Patent Publication Number: US-2013241134-A1

Title: Sheet feeding device capable of skew correction and image forming apparatus including the same

Description:
This application is based on an application No. 2012-57584 filed in Japan, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates a sheet feeding device and an image forming apparatus provided with the sheet feeding device. 
     (2) Description of the Related Art 
     In general, an image forming apparatus, such as a photocopier, printer, fax machine, or MFP (Multi Function Peripheral), has a configuration where a toner image formed on an image carrier, such as a photoconductive drum or an intermediate transfer belt, is transferred onto a recording sheet conveyed from a sheet feeder along a conveyance path. After transfer of the toner image onto the recording sheet, the toner image is fixed by a fixing unit. 
     Generally, in the above type of image forming apparatus, recording sheets are stacked in a feeder tray and an uppermost recording sheet among the stacked recording sheets is conveyed from the feeder tray by a pick-up roller and along a conveyance path by a feeder roller pair. Subsequently, skew correction of the recording sheet is performed by a resist roller pair positioned upstream of a transfer position. There above type of configuration is recited in Japanese Patent Publication No. 2002-244526. 
     In the type of configuration above, skew correction is performed by formation of a loop in the recording sheet. A leading part of the recording sheet being conveyed by the feeder roller pair, is impacted against a nip of the resist roller pair which are in a state of non-rotation, thus causing formation of the loop. When the loop is formed, stiffness of the recording sheet causes an edge of the leading part of the recording sheet to be pressed against the nip so as to become parallel to an axis of the resist roller pair. Once in the state described above, rotation of the resist roller pair causes the recording sheet to pass through the nip in a skew corrected state. 
     In recent years, in order to allow production of more compact image forming apparatuses, there has been a demand to reduce separation between configuration elements. Consequently, separation between the feeder tray and the resist roller pair should preferably be as small as possible. 
     Unfortunately, during skew correction by the resist roller pair, sufficient separation between the feeder tray and the resist roller pair is required in order that the loop can be formed in the recording sheet. Therefore, there is a problem that if separation between the feeder tray and the resist roller pair is reduced, the separation may be insufficient for the loop to be formed, and thus skew correction may be complicated. 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In response to the above problem, the present invention aims to provide a sheet feeding device and an image forming apparatus including the sheet feeding device, wherein spatial efficiency is improved while also ensuring reliable loop formation in sheets during skew correction. 
     Means for Solving the Problems 
     In order to achieve the above aim, one aspect of the present invention is an image forming apparatus for forming an image, including a sheet feeding device that corrects skew of a sheet before feeding the sheet to a transfer position of a toner image for image formation, the sheet feeding device comprising: a driver; a feeder roller unit provided with a feeder roller that is rotationally driven by the driver, and a pressing member that presses against a circumferential surface of the feeder roller forming a first nip; and at least two resist roller units that cause formation of a loop in the sheet between the resist roller units and the feeder roller unit during skew correction, each resist roller unit provided with a first resist roller and a second resist roller that press against one another forming a second nip, wherein the first resist roller and the feeder roller are positioned so that (i) when viewed in a direction of a rotational axis of the feeder roller, the first resist roller and the feeder roller overlap at least partially, and (ii) the first resist roller and the feeder roller occupy different positions with respect to the direction of the rotational axis, and the pressing member and the second resist roller are positioned so that when viewed in the direction of the rotational axis, the pressing member and the second resist roller do not overlap. 
     In order to achieve the above aim, another aspect of the present invention is a sheet feeding device for feeding a sheet and correcting skew thereof, the sheet feeding device comprising: a driver; a feeder roller unit provided with a feeder roller that is rotationally driven by the driver, and a pressing member that presses against a circumferential surface of the feeder roller forming a first nip; and at least two resist roller units that cause formation of a loop in the sheet between the resist roller units and the feeder roller unit during skew correction, each resist roller unit provided with a first resist roller and a second resist roller that press against one another forming a second nip, wherein the first resist roller and the feeder roller are positioned so that (i) when viewed in a direction of a rotational axis of the feeder roller, the first resist roller and the feeder roller overlap at least partially, and (ii) the first resist roller and the feeder roller occupy different positions with respect to the direction of the rotational axis, and the pressing member and the second resist roller are positioned so that when viewed in the direction of the rotational axis, the pressing member and the second resist roller do not overlap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and the other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention. 
       In the drawings: 
         FIG. 1  is a schematic diagram showing configuration of a printer including a sheet feeding device relating to a first embodiment of the present invention; 
         FIG. 2  is a partially cutaway perspective view showing configuration of the sheet feeding device included in the printer; 
         FIG. 3  is a partially cutaway perspective view showing configuration of a reverse drive charging unit included in the sheet feeding device; 
         FIG. 4A ,  FIG. 4B  and  FIG. 4C  show the sheet feeding device during operation; 
         FIG. 5  is a partially cutaway perspective view for explaining configuration of a sheet feeding device relating to a second embodiment; 
         FIG. 6A ,  FIG. 6B  and  FIG. 6C  show the sheet feeding device relating to the second embodiment during operation; 
         FIG. 7  is a partially cutaway perspective view showing configuration of a sheet feeding device relating to a third embodiment; 
         FIG. 8  is a lateral view of the sheet feeding device relating to the third embodiment; 
         FIG. 9  is a broken down perspective view showing main elements of a sheet feeding device relating to a first modified example; and 
         FIG. 10  is a broken down perspective view showing main elements of a sheet feeding device relating to a second modified example. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     The following describes, with reference to the drawings, an image forming apparatus including a sheet feeding device relating to a first embodiment of the present invention. 
     Image Forming Apparatus Configuration 
       FIG. 1  is a schematic diagram for explaining configuration of a printer which is one example of the image forming apparatus including the sheet feeding device relating to the first embodiment of the present invention. The printer is for forming a monochrome toner image on a recording sheet, such as a paper sheet or an OHP sheet. 
