Patent Publication Number: US-2006017221-A1

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an image forming apparatus. More specifically, the present invention relates to an image forming apparatus that has a feed roller and a feed roller bearing.  
      2. Background Information  
      Image forming apparatuses that include a feed roller and a feed roller bearing are well known.  
      A heat transfer printer is one known example of an image forming apparatus.  FIG. 10  is a perspective view showing the entire configuration of an example of a conventional heat transfer printer.  FIG. 11  is a plan view of the conventional heat transfer printer shown in  FIG. 10 .  FIGS. 12 through 14  are diagrams for describing the detailed structure of the conventional heat transfer printer shown in  FIG. 10 . The structure of the conventional heat transfer printer will now be described with reference to  FIGS. 10 through 14 .  
      As shown in  FIGS. 10-14 , a conventional heat transfer printer includes a metal chassis  101 , a metallic feed roller  102  designed to feed paper  125 , a metallic press roller  103  for pressing on the feed roller  102  with a specific amount of pressure, separating members  104 , a resinous feed roller bearing  105  for rotatably supporting one end of the feed roller  102 , a resinous feed roller bearing  106  for rotatably supporting the other end of the feed roller  102 , resinous press roller bearings  107  for rotatably supporting the press roller  103 , a feed roller gear  108  mounted on the feed roller  102 , metallic bearing support plates  109  for supporting the press roller bearings  107 , tension coil springs  110 , a motor  111  for driving the feed roller  102  and the like, a thermal head  112  for printing, a platen roller  113  shown in  FIG. 11 , a resinous platen roller bearing  114  for rotatably supporting the platen roller  113 , a torsion coil spring  115 , a motor  116  for driving the thermal head  112 , an ink sheet take-up gear  117 , a roller axle  118 , a rubber paper supply/eject roller  119  mounted on the roller axle  118 , a roller axle gear  120  mounted on the roller axle  118 , a motor bracket  121 , and intermediate gears  122 ,  123 , and  124 .  
      As shown in  FIG. 10 , the motor bracket  121  is mounted on the first side surface  101   a  of the chassis  101 . Also, an ink sheet insertion part  101   c  for mounting ink sheets (not shown) is provided to the second side surface  101   b  of the chassis  101 . A spring mounting hole  101   d  in which one end of the tension coil spring  110  is mounted is formed in the second side surface  101   b  of the chassis  101 . Also, as shown in  FIG. 12 , an opening  101   e  for supporting the feed roller bearing  105  is formed in the first side surface  101   a  of the chassis  101 . An opening  101   f  for supporting the feed roller bearing  106  is formed in the second side surface  101   b  of the chassis  101 .  
      The feed roller  102  has an outer peripheral surface  102   a  on the side of the first side surface  101   a  of the chassis  101 , an outer peripheral surface  102   b  on the side of the second side surface  101   b  of the chassis  101 , a gear insertion part  102   c,  and a paper conveying portion  102   d,  as shown in  FIG. 12 . The feed roller  102  is rotatably supported by the feed roller bearings  105  and  106 . Also, the outer peripheral surface  102   b  of the feed roller  102  is supported in a linear fashion by a feed roller support portion  106   a  of the feed roller bearing  106  in a direction perpendicular to the surface of the paper  125 , as shown in  FIG. 14 . Also, convex portions having a specific height are formed by roll forming on the surface of the paper conveying portions  102   d  of the feed roller  102 .  
      The metallic separating members  104  have a larger diameter than that of the press roller  103 , and are attached to the outer peripheral surfaces  103   a  and  103   b  of the press roller  103 . As a result of the separating members  104  coming into contact with the outermost peripheral surface of the feed roller  102 , the distal ends of the convex portions on the surface of the paper conveying portions  102   d  of the feed roller  102  are prevented from coming into contact with the press roller  103 . Also, the outer peripheral surfaces  103   a  and  103   b  of the press roller  103  are rotatably supported by the press roller bearings  107 . The press roller bearings  107  are mounted on the bearing support plate  109  that are mounted to the inner sides of both the first side surface  101   a  and the second side surface  101   b  of the chassis  101 . The bearing support plates  109  are mounted to the first side surface  101   a  and the second side surface  101   b  of the chassis  101  so as to be pivotable around supporting portions  109   a,  as shown in  FIG. 10 . Also, the other end of the tension coil spring  110  that urges the press roller  103  to press on the feed roller  102  is attached to a spring mounting portion  109   b  of the bearing support plate  109 .  
