Patent Publication Number: US-8532532-B2

Title: Image forming apparatus having a controllable drive coupling

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 12/415,822, filed Mar. 31, 2009, which claims priority to Japanese Patent Application No. 2009-053711, filed Mar. 6, 2009, and Japanese Patent Application No. 2008-095125, filed Apr. 1, 2008, each of which are hereby incorporated by reference herein in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an image forming apparatus using an electrophotographic recording method, such as a laser printer, a copying machine, a facsimile machine, or the like. 
     2. Description of the Related Art 
     Conventionally, image forming apparatuses using an electrophotographic method include a plurality of photosensitive drums that contribute to enhancing the speed of image forming. Various types of methods have been proposed for sequentially transferring toner images having different colors onto a recording material conveyed by an intermediate transfer belt or a conveying belt. 
     In such an image forming apparatus, a cartridge including a photosensitive drum or an intermediate transfer unit including an intermediate transfer belt is in a removable configuration. A driving force of a motor that is a drive unit of the main body of the image forming apparatus is transmitted to the photosensitive drum or a drive roller that is a drive member for the intermediate transfer belt or the conveying belt via a coupling unit. The coupling unit includes a first coupling provided on the side of the main body and a second coupling provided on the side of the photosensitive drum or the drive roller. The first coupling and the second coupling are configured such that a disconnected state as well as a connected state can be assumed. 
     When the first coupling and the second coupling are connected, the surface speed of the photosensitive drum or the belt is typically proportional to the speed of the motor. However, if the motor is started when the state of the couplings is changing from the disconnected state to the connected state, the photosensitive drum or the belt does not rotate (i.e., the surface speed is zero) even though the motor is activated. If this state continues, the surface of the photosensitive drum or the belt may be damaged, which may lead to image quality issues. 
     In these circumstances, Japanese Patent Application Laid-Open No. 2002-182537 discusses a method in which a low motor speed, compared to the speed that is used when an image is formed, is used until one of the couplings, which is used for transmitting a driving force generated by a motor to a photosensitive drum, is connected to the other coupling. In other words, the motor speed will be low during the connection time of the couplings. This time is based on the couplings that take the longest time in the connection. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, an image forming apparatus for forming an image on a recording medium includes a photosensitive member, a motor, a belt configured to contact the photosensitive member, a drive member configured to rotate the belt and including a driven coupling, a drive coupling configured to rotate by a driving force generated by the motor, the drive coupling being movable in an axial direction such that the drive coupling can take an engagement position, at which the driving force is transmittable to the driven coupling while in engagement with the driven coupling, and a release position, at which the engagement with the driven coupling is released, a transfer member configured to transfer a toner image formed on the photosensitive member onto the recording medium carried on the belt or onto the belt, a voltage application unit configured to apply a voltage to the transfer member, and a control unit configured to execute control for causing the voltage application unit to apply the voltage to the transfer member after the drive coupling moves to the engagement position by drive of the motor and the driving force generated by the motor is transmitted to the driven coupling. 
     According to another aspect of the present invention, an image forming apparatus for forming an image on a recording medium includes a photosensitive member, a charging member configured to charge the photosensitive member, a motor, a belt configured to contact the photosensitive member, a drive member configured to rotate the belt and including a driven coupling, a drive coupling configured to rotate by a driving force generated by the motor, the drive coupling being movable in an axial direction such that the drive coupling can take an engagement position, at which the driving force is transmittable to the driven coupling while in engagement with the driven coupling, and a release position, at which the engagement with the driven coupling is released, a voltage application unit configured to apply a voltage to the charging member, and a control unit configured to execute control for causing the voltage application unit to apply the voltage to the charging member such that a portion charged by the charging member on the photosensitive member comes to a contact position where the portion contacts the belt after the drive coupling moves to the engagement position by drive of the motor and the driving force generated by the motor is transmitted to the driven coupling. 
     According to yet another aspect of the present invention, an image forming apparatus for forming an image on a recording medium includes a motor, a main-body coupling configured to rotate by a driving force generated by the motor, a process cartridge removable from a main body of the image forming apparatus and including a photosensitive drum and a drum coupling configured to transmit the driving force to the photosensitive drum by engaging with the main-body coupling at a predetermined phase angle when the process cartridge is mounted into the main body, a belt configured to contact the photosensitive member, a drive member configured to rotate the belt and including a driven coupling, a drive coupling configured to rotate by the driving force generated by the motor, the drive coupling being movable in an axial direction such that the drive coupling can take an engagement position, at which the driving force is transmittable to the driven coupling while in engagement with the driven coupling at a phase angle larger than the predetermined phase angle, and a release position, at which the engagement with the driven coupling is released, a transfer member configured to transfer a toner image formed on the photosensitive member onto the recording medium carried on the belt or onto the belt, a voltage application unit configured to apply a voltage to the transfer member, and a control unit configured to execute control for causing the voltage application unit to apply the voltage to the transfer member after the drive coupling moves to the engagement position by drive of the motor and the driving force generated by the motor is transmitted to the driven coupling. 
     According to yet another aspect of the present invention, an image forming apparatus for forming an image on a recording medium includes a motor, a main-body coupling configured to rotate by a driving force generated by the motor, a process cartridge removable from a main body of the image forming apparatus and including a photosensitive drum, a charging member configured to charge the photosensitive drum, and a drum coupling configured to transmit the driving force to the photosensitive drum by engaging with the main-body coupling at a predetermined phase angle when the process cartridge is mounted into the main body, a belt configured to contact the photosensitive member, a drive member configured to rotate the belt and including a driven coupling, a drive coupling configured to rotate by the driving force generated by the motor, the drive coupling being movable in an axial direction such that the drive coupling can take an engagement position, at which the driving force is transmittable to the driven coupling while in engagement with the driven coupling at a phase angle larger than the predetermined phase angle, and a release position, at which the engagement with the driven coupling is released, a voltage application unit configured to apply a voltage to the charging member, and a control unit configured to execute control for causing the voltage application unit to apply the voltage to the charging member such that a portion charged by the charging member on the photosensitive member comes to a contact position where the portion contacts the belt after the drive coupling moves to the engagement position by drive of the motor and the driving force generated by the motor is transmitted to the driven coupling. 
     Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  illustrates an example of a configuration of an image forming apparatus according to a first exemplary embodiment of the present invention. 
         FIG. 2  illustrates an example of a drive transmission system of photosensitive drums and an intermediate transfer belt used in the first exemplary embodiment of the present invention. 
         FIG. 3  illustrates an example of a drive transmission system of a photosensitive drum according to the first exemplary embodiment of the present invention. 
         FIG. 4  illustrates an example of a drive transmission system of the intermediate transfer belt according to the first exemplary embodiment of the present invention. 
         FIG. 5  illustrates an example of coupling members of a drive roller according to the first exemplary embodiment of the present invention. 
         FIGS. 6A and 6B  illustrate an example of voltage application timing according to the first exemplary embodiment of the present invention. 
         FIG. 7  illustrates an example of the image forming apparatus according to the first exemplary embodiment of the present invention. 
         FIG. 8  illustrates an example of a drive transmission system of photosensitive drums and an intermediate transfer belt used in a second exemplary embodiment of the present invention. 
