Patent Publication Number: US-9897961-B2

Title: Image forming apparatus provided with transmission mechanism capable of interrupting transmission of rotational force to reconveying roller

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2015-096729 filed May 11, 2015. The entire content of the priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an image forming apparatus capable of forming images on both sides of sheets (hereinafter referred to as “duplex printing”). 
     BACKGROUND 
     There is conventionally known an image forming apparatus that is provided with a pendulum gear mechanism. The pendulum gear mechanism switches the transmission path for a rotational force generated by an electric motor to a path for transmitting the force to reconveying rollers. In this way, the image forming apparatus can transmit a unidirectional rotational force to the reconveying rollers, regardless of the direction in which the electric motor rotates. 
     Here, the reconveying rollers are conveying rollers used in a duplex printing operation for conveying a sheet having an image formed on one surface back to an image-forming mechanism to have an image formed on the other surface. The pendulum gear mechanism is a gear mechanism that can switch the state of a planetary gear or other pendulum gear between a state engaged directly with an output gear for transmitting the rotational force to the output gear, and a state engaged with an intermediate gear (idle gear) for transmitting the rotational force to the output gear through the intermediate gear. 
     SUMMARY 
     However, in the above-described conventional image forming apparatus, the rotational force is transmitted to the reconveying rollers even when performing simplex (single-sided) printing to form an image on only one side of the sheet. Thus, in the conventional image forming apparatus described above, the reconveying rollers rotate even when such rotation is unnecessary. 
     This unnecessary rotation of the reconveying rollers not only generates noise, but also leads to premature wear in the reconveying rollers and bearings and the like associated with the reconveying rollers. 
     In view of the foregoing, it is an object of the disclosure to provide an image forming apparatus capable of reducing noise generated by rotation of reconveying rollers. 
     In order to attain the above and other objects, according to one aspect, the disclosure provides an image forming apparatus configured to form images on both sides of a sheet, the image forming apparatus comprising: an image-forming mechanism; a discharge tray; a switchback roller; a reconveying roller; an electric motor; and a transmission mechanism. The image-forming mechanism is configured to form an image on a sheet while conveying the sheet. The discharge tray is configured to receive a sheet on which an image is formed. The switchback roller is configured to rotate in one of a normal rotation mode and a reverse rotation mode. The switchback roller in the normal rotation mode conveys a sheet discharged from the image-forming mechanism toward the discharge tray. The switchback roller in the reverse rotation mode conveys the sheet conveyed toward the discharge tray back toward the image-forming mechanism. The reconveying roller is configured to rotate while contacting a sheet to be conveyed back toward the image-forming mechanism to reconvey the sheet toward the image-forming mechanism. The electric motor is configured to supply the image-forming mechanism, the switchback roller, and the reconveying roller with a rotational force for conveying a sheet. The transmission mechanism is configured to operate in one of a reverse-rotation transmission mode and a normal-rotation transmission mode. The transmission mechanism in the reverse-rotation transmission mode reverses a direction of the rotational force received from the electric motor to transmit the reversed rotational force to the reconveying roller. The transmission mechanism in the normal-rotation transmission mode transmits the rotational force received from the electric motor to the reconveying roller without reversing the direction of the rotational force received from the electric motor. The transmission mechanism includes an interrupting unit configured to interrupt transmission of a rotational force from the reconveying roller to the electric motor at least when a sheet is being conveyed in the image-forming mechanism. The transmission mechanism is configured to interrupt transmission of the rotational force received from the electric motor to the reconveying roller when a sheet is being conveyed in the image-forming mechanism. 
