Patent Publication Number: US-10310436-B2

Title: Sheet discharge device

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to a sheet discharge device, particularly relates to a sheet discharge device including a decurl unit for removing curl from a sheet. 
     Description of the Related Art 
     A conventional electro-photographic image forming apparatus, such as a printer, transfers a toner image to a sheet, fixes the toner image by applying heat and pressure to the sheet, and eventually discharges the sheet on which the toner image is fixed to a discharge tray through a sheet discharge device. Generally, a sheet is curled when heat and pressure are applied to the sheet, so that a sheet stacking property will be lowered if such a curled sheet is discharged to the discharge tray. 
     Therefore, the sheet discharge device conventionally includes a decurl unit for correcting curl before discharging the sheet. As the above-described decurl unit, there is a decurl unit which has a discharge roller and two pinch rollers disposed in a sheet discharge direction, and corrects curl by pressing a curled sheet against an outer circumferential face of the discharge roller with the two pinch rollers. 
     Sheet curvature varies considerably depending on a sheet type, a sheet thickness, an environment, and the like. Therefore, if a decurl force for pressing the sheet against the outer circumferential face of the discharge roller with the two pinch rollers is constant, and, for example, in a case where sheet curvature is small, the sheet may curl in an inverse direction. 
     Therefore, as a conventional decurl unit, Japanese Patent No. 5381750 discusses a sheet discharge device having a decurl unit which is configured such that an upstream side pinch roller and a downstream side pinch roller are rotatably supported with a holder and a decurl force is adjusted by moving the holder according to sheet curvature. In this decurl unit, when sheet curl with small curvature is to be corrected, the holder is moved toward the upstream side in the sheet discharge direction, and the upstream side pinch roller is separated away from a discharge roller. With this configuration, the decurl force is lowered because the sheet is pressed against the discharge roller only with the downstream side pinch roller, so that the sheet curl can be corrected appropriately even if the sheet curvature is small. 
     In addition, in the sheet discharge device, a sheet discharge direction after decurl is approximately orthogonal to a line that connects a rotation center of the discharge roller and a rotation center of the downstream side pinch roller. If an angle with respect to a direction horizontal to the sheet discharge direction (hereinafter, referred to as “discharge angle”) is too large, the sheet is not discharged to the sheet discharge tray because a trailing end thereof leans on a discharge mechanism, so that a stacking failure will occur. Further, if the discharge angle is too small, a sheet that is being discharged is strongly rubbed with a sheet already stacked on the sheet discharge tray to push out the sheet, so that the sheet stacking property will be lowered. Therefore, the discharge angle has to be set appropriately. 
     However, in the sheet discharge device including the conventional decurl unit, when the holder is moved, the downstream side pinch roller also moves to the upstream side in the sheet discharge direction, and the rotation center of the downstream side pinch roller also moves to the upstream side in the sheet discharge direction. When the rotation center of the downstream side pinch roller moves to the upstream side in the sheet discharge direction, a discharge angle becomes smaller because a position of a rotation center of the discharge roller is not changed. The sheet stacking property will be therefore lowered as described above. Further, if a position of the downstream side pinch roller is set to make the discharge angle become an appropriate angle after moving the holder, the discharge angle will be too large when sheet curl with a regular curvature is to be corrected, and thus the sheet stacking property will be lowered as described above. 
     Furthermore, if the holder is moved and the upstream side pinch roller is separated away from the discharge roller, a nip between the discharge roller and the upstream side pinch roller is cleared, so that the sheet passes over the upstream side pinch roller and enters a nip portion between the downstream side pinch roller and the discharge roller. In this case, if the curvature is large, the sheet cannot enter the nip portion between the downstream side pinch roller and the discharge roller but enters a space between the upstream side pinch roller and the downstream side pinch roller thereby to cause a paper jam. As described above, if a decurl function of the decurl unit is changed according to sheet curvature, a paper jam may occur or a sheet stacking property may be lowered. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present disclosure, a sheet discharge device includes a discharge roller, a first roller configured to form a first nip portion by contacting the discharge roller, a second roller configured to form a second nip portion by contacting the discharge roller on a downstream side of the first roller in a rotation direction of the discharge roller, an urging unit configured to urge the first roller and the second roller toward the discharge roller, and an adjustment unit configured to adjust a ratio of a pressure of the second nip portion to a pressure of the first nip portion in a state in which the first roller and the second roller are kept in contact with the discharge roller by the urging unit, wherein a sheet is conveyed while being simultaneously held by the first nip portion and the second nip portion. 
     Further features of the present disclosure 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 diagram illustrating a schematic configuration of an electro-photographic printer as an example of an image forming apparatus having a sheet discharge device according to a first exemplary embodiment. 
         FIG. 2  is a diagram illustrating a configuration of the sheet discharge device. 
         FIG. 3  is a diagram illustrating a configuration of a decurl unit provided on the sheet discharge device. 