      The thermal head  112  is mounted in between the inner sides of the first side surface  101   a  and the second side surface  101   b  of the chassis  101  so as to be pivotable around a supporting axle  112   a,  as shown in  FIGS. 10 and 11 . One end of the torsion coil spring  115  described above and illustrated in  FIG. 11  is coupled to the supporting axle  112   a.  The torsion coil spring  115  functions to urge the thermal head  112  in a direction away from the platen roller  113 . Also, during the printing operation, the thermal head  112  pivots in a direction of pressing on the platen roller  113  (the direction of the arrow E in  FIG. 10 ) and presses on the platen roller  113 . The thermal head  112  is disposed so as to face the platen roller  113 .  
      Also, the driving force from the motor  111  is transmitted to the feed roller gear  108  via the intermediate gear  122 , as shown in  FIGS. 10 and 11 . The feed roller gear  108  engages the intermediate gears  122  and  123 . The intermediate gear  123  transmits the driving force from the feed roller gear  108  to the ink sheet take-up gear  117  fitted on an ink sheet take-up member (not shown). The intermediate gears  123  also transmit the driving force from the feed roller gear  108  to the roller axle  118  via the roller axle gear  120 . The paper supply/eject roller  119  is mounted on the roller axle  118 .  
      Next, the operation of feeding paper  125  in the conventional heat transfer printer will be described with reference to  FIGS. 10 and 11 . As shown in  FIGS. 10 and 11 , the driving force of the motor  111  is transmitted to the feed roller gear  108  via the intermediate gear  122 . The feed roller  102  thereby rotates. Also, the driving force of the motor  111  is transmitted from the feed roller gear  108  to the ink sheet take-up gear  117  via the intermediate gear  123 . The ink sheet take-up member (not shown) thereby rotates. Also, the driving force of the motor  111  is transmitted from the feed roller gear  108  to the roller axle gear  120  via the intermediate gears  123 . The roller axle  118  thereby rotates.  
      During the paper supply operation, the thermal head  112  is pivoted in a direction away from the platen roller  113  (the direction of the arrow F in  FIG. 10 ) by the torsion coil spring  115 . Also, during the paper supply operation, the paper  125  is conveyed in the paper supplying direction (the direction of the arrow H in  FIG. 10 ) by the feed roller  102  and the paper supply/eject roller  119 .  
      During the printing operation, the thermal head  112  is pivoted in the direction of pressing on the platen roller  113  (the direction of the arrow E in  FIG. 10 ) by the motor  111 , and presses against the platen roller  113 . During the printing operation, the paper  125  and the ink sheet (not shown) are conveyed while being held between the thermal head  112  and the platen roller  113 . During the printing operation, the paper  125  is conveyed in the paper ejecting direction (the direction of the arrow G in  FIG. 10 ) by the feed roller  102 , the press roller  103 , and the paper supply/eject roller  119 . During the printing operation, the ink sheet (not shown) is conveyed in the paper ejecting direction (the direction of the arrow G in  FIG. 10 ) by the feed roller  102 , the press roller  103 , and the ink sheet take-up member (not shown).  
      The conventional heat transfer printer shown in  FIGS. 10 through 14  has problems in that when the load of the paper  125  is received, the position where the feed roller  102  contacts the feed roller bearing  106  shifts from the feed roller support portion  106   a  in the direction of the load of the paper  125  (the conveyance direction of the arrow H in  FIG. 14 ) due to the moment produced by the load of the paper  125 , which applies mainly to the feed roller support portion  106   a.  These problems will now be described in detail with reference to  FIG. 14 .  
