Patent Publication Number: US-9885988-B2

Title: Sheet conveying apparatus and image forming apparatus including same

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This disclosure relates to an image forming apparatus configured to form an image on a sheet. 
     Description of the Related Art 
     Hitherto, an image forming apparatus such as a printer including a first discharge roller that discharges a sheet, to which toner is fixed, to a sheet discharge tray and a second discharge roller that is able to switchback the sheet, for example, for duplex printing on the sheet is known. 
     As an example of such an image forming apparatus, an image forming apparatus configured to allow a second discharge roller to be normally and reversely rotatable by a stepping motor has been proposed in JP-A-2003-215874. The image forming apparatus conveys the sheet by normally rotating the second discharge roller until a trailing edge of the sheet passes through a reverse sensor and then conveys the sheet to a duplex sheet re-feed path by reversely rotating the second discharge roller. 
     However, the image forming apparatus described in JP-A-2003-215874 needs to reverse a rotating direction of the stepping motor and accelerate the stepping motor after decelerates and stops the stepping motor when switching a rotating direction of the second discharge roller. Therefore, a downtime of the stepping motor becomes long and throughput is reduced when performing duplex printing. 
     In addition, it is considered that the stepping motor is configured to normally and reversely rotate a drive force transmitted to the second discharge roller by a gear train and the like while maintaining a state in which the stepping motor is rotated in one direction. However, even in such a configuration, when the rotating direction of the second discharge roller is switched by the gear train, a large load is applied in a direction in which the rotation of the stepping motor that is a drive source is hindered. 
     As a result, there is a problem that time for switching the rotating direction of the stepping motor becomes long and the throughput is reduced. 
     SUMMARY OF THE INVENTION 
     According to an aspect of this disclosure, there is provided an image forming apparatus including an image forming portion configured to forma toner image on a sheet, a fixing portion configured to fix the toner image, formed on the sheet by the image forming portion, to the sheet, a sheet discharge portion, comprising a discharge roller, configured to discharge the sheet, on which the toner image has been formed, to an outside of the apparatus, a reverse portion, comprising a reverse roller, configured to convey the sheet on which the toner image have been fixed on a first surface thereof by the fixing portion to the image forming portion again to form a toner image on a second surface opposite to the first surface thereof while the reverse roller rotating in a reverse rotating direction after rotating in a forward rotating direction, a drive source, a drive unit comprising a first drive train through which a driving force from the drive source is transmitted to the discharge roller, and a second drive train through which a driving force from the drive source is transmitted to the reverse roller, and a switching mechanism, provided on the second drive train, configured to switch a rotating direction of the reverse roller between the forward rotating direction and the reverse rotating direction with the discharge roller rotating in one direction. 
     The switching mechanism has a first planetary gear, a second planetary gear and a stop unit. The first planetary gear unit has a first internal gear to which the drive force is transmitted from the drive source, a first sun gear, a first planetary gear engaging with the first internal gear and the first sun gear, and configured to rotate around the first sun gear, and a first planetary carrier supporting the first planetary gear rotatably, transmitting the drive force to the reverse roller, and configured to rotate around a rotating axis of the first sun gear together with the first planetary gear. The second planetary gear unit has a second internal gear to which the drive force is transmitted from the drive source and which rotates in the same direction as that of the first internal gear, a second sun gear, a second planetary gear engaging with the second internal gear and the second sun gear, and configured to rotate around the second sun gear, and a second planetary carrier supporting the second planetary gear rotatably, engaging with the first planetary carrier, and configured to rotate around a rotating axis of the second sun gear together with the second planetary gear. Lastly, the stop unit selectively stops the first sun gear and the second sun gear. 
     In a case where the first sun gear is stopped by the stop unit, the first planetary carrier rotates in a first rotating direction so that the reverse roller rotates in the forward rotating direction, and in a case where the second sun gear is stopped by the stop unit, the first planetary carrier rotates in a second rotating direction opposite to the first rotating direction so that the reverse roller rotates in the reverse rotating direction. 
     In other respects there is provided the sheet conveying apparatus of the image forming apparatus. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic section view illustrating a printer according to an embodiment of this disclosure. 