     The image forming apparatus shown in  FIG. 1  includes a photoconductive drum  11  that is driven in a rotational direction shown by an arrow A. The photoconductive drum  11  is held horizontally level between a front side and a rear side of the image forming apparatus (between a near side and a far side of  FIG. 1 ). 
     In order to form the toner image on the recording sheet through an electrophotographic method, a charger  12 , an optical unit  13 , a developer  14  and a transfer roller  15  are provided around the photoconductive drum  11  in respective order in the rotational direction of the photoconductive drum  11  (anti-clockwise direction in  FIG. 1 ). 
     In the printer, a control unit  40  converts image data input from an external device into a drive signal for a laser diode, and a laser diode provided in the optical unit  13  is driven by the drive signal. 
     As a result of the above, a surface of the photoconductive drum  11  is irradiated by laser light L from the optical unit  13 , in accordance with the image data. 
     The surface of the photoconductive drum  11  is charged in advance to a determined electrical potential by the charger  12  so that, when exposed to the laser light L from the optical unit  13 , an electrostatic latent image is formed on the surface. The electrostatic latent image is developed by the developer  14  using a toner, thus forming a toner image. 
     A sheet feeder  20  is positioned below the photoconductive drum  11 . The sheet feeder  20  includes a feeder tray  21 , on which a plurality of recording sheets such as paper or OHP sheets are stacked, and a feeder mechanism  41  (the sheet feeding device). 
     In the first embodiment, the feeder mechanism  41  picks-up a single uppermost recording sheet among the plurality of recording sheets stacked in the feeder tray  21  (below the uppermost recording sheet is referred to as recording sheet S 1 ). The feeder mechanism  41  performs skew correction on the recording sheet S 1  and conveys the recording sheet S 1  to a conveyance path  23  that leads towards the photoconductive drum  11 . 
     The feeder tray  21  has a sheet stacking surface  21  a, which is raised and lowered by a driver (omitted in  FIG. 1 ). 
     A transfer roller  15 , which rotates in a direction shown by arrow B, is positioned horizontally adjacent to and in pressure contact with the photoconductive drum  11 . The pressure contact between the transfer roller  15  and the photoconductive drum  11  forms a transfer nip  25 . The recording sheet S 1 , after being conveyed along the conveyance path  23 , is conveyed into the transfer nip  25 . 
     Thus, the recording sheet S 1  supplied from the sheet feeder  20  to the conveyance path  23 , is conveyed directly to the transfer nip  25  by the sheet feeder  20 . 
     The recording sheet S 1  enters the transfer nip  25 , and while passing therethrough the toner image on the photoconductive drum  11  is transferred onto the recording sheet S 1 , due to a transfer electric field created by a transfer voltage applied against the transfer roller  15 . 
     The recording sheet S 1 , with the toner image formed thereon, is separated from the photoconductive drum  11  by a separation claw  16 , and conveyed to a fixing unit  30 . 
     After the toner image has been transferred to the recording sheet S 1 , the photoconductive drum  11  is cleaned by a cleaning unit  17 . 
     The fixing unit  30  includes a heating roller  31  and a fixing roller  32 , that are arranged horizontally level with one another, and a fixing belt  33  which is wound and cyclically driven around the heating roller  31  and the fixing roller  32 . The fixing unit  30  also includes a pressing roller  34  which is in an opposing position to the fixing roller  32  and horizontally level therewith. The fixing roller  32  and the pressing roller  34  sandwich the fixing belt  33  therebetween. 
     A heating lamp (halogen lamp) is provided within the heating roller  31 , and the fixing belt  33  wound around the heating roller  31  is heated by the heating lamp. At a position where the fixing belt  33  and the pressing roller  34  are in pressure contact a fixing nip is formed, through which the recording sheet S 1  with the toner image formed thereon passes. 
     As the recording sheet S 1  passes through the fixing nip, the toner image on the recording sheet S 1  is heated to a predetermined fixing temperature by the fixing belt  33 , and thus the toner image is fixed on the recording sheet S 1 . 
     After passing through the fixing nip, the recording sheet S 1  is conveyed to ejection rollers  24  by the fixing belt  33  and the pressing roller  34 . The recording sheet S 1  is subsequently ejected onto an ejection tray  19  by the ejection rollers  24 . 
     Feeder Mechanism Configuration 
     The feeder mechanism  41  is positioned at a feeding inlet of the feeder tray  21 , and is configured to pick-up the uppermost recording sheet S 1  stacked in the feeder tray  21  (refer to  FIG. 1 ), perform skew correction on the recording sheet S 1 , and convey the recording sheet S 1  to the conveyance path  23 . 
       FIG. 2  is a partially cutaway perspective diagram for explaining configuration of main elements of the feeder mechanism  41 . For convenience of drawing, parts further left than feeder roller  170  are shown in a partially cutaway form at the top left of  FIG. 2 . 
     As shown in  FIG. 2 , the feeder mechanism  41  includes supporting members  140 , primary resist rollers  150 , coupling units  160 , a feeder roller  170 , secondary resist rollers  180 , a separation roller  190 , and a reverse drive charging unit  200 . The feeder roller  170  is fixed approximately centrally on a primary roller axle  171 . The primary resist rollers  150  are positioned one each at opposite ends of the feeder roller  170  along the primary roller axle  171 , and each of the primary resist rollers  150  is held freely rotatably by a pair of the supporting members  140 . The four supporting members  140  are each provided with a through hole  145 , into which the primary roller axle  171  is moveably inserted. 
     The feeder roller  170  is in contact with the uppermost recording sheet S 1  stacked in the feeder tray  21 , and functions as a pick-up roller by picking-up recording sheets one by one. The feeder roller  170  is formed from the primary roller axle  171 , a core part  172  and a peripheral part  173 . 
