Patent Publication Number: US-6212356-B1

Title: Fixing apparatus

Description:
FIELD OF TECHNOLOGY 
     The present invention relates to a fixing apparatus which is used for fusing and pressing an unfixed toner carried on a recording medium to fix the toner to the recording medium. 
     TECHNICAL BACKGROUND 
     Recently, a fixing apparatus for use in an electric image forming apparatus has been developed which is disclosed in Japanese Patent Laid-Open Publication HEI 9-73243, for example, not employing a conventional heating roller type fixing system. The conventional fixing process is provided with a heat accumulation step of accumulatively heating a thin heating medium on upstream side of a fixing stage with respect to a feeding direction of the heating medium which is fed along the fixing stage, a heat/press step of clamping the heating medium and a recording medium to contact with each other, being sandwiching an unfixed toner image on the recording medium therebetween in the fixing stage after the heat accumulating step to heat and fuse the unfixed toner image on the recording medium using the heat accumulated in the heating medium as well as to press it to fix the toner image onto the recording medium, a peeling step of peeling off the fixed recording medium from the heating medium after the heat/press step. The aim to constitute such a conventional fixing process is to enable to quickly start, to correspond to a high-speed type machine, to eliminate an offset phenomenon in a case of full-color image, to obtain the color image with good quality, to reduce the electric power consumption, and to respond to a toner image having a different glossiness. Especially, in the conventional fixing process, it is disclosed that a preheat step of preheating the recording medium on the upstream side of the fixing stage with respect to the feeding direction of the recording medium is further provided. 
     In the conventional fixing apparatus as described above, the heating medium is constructed so as not to discharge the heat energy being accumulating the heat energy therein prior to the fixing stage, and to heat the toner image on the recording medium using the heat energy accumulated in the heating medium at the heat/press step (fixing step). Accordingly, with respect to a speed of heat transfer which is defined from the heater housed in the heating roller through the fixing belt (heat transfer belt) to the recording medium at the heat/press step (fixing step), it was difficult to accelerate a transfer speed of the recording medium which passes through the fixing stage. Even though the recording medium has been preheated prior to the fixing stage, it was also difficult to accelerate the transfer speed of the recording medium. Thus, it was difficult to accelerate the transfer speed of the recording medium to increase the copying number per a unit time (i.e., per one minute) even though the conventional technical skill disclosed in the aforesaid Laid-Open Publication was applied. As a result, it causes to set a nip width at the fixing stage to be large, whereby it was in danger of occurring the other bad influence in the fixing apparatus. 
     On the other hand, a fixing apparatus for an electro-photographic machine disclosed in Patent Laid-Open Publication HEI 8-2865335 is known as a preceding patent application with the same applicant as the present patent application. In the preceding patent application, a fixing belt is trained around the fixing roller and the heating roller and is constructed to preheat the unfixed toner formed on the recording sheet prior to pass a nip portion defined between the fixing roller and the pressing roller, using the heat radiation from the fixing belt which is heated by the heat conduction from the heating roller. Since the temperature of the heating roller is so controlled as to maintain the temperature thereof to a constant value, it is possible to accomplish the preheat effect certainly. 
     However, the atmosphere temperature in a preheat space which is defined from a space between the fixing belt and a guide plate for guiding the sheet with the unfixed toner image thereon to the nip portion and being provided under the fixing belt varies widely due to the influence of the temperature of the guide plate. Furthermore, the sheet with the unfixed toner image thereon guided on the guide plate would be preheated by a contact with the guide plate. However, the effect of the preheat from the guide plate will depend on the temperature of the guide plate. Please note that the temperature of the guide plate is different between a case just after a warming up operation is finished and a case in a continuous feeding period, depending on feed modes, as shown in a Table 1. Also, the temperature of the guide plate is different between at least a case in which the image is of mono-color and a case in which the image is of full-color, depending on the thickness (t) of the guide plate, as shown in FIG.  16 . Thus, the preheat effect by the guide plate is not maintained certainly, but conversely a problem due to the bad influence of the preheat by the guide plate has been pointed out. It should be noted that the numerals indicated in the table 1 are temperature (°C.) at each measuring point. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                 mono- 
               
               
                   
                   
                 full-color 
                 color 
               
               
                   
                   
                 mode 
                 mode 
               
               
                   
                 idle rotating time after warming up 
                 feeding 
                 feeding 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 object to be 
                 initial 
                 just after 
                 1 minute 
                 3 minutes 
                 5 minutes 
                 7 minutes 
                 10 minutes 
                 speed 
                 speed 
               
               
                   
                 measured 
                 time 
                 warming up 
                 later 
                 later 
                 later 
                 later 
                 later 
                 (5 cpm) 
                 (20 cpm) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Guide plate 
                 at first 
                 heating roller 
                 28 
                 155 
                 158 
                 161 
                 161 
                 162 
                 162 
                   
                   
               
               
                 (Thickness 
                 in 
                 fixing roller 
                 28 
                 155 
                 160 
                 163 
                 165 
                 166 
                 167 
               
               
                 1.6 mm) 
                 morning 
                 pressing rollerg 
                 28 
                 130 
                 140 
                 149 
                 153 
                 155 
                 157 
               
               
                   
                   
                 guide plate 
                 28 
                  44 
                  58 
                  72 
                  77 
                  80 
                  83 
               
               
                   
                 stand-by 
                 heating roller 
                 173  
                 159 
                 160 
                 161 
                 162 
                 162 
                 162 
                 163 
                 156 
               
               
                   
                   
                 fixing roller 
                 70 
                 155 
                 160 
                 163 
                 166 
                 167 
                 168 
                 159 
                 154 
               
               
                   
                   
                 pressing roller 
                 53 
                 126 
                 141 
                 150 
                 154 
                 156 
                 158 
                 151 
                 102˜116 
               
               
                   
                   
                 guide plate 
                 51 
                  56 
                  66 
                  78 
                  82 
                  84 
                  85 
                  64 
                  49 
               
               
                 Guide plate 
                 at first 
                 heating roller 
                 26 
                 156 
                 159 
                 159 
                 159 
                 159 
                 161 
                 156 
                 155 
               
               
                 (Thickness 
                 in 
                 fixing roller 
                 26 
                 148 
                 155 
                 158 
                 159 
                 161 
                 161 
                 154 
                 147 
               
               
                 0.5 mm) 
                 morning 
                 pressing roller 
                 26 
                 128 
                 137 
                 146 
                 150 
                 152 
                 153 
                 144 
                 102 
               
               
                   
                   
                 guide plate 
                 26 
                  74 
                 100 
                 109 
                 110 
                 112 
                 113 
                  79 
                  45 
               
               
                   
                 stand-by 
                 heating roller 
                 168  
                 160 
                 160 
                 159 
                 160 
                 160 
                 160 
                 155 
                 155 
               
               
                   
                   
                 fixing roller 
                 62 
                 149 
                 157 
                 161 
                 163 
                 163 
                 163 
                 153 
                 149 
               
               
                   
                   
                 pressing roller 
                 47 
                 139 
                 141 
                 150 
                 154 
                 156 
                 157 
                 144 
                 118 
               
               
                   
                   
                 guide plate 
                 58 
                  80 
                 112 
                 114 
                 115 
                 115 
                 116 
                  78 
                  48 
               
               
                 Guide plate 
                 at first 
                 heating roller 
                 32 
                 157 
                 159 
                 161 
                 162 
                 162 
                 163 
                   
                 157 
               
               
                 (Thickness 
                 in 
                 fixing roller 
                 32 
                 156 
                 161 
                 164 
                 166 
                 167 
                 167 
                   
                 154 
               
               
                 0.2 mm) 
                 morning 
                 pressing roller 
                 32 
                 129 
                 140 
                 149 
                 154 
                 156 
                 157 
                   
                  96˜109 
               
               
                   
                   
                 guide plate 
                 32 
                  98 
                 131 
                 138 
                 140 
                 141 
                 142 
                   
                 48˜52 
               
               
                   
                 stand-by 
                 heating roller 
                 171  
                 160 
                 161 
                 162 
                 162 
                 163 
                 163 
                 163 
                 160 
               
               
                   
                   
                 fixing roller 
                 73 
                 156 
                 162 
                 165 
                 167 
                 168 
                 168 
                 166 
                 156 
               
               
                   
                   
                 pressing roller 
                 59 
                 127 
                 143 
                 151 
                 154 
                 156 
                 157 
                 152 
                 120 
               
               
                   
                   
                 guide plate 
                 85 
                 102 
                 131 
                 140 
                 141 
                 142 
                 142 
                 104 
                 48˜54 
               
               
                   
               
            
           
         
       
     
