Abstract:
An image forming apparatus includes: an image bearing member (drum); a movable belt; an operating lever movable between a contact position (state) between the belt and the drum and a second position (state) between the belt and the drum, wherein between the contact position and the spaced position, a passing position is located; a cam for transiting the belt and the drum between the contact and spaced states by transmitting thereto a driving force for moving the operating lever to rotate the cam; a driving force transmitting mechanism; and a rotation stopping mechanism for stopping, when the operating lever is moved from the first position to the third position, rotation of the cam to maintain the spaced state so that the belt and drum are prevented from being transited from the spaced state to the contact state by continuation of the rotation of the cam by inertia.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    In an image forming apparatus, such as a copying machine or a printer, to which electrophotography is applied, as a form of a transfer means, a transfer device of a belt transfer type in which a toner image on an image bearing member is transferred onto a sheet while attracting and conveying the sheet by a transfer belt which is a belt member has been used. The image forming apparatus including the transfer device of the belt transfer type includes a transfer belt moving mechanism, using a motor, a solenoid or the like as a driving source, for moving the transfer belt toward and away from the image bearing member with desired timing such as an image forming operation or during jam occurrence. 
         [0002]    On the other hand, in order to facilitate jam clearance or a maintenance operation, an image forming apparatus including a conveying unit, on which a transfer device, a fixing device and the like which are provided in a conveying path are mounted, capable of being pulled out from an apparatus main assembly has been widely known. In the image forming apparatus in which the transfer device of the belt transfer type is mounted on the conveying unit capable of being pulled out from the apparatus main assembly, in order to prevent mutual friction between the image bearing member and the transfer device, there is a need to effect mounting and demounting of the conveying unit after the transfer belt is retracted from the image bearing member with reliability. 
         [0003]    However, the transfer belt cannot be reliably retracted from the image bearing member in some cases including the case where supply of energy to the motor, the solenoid or the like as the driving source for the transfer belt moving mechanism is blocked and the case where the driving source caused an abnormal operation. 
         [0004]    Therefore, in Japanese Laid-Open Patent Application (JP-A) 2011-180370, an image forming apparatus including a transfer belt moving mechanism capable of retracting a transfer belt from a photosensitive drum during mounting and demounting of a conveying unit irrespective of an energy supply state has been proposed. In this image forming apparatus, the transfer belt held by a transfer unit can be retracted from the photosensitive drum by rotating an urging cam for raising and lowering the transfer unit in interrelation with a releasing operation of a handle (operating portion) for releasing a lock mechanism for the conveying unit. 
         [0005]    In the case of the image forming apparatus described in JP-A 2011-180370, rotation (rotational movement) motion of a lock-releasing handle for the conveying unit is converted into rotation motion of integrally rotatable urging cam and gear having partly omitted teeth via linear motion of a slidable member including a rack portion. Accordingly, a rotation speed and kinetic energy of the urging cam and the gear having partly omitted teeth which are rotated in interrelation with the releasing operation of the handle depend on a condition such as an operating speed or an operating force during the operation of the handle. 
         [0006]    For that reason, e.g., in the case where an abrupt handle operation is performed, the urging cam and the gear having partly omitted teeth which are rotated quickly and increased in inertia cannot stop at a stop position which is a position where the transfer unit is lowered, thus overrunning the stop position. When the urging cam overruns the stop position, the transfer unit is raised again, so that there is a possibility that the transfer and the photosensitive drum are contacted to each other again during the mounting and demounting of the conveying unit. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention has been accomplished in view of the above-described circumferences. A principal object of the present invention is to provide an image forming apparatus including a structure capable of preventing re-contact of a belt member with an image bearing member caused by overrun of a gear when an operating portion is operated so as to space the belt member from the image bearing member. 
         [0008]    According to an aspect of the present invention, there is provide an image forming apparatus comprising: an image bearing member; a belt member provided movably toward and away from the image bearing member; an operating lever movable between a first position where the belt member and the image bearing member are in a contact state and a third position where the belt member and the image bearing member are in a spaced state, wherein between the first position and the third position, a second position where the belt member and the image bearing member are switched between the contact state and the spaced state is located; a cam for transiting the belt member and the image bearing member between the contact state and the spaced state by transmitting thereto a driving force for moving the operating lever to rotate the cam; a transmitting mechanism for transmitting the driving force for moving the operating lever, wherein when the operating lever is moved between the first position and the third position, the transmitting mechanism transmits the driving force to the cam during passing of the operating lever between the first position and the second position and eliminates transmission of the driving force to the cam during passing of the operating lever between the second position and the third position; and a rotation stopping mechanism for stopping, when the operating lever is moved from the first position to the third position, rotation of the cam to maintain the spaced state so that the belt member and the image bearing member are prevented from being transited from the spaced state to the contact state by continuation of the rotation of the cam by inertia. 
         [0009]    According to the present invention, the rotation stopping mechanism for stopping the rotation of the gear is provided and therefore when the operating portion is operated so as to space the belt member from the image bearing member, it is possible to prevent the re-contact of the belt member with the image bearing member caused by the overrun of the gear. 
         [0010]    These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic illustration of an image forming apparatus according to First Embodiment of the present invention. 
           [0012]      FIG. 2  is a schematic perspective illustration showing a state in which a conveying unit is pulled out from an apparatus main assembly in First Embodiment. 
           [0013]      FIG. 3  is a perspective view for illustrating a contact and separation (spacing) mechanism of a belt unit with respect to a photosensitive drum in First Embodiment. 
