Abstract:
The invention addresses the problem of providing a steady drive to the photosensitive drum when all loads are driven from a common motor. To address the issue, the invention includes three drive trains. A first drive train that extends from the drive shaft of the motor to the photosensitive drum; a second drive train that extends from the drive shaft of the motor to the heating mechanism; and a third drive train that extends from the drive shaft of the motor to the toner cassette and drives the toner feed elements contained therein. A first drive element for each of the first and second drive trains is independently rotatably mounted on a common axis. Positioned on substantially an opposite side of the drive shaft is a first drive element of the third drive mechanism. As a result, the forces applied to the drive shaft as reaction forces to driving the various loads are substantially counterbalanced. Further, because the first drive element of each of the first and second drive trains are separate from one another in rotation, the second drive train has no effect on the first drive train driving the photosensitive drum.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention is related to an image forming apparatus for providing stable rotation of an image bearing body. 
   2. Description of Related Art 
   Conventionally, in an image forming apparatus using an electrophotographic method, such as a laser printer, a static latent image is formed on a photosensitive drum, and the electrostatic latent image is developed by a developing roller where toner is supplied from a toner supply roller. The developed toner image is transferred to a recording medium and fixed onto the recording medium by applying high temperature thereto by a fixing roller. The recording medium is transported to the photosensitive drum by a recording medium transporting roller. 
   Therefore, in the image forming apparatus, the photosensitive drum, the developing roller, the toner supply roller, the fixing roller, the recording medium transporting roller and other components are driven by a motor. 
   Recently, an image forming apparatus having one motor that drives the rollers including the photosensitive drum has been known, as disclosed in Japanese Unexamined Patent Publication No. 8-137180. In this image forming apparatus, a plurality of gears are used for transferring driving force from a motor shaft to the rollers including the photosensitive roller. 
   SUMMARY OF THE INVENTION 
   The invention addresses the problem of providing a steady drive to the photosensitive drum when all loads are driven from a common motor. To address the issue, the invention includes three drive trains. A first drive train that extends from the drive shaft of the motor to the photosensitive drum; a second drive train that extends from the drive shaft of the motor to the heating mechanism; and a third drive train that extends from the drive shaft of the motor to the toner cassette and drives the toner feed elements contained therein. 
   A first drive element for each of the first and second drive trains is independently rotatably mounted on a common axis. Positioned on substantially an opposite side of the drive shaft is a first drive element of the third drive mechanism. As a result, the forces applied to the drive shaft as reaction forces to driving the various loads are substantially counterbalanced. Further, because the first drive element of each of the first and second drive trains are separate from one another in rotation, the second drive train has no effect on the first drive train driving the photosensitive drum. 
   The first drive elements for each of the first, second and third drive trains are helical gears and engage a gear that is either formed in the drive shaft of the drive motor or a gear fixedly mounted to the drive shaft of the drive motor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with reference to the drawings, in which: 
       FIG. 1  is a side cross-sectional view of a laser printer; 
       FIG. 2  is a side cross-sectional view of an image forming portion; 
       FIG. 3  is a side view of a process cartridge; 
       FIG. 4  is a horizontal cross sectional view of a driving mechanism of the laser printer; 
       FIG. 5  is a view of the driving mechanism of  FIG. 4  seen from the side; 
       FIG. 6  is a plane view of a main gear unit as viewed in the same direction as in  FIG. 5 ; 
       FIG. 7  is a view seen from a rear side in  FIG. 6 ; 
       FIG. 8  is a cross sectional view of a drum driving gear and a thin gear; 
       FIG. 9  is a view of a portion of a discharge gear unit that is arranged in the driving mechanism shown in  FIG. 5 ; 
       FIG. 10  is a view of the discharge gear unit shown in  FIG. 9 ; 
       FIG. 11  is a central cross-sectional view of a twelfth idle gear; and 
       FIG. 12  is a view showing the twelfth idle gear is mounted in a fourth frame. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The structure of a laser printer  1  will be explained with reference to FIG.  1 .  FIG. 1  is a central cross sectional view of a laser printer of the embodiment. As shown in  FIG. 1 , the laser printer  1  comprises a casing  2  and in the casing  2 , a feeder portion  4  for supplying a paper  3  as a recording medium and an image forming portion  5  for forming a predetermined image onto the supplied paper  3 . The left side in  FIG. 1  is the front side of the laser printer  1 . 
