness of the peripheral wall of the heat roller is too small, the rigidity of the heat roller becomes low. Thus, the pressure of engagement of the pressure roller against the heat roller may deform the heat roller, in which case an appropriate nip area cannot be ensured. To cope with this, in the case of making the heat roller from aluminum, the peripheral wall of the heat roller is designed to have a thickness of 0.55 mm or more. In the case of making the heat roller from stainless steel, the peripheral wall of the heat roller is designed to have a thickness of 0.3 mm or more. In these manners, the rigidity of the heat roller are ensured. 
     Alternatively, a member configured to be pressed against the inner peripheral surface of the heat roller may be provided to prevent deformation of the heat roller and ensure a nip area between the heat roller and the pressure roller. 
     For example, a fixing device is known in which a fixed pad is pressed against the inner peripheral surface of a fixing roller (an equivalent of the heat roller), more specifically against a region thereof corresponding to a nip area, thus preventing deformation of the fixing roller. 
     Likewise, a fixing device is also known in which a second pressing member is pressed against a region of the inner peripheral surface of a fixing roller corresponding to a nip area to prevent deformation of the fixing roller. Also, there is a fixing device configured to prevent deformation of a heat roller by pressing a pressing member against near a region of the inner peripheral surface of the heat roller corresponding to a nip area. 
     Furthermore, there is known a fixing device configured to press a metal roller against a region of the inner peripheral surface of a fixing roller corresponding to a nip area. This metal roller is provided to homogenize the temperature distribution of the fixing roller but can also be expected to have the effect of preventing deformation of the fixing roller. 
     Likewise, there is also known a fixing device configured to press a highly thermally conductive roller against near a region of the inner peripheral surface of a heat roller corresponding to a nip area. This highly thermally conductive roller is also provided to homogenize the temperature distribution of the heat roller but can be expected to have the effect of preventing deformation of the heat roller. 
     In the case where, as in each of the above fixing devices, deformation of the heat roller is prevented by pressing a member against the inner peripheral surface of the heat roller, the thickness of the peripheral wall of the heat roller can be reduced to 100 μm to 200 μm. In this relation, a 0.55 mm or larger thickness of the aluminum-made heat roller or a 0.3 mm or larger thickness of the stainless steel-made heat roller, which are examples of enough thicknesses of the peripheral wall to ensure the rigidity of these heat rollers, can be achieved by cutting. However, a 100 μm to 200 μm thickness is difficult to achieve by cutting and has to be provided by rolling. 
     SUMMARY 
     A technique improved over the aforementioned techniques is proposed as one aspect of the present disclosure. 
     A fixing device according to an aspect of the present disclosure includes a heat roller, a heating element, a pressure roller, and an auxiliary roller. 
     The heat roller has a cylindrical shape. 
     The heating element is provided inside the heat roller and configured to apply heat to the heat roller. 
     The pressure roller is configured to be pressed against the heat roller to form, with the heat roller, a nip area where a recording paper sheet is to be nipped. 
     The auxiliary roller is provided inside the heat roller and configured to be pressed toward the pressure roller with a peripheral wall of the heat roller in between. 
     An outer peripheral surface of the auxiliary roller pressed against an inner peripheral surface of the heat roller is formed into an inverted crown shape so that an outside diameter of the auxiliary roller gradually decreases from both axial ends of the auxiliary roller toward an axial center thereof. 
     An image forming apparatus according to another aspect of the present disclosure includes the above-described fixing device and an image forming section. 
     The image forming section is configured to form an image on the recording paper sheet. 
     The fixing device is configured to nip the recording paper sheet in a nip area between the heat roller and the pressure roller and fix the image on the recording paper sheet by heat and pressure applied from the heat roller and the pressure roller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front cross-sectional view showing the structure of an image forming apparatus to which a fixing device according to a first embodiment of the present disclosure is applied. 
         FIG. 2  is a perspective view showing the fixing device according to the first embodiment. 
         FIG. 3  is a side view showing the fixing device according to the first embodiment. 
         FIG. 4  is a transverse cross-sectional view showing the fixing device according to the first embodiment. 
         FIG. 5  is a partly cross-sectional view of a heat roller and an auxiliary roller of the fixing device according to the first embodiment as viewed from the side, wherein only the heat roller is shown in broken section. 
