Patent Publication Number: US-8523173-B2

Title: Sheet feeder and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet feeder and an image forming apparatus and, more particularly, to a configuration in which a sheet feeding force can be prevented from being markedly reduced even in the case where an amount of stacked sheets is increased or sheets having a great density are stacked. 
     2. Description of the Related Art 
     An image forming apparatus, such as a printer or a copying machine, has been conventionally provided with a sheet feeder including a sheet feeding cassette serving as a sheet container of a feeding type, on which sheets are stacked, and a feeding portion which feeds the sheets contained in the sheet feeding cassette one by one in separation. 
     The above-described sheet feeder is exemplified by a sheet feeder provided with a feed roller which feeds sheets and a separation roller which separates the sheets in abutment against the feed roller. In a sheet feeding cassette disclosed in U.S. Patent Application Publication No. 2006/237895 A1, a lifter plate having sheets stacked thereon is disposed in a removable manner in a vertical direction, and further, the lifter plate is urged by a spring to bring the sheet into press-contact with the feed roller, thereby generating a feeding force. When the sheet is fed, the feed roller is rotated in press-contact with an uppermost sheet stacked on the lifter plate so that the uppermost sheet is fed. After that, the fed uppermost sheet is allowed to pass a nip interposed between the feed roller and the separation roller. In this manner, the sheets are fed one by one in separation. 
     In the above-described sheet feeder provided with the sheet feeding cassette such configured that the lifter plate is urged so as to generate the sheet feeding force, the weight of the sheets stacked on the lifter plate is increased in the case where an amount of the stacked sheets is increased or sheets having a great density are stacked. When the weight of the sheets is increased in such a manner, the weight reduces the resiliency of the spring, and therefore, a pressing pressure between the feed roller and the sheet is decreased, thereby raising a fear of instability of the sheet feeding force of the feed roller. 
     In view of the above-described present circumstances, the present invention has been accomplished to solve the above-described problems experienced in the prior art. Therefore, the present invention provides a sheet feeder and an image forming apparatus, in which a sheet feeding force can be prevented from being markedly reduced even in the case where an amount of stacked sheets is increased or sheets having a great density are stacked. 
     SUMMARY OF THE INVENTION 
     A sheet feeder according to the present invention includes: a sheet containing portion that has a sheet stacking portion to be lifted and lowered and that is detachably attached to an apparatus body; a feed roller that is located above the sheet stacking portion in a manner movable in a vertical direction so as to feed sheets stacked on the sheet stacking portion; a feed roller biasing portion that applies a force to the feed roller in a direction in which the feed roller is brought into press-contact with the sheets stacked on the sheet stacking portion; a separation member that is brought into press-contact with the feed roller so as to separate the sheets fed by the feed roller one by one; a separating portion that separates the separation member from the feed roller in association with a pulling operation of the sheet containing portion from the apparatus body; a lifting and lowering portion that lifts the sheet stacking portion in such a manner that an upper surface of the sheet is located at a predetermined height during a sheet feeding operation whereas lowers the sheet stacking portion in association with the pulling operation of the sheet containing portion when the sheet containing portion is pulled out of the apparatus body; and a restricting portion that restricts the feed roller being applied a force by the feed roller biasing portion from following the separation member separated from the feed roller and the lowering sheet stacking portion when the sheet containing portion is pulled out of the apparatus body. 
