Abstract:
An image heating device according to an embodiment of the present invention includes a heat generator that has an outer surface and that generates heat by induction heating and a heater positioned close to the outer surface of the heat generator, the heater being configured to heat the heat generator by induction heating. A positioner is located close to an end of the heater, the positioner being configured to position the heater with respect to the heat generator. A vibration absorber is attached to the positioner and is configured, in one embodiment, to viscoelasticly absorb vibration of the heater produced by a vibration caused by an electromagnetic repulsive force acting between the heat generator and the heater when the heater heats the heat generator by induction heating.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image heating device, or more specifically, to an image heating device which is preferably applicable to fixing of a non-fixed image used for an electrophotographic device or electrostatic recording device, etc., through heating. 
   2. Description of the Related Art 
   As this type of image heating device, there is a conventional proposal on an image heating device using electromagnetic induction. One example of this is an image heating device disclosed in the Unexamined Japanese Patent Publication No.HEI 10-232575. 
   As shown in  FIG. 1 , an image heating device  1  is constructed of a fixing member  2  and a pressure roller  3 . The fixing member  2  is provided with a stay  4  and a fixing film  5  is attached to this stay  4  in such a way that it is rotatable around the stay  4  in the direction indicated by an arrow. The stay  4  contains an exciting coil  6  and an induction heating plate  7 . The exciting coil  6  consists of a core  6   a  made of a ferromagnetic substance and a winding  6   b  which is wound around the core  6   a . In this way, by passing a high-frequency AC through the winding  6   b  and generating an alternating field, it is possible to generate an eddy current in the induction heating plate  7  and thereby heat the induction heating plate  7 . Furthermore, a temperature sensor  8  is provided close to the induction heating plate  7 . The high-frequency current is controlled according to the temperature detection result obtained through the temperature sensor  8  and the temperature of the induction heating plate  7  is set to a desired value. 
   In the image heating device  1 , with the induction heating plate  7  being heated, the pressure roller  3  rotates while contacting the induction heating plate  7  under pressure through the fixing film  5  and carries a recording sheet into a nip section of the fixing film  5  which rotates driven by the pressure roller  3 . As a result, toner on the recording sheet is heated and pressurized and thereby fixed to the recording sheet. 
   In addition to such a configuration, the image heating device  1  is provided with a vibration absorption member  9  between the induction heating plate  7  and exciting coil core  6   a . This prevents image disturbance caused by vibration due to electromagnetic induction. 
   However, in the above described conventional configuration, the vibration absorption member  9  is placed in an area which is directly heated by electromagnetic induction. For this reason, a highly heat-resistant material needs to be used for the vibration absorption member  9 , which increases the cost of the device. 
   Furthermore, the exciting coil  6  and vibration absorption member  9  are placed inside the fixing film  5  which is heated to a high temperature. For this reason, these components are required to have high heat resistance. 
   Furthermore, the pressure from the fixing nip is received by the thin induction heating plate  7 . This causes a great pressure to act on the vibration absorption member  9  and exciting coil  6 . To withstand this great pressure, the vibration absorption member  9  needs to be placed over the entire width, which requires the use of a large amount of the costly vibration absorption member  9 . 
   Moreover, because of the great pressure acting thereupon, it is not easy for the thin vibration absorption member  9  to absorb vibration sufficiently. In the case of insufficient vibration absorption, vibration produced when a high-frequency current passes through the exciting coil  6  is transmitted to the fixing film  5 , which disturbs a toner image on the recording sheet in the fixing nip section and may change the rotation speed of the fixing film  5 , thus generating jitter on the image in extreme cases. 
   This tendency may become more noticeable when a high-frequency current not lower than approximately 50 kHz is passed to heat a copper or aluminum material having low magnetic permeability and resistivity. 
   Furthermore, since it is difficult to fix the exciting coil  6  to the induction heating plate  7  firmly, there is a problem that the positioning reliability deteriorates. As a result, when the distance between the exciting coil  6  and induction heating plate  7  fluctuates, the magnetic coupling condition between the exciting coil  6  and induction heating plate  7  changes, which makes it difficult to perform stable power supply control and prevents accurate temperature control. Consequently, the toner fixing state changes, causing an uneven luster or fixing defect. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an image heating device having a simple structure capable of reliably positioning a member to be heated and an exciting coil and preventing vibration transmission from the exciting coil to the member to be heated. 
