JPH08239154A - Paper feeder - Google Patents

Abstract

PURPOSE: To enable paper to be switched back smoothly without increasing the number of rollers by rotating a first roller in time opposite direction to first direction when switching pawls are in second position and when the paper is to be advanced into a switchback passage, and rotating the first roller in the first direction when the paper is to be moved back. CONSTITUTION: When switching claws 32, 33 are in second position indicated by the full line, the rotating direction of a first roller 21a is changed as follows: first, paper is fed from a pair of feed rollers 29, 30, and when the paper advances into a switchback passage 22, the first roller 21a is rotated in a direction for accelerating the advance of the paper, i.e., in the opposite direction to first direction in which it is rotated clockwise in the figure; after the paper has advanced into the switchback passage 22, the paper is switched back, and when it is desired to feed the paper into a re-supply passage 23 via a pair of feed rollers 30, 31 with the rear end first, i.e., to move the paper back from the switchback passage 22, the first roller 21a is rotated in the first direction in which it is rotated counterclockwise.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sheet conveying device for an apparatus for copying or printing an image on a sheet such as a copying machine or a printer. In particular, a paper transport device having a transport mechanism for switching back the paper in order to copy or print the image on the back surface (or front surface) of the paper on which the image is copied or printed on the front surface (or back surface) of the paper. Regarding

[0002]

2. Description of the Related Art Copiers capable of copying images on both the front and back sides of paper are known. Such a double-sided copying machine is equipped with a transport mechanism for switching back the paper. After the paper on which the image is copied on one side is discharged from the fixing device, the paper enters the switchback passage from the leading end side thereof and is retracted from the rear end side through the switchback passage. This allows the paper to be re-supplied to the image transfer unit so that an image is formed on the other surface of the paper. In the conventional paper transport device including the switchback mechanism, there was a demand to reduce the number of paper transport rollers as much as possible.
There was no dedicated feed roller for entering the paper into the switchback passage. On the other hand, a discharge roller for discharging the paper out of the machine is also used as a roller for retracting the paper from the switchback passage.

[0003]

As described above, when the sheet is retracted from the switchback passage without providing a dedicated roller for advancing the sheet into the switchback passage,
In the configuration that uses the conveying force of the discharge roller, the positional relationship between the guide plate that forms the switchback passage and the discharge roller must be strictly adjusted. because,
Since the discharge roller rotates in the direction of retracting the paper in the switchback passage, if the discharge roller protrudes too much into the switchback passage, it becomes difficult for the paper to enter the switchback passage. On the other hand, if the discharge roller is too far from the switchback passage, the conveying force from the discharge roller cannot be obtained when the paper is retracted from the switchback passage.

As a result, in the conventional sheet conveying apparatus including the switchback mechanism, it is necessary to strictly adjust the mounting position of the apparatus in order to smoothly switch back the sheet. There was a problem that it was difficult. Therefore, it is an object of the present invention to provide a paper transporting device that can favorably switch back paper without increasing the number of rollers for transporting the paper.

[0005]

From a certain aspect, the present invention is used in an apparatus having an image transfer section and an image fixing section, in which an image is transferred onto a sheet by the image transferring section and the image is fixed on the sheet by the image fixing section. In the paper transporting device for transporting the paper discharged from the image fixing unit after the image is fixed, the first position for guiding the paper discharged from the fixing device to the discharge path to the outside of the machine, or the paper In order to re-feed the image to the image transfer unit, there is provided a switching claw that is switched to a second position for leading to a switchback path for switching back the sheet, a first roller and a second roller whose peripheral surfaces are in contact with each other, and A sheet of paper that enters the passage can be fed while being sandwiched between the first roller and the second roller, and a part of the peripheral surface of the first roller faces the switchback passage so that the paper passing through the passage has a peripheral surface. A pair of discharge rollers that can come into contact with each other, a pressing means for pressing the sheet against the first roller when the sheet passes through the switchback passage, and a pair of discharge rollers when the switching claw is in the first position. When the sheet is rotated in the first direction, which is the direction in which the sheet is ejected outside the machine, the switching pawl is in the second position, and when the sheet enters the switchback path, the first roller is moved to the first position.
Rotation direction switching drive means for rotating the first roller in the first direction when the switching pawl is in the second position and the paper is retracted from the switchback path. It is characterized by including.

