Abstract:
A switchback transport mechanism includes a first transport section, a second transport section, a third transport section, and a transport control section. The first transport section applies propelling force to a sheet in the guiding path. The second transport section applies propelling force to a sheet in the ejecting path. The third transport section has a first transport member and a second transport member placed so as to be attached to and detached from each other. The third transport section selectively applies propelling forces in a frontward direction and a backward direction to a sheet in the switchback transport path through the first and second transport members. The transport control section controls operations of the first, second, and third transport sections. The transport control section detaches the first and second transport members from each other in a time period when no sheet is being transported by the third transport section.

Description:
CROSS REFERENCE 
     This Nonprovisional application claims priority under 35 U.S.C. § 119 (a) on Patent Application No. 2005-327630 filed in Japan on Nov. 11, 2005, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a switchback transport mechanism for switching back a sheet being transported along a transport path. The invention further relates to an image forming apparatus provided with such a switchback transport mechanism. 
     There has been a growth in the number of image forming apparatus provided with a switchback transport mechanism that switches back a sheet by transporting the sheet forwards and then backwards. In image forming apparatus with duplex-printing features, for example, a sheet, after passing through an image forming section, is switched back by a switchback transport mechanism and then guided again to the image forming section. Such image forming apparatus use various different methods devised of switching back a sheet by the switchback transport mechanism. JP S58-207247A discloses a switchback transport mechanism having a half-moon roller that is arranged along a switchback transport path for the purpose of facilitating sheet switching-back. 
     With the prior art mechanism, however, it is impossible to guide a sheet into a switchback section until an immediately preceding sheet is switched back and ejected out of the switchback section. This results in relatively long intervals at which a series of sheets to be successively switched back are transported, thereby preventing an image forming process from being speeded up. 
     In view of the foregoing problems, a feature of the invention is to provide a switchback transport mechanism that allows sheets to be transported, and switched back, with improved efficiency, and an image forming apparatus provided with such switchback mechanism. 
     SUMMARY OF THE INVENTION 
     A switchback transport mechanism according to an aspect of the invention switches back a sheet by guiding the sheet from a guiding path to a switchback transport path through a connecting point and then ejecting the sheet from the switchback transport path to the ejecting path through the connecting point. The mechanism includes a first transport section, a second transport section, a third transport section, and a transport control section. The first transport section applies propelling force to a sheet in the guiding path. The second transport section applies propelling force to a sheet in the ejecting path. 
     The third transport section has a first transport member and a second transport member, placed in such a manner as to be selectively attached to and detached from each other. An example of the first and second transport members is the combination of a first roller that has a circumferential surface with a cutout portion and a second roller placed in contact with the circumferential surface of the first roller. Another example of the first and second transport members is a pair of rollers pressed against each other and supported in such a manner that a first roller is detachable from a second roller. 
     The third transport section selectively applies propelling forces in a frontward direction and a backward direction to a sheet in the switchback transport path through the first and second transport members. 
     The transport control section controls operations of the first, second, and third transport sections. The transport control section detaches the first and second transport members from each other in a time period when no sheet is being transported by the third transport section. This is because a space formed between the first and second transport members reduces sheet transport failures even when two sheets are passing each other in the switchback transport path. 
     In switching back a sheet, it becomes unnecessary for the third transport section to apply propelling force to a sheet when a leading end of the sheet reaches the second transport section. In other words, a space formed between the circumferential surfaces of the first and second rollers does not prevent transport of the sheet. This allows guiding a sheet into the switchback transport path without waiting for a preceding sheet to be ejected out of the switchback transport path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a schematic configuration of an image forming apparatus; 
         FIG. 2  is a diagram illustrating a configuration of a sheet transport path provided in the apparatus; 
         FIG. 3  is a schematic diagram illustrating a configuration of part of the sheet transport path near a switchback section; 
         FIG. 4  is a block diagram illustrating a schematic configuration of the apparatus; 
         FIG. 5  is a schematic diagram illustrating an example of a first type of sheet transport operation; 
         FIG. 6  is a schematic diagram illustrating another example of the first type of sheet transport operation; 
         FIG. 7  is a schematic diagram illustrating an example of a second type of sheet transport operation; 
         FIG. 8  is a flowchart illustrating steps of a process performed in duplex-printing operation by a CPU; 
         FIGS. 9A and 9E  are diagrams illustrating operating conditions of a reversing roller in a switchback operation; and 
         FIG. 10  is a timing chart illustrating the operating conditions of the reversing roller in the switchback operation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic front cross-sectional view illustrating a configuration of an image forming apparatus according to an embodiment of the invention, such as an apparatus  100 . The apparatus  100  includes an image reading unit  200 , an image forming unit  300 , and a sheet feeding unit  400 . 
