Patent Publication Number: US-8995881-B2

Title: Image forming apparatus and image carrier including a backward-rotation-suppressing mechanism

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2013-065005 filed Mar. 26, 2013. 
     BACKGROUND 
     The present invention relates to an image forming apparatus and an image carrier. 
     SUMMARY 
     According to an aspect of the invention, an image forming apparatus includes a driving portion configured to generate a driving force, a first magnet configured to rotate when receiving the driving force from the driving portion, a second magnet that faces the first magnet with a gap interposed therebetween and is configured to rotate together with the first magnet while attracting and being attracted by the first magnet with magnetism, a rotating member configured to rotate in a predetermined direction when receiving the driving force from the second magnet, and a backward-rotation-suppressing mechanism configured to suppress rotation of the rotating member in a direction opposite to the predetermined direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  illustrates an overall configuration of an image forming apparatus according to the exemplary embodiment; 
         FIG. 2  is a schematic diagram illustrating a rear-side end of a photoconductor drum according to the exemplary embodiment; 
         FIG. 3  is a sectional view taken along line III-III illustrated in  FIG. 2 ; 
         FIGS. 4A to 4C  are schematic diagrams of a coupling pin; 
         FIGS. 5A and 5B  illustrate configurations of a drum-side magnet and a gear-side magnet; 
         FIGS. 6A and 6B  illustrate a configuration of a link mechanism; 
         FIG. 7  illustrates a state of a photoconductor-drum-driving mechanism realized when a covering is closed; 
         FIG. 8  illustrates a state of the photoconductor-drum-driving mechanism realized when the covering is open; and 
         FIGS. 9A and 9B  are graphs illustrating changes in the movement of a photoconductor drum body that are observed without and with a one-way clutch, respectively. 
     
    
    
     DETAILED DESCRIPTION 
     Configuration of Image Forming Apparatus  1   
     Referring to  FIG. 1 , a configuration of an image forming apparatus  1  according to an exemplary embodiment of the present invention will first be described.  FIG. 1  illustrates an overall configuration of the image forming apparatus  1  according to the exemplary embodiment. 
     The image forming apparatus  1  includes an image forming section  10  that forms a toner image on each piece of paper P, a fixing section  20  that fixes the toner image formed on the piece of paper P by the image forming section  10 , and a paper transport system  30  that supplies each piece of paper P to the image forming section  10 . 
     The image forming apparatus  1  further includes a toner transport section  40  that transports toner to the image forming section  10 , a toner cartridge  50  that is provided in the toner transport section  40  and stores toner to be supplied to the image forming section  10 , and a toner collecting device  60  that collects residual toner (to be described below) on a photoconductor drum  11  provided in the image forming section  10 . 
     The image forming apparatus  1  further includes a controller  100  and a user interface (UI)  200 . The controller  100  controls all operations performed by the image forming section  10 , the fixing section  20 , the paper transport system  30 , the toner transport section  40 , the toner cartridge  50 , and the toner collecting device  60 . The UI  200  includes a display panel, through which the UI  200  receives instructions from the user and displays messages and so forth to the user. The image forming apparatus  1  further includes a housing  70  that supports the above elements. 
     Hereinafter, the near side and the far side of the image forming apparatus  1  illustrated in  FIG. 1  are also referred to as “front side” and “rear side”, respectively. Furthermore, the horizontal direction and the vertical direction of the image forming apparatus  1  illustrated in  FIG. 1  are also simply denoted as “horizontal direction H” and “vertical direction V”, respectively. Furthermore, the direction of the rotational axis of the photoconductor drum  11  (to be described below) included in the image forming apparatus  1  is also simply referred to as “axial direction”. 
     The image forming section  10  includes the photoconductor drum  11 , a charging device  12  that charges the photoconductor drum  11 , an exposure device  13  that performs exposure on the photoconductor drum  11 , a developing device  14  that performs development on the photoconductor drum  11  that has been charged, a transfer device  15  that transfers a toner image formed on the photoconductor drum  11  to a piece of paper P, and a cleaning member  16  that cleans the photoconductor drum  11  after the transfer. 
     The photoconductor drum  11  includes a photosensitive layer (not illustrated) on the outer circumference thereof and rotates in a forward direction (see arrow D0 in  FIG. 1 ). The photoconductor drum  11  is detachably attached to the housing  70 . The attaching and detaching of the photoconductor drum  11  are performed through an open portion (not illustrated) that appears when a covering  71  included in the housing  70  is opened. 
     The housing  70  includes urging portions (not illustrated) such as springs that urge the photoconductor drum  11  in the horizontal direction H and the vertical direction V, and pressed portions (not illustrated) provided inside the housing  70  and against which the photoconductor drum  11  urged by the urging portions is pressed. The urging portions and the pressed portions in combination determine the position of the photoconductor drum  11  in the horizontal direction H and the vertical direction V. Hence, even if a gear-side magnet  125  or a drum-side magnet  117  (to be described below) is attached to a deflected position, vibrations that may occur in the photoconductor drum  11  because of the deflection are suppressed. The positioning of the photoconductor drum  11  in the axial direction will be described separately below. 
     The charging device  12  includes a charging roller provided in contact with the photoconductor drum  11  and charges the photoconductor drum  11  to a predetermined potential. 
     The exposure device  13  applies a laser beam to the photoconductor drum  11  so as to selectively perform exposure on the photoconductor drum  11  that has been charged by the charging device  12 , whereby the exposure device  13  forms an electrostatic latent image on the photoconductor drum  11 . 
