Abstract:
An image forming includes a plurality of photosensitive bodies and a drive unit. Each photosensitive body forms an image having a different color. The drive unit selectively switches between forward drive and reverse drive. The drive unit uses forward drive to selectively drive a particular one of the plurality of photosensitive bodies and uses reverse drive to selectively drive another one of the plurality of photosensitive bodies.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming device such as a color laser printer. 
     2. Description of the Related Art 
     A tandem laser printer is one type of laser printer for forming full-color images. One example of a tandem laser printer includes an image forming unit for each color. Each image forming unit includes a developing roller, a photosensitive drum, a charge unit, and an exposure unit. The developing roller, the charge unit, and the exposure unit are disposed in confrontation with the photosensitive drum. The charge unit forms a uniform charge on the surface of the photosensitive drum. The exposure unit selectively exposes portions of the charged surface to form a latent static-electric image on the surface of the photosensitive drum. The developing roller bears toner on its surface and develops the latent static-electric image using the toner. 
     The visible toner images developed for each color are transferred one at a time in order onto a transfer belt so that a full-color image can be formed at substantially the same speed as a monochrome image. 
     Some tandem color laser printers can selectively switch between forming color images and monochrome images. That is, to form a color image, all four photosensitive drums are driven to form images in the four colors of yellow, magenta, cyan, and black. The different color images are transferred one at a time into a stacked condition onto a transfer belt to form a color image. On the other hand, to form a monochrome image, only the photosensitive drum for forming black image is driven so that only a black image is transferred onto the transfer belt to form a monochrome image. 
     Four motors are provided, one for each photosensitive drum in order to enable selective switching between multi-color and monochrome image formation. All four of the motors are driven when a multi-color image is to be formed and only the motor that corresponds to the black photosensitive drum is driven when a monochrome image is to be formed. However, providing four motors in this manner increases production costs. Also, the control circuit must be able to control drive of all the motors, which increases the complexity of the printer. 
     SUMMARY Of THE INVENTION 
     It is conceivable to drive all four photosensitive drums using a single motor in order to reduce production costs and simplify configuration. To achieve this, it is conceivable to provide an electromagnetic clutch between the single motor and the photosensitive drums that can be switched to selectively transmit drive force from the motor to one or all of the four photosensitive drums. Monochrome images can be formed when only one of the photosensitive drum is driven and multi-color images can be formed when all four photosensitive drums are driven. By providing this electromagnetic clutch, there is no need to provide a separate motor for all of the four photosensitive drums. 
     However, with this conceivable configuration, the electromagnetic clutch itself as well as circuitry for controlling the switching operation of the electromagnetic clutch must be provided, thereby increasing production costs and complexity of the printer. Also, a large torque is required to rotate all four of the photosensitive drums. As a result, a great deal of power would be required to prevent the electromagnetic clutch from slipping while a multi-color image is being formed. This would greatly increase running costs. 
     It is an objective of the present invention to overcome the above-described problems and to provide an image forming device with low production coats and a simple configuration capable of selectively switching drive of a plurality of photosensitive bodies and selectively forming multi-color and monochrome images. 
     To achieve the above-described objectives, an image forming device according to one aspect of the present invention includes a plurality of photosensitive bodies and a single drive unit. The plurality of photosensitive bodies each forms an image having a different color. The single drive unit switches between driving at least one of the photosensitive bodies and at least a different one of the photosensitive bodies. 
     An image forming device according to another aspect of the present invention includes a plurality of developing units, a plurality of photosensitive bodies, a transfer unit, a drive unit, and a transmission mechanism. Each of the developing units is provided for a different one of a plurality of colors. The photosensitive bodies are provided in correspondence with the developing units. The transfer unit is disposed in confrontation with the photosensitive bodies. The drive unit generates drive force. The transmission mechanism switches transmission of the drive force from the drive unit to photosensitive bodies selected in accordance with drive condition of the drive unit. 
     According to still another aspect of the present invention, an image forming device includes a plurality of developing units, a plurality of photosensitive bodies, a transfer unit, a drive unit, and a transmission mechanism. Each developing unit is provided for a different one of a plurality of colors. The photosensitive bodies are provided in correspondence with the developing units. The transfer unit is disposed in confrontation with the photosensitive bodies. The drive unit switchingly generates forward drive force and reverse drive force. The transmission mechanism transmits drive force from the drive unit to the photosensitive bodies. The transmission mechanism transmits the same direction of drive force to the photosensitive bodies regardless of whether the drive unit generates forward drive force or reverse drive force. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will become more apparent from reading the following description of the embodiment taken in connection with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view showing essential components of a color laser printer according to an embodiment of the present invention; 
     FIG. 2 is a perspective view showing a cyan developing process portion as an example of developing process portions in the color laser printer of FIG. 1; 
     FIG. 3 is a frontal cross-sectional view showing configuration of the color laser printer of FIG. 1 for transmitting drive force and a side view showing details of a reverse direction transmission mechanism; 
     FIG.  4 ( a ) is a cross-sectional view showing condition of a first one-way clutch mechanism during forward direction drive of a drive shaft; 
     FIG.  4 ( b ) is a cross-sectional view showing condition of the first one-way clutch mechanism during reverse direction drive of the drive shaft; 
     FIG.  5 ( a ) is a cross-sectional view shoving condition of a second one-way clutch mechanism during forward direction drive of a drive shaft; and 
     FIG.  5 ( b ) is a cross-sectional view showing condition of the second one-way clutch mechanism during reverse direction drive of the drive shaft. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Next, a color laser printer  1  according to an embodiment of the present invention will be described while referring to the attached drawings. As shown in FIG. 1, the laser printer  1  includes a casing  2 , an upper cover  18 , a sheet-feed portion  4 , and an image forming portion  5 . The casing  2  houses the sheet-feed portion  4  and the image forming portion  5 . The sheet-feed portion  4  is for feeding out sheets  3  one sheet at a time. The image forming portion  5  is for forming images on the fed out sheets  3 . 
