Patent Publication Number: US-11034540-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. 2018-183480 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 No. 5479984 ([0033] to [0035] and FIGS. 4 and 5) describes a known technology relating to an image forming apparatus, 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 No. 5479984 describes a structure that operates a branch member (220), which switches the direction in which sheets are transported, using a solenoid mechanism (30). In Japanese Patent No. 5479984, a buffer pad (241) and a resin sheet (251) are disposed on a sheet transport path to reduce the impulsive tone caused by transporting sheets with high speed. 
     SUMMARY 
     Aspects of non-limiting embodiments of the present disclosure relate to a measure to reduce erroneous guide of media compared to the case where a guide member that guides media are held in a predetermined position with a spring. 
     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 medium guide member, a driving source, a first movement member, a movement restricting member, and a second moving member. The medium guide member is movable between a first guide position, in which the medium guide member guides a medium toward a receiving member on which the medium is loaded, and a second guide position, in which the medium guide member guides the medium from the receiving member into the device. The driving source moves the medium guide member. The first movement member moves the medium guide member in accordance with an operation of the driving source. The movement restricting member is movable together with the medium guide member. The second moving member is movable between a first position, in which the second moving member is located close to the movement restricting member to restrict the medium guide member from moving from the second guide position to the first guide position, and a second position, in which the second moving member is spaced apart from the movement restricting member to allow the medium guide member to move to the first guide position. The second moving member moves from the first position to the second position when the medium guide member moves toward the first guide position in response to an operation of the driving source. 
    
    
     
       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 an image recording portion according to the example 1; 
         FIG. 3  illustrates a gate driving mechanism according to the example 1, in the state where a second moving member is moved to a first position; 
         FIG. 4  is a perspective view of the driving mechanism in the state illustrated in  FIG. 3 ; 
         FIG. 5  illustrates the gate driving mechanism according to the example 1, in the state where the second moving member is moving from the first position to a second position; 
         FIG. 6  is a perspective view of the driving mechanism in the state illustrated in  FIG. 5 ; 
         FIG. 7  illustrates the gate driving mechanism according to the example 1, in the state where the second moving member is moved to the second position; 
         FIGS. 8A and 8B  illustrate the gate according to the example 1, where  FIG. 8A  illustrates the gate in the first gate position, and  FIG. 8B  illustrates the gate in the second gate position; 
         FIGS. 9A and 9B  illustrate a gate positioning mechanism according to the example 1, where  FIG. 9A  illustrates a positioning member in a first gate position, and  FIG. 9B  illustrates the positional relationship between the gate and a downstream portion of a fixing device in the state illustrated in  FIG. 9A ; 
         FIGS. 10A and 10B  illustrate the gate positioning mechanism according to the example 1, where  FIG. 10A  illustrates the positioning member while a cover is opened, and  FIG. 10B  illustrates the positional relationship between the gate and a downstream portion of a fixing device in the state illustrated in  FIG. 10A ; 
         FIG. 11  illustrates a gate driving mechanism according to an example 2, where a second moving member is moved to the first position; 
         FIG. 12  illustrates a gate driving mechanism according to the example 2, in the state where a second moving member is moving from the first position to the second position; and 
         FIG. 13  illustrates the gate driving mechanism according to the example 2, where the second moving member is moved to the second 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 an 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. 
     Light reflected off a document transported over the original read surface PG by an original transporting device or a document manually placed on the original read surface PG is converted into electric signals of red R, green G, and blue B by a solid-state image sensor CCD via an exposure optical system A. 
     In a first multifunctional device, an information converting unit IPS converts the R, G, and B electric signals input from a solid-state image sensor CCD and print information transmitted from a client personal computer PC into image information for black K, yellow Y, magenta M, and cyan C, and temporarily stores the image information. The information converting unit IPS outputs the image information to a write circuit DL as image information for forming latent images at predetermined timing. 
     When the document image is a single-color image or a monochrome image, image information for only black K is input to the write circuit DL. 
       FIG. 2  illustrates a related portion of an image recording unit according to the example 1. 
     The image processor GS is electrically connected to the write circuit DL of the printer unit U 1 . The write circuit DL is electrically connected to an 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. 