     The unit  200  has an automatic document feeder (ADF)  201 , a first document platen  202 , a second document platen  203 , a first mirror base  204 , a second mirror base  205 , a lens  206 , and a charge coupled device (CCD)  207 . 
     The ADF  201  feeds an original document, sheet by sheet, from a document tray  211  to the platen  203 . The ADF  201  serves as a document cover that covers the platens  202  and  203 . Each of the platens  202  and  203  includes a hard glass plate. 
     The bases  204  and  205  are located below the platens  202  and  203 . The bases  204  and  205  are supported reciprocally along a horizontal direction. The base  205  moves half as fast as the base  204  does. On the base  204 , a light source and a first mirror are mounted. On the base  205 , a second mirror and a third mirror are mounted. 
     In reading an image of original document that is being transported by the ADF  201 , the base  204  is held still below the platen  203 . In reading an image of original document placed on the platen  202 , the bases  204  and  205  are moved horizontally below the platen  202 . 
     In either case, light reflected from an image-bearing surface of the original document strikes the CCD  207  via the bases  204  and  205  and the lens  206 . The CCD  207  outputs an electric signal according to an amount of light reflected from the image-bearing surface of original document. The electric signal is input to the unit  300  as image data. 
     The unit  300  is provided with an image forming section  30 . The section  30  has a photoreceptor drum  31 , a charging device  32 , an exposure device  33 , a developing device  34 , a transfer belt  35 , a cleaner  36 , and a fusing device  37 . 
     The drum  31 , which has an outer photoreceptive surface, is rotatable in a direction indicated by an arrow in  FIG. 1 . The charging device  32  applies, to the surface of the drum  31 , such a voltage as to allow the surface to have a uniform electric potential. The device  32  may be either a noncontact charger, or a contact charger of roller or brush type. 
     The exposure device  33  irradiates the surface of the drum  31  with light modulated according to image data, so that an electrostatic latent image is formed on the surface. As the device  33 , a laser scanning unit is used in the present embodiment. Alternatively, a writing unit provided with an array of luminous elements such as ELs or LEDs may be used as the device  33 . The developing device  34  supplies toner to the surface of the drum  31  to form a toner image on the surface. 
     Under the drum  31 , the transfer belt  35  is looped over a plurality of rollers. The belt  35  has a resistance of 1×10 9  Ω·cm to 1×10 13  Ω·cm. 
     Positioned inside the loop of the belt  35  is a transfer roller  35 A for transferring a toner image from the surface of the drum  31  to a sheet. The roller  35 A is pressed against the drum  31  through the belt  35 . A predetermined amount of transfer voltage is applied to the roller  35 A when a toner image is to be transferred from the drum  31  to a sheet. 
     The cleaner  36  removes residual toner from the surface of the drum  31  after transfer of a toner image to a sheet. The fusing device  37  has a heat roller  37 A and a pressure roller  37 B. The roller  37 A is provided with an internal heater for heating an outer surface thereof. The roller  37 B is pressed against the roller  37 A at a predetermined pressure. The device  37  heats and pressurizes a sheet passing between the rollers  37 A and  37 B, thereby fixing a toner image to the sheet. After passing through the device  37 , the sheet is output to an output tray  38  mounted on a side surface of the apparatus  100 . 
