Abstract:
An image carrier is adapted such that an electrostatic latent image is formed thereon. A rotary member has a plurality of loading sections each of which is adapted to accommodate a developing device which is operable to develop the electrostatic latent image. A motor is operable to rotate the rotary member so that each of the loading sections is sequentially opposed to the image carrier, and operable to exert a detent torque when no power is supplied thereto. A home position is defined, in a case where only one of the loading sections accommodates the developing device, such that the one of the loading sections is placed in a position lower than any other loading sections. The motor rotates the rotary member to the home position at least when the development of the electrostatic latent image is not performed. The rotary member is retained in the home position with the detent torque exerted by the motor.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to an image forming apparatus and an image forming system incorporating the same.  
         [0002]     An image forming apparatus, such as a laser beam printer, has already been well known. Such an image forming apparatus comprises, for example, an image carrier for carrying a latent image thereon; developing devices, each of which develops the latent image carried on the image carrier with use of a developing agent; a rotary member having a plurality of loading sections arranged in a circumferential direction thereof and allowing removable loading of the developing devices; and a motor for rotating the rotary member. The image forming apparatus can perform image forming operation with a single developing device loaded in one of the loading sections.  
         [0003]     When an image signal is transmitted from an external apparatus, such as a host computer, to the image forming apparatus, the rotary member situated at a home position, where the rotary member awaits execution of image forming operation, is rotated by driving force of the motor, so that the developing device is placed at the development position opposing the image carrier. The latent image carried on the image carrier is developed with the developing agent stored in the developing device, to thus form a visible image, and the visible image is transferred onto a medium, thereby eventually forming an image. Such an image forming apparatus is disclosed in Japanese Patent Publication No. 2005-24859A, for example.  
         [0004]     When the developing device remains loaded only in one of the plurality of loading sections, the distribution of load (weight) of the rotary member becomes uneven, to thus bring imbalance to the rotary member. Therefore, when the rotary member is situated at the home position, there may arise a case where the rotary member becomes displaced (rotated) under the own weight of the developing device, depending on the position of the one loading section to which the developing device is loaded. In a case where an impact is inflicted on the rotary member when the rotary member is situated at the home position, the rotary member is displaced (rotated) by the impact.  
         [0005]     In order to prevent displacement of the rotary member, the related-art printer is equipped with a stopper which comes in contact with the rotary member situated in the home position. However, in order to cause the stopper to duly exhibit its function, the strength of the stopper must be sufficiently ensured.  
       SUMMARY OF THE INVENTION  
       [0006]     It is therefore an object of the invention to provide an image forming apparatus which can prevent undesirable displacement of a rotary member.  
         [0007]     It is also an object of the invention to provide an image forming system incorporating such an image forming apparatus.  
         [0008]     In order to achieve at least one of the above objects, according to the invention, there is provided an image forming apparatus, comprising:  
         [0009]     an image carrier, adapted such that an electrostatic latent image is formed thereon;  
         [0010]     a rotary member, having a plurality of loading sections each of which is adapted to accommodate a developing device which is operable to develop the electrostatic latent image; and  
         [0011]     a motor, operable to rotate the rotary member so that each of the loading sections is sequentially opposed to the image carrier, and operable to exert a detent torque when no power is supplied thereto; wherein:  
         [0012]     a home position is defined, in a case where only one of the loading sections accommodates the developing device, such that the one of the loading sections is placed in a position lower than any other loading sections;  
         [0013]     the motor rotates the rotary member to the home position at least when the development of the electrostatic latent image is not performed; and  
         [0014]     the rotary member is retained in the home position with the detent torque exerted by the motor.  
         [0015]     Alternatively, there is provided an image forming apparatus, comprising:  
         [0016]     an image carrier, adapted such that an electrostatic latent image is formed thereon;  
         [0017]     a rotary member, having a plurality of loading sections each of which is adapted to accommodate a developing device which is operable to develop the electrostatic latent image; and  
         [0018]     a motor, operable to rotate the rotary member so that each of the loading sections is sequentially opposed to the image carrier, and operable to exert a detent torque when no power is supplied thereto; wherein:  
         [0019]     a home position is defined, in a case where only one of the loading sections accommodates the developing device, such that the one of the loading sections is placed in a position higher than any other loading sections;  
         [0020]     the motor rotates the rotary member to the home position at least when the development of the electrostatic latent image is not performed; and  
         [0021]     the rotary member is retained in the home position with the detent torque exerted by the motor.  
         [0022]     In a case where the one of the loading sections is located at the highest or lowest position among the plurality of loading sections when the rotary member is situated at the home position, the rotary member becomes stable. Moreover, in a case where the motor retains the rotary member at the home position with the detent torque, the detent torque acts as retaining force for retaining the rotary member. When these two matters are combined together, the rotary member situated at the home position is retained more stably. Accordingly, an image forming apparatus capable of preventing displacement (rotation) of the rotary member situated at the home position can be provided.  
         [0023]     The motor may be a stepping motor. Since the detent torque of the stepping motor is relatively high, the rotary member situated at the home position is retained more stably.  
         [0024]     The rotary member may be retained in the home position without any locking member. In this case, the number of components can be diminished.  
         [0025]     The motor may rotate the rotary member to the home position when the image forming apparatus is deactivated.  
         [0026]     The motor may rotate the rotary member to a replacement position in which the one of the loading sections equipped with the developing device is placed in a position higher than any other loading sections, when replacement of the developing device is performed. The rotary member may be retained in the replacement position with the detent torque exerted by the motor.  
