Abstract:
An image forming apparatus includes a disengagement unit that disengages all of plurality of image carrier gears and plurality of driving, gears, and allows an image carrier unit to be removed outside of an unit containing portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Japanese Patent Application No. 2006-053065 filed Feb. 28, 2006 in the Japan Patent Office, the disclosure of which is incorporated herein by reference. 
   BACKGROUND 
   The present invention relates to an image forming apparatus including: an image carrier unit integrally having a plurality of image carriers, which respectively carries an image; and a unit containing portion which removably contains the image carrier unit. The present invention also relates to a technique associated with the image forming apparatus. 
   Some type of a conventional laser printer includes a photoreceptor unit, having a plurality of photoreceptor drums integrally aligned in one direction, and a unit containing portion, wherein the photoreceptor unit can be removably contained along the alignment direction of the photoreceptor drums. 
   In this type of laser printer, drum gears are provided and respectively connected to the photoreceptor drums so as to transmit driving force to the photoreceptor drums for rotating the drums. Moreover, a plurality of driving gears is provided in the unit containing portion. The driving gears are engaged with the respective drum gears so as to transmit driving force to the drum gears from a motor. 
   Generally, in this type of laser printer, when the photoreceptor unit is removed while a user lifting an end portion of the photoreceptor unit in a downstream side of the removal direction, the drum gears and the driving gears are disengaged in a consecutive manner from the downstream side. 
   The drum gears and the driving gears are formed in such a manner that the center of each circle works as the center of rotation. However, in a precise sense, the center of rotation slightly deviates from the center of the circles. Therefore, the displacement rate on the outer circumference of the gears at the time of rotation (the displacement amount on the outer circumference per unit time) is not constant. 
   When images, carried by respective the photoreceptor drums, are sequentially superposed on a sheet of paper conveyed in the above-described laser printer, the respective images are misaligned due to the dislocation of the center of gear rotation. A solution is required so as to inhibit misalignment of images carried by the photoreceptor drums. 
   For this purpose, in the above-described laser printer, a phase reference point is predetermined for the respective gears based on the dislocation of the rotational center, and the phase differences between the adjacent drum gears and between the adjacent driving gears are set so as to be respectively constant. 
   SUMMARY 
   However, in the above-described laser printer, when the photoreceptor unit is removed and the drum gears and the driving gears are disengaged, the drum gears and the driving gears, especially in the upstream side of the removal direction, sometimes interfere with each other and are unintentionally rotated. As a result, a problem is caused wherein the phase differences between the adjacent drum gears and between the adjacent driving gears are changed. 
   One aspect of the present invention preferably provides a technique wherein, when an image carrier unit having a plurality of image carriers respectively carrying images is removed, image carrier gears, connected to rotational shafts of the image carriers, and driving gears, which transmit driving force to the image carrier gears, are inhibited from interfering from each other. 
   In one aspect of the present invention, an image forming apparatus includes an image carrier unit, a unit containing portion, a plurality of driving gears, and a disengagement unit. The image carrier unit includes a plurality of image carriers that is integrally disposed in the image carrier unit, respectively carries images, and respectively has rotational shafts. The image carrier unit further includes a plurality of image carrier gears connected to the rotational shafts. The plurality of image carrier gears transmits driving force to the plurality of image carriers so as to rotate the plurality of image carriers. The unit containing portion removably contains the image carrier unit. The plurality of driving gears is disposed in the unit containing portion, and respectively corresponds to the plurality of image carrier gears. Each of the plurality of driving gears is engaged with one of the plurality of image carrier gears that corresponds thereto so as to transmit driving force to the plurality of image carrier gears from a driving source. The disengagement unit disengages all the plurality of image carrier gears and the plurality of driving gears, and allows the image carrier unit to be removed outside of the unit containing portion. 
   In the image forming apparatus configured as above, the image carrier unit may become removal when all the image carrier gears and the driving gears are disengaged by the disengagement unit. 
   Therefore, the image carrier gears and the driving gears do not interfere with each other when the image carrier unit is removed. 
   In another aspect of the present invention, an image carrier unit includes a plurality of image carriers, a housing, a plurality of image carrier gears, and guided members. The plurality of image carriers respectively carries images, and has rotational shafts. The housing integrally supports the plurality of image carriers such that the plurality of image carriers are aligned along one direction and the rotational shafts are disposed in parallel to one another. The plurality of image carrier gears is respectively connected to the rotational shafts, and respectively engaged with a plurality of driving gears disposed in a unit containing portion of an image forming apparatus so as to transmit driving force for rotating the plurality of image carriers from a driving source to the plurality of image carriers via the plurality of driving gears. The guided members are guided by guide members disposed in the unit containing portion along a centerline direction directed from a rotational center of one of the plurality of driving gears to a rotational center of one of the plurality of image carrier gears that corresponds to the one of the plurality of drive gears. 
   The image carrier unit configured as above may be moved in the centerline direction by the guided members being guided by the guide members. 
   Therefore, by the above-described image carrier unit, the image carrier gears and the driving gears may be disengaged without the teeth of these gears interfering with each other. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described below, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a perspective view showing an exterior appearance of a printer according to a first embodiment of the present invention; 
