Patent Publication Number: US-8991824-B2

Title: Sheet conveying apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a sheet conveying apparatus which conveys a sheet within a sheet conveying path. 
     2. Description of the Related Art 
     In the related art, an image forming apparatus of an electrophotographic system, such as a printer, a copying machine, or a facsimile is configured so as to form an image on double sides of a sheet by forming the image one side of the sheet with an image forming portion, reversing the sheet with a reverse portion, and conveying once again the reversed sheet to the image forming portion. 
     In such an apparatus, a switching member, in which a flapper-shaped member is swingingly configured, is provided to make a switching such that the sheet is conveyed to a discharge conveying path, which discharges the sheet into the outside of the apparatus, or a reverse conveying path, which is provided with the reverse portion. A solenoid is used as a switching unit which switches the switching member to receive instruction from a controller within an image forming apparatus body, and thus the conveying path is selectively switched. 
     Generally, when a preceding sheet is conveyed to the discharge conveying path side and a succeeding sheet is conveyed to the reverse conveying path side, after a trailing end of the preceding sheet passes through the switching member, the switching member starts a switching operation of the conveying path as the solenoid operates by receiving the instruction from the controller. Then, the switching operation of the conveying path is completed while a leading end of the succeeding sheet reaches the switching member. 
     However, a speeding-up of the image forming apparatus has been advanced in recent years, and the speeding-up is achieved by shortening an interval between sheets to be conveyed continuously. Meanwhile, the conventional switching operation of the switching member has a certain amount of variation due to a response delay of the solenoid. When the interval between the sheets to be conveyed continuously is gradually shortened, there is a possibility that the interval is not in time between the start and completion of the operation of the switching member due to the variation. In the worst case, the leading end of the succeeding sheet reaches the switching member before the switching operation of the switching member is completed, and thus a jam may occur or the sheet may be damaged. 
     In order to solve these problems, a following configuration is disclosed in Japanese Patent Laid-Open No. 2001-106409. That is, the solenoid is in ON before the trailing end of the preceding sheet passes through a double-side switching member, and a semi-closed position state is provided to convey while interposing the preceding sheet with the double-side switching member and a conveying guide. Then, when the trailing end of the preceding sheet passes through the double-side switching member, the double-side switching member of the semi-closed position state becomes a completely-closed position. 
     Thereby, even when the interval between the sheets is short, a conveying path switching operation can be performed. Further, a slip does not occur in such a manner that a conveying force of a reverse roller which reverses the preceding sheet at a downstream side of the double-side switching member is sufficiently larger than a conveying resistance due to an abutting pressure between the double-side switching member and the conveying guide. 
     In addition, a configuration, which switches the switching member to select three conveying paths, has been proposed Japanese Patent Laid-Open No. 5-286627. 
     As described above, in Japanese Patent Laid-Open No. 2001-106409, even when the sheet is interposed between the switching member and the conveying member, the slip does not occur in such a manner that the conveying force of the conveying roller which conveys at the downstream side is sufficiently larger than the conveying resistance due to the abutting pressure between the switching member and the conveying guide. However, as the conveying force increases, a size of a motor becomes larger, resulting in increasing the cost. In addition, a roller having a small conveying force, for example, a roller that conveys without a nip, such as a comb-tooth discharge roller is not disposed just behind the switching member. 
     In addition, due to high image quality in recent years, when the sheet is interposed with the switching member and the guide at a state in which the sheet temperature immediately after fixing is high and a fixed image surface and a conveying rib come in contact with each other, there is a conspicuous problem in that unevenness density of the image between a contact surface and a non-contact surface with the conveying rib is caused. 
     In addition, since the configuration disclosed in Japanese Patent Laid-Open No. 5-286627 is complicated, a space is required and a manufacturing cost is expensive. 
     SUMMARY OF THE INVENTION 
     The invention has been made to solve the above-described problems. The invention is desirable to provide an apparatus that conveys a sheet with a simple and inexpensive configuration to cope with a case in which an interval between sheets to be conveyed continuously is short. 
     According to the invention, a sheet conveying apparatus which conveys a sheet includes: a sheet conveying apparatus which conveys a sheet, comprising: a sheet conveying path which diverges into a first conveying path and a second conveying path at a diverging point; a guide member which is movable between a first position where the guide member guides the sheet to the first conveying path and a second position where the guide member guides the sheet to the second conveying path; a driving portion which is configured to move the guide member to the second position from the first position; and a holding portion which, in a case that the guide member is driven by the driving portion to move to the second position from the first position, provides a load to the driving by the driving portion and holds the guide member at a third position between the first position and the second position. 
     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 an example of a sheet conveying apparatus according to a first embodiment of the invention and a cross-sectional view illustrating a configuration of an image forming apparatus of a color electrophotographic system; 
         FIG. 2  is a cross-sectional view illustrating a detailed configuration of a periphery a double-side switching member according to the first embodiment of the invention; 
         FIG. 3  is a diagram illustrating in detail a switching configuration of the double-side switching member according to the first embodiment of the invention; 
         FIG. 4  is a diagram illustrating schematically a conveying path switching unit according to the first embodiment of the invention; 
         FIG. 5  is a graph illustrating a relation between a value of a current flowing in a solenoid according to the first embodiment of the invention and time; 
         FIG. 6  is a graph illustrating a relation between a force to operate the double-side switching member in a second position and a force to operate the double-side switching member in a first direction in each position of the double-side switching member according to the first embodiment of the invention; 
         FIGS. 7A to 7C  are diagrams illustrating a relation between a preceding sheet and a reversing sheet in each position of the double-side switching member according to the first embodiment of the invention; 
         FIG. 8  is a graph illustrating a relation between a stroke amount and a driving force of an actuator of the solenoid according to the first embodiment of the invention; 
         FIG. 9  is a diagram illustrating a schematic configuration a conveying path switching mechanism according to a second embodiment of the invention; 
         FIG. 10  is a graph illustrating a relation between a deflection amount and a load of an unequal pitch spring according to the second embodiment of the invention; 
         FIG. 11  is a graph illustrating a relation between a force to operate the double-side switching member in a second position and a force to operate the double-side switching member in a first direction in each position of the double-side switching member according to the second embodiment of the invention; 
         FIG. 12  is a graph illustrating a difference in effects between a case in which a switching member return compression spring according to the second embodiment of the invention has a linear characteristic and a case in which the switching member return compression spring has a non-linear characteristic; 
         FIG. 13  is a diagram illustrating a schematic configuration of a conveying path switching mechanism according to a third embodiment of the invention; 
         FIGS. 14A to 14C  are diagrams illustrating a shape of a cam according to the third embodiment of the invention; 
         FIGS. 15A to 15C  are diagrams illustrating a relation between the cams in each position of the double-side switching member according to the third embodiment of the invention; and 
         FIG. 16  is a graph illustrating a relation between a force to operate the double-side switching member in a second position and a force to operate the double-side switching member in a first direction in each position of the double-side switching member according to the third embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, with reference to the drawings, an embodiment in which an electrophotographic printer is applied as an example of a sheet discharge apparatus and an image forming apparatus provided with the sheet discharge apparatus according to the invention is specifically described. Unless otherwise specified, scope of the invention should not be construed restrictively in terms of dimensions, materials, and shapes of components, and relative arrangement thereof, which are described in these embodiments. 