         [0027]     Alternatively, the motor may rotate the rotary member to a replacement position in which the one of the loading sections equipped with the developing device is placed in a position lower than any other loading sections, when replacement of the developing device is performed. The rotary member may be retained in the replacement position with the detent torque exerted by the motor.  
         [0028]     In any of the cases, since the rotary member situated at the replacement position is retained stably, the operator can smoothly perform the replacement of the developing device.  
         [0029]     In order to achieve at least one of the objects, according to the invention, there is also provided an image forming system, comprising:  
         [0030]     the above image forming apparatus; and  
         [0031]     a computer, communicatively connected to the image forming apparatus and operable to cause the image forming apparatus to execute an image forming operation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]     The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein:  
         [0033]      FIG. 1  is a schematic section view of a printer according to one embodiment of the invention, showing a state that the printer is used as a full-color printer;  
         [0034]      FIG. 2  is a schematic section view of the printer, showing a state that the printer is used as a monochrome printer;  
         [0035]      FIG. 3  is a block diagram of a control unit in the printer;  
         [0036]      FIG. 4  is a perspective view of a black developing device in the printer;  
         [0037]      FIG. 5  is a vertical section view of the black developing device;  
         [0038]      FIG. 6  is a schematic view showing a state that the black developing device is placed in a home position;  
         [0039]      FIG. 7  is a schematic view showing a state that the black developing device is placed in a developing position;  
         [0040]      FIG. 8  is a schematic view showing a state that the black developing device is placed in a replacement position;  
         [0041]      FIG. 9  is a perspective view of a rotary in the printer;  
         [0042]      FIG. 10  is a schematic view showing a state that the black developing device is placed in the home position, according to another example;  
         [0043]      FIG. 11  is a schematic view showing a state that the black developing device is placed in the replacement position, according to another example;  
         [0044]      FIG. 12  is a perspective view of an image forming system incorporating the printer; and  
         [0045]      FIG. 13  is a block diagram of the image forming system.  
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0046]     Embodiments of the invention will be described below in detail with reference to the accompanying drawings.  
         [0047]     As illustrated in  FIGS. 1 and 2 , a printer  10  according to one embodiment of the invention has, along a rotational direction of a photosensitive body  20  serving as an image carrier for carrying a latent image thereon, a charging unit  30 , an exposing unit  40 , a developing device holder  48 , a primary transferring unit  60 , an intermediate transferring member  70 , and a cleaning unit  75 . The printer  10  further has a secondary transferring unit  80 ; a fusing unit  90 ; a display unit  95  provided with a liquid crystal panel or the like; and a control unit  100  which controls these units and manages operation of the printer.  
         [0048]     The photosensitive body  20  has a cylindrical, conductive substrate and a photosensitive layer formed on the circumferential surface thereof. The photosensitive body  20  can rotate about a center axis; in the present embodiment, can rotate clockwise as indicated by an arrow in each of  FIGS. 1 and 2 .  
         [0049]     The charging unit  30  is a unit for charging the photosensitive body  20 . The exposing unit  40  is a unit for irradiating the charged photosensitive body  20  with a laser beam to thereby form a latent image thereon. The exposing unit  40  comprises a semiconductor laser, a polygon mirror, an F-θ lens, and the like, and emits a laser beam which has been modulated in accordance with image information input from an unillustrated external device such as a personal computer, a word processor.  
         [0050]     The developing device holder  49  has a rotary  50 . The rotary  50  is equipped with a plurality of loading sections  50   a,    50   b,    50   c,  and  50   d,  each of which allows removable loading of a developing device for developing a latent image formed on the photosensitive body  20  through use of toner T stored therein.  
         [0051]     When the plurality of loading sections  50   a,    50   b,    50   c,  and  50   d  are respectively equipped with the developing devices, the printer  10  of the present embodiment can be used as a full-color printer which develops a latent image formed on the photosensitive body  20  with use of the toner T stored in the respective developing devices, to thus form a color image. When the developing device is loaded to only one of the plurality of loading sections  50   a,    50   b,    50   c,  and  50   d,  the printer  10  can be used as a monochrome printer which develops the latent image carried on the photosensitive body  20  with the toner T stored in that developing device, to thereby form a monochrome image.  
         [0052]     In order to enable formation of a full-color image at any time, a conventional printer is arranged to not form even a monochrome image unless a plurality of developing devices storing toner of different colors are loaded to the developing device holder. In contrast, the printer  10  of the present embodiment operates also as a monochrome printer which enables formation of a monochrome image on a medium through use of an loaded black developing device  51  even if only the black developing device  51  is loaded to the developing device holder  49 .  
         [0053]     When the printer  10  is used as a full-color printer, four developing devices; namely, the black developing device  51 , a magenta developing device  52 , a cyan developing device  53 , and a yellow developing device  54 , are inserted respectively to the plurality of loading sections  50   a,    50   b,    50   c,  and  50   d  of the rotary  50 , as illustrated in  FIG. 1 . The latent image formed on the photosensitive body  20  is then developed with the toner T stored in the respective developing devices.  
         [0054]     The rotary  50  can move the four developing devices  51 ,  52 ,  53 , and  54  by rotation. More specifically, the four developing devices  51 ,  52 ,  53 , and  54  can rotate around a rotary shaft  50   e  while maintaining their relative positions. Every time formation of an image of one page is completed, the developing device selectively opposes the photosensitive member  20 , and the latent image formed on the photosensitive member  20  is sequentially developed with the toner T stored in the developing devices  51 ,  52 ,  53 , and  54 .  