         FIG. 2  is a rear view illustrating a fixing unit. 
         FIG. 3  is a perspective view illustrating an actuator unit. 
         FIG. 4  is an exploded perspective view illustrating the actuator unit. 
         FIG. 5A  is a section view that is taken along line VA-VA of  FIG. 2  illustrating the fixing unit. 
         FIG. 5B  is a section view that is taken along line VB-VB of  FIG. 2  illustrating the fixing unit. 
         FIG. 6  is a section view that is taken along line VI-VI of  FIG. 2  illustrating the fixing unit when a guide member is positioned in a discharge position. 
         FIG. 7  is a section view illustrating the fixing unit when the guide member is positioned in a reverse position. 
         FIG. 8  is a section view illustrating the fixing unit. 
         FIG. 9  is an explanatory view illustrating a drive transmission route of a drive force of a motor. 
         FIG. 10A  is a front perspective view illustrating a switching mechanism. 
         FIG. 10B  is a rear perspective view illustrating the switching mechanism. 
         FIG. 11  is an exploded perspective view illustrating first and second planetary gear mechanisms. 
         FIG. 12  is a section view illustrating a rotating direction of each gear when a guide member is positioned in a discharge position. 
         FIG. 13  is a section view illustrating the rotating direction of each gear when the guide member is positioned in a reverse position. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, an embodiment of this disclosure will be described in detail with reference to  FIGS. 1 to 13 . A printer  1  according to the embodiment of this disclosure is an electro-photographic system color laser beam printer which is an example of an image forming apparatus. As illustrated in  FIG. 1 , the printer  1  has a cassette  31 , a sheet feeding unit  30 , an image forming portion  12  that forms a toner image on a sheet, a fixing unit  51 , a discharge roller pair  52 , and a reverse roller pair  53 . 
     If a command of image formation is output to the printer  1 , an image formation process is started by the image forming portion  12  based on image information input from an external computer and the like connected to the printer  1 . The image forming portion  12  includes a laser scanner unit  2  and four process cartridges  12 Y,  12 M,  12 C, and  12 K which form four colors of yellow (Y), magenta (M), cyan (C), and black (Bk). It is noted that the four process cartridges  12 Y,  12 M,  12 C, and  12 K have the same configuration except that the colors of an image to be formed are different, only the image formation process of the process cartridge  12 Y is described, and description of the process cartridges  12 M,  12 C, and  12 K will be omitted. 
     The laser scanner unit  2  applies a laser beam to a photosensitive drum  11   a  of the process cartridge  12 Y based on input image information. In this case, the photosensitive drum  11   a  is charged in advance by a charger  12   a  and an electrostatic latent image is formed on the photosensitive drum  11   a  by applying the laser beam to the photosensitive drum  11   a . Thereafter, the electrostatic latent image is developed by a developing roller  13   a   1  provided on the inside of a developer  13   a  and a toner image of yellow (Y) is formed on the photosensitive drum  11   a.    
     Similarly, toner images of magenta (M), cyan (C), and black (Bk) are formed on the photosensitive drums of the process cartridges  12 M,  12 C, and  12 K. The toner image of each color formed on each photosensitive drum is transferred to an intermediate transfer belt  21  by primary transfer rollers  25   a ,  25   b ,  25   c , and  25   d . The endless intermediate transfer belt  21  having dielectric property and flexible property is wound around a drive roller  22 , a turn roller  23 , and a tension roller  24 . The intermediate transfer belt  21  is rotated by the drive roller  22 , whereby the toner image on the intermediate transfer belt  21  is conveyed to a secondary transfer roller  26 . It is noted that the image formation process of each color is performed at a timing of superimposing a toner image of each color on the toner image of an upstream that is primarily transferred onto the intermediate transfer belt  21 . 
     Sheets stacked in the cassette  31  are fed by the sheet feeding unit  30  in parallel in the image formation process described above. The cassette  31  has an intermediate plate  31   a  that is pivotably supported, the intermediate plate  31   a  pivots, whereby the uppermost sheet S in a sheet bundle stacked on the intermediate plate  31   a  abuts against a sheet feeding roller  33 . In this state, the sheets S are fed by the sheet feeding roller  33  and are separated one by one by a conveyance roller  35  and a separating roller  34 . 