     The developing device  14  stores two-component developer containing, for example, toner that is negatively charged and a carrier that is positively charged. The developing device  14  develops, with the toner, the electrostatic latent image formed on the photoconductor drum  11  with the aid of a developing roller  14 A, thereby forming a toner image on the photoconductor drum  11 . 
     The transfer device  15  includes a roller member and transfers the toner image on the photoconductor drum  11  to a piece of paper P by producing an electric field at a position (transfer part Tp) between the transfer device  15  and the photoconductor drum  11 . 
     The cleaning member  16  is a plate-like member made of an elastic material such as thermosetting urethane rubber and having a predetermined thickness. The cleaning member  16  extends in the axial direction and is in contact with the surface of the photoconductor drum  11 . The cleaning member  16  removes toner and so forth (hereinafter referred to as residual toner) remaining on the photoconductor drum  11  after the transfer of the toner image. 
     In the exemplary embodiment, the cleaning member  16  is provided on the downstream side with respect to the transfer device  15  in the direction of rotation of the photoconductor drum  11  and is in contact with the surface of the photoconductor drum  11  along the tangent line to the photoconductor drum  11 . 
     The fixing section  20  includes a pressure roller and a heat roller (both not illustrated). The piece of paper P having the toner image transferred thereto is made to pass through the nip between the rollers, whereby the fixing section  20  fixes the toner image on the piece of paper P through a fixing process using heat and pressure. 
     The paper transport system  30  includes a paper storing portion  31  that stores plural pieces of paper P, a paper transport path  32  along which each piece of paper P is transported and that extends from the paper storing portion  31  through the transfer part Tp and the fixing section  20  to a paper stacking portion S on which the piece of paper P is to be stacked, and a reversal transport path  33  in which the piece of paper P having passed through the fixing section  20  is turned upside down and is supplied to the transfer part Tp again. 
     The paper transport system  30  includes a pickup roller  34  that picks up some pieces of paper P from the paper storing portion  31  storing plural pieces of paper P, and a pair of separating rollers  35  that separate one of the pieces of paper P picked up by the pickup roller  34  from the others and transport the piece of paper P toward the transfer part Tp. 
     The paper transport system  30  further includes a pair of registration rollers  36  that temporarily stop the transportation of the piece of paper P when not rotated, and supply the piece of paper P to the transfer part Tp by rotating at a predetermined timing while registering the piece of paper P. 
     The paper transport system  30  further includes a pair of transport rollers  37  that are provided on the reversal transport path  33  and transport the piece of paper P, and a pair of discharge rollers  38  that are provided on the downstream side in a paper transport direction with respect to a position where the paper transport path  32  and the reversal transport path  33  merge. The pair of discharge rollers  38  discharge the piece of paper P having undergone fixing toward the paper stacking portion S, or transport the piece of paper P toward the reversal transport path  33  when images are to be formed on both sides of the piece of paper P. 
     The toner transport section  40  holds the toner cartridge  50 , which is interchangeable. The toner transport section  40  transports toner supplied thereto from the toner cartridge  50  toward the developing device  14  included in the image forming section  10 . 
     The toner cartridge  50  includes a toner container  51  and a storage medium  52 . The toner container  51  contains toner. The storage medium  52  is an electrically erasable and programmable read-only memory (EEPROM) or the like. The storage medium  52  stores information indicating the type of the toner cartridge  50 , and information on the usage condition of the toner cartridge  50  such as the number of revolutions of a rotating member provided in the toner container  51  and that rotates and thus stirs the toner. If, for example, the toner in the toner container  51  runs out, the toner cartridge  50  is replaced with another toner cartridge  50 . 
     The toner collecting device  60  collects and stores the residual toner removed from the photoconductor drum  11  by the cleaning member  16  after the transfer. 
     The controller  100  receives image data and printing instructions for image formation from a personal computer (PC) or the like that is connected to the image forming apparatus  1  over a network or the like. Furthermore, the controller  100  processes the image data thus received and sends the processed image data to the exposure device  13 . 
     The controller  100  according to the exemplary embodiment includes a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM), and a hard disk drive (HDD) (all not illustrated). The CPU executes processing programs. The ROM stores programs, tables, parameters, and so forth. The RAM is used as a work area or the like when any of the programs is executed by the CPU. 
     Operation Performed by Image Forming Apparatus  1   
     An image forming operation performed by the image forming apparatus  1  according to the exemplary embodiment will now be described. 
     When image data generated by the PC or the like (not illustrated) is received by the controller  100 , the controller  100  processes the image data. The image data thus processed is output to the exposure device  13 . The exposure device  13  that have acquired the image data selectively performs exposure on the photoconductor drum  11  that has been charged by the charging device  12 , thereby forming an electrostatic latent image on the photoconductor drum  11 . The electrostatic latent image on the photoconductor drum  11  is developed into a toner image in, for example, black (K) by the developing device  14 . 
     Meanwhile, in the paper transport system  30 , the pickup roller  34  rotates in accordance with the timing of image formation, and some pieces of paper P are picked up from the paper storing portion  31 . One of the pieces of paper P that has been separated from the others by the pair of separating rollers  35  is transported to the pair of registration rollers  36 , where the piece of paper P is temporarily stopped. The pair of registration rollers  36  rotate in accordance with the timing of rotation of the photoconductor drum  11 , whereby the piece of paper P is supplied to the transfer part Tp, where the toner image formed on the photoconductor drum  11  is transferred to the piece of paper P by the transfer device  15 . 
