Abstract:
A sheet feeding device includes a loading portion configured to be loaded with a sheet, a lifting mechanism that lifts the loading portion in an operating state and stops the loading portion in an stopped state, a cam configured to be rotated alternately to a first rotational position for setting the lifting mechanism to the operating state and to a second rotational position for setting the lifting mechanism to the stopped state, a detecting unit having a contact portion movable up and down while contacting the sheet on the loading portion, which detects whether the sheet reaches a first height based upon the movement of the contact portion thereof, and a control unit configured to rotate the cam before the detecting unit detects that the sheet reaches the first height and to stop the cam in the second rotational position in response to the detection by the detecting unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. §119 from Japanese Patent Application No. 2007-176317 filed on Jul. 4, 2007. The entire subject matter of the application is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The following description relates to one or more sheet feeding techniques for an image forming apparatus. 
         [0004]    2. Related Art 
         [0005]    Among image forming apparatuses, for example, there is an apparatus provided with a loading portion configured to be loaded with a sheet, a lifting mechanism configured to lift the loading portion, and a detecting unit configured to detect whether the loading portion reaches a predetermined height. It is noted that the predetermined height represents such a height that a sheet on the loading portion can be fed to a carrying route inside a main body of the apparatus. 
         [0006]    In the meantime, a configuration has been known, in which ON/OFF control of a lifting operation by the lifting mechanism is mechanically performed with, for example, a cam (see Japanese Patent Provisional Publication No. 2006-151655). Specifically, the aforementioned configuration includes a cam and a control unit configured to control a rotational position of the cam. The control unit holds the cam in a first rotational position to set the lifting operation ON until the loading portion reaches the predetermined height, and rotates the cam to a second rotational position to set the lifting operation OFF when the loading portion reaches the predetermined height. 
       SUMMARY 
       [0007]    In the aforementioned configuration, the lifting operation is maintained until the cam is rotated to the second rotational position from the first rotational position even after the loading portion reaches the predetermined height. A time-lag until the cam is rotated to the second rotational position from the first rotational position (hereinafter referred to as a first time-lag) cannot be avoided as far as the aforementioned configuration is applied, in which the ON/OFF control of the lifting operation by the lifting mechanism is mechanically performed with a cam mechanism. 
         [0008]    However, in the aforementioned configuration, in addition to the first time-lag mechanically caused, there is caused a time-lag until the rotation of the cam to the second rotational position is actually started after it is detected that the loading portion reaches the predetermined height (hereinafter referred to as a second time-lag). Therefore, it is unfortunate that the lifting operation might be further continued even after the loading portion reaches the predetermined height and a sheet feeding performance might be worsened. 
         [0009]    Aspects of the present invention is advantageous in that there are provided one or more improved sheet feeding devices and image forming apparatuses that make it possible to restrain worsening of sheet feeding performance even though a mechanical configuration is applied so as to control a lifting operation for a loading portion loaded with a sheet. 
         [0010]    According to aspects of the present invention, there is provided a sheet feeding device, which includes a loading portion configured to be loaded with a stack of sheets and to be movable up and down, a lifting mechanism configured to lift the loading portion in an operating state and to stop the lifting of the loading portion in an stopped state, a cam configured to be rotated alternately to a first rotational position for setting the lifting mechanism to the operating state and to a second rotational position for setting the lifting mechanism to the stopped state, a detecting unit having a contact portion configured to be movable up and down while contacting a top sheet of the stack on the loading portion, the detecting unit being configured to detect whether the top sheet reaches a first height based upon the movement of the contact portion thereof, and a control unit configured to rotate the cam before the detecting unit detects that the top sheet reaches the first height and to stop the cam in the second rotational position in response to the detecting unit detecting that the top sheet reaches the first height. 
         [0011]    In some aspects of the present invention, the cam is driven and rotated before the top sheet on the loading portion reaches the first height. Thereafter, until the top sheet reaches the first height, the lifting mechanism is repeatedly set alternately to the operating state and the stopped state, as the cam is rotated alternately to the first rotational position and the second rotational position. Thereby, the loading portion is intermittently lifted. After that, in response to the top sheet reaching the first height, an operation of stopping the cam being rotated is started, and the cam is then stopped. It is noted that, even in some aspects of the present invention, the first time-lag has to be caused that is taken for the cam to be rotated from the first rotational position to the second rotational position. However, in some aspects of the present invention, the cam has already been rotating before the sheet on the loading portion reaches the first height. Therefore, the lifting mechanism is stopped the first time-lag after the sheet reaches the first height. Thus, the second time-lag is not caused that is a time period until a start time to cause the cam rotate after it is detected that the sheet on the loading portion reaches the first height. Thereby, it is possible to prevent the worsening of sheet feeding performance better than the aforementioned conventional configuration. 
         [0012]    According to another aspect of the present invention, there is provided an image forming apparatus, which a sheet feeding unit configured to feed a sheet, and an image forming unit configured to form an image on the sheet fed by the sheet feeding unit. The sheet feeding unit includes a loading portion configured to be loaded with a stack of sheets and to be movable up and down, a lifting mechanism configured to lift the loading portion in an operating state and to stop the lifting of the loading portion in an stopped state, a cam configured to be rotated alternately to a first rotational position for setting the lifting mechanism to the operating state and to a second rotational position for setting the lifting mechanism to the stopped state, a detecting unit having a contact portion configured to be movable up and down while contacting a top sheet of the stack on the loading portion, the detecting unit being configured to detect whether the top sheet reaches a first height based upon the movement of the contact portion thereof, and a control unit configured to rotate the cam before the detecting unit detects that the top sheet reaches the first height and to stop the cam in the second rotational position in response to the detecting unit detecting that the top sheet reaches the first height. 
         [0013]    According to the image forming apparatus configured as above, the same effect as the aforementioned sheet feeding unit can be provided. 
     
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         [0014]    Illustrative aspects of the invention will be described in detail with reference to the following figures in which like elements are labeled with like numbers and in which: 
           [0015]      FIG. 1  is a cross-sectional side view showing a configuration of a laser printer in an embodiment according to one or more aspects of the present invention. 