       FIG. 2  is a perspective view showing the exterior appearance of the printer and an image forming unit in which the image forming unit is removed outside of the printer; 
       FIG. 3  is a perspective view showing an exterior appearance of a drum unit from which all development cartridges are removed; 
       FIG. 4  is a cross sectional view showing an internal structure of the printer in which the image forming unit is installed; 
       FIG. 5  is a schematic view showing a structure of a drive mechanism for driving various parts of the image forming unit as installed in a body frame of the printer, and a removal mechanism for removing the image forming unit from the body frame; 
       FIGS. 6A and 6B  are explanatory views illustrating a removal operation for removing the image forming unit from the body frame; 
       FIGS. 7A and 7B  are explanatory views illustrating the removal operation for removing the image forming unit from the body frame; 
       FIG. 8  is an explanatory view showing an engagement between a drum gear and an inner gear of the printer; 
       FIG. 9  is a schematic view showing a structure of a disengagement mechanism of a printer according to a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   First Embodiment 
   &lt;External Structure of Printer  100  in First Embodiment&gt; 
   In the following, when the disposition order of a plurality of constituents is indicated, the start point of the order is set in the front side of a printer  100 . That is, the constituent disposed in the moot front side of the printer  100  is referred to as the first one among the plurality of constituents. 
   As shown in  FIG. 1 , a body  110  of the printer  100  includes a body casing  111  and a body frame  112  contained inside of the body casing  111 . 
   The body casing  111  is made of synthetic resin, and formed approximately in a rectangular parallelepiped shape. On a top surface  111 A of the body casing  111 , a paper discharge tray  111 B is formed. The paper discharge tray  111 B is downwardly inclined from the front side of the body casing  111  toward the rear side thereof. A paper discharge opening  111 C is disposed in an upper portion of the body casing  111  above the rear end portion of the paper discharge tray  111 B. Paper is discharged through the paper discharge opening  111 C on to the paper discharge tray  111 B. 
   In a front side of the body casing  111 , a front opening  111 D is formed. A front cover  111 E is disposed in an openable/closable manner for closing the front opening  111 D. The lower end of the front cover  111 E is supported by the body casing  111 . 
   The body frame  112  is configured so as to support various members provided for an image forming operation inside of the body  110 . Driving sources and driving force transmission mechanisms are disposed inside of the body frame  112  for rotating and driving the various members. 
   The left inner wall and the right inner wall of the body frame  112  are respectively provided with guide rails  112 A. More specifically, the respective guide rails  112 A are disposed approximately horizontally from the front side of the printer  100  toward the rear side thereof. 
   In the body frame  112 , an image forming unit  120  is installed such that the image forming unit  120  can be removed in a direction (shown with Arrow S in the figure) from the rear side of the printer  100  toward the front side thereof. In other words, the rear side of the printer  100  corresponds to the upstream side of the removal direction of the image forming unit  120 , and the front side of the printer  100  corresponds to the downstream side of the removal direction. 
   &lt;External Structure of Image Forming Unit  120 &gt; 
   As shown in  FIG. 2 , the image forming unit  120  includes a drum unit  130 , and four development cartridges  140 . 
   The drum unit  130  includes a frame forming approximately a quadrangular plane with a front beam  131 , a left supporting plate  132 , a right supporting plate  133 , and a rear beam  134 . 
   More specifically, the front beam  131  and the rear beam  134 , respectively provided in the front end side and in the rear end side of the drum unit  130 , are disposed in parallel to each other. To the left and right ends of the respective front beam  131  and the rear beam  134 , the left supporting plate  132  and the right supporting plate  133  are connected. In the inner side of the left supporting plate  132 , four left plates  135  are aligned along the left supporting plate  132 . In the inner side of the right supporting plate  133 , four right plates  136  are aligned along the right supporting plates  133 . 
   Between the left plates  135  and the right plates  136 , the above-described development cartridges  140  are aligned from the front side of the drum unit  130  toward the rear side thereof. The development cartridges  140  are respectively supported by the left plates  135  and the right plates  136  in an attachable/detachable manner. 
   In the upper end portions of the left supporting plate  132  and the right supporting plate  133 , flange portions  132 A and  133 A are respectively formed. The flange portions  132 A and  133 A are engaged with the above-described guide rails  122 A when the drum unit  130  is inserted into the body frame  112 . 
   In the upper portion on the respective outer walls of the left supporting plate  132  and the right supporting plate  133  (beneath the flange portions  132 A,  133 A), rollers  137 A,  137 B, and  137 C are rotatably supported. ( FIG. 2  shows only the rollers  137 A,  137 B, and  137 C provided on the left supporting plate  132 .) 
   More specifically, the rollers  137 A,  137 B, and  137 C are respectively disposed in the front end portion, the center portion, and the rear end portion in the upper portion of the left supporting plate  132  and the right supporting plate  133 . That is, the drum unit  130  is guided along the guide rails  112 A in the front-to-rear direction of the body frame  112  by the rollers  137 A,  137 B, and  137 C being rotated on the guide rails  112 A. The diameters of the rollers  137 A,  137 B, and  137 C are determined such that the diameters become larger in the order from the roller  137 A,  137 B, and  137 C. 
   On the respective outer walls of the left supporting plate  132  and the right supporting plate  133 , projection members  138 A and  138 B are disposed. ( FIG. 2  shows only the projection members  138 A and  138 B provided on the left supporting plate  132 .) 