     Skew of the sheet S conveyed by the conveyance roller  35  and the separating roller  34  is corrected by a registration roller pair  92 . A full color toner image on the intermediate transfer belt  21  is transferred on the sheet S, which is conveyed at a predetermined conveying timing by the registration roller pair  92 , by the secondary transfer roller  26 . After the toner image is transferred onto the sheet by the secondary transfer roller  26 , toner remaining on the intermediate transfer belt  21  is recovered by a belt cleaning unit  27 . 
     It is noted that a fixing unit  50  is provided on a downstream of the secondary transfer roller  26  in a direction of conveyance. The fixing unit  50  has the fixing unit  51 , the discharge roller pair  52 , the reverse roller pair  53  which is able to normally rotate and reversely rotate, a guide member  54 , an actuator unit  55 , and a discharge reverse guide  59 , and these are unitized as illustrated in  FIG. 2 . 
     The sheet S to which the toner image is transferred is conveyed to the fixing unit  51  through a conveyance path  42 . Then, predetermined heat and pressure are applied to the sheet S by the fixing unit  51  and toner is melted and fixed to the sheet S. The guide member  54 , which is able to move to a discharge position and a reverse position, is provided in the downstream of the fixing unit  51  in the direction of conveyance. In a state in which the guide member  54  is positioned in the discharge position, i.e., first position, the sheet S is guided to the discharge roller pair  52  via a discharge conveyance path  56 . Then, the sheet S is discharged to a sheet discharge tray  57  provided on an upper surface of an apparatus body  1 A by the discharge roller pair  52 . It is noted that the discharge reverse guide  59  pivotably supports the guide member  54  and configures a part of the discharge conveyance path  56  and a reverse conveying path  58 . 
     In addition, for example, if duplex printing is performed on the sheet S, the guide member  54  is positioned in the reverse position, i.e., second position. Then, the toner image transferred onto the first surface is fixed by the fixing unit  51  and the sheet S passing through the fixing unit  51  is guided to the reverse roller pair  53  via the reverse conveying path  58 . 
     The reverse roller pair  53  is reversed, whereby the sheet S conveyed to the reverse roller pair  53  is switched back at a predetermined timing (described in detail later) and the sheet S is conveyed to a re-conveyance unit  70  disposed on a side of the fixing unit  50 . The re-conveyance unit  70  has a re-conveyance path  71  and a re-conveyance roller pair  72 , and is pivotably supported on the apparatus body  1 A around a pivotal shaft  70   a . The sheet S is conveyed on the re-conveyance path  71  by the re-conveyance roller pair  72  and reaches the registration roller pair  92  again. It is noted that the re-conveyance unit  70  on the apparatus body  1 A is opened whereby the re-conveyance path  71  is exposed and it is possible to easily remove a jammed sheet in the re-conveyance path  71 . 
     Then, the sheet S is conveyed to the secondary transfer roller  26  at a predetermined timing by the registration roller pair  92 , the toner image is transferred onto the second surface by the secondary transfer roller  26 , and the toner image is fixed by the fixing unit  51 . The sheet S is conveyed to the discharge roller pair  52  by the guide member  54  positioned in the discharge position and is discharged to the sheet discharge tray  57  by the discharge roller pair  52 . 
     Next, the actuator unit  55  provided in the fixing unit  50  will be described. As illustrated in  FIGS. 3 and 4 , the actuator unit  55  has a solenoid retaining member  552  that is fixed to a frame (not illustrated) of the fixing unit  50  and a mounting member  554  that is fixed to the apparatus body  1 A and is mounted on the solenoid retaining member  552 . In addition, the actuator unit  55  has a lever  553 , i.e., moving portion, that is pivotably supported on a bearing portion  552   a  of the solenoid retaining member  552  and a bearing portion  554   a  of the mounting member  554 , and a solenoid  551  that is mounted on the solenoid retaining member  552 . 
