Patent Publication Number: US-8123218-B2

Title: Sheet conveyance apparatus and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 12/245,125, filed on Oct. 3, 2008, which claims priority to Japanese Patent Application No. 2007-262477, filed on Oct. 5, 2007, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet conveyance apparatus capable of correcting skew of a sheet while conveying the sheet. 
     2. Description of the Related Art 
     An image forming apparatus includes a sheet conveyance apparatus configured to correct skew of a sheet by conveying a plurality of sheets speedily and successively along a reference surface while maintaining a short interval between a trailing edge of a preceding sheet and a leading edge of a following sheet. 
     As discussed in Japanese Patent Application Laid-Open No. 11-189355, an electrophotographic image forming apparatus includes a sheet conveyance apparatus disposed on the upstream side of a toner image transfer unit. A reference member, positioned near a sheet conveyance path, has a reference surface parallel to a conveyance direction of sheets. 
     Two or more skew rollers, located close to the reference surface, obliquely convey a sheet toward the reference surface. When the sheet collides with the reference surface, the sheet rotates to correct its orientation and starts moving straight along the reference surface. A positioning roller, located on the downstream side of the reference surface, is movable in a direction perpendicular to the sheet conveyance direction. When the positioning roller conveys a skew-corrected sheet in a direction perpendicular to the sheet conveyance direction, the sheet reaches a predetermined set position. 
     As discussed in Japanese Patent Application Laid-Open No. 2003-146489, an electrophotographic image forming apparatus includes a sheet conveyance apparatus disposed near a conveyance path used for reverse surface printing. The sheet conveyance apparatus includes a first skew roller positioned close to a reference surface and a second skew roller disposed on the upstream side of the first skew roller and positioned far from the reference surface. A skew amount set for the second skew roller is comparable to that for the first skew roller. Therefore, when the first skew roller starts conveying a sheet, the sheet does not rotate around its centroid. 
     A sheet conveyance apparatus discussed in Japanese Patent Application Laid-Open No. 2005-104712 includes a first skew roller and a second skew roller positioned on the upstream side of the first skew roller. The first skew roller contacts a surface of a sheet at a position closer to a reference surface than the centerline of a sheet parallel to the sheet conveyance direction. The second skew roller contacts a surface of a sheet at a position far from the reference surface than the centerline. 
     Recent image forming apparatuses are required to perform image formation on various types of sheets which are different from commonly used plain papers and coated papers in grammage, coefficient of friction, size, and conveyance orientation. According to the above-described conventional sheet conveyance apparatus, when the upstream skew roller obliquely conveys a sheet toward a reference surface, the sheet cannot stably maintain its orientation until the sheet reaches the downstream skew roller. 
     Although more details will be described in comparative examples, while only the upstream skew roller nips a conveyed sheet, the sheet rotates around the upstream skew roller. For example, if the lower surface of a lightweight sheet has a large coefficient of friction and the sheet is short in the direction parallel to a reference surface, the sheet rotates with a large angle and the amount of skew becomes larger. 
     When the amount of skew is excessively large, the downstream skew roller positioned near the reference surface may not be able to accomplish skew correction before the sheet thoroughly passes the reference surface. In other words, if a skew roller rotates a sheet, the rotating sheet cannot smoothly move toward the reference surface as intended and rather makes it difficult to accomplish the skew correction. 
     Furthermore, it is desirable that skew rollers do not interfere with a sheet during the skew correction or do not buckle the sheet in the process of aligning the sheet along the reference surface. To this end, when skew rollers obliquely convey a sheet, the rollers allow the sheet to freely slide and rotate. In a state where a sheet is obliquely conveyed by a pair of (upstream and downstream) skew rollers, if two rollers have differences in conveyance resistance (friction) acting between a sheet and a guide surface, the sheet rotates unwontedly and the orientation of the conveyed sheet becomes unstable. 
     SUMMARY OF THE INVENTION 
     Exemplary embodiments of the present invention are directed to a sheet conveyance apparatus capable of stabilizing the orientation of a conveyed sheet to ensure skew correction. Components and a control method of the sheet conveyance apparatus are commonly applicable to various sheets. 
     According to an aspect of the present invention, a sheet conveyance apparatus includes a reference member disposed in a sheet conveyance direction so as to align a side of a sheet along the reference member, a first skew rotary member configured to obliquely convey the sheet toward the reference member, a second skew rotary member positioned on an upstream side of the first skew rotary member and farther from the reference member than the first skew rotary member, at a position corresponding to a central position of the sheet in a direction perpendicular to the sheet conveyance direction, and configured to obliquely convey the sheet toward the reference member, a contact/separation mechanism configured to bring the second skew rotary member into contact with the sheet or separate the second skew rotary member from the sheet, and a control unit configured to control the contact/separation mechanism to cause the second skew rotary member to contact the sheet at a timing when the sheet, which is separated from the second skew rotary member and is being conveyed in the sheet conveyance direction, reaches the first skew rotary member. 
     Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments and features of the invention and, together with the description, serve to explain at least some of the principles of the invention. 
         FIG. 1  illustrates a longitudinal cross-sectional view of an image forming apparatus according to a first exemplary embodiment of the present invention. 
         FIG. 2  illustrates a plan view of a conveyance unit including a skew registration device according to the first exemplary embodiment. 
         FIG. 3  illustrates a driving mechanism of the skew registration device. 
         FIG. 4  is a flowchart illustrating an example conveyance control operation. 
         FIGS. 5A to 5C  illustrate various phases of the conveyance control operation. 
         FIG. 6  illustrates a plan view of a skew registration device according to a first comparative example. 
         FIGS. 7A and 7B  illustrate various phases of a conveyance control operation according to the first comparative example. 
         FIGS. 8A to 8C  illustrate various phases of a conveyance control operation performed by a skew registration device according to a second comparative example. 
         FIGS. 9A to 9C  illustrate various phases of a conveyance control operation performed by a skew registration device according to a third comparative example. 
         FIG. 10  illustrates a plan view of a skew registration device according to a second exemplary embodiment of the present invention. 
         FIG. 11  illustrates a plan view of a skew registration device according to a third exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following description of exemplary embodiments is illustrative in nature and is in no way intended to limit the invention, its application, or uses. It is noted that throughout the specification, similar reference numerals and letters refer to similar items in the following figures, and thus once an item is described in one figure, it may not be discussed for following figures. Exemplary embodiments will be described in detail below with reference to the drawings. 
     The following are exemplary embodiments of the present invention described with reference to the drawings. In the exemplary embodiments of the present invention, in as far as the distance between an upstream skew roller and a reference surface is variable according to the size of sheets, a part or all of components of the exemplary embodiments can be replaced by alternative components. 