     [First Embodiment]  FIG. 1  is an example of a sheet conveying apparatus according to a first embodiment of the invention and a cross-sectional view illustrating a configuration of an image forming apparatus of a color electrophotographic system, and  FIG. 2  is a cross-sectional view illustrating a detailed configuration of a periphery a double-side switching member, which is a conveying path switching member. 
     (Overall Configuration of Image Forming Apparatus) An image forming apparatus  1  is provided with four drum-shaped image bearing members which are juxtaposedly arranged in a substantially horizontal direction as an image bearing member, that is, photosensitive drums  2  ( 2   a ,  2   b ,  2   c , and  2   d ). The photosensitive drum  2  is rotationally driven in a clockwise direction in  FIG. 1 . 
     In addition, a charging device  3  ( 3   a ,  3   b ,  3   c , and  3   d ) is provided to uniformly charge a surface of the photosensitive drum  2 . Moreover, a scanner unit  4  ( 4   a ,  4   b ,  4   c , and  4   d ) is provided to form an electrostatic latent image on each photosensitive drum  2  by irradiation of a laser beam based on image information. 
     In addition, a developing device  5  ( 5   a ,  5   b ,  5   c , and  5   d ) is provided to develop the electrostatic latent image as a toner image by attaching a toner including a developer to the electrostatic latent image. Further, a cleaning device  6  ( 6   a ,  6   b ,  6   c , and  6   d ) is provided to remove a transfer residual toner remaining on the surface of the photosensitive drum  2  after a transfer. 
     The photosensitive drum  2 , the charging device  3 , the developing device  5 , and the cleaning device  6  is integrated as a cartridge unit to form an image of different colors (yellow, cyan, magenta, and black colors) by a electrophotographic recording system, respectively. 
     A primary transfer roller  7  ( 7   a ,  7   b ,  7   c , and  7   d ) is abutted on the photosensitive drum  2  through an intermediate transfer belt  8 , and the toner image on the photosensitive drum  2  is transferred to the intermediate transfer belt  8 . The intermediate transfer belt  8  is tensioned between a driving roller  9  and a tension roller  10  to rotate in counterclockwise direction by driving of the driving roller  9 . A secondary transfer roller  11 , which is provided at a position opposite to the driving roller  9  through the intermediate transfer belt  8 , transfers the tonner image transferred to the intermediate transfer belt  8  to a sheet S. In addition, an intermediate transfer belt cleaning device  12  is provided at a position opposite to the tension roller  10  through the intermediate transfer belt  8  to remove and recover the transfer residual toner remaining on the surface of the intermediate transfer belt  8 . 
     In order to feed and convey the sheet S, a sheet cassette  13 , a multi-tray  17 , and a pair of resister rollers  20  are provided to convey the sheet along a sheet conveying path  200 . The sheet cassette  13  is provided at the lowermost portion of the image forming apparatus  1 , and the multi-tray  17  is provided at a lower right of the apparatus to correct a skew feeding of the sheet S. 
     A fixing portion  21  fixes the toner image, which is formed on the sheet S through the intermediate transfer belt  8 , by the image forming portion of each color. 
     A diverging point B is provided on the sheet conveying path  200 , and a discharge conveying path  23  (first conveying path) and a reverse conveying path  26  (second conveying path) are provided to diverge into a downstream side in a conveying direction of the sheet S from this diverging point. 
     A double-side switching member  22  is a flapper-shaped member which is swingingly configured and is provided in the vicinity of the diverging point B. The double-side switching member  22  is waiting at a first position X which guides the sheet S to the discharge conveying path  23  during a normal state. In order to form the toner image on a second side of the sheet S, then, when a solenoid  50  to be described later is turned ON, the double-side switching member  22  is switched to a second position Y indicated by a dotted line portion of  FIG. 3  so as to guide the sheet S to the reverse conveying path  26  side. As described above, the double-side switching member  22  is movable from the first position X to the second position Y. 
     A pair of discharge rollers  24  is a roller that discharges the sheet S to a discharge tray  25  acting as a sheet stacking portion. 
     Further, in order to form the toner image on the second side of the sheet S, a pair of reverse rollers  27 , which switches back the sheet S, and a pair of double-side conveying rollers  28 ,  29 , and  30 , which again conveys the sheet to the pair of resister rollers  20 , are equipped. In addition, a pre-discharge roller  31  ( FIG. 2 ), in which a silicon sponge is covered with a PFA tube, is provided on the discharge conveying path  23  so as to prevent the image from being scratched when the sheet S is tensioned between the fixing portion  21  and the pair of discharge rollers  24 . 
     Next, an operation of forming the image by the image forming apparatus  1  will be schematically described. 
     The predetermined number of sheets S stacked on the sheet cassette  13  is separated one by one by a feeding roller  14  and a separation roller  15  and is then conveyed to a feeding pull-out roller  16 . In addition, the predetermined number of sheets S stacked on the multi-tray  17  is separated one by one by a multi-feeding roller  18  and a multi-separation roller  19  and is then conveyed to the feeding pull-out roller  16 . 
     The sheet S conveyed by the feeding pull-out roller  16  is conveyed to the pair of resister rollers  20  to correct the skew feeding and is then conveyed to an abutting portion between the intermediate transfer belt  8  and the secondary transfer roller  11 . The toner image transferred onto the intermediate transfer belt  8  from the image forming portion of each color is transferred onto the sheet S from the abutting portion between the intermediate transfer belt  8  and the secondary transfer roller  11  to form a color image, and then the sheet S is conveyed to the fixing portion  21 . 
     The fixing portion  21  includes a fixing sleeve  21   a  as a heat source and a pressure roller  21   b  which is pressed against the fixing sleeve  21   a  to apply pressure to the sheet S, and heat and pressure are applied to the sheet S passing through the fixing portion  21 , which is conveyed by the fixing portion  21 . The sheet S, on which the toner image of plural colors is fixed, is guided to the double-side switching member  22  situated at the first position X by the fixing portion  21  to be guided to the discharge conveying path  23  side, then, is discharged to the discharge tray  25  through the pair of discharge rollers  24 . 
     In addition, during a double-side printing, the double-side switching member  22  is switched to the second position Y guiding to the reverse conveying path  26  by turning-On the solenoid  50  ( FIG. 4 , details will be described later). The sheet S, in which the image is fixed on a first side by the fixing portion  21 , is guided to the reverse conveying path  26  side by the double-side switching member  22  situated at the second position Y and is then conveyed to the pair of reverse rollers  27 . 