   More specifically, the projection member  138 A is disposed below and behind the roller  137 A. The projection member  138 B is disposed below and anterior to the roller  137 C. 
   The drum unit  130  is provided with a front handle  131 A in the front surface of the front beam  131 . The drum unit  130  can be easily removed from the body frame  112  by a user pulling the front handle  131 A toward the front side of the body frame  112 . 
   The drum unit  130  is also provided with a rear handle  134 A in the upper end portion of the rear beam  134 . The drum unit  130  can be easily carried by a user holding the front handle  131 A and the rear handle  134 A. 
   As shown in  FIG. 3 , the left supporting plate  132  of the drum unit  130  is provided with four coupling insertion holes  132 B along the disposition direction of the development cartridges  140  such that the respective coupling insertion holes  132 B face the respective development cartridges  140 . 
   Each of the left plates  135  is provided with a coupling exposure hole  135 B in a position so that the coupling exposure hole  135 B faces the coupling insertion hole  132 B. 
   The coupling insertions holes  132 B and the coupling exposure holes  135 B are provided for inserting driving shafts (not shown) disposed within the body frame  112  so as to apply driving force to coupling receiving gears (not shown) disposed in the development cartridges  140 . 
   On the respective inner walls of the left plates  135  and the right plates  136 , guide grooves  135 A,  136 A are formed for guiding the development cartridges  140  in the up-and-down direction. 
   In the bottom portion of the drum unit  130 , four drum portions  150  are disposed along the disposition direction of the development cartridges  140  (only first three drum portions  150  from the front side are shown in  FIG. 3 ). 
   &lt;Internal Structure of Printer  100 &gt; 
   As shown in  FIG. 4 , in the body  110  of the printer  100 , the image forming unit  120  is disposed in the center portion thereof, and the paper discharge unit  193  is disposed behind the image forming unit  120 . 
   The body  110  further includes a scanner unit  160 , a transfer unit  170 , and a feeder unit  180 . The scanner unit  160  is disposed above the image forming unit  120 . The transfer unit  170  is disposed below the image forming unit  120 . The feeder unit  180  is disposed below the transfer unit  170 . 
   &lt;&lt;Structure of Feeder Unit  180 &gt;&gt; 
   The feeder unit  180  includes a feeder case  181 , a feed roller  183 , a separation roller  184 , a separation pad  185 , and a paper dust removal roller  187 . 
   The feeder case  181  is formed in such a manner that sheets of paper P can be stacked inside thereof. In the feeder case  181 , a paper pressing plate  182  is disposed. A rear end portion  182 A of the paper pressing plate  182  is rotatably supported inside of the feeder case  181 . That is, in the feeder case  181 , a front end portion  182 B of the paper pressing plate  182  is swayed approximately in the up-and-down direction in the figure. 
   The feed roller  183  is made of synthetic rubber. The feed roller  183  is rotatably supported above the front end portion  182 B of the paper pressing plate  182  by the body frame  112 . The feed roller  183  is driven so as to rotate in the counterclockwise direction in the figure, and conveys a sheet of paper P, stacked on the top inside of the feeder case  181 , toward the front side of the feed roller  183 . 
   The separation roller  184  is made of synthetic rubber in the same manner as the feed roller  183 . The separation roller  184  is rotatably supported by the body frame  112  in the front side of the feed roller  183 . The separation roller  184  is driven so as to rotate in the same direction as the feed roller  183 , and conveys the sheet of paper P toward the front side thereof. 
   The separation pad  185  is disposed so as to face the separation roller  184 . A separation surface  185 A of the separation pad  185  facing the separation roller  184  is made of a material having a high friction coefficient, such as synthetic rubber, felt, and so on. Below the separation pad  185 , a separation pad biasing spring  186  is disposed. By the separation pad biasing spring  186  biasing the separation pad  185  toward the separation roller  184 , the separation roller  184  and the separation pad  185  are pressed against each other. 
   The paper dust removal roller  187  removes paper dust adhered to the sheet of paper P. The paper dust removal roller  187  is disposed above and in the front side of the separation roller  184  so as to face a pinch roller  188 , and rotatably supported by the body frame  112 . 
   &lt;&lt;Structure of Image Forming Unit  120 &gt;&gt; 
   In the image forming unit  120 , the four development cartridges  140  ( 140 Y,  140 M,  140 C, and  140 K) are aligned from the front side of the printer  100  toward the rear side thereof. Beneath the development cartridges  140 , the four drum portions  150  are aligned from the front side of the printer  100  toward the rear side thereof, so as to face the development cartridges  140 . 
   The four development cartridges  140 Y,  140 M,  140 C, and  140 K respectively contain toners (developers) in some colors different from one another, such as yellow, magenta, cyan, and black. Although the development cartridges  140 Y,  140 M,  140 C, and  140 K contain toners of different colors, the structures thereof are exactly the same. 
   More specifically, the development cartridges  140  respectively contain toners, which are developers for developing electrostatic latent images, in respective cartridge cases  141 . The development cartridges  140  respectively include agitators  142 , supply rollers  143 , development rollers  144 , and blades  145 . 
   The agitator  142  stirs a toner contained in the cartridge case  141 , and is rotatably supported by the cartridge case  141 . 
   The supply roller  143  is made of a sponge roller, and rotatably supported by the cartridge case  141 . 
   The development roller  144  is made of a rubber roller, and rotatably supported by the cartridge case  141 . The supply roller  143  and the development roller  144  are disposed such that the supply roller  143  and the development roller  144  face each other and the periphery surfaces thereof contact with each other. The supply roller  143  is driven so as to rotate in the counterclockwise direction in the figure, and supplies an electrically-charged toner to the periphery surface of the development roller  144 . 
   The blade  145  is disposed so as to abut on the periphery surface of the development roller  144 . The blade  145  adjusts the amount of the toner supplied on to the periphery surface of the development roller  144 , which is driven so as to rotate in the counterclockwise direction in the figure. 