     On the other hand, where fixing apparatuses each of which has temperature characteristics of the heating roller, the fixing roller and the pressing roller as shown in FIGS. 4 through 6, in a case that the sheet feeding speed is changed among 40 mm/sec, 80 mm/sec and 120 mm/sec, are utilized, a glossiness of the fixed image on the sheet after the nip portion is totally different among a case just after a warm up operation in the morning (that is, from a cold condition), a case just after a warm up operation from a stand-by mode (that is, from a condition in which the temperature of the fixing roller  22  is set below 60 (°C.)), and a case of a stable feeding mode (that is, just after the fixing apparatus has been actuated for 6 minutes), as shown in FIGS. 17 through 19, respectively. In other words, there is problems in which the glossiness of the fixed toner image is not stable, the quality of the fixed toner image could not be maintained to be good, when the feeding speed of the sheet is changed. 
     When the temperature of the heating roller is maintained in a constant value so that a predetermined glossiness would be given just after the warm up operation (that is, in a case where the temperature of the guide plate is set to be low) is finished, the excessive heat will be given to the unfixed toner and the recording medium at the stable feeding mode (that is, the temperature of the guide plate is set to be high). Accordingly, the fixing apparatus has not a fixable area in reserve with respect to the high-temperature offset. As a result, it is necessary to apply an oil onto the outer peripheral surface of the fixing belt. Therefore, it has been desired to obviate this drawback. 
     Accordingly, it is a primary object of the present invention to provide a fixing apparatus which is capable of ensuring the increase of the feeding speed of the recording medium, by effectively accomplishing to preheat the recording medium. 
     It is another object of the present invention to provide a fixing apparatus which is capable of enhancing the preheat effect by preheating uniformly the recording medium. 
     It is different object of the present invention to provide a fixing apparatus which is capable of maintaining the good quality of the toner image even though the feeding speed of the recording medium is increased. 
     DISCLOSURE OF THE INVENTION 
     In order to solve the aforementioned problems and accomplish the above object, an inventive fixing apparatus comprises a fixing roller, and a pressing roller being rollingly contacted the fixing roller by a predetermined pressure, wherein an unfixed toner image which is carried on a sheet is fixed thereon by passing the sheet, on which the toner image is carried, through a nip portion between the fixing roller and the pressing roller along one direction, and further comprises a first heat generating means provided so as to be remote from said fixing roller, a heating roller provided so as to be remote from said fixing roller, a fixing belt endlessly trained around said heating roller and said fixing roller, for heating said sheet passing through said nip portion upon receiving the heat from said first heat generating means, a supporting member fixed on an upstream side of said pressing roller with respect to said one direction, for supporting the undersurface of said sheet prior to passing through said nip portion, and second heat generating means for heating said supporting member to preheat said sheet prior to pass through said nip portion. 
     In the inventive fixing apparatus, said supporting member includes one end neighboring said nip portion and the other end so as to be remote from said nip portion, and said the other end of the supporting member is set to be lower than said one end thereof. 
     In the inventive fixing apparatus, said supporting member is formed so as to be substantially flat. 
     In the inventive fixing apparatus, said supporting member is electrically grounded. 
     In the inventive fixing apparatus, said first heat generating means is housed in said heating roller. 
     In the inventive fixing apparatus, said first heat generating means includes a halogen lamp. 
     In the inventive fixing apparatus, said first heat generating means include a plate-like heat generating member attached to an inner peripheral surface of said heating roller. 
     In the inventive fixing, said first heat generating means is arranged so as to be out of said heating roller and opposing said fixing belt. 
     In the inventive fixing apparatus, said first heat generating means includes a plate-like heat generating member attached to an outer peripheral surface of said heating roller. 
     In the inventive fixing apparatus, said first heat generating means includes an induction heating means for heating said fixing belt by induction heating. 
     In the inventive fixing apparatus, said induction heating means is provided between said fixing roller and said heating roller, and heating that portion of said fixing belt which is positioned upstream side of said nip portion with respect to said running direction of said fixing belt. 
     In the inventive fixing apparatus, said fixing belt includes an endless belt and a separating layer coated on an outer circumferential surface of the endless belt base. 
     In the inventive fixing apparatus, said endless belt base is made of metal belt. 
     In the inventive fixing apparatus, said endless belt base is made of electro-formed nickel. 
     In the inventive fixing apparatus, said first heat generating means includes a halogen lamp opposing said fixing belt. 
     In the inventive fixing apparatus, said second heat generating means is constructed so as to heat said supporting member substantially over the surface thereof. 
     In the inventive fixing apparatus, said second heat generating means is housed in said supporting member. 
     In the inventive fixing apparatus, said second heat generating means is arranged so as to closely contact with the surface of said supporting member. 
     In the inventive fixing apparatus, said second heat generating means is attached to the under surface of said supporting member. 
     In the inventive fixing apparatus, said second heat generating means is attached to the upper surface of said supporting member. 
     In the inventive fixing apparatus, said second heat generating means is coated by a separating layer on its upper surface. 
     In the inventive fixing apparatus, said separating layer has an electro-conductivity. 
     In the inventive fixing apparatus, said second heat generating means is arranged below and remote from said supporting member. 
     In the inventive fixing apparatus, said second heat generating means is formed of a material which is capable of maintaining a predetermined shape, and said supporting member is constructed by said second heat generating means. 
     In the inventive fixing apparatus, said second heat generating means is coated by a separating layer on its upper surface. 
     In the inventive fixing apparatus, said separating layer has an electro-conductivity. 
     In the inventive fixing apparatus, said heating roller is arranged in a preheat effecting area in which the heat radiated from said fixing belt can preheat said sheet prior to pass through said nip portion. 
     In the inventive fixing apparatus, said heating roller is arranged out of a preheat effecting area in which the heat radiated from said fixing belt can preheat said sheet prior to pass through said nip portion. 
     In the inventive fixing apparatus, said fixing belt includes an endless belt and a separating layer coated on an outer circumferential surface of the endless belt base. 
     In the inventive fixing apparatus, said endless belt base is made of metal belt. 
     In the inventive fixing apparatus, said endless belt base is made of electro-formed nickel. 
     In the inventive fixing apparatus, said endless belt base is made of synthetic resin. 
     In the inventive fixing apparatus, said synthetic resin has an electro-conductivity. 
     In the inventive fixing apparatus, said endless belt base is made of polyimide. 
     In the inventive fixing apparatus, said polyimide has an electro-conductivity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front sectional view schematically showing a total construction of the fixing apparatus according to one embodiment of the present invention; 
     FIG. 2 is a side view of the heating roller which is used in the fixing apparatus shown in FIG. 1; 
     FIG. 3A is a front view showing a positional relationship of the supporting member which is used in the fixing apparatus shown in FIG.  1 : 
     FIG. 3B is a front view of the supporting member shown in FIG.  3 A: 
     FIG. 4 is a graph showing temperature-time characteristics of the heating roller, the fixing roller and the pressing roller where the sheet feeding speed is set to be 40 mm/sec, in a case just after a warm up operation in the morning first is finished, and a case just after a warm up operation from a stand-by mode is finished, in a fixing apparatus with the fixing belt: 
     FIG. 5 is a graph showing temperature-time characteristics of the heating roller, the fixing roller and the pressing roller where that the sheet feeding speed is set to be 80 mm/sec, in a case just after a warm up operation in the morning first is finished, and in a case just after a warm up operation from a stand-by mode is finished, in a fixing apparatus with the fixing belt: 
     FIG. 6 is a graph showing temperature-time characteristics of the heating roller, the fixing roller and the pressing roller where the sheet feeding speed is set to be 120 mm/sec, in a case just after a warm up operation in the morning first is finished, and a case just after a warm up operation from a stand-by mode is finished, in a fixing apparatus with the fixing belt: 
     FIG. 7 is a graph showing a change of the glossiness of the sheet where the sheet feeding speed is set to be 40 mm/sec and the temperature of the heating roller is changed, in a case just after a warm up operation in the morning first is finished, a case just after a warm up operation from a stand-by mode is finished, and a case of stably feeding of the sheet, in the fixing apparatus according to the present invention: 
     FIG. 8 is a graph showing a change of the glossiness of the sheet where the sheet feeding speed is set to be 80 mm/sec and the temperature of the heating roller is changed, in a case just after a warm up operation in the morning first is finished, a case just after a warm up operation from a stand-by mode is finished, and a case of stably feeding of the sheet, in the fixing apparatus according to the present invention: 
     FIG. 9 is a graph showing a change of the glossiness of the sheet where the sheet feeding speed is set to be 120 mm/sec and the temperature of the heating roller is changed, in a case just after a warm up operation in the morning first is finished, a case just after a warm up operation from a stand-by mode is finished, and a case of stably feeding of the sheet, in the fixing apparatus according to the present invention: 
     FIG. 10A is a front view of a first modification of the second heat generating means installed in the fixing apparatus according to the present invention: 
     FIG. 10B is a front view of a second modification of the second heat generating means installed in the fixing apparatus according to the present invention: 
     FIG. 10C is a front view of a third modification of the second heat generating means installed in the fixing apparatus according to the present invention: 
     FIG. 10D is a front view of a fourth modification of the second heat generating means installed in the fixing apparatus according to the present invention: 
     FIG. 11 is a sectional front elevational view of a fixing apparatus according to the other embodiment of the present invention; 
     FIG. 12 is a schematic view showing the manner in which a fixing roller and a pressing roller are held in rolling contact with each other in the other embodiment shown in FIG. 11; 
     FIG. 13 is a schematic front elevational view of an actuating mechanism in the other embodiment shown in FIG. 11; 
     FIG. 14 is a diagram showing angles employed in an experiment conducted to check an allowable range of positions of a heating roller with respect to the fixing roller in the other embodiment shown in FIG. 11; 
     FIG. 15A is a front view of the first heat generating means according to a first modification of the present invention; 
     FIG. 15B is a front view of the first heat generating means according to a second modification of the present invention: 
     FIG. 15C is a front view of the first heat generating means according to a third modification of the present invention; 
     FIG. 15D is a front view of the first heat generating means according to a fourth modification of the present invention; 
     FIG. 15E is a front view of the first heat generating means according to a fifth modification of the present invention; 
     FIG. 16 is a diagram showing a relationship between the drive condition of a conventional fixing apparatus and the temperature of the guide plate: 
     FIG. 17 is a graph showing a change of the glossiness of the sheet where the sheet feeding speed is set to be 40 mm/sec and the temperature of the heating roller is changed, in a case just after a warm up operation in the morning first is finished, a case just after a warm up operation from a stand-by mode is finished, and a case of stably feeding of the sheet, in a conventional fixing apparatus: 
     FIG. 18 is a graph showing a change of the glossiness of the sheet where the sheet feeding speed is set to be 80 mm/sec and the temperature of the heating roller is changed, in a case just after a warm up operation in the morning first is finished, a case just after a warm up operation from a stand-by mode is finished, and a case of stably feeding of the sheet, in a conventional fixing apparatus: and 
     FIG. 19 is a graph showing a change of the glossiness of the sheet where the sheet feeding speed is set to be 120 mm/sec and the temperature of the heating roller is changed, in a case just after a warm up operation in the morning first is finished, a case just after a warm up operation from a stand-by mode is finished, and a case of stably feeding of the sheet, in a conventional fixing apparatus. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, a construction of one embodiment of a fixing apparatus according to the present invention will be described in detail with reference to the accompanying drawings. 
     {General description of fixing apparatus  10 } 
     At first, as shown in FIG. 1, a fixing apparatus according to one embodiment of the present invention has a housing  12 , as a housing structure, to be fixed to a frame of an electronic image forming apparatus such as an electronic copying machine (not shown). The housing  12  is provided with a base plate  14  to be fixed directly to the frame, a pair of vertical mounting stays  16  directly erected from respective side edges of the base plate  14 , and an ceiling plate  18  connecting between the upper portions of the mounting stays  16 . 
     It should be noted that the left direction indicated by an arrow in the drawing is defined as a feeding direction in which an unfixed sheet S on which an unfixed toner image is carried is fed. That is to say, the aforementioned mounting stays  16  are positioned so as to stand on right and left sides of the unfixed sheet S which is to be fed in the feeding direction and each of the mounting stays  16  extends in the feeding direction. 