      In  FIG. 14 , the magnitude of the load of the paper  125  is denoted by W, and the perpendicular distance from the feed roller support portion  106   a  to the line along which the load of the paper  125  is applied is denoted by a. Here, a moment having the magnitude of W×a generated by the load of the paper  125  acts on the feed roller support portion  106   a.  In this case, the feed roller support portion  106   a  is positioned on the line along which the pressure of the press roller  103  is applied. Therefore, a moment from the pressure of the press roller  103  does not act on the feed roller support portion  106   a.  Since the moment from the load of the paper  125  acts on the feed roller support portion  106   a,  the position where the feed roller  102  contacts the feed roller bearing  106  shifts from the feed roller support portion  106   a  in the direction in which the load of the paper  125  is applied (the direction of the arrow H in  FIG. 14 ) due to the moment having the magnitude W×a and generated by the load of the paper  125 .  
      Japanese Patent Application Publication No. 2004-25587 discloses an image forming apparatus wherein a feed roller is supported by two contact portions provided to a feed roller bearing. The positions of the two contact portions are determined such that the feed roller does not move, in spite of the force acting on the feed roller.  
      In the structure disclosed in Japanese Patent Application Publication No. 2004-25587, the two contact portions provided to the feed roller bearing support the feed roller in a linear fashion, so the surface areas that support the feed roller are extremely small. The contact portions with a small supporting surface areas become abraded easily by the dynamic friction generated due to the pressure applied from the feed roller and the rotation of the feed roller. Abrasion of the contact portions of the feed roller bearing is problematic because the positions at which the contact portions of the feed roller bearing come into contact with the feed roller tend to become misaligned. In the structure disclosed in Japanese Patent Application Publication No. 2004-25587, since the feed roller is prevented from shifting by positioning the contact portions appropriately in the feed roller bearing, the feed roller cannot be prevented from shifting when the positions of the contact portions of the feed roller bearing are misaligned.  
      In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved image forming apparatus that overcomes the problems of the conventional art. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.  
     SUMMARY OF THE INVENTION  
      An object of this invention is to provide an image forming apparatus that is capable of preventing the feed roller from shifting.  
      The image forming apparatus according to the first aspect of the present invention has a chassis, a feed roller rotatably supported by the chassis and configured to convey paper in a first direction with a first load, a press roller rotatably supported by the chassis and configured to press on the feed roller with a predetermined pressing force, and feed roller bearings coupled to the chassis for rotatably supporting the feed roller. At least one of the feed roller bearings has a feed roller support surface that is at least partially formed into an arc shape for supporting an outer peripheral surface of the feed roller, an inner diameter of the arc-shaped portion of the feed roller support surface being substantially equal to an outer diameter of a portion of the feed roller that is supported by the feed roller support surface. A first direction end of the feed roller support surface is configured to be positioned such that a moment on the first direction end generated by the pressing force of the press roller is equal to or greater than a moment on the first direction end generated by the first load of the paper.  
      In the image forming apparatus described above, the feed roller bearing is formed with a feed roller support surface for supporting the outer peripheral surface of the feed roller, into an arc shape having the inner diameter substantially equal to the outer diameter of the portion of the feed roller supported by the feed roller bearing. Therefore, the outer peripheral surface of the feed roller can be supported by a surface, as opposed to in linear fashion.  
      Accordingly, abrasion of the feed roller support surface can be effectively reduced because the area at which the feed roller is supported increases as compared with the cases in which the feed roller is supported in linear fashion. As a result, it is possible to suppress the misalignment of the position where the feed roller support surface comes into contact with the feed roller due to the abrasion of the feed roller support portion.  
      Also, since the feed roller bearing is provided with a feed roller support surface that protruding inward toward the outer peripheral surface of the feed roller, it is possible to form the feed roller support surface into an arc shape easily. The first direction end of the feed roller support surface on the side is positioned such that the magnitude of the moment generated by the pressing force on the feed roller at the first direction end is equal to or greater than the magnitude of the moment generated by the load of the paper at the first direction end. Therefore, it is possible to prevent the feed roller from pivoting around the first direction end in the direction of the paper load. It is thereby possible to prevent the feed roller from shifting in relation to the feed roller bearing.  
      In the image forming apparatus according to the second aspect of the present invention, it is preferable the first direction end be positioned so as to satisfy 
 
 W×a≦P×b  
 
 where W is the first load of the paper, a is a perpendicular distance from the first side end to the first load of the paper, P is the pressing force from the press roller, and b is a perpendicular distance from the first direction end to the pressing force. 