           [0016]      FIG. 2  is a perspective view of a storage cassette of the laser printer in the embodiment according to one or more aspects of the present invention. 
           [0017]      FIG. 3  is a perspective view showing a mechanism for lifting a pressing plate in a case where the pressing plate is in a sheet loading state in the embodiment according to one or more aspects of the present invention. 
           [0018]      FIG. 4  is a perspective view showing the mechanism for lifting the pressing plate in a case where the pressing plate is between the sheet loading state and a sheet feeding state in the embodiment according to one or more aspects of the present invention. 
           [0019]      FIG. 5  is a perspective view showing the mechanism for lifting the pressing plate in a case where the pressing plate is in the sheet feeding state in the embodiment according to one or more aspects of the present invention. 
           [0020]      FIG. 6  is a perspective view of a gear mechanism viewed from a left side of the laser printer in the embodiment according to one or more aspects of the present invention. 
           [0021]      FIG. 7  is a perspective view of the gear mechanism viewed from a right side of the laser printer in the embodiment according to one or more aspects of the present invention. 
           [0022]      FIG. 8A  is a right side view of the gear mechanism in the case where a sheet feeding roller is in the lowest position in the embodiment according to one or more aspects of the present invention. 
           [0023]      FIG. 8B  is a rear side view of the gear mechanism in the case where the sheet feeding roller is in the lowest position in the embodiment according to one or more aspects of the present invention. 
           [0024]      FIG. 8C  is a left side view of the gear mechanism in the case where the sheet feeding roller is in the lowest position in the embodiment according to one or more aspects of the present invention. 
           [0025]      FIG. 9A  is a right side view of the gear mechanism in a case where the sheet feeding roller is in a second release position in the embodiment according to one or more aspects of the present invention. 
           [0026]      FIG. 9B  is a rear side view of the gear mechanism in the case where the sheet feeding roller is in the second release position in the embodiment according to one or more aspects of the present invention. 
           [0027]      FIG. 9C  is a left side view of the gear mechanism in the case where the sheet feeding roller is in the second release position in the embodiment according to one or more aspects of the present invention. 
           [0028]      FIG. 10A  is a right side view of the gear mechanism in a case where a cam gear is engaged with a differential gear in the embodiment according to one or more aspects of the present invention. 
           [0029]      FIG. 10B  is a rear side view of the gear mechanism in the case where the cam gear is engaged with the differential gear in the embodiment according to one or more aspects of the present invention. 
           [0030]      FIG. 10C  is a left side view of the gear mechanism in the case where the cam gear is engaged with the differential gear in the embodiment according to one or more aspects of the present invention. 
           [0031]      FIG. 11A  is a right side view of the gear mechanism in a case where the sheet feeding roller is in a first release position in the embodiment according to one or more aspects of the present invention. 
           [0032]      FIG. 11B  is a rear side view of the gear mechanism in the case where the sheet feeding roller is in the first release position in the embodiment according to one or more aspects of the present invention. 
           [0033]      FIG. 11C  is a left side view of the gear mechanism in the case where the sheet feeding roller is in the first release position in the embodiment according to one or more aspects of the present invention. 
           [0034]      FIG. 12A  is a right side view of the gear mechanism in a case where the sheet feeding roller is between the first release position and the second release position in the embodiment according to one or more aspects of the present invention. 
           [0035]      FIG. 12B  is a rear side view of the gear mechanism in the case where the sheet feeding roller is between the first release position and the second release position in the embodiment according to one or more aspects of the present invention. 
           [0036]      FIG. 12C  is a left side view of the gear mechanism in the case where the sheet feeding roller is between the first release position and the second release position in the embodiment according to one or more aspects of the present invention. 
           [0037]      FIG. 13  schematically shows side views of the gear mechanism in a case where a storage cassette is inserted in the embodiment according to one or more aspects of the present invention and a comparative example. 
           [0038]      FIG. 14  schematically shows side views of the gear mechanism in a case where the number of sheets is reduced through a sheet feeding operation in the embodiment according to one or more aspects of the present invention and the comparative example. 
           [0039]      FIG. 15  shows time dependency of height of the sheet feeding roller in each the embodiment according to one or more aspects of the present invention and the comparative example. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    It is noted that various connections are set forth between elements in the following description. It is noted that these connections in general and, unless specified otherwise, may be direct or indirect and that this specification is not intended to be limiting in this respect. 
         [0041]    Hereinafter, an embodiment according to aspects of the present invention will be described with reference to the accompany drawings. It is noted that, in each drawing, an outline arrow denotes a direction in which a sheet feeding roller  9  or each gear is moved or rotated. 
       Overall Configuration in Embodiment 
       [0042]      FIG. 1  is a cross-sectional side view showing a configuration of a laser printer  1  (hereinafter, simply referred to as a printer  1 ).  FIG. 2  is a perspective view of a storage cassette  6 .  FIGS. 3 to 5  are perspective views showing respective states in process of lifting a pressing plate  11 . 
         [0043]    The printer  1  is provided with a main body casing  2 , a feeder unit  4  configured to feed a sheet  3 , and an image forming unit  5  configured to form an image on the sheet  3 . 
         [0044]    1. Feeder Unit 
         [0045]    The feeder unit  4  is provided at a bottom portion in the main body casing  2 , and provided with a storage cassette  6  set therein to be drawn. In a following description, a side to which the storage cassette  6  is drawn is defined as a front side of the printer  1 . The feeder unit  4  further includes a separation roller  7  and a separation pad  8  provided at an upper front portion of the storage cassette  6 , and a sheet feeding roller  9  provided at a rear side of the separation roller  7  (at an upstream side of the separation pad  8  in a carrying direction in which the sheet  3  is conveyed). 
         [0046]    The carrying route is fold back in a U-shape at a downstream side of the separation roller  7  and separation pad  8  in the carrying direction. At a further downstream side in the carrying direction, a pair of registration rollers  10  is provided. 