         FIG. 9  illustrates an example of a drive transmission system of a photosensitive drum according to the second exemplary embodiment of the present invention. 
         FIG. 10  illustrates an example of a drive transmission system of the intermediate transfer belt according to the second exemplary embodiment of the present invention. 
         FIG. 11  illustrates an example of coupling members of a drive roller according to the second exemplary embodiment of the present invention. 
         FIGS. 12A and 12B  illustrate an example of voltage application timing according to the second exemplary embodiment of the present invention. 
         FIG. 13  illustrates an example of an image forming apparatus according to another exemplary embodiment of the present invention. 
         FIG. 14  illustrates an example of coupling members of the drive roller according to the first exemplary embodiment of the present invention. 
         FIG. 15  illustrates an example of coupling members of the drive roller according to the first exemplary embodiment of the present invention. 
         FIG. 16  illustrates an example of coupling members of the drive roller according to the first exemplary embodiment of the present invention. 
         FIG. 17  illustrates an example of a block diagram of a control unit according to the first exemplary embodiment of the present invention. 
         FIG. 18  illustrates an example of coupling members of a drum according to the second exemplary embodiment of the present invention. 
         FIG. 19  illustrates an example of coupling members of the drum according to the second exemplary embodiment of the present invention. 
         FIG. 20  illustrates an example of coupling members of the drum according to the second exemplary embodiment of the present invention. 
         FIG. 21  illustrates an example of coupling members of the drum according to the second exemplary embodiment of the present invention. 
         FIGS. 22A ,  22 B, and  22 C illustrate an example of coupling members of the drive roller according to the second exemplary embodiment of the present invention. 
         FIGS. 23A ,  23 B, and  23 C illustrate an example of coupling members of the drive roller according to the second exemplary embodiment of the present invention. 
         FIG. 24  illustrates an example of a block diagram of a control unit according to the second exemplary embodiment of the present invention. 
         FIGS. 25A and 25B  illustrate an example of voltage application timing according to the first exemplary embodiment of the present invention. 
         FIGS. 26A and 26B  illustrate an example of voltage application timing according to the second exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings. 
       FIG. 1  illustrates an example of a configuration of an image forming apparatus according to a first exemplary embodiment of the present invention. 
     According to this embodiment, an image forming apparatus  100  includes four process cartridges  9   a ,  9   b ,  9   c , and  9   d , which are removable from an apparatus main body  100 A. The process cartridges  9   a  to  9   d  are used for forming images of yellow (Y), magenta (M), cyan (C), and black (K), respectively. 
     The process cartridge  9   a  to  9   d  is an all-in-one cartridge including an organic photosensitive (OPC) drum  1   a  to  1   d  as a photosensitive member, a charging roller  2   a  to  2   d  as a charging member, a cleaning unit  3   a  to  3   d  used for removing residual toner on the photosensitive drum  1   a  to  1   d , and a developing unit  8   a  to  8   d . The developing unit  8   a  to  8   d  includes a developing sleeve  4   a  to  4   d , nonmagnetic one-component developer (toner)  5   a  to  5   d , and a developer blade  7   a  to  7   d . The process cartridges  9   b  to  9   d  have a similar configuration as the process cartridge  9   a  except that they include developers  5   b  to  5   d  having respective different colors. 
     Exposure units  11   a  to  11   d  are provided above the process cartridges  9   a  to  9   d . Each of the exposure units  11   a  to  11   d  includes a scanner unit configured to direct laser beams onto a polygonal mirror for scanning or a light emitting diode (LED) array. Scanning beams  12   a  to  12   d , which are modulated by image signals, are directed on the photosensitive drums  1   a  to  1   d , respectively. 
     Further, an intermediate transfer belt  13 , which contacts all of the four photosensitive drums  1   a  to  1   d , is arranged under the process cartridges  9   a  to  9   d . The intermediate transfer belt  13  is stretched and supported by a secondary transfer counter roller  24 , a drive roller  14  as a drive member, and a tension roller  15 . These three rollers provide appropriate tension to the intermediate transfer belt  13 . According to drive of the drive roller  14 , the intermediate transfer belt  13  moves in the direction indicated by the arrow B, which is the same as the movement direction indicated by the arrow A of the photosensitive drums  1   a  to  1   d , at a speed approximately the same as that of the drive roller  14 . 
     According to the present exemplary embodiment, the intermediate transfer belt  13  is a 100 micrometer-thick polyvinylidene fluoride (PVDF) belt having a volume resistivity of 10 10  ohm cm. The drive roller  14  as the stretching member is a 20-mm diameter roller with an aluminum cored bar coated with a 1.0 mm-thick layer of ethylene propylene diene M-class (EPDM) rubber which has an electric resistance of 10 4  ohms and in which carbon is distributed as an electroconductive agent. The tension roller  15  as the stretching member is a 20-mm diameter aluminum metal bar and provides a tension of 19.6 N for each one side and 39.2 N in total. The secondary transfer counter roller  24  as the stretching member is a 20-mm diameter roller with an aluminum cored bar coated with a 1.5 mm-thick layer of EPDM rubber which has an electric resistance of 10 4  ohms and in which carbon is distributed as an electroconductive agent. 
     Further, primary transfer rollers  10   a  to  10   d  as primary transfer members are arranged opposite the photosensitive drums  1   a - 1   d  with the intermediate transfer belt  13  in between. According to the present exemplary embodiment, each of the primary transfer rollers  10   a  to  10   d  is a roller having an outside diameter of 14 mm and includes a 6-mm-diameter nickel plated steel bar coated with a 4-mm thick elastic layer of nitrile butadiene rubber (NBR) foam sponge having an electric resistance of 10 7  ohms. 
     Further, the apparatus main body  100 A includes a charge bias power supply unit  20   a  to  20   d  as a voltage supply unit for supplying a bias voltage to the charging roller  2   a  to  2   d , a developing bias power supply unit  21   a  to  21   d  as a voltage supply unit for supplying a bias voltage to the developing sleeve  4   a  to  4   d , and a primary transfer bias power supply unit  22   a  to  22   d  as a voltage supply unit for supplying a bias voltage to the primary transfer roller  10   a . The intermediate transfer belt  13 , the drive roller  14 , the tension roller  15 , the secondary transfer counter roller  24 , the primary transfer rollers  10   a - 10   d , and a toner charge unit  27  constitute an all-in-one belt unit  103 . The belt unit  103  is removably mounted in the apparatus main body  100 A. 
     Next, an example of an image forming operation will be described. When the image forming operation is started, the photosensitive drums  1   a  to  1   d  and the intermediate transfer belt  13  start to rotate in the direction indicated by the arrows A and B at a predetermined process speed. By the power supplied by the charge bias power supply unit  20   a , the charging roller  2   a  charges the photosensitive drum  1   a  to a uniform negative polarity at its surface. Then, an electrostatic latent image corresponding to image information is formed on the photosensitive drum  1   a  by the scanning beam  12   a  emitted from the exposure unit  11   a . According to the present exemplary embodiment, a voltage of −1000 V is applied to the charging roller  2   a  by the charge bias power supply unit  20   a  so that the surface of the photosensitive drum  1   a  is electrically charged to −500 V. 
