Abstract:
An image forming apparatus includes a plurality of photosensitive drums and a plurality of developing rollers for developing latent images formed on the photosensitive drums, respectively. A switching unit switches positions of the plurality of developing rollers relative to the photosensitive drums, respectively, between developing positions for developing the latent images and spaced positions retracted from the developing positions. The developing rollers are capable of taking (i) a full spaced state in which all of the developing rollers are in the spaced positions, (ii) a partly developing state in which a portion of the developing rollers are in the developing positions, while the other is in the spaced position, and (iii) a full developing state in which all of the developing rollers are in the developing positions. The switching unit switches the positions of the developing rollers among the three states in a predetermined fixed order.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image forming apparatus, such as a copying machine, a printer, a facsimile apparatus, a multifunction apparatus, etc. 
     As one of the examples of an image forming apparatus which uses an electrophotographic process, a color image forming apparatus of the inline type. A color image forming apparatus of the inline type employs multiple process cartridges, which are juxtaposed in a straight line in the main assembly of the color image forming apparatus. A process cartridge is made up of a photosensitive drum, and one or more processing means which process a photosensitive drum, and a cartridge in which the photosensitive drum and processing means are integrally disposed. A processing means includes a charging means, a developing means, a cleaning means, etc. The charge roller is a means for applying charge bias voltage to a photosensitive drum. The developing means is a means for developing a latent image formed on a photosensitive drum, using developer (toner). 
     Generally speaking, there are two types of developing methods: contact developing method and noncontact developing method. In the contact developing method, a development roller is placed in contact with a photosensitive drum, whereas in the noncontact developing method, a preset amount of gap is kept between the peripheral surface of a development roller and the peripheral surface of a photosensitive drum. 
     In the case of the contact developing method, the peripheral surface of a photosensitive drum is shaved as it is rubbed by the peripheral surface of the development roller. Further, the development rollers in the cartridges which are not involved in the ongoing developing operation are also rotated. Therefore, an image forming apparatus in accordance with the prior art sometimes suffered from the problems that the internal components of a cartridge are unnecessary worn; recording paper is soiled by toner; a nonuniform image, the nonuniformity of which is attributable to the deformation of the surface layer of a development roller is formed; etc. 
     Thus, in order to solve the above described problems, the applicants of the present invention proposed the image forming apparatus stated in Japanese Laid-open Patent Application 2003-215876. In this image forming apparatus, multiple photosensitive drums are always kept in contact with a transfer belt, and are rendered different in the timing with which a developing means is placed in contact with a photosensitive drum. Further, the transmission of driving force to each developing means is synchronized with the timing with which the developing means is placed in contact with the corresponding photosensitive drum. Thus, during an image forming operation, all the photosensitive drums are driven along with the transfer belt, whereas the developing means are selectively driven; only the developing means necessary for the ongoing image forming operation is driven. After the completion of the image forming operation, the developing means is separated from the photosensitive drum, and the transmission of driving force to this developing means is stopped. Then, the driving of all the photosensitive drums and the transfer belt is also stopped. 
     However, it has long been desired to simplify an image forming apparatus such as the above described one, in the structure for separating the developing means from the corresponding photosensitive drum, and also, to simplify the mechanism for transmitting driving force to the development roller. 
     SUMMARY OF THE INVENTION 
     The primary object of the present invention is to provide an image forming apparatus which is substantially simpler in the structure for transmitting driving force to a development roller than an image forming apparatus in accordance with the prior art. 
     Another object of the present invention is to provide an image forming apparatus which is substantially simpler in the structure for separating a development roller from a photosensitive drum or placing a development roller in contact with a photosensitive drum than an image forming apparatus in accordance with the prior art. 
     Another object of the present invention is to provide an image forming apparatus which is substantially smaller in the amount by which the life of a process cartridge is unnecessarily reduced than an image forming apparatus in accordance with the prior art. 
     Another object of the present invention is to provide an image forming apparatus which is superior to an image forming apparatus in accordance with the prior art, in terms of image quality. 
     According to an aspect of the present invention, there is provided an image forming apparatus for forming a color image on a recording material, comprising i) mounting means for detachably mounting a plurality of process cartridges each including a photosensitive drum and a developing roller for developing an electrostatic latent image formed on said photosensitive drum, said process cartridges including a black process cartridge containing a black color developer and a non-black process cartridge containing a non-black developer; (ii) a driving source; (iii) a first clutch for connecting and disconnecting between said driving source and said developing roller for selective driving force transmission to the black process cartridge; (iv) a second clutch for connecting and disconnecting between said driving source and said developing roller for selective driving force transmission to the non-black process cartridge; (v) a movable first member actable on said first clutch for switching between an operation state for transmitting the driving force to said developing rollers and a non-operation state not transmitting the driving force thereto; (vi) a movable second member, actable on said second clutch, for switching between an operation state for transmitting the driving force to said developing rollers and a non-operation state not transmitting the driving force thereto; (vii) a switching member, movable by the driving force of said driving source and actable on said first member and second member, for switching among a first mode for transmitting the driving force to said developing rollers of all of said process cartridges, a second mode for not transmitting the driving force to any one of said developing rollers, and a third mode for transmitting the driving force only to said developing roller of said black process cartridge. 
     According to another aspect of the present invention, there is provided an image forming apparatus for forming a color image on a recording material, comprising (i) mounting means for detachably mounting a plurality of process cartridges each including a photosensitive drum, a developing roller for developing an electrostatic latent image formed on said photosensitive drum, a first frame for rotatably supporting said photosensitive drum and a second frame for rotatably supporting said developing roller, said second frame being movable relative to said first frame to contact said developing roller to said photosensitive drum and spacing said developing roller from said photosensitive drum, said process cartridges including a black process cartridge containing a black color developer and a non-black process cartridge containing a non-black developer; (ii) a driving source; (iii) a first clutch for connecting and disconnecting between said driving source and said developing roller for selective driving force transmission to the black process cartridge; (iv) a second clutch for connecting and disconnecting between said driving source and said developing roller for selective driving force transmission to the non-black process cartridge; (v) a first member for switching between an operation state for acting on said first clutch and on said second frame of said black process cartridge to contact said photosensitive drum and said developing roller to each other and transmit the driving force to said developing roller in said black process cartridge, and a non-operation state not transmitting the driving force thereto; (vi) a second member for switching between an operation state for acting on said second clutch and on said second frame of said non-black process cartridge to contact said photosensitive drum and said developing roller to each other and transmit the driving force to said developing roller in said non-black process cartridge, and a non-operation state not transmitting the driving force thereto; (vii) a switching member, movable by the driving force of said driving source and actable on said first member and second member, for switching among a first mode for contacting said developing rollers to said photosensitive drums, respectively and for transmitting the driving force to said developing rollers of all of said process cartridges, a second mode for spacing said developing rollers from said photosensitive drums and for not transmitting the driving force to any one of said developing rollers, and a third mode for contacting said developing roller to said photosensitive drum and for transmitting the driving force only to said developing roller of said black process cartridge. 
     These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view of the image forming apparatus in the first embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of the process cartridge in the first embodiment of the present invention, showing the internal structure thereof. 
         FIG. 3  is a partially exploded perspective view of the process cartridge in the first embodiment. 
         FIG. 4  is a perspective view of the main frame of the image forming apparatus and one of the process cartridges, in the first embodiment, showing how the process cartridge is mounted into the apparatus main assembly. 
         FIGS. 5(   a ) and  5 ( b ) are a cross-sectional view of the portions of the apparatus main assembly, and a cartridge therein, in the first embodiment, and a side view of the cartridge positioning portion of the apparatus main assembly, and the cartridge therein, respectively, showing how the cartridge is accurately positioned relative to the apparatus main assembly. 
         FIG. 6  is an oblique sectional view of all the process cartridge bays, and all the process cartridges therein, in which the cartridge has been separated from the photosensitive drum. 
         FIG. 7  is a schematic perspective view of the process cartridges in the apparatus main assembly, and their adjacencies, showing that the development roller of the cartridge for black color is in contact with the corresponding photosensitive, whereas the development roller in each of the cartridges for yellow, magenta, and cyan colors, is remaining separated from the corresponding photosensitive drum. 
         FIG. 8  is a schematic perspective view of the process cartridges in the apparatus main assembly, and their adjacencies, showing that the development rollers in all cartridges are remaining separated from the corresponding photosensitive drums. 