   A image heating device according to an aspect of the invention comprises a heat generating section that has an outer surface and generates heat by induction heating, a heating section placed close to the outer surface of the heat generating section that heats the heat generating section by induction heating, a positioning section placed close to the end of the heating section that positions the heating section with respect to the heat generating section and a vibration absorption section attached to the positioning section that absorbs vibration of the heating section produced when the heating section heats the heat generating section by induction heating. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the invention will appear more fully hereinafter from a consideration of the following description taken in connection with the accompanying drawing wherein one example is illustrated by way of example, in which; 
       FIG. 1  is a partial cross-sectional view showing a configuration of a conventional image heating device; 
       FIG. 2  is a plan view showing an overall configuration of an image formation device to which the image heating device of the present invention is applied; 
       FIG. 3  is a partial cross-sectional view showing a configuration of an image heating device according to Embodiment 1; 
       FIG. 4  illustrates the operation of induction heating by the image heating device; 
       FIG. 5  is a perspective view of principal components of the image heating device viewed from the direction indicated by the arrow E in  FIG. 3 ; 
       FIG. 6  illustrates a mounting structure of the exciting unit and heat generating roller; 
       FIG. 7  is a partial cross-sectional view along a line B–B′ showing details of the mounting structure of the exciting unit and heat generating roller; 
       FIG. 8  illustrates a characteristic of loss factor regarding one example of a vibration absorption material used as a shock-absorbing member; 
       FIG. 9  is a partial cross-sectional view showing a configuration of an image heating device according to Embodiment 2; and 
       FIG. 10  illustrates a mounting structure of an exciting unit, auxiliary roller and fixing roller according to Embodiment 2. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An essence of the present invention is to provide an exciting unit having an exciting coil outside a heat generating member, a positioning section that keeps the distance between this exciting unit and heat generating member to a predetermined distance and a shock-absorbing member at the position of this positioning section. 
   With reference now to the attached drawings, embodiments of the present invention will be explained in detail below. 
   (Embodiment 1) 
   (1) Overall configuration 
     FIG. 2  shows an overall configuration of an image formation device. An image formation device  10  outputs four laser beams  12 Y,  12 M,  12 C and  12 Bk according to an image signal from a photolithography device  11 . In this way, latent images of the laser beams  12 Y,  12 M,  12 C and  12 Bk are formed on photosensitive members  13 Y,  13 M,  13 C and  13 Bk. Developing devices  14 Y,  14 M,  14 C and  14 Bk apply toner to the latent images on the photosensitive members  13 Y,  13 M,  13 C and  13 Bk to make the images visible. There are four combinations of these photosensitive members and developing devices; Y, M, C and Bk, and the developing devices  14 Y,  14 M,  14 C and  14 Bk contain toner of four colors of yellow, magenta, cyan and black respectively. The reference numerals denoting the above described members of the respective colors are accompanied by characters Y, M, C and Bk. 
   Toner images  18  of four colors formed on the photosensitive members  13 Y,  13 M,  13 C and  13 Bk are superimposed one atop another on the surface of an intermediate transfer belt  15  held by support shafts and made to move in the direction indicated by an arrow in the figure. This toner image  18  is transferred to a recording sheet  17  at the position of a secondary transfer roller  16 . 
   The secondary transfer roller  16  is provided so as to be contiguous to the intermediate transfer belt  15 . Furthermore, by applying an electric field to the secondary transfer roller  16  pressed against the intermediate transfer belt  15  with the recording sheet  17  sandwiched in between, the secondary transfer roller  16  transfers the toner image  18  superimposed on the intermediate transfer belt  15  to the recording sheet  17 . A paper feed unit  19  feeds the recording sheet  17  at appropriate timings. 
   The recording sheet  17  to which the toner image  18  has been transferred is sent to an image heating device  20 . The image heating device  20  heats and pressurizes the recording sheet  17  with the transferred toner image  18  preferably at a fixing temperature of approximately 170° C. to thereby fix the toner image  18  to the recording sheet  17 . 
   (2) Configuration of Image Heating Device 
     FIG. 3  shows a configuration of the image heating device  20  according to this embodiment. The image heating device  20  is constructed of a heat generating roller  21  supported by a rotation axis (not shown) in a rotatable manner, a pressure roller  22  that presses the recording sheet  17  sandwiched between the pressure roller  22  and the heat generating roller  21  and an exciting unit  23  provided along the outer surface of the heat generating roller  21  and containing an exciting coil  24  for induction-heating the heat generating roller  21 . 
