Patent Publication Number: US-8523179-B2

Title: Document device and image forming apparatus with offset driven roller

Description:
INCORPORATION BY REFERENCE 
     This application is based on and claims the benefit of priority from Japanese Patent Application No. 2011-049139, filed on 7 Mar. 2011, the content of which is incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a document feeding device that conveys a document to a document reader, and an image forming apparatus provided with the document feeding device. 
     A document feeding device generally causes a document to be nipped at nips formed between a plurality of drive rollers disposed at predetermined intervals in a direction orthogonal to a feeding direction and a plurality of driven rollers disposed to face the drive rollers, respectively, feeding the document to a document reader through the rotation driven by the drive rollers. If the nips formed between a plurality of facing pairs of rollers are aligned in a direction orthogonal to the feeding direction, a rear end of the fed document will be released from the nips simultaneously or substantially simultaneously, when the document is fed by such a document feeding device. Since a feeding load drastically changes (reduces) at the moment of the rear end of the document being released from the nips, a feeding speed of the document temporarily accelerates. This may affect the feeding speed of the document in the document reader, leading to defective reading. 
     In order to reduce such change in the feeding load, a document feeding device has been conventionally proposed in which nips are formed not in line with each other in a direction orthogonal to a feeding direction. This is implemented by arranging respective rotational shafts of a plurality of rollers, either a plurality of drive rollers or a plurality of driven rollers, offset with respect to the feeding direction. 
     SUMMARY 
     However, the conventional document feeding device requires that the rotational shafts of the plurality of rollers and supporting members for supporting the rotational shafts be independently provided. This may lead to an increase in the number of parts, rendering the structure and assembly of an apparatus to be complex, and an increase in cost. In addition, as the plurality of rollers needs to be assembled individually, a problem arises in that variations may tend to occur in accuracies of positions of the rollers and distances (dimensions) therebetween, which may prevent stable feeding of a document. 
     The present disclosure provides a document feeding device that realizes stable feeding of documents by suppressing sudden change in the feeding load with a simple structure of a reduced number of parts. 
     The present disclosure also provides an image forming apparatus provided with such a document feeding device. 
     The present disclosure relates to a document feeding device, which includes a document reader, a drive roller and a plurality of driven rollers. The document reader is configured to read a document fed in a feeding direction. The drive roller is disposed upstream of the document reader in the feeding direction. The plurality of driven rollers is disposed opposite to the drive roller to form nips therebetween. The plurality of driven rollers is supported by one common shaft extending in a direction orthogonal to the feeding direction. A position of at least one nip formed by at least one of the plurality of driven rollers is offset downstream or upstream of other nips formed by other driven rollers. 
     The present disclosure also relates to an image forming apparatus, which includes the document feeding device described above and an image forming unit. The image forming unit is configured to form an image on an image recording medium shaped like a sheet based on image information acquired by the document reader in reading a document. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a left side view illustrating an arrangement of respective components of a copy machine  1  as an embodiment of an image forming apparatus of the present disclosure; 
         FIG. 2  is a schematic view illustrating a positional relationship between a document feeding mechanism  44  and a document lifting member  32  in a state in which a downstream end of the document lifting member  32  is lowered in the document feeding device  700  shown in  FIG. 1 ; 
         FIG. 3  is a schematic view illustrating a positional relationship between the document feeding mechanism  44  and the document lifting member  32  in a state in which the downstream end of the document lifting member  32  is raised in the document feeding device  700  shown in  FIG. 1 ; 
         FIG. 4  is a perspective view of respective roller groups arranged along a feeding path L 10  for reading a document, when viewed from inside a feeding mechanism housing  701 ; 
         FIG. 5  is a plan view illustrating an overall configuration of a pair of registration rollers  70  and a pair of first feeding rollers  90  in a state in which a cover member  702  is removed; 
         FIG. 6  is an enlarged cross-sectional view illustrating the vicinity of the pair of first feeding rollers  90 ; 
         FIG. 7  is an enlarged perspective view illustrating the vicinity of the pair of first feeding rollers  90 ; 
         FIG. 8  is an overall perspective view illustrating driven rollers  902  and a central driven roller  903  of the pair of first feeding rollers  90 ; and 
         FIG. 9  is a perspective view illustrating a detailed configuration of the central driven roller  903 . 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present disclosure will be described hereinafter with reference to the drawings. First, the overall structure of a copy machine  1  as an embodiment of an image forming apparatus according to the present disclosure will be described with reference to  FIGS. 1 to 3 .  FIG. 1  is a left side view illustrating an arrangement of respective components of the copy machine  1  as an embodiment of an image forming apparatus of the present disclosure.  FIG. 2  is a schematic view illustrating a positional relationship between a document feeding mechanism  44  and a document lifting member  32  in a state in which a downstream end of the document lifting member  32  is lowered in a document feeding device  700  shown in  FIG. 1 .  FIG. 3  is a schematic view illustrating a positional relationship between the document feeding mechanism  44  and the document lifting member  32  in a state in which the downstream end of the document lifting member  32  is raised in the document feeding device  700  shown in  FIG. 1 . 
