Patent Publication Number: US-11649125-B2

Title: Sheet feed device

Description:
FIELD 
     Embodiments described herein relate generally to a sheet feed device, an image forming apparatus containing the sheet feed device, and methods related thereto. 
     BACKGROUND 
     There is a sheet feed device that sandwiches a sheet between a pair of rollers and conveys the sheet in a first direction. One of the pair of rollers is supported by a torque limiter. Even when two sheets are simultaneously conveyed between the pair of rollers, only one of the two sheets is conveyed downstream of the pair of rollers. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic configuration diagram of an image forming apparatus according to a first embodiment; 
         FIG.  2    is a perspective view of a sheet feed device according to the first embodiment; 
         FIG.  3    is a front view when one sheet is conveyed to first and second rollers in the sheet feed device according to the first embodiment; 
         FIG.  4    is a front view in which states where a holder is rotated overlap each other in the sheet feed device according to the first embodiment; 
         FIG.  5    is a hardware configuration diagram of the image forming apparatus according to the first embodiment; 
         FIG.  6    is a front view when two sheets are conveyed to the first and second rollers in the sheet feed device according to the first embodiment; 
         FIG.  7    is a diagram illustrating changes in a dynamic pressing force with respect to the fulcrum angle in the sheet feed device according to the first embodiment; 
         FIG.  8    is a front view when the fulcrum angle is 40° in a sheet feed device according to a second embodiment; 
         FIG.  9    is a front view when the fulcrum angle is 35° in the sheet feed device according to the second embodiment; and 
         FIG.  10    is a front view when a fulcrum angle is 45° in the sheet feed device according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, the sheet feed device includes a first roller, a second roller, a torque limiter, a holder, and a driving unit. The first roller conveys a sheet in a first direction while being in contact with a first surface of the sheet. The second roller faces the first roller and is in contact with a second surface which is the back side of the first surface of the sheet in a state of sandwiching the sheet between the second roller and the first roller. The torque limiter imparts an anti-torque to the second roller in order to generate a force on the second surface in a direction opposite to the first direction. The holder rotatably supports the second roller. The driving unit moves the holder and biases the holder toward the first roller. According to another embodiment, a sheet feeding method involves conveying a sheet in a first direction by a first roller in contact with a first surface of the sheet and a second roller configured to face the first roller and contact with a second surface the sheet in a state of sandwiching the sheet between the second roller and the first roller; imparting an anti-torque to the second roller in order to generate a force on the second surface of the sheet in a direction opposite to the first direction; and a driver configured to moving a holder configured to rotatably support the second roller and biasing the holder toward the first roller. 
     Hereinafter, the sheet feed device according to the embodiment will be described with reference to the drawings. 
     First Embodiment 
     In the present embodiment, an example in which the sheet feed device is used in a sheet supply unit of image processing device will be described. The sheet feed device may be used in a manual feed tray of the image processing device, and the like. 
       FIG.  1    is a schematic configuration diagram of the image processing device according to the embodiment. The image processing device according to the embodiment is an image forming apparatus  1 . The image forming apparatus  1  performs a process of forming an image on a sheet S. 
     The image forming apparatus  1  includes a housing  10 , a scanner unit  2 , an image forming unit  3 , a sheet supply unit  4 , a conveyance unit  5 , a sheet feed tray  7 , a reversing unit  9 , a control panel  8 , and a control unit  6 . 
     The housing  10  forms the external appearance of the image forming apparatus  1 . A humidity sensor  11  that detects the humidity of the air outside the image forming apparatus  1  is fixed to the housing  10 . The humidity sensor  11  transmits the detection result to the control unit  6 . 
     The scanner unit  2  reads image information to be copied based on brightness and darkness of light to generate an image signal. The scanner unit  2  outputs the generated image signal to the image forming unit  3 . 
     The image forming unit  3  forms an output image using a recording agent such as a toner based on an image signal received from the scanner unit  2  or an image signal received from the outside. Hereinafter, an output image will be referred to as a toner image. The image forming unit  3  transfers a toner image onto the surface of a sheet S. The image forming unit  3  heats and pressurizes the toner image on the surface of the sheet S to fix the toner image onto the sheet S. 
     The sheet supply unit  4  supplies the sheets S to the conveyance unit  5  one by one in accordance with the timing at which the image forming unit  3  forms a toner image. The sheet supply unit  4  includes a sheet accommodation unit  20 , a pick-up roller  21 , and a sheet feed device  22 . 