 
      With such a configuration, it is possible to ensure that the feed roller does not shift relative to the feed roller bearing. It is also possible to easily determine the position of the first direction end using the above formula.  
      In the image forming apparatus according to the third aspect of the present invention, the chassis has an opening through which the feed roller is inserted, and a width of the opening of the chassis is set to be equal to or less than a width of one of the feed roller bearings. With such a configuration, there is no need to secure a tolerance between the feed roller bearing and the opening in the chassis, and the feed roller bearing can therefore be prevented from shifting horizontally relative to the chassis. Accordingly, the feed roller can thereby be prevented from shifting horizontally.  
      In the image forming apparatus according to the fourth aspect of the present invention, the arc-shaped portion of the feed roller support surface is formed on an inner side of the feed roller bearing extending along an angle range of 180° or less. With such a configuration, the outer peripheral surface of the feed roller can be supported within an angle range of 180° or less. It is therefore possible to prevent problems wherein the outer peripheral surface of the feed roller does not fit the arc-shaped feed roller support surface even though the inner diameter of the arc-shaped feed roller support surface is formed to be substantially equal to the outer diameter of the feed roller. As a result, since the outer peripheral surface of the feed roller can be fitted to the arc-shaped feed roller support surface, the feed roller can be coupled to the feed roller bearing.  
      In the image forming apparatus according to the fifth aspect of the present invention, the feed roller is configured to convey paper in a second direction with a second load, and a second direction end of the feed roller support surface is positioned above a lowest position of the feed roller support surface.  
      In the image forming apparatus according to the sixth aspect of the present invention, the second direction end is positioned so as to satisfy: 
 
 W′×a′≦P×b′ 
 
 where W′ is the second load of the paper, a′ is a perpendicular distance from the second side end to the second load of the paper, P is the pressing force from the press roller, and b′ is a perpendicular distance from the second direction end to the pressing force. 
 
      These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Referring now to the attached drawings which form a part of this original disclosure:  
       FIG. 1  is a perspective view showing the entire configuration of the heat transfer printer according to one embodiment of the present invention;  
       FIG. 2  is a schematic front view showing the motor and the gears of the heat transfer printer according to the embodiment shown in  FIG. 1 ;  
       FIG. 3  is a plan view of the heat transfer printer according to the embodiment shown in  FIG. 1 ;  
       FIG. 4  is a partial perspective view showing the mechanism by which the press roller presses on the feed roller in the heat transfer printer according to the embodiment shown in  FIG. 1 ;  
       FIG. 5  is a cross-sectional view of the heat transfer printer according to the embodiment shown in  FIG. 1 , viewed along the line  5 - 5  of  FIG. 3 ;  
       FIG. 6  is a perspective view of the feed roller bearing and the opening of the heat transfer printer according to the embodiment shown in  FIG. 1 ;  
       FIG. 7  is a cross-sectional view of the feed roller bearing of the heat transfer printer according to the embodiment shown in  FIG. 1 , viewed along the line  7 - 7  of  FIG. 5 ;  
       FIG. 8  is a schematic view for describing the steps of assembling the feed roller bearings of the feed roller in the heat transfer printer according to the embodiment shown in  FIG. 1 ;  
       FIG. 9  is a schematic view for describing the steps of assembling the feed roller bearings of the feed roller in the heat transfer printer according to the embodiment shown in  FIG. 1 ;  
       FIG. 10  is a perspective view showing the entire configuration of a conventional heat transfer printer;  
       FIG. 11  is a plan view of the conventional heat transfer printer shown in  FIG. 10 ;  
       FIG. 12  is a perspective view along the line  12 - 12  in  FIG. 11 ;  
       FIG. 13  is a perspective view of the feed roller bearing and the opening of the conventional heat transfer printer shown in  FIG. 10 ; and  
       FIG. 14  is a cross-sectional view of the feed roller bearing of the conventional heat transfer printer shown in  FIG. 10 , viewed along the line  14 - 14  of  FIG. 12 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Selected embodiments of the present invention will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.  
      Embodiments of the present invention will now be described with reference to the diagrams.  