     The double-side switching member  22  is switched to the first position X by turning-OFF the solenoid  50  when the trailing end of the sheet S passes through. The pair of reverse rollers  27  is reversed by receiving a signal from a controller (not illustrated) before the trailing end of the sheet S passes through the pair of reverse rollers  27 . The sheet S passes through the reverse conveying path  26  by setting the trailing end of the sheet S to the leading position and is then conveyed to the pair of double-side conveying rollers  28 . And then, the sheet S is conveyed to the pair of resister rollers  20  by each of the pair of double-side conveying rollers  29  and  30  by joining the conveying path from the sheet cassette  13  and the multi-tray  17 . 
     Then, the image is formed on the second side of the sheet S through the intermediate transfer belt  8  and the secondary transfer roller  11 . Then, the second side of the sheet S is fixed in the fixing portion  21 , and the sheet S is guided to the discharge conveying path  23  side by the double-side switching member  22  situated at the first position X and is then discharged onto the discharge tray  25  by the pair of discharge rollers  24 . 
     (Switching Configuration of Switching Member)  FIG. 3  is a diagram illustrating a detailed switching configuration of the double-side switching member  22  of the image forming apparatus  1  according to the first embodiment of the invention. The switching configuration of the conveying path will schematically be described with reference to  FIG. 3 . Each arrow in  FIG. 3  indicates an operation direction of each configuration in the case in which the double-side switching member  22  is switched from the first position X to the second position Y. 
     The solenoid  50  is a driving portion, and incorporates one end of a plunger  50   a . Moreover, the other end of the plunger  50   a  is pivotally supported on an arm  51 . The arm  51  abuts on a connection plate  52  at a connection plate abutting portion  51   b , and the connection plate  52  abuts on an arm portion  22   a  of the double-side switching member  22  at a switching member abutting portion  52   b.    
     The double-side switching member  22  as a guide member moves from the first position X to the second position Y around a double-side switching member rotating shaft  53 . A double-side switching member return spring  54  as a force applying portion to the first position X is configured in such a manner that one end of the arm is fixed to a return spring holding portion  22   b  of the double-side switching member  22  and the other end is fixed to a return spring holding portion (not illustrated) of the discharge conveying path  23 , thereby pressing in a direction in which the double-side switching member  22  rotates toward the first position X. The pressing force of the double-side switching member return spring  54  is greater than a force to rotate toward the second position Y by its own weight of the double-side switching member  22 . 
     A second position abutting portion  55  made of polone having excellent impact absorption is provided at a discharge conveying lower guide  56  constituting the discharge conveying path  23 , and a first position abutting portion  57  made of polone is provided in a non-sheet passing area of a reverse conveying guide (not illustrated) constituting the reverse conveying path  26 . 
     (Switching Operation between First Position and Second Position of Switching Member) Next, an operation when the double-side switching member  22  is switched from the first position X to the second position Y will be described. 
     First, a current is applied to the solenoid  50  by reception of an ON signal from the controller  71 . Then, the plunger  50   a  is driven to the solenoid  50  side (left side in  FIG. 3 ) such that the arm  51  is tensioned, and the arm  51  rotates in a clockwise direction around the arm rotating shaft  51   a . When the connection plate abutting portion  51   b  of the arm  51  pushes the connection plate  52 , the connection plate  52  rotates in a counterclockwise direction around a connection plate rotating shaft  52   a , and the switching member abutting portion  52   b  of the connection plate  52  pushes up the arm portion  22   a  of the double-side switching member  22 . Then, the double-side switching member  22  rotates to the second position Y around the double-side switching member rotating shaft  53 , the switching operation is completed at the position where the leading end of the double-side switching member  22  abuts on the second position abutting portion  55 . 
     On the other hand, when the double-side switching member  22  returns from the second position Y to the first position X, the current application to the solenoid  50  side is stopped by reception of an OFF signal from the controller  71 . Then, a force to drive the plunger  50   a  to the solenoid  50  side is eliminated, and the double-side switching member  22  situated at the second position Y rotates in the counterclockwise direction by the force of the double-side switching member return spring  54 . Then, the double-side switching member  22  rotates to the position where the leading end of double-side switching member  22  abuts on the first position abutting portion  57 . According to the rotation, the connection plate  52  rotates in the clockwise direction, the arm  51  rotates in the counterclockwise direction, and the plunger  50   a  operates in a direction away from the solenoid  50 , thereby returning to the state of  FIG. 3 . 
     It is assumed that the description of the operation from the first position X to the second position Y of the above-described double-side switching member  22  is the same as that of the schematic operation of the double-side switching member  22 . In fact, as will be described below, the double-side switching member  22  moves through a third position Z. 
     (Switching Operation to Third Position of Switching Member) A switching operation of the double-side switching member  22  from the first position X to the second position Y and the third position Z will be described below. 
       FIG. 4  is a perspective view of the conveying path switching configuration in the image forming apparatus  1  according to the first embodiment. With reference to  FIG. 4 , a configuration for holding the double-side switching member  22  at the third position Z (illustrated in  FIG. 7B ) positioned between the first position X and the second position Y will be described. 
     As illustrated in  FIG. 4 , a plate spring  58  is provided as a holding portion, which temporarily holds, at the third position. The plate spring  58  is supported by a plate spring supporting member  59 . Moreover, a plate spring biasing portion  59   a  is provided at the plate spring supporting member  59  to apply an appropriate biasing force to the plate spring  58 . A plate spring abutting portion  52   c  of the connection plate  52  is disposed to come in contact with the plate spring  58  at a certain position. 
       FIG. 5  illustrates a relation between a current value to be applied to the solenoid  50  and time. In  FIG. 5 , the vertical axis represents the current value; the horizontal axis represents the time; and a lower side in  FIG. 5  represents a time change in the present or absence of sheet at the tip portion of the double-side switching member  22 . Further, the solenoid  50  pulses the current flowing therethrough to change a pulse period, thereby changing the force to operate the double-side switching member  22 . A current-carrying control to the solenoid  50  is performed by the controller  71 . 
     Each of points a, b, and c on the vertical axis of  FIG. 5  indicates the current value to be applied to the solenoid  50  when the solenoid  50  receives the ON signal from the controller  71 . T 1  on the horizontal axis indicates the time at which the double-side switching member  22  starts to operate in the direction from the first position X to the third position Z, and T 2  indicates the time at which the double-side switching member  22  starts to operate in the direction from the third position Z to the second position Y. T 3  indicates the time required to reduce the current value flowing into the solenoid  50  so as to prevent a temperature rise of the solenoid  50 , and T 4  indicates the time at which the double-side switching member  22  starts to operate in the direction from the second position Y to the first position X when the solenoid  50  receives the OFF signal from the controller  71 . 