         [0055]     Meanwhile, when the printer  10  is used as a monochrome printer, the developing device is loaded to only one of the plurality of loading sections of the rotary  50 , as illustrated in  FIG. 2 . The latent image formed on the photosensitive body  20  is developed with the toner T housed in that developing device. Specifically, when the black developing device  51  is loaded to one loading section  50   a  of the four loading sections  50   a,    50   b,    50   c,  and  50   d  to thus form an image, the black developing device  51  moves to the position opposing the photosensitive body  20  as a result of rotation of the rotary  50 . The latent image formed on the photosensitive body  20  is developed with black toner T stored in the black developing device  51 .  
         [0056]     The developing devices and the loading sections, all of which pertain to the present embodiment, are configured so that each of the developing devices can be physically loaded solely to one corresponding loading section.  
         [0057]     Specifically, of the four loading sections  50   a,    50   b,    50   c,  and  50   d,  only the loading section  50   a  enables loading of the black developing device  51 ; only the loading section  50   b  enables loading of a magenta developing device  52 ; only the loading section  50   c  enables loading of a cyan developing device  53 ; and only the loading section  50   d  enables loading of a yellow developing device  54 .  
         [0058]     The developing device holder  49  and the developing devices will be described in detail later.  
         [0059]     The primary transferring unit  60  is a unit for transferring, onto the intermediate transferring unit  70 , a toner image formed on the photosensitive body  20 .  
         [0060]     The intermediate transferring unit  70  is a multilayer endless belt formed by providing a tin-deposited layer on the surface of a PET film, and further forming a semi-conductive coating layer on the surface of the tin-deposited layer. The intermediate transferring member  70  is circulated at a velocity which is substantially the same as a circumferential velocity of the photosensitive body  20 .  
         [0061]     The secondary transferring unit  80  is a unit for transferring onto a medium, such as paper, a film, or cloth, a toner image formed on the intermediate transferring unit  70 .  
         [0062]     The fusing unit  90  is a unit for fusing the toner image transferred onto the medium, thereby rendering an image permanent.  
         [0063]     The cleaning unit  75  is disposed between the primary transferring unit  60  and the charging unit  30 , and has a rubber cleaning blade  76  remaining in contact with the surface of the photosensitive body  20 . The cleaning unit  75  is a unit for, after a toner image has been transferred onto the intermediate transferring unit  70  by the primary transferring unit  60 , scraping off and removing the toner T still remaining on the photosensitive body  20  by the cleaning blade  76 .  
         [0064]     As shown in  FIG. 3 , the control unit  100  has a main controller  101  and a unit controller  102 . An image signal and a control signal are input to the main controller  101 , and the unit controller  102  controls the respective units, and the like, in accordance with the image signal and the control signal, to thus form an image.  
         [0065]     The main controller  101  includes a CPU  111 ; an interface  112  used for establishing connection with an unillustrated computer; an image memory  113  for storing an image signal, and the like, input from the computer; and a main controller memory  114 . The main controller memory  114  comprises an electrically-rewritable EEPROM  114   a,  a RAM  114   b,  a program ROM provided with programs for use in various control operations, and the like.  
         [0066]     The CPU  111  of the main controller  101  manages control of operation for writing or reading the image data, which are input by way of an interface, into or from the image memory  113  as well as control of the entire image forming apparatus in synchronism with a CPU  120  of the unit controller  102  in accordance with a control signal input from the computer.  
         [0067]     The EEPROM  114   a  stores mode data indicating whether the printer  10  is to be used as a full-color printer or a monochrome printer. The CPU  111  receives, at given timing from the unit controller  102 , data pertaining to loading of a developing device (hereinafter called “developing device loading data”) indicating which one of the four loading sections is equipped with the developing device. The CPU  111  rewrites the mode data in the EEPROM  114 a in accordance with the loading data, as required. The mode data have already been written in the EEPROM  114   a  in the form of 1-bit data. When the value of the data is 0, the data indicate a full-color printer. When the value of the data is 1, the data indicate a monochrome printer. When the printer  10  is activated, the main controller  101  detects the power supply and loads the mode data from the EEPROM  114   a  to the RAM  114   b.  Even when the printer  10  is not activated, the mode data are stored in the EEPROM  114   a.    
         [0068]     The unit controller  102  comprises the CPU  120 ; a unit controller memory  116  including an electrically-rewritable EEPROM  116   a,  a RAM, a program ROM provided with programs for use in various control processes, or the like; and various driving circuits for controlling operations of the respective units of the main body of the printer (i.e., a charging unit driving circuit  130 , a exposing unit driving circuit  140 , a holder driving circuit  149 , a primary transferring unit driving circuit  160 , a cleaning unit driving circuit  175 , a secondary transferring unit driving circuit  180 , and a fusing unit driving circuit The CPU  120  of the unit controller  102  is electrically connected to the respective driving circuits, and controls the respective driving circuits in accordance with control signals output from the CPU  111  of the main controller  101 . More specifically, the CPU  120  receives signals output from sensors provided in the respective units, thereby controlling the respective units in accordance with the signals input from the main controller  101  while detecting the statuses of the respective units.  
         [0069]     The CPU  120  also controls the respective driving circuits in accordance with the mode data. Specifically, when the value of the mode data is 0, the CPU  120  controls the respective units of the printer  10  in such a way that the printer functions as a full-color printer. When the value of the apparatus data is 1, the CPU  120  controls the respective units of the printer  10  in such a way that the printer functions as a monochrome printer.  
