Abstract:
The present device includes a conveying path, a contact member coming in contact with the front of a printing medium to stop the conveyance of the medium, a hold member holding the front of the medium having contacted with the contact member, a first conveying unit conveying the medium so as to abut the contact member to form slack on the medium between itself and the hold member holding the medium, and a second conveying unit being provided nearer to the hold member than the first conveying unit in the conveying direction and conveying the medium so as to abut the contact member to form the slack. The second conveying unit is movable so that a part thereof existing on the side where the slack is formed on the medium in the conveying path is moved to the outside of the conveying path.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2012-125343 filed in Japan on May 31, 2012, Japanese Patent Application No. 2012-125344 filed in Japan on May 31, 2012 and Japanese Patent Application No. 2013-088609 filed in Japan on Apr. 19, 2013. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a printing medium conveying device and an image forming apparatus. 
         [0004]    2. Description of the Related Art 
         [0005]    In conventional image forming apparatuses, sheets such as documents and recording sheets are sequentially conveyed by a printing medium conveying device from a paper cassette to a sheet ejection unit. The printing medium conveying device includes a conveying path including a plurality of guiding members and a plurality of rollers that are driven to rotate, through which sheets are conveyed. 
         [0006]    Once a printing operation is started, toner images formed in four colors (yellow, magenta, cyan, and black) image carriers are transferred to an intermediate transfer belt. Sheets that have been fed from the paper cassette one by one are conveyed through the conveying path to a secondary transfer device, where the toner images on the intermediate transfer belt are transferred to the sheet. The toner images are then fixed to the sheet in a fixing unit, and after that the sheet is ejected from the printing medium conveying device to outside the apparatus. 
         [0007]    A skew correction device is provided in the conveying path for correcting the skew of the sheet occurred during the conveyance. The skew correction device abuts the leading edge of the sheet to the guiding member to align the position of the leading edge of the sheet to correct the position of the sheet so as to be parallel to a secondary transfer device. Specifically, by feeding the sheet excessively between a roller for the skew correction and a conveying roller located in the upstream thereof, the leading edge of the sheet is abutted to the guiding member. To align the sheet as described above, a space is required for forming slack on the sheet that has been fed due to evacuation of the sheet from the conveying path. For that purpose, sufficient distances and spaces need to be ensured between the rollers. In addition, if the used sheet is more rigid and thicker, larger distances between the rollers need to be ensured. 
         [0008]    In recent years, however, the need has increased for a printer and other image forming apparatuses to cope with various types of sheet, print on a rigid thick sheet and print on a short-sized sheet such as a post card with a single printer or apparatus. 
         [0009]    When slack is formed on a sheet between rollers using a skew correction device, for example, if the rollers are arranged with a short distance interposed therebetween, the conveying roller cannot feed a rigid thick sheet excessively due to the hardness of the sheet. To address such an issue, as disclosed in Japanese Patent Application Laid-open No. 2008-024507, for example, a mechanism is provided to separate a pair of rollers in the vertical direction and convey the sheet using other conveying rollers provided on the upstream thereof. With this structure, the sheet does not contact with the separated rollers, whereby slack is formed on the sheet along the long pitch between the upstream rollers and the rollers for the skew correction. The separation operation of rollers is performed, however, in a short time and typically for a few millimeters. The separation distance is too short to ensure a space to form sufficient slack on the sheet, causing an obstacle for feeding the sheet. 
         [0010]    In view of the circumstances above, there is needed to provide a printing medium conveying device capable of ensuring a space for feeding sheets of different lengths to correct the direction of the sheets when printing media are conveyed. 
       SUMMARY OF THE INVENTION 
       [0011]    It is an object of the present invention to at least partially solve the problems in the conventional technology. 
         [0012]    According to the present invention, there is provided: a printing medium conveying device comprising: a conveying path serving as a path through which a printing medium is conveyed; a contact member configured to come in contact with the front part of the printing medium in the sheet conveying direction to stop conveyance of the printing medium; a hold member configured to hold the front part of the printing medium that has contacted with the contact member; a first conveying unit configured to convey the printing medium so as to abut the contact member between the first conveying unit and the hold member that holds the printing medium; and a second conveying unit configured to be provided nearer to the hold member than the first conveying unit in the sheet conveying direction and convey the printing medium so as to abut the contact member between the second conveying unit and the hold member that holds the printing medium. 