     The primary roller axle  171  is a shaft formed from a rigid metal or the like, and is fixed to the feeder roller  170  either through forceful insertion into an axle hole  172   a  in the core part  172 , or through use of an adhesive. 
     The core part  172  is provided with a long arc-shaped hole  174 , which is concentric to the feeder roller  170 , and runs through the core part  172  in an axial direction between opposite ends of the feeder roller  170 . The long arc-shaped hole  174  serves a function in causing rotational movement of the primary resist rollers  150  coupled to rotational movement of the feeder roller  170 . 
     The peripheral part  173  is formed from an elastic material, such as rubber, of uniform thickness, which covers an outer circumferential surface of the core part  172 . 
     The separation roller  190  is in pressure contact with an outer circumferential surface of the feeder roller  170 , and has a function of separating recording sheets picked-up by the feeder roller  170  into single sheets. 
     A torque limiter (omitted in  FIG. 2 ) is attached to an axle of the separation roller  190 , and is configured so that a predetermined torque arises when the axle is rotationally driven. 
     The above configuration ensures that when a plurality of recording sheets become sandwiched between the feeder roller  170  and the separation roller  190 , only the uppermost recording sheet S 1  is picked-up. 
     Each of the primary resist rollers  150  includes a wheel part  152 , which is a hollow cylinder. An outer skin  151  of uniform thickness is formed on a circumferential surface of the wheel part  152  by an elastic material such as rubber. At each end of the wheel part  152  in terms of an axial direction, an inner circumference gear  153  is formed on an inner circumferential surface of the wheel part  152 . A diameter D 2  of the outer skin  152  is equal to a diameter D 1  of the feeder roller  170 . 
     Axle holes  141 ,  142 , and  143  are provided in each of the pairs of supporting members  140  holding the primary resist rollers  150 . The axle holes  141 ,  142 , and  143  respectively hold and allow free rotation of internal gears  154   a,    154   b  and  154   c,  each being identical in shape and teeth number. 
     The internal gears  154   a,    154   b  and  154   c  mesh with the inner circumference gears  153  of the primary resist roller  150 , therefore ensuring the primary resist roller  150  is held by the pair of supporting members  140 , but is able to rotate freely in relation to the pair of supporting members  140 . 
     Positioning of the axle holes  141 ,  142 , and  143  in each of the supporting members  140  is determined so that a center of rotation of the primary resist rollers  150  (resist roller axis) is offset by approximately 2-3 mm downstream in a direction of recording sheet conveyance, compared to a center of rotation of the feeder roller  170  (feeder roller axis). 
     The feeder roller axis is positioned slightly lower than the resist roller axis, and the feeder roller  170  is closer than the primary resist rollers  150  to the feeder tray  21 . Through the above configuration, only the outer circumferential surface of the feeder roller  170  is applied against an upper surface of the uppermost recording sheet S 1  in the feeder tray  21 , therefore ensuring a smooth pick-up movement of the uppermost recording sheet S 1 . 
     The internal gear  154   a  is formed from two gear wheels  156   a  provided one each at opposite ends of an axle  155   a.  More specifically, the gear wheels  156   a  are positioned slightly towards a center point of the axle  155   a  from respective ends of the axle  155   a.  The gear wheels  156   a  are fixed on the axle  155   a,  for example by forceful insertion of the axle  155   a  therethrough, or use of an adhesive. 
     The inner gear  154   b  has the same configuration as the inner gear  154   a.    
     Compared to the inner gear  154   a,  the inner gear  154   c  has a configuration where an end of the axle  155   a  closest to the feeder roller  170  is extended, and a gear wheel  158  is additionally provided thereon. 
     The gear wheel  158  is identical to each of the gear wheels  156   a,  and as shown in  FIG. 2 , the gear wheel  158  is positioned so as to be on an opposite side of the supporting member  140  to the gear wheels  156   a,  sandwiching the supporting member  140  therebetween. The gear wheel  158  has a function of transmitting rotational movement of a central wheel  162  of a corresponding coupling unit  160  to the primary resist roller  150  (explained below in more detail). 
     Each of the secondary resist rollers  180 , having a smaller diameter than the diameter D 2  of each of the primary resist rollers  150 , is pressed against a corresponding primary resist roller  150  forming a nip. 
     Positioning of each of the secondary resist rollers  180  in relation to the corresponding primary resist roller  150  is identical. The secondary resist roller  180  presses against the primary resist roller  150 , and is driven by movement thereof. 
     As explained above, the diameter D 1  of the feeder roller  170  and the diameter D 2  of each of the primary resist rollers  150  are equal. Therefore, conveyance speed of the recording sheet S 1  is identical for a pairing of the feeder roller  170  with the separation roller  190 , and for pairings of each of the primary resist rollers  150  with the corresponding secondary resist roller  180 . 
     Furthermore, a torque limiter (omitted in  FIG. 2 ) is provided on an axle of each of the secondary resist rollers  180 , and is configured so that when rotational drive is applied in one direction, a predetermined amount of torque arises in a direction resisting the rotational drive. The above configuration ensures that during skew correction, the primary resist rollers  150  do not rotate prematurely before skew correction is complete. 
     (Coupling Units  160 ) 
     Each of the coupling units  160  is configured as an intermediate for transmitting rotational drive of the feeder roller  170  to a corresponding primary resist roller  150  with a delay. Two coupling units  160  are provided one at each end of the feeder roller  170  along a Y-axis (rotational axis) thereof, thus each coupling unit  160  is positioned between the feeder roller  170  and the corresponding primary resist roller  150 . 
     Each of the coupling units  160  is formed from an intermediate wheel  162  and a drive transmission shaft  165 . 