         FIG. 6A  is a plan view showing an auxiliary roller of the fixing device according to the first embodiment. 
         FIGS. 6B and 6C  are plan views showing modifications of the auxiliary roller. 
         FIG. 7  is a perspective view showing a modification of the fixing device according to the first embodiment. 
         FIG. 8  is a perspective view showing a fixing device according to a second embodiment. 
         FIG. 9  is a side view showing the fixing device according to the second embodiment. 
         FIG. 10A  is a plan view showing an auxiliary roller of the fixing device according to the second embodiment. 
         FIGS. 10B and 10C  are plan views showing modifications of the auxiliary roller. 
         FIG. 11  is a perspective view showing a modification of the fixing device according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a description will be given of embodiments of the present disclosure with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a front cross-sectional view showing the structure of an image forming apparatus to which a fixing device according to a first embodiment of the present disclosure is applied. This image forming apparatus  1  is a multifunction peripheral having a plurality of functions including, for example, a copy function, a print function, a scan function, and a facsimile function. The image forming apparatus  1  is made up so that an apparatus body  2  is provided with an image scanner unit (ISU)  5 , an operating section  47 , an image forming section  120 , a fixing device  13  according to the first embodiment, a sheet feed section  14 , and so on. 
     In performing an image forming operation, the image forming section  120  of the image forming apparatus  1  forms a toner image on a recording paper sheet P serving as a recording medium fed from the sheet feed section  14 , based on image data generated by image scanning, image data received from a network-connected computer or a user terminal device, such as a smartphone, image data stored on an internal HDD, or other image data. 
     Each of image forming units  12 M,  12 C,  12 Y, and  12 Bk of the image forming section  120  includes a photosensitive drum  122 , a charging device operable to uniformly charge the surface of the photosensitive drum  122 , a laser scanning unit (LSU)  123  operable to expose the surface of the photosensitive drum  122  to laser light to form an electrostatic latent image on the surface thereof, a developing device operable to develop the electrostatic latent image on the surface of the photosensitive drum  122  into a toner image, and a primary transfer roller  126 . 
     In performing color printing, the image forming unit  12 M for magenta, the image forming unit  12 C for cyan, the image forming unit  12 Y for yellow, and the image forming unit  12 Bk for black in the image forming section  120  uniformly charge the surfaces of their respective photosensitive drums  122 , then expose them to laser light to form respective electrostatic latent images corresponding to images of their color components on the surfaces, develop the electrostatic latent images on the surfaces of the photosensitive drums  122  with toners of their color components to form respective toner images on the photosensitive drums  122 , and then allow their respective primary transfer rollers  126  to primarily transfer the toner images to an intermediate transfer belt  125  mounted around a drive roller  125 A and a driven roller  125 B. 
     The toner images of different color components transferred to the intermediate transfer belt  125  are superposed each other on the intermediate transfer belt  125  by controlling their transfer timings, resulting in a multicolor toner image. A secondary transfer roller  210  is configured to secondarily transfer the multicolor toner image formed on the surface of the intermediate transfer belt  125 , at a nip N between the secondary transfer roller  210  and the intermediate transfer belt  125 , to a recording paper sheet P conveyed from the sheet feed section  14  along a conveyance path  190 . 
     Thereafter, the fixing device  13  applies heat and pressure to the recording paper sheet P, thus fixing the toner image on the recording paper sheet P by heat and pressure. Then, the recording paper sheet P is discharged through an output roller pair  159  to a sheet output tray  151 . 
     Next, a detailed description will be given of the fixing device  13  according to the first embodiment.  FIG. 2  is a perspective view showing the fixing device  13  according to the first embodiment. Furthermore,  FIG. 3  is a side view showing the fixing device  13  according to the first embodiment and  FIG. 4  is a transverse cross-sectional view showing the fixing device  13  according to the first embodiment. 
     As shown in  FIGS. 2 to 4 , the fixing device  13  includes a heat roller  31 , a heating element  32 , a pressure roller  33 , an auxiliary roller  34 , and so on. 