   Thus, the image heating device  20  according to this embodiment provides the exciting unit  23  outside the heat generating roller  21  so that the external exciting unit  23  induction-heats the heat generating roller  21 . 
   Then, more specific configurations of the heat generating roller  21 , pressure roller  22  and exciting unit  23  will be explained. The heat generating roller  21  has a laminated structure formed of a hollow cored bar  21   a  made of aluminum or the like, a magnetic layer  21   b  made of an insulating material and a sponge layer  21   c  having high thermal insulating property and elasticity. 
   Furthermore, a heat generating belt  21   d  is provided on the surface of the heat generating roller  21 . The heat generating belt  21   d  consists of an aluminum base material as a dielectric heat generating layer with an elastic layer and mold releasing layer formed thereupon in that order. It is also possible to use any one of metal materials such as copper, silver, nickel, stainless steel and iron instead of using aluminum as the base material of the heat generating belt  21   d . Or it is also possible to use a composite material made up of a plurality of these metal materials. Or it is also possible to use a composite material made up of at least one of these metal materials and resin such as polyimide. 
   The heat generating belt  21   d  may also be adhered to the sponge layer  21   c  as one body or may be simply attached onto the outer surface of the sponge layer  21   c . Moreover, an induction heating layer may also be formed directly on the sponge layer  21   c.    
   The pressure roller  22  is constructed of a cored bar  22   a  and a silicon rubber layer  22   b  and pressed against the heat generating belt  21   d  to form a fixing nip section. The pressure roller  22  is rotated and driven by a driving section (not shown) of the device body. In this way, the heat generating roller  21  rotates driven by the rotation of the pressure roller  22  and the recording sheet  17  sandwiched between the heat generating roller  21  and pressure roller  22  is moved in the direction indicated by an arrow a in the figure. At this time, the toner image  18  on the recording sheet  17  is heated by the heat generating belt  21   d , pressurized by the heat generating roller  21  and pressure roller  22  and thereby fixed. 
   The exciting unit  23  has an arc-shaped cross section as a whole. Aback core  25  is provided on its outer surface and a coil holding member  26  is provided on its inner surface and an exciting coil  24  is provided between the back core  25  and coil holding member  26 . 
   The exciting coil  24  is formed of a predetermined number of surface-insulated conductive wire members bundled together and extended around the heat generating roller  21  in the axial direction. In other words, the exciting coil  24  is provided in the circumferential direction of the heat generating belt  21   d  so as to cover and surround the heat generating belt  21   d  in close contact with each other. The ends of the exciting coil  24  are heaped with the overlapped wire bundle and look saddle-shaped as a whole. The exciting coil  24  is preferably placed at a distance of approximately 3 mm from the outer surface of the heat generating belt  21   d.    
   The back core  25  is principally made of ferrite and consists of a central core  25   a  placed on the inner surface around the coil, an arch-shaped arch core  25   b  and an end core  25   c  placed on the outer surface of the exciting coil  24 . As shown in  FIG. 5  which is viewed from the direction indicated by an arrow E in  FIG. 3 , a predetermined number (e.g., 7) of arch cores  25   b  are arrayed on the back of the exciting coil  24  with a certain space in between. The central core  25   a , end core  25   c  and arch core  25   b  which are continuous in the axial direction each consist of a combination of a plurality of members. As the material of the back core  25 , a material with high magnetic permeability and high resistivity such as permalloy is preferable in addition to ferrite. 
   The coil holding member  26  is made of resin with high heatproof temperature such as PEEK (polyether ether ketone) material or PPS (polyphenylene sulfide) preferably of approximately 1.5 mm in thickness and holds the exciting coil  24 . 
   Here, the induction heating operation of the heat generating belt  21   d  by the exciting unit  23  will be explained using  FIG. 4  and  FIG. 5 . 
   An AC current having a predetermined frequency is applied from an exciting circuit  27  ( FIG. 5 ) to the exciting coil  24 . This frequency is selected preferably from a frequency range of approximately 20 to 100 kHz according to the material of the base material of the heat generating belt  21   d . For example, in the case where the heat generating belt  21   d  is an aluminum base material, a frequency of approximately 60 kHz is selected. 