     The fixing apparatus  10  also has a roller assembly including a fixing roller  22  rotatably supported on the mounting stays  16  for being rotatable about its own central axis, a pressing roller  24  positioned below the fixing roller  22  in rolling contact therewith, rotatably supported on the mounting stays  16  for rotation about its own central axis and movably supported to the mounting stays  16  in a vertical direction, and a heating roller  28  arranged on the upstream side of the fixing roller  22  with respect to the feeding direction of the unfixed sheet S, being movably supported to the mounting stays  16  in the feeding direction and having a heating source  26  such as a halogen lamp therein. 
     Furthermore, the fixing apparatus  10  is provided with a supporting member  30  which is fixed on the upstream side of the pressing roller  24  with respect to the feeding direction, for supporting the under surface of the unfixed sheet S (that is, a surface on which the unfixed toner image is not formed) and guiding the unfixed sheet S to a rolling contact region (nipping region) defined between the fixing roller  22  and the pressing roller  24 , and an endless fixing belt  32  which is trained around the fixing roller  22  and the heating roller  28 . The gap between the supporting member  30  and the fixing belt  32  serves as a preheating passage P for preheating the unfixed sheet S from above and below. 
     The fixing apparatus  10  is also provided with an oil applying roller  36  for applying silicone oil to an outer circumferential surface of the fixing belt  32  and cleaning the outer surface of the fixing belt  32 , a tension lever  38  for urging the heating roller  28  so as to separate from the fixing roller  22 , thereby applying a predetermined tension to the fixing belt  32  in cooperation with the oil applying roller  36 , and a pressure lever  40  for urging the pressing roller  24  so as to approach it to the fixing roller  22 , thereby pressing the pressing roller  24  to the fixing roller  22  through the fixing belt  32 . 
     In the fixing apparatus  10  with such a construction, the unfixed sheet S which are transferred on the supporting plate  30  is further transferred to the rolling contact region in such a manner that the under surface on which the unfixed toner image is not formed is supported and guided by the supporting plate  30 . Since the unfixed sheet S is passing through the rolling contact region with a high pressure, the unfixed toner on the sheet S is fused and pressed onto the sheet S to be fixed thereon. 
     {Description of fixing roller  22 } 
     The fixing roller  22  includes a core  22 A rotatably supported in circular first supporting holes  16 A formed on the mounting stays  16  by bearings and a roller body  22 B which is fitted coaxially over the core  22 A and around which the fixing belt  32  is trained. The fixing roller  22  has an outside diameter of 20 mm in this embodiment. The core  22 A is formed by a shaft of iron having a diameter of 12 mm, and the roller body  22 B is made of a heat-resistant resilient material of silicone rubber (specifically, silicone rubber sponge having ASKER C hardness of 30 degrees) having a wall thickness of 4 mm. 
     It should be noted that a first driven gear (not shown) is fixed to the one end of the core  22 A being coaxial thereto. A driving gear of a driving mechanism (not shown) is meshed with the first driven gear. A driving force from the driving mechanism is transmitted to the first driven gear through the driving gear to rotate the fixing roller  22 . 
     {Description of pressing roller  24 } 
     The pressing roller  24  includes a core  24 A rotatably supported in elongated and arcuate second supporting holes  16 B, which is formed on the mounting stays  16  so as to extend vertically, by bearings and movable in the extending direction thereof, and a roller body  24 B fitted coaxially over the core  24 A. The pressing roller  24  has an outside diameter of 20 mm in this embodiment. The core  24 A comprises a shaft of iron having a diameter of 14 mm, and the roller body  24 B is made of a heat-resistant resilient material of silicone rubber (specifically, the roller body  24 B is made of silicone rubber sponge having a JIS Model A hardness of 40 degrees, which is harder than the fixing roller  22 ) having a wall thickness of 3 mm. 
     It should be noted that a second driven gear (not shown) is coaxially fixed to the one end of the core  24 A. The first driven gear is meshed with the second driven gear. The driving force is transmitted from the first driven gear to the second driven gear to rotate the pressing roller  24  in a direction opposite to the fixing roller  22  with the same rotating speed. 
     Here, in the fixing apparatus  10 , the transferring speed of the unfixed sheet S is set to be within a range from 10 mm/sec to 400 mm/sec, and the gap of the preheating passage P is set to be within a range from 0.5 mm to 10 mm, and various values are settled so as to satisfy condition wherein a preheating time for passing through a distance between the centers of fixing roller  22  and heating roller  28  is to be set within a range from 0.1 sec to 4 sec. 
     {Description of heating roller  28 } 
     In this embodiment, as the heat source  26  in the heating roller  28 , there is provided a 600W halogen lamp which is designed to emit light at an intensity that is 50% greater in opposite end regions than in a central region thereof. The heating roller  28  includes a core in the form of an aluminum pipe having a diameter of 20 mm and a wall thickness of 1.75 mm. The outer circumferential surface of the core is treated by a hard alumite. As shown in FIG. 2, a circular collar  48  made of heat-resistant polyetheretherketone (PEEK) and having a diameter of 24 mm is press-fitted over each of opposite supporting shaft portions of the heating roller for preventing the fixing belt  32  from being tortured or displaced out of position on the heating roller  28 . 
     {Description of supporting member  30 } 
     The fixing apparatus  10  is provided with the supporting plate  30  which serves as a guide plate for guiding the unfixed sheet S transferred by the transferring mechanism (not shown) to the rolling contacting region between the fixing roller  22  and the pressing roller  24 . The supporting member  30  is positioned on the left side of the pressing roller  24 , that is, on the upstream side of the pressing roller  24  with respect to the feeding direction of the unfixed sheet S, being fixed to the mounting stays  16 . 
     As shown in FIG. 3A, The supporting member  30  is settled in such a manner that a gap L 1  between the upper surface the supporting member  30  and the lower run of the fixing belt  32  is set to 3 mm at a central position M which is defined by a crossing point of the upper surface of the supporting member  30  and the vertical line passing through an intermediate point between the heating roller  28  and fixing roller  22 . Note that, in the embodiment, the distance L 3  between the heating roller  28  and fixing roller  22  is set to 47 mm and a distance L 2  between the central position M and a vertical line passing through the center of the fixing roller  22  is set to 23.5 mm. 
     As shown in FIG. 3B, the supporting member  30  is provided with a separating layer  62  formed of a fluoroplastics such as a PTFE (Polytetrafluoroethylene) etc., thereon. Although the supporting member  30 , as shown in FIGS. 1 and 3A, is grounded in order to disturb the unfixed toner image, this is not limited to such a construction but it will be possible to use an electrically erasing brush, a varistor element, and a diode etc., instead of the grounding. 
     On the other hand, as shown in FIG. 1, it is settled that the lower run of the fixing belt  32  trained around the fixing roller  22  and heating roller  32  travels towards substantially in the horizontal direction, that is, to the left in the drawing. A distance between the supporting member  30  and the fixing belt  32  is set to be within a range of about 5 mm to 20 mm at a position just below the heating roller  28 . That is to say, the supporting plate  30  is set to be slant to the horizontal lower run of the fixing belt  32  such that it extends to the nip region in a slanted manner. In other words, the supporting member  30  is formed of a substantially planer shape, with its one end (that is, an end on the side of nipping region) bing lower than the nipping region, and higher than its the other end (that is, an end opposite to the nipping region). 
     It should be noted that the lower run of the fixing belt  32  and the supporting member  30  define a so-called wedge shaped transferring space having the nip position as an apex. Accordingly, the unfixed sheet S is certainly transferred to the nip position between the fixing belt  32  and the pressing roller  24  upon the conveyance on the guide plate  34 . In the nipping region, the unfixed sheet S which has been already preheated by a heat radiation from the fixing belt  32  is pressed by a predetermined pressure Fl and heated by the heat of the fixing belt  32 , whereby the unfixed toner on the unfixed sheet S it heatedly fixed thereon. 
     On the other hand, a plate-like heater  34  (so-called panel heater) for heating the supporting member  30  totally is arranged on the under surface of the supporting member as a second heat generating means. More specifically, in the present embodiment, SAMICON HEATER (a trade name; 230:100W, manufactured by SAKAGUCHI ELECTRIC HEATING Co.) with a size of 50×200 mm is used as the plate-like heater  34 . The plate-like heater  34  is closely fitted on the under surface of the supporting member  30 . 
     Since the unfixed sheet S is preheated from below by the supporting member  30  which is directly heated by the plate-like heater  34 , it would be possible to increase the feeding speed of the unfixed sheet S in cooperation with the preheat by the heat radiation from the fixing belt  32 . This effect will be described in detail more with reference to results of experiments. 
     {Description of fixing belt  32 } 
     The fixing belt  32  preferably has a heat capacity of 0.002 cal/° C.-0.025 cal/° C. per cm 2  so as to be able to preheat the unfixed toner on the unfixed sheet S to a fixing temperature through heat radiation for thereby fixing the toner. In this embodiment, the fixing belt  32  includes an endless belt base made of electro-formed nickel having a thickness of 40 μm and an inner diameter of 50 mm and a heat-resistant resilient separating layer of silicone rubber that is coated to a thickness of 150 μm on an outer circumferential surface of the endless belt base. 
     {Description of oil applying roller  36 } 
     The fixing apparatus  10  is provided with an oil applying roller  36  which serves to apply a small amount of silicone oil to the outer circumferential surface of the fixing belt  32  for separating the sheet S easily from the fixing belt  32 . The oil applying roller  36  includes a support shaft  36 A rotatably supported in a mounting stays  16  through bearings (not shown) and a heat-resistant layer  36 B of paper fitted over the support shaft  36 A and impregnated with silicone coil. In this embodiment, the support shaft  36 A comprises a shaft of iron having a diameter of 8 mm, and the heat-resistant layer  36 B of paper is covered with a synthetic resin film  36 C of porous fluoroplastics having a thickness of 100 μm. The oil applying roller  36  has a diameter of 14 mm. The oil app lying roller  36  thus constructed is capable of stably applying a small amount of silicone oil to the outer circumferential surface of the fixing belt  32 . 
     {Description of tensioning mechanism for the fixing belt  32 } 
     As a mechanism for tensioning the fixing belt  32 , the fixing apparatus  10  is provided with a tension lever. The tension lever  38  is swingably supported to the mounting stays  16  at its lower end, and connected to one end of the mounting stays  16  at its upper end through a tension spring  52 . By the urging force of the tension spring  52 , the tension lever  38  is urged to rotate so as to contact with the outer circumferential surface of the heating roller  28  at its intermediate portion by a predetermined urging force F 2 . 
     Namely, the heating roller  28  urged by the urging force of the tension spring  52  through the tension lever  38  is biased to move so as to separate from the fixing roller  22 . Thus, the upper run of the fixing belt  32  trained around the heating roller  28  and fixing roller  22  is restricted by the tension roller  36 , thereby tensioning the fixing belt  32 . When the fixing belt  32  is driven by the fixing roller  22 , the fixing belt  32  is stably traveled without slipping or sagging upon the rotation of the fixing roller  22 . 
     {Description of pressing mechanism between fixing roller  24  and pressing roller  24 } 
     Next, a description of a pressing mechanism  54  for pressing the fixing belt  32  and pressing roller  24  with each other by a predetermined urging force at a position where the fixing belt  32  starts to be trained around the fixing roller  22 , will be given. The pressing mechanism  54  is provided with a pressure lever  40  which is rotatably supported to the mounting stays  16  at one end through a supporting shaft  56  and capable of abutting against the core  24 A of the pressing roller  24  from below, and a pressure spring  58  a lower end of which is coupled to the other end of the pressure lever  50  and an upper end of which is coupled to the upper portion of the mounting stay  16 . By the urging force of the pressure spring  58 , the pressure lever  40  contacts the outer circumferential surface of the core  24 A of the pressing roller  24  by a predetermined force F 1 . As a result, the fixing belt  28  trained around the fixing roller  22  and the pressing roller  24  are contacted with each other by a predetermined urging force. That is to say, the fixing belt  32  and the pressing roller are rolling contacted with each other having a predetermined nip width. 
     Here, in the embodiment, a positional relationship between the fixing roller  22  and pressing roller  24  is set so that the pressing roller  24  is inclined by a predetermined angle in clockwise direction from a vertical line passing through the central position of the fixing roller  22 . Thus, the nipping position between the fixing belt  32  and pressing roller  24  (that is, the central position of the nip width) is defined so as to biased forward (that is, to the left direction in drawing) in the feeding direction of the unfixed sheet S from a position just below the central position of the fixing roller  22  slightly. 
     {Description of thermistor} 
     The fixing apparatus  10  is provided with a thermistor  60  for controlling the temperature of the fixing belt  32 . In the embodiment, the thermistor  60  is constructed to contact a non-passage portion of the fixing belt  32 , that is, the right side portion of the fixing belt  32  which it directly trained around the heating roller  28 , to detect the surface temperature thereof. In other words, the thermistor  60  is electrically connected to a control unit (not shown) to transmit the temperature information of the fixing belt  32  to the control unit. 
     {Description of experiment 1 where supporting member  30  is heated} 
     The preheat effect by the supporting member  30  to a fixibility of the unfixed toner at a condition where the heating roller  28  and pressing roller  24  are in a predetermined temperature condition in a case where the supporting member  30  was heated by the plate-like heater  34  was examined by an experiment 1 while measuring temperatures of the heating roller  28  and the pressing roller  24 . In the experiment 1, date (that is, the glossiness) was measured where the feeding speed of the unfixed sheet S is set to be 120 mm/sec, the fixing pressure (that is, the pressure F 1 ) is set to be 8 kgf/each side, and the width of the nipping region is set to be 4.5 mm. 
     The results from the experiment 1 is indicated in Table 2. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Tem- 
                 Tem- 
                   
