Patent Publication Number: US-6671480-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an image forming apparatus to which electrophotography is applied and in which developing devices are used, switched from one to anther. 
     2. Description of the Related Art 
     Generally, an image forming apparatus to which electrophotography is applied have a photosensitive member, a charging device, an exposing device, a developing device, a transfer device and a fixing device. The photosensitive member may have an electrically charged area. This area is electrically discharged when light is applied to the photosensitive member. The charging device charges the photosensitive member. As the exposing device applies light to the photosensitive member, an electrostatic latent image is formed on the photosensitive member. As the developing device makes the electrostatic latent image attract toner, a visible toner image is formed on the surface of the photosensitive member. The transfer device transfers the toner image from the surface of the photosensitive member onto a recording sheet. The fixing device fixes the toner image that the transfer device has transferred onto the recording sheet. 
     Multi-color image forming apparatus have a developing device assembly that comprises a plurality of developing devices. The assembly is a means for forming multi-color images. Each developing device has a toner container. The toner containers store toners of different colors. In the multi-color image forming apparatus, the developing device assembly is rotated like a cylinder of a revolver to one developing device to another. The multi-color image forming apparatus forms a multi-color image on the surface of a recording sheet by repeating the image forming routine at the photosensitive drum, for the respective developing devices. Note that the image forming routine is a sequence of charging, exposing, developing and transferring. Since the image forming procedure is repeated as many times as the number of the developing devices, the multi-color image forming apparatus needs more time to form a complete image than the mono-color image forming apparatus. 
     Recently, there is a strong demand for multi-color image forming apparatus that can form images at high speed. One of the methods devised to meet the demand is to operate each device at high speed. 
     The developing device assembly is rotated intermittingly to switch one developing device to the next one. Each time the assembly starts or stops rotating, it vibrates due to its own moment of inertia. The vibration affects the image forming operation of the multi-color image forming apparatus. 
     If the exposing device vibrates, the light beam irradiating the surface of the photosensitive member may miss the target. If the developing device assembly vibrates, the distance between the developing device and the photosensitive member will change incessantly. The toner will be attracted, inevitably in uneven density, to the surface of the photosensitive member due to the electrostatic force. The faster the developing device assembly rotates, the greater the kinetic energy of the developing device assembly. Consequently, the vibration of the developing device assembly increases. 
     The vibrations of the devices result in misregistration of the color layers laid one on another. In order to avoid misregistration of colors, each device needs to wait until it ceases to vibrate, before it starts operating. 
     BRIEF SUMMARY OF THE INVENTION 
     An image forming apparatus according to an embodiment of the present invention is designed to reduce the vibration that occurs in each stop after switching one developing device to another in the developing device assembly that comprises a plurality of developing devices. 
     An image forming apparatus according to an aspect of the present invention comprises a developing device assembly, an inertia acceptor and a setting mechanism. The developing device assembly has a plurality of developing devices for developing a latent image formed on a photosensitive member. The assembly can rotate to switch one developing device to the next one. The energy of rotational inertia is shifted from the developing device assembly to the inertia acceptor, in order to stop the rotation of the developing device assembly. The setting mechanism controls the movement of the inertia acceptor that has received the energy of the rotational inertia from the developing device assembly. 
     An image forming apparatus according to another embodiments comprises a developing device assembly and an inertia acceptor. The developing device assembly has a plurality of developing devices for developing a latent image formed on a photosensitive member. The assembly can rotate to switch one developing device to the next one. The inertia acceptor shifts the energy of rotational inertia to the developing device assembly to make the assembly start rotating, and is shifted the energy of rotational inertia from the developing device assembly in order to stop the rotation of the developing device assembly. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serves to explain the principles of the invention. 
