Patent Publication Number: US-11027938-B2

Title: Medium transport 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. 2018-183481 filed Sep. 28, 2018. 
     BACKGROUND 
     (i) Technical Field 
     The present disclosure relates to a medium transport device and an image forming apparatus. 
     (ii) Related Art 
     Japanese Patent Application Publication No. 6-16327 ([0027] to [0032] and  FIGS. 1 and 2 ) describes a known technology relating to image forming apparatuses, such as a copying machine, a printer, or a FAX machine, including a switching device for switching the direction in which media are transported. 
     Japanese Patent Application Publication No. 6-16327 describes a technology of a sorter for sorting the recording sheets. The sorter vertically moves an indexer ( 21 ) by driving a motor ( 10   m ), and transports sheets by winding up or rewinding a wire ( 24 ) with the driving of the motor ( 10   m ). Specifically, Japanese Patent Application Publication No. 6-16327 describes a technology of switching the destination of the sheets and transporting the sheets with one motor ( 10   m ). 
     SUMMARY 
     Aspects of non-limiting embodiments of the present disclosure relate to a medium transport device that includes fewer driving sources than a structure including a driving source dedicated for switching transport paths. 
     Aspects of certain non-limiting embodiments of the present disclosure address the above advantages and/or other advantages not described above. However, aspects of the non-limiting embodiments are not required to address the advantages described above, and aspects of the non-limiting embodiments of the present disclosure may not address advantages described above. 
     According to an aspect of the present disclosure, there is provided a medium transport device that includes a transporting member, a medium offsetting member, a guide member, and an interlocking member. The transporting member transports a medium toward a medium accommodating member. The medium offsetting member moves the transporting member in a medium width direction to transport the medium to the medium accommodating member at positions shifted in the medium width direction. The guide member is located upstream of the transporting member in a medium transport direction, the guide member being movable between a first position, in which the guide member guides the medium toward the medium accommodating member, and a second position, in which the guide member guides the medium to a destination different from the medium accommodating member, to guide the medium. The interlocking member moves the guide member between the first position and the second position in conjunction with a movement of the medium offsetting member in a width direction of the medium offsetting member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present disclosure will be described in detail based on the following figures, wherein: 
         FIG. 1  illustrates the entirety of an image forming apparatus according to an example 1; 
         FIG. 2  illustrates a related portion of the image recording portion according to the example 1; 
         FIG. 3  is a perspective view of a related portion of a medium transport device according to the example 1; 
         FIG. 4  is an enlarged view of a rear end portion of the medium transport device according to the example 1; 
         FIGS. 5A and 5B  illustrate an interlocking member according to the example 1, where  FIG. 5A  illustrates the interlocking member in a first gate position, and  FIG. 5B  illustrates the interlocking member in a second gate position; 
         FIGS. 6A and 6B  are enlarged views of a related portion illustrated in  FIGS. 5A and 5B , where  FIG. 6A  illustrates the interlocking member in the first gate position, and  FIG. 5B  illustrates the interlocking member in the second gate position; and 
         FIGS. 7A and 7B  illustrate a gate and a discharging roller according to the example 1, where  FIG. 7A  illustrates the gate and the discharging roller in the first gate position, and  FIG. 7B  illustrates the gate and the discharging roller in the second gate position. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the drawings, specific examples (referred to as examples, below) of exemplary embodiments of the present disclosure will be described. The present disclosure is not limited to the following examples. 
     For easy understanding of the following description, throughout the drawings, an X axis direction denotes the front-rear direction, a Y axis direction denotes the lateral direction, and a Z axis direction denotes the vertical direction. The directions or sides denoted with arrows X, −X, Y, −Y, Z, and −Z are respectively referred to as forward, rearward, rightward, leftward, upward, and downward, or a front side, a rear side, a right side, a left side, an upper side, and a lower side. 
     Throughout the drawings, an encircled dot denotes an arrow directing from the back to the front of the sheet, and an encircled cross denotes an arrow directing from the front to the back of the sheet. 
     In the description with reference to the drawings, components other than those needed for the description are appropriately omitted for ease of understanding. 
     Example 1 
       FIG. 1  illustrates the entirety of an image forming apparatus according to an example 1. 
     In  FIG. 1 , a copying machine U, which is an example of an image forming apparatus according to the example 1 of the present disclosure, includes a printer unit U 1 , which is an example of a recording unit and an example of an image recording device. The printer unit U 1  supports, on its upper side, a scanner unit U 2 , which is an example of a reading unit and an example of an image reading device. The scanner unit U 2  supports, on its upper side, an auto-feeder U 3 , which is an example of a document transporting device. 