     On the lower left of the copying machine U, a manual tray TR 0 , which is an example of a medium accommodating member, is disposed. On the upper right 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), at 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), at 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  illustrates a gate driving mechanism according to the example 1, in the state where a second moving member is moved to a first position. 
       FIG. 4  is a perspective view of the driving mechanism in the state illustrated in  FIG. 3 . 
       FIG. 5  illustrates the gate driving mechanism according to the example 1, in the state where the second moving member is moving from the first position to a second position. 
       FIG. 6  is a perspective view of the driving mechanism in the state illustrated in  FIG. 5 . 
       FIG. 7  illustrates the gate driving mechanism according to the example 1, in the state where the second moving member is moved to the second position. 
       FIGS. 8A and 8B  illustrate the gate according to the example 1, where  FIG. 8A  illustrates the gate in the first gate position, and  FIG. 8B  illustrates the gate in the second gate position. 
     In  FIGS. 3 to 8B , 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 . 
     A rear shaft  3   a  is rotatably supported by a shaft bearing  4  of a frame Ua. 
     At the rear of the rear (first end) shaft  3   a,  a lock piece  6 , which is an example of a movement restricting member, is disposed. The lock piece  6  according to the example 1 has a plate shape extending in the radial direction of the shaft  3 . 
     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  according to the example 1 has a plate shape extending downward. 
     To an end portion of the gate GT 1  at the rear of the positioning plate  7 , a first end of a gate spring  8 , which is an example of a first urging member, is coupled. The gate spring  8  has the other end supported by a spring supporter  9  of the frame Ua. The gate spring  8  according to the example 1 exerts a force directing to move the gate GT 1  to the first gate position (second guide position). 
     At the rear end portion of the frame Ua, a solenoid  11 , which is an example of a driving source, is supported. The solenoid  11  includes a plunger  11   a , which is an example of a retractable portion. The plunger  11   a  is capable of expanding beyond and contracting into the solenoid  11 , and retracted into the solenoid  11  when the solenoid  11  is in operation (turned on). 
     At the tip end of the plunger  11   a , a first link  12 , which is a first connection member, is rotatably supported. 
     At the front end of the first link  12 , an upper end of a second link  13 , which is an example of a second moving member, is rotatably supported. 
     The second link  13  is rotatably supported by a rotation shaft  13   a  disposed on the frame Ua. Thus, the second link  13  is supported to be movable between the locked position illustrated in  FIG. 3 , which is an example of a first position, and an unlocked position illustrated in  FIG. 6 , which is an example of a second position. 
     At the left of the lower end of the second link  13 , a locking portion  13   b,  which is an example of a restricting member, is disposed. The locking portion  13   b  protrudes rightward. At the front portion of the locking portion  13   b , a lock surface  13   b   1 , which is an example of a stop surface, extends in the front-rear direction. At the rear portion of the locking portion  13   b,  an inclined surface  13   b   2 , which is inclined to the left as it extends rearward, is disposed. 
     The position, the dimensions, and the shape of the locking portion  13   b  according to the example 1 are determined so that the lock surface  13   b   1  faces or adjoins the lock piece  6  when the gate is in the first gate position illustrated in  FIG. 3  and  FIG. 4 . The position and the shape of the locking portion  13   b  are determined so that the locking portion  13   b  is spaced apart from the lock piece  6 , specifically, shifted from the lock piece  6  in the front-rear direction when the gate is in the second gate position illustrated in  FIG. 7 . 
     The second link  13  includes a coupling portion  13   c , which extends rearward from the position of the rotation shaft  13   a.    
     The coupling portion  13   c  has a long hole  13   d,  which extends in the front-rear direction. 
     An upper end portion  14   a  of a third link  14 , which is an example of a first movement member, is coupled to the long hole  13   d.  The third link  14  is supported by the upper end portion  14   a  to be movable along the long hole  13   d  and rotatable. Thus, the third link  14  is coupled to the second link  13  via the long hole  13   d.    