         [0013]    In the above-mentioned printing medium conveying device, the second conveying unit is configured to be movable so that a part thereof existing on a side where slack is formed on the printing medium due to the conveyance in the conveying path is moved to outside of the conveying path. 
         [0014]    The present invention also provides an image forming apparatus comprising the above-mentioned printing medium conveying device. 
         [0015]    The present invention also provides a printing medium conveying device comprising: a conveying path serving as a path through which a printing medium is conveyed; a contact member configured come in contact with the front part of the printing medium in the sheet conveying direction to stop conveyance of the printing medium; a hold member configured to hold the front part of the printing medium that has contacted with the contact member; a first conveying unit configured to convey the printing medium so as to abut the contact member to form slack on the printing medium between the first conveying unit and the hold member that holds the printing medium; and a detachable unit in which a second conveying unit is detachable and provided nearer to the hold member than the first conveying unit in the sheet conveying direction and conveys the printing medium so as to abut the contact member to form slack on the printing medium between the detachable unit and the hold member that holds the printing medium. 
         [0016]    The present invention also provides an image forming apparatus comprising the above-mentioned printing medium conveying device. 
         [0017]    The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a side view of an image forming apparatus according to a first embodiment of the present invention; 
           [0019]      FIG. 2  is a side view of a skew correction device according to the first embodiment; 
           [0020]      FIG. 3  is a side view of the skew correction device when a second conveying unit according to the first embodiment has been moved to another position; 
           [0021]      FIG. 4  is a side view of the skew correction device when a second conveying unit according to the first embodiment has been moved to another position; 
           [0022]      FIG. 5  is a side view of the skew correction device when a second conveying unit according to the first embodiment has been moved to another position; 
           [0023]      FIG. 6  is a perspective view of an end of the second conveying unit according to the first embodiment; 
           [0024]      FIG. 7  is a cross-sectional structural view of a bent member of a guide plate of the second conveying unit according to the first embodiment viewed from the arrow A illustrated in  FIG. 6 ; 
           [0025]      FIG. 8  is a table listing conditions of the second conveying unit of the image forming apparatus according to the first embodiment, used for each type of sheet; 
           [0026]      FIG. 9  is a side view of a skew correction device according to a second embodiment of the present invention; 
           [0027]      FIG. 10  is a side view of the skew correction device when a second conveying unit according to the second embodiment has been removed; 
           [0028]      FIG. 11  is a side view of the skew correction device when the second conveying unit according to the second embodiment has been removed; and 
           [0029]      FIG. 12  is a side view of the skew correction device when the second conveying unit according to the second embodiment has been removed. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]      FIG. 1  is a cross-sectional side view of an image forming apparatus including a printing medium conveying device according to an embodiment of the present invention. As illustrated in  FIG. 1 , an image forming apparatus  100  includes a feeding device  200 , a printing medium conveying device  300 , a skew correction device  400 , a secondary transfer device  500 , an image carrier  600 , and a fixing unit  700 . 
         [0031]    The image carrier  600  includes four colors image carriers  600 Y (yellow),  600 M (magenta),  600 C (cyan), and  600 Bk (black). The image forming apparatus  100  includes an intermediate transfer belt  501 . Toner images formed on the image carriers  600 Y,  600 M,  600 C, and  600 Bk are transferred to the intermediate transfer belt  501 . 
         [0032]    The feeding device  200  feeds a sheet S from a paper cassette  201   a  or a paper cassette  201   b  to the printing medium conveying device  300 . A plurality of sheets S are stacked in the paper cassettes  201   a  and  201   b . The sheet S fed from the feeding device  200  is conveyed to the printing medium conveying device  300  that includes a conveying path  301  and a pair of conveying rollers  302 . The sheet S is conveyed through the conveying path  301  formed by guide plates. The conveying path  301  has a structure that sandwiches the sheet S between the guide plates from above and below the sheet S. A plurality of pairs of conveying rollers  302  are provided along the conveying path  301  and driven to rotate around a rotating shaft, thereby conveying the sheet S held between the pairs of conveying rollers. 