     FIG. 1 is a schematic sectional view of the image forming apparatus according to the first embodiment of the present invention; 
     FIG. 2 is a schematic perspective view of the developing device assembly and the mechanisms provided around the assembly in the image forming apparatus of FIG. 1; 
     FIG. 3 is a schematic plan view of the developing device assembly and the like, shown in FIG. 2; 
     FIG. 4 is a timing chart showing how the operating states of the developing device assembly and the like, illustrated in FIG. 3, change with time; 
     FIG. 5 is a schematic sectional view taken along line A—A in FIG. 3, representing the positional relation between the developing device assembly and the inertia acceptor, at K 1  in FIG. 4; 
     FIG. 6 is a schematic sectional view taken along line A—A in FIG. 3, showing the positional relation between the developing device assembly and the inertia acceptor, at K 2  in FIG. 4; 
     FIG. 7 is a schematic sectional view taken along line A—A in FIG. 3, illustrating the positional relation between the developing device assembly and the inertia acceptor, at K 3  in FIG. 4; 
     FIG. 8 is a schematic plan view of the developing device assembly and some other components of the image forming apparatus according to the second embodiment of the present invention; 
     FIG. 9 is a schematic cross sectional view taken along line B—B in FIG. 8, showing a state in which the developing device assembly collides with the inertia acceptor, at a strike portion and a stricken portion; 
     FIG. 10 is a schematic sectional view taken along line B—B in FIG. 8, showing a state in which the inertia acceptor is shifted the energy of rotational inertia from the developing device assembly and starts rotating; 
     FIG. 11 is a schematic sectional view taken along line B—B in FIG. 8, showing a state in which the inertia acceptor is located at the next setting position and the developing device assembly is rotating to switch one developing device to the next one; 
     FIG. 12 is a timing chart illustrating how the operating states of the developing device assembly and the like of the image forming apparatus according to the third embodiment of the present invention change with time; 
     FIG. 13 is a schematic view illustrating the positional relation between the developing device assembly and the inertia acceptor, at V 1  in FIG. 12; 
     FIG. 14 is a schematic view showing the positional relation between the developing device assembly and the inertia acceptor, at V 2  in FIG. 12; 
     FIG. 15 is a schematic view showing the positional relation between the developing device assembly and the inertia acceptor, at V 3  in FIG. 12; 
     FIG. 16 is a schematic view representing the positional relation between the developing device assembly and the inertia acceptor, at V 4  in FIG. 12; and 
     FIG. 17 is a schematic view showing the positional relation between the developing device assembly and the inertia acceptor, at V 5  in FIG.  12 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An image forming apparatus  1  according to the first embodiment of the present invention will be described, with reference to FIGS. 1 to  7 . As FIG. 1 shows, the image forming apparatus  1  forms an image on a recording sheet  2  by performing electrophotography. The image forming apparatus  1  comprises a photosensitive drum  3  for a photosensitive member, a developing device assembly  4 , an intermediary transfer belt  5 , an charging device, an exposing device  41 , a transfer roller  42 , and a fixing device  43 . The photosensitive drum  3 , developing device assembly  4  and intermediary transfer belt  5  are arranged in parallel to each other. The photosensitive drum  3  and the intermediary belt  5  contact each other. 
     The charging device electrically charges the photosensitive drum  3 . An invisible electrostatic latent image is formed on the surface  3   a  of the photosensitive drum  3 , which is irradiated with light by the exposing device  41 . 
     The developing device assembly  4  has a plurality of developing devices  6 , or four devices  6  in this embodiment. Each developing device  6  has a developing roller  7 . The developing roller  7  causes the electrostatic latent image formed on the surface  3   a  of the photosensitive drum  3  to attract toner by the electrostatic force. As a result, a toner image is formed on the surface  3   a  of the photosensitive drum  3 , which has the electrostatic latent image. The developing devices  6  are arranged parallel relative to one another, with the developing rollers  7  facing outside. The developing device assembly  4  positions the developing roller  7  of one of the developing devices  6  at developing position P 1 , holding the roller  7  facing the photosensitive drum  3 . The developing device assembly  4  switches the developing device  6  to the next one when it is rotated. The developing devices  6  contain toners of different colors, including yellow, magenta, cyan and black in this embodiment. The image forming apparatus  1  can therefore form multi-color images. 
     The toner image that each of the developing device  6  has formed on the surface  3   a  of the photosensitive drum  3  is transferred from the photosensitive drum  3  onto the surface  5   a  of the intermediary transfer belt  5 . The toner images are formed, sequentially one upon another, on the surface  5   a  of the intermediary transfer belt  5 . They are collectively transferred onto the recording sheet  2  that has been conveyed along arrow R shown in FIG.  1 . The recording sheet  2 , which has transferred a toner image, is moved further along the arrow R, passes by the fixing device  43 , and leaves the image forming apparatus  1 . 