     In  FIGS. 8A and 8B , the third link  14  includes a gate body  14   b,  which extends vertically. At the lower end of the gate body  14   b,  a press-down portion  14   c,  which extends rightward, is disposed as an example of a contact portion. On the lower surface of the press-down portion  14   c,  an inclined portion  14   d,  which is inclined upward as it extends leftward, is disposed. 
     To the upper end of the third link  14 , the lower end of a return spring  15 , which is an example of the second urging member, is coupled. The upper end of the return spring  15  is supported by a spring supporter of the frame Ua, which is not illustrated. The return spring  15  exerts a force of urging the third link  14  upward. 
     As illustrated in  FIG. 3  and  FIG. 8A , the third link  14  according to the example 1 is spaced apart from the gate GT 1  when the gate GT 1  is in the first gate position. As illustrated in  FIG. 7  and  FIG. 8B , when the solenoid  11  is turned on, the third link  14  comes into contact with a contacted surface  16  of the gate GT 1  to press down the gate GT 1 . Thus, the gate GT 1  rotates by being pressed by the third link  14  and moves to the second gate position. 
     The contacted surface  16  according to the example 1 is curved out upward. Compared to the one having a flat surface, the contacted surface  16  facilitates sliding with the third link  14  when in contact with the third link  14 , and the frictional resistance at the contact, which serves as rotation resistance of the gate GT 1 , is reduced. 
       FIGS. 9A and 9B  illustrate a gate positioning mechanism according to the example 1, where  FIG. 9A  illustrates a positioning member in a first gate position, and  FIG. 9B  illustrates the positional relationship between the gate and a downstream portion of a fixing device in the state illustrated in  FIG. 9A . 
     In  FIGS. 9A and 9B , the copying machine U according to the example 1 includes an open-close cover Ub, which is an example of an openable member, supported on the side surface of the copying machine U. The open-close cover Ub renders the reverse path SH 6  open or closed when the reverse path SH 6  has a paper jam. A stopblock  21 , which is an example of a positioning member, is disposed on the open-close cover Ub according to the example 1. When the open-close cover Ub illustrated in  FIG. 9A  is closed, the gate GT 1  is fixed in the first gate position with the positioning plate  7  coming into contact with the stopblock  21 . Here, as illustrated in  FIG. 9B , the comb-shaped gate bodies  1  and comb-shaped guide portions  22  on the downstream side of the fixing device F are alternately arranged in the front-rear direction. Specifically, as illustrated in  FIG. 9B , when viewed in the front-rear direction (medium width direction), the gate bodies  1  and the guide portions  22  are arranged to overlap partially. 
       FIGS. 10A and 10B  illustrate the gate positioning mechanism according to the example 1, where  FIG. 10A  illustrates the positioning member while the cover is opened, and  FIG. 10B  illustrates the positional relationship between the gate and a downstream portion of the fixing device in the state illustrated in  FIG. 10A . 
     In  FIGS. 10A and 10B , when the open-close cover Ub is opened, the stopblock  21  is spaced apart from the positioning plate  7 . Thus, the gate GT 1  rotates with the elastic force of the gate spring  8  to be in the state illustrated in  FIG. 10A . In this state, the gate bodies  1  and the guide portions  22  are apart from each other. In this state, in the 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. 
     Particularly, in the example 1, the third link  14  has the inclined portion  14   d  at the lower end. In a structure excluding the inclined portion  14   d,  the contacted surface  16  would come into contact with the lower end of the third link  14  before the gate GT 1  moves to the state illustrated in  FIGS. 10A and 10B , so that the range in which the gate GT 1  is movable is reduced. In contrast, in the example 1, the inclined portion  14   d  is prevented from coming into contact with the gate GT 1 , so that the gate GT 1  is allowed to move widely. This structure thus further facilitates visual check or removal of a paper jam. 
     In this state, the positioning plate  7  is apart from the stopblock  21 , and thus the gate GT 1  is movable freely. Unlike the case where the gate GT 1  is fixed in position during removal of jammed paper, no load is born on the movable gate GT 1 , and the gate GT 1  is thus prevented from being broken. 
     The reverse roller Rb, the gate GT 1 , and the components denoted with  1  to  21  constitute a medium transport device according to the example 1. 