     A preceding sheet Sa proceeds toward the discharge conveying path  23  ( FIG. 2 ), and a trailing end sheet Sb moves toward the reverse conveying path  26  ( FIG. 3 ). A trailing end S 1  is a trailing end of the preceding sheet Sa, a leading end S 2  is a leading end of the reversing sheet Sb, and time S 3  is time when the trailing end of the reversing sheet passes through the tip portion of the double-side switching member  22 . 
       FIG. 6  is a diagram in which forces to rotate the double-side switching member  22  are summarized. 
     A solid line indicates the driving force caused by the solenoid  50  ( FIG. 4 ) and the force by its own weight of the double-side switching member  22 , which are forces to operate the double-side switching member  22  from the first position X to the third position Z and the second position Y. On the other hand, a dashed-dotted line indicates the force by the double-side switching member return spring  54  and the plate spring  58 , which is a force to operate the double-side switching member  22  from the second position Y to the first position X. 
     A force f 1  on the vertical axis indicates the sum of the driving force and the force by its own weight when a current value “a” is applied to the solenoid  50 , and a force f 1   a  indicates the sum of the force when the double-side switching member  22  moves to the third position Z at the current value “a” and the force by its own weight. Furthermore, since the solenoid  50  has the characteristics that the driving force rises when the stroke amount of the plunger  50   a  and the solenoid  50  is shorter, it satisfies the relation of f 1  &gt;f 1   a.    
     The sum of forces f 2  is the sum of the driving force and the force by its own weight when a current value “b” is applied to the solenoid  50  at the third position Z, and the sum of forces f 2   a  is the sum of the driving force and the force by its own weight when the current value “b” is applied to the solenoid  50  at the second position Y. A force r 1  is a force by the double-side switching member return spring  54  ( FIG. 5 ) at the first position X, and a force r 1   a  is a force by the double-side switching member return spring  54  at the third position Z. The sum of forces r 2  is the sum of the biasing force of the double-side switching member return spring  54  at the third position Z and the force by the load of the plate spring  58  ( FIG. 5 ), and the sum of forces r 2   a  is the sum of the force by the double-side switching member return spring  54  and the force by the plate spring  58  at the second position Y. 
       FIGS. 7A to 7C  are diagrams illustrating states of the preceding sheet Sa and the reversing sheet Sb at each position of the double-side switching member  22 .  FIG. 7A  illustrates the relation between the preceding sheet Sa and the reversing sheet Sb when the double-side switching member  22  is positioned at the first position X,  FIG. 7B  illustrates the relation between the preceding sheet Sa and the reversing sheet Sb when the double-side switching member  22  is positioned at the third position Z, and  FIG. 7C  illustrates the relation between the preceding sheet Sa and the reversing sheet Sb when the double-side switching member  22  is positioned at the second position Y. 
     Next, the state in which the preceding sheet Sa and the reversing sheet Sb are conveyed to the conveying path selected by the double-side switching member  22  will be described. 
     First, a case in which the double-side switching member  22  is rotatably moved from the first position X to the third position Z will be described. As illustrated in  FIG. 7A , when the double-side switching member  22  is positioned at the first position X, the preceding sheet Sa conveyed by the fixing portion  21  passes through the discharge conveying path  23  and is then discharged to the discharge tray  25  through the pair of discharge rollers  24 . 
     After the elapse of a certain time from the point where the leading end of the preceding sheet Sa passes through a sheet detecting sensor provided at a downstream side in a conveying direction of the fixing portion  21 , a voltage is applied such that the current of the current value “a” ( FIG. 5 ) flows through the solenoid  50  as the ON signal is received from the controller  71  at the time T 1  ( FIG. 5 ). 
     When the current of the current value “a” flows through the solenoid  50 , the plunger  50   a  ( FIG. 3 ) is driven into the solenoid  50  side. Thus, the resultant force f 1  of the force to rotate the double-side switching member  22  in the clockwise direction ( FIG. 3 ) through the arm  51  ( FIG. 4 ) and the connection plate  52  ( FIG. 4 ), and the force by its own weight of the double-side switching member  22  is applied to the double-side switching member  22 . When the double-side switching member  22  is positioned at the first position X, the force r 1  is a force to rotate the double-side switching member  22  in the counterclockwise direction by the double-side switching member return spring  54  ( FIG. 3 ). At this time, since the relation of f 1 &gt;r 1  is set, the double-side switching member  22  starts to rotate in the clockwise direction. 
     Next, a case in which the double-side switching member  22  is held at the third position Z ( FIG. 7B ) will be described. At the third position Z, since the stroke amount of the plunger  50   a  is shortened, the force becomes the force f 1   a  ( FIG. 6 ) to cause the double-side switching member  22  to be rotated in the clockwise direction when the current of the solenoid  50  has the current value “a”. 
     When the double-side switching member  22  is positioned at the third position Z, the plate spring abutting portion  52   c  ( FIG. 4 ) of the connection plate  52  rotates in the counterclockwise direction around the connection plate rotating shaft  52   a  to abut on the plate spring  58  ( FIG. 4 ). Here, the plate spring  58  is in a state in which a pressure is applied in advance by the plate spring biasing portion  59   a  ( FIG. 4 ) of the plate spring supporting member  59 . For this reason, when the plate spring abutting portion  52   c  of the connection plate  52  abuts on the plate spring  58 , a force to rotate the connection plate  52  in the clockwise direction (a force to rotate the double-side switching member  22  in the counterclockwise direction) is generated. This force acts as a load against the driving force of the solenoid  50 . 
     The force r 2  to rotate the double-side switching member  22  in the counterclockwise direction is generated by joining the force r 1   a  to rotate the double-side switching member  22  in the counterclockwise direction at the third position Z by the double-side switching member return spring  54  ( FIG. 3 ) in addition to the force caused by the plate spring  58 . Thus, the forces to rotate the double-side switching member  22  satisfy the relation of f 1   a &lt;r 2 . Therefore, since the double-side switching member  22  is held at a state in which the plate spring abutting portion  52   c  of the connection plate  52  abuts on the plate spring  58  when the current value “a” flows through the solenoid  50 , the double-side switching member  22  is held at the third position Z. 
     Further, as illustrated in  FIG. 7B , the double-side switching member  22 , which is held at the third position Z, is positioned so as not to press the preceding sheet Sa without causing the preceding sheet Sa to be interposed with the discharge conveying lower guide  56 . For this reason, there is no occurrence of density unevenness of the image to be generated when the preceding sheet Sa is in contact with the double-side switching member  22  or the discharge conveying lower guide  56  immediately after the fixing. Further, in the state of  FIG. 7A , since a conveying speed of the pair of discharge rollers  24  is faster than that of the fixing portion  21 , the preceding sheet Sa is in the tensioned state between the fixing portion  21 , the pre-discharge roller  31 , and the pair of discharge rollers  24  and is out of contact with the guide portion of the discharge conveying path  23 . 