   A discharge tray  46  is formed in a recess portion at an upper rear side in the casing  2  so as to stack the printed papers  3 . A cartridge accommodation portion  57  is arranged at an upper front side in the casing  2 . The cartridge accommodation portion  57  is a space when its upper surface is open. A process cartridge  17  is mounted in or removed from the cartridge accommodation portion  57 . The cartridge accommodation portion  57  is covered with an upper cover  54  that is rotated up and down around a support shaft  54   a  arranged at a front end of the discharge tray  46 . The position of the upper cover  54  when it is open is shown by a double dotted line in FIG.  1 . 
   A discharge path  44  is arranged at a rear side (a right side in  FIG. 1 ) in the casing  2 . The discharge path  44  is formed in an arc in an up-down direction along a rear surface of the casing so that a paper  3  is discharged from a fixing device  18  to the discharge tray  46 . The fixing device  18  is arranged at a lower rear side in the casing  2  and the discharge tray  46  is arranged at an upper rear side in the casing  2 . A discharge roller  45  is arranged on the discharge path  44  for transporting the paper  3 . 
   Because the discharge path  44  is formed in an arc, the paper  3 , whose upper surface has a printed image, is discharged onto the discharge tray  46  with its upper surface facing down. This discharge method is called face-down method. When a plurality of papers are printed, the papers  3  are stacked with their printed surfaces facing down in a discharged order and the printed papers  3  are ordered in the printed order. 
   The feeder portion  4  comprises a supply roller  8 , a supply tray  6 , a paper pressing plate  7 , a separation pad  9 , transporting rollers  11 , paper powder removing rollers  10  and resist rollers  12 . The supply roller  8  is arranged at a bottom in the casing  2 . The supply tray  6  is detachably arranged in the casing  2 . The paper pressing plate  7  is arranged in the supply tray  6  so as to stack the papers  3  thereon and press the papers  3  to the supply roller  8 . The separation pad  9  is arranged at one side end of the supply tray  6  so as to be pressed toward the supply roller  8 . The separation pad  9  presses a paper  3  with the supply roller  8  for transporting the paper  3  and preventing more than one paper  3  being fed at a time. The transporting rollers  11  are arranged at two points in a lower stream side of a paper  3  transporting direction with respect to the supply roller  8  for transporting the paper  3 . The paper powder removing rollers  10  remove paper powder via the paper  3  by opposing the paper transporting rollers  11  that transport the paper  3  in cooperation with the opposing transporting rollers  11 . The resist rollers  12  are arranged downstream in the paper  3  transporting direction with respect to the transporting rollers  11 . The resist rollers  12  adjust a timing for feeding a paper  3  during the printing operation. 
   The paper pressing plate  7  has papers  3  stacked thereon. A support shaft  7   a  of the paper pressing plate  7 , arranged at an end away from the paper feeding roller  8 , is supported by the bottom surface of the supply tray  6  and the end closest to the paper feeding roller  8  is movable up and down around the support shaft  7   a . The paper pressing plate  7  is urged toward the paper feeding roller  8  by a spring (not shown) from the rear side of the paper pressing plate  7 . 
   The paper pressing plate  7  is moved downwardly around the support shaft  7   a  against the urging force of the spring as the stacked amount of the papers  3  increases. The supply roller  8  and the separation pad  9  are arranged to face one another. The separation pad  9  is pressed toward the supply roller  8  by a spring  13  arranged at a rear side of the separation pad  9 . 