     The image forming apparatus  1  further comprises an inertia acceptor  8 , a setting mechanism  9 , and a positioning mechanism  10 . The mechanism  10  is used for the developing device assembly  4 . FIG. 2 shows the photosensitive drum  3 , developing device assembly  4 , intermediary transfer belt  5 , inertia acceptor  8 , setting mechanism  9  and positioning mechanism  10  of the developing device assembly  4 . FIG. 3 is a plan view of the developing device assembly  4 , inertia acceptor  8 , setting mechanism  9  and positioning mechanism  10 , all shown in FIG.  2 . The inertia acceptor  8  and the setting mechanism  9  are arranged at an end of the developing device assembly  4 . The positioning mechanism  10  of the developing device assembly  4  is arranged at the other end of the developing device assembly  4 . 
     The inertia acceptor  8  is arranged on the axis C of the rotary shaft  11  of the developing device assembly  4 . The inertia acceptor  8  can revolve around the axis C, independently of the developing device assembly  4 . The inertia acceptor  8  has a mass sufficient to stop the rotation of the developing device assembly  4  when it receives the energy of rotational inertia from the developing device assembly  4 . Once the developing device assembly  4  shifts the energy of rotational inertia to the inertia acceptor  8 , it stops without vibrating at all. 
     The developing device assembly  4  has a strike portion  12 . The inertia acceptor  8  has a stricken portion  13 . The energy of rotational inertia is shifted from the developing device assembly  4  to the inertia acceptor  8  when the strike portion  12  is made to collide with the stricken portion  13 . 
     The setting mechanism  9  has a motor  14 , a gear  15 , a disk  16 , a detector  17 , and a braking mechanism  18 . The motor  14  is in mesh with the gear  15 . The gear  15  and the disk  16  are arranged to rotate with the inertia acceptor  8 . The motor  14  rotates the inertia acceptor  8  by way of the gear  15 . The disk  16  has marks  19  for detecting the setting positions P 2 , P 3 , P 4  and P 5  of the inertia acceptor  8 . The marks  19  are, for example, notches formed along the outer periphery of the disk  16  as is illustrated in FIG.  2 . The detector  17  detects the marks  19  on the disk  16 . The braking mechanism  18  has a pad  20 . The pad  20  is pushed against the inertia acceptor  8 , dampening and finally stopping the rotation of the inertia acceptor  8 . 
     The positioning mechanism  10  has a gear  21 , a disk  22 , a detector  23 , and a drive unit  24 . The gear  21  and the disk  22  are arranged to rotate with the developing device assembly  4 . The disk  22  has marks  25 , for detecting the developing position P 1  where the each developing devices  6  of the developing device assembly faces the photosensitive drum  3 . The marks  25  are, example, notches formed along the outer periphery of the disk  22  as is shown in FIG.  2 . The detector  23  detects the marks  25  on the disk  22 . The drive unit  24  meshes with the gear  21  as shown in FIG. 3, and rotates the developing device assembly  4 . Any other positioning mechanism can be used in this embodiment, if positions the developing device assembly  4  with respect to the photosensitive drum  3  so that each developing devices  6  may sufficiently develop a latent image. The positioning mechanism  10  may be arranged at the same side of the developing device assembly  4  as the inertia acceptor  8  is arranged. 
     As FIG. 3 shows, a controller  26  controls the setting mechanism  9  and the positioning mechanism  10 . The controller  26  controls the motor  14 , drive unit  24  and braking mechanism  18  in accordance with the developing position P 1  of the developing device assembly  4 , detected by the detector  23 , and the setting position P 2 , P 3 , P 4  or P 5  of the inertia acceptor  8 , detected by the detector  17 . Thus, the controller  26  places the developing device assembly  4  at the developing position P 1  and the inertia acceptor  8  at one of the setting positions P 2 , P 3 , P 4  and P 5 , by controlling the motor  14 , drive unit  24  and the braking mechanism  18 , as the detectors  17  and  23  detect the marks  19  and  25 , respectively. The controller  26  synchronizes the movement of the developing device assembly  4  and the inertia acceptor  8 . 