         [0047]    As shown in  FIGS. 1 and 2 , a pressing plate  11  on which the sheet  3  is placed in an accumulated manner is provided inside the storage cassette  6 . The pressing plate  11  is supported at a supporting portion  11 A at a rear end thereof, swingably between a sheet loading state where a front end portion  11 B of the pressing plate  11  is disposed at a lower side and the pressing plate  11  is along a bottom plate  12  of the storage cassette  6  (as shown in  FIGS. 1 to 3 ) and a sheet feeding state where the front end portion  11 B of the pressing plate  11  is disposed at an upper side and the pressing plate  11  is tilted up from the bottom plate  12  of the storage cassette  6  (namely, a state where the sheet feeding roller  9  contacts the uppermost sheet  3  placed on the pressing plate  11  as shown in  FIG. 5 , and the sheet  3  is fed by the sheet feeding roller  9  being rotated). 
         [0048]    In addition, at a front end portion of the storage cassette  6 , a lever  13  is provided to lift up the front end portion  11 B of the pressing plate  11 . The lever  13  is disposed under the front end portion  11 B of the pressing plate  11  and supported such that a rear end portion  13 A of the lever  13  is swingable between a lying down state where a front end portion  13 B of the lever  13  is lying down on the bottom plate  12  of the storage cassette  6  (as shown in  FIGS. 1 to 3 ) and a tilted state where the front end portion  13 B is lifting the pressing plate  11  (as shown in  FIG. 5 ). When a rotational driving force is applied to the rear end portion  13 A of the lever  13  in a clockwise direction of the figure, the lever  13  is rotated around the rear end portion  13 A, and the front end portion  13 B of the lever  13  lifts the front end portion  11 B of the pressing plate  11  to set the pressing plate  11  in the sheet feeding state. At a side end of the lever  13 , a fan-shaped gear  43  is fixed as shown in  FIG. 2 . The gear  43  is linked, via a gear  44 , with a linking gear  45  to which the driving force is transmitted from a driving source (not shown). 
         [0049]    When the pressing plate  11  is set in the sheet feeding state, the uppermost sheet  3  placed on the pressing plate  11  is pressed by the sheet feeding roller  9 , and sheet feeding is caused by rotation of the sheet feeding roller  9  to feed the sheet  3  toward a separation position between the separation roller  7  and separation pad  8 . 
         [0050]    Meanwhile, when the storage cassette  6  is drawn from the feeder unit  4 , the linkage of the driving source (not shown) with the linking gear  45  is released. Then, the pressing plate  11  is set into the sheet loading state where a user can place the sheet  3  on the pressing plate  11  in an accumulated manner, with the front end portion  11 B thereof moving down due to its own weight. It is noted that the separation pad  8 , pressing plate  11 , and lever  13  are disposed in the storage cassette  6 , and that the sheet feeding roller  9 , separation roller  7 , registration rollers  10  are disposed in the main body casing  2 . 
         [0051]    When the sheet  3  is fed by the sheet feeding roller  9  toward the separation position and pinched in the separation position between separation roller  7  and separation pad  8 , the sheet  3  is separated and fed on a sheet-by-sheet basis. The sheet  3  as fed is turned down along the U-shaped carrying route and conveyed to the registration rollers  10 . 
         [0052]    The registration rollers  10  are configured to adjust skewing of the sheet  3  and then convey the sheet  3  into a transfer position between a below-mentioned photoconductive drum  29  and transfer roller  32 , where a toner image on the photoconductive drum  29  is transferred onto the sheet  3 . 
         [0053]    2. Image Forming Unit 
         [0054]    The image forming unit  5  is provided with a scanner unit  19 , a process cartridge  20 , and a fixing unit  21 . The scanner unit  19  is provided at an upper portion inside the main body casing  2 . A laser light source (not shown) emits a laser beam based upon image data. As indicated by a chain line in  FIG. 1 , the laser beam is deflected by a polygon mirror  22 , transmitted through an fD lens  23 , bent by a reflective mirror  24 , transmitted through a lens  25 , and bent downward by a reflective mirror  26 . Thereby, the laser beam is incident onto a surface of the below-mentioned photoconductive drum  29  of the process cartridge  20 . 
         [0055]    The process cartridge  20  is provided under the scanner unit  19 , and includes, in a housing thereof, the photoconductive drum  29 , a scorotron-type charger  30 , a development cartridge  31 , and the transfer roller  32 . In addition, the development cartridge  31  includes a toner container  39 , a toner supply roller  40 , and a development roller  41 . 
         [0056]    Toner provided from the toner container  39  is supplied to the development roller  41  through rotation of the toner supply roller  40 . At this time, the toner is positively friction-charged between the toner supply roller  40  and the development roller  41 . The toner supplied onto the development roller  41  is held on the development roller  41  as a thin layer with a constant thickness. 
         [0057]    First, the surface of the photoconductive drum  29  is evenly and positively charged by the charger  30  through rotation of the photoconductive drum  29 . Thereafter, the surface of the photoconductive drum  29  is exposed by fast scanning of the laser beam emitted from the scanner unit  19 , and an electrostatic latent image corresponding to an image to be formed on the sheet  3  is formed thereon. 
         [0058]    Subsequently, the toner held on the development roller  41  is supplied to the electrostatic latent image formed on the surface of the photoconductive drum  29 , namely, exposed portions on the photoconductive drum  29  as evenly and positively charged that have been exposed by the laser beam to have a lowered electric potential, when facing and contacting the photoconductive drum  29  through rotation of the development roller  41 . Thereby, the electrostatic latent image formed on the photoconductive drum  29  is visualized with the toner image due to inversion development being held on the photoconductive drum  29 . 
         [0059]    After that, as shown in  FIG. 1 , the toner image held on the surface of the photoconductive drum  29  is transferred onto the sheet  3  by a transfer bias applied to the transfer roller  32  while the sheet  3  conveyed by the registration rollers  10  is passing through the transfer position between the photoconductive drum  29  and the transfer roller  32 . The sheet  3  with the toner image transferred thereon is carried to the fixing unit  21 . The fixing unit  21  thermally fixes the toner transferred onto the sheet  3  in the transfer position while the sheet  3  is passing through between a heating roller  33  and pressing roller  34 . The sheet  3  with the toner fixed thereon is conveyed to a sheet discharge path  35  extending toward the upper face of the main body casing  2  along a vertical direction. The sheet  3  carried to the sheet discharge path  35  is discharged, by a sheet discharge roller  36  provided above the sheet discharge path, onto a catch tray  37  formed on the upper face of the main body casing  2 . 