     The sheet feeding unit  400  corresponds to the sheet feeding section of the present embodiment. The unit  400  has sheet cassettes  401 ,  402 ,  403 , and  404 , and a manual sheet feeding tray  405 . The unit  400  feeds sheets, one by one, to the section  30  from any one of the cassettes  401  to  404  and the tray  405 . 
       FIG. 2  is a diagram illustrating a configuration of a sheet transport path  1  provided in the apparatus  100 . The path  1  is located inside the image forming unit  300 . The path  1  includes a first path  11 , a second path  12 , a third path  13 , a fourth path  14 , and a fifth path  15 . In the present embodiment, the paths  11 ,  12 , and  13  correspond to the first path, the guiding path, and the ejecting path, respectively. 
     The path  11  leads from the unit  400  to the tray  38 , through a first confluence  21 , the section  30 , a first bifurcation  24 , and a second confluence  22  in that order. Arranged along the path  11  are transport rollers  61 ,  62 , and  63 , registration rollers  51 , and output rollers  52 . 
     The path  11  extends substantially horizontally in the section  30 , for stable transfer of a toner image from the drum  31  to a sheet and for stable transport of a sheet carrying a pre-fusion toner image, to the device  37 . 
     The path  12  guides a sheet from the bifurcation  24  to a first switchback section  2 . The path  12  leads from the bifurcation  24  to the section  2 , through a second bifurcation  25  and a third bifurcation  26  in that order. Transport rollers  59  are arranged with the path  12  sandwiched therebetween. The rollers  59  transport a sheet toward the section  2 . The rollers  59  correspond to the first transport section of the present embodiment. 
     It is to be noted that, in the present embodiment, the first bifurcation  24  and the third bifurcation  26  correspond to the bifurcation and the connecting point, respectively. 
     The section  2  is provided with a switchback transport path  12 A that extends substantially horizontally. Reversing rollers  53  and  58  are arranged with the path  12 A sandwiched therebetween. The roller  58  is a half-moon roller. More specifically, the roller  53  has a circumferential surface with a flat portion oriented along a rotation axis thereof, and thus is half-moon shaped in cross section perpendicular to the rotation axis. As the roller  58 , a conventional half-moon roller for general purpose use is usable. It is preferable that a circumferential length of the roller  58 , excluding length of the flat portion, is longer than a distance between the bifurcation  26  and transport rollers  54 . This allows a switched-back sheet to be delivered to the rollers  54  by rotating the roller  58  a turn in the backward direction. 
     In the configuration of the present embodiment, thus, the circumferential length of the roller  58 , excluding the length of the flat portion, is longer than the distance between the bifurcation  26  and the rollers  54 . It is to be noted that the roller  58  includes, but is not limited to, a half-moon roller. As the roller  58 , any roller will suffice that has nonconstant distance between its rotation axis and its circumferential surface. In the present embodiment, the roller  58  corresponds to the first roller. The roller  53  is positioned in such a manner that a circumferential surface thereof is in contact with a portion of the circumferential surface of the roller  58  other than the flat portion. In the present embodiment, the roller  53  corresponds to the second roller. 
     The third path  13  leads from the third bifurcation  26  to the first confluence  21 , via a third confluence  23 . Along the path  13 , transport rollers  54 ,  55 ,  56 , and  57  are arranged. In the present embodiment, the rollers  54  to  57  collectively correspond to the second transport section. The fourth path  14  leads from the bifurcation  25  to the confluence  23 . The fifth path  15  leads from the bifurcation  25  to the confluence  22 . 
       FIG. 3  is a schematic diagram illustrating a configuration of the first bifurcation  24 , the second bifurcation  25 , and the third bifurcation  26 , in the sheet transport path  1 . A guide  41  is provided at the bifurcation  24 . The guide  41  is selectively moved between two respective positions indicated by a solid line and a dashed line shown in  FIG. 3 , to guide a sheet on the path  11  to either one of the tray  38  and the section  2 . 
     Guides  42  and  43  are provided at the bifurcation  25 . With no external force acting thereon, the guide  42  is in a position, indicated by a solid line shown in  FIG. 3 , to guide a sheet being transported upward along the path  12  or the path  14 , into the path  15 . The guide  42  is moved to a position indicated by a dashed line shown in  FIG. 3 , by contact with a sheet that is being transported downward from the bifurcation  24  along the path  12 . The guide  43  is supported pivotably between two respective positions indicated by a solid line and a dashed line shown in  FIG. 3 . 