         FIG. 4  is a diagram illustrating a configuration of a pressure force changing unit provided on the sheet discharge device. 
         FIG. 5A  is a diagram illustrating a state before an operation lever provided on the pressure force changing unit is operated, and  FIG. 5B  is a diagram illustrating a state after the operation lever is operated. 
         FIG. 6A  is a diagram illustrating a state of a holding unit provided on the pressure force changing unit before the operation lever is operated, and  FIG. 6B  is a diagram illustrating a state of the holding unit after the operation lever is operated. 
         FIG. 7  is a diagram illustrating a state where a decurl function of the decurl unit is reduced. 
         FIG. 8A  is a diagram illustrating a state where a leading end of a sheet has reached an upstream side nip when the decurl unit is in a decurl setting for large curl, and  FIG. 8B  is a diagram illustrating a state where a leading end of a sheet has passed through a downstream side nip when the decurl unit is in the decurl setting for large curl. 
         FIG. 9A  is a diagram illustrating a state where a leading end of a sheet has reached the upstream side nip when the decurl unit is in a decurl setting for small curl, and  FIG. 9B  is a diagram illustrating a state where a leading end of a sheet has passed through the downstream side nip when the decurl unit is in the decurl setting for small curl. 
         FIG. 10A  is a diagram illustrating another configuration of the holding unit provided on the pressure force changing unit, and  FIG. 10B  is a diagram illustrating another configuration of the downstream side pinch roller provided on the decurl unit. 
         FIG. 11A  is a diagram illustrating a state where a decurl unit provided on a sheet discharge device according to a second exemplary embodiment is in a decurl setting for large curl, and  FIG. 11B  is a diagram illustrating a state where the decurl unit is in a decurl setting for small curl. 
         FIG. 12A  is a diagram illustrating a state where a decurl unit provided on a sheet discharge device according to a third exemplary embodiment is in a decurl setting for large curl, and  FIG. 12B  is a diagram illustrating a state where the decurl unit is in a decurl setting for small curl. 
         FIG. 13  is a diagram illustrating another configuration of the sheet discharge device according to the first exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, an exemplary embodiment will be described in detail with reference to the appended drawings.  FIG. 1  is a diagram illustrating a schematic configuration of an electro-photographic printer as an example of an image forming apparatus having a sheet discharge device according to a first exemplary embodiment. 
     As illustrated in  FIG. 1 , an image forming unit  1 B for forming an image through an electro-photographic method, a sheet feeding device  1 C for feeding a sheet S to the image forming unit  1 B, and a sheet discharge device  10  for discharging the sheet S on which an image is formed are provided on a printer main body  1 A of a printer  1 . 
     The image forming unit  1 B includes a photosensitive drum  6   a  for forming a toner image, a development unit  6   b,  a discharge roller (not illustrated) for uniformly charging a surface of the photosensitive drum  6   a,  and a process cartridge  6  detachably attached to the printer main body  1 A. The image forming unit  1 B further includes a laser scanner  7  and a transfer roller  5  for transferring a toner image formed on the photosensitive drum  6   a  onto the sheet S. The sheet feeding device  1 C includes a feeding roller  2  for feeding sheets S stacked on and stored in a tray (not illustrated) provided on the printer main body  1 A. 
     Then, when a printing start signal is input to the printer  1  having the above-described configuration, the feeding roller  2  rotates to feed out an uppermost sheet Sa of the sheets S stacked on and stored in the tray with the friction between the sheet Sa and the feeding roller  2 . The fed sheet Sa is separated from the sheets S by a separation pad  3  one by one and conveyed to the image forming unit  1 B by a conveyance roller pair  4 . 
     Subsequently, when conveyance of the sheet Sa is started, the laser scanner  7  irradiates the photosensitive drum  6   a,  a surface of which is charged uniformly, with laser light corresponding to image information at a predetermined timing, whereby an electrostatic latent image is formed on the photosensitive drum  6   a.  Then, the electrostatic latent image formed on the photosensitive drum  6   a  is developed and visualized as a toner image by the development unit  6   b  and transferred to the sheet Sa by the transfer roller  5 . Next, the sheet Sa on which the toner image has been transferred is conveyed to a fixing device  9  configured of a pressure roller  9   a  and a fixing roller  9   b,  so that the toner image is fixed onto the sheet Sa by the heat and pressure applied by the fixing device  9 . After the toner image is fixed thereon, the sheet Sa is discharged to a discharge stacking tray  11  provided on an upper face of the printer main body  1 A by the sheet discharge device  10 . 