     The auto-feeder U 3  includes, at an upper portion, a document tray TG 1 , which is an example of a medium accommodating member. The document tray TG 1  is capable of accommodating a stack of multiple documents Gi that are to be copied. A document output tray TG 2 , which is an example of a document discharge portion, is disposed below the document tray TG 1 . Document transport rollers U 3   b  are disposed along a document transport path U 3   a  connecting the document tray TG 1  and the document output tray TG 2 . 
     On the upper surface of the scanner unit U 2 , a platen glass PG, which is an example of a transparent document table, is disposed. The scanner unit U 2  according to the example 1 includes a reading unit U 2   a , which is an example of the reading unit, under the platen glass PG. The reading unit U 2   a  according to the example 1 is supported to be movable in the lateral direction, which is an example of a sub-scanning direction, along the lower surface of the platen glass PG. The reading unit U 2   a  is stationary in a normal state in an initial position drawn with a solid line in  FIG. 1 . The reading unit U 2   a  is electrically connected to an image processor GS. 
       FIG. 2  illustrates a related portion of an image recording unit according to the example 1. 
     The image processor GS is electrically connected to a write circuit DL of the printer unit U 1 . The write circuit DL is electrically connected to exposure devices LHy, LHm, LHc, and LHk, which are an example of a latent image forming member. 
     The exposure devices LHy to LHk according to the example 1 are formed from, for example, LED heads each including multiple LEDs arranged in the main scanning direction. The exposure devices LHy to LHk are capable of outputting write light, corresponding to the colors Y, M, C, and K in response to signals input from the write circuit DL. 
     The write circuit DL or a power circuit E has write timing or power feed timing controlled in accordance with control signals from a controller C, which is an example of a controller. 
     In  FIG. 1 , photoconductors PRy, PRm, PRc, and PRk, which are an example of an image carrier, are disposed above the exposure devices LHy to LHk. In  FIGS. 1 and 2 , the areas of the photoconductors PRy to PRk respectively irradiated with the write light constitute write areas Q 1   y , Q 1   m , Q 1   c , and Q 1   k.    
     Upstream of the write areas Q 1   y  to Q 1   k  in the rotation direction of the photoconductors PRy, PRm, PRc, and PRk, charging rollers CRy, CRm, CRc, and CRk, which are an example of a charging member, are disposed. The charging rollers CRy to CRk according to the example 1 are supported to be driven to rotate in contact with the photoconductors PRy to PRk. 
     Downstream of the write areas Q 1   y  to Q 1   k  in the rotation direction of the photoconductors PRy to PRk, developing devices Gy, Gm, Gc, and Gk, which are an example of a developing member, are disposed. The areas over which the photoconductors PRy to PRk and the developing devices Gy to Gk face each other constitute development areas Q 2   y , Q 2   m , Q 2   c , and Q 2   k.    
     Downstream of the developing devices Gy to Gk in the rotation direction of the photoconductors PRy to PRk, first transfer rollers T 1   y , T 1   m , T 1   c , and T 1   k , which are an example of a first transfer member, are disposed. The areas over which the photoconductors PRy to PRk and the first transfer rollers T 1   y  to T 1   k  face each other constitute first transfer areas Q 3   y , Q 3   m , Q 3   c , and Q 3   k.    
     Downstream of the first transfer rollers T 1   y  to T 1   k  in the rotation direction of the photoconductors PRy to PRk, photoconductor cleaners CLy, CLm, CLc, and CLk, which are an example of a cleaner, are disposed. 
     The photoconductor PRy, the charging roller CRy, the exposure device LHy, the developing device Gy, the first transfer roller T 1   y , and the photoconductor cleaner CLy for the color Y constitute an image forming unit Uy for the color Y, which is an example of a visible image forming member for the color Y according to the example 1 that forms toner images for the color Y. Similarly, the photoconductors PRm, PRc, and PRk, the charging rollers CRm, CRc, and CRk, the exposure devices LHm, LHc, and LHk, the developing devices Gm, Gc, and Gk, the first transfer rollers T 1   m , T 1   c , and T 1   k , and the photoconductor cleaners CLm, CLc, and CLk constitute image forming units Um, Uc, and Uk for the colors M, C, and K. 
     Above the photoconductors PRy to PRk, a belt module BM, which is an example of an intermediate transfer device, is disposed. The belt module BM is an example of an image carrier, and includes an intermediate transfer belt B, which is an example of an intermediate transfer member. The intermediate transfer belt B is formed from an endless belt member. 
