Abstract:
An image forming device includes a main body unit; an image forming device disposed in the main body unit; a first structural member capable of dividing and moving relative to a third structural member; a second structural member capable of dividing and moving relative to the third structural member and the first structural member; a first transport path situated between the first structural member and the second structural member; a second transport path situated between the second structural member and the third structural member; a sensor for detecting the medium; a first positioning unit for restricting a movement of the second structural member; a second positioning unit for engaging the first structural member with the third structural member; and a control unit for sending a signal to restrict a movement of the first structural member to a specific position based on a detection result of the sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a divisional application of the prior application Ser. No. 11/188,758, filed Jul. 26, 2005, allowed. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to an image forming device such as a printer capable of automatic duplex printing. 
         [0003]    In a conventional image forming device of an electric photography type such as a printer, a copier, and a facsimile, a plurality of image forming units is arranged along a medium transporting member in a direction that the medium transporting member transports a medium, and the medium transporting member transports the medium to the image forming units. Each of the image forming units is provided with a detachable print process cartridge for forming a toner image. 
         [0004]    The print process cartridge includes a photo-sensitive member for forming an image on a surface thereof; a charging unit for charging the photo-sensitive member; a developing unit for attaching toner on a static latent image on the surface of the photo-sensitive member; a cleaning unit for cleaning remaining toner on the surface of the photo-sensitive member; a toner cartridge for supplying toner to the developing unit; and a drive transmitting unit for driving the charging unit, the photo-sensitive member, and the developing unit. 
         [0005]    Each of the image forming units includes an exposing unit for exposing the surface of the photo-sensitive member to form the static latent image on the surface of the photo-sensitive member. The developing unit of the print process cartridge includes a developing roller for pressing the surface of the photo-sensitive member and attaching toner to the static latent image to form a toner image; a toner supply roller for supplying the toner received from the toner cartridge to the developing roller. A transfer unit is disposed to face each of the image forming units for transferring the toner image on the photo-sensitive member to a recording medium while transporting the recording medium. 
         [0006]    Each of the image forming units further includes a fixing unit for fixing the toner image transferred at the transfer unit while transporting the recording medium. The fixing unit includes a heat roller for heating and transporting the recording medium, and a pressure roller for pressing the recording medium against the heat roller. A duplex transport path is provided for transporting the recording medium toward a downstream side of the photo-sensitive member in a transport direction to transfer another toner image on the photo-sensitive member after the fixing unit fixes the toner image. 
         [0007]    When the image forming device performing the duplex printing to a plurality of media, while a first sheet is transported in a switchback path, a next sheet and a sheet after the next sheet are transported and printed in the image forming device concurrently. 
         [0008]    When a sheet is jammed during transportation, in an image forming device disclosed in Japanese Patent Publication No. 08-151166, a housing is divided into an upper housing and a lower housing (device main body), so that the upper housing is opened around a hinge mechanism disposed on a side surface of the lower housing to remove the jammed sheet. That is, after a sheet transport path is opened, the jammed sheet in the sheet transport path is identified and removed. Accordingly, after the sheet transport path is opened, it is necessary to move a part of the sheet transport path so that an operator can find the jammed sheet, thereby making the operation troublesome. 
         [0009]    In view of the problem described above, an object of the present invention is to provide an image forming device, in which it is possible to easily remove a jammed sheet. 
         [0010]    Further objects and advantages of the invention will be apparent from the following description of the invention. 
       SUMMARY OF THE INVENTION 
       [0011]    In order to attain the objects described above, according to the present invention, an image forming device comprises a transport path formed of a plurality of structural members connected with each other; a plurality of jam detection devices disposed at proper locations along the transport path; and a dividing and opening control device for dividing and opening the transport path according to a location where a jam occurs when one of the jam detection devices detects the jam, so that the jam is recovered at the location. 
         [0012]    In the invention, when a jam occurs during printing, one of the jam detection devices detects a location of the jam in the transport path. The dividing and opening control device divides and opens the transport path, so that the jam is easily recovered at the location. Accordingly, it is possible to easily identify the jammed sheet and recover the jam. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]      FIG. 1  is a sectional view showing an image forming device according to a first embodiment of the present invention; 
           [0014]      FIG. 2  is a schematic block diagram showing a control system according to the first embodiment of the present invention; 
           [0015]      FIG. 3  is a flow chart showing a jam recovery process according to the first embodiment of the present invention; 
           [0016]      FIG. 4  is a schematic sectional view showing the image forming device according to the first embodiment of the present invention; 
           [0017]      FIG. 5  is a sectional view showing an operation (No. 1) of the image forming device according to the first embodiment of the present invention; 
           [0018]      FIG. 6  is a sectional view showing an operation (No. 2) of the image forming device according to the first embodiment of the present invention; 
           [0019]      FIG. 7  is a sectional view showing an image forming device according to a second embodiment of the present invention; 
           [0020]      FIG. 8  is an enlarged view of a part  8  in  FIG. 7 ; 
           [0021]      FIG. 9  is a schematic block diagram showing a control system according to the second embodiment of the present invention; 
           [0022]      FIG. 10  is a flow chart showing a jam recovery process according to the second embodiment of the present invention; 
           [0023]      FIG. 11  is a sectional view showing an image forming device according to a third embodiment of the present invention; 
           [0024]      FIG. 12  is a view showing an essential part of the image forming device according to the third embodiment of the present invention; 
           [0025]      FIG. 13  is a schematic block diagram showing a control system according to the third embodiment of the present invention; 
           [0026]      FIG. 14  is a flow chart showing a jam recovery process according to the third embodiment of the present invention; 
           [0027]      FIG. 15  is a sectional view showing an operation (No. 1) of the image forming device according to the third embodiment of the present invention; 
           [0028]      FIG. 16  is a sectional view showing an operation (No. 2) of the image forming device according to the third embodiment of the present invention; and 
           [0029]      FIG. 17  is a sectional view showing an operation (No. 3) of the image forming device according to the third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0030]    Hereunder, embodiments of the present invention will be explained with reference to the accompanying drawings. An image forming device includes a plurality of structural members with a same rotational center, so that the structural members can be divided and opened in a same rotational direction. Further, the image forming device includes a dividing and opening device formed of a link for connecting the structural members to be slidable and an electromagnetic clutch for controlling the structural members to slide, thereby reducing the number of parts. 
       FIRST EMBODIMENT 
       [0031]      FIG. 1  is a sectional view showing an image forming device  100  according to a first embodiment of the present invention. The image forming device  100  includes a cassette  1  for storing printing sheets P; a sheet supply auxiliary roller  3 ; a sheet supply roller  4 ; a printing sheet receiving plate  2  for pressing the printing sheet P against the sheet supply auxiliary roller  3  and the sheet supply roller  4 ; a separation roller  5  for separating the printing sheets P transported with the sheet supply auxiliary roller  3  and the sheet supply roller  4 ; and a resist roller  6  for collecting skew of the printing sheet P and transporting the printing sheet P to a transfer belt  11 . 