     The intermediate wheel  162  is cylindrical with a base, and has an inner circumference gear  164  formed on an inner circumferential surface thereof. The inner circumference gear  164  has identical pitch and number of teeth to each of the inner circumference gears  153  of the corresponding primary resist roller  150 . The inner circumference gear  164  meshes with the gear wheel  158  provided at the extended end of the gear axle  155   a  of a corresponding inner gear  154   c.    
     The intermediate wheel  162  has a boss part  163  positioned centrally on a base surface thereof. The primary roller axle  171  inserts into the boss part  163 , and thus the intermediate wheel  162  is supported by and freely rotatable around the primary roller axle  171 . 
     When viewed in a Y-axis direction, a center point of the boss part  163  and a center of rotation of the inner circumference gear  164  are identical. 
     The inner circumference gear  164  of the coupling unit  160  is identical in terms of shape and number of teeth to each of the inner circumference gears  153  of the corresponding primary resist roller  150 . Also, the inner circumference gear  164  meshes with the gear wheel  158  provided on the same axle as the corresponding inner gear  154   c.  Consequently, the intermediate wheels  162  each rotate at the same angular velocity as the corresponding primary resist roller  150 . 
     The drive transmission shaft  165  may for example be a metal shaft. The drive transmission shaft  165  extends from a base part  161  of one of the intermediate wheels  162  in a direction perpendicular to the base part  161 . The drive transmission shaft  165  extends through the long arc-shaped hole  174 , provided in the core part  172  of the feeder roller  170 , and extends to a base part  161  of the other intermediate wheel  162 . 
       FIG. 2  shows the drive transmission shaft  165  in a state of contact with an inner wall  172   b  of the long arc-shaped hole  174  (initial engagement state). The drive transmission shaft  165  is configured so that when driving force from the primary roller axle  171  causes the feeder roller  170  to rotate in the anticlockwise direction, the drive transmission shaft  165  is disengaged from the initial engagement state and moved into a state of contact with an inner wall  172   c  of the long arc-shaped hole  174 . Through the above configuration, once the drive transmission shaft  165  is in contact with the inner wall  172   c,  the intermediate wheels  162  rotate at an identical velocity to the feeder roller  170 . 
     Rotation of each of the intermediate wheels  162  is transmitted to the corresponding primary resist roller  150  through the inner circumference gear  164 , the inner gear  154   c  and the inner circumference gear  153 . As a result, once the feeder roller  170  commences rotation, the primary resist rollers  150 , after a delay corresponding to magnitude of a central angle of the long arc-shaped hole  174 , each commence rotation at an identical velocity to the feeder roller  170 . 
     In other words, the long arc-shaped hole  174  and the drive transmission shaft  165  act in cooperation to transmit rotational drive of the feeder roller  170  to the primary resist rollers  150  after a predetermined delay. Thus, the long arc-shaped hole  174  and the drive transmission shaft  165  function together as a delayed drive transmission unit. 
     Through the above configuration, after the recording sheet S 1  is picked-up from the feeder tray  21 , a loop is formed in the recording sheet S 1  upstream of the nip between each of the primary resist rollers  150  and the corresponding secondary resist roller  180 , thus allowing skew correction to be performed. 
     For the two primary resist rollers  150 , positioned one each at opposite ends of the feeder roller  170 , a distance therebetween (the shortest distance in the Y-axis direction between the outer skin  151  of each of the primary resist rollers  150 ) is set as smaller than the smallest expected width for the recording sheet S 1 . Through the above configuration, skew correction can be performed reliably even when recording sheets of a small size are used. 
     Once the recording sheet S 1  has been conveyed out of the feeder mechanism  41 , engagement between the drive transmission shaft  165  and the long arc-shaped hole  174  must be returned to the initial engagement state so that skew correction can be performed on a next recording sheet picked-up from the feeder tray  21 . In the present embodiment, the reverse drive charging unit  200  is included in the feeder mechanism  41  in order to achieve the above. 
       FIG. 3  shows configuration of the reverse drive charging unit  200 . 
     As shown in  FIG. 3 , the reverse drive charging unit  200  is formed from a drive axle  201 , a spiral spring  203  and a casing  202 . 
     The drive axle  201  is connected to a driver (omitted in  FIG. 3 ), and is rotationally driven thereby. An inner end of the spiral spring  203  is joined to the primary roller axle  171 , and the other end (outer end) of the spiral spring  203  is joined to an inner surface of a body  202   a  of the casing  202 . 
     The casing  202  is formed from the body  202   a,  a top plate  202   b  and a bottom plate  202   d.  The body  202   a  is a hollow cylinder, and the top plate  202   b  and the bottom plate  202   d  block openings at respective ends of the cylinder in an axial direction thereof. The spiral spring  203  is housed in the casing  202 . 
     A hole  202   c  is provided in the top plate  202   b  so that one end of the primary roller axle  171  can be inserted into the casing  202 . The drive axle  201  is connected centrally to the bottom plate  202   d  of the casing  202 , so that the drive axle  201  is positioned on the same axis as the primary roller axle  171 . 
     Through the above configuration, when driving of the feeder mechanism  41  commences, a portion of driving force from the driver is used for winding of the spiral spring  203  in the reverse drive charging unit  200 , thus causing charging of elastic energy in the spiral spring  203 . Once the recording sheet has been conveyed from the feeder mechanism  41 , the driver is suspended, and the spiral spring  203  attempts to return to a pre-winding state causing clockwise rotation of the primary roller axle  171 . The above causes a return to the initial engagement state of the long arc-shaped hole  174  and the drive transmission shaft  165 . 
     (Feeder Mechanism  41  Operation) 
     Feeding and loop formation operations in the feeder mechanism  41  are explained below with reference to  FIGS. 4A-4C , which each show a side view of main elements of the feeder mechanism  41 . 
     For ease of explanation of rotational movement, points C and E are marked on circumferential surfaces respectively of the feeder roller  170  and each of the primary resist rollers  150 . 