     According to the present invention, a sheet stack portion is lowered as a force is applied to a feed roller such that a sheet feeding force of the feed roller can be prevented from being markedly reduced even in the case where an amount of stacked sheets is increased or sheets having a great density are stacked. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view schematically illustrating the configuration of a full-color laser beam printer exemplifying an image forming apparatus provided with a sheet feeder in a first embodiment according to the present invention; 
         FIG. 2A  is a first view illustrating the configuration of the sheet feeder; 
         FIG. 2B  is the first view illustrating the configuration of the sheet feeder; 
         FIG. 3A  is a view illustrating a sheet feeder of a feed roller pressurizing system in the prior art; 
         FIG. 3B  is a view illustrating the sheet feeder of the feed roller pressurizing system in the prior art; 
         FIG. 3C  is a view illustrating the sheet feeder of the feed roller pressurizing system in the prior art; 
         FIG. 4A  is a view illustrating a separating mechanism which separates a feed roller and a Separation roller from each other in the sheet feeder of the feed roller pressurizing system in the prior art; 
         FIG. 4B  is another view illustrating the separating mechanism which separates the feed roller and the separation roller from each other in the sheet feeder of the feed roller pressurizing system in the prior art; 
         FIG. 5A  is a second view illustrating the configuration of the sheet feeder; 
         FIG. 5B  is the second view illustrating the configuration of the sheet feeder; 
         FIG. 6A  is a first view illustrating the configuration of a sheet feeder in a second embodiment according to the present invention; 
         FIG. 6B  is the first view illustrating the configuration of the sheet feeder in the second embodiment according to the present invention; 
         FIG. 7A  is a second view illustrating the configuration of the sheet feeder; and 
         FIG. 7B  is the second view illustrating the configuration of the sheet feeder. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A detailed description of embodiments according to the present invention with reference to the attached drawings will be given below.  FIG. 1  is a view schematically illustrating the configuration of a full-color laser beam printer exemplifying an image forming apparatus provided with a sheet feeder in a first embodiment according to the present invention. 
     In  FIG. 1 , a full-color laser beam printer (hereinafter referred to as a “printer”)  1  includes a printer body  1 A serving as an image forming apparatus body and an image forming portion  1 B which forms an image on a sheet. An image reading apparatus  2  is disposed substantially horizontally above the printer body  1 A. A sheet discharging space D is defined between the image reading apparatus  2  and the printer body  1 A. 
     Furthermore, the printer  1  further includes sheet feeders  20  which feed sheets and toner cartridges  15 . 
     The image forming portion  1 B is of a four-drum full color system, including a laser scanner  10  and four process cartridges  11  which form toner images with four colors, that is, yellow (Y), magenta (M), cyan (C), and black (K). Here, each of the process cartridges  11  is provided with a photosensitive drum  12 , a charger  13  serving as an electric charging portion, and a development device  14  serving as a developing portion. Moreover, an intermediate transfer unit  1 C is disposed above the process cartridges  11 . 
     The intermediate transfer unit  1 C is provided with an intermediate transfer belt  16  stretched between a drive roller  16   a  and a tension roller  16   b  and primary transfer rollers  19  which are disposed inside of the intermediate transfer belt  16  and abut against the intermediate transfer belt  16  at positions facing the photosensitive drums  12 . The intermediate transfer belt  16  is formed of a film-like member in contact with each of the photosensitive drums  12 , and further, is designed to be rotated in a direction indicated by an arrow by the drive roller  16   a  to be driven by a drive portion, which not illustrated. 
     A transfer bias of a positive polarity is applied to the intermediate transfer belt  16  by the primary transfer rollers  19 , and thus, the toner images of the colors having a negative polarity on the photosensitive drums are sequentially transferred onto the intermediate transfer belt  16  in superimposition. At a position opposite to the drive roller  16   a  in the intermediate transfer unit  1 C, a secondary transfer roller  17  constituting a secondary transfer portion, in which the color images formed on the intermediate transfer belt are transferred onto a sheet S, is disposed. 
     Additionally, a fixing portion  200  is disposed above the secondary transfer roller  17 . Furthermore, at an upper left portion of the fixing portion  200 , a pair of first discharge rollers  25   a , a pair of second discharge rollers  25   b , and a double-sided reversing portion  1 D are disposed. In the double-sided reversing portion  1 D, there are provided a pair of reverse rollers  22  which can be rotated forward and reversely, a re-conveyance path R on which the sheet having the image on one side thereof is conveyed to the image forming portion  1 B again, and the like. 