     The sheet-feed portion  4  includes a sheet-feed tray  6  and a sheet-feed roller  7 . The sheet-feed tray  6  is stacked with sheets  3 . The sheet-feed roller  7  feeds out the highest sheet  3  on the sheet-feed tray  6  in order to supply one sheet at a time to the image forming portion  5 . 
     The image forming portion  5  includes four process portions  8 K,  8 C,  8 M, and  8 Y, an intermediate transfer mechanism  9 , a secondary transfer roller  10 , and a fixing portion  11 . The four process portions are located in the upper portion of the casing  2  and will be referred to collectively as the “process portions  8 ” to simplify explanation. 
     The four process portions a include a yellow developing process portion  8 Y, a magenta developing process portion  8 M, a cyan developing process portion  8 C, and a black developing process portion  8 K. The four process portions  8  are aligned in the horizontal direction, separated by a predetermined spacing. Each developing process portion  8  has substantially the same configuration and surrounding components, so the configuration of the cyan developing process portion  8 C and surrounding components will be described as a representative example. 
     As shown in FIG. 2, the cyan developing process portion  8 C includes a process cartridge  12 , an LED array  14 , and a scorotron charge unit  15 . The process cartridge  12  is freely detachably mounted to the casing  2  as indicated by two-dot chain line in FIGS. 1 and 2. As shown in FIG. 2, the process cartridge  12  includes a photosensitive drum  13  and a developing cartridge  16 . The developing cartridge  16  is mounted on the photosensitive drum  13 . The developing cartridge  16  includes a developing roller  17  and, although not shown in the drawings, a layer-thickness regulating blade, a toner-supply roller, and a toner box. 
     The toner box of the developing cartridge  16  is filled with non-magnetic, single-component toner that charges to a positive charge. Because the cyan developing process portion  8 C is being described in this example, the toner box in the developing cartridge  16  is filled with cyan-colored toner. However, the toner box in the developing cartridge  16  of the yellow developing process portion  8 Y is filled with yellow-colored toner, the toner box in the developing cartridge  16  of the magenta developing process portion  8 M is filled with magenta-colored toner, and the toner box in the developing cartridge  16  of the black developing process portion  8 Y is filled with black-colored toner. 
     The toner-supply roller is rotatably disposed below the toner box. The toner-supply roller includes a metal roller shaft that is covered by a conductive foam roller. The developing roller  17  is rotatably disposed below the toner-supply roller in pressing contact with the toner-supply roller. The developing roller  17  includes a metal roller shaft that is covered by a conductive rubber roller. 
     The layer-thickness regulating blade is disposed adjacent to the developing roller  17 . The layer-thickness regulating blade includes a blade body and a pressing portion. The blade body is made from a metal plate spring and is supported at one end by the developing cartridge  16  at a position near the developing roller  17 . The pressing portion is provided on the free end of the blade body, that is, at the end opposite from the end supported by the developing cartridge  16 . The pressing portion is made from silicon rubber that has electrically insulating properties and is formed in a half-circle shape when viewed in cross-section. Resilient force of the blade body presses the pressing portion onto the developing roller  17 . 
     Rotation of the toner-supply roller supplies the toner from the toner box to the developing roller  17 , where friction between the toner-supply roller and the developing roller  17  charges the toner to a positive charge. As the developing roller  17  rotates, the layer-thickness regulating blade operates to regulate the toner on the developing roller  17  to a fixed thickness and to sufficiently charge the toner. 
     The photosensitive drum  13  is attached to the developing cartridge  16  in a condition below and in confrontation with the developing roller  17 . The photosensitive drum  13  is driven to rotate clockwise as indicated by arrows in FIG.  1 . The photosensitive drum  13  includes a cylindrical drum body that is connected to ground. The outer circumferential surface of the photosensitive drum  13  is made from an organic photosensitive material including polycarbonate. 
     The upper cover  18  covers the upper portion of the casing  2 . The upper cover  18  is pivotably attached to a side wall of the casing  2  by a hinge  19 . A downward-extending attachment frame  20  for each process cartridge  12  is provided integrally with the upper cover  18 . The LED array  14  and the scorotron charge unit  15  are attached to the attachment frame  20  so that by opening the upper cover  18  the process cartridge  12  can be attached and removed as indicated in two-dot chain line in FIGS. 1 and 2. 