         FIG. 9  is a perspective view of the cams and development roller separating plates in the first embodiment. 
         FIG. 10  is a perspective view of the process cartridge driving portion in the first embodiment. 
         FIG. 11  is a side view of the gear train in this embodiment, showing the role of the gear with a toothless range (which hereafter may be referred to as partially toothless gear). 
         FIG. 12  is a schematic side view of one of the cams in the first embodiment, showing the movement of the cam. 
         FIG. 13  is a table showing the mode switching order in the first example of mode switching sequence. 
         FIG. 14  is a table showing the mode switching order in the second example of mode switching sequence. 
         FIG. 15  is a table showing the mode switching order in the third example of mode switching sequence. 
         FIG. 16  is a perspective exploded view of one of the development roller clutches. 
         FIG. 17  is a schematic drawing of the development roller separating mechanism in the full-color mode. 
         FIG. 18  is a schematic drawing of the development roller separating mechanism in the black-and-white mode. 
         FIG. 19  is a schematic drawing of the development roller separating mechanism in the home mode. 
         FIG. 20  is a perspective view of the modified versions of the development roller separating plates and development roller separating cam in the first embodiment. 
         FIG. 21  is a perspective view of the development roller separating plates and development roller separating cams in the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     Hereinafter, the preferred embodiments of the present invention will be described in detail with reference to the appended drawings. 
     (Image Forming Apparatus) 
       FIG. 1  is a sectional view of a full-color laser beam printer, as an example of an image forming apparatus, showing the overall structure of the main assembly  100  of the full-color laser beam printer (which hereafter will be referred to simply as printer main assembly  100 ). In this printer main assembly  100 , multiple process cartridges  7   a ,  7   b ,  7   c , and  7   d  (which hereafter may be referred to simply as cartridge), are removably mounted, being juxtaposed in a virtually vertical straight line. The cartridges  7   a ,  7   b ,  7   c , and  7   d  contain yellow (Y), magenta (M), cyan (C), and black (BK) toners, respectively. Each of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d  is made up of a cylinder, and an organic photoconductive layer (OPC) coated on the peripheral surface of the cylinder. Each of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d  rotates in the counterclockwise direction by receiving rotational driving force from a motor as a driving force source, through one of the lengthwise ends of its cylinder. The photosensitive drum  1  is processed by the following processing means, which will be described in the order they process the photosensitive drum  1 , in terms of the rotational direction of the photosensitive drum  1 . 
     First, the charge rollers  2   a ,  2   b ,  2   c , and  2   d  uniformly charge the peripheral surfaces of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , respectively. As examples of charging apparatuses, electrically conductive charge rollers  2   a ,  2   b ,  2   c , and  2   d  are used. The peripheral surfaces of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d  are uniformly charged by applying charge bias to these electrically conductive charge rollers  2   a ,  2   b ,  2   c , and  2   d  while keeping the electrically conductive charge rollers  2   a ,  2   b ,  2   c , and  2   d  in contact with the peripheral surfaces of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , respectively. 
     Scanner units  3   a ,  3   b ,  3   c , and  3   d  form an electrostatic latent image on the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d  by projecting a beam of laser light onto the peripheral surfaces of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , while modulating the beam of laser light with picture information. The scanner units  3   a ,  3   b ,  3   c , and  3   d  are disposed at roughly the same levels as the axial lines of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , respectively. The beams of image formation light, that is, the beams of laser light emitted by laser diodes while being modulated with picture signals, are projected onto polygon mirrors  9   a ,  9   b ,  9   c , and  9   d  which are being rotated at a high speed by scanner motors (unshown). The beams of image formation light reflected by these polygon mirrors  9   a ,  9   b ,  9   c , and  9   d  are focused onto the uniformly charged areas of the peripheral surfaces of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , through focal lenses  10   a ,  10   b ,  10   c , and  10   d , selectively exposing numerous points on the uniformly charged areas of the peripheral surfaces of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , respectively. As a result, electrostatic latent images are effected on the peripheral surfaces of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , one for one. Referring to  FIGS. 5(   a ) and  5 ( b ), the length of each scanner unit  3  is greater than the distance between the left and right lateral panels  32 , in terms of the lengthwise direction of the scanner unit  3   a . Thus, the scanner unit  3   a  is attached to the main frame of the apparatus main assembly so that a pair of protrusions  33   a , with which the scanner unit  3   a  are provided protrude outward beyond the left and right lateral panel  32  through a pair of openings  35   a  which the left and right panel  32  are provided. While the scanner unit  3   a  is in the apparatus main assembly, it is kept pressed in the slantingly downward direction, indicated by an arrow mark G, by roughly 1 kgf of pressure applied by a compression spring  36   a . Therefore, it is assured that the scanner unit  3   a  is kept pressed upon a pair of scanner unit positioning protrusions  35   a   1  and  35   a   2 , remaining thereby accurately positioned. Incidentally, scanner units  3   b ,  3   c , and  3   d  are also fastened to the lateral panels  32  in the same manner as the scanner unit  3   a.    
     Each of the developing apparatuses  4   a ,  4   b ,  4   c , and  4   d  develops an electrostatic latent image into a toner (developer) image, that is, an image formed of toner (developer), by adhering toner to the electrostatic latent image. Referring to  FIG. 2 , the developing apparatuses have toner containers  41   a ,  41   b ,  41   c , and  41   d , which store the toners of the abovementioned colors, that is, Y, M, C, and BK colors, respectively. The toner in the toner container  41   a  is sent to a toner supply roller  43   a  by a toner sending mechanism  42   a . Then, the toner is coated on the peripheral surface of the development roller  40   a , while being given electric charge, by the developer supply roller  43   a , and a development blade  44   a  which is kept pressed upon the peripheral surface of the development roller  40   a . The development roller  40   a  is disposed so that its peripheral surface opposes the peripheral surface of the photosensitive drum  1   a , on which an electrostatic latent image is formed. The latent image formed on the peripheral surface of the photosensitive drum  1   a  is developed into a toner image by applying development bias to this development roller  40   a . The functions and operations of each of the development apparatuses  4   b ,  4   c , and  4   d  are the same as those of the developing apparatus  4   a.    
     An electrostatic transferring apparatus  5  is an apparatus which transfers the toner images formed on the peripheral surface of each of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d  onto transfer medium (sheet S of recording paper). More specifically, the electrostatic transferring apparatus  5  is provided with an electrostatic transfer belt  11 , which is disposed so that it opposes all the photosensitive drums  1  and circularly moves in contact with all the photosensitive drums  1 . The electrostatic transfer belt  11  is suspended by being stretched around four rollers, which are a driver roller  13 , follower rollers  14   a  and  14   b , and a tension roller  15 . It moves the sheet S to place the sheet S in contact with the photosensitive drums  1 , by electrostatically adhering the sheet S to the outwardly facing surface of the left side portion of the electrostatic transfer belt, in terms of the loop the belt  11  forms. In operation, while the sheet S is conveyed from the position of the follower roller  14   a  to the position of the driver roller  13  by the electrostatic transfer belt  11 , the toner images on the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d  are transferred onto the sheet S at the corresponding transfer positions. The electrostatic transferring apparatus  5  is also provided with transfer rollers  12   a ,  12   b ,  12   c , and  12   d , which are disposed in contact with the inward side of the electrostatic transfer belt  11 , in terms of the loop which the belt  11  forms, opposing the photosensitive drums  1 , one for one, at the locations where the photosensitive drums  1  are in contact with the outward side of the belt  11 . Through the electrostatic transfer belt  11 , positive electric charge is applied to the sheet S from these transfer rollers  12   a ,  12   b ,  12   c , and  12   d , generating thereby electric fields. As a result, the toner images, which are negative in polarity, are transferred by the electric fields onto the sheet S while the sheet S is in contact with the photosensitive drums  1 . 
     Cleaning apparatuses  6   a ,  6   b ,  6   c , and  6   d  are apparatuses for removing the toner remaining on the peripheral surfaces of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d  after the transfer of the toner images. 