   A manual tray  14  is arranged at a front surface side (a left side in  FIG. 1 ) of the casing  2 . The manual tray  14  comprises a tray  14   b  and a cover  14   c . The tray  14   b  is open and closed in a front and rear direction (left and right direction in  FIG. 1 ) around a support shaft  14   a . The tray  14   b  holds stacked papers  3  when opened. The cover  14   c  slides with respect to the tray  14   b  and becomes a part of the casing  2  when the tray  14   b  is closed. A manual roller  15  and a separation pad  25  are arranged in the vicinity of the manual tray  14 . The manual roller  15  feeds the papers  3  stacked on the tray  14   b  of the manual tray  14 . The separation pad  25  prevents more than one paper  3  being fed at a time. 
   The manual roller  15  and the separation pad  25  are arranged to face one another and the separation pad  25  is pressed toward the manual roller  15  by a spring (not shown) arranged at a rear side of the separation pad  25 . The papers  3  stacked on the manual tray  14  are separated by the separation pad  25  one by one and the separated paper  3  is transported to the resist rollers  12  by the manual roller  15 . 
   The structure of the image forming portion  5  will be explained with reference to  FIGS. 2 and 3 .  FIG. 2  is a cross-sectional view of the image forming portion  5  seen from the side.  FIG. 3  is a side view of the process cartridge  17 . As shown in  FIG. 2 , the image forming portion  5  comprises a scanner unit  16 , the process cartridge  17  and the fixing device  18  so as to form an image on the paper  3  that is transported by the feeder portion  4 . 
   The scanner unit  16  is arranged at a lower side of the discharge tray  46  in the casing  2 . The scanner unit  16  comprises a laser emission portion (not shown), a polygon mirror  19 , a fθ lens  20 , a reflection mirror  21  and a relay lens  22 . The laser emission portion emits a laser beam. The polygon mirror  19  is rotated to scan the laser beam, that is emitted from the laser emission portion, in a main scanning direction. The fθ lens  20  keeps the scanning speed of the laser beam constant. The reflection mirror  21  reflects the scanned laser beam. The relay lens  22  adjusts a focus position so as to form an image on the photosensitive drum  27  by the laser beam reflected by the reflection mirror  21 . 
   The laser beam emitted from the laser emission portion, based on predetermined image data, is passed through or reflected by the polygon mirror  19 , the fθ lens  20 , the reflection mirror  21  and the relay lens  22 , in order, as shown by a dash and dot line A. Accordingly, the laser beam is irradiated to the surface of a photosensitive drum  27  of the process cartridge  17 . 
   The process cartridge  17  comprises the photosensitive drum  27 , a scorotron type charger  29 , a developing roller  31 , a toner supply roller  33 , a toner box  34 , a transfer roller  30 , a cleaning roller  51  and a secondary roller  52 . The photosensitive drum  27  is rotatably arranged at a side of the developing roller  31  so that a rotational shaft of the photosensitive drum  27  is parallel to the rotational shaft of the developing roller  31 . The photosensitive drum  27  is rotated in a counterclockwise direction shown by an arrow in  FIG. 2 , with contact with the developing roller  31 . The rotational speed of the photosensitive drum  27  is different from that of the developing roller  31  so that there is a rotational speed difference between the photosensitive drum  27  and the developing roller  31 . Due to the rotational speed difference, the load for rotating the developing roller  31  and the photosensitive roller  27  is increased. 
   A charge generation layer and a charge transporting layer are laminated on a conductive substrate to form the photosensitive drum  27 . An organic beam electric conductive body, such as an azo pigment or a phthalocyanine pigment, is dispersed in a binder resin as a charge generation material to form the charge generation layer. A compound, such as a hydrazone type or an aryl amine type, is mixed in a resin, such as polycarbonate, to form the charge transporting layer. 