     Next, a case in which the double-side switching member  22  rotates from the third position Z to the second position Y will be described. Until the time S 1  ( FIG. 5 ) at which the trailing end of the preceding sheet Sa passes through the tip portion of the double-side switching member  22 , the double-side switching member  22  is held at the third position Z. 
     At the time T 2  ( FIG. 5 ) after the time S 1  at which the sheet detecting sensor  72  provided at the downstream side in the conveying direction of the fixing portion  21  detects the trailing end of the preceding sheet Sa, the current value “b” ( FIG. 5 ) flows through the solenoid  50  as the ON signal is received from the controller  71 . When the current value “b” ( FIG. 5 ) flows through the solenoid  50 , the force to drive the plunger  50   a  into the solenoid  50  side increases, and the force to rotate the double-side switching member  22  in the clockwise direction becomes the resultant force f 2  ( FIG. 6 ) with the force by its own weight of the double-side switching member  22 . The resultant force of the force to rotate the double-side switching member  22  in the counterclockwise direction by the double-side switching member return spring  54  ( FIG. 3 ) and the plate spring  58  ( FIG. 4 ) when the double-side switching member  22  is positioned at the third position Z is the resultant force r 2  ( FIG. 6 ) as described above. 
     The current value “b” is set such that the resultant force f 2  to rotate the double-side switching member  22  in the clockwise direction satisfies the relation of f 2 &gt;r 2 . When the relation of f 2 &gt;r 2  is satisfied, the plate spring abutting portion  52   c  of the connection plate  52 , which is held by abutting on the plate spring  58 , pushes up the plate spring  58  with a force greater than the pressure that is applied to the plate spring  58  in advance. In this way, the double-side switching member  22  starts to rotate in the clockwise direction. 
     The double-side switching member  22  is held at the second position Y ( FIG. 7C ) in the place where the double-side switching member  22  rotates in the clockwise direction from the third position Z to abut on the second position abutting portion  55  ( FIG. 3 ). The force to rotate the double-side switching member  22  in the clockwise direction, with the current value “b” at the second position Y, since the stroke amount of the plunger  50   a  is shortened, increases to become the resultant force f 2   a  ( FIG. 6 ) with the force by its own weight of the double-side switching member  22 . On the other hand, the resultant force r 2   a  ( FIG. 6 ) is the force to rotate the double-side switching member  22  in the counterclockwise direction by double-side switching member return spring  54  ( FIG. 3 ) and the plate spring ( FIG. 4 ) at the second position Y. 
     At this time, since the current value “b” is set such that the forces satisfy the relation of f 2 &gt;r 2   a , the double-side switching member  22  is held at the second position Y. The leading end of the reversing sheet Sb reaches the tip portion of the double-side switching member  22  at the time S 2  of  FIG. 6  after completing the operation to the second position Y and is in a state illustrated in  FIG. 7C . 
     In this manner, the double-side switching member  22  is switched to the third position Z in advance since the preceding sheet Sa passes through the double-side switching member  22  toward the discharge conveying path  23  side. By this, it is possible to quickly perform the switching up to the second position Y, compared to the operation of switching from the first position X to the second position Y at once. From the above, it is possible to shorten the distance between sheets while maintaining a status in which the preceding sheet Sa is out of contact with the double-side switching member  22 . 
     Next, an operation after the double-side switching member  22  is switched to the second position Y will be described. A current value “c” ( FIG. 5 ) is set in response to a temperature rise of the solenoid at the time T 3  ( FIG. 6 ) after the elapse of a certain time from the time S 2  ( FIG. 5 ) at which the leading end of the reversing sheet Sb reaches the tip portion of the double-side switching member  22 . At this time, the current value “c” is set within the range in which the force to rotate the double-side switching member  22  in the clockwise direction ( FIG. 7 ) is not less than the resultant force r 2   a  ( FIG. 6 ) of the force to rotate the double-side switching member  22  in the counterclockwise direction by the double-side switching member return spring  54  ( FIG. 3 ) and the plate spring  58  ( FIG. 4 ) at the second position Y. 
     Thereafter, when the sheet detecting sensor (not illustrated), which is provided at the downstream side in the conveying direction of the fixing portion  21 , detects the trailing end of the reversing sheet Sb, the OFF signal is received from the controller  71  at the time T 4  ( FIG. 5 ) after the elapse of a certain time from the time S 3  ( FIG. 5 ) at which the trailing end of the reversing sheet Sb passes through the tip portion of the double-side switching member  22 . Thus, the application of current to the solenoid  50  is stopped. Then, the force to drive the plunger  50   a  ( FIG. 3 ) by the solenoid  50  becomes zero, and the double-side switching member  22  positioned at the second position Y rotates in the counterclockwise direction due to the force of the double-side switching member return spring  54  and the plate spring  58  ( FIG. 4 ). Further, the double-side switching member  22  is held at the first position X by abutting the leading end of double-side switching member  22  on the first position abutting portion  57  ( FIG. 3 ). 
     Furthermore, the force to rotate the double-side switching member  22  in the counterclockwise direction by the double-side switching member return spring  54  and the plate spring  58  is illustrated as a vertically standing shape at the third position Z of the graph of  FIG. 6 . That is, the third position Z is constantly positioned for the period in which a press force does not exceed the force r 2  after the plate spring abutting portion  52   c  of the connection plate  52  abuts on the plate spring  58 . Therefore, even though the force applied to the double-side switching member  22  is varied in some degree by variation of the driving force of the solenoid  50 , the double-side switching member  22  can be exactly positioned at the third position Z. 
     For example, even when the force applied to the double-side switching member  22  is varied from the force f 1   a  to the force F 1  ( FIG. 6 ) due to the variation of the driving force of the solenoid  50 , the double-side switching member  22  can be exactly positioned at the third position Z. 
       FIG. 8  is a diagram illustrating an example of a relation between the driving force of the solenoid  50  and the stroke amount of the plunger  50   a . In  FIG. 8 , a solid line illustrates the driving force when the current value applied to the solenoid  50  is high, and a dotted line illustrates the driving force when the current value applied to the solenoid  50  is low. 
     In the ratio of the current value applied to the solenoid  50 , the current value “a” ( FIG. 5 ) is 45% when the current value “b” ( FIG. 5 ) is 100%. In the case of the current value “a”, when the double-side switching member  22  is positioned at the first position X, the driving force to attract the plunger  50   a  by the solenoid  50  becomes 75 gf. In the case of the current value “b”, when the double-side switching member  22  is positioned at the third position Z, since the current value increases and the stroke amount is shorten, the driving force is 180 gf. In this case, when the double-side switching member  22  is positioned at the second position Y, the current value is constant, but the stroke amount is shorten. Accordingly, the driving force is 210 gf. These driving force become the force to rotate the double-side switching member  22  through the arm  51  and the connection plate  52 . In addition, the force by its own weight of the double-side switching member  22  is 22 gf. 