     Therefore, the present invention is applicable to various image forming apparatuses including electrophotographic image forming apparatuses, offset print systems, and inkjet print systems. An example electrophotographic image forming apparatus is a tandem type including a plurality of image forming units disposed straight in a predetermined order or a rotary type including image forming units disposed around a drum. An example electrophotographic image forming method includes primarily transferring an image onto an intermediate transfer member and secondarily transferring the image from the intermediate transfer member to a sheet. Another electrophotographic image forming method includes directly transferring a toner image from a photosensitive member to a sheet. 
     First Exemplary Embodiment 
       FIG. 1  is a longitudinal cross-sectional view of an image forming apparatus according to a first exemplary embodiment of the present invention. As illustrated in  FIG. 1 , an image forming apparatus  60  according to the first exemplary embodiment is a full-color multifunction peripheral including four-color electrophotographic image forming units arrayed along a straight part of an intermediate transfer belt  606 . The intermediate transfer type is different from a direct transfer type in that a transfer drum or a transfer belt does not hold a sheet. 
     In this respect, the image forming apparatus  60  can perform printing on various types of sheets including thick papers and coated papers. The image forming apparatus  60  can realize parallel processing using a plurality of image forming units and can perform collective transfer of full-color images. The characteristics of an intermediate transfer and tandem type enable the image forming apparatus  60  to attain higher productivity. A paper feeding device  61  includes a lift-up member  62  that can lift a plurality of sheets. A paper feeding unit  63  is configured to feed an uppermost sheet S from the paper feeding device  61  to a conveyance unit  64 . 
     For example, the paper feeding unit  63  is configured as a friction type that includes a feeding roller to separate a paper or as an air type that can use a suction force to hold and separate a sheet. The paper feeding unit  63  according to the first exemplary embodiment is an air-type. 
     The sheet S, fed from the paper feeding unit  63 , passes a conveyance path  64   a  provided in the conveyance unit  64  and reaches a skew registration device  65 . The skew registration device  65  performs skew correction for the sheet S and timing correction for synchronizing the sheet S with a toner image formed on the intermediate transfer belt  606 . Then, the skew registration device  65  conveys the sheet S to a secondary transfer unit. 
     The secondary transfer unit includes an inner secondary transfer roller  603  and an external secondary transfer roller  66 , which are disposed in an opposed relationship to press the intermediate transfer belt  606  from both sides. The secondary transfer unit is configured to transfer a toner image formed on an intermediate transfer member  606  to the sheet S under a pressing force and a transfer field while the sheet S moves together with the intermediate transfer belt  606 . 
     Four image forming units  613  configured to form toner images of yellow (Y), magenta (M), cyan (C), and black (Bk) are disposed along the intermediate transfer belt  606 . Four image forming units  613  are similar in arrangement except for the color of toner stored in a developing device  610 . The image forming unit  613  of yellow (Y), disposed at the most upstream side, has the following arrangement. The number of toner colors is not limited to four. The order of toner colors is arbitrarily changeable. 
     The image forming unit  613  includes a photosensitive drum  608  that rotates in the direction indicated by arrow “a” in  FIG. 1 . Peripheral devices disposed around the photosensitive drum  608  are a charging device (not illustrated), an exposure apparatus  611 , the developing device  610 , a primary transfer roller  607 , and a drum cleaner  609 . 
     The charging device in the exposure apparatus  611  uniformly charges the surface of the photosensitive drum  608 . The exposure apparatus  611  generates a laser beam modulated according to image data. A mirror  612  reflects the laser beam toward the photosensitive drum  608 . Namely, the exposure apparatus  611  and the mirror  612  realize exposure scanning using a laser beam for forming an electrostatic latent image on the surface of the photosensitive drum  608 . Under an electrostatic force, the developing device  610  applies toner particles to the electrostatic latent image formed on the photosensitive drum  608 . Thus, a toner image appears on the photosensitive drum  608 . 
     The primary transfer roller  607  and the photosensitive drum  608 , pressing the intermediate transfer belt  606  from both sides, constitute a primary transfer unit configured to transfer a toner image formed on the photosensitive drum  608  to the intermediate transfer belt  606  by applying a predetermined pressing force and a transfer field to the toner image. The drum cleaner  609  collects toner particles remaining on the surface of the photosensitive drum  608  after the toner image is transferred to the intermediate transfer belt  606 . Thus, the photosensitive drum  608  can stand by for the next image forming processing. 
     The intermediate transfer belt  606 , which is entrained around a driving roller  604 , a tension roller  605 , and the inner secondary transfer roller  603 , can rotate in the direction indicated by arrow “b”, as illustrated in  FIG. 1 . The Y, M, C, and Bk image forming units  613  perform parallel image forming processes of respective colors at predetermined timing to accurately overlap images with upstream toner images already transferred on the intermediate transfer belt  606 . As a result, the intermediate transfer belt  606  conveys a full-color toner image finally formed on the intermediate transfer belt  606  to the secondary transfer unit. As described above, a full-color toner image formed on the intermediate transfer belt  606  is transferred onto the sheet S fed to the secondary transfer unit. Then, a pre-fixing conveyance unit  67  conveys the sheet S to a fixing device  68 . 
     The fixing device  68  includes a pair of opposed rollers or belts that can apply a predetermined pressing force to the sheet S and a heat source (e.g., a heater) that can generate heat to melt and fix a toner image formed on the sheet S. A diverging conveyance device  69  receives the sheet S carrying a fixed image formed thereon and directly discharges the sheet S to a discharge tray  600 . When the image forming apparatus  60  performs two-sided image formation, the diverging conveyance device  69  can switch its conveyance path to convey the sheet S toward a reversing conveyance device  601 . 
     After completing a switchback motion in the reversing conveyance device  601 , the sheet S enters a two-sided conveyance device  602  with leading and trailing edges switched each other. Then, in synchronism with a conveyance gap between two sheets conveyed by the paper feeding device  61  instructed according to a following job, the conveyance unit  64  causes the sheet S coming from a re-feeding path  64   b  to enter the sheet conveyance path connected to the secondary transfer unit for two-sided image formation. Image forming processing for the reverse surface (second surface) is similar to the above-described processing for the front surface (first surface). 
     The switchback mechanism for the reversing conveyance device  601  is relatively simple in configuration and does not require a large space for reversing the sheet S. However, the switchback mechanism switches the leading and trailing edges of the sheet S in the process of reversing the sheet S. To ensure positioning of images formed on front and rear surfaces of the sheet S, it is necessary to set a common reference edge that regulates the position in a direction perpendicular to the sheet conveyance direction. 