     The heat roller  31  has a hollow, cylindrical shape. Both ends of this heat roller  31  are rotatably supported in a pair of bearings  35  fixed to a frame (not shown) of the fixing device  13 . Furthermore, a rotating gear  36  is fixed to one end of the heat roller  31 . When this rotating gear  36  is meshed with a drive gear (not shown) of the fixing device  13  and rotated, the heat roller  31  is driven into rotation in the direction of the arrow. 
     The heating element  32  is provided inside the heat roller  31  along the axial direction (longitudinal direction) of the heat roller  31 . For example, the heating element  32  is a halogen lamp. 
     The pressure roller  33  is provided in parallel with the heat roller  31  and pressed against the outer peripheral surface of the heat roller  31  to form a nip area N 1  with the heat roller  31 . Both ends of a shaft  38  of this pressure roller  33  are rotatably supported in a pair of bearings (not shown) fixed to the frame of the fixing device  13 . 
     The auxiliary roller  34  is provided, inside the heat roller  31 , in abutment with the inner peripheral surface of the heat roller  31  and disposed in a region corresponding to the nip area N 1 . Therefore, the auxiliary roller  34  receives a pressure of engagement from the pressure roller  33  with the peripheral wall of the heat roller  31  in between and is thus pressed against the inner peripheral surface of the heat roller  31 . Both ends of a shaft  37  of this auxiliary roller  34  are rotatably supported in a pair of bearings (not shown) fixed to the frame of the fixing device  13 . 
     Each of the outer peripheral surfaces of the heat roller  31 , the pressure roller  33 , and the auxiliary roller  34  are subjected to an appropriate surface treatment. 
     When in the fixing device  13  having the above structure the rotating gear  36  of the heat roller  31  is rotated, so that the heat roller  31  is driven into rotation in the direction of the arrow, the pressure roller  33  pressed against the outer peripheral surface of the heat roller  31  follows the rotation of the heat roller  31  to rotate in the direction opposite to the heat roller  31  and the auxiliary roller  34  pressed against the inner peripheral surface of the heat roller  31  follows the rotation of the heat roller  31  to rotate in the same direction as the heat roller  31 . 
     Furthermore, the heating element  32  is configured to, when electric current passes therethrough, generate heat and apply heat directly to the heat roller  31  and the auxiliary roller  34 . In addition, the heat roller  31  is indirectly heated by heat conduction from the auxiliary roller  34  to the heat roller  31 . Thus, the heat roller  31  is heated to a specified fixing temperature. 
     When in this state the recording paper sheet P is conveyed to the nip area N 1  between the heat roller  31  and the pressure roller  33 , heat and pressure are applied to the recording paper sheet P in the nip area N 1 , so that the toner image on the recording paper sheet P is fixed by heat and pressure. 
     Next, a detailed description will be given of the shapes and so on of the heat roller  31 , the pressure roller  33 , and the auxiliary roller  34 .  FIG. 5  is a partly cross-sectional view of the heat roller  31  and the auxiliary roller  34  of the fixing device  13  according to the first embodiment as viewed from the side, wherein only the heat roller  31  is shown in broken section.  FIG. 6A  is a plan view showing the auxiliary roller  34 . In  FIG. 5 , for ease of understanding of the shapes of the outer peripheral surfaces of the heat roller  31  and the auxiliary roller  34 , the radii of curvature of these outer peripheral surfaces are shown to be larger than in reality. The same applies to the radius of curvature of the outer peripheral surface of the auxiliary roller  34  in  FIG. 6A . 
     As shown in  FIG. 5 , the outer peripheral surface of the heat roller  31  has an inverted crown shape so that the outside diameter thereof gradually decreases from both axial (longitudinal) ends of the heat roller  31  toward an axial center thereof. 
     Unlike the above, the pressure roller has a straight cylindrical shape so that its outside diameter is constant from one end to the other end of the pressure roller  33 . 
     Therefore, the pressure of engagement of the pressure roller  33  against the heat roller  31  is larger in regions a (see  FIG. 3 ) located at both ends of the rollers than in the other regions. Thus, the recording paper sheet P can be promptly passed through the nip area N 1  between the heat roller  31  and the pressure roller  33  without getting wrinkles or the like. 
