Sheet accommodating device

Information

  • Patent Grant
  • 6364309
  • Patent Number
    6,364,309
  • Date Filed
    Wednesday, August 23, 2000
    24 years ago
  • Date Issued
    Tuesday, April 2, 2002
    22 years ago
Abstract
A sheet accommodating device is provided which always performs stable sheet feeding in accordance with sizes of sheets or the number of sheets to be stacked. A holder member that is movable back and forth supports a sheet pressing plate near the center of gravity of the sheets. The holder member is movable in accordance with the sizes of the sheets or the amount of stacked sheets, so that the sheets can be stacked and fed with stability.
Description




BACKGROUND OF THE INVENTION




1. Field of Invention




The invention relates to a sheet accommodating device for accommodating a stack of sheets in an image forming apparatus.




2. Description of Related Art




Japanese Utility Model Publication JP-U-62-181530 discloses a sheet cassette that is to be mounted in a printer. A structure of this sheet cassette is shown in FIG.


10


. The sheet cassette


101


includes a sheet pressing plate


102


where a stack of sheets are placed thereof, a spring


104


for urging a front end of the sheet pressing plate


102


against a sheet feed roller


103


, and a support member


105


for supporting the sheet pressing plate


102


at the center of gravity of the sheets to be stacked so that the sheet pressing plate


102


is swingable vertically. As the front end of the sheet pressing plate


102


is swung upward on the support member


105


, a rear end of the sheet pressing plate


102


is swung downward, like a seesaw. The sheet pressing plate


102


is provided with an end guide


106


for supporting rear edges of the sheets to be stacked by moving back and forth.




However, in the seesaw type sheet cassette shown in

FIG. 10

, the center of gravity of the sheet deviates from the position where the support member


105


supports the sheet pressing plate


102


because the centers of gravity of the sheets vary with the size of the sheets to be stacked. As a result, the sheet pressing plate


102


is not swung with stability.




Further, there is a large difference (Dh) between a height from a bottom of the sheet cassette to the rear end of the sheet pressing plate


102


when few sheets are stacked (h1) and the height when a large number of sheets are stacked (h2). Therefore, there is a problem that the printer becomes large in size.




In particular, in order for the sheet cassette to accommodate a large amount of sheets, the difference Dh becomes larger, so that the printer becomes larger in size. Consequently, it is difficult to make the sheet cassette small in size.




The seesaw type sheet cassette can minimize variations of a pressing force from the spring


104


traceable to a weight change that occurs due to variations in size of the sheets. However, the amount of compression of the spring


104


varies in accordance with the amount of stacked sheets, so that the pressing force from the spring


104


varies and sheet feeding operation becomes unstable.




SUMMARY OF THE INVENTION




According to the invention, a sheet accommodating device is provided which can always perform sheet feeding with stability in accordance with the size of sheets or the amount of sheets to be stacked.




In the invention, the sheet accommodating device includes a stacking portion holding a sheet thereon, a first urging member urging one end of the stacking portion upward, and a support member that is movable back and forth relative to the stacking portion and supports the stacking portion near a center of gravity of the sheet to be stacked on the stacking portion.




According to this structure, the support member moves back and forth relative to the stacking portion so as to support the stacking portion near the center of gravity of the sheet to be stacked. Therefore, the support member can be moved back and forth even when the size of the sheets to be stacked on the stacking portion is changed, and thus the support member can support the stacking portion near the center of gravity of the sheet at all times and the stacking portion is swung with stability.




The sheet accommodating device further includes a rear edge support member that supports a rear edge of the sheet and is disposed at a rear end of the stacking portion so as to be movable in accordance with the size of the sheet, and a link mechanism that moves the support member back and forth in accordance with a movement of the rear edge support member.




According to this structure, when the rear edge support member is moved back and forth in accordance with the size of the sheet to be stacked, the link mechanism moves the support member back and forth relative to the stacking member so that the support member supports the stacking member near the center of gravity of the sheets, in synchronization with the movement of the rear edge support member. Therefore, the stacking portion can be supported by the support member at a position near the center of gravity at all times by a simple operation such as moving the rear edge support member in accordance with the sheet size.




The link mechanism acts so that the amount of travel of the support member becomes half distance of the rear edge support member. The link mechanism includes a pinion gear and a rack and the amount of travel is determined by arranging the number of teeth of the pinion gear and the rack. With such a link mechanism, the support member can support the stacking portion at the center of gravity of the sheet.




Further, a second urging member that urges the stacking portion upward may be provided near the support member. When a large number sheets are stacked on the stacking portion, the weight of the stock of the sheets overcomes the urging force from the spring and thus the sheet pressing plate


53


is moved downward. In accordance with the downward movement, the other end of the stacking portion is also moved downward.




On the other hand, when few sheets are stacked, the urging force from the spring overcomes the weight of the stack of sheets and thus the sheet pressing plate is moved upward. In accordance with the upward movement, the other end of the stacking portion is moved also upward.