   The four drum portions  150  are configured exactly in the same manner. The drum portions  150  respectively include photoreceptor drums  151  and scorotron chargers  152 . 
   The photoreceptor drum  151  has a photoreceptive layer, made of a photoconductor, on the periphery surface thereof. The photoreceptor drum  151  is disposed such that the periphery surface thereof faces the periphery surface of the development roller  144  in the development cartridge  140 . 
   The photoreceptor drum  151  is rotatably supported by the drum unit  130 , and driven so as to rotate in the clockwise direction in the figure by a drive mechanism to be described later. However, the direction of the rotational shafts  151 C of all the photoreceptor drums  151  is set to be perpendicular to the installation direction of the drum unit  130  in a horizontal plane (a direction perpendicular to the surface of the drawing). That is, all the rotational shafts  151 C are disposed in parallel to one another. 
   The scorotron charger  152  is constituted so as to uniformly charge the periphery surface of the photoreceptor drum  151 . The scrotron charger  152  is disposed above the photoreceptor drum  151  so as to face the periphery surface of the photoreceptor drum  151 . 
   &lt;&lt;Structure of Scanner Unit  160 &gt;&gt; 
   The scanner unit  160  is constituted  80  as to emit laser beam, generated based on image data, from a laser emission portion (not shown) on to the periphery surfaces of the photoreceptor drums  151 . Also the scanner unit  160  is constituted so as to scan laser beam emitted therefrom in the width direction of the printer  100  (the direction perpendicular to the surface of  FIG. 4 ). 
   &lt;&lt;Structure of Transfer Unit  170 &gt;&gt; 
   The transfer unit  170  includes a belt driving roller  171 , a driven roller  172 , a conveyance belt  173 , four transfer rollers  174 , and a belt cleaner  175 . 
   The belt driving roller  171  is disposed below and behind the drum portion  150  facing the development cartridge  140 K disposed in the most rear side among the four development cartridges  140 . The belt driving roller  171  is rotatably supported by the body frame  112 . 
   The driven roller  172  is disposed below and in the front side of the drum portion  150  facing the development cartridge  140 Y disposed in the most front side among the four development cartridges  140 . The driven roller  172  is rotatably supported by the body frame  112 . 
   The conveyance belt  173  is an endless belt made of a resin film, such as conductive polycarbonate or polyimide, wherein conductive particles, such as carbon, are dispersed. The conveyance belt  173  runs between the belt driving roller  171  and the driven roller  172 . 
   The conveyance belt  173  is moved in the counterclockwise direction in the figure by the belt driving roller  171  being driven so as to rotate in the counterclockwise direction. The conveyance belt  173  conveys the sheet of paper P placed thereon along the disposition direction of the development cartridges  140 . 
   The transfer rollers  174  are respectively disposed beneath the respective photoreceptor drums  151  so as to face the photoreceptive drums  151  of the drum portions  150  with the conveyance belt  173  inbetween. The transfer rollers  174  are rotatably supported by the body frame  112 , and rotated corresponding to the conveyance belt  173  moving counterclockwise. 
   That is, each of the transfer rollers  174  holds the sheet of paper P by sandwiching the sheet of paper P with the photoreceptor drum  151  facing thereto, and transfers an image (a toner image) carried on the periphery surface of the photoreceptor drum  151  to the surface of the sheet of paper P. Furthermore, the transfer rollers  174  convey the sheet of paper P together with the photoreceptor drums  151  toward the rear side the printer  100 . 
   The belt cleaner  175  is disposed beneath the second transfer roller  174  among the four transfer rollers  174 . The belt cleaner  175  removes toner and paper dust adhered to the surface of the conveyance belt  173  by a pair of cleaning rollers  175 A,  175 B. 
   &lt;&lt;Structure of Paper Discharge Unit  193 &gt;&gt; 
   The paper discharge unit  193  includes a heat roller  193 A and a pressure roller  193 B. 
   The heat roller  193 A is constituted with a metal cylinder, having a surface treated with as mold release process, and a halogen lamp contained in the cylinder. The heat roller  193 A is rotatably supported by the body frame  112 . 
   The pressure roller  193 B is made of silicone rubber, and disposed so as to be pressed against the heat roller  193 A at predetermined pressure. The pressure roller  193 B is rotatably supported by the body frame  112 . 
   In the paper discharge unit  193 , when the heat roller  193 A is driven so as to rotate in the clockwise direction in the figure, the pressure roller  193 B is correspondingly rotated in the counterclockwise direction. The sheet of paper P, conveyed from the transfer unit  170 , is fed between the heat roller  193 A and the pressure roller  193 B, and conveyed behind the rollers  193 A and  193 B. As a result, a toner on the sheet of paper P melts and adheres (is fixed) to the sheet of paper P. Then, the sheet of paper P is conveyed toward the paper discharge opening  111 C. 
   The paper discharge unit  193  furthermore includes a conveyance roller  194  for conveying the sheet of paper P on which toner is adhered, and a pinch roller  195 . The conveyance roller  194  and the pinch roller  195  are disposed behind the heat roller  193 A and the pressure roller  193 B. 
   The conveyance roller  194  is rotatably supported by the body frame  112 . 
   The pinch roller  195  is disposed so as to face the conveyance roller  194 , and rotatably supported by the body frame  112 . 
   By the conveyance roller  194  being driven so as to rotate in the clockwise direction in the figure, the pinch roller  195  is correspondingly rotated in the counterclockwise direction in the figure. As a result, the sheet of paper P is conveyed toward the paper discharge opening  111 C. 
   The paper discharge unit  193  still further includes paper guides  196 A,  196 B disposed above the conveyance roller  194  and the pinch roller  195 , for guiding the sheet of paper P with a toner adhered thereon. 
   The paper guides  196 A and  196 B guide the sheet of paper P, conveyed by the conveyance roller  194  and the pinch roller  195 , toward the paper discharge opening  111 C. 
   The paper discharge unit  193  further includes a paper discharge roller  197  and a paper discharge driven roller  198  both disposed in the vicinity of the paper discharge opening  111 C. 