     Operations 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  11  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  8 . The gate GT 1  is fixed in the first gate position with the positioning plate  7  and the stopblock  21  coming into contact with each other. 
     In this state, the locking portion  13   b  faces the lock piece  6 . Thus, the gate GT 1  that is to rotate to the second gate position is prevented from moving as a result of the lock piece  6  coming into contact with the locking portion  13   b  (in other words, the movement of the gate GT 1  is restricted or limited). Specifically, the gate GT 1  is locked to be unmovable. 
     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  11  is turned on (in the operational state). When the solenoid  11  is turned on, the first link  12  is pulled rearward. When the first link  12  is pulled rearward, the lower end of the second link  13  rotates forward, and concurrently, the coupling portion  13   c  rotates downward. With the movement of the lower end of the second link  13 , the locking portion  13   b  moves away from the lock piece  6 . Thus, the gate GT 1  becomes movable toward the second gate position, or, becomes unlocked. 
     When the coupling portion  13   c  moves downward, the third link  14  moves downward. When the third link  14  moves downward, the press-down portion  14   c  presses down the contacted surface  16 . Thus, the gate GT 1  moves to the second gate position. 
     In the example 1, the third link  14  is coupled to the second link  13  with the long hole  13   d.  A time lag occurs after the second link  13  starts moving and before the third link  14  starts moving. A time lag also occurs after the third link  14  starts moving and before the third link  14  starts pressing the gate GT 1 , since the lower end of the third link  14  is spaced apart from the contacted surface  16 . These time lags allow the locking portion  13   b  to be fully spaced apart from the lock piece  6  before the gate GT 1  starts moving toward the second gate position. Compared to the structure in which unlocking occurs concurrently with the start of the movement of the gate GT 1 , this structure reduces movement errors of the gate GT 1  or erroneous guide of recording sheets S due to movement errors of the gate GT 1 . 
     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  11  is switched from on to off. Thus, the gate GT 1  rotates toward the first gate position with the force of the gate spring  8 . In addition, the force from the solenoid  11  is no longer exerted, and the third link  14  is pushed upward with the force of the return spring  15 . Accordingly, the second link  13  rotates rearward and the first link  12  moves forward. Thus, the locking portion  13   b  faces the lock piece  6 , and the gate GT 1  is returned to be locked in the first gate position. 
     Even if the gate GT 1  is returned to the first gate position with delay, the inclined surface  13   b   2  of the locking portion  13   b  comes into contact with the lock piece  6 , and the lock piece  6  is pressed by the inclined surface  13   b   2 , which supports the gate GT 1  in its return to the first gate position. Thus, the gate GT 1  is surely returnable to the first gate position. 
     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  11 . 
     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, also in an existing structure, 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 lock piece  6  and the locking portion  13   b  are held while being close to each other 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 guide position, the locking portion  13   b  comes into contact with the lock piece  6  and blocks or restricts 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. Thus, the structure according to the example 1 reduces erroneous guide of the recording sheets S while restricting an increase of costs such as the running cost, compared to the existing structure in which the gate is held in the first gate position with only a spring. 
     Particularly, in the example 1, a spring exerting a force of returning the gate GT 1  to the first gate position or returning the third link  14  to the upper position is sufficient for the gate spring  8  or the return spring  15 , 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 an inexpensive spring having lower elasticity than a spring for an existing technology for holding the gate in the first gate position using only the spring. Thus, the manufacturing cost is reduced, and noise is reduced. 
     The gate GT 1  returns from the second gate position to the first gate position with the gate spring  8 . Thus, the example 1 does not involve the use of electric power for the return to enable reduction of the running cost. 
     Particularly, in the example 1, the gate GT 1  is allowed to be held in the frequently placed first guide position without operating the solenoid  11 . Thus, the running cost is further reduced than in the case of holding the gate by operating the solenoid  11 . 
     EXAMPLE 2 
       FIG. 11  illustrates a gate driving mechanism according to an example 2, where a second moving member is moved to the first position. 
       FIG. 12  illustrates the gate driving mechanism according to the example 2, in the state where a second moving member is moving from the first position to the second position. 