Therefore, variations in the position of the other end of the stacking portion between a case when a large number sheets are stacked and a case when few sheets are stacked become small. Accordingly, the sheet accommodating device does not need to be large in size and thus it can be compact in size even when the amount of the sheets that can be accommodated in the sheet accommodating device is increased.




A spring constant of the second urging member may be equal to a weight per unit thickness of the stack of sheets on the stacking portion. That is, the second urging member acts to move the stacking portion downward by an amount corresponding to a thickness of the sheets added. As the stacked sheets are removed, the second urging member acts to move the stacking portion upward by the amount corresponding to the thickness of the sheets removed.




Therefore, even when the stacked sheets are added or removed, the stacking portion is vertically moved by an amount corresponding to the thickness of the sheets that have been added or removed. Consequently, the sheets on the stacking portion can be held at a certain position at all times, so that there is little variation in the vertical movement and the sheets can be fed with stability.




Further, the support member can support the stacking portion so that a pressing force acting on one end of the stacking portion by the first urging member becomes constant regardless of the number of the sheets stacked on the stacking portion.




Therefore, even when the thickness or weight of the sheets to be stacked on the stacking portion is changed, the pressing force acting on the one end of the stacking portion becomes nearly constant regardless of the weight of the sheets to be stacked on the stacking portion. That is, the stacking portion always presses the sheets upward with a nearly constant pressing force by the urging force from the first urging member, so that the sheets can be fed with stability.




In particular, the support member supports the stacking portion at a position expressed by X that satisfies an equation below.






Y−2XZ=(F=nearly constant)






wherein:




Y is the urging force from the first urging member;




X is the offset from the center of gravity in a back and forth direction of the sheet;




Z is the weight per unit length of the stack of sheets;




F is the pressing force acting on one end of the stacking portion.




Even when the urging force Y from the first urging member is changed in accordance with change of the weight per unit length Z of the sheet caused by changing the number of sheets, the stacking portion is supported by the support member at a position where the pressing force F acting on the one end of the stacking portion becomes nearly constant at all times. Therefore, the sheet stacking portion presses the sheet upward with a constant pressing force by the urging force from the first urging member, so that the sheets can be fed with stability.




In other words, in accordance with the weight change of the sheets to be stacked on the stacking portion, the pressing force acting on the one end of the stacking portion is maintained at nearly constant value by changing a position where the support member supports the sheet staking member as necessary, so that the sheets can be fed with stability regardless of the number of the sheets.




Further, when the variation of the pressing force acting on the one end of the stacking portion is determined ±10%, the sheets can be fed more stably.




Furthermore, when the pressing force acting on the one end of the stacking portion is between 100-600 gf, the pressing force acts on the one end of the stacking portion at all times, so that the sheets can be fed with stability.











BRIEF DESCRIPTION OF THE DRAWINGS




Various exemplary embodiments of the invention will be described in detail with reference to the following figures wherein:





FIG. 1

a side sectional view of a laser beam printer;





FIG. 2

is a plan view of a sheet cassette provided in the laser beam printer of

FIG. 1

;





FIG. 3

is a partially enlarged plan view of the sheet cassette of

FIG. 2

;





FIG. 4

is a partially enlarged cross sectional view of the sheet cassette of

FIG. 3

;





FIG. 5

is a side view of a state where a maximum number of large sized sheets are stacked in the sheet cassette of

FIG. 2

;





FIG. 6

is a side view of a state where no sheets are stacked in the sheet cassette of

FIG. 5

;





FIG. 7

is a side view of a state where an maximum amount of small sized sheets are staked in the sheet cassette of

FIG. 2

;





FIG. 8

is a side view of a state where no sheets are stacked in the sheet cassette of

FIG. 7

;





FIG. 9

is a side view of a modification of the sheet cassette; and





FIG. 10

is a side view of a conventional seesaw type sheet cassette.











DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS





FIG. 1

is a side sectional view showing an embodiment of a laser beam printer provided with a sheet accommodating device according to the invention.




In

FIG. 1

, a laser beam printer


1


includes a feeder unit


4


, an image forming unit


5


for forming a predetermined image on a sheet


3


fed from the feeder unit


4


, and the like, in a main casing


2


.




The feeder unit


4


includes a sheet cassette accommodating portion


51


formed at a bottom of the main casing


2


, a sheet cassette


52


detachably attached to the sheet cassette accommodating portion


51


, a sheet feed roller


7


disposed above one end of the sheet cassette


52


, and resist rollers


9


disposed downstream of a feed direction of the sheet


3


with respect to the sheet feed roller


7


.




As described later, the sheet cassette


52


includes a sheet pressing plate


53


where the sheets


3


are to be stacked, springs


54


, a separation pad


8


, and a spring


10


that urges the separation pad


8


. The springs


54


upwardly urge a front end portion of the sheet pressing plate


53


, more particularly, the end portion of the sheet pressing plate


53


near the sheet feed roller


7


, from the reverse side of the sheet pressing plate


53


. The separation pad


8


and the spring


10


are illustrated in only

FIG. 1

, in other words, they are omitted in

FIGS. 2

trough


9


.