     Next, a description of an image forming operation by the printer  1  such configured as described above will be given. First, the image reading apparatus  2  reads image information on a document. The image information is then subjected to image processing, is subsequently converted into an electric signal, and then, is transmitted to the laser scanner  10  in the image forming portion  1 B. Here, the image information may be input into the image forming portion  1 B from outside equipment such as a personal computer, which is not illustrated. 
     In the image forming portion  1 B, a surface of the photosensitive drum  12  in each of the process cartridges  11  is scanned with laser beams which are emitted from the laser scanner  10  and correspond to the image information on yellow, magenta, cyan, and black component colors. In this manner, the surface of the photosensitive drum  12  which is uniformly charged to a predetermined polarity and a predetermined potential by the charger  13  is sequentially exposed to the light beams. Thus, electrostatic latent images of yellow, magenta, cyan, and black colors are sequentially formed on the photosensitive drum of each of the process cartridges  11 . 
     Thereafter, the electrostatic latent images are visually developed with toners of yellow, magenta, cyan, and black colors, and further, the toner images of the colors formed on the photosensitive drums are superimposed in sequence onto the intermediate transfer belt  16  by a primary transfer bias applied to the primary transfer rollers  19 . As a consequence, the toner images are formed on the intermediate transfer belt  16 . 
     In parallel to the above-described toner image forming operation, the sheet S is fed from the sheet feeder  20 . The fed sheet S is conveyed to a pair of registration rollers  400 , and then, conveyed to a secondary transfer portion while skew feeding is corrected by the pair of registration rollers  400 . Thereafter, the toner images are transferred onto the sheet S by one operation by a secondary transfer bias, which is applied to the secondary transfer roller  17 , in the secondary transfer portion. 
     Next, the sheet S having the toner images transferred thereonto in the above-described manner is conveyed to the fixing portion  200 . When the sheet S receives heat and pressure in the fixing portion  200 , the toners of the colors are fused and mixed so that color images are fixed onto the sheet S. After that, the sheet S having the images fixed thereto is discharged to the discharge space D by the pair of first discharge rollers  25   a  disposed downstream of the fixing portion  200 , and then, is stacked on a stack portion  23  disposed in such a manner as to project at the bottom surface of the discharge space D. 
     When the images are formed on both sides of the sheet S, the sheet S having the image formed on either side thereof passes the fixing portion  200 , and then, passes the fixing portion  200  by the pair of reverse rollers  22 . Thereafter, the pair of reverse rollers  22  is rotated reversely, so that the sheet S is conveyed reversely onto the re-conveyance path R, and then, is conveyed again to the pair of registration rollers  400 . After the sheet S is subjected to the image forming and fixing operations on the other side thereof, the sheet S is discharged to the discharge space D by the pair of first discharge rollers  25   a  to be thus stacked on the stack portion  23 . 
     Subsequently, a description of the configuration of the sheet feeder  20  in the present embodiment will be given with reference to  FIGS. 2 to 5 . Cassettes  30  are provided in the printer body  1 A serving as the sheet feeder body in such a manner as to be freely (detachably) pulled. As illustrated in  FIGS. 2A and 2B , a lifter plate  37  serving as the sheet stack portion, on which the sheets S are stacked, is disposed inside of each of the cassettes  30  in such a manner as to be freely turned (lifted and lowered) in a vertical direction on a fulcrum  37   a . Moreover, the sheet feeder  20  includes a feed roller  31  that is disposed above the lifter plate  37  in such a manner as to be freely moved in the vertical direction and feeds the sheets stacked on the lifter plate  37 . 
     As illustrated in  FIGS. 2A ,  2 B,  5 A, and  5 B, a separation roller  32 , which serves as a separation member provided with a torque limiter and separates the sheets one by one, is disposed downstream in a sheet feed direction of the feed roller  31  in such a manner as to be brought into press-contact with the feed roller  31 . Here, a separation roller pressurizing spring  33  applies a force to the separation roller  32  toward the feed roller side, and further, a separation roller holder  34 , which can be turned on a turn shaft  34   a , rotatably supports the separation roller  32 . The separation roller  32  and the feed roller  31  constitute a separation portion at which the sheets are separated and fed one by one. 