     Therefore, the skew registration device  65  has a reference surface extending in a direction parallel to the sheet conveyance direction. The skew registration device  65  performs skew correction on the sheet S by causing one end of the sheet S to move along the reference surface. The skew registration device  65  is capable of accurately adjusting the transfer position of toner images formed on front and reverse surfaces of the sheet S. 
     In a sheet conveyance apparatus, if any skew or positional deviation of a sheet occurs during conveyance of the sheet, print accuracy may deteriorate when printing is performed on the sheet. Similarly, a conveyance path provided in an image forming apparatus is required to assure positioning accuracy for an image formed on a sheet, in particular, when image formation is performed on front and reverse surfaces of a sheet. 
     Accordingly, the image forming apparatus  60  includes the skew registration device  65  positioned immediately before (on the upstream side of) the secondary transfer unit that forms an image on a sheet surface. The skew registration device  65  corrects a skew or a positional deviation of a sheet occurring during conveyance of the sheet in a long conveyance path extending from the paper feeding device  61 . In other words, the skew registration device  65  assures higher accuracy in positioning an image. 
     The skew registration device  65  obliquely conveys a sheet toward an abutment reference member  71  while speedily conveying the sheet. Therefore, a side of the sheet contacts the abutment reference member  71 . Then, the sheet moves straight along the abutment reference member  71  (as illustrated in  FIG. 2 ). In other words, the skew registration device  65  can adjust the orientation of a conveyed sheet to be parallel to the sheet conveyance direction. 
     The skew registration device  65  contributes to a product excellent in image positioning accuracy. The skew registration device  65  can be applied to a high-end machine as a printing machine. However, such a high-end machine is often required to perform printing on a wide variety of sheets. Therefore, the skew registration device  65  is required to assure stable performances in aligning conveyed sheets, considering various material parameters (sheet size, thickness, grammage, friction coefficient, smoothness, etc.) as well as environmental parameters (temperature, humidity, etc.). 
       FIG. 2  illustrates a plan view of the conveyance unit including the skew registration device according to the first exemplary embodiment.  FIG. 3  illustrates a driving mechanism for the skew registration device.  FIG. 4  is a flowchart illustrating an example conveyance control operation.  FIG. 5  illustrates various phases of the conveyance control operation. 
     As illustrated in  FIG. 2 , the skew registration device  65  receives a sheet conveyed from a pre-registration conveyance unit  64 R (an upstream device) in the sheet conveyance direction indicated by an arrow “A” and sends the received sheet to a secondary transfer unit  66 P (a downstream device). The skew registration device  65  includes a skew registration unit  65 P configured to perform skew correction and side edge alignment for a received sheet and a slide unit  65 R configured to convey a sheet by a predetermined amount in a direction perpendicular to the sheet conveyance direction. Thus, the skew registration device  65  has a function of positioning a sheet in the axial direction of the external secondary transfer roller  66 . 
     The pre-registration conveyance unit  64 R includes pre-registration conveyance rollers  73  and  74  that receive a sheet conveyed along the conveyance guide  75  and convey the sheet in the direction indicated by arrow “A.” The pre-registration conveyance rollers  73  and  74  can rotate when the rollers  73  and  74  contact driven rollers (not illustrated) via openings formed on the conveyance guide  75  (see  FIG. 1 ). The pre-registration conveyance unit  64 R and the skew registration unit  65 P constitute a sheet conveyance apparatus that sets a conveyance reference on the center line extending in the sheet conveyance direction. 
     A pre-skew roller  12 , positioned on the center line of the sheet conveyance path, receives a sheet from the pre-registration conveyance roller  73  and obliquely conveys the sheet toward the abutment reference member  71 . The distance between a nip portion of a most upstream side skew roller  70   a  and a nip portion of the pre-skew roller  12  in the sheet conveyance direction is set to distance D 1 , as described below. 
     The skew registration unit  65 P includes a movable guide  11  and a stationary guide  10 , which cooperatively receive a sheet conveyed from the pre-registration conveyance unit  64 R. The skew registration unit  65 P includes three skew rollers  70   a ,  70   b , and  70   c , which can obliquely convey a sheet toward the abutment reference member  71  and bring a side of the sheet into line contact with the abutment reference member  71 . When the sheet starts sliding along the abutment reference member  71 , the sheet has an aligned orientation that coincides with the sheet conveyance direction. 
     The skew registration unit  65 P includes a movable unit  11 U and a stationary unit  10 U. The movable unit  11 U integrates the movable guide  11 , three skew rollers  70   a ,  70   b , and  70   c , and the abutment reference member  71 . The stationary unit  10 U includes the stationary guide  10 . The position of the movable unit  11 U is determined according to the size of sheets received from the pre-registration conveyance rollers  73  and  74 . 
     The pre-skew roller  12  and the skew rollers  70   a ,  70   b , and  70   c  obliquely convey a received sheet. A side of the obliquely conveyed sheet collides with the abutment reference member  71  (an example reference member). 
     The slide unit  65 R includes a conveyance guide  79 , a slide roller  7 , a pre-slide sensor  77 , and a post-slide sensor  78 . The slide roller  7  can slide in the direction indicated by arrow “B” while it rotates to convey a sheet. The slide roller  7  can adjust the thrust position of a sheet, which is skew-corrected by the skew registration unit  65 P, according to the image position on the intermediate transfer belt  606 . 
     As illustrated in  FIG. 1 , the secondary transfer unit  66 P includes the external secondary transfer roller  66 , the inner secondary transfer roller  603 , and the intermediate transfer belt  606 . Transfer timing of a toner image on the intermediate transfer belt  606  is adjustable by controlling arrival timing of a leading edge of a sheet based on a detection signal obtained by the post-slide sensor  78 . 
     As illustrated in  FIG. 3 , the setup position of three skew rollers  70   a ,  70   b , and  70   c  is sufficiently close to the abutment reference member  71  so as to prevent the sheet from generating any buckling and floating when a sheet is pressed against the abutment reference member  71 . If the distance from the sheet nipped by the skew rollers  70   a ,  70   b , and  70   c  to the abutment reference member  71  is long, the side edge alignment may be incomplete when a sheet thoroughly passes the abutment reference member  71 . 
     The skew rollers  70   a ,  70   b , and  70   c  (first skew rotary members) are in a fixed positional relationship with the abutment reference member  71 . The skew rollers  70   a ,  70   b , and  70   c  can contact driven rollers  70   i ,  70   j , and  70   k  via openings  11   h  formed on the movable guide  11 . A frictional force generated by the driven rollers  70   i ,  70   j , and  70   k , which contact a sheet, is smaller than a frictional force generated by the skew rollers  70   a ,  70   b , and  70   c . The altitudinal position where the driven rollers  70   i ,  70   j , and  70   k  contact the skew rollers  70   a ,  70   b , and  70   c  is slightly higher than a flat surface of the movable guide  11 . 