         [0070]     The EEPROM  116   a  stores the developing device loading data indicating which one of the four loading sections is equipped with the developing device. The CPU  120  determines whether or not a developing device is loaded to the loading section assigned to the removed/loaded developing device after removal/loading of the developing device. On the basis of the result of determination, the loading data in the EEPROM  116   a  are rewritten, as required. The loading data have already been written in the EEPROM  116   a  as 1-bit data; i.e., a total of four bits of data, for the loading sections. When the loading data assume a value of 0, the data show that no developing device is loaded. In contrast, when the loading data assume a value of 1, the data show that the developing device has been loaded.  
         [0071]     The CPU  120  of the unit controller  102  is connected to a nonvolatile main memory  122  such as serial EEPROM, by way of a serial interface  121 . This main memory  122  stores data required for controlling the apparatus. The CPU  120  is connected to memories  51   a,    52   a,    53   a,  and  54   a  (hereinafter called “developing device memories”) provided in the respective developing devices  51 ,  52 ,  53 , and  54  by way of the serial interface  121 , as well as to the main memory  122 . Data can be transferred between the main memory  122  and the developing device memories  51   a,    52   a,    53   a,  and  54   a.  Further, a chip select signal CS can be input to the respective developing device memories  51   a,    52   a,    53   a,  and  54   a  by way of an input/output port  123 . The CPU  120  is connected to a home position detector  31 , as well, by way of the input/output port  123 .  
         [0072]     The rotary  50  can be equipped with the black developing device  51  containing black (K) toner, the magenta developing device  52  containing magenta (M) toner, the cyan developing device  53  containing cyan (C) toner, and the yellow developing device  54  containing yellow (Y) toner. Since the respective developing devices have the same configuration, the configuration of the black developing device  51  will now be described.  
         [0073]     As shown in  FIGS. 4 and 5 , the black developing device  51  comprises the developing roller  510 , a sealing member  520 , a toner storage section  530 , a housing  540 , a toner supplying roller  550 , a control blade  560 , a positioning pin  588 .  
         [0074]     The developing roller  510  carries thereon and transports toner T to the developing position opposing the photosensitive body  20 . This developing roller  510  is made from metal; that is, manufactured from an aluminum alloy such as a 5056 aluminum alloy or a 6063 aluminum alloy, a ferroalloy such as STKM, and the like. The metal roller is plated with nickel or chromium, as required.  
         [0075]     As shown in  FIG. 4 , the developing roller  510  is supported at both longitudinal ends thereof and can rotate around the center axis thereof. As shown in  FIG. 5 , the developing roller  510  rotates in a direction (the counterclockwise direction in this figure) opposite the rotational direction (the clockwise direction in this figure) of the photosensitive body  20 . The center axis of the developing roller  510  is located at a position that is lower than the center axis of the photosensitive body  20 . A gap exists between the developing roller  510  and the photosensitive body  20 , with the black developing device  51  opposing the photosensitive body  20 . In short, the black developing device  51  develops the latent image formed on the photosensitive body  20  in a non-contact manner. When the latent image formed on the photosensitive body  20  is developed, an alternating electric field develops between the developing roller  510  and the photosensitive body  20 .  
         [0076]     The sealing member  520  prevents leakage of the toner T out of the black developing device  51 , and recovers the toner T on the developing roller  510 , which has passed by the developing position, into the developing device without scraping it off the developing roller. The sealing member  520  is a seal which is made of a polyethylene film, or the like. The sealing member  520  is supported by a supporting plate  522 , and is mounted on the housing  540  by way of this supporting plate  522 . A side of the sealing member  520  opposite to the side thereof opposing the development roller  510  is provided with an impelling member  524  made of mortoprain, or the like. The sealing member  520  is pressed against the developing roller  510  by elastic force of the impelling member  524 . The location where the sealing member  520  comes into contact with the developing roller  510  is higher than the center axis of the developing roller  510 .  
         [0077]     The housing  540  is manufactured by welding together a plurality of integrally-molded housing members; that is, an upper housing member  542  and a lower housing member  544 . The inside of the housing  540  is divided into two toner storage sections  530 ; namely, a first toner storage section  530 a and a second toner storage section  530   b,  by a partition  545  which vertically protrudes from an inner wall for partitioning the toner T. An opening  572  is formed in a lower portion of the housing  540 , and the developing roller  510  is arranged in the opening  572 , with a portion of the developing roller  510  being exposed.  
         [0078]     A stirring member for stirring the toner T may be provided in the toner storage section  530 . However, in the present embodiment, the respective developing devices (the black developing device  51 , the magenta developing device  52 , the cyan developing device  53 , and the yellow developing device  54 ) rotate in association with the rotation of the developing unit holding unit  49 . Thus, the toner T in the respective developing devices is stirred, and hence the stirring member is not provided in the toner storage section  530 .  
         [0079]     The toner supplying roller  550  is disposed in the first toner storage section  530   a,  and supplies to the developing roller  510  the toner T stored in the first toner storage section  530   a.  The toner supplying roller  550  is made of polyurethane foam, or the like, and remains in contact with the developing roller  510  while being elastically deformed. The toner supplying roller  550  is provided in a lower portion of the toner storage section  530 . The toner T stored in the toner storage section  530  is supplied to the developing roller  510  by the toner supplying roller  550  at the lower portion of the toner storage section  530 .  
         [0080]     As shown in  FIG. 5 , the toner supplying roller  550  is capable of rotating about the center axis thereof, and the center axis is located in a position that is lower than the center axis of the developing roller  510 . The toner supplying roller  550  rotates in a direction (the clockwise direction in this figure) opposite the rotational direction of the development roller  510  (the counterclockwise direction in this figure). The toner supplying roller  550  has the function of stripping off the toner T, which still remains on the developing roller  510  after developing operation, from the developing roller  510  as well as the function of supplying to the developing roller  510  the toner T stored in the toner storage section  530 .  