               
               
                 pera- 
                 pera- 
               
               
                 ture 
                 ture 
               
               
                 of 
                 of 
               
               
                 heat- 
                 press- 
               
               
                 ing 
                 ing 
               
               
                 roll- 
                 roll- 
               
               
                 er 
                 er 
                 Temperature of supporting member (° C.) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 (° C.) 
                 (° C.) 
                 22 
                 50 
                 80 
                 110 
                 140 
               
               
                   
               
               
                 140 
                 22 
                 X 
                 18 
                 23 
                 {circle around (28)} 
                 {circle around (36)} 
               
               
                   
                 130 
                 X 
                 17 
                 19 
                 {circle around (24)} 
                 {circle around (28)} 
               
               
                   
                   
                   
                   
                 (11) 
                 (21) 
                 ({circle around (25)}) 
               
               
                 150 
                 22 
                 X 
                 26 
                 27 
                 {circle around (29)} 
                 {circle around (33)} 
               
               
                   
                 140 
                 X 
                 16 
                 20 
                 {circle around (25)} 
                 {circle around (27)} 
               
               
                   
                   
                   
                   
                   
                 ({circle around (24)}) 
                 ({circle around (27)}) 
               
               
                 160 
                 22 
                 — 
                 {circle around (28)} 
                 {circle around (30)} 
                 {circle around (33)} 
                 {circle around (42)} 
               
               
                   
                 150 
                 21 
                 {circle around (24)} 
                 {circle around (23)} 
                 {circle around (29)} 
                 {circle around (35)} 
               
               
                   
                   
                   