     The pre-skew roller  12  (second skew rotary member) is similar to the skew rollers  70   a ,  70   b , and  70   c  in configuration. The pre-skew roller  12  can contact a driven roller  12   j  via an opening  75   h  formed on the conveyance guide  75 . A frictional force generated by the driven roller  12   j , which contacts a sheet, is smaller than a frictional force generated by the pre-skew roller  12 . The altitudinal position where the driven roller  12   j  contacts the pre-skew roller  12  is slightly higher than an upper surface of the conveyance guide  75 . 
     An exemplary embodiment includes appropriate friction coefficients and cross-sectional shapes for the skew rollers  70   a ,  70   b , and  70   c  and the pre-skew roller  12 , which are employed so that a sheet can rotate along the abutment reference member  71  in a state where two or more of the skew rollers  70   a ,  70   b , and  70   c  and the pre-skew roller  12  obliquely convey the sheet. 
     A motor M 3  rotates the skew rollers  70   a ,  70   b  and  70   c , the pre-skew roller  12 , the pre-registration conveyance rollers  73  and  74  (see  FIG. 2 ), and the slide roller  7  (see  FIG. 2 ). The motor M 3  can convey a sheet, via the rollers, at a uniform speed in the direction indicated by arrow “A.” 
     A motor M 2 , serving as a contact/separation mechanism, elevates the skew rollers  70   a ,  70   b , and  70   c , the pre-skew roller  12 , and the pre-registration conveyance rollers  73  and  74  (see  FIG. 2 ) to enable the rollers to come into contact with or separate from the driven rollers  70   i ,  70   j ,  70   k , and  12   j.    
     When the skew rollers  70   a ,  70   b , and  70   c , and the pre-skew roller  12  move downward to contact a sheet, the pre-registration conveyance rollers  73  and  74  (see  FIG. 2 ) separate from the sheet. A motor M 1 , serving as a first adjustment unit, drives the movable unit  11 U. The movable unit  11 U is movable in the direction indicated by arrow “C.” In other words, the abutment reference member  71 , serving as a reference member, is movable in the sheet width direction. The motor M 1  can change the distance between the abutment reference member  71  and the pre-skew roller  12  according to the size of sheets. 
     A control unit  9  (an example control unit) performs control processing according to the flowchart illustrated in  FIG. 4 . When a pre-registration conveyance sensor  76  detects the leading edge of a sheet, the control unit  9  stops the sheet at a position corresponding to the pre-registration conveyance roller  73 . This is to reset a cumulative error with respect to passing timing of the leading edge, if such an error occurs during the conveyance of a paper. 
     In step S 11 , the control unit  9  acquires the size of a sheet. The control unit  9  determines a distance D 2  between the nip portion of the skew rollers  70   a ,  70   b , and  70   c  and the nip portion of the pre-skew roller  12  in the width direction of the sheet. More specifically, the control unit  9  sets the distance D 2  to be a half of the width of a conveyed sheet. 
     In step S 12 , the control unit  9  controls the motor M 1  to cause the movable unit  11 U to slide in the direction indicated by arrow “C” (in the width direction of the sheet). Thus, the control unit  9  stops the movable unit  11 U to stay at the position where the distance between the skew rollers  70   a ,  70   b , and  70   c  and the pre-skew roller  12  becomes D 2 . 
     In step S 13 , the control unit  9  controls the motor M 3  to cause the pre-registration conveyance rollers  73  and  74  to start conveying a sheet. In step S 14 , the control unit  9  determines, with reference to, for example, a timer count value, whether the sheet conveyed by the pre-registration conveyance rollers  73  and  74  has reached a position corresponding to the skew roller  70   a.    
     If the sheet has reached the skew roller  70   a  (YES in step S 14 ), the processing proceeds to step S 15 . In step S 15 , the control unit  9  causes the motor M 2  to lift the pre-registration conveyance rollers  73  and  74  to release a nipping force applied to the sheet. If the rollers  73  and  74  continuously nip the sheet, the sheet travels straight even after the skew roller  70   a  starts obliquely conveying the sheet. The skew roller  70   a  cannot smoothly convey the sheet obliquely. 
     On the other hand, the pre-skew roller  12  moves downward to nip the center of the sheet and starts obliquely conveying the sheet. Subsequently, the skew roller  70   a  nips an edge of the sheet and starts conveying the sheet. When the skew roller  70   a  starts conveying the sheet, a nipping force generated by the pre-skew roller  12  prevents the sheet from rotating. 
     In step S 16 , the pre-skew roller  12  and the skew roller  70   a  start obliquely conveying the sheet. Then, the skew rollers  70   b  and  70   c  successively start obliquely conveying the sheet toward the abutment reference member  71 . When the sheet collides with the abutment reference member  71 , the sheet starts rotating until the orientation of the sheet corresponds to the sheet conveyance direction. Namely, in step S 17 , the control unit  9  causes the skew registration device  65  to perform skew correction and side edge alignment. 
     In step S 18 , the control unit  9  determines whether the sheet has reached a position corresponding to the slide roller  7 . If the pre-slide sensor  77  detects the leading edge of the sheet, the control unit  9  determines that the sheet has reached the position corresponding to the slide roller  7  (YES in step S 18 ). In step S 19 , the control unit  9  causes the motor M 2  (an example contact/separation mechanism) to lift the pre-skew roller  12  and the skew rollers  70   a ,  70   b , and  70   c  to release the nipping force applied to the sheet. If the rollers  12 ,  70   a ,  70   b , and  70   c  continuously nip the sheet, the sheet rotates and inclines when the slide roller  7  slides. 
     In step S 20 , the control unit  9  causes the slide roller  7  to slide in the direction indicated by arrow “B” to adjust the sheet position according to a toner image of the intermediate transfer belt  606 . In step S 21 , the control unit  9  causes the slide roller  7  to convey the sheet to the secondary transfer unit. In step S 22 , the control unit  9  determines whether the job is complete. If the control unit  9  determines that the job is incomplete (NO in step S 22 ), the control unit  9  repeats the processing of steps S 11  through S 22 . 
       FIGS. 5A to 5C  illustrate various phases corresponding to the processing performed in steps S 13  to S 17  of  FIG. 4 , in which the sheet length in the conveyance direction is twice as much as the above-described distance D 1 .  FIG. 5A  illustrates a state immediately after the pre-registration conveyance roller  73  starts conveying the sheet S after the pre-registration conveyance roller  73  once stops the sheet S. At this moment, the pre-skew roller  12  does not give any nipping force to the sheet S. Only the pre-registration conveyance roller  73  gives a conveyance force F 1  to the sheet S. Thus, the sheet S travels in the direction indicated by arrow “A.” 