An uppermost sheet


3


in the stack on the sheet pressing plate


53


is pressed against the sheet feed roller


7


by the urging force from the springs


54


, from the reverse side of the sheet pressing plate


53


. As the sheet feed roller


7


rotates, the sheet


3


is pinched between the sheet feed roller


7


and the separation pad


8


. The sheets


3


are fed in one sheet at a time. The resist rollers


9


include a drive roller and a driven roller. The resist rollers


9


temporarily stop the sheet


3


fed from the sheet feed roller


7


to adjust a deviation of the sheet


3


and then feed the sheet


3


to the image forming unit


5


.




The image forming unit


5


includes a scanning unit


11


, a developing unit


12


, and a fixing unit


13


.




The scanning unit


11


is provided in an upper portion of an internal space of the main casing


2


. The scanning unit


11


has a laser emitting portion (not shown), a rotatable polygon mirror


14


, lenses


15


,


16


, and reflecting mirrors


17


,


18


,


19


. A laser beam that is emitted from the laser emitting portion based on predetermined image data sequentially passes through or is reflected by the polygon mirror


14


, the lens


15


, the reflecting mirrors


17


,


18


, the lens


16


, and the reflecting mirror


19


in that order as indicated by a dot and dashed line. The laser beam is thus directed to and high-speed scanned over a photosensitive drum


21


of the developing unit


12


for irradiation of the surface of the photosensitive drum


21


.




The developing unit


12


is disposed below the scanning unit


11


. The developing unit


12


includes the photosensitive drum


21


, a developing cartridge


36


, a scorotron electrical charging device


25


, and a transfer roller


26


in a drum cartridge


20


that is detachably attached to the main casing


2


.




An internal space of the developing cartridge


36


is divided into a developing chamber


37


that contains the developing roller


22


, a layer thickness-regulating blade


23


, and a supply roller


24


, and into a toner box


27


containing toner. The toner box


27


contains positively electrically charged toner of a single non-magnetic component. The toner is agitated by an agitator


29


provided at a center of the toner box


27


, and is discharged into the developing chamber


37


. In the developing chamber


37


, the supply roller


24


is rotatably disposed at the toner box


27


side. The developing roller


22


is rotatably disposed facing the supply roller


24


. The supply roller


24


and the developing roller


22


are disposed in contact with each other so that they are press-deformed against each other to an appropriate extent. The supply roller


24


is formed by covering a metallic roller shaft with a roller part formed from an electrically conductive foam material. The developing roller


22


is formed from by covering a metallic roller shaft with a roller part formed by an electrically conductive rubber material. The developing roller


22


is applied a bias so as to produce an electric potential difference between the developing roller


22


and the photosensitive drum


21


. The layer thickness-regulating blade


23


that regulates a thickness of toner on the developing roller


22


is disposed near the developing roller


22


.




Toner discharged from the toner box


27


into the developing chamber


37


is supplied to the developing roller


22


as the supply roller


24


rotates. At this time, toner is positively electrically charged between the supply roller


24


and the developing roller


22


due to friction. After being supplied onto the developing roller


22


, toner enters a gap between the layer thickness-regulating blade


23


and the developing roller


22


as the developing roller


22


rotates. Toner becomes sufficiently electrically charged therebetween due to friction, and is formed into a thin layer of a predetermined thickness on the developing roller


22


.




The photosensitive drum


21


is rotatably disposed beside the developing roller


22


so that the photosensitive drum


21


faces the developing roller


22


. A drum body of the photosensitive drum


21


is grounded, and its surface is formed from a positively electrically charged organic photosensitive material containing a polycarbonate as a main component. The scorotron electrical charging device


25


is disposed at a predetermined interval upward from the photosensitive drum


21


. The scorotron electrical charging device


25


produces corona discharge from a tungsten wire and positively charges the surface of the photosensitive drum


21


uniformly.




After the surface of the photosensitive drum


21


is uniformly positively charged by the scorotron electrical charging device


25


, the surface of the photosensitive drum


21


is exposed to a laser beam emitted from the scanning unit


11


so that an electrostatic latent image is formed based on predetermined image data. The electrostatic latent image is portions of the uniformly positively charged surface of the photosensitive drum


21


that have a reduced electric potential due to exposure to the laser beam. When positively charged toner carried on the developing roller


22


come to face and contact the photosensitive drum


21


as the developing roller


22


rotates, the toner is selectively transferred and deposited onto the electrostatic latent image formed on the surface of the photosensitive drum


21


, so that the image is visualized. Thus, image development (reversal development) is accomplished.




The transfer roller


26


is rotatably disposed below the photosensitive drum


21


, facing the photosensitive drum


21


. The transfer roller


26


is formed by covering a metallic roller shaft with a roller part formed from an electrically conductive rubber material. A predetermined transfer bias is applied to the transfer roller


26


. Therefore, the toner image developed on the photosensitive drum


21


is transferred to the sheet


3


due to the transfer bias when the sheet


3


is passed between the photosensitive drum


21


and the transfer roller


26


.