     As shown in  FIG. 1 , the copy machine  1  according to the present embodiment includes a document reading device  200  and an apparatus main body M. The document reading device  200  includes a document reader  201  and a document feeding device  700 . The document feeding device  700  is disposed perpendicularly above the document reader  201  and automatically feeds a document G to the document reader  201 . The apparatus main body M is disposed perpendicularly below the document reading device  200  and forms a toner image on a sheet of paper T (recording medium shaped like a sheet) based on image information of the document G read by the document reader  201 . 
     First, the document feeding device  700  will be briefly described. Thereafter, the document reader  201  of the document reading device  200  and the apparatus main body M will be described, and finally the document feeding device  700  will be described in detail. As illustrated in  FIGS. 1 to 3 , the document feeding device  700  includes a feeding mechanism housing  701 , a document placement portion  710 , and a document collecting portion  720 . 
     The feeding mechanism housing  701  houses a feed roller unit  40  (described later) and a document feeding mechanism  44  (described later) in the document feeding device  700 . The document placement portion  710  is where documents G are stacked, images of which are to be read. The document collecting portion  720  is disposed perpendicularly below the document placement portion  710 , and collects documents G having passed predetermined document reading points J 1 , J 2  (described later). 
     As shown in  FIGS. 2 and 3 , the document feeding device  700  of the present embodiment sequentially feeds the documents G placed on the document placement portion  710  to the first document reading point J 1  lying upstream in a feeding direction D 1  and the second document reading point J 2  lying downstream by means of the document feeding mechanism  44  provided inside the feeding mechanism housing  701 . The document feeding device  700  of the present embodiment discharges a document G on which an image reading process has been completed by the document reader  201  to a document discharging space K. 
     The detailed configuration of the document placement portion  710 , and the configuration of the feed roller unit  40  and the document feeding mechanism  44  inside the feeding mechanism housing  701  will be described after explanation of the apparatus main body M. 
     Next, the document reader  201  will be described. As shown in  FIGS. 2 and 3 , the document reader  201  includes a first reading unit  201 A which is composed of a contact image sensor (CIS), and a second reading unit  201 B. The first reading unit  201 A is disposed at the first document reading point J 1  lying upstream in the feeding direction D 1  of the document feeding mechanism  44 , and reads image information on a back face of a document G. The second reading unit  201 B is disposed at the second document reading point J 2  lying downstream in the feeding direction D 1 , and reads image information on a front face of the document G. The second reading unit  201 B lying downstream in the feeding direction D 1  includes an illumination unit including a light source, a plurality of mirrors forming light paths, an imaging lens, a CCD, and a CCD board. 
     The document reader  201  is configured to read the image information of front and back faces of the document G by the first reading unit  201 A and the second reading unit  201 B without turning over the document G and to perform a predetermined process on the read image information of the front and back faces of the document G. Also the document reader  201  is configured to output the image information thus processed to an image forming unit GK in the apparatus main body M. 
     Next, the apparatus main body M will be described. As illustrated in  FIG. 1 , the apparatus main body M includes the image forming unit GK that forms an image on a sheet of paper T based on the image information sent from the document reader  201 , and a paper feeding/discharging unit KH that supplies a sheet of paper T to the image forming unit GK and discharges the sheet of paper T with an image formed thereon. 
     As illustrated in  FIG. 1 , the image forming unit GK includes a photoreceptor drum  2 , a charging unit  10 , a laser scanner unit  4 , a developing unit  16 , a toner cartridge  5 , a toner supply device  6 , a drum cleaning device  11 , a transfer roller  8 , and a fixing device  9 . 
     The paper feeding/discharging unit KH includes a paper feeding cassette  52 , a manual feeding portion  64 , a pair of registration rollers  80 , and a feeding path L for a sheet of paper T. 
     The photoreceptor drum  2  is composed of a cylindrical member, and functions as an image bearing member. The photoreceptor drum  2  is disposed rotatably about an axis perpendicular to a paper surface of  FIG. 1  in the apparatus main body M. An electrostatic latent image is formed on a surface of the photoreceptor drum  2 . 