     The printer main assembly  100  is also provided with other members and apparatuses than the above described ones. That is, the printer main assembly  100  is provided with a sheet feeding-and-conveying portion, which feeds the sheets S stored in layers in a sheet feeder cassette  17 , into the printer main assembly  100 , and then, conveys them toward the image forming portion. In an ordinary image forming operation, the sheet feeding-and-conveying roller  18 , the cross-section of which is in the form of a half moon, or roughly D-shaped, and a pair of registration rollers  19  rotates, feeding thereby the sheets S from the sheet feeder cassette  17  into the apparatus main assembly while separating them one by one. As the leading edge of each sheet S comes into contact with the interface between the pair of registration rollers  19 , the sheet S is temporarily held by the pair of registration rollers  19 , while being made to curve so that the center portion of the sheet S, in terms of the direction in which the sheet S is conveyed, separates from the transfer belt  11 . Then, the sheet S is released by the pair of registration rollers  19  toward the electrostatic transfer belt  11  with such timing that the writing start line of the sheet S arrives at the interface between the photosensitive drum  1  and transfer belt  11  at the same time as the image formation start line on the peripheral surface of the photosensitive drum  1 . The printer main assembly  100  is also provided with a fixing portion  20  for fixing the multiple monochromatic toner images, which are different in color and have just been transferred onto the sheet S, to the sheet S. This fixing portion  20  has a heat roller  21   a  and a pressure roller  21   b . The pressure roller  21   b  is kept pressed upon the heat roller  21   a  to apply heat and pressure to the sheet S. Therefore, while the sheet S is conveyed through the fixing portion  20  after the transfer of the toner images on the photosensitive drums  1 , the sheet S is subjected to heat and pressure while being conveyed by the fixation rollers  21 . As a result, the toner images, different in color, on the sheet S are fixed to the surface of the sheet S. 
     In an image forming apparatus, the cartridges  7   a ,  7   b ,  7   c , and  7   d  for Y, M, C, and BK colors, respectively, are sequentially driven in synchronization with the printing timing. As cartridges  7  are driven, the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d  in the cartridges  7   a ,  7   b ,  7   c , and  7   d , respectively, rotate in the counterclockwise direction. With the same timing as the cartridges  7   a ,  7   b ,  7   c , and  7   d , the scanner units  3   a ,  3   b ,  3   c , and  3   d  are sequentially driven, and the charge rollers  2   a ,  2   b ,  2   c , and  2   d  uniformly charge the peripheral surface of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , respectively. The scanner units  3   a ,  3   b ,  3   c , and  3   d  expose the charged areas of the peripheral surfaces of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d  in accordance with the picture signals. As a result, an electrostatic latent image is effected on the charged area of the peripheral surface of each of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d . The development rollers  40   a ,  40   b ,  40   c , and  40   d  transfer toner onto the numerous low potential level points of the electrostatic latent image; they develop (form) toner images on the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d.    
     The rotation of the pair of registration rollers  19  is started with such a timing that the arrival of the leading edge of the toner image formed on the peripheral surface of the most upstream photosensitive drum  1 , that is, photosensitive drum  1   a , at the interface between the peripheral surface of the photosensitive drum  1   a  coincides with the arrival of the printing (writing) start line of the sheet S at the interface, and the sheet S is conveyed to the electrostatic transfer belt  11 . After being released by the pair of registration rollers  19 , the sheet S is conveyed between an electrostatic adhesion roller  22  and the electrostatic transfer belt  11 , while remaining pinched by the roller  22  and belt  11 , being thereby pressed upon the outward surface of the electrostatic transfer belt  11  in terms of the aforementioned belt loop. Further, while the sheet S is conveyed between the rollers  22  and belt  11 , voltage is applied between the sheet S and electrostatic transfer belt  11 , inducing thereby electric charge between the sheet S and electrostatic transfer belt  11 , which are dielectric. As a result, the sheet S is electrostatically adhered to the outward surface of the electrostatic transfer belt  11 , ensuring that the sheet S remains satisfactorily adhered to the electrostatic transfer belt  11  while it is conveyed to the most upstream transfer portion. 
     While the sheet S is conveyed as described above, the toner images are sequentially transferred onto the sheet S by the electric fields formed between the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d  and transfer rollers  12   a ,  12   b ,  12   c , and  12   d , respectively. After the transfer of the Y, M, C, and BK color toner images onto the sheet S, the sheet S is separated from the electrostatic transfer belt  11  by the curvature of the belt driver roller  13 , and is conveyed into the fixing portion  20 , in which the toner images are thermally fixed. After the fixation of the toner images, the sheet S is discharged from the printer main assembly  100 , with its image bearing surface facing downward, through the sheet discharge portion by a pair of sheet discharge rollers  23 . 
     (Process Cartridge) 
     The cartridges  7   a ,  7   b ,  7   c , and  7   c  shown in  FIGS. 2 and 3  are the same in structure. Thus, the cartridge  7  will be described with reference to the cartridge  7   a . The cartridge  7   a  is an integration of the photosensitive drum  1   a , and the processing means, such as the charging apparatus  2   a , developing apparatus  4   a , and cleaning apparatus  6   a , etc. In this embodiment, the cartridge  7   a  is made up of a photosensitive drum unit  50   a  (image bearing member, and a developing apparatus  4   a.    
     First, the photosensitive drum unit  50   a  will be described. 
     The photosensitive drum  1   a  is rotatably supported by a cleaning means frame  51 , with bearings  31   a   1  and  31   a   2  placed between the photosensitive drum  1  and frame  51 . Referring to  FIG. 10 , when the cartridge  7   a  is mounted into the printer main assembly  100 , a coupler  1   a   1  with which one of the lengthwise ends of the photosensitive drum  1   a  is provided engages with the coupler  107  on the main assembly side, making it possible for the rotational force of a motor  103  to be transmitted to the photosensitive drum  1   a  through the coupling  107  to rotate the photosensitive drum  1   a  in the counterclockwise direction for image formation. In the adjacencies of the peripheral surface of the photosensitive drum  1   a , the charging apparatus  2   a  and cleaning blade  6   a  are disposed in contact with the peripheral surface of the photosensitive drum  1   a . The cleaning blade  6   a  is disposed so that as the photosensitive drum  1   a  is rotated, it removes the toner remaining on the peripheral surface of the photosensitive drum  1   a  by scraping the peripheral surface of the photosensitive drum  1   a . As the residual toner is removed by the cleaning blade  6   a , it is sent by a residual toner sending mechanism to a waste toner chamber  53   a  located in the rear portion of the cleaning means frame  51   a.    
     The developing apparatus  4   a  is made up of the development roller  40   a , as a developing means, which rotates in contact with the photosensitive drum  1   a  in the direction indicated by an arrow symbol Y, a toner container  41   a , developing means frame  54   a , etc. The development roller  40   a  is rotatably supported, by its axle, by the developing means frame  45   a , with the bearing disposed between its axle and the developing means frame  45   a . The developing apparatus  4   a  is also provided with a toner supply roller  43   a , which rotates in contact with the peripheral surface of the development roller  40   a  in the direction indicated by an arrow mark Z, and a development blade  44   a . Within the toner container  41   a , a toner moving mechanism  42   a  is provided, which is for feeding the toner supply roller  43   a  with the by moving the toner toward the toner supply roller  43   a  while stirring the toner. Also referring to  FIG. 10 , when the cartridge  7   a  is mounted into the printer main assembly  100 , a gear  60   a  with which the developing apparatus  4   a  is provided engages with a gear  121  on the apparatus main assembly side, making it possible for the rotational force of the motor  103  to rotate the supply roller  43   a , by being transmitted from the gear  60   a  to  a  gear  65   a  with which one of the lengthwise ends of the supply roller  43   a  is provided, through a gear  61   a ,  62   a , and  63   a  with which the developing apparatus  4   a  is provided. The gear  65   a  is in mesh with the gear  64   a  with which one of the lengthwise ends of the development roller  40   a  is provided. Therefore, the rotational force is transmitted to the development roller  40   a , rotating thereby the development roller  40   a.    
     The cartridge  7   a  is structures so that the entirety of the developing apparatus  4   a  is allowed to rotate relative to the photosensitive drum unit  50   a , in an oscillatory fashion, about the axial line of the joint between the developing apparatus  4   a  and photosensitive drum unit  50   a . That is, at one of the lengthwise ends of the cartridge  7   a , a pin  49   a   1  is fitted in a hole  51   a   1  with which the cleaning means frame  51   a  is provided, and a supportive hole  47   a   1  with which a bearing member  47   a  of the developing apparatus  4   a  is provided, whereas at the other lengthwise end of the cartridge  7   a , a pin  49   a   2  is fitted in a hole  51   a   2  with which the cleaning means frame  51   a  is provided, and a supportive hole  48   a   1  with which a bearing member  48   a  of the developing apparatus  4   a  is provided. 