     A guide  44  is provided at the bifurcation  26 . The guide  44  is supported pivotably between two respective positions indicated by a solid line and a dashed line shown in  FIG. 3 . 
       FIG. 4  is a block diagram illustrating a configuration of a control section  70  provided in the apparatus  100 . In the present embodiment, the section  70  corresponds to the transport control section. The control section  70  has a CPU  71 , a ROM  72 , a RAM  73 , motor drivers  74 ,  75 , and  76 , solenoid drivers  77  and  78 , clutch drivers  80  and  81 , and a sensor section  82 . 
     The section  82  has a plurality of sensors arranged in the sheet transport path  1 . The sensors detect presence of a sheet at respective different locations in the path  1  and send detection signals to the CPU  71  according to the detection results. 
     The CPU  71  executes programs prestored in the ROM  72 . For example, the CPU  71  controls the motor drivers  74  to  76 , the solenoid drivers  77  and  78 , and the clutch drivers  80  and  81 , according to the detection signals received from the section  82 . 
     The driver  74  drives a first motor  83 . The motor  83  is used to rotate the transport rollers  61  to  63 , the registration rollers  51 , the output rollers  52 , and the transport rollers  59 . The driver  75  drives a second motor  84 . The motor  84  is used to rotate the reversing roller  58 . The driver  76  drives a third motor  85 . The motor  85  is used to rotate the transport rollers  54  to  57 . 
     The driver  77  activates a first solenoid  86 . The solenoid  86  actuates the guide  41 . The driver  78  activates a second solenoid  87 . The solenoid  87  actuates the guide  43 . 
     The driver  80  activates a first connecting mechanism  89 . The mechanism  89  is connected to each of the motor  84  and the roller  58 . In a deactivated state, the mechanism  89  directly transmits rotation of the motor  84  to the roller  58 , so that the roller  58  is rotated in a forward direction to guide a sheet into the section  2 . In an activated state, meanwhile, the mechanism  89  transmits, to the roller  53 , rotation in an opposite direction to a rotational direction of the motor  84 . Thus, the roller  58  is rotated in a backward direction to eject a sheet from the section  2 . Alternatively, a motor that is rotatable in forward and backward directions may be directly connected to the roller  58  in order to allow the roller  58  to be rotated in forward and backward directions. 
     The driver  81  activates a second connecting mechanism  90 . The mechanism  90  is connected to each of the motor  85  and the rollers  54  to  57 . In a deactivated state, the mechanism  90  directly transmits rotation of the motor  85  to the rollers  54  to  57 . In an activated state, meanwhile, the mechanism  90  transmits, to the rollers  54  to  57 , rotation in an opposite direction to a rotational direction of the motor  85 . 
     The apparatus  100  selectively performs a face-up transport operation, a face-down transport operation, and a duplex printing operation. In the face-up transport operation, a sheet with an image formed on a single side is output to the tray  38 , with the image-formed side facing upward. In the face-down transport operation, a sheet with an image formed on a single side is output to the tray  38 , with the image-formed side facing downward. In the duplex printing operation, an image is formed on each side of a sheet. 
     When an original document is to be copied onto a sheet, the face-up transport operation is performed in which the sheet is output to the tray  38 , with the image-formed side facing upward. This is because the operator is near the apparatus  100  and ready to check the copied image on the sheet. 
     In the face-up transport operation, the CPU  71  rotates the motor  83  through the driver  74 . A sheet fed from the unit  400  is transported along the path  11  by the transport rollers  61  to  63 , the registration rollers  51 , and the output rollers  52 . A toner image is formed on an upper side of the sheet while the sheet is being passed through the image forming section  30 . The sheet is output to the tray  38  with the image-formed side facing upward. 
