Abstract:
A sheet cassette is adapted to be detachably attached to a cassette receiver of an information processing apparatus. A cassette body is formed with an opened space adapted to contain sheets therein. A stopper is operable to prevent a sheet from moving in a first direction to enter the opened space when a volume of the sheets contained in the opened space reaches a maximum containing capacity, while allowing the sheets contained in the opened space to move in a second direction to be fed to the information processing apparatus.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a sheet cassette in which sheets of paper can be stacked over another and which is removably mounted in a cassette containing portion of a main body of an apparatus that uses the sheets, and the invention relates to an information processing apparatus having a cassette containing portion in and from which the sheet cassette can be mounted and removed.  
         [0002]     An information processing apparatus such as a printer, copying machine, or facsimile generally includes a sheet conveying portion for conveying a sheet of paper to an information processing portion to allow information processing such as a printing process to be performed on the sheet. Conventional information processing apparatuses generally have a configuration in which a sheet cassette for containing sheets of paper to be conveyed by such a sheet conveying portion is removably mounted in a main body of the apparatus.  
         [0003]     Various types of sheet feeding mechanisms for such information processing apparatus have been proposed, the mechanisms having a sheet cassette which is removably mounted in a cassette containing portion of an apparatus main body with sheets of papers stacked therein one over another and which has a guide wall for regulating a sheet feeding direction provided at the end thereof in the mounting direction, a sheet feeding unit which is provided on the apparatus main body such that it can contact a surface of the uppermost sheet in the cassette containing portion and which has a pickup roller for exerting a feeding force acting toward the guide wall to the surface of the sheet in contact therewith, and a control unit for controlling the operation of the sheet feeding unit. The mechanisms are configured such that the sheets contained in the sheet cassette are supplied to a conveying mechanism provided ahead of the guide wall one at a time starting with the uppermost sheet. Another proposed configuration of a sheet cassette includes a base plate for lifting up sheets of paper stacked in the sheet cassette while urging them toward a pickup roller.  
         [0004]     A maximum amount of sheets to be stacked in the sheet cassette is set in advance. When the cassette is in an overloaded state in which the number of sheets of paper exceed the specified amount, an excessive load is likely to be generated at a sheet feeding unit for feeding the sheets one at a time from the uppermost sheet. When an excessive load is generated at the sheet feeding unit, a part for driving the sheet feeding unit can be damaged, and there will be an increase in the contact pressure on the sheets. Thus, the sheets are more liable to be wrinkled or torn, which can result in errors in sheet feeding.  
         [0005]     Sheet cassette configurations have been proposed for use in sheet cassettes having a base plate as described above in order to prevent the generation of an excessive load to a sheet feeding unit by reducing the urging force of a base plate even when sheets of paper are set in the excess of the specified amount (for example, see Japanese Patent Publication No. 2000-335755A).  
         [0006]     The technique disclosed in Japanese Patent Publication No. 2000-335755A can be adopted only in sheet cassettes having a base plate for lifting up sheets of paper and cannot accommodate sheet cassettes of other types.  
         [0007]     Although most sheet cassettes have a mark indicating such a specified amount in general, they can be easily loaded with sheets of paper in the excess of the specified amount and can be mounted in an apparatus main body even in such an overloaded state. When a sheet cassette contains sheets of paper in the excess of a specified amount, an excessive load is generated at a sheet feeding unit as described above, and problems can occur including damage to a part for driving the sheet feeding unit and wrinkling or tearing of paper.  
         [0008]     Some sheet cassettes are provided with a cover to close a part of a top side of the same that is open in order to regulate a sheet containing space above the bottom of the cassette and prevent sheets of paper from being loaded in the excess of a specified amount. However, even such a cassette may be forcibly loaded with sheets of paper in the excess of the specified amount. In such a case, since the pile of paper is sandwiched between the cover and the bottom of the cassette, an error or jam can sometimes be caused by a heavy load generated when a sheet is fed even though a feeding force is exerted on the sheet by the sheet feeding unit during sheet feeding.  
         [0009]     While a sheet cassette is liable to have problems when overloaded as described above, it has not been possible to prevent sheets of paper from being set in a sheet cassette in the excess of a specified amount.  