       FIG. 5B  illustrates a state where the leading edge of the sheet S has reached a nip portion of the registration roller  70   a . At this moment, the pre-registration conveyance rollers  73  and  74  do not give any nipping force to the sheet S. On the other hand, the pre-skew roller  12  gives a nipping force to the sheet S. Thus, two rollers (the skew roller  70   a  and the pre-skew roller  12 ) convey the sheet S. 
     The following is the relationship of forces acting on the sheet Sat this moment. The conveyance guide  75  generates a frictional resistance R acting on the sheet S when the skew roller  70   a  gives a conveyance force F 3  to the sheet S. The frictional resistance R concentrates on the centroid of the sheet S. 
     Therefore, if the relationship “sheet length in the sheet conveyance direction=distance D 1 ×2” is satisfied, the pre-skew roller  12  gives a conveyance force F 2  to the centroid of the sheet S. The conveyance force F 2  cancels the frictional resistance R. As a result, a rotational component R 1  decreases. The sheet S approaches the abutment reference member  71  without greatly changing its orientation as illustrated in  FIG. 5C . As described above, the example illustrated in  FIGS. 5A to 5C  satisfies the relationship “sheet length in the sheet conveyance direction=distance D 1 ×2” and can effectively reduce the rotational component R 1 . 
     The following is an example that does not satisfy the relationship “sheet length in the sheet conveyance direction=distance D 1 ×2.” In this case, the nip portion of the pre-skew roller  12  is offset from the centroid of the sheet S. A significant amount of rotational component R 1  is generated regardless of sheet size, because the setup position of the skew roller  70   a  is close to the abutment reference member  71  to prevent the sheet S from buckling when the sheet S collides with the abutment reference member  71 . 
     However, an exemplary embodiment can reduce the rotational component R 1  acting on each conveyed sheet by positioning the pre-skew roller  12  on a line passing the centroid of the sheet S and extending in the sheet conveyance direction. More specifically, the control unit  9  moves the movable unit  11 U to a position where the distance D 2  ( FIG. 2 ) becomes a half of the sheet length in the direction perpendicular to the sheet conveyance direction (i.e., sheet size in the width direction). 
     To reduce the rotational component R 1 , it is ideal that the relationship “sheet length in the sheet conveyance direction=distance D 1 ×2” can be satisfied for various types of sheets having different lengths in the sheet conveyance direction. To this end, an exemplary embodiment provides a second adjustment unit configured to move the pre-skew roller  12  and the driven roller  12   j  in the sheet conveyance direction. The second adjustment unit moves the pre-skew roller  12  and the driven roller  12   j  to a position corresponding to a half of the length of the conveyed sheet S in the sheet conveyance direction. 
     In other words, the second adjustment unit can change the distance D 1  between the nip portion of the skew roller  70   a  and the nip portion of the pre-skew roller  12  in the sheet conveyance direction. The pre-skew roller  12  starts conveying the sheet S when the center of the sheet S in the conveyance direction reaches the pre-skew roller  12 . As indicated by a dotted line in  FIG. 2 , the second adjustment unit can move the pre-skew roller  12  to an appropriate position corresponding to one of predetermined distances D 1 , D 1 ′, . . . . 
     As described above, the first exemplary embodiment can reduce the rotational component R 1  acting on the sheet S to enable the sheet S to collide with the abutment reference member  71  at an appropriate (moderate) angle. Therefore, compared to first to third comparative examples described below, the first exemplary embodiment can prevent the leading or trailing edge of the sheet S from colliding at a steep angle with the abutment reference member  71 . Thus, the first exemplary embodiment does not damage an abutting edge of the sheet S and eliminates defective abutment caused when an abutting operation is erroneous. The sheet conveyance apparatus and the image forming apparatus according to the first exemplary embodiment can flexibly perform print processing on various print media. 
     Furthermore, the first exemplary embodiment can stabilize the orientation of a conveyed sheet so that the sheet can smoothly contact the abutment reference member  71 . Therefore, compared to the first to third comparative examples, the first exemplary embodiment can locate the skew rollers  70   a ,  70   b , and  70   c  close to the abutment reference member  71  while setting a required margin for a rotated sheet. Thus, the first exemplary embodiment can reduce the length of the abutment reference member  71  and can downsize the skew registration device  65 . Thus, the image forming apparatus according to the first exemplary embodiment can perform printing on a sheet having lower rigidity. 
     Furthermore, in the first exemplary embodiment, the skew roller  70   a  (first skew rotary member) and the pre-skew roller  12  (second skew rotary member) move to predetermined positions corresponding to an edge and the center of a conveyed sheet in the direction perpendicular to the sheet conveyance direction. Therefore, the first exemplary embodiment can reduce a rotational component acting on a sheet and prevent the sheet from rotating. The setup position of each sensor is not limited to the example illustrated in  FIG. 2 . The control timing is not limited to the example illustrated in  FIG. 4 . 
     An exemplary embodiment includes descriptions (e.g., “centroid” and “half”) designating definite positions. However, any description relating to the position does not intend to limit the scope of the present invention. In fact, actual “centroid” and “half” positions tend to deviate from designed positions due to differences in tolerance or conveyance accuracy, and similar effects of the present invention can be assured. Thus, the positional description in an exemplary embodiment does not narrowly limit the present invention. 
     In the first exemplary embodiment, the pre-skew roller  12  and the skew rollers  70   a ,  70   b  and  70   c  are driving rollers, and the driven rollers  12   j ,  70   i ,  70   j , and  70   k  are driven rollers. However, the pre-skew roller  12  and the skew rollers  70   a ,  70   b , and  70   c  can be driven rollers, and the driven rollers  12   j ,  70   i ,  70   j , and  70   k  also can be driving rollers. Alternatively, all of them can be driving rollers. 
     In the first exemplary embodiment, the pre-skew roller  12  and the skew rollers  70   a ,  70   b  and  70   c  nip one surface of a sheet and the driven rollers  12   j ,  70   i ,  70   j , and  70   k  nip the other surface of the sheet. However, the driven rollers  12   j ,  70   i ,  70   j , and  70   k  are replaceable with a flat surface of a member with a small friction. The pre-skew roller  12  and the skew rollers  70   a ,  70   b , and  70   c  can be friction rollers capable of rotating the surfaces opposing thereto in an idling state. The above-described modifications are applicable to second and third exemplary embodiments described below. 