         [0081]     The control blade  560  electrically charges the toner T carried by the developing roller  510 , and regulates the thickness of the toner T carried by the developing roller  510 . This control blade  560  has a rubber section  560   a  and a supporting section  560   b.  The rubber section  560   a  is made of a silicone rubber, a urethane rubber, or the like. The supporting section  560   b  is an elastic plate member which is made of phosphor bronze, stainless steel, or the like. The rubber section  560   a  is supported by the supporting section  560   b.  The supporting section  560   b  is fastened to the housing  540  via a supporting plate  562 . In addition, an impelling member  570  made of mortoprain, or the like, is disposed on a side of the control blade  560  opposite the side thereof facing the developing roller  510 .  
         [0082]     The rubber section  560   a  is pressed against the developing roller  510  by elastic force stemming from flexure of the supporting section  560   b.  In addition, the impelling member  570  prevents the toner T from entering between the rubber support section  560   b  and the housing  540  to thereby stabilize the elastic force stemming from flexure of the supporting section  560   b;  and impels the rubber section  560   a  toward the developing roller  510  from the right behind of the rubber section  560   a,  thereby pressing the rubber section  560   a  against the developing roller  510 . Therefore, the impelling member  570  enhances uniform contact of the rubber section  560   a  against the developing roller  510 .  
         [0083]     The side of the control blade  560  opposite the side thereof supported by the supporting plate  562 ; namely, the edge of the control blade  560 , remains out of contact with the developing roller  510 . A portion of the control blade  560 , which is spaced a predetermined distance away from the edge thereof, remains, over a width, in contact with the developing roller  510 . Specifically, the edge of the control blade  560  does not remain in contact with the development roller  510 , but the body of the control blade  560  remains in contact with the developing roller  510 . The control blade  560  is also arranged such that the edge thereof is oriented upstream with respect to the rotational direction of the developing roller  510 . Thus, the control blade  560  remains in a so-called counter contact with the developing roller. The position where the control blade  560  contacts, the developing roller  510  is lower than the center axis of the developing roller  510 , as well as being lower than the center axis of the toner supplying roller  550 .  
         [0084]     The positioning pin  588  is for positioning the black developing device  51  onto the rotary  50  (more specifically, the loading section  50   a ) and fit into a hole  59   b  (described later).  
         [0085]     In the black developing device  51  configured as above, the toner supplying roller  550  supplies the toner T stored in the toner storage section  530  to the developing roller  510 . In association with rotation of the developing roller  510 , the toner T supplied to the developing roller  510  reaches a contact position of the control blade  560 . During the course of passing by the abutting position, the toner T is electrically charged, and subjected to layer thickness regulation. By further rotation of the developing roller  510 , the toner T on the developing roller  510 , which has undergone layer thickness regulation, arrives at the developing position opposing the photosensitive body  20 . In the developing position, the toner T is used for developing the latent image formed on the photosensitive body  20  under the alternating electric field. By still further rotation of the developing roller  510 , the toner T on the developing roller  510 , which has passed by the developing position, passes by the sealing member  520 , and is collected into the developing device without being scraped off by the sealing member  520 . The toner T still remaining on the developing roller  510  can be scraped off by the toner supplying roller  550 .  
         [0086]     As described above, the developing device holder  49  comprises the rotary  50  and a driving motor  401  for rotating the rotary  50 . The rotary  50  is rotated by the driving force of the driving motor  401 , to thus reach a predetermined position. This predetermined position includes a home position, a developing position, and a replacement position.  
         [0087]     As shown in  FIG. 9 , the rotary  50  comprises a rotary shaft  50   e  located in the center of the rotary, a frame  58  for retaining the developing devices, and a circular plate  59  used for rotating the rotary  50 .  
         [0088]     The rotary shaft  50   e  extends horizontally between two frame side plates (not shown) forming an enclosure of the printer  10 , and is supported at both ends thereof. The supporting frame  58  is mounted to the rotary shaft  50   e.  The four loading sections  50   a,    50   b,    50   c,  and  50   d,  which removably retain the above-described developing devices  51 ,  52 ,  53 , and  54  around the rotary shaft  50   e,  are provided on the supporting frame  58  while being spaced apart from each other through 90 degrees in the circumferential direction.  
         [0089]     The circular plate  59  is fastened to one axial end of the rotary shaft  50   e.  Teeth  59   a  are formed in an outer peripheral surface of the circular plate  59 . The teeth  59   a  mesh with a gear  402  (which will be described later), and can rotate in the counterclockwise direction (a direction of Z) in  FIG. 6 . Accordingly, the circular plate  59  has the function of a gear which rotates the rotary  50 . The circular plate  59  also has the positioning recess sections  59   b  into which the positioning pins  588  ( FIG. 4 ) of the respective developing devices fit.  
         [0090]     The driving motor  401  is a stepping motor. As shown in  FIG. 6 , the driving motor  401  has a motor shaft  401   a  provided with a gear  402 . The gear  402  meshes with the teeth  59   a  of the circular plate  59  to thus transmit power to the circular plate  59 . This gear  402  rotates in the clockwise direction shown in  FIG. 6 . The driving motor  401  is further provided with a connector  403  connected to the printer main body  10   a,  and power is supplied from the printer main body  10   a  to the driving motor  401  by way of the connector  403 .  