       SUMMARY OF THE INVENTION  
       [0010]     It is therefore an object of at least one embodiment of the invention to provide a sheet cassette which can be reliably prevented from being loaded with sheets of paper in excess of a specified amount and which is capable of feeding sheets smoothly without any problem in a sheet feeding operation and to provide an information processing apparatus having the same.  
         [0011]     In order to attain the above described object, according to at least one embodiment of the invention, there is provided a sheet cassette adapted to be detachably attached to a cassette receiver of an information processing apparatus, the sheet cassette comprising:  
         [0012]     a cassette body, formed with an opened space adapted to contain sheets therein; and  
         [0013]     a stopper, being movable relative to the cassette body and operable to prevent a sheet from moving in a first direction to enter the opened space when a volume of the sheets contained in the opened space reaches a maximum containing capacity, while allowing the sheets contained in the opened space to move in a second direction to be fed to the information processing apparatus.  
         [0014]     With this configuration, the sheet cassette can be reliably prevented from being overloaded. It is therefore possible to reliably prevent problems resulting from overloading such as damage to a part for driving a sheet feeding unit of the information processing apparatus and wrinkling or tearing of paper caused by an excessive load generated at the sheet feeding unit.  
         [0015]     Further, with this configuration, an operation of feeding a sheet to the apparatus can be smoothly performed.  
         [0016]     The sheet cassette may further comprise a cassette cover, partly covering the opened space of the cassette body, wherein the stopper may be provided with the cassette cover.  
         [0017]     With this configuration, the cassette cover makes it possible to prevent sheets from being loaded in the sheet cassette when the cassette is attached to the cassette receiver. When the top side of the sheet cassette attached to the cassette receiver opens on the outside of the apparatus, sheets can be loaded from the opened space. However, the sheets can cause a feeding error or jam because they may be skewed in the sheet cassette when loaded through the narrow opening. Since the cassette cover is provided, the sheet cassette must be removed from the cassette receiver to load it with sheets. Thus, the sheets are properly set in the sheet cassette. In addition, since the stopper is provided on the cassette cover, sheets will not be caught between the cassette cover and the bottom of the cassette by being forcibly loaded in the excess of the specified amount. Thus, the loading of excess sheets can be prevented by the stopper.  
         [0018]     The first direction may be opposite to the second direction.  
         [0019]     With this configuration, a number of sheets within the specified amount can be smoothly fed while preventing excess sheets from being loaded.  
         [0020]     The stopper may comprise:  
         [0021]     a roller, adapted to come in contact with an uppermost one of the sheets contained in the space when the volume of the sheets reaches the maximum containing capacity;  
         [0022]     a support shaft, rotatably supporting the roller and movable in the first direction and the second direction; and  
         [0023]     a rotation preventing member, adapted to come in contact with the roller when the support shaft is moved in the first direction, thereby preventing the roller from rotating.  
         [0024]     With this configuration, when the loading of an excess sheet is about to occur, the outer circumferential surface of the stopper comes into contact with the excess sheet, and the roller is moved in the first direction by the frictional force generated between the excess sheet and itself. When the support shaft of the roller moves in the first direction, the rotation preventing member prevents rotation of the roller. When the roller stops rotating, since the excess sheet in contact with the outer circumferential surface of the roller cannot move in the first direction because of the frictional force, the loading of the sheet into the sheet cassette is prevented. When a sheet is about to move in the sheet feeding direction to be fed while sheets of paper within the specified amount are contained in the sheet cassette, although the outer circumferential surface of the roller is in contact with the sheet, the roller moves in the second direction because of the frictional force between the sheet and itself and becomes rotatable as the outer circumferential surface of the roller leaves the rotation preventing member. Thus, substantially no resistive force is generated against the movement of the sheet in the second direction.  
         [0025]     Thus, the stopper configured as described above reliably prevents the loading of an excess sheet and allows sheets contained within the specified amount to be smoothly fed, and it therefore constitutes a one-way stopper mechanism which works effectively in a simple configuration.  