The fixing unit


13


is disposed beside the developing unit


12


, that is downstream thereof, as shown in FIG.


1


. The fixing unit


13


includes a heat roller


32


, a pressing roller


31


pressed against the heat roller


32


, and a pair of conveying rollers


33


disposed downstream of the heat roller


32


and the pressing roller


31


. The heat roller


32


is a hollow-roller made from metal and is equipped with a heating halogen lamp. While the sheet


3


is being passed between the heat roller


32


and the pressing roller


31


, toner transferred on the sheet


3


melts and becomes fixed due to heat. Then, the sheet


3


is conveyed to a pair of sheet ejecting rollers


34


by the conveying rollers


33


. The sheet


3


is then ejected on an output tray


35


by the sheet ejecting rollers


33


.




A structure of the sheet cassette


52


will be described below.




As shown in

FIGS. 1 and 2

, the sheet cassette


52


is formed in a generally rectangular box shape having an upper open structure. The sheet cassette


52


is formed by side plates


55


,


56


disposed on both sides of the sheet cassette


52


in a width direction so as to face each other, a grip portion


57


provided at the front end in a feed direction of the sheet


3


, a rear plate


58


provided at the rear end, and a bottom plate


59


.




In the sheet cassette


52


, there are the sheet pressing plate


53


, the springs


54


, side guides


60


, an end guide


61


, and a holder member


62


.




The sheet pressing plate


53


includes a front plate


63


that receives the front portion of the sheet


3


and a rear plate


64


that receives the rear portion of the sheet


3


.




The front plate


63


is formed in a generally rectangular shape. Side openings


65


are defined at each side portion of the front plate


63


by concavely and inwardly carving out in the width direction of the sheet cassette


52


from each side edge of the front plate


63


. A holder member guide groove


79


that slidably receives slide guides


80


(described later) is formed in a middle portion of the front plate


63


in the width direction so as to extend in a back and forth direction.




The rear plate


64


having a generally U-shaped rectangular shape is narrower than the front plate


63


and extends in the back and forth direction. The rear plate


64


has a pair of side portions


64




a


and


64




b


that extend in the back and forth direction in parallel each other with a rear plate guide member


78


sandwiched therebetween and a rear portion


64




c


by which the side portions


64




a


and


64




b


are connected to each other. As shown in

FIG. 4

, the side portion


64




a


is formed in a rectangular shape in cross section, and the side portion


64




b


is formed in a generally L-shape in cross section. The side portion


64




b


is formed by a bottom wall


83


and a side wall


84


that stands from outside in the width direction of the bottom wall


83


. A rack


68


that engages a first pinion gear


85


(described later) is formed on the internal surface of the side wall


84


across the back and forth direction.




The rear plate


64


overlaps the front plate


63


at a middle portion in the width direction of the front plate


63


so that the bottom surface of the front plate


63


can slide on the upper surface of the rear plate


64


. The rear plate


64


is disposed so as to extend toward the rear from the position where the front plate


63


and the rear plate


64


overlap each other.




The springs


54


are mounted on two positions (right and left) in the width direction of the front end of the bottom plate


59


and are opposed to the reverse side of the front end of the front plate


63


. The front end of the front plate


63


is urged against the sheet feed roller


7


by the two springs


54


.




Each side guide


60


is provided at a position facing each side opening


65


of the front plate


65


. Each side guide


60


has a generally rectangular shaped side edge contact member


69


for contacting both sides of the sheets


3


in the width direction and a side edge slide member


71


for supporting the side edge contact member


69


. The side edge slide member


71


is provided with protrusions


70


on its reverse side. Side guide guiding grooves


72


that guide the side guides


60


along the width direction are formed in the width direction of the sheet cassette


52


at positions opposed to the side edge slide members


71


of each side guide


60


.




Each side guide


60


can be slid either outward or inward in the width direction along each side guide guiding groove


72


by engaging the protrusions


70


of each side edge slide member


71


with each side guide guiding groove


72


. When large sized sheets


3


, e.g. A3- or B3-size sheets, are stacked in the sheet cassette


52


, the side guides


60


are slid outward in the width direction so as to regulate the side edges of the sheets


3


. On the other hand, when small sized sheets


3


, e.g., A4- or B5-size sheets, are stacked in the sheet cassette


52


, the side guides


60


are slid inward in the width direction so as to regulate the side edges of the sheets


3


.




The end guide


61


stands from the rear end of the rear plate


64


. The end guide


61


has a generally rectangular shape and moves back and forth together with the rear plate


64


to support the rear edge of the sheet


3


, in accordance with size of the stack of sheets


3


.




The holder member


62


is disposed near the center of gravity of the sheet


3


in the halfway of the length of the sheet pressing plate


53


. The holder member


62


can slide back and forth relative to the front plate


63


and supports the sheet pressing plate


53


so that the sheet pressing plate


53


can be swung vertically. The holder member


62


includes a holder frame


73


attached to the reverse surface of the front plate


63


and a holder arm


75


swingably attached to the holder frame


73


.