     The charging unit  10  is disposed on a vertically upper side (upward) of the photoreceptor drum  2 . The charging unit  10  positively charges (in a positive polarity) the surface of the photoreceptor drum  2  uniformly. 
     The laser scanner unit  4  is spaced away from the photoreceptor drum  2  on a vertically upper side (upward) of the photoreceptor drum  2 . The laser scanner unit  4  includes a laser light source, a polygon mirror, a polygon mirror driving motor and the like, which are not illustrated. 
     The laser scanner unit  4  scans and exposes the surface of the photoreceptor drum  2  based on image information output by the image reading device  200 . By being scanned and exposed by the laser scanner unit  4 , an electric charge charged onto the surface of the photoreceptor drum  2  is removed. In this way, an electrostatic latent image is formed on a surface of the photoreceptor drum  2 . 
     The developing unit  16  is disposed in front of the photoreceptor drum  2  (on the right side of  FIG. 1 ). The developing unit  16  develops a single color (usually black) toner image on the electrostatic latent image formed on the photoreceptor drum  2 . The developing unit  16  includes a developing roller  17  that can be disposed facing the photoreceptor drum  2 , and a stirring roller  18  for stirring toner. 
     The toner cartridge  5  contains toner that is to be supplied to the developing unit  16 . The toner feeding device  6  supplies the toner stored in the toner cartridge  5  to the developing unit  16 . 
     The drum cleaning device  11  is disposed behind the photoreceptor drum  2  (on the left side as shown in  FIG. 1 ). The drum cleaning device  11  removes remaining toner and attached matters on the surface of the photoreceptor drum  2 . 
     The transfer roller  8  causes a toner image developed on the surface of the photoreceptor drum  2  to be directly transferred to a sheet of paper T. A voltage application means (not illustrated) applies a transfer bias to the transfer roller  8 , the bias being for transferring the toner image developed on the photoreceptor drum  2  to the sheet of paper T. The toner image developed on the surface of the photoreceptor drum  2  can be transferred to the sheet of paper T indirectly through an intermediate transfer belt or the like. 
     The transfer roller  8  is configured to be movable between a contact position and a separated position. That is to say, the transfer roller  8  is in contact with or spaced away from the photoreceptor drum  2  at the contact position or the separated position, respectively. In particular, the transfer roller  8  is moved to the contact position for transferring the toner image developed on the photoreceptor drum  2  to the sheet of paper T, and to the separated position in all other circumstances. The sheet of paper T is sandwiched by the photoreceptor drum  2  and the transfer roller  8 , and is pressed onto the surface of the photoreceptor drum  2  (a side on which the toner image is developed). A transfer nip N for transferring an image is thus formed and the toner image developed on the photoreceptor drum  2  is transferred to the sheet of paper T. 
     The fixing device  9  fuses toner and fixes the toner image transferred to the sheet of paper T. The fixing device  9  includes a heating roller  9   a  heated by a heater, and a pressurizing roller  9   b  in pressure contact with the heating roller  9   a . The heating roller  9   a  and the pressurizing roller  9   b  sandwich and convey the sheet of paper T onto which the toner image has been transferred. As the sheet of paper T is conveyed while being sandwiched between the heating roller  9   a  and the pressurizing roller  9   b , the toner transferred to the sheet of paper T is fused and fixed. 
     The paper feeding cassette  52  is disposed in a lower portion (lower side in a vertical direction) in the apparatus main body M. The paper feeding cassette  52  is disposed horizontally slidable toward a front side (the right side in  FIG. 1 ) of the apparatus main body M. The paper feeding cassette  52  includes a placing board  60  on which sheets of paper T are placed. The sheets of paper T stacked on the placing board  60  are stored in the paper feeding cassette  52 . A cassette feeding portion  51  is disposed in an end portion of the paper feeding cassette  52  on a side of discharging a sheet of paper T (in a right end portion of  FIG. 1 ). The cassette feeding portion  51  feeds out the sheets of paper T stored in the paper feeding cassette  52  to a paper path L. 
     The cassette feeding portion  51  includes a double feed preventing mechanism composed of a forward feed roller  61  for picking up the sheets of paper T placed on the placing board  60  and a pair of rollers  63  for feeding the sheets of paper T one sheet at a time to the paper path L. 
     The paper path L for feeding a sheet of paper T is formed between a discharge portion  50  and the cassette feeding portion  51  or the manual feeding portion  64 . The paper path L includes a first feeding path L 1  from the cassette feeding portion  51  to a first junction P 1 , a second feeding path L 2  from the first junction P 1  to the pair of registration rollers  80 , a third feeding path L 3  from the pair of registration rollers  80  to the transfer roller  8 , a fourth feeding path L 4  from the transfer roller  8  to the fixing device  9 , a fifth feeding path L 5  from the fixing device  9  to a branch portion P 3 , and a sixth feeding path L 6  from the branch portion P 3  to the discharge portion  50 . 