                 ({circle around (20)}) 
                 ({circle around (23)}) 
                 ({circle around (28)}) 
                 ({circle around (31)}) 
               
               
                 170 
                 22 
                 X 
                 {circle around (30)} 
                 {circle around (34)} 
                 {circle around (42)} 
                 {circle around (57)} 
               
               
                   
                 160 
                 24 
                 {circle around (24)} 
                 {circle around (33)} 
                 {circle around (37)} 
                 {circle around (49)} 
               
               
                   
                   
                   
                 ({circle around (25)}) 
                 ({circle around (29)}) 
                 ({circle around (31)}) 
                 ({circle around (38)}) 
               
               
                 180 
                 22 
                 — 
               
               
                   
                 170 
                 {circle around (29)} 
               
               
                   
                   
                 ({circle around (29)}) 
               
               
                   
               
            
           
         
       
     
     In the Table 2, numerals denote the measured glossiness, and numerals circled by ◯ indicate results in which the fixibility is good (the fixibilith is OK). Symbols × show results in which the low-temperature offset phenomenon was occurred. Furthermore, in a case where two-line numeral is described in one column, the upper numeral indicates measuring results of glossiness in a case of using sheets with a thickness corresponding to 55 kg/1,000 sheets, and the lower numeral denotes measuring results of glossiness in a case of using sheets with a thickness corresponding to 64 kg/1,000 sheets. 
     It should be noted that the glossiness is a mirror glossiness in 75 degrees in the present embodiment, measured by using glossimeter (GMX-type 202) manufactured by MURAKAMI SHIKISAI GIJUTSU KENKYUSHO Co., based on the method 2 defined in JIS Z8741. 
     From the results shown in the Table 2, it would be possible to lower the temperature of the heating roller  28  by 10° C. whereby the supporting member  30  is heated so that the temperature thereof is raised up by 30° C. Accordingly, it has been clear that it is possible to increase the feeding speed of the unfixed sheet S. 
     On the other hand, as well as previously described in the technical background of the present specification with reference to FIGS. 4 through 6, where fixing apparatuses each of which has temperature characteristics of the heating roller  28 , the fixing roller  22  and the pressing roller  24 , in a case that the sheet feeding speed is changed among 40 mm/sec, 80 mm/sec and 120 mm/sec, are utilized, the difference of glossiness of the fixed image on the sheets after passing through the nip portion among a case just after a warm up operation in the morning (that is, from a cold condition), a case just after a warm up operation from a stand-by mode (that is, from a condition in which the temperature of the fixing roller  22  is set below 60 (°C.)), and a case of a stable feeding mode (that is, just after the fixing apparatus has been actuated for 6 minutes), is little, as shown in FIGS. 7 through 9, respectively. 
     It should be noted that “Acolor” in FIGS. 7 through 9 shows the data in a case where the toner which is used in a copying machine of Acolor (Trade name of FUJI XEROX Co.) is utilized, while “Pretail” indicates the data in a case where the toner which is used in a copying machine of Pretail (Trade name of RICOH Co.) is utilized. 
     In other words, it is possible to accomplish the stable glossiness by preheating the supporting member  30  by the plate-like heater  34 , even though the feeding speed is increased. That is to say, in the present embodiment, it is possible to increase the feeding speed with maintaining the good image quality, by preheating the supporting member  30 . 
     Note that data shown in FIGS. 7 through 9 is a value in a case of using paper sheets with a thickness corresponding to 80 g/m 2 , and the fixing pressure (that is, the pressure F 1 ) is set to 6 kgf/each side, in the present experiment. 
     {Description of a modification of one embodiment} 
     It should be noted that the present invention is not limited to such a construction of the aforesaid embodiment, and a variety of modifications can be made without departing from the spirit and scope of the present invention. 
     For example, the fixing apparatus  10  has been described as being used in an electrophotographic copying system. However, the principles of the present invention are not limited to such an application, but are also applicable to other electronic image forming systems including an electronic facsimile machine, an electronic printer, etc. 
     The plate-like heater  34  as the second heat generating means has been described as being closely fitted to the undersurface of the supporting member  30 . However, the principles of the present invention are not limited to such a construction, but are also closely fittable to the upper surface of the supporting member  30 , as a first modification shown in FIG.  10 A. In this case, the upper surface of the supporting member  30  is coated by an electro-conductive separating layer  62  such as PFA or PTFE. It should be noted that the same or similar member as the above embodiment will be added the same or similar reference numeral and the description thereof will be omitted. 
     The plate-like heater  34  has been described as being closed fitted to the surface of the supporting member  30 . However, the principles of the present invention are not limited to such a construction, but may be embedded in the supporting member  30 , as a second modification shown in FIG.  10 B. 
     The second heat generating means has been described as being constructed by the plate-like heater  34 . However, the principles of the present invention are not limited to such a construction, but may comprise a halogen lamp  34 A which is arranged being remote from the undersurface of the supporting member  30 , and a reflecting plate  34 B which is capable of reflecting the heat from the halogen lamp  34 A to the undersurface of the supporting member  30 , as a third modification shown in FIG.  10 C. 
     The supporting member  30  and the heat generating source  34  have been described as being separated from each other. However, the principles of the present invention are not limited to such a construction, but the supporting member  30  may be formed by the plate-like heater itself, while the plate-like heater  34  as the second heat generating means may be formed in such a manner that the plate-like heater  34  can hold the predetermined shape thereof, as a fourth modification shown in FIG.  10 D. In this case, the upper surface of the plate-like heater  34  as the supporting member  30  is coated by the electro-conductive separating layer  62 . 
     Further in the embodiment, the fixing belt  32  has been described as including an endless belt base made of electro-formed nickel and a heat-resistant resilient separating layer of silicone rubber that is coated on an outer circumferential surface of the endless belt base. However, the principles of the present invention are not limited to such a construction, but the fixing belt may include an endless belt base made of a synthetic resin such as a polyimide and a separating layer added to an outer circumferential surface of the endless belt base. In this case, the endless belt base may be formed of an electro-conductive synthetic resin such as an electro-conductive polyimide. 
     (Description of other embodiment} 
     Further in the above-mentioned embodiment, the heating roller  28  has been described as being arranged so as to be on a side of the fixing roller  22  (that is to say, on an upstream side of the fixing roller  22  with respect to the feeding direction of the unfixed sheet S), that is, so that a space between the supporting member  30  and the fixing belt  32  functions as a preheating passage P for preheating the unfixed sheet S. In other words, the heating roller  28  is positioned in such a manner that the unfixed sheet S passing through the preheating passage P is transferred within an area for affecting the preheat by heat radiated from the fixing belt  32 . However, the principles of the present invention are not limited to such a construction, but the heating roller  28  may be positioned above the fixing roller  22  relative to the pressing roller  24 , in other words, the heating roller  28  may be positioned in such a manner that the unfixed sheet is transferred out of the area for affecting the preheat by heat radiated from the fixing belt  32 . 
     Hereinafter, a description will be given to a fixing apparatus  10 ′ of the other embodiment according to the present invention in which the heating roller  28  is positioned above the fixing roller  22  (that is, so as to be remote from the fixing roller  22  in a direction crossing to the feeding direction of the unfixed sheet S), with reference to the accompanying FIGS. 11 through 14. 
     As shown in FIG. 11, a fixing apparatus  10 ′ according to the other embodiment of the present invention has a housing  12  which is the same as in the above one embodiment. The housing  12  comprises a base plate  14  to be fixed directly to the frame, a pair of vertical side plates (main mounting stays)  16  erected from respective side edges of the base plate  14 , auxiliary mounting stays which are the same in the above one embodiment, an upper cover  72  mounted on the side plates  16  to cover upper right regions of the side plates  16 , and a left cover  74  mounted on the side plates  16  to cover left side regions of the side plates  16 . 
     The upper cover  72  is fixedly mounted on the side plates  16 . A swing lever  76  is swingably supported on right portions of the side plates  16  by a first pivot shaft  78  positioned on a right end of the swing lever  76 , for swinging movement about the first pivot shaft  78  to provide an open space at a left end of the swing lever  76 . The left cover  74  is swingably supported on the side plates  16  by a second pivot shaft  80  positioned on a lower end of the left cover  74 , for swinging movement about the second pivot shaft  78  to provide an open space at an upper end of the left cover  74 . 
     The fixing apparatus  10 ′ has a roller assembly including a fixing roller  22  rotatably supported on the side plates  16  for rotation about a fixed axis, a pressing roller  24  positioned obliquely downwardly of the fixing roller  22  in rolling contact with the fixing roller  22  and rotatably supported on the side plates  16  for rotation about a fixed axis parallel to the fixed axis of the fixing roller  22 , and a heating roller  28  positioned obliquely upwardly of the fixing roller  22  and rotatably supported on the swing lever  76  for rotation about its own axis. The heating roller  28  has a heater  26  such as a halogen lamp or the like disposed therein. 