According to another aspect of the present invention, in the above-described sheet conveying device, the first clutch that transmits the driving force to the first output gear at the time of turning on or off, and the driving force that always transmits the driving force to the second output gear. It has an idle gear meshed with the two output gears and a first input gear meshed with the first output gear, and always transmits the driving force from the first output gear to the output shaft through the first input gear and A second clutch that has a second input gear meshed with the gear, and transmits the driving force from the idle gear to the output shaft via the second input gear when on or off; Switching means to turn on / off in a linked manner,
It is characterized by including.

Viewed from still another aspect, the present invention is
In the paper transporting device, the rotation direction switching drive means, when transmitting the drive force from the drive shaft to the first roller,
The rotation direction of the first roller is switched depending on whether the driving force is transmitted through an odd number of transmission gears or the driving force is transmitted through an even number of transmission gears. Viewed from still another aspect, the present invention uses the odd number of driving force transmission gears in the sheet conveying device,
Whether to use an even number of driving force transmission gears is characterized by being switched by a clutch.

Viewed from still another aspect, the present invention is
The sheet conveying device is provided with a detecting unit which is provided on the outlet side of the fixing unit and detects the leading edge of the sheet discharged from the fixing unit. After a predetermined time, the rotation direction switching drive means performs the switching operation.

[0009]

According to the present invention, the rotation direction switching drive means switches the rotation direction of the discharge roller pair, especially the first roller of the discharge roller pair. That is, in the state where the switching claw is at the first position and the paper discharged from the fixing device is guided to the discharge passage, the discharge roller pair is rotated in the first direction which is the rotation direction for discharging the paper to the outside of the machine.

On the other hand, when the switching pawl is in the second position and the sheet discharged from the fixing device is guided to the switchback path, the rotation direction of the first roller is switched as follows. That is, when the sheet discharged from the fixing device enters the switchback path, the first roller is rotated in the direction opposite to the first direction. As a result, the sheet entering the switchback path is conveyed in the entering direction by the first roller. Further, when the paper is retracted from the switchback passage, the first roller is rotated in the first direction. As a result, the sheet in the switchback path is conveyed in the backward direction by the first roller (claim 1).

When the rotation direction is switched by the rotation direction switching drive means by the first and second clutches and the idle gear inserted between the clutches, the first rotation is surely performed.
The rollers can be switched (claims 2 and 4).
Further, the rotation direction of the first roller is switched depending on whether the driving force is transmitted by an odd number of gears or the driving force is transmitted by an even number of gears, that is, the number of gears is increased or decreased by one. If the rotation direction is switched to the opposite direction by doing so, the rotation direction can be switched with a simple configuration, and idling and torque shortage will not occur when switching the rotation direction (claim 3).

Further, by detecting the leading edge of the sheet discharged from the fixing device and switching the first roller after a predetermined time corresponding to the sheet size after the detection, the first roller is switched at the optimum timing according to the movement of the sheet. The rotation direction of the roller can be switched (claim 5).

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a schematic sectional view showing the overall construction of a copying machine in which a sheet conveying device according to an embodiment of the present invention is incorporated. This copier has
A copying machine main body 1, an automatic document feeder 2 mounted on the copying machine main body 1, and a sorter 3 connected to the paper discharge side of the copying machine main body 1 are included. The automatic document feeder 2 is
This is for automatically setting a document on a document setting surface 4 provided on the upper surface of the copying machine main body 1. Copier body 1
In order to form an original image on the basis of the optical system 5 for illuminating and scanning the original set on the original setting surface 4 and guiding the reflected light of the original, and the reflected light from the original guided by the optical system 5. Image forming unit 6 and paper trays 7, 8, 9,
A paper supply path 12 is provided for supplying the paper from the paper deck 10 or the manual paper feed port 11 to the image forming unit 6. In the image forming unit 6, the toner image formed on the photoconductor drum 13 is transferred to the supplied paper by the transfer charger 14. The sheet on which the toner image is transferred is sent to the fixing device 16 by the conveyor belt 15, and the toner image on the sheet is heated and pressed to be fixed.