     Subsequently, the piece of paper P having the toner image transferred thereto undergoes the fixing process in the fixing section  20 , and is discharged to the paper stacking portion S by the pair of discharge rollers  38 . 
     If another image is to be formed on a second side of the piece of paper P in addition to a first side of the piece of paper P (if images are to be formed on both sides of the piece of paper P), the piece of paper P that has passed through the fixing section  20  is transported into the reversal transport path  33  by the pair of discharge rollers  38  and is supplied to the transfer part Tp again by the pair of transport rollers  37 . Then, another toner image formed on the photoconductor drum  11  is transferred to the second side of the piece of paper P at the transfer part Tp. The piece of paper P having the toner image transferred also to the second side thereof undergoes the fixing process in the fixing section  20  and is discharged onto the paper stacking portion S by the pair of discharge rollers  38 . 
     After the above image formation is performed by the image forming section  10  and the toner image on the photoconductor drum  11  is transferred to the piece of paper P, the photoconductor drum  11  may have some residual toner. Such residual toner on the photoconductor drum  11  is removed by the cleaning member  16 . The residual toner thus removed is collected by the toner collecting device  60 . 
     Photoconductor Drum  11   
     Referring now to  FIG. 2 , a configuration including the photoconductor drum  11  and peripheral elements according to the exemplary embodiment will be described.  FIG. 2  is a schematic diagram illustrating a rear-side end of the photoconductor drum  11  according to the exemplary embodiment. 
     As illustrated in  FIG. 2 , the photoconductor drum  11 , which is an exemplary image carrier, includes a photoconductor drum unit  110  and a photoconductor-drum-driving mechanism  120  that transmits a driving force to the photoconductor drum unit  110 . The photoconductor drum  11  includes a link mechanism  80  (to be described below) configured to cut the transmission of the driving force from the photoconductor-drum-driving mechanism  120 . 
     Photoconductor Drum Unit  110   
     The photoconductor drum unit  110  includes a cylindrical photoconductor drum body (rotating member)  111  configured to carry a toner image on the outer circumferential surface thereof, a shaft  113  functioning as a rotating shaft of the photoconductor drum body  111 , a drum-side-magnet-supporting member  115  provided at the rear-side end of the shaft  113  and rotating together with the shaft  113 , and a drum-side magnet  117  (to be described below) supported by the drum-side-magnet-supporting member  115  coaxially with the shaft  113 . 
     The photoconductor drum unit  110  includes a one-way clutch  119 , which is a mechanism that transmits a rotational force only in one direction. The one-way clutch  119  is provided at the rear-side end of the photoconductor drum unit  110  and between the shaft  113  and the photoconductor drum body  111 . The photoconductor drum unit  110  further includes a bearing (not illustrated) provided at the front-side end thereof and between the shaft  113  and the photoconductor drum body  111 . At the front-side end of the photoconductor drum unit  110 , the shaft  113  and the photoconductor drum body  111  are freely rotatable with respect to each other. 
     The above elements integrally form the photoconductor drum unit  110 . The photoconductor drum unit  110  is detachably attached to the housing  70 . More specifically, the photoconductor drum unit  110  illustrated in  FIG. 2  is attached to the housing  70  in a direction intersecting the axial direction of the photoconductor drum unit  110  and from the near side toward the far side in  FIG. 2 . A driving force generated by the photoconductor-drum-driving mechanism  120  causes the photoconductor drum unit  110  as a whole to rotate about the shaft  113 . 
     Referring now to  FIG. 3 , the one-way clutch  119  will be described.  FIG. 3  is a sectional view taken along line III-III illustrated in  FIG. 2 . 
     As illustrated in  FIG. 3 , the one-way clutch  119 , which is an exemplary backward-rotation-suppressing mechanism, is an annular member and is provided between the shaft (rotating shaft)  113  and the photoconductor drum body (outer circumferential member)  111  in the diametrical direction of the shaft  113 . That is, the shaft  113  is provided on the inner circumferential side of the one-way clutch  119 , and the photoconductor drum body  111  is provided on the outer circumferential side of the one-way clutch  119 . 
     The one-way clutch  119  transmits the driving force to the photoconductor drum body  111  when the shaft  113  rotates in the forward direction (see arrow D 2  in  FIG. 3 ), but does not transmit the driving force to the photoconductor drum body  111  when the shaft  113  rotates in the backward direction (see arrow D 3  in  FIG. 3 ). Furthermore, in the exemplary embodiment, since the one-way clutch  119  is provided, the shaft  113  is capable of causing the photoconductor drum body  111  to rotate in the forward direction (see arrow D 0  in  FIG. 3 ) but is not capable of causing the photoconductor drum body  111  to rotate in the backward direction (see arrow D 1  in  FIG. 3 ). 
     With the one-way clutch  119 , the configuration of a driving path (the photoconductor-drum-driving mechanism  120 , for example) for driving the photoconductor drum body  111  may be simplified. Furthermore, since the one-way clutch  119  is provided on the inner side of the photoconductor drum body  111 , the configuration around the photoconductor drum unit  110  may be simplified. 
     Photoconductor-Drum-Driving Mechanism  120   
     Referring now to  FIG. 2 , the photoconductor-drum-driving mechanism  120  will be described. 