     The sheet accommodation unit  20  accommodates the sheets S of predetermined sizes and types. 
     The pick-up roller  21  picks up the sheets S one by one from the sheet accommodation unit  20 . The sheet feed device  22  supplies the sheets S picked up by the pick-up roller  21  to the conveyance unit  5 . 
     In the present embodiment, the description will be given on the assumption that a rotation shaft CB of a second roller  26  does not move even when a fulcrum angle θ changes according to the theory. As illustrated in  FIGS.  2  and  3   , the sheet feed device  22  includes a first roller  25 , the second roller  26 , a torque limiter  27 , a holder  28 , and a driving unit  29 . 
     For example, the first roller  25  has a cylindrical shape. The first roller  25  is rotatably supported around a rotation shaft CA of the first roller  25  by a supporting member. For example, the first roller  25  is configured such that the rotation shaft CA is aligned with a horizontal plane. The first roller  25  conveys the sheet S in a first direction (the downstream side in the conveyance direction) XA while being in contact with a first surface SA of the sheet S. The direction opposite to the first direction XA will be referred to as a second direction (the upstream side in the conveyance direction) XB. 
     For example, the second roller  26  has a cylindrical shape. The second roller  26  faces the first roller  25  below the first roller  25 . The second roller  26  is configured such that the rotation shaft CB of the second roller  26  is aligned with a horizontal plane. The second roller  26  sandwiches the sheet S together with the first roller  25 . The nip formed between the first roller  25  and the second roller  26  will be referred to as a nip NA. 
     The second roller  26  is in contact with a second surface SB which is the back side of the first surface SA of the sheet S. The rotation shafts CA and CB have a length in a third direction Y perpendicular to the first direction XA. The third direction Y may be a direction intersecting the first direction XA. 
     For example, the torque limiter  27  is provided in the second roller  26 . The torque limiter  27  is coaxial with the second roller  26 . In the torque limiter  27 , a support shaft  33  protrudes in the third direction Y with respect to a main body  32 . If a torque having a value equal to or less than a threshold value determined in advance acts between the second roller  26  and the torque limiter  27 , the torque limiter  27  is integrated with the second roller  26  and rotates around the rotation shaft CB. If a torque having a value exceeding the torque threshold value acts between the second roller  26  and the torque limiter  27 , the torque limiter  27  slides against the second roller  26  to rotatably support the second roller  26  around the rotation shaft CB with an anti-torque equivalent to the torque threshold value. That is, the torque limiter  27  imparts an anti-torque to the second roller  26  in order to generate a force in the second direction XB on the second surface SB of the sheet S. 
     A torque exceeding the torque threshold value is a torque for rotating an end portion of the second roller  26  on the first roller  25  side in the first direction XA with respect to the torque limiter  27 . 
     The torque limiter may not be coaxial with the second roller  26  as long as a torque can be cut off if an excessive torque acts on the second roller  26 . 
     The holder  28  includes a main body  36 , a pair of first support pieces  37 , and a second support piece  38 . 
     The main body  36  has a length in the third direction Y. The main body  36  is positioned below the second roller  26 . The pair of first support pieces  37  has a length upward from each end portion of the main body  36  in the third direction Y. The support shaft  33  of the torque limiter  27  is fixed to each of the first support piece  37 . The holder  28  rotatably supports the second roller  26 . 
     The second support piece  38  is fixed to an intermediate portion of the main body  36  in the third direction Y. As illustrated in  FIG.  4   , a convex portion  39  is fixed to a lower surface of the second support piece  38 . The convex portion  39  has a truncated conical shape. 
     The driving unit  29  rotates and moves the holder  28 . The driving unit  29  biases the holder  28  toward the first roller  25 . As illustrated in  FIG.  2   , the driving unit  29  includes a moving mechanism  42  and a biasing member  43 . The moving mechanism  42  rotates and moves the holder  28 . The moving mechanism  42  rotates the holder  28  around a first rotation shaft CC to move the holder  28 . The moving mechanism  42  includes a motor  45 , a cam  46 , and a shaft  47 . 
     A stepping motor or the like is used as the motor  45 . The motor  45  includes a main body  48  and a driving shaft  49 . The shaft member  52  is supported by the main body  48  to be rotatable around the first rotation shaft CC of the shaft member  52 . A plurality of teeth are provided on the outer circumferential surface of the driving shaft  49 . 
