Patent Publication Number: US-6661987-B2

Title: Image forming apparatus for forming color images

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to such image forming apparatus as electrophotographic apparatus, printers, and the like to obtain color developer images using a revolver type-developing unit. 
     2. Description of the Related Art 
     The image forming apparatus produces a full-color image using 4 color toners of yellow (Y), magenta (M), cyan (C) and black (BK). In this image forming apparatus, a photoconductor is rotated by plural turns and latent images in respective colors are formed successively on the photoconductor for every turn. After forming the latent images on the photoconductor, these latent images are made visible by supplying toners in different colors to the photoconductor by plural developing units and the formed toner images are superposed on each other on the photoconductor or an intermediate transfer body. Thus, the image forming apparatus is made small in size. 
     As developing units to supply toners in different colors successively to latent images formed on a photoconductor for every turn of the photoconductor, a 4-color revolver type developing unit for developing latent images on the photoconductor by arranging developing units opposing to the photoconductor for supplying developers in specified colors successively by revolving a single revolving holder equipped with four developing units having four color toners of yellow (Y), magenta (M), cyan (C) and black (BK) is disclosed in Japanese Patent Publication No. 2001-24952. However, this 4-color revolver type developing unit is small in size but sizes of developing units that are incorporated in a revolving-type holder are limited. Therefore, a black developing unit to supply black (BK) toner only consumes much black toner fast and replenishment of black toner or exchange of a black developing unit is frequently demanded. 
     Therefore, an apparatus equipped with three developing units only to supply three colors of yellow (Y), magenta (M) and cyan (C), and a large capacity black developing unit for supplying much consuming black (BK) toner arranged separately from a revolver type developing unit for extending the life of the black developing unit is disclosed in Japanese Patent Publication No. 6-19271. 
     However, in the case of this 3-color revolver type developing unit equipped with 3-color developing units of yellow (Y), magenta (M) and cyan (C) only and a black developing unit provided separately, 3-color developing units  1 Y,  1 M, and  1 C for supplying yellow (Y), magenta (M) and cyan (C) are arranged at an angle 120° to a revolving shaft  1   a  of the 3-color revolver type developing unit as shown in FIG.  1 . That is, in this 3-color revolver type developing unit  1 , to switch the yellow (Y) developing unit  1 Y to the next magenta (M) developing unit  1 M after completing the yellow toner image development, it is necessary to revolve the 3-color revolver type developing unit  1  by 120°. 
     In the case of a 4-color revolver type developing unit  2  shown in FIG. 2, four color developing units of yellow  2 Y, magenta  2 M, cyan  2 C and black 2BK are arranged at an angle 90° to the revolving shaft  2   a . Therefore, the developing units can be switched by revolving the 4-color revolver type developing unit  2  only by 90° but the 3-color revolver type developing unit  1  requires 4/3 times of revolving amount. 
     For this reason, when the printing velocity of the 3-color revolver type developing unit  1  is made at least the same as that of the 4-color revolver type developing unit  2 , it is necessary to revolve the 3-color revolver type developing unit at a higher speed of 4/3 times than the revolving speed of the 4-color revolver type developing unit  2 . 
     Moreover, the further high-speed process is demanded in recent years for the increase in the number of image forming sheets per unit time resulting from the pursuit of high productivity of an image forming apparatus. This high productivity of the number of image forming sheets is achieved by increasing the processing speed and decreasing the supply interval of recording paper during the image forming process. In order to reduce the supply interval of recording paper, a switching time of developing units is more and more reduced, and the more increase of revolving velocity in switching the developing units is demanded for the revolver type developing unit. 
     For such the increase of switching velocity of the developing units, a driving motor and a power source for revolving the revolver type developing unit become large and the development of small-sized and energy saving image forming apparatus is impeded and further, vibration is generated when the revolver type developing unit is stopped after the high-speed revolving, and this vibration may possibly adversely affect images. 