         [0060]    (Configurations of Sheet Feeding Roller and Separation Roller) 
         [0061]      FIG. 6  is a perspective view of a gear mechanism  60  viewed from a front side of the printer. 
         [0062]      FIG. 7  is a perspective view of the gear mechanism  60  viewed from a rear side of the printer. 
         [0063]    As shown in  FIG. 6 , the sheet feeding roller  9  and the separation roller  7  are provided to a bearing member  50  so as to be aligned in a front-to-rear direction of the printer  1 . Specifically, the sheet feeding roller  9  and the separation roller  7  are borne by the bearing member  50  via rotational shaft bodies  51  and  52  along a right-to-left direction, respectively. One end of the rotational shaft body  52  of the separation roller  7  penetrates a side wall portion  50 A provided at a left side of the bearing member  50 . Further, a separation roller gear  53  is provided at a distal end portion of the rotational shaft body  52  so as to be rotated integrally with the rotational shaft body  52 . When the separation roller gear  53  receives a driving force from a below-mentioned gear mechanism  60 , the rotational shaft body  52  is rotated, and thereby the separation roller  7  is rotated integrally with the rotational shaft body  52 . 
         [0064]    Additionally, the bearing member  50  is configured such that a portion thereof at a side of the sheet feeding roller  9  can be swung around the rotational shaft body  52  of the separation roller  7  (along a direction indicated by an outline double-sided arrow in  FIG. 1 ). While the pressing plate  11  is lifted with the lever  13  being turned, the sheet feeding roller  9  contacts a surface of the uppermost sheet  3  of sheets placed on the pressing plate  11  from a lower side of the sheet feeding roller  9  and is swung up. 
         [0065]    Additionally, gears  54  and  55 , respectively rotating integrally with the rotational shaft bodies  51  and  52 , are provided concentrically with the sheet feeding roller  9  and the separation roller  7 , respectively. The gears  54  and  55  are linked via a linking gear  56 . Thereby, the sheet feeding roller  9  is driven by the rotation of the separation roller  7 . 
         [0066]    As shown in  FIG. 6 , an arm member  57  with a center portion  57 A thereof rotatably supported is provided in parallel with the rotational shaft body  52  at a rear side of the rotational shaft body  52 . A hole provided at one end portion  57 B of the arm member  57  is engaged with a protrusion  50 B provided at a swingable end side of the bearing member  50  at which the sheet feeding roller  9  is provided. Further, the other end portion  57 C of the arm member  57  is engaged with the gear mechanism  60 . 
         [0067]    (Configuration of Gear Mechanism) 
         [0068]    The gear mechanism  60  is provided with a plurality of gears configured to be rotated by a driving force transmitted by a driving motor (not shown) provided at a main body casing  2  side. The gear mechanism  60  is configured to mainly control the lifting operation of the pressing plate  11  and the rotating operation of the separation roller  7 . 
         [0069]    Specifically, the gear mechanism  60  includes the aforementioned separation roller gear  53 , a first engaging lever  61 , a second engaging lever  62 , a cam gear  63 , and differential gears  64  and  65 . 
         [0070]    The first engaging lever  61  and second engaging lever  62  are arranged side by side in the right-to-left direction, and provided swingably around the same rotational shaft  66  extending along the right-to-left direction. The first engaging lever  61  and the second engaging lever  62  are formed with locking claws  61 A and  62 A at rear sides thereof and arm portions  61 B and  62 B at front sides thereof, respectively. The arm portions  61 B and  62 B are provided so as to pinch the end portion  57 C of the arm member  57  therebetween. Thereby, when the arm portions  61 B and  62 B are moved in response to a swing motion of the end portion  57 C of the arm member  57 , respective sides of the locking claws  61 A and  62 A of the first engaging lever  61  and second engaging lever  62  are swung. 
         [0071]    The cam gear  63  is formed with a cam  67  and a tooth-lacking gear  68  being concentrically integrated. The cam  67  is formed such that a portion thereof corresponding to about one forth as long as an entire circumference of the cam  67  has a smaller radius than that of the other portion of the cam  67  (for example, see  FIG. 8A ). In addition, on an outer circumferential surface of the cam  67 , there are formed a first engaged portion  71  configured to be engaged with the locking claw  61  A of the first engaging lever  61  and a second engaged portion  72  configured to be engaged with the locking claw  62 A of the second engaging lever  62 . It is noted that the first engaged portion  71  is provided in a position slightly shifted from the second engaging lever  62  in the right-to-left direction so as to comply with the positional relationship between the first engaging lever  61  and second engaging lever  62 . 
         [0072]    As illustrated in  FIG. 8C , the tooth-lacking gear  68  is provided with a tooth-lacking portion  68 A with no tooth that corresponds to about one forth as long as an entire circumference of the tooth-lacking gear  68 . Additionally, a lower end  69 A of a spring  69  is fixed at a side of the main body casing  2 , and a free end  69 B of the spring  69  is hooked with a protrusion  68 B protruded at a point off a rotational center of the tooth-lacking gear  68  in an axial direction of the tooth-lacking gear  68 . The differential gear  64 , which is configured to be rotated due to a driving force transmitted from a main motor (not shown), is provided in the vicinity of the tooth-lacking gear  68 . The tooth-lacking gear  68  is rotated while teeth thereof are being engaged with the differential gear  64 . Meanwhile, the tooth-lacking gear  68  is not driven and stopped while the tooth-lacking portion  68 A thereof is facing the differential gear  64 . 