           [0014]      FIG. 4  is a perspective view for illustrating a structure for operating the contact and separation mechanism by a handle operation in First Embodiment. 
           [0015]      FIG. 5  is a control block diagram of a driving motor in First Embodiment. 
           [0016]    Part (a) of  FIG. 6  is a schematic illustration showing a state in which a belt member is contacted to the photosensitive drum in First Embodiment, and (b) of  FIG. 6  is a schematic illustration showing a state in which the belt member is spaced from the photosensitive drum in First Embodiment. 
           [0017]    Part (a) of  FIG. 7  is a schematic illustration showing a state in which a gear having partly omitted teeth is driven by an operating portion in First Embodiment, and (b) of  FIG. 7  is a schematic illustration showing a state in which rotation of the gear having partly omitted teeth is stopped in First Embodiment. 
           [0018]      FIG. 8  is a schematic perspective illustration for illustrating a structure for operating a contact and separation mechanism for a belt unit by a handle operation according to Second Embodiment of the present invention. 
           [0019]    Parts (a) to (d) of  FIG. 9  are schematic illustrations sequentially showing an operation for contacting and separating a belt member and a photosensitive drum in Second Embodiment. 
           [0020]    Parts (a) and (b) of  FIG. 10  are schematic illustrations sequentially showing motions of respective portions in the case where the contact and separation mechanism for the belt unit is operated by the handle operation in Second Embodiment. 
           [0021]      FIG. 11  is a schematic perspective illustration for illustration a structure for operating a contact and separation mechanism for a belt unit by a handle operation according to Third Embodiment of the present invention. 
           [0022]    Parts (a) and (b) of  FIG. 12  are schematic illustration showing a state in which rotation of a gear having partly omitted teeth is stopped and a state in which the rotation of the gear having partly omitted teeth is not stopped, respectively, in Third Embodiment. 
           [0023]    Parts (a) and (b) of  FIG. 13  are schematic illustrations sequentially showing motions of respective portions in the case where the contact and separation mechanism for the belt unit is operated by the handle operation in Third Embodiment. 
           [0024]      FIG. 14  is a schematic perspective illustration for illustrating a structure for operating a contact and separation mechanism for a belt unit by a handle operation according to Fourth Embodiment of the present invention. 
           [0025]    Parts (a) and (b) of  FIG. 15  are schematic illustrations showing a state in which rotation of a gear having partly omitted teeth is stopped and a state in which the rotation of the gear having partly omitted teeth is not stopped, respectively, in Fourth Embodiment. 
           [0026]      FIG. 16  is a flow chart showing a flow of one rotation operation of the gear having partly omitted teeth in Fourth Embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0027]    First Embodiment of the present invention will be described with reference to  FIG. 1  to  FIG. 7 . First, a general structure of an image forming apparatus including a belt unit in this embodiment will be described with reference to  FIGS. 1 and 2 . 
       [Image Forming Apparatus] 
       [0028]    As shown in  FIG. 1 , a digital printer  10  as the image forming apparatus of an electrophotographic type includes an image forming portion  11 , a sheet feeding portion  12 , a conveying unit as a belt unit, and a controller (control portion)  14 . 
         [0029]    At the image forming portion  11 , toner image formation which starts from lower exposure depending on an image signal and which uses an electrophotographic process is effected on a photosensitive drum (photosensitive member)  15  as an image bearing member provided inside an apparatus main assembly. The sheet feeding portion feeds a recording material P, stacked in a sheet cassette  16 , toward the conveying unit  13 . The conveying unit  13  including a registration device  17 , a transfer unit  30  and a fixing device  18  and discharges the recording material P after an image is transferred and fixed on the recording material P fed from the sheet feeding portion  12 . 
         [0030]    The registration device  17  sends the recording material to the transfer unit  30  in synchronism with the toner image on the photosensitive drum  15 . The transfer unit  30  attracts and conveys the recording material P on a transfer belt  31 , as a belt member, which is stretched by a driving roller  33  and a stretching roller  34  and which is moved in an arrow A direction in  FIG. 1 . The transfer belt  31  is urged together with the recording material P toward the photosensitive drum  15  by a transfer roller  35  urged by an urging means (not shown), thus forming a transfer nip between itself and the photosensitive drum  15 . 
         [0031]    Further, to the transfer roller  35 , a voltage of an opposite polarity to that of the toner image formed on the photosensitive drum  15 , so that the toner image is transferred at the transfer nip onto the recording material P attracted and conveyed on the transfer belt  31 . The recording material P on which the toner image is transferred is delivered from the transfer unit  30  to the fixing device  18 , and then the fixing device  18  fixes the toner image on the recording material P by heat and pressure of a pair of fixing rollers  19   a  and  19   b.    
         [0032]    The controller  14  controls operations of respective portions of the digital printer  10  in order to form the image on the recording material P as described above. 
         [0033]    The conveying unit  13  at least enables the transfer belt  31  as the belt member to be pulled out from an apparatus main assembly  20 . In an example shown in  FIG. 2 , the transfer unit  13  is supported mountable into demountable from the apparatus main assembly  20  via slide rails  21   a  and  21   b  with respect to a pulling-out direction. 
         [0034]    Further, the conveying unit  13  includes a locking mechanism (not shown), for the conveying unit  13 , which operates in interrelation with a rotation (movement) operation of a locking handle  22  which is rotatably supported operating portion. As shown in  FIG. 2 , the locking mechanism fixes the conveying unit  13  to the apparatus main assembly  20  at a locking position  221  of the locking handle  22  and releases (eliminates) the fixing of the conveying unit  13  to the apparatus main assembly  20  at a releasing position  222  of the locking handle  22 . 