         [0032]    The image forming device  100  further includes an exposure device  7  for irradiating a laser beam; a plurality of photo-sensitive drums  8  for receiving the laser beam of the exposure device  7  to form a static latent image; a plurality of developing units  9  for attaching toner to the static latent image formed on surfaces of the photo-sensitive drums  8  with the laser beam of the exposure device  7 ; a frame  10  for positioning the developing units  9  to be detachable; and the transfer belt  11  for transferring the image to the printing sheet P transported from the photo-sensitive drums  8 . The transfer belt  11  includes a plurality of transfer rollers  12  disposed therein and facing the photo-sensitive drums  8 , respectively. Accordingly, the frame  100  is provided with an image forming transport path  106  between the photo-sensitive drums  8  and the transfer belt  11  for transporting the printing sheets P. 
         [0033]    The image forming device  100  further includes a fixing device  14  for fixing the toner image transferred on the printing sheet P with the transfer belt  11  while transporting the printing sheet P; a discharge roller  15  for transporting the printing sheet P passing through the fixing device  14 ; a discharge tray  16  for stacking the printing sheet P transported from the discharge roller  15 ; a sheet guide  35  disposed on the frame  10  between the sheet supply roller  4  and the resist roller  6 ; a sheet guide  37  disposed at a downstream side of the fixing device  14  for guiding the printing sheet P from the fixing device  14  to a transport path switching guide  17 . The sheet guide  35  is provided with a sensor  36  for detecting the printing sheet P moving toward the resist roller  6 . The sheet guide  37  is also provided with a sensor  38  for detecting the printing sheet P transported from the fixing device  14 . 
         [0034]    The image forming device  100  further includes a duplex transport path  18  for transporting the printing sheet P switched back with the discharge roller  15  rotating in reverse during duplex printing. The duplex transport path  18  is provided with a plurality of transport rollers  19  and a sheet re-supply roller  20  for transporting the printing sheet P to the resist roller  6  one more time. The duplex transport path  18  is also provided with a sheet guide  21   a  disposed on a first structure member  21  situated outside the device. The first structure member  21  is provided with the discharge rollers  19 , and sensors  22  ( 22 - 1  and  22 - 2 ) for detecting the printing sheet P in addition to the sheet guide  21   a.    
         [0035]    The first structural member  21  is supported on the frame  10  with a rotational pivot  23 . A handle  24  disposed at an upper portion of the first structural member  21  engages a protrusion  25  disposed on the frame  10  to fix the first structural member  21 . The duplex transport path  18  has a sheet guide  26   a  disposed on a second structural member  26  situated inside the device. The second structure member  26  is provided with the transfer belt  11  in addition to the sheet guide  26   a.  The second structural member  26  is supported on the frame  10  with the rotational pivot  23 , and is fixed and sandwiched between the first structural member  21  and the developing units  9  having the photo-sensitive drums  8 . 
         [0036]    The second structural member  26  has solenoids  28  and  29  and a sensor  27  for detecting the printing sheet P. When the solenoid  28  moves reciprocally, a latch  30  rotates, so that the second structural member  26  engages or disengages from a boss  31  disposed on the first structural member  21 , thereby connecting or opening the first structural member  21 . When the solenoid  29  moves reciprocally, a link  33  is driven reciprocally to rotate latches  32 , so that the second structural member  26  engages or disengages from bosses  34  disposed on the frame  10 , thereby connecting or opening the second structural member  26  relative to the frame  10 . 
         [0037]    An operation of connecting and opening the first structural member  21  and the second structural member  26 , and an operation of connecting and opening the second structural member  26  and the frame  10  will be described in more detail later. A control unit  101  is electrically connected through an electrical signal system for controlling the device as a whole including the operations. 
         [0038]      FIG. 2  is a schematic block diagram showing connection of the control unit  101  according to the first embodiment of the present invention. The control unit  101  is connected to a display unit  102 ; an image data receiving unit  105 ; a timer  103 ; and sensors  36 ,  38 ,  27 ,  22 - 1 , and  22 - 2  for detecting a state of the printing sheet P during transportation. Further, the control unit  101  is connected to a drive source  104  and a sheet supply drive source  107  to be controlled. 
         [0039]    The control unit  101  controls the sheet supply drive source  107  to rotate the sheet supply roller  4 , the sheet supply auxiliary roller  3 , and the separation roller  5  to transport the printing sheet P to the photo-sensitive drum one at a time in a specific interval. The control unit  101  controls the drive source  104  to transmit drive to the photo-sensitive drums  8 , the developing units  9 , the transfer roller  12 , the belt roller  13 , the fixing device  14 , the discharge roller  15 , the discharge rollers  19  and the sheet re-supply roller  20  to form an image on the printing sheet P while transporting the printing sheet P. 
         [0040]    The control unit  101  is also connected to the exposure device  7  to expose the photo-sensitive drums  8 , so that a static latent image is formed on the photo-sensitive drums  8  according to image data received at the image data receiving unit  105 , positional information of the printing sheet P from each sensor, and timing information of the timer  103 . A belt roller  13  is provided for rotating the transfer belt  11  along with a transfer roller  12 . The control unit  101  is also connected to solenoids  28  and  29 . 
         [0041]    An operation of the image forming device according to the first embodiment will be explained next. First, with reference to  FIG. 1 , an operation of the image forming device as a whole will be explained. Then, with reference to other drawings, an operation of detecting the jam during the transportation of the printing sheet P and a process of recovering the jam will be explained in detail. 
         [0042]    When the image forming device is not turned on, or is in an initial state, the solenoid  28  is not turned on, so that the solenoid  28  is in a stretched state due to gravity. In the stretched state of the solenoid  28 , the latch  30  connected to the solenoid  28  rotates to engage the boss  31 , so that the first structural member  21  and the second structural member  26  are in a locked state. When the image forming device is in the initial state, the solenoid  29  is also not turned on, so that the solenoid  29  is in a stretched state due to gravity. In the stretched state of the solenoid  29 , a latch  32 - 1  connected to the solenoid  29  rotates to engage a boss  34 - 1  and a latch  32 - 2  rotates to engage a boss  34 - 2 , so that the first structural member  21  and the second structural member  26  are in a locked state. 
         [0043]    When the image forming device starts printing, the sheet supply auxiliary roller  3  picks up the printing sheet P on the cassette  1 , and the sheet supply roller  4  and the separation roller  5  sandwich the printing sheet P, so that the printing sheet P is transported to the resist roller  6  in a separated state one at a time. The printing sheet P transported with the sheet supply roller  4  abuts against the resist roller  6 , thereby correcting skew. The printing sheet P is transported to the transfer roller  12  at such a timing that the toner image formed on a surface of the photo-sensitive drums  8  approaches the transport path with rotation of the photo-sensitive drums  8 . The toner image is transferred from the surface of the photo-sensitive drums  8  to the printing sheet P, and is fixed when the printing sheet P passes through the fixing device  14 . When the image forming device is not performing the duplex printing, or an image is formed on the printing sheet P for the second time, the discharge roller  15  discharges the printing sheet P to the discharge tray  16 . 
         [0044]    When the image forming device is performing the duplex printing, the discharge roller  15  rotates forward to transport the printing sheet P until a trailing edge thereof passes through the transport path switching guide  17 . At this time, in a state that the discharge roller  15  holds the printing sheet P, the discharge roller  15  reverses the rotation from the forward rotation to the reverse rotation to switch back the printing sheet P. At the same time, the transport path switching guide  17  switches the transport path, so that the printing sheet P is transported to the duplex transport path  18  through the reverse rotation of the discharge roller  15 . Then, the discharge rollers  19  and the sheet re-supply roller  20  transport the printing sheet P to the resist roller  6  one more time to form an image for the second time. After the image is formed on the printing sheet P for the second time and the printing sheet P passes through the fixing device  14 , the discharge roller  15  discharges the printing sheet P to the discharge tray  16 , thereby completing the second image formation. 