         [0073]    As shown in  FIG. 8A , a switching tilting member  75  for taking ON/OFF control of the lifting operation for the pressing plate  11  is disposed behind the cam gear  63 . The switching tilting member  75  is tiltably supported at a center portion thereof by a rotational shaft  75 C extending along the right-to-left direction. The switching tilting member  75  includes a front end portion  75 A that is configured to contact the outer circumferential surface of the cam  67 , and a locking claw  75 B that is integrally provided at a distal end of a rear end portion of the switching tilting member  75 . Additionally, a spring  76  is formed with a front end  76 A thereof being fixed at a side of the main body casing  2  and a free end  76 B thereof being linked with a lower end of the switching tilting member  75 . Thereby, the switching tilting member  75  is biased by the spring  76  in a clockwise direction of  FIG. 8A . 
         [0074]    The differential gear  65  is provided with a locked gear  80 , an input gear  81 , and an output gear  82 . The locked gear  80  is disposed to be latched with the locking claw  75 B of the switching tilting member  75 . The input gear  81  is always driven and rotated by the driving force received from the main motor. The output gear  82  is in an idling state until the locked gear  80  is latched with the locking claw  75 B of the switching tilting member  75 . Meanwhile, in response to the locked gear  80  being latched with the locking claw  75 B of the switching tilting member  75 , the output gear  82  is driven and rotated in conjunction with the input gear  81 . The output gear  82  is linked to a linking gear  86  via gears  83  to  85 . The linking gear  86  is provided in such a position as to be linked with the linking gear  45  at a side of the storage cassette  6  in a state where the storage cassette  6  is inserted and attached into the feeder unit  4  (in a state shown in  FIG. 1 ). On the other hand, when the storage cassette  6  is drawn out from the feeder unit  4 , the linkage between the linking gears  45  and  86  is released, and thus, as described above, the pressing plate  11  is set into the sheet loading state, with the front end portion  11 B thereof moving down due to its own weight. 
         [0075]    (Operation of Gear Mechanism) 
         [0076]    Next, operation of the gear mechanism  60  will be described with reference to  FIGS. 8A to 12C . Each figure indicated by a number with a suffix “A” is a right side view of a part of the gear mechanism  60 . Each figure indicated by a number with a suffix “B” is a rear side view of a part of the gear mechanism  60 . Each figure indicated by a number with a suffix “C” is a left side view of a part of the gear mechanism  60 . 
         [0077]    1. Continuous Lifting Mode 
         [0078]    The gear mechanism  60  continuously lifts the pressing plate  11  until the sheet feeding roller  9  reaches a second release position X 2  from the lowest position O. 
         [0079]    As described above, for example, when the storage cassette  6  is drawn out from the feeder unit  4 , the pressing plate  11  is shifted in the sheet loading state due to its own weight. At this time, the sheet feeding roller  9  (the end portion  57 B of the arm member  57 , or strictly, a bottom face of the sheet feeding roller  9 ) is moved to the lowest position O as illustrated in  FIG. 8B . Thereby, the arm member  57  is tilted like a seesaw, and on the contrary, the end portion  57 C thereof is moved to the highest position. Thereafter, for example, when the storage cassette  6  is inserted and re-attached into the feeder unit  4  after the sheet  3  is re-supplied (see  FIG. 3 ), the gear mechanism  60  is set into a state shown in  FIGS. 8A and 8C . When the end portion  57 C is moved up, the locking claw  61 A of the first engaging lever  61  is spaced from the first engaged portion  71 . Further, the locking claw  62 A of the second engaging lever  62  is engaged with the second engaged portion  72  of the cam gear  63 . Thereby, the rotation of the cam gear  63  is stopped against a biasing force of the spring  69 . At this time, the tooth-lacking gear  68  is in a state where the tooth-lacking portion  68 A faces the differential gear  64  and thus the driving force from the differential gear  64  is not transmitted to the tooth-lacking gear  68 . 
         [0080]    In addition, since the front end portion  75 A of the switching tilting member  75  is within the smaller-radius portion of the cam  67 , the locking claw  75 B is latched with the locked gear  80  owing to a biasing force of the spring  76 . Thereby, in accordance with a mechanical property of the differential gear  65 , the output gear  82  is rotated and set into an operating state for lifting the pressing plate  11 , while the locked gear  80  is stopped (a rotational position of the cam  67  shown in  FIGS. 8A to 8C  and  9 A to  9 C is defined as a first rotational position in the present embodiment). As illustrated in  FIGS. 9A to 9C , the gear mechanism  60  maintains the operating state until the sheet feeding roller  9  reaches the second release position X 2  from the lowest position O. Namely, during the operating state, the output gear  82  of the differential gear  65  is always rotated, and the pressing plate  11  is continuously lifted. 
         [0081]    Specifically, when the pressing plate  11  is lifted to a certain height, the uppermost sheet  3  of sheets stacked on the pressing plate  11  contacts the sheet feeding roller  9 . Thereby, the sheet feeding roller  9  is lifted up from the lowest position O along with the rising of the pressing plate  11  (see  FIG. 5 ). When the sheet feeding roller  9  is lifted up, the end portion  57 C of the arm member  57  is reversely moved down so as to move the first engaging lever  61  and the second engaging lever  62 , which are disposed to pinch the end portion  57 C therebetween. Then, when the sheet feeding roller  9  reaches the second release position X 2  (see  FIG. 9B ), the second engaging lever  62  is disengaged from the second engaged portion  72  as shown in  FIG. 9A . 
         [0082]    2. Intermittent Lifting Mode 
         [0083]    Subsequently, when the second engaging lever  62  is disengaged from the second engaged portion  72 , the operation of the gear mechanism  60  shifts to an intermittent lifting mode from the continuous lifting mode. Specifically, the cam gear  63  is enforcedly rotated by the biasing force of the spring  69  in a counterclockwise of  FIG. 9C . Then, as shown in  FIG. 10C , the teeth of the tooth-lacking gear  68  of the cam gear  63  are engaged with the differential gear  64 . Thereby, the cam gear  63  is driven by a driving force from the differential gear  64  in a counterclockwise direction of  FIG. 10C . It is noted that, at this time, since the front end portion  75 A of the switching tilting member  75  is still within the smaller-radius portion of the cam  67 , the aforementioned operating state is maintained (see  FIG. 10A ), and the sheet feeding roller  9  is further lifted up (see  FIG. 10B ). 