     Next, the sheet discharge device  10  will be described with reference to  FIGS. 2 and 3 . The sheet discharge device  10  includes a discharge roller  12 , an upstream side pinch roller  13  (first roller) that is press-contactable with and separable from the discharge roller  12 , and a downstream side pinch roller  14  (second roller) that is press-contactable with and separable from the discharge roller  12  in a downstream side in the sheet discharge direction with respect to the upstream side pinch roller  13  (i.e., a rotation direction of the discharge roller  12 ). Then, the discharge roller  12 , the upstream side pinch roller  13  as an upstream side roller, and the downstream side pinch roller  14  as a downstream side roller are included in a decurl unit  10 A that discharges the sheet Sa while removing curl from the sheet Sa. 
     A surface of the discharge roller  12  is formed of a material, such as rubber, having a high friction coefficient. In the present exemplary embodiment, two discharge rollers  12  are attached to a discharge roller shaft  21 . The discharge roller shaft  21  is rotatably held by a main unit frame  18  via shaft bearings  23 , and a discharge roller gear  22  is attached to one end portion of the discharge roller shaft  21 . Then, the discharge roller gear  22  is driven and rotated by a motor (not illustrated), so that the discharge roller shaft  21  rotates. The discharge roller  12  is rotated in a direction indicated by an arrow in  FIG. 3 . 
     The upstream side pinch roller  13  includes roller shaft portions  13   a  at both end portions in an axis direction. The roller shaft portions  13   a  are respectively attached to positioning sliding portions  18   b   1  provided on upper portions of upstream side positioning ribs  18   a   1  serving as upstream side supporting members provided on the main unit frame  18 . Then, each of the upstream side pinch rollers  13  is supported by the corresponding upstream side positioning rib  18   a   1  that is included in a supporting unit, so as to be rotatable with respect to the main unit frame  18  and linearly separable with respect to the corresponding discharge roller  12 . 
     The downstream side pinch roller  14  includes roller shaft portions  14   a  at both end portions in an axis direction. These roller shaft portions  14   a  are respectively attached to positioning sliding portions  18   b   2  provided on upper portions of positioning ribs  18   a   2  serving as supporting members provided on the main unit frame  18 , so that each of the downstream side pinch rollers  14  is supported rotatably and slidably toward the corresponding discharge roller  12 . 
     As illustrated in  FIG. 4 , central shaft portions  13   c  and  14   c  are respectively formed on a central portion in the axis direction of the upstream side pinch roller  13  and a central portion in the axis direction of the downstream side pinch roller  14 . In the present exemplary embodiment, the upstream side pinch roller  13  and the downstream side pinch roller  14  are narrowed at the central portions in the axis direction, and the central shaft portions  13   c  and  14   c  are formed by these narrowed portions. Each of the central shaft portions  13   c  and  14   c  may be formed by arranging two pinch rollers on a shaft with a space at a central portion thereof. 
     The central shaft portions  13   c  and  14   c  are coupled by a pressure transmission member  15  serving as a pivoting member. A holding portion  15   a  that holds the central shaft portion  13   c  of the upstream side pinch roller  13  with play in the horizontal direction is formed on an upstream end in the sheet discharge direction of the pressure transmission member  15 . A fitting portion  15   b  into which the central shaft portion  14   c  of the downstream side pinch roller  14  fits is formed on a downstream end portion in the sheet discharge direction as a pivoting end portion of the pressure transmission member  15 . With the above-described configuration, the pressure transmission member  15  is pivotably provided on the upstream side pinch roller  13 , and the downstream side pinch roller  14  is rotatably supported by the pressure transmission member  15 . 
     A swing member  17  is provided below the pressure transmission member  15  so as to be swingable in a direction in which the swing member  17  is in contact with or separated from the downstream side pinch roller  14 . As illustrated in  FIG. 2 , the swing member  17  is attached to a swing member coupling shaft  16  serving as a swing shaft which is rotatably held by the main unit frame  18  via bearing shafts  24 . Pressure springs, such as an upstream side pressure spring  26  serving as a first urging member and a downstream side pressure spring  27  serving as a second urging member, are arranged in a space between the swing member  17  and the pressure transmission member  15 . In the present exemplary embodiment, identical springs having the same urging forces are used as the upstream side pressure spring  26  and the downstream side pressure spring  27 . Further, the upstream side pressure spring  26  is arranged on the upstream side in the sheet discharge direction with respect to the swing member coupling shaft  16 , whereas the downstream side pressure spring  27  is arranged on the downstream side in the sheet discharge direction with respect to the swing member coupling shaft  16 . 
     By arranging the upstream side pressure spring  26  and the downstream side pressure spring  27  as described above, the upstream side pinch roller  13  is pressed and in contact with the discharge roller  12  mainly by the upstream side pressure spring  26  via the pressure transmission member  15  thereby to form an upstream side nip portion (first nip portion) N 1 . Further, the downstream side pinch roller  14  is pressed and in contact with the discharge roller  12  mainly by the downstream side pressure spring  27  via the pressure transmission member  15  thereby to form a downstream side nip portion (second nip portion) N 2 . As described above, the spring forces of the upstream side pressure spring  26  and the downstream side pressure spring  27  are applied to the upstream side pinch roller  13  and the downstream side pinch roller  14  via the pressure transmission member  15 . 