   The scorotron type charger  29  is arranged above and apart from the photosensitive drum  27  by a predetermined space therebetween so as not to contact the photosensitive drum  27 . The scorotron type charger  29  is for positive charging. In the positive charging, corona discharge is generated from a discharge wire made of tungsten. The scorotron type charger  29  positively charges the surface of the photosensitive drum  27  uniformly. 
   When the photosensitive drum  27  is irradiated by the laser beam, a charge is generated in the charge generation layer by light absorption and the charge is transported to the surface of the photosensitive drum  27  by the charge transporting layer. The charge transported by the charge transporting layer nullifies the surface potential that is charged by the scorotron type charger  29 . Accordingly, a potential difference is generated between a potential of an irradiated portion and a potential of a non-irradiated portion. The potential difference forms an electrostatic latent image. 
   The developing roller  31  is arranged at a down stream side of the scorotron type charger  29  with respect to the rotational direction (a counterclockwise direction in FIG.  2 ) of the photosensitive drum  27 . The developing roller  31  is rotatable in a clockwise direction as shown by an arrow in FIG.  2 . The developing roller  31  is formed by covering a metal roller shaft with a roller portion of a conductive rubber material and a developing bias is applied to the developing roller  31  from a developing bias applying electric source (not shown). 
   The toner supply roller  33  is aligned with the developing roller  31  and is arranged rotatably at a position opposite to the photosensitive drum  27  with respect to the developing roller  31 . The supply roller  33  is in contact with the developing roller  31  in a compressed condition. There is a speed difference between the rotational speed of the toner supply roller  33  and that of the developing roller  31 . 
   The supply roller  33  is formed by covering a metal roller shaft with a roller portion of a conductive foaming material. Toner is charged by frictional force that is generated by the developing roller  31  and the toner supply roller  33 . The toner box  34  is arranged near the toner supply roller  33  and stores therein toner that is supplied to the developing roller  31  via the toner supply roller  33 . 
   In this embodiment, the positive charged non-magnetic one component polymerized toner is used as a developer. The toner is polymerized toner that is obtained by copolymerizing a polymerization monomer, such as styrene monomer, or an acrylic monomer, such as acryl acid, alkyl (C1-C4) acrylate, and alkyl (C1-C4) methaacrylate, by a known polymerization method such as suspension polymerization. A particle diameter of the polymerization toner is approximately 6-10 μm. A coloring agent, such as carbon black or wax, is mixed with the polymerization toner and an additive, such as silica, is added to the polymerization toner for improving fluidity. 
   An agitator  36  is supported by a rotational shaft  35  and is rotatably arranged at a center of the toner box  34 . When the agitator  36  is rotated in a counterclockwise direction, shown by an arrow in  FIG. 2 , the toner in the toner box  34  is agitated and fed toward the toner supply roller. A window  38  is arranged on a side wall of the toner box  34  for detecting the remaining amount of toner. The window  38  is cleaned by a cleaner  39  that is supported by the rotational shaft  35 . 
   The transfer roller  30  is arranged at a lower stream side from the developing roller  31  and at a lower side of the photosensitive drum  27  with respect to the rotational direction of the photosensitive drum  27 . The transfer roller  30  is supported rotatably in a clockwise direction shown by an arrow in FIG.  2 . The transfer roller  30  is formed by covering a metal roller shaft with a roller of an ion conductive rubber material. Transfer bias is applied to the transfer roller  30  from the transfer bias applying electric source during a transfer operation. 
   The cleaning roller  51  is arranged near the photosensitive drum  27 . The cleaning roller  51  is positioned at a downstream side from the transfer roller  30  and an upstream side from the scrotron type charger  29  with respect to the rotational direction of the photosensitive drum  27 . A secondary roller  52  is arranged at an opposite side of the cleaning roller  51  from the photosensitive drum  27 , which is positioned therebetween, so as to contact the cleaning roller  51 . A wiping member  53  contacts the secondary roller  52 . 