       FIG. 1  is a perspective view showing the entire configuration of a heat transfer printer according to one embodiment of the present invention.  FIG. 2  is a schematic front view showing the motor and the gears of the heat transfer printer according to the embodiment shown in  FIG. 1 .  FIG. 3  is a plan view of the heat transfer printer.  FIGS. 4 through 9  are various views illustrating the feed roller and the feed roller bearing of the heat transfer printer shown in  FIG. 1 .  
      The structure of the heat transfer printer according to one embodiment of the present invention will now be described with reference to  FIGS. 1 through 9 . In the present embodiment, a heat transfer printer will be described as one example of the image forming apparatus of the present invention.  
      As shown in  FIGS. 1 through 3 , the heat transfer printer according to one embodiment of the present invention includes a metal chassis  1 , a metallic feed roller  2  that conveys paper  25 , a metallic press roller  3  that presses on the feed roller  2  with a specific amount of pressure, a separating member  4 , a side plate  5  that has a resinous feed roller bearing  5   a  and a platen roller bearing  5   b,  a side plate  6  that has a resinous feed roller bearing  6   a  and a platen roller bearing  6   b,  resinous press roller bearings  7  that rotatably support the press roller  3 , a feed roller gear  8  mounted on the feed roller  2 , metallic bearing support plates  9  that support the press roller bearings  7 , tension coil springs  10 , a motor  11  for driving the feed roller  2  and the like, a thermal head  12  for printing, a platen roller  13 , a torsion coil spring  14 , a motor  15  for driving the thermal head  12 , an ink sheet take-up gear  16 , a roller axle  17 , a rubber paper supply/ejection roller  18  mounted on the roller axle  17 , a roller axle gear  19  mounted on the roller axle  17 , a motor bracket  20 , and intermediate gears  21  through  24 .  
      As shown in  FIG. 1 , the motor bracket  20  is mounted on the first side surface  1   a  of the chassis  1 . Also, an ink sheet insertion part  1   c  through which ink sheets (not shown) are mounted is provided to the second side surface  1   b  of the chassis  1 . A spring mounting hole  1   d  to which one end of the tension coil spring  10  is mounted is formed in the second side surface  1   b  of the chassis  1 . Also, an opening  1   e  to which the feed roller bearing  5   a  is coupled is formed in the first side surface  1   a  of the chassis  1 . An opening  1   f  in which the feed roller bearing  6   a  is coupled is formed in the second side surface  1   b  of the chassis  1 .  
      The feed roller  2  has an outer peripheral surface  2   a  on the side of the first side surface  1   a  of the chassis  101 , an outer peripheral surface  2   b  on the side of the second side surface  1   b  of the chassis  101 , a gear insertion part  2   c,  and a paper conveying portion  2   d,  as shown in  FIG. 5 . One end of the feed roller  2  is rotatably supported by the feed roller bearing  5   a  formed integrally with the side plate  5 . The other end of the feed roller  2  is rotatably supported by the feed roller bearing  6   a  formed integrally with the side plate  6 . Also, convex portions having a specific height are formed by the roll forming on the surface of the paper conveying portion  2   d  of the feed roller  2 , as shown in  FIG. 5 .  
      In the present embodiment, as shown in  FIGS. 6 and 7 , the feed roller bearing  6   a  has a semi-cylindrical shape that opens at the top. The inner surface of the feed roller bearing  6   a  is provided with a feed roller support surface  6   c  that supports the outer peripheral surface  2   b  of the feed roller  2 . The feed roller support surface  6   c  is formed in an arc shape with the inner diameter of the feed roller support surface  6   c  being substantially equal to the outer diameter of the portion of the feed roller  2  supported by the feed roller bearing  6   a.    
      Also, the feed roller support surface  6   c  is formed so as to protrude toward the outer peripheral surface  2   b  of the feed roller  2 . Paper supply and paper eject ends  6   d  and  6   e  are formed at the end portions of the feed roller support surface  6   c.  Where the magnitude of the load of the paper  25  in the paper supply (first) direction (direction of the arrow D in  FIG. 1 ) is denoted by W, the perpendicular distance from the paper supply end  6   d  to the line along which the load of the paper  25  is applied is denoted by a, and the magnitude of the pressure from the press roller  3  is denoted by P, then the perpendicular distance b from the paper supply end  6   d  to the line on which the pressure is applied is set such that the following formula is satisfied. 