         [0045]    The process of detecting the jam and the process of recovering the jam will be explained next. With reference to  FIG. 3 , the process of detecting the jam and the process of recovering the jam will be explained in detail from step S 1 - 1  to step S 1 - 14  during the transportation of the printing sheet P. Symbols in the flow chart are defined as follows. T 1 (s) is an estimated time from when the sheet supply roller  4  starts rotating to transport the printing sheet P to when the printing sheet P passes through the sensor  27  ( FIG. 1 ). T 2 (s) is an estimated time from when the sheet supply roller  4  starts rotating to transport the printing sheet P to when the printing sheet P passes through the sensor  38  ( FIG. 1 ). T 3 (s) is an estimated time from when the sheet supply roller  4  starts rotating to transport the printing sheet P to when the printing sheet P passes through the sensor  22 - 1  ( FIG. 1 ). T 4 (s) is an estimated time from when the sheet supply roller  4  starts rotating to transport the printing sheet P to when the printing sheet P passes through the sensor  22 - 2  ( FIG. 1 ). The estimated times T 1 (s) to T 4 (s) are set in the timer  103  in the control unit  101  in advance. 
       Step S 1 - 1   
       [0046]    The printing process starts, and the printing sheet P starts being transported from the cassette  1  ( FIG. 1 ). At the same time, the timer  103  of the control unit  101  starts measuring time. 
       Step S 1 - 2   
       [0047]    When the sensor  27  ( FIG. 1 ) does not detect the printing sheet P within T 1 (s) since the sheet supply roller  4  starts rotating, the process proceeds to step S 1 - 3 . When the sensor  27  detects the printing sheet P, the process proceeds to step S 1 - 5 . 
       Step S 1 - 3   
       [0048]    Since the sensor  27  ( FIG. 1 ) does not detect the printing sheet P within T 1 (s) since the sheet supply roller  4  starts rotating, it is determined that the jam occurs during the transportation and the printing sheet P remains in the image forming transport path  106  between the developing units  9  ( FIG. 1 ) and the transfer belt  11  ( FIG. 1 ). The control unit  101  stops the drive source  104  and the sheet supply drive source  107  to stop the image formation process, and drives the solenoid  29  ( FIG. 4 ) to rotate the latches  32 - 1  and  32 - 2  ( FIG. 4 ) through the reciprocal movement of the link  33  ( FIG. 4 ), thereby releasing the latches  32 - 1  and  32 - 2  from the bosses  34 - 1  and  34 - 2  ( FIG. 4 ). 
       Step S 1 - 4   
       [0049]    The control unit  101  controls the display unit  102  to display the occurrence of the jam to notify an operator, thereby requesting removal of the printing sheet P remaining due to the jam. When the operator pulls up the handle  24  ( FIG. 1 ) to remove the printing sheet P causing the jam, the handle  24  ( FIG. 1 ) is released from the protrusion  25  ( FIG. 1 ) disposed on the frame  10  ( FIG. 1 ). Accordingly, as shown in  FIG. 5 , the first structural member  21  and the second structural member  26  rotate around the rotational pivot  23  and open. At this time, the latch  30  of the second structural member  26  engages the boss  31  of the first structural member  21 , so that the first structural member  21  and the second structural member  26  are integrated. As described above, when the printing sheet P is jammed in the image forming transport path  106  between the developing units  9  ( FIG. 1 ) and the transfer belt  11  ( FIG. 1 ), the location of the jam where the printing sheet P remains is opened to a large extent with the single handle operation, thereby making it easy to find the jammed sheet and recover the jam. 
       Step S 1 - 5   
       [0050]    When the sensor  38  ( FIG. 1 ) does not detect the printing sheet P within T 2 (s) since the sheet supply roller  4  starts rotating, it is determined that the jam occurs during the transportation between the developing units  9  ( FIG. 1 ) and the transfer belt  11  ( FIG. 1 ) and the printing sheet P remains in the image forming transport path  106  between the developing units  9  ( FIG. 1 ) and the transfer belt  11  ( FIG. 1 ). Accordingly, similar to the case when the sensor  27  ( FIG. 1 ) does not detect the printing sheet P within T 1 (s), the process proceeds to steps S 1 - 3  and S 1 - 4  to recover the jam. When the sensor  38  ( FIG. 1 ) detects the printing sheet P within T 2 (s), the process proceeds to step S 1 - 6 . 
       Step S 1 - 6   
       [0051]    When the image forming device is not performing the duplex printing (printing one side), the process proceeds to step S 1 - 7 . When the image forming device is performing the duplex printing (printing both sides), the process proceeds to step S 1 - 9 . 
       Step S 1 - 7   
       [0052]    When there is next print data, the process proceeds to step S 1 - 8 . When there is no print data, the process stops. 
       Step S 1 - 8   
       [0053]    The control unit  101  sets the timer  103  to zero, and the process returns to step S 1 - 1  for starting the next printing process. 
       Step S 1 - 9   
       [0054]    In a case that the duplex printing needs to be performed and both sides are printed, the process proceeds to step S 1 - 7 , so that the process of printing one side in step S 1 - 6  is performed. On the other hand, in a case that the duplex printing needs to be performed and only one side is printed, the process proceeds to step S 1 - 10 . 
       Step S 1 - 10   
       [0055]    When the sensor  22 - 1  ( FIG. 1 ) does not detect the printing sheet P within T 3 (s) since the sheet supply roller  4  starts rotating, the process proceeds to step S 1 - 11 . When the sensor  22 - 1  detects the printing sheet P, the process proceeds to step S 1 - 13 . 
       Step S 1 - 11   
       [0056]    Since the sensor  22 - 1  ( FIG. 1 ) does not detect the printing sheet P within T 3 (s) since the sheet supply roller  4  starts rotating, it is determined that the jam occurs at an entrance of the duplex transport path  18  and the printing sheet P remains in the duplex transport path  18 . The control unit  101  stops the drive source  104  and the sheet supply drive source  107  to stop the image formation process, and drives the solenoid  28  ( FIG. 4 ) to rotate the latch  30  ( FIG. 4 ), thereby releasing the latch  30  from the boss  31  ( FIG. 4 ). 
       Step S 1 - 12   
       [0057]    The control unit  101  controls the display unit  102  to display the occurrence of the jam to notify the operator, thereby requesting removal of the jammed sheet. When the operator pulls up the handle  24  ( FIG. 1 ) to remove the jammed sheet, the handle  24  ( FIG. 1 ) is released from the protrusion  25  ( FIG. 1 ) disposed on the frame  10  ( FIG. 1 ). Accordingly, as shown in  FIG. 6 , the first structural member  21  rotates around the rotational pivot  23  and open. At this time, the latches  32 - 1  and  32 - 2  ( FIG. 4 ) of the second structural member  26  engage the bosses  34 - 1  and  34 - 2  ( FIG. 4 ) of the frame  10 , so that the frame  10  and the second structural member  26  are integrated. As described above, when the printing sheet P is jammed in the duplex transport path  18 , the location of the jam where the printing sheet P remains is opened to a large extent with the single handle operation, thereby making it easy to find the jammed sheet and recover the jam. 