     The lifter plate  37  is vertically turned on the fulcrum  37   a  by a lifter arm  36  which is vertically turned on a lifter arm shaft  36   a  by the drive portion, which is not illustrated. 
     Here, the lifter arm  36  is designed to be connected to a lifter lever  38  illustrated in  FIGS. 3A to 3C  via the lifter arm shaft  36   a , so that the lifter arm  36  is turned on the lifter arm shaft  36   a  to lift the lifter plate  37 . 
     Furthermore, a driving force is transmitted to the lifter lever  38  from a lifter motor  41  disposed in the printer body  1 A via a lifter idler gear  40  illustrated in  FIGS. 3A to 3C , thus lifting and lowering the lifter plate  37  via the lifter lever  38  and the lifter arm  36 . In this manner, a lifting and lowering portion that lifts and lowers the lifter plate  37  comprises the lifter arm  36 , the lifter lever  38 , and the lifter motor  41 . 
     As illustrated in  FIGS. 3A to 3C , a sheet height detecting sensor  46  which detects a height of the sheets is disposed in the vicinity of the abutment position of the feed roller  31  against the sheets. A controller, which is not illustrated, is adapted to control the lifter motor  41  based on a signal output from the sheet height detecting sensor  46  in such a manner that the upper surface of an uppermost sheet S 1  is in a height equal to a predetermined height. 
     Here,  FIG. 3A  illustrates the state in which the cassette  30  is inserted into the printer body. When a user inserts the cassette  30  into the printer body, the lifter motor  41  is rotated, and the lifter plate  37  is lifted as illustrated in  FIG. 3B . When the sheet height detecting sensor  46  detects the upper surface of the uppermost sheet S 1 , the lifter motor  41  is stopped. Incidentally, the sheet height detecting sensor  46  is such configured as to detect a position at which the feed roller  31  abuts against the uppermost sheet S 1  to push up the feed roller pressurizing spring  43  at a sheet feed pressure which can generate a proper feeding force. 
     Next, a simple description of the feeding operation by the sheet feeder in the present exemplary embodiment will be given. When a signal indicating a start of sheet feeding is output from the controller, which is not illustrated, in the printer body, the uppermost sheet S 1  is fed along the rotation of the feed roller  31  to be thus introduced into a nip portion between the feed roller  31  and the separation roller  32  disposed downstream, as illustrated in  FIG. 3B . At this time, in the case of one sheet S 1  is fed, the separation roller  32  is rotated following the feed roller  31  and the sheet S 1 , and further, conveys the sheet S 1  to withdrawal rollers  42  disposed downstream. In contrast, in the case of a plurality of sheets S 1  is fed by the feed roller  31 , a second sheet onwards are stopped at the nip portion between the feed roller  31  and the separation roller  32  by the effect of the torque limiter housed inside of the separation roller  32 . 
     In this manner, only the uppermost sheet S 1  is conveyed in separation. 
     Here, the lifting and lowering portion lifts the lifter lever  38  in feeding the sheet. When the cassette  30  is pulled out, as described later, the lifting and lowering portion is disconnected from the drive portion in association with a pulling operation of the cassette  30 , and thus, the lifter lever  38  is lowered by its own weight or the load of the sheet. 
     Additionally, the lifting and lowering portion is controlled in such a manner as to lift the lifter plate  37  such that the uppermost sheet S 1  can be fed at the predetermined height when the height of the uppermost sheet S 1  becomes low after the sheets S are fed during the sheet feeding operation. Incidentally, when a sheet presence detecting sensor, which is not illustrated, detects that all of the sheets are fed, the controller, which is not illustrated, stops the sheet feeding operation, and displays an alarm of sheet replenishment on a panel or the like. The user pulls the cassette  30  and replenishes sheets on the cassette  30  based on the display. 