     The LED array  14  is configured from a plurality of LEDs aligned in a row disposed above the photosensitive drum  13  when the upper cover  18  is closed. The LEDs selectively emit light based on image data to selectively irradiate the surface of the photosensitive drum  13 . 
     The scorotron charge unit  15  is disposed, that is, when the upper cover  18  is closed, to the side of the photosensitive drum  13  at a position separated from the photosensitive drum  13  so as not to contact the photosensitive drum  13 . The scorotron charge unit  15  is a positively-charging scorotron type charge unit that generates a corona discharge from a charge wire made from tungsten, for example. The scorotron charge unit  15  charges the surface of the photosensitive drum  13  to a uniform positive charge. 
     After the scorotron charge unit  15  charges the surface of the photosensitive drum  13  to a uniform positive charge, the LED array  14  emits light based on image data to selectively expose the charged surface of the photosensitive drum  13 . The electric potential of the uniform charge on the surface of the photosensitive drum  13  drops where exposed by light from the LED array  14 . The portions at the surface with electric potential lowered in this manner form a latent static-electric image. 
     As mentioned previously, the toner borne on the surface of the developing roller  17  is charged to a positive charge. When the toner on the surface of the developing roller  17  moves into confrontation with the surface of the photosensitive drum  13 , the toner is selectively borne on the latent static-electric image, thereby developing the latent static-electric image into a visible toner image. This visible toner forming process is performed separately for each different color the process portions  8 K,  8 C,  8 M, and  8 Y. Accordingly, inverse development is achieved for each color. The visible image borne on the photosensitive dram  13  is transferred onto the endless belt  22  as the corresponding portion of the endless belt  22  moves into and out of confrontation with the photosensitive drum  13  by circulating movement of the endless belt  22 . 
     As shown in FIG. 1, the intermediate transfer mechanism  9  is disposed in confrontation with all of the photosensitive drums  13  from a position below the photosensitive drums  13 . The intermediate transfer mechanism  9  includes the endless belt  22  and three rollers, that is, a first roller  23 , a second roller  24 , and a third roller  25 . The first roller  23  is provided downstream from the sheet-feed roller  7  with respect to the transport direction of sheets  3 . The second roller  24  is disposed above the first roller  23  at a position upstream from the yellow developing process unit  8 Y with respect to the movement direction of the endless belt  22 . The third roller  25  is disposed substantially beneath the black developing process unit  8 K separated from the second roller  24  by a predetermined distance in the horizontal direction. The first through third rollers  23  to  25  are disposed at the corners of an imaginary inverted triangle. The upper edge of the imaginary triangle is formed by an imaginary horizontal line that connects the upper edges or the second roller  24  and the third roller  25  and contacts the lower edge of between the photosensitive drums  13 . Another edge of the imaginary triangle extends diagonally downward and frontward from the third roller  25  to the first roller  23  and still another extends diagonally upward and forward the first roller  23  to the second roller  24 . 
     The endless belt  22  is wound around the outer periphery of the first through third rollers  23  to  25 . The endless belt  22  moves between the second and third rollers  24 ,  25  in a direction indicated by arrows in FIG. 1, pressed against the lower edge of the photosensitive drums  13  by a predetermined pressing force. It should be noted that the endless belt  22  is made from conductive resin, such as polycarbonate or polyimide, dispersed with conductive particles, such as carbon. 
     Rotation of the first through third rollers  23  to  25  brings the endless belt  22  sequentially into confrontation with the photosensitive drums  13  so that visible toner images formed in different colors by the different photosensitive drums  13  are transferred onto the endless belt  22  one at a time in order, and overlap to form a full-color image. For example, first a yellow visible image, which was formed on the corresponding photosensitive drum  13  from yellow toner that fills the developing cartridge  16  of the yellow process portion  8 Y, is transferred onto the endless belt  22 , then a magenta visible image, which was formed on the magenta photosensitive drum  13 M from magenta toner that fills the developing cartridge  16  of the magenta process portion  8 M, is transferred onto the endless belt  22  on top of the previously transferred yellow image. By the same operation, the cyan visible image, which was formed on the cyan photosensitive drum  13 C from cyan toner that fills the developing cartridge  16  of the cyan process portion  8 C, and the black visible image, which was formed on the black photosensitive drum  13 B from black toner that fills the developing cartridge  16  of the black process portion  8 B, are also transferred onto the endless belt  22  in an overlapping condition with the yellow visible image and the magenta visible image so that a color image is formed on the endless belt  22 . 
     The secondary transfer roller  10  is rotatably disposed at a position in confrontation with the first roller  23  of the intermediate transfer mechanism  9  through a sheet  3 . The secondary roller  10  includes a metal roller shaft and a conductive rubber roller. The roller covers the metal roller shaft. The secondary roller  10  is applied with a predetermined transfer bias. The color image formed on the endless belt  22  is transferred all at once onto the sheet  3  passing between the endless belt  22  and the secondary transfer roller  10 . 