       FIG. 5  is a schematic diagram illustrating an example of face-down transport operation. In a case where the operator is not around the apparatus  100 , the face-down transport operation is performed so that an image-formed side of the output sheet cannot be seen. When images on consecutive pages of an original document are to be formed on sheets of paper, the face-down transport operation is also performed for the purpose of eliminating the need for collating the pages of the output sheets. 
     In the face-down transport operation, a sheet, after passing through the section  30 , is guided from the bifurcation  24  into the path  12 . Subsequently, the sheet is switched back in the first switchback section  2 , and then output to the tray  38  via the paths  12  and  15 . 
       FIG. 6  is a schematic diagram illustrating another example of face-down transport operation. Here too, a sheet, after passing through the section  30 , is first guided from the bifurcation  24  into the path  12 . Then, the sheet is guided into the path  13  via the path  14 . Subsequently, the sheet is output to the tray  38 , via the path  13  and then the path  15 . In this example, the path  13  and the rollers  54  to  57  make up a second switchback section. 
       FIG. 7  is a schematic diagram illustrating an example of duplex-printing transport operation. When an image is to be formed on each side of a sheet, the duplex printing operation is performed as follows. First, an image is formed on a first side of the sheet in the section  30 . Next, the sheet is reversed and returned to the section  30  where an image is formed on a second side of the sheet. And finally, the sheet is output to the tray  38 . 
     In the duplex printing operation, a sheet, after passing through the section  30 , is guided from the bifurcation  24  into the path  12 . Next, the sheet is switched back in the section  2  and then guided from the bifurcation  26  into the path  13 . Subsequently, the sheet is guided from the path  13  into the path  11 , and finally output to the tray  38  via the section  30 . 
     The CPU  71  drives the second motor  84  through the motor driver  75  by the time a leading end of the sheet passes through the bifurcation  25 . At the time, the first connecting mechanism  89  is not activated. Thus, the reversing rollers  53  and  58  are rotated in the forward directions. 
     Consequently, the sheet is guided from the bifurcation  24  into the path  12 , and into the section  2 . It is to be noted that the guide  42  is pivoted to the position indicated by the dashed line by contact with the leading end of the sheet being transported downward through the bifurcation  25 , thereby allowing downward passage of the sheet along the path  12 . 
     As the sheet is transported downward through the bifurcation  26 , a tail end of the sheet becomes nipped by the reversing rollers  53  and  58 . It is when the CPU  71  activates the mechanism  89  through the driver  80  and, at the same time, deactivates the solenoid  87 . Further, the CPU  71  drives the motor  85  through the driver  76 . At the time, the mechanism  90  is not activated. Thus, the rollers  53  and  58  are rotated in the reverse directions. Simultaneously, the rollers  54 ,  55 ,  56 , and  57  are rotated in the forward directions, and the guide  44  is pivoted to the position indicated by the solid line as in  FIG. 3 . 
     With the tail end leading, the sheet is transported, upward from the section  12 A, along the path  12  and is guided into the path  13  at the bifurcation  26 . Next, the sheet is transported along the path  13  toward the first confluence  21 . Then, the sheet is guided into the path  11  at the confluence  21 , and is transported along the path  11  to the section  30  with a second side facing the drum  31 . 
     By the time the leading end of the sheet with the second side facing upward passes through the section  30 , the CPU  71  deactivates the solenoid  86 . Thus, the guide  41  is pivoted to the position indicated by the solid line shown in  FIG. 3 . After an image is formed on the second side in the section  30 , the sheet is transported through the bifurcation  24  and output to the tray  38  by the rollers  52 . 
       FIG. 8  is a flowchart illustrating steps of a process performed in the duplex-printing operation by the CPU  71 . The CPU  71  rotates the reversing roller  58  in the forward direction in a time period between the instant when a tail end of a sheet passes between the rollers  59  and the instant when the tail end passes through the bifurcation  26  (step S 1 ). Thus, the sheet is guided into the section  2  as shown in  FIG. 9A . In the step S 1 , the CPU  71  controls the motor  84  and the mechanism  89  to determine the direction, and amount, of rotation of the roller  58 . Next, the CPU  71  waits until the roller  58  is rotated a predetermined amount (step S 2 ). When the roller  58  is rotated the predetermined amount, the CPU  71  determines that the tail end has passed through the bifurcation  26 , and brings the roller  58  to a temporary stop as shown in  FIG. 9B . When the step S 2  is completed, it becomes possible to guide the sheet into the path  13 , the tail end first. 