     Here, the sheet feeder  20  in the present embodiment includes a separation mechanism  30 A serving as a separation portion which separates the separation roller  32  from the feed roller  31  in association with the pulling operation of the cassette  30 , as illustrated in  FIGS. 6A and 6B . The separation mechanism  30 A will be described below.  FIG. 4A  illustrates the state in which the cassette  30  is mounted in the printer body. Here, when the cassette  30  is mounted in the printer body, a separation roller separating projection  30   a  formed in the cassette  30  is adapted to press a separation roller support lever  35 , which is slidably disposed in the printer body, in a mount direction against a biasing force of a biasing member, which is not illustrated. 
     When the separation roller separating projection  30   a  presses the separation roller support lever  35  in the above-described manner, the separation roller support lever  35  presses a separation roller holder projection  34   b  projecting from a side wall surface of the separation roller holder  34 . Consequently, the separation roller holder  34  is supported in movement toward the feed roller on the turn shaft  34   a , so that the separation roller  32  can be brought into press-contact with the feed roller  31 . 
     In the meantime, when the cassette  30  is pulled out in a direction indicated by an arrow F in  FIG. 4B , the pressing by the separation roller separating projection  30   a  is released, and the separation roller support lever  35  is moved in a direction indicated by an arrow F 1  together with the cassette  30  by the biasing force of the biasing member, which is not illustrated. In this manner, the support for the separation roller holder projection  34   b  is released, and the separation roller holder  34  is turned in a direction indicated by an arrow G on the turn shaft  34   a , thereby separating the separation roller  32  from the feed roller  31 . Here,  FIG. 3C  illustrates the state in which the sheet S is separated from the feed roller  31  and the feed roller  31  is separated from the separation roller  32 . 
     As illustrated in  FIG. 5B , when the cassette  30  is pulled out by the separation mechanism  30 A in a direction indicated by an arrow K, the pressing by the separation roller separating projection  30   a  is released, and the separation roller support lever  35  is moved in the direction indicated by the arrow together with the cassette  30  by the biasing force of the biasing member, which is not illustrated. In this manner, the support for the separation roller holder projection  34   b  is released, and the separation roller holder  34  is turned in the direction indicated by the arrow G, thereby separating the separation roller  32  from the feed roller  31 . 
     The feed roller  31  in the present embodiment is disposed in such a manner as to be vertically movable, and receives the force downward from the feed roller pressurizing spring  43 , as described above. Therefore, there is a fear that the feed roller  31  follows the sheet S and the separation roller  32  that are to be separated even if the sheet S and the separation roller  32  are to be separated in association with the pulling operation of the cassette  30 , so that the sheet S and the separation roller  32  cannot be separated or a separation timing is largely delayed. 
     In such a case, breakage or bending of the sheet occurs or the sheet remains inside of the apparatus. In other words, when the sheet containing portion (i.e., the cassette) is pulled out, the feed roller follows the sheet and the separation roller that are to be separated, and thus, the sheet is kept to be nipped between the feed roller and the separation portion, thereby generating the breakage or bending of the sheet or making the sheet remain inside of the apparatus. 
     In view of the foregoing, in the present embodiment, there is a restricting portion that restricts the feed roller  31  from following the separation roller  32  that is to be separated. Description on the restricting portion will be given below. 
     In  FIGS. 2A ,  2 B,  5 A and  5 B, a feed roller separating arm  44  serves as a moving portion that moves the feed roller  31  upward. The feed roller separating arm  44  is supported by a frame, which is not illustrated, in the printer body  1 A in such a manner as to be freely turned in the vertical direction on separation arm shafts  44   a  and  44   b . Further, The feed roller separating arm  44  is applied a force upward by a feed roller separating spring  45  serving as a moving portion biasing member. 