     The intermediate transfer belt B according to the example 1 is rotatably supported by a tension roller Rt, which is an example of a tension member, a walking roller Rw, which is an example of an imbalance correcting member, an idler roller Rf, which is an example of a driven member, a backup roller T 2   a , which is an example of a member opposing the second transfer area, the first transfer rollers T 1   y , T 1   m , T 1   c , and T 1   k , and a driving roller Rd, which is an example of a driving member. In the example 1, the intermediate transfer belt B rotates when the driving roller Rd receives a driving force. 
     At the position opposing the backup roller T 2   a  across the intermediate transfer belt B, a second transfer roller T 2   b , which is an example of a second transfer member, is disposed. The backup roller T 2   a , the second transfer roller T 2   b , and other components constitute a second transfer device T 2  according to the example 1, which is an example of a transfer device. The area over which the second transfer roller T 2   b  and the intermediate transfer belt B come into contact with each other forms a second transfer area Q 4 . 
     Downstream of the second transfer area Q 4  in the rotation direction of the intermediate transfer belt B, a belt cleaner CLb, which is an example of a device for cleaning an intermediate transfer body, is disposed. 
     The first transfer rollers T 1   y  to T 1   k , the intermediate transfer belt B, the second transfer device T 2 , and other components constitute a transfer device T 1 +T 2 +B according to the example 1, which is an example of a transfer member. The image forming units Uy to Uk and the transfer device T 1 +T 2 +B constitute an image recording unit Uy+Um+Uc+Uk+T 1 +T 2 +B according to the example 1. 
     In  FIG. 1 , below the image forming units Uy to Uk, four pairs of left and right guide rails GR, which are an example of a guide member, are disposed on four levels. Each guide rail GR supports a corresponding one of sheet feed trays TR 1  to TR 4 , which are an example of a medium accommodating member, while allowing the sheet feed tray to be inserted thereinto or removed therefrom in the front-rear direction. The sheet feed trays TR 1  to TR 4  accommodate recording sheets S, which are an example of a medium. 
     On the upper left of each of the sheet feed trays TR 1  to TR 4 , a pickup roller Rp, which is an example of a pickup member, is disposed. Downstream of each pickup roller Rp in the direction in which the recording sheets S are transported, separation rollers Rs, which are an example of a separation member, are disposed. Downstream of the separation rollers Rs in the direction in which the recording sheets S are transported, a sheet feed path SH 1 , which is an example of a medium transport path, extends upward. On the sheet feed path SH 1 , multiple transport rollers Ra, which are an example of a transport member, are disposed. 
     At a lower left portion of the copying machine U, a manual tray TR 0 , which is an example of a medium accommodating member, is disposed. At an upper right portion of the manual tray TR 0 , pickup rollers Rp 0  are disposed, and a manual feed path SH 0  extends from the pickup rollers Rp 0 . The manual feed path SH 0  is merged with the sheet feed path SH 1 . 
     Registration rollers Rr, which are an example of a transport timing adjusting member, are disposed on the sheet feed path SH 1  upstream of the second transfer area Q 4 . A transport path SH 2  extends from the registration rollers Rr to the second transfer area Q 4 . 
     Downstream of the second transfer area Q 4  in the direction in which the recording sheets S are transported, a fixing device F, which is an example of a fixing member, is disposed. The fixing device F includes a heating roller Fh, which is an example of a heating fixing member, and a pressing roller Fp, which is an example of a pressing fixing member. The area over which the heating roller Fh and the pressing roller Fp come into contact with each other constitutes a fixing area Q 5 . 
     On the upper surface of the printer unit U 1 , a lower paper output tray TRh, which is an example of a medium output portion, is disposed. A paper output path SH 3 , which is an example of a medium transport member, extends toward the lower paper output tray TRh above the fixing device F. At the downstream end of the paper output path SH 3 , output rollers Rh, which are an example of a medium transport member, are disposed. 
     Above the lower paper output tray TRh, an upper paper output tray TRh 2 , which is an example of a medium output portion, is disposed. Above the fixing device F, an upper transport path SH 4 , which diverges from the paper output path SH 3 , extends toward the upper paper output tray TRh 2 . 
     On the upper transport path SH 4 , reversing rollers Rb rotatable forward and rearward, which are an example of a medium transport member, are disposed. Above the point of divergence between the paper output path SH 3  and the upper transport path SH 4 , a reverse path SH 6 , which is an example of a medium transport path, diverges downward to the left from the upper transport path SH 4 . A gate GT 1 , which is an example of a switching member, is disposed across the point of divergence between the paper output path SH 3  and the upper transport path SH 4  and the point of divergence between the upper transport path SH 4  and the reverse path SH 6 . The gate GT 1  is supported to be switchable between a first guide position (second position), in which it guides a recording sheet S from the fixing device F toward the lower paper output tray TRh and guides a recording sheet S from the upper transport path SH 4  to the reverse path SH 6 , and a second guide position (first position), in which it guides a recording sheet S from the fixing device F to the upper transport path SH 4 . 