In order to convey the sheet discharged from the fixing device 16, the sheet conveying device 20 according to this embodiment is provided. That is, the paper discharged from the fixing device 16 is sent to the sorter 3 outside the copying machine main body 1 by the discharge roller pair 21, and is discharged to a predetermined storage bin in the sorter 3. Further, in the case where copying is performed on both the front surface and the back surface of the sheet and the sheet on which the image on one side is transferred and fixed is discharged from the fixing device 16, first,
The paper enters the switchback passage 22 from the front side,
The entered sheet is retracted from the rear end side through the switchback passage 22 and sent to the resupply path 23, and is temporarily stored in the intermediate tray 24. Then, after that, the paper is fed again from the intermediate tray 24 through the paper supply device 12 to the image forming unit 6 and the fixing device 16, and the image is formed on the remaining surface.

FIG. 2 is a partially enlarged view showing the structure of the downstream side portion of the fixing device 16 with reference to the sheet conveying direction inside the copying machine main body 1 shown in FIG. With reference to FIG. 2, the features of the sheet conveying device according to this embodiment incorporated in the copying machine body 1 will be described below. Guide plates 25 and 26 for guiding the paper discharged from the fixing device 16 are provided on the exit side of the fixing device 16. Guide plate 25,
A sheet detection actuator 27 of a detection switch (not shown) is arranged at, for example, a substantially central position of the sheet passage formed by 26 in the sheet conveyance direction. The actuator 27 rotates counterclockwise around the fulcrum 28 when the sheet passes through the passage formed by the guide plates 25 and 26. When the actuator 27 rotates as described above, a detection switch (not shown) is turned on, for example. A pair of feed rollers 2 is provided on the downstream end side of the paper path formed by the guide plates 25 and 26 when viewed in the paper transport direction.
9, 30 are provided. These feed roller pairs 29,
Reference numeral 30 is for feeding the paper conveyed through the passage formed by the guide plates 25 and 26 further to the downstream side in the conveying direction.

The feed roller pair 29, when viewed in the sheet conveying direction,
On the downstream side of 30, the switching claws 32, 33 for switching whether the sheet conveyed by the pair of feed rollers 29, 30 is guided to the pair of discharge rollers 21 a, 21 b and discharged to the outside of the machine, or guided to the switchback passage 22. Is provided. The switching claws 32 and 33 are rotationally displaced in conjunction with the fulcrums 34 and 35, respectively. The switching displacement of the switching claws 32, 33 is performed by a solenoid (not shown). Switching claws 32, 3
At the first position 3 indicated by the chain line, the feed roller pair 29,
The sheet sent by 30 is given to the pair of discharge rollers 21a and 21b as shown by the one-dot chain line arrow, and discharged through the discharge passage 36 to the outside of the machine.

On the contrary, the switching claws 32 and 33 are the second ones shown by the solid lines.
At the position, the sheet delivered by the pair of feed rollers 29 and 30 enters the switchback passage 22 as indicated by the solid arrow. Further, when the switching claws 32 and 33 are in the second position shown by the solid line, when the sheet that has entered the switchback passage 22 is retracted from the switchback passage 22,
The switching claw 33 functions as a guide for the sheet, and the sheet is fed to the feed roller pair 30, 31 as indicated by a dashed arrow, and is fed to the re-feed path 23 by the feed roller pair 30, 31.