         [0091]     However, in terms of characteristics, the driving motor  401  has three types of torque (rotation torque, holding torque and detent torque) in accordance with operating conditions.  
         [0092]     The rotation torque is one that develops when the motor shaft  401   a  of the driving motor  401 , which is supplied with power by way of the connector  403 , rotates. The holding torque is one that develops when the motor shaft  401   a  of the driving motor  401  that is supplied with power is controlled so as not to rotate. The detent torque is one that develops when no power is supplied to the driving motor  401 .  
         [0093]     The predetermined positions of the rotary  50 ; that is, the home position, the developing position, and the replacement position, will now be described.  
         [0094]     The home position will first be described by reference to  FIG. 6 . The rotary  50  is situated at the home position when the printer  10  awaits execution of image forming operation. Even when no power is supplied to the printer  10 , the rotary  50  is situated at the home position.  
         [0095]      FIG. 6  shows the home position of the rotary  50  achieved when execution of image forming operation is awaited with the developing device (the black developing device  51 ) being loaded to one loading section; namely, the loading section  50   a,  among the plurality of loading sections  50   a,    50   b,    50   c,  and  50   d  (i.e., the printer  10  is used as a monochrome printer). The home position is a position where the loading section  50   a  is situated in the lowermost position among the plurality of loading sections  50   a,    50   b,    50   c,  and  50   d  with reference to the vertical direction.  
         [0096]     The home position detector  31  for detecting the home position is provided at one axial end of the rotary shaft  50   e  of the rotary  50 . The home position detector  31  comprises a disk, that is fastened to one end of the rotary shaft  50   e  and is to be used for generating a signal and a home position sensor consisting of a photo-interrupter having a light-emitting section and a light-receiving section, or the like.  
         [0097]     A brim portion of the disk is arranged to reach a position between the light-emitting section and the light-receiving section of the home position sensor. When slit sections formed in the disk have moved to the detecting position of the home position sensor, a signal output from the home position sensor changes from “L” to “H.” The home position of the rotary  50  is detected on the basis of the change in the signal level and the number of pulses of the pulse motor. The respective developing devices can be placed in the developing position, or the like, by taking the home position as a reference.  
         [0098]     When the rotary  50  is situated in the home position in the printer  10 , the driving motor  401  is not supplied with power. In such a situation, detent torque develops in the driving motor  401 . Since the gear  402  of the driving motor  401  meshes with the teeth  59   a  provided on the rotary  50 , the detent torque is exerted on the rotary  50 . The printer  10  is configured such that the driving motor  401  retains the rotary  50  in the home position by the detent torque.  
         [0099]     The developing position will now be described by reference to  FIG. 7 . The developing position is one used for placing the developing device loaded to the loading section in the position opposing the photosensitive body  20 . The developing position of the rotary  50  is allocated to each of the developing devices loaded to the four loading sections  50   a,    50   b,    50   c,  and  50   d.  Namely, the developing positions are a black developing position of the rotary  50  acquired when the black developing device  51  loaded to the loading section  50   a  is placed in the developing position; a magenta developing position acquired when the magenta developing device  52  loaded to the loading section  50   b  is placed in the developing position; a cyan developing position when the cyan developing device  53  loaded to the loading section  50   c  is placed in the developing position; and a yellow developing position acquired when the yellow developing device  54  loaded to the loading section  50   d  is placed in the developing position.  
         [0100]      FIG. 7  shows the black developing position of the rotary  50  achieved when the black developing device  51  is situated at the developing position opposing the photosensitive body  20 . When the black developing device  51  has reached the developing position, the developing roller  510  loaded to the black developing device  51  opposes the photosensitive body  20 . As a result, the latent image carried by the photosensitive body  20  can be developed with the black toner T carried by the developing roller  510 .  
         [0101]     When the rotary  50  is situated in the black developing position in the printer  10 , the motor shaft  401   a  of the driving motor  401  supplied with power by way of the connector  403  is controlled so as not to rotate. Therefore, in such a case, holding torque develops in the driving motor  401 .  
         [0102]     The replacement position will now be described by reference to  FIG. 8 . The replacement position is a position for placing the loading section, to or from which the developing device is loaded or removed, in a position corresponding to a replacement opening  10   e.  The replacement position of the rotary  50  is allocated to each of the four loading sections  50   a,    50   b,    50   c,  and  50   d.  Namely, the replacement positions include a black replacement position of the rotary  50  achieved when the loading section  50   a,  to or from which the black developing device  51  is loaded or removed, is placed in the position opposing the replacement opening  10   e;  a magenta replacement position of the rotary  50  achieved when the loading section  50   b,  to or from which the magenta developing device  52  is loaded or removed, is placed in the position opposing the replacement opening  10   e;  a cyan replacement position of the rotary  50  achieved when the loading section  50   c,  to or from which the cyan developing device  53  is loaded or removed, is placed in the position opposing the replacement opening  10   e;  and a yellow replacement position of the rotary  50  achieved when the loading section  50   d,  to or from which the yellow developing device  54  is loaded or removed, is placed in the position opposing the replacement opening  10   e.    
         [0103]      FIG. 8  shows the black replacement position achieved when the loading section  50   a  is situated in the position opposing the replacement opening  10   e.  When the rotary  50  is situated in the black replacement position; namely, when the loading section  50   a  is situated in the position opposing the replacement opening  10   e,  the black developing device  51  is removably loaded to the loading section  50   a  by way of the replacement opening  10   e.  The black replacement position is a position where the loading section  50   a  among the plurality of loading sections  50   a,    50   b,    50   c,  and  50   d  is situated in the highest position with reference to the vertical direction.  