     On the reverse path SH 6 , multiple transport rollers Ra, which are an example of a medium transport member, are disposed. The reverse path SH 6  has its downstream end merged to the sheet feed path SH 1  at a portion upstream of the registration rollers Rr. 
     Description of Image Forming Operation 
     When an operator manually places a document Gi on the platen glass PG of the copying machine U according to the example 1 having the above structure for photocopying, the reading unit U 2   a  moves in the lateral direction from the initial position to scan the document Gi on the platen glass PG while exposing the document Gi to light. When the auto-feeder U 3  is used to automatically transport the documents Gi for photocopying, the reading unit U 2   a  moves from the initial position to a document read position, indicated with a broken line in  FIG. 1 , and remains stationary. Thereafter, the multiple documents Gi accommodated in the document tray TG 1  are sequentially transported to the document read position on the platen glass PG, and then passes the document read position to be discharged onto the document output tray TG 2 . The documents Gi that sequentially pass the read position on the platen glass PG are exposed to light and scanned by the stationary reading unit U 2   a . Light reflected off the documents Gi is received by the reading unit U 2   a . The reading unit U 2   a  converts the received light reflected off the documents Gi into electric signals. To perform double-sided reading of a document Gi, a read sensor U 3   d  also reads the document Gi. 
     The image processor GS receives electric signals output from the reading unit U 2   a . The image processor GS converts the electric signals of images of the colors R, G, and B read by the reading unit U 2   a  into image information of yellow Y, magenta M, cyan C, and black K for latent image formation. The image processor GS outputs the converted image information to the write circuit DL of the printer unit U 1 . The image processor GS outputs the image information for only black K to the write circuit DL when an image is a single-color image, or a monochrome image. 
     The write circuit DL outputs control signals corresponding to the input image information to the exposure devices LHy to LHk. The exposure devices LHy to LHk output the write light corresponding to the control signals. 
     The photoconductors PRy to PRk rotate in response to the start of image formation. The charging rollers CRy to CRk receive a charging voltage from the power circuit E. Thus, the photoconductors PRy to PRk have their surfaces electrically charged by the charging rollers CRy to CRk. Electrostatic latent images are formed in the write areas Q 1   y  to Q 1   k  on the surfaces of the electrically charged photoconductors PRy to PRk with the laser beams Ly to Lk. The electrostatic latent images on the photoconductors PRy to PRk are developed into toner images, which are an example of a visible image, by the developing devices Gy, Gm, Gc, and Gk in the development areas Q 2   y  to Q 2   k.    
     The developed toner images are transported to the first transfer areas Q 3   y , Q 3   m , Q 3   c , and Q 3   k , at which they come into contact with the intermediate transfer belt B, which is an example of an intermediate transfer body. In the first transfer areas Q 3   y , Q 3   m , Q 3   c , and Q 3   k , the first transfer rollers T 1   y  to T 1   k  receive, from the power circuit E, a first transfer voltage having a polarity opposite to the polarity with which the toner is charged. Thus, the toner images on the photoconductors PRy to PRk are transferred to the intermediate transfer belt B by the first transfer rollers T 1   y  to T 1   k . To form a multi-color toner image, a toner image on the downstream side is transferred to the intermediate transfer belt B to be superposed on a toner image that has been transferred to the intermediate transfer belt B in the upstream first transfer area. 
     Remnants or deposits left on the photoconductors PRy to PRk after a first transfer are respectively removed by the photoconductor cleaners CLy to CLk. The surfaces of the cleaned photoconductors PRy to PRk are respectively electrically recharged by the charging rollers CRy to CRk. 
     Single-color or multi-color toner images transferred onto the intermediate transfer belt B by the first transfer rollers T 1   y  to T 1   k  in the first transfer areas Q 3   y  to Q 3   k  are transported to the second transfer area Q 4 . 
     Recording sheets S on which images are to be recorded are picked up by the pickup roller Rp of an appropriate one of the sheet feed trays TR 1  to TR 4 . The recording sheets S picked up by the pickup roller Rp while being stacked together are separated one from another by the separation rollers Rs. The recording sheets S separated by the separation rollers Rs are transported along the sheet feed path SH 1  by the transport rollers Ra. The recording sheets S transported along the sheet feed path SH 1  are fed to the registration rollers Rr. The recording sheets S placed on the manual tray TR 0  are also fed to the sheet feed path SH 1  through the manual feed path SH 0  by the pickup rollers Rp 0 . 