         [0033]      FIG. 2  is a detailed side view of the skew correction device  400 . As illustrated in  FIG. 1 , the printing medium conveying device  300  includes the skew correction device  400 . The skew correction device  400  includes a first conveying unit  303 , a second conveying unit  304 , a contact member  401 , and a hold member  402 . 
         [0034]    The first conveying unit  303  and the second conveying unit  304  convey the sheet S for aligning the sheet S. The first conveying unit  303  includes rollers  303   a  and  303   b.  The rollers  303   a  and  303   b  protrude into the conveying path  301  from holes formed in the guide plates forming the conveying path  301 . The second conveying unit  304  includes rollers  304   a  and  304   b,  and guiding members  305   a  and  305   b.  The guiding members  305   a  and  305   b  guide the sheet S from above and below the sheet S and control the direction of the sheet S in the same manner as the guide plates forming the conveying path  301 . The guiding members  305   a  and  305   b  are standalone, that is to say, disconnected from the guide plates of the conveying path  301 . Holes are formed in the guiding members  305   a  and  305   b , from which the rollers  304   a  and  304   b  protrude into the conveying path  301 . 
         [0035]    The guiding members  305   a  and  305   b  of the second conveying unit  304  are coupled to each other. The second conveying unit  304  is supported rotatably around the roller  304   b  on the opposite side from the side where slack is formed on the sheet S in the conveying path  301 . 
         [0036]    The first conveying unit  303  and the second conveying unit  304  are driven by a common driving force transmission unit  307 . The driving force transmission unit  307  contacts both one of the rollers of the first conveying unit  303  and one of the rollers of the second conveying unit  304 , whereby the rotational force of the driving force transmission unit  307  is transmitted to both one of the rollers of the first conveying unit  303  and one of the rollers of the second conveying unit  304 . It is permissible that the driving force transmission unit  307  itself does not rotate, instead, the rotation of the first conveying unit  303  is transmitted through the driving force transmission unit  307  to the second conveying unit  304 . 
         [0037]    The contact member  401  comes in contact with the front part of the sheet S in the sheet conveying direction, thereby stopping the conveyance of the sheet S. In the present embodiment, the contact member  401  is formed in such a shape that the guide plate is bent to protrude into the conveying path  301 . After the sheet S comes in contact with the contact member  401 , the first conveying unit  303  or the second conveying unit  304  conveys the sheet S for a predetermined distance and temporarily stops the conveyance of the sheet S. The first conveying unit  303  or the second conveying unit  304  resumes the conveyance of the sheet S in synchronization with the conveyance of the images transferred on the intermediate transfer belt  501  to their predetermined positions. At this time, the contact member  401  evacuates downward, whereby the sheet S is conveyed. The contact member can also be achieved as a “nip abutting structure” in which the contact point of a later-described hold member  402  comes in contact with the sheet S. With this structure, a common member is used for both the contact member and the hold member. 
         [0038]    The hold member  402  is a pair of rollers that holds the front part of the sheet S in the sheet conveying direction. The skew correction device  400  forms slack on the sheet between the hold member  402  and the first conveying unit  303  or the hold member  402  and the second conveying unit  304  when the first conveying unit  303  or the second conveying unit  304  conveys the sheet S so as to abut the contact member  401 . This corrects skew feed of the sheet. As illustrated in  FIG. 2 , the second conveying unit  304  is provided so that slack is formed on the sheet S within the distance L 2  between the hold member  402  and the second conveying unit  304 . This aligns the sheet  5 , which will be further conveyed to the secondary transfer device  500 . The secondary transfer device  500  transfers the toner images formed on the intermediate transfer belt  501  to the sheet S. The fixing unit  700  then fixes the toner images transferred on the sheet S, and the sheet S is ejected from the image forming apparatus  100 . 
         [0039]      FIG. 3  is a side view of the printing medium conveying device  300  when the second conveying unit  304  has been moved to the outside of the conveying path  301 . When printing is performed and if the length of the sheet S selected as a print sheet for the image forming apparatus  100  is larger than the roller pitch L 1  between the hold member  402  and the first conveying unit  303 , the roller  304   a  and the guiding member  305   a  of the second conveying unit  304  are rotated around the rotating shaft of the roller  304   b  forward in the sheet conveying direction. The rotation of the second conveying unit  304  moves the roller  304   a  and the guiding member  305   a  to the outside of the conveying path  301  from the side where the slack is formed on the sheet S in the conveying path  301 . After the second conveying unit  304  is rotated, the other surfaces of the guiding members  305   a  and  305   b  face the conveying path  301 , which is capable of guiding the sheet S being conveyed in the conveying path  301 .  FIG. 3  illustrates that the second conveying unit  304  is rotated by approximately 90 degrees, however, the angle is not limited to this example. Other angles may be used by appropriately setting a folded and bent member of the guiding member or the guiding members. 