   The AC current applied to the exciting coil  24  is controlled by a temperature signal obtained from a temperature sensor  28  ( FIG. 5 ) so that the surface of the heat generating belt  21   d  is set to approximately 170° C. which is a predetermined fixing set temperature. 
   Here, magnetic flux generated by the exciting coil  24  through the AC current from the exciting circuit  27  penetrates the heat generating belt  21   d  from the end core  25   c  and reaches the magnetic layer  21   b  as shown by a dashed line M in  FIG. 4 . Due to the magnetism of the magnetic layer  21   b , the magnetic flux M penetrates the magnetic layer  21   b  in the circumferential direction. Then, the magnetic flux M forms an alternating field forming a loop which penetrates the heat generating belt  21   d  again and passes through the central core  25   a . The induced current generated by the change of this magnetic flux passes through the base material layer of the heat generating belt  21   d  and generates joule heat. Since the magnetic layer  21   b  has insulating properties, it is not induction-heated. 
   Furthermore, since the magnetic flux M does not reach the cored bar  21   a  of the heat generating roller  21 , induction heating energy is never used directly for heating of the cored bar  21   a . Furthermore, since the heat generating belt  21   d  is held with the highly thermal insulating sponge layer  21   c , less heat leaks from the heat generating belt  21   d . For this reason, the thermal capacity of the heated part is small and has low thermal conductivity, and it is therefore possible to heat the heat generating belt  21   d  up to a desired temperature (e.g., fixing set temperature) in a short time. 
   Then, the mounting structure of the exciting unit  23  and heat generating roller  21  of this embodiment will be explained using  FIG. 6 .  FIG. 6  shows the cross section along a line A–A′ in  FIG. 5  of the exciting unit  23  as well as the mounting part of the exciting unit  23  and heat generating roller  21 . 
   The heat generating roller  21  has a structure in which its rotation axis  21   e  is supported by a bearing  31  which is fixed to a unit chassis  30  of the image heating device  20  in a rotatable manner. The unit chassis  30  also holds the pressure roller  22  as one unit and forms a fixing unit detachable to the body of the device. A positioning section  32  is provided at the end of the exciting unit  23 . The positioning section  32  and bearing  31  determine the position of the exciting unit  23  relative to the position of the heat generating roller  21 . 
   In addition to such a configuration, a shock-absorbing member  34  is provided between the bearing  31  and positioning section  32 . As this shock-absorbing member  34 , for example, fluorine-based or silicon-based heat resistant rubber is used. The material of the shock-absorbing member  34  will be described in detail later. 
   The exciting unit  23  is placed under pressure of a pressure spring  33  attached to the positioning section  32  in such a way as to approach the heat generating roller  21 . In this way, the distance between the heat generating roller  21  and exciting unit  23  is determined by the positions of the bearing  31  and positioning section  32  which are pressed against each other through the shock-absorbing member  34  with the exciting unit  23  being pressed by the pressure spring  33 . The heat generating roller  21  and exciting unit  23  are actually positioned in such a way that the distance between the surface of the heat generating roller  21  (that is, heat generating belt  21   d ) and the exciting coil  24  in the exciting unit  23  is preferably approximately 3 mm. 
   This positioning structure will be explained in further detail using  FIG. 7 .  FIG. 7  shows a cross section along a line B–B′ in  FIG. 6 . The positioning section  32  is regulated by a slide guide  35  provided in the body of the image heating device  20  and movable only in the direction in which it approaches the heat generating roller  21 , in other words, only in the direction of the radius of the heat generating roller  21 . 
   Furthermore, the surface of the bearing  31  (that is, the surface facing the positioning section  32  with the shock-absorbing member  34  in between) and the surface of the positioning section  32  facing the bearing  31  with the shock-absorbing member  34  in between form a circumferential surface along the circumferential surface of the heat generating roller  21  (that is, heat generating belt  21   d ), that is, the circumferential surface parallel to the circumferential surface of the heat generating roller  21  (or heat generating belt  21   d ). Then, the shock-absorbing member  34  is provided between the circumferential surfaces of the bearing  31  and positioning section  32 . In this way, even if the positioning section  32  is shifted slightly in the direction indicated by an arrow d or arrow d′, it is possible to prevent the exciting unit  23  from contacting the heat generating belt  21   d  and keep the distance between the exciting unit  23  and the heat generating belt  21   d  to a predetermined distance. 