       FIG. 13  illustrates the gate driving mechanism according to the example 2, where the second moving member is moved to the second position. 
     An example 2 of the disclosure is described now. In the description of the example 2, components corresponding to the components according to the example 1 are denoted with the same reference signs, and not described in detail. 
     The example 2 is different from the example 1 in the following points, but the same in the other points. 
     In  FIG. 11  to  FIG. 13 , unlike in the example 1, in the medium transport device according to the example 2, the plunger  11   a  of the solenoid  11  is vertically retractable. In the example 2, a return spring  15 ′, which is an example of a second return member that urges the plunger  11   a  upward, is attached to the plunger  11   a.    
     The example 2 includes one link  31 , unlike in the example 1 that includes the three links  12  to  14 . The link  31  includes an upper portion  31   a,  which extends horizontally. A first end of the upper portion  31   a  is coupled to the plunger  11   a.  A link body  31   b  extends downward from a second end of the upper portion  31   a.    
     At the lower end of the link body  31   b,  a press-down portion  31   c,  which is an example of a first movement member, is disposed. The press-down portion  31   c  according to the example 2 has a curved lower surface, and is held apart from the contacted surface  16  when the gate is in the first gate position. 
     Above the press-down portion  31   c,  a locking portion  31   d , which protrudes rightward, is disposed as an example of a second moving member. The locking portion  31   d  has a curved right surface. 
     At a lower portion of a lock piece  6 ′ according to the example 2, an inclined surface  6   a ′ is disposed as a component corresponding to the inclined surface  13   b   2  according to the example 1. The inclined surface  6   a ′ is inclined rightward as it extends downward to correspond to the position and the movement direction (vertical direction) of the locking portion  31   d.    
     Operations of Example 2 
     In the medium transport device according to the example 2 having the above structure, the locking portion  31   d  faces or adjoins to the lock piece  6 ′ while the solenoid  11  is turned off, as illustrated in  FIG. 11 . Thus, the gate GT 1  in the first gate position is locked with the locking portion  31   d  and the lock piece  6 ′. 
     When the solenoid  11  is turned on, the locking portion  31   d  moves away from the lock piece  6 ′ in the state illustrated in  FIG. 12  to unlock the gate GT 1 , and the press-down portion  31   c  presses the contacted surface  16  downward. Thus, as illustrated in  FIG. 13 , the gate GT 1  moves to the second gate position. 
     Thus, as in the case of the example 1, the medium transport device according to the example 2 also reduces erroneous guide of the recording sheets S while reducing an increase of costs such as the running cost, compared to the existing structure in which the gate is held in the first gate position with only a spring. 
     Modified Examples 
     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 H09 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  11  as an example of a driving source and the links  12  to  14  for transmitting a driving force. 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 
     In the above example, desirably, the lower end of the third link  14  or the press-down portion  31   c  is kept apart from the contacted surface  16  while the gate is in the first gate position. This is not the only possible structure, however. The disclosure is also applicable to a structure where they are in contact with each other or the gate GT 1  and the third link  14  or other components are coupled together via, for example, a long hole as long as this structure secures a time lag for lock release after the gate GT 1  starts moving and before the lock piece  6  or  6 ′ comes into contact with the locking portion  13   b  or  31   d.    
     H05 
     In the above example, desirably, the solenoid  11  is operated to move the gate to the second gate position, but the solenoid 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  21 . 
     This is not the only possible structure, however. For example, the frame Ua may include the stopblock  21 . 
     H07 
     In the above example, desirably, the contacted surface  16  is curved, but may be flat. 
     H08 
     The above example has described a structure, by way of example, in which the gate GT 1  is returned to the first gate position with the gate spring  8 . This is not the only possible structure, however. For example, the disclosure is also applicable to a structure in which the gate GT 1  is returned to the first gate position with its weight in relation to the center of gravity of the gate GT 1  and the shaft  3  without including the gate spring  8 . 
     H09 
     The above example desirably includes the inclined surface  13   b   2  or  6   a ′, but may not include the inclined surface  13   b   2  or  6   a′.    
     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.