Patent Publication Number: US-2013234386-A1

Title: Transport device, transfer device, and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2012-049253 filed Mar. 6, 2012. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a transport device, a transfer device, and an image forming apparatus. 
     SUMMARY 
     According to an aspect of the invention, a transport device includes a transporting member that transports an object that is wound around an outer peripheral surface thereof by rotating; a holding unit that is rotatably supported by the transporting member at a first end thereof, the holding unit holding a leading end of the object in a transport direction by gripping the leading end between a second end thereof and the outer peripheral surface when the object enters a space between the second end and the outer peripheral surface; and a positioning unit disposed on the transporting member or the holding unit so as to be movable downstream in a direction in which the object enters the space. The positioning unit contacts the leading end of the object that has entered the space and positions the object in the transport direction when the holding unit moves to hold the object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic view illustrating the structure of an image forming apparatus; 
         FIG. 2  is a schematic view illustrating the structure of a leading end gripper; 
         FIG. 3  is a perspective view illustrating the structure of the leading end gripper; 
         FIGS. 4A and 4B  illustrate the structure of the leading end gripper; 
         FIGS. 5A to 5D  illustrate a gripping operation performed by the leading end gripper; 
         FIGS. 6A to 6C  illustrate the gripping operation performed by the leading end gripper; 
         FIG. 7  is a schematic view illustrating an appropriate retracted position of a positioning unit of the leading end gripper; and 
         FIG. 8  is a schematic view illustrating the structure of a leading end gripper according to a comparative example. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an exemplary embodiment of the present invention will be described with reference to the drawings. Structure of Image Forming Apparatus according to Present 
     Exemplary Embodiment 
     First, the structure of an image forming apparatus according to the present exemplary embodiment will be described.  FIG. 1  is a schematic view illustrating the structure of an image forming apparatus  50  according to the present exemplary embodiment. 
     As illustrated in  FIG. 1 , the image forming apparatus  50  includes an image forming apparatus body  62  and various components disposed in the image forming apparatus body  62 . An image forming unit  10 , a recording medium feeder  40 , and a controller  60  are disposed in the image forming apparatus body  62 . The image forming unit  10  forms an image on a recording medium P, which is an example of an object. The recording medium feeder  40  feeds the recording medium P to the image forming unit  10 . The controller  60  controls the operation of each component of the image forming apparatus  50 . A recording medium output unit  63  is disposed on an upper part of the image forming apparatus body  62 . After an image has been formed on the recording medium P by the image forming unit  10 , the recording medium P is output to the recording medium output unit  63 . 
     The recording medium feeder  40  includes a recording medium container  41  and a transport unit  45 . The recording medium container  41  contains the recording medium P. The transport unit  45  transports the recording medium P from the recording medium container  41  to the image forming unit  10 . The transport unit  45  includes a feed roller  42  and plural transport rollers  44 . The feed roller  42  feeds the recording medium P contained in the recording medium container  41 . The transport rollers  44  are arranged along a transport path  51  of the recording medium P and transport the recording medium P, which has been fed by the feed roller  42 , to the image forming unit  10 . 
     The image forming unit  10  includes a photoconductor drum  11  and a transfer device  20 . The photoconductor drum  11  is an example of an image carrier that carries an image. The transfer device  20  transfers the image (toner image), which is carried by the photoconductor drum  11 , to the recording medium P. The photoconductor drum  11  rotates in one direction (for example, the direction of arrow A in  FIG. 1 ). 
     A charging roller  12 , an exposure device  13 , a rotary developing device  14 , and a cleaner  15  are arranged around the photoconductor drum  11  in the rotation direction of the photoconductor drum  11 . The charging roller  12 , which is an example of a charger, charges the photoconductor drum  11 . The exposure device  13  exposes the photoconductor drum  11 , which has been charged by the charging roller  12 , with light, and thereby forms an electrostatic latent image on the photoconductor drum  11 . The rotary developing device  14 , which is an example of a developing device, develops the electrostatic latent image, which has been formed on the photoconductor drum  11  by the exposure device  13 , and thereby forms a toner image. The cleaner  15  cleans residual toner (developer) remaining on the photoconductor drum  11 . 