   The paper discharge roller  197  and the paper discharge driven roller  198  are disposed so as to face each other in the up-and-down direction in the figure, and respectively supported by the body frame  112  in a rotatable manner. 
   By the paper discharge roller  197  being driven so as to rotate in the counterclockwise direction in the figure, the paper discharge driven roller  198  is correspondingly rotated in the clockwise direction. As a result, the sheet of paper P is discharged outside of the body  110  from the paper discharge opening  111 C. 
   &lt;&lt;Structure of Drive Mechanism and Removal Mechanism&gt;&gt; 
   As shown in  FIG. 5 , the left outer walls of respective development cartridges  140  are provided with a supply roller driving gear  143 A and a development roller driving gear  144 A. The supply roller driving gear  143 A is connected to the rotational shaft of the supply roller  143 . The development roller driving gear  144 A is connected to the rotational shaft of the development roller  144 . 
   The respective teeth of the supply roller driving gear  143 A and the development roller driving gear  144 A are engaged with the teeth of the above-described coupling receiving gear. When driving force is applied from the above-described driving shaft to the coupling receiving gear, the supply roller driving gear  143 A and the development roller driving gear  144 A are correspondingly rotated. In other words, the supply roller driving gear  143 A and the development roller driving gear  144 A transmit driving force applied from the above-described driving axis to the supply roller  143  and the development roller  144 . 
   Inside of the drum unit  130 , four drum gears  151 A are disposed so as to be respectively connected to the rotational shafts  151 C of the photoreceptor drums  151 . Phases of the respective drum gears  151 A, which indicate rotational angles thereof, are determined with respect to reference rotational positions thereof. The reference rotational positions are set based on the locations of the rotational centers of the respective drum gears  151 A. The rotational orientations of the respective drum gears  151 A are set such that phase differences between the first and second drum gears  151 A, between the second and third drum gears  151 A, and between the third and fourth drum gears  151 A are predetermined phase differences. The predetermined phase differences may be all the same, or be partly the same, or be different from each other. 
   The body frame  112  (not shown in  FIG. 5 ) is provided with four body gears  113 , constituted with two-stage gears: an inner gear  113 A and an outer gear  113 B. The body gears  113  are disposed beneath the drum unfit  130 , and rotatably supported by the body frame  112 . 
   More specifically, the body gears  113  are disposed along the disposition direction of the drum gears  115 A. Each of the inner gears  113 A of the body gears  113  is engaged with the teeth of the drum gear  151 A disposed above and behind the inner gear  133 A. In other words, each of the drum gears  151 A is engaged with the inner gear  113 A disposed below and in the front side of the drum gear  151 A. 
   Phases of the respective body gears  113 , which indicate rotational angles thereof, are determined with respect to reference rotational positions thereof. The reference rotational positions are set based on the locations of the rotational centers of the respective body gears  113 . The rotational orientations of the respective body gears  113  are set such that phase differences between the first and second body gears  113 , between the second and third body gears  113 , and between the third and fourth body gears  113  are predetermined phase differences. The predetermined phase differences may be all the same, or be partly the same, or be different from each other. 
   The drum gears  151 A receive reaction force from the inner gears  113 A when the inner gears  113 A are rotated. The reaction force has a direction at a predetermined angle (pressure angle: 20° in the present embodiment) with respect to a tangent line passing through the contact point between the pitch circle of the drum gear  151 A and the pitch circle of the inner gear  113 A. 
   If the inner gear  113 A is disposed beneath the drum gear  151 A in the perpendicular direction, the reaction force is applied to the drum gear  151 A, which is directed toward 20° in the upper rear side with respect to the horizontal direction. As a result, the reaction force applied to the photoreceptor drum  151  lifts the photoreceptor drum  151 . Therefore, maintaining suitable nip pressure between the photoreceptor drum  151  and the transfer roller  174  becomes difficult. 
   In the present embodiment, the inner gear  113 A is disposed below and in the front side of the drum gear  151 A as described above. Therefore, the reaction force can be directed along the conveyance direction of the sheet of paper P, and suitable nip pressure can be maintained between the photoreceptor drum  151  and the transfer roller  174 . 
   Beneath the body gears  113 , four drive motors  114  are disposed for the respective body gears  113 . Motor gears  114 A are connected to the rotational shafts of the respective drive motors  114 . The teeth of the respective motor gears  114 A are engaged with the teeth of the outer gears  113 B of the corresponding body gears  113 . 
   That is, when the drive motors  114  are driven, the body gears  113  and the drum gears  151 A are correspondingly rotated and transmit the driving force, applied from the drive motors  114 , to the photoreceptor drums  151 . 
   On the respective guide rails  112 A, holes are respectively formed in the front side, in the center portion, and in the rear side. The rollers  137 A,  137 B, and  137 C are engaged with these holes. 
   The size of the respective holes in the front-to-rear direction is determined so as to be approximately equivalent to the diameter of the roller to be engaged therein. Therefore, when the rollers  137 A,  137 B, and  137 C are rotated on the guide rails  112 A, the rollers  137 B and  137 C do not become engaged with the hole for the roller  137 A. The roller  137 C likewise does not become engaged with the hole for the roller  137 B. 
   Beneath the respective guide rails  112 A, disengagement mechanisms  115  are disposed (only the disengagement mechanism  115  on the left side of the printer  100  is shown in  FIG. 5 ) for disengaging all the drum gears  151 A and the inner gears  113 A. 
   More specifically, the respective disengagement mechanisms  115  include links  116 , cover support members  117 , operation gears  118 , and disengagement guides  119 A,  119 B. 
   The links  116  are made of a rod-shaped member, and supported by the body frame  112  so as to be movable in the front-to-rear direction. 