     In this way, the visible toner images borne on the different photosensitive drums  13  are temporarily transferred onto the endless belt  22  of the intermediate transfer mechanism  9 . After a color image is formed on the endless belt  22  by stacking the different colored images onto the endless belt  22 , the full color image is transferred in a single action from the endless belt  22  onto the secondary transfer roller  10 . 
     The fixing portion  11  is disposed downstream from the secondary transfer roller  10  with respect to the transport direction of the sheet  3 . The fixing portion  11  includes a thermal roller  26  and a pressing roller  27 . The pressing roller  27  presses against the thermal roller  26 . The thermal roller  26  is made from metal and includes a halogen lamp for heating the metal. The thermal roller  26  thermally fixes the color image that was transferred by the secondary transfer roller  10  onto the sheet  3  as the sheet  3  passes between the thermal roller  26  and the pressing roller  27 . Afterward, the sheet  3  is discharged from the casing  2 . 
     In this way, the color laser printer  1  includes a photosensitive drum  13  for each color so that using a tandem type mechanism, a full color image can be formed with substantially the same speed as a monochrome image. 
     The color laser printer  1  includes a first worm gear  31 , and two support rollers  32   a ,  32   b  for each photosensitive drum  13 . The two support rollers  32   a ,  32   b  will be alternately referred to collectively as support rollers  32  hereinafter. Each set of first worm gear  31  and the support rollers  32  supports the corresponding photosensitive drum  13  in a rotatable manner. 
     Two drive shafts  25  extend in the direction followed the upper portion of the endless belt  22 . Although only one is shown in the drawings, one of the drive shafts  25  is provided on either axial side of the photosensitive drums  13 . The drive shafts  25  serve as a common drive source for all of the photosensitive drums  13 . The first worm gears  31  are provided on the drive shafts  25  at positions in confrontation with the corresponding photosensitive drums  13 . 
     Two disk-shaped bearing members  33  and two first worm wheels  34  are provided on the outer peripheral surface of each photosensitive drum  13 . One of the disk-shaped bearing members  33  and one of the first worm wheels  34  are provided at each axial end of the photosensitive drum  13 . As shown in FIG. 2, the first worm wheels  34  are disposed nearer the axial ends of the photosensitive drum  13  than the disk-shaped bearing members  33 . Each first worm wheel  34  is meshingly engaged with the corresponding first worm gear  31 . 
     A single reversible motor M is provided for driving rotation of the drive shaft  35  that is visible in FIG.  1 . The motor M is a reversible motor and so can selectively rotate the drive shaft  35  in forward or reverse directions. 
     A pair of support rollers  32  is provided for each photosensitive drum  13 . As shown in FIG. 2, the first support roller  32   a  and the second support roller  32   b  are located at the upper portion of each bearing member  33  separated from each other by a predetermined distance. Although not shown detail in the drawings, each set of first and second support rollers  32   a ,  321   b  is provided on the attachment frame  20  of the upper cover  18  so as to swing away from and toward the corresponding photosensitive drum  13  with opening and closing movement of the upper cover  18 . When one of the process cartridges  12  is to be removed from the casing  2 , the upper cover  18  is opened up to swing the corresponding set of first and second support rollers  32   a ,  32   b  away from the corresponding photosensitive drum  13 . On the other hand, after one of the process cartridges  12  is newly mounted into the casing  2 , the upper cover  18  is closed up to swing the corresponding set of first and second support rollers  32   a ,  32   b  into pressing contact with the bearing members  33  at both axial ends of the corresponding photosensitive drum  13 , while separated from each other by the predetermined distance. 
     Each axial end photosensitive drum  13  is supported at a total of three positions, that is, by the corresponding first worm gear  31  and two support rollers  32 . One of the first worm gears  31  supports an axial end of the corresponding photosensitive drum  13  from below through the corresponding first worm wheel  34 . Each pair of support rollers  32  are swingable, via the cover  18 , into pressing contact with an axial end of the corresponding photosensitive drum  13  to support the photosensitive drum  13  from above. 
     With this configuration, each photosensitive drum  13  is supported at three positions, by two support rollers  32  and the drive shaft  35 , at both axial ends on its outer peripheral surface, which is formed with extremely high precision. Therefore, the photosensitive drums  13  can be rotated precisely without any eccentricity of rotation. Visible images formed on the photosensitive drums  13  can be transferred at the same speed onto the endless belt  22 . Eccentric rotation of the photosensitive drum  13  can be reliably and easily prevented and good images can be formed. 
     Power from the single motor M is transmitted to drive the drive shaft  35  to rotate. The first worm gears  31  provided on the drive shaft  35  rotate as a result. Therefore, the photosensitive drums  13  are driven to rotate by their first worm wheels  31 , which are in meshing engagement with the worm gears  31 . Therefore, the photosensitive drums  13  can be reliably rotated using a simple configuration. 
     All of the photosensitive drums  13  can be driven to rotate by driving the drive shaft  35  to rotate using the single motor M. There is no need to provide a gear train transmission system or a motor for each photosensitive drum  13 . Therefore the photosensitive drums  13  can be reliably driven with a simple configuration. 