         [0040]      FIG. 4  is a side view of another structure of the second conveying unit. As illustrated in  FIG. 4 , the second conveying unit  1304  includes rollers  1304   a  and  1304   b,  and a guiding member  1305   a.  In the structure illustrated in  FIG. 4 , the roller  1304   b  protrudes from the guide plate included in the conveying path  301 . The roller  1305   a  is coupled to the roller  303   a  of the first conveying unit  303  by a guide plate  310 . The guide plate  310  can rotate around the rotating shaft of the roller  303   a  of the first conveying unit  303 . As illustrated in  FIG. 4 , the roller  1304   a  and the guiding member  1305   a  are moved to the upper side (e.g., rotated by approximately 45 degrees as illustrated in  FIG. 4 ) so as not to obstruct the formation of the slack on the sheet S. 
         [0041]      FIG. 5  is a side view of still another structure of the second conveying unit after moved. As illustrated in  FIG. 5 , the second conveying unit  2304  includes rollers  2304   a  and  2304   b , and guiding members  2305   a  and  2305   b . In the structure illustrated in  FIG. 5 , the roller  2304   b  protrudes from the guide plate included in the conveying path  301 . The rollers  2304   a  and  2304   b  of the second conveying unit  2304  are coupled to each other. The second conveying unit  2304  is supported rotatably around the roller  2304   b  on the opposite side from the side where slack is formed on the sheet S in the conveying path  301 . The angle of this rotation is larger than the example illustrated in  FIG. 3 . After the guiding member  2305   b  is moved, the rotation angle is adjusted so that the guiding member  2305   b  is positioned along the conveying path  301 . In addition, the guiding member  2305   b  can be removed, for example, so as not to obstruct the rotation of the second conveying unit  2304 . 
         [0042]    The unit for moving the second conveying unit will now be described with reference to  FIGS. 6 and 7 .  FIG. 6  is a perspective view of an end of the second conveying unit  304 .  FIG. 7  is a cross-sectional structural view of a bent member  305   c  of the guiding member  305   b  viewed from the arrow A illustrated in  FIG. 6 . As illustrated in  FIG. 6 , the bent member  305   c  is formed, and a pulley  323  and a shaft bearing  322  are provided on an end of the guiding member  305   b . The pulley  323  and shaft bearing  322  are formed integrally by press fitting in advance. As illustrated in the cross-sectional view of  FIG. 7 , the flange of the pulley  323  is fitted and inserted into the hole of the bent member  305   c  and fixed with a fixing member  324  such as a screw. A shaft  304   c  of the second conveying roller  304   b  is inserted into the shaft bearing  322  and supported by the frame  400   a  of the skew correction device  400 . In the same manner, on the opposite side, the shaft  304   c  of the second conveying roller  304   b  is inserted into a bent member  305   e  through the shaft bearing  322 , and a gear  326  is fastened to the shaft  304   c  or inserted detently into the shaft  304   c.  The shaft  304   c  is supported by the frame  400   a  of the skew correction device  400  at the further back position through the shaft bearing  322 . The second conveying unit  304  is supported rotatably around the shaft  304   c.    
         [0043]    For driving to convey a printing medum, a driving unit  328  is mounted on the frame  400   a  of the skew correction device  400 . The rotating drive force is transmitted from the driving unit  328  through a belt  321  to a gear  327 . This drives the first conveying roller  303   b  of the first conveying unit  303  to convey the sheet S. The rotating drive force is also transmitted from the driving unit  328  through a gear  307  to the gear  326 . This drives the second conveying roller  304   b  to rotate to convey the sheet S. In this manner, the first conveying unit  303  and the second conveying unit  304  are driven to convey the sheet S at the same time. 