     Before the cartridge  7   a  is mounted into the printer main assembly  100 , that is, while the cartridge  7   a  is left alone outside the printer main assembly  100 , the developing apparatus  4   a  remains kept pressed by a compression spring  54   a  so that the development roller  40   a  remains kept in contact with the photosensitive drum  1   a . The toner container  41   a  is provided with a rib  46   a , which protrudes outward from the external surface of the toner container  41   a . That is, a development roller separating mechanism, with which the printer main assembly  100  is provided comes into contact with the rib  46   a  and pushes it up, causing the development roller  40   a  to separate from the photosensitive drum  1   a . The development roller separating mechanism will be described next. 
     (Development Roller Separating Mechanism) 
     At this time, referring to  FIGS. 6-8 , the development roller separating mechanism (separating means) with which the printer main assembly  100  is provided will be described. 
     The development roller separating mechanism, which is made up of various members, which will be described next, is located in the rear portion in the printer main assembly  100 . It separates the development rollers  40   a ,  40   b ,  40   c , and  40   d  from the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , respectively, against the force generated by the resiliency of the abovementioned springs. 
     First, referring to  FIGS. 8 and 9 , the printer main assembly  100  is provided with first and second plates  81  and  80 , as the first and second members, respectively, for pushing up the ribs  46   a ,  46   b ,  46   c , and  46   d  with which the developing apparatuses  4   a ,  4   b ,  4   c , and  4   d  are provided, respectively. The first plate  81  is involved with only the developing apparatus  4   d , that is, the developing apparatus containing the black toner. The second plate  80  is involved with the developing apparatuses other than the developing apparatus containing black (BK) toner, that is, the developing apparatuses  4   a ,  4   b , and  4   c  containing yellow (Y), magenta (M), and cyan (C) toners, respectively. The second plate  80  is provided with first engaging portions  80   a   1 ,  80   b   1 , and  80   c   1 , which are in the form of a protrusion, and second engaging portions  80   a   2 ,  80   b   2 , and  80   c   2 , each of which is made up of a pair of protrusions. The first and second engaging portions perpendicularly protrude from the surface of the second plate  80 . As the second plate  80  is vertically moved upward, the first engaging portions  80   a   1 ,  80   b   1 , and  80   c   1  move upward, pushing up the ribs  46   a ,  46   b , and  46   c , respectively, of the developing apparatuses  4   a ,  4   b , and  4   c , respectively. As a result, the developing apparatuses  4   a ,  4   b , and  4   c  rotate about the abovementioned pins  49   a   1  and  49   a   2 , causing the development roller  40   a ,  40   b , and  40   b , which are in the leading end portions of the development units  4   a ,  4   b , and  4   c , to separate from the photosensitive drums  1   a ,  1   b , and  1   c , respectively. Hereafter, these positions in which developing apparatuses  4   a ,  4   b , and  4   c  are after the development rollers  40   a ,  40   b , and  40   c  are separated from the photosensitive drums  1   a ,  1   b , and  1   c , respectively, will be referred to as separation positions. 
     On the other hand, as the second plate  80  is moved downward, the second engaging portions  80   a   2 , and  80   b   2 , and  80   c   2  come into contact with the levers portions  116   a ,  116   b  and  116   c  of clutch controlling members  116 , with which clutches  92   a ,  92   b , and  92   c  (driving force transmission controlling means) are provided, and move them downward, connecting thereby the clutches  92  so that the rotational force from the motor  103  is transmitted to each of the developing apparatuses  4   a ,  4   b , and  4   c . The clutch  92  will be described later in more detail. The levers portion  116   a  of clutch controlling member  116  extends from the clutch  92  in the direction perpendicular to the axial line of the clutch  92 . The first plate  81  is provided with a first engaging portion  81   d   1 , and a second engaging portion  81   d   2  which is made up of a pair of protrusions. The roles which the first and second engaging portions  81   d   1  and  81   d   2 , respectively, of the first plate  81  play are the same as those which the first and second engaging portions of the second plate  80  play. 
     Referring to  FIG. 10 , the first and second plates  81  and  80  vertically move upward or downward by receiving the rotational force from the motor  103  as a driving force source. More specifically, referring to  FIG. 9 , a shaft  90  is rotated by the driving force from the motor  103 , transmitting thereby the driving force to cams  94  and  93 , as first and second cams, respectively, which are solidly attached to the shaft  90  and are shaped and positioned to move upward or downward the plates  81  and  80 , respectively. 
     (Cartridge Driving Mechanism) 
       FIG. 10  shows the mechanism for driving the cartridges. 
     This driving mechanism is provided with multiple motors  103 , which are for driving the cartridges  7   a ,  7   b ,  7   c , and  7   d , one for one. The driving force outputted by each motor  103  is divided into two portions; it is transmitted to a drum gear  101  which drives the photosensitive drum  1 , and a gear  102  which is a part of the clutch through which the driving force is transmitted to the development roller  40 . The rotational shaft attached to the gear  102  is provided with clutch  92  ( 92   a ,  92   b ,  92   c , or  92   d ). Thus, even when the photosensitive drum  1  is rotating, the transmission of the driving force to the development roller  40  can be interrupted or restored. 
     The engagement or disengagement of the clutch  92  is achieved by moving upward or downward the first and second plates  81  and  80 . That is, the clutch  92  becomes engaged or disengaged as the engaging portions  80   a   2 ,  80   b   2 ,  80   c   2 , and  80   d   2  push up or down the levers portions  116   a ,  116   b ,  116   c , and  116   d  of the clutch controlling members  116  of the clutches  92   a ,  92   b ,  92   c , and  92   d , respectively. That is, when the lever portion  116   a  ( 116   b ,  116   c ) is in the top position into which it is pushed up, the clutch  92  remains disengaged, and therefore, the rotational force of the motor  103  is not transmitted to the development roller  40 , and also, the developer roller  40  remains separated from the photosensitive drum  1 . Hereafter, the state of the first and second plates  81  and  80 , in which the clutch  92  remains disengaged as described above will be referred to as non-operational state. 
     On the contrary, when the lever portion  116   a  is in the bottom position into which it is pushed down, the clutch  92   a  remains engaged, and therefore, the rotational force of the motor  103  is transmitted to the development roller  40 , rotating thereby the development roller  40 , and also, the developer roller  40  remains in contact with the photosensitive drum  1 . Incidentally, when the image forming apparatus is in the standby mode, shown in  FIG. 8 , which will be described later in more detail, the first and second plates  81  and  80  are in the top positions into which they are pushed up, and the development rollers  40   a ,  40   b ,  40   c , and  40   d , which correspond to Y, M, C, and BK colors, respectively, remain separated from the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , respectively. In this mode, the clutches  92   a ,  92   b ,  92   c , and  92   d  remain disengaged. Hereafter, the state of the first and second plates  81  and  80 , in which the clutches  92  remain disengaged as described above will be referred to as operational state. 
     (Clutch) 
     Next, referring to  FIG. 16 , the details of the clutch  92   a  of the driving apparatus will be described. 
     The gear  102 , which meshes with the gear attached to the output shaft of the motor  103 , is rotatably fitted around a shaft  118 . The positional relationship between the gear  102  and shaft  118  in terms of the direction of the axial line of the shaft  118  is preset. The gear  102  is hollowed on the opposite side from the motor  103 , except for the boss  102   a , which is located at the center of the gear  102  in terms of the radius direction of the gear  102 . The internal surface of the boss  102   a  constitutes the surface by which the gear  102  slide on the rotational shaft  118  (driving force receiving side) to be accurately positioned in terms of the axial direction of the shaft  118  and/or rotate around the shaft  118 . The peripheral surface of the boss  102   a  constitutes the surface on which the coupler  113  slides to be accurately positioned in terms of the axial direction of the coupler  113  (boss  102   a ) and/or rotate. The lateral wall of the hollow of the gear  102  is provided with four rotation control recesses  102   b , which constitute means for preventing the coupler  113  from rotating relative to the gear  102  while the coupler  113  is in the abovementioned hollow of the gear  102 . 