         [0026]     The stopper also may comprise:  
         [0027]     a pivot member; and  
         [0028]     a support shaft, supporting the pivot member so as to be pivotable between a first position and a second position, wherein:  
         [0029]     the pivot member comes in contact with an uppermost one of the sheets contained in the space when the volume of the sheets reaches the maximum capacity to prevent the sheet from entering the space when the pivot member is placed in the first position; and  
         [0030]     the pivot member is separated from the uppermost sheet to allow the uppermost sheet to move in the second direction when the pivot member is placed in the second position.  
         [0031]     With this configuration, when the loading of an excess sheet is about to occur, the excess sheet comes into contact with the rotating member, and the sheet is prevented from moving further in the first direction. When a sheet is about to move in the second direction to be fed while sheets within the specified amount are contained in the sheet cassette, although the rotating member contacts the sheet, the rotating member can rotate in the direction of retracting from the sheet moving in the sheet feeding direction. Therefore, substantially no resistive force is generated against the movement of the sheet in the second direction. Thus, the stopper configured as described above reliably prevents the loading of an excess sheet and allowing sheets contained within the specified amount to be smoothly fed, and it therefore constitutes a one-way stopper mechanism which works effectively in a very simple configuration.  
         [0032]     The first direction maybe the same direction as the second direction.  
         [0033]     The stopper also may comprise: 
        a roller, adapted to come in contact with an uppermost one of the sheets contained in the space when the volume of the sheets reaches the maximum containing capacity;     a support shaft rotatably supporting the roller and movable between the first position and the second position;     a locking member, adapted to engage with the support shaft so as to prevent the roller from rotating when the support shaft is moved to the first position; and     an urging member, urging the support shaft to the first position; and        
 
         [0038]     the support shaft may be disengaged from the locking member when the support shaft is moved to the second position, thereby allowing the roller to rotate.  
         [0039]     With this configuration, the support shaft is normally urged by the urging member toward the first position, and the locking member and the support shaft are engaged with each other to stop the rotation of the roller. The roller can be easily switched from the rotation disabled condition to the rotatable condition by moving the support shaft. Therefore, when the loading of a sheet is about to occur, an excess sheet can be prevented from moving in the first direction by a frictional force originating from the roller that remains in the rotation disabled condition. When a sheet is to be fed, the roller can be switched to the rotatable condition to allow the sheet to be smoothly moved in the second direction.  
         [0040]     The support shaft may be moved in the second direction when the sheet cassette is attached to the cassette receiver.  
         [0041]     With this configuration, the stopper can be switched to the operating condition for preventing a sheet from moving and to the non-operating condition for allowing the sheet to move when the first direction and the second direction are the same. Therefore, by switching the mechanism to the operating condition when a sheet is to be loaded and to the non-operating condition when a sheet is to be fed, it is possible to feed sheets contained within the specified amount smoothly while preventing the loading of an excess sheet.  
         [0042]     According to at least one embodiment of the invention, there is also provided an information processing apparatus, comprising:  
         [0043]     a cassette receiver; and  
         [0044]     a sheet cassette, detachably attached to the cassette receiver and comprising: 
        a cassette body, formed with an opened space adapted to contain sheets therein; and     a stopper, being movable relative to the cassette body and operable to prevent a sheet from moving in a first direction to enter the space when a volume of the sheets contained in the space reaches a maximum containing capacity, while allowing the sheets contained in the space to move in a second direction to be fed to the information processing apparatus.        