         [0044]    A driving unit  325  (a drive unit) and a sensor  320  are fixed to the frame  400   a  of the skew correction device  400  or the frame (not illustrated) of the secondary transfer device  500  and independent from the rotatable second conveying unit. The rotating drive force of the driving unit  325  drives the pulley  323  to rotate through the belt  321 , whereby the second conveying unit  304  can rotate. The bent member  305   c  has a feeler member  305   d  serving as a shield plate for the sensor  320  in response to the rotation of the second conveying unit  304 . The driving unit  325  drives or stops the forward and backward rotation of the driving unit  325  depending on whether the signal of the sensor is shielded or not. As a result, the driving unit  325  controls the second conveying unit  304  to move to either the state illustrated in  FIG. 2  or the state illustrated in  FIG. 3 . Specifically, when the sensor  320  detects the feeler member  305   d  the signal is shielded. This controls the second conveying unit  304  to be moved to the outside of the conveying path  301 . 
         [0045]    The control method for the driving unit  325  is not limited to this example. When an AC stepping motor is used, the driving unit  325  may be controlled by counting pulses determined in advance during the forward or backward rotation of the driving unit  325 . When the rotation angle of the second conveying unit  304  can be physically restricted in the state illustrated in  FIG. 2  or the state illustrated in  FIG. 3  with a stopper and so on, the driving unit  325  may be controlled by being driven or being stopped in response to detection of overcurrent of a DC motor. 
         [0046]    When a typical printing operation is started, an operator selects the size, the thickness, the type (e.g. a plain sheet, a coated sheet, an envelope, a punched sheet) of the sheet S that has been loaded in the feeding device  200  through a not-illustrated operation panel in advance. When a typical standard size sheet is used such as an A4-sized sheet or an A3-sized sheet, the width and length of the sheet can be determined from the size information selected by the operator. When a non-standard form size sheet is used, the width and length of the sheet can be directly input by the operator. According to the length information of the sheet, the second conveying unit  304  selects the state illustrated in  FIG. 2  or the state illustrated in  FIG. 3  to be used for starting a printing operation.  FIG. 8  is a table that lists positional information of the second conveying unit  304  that is determined according to the combination of the thickness, the length, and the type of the sheet. This table is stored in a hard disk or other storage in the image forming apparatus. 
         [0047]    For example, as illustrated in  FIG. 8 , when the length of the sheet is L and the length of the slack formed during the skew correction is Lbf, and L&gt;L 1 +Lbf, the state illustrated in  FIG. 3  is selected. When L≧L 1 +Lbf, the state illustrated in  FIG. 2  is selected. The state can be selected according to the combination of the thickness of the sheet and the type of the sheet in addition to the length of the sheet L. For example, when the thickness of the sheet is T and the type of the sheet is K, the matrix illustrated in  FIG. 8  is stored in advance in a not-illustrated storage device. According to the obtained sheet information, the matching condition is selected from the conditions listed in  FIG. 8 , whereby the state illustrated in  FIG. 2  or the state illustrated in  FIG. 3  is selected to be used for starting a printing operation. The information on the length, the thickness, and the types of the sheet can be obtained according to the information selected by a user, and can alternatively be obtained by measurement. 
         [0048]    In the printing medium conveying device  300  as described above, if it is not required to print on a short-sized sheet such as a post card, the skew correction device  400  can be used in a state where the second conveying unit  304  is moved to the outside of the conveying path  301 . In this state, a sheet whose length is smaller than the roller pitch L 1  between the first conveying unit  303  and the hold member  402  cannot be conveyed. When the length of a sheet is equal to or larger than L 1 , the sheet can be conveyed. The guiding member  305   a  provides sufficient space for forming slack on the sheet, whereby the skew correction of the sheet can be performed even if a rigid sheet is used. 
         [0049]    When printing on a short-sized sheet, the skew correction device  400  can be used in a state where the second conveying unit  304  is moved to the inside of the conveying path  301 . In this state, the sheet S whose length is equal to or smaller than the roller pitch L 2  between the second conveying unit  304  and the hold member  402  can be conveyed. The short-sized sheet that could not be conveyed without the second conveying unit  304  can be, therefore, conveyed. 