     The fixing apparatus  10 ′ has a belt assembly including an endless fixing belt (heat transfer belt)  32  which is trained around the fixing roller  22  and the heating roller  28 . 
     Here, although the detail will be described later, the fixing roller  22  comprises a resilient roller, and the pressing roller  24  comprises a roller harder than the fixing roller  22 . As shown in FIG. 12, the fixing roller  22  and the pressing roller  24  have respective centers O 1 , O 2  spaced from each other by a distance D which is slightly smaller than the sum (R 1 +R 2 ) of their radii R 1 , R 2  (that is, D&lt;R 1 +R 2 ). In a rolling contact region (nipping region) between the fixing roller  22  and the pressing roller  24 , the fixing roller  22  and the pressing roller  24  are held in rolling contact with each other under a predetermined pressure P 1 , so that the fixing roller  22  has an outer circumferential surface made partly concave by the pressing roller  24  held in rolling contact therewith, thus providing a sufficient nipping width in a direction across the axes of the fixing roller  22  and the pressing roller  24 . 
     The fixing apparatus  10 ′ also has an oil applying roller  36  for applying silicone oil to an outer circumferential surface of the fixing belt  32  and cleaning the outer surface of the fixing belt  32 , a first helical spring  82  for normally pressing the oil applying roller  36  against the fixing belt  32  perpendicularly thereto to tension the fixing belt  32 , and a second helical spring  84  for normally urging the heating roller  28  in a direction away from the fixing roller  22  to tension the fixing belt  32  in coaction with the first helical spring  82 . 
     A sheet discharge passage  86  is defined above the left cover  74  for discharging a sheet with a toner image fixed thereto with heat and pressure by the fixing roller  22  and the pressing roller  24  in the rolling contact region. Such a sheet will hereinafter be referred to as a “fixed toner sheet”. The sheet discharge passage  86  is oriented such that it discharges the fixed toner sheet substantially upwardly along a vertical plane. 
     A lower discharge roller  88  is rotatably mounted on the left cover  74  between the sheet discharge passage  86  and the rolling contact region. The lower discharge roller  88  is actuated by an actuating mechanism  90  (described later on) to rotate at a speed greater than the pressing roller  24 , i.e., at a speed which is 5% greater than the speed at which the pressing roller  24  rotates. An upper discharge roller  92  is positioned obliquely upwardly of the lower discharge roller  88  and held in rolling contact with the lower discharge roller  88  under resilient forces from a leaf spring  94 . It should be noted the upper discharge roller  92  is positioned with respect to the lower discharge roller  88  such that a line interconnecting the centers of the upper and lower discharge rollers  92 ,  88  extends substantially perpendicularly across a sheet discharge passage along which the fixed toner sheet is delivered from the rolling contact region to the sheet discharge passage  86 . 
     In the fixing apparatus  10 ′ thus constructed, the unfixed toner sheet S fed onto the supporting member  30  by the endless belt EB has its lower surface, opposite to the unfixed toner image, borne by supporting member  30 , and is guided by the supporting member  30  toward the rolling contact region (nipping region) between the fixing roller  22  and the pressing roller  24 , with the fixing belt  32  being trained around the fixing roller  22 . When the unfixed toner sheet S passes under pressure between the fixing roller  22  and the pressing roller  24 , the unfixed toner image is fixed to the sheet S with heat and pressure. 
     {Description of fixing roller  22 } 
     The fixing roller  22  comprises a core  22 A rotatably supported on the side plates  16  by bearings (not shown) and a roller sleeve  22 B fitted coaxially over the core  22 A. The fixing belt  32  is trained around the roller sleeve  22 B. The fixing roller  22  has an outside diameter of 38.5 mm in this embodiment. The core  22 A comprises a shaft of iron having a diameter of 25 mm, and the roller sleeve  22 B is made of a heat-resistant resilient material of silicone rubber having a wall thickness of 6.75 mm. Specifically, the roller sleeve  22 B is made of silicone rubber sponge having an JIS A hardness of 15 degrees. 
     As shown in FIG. 13, the core  22 A has an end combined with a shaft which is coaxially coupled to a first driven gear  96  through a one-way clutch  98  (described later on). The first driven gear  96  is held in mesh with a transmission gear  100  of the actuating mechanism  90 . Drive forces produced by the actuating mechanism  90  are transmitted through the transmission gear  100  to the first driven gear  96  which is rotated clockwise to rotate the fixing roller  22  through the one-way clutch  98 . 
     {Description of pressing roller  24 } 
     The pressing roller  24  comprises a core  24 A rotatably supported on the side plates  16  by bearings (not shown) and a roller sleeve  24 B fitted coaxially over the core  24 A. The pressing roller  24  has an outside diameter of 35 mm in this embodiment. The core  24 A comprises a shaft of iron having a diameter of 32 mm, and the roller sleeve  24 B is made of a heat-resistant resilient material of silicone rubber having a wall thickness of 1.5 mm. Specifically, the roller sleeve  24 B is made of silicone rubber sponge having a JIS A hardness of 20 degrees, which is harder than the roller sleeve  22 B of the fixing roller  22 . The outer circumferential surface of the roller sleeve  24 B is covered with a tube of fluoroplastics having a wall thickness of 50 μm. 
     The core  24 A has an end combined with a shaft which is coaxially coupled to a second driven gear  102  which is held in mesh with the first driven gear  96 . Drive forces are transmitted from the first driven gear  96  to the second driven gear  102 , which rotates the pressing roller  24  counterclockwise. 
     In this embodiment, the pressing roller  24  is used as a primary drive roller for establishing a speed at which the unfixed toner sheet is fed through the nipping region. The ratio of gear teeth of the first and second drive gears  96 ,  102  is selected such that the peripheral speed of the fixing roller  22  as it is thermally expanded is not greater than the peripheral speed of the pressing roller  24 , as shown in FIG.  13 . Specifically, the speed at which the fixing roller  22  is rotated by the first driven gear  96  is slightly lower than the speed at which it is rotated in frictional engagement with the pressing roller  24  through the fixing belt  32 . 
     On the other hand, the pressing roller  24  is not positioned directly downwardly of the fixing roller  22 , but is displaced downstream in the feed direction of a position directly downward of the fixing roller  22 . Specifically, the pressing roller  24  is positioned with respect to the fixing roller  22  such that an acute angle is formed between a vertical line passing through the center of the fixing roller  22  and a line segment passing through the centers of the fixing roller  22  and the pressing roller  24 . The line segment passing through the centers of the fixing roller  22  and the pressing roller  24  extends perpendicularly to the feed direction across the rolling contact region. 
     {Description of supporting member  30 } 
     The upper cover  72  has a right lower portion bent inwardly into the housing  12 . A supporting member  30  is positioned below and largely spaced from the bent right lower portion of the upper cover  72 . The supporting member  30  and the bent right lower portion of the upper cover  72  jointly define an inlet port  46  therebetween for introducing therethrough a sheet S with an unfixed toner image carried thereon into the housing  12  in the direction (feed direction) indicated by the arrow in FIG.  11 . 
     The supporting member  30  is inclined obliquely upwardly to the left such that the height of the supporting member  30  progressively increases into the housing  12 . The supporting member  30  has an inlet end, i.e., a right end, positioned in confronting relation to an outlet end of a sheet feeding endless belt EB that is positioned in the electrophotographic copying system adjacent to the right end of an inlet port. The supporting member  30  has an outlet end, i.e., a left end, positioned in confronting relation to the rolling contact region (nipping region) between the fixing roller  22  and the pressing roller  24 . 
     When the unfixed toner sheet S is fed in the feed direction indicated by the arrow toward the toner image fixing apparatus  10 ′ by the endless belt EB, the leading end of the unfixed toner sheet S contacts the supporting member  30 , and is then guided thereby to travel obliquely upwardly into the rolling contact region between the fixing roller  22  and the pressing roller  24 . 
     A plate-like heater  34  (so-called panel heater) for heating the supporting member  30  is arranged on the under surface of the supporting member  30  as a second heat generating means, with the same manner as the first embodiment. 
     {Description of one-way clutch  98 } 
     The one-way clutch  98  allows the fixing roller  22  to rotate clockwise relatively to the first driven gear  96 , but prevents the fixing roller  22  from rotating counterclockwise relatively to the first driven gear  96 , i.e., rotates the fixing roller  22  and the first driven gear  96  in unison with each other. Specifically, when the fixing roller  22  is cold, i.e., when the fixing roller  22  and the fixing belt  32  are driven by the pressing roller  24  while the fixing belt  32  is held in frictional engagement with the pressing roller  24  and the fixing roller  22  is held in frictional engagement with the fixing belt  32 , the peripheral speed of the fixing roller  22  upon clockwise rotation thereof is the same as the peripheral speed of the pressing roller  24 , and hence is slightly higher than the peripheral speed of the first driven gear  96 . The difference between the peripheral speeds of the fixing roller  22  and the first driven gear  96  is absorbed by the one-way clutch  98 . 
     When the heating roller  28  is heated by the heater  26  and the fixing roller  22  is heated through the fixing belt  32 , the outside diameter of the fixing roller  22  is increased as it is thermally expanded, and the peripheral speed of the fixing roller  22  increases. Since the peripheral speed of the fixing roller  22  does not become higher than the peripheral speed of the pressing roller  24 , the increase in the peripheral speed of the fixing roller  22  is absorbed by the one-way clutch  98 . 