     On the other hand, the heat roller  31  is made by forming a metal by rolling, as represented by spindle working, wherein the peripheral wall of the heat roller  31  is designed to have a thickness of 100 μm to 200 μm. Thus, the heat capacity of the heat roller  31  can be reduced, achieving a reduced heating time of the heat roller  31  and saved energy. 
     However, since as just described the peripheral wall of the heat roller  31  is designed to have a thickness of 100 μm to 200 μm, further reduction in thickness of the central portion of the peripheral wall of the heat roller  31  makes it difficult to ensure a required rigidity of the heat roller  31 . For this reason, the peripheral wall of the heat roller  31  is designed to have a constant thickness from one end to the other end of the heat roller  31  in the axial (longitudinal) direction. Therefore, in conformity to the inverted crown-shaped outer peripheral surface of the heat roller  31 , the inner peripheral surface of the heat roller  31  has a crown shape as viewed from the inside of the heat roller  31  so that the inside diameter of the heat roller  31  gradually decreases from both axial (longitudinal) ends of the heat roller  31  toward the axial center thereof. 
     The outer peripheral surface of the auxiliary roller  34  pressed against the inner peripheral surface of the heat roller  31  is formed into an inverted crown shape so that the outside diameter of the auxiliary roller  34  gradually decreases from both axial ends of the auxiliary roller  34  toward the axial center thereof. 
     In this case, as shown in  FIG. 5 , the shapes of the outer peripheral surface of the auxiliary roller  34  and the inner peripheral surface of the heat roller  31  are designed so that the inverted crown shape of the outer peripheral surface of the auxiliary roller  34  and the crown shape of the inner peripheral surface of the heat roller  31  (as viewed from the inside of the heat roller  31 ) conform and fit to each other. Therefore, when the auxiliary roller  34  is pressed against the inner peripheral surface of the heat roller  31 , the inverted crown shape of the outer peripheral surface of the auxiliary roller  34  fits closely to the crown shape of the inner peripheral surface of the heat roller  31 , so that the outer peripheral surface of the auxiliary roller  34  is pressed uniformly against the inner peripheral surface of the heat roller  31 . Hence, even if the auxiliary roller  34  is pressed toward the pressure roller  33  with the peripheral wall of the heat roller  31  in between, the central portion of the peripheral wall of the heat roller  31  can be prevented from being pushed outward, so that the inverted crown shape of the outer peripheral surface of the heat roller  31  can be maintained. Thus, the above sheet passage effect can be maintained of promptly passing the recording paper sheet P through the nip area N 1  without causing wrinkles or the like in the recording paper sheet P. 
     Furthermore, as shown in  FIG. 3 , the length A of an engagement region within which the auxiliary roller  34  is pressed against the inner peripheral surface of the heat roller  31  and which extends along the axial direction of the auxiliary roller  34  is designed to be longer than the length B of an engagement region within which the pressure roller  33  is pressed against the outer peripheral surface of the heat roller  31  and which extends along the axial direction of the auxiliary roller  34 . Therefore, in the engagement region of a length B receiving a pressure of engagement from the pressure roller  33 , the outer peripheral surface of the auxiliary roller  34  certainly fits to the inner peripheral surface of the heat roller  31 , so that the heat roller  31  can be certainly prevented from being deformed. 
     Furthermore, the auxiliary roller  34  is made of a highly thermally conductive material, for example, a metal material such as aluminum or stainless steel. Therefore, by heat conduction from the auxiliary roller  34  to the heat roller  31 , the heat roller  31  can be effectively and uniformly heated to homogenize the temperature distribution on the surface of the heat roller  31 . Thus, the toner image on the recording paper sheet P can be fixed well. 
     As thus far described, in the first embodiment, the peripheral wall of the heat roller  31  is designed to have a thickness of 100 μm to 200 μm, so that the inverted crown-shaped outer peripheral surface of the auxiliary roller  34  can receive a pressure of engagement from the pressure roller  33  with the peripheral wall of the heat roller  31  in between while a reduced heating time of the heat roller  31  and saved energy are achieved. Therefore, a nip area N 1  can be formed between the heat roller  31  and the pressure roller  33  while the outer peripheral surface of the heat roller  31  maintains an inverted crown shape, so that the sheet passage effect can be maintained of promptly passing the recording paper sheet P through the nip area N 1 . Furthermore, the auxiliary roller  34  can homogenize the temperature distribution on the surface of the heat roller  31 , so that the toner image on the recording paper sheet P can be fixed well. 