 
 W×a≦P×b  
 
      The position of the paper supply end  6   d  is thereby determined. This allows the magnitude of the moment generated by the pressure from the press roller  3  about the paper supply end  6   d  to be equal to or greater than the magnitude of the moment generated by the load of the paper  25  about the paper supply end  6   d.  As a result, the feed roller  2  is prevented from pivoting around the paper supply end  6   d  in the direction of the load of the paper  25 .  
      Also, in the present embodiment, the width of the opening  1   f  in the chassis  1  shown in  FIG. 7  is set to be equal to or less than the width of the feed roller bearing  6   a.  The feed roller bearing  6   a  is thereby mounted between the inner side surfaces of the opening  1   f  in the chassis  1  without any gaps, and the feed roller bearing  6   a  is therefore supported at three locations: the two inner side surfaces and the bottom surface of the opening  1   f  in the chassis  1 .  
      Furthermore, the feed roller support surface  6   c  is formed on the inner surface of the feed roller bearing  6   a  extending along an angle range of less than 180°. Particularly, the paper ejection side end  6   e  in the paper ejection direction (the direction of the arrow C in  FIG. 7 ) of the feed roller support surface  6   c  shown in  FIGS. 6 and 7  is located farther along the paper ejection direction (the direction of the arrow C in  FIG. 7 ) than the lowest point  6   f  of the feed roller support surface  6   c.  The lowest point  6   f  is positioned on the line at which the pressure from the press roller  3  is applied. The paper ejection side end  6   e  is also located at a position higher than the lowest point  6   f  of the feed roller support surface  6   c.    
      Particularly, where the magnitude of the load of the paper  25  in the paper ejection (second) direction (direction of the arrow C in  FIG. 1 ) is denoted by W′, the perpendicular distance from the paper ejection side (second direction) end  6   e  to the line along which the paper ejection direction load of the paper  25  is applied is denoted by a′, and the magnitude of the pressure from the press roller  3  is denoted by P, then the perpendicular distance b′ from the paper ejection side end  6   e  to the line on which the pressure is applied is set such that the following formula is satisfied. 
 
 W′×a′≦P×b′ 
 
 The position of the paper ejection side end  6   e  is thereby determined. The feed roller  2  is thereby prevented from being moved in the paper ejection direction (the direction of the arrow C in  FIG. 7 ) by the weight of the feed roller  2  and the pressure from the press roller  3 . 
 
      The metallic separating members  4  having a larger diameter than that of the press roller  3  are coupled to the outer peripheral surfaces  3   a  and  3   b  of the press roller  3 , as shown in  FIG. 5 . By bringing the separating member  4  into contact with the outermost peripheral surface of the feed roller  2 , the distal ends of the convex portions on the paper conveying portion  2   d  of the feed roller  2  are prevented from coming into contact with the press roller  3 . Also, the outer peripheral surfaces  3   a  and  3   b  of the press roller  3  are rotatably supported by the press roller bearing  7 .  
      Each of the press roller bearings  7  is open at the portion facing the feed roller  2 . The press roller bearings  7  are mounted on the bearing support plates  9  provided to the inner sides of both the first side surface  1   a  and the second side surface  1   b  of the chassis  1 . The bearing support plates  9  are mounted on the first side surface  1   a  and second side surface  1   b  of the chassis  1  so as to be rotatable around support portions  9   a,  as shown in  FIG. 4 .  
      The first ends of the tension coil springs  10  that urge the press roller  3  in the direction of applying pressure on the feed roller  2  are mounted in the spring mounting holes  1   d  (see  FIG. 1 ) of the chassis  1 , as described above. Also, second ends of the tension coil spring  10  are mounted on spring mounting portions  9   b  of the bearing support plates  9 .  