     In addition, the pressure, which is applied to the plate spring  58  in advance, is 25 g, the force of the plate spring  58  is 35 g when the double-side switching member  22  is positioned at the second position Y, and the pressure of the double-side switching member return spring  54  is 30 g, 35 g, and 45 g at the first position X, the third position Z, and the second position Y, respectively. 
     When the above-described forces are applied to each configuration, moments applied to the double-side switching member  22  around the double-side switching member rotating shaft  53  are that the force f 1  is 280 gf·mm and the force r 1  is 175 gf·mm at the first position X to satisfy the relation of f 1 &gt;r 1 , and thereby the double-side switching member  22  starts to rotate in the direction of the second position Y. 
     In addition, when the double-side switching member  22  is held at the third position Z, the force f 1   a  is 290 gf·mm and the force r 2  is 350 gf·mm to satisfy the relation of f 1   a &lt;r 2 , and thus the double-side switching member  22  is held at the third position Z. When the double-side switching member  22  starts to operate from the third position Z to the second position Y, the force f 2  is 460 gf·mm and the force r 2  is 350 gf·mm without being changed to satisfy the relation of f 2 &gt;r 2 , and thus the double-side switching member  22  starts to rotate. When the double-side switching member  22  abuts on the second position abutting portion  55 , the force f 2   a  is 575 gf·mm and the force r 2   a  is 448 gf·mm to satisfy the relation of f 2   a &gt;r 2   a , and thus the double-side switching member  22  is held at second position Y. 
     Further, since the values described above are intended to illustrate one example that implements the first embodiment, values for implementing the first embodiment are not limited to these values. 
     Incidentally, the first embodiment is configured such that the plate spring  58  is used as a third position holding member, but may be configured such that a constant pressure is applied to other force applying portions such as a compression spring, tension spring, or torsion coil spring in advance. 
     The above-described embodiment describes an example in which the its own weight of the double-side switching member  22  acts so as to apply a force of the double-side switching member  22  from the first position X to the direction of the second position Y. However, it may be configured such that the double-side switching member  22  acts from the second position Y to the first position X by its own weight. In this case, its own weight acts in a direction to reduce the driving force. Furthermore, in the configuration in which the conveying path is diverged in the horizontal direction and the double-side switching member  22  rotates in a right-and-left direction, its own weight is not loaded on the driving force. 
     [Second Embodiment] Another embodiment of the invention will be described below with reference to  FIGS. 9 to 11 . A configuration of an image forming apparatus in the second embodiment is the same as that in the first embodiment, and the same or similar components as those in the first embodiment are denoted by the same reference numerals and the repeated description will not be presented. Hereinafter, parts different from the first embodiment will be mainly described. 
       FIG. 9  is a diagram illustrating a detailed switching configuration of the double-side switching member  22  of the image forming apparatus  1  according to the second embodiment. A switching member return compression spring  60  is an unequal pitch spring, and a pitch is different in the middle of spring. A compression spring holding portion  61  is provided in a part of the sheet guide portion of the reverse conveying path  26  ( FIGS. 7A to 7C ) to hold the switching member return compression spring  60 . 
     Arrows in  FIG. 9  illustrate operational directions when the force to rotate the double-side switching member  22  in the clockwise direction is applied in the case in which the current flows through the solenoid  50 . 
     When the voltage is applied and the current flows to/through the solenoid  50 , the force to rotate the double-side switching member  22  in the clockwise direction to the arm portion  22   a  of the double-side switching member  22  through the plunger  50   a , the arm  51 , and the connection plate  52  is generated. The switching member return compression spring  60  acting as an unequal pitch spring is pressed against the arm portion  22   a  of the double-side switching member  22  in the direction to rotate the double-side switching member  22  in the counterclockwise direction. Further, the other end of the switching member return compression spring  60  is fixed to the compression spring holding portion  61  provided in the reverse conveying path  26 . 
       FIG. 10  illustrates a relation between a load and a deflection of the switching member return compression spring  60  acting as the unequal pitch spring. In this unequal pitch spring, since the number of effective turns acting as a spring is changed when the load is applied to the spring, the unequal pitch spring has a non-linear characteristic as illustrated in  FIG. 10 . 
       FIG. 11  is a diagram in which forces to rotate the double-side switching member  22  during a conveying path switching according to the second embodiment are summarized. A solid line indicates a driving force caused by the solenoid  50  ( FIG. 9 ) and a force by its own weight of the double-side switching member  22 , which are forces to operate the double-side switching member  22  from the first position X ( FIGS. 7A to 7C ) to the third position Z and the second position Y. On the other hand, a dashed-dotted line indicates a force by the switching member return compression spring  60 , which is a force to operate the double-side switching member  22  from the second position Y to the first position X. 
     The current value and timing for applying the current to the solenoid  50  are the same as those in the first embodiment. In  FIG. 11 , a force f 3  indicates the sum of forces due to the solenoid  50  and its own weight of the double-side switching member  22  when the current value “a” is applied at the first position X, a force f 3   a  indicates the sum of forces due to the solenoid  50  and its own weight of the double-side switching member  22  when the current value “a” is applied at the third position Z, a force f 4  indicates the sum of forces due to the solenoid  50  and its own weight of the double-side switching member  22  when the current value “b” is applied at the third position Z, and a force f 4   a  indicates the sum of forces due to the solenoid  50  and its own weight of the double-side switching member  22  when the current value “b” is applied at the second position Y. A force r 3  indicates a force by the switching member return compression spring  60  at the first position X, a force r 3   a  indicates a force by the switching member return compression spring  60  when a spring constant is changed by the change of the number of effective turns of the switching member return compression spring  60 , a force r 4  indicates a force by the switching member return compression spring  60  at the third position Z, and a force r 5  indicates a force by the switching member return compression spring  60  at the second position Y. 
     Next, the relation between the forces to operate the double-side switching member  22  will be described with reference to  FIG. 11 . When the current value “a” ( FIG. 5 ) flows through the solenoid  50  at the first position X, since the relation of f 3 &gt;r 3  is satisfied as illustrated in  FIG. 11 , the double-side switching member  22  starts to rotate in the clockwise direction. Thereafter, when the force reaches the force r 3   a  by compressing a certain amount of the switching member return compression spring  60  ( FIG. 9 ), the spring constant is higher by the change of the number of effective turns. When the switching member return compression spring  60  is further compressed, the relation of f 3   a =r 4  is satisfied, and thus the double-side switching member  22  is held at the third position Z. Thereafter, when the current value “b” flows through the solenoid  50 , the relation of f 4 &gt;r 4  is satisfied, and thus the double-side switching member  22  starts to rotate to the second position Y. When the double-side switching member  22  abuts on the second position abutting portion  55  ( FIG. 3 ), since the relation of f 4   a &gt;r 5  is satisfied, the double-side switching member  22  is held at the second position Y. 