     The exposure device  13  forms an electrostatic latent image on the basis of an image signal sent from the controller  60 . Examples of an image signal sent from the controller  60  include an image signal received by the controller  60  from an external apparatus. 
     The rotary developing device  14  includes a rotation shaft  14 A; and developing units  14 Y,  14 M,  14 C, and  14 K for yellow (Y), magenta (M), cyan (C), and black (K), which are arranged around the rotation shaft  14 A in the circumferential direction of the rotation shaft  14 A. The rotary developing device  14  rotates around the rotation shaft  14 A in the direction of arrow C. When one of the developing units  14 Y,  14 M,  14 C, and  14 K of the rotary developing device  14  is located at a facing position at which the developing unit faces the photoconductor drum  11 , an electrostatic latent image formed on the photoconductor drum  11  is developed by using a color toner corresponding to the developing unit located at the facing position, and thereby a toner image is formed. 
     The transfer device  20  includes a transfer drum  21 , which is an example of a transporting member. The transfer drum transports the recording medium P, which is wound around the outer peripheral surface thereof, by rotating. In the transfer device  20 , the recording medium P, which has been transported by the transport unit  45 , is wound around the outer peripheral surface of the transfer drum  21  (to be specific, the outer peripheral surface of an elastic layer  21 B described below), and rotates together with the photoconductor drum  11 . As a result, the recording medium P is transported to a transfer position Tr (transfer region) between the transfer drum  21  and the photoconductor drum  11 , and the transfer drum  21  transfers a toner image from the photoconductor drum  11  to the recording medium P. That is, the transfer drum  21  also functions as an example of a transfer member and transfers a toner image from the photoconductor drum  11  to the recording medium P. As described below, a leading end gripper  23  and a trailing end gripper  27  respectively grip the leading end and the trailing end of the recording medium P in the transport direction, and thereby the recording medium P is wound around the outer peripheral surface of the transfer drum  21 . 
     The recording medium P is supplied to the transfer drum  21  at a supply position Pa shown in  FIG. 1  (a holding-start position at which the leading end gripper  23  (described below) starts holding the recording medium P). The recording medium P is peeled off the transfer drum  21  at a peel-off position Pb (a holding-finish position at which the leading end gripper  23  finishes holding the recording medium P) shown in  FIG. 1 . The structure of the transfer device  20  will be described below in detail. 
     A fixing unit  30  is disposed downstream of (in  FIG. 1 , above) the transfer position Tr along the transport path  51 . The fixing unit  30  fixes the toner image, which has been transferred to the recording medium P by the transfer device  20 , to the recording medium P. The fixing unit  30  includes a heating roller  31  and a pressing roller  32 . The heating roller  31  heats the toner image on the recording medium P. The pressing roller  32  presses the recording medium P as a result of being pressed against the heating roller  31 . 
     In the fixing unit  30 , the recording medium P is pressed and heated by the pressing roller  32  and the heating roller  31 , which rotate with the recording medium P therebetween while transporting the recording medium P downstream (in  FIG. 1 , upward) along the transport path  51 . As a result, the toner image is fixed to the recording medium P. 
     Output rollers  46  are disposed downstream of the fixing unit  30  along the transport path  51 . The output rollers  46  output the recording medium P, to which the toner image has been fixed, to the recording medium output unit  63 . Structure of Transfer Device  20  according to Present 
     Exemplary Embodiment 
     Next, the structure of the transfer device  20  according to the present exemplary embodiment will be described. 
     As illustrated in  FIG. 1 , the transfer device  20  includes the transfer drum  21 , the leading end gripper  23 , and the trailing end gripper  27 . The leading end gripper  23 , which is an example of a leading end holding member, is disposed on the transfer drum  21  and grips the leading end of the recording medium P in the transport direction. The trailing end gripper  27 , which is an example of a trailing end holding member, is disposed on the transfer drum  21  and grips the trailing end of the recording medium P in the transport direction. 