The switchback passage 22 has a guide plate 37.
And a guide plate 38, which is formed therebetween. The upper first roller 21a of the pair of discharge rollers 21a and 21b faces the switchback passage 22 through a notch (not shown) formed in the guide plate 37 at a part of its peripheral surface. Therefore, the sheet passing through the switchback passage 22 can come into contact with the peripheral surface of the first roller 21a.

In this case, the amount by which the peripheral surface of the first roller 21a projects from the guide plate 37 toward the switchback passage 22 does not need to be adjusted very strictly. Therefore, positioning adjustment when attaching the first roller 21a and the guide plate 37 is easy. This is a significant improvement over the configuration of the conventional sheet conveying device. This is because, in the case of the conventional sheet conveying device, the discharge roller pair 21a, 21b
Is always rotated in the direction in which the paper is ejected (the first roller is clockwise and the second roller 21b is counterclockwise in FIG. 2). Therefore, the peripheral surface of the first roller 21a has the guide plate 37.
If it protrudes too much from the switchback passage 22 side,
This will prevent the sheet from entering the switchback passage 22. Therefore, in the conventional device, the peripheral surface of the first roller 21a facing the switchback passage 22 is strictly adjusted so as to be substantially flush with the guide surface of the guide plate 37. On the other hand, in this embodiment, such adjustment is unnecessary.

In addition to the switchback passage 22, a tapping roller 39 is further provided as a pressing means for pressing the sheet against the first roller 21a. The tapping roller 39 is rotatably held on one end of the longitudinal support plate 40. The support plate 40 is swingable around a fulcrum not shown. The support plate 40 is swung by, for example, a solenoid (not shown), and is switched between a solid line state and a state in which the sheet indicated by the alternate long and short dash line is pressed against the first roller 21a. In the state where the tapping roller 39 is indicated by the alternate long and short dash line, the sheet is pressed against the peripheral surface of the first roller 21a by the tapping roller 39. Therefore, the rotation of the first roller 21a ensures that the paper in the switchback passage 22 is conveyed.

When the switching pawls 32 and 33 are in the second position shown by the solid line, the rotation direction of the first roller 21a is switched as follows. First, when the paper is sent out from the pair of feed rollers 29 and 30, and the paper enters the switchback path 22, the first roller 21a moves in a direction that promotes the entrance of the paper.
That is, it is rotated counterclockwise in the figure. Further, after the sheet has finished entering the switchback passage 22, the sheet is switched back, and the rear end side of the sheet is set to the leading end of the feed roller pair 30,
When it is desired to feed the sheet to the re-supply path 23 via 31, that is, when the sheet is retracted from the switchback path 22, the first roller 21a is rotated in the first direction which is the right rotation.

The switching timing of the rotation of the above-mentioned first roller 21a is based on the time when the leading edge of the paper discharged from the fixing device 16 is detected by the actuator 27, and after a predetermined time corresponding to the paper size from that time. is there. Therefore, regardless of the paper size, the switchback passage 22
Can be used to reliably switch back the paper.

FIG. 3 is an illustrative view showing a structure of a gear mechanism for realizing the switching drive of the first roller 21a shown in FIG. The arrangement of gears in FIG. 3 is shown in association with the arrangement of rollers shown in FIG. To clarify this positional relationship, in FIG. 3, the fixing device 16, the feed roller 30, and the first roller 21a are shown by a chain line. FIG. 4 is a plan view of the gear mechanism shown in FIG. 3 in a developed state.

Next, with reference to FIGS. 3 and 4, a configuration for switching the rotation direction of the first roller 21a in the discharge roller pair 21 will be described. In this embodiment, in the discharge roller pair 21, the first roller 21a is a driving roller and the second roller 21b is a driven roller. Therefore, when the rotation direction of the first roller 21a is switched, the rotation direction of the second roller 21b is also switched accordingly.