         [0084]    Thereafter, when the cam gear  63  is further rotated, the front end portion  75 A of the switching tilting member  75  runs upon the larger-radius portion of the cam  67 . Therefore, the switching tilting member  75  is rotated against the biasing force of the spring  76 , in the counterclockwise of  FIG. 10A , and the locking claw  75 B is disengaged from the locked gear  80 . Thereby, in accordance with a mechanical property of the differential gear  65 , power transmission to the output gear  82  is stopped even while the locked gear  80  keeps its rotation. Thus, the gear mechanism  60  is set into a stopped state for stopping the rising of the pressing plate  11  from the aforementioned operating state. 
         [0085]    The second engaging lever  62  is in a position away from the cam  67  and never engaged with the second engaged portion  72  whenever the sheet feeding roller  9  is positioned higher than the second release position X 2 . Accordingly, when the sheet feeding roller  9  is positioned higher than the second release position X 2 , the cam gear  63  alternately receives the driving force from the differential gear  64  and the biasing force of the spring  69  and keeps the rotation thereof as far as the first engaging lever  61  is not engaged with the first engaged portion  71 . Thus, since the gear mechanism  60  alternately repeats the operating state and the stopped state, the pressing plate  11  is intermittently lifted up. 
         [0086]    As illustrated in  FIG. 11B , when the sheet feeding roller  9  reaches the first release position X 1 , the first engaging lever  61  contacts the cam  67  and waits for the first engaged portion  71  to come. Therefore, as shown in  FIG. 11A , the first engaging lever  61  is engaged with the first engaged portion  71 , and the cam gear  63  is stopped against the biasing force of the spring  69  (a rotational position of the cam  67  shown in  FIGS. 11A to 11C  is defined as a second rotational position in the present embodiment). At this time, since the front end portion  75 A of the switching tilting member  75  runs upon the larger-radius portion of the cam  67 , the pressing plate  11  is not lifted. In addition, as illustrated in  FIG. 11C , the tooth-lacking gear  68 , of which the tooth-lacking portion  68 A is facing the differential gear  64 , does not receive the driving force from the differential gear  64 . It is noted that there is caused even in the present embodiment, a first time-lag until the cam  67  takes the second rotational position after the sheet feeding roller  9  reaches the first release position X 1 . 
         [0087]    When the gear mechanism  60  is set into a state shown in  FIG. 11B , the pressing plate  11  is set into the sheet feeding state, in which a sheet feeding operation is permitted. Then, based upon a sheet feeding command issued by a CPU (not shown), the sheet feeding roller  9  is rotated, the sheet  3  is fed to the image forming unit  5 , and a printing operation is performed. After that, the sheet feeding operation is repeated, and when the number of sheets placed on the pressing plate  11  is reduced, for example, by about 10 sheets, the sheet feeding roller  9  is moved down to a position between the first release position X 1  and the second release position X 2  as shown in  FIG. 12B . Then, as illustrated in  FIG. 12A , the locking claw  61 A of the first engaging lever  61  and the locking claw  62 A of the second engaging lever  62  are slightly rotated in a clockwise direction of the figure, and the first engaging lever  61  is disengaged from the first engaged portion  71 . It is noted that, at this time, the second engaging lever  62  is still in the position away from the cam  67  and is not engaged with the second engaged portion  72 . Accordingly, the gear mechanism  60  again begins the operation of alternately repeating the operating state and stopped state in the intermittent lifting mode (in the state shown in  FIGS. 10A to 10C ). Thereafter, the gear mechanism  60  repeatedly performs operations of shifting to the state shown in  FIGS. 12A to 12C  from the state shown in  FIGS. 10A to 10C  (in the intermittent lifting mode). Further, it is noted that when the storage cassette  6  is drawn out from the feeder unit  4  and then re-attached thereinto, the gear mechanism  60  is set into the state shown in  FIG. 9  and the continuous lifting mode is restarted. 
       SUMMARY OF OPERATIONS 
       [0088]    Positions of the sheet feeding roller  9  depending on time will be described with reference to  FIGS. 13 to 15 .  FIGS. 13 and 14  are schematic side views of the gear mechanism  60  for explaining operations of the cam gear  63 , the first engaging lever  61 , and the second engaging lever  62  when the storage cassette  6  is inserted and when the number of the sheets  3  on the pressing plate  11  is reduced through the sheet feeding operation, respectively. In  FIGS. 13 and 14 , operations in the present embodiment are shown on left sides of the figures, while operations in a comparative example are shown on right sides of the figures.  FIG. 15  shows time dependency of height of the sheet feeding roller  9 , where a solid line denotes the present embodiment, and a chain line denotes the comparative example. 
         [0089]    1. Regarding when the Storage Cassette is Inserted 
       (1) Operations in Embodiment 
       [0090]    In the present embodiment, as shown in  FIG. 13 , when the storage cassette  6  is inserted, in the same manner as the comparative example, the second engaging lever  62  is engaged with the second engaged portion  72  of the cam gear  63  (see “pressing plate is being lifted up” of the present embodiment in  FIG. 13 ). Thereby, the gear mechanism  60  is set in the operating state for lifting the pressing plate  11  as shown in  FIG. 8A . 
         [0091]    The sheet feeding roller  9  is in the lowest position O ( FIG. 8B ) at a time t 0  ( FIG. 15 ), and therefrom shifted up to the second release position X 2  ( FIG. 9B ) at a time t 1 , where the engagement of the second engaging lever  62  with the second engaged portion  72  is released, and the cam gear  63  is rotated in a clockwise direction of  FIG. 13  (see “sheet feeding roller reaches second release position X2” of the present embodiment in  FIG. 13 ). At this time, the first engaging lever  61  does not contact the cam gear  63 , yet gradually approaches the cam gear  63 . Then, when the sheet feeding roller  9  reaches the second release position X 1  ( FIG. 11B ) (at a time t 2  in  FIG. 15  at when an instruction for stopping lifting the pressing plate  11  is instructed), the first engaging lever  61  contacts the cam gear  63  (see “sheet feeding roller reaches first release position X1” of the present embodiment in  FIG. 13 ). 