     The photoconductor-drum-driving mechanism  120  includes a motor (driving portion) M 1  as a drive source, a train of gears (not illustrated) each rotating with the driving force transmitted thereto from the motor M 1 , a coupling gear  121  rotating with the driving force transmitted thereto from the train of gears, a gear-side-magnet-supporting member  123  rotating with the driving force transmitted thereto from the coupling gear  121 , and a gear-side magnet  125  supported by the gear-side-magnet-supporting member  123 . The coupling gear  121 , the gear-side-magnet-supporting member  123 , and the gear-side magnet  125  are coaxial with the shaft  113 . 
     The coupling gear  121  includes a gear body  127 , a coupling pin  129  coaxial with the gear body  127  and movable in the axial direction, and a spring  131  urging the coupling pin  129  in a direction of the rotational axis of the gear body  127  toward the photoconductor drum  11  (toward the front side). 
     The gear body  127  has a coupling-pin-receiving hole  127 A that is open from the front-side face thereof and extending coaxially with the gear body  127 . The coupling-pin-receiving hole  127 A receives the coupling pin  129  and the spring  131  and has dimensions that allow the coupling pin  129  to move along the rotational axis of the gear body  127 . 
     The gear body  127  has grooves  127 B (see  FIG. 4B  to be referred to below) provided on the inner circumferential surface of the coupling-pin-receiving hole  127 A and extending along the rotational axis of the gear body  127 . In an exemplary configuration illustrated in  FIG. 4B , two grooves  127 B are provided across the rotational axis of the gear body  127  from each other. 
     The gear body  127  includes a holding member  127 C that is a substantially columnar member extending in the coupling-pin-receiving hole  127 A and coaxially with the coupling-pin-receiving hole  127 A. A portion of the spring  131  is wound around and is thus held by the holding member  127 C. 
     Referring now to  FIGS. 2 and 4A  to  4 C, the coupling pin  129  will be described.  FIGS. 4A to 4C  are schematic diagrams of the coupling pin  129 . More specifically,  FIG. 4A  is a perspective view of the coupling pin  129  seen from the front side.  FIG. 4B  is a sectional view taken along line IVB-IVB in  FIG. 2  and illustrates the relationship between the coupling pin  129  and the gear body  127 .  FIG. 4C  is a sectional view taken along line IVC-IVC in  FIG. 2  and illustrates the relationship between the coupling pin  129  and the gear-side-magnet-supporting member  123 . 
     As illustrated in  FIGS. 2 and 4A , the coupling pin  129  is a substantially cylindrical member with one end (the rear-side end) thereof fitted in the coupling-pin-receiving hole  127 A provided in the gear body  127 . In the exemplary configuration illustrated in  FIGS. 2 and 4A , the coupling pin  129  has a spring-receiving hole  129 A extending in the axial direction thereof from a surface thereof facing the gear body  127  (facing toward the rear side). Furthermore, the coupling pin  129  includes a limiting portion  129 B that is a substantially columnar member extending in the axial direction of the coupling pin  129  and provided at the bottom of the spring-receiving hole  129 A. The limiting portion  129 B limits the movement of the spring  131 , provided in the spring-receiving hole  129 A, in the radial direction. 
     The coupling pin  129  further includes projections  129 C provided near the rear-side end on the outer circumferential surface thereof, a flange  129 D (to be described in detail below) provided around the outer circumference thereof and at a position nearer to the front side than the projections  129 C, and catches  129 E projecting from a front-side end facet  129 F thereof toward the rear side. 
     In the exemplary configuration illustrated in  FIGS. 4A and 4B , two projections  129 C are provided across the rotational axis of the coupling pin  129  from each other, and the projections  129 C each have a substantially hemispherical shape on the outer circumferential surface of the coupling pin  129 . 
     In the state where the coupling pin  129  is set in the coupling-pin-receiving hole  127 A of the gear body  127  as illustrated in  FIG. 4B , the projections  129 C of the coupling pin  129  reside in the respective grooves  127 B of the gear body  127 . Hence, in the coupling-pin-receiving hole  127 A of the gear body  127 , the movement (relative movement) of the coupling pin  129  in the circumferential direction is limited while the movement (relative movement) of the coupling pin  129  in the axial direction is not limited. 
     Referring to  FIG. 4A  again, the catches  129 E will be described. In the exemplary configuration illustrated in  FIG. 4A , two catches  129 E are provided across the rotational axis of the coupling pin  129  from each other. The catches  129 E are each a plate-like member provided on the front-side end facet  129 F of the coupling pin  129  and curving in the circumferential direction of the coupling pin  129 . The catches  129 E each have, at one end thereof in the circumferential direction, a sloping portion  129 G sloping in a direction intersecting the front-side end facet  129 F of the coupling pin  129 . 
     Referring to  FIG. 4C , the catches  129 E of the coupling pin  129  are set in a recess  123 A (to be described below) provided in the gear-side-magnet-supporting member  123 . When the coupling pin  129  is driven to rotate about the rotational axis thereof, ends  129 H of the respective catches  129 E that are opposite the sloping portions  129 G push a rib  123 B (to be described below) provided in the recess  123 A, thereby rotating the gear-side-magnet-supporting member  123  (see arrow D 0  in  FIG. 4C ). In this manner, the driving force is transmitted from the coupling pin  129  to the gear-side-magnet-supporting member  123 . 
     Referring now to  FIG. 2 , the gear-side-magnet-supporting member  123  and the gear-side magnet  125  will be described. 
     As illustrated in  FIG. 2 , the gear-side-magnet-supporting member  123  has, at the rear-side end thereof, the recess  123 A in a region facing the coupling pin  129 . Furthermore, the rib  123 B is provided in the recess  123 A. The rib  123 B projects from the bottom of the recess  123 A and extends in the diametrical direction. The rib  123 B resides in a path along which the catches  129 E move with the rotation of the coupling pin  129 . 