As shown in

FIGS. 3 and 4

, the holder frame


73


is made up of a housing portion


74


that is concavely formed toward the bottom surface of the front plate


63


and collar portions


77


that are formed outwardly in the width direction of the housing portion


74


. A rectangular rear plate guide member


78


extending in the back and forth direction protrudes from the middle portion in the width direction of the housing portion


74


. The inside of the housing portion


74


is partitioned off to make two rectangular rooms by the rear plate guide member


78


.




Slide guides


80


that are engaged with the holder member guide groove


79


protrudes from the upper surface of the rear plate guide member


78


. Each collar portion


77


is slidably in contact with the bottom surface of the front plate


63


. With this structure, the holder frame


73


supports the front plate


63


in a state where the holder frame


73


can be slid back and forth relative to the front plate


63


while guided along the holder member guide groove


79


. Each collar portion


77


has a circular spring pressing portion


76


to make contact with a spring


89


(described later).




The rear plate guide member


78


extends toward the rear from the housing portion


74


to make a rectangular shape and a stepped portion


81


is formed on the rear plate guide member


78


in its back and forth direction. A rack


82


that engages a second pinion gear


86


(described later) is formed on the surface of the side wall of the stepped portion


81


along the stepped portion


81


.




Within the housing portion


74


, the side portion


64




a


, having a rectangular shape in cross section, of the rear plate


64


is inserted in one room partitioned by the rear plate guide member


78


and the side portion


64




b


having a generally rectangular shape in cross section is inserted in another room. Therefore, the rear plate


64


is supported by the holder member


62


in a state where the rear plate


64


can be slid back and forth relative to the holder member


62


while guided along the rear plate guide member


78


.




In the state where the side portion


64




a


is inserted in the room in the housing portion


74


, the first and second pinion gears


85


,


86


are provided between the racks


68


and


82


that are opposed to each other. The first pinion gear


85


is rotatably supported at its shaft by a recess


87


formed in the front plate


63


, at a position where the first pinion gear engages the rack


68


. Similarly, the second pinion gear


86


is rotatably supported at it shaft by a recess


88


formed in the front plate


63


, at a position where the second pinion gear


86


engages the rack


82


and the first pinion gear


85


. A reduction ratio of the first pinion gear


85


to the second pinion gear


86


is set to 2:1.




Because the first and second pinion gears


85


,


86


are rotatably supported by the front plate


63


, as described above, a predetermined gap is produced between the first and second gears


85


,


86


and the bottom wall of the housing portion


74


. In this gap, the bottom wall


83


of the side portion


64




b


can move back and forth.




As the rear plate


64


is slid back and forth relative to the holder member


62


, the holder member


62


is slid back and forth relative to the front plate


63


via the rack


68


, the first pinion gear


85


, the second pinion gear


86


and the rack


82


. In particular, the reduction ratio of the first pinion gear


85


to the second pinion gear


86


is set to 2:1. Accordingly, when the rear plate


64


is slid forward relative to the holder member


62


, the holder member


62


is slid forward by a half distance traveled forward by the rear plate


64


.




The holder arm


75


has arm support portions


91


that protrude outward in the width direction from each front edge of the housing portion


74


and swing arms


92


that are supported by the arm support portions


91


. One end of each swing arm


92


is swingably supported by the arm support portion


91


. Leg portions


94




a


,


94




b


extending outward in the width direction are formed at another ends. The front plate


63


and the rear plate


64


supported by the holder member


62


can be swung relative to the swing arms


92


on each arm support portion


91


.




An engagement protrusion


96


protruding outward in the width direction is formed at the leg portion


94




b


. Guide members


93




a


and


93




b


that extend in parallel to the back and forth direction of the sheet cassette


52


are provided at positions each opposed to the leg portion


94




a


and


94




b


. The guide members


93




a


,


93




b


are omitted in

FIG. 1

, and the guide member


93




b


is shown by a phantom line in FIG.


6


. The guide member


93




b


is formed in a C-shape in cross section and has a guide groove


95


. The protrusion


96


of the leg portion


94




b


is engaged with the guide groove


95


. Further, the leg portion


94




a


contacts the guide member


93




a


. When the holder frame


73


is slid back and forth relative to the front plate


63


under this condition, the holder arm


75


is guided back and forth along the guide members


93




a


,


93




b.






Spring rests


97


that have a generally round shape and protrude in expanded condition are formed at positions opposed to each spring pressing portion


76


of the swing arms


92


. The springs


89


urging the sheet pressing plate


53


(the front and rear plates


63


,


64


)upward are provided between each spring rest


97


and spring pressing portion


76


. The urging force from those springs


89


acts in a direction that the holder frame


73


and the swing arm


92


are apart from each other. The swing arms


92


are swung on the arm support portion


91


, so that the sheet pressing plate


53


is moved upward.