     The developing device assembly  4 , inertia acceptor  8 , setting mechanism  9  and positioning mechanism  10  will be described, for their respective movements, with reference to FIGS. 4 to  7 . FIG. 4 shows how these components move with time. In FIG. 4, (A) indicates the operating state of the developing devices; ON means the active state, and OFF means the inactive state. In FIG. 4, (B) shows the state of the drive unit; ON means that the devices are operating, and OFF means that they are at stop. In FIG. 4, (C) shows the detecting operation of the detector  23  of the positioning mechanism  10 ; ON means that the developing device  6  is located at the developing position, and OFF means that the developing device  6  is located outside the developing position. In FIG. 4, (D) indicates the rotational speed of the developing device assembly  4 . In FIG. 4, (E) indicates the rotational speed of the inertia acceptor  8 . In FIG. 4, (F) depicts the detecting condition of the detector  17  of the setting mechanism  9 ; ON means that the inertia acceptor  8  is located at the setting position, and OFF means that the inertia acceptor  8  is located outside the setting position. In FIG. 4, (G) shows the state of the braking mechanism  18 ; ON means that mechanism  18  is operating, and OFF means the mechanism  18  is at stop. In FIG. 4, (H) indicates the operating state of the motor  14  of the setting mechanism  9 ; ON means that the motor  14  is operating, and OFF means that the motor  14  is at stop. 
     FIG. 5 represents the positional relation that the developing device assembly  4  and the inertia acceptor  8  have at time K 1  in FIG.  4 . At time K 1 , the strike portion  12  of the developing device assembly  4  collides with the stricken portion of the inertia acceptor  8 . FIG. 6 shows the positional relation that the developing device assembly  4  and the inertia acceptor  8  have at time K 2  in FIG.  4 . At time K 2 , the developing device assembly  4  shifts the energy of rotational inertia to the inertia acceptor  8  and then stops operating. The inertia acceptor  8 , which has received the energy of rotational inertia, rotates. FIG. 7 shows the positional relation that the developing device assembly  4  and the inertia acceptor  8  have at time K 3  in FIG.  4 . The braking mechanism  18  holds the inertia acceptor  8  at the next setting position. After finishing the second development, the developing device assembly  4  rotates to set the next developing device in the developing position. 
     How the developing device assembly  4 , inertia acceptor  8 , setting mechanism  9  and positioning mechanism  10  operate in sequence will be described. The developing device assembly  4  starts to rotate at S 1  when the drive unit  24  starts operating to switch to the second developing device  6 , after the first developing device  6  has been finished the development. The detector  23  of the positioning mechanism  10  of the developing device assembly  4  indicates that the developing device assembly  4  is off the developing position P 1  at S 4 . As indicated by S 5 , the inertia acceptor  8  stops at the setting position P 2  to stop the next developing device  6  of the developing device assembly  4  at the developing position P 1 . The detector  17  of the setting mechanism  9  indicates that the inertia acceptor  8  stays at the setting position P 2  as indicated by S 6 . The braking mechanism  18  holds the inertia acceptor  8  at the setting position P 2  as indicated by S 7 . 
     When the strike portion  12  collides with the stricken portion  13  at S 8  as shown in FIG. 5 after the developing device assembly  4  has rotated, the energy of rotational inertia is shifted from the developing device assembly  4  to the inertia acceptor  8 . At this moment, the detector  23  detects, as indicated by S 9 , that the second developing device  6  has been set in the developing position P 1 . The braking mechanism  18  releases the inertia acceptor  8  at S 10  on the basis of the signal from the detector  23 . The drive unit  24  stops operating at S 11  on the basis of the signal from the detector  23 . 
     The developing device assembly  4 , which has shifted the energy of rotational inertia, stops as indicated by S 12 . On the other hand, the inertia acceptor  8 , which has received the energy of rotational inertia, starts rotating as shown by S 13 . The detector  17  detects that the inertia acceptor  8  has started to rotate as indicated by S 14 . The second developing device  6  starts a developing operation as indicated by S 15  on the basis of the signal from the detector  23 . The controller  26  decelerates the rotating inertia acceptor  8  at a rate that is sufficient but does not vibrate the inertia acceptor  8 , as is indicated by S 16  by controlling the braking mechanism  18  as indicated by S 18  on the basis of the signal from the detector  17 . The controller  26  then stops the acceptor  8  at S 17 . As S 19  indicates, the motor  14  drives the inertia acceptor  8  to the setting position P 3  that make the third developing device  6  stop at the developing position P 1 , during a period between S 20  when the second developing device  6  completes the developing and S 21  when the third developing device  6  turns to the developing position P 1 . The sequence of operation is repeated for each developing device  6 . The position, where the strike portion  12  and the stricken portion  13  collide, moves as the developing device  6  is switched to the next one. 