     The paper path L further includes a seventh feeding path L 7  from the manual feeding portion  64  to the first junction P 1 . The first junction P 1  is where the first feeding path L 1  for feeding a sheet of paper T from the cassette feeding portion  51  merges with the seventh feeding path L 7  for feeding a sheet of paper T from the manual feeding tray  65 . A second junction P 2  is provided midway in the second feeding path L 2 . In addition, the paper path L has a return feeding path Lb from the branch portion P 3  to the second junction P 2 . The second junction P 2  is where the second feeding path L 2  merges with the return feeding path Lb. 
     Here, the pair of registration rollers  80  is disposed upstream of the transfer roller  8 , in a direction feeding a sheet of paper T (the right side in  FIG. 1 ). The pair of registration rollers  80  performs skew correction of a sheet of paper T and timing adjustment with respect to a toner image. 
     The return feeding path Lb is provided for causing a surface of a sheet of paper T (unprinted surface) opposite to a surface that has been already printed to face the photoreceptor drum  2  when duplex printing of the sheet of paper T is performed. The return feeding path Lb can reverse and return the sheet of paper T, conveyed from the branch portion P 3  toward the paper discharge portion  50 , to the second feeding path L 2 . A toner image is transferred by the photoreceptor drum  2  to the unprinted surface of the sheet of paper T that is reversed by the return feeding path Lb. 
     The manual feeding portion  64  is provided in front (the right side in  FIG. 1 ) of the apparatus main body M, above the feeding cassette  52 . The manual feeding portion  64  feeds a sheet of paper T placed on the open manual feeding tray  65  to the second feeding path L 2  via the seventh feeding path L 7  and the first junction P 1 . The manual feeding portion  64  includes the manual feeding tray  65  and a feeding roller  66 . A base end portion of the manual feeding tray  65  is rotatably (openably and closably) connected in the vicinity of an entrance to the seventh feeding path L 7 . The manual feeding tray  65  makes a portion of a front surface of the apparatus main body M when the manual feeding tray  65  is closed. The feeding roller  66  picks up a sheet of paper T placed on the manual feeding tray  65  and feeds it toward the seventh feeding path L 7 . 
     The paper discharge portion  50  is formed at an end of the sixth feeding path L 6 . The paper discharge portion  50  is disposed in an upper portion of the apparatus main body M. The paper discharge portion  50  has an opening toward a front of the apparatus main body M (right side in  FIG. 1 ). The paper discharge portion  50  discharges a sheet of paper T, on which the toner is fixed by the fixing device  9 , to the outside of the apparatus main body M. 
     A discharged paper collecting portion M 1  is provided on a side of an open port of the paper discharge portion  50 . The discharged paper collecting portion M 1  is provided on an upper face (outer face) of the apparatus main body M. The discharged paper collecting portion M 1  is where the upper surface of the apparatus main body M is recessed (vertically downward). A bottom face of the discharged paper collecting portion M 1  constitutes a part of the upper face of the apparatus main body M. Sheets of paper T discharged from the discharge portion  50 , to which images have been transferred, are stacked and collected at the discharged paper collecting portion M 1 . 
     The image information delivered by the document reader  201  is sent to the apparatus main body M through the document reading device  200 . The image information received by the apparatus main body M is sent to an image forming control unit (not illustrated). Based on the image information, the image forming control unit controls the photoreceptor drum  2  as an image bearing member, the charging unit  10 , the laser scanner unit  4 , the developing unit  16 , and the like, which compose the image forming unit GK, respectively. A predetermined toner image is formed on the photoreceptor drum  2  based on the image information. 
     Based on the image information, an image equivalent to the image of a document G is transferred to a sheet of paper T, which is conveyed to the transfer nip N formed by the photoreceptor drum  2 . The sheet of paper T, on which the image is formed, is discharged from the paper discharge portion  50  to the discharged paper collecting portion M 1 . 
     Next, the document placement portion  710  of the document feeding device  700  and the document feeding mechanism  44  in the feeding mechanism housing  701  of the present embodiment are described with reference to  FIGS. 2 and 3 . First, the schematic configuration of the document feeding device  700  is described with reference to  FIGS. 2 and 3 . 