     The feed roller separating arm  44  includes feed roller bearings  47  ( 47   a  and  47   b ) and shaft supporting portions  44   c  and  44   d  that support a feed roller shaft  50  from below in such a manner as to lift the feed roller  31  when the feed roller separating arm  44  is lifted, as described later. Here, in the present embodiment, the feed roller bearings  47  are applied a force toward the lifter plate  37  by feed roller pressurizing springs  43  ( 43   a  and  43   b ) serving as feed roller biasing portions. The feed roller  31  is applied a force toward the lifter plate  37  by the feed roller pressurizing springs  43  via the feed roller separating arm  44 . 
     In this manner, when the upper surface of the uppermost sheet is lowered by lowering the position of the lifter plate  37 , the feed roller  31  is lowered accordingly, so that the feed roller  31  can be brought into press-contact with the uppermost sheet. 
     As a consequence, even if the lifter plate  37  is lowered against the biasing force of the spring as a result of an increase in weight of the sheets, the sheet feeding force can be stably held. Specifically, the feed roller  31  is lowered as the lifter plate  37  is lowered, thereby preventing the sheet feeding force from being markedly reduced even in the case where the amount of stacked sheets is increased or the sheets having a great density are stacked. 
     Moreover, the feed roller separating arm  44  includes a locking portion  44   f  that extends in a vertical direction and has a separation arm positioning hole  44   e , to which a separation arm positioning boss  30   b  formed at the cassette  30  is fitted. When the cassette  30  is inserted into the printer body  1 A, the separation arm positioning boss  30   b  formed at the cassette  30  is fitted to the separation arm positioning hole  44   e  formed in the locking portion  44   f , as illustrated in  FIG. 5A . 
     Consequently, the feed roller separating arm  44  that receives a tension upward from the feed roller separating spring  45  is positioned against the biasing force of the feed roller separating spring  45 . In this manner, a stopper that restricts the upward motion of the feed roller separating arm  44  against the biasing force of the feed roller separating spring  45  is constituted of the separation arm positioning boss  30   b  formed at the cassette  30  and the separation arm positioning hole  44   e  formed in the locking portion  44   f . When the feed roller separating arm  44  is positioned in the above-described manner, the shaft supporting portions  44   c  and  44   d  formed at the feed roller separating arm  44  are brought out of contact with the feed roller shaft  50 , and therefore, the feed roller  31  cannot be prevented from being rotated. 
     When the cassette  30  is pulled out of the printer body  1 A in the direction indicated by the arrow K, the separation arm positioning boss  30   b  is released from the separation arm positioning hole  44   e . At this instant, the restriction by the separation arm positioning boss  30   b  is released, and then, the feed roller separating arm  44  is turned on the separation arm shafts  44   a  and  44   b  upward in a direction indicated by an arrow H in  FIG. 2B  by the tension of the feed roller separation spring  45 . 
     When the feed roller separating arm  44  is turned upward in the above-described manner, the shaft supporting portions  44   c  and  44   d  formed at the feed roller separating arm  44  lift the feed roller shaft  50 . Accordingly, the feed roller  31  is lifted as indicated by an arrow I. Incidentally, in the present embodiment, the feed roller shaft  50  is connected to the motor, which is not illustrated, via a universal joint  51 , as illustrated in  FIGS. 5A and 5B , and therefore, the driving can be achieved even when the feed roller shaft  50  is lifted in the above-described manner. 
     As described above, when the cassette  30  is pulled out, the separation roller  32  is turned in the direction, in which the separation roller  32  is separated from the feed roller  31 , in association with the movement of the cassette  30  by the separation mechanism  30 A, and further, the feed roller  31  is lifted. In other words, when the cassette  30  is pulled out, the feed roller  31  is restricted from following the separation roller  32  that is to be separated by the restricting portion constituted of the feed roller separation arm  44  and the feed roller separation spring  45 . Here, at the timing when the feed roller  31  is lifted, the lifter plate  37  is disconnected from the lifter idler gear  40 , and is thus lowered down to a lowest position. Consequently, the sheet S and the feed roller  31  are separated from each other, too. 