       Step S 1 - 13   
       [0058]    When the sensor  22 - 2  ( FIG. 1 ) does not detect the printing sheet P within T 4 (s) since the sheet supply roller  4  starts rotating, it is determined that the jam occurs during the transportation in the duplex transport path  18  ( FIG. 1 ) and the printing sheet P remains in the duplex transport path  18  ( FIG. 1 ). Accordingly, similar to the case when the sensor  22 - 1  ( FIG. 1 ) does not detect the printing sheet P within T 3 (s), the process proceeds to steps S 1 - 11  and S 1 - 13  to recover the jam. When the sensor  22 - 2  ( FIG. 1 ) detects the printing sheet P within T 4 (s), the process proceeds to step S 1 - 14 . 
       Step S 1 - 14   
       [0059]    The control unit  101  resets the timer, and the process returns to step S 1 - 2  to print the second sheet. 
         [0060]    When a plurality of printing sheets P is transported and jammed, it is possible that the printing sheets remain both between the developing units  9  and the transfer belt  11  and in the duplex transport path  18 . In this case, once the printing sheet P is removed from the jam location, other printing sheets P are transported and discharged to the discharge tray  16 , thereby making it possible to resume the printing process. 
         [0061]    As described above, when the jam occurs during the transportation, the jam location where the printing sheet remains is detected and opened to a large extent with the single handle operation, thereby making it easy to find the jammed sheet and recover the jam. 
       SECOND EMBODIMENT 
       [0062]    In the first embodiment, it is possible to deal with the case that several printing sheets P are transported sequentially. However, the first embodiment does not consider the jam recovery in a case that the printing sheet remains both between the developing units  9  and the transfer belt  11 , and in the duplex transport path  18 . In the second embodiment, it is possible to deal with the jam recovery in a case that the printing sheet remains one of between the developing units  9  and the transfer belt  11 , and in the duplex transport path  18 , and a case that the printing sheet remains both between the developing units  9  and the transfer belt  11 , and in the duplex transport path  18  substantially at the same time. 
         [0063]      FIG. 7  is a sectional view showing an image forming device according to the second embodiment of the present invention.  FIG. 8  is an enlarged view of a part  8  in  FIG. 7 . Only differences from the first embodiment will be explained. Components same as those in the first embodiment are designated with the same reference numerals, and explanations thereof are omitted. 
         [0064]    As shown in  FIG. 8 , a solenoid  40  fixed to the frame  10  and a latch  41  rotating around a rotational pivot  42  disposed on the frame  10  when the solenoid  40  moves reciprocally are added to the image forming device in the first embodiment. As shown in  FIG. 8 , the latch  41  is arranged to engage and disengage from a projection  39  disposed near the rotational pivot  23  of the second structural member  26 . 
         [0065]      FIG. 9  is a schematic block diagram showing connection of a control system according to the second embodiment of the present invention. The control unit  101  is connected to the display unit  102  for displaying a message; the image data receiving unit  105 ; the timer  103 ; and sensors  36 ,  38 ,  27 ,  22 - 1 , and  22 - 2  for detecting transportation state of the printing sheet P. Further, the control unit  101  is connected to the drive source  104  and the sheet supply drive source  107  to be controlled. 
         [0066]    The control unit  101  controls the sheet supply drive source  107  to rotate the sheet supply roller  4 , the sheet supply auxiliary roller  3 , and the separation roller  5  to transport the printing sheet P to the photo-sensitive drum one at a time in a specific interval. The control unit  101  controls the drive source  104  to transmit drive to the photo-sensitive drums  8 , the developing units  9 , the transfer roller  12 , the belt roller  13 , the fixing device  14 , the discharge roller  15 , the discharge rollers  19  and the sheet re-supply roller  20  to form an image on the printing sheet P while transporting the printing sheet P. 
         [0067]    The control unit  101  is also connected to the exposure device  7  to expose the photo-sensitive drums  8 , so that a static latent image is formed on the photo-sensitive drums  8  according to image data received at the image data receiving unit  105 , positional information of the printing sheet P from each sensor, and timing information of the timer  103 . The belt roller  13  is provided for rotating the transfer belt  11  along with the transfer roller  12 . The control unit  101  is also connected to the solenoids  28 ,  29  and  40 . 
         [0068]    An operation of the image forming device according to the second embodiment will be explained next. An operation of the image forming device as a whole is the same as that in the first embodiment, and explanation thereof is omitted. An operation of detecting the jam during transportation of the printing sheet P and a process of recovering the jam will be explained in detail. 
         [0069]      FIG. 10  is a flow chart showing a jam recovery process according to the second embodiment of the present invention. With reference to  FIG. 10 , the process of detecting the jam and the process of recovering the jam will be explained in detail from step S 2 - 1  to step S 2 - 20  during the transportation of the printing sheet P. Symbols in the flow chart are defined as follows. T 1 (s) is an estimated time from when the sheet supply roller  4  starts rotating to transport the printing sheet P to when the printing sheet P passes through the sensor  27  ( FIG. 1 ). T 2 (s) is an estimated time from when the sheet supply roller  4  starts rotating to transport the printing sheet P to when the printing sheet P passes through the sensor  38  ( FIG. 1 ). T 3 (s) is an estimated time from when the sheet supply roller  4  starts rotating to transport the printing sheet P to when the printing sheet P passes through the sensor  22 - 1  ( FIG. 1 ). T 4 (s) is an estimated time from when the sheet supply roller  4  starts rotating to transport the printing sheet P to when the printing sheet P passes through the sensor  22 - 2  ( FIG. 1 ). T 5 (s) is a specific time since when the printing sheet P passes through the sensor  22 - 1  ( FIG. 1 ) or the sensor  22 - 2  ( FIG. 1 ). T 6 (s) is a specific time since when the printing sheet P passes through the sensor  27  ( FIG. 1 ) or the sensor  38  ( FIG. 1 ). The estimated times T 1 (s) to T 6 (s) are set in the timer  103  in the control unit  101  in advance. 
       Step S 2 - 1   
       [0070]    The supply roller  4  starts rotating to transport the printing sheet P 0 , and then the printing sheet P 1  from the cassette  1  ( FIG. 1 ). At the same time, the timer  103  of the control unit  101  starts measuring time. 
       Step S 2 - 2   
       [0071]    When the sensor  27  ( FIG. 1 ) does not detect the printing sheet P within T 1 (s) since the sheet supply roller  4  starts rotating, the process proceeds to step S 2 - 3 . When the sensor  27  detects the printing sheet P 1 , the process proceeds to step S 2 - 8 . 
       Step S 2 - 3   
       [0072]    When the sensor  22 - 1  or the sensor  22 - 2  detects the printing sheet P 0  and it is within T 5 (s), the process proceeds to step S 2 - 4 , otherwise proceeds to step S 2 - 6 . 