     The document feeding device  700  of the present embodiment includes, as shown in  FIGS. 2 and 3 , a document mounting part  31 , a document lifting member  32 , a lift operating member  35 , a feeding roller unit  40 , a document feeding mechanism  44 , and a motor (not illustrated). The feeding roller unit  40  is composed of a pickup roller  34  and a belt-type feeding means  413 . 
     The document mounting part  31  is a tray that constitutes a main part of the document placement portion  710 , on which a document G to be fed is placed. A downstream end  31   b  of the document mounting part  31  is rotatably connected to the feeding mechanism housing  701  such that an upstream end  31   a  thereof is movable upward as indicated by an arrow U 1  in  FIG. 2 . 
     Operation to move upward the upstream end  31   a  of the document mounting part  31  is necessary for facilitating troubleshooting of a paper jam occurring in the document discharging space K, for example, which is positioned below the document mounting part  31 . 
     As shown in  FIGS. 2 and 3 , the document lifting member  32  is attached to an upper face at a downstream portion of the document mounting part  31  so as to be positioned on a lower face at a downstream portion of a document G placed on the document mounting part  31 . An upper face  32   a  of the document lifting member  32  serves as a placement face on which a downstream portion of the document G is placed, which is on the document mounting part  31 . 
     The document lifting member  32  includes a pair of movable side walls  321  that restricts in a width direction the positions of both sides of the document G placed thereon (only one of the movable side walls is illustrated in  FIGS. 2 and 3 ). The pair of movable side walls  321  is movable to be spaced apart from each other or to approach each other, thereby allowing positioning of both ends of various sizes of documents G. 
     In response to an upward rotational operation of the lift operating member  35 , the document lifting member  32  rotates to lift the downstream end  32   b  as shown in  FIG. 3 . This lifts the downstream end of a document G placed on the document mounting part  31  and the document lifting member  32 . On the other hand, in response to a downward rotational operation of the lift operating member  35 , the document lifting member  32  rotates to lower the downstream end  32   b  as shown in  FIG. 2 . This lowers the downstream end of the document G placed on the document mounting part  31  and the document lifting member  32 .  FIG. 2  shows the document lifting member  32  that is lowered to the lowest position. 
     Although not illustrated, the document lifting member  32  rotatably engages with the document mounting part  31  at the upstream end  32   c . In response to an upward rotational operation of the lift operating member  35 , the document lifting member  32  rotates about the upstream end  32   c  to cause the downstream end  32   b  to move upward, as indicated by an arrow R 11  in  FIG. 3 . This rotational movement is referred to as “forward rotational movement” and another rotational movement in the opposite direction is referred to as “reverse rotational movement.” 
     In other words, the document lifting member  32  lifts in response to the forward rotational movement and lowers in response to the reverse rotational movement. 
     The pickup roller  34  is arranged opposite to the upper face of the document lifting member  32 , as shown in  FIGS. 2 and 3 . When the downstream end  32   b  of the document lifting member  32  is lifted by the lift operating member  35  and a document G is lifted by the document lifting member  32 , the pickup roller  34  comes into contact with an upper face of the document G placed on the document lifting member  32 . The pickup roller  34  in contact with the upper face of the document G rotates, so that it is possible to feed the document G from the document lifting member  32  to the document feeding mechanism  44  in the feeding mechanism housing  701 . The pickup roller  34  is composed of a hollow rubber tube. 
     As shown in  FIGS. 2 and 3 , the feeding roller unit  40 , the document feeding mechanism  44  and the like are provided in the feeding mechanism housing  701 . The pickup roller  34  is disposed inside the feeding roller unit  40 . The document feeding mechanism  44  discharges the document G, which is fed by the feeding roller unit  40 , to the document discharging space K via the first document reading point J 1  and the second document reading point J 2 . Configurations of the feed roller unit  40  and the document feeding mechanism  44  inside the feeding mechanism housing  701  are described later. 
     As shown in  FIGS. 2 and 3 , the lift operating member  35  includes a first rotational shaft member  351  and a lifting member  352 . The first rotational shaft member  351  extends in the width direction of the document G. Both ends of the first rotational shaft member  351  are rotatably supported by a bearing portion  313  provided on a bottom wall  312  of the document mounting part  31 . 
     The lifting member  352  is disposed below the document lifting member  32 . A base end of the lifting member  352  is connected to the first rotational shaft member  351  and rotates about an axial center of the first rotational shaft member  351 . The lifting member  352  is fixed to two points separated in an axial direction of the first rotational shaft member  351 . In a case of the forward rotational movement (rotational movement in the direction of the arrow R 12  in  FIG. 3 ), the lifting member  352  rotates to lift the downstream end  32   b  of the document lifting member  32 . On the other hand, in a case of the reverse rotational movement (rotational movement in the direction of the arrow R 13  in  FIG. 3 ), the lifting member  352  rotates to lower the downstream end  32   b  of the document lifting member  32 . 