   Here, suppose a case where the surface of the bearing  31  and the surface of the positioning section  32  are formed of flat surfaces. In this case, if a positional shift is produced in the direction orthogonal to the rotation axis of the heat generating roller  21 , the end of the exciting unit  23  may possibly contact the surface of the heat generating belt  21   d , damaging the heat generating belt  21   d . This is because the heat generating belt  21   d  and the surface of the exciting unit  23  facing the heat generating belt  21   d  have mutually circumferential shapes. Therefore, this embodiment forms the surface of the bearing  31  and the surface of the positioning section  32  facing the bearing  31  with the shock-absorbing member  34  in between in a shape conforming to the circumferential surface of the heat generating roller  21 . This can reliably avoid the above described trouble. 
   Both the shock-absorbing member  34  and sponge layer  21   c  have elasticity and the relationship between coefficients of these elastic moduli is preferably shock-absorbing member&gt;sponge layer. With regard to hardness, a material with a hardness level of approximately 20 degrees to 80 degrees according to the JIS (Japanese Industrial Standards)-A can be used for the shock-absorbing member  34  and approximately 30 degrees to 70 degrees is preferably used. When the shock-absorbing member  34  is too soft (e.g., when hardness is smaller than approximately 20 degrees), the gap between the exciting coil  24  and the heat generating layer of the heat generating belt  21   d  is liable to fluctuate, and on the contrary when the shock-absorbing member  34  is too hard (e.g., when hardness is greater than approximately 80 degrees), the buffering action decreases. 
   On the other hand, a material with a degree of hardness of approximately 20 degrees to 50 degrees according to Asker-C (hardness specified by the standard of the Society of Rubber Industry, Japan) can be used for the sponge layer  21   c  and approximately 30 degrees to 50 degrees is preferable. If the sponge layer  21   c  is too soft (e.g., when hardness is lower than approximately 20 degrees), it is not possible to apply a sufficient pressure at the fixing nip section and when the sponge layer  21   c  is too hard (e.g., when hardness is greater than approximately 50 degrees), it is not possible to secure the sufficient nip width. 
   (3) Operation of Embodiment 
   In the above described configuration of the image heating device  20 , the positioning section  32  and the bearing  31  are pressed against each other through the shock-absorbing member  34  in between, and in this way the distance between the heat generating belt  21   d  to be heated and the exciting unit  23  is determined. Since the bearing  31  of the heat generating roller  21  is used for positioning, the relative positions of the heat generating roller  21  and exciting unit  23  remain unchanged, the distance between the heat generating roller  21  and exciting unit  23  can be kept to a predetermined distance. 
   This makes it possible to keep the distance between the exciting coil  24  in the exciting unit  23  and the heat generating belt  21   d  constant, allow magnetic flux generated at the exciting coil  24  to enter the heat generating belt  21   d  efficiently and accurately and heat the heat generating belt  21   d  efficiently and accurately. 
   In this condition, if a high-frequency current is passed from the exciting circuit  27  into the exciting coil  24  as an exciting current, the heat generating belt  21   d  is induction-heated by an alternating field. At this time, an induced current flows through the heat generating belt  21   d  in the direction opposite the direction of the exciting current which is always passing through the exciting coil  24  due to a mutual induction action. Then, because of Fleming&#39;s left-hand rule, repulsive forces which act in mutually repelling directions are generated between the exciting coil  24  and heat generating belt  21   d . The magnitude of the repulsive forces is proportional to the square of the exciting current. Furthermore, the vibration force caused by this electromagnetic repulsive force has a frequency approximately twice the frequency of the exciting current (exciting frequency). 
   Furthermore, when the DC power supply output voltage of the power supply of the exciting current is a pulsating current including a ripple component, the exciting current is subjected to amplification modulation by the ripple component, and therefore a component having substantially the same frequency as the ripple frequency is also generated in the vibration force. The ripple frequency varies depending on the circuit configuration of the DC power supply. In the case of a DC power supply using a full-wave rectifying circuit, the ripple frequency is double the AC input frequency and in the case of a DC power supply using a half-wave rectifying circuit, the ripple frequency is the same as the AC input frequency. 
   For example, suppose the exciting circuit  27  generates a high-frequency current of 20 kHz using a DC power supply resulting from full-wave rectification of 60 Hz AC input, supplies the high-frequency current to the exciting coil  24  and thereby drives the exciting coil  24 . In this case, a vibration force of 120 Hz (same frequency as the ripple frequency caused by the ripple component of the high-frequency current) and a vibration force of 40 kHz (frequency double the exciting frequency caused by electromagnetic repulsive force) are generated between the exciting coil  24  and heat generating belt  21   d.    