     For example, in the fixing device shown in BACKGROUND in which the fixed pad pressed against the inner peripheral surface of the heat roller has a curved shape so that its central portion rises toward the pressure roller, thus preventing the central portion of the heat roller  31  from being depressed, the raised central portion of the fixed pad strongly presses against the central portion of the inner peripheral surface of the heat roller. Thus, the central portion of the heat roller is pushed outward, so that the inverted crown shape of the outer peripheral surface of the heat roller is eliminated, which presents a problem with the passage of the recording paper sheet. 
     On the other hand, in the other fixing devices shown in BACKGROUND, the member pressed against the inner peripheral surface of the heat roller extends linearly in the axial direction of the heat roller. Furthermore, in the case of producing the heat roller by cutting as described previously, the central portion of the heat roller is deeply cut to form the outer peripheral surface of the heat roller into an inverted crown shape. In this case, the heat roller has a constant inside diameter so that the inner peripheral surface of the heat roller extends linearly in the axial direction of the heat roller. Therefore, so long as the member extending linearly in the axial direction of the heat roller is pressed against the inner peripheral surface of the heat roller, the inverted crown shape of the outer peripheral surface of the heat roller is not eliminated, which does not present the problem with the passage of the recording paper sheet. 
     However, in the case of reducing the thickness of the peripheral wall of the heat roller to 100 μm to 200 μm by rolling as described above, the central portion of the peripheral wall cannot be reduced in thickness because the heat roller must ensure rigidity required as a roller. In this case, the thickness of the peripheral wall of the heat roller can only be made constant. Therefore, when the outer peripheral surface of the heat roller is formed into an inverted crown shape, the inside diameter of the heat roller cannot be made constant and the inner peripheral surface of the heat roller has a crown shape in which the inside diameter thereof gradually decreases from both axial ends thereof toward the axial center thereof. 
     Thus, when the member extending linearly in the axial direction of the heat roller is pressed against the inner peripheral surface of the heat roller, the central portion of the inner peripheral surface of the heat roller is strongly pushed outward. Also in this case, the inverted crown shape of the outer peripheral surface of the heat roller is eliminated, which presents a problem with the passage of the recording paper sheet. 
     In contrast, in accordance with this embodiment, the heat roller  31  can be prevented from being deformed while the thickness of the peripheral wall of the heat roller  31  can be reduced. 
     Modification 1 
       FIG. 7  is a perspective view showing Modification 1 of the fixing device  13  according to the first embodiment. The fixing device  13  of Modification 1 is different from the first embodiment in that the rotating gear  36  at one end of the heat roller  31  is eliminated and a rotating gear  41  is provided instead at one end of the shaft  38  of the pressure roller  33 , and the other structures thereof are the same as in the first embodiment. 
     The rotating gear  41  can be rotated in meshing engagement with a drive gear (not shown) of the fixing device  13 , so that the pressure roller  33  is driven into rotation in the direction of the arrow. With this rotation, the heat roller  31  pressed against the outer peripheral surface of the pressure roller  33  follows the rotation of the pressure roller  33  to rotate in the reverse direction and the auxiliary roller  34  pressed against the inner peripheral surface of the heat roller  31  follows the rotation of the heat roller  31  to rotate in the same direction as the heat roller  31 . 
     Furthermore, the heat roller  31  and the auxiliary roller  34  are heated by the heating element  32 . When in this state the recording paper sheet P is conveyed to the nip area N 1  between the heat roller  31  and the pressure roller  33  and nipped in the nip area N 1 , the toner image on the recording paper sheet P is fixed by the application of heat and pressure. 
       FIGS. 6B and 6C  show other modifications of the auxiliary roller  34 . Like  FIG. 6A , also in  FIGS. 6B and 6C , for ease of understanding of the shape of the outer peripheral surface of the auxiliary roller  34 , the radius of curvature of the outer peripheral surface is shown to be larger than in reality. 