     The one-way clutch  98  offers the following advantages: If the one-way clutch  98  were not employed, when a sheet with a glossy and slippery surface, such as a coated sheet, is fed as an unfixed toner sheet into the rolling contact region, the fixing belt  32  would slip against the unfixed toner sheet, and drive forces would not be transmitted from the pressing roller  24  to the fixing belt  32  and the fixing roller  22 , which would not then be driven by the pressing roller  24 . Therefore, the unfixed toner sheet would be jammed in the rolling contact region, or even if the unfixed toner sheet passed through the rolling contact region, the unfixed toner image on the unfixed toner sheet would be abraded and disturbed by the fixing belt  32  kept at rest. 
     In this embodiment, however, since the one-way clutch  98  is connected between the fixing roller  22  and the first driven gear  96 , even if drive forces from the pressing roller  24  are not transmitted to the fixing belt  32 , the fixing roller  22  is rotated clockwise by the first driven gear  96  through the one-way clutch  98  when the peripheral speed of the fixing roller  22  starts being lower than the peripheral speed of the first driven gear  96 . Therefore, the unfixed toner sheet passes reliably through the rolling contact region for effective protection against a sheet jam in the nipping region and toner image disturbance on the sheet. 
     {Description of fixing belt  32 } 
     In this embodiment, the fixing belt  32  comprises an endless belt base of an electro-conductive polyimide having a thickness of 100 μm and an inner diameter of 60 mm, and a heat-resistant resilient separating layer of silicone rubber that is coated to a thickness of 200 μm on an outer circumferential surface of the endless belt base. 
     Alternatively, the fixing belt  32  may comprise an endless belt base of electroformed nickel and a heat-resistant resilient separating layer of silicone rubber that is coated on an outer circumferential surface of the endless belt base. 
     {Description of tensioning mechanism for the fixing belt  32 } 
     As described above, a mechanism for tensioning the fixing belt  32  has the first helical spring  82  for normally pressing the oil applying roller  36  against the fixing belt  32  perpendicularly thereto to tension the fixing belt  32 , and the second helical spring  84  for normally urging the heating roller  28  in a direction away from the fixing roller  22  to tension the fixing belt  32  in coaction with the first helical spring  82 . 
     The first helical spring  82  is attached to the left cover  74  for normally urging the casing  104 , on which the oil applying roller  36  is rotatably supported, toward the fixing belt  32 . The casing  104  is movably supported by a guide rib  106  on one of the side plates  16  for movement toward and away from the fixing belt  32 . When the left cover  74  is swung open to the left about the second pivot shaft  80 , the first helical spring  82  is disengaged from the casing  104 , releasing the oil applying roller  36  from the fixing belt  32 . When the left cover  74  is swung to the right about the second pivot shaft  80 , the first helical spring  82  pushes the casing  104  under a pressing force P 2 , causing the oil applying roller  36  to press the fixing belt  32  under a certain tension. 
     On the other hand, the second helical spring  84  is connected between the left end of the swing lever  76  and the side plate  16  for normally urging the swing lever  76  to turn clockwise about the first pivot shaft  78 , i.e., to push the heating roller  28  on the swing lever  76  under a third pressing force P 3  in a direction away from the fixing roller  22 . In this manner, the fixing belt  32  is given a desired tension. 
     Therefore, the heating roller  28  is displaced away from the fixing roller  22  by the swing lever  76  under the bias of the second helical spring  84 , tensioning the fixing belt  32  trained around the heating roller  28  and the fixing roller  22 . 
     The fixing belt  32  thus tensioned by the first and second helical springs  40 ,  42  is held in frictional engagement with the pressing roller  24  and driven thereby. When the fixing belt  32  is driven by the pressing roller  24 , the fixing roller  22  is stably driven thereby without slipping or sagging with respect to the fixing belt  32 . 
     {Description of actuating mechanism  90 } 
     As shown in FIG. 13, the actuating mechanism  90  for rotatably driving the pressing roller  24  includes the transmission gear  100  which is held in mesh with an output gear GE that is connected through a gear train (not shown) to an actuator in the electrophotographic copying system when the fixing apparatus  10 ′ is installed in the electrophotographic copying system. The transmission gear  100  can be driven to rotate by the output gear GE. The actuating mechanism  90  also has, in addition to the transmission gear  100 , the first driven gear  96  held in mesh with the transmission gear  100  and coupled to the fixing roller  22  through the one-way clutch  98 , and the second driven gear  102  held in mesh with the first driven gear  96  and fixed coaxially to the pressing roller  24 . 
     The actuating mechanism  90  also has an idler gear  108  held in mesh with the transmission gear  100 . The idler gear  108  is also held in mesh with a third driven gear  110  fixed coaxially to the lower discharge roller  88  for rotating the lower discharge roller  88  at a speed equal to or higher than the rotational speed of the pressing roller  24 . 
     {Description of other structural details} 
     As shown in FIG. 11 again, the fixing apparatus  10 ′ has a peeler blade  112  for peeling the fixed toner sheet off the outer circumferential surface of the pressing roller  24 , and a sheet sensor  114  for detecting the leading end of the fixed toner sheet as it is fed to a rolling contact region between the upper and lower discharge rollers  88  and  92 . 
     The fixing apparatus  10 ′ further includes a thermistor  60  for detecting the temperature of that portion of the fixing belt  32  which is trained around the heating roller  28 , a second thermistor  116  for detecting the temperature of that portion of the outer circumferential surface of the fixing belt  32  which is trained around the fixing roller  36 , a third thermistor  118  for detecting the temperature of the outer circumferential surface of the pressing roller  84 , and a control circuit (not shown) for controlling the heater  26  and the plate-like heater  34  based on the temperatures detected by the first, second, and third thermistors  60 ,  116  and  118 . 
     {Description of position of the heating roller  28 } 
     As described above, the heating roller  34  is positioned substantially upwardly of the fixing roller  22 . Therefore, the fixing belt  32  that is trained around the fixing roller  22  and the heating roller  28  is so spaced from the supporting member  30  that the unfixed toner sheet fed on the supporting member  30  will not be brought into contact with the fixing belt  32 . Stated otherwise, the fixing belt  32  is disposed in a position outside of a region where the unfixed toner sheet fed on the supporting member  30  possibly passes. 
     Because the heating roller  34  is positioned substantially upwardly of the fixing roller  22 , the unfixed toner sheet S carried on the upper surface of the supporting member  30  is reliably prevented from contacting the fixing belt  32  irrespective of how the unfixed toner sheet being fed may be curled. Consequently, the unfixed toner sheet S can be led to the rolling contact region between the fixing roller  22  and the pressing roller  24  without disturbing the unfixed toner image on the unfixed toner sheet S, so that the unfixed toner image on the unfixed toner sheet S can reliably be fixed to the unfixed toner sheet S in the rolling contact region. 
     {Description of angle of the heating roller  28 } 
     The fact that the heating roller  28  is positioned substantially upwardly of the fixing roller  22  offers advantages inherent in the fixing apparatus  10 ′. An experiment to determine an optimum angular range in which the heating roller  28  can be positioned substantially upwardly of the fixing roller  22  by changing the angle of the heating roller  28  as shown in FIG. 14 will be described below. 
     In the experiment, a straight line passing through the centers of the fixing roller  22  and the pressing roller  24  was defined as a reference line B, and an angle θ was defined between the reference line B and a line segment L interconnecting the centers of the fixing roller  22  and the heating roller  28 . The angular position of the heating roller  28  with respect to the fixing roller  22  was changed to change the angle θ between 90° and 180°, and the frequency of rubbed states of toner images at the inlet of the rolling contact region between the fixing roller  22  and the pressing roller  24  and also the frequency of defects of toner images at the outlet of the rolling contact region between the fixing roller  22  and the pressing roller  24  were measured when the toner images were copied on one side and both sides of sheets. 
     The angle θ was defined as a positive angle when measured counterclockwise from the reference line B, and as a negative angle when measured clockwise from the reference line B. Therefore, the heating roller  28  positioned at the angle θ=+180° and the heating roller  28  positioned at the angle θ=−180° were in the same angular position, and the heating roller  28  positioned at the angle θ=+105° and the heating roller  28  positioned at the angle θ=−255° were in the same angular position. Defects of toner images at the outlet of the rolling contact region represent sheet offsets or sheet jams. 
     The experiment was conducted under the following conditions: 
     The nipping width in the rolling contact region was set to 8 mm, and the pressing roller  24  applied a pressure P 1  of 24 kgf to one side of the unfixed toner sheet S. The temperature of the fixing belt  32  trained around the fixing roller  22  was set to 160° C. The surface temperature of the pressing roller  24  was set to 140° C. The speed at which to feed the unfixed toner sheet S was set to 180 mm/sec. The pressing roller  24  was rotated in synchronism with the speed of 180 mm/sec. The toner used was an Acolor toner manufactured by Fuji Xerox Co. The sheet S used was plain paper having a weight of 64 g/m 2 . 
     The experiment was made for nine angles θ of 90°, 105°, 120°, 150°, 180°, −150°, −120°, −105°, and −90°. 
     The results of the experiment are given in Table 3 shown below. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Copied on one 
                   