     The upstream side pressure spring  26  is in charge of a pressure force (press-contact force) of the upstream side pinch roller  13  by mainly urging the upstream side pinch roller  13 , whereas the downstream side pressure spring  27  is in charge of a pressure force of the downstream side pinch roller  14  by mainly urging the downstream side pinch roller  14 . The pressure forces of the upstream side pinch roller  13  and the downstream side pinch roller  14  are used for removing curl from a sheet in such a manner that the curled sheet is pressed against the discharge roller  12 . Further, in the present exemplary embodiment, because the central portions of the upstream side pinch roller  13  and the downstream side pinch roller  14  are pressurized via the pressure transmission member  15 , the upstream side pinch roller  13  and the downstream side pinch roller  14  can uniformly apply the pressure forces to the discharge roller  12  in the axis direction. Therefore, the sheet S can be pressed against the discharge roller  12  in a well-balanced manner. 
     As illustrated in  FIGS. 5A and 5B , a manually-operated operation lever  25  is attached to the central portion in the axis direction of the swing member coupling shaft  16 . Then, as described below, when the decurl function of the decurl unit  10 A is to be lowered, the operation lever  25  is operated in a direction indicated by an arrow F to make a downstream side of the swing member  17  in the sheet discharge direction swing and move close to the downstream side pinch roller  14 , as illustrated in  FIGS. 9A and 9B  described below. 
     Further, as illustrated in  FIGS. 6A and 6B , a cam  31  as a latched member which pivots integrally with the swing member  17  is attached to one end portion of the swing member coupling shaft  16 , and cam concave portions  31   a   1  and  31   a   2  as two latched portions are provided on a pivoting end portion of the cam  31 . A click spring  28  as an elastic member, which elastically engages with the cam concave portions  31   a   1  and  31   a   2  when the cam  31  is rotated, is provided on the main unit frame  18  (not illustrated). Further, an upper stopper  29  for regulating the upward rotation of the cam  31  is provided on the upper side of the cam  31 , and a lower stopper  30  for regulating the downward rotation of the cam  31  is provided on the lower side of the cam  31 . A pivoting amount of the cam  31  in an upper/lower direction, i.e., an operation amount of the operation lever  25  in an upper/lower direction, is limited by the upper stopper  29  and the lower stopper  30 . 
       FIG. 6A  is a diagram illustrating a state of the cam  31  when the operation lever  25  is not operated as illustrated in  FIG. 5A . In this state, a latching portion  28   a  of the click spring  28  engages with the cam concave portion  31   a   1  positioned on the upper portion, so that the swing member coupling shaft  16  is held in a position where the operation lever  25  is not operated. 
     Further, when the operation lever  25  is operated as illustrated in  FIG. 5B , the swing member coupling shaft  16  is rotated, so that the cam  31  pivots according to  FIG. 6B . Then, when the cam  31  pivots, the latching portion  28   a  of the click spring  28  engages with the cam concave portion  31   a   2  positioned on the lower portion. With this configuration, the swing member coupling shaft  16  is held in a position where the operation lever  25  is operated. In other words, in the present exemplary embodiment, the pivot member  17  is held by the click spring  28  and the cam  31  in a position according to the operation of the operation lever  25  as illustrated in  FIG. 7 . 
     When the operation lever  25  is operated, the swingable swing member  17  swings in such a manner that a side of the swing member  17  on the downstream side pinch roller  14  (i.e., a downstream side in the sheet discharge direction) is positioned higher than a side of the swing member  17  on the upstream side pinch roller  13 . Then, when the swing member  17  swings as described above, the pressure force of the upstream side pressure spring  26  to the upstream side pinch roller  13  becomes weaker, and the pressure force of the downstream side pressure spring  27  to the downstream side pinch roller  14  becomes stronger. 
     As described above, by swinging the swing member  17  through the operation of the operation lever  25 , it is possible to change a balance between the pressure forces, of the upstream side pressure spring  26  and the downstream side pressure spring  27 , which are applied to the upstream side pinch roller  13  and the downstream side pinch roller  14 . In other words, it is possible to change (adjust) a ratio of the pressure of the downstream side nip portion N 2  to the pressure of the upstream side nip portion N 1 . Therefore, in the present exemplary embodiment, the swing member  17  and the operation lever  25  are included in a pressure force changing unit (adjustment unit)  20  as a changing unit for changing a balance between the urging forces to the upstream side pinch roller  13  and the downstream side pinch roller  14  via the pressure transmission member  15 . 