     According to another aspect, the disclosure provides a transmission mechanism including: a sun gear; an output gear; a first intermediate gear; a second intermediate gear; and a locking mechanism. The sun gear is configured to receive a rotational force and configured to rotate about a rotational axis in a first rotational direction and in a second rotational direction opposite to the first rotational direction. The output gear is configured to output a rotational force. The first intermediate gear meshes with the output gear. The second intermediate gear, being a sector gear, includes: a toothed part in which teeth are formed; a toothless part in which no teeth are formed; and an engagement part. The interrupting unit is configured to be coupled to the first intermediate gear and the second intermediate gear. The interrupting unit is configured to transmit a rotational force from the second intermediate gear to the first intermediate gear and configured to interrupt a rotational force from the first intermediate gear to the second intermediate gear. The locking mechanism includes: a revolving member; a first planetary gear; a second planetary gear; and a spring. The revolving member has an engaging part engageable with the engagement part of the second intermediate gear. The revolving member is configured to move between an engaging position where the engaging part is engageable with the engagement part of the second intermediate gear and a release position where the engaging part is separated from the engagement part of the second intermediate gear. The revolving member moves to the engaging position in response to the rotation of the sun gear in the first rotational direction and moves to the release position in response to the rotation of the sun gear in the second rotational direction. The first planetary gear meshes with the sun gear and is supported at the revolving member. The first planetary gear is configured to revolve about the rotational axis of the sun gear along with the revolving member. The first planetary gear is configured to mesh with the output gear in response to the rotation of the sun gear in the second rotational direction. The second planetary gear meshes with the sun gear and is supported at the revolving member. The second planetary gear is configured to revolve about the rotational axis of the sun gear along with the revolving member. The second planetary gear is configured to move to a position capable of meshing with the toothed part of the second intermediate gear in response to the rotation of the sun gear in the first rotational direction. The spring is configured to apply a force to the second intermediate gear to rotate the second intermediate gear in the first rotational direction. While the sun gear rotates in the first rotational direction and the second planetary gear faces the toothless part of the second intermediate gear, the revolving member is in the engaging position, and a rotational force of the sun gear rotating in the first rotational direction is transmitted to the second planetary gear facing the toothless part of the second intermediate gear. When the sun gear changes the rotational direction from the first rotational direction to the second rotational direction, the first planetary gear and the second planetary gear revolve in the second rotational direction about the rotational axis of the sun gear to move the revolving member from the engaging position to the release position, and the spring rotates the second intermediate gear to a position where the toothed part of the second intermediate gear is capable of meshing with the second planetary gear when the revolving member is returned from the release position to the engaging position. While the revolving member is in the release position, the first planetary gear meshes with the output gear, and a rotational force of the sun gear rotating in the second rotational direction is transmitted to the output gear through the first planetary gear. When the sun gear changes the rotational direction from the second rotational direction to the first rotational direction, the first planetary gear and the second planetary gear revolve in the first rotational direction about the rotational axis of the sun gear to move the revolving member from the release position to the engaging position, and the second planetary gear meshes with the toothed part of the second intermediate gear to rotate the second intermediate gear until the toothless part of the second intermediate gear faces the second planetary gear, and a rotational force of the sun gear rotating in the first rotational direction is transmitted to the output gear through the second planetary gear, the toothed part of the second intermediate gear, the interrupting unit, and the first intermediate gear while the second planetary gear meshes with the toothed part of the second intermediate gear. When the toothless part of the second intermediate gear has returned to a position facing the second planetary gear as a result of the rotation of the second intermediate gear through meshing with the secondary planetary gear while the sun gear rotates in the first rotational direction, transmission of the rotational force of the sun gear rotating in the first rotational direction to the output gear is interrupted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the embodiment(s) as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic central cross-sectional view of an image forming apparatus  1  according to one embodiment; 
         FIG. 2A  is a perspective view of a transmission mechanism  30  provided in the image forming apparatus  1  according to the embodiment; 
         FIG. 2B  is an exploded perspective view of the transmission mechanism  30 ; 
         FIG. 3A  is a front view of the transmission mechanism  30 ; 
         FIG. 3B  is a left side view of the transmission mechanism  30 ; 
         FIG. 3C  is a cross-sectional view of the transmission mechanism  30  taken along a line C-C in  FIG. 3B ; 
         FIG. 4A  is a front view of the transmission mechanism  30 ; 
         FIG. 4B  is a left side view of the transmission mechanism  30 ; 
         FIG. 4C  is a cross-sectional view of the transmission mechanism  30  taken along a line C-C in  FIG. 4B ; 
         FIG. 5A  is a front view of the transmission mechanism  30 ; 
         FIG. 5B  is a left side view of the transmission mechanism  30 ; 
         FIG. 5C  is a cross-sectional view of the transmission mechanism  30  taken along a line C-C in  FIG. 5B ; 
         FIG. 6  is a timing chart of the image forming apparatus  1 ; 
         FIG. 7  is a schematic central cross-sectional view of the image forming apparatus  1 , illustrating a sheet conveying state at a region A in  FIG. 6 ; 
         FIG. 8  is a schematic central cross-sectional view of the image forming apparatus  1 , illustrating a sheet conveying state following the state shown in  FIG. 7 ; 
         FIG. 9  is a schematic central cross-sectional view of the image forming apparatus  1 , illustrating a sheet conveying state following the state shown in  FIG. 8 ; and 
         FIG. 10  is a view of an input-side intermediate gear  35  of the transmission mechanism  30  illustrating a toothless part  35 A and a toothed part  35 B thereof. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiment 
     An image forming apparatus  1  according to one embodiment will be described with reference to the accompanying drawings, wherein like parts and components are designated by the same reference numerals to avoid duplicating description. 