     When the double-side switching member  22  returns to the first position X, the switching operation thereof is completed by stopping the application of the current to the solenoid  50  and by abutting the double-side switching member  22  on the first position abutting portion  57  ( FIG. 3 ) with the force of the switching member return compression spring  60 . 
     According to the second embodiment, it is possible to realize the effect, which is obtained by the double-side switching member return spring  54  ( FIG. 3 ) and the plate spring  58  ( FIG. 4 ) of the first embodiment, by a single component using the switching member return compression spring  60  acting as the unequal pitch spring having the non-linear characteristic. 
       FIG. 12  is a graph illustrating effect in the case in which the switching member return compression spring  60  has the non-linear characteristic in relation to a case in which the switching member return compression spring  60  has a linear characteristic. In  FIG. 12 , a two-dot chain line “g” indicates the relation between a displacement and a force applied to the switching member return compression spring  60  in the case in which the switching member return compression spring  60  has the linear characteristic. 
     In the second embodiment, the third position Z becomes a position where the force f 3   a  and the force r 4  are balanced. Therefore, when variation occurs in the driving force of the solenoid  50 , the spring having the non-linear characteristic in which an angle formed with the horizontal axis is large (spring constant is large) in the vicinity of the third position Z is smaller in the positional deviation of the third position Z than the spring having the linear characteristic in which an angle formed with the horizontal axis is small (spring constant is small). In the second embodiment, that is, the spring having the non-linear characteristic, which is configured such that the spring constant of a predetermined region D 1  including the third position Z becomes larger than that of another region D 2 , is used as the switching member return compression spring  60 . 
     In  FIG. 12 , for example, when the driving force F 2  applied instead of the force f 3   a  due to the variation of the driving force of the solenoid  50 , the balanced position becomes a point P 2  in the case of the spring having the linear characteristic. At this time, a position Z 2  becomes the third position Z, whereas the balanced position becomes a point P 1  in the case of the spring having the non-linear characteristic, and thus a position Z 1  smaller in fluctuation than the position Z 2  becomes the third position Z. Therefore, the double-side switching member  22  is exactly positioned at the third position Z. 
     In the second embodiment, the unequal pitch spring is used as the switching member return compression spring  60 , but the spring having the non-linear characteristic, for example, a conical spring and the like may be used. 
     [Third Embodiment] Next, further another embodiment of the invention will be described with reference to  FIGS. 13 to 16 . A configuration of an image forming apparatus in a third embodiment is the same as that in the first embodiment, and the same or similar components as those in the first embodiment are denoted by the same reference numerals and the repeated description will not be presented. Hereinafter, parts different from the first embodiment will be mainly described. 
       FIG. 13  is a diagram illustrating a detailed switching configuration of the double-side switching member  22  according to a third embodiment. 
     A switching member abutting portion  62   a  of a cam-attached connection plate  62  abuts on the arm portion  22   a  of the double-side switching member  22 . A cam  63  (cam-shaped member) is pressed against the cam-attached connection plate  62  side by a cam pressure spring  64 . In addition, the cam pressure spring  64  is held by a pressure spring holding portion  65 . Further, the cam-attached connection plate  62  and the cam  63  is a state formed by coating grease on a polyacetal resin having small sliding resistance, and the sliding resistance of the cam-attached connection plate  62  and the cam  63  is so small as to be negligible. Arrows in  FIG. 13  indicate directions of forces when the force to rotate in a direction to move the double-side switching member  22  from the first position X to the third position Z is applied in the case in which the current flows through the solenoid  50 . 
       FIGS. 14A to 14C  are diagrams illustrating a detailed diagram ( FIG. 14A ) of the cam-attached connection plate  62 , a detailed diagram ( FIG. 14B ) of the cam  63 , a schematic diagram ( FIG. 14C ) illustrating a relation between the cam-attached connection plate  62  and the cam  63 . 
     As illustrated in  FIG. 14C , when the double-side switching member  22  is positioned at the first position X, the cam-attached connection plate  62  includes a first position contacting portion  62   b  which comes in contact with the cam  63 . In addition, when the double-side switching member  22  is positioned at the third position Z, the cam-attached connection plate  62  includes a third position contacting portion  62   c  which comes in contact with the cam  63 . Moreover, when the double-side switching member  22  is positioned at the second position Y, the cam-attached connection plate  62  includes a second position contacting portion  62   d  which comes in contact with the cam  63 . Furthermore, the second position contacting portion  62   d  is a face that is protruded in a convex shape from the surface of the first position contacting portion  62   b . In addition, the third position contacting portion  62   c  has an inclined-plane shape that is continuously formed from the surface of the first position contacting portion  62   b  to the surface of the second position contacting portion  62   d.    
     As illustrated in  FIG. 14A , those having the same shape as the first position contacting portion  62   b , the third position contacting portion  62   c , and the second position contacting portion  62   d  are provided at a position rotated by 180°. That is, the first position contacting portions  62   b , the third position contacting portions  62   c , and the second position contacting portions  62   d  are provided in pairs at positions opposite to each other. 
     As illustrated in  FIG. 14C , the cam  63  side is provided with a contacting portion  63   b , which comes in contact with the first position contacting portion  62   b  and the second position contacting portion  62   d  of the cam-attached connection plate  62  when the double-side switching member  22  is positioned at the first position X and the second position Y, and a contacting portion  63   c  of the inclined-plane shape, which comes in contact with the third position contacting portion  62   c  of the cam-attached connection plate  62  when the double-side switching member  22  is positioned at the third position Z. Similar to the connection plate  62 , as illustrated in  FIG. 14B , those having the same shape as the contacting portions  63   b  and  63   c  are provided at a position rotated by 180° even in the cam  63 . 
       FIGS. 15A to 15C  are diagrams illustrating a relation between the cam-attached connection plate  62  and the cam  63  when the double-side switching member  22  is positioned at the first position X ( FIG. 15A ), at the third position Z ( FIG. 15B ), and at the second position Y ( FIG. 15C ), respectively. Further, those having the same shape as the cam-attached connection plate  62  and the cam  63  are disposed at a position rotated by 180°, but only one cam-attached connection plate  62  and cam  63  are displayed for simplicity. 
       FIG. 16  is a diagram in which forces to rotate the double-side switching member  22  during a conveying path switching according to the third embodiment are summarized. A solid line indicates a driving force caused by the solenoid  50  ( FIG. 13 ) and a force by its own weight of the double-side switching member  22 , which are forces to operate the double-side switching member  22  from the first position X to the third position Z and the second position Y. On the other hand, a dashed-dotted line indicates a force by the double-side switching member return spring  54 , which is a force to operate the double-side switching member  22  from the second position Y to the first position X. 