         [0104]     In the printer  10 , when the rotary  50  is situated in the black replacement position, no power is supplied to the driving motor  401 . Therefore, in such a case, detent torque is exerted in the driving motor  401 . The printer  10  is configured such that the driving motor  401  retains the rotary  50  in the black replacement position by the detent torque.  
         [0105]     The printer  10  of the present embodiment does not have any locking member which locks the rotary  50  by contacting the same when the rotary  50  is situated in the home position or the black replacement position. Namely, when the rotary  50  is situated in the home position or the black replacement position, the rotary  50  is retained in the home position or the black replacement position by only the detent torque of the driving motor  401 .  
         [0106]     An example operation of the printer  10  will now be described. A monochrome image forming operation, by which the printer  10  forms a monochrome image when being used as a monochrome printer, will be described hereinbelow. Moreover, there will be provided a description about replacement operation by which the black developing device  51  is loaded to or removed from the loading section  50   a.    
         [0107]     When the operator activates the printer  10 , power is supplied to the printer  10 , and the printer  10  awaits execution of image forming operation. At that time, the rotary  50  is situated in the home position shown in  FIG. 6 . In a state where the printer  10  awaits execution of image forming operation, no power is supplied to the driving motor  401 . Accordingly, detent toque develops in the driving motor  401 . The rotary  50  that awaits execution of image forming operation is retained in the home position by the detent torque.  
         [0108]     When an image signal and a control signal, both of which have been output from an unillustrated computer, are input to the main controller  101  of the printer  10  by way of the interface  112 , the photosensitive body  20 , the rotary  50 , and the intermediate transferring unit  70  rotate under control of the unit controller  102  on the basis of the command from the main controller  101 .  
         [0109]     The photosensitive body  20  is sequentially charged, while being rotated, by the charging unit  30  in the charging position. A charged area of the photosensitive body  20  reaches the exposure position in association with rotation of the photosensitive body  20 , and a latent image corresponding to image data is formed in that area by the exposing unit  40 .  
         [0110]     The rotary  50  situated in the home position shown in  FIG. 6  is rotated by the driving force of the driving motor  401  that is supplied with power from the printer main body by way of the connector  403 , and reaches the black developing position shown in  FIG. 7 . In that case, the black developing device  51  loaded to the loading section  50   a  is situated in the position opposing the photosensitive body  20 . After the rotary  50  has stopped its rotation to the black developing position, the developing roller  510  of the black developing device  51  rotates. When the rotary  50  has reached the black developing position, the motor shaft  401   a  of the driving motor  401  supplied with power is controlled so as not to rotate. Therefore, holding torque is exerted in the driving motor  401 . The rotary  50  is retained in the black developing position by the holding torque.  
         [0111]     The latent image formed on the photosensitive body  20  reaches a developable position in association with rotation of the photosensitive body  20 , and is developed by the developing roller  510  of the black developing device  51 . Thereby, a toner image is formed on the photosensitive body  20 .  
         [0112]     In association with rotation of the photosensitive body  20 , the toner image formed on the photosensitive body  20  reaches a primary transfer position, and is transferred onto the intermediate transferring unit  70  by the primary transferring unit  60 . In this case, a primary transfer voltage, whose polarity is opposite that of the electrical charges on the toner, is applied to the primary transferring unit  60 . During this time, the secondary transferring unit  80  is spaced apart from the intermediate transferring member  70 .  
         [0113]     The toner image formed on the intermediate transferring unit  70  reaches a secondary transfer position in association with rotation of the intermediate transferring unit  70 , and is transferred onto a medium by the secondary transferring unit  80 . The medium is transported to the secondary transferring unit  80  from the sheet feeding tray  92  by way of a sheet feeding roller  94  and registration rollers  96 . When the transfer operation is performed, the secondary transferring unit  80  is pressed against the intermediate transferring unit  70 , and a secondary transfer voltage is applied to the secondary transferring unit  80 .  
         [0114]     The toner image having been transferred onto the medium is heated and pressed by the fusing unit  90  to thus be fused onto the medium.  
         [0115]     Meanwhile, after the photosensitive body  20  has passed by the primary transfer position, the toner T sticking to the surface of the photosensitive body  20  is scraped off by the cleaning blade  76  supported on the cleaning unit  75 , whereby turning the photosensitive body  20  into a standby state for the formation of next latent image. The thus-scraped toner is recovered in a residual-toner recovering section provided in the cleaning unit  75 .  
         [0116]     After the toner image has been formed on the photosensitive body  20 , the rotary  50  situated in the black developing position shown in  FIG. 7  rotates to reach the home position shown in  FIG. 6 . The printer  10  awaits execution of image forming operation. In that case, detent toque is exerted in the driving motor  401 . The rotary  50  is retained in the home position by this detent torque.  
         [0117]     Next, when the operator deactivates the printer  10 , power supply to the printer  10  is stopped. Even when no power is supplied to the printer  10 , the rotary  50  is maintained at the home position shown in  FIG. 6 . The rotary  50  is retained in the home position by detent torque.  
         [0118]     Removal/loading operation of the black developing device  51  will now be described. In the printer  10 , the black developing device  51  is loaded to the loading section  50   a,  and the rotary  50  is situated in the home position shown in  FIG. 6 .  
         [0119]     First, the rotary  50  situated in the home position is rotated by the driving force of the driving motor  401 , to thus reach the black replacement position shown in  FIG. 8 . When the rotary  50  remains positioned in the black replacement position, no power is supplied to the driving motor  401 . Hence, detent torque is exerted in the driving motor  401 . When being positioned in the black replacement position, the rotary  50  is retained at the black replacement position by detent torque of the driving motor  401 .  