     The registration rollers Rr transport a recording sheet S to the second transfer area Q 4  at the timing when a toner image formed on the intermediate transfer belt B is transported to the second transfer area Q 4 . The second transfer roller T 2   b  receives, from the power circuit E, a second transfer voltage having a polarity opposite to the polarity with which toner is charged. Thus, the toner image on the intermediate transfer belt B is transferred to the recording sheet S from the intermediate transfer belt B. 
     After the second transfer, the intermediate transfer belt B is cleaned by the belt cleaner CLb to remove deposits or other matters adhering to the surface. 
     The recording sheet S to which the toner image has been second-transferred is heated to have the toner image fixed while passing the fixing area Q 5 . 
     When the recording sheet S having an image fixed thereto is discharged to the lower paper output tray TRh, the gate GT 1  is moved to the first guide position. The recording sheet S discharged from the fixing device F is thus transported along the paper output path SH 3 . The recording sheet S transported along the paper output path SH 3  is discharged to the lower paper output tray TRh by the output rollers Rh. 
     When the recording sheet S is to be discharged to the upper paper output tray TRh 2 , the gate GT 1  is moved to the second guide position to allow the recording sheet S to be discharged to the upper paper output tray TRh 2 . 
     When the recording sheet S is to be subjected to double-side printing, the gate GT 1  is moved to the second guide position. When the recording sheet S has its trailing end passing the gate GT 1 , the gate GT 1  is moved to the first guide position, and the reversing rollers Rb rotate rearward. Thus, the recording sheet S is guided to the gate GT 1 , and transported to the reverse path SH 6 . 
     Description of Gate Movement Mechanism 
       FIG. 3  is a perspective view of a related portion of a medium transport device according to the example 1. 
       FIG. 4  is an enlarged view of a rear end portion of the medium transport device according to the example 1. 
       FIGS. 5A and 5B  illustrate an interlocking member according to the example 1, where  FIG. 5A  illustrates the interlocking member in a first gate position, and  FIG. 5B  illustrates the interlocking member in a second gate position. 
       FIGS. 6A and 6B  are enlarged views of a related portion illustrated in  FIGS. 5A and 5B , where  FIG. 6A  illustrates the interlocking member in the first gate position, and  FIG. 5B  illustrates the interlocking member in the second gate position. 
       FIGS. 7A and 7B  illustrate a gate and a discharging roller according to the example 1, where  FIG. 7A  illustrates the gate and the discharging roller in the first gate position, and  FIG. 7B  illustrates the gate and the discharging roller in the second gate position. 
     In  FIGS. 3 to 7 , the gate GT 1  according to the example 1 includes multiple plate-shaped gate bodies  1  arranged at intervals in the width direction of the recording sheet S. The gate bodies  1  are coupled together with a coupling portion  2 , extending in the width direction of the recording sheet S. The gate bodies  1  thus form a comb shape. Shafts  3  are disposed at the outer ends of the coupling portion  2 . 
     The shafts  3  are rotatably supported by shaft bearings of the frame Ua. 
     To the rear of the gate GT 1 , an end of a gate spring  6 , which is an example of an urging member, is coupled. The gate spring  6  has the other end supported by the frame Ua. In  FIG. 4 , the gate spring  6  urges the gate GT 1  in a direction of arrow  6   a , that is, in a direction from the second gate position to the first gate position. 
     At the rear end portion of the gate GT 1 , a positioning plate  7 , which is an example of a positioned member, is disposed. The positioning plate  7  comes into contact with a stopblock  8  on an open-close cover Ub, which renders the reverse path SH 6  open or closed, and fixes the position of the gate GT 1  at the first gate position. 
     When the open-close cover Ub is opened, the positioning plate  7  is no longer in contact with the stopblock  8 , which is an example of a positioning member, and the gate GT 1  is allowed to rotate in the direction of arrow  6   a  from the first gate position with the force of the gate spring  6 . In this state, in case of a paper jam caused around the gate GT 1 , visual check of the jammed sheet or removal of the jammed sheet is facilitated by opening the open-close cover Ub. 
     In  FIGS. 3 and 5 to 7 , the front shaft  3  includes a guide rail  11 , which is an example of a second interlocking member. The guide rail  11  includes an upper rail  12  and a lower rail  13 . The upper rail  12  includes an inclined portion  12   a , which is inclined with respect to the axial direction of the shaft  3  and protrudes toward the output roller Rh, and a horizontal portion  12   b , which extends from the front end of the inclined portion  12   a  in the axial direction. The inclined portion  12   a  and the horizontal portion  12   b  are smoothly connected together to form a curved surface, or, a round shape. Thus, an interlock hook  33 , described below, moves smoothly. 