     A detection sensor  25  for detecting passage of the recording medium P is disposed so as to face the outer peripheral surface of the transfer drum  21 . The detection sensor  25  is disposed upstream of a standby position (described below) of the trailing end gripper  27  (the position of the trailing end gripper  27  shown in  FIG. 1 ) in the transport direction of the recording medium P. 
     Transfer Drum  21   
     As illustrated in  FIG. 1 , the transfer drum  21  is disposed in the image forming apparatus body  62  so as to face the photoconductor drum  11  and so as to be rotatable around a rotation shaft  21 D. The transfer drum  21  includes a base member  21 A having a cylindrical shape and the elastic layer  21 B formed on the outer peripheral surface of the base member  21 A. 
     The base member  21 A is electroconductive and is made of, for example, a metal material. The elastic layer  21 B is made of a semi-conductive elastic material (for example, a resin material such as a polyurethane resin). 
     The transfer drum  21  rotates in the direction of arrow B in synchronism with the rotation of the photoconductor drum  11  while the elastic layer  21 B is in contact with the photoconductor drum  11 . A part of the elastic layer  21 B that contacts the photoconductor drum  11  becomes elastically deformed. 
     A voltage (transfer bias) having a polarity opposite to that of toner is applied to the base member  21 A of the transfer drum  21 , and thereby a toner image is transferred from the photoconductor drum  11  to the recording medium P, which is wound around the elastic layer  21 B, at the transfer position Tr. 
     A portion of the outer peripheral surface of the base member  21 A in the circumferential direction is not covered with the elastic layer  21 B. This portion, in which the base member  21 A is exposed, will be referred to as an exposed portion  21 C. Even when the exposed portion  21 C faces the photoconductor drum  11 , the exposed portion  21 C does not contact the photoconductor drum  11 . 
     In  FIGS. 1 and 2 , the outer periphery of the transfer drum  21  when the elastic layer  21 B is elastically deformed is shown by two-dot chain line K. For clarity, the two-dot chain line K is drawn around the entire circumference of the transfer drum  21 . 
     Trailing End Gripper  27   
     As illustrated in  FIG. 1 , the trailing end gripper  27  straddles the outer peripheral surface of the transfer drum  21  in the axial direction of the transfer drum  21 . The trailing end gripper  27  is supported by support portions  27 A disposed at ends of the transfer drum  21  in the axial direction so that the trailing end gripper  27  is capable of contacting and becoming separated from the outer peripheral surface of the transfer drum  21 . 
     The support portions  27 A are rotatably supported by a rotation shaft  27 D, which is coaxial with the rotation shaft  21 D of the transfer drum  21 . The trailing end gripper  27  rotates around the transfer drum  21  independently of the transfer drum  21 . 
     The trailing end gripper  27  is located at a predetermined standby position (the position of the trailing end gripper  27  shown in  FIG. 1 ) on the outer periphery of the transfer drum  21  before gripping the recording medium P. The standby position is, for example, a position between the transfer position Tr (transfer region) and the supply position Pa in the rotation direction of the transfer drum  21 . 
     The trailing end gripper  27  is made of a resin material (such as PET, a polyimide resin, or a fluorocarbon resin) and has a plate-like shape extending in the axial direction of the transfer drum  21 . The length of the trailing end gripper  27  in the axial direction is larger than the maximum width of the recording medium P (in the axial direction of the transfer drum  21 ), which is wound around the transfer drum  21 . Alternatively, the trailing end gripper  27  may have a wire-like shape, a solid cylindrical shape, or the like. 
     When the detection sensor  25  detects passage of the trailing end of the recording medium P in the transport direction, the trailing end gripper  27  moves from a position in which the trailing end gripper  27  is separated from the transfer drum  21  to a position in which the trailing end gripper  27  contacts the transfer drum  21 , and thereby holds the trailing end of the recording medium P by gripping the trailing end between the trailing end gripper  27  and the outer peripheral surface of the transfer drum  21  (to be specific, the outer peripheral surface of the elastic layer  21 B). 