       Step S 2 - 4   
       [0073]    Since the sensor  27  ( FIG. 1 ) does not detect the printing sheet P within T 1 (s) since the sheet supply roller  4  starts rotating, it is determined that the jam occurs during the transportation and the printing sheet P 1  remains in the image forming transport path  106  between the developing units  9  ( FIG. 1 ) and the transfer belt  11  ( FIG. 1 ). The control unit  101  stops the drive source  104  and the sheet supply drive source  107  to stop the image formation process, and drives the solenoid  28  and the solenoid  29  ( FIG. 4 ) to separate the first structural member  21 , the second structural member  26 , and the frame  10 . Further, the control unit  101  drives the solenoid  40 , so that the projection of the second structural member  26  can engage the latch  41 . 
       Step S 2 - 5   
       [0074]    The control unit  101  controls the display unit  102  to display the occurrence of the jam to notify an operator, thereby requesting removal of the jammed sheet. When the operator pulls up the handle  24  ( FIG. 1 ) to remove the jammed sheet, the handle  24  ( FIG. 1 ) is released from the protrusion  25  ( FIG. 1 ) disposed on the frame  10  ( FIG. 1 ). Accordingly, as shown in  FIG. 5 , the first structural member  21  and the second structural member  26  rotate around the rotational pivot  23  and open. At this time, when the second structural member  26  rotates by a specific angle, the projection  39  of the second structural member  26  engages the latch  41  ( FIG. 8 ), and the second structural member  26  stops. The first structural member  21  rotates by the specific angle and stops similar to the first embodiment. As a result, as shown in  FIG. 7 , the frame  10 , the first structural member  21 , and the second structural member  26  are opened in the separated state, respectively. As described above, when the printing sheet P is jammed both between the developing units  9  ( FIG. 1 ) and the transfer belt  11  ( FIG. 1 ) and in the duplex transport path  18  ( FIG. 1 ), the locations of the jams where the printing sheets P 0  and P 1  remain are opened to a large extent with the single handle operation, thereby making it easy to find the jammed sheet and recover the jam. 
       Step S 2 - 6   
       [0075]    Since the sensor  27  ( FIG. 1 ) does not detect the printing sheet P 1  within T 1 (s), it is determined that the jam occurs during the transportation and the printing sheet P 1  remains in the image forming transport path  106  between the developing units  9  ( FIG. 1 ) and the transfer belt  11  ( FIG. 1 ). Also, it has passed T 5 (s) since the sensor  22 - 1  or  22 - 2  detects the printing sheet P 0 , so that it is determined that the printing sheet P 0  does not remain in the duplex transport path  18  (FIG.  1 ). The control unit  101  stops the drive source  104  and the sheet supply drive source  107  to stop the image formation process, and drives the solenoid  29  ( FIG. 4 ) to rotate the latches  32 - 1  and  32 - 2  ( FIG. 4 ) through the reciprocal movement of the link  33  ( FIG. 4 ), thereby releasing the engagement with the latches  32 - 1  and  32 - 2  ( FIG. 4 ). 
       Step S 2 - 7   
       [0076]    The control unit  101  controls the display unit  102  to display the occurrence of the jam to notify and requests the operator to remove the jammed sheet P 1 . When the operator pulls up the handle  24  ( FIG. 1 ) to remove the jammed sheet, the handle  24  ( FIG. 1 ) is released from the protrusion  25  ( FIG. 1 ) disposed on the frame  10  ( FIG. 1 ). Accordingly, similar to the first embodiment shown in  FIG. 5 , the first structural member  21  and the second structural member  26  rotate around the rotational pivot  23  and open. Accordingly, when the jam occurs only between the developing units  9  ( FIG. 1 ) and the transfer belt  11  ( FIG. 1 ), the location of the jam where the printing sheet P 1  remains is opened to a large extent with the single handle operation, thereby making it easy to find the jammed sheet and recover the jam. 
       Step S 2 - 8   
       [0077]    When the sensor  38  ( FIG. 1 ) does not detect the printing sheet P 1  within T 2 (s), it is determined that the jam occurs during the transportation between the developing units  9  ( FIG. 1 ) and the transfer belt  11  ( FIG. 1 ) and the printing sheet P 1  remains in the image forming transport path  106  between the developing units  9  ( FIG. 1 ) and the transfer belt  11  ( FIG. 1 ). Accordingly, similar to step S 2 - 2  when the sensor  27  ( FIG. 1 ) does not detect the printing sheet P 1  within T 1 (s), the process proceeds to steps S 2 - 4  or S 2 - 6  through step  2 - 3  to recover the jam. When the sensor  38  ( FIG. 1 ) detects the printing sheet P 1  within T 2 (s), the process proceeds to step S 2 - 9 . 
       Step S 2 - 9   
       [0078]    When the image forming device is not performing the duplex printing (printing one side), the process proceeds to step S 2 - 10 . When the image forming device is performing the duplex printing (printing both sides), the process proceeds to step S 2 - 12 . 
       Step S 2 - 10   
       [0079]    When there is next print data, the process proceeds to step S 2 - 11 . When there is no print data, the process stops. 
       Step S 2 - 11   
       [0080]    The control unit  101  resets the timer  103 , and the process returns to step S 2 - 1  for starting the next printing process. 
       Step S 2 - 12   
       [0081]    In a case that the duplex printing needs to be performed and both sides are printed, the process proceeds to step S 2 - 10 , so that the process of printing one side in step S 2 - 9  is performed. On the other hand, in a case that the duplex printing needs to be performed and only one side is printed, the process proceeds to step S 2 - 13 . 
       Step S 2 - 13   
       [0082]    When the sensor  22 - 1  ( FIG. 1 ) does not detect the printing sheet P within T 3 (s) since the sheet supply roller  4  starts rotating, the process proceeds to step S 2 - 14 . When the sensor  22 - 1  detects the printing sheet P, the process proceeds to step S 2 - 19 . 
       Step S 2 - 14   
       [0083]    When it is within T 6 (s) since the sensor  27  or the senor  38  detects the printing sheet P 2 , the process proceeds to step S 2 - 15 , otherwise proceeds to step S 2 - 17 . 
       Step S 2 - 15   
       [0084]    Since the sensor  22 - 1  ( FIG. 1 ) does not detect the printing sheet P 1  within T 3 (s) since the sheet supply roller  4  starts rotating, it is determined that the jam occurs in the duplex transport path  18  ( FIG. 1 ) and the printing sheet P 1  remains in the duplex transport path  18  ( FIG. 1 ). Further, since it is within T 6 (s) since the sensor  27  or the sensor  38  detects the printing sheet P 2 , it is determined that the printing sheet P 2  still remains in the image forming transport path  106  between the developing units  9  ( FIG. 1 ) and the transfer belt  11  ( FIG. 1 ). The control unit  101  stops the drive source  104  and the sheet supply drive source  107  to stop the image formation process, and drives the solenoid  28  and the solenoid  29  to separate the first structural member  21 , the second structural member  26 , and the frame  10 , respectively. The control unit  101  also drives the solenoid  40 , so that the projection  39  of the second structural member  26  can engage the latch  41 . 