         [0035]    Accordingly, when the conveying unit  13  is pulled out from the apparatus main assembly  20 , the locking handle  22  located at the locking position  221  is rotated in an arrow B direction in  FIG. 2  to be moved to the releasing position  222 , so that the fixing of the conveying unit  13  is released to enable the pulling-out of the conveying unit  13 . On the other hand, after the conveying unit  13  is inserted into the apparatus main assembly  20  until a mounting position, the locking handle  22  located at the releasing position  222  is rotated in a direction opposite from the arrow B direction to be moved to the locking position  221 , so that the conveying unit  13  can be fixed to the apparatus main assembly  20 . 
         [0036]    Further, the conveying unit  13  includes a transfer belt moving mechanism  40  for performing an operation in which the transfer belt  31  is contacted to or retracted from the photosensitive drum  15  (contact or separation operation). In other words, the transfer belt  31  is disposed movably toward and away from the photosensitive drum  15  by the transfer belt moving mechanism  40 . The transfer belt moving mechanism  40  performs a contact and retraction spacing) operates of the transfer belt  31  depending on the image forming operation and performs a retraction operation of the transfer belt  31  in interrelation with the releasing operation of the locking handle  22 . 
       [Transfer Belt Moving Mechanism] 
       [0037]    Next, with reference to  FIGS. 3 and 4 , the transfer belt moving mechanism  40  will be described in detail. The transfer unit  30  is, as shown in  FIG. 3 , constituted by a frame  32 , rollers consisting of the driving roller  32 , the stretching roller  34  and the transfer roller  35  which are supported by the frame  32 , and the transfer belt  31  stretched by the driving roller  33  and the stretching roller  34 . On a supporting shaft  33   a  of the driving roller  33 , a gear  33   b  is integrally provided, and the driving roller  33  is rotationally driven by receiving a driving force from a transfer belt driving motor (not shown) by the gear  33   b , thus moving the transfer belt  31  in the arrow A direction. Further, the supporting shaft  33   a  of the driving roller  33  is rotatably supported by a supporting member (not shown) provided to the conveying unit  13 . 
         [0038]    The transfer belt moving mechanism  40 , as shown in  FIG. 4 , constituted by a transfer unit urging portion  100 , a motor driving portion  200  and a handle-interrelated driving portion  300 . 
         [0039]    The transfer unit urging portion  100  includes cam shaft  41  to be rotatably supported, and an urging cam  42  which is a contact and separation member rotatable integrally with the cam shaft  41 . The urging cam  42  supports the supporting portion  32   a  of the frame  32  via a cam follower  43  and an urging member  44 . Further, the urging cam  42  is rotationally driven to move the transfer belt  31  toward and away from the photosensitive drum  15 . That is, when the urging cam  42  is rotated and its phase is moved to the top dead center, the supporting portion  32   a  is raised, so that the transfer belt  31  is contacted to the photosensitive drum  15 . On the other hand, when the phase is moved to the bottom dead center, the supporting portion  32   a  is lowered, so that the transfer belt  31  is retracted (spaced) from the direction  15 . 
         [0040]    Incidentally, as the contact and separation member, in place of the urging cam  42 , e.g., one such that rotation power is converted into an operation toward a contact and separation direction with respect to the photosensitive drum  15  by a gear mechanism may also be used. In short, the contact and separation member may only be required that it operates so that the transfer belt  31  is moved toward and away from the photosensitive drum  15  by being rotationally driven. 
         [0041]    Thus, the transfer unit urging portion  100  rotates the transfer unit  30  around the driving roller  33  with the rotation of the urging cam  42 , so that it can perform the contact and separation operation of the transfer belt  31  with respect to the photosensitive drum  15 . 
         [0042]    The motor driving portion  200  includes a driving motor  47  for rotationally driving the urging cam  42  and a one-way clutch  45 , provided between the urging cam  42  and the driving motor  47 , for transmitting unidirectional rotation. The rotation of the driving motor  47  is transmitted to the cam shaft  41  via an input gear  46  and the one-way clutch  45 , so that the urging cam  42  is rotationally driven. Drive transmission between the driving motor  47  and the input gear  46  is effected via an idler gear  48  and a timing belt  49 . 
         [0043]    Here, the one-way clutch  45  acts so that rotation of the input gear  46  in an arrow C direction in  FIG. 4  is transmitted to the cam shaft  41  but is not transmitted to the cam shaft  41  by idling the input gear  46  in an opposite direction to the arrow C direction. Incidentally, in place of the one-way clutch, e.g., another clutch such as an electromagnetic clutch by which power is transmitted during energy supply but is turned off during non-energy supply may also be used. 
         [0044]    Further, the motor driving portion  200  includes a phase-detecting sensor (not shown) for detecting the phase of the urging cam  42 . The controller  14  controls, as shown in  FIG. 5 , an operation of the driving motor  47  on the basis of a detection result of the phase-detecting sensor, so that the transfer belt  31  is moved toward and away from the photosensitive drum  15  depending on an image forming operation. 
         [0045]    The handle-interrelated driving portion  300  includes a gear having partly omitted teeth  51  which is a gear rotatable together with the urging cam  42 , a slide rack  50  which is a slidable member which is provided slidably with respect to the gear having partly omitted teeth  51 , and the locking handle  22  as the operating portion. The gear having partly omitted teeth  51  includes a toothed portion  51   a  where there are a plurality of teeth at a portion with respect to a rotational direction and includes a non-toothed portion  51   b  where there is no tooth at another (remaining) portion with respect to the rotational direction. In this embodiment, the gear having partly omitted teeth  51  is fixed to the cam shaft  41  and is rotated together with the cam shaft  41  and the urging cam  42 . 