       FIG. 4A  shows the main elements of the feeder mechanism  41  when in the initial engagement state. In  FIG. 4A , the drive transmission shaft  165  is in contact with the inner wall  172   b  of the long arc-shaped hole  174 . Point C shows a lowest point on the circumferential surface of the feeder roller  170 , and point E shows a contact position between the primary resist roller  150  and the corresponding secondary resist roller  180 . 
     When a recording sheet is to be fed-in, in accordance with an instruction from the control unit  40 , the sheet stacking surface  21   a  is raised by an actuator (omitted in  FIGS. 4A-4C ) so that the upper surface of the uppermost recording sheet S 1  is in contact with the circumferential surface of the feeder roller  170 , and the drive axle  201  (refer to  FIG. 2 ) is rotationally driven in the anticlockwise direction by the driver (omitted in  FIGS. 4A-4C ). 
     Driving force is transmitted to the primary roller axle  171  through the spiral spring  203 , and thus the primary roller axle  171  attempts to rotate the feeder roller  170 . However, due to the recording sheet S 1  and the torque limiter of the separation roller  190  that presses against the feeder roller  170 , a certain amount of torque load is applied against the primary roller axle  171 . 
     Due to the above, at least a portion of the driving force is used to power winding of the spiral spring  203 , and therefore the portion of the driving force is converted to and stored as elastic energy. 
     Once there has been a certain amount of winding of the spiral spring  203 , the driving force is transmitted to the main roller axle  171 , and the feeder roller  170  rotates in the anticlockwise direction. 
     Through the above movement, the recording sheet S 1  is conveyed in a rightwards direction of  FIGS. 4A-4C , and passes through point F as shown in  FIG. 4B . Point F is an intersection point of outer contour lines of the circumferential surfaces of the feeder roller  170  and the primary resist roller  150 , when viewed as in  FIG. 4B . 
     A portion of the recording sheet S 1  that has passed through point F slides across the circumferential surface of the primary resist roller  150 , which is temporarily stationary, as it is conveyed. Eventually, the recording sheet S 1  contacts with a nip N formed between the primary resist roller  150  and the corresponding secondary resist roller  180  at point E. Due to the torque limiter (omitted in  FIGS. 4A-4C ) provided on the rotational axle of the corresponding secondary resist roller  180 , a leading part of the recording sheet S 1  pushing against the nip N is insufficient to cause rotation of the primary resist roller  150  and the corresponding secondary resist roller  180 . 
     Once the above situation has been reached, the recording sheet S 1  continues to be conveyed by the feeder roller  170 , causing formation of a loop L in the recording sheet S 1  as shown in  FIG. 4B . 
     When the loop L forms, stiffness of the recording sheet S 1  causes an edge of the leading part of the recording sheet S 1  to align with the nip N (parallel to the axial direction of the primary resist roller  150 ). 
     Rotation of the feeder roller  170  moves the drive transmission shaft  165  into contact with the inner wall  172   c  of the long arc-shaped hole  174 , thus causing the primary resist roller  150  to commence rotation in the anticlockwise direction at an identical velocity to the feeder roller  170 . Through the above configuration, the recording sheet S 1  is conveyed further downstream in a skew corrected state (refer to  FIG. 4C ). 
     A conveyance speed of the recording sheet S 1  when passing between the feeder roller  170  and the separation roller  190 , is set as equal to a conveyance speed of the recording sheet S 1  when passing between the primary resist roller  150  and the corresponding secondary resist roller  180 . The above ensures that there is no excessive tension applied to or slackness of the recording sheet S 1  during conveyance. 
     In order to ensure that the next recording sheet is not fed-in while the loop is being formed in the recording sheet S 1  and while a trailing part of the recording sheet S 1  has not yet been conveyed through the feeder mechanism  41 , the control unit  40  lowers the sheet stacking surface  21   a  of the feeder tray  21  using the driver. 
     In order to achieve the above, lowering of the sheet stacking surface  21   a  should preferably be performed while the trailing part of the recording sheet S 1  is still positioned between the circumferential surface of the feeder roller  170  and the next recording sheet S. To ensure correct timing, the lowering may for example be performed a predetermined amount of time after the drive axle  201  commences rotation, or alternatively a reflective photosensor may be provided at a point downstream of the nip N in the conveyance direction of the recording sheet S 1 , and the lowering may be performed when the reflective photosensor detects the leading part of the recording sheet S 1 . 
     The control unit  40  stops rotational drive of the primary roller axle  171  at a point in time when the trailing part of the recording sheet S 1  has passed through the nip N, or at a time thereafter. 
     For example, if the reflective photosensor is provided at the point downstream of the nip N, rotational drive of the primary roller axle  171  may be stopped when the reflective photosensor detects the trailing part of the recording sheet S 1 . Alternatively, if no reflective photosensor is provided, rotational drive of the primary roller axle  171  may be stopped a predetermined amount of time after the drive axle  201  commences rotation. 
     When rotational drive of the primary roller axle  171  is stopped, the elastic energy charged in the reverse drive charging unit  200  is released, causing reverse rotation of the primary roller axle  171  in a clockwise direction shown in  FIG. 4C . 
     When the primary roller axle  171  commences reverse rotation, only the feeder roller  170  and the separation roller  190  are rotated, therefore the reverse drive charging unit  200  is able to overcome torque produced by the torque limiter of the separation roller  190 , causing reverse rotation of the feeder roller  170 . The above causes the drive transmission shaft  165  to move into contact with the inner wall  172   b  of the long arc-shaped hole  174 . 
     If the reverse drive charging unit  200  is to cause further reverse rotation of the feeder roller  170 , beyond the point described above, the coupling units  160 , the primary resist rollers  150  and the secondary resist rollers  180  must also be reverse rotated. In particular, torque is produced by the torque limiter of each of the secondary resist rollers  180 . If enough energy remains charged in the reverse drive charging unit  200  to overcome the above torque, there is further reverse rotation due to the remaining energy. 