       Step S 2 - 16   
       [0085]    The control unit  101  controls the display unit  102  to display the occurrence of the jam to notify and requests the operator to remove the jammed sheet (printing sheet P). When the operator pulls up the handle  24  ( FIG. 1 ) to remove the jammed sheet, the handle  24  ( FIG. 1 ) is released from the protrusion  25  ( FIG. 1 ) disposed on the frame  10  ( FIG. 1 ). Accordingly, similar to the first embodiment shown in  FIG. 5 , the first structural member  21  and the second structural member  26  rotate around the rotational pivot  23  and open. At this time, when the second structural member  26  rotates by a specific angle, the projection  39  of the second structural member  26  engages the latch  41 , and the second structural member  26  stops at the specific angle. The first structural member  21  rotates by the specific angle and stops similar to the first embodiment. As a result, as shown in  FIG. 7 , the frame  10 , the first structural member  21 , and the second structural member  26  are opened in the separated state, respectively. As described above, when the jams occur both between the developing units  9  ( FIG. 1 ) and the transfer belt  11  ( FIG. 1 ) and in the duplex transport path  18  ( FIG. 1 ), the locations of the jams where the printing sheets P 1  and P 2  remain are opened to a large extent with the single handle operation, thereby making it easy to find the jammed sheet and recover the jam. 
       Step S 2 - 17   
       [0086]    Since the sensor  22 - 1  ( FIG. 1 ) does not detect the printing sheet P within T 3 (s) since the sheet supply roller  4  starts rotating, it is determined that the jam occurs at an entrance of the duplex transport path  18  and the printing sheet P 1  remains in the duplex transport path  18 . Further, with the sensor  27  or the sensor  38 , it is determined that the printing sheet P 2  does not remain in the image forming transport path  106  between the developing units  9  ( FIG. 1 ) and the transfer belt  11  ( FIG. 1 ). The control unit  101  stops the drive source  104  and the sheet supply drive source  107  to stop the image formation process, and drives the solenoid  28  ( FIG. 4 ) to rotate the latch  30  ( FIG. 4 ), thereby releasing the latch  30  from the boss  31  ( FIG. 4 ). 
       Step S 2 - 18   
       [0087]    The control unit  101  controls the display unit  102  to display the occurrence of the jam to notify the operator, thereby requesting removal of the jammed sheet. When the operator pulls up the handle  24  ( FIG. 1 ) to remove the jammed sheet, the handle  24  ( FIG. 1 ) is released from the protrusion  25  ( FIG. 1 ) disposed on the frame  10  ( FIG. 1 ). Accordingly, as shown in  FIG. 6 , the first structural member  21  rotates around the rotational pivot  23  and opens. At this time, the latches  32 - 1  and  32 - 2  ( FIG. 4 ) of the second structural member  26  engage the bosses  34 - 1  and  34 - 2  ( FIG. 4 ) of the frame  10 , so that the frame  10  and the second structural member  26  are integrated. As described above, when the jam occurs in the duplex transport path  18 , the location of the jam where the printing sheet P remains is opened to a large extent with the single handle operation, thereby making it easy to find the jammed sheet and recover the jam. 
       Step S 2 - 19   
       [0088]    When the sensor  22 - 2  ( FIG. 1 ) does not detect the printing sheet P 1  within T 4 (s) since the sheet supply roller  4  starts rotating, it is determined that the jam occurs in the duplex transport path  18  ( FIG. 1 ) during the transportation and the printing sheet P 1  remains in the duplex transport path  18  ( FIG. 1 ). Accordingly, similar to step S 2 - 13  in which the sensor  22 - 1  ( FIG. 1 ) does not detect the printing sheet P 1  within T 3 (s) since the sheet supply roller  4  starts rotating, the process proceeds to step S 2 - 15  or step S 2 - 17  through step S 2 - 14  for similar jam recovery. When the sensor  22 - 2  ( FIG. 1 ) detects the printing sheet P within T 4 (s), the process proceeds to step S 2 - 20 . 
       Step S 2 - 20   
       [0089]    The control unit  101  resets the timer to zero, and the process returns to step S 2 - 2  to print the second sheet. 
         [0090]    As described above, the jam location where the printing sheet remains is detected and the structural members are connected or opened. Accordingly, when the jams occur at several locations, or the jam occurs at one location and the printing sheet at other location needs to be removed, it is easy to find the jammed sheet and recover the jam. 
       THIRD EMBODIMENT 
       [0091]    In the first embodiment, as shown in  FIG. 3 , the second structural member  26  is provided with the solenoids  28  and  29 . Accordingly, it is possible to connect the first structural member  21  and the second structural member  26  to the frame  10  and open in the state that the first structural member  21  and the second structural member  26  are integrated, or to connect the second structural member  26  to the first structural member  21  and open in the state that the second structural member  26  and the frame  10  are integrated. In the second embodiment, as shown in  FIG. 8 , in addition to the configuration in the first embodiment, the frame  10  is provided with the solenoid  40 . Accordingly, it is possible to open between the first structural member  21  and the second structural member  26 , and between the second structural member  26  and the frame  10  at the same time. In the third embodiment, instead of the solenoids  28 ,  29 , and  40 , electromagnetic clutches are disposed between the first structural member  21  and the second structural member  26 , and between the second structural member  26  and the frame  10 , respectively, to constitute controllable links, thereby obtaining effects of the first and second embodiments. 
         [0092]      FIG. 11  is a sectional view showing an image forming device according to the third embodiment of the present invention.  FIG. 12  is an enlarged view showing an essential part of the image forming device according to the third embodiment of the present invention. Only differences from the first embodiment will be explained. Components same as those in the first embodiment are designated with the same reference numerals, and explanations thereof are omitted. 
         [0093]    As shown in  FIG. 11 , an image forming device  101  is provided with a frame  55  for mounting each component. The frame  55  is provided with a first structural member  43 . The first structural member  43  is supported on the frame  55  with the rotational pivot  23 . The handle  24  disposed at an upper portion of the first structural member  43  engages the protrusion  25  disposed on the frame  55  to fix the first structural member  43 . The frame  55  is also provided with a second structural member  44 . The second structural member  44  is supported on the frame  55  with the rotational pivot  23 , and fixed between the first structural member  43  and the developing units  9 . 
         [0094]    As shown in  FIG. 12 , the second structural member  44  is provided with an electromagnetic clutch  46 . The electromagnetic clutch  46  is attached to a shaft  45  fixed to the second structural member  44 , and the shaft  45  has a gear portion rotatable around the shaft  45  in a non-energized state. The electromagnetic clutch  46  has a gear and engages a link  48 . The link  48  has a lack gear  47  to sandwich the electromagnetic clutch  46  and engage the gear of the electromagnetic clutch  46 . The link  48  is rotatably attached to a boss  49  having one end portion disposed on the first structural member  43 . The frame  55  has a shaft  50  fixed to the frame  55  and an electromagnetic clutch  51  having a gear portion rotatable around the shaft  50  in a non-energized state. The electromagnetic clutch  51  has a gear and engages a link  52 . The link  52  has a lack gear  53  to sandwich the electromagnetic clutch  51  and engage the gear of the electromagnetic clutch  51 . The link  52  is rotatably attached to a boss  54  having one end portion disposed on the second structural member  44 . 
         [0095]    An operation of the image forming device according to the third embodiment will be explained next. An operation of the image forming device as a whole is the same as that in the first embodiment, and explanation thereof is omitted. An operation of detecting the jam during transportation of the printing sheet P and a process of recovering the jam will be explained in detail. 