     Accordingly, in an image forming apparatus to obtain color developed images using a revolver type developing unit, it is expected to develop a small-sized driving motor and a power source for a revolver type developing unit to realize the high-speed development without impeding the maintenance or the long life of a black developing unit and to obtain images of good quality without being adversely affected by vibration when stopping the revolver type developing unit. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide an image forming apparatus with a driving motor and a power source small in size without impeding the high-speed image forming process by a revolver type developing unit with a black developing unit arranged separately, capable of revolving in a short time and obtaining good images without generating vibration. 
     According to the embodiment of this invention, an image forming apparatus is provided. This image forming apparatus comprises an image carrier; a latent image forming unit to form latent images on the image carrier; a black developing unit arranged near the image carrier to supply a black developer to the image carrier; and a revolving-type developing unit. This revolving-type developing unit is arranged at a position near the image carrier and differing from the black developing unit, comprising developing units of plural color developing units which have different color developers arranged at equal space of a first distance along the circumference of a revolving body in contact with the image carrier. On the other hand, the developing unit at the last part in the revolving direction of the revolving body to the developing unit at the foremost part in the revolving direction are separated at a second distance that is longer than the first distance, and anyone of the plural color developing units is arranged opposing to the image carrier and supplies a color developer, and it is characterized in that for the area from the developing unit at the last part of the revolving-type developing unit to the foremost developing unit separated by the second distance opposing to the image carrier, a black developer is supplied to the image carrier by the black developing unit. 
     Further, according to the embodiment of this invention, an image forming apparatus is provided. This image forming apparatus comprises an image carrier; a latent image forming unit to form latent images on the image carrier; a black developing unit arranged near the image carrier to supply a black developer to the image carrier; and a revolver type developing unit arranged at a position differing from the black developing unit. This revolver type developing unit is composed of plural color developing units having different color developers, and developing rollers of these plural color developing units are arranged along the circumference of a revolving holder in contact with the image carrier at an equal space of the first distance and on the other hand, those developing rollers from the roller at the last part in the the revolving direction of the revolving body to the roller at the foremost part in the revolving direction are separated at a second distance that is longer than the first distance, and one of the plural color developing units is arranged opposing to the image carrier sequentially and supplies a color developer to the image carrier, and it is characterized in that the area ranging from the developing roller at the last part of the revolving-type developing unit to the foremost developing roller separated by the second distance opposing to the image carrier, a black developer is supplied to the image carrier by the black developing unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing the structure of a conventional 4-color revolver type developing unit; 
     FIG. 2 is a schematic diagram showing the structure of a conventional 3-color revolver type developing unit; 
     FIG. 3 is a schematic diagram showing an image-forming unit in an embodiment of this invention; 
     FIG. 4 is a schematic diagram for explaining the state of a black developing unit in contact with a photosensitive drum; 
     FIG. 5 is a schematic diagram for explaining the state of the black developing unit separated from the photosensitive drum in the embodiment of this invention; 
     FIG. 6 is a schematic diagram showing a 3-color revolver type developing unit in an embodiment of this invention; 
     FIG. 7 is a schematic diagram for explaining the state of the development by a first 3-color revolver type developing unit in an embodiment of this invention; 
     FIG. 8 is a schematic diagram for explaining the state of the development by a second 3-color revolver type developing unit in an embodiment of this invention; 
     FIG. 9 is a schematic diagram for explaining the state of the development by a third 3-color revolver type developing unit in an embodiment of this invention; 
     FIG. 10 is a schematic diagram for explaining the ready state of the 3-color revolver type developer unit in an embodiment of this invention; 
     FIG. 11 is a timing chart showing the developing process in an embodiment of this invention; 
     FIG. 12 is a timing chart showing the developing process in a first modified example of this invention; 
     FIG. 13 is a timing chart showing the developing process in a second modified example of this invention; and 
     FIG. 14 is a schematic structural diagram showing the 3-color revolver type developing unit in the second modified example of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of this invention will be described below in detail referring to the attached drawings. FIG. 3 is a schematic diagram showing the structure of an image forming unit  10  of an image forming apparatus such as color printers that is an embodiment of this invention. Around a photosensitive drum  11  which is an image carrier of the image forming unit  10  and the back of its photosensitive layer (not shown) is grounded to 0 [V] (GND), there are arranged a charger  12  which uniformly charges the photosensitive drum  11  to about −700 [V] gradually in the revolving allow direction s, a laser writing unit  13  which is a latent image forming unit for forming latent images on the charged photosensitive drum  11 , a black developing unit  14  which is a black developing device having black (BK) toner, a 3-color revolver type developing unit  20  which is a revolving type developing device equipped with a first through third developing units  16 - 18  which are color developing devices having three color developers; yellow (Y), magenta (M) and cyan (C) three color toners; an intermediate transfer device  21  having an intermediate transfer belt  21   a , a cleaner  22 , and a change eliminating lamp  23 . 