     Further, by switching between forward drive and reverse drive of the drive shaft  35  using the motor M, either all or only one of the photosensitive drums  13  can be selectively driven. In order to form a multi-color image, all four photosensitive drums  13 , that is, the yellow photosensitive drum  13 Y, the magenta photosensitive drum  13 M, the cyan photosensitive drum  13 C, and the black photosensitive drum  13 K, are driven to rotate by forward drive of the drive shaft  35 . On the other hand, in order to form a monochrome image, only one of the photosensitive drums  13 , that is, the black photosensitive drum  13 K, is driven to rotate by reverse drive of the drive shaft  35 . 
     Configuration for achieving this selective rotational drive will be described next. As shown in FIGS. 3,  4 ( a ), and  4 ( b ), a first one-way clutch mechanism  36  is interposed between the drive shaft  35  and each of the first worm gears  31 . As a result, four first one-way clutch mechanisms  36  are provided in total along the drive transmission path between the drive shaft  35  and the four photosensitive drums  13 . The first one-way clutch mechanisms  36  transmit drive force only during forward drive of the drive shaft  35 . In addition, a reverse direction transmission mechanism  50  is provided along the drive transmission path between the drive shaft  35  that is visible in FIG.  1  and the black photosensitive drum  13 K. The reverse direction transmission mechanism  50  transmits drive force from the drive shaft  35  only during reverse drive of the drive shaft  35 . With this configuration, three of the photosensitive drums  13 , that is, the yellow photosensitive drum  13 Y, the magenta photosensitive drum  13 M, and the cyan photosensitive drum  13 C are only driven during forward drive of the drive shaft  35 , and one of the photosensitive drums  13 , that is, the black photosensitive drum  13 K, is driven both during forward and reverse drive of the drive shaft  35 . 
     The first one-way clutch mechanisms  36  are provided at the outer periphery of the drive shaft  35 , within the first worm wheels  31  of each of the four photosensitive drums  13 . As shown in FIGS.  4 ( a ) and  4 ( b ), each first one-way clutch mechanism  36  includes a first sleeve  42 , first rollers  44 , and springs  45 . Each first sleeve  42  is provided so that its inner peripheral surface is slidable with respect to the drive shaft  35  and so that it outer peripheral surface moves integrally with the inner peripheral surface of the corresponding worm gear  31 . Said differently, each first sleeve  42  is provided incapable of relative movement with respect to the corresponding worm gear  31 . Each first sleeve  42  is formed with a plurality of first grooves  43 . One of the first rollers  44  and one of the springs  45  is disposed in each of the first grooves  43 . 
     Each first sleeve  42  has a tube shape that follows the axial direction of the corresponding worm gear  31 . 
     Six first grooves  43  are formed in the outer peripheral surface of each first sleeve  42 , spaced at a predetermined interval following around the circumference of the first sleeve  43 . The first grooves  43  are formed as openings in the inner peripheral surface of each first sleeve  42  and follow the axial direction of the corresponding first worm gear  31 . Although each first groove  43  is substantially rectangular in cross section as can be viewed in FIGS.  4 ( a ) and  4 ( b ), each first groove  43  includes a broad space  43   a  and a narrow space  43   b . Each broad space  43   a  is located at the upstream side of the corresponding groove  43  with respect to the forward drive direction of the drive shaft  35 , that is, the counterclockwise direction as indicated by an arrow in FIG.  4 ( a ), and is formed sufficiently large to enable the corresponding first roller  44  to move freely between the first sleeve  42  and the outer peripheral surface of the drive shaft  35 . On the other hand, each narrow space  43   b  is located at the downstream side of the corresponding groove  43  with respect to the forward drive direction of the drive shaft  35 , and is formed sufficiently small to firmly sandwich the corresponding first roller  44  between the first sleeve  42  and the outer peripheral surface of the drive shaft  35 . 
     That is, the broad space  43   a  of each first groove  43  is formed into the first sleeve  42  to an average depth from the inner peripheral surface of the first sleeve  42  that is larger than the diameter of the first roller  44 . The narrow space  43   b  of each first groove  43  tapers so that its depth from the inner peripheral surface of the first sleeve  42  gradually diminishes from its rear upstream side, where it connects to the corresponding broad space  43   a , to its front upstream side, where it is shallower than the diameter of the corresponding first roller  44 . 
     Each first roller  44  has a rod shape and is disposed in the corresponding first groove  43  so as to extend following the axial direction of the corresponding first worm gear  31 . Each first spring  45  is positioned in the rear end upstream side of the broad space  43   a  of the corresponding first groove  43 . The springs  45  constantly urge the corresponding first roller  44  toward the front end downstream side of the corresponding narrow space  43   b.    
     Next, operation of the first one-way clutch mechanisms  36  will be described. During forward drive of the drive shaft  35  as shown in FIG.  4 ( a ), the urging force of the first springs  45  move the first rollers  44  toward the narrow spaces  43   b  in association with the forward rotation of the drive shaft  35  so that the first rollers  44  become firmly sandwiched between the first sleeve  42  and drive shaft  35  and restrict relative movement between the first sleeve  42  and the drive shaft  35 . As a result, forward drive of the drive shaft  35  is transmitted through the first one-way clutch mechanisms  36  to the first worm gears  31  so that the first worm gears  31  rotate with the drive shaft  35 . 