     The toner  5   a  in the developing unit  8   a  is coated onto the developing sleeve  4   a . The toner  5   a  on the developing sleeve  4  is regulated to have a predetermined thickness by the developer blade  7   a . A predetermined bias voltage is supplied to the developing sleeve  4   a  from the developing bias power supply unit  21   a . When the electrostatic latent image formed on the photosensitive drum  1   a  reaches the developing sleeve  4   a  by the rotation of the photosensitive drum  1   a , the electrostatic latent image is made visible with the negatively chargeable toner. Accordingly, a toner image of the first color (yellow (Y), in this embodiment) is formed on the photosensitive drum  1   a . Since the configuration of the process cartridges  9   b ,  9   c , and  9   d  is similar to that of the process cartridge  9   a , their description will be omitted. 
     An electrostatic image is formed on each of the photosensitive drums  1   a  to  1   d  according to exposure by the exposure units  11   a  to  11   d  while a controller as a control unit outputs a writing signal which is delayed depending on each primary transfer position of the corresponding color. Then, by each of the developing units  8   a  to  8   d , an electrostatic latent image is developed into a toner image. Further, a bias voltage of a polarity opposite to a charge polarity of the toner is applied to the primary transfer rollers  10   a  to  10   d  by the primary transfer bias power supply units  22   a  to  22   d , respectively. Through the above-described processes, toner images can be successively transferred onto the intermediate transfer belt  13 , and a multiple superimposed image can be formed on the intermediate transfer belt  13 . 
     Subsequently, in synchronization with the image formation, a transfer material P stacked in a transfer material cassette  16  is picked up by a feeding roller  17  and conveyed to registration rollers  18 . Then, the transfer material P is further conveyed to an abutment portion which is formed between the intermediate transfer belt  13  and a secondary transfer roller  25  in synchronization with the toner image formed on the intermediate transfer belt  13 . Subsequently, a bias voltage opposite in polarity to the toner is applied to the secondary transfer roller  25  by a secondary transfer bias power supply unit  26 , so that the four-color superimposed images carried on the intermediate transfer belt  13  are simultaneously secondary-transferred onto the transfer material P. According to this exemplary embodiment, the secondary transfer roller  25  is a roller having an outside diameter of 18 mm and includes a 8-mm-diameter nickel plated steel bar coated with a 5-mm thick elastic layer of NBR foam (sponge) having an electric resistance of 10 8  ohms. 
     On the other hand, after the secondary transfer is completed, residual toner remaining on the intermediate transfer belt  13  is positively charged by the toner charge unit  27  which abuts the intermediate transfer belt  13 . Then, by the primary transfer process in which a bias voltage of positive polarity is applied to each of the primary transfer members  10   a  to  10   d , the residual toner is transferred onto the photosensitive drums  1   a  to  1   d  from the surface of the intermediate transfer belt  13  and collected by the cleaning units  3   a  to  3   d.    
     According to the image forming apparatus of the present exemplary embodiment, a roller member including a 6-mm-diameter nickel plated steel bar  27   a  coated with a foam sponge layer  27   b  of EPDM rubber in which carbon black is distributed, and with a protective layer  27   c  made of water-soluble nylon as a surface layer, is used as the toner charge unit  27 . Further, the toner charge unit  27  is connected to a toner charge bias power supply unit  28 . In order to positively charge the untransferred residual toner, a direct voltage of 1 kV superposed on an alternating voltage of 2.5 kVpp is applied to the toner charge unit  27  by the toner charge bias power supply unit  28 . 
     The transfer material P onto which the secondary-transfer-completed toner image is transferred is conveyed to a fixing unit  19 . After the toner image is fixed by the fixing unit  19 , the transfer material P is discharged to a discharging portion  112 . 
     As illustrated in the example shown in  FIG. 2 , a driving force is transmitted from motors  30   a  to  30   d , which are independent of one another, to the photosensitive drums  1   a  to  1   d  having respective colors via reduction gears  34   a  to  34   d . Further, a driving force is transmitted to the drive roller  14 , which allows the intermediate transfer belt  13  to rotate via a reduction gear group  40  and also via a driven coupling  43  arranged on one end of the drive roller  14 . The intermediate transfer belt  13  contacts each surface of the photosensitive drums  1   a - 1   d . Each of the photosensitive drums  1   a - 1   d  rotates in the same direction at approximately the same speed. 
     Now, the transmission of driving force to the photosensitive drum  1   a  in the process cartridge  9   a  will be described referring to the example shown in  FIG. 3 . According to the example as shown, a driving force of a motor  30   a  is reduced by a reduction gear group  34   a  and transmitted to the photosensitive drum  1   a  via a gear  36   a  provided on a drive shaft  35   a . The reduction gear group  34   a  includes gears  34   a   1  to  34   a   4  and a shaft  34   a   5  that provide a predetermined speed reduction ratio to the reduction gear group  34   a . A gear  37   a , which is provided at one end of the photosensitive drum  1   a  in the longitudinal direction, meshes with the gear  36   a . The photosensitive drum  1   a  starts rotating when the gear  37   a  receives the driving force from the gear  36   a . The gear  37   a  connected to the photosensitive drum  1   a  as well as the gear  36   a  connected to the drive shaft  35   a  may use a spur gear. When the process cartridge  9   a  is mounted in the apparatus main body  100 A, the gear  37   a  meshes with the gear  36   a . The insertion direction is the axial direction of the photosensitive drum  1   a . Each of the photosensitive drums  1   b - 1   d  may also perform the drive transmission according to a similar configuration. 
     Next, the transmission of driving force to the intermediate transfer belt  13  will be described referring to the example shown in  FIG. 4 . A driving force of a motor  31  is reduced by a reduction gear group  40  and transmitted to the drive roller  14 , which makes the intermediate transfer belt  13  rotate, via a drive coupling  42  provided on a drive shaft  41 . The reduction gear group  40  includes gears  40   a  to  40   d  and a shaft  40   e  that provide a predetermined speed reduction ratio to the reduction gear group  40 . The driven coupling  43 , which is fixed on one end of a shaft  14   a  of the drive roller  14  in the longitudinal direction, is connected to the drive coupling  42 . The drive roller  14  starts rotating when the driven coupling  43  receives the driving force from the drive coupling  42 . 
       FIG. 5  illustrates an example of a configuration of the drive coupling  42  and the driven coupling  43 . The driven coupling  43  is provided at one end of the drive roller  14  and includes a triangular recessed portion  43   a  on its side. The recessed portion  43   a  is twisted (i.e., recessed) in the axial direction. Further, the drive coupling  42  provided on the drive shaft  41  includes a triangular raised portion  42   a  on its side. The raised portion  42   a  is also twisted (i.e. raised) in the axial direction. Thus, when the raised portion  42   a  fits into the recessed portion  43   a  and the drive coupling  42  starts rotating, the driving force is transmitted to the driven coupling  43  and, at the same time, a force that draws the couplings to each other is generated. It is to be noted that, as an example of an alternative configuration, the drive coupling can have a triangular recessed portion and the driven coupling can have a raised portion. In other words, the shapes of the couplings are not limited as long as one coupling has a triangular protrusion section and the other has a triangular hole section, into which the triangular projection is fittable. 