     The coupler  113  has four protrusions  113   c  which protrude from its peripheral surface. The coupler  113  is shaped so that it fits in the hollow of the gear  102 , while being allowed to slide in the axial direction of the gear  102  (coupler  113 ). As the rotation control protrusions  113   b  on the peripheral surface of the coupler  113  fit in the rotation control recesses  102   b , one for one, of the gear  102 , the coupler  113  rotates with the gear  102 . Further, the coupler  113  is provided with four protrusions  113   c , whereas the coupler  114  on the driving force receiving side is provided with protrusions  114   c . Thus, the meshing between the protrusions  113   c  and protrusions  114   c  makes it possible for the abovementioned rotational force to be transmitted. 
     The driving force transmitting surface  113   c   1  of each protrusion  113   c  is slanted so that as the coupler  113  is rotated, the component in contact with the surface  113   c   1  is pulled toward the coupler  113 . Thus, it is assured that as the clutch  92  is engaged, the gear  102  engages with the coupler  113 , and also, that even if a large amount of torque bears on the gear  102 , “skipping” does not occur. Further, the adjacent driving force transmitting surfaces  113   c   1  are connected with a gently slanted surface  113   c   2 . Therefore, even when the clutch  92  is engaged while the gear  102  is rotating, the engagement occurs very smoothly. 
     A surface  113   d  of the coupler  113 , which is on the opposite side from the motor  103  constitutes the surface which rubs a release ring  115  (which will be described later) in the rotational direction. The coupler  113  is kept pressed toward the coupler  114 , that is, the coupler on the driving force receiving side, by a coil spring  112 , that is, an elastic member. 
     The coupler  114  is provided with a center hole and a groove  114   b . The groove  114   b  extends in the diameter direction of the coupler  114 , and the center of the groove  114   b  in terms of the diameter direction of the coupler  114  coincides with the axial line of the center hole. The shaft  118  fits in the center hole, and a pin  119  fits in the groove  114   b . The coupler  114  is provided with the abovementioned four protrusions  114   c . When these protrusions  114   c  are in engagement with the protrusions  113   c  of the coupler  113 , one for one, the abovementioned driving force can be transmitted. The driving force transmitting surface  114   c   1  of each protrusion  114  is slanted so that as the coupler  113  is rotated, the coupler  114  is pulled into the coupler  113 . Further, the adjacent two driving force transmitting surfaces  114   c   1  are connected by a gently slanted surface  114   c   2 . Further, the coupler  113 , coupler  114 , and coil spring  112  are fitted in the abovementioned hollow of the gear  102 , to reduce the size of the image forming apparatus by more effectively using the internal space of the apparatus main assembly. In addition, the rotational force transmitted through the surface of each tooth of the gear  102  is transmitted straightly inward of the cartridge  7 . Therefore, it does not occur that such force that acts in the direction to twist and/or fell the couplers is generated. Thus, the above described structural design makes it easier to ensure that the abovementioned components are strong enough for their roles, and also, to transmit a substantially larger amount of torque than that transmittable by couplers in accordance with the prior art. 
     A clutch controlling member  116  is fitted around the shaft  118  so that it is rotatable about the shaft  118 . The engagement between the lever portion  116   a  and the second engaging portion  80   a   2  ( 80   b   2 ,  80   c   2 ) causes the clutch controlling member  116  to rotate. The clutch controlling member  116  is provided with a cam portion  116   c , which comes into contact with the cam portion  115   c  of the release ring  115  to move the release ring  115  in the direction of the axial line. The release ring  115  is provided with multiple pairs of cam portions  115   a , the cam portions in each pair being symmetrically positioned with reference to the axial line of the release ring  115 , and the clutch control lever are provided multiple pairs of cam portions  116   a , the cam portions of each pair being symmetrically positioned with reference to the axial line of the clutch controlling member  116 . 
     While the cam portions  116   c  of the clutch controlling member  116  are in contact with the cam portions  115   c  of the release ring  115 , the release ring  115  is kept pressed toward the gear  102 . That is, the surface  115   b  of the release ring  115  comes into contact with the surface  113   d  of the coupler  113 , pushing the coupler  113  away from the coupler  114  against the spring  112 , making it impossible for the rotational force from the motor  103  to be transmitted to the shaft  118 . 
     On the other hand, the cam portions  116   c  of the clutch controlling member  116  can be separated from the cam portions  115   c  of the release ring  115 , by rotating the clutch controlling member  116 . While the cam portions  116   c  of the clutch controlling member  116  remain separated from the cam portion  115   c  of the release ring  115 , the release ring  115  moves toward the gear  121 , that is, the gear on the driving force receiving side. Here, referring to  FIGS. 3 and 10 , the gear  121  is a gear which transmits the rotational force to the developing apparatus  4   a  by meshing with the gear  60   a , with which the developing apparatus  4   a  is provided, when the cartridge  7   a  is in the printer main assembly  100 . That is, the coupler  113  is pressed by the pressure generated by the resiliency of the spring  112 , being thereby caused to engage with the coupler  114 . Thus, the rotational force from the motor  103  is transmitted to the shaft  118 . Incidentally, the clutch  92   a  may be modified in structure so that the couplers  114 , or the coupler on the driving force receiving side, and the coupler  113 , or the coupler on the driving force transmitting side, are switched in position. The structures of other clutches  92   b ,  92   c , and  92   d  are the same as the above described structure of the clutch  92   a.    
     (Driving Force Transmission to Mode Switching Member) 
     Referring to  FIG. 11 , to the cams  93  and  94 , the driving force is transmitted through the gear  102 . That is, the rotational force of the gear  102  is first transmitted to a gear  131  with a toothless range (third clutch), and then, is transmitted to a cam gear  133  through a gear  132  which is on the same axle as the gear  131  with a toothless range. The gear  133  is provided with a shaft  133   a  which rotates with the gear  133 . It is to this shaft  133   a  that the cams  93  and  94  are attached. Thus, as the gear  133  rotates, the cams  93  and  94  also rotate. The gear  131  with a toothless range is provided with an engaging portion  131   a , with which the lever  130   a  of a solenoid  130  as the actuator of the third clutch engages. While the lever  130   a  is in engagement with this engaging portion  131   a , the gear  131  with a toothless range remains stationary, with its toothless range  131   b  opposing the gear  102 . Therefore, while the lever  130   a  is in engagement with the engaging portion  131   a , the rotational force of the gear  102  does not transmit to the gear  131  with a toothless range. However, as the solenoid  130  is activated, and therefore, the lever  130   a  is pulled, the lever  130   a  is disengaged from the engaging portion  131   a , making it possible for the gear  131  with a toothless range to rotate. Since gear  131  with a toothless range is kept pressured to rotate in a direction A, the gear  131  with a toothless range rotates in the direction A, meshes again with the gear  102 , being thereby rotated by the gear  102 . Then, as the following full rotation of the gear  131  with a toothless range causes the lever  130   a  to engage with the engaging portion  131   a , the rotation of the gear  131  with a toothless range stops. 
     The gears  132  and  133  are designed so that a single full rotation of the gear  132  causes the gear  133  to rotate 90°. Thus, a single full rotation of the gear  132  changes the rotational phase of the cam  93  by 90°. 
       FIGS. 12(   a )- 12 ( d ) are drawings for showing the states in which the first cam  93  for moving upward or downward the second plate  80  which is involved with three colors Y, M, and C, can be. That is, each time the solenoid  130  is activated, the cam  93  is rotated by 90°, causing the separation plate  80  to move upward or downward, because of the profile of the cam  93 . Incidentally, the relationship between the first plate  81 , which is involved with black color, and the cam  94 , is the same as the above described relationship between the second plate  80  and cam  93 . 
     That is, when the cam  93  is in the state shown in  FIG. 12(   a ), the second plate  80  is in contact with the cam surface  93   a  of the cam  93 , being thereby held at its highest position. Each time the lever  130   a  of the solenoid  130  is pulled, the cam  93  rotates by 90°. Also when the cam  93  is in the state shown in  FIG. 12(   b ) or  12 ( c ), the second plate  80  is in contact with the cam surface  93   a  of the cam  93 , being thereby held at its highest position as in the state shown in  FIG. 12(   a ). However, as the cam  93  moves into the position shown in  FIG. 12(   d ), the cam surface  93   a  of the cam  93  becomes separated from the second plate  80 , allowing the second plate  80  to move downward. The cam  94 , which is for moving the first plate  81  for the BK color, is contoured as shown in  FIG. 9 . Therefore, each time the cam  94  rotates by 90°, it changes the position of the first plate  81 . As described above, the positions of the first and second plates  81  and  80  are changed by the rotation of the cams  93  and  94 , respectively. 