     In this manner, in the present embodiment, when the cassette  30  is pulled out, the feed roller  31  is lifted, and further, the separation roller  32  is moved in the direction in which the separation roller  32  is separated from the feed roller  31 . Moreover, the lifter plate  37  is lowered, so that the sheet S and the feed roller  31  are separated from each other. Accordingly, at feed standby time illustrated in  FIGS. 2A and 5A , even when the sheet is nipped by the feed roller  31 , the lifter plate  37 , and the separation roller  32 , the cassette  30  is pulled out while avoiding the sheet from being bent, broken, or remaining inside of the apparatus. 
     When the cassette  30  is mounted in the printer body  1 A, the separation roller separating projection  30   a  formed in the cassette  30  presses the separation roller supporting lever  35 , which then presses the separation roller holder projection  34   b  projecting from the side wall surface of the separation roller holder  34 . Consequently, the separation roller holder  34  is moved toward the feed roller, and thus, the separation roller  32  is brought into press-contact with the feed roller  31 . 
     Additionally, when the cassette  30  is mounted, the separation arm positioning boss  30   b  is fitted to the separation arm positioning hole  44   e  shifted laterally, as illustrated in  FIG. 2B , at the time of disengagement since the tip of the separation arm positioning boss  30   b  is formed into a tapered shape. Along with the fitting of the separation arm positioning boss  30   b , the feed roller separation arm  44  is turned downward on the separation arm shafts  44   a  and  44   b  against the tension of the feed roller separation spring  45 , thereby being turnably restored to the position illustrated in  FIG. 2A . 
     As described above, in the present embodiment, the feed roller  31  is configured to be applied a force to follow the lowering lifter plate  37 . As a consequence, even when the amount of stacked sheets is increased or the sheets having a great density are stacked, the sheet feeding force by the feed roller  31  can be prevented from being markedly reduced. 
     In addition, when the cassette  30  is pulled out, the feed roller  31  is lifted such that the feed roller  31  cannot follow either of the lifter plate  37  and the separation roller  32  that are to be separated. In this manner, even when the cassette  30  is pulled out in the state in which the uppermost sheet S 1  is nipped between the feed roller  31  and the lifter plate  37  and between the feed roller  31  and the separation roller  32 , the sheet can be avoided from being bent, broken, or remaining inside of the apparatus. 
     Next, a description of a second exemplary embodiment according to the present invention will be given.  FIGS. 6A ,  6 B,  7 A, and  7 B are views illustrating the configuration of a sheet feeder in the present embodiment. In  FIGS. 6A ,  6 B,  7 A, and  7 B, the same reference numerals as those in  FIGS. 2A ,  2 B,  5 A and  5 B designate the same or corresponding constituent elements. 
     In  FIGS. 6A ,  6 B,  7 A and  7 B, a feed roller stopper  48  serves as a holding portion that holds a feed roller  31  in such a manner as to restrict the feed roller  31  from following, in a downward direction, a sheet S (i.e., a lifter plate  37 ) and a separation roller  32  that are to be separated when a cassette  30  is pulled out. The feed roller stopper  48  is slidably supported in a cassette mounting direction by a frame, which is not illustrated, and further as well as provided with pressing portions  48   a  and  48   b  which are freely brought into or out of contact with feed roller bearings  47  ( 47   a  and  47   b ), respectively, at both ends in the cassette mounting direction. 
     A feed roller stopper spring  49  is interposed between a locking portion  48   b  downstream in the cassette mounting direction in the feed roller stopper  48  and a side plate  53  downstream in the cassette mounting direction in a printer body  1 A. The feed roller stopper spring  49  applies a force to the feed roller stopper  48  in a cassette pulling direction. Moreover, a feed roller support plate  52  is provided at both ends thereof with roller supporting portions  52   a  and  52   b  that turnably support the feed roller  31  in a manner movable in a vertical direction. 