     After the last developing device  6  has completed the developing operation at S 22 , the developing device assembly  4  rotates to bring the first developing device  6  to the developing position P 1  as indicated by S 23 , in preparation for prepare the next cycle of image forming operation. When the first developing device  6  reaches the developing position P 1 , the developing device assembly  4  collides with the inertia acceptor  8  that is located at the setting position P 5 . The first developing device  6  stops at S 24 . As S 25  indicates, the inertia acceptor  8  is moved, from the setting position P 5  since it has collided with the developing device assembly  4 . The controller  26  may set the inertia acceptor  8 , moved from the setting position P 5 , at the next setting position P 2 . The operation of setting the inertia acceptor  8  at the setting position P 2  as shown by S 26  is performed after the next image forming cycle starts and before the first developing device  6  is switched to the second developing device  6 . As described above, the image forming apparatus  1  has two pair of the strike portion  12  and stricken portion  13 . Nonetheless, the present embodiment may have one pair, three pairs or four pairs of a strike portion  12  and a stricken portion  13 . The energy of rotational inertia may be shifted from the developing device assembly  4  to the inertia acceptor  8  by means of a clutch, a cam or the like, not by collision between the assembly  4  and the acceptor  8 . 
     The time between the stop of the rotation of the developing device assembly  4  and the start of the next developing sequence is shortened, because the energy of rotational inertia is shifted from the developing device assembly  4  to the inertia acceptor  8 . 
     An image forming apparatus according to the second embodiment of the present invention will be described, with reference to FIG.  4  and FIGS. 8 to  11 . The components identical or similar to those of the image forming apparatus  1  according to the first embodiment are designated at the same reference numerals and will not be described. The sequence of operation of the developing device assembly  4 , inertia acceptor  8 , setting mechanism  9  and positioning mechanism  10  of the image forming apparatus according to the second embodiment may be described with reference to FIG. 4, too. FIG. 9 shows the positional relation between the developing device assembly  4  and the inertia acceptor  8 , at K 1  in FIG.  4 . FIG. 10 shows the positional relation between the developing device assembly  4  and the inertia acceptor  8 , at K 2  in FIG.  4 . FIG. 11 shows the positional relation between the developing device assembly  4  and the inertia acceptor  8 , at K 3  in FIG.  4 . 
     The image forming apparatus according to the second embodiment has an inertia acceptor  31  arranged to rotate around axial line D that is parallel to the axis C of the developing device assembly  4 . The strike portion  32  of the developing device assembly  4  and the stricken portion  33  of the inertia acceptor  31  collides with each other at a position. 
     The detector  23  for detecting the developing position P 1  of the developing device assembly  4  is designed to detect the strike portion  32 . The strike portion  32  works as a mark for the detector  23 . Similarly, the detector  17  for detecting a setting position P 6  of the inertia acceptor  31  is designed to detect the stricken portion  33 . The stricken portion  33  works as a mark for the detector  17  for detecting the setting position P 6  of the inertia acceptor  31 . 
     Motor  34  engages with the gear  15  arranged on the inertia acceptor  31  and makes the inertia acceptor  31  rotate, decelerate and stop. The motor  34  also functions as a braking mechanism. The motor  34  operates for a combination of (G) and (H) in FIG.  4 . In the inoperative state S 27  in both (G) and (H) in FIG. 4, the inertia acceptor  8  can freely rotate. 
     All other components are identical to their counterparts of the image forming apparatus according to the first embodiment, they are designated at the same reference numerals and will not be described. 
     In the image forming apparatus described above, the controller  26  controls the developing device assembly  4 , inertia acceptor  31 , setting mechanism  9  and positioning mechanism  10 . However, after the collision as shown by S 28 , the operation of S 26  of setting the inertia acceptor  31  to the setting position P 6  is performed during a period between the start of the next developing sequence and the S 8  when the second developing device  6  reaches the developing position P 1 . 