         [0092]    After that, when the cam gear  63  takes the second rotational position, as shown in  FIG. 13 , the first engaging lever  61  is engaged with the first engaged portion  71  (see “pressing plate is stopped” of the present embodiment in  FIG. 13 ). At this time, as illustrated in  FIG. 15 , the sheet feeding roller  9  is lifted up to a height X 3  and stopped at a time t 3 . Here, a time period between t 2  and t 3  taken for the sheet feeding roller  9  to be lifted from the first release position X 1  to the height X 3  corresponds to the first time-lag in the present embodiment. 
       (2) Operations in Comparative Example 
       [0093]    Unlike the present embodiment, in the comparative example, at the time (t 1  in  FIG. 15 ) when the sheet feeding roller  9  reaches the second release position X 2 , the engagement of the second engaging lever  62  with the second engaged portion  72  is not released (see “sheet feeding roller reaches second release position X2” of the comparative example in  FIG. 13 ). Namely, the cam gear  63  is maintained in the first rotational position. Thereafter, when the sheet feeding roller  9  is further lifted up to the first release position X 1  (at t 2  in  FIG. 15 ), as shown in “sheet feeding roller reaches first release position X1” of the comparative example in  FIG. 13 , firstly the first engaging lever  61  contacts the cam gear  63 , and then the engagement of the second engaging lever  62  with the second engaged portion  72  is released around a time t 2 ′. Thereby, finally the cam gear  63  is rotated in the clockwise direction of  FIG. 13  toward the second rotational position. A time period between t 2 ′ and t 3  corresponds to the second time-lag. 
         [0094]    Thus, in the comparative example, when the sheet feeding roller  9  reaches the first release position X 1 , the first engaging lever  61  contacts the cam gear  63 , and thereafter the engagement of the second engaging lever  62  with the second engaged portion  72  is released. This is because in the comparative example, unlike the present embodiment, when the sheet feeding roller  9  reaches the first release position X 1 , the cam gear  63  is still latched in the first rotational position by the second engaging lever  62 . Therefore, in order to stop the rising of the pressing plate  11 , firstly the engagement of the second engaging lever  62  with the second engaged portion  72  has to be released. Here, supposing that the comparative example is configured such that the engagement of the second engaging lever  62  with the second engaged portion  72  is released before the first engaging lever  61  contacts the cam gear  63 , the first engaged portion  71  of the cam gear  63  may pass without being engaged with the first engaging lever  61  as still spaced from the cam gear  63 . Thus, an undesired situation may be caused that it is delayed to stop the cam gear  63  in the second rotational position. To avoid such a situation, in the comparative example, when the sheet feeding roller  9  reaches the first release position X 1 , firstly the first engaging lever  61  is brought into contact with the cam gear  63 , and then the engagement of the second engaging lever  62  with the second engaged portion  72  is released. The second time-lag is set in view of dimensional tolerances of the cam gear  63 , the first engaging lever  61 , and the second engaging lever  62 . Then the first time-lag is caused after the second time-lag has elapsed. Consequently, the sheet feeding roller  9  is lifted up to a height X 4  at a time t 4  as shown in  FIG. 15 , and the pressing plate  11  is shifted up to a higher position than that in the present embodiment. 
         [0095]    Hereinabove, in the comparative example, the second time-lag is caused as well as the first time-lag until the rising of the pressing plate  11  is actually stopped after the sheet feeding roller  9  reaches the first release position X 1 . On the contrary, in the present embodiment, before the sheet feeding roller  9  reaches the first release position X 1 , the engagement of the second engaging lever  62  with the second engaged portion  72  has already been released, and the cam gear  63  has been driven and rotated. Accordingly, in the present embodiment, the second time-lag is not caused at the time when the pressing plate  11  reaches the first release position X 1 . Furthermore, in the present embodiment, the engagement of the second engaging lever  62  with the second engaged portion  72  is released earlier than in the comparative example. Hence, a rising height amount of the pressing plate  11  is more constrained in the present embodiment than in the comparative example. Thus sheet feeding performance can be improved. 
         [0096]    2. Regarding when Sheet Count is Reduced through Sheet Feeding Operation 
       (1) Operations in Embodiment 
       [0097]    As shown in “while pressing plate is stopped in sheet feeding operation” of the present embodiment in  FIG. 14 , while the sheet feeding operation is performed with the sheet feeding roller  9 , the first engaging lever  61  is engaged with the first engaged portion  71  of the cam gear  63 . The sheet feeding roller  9  is moved lower along with decrease of the number of the sheets  3 , as shown between a time t 5  and a time t 6  in  FIG. 15 . 
         [0098]    At the time t 6  when the sheet feeding roller  9  is shifted down to the height X 5 , as shown in “pressing plate is caused to be lifted up” of the present embodiment in  FIG. 14 , the engagement of the first engaging lever  61  with the first engaged portion  71  is released (see  FIGS. 12A to 12C ), and the cam gear  63  is rotated in a clockwise direction of  FIG. 14 . Thereby, the pressing plate  11  begins to intermittently rise, and when the sheet feeding roller  9  reaches the first release position X 1  (at a time t 7  in  FIG. 15 ), the first engaging lever  61  contacts the cam gear  63  (see “sheet feeding roller reaches first release position X1” of the present embodiment in  FIG. 14 ). Thereafter, when the cam gear  63  reaches the second rotational position, as illustrated in “pressing plate is stopped” of the present embodiment in  FIG. 14 , the first engaging lever  61  is engaged with the first engaged portion  71 . At this time, as shown in  FIG. 15 , the sheet feeding roller  9  is lifted up to the height X 3  and stopped at a time t 8 . Here, a time period between t 7  and t 8  taken for the sheet feeding roller  9  to be lifted from the first release position X 1  to the height X 3  corresponds to the first time-lag in the present embodiment. 