     As illustrated in  FIG. 4 , if the solenoid  551  is turned on by a control portion  90  (see  FIG. 1 ) provided in the apparatus body  1 A, a plunger  5510  is attracted in a Z direction and if the solenoid  551  is turned off by the control portion  90 , the plunger  5510  falls in a −Z direction, i.e., direction, opposite to the Z direction, by its own weight. The plunger  5510  is configured such that a nipped portion  5510   a  that is nipped by a nipping portion  553   a  of the lever  553  is formed in a lower end. 
     As illustrated in  FIGS. 5A and 5B , the guide member  54  has a spring hooking portion  54   a  and is biased in a clockwise (hereinafter referred to as “CW”) direction by a spring  60  stretching between a spring hooking portion  59   a  formed in the discharge reverse guide  59  and the spring hooking portion  54   a . Then, the guide member  54  is retained in the discharge position by abutting against a guide  61 . In addition, the guide member  54  has a pressed portion  54   c  abutting against the pressing portion  553   c  (see  FIG. 4 ) formed in the end portion of the lever  553 . It is noted that  FIG. 5A  is a section view that is taken along line VA-VA in  FIG. 2  and  FIG. 5B  is a section view that is taken along line VB-VB in  FIG. 2 . 
     Next, operations of the solenoid  551  and the guide member  54  will be described. If the control portion  90  turns off the solenoid  551 , as illustrated in  FIGS. 5A and 5B , the guide member  54  is retained in the discharge position by the spring  60  and the weight of the solenoid  551  itself. If the control portion  90  turns on the solenoid  551  from this state, the solenoid  551  is attracted in the Z direction illustrated in  FIG. 4  and as illustrated in  FIG. 6 , the lever  553  pivots in the CW direction. The lever  553  pivots in the CW direction, whereby the pressing portion  553   c  of the lever  553  presses the pressed portion  54   c  of the guide member  54  (see  FIG. 7 ). Therefore, the guide member  54  pivots in a counterclockwise (hereinafter referred to as “CCW”) direction against a biasing force of the spring  60 . 
     As illustrated in  FIG. 7 , in this state, the guide member  54  receives a force F 1  from a pressing portion of the lever  553  and receives a force Fsp from the spring  60 . Then, in the embodiment, a torque of the solenoid  551  and a resilient force of the spring  60  are set and the guide member  54  is configured to be pivotable in the CCW direction so as to satisfy a relationship of F 1 &gt;Fsp. As described above, the guide member  54  pivots in the CCW direction, thereby abutting against a stopper (not illustrated) and being retained in the reverse position. It is noted that a configuration, in which a current flowing through the solenoid  551  is detected, the current is a predetermined value, whereby the guide member  54  is positioned in the reverse position, may be used without providing the stopper. In a state in which the guide member  54  is positioned in the reverse position, if the control portion  90  turns off the solenoid  551 , the guide member  54  returns to the discharge position by its own weight and the biasing force of the spring  60 . 
     The solenoid  551  is turned on or off, whereby the guide member  54  moves between the discharge position and the reverse position as described above. Therefore, in a case of simplex printing, as illustrated in  FIG. 8 , the control portion  90  turns off the solenoid  551  and positions the guide member  54  in the discharge position. Here, the fixing unit  51  has a fixing roller  511  that drives in one direction, a heating unit  512  that heats the fixing roller  511 , and a pressing unit  513  that comes into pressure contact with the fixing roller  511 . The sheet S is pressed and heated by the nip of the fixing roller  511  and the pressing unit  513 , the toner image is fixed onto the sheet S, and then the sheet S is conveyed to the guide member  54 . The sheet S is guided to the discharge conveyance path  56  by the guide member  54  and is discharged to the sheet discharge tray  57  by the discharge roller pair  52 . 
     In a case of duplex printing, the control portion  90  detects the position of a leading end of the sheet S by a sensor (not illustrated) and turns on the solenoid  551  at a timing when the leading end of the sheet S is in an upstream from a conveyance path branching point B that is a locus of a tip end portion of the guide member  54 . Therefore, the guide member  54  is positioned in the reverse position and the sheet S is guided to the reverse conveying path  58  by the guide member  54 . 