     Here in the present embodiment, the feed roller bearings  47  ( 47   a  and  47   b ) are applied a force by feed roller pressurizing springs  43  ( 43   a  and  43   b ), which apply a force to the feed roller  31  toward the lifter plate  37 . Moreover, the feed roller bearings  47  are supported by the roller supporting portions  52   a  and  52   b  in such a manner as to be movable in the vertical direction. Consequently, the feed roller  31  is supported by the roller supporting portions  52   a  and  52   b  via the feed roller bearings  47  in such a manner as to be movable in the vertical direction indicated by an arrow J. As a consequence, when the lifter plate  37  is lowered and accordingly the upper surface of an uppermost sheet is lowered, the feed roller  31  follows them in a downward direction so as to be brought into press-contact with the uppermost sheet. 
     As illustrated in  FIG. 7A , when the cassette  30  is inserted into the printer body  1 A, the cassette  30  pushes the pressing portion  48   a  of the feed roller stopper  48 , thereby bringing the feed roller stopper  48  and the feed roller bearings  47  out of contact. In this state, the feed roller  31  is applied a force by the feed roller pressurizing spring  43  via the feed roller bearings  47  together with the feed roller shaft  50 , and therefore, it can be moved downward. 
     In contrast, when the cassette  30  is pulled out of the printer body  1 A in a direction indicated by an arrow K, the feed roller stopper  48  is pressed by the feed roller stopper spring  49 , and then, slides in a direction indicated by an arrow K 1  together with the cassette  30 . In this manner, when the feed roller stopper  48  is moved, the pressing portions  48   a  and  48   b  of the feed roller stopper  48  and the roller supporting portions  52   a  and  52   b  of the feed roller support plate  52  nip the feed roller bearings  47   a  and  47   b  therebetween, respectively. 
     Consequently, the feed roller bearings  47   a  and  47   b  are resiliently held at the positions. Additionally, the feed roller  31  supported by the feed roller bearings  47  also is held at its current position at this time together with the feed roller shaft  50 . Incidentally, the separation roller  32  is turned in a direction in separation from the feed roller  31  in association with the movement of the cassette  30  by a separation mechanism  30 A; like in the above-described first embodiment. 
     In the present embodiment, a timing when the feed roller stopper  48  nips the feed roller bearings  47  is set to be earlier than a separation timing of the separation roller  32 . Consequently, during a separating operation of the separation roller  32 , the feed roller  31  is held at that position. Thus, even if the separation roller  32  is turned, the feed roller  31  cannot follow the separation roller  32 . Therefore, when the separation roller  32  is turned, the separation roller  32  is separated from the feed roller  31 . At this timing, the lifter plate  37  is lowered in disconnection from a lifter idler gear  40  so that the sheet and the feed roller  31  also are separated from each other, like in the above-described first embodiment. 
     In this manner, in the present embodiment, when the cassette  30  is pulled out, the feed roller  31  is held at its current position and further the separation roller  32  is moved in the direction where it is separated from the feed roller  31 . The lifter plate  37  is lowered to separate the sheet S from the feed roller  31 . Thus, even if the cassette  30  is pulled out when an uppermost sheet S 1  is held by the feed roller  31 , the lifter plate  37 , and the separation roller  32  during the feed standby time illustrated in  FIGS. 6A and 7A , the sheet can be avoided from being bent, broken, or remaining inside of the apparatus. 
     The above description of the system, in which the feed roller is applying pressure downward, has been given. However, the present invention may be applied to a system in which a feed roller is supported by a turnable arm that is applied a force in a turning direction. Alternatively, although the roller provided with the torque limiter has served as the separation member, a friction pad including a friction member may be used. In addition, although the separation member is separated by the turn system, the same effect can be produced by a vertical slide system. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2011-140345, filed Jun. 24, 2011, which is hereby incorporated by reference herein in its entirety.