      The thermal head  12  is mounted on the inner sides of the first side surface  1   a  and the second side surface  1   b  of the chassis  1  so as to be pivotable around a supporting axle  12   a,  as shown in  FIGS. 1 and 3 . The torsion coil spring  14  is mounted on the supporting axle  12   a  next to the first side surface  1   a  of the chassis  1 . The torsion coil spring  14  urges the thermal head  12  in a direction away from the platen roller  13 . During the printing operation, the thermal head  12  pivots in a direction in which pressure is exerted on the platen roller  13  (the direction of the arrow A in  FIG. 1 ), and presses on the platen roller  13 . The thermal head  12  is disposed so as to face the platen roller  13 .  
      The driving force from the motor  11  is transmitted to the feed roller gear  8  via the intermediate gear  21 , as shown in  FIGS. 2 and 3 . The feed roller gear  8  engages both the intermediate gears  22  and  23 . The intermediate gear  22  transmits the driving force from the feed roller gear  8  to the ink sheet take-up gear  16  fitted over the ink sheet take-up member (not shown). Also, the intermediate gears  23  transmit the driving force from the feed roller gear  8  to the roller axle  17  on which the paper supply/ejection roller  18  is mounted via the roller axle gear  19 .  
       FIGS. 8 and 9  are cross-sectional views illustrating the steps of assembling the feed roller bearings  5   a  and  6   a  of the feed roller  2  in the heat transfer printer according to the embodiment of the present invention. The steps of assembling the feed roller  2  will now be described with reference to  FIGS. 8 and 9 . First, as shown in  FIG. 8 , the gear insertion part  2   c  of the feed roller  2  is fitted in an insertion hole  8   a  of the feed roller gear  8 . From this state, the other end of the feed roller  2  is inserted into the opening  1   e  and the feed roller bearing  5   a  from the outer side of the side plate  5 . Then, the feed roller  2  is coupled on the feed roller bearings  5   a  and  6   a  by moving the feed roller  2  toward the bottom surface of the chassis  1 . This results in the state shown in  FIG. 9 .  
      OPERATION  
      Next, the operation of feeding the paper  25  in the heat transfer printer according to one embodiment of the present invention will be described with reference to  FIGS. 1 through 3 . The driving force from the motor  11  is transmitted to the feed roller gear  8  via the intermediate gear  21 , as shown in  FIGS. 2 and 3 . The feed roller  2  thereby rotates. Also, the driving force from the motor  11  is transmitted to the ink sheet take-up gear  16  via the intermediate gear  22 . The ink sheet take-up member (not shown) thereby rotates. The driving force from the motor  11  is also transmitted to the roller axle gear  19  via the plurality of intermediate gears  23 . The paper supply/ejection roller  18  thereby rotates.  
      As shown in  FIG. 1 , when paper is supplied, the thermal head  12  is pivoted in a direction away from the platen roller  13  (the direction of the arrow B in  FIG. 1 ) by the torsion coil spring  14 . Also, when paper is supplied, the paper  25  is delivered in the paper supply direction (the direction of the arrow D in  FIG. 1 ) by the feed roller  2  and the paper supply/ejection roller  18 .  
      During the printing operation, the thermal head  12  is pivoted in a direction in which pressure is exerted on the platen roller  13  (the direction of the arrow A in  FIG. 1 ) by the motor  15 , and presses on the platen roller  13 . Also during the printing operation, the paper  25  and the ink sheet (not shown) are conveyed while held between the thermal head  12  and the platen roller  13 . The paper  25  is conveyed in the paper ejecting direction (the direction of the arrow C in  FIG. 1 ) by the feed roller  2 , the press roller  3 , and the paper supply/ejection roller  18 . The ink sheet (not shown) is conveyed in the paper ejecting direction (the direction of the arrow C in  FIG. 1 ) by the feed roller  2 , the press roller  3 , and the ink sheet take-up member (not shown).  