     1. Structure of Image Forming Apparatus 
     1.1 Overall Structure 
       FIG. 1  shows a monochromatic, electrophotographic image forming apparatus  1 . The image forming apparatus  1  includes a casing  3 , and an image-forming mechanism  5  accommodated in the casing  3 . 
     In the following description, arrows indicating directions and the like in the drawings are intended to facilitate understanding of how the drawings relate to each other, but the disclosure is not limited to the specified directions. 
     Further, the image forming apparatus  1  according to the embodiment is provided with at least one of each part and component designated with a symbol or reference numeral, except when the numbers of the parts and components are specifically stated as being a “plurality,” “two or more,” or the like. 
     The image-forming mechanism  5  is adapted to form images on sheets of paper or another recording medium. The image-forming mechanism  5  includes a developing cartridge  7 , a photosensitive drum  8 , an exposure unit  9 , a fixing unit  11 , and a pair of registration rollers  19 . 
     The developing cartridge  7  includes a developing roller  7 A, and a storage section  7 B. The photosensitive drum  8  is adapted to carry developer images on its circumferential surface. After a charger  8 A applies a charge to the circumferential surface of the photosensitive drum  8 , the exposure unit  9  exposes the charged photosensitive drum  8 , forming an electrostatic latent image on the circumferential surface of the photosensitive drum  8 . 
     The developing roller  7 A is adapted to supply developer accommodated in the storage section  7 B to the photosensitive drum  8 , forming a developer image on the photosensitive drum  8 . A transfer roller  13  is disposed at a position confronting the photosensitive drum  8 . 
     The transfer roller  13  is adapted to transfer the developer image carried on the photosensitive drum  8  to a sheet passing between the photosensitive drum  8  and the transfer roller  13 . 
     The fixing unit  11  is adapted to fix the transferred developer to the sheet. Specifically, the fixing unit  11  includes a heating roller  11 A, and a pressure roller  11 B. 
     The heating roller  11 A is adapted to directly or indirectly heat the developer image on the sheet, while the pressure roller  11 B presses the sheet against the heating roller  11 A. Subsequently, the fixing unit  11  conveys the sheet toward a discharge tray  3 A formed on a top surface of the casing  3 . Thus, sheets are received in the discharge tray  3 A after images have been formed thereon. 
     The image forming apparatus  1  also includes a feeding mechanism  15  disposed upstream of the image-forming mechanism  5  in a sheet-conveying direction. The feeding mechanism  15  is adapted to feed sheets one at a time from a paper tray  17  toward the image-forming mechanism  5 . 
     The paper tray  17  is removably mounted in the casing  3 . A user can remove the paper tray  17  from the casing  3  in order to load sheets into the paper tray  17  or change the types of sheets loaded therein. 
     The sheets placed on the paper tray  17  are conveyed along a conveying path L 1  leading from the paper tray  17  to the discharge tray  3 A via the image-forming mechanism  5 . The pair of registration rollers  19  is provided on the conveying path L 1  at a position upstream of the photosensitive drum  8  in the sheet-conveying direction. 
     The pair of registration rollers  19  is adapted to correct the orientation of sheets to be fed into the image-forming mechanism  5 . More specifically, rotation of the registration rollers  19  is halted when a leading edge of a sheet relative to the sheet-conveying direction arrives at the registration rollers  19  or just prior to the leading edge arriving at the registration rollers  19 . 
     Consequently, the leading edge of the sheet contacts outer circumferential surfaces of the non-rotating registration rollers  19 , causing the sheet&#39;s orientation to be corrected until the leading edge of the sheet is aligned with the outer circumferential surfaces of the registration rollers  19 . Subsequently, the registration rollers  19  resume rotating, drawing the sheet into the image-forming mechanism  5  (to the photosensitive drum  8 ). 
     The image forming apparatus  1  is further provided with a pre-registration sensor S 1  positioned upstream of the registration rollers  19  in the sheet-conveying direction, and a post-registration sensor S 2  positioned downstream of the registration rollers  19  in the sheet-conveying direction. The pre-registration sensor S 1  and the post-registration sensor S 2  (hereinafter also collectively referred to as registration sensors S) are adapted to output signals indicating whether a sheet is present at their respective positions. 
     That is, the signals outputted by the registration sensors S correspond to whether a sheet is present or not. Specifically, the registration sensors S output a Lo level signal when a sheet has been detected, and a Hi level signal when a sheet has not been detected, for example. 
     In other words, the registration sensors S output a Lo level signal when the leading edge of a sheet relative to the sheet-conveying direction arrives at their positions, and output a Hi level signal when the trailing edge of the sheet relative to the conveying direction arrives at their positions. 