         [0096]    With reference to  FIG. 14 , the process of detecting the jam and the process of recovering the jam will be explained in detail from step S 3 - 1  to step S 3 - 18  during the transportation of the printing sheet P. Symbols in the flow chart are defined as follows. T 1 (s) is an estimated time from when the sheet supply roller  4  starts rotating to transport the printing sheet P to when the printing sheet P passes through the sensor  27  ( FIG. 11 ). T 2 (s) is an estimated time from when the sheet supply roller  4  starts rotating to transport the printing sheet P to when the printing sheet P passes through the sensor  38  ( FIG. 11 ). T 3 (s) is an estimated time from when the sheet supply roller  4  starts rotating to transport the printing sheet P to when the printing sheet P passes through the sensor  22 - 1  ( FIG. 11 ). T 4 (s) is an estimated time from when the sheet supply roller  4  starts rotating to transport the printing sheet P to when the printing sheet P passes through the sensor  22 - 2  ( FIG. 11 ). T 5 (s) is a specific time since when the printing sheet P passes through the sensor  22 - 1  ( FIG. 11 ) or the sensor  22 - 2  ( FIG. 11 ). T 6 (s) is a specific time since when the printing sheet P passes through the sensor  27  ( FIG. 11 ) or the sensor  38  ( FIG. 11 ). The estimated times T 1 (s) to T 6 (s) are set in the timer  103  in the control unit  101  in advance. 
       Step S 3 - 1   
       [0097]    The printing sheet P 0  and then the printing sheet P 1  start being transported from the cassette  1  ( FIG. 11 ). At the same time, the timer  103  starts measuring time. 
       Step S 3 - 2   
       [0098]    When the sensor  27  ( FIG. 11 ) does not detect the printing sheet P within T 1 (s) since the sheet supply roller  4  starts rotating, the process proceeds to step S 3 - 3 . When the sensor  27  detects the printing sheet P 1 , the process proceeds to step S 3 - 7 . 
       Step S 3 - 3   
       [0099]    When the sensor  22 - 1  or the sensor  22 - 2  detects the printing sheet P 0  and it is within T 5 (s), the process proceeds to step S 3 - 4 , otherwise proceeds to step S 3 - 5 . 
       Step S 3 - 4   
       [0100]    Since the sensor  27  ( FIG. 11 ) does not detect the printing sheet P within T 1 (s) since the sheet supply roller  4  starts rotating, it is determined that the jam occurs during the transportation and the printing sheet P 1  remains in the image forming transport path  106  between the developing units  9  ( FIG. 11 ) and the transfer belt  11  ( FIG. 11 ). Further, if it is within T 5 (s) since the sensor  22 - 1  or the sensor  22 - 2  detects the printing sheet P 0 , it is determined that the printing sheet P 0  remains in the duplex transport path  18  ( FIG. 11 ) regardless of the occurrence of the jam. The control unit  101  stops the drive source  104  and the sheet supply drive source  107  to stop the image formation process, and controls the electromagnetic clutch  46  and the electromagnetic clutch  51  to be in the non-energized state. 
         [0101]    The control unit  101  controls the display unit  102  to display the occurrence of the jam to notify an operator, thereby requesting removal of the jammed sheet. In this state, the first structural member  43 , the second structural member  44 , and the frame  55  are separated. When the operator pulls up the handle  24  ( FIG. 11 ) to remove the jammed sheet, the handle  24  ( FIG. 11 ) is released from the protrusion  25  ( FIG. 11 ) disposed on the frame  55  ( FIG. 11 ). Accordingly, as shown in  FIG. 15 , the first structural member  43  and the second structural member  44  rotate around the rotational pivot  23  and open. 
         [0102]    As shown in  FIG. 15 , in the second structural member  44 , the electromagnetic clutch  51  rotates freely, so that the link  52  slides and an end portion of the link  52  rotates until the end portion abuts against the gear of the electromagnetic clutch  51 . In the first structural member  43 , the electromagnetic clutch  46  rotates freely, so that the link  48  slides and an end portion of the link  48  rotates until the end portion abuts against the gear of the electromagnetic clutch  46 . As a result, the frame  55 , the first structural member  43 , and the second structural member  44  are opened in the separated state, respectively. Accordingly, when the jams occur both between the developing units  9  ( FIG. 11 ) and the transfer belt  11  ( FIG. 11 ) and in the duplex transport path  18  ( FIG. 11 ), the locations of the jams where the printing sheets P 0  and P 1  remain are opened to a large extent with the single handle operation, thereby making it easy to find the jammed sheets and recover the jams. 
       Step S 3 - 5   
       [0103]    Since the sensor  27  ( FIG. 11 ) does not detect the printing sheet P 1  within T 1 (s) since the sheet supply roller  4  starts rotating, it is determined that the jam occurs during the transportation and the printing sheet P 1  remains in the image forming transport path  106  between the developing units  9  ( FIG. 1 ) and the transfer belt  11  ( FIG. 1 ). Also, it has passed T 5 (s) since the sensor  22 - 1  or the sensor  22 - 2  detects the printing sheet P 0 , so that it is determined that the printing sheet P 0  does not remain in the duplex transport path  18  ( FIG. 1 ). The control unit  101  stops the drive source  104  and the sheet supply drive source  107  to stop the image formation process, and energizes the electromagnetic clutch  46 . 
       Step S 3 - 6   
       [0104]    The control unit  101  controls the display unit  102  to display the occurrence of the jam to notify and requests the operator to remove the jammed sheet P 1 . When the operator pulls up the handle  24  ( FIG. 11 ) to remove the jammed sheet, the handle  24  ( FIG. 11 ) is released from the protrusion  25  ( FIG. 11 ) disposed on the frame  55  ( FIG. 11 ). Accordingly, the first structural member  43  and the second structural member  44  rotate around the rotational pivot  23  and open shown in  FIG. 16 . As shown in  FIG. 16 , the gear of the electromagnetic clutch  46  does not rotate, so that the link  48  does not slide and is fixed. Therefore, the first structural member  43  and the second structural member  44  are integrated. Accordingly, when the jam occurs only between the developing units  9  ( FIG. 11 ) and the transfer belt  11  ( FIG. 11 ), the location of the jam where the printing sheet P 1  remains is opened to a large extent with the single handle operation, thereby making it easy to find the jammed sheet and recover the jam. 
       Step S 3 - 7   
       [0105]    When the sensor  38  ( FIG. 11 ) does not detect the printing sheet P 1  within T 2 (s), it is determined that the jam occurs during the transportation between the developing units  9  ( FIG. 11 ) and the transfer belt  11  ( FIG. 11 ) and the printing sheet P 1  remains in the image forming transport path  106  between the developing units  9  ( FIG. 11 ) and the transfer belt  11  ( FIG. 11 ). Accordingly, similar to step S 3 - 2  when the sensor  27  ( FIG. 11 ) does not detect the printing sheet P 1  within T 1 (s) since the sheet supply roller  4  starts rotating, the process proceeds to steps S 3 - 4  or S 3 - 5  through step S 3 - 3  to recover the jam. When the sensor  38  ( FIG. 11 ) detects the printing sheet P 1  within T 2 (s), the process proceeds to step S 3 - 8 . 