     The gear-side-magnet-supporting member  123  includes a holding portion  123 C provided at the front-side end thereof and that holds the gear-side magnet  125 . In the exemplary configuration illustrated in  FIG. 2 , the holding portion  123 C is an annular member that holds the gear-side magnet  125  at the inner circumference thereof. 
     The gear-side magnet  125  that is held by the gear-side-magnet-supporting member  123  is coaxial with the drum-side magnet  117  and faces the drum-side magnet  117 . 
     Referring now to  FIGS. 5A and 5B , configurations of the drum-side magnet  117  and the gear-side magnet  125  will be described.  FIGS. 5A and 5B  illustrate the configurations of the drum-side magnet  117  and the gear-side magnet  125 . More specifically,  FIG. 5A  illustrates the positional relationship between the drum-side magnet  117  and the gear-side magnet  125  that is realized when the two rotate together.  FIG. 5B  schematically illustrates the drum-side magnet  117  seen in a direction indicated by arrows VB in  FIG. 5A . 
     The drum-side magnet (a second magnet, or a follower magnet)  117  and the gear-side magnet (a first magnet, or a driving magnet)  125  are each an annular plate-type magnet. As illustrated in  FIG. 5B , the drum-side magnet  117  includes magnets arranged in the circumferential direction thereof such that the magnetic poles of adjacent magnets have opposite polarities. While three pairs of opposite magnetic polarities, the north pole and the south pole, are provided in the circumferential direction in the exemplary configuration illustrated in  FIG. 5B , the number of pairs is not limited to three. The gear-side magnet  125  has the same configuration as the drum-side magnet  117 , although not illustrated. 
     The gear-side magnet  125  and the drum-side magnet  117  in combination form a so-called magnet coupling. More specifically, each magnetic pole of each of the gear-side magnet  125  and the drum-side magnet  117  faces the opposite magnetic pole of the other with a gap (denoted by G in  FIG. 5A ) interposed therebetween. The gap G falls within a range in which the magnets  125  and  117  attract each other. When the gear-side magnet  125  is driven to rotate by the motor M 1  of the photoconductor-drum-driving mechanism  120 , the drum-side magnet  117  rotates. In this manner, the driving force from the motor M 1  of the photoconductor-drum-driving mechanism  120  is transmitted to the photoconductor drum  11 . 
     In the exemplary embodiment, the driving force is transmitted between the gear-side magnet  125  and the drum-side magnet  117  that are not in contact with each other. Therefore, noise is smaller and recycling is easier than in the case of a contact-type coupling. Furthermore, the deterioration of image quality, such as density nonuniformity (banding) caused by a resonance that may occur in the photoconductor drum  11  because of torsional rigidity, may be suppressed. 
     Since the gear-side magnet  125  and the drum-side magnet  117  attract each other with their magnetism, the position of the photoconductor drum unit  110  in the axial direction is determined. More specifically, since a flange  115 A (see  FIG. 2 ) included in the drum-side-magnet-supporting member  115  is pressed against a positioning portion  73 P (see  FIG. 2 ) included in the housing  70 , the position of the photoconductor drum unit  110  in the axial direction is determined. 
     Housing  70   
     Referring now to  FIG. 2 , how the housing  70  according to the exemplary embodiment supports the photoconductor drum unit  110  and the photoconductor-drum-driving mechanism  120  will be described. 
     As illustrated in  FIG. 2 , the housing  70  includes a first bearing (not illustrated), a second bearing  73 , a third bearing  75 , and a fourth bearing  77  that support the photoconductor drum unit  110  and the photoconductor-drum-driving mechanism  120  while allowing the rotation of the two. The first bearing (not illustrated), the second bearing  73 , the third bearing  75 , and the fourth bearing  77  are provided in that order in the axial direction from the front side toward the rear side and are each a sliding bearing (oil-less bearing) made of resin or the like. 
     The first bearing (not illustrated) and the second bearing  73  support the front-side end and the rear-side end, respectively, of the shaft  113 . 
     The second bearing  73  supports the drum-side-magnet-supporting member  115  while allowing the rotation of the drum-side-magnet-supporting member  115 . The second bearing  73  includes the above-mentioned positioning portion  73 P against which the flange  115 A of the drum-side-magnet-supporting member  115  is pressed. 
     The third bearing  75  supports the gear-side-magnet-supporting member  123  while allowing the rotation of the gear-side-magnet-supporting member  123  and in such a manner as to hold both axial ends of the gear-side-magnet-supporting member  123 . In this manner, the third bearing  75  suppresses the movement of the gear-side-magnet-supporting member  123  in the axial direction. 
     The fourth bearing  77  supports the coupling gear  121  while allowing the rotation of the coupling gear  121  and in such a manner as to hold both axial ends of the coupling gear  121 . In this manner, the fourth bearing  77  suppresses the movement of the coupling gear  121  in the axial direction. The fourth bearing  77  also functions as a covering portion covering the coupling gear  121  and includes projecting portions  77 A (see  FIG. 6A  to be referred to below) that guide the movement of a link  85  (to be described below). 
     Link Mechanism  80   
     Referring now to  FIGS. 6A and 6B , the link mechanism  80  according to the exemplary embodiment will be described.  FIGS. 6A and 6B  illustrate a configuration of the link mechanism  80 . More specifically,  FIG. 6A  is a perspective view illustrating the link mechanism  80 , the coupling pin  129 , and peripheral elements.  FIG. 6B  illustrates the link mechanism  80  and the coupling pin  129  seen in a direction of arrow VIB in  FIG. 6A . 