         [0046]    The slide rack  50  includes a rack portion  50   a  where there are a plurality of teeth as a part of the rack portion  50   a  with respect to a sliding direction. The rack portion  50   a  is slid while being engaged with the gear portion  51   a  of the gear having partly omitted teeth  51 , so that the urging cam  42  is rotated via the gear having partly omitted teeth  51 . 
         [0047]    The locking handle  22  is operated by a user to move the slide rack  50  between from a locking position until a releasing position via an engaging position. That is, the slide rack  50  is connected with a handle shaft  52 , which is rotated integrally with the locking handle  22 , via a link member  53 , and is moved between a locking position  501  and a releasing position  502  in interrelation with a rotation operation of the locking handle  22 . 
         [0048]    Here, the locking position  501  is a position where the rack portion  50   a  and the gear portion  51   a  are not engaged in a state in which the transfer belt  41  is contacted to the photosensitive drum  15 . The engaging position is a position where the rack portion  50   a  and the gear portion  51   a  are engaged with each other. The releasing position  502  is a position where the rack portion  50   a  and the gear portion  51   a  are not engaged in a state in which the transfer belt  41  is spaced from the photosensitive drum  15 . Accordingly, the rack portion  50   a  is provided at a position where it is engaged with the gear portion  51   a  when the slide rack  50  is moved from the locking position  501  to the releasing position  502  but is not engaged with the gear portion  51   a  when the slide rack  50  is located at the locking position  501 . 
         [0049]    As shown in (a) of  FIG. 6 , the gear portion  51   a  of the gear having partly omitted teeth  51  is disposed so as to be engaged with the movable rack portion  50   a  when the phase of the urging cam  42  is located in the neighborhood of the top dead center and the transfer belt  31  is contacted or close to the photosensitive drum  15 . On the other hand, as shown in (b) of  FIG. 6 , when the urging cam  42  is moved to the bottom dead center where the transfer belt  31  is retracted from the photosensitive drum  15 , also the gear having partly omitted teeth  51  is rotated integrally with the urging cam  42 , so that the gear having partly omitted teeth  51  is moved to the engaging position through which the rack portion  50   a  passes. 
         [0050]    Here, as shown in (a) of  FIG. 7 , in the case where the phase of the urging cam  42  is located in the neighborhood of the top dead center, when the locking handle  22  is rotated from the locking position  221  to the releasing position  222 , the rack portion  50   a  moved in an arrow D direction is engaged with the gear portion  51   a . Then, with the movement of the rack portion  50   a  in the arrow D direction, the gear having partly omitted teeth  51  is rotated in the arrow C direction. 
         [0051]    At this time, by the action of the one-way clutch  45 , idling is generated between the cam shaft  41  and the input gear  46  and therefore a rotational load of the motor driving portion  200  is not exerted on the cam shaft  41 . That is, the one-way clutch  45  prevents the transmission of the rotation of the urging cam  42  to the driving motor  47  in the case where the slide rack  50  is slid from the locking position to the releasing position. 
         [0052]    Further, arrangement and the number of teeth of the rack portion  50   a  and the gear portion  51   a  are set so that the engagement is released to eliminate the drive (driving force) transmission until the gear having partly omitted teeth  51  reaches a stop position. Incidentally, the stop position referred to herein is a position where the phase of the urging cam  42  is moved to the bottom dead center or its neighborhood so that the transfer belt  31  can be sufficiently retracted from the photosensitive drum  15 . 
         [0053]    Then, as shown in (b) of  FIG. 7 , the gear having partly omitted teeth  51  to which the drive transmission from the rack portion  50   a  is eliminated is rotated, integrally with the urging cam  42  subjected to a load of the transfer unit  30 , to the stop position where the gear having partly omitted teeth  51  stops. At this time, when the gear having partly omitted teeth  51  does not stop but overruns, the urging cam  42  is rotated again toward the top dead center, so that there is a possibility that the transfer belt  31  is contacted again to the photosensitive drum  15 . 
         [0054]    Further, even when the urging cam  42  is not rotated to the top dead center by the overrun of the gear having partly omitted teeth  51 , there is a possibility that the gear portion  51   a  enters a passing region of the rack portion  50   a  of the slide rack  50 . In the case where there is the gear portion  51   a  in the passing region of the rack portion  50   a , thereafter when the slide rack  50  is moved to the locking position by operating the locking handle  22 , the rack portion  50   a  of the slide rack  50  image forms with the gear portion  51   a.    
         [0055]    Therefore, in the case of this embodiment, a rotation stopping mechanism  500  for preventing the gear having partly omitted teeth  51  and the urging cam  42 , which are rotated in interrelation with an abrupt releasing operation of the locking handle  22 , from overrunning the stop position is provided. 
       [Rotation Stopping Mechanism] 
       [0056]    In the following, the rotation stopping mechanism (overrun-preventing mechanism for the gear having partly omitted teeth)  500  will be described. The rotation stopping mechanism  500  stops the rotation of the gear having partly omitted teeth  51  so that the transfer belt  31  is not contacted to the photosensitive drum  15  in the case where the slide rack  50  is slid from the engaging position to the releasing position. Particularly in this embodiment, the rotation stopping mechanism  500  is configured to stop the rotation of the gear having partly omitted teeth  51  so that the gear having partly omitted teeth  51  opposes the passing region of the rack portion  50   a  in the case where the slide rack  50  is slid from the engaging position to the releasing position. 