       Step S 3 - 8   
       [0106]    When the image forming device is not performing the duplex printing (printing one side), the process proceeds to step S 3 - 9 . When the image forming device is performing the duplex printing, the process proceeds to step S 3 - 11 . 
       Step S 3 - 9   
       [0107]    When there is next print data, the process proceeds to step S 3 - 10 . When there is no print data, the process stops. 
       Step S 3 - 10   
       [0108]    The control unit  101  resets the timer  103  to zero, and the process returns to step S 3 - 1  for starting the next printing process. 
       Step S 3 - 11   
       [0109]    In a case that the duplex printing needs to be performed and both sides are printed, the process proceeds to step S 3 - 9 , so that the process of printing one side in step S 3 - 8  is performed. On the other hand, in a case that the duplex printing needs to be performed and only one side is printed, the process proceeds to step S 3 - 12 . 
       Step S 3 - 12   
       [0110]    When the sensor  22 - 1  ( FIG. 11 ) does not detect the printing sheet P within T 3 (s) since the sheet supply roller  4  starts rotating, the process proceeds to step S 3 - 13 . When the sensor  22 - 1  detects the printing sheet P, the process proceeds to step S 3 - 17 . 
       Step S 3 - 13   
       [0111]    When it is within T 6 (s) since the sensor  27  or the sensor  38  detects the printing sheet P 2 , the process proceeds to step S 3 - 14 , otherwise proceeds to step S 3 - 15 . 
       Step S 3 - 14   
       [0112]    Since the sensor  22 - 1  ( FIG. 11 ) does not detect the printing sheet P 1  within T 3 (s) since the sheet supply roller  4  starts rotating, it is determined that the jam occurs in the duplex transport path  18  ( FIG. 11 ) and the printing sheet P 1  remains in the duplex transport path  18  ( FIG. 11 ). Further, since it is within T 6 (s) since the sensor  27  ( FIG. 11 ) or the sensor  38  ( FIG. 11 ) detects the printing sheet P 2 , it is determined (regardless of the occurrence of the jam) that the printing sheet P 2  still remains in the image forming transport path  106  between the developing units  9  ( FIG. 11 ) and the transfer belt  11  ( FIG. 11 ). The control unit  101  stops the drive source  104  and the sheet supply drive source  107  to stop the image formation process, and controls the electromagnetic clutches  46  and  51  to be in the non-energized state, respectively. 
         [0113]    The control unit  101  controls the display unit  102  to display the occurrence of the jam to notify and requests the operator to remove the jammed sheet (printing sheet P). In this state, the second structural member  43 , the second structural member  44 , and the frame  55  are separated. When the operator pulls up the handle  24  ( FIG. 11 ) to remove the jammed sheet, the handle  24  ( FIG. 11 ) is released from the protrusion  25  ( FIG. 11 ) disposed on the frame  55  ( FIG. 11 ). Accordingly, as shown in  FIG. 15 , the first structural member  43  and the second structural member  44  rotate around the rotational pivot  23  and open. 
         [0114]    As shown in  FIG. 15 , in the second structural member  44 , the electromagnetic clutch  51  rotates freely, so that the link  52  slides and the end portion of the link  52  rotates until the end portion abuts against the gear of the electromagnetic clutch  51 . In the first structural member  43 , the electromagnetic clutch  46  rotates freely, so that the link  48  slides and the end portion of the link  48  rotates until the end portion abuts against the gear of the electromagnetic clutch  46 . As a result, the frame  55 , the first structural member  43 , and the second structural member  44  are opened in the separated state, respectively. Accordingly, when the jams occur both between the developing units  9  ( FIG. 11 ) and the transfer belt  11  ( FIG. 11 ) and in the duplex transport path  18  ( FIG. 11 ), the locations of the jams where the printing sheets P 0  and P 1  remain are opened to a large extent with the single handle operation, thereby making it easy to find the jammed sheets and recover the jams. 
       Step S 3 - 15   
       [0115]    Since the sensor  22 - 1  ( FIG. 11 ) does not detect the printing sheet P 1  within T 3 (s) since the sheet supply roller  4  starts rotating, it is determined that the jam occurs in the duplex transport path  18  ( FIG. 11 ) and the printing sheet P 1  remains in the duplex transport path  18  ( FIG. 1 ). Also, it has passed T 6 (s) since the sensor  27  or the sensor  38  detects the printing sheet P 2 , so that it is determined that the printing sheet P 2  does not remain in the image forming transport path  106  between the developing units  9  ( FIG. 11 ) and the transfer belt  11  ( FIG. 1 ). The control unit  101  stops the drive source  104  and the sheet supply drive source  107  to stop the image formation process, and energizes the electromagnetic clutch  51  ( FIG. 11 ). 
       Step S 3 - 16   
       [0116]    The control unit  101  controls the display unit  102  to display the occurrence of the jam to notify and requests the operator to remove the jammed sheet. When the operator pulls up the handle  24  ( FIG. 11 ) to remove the jammed sheet, the handle  24  ( FIG. 11 ) is released from the protrusion  25  ( FIG. 11 ) disposed on the frame  55  ( FIG. 11 ). Accordingly, the first structural member  43  ( FIG. 11 ) rotates around the rotational pivot  23  ( FIG. 11 ) and opens as shown in  FIG. 17 . As shown in  FIG. 16 , the gear of the electromagnetic clutch  51  does not rotate, so that the link  52  does not slide and is fixed. Therefore, the second structural member  44  and the frame  55  are integrated. Accordingly, when the jam occurs between the developing units  9  ( FIG. 11 ) and the transfer belt  11  ( FIG. 11 ), the location of the jam where the printing sheet P 1  remains is opened to a large extent with the single handle operation, thereby making it easy to find the jammed sheet and recover the jam. 
       Step S 3 - 17   
       [0117]    When the sensor  22 - 2  ( FIG. 11 ) does not detect the printing sheet P 1  within T 4 (s), it is determined that the jam occurs during the transportation in the duplex transport path  18  ( FIG. 11 ) and the printing sheet P 1  remains in the duplex transport path  18  ( FIG. 11 ). Accordingly, similar to step S 3 - 12  when the sensor  22 - 1  ( FIG. 11 ) does not detect the printing sheet P 1  within T 3 (s) since the sheet supply roller  4  starts rotating, the process proceeds to steps S 3 - 14  or S 3 - 15  through step  3 - 13  to recover the jam. When the sensor  22 - 2  ( FIG. 11 ) detects the printing sheet P 1  within T 4 (s), the process proceeds to step S 3 - 18 . 
       Step S 3 - 18   
       [0118]    The control unit  101  resets the timer  103  to zero, and the process returns to step S 3 - 2  for starting the next printing process. 
         [0119]    As described above, the jam location where the printing sheet remains is detected and the structural members are connected or opened. Accordingly, when the jams occur at several locations, or the jam occurs at one location and the printing sheet at other location needs to be removed, it is easy to find the jammed sheet and recover the jam similar to the second embodiment with the simple structure having a small number of components. 
         [0120]    In the embodiments, the image forming device is applied to the printer, and the invention is not limited thereto. The image forming device is applicable to a fax machine, a copier, and a plotter. 
         [0121]    The disclosure of Japanese Patent Application No. 2004-217454, filed on Jul. 26, 2004, is incorporated in the application. 
         [0122]    While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.