     Leading End Gripper  23   
     As illustrated in  FIG. 2 , the leading end gripper  23  is disposed in the exposed portion  21 C of the transfer drum  21 . The leading end gripper  23  rotates together with the transfer drum  21 . 
     The leading end gripper  23  includes a holding unit  22  and a positioning unit  24 . The holding unit  22  is rotatably supported by the transfer drum  21  (to be specific, the base member  21 A) at a base end of the holding unit  22 . The holding unit  22  holds the leading end of the recording medium P in the transport direction by gripping the leading end between a second end of the holding unit  22  and the outer peripheral surface of the transfer drum  21 . The positioning unit  24  is disposed on the transfer drum  21 . The positioning unit  24  contacts the leading end of the recording medium P and positions the recording medium P in the transport direction while the recording medium P is held by the holding unit  22 . 
     A facing member  29 , which faces the holding unit  22 , is disposed in the exposed portion  21 C on the base member  21 A of the transfer drum  21 . The facing member  29  has a facing surface  29 A that faces the holding unit  22 . The facing surface  29 A is located inside of the outer peripheral surface of the elastic layer  21 B (to be specific, inside of the outer periphery (two-dot chain line K) of the elastic layer  21 B in an elastically deformed state). In the present exemplary embodiment, the facing surface  29 A is a part of the outer peripheral surface of the transfer drum  21 , and the holding unit  22  grips the leading end of the recording medium P in the transport direction between the holding unit  22  and the facing surface  29 A. Alternatively, the holding unit  22  may grip the leading end of the recording medium P in the transport direction between the holding unit  22  and the elastic layer  21 B or between the holding unit  22  and the base member  21 A. 
     The holding unit  22  has a plate-like shape extending in the axial direction of the transfer drum  21 . The length of the holding unit  22  in the axial direction is larger than the maximum width of the recording medium P, which is wound around the transfer drum  21 , in the axial direction of the transfer drum  21 . As illustrated in  FIG. 2 , when seen from an end in the longitudinal direction, the holding unit  22  has a shape that is bent so as to be convex outward in the radial direction of the transfer drum  21  (diagonally toward the upper left side in  FIG. 2 ). 
     As illustrated in  FIG. 3 , at a tip end of the holding unit  22 , plural protruding portions  227  are arranged in the longitudinal direction X of the holding unit  22 . The protruding portions  227  protrude upstream in the rotation direction of the transfer drum  21 . The protruding portions  227  of the holding unit  22  hold the leading end of the recording medium P in the transport direction. Plural cutout portions  223 , which extend from the tip end toward the base end of the holding unit  22 , are formed between the protruding portions  227  so as to be arranged in the longitudinal direction X of the holding unit  22 . Due to the presence of the cutout portions  223 , the positioning unit  24  is movable toward the base end of the holding unit  22  (downstream in the rotation direction of the transfer drum  21 ). 
     As illustrated in  FIGS. 4A and 4B , the base end of the holding unit  22  is supported by a swing shaft  22 C so as to be swingable with respect to the base member  21 A of the transfer drum  21 . Thus, the holding unit  22  is movable between a contact position (see  FIG. 4A ) in which the tip end of the holding unit  22  (an upstream end in the rotation direction of the transfer drum  21 ) contacts the facing surface  29 A of the facing member  29  and a separated position (see  FIG. 4B ) in which the tip end is separated from the facing surface  29 A of the facing member  29 . 
     When the holding unit  22  is located in the separated position, the holding unit  22  allows the recording medium P to enter a space S between the holding unit  22  and the facing surface  29 A of the facing member  29 . When the holding unit  22  is located in the contact position, the holding unit  22  is capable of holding the recording medium P, which has entered the space S, by holding the recording medium P between the holding unit  22  and the facing surface  29 A of the facing member  29 . That is, the contact position of the holding unit  22  is, in other words, a gripping position in which the holding unit grips the recording medium P. 
     When the holding unit  22  is located in the gripping position, the holding unit  22  is capable of passing the transfer position Tr without contacting the photoconductor drum  11 . That is, when the holding unit  22  is located in the gripping position, the holding unit  22  is located inside of the two-dot chain line K in  FIG. 2 . 