     A setting in which the pressure force caused by the upstream side pressure spring  26  becomes weaker whereas the pressure force caused by the downstream side pressure spring  27  becomes stronger is referred to as a decurl setting for small curl in which the decurl unit  10 A processes a sheet having a curvature amount smaller than a predetermined amount. A mode in which a sheet is conveyed by the decurl unit  10 A which operates in the decurl setting for small curl is referred to as a first mode. In the present exemplary embodiment, even in the decurl setting for small curl, the pressure force of the upstream side pressure spring  26  will not be reduced to zero, so that the upstream side pinch roller  13  is pressed and in contact with the discharge roller  12  with a small pressure force. 
     The state illustrated in  FIG. 4  is referred to as a decurl setting for large curl in which the decurl unit  10 A corrects sheet curl from a sheet having a curvature amount of a predetermined amount or more. In the decurl setting for large curl, a ratio of the pressure of the downstream side nip portion N 2  to the pressure of the upstream side nip portion N 1  is set to be smaller than that of the decurl setting for small curl. A mode in which a sheet is conveyed by the decurl unit  10 A which operates in the decurl setting for large curl is referred to as a second mode. 
     Further, in the present exemplary embodiment, because a total of the pressure forces of the upstream side pressure spring  26  and the downstream side pressure spring  27  is approximately the same in the decurl setting for small curl and the decurl setting for large curl, the conveyance force of the sheet discharge device  10  is approximately the same in any of the settings. Therefore, sliding marks of the rollers will not be formed on a sheet because the conveyance force is not too strong, and a sheet can be conveyed reliably because the conveyance force is not too weak, and thus the sheet can be discharged stably. As illustrated in  FIG. 4 , a concave-shaped guide portion  15   c  curved along the outer circumferential face of the discharge roller  12  is formed on an upper face of the pressure transmission member  15 . With this guide portion  15   c,  a sheet conveyed through conveyance guides  19   a  and  19   b  can be guided from the upstream side nip portion N 1  to the downstream side nip portion N 2  without entering a space between the upstream side pinch roller  13  and the downstream side pinch roller  14 . 
     Next, a decurl operation of the decurl unit  10 A configured as the above will be described.  FIGS. 8A and 8B  are diagrams each illustrating a decurl operation performed in the decurl setting for large curl (second mode). In this state, the swing member  17  is positioned horizontally without swinging because the operation lever  25  is not operated. Therefore, both of the upstream side pressure spring  26  and the downstream side pressure spring  27  pressurize the upstream side pinch roller  13  and the downstream side pinch roller  14  via the pressure transmission member  15  with approximately the same pressure forces. 
     As illustrated in  FIG. 8A , when the conveyed sheet S is further conveyed after a leading end thereof has reached the upstream side nip portion N 1 , the leading end of the sheet S is lead to the downstream side nip portion N 2  while being guided by the guide portion  15   c  of the pressure transmission member  15 . Then, after the leading end of the sheet S has passed through the downstream side nip portion N 2 , the sheet S is discharged at a discharge angle θ 1  as illustrated in  FIG. 8B . 
     When the sheet S passes through the upstream side nip portion N 1  and the downstream side nip portion N 2 , the upstream side pinch roller  13  and the downstream side pinch roller  14  press the sheet S against the discharge roller  12  with approximately the same forces at the upstream side nip portion N 1  and the downstream side nip portion N 2 . With this operation, curl of the sheet S is corrected because the sheet S is pressed and stretched at curvature of the discharge roller  12 . Herein, a direction in which the sheet S is discharged, i.e., the discharge angle θ 1 , is approximately orthogonal to a line that connects the rotation center of the discharge roller  12  and the rotation center of the downstream side pinch roller  14 . This discharge angle θ 1  is set as appropriate in such a manner that the sheet stacking property is not lowered. 
     Next, a decurl operation, of the decurl unit  10 A, which is performed on a sheet having small curvature will be described. In this case, the decurl unit  10 A corrects curl in the decurl setting for small curl (first mode). In this operation, the operation lever  25  that is positioned as illustrated in  FIG. 5A  is operated in a direction indicated by the arrow F and moved to a position illustrated in  FIG. 5B . 
     When the operation lever  25  is moved thereto, the swing member  17  swings integrally with the operation lever  25  from a horizontal state, and the downstream side of the swing member  17  moves close to the downstream side pinch roller  14  as illustrated in  FIG. 9A . When the swing member  17  swings, the pressure force of the upstream side pressure spring  26  to the upstream side pinch roller  13  becomes weaker than in the case of the pressure force in the decurl setting for large curl, and the pressure force of the downstream side pressure spring  27  to the downstream side pinch roller  14  becomes stronger than in the case of the pressure force in the decurl setting for large curl. In other words, the sheet discharge device  10  can operate in the first mode in which the sheet S is conveyed in a state where the pressure of the upstream side nip portion N 1  is set to a first pressure while the pressure of the downstream side nip portion N 2  is set to a second pressure, and in the second mode in which the sheet S is conveyed in a state where the pressure of the upstream side nip portion N 1  is set to a third pressure that is greater than the first pressure while the pressure of the downstream side nip portion N 2  is set to a fourth pressure that is smaller than the second pressure. 