     Then, the CPU  71  controls the mechanism  89  to rotate the roller  58  in the backward direction as shown in  FIG. 9C  (step S 3 ). As described earlier, the circumferential length of the roller  58 , excluding the length of the flat portion, is longer than the distance between the bifurcation  26  and the rollers  54 . Thus, rotating the roller  58  a turn in the backward direction delivers the tail end of the sheet to the rollers  54 . In the present embodiment, the CPU  71  waits until the roller  58  is rotated a turn (step S 4 ). When the step S 4  is completed, the sheet reaches such a position as to be propelled by the rollers  54 . 
     When the leading end of the sheet reaches the roller  54   s , it becomes possible for the sheet to be ejected from the section  2  without being propelled by the rollers  53  and  58 . Thus, the CPU  71  brings the roller  58  to a stop with the flat portion of the roller  58  facing the roller  53  (step S 5 ). Thus, there is a space formed between the rollers  53  and  58  during a period of time when it is not necessary for the rollers  53  and  58  to transport a sheet. This allows a subsequent sheet to be guided into the section  2 , as shown in  FIG. 9E , before a preceding sheet is ejected out of the path  12 A. This shortens intervals at which a series of sheets are guided into the section  2  to be successively switched back. 
     In the present embodiment, the roller  58  is controlled in such a manner as to apply, to a sheet, a minimum propelling force required for switching back the sheet. Thus, the space formed between the rollers  53  and  58  is maintained for a long time period. This facilitates guiding a sheet into the path  12 A when a preceding sheet is ejected from the path  12 A. 
       FIG. 10  is a timing chart illustrating operating conditions of the transport rollers  59 , the reversing roller  58 , and the transport rollers  54  in a case where three sheets are successively switched back in the first switchback section  2 . 
     In the figure, legends X 1 , X 2 , and X 3  depict respective time periods when a first sheet, a second sheet, and a third sheet are being transported toward the section  2  by the rollers  59 . Legends F 1 , F 2 , and F 3  depict respective time periods when the first sheet, the second sheet, and the third sheet are being guided into the section  2  by rotation of the roller  58  in the forward direction. Legends R 1 , R 2 , and R 3  depict respective time periods when the first, second, and third sheets are being ejected from the section  2  by rotation of the roller  58  in the backward direction. Legends Y, Y 2 , and Y 3  depict respective time periods when the first, second, and third sheets are being transported toward the first confluence  21  by the rollers  54 . 
     Further, legend Z 1  depicts a time period when the first and second sheets are passing each other in the space between the rollers  53  and  58 , and legend Z 2  depicts a time period when the second and third sheets are passing each other in the space. 
     Thus, the space formed between the rollers  53  and  58  allow two sheets to pass each other in a single transport path. This is effective not only in the duplex-printing operation, but also in the face-down transport operation where a sheet is output face-down to the tray  38  via the section  2 . Also, application of a half-moon roller to the transport rollers  54  allows sheets to pass each other also in the second switchback section. 
     It is to be noted that the CPU  71  may alternatively bring the roller  58  to a stop, with the flat portion of the roller  58  facing the roller  53 , in a time period when at least both of the transport rollers  59  and the transport rollers  54  are being rotated, i.e., in a time period when a first sheet is being ejected from the path  12 A and a second sheet immediately following the first sheet is being guided into the path  12 A. Although the first and second sheets are more likely to pass each other in the path  12 A in this particular time period, the space formed between the rollers  58  and  53  reduces sheet transport failures. 
     In addition, it is preferable that the CPU  71  controls the rollers  59  in such a manner that the second sheet is delivered to between the rollers  58  and  53  at a time when the first sheet reaches the roller  54 . Such control allows a minimum interval at which the first and second sheets are guided into the section  2 . 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.