     An attachment portion  225  is integrally formed with the base end of the holding unit  22 . A tension spring  22 B, which is an example of an elastic member (urging member), is attached to the attachment portion  225 . The attachment portion  225  extends downward from the swing shaft  22 C in FIG.  4 A (toward the rotation shaft  21 D of the transfer drum  21  (see  FIG. 1 )). The tension spring  22 B pulls the attachment portion  225  with its elastic force, and thereby the holding unit  22  is urged toward the gripping position (contact position). 
     A cam  22 D is disposed below the attachment portion  225  in  FIGS. 4A and 4B  (between the attachment portion  225  and the rotation shaft  21 D of the transfer drum  21  (see  FIG. 1 )). The cam  22 D functions to move the holding unit  22  to the separated position. The cam  22 D is fixed to a cam shaft  26 , rotates together with the cam shaft  26 , contacts the attachment portion  225 , and moves the holding unit  22  to the separated position against the elastic force of the tension spring  22 B. 
     The positioning unit  24  has a plate-like shape extending in the axial direction of the transfer drum  21 . The length of the positioning unit  24  in the axial direction is larger than the maximum width of the recording medium P, which is wound around the transfer drum  21 , in the axial direction of the transfer drum  21 . As illustrated in  FIG. 4A , when seen from an end in the longitudinal direction, the positioning unit  24  has a shape that is bent so as to be concave toward the holding unit  22  (diagonally toward the upper left side in  FIG. 4A ). 
     As illustrated in  FIG. 3 , at a tip end of the positioning unit  24 , plural protruding portions  243  are arranged in the longitudinal direction. The protruding portions  243  protrude into the cutout portions  223  of the holding unit  22 . The protruding portions  243  are disposed so as to be substantially perpendicular to the holding unit  22 . The protruding portions  243  are movable through the cutout portions  223  from the tip end of the holding unit  22  toward the base end of the holding unit  22 . The protruding portions  243  of the positioning unit  24  contacts the leading end of the recording medium P. 
     As illustrated in  FIGS. 4A and 4B , a base end of the positioning unit  24  (a downstream end in the rotation direction of the transfer drum  21 ) is supported by a swing shaft  24 C so as to be swingable with respect to the transfer drum  21  (to be specific, the base member  21 A). Thus, the positioning unit  24  is movable between a positioning position and a retracted position. When the holding unit  22  is located in the contact position, the positioning unit  24  is in the positioning position and positions the recording medium P in the transport direction (an entry direction (the rotation direction of the transfer drum  21 )). When the holding unit  22  is located in the separated position, the positioning unit  24  is in the retracted position, in which the positioning unit  24  is retracted from the positioning position toward the base end of the holding unit  22  (the downstream end in the transport direction of the recording medium P). 
     As the positioning unit  24  moves from the positioning position toward the retracted position, the recording medium P is allowed to enter more deeply into the space S between the holding unit  22  and the facing surface  29 A of the facing member  29 . 
     An attachment portion  245  is integrally formed with the positioning unit  24 . A tension spring  24 B, which is an example of an elastic member (urging member), is attached to the attachment portion  245 . The attachment portion  245  extends downward from the swing shaft  24 C in  FIG. 4A  (toward the rotation shaft  21 D of the transfer drum  21  (see  FIG. 1 )). The tension spring  24 B pulls the attachment portion  245  with its elastic force, and thereby the positioning unit  24  is urged toward the positioning position. 
     A cam  24 D is disposed on the left side the attachment portion  245  in  FIGS. 4A and 4B  (on the downstream side in the rotation direction of the transfer drum  21 ). The cam  24 D functions to move the positioning unit  24  to the retracted position. As with the cam  22 D, the cam  24 D is fixed to the cam shaft  26 , rotates together with the cam shaft  26 , contacts the attachment portion  245 , and moves the positioning unit  24  to the retracted position against the elastic force of the tension spring  24 B. 