     Further, as illustrated in the examples shown in  FIGS. 14 and 15 , the drive coupling  42  may be configured to be movable in the axial direction of the drive shaft  41  in synchronization with the open/close operation of an open/close door  102 . As illustrated in the example shown in  FIG. 7 , the open/close door  102  is movably provided on the apparatus main body  100 A in such a manner that it can take either a closed state when an opening  101  (see, e.g.,  FIG. 1 ) provided on the apparatus main body  100 A is closed, or an open state when the opening  101  is open. The opening  101  may be used, for example, when any of the process cartridges  9   a  to  9   d  is removed, the belt unit  103  including the intermediate transfer belt  13  is replaced, or jammed paper is removed. 
     As illustrated in the example shown in  FIG. 14 , when the open/close door  102  is closed, a force is applied to the drive coupling  42  in the direction indicated by the arrow C by a spring  44  provided between a main body frame  100 F and the drive coupling  42 . The drive coupling  42  and the driven coupling  43  may thus be connected in such a manner that the driving force of the motor  31  can be transmitted to the driven coupling  43 . Further, as illustrated in the example shown in  FIG. 15 , when the open/close door  102  is opened, a flange portion  42   c  is pressed, and accordingly a release member  45  moves in the direction indicated by the arrow D. According to this movement, the drive coupling  42  moves to a release position where the connection with the driven coupling  43  is released. The connection portion of the drive coupling  42  and the driven coupling  43  is “D-shaped”. Although, the drive coupling  42  is movable in the axial direction of the drive shaft  41 , the rotation of the drive shaft  41  is transmitted to the drive coupling  42 . 
     When the open/close door  102  is closed from an open state, a force is applied to the drive coupling  42  by the spring  44  in the direction indicated by the arrow E. However, since the recessed portion  43   a  and the raised portion  42   a  of this embodiment are triangular in section as illustrated in the example shown in  FIG. 5 , if phase angles of the raised portion  42   a  and the recessed portion  43   a  do not match, then, as illustrated in the example shown in  FIG. 16 , an end face  42   b  of the drive coupling  42  will contact an end face  43   b  of the driven coupling  43 , and the couplings will be in an abutting position. When the drive coupling  42  rotates to a maximum angle of 120 degrees, phase angles of the raised portion  42   a  and the recessed portion  43   a  match, and the raised portion  42   a  fits into the recessed portion  43   a . In this way, the drive coupling  42  and the driven coupling  43  will be in an engagement position. 
     As illustrated in the block diagram shown in the example of  FIG. 17 , a controller  104  as a control unit according to the present exemplary embodiment is electrically connected to the charge bias power supply units  20   a  to  20   d , the primary transfer bias power supply units  22   a  to  22   d , the secondary transfer bias power supply unit  26 , the toner charge bias power supply unit  28 , the motors  30   a  to  30   d  and  31 , and a sensor  105  configured to detect whether the open/close door  102  is opened. 
     According to the image forming apparatus  100  of the present exemplary embodiment, since the resistance value of each of the intermediate transfer belt  13 , the transfer rollers  10   a  to  10   d , and the charging roller  2   a  varies depending on the environment, a preparation operation may be performed before the image forming operation. The preparation operation is an operation by which a bias voltage that is to be applied is corrected. This preparation operation may be performed when the open/close door  102  is opened or closed, or when the power of the image forming apparatus  100  is turned on. 
     However, since the movement of the drive coupling  42  is in synchronization with the movement of the open/close door  102  as described above, if the open/close door  102  is opened, the drive coupling  42  moves to the release position. On the other hand, if the open/close door  102  is closed, the drive coupling  42  also moves, but will be at the abutting position if it is not in phase with the driven coupling  43 . 
     In this state, if the driving forces of the motors  30   a  to  30   d  and  31  are transmitted to the photosensitive drums  1   a  to  1   d  before they are transmitted to the intermediate transfer belt  13  in a preparation stage, then the photosensitive drum  1   a  will rotate while the intermediate transfer belt  13  is not rotating. In this case, if a bias voltage is applied to the charging roller  2   a , a potential difference is generated between the surface of the photosensitive drum  1   a  and the intermediate transfer belt  13 , which may then cause the photosensitive drum  1   a  to electrostatically attract the intermediate transfer belt  13 . Accordingly, the photosensitive drum  1   a  may make the intermediate transfer belt  13  follow its movement in the same direction. Thus, the intermediate transfer belt  13  may rotate. This rotation may also cause the drive roller  14 , to which the driven coupling  43  is arranged, to rotate. As a result, the state where the drive coupling  42  is not in phase with the driven coupling  43  (i.e., the unconnected state of the drive coupling  42  and the driven coupling  43 ) may continue. Thus, even if the image forming operation of the image forming apparatus  100  is started, the intermediate transfer belt  13  may rotate while being attracted to the photosensitive drum  1   a , and the toner image will be formed on the photosensitive drum  1   a  in such a state. 
     When the toner image reaches a nip portion of the intermediate transfer belt  13  and the photosensitive drum  1   a , the attracting force between the photosensitive drum  1   a  and the intermediate transfer belt  13  is reduced by the influence of the toner image. Thus, the rotation of the intermediate transfer belt  13  due to the electrostatic attraction between the photosensitive drum  1   a  and the intermediate transfer belt  13  may be stopped, but the transfer of the toner image onto the intermediate transfer belt  13  from the photosensitive drum  1   a  is continued. Then, the phase angle of the drive coupling  42  which is rotating according to the driving force generated by the motor  31  may match the phase angle of the driven coupling  43  which is not rotating. Subsequently, the couplings  42  and  43  are connected and move to the engagement position. Then, the drive roller  14  starts rotating as it receives the driving force of the motor  31 . In such a state, the toner image is superposed on the intermediate transfer belt  13  and transferred, and thereby a defective image may be generated. 
     Further, a similar state may occur if the driving forces of the motors  30   a  to  30   d  and  31  are transmitted to the photosensitive drums  1   a  to  1   d  before they are transmitted to the intermediate transfer belt  13  in the preparation stage, and a bias voltage is applied to the charging roller  2   a . That is, if the surface of the photosensitive drum  1   a  is charged, a potential difference may be generated between the surface of the photosensitive drum  1   a  and the intermediate transfer belt  13 , which may then cause the photosensitive drum  1   a  to electrostatically attract the intermediate transfer belt  13 . Accordingly, the photosensitive drum  1   a  may make the intermediate transfer belt  13  follow its movement in the same direction. Thus, the intermediate transfer belt  13  may rotate. This rotation can also cause the drive roller  14 , to which the driven coupling  43  is arranged, to rotate. As a result, the state where the drive coupling  42  is not in phase with the driven coupling  43  (i.e., the unconnected state of the drive coupling  42  and the driven coupling  43 ) may continue. Thus, the toner image may be formed on the photosensitive drum  1   a  while the intermediate transfer belt  13  rotates while being attracted to the photosensitive drum. 