     In other words, in a case of the forward rotational movement (rotational movement in the direction of the arrow R 12  in  FIG. 3 ), the lifting member  352  operates to lift the document lifting member  32 . On the other hand, in a case of the reverse rotational movement (rotational movement in a direction of an arrow R 13  in  FIG. 3 ), the lifting member  352  operates to lower the document lifting member  32 . 
     In the present embodiment, as shown in  FIGS. 2 and 3 , the belt-type feeding means  413  is supported more downstream than the pickup roller  34 . The belt-type feeding means  413  includes a pair of rollers  414 ,  415  disposed separated from each other in the feeding direction of a document G, a feeding belt  416  stretched around these rollers, and a pressing roller  417  that is pressed against the feeding belt  416  from below. The belt-type feeding means  413  feeds downstream a document G by driving the feeding belt  416  in the direction indicated by an arrow X 9  in  FIG. 2 , while the feeding belt  416  and the pressing roller  417  sandwich the document G. 
     The document feeding mechanism  44  has a feeding path L 10  for reading a document and a feeding path L 11  for discharging a document inside the feeding mechanism housing  701 . The feeding path L 10  for reading a document feeds a document G from the feeding roller unit  40  to the second reading unit  201 B (the second document reading point J 2 ) via the first reading unit  201 A (the first document reading point J 1 ) of the document reader  201 . The feeding path L 11  for discharging a document feeds the document G having undergone a reading process from the second reading unit  201 B (the second document reading point J 2 ) to the document collecting portion  720 . 
     In the feeding path L 10  for reading a document, a pair of registration rollers  70  composed of a first registration roller  71  and a second registration roller  72 , a pair of first feeding rollers  90 , a CIS roller  91  facing the first reading unit  201 A, and a pair of second feeding rollers  92  are provided along the feeding direction D 1 . In the feeding path L 11  for discharging a document, a pair of third feeding rollers  93  and a pair of discharging rollers  94  are provided along the feeding direction D 1 . 
     Next, the document feeding mechanism  44 , which is a characterizing feature of the document feeding device  700  of the copy machine  1  according to the present embodiment, is described in detail with reference to  FIGS. 4 to 9 .  FIG. 4  is a perspective view illustrating respective roller groups arranged along the feeding path L 10  for reading a document, when viewed from inside the feeding mechanism housing  701 .  FIG. 5  is a plan view illustrating an overall configuration of the pair of registration rollers  70  and the pair of first feeding rollers  90  in a state in which a cover member  702  is removed.  FIG. 6  is an enlarged cross-sectional view illustrating the vicinity of the pair of first feeding rollers  90 .  FIG. 7  is an enlarged perspective view illustrating the vicinity of the pair of first feeding rollers  90 .  FIG. 8  is an overall perspective view illustrating driven rollers  902  and a central driven roller  903  of the pair of first feeding rollers  90 .  FIG. 9  is a perspective view illustrating a detailed configuration of the central driven roller  903 . 
     As shown in  FIGS. 4 to 9 , upper portions of the pair of registration rollers  70  and the pair of first feeding rollers  90  are covered by the cover member  702  detachably installed inside the feeding mechanism housing  701 . The pair of first feeding rollers  90 , which is positioned upstream of the first reading unit  201 A in the feeding direction D 1 , includes a plurality (five) of drive rollers  901  that are arranged at predetermined intervals in a direction D 2  orthogonal to the feeding direction D 1  and a plurality (five) of driven rollers  902  and  903  arranged opposite to the drive rollers  901 . Between each drive roller  901  and each driven roller  902 , a nip N 2  (see  FIG. 6 ) is formed. Between a center drive roller  901  and a center driven roller  903 , which is an offset driven roller, a nip N 3  (see  FIG. 6 ) is formed. 
     The five drive rollers  901  are fixed to one drive shaft  904  extending in the direction D 2  orthogonal to the feeding direction D 1 . An end of the drive shaft  904  projects outside a side portion of the feeding mechanism housing  701  and is connected to a motor (not illustrated). As the motor drives the drive shaft  904 , the five drive rollers  901  are rotationally driven in the direction of the arrow R 1  shown in  FIGS. 6 and 7 . 
     As shown in  FIG. 8 , the drive rollers  901  rotationally drive the five driven rollers  902  and  903  to rotate in a direction of an arrow R 2  shown in  FIG. 6 . Among the five driven rollers  902  and  903 , the center driven roller  903  arranged at a central position in the direction D 2  orthogonal to the feeding direction D 1  is designed to be smaller in diameter and greater in length than other four driven rollers  902 . 