   The photosensitive drum  27  of the laser printer  1  is cleaned by a cleanerless method described below. After toner is transferred from the photosensitive drum  27  to a paper  3 , the toner or the paper powder remaining on the surface of the photosensitive drum  27  is electrically attracted by the cleaning roller  51 . Only the paper powder that is electrically attracted by the cleaning roller  51  is electrically attracted to the secondary roller  52 . The paper powder attracted by the secondary roller  52  is wiped by the wiping member  53 . The toner that is attracted by the cleaning roller  51  is returned to the photosensitive drum  27  and collected by the developing roller  31 . 
   In the process cartridge  17 , an exposure window  69  is arranged above the photosensitive drum  27  so that the laser beam from the scanner unit  16  is irradiated to the photosensitive drum  27 . The exposure window  69  is arranged on an upper surface of a case  40  of the process cartridge  17  and at the toner box  34  side from the opening  171  of the scorotron type charger  29 . The photosensitive drum  27  communicates with outside of the process cartridge  17  via the exposure window  69 . 
   As shown in  FIG. 3 , the drive shaft  27   a  of the photosensitive drum  27  extends from the left and right sides of the case  40 . A transfer gear  27   b  is fixed to the drive shaft  27   a . A gear surface of the transfer gear  27   b  is partially exposed on one side of the case  40 . A small gear portion  112   a  ( FIG. 4 ) of a drum drive gear  112 , that is interlocked with the transfer gear  27   b , is exposed to the cartridge accommodation portion  57  (FIG.  1 ). 
   A guide plate  60  is arranged in the vicinity of the drive shaft  27   a  in the case  40  of the process cartridge  17 . When the process cartridge  17  is mounted in the casing  2 , the drive shaft  27   a  and the guide plate  60  are engaged to a guide groove that is formed in the casing  2 . Accordingly, the process cartridge  17  is smoothly mounted in and removed from the casing  2 . 
   A driving force input portion  70  is arranged almost centrally on a side surface of the case  40  of the process cartridge  17  where the transfer gear  27   b  is arranged. A cylindrical shaft receiving member  70   a  is formed in the driving force input portion  70 . Two projections are arranged on an inner wall of the shaft receiving member  70   a  so as to be extended toward the center shaft and to face each other. The side surface where the driving force input portion  70  is arranged is a right side surface with respect to the insertion direction of the process cartridge  17  into the casing  2 . 
   As shown in  FIG. 2 , the fixing device  18  is arranged at a lower stream side of the process cartridge  17 . The fixing device  18  comprises a heat roller  41 , a pressure roller  42  for pressing the heat roller  41 , and a pair of transporting rollers  43  arranged at a downstream side of the heat roller  41  and the pressure roller  42 . The heat roller  41  is made of a cylindrical metal roller and has a halogen lamp in the roller as a heating source. 
   Toner that is transferred to a paper  3  in the process cartridge  17  is melted by the heat and pressed and fixed onto the paper  3  when the paper  3  passes between the heat roller  41  and the pressure roller  42 . Afterwards, the paper  3  is transported to the paper discharge path  44  by the transporting roller  43 . 
   The driving mechanism  100  of the laser printer  1  will be explained with reference to  FIGS. 4 and 5 .  FIG. 4  is a horizontal cross sectional view of the driving mechanism  100  of the laser printer  1  and  FIG. 5  is a view of the driving mechanism  100  shown in  FIG. 4  seen from the left side. 
   As shown in  FIG. 4 , a gear tooth is integrally formed with a drive shaft  111  of a DC brushless motor  110 . A drum drive gear  112  for driving the photosensitive drum  27 , a thin gear  113  and a first idle gear  114  are interlocked with the drive shaft  111 . The thin gear  113  is arranged on the same shaft as the drum drive gear  112 . 
