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

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.

The present disclosure relates to an image forming apparatus capable of forming images on both sides of sheets (hereinafter referred to as “duplex printing”).

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.

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.

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.

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'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

<First Interrupting Function>

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 .

<Second Interrupting Function>

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 .

<Sun Gear and Output Gear>

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 .

<Output-Side Intermediate Gear and Input-Side Intermediate Gear>

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.

<Planetary Gears>

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.

<Locking Mechanism>

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.

<Variations of the Embodiment>

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.