     The four driven rollers  902  except for the center driven roller  903  are rotatably supported concentrically about one common shaft  905  extending in the direction D 2  orthogonal to the feeding direction D 1 . The common shaft  905  is biased downward by a plurality of coil springs  907  provided between the cover member  702  and the common shaft  905 . In this manner, the four driven rollers  902  are elastically pressed against peripheral surfaces of the corresponding four drive rollers  901 , respectively, thereby forming linear nips N 2 . 
     On the other hand, the center driven roller  903  is rotatably supported by a supporting member  906  that is positioned about the penetrating common shaft  905  at a central position in an axial direction thereof. As shown in  FIG. 9 , the supporting member  906  includes two side plates  906   a  in a substantially isosceles triangle shape, a connecting plate  906   b  that connects the two side plates  906   a  at first corners thereof, and a fixed shaft  906   c  fixed between the two side plates  906   a  at second corners thereof. 
     At top portions of the two side plates  906   a  of the supporting member  906 , penetrating holes  906   d  are formed, through which the common shaft  905  penetrates. The center driven roller  903  of the smaller diameter is rotatably supported about the penetrating fixed shaft  906   c  of the supporting member  906 . In addition, both end portions of a torsion spring  908  coiled around the common shaft  905  are in contact not only with the connecting plate portion  906   b  of the supporting member  906 , but also with a fixed plate portion  703  inside the feeding mechanism housing  701 . With a biasing force applied by the torsion spring  908 , the center driven roller  903  is elastically pressed against a peripheral surface of a center drive roller  901 , which faces the driven roller  903 , thereby forming a nip N 3 . 
     With the above-described configuration, the position of the nip N 3  formed between the center driven roller  903  and the corresponding center drive roller  901  is offset downstream in the feeding direction D 1  with respect to positions of nips N 2  formed between the other four driven rollers  902  and the corresponding four drive rollers  901 . The amount of offset between the nip N 3  and the nips N 2  is 1 to 5 mm, for example. 
     As shown in  FIG. 5 , a plurality of coil springs  73  elastically presses second registration rollers  72  (upper side) to first registration rollers  71  (lower side) of pairs of registration rollers  70 . The coil springs  73  are provided between the cover member  702  and a shaft  74  that rotatably supports the second registration rollers  72 . 
     In the document feeding mechanism  44  of the document feeding device  700  thus configured, a document G fed from the feeding roller unit  40  is sequentially conveyed to the pairs of registration rollers  70 , to the pair of first feeding rollers  90 , and then to the CIS roller  91 , along the feeding path L 10  for reading a document toward the first document reading point J 1 . Upon the document G passing the first document reading position J 1 , an image on the back face of the document G is read by the first reading unit  201 A. 
     Thereafter, while the image on the back face thereof is read, the document G is conveyed from the CIS roller  91  to the pairs of second feeding rollers  92 , along the feeding path L 10  for reading a document toward the second document reading point J 2 . Upon the document G passing the second document reading position J 2 , an image on the front face of the document G is read by the second reading unit  201 B. 
     After the image on the back face of the document G is read at the first document reading point J 1  and the image on the front face of the document G is read at the second document reading point J 2 , the document G is conveyed along the feeding path L 11  for discharging a document from the pair of third feeding rollers  93  to the pair of discharging rollers  94 . Subsequently, the document G is conveyed to the document collecting portion  720  and discharged to the document discharging space K. 
     During the abovementioned feeding operation and reading operation of the document G, the position of the nip N 3  formed by the center driven roller  903 , among the five driven rollers  902 ,  903  of the pair of first feeding rollers  90  that feed the document G to the first document reading point J 1 , is offset downstream in the feeding direction D 1  with respect to the position of the nips N 2  formed by the four other driven rollers  902 . In other words, the nip N 3  is configured to be not in line with the nips N 2  along the direction D 2  orthogonal to the feeding direction D 1 . 
     Given this, when a rear end of the document G in the feeding direction is released from the pair of first feeding rollers  90 , an entirety of the rear end is not simultaneously or substantially simultaneously released, but rather is released with a time lag corresponding to the amount of offset between the nips N 2  and the nip N 3 . This can reduce a change (reduction) in the feeding load upon release of the rear end of the document G from the nips N 2 , N 3  and a resulting temporary change (acceleration) in the feeding speed of the document G. As a result, it is possible to feed the document G at a constant speed so that it is read properly. 