   However, since the vibration caused by this vibration force is absorbed by the shock-absorbing member  34 , the vibration amplitude of the heat generating roller  21  is prevented from expanding. When the loss factorloss factor (a kind of index indicating vibration absorption performance) of the shock-absorbing member  34  at the vibration frequency falls below approximately 0.01, almost no vibration is absorbed by the shock-absorbing member  34 . Therefore, in order for optimal vibration absorption by the shock-absorbing member  34  to take place, the loss factorloss factor should be approximately 0.01 or above or preferably approximately 0.1 or above. A loss factorloss factor for each specific preferred material is approximately 0.05 to approximately 0.15 for natural rubber, approximately 0.15 to approximately 0.3 for chloroprene, approximately 0.25 to approximately 0.4 for nitrile rubber, approximately 0.15 to approximately 0.3 for styrene-butadiene rubber and approximately 0.25 to approximately 0.4 for butyl rubber, etc. In addition, various resin materials or viscoelastic materials having a loss factor of approximately 0.01 or above can be used as materials for the shock-absorbing member  34 . 
   However, when the material is actually selected, it is necessary to consider influences of the operating temperature. In the case of the shock-absorbing member  34  used for the image heating device  20 , a temperature rise during operation is unavoidable due to heat transmission from the heat generating belt  21   d . Therefore, a resin material or viscoelastic material displaying vibration absorption performance of a certain level or higher (having a loss factor of approximately 0.01 or above in this embodiment) at an arbitrary operating temperature is selected. 
   A high polymer material such as rubber and resin, etc., generally shows similar frequency characteristics of viscoelasticity when the temperature rises and when the vibration frequency decreases. Here,  FIG. 8  shows a frequency characteristic of a loss factor of a vibration absorption material principally composed of styrene-butadiene rubber. The data shown in  FIG. 8  is obtained by converting a test result on a loss factor of styrene-butadiene rubber described in “Elastomers for damping over wide temperature ranges” (Owens, F. S., AFML-TR-68-179, Wright-Patterson AFB, Ohio, 1968) to a case of approximately 20° C. and a case of approximately 50° C. The horizontal axis X 1  in  FIG. 8  corresponds to the vibration frequency in the case of approximately 20° C. The frequency characteristic graph (curve) shifts rightward as the temperature rises. This is equivalent to the frequency axis shifting leftward as the temperature rises. Therefore, in  FIG. 8 , the horizontal axis X 2  corresponds to the vibration frequency in the case of approximately 50° C. 
   In  FIG. 8 , point P indicates the loss factor in the case of approximately 20° C. corresponding to the vibration force having a frequency (e.g., 40 kHz) approximately double the exciting frequency. Point Q indicates the loss factor in the case of approximately 20° C. corresponding to the vibration force having the loss factor substantially the same as the ripple frequency (e.g., 120 Hz). Both the loss factor at point P and the loss factor at point Q exceed 0.01 and the loss factor at point Q even exceeds 0.1. Therefore, the vibration absorption material principally composed of styrene-butadiene rubber (or other material having a similar nature) at least at a normal temperature (e.g., approximately 20° C.) can carry out the vibration absorption function of the shock-absorbing member  34  sufficiently. It effectively functions for vibration caused by the ripple component of a high-frequency current in particular. 
   Here, suppose the temperature of the shock-absorbing member  34  has risen to approximately 50° C. which is a somewhat higher temperature than a normal temperature due to heat transmission from the heat generating belt  21   d . Point P′ and point Q′ indicate loss factors for this case. Point P′ indicates a loss factor in the case of 50° C. corresponding to a vibration force having a frequency approximately twice the exciting frequency (e.g., 40 kHz) and point P′ indicates a loss factor in the case of approximately 50° C. corresponding to a vibration force having a frequency (e.g., 120 Hz) substantially the same as the ripple frequency. Both exceed the loss factor in the case of approximately 20° C. (that is, loss factor at point P and loss factor at point Q). 
   That is, in the case of a vibration absorption material principally composed of styrene-butadiene rubber (or other material having a similar nature), even if the temperature increases at least near a normal temperature, the vibration absorption performance can be expected to improve. This can be realized when a vibration absorption material characterized in that the frequency giving a maximum loss factor at a normal temperature is smaller than the frequency of the vibration force is adopted. 