     The current value and timing for applying the current to the solenoid  50  are the same as those in the first embodiment and are performed as illustrated in  FIG. 5 . A force f 5  indicates the sum of forces due to the driving force of the solenoid  50  and its own weight of the double-side switching member  22  when the current value “a” ( FIG. 5 ) is applied at the first position X, a force f 5   a  indicates the sum of forces due to the driving force of the solenoid  50  and its own weight of the double-side switching member  22  when the current value “a” is applied at the third position Z, a force f 6  indicates the sum of forces due to the driving force of the solenoid  50  and its own weight of the double-side switching member  22  when the current value “b” is applied at the third position Z, and a force f 6 a indicates the sum of forces due to the driving force of the solenoid  50  and its own weight of the double-side switching member  22  when the current value “b” is applied at the second position Y. 
     A force r 6  indicates a force by the double-side switching member return spring  54  at a position immediately before the third position contacting portion  62   c  of the cam-attached connection plate  62  and the contacting portion  63   c  of the cam  63  start to come in contact with each other when the double-side switching member  22  is positioned at the first position X, and a force r 6   a  indicates a force indicates a force by the double-side switching member return spring  54  at a position immediately before the third position contacting portion  62   c  of the cam-attached connection plate  62  and the contacting portion  63   c  of the cam  63  start to come in contact with each other when the double-side switching member  22  is positioned at the third position Z. 
     A force r 7  indicates a force by the double-side switching member return spring  54  at a position where the second position contacting portion  62   d  of the cam-attached connection plate  62  and the contacting portion  63   b  of the cam  63  start to come in contact with each other when the double-side switching member  22  is positioned at the third position Z. A force r 7 a indicates a force by the double-side switching member return spring  54  when the double-side switching member  22  is positioned at the second position Y. 
     A force r 8  indicates the sum of the force of the double-side switching member return spring  54  and a force to press the cam pressure spring  64  of a state immediately before the contacting portion  63   b  of the cam  63  and the second position contacting portion  62   d  of the cam-attached connection plate  62  come in contact with each other at the state in which the contacting portion  63   c  of the cam  63  and the third position contacting portion  62   c  of the cam-attached connection plate  62  come in contact with each other. 
     Next, an operation of the double-side switching member  22  according to the third embodiment will be described with reference to  FIGS. 15A to 15C  and  FIG. 16 . 
     In the case in which the double-side switching member  22  is positioned at the first position X, when the current value “a” ( FIG. 5 ) flows though the solenoid  50 , as illustrated in  FIG. 16 , since the relation of f 5 &gt;r 6  is satisfied, the double-side switching member  22  starts to rotate in the clockwise direction. At this time, as illustrated in  FIG. 15A , the first position contacting portion  62   b  of the cam-attached connection plate  62  and the contacting portion  63   b  of the cam  63  are in a contact state. 
     Thereafter, as illustrated in  FIG. 15B , the double-side switching member  22  rotates, and the third position contacting portion  62   c  of the cam-attached connection plate  62  and the contacting portion  63   c  of the cam  63  abut on each other. Further, at this time, the inclined-plane shape of the third position contacting portion  62   c  of the cam-attached connection plate  62  comes in contact with the inclined-plane shape of the contacting portion  63   c  of the cam  63 . 
     After the cam portion is in the state of  FIG. 15B , when the double-side switching member  22  further rotates in the clockwise direction, the third position contacting portion  62   c  of the cam-attached connection plate  62  requires the force to move the cam  63  to the left side in  FIG. 15B  with the force greater than the force to press the cam against the right side in  FIG. 15B  by the cam  63  pressure spring  64 . When the force applied to the double-side switching member  22  is greater than the force f 5   a  but is less than the force r 8 , the double-side switching member  22  is held at the third position Z. 
     Thereafter, when the current value “b” ( FIG. 5 ) is applied to the solenoid  50 , since the relation of f 6 &gt;r 8  is satisfied, the double-side switching member  22  starts to rotate in the second position Y. Then, as illustrated in  FIG. 15C , the second position contacting portion  62   d  of the cam-attached connection plate  62  and the contacting portion  63   b  of the cam are in a contact state. At this time, since the force to push up the cam  63  in an arrow direction illustrated in  FIG. 15C  disappears, the force to rotate the double-side switching member  22  in the counterclockwise direction is lowered to the force r 7  from the force r 8 . Thereafter, when the double-side switching member  22  abuts on the second position abutting portion  55  ( FIG. 3 ), since the relation of f 6   a &gt;r 7   a  is satisfied, the double-side switching member  22  is held at the second position Y. 
     When the double-side switching member  22  returns from the second position Y to the third position Z and the first position X, the current flowing through the solenoid  50  is stopped. And then, the double-side switching member  22  rotates in the counterclockwise direction in a sequential order from  FIG. 15C  to  FIG. 15B  and  FIG. 15A  due to the force of the double-side switching member return spring  54 , and the relation between the cam-attached connection plate  62  and the cam  63  will be also changed at any time. Eventually, the double-side switching member  22  abuts on the first position abutting portion  57  ( FIG. 3 ) to complete the switching operation. 
     In the third embodiment, when the double-side switching member  22  is operated from the third position Z to the second position Y, it is configured such that the maximum force r 8  to rotate the double-side switching member  22  in the counterclockwise direction is generated. Therefore, the third embodiment is configured such that the load is reduced by generating the force r 8 , which is the peak of the load with respect to the driving force of the solenoid  50 . 
     Thereby, the force to hold the double-side switching member  22  at the second position Y can be reduced. In order to prevent the temperature rise of the solenoid  50 , for example, the current flowing through the solenoid  50  is lowered to the current value “c” at the timing (after the elapse of a certain time from the time at which the reversing sheet Sb reaches the tip portion of the double-side switching member  22 ) of the time T 3  illustrated in  FIG. 5 , but the value of the current value “c” can be set to a lower value. 
     In a graph of the dashed-dotted line portion illustrated in  FIG. 16 , furthermore, a slope of force variation from the force r 6   a  to the force r 8  can be changed by varying the spring constant of the cam pressure spring  64 , for example. 
     In the third embodiment, the force r 8  of the maximum load illustrated in  FIG. 16  is generated using the cam pressure spring  64 , but the invention is not limited thereto and may also obtain the same effect by deforming the cam shape without using the cam pressure spring  64 . 
     &lt;Other Configurations&gt; In the first, second and third embodiments, as described above, when the double-side switching member  22  is switched from the first position X to the second position Y, the double-side switching member  22  is switched to the third position Z in advance. However, the sheet passing through the tip of the double-side switching member  22  is the order of the reversing sheet Sb and the sheet proceeding toward the discharge conveying path  23 , and when the interval between the sheets is short, it may be as follows. That is, even when the double-side switching member  22  is switched from the second position Y to the first position X, it may be switched to the first position X after being moved to the third position Z in advance by reducing the current value applied to the solenoid  50 , as opposed to suddenly setting the current flowing through the solenoid  50  to zero. 
     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 modifications, equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2012-271400, filed Dec. 12, 2012, which is hereby incorporated by reference herein in its entirety.