     The cam  46  includes a first portion  54  and a second portion  55 . The first portion  54  has an elongated circular shape. The second portion  55  has a quadrantal shape. The second portion  55  is a portion different from the first portion  54  in the cam  46 . A plurality of teeth are provided on a side surface having an arc shape in the second portion  55 . The plurality of teeth are fitted to the plurality of teeth of the driving shaft  49  of the motor  45 . An apex having an interior angle of approximately 90 degrees in the second portion  55  communicates with a first end portion of the first portion  54 . 
     A central portion of the first portion  54  in the longitudinal direction is fixed to the shaft member  52 . The first portion  54  of the cam  46  is supported by the motor  45  to be rotatable around the first rotation shaft CC. 
     The shaft  47  has a length in the third direction Y. Here, the central axis of the shaft  47  is referred to as a second rotation shaft CD. 
     The first end portion of the shaft  47  is fixed to the second portion  55  of the cam  46 . A second end portion opposite to the first end portion in the shaft  47  is fixed to a second cam  57 . The second cam  57  is supported by the housing  10  to be rotatable around the first rotation shaft CC. An intermediate portion of the shaft  47  in the third direction Y is connected to the first support piece  37  of the holder  28  to be rotatable around the second rotation shaft CD of the shaft  47 . The holder  28  is rotatably connected to the second rotation shaft CD of the shaft  47 . The second rotation shaft CD is a rotation shaft to which the holder  28  is directly connected. The holder  28  rotates around the shaft  47 . The shaft  47  rotates around the first rotation shaft CC together with the cam  46 . 
     As illustrated in  FIGS.  2  and  3   , for example, the shaft  47  is provided in the first direction XA with respect to the rotation shaft CB. 
     The first rotation shaft CC and the second rotation shaft CD of the moving mechanism  42  are rotation shafts if the moving mechanism  42  rotates and moves the holder  28 . The moving mechanism  42  includes two rotation shafts CC and CD. 
     As illustrated in  FIG.  3   , the first rotation shaft CC is preferably formed coaxially with the rotation shaft CB of the second roller  26 . 
     For example, the biasing member  43  is a helical spring. 
     A first end portion of the biasing member  43  is in contact with the second support piece  38  of the holder  28  from below the second support piece  38 . The convex portion  39  is provided in the first end portion of the biasing member  43 . A second end portion of the biasing member  43  is disposed on the housing  10 . 
     The biasing member  43  generates a static pressing force P upward in the second support piece  38  of the holder  28 . The biasing member  43  biases the holder  28  toward the first roller  25 . 
     The biasing member may be a torsion spring or a weight. 
     Here, the operation of the sheet feed device  22  configured as described above will be described. When viewed along the rotation shaft CA of the first roller  25  and the rotation shaft CB of the second roller  26 , the angle formed between a line connecting the second rotation shaft CD and the nip NA and the first direction XA and closer to the rotation shaft CB side of the second roller  26  than the first direction XA will be referred to as a fulcrum angle θ. The fulcrum angle θ can be referred to as an angle formed between the line and a direction from the nip NA toward the first direction XA and closer to the rotation shaft CB side than the first direction XA. For example, the fulcrum angle θ is 39° in a state indicated by a solid line in  FIGS.  3  and  4   . 
     If a voltage is applied to the motor  45  in a predetermined direction, the driving shaft  49  rotates in a predetermined direction with respect to the main body  48 . As illustrated in  FIG.  2   , the cam  46  fitted to the driving shaft  49  rotates in a direction DA around the first rotation shaft CC. The driving shaft  49  of the motor  45  rotates the cam  46  around the first rotation shaft CC. For example, the holder  28  moves to a position indicated by an alternate two-dot chain line in  FIG.  4   . The fulcrum angle θ is 5°. 
     If a voltage is applied to the motor  45  in a direction opposite to the predetermined direction from a state indicated by a solid line in  FIGS.  3  and  4   , the driving shaft  49  rotates in the direction opposite to the predetermined direction with respect to the main body  48 . As illustrated in  FIG.  2   , the cam  46  fitted to the driving shaft  49  rotates in a direction DB around the first rotation shaft CC. For example, the holder  28  moves to a position indicated by a dashed line in  FIG.  4   . The fulcrum angle θ is 45°. 
     The static pressing force P has substantially a fixed value regardless of the fulcrum angle θ due to the position of the biasing member  43  contacting the convex portion  39 , or the like. 
     In the first embodiment, even when the fulcrum angle θ changes due to the holder  28  rotating around the first rotation shaft CC, the rotation shaft CB of the second roller  26  does not move. 
     As illustrated in  FIG.  1   , the conveyance unit  5  conveys the sheet S supplied from the sheet supply unit  4  to the image forming unit  3 . The conveyance unit  5  includes conveyance rollers  61  and resist rollers  62 . 
     The conveyance rollers  61  convey the sheet S supplied from the sheet feed device  22  to the resist rollers  62 . The conveyance rollers  61  abut the leading end of the sheet S in the conveyance direction against a nip NB of the resist rollers  62 . 