     At this time, referring to  FIG. 6 , an image forming operation carried out by the image forming apparatus when the apparatus is in the full-color mode will be described. When the image forming apparatus is in the full-color mode, the developing process is carried out by all the developing apparatuses  40   a ,  40   b ,  40   c , and  40   d , which correspond to Y, M, C, and BK colors, respectively. In other words, if the first and second plate  81  and are being held at their top positions by the cams  93  and  94  when the image forming operation is started, the cams  93  and  94  are rotated into the positions in which they cannot contact the first and second plates  81  and  80 , respectively. Thus, the development roller  40   a ,  40   b ,  40   c , and  40   d  come into contact with the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , respectively. Also in the full-color mode, the clutches  92   a ,  92   b ,  92   c , and  92   d  are in the states shown in  FIG. 17 , in which the cam portions  116   c  of the clutch controlling member  116  are not in contact with the cam portions  115   c  of the release ring  115 , and therefore, the coupler  113  is kept engaged with the coupler  114  by the pressure generated by the resiliency of the spring  112 . Therefore, the rotational force from the motor  103  is transmittable to each of the development rollers  40   a ,  40   b ,  40   c , and  40   d.    
       FIG. 7  shows the states of the essential portions of the image forming apparatus which is in the black-and-white mode in which the development process is carried out only by the developing apparatus  4   d  for the black color. In this mode, the second plate  80  is kept in its top position, into which it is pushed up by the cam  93 , to keep the development rollers  40   a ,  40   b , and  40   c  for Y, M, and C colors separated from the photosensitive drums  1   a ,  1   b , and  1   c , respectively, and keep the development roller  40   d  for the BK color in contact with the photosensitive drum  1   d . Also in the black-and-white mode, the clutches  92   a ,  92   b ,  93   c , and  93   d  are kept in the states shown in  FIG. 18 . That is, in the clutch  92   d , which corresponds to the developing apparatus  4   d , which is for BK color, the cam portions  116   a  of the clutch controlling member  116  are not in contact with the cam portions  115   c  of the release ring  115 , and therefore, the coupler  113  is kept engaged with the coupler  114  by the pressure generated by the resiliency of the spring  112 . Therefore, the rotational force from the motor  103  is transmitted to the developing apparatus  4   d . In the other clutches, that is, the clutches  92   a ,  92   b , and  92   c , however, the cam portions  116   c  of the clutch controlling member  116  are in contact with the cam portions  115   c  of the release ring  115 , keeping thereby the coupler  113  separated from the coupler  114  against the resiliency of the spring  112 . Therefore, the rotational force from the motor  103  is not transmitted to the development rollers  40   a ,  40   b , and  40   c.    
     Shown in  FIG. 8  is the state of the essential portion of the image forming apparatus, in which the apparatus is in the home mode (on standby). When the apparatus is in this state, the first and second plates  81  and  80  are kept in their top positions by the cams  93  and  94 , respectively, to keep the development rollers  40   a ,  40   b ,  40   c , and  40   d  separated from the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , respectively. When the image forming apparatus is in the home mode, the clutches  92   a ,  92   b ,  92   c , and  92   d  are kept in the states shown in  FIG. 19 . That is, in all clutches  92   a ,  92   b ,  92   c , and  92   d , the cam portions  116   c  of the clutch controlling member  116  are in contact with the cam portions  115   c  of the release ring  115 , keeping thereby the coupler  113  separated from the coupler  114  against the resiliency of the spring  112 . Therefore, the rotational force from the motor  103  is transmitted to none of the development rollers  40   a ,  40   b ,  40   c , and  40   d.    
     That is, the development roller separating mechanism moves the first plate  81  and/or second plate  80  to select one of the abovementioned three modes, that is, the full-color mode, black-and-white mode, or home mode. 
     As will be evident from the description given above, in this embodiment, only a single motor, or the motor  103  ( FIG. 10 ), is used as multiple driving force sources, that is, the driving force source for rotating the photosensitive drums  1 , the driving force source for rotating the development rollers  40 , and the driving force source for operating the development roller separating mechanism. Therefore, it is possible for the rotation of each photosensitive drum  1  to be independently controlled from those of the others. Therefore, the image forming apparatus in this embodiment is far less likely to suffer from the long standing problems in the field of a full-color image forming apparatus of the inline type, that is, the image deviation in terms of position and/or color, than an image forming apparatus in accordance with the prior art. Obviously, the cost of providing an image forming apparatus with the clutches  92  is much smaller than the cost of providing an image forming apparatus with motors dedicated to the driving of the development rollers  40  in addition to the motor dedicated to the driving of the photosensitive drums  1 . 
     (Operation for Mounting Process Cartridge) 
     Next, the operation for mounting the process cartridge(s)  7  into the printer main assembly  100  will be described. 
     Referring to  FIG. 4 , the cartridge  7  is to be inserted into the printer main assembly  100  from the direction indicated by an arrow mark in the drawing, so that it will be precisely placed in the preset position in the printer main assembly  100 . Incidentally, in order to prevent the description of this cartridge mounting operation from becoming excessively complicated, only single photosensitive drum  1 , that is, the photosensitive drum  1   a  and a single bearing  31 , that is, the bearing  31   a , are shown in  FIG. 4 . 
     Referring to  FIG. 2 , while the cartridge  7  is outside the apparatus main assembly, and is left alone, the development roller  40   a  in the cartridge  7   a  remains in contact with the photosensitive drum  1   a  in the cartridge  7   a . The cartridge  7   a  is to be inserted into the apparatus main assembly in the direction by the arrow mark in  FIG. 4 , with the photosensitive drum bearings  31  being guided by a pair of first guiding grooves  34   a  ( 34   b ,  34   c , or  34   d ). Referring to  FIG. 5(   b ), as the bearings  31   a  come into contact with the bearing catching surfaces  37   a  and  38   a  of the guiding groove  34   a , and are pressed against the surfaces  37   a  and  38   a , the cartridge  7   a  is accurately positioned relative to the printer main assembly  100 . Incidentally, the other cartridges  7   b ,  7   c , and  7   d  are also accurately positioned relatively to the printer main assembly  100  in the same manner as the cartridge  7   a.    
     The structural arrangement for keeping the cartridge  7   a  pressured in the printer main assembly  100  is as follows. That is, referring to  FIG. 5(   a ), the printer main assembly  100  is provided with a pair of shafts  39   a , which are attached to the left and right panels  32  of the printer main assembly  100 , one for one, by crimping, and a pair of return springs (coil springs)  30   a , which are fitted around the pair of shafts  39   a , one for one. The return coil spring  30   a  is held to the corresponding panel  32  by fitting one end  30   a   1  of the return coil spring  30   a  in the return coil spring anchoring hole  23   a   1  of the panel  32 . Before the cartridge  7   a  is mounted into the printer main assembly  100 , the return coil spring  30   a  is prevented from rotating, by a stopper  32   b  formed by cutting and bending a part of the side panel  32 . However, during the insertion of the cartridge  7   a  into the printer main assembly  100 , the return coil spring  30   a  is rotationally wound in the counterclockwise direction against its own resiliency, until it slides over the bearing  31   a  which supports the photosensitive drum  1   a . After the return coil spring  30   a  slides over the bearing  31   a , it presses the bearing  31   a  in the direction indicated by an arrow mark F shown in  FIG. 5(   a ). The other cartridges  7   b ,  7   c , and  7   d  are also kept pressed in the same manner. 
     Referring to  FIG. 10 , when the cartridge  7   a  is mounted into the printer main assembly  100 , the coupler  1   a   1 , with which one of the lengthwise ends of the photosensitive drum  1   a  is provided, engages with the coupler  107  on the printer main assembly  100  side. To the coupler  107  the rotational force of the motor  103  is transmitted through gears  101 ,  105 , and  106 . The printer main assembly  100  is structured so that the gear  106  and coupler  107  are movable in the direction of the axial line of the gear  106 , that is, the directions indicated by arrow marks J 1  and J 2 . More specifically, during the insertion of the cartridge  7   a  into the printer main assembly  100 , the gear  106  and coupler  107  remain in their home positions, into which they had been retracted in the direction J 1 . However, as the door  100   a  ( FIG. 1 ) of the printer main assembly  100  is moved from its open position to closed position after the insertion of the cartridge  7   a  into the printer main assembly  100 , the gear  106  and coupling  107  are moved in the direction J 2 . As a result, the coupler  107 , or the coupler on the main assembly side, engages with the coupler  1   a   1 . 