     The above causes relative positions of the feeder roller  170  and the primary resist rollers  150  to return to the initial engagement state shown in  FIG. 4A . Thus, feeding of the next recording sheet is now possible. 
     Through the above configuration the feeder roller  170  and each of the primary resist rollers  150  can be caused to return to a standard position. Consequently, there is no need to control the driver in order to cause reverse rotation of the primary roller axle  171 . 
     In the first embodiment, through delayed transmission of rotational drive of the feeder roller  170  to the primary resist rollers  150 , a single driver can be used for both the feeder roller  170  and the primary resist rollers  150 . The above is achieved in the first embodiment while also allowing reliable loop formation, skew correction and conveyance of the recording sheet. 
     When viewed in the direction of the rotational axis, the outer circumference  173  of the feeder roller  170  overlaps almost completely with the outer skin  151  of each of the primary resist rollers  150 . Therefore, separation between the feeder roller  170  and the primary resist rollers  150  can be small, allowing the image forming apparatus to be compact in size. 
     Second Embodiment 
     Configuration of a feeder mechanism relating to a second embodiment is largely the same as configuration of the feeder mechanism  41  relating to the first embodiment. However, configuration of the feeder mechanism relating to the primary resist rollers  150 , and the intermediate wheel  162  of each of the coupling units  160 , differs from configuration of the feeder mechanism  41  relating to the first embodiment. 
     Configuration elements that are the same as in the first embodiment are referred to below using the same reference symbols, and description thereof is omitted or abbreviated in order to focus on configuration elements that are different. 
       FIG. 5  is a partially cutaway perspective diagram showing configuration of main elements of the feeder mechanism relating to the second embodiment. 
     As shown by  FIG. 5 , in the feeder mechanism  241  relating to the second embodiment there is no configuration corresponding to the coupling units  160 . Furthermore, two primary resist rollers  350 , corresponding to the two primary resist rollers  150  in the first embodiment, are supported by and freely rotatable around the primary roller axle  171  of the feeder roller  170 . In other words, the feeder roller  170  and each of the primary resist rollers  350  are positioned on the same axis. In contrast, each of the primary resist rollers  150  in the first embodiment is supported by the three internal gears  154   a - 154   c.    
     In contrast to the feeder roller  170 , which is fixed on the primary roller axle  171 , each of the primary resist rollers  350  is supported by the primary roller axle  171  without fixation thereon. 
     Consequently, driving force from the driver connected to the primary roller axle  171  is only directly transmitted to the feeder roller  170 . 
     One end of the drive transmission shaft  165  is joined to an end surface of one of the primary resist rollers  350 , and the other end of the drive transmission shaft  165  is joined to an end surface of the other of the primary resist roller  350 . 
     In the above configuration where the feeder roller  170  and the primary resist rollers  350  are positioned on the same axis, preferably a diameter D 3  of each of the primary resist rollers  350  should be set marginally smaller than the diameter D 1  of the feeder roller  170 . 
     Reasoning behind the above is that particularly in a configuration where the feeder roller  170  also functions as a pick-up roller, if the primary resist rollers  350  are equal in diameter to the feeder roller  170 , the primary resist rollers  350  may also contact with the uppermost recording sheet S 1  (refer to  FIG. 1 ) stacked in the feeder tray  21 . In the above situation, when the feeder roller  170  attempts to pick-up the recording sheet S 1 , rotation of the primary resist rollers  350  may be caused by friction with the recording sheet S 1 . The above rotation of the primary resist rollers  350  means that the recording sheet S 1  might be conveyed in a non-skew corrected state. 
     Preferably, a difference between the diameter D 1  of the feeder roller  170  and the diameter D 3  of each of the primary resist rollers  350  should be small. 
     Reasoning behind the above is that the larger the diameter D 1  is compared to the diameter D 3 , the larger conveyance speed of the recording sheet S 1  by the feeder roller  170  is compared to conveyance speed of the recording sheet S 1  by the primary resist rollers  350 . If the conveyance speed by the feeder roller  170  is significantly larger, once the leading part of the recording sheet S 1  has passed through the nip formed between each of the primary resist rollers  350  and the corresponding secondary resist roller  180 , the loop formed in the recording sheet S 1  in order to perform skew correction may become increasingly large. If the loop becomes too large, the loop may become caught in the nip between each of the primary resist rollers  350  and the corresponding secondary resist roller  180 , thus preventing correct sheet feeding. 
     In the second embodiment, through setting the diameter D 3  of each of primary resist rollers  350  as marginally smaller than the diameter D 1  of the feeder roller  170 , the feeder roller  170  is able to rotate while in contact with an inner surface the recording sheet S 1 , even when the primary resist rollers  350  are stationary. 
     In contrast to the feeder roller  170 , each of the primary resist rollers  350  is pressed against by the corresponding secondary resist roller  180 , which is coupled to the torque limiter (omitted in  FIG. 5 ). Through setting a torque value of the torque limiter sufficiently high, friction from the recording sheet S 1  and the feeder roller  170  can be counteracted. In other words, rotation of the primary resist rollers  350  due to transmission of driving force through the recording sheet S 1  is prevented. 
       FIGS. 6A-6C  show operation of the feeder mechanism  241  relating to the present embodiment. 
     For ease of explanation of rotational movement of the feeder roller  170  and each of the primary resist rollers  350 , points C and G are marked on respective circumferential surfaces thereof. 
     As shown in  FIG. 6A , the feeder roller  170  and the primary resist roller  350  are positioned on the same axis. Furthermore, the feeder roller  170  is larger in diameter than the primary resist roller  350 , therefore when viewed in the direction of the rotational axis as in  FIG. 6A , a contour line of an outer skin of the primary resist roller  350  is contained completely within a contour line of an outer skin of the feeder roller  170 . 