     The control portion  90  detects the position of the trailing edge of the sheet S by a sensor  611  provided in the guide  61 . Then, the control portion  90  turns off the solenoid  551  and positions the guide member  54  in the discharge position when the trailing edge of the sheet S is in a reversible area R from a position, in which the trailing end of the sheet S passes through a tip end  61   a  of the guide  61 , to the reverse roller pair  53 . Accordingly, as described below, the reverse roller pair  53  is reversed, and the sheet S is switched back and is guided to the re-conveyance unit  70  by the guide member  54 . 
     Next, a switching mechanism that switches the rotating direction of the reverse roller pair  53  according to the embodiment will be described. It is noted that the reverse roller pair  53  is configured of a reverse roller  531  and a reverse driven roller  532 , and the discharge roller pair  52  is configured of a discharge roller  521  and a discharge driven roller  522 . 
       FIG. 9  is an explanatory view of a drive transmission route  80 , i.e., drive unit, in the embodiment. First, as illustrated in  FIG. 9 , the drive force generated by a motor D that is the drive source is transmitted to a fixing roller gear  514  provided coaxially with the fixing roller  511 . The drive transmission route  80  has a first drive transmission route  81 , i.e., first drive train, and a second drive transmission route  82 , i.e., second drive train, which are respectively branched from the fixing roller gear  514 . That is, the motor D transmits the drive force to the fixing unit  51  in the upstream of the first drive transmission route  81  and the second drive transmission route  82  in the drive transmission direction. 
     The first drive transmission route  81  transmits the drive force transmitted to the fixing roller gear  514  to a discharge roller gear  523  provided coaxially with the discharge roller  521 . The second drive transmission route  82  transmits the drive force transmitted to the fixing roller gear  514  to a reverse roller gear  533  provided coaxially with the reverse roller  531 . Then, a switching mechanism  83  that switches the rotating direction of the reverse roller  531  is provided on the second drive transmission route  82 . 
     Next, particularly, the switching mechanism  83  will be described in detail. As illustrated in  FIGS. 10A and 10B , the drive force of the motor D is transmitted to the fixing roller gear  514  via gears  801 ,  802 , and  803 . It is noted that, in  FIGS. 10A and 10B , a specific configuration of the second drive transmission route  82  is omitted and the drive force of the fixing roller gear  514  is transmitted to the discharge roller gear  523  via a gear train (not illustrated). 
     A gear  804  is in engagement with the fixing roller gear  514 , and a gear  805  is provided coaxially with the gear  804 . A gear  806  is in engagement with the gear  805 , and a first planetary gear mechanism  807 , i.e., first planetary gear portion, and a second planetary gear mechanism  808 , i.e., second planetary gear portion, are in engagement with the gear  806 . A planetary carrier  807   b  (described below) of the first planetary gear mechanism  807  is in engagement with a gear  809  and a drive force of the gear  809  is transmitted to the reverse roller gear  533  via a gear  810 . 
     The motor D, the gears  801  to  806 ,  809 , and  810 , the fixing roller gear  514 , the first planetary gear mechanism  807 , the second planetary gear mechanism  808 , and the reverse roller gear  533  are unitized by a drive base  811 , a drive cover  812  and a plurality of shafts  813 , and are mounted on the apparatus body. 
     As illustrated in  FIG. 11 , two adjacent planetary gear mechanisms  807  and  808  are respectively configured of sun gears  807   a  and  808   a , planetary carriers  807   b  and  808   b , two planetary gears  807   c  and  808   c , and internal gears  807   d  and  808   d . In addition, the planetary gear mechanisms  807  and  808  configure a planetary gear apparatus  878 . 
     As described above, two internal gears  807   d  and  808   d , i.e., first rotation element and fourth rotation element, are in engagement with the gear  806  driven by the motor D and are rotated in the same direction. The internal gear  807   d , i.e., first rotation element, is in engagement with two planetary gears  807   c  and  807   c  which are rotatably mounted on the planetary carrier  807   b , i.e., second rotation element. In addition, the planetary gears  807   c  and  807   c  are in engagement with the sun gear  807   a , i.e., third rotation element. 