     When the developing device  6  of the developing device assembly  4  is switched to the next developing device  6 , the energy of rotational inertia is shifted from of the developing device assembly  4  to the inertia acceptor  31 , by causing the strike portion  32  to collide with the stricken portion  33  as shown in FIG.  9 . The developing device assembly  4  stops as shown in FIG. 10, because the energy of rotational inertia has been shifted to the inertia acceptor  31 . As FIG. 10 shows, the inertia acceptor  31  rotates when it receives the energy of rotational inertia from the developing device assembly  4 . 
     While the developing device assembly  4  is rotating further to switch from the developing device  6  to the next, the inertia acceptor  31  is set at the setting position P 6  by the setting mechanism  9  as shown in FIG.  11 . The sequence of operation shown in FIGS. 9 to  11  is repeated as many times as the developing devices  6  provided. After the image forming sequence is finished, the developing device assembly  4  and the inertia acceptor  31  wait, maintaining the positional relation as shown in FIG.  10 . At least a stricken portion  33  may be provided to shift the energy of rotational inertia from the developing device assembly  4  to the inertia acceptor  31  when the strike portion  32  rotates. 
     The image forming apparatus according to the third embodiment of the present invention will be described by referring to FIGS. 12 to  17 . The components that are identical to the counterparts of the image forming apparatus according to the first embodiment are designated at the same reference numerals and will not be described. 
     FIG. 12 illustrates how each component operates with time. In FIG. 12, (K) indicates the operating state of one of the developing devices; ON means the active state, and OFF means the inactive state. In FIG. 12, (L) shows the operating state of the braking mechanism  18 ; ON means the active state, and OFF means the inactive state. In FIG. 12, (M) depicts the operating state of the motor  14  of the setting mechanism  9 : ON means the operating, and OFF means the stop. In FIG. 12, (N) indicates the detecting condition of the detector  17  of the setting mechanism  9 ; ON means that the inertia acceptor  8  is located at the setting position, and OFF means that the inertia acceptor  8  is not located at the setting position. In FIG. 12, (P) indicates the rotational speed of the inertia acceptor  8 . In FIG. 12, (Q) indicates the rotational speed of the developing device assembly  4 . In FIG. 12, (R) shows the operation of the drive unit; ON means operating, and OFF means stop. In FIG. 12, (S) illustrates the detecting condition of the detector  23  of the positioning mechanism  10 ; ON means that the developing device  6  is located at the developing position, and OFF means that the developing device  6  is not located at the developing position. 
     As FIG. 13 shows, the image forming apparatus according to the third embodiment has the first strike portion  35  and the second stricken portion  36  at the inertia acceptor  8 , and further has the second strike portion  37  and the first stricken portion  38  at the developing device assembly  4 . In other words, the developing device assembly  4  and the inertia acceptor  8  have the strike portions  35  and  37 , respectively, and the stricken portions  36  and  38 , respectively. When the first strike portion  35  collides with the first stricken portion  38 , the energy of rotational inertia is shifted from the inertia acceptor  8  to the developing device assembly  4 . When the second strike portion  37  collides with the second stricken portion  36 , the energy of rotational inertia is shifted from the developing device assembly  4  to the inertia acceptor  8 . 
     FIG. 13 shows the positional relation between the developing device assembly  4  and the inertia acceptor  8 , at V 1  in FIG.  12 . At V 1 , both the developing device assembly  4  and the inertia acceptor  8  wait. FIG. 14 shows the positional relation between the developing device assembly  4  and the inertia acceptor  8 , at V 2  in FIG.  12 . At V 2 , the first strike portion  35  of the inertia acceptor  8  collides with the first stricken portion  38  of the developing device assembly  4 . FIG. 15 shows the positional relation between the developing device assembly  4  and the inertia acceptor  8 , at V 3  in FIG.  12 . At V 3 , the inertia acceptor  8  is held at the setting position P 7 , stopping the developing device assembly  4  to set the second developing device  6  at the developing position. After the first developing operation is completed, the developing device assembly  4  is rotated to set the second developing device to the developing position. FIG. 16 shows the positional relation between the developing device assembly  4  and the inertia acceptor  8 , at V 4  in FIG.  12 . At V 4 , the second strike portion  37  of the developing device assembly  4  collides with the second stricken portion  36  of the inertia acceptor  8 . FIG. 17 shows the positional relation between the developing device assembly  4  and the inertia acceptor  8 , at V 5  in FIG.  12 . At V 5 , the developing device assembly  4  is held at the developing position after shifting the energy of rotational inertia to the inertia acceptor  8 . The inertia acceptor  8 , which has received the energy of rotational inertia from the developing device assembly  4 , rotates. 