     First Comparative Example 
       FIG. 6  illustrates a plan view of a skew registration device according to a first comparative example (an example background art).  FIGS. 7A and 7B  illustrate various phases of a conveyance control operation according to the first comparative example. The first comparative example includes a skew registration device  65 E, which is replaceable with the skew registration device  65  illustrated in  FIG. 1 . The same reference numerals denote similar or common components illustrated in  FIGS. 1 and 6 . 
     As illustrated in  FIG. 6 , the pre-registration conveyance unit  64 R receives a sheet placed on the conveyance guide  75 . The pre-registration conveyance rollers  73  and  74  convey the sheet in the direction indicated by arrow “A.” The pre-registration conveyance unit  64 R and the skew registration unit  65 P constitute a sheet conveyance apparatus that sets a conveyance reference on the center line extending in the sheet conveyance direction. The skew registration unit  65 P includes skew rollers  70   a ,  70   b , and  70   c  inclined by an angle α relative to the sheet conveyance direction, which can obliquely convey a sheet S placed on a stationary conveyance guide  72  toward the abutment reference member  71 . 
     While the sheet S speedily moves in the conveyance direction, the skew rollers  70   a ,  70   b , and  70   c  obliquely convey the sheet S toward the abutment reference member  71 . When the sheet S collides with the abutment reference member  71 , the sheet S starts rotating to change its orientation. Namely, the skew rollers  70   a ,  70   b , and  70   c  and the abutment reference member  71  cooperatively perform skew correction to align a side of the conveyed sheet S along the abutment reference member  71 . At this moment, the pre-registration conveyance rollers  73  and  74  located on the upstream side do not give any nipping force to the sheet S. In other words, the pre-registration conveyance rollers  73  and  74  do not interface with the skew rollers  70   a ,  70   b , and  70   c  that obliquely convey the sheet S. 
     As illustrated in  FIG. 7A , the sheet S receives a conveyance force F 3  from the skew roller  70   a  in a state where the skew roller  70   a  nips the sheet S. The skew roller  70   a  obliquely conveys the sheet S in the direction corresponding to the conveyance force F 3 . A resistance force R acts on a centroid G of the sheet S due to friction between the conveyance guide  75  and the sheet S. The resistance force R and the conveyance force F 3  are mutually opposite. 
     At this moment, the resistance force R is a sum of a component R 1  and a component R 2 . The component R 1  is a rotational component causing the sheet S to rotate around the skew roller  70   a . The component R 2  resists the movement of the sheet S obliquely conveyed. A moment M generated by the rotational component R 1  rotates the conveyed sheet S. As a result, an abutting reference edge of the sheet S collides with an inlet edge E of the abutment reference member  71  as illustrated in  FIG. 7B . 
     According to the first comparative example, the moment M constantly appears irrespective of the size of the sheet S in the state illustrated in  FIG. 7A . In particular, when the sheet S is an A4-size sheet conveyed with a short side aligned in the conveyance direction, the length of an abutting reference edge becomes shorter than the sheet width. Thereby, the rotational component R 1  becomes larger. In addition, when the sheet S is a lightweight sheet, the sheet may be damaged. When the sheet S rotates unnecessarily, the sheet S is damaged and conveyance jam may occur. Accuracy in the sheet skew alignment deteriorates, and the quality of a printing product deteriorates. 
     Second Comparative Example 
       FIGS. 8A to 8C  illustrate various phases of a conveyance control operation performed by a skew registration device according to a second comparative example (an example background art). The second comparative example includes a skew registration device  65 F, which is replaceable with the skew registration device  65  illustrated in  FIG. 1 . The same reference numerals common to  FIGS. 8A to 8C  and  FIG. 2  denote similar or common components. 
     As illustrated in  FIG. 8A , the skew registration device  65 F includes skew rollers  80  and  81 , which are inclined with respect to the conveyance direction. The skew rollers  80  and  81  obliquely convey the sheet S placed on the stationary conveyance guide  72  toward the abutment reference member  71 . In the second comparative example, a distance x 1  between the abutment reference member  71  and the skew roller  80  is greater than a distance x 2  between the abutment reference member  71  and the skew roller  81 . 
     As illustrated in  FIG. 8A , when the skew roller  80  nips the sheet S, friction between the sheet S and the conveyance guide  75  generates a resistance force R. A rotational component R 1  of the resistance force R generates a moment M 1 , which causes the sheet S to rotate in the clockwise direction. Thus, the trailing edge of the sheet S moves toward the abutment reference member  71 . Then, as illustrated in  FIG. 8B , two skew rollers  80  and  81  nip the inclined sheet S. A moment M 2  generated at this moment causes the sheet S to rotate in the counterclockwise direction. The sheet S quickly approaches the abutment reference member  71 . 
     As a result, as illustrated in  FIG. 8C , the leading edge of the sheet S abuts the abutment reference member  71 . A moment M 3  generated at this moment causes the sheet S to rotate in the clockwise direction. Then, the orientation of the sheet S is aligned along the abutment reference member  71 . The second comparative example satisfies the relationship distance x 1 &gt;distance x 2 . The upstream skew roller  80  conveys the sheet S at a position closer to the centroid. Therefore, the rotational component R 1  generated in the second comparative example is smaller than the rotational component R 1  generated in the first comparative example. 
     However, if the sheet S is an A4-size sheet conveyed with a short side aligned in the conveyance direction, the sheet S may collide with the abutment reference member  71  as illustrated in  FIG. 7B . According to the relationship distance x 1 &gt;distance x 2 , the leading edge of the sheet S rotates in a direction departing from the abutment reference member  71  in the state illustrated in  FIG. 8A . Therefore, behavior of the sheet S becomes unstable due to alternate changes in the rotational direction after the sheet S reaches the skew roller  80  and until the skew correction is completed. 
     Third Comparative Example 
       FIGS. 9A to 9C  illustrate various phases of a conveyance control operation performed by a skew registration device according to a third comparative example (an example background art). The third comparative example includes askew registration device  65 G, which is replaceable with the skew registration device  65  illustrated in  FIG. 1 . The same reference numerals common to  FIGS. 9A to 9C  and  FIG. 2  denote similar or common components. 
     As illustrated in  FIG. 9A , the skew registration device  65 G includes skew rollers  91  and  92 , which are inclined with respect to the conveyance direction. The skew rollers  91  and  92  obliquely convey the sheet S placed on the stationary conveyance guide  72  toward the abutment reference member  71 . The skew registration device  65 G according to the third comparative example constitutes a sheet conveyance apparatus that sets a conveyance reference on the center line extending in the sheet conveyance direction. The skew rollers  91  and  92  are located at both sides of the conveyance center  90  of the sheet S extending in the sheet conveyance direction. The upstream skew roller  91  is farther from the abutment reference member  71  than the conveyance center  90 . The downstream skew roller  92  is closer to the abutment reference member  71  than the conveyance center  90 . 