   The length of the links  116  is such that the links  116  extend between the vicinity of the front end portion of the body frame  112  and the vicinity of the rear end portion thereof. 
   In the front end portion of the respective links  116 , front bend portions  116 A are formed. The front bend portion  116 A is formed by the front end portion of the link  116  being bent downward and then bent backward. Teeth are provided on the bottom circumference of the front bend portion  116 A. 
   The links  116  are also respectively provided with front inclined portions  116 B in front of the projection member  138 A of the drum unit  130  installed in the body frame  112 . The front inclined portion  116 B is made with a portion of the link  116  extending from in the front side of the projection member  138 A to the position where the projection member  138 A is disposed. This portion of the link  116  is inclined downward so as to form the front inclined portion  116 B. 
   Furthermore, the links  116  are respectively provided with rear inclined portions  116 C in front of the projection member  138 B of the drum unit  130  installed in the body frame  112 . The rear inclined portion  116 C is made with the rear portion of the link  116  bent so as to be inclined upwardly toward the front side, and then bent such that the leading end of the rear end portion is directed toward the front side. 
   The cover support members  117  are formed in an arc shape, and disposed beneath the front bend portion  116 A of the link  116 . 
   The arcs of the respective cover support members  117  is directed toward the upper rear side, and provided with a plurality of teeth thereon. The front ends of the respective cover support members  117  are connected to the bottom portion of the front cover  111 E. The cover support members  117  are rotated on the support shaft  111 F of the front cover  111 E when the front cover  111 E is moved so as to open/close the front opening  111 D. 
   The operation gears  118  are rotatably supported by the body frame  112  between the cover support member  117  and the front bend portion  116 A of the link  116 . The teeth of the operation gears  118  are respectively engaged with the teeth of the cover support members  117 , and with the teeth of the front bend portions  116 A of the links  116 . 
   The disengagement mechanisms  119 A,  119 B are respectively disposed in the vicinity of the projection members  138 A,  138 B of the drum unit  130  installed in the body frame  112 . 
   More specifically, the disengagement mechanisms  119 A,  119 B are configured with a pair of plate members sandwiching the projection members  138 A and  138 B. The direction of the plate members is determined so as to be in parallel to the direction of the center line extending from the center of the rotation of the body gear  113  (the center of the rotation of the inner gear  113 A) to the center of the rotation of the drum gear  151 A. 
   That is, the disengagement guides  119 A,  119 B respectively guide the projection members  138 A,  138 B along the direction of the center line. 
   By the disengagement mechanisms  115  configured as above, the image forming unit  120  is removed from the body frame  112  as follows. 
   The following describes the removal operation for removing the image forming unit  120  from the body frame  112  with reference to  FIGS. 6A ,  6 B,  7 A and  7 B. It is to be noted that the front cover  111 E, the cover support member  117 , and the operation gear  118  are not shown in  FIGS. 7A and 7B  so as to simplify the description. 
   As shown in  FIG. 6A , as a user starts opening the front cover  111 E, the cover support members  117  are rotated in the clockwise direction in the figure, and the operation gears  118  are rotated in the counterclockwise direction. Correspondingly, the links  116  are moved toward the rear side. 
   Then, as shown in  FIG. 6B , as the user further opens the front cover  111 E, the links  116  are further moved toward the rear side. The front inclined portions  116 B are abutted, on the projection members  138 A. Simultaneously, the rear inclined portions  116 C are abutted on the projection members  138 B. 
   As shown in  FIG. 7A , when the user completely opens the front cover  111 E, the links  116  are furthermore moved toward the rear side. The image forming unit  120  is guided by the disengagement guides  119 A,  119 B and lifted in the above-described centerline direction. As a result, all the drum gears  151 A and the inner gears  113 A are disengaged, and the rollers  137 A,  137 D, and  137 C are removed from the above-described holes provided with the guide rails  112 A. 
   Subsequently, as shown in  FIG. 7B , when the user pulls the image forming unit  120  toward the front side, the rollers  137 A,  137 B, and  137 C are rotated on the guide rails  112 A. The image forming unit  120  is guided by the guide rails  112 A, and removed to the outside of the body frame  112 . 
   Even when the image forming unit  120  is removed and all the drum gears  151 A and the inner gears  113 A are disengaged, if the teeth of the drum gears  151 A and the inner gears  113 A come in contact, the phase differences between the adjacent drum gears  151 A and between the adjacent the body gears  113  become out of the predetermined phase differences. 
   Therefore, the inventor of the present invention calculated the minimum travel distance X for the disengagement mechanisms  116  to move the image forming unit  120 . 
   The calculation will be described below with reference to  FIG. 8 . 
   Here, the pitch circle diameter of the drum gear  151 A (the diameter of the circle with dotted line in the drum gear  151 A) is represented as D 1 . The pitch circle diameter of the inner gear  113 A (the diameter of the circle with dotted line in the inner gear  113 A) is represented as D 2 . 
   The size of the module of the drum gear  151 A (the distance between the circle with the dotted line in the drum gear  151 A and the circle with the full line) and the size of the module of the inner gear  113 A (the distance between the dotted line in the inner gear  113 A and the circle with the full line) are determined to be equivalent, and represented as M. 
   The angle between the centerline direction and the direction perpendicular to the removal direction of the image forming unit  120  is θ. 
   With using the various parameters determined as above, the distance W 1  in the perpendicular direction, as shown in  FIG. 8 , between the straight line, drawn from the center of the drum gear  151 A in parallel to the removal direction, and the dot-dash line, drawn from the center of the inner gear  113 A in parallel to the removal direction, is obtained from:
 