     Start and stop of rotation of the registration rollers  19  are controlled by selectively supplying an interrupting current to an electromagnetic clutch (not shown). Specifically, rotation of the registration rollers  19  is halted when a prescribed time has elapsed after the post-registration sensor S 2  detects the trailing edge of a sheet. Rotation of the registration rollers  19  is resumed when a prescribed time has elapsed after the pre-registration sensor S 1  detects the leading edge of a sheet. 
     A discharge roller  21  is provided at a position downstream of the fixing unit  11  in the sheet-conveying direction. The discharge roller  21  functions to discharge sheets into the discharge tray  3 A. In addition to this function, the discharge roller  21  also has a reconveying function for reversing the conveyed direction of a sheet exiting the fixing unit  11  and reconveying the sheet back to the photosensitive drum  8 . 
     That is, the image forming apparatus  1  according to the embodiment can selectively execute one of two printing modes: a simplex printing mode for forming an image on only one side of a sheet; and a duplex printing mode for forming images on both sides of a sheet. Hereinafter, the discharge roller  21  will also be referred to as a “switchback roller  21 .” A pinch roller  21 A is disposed in confrontation with the switchback roller  21  for pressing a sheet against the switchback roller  21  while following the rotation of the switchback roller  21 . 
     In the following description, a mode in which the switchback roller  21  rotates for conveying a sheet toward the discharge tray  3 A will be referred to as a normal rotation mode, while a mode in which the switchback roller  21  rotates for conveying a sheet back toward the image-forming mechanism  5  will be referred to as a reverse rotation mode. 
     When the image forming apparatus  1  is operating in the duplex printing mode, the switchback roller  21  reverses the conveyed direction of a sheet after an image has been formed on one side thereof and reconveys the sheet along a reconveying path L 2 . The reconveying path L 2  is a sheet-conveying path leading from the switchback roller  21  toward the photosensitive drum  8 . 
     1.2 Structure of Reconveying Path 
     The reconveying path L 2  branches from the conveying path L 1  in an area downstream in the sheet-conveying direction of the fixing unit  11  that will be referred to as a branch point L 3 , and rejoins the conveying path L 1  in an area upstream in the sheet-conveying direction of the pre-registration sensor S 1  that will be referred to as a rejoining point L 4 . The reconveying path L 2  includes a conveying path L 5  between the branch point L 3  and the rejoining point L 4 . The conveying path L 5  is offset below the image-forming mechanism  5  including the photosensitive drum  8 . 
     Curved conveying paths L 6  and L 7  are respectively provided on upstream and downstream of the conveying path L 5  for connecting the conveying path L 5  to the conveying path L 1 . The curved conveying paths L 6  and L 7  function to change the direction of the conveyed sheet after the sheet have been discharged from the fixing unit  11 . 
     A pair of first reconveying rollers  23  and a pair of second reconveying rollers  25  are provided on the conveying path L 5 . The pair of first reconveying rollers  23  is disposed on the exit side of the curved conveying path L 6  and convey sheets downstream of the conveying path L 5 . 
     The pair of second reconveying rollers  25  is disposed on the entrance side of the curved conveying path L 7  and reconveys the sheets toward the pair of registration rollers  19 . The pair of registration rollers  19  is disposed on the exit side of the curved conveying path L 7 . In the following description, the pair of first reconveying rollers  23  and the pair of second reconveying rollers  25  will be collectively referred to as reconveying rollers  24 . 
     2. Rotation Control of Switchback Roller and Reconveying Rollers 
     2.1 Overview of Rotation Control 
     As shown in  FIG. 1 , the image forming apparatus  1  is provided with a single electric motor  27  (an example of an electric motor) that is adapted to supply a rotational force to the image-forming mechanism  5 , the switchback roller  21 , and the reconveying rollers  24 . The image forming apparatus  1  is also provided with a gear mechanism  29  having a plurality of gears and the like for transmitting the rotational force generated by the electric motor  27  to the switchback roller  21  and the like. 
     A control unit  27 A is provided in the image forming apparatus  1  for controlling the rotation of the electric motor  27 , and specifically for controlling when the electric motor  27  rotates in the normal direction and the reverse direction and when the electric motor  27  is halted. The control unit  27 A controls the rotation of the electric motor  27  according to a pre-stored program (software) and based on signals from the pre-registration sensor S 1 . 
     The control unit  27 A is configured of a microcomputer having a CPU, ROM, RAM, and the like. The program used for implementing control is stored in the ROM or another nonvolatile storage unit. 
     The rotating directions of the switchback roller  21 , the pair of registration rollers  19 , the photosensitive drum  8 , the heating roller  11 A, and the like are linked to the rotating direction of the electric motor  27 . Specifically, when the electric motor  27  rotates in the normal direction, the switchback roller  21 , the pair of registration rollers  19 , the photosensitive drum  8 , the heating roller  11 A, and the like rotate in directions for conveying sheets toward the discharge tray  3 A (hereinafter referred to as normal rotation). 