When small sized sheets


3


, e.g., A4- or B5-size sheets, are accommodated in the sheet cassette


52


structured as described above, as shown in

FIG. 7

, the sheets


3


are stacked on the sheet pressing plate


53


and the end guide


61


is slid forward to make contact with the rear edges of the sheets so as to support the rear portion of the sheets


3


.




Then, the rear plate


64


moves forward together with the end guide


61


. In synchronization with this movement, the holder member


62


moves forward relative to the front plate


63


by the half distance traveled forward by the rear plate


64


, via the rack


68


, the first pinion gear


85


, the second pinion gear


86


, and the rack


82


. That is, when the end guide


61


is moved according to the size of the sheets


3


, the holder member


62


is moved relative to the sheet pressing plate


53


and supports the sheet pressing plate


53


near the center of gravity of the sheets


3


.




On the other hand, when large sized sheets


3


, e.g., A3- or B4-size sheets, are accommodated in the sheet cassette


52


, as shown in

FIG. 5

, the end guide


61


is slid backward, the sheets


3


are stacked on the sheet pressing plate


53


and then the end guide


61


is made to contact with the rear edges of the sheets


3


so as to support the rear portion of the sheets


3


.




Then, the rear plate


64


moves backward together with the end guide


61


. In synchronization with this movement, the holder member


62


moves backward relative to the front plate


63


by the half distance traveled backward by the rear plate


64


, via the rack


68


, the first pinion gear


85


, the second pinion gear


86


and the rack


82


. That is, when the end guide


61


is moved according to the size of the sheets


3


, the holder member


62


is moved relative to the sheet pressing plate


53


and supports the sheet pressing plate


53


near the center of gravity of the sheets


3


.




Even when the size of the sheets


3


to be accommodated in the sheet cassette


52


is changed, the holder member


62


supports the sheet pressing plate


53


near the center of gravity of the sheets


3


at all times. Therefore, the sheet pressing plate


53


can be swung with stability at all times.




The holder member


62


moves back and forth relative to the sheet pressing plate


53


in synchronization with the movement of the end guide


61


. With such an extremely simple operation, the holder member


62


can support the sheet pressing plate


53


near the center of gravity of the sheets


3


at all times.




The reduction ratio of the first pinion gear


85


to the second pinion gear


86


is set to 2:1. Therefore, the holder member


62


moves back and forth relative to the sheet pressing plate


53


by the half distance traveled back and forth by the end guide


61


, so that the holder member


62


surely supports at the center of gravity of the sheets


3


.




In the laser beam printer


1


provided with the sheet cassette


52


structured as described above, sheet feeding can be stably and surely performed at all times even when the size of the sheets


3


to be accommodated in the sheet cassette


52


is changed.




In the sheet cassette


52


, the sheet pressing plate


53


can be moved vertically by swinging the swing arms


92


and is urged upward by the springs


89


. Therefore, when the weight of the stack of sheets


3


is heavy because a large number of sheets


3


are stacked, the weight of the stack of sheets overcomes the urging force from the springs


89


and thus the sheet pressing plate


53


is moved downward. In accordance with the downward movement of the sheet pressing plate


53


, the rear end of the sheet pressing plate


53


is also moved downward. This state is shown in

FIG. 5

that shows a state where the maximum number of sheets


3


are stacked.




On the other hand, when the weight of the stack of sheets


3


is light because few sheets


3


are stacked, the urging force from the springs


89


overcomes the weight of the stack of sheets


3


and thus the sheet pressing plate


53


is moved upward. At that time, the pressing plate


53


is moved upward. However, the urging force from the springs


54


is stronger than the urging force from the springs


89


, so that the front end of the front plate


63


is lifted upward and the rear end of the rear plate


64


is not so much moved upward as much as the front end. This state is shown in

FIG. 6

that shows a state where no sheets


3


are stacked.




That is, there is little variation in the position of the rear end of the sheet pressing plate


53


between a case when a large number of sheets


3


are stacked and a case when few sheets


3


are stacked. Accordingly, a vertical stroke of the rear end of the sheet pressing plate


53


can be small, so that the sheet cassette


52


and the laser beam printer


1


can be made compact in size.




In the case where a spring constant of the spring


89


is the same value as the weight per unit thickness of the stack of sheets


3


, the spring


89


acts to move the sheet pressing plate


53


downward by the amount corresponding to a thickness of the sheets


3


added. Therefore, an uppermost sheet


3


in the stack on the sheet pressing plate


53


can be held at a certain position at all times. Consequently, there is little variation in the vertical movement and stable sheet feeding can be achieved.




Further, as described above, the springs


89


are structured to urge the sheet pressing plate


53


at all times near the center of gravity of the stack of sheets


3


, so that the urging force from the springs


89


can most accurately act on the sheet pressing plate


53


and the sheet pressing plate


53


can be moved with stability.




In particular, as shown in

FIG. 5

, the holder member


62


is disposed to support the sheet pressing plate


53


at a position which is apart from the center of gravity


98


of the stack of sheets


3


on the sheet pressing plate


53


in the back and forth direction and at a position


99


where a pressing force acting on the front end of the sheet pressing plate


53


by the urging force from the spring


54


becomes nearly constant regardless of the weight of the stack of sheets


3


on the sheet pressing plate


53


.