     Thus, the cam  24 D for moving the positioning unit  24  and the cam  22 D for moving the holding unit  22  are fixed to the single cam shaft  26 , and the cam shaft  26  is rotated by the single drive motor  28  (driving unit), which is controlled by the controller  60 . When the cam shaft  26  is rotated by the drive motor  28 , the cams  24 D and  22 D rotate together, and the cams  24 D and  22 D respectively contact the attachment portion  245  of the positioning unit  24  and the attachment portion  225  of the holding unit  22 . 
     The phase difference between the cams  24 D and  22 D is set so that the positioning unit  24  moves from the positioning position to the retracted position when the holding unit  22  moves toward the separated position and so that the positioning unit  24  returns to the positioning position when the holding unit  22  moves from the separated position to the contact position. Thus, the movement of the positioning unit  24  is linked to the movement of the holding unit  22 . That is, the cams  24 D and  22 D also function as examples of a linkage unit and link the movement of the positioning unit  24  to the movement of the holding unit  22 . 
     As heretofore described, the trailing end gripper  27  and the leading end gripper  23  respectively grip the trailing end and the leading end of the recording medium P in the transport direction, and thereby the recording medium P is wound around the outer peripheral surface of the transfer drum  21 . The positioning unit  24  and the holding unit  22  are each made of a metal material such as a stainless steel (SUS). 
     Image Forming Operation of Image Forming Apparatus  50   
     Next, an image forming operation of forming a toner image on the recording medium P, which is performed by the image forming apparatus  50  according to the present exemplary embodiment, will be described. 
     Before an image forming operation is started, the trailing end gripper  27  is located at the standby position between the transfer position Tr and the supply position Pa in the rotation direction of the transfer drum  21 . 
     When the image forming operation is started, the feed roller  42  feeds the recording medium P from the recording medium container  41 , and the transport rollers  44  transport the recording medium P toward the transfer drum  21 . While the recording medium P is transported toward the transfer drum  21 , the transfer drum  21  rotates and the leading end gripper  23  moves toward the supply position Pa. 
     When the recording medium P has been transported to the transfer drum  21  by the transport rollers  44 , the leading end gripper  23  grips the leading end of the recording medium P in the transport direction at the supply position Pa. While the transfer drum  21  rotates, the leading end of the recording medium P in the transport direction, which is gripped by the leading end gripper  23 , passes through the space Sa between the transfer drum  21  and the trailing end gripper  27  located at the standby position. After the leading end of the recording medium P in the transport direction has passed through the space Sa, the trailing end gripper  27  grips the trailing end of the recording medium P in the transport direction. Thus, the leading end gripper  23  and the trailing end gripper  27  respectively grip the leading end and the trailing end of the recording medium P in the transport direction, and thereby the recording medium P is wound around the outer peripheral surface of the transfer drum  21 . 
     The transfer drum  21  and the trailing end gripper  27  rotate in synchronism with each other while the recording medium P is wound around the outer peripheral surface of the transfer drum  21 , and thereby the recording medium P is rotated. Thus, the recording medium P is transported to the transfer position Tr. 
     In the image forming unit  10 , the photoconductor drum  11  is charged by the charging roller  12  (charging) and exposed to light by the exposure device  13  (exposure), and thereby an electrostatic latent image is formed on the photoconductor drum  11 . The electrostatic latent image is developed by the developing unit  14 Y that faces the photoconductor drum  11 , and thereby a yellow toner image is formed on the photoconductor drum  11  (development). The yellow toner image is transferred by the transfer drum  21  to the recording medium P, which has been transported to the transfer position Tr. 
     The transfer drum  21  and the trailing end gripper  27  rotate in synchronism with each other, and thereby rotate the recording medium P. The rotary developing device  14  rotates to a position at which the developing unit  14 M faces the photoconductor drum  11 . Charging, exposure, and development are performed as described above to form a magenta toner image on the photoconductor drum  11 . The magenta toner image is transferred to the recording medium P, which is transported to the transfer position Tr again while the transfer drum  21  and the trailing end gripper  27  rotate. Likewise, cyan (C) and black (K) toner images are successively transferred to the recording medium P in an overlapping manner. 