     As illustrated in  FIG. 9A , when the upstream skew roller  91  nips the sheet S, a resistance force R acts on the centroid G of the sheet S. A rotational component R 1  of the resistance force R generates a moment M 1 , which causes the sheet S to rotate in the counterclockwise direction. Thus, the leading edge of the sheet S moves toward the abutment reference member  71 . 
     Then, as illustrated in  FIG. 9B , two skew rollers  91  and  92  nip the sheet S. At this moment, the rotational component R 1  of the resistance force R, which acts on the centroid G of the sheet S, generates a moment M 2  to rotate the sheet in the same direction (counterclockwise direction). The moment M 2  causes the leading edge of the sheet S to quickly approach the abutment reference member  71 . 
     As a result, the leading edge of the sheet S collides with the abutment reference member  71 . At this moment, the sheet is greatly inclined as illustrated in  FIG. 9C . A large rotational angle is required to bring the sheet S into a state where a side of the sheet S is aligned along the abutment reference member  71 . 
     In the third comparative example, until the leading edge of the sheet S collides with the abutment reference member  71 , the moments M 1  and M 2  act on the sheet S in the same direction. Therefore, the leading edge of the sheet S smoothly approaches the abutment reference member  71 . After the leading edge of the sheet S collides with the abutment reference member  71 , the moment M 3  acts on the sheet S in the opposite direction. Therefore, the trailing edge of the sheet S can approach the abutment reference member  71 . 
     As a result, compared to the second comparative example, behavior of the sheet S is stable. The sheet S does not collide with the edge of the abutment reference member  71  (see  FIG. 7B ). However, compared to the second comparative example, the sheet S requires a large rotational angle to reach the abutment reference member  71  at its leading edge. The distance  51  illustrated in  FIG. 9B  gradually becomes larger toward the distance S 2  illustrated in  FIG. 9C . 
     Therefore, in the skew correction (the state illustrated in  FIG. 9C ), the moment M 3  may be insufficient to completely align the sheet S along the abutment reference member  71 . Such a problem arises when the friction between the conveyance guide  75  and the sheet S is large, or when the sheet S is a thick paper or any other sheet having a large grammage. 
     Second Exemplary Embodiment 
       FIG. 10  illustrates a plan view of a skew registration device according to a second exemplary embodiment of the present invention. The second exemplary embodiment includes a skew registration device  65 B, which is replaceable with the skew registration device  65  illustrated in  FIG. 1 . The same reference numerals common to  FIG. 10  and  FIG. 2  denote similar or common components. 
     As illustrated in  FIG. 10 , the skew registration device  65 B performs skew correction on a sheet received from the pre-registration conveyance rollers  73  and  74  and conveyed in the direction indicated by arrow “A”, and sends the skew-corrected sheet to the slide roller  7 . 
     The second exemplary embodiment includes a slide unit  65 R and a secondary transfer unit  66 P, which are similar to those described in the first exemplary embodiment. The slide roller  7  receives a skew-corrected sheet from the skew registration device  65 B and slides in the direction indicated by arrow “B”, to adjust the thrust position of the skew-corrected sheet held by the slide roller  7  according to a toner image on the intermediate transfer belt  606  (illustrated in  FIG. 1 ). 
     In the skew registration device  65 B, the skew rollers  70   a ,  70   b , and  70   c  and the pre-skew roller  12  obliquely convey a sheet placed on the conveyance guide  72  toward the reference member  71  to align a side of the sheet along the reference member  71 . 
     A sheet conveyance apparatus including the skew registration device  65 B according to the second exemplary embodiment sets a conveyance reference on one end (the abutment reference member  71 ) of the apparatus. Therefore, the center-line position of a sheet is variable according to the sheet size in the direction perpendicular to the sheet conveyance direction (sheet width). The conveyance guide  72  used for the skew registration device  65 B is a stationary type, which is different from the conveyance guide of the first exemplary embodiment including two separated parts (the stationary guide  10  and the movable guide  11 ). 
     In the second exemplary embodiment, the pre-skew roller  12  and the driven roller  12   j  (illustrated in  FIG. 3 ) are integrally movable as a unit in the direction perpendicular to the sheet conveyance direction (the direction indicated by arrow “C”) to set a predetermined distance between the pre-skew roller  12  and the conveyance guide  72  according to the size of sheets. The opening  75   h  formed on the conveyance guide  75  illustrated in  FIG. 3  has a length comparable to the maximum stroke of the conveyance roller  12  and the driven roller  12   j.    
     The control unit  9  (illustrated in  FIG. 3 ) controls a driving mechanism configured to move the integrated unit of the pre-skew roller  12  and the driven roller  12   j  in the direction indicated by arrow “C” to place the pre-skew roller  12  on the center line of a sheet extending in the sheet conveyance direction. More specifically, D 2  represents the distance between a nip portion of the skew rollers  70   a ,  70   b , and  70   c  and a nip portion of the pre-skew roller  12  in the direction perpendicular to the sheet conveyance direction. The control unit  9  equalizes the distance D 2  with a half of the width of a conveyed sheet by adjusting the position of the pre-skew roller  12 . 
     The pre-skew roller  12  and the skew rollers  70   a ,  70   b , and  70   c  perform skew correction on a conveyed sheet according to a method similar to that described in the first exemplary embodiment with reference to  FIGS. 4 and 5A  to  5 C. However, as described above, the conveyance reference is set on one side (the abutment reference member  71 ). Therefore, the second exemplary embodiment differs from the first exemplary embodiment in that the pre-skew roller  12  moves to a position where the relationship “distance D 2 =half of sheet width” is satisfied. 
     Accordingly, in the second exemplary embodiment,  FIGS. 5A to 5C  illustrate the phases of the conveyance control operation performed after the pre-skew roller  12  has already moved to the predetermined position according to the selected size of a conveyed sheet. The relationship of forces acting on the sheet S and the timing for applying a nipping force by the pre-skew roller  12  are similar to those described in the first exemplary embodiment. 
     Similar to the first exemplary embodiment, a distance between the nip portion of the skew roller  70   a  and the nip portion of the pre-skew roller  12  in the sheet conveyance direction is equal to the distance D 1 . More specifically, the second exemplary embodiment can effectively reduce the rotational component R 1  by satisfying the relationship “sheet length in the sheet conveyance direction=distance D 1 ×2.” 
     However, even if the above-described relationship is not satisfied, the second exemplary embodiment can effectively reduce the rotational component R 1  for a wide variety of sheets having different sheet sizes, because the second exemplary embodiment equalizes the distance D 2  with a half of the sheet width and causes the pre-skew roller  12  to start obliquely conveying a sheet at the predetermined position (on a line passing the centroid G of the sheet and extending in the sheet conveyance direction). 