     When the toner image reaches a nip portion of the intermediate transfer belt  13  and the photosensitive drum  1   a , the attracting force between the photosensitive drum  1   a  and the intermediate transfer belt  13  may be reduced by the influence of the toner image. Thus, the rotation of the intermediate transfer belt  13  due to the electrostatic attraction between the photosensitive drum  1   a  and the intermediate transfer belt  13  may be stopped, but the transfer of the toner image onto the intermediate transfer belt  13  from the photosensitive drum  1   a  may be continued. Then, the phase angle of the drive coupling  42  which is rotating according to the driving force generated by the motor  31  may be brought to match the phase angle of the driven coupling  43  which is not rotating. Subsequently, the couplings  42  and  43  are connected and move to the engagement position. Then, the drive roller  14  starts rotating as it receives the driving force of the motor  31 . In such a state, the toner image may be superposed on the intermediate transfer belt  13  and transferred, and thereby a defective image may be generated. 
     Thus, according to the present exemplary embodiment, a bias voltage is not applied to the primary transfer rollers  10   a  to  10   d  during the connection time, which is the time for the connection of the drive coupling  42  and the driven coupling  43 , after the motors  30   a  to  30   d  and  31  are started. That is, the controller  104  drives the motor  31  so that the drive coupling  42  is moved to the engagement position, and after a driving force of the motor  31  is transmitted to the driven coupling  43 , the controller  104  controls each of the primary transfer bias power supply units  22   a  to  22   d  as a voltage application unit so that the voltage is applied to each of the transfer rollers  10   a  to  10   d.    
     Details of the control will now be described referring to the examples shown in  FIGS. 6A and 6B . If the power supply of the image forming apparatus  100  is turned on, or if the open/close door  102  is closed from an open state, the controller  104  performs the preparation operation for receiving a print signal for starting the image forming operation of the image forming apparatus  100 . For example, the resistance value of each of the intermediate transfer belt  13 , the transfer rollers  10   a  to  10   d , and the charging rollers  2   a  to  2   d  may vary depending on a use environment of the image forming apparatus  100 . Thus, the optimum bias voltage to be applied to the transfer rollers  10   a  to  10   d  and the charging rollers  2   a  to  2   d  may be determined in this preparation operation. It is to be noted that “the state in which the power supply of the image forming apparatus  100  is turned on” is a state where the charge bias power supply units  20   a  to  20   d , the primary transfer bias power supply units  22   a  to  22   d , the secondary transfer bias power supply unit  26 , and the toner charge bias power supply unit  28  are electrically connected to a commercial power source. Further, the open/close door  102  may be typically opened, for example, when any of the process cartridges  9   a - 9   d  is removed, the belt unit  103  including the intermediate transfer belt  13  is replaced, or a paper jam is cleared. 
     The controller  104  outputs signals for starting the motors  30   a  to  30   d  and  31   a  time Tm after the power is turned on or the open/close door  102  is closed. Further the controller  104  outputs signals for starting application of voltage to the primary transfer bias power supply units  22   a  to  22   d  to start applying voltages to the primary transfer rollers  10   a  to  10   d  a time Td after the signals for starting the motors  30   a  to  30   d  and  31  are output. Here, the time Td is longer than a maximum connection time Tc which is a maximum time for connecting the drive coupling  42  to the driven coupling  43 . According to the present exemplary embodiment, the maximum connection time Tc is a time for drive coupling  42  to rotate approximately 120 degrees at the maximum, since the recessed portion  43   a  and the raised portion  42   a  are triangular. If the process speed Vps (mm/sec) is defined using the middle point of the thickness of the intermediate transfer belt  13 , since the outer diameter of the drive roller  14  of the present exemplary embodiment is 20 mm, T equals to or greater than 1000×120/360×(20π+50/1000)/Vps (msec). Further, the motors  30   a  to  30   d  and  31  are stopped and the voltage application to the primary transfer rollers  10   a  to  10   d  is stopped at a time Ts after the application of the voltage to the primary transfer rollers  10   a  to  10   d  is started. Then, the controller  104  may enter into a print signal waiting state and wait until it receives the print signal used for image forming. 
     According to the configuration of the present exemplary embodiment, when the process cartridge  9   a  is mounted in the apparatus main body  100 A, the gear  37   a  which is provided at one end of the photosensitive drum  1   a  meshes with the gear  36   a  on the side of the main body. As described above, in some cases, however, the end face  42   b  of the drive coupling  42  illustrated in the example shown in  FIG. 16  contacts the end face  43   b  of the driven coupling  43  provided on the drive roller  14 , and thus the couplings are not connected when the open/close door  102  is closed from an open state. In this case, the photosensitive drum  1   a  will start rotating before the intermediate transfer belt  13 . Thus, the supply of a bias voltage to the primary transfer roller  10   a  by the primary transfer bias power supply unit  22   a  will be started after the motors  30   a  and  31  are started. In this way, the intermediate transfer belt  13  can be prevented from being attracted to the photosensitive drum  1   a  due to the electrostatic attraction force that is generated by the electric potential difference between the surface of the photosensitive drum  1   a  and the intermediate transfer belt  13 . Since the intermediate transfer belt  13  is not attracted and the drive roller  14  is not affected by the rotation of the intermediate transfer belt  13 , the drive coupling  42  is connected to the driven coupling  43  before the drive coupling  42  rotates more than 120 degrees at the maximum. 
     When the above-described preparation operation is finished, the driven coupling  43  and the drive coupling  42  are generally relatively securely connected. This may help improve throughput of the image forming operation. Further, since the image forming operation is started based on the securely-connected couplings, the generation of a defective image due to poor connection of the couplings may be prevented. 
     Since the voltage application to the primary transfer roller  10   a  has been described above, now, timing of voltage application to the charging roller  2   a  will be described referring to the examples shown in  FIGS. 25A and 25B . If voltage is applied to the charging roller  2   a  at the same time the signals used for starting the motors  30   a  and  31  are output, the portion of the photosensitive drum  1   a  that has been charged by the charging roller  2   a  may reach a position where the photosensitive drum  1   a  contacts the intermediate transfer belt  13  at a predetermined time Te earlier. At this time, by the electrostatic attraction force that is generated by the electric potential difference between the photosensitive drum  1   a  and the intermediate transfer belt  13 , the intermediate transfer belt  13  may be attracted to the photosensitive drum  1   a . In order to prevent this from occurring, the voltage application to the charging roller  2   a  will be started after the signals used for starting the motors  30   a  and  31  are output. In this way, the intermediate transfer belt  13  may be prevented from being attracted to the photosensitive drum  1   a . That is, voltage application to the primary transfer roller  10   a  by the primary transfer bias power supply unit  22   a  will not be started during the connection time of the driven coupling  43  and the drive coupling  42  after the motors  30   a  and  31  are started. 
     However, if a voltage is applied to the charging roller  2   a  by the charge bias power supply unit  20   a , the portion of the photosensitive drum  1   a  that has been charged by the charging roller  2   a  takes the time Te to reach the position where the photosensitive drum  1   a  contacts the intermediate transfer belt  13 . This means that timing Tf, which is the time the voltage application to the charging roller  2   a  is started after the signals for starting the motors  30   a  and  31  are output, may be set so that it comes after the time obtained by subtracting the time Te from the time Tc, which is a maximum connection time for the driven coupling  43  and the drive coupling  42 , has passed after the start of the motors. Further the controller  104  may control the charge bias power supply unit  20   a  to start applying a voltage to the charge roller  2   a  so that the portion of the photosensitive drum  1   a  charged by the charge roller  2   a  does not reach the position where the portion contacts the intermediate transfer belt  13  until the driving force is transmitted to the driven coupling  43  after the motors  30   a  and  31  are started. In other words, the controller  104  may drive the motor  31  so that the drive coupling  42  is moved to the engagement position, and may control the charge bias power supply unit  20   a  to apply a voltage to the charging roller  2   a  so that a portion of the photosensitive drum  1   a  charged by the charging roller  2   a  reaches the position where the portion contacts the intermediate transfer belt  13  after the driving force is transmitted to the driven coupling  43  by the driving force of the motor  31 . 