A drive shaft 41 rotated by, for example, a motor is arranged on the rotation axis of the feed roller 30.
The drive roller 41 rotates the feed roller 30. A gear 42 is fixed to one end of the drive shaft 41.
A gear 44 fixed to one end of the input shaft 43 meshes with the gear 42. A gear 45 is fixed to the other end of the input shaft 43. A second output gear 47 of the first clutch 46 meshes with the gear 45. The first clutch 46 includes a first clutch shaft 48, a first output gear 49 fixed to one end of the first clutch shaft 48, and a switching solenoid 50, and the second output gear 47 is the first clutch shaft. It is loosely fitted in 48. Then, only when the solenoid 50 is in the ON state, the second output gear 47 is fixed to the first clutch shaft 48 and rotates together.

A second clutch 51 is arranged adjacent to the first clutch 46. The second clutch 51 is provided with a second clutch shaft 52 and a first input gear 53 fixed to one end of the second clutch shaft 52. The first input gear 53 meshes with the first output gear 49. Note that FIG.
In the developed view, the two are shown as separated from each other, but in reality, as shown in FIG.
9 and the first input gear 53 always mesh with each other. A second input gear 54 is further loosely fitted on the second clutch shaft 52. The second input gear 54 rotates together with the second clutch shaft 52 only when the switching solenoid 55 is on. An idle gear 56 is connected between the second input gear 54 and the aforementioned second output gear 47.

A gear 57 is fixed to the other end of the second clutch shaft 52. This gear 57 has a shaft 58
The gear 59 attached to the gear is meshed. further,
The gear 59 meshes with a gear 61 fixed to an output shaft 60 that gives a driving force to the first roller 21a of the discharge roller pair 21. FIG. 5 is a timing chart for explaining the switching operation of the gear mechanism shown in FIGS. 3 and 4. Next, referring to FIGS. 2 to 5, the switching claw 3 will be described.
2, the first position indicated by the alternate long and short dash line in FIG.
The discharge roller pair 21 is always rotated in the first direction (the first roller 21a is in the clockwise direction). In order to rotate the first roller 21a in this manner, the first clutch 46 (the solenoid 50 thereof) is turned on, and the second clutch 51 (the solenoid 55 thereof) is turned off. That is, in the first clutch 46, the second output gear 47 is fixed to the first clutch shaft 48. On the other hand, in the second clutch 51, the second input gear 54 is loosely fitted to the second clutch shaft 52. Therefore, for example, when the drive shaft 41 is rotated counterclockwise, the gear 42 is also rotated counterclockwise, and the gear 44, the input shaft 43, and the gear 45 are rotated clockwise. The right rotation of the gear 45 is transmitted to the second output gear 47 to rotate the gear 47 counterclockwise. The left rotation of the gear 47 is transmitted to the first clutch shaft 48 when the solenoid 50 is turned on, and the first output gear 49 is rotated left. Therefore, the first input gear 53 is rotated clockwise, and the second clutch shaft 52 is also rotated clockwise.

In this case, the idle rotation of the second output gear 47 to the left causes the idle gear 56 to rotate to the right, and the rotation causes the second input gear 54 to rotate to the left. However, since the solenoid 55 is off, the second input gear 54 is loosely fitted to the second clutch shaft 52, and the right rotation of the second input gear 54 is not transmitted to the second clutch shaft 52. Therefore, the gear 57 fixed to the second clutch shaft 52 rotates to the right, which causes the gear 59 to rotate to the left. As a result, the gear 61 is rotated to the right, and its rotational force is transmitted via the output shaft 60 to the first roller 2
Rotate 1a to the right.