     To reduce the rotational component R 1 , it is ideal that the relationship “sheet length in the sheet conveyance direction=distance D 1 ×2” can be satisfied for two or more sheets having different lengths in the sheet conveyance direction. More specifically, an exemplary embodiment provides a second adjustment unit configured to move the pre-skew roller  12  and the driven roller  12   j  in the sheet conveyance direction. The second adjustment unit moves the pre-skew roller  12  and the driven roller  12   j  to a position corresponding to a half of the length of the conveyed sheet S in the sheet conveyance direction. 
     In other words, the second adjustment unit can change the distance D 1  between the nip portion of the skew roller  70   a  and the nip portion of the pre-skew roller  12  in the sheet conveyance direction. The pre-skew roller  12  starts conveying the sheet S when the center of the sheet S in the conveyance direction reaches the pre-skew roller  12 . As indicated by a dotted line in  FIG. 10 , the second adjustment unit can move the pre-skew roller  12  to an appropriate position corresponding to one of predetermined distances D 1 , D 1 ′, . . . . 
     Although the second exemplary embodiment does not set the conveyance reference to the center, the sheet conveyance apparatus and the image forming apparatus according to the second exemplary embodiment can flexibly perform print processing on various print media. As a modified embodiment, the sheet conveyance apparatus illustrated in  FIG. 10  can set the conveyance reference to the other side far from the abutment reference member  71 . 
     Third Exemplary Embodiment 
       FIG. 11  illustrates a plan view of a skew registration device according to a third exemplary embodiment of the present invention. The third exemplary embodiment includes a skew registration device  65 C, which is replaceable with the skew registration device  65  illustrated in  FIG. 1 . The same reference numerals common to  FIG. 11  and  FIG. 2  denote similar or common components. 
     As illustrated in  FIG. 11 , the skew registration device  65 C performs skew correction on a sheet received from the pre-registration conveyance rollers  73  and  74  and conveyed in the direction indicated by arrow “A”, and sends the skew-corrected sheet to the slide roller  7 . 
     The third exemplary embodiment includes a slide unit  65 R and a secondary transfer unit  66 P, which are similar to those described in the first exemplary embodiment. The slide roller  7  receives a skew-corrected sheet from the skew registration device  65 C and slides in the direction indicated by arrow “B”, to adjust the thrust position of the skew-corrected sheet held by the slide roller  7  according to a toner image on the intermediate transfer belt  606  (illustrated in  FIG. 1 ). 
     The pre-registration conveyance unit  64 R and the skew registration unit  65 P according to the third exemplary embodiment constitute a sheet conveyance apparatus that sets a conveyance reference on the center line extending in the sheet conveyance direction. The skew registration device  65 C according to the third exemplary embodiment is similar to the skew registration device  65  described in the first exemplary embodiment. The skew registration device  65 C obliquely conveys a sheet placed on the movable guide  11  of the movable unit  11 U and the stationary guide  10  toward the reference member  71  to align a side of the sheet along the reference member  71 . 
     The movable unit  11 U can move in the direction indicated by arrow “C” to a predetermined position determined according to the size of a conveyed sheet. More specifically, D 2  represents the distance between a nip portion of the skew rollers  70   a ,  70   b , and  70   c  and a nip portion of the pre-skew roller  12  in the direction perpendicular to the sheet conveyance direction. The control unit  9  ( FIG. 3 ) sets the position of the movable unit  11 U to equalize the distance D 2  with a half of the width of a conveyed sheet. 
     In the third exemplary embodiment, three pre-skew rollers  12 ,  13 , and  14  are positioned on the center line (conveyance reference) of the pre-registration conveyance unit  64 R. Each of the three pre-skew rollers  12 ,  13 , and  14  is independently elevatable. D 1 , D 1 ′, and D 1 ″ represents distances from the nip portions of respective pre-skew rollers  12 ,  13 , and  14  to the nip portion of the skew roller  70   a  in the sheet conveyance direction. Therefore, the pre-skew rollers  12 ,  13 , and  14  can contact a sheet at different positions in the sheet conveyance direction. 
     For example, the distances D 1 , D 1 ′, and D 1 ″ are equal to half-lengths of A4, A4R, and A3 sheets, respectively, in the sheet conveyance direction. The control unit  9  can select an appropriate one of the pre-skew rollers  12 ,  13 , and  14  according to the centroid position of a conveyed sheet. More specifically, each of the pre-skew rollers  12 ,  13 , and  14  (second adjustment unit) includes a nip releasing mechanism. The control unit  9  selects an optimum pre-skew roller according to the size of a conveyed sheet so that the selected pre-skew roller can nip a portion closest the centroid of the sheet. 
     The pre-skew rollers  12 ,  13 , and  14  and the skew rollers  70   a ,  70   b , and  70   c  perform skew correction on a sheet according to a method similar to that described with reference to  FIGS. 4 and 5A  to  5 C in the first exemplary embodiment. However, as described above, the third exemplary embodiment differs from the second exemplary embodiment in that the control unit  9  selects an optimum one from among a plurality of pre-skew rollers  12 ,  13 , and  14 . Therefore, the third exemplary embodiment replaces the pre-skew roller  12  described in the first exemplary embodiment with the pre-skew roller  13  or the pre-skew roller  14  according to the size of a sheet. However, the relationship of forces acting on the sheet S and the timing for applying a nipping force by the selected pre-skew roller ( 12 ,  13 , or  14 ) are similar to those described in the first exemplary embodiment. 
     The third exemplary embodiment can greatly reduce the rotational component R 1  regardless of the size of a conveyed sheet, and enables the sheet to approach the abutment reference member  71  smoothly. The number of pre-skew rollers illustrated in  FIG. 11  is not limited to three and can be four or above suitable for the sizes of sheets. 
     If the number of installable pre-skew rollers is limited, it is desirable to locate the pre-skew rollers at positions corresponding to the sizes of frequently used sheets. In this case, if a conveyed sheet has a non-defined size, the control unit  9  selects an optimum pre-skew roller positioned closest to the centroid of the sheet to reduce the rotational component R 1 . 
     The arrangement of a plurality of pre-skew rollers, employed for the sheet conveyance apparatus illustrated in  FIG. 11 , is applicable to a sheet conveyance apparatus that sets a conveyance reference on one side as described in the second exemplary embodiment. In this case, as described in the second exemplary embodiment, the sheet conveyance apparatus includes a mechanism for moving a plurality of pre-skew rollers in the direction perpendicular to the sheet conveyance direction. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures, and functions.