     As illustrated in  FIG. 6A , the link mechanism  80  includes the link  85  connected to the coupling pin  129 , a solenoid S 1  functioning as a driving source that drives the link  85 , and a sensor (not illustrated) provided at a position facing the covering  71  (see  FIG. 1 ) and that is configured to detect the opening of the covering  71 . The solenoid S 1  is activated by, for example, receiving a control signal from the controller  100  that has received a detection signal from the sensor. 
     The link  85  is a substantially rectangular plate-like member and is connected to the solenoid S 1  at one end thereof. Furthermore, the link  85  has a slit  85 B at the other end thereof. The coupling pin  129  is held in the slit  85 B. As illustrated in  FIG. 6B , the link  85  has a sloping portion  85 C on a rear-side face  85 S thereof. The sloping portion  85 C slopes toward the other end (the upper end in  FIG. 6B ) of the link  85  and in a direction away from a front-side face  85 F of the link  85 . 
     Since the coupling pin  129  is held in the slit  85 B of the link  85 , the rear-side face  85 S of the link  85  is pressed against the flange  129 D of the coupling pin  129 . When the solenoid S 1  is activated, the link  85  moves down or up in  FIG. 6B  (see the double-headed arrow in  FIG. 6B ). With the movement of the link  85 , the flange  129 D is pushed by the sloping portion  85 C, whereby the coupling pin  129  moves in the axial direction (the lateral direction in  FIG. 6B ). 
     Movement of Photoconductor-Drum-Driving Mechanism  120   
     Referring now to  FIGS. 7 and 8 , how the photoconductor-drum-driving mechanism  120  moves with the above movement of the link mechanism  80  will be described.  FIG. 7  illustrates a state of the photoconductor-drum-driving mechanism  120  realized when the covering  71  is closed.  FIG. 8  illustrates a state of the photoconductor-drum-driving mechanism  120  realized when the covering  71  is open. 
     First, a movement of the coupling pin  129  that is made along with the movement of the link  85  of the link mechanism  80  will be described. As illustrated in  FIG. 7 , in the state where the covering  71  is closed, the solenoid S 1  is not activated and the coupling pin  129  projects from the gear body  127 . When the covering  71  is opened, the solenoid S 1  is activated, whereby the sloping portion  85 C of the link  85  pushes the flange  129 D of the coupling pin  129  toward the rear side. Consequently, as illustrated in  FIG. 8 , the coupling pin  129  is embedded into the gear body  127 . 
     Although detailed description is omitted, when the covering  71  that is in the open state is closed, the coupling pin  129  connected to the link  85  of the link mechanism  80  moves from the position of being embedded in the gear body  127  (see  FIG. 8 ) to the position of projecting from the gear body  127  (see  FIG. 7 ). 
     How the states of connections among the elements included in the photoconductor-drum-driving mechanism  120  change with the movement of the coupling pin  129  will now be described. As illustrated in  FIG. 7 , in the state where the covering  71  (see  FIG. 1 ) is closed, the coupling pin  129  is at the position of projecting from the gear body  127 . In this state, the rib  123 B of the gear-side-magnet-supporting member  123  resides in the path along which the catches  129 E move with the rotation of the coupling pin  129  about the rotational axis. 
     When the covering  71  is open as illustrated in  FIG. 8 , the coupling pin  129  is at the position of being embedded in the gear body  127 . In this state, the coupling pin  129  is retracted from the gear-side-magnet-supporting member  123 , and the rib  123 B of the gear-side-magnet-supporting member  123  is retracted from the path along which the catches  129 E moves with the rotation of the coupling pin  129 . That is, the catches  129 E are out of engagement with the rib  123 B. 
     In such a state where the catches  129 E and the rib  123 B are not pressed against each other, the gear-side-magnet-supporting member  123  is rotatable independently of the coupling pin  129  or the coupling gear  121 . More specifically, in the exemplary embodiment, in the state where the covering  71  is open, the gear-side-magnet-supporting member  123  and the gear-side magnet  125  are capable of rotating idly while being disconnected from the photoconductor-drum-driving mechanism  120  including the motor M 1 . 
     Hence, when, for example, the photoconductor drum unit  110  is exchanged with a new one, the gear-side-magnet-supporting member  123  and the gear-side magnet  125  are rotated idly, whereby the backward rotation of the photoconductor drum unit  110  may be suppressed. 
     In other words, the photoconductor-drum-driving mechanism  120  according to the exemplary embodiment may be regarded as a unit including an idling mechanism that allows only some members to rotate idly, or a unit including a connecting-and-disconnecting mechanism that switches between a connected state (a fixed state or a transmittable state) realized when the covering  71  is closed and a disconnected state (an idle state) realized when the covering  71  is open. 
     The above description concerns a configuration in which the link  85  is moved by using the solenoid S 1 . If the link  85  is connected to the covering  71  instead of being connected to the solenoid S 1 , the coupling pin  129  is movable by the link mechanism  80  with the opening of the covering  71 . 
     Stopping Rotation of Photoconductor Drum Unit  110   
     Referring now to  FIG. 2 , how individual members move when the rotation of the photoconductor drum unit  110  is stopped will be described. 