     When the electric motor  27  rotates in the reverse direction, the switchback roller  21 , the pair of registration rollers  19 , the photosensitive drum  8 , the heating roller  11 A, and the like rotate in the directions opposite their normal rotations (hereinafter referred to as reverse rotation). 
     The gear mechanism  29  includes a transmission mechanism  30  that transmits the rotational force of the electric motor  27  to the reconveying rollers  24 . Accordingly, the rotating direction of the reconveying rollers  24  does not always match the rotating direction of the electric motor  27 , depending on transmission modes of the transmission mechanism  30 . 
     More specifically, the transmission mechanism  30  can operate in one of a reverse-rotation transmission mode and a normal-rotation transmission mode. In the reverse-rotation transmission mode, the transmission mechanism  30  reverses the direction of the rotational force received from the electric motor  27  and transmits this reversed force to the reconveying rollers  24 . In the normal-rotation transmission mode, the transmission mechanism  30  transmits the rotational force received from the electric motor  27  to the reconveying rollers  24  without reversing the direction of the rotational force. 
     The transmission mechanism  30  also has a first interrupting function and a second interrupting function. The first interrupting function serves to interrupt transmission of a rotational force from the reconveying rollers  24  to the electric motor  27  at least while a sheet is being conveyed in the image-forming mechanism  5 . The second interrupting function serves to interrupt transmission of a rotational force to the reconveying rollers  24  while a sheet is being conveyed in the image-forming mechanism  5 . 
     2.2 Configuration of Transmission Mechanism 
     &lt;First Interrupting Function&gt; 
     The first interrupting function is implemented using an interrupting unit  31  shown in  FIG. 2B . The interrupting unit  31  includes a ratchet gear  31 A, a pawl gear  31 B, and a spring (not shown) that presses the ratchet gear  31 A toward the pawl gear  31 B. 
     The ratchet gear  31 A is displaceable in a direction of its rotational axis. The pawl gear  31 B is capable of engaging with the ratchet gear  31 A. When a rotational force is inputted into the ratchet gear  31 A, the ratchet gear  31 A and the pawl gear  31 B are maintained in an engaged state. Hence, the rotational force is transmitted from the ratchet gear  31 A to the pawl gear  31 B. 
     When a rotational force is inputted into the pawl gear  31 B, a force in the axial direction is generated by engaged parts of the ratchet gear  31 A and the pawl gear  31 B, forcing the ratchet gear  31 A and the pawl gear  31 B to separate. Accordingly, transmission of the rotational force from the pawl gear  31 B to the ratchet gear  31 A is interrupted. 
     The interrupting unit  31  is placed in its interrupting state for interrupting transmission of a rotational force when at least the pair of registration rollers  19  and the reconveying rollers  24  are simultaneously contacting the same sheet, as illustrated in  FIG. 9 . 
     In this state, the sheet receives a conveying force from the pair of registration rollers  19 , while the reconveying rollers  24  rotate to follow the conveyance of the sheet. Therefore, the reconveying rollers  24  do not hinder conveyance of a sheet when the sheet is being conveyed by the pair of registration rollers  19 . 
     &lt;Second Interrupting Function&gt; 
     As shown in  FIG. 2A , the second interrupting function is implemented by a gear mechanism that uses a planetary gear mechanism. 
     That is, the mechanical components of the transmission mechanism  30  that implement the second interrupting function include a sun gear  32 , an output gear  33 , an output-side intermediate gear  34  (an example of a first intermediate gear), an input-side intermediate gear  35  (an example of a second intermediate gear), a first planetary gear  36 A, a second planetary gear  36 B, and a locking mechanism  37 . In the following description, the first planetary gear  36 A and the second planetary gear  36 B will also be collectively referred to as planetary gears  36 . 
     &lt;Sun Gear and Output Gear&gt; 
     The sun gear  32  rotates when a rotational force supplied from the electric motor  27  is inputted into the sun gear  32 . A rotational center of the sun gear  32  is fixed relative to a reconveying unit (not shown) constituting the reconveying path L 2 . The output gear  33  outputs a rotational force to the reconveying rollers  24 . 
     After the output gear  33  outputs a rotational force to the pair of second reconveying rollers  25 , a portion of the rotational force is diverted to the pair of first reconveying rollers  23 . The reconveying unit is provided with a diverting mechanism (not shown) for diverting the rotational force outputted from the output gear  33  to the pair of first reconveying rollers  23  and the pair of second reconveying rollers  25 . 