         [0057]    For this purpose, the rotation stopping mechanism  500  includes a projection  51   c  as an engaging portion and a rotation stopping member  54  as a portion-to-be-engaged. The projection  51   c  is provided to the gear having partly omitted teeth  51  and is rotated together with the gear having partly omitted teeth  51 . The rotation stopping member  54  is constituted so that it is engaged with the projection  51   c  at the releasing position to stop the rotation of the gear having partly omitted teeth  51  but does not stop the rotation of the gear having partly omitted teeth  51  at the locking position and the engaging position. Such a rotation stopping member  54  operates in interrelation with the slide rack  50  so that it is engaged with the projection  51   c  at the releasing position but is not engaged with the projection  51   c  at the locking position and the engaging position. 
         [0058]    Description will be made specifically. The rotation stopping member  54  is supported, by the transfer belt moving mechanism  40 , slidably relative to the gear having partly omitted teeth  51  in a direction of movement toward and away from the gear having partly omitted teeth  51 . Therefore, in the rotation stopping member  54 , an elongated hole  54   a  extending in the sliding direction is formed, and a supporting shaft  55  provided to the transfer belt moving mechanism  40  is inserted into the elongated hole  54   a . As a result, the rotation stopping member  54  is slid within an engaging range between the elongated hole  54   a  and the supporting shaft  55 . Further, the rotation stopping member  54  is urged by a spring  56  as an urging member in an arrow E direction which is a direction in which the rotation stopping member  54  approaches the gear having partly omitted teeth  51 . 
         [0059]    Here, shapes and positions of the rotation stopping member  54  and the projection  51   c  are set so that the rotation stopping member  54  contacts the projection  51   c  in a section from the passing of the urging cam  42  through the bottom dead center until the gear portion  51   a  does not reach the engaging position which is the passing region of the rack portion  50   a.    
         [0060]    For this purpose, the rotation stopping member  54  includes a contact portion  54   b  contactable with a limiting (regulating) portion  50   b  as an end of the slide rack  50 . Further, as shown in (a) and (b) of  FIG. 6 , the contact portion  54   b  contacts the limiting portion  50   b  of the slide rack  50  located at the locking position  501 , and stops at the retracted position where the rotation of the gear having partly omitted teeth  51  is not prevented. On the other hand, when the slide rack  50  is moved from the locking position  501  to the releasing position  502 , the rotation stopping member  54  is moved in the arrow E direction with the movement of the slide rack  50 . Thereafter, as shown in (a) of  FIG. 7 , the rotation stopping member  54  enters a movement region of the projection  51   c , and moves to an operation position where an end of the elongated hole contacts the supporting shaft  55  to stop the rotation stopping member  54 . 
         [0061]    Accordingly, in the case where the gear having partly omitted teeth  51  rotated with the releasing operation of the locking handle  22  passes the stop position and continues its rotation, as shown in (b) of  FIG. 7 , the projection  51   c  contacts the rotation stopping member  54  moved to the operation position. As a result, it is possible to stop the rotation of the gear having partly omitted teeth  51  and the urging cam  42 . In this embodiment, in a state in which the rotation of the gear having partly omitted teeth  51  is stopped, the gear having partly omitted teeth  51  is configured to oppose the passing region of the rack portion  50   a.    
         [0062]    Incidentally, when the slide rack  50  is located at the locking position  501 , the rotation stopping member  54  is pushed by the slide rack  50  by the contact between the limiting portion  50   b  and the contact portion  54   b , thus being retracted from the movement region of the projection  51   c . For this reason, at the locking position, the rotation stopping member  54  does not prevent a normal operation for rotationally driving the urging cam  42  by the driving motor  47 . 
         [0063]    In the case of this embodiment, the rotation stopping mechanism  500  for stopping the rotation of the gear having partly omitted teeth as described above is provided. For this reason, when the locking handle  22  is operated so that the transfer belt  31  is spaced from the photosensitive drum  15 , it is possible to prevent the gear having partly omitted teeth  31  to overrun to bring the transfer belt  31  into contact with the photosensitive drum  15  again. 
         [0064]    Further, in the case of this embodiment, even in the case where the gear having partly omitted teeth  51  overruns the stop position, the rotation of the gear having partly omitted teeth  51  can be stopped before the gear portion  51   a  reaches the engaging position with the rack portion  50   a . For that reason, when the locking handle  22  is rotationally moved from the releasing position  222  to the locking position  221 , it is possible to prevent the contact between the overrun gear having partly omitted teeth  51  and the moving slide rack  50 . 
         [0065]    Incidentally, if only the re-contact between the transfer belt  31  and the photosensitive drum  15  by the overrun is intended to be prevented, the shapes and positions of the rotation stopping member  54  and the projection  51   c  may also be those other than the above-described constitutions. That is, before the urging cam  42  reaches the top dead center, it is only required that the rotation stopping member  54  and the projection  51   c  contact each other. Further, in this case, the gear having partly omitted teeth  51  may also be replaced with a gear having teeth on its full circumference. 
         [0066]    Further, the digital printer  10  includes, as the belt member, the transfer belt  31  for transferring the toner image while attracting and conveying the recording material P. However, even in another embodiment in which the belt member is an intermediary transfer belt for transferring the toner image from the image bearing member such as the photosensitive drum, the present invention is applicable. 
       Second Embodiment 
       [0067]    Second Embodiment of the present invention will be described with reference to  FIG. 8  to  FIG. 10 . In the case of this embodiment, the rotation stopping mechanism for the transfer belt moving mechanism is different from that in First Embodiment described above. Other constitutions and actions are the same as those in First Embodiment and therefore in the following, redundant description and illustration are omitted or simplified and repeated reference numeral or symbols and repeated names are used as they are, and a different portion will be principally described. 