   The drive shaft  11  is made of stainless steel. The drum drive gear  112  and the thin gear  113  and a first idle gear  114  are arranged symmetrically with respect to the drive shaft  111 . The drum drive gear  112 , the thin gear  113  and the first idle gear  114  are interlocked with the drive shaft  111  and are helical tooth gears, i.e., the gear tooth thereof is not symmetrical with respect to a line perpendicular to the rotational shaft of each gear. Thus, each gear is rotated only in one direction, and only one side of the gear tooth is necessarily formed with certain precision. Accordingly, the manufacturing cost of the gears is decreased. 
   A second idle gear  115  is interlocked with the first idle gear  114 . A third idle gear  116  is interlocked with the second idle gear  115 . An electromagnetic clutch  117  is arranged on the third idle gear  116  and the third idle gear  116  is connected to the fourth idle gear  118  via the electromagnetic clutch  117 . The fourth idle gear  118  is interlocked with an input gear  119 . 
   When the third idle gear  116  is rotated with the electromagnetic clutch  117  being in a connected condition, the fourth idle gear  118  is rotated and the input gear  119  is rotated. A driving force input portion  120  is arranged at the distal end of the rotational shaft of the input gear  119 . 
   An input terminal  122 , that is urged by a spring  123  into an engagement position, is arranged at the distal end of the driving force input portion  120 . As shown in  FIG. 4 , the input terminal  122  is detachably fitted to a shaft receiving member  70   a  that is arranged on the process cartridge  17 . 
   When the input gear  119  is rotated during the fitted condition, the input terminal  122  transfers the driving force to the driving force input portion  70 , shown in  FIG. 3 , to drive the developing roller  31 , the supply roller  33  and the agitator  36 . 
   U.S. patent application Ser. No. 10/397,371, filed concurrently and identified by Attorney Reference No. 115213, having the same application date as the Japanese Priority Patent Application for the instant U.S. Patent Application, discloses the structure of the process cartridge  17  wherein the driving force that is transferred to the driving force input portion  70  drives the developing roller  31 , the supply roller  33  and the agitator  36 . The disclosure of U.S. patent application No. 10/397,371, filed concurrently and identified by Attorney Reference No. 115213, is incorporated by reference herein in its entity. 
   The drum drive gear  112  is a two stage gear and includes a small gear portion  112   a . Because the diameter of the small gear portion  112   a  is smaller than that of the drum drive gear  112 , the small gear portion  112   a  rotates at slower rotational speed than the drum drive gear  112 . The small gear portion  112   a  is interlocked with the transfer gear  27   b  that is formed at the end of the photosensitive drum  27 . Therefore, when the drum drive gear  112  is rotated, the photosensitive drum  27  rotates. 
   The thin gear  113  is arranged on the same shaft as the drum drive gear  112 . However, it is independently rotatable. A fifth idle gear  130  is also interlocked with the thin gear  113 . The fifth idle gear  130  is interlocked with a sixth idle gear  141 . An electromagnetic clutch  140  is arranged on the sixth idle gear  141 . The driving of the sixth idle gear  141  is transferred to a rotational shaft  142  of the resist roller  12  via the electromagnetic clutch  140 . 
   The driving mechanism  100  comprises a first frame  101 , an upper first frame  101   a  ( FIG. 6 ) and a second frame  102 . The first frame  101 , the upper first frame  101   a  and the second frame  102  are flat plates and arranged parallel to each other. One end of the rotational shaft of each of the above described gears is supported by one of the three frames and the other end of each rotational shaft is supported by a frame that faces the frame that supports the one end of the rotational shaft. That is, because the two ends of the rotational shaft of each gear are supported, the rotational shaft is prevented from becoming slanted. 