 
         [0047]     With this configuration, the sheet cassette is not loaded with sheets in the excess of the specified amount, and it is possible to reliably prevent problems such as damage to a part for driving the sheet feeding unit and wrinkling or tearing of paper caused by an excessive load generated at the sheet feeding unit. Further, since the stopper of the sheet cassette does not prevent a sheet from moving in the first direction when the sheet has been once contained within the specified amount, the sheet feeding operation can be smoothly performed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0048]     The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein:  
         [0049]      FIG. 1  is an external perspective view of an embodiment of an information processing apparatus in which a sheet cassette according to the invention is mounted;  
         [0050]      FIG. 2  is a perspective view of the information processing apparatus shown in  FIG. 1 , the apparatus being shown with a top case thereof removed;  
         [0051]      FIG. 3  is a side sectional view of the information processing apparatus shown in  FIG. 1  showing a state of the same when a sheet is fed;  
         [0052]      FIG. 4  is a perspective view of a sheet cassette mounted in a cassette containing portion of the information processing apparatus shown in FIGS.  1  to  3 ;  
         [0053]      FIG. 5  is a sectional view of the sheet cassette shown in  FIG. 4 ;  
         [0054]      FIG. 6  is a perspective view of a cassette cover which is shown in  FIGS. 4 and 5 , obliquely taken from below the same;  
         [0055]      FIG. 7  is a perspective view of a stopper mechanism which is shown in  FIG. 4 , the mechanism being shown with a roller cover thereof removed;  
         [0056]      FIG. 8  is an enlarged sectional view of the stopper mechanism shown in  FIG. 5  in a state in which the mechanism prevents the loading of an excess sheet;  
         [0057]      FIG. 9  is an enlarged sectional view of the stopper mechanism shown in  FIG. 5  in a state in which the mechanism allows a sheet of paper to move when the sheet is to be fed;  
         [0058]      FIG. 10  is an enlarged sectional view of another embodiment including a configuration of the stopper mechanism that can be used in the sheet cassette according to the invention showing a state of the same in which the mechanism prevents the loading of an excess sheet;  
         [0059]      FIG. 11  is an enlarged sectional view of the stopper mechanism shown in  FIG. 10  in a state in which the mechanism allows a sheet of paper to move when the sheet is to be fed;  
         [0060]      FIG. 12  is an enlarged sectional view of another embodiment including a configuration of a stopper mechanism switchable between an operating condition and a non-operating condition that can be used in the sheet cassette according to the invention showing a state of the same in which the mechanism prevents the loading of an excess sheet;  
         [0061]      FIG. 13  is an enlarged sectional view of the stopper mechanism shown in  FIG. 12  in a state in which the mechanism allows a sheet of paper to move when the sheet is to be fed. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0062]     An embodiment of a sheet cassette and information processing apparatus according to the invention will now be described in detail with reference to the accompanying drawings.  
         [0063]     In the present embodiment, reference is made to an inkjet printer which performs a printing operation on paper as an example of the information processing apparatus. For example, information processing apparatus which can carry a sheet cassette according to the invention include printers, copying machines, and facsimiles.  
         [0064]      FIG. 1  is an external perspective view of an embodiment of an information processing apparatus in which a sheet cassette according to the invention is mounted.  FIG. 2  is a perspective view of the information processing apparatus shown in  FIG. 1 , the apparatus being shown with a top case thereof removed.  FIG. 3  is a side sectional view of the information processing apparatus shown in  FIG. 1  showing a state of the same when a sheet is fed.  FIG. 4  is a perspective view of a sheet cassette mounted in a cassette containing portion of the information processing apparatus shown in FIGS.  1  to  3 .  FIG. 5  is a sectional view of the sheet cassette shown in  FIG. 4 .  FIG. 6  is a perspective view of a cassette cover which is shown in  FIGS. 4 and 5 , obliquely taken from below the same.  FIG. 7  is a perspective view of a stopper mechanism which is shown in  FIG. 4 , the mechanism being shown with a roller cover thereof removed.  FIG. 8  is an enlarged sectional view of the stopper mechanism shown in  FIG. 5  in a state in which the mechanism prevents the loading of an excess sheet.  FIG. 9  is an enlarged sectional view of the stopper mechanism shown in  FIG. 5  in a state in which the mechanism allows a sheet of paper to move when the sheet is to be fed.  
         [0065]     A printer  1  of the present embodiment is a printer for business use of a front feed/discharge type. As shown in  FIG. 1 , a cassette containing portion  17  opens in the middle of a front face of an apparatus case  4  including a top case  2  and a bottom case  3 , and a substantially box-like sheet cassette  5  capable of containing sheets stacked over another is removably mounted in the cassette containing portion  17 .  
         [0066]     A discharge tray  6  for receiving a printed sheet is mounted above the sheet cassette  5 . Display portions  7  including LED lamps for displaying states of operation are provided on both sides of the front face of the apparatus case  4 .  
         [0067]     A black ink cover  8   a  for covering the front of a black ink containing portion containing a black ink cartridge and a color ink cover  8   b  for covering the front of a color ink containing portion containing a plurality of color ink cartridges are provided in left and right positions, respectively, under the display portions  7 .  