         [0068]    A handle-interrelated driving portion  300   a  in this embodiment includes a slide rack  60  which is slidably supported and which is moved in interrelation with the rotation operation of the locking handle  22 , and includes a gear having partly omitted teeth  61  rotatable integrally with the cam shaft  41 . The slide rack  60  is moved in the arrow D direction with the rotation of the locking handle  22  when the releasing operation of the locking handle  22  is performed. At this time, when the transfer belt  31  is located at the contact position with the photosensitive drum  15 , a rack portion  60   a  provided to the locking handle  22  engages with a gear portion  61   a  of the gear having partly omitted teeth  61  to rotate the gear having partly omitted teeth  61  in the arrow C direction. Further, the phase of the urging cam  42  rotating integrally with the gear having partly omitted teeth  61  is moved from the top dead center to the bottom dead center, so that the transfer belt  31  is retracted from the photosensitive drum  15 . 
         [0069]    Further, the handle-interrelated driving portion  300   a  includes a rotation stopping mechanism  600  and prevents the overrun of the gear having partly omitted teeth  61  and the urging cam  42 . The rotation stopping mechanism  600  includes a rotation stopper  62  as a stopper portion and a stopper contact portion  60   b  as a contact portion. 
         [0070]    The rotation stopper  62  is rotatably supported by the gear having partly omitted teeth  61 . The rotation stopper  62  is rotatably supported by the cam shaft  41  to which the gear having partly omitted teeth  61  is fixed. To the rotation stopper  62 , a projection (boss)  62   a  as the portion-to-be-engaged is provided. The projection  62   a  is inserted into a limiting hole  61   c  provided as the engaging portion in the gear having partly omitted teeth  61 . The limiting hole  61   c  is an elongated hole along the rotational direction of the gear having partly omitted teeth  61 , and limits the movement of the projection  62   a  in a region of a non-toothed portion  61   b  of the gear having partly omitted teeth  61 . For this reason, the gear having partly omitted teeth  61  and the rotation stopper  62  permit movement of the projection  62   a  in the limiting hole  61   c  while relative movement thereof is limited (prevented). 
         [0071]    The stopper contact portion  60   b  is formed on the slide rack  60  so as to satisfy the following condition. That is, the stopper contact portion  60   b  contacts, at the releasing position, a stopper portion  62   b  provided to the rotation stopper  62  to stop the rotation of the rotation stopper  62  in the same direction as the rotational direction of the gear having partly omitted teeth  61  in the case where the slide rack  60  is slid from the engaging position to the releasing position. On the other hand, the stopper contact portion  60   b  is configured not to contact the stopper  62   b  at the locking position and the engaging position. In an example of  FIG. 8 , the stopper contact portion  60   b  is formed so that its contact surface with the stopper  62   b  is located at a position higher than a tooth top of the rack portion  60   a  in a range ranging to the end portion of the slide rack  60  including the rack portion  60   a . Incidentally, the stopper contact portion  60   b  is formed at a position deviated from the gear having partly omitted teeth  61  with respect to a rotational axis direction of the gear having partly omitted teeth  61 , so that it contacts the stopper  62   b  but does not image form with the gear having partly omitted teeth  61 . 
         [0072]    An operation of the thus-constituted rotation stopping  600  will be described with reference to  FIGS. 9 and 10 . Part (a) of  FIG. 9  shows a state in which the transfer belt  31  is contacted to the photosensitive drum  15 , and (c) of  FIG. 9  shows a state in which the transfer belt  31  is retracted from the photosensitive drum  15 . Further, (b) and (d) of  FIG. 9  show a transition process from the state of (a) of  FIG. 9  to the state of (c) of  FIG. 9  and a transition process from the state of (c) of  FIG. 9  to the state of (a) of  FIG. 9 , respectively. 
         [0073]    The gear having partly omitted teeth  61  is rotated in the arrow C direction integrally with the urging cam  42 , rotationally driven by the driving motor  47 , in order to move the transfer belt  31  toward and away from the photosensitive drum  15  depending on the image forming operation. Further, the rotation stop per  62  is rotated in the arrow C direction with the rotation of the gear having partly omitted teeth  61  while repeating the contact and separation of its projection  62   a  with respect to the inner surface of the limiting hole  61   c . On the other hand, the slide rack  60  maintains the stop state at the locking position. 
         [0074]    The shape of the stopper  62   b  provided to the rotation stopper  62  and the shape of the stopper contact portion  60   b  provided to the slide rack  60  are set so that the portions  62   b  and  60   b  do not contact each other in the case where the slide rack  60  is located at the locking position. 
         [0075]    Further, in the state shown in (c) of  FIG. 9  in which the transfer belt  31  is retracted from the photosensitive drum  15 , the rotation stopper  62  enters the movement path (the passing region) of the rack portion  60   a  of the slide rack  60  and then stops. In such a case, the stopper contact portion  60   b  of the slide rack  60  moving in the arrow D direction interferes with the rotation stopper  62 . 
         [0076]    However, as shown in (a) of  FIG. 10 , the rotation stopper  62  with which the stopper contact portion  60   b  interferes is rotated in the limiting hole  60   c  in the arrow C direction to run up onto the stopper contact portion  60   b , so that the rotation stopper  62  can avoid the interference. Then, in the case where the gear having partly omitted teeth  61  overruns the stop position when the releasing operation of the locking handle  22  is performed, as shown in (b) of  FIG. 10 , the stopper  62   b  contacts the stopper contact portion  60   b . Further, the inner surface of the limiting hole  61   c  contacts the projection  62   a , so that the rotation of the gear having partly omitted teeth  61  and the urging cam  42  is stopped. The position where the rotation of the gear having partly omitted teeth  61  and the urging cam  42  is the same as that in First Embodiment. 