   Furthermore, in the case of a vibration absorption material principally composed of styrene-butadiene rubber (or other material having a similar nature), even if the temperature further rises and the frequency axis is further shifted leftward, the loss factor never falls below 0.01. For this reason, the vibration absorption material principally composed of styrene-butadiene rubber (or other material having a similar nature) has excellent vibration absorption performance for a further temperature rise. Thus, when a material having a loss factor of approximately 0.01 or above in a frequency area lower than the frequency of the vibration force in question at a normal temperature is used, it is possible to maintain an excellent vibration absorption effect even if a drastic temperature rise occurs. 
   As a result, the image heating device  20  is free of such problems that the heat generating belt  21   d  vibrates and disturbs a non-fixed toner image  18  or changes the rotation speed causing jitter, and is therefore easy to handle and can provide a high definition image. 
   Furthermore, the shock-absorbing member  34  can be placed in a location apart from the heat generating area, and therefore it is possible to provide the shock-absorbing member  34  with relatively low heat resistance (that is, low heat resistance compared to the vibration absorption member placed in the area which is directly heated by electromagnetic induction) and therefore use a relatively inexpensive material for the shock-absorbing member  34 . 
   When aged deterioration, etc., occurs in the heat generating belt  21   d , the image heating device  20  of this embodiment is designed to be able to leave the exciting unit  23  in the main body and remove and replace the heat generating roller  21  and pressure roller  22  together with the bearing  31  and unit chassis  30  as a fixing unit. When these components are replaced, the elastic force of the pressure spring  33  of the exciting unit  23  is held by a stopper (not shown). In this condition, the fixing unit including the heat generating roller  21  is removed and a new fixing unit is attached instead of this. When the new fixing unit is attached, it is set in a predetermined position while pressing the shock-absorbing member  34  by means of the bearing  31 . In this condition, positioning is performed with the shock-absorbing member  34  contacting the bearing  31  by the pressure spring  33 . 
   In this way, with the image heating device  20 , it is possible to leave the exciting unit  23  in the main body and easily replace the fixing unit including the heat generating roller  21 . Even after the replacement, the respective components can be positioned at exact positions through the positioning section  32 , bearing  31  and pressure spring  33 . 
   (4) Effects of Embodiment 
   According to the above described configuration, by providing the exciting unit  23  outside the heat generating roller  21  provided with the heat generating belt  21   d  to be heated, forming the positioning section  32  at the end of this exciting unit  23 , further providing the shock-absorbing member  34  between the positioning section  32  and contact member (bearing  31  in this embodiment) to thereby keep the distance between the exciting unit  23  and heat generating roller  21  to a predetermined distance, it is possible to realize the image heating device  20  in a simple configuration capable of reliably keeping the distance between the exciting coil  24  provided inside the exciting unit  23  and the heat generating belt  21   d  provided in the heat generating roller  21  to a predetermined value and reliably preventing vibration transmission from the exciting unit  23  to the heat generating roller  21 . 
   (Embodiment 2) 
     FIG. 9  shows a configuration an image heating device according to Embodiment 2 of the present invention. A image heating device  40  shown in  FIG. 9  has a basic configuration similar to that of the image heating device  20  in  FIG. 3  explained in Embodiment 1 and the same or corresponding components are assigned the same reference numerals and detailed explanations thereof will be omitted. 
   In the image heating device  40 , a heat generating belt  43  is not formed so as to be wound around the surface of an auxiliary roller  41 , but formed so as to be run between an auxiliary roller  41  and a fixing roller  42 . That is, the heat generating belt  43  is induction-heated at the position of the auxiliary roller  41  by an exciting unit  23  and the heated heat generating belt  43  is designed to heat a toner image  18  on a recording sheet  17  at the position of the fixing roller  42 . 
   For the auxiliary roller  41 , it is possible to use induction-heated magnetic metal such as iron and SUS, insulating material such as heat-resistant resin or highly resistant or insulating magnetic material such as ferrite. The fixing roller  42  has a structure with sponge made of foamed silicon rubber laminated on a cored bar. 
     FIG. 10  shows a mounting structure of the exciting unit  23 , auxiliary roller  41  and fixing roller  42  of this embodiment.  FIG. 10  shows the mounting part of the exciting unit  23 , auxiliary roller  41  and fixing roller  42  in addition to the cross section along a line C–C′ in  FIG. 9  of the exciting unit  23 , auxiliary roller  41  and fixing roller  42 . 