         [0068]     Each of the black ink cover  8   a  and the color ink cover  8   b  is mounted such that it can be opened and closed. The ink cartridges which are removably mounted therein can be accessed by opening the respective covers  8   a  and  8   b.    
         [0069]     As shown in  FIG. 2 , a button type power switch  9  for turning power to the printer  1  on and off is provided under the black ink cover  8   a.  When the power switch  9  is turned on, power is supplied to a conveying mechanism  10 , a printing process mechanism, a processing gap adjusting mechanism, a plurality of sensors, and control means to enable the printer  1  for operation.  
         [0070]     As shown in  FIG. 2 , the apparatus case  4  contains the conveying mechanism  10  which conveys sheets of paper fed one at a time from the sheet cassette  5  by a pickup roller of a sheet feeding mechanism along a conveying path, a carriage  11  which is provided on the conveying path of the conveying mechanism  10  such that it can be moved back and forth in a direction orthogonal to the sheet conveying direction, a printing head (see  FIG. 3 ) carried on a bottom surface of the carriage  11  for ejecting microscopic particles of recording ink onto a sheet of paper, and ink cartridges  12  for reserving the recording ink supplied to the printing head.  
         [0071]     As shown in  FIG. 3 , the conveying mechanism  10  forms the conveying path which extends from the sheet cassette  5  to the discharge tray  6  via an arcuate conveying path. The conveying mechanism  10  includes an ASF (automatic sheet feeder) portion and a PF (paper feeder) portion.  
         [0072]     The ASF portion includes a sheet feeding unit  22  for feeding sheets of paper P contained in the sheet cassette  5  one at a time toward the rear of the apparatus and an intermediate roller  31  and a guide portion  34  forming an arcuate conveying section which conveys the sheet P fed by the sheet feeding unit  22  along the arcuate conveying path.  
         [0073]     The PF portion includes a pair of sheet conveying rollers  41  and  42  for sandwiching the sheet P conveyed from the arcuate conveying section to convey it to a printing position (processing position).  
         [0074]     The ASF portion and the PF portion are dynamically coupled through a plurality of gears and can be driven by a single conveying motor (not shown).  
         [0075]     The sheet feeding unit  22  is configured such that a rear end of a frame  25  where a pickup roller  21  for feeding a sheet is provided can be rotated in the vertical direction in the figure by a support pin  24 . The pickup roller  21  of the sheet feeding unit  22  can be put in contact with the uppermost sheet of paper P in the sheet cassette  5  by a swing of the frame  25  in the vertical direction. When the printer  1  is at rest, the frame  25  is kept horizontal as shown in  FIG. 2 , and the pickup roller  21  of the sheet feeding unit  22  is thus kept away from the sheet P on the sheet cassette  5 . When the sheet is to be fed, a downward rotation of the frame  25  about the support pin  24  urges the pickup roller  21  against the sheet P on the sheet cassette  5 , and the rotation of the pickup roller  21  causes the sheet P on the sheet cassette  5  to abut on an inclined friction pad  23 . The sheets of paper P are separated one at a time due to the frictional force and the viscosity of the paper P and fed into a gap between the intermediate roller  31  and the guide portion  34 . When the printing portion  43  starts printing after a sheet is fed, the frame  25  is returned to the initial horizontal state as shown in  FIG. 3 .  
         [0076]     The arcuate conveying section includes the guide portion  34  which is provided outside the section (on the side of the rear of the printer  1 ) and which is curved in the form of an arc and the intermediate roller  31  which is provided inside the section (on the side of the front of the printer  1 ) and which sends a sheet of paper P upward from below the guide portion  34 . The guide portion  34  includes a retard roller  32  provided on the bottom side thereof and an assist roller  33  provided on the top side thereof. A sheet of paper P fed by the pickup roller  21  is sandwiched and conveyed by the intermediate roller  31  and the retard roller  32  and is further sandwiched and conveyed by the intermediate roller  31  and the assist roller  33 . Thus, the sheet P is conveyed along the arcuate conveying path that is formed as a gap between the intermediate roller  31  and the guide portion  34 , and the sheet is sent into a U-shaped conveying path with the leading edge thereof directed to the front of the apparatus.  