     In the image forming apparatus described in above, the controller  26  controls the operation sequence as shown in FIG.  12 . When the first developing device  6  of the developing device assembly  4  starts a developing operation at T 1 , the developing device assembly  4  and the inertia acceptor  8  are located as shown in FIG.  13 . The controller  26  releases the braking mechanism  18  at T 2  and simultaneously starts to drive the motor  14  at T 3  to rotate the inertia acceptor  8  at T 4  that precedes T 5 , when the first developing device  6  completes its developing operation. The inertia acceptor  8  rotates to shift the energy of rotational inertia to the developing device assembly  4  after T 5  when the first developing device  6  finishes the developing operation. 
     The inertia acceptor  8  collides with the developing device assembly  4 , exactly at the next setting position P 7  as shown in FIG.  14 . The inertia acceptor  8  shifts the energy of rotational inertia to the developing device assembly  4  at T 6  and stops at the same time. The controller  26  makes the motor  14  stop immediately at T 8  before inertia acceptor  8  collides with the developing device assembly  4 , as soon as the detector  17  detects that the inertia acceptor  8  rotates to the setting position P 7  at T 9  just before the inertia acceptor  8  collides with the developing device assembly  4 . The braking mechanism  18  is started to operate at T 7 , after the inertia acceptor  8  has collided with the developing device assembly  4 . The developing device assembly  4  starts to rotate at T 10  when it has stricken, because the first strike portion  35  and the first stricken portion  38  have shifted the energy of rotational inertia from the inertia acceptor  8  to the developing device assembly  4 . The controller  26  starts to operate the drive unit  24  at T 11  after the signal from the detector  23  is no longer detected at T 12 . 
     The developing device assembly  4  strikes the inertia acceptor  8 , exactly when the next developing device  6  sets to the developing position P 1  at T 13  as a result of its rotary movement as shown in FIG.  16 . At T 14  the detector  23  detects that the developing device assembly  4  is located at the developing position P 1 . The controller  26  stops the drive unit  24  at T 15 , immediately before the developing device assembly  4  strikes the inertia acceptor  8 . At T 16  the controller  26  releases the braking mechanism  18  that has been holding the inertia acceptor  8 , on the basis of the signal from the detector  23 . The second strike portion  37  and the second stricken portion  36  shifts the energy of rotational inertia from the developing device assembly  4  to the inertia acceptor  8  when the developing device assembly  4  has stricken the inertia acceptor  8 . As a result, the developing device assembly  4  that has shifted the energy of rotational inertia to the inertia acceptor  8  stops at T 17 , or at the time of collision. The inertia acceptor  8  that has received the energy of rotational inertia from the developing device assembly  4  starts to rotate at T 18 , or at the time of collision. The developing operation of the second developing device  6  starts at T 19  on the basis of the signals from the detector  23  and detector  17 . The controller  26  starts to control the motor  14  at T 20  when the detector  17  ceases to output signals at T 21 . The controller  26  makes the inertia acceptor  8  rotate toward to the next setting position P 8 . After the developing device  6  has finished the developing operation at T 22 , the inertia acceptor  8  is rotated to reach the setting position P 8  shown in FIG.  17 . Then, the inertia acceptor  8  strikes again the developing device assembly  4  at the setting position P 8 . The first strike portion  35  and the first stricken portion  38  therefore shift the energy of rotational inertia to the developing device assembly  4 . 
     The image forming apparatus according to the third embodiment sequentially shifts the energy of rotational inertia between the developing device assembly  4  and the inertia acceptor  8 . Therefore, the image forming apparatus saves a power loss and is, therefore, economical. Additionally, all time spent for forming a multi-color image can be shortened since the loss of time caused by acceleration at the beginning of rotation and deceleration at the stopping of rotation is decreased. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.