     Below the image forming unit  10 , there are provided a paper cassette  24  to house recording paper (not shown) that is transfer material, an aligning roller pair  27  to bring the leading edge of a recording paper taken out from the paper cassette  24  and conveyed along a conveying path  26  to synchronize with the leading edge of a toner image transferred on the intermediate transfer belt  21   a , a secondary transfer roller  28  to which +1-2 [kV] bias voltage is applied for secondarily transferring a toner image from the intermediate transfer belt  21   a  on a recording paper, and a fixing roller pair  30  to fix a toner image on a recording paper. Further, the speed of the image forming process in this image forming unit  10  is 180 [mm/sec]. 
     The laser writing unit  13  forms an electrostatic latent image on the photosensitive drum  11  by applying a laser beam corresponding to writing signals for all colors according to image data input from external computer terminals. 
     The black developing unit  14  is capable of revolving centering around a supporting point  14   b . A developing roller  14   a  is revolved in the arrow direction t 1  or t 2  when a cam  32  is revolved centering around a shaft  32   a  and contacts or separates from the photosensitive drum  11 .  33  is a mark for stopping the revolution of the cam  32 , and  34  is a sensor to detect this mark  33 . A toner is supplied to the black developing unit  14  by a black (BK) toner cartridge that is not shown. 
     The first through third developing units  16 - 18  have first through third non-magnetic one-component type developing cartridges  16   b - 18   b  and first through third toner cartridges  16   c - 18   c  which supply yellow (Y), magenta (M), and cyan (C) toners to the first through third developing cartridges  16   b - 18   b , respectively. The first through third developing cartridges  16   b - 18   b  and the first through third toner cartridges  16   c - 18   c  are connected to each other with first through third pipes  16   d - 18   d.    
     The 3-color revolver type developing unit  20  has a revolving holder  37  which is revolved centering around a revolving shaft  36  at a peripheral velocity 180-200 [mm/sec] by a driving motor (not shown). The revolving holder  37  revolves at an angle of 30° to the revolving shaft  36  between the first and second developing cartridges  16   b  and  17   b  and the second and third developing cartridges  17   b  and  18   b , and supports the first through third developing units  16 - 18  so that an angle to the revolving shaft  36  becomes 300° between the third and first developing cartridges  18   b  and  16   b . Thus, first through third developing rollers  16   a - 18   a  are arranged centering around the revolving shaft  36  of the circumference  38  in a diameter 180 [mm] in contact with the photosensitive drum  11  in the arrow direction v at equal space 47.1 [mm]=L 1  that is a first distance. On the other hand, the developing rollers  16   a - 18   a  from the last third developing roller  18   a  to the front developing roller  16   a  in the arrow direction v are arranged at equal distance 471.2 [mm]=L 2  that is longer than L 1 . 
     Further, the first through third toner cartridges  16   c - 18   c  of the first through third developing units  16 - 18  are arranged centering around the revolving shaft  36  symmetrically with the first through the third developing cartridges  16   b - 18   b.    
     In FIGS. 7-10,  46  is a mark for stopping the rotation formed on the revolving holder  37  and  47  is a holder sensor to detect the mark  46 . When the 3-color revolver type developing unit  20  stops at the position where this mark  46  is detected by the holder sensor  47 , the first developing cartridge  16   b  stops at the position opposing to the photosensitive drum  11 , that is the developing position. Hereafter, the revolving holder  37  is revolved successively for a specified time in the arrow direction v and the second and third developing cartridges  17   b  and  18   b  are arranged to face the photosensitive drum  11 . 