         [0077]     The pair of sheet conveying rollers  41  and  42  is provided on the front side of the apparatus in a position at a predetermined distance from an exit end of the arcuate conveying path (a downstream end of the guide portion  34 ), and the rollers sandwich the sheet P sent from the arcuate conveying path through the substantially linear path and convey it to a position under the printing portion  43  or the printing position. The sheet P can be positioned relative to the printing portion  43  by controlling the amount in which the sheet conveying rollers are driven for rotation.  
         [0078]     The printing portion  43  as a processing portion includes the carriage  11  which has a printing head  44 . The carriage  11  is supported such that it can move in the width direction of the paper on a carriage shaft  13  extending in the width direction of the paper. The carriage is also secured to a timing belt  15  driven by a carriage motor  14  and moved back and forth in the width direction of the paper as the timing belt  15  travels (see  FIG. 2 ).  
         [0079]     The printing head  44  has an ejection nozzle  46  for ejecting ink toward the sheet of paper P. A plurality of guide rollers  48  for sandwiching the sheet of paper P to guide it to the discharge tray  6  on the front side of the apparatus is provided downstream of the printing portion  43 .  
         [0080]     The printer  1  records (prints) characters and graphics on the sheet by ejecting ink from the printing head  44  carried on the carriage  11  in a sub-scanning direction that is the direction in which the sheet P is conveyed and a main scanning direction that is the moving direction of the carriage  11 .  
         [0081]     A description will now be made on the sheet cassette  5  removably mounted in the cassette containing portion  17  of the printer  1 .  
         [0082]     As shown in  FIGS. 4 and 5 , the sheet cassette  5  has a box-like structure which includes a bottom wall  51 , a left side wall  52  and a right side wall  53  erected from left and right edges of the bottom wall  51 , a front wall  54 , and a rear wall  55  and which is open on the top side thereof. As shown in  FIG. 3 , a guide wall  23  for regulating a feeding direction of sheets of paper P contained in the sheet cassette  5  is formed inside and integrally with the rear wall  55  to which the leading edge of the cassette is directed when inserted in the cassette containing portion  17 .  
         [0083]     In order to send only the uppermost sheet of paper P to the conveying mechanism  10  (see  FIG. 2 ) utilizing a difference between frictional forces acting on the top and bottom surfaces of the sheet P and the viscosity of the sheet P, as shown in  FIG. 3 , the guide wall  23  is inclined at a predetermined angle, and a part of the surface of the wall to be put in contact with the sheet is formed by a cork board having high contact friction.  
         [0084]     A cutout  56  is formed on the outside of the rear wall  55  in a position associated with cassette detecting means (not shown) provided in the cassette containing portion  17 . The cassette detecting means includes a plurality of contact-type detection sensors and detects that the sheet cassette  5  is mounted when any of the sensors contacts the sheet cassette  5 . The cutout  56  is set to be variable in shape depending on the type of the sheet cassette  5  (e.g., the type of the same depending on the size of sheets), and the plurality of detection sensors contact a sheet or not depending on the pattern of the cutout. Thus, some of the plurality of sensors will contact the rear wall  55  and some of the sensors will enter the cutout  56  instead of contacting the wall, which allows the type of the sheet cassette  5  formed with the cutout  56  in a predetermined shape to be determined.  
         [0085]     Insertion guide structures (not shown) for the mounting of the sheet cassette  5  into the cassette containing portion  17  are formed on the outside of the left side wall  52  and the right side wall  53  to allow the sheet cassette  5  to be easily and straightly inserted to a proper position.  
         [0086]     Marks  57  indicating a specified containable amount of paper which is set in advance are provided on the inside of the left side wall  52  and the right side wall  53 . That is, a user can visually determine whether the top of sheets of paper contained one over another in the sheet cassette  5  reaches the height of the marks  57 , whereby sheets of paper can be loaded such that they will not exceed the specified amount.  