         [0050]    With the movable structures as described above, there is no need to provide a drive system or a structure to perform control for changing the position of a conveying unit. The second conveying unit  304  can be moved according to a user&#39;s demand without increasing the cost of the device, whereby printing on a short-sized sheet such as a post card and a rigid thick sheet can be achieved. As a result, the image forming apparatus capable of coping with various lengths of the sheet can be provided. 
         [0051]    In addition to the movable structures as described above in which the second conveying unit  304  is rotated, other methods can be applied. For example, the second conveying unit can be moved upward or to the right or to the left to be evacuated from the position where the slack is formed on the conveying path  301 . The space for the second conveying unit to move in the image forming apparatus need to be ensured, with this structure. 
         [0052]    The second conveying unit can be controlled to move rather than manually moved. This can be achieved by determining whether the second conveying unit is moved according to the size of the sheet specified when printing is instructed by a user and controlling the driving device of the second conveying unit as necessary. 
       Second Embodiment 
       [0053]    A second embodiment will now be described. The second embodiment differs from the first embodiment in that the second conveying unit is provided detachably. The second embodiment is described hereinafter with reference to the drawings.  FIG. 9  is a detailed side view of a skew correction device  3400 . As illustrated in  FIG. 9 , the printing medium conveying device  300  includes a first conveying unit  3303 , a second conveying unit  3304 , and a skew correction device  3400 . The skew correction device  3400  includes the first conveying unit  3303 , the second conveying unit  3304 , a contact member  3401 , and a hold member  3402 . 
         [0054]    The first conveying unit  3303  and the second conveying unit  3304  convey the sheet S for aligning the sheet S. The first conveying unit  3303  includes rollers  3303   a  and  3303   b.  The rollers  3303   a  and  3303   b  protrude into the conveying path  301  from the holes formed in the guide plates forming the conveying path  301 . The second conveying unit  3304  includes rollers  3304   a  and  3304   b,  and guiding members  3305   a  and  3305   b.  The guiding members  3305   a  and  3305   b  guide the sheet S from above and below in the same manner as the guide plates forming the conveying path  301 . The guiding members  3305   a  and  3305   b  are standalone, that is to say, disconnected from the guide plates of the conveying path  301 . Holes are formed in the guiding members  3305   a  and  3305   b,  from which the rollers  3304   a  and  3304   b  protrude into the conveying path  301 . 
         [0055]    A detachable mechanism  3306  with the detachable second conveying unit  3304  is provided in the printing medium conveying device  300 . In the detachable mechanism  3306 , a guide plate is not provided on the part where the second conveying unit  3304  is mounted in the conveying path  301 , whereby a space is formed into which the second conveying unit  3304  is fitted. The second conveying unit  3304  is fixed to the printing medium conveying device  300  by a fastening unit (not-illustrated) such as a screw, for example. The second conveying unit  3304 , therefore, can be removed and mounted manually. 
         [0056]    The first conveying unit  3303  and the second conveying unit  3304  are driven by a common driving force transmission unit  3307 . The driving force transmission unit  3307  contacts one of the rollers of the first conveying unit  3303  and one of the rollers of the second conveying unit  3304 , whereby the rotational force of the driving force transmission unit  3307  is transmitted to both one of the rollers of the first conveying unit  3303  and one of the rollers of the second conveying unit  3304 . This structure with a common drive unit can be achieved without an additional driving unit or control unit, thereby reducing the cost on the device. It is permissible that the driving force transmission unit  3307  itself does not rotate, instead, the rotation of the first conveying unit  3303  is transmitted through the driving force transmission unit  3307  to the second conveying unit  3304 . 
         [0057]    The contact member  3401  comes in contact with the front part of the sheet S in the sheet conveying direction, thereby aligning the leading edge of the sheet S so as to be parallel to the contact member  3401 . In the present embodiment, the contact member  3401  is formed in such a shape that the guide plate is bent to protrude into the conveying path  3301 . After the sheet S comes in contact with the contact member  3401 , the first conveying unit  3303  or the second conveying unit  3304  conveys the sheet S for a predetermined distance and temporarily stops the conveyance of the sheet S. The first conveying unit  3303  or the second conveying unit  3304  resumes the conveyance of the sheet S in synchronization with the conveyance of the images transferred on the intermediate transfer belt  501  to their predetermined positions. At this time, the contact member  3401  evacuates downward, whereby the sheet S is conveyed. The contact member can also be achieved as a “nip abutting structure” in which the contact point of a later-described hold member  3402  comes in contact with the sheet S. With this structure, a common member is used for both the contact member  3401  and the hold member  3402 . 