         [0077]    Here, a relative movable range of the limiting hole  61   c  and the projection  62   a  is set so that it does not interfere with the stopper contact portion  60   b  to which the rotation stopper  62  moves and so that the gear portion  61   a  of the gear having partly omitted teeth  61  stopped during the overrun does not enter the engaging position. 
         [0078]    In the case of this embodiment, the rotation of the overrun gear having partly omitted teeth  61  can be stopped by the contact between the stopper  62   b  and the stopper contact portion  60   b  and by the contact between the inner surface of the limiting hole  61   c  and the projection  62   a . Further, even in the case where the slide rack  60  interferes with the rotation stopper  62  during the movement, the rotation stopper  62  runs up onto the stopper contact portion  60   b , so that the interference with the slide rack  60  can be avoided. 
       Third Embodiment 
       [0079]    Third Embodiment of the present invention will be described with reference to  FIG. 11  to  FIG. 13 . In the case of this embodiment, the rotation stopping mechanism for the transfer belt moving mechanism is different from that in First Embodiment described above. Other constitutions and actions are the same as those in First Embodiment and therefore in the following, redundant description and illustration are omitted or simplified and repeated reference numeral or symbols and repeated names are used as they are, and a different portion will be principally described. 
         [0080]    A handle-interrelated driving portion  300   b  includes a slide rack  70  which is slidably supported and which is moved in interrelation with the rotation operation of the locking handle  22 , and includes a gear having partly omitted teeth  71  rotatable integrally with the cam shaft  41 . The gear having partly omitted teeth  71  includes a gear portion  71   a  and a non-toothed portion  71   b.    
         [0081]    Further, also in this embodiment, the handle-interrelated driving portion  300   b  includes a rotation stopping mechanism  700  and prevents the overrun of the gear having partly omitted teeth  71  and the urging cam  42 . The rotation stopping mechanism  700  includes a stop per portion  71  as the engaging portion and a stopper contact portion (rotation stopping portion)  70   b  as the portion-to-be-engaged. 
         [0082]    The stopper portion  71   c  is fixed on a side surface of the gear having partly omitted teeth  71  and is rotated together with the gear having partly omitted teeth  71 . Such a stopper portion  71   c  includes a toothed surface shape portion  71   e  disposed in the same phase as a starting tooth  71   d  in the upstream side with respect to the rotational direction (arrow C direction) of the gear portion  71   a  of the gear having partly omitted teeth  71 . At least an end portion of the toothed surface shape portion  71   e  at the downstream surface with respect to the rotational direction is formed so that it is flush with a downstream tooth surface of the tooth  71   d  with respect to the rotational direction. 
         [0083]    The stopper contact portion  70   b  is formed on the slide rack  70  so as to satisfy the following condition. That is, the stopper contact portion  70   b  contacts, at the releasing position, the stopper portion  71   c  to stop the rotation of the stopper portion  71   c  in the same direction as the rotational direction of the gear having partly omitted teeth  71  in the case where the slide rack  70  is slid from the engaging position to the releasing position. On the other hand, the stopper contact portion  70   b  is configured not to contact the stopper portion  71   c  at the locking position and the engaging position. In an example of  FIG. 11 , the stopper contact portion  70   b  is formed so that its contact surface with the stopper portion  71   c  is located at a position higher than a tooth top of the rack portion  70   a  in a range ranging to the end portion of the slide rack  70  including the rack portion  70   a . Incidentally, the stopper contact portion  70   b  is formed at a position deviated from the gear having partly omitted teeth  71  with respect to a rotational axis direction of the gear having partly omitted teeth  71 , so that it contacts the stopper portion  71   c  but does not image form with the gear having partly omitted teeth  71 . 
         [0084]    Further, the stopper contact portion  70   b  has an inclined surface  70   c  which is aligned with the tooth surface of one of racks of the rack portion  70   a  in the movement direction (arrow D direction) side during the releasing operation of the locking handle  22 . In the example of  FIG. 11 , the inclined surface  70   c  is aligned with the tooth surface of the end of the rack portion  70   a  with respect to the movement direction during the releasing operation. 
         [0085]    An operation of the thus-constituted rotation stopping mechanism  700  will be described with reference to  FIGS. 12 and 13 . 
         [0086]    In the case where the gear having partly omitted teeth  71  overruns when the releasing operation of the locking handle  22  is performed, as shown in (a) of  FIG. 12 , the stopper portion  71   c  contacts the stopper contact portion  70   b , thus stopping the rotation of the gear having partly omitted teeth  71  and the urging cam  42 . The position where the rotation of the gear having partly omitted teeth  71  and the urging cam  42  is the same as that in First Embodiment. 
         [0087]    On the other hand, as shown in (b) of  FIG. 12 , the case where when the stopper portion  71   c  stops at the position such that it interferes with the stopper contact portion  70   b  moving in the arrow D direction, the releasing operation of the locking handle  22  will be described. 
         [0088]    As shown in (a) and (b) of  FIG. 13 , the inclined surface  70   c  of the rack having the toothed surface shape is moved while contacting the toothed surface shape portion  71   e  of the stopper portion  71   c , so that the rack portion  70   a  and the gear portion  71   a  are engaged with each other and thus the transfer belt  31  can be moved to the retracted position. 