         [0087]     A cassette cover  58  is mounted on the sheet cassette  5  to cover the open top side of the sheet cassette  5  when the cassette is mounted in the cassette containing portion  17 . When the top side of the sheet cassette  5  mounted in the cassette containing portion  17  opens on the outside of the printer  1 , sheets of paper can be loaded through the narrow opening while the sheet cassette  5  is kept mounted. The sheets can cause a feeding error or jam because they may be skewed in the sheet cassette  5  when loaded through the narrow opening. The cassette cover  58  prevents sheets of paper from being loaded while the sheet cassette  5  is kept mounted in the cassette containing portion  17 . When the cassette cover  58  is mounted, the sheet cassette  5  must be removed from the cassette containing portion  17  to load it with sheets of paper. Thus, the sheets of paper are properly set in the sheet cassette  5 .  
         [0088]     Since the cassette cover  58  is mounted, a front part of the top side of the sheet cassette  5  (the rear side of the cassette in the direction of insertion of the same into the cassette containing portion  17 ) is covered. Therefore, when the sheet cassette  5  is loaded with sheets of paper, the sheets are inserted from the front side of the sheet cassette  5 , which is open, toward the region under the cassette cover  58 . That is, the sheet loading direction is a direction from the right side of  FIG. 5  to the left side of the same. The direction in which sheets of paper are fed by the sheet feeding unit  22  is a direction from the left side of  FIG. 5  to the right side of the same. Thus, the sheet loading direction and the sheet feeding direction are opposite to each other.  
         [0089]     As shown in  FIGS. 5 and 6 , the cassette cover  58  is formed with inclined guide surfaces  59  for inserting sheets of paper under the cassette cover  58 , and sheets of paper are thus smoothly guided toward the region under the cassette cover  58 . As shown in  FIG. 6 , engaging portions  60  and  61  which are positioned by being engaged near the rear end of the sheet cassette  5  are formed on the bottom of the cassette cover  58 , and they allow the cassette cover  58  to be positioned and secured on the sheet cassette  5 .  
         [0090]     Stopper mechanisms  65  are provided in positions which are adjacent to the guide surfaces  59  of the cassette cover  58  in the sheet loading direction, the mechanisms preventing an excess sheet which is about to be loaded in the excess of the specified amount for the sheet cassette  5  from moving in the sheet loading direction and allowing a sheet of paper to move in the sheet feeding direction.  
         [0091]     As shown in FIGS.  5  to  9 , the stopper mechanisms  65  includes rollers  66  which contact the excess sheet on outer circumferential surfaces thereof to generate a frictional force, support shafts  67  which constitute rotating shafts of the rollers  66  and for which a movable range A of a predetermined distance is set in the sheet loading direction and the sheet feeding direction, and rotation stopping portions  68  which contact the outer circumferential surfaces of the rollers  66  to prevent the rollers  66  from rotating when the support shafts  67  are at the end of the movable range A in the sheet loading direction.  
         [0092]     The rollers  66  are provided in openings  71  (see  FIG. 8 ) formed on the cassette cover  58  such that they can be put in contact with sheets of paper P inserted to the region under the cassette cover  58 . Lower ends of the rollers  66  are in positions which substantially agree with the specified height of the sheets of paper P, and they contact an excess sheet P 1  which is about to be loaded in the excess of the specified amount. The outer circumferential surfaces of the rollers  66  are lined with rubber, and the outer circumferential surfaces generate a frictional force between a sheet of paper and themselves when they contact the sheet of paper. The support shafts  67  constituting rotating shafts of the rollers  66  are supported in recesses  70  formed on the cassette cover  58  so as to extend in the direction of the axes of the rollers  66 . The recesses  70  support the support shafts  67  such that the shafts can move in the sheet loading direction and the sheet feeding direction.  
         [0093]     The rotation stopping portions  68  are positioned so as to abut on the outer circumferential surfaces of the rollers  66  when the support shafts  67  are moved in the recesses  70  to predetermined positions in the sheet loading direction, and they stop the rotation of the rollers  66  with a frictional force when they abut on the rollers  66 . The rotation stopping portions  68  have inclined surfaces in positions thereof in contact with the rollers  66  to exert a force which moves the rollers  66  toward the recesses  70  when they abut on the rollers. Thus, the rotation of the rollers  66  is prevented and, in addition, the support shafts  67  are urged against the recesses  70  to prevent the rollers  66  from being lifted up by a sheet of paper. Therefore, the rollers  66  can be kept stable in the rotation disabled state.  