     (Printing Operation of Image Forming Apparatus) 
     When the image forming apparatus is in the home mode shown in  FIG. 8 , the first and second plates  81  and  80  of the printer main assembly  100  are in their top positions to which they were pushed up, and therefore, each development roller  40  remains separated from the corresponding photosensitive drum  1 . That is, when the image forming apparatus is in the state shown in  FIG. 8 , the power supply to the apparatus is off, or the developing process is not carried out. It is when the image forming apparatus is in this state that the cartridges  7   a ,  7   b ,  7   c , and  7   d  are to be mounted into the printer main assembly  100  one by one. Also when the image forming apparatus is in the abovementioned state, the ribs  46   a ,  46   b ,  46   c , and  46   d  of the developing apparatuses  4   a ,  4   b ,  4   c , and  4   d  are borne by the first engaging portions  80   a   1 ,  80   b   1 ,  80   c   1 , and  81   d   1 , respectively. 
     The cartridge  7  is mounted into the printer main assembly  100  as described above. Sometimes, the cartridges are left in the printer main assembly  100 , without being used, for a substantial length of time. With the employment of the above described structural arrangement, however, each development roller  40  is kept separated from the photosensitive drum  1  while it is not involved in the ongoing the image forming operation. Therefore, the image forming apparatus in this embodiment does not suffer from the problem that the surface layer of the development roller  40  is permanently deformed by the unnecessary contact between the development roller  40  and photosensitive drum  1 . 
     (Mode Switching Sequence 1) 
       FIG. 13  is a table showing the mode switching sequence 1. Each time the lever  130   a  is pulled by activation of the solenoid  130  when the image forming apparatus is in the home mode, the cams  93  and  94  are changed in phase angle by 90°. Thus, the first and second plate  81  and  80  are moved upward or downward, because of the profiles of the cams  93  and  94 . Therefore, the clutches  92  are engaged or disengaged according to the positions into which the first and second plate  81  and  80  are moved up or down by the rotation of the cams  93  and  94 . 
     In mode switching sequence 1, the operational mode is switched in the order of (1) home mode-(2) black-and-white mode-(3) home mode-(4) full-color mode. As the lever  130   a  is pulled one more time by the activation of the solenoid  130  (which hereafter may be referred to as pulling operation) when the image forming apparatus is in the full-color mode (4), the cams  93  and  94  finish rotating 360° relative to the home position in which they were before they began to be rotated; in other words, they return to their home positions [home mode (1)]. 
     As a printing operation is started by a print signal when the image forming apparatus is in the black-and-white mode, the motor  103  for driving the cartridges  7 , and the motor (unshown) for driving the transfer belt  11 , begin to rotate. However, the clutches  92  have been disengaged. Therefore, the development rollers  40   a ,  40   b ,  40   c , and  40   d  do not rotate. Then, the solenoid  130  is activated once. As the solenoid  130  is activated to pull the lever  130   a , the cams  93  and  94  rotate by 90°. As the cams  93  and  94  rotate 90°, the first plate  81  moves downward, allowing the clutch  92   d , or the clutch corresponding to the BK color, to engage. Therefore, only the development roller  40   d , or the development roller for the BK color, is rotated. That is, the force which the first plate  81  has been applying upward is removed. Therefore, the development roller  40   a  is allowed to come into contact with the photosensitive drum  1   a , making it possible for the image forming apparatus to print a black-and-white image; the image forming apparatus is placed in the black-and-white mode (2). 
     As the solenoid  130  is activated once more to pull lever  130   a  when the image forming apparatus is in the black-and-white mode, the cams rotate 90°, causing the first plate  81  to move upward. As a result, the development roller  40   d , or the development roller for the BK color, is separated from the photosensitive drum  1   d . Then, the rotation of the development roller  40  is stopped, and the cartridge driving motor  103 , and the driving of the transfer belt  11 , are stopped; in other words, the image forming apparatus is placed in the home mode (3). 
     In the case of the full-color mode, as the solenoid  130  is activated once to pull the lever  130   a  when the image forming apparatus is in the home mode (3), the cams  93  and  94  rotate 90°. As the cams  93  and  94  rotate 90°, the first and second plates  81  and  80  moves downward, allowing the clutches  92   a ,  92   b , and  92   c , or the clutches for Y, M, and C colors, and the clutch  92   d , or the clutch for the BK color, to engage. Therefore, the development rollers  40   a ,  40   b ,  40   c , and  40   d  are rotated. That is, the force which has been applied upward by the first plate  81 , is removed. Therefore, the development rollers  40   a ,  40   b ,  40   c , and  40   d , or the development rollers for all colors, are allowed to come into contact with the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , making it possible for the image forming apparatus to print in full color; the image forming apparatus is placed in the full-color mode (4). 
     As the solenoid  130  is activated once to pull the lever  130   a  when the image forming apparatus is in the full-color mode, the cams  93  and  94  rotate 90°, causing the first and second plates  81  and  80  to move upward. As a result, all development rollers  40 , or the development rollers for all colors, are separated from the corresponding photosensitive drums  1 . Then, the rotation of the development roller  40  is stopped, and the driving of the cartridge driving motor  103  and the driving of the transfer belt  11 , are stopped; in other words, the image forming apparatus is placed in the home mode (1). 
     The image forming process carried out by an image forming apparatus such as the above described one includes the so-called pre-rotation step, which is carried out before the formation of an electrostatic latent image by the scanner unit  3 , to ensure that the peripheral surface of a photosensitive drum is uniformly charged, the so-called post-rotation step, which is carried out after the development of the electrostatic latent image into a toner image, to clear the peripheral surface of the photosensitive drum  1  of potential, etc. 
     During these steps, the photosensitive drums  1  are rotated. As described above, in this embodiment, the image forming apparatus is structured so that the separation of the development roller is ended with the same timing as the timing with which the development operation is started. Therefore, during the pre-rotation step and post-rotation step, the development roller  40  remains separated from the corresponding photosensitive drum  1 . Therefore, the image forming apparatus in this embodiment is substantially smaller, in the amount by which the surface layer of the photosensitive drum  1  is shaved by the friction between the peripheral surfaces of the photosensitive drum  1  and development roller  40  during the pre-rotation and post-rotation steps, as well as the step in which the photosensitive drum  1  is rotated for actual image formation, than an image forming apparatus in accordance with the prior art. 
     In the mode switching sequence 1, there are two home modes: home mode (1) and home mode (3). In this case, either home mode (1) or (3) may be designated as the normal home mode. For example, if the home mode (1) is selected as the normal home mode, all that is necessary to switch to the black-and-white mode is to activate the solenoid once to pull the lever. However, in order to switch to the full-color mode, the solenoid must be activated three times to pull the lever three times, requiring more time. 
     On the other hand, if the home mode (3) is selected as the normal home mode, all that is necessary to switch to the full-color mode is to activate the solenoid once to pull the lever once. However, in order to switch to the black-and-white mode, the solenoid must be activated three times to pull the lever three times, requiring more time. 
     Therefore, which of the two home modes, that is, the home mode (1) or home mode (3), should be selected as the normal home mode may be determined according to the frequency at which the image forming apparatus is used in the black-and-white or full-color mode by a user. That is, a user is allowed to set the home mode to minimize the length of time necessary to switch the operational mode. For example, a user who more frequently uses the image forming apparatus in the full-color mode than the black-and-white mode may select the home mode (3) as the normal home mode. 
     (Mode Switching Sequence 2) 
     Mode switching sequence 2 is different from mode switching sequence 1 in the shape of the cams  93  and  94  and the order in which the image forming apparatus is switched in operational mode. Referring to  FIG. 14 , in this mode switching sequence, the mode is switched in the sequence of (1) home mode-(2) black-and-white mode-(3) full-color mode-(4) black-and-white mode. 
     In this mode switching sequence, in order to separate the development roller, which is in contact with the photosensitive drum  1 , from the photosensitive drum  1 , it is necessary to rotate the development unit by applying upward pressure to the rib of the development unit against a pressure application spring  54 . Therefore, the power source for the mode switching is subjected to a heavy load when separating the development roller  40  from the photosensitive drum  1 . 
     First, this mode switching sequence 2 is compared to mode switching sequence 1 in terms of the process of separating the development roller  40 , which is in contact with the photosensitive drum  1 , from the photosensitive drum  1 , and the number of cartridges which are operated at the same time. 