         [0089]    In the case of this embodiment, the rotation of the overrun gear having partly omitted teeth  71  can be stopped by the contact between the stopper portion  71   c  and the stopper contact portion  70   b . Further, in the case where the stopper contact portion  70   b  interferes with the stopper portion  71   c  during the movement, of the slide rack  70 , the rack portion  70   a  and the gear portion  71   a  are engaged with each other, so that the gear having partly omitted teeth  71  can be rotated. 
       Fourth Embodiment 
       [0090]    Fourth Embodiment of the present invention will be described with reference to  FIG. 14  to  FIG. 16  in combination with  FIG. 4 . In the case of this embodiment, the rotation stopping mechanism for the transfer belt moving mechanism is different from that in First Embodiment described above. Other constitutions and actions are the same as those in First Embodiment and therefore in the following, redundant description and illustration are omitted or simplified and repeated reference numeral or symbols and repeated names are used as they are, and a different portion will be principally described. 
         [0091]    A handle-interrelated driving portion  300   c  includes a rotation stopping mechanism  800  and prevents overrun of the urging cam  42 . In the case of this embodiment, the rotation stopping mechanism  800  includes a solenoid  80  as an actuator. The solenoid  80  moves a solenoid arm  81   a  as the portion-to-be-engaged to a position, in a non-energy supply state, where the solenoid arm  81   a  is engageable with the projection  42   a  as the engaging portion, and moves the solenoid arm  81   a  to a position, in an energy supply state, where the solenoid arm  81   a  is not engageable with the projection  42   a.    
         [0092]    The solenoid  80  is fixed on the conveying unit  13  and includes the solenoid arm  81   a  and an elastic member  81   b  for uniting the solenoid arm  81   a  in the arrow D direction. The urging cam  42  is fixed to the cam shaft  41  and includes the projection  42   a  rotatable together with the urging cam  42  and the cam shaft  41 . The projection  42   a  is formed so as to project in a diameter direction of the cam shaft  41 . Further, the projection  42   a  is engaged with the solenoid arm  81   a , so that the rotation of the gear having partly omitted teeth  51  and the urging cam  42  is stopped. 
         [0093]    The shapes and positions of the solenoid arm  81   a  and the projection  42   a  are set so that the solenoid arm  81   a  contacts the projection  42   a  in a section from the passing of the urging cam  42  through the bottom dead center until the gear portion  51   a  does not reach the engaging position with the rack portion  50   a.    
         [0094]    An operation of the thus-constituted rotation stopping mechanism  800  will be described with reference to  FIG. 15 . As shown in (a) of  FIG. 15 , only when the urging cam  42  is moved from the bottom dead center to the top dead center, the solenoid  80  is placed in the energy supply state. As a result, the solenoid arm  81   a  is attracted, so that the solenoid arm  81   a  is moved to a position  83  and thus is retracted from the movement range of the projection  42   a . Then, the urging cam  42  can be freely rotated. 
         [0095]    On the other hand, as shown in (b) of  FIG. 15 , when the urging cam  42  is moved to the top dead center, the energy supply to the solenoid  80  is turned off (the non-energy supply state). As a result, the solenoid arm  81   a  is moved to the position  82  again, thus entering the movement range of the projection  42   a.    
         [0096]    In this case, when an abrupt operation of the locking handle  22  is performed in a process in which the urging cam  42  is moved from the top dead center to the bottom dead center, the gear having partly omitted teeth  51  and the urging cam  42  which are rotated in interrelation with motion of the locking handle  22  overrun. However, the projection  42   a  contacts the solenoid arm  81   a  moved to the position  82 , so that the rotation of the gear having partly omitted teeth  51  can be stopped. 
         [0097]    A sequential operation in the case where the urging cam  42  is rotated through one-full-circumference from the bottom dead center is shown in  FIG. 16 . First, when the urging cam  42  is positioned at the bottom dead center (S 1 ), energy (electric power) is supplied to the solenoid  80  (S 2 ), so that the solenoid arm  81   a  is moved to the position  83  (S 3 ). As a result, the rotation of the urging cam  42  is not stopped, so that the urging cam  42  can be moved to the top dead center (S 4 ), and thus the transfer belt  31  can be contacted to the photosensitive drum  15  (S 5 ). In this state, the energy supply to the solenoid  80  is turned off (S 6 ). Thus, the solenoid arm  81   a  is moved to the position  82  (S 7 ). Next, in order to space the transfer belt  31  from the photosensitive drum  15 , the locking handle  22  is operated to rotate the urging cam  42 , so that the urging cam  42  is moved to the bottom dead center (S 8 ). At this time, even when the urging cam  42  overruns, the projection  42   a  contacts the solenoid arm  81   a , so that the urging cam  42  is prevented from being further rotated. 
         [0098]    In the case of this embodiment, by providing the rotation stopping mechanism  800 , even in the case where the gear having partly omitted teeth  51  overruns the stop position, the rotation of the gear having partly omitted teeth  51  can be stopped before the gear portion  51   a  reaches the engaging position with the rack portion  50   a . Further, by using the solenoid  80 , the rotation of the urging cam  42  can be limited irrespective of a state of the machine so long as intentional energy supply to the solenoid  80  is not effected, so that it is possible to prevent the overrun of the gear having partly omitted teeth  51  caused by the abrupt handle operation or the like. 
         [0099]    While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims. 
         [0100]    This application claims priority from Japanese Patent Application No. 044513/2012 filed Feb. 29, 2012, which is hereby incorporated by reference.