     &lt;Output-Side Intermediate Gear and Input-Side Intermediate Gear&gt; 
     The output-side intermediate gear  34  is meshingly engaged with the output gear  33  at all times. Hence, the output gear  33  rotates when a rotational force is inputted into the output-side intermediate gear  34 . Conversely, the output-side intermediate gear  34  rotates when a rotational force is inputted into the output gear  33 . 
     The input-side intermediate gear  35  is coupled to the output-side intermediate gear  34  via the interrupting unit  31 . Hence, a rotational force can be transmitted from the input-side intermediate gear  35  to the output-side intermediate gear  34 , but cannot be transmitted from the output-side intermediate gear  34  to the input-side intermediate gear  35 . 
     As shown in  FIGS. 3C, 4C, and 5C , the input-side intermediate gear  35  includes a toothless part  35 A constituting a part of its circumferential surface in which no teeth are provided, and a toothed part  35 B constituting a remaining part of the circumferential surface in which teeth are provided (see also  FIG. 10 ). That is, the input-side intermediate gear  35  is a sector gear (i.e. partially-toothless gear). Hence, when teeth of the second planetary gear  36 B are positioned within the toothless part  35 A (see  FIG. 3C ), transmission of a rotational force from the second planetary gear  36 B to the input-side intermediate gear  35  is interrupted. 
     &lt;Planetary Gears&gt; 
     As shown in  FIG. 2A , the planetary gears  36  are meshingly engaged with the sun gear  32  at all times and rotate upon rotation of the sun gear  32 . A rotational force transmitted from the sun gear  32  to the planetary gears  36  acts as a revolving force for displacing the rotational centers of the planetary gears  36  by revolving the planetary gears  36  about the rotational center of the sun gear  32 . Note that the term “revolve” is used here interchangeably with “orbitally move”. Further, in the following description, the rotations of the planetary gears  36  about their own rotational centers will be referred to as “rotation.” 
     That is, the planetary gears  36  are meshingly engaged with the sun gear  32  at all times. Therefore, when the sun gear  32  rotates, the planetary gears  36  receive the rotational force of the sun gear  32 . At this time, if the rotational centers of the planetary gears  36  are in a nondisplaceable state, the rotational force supplied from the sun gear  32  serves as a rotating force for rotating the planetary gears  36 . 
     However, if the rotational centers of the planetary gears  36  are in a displaceable state, the rotational force supplied from the sun gear  32  serves as a revolving force for revolving the planetary gears  36  about the rotational center of the sun gear  32 . Consequently, the revolving direction of the planetary gears  36  matches the rotating direction of the sun gear  32 . 
     When receiving a revolving force from the sun gear  32 , the planetary gears  36  can be displaced by revolving between a first position in which the planetary gear  36 A is meshingly engaged with the output gear  33  and a second position in which the planetary gear  36 B is meshingly engageable with the input-side intermediate gear  35 . 
     More specifically, the first planetary gear  36 A can be displaced by revolving between the first position (the position shown in  FIG. 4A ) in which the first planetary gear  36 A is meshingly engaged with the output gear  33  and a third position (the position shown in  FIGS. 3A and 5A ) offset from the first position and different from the second position. 
     The second planetary gear  36 B can be displaced by revolving between the second position (the position shown in  FIGS. 3A and 5A ) in which the second planetary gear  36 B is meshingly engageable with the input-side intermediate gear  35  and a fourth position (the position shown in  FIG. 4A ) offset from the second position and different from the first position. 
     The first planetary gear  36 A and the second planetary gear  36 B are revolvably and rotatably supported by a revolving member  37 A (described later). Accordingly, the first planetary gear  36 A and the second planetary gear  36 B revolve as a unit. 
     Thus, the second planetary gear  36 B is in the fourth position when the first planetary gear  36 A is in the first position. The first planetary gear  36 A is in the third position when the second planetary gear  36 B is in the second position. 
     If the sun gear  32  is rotated counterclockwise in  FIG. 3A  while the first planetary gear  36 A is in the third position (see  FIG. 3A , for example), the first planetary gear  36 A revolves counterclockwise together with the sun gear  32  to be displaced from the third position to the first position (see  FIG. 4A ). 
     When the first planetary gear  36 A is meshingly engaged with the output gear  33 , the rotational center of the first planetary gear  36 A is in a nondisplaceable state. This arrangement halts the revolution of the first planetary gear  36 A, and the rotational force supplied from the sun gear  32  acts as a rotating force for rotating the first planetary gear  36 A. As a result, the rotational force of the sun gear  32  is transmitted to the output gear  33  via the first planetary gear  36 A. 
     If the sun gear  32  is rotated clockwise in  FIG. 4A  while the second planetary gear  36 B is in the fourth position (see  FIG. 4A , for example), the second planetary gear  36 B revolves clockwise together with the sun gear  32  to be displaced from the fourth position to the second position (see  FIGS. 3A and 5A ). 