         [0120]     Next, the operator removes the black developing device  51  loaded to the loading section  50   a,  by way of the replacement opening  10   e.  A new black developing device  51  is loaded to the loading section  50   a  by the operator by way of the replacement opening  10   e.  After the new developing device  51  has been loaded to the loading section  50   a,  the rotary  50  situated in the black replacement position is rotated by the driving force of the driving motor  401 , to thus reach the home position shown in  FIG. 6 . The printer  10  awaits execution of image forming operation.  
         [0121]     As shown in  FIG. 6 , in a case where the rotary  50  is situated in the home position, since the one loading section  50   a  equipped with the black developing device  51  is located at the lowest position among the plurality of loading sections  50   a,    50   b,    50   c,  and  50   d  with reference to the vertical direction, the rotary  50  becomes stable. Moreover, since the driving motor  401  retains the rotary  50  at the home position with detent torque when the rotary  50  is situated at the home position, the detent torque serves as retention force for retaining the rotary  50 . When the above two matters are combined together, the rotary  50  situated at the home position is retained more stably. Consequently, according to the present embodiment, the printer  10  capable of preventing displacement of the rotary  50  situated at the home position can be realized.  
         [0122]     As shown in  FIG. 8 , when the black developing device  51  is removed from or loaded to the loading section  50   a,  the rotary  50  is located in the black replacement position where the loading section  50   a  is located at the highest position among the plurality of loading sections  50   a,    50   b,    50   c,  and  50   d  with reference to the vertical direction.  
         [0123]     However, as shown in  FIG. 10 , the home position may be a position where the one loading section  50   a  is located at the highest position among the plurality of loading sections  50   a,    50   b,    50   c,  and  50   d  with reference to the vertical direction. Even in such a case, the rotary  50  is retained stably, and hence it is possible to prevent displacement of the rotary  50  situated in the home position.  
         [0124]     In addition, as shown in  FIG. 11 , the black replacement position may be a position where the loading section  50   a  is located at the lowest position among the plurality of loading sections  50   a,    50   b,    50   c,  and  50   d  with reference to the vertical direction.  
         [0125]     The image forming apparatus, and the like, according to the present invention has hitherto been described by reference to the embodiment. However, the above-described embodiment of the invention aims at facilitating understanding of the invention, and should not be construed as limiting the range of the invention. As a matter of course, the invention can be changed and modified without departing from the scope of the invention, and equivalents thereof are included in the invention.  
         [0126]     The above embodiment has been described while an intermediate transferring type full-color laser beam printer is taken as an image forming apparatus. However, the present invention can also be applied to a variety of types of image forming apparatus, such as a full-color laser beam printer of a type other than the intermediate transfer type, a monochrome laser beam printer, a copying machine, or a facsimile.  
         [0127]     In the above embodiment, the photosensitive body serving as an image carrier has been described as having a configuration of a photosensitive layer being provided on an outer circumferential surface of the cylindrical conductive substrate. However, the photosensitive body may be a so-called photosensitive belt having a configuration of a photosensitive layer being provided on the surface of a belt-shaped conductive substrate.  
         [0128]     In the above embodiment, the driving motor  401  is a stepping motor. However, the driving motor  401  may be a DC motor.  
         [0129]     In the above embodiment, the rotary  50  is situated at the home position when no power is supplied to the printer  10 . However, the rotary  50  may be situated at the home position when execution of image forming operation is awaited. When no power is supplied to the printer  10 , the rotary  50  may be situated at a position differing from the home position.  
         [0130]     Next, an image forming system incorporating the above image forming apparatus will be described.  
         [0131]     As shown in  FIG. 12 , an image forming system  700  comprises a computer  702 , a display device  704 , the printer  10 , input devices  708 , and reading devices  710 .  
         [0132]     The computer  702  of the present embodiment is enclosed in a mini-tower-type enclosure; however, the configuration is not limited to this type. A CRT (cathode ray tube), a plasma display, a liquid crystal display device, or the like, is generally employed for the display device  704 ; however, the display device is not limited to them. A keyboard  708 A and a mouse  708 B are employed in the embodiment as the input devices  708 ; however, the input devices are not limited to them. A flexible disk drive device  710 A and a CD-ROM drive device  710 B are employed in the embodiment as the reading devices  710 ; however, the reading devices are not limited to them. For instance, another device, such as an MO (magneto optical) disk drive, or a DVD (digital versatile disk), or the like may also be employed.  
         [0133]     As shown in  FIG. 13 , an internal memory  802  such as a RAM and an external memory such as a hard disk drive unit  804  are further provided in the enclosure where the computer  702  is enclosed.  
         [0134]     The above description has described the example where the printer  10  is connected to the computer  702 , the display device  704 , the input devices  708 , and the reading devices  710 , thereby forming the image forming system. However, the present invention is not limited to this example. For instance, the image forming system may be formed from the computer  702  and the printer  10 , or the image forming system may not include any of the display device  704 , the input devices  708 , and the reading devices  710 .  
         [0135]     Alternatively, e.g., the printer  10  may include portions of functions or mechanisms of the computer  702 , the display device  704 , the input devices  708 , and the reading devices  710 . As an example configuration, the printer  10  may include an image forming section for effecting image processing, a display section for performing a variety of display operations, a recording medium loading section for removably loading a recording medium where image data having been captured by a digital camera, or the like, are recorded, and the like.  
         [0136]     The image forming system realized as described above becomes superior to the related-art system in terms of overall execution.