     The lower rail  13  includes an inclined portion  13   a  and a front horizontal portion  13   b , spaced apart from and extending parallel to the inclined portion  12   a  and the horizontal portion  12   b  of the upper rail  12 . The lower rail  13  of the example 1 includes a rear horizontal portion  13   c , which extends rearward from the rear end of the inclined portion  13   a  in the axial direction. Thus, in the example 1, the upper rail  12  does not have a portion opposing the rear horizontal portion  13   c . The inclined portion  13   a , the front horizontal portion  13   b , and the rear horizontal portion  13   c  of the lower rail  13  are smoothly connected together to form a rounded shape. 
     Thus, the guide rail  11  according to the example 1 is formed helically around the shaft  3 . 
     On the right side of the gate GT 1 , that is, downstream of the gate GT 1  in the sheet transport direction, the output roller Rh is disposed. The output roller Rh includes a rotation shaft  21 , and roller bodies  22 , arranged at intervals in the sheet width direction along the rotation shaft  21 . Driven rollers of the output roller Rh (disposed above the roller bodies  22 ) are not illustrated. 
     The rotation shaft  21  of the output roller Rh is supported to be rotatable relative to the frame Ua and movable in the axial direction (sheet width direction). A gear  23  is supported at the rear end of the rotation shaft  21 . The gear  23  is engaged with an intermediate gear  24 . The gear  23  according to the example 1 is longer in the axial direction than the intermediate gear  24 . The gears  23  and  24  are spur gears. Thus, while the gears  23  and  24  are engaged together, the rotation shaft  21  and the gear  23  are movable in the axial direction and the driving force is transmittable when they move in the axial direction. The intermediate gear  24  receives a driving force from a motor  26 , which is an example of a driving source, via a gear train not illustrated. 
     Below the rotation shaft  21 , an offset frame  31 , which is an example of a movable frame, is disposed. The offset frame  31  rotatably supports the rotation shaft  21 , and is movable in the sheet width direction together with the rotation shaft  21 . The offset frame  31  is rendered movable by a solenoid  32 , which is an example of a driving source, in the front-rear direction. In response to driving of the solenoid  32 , the offset frame  31  moves in the front-rear direction, and the rotation shaft  21  moves in the front-rear direction integrally with the movement of the offset frame  31 . 
     Thus, when the recording sheet S is to be discharged to the lower paper output tray TRh, if the solenoid  32  operates while the rear end portion of the recording sheet S is held between the output roller Rh, that is, while the trailing end of the recording sheet S is passing through the gate GT 1 , the recording sheet S is discharged to the lower paper output tray TRh while moving in the width direction. In other words, recording sheets S are discharged in an offset manner. Thus, recording sheets S are sorted to the front side (near side) and the rear side (far side) on the lower paper output tray TRh. 
     In  FIGS. 5A, 5B, 6A, and 6B , at the front portion of the offset frame  31 , the interlock hook  33 , which is an example of a first interlocking member, is disposed. The interlock hook  33  is disposed to face or adjacent to the rear horizontal portion  13   c  of the lower rail  13  while the offset frame  31  is moved to the first offset position on the rear side. The interlock hook  33  passes between the upper rail  12  and the lower rail  13  when the offset frame  31  is moved to the front. 
     The components including the offset frame  31 , the solenoid  32 , and the interlock hook  33  constitute an offset mechanism  31 + 32 + 33 , which is an example of a medium offsetting member according to an example. The guide rail  11  and the interlock hook  33  constitute an interlocking member  11 + 33  of the example 1. The output roller Rh, the gate GT 1 , and the components denoted with  1  to  33  constitute the medium transport device according to the example 1. 
     Effects of Example 1 
     In the copying machine U according to the example 1 having the above structure, the gate GT 1  has to move to the first gate position when the recording sheet S is to be discharged to the lower paper output tray TRh. Here, the solenoid  32  is kept off (in the nonoperational state), and the gate GT 1  is held in the first gate position with the force of the gate spring  6 . The gate GT 1  is fixed in the first gate position with the positioning plate  7  and the stopblock  8  coming into contact with each other. 
     In this state, the interlock hook  33  faces and is adjacent to the rear horizontal portion  13   c . Thus, the gate GT 1  that is to rotate to the second gate position is prevented from moving as a result of the rear horizontal portion  13   c  coming into contact with the interlock hook  33  (in other words, the movement of the gate GT 1  is restricted or limited). 