That is, the holder member


62


supports the sheet pressing plate


53


at the position


99


expressed by X that satisfies an equation (1) below.




 Y−2XZ=F (F=nearly constant)  (1)




wherein:




Y is the urging force from the spring


54


;




X is the offset from the center of gravity in the back and forth direction of the sheet


3


;




Z is the weight per unit length of the stack of sheets


3


; and




F is the pressing force acting on the front end of the sheet pressing plate


53


.




When the sheet pressing plate


53


is supported at such a position


99


, for example, the weight per unit length Z of the stack of sheets


3


changes as the number of sheets


3


changes. Even when the urging force Y from the spring


54


changes in accordance with this change, the pressing force F acting on the front end of the sheet pressing plate


53


is nearly constant at all times.




A concrete example will be described below. It is assumed that an entire length of the sheet pressing plate


53


is 354 mm, a maximum stack weight of the sheets


3


is 3400 g, and the spring that produces the urging force of 400 gf in the most compressed state and the urging force of 200 gf in the most stretched state is used as the spring


54


. When the holder member


62


is disposed so as to support the sheet pressing plate


53


at the position


99


offset 10 mm backward from the center of gravity


98


of the stack of sheets


3


, the pressing force F acting on the front end of the sheet pressing plate


53


when the maximum amount of sheets


3


are stacked (a state shown in

FIG. 5

) is 400 gf−2 ×10 mm×(3400 gf/354 mm)=209 gf that is derived from the equation (1) above.




After that, in accordance with a decrease in the number of the sheets


3


by feeding, each swing arm


92


is swung on the arm support portion


91


and thus the sheet pressing plate


53


is moved upward by the urging force from the springs


89


. Therefore, the offset position


99


gradually approaches the center of gravity


98


of the sheets


3


as the sheets


3


are decreased in quantity. When the sheets


3


are all fed and no sheet


3


remains on the sheet pressing plate


53


(a state shown in FIG.


6


), the holder member


62


supports the sheet pressing plate


53


at the center of gravity


98


of the sheet


3


, so that the offset becomes zero. At the time, the pressing force F acting on the front end of the sheet pressing plate


53


is 200 gf−0=200 gf that is derived from the equation (1) above. A difference between two values that are the pressing force F in a case when the maximum amount of sheets


3


are stacked and the pressing force F in a case when no sheets are stacked is within 5%. It may be accepted that this value is nearly constant.




In this example, it is assumed that the offset X is 10 mm. However, the offset X is changed in accordance with size or density of the sheets


3


or the urging force from the spring


54


, as necessary.




As described above, if the holder member


62


supports the sheet pressing plate


53


at the position that is offset a predetermined amount from the center of gravity


98


of the sheet


3


, the pressing force F acting on the front end of the sheet pressing plate


53


becomes nearly constant even when the urging force Y from the springs


54


is changed in accordance with the amount of stacked sheets


3


. Consequently, stable sheet feeding can be achieved.




Further, the pressing force F acting on the front end of the sheet pressing plate


53


is preferably constant within ±10%. By making the urging force from the springs


54


act on the sheet pressing plate


53


with the constant pressing force within ±10%, the sheets


3


can be fed with stability.




The constant pressing force acting on the front end of the sheet pressing plate


53


is, in particular, 100-600 gf, and preferably 200-400 gf. That is, when the spring


54


is structured so that its urging force acts on the sheet pressing plate


53


with the constant pressing force of within 100-600 gf, the sheet pressing plate


53


can press the sheet


3


against the sheet feed roller


7


by a suitable pressing force at all times. Accordingly, the sheets


3


can be fed with stability.




In this embodiment, the offset X is changed in accordance with the weight per unit length Z of the stack of sheets


3


in the equation (1) described above. However, it should be appreciated that, the urging force Y may be provided from a plurality of springs


54


. Further, the urging force F from the spring


54


and the offset X may be fixed in accordance with the weight range of the stack of sheets


3


.




As described above, the rear plate


64


doubles as the holder of the end guide


61


, and the rear plate


64


and the end guide


61


move together back and forth. However, the sheet pressing plate


53


may be formed by integrating the front plate


63


with the rear plate


64


, and the end guide


61


may be disposed on the sheet pressing plate


53


so as to be slidable back and forth. In this case, the holder member


62


may be structured to slide back and forth relative to the sheet pressing plate


53


in synchronization with the slide movement of the end guide


61


.




The spring constant of the spring


89


is set to the same value as the weight per unit thickness of the stack of sheets


3


. However, it is to be understood that the invention is not restricted to the particular forms described above. According to purposes and uses, a spring that has any appropriate spring constant may be used.