      In the present embodiment, as described above, the feed roller bearing  6   a  is provided with a feed roller support surface  6   c  that is formed in an arc shape with an inner diameter that is substantially equal to the outer diameter of the portion of the feed roller  2  supported by the feed roller bearing  6   a.  Also, the feed roller support surface  6   c  supports the outer peripheral surface  2   b  of the feed roller  2 . Therefore the outer peripheral surface  2   b  of the feed roller  2  can be supported in planar fashion, as opposed to in linear fashion. It is therefore possible to better prevent abrasions in the feed roller support surface  6   c,  because the feel roller support surface  6   c  supports the feed roller  2  with a greater surface area than when the roller is supported in linear fashion. As a result, it is possible to prevent friction-induced misalignments of the position at which the feed roller support surface  6   c  comes into contact with the feed roller  2 . Also, since the feed roller support surface  6   c  is formed so as to protrude toward the outer peripheral surface  2   b  of the feed roller  2 , it is possible to easily form the feed roller support surface  6   c  into an arc shape.  
      Also, in the present embodiment, the feed roller  2  can be prevented from pivoting around the paper supply end  6   d  in the direction of the load of the paper since the paper supply end  6   d  of the feed roller support surface  6   c  is formed at such a position that the magnitude of the moment generated by the pressure applied to the feed roller  2  about the paper supply end  6   d  is equal to or greater than the magnitude of the moment generated by the load of the paper  25  about the paper supply end  6   d.  The feed roller  2  can thereby be prevented from shifting relative to the feed roller bearing  6   a.    
      Also, in the present embodiment, the width of the opening  1   f  in the chassis  1  through which the feed roller bearing  6   a  is inserted is set to be equal to or less than the width of the feed roller bearing  6   a.  Therefore, there is little gap between the feed roller bearing  6   a  and the inner surface of the opening If in the chassis  1 , and the feed roller bearing  6   a  can be prevented from shifting horizontally relative to the chassis  1 . The feed roller  2  can thereby be prevented form shifting horizontally.  
      Also, in the present embodiment, the arc-shaped feed roller support surface  6   c  is formed to extend on the inner surface of the feed roller bearing  6   a  within an angle range of less than 180°. Thus, the outer peripheral surface  2   b  of the feed roller  2  can be supported within an angle range of less than 180°. Therefore, it is possible to prevent problems wherein the outer peripheral surface  2   b  of the feed roller  2  does not engage with the arc-shaped feed roller support surface  6   c  even though the arc-shaped feed roller support surface  6   c  is formed to have an inner diameter substantially equal to the outer diameter of portion of the feed roller  2  to be supported by the feed roller bearing  6   a.  As a result, the outer peripheral surface  2   b  of the feed roller  2  can be securely supported by the arc-shaped feed roller support surface  6   c,  and the feed roller  2  can therefore be coupled to the feed roller bearing  6   a.    
      The embodiment currently disclosed should be considered as merely an example in all respects and not as being restrictive. The range of the present invention is expressed by the patent claims and not by the above descriptions of the embodiment, and further includes meanings equivalent to the range of the patent claims and all variations thereof.  
      For example, although a heat transfer printer is given as an example of an image forming apparatus in the embodiment described above, the present invention is not limited thereto. The present invention can also be applied to image forming apparatuses other than heat transfer printers as long as such image forming apparatus has a feed roller.  
      Also, in the embodiment described above, the present invention is applied to the feed roller bearing  6   a  and the opening  1   f  on the second side surface  1   b  of the chassis  1 , but the present invention is not limited thereto. The present invention may also be applied to the feed roller bearing  5   a  and the opening  1   e  on the first side surface  1   a  of the chassis  1 .  
      Also, in the embodiment described above, the feed roller bearing  6   a  is formed in a semi-cylindrical shape that opens at the top as shown in  FIG. 4 , but the present invention is not limited thereto. The feed roller bearing  6   a  may also be formed in a cylindrical shape.  
      As used herein, the following directional terms “forward, rearward, above, downward, vertical, horizontal, below and transverse” as well as any other similar directional terms refer to those directions of a device equipped with the present invention. Accordingly, these terms, as utilized to describe the present invention should be interpreted relative to a device equipped with the present invention.  
      The term “configured” as used herein to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function.  
      Moreover, terms that are expressed as “means-plus function” in the claims should include any structure that can be utilized to carry out the function of that part of the present invention.  
      The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies.  
      This application claims priority to Japanese Patent Application No. 2004-203138. The entire disclosure of Japanese Patent Application No. 2004-203138 is hereby incorporated herein by reference.  
      While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. Thus, the scope of the invention is not limited to the disclosed embodiments.