     The auxiliary roller  34  in this modification is made of a metal material, such as aluminum or stainless steel and smooth asperities, i.e., asperities of irregularities, are formed on the outer peripheral surface thereof. For example, reticular shallow grooves are formed in the outer peripheral surface of the auxiliary roller  34  as shown in  FIG. 6B  or the outer peripheral surface of the auxiliary roller  34  is roughened as shown in  FIG. 6C . Thus, the frictional resistance of the outer peripheral surface of the auxiliary roller  34  to the inner peripheral surface of the heat roller  31  is increased, so that the auxiliary roller  34  can certainly follow the rotation of the heat roller  31  to rotate. Furthermore, since the asperities formed on the outer peripheral surface of the auxiliary roller  34  are smooth, it can be avoided that the inner peripheral surface of the heat roller  31  is damaged by contact with the outer peripheral surface of the auxiliary roller  34 . 
     The outer peripheral surface of the auxiliary roller  34  is more preferably subjected to a black surface treatment. Examples of the surface treatment include a surface treatment using a heat-resistant resin and a surface treatment using an inorganic mineral. Thus, the auxiliary roller  34  can efficiently absorb heat from the heating element  32 , such as a halogen lamp, and, in addition, the heat conduction from the auxiliary roller  34  to the heat roller  31  can more effectively and uniformly heat the heat roller  31 . 
     Second Embodiment 
     Next, a detailed description will be given of a fixing device  13  according to a second embodiment.  FIG. 8  is a perspective view showing the fixing device  13  according to the second embodiment.  FIG. 9  is a side view showing the fixing device  13  according to the second embodiment.  FIG. 10A  is a plan view showing an auxiliary roller  34  of the fixing device  13  according to the second embodiment. In  FIG. 10A , for ease of understanding of the shape of the outer peripheral surface of the auxiliary roller  34 , the radius of curvature of the outer peripheral surface is shown to be larger than in reality. 
     The fixing device  13  according to the second embodiment is different from the first embodiment in that a shaft  37  of the auxiliary roller  34  is additionally provided at both end portions with their respective position restricting members  42 , and the other structures thereof are the same as in the first embodiment. 
     The position restricting members  42  are flange-shaped members provided at both the end portions of the shaft  37  of the auxiliary roller  34  which extend outwardly of both ends of the heat roller  31  and they face and come close to or in contact with both the ends of the heat roller  31 . 
     In this structure, when the position of the heat roller  31  comes nearly to deviate in the axial direction of the heat roller  31 , either one of the position restricting members  42  of the auxiliary roller  34  abuts on the adjacent end of the heat roller  31 , so that the deviation of the heat roller  31  can be prevented. 
     Therefore, the nip area N 1  between the heat roller  31  and the pressure roller  33  can be appropriately held and the inverted crown shape of the outer peripheral surface of the heat roller  31  can be maintained, so that the sheet passage effect can be maintained of promptly passing the recording paper sheet P through the nip area N 1 . 
     Furthermore, since the auxiliary roller  34  and the position restricting members  42  rotate in the same direction as the heat roller  31 , the frictional resistance of the position restricting members  42  to both the ends of the heat roller  31  is small. Therefore, it can be avoided that both the ends of the heat roller  31  are worn or damaged by contact with the position restricting members  42  of the auxiliary roller  34  to reduce the life of the heat roller  31 . 
     In the case where, as in this structure, the auxiliary roller  34  and the heating element  32  are disposed inside the heat roller  31 , the space for placement of the auxiliary roller  34  becomes narrow and, therefore, the outside diameter of the auxiliary roller  34  needs to be, for example, about 10 mm. Therefore, the shaft  37  of the auxiliary roller  34  comes very close to the edges of the heat roller  31  so that there is almost no space between the shaft  37  of the auxiliary roller  34  and both the ends of the heat roller  31 . Hence, the heat roller  31  cannot be provided at both the ends with stop rings for preventing axial deviation of the heat roller  31 . 
     In the second embodiment, however, the position restricting members  42  of the auxiliary roller  34  function as the above stop rings. Therefore, there is no need to provide the stop rings. 