     When the sheet S is conveyed toward the upstream side nip portion N 1  in this state, the sheet S enters the upstream side nip portion N 1  while being guided appropriately because the upstream side pinch roller  13  abuts on the discharge roller  12  at the upstream side nip portion N 1 . Thereafter, when the sheet S that has reached the upstream side nip portion N 1  is conveyed further, a leading end of the sheet S is lead to the downstream side nip portion N 2  while being guided by the guide portion  15   c  of the pressure transmission member  15 . Then, the leading end of the sheet S passes through the downstream side nip portion N 2 . 
     In this state, although the downstream side pinch roller  14  abuts on the discharge roller  12  at the downstream side nip portion N 2 , as illustrated in  FIG. 9B , the pressure force of the upstream side pinch roller  13  has become smaller. It is because that the upstream side pinch roller  13  moves in a lower direction due to the rigidity of the sheet S while making the pressure transmission member  15  pivot. Then, because the upstream side pinch roller  13  moves in the lower direction, winding of the sheet S around the discharge roller  12  is weakened, and thus the decurl function of the decurl unit  10 A is reduced. 
     Further, as described above, the holding portion  15   a  of the pressure transmission member  15  has play in the horizontal direction with respect to the central shaft portion  13   c  of the upstream side pinch roller  13  as illustrated in  FIGS. 9A and 9B . With this configuration, the upstream side pinch roller  13  is held by the pressure transmission member  15  so as to be movable in the horizontal direction. Therefore, even in a case where the swing member  17  swings to make the pressure transmission member  15  tilt, the upstream side pinch roller  13  linearly moves in the lower direction along the upstream side positioning rib  18   a   1  while moving in a direction separating away from the downstream side pinch roller  14  along the holding portion  15   a.    
     Then, after the curl is corrected from the sheet S by the decurl unit  10 A having the decurl function according to the curvature, the sheet S is discharged at a discharge angle θ 2 . In this process, because the pressure force is increased by the downstream side pressure spring  27 , the downstream side pinch roller  14  is pressed and in contact with the discharge roller  12  while maintaining the press-contact position constant with respect to the discharge roller  12  without separating from the discharge roller  12 . Therefore, the discharge angle θ 2  when the decurl unit  10 A is in the decurl setting for small curl is equivalent to the discharge angle θ 1  when the decurl unit  10 A is in the decurl setting for large curl. Accordingly, even if the decurl unit  10 A discharges the sheet S in the decurl setting for small curl, the sheet stacking property will not be lowered. For example, if the discharge angle is too small, a sheet that is being discharged is strongly rubbed with a sheet already stacked on the sheet discharge tray to push out the already stacked sheet, so that the sheet stacking property will be lowered. Since the discharge angle θ 2  when the decurl unit  10 A is in the decurl setting for small curl is equivalent to the discharge angle θ 1  when the decurl unit  10 A is in the decurl setting for large curl, the discharge angle is set appropriately and the sheet stacking property will not be lowered. 
     As described above, in the present exemplary embodiment, the decurl unit  10 A includes the pressure force changing unit  20  for changing a balance of magnitudes of the urging forces to the upstream side pinch roller  13  and the downstream side pinch roller  14 . Then, in a state where the upstream side pinch roller  13  and the downstream side pinch roller  14  are pressed and in contact with the discharge roller  12 , the balance of the magnitudes of the urging forces to the upstream side pinch roller  13  and the downstream side pinch roller  14  is changed by the pressure force changing unit  20 . With this configuration, the decurl function can be changed without generating a paper jam or lowering a sheet stacking property. 
     The above description has been given for an exemplary embodiment in which a setting of the decurl function of the decurl unit  10 A is changed from the large curl setting to the small curl setting when the curvature amount of the sheet S is smaller than a predetermined amount. Alternatively, an initial state of the decurl function of the decurl unit  10 A may be set as the decurl setting for small curl, and the decurl function of the decurl unit  10 A may be changed to the decurl setting for large curl when the curvature amount of the sheet S is a predetermined amount or more. 
     Although the above description has been given for an exemplary embodiment in which two cam concave portions  31   a   1  and  31   a   2  are provided on the cam  31 , the number of cam concave portions may be three or more, and thus four cam concave portions  31   a   1 ,  31   a   2 ,  31   a   3 , and  31   a   4  may be provided on the cam  31  in the rotation direction of the cam  31  as illustrated in  FIG. 10A . Then, the click spring  28  may be latched with any one of the four cam concave portions  31   a   1 ,  31   a   2 ,  31   a   3 , and  31   a   4  according to a swinging amount of the swing member  17 . Further, the cam  31  may be stopped at any positions. By arranging a plurality of cam concave portions or making the cam  31  be stopped at any positions as described above, the decurl function can be changed more precisely, and thus the curl can be corrected more reliably. 