     The above-described control may be performed when the power is turned on, for example since the open/close door  102  can be opened and closed while the power is turned off. According to the present exemplary embodiment, a voltage is applied to at least one of the charging roller  2  and the primary transfer roller  10  at a certain time after the signals for starting the motors  30  and  31  are output. However, voltage can be also be applied to both the charging roller  2  and the primary transfer roller  10  a certain time after the signals for starting the motors  30  and  31  are output. 
     Next, a second exemplary embodiment of the present invention will be described. 
     According to the present exemplary embodiment, components similar to those in the first exemplary embodiment are denoted by the same reference numerals and their description is omitted for simplification. 
     According to the first exemplary embodiment, independent motors  30   a  to  30   d  are provided for the photosensitive drums  1   a  to  1   d  for CMYK colors, respectively, and the motor  31  is provided for the drive roller  14  of the intermediate transfer belt  13 . 
     As illustrated in the example shown in  FIG. 8 , according to the present exemplary embodiment, the photosensitive drums  1   a  to  1   d  and the drive roller  14  are driven by a common motor  70 . Driving force is transmitted to the photosensitive drums  1   a  to  1   d  from the motor  70  via reduction gear groups  71   a  to  71   d . Further, a driving force is transmitted to the drive roller  14  of the intermediate transfer belt  13  from the motor  70  via reduction gear groups  72 . The intermediate transfer belt  13  contacts the surfaces of the photosensitive drums  1   a  to  1   d , each of which rotates in the same direction at approximately the same speed. 
     Next, the transmission of driving force to the photosensitive drum  1   a  of the process cartridge  9   a  will be described in detail referring to the example shown in  FIG. 9 . The driving force of the motor  70  is transmitted to a drum coupling  82 , which is provided at one end of a shaft  1   a   1  of the photosensitive drum  1   a , via a reduction gear group  71  and a main-body coupling  81 , and thus transmitted to the photosensitive drum  1   a . A driving force of the motor  70  is reduced by the reduction gear group  71  and transmitted to the drum coupling  82  and further to the photosensitive drum  1   a  via the main-body coupling  81  provided on an end of a drive shaft  80 . The reduction gear group  71  includes gears  71   a  to  71   d  and a shaft  71   e , and is configured to have a predetermined speed reduction ratio. The drum coupling  82 , which is fixed on one end of the photosensitive drum  1   a  in the longitudinal direction, is connected to the main-body coupling  81 . The photosensitive drum  1   a  starts rotating when the drum coupling  82  receives the driving force from the main-body coupling  81 . 
       FIG. 18  illustrates an example of the configuration of the main-body coupling  81  and the drum coupling  82 . The main-body coupling  81  is provided at one end of the drive shaft  80  and includes a triangular raised portion  81   a  on its side. The raised portion  81   a  is twisted (i.e. raised) in the axial direction. Further, the drum coupling  82  provided on the photosensitive drum  1   a  includes a triangular recessed portion  82   a  on its side. The recessed portion  82   a  is also twisted (i.e., recessed) in the axial direction. Thus, when the raised portion  81   a  fits into the recessed portion  82   a  and the main-body coupling  81  starts rotating, the driving force is transmitted to the drum coupling  82  and, at the same time, a force that draws the couplings to each other is generated. Further, driving forces are transmitted according to a similar configuration of each of the photosensitive drums  1   b  to  1   d . Alternatively, the main-body coupling can have a triangular recessed portion and the drum coupling can have a raised portion. In other words, the shapes of the couplings are not limited so long as one coupling has a triangular protrusion section and the other has a triangular hole section, into which the triangular projection is fittable. 
     Further, as illustrated in the examples shown in  FIGS. 19 and 20 , the main-body coupling  81  may be configured to be movable in the axial direction of the drive shaft  80  in synchronization with the open/close operation of the open/close door  102 . As illustrated in the example shown in  FIG. 19 , when the open/close door  102  is closed, a force is applied to the main-body coupling  81  in the direction indicated by the arrow F by a spring  144  provided between a main body frame  200 F and the main-body coupling  81 . The main-body coupling  81  and the drum coupling  82  may thus be connected in such a manner that the driving force of the motor  70  can be transmitted to the drum coupling  82 . Further, as illustrated in the example shown in  FIG. 20 , when the open/close door  102  is opened, a flange portion  81   c  is pressed, and accordingly a release member  145  moves in the direction indicated by the arrow G. According to this movement, the main-body coupling  81  moves to a release position where the fitting with the drum coupling  82  is released. The connection portion of the drive shaft  80  and the main-body coupling  81  is “D-shaped”. Although, the main-body coupling  81  may be movable in the axial direction of the drive shaft  80 , and the rotation of the drive shaft  80  may be transmitted to the main-body coupling  81 . 
     When the open/close door  102  is closed from an open state, a force is applied to the main-body coupling  81  in the direction of the drum coupling  82  by the spring  144 . However, since the raised portion  81   a  and the recessed portion  82   a  are triangular in section as illustrated in the example shown in  FIG. 18 , if phase angles of the recessed portion  82   a  and the raised portion  81   a  do not match, then, as illustrated in the example shown in  FIG. 21 , an end face  81   b  of the main-body coupling  81  will contact an end face  82   b  of the drum coupling  82 . Thus, the main-body coupling  81  and the drum coupling  82  will be in an abutting position. When the main-body coupling  81  rotates at a maximum angle of 120 degrees, phase angles of the raised portion  81   a  and the recessed portion  82   a  can be made to match, and the raised portion  81   a  may fit into the recessed portion  82   a . In this way, the main-body coupling  81  is connected to the drum coupling  82 . The main-body coupling  81  and the drum coupling  82  will be in the engagement position. 
     Further, as illustrated in the example shown in  FIG. 10 , a driving force of the motor  70  is transmitted to a driven coupling  92 , which is provided at one end of the drive roller  14  that drives the intermediate transfer belt, via a reduction gear group  72  and a drive coupling  91 . In this way, the driving force is transmitted to the drive roller  14 . The reduction gear group  72  includes gears  72   a  to  72   d  and a shaft  72   e  that provide a predetermined speed reduction ratio to the reduction gear group  72 . 
       FIG. 11  illustrates examples of configurations of the drive coupling  91  and the driven coupling  92 . The drive coupling  91  is provided at one end of the drive shaft  80  and includes a triangular raised portion  91   a  on its side. Further, the driven coupling  92  provided at one end of the drive roller  14  includes a triangular recessed portion  92   a  on its side. 
     Further, the raised portion  91   a  of the drive coupling  91  includes a protrusion  91   c  formed on its side so that the drive coupling  91  and the driven coupling  92  are connected at a phase angle of 360 degrees. Further, the driven coupling  92  includes a notch  92   b . The protrusion  91   c  fits into the notch  92   b . When the drive coupling  91  and the driven coupling  92  are connected, the driving force is transmitted to the driven coupling  92  by the rotation of the drive coupling  91 . 