 W 1=( D 1+ D 2)cos θ/2
 
   The distance W 2  in the perpendicular direction between the straight line, drawn from the center of the drum gear  151 A in parallel to the removal direction, and the tangent line, drawn tangentially to the circumference of the inner gear  131 A in parallel to the removal direction, is obtained from: 
   
     
       
         
           
             
               
                 
                   W 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
                 = 
                 
                   
                     W 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   - 
                   
                     { 
                     
                       
                         ( 
                         
                           D 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             2 
                             / 
                             2 
                           
                         
                         ) 
                       
                       + 
                       M 
                     
                     } 
                   
                 
               
             
           
           
             
               
                 = 
                 
                   [ 
                   
                     
                       { 
                       
                         
                           ( 
                           
                             
                               D 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                             + 
                             
                               D 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                             
                           
                           ) 
                         
                         ⁢ 
                         cos 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           θ 
                           / 
                           2 
                         
                       
                       } 
                     
                     - 
                     
                       { 
                       
                         
                           ( 
                           
                             D 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               2 
                               / 
                               2 
                             
                           
                           ) 
                         
                         + 
                         M 
                       
                       } 
                     
                   
                   ] 
                 
               
             
           
         
       
     
   
   The distance W 3  in the perpendicular direction between the above-described tangent line, drawn tangentially to the circumference of the inner gear  113 A, and the tangent line, drawn tangentially to the circumference of the drum gear  151 A in parallel to the removal direction, is obtained from: 
   
     
       
         
           
             
               
                 
                   W 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
                 = 
                 
                   
                     ( 
                     
                       D 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         1 
                         / 
                         2 
                       
                     
                     ) 
                   
                   + 
                   M 
                   - 
                   
                     W 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                 
               
             
           
           
             
               
                 = 
                 
                   
                     { 
                     
                       
                         ( 
                         
                           
                             D 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           + 
                           
                             D 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                           + 
                           
                             4 
                             ⁢ 
                             M 
                           
                         
                         ) 
                       
                       - 
                       
                         
                           ( 
                           
                             
                               D 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                             + 
                             
                               D 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                             
                           
                           ) 
                         
                         ⁢ 
                         cos 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         θ 
                       
                     
                     } 
                   
                   / 
                   2 
                 
               
             
           
         
       
     
   
   Therefore, the minimum travel distance X is obtained from: 
   
     
       
         
           
             
               
                 X 
                 = 
                 
                   W 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     3 
                     / 
                     cos 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   θ 
                 
               
             
           
           
             
               
                 = 
                 
                   
                     
                       { 
                       
                         
                           ( 
                           
                             
                               D 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                             + 
                             
                               D 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                             
                             + 
                             
                               4 
                               ⁢ 
                               M 
                             
                           
                           ) 
                         
                         - 
                         
                           
                             ( 
                             
                               
                                 D 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                               + 
                               
                                 D 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                             
                             ) 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           cos 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           θ 
                         
                       
                       } 
                     
                     / 
                     2 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   cos 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   θ 
                 
               
             
           
         
       
     