     On the other hand, during reverse drive of the drive shaft  35 , that is, when the drive shaft  35  is driven by the motor M to rotate in the clockwise direction indicated by arrows in FIG.  4 ( b ), rotation of the drive shaft  35  moves the first rollers  44  against the urging force of the first springs  45  into the broad spaces  43   a  so that the first rollers  44  move freely between the first sleeve  42  and drive shaft  35 . Thus, relative movement between the first sleeve  42  and the drive shaft  35  is allowed and reverse drive from the drive shaft  35  is not transmitted through the first one-way clutch mechanisms  36  to the first worm gears  31 . The drive shaft  35  rotates idly with respect to the first worm gears  31 . 
     The reverse direction transmission mechanism  50  is disposed along the power transmission path between the drive shaft  35  and the black photosensitive drum  13 K. As shown in FIG. 3, the reverse direction transmission mechanism  50  includes a rotation shaft  51 , a second worm gear  40 , a second worm wheel  41 , a first gear  37 , and a second gear  38 . 
     The second worm gear  40  is provided around the periphery of the drive shaft  35  at an axial end of the drive shaft  35 , further to the axial end than the first worm gear  31  that is in meshing engagement with the first worm wheel  34  of the black photosensitive drum  13 K. 
     The rotation shaft  51  is rotatably supported on the casing  2  at a position that is above and in confrontation with the second worm gear  40 . The second worm wheel  41  and the second gear  38  are formed integrally with the axial end of the rotation shaft  51 . The second worm wheel  41  is formed further from the axial end of the rotation shaft  51  than is the second gear  38  at a position in confrontation with and in meshing engagement with the second worm gear  40 . The second worm wheel  41  has substantially the same outer diameter as the first worm wheel  34 . 
     The second gear  38  is disposed in meshing engagement with the first gear  37  at a position outside from the second worm wheel  41  in the axial direction of the rotation shaft  51 . 
     The first gear  37  is formed at the outer peripheral surface of the black photosensitive drum  13 K to have substantially the same outer diameter as the second gear  38 . The first gear  37  is disposed on the axial end of the black photosensitive drum  13 K at a position further outside than the first worm wheel  34  in the axial direction of the black photosensitive drum  13 K. The first gear  37  is in meshing engagement with the second gear  38 . 
     The reverse direction transmission mechanism  50  further includes a second one way clutch mechanism  39  disposed in the second worm gear  40 . As shown in FIGS.  5 ( a ) and  5 ( b ), the second one way clutch mechanism  39  has a configuration similar to the first one way clutch mechanisms  36  and includes a second sleeve  46 , second rollers  48 , and springs  49 . The second sleeve  46  is provided capable of sliding over the outer peripheral surface of the drive shaft  35 . Second grooves  47  are formed in the inner peripheral surface of the second sleeve  46 . A set of one second roller  48  and one spring  49  is disposed in each of the second grooves  47 . 
     Each second groove  47  includes a broad space  47   a  and a narrow space  47   b . However, compared with the broad space  43   a  and the narrow space  43   b  of each first groove  43 , the broad space  47   a  and the narrow space  47   b  of each second groove  47  have the opposite orientation with respect to the rotational direction of the drive shaft  35 . That is, each broad space  47   a  is located at the downstream side of the corresponding groove  47  with respect to the forward drive direction, that is, the counterclockwise direction as indicated by an arrow in FIG.  5 ( a ), and each narrow space  47   b  is located at the upstream side of the corresponding groove  47  with respect to the forward drive direction. 
     Next, operation of the second one-way clutch mechanism  39  will be described. During forward drive of the drive shaft  35  as shown in FIG.  5 ( a ), rotation of the drive shaft  35  moves the second rollers  48  against the urging force of the second springs  49  into the broad spaces  47   a , so that the second rollers  48  move freely between the second sleeve  46  and the drive shaft  35  and relative movement between the second sleeve  46  and the drive shaft  35  is allowed. As a result, forward drive from the drive shaft  35  is not transmitted through the second one-way clutch  39  to the second worm gear  40 . The drive shaft  35  therefore rotates idly with respect to the second worm gear  40 . 
     On the other hand, during reverse drive of the drive shaft  35  as shown in FIG.  5 ( b ), the reverse Notation of the drive shaft  35  and the urging force of the second springs  49  move the second rollers  48  toward the narrow spaces  47   b , so that the second rollers  48  become firmly sandwiched between the second sleeve  46  and the drive shaft  35  and restrict relative movement between the first sleeve  42  and the drive shaft  35 . As a result, reverse drive of the drive shaft  35  is transmitted through the second one-way clutch  39  to the second worm gear  40  so that the second worm gear  40  rotates with the drive shaft  35 . 