     Furthermore, if the position restricting members  42  were configured not to rotate, the frictional resistance of the position restricting members  42  to both the ends of the heat roller  31  might be large. In addition, because the peripheral wall of the heat roller  31  is thin and the pressure per unit area on both the ends of the heat roller  31  from the position restricting members  42  is high, both the ends of the heat roller  31  would be easily worn or damaged to reduce the life of the heat roller  31 . Alternatively, if the position restricting members  42  that may abut on both the ends of the heat roller  31  are made of a soft material, such as resin, it may be possible to prevent wear or damage of both the ends of the heat roller  31 . In this case, however, the life of the position restricting members  42  will be reduced. 
     Unlike the above, since in the second embodiment the auxiliary roller  34  and the position restricting members  42  rotate in the same direction as the heat roller  31 , it can be avoided that the life of the heat roller  31  or the position restricting members  42  is reduced in the above manner. 
     Specifically, although the placement of the auxiliary roller  34  inside of the heat roller  31  makes it impossible to provide stop rings at both the ends of the heat roller  31 , the alternative provision of the position restricting members  42  on both the end portions of the shaft  37  of the auxiliary roller  34  makes it possible to prevent axial deviation of the heat roller  31 . In addition, since the auxiliary roller  34  and the position restricting members  42  rotate in the same direction as the heat roller  31 , it can be avoided that the lives of the heat roller  31  and the position restricting members  42  are reduced. 
     Modification 2 
       FIG. 11  is a perspective view showing Modification 2 of the fixing device  13  according to the second embodiment. The fixing device  13  of Modification 2 is different from the second embodiment in that the rotating gear  36  at one end of the heat roller  31  is eliminated and a rotating gear  41  is provided instead at one end of the shaft  38  of the pressure roller  33 , and the other structures thereof are the same as in the second embodiment. 
     The rotating gear  41  can be rotated in meshing engagement with a drive gear (not shown) of the fixing device  13 , so that the pressure roller  33  is driven into rotation in the direction of the arrow. With this rotation, the heat roller  31  follows the rotation of the pressure roller  33  to rotate in the reverse direction and the auxiliary roller  34  pressed against the inner peripheral surface of the heat roller  31  follows the rotation of the heat roller  31  to rotate in the same direction as the heat roller  31 . 
     When the recording paper sheet P is conveyed to the nip area N 1  between the heat roller  31  and the pressure roller  33  and nipped in the nip area N 1  while the heat roller  31  and the auxiliary roller  34  are heated by the heating element  32 , the toner image on the recording paper sheet P is fixed by the application of heat and pressure. 
       FIGS. 10B and 10C  show other modifications of the auxiliary roller  34 . Like  FIG. 10A , also in  FIGS. 10B and 10C , the radius of curvature of the outer peripheral surface of the auxiliary roller  34  is shown to be larger than in reality. 
     The auxiliary roller  34  in this modification is a roller made of a metal material, such as aluminum or stainless steel, and having an outer peripheral surface formed of smooth asperities, for example, by forming reticular shallow grooves in the outer peripheral surface or roughening the outer peripheral surface. Therefore, the frictional resistance of the outer peripheral surface of the auxiliary roller  34  to the inner peripheral surface of the heat roller  31  is increased, so that the auxiliary roller  34  can certainly follow the rotation of the heat roller  31  to rotate. 
     Furthermore, the outer peripheral surface of the auxiliary roller  34  is subjected to a black surface treatment using a heat-resistant resin or an inorganic mineral. Thus, the auxiliary roller  34  can efficiently absorb heat from the heating element  32 , such as a halogen lamp, and, in addition, the heat conduction from the auxiliary roller  34  to the heat roller  31  can more effectively and uniformly heat the heat roller  31 . 
     Although the description of the above embodiments is given taking a color printer as an example of the image forming apparatus according to the present disclosure, the example is merely illustrative and the image forming apparatus may be any other image forming apparatus, including a black-and-white printer and other electronic devices, such as a multifunction peripheral, a copier, and a facsimile machine. 
     The structures and processing described with reference to  FIGS. 1 to 11  are merely illustrative of the present disclosure and the present disclosure is not intended to be limited to the above structures and processing. 
     Various modifications and alterations of this disclosure will be apparent to those skilled in the art without departing from the scope and spirit of this disclosure, and it should be understood that this disclosure is not limited to the illustrative embodiments set forth herein.