     The resist rollers  62  bend the sheet S in the nip NB to align the position of the leading end of the sheet S in the conveyance direction. The resist rollers  62  convey the sheet S in accordance with a timing at which the image forming unit  3  transfers a toner image onto the sheet S. 
     The image forming unit  3  will be described. 
     The image forming unit  3  includes a plurality of image forming portions  65 , a laser scanning unit  66 , an intermediate transfer belt  67 , a transfer unit  68 , and a fixing device  69 . 
     Each of the image forming portions  65  includes a photoconductor drum  70 . The image forming portion  65  forms a toner image corresponding to an image signal received from the scanner unit  2  or from the outside on a photoconductor drum  70 . The plurality of image forming portions  65  form a toner image using yellow, magenta, cyan, and black toners, respectively. 
     A charger, a developing device, and the like are disposed around the photoconductor drum  70 . The charger charges the surface of the photoconductor drum  70 . The developing device stores a developer which contains toners of yellow, magenta, cyan, and black colors, respectively. The developing device develops an electrostatic latent image on the photoconductor drum  70 . Toner images based on toners of respective colors are formed on the photoconductor drums  70 . 
     The laser scanning unit  66  scans the charged photoconductor drums  70  with laser beams L to expose the photoconductor drums  70 . The laser scanning unit  66  exposes the photoconductor drums  70  of the image forming portions  65  of the respective colors by different laser beams LY, LM, LC, and LK. The laser scanning unit  66  forms electrostatic latent images on the photoconductor drums  70 . 
     The toner images on the surfaces of the photoconductor drums  70  are primarily transferred onto the intermediate transfer belt  67 . 
     The transfer unit  68  transfers the toner image primarily transferred onto the intermediate transfer belt  67  onto the surface of the sheet S at a secondary transfer position. 
     The fixing device  69  heats and pressurizes the toner image transferred onto the sheet S to fix the toner image to the sheet S. 
     The reversing unit  9  reverses the sheet S in order to form an image on the back surface of the sheet S. The reversing unit  9  reverses the front and back of the sheet S discharged from the fixing device  69  through switching back. The reversing unit  9  conveys the reversed sheet S toward the resist rollers  62 . 
     The sheet feed tray  7  places the discharged sheet S on which an image was formed. 
     The control panel  8  is a portion of an input unit that inputs information for an operator to operate the image forming apparatus  1 . The control panel  8  includes a touch panel and various hardware keys. 
     The control unit  6  controls each portion of the image forming apparatus  1 . 
       FIG.  5    is a hardware configuration diagram of the image forming apparatus  1  according to the embodiment. The image forming apparatus  1  includes a central processing unit (CPU)  91 , a memory  92 , an auxiliary storage device  93 , and the like that are connected to each other through a bus and executes programs. The image forming apparatus  1  functions as a device that includes the scanner unit  2 , the image forming unit  3 , the sheet supply unit  4 , the conveyance unit  5 , the reversing unit  9 , the control panel  8 , and a communication unit  90  by executing the programs. 
     The CPU  91  functions as the control unit  6  by executing programs stored in the memory  92  and the auxiliary storage device  93 . The control unit  6  controls the operation of the respective functional units of the image forming apparatus  1 . Specifically, the control unit  6  controls the driving unit  29  based on detection results obtained by the humidity sensor  11 . 
     The auxiliary storage device  93  is configured using a storage device such as a magnetic hard disk device or a semiconductor storage device. The auxiliary storage device  93  stores information. The auxiliary storage device  93  stores a humidity threshold value determined in advance, and the like. 
     The communication unit  90  is configured to include a communication interface for connecting the image forming apparatus  1  to an external device. The communication unit  90  communicates with the external device through the communication interface. 
     A mechanism in which the sheet supply unit  4  supplies one sheet S will be described. 
     As illustrated in  FIG.  3   , the radius of the outer circumferential surface of the second roller  26  is set to be “r”. For example, the unit of the radius “r” is “cm”. A torque threshold value of the torque limiter  27  is set to be “TL”. For example, the unit of the torque threshold value “TL” is “cN·m (centinewton meter)”. For example, the torque threshold value is a value equal to or greater than 2.94 cN·m and equal to or less than 4.9 cN·m. 
     A returning force in the second direction XB which is generated by the torque limiter  27  is set to be “F tl ”. For example, the unit of the returning force F tl  is “N”. For example, the unit of the static pressing force P is “N”. 
     A case where one sheet S is conveyed to the first roller  25  and the second roller  26  as illustrated in  FIG.  3    will be described. 
     The returning force F tl  is obtained by Equation (1).
 