     Similarly, the internal gear  808   d , i.e., fourth rotation element, is in engagement with two planetary gears  808   c  and  808   c  which are rotatably mounted on the planetary carrier  808   b , i.e., fifth rotation element. In addition, the planetary gears  808   c  and  808   c  are in engagement with the sun gear  808   a , i.e., sixth rotation element. In addition, the planetary carriers  807   b  and  808   b  are in engagement with each other and the planetary carrier  807   b  outputs the drive force to the gear  809 . 
     As illustrated in  FIGS. 10A and 10B , a stopper  814  is provided coaxially with the gear  806  to be relatively rotatable with respect to the gear  806 . The stopper  814  has a first engagement portion  814   a  that is able to engage and stop the sun gear  807   a , a second engagement portion  814   b  that is able to engage and stop the sun gear  808   a , and a nipped portion  814   d . The stopper  814  is biased by a spring  816  of which one end is fixed to a spring hooking portion  811   a  of the drive cover  812  so that the first engagement portion  814   a  engages with the sun gear  807   a . It is noted that when the first engagement portion  814   a  engages with the sun gear  807   a , the second engagement portion  814   b  does not engage with the sun gear  808   a  and the guide member  54  is positioned in the discharge position. 
     In addition, as illustrated in  FIG. 4 , the lever  553  has a nipping portion  553   d  that nips the nipped portion  814   d  of the stopper  814 . The solenoid  551  is turned on and the lever  553  pivots, whereby the stopper  814  is pivoted against the biasing force of the spring  816 . Therefore, the first engagement portion  814   a  moves away from the sun gear  807   a , the second engagement portion  814   b  is in engagement with the sun gear  808   a , and the guide member  54  is positioned in the reverse position. That is, the stopper  814  selectively stops the sun gear  807   a  and the sun gear  808   a . It is noted that, in the embodiment, a stop unit  800  is configured of the actuator unit  55  and the stopper  814 . The switching mechanism  83  has the first planetary gear mechanism  807 , the second planetary gear mechanism  808 , and the stop unit  800 . 
     Next, a switching operation of the rotating direction of the reverse roller  531  by the switching mechanism  83  will be described.  FIG. 12  illustrates the rotating direction of each gear on the first drive transmission route  81  when the guide member  54  is positioned in the discharge position.  FIG. 13  illustrates the rotating direction of each gear on the first drive transmission route  81  when the guide member  54  is positioned in the reverse position. It is noted that  FIGS. 12 and 13  are section views that are respectively taken along line XII-XII and line XIII-XIII of  FIG. 2 . 
     When the solenoid  551  is turned off, as illustrated in  FIG. 12 , the reverse roller gear  533  and the reverse roller  531  are rotated in the CW direction, i.e., first rotating direction. Specifically, as described above, the first engagement portion  814   a  is in engagement with the sun gear  807   a  and the sun gear  807   a  is stopped. The internal gears  807   d  and  808   d  are rotated in the CCW direction, and the planetary carrier  807   b  is decelerated and is rotated in the CCW direction by the gear  806  rotating in the CW direction. The rotation of the planetary carrier  807   b  rotating in the CCW direction is transmitted to the reverse roller  531  via the gears  809  and  810  and the reverse roller gear  533 , and the reverse roller  531  is rotated in the CW direction. It is noted that, in this case, the guide member  54  is positioned in the discharge position. The motor D, the fixing roller  511 , and the discharge roller  521  are rotated in the CW direction. 
     Then, when switching the reverse roller gear  533  and the reverse roller  531  to be rotated in the CCW direction, the control portion  90  turns on the solenoid  551 . Then, as illustrated in  FIG. 13 , the lever  553  pivots and the stopper  814  pivots against the biasing force of the spring  816 . Therefore, the first engagement portion  814   a  moves away from the sun gear  807   a , the second engagement portion  814   b  is in engagement with the sun gear  808   a , and the sun gear  808   a  is in a stop state. 