     Next, referring to  FIGS. 22A to 22C  and  FIGS. 23A  to  23 C, examples of detailed configurations of the drive coupling  91  will be described.  FIG. 22A  is a sectional view taken along the longitudinal direction of the drive coupling  91  and the driven coupling  92  in a connected state.  FIG. 22B  is a sectional view taken along line Sa-Sa in  FIG. 22A . Further,  FIG. 22C  is a sectional view taken along line Sb-Sb in  FIG. 22A . 
     The example of the drive coupling  91  as shown includes an intermediate part  91   e  and a cap  91   g . The intermediate part  91   e  is fixed to the drive shaft  81  by a pin  84 . A force is applied to the cap  91   g  by a spring  91   f  against the intermediate part  91   e  in the axial direction. A raised portion  91   b  is provided on one end of the cap  91   g . A flange  91   d  is provided on the other end. The flange  91   b  is connected to a release member  146  described below. Further, as illustrated in the example shown in  FIG. 22C , when the raised portion  91   a  fits into the recessed portion  92   a , the rotation of the drive shaft  81  may be transmitted to the drive roller  14  as the intermediate part  91   e  is engaged with ribs  91   g   1  and  91   g   2  in the cap  91   g . Further, a leaf spring  91   g   3  is provided in the cap  91   g . The leaf spring  91   g   3  may apply force to the intermediate part  91   e  in a direction to move the intermediate part  91   e  away from the ribs  91   g   1  and  91   g   2 . 
     Next, an example of a state where the open/close door  102  is opened and the connection of the drive coupling  91  and the driven coupling  92  is released will be described.  FIG. 23A  is a sectional view of an example of the drive coupling  91  and the driven coupling  92  in a released state taken along the longitudinal direction.  FIG. 23B  is a sectional view taken along line Sc-Sc in  FIG. 23A . Further,  FIG. 23C  is a sectional view taken along line Sd-Sd in  FIG. 23A . As illustrated in  FIG. 23A , when the open/close door  102  is opened, a flange portion  91   d  is pressed, and accordingly the release member  146  moves in the direction indicated by the arrow J despite the force applied by the spring  91   f . According to this movement, the drive coupling  91  moves to a position where the drive coupling  91  is disconnected from the driven coupling  92 . At this time, as illustrated in  FIG. 23B , by the leaf spring  91   g   3  provided in the cap  91   g , the cap  91   g  rotates in a counterclockwise direction until a rib  91   g   4  in the cap  91   g  contacts the intermediate part  91   e . As illustrated in the example shown in  FIG. 23C , the rotation angle of the cap  91   g  may be such that the protrusion  91   b  does not fit in the notch  92   b . The configuration according to the present exemplary embodiment is designed such that when the open/close door  102  is opened, the connection of the main-body coupling  81  to the drum coupling  82 , as well as the connection of the drive coupling  91  to the driven coupling  92 , is released. Further, the configuration is mechanically designed such that when the open/close door  102  is closed, the drive coupling  91  is connected to the driven coupling  92  after the main-body coupling  81  is connected to the drum coupling  82 . 
     With this configuration, the photosensitive drum  1   a  may rotate before the rotation of the intermediate transfer belt  13  is started without exception. According to the present exemplary embodiment, although the photosensitive drum  1   a  slides over the intermediate transfer belt  13  which is in a stop state, the damage of the photosensitive drum  1   a  will be smaller compared to when the intermediate transfer belt  13  slides over the photosensitive drum  1   a  which is in a stop state. This is because, if the intermediate transfer belt  13  slides over the photosensitive drum  1   a  in a stopped state, a particular portion of the photosensitive drum  1   a  will intensively receive friction contact. However, if the photosensitive drum  1   a  slides over the intermediate transfer belt  13  in a stop state, the whole circumference of the photosensitive drum  1   a  will receive the friction, and thus the damage will be smaller. 
     According to the present exemplary embodiment, the photosensitive drum  1   a  will rotate before the intermediate transfer belt  13  starts rotating as is with the first exemplary embodiment. 
     As illustrated in the block diagram shown in the example of  FIG. 24 , a controller  204  as a control unit according to the present exemplary embodiment is electrically connected to the charge bias power supply units  20   a  to  20   d , the primary transfer bias power supply units  22   a  to  22   d , the secondary transfer bias power supply unit  26 , the toner charge bias power supply unit  28 , the motor  70 , and the sensor  105  configured to detect whether the open/close door  102  is opened. Thus, according to the present exemplary embodiment, bias voltage is not applied to the primary transfer rollers  10   a  to  10   d  during the connection time, which is the time for the connection of the drive coupling  91  and the driven coupling  92 , after the motor  70  is started. That is, the controller  204  drives the motor  70  so that the drive coupling  91  is moved to the engagement position, and after a driving force of the motor  70  is transmitted to the driven coupling  92 , the controller  204  controls each of the primary transfer bias power supply units  22   a  to  22   d  as a voltage application unit so that the voltage is applied to each of the transfer rollers  10   a  to  10   d.    
     Details of the control will now be described referring to the examples shown in  FIGS. 12A and 12B . If the power supply of the image forming apparatus  100  is turned on, or if the open/close door  102  is closed from an open state, the controller  204  performs the preparation operation for receiving a print signal for starting the image forming operation of the image forming apparatus  100 . For example, the resistance value of each of the intermediate transfer belt  13 , the transfer rollers  10   a  to  10   d , the charging rollers  2   a  to  2   d  may vary depending on an environment in which the image forming apparatus  100  is used. Thus, an optimum bias voltage to be applied to the transfer rollers  10   a  to  10   d  and the charging rollers  2   a  to  2   d  may be determined in this preparation operation. It is to be noted that “the state in which the power supply of the image forming apparatus  100  is turned on” is a state where the charge bias power supply units  20   a  to  20   d , the primary transfer bias power supply units  22   a  to  22   d , the secondary transfer bias power supply unit  26 , and the toner charge bias power supply unit  28  are electrically connected to a commercial power source. Further, the open/close door  102  may typically be opened, for example, when any of the process cartridges  9   a  to  9   d  is removed, the belt unit  103  including the intermediate transfer belt  13  is replaced, or paper jam is cleared. 
     According to this example, the controller  204  outputs a signal for starting the motor  70  a time Tm after the power is turned on or the open/close door  102  is closed. Further the controller  104  outputs signals for starting application of voltage to the primary transfer bias power supply units  22   a  to  22   d  to start applying voltages to the primary transfer rollers  10   a  to  10   d  a time Td after the signal for starting the motor  70  is output. Here, the time Td comes after a maximum connection time Tc, which is a maximum time for connecting the drive coupling  91  to the driven coupling  92 , has passed. 