 F   tl   =TL/r   (1)
 
     The rotating force around the second rotation shaft CD which is generated by the returning force F tl  of the torque limiter  27  when one sheet S was conveyed is set to be “P 1 ”. For example, the unit of the rotating force P 1  is “N”. 
     In this case, the rotating force P 1  is obtained by Equation (2).
 
 P 1= F   tl ×tan θ  (2)
 
     The dynamic pressing force PF 1  when one sheet S is conveyed is obtained by Equation (3). The dynamic pressing force PF 1  is a force acting perpendicularly to the sheet S. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           PF 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                         = 
                         
                           
                             
                               F 
                               
                                 t 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                             
                             × 
                             tan 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θ 
                           
                           + 
                           P 
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             P 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           + 
                           P 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     A static friction coefficient between the first roller  25  and the sheet S is set to be “μ f ”. The conveying force F f  of the first roller  25  is obtained by Equation (4).
 
 F   f =μ f   ×P   (4)
 
     The condition that one sheet S is conveyed in the first direction XA from the first roller  25  and the second roller  26  is obtained by Equation (5).
 
 F   f   &gt;F   tl   (5)
 
     In this case, the torque limiter  27  slides. The second roller  26  rotates in a direction DC around the rotation shaft CB with respect to the torque limiter  27 . Since the second roller  26  is supported by the torque limiter  27 , the holder  28  rotates around the second rotation shaft CD. The second roller  26  is pressed against the sheet S side and the dynamic pressing force PF 1  in which the second roller  26  bites into the first roller  25  is generated. The dynamic pressing force PF 1  is also a biting force. The sheet S and the second roller  26  rotate together and one sheet S is conveyed in the first direction XA from the first roller  25  and the second roller  26 . 
     For example, if the static friction coefficient μ f  is reduced, the conveying force F f  is decreased. Equation (5) is not satisfied, and thus there is a concern that a conveyance defect of the sheet S may occur. In this case, the dynamic pressing force PF 1  is increased by increasing the fulcrum angle θ. Equation (5) is satisfied, and thus the concern that a conveyance defect of the sheet S may occur is removed. 
     As illustrated in  FIG.  6   , a case where two sheets S are conveyed to the first roller  25  and the second roller  26  will be described. 
     The static friction coefficient between the sheets S is set to be “μ pp ”. The returning force generated by the static friction coefficient μ pp  and the static pressing force P is set to be “F pp ”. The returning force F pp  is obtained by Equation (11).
 
 F   pp =μ pp   ×P   (11)
 
     The rotating force around the second rotation shaft CD which is generated by the static friction coefficient μ pp  and the static pressing force P when two sheets S were conveyed is set to be “P 2 ”. 
     The rotating force P 2  is obtained by Equation (12).
 
 P 2= F   pp ×tan θ  (12)
 
     The dynamic pressing force PF 2  when two sheets S are conveyed is obtained by Equation (13). The dynamic pressing force PF 2  is a force acting perpendicularly to the sheet S. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           PF 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                         = 
                         
                           
                             
                               F 
                               pp 
                             
                             × 
                             tan 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             θ 
                           
                           + 
                           P 
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             P 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                           + 
                           P 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     The condition that one sheet S is conveyed in the first direction XA from the first roller  25  and the second roller  26  is obtained by Equation (14) and Equation (15).
 