     When the recording sheet S is to be discharged to the upper paper output tray TRh 2  or to be subjected to double-side printing, the gate GT 1  has to move to the second gate position. Here, the solenoid  32  is turned on (in the operational state). When the solenoid  32  is turned on, the offset frame  31  moves forward, and the interlock hook  33  moves forward. When the interlock hook  33  moves forward, the helical guide rail  11 , through which the interlock hook  33  passes, is pushed to rotate the shaft  3  and move the gate GT 1  to the second gate position. 
     Subsequently, to transport the recording sheet S toward the reverse path SH 6  for double-side printing, the gate GT 1  needs to move to the first gate position. When the recording sheet S is to be transported to the reverse path SH 6 , the solenoid  32  is switched from on to off. Thus, the gate GT 1  rotates toward the first gate position with the force of the gate spring  6 . With the rotation of the gate GT 1 , the guide rail  11  is rotated in the direction of arrow  6   a , the interlock hook  33  is pushed, and the offset frame  31  and the rotation shaft  21  are moved rearward. 
     Thus, in the medium transport device according to the example 1, the gate GT 1  moves between the first gate position and the second gate position in conjunction with the operation or the stop of the solenoid  32 . Specifically, the gate GT 1  moves in conjunction with the offset mechanism. 
     Here, in the structure of an existing gate, the gate is held in the first gate position according to the example 1 with only the force of spring. This is because, usually, discharging the recording sheets to the lower paper output tray in a single-side printing is more frequently performed than the double-side printing. Thus, the use of a spring is reasonable to hold the gate in the frequently placed first gate position without electric power, and to move the gate to the second gate position with the operation of a driving source (with electric power) such as a motor or a solenoid for double-side printing, which is performed less frequently. 
     Here, the recording sheets are guided while being in contact with the gate held with the force of the spring. If the recording sheets are stiff media, such as cardboard, such recording sheets press the gate with a strong force. Particularly, the leading ends of the recording sheets in the transport direction collide against the gate with a strong force. 
     In the existing technology for holding the gate in the first gate position with only a spring, the spring force may be insufficient and allow the gate to rotate toward the second gate position if the gate receives a strong force from the recording sheet in the structure, as in the example 1 where the gate in the first gate position also guides the recording sheet to the reverse path. When the gate rotates, the recording sheet may be transported rearward to the fixing device, instead of the reverse path, and may be guided erroneously. 
     Continuously operating a motor or using a highly elastic spring as a gate spring to transport a recording sheet to the reverse path to avoid erroneous guide may increase the running cost or manufacturing cost. Moreover, a highly elastic spring allows the gate to forcibly rotate when the solenoid is turned off, and to be returned to the first gate position and stop by colliding against the stopblock with a large noise (unusual sound). 
     To avoid these, in the example 1, the interlock hook  33  is located close to the rear horizontal portion  13   c  when the gate GT 1  is moved to the first gate position. Thus, when the gate GT 1  is pressed by the recording sheet S to move toward the second gate position, the rear horizontal portion  13   c  comes into contact with the interlock hook  33  and blocks rotation of the gate GT 1 . The gate GT 1  is thus prevented from moving from the first gate position, so that erroneous guide of the recording sheet S is prevented. 
     Particularly, in the example 1, a spring exerting a force of returning the gate GT 1  to the first gate position is sufficient for the gate spring  6 , and a strong spring resistant to the impact caused when the recording sheet S collides against the gate GT 1  is not needed. This structure thus employs a spring having lower elasticity than a spring for an existing technology for holding the gate in the first gate position using only the spring. 
     The gate GT 1  returns from the second gate position to the first gate position with the gate spring  6 . Thus, the example 1 does not involve the use of electric power for the return. Particularly, in the example 1, the gate GT 1  is allowed to be held in the frequently placed first gate position without operating the solenoid  32 . 
     The example 1 allows the gate GT 1  to move between the first gate position and the second gate position using the solenoid  32 , which is a driving source of an offset discharging mechanism. An existing structure includes a separate driving source for moving the gate GT 1 . In the example 1, in contrast, the solenoid  32 , serving as the driving source of the offset discharging mechanism, is also used for moving the gate GT 1 . 