     When the reversible motor M drives the drive shaft  35  in the forward direction, the first one way clutch mechanisms  36  corresponding to all four photosensitive drums  13 , that is, to the yellow photosensitive drum  13 Y, the magenta photosensitive drum  13 M, the cyan photosensitive drum  13   c , and the black photosensitive drum  13 K, transmit the drive force to the first worm gears  31 . Therefore, the first worm gears  31  rotate with the rotation of the drive shaft  35 , so that the sour photosensitive drums  13 , that is, the yellow photosensitive drum  13 Y, the magenta photosensitive drum  13 M, the cyan photosensitive drum  13 C, and the black photosensitive drum  13 K, all rotate. 
     However, during forward drive of the drive shaft  35 , the second one way clutch mechanism  39  of the reverse direction transmission mechanism  50  does not transmit drive force to the second worm gear  40 . Therefore, the drive shaft  35  rotates idly with respect to the second worm gear  40 . It should be noted that at this time, the first gear  37  is driven to rotate in association with rotational drive of the black photosensitive drum  13 K and, consequently, the second worm wheel  40  is driven to rotate in the opposite direction from the forward drive direction of the drive shaft  35  through the second gear  38  and the second worm wheel  41 . However, even though the second worm wheel  40  is driven to rotate in the opposite direction from the forward drive direction of the drive shaft  35 , the second one way clutch mechanism  39  prevents the drive force from being transmitted to the drive shaft  35 , so the drive shaft  35  rotates smoothly in the forward direction. 
     Accordingly, by driving the motor M to drive in the forward direction so that the drive shaft  35  rotates in the forward direction, all of the photosensitive drums  13 , that is, the yellow photosensitive drum  13 Y, the magenta photosensitive drum  13 M, the cyan photosensitive drum  13 C, and the black photosensitive drum  13 K, can be driven to smoothly rotate and a good-quality color image can be formed. 
     One the other hand, by driving the motor M to drive in the reverse direction so that the drive shaft  35  rotates in the reverse direction, the second one way clutch mechanism  39  of the reverse direction transmission mechanism  50 , which is provided only to a single photosensitive drum  13 , that is, the black photosensitive drum  13 K, transmits the drive force to the second worm gear  40 . Therefore, because the second worm gear  40  rotates with the drive shaft  35 , the second worm wheel  41  in meshing engagement with the second worm gear  40  is driven so that, consequently, the black photosensitive drum  13 K is driven to rotate through the second gear  38  and the first gear  37 . It should be noted that even when the drive shaft  35  rotates in reverse, the black photosensitive drum  13 K is driven through the reverse direction transmission mechanism  50  to rotate in the same rotational direction as during forward drive of the drive shaft  35 , so that image formation can be smoothly achieved. 
     Also, during reverse drive of the drive shaft  35 , the first one way clutch mechanisms  36  do not transmit drive force to the first worm gears  31 . Therefore, the drive shaft  35  will merely rotate idly with respect to the first worm gear  31 . For this reason, the other three photosensitive drums  13 , that is, the yellow photosensitive drum  13 Y, the magenta photosensitive drum  13 M, and the cyan photosensitive drum  13 C, will not rotate because of engagement between the first worm wheel  34  and the first worm gear  31 , for example. 
     Also, although during reverse drive of the drive shaft  35  the first worm wheel  34  rotates in association with rotational drive of the black photosensitive drum  13 K and, by its meshing engagement with the first worm wheel  34 , the first worm gear  31  is driven to rotate in the opposite direction from the reverse rotation direction of the drive shaft  35 , the first one-way clutch mechanism  36  that corresponds to the black photosensitive drum  13 K prevents the drive force from being transmitted to the drive shaft  35 . Therefore, smooth reverse drive of the drive shaft  35  can be achieved. 
     Accordingly, by driving the motor M in reverse so that the drive shaft  35  rotates in reverse, the black photosensitive drum  13 K can be smoothly driven to rotate while the yellow photosensitive drum  13 Y, the magenta photosensitive drum  13 M, and the cyan photosensitive drum  13 C are stopped. A high-quality monochrome image can be formed. 
     In this way, when a full color image is to be formed, the drive shaft  35  is driven in the forward direction so that all of the photosensitive drums  14  are driven to rotate through the first worm gears  31  and the first worm wheels  34 . On the other hand, when a monochrome image is to be formed, the drive shaft  35  driven to rotate in the reverse direction so that only the black photosensitive drum  13 K is driven to rotate through the second worm gear  40 , the second worm wheel  41 , the second gear  33 , and the first gear  37 . That is, all four photosensitive drums  13  for forming a full color image or only the black photosensitive drum  13 K for forming a monochrome image can be selected by merely switching drive direction of the drive shaft  35 . With this configuration, color images and monochrome images can be selectively formed using a simpler configuration that is less costly to produce than other configurations, for example, than a configuration that provides a separate motor for each photosensitive drum or an electromagnetic clutch along the drive transmission path for transmitting force to the photosensitive drums. Moreover, because the drive direction of the drive shaft  35  is merely switched between forward and reverse, there is no need to provide a large drive as would be the case were an electromagnetic clutch provided. Therefore, running costs can be reduced. 