 F   tl   &gt;F   pp   (14)
 
 F   f   &gt;−F   pp   (15)
 
     When Equation (14) is satisfied, the torque limiter  27  does not slide. The second roller  26  rotates in the direction DC around the rotation shaft CB with respect to the torque limiter  27 . Also, in this case, the dynamic pressing force PF 2  in which the second roller  26  bites into the first roller  25  is generated. The end of the lower sheet S in the first direction XA stops at the nip NA. The lower sheet S is separated from the upper sheet S. When Equation (15) is satisfied, the upper sheet S is conveyed in the first direction XA from the first roller  25  and the second roller  26 . 
     The returning force F pp  may be increased due to characteristics of the sheet S such as the rigidity of the sheet S, the static friction coefficient μ pp , and the smoothness of the sheet S, and the environment such as humidity. In this case, Equation (14) is not satisfied, and thus the torque limiter  27  slides, which leads to a concern that the lower sheet S may be conveyed in the first direction XA. 
     In this case, the dynamic pressing force PF 2  is decreased by reducing the fulcrum angle θ. Equation (14) is satisfied, and thus the torque limiter  27  does not slide. A concern that a conveyance defect of the sheet S may occur is removed. 
     Results obtained by measuring the dynamic pressing forces PF 1  and PF 2  through experiments are illustrated in  FIG.  7   . In  FIG.  7   , the horizontal axis represents the fulcrum angle θ and the vertical axis represents the dynamic pressing forces PF 1  and PF 2 . The curve L 1  represents the dynamic pressing force PF 1  if one sheet S is conveyed. The curve L 2  represents the dynamic pressing force PF 2  if two sheets S are conveyed. The curve L 2  is the dynamic pressing force PF 2  for the upper sheet S. The fulcrum angle θ is changed from 5° to 45°. 
     With respect to each of the dynamic pressing forces PF 1  and PF 2 , the dynamic pressing forces PF 1  and PF 2  increase gradually as the fulcrum angle θ becomes larger. The dynamic pressing force PF 1  is greater than the dynamic pressing force PF 2  with respect to a fixed fulcrum angle θ. If the dynamic pressing force PF 1  changes, the conveyance force F f  for conveying the sheet S in the first direction XA also changes. If the dynamic pressing force PF 2  changes, the conveyance force F f  for conveying the upper sheet S in the first direction XA changes. 
     In the image forming apparatus  1 , if the detection result obtained by the humidity sensor  11  exceeds a humidity threshold value, the control unit  6  rotates and moves the holder  28  by the driving unit  29  so that the fulcrum angle θ becomes smaller. 
     As described above, in the sheet feed device  22  of the present embodiment, the driving unit  29  biases the holder  28  toward the first roller  25 , and thus the sheet S can be reliably sandwiched between the first roller  25  and the second roller  26 . If the driving unit  29  rotates and moves the holder  28 , the fulcrum angle θ changes, and the dynamic pressing force PF 2  changes. It is possible to prevent two sheets from being conveyed at once in the first direction XA from the first roller  25  and the second roller  26  by appropriately adjusting the dynamic pressing force PF 2 . 
     The driving unit  29  includes the moving mechanism  42  and the biasing member  43 . The movement of the holder  28  and the biasing of the holder  28  toward the first roller  25  can be performed separately by the moving mechanism  42  and the biasing member  43 . 
     The moving mechanism  42  rotates the holder  28 . The rotation can be easily performed using a rotation shaft of a general motor, or the like, as compared with the parallel movement. 
     The first rotation shaft CC when the moving mechanism  42  rotates and moves the holder  28  is coaxial with the rotation shaft CB of the second roller  26 . Even when the holder  28  rotates around the first rotation shaft CC, it is possible to suppress a change in the position of the nip NA due to the movement of the rotation shaft CB of the second roller  26 . 
     The moving mechanism  42  includes the motor  45 , the cam  46 , and the shaft  47 . The holder  28  can be rotated and moved with a simple configuration including the motor  45 , the cam  46 , and the shaft  47 . 
     The motor  45  includes the driving shaft  49  that rotates the cam  46  around the first rotation shaft CC. The cam  46  can be rotated around the first rotation shaft CC without using a gear and the like other than the motor  45  and the cam  46 . 
     In the image forming apparatus  1  of the present embodiment, it is possible to configure the image forming apparatus  1  by using the sheet feed device  22  in which two sheets S are prevented from being conveyed at once in the first direction XA from the first roller  25  and the second roller  26 . 