     In mode switching sequence 1, when switching from (3) home mode to (4) full-color mode, the separation of the development roller  40 , which is in contact with the photosensitive drum  1 , from the photosensitive drum  1  occurs in all cartridges, that is, the cartridge for black toner, and all cartridges for the color toners, whereas in mode switching sequence 2, when switching from (3) full-color mode to (4) black-and-white mode, a total of three development rollers  40 , that is, the development rollers in the cartridges for the three colors, are separated from the corresponding photosensitive drums  1 . 
     Thus, the amount of load which must be borne by the motor  103 , as the mode switching power source, in mode switching sequence 2 is roughly 75% of that in mode switching sequence 1; mode switching sequence 2 is smaller in the amount of load which must be borne by the motor  103 . Therefore, mode switching sequence 2 makes it possible to reduce in size the motor as the mode switching power source. 
     (Mode Switching Sequence 3) 
     Not only is this mode switching sequence 3 different from the mode switching sequence in the first embodiment in the shapes of the cams  93  and  94  and the mode switching order, but also, in that in this sequence, the speed reduction ratios between the gear  132  and cam gear  133  is set to 3. That is, each time the solenoid  130  is activated, the cam gear  133  is rotated 120°. Referring to  FIG. 15(   a ), the operational mode is switched in the order of (1) home mode-(2) black-and-white mode-(3) full-color mode. 
     Mode switching sequence 3 is smaller in the number of mode switching steps than mode switching sequences 1 and 2, and therefore, is shorter in the total length of time necessary to switch to the full-color mode or black-and-white mode, and then, back to the home mode; it can minimize the total length of time necessary for the mode switching. Referring to  FIG. 15(   b ), the operational mode may be switched in the order of (1) home mode-(2) full-color mode-(3) black-and-white mode. 
     (Another Development Roller Separating Mechanism) 
     In the first embodiment described above, the cam  94  as the first cam, and the cam  93  as the second cam, are two different components as shown in  FIG. 9 . However, the two cams  93  and  94  may be two different portions of the same component, as shown in  FIG. 20 . That is, a cam  95  has a cam portion  95   a , which is equivalent to the cam  94  as the first cam, and a cam portion  95   b , which is equivalent to the cam  93  as the second cam. The cam  95  is solidly attached to a shaft  95   c , which is rotated by the rotational force from the motor  103 . The first plate  81 , in this mechanism, on which the cam portion  95   a  acts, is the same in structure as the first plat  81  in the first embodiment, which is shown in  FIG. 9 . However, the second plate  96  in this embodiment is different from the second plate  80  shown in  FIG. 9 , in that the engaging portion  96   d  of the second plate  96 , on which the cam portion  95   b  acts, is on the opposite side of the second plate from the engaging portion of the second plate  80 , shown in  FIG. 9 , in terms of the direction of the axial line of the shaft  95   c  of the cam  95 . Further, the first engaging portions  96   a   1 ,  96   b   1 , and  96   c   1 , which are in the form of a protrusion, and the second engaging portions  96   a   2 ,  96   b   2 , and  96   c   2 , which are in the form of a pair of protrusions, protrude from the surface of the second plate  96 . They are the same in function as the counterparts of the first plate  80  shown in  FIG. 9 . 
     Further, the structural arrangements in the second embodiment other than the above described one are the same as those in the first embodiment, and the effects obtainable by this embodiment are the same as those obtainable by the first embodiment. 
     Embodiment 3 
     In the first and second embodiments, the protrusions which act on the clutches, one for one, and the protrusions which act on the developing apparatuses, one for one, protrude from the surface of the same plate. However, an image forming apparatus may be structured so that the plate from which the protrusions which act on the clutches, one for one, protrude, may be different from the plate from which the protrusions which act on the developing apparatuses, one for one, protrude, as shown in  FIG. 21 . That is, a first plate  99 , as the first member, has an engaging portion  99   d   2 , which acts on the first clutch  92   d . A second plate  97 , as the second member, has engaging portions  97   a   2 ,  97   b   2 , and  97   c   2 , which act on the second clutches  92   a ,  92   b , and  92   c . It is a cam  98   b , as the first mode switching member, that acts on the first and second plates  99  and  97 . The cam  98   b  is solidly attached to a shaft  98   c , which rotates by receiving the driving force from the motor  103 . The cam  98   b  has a first portion  98   b   1 , which comes into contact with the first plate  99  and moves it, and a second portion  98   b   2  which comes into contact with the second plate  97  and moves it. The shape of the cam  98   b  is the same as that in the second embodiment, and the movements of the first and second plates  99  and  97 , which are caused by the cam  98   b , are the same as those in the first and second embodiments. 
     The printer main assembly  100  has a third plate  141  as the third member, and a fourth plate  140  as the fourth member. The third plate  141  acts on only the rib  46   d  with which the developing apparatus  4   d , which contains the toner of black color, is provided. The fourth plate  140  acts on the ribs  46   a ,  46   b , and  46   c , that is, the ribs with which the developing apparatuses  4   a ,  4   b , and  4   c , that is, the developing apparatuses which contain the toners of the colors (Y, M, and C) other than black, are provided, respectively. The fourth plate  140  is provided with engaging portions  140   a   1 ,  140   b   1 , and  140   c   1 , which are in the form of a protrusion and protrude from the surface of the fourth plate  140 . The engaging portions  140   a   1 ,  140   b   1 , and  140   c   1  move the developing apparatuses  4   a ,  4   b , and  4   c  by coming into contact with the ribs  46   a ,  46   b , and  46   c , respectively. The third plate  141  is provided with engaging portion  141   d   1 , which is in the form of a protrusion and protrudes from the surface of the third plate  141 . The engaging portion  141   d   1  moves the developing apparatus  4   d  by coming into contact with the rib  46   d . It is cam  98   a , as the second mode switching member, that acts on the third and fourth plates  141  and  140 . The cam  98   a  is solidly attached to a shaft  98   c  which rotates by receiving the driving force from the motor  103 . Further, the cam  98   a  is provided with a third portion portion  98   a   1 , which moves the third plate  141  by coming in contact with the third plate  141 , and a fourth portion  98   a   2 , which moves the fourth plate  140  by coming into contact with the fourth plate  140 . The shape of the cam  98   a  is the same as that in the second embodiment, and the movements of the third and fourth plates  141  and  140 , which are caused by the cam  98   a , are the same as those in the first and second embodiments. With the employment of the above described structural arrangement, the same home mode, black-and-white mode, and full-color mode as those in the first embodiment can be carried out. Further, the structural arrangements in the third embodiment other than the above described one are the same as those in the first embodiment, and the effects obtainable by this embodiment are the same as those obtainable by the first embodiment. 
     In the first to third embodiments described above, the image forming apparatuses were structured so that the cartridges  7   a ,  7   b ,  7   c , and  7   d  were removably juxtaposed in vertical straight line in the printer main assembly  100 . However, these embodiments are not intended to limit the present invention in scope. That is, the present invention is also applicable to an image forming apparatus (printer) structured so that the cartridges  7   a ,  7   b ,  7   c , and  7   d  are removably juxtaposed in virtually horizontal straight line in the apparatus main assembly (printer main assembly  100 ). In such a case, the apparatus main assembly is structured so that the first plate as the first member, and the second plate as the second member, are horizontally moved. Further, the present invention is also applicable to an image forming apparatus (printer) structured so that the cartridges  7   a ,  7   b ,  7   c , and  7   d  are removably juxtaposed in the slanted straight line (relative to horizontal direction) in the apparatus main assembly (printer main assembly  100 ). In such a case, the apparatus main assembly is structured so that the first plate as the first member, and the second plate as the second member, are moved in the slanted direction (relative to horizontal direction). 
     Also in the first to third embodiments described above, in order to transmit to each of the cartridges  7   a ,  7   b ,  7   c , and  7   d  a driving force that is independently from the driving force transmitted to the rest, the image forming apparatuses were provided with multiple motors  103 , that is, one for each cartridge. However, an image forming apparatus may be structured so that the rotational force from a single motor is transmitted to each of the cartridge  7   a ,  7   b ,  7   c , and  7   d.    
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Applications Nos. 004104/2006 and 346204/2006 filed Jan. 11, 2006 and Dec. 22, 2006, respectively, which are hereby incorporated by reference.