     As described above, in the exemplary embodiment, the gear-side magnet  125  and the drum-side magnet  117  attract each other with their magnetism. Hence, when the gear-side magnet  125  rotates, the drum-side magnet  117  rotates. Thus, the driving force from the motor M 1  included in the photoconductor-drum-driving mechanism  120  is transmitted to the photoconductor drum unit  110 , and the photoconductor drum unit  110  rotates. 
     To stop the rotation of the photoconductor drum unit  110 , the motor M 1  is stopped. The gear-side magnet  125 , which is mechanically connected to the motor M 1 , is also stopped together with the motor M 1 . Meanwhile, the drum-side magnet  117 , which is spaced apart from the gear-side magnet  125  and is not mechanically connected to the motor M 1 , may continue to rotate with inertia even after the motor M 1  is stopped. Furthermore, there may be a difference in the speed of rotation between the gear-side magnet  125  and the drum-side magnet  117 . 
     When there is a difference in the speed of rotation between the gear-side magnet  125  and the drum-side magnet  117  because the speed of rotation of the gear-side magnet  125  is reduced (the gear-side magnet  125  stops rotating), the phase relationship between the gear-side magnet  125  and the drum-side magnet  117  may change from the predetermined phase relationship. 
     More specifically, the relationship between each of the magnetic poles of the gear-side magnet  125  and a corresponding one of the magnetic poles of the drum-side magnet  117  may change from the predetermined relationship (see  FIG. 5A ), that is, not a state where opposite magnetic poles face each other but a state where, for example, the same magnetic poles face each other. In such a case, a force acting to change the relative positions of the gear-side magnet  125  and the drum-side magnet  117  (a force that tends to restore the predetermined phase relationship) occurs with the magnetism. 
     The force acting to change the relative positions of the gear-side magnet  125  and the drum-side magnet  117  acts as a force causing the gear-side magnet  125  and the drum-side magnet  117  to rotate. Depending on the positional relationship between the gear-side magnet  125  and the drum-side magnet  117 , the force may cause the photoconductor drum body  111  to rotate in the backward direction (see arrow D1 in  FIG. 3 ). 
     If the photoconductor drum body  111  rotates in the backward direction (see arrow D 1  in  FIG. 3 ), the cleaning member  16  provided in contact with the surface of the photoconductor drum  11  so as to clean the photoconductor drum body  111  after the transfer may be rolled up. Furthermore, in an area where the photoconductor drum body  111  and the developing roller  14 A of the developing device  14  face each other, the developer carried by the developing roller  14 A may be removed from the developing roller  14 A. Consequently, the transferability may be deteriorated at the start of rotation of the photoconductor drum body  111 . Such phenomena lead to some deterioration in the quality of an image to be formed on the piece of paper P. 
     In the exemplary embodiment, however, the one-way clutch  119  is provided between the shaft  113  and the photoconductor drum body  111  as described above, and the one-way clutch  119  does not transmit the driving force acting in such a direction that the shaft  113  tends to cause the photoconductor drum body  111  to rotate in the backward direction. 
     Hence, even if the drum-side magnet  117  that has received a magnetic force from the gear-side magnet  125  acts to rotate the photoconductor drum body  111  in the backward direction, the driving force is not transmitted to the photoconductor drum body  111 . Therefore, the photoconductor drum body  111  does not rotate in the backward direction. In this state, the magnetic force generated between the gear-side magnet  125  and the drum-side magnet  117  causes the drum-side magnet  117 , the drum-side-magnet-supporting member  115 , and the shaft  113  to rotate idly without causing the photoconductor drum body  111  to rotate. 
     Results of Measurement 
     Referring now to  FIGS. 9A and 9B , changes in the movement of the photoconductor drum body  111  that are observed in a case where the one-way clutch  119  is not provided and in a case where the one-way clutch  119  is provided will be described. 
       FIGS. 9A and 9B  are graphs illustrating changes in the movement of the photoconductor drum body  111  that are observed without and with the one-way clutch  119 , respectively. In the each of the graphs, the horizontal axis represents time, and the vertical axis represents the number of revolutions. Furthermore, the solid-line curve for the input side represents the rotation of the gear-side magnet  125 , and the broken-line curve for the output side represents the rotation of the photoconductor drum body  111 . Furthermore, in each of the graphs, the number of revolutions of the photoconductor drum body  111  in the forward direction (see arrow D0 in  FIG. 3 ) is positive, and the number of revolutions of the photoconductor drum body  111  in the backward direction (see arrow D1 in  FIG. 3 ) is negative. 
     Referring to  FIG. 9A , the case where the one-way clutch  119  is not provided, unlike the exemplary embodiment, will be described. As is seen from the graph in  FIG. 9A , stopping the rotation of the gear-side magnet  125  on the input side is accompanied by a period in which the number of revolutions of the photoconductor drum body  111  (represented by the broken-line curve in the graph) is negative (see the ovally enclosed area in the graph). That is, in the graph in  FIG. 9A , the photoconductor drum body  111  rotates in the backward direction (see arrow D1 in  FIG. 3 ). 
     In contrast, in the case of the graph illustrated in  FIG. 9B  where the one-way clutch  119  is provided, there is no period in which the number of revolutions of the photoconductor drum body  111  is negative. That is, providing the one-way clutch  119  suppresses the rotation of the photoconductor drum body  111  in the backward direction (see arrow D1 in  FIG. 3 ). 