       (2) Operations in Comparative Example 
       [0099]    In the comparative example, at a time t 9  when the sheet feeding roller  9  is moved down to the height X 5 , as shown in “pressing plate is caused to be lifted up” of the comparative example in  FIG. 14 , the second engaging lever  62  contacts the cam gear  63 , and the engagement of the first engaging lever  61  with the first engaged portion  71  is released. Thereby, the cam gear  63  is held in the first rotational position, and the pressing plate  11  begins to rise. After that, in the same manner as from “sheet feeding roller reaches first release position X1” to “pressing plate is stopped” of the comparative example in  FIG. 13 , the second time-lag is caused as well as the first time-lag until the rising of the pressing plate  11  is actually stopped after the sheet feeding roller  9  reaches the first release position X 1  (see “sheet feeding roller reaches first release position” to “pressing plate is stopped” of the comparative example in  FIG. 14 , and a time period from t 10  to t 12  in  FIG. 15 ). 
         [0100]    Hereinabove, according to the present embodiment, the second time-lag is not caused as well when the number of the sheets  3  is reduced through the sheet feeding operation. Thus, the rising height amount of the pressing plate  11  is more constrained in the present embodiment than in the comparative example. Therefore the sheet feeding performance can be improved. 
       Effects of Embodiment 
       [0101]    In the comparative example, even though a loading portion (which corresponds to the pressing plate  11  of the present embodiment) reaches a predetermined height (which corresponds to the first release position X 1  of the present embodiment), a continuous lifting operation is maintained while a cam (which corresponds to the cam gear  63  of the present embodiment) is rotated from the first rotational position (a rotational position where a lifting mechanism is set to an operating state) to the second rotational position (a rotational position where a lifting mechanism is set to a stopped state). Further, since the height of the loading portion is mechanically detected, the second time-lag is caused until the cam is actually rotated after it is detected that the loading portion reaches the predetermined height. Accordingly, even though the loading portion reaches the predetermined height, the lifting operation is further maintained. Hence, an undesired situation might be caused in which a contact pressure between the sheet feeding roller and a sheet is increased too much and thus sheet feeding performance is deteriorated such that two or more sheets are fed together or that no sheet is fed. 
         [0102]    On the contrary, in the present embodiment, the gear mechanism  60  alternately repeats the operating state and stopped state until the pressing plate  11  reaches the first release position X 1 . Thereby, the pressing plate  11  intermittently rises (intermittent lifting mode). Then, when the pressing plate  11  reaches the first release position X 1 , the cam  67  is stopped in a rotational position where the gear mechanism  60  is set to the stopped state (see  FIGS. 11A to 11C ). Even in the gear mechanism  60  of the present embodiment, it may be delayed that the cam  67  is rotated to the rotational position (see  FIGS. 11A to 11C ) where the gear mechanism  60  is set in the stopped state, with respect to the time when the sheet feeding roller  9  reaches the first release position X 1  (the first time-lag). However, when the sheet feeding roller  9  reaches the first release position X 1 , and the engagement of the first engaging lever  61  with the first engaged portion  71  is released, the second engaging lever  62  is still in the position away from the cam  67 , and is not engaged with the second engaged portion  72 . Accordingly, the second time-lag as caused in the comparative example is not caused in the present embodiment. Namely, the cam  67  is driven and rotated immediately after the sheet feeding roller  9  reaches the first release position X 1 , and it can be avoided to lift the pressing plate  11  too high. Further, the pressing plate  11  is intermittently lifted. Therefore, it can be prevented that the sheet feeding roller  9  is lifted too far from the first release position X 1 . 
         [0103]    Further, a rising speed of the pressing plate  11  in the intermittent lifting mode is lower than that in the continuous lifting mode. In addition, a sound is emitted in the intermittent lifting mode when the engagement of the first engaging lever  61  with the first engaged portion  71  is established and released. On the contrary, in the continuous lifting mode, engagement of any lever  61  or  62  is not established or released. In the present embodiment, the continuous lifting mode is implemented until the pressing plate  11  reaches the second release position X 2 , and the mode is switched to the intermittent lifting mode when the pressing plate  11  is lifted in or higher than the second release position X 2 . Thereby, it is possible to increase the rising speed of the pressing plate  11  and reduce noises until the pressing plate  11  reaches the second release position X 2 . 
         [0104]    Hereinabove, the embodiments according to aspects of the present invention have been described. The present invention can be practiced by employing conventional materials, methodology and equipment. Accordingly, the details of such materials, equipment and methodology are not set forth herein in detail. In the previous descriptions, numerous specific details are set forth, such as specific materials, structures, chemicals, processes, etc., in order to provide a thorough understanding of the present invention. However, it should be recognized that the present invention can be practiced without reapportioning to the details specifically set forth. In other instances, well known processing structures have not been described in detail, in order not to unnecessarily obscure the present invention. 
         [0105]    Only exemplary embodiments of the present invention and but a few examples of its versatility are shown and described in the present disclosure. It is to be understood that the present invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. 
         [0106]    (Modifications) 
         [0107]    In the aforementioned embodiment, the first engaging lever  61  and the second engaging lever  62  are configured as different members, respectively. However, they may be configured to be swung as a single integrated member. 
         [0108]    The aforementioned “sheet” may include an OHP sheet and a banknote as well as a recording medium. 
         [0109]    The aforementioned “feeder unit” may be a unit configured to be detachably attached or a unit configured to be un-detachably attached to a main body of an image forming apparatus (e.g., printer, facsimile machine, and multi function peripheral with a printer function and a scanner function). Further, the aforementioned “feeder unit” is not limited to a unit configured to feed a sheet to the main body of the image forming apparatus. The aforementioned “feeder unit” may be incorporated in an apparatus configured to count the number of sheets such as banknotes. 
         [0110]    In the aforementioned embodiment, the feeder unit according to aspects of the present invention is applied to the laser printer. However, the feeder unit may be applied to various image forming apparatuses such as a printer having an LED as an exposure unit and an inkjet printer. Furthermore, the feeder unit may be applied to a sheet feeding apparatus configured to feed a sheet such as a banknote.