Further, as shown in

FIG. 9

, the sheet cassette may be structured such that the holder member


62


supports the sheet pressing plate


53


, a guide rail


100


for guiding the springs


89


along the up and down direction is provided to the holder member


62


, and the spring is inserted in the guide rail


100


, so that the sheet pressing plate


53


can be swung near its center of gravity and can be vertically moved.




Although the invention has been described as embodied in a laser beam printer, is should be appreciated that the invention is applicable to all image forming apparatus in which sheets of recording medium are fed to an image forming engine. It should also be appreciated that the invention is applicable to any apparatus that utilizes a feeder of stacked sheets.




While this invention has been described in conjunction with the exemplary embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the exemplary embodiments of the invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.



Claims
  • 1. A sheet accommodating device, comprising:a stacking portion holding at least sheet thereon; a first urging member urging one end of the staking portion upward; and a support member movable back and forth relative to the stacking portion and supporting the stacking portion near a center of gravity of the at least one sheet on the stacking portion.
  • 2. The sheet accommodating device according to claim 1, further comprising:a rear edge support member supporting a rear edge of the sheet and movably disposed at a rear end of the stacking portion in accordance with sheet size; and a link mechanism moving the support member back and forth in accordance with a movement of the rear edge support member.
  • 3. The sheet accommodating device according to claim 2, wherein the link mechanism acts so that the amount of travel of the support member becomes half of an amount of travel of the rear edge support member.
  • 4. The sheet accommodating device according to claim 1, further comprising:a second urging member urging the stacking portion upward near the support member.
  • 5. The sheet accommodating device according to claim 4, wherein a spring constant of the second urging member is equal to a weight per unit thickness of the at least one sheet on the stacking portion.
  • 6. The sheet accommodating device according to claim 1, wherein the support member supports the stacking portion so that a pressing force acting on one end of the stacking portion by the first urging member becomes constant regardless of the number of sheets stacked on the stacking portion.
  • 7. The sheet accommodating device according to claim 6, wherein the support member supports the stacking portion at a position expressed by X that satisfies:Y−2XZ=F, wherein is the urging force from the first urging member; X is the offset from the center of gravity in a back and forth direction of the at least one sheet; Z is the weight per unit length of the at least one sheet; and F is a nearly constant pressing force acting on one end of the stacking portion.
  • 8. The sheet accommodating device according to claim 7, wherein a variation in the pressing force acting on the one end of the stacking portion according to a weight change of the at least one sheet on the stacking portion is ±10%.
  • 9. The sheet accommodating device according to claim 8, wherein the pressing force acting on the one end of the stacking portion is between about 100-600 gf.
  • 10. The sheet acommodating device according to claim 8, wherein the pressing force acting on the one end of the stacking portion is between about 200-400 gf.
  • 11. An image forming apparatus including sheet accommodating device, the sheet accommodating device comprising:a stacking portion holding at least sheet thereon; a first urging member urging one end of the staking portion upward; and a support member movable back and forth relative to the stacking portion and supporting the stacking portion near a center of gravity of the at least one sheet on the stacking portion.
  • 12. The image forming apparatus according to claim 11, further comprising:a rear edge support member supporting a rear edge of the sheet and movably disposed at a rear end of the stacking portion in accordance with sheet size; and a link mechanism moving the support member back and forth in accordance with a movement of the rear edge support member.
  • 13. The image forming apparatus according to claim 12, wherein the link mechanism acts so that the amount of travel of the support member becomes half of an amount of travel of the rear edge support member.
  • 14. The image forming apparatus according to claim 11, further comprising:a second urging member urging the stacking portion upward near the support member.
  • 15. The image forming apparatus according to claim 14, wherein a spring constant of the second urging member is equal to a weight per unit thickness of the at least one sheet on the stacking portion.
  • 16. The image forming apparatus according to claim 11, wherein the support member supports the stacking portion so that a pressing force acting on one end of the stacking portion by the first urging member becomes constant regardless of the number of sheets stacked on the stacking portion.
  • 17. The image forming apparatus according to claim 16, wherein the support member supports the stacking portion at a position expressed by X that satisfies:Y−2XZ=F, wherein Y is the urging force from the first urging member; X is the offset from the center of gravity in a back and forth direction of the at least one sheet; Z is the weight per unit length of the at least one sheet; and F is a nearly constant pressing force acting on one end of the stacking portion.
  • 18. The image forming apparatus according to claim 17, wherein a variation in the pressing force acting on the one end of the stacking portion according to a weight change of the at least one sheet on the stacking portion is ±10%.
  • 19. The image forming apparatus according to claim 18, wherein the pressing force acting on the one end of the stacking portion is between about 100-600 gf.
  • 20. The image forming apparatus according to claim 18, wherein the pressing force acting on the one end of the stacking portion is between about 200-400 gf.
Priority Claims (3)
Number Date Country Kind
11-243218 Aug 1999 JP
11-243219 Aug 1999 JP
11-243220 Aug 1999 JP
US Referenced Citations (3)
Number Name Date Kind
1951067 Shepherd Mar 1934 A
3533617 Collins Oct 1970 A
4033577 Godard et al. Jul 1977 A