         [0058]    The hold member  3402  is a pair of rollers that holds the front part of the sheet S in the sheet conveying direction. The skew correction device  3400  forms slack on the sheet between the hold member  3402  and the first conveying unit  3303  or the hold member  3402  and the second conveying unit  3304  when the first conveying unit  3303  or the second conveying unit  3304  conveys the sheet S so as to abut the contact member  3401 . This corrects skew feed of the sheet. As illustrated in  FIG. 9 , the second conveying unit  3304  is provided so that the slack is formed on the sheet S within the distance L 2  between the hold member  3402  and the second conveying unit  3304 . This aligns the sheet S, which will be further conveyed to the secondary transfer device  500 . The secondary transfer device  500  transfers the toner images formed on the intermediate transfer belt  501  to the sheet S. The fixing unit  700  then fixes the toner images transferred on the sheet S, and the sheet S is ejected from the image forming apparatus  100 . 
         [0059]      FIG. 10  is a side view of the skew correction device  3400  when the second conveying unit  3304  has been removed. As illustrated in  FIG. 10 , after removing the second conveying unit  3304 , another member, that is, a guiding member  3308  is mounted so as to cover the space in the detachable mechanism  3306 . The guiding member  3308  is formed nearly parallel to the conveying path  301 , however, the guiding member  3308   a  may be formed in a shape protruding to the side where the slack is formed on the sheet S, as illustrated in  FIG. 11 . With this structure, the slack can be readily formed on the sheet S with the guiding member  3308   a  as a starting point. 
         [0060]    In the example illustrated in  FIG. 10 , the second conveying unit  3304  does not exist, therefore, the slack is formed on the sheet S within the distance L 1  between the first conveying unit  3303  and the hold member  3402 . The distance L 1  is larger than the L 2  illustrated in  FIG. 9 , therefore, the slack can also be formed on the sheet S whose length is larger than in a state where the second conveying unit  3304  exists. 
         [0061]      FIG. 12  is a side view of another removal method of the second conveying unit. As illustrated in  FIG. 12 , the second conveying unit  4304  includes the rollers  4304   a  and  4304   b,  and guiding members  4305   a  and  4305   b  in the printing medium conveying device. The guiding member  4305   b  is formed integrally with the guide plate included in the conveying path  301 . The roller  4304   b  and the guiding member  4305   b  cannot be removed from a detachable unit  4306 . On the other hand, the roller  4304   a  and the guiding member  4305   a  can be removed from the detachable unit  4306 . That is, in this example, only the part of the second conveying unit  4304  on the side where the slack is formed on the sheet S can be removed. 
         [0062]    In the printing medium conveying device  300  as described above, if it is not required to print on a short-sized sheet such as a post card, the skew correction device  3400  can be used in a state illustrated in  FIG. 10 . In this state, the sheet whose length is smaller than the roller pitch L 1  between the first conveying unit  3303  and the hold member  3402  cannot be conveyed. When the length of a sheet is equal to or larger than L 1 , the sheet can be conveyed. The guiding member  3308  provides sufficient space for forming slack on the sheet, whereby the skew correction can be performed even if a rigid sheet is used. 
         [0063]    When printing on a short-sized sheet, the skew correction device  3400  can be used with the structure illustrated in  FIG. 9 . With this structure, the sheet S whose length is equal to or smaller than the roller pitch L 2  between the second conveying unit  3304  and the hold member  3402  can be conveyed. The short-sized sheet that could not be conveyed without the second conveying unit  3304  can be, therefore, conveyed. 
         [0064]    With the detachable structure as described above, there is no need to provide a drive system or the structure to perform control for changing the position of the conveying unit. The second conveying unit  3304  can be moved according to a user&#39;s demand without increasing the cost of the device, whereby printing on a short-sized sheet such as a post card and printing on a rigid thick sheet can be achieved. As a result, the image forming apparatus capable of coping with various lengths of the sheet can be provided. 
         [0065]    According to a printing medium conveying device of the present invention, a space for feeding sheets of different lengths can be ensured to correct the direction of the sheets when printing media are conveyed. 
         [0066]    Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.