     Thus, yellow (Y), magenta (M) and cyan (C) toners are supplied to the photosensitive drum  11  in order by the first through third developing rollers  16   a - 18   a . Further, when the revolution of the 3-color revolver type developing unit  20  is revolved in the arrow direction v after completing the development by the third developing roller  18   a , the black developing unit  14  develops images on the photosensitive drum  11  as long as the area from the 
     third developing roller  18   a  to the first developing cartridge  16   b  faces the photosensitive drum  11 . 
     A length Lb of the intermediate transfer belt  21   a  of the intermediate transfer device  21  is (216+51)×2=534 [mm]) so that an image requiring 2 sheets of letter size paper (216×279 [mm]) is formed at a space 51 [mm] between recording sheets of paper and the intermediate transfer belt  21   a  is put over a primary transfer roller  40   a  to which +1 [kV] transfer bias is applied, an opposed secondary transfer roller  40   b  that is grounded at 0 [V], and a tension roller  40   c . On the intermediate transfer belt  21   a , an exposure start mark  41  for positioning when image patterns in different colors are put over each other and a sensor  42  for detecting the exposure start mark  41  is provided near the intermediate transfer belt  21   a . Further,  43  is a belt cleaner. 
     The intermediate transfer belt  21   a  contacts the photosensitive drum  11  at the position where it is supported by the primary transfer roller  40   a  and a toner image formed on the photosensitive drum  11  is transferred primarily, and is opposed to the secondary transfer roller  28  at the position where it is supported by the opposed secondary transfer roller  40   b  and transfers the toner image secondarily on a recording paper passing between the secondary transfer roller  28  and the opposed secondary transfer roller  40   b.    
     Next, the full-color image forming process by the image forming unit  10  will be described. In this image forming unit  10 , a full-color image is formed by superposing yellow (Y), magenta (M), cyan (C) and black (BK) toner images over each other in that order. When image forming is started, the 3-color revolver type developing unit  20  is stopped at a position where the holder sensor  47  detects the mark  46  as shown in FIG.  7 . Then, the first developing cartridge  16   b  having yellow (Y) toner is arranged at the developing position opposite to the photosensitive drum  11 . The first developing roller  16   a  contacts the photosensitive drum  11  at this position. 
     At this time, as the cam  32  is kept stopped at the position where the sensor  34  detected the mark  33  as shown in FIG. 5, the developing roller  14   a  of the black developing unit  14  was revolved centering around the supporting point  14   b  in the arrow direction t 1  by its dead weight and the developing roller  14   a  is separated from the photosensitive drum  11 . 
     When an image forming process starts under such a state, the photosensitive drum  11  is revolved in the arrow direction s at the velocity of 180 [mm/sec] and uniformly charged to −700 [V] by the charger  12  with its revolution. Hereafter, the electrostatic latent image forming and the developing image forming are executed at the timings shown in the timing chart in FIG.  11 . That is, when the sensor  42  detects the passage of the exposure start mark  41  on the intermediate transfer belt  21   a  at the timing ( 1 )−y,the laser beam corresponding to the yellow image signal out of separated yellow, magenta, cyan and black signals is applied to the photosensitive drum  11  by the laser writing unit  13  at the timing ( 2 ). Thus, an yellow electrostatic latent image of about −100 [V] of the laser beam applied part is formed on the photosensitive drum  11 . Hereafter, the photosensitive drum  11  reaches the 3-color revolver type developing unit  20 . 
     In the 3-color revolver type developing unit  20 , the first developing roller  16   a  is revolved in the arrow direction w by a developing roller revolving motor (not shown) as shown by the timing ( 3 ) in consonance with the arrival of the yellow electrostatic latent image as shown by the timing ( 3 ). Bias voltage −300 V or AC when necessary is applied to the first developing roller  16   a  and the yellow toner of the yellow toner image on the photosensitive drum  11  is electrostatically transferred while it passes through the nip between the photosensitive drum  11  and the first developing roller  16   a  and an yellow (Y) toner image is formed. 