   With reference to  FIGS. 4 and 5 , the gear mechanism for rotating the heat roller  41  of the fixing device  18  will be explained. As shown in  FIG. 4 , the rotation of the thin gear  113  is transferred to a seventh idle gear  132  that is supported between a fourth frame  106  and a fifth frame  107 . The thin gear  113  and the seventh idle gear  132  appear not to be interlocked with each other, as shown in  FIG. 4 , however, they are actually interlocked with each other as shown in FIG.  5 . The seventh idle gear  132  is a two stage gear and a small gear  132   a  is integrally formed with the rotational shaft of the seventh idle gear  132 . An eighth idle gear  133  is interlocked with the small gear  132   a.    
   A ninth idle gear  138  is interlocked with the eighth idle gear  133 . A tenth idle gear  134  is interlocked with the ninth idle gear  138 . An eleventh idle gear  135  is interlocked with the tenth idle gear  134 . A twelfth idle gear  136  is interlocked with the eleventh idle gear  135 . A heat roller gear  41   a , that is arranged at the end of the heat roller  41  of the fixing device  18 , is interlocked with the twelfth idle gear  136 . 
   The structure of a main gear unit  150  will be explained with reference to  FIGS. 6 and 7 . The main gear unit  150  is a part of the driving mechanism  100  shown in FIG.  5 .  FIG. 6  is a view of the main gear unit  150  seen from the same side as FIG.  5 .  FIG. 7  is a view showing the main gear unit  150  of  FIG. 6  seen from the rear side or inner case  2  sides. 
   As shown in  FIG. 6 , the main gear unit  150  comprises the first frame  101  and the upper first frame  101   a  that is fixed to the first frame  101  with screws. The first frame  101  and the upper first frame  101   a  are formed by cutting a metal plate into a predetermined shape and bent. 
   The DC brushless motor  110  is arranged at a center of the first frame  101 . An insulating member  151 , formed of a resin film, is arranged on a part of the first frame  101  as shown by shading in  FIG. 6. A  harness (not shown) is arranged on the insulating member  151 . Therefore, even if the harness rubs against the insulating member  151  and is damaged, the harness is not electrically shorted. 
   As shown in  FIG. 7 , the drum drive gear  112  and the idle gear  114  are arranged on the rear side of the main gear unit  150  so as to be symmetrical with respect to the drive shaft  111  of the DC brushless motor  110 . This arrangement prevents a force that could slant the drive shaft  111  in one direction from affecting the drive shaft  111 . 
   Only the transfer gear  27   b , that is interlocked with the small gear portion  112   a , is connected to the drum drive gear  112 , that is interlocked with the drive shaft  111  of the motor  110 . The rotation of the drum drive gear  112  is transferred only to the photosensitive drum  27  and not transferred to other loads. Therefore, the drum drive gear  112  is not influenced by the other loads and the driving force from the drive shaft  111  of the DC brushless motor  110  is stably converted to the rotation of the photosensitive drum  27 . Accordingly, the photosensitive drum  27  is rotated stably and images of high quality are obtained. 
   Because the line length of a line connecting a position where the drum drive gear  112  is interlocked with the drive shaft  111  and a position where the idle gear  114  is interlocked with the drive shaft  111  is almost equal to the diameter of the drive shaft  111 , no unnecessary moment is applied to the drive shaft  111 . 
   The structure of the drum drive gear  112  and the thin gear  113  will be explained with reference to FIG.  8 .  FIG. 8  is a cross sectional view of the drum drive gear  112  and the thin gear  113 . The drum drive gear  112  and the thin gear  113  are made of polyphenylene sulfaide resin. The drum drive gear  112  and the thin gear  113  are supported by a shaft  112   d , that extends from the first frame  101 , so as to be rotatable independently of each other and are fixed by a washer  112   b  and a snap ring  112   c.    
   As described above, the drum drive gear  112  transfers the driving force only to the photosensitive drum  27  and the thin gear  113  transfers the driving force only to the resist roller  12 . Because the gears that transfer the driving force to different loads are arranged on the same shaft, the space required is small. 