     Further, as illustrated in  FIG. 10B , a pinch roller  14  having sheet kick-out protrusions  14   d  at both end portions in the axis direction thereof may be used as the downstream side pinch roller  14 . By using the above-described downstream side pinch roller  14 , the sheet S can be discharged without making the trailing end lean on the discharge mechanism, and thus the stacking property of the sheet S can be improved. 
     A second exemplary embodiment will be described.  FIGS. 11A and 11B  are diagrams illustrating a configuration of a decurl unit provided on a sheet discharge device according to the present exemplary embodiment. In  FIGS. 11A and 11B , reference numerals which are the same as those illustrated in  FIG. 3  represent the same or corresponding portions. 
     As illustrated in  FIG. 11A , an upstream side pinch roller  13  and a downstream side pinch roller  14  are coupled to each other by a pressure transmission member  38  serving as a pivoting member. Supporting portions  38   d  are arranged opposite to each other on both end portions in the axis direction of the pressure transmission member  38 . Then, a holding portion  38   a  for holding an upstream side pinch roller shaft portion  13   a  with play in the horizontal direction is formed on the upstream end portion in the sheet discharge direction of each of the supporting portions  38   d,  and a fitting portion  38   b  into which a downstream side pinch roller shaft portion  14   a  fits is formed on a downstream end portion in the sheet discharge direction. 
     With this configuration, the pressure transmission member  38  is pivotably provided on the upstream side pinch roller  13 , and the downstream side pinch roller  14  is rotatably supported by the pressure transmission member  38  so as to be movable in the horizontal direction. Further, the upstream side pinch roller shaft portion  13   a  and the downstream side pinch roller shaft portion  14   a  are held rotatably while being held away from the discharge roller  12  by the positioning ribs  18   a   1  and  18   a   2  of the main unit frame  18  illustrated in  FIG. 3 . 
     Furthermore, a pressure intermediate transmission arm  32  as a press-contact member is provided on the lower side of the pressure transmission member  38 . A pressure intermediate transmission arm pressure portion  32   a  serving as a press-contact portion, which is pressed and in contact with a lower face  38   c  of the pressure transmission member  38 , is protruded from an upper face of a downstream end portion in the sheet discharge direction of the pressure intermediate transmission arm  32 . Further, a pressure spring  35  as an urging unit is provided on a space between the main unit frame  18  and the pressure intermediate transmission arm  32 . Then, the pressure intermediate transmission arm  32  is pressurized in an upper direction by the pressure spring  35 , so as to be pressed and in contact with the pressure transmission member  38  pivotably held by the downstream side pinch roller shaft portion  14   a.  Further, a concave-shaped guide portion  38   e  curved along the outer circumferential face of the discharge roller  12  is formed on the upper face of the pressure transmission member  38 . 
     Furthermore, an eccentric cam  33  as a moving unit fixed to an eccentric cam shaft  34 , which is rotated by a motor (not illustrated), is attached to the upstream end portion in the sheet discharge direction of the pressure intermediate transmission arm  32 . Then, the pressure intermediate transmission arm  32  is moved by rotation of the eccentric cam  33 . 
       FIG. 11A  is a diagram illustrating a state where the decurl unit  10 A is in the decurl setting for large curl. In this state, the pressure intermediate transmission arm pressure portion  32   a  is set so that the pressure intermediate transmission arm pressure portion  32   a  is positioned approximately in the middle of the upstream side pinch roller  13  and the downstream side pinch roller  14 . In this state, when the eccentric cam  33  is rotated by approximately 180-degree in a direction G, the pressure intermediate transmission arm  32  is moved in a direction H as illustrated in  FIG. 11B . 
     With this operation, the pressure intermediate transmission arm pressure portion  32   a  moves so as to be closer to the downstream side pinch roller  14  along the lower face  38   c  of the pressure transmission member  38 . The moving amount of the pressure intermediate transmission arm  32  is set in such a state that the pressure intermediate transmission arm pressure portion  32   a  does not move beyond a position immediately beneath the downstream side pinch roller  14 . 
     By movement of the pressure intermediate transmission arm  32 , the pressure force of the pressure spring  35  to the upstream side pinch roller  13  via the pressure transmission member  38  and the pressure intermediate transmission arm  32  becomes smaller than the pressure force to the downstream side pinch roller  14 . As described above, by movement of the pressure intermediate transmission arm  32 , a state of the decurl unit  10 A is changed from the decurl setting for large curl to the decurl setting for small curl. Therefore, in the present exemplary embodiment, the pressure intermediate transmission arm  32  and the eccentric cam  33  are included in a pressure force changing unit  20  as a changing unit for changing a balance between the urging forces to the upstream side pinch roller  13  and the downstream side pinch roller  14  via the pressure transmission member  38 . 