     The internal gears  807   d  and  808   d  are rotated in the CCW direction and the planetary carrier  807   b  is decelerated and is rotated in the CW direction by the gear  806  rotating in the CW direction. The rotation of the planetary carrier  807   b  rotating in the CW direction is transmitted to the reverse roller  531  via the gears  809  and  810 , and the reverse roller gear  533 . The reverse roller  531  is rotated in the CCW direction, i.e., second rotating direction, that is the direction opposite to the CW direction. It is noted that, in this case, the guide member  54  is positioned in the reverse position, and the motor D, the fixing roller  511 , and the discharge roller  521  are rotated in the CW direction. 
     Furthermore, if the solenoid  551  is turned off from on, a procedure reverse to the procedure described above is followed and the rotating direction of the discharge roller  521  is switched from in the CW direction to in the CCW direction. Also, in this case, the motor D, the fixing roller  511 , and the discharge roller  521  are still rotated in the CW direction. It is noted that, in a moment when the rotating direction of the reverse roller  531  is changed, a load is applied to the motor D in a direction opposite to the direction in which the motor D is rotated. 
     Here, an equivalent mass of the motor D continuously rotated in the CW direction and the fixing roller  511  on conveyance is M 1  and an equivalent mass of the discharge roller  521  on conveyance is M 2 . When a total equivalent mass on a side on which rotation is always provided in the CW direction is M, M=M 1 +M 2  is satisfied. If an equivalent mass of the reverse roller  531  that is normally and reversely rotated on conveyance is m, in general, since the equivalent mass M is greater than the equivalent mass m, an equivalent mass ratio to rotate in the CW direction is increased. As a result, it is possible to reduce the load applied to the motor D when the reverse roller  531  is reversely rotated. In addition, as M/m is larger, it is possible to reduce the load applied to the motor D. Here, the equivalent mass is obtained by converting the moment of inertia as a mass to be an inertial force equivalent in speed on the conveyance. 
     In other words, in the embodiment, the fixing roller  511  and the discharge roller  521  are always rotated in one direction (CW direction) and only the rotating direction of the reverse roller  531  is switched by the switching mechanism  83 . Therefore, the fixing roller  511  and the discharge roller  521  act as inertia on the motor D side and when the rotating direction of the reverse roller  531  is switched, even if the load is applied to the motor D, a constant speed of the motor is maintained by the inertia. 
     Therefore, it is possible to reduce a load hindering the rotation generated by the motor D and it is possible to reduce time during switch-back of the sheet S. As a result, it is possible to provide the image forming apparatus capable of improving the throughput and capable of performing duplex printing with high productivity. 
     In addition, the first planetary gear mechanism  807  and the second planetary gear mechanism  808  have the same configuration, can use common parts, and can reduce costs. In addition, it is possible to downsize the mechanism by using the planetary gear mechanism compared to a gear train in which spur gears are arranged in a radius direction. In addition, even if two planetary gear mechanisms are used, since the drive force is output from the planetary carrier  807   b  that is always the same rotation element, it is possible to transmit stable rotation to the reverse roller  531 . 
     It is noted that, in the embodiment, a configuration, in which the fixing roller  511 , the discharge roller  521 , and the reverse roller  531  are driven by the motor D, is used, but the invention is not limited to the embodiment. For example, the fixing roller  511  may be driven by another motor. 
     In addition, in the embodiment, switching of the rotating direction of the reverse roller  531  and the pivot of the guide member  54  are performed by the common solenoid  551 , but another solenoid may be used. 
     In addition, in the embodiment, the planetary gear mechanisms  807  and  808  input the drive force from the internal gears  807   d  and  808   d , and the drive force is output from the planetary carrier  807   b , but the invention is not limited to the embodiment. That is, three rotation elements of the sun gear, the planetary gear, and the internal gear may appropriately be used to be allocated to an input element, a fixing element, and an output element. 
     In addition, in the embodiment, the switching mechanism  83  has two planetary gear mechanisms  807  and  808 , but the invention is not limited to the embodiment. For example, the drive force may be reversely transmitted from the sun gear or the internal gear to the reverse roller  531  by using one planetary gear and a clutch. In addition, the drive force may be reversely transmitted from the motor D to the reverse roller  531  by using the gear train and two clutches without using the planetary gear mechanism. 
     Other Embodiments 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2015-108835, filed May 28, 2015, which is hereby incorporated by reference herein in its entirety.