   The structure of a discharge gear unit  170  for transferring the driving force to the heat roller  41  will be explained with reference to  FIGS. 9 and 10 .  FIGS. 9 and 10  are views of the discharge gear unit that is arranged in the driving mechanism  100  as shown in FIG.  5 . 
   As shown in  FIG. 10 , the third frame  105  and the fourth frame  106  are arranged in the discharge gear unit  170  so as to be parallel to each other. The tenth idle gear  134 , the eleventh idle gear  135  and the twelfth idle gear  136  are supported by shafts between the third frame  105  and the fourth frame  106 . As shown in  FIG. 9 , a support shaft  108   a  and a hollow support shaft  108   b  are arranged between the fourth frame  106  and the subframe  108 . The tenth idle gear  134  is supported by the hollow support shaft  108   b.    
   The support shafts  108   a ,  108   b  are supported so that the two ends of each support shaft  108   a ,  108   b  are supported by the fourth frame  106  and the subframe  108 . The support shaft  106   a  is supported so that its two ends are supported by the third frame  105  and the fourth frame  106 . Therefore, the support shafts  106   a ,  108   a ,  108   b  are prevented from being slanted. A support shaft  138   a  extends from the fourth frame  106  to support the ninth idle gear  138 . 
   The structure of the twelfth idle gear  136  will be explained with reference to  FIGS. 11 and 12 .  FIG. 11  is a central cross sectional view of the twelfth idle gear  136  and  FIG. 12  is a view showing that the twelfth idle gear  136  fitted to the support shaft  106   a  that is arranged on the fourth frame  106 . 
   The twelfth idle gear  136  is interlocked with the heat roller gear  41   a  that is arranged at the end of the heat roller  41  so as to transfer the power for rotating the heat roller  41 . When the halogen lamp is turned on, the heat roller  41  has high temperature of 170° C. and the twelfth idle gear  136  is subjected to the heat. Therefore, the twelfth idle gear  136  must be heat resistant. 
   The shaft receiving member  136   a  rotatably received on the support shaft  106   a  is formed of polyphenylene sulfaide resin having superior heat resistance, and the metal tooth member  136   b  arranged around the shaft receiving member  136   a  is made by sintering iron or copper powder. 
   As explained above, in the laser printer  1  of this embodiment, the drum drive gear  112 , that drives the photosensitive drum  27 , is interlocked with the drive shaft  111  of the DC brushless motor  110  and the drum drive gear  112  does not transfer the driving force to loads other than the photosensitive drum  27 . Accordingly, the photosensitive drum  27  is not influenced by the other loads and rotates stably. Therefore, images of high quality can be obtained. 
   The developing roller  31  and the supply roller  33  rotate in contact with each other with a rotational speed difference between the developing roller  31  and the supply roller  33 . Therefore, when the developing roller  31  and the supply roller  33  rotate, a large load is generated. However, the load fluctuation is not transferred to the drum drive gear  112  and the photosensitive drum  27  rotates stably. 
   When a paper  3  is supplied to the resist roller  12  or the heat roller  41 , the load applied to the resist roller  12  or the heat roller  41  is increased compared to before the paper  3  is so supplied. However, because the load fluctuation is not transferred to the drum drive gear  112 , the photosensitive drum  27  rotates stably. 
   Although the described invention has been described in detail and with reference to a specific embodiment thereof, it would be apparent to those skilled in the art that various changes, arrangements and modifications may be applied therein without departing from the spirit and scope of the invention. 
   In the embodiment, a tooth is formed by cutting the drive shaft  111  of the DC brushless motor  110  to obtain a gear, however, another gear may be fixed to the drive shaft  111 . Further, in the embodiment, the small gear portion  112   a  of the drum drive gear  112  directly drives the transfer gear  27   b , that is formed at the end of the photosensitive drum  27 , to rotate the photosensitive drum  27 . However, another driving force transferring mechanism may be arranged between the small gear portion  112   a  and the transfer gear  27   b.