     The present embodiment provides the following effects, for example. 
     In the document feeding device  700  of the present embodiment, the pair of first feeding rollers  90 , which is positioned more upstream than the first document reading point J 1  in the feeding direction D 1 , include the plurality (five) of drive rollers  901  and the plurality (five) of driven rollers  902  and  903 . The drive rollers  901  are arranged at the predetermined intervals in the direction D 2  orthogonal to the feeding direction D 1 . The driven rollers  902  and  903  are arranged to face the drive rollers  901  and form the plurality of nips N 2 , N 3  with the drive rollers  901 . In addition, the plurality of driven rollers  902 ,  903  is supported by one common shaft  905  extending in the direction D 2  orthogonal to the feeding direction D 1 . The position of the nip N 3  formed by the center driven roller  903 , among the plurality of driven rollers  902 ,  903 , is offset more downstream in the feeding direction D 1  with respect to the positions of the nips N 2  formed by other driven rollers  902 . 
     Accordingly, the present embodiment does not require the rotational shafts and the supporting member thereof to be individually and independently provided, differing from the prior art. In the prior art, the rotational shafts of the plurality of driven rollers  902  and the center driven roller  903  have been arranged offset with respect to each other along the feeding direction D 1 , so that the nips N 2 , N 3  have been formed nonlinearly along the direction D 2  orthogonal to the feeding direction D 1 . In addition, it is possible to support all of the driven rollers  902 ,  903  at the predetermined positions by employing one common shaft  905  and one supporting member supporting the common shaft  905 . This can reduce the number of components in the apparatus, simplify a structure thereof, facilitate assembly thereof, and reduce manufacturing cost thereof. 
     Furthermore, it is not necessary to individually assemble the plurality of driven rollers  902 ,  903 . It is sufficient only to assemble the one common shaft  905  into these components. This can increase the positional accuracy of the driven rollers  902 ,  903  and the distance (dimensional) accuracy between these rollers. It is possible to perform stable feeding of a document by suppressing the feeding load through the offset between the nips N 2  and nip N 3  in addition to proper feeding operation by the driven rollers  902 ,  903 . 
     In addition, in the present embodiment, the position of the nip N 3  formed by the center driven roller  903  arranged at the central position in the direction D 2  orthogonal to the feeding direction D 1 , among the plurality of driven rollers  902 ,  903 , is offset more downstream in the feeding direction D 1  than the positions of the nips N 2  formed by the other driven rollers  902 . 
     Therefore, the central portion in the width direction of the rear end of the document G in the feeding direction D 1  is released from the center driven roller  903 , with a time delay with respect to the both end portions in the width direction. Accordingly, since an excessive rotational moment is not applied to the document G, it is possible to cause the document G to be released straight from the driven rollers, thereby further stabilizing the feeding of the document G. 
     A preferred embodiment of the present disclosure has been described above. However, the present disclosure is not limited thereto and can be carried out in various modes. 
     The copy machine  1  is exemplified in the present embodiment as an image forming device. However, the present disclosure is not limited thereto and can be a color copy machine, a printer, a facsimile machine, and a multi-functional appliance having functions thereof, for example. 
     The offset driven roller, of which the position of the nip N 3  is offset, is not limited to the center driven roller located at the central position in the direction D 2  orthogonal to the feeding direction D 1 . It may be one or more driven rollers not located at the central position in the direction D 2  orthogonal to the feeding direction D 1 . 
     In addition, in the present embodiment, the five drive rollers  901  and five driven rollers  902 ,  903  are provided at the predetermined intervals in the direction D 2  orthogonal to the feeding direction. However, the present disclosure is not limited thereto. It may be that the number of the driven rollers  902 ,  903  is an arbitrary plural number, two or more. The drive roller  901  may be composed of one or more rollers. The drive roller  901  has a length corresponding to the plurality of driven rollers and is disposed at positions corresponding to them. 
     In the present embodiment, the position of the nip N 3  formed by the center driven roller  903  is offset more downstream in the feeding direction D 1  with respect to the position of the nips N 2  formed by the other driven rollers  902 . However, the present disclosure is not limited thereto and the position of the nip N 3  may be offset more upstream in the feeding direction D 1 . The amount of an offset is 1 to 5 mm, for example. 
     In addition, in the present embodiment, the document reader  201  has the first reading unit  201 A and the second reading unit  201 B, which read images on the front and back faces of the document G sequentially while the document G is being fed in one direction without turning over the document G, disposed upstream and downstream in the feeding direction D 1 . However, the present disclosure is not limited thereto. It may be that one document reader is configured to read images on front and back faces of a document G by turning it over.