     Because the three photosensitive drums  13 Y,  13 M and  13 C are driven by forward drive of the drive shaft  35  and the single black photosensitive drum  13 K is driven by forward and reverse drive of the drive shaft  35 , when the drive shaft  35  drives in the forward direction, then all of the photosensitive drums  13  are driven. On the other hand, when the drive shaft  35  drives in the reverse direction, then only the black photosensitive drum  13 K is driven to rotate. The four photosensitive drums  13 K can be selectively driven in a reliable manner with a simple configuration by merely switching between forward and reverse drive of the drive shaft  35  Moreover, the black photosensitive drum  13 K is is driven to rotate in the same direction as the other three photosensitive drums  13 Y,  13 M, and  13 C during both forward and reverse drive of the drive shaft  35 . Therefore, images can be formed in a smooth manner. 
     First one-way clutch mechanisms  36 , which transmit drive force only during forward drive of the drive shaft  35 , are provided along the drive transmission path between the drive shaft  35  and the yellow photosensitive drum  13 Y, the magenta photosensitive drum  13 M, and the cyan photosensitive drum  13 C. Another of the first one-way clutch mechanisms  36  and also a second one-way clutch mechanism  39 , which transmits drive force only during reverse drive of the drive shaft  35 , are provided along the drive transmission path between the drive shaft  35  and black photosensitive drum  13 K. With this configuration, when the drive shaft  35  is driven in the forward direction, the drive force is transmitted through the first one-way clutch mechanisms  36  to drive the yellow photosensitive drum  13 Y, the magenta photosensitive drum  13 M, the cyan photosensitive drum  13 C, and the black photosensitive drum  13 K to form a color image. Also, when the drive shaft  35  is driven to rotate in the reverse direction, then the drive force is transmitted through the second one-way clutch mechanism  39  to drive only the black photosensitive drum  13 K. Drive force can be reliably and selectively transmitted to the photosensitive drums for forming color images and to the photosensitive drum for forming a monochrome image using a simple configuration for switching between driving the drive shaft  35  in the forward and reverse directions. 
     Further, because drive force is transmitted unit-directionally using the first one-way clutch mechanisms  36  and the second one-way clutch mechanism  39 , drive force can be simply and reliably transmitted in one direction. Manufacturing costs can be reduced and selective transmission of drive force can be reliably performed. 
     Although not show in the drawings, the color laser printer  1  includes a central processing unit (CPU) that judges whether to drive the motor M and the drive shaft  35  forward or in reverse, that is, in order to print multi-color or monochrome images, based on image data input to the color laser printer  1 . 
     Although not shown in the drawings, a cam mechanism is provided for moving the endless belt  22  selectively into contact with all of the photosensitive drums  13  or just the black photosensitive drum  13 K depending on whether a monochrome image or a multi-color image is being formed. That is, when a monochrome image is to be formed, the can mechanism is driven by reverse drive of the drive shaft  35  to move the second roller  24  downward from a first position indicated in FIG. 1 by solid line to a second position indicated in FIG. 1 in two-dot chain line. In this condition, the endless belt  22  is in contact with only the black photosensitive drum  13 K. The yellow photosensitive drum  13 Y, the magenta photosensitive drum  13 M, and the cyan photosensitive drum  13 C are separated from the endless belt  22 . On the other hand, when a multi-color image is to be formed, the cam mechanism is driven by forward drive of the drive shaft  35  to move the second roller  24  upward from the second position to the first position. In this condition, the endless belt  22  is in contact with all of the photosensitive drums  13  as indicated by solid line in FIG.  1 . With this configuration, images from either all of the photosensitive drums  13  or just the black photosensitive drum  13 Y can be selectively transferred onto the endless belt  22  by switching merely between driving the drive shaft  35  forward and reverse. As a result, the images formed by driving either all the photosensitive drums  13  to form a multi-color image or just the black photosensitive drum  13 K to form a monochrome image can be selectively transferred onto the endless belt  22  simply and reliably. 
     While the invention has been described in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the attached claims. 
     For example, the intermediate transfer mechanism  9  need not be provided, depending on the objectives and use the color laser printer  1 . That is, the embodiment described using the intermediate transfer mechanism  1  for transferring the different color images formed by the different photosensitive drums  13  one at a time onto the endless belt  22  and then, after a multi-color image is formed on the endless belt  22 , transferring the multi-color image in a single action onto the sheet  3 . However, the intermediate transfer mechanism  9  need not be provided. Instead, a transfer roller can be disposed in confrontation with each of the photosensitive drums, and the visible images formed at each of the photosensitive drums can be transferred directly onto a sheet  3  that passes between the photosensitive drums and the transfer rollers. 
     Also, the switching operation achieved by the first one-way clutch mechanisms  36  and the reverse clutch mechanism  39  is not limited to switching between multi-color and monochrome image formation. For example, the first one-way clutch mechanisms  36  and the reverse clutch mechanism  39  can be used for switching to two-color or to three-color image formation instead. Also, the first one-way clutch mechanisms  36  and the reverse clutch mechanism  39  can be used for switching between two different types of monochrome image formation, such as from black image to red image formation. 
     Also, in the embodiment, the second roller  24  was moved up and down by a cam mechanism driven by forward and reverse drive of the drive shaft  35 . However, the endless belt  22  can be switched between the first and second contact positions using other configurations, such as a solenoid and plunger.