     Then, when the toner image on the photosensitive drum  11  reaches the position to contact the intermediate transfer belt  21   a  that is revolved in the arrow direction x, the yellow toner image is primarily transferred on the intermediate transfer belt  21   a  by the transfer bias of about +1 [kV] applied to the primary transfer roller  40   a . After the primarily transfer, residual toner remained on the photosensitive drum  11  is cleaned and the surface charge is eliminated by the charge eliminating lamp  23 . 
     Hereafter, the magenta, cyan and black toner image forming processes are repeated to the photosensitive drum  11  successively likewise the yellow toner image forming process described above, and by transferring the yellow (Y), magenta (M), cyan (C) and black (BK) toner images 
     successively on the intermediate transfer belt  21   a  from the photosensitive drum  11 , a full-color toner image with 4 color toner images put over each other is formed on the intermediate transfer belt  21   a.    
     During this period, in the 3-color revolver type developing unit  20 , when the trailing edge of the yellow electrostatic latent image passed and the yellow toner image development is completed, the first developing roller  16   a  is stopped to revolve and the first developing cartridge  16   b  at the developing position is switched to the second developing cartridge  17   b  until the leading edge of the next magenta electrostatic latent image arrives as shown by the timing ( 4 ) and therefore, the second developing cartridge  17   b  is moved to a developing position facing to the photosensitive drum  11 . That is, as shown in FIG. 8, the revolving holder  37  is revolved in the arrow direction v for a specified 0.262 [sec] at a mean moving velocity 180 [mm/sec] so as to move for the distance L 1  on the circumference  38  from the position where the holder sensor  47  detects the mark  46 . The second developing cartridge  17   b  reaches the developing position and the second developing roller  17   a  contacts the photosensitive drum  11  and the development by the magenta toner becomes possible. 
     Then, when the sensor  42  detects the passing of the exposure start mark  41  by the intermediate transfer belt  21   a  at the timing ( 1 )−m, a magenta electrostatic latent image is formed on the photosensitive drum  11  by the laser writing unit  13 . Thereafter, the electrostatic latent image on the photosensitive drum  11  is developed by a magenta (M) toner while passing the nip between the second developing roller  17   a  to which a bias voltage is applied and revolved in the arrow direction w and a magenta toner image is formed on the photosensitive drum  11 . 
     Hereafter, when the trailing edge of the magenta electrostatic latent image passed and the development of the magenta toner image is completed, the 3-color revolver type developing unit  20  switches the second developing cartridge  17   b  to the third developing cartridge  18   b  before the leading edge of a next cyan electrostatic latent image reaches the developing position likewise the switching from the first developing cartridge  16   b  to the second developing cartridge  16   b . That is, the revolving holder  37  is revolved for a specified 0.262 sec. in the arrow direction v at a mean moving velocity 180 [mm/sec] so as to further move for a distance L 1  on the circumference  38  as shown in FIG.  9 . Thus, the third developing roller  18   a  contacts the photosensitive drum  11  and the development by a cyan (c) toner becomes possible. 
     Then, when the cyan electrostatic latent image formed on the photosensitive drum  11  passes the nip with the third developing roller  18   a , the electrostatic latent image is developed with a cyan (c) toner and a cyan toner image is formed on the photosensitive drum  11 . 
     Hereafter, when the trailing edge of the cyan electrostatic latent image passes the developing position and the cyan toner image development is completed, the 3-color revolver type developing unit  20  revolves the revolving holder  37  in the arrow direction v at a mean moving velocity 180 [mm/sec] at the timing ( 10 ). Then, when the image forming process is executed successively, the revolving holder  37  is stopped at a position where the holder sensor  47  detects the mark  46  as shown in FIG.  7 . On the other hand, when there is no succeeding image forming process, after revolving the revolving holder  37  for a specified time and stopping it, the 3-color revolver type developing unit  20  becomes the ready state as shown in FIG.  10 . 
     At this time, by decelerating the moving velocity of the revolving holder  37 , the first developing unit  16  having a yellow (Y) toner is prevented from reaching the developing position before completing the developing by the black developing unit  14  as described later and disturbing a black toner image formed on the photosensitive drum  11  during the successive image forming processes. 