On the other hand, when the switching pawls 32 and 33 are in the second position shown by the solid line in FIG. 2, the gear mechanism is switched as follows. When the leading edge of the sheet discharged from the fixing device 16 is detected by the actuator 27 shown in FIG. 2 and the detection switch is turned on, the first clutch 46 (the solenoid 50 of the first clutch 46) is turned off after a predetermined time determined by the sheet size. , The second clutch 51 (the solenoid 55 thereof) is turned on. Then, as described above, the second output gear 47 is loosely fitted to the first clutch shaft 48, and conversely, the second input gear 54 is fixed to the second clutch shaft 52. In this state, the left rotation of the second output gear 47 is not transmitted to the first clutch shaft 48, and therefore the first output gear 49 does not rotate. On the other hand, the left rotation of the second output gear 47 causes the idle gear 56 to rotate right, and the right rotation of the idle gear 56 causes the second input gear 54 to rotate left. Therefore, the second clutch shaft 52 also rotates counterclockwise, and the gear 57 rotates counterclockwise. Therefore, the gear 59 rotates right, the gear 61 rotates left, and the first roller 21a rotates left. That is, the first roller 21
The sheet a entering the switchback passage 22 rotates counterclockwise to promote the entry.

Then, as shown in FIG. 5, the first roller 21
After “a” starts rotating counterclockwise, after a predetermined time, that is, after the leading edge of the sheet has entered the switchback path 22, the hit roller 39
Is displaced to the position indicated by the alternate long and short dash line in FIG. Therefore the first
The paper is nipped between the roller 21a and the hitting roller 39,
The sheet is fed so as to enter the switchback passage 22 by the left rotation of the first roller 21a.

After that, the first clutch 46 and the second clutch 51 are switched on and off again after a lapse of a predetermined time determined by the sheet size from the rise of the detection switch. That is, (the solenoid 50 of) the first clutch 46 is turned on, and (the solenoid 55 of) the second clutch 51 is turned off. As a result, the first roller 21a is switched to the right rotation. When the first roller 21a rotates to the right, the sheet that has entered the switchback path 22 is sent in the backward direction. At this time, since the switching claws 32 and 33 are in the second position shown by the solid line in FIG. 2, the switching claw 33 guides the sheet retracting from the switchback passage 22 as described above,
Guide between the feed rollers 30 and 31, and feed rollers 30 and 31
The paper is sent to the re-supply path 23 by.

The sheet conveying apparatus according to the present invention is not limited to the structure of the gear mechanism shown in FIGS. 3 and 4, but any other gear mechanism can be used as long as it can change the rotation direction of the first roller 21a. The configuration may be. 3 and 4 to switch the rotation direction of the first roller 21a.
If you use a gear mechanism such as shown in, or other gear mechanism, the rotation direction of the roller can be switched very smoothly.
And there is an advantage that it can be done reliably.

Although the above-described embodiment has been described by taking the paper transporting device incorporated in the copying machine main body 1 as shown in FIG. 1 as an example, other paper transporting devices such as copying machines and printers and printing machines. The present invention can also be applied to. Besides, the present invention can be variously modified within the scope of the claims.

[0034]

According to the present invention, the first roller for conveying the paper is appropriately switched between the first direction and the reverse direction by the rotation direction switching drive means in accordance with the discharge state of the paper discharged from the fixing device. Therefore, it is possible to smoothly discharge the paper to the outside of the machine and switch the paper back (claim 1).

Particularly, when the sheet discharged from the fixing device is guided to the switchback passage by the switching claw, when the sheet enters the switchback passage, the first roller is rotated in a direction that assists the entry, and the sheet is switched. When it is retracted because it is fed from the back path to the re-supply path, the rotation direction of the first roller is switched and rotated in the direction that assists the retraction of the sheet. Therefore, it is possible to provide a paper transporting device in which the paper is smoothly transported when the paper is switched back, and the frequency of occurrence of the paper jam is reduced (claim 1).

Further, since the switching of the first roller is performed by the gear mechanism and the clutch, the switching is reliable and smooth (claims 2, 3 and 4). Furthermore, since the rotation direction of the first roller is switched after a lapse of a predetermined time after the leading edge of the paper discharged from the fixing device is detected, the time corresponding to the paper size can be set by appropriately setting the predetermined time. By setting, it is possible to smoothly switch back the paper regardless of the paper size (claim 5).