     After the toner images have been transferred to the recording medium P in an overlapping manner, the leading end gripper  23  releases the leading end of the recording medium P in the transport direction at the peel-off position Pb, and thereby the recording medium P is peeled off the transfer drum  21 . 
     The recording medium P, which has been peeled off the transfer drum  21 , is transported to the fixing unit  30 , and the toner images are fixed to the recording medium P by the fixing unit  30 . The recording medium P, on which the toner images have been fixed, is output to the recording medium output unit  63  by the output rollers  46 . An image forming process is performed as heretofore described. 
     Function of Present Exemplary Embodiment 
     Next, a gripping operation performed by the leading end gripper  23  to hold the recording medium P, which is a function of the present exemplary embodiment, will be described. 
     As illustrated in  FIG. 5A , before the gripping operation is started, the holding unit  22  is located in the contact position, in which the holding unit  22  is in contact with the facing surface  29 A of the facing member  29  (that is, the gripping position, in which the holding unit  22  is to grip the recording medium P). At this time, the positioning unit  24  is located in the positioning position, in which the positioning unit  24  is to position the recording medium P in the transport direction. 
     When the gripping operation is started, the drive motor  28 , which is controlled by the controller  60 , rotates the cam shaft  26  (forward) and thereby the cams  24 D and  22 D rotate (forward). Then, the cams  24 D and  22 D respectively contact the attachment portion  245  of the positioning unit  24  and the attachment portion  225  of the holding unit  22  (see  FIG. 4B ). 
     Thus, as illustrated in  FIG. 5B , the holding unit  22  starts moving from the contact position (see  FIG. 5A ) toward the separated position, in which the holding unit  22  is separated from the facing surface  29 A of the facing member  29 . At the same time, the positioning unit  24  starts moving from the positioning position toward the retracted position, in which the positioning unit  24  is retracted toward the base end of the holding unit  22 . 
     As illustrated in  FIG. 5C , when the holding unit  22  reaches the separated position, the positioning unit  24  reaches the retracted position. At this time, the angle θ 1  between the holding unit  22  and an outer peripheral surface  21 E of the transfer drum  21  (to be specific, the elastic layer  21 B) is smaller than 90 degrees. 
     As illustrated in  FIG. 7 , the retracted position of the positioning unit  24  may be located further toward the base end of the holding unit  22  (toward the center of the transfer drum  21 ) from an intersection E of the holding unit  22  and the outer peripheral surface  21 E of the transfer drum (to be specific, the elastic layer  21 B). In this case, when the recording medium P enters the space S between the holding unit  22  and the facing surface  29 A of the facing member  29 , the recording medium P is more likely to first contact the holding unit  22  than the positioning unit  24 . 
     As illustrated in  FIG. 5D , the recording medium P enters the space S between the holding unit  22  and the facing surface  29 A of the facing member  29  when the holding unit  22  is located in the separated position and the positioning unit  24  is located in the retracted position. 
     As illustrated in  FIG. 6A , the leading end of the recording medium P, which has entered the space S between the holding unit  22  and the facing surface  29 A of the facing member  29 , first contacts the holding unit  22 , because the positioning unit  24  is located in the retracted position. Then, the leading end of the recording medium P is guided along the holding unit  22  that is inclined at the angle θ 1 , which is relatively small, toward the base end of the holding unit  22 . 
     Next, the drive motor  28 , which is controlled by the controller  60 , rotates the cam shaft  26  (backward) and thereby the cams  24 D and  22 D rotate together (backward). As a result, the cams  24 D and  22 D respectively become separated from the attachment portion  245  of the positioning unit  24  and the attachment portion  225  of the holding unit  22  (see  FIG. 4A ). 
     Thus, as illustrated in  FIG. 6B , the holding unit  22  starts moving from the separated position toward the gripping position (contact position). At the same time, the positioning unit  24  starts moving from the retracted position toward the positioning position. Thus, the recording medium P is pushed back toward the tip end of the holding unit  22 . 