     Before the main roller axle  171  commences rotation the drive transmission shaft  165  is in contact with the inner wall  172   b  of the core part  172 . 
     In the above situation, point C shows a lowest point on the feeder roller  170  and point G shows a point of contact the primary resist roller  350  and the corresponding secondary resist roller  180 . 
     When a recording sheet is to be fed-in, in accordance with an instruction from the control unit  40 , the sheet stacking surface  21   a  is raised by the actuator so that the upper surface of the uppermost recording sheet S 1  is in contact with the circumferential surface of the feeder roller  170 , and the primary roller axle  171  is rotationally driven in the anticlockwise direction by the driver (omitted in  FIGS. 6A-6C ). 
     Through the above, the recording sheet S 1  is picked-up and conveyed in a direction corresponding to rightwards movement in  FIG. 6B . 
     When the feeder roller  170  rotates, the core part  172  thereof also rotates in the anticlockwise direction. The long arc-shaped hole  174  extends in the direction of rotation, therefore driving force is not transmitted to the drive transmission shaft  165  inserted therethrough until the drive transmission shaft  165  is brought into contact with the inner wall  172   c  of the core part  172 . 
     Consequently, each of the primary resist rollers  350  connected to the drive transmission shaft  165  remain stationary until the drive transmission shaft is in contact with the inner wall  172   c.  In other words, there is no change in position of point G 
     As shown in  FIG. 6B , by the time the drive transmission shaft  165  is in contact with the inner wall  172   c,  point C on the feeder roller  170  has moved to a new position slightly beyond point G 
     Through the above, the leading part of the recording sheet S 1  is conveyed towards a point approximately equivalent to point C, however partway through the above movement the leading part of the recording sheet S 1  is pressed against the nip N formed between the primary resist roller  350  and the corresponding secondary resist roller  180 . 
     Even once the leading part of the recording sheet S 1  is pressed against the nip N, the trailing part of the recording sheet S 1  continues to be conveyed, and therefore the loop L is formed in the recording sheet S 1  as shown in  FIG. 6B . 
     The diameter D 1  of the feeder roller  170  is marginally larger than the diameter D 3  of each of the primary resist rollers  350 , therefore when the loop L is formed in the recording sheet S 1 , at point G two opposite side edge sections of the recording sheet S 1 , in terms of a width direction thereof, are respectively in contact with the two primary resist rollers  350 . A central section of the recording sheet S 1 , in terms of the width direction thereof, is in contact with the feeder roller  170 . Through the above, the edge of the leading part of the recording sheet S 1  becomes approximately parallel to the axial direction of the primary resist rollers  350 . 
     In the above situation, once the drive transmission shaft  165  is brought into contact with the inner wall  172   c,  rotational driving force is applied against the drive transmission shaft  165  in the anticlockwise direction as shown in  FIG. 6C , thus also causing rotation of the primary resist rollers  350 . 
     Rotation of the primary resist rollers  350  causes conveyance of the leading part of the recording sheet S 1 , and thus the recording sheet S 1  is conveyed downstream in the skew corrected state as in the first embodiment. 
     During the above operations, the control unit  40  lowers the sheet stacking surface  21   a  of the feeder tray  21  using the driver (omitted in  FIGS. 6A-6C ), in order to ensure that the feeder roller  170  is not in contact with recording sheets on the sheet stacking surface  21   a.  The above prevents the next recording sheet being fed-in prematurely. 
     In the above configuration, when viewed in the direction of the rotational axis the feeder roller  170  and the primary resist rollers  150  each have a center of rotation at the same position. In other words, the outer skin of the feeder roller  170  overlaps the entire circumference of the outer skin of each of the primary resist rollers  350 . Consequently, the image forming apparatus including the feeder mechanism  241  can be more compact in terms of size. 
     In the second embodiment there is delayed transmission of the rotational drive causing rotation of the feeder roller  170  to the primary resist rollers  350 . Thus, a single driver can be used for both the feeder roller  170  and the primary rollers  350 , while also ensuring reliable loop formation, skew correction and conveyance of the recording sheet. The above configuration allows a reduction in cost of the image formation apparatus including the feeder mechanism  241 . 
     Third Embodiment 
     Configuration of a feeder mechanism relating to a third embodiment is generally the same as configuration of the feeder mechanism  41  relating to the first embodiment. However, configuration of the primary resist rollers  150  differs from in the feeder mechanism  41 . 
     Configuration elements that are the same as in the first embodiment are referred to below using the same reference symbols, and description thereof is omitted or abbreviated in order to focus on configuration elements that are different. 
       FIG. 7  is a perspective diagram showing configuration of main elements of a feeder mechanism  441  relating to the third embodiment. 
     As shown in  FIG. 7 , the feeder mechanism  441  relating to the third embodiment includes two coupling units  460   a  that have a different configuration to the two coupling units  160  in the first embodiment. Furthermore, the feeder mechanism  441  includes two primary resist rollers  450 , which correspond to the two primary resist rollers  150  in the first embodiment. The primary resist rollers  450  are not each supported by three internal gears as in the first embodiment, but are instead supported by a secondary roller axle  455  that is adjacent and extending in parallel to the primary roller axle  171 , which supports the feeder roller  170 . In the first embodiment coupling between each of the coupling units  160  and the corresponding primary resist roller  150  is through gears, but in the third embodiment, each of the coupling units  460   a  is coupled to a corresponding primary resist roller  450  through a belt  454 . 
     In the same way as for the coupling units  160  in the first embodiment, the two coupling units  460   a  are positioned one each at two ends of the feeder roller  170  in terms of the Y-axis direction. The drive transmission shaft  165  extends from one of the coupling units  460   a  to the other of the coupling units  460   a.    