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the overall configuration of a copying machine in which a sheet conveying device according to an embodiment of the present invention is incorporated.

FIG. 2 is a partially enlarged cross-sectional view showing a characteristic portion of a sheet conveying apparatus according to an embodiment of the present invention in the configuration of FIG.

FIG. 3 is a schematic diagram of a gear mechanism for switching rollers drawn corresponding to FIG.

FIG. 4 is a plan view showing the gear mechanism of FIG. 3 in a developed state.

5 is a timing chart showing the switching operation timing of the first roller 21a in the paper transport device incorporated in the copying machine of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Copier main body 2 Image forming part 16 Fixing device 20 Paper transporting devices 21, 21a, 21b Ejection roller pair 21a First roller 21b Second roller 22 Switchback passage 27 Actuator 32, 33 Switching claw 36 Ejection passage 37, 38 Guide plate 39 Beating Roller 46 First Clutch 47 Second Output Gear 48 First Clutch Shaft 49 First Output Gear 50 Solenoid 51 Second Clutch 52 Second Clutch Shaft 53 First Input Gear 54 Second Input Gear 55 Solenoid 56 Idle Gear

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasunori Ueno 1-2-2 Tamatsukuri Chuo-ku, Osaka City, Osaka Prefecture Mita Industry Co., Ltd.

Claims (5)

[Claims] 1. An apparatus having an image transfer section and an image fixing section, wherein an image is transferred onto a sheet at the image transferring section, the image on the sheet is fixed at the image fixing section, and then the image is discharged from the image fixing section. In a paper transport device for transporting a sheet of paper, a switchback of the paper is carried out in order to guide the paper discharged from the fixing device to the discharge path for discharging the paper to the outside of the machine or to re-supply the paper to the image transfer portion. A switching claw for guiding the switchback path to a second position, a first roller and a second roller whose peripheral surfaces are in contact with each other, and a sheet entering the discharge path is placed between the first roller and the second roller. The first roller has a part of its peripheral surface facing the switchback path, and the peripheral surface of the first roller can contact the sheet passing through the path. When the paper passes through the back path, the pressing means for pressing the paper against the first roller, and when the switching claw is in the first position, the discharge roller pair are in a direction for discharging the paper to the outside of the machine. When the switching pawl is in the second position and the sheet is to enter the switchback path, the first roller is rotated in the direction opposite to the first direction, and the switching pawl is in the second position. And a rotation direction switching drive means for rotating the first roller in the first direction when the paper is retracted from the switchback path. 2. The sheet conveying device according to claim 1, wherein the first clutch and the second output gear are meshed with each other to transmit the driving force to the first output gear while constantly transmitting the driving force to the second output gear when turned on or off. And a first input gear meshed with the first output gear. The drive force from the first output gear is always transmitted to the output shaft via the first input gear and meshed with the idle gear. A second clutch that has a second input gear and transmits the driving force from the idle gear to the output shaft via the second input gear when on or off, and the first clutch and the second clutch are interlocked in a predetermined relationship. A sheet conveying device comprising a switching means for turning on and off. 3. The sheet conveying apparatus according to claim 1, wherein the rotation direction switching driving means transmits the driving force from an odd number of transmission gears when transmitting the driving force from the driving shaft to the first roller. A sheet conveying device characterized in that the rotation direction of the first roller is switched by transmitting the driving force or transmitting the driving force through an even number of transmission gears. 4. The sheet conveying apparatus according to claim 3, wherein a clutch is used to switch between the odd number of driving force transmitting gears and the even number of driving force transmitting gears. apparatus. 5. The sheet conveying device according to claim 1, further comprising a detection unit provided on the outlet side of the fixing device for detecting the leading edge of the sheet discharged from the fixing device. A sheet conveying device characterized in that the rotation direction switching drive means performs a switching operation after a predetermined time corresponding to the sheet size after the detection output of the detection means is given.