     On the other hand, when the trailing edge of a cyan electrostatic latent image passes the developing position of the black developing unit  14 , the cam  32  of the black developing unit  14  revolves in the arrow direction y for a specified time as shown by the timing ( 7 ). Then, the developing roller  14   a  of the black developing unit  14  is pushed up in the arrow direction t 2  and contacts the photosensitive drum  11  as shown in FIG. 4, and the development becomes possible. 
     Then, when the passing of the exposure start mark  41  of the intermediate transfer belt  21   a  is detected by the sensor  42  as shown by the timing ( 5 ), a black electrostatic latent image is formed on the photosensitive drum  11  by the laser writing unit  13  at the timing ( 6 ). Then, the black electrostatic latent image is developed while passing through the nip with the developing roller  14   a  that is applied with a bias voltage and revolved in the arrow direction z, and a black toner image is formed on the photosensitive drum  11 . 
     When the trailing edge of the black electrostatic latent image passes and the black toner development is completed, in the black developing unit  14 , the developing roller  14   a  stops to revolve, the cam  32  is revolved again in the arrow direction y as shown by the timing ( 9 ) and stops at the position where the sensor  34  detects the mark  33  and the developing roller  14   a  is separated from the photosensitive drum  11 . 
     Thereafter, when the image forming process is executed successively, the revolution of the revolving holder  37  started at the timing ( 10 ) continuous as long as the development is made by the black developing unit  14 , and the revolving holder  37  stops to revolve when the mark  46  is detected by the holder sensor  47  and the first developing cartridge  16   b  is arranged at the developing position. Thereafter, when the exposure start mark  41  is detected by the sensor  42  at the intermediate transfer belt  21   a , the laser beam corresponding to the yellow image signal is applied to the photosensitive drum  11  by the laser wiring unit  13  in order to form a full-color image on a second sheet as shown by the timing ( 11 ). 
     Hereafter, the similar operations are repeated and toner images formed on the photosensitive drum  11  are intermediately transferred on the intermediate transfer belt  21   a  successively. That is, while the development is being executed by the black developing unit  14 , the 3-color revolver type developing unit  20  moves for the distance L 2 , the first developing cartridge  16   b  is moved to the developing position for the third developing cartridge  18   b  and the next continuous image forming process becomes ready. 
     On the other hand, yellow (Y), magenta (M), cyan (C) and black (BK) full-color toner images superposed on the intermediate transfer belt  21   a  are collectively transferred on a recording paper by the electric field formed at the position of the secondary transfer roller  28  by the secondary transfer roller  28  to which +1-2 [kV] bias voltage is applied and the opposing secondary transfer roller  40   b  which is grounded to 0 [V]. Further, a recording paper is taken out in the arrow direction r from the paper cassette  24  and conveyed to the position of the secondary transfer roller  28  synchronizing with the full-color image. Thereafter, the full-color toner image is fixed on the recording paper by the fixing roller pair  30  and a color image is completed. On the other hand, after completing the transfer on a recording paper, residual toners remained on the intermediate transfer belt  21   a  arw cleaned by the belt cleaner  43 . 
     According to this embodiment, the first through third developing units  16 - 18  are adjacent to the revolving shaft  36  at an angle 30° and arranged in a space as short as L=47.1 [mm] on the circumference  38  as shown in FIG.  6 . Therefore, a switching time of the developing units  16 - 18  can be made short even when not revolving the 3-color revolver type developing unit  20  at a high speed, and the developing units  16 - 18  can be switched before a next electrostatic latent image reaches the developing position after a preceding electrostatic latent image is developed. Further, the third developing unit  18  can be switched to the first developing unit  16  when the development is being made by the black developing unit  14  and therefore, even when a distance set between the third developing unit  18  and the first developing unit  16  is as long as L 2 =471.2 [mm], the switching can be sufficiently made before starting the next development by the first developing unit  16  even when the 3-color revolver type developing unit  20  is not revolved at a high speed. 