   The auxiliary roller  41  is mounted on a bearing  50  in a rotatable manner and the fixing roller  42  is mounted on a bearing  51  in a rotatable manner. Furthermore, the bearing  50  and bearing  51  are biased by a spring  52  in the direction in which both bearings go away from each other. Through the spring force of the spring  52 , the heat generating belt  43  is run between the auxiliary roller  41  and fixing roller  42  without flexure. 
   In addition to such a configuration, as in the case of Embodiment 1, the auxiliary roller  41  and exciting unit  23  are placed in such a way that the bearing  50  of the auxiliary roller  41  and a positioning section  32  are pressed against each other through a shock-absorbing member  34 . This makes it possible to hold the distance between the auxiliary roller  41  and exciting unit  23  to a predetermined distance and prevent transmission of micro vibration from the exciting unit  23  to the fixing roller  42 . 
   That is, in the image heating device  40  according to this embodiment, the auxiliary roller  41  and fixing roller  42  are connected through the heat generating belt  43  and the respective bearings  50  and  51 , and transmission of micro vibration from the exciting unit  23  to the fixing roller  42  is prevented by the shock-absorbing member  34 . In this way, when the auxiliary roller  41  receives vibration from the exciting unit  23 , it is possible to reduce the possibility that the fixing roller  42  may vibrate and disturb a non-fixed toner image or the possibility that the rotation speed may fluctuate and produce jitter. 
   When the heat generating belt  43  to be heated is run between the auxiliary roller  41  and the fixing roller  42 , and this heat generating belt  43  is induction-heated with the exciting unit  23  provided outside the auxiliary roller  41 , the above described configuration forms the positioning section  32  at the end of this exciting unit  23 , provides the shock-absorbing member  34  between the positioning section  32  and contact member (bearing  50  in this embodiment) and keeps the distance between the exciting unit  23  and auxiliary roller  41  to a predetermined distance, and can thereby realize the image heating device  40  in a simple configuration capable of reliably keeping the distance between the exciting coil  24  and heat generating belt  43  provided inside the exciting unit  23  to a predetermined value and reliably prevent vibration transmission from the exciting unit  23  to the fixing roller  43 . 
   (Other Embodiments) 
   Embodiments 1 and 2 have described the case where the bearings  31  and  50  of the heat generating roller  21  and the auxiliary roller  41  are used as the contact members which the positioning section  32  contacts through the shock-absorbing member  34 . However, the contact members which contact the positioning section  32  are not limited to the bearings of the heat generating roller  21  and the auxiliary roller  41 . In brief, any member is acceptable if it can at least keep the distance between the heat generating roller  21  or auxiliary roller  41  and the exciting unit  23  to a predetermined value when the positioning section  32  contacts it through the shock-absorbing member  34 . 
   Furthermore, Embodiments 1 and 2 have shown the case where the shock-absorbing member  34  is provided between the positioning section  32  and the bearings  31  and  50 . However, the present invention is not limited to this. For example, it is also possible to provide the shock-absorbing member  34  in the joint between the exciting unit  23  and positioning section  32 . In this case, too, it is possible to prevent vibration transmission from the exciting unit  23  to the heat generating belts  21   d  and  43  or fixing roller  42 . Furthermore, in this case, there is no need to place the shock-absorbing member  34  between the positioning section  32  and contacting member (bearings  31  and  50  in Embodiments 1 and 2) Therefore, it is possible to keep the distance between the exciting unit  23  and heat generating roller  21  or auxiliary roller  41  more accurately and stably. 
   As described above, according to the present invention, by providing the exciting unit having the exciting coil outside the heat generating member, providing the positioning section which keeps the distance between this exciting unit and heat generating member and providing the shock-absorbing member at the position of this positioning section, it is possible to realize an image heating device in a simple configuration capable of accurately and reliably positioning the distance between the heat generating member and exciting coil and reliably preventing vibration transmission from the exciting coil to the heat generating member. 
   The present invention is not limited to the above described embodiments, and various variations and modifications may be possible without departing from the scope of the present invention. 
   This application is based on the Japanese Patent Application No.2003-040823 filed on Feb. 19, 2003, the entire content of which is expressly incorporated by reference herein.