     When the second planetary gear  36 B is meshingly engaged with the input-side intermediate gear  35 , the rotational center of the second planetary gear  36 B is in a nondisplaceable state. This arrangement halts the revolution of the second planetary gear  36 B, and the rotational force supplied from the sun gear  32  acts as a rotating force for rotating the second planetary gear  36 B. 
     If the second planetary gear  36 B is offset from the position of the toothless part  35 A at this time (see  FIG. 5C ), the rotational force of the sun gear  32  is transmitted to the input-side intermediate gear  35  via the second planetary gear  36 B. This rotational force transmitted to the input-side intermediate gear  35  is then relayed to the output gear  33  via the interrupting unit  31  and the output-side intermediate gear  34 . 
     If the teeth of the second planetary gear  36 B are positioned in the toothless part  35 A while the second planetary gear  36 B is in the second position (see  FIG. 3C ), the rotational force of the sun gear  32  is not transmitted to the output gear  33 . 
     Note that the rotating direction of the output gear  33  when a rotational force is transmitted to the output gear  33  via the first planetary gear  36 A is identical to the rotating direction of the output gear  33  when a rotational force is transmitted to the output gear  33  via the second planetary gear  36 B. 
     &lt;Locking Mechanism&gt; 
     The locking mechanism  37  functions to halt rotation of the input-side intermediate gear  35  and to situate the second planetary gears  36 B in the second position within the area of the toothless part  35 A (see  FIG. 3C ) while a sheet is being conveyed in the image-forming mechanism  5 . As shown in  FIG. 2B , the locking mechanism  37  includes the revolving member  37 A, and a spring  37 D. 
     The revolving member  37 A has an engaging part  37 B that engages with an engagement part  37 C provided on the input-side intermediate gear  35 . The revolving member  37 A can be displaced by revolving between an engaging position (see  FIG. 3A ) in which the engaging part  37 B is engaged with the engagement part  37 C, and a release position (see  FIGS. 4A and 5A ) in which the engaging part  37 B is disengaged from the engagement part  37 C. 
     The spring  37 D exerts a resilient force on the input-side intermediate gear  35  for rotating the input-side intermediate gear  35  to a position meshingly engaged with the second planetary gear  36 B in the second position. As shown in  FIG. 3A , a cam  37 E that is substantially triangular in shape is provided on a portion of the input-side intermediate gear  35  that slides in contact with (or “slidingly contacts”) the spring  37 D. 
     The spring  37 D presses the cam  37 E while slidingly contacting a portion of the cam  37 E offset from the rotational center of the input-side intermediate gear  35 . Through this arrangement, the spring  37 D applies a rotational force to the input-side intermediate gear  35 . 
     When the engaging part  37 B and the engagement part  37 C are in an engaged state, the input-side intermediate gear  35  cannot rotate even when the spring  37 D applies its rotational force to the input-side intermediate gear  35 . When the revolving member  37 A is displaced by revolving from its engaging position to its release position, the input-side intermediate gear  35  is rotated by the rotational force applied from the spring  37 D and becomes meshingly engaged with the second planetary gear  36 B, as shown in  FIG. 5C . 
     2.3 Operations of Electric Motor and Transmission Mechanism 
     When the image forming apparatus  1  performs an image-forming operation on a sheet in either the simplex printing mode or the duplex printing mode, the electric motor  27  is rotated in the normal direction (indicated by a region A in  FIG. 6 ). At this time, the sun gear  32  rotates in the normal direction (clockwise in  FIG. 3A , an example of a first rotational direction) and the revolving member  37 A is in the engaging position. 
     Since the teeth of the second planetary gear  36 B are positioned within the toothless part  35 A while the second planetary gear  36 B is in the second position (see  FIG. 3C ), the rotational force of the sun gear  32  is not transmitted to the output gear  33 , and the reconveying rollers  24  remain halted. 
     After an image-forming operation has been completed for one side of a sheet in the duplex printing mode, and specifically once a prescribed time has elapsed after the post-registration sensor S 2  has detected the trailing edge of the sheet relative to the sheet-conveying direction so that the trailing edge of the sheet has passed through the branch point L 3 , the electric motor  27  is shifted from normal rotation to reverse rotation. 
     Consequently, the switchback roller  21  shifts from the normal rotation mode to the reverse rotation mode, and the sun gear  32  begins rotating counterclockwise in  FIG. 3A  (an example of a second rotational direction). Since a counterclockwise revolving force is applied to the planetary gears  36  (first planetary gear  36 A and second planetary gear  36 B), the revolving member  37 A is displaced by revolving from the engaging position to the release position, as illustrated in  FIG. 4A . 