     Even if the decurl setting is changed to the decurl setting for small curl, the downstream side pinch roller  14  is pressed and in contact with the discharge roller  12  at a constant position without separating away from the discharge roller  12 . Accordingly, in the present exemplary embodiment, similar to the first exemplary embodiment described above, because a discharge angle can be also kept constant in the decurl setting for small curl, the decurl function can be changed without generating a paper jam or lowering a sheet stacking property. 
     A third exemplary embodiment will be described.  FIGS. 12A and 12B  are diagrams illustrating a configuration of a decurl unit provided on a sheet discharge device according to the present exemplary embodiment. In  FIGS. 12A and 12B , reference numerals which are the same as those illustrated in  FIG. 3  represent the same or corresponding portions. 
     As illustrated in  FIG. 12A , an upstream side pinch roller  13  and a downstream side pinch roller  14  are held by a pinch roller holding member  36  serving as a pivoting member. Supporting portions  36   d  are arranged opposite to each other on both end portions in the axis direction of the pinch roller holding member  36 . Then, a fitting portion  36   b  into which an upstream side pinch roller shaft portion  13   a  fits is formed on an upstream end in the sheet discharge direction of each of the supporting portions  36   d,  and a fitting portion  36   c  into which a downstream side pinch roller shaft portion  14   a  fits is formed on a downstream end in the sheet discharge direction. Further, a concave-shaped guide portion  36   e  curved along the outer circumferential face of the discharge roller  12  is formed on an upper face of the pinch roller holding member  36 . 
     A positioning protrusion  36   a  is provided on each of the supporting portions  36   d  of the pinch roller holding member  36 , and positioning ribs  18   c  as counter supporting members are provided opposite to each other in the axis direction on the main unit frame  18 . A sliding elongate hole  18   d  in which the positioning protrusion  36   a  of the pinch roller holding member  36  is latched slidably is formed on each of the positioning ribs  18   c.  In addition, the sliding elongate hole  18   d  is formed into an arc-like shape with the downstream side pinch roller  14  as a center. 
     When the positioning protrusion  36   a  is latched in the sliding elongate hole  18   d,  the pinch roller holding member  36  is supported so that the pinch roller holding member  36  pivotably moves in a direction indicated by an arrow K along the sliding elongate hole  18   d.  As described above, in the present exemplary embodiment, the sliding elongate hole  18   d  and the positioning protrusion  36   a  are included in a guiding unit  39  that guides the movement of the upstream side pinch roller  13  by guiding the pivotal movement of the pinch roller holding member  36 . 
     Then, similar to the first exemplary embodiment, the operation lever  25  is operated when the decurl function of the decurl unit  10 A is changed. In addition,  FIG. 12A  is a diagram illustrating a state where the decurl unit  10 A is in the decurl setting for large curl. By the operation of the operation lever  25 , the decurl setting of the decurl unit  10 A becomes the decurl setting for small curl illustrated in  FIG. 12B . 
     When the operation lever  25  is operated, the upstream side pinch roller  13  moves in a direction separating away from the discharge roller  12  by making the downstream side pinch roller  14  as a center together with the pinch roller holding member  36  that pivots around the downstream side pinch roller  14 . However, in this operation, because the pinch roller holding member  36  pivots around the downstream side pinch roller  14 , the downstream side pinch roller  14  is pressed and in contact with the discharge roller  12  at a constant position without separating away from the discharge roller  12 . Accordingly, in the present exemplary embodiment, similar to the first and the second exemplary embodiments described above, because the discharge angle can be also kept constant in the decurl setting for small curl, the decurl function can be changed without generating a paper jam or lowering a sheet stacking property. 
     Furthermore, in the present exemplary embodiment, although description has been given to an exemplary embodiment in which the sliding elongate hole  18   d  is provided on the positioning rib  18   c,  and the positioning protrusion  36   a  is provided on the pinch roller holding member  36 , the present invention is not limited thereto. The sliding elongate hole  18   d  may be provided on the pinch roller holding member  36 , and the positioning protrusion  36   a  may be provided on the positioning rib  18   c.  In other words, the positioning protrusion  36   a  may be provided on any one of the pinch roller holding member  36  and the positioning rib  18   c,  and the sliding elongate hole  18   d  may be provided on another one of the pinch roller holding member  36  and the positioning rib  18   c.    
     Further, depending on the configuration of the image forming apparatus, sheet curl may be formed in an inverse direction with respect to the one described above. In this case, as illustrated in  FIG. 13 , for example, the sheet discharge device  10  according to the first exemplary embodiment may be arranged on the printer main body  1 A by making the entire configuration upside down. With this configuration, sheet curl in the inverse direction can be corrected because the upstream side pinch roller  13  and the downstream side pinch roller  14  are positioned on the upper side of the discharge roller  12 . 
     While the present disclosure 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. 2015-256749, filed Dec. 28, 2015, which is hereby incorporated by reference herein in its entirety.