                 Copied on both 
                   
               
               
                   
                 side 
                   
                 sides 
               
            
           
           
               
               
               
               
               
               
            
               
                 Angles 
                 A 
                 B 
                 A 
                 B 
                 Evaluation 
               
               
                   
               
               
                 90° 
                 3/5 
                 0/5 
                 5/5 
                 0/5 
                 Not acceptable 
               
               
                 105°  
                 0/5 
                 0/5 
                 1/5 
                 0/5 
                 Partly 
               
               
                   
                   
                   
                   
                   
                 acceptable 
               
               
                 120°  
                 0/5 
                 0/5 
                 0/5 
                 0/5 
                 Acceptable 
               
               
                 150°  
                 0/5 
                 0/5 
                 0/5 
                 0/5 
                 Acceptable 
               
               
                 ±180°    
                 0/5 
                 0/5 
                 0/5 
                 0/5 
                 Acceptable 
               
               
                 −150°    
                 0/5 
                 0/5 
                 0/5 
                 0/5 
                 Acceptable 
               
               
                 −120°    
                 0/5 
                 0/5 
                 0/5 
                 0/5 
                 Acceptable 
               
               
                 −105°    
                 0/5 
                 2/5 
                 0/5 
                 3/5 
                 Not acceptable 
               
               
                 −90°   
                 0/5 
                 5/5 
                 0/5 
                 5/5 
                 Not acceptable 
               
               
                   
               
            
           
         
       
     
     A: The frequency of rubbed states of toner images at the inlet of the rolling contact region. 
     B: The frequency of defects of toner images at the outlet of the rolling contact region. 
     As can be seen from Table 3, when the angle θ is greater than 105° and smaller than −105°, i.e., when the angle θ is in a range from 105° to 255° as measured only counterclockwise, toner images were neither rubbed at the inlet of the rolling contact region and nor defective at the outlet of the rolling contact region, indicating a good toner image fixing process. However, when the angle θ is equal or smaller than 105°, toner images were either rubbed at the inlet of the rolling contact region and or defective at the outlet of the rolling contact region, indicating a poor toner image fixing process. 
     As described above in detail, in the above embodiment, the heating roller  28  is positioned substantially upwardly of the fixing roller  22 , i.e., the heating roller  28  is angularly positioned with respect to the fixing roller  22  such that the angle formed between the line segment L interconnecting the center of the heating roller  28  and the center of the fixing roller  22  and the reference line B interconnecting the center of the fixing roller  22  and the center of the pressing roller  24  lies in a range from about 105° to about 255°. Therefore, the fixing belt  32  that is trained around the fixing roller  22  and the heating roller  28  is so spaced from the supporting member  30  that the unfixed toner sheet fed on the supporting member  30  will not be brought into contact with the fixing belt  32 . Stated otherwise, the fixing belt  32  is disposed in a position outside of a region where the unfixed toner sheet fed on the supporting member  30  possibly passes. 
     Consequently, no matter how the unfixed toner sheet being fed is curled due to jumping or sagging on account of the speed difference between a speed difference between the fixing apparatus  10 ′ and a preceding toner image transferring apparatus, the unfixed toner image on the upper surface of the unfixed toner sheet is reliably prevented from touching the fixing belt  32 , and can be led, without being disturbed, into the rolling contact region between the fixing roller  22  and the pressing roller  24 , so that the toner image can reliably be fixed to the sheet by the fixing roller  22 . 
     In the above mentioned embodiment, the fixing roller  22  comprises a resilient roller, and the pressing roller  24  comprises a roller harder than the fixing roller  22 . Therefore, the fixing roller  22  can provide an upwardly concave surface in the nipping region, unlike the conventional structure. The upwardly concave nipping region provided by the fixing roller  22  produces forces tending to separate a sheet carrying a fixed toner image from the fixing belt  32 . Even though the toner is carried on the surface of the sheet held in contact with the fixing belt  32 , because the sheet can easily be separated from the fixing belt  32  due to the upwardly concave nipping region, the amount of oil applied to the fixing belt  32  by the oil applying roller  36  for preventing sheet offsets and jams may be relatively small. Actually, the upwardly concave nipping region provided by the fixing roller  22  is effective to avoid sheet offsets and jams between the fixing roller  22  and the pressing roller  24  even without the application of oil to the fixing belt  32  by the oil applying roller  36 . 
     Furthermore, the fixing belt  32  is made of a material having a small heat capacity, trained around the heating roller  28  at a large contact angle, and held in intimate contact with the heating roller  28 . As a result, even when sheets are passed at a high speed, i.e., even when a large number of sheets are passed in a unit time, through the nipping region, the temperature necessary to fix toner images to the sheets can reliably be maintained in the rolling contact region between the fixing roller  22  and the pressing roller  24 . 
     In the embodiment, the resilient fixing roller  22  does not house any heater, but the heating roller  28  spaced from the fixing roller  22  houses the heater  26  therein. Thus, it is possible to sufficiently increase the thickness of the roller sleeve  28 B that is made of a heat-resistant resilient material. Consequently, the nipping width in the rolling contract region can be sufficiently large while at the same time the fixing roller  22  may be relatively small in diameter. 
     In addition, the one-way clutch  98  disposed between the first driven gear  96  and the fixing roller  22  allows the pressing roller  24 , rather than the fixing roller  22 , as a primary drive roller for establishing a speed at which the unfixed toner sheet is fed through the nipping region. Therefore, even when the fixing roller  22  is heated in the fixing process and thermally expanded to increase its diameter, since the speed at which the unfixed toner sheet is fed through the nipping region is not established by the fixing roller  22 , it is not varied by the thermal expansion of the fixing roller  22 , but is maintained at a constant level. Consequently, the fixing belt  32  is maintained at a constant linear velocity to prevent toner images from being displaced or rubbed. 
     {Description of modifications in the other embodiment} 
     In the above embodiment, the unfixed toner sheet is introduced laterally into the fixing apparatus  10 ′. However, the unfixed toner sheet may be introduced vertically, e.g., upwardly, into the fixing apparatus  10 ′. In such a modification, the pressing roller  24  is disposed laterally of the fixing roller  22 , and the heating roller  28  is disposed on one side of the fixing roller  22  which is opposite to the pressing roller  24 . 
     In the above embodiment, the third thermistor  118  is provided to detect the temperature of the outer circumferential surface of the pressing roller  24 . 
     Furthermore in the embodiment, the heat generating source  26  has been described as being constructed from the halogen lamp which is installed in the heating roller  28 . However, the principles of the present invention are not limited to such a construction, but the heat generating source may constructed from a plate-like heater  26 A which is closely attached to the inner circumferential surface of the heating roller  28 , as a first modification of the other embodiment shown in FIG.  15 A. 
     The heat generating source  26  is not limited to such a construction so that it is installed in the heating roller  28 , but it may be constructed from a plate-like heater  26 B which is located out of the heating roller  28 , for example, closely attached to the outer circumferential surface of the heating roller  28 , as a second modification of the other embodiment shown in FIG.  15 B. 
     In the construction of which the heat generating source  26  is arranged out of the heating roller  28 , it may be constructed from an induction heating mechanism  120  for directly heating the fixing belt  32  by an induction heating, as a third modification of the other embodiment shown in FIG. 15C or a mechanism including a halogen lamp  122  opposed to the outer circumferential surface of the fixing belt  32 , for directly heating it and a reflector for reflecting the heat from the halogen lamp  122  to the fixing belt  32 , as a fourth modification of the other embodiment shown in FIG.  15 D. 
     In the third and fourth modifications shown in FIGS. 15C and 15D, respectively, the location of the induction heating mechanism  120  and the halogen lamp  122  is set to be between the fixing roller  22  and heating roller  28  and arranged in such a manner that they heat a portion of the fixing belt  32  which is positioned on the upstream side of the nipping portion with respect to the traveling direction of the fixing belt  32 . 
     It should be noted that the fixing belt  32  must include the endless belt base of electroformed nickel, not of the synthetic resin such as a polyimide, in the third modification shown in FIG.  15 C. 
     In the afore-mentioned other embodiment, the heat generating source  26  has been described as being arranged at a single location. However, the principles of the present invention are not limited to such a construction, but an additional heat generating source  126  may be installed in the pressing roller  24 . In this case, it is necessary to provide the third thermistor  118  for detecting the temperature of the outer circumferential surface of the heating roller  24 , being differed from the other embodiment. 
     Industrial Applicability 
     As described in detail above, the present invention can provide a fixing apparatus which is capable of ensuring the increase of the feeding speed of the recording medium, by effectively accomplishing to preheat the recording medium. Also, the present invention can provide a fixing apparatus which is capable of enhancing the preheat effect by preheating uniformly the recording medium. Further, the present invention can provide a fixing apparatus which is capable of maintaining the good quality of the toner image even though the feeding speed of the recording medium is increased. Thus, the fixing apparatus according to the present invention is suitable to be installed in the image forming apparatus such as an electrical copying machine, electrical printing machine, electrical facsimile machine, etc.