   
   In the disengagement mechanisms  115 , the height of the inclination of the front inclined portion  116 B and the rear inclined portions  116 C, and the length of the disengagement guides  119 A,  119 B are determined such that the image forming unit  120  is moved for at least the minimum travel distance X. 
   &lt;Effects of Printer  100 &gt; 
   In the printer  100  according to the first embodiment, the image forming unit  120  becomes removable after all the dram gears  151 A and the inner gears  113 A are disengaged by the disengagement mechanisms  115 . Therefore, the dram gears  151 A and the body gears  113  do not interfere with each other when the image forming unit  120  is removed. 
   As a result, according to the printer  100 , the phase differences between the adjacent dram gears  151 A and between the adjacent body gears  113  can be inhibited from becoming out of the predetermined phase differences when the image forming unit  120  is removed. 
   Moreover, in the printer  100  according to the first embodiment, the dram gears  151 A and the inner gears  113 A are disengaged by moving the image forming unit  120  along the centerline direction. Therefore, the engagement can be performed without the teeth of the dram gears  151 A and the inner gears  113 A becoming in contact with each other. As a result, the phase differences can be reliably inhibited from becoming out of the predetermined phase differences. Additionally, the teeth of these gears can be inhibited from being worn away. 
   Furthermore, in the printer  100  according to the first embodiment, the disengagement mechanism  115  moves the image forming unit  120  at least for the above-described minimum travel distance X in the centerline direction. Therefore, once the disengagement of these gears is performed, the teeth of these gears do not contact with each other. As a result, the phase differences can be inhibited from becoming out of the predetermined phase differences, which may be caused by the teeth of these gears being in contact with each other when the image forming unit  120  is removed. 
   Still furthermore, in the printer  100  according to the first embodiment, the inner gears  113 A are disposed below and in front of the drum gears  151 A. Therefore, the angle, in which pressure is applied from the inner gears  113 A to the drum gears  151 A, conforms with the direction of conveyance of a sheet of paper P. As a result, in the printer  100  according to the present embodiment, suitable nip pressure can be maintained between the photoreceptor drum  151  and the transfer roller  174 . 
   Moreover, in the printer  100  according to the present embodiment, the projection members  138 A,  138 B of the drum unit  130  are guided by the disengagement guides  119 A,  119 B in the body frame  112 . Therefore, the image forming unit  120  can be reliably moved along the centerline direction. 
   In addition, the projection members  138 A,  138 B are provided respectively in the front side and rear side of the drum unit  130 , and the disengagement guides  119 A,  119 B are provided respectively in the front side and the rear side in the body frame  112 . Therefore, the image forming unit  120  can be stably moved along the centerline direction. 
   Moreover, in the printer  100  according to the present embodiment, guide rails  112 A are provided in the body frame  112 , and the rollers  137 A,  137 B, and  137 C, rotated on the guide rails  112 A, are provided in the drum unit  180 . Therefore, the image forming unit  120  can be stably removed outside of the body frame  112 . 
   Furthermore, in the printer  100  according to the present embodiment, all the drum gears  115 A and the inner gears  113 A are simultaneously disengaged. Therefore, the image forming unit  120  can be efficiently removed from the body frame  112 . 
   Additionally, in the printer  100  according to the present embodiment, all the drum gears  151 A and the inner gears  113  are disengaged at the same time when a user opens the front cover  111 E. Therefore, a user can remove the image forming unit  120  immediately after opening the front opening  111 D. 
   Second Embodiment 
   A printer  200  according to a second embodiment can be simply obtained by partially modifying the structure of the above-described printer  100  according to the first embodiment. Accordingly, the same reference numbers are used to components that are the same as in the printer  100  according to the first embodiment, and the descriptions thereof are not repeated here. 
   &lt;Structure of Disengagement Mechanism&gt; 
   The following describes the structure of the disengagement mechanism in the printer  200  with reference to  FIG. 9 . It is to be noted that the front cover  111 E, the cover support member  117 , and the operation gears  118  are not shown in  FIG. 9 , in order to simplify the description. 
   As shown in  FIG. 9 , the printer  200  includes links  216 , instead of the links  116  of the printer  100  according to the first embodiment. 
   The links  216  are different from the links  116  in a way that the length thereof is shorter than the links  116 . 
   In the printer  200  configured as above, when the links  216  are pushed toward the rear side by a user opening the front cover  111 E, firstly, the front inclined portions  216 B of the links  216  are abutted on the projection members  138 A. Then, the front side of the image forming unit  120  is guided by the disengagement guide  119 A, and lifted in the centerline direction. 
   When the user further opens the front cover  111 E and thereby pushes the links  216  toward the rear side, the rear inclined portions  216 C are abutted on the projection members  138 B. Subsequently, the rear side of the image forming unit  120  is guided by the disengagement guides  119 B, and lifted in the above-described centerline direction. 
   Therefore, in the printer  200 , the drum gears  151 A and the inner gears  113 A are sequentially disengaged from the front side of the drum unit  130 . 
   &lt;Effect of Printer  200 &gt; 
   In the printer  200  according to the second embodiment, the drum gears  151 A and the inner gears  113 A are sequentially disengaged from the front side. Therefore, even if the image forming unit  120  is heavy, a large load is not applied on the link  216  at a time. As a result, a user can easily disengage these gears. 
   [Variation] 
   Although specific embodiments have been illustrated and described herein, it is to be understood that the above description is intended to be illustrative, and not restrictive. Combinations of the above embodiments and other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention includes any other applications in which the above structures are used. Accordingly, the scope of the invention should only be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 
   For example, the inner gears  113 A of the above described printers  100  and  200  are disposed below and in front of the drum gears  151 A. However, the inner gears  113 A can be disposed in an alternative position, such as below and behind the drum gears  151 A.