     Modifications 
     The exemplary configuration illustrated in the drawings concerns a case where the one-way clutch  119  is provided between the shaft  113  and the photoconductor drum body  111 . The one-way clutch  119  only needs to suppress the transmission of the driving force to the photoconductor drum body  111  when the one-way clutch  119  has received the driving force acting in such a direction that the drum-side magnet  117  tends to cause the photoconductor drum body  111  to rotate in the backward direction (see arrow D1 in  FIG. 3 ). 
     Hence, the one-way clutch  119  only needs to be provided in a path along which the driving force is transmitted from the drum-side magnet  117  to the photoconductor drum body  111 . For example, the one-way clutch  119  may be provided between the drum-side magnet  117  and the drum-side-magnet-supporting member  115  or between the drum-side-magnet-supporting member  115  and the shaft  113 . Alternatively, the one-way clutch  119  may be provided as a substituted for the second bearing  73 , with the drum-side-magnet-supporting member  115  being provided on the inner circumferential side of the one-way clutch  119  such that the drum-side-magnet-supporting member  115  is allowed to rotate in the forward direction but is not allowed to rotate in the backward direction. 
     The exemplary configuration illustrated in the drawings concerns a case where the photoconductor drum unit  110  includes the one-way clutch  119  at the rear-side end thereof and the bearing (not illustrated) at the front-side end thereof. The present invention is not limited to such a case. For example, the photoconductor drum unit  110  may include the one-way clutch  119  at the front-side end thereof and the bearing at the rear-side end thereof. For another example, the one-way clutch  119  may be provided at each of the rear-side end and the front-side end of the photoconductor drum unit  110 . If the one-way clutch  119  is provided at the end of the photoconductor drum unit  110  having the drum-side magnet  117  (the rear-side end in the exemplary configuration illustrated in the drawings), the backward rotation of the photoconductor drum unit  110  due to torsion may be suppressed more than in the case where the one-way clutch  119  is provided at the front-side end of the photoconductor drum unit  110 . 
     While the above exemplary embodiment employs the one-way clutch  119 , any other mechanism may alternatively be employed as long as the mechanism suppresses the rotation of the photoconductor drum unit  110  in the backward rotation (see arrow D 1  in the drawings) that may occur when the rotation of the photoconductor drum unit  110  is stopped. 
     Specifically, the one-way clutch  119  may be substituted by a mechanism in which the link  85  is moved by the solenoid S 1 . In such a mechanism, the controller  100  actuates the solenoid S 1  before stopping the motor M 1  so as to stop the rotation of the photoconductor drum unit  110 . Subsequently, the link  85  driven by the solenoid S 1  moves the coupling pin  129 , whereby the coupling pin  129  moves to the position of being embedded in the gear body  127  as illustrated in  FIG. 8 . This allows the gear-side-magnet-supporting member  123 , the coupling pin  129 , and the coupling gear  121  to rotate independently of one another. 
     Subsequently, the motor M 1  is stopped, whereby the coupling pin  129  and the coupling gear  121  that are mechanically connected to the motor M 1  are stopped. Meanwhile, the gear-side-magnet-supporting member  123  and the photoconductor drum unit  110  that have been disconnected from the coupling pin  129  and associated elements continue to rotate by inertia. In this state, the gear-side magnet  125  and the drum-side magnet  117  maintain to attract each other with their magnetism. Subsequently, because of the friction between the photoconductor drum unit  110  and the cleaning member  16  and so forth, the gear-side-magnet-supporting member  123  and the photoconductor drum unit  110  stop rotating. 
     Since the gear-side magnet  125  and the drum-side magnet  117  maintain to attract each other with their magnetism until the photoconductor drum unit  110  stops rotating, the gear-side magnet  125  and the drum-side magnet  117  tend to be prevented from becoming out of phase. In this manner, the rotation of the photoconductor drum unit  110  in the backward direction (see arrow D 1  in the drawings) is suppressed. In this case, a group of the solenoid S 1 , the link  85 , the coupling pin  129 , and the gear-side-magnet-supporting member  123  is regarded as a backward-rotation-suppressing mechanism. 
     In the above case, the controller  100  actuates the solenoid S 1  before stopping the motor M 1 . Alternatively, for example, the controller  100  may actuate the solenoid S 1  after the motor M 1  starts to decelerate so as to stop the photoconductor drum unit  110  and when the speed of the motor M 1  has been reduced to or below a predetermined value. 
     The above description concerns a case where the rotation of the photoconductor drum body  111  in the backward direction (see arrow D 1  in  FIG. 3 ) is suppressed when the rotation of the photoconductor drum unit  110  is stopped. The exemplary embodiment is also applicable to a case where, for example, the photoconductor drum body  111  tends to rotate in the backward direction (see arrow D 1  in  FIG. 3 ) because the gear-side magnet  125  and the drum-side magnet  117  go out of phase when the rotation of the photoconductor drum unit  110  is accelerated or when the speed of rotation of the photoconductor drum unit  110  is changed by disturbance or the like. 
     While the exemplary configuration illustrated in the drawings concerns the photoconductor drum  11 , the exemplary embodiment is also applicable to any other rotating member included in the image forming apparatus  1 . For example, the exemplary embodiment is applicable to any of the developing device  14 , the transfer device  15 , the fixing section  20 , the toner cartridge  50 , and other rotating members. 
     In the exemplary configuration illustrated in the drawings, the drum-side magnet  117  and the gear-side magnet  125  are each an annular plate-type magnet. Alternatively, the magnet coupling may include, for example, an annular magnet and a cylindrical magnet that encloses the annular magnet. 
     The foregoing description of the exemplary embodiment of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiment was chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.