     Accordingly, it is not needed to use a large power source to supply electric power to a large torque driving motor or ordinary driving motor for revolving the 3-color revolver type developing unit  20  with a separately provided black developing unit  14 , and a small sized and energy saving type image forming apparatus capable of extending the life of the black developing unit  14  without impeding a high speed image forming processing property can be obtained. Furthermore, as it is not needed to revolve the 3-color revolver type developing unit  20  at a higher speed, it is possible to reduce the vibration of the 3-color revolver type developing unit generated when it stops to revolve, prevent adverse effects of the vibration to images and obtain a color image of good quality. 
     Further, the first through third developing units  16 - 18  are so arranged that the first through third toner cartridges  16   c - 18   c  and the first through third developing toner cartridges  16   b - 18   b  become symmetrical centering around the revolving shaft  36  and therefore, the first through third developing units  16 - 18  can be formed in the same structure except the pipes  16   d - 18   d  and manufacturing costs can be reduced by manufacturing commonly usable component parts. 
     Further, this invention is not limited to the embodiment described above but can be modified variously within the spirit and scope thereof. For example, the first distance or the second distance between the developing units in a revolving type developing device is not limited; for example, the first distance cannot be made extremely short because of the restriction of a size of the developing device but a switching time of the developing devices can be further shortened and it becomes possible to achieve the higher speed of the image forming process without revolving a revolving type developing device at high speeds. 
     Further, the timing for revolving the rotary type-developing device is also optional. In the above embodiment, the revolving velocity of the 3-color revolver type developing unit  20  is slow so as to prevent a black toner image on the photosensitive drum  11  from being disturbed by contact with the first developing roller  16   a . However, another method not to carry out such velocity control may be used. 
     For example, as a first modified example, in the above embodiment, after completing the development of the cyan toner image at the timing ( 12 ) as shown in the timing chart in FIG. 12, the 3-color revolver type developing unit  20  may be revolved for a specified time at the timing ( 13 ) and then, once stopped to revolve and again revolved at the timing ( 14 ) immediately before starting the development of an yellow toner image and the first developing unit  16  may be moved to the developing position. Thus, when the 3-color revolver type developing unit  20  is revolved by dividing into two times and again revolved immediately before starting the development of an yellow toner image so that the first developing unit  16  reaches the developing position, it is possible to prevent a black toner image from being disturbed by the contact with the first developing roller  16   a.    
     Further, as a second modified example, in the above embodiment, the disturbance of a black toner image caused by contact with the first developing roller  16   a  can be prevented by starting the revolving holder  37  to revolve after waiting a specified time as shown by the timing ( 17 ) after completing the development of a cyan toner image as shown by the timing ( 16 ) so that the first developing unit  16  reaches the developing position immediately before starting the development of an yellow toner image. 
     Further, the structure of the revolving type developing device is also optional as shown in FIG. 14 as a third modified example. Fourth-through sixth developing units  51 - 53  includes fourth through sixth developing cartridges  51   b - 53   b  having yellow (Y), magenta (M), cyan (C) toners, respectively and fourth through sixth developing rollers  51   a - 53   a . The fourth through sixth developing cartridges  51   b - 53   b  may be arranged in parallel with each other in a 3-color revolver type developing unit  50 . The fourth through sixth developing rollers  51   a - 53   a  may be arranged consecutively at an equal space of L 3  along the circumference  54 . On the other hand, the sixth developing roller  53   a  and the fourth developing roller  5 l a  may be set at an equal space of L 4  which is longer than L 3 . 
     As described above in detail, according to this invention, the developing units of a revolving type developing device equipped with plural color developing units only except a black developing unit are arranged consecutively at a first distance and the developing unit at the rearmost part in the revolving direction, the developing unit at the foremost part are arranged at a second distance which is longer than the first distance. Thus, the first distance is set short and a time required for switching color developing units separated at the first distance can be made short. Accordingly, it is not necessary to use a large driving motor or a large power source for revolving the revolving type developing device and a small-sized and energy saving image forming apparatus for extending the life of the black developing device without impeding a high speed quality can be obtained. 
     Further, in the area separated by the second distance, a time is needed for switching color image developing devices; however, during this time interval, the development by the black developing device is made and therefore, there is no possibility for decreasing the image forming process speed. Further, since it is not necessary to revolve the revolving type image developing device at a high speed, vibration generated when stopping the revolving type image developing device can be reduced and the image quality can be improved.