     As illustrated in  FIG. 6C , when the holding unit  22  reaches the contact position, the positioning unit  24  reaches the positioning position. Thus, the holding unit  22  holds the recording medium P by gripping the recording medium P between the holding unit  22  and the facing surface  29 A of the facing member  29 , and the positioning unit  24  positions the recording medium P in the transport direction while the recording medium P is held by the holding unit  22 . The holding unit  22  holds the recording medium P at the supply position Pa illustrated in  FIG. 1 . 
     As heretofore described, with the present exemplary embodiment, because the positioning unit  24  moves to the retracted position, the recording medium P is allowed to enter more deeply into the space S between the holding unit  22  and the facing surface  29 A of the facing member  29 . As a result, the recording medium P is unlikely to be unintentionally removed from the holding unit  22 . Therefore, with the structure according to the present exemplary embodiment, occurrence of insufficient gripping of the leading end of the recording medium P in the transport direction by the holding unit  22  is reduced as compared with a structure in which the positioning unit  24  does not move. 
     Since occurrence insufficient gripping of a recording medium by the holding unit  22  is reduced, displacement of a transfer position (image formation position) on a recording medium due to such insufficient gripping by the holding unit  22  is reduced. 
     With the present exemplary embodiment, when the recording medium P enters the space S between the holding unit  22  and the facing surface  29 A of the facing member  29 , the leading end of the recording medium P first contacts the holding unit  22  and then the leading end is guided along the holding unit  22 . At this time, the leading end of the recording medium P is not likely to be bent, because the angle θ 1  between the holding unit  22  and the outer peripheral surface  21 E of the transfer drum  21  (to be specific, the elastic layer  21 B) is smaller than an angle θ 2  between the positioning unit  24  and the outer peripheral surface  21 E of the transfer drum  21  (to be specific, the elastic layer  21 B). 
       FIG. 8  illustrates a comparative example in which the positioning unit  24  does not retract and the leading end of the recording medium P first contacts the positioning unit  24  when the recording medium P enters the space S between the holding unit  22  and the facing surface  29 A of the facing member  29 . In this case, the angle θ 2  is larger than 90 degrees (cf.  FIG. 5C ), and therefore the recording medium P rises sharply along the positioning unit  24 . As a result, the leading end of the recording medium P is likely to become bent. 
     With the present exemplary embodiment, the positioning unit  24  returns to the positioning position when the holding unit  22  grips the recording medium P. Therefore, the holding unit  22  grips a smaller portion (area) of the recording medium P than in a case where the positioning unit  24  does not return to the positioning position. Thus, the margin of the recording medium P is reduced, and an image may be formed in a larger region. 
     With the present exemplary embodiment, a driving force is transmitted from the single drive motor  28  to the holding unit  22  and the positioning unit  24 , and thereby the movement of the positioning unit  24  is linked to the movement of the holding unit  22 . Therefore, an error in the movements of the holding unit  22  and the positioning unit  24  is smaller than in a case where the movements of the holding unit  22  and the positioning unit  24  are controlled by driving the holding unit  22  and the positioning unit  24  by using different driving units. 
     Modifications 
     In the present exemplary embodiment, the positioning unit  24  is disposed on the transfer drum  21 . However, this is not necessarily the case. For example, the positioning unit  24  may be disposed on the holding unit  22 . 
     In the present exemplary embodiment, the movement of the holding unit  22  is linked to the movement of the positioning unit  24 . However, this is not necessarily the case. In the present exemplary embodiment, the holding unit  22  and the positioning unit  24  start moving at the same timing. However, they may start moving at different timings. 
     In the present exemplary embodiment, a driving force is transmitted from the single drive motor  28  to the positioning unit  24  and the holding unit  22  and the movement of the positioning unit  24  is linked to the movement of the holding unit  22 . However, this is not necessarily the case. For example, the movement of the holding unit  22  and the movement of the positioning unit  24  may be controlled by driving the holding unit  22  and the positioning unit  24  by using different driving units. 
     The present invention is not limited to the exemplary embodiment described above and may be modified, changed, and improved in various ways. For example, the modifications described above may be used in combination. 
     The foregoing description of the exemplary embodiment of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiment was chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.