     In the example 1, the upper rail  12  does not face the interlock hook  33  from above when the gate GT 1  is moved to the first gate position. In other words, the upper rail  12  has its rear portion open to the lower rail  13 . Thus, when the open-close cover Ub is opened and the gate GT 1  is to rotate in the direction of arrow  6   a , or away from the second gate position, with the force of the gate spring  6 , the interlock hook  33  does not come into contact with the upper rail  12 , and the guide rail  11  is capable of passing or rotating downward. Thus, the gate GT 1  is rendered rotatable in the direction of arrow  6   a . The area near the gate GT 1  is allowed to be open widely. The wide-open area further facilitates removability of jammed sheets than the structure including an upper rail  12  having a portion opposing the interlock hook  33  when the gate GT 1  is moved to the first gate position. 
     Modified Example 
     Thus far, the examples of the present disclosure have been descried in detail. However, the disclosure is not limited to the above-described examples, and may be modified in various manners within the scope of the gist of the present disclosure described in the scope of claims. Modified examples H01 to H11 of the present disclosure are described, below, by way of examples. 
     H01 
     In the above examples, the copying machine U has been described as an example of an image forming apparatus. The present disclosure is not limited to this, however. The image forming apparatus is applicable to a FAX machine, or a multifunctional printer having multiple functions such as a FAX machine, a printer, and a copying machine. The image forming apparatus is not limited to an electrophotographic image forming apparatus, and is applicable to an image forming apparatus of any image forming form such as ink jet printing, or photolithographic printing including thermal head printing. In addition, the image forming apparatus is not limited to an image forming apparatus for multi-color development, and may be an image forming apparatus for forming single-color or monochrome images. 
     H02 
     The above example has described a structure, by way of example, including the paper output trays TRh and TRh 2  vertically arranged in two levels. However, the structure may include paper output trays arranged in three or more levels. The above example has described a structure, by way of example, including a medium transport device disposed in the printer unit U 1 . This is not the only possible structure, however. The disclosure is also applicable to a structure for a postprocessor including a transport path including a gate. 
     H03 
     The above example has described a structure, by way of example, including the solenoid  32  as an example of a driving source. This is not the only possible structure, however. The disclosure is also applicable to a structure including, for example, a motor, a gear, a pinion, and a rack. 
     H04 
     The above example has described a structure, by way of example, in which the gate spring  6  is used to return the offset mechanism from the second offset position on the front side to the first offset position on the rear side. This is not the only possible structure, however. The offset mechanism may be returned to the first offset position with a spring or the offset mechanism may be operated by a motor. 
     H05 
     In the above example, desirably, the solenoid  32  is operated to move the gate to the second gate position, but the solenoid  32  may be operated to move the gate to the first gate position. 
     H06 
     The above example has described a structure including the open-close cover Ub that includes the stopblock  8 . This is not the only possible structure, however. For example, the frame Ua may include the stopblock  8 . 
     H07 
     The above example has described a structure, by way of example, in which the guide rail  11  includes the inclined portions  12   a  and  13   a  and the horizontal portions  12   b ,  13   b , and  13   c . This is not the only possible structure, however. For example, the guide rail may have curved surfaces helically continuing around the shaft  3 . The guide rail desirably includes the horizontal portions  12   b ,  13   b , and  13   c , but may not include the horizontal portions  12   b ,  13   b , and  13   c . The above example has described a structure, by way of example, in which the guide rail  11  and the interlock hook  33  are disposed on the front side in the axial direction. However, the guide rail  11  and the interlock hook  33  may be disposed appropriately in accordance with the design or specifications, such as disposed on the rear side in the axial direction, or multiple guide rails and interlock hooks may be disposed, instead. 
     H08 
     In the above example, desirably, the upper rail  12  does not have a rear horizontal portion, but may include a rear horizontal portion. 
     H09 
     In the above example, the guide rail and the interlock hook may be interchanged, that is, the offset frame  31  may have a shape of a guide rail, and the gate GT 1  may include an interlock hook. 
     H010 
     In the above example, the gate GT 1  is in a first orientation when the offset frame  31  is located at the front, and the gate GT 1  is in a second orientation when the offset frame  31  is located at the rear. However, this arrangement may be reversed: the gate GT 1  may be in the first orientation when the offset frame  31  is located at the rear, and the gate GT 1  may be in the second orientation when the offset frame  31  is located at the front. 
     H011 
     In the above example, a spring exerting a force to hold the gate GT 1  is located at the front, and the guide rail  11  and the interlock hook  33  are located at the rear. However, this is not the only possible arrangement. Specifically, a spring may be located at the rear, and the guide rail  11  and the interlock hook  33  may be located at the front, or the spring, the guide rail  11 , and the interlock hook  33  may be collectively located at the front or rear. 
     The foregoing description of the exemplary embodiments of the present disclosure has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the disclosure and its practical applications, thereby enabling others skilled in the art to understand the disclosure for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the disclosure be defined by the following claims and their equivalents.