     According to the present exemplary embodiment, the maximum connection time Tc is the time for the drive coupling  91  to rotate approximately 360 degrees at the maximum, since the raised portion  81   a  fits into the recessed portion  82   a  at a phase angle of 360 degrees. If the process speed Vps (mm/sec) is defined using the middle point of the thickness of the intermediate transfer belt  13 , since the outer diameter of the drive roller  14  of the present exemplary embodiment is 20 mm, T equals to or greater than 1000×360/360×(20π+50/1000)/Vps (msec). Further, the motor  70  is stopped and the voltage application to the primary transfer rollers  10   a  to  10   d  is stopped at a time Ts after the application of the voltage to the primary transfer rollers  10   a  to  10   d  is started. Then, the controller  204  enters into a print signal waiting state and waits until it receives the print signal used for image forming. 
     According to the configuration of the present exemplary embodiment, as illustrated in the examples shown in  FIGS. 12A and 12B , when the open/close door  102  is closed or the power is turned on, a force is applied to the drum coupling  82  provided on one end of each of the photosensitive drums  1   a  to  1   d  from the main-body coupling  81  by the spring  144 . However, as illustrated in  FIG. 18 , since the raised portion  81   a  and the recessed portion  82   a  are triangular in section, if the phase angles do not match, the main-body coupling  81  is not connected to the drum coupling  82 . 
     The main-body coupling  81  rotates 120 degrees at the maximum before it is connected to the drum coupling  82 . On the other hand, the driven coupling  92  provided on the drive roller  14  is not connected to the driven coupling  92  unless the drive coupling  91  rotates approximately 360 degrees if the open/close door  102  is closed or the power is turned on. This means that each of the photosensitive drums  1   a  to  1   d  starts rotating prior to the intermediate transfer belt  13 . Thus, at that time, the timing of voltage application to each of the primary transfer rollers  10   a  to  10   d  will be delayed. In this way, the intermediate transfer belt  13  can be prevented from being attracted to the photosensitive drums  1   a  to  1   d  due to the electrostatic attraction force that is generated by the electric potential difference between the surface of each of the photosensitive drums  1   a  to  1   d  and the intermediate transfer belt  13 . Since the intermediate transfer belt  13  is not attracted, and since the drive roller  14  is not affected by the rotation of the intermediate transfer belt  13 , the drive coupling  91  is connected to the driven coupling  92  before the drive coupling  91  rotates 360 degrees at the maximum. 
     Since the driven coupling  92  and the drive coupling  91  can be relatively securely connected in the above-described preparation operation, throughput of the image forming operation can be improved. Further, since the image forming operation may be started based on the securely-connected couplings, the generation of defective images due to poor connection of the couplings may be prevented. 
     Since the voltage application to the primary transfer roller  10   a  from the primary transfer bias power supply unit  22   a  has been described above, now, timing of voltage application to the charging roller  2   a  will be described referring to the examples shown in  FIGS. 26A and 26B . If a voltage is applied to the charging roller  2   a  at the same time the signal used for starting the motor  70  is output, the portion of the photosensitive drum  1   a  that has been charged by the charging roller  2   a  reaches a position where the photosensitive drum  1   a  contacts the intermediate transfer belt  13  at a predetermined time Te earlier. At this time, by the electrostatic attraction force that is generated by the electric potential difference between the photosensitive drum  1   a  and the intermediate transfer belt  13 , the intermediate transfer belt  13  may be attracted to the photosensitive drum  1   a . In order to prevent this from occurring, the voltage application to the charging roller  2   a  may be started after the signal used for starting the motor  70  is output. In this way, the intermediate transfer belt  13  can be prevented from being attracted to the photosensitive drum  1   a.    
     That is, according to this example, voltage application to the primary transfer roller  10   a  by the primary transfer bias power supply unit  22   a  may not be started until the driven coupling  92  and the drive coupling  91  are connected after the motor  70  is started. However, if a voltage is applied to the charging roller  2   a  by the charge bias power supply unit  20   a , the portion of the photosensitive drum  1   a  that has been charged by the charging roller  2   a  takes the time Te to reach the position where the photosensitive drum  1   a  contacts the intermediate transfer belt  13 . This means that timing Tf, which is the time the voltage application to the charging roller  2   a  is started after the signals for starting the motor  70  is output, is set so that it comes after the time obtained by subtracting the time Te from the time Tc, which is a maximum connection time of the driven coupling  92  and the drive coupling  91 , has passed after the start of the motors. Further, the controller  204  may control the charge bias power supply unit  20   a  to start applying a voltage to the charge roller  2   a  so that the portion of the photosensitive drum  1   a  charged by the charge roller  2   a  does not reach the position where the portion contacts the intermediate transfer belt  13  until the driving force is transmitted to the driven coupling  92  after the motor  70  is started. In other words, the controller  204  drives the motor  70  so that the drive coupling  91  is moved to the engagement position, and controls the charge bias power supply unit  20   a  to apply a voltage to the charging roller  2   a  so that a portion of the photosensitive drum  1   a  charged by the charging roller  2   a  reaches the position where the portion contacts the intermediate transfer belt  13  after the driving force is transmitted to the driven coupling  92  by the driving force of the motor  70 . 
     It is to be noted that the above-described control may be performed when the power is turned on since the open/close door  102  can be opened and closed while the power is turned off. 
     According to the present exemplary embodiment, a voltage is applied to the charging roller  2  or the primary transfer roller  10  a certain time after the signal for starting the motor  70  is output. However, the voltage can be applied to both the charging roller  2  and the primary transfer roller  10  a certain time after the signal for starting the motor  70  is output. 
     According to the first and the second exemplary embodiments, the image forming apparatus  100  including the intermediate transfer belt  13  to which a toner image on the photosensitive drum  1  is directly transferred to form a superimposed image has been described. 
     According to exemplary embodiments of the invention, an image forming apparatus may be provided having a relatively simple configuration used for enabling fairly secured connection of a coupling provided on a photosensitive member or a drive member for or a belt to a coupling provided on a main body of the image forming apparatus. Aspects according to the present invention may provide an image forming apparatus capable of inhibiting and even preventing defective images due to connection failure of the couplings. 
     According to another exemplary embodiment of the present invention, as illustrated in the example shown in  FIG. 13 , an image forming apparatus  200  includes a conveying belt  110  used for carrying and conveying paper as a recording medium in place of an intermediate transfer belt. According to the present exemplary embodiment, when voltages from power sources  112   a  to  112   d  are applied to primary transfer rollers  11   a  to  111   d , respectively, toner images formed on the respective photosensitive drums  1   a  to  1   d  may be directly multi-layer transferred onto a recording medium conveyed by the conveying belt  110 . Thus, the secondary transfer roller  25 , the secondary transfer counter roller  24 , and the secondary transfer bias power supply unit  26  may not be included in the present exemplary embodiment. Other configurations may be similar to those of the second exemplary embodiment. 
     As describe above, according to an exemplary embodiment of the present invention, a voltage is applied to the transfer member after the drive of the motor is started and the driving force is transmitted to the couplings. Further, a voltage is applied to the charging member so that a portion of the photoreceptor charged by the charging member is moved to the position where that portion contacts the belt after the drive of the motor is started and the driving force is transmitted to the couplings. Accordingly, the belt can be prevented from being attracted to the photosensitive member due to electrostatic attraction force that is generated between the surface of the photosensitive member and the belt. Further, since the driven coupling and the drive coupling can be relatively securely connected, it may be possible to prevent defective images due to poor connection of the couplings. Further, by performing the control in the preparation operation, enhanced throughput considering image forming can be achieved. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures, and functions.