     The image forming apparatus  1  includes the humidity sensor  11  and the control unit  6 . If the detection result obtained by the humidity sensor  11  exceeds a humidity threshold value, the driving unit  29  can rotate and move the holder  28  so that the fulcrum angle θ becomes smaller. If the fulcrum angle θ becomes smaller, the dynamic pressing force PF 2  is reduced. Thus, if two sheets S are conveyed, the two sheets S can be easily separated from each other. 
     The configurations of the sheet feed device  22  and the image forming apparatus  1  of the present embodiment can be modified in various ways as described below. 
     As illustrated in  FIG.  3   , a first rotation shaft CE when the moving mechanism  42  rotates and moves the holder  28  may be positioned on a reference surface SC including the rotation shaft CA of the first roller  25  and the rotation shaft CB of the second roller  26 . For example, the first rotation shaft CE is positioned between the rotation shaft CA and the rotation shaft CB except for the rotation shafts CA and CB on the reference surface SC. 
     According to such a configuration as in the modification, if the holder  28  rotates around the first rotation shaft CE, it is possible to suppress a change in the position of the nip NA due to the movement of the rotation shaft CB of the second roller  26  within a fixed range. 
     As illustrated in  FIG.  1   , an image forming apparatus  101  may include a mass sensor  12  that detects the mass of the sheet S. In this case, an auxiliary storage device  93  of the control unit  6  may store a mass threshold value determined in advance, and the like. The control unit  6  controls the driving unit  29  based on the detection result obtained by the mass sensor  12 . 
     For example, the image forming apparatus  101  of the present modification example is used to form an image on the sheet S having a large thickness and a relatively large mass. The mass sensor  12  may be a timer that measures the time required for the sheet S to move through a predetermined conveyance path. This is because the time required for the sheet S to move through a predetermined conveyance path generally increases as the mass of the sheet S increases. 
     The control unit  6  in the image forming apparatus  101  rotates and moves the holder with the driving unit  29  so that the fulcrum angle θ becomes larger if the detection result obtained by the mass sensor  12  exceeds the mass threshold value. Since the dynamic pressing force PF 2  increases if the fulcrum angle θ becomes larger, the sheet S can be reliably conveyed in the first direction XA from the first roller  25  and the second roller  26  even when the mass of the sheet S is relatively large. 
     Second Embodiment 
     In the present embodiment, the description will be given on the assumption that the rotation shaft CB of the second roller  26  is moved if the fulcrum angle θ changes as in a sheet feed device of an actual machine. 
     In a sheet feed device  71  illustrated in  FIG.  8   , unlike the sheet feed device  22  of the first embodiment, the first rotation shaft is disposed at a position different from the rotation shaft CB of the second roller  26 . The position of the first rotation shaft changes complicatedly due to components of the image forming apparatus. 
       FIG.  8    illustrates a state where a fulcrum angle θ is 40° in the sheet feed device  71 . In this state, the nip NA is positioned on the reference surface SC. 
       FIG.  9    illustrates a state where a fulcrum angle θ is 35° in the sheet feed device  71 . In this state, the nip NA is positioned in the second direction XB with respect to the reference surface SC. 
       FIG.  10    illustrates a state where a fulcrum angle θ is 45° in the sheet feed device  71 . In this state, the nip NA is positioned in the first direction XA with respect to the reference surface SC. 
     Also in the sheet feed device  71  configured in this manner, it can be understood that the dynamic pressing force changes if the fulcrum angle θ changes. 
     Also in the sheet feed device  71  according to the second embodiment, it is also possible to exhibit the same effects as those in the sheet feed device  22  according to the first embodiment. 
     In the first and second embodiments and the modification, the moving mechanism may be a mechanism that moves the holder  28  in parallel. 
     An image processing device is configured as the image forming apparatus  1 . The image processing device may be a device that forms an image on the sheet S using a decolorable toner. 
     The image processing device is configured to include the control unit  6  but the sheet feed device  71  may be configured to include the control unit  6 . 
     According to at least one of the above-described embodiments, two sheets S can be prevented from being conveyed at once in the first direction XA from the first roller  25  and the second roller  26  by including the driving unit  29 . 
     While certain embodiments have been described these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms: furthermore various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.