     Thus, the first planetary gear  36 A is displaced from the third position to the first position, and the second planetary gear  36 B is displaced from the second position to the fourth position. Therefore, since the rotational force of the sun gear  32  is transmitted to the output gear  33  via the first planetary gear  36 A, the reconveying rollers  24  begin to rotate. 
     Since the revolving member  37 A is in the release position, the input-side intermediate gear  35  receives the rotational force from the spring  37 D and rotates a prescribed angle (see  FIG. 4C ). In this description, the prescribed angle denotes a rotational angle at which the second planetary gear  36 B must shift from the toothless part  35 A in order to meshingly engage with the teeth (i.e. toothed part  35 B) of the input-side intermediate gear  35  (see  FIG. 5C ). 
     After being rotated in the reverse direction for a prescribed time, the electric motor  27  is shifted to normal rotation (see  FIG. 6 ). The timing at which the electric motor  27  is shifted from reverse rotation to normal rotation occurs before the leading edge of the sheet relative to the sheet-conveying direction arrives at the pair of registration rollers  19 , as illustrated in  FIG. 8 . 
     When the electric motor  27  shifts from reverse rotation to normal rotation, the sun gear  32  reverses from counterclockwise to clockwise in rotation in  FIG. 4A . Consequently, the first planetary gear  36 A is shifted to the third position, and the second planetary gear  36 B is shifted to the second position so as to meshingly engage with the toothed part  35 B of the input-side intermediate gear  35  (see  FIG. 5C ). 
     Since the rotational force of the sun gear  32  is transmitted to the input-side intermediate gear  35  via the second planetary gear  36 B, the input-side intermediate gear  35  begins to rotate, rotating the output gear  33  through the output-side intermediate gear  34 . 
     When the input-side intermediate gear  35  has rotated one complete turn (approximately 360 degrees), the toothless part  35 A returns to its position confronting the second planetary gear  36 B (see  FIG. 3C ), and the revolving member  37 A returns to its engaging position (see  FIG. 3A ). 
     At this time, the leading edge of the sheet relative to the conveying direction passes through the pair of registration rollers  19  and begins receiving a conveying force from the image-forming mechanism  5 , as shown in  FIG. 9 . The rotational force from the electric motor  27  is not supplied to the pair of second reconveying rollers  25  at this time. Further, the interrupting unit  31  interrupts transmission of the rotational force from the reconveying rollers  24  to the electric motor  27 . The pair of second reconveying rollers  25  rotates while following the movement of the sheet (indicated by a dashed line in a region B of  FIG. 6 ). 
     3. Features of the Image-Forming Device According to the Embodiment 
     In the embodiment described above, the image forming apparatus  1  can prevent the reconveying rollers  24  from rotating when a sheet is being conveyed through the image-forming mechanism  5 , i.e., when the image forming apparatus  1  is performing simplex printing or is forming an image on the first side of a sheet in duplex printing. Therefore, the structure of the image forming apparatus  1  can reduce noise generated by the rotating reconveying rollers  24 . 
     Further, when a sheet is being conveyed through the image-forming mechanism  5 , the image forming apparatus  1  interrupts transmission of a rotational force to the reconveying rollers  24  while the interrupting unit  31  interrupts transmission of a rotational force from the reconveying rollers  24  to the electric motor  27 . 
     Accordingly, a sheet that is being reconveyed receives a conveying force from the image-forming mechanism  5 , and the reconveying rollers  24  rotate to follow the conveyed movement of the sheet. Thus, the sheet can be suitably reconveyed despite transmission of the rotational force to the reconveying rollers  24  being interrupted. 
     &lt;Variations of the Embodiment&gt; 
     In the embodiment described above, the planetary gears  36  are configured of the first planetary gear  36 A and the second planetary gear  36 B. However, a structure that includes a single planetary gear  36  may be available. In this case, the single planetary gear  36  is configured to revolve in an area opposite the output gear  33  with respect to the sun gear  32 . 
     In the embodiment described above, the interrupting unit  31  is disposed between the output-side intermediate gear  34  and the input-side intermediate gear  35  on the transmission path of the rotational force. However, for example, the interrupting unit  31  may be provided on the transmission path of the rotational force at a position closer to the reconveying rollers  24  than the output gear  33  to the reconveying rollers  24 . 
     Further, the specific structures of the locking mechanism  37  and the interrupting unit  31  are not limited to those given in the embodiment described above. Other structures may be available for the locking mechanism  37  and the interrupting unit  31 . 
     While the description has been made in detail with reference to the embodiment thereof, it would be apparent to those skilled in the art that many modifications and variations may be made therein without departing from the spirit of the disclosure.