     Two supporting members  440 , which are narrower in terms of an X-axis direction than the supporting members  140  in the first embodiment, support the primary roller axle  171 . The supporting members  440  are narrower in order to avoid interference with the primary resist rollers  450 . 
     As described above, the primary resist rollers  450  are supported by the secondary roller axle  455  which is adjacent and parallel to the primary roller axle  171  supporting the feeder roller  170 . Each of the primary resist rollers  450  is formed from the outer skin  151 , a core part  450   c  corresponding to the core part  172  in the first embodiment, and a pulley part  450   a,  which is provided on one end of the core part  450   c  in terms of the Y-axis direction. 
     The pulley part  450   a  is a solid cylinder, having a groove  454   b  in an outer circumference thereof against which the belt  454  of the corresponding coupling unit  460   a  winds. 
       FIG. 8  shows the feeder mechanism  441  viewed in the direction of the rotational axis of the feeder roller  170  (viewed from a side Y′ shown in  FIG. 7 ). 
     The belt  454  is omitted in  FIG. 8 . 
     The feeder roller  170  and each of the primary resist rollers  450  are positioned so that when viewed in the direction of the rotational axis, a portion of the outer circumference  173  of the feeder roller  170  overlaps with a portion of the outer skin  151  of the primary resist roller  450 . Therefore, the image forming apparatus including the feeder mechanism  441  can be made more compact in size. 
     Furthermore, the drive transmission shaft  165  and the long arc-shaped hole  174  function as a delayed drive transmission unit in the same way as in the first embodiment. Therefore, a single driver can be used to cause the feeder roller  170  and the primary resist rollers  450  to commence rotation at different times, thus skew correction can be performed and costs can be reduced through use of just the single driver. 
     Modified Examples  
     The present invention is not limited to the embodiments given above, and alternatively may be realized as described in modified examples given below. 
     (1) In the first embodiment the separation roller  190  presses against the feeder roller  170 , but alternatively the separation roller  190  may be replaced by any pressing member that presses against the feeder roller  170 . For example the pressing member may be a fixed pressing pad that does not rotate. 
     (2) In the first embodiment the feeder roller  170  also functions as a pick-up roller. Alternatively, a pick-up roller may be provided in addition to the feeder roller  170 . 
     (3) In the first embodiment, the diameter D 1  of the feeder roller  170  is equal to the diameter D 2  of each of the primary resist rollers  150 . Alternatively, the diameter D 1  may be different to the diameter D 2 , so long as the difference does not cause creasing of or excessive tension on the recording sheet. 
     For example, the diameter D 1  may differ from the diameter D 2  so long as the diameter D 2  is not so large that the primary resist rollers  150  are in contact with the uppermost recording sheet S 1  when the sheet stacking surface  21   a  of the sheet feeder  21  is raised as in  FIG. 4A . 
     If the diameter D 1  is different to the diameter D 2 , preferably the inner gears  154   c  should each have a different gear ratio at a side of the corresponding coupling unit  160  compared to a side of the corresponding primary resist roller  150 . The above is in order to ensure that a circumferential surface (outer circumference of the circumferential part  173 ) of the feeder roller  170  and a circumferential surface (outer circumference of the outer skin  151 ) of each of the primary resist rollers  150  are equal in terms of rotational velocity. 
     (4) In the first embodiment the long arc-shaped hole  174  is provided on the feeder roller  170 , and the drive transmission shaft  165  is attached to the coupling units  160 . However, the above is not a limitation on the present invention. 
     For example, as shown in  FIG. 9 , alternatively a long arc-shaped hole  265 , corresponding to the long arc-shaped hole  174  in the first embodiment, may be provided on each of two coupling units  260 , corresponding to the coupling units  160  in the first embodiment. Furthermore, a drive transmission shaft  274 , corresponding to the drive transmission shaft  165  in the first embodiment, may be provided extending in the Y-axis direction from both ends of a core part  272  of a feeder roller  270 , corresponding to feeder roller  170  in the first embodiment. 
     (5) In the first embodiment, the delayed drive transmission unit is configured as the long arc-shaped hole  174  and the drive transmission shaft  165 . However, the above is not a limitation on the present invention. 
     For example, the delayed drive transmission unit may alternatively be configured as shown in  FIG. 10 . A first engaging part  374   a  and a second engaging part  374   b  are provided at each end, in terms of the Y-axis direction, of a core part  372  of a feeder roller  370 , corresponding to the feeder roller  170  in the first embodiment. The first engaging part  374   a  and the second engaging part  374   b  are positioned so that when viewed in the direction of the rotational axis (Y-axis direction), the first engaging part  374   a  and the second engaging part  374   b  are separated from one another by a predetermined angle measured from the primary roller axle  171  of the feeder roller  370 . A protrusion  165   a  is provided on an end, in terms of the Y-axis direction, of each of two coupling units  160   a,  which correspond to the coupling units  160  in the first embodiment. Each of the protrusions  165   a  engages selectively with the first engaging unit  374   a  and the second engaging unit  374   b  at a corresponding end of the feeder roller  370 , depending on a state of rotation of the feeder roller  370 . 
     (6) The image forming apparatus in the first embodiment is a monochrome image forming apparatus. However, the above is not a limitation on the present invention. Alternatively, the image forming apparatus may be four-cycle type image forming apparatus, or a tandem type color printer for forming a full-color image. Configuration of the present invention is not limited to printers, and may also be applicable for photocopiers, fax machines, MFPs and the like. 
     The feeder mechanism  41  may also be applicable for skew correction in an ADF (Auto Document Feeder). 
     In the above type of apparatus, usually original documents stacked in a feeder tray are picked-up in a downwards direction, therefore the feeder roller  170  should preferably be positioned so as to be in contact with a lower surface of a lowermost original document. 
     The present invention may also be configured as any appropriate combination of the embodiments and the modified examples described above. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.