         [0094]     When the rollers  66  abut on the rotation stopping portions  68 , the movement of the support shafts  67  is obviously limited. Specifically, when the rollers  66  abut on the rotation stopping portions  68 , the support shafts  67  are located at the extremities in the sheet loading direction up to which the shafts  67  can move. The extremities to which the support shafts  67  can move are determined by the end of the recesses  70  (the right ends in  FIG. 8 ). As thus described, the movable range A of a predetermined distance is set for the support shafts  67  in the sheet loading direction and sheet feeding direction.  
         [0095]     In order to prevent the support shafts  67  from coming out the recesses  70  and the rollers  66  from jumping up obliquely to the sheet feeding direction (upward and to the right in  FIG. 8 ) consequently, the stopper mechanisms  65  are provided with roller covers  69 .  FIG. 7  shows the mechanisms with the roller covers  69  removed.  
         [0096]     When the excess sheet P 1  is about to be loaded, the outer circumferential surfaces of the rollers  66  of the stopper mechanisms  65  contact the excess sheet P 1 , and the rollers  66  are rotated by a frictional force between the excess sheet P 1  and themselves to move in the sheet loading direction. When the support shafts  67  of the rollers  66  move to the end of the movable range A in the sheet loading direction, the outer circumferential surfaces of the rollers  66  abut on the rotation stopping portions  68  to stop the rotation of the rollers  66 . When the rollers  66  stop rotating, the excess sheet P 1  in contact with the outer circumferential surfaces of the rollers  66  is prevented by the frictional force from moving any further in the sheet loading direction, whereby the loading of the sheet into the sheet cassette  5  is prevented. It is therefore possible to reliably prevent problems resulting from overloading such as damage to the part for driving the sheet feeding unit  22  and wrinkling or tearing of the paper P being fed caused by an excessive load generated at the sheet feeding unit  22 .  
         [0097]     When the uppermost sheet of paper P is about to move in the sheet feeding direction to be fed while sheets of paper P within the specified amount are contained in the sheet cassette  5 , although the outer circumferential surfaces of the roller  66  are in contact with the sheet P, the rollers move in the sheet feeding direction because of the frictional force between the sheet P and the rollers and become rotatable as the outer circumferential surfaces of the rollers  66  leave the rotation stopping portions  68 . Even when the support shafts  67  are moved to the end of the movable range A in the sheet feeding direction, the rollers  66  are kept in a rotatable state in that position. Thus, the stopper mechanisms  65  generate substantially no resistive force against the movement of the sheet P in the sheet feeding direction.  
         [0098]     Thus, the stopper mechanisms  65  configured as described above reliably prevent the loading of an excess sheet P 1  and allows smooth feeding of sheets of paper P which are contained within the specified amount, and they constitute one-way stopper mechanisms which work effectively in a simple configuration.  
         [0099]     Since the stopper mechanisms  65  of the present embodiment are provided on the cassette cover  58 , an excess sheet P 1  will not be caught between the cassette cover  58  and the bottom wall  51  by being forcibly loaded in the excess of the specified amount. Thus, the loading of the excess sheet P 1  can be reliably prevented.  
         [0100]     As described above, when the sheet loading direction and the sheet feeding direction are opposite to each other, the one-way stopper configuration may be employed to prevent the excess sheet P 1  from moving in the sheet loading direction. Then, sheets of paper P contained within the specified amount can be smoothly fed while preventing an excess sheet P 1  from being loaded. The configuration of the one-way stopper mechanism is not limited to that of the above-described stopper mechanism  65 , and stopper mechanisms in various configurations maybe employed. For example, a roller incorporating a one-way clutch may be used as a stopper mechanism.  
         [0101]      FIGS. 10 and 11  show an example of a one-way stopper mechanism which can be used in the sheet cassette according to another embodiment of the invention.  
         [0102]     A stopper mechanism  75  shown in  FIGS. 10 and 11  includes a rotating member  76  which is rotatably pivoted and a support member  78  for supporting the rotating member  76  in a position where the rotating member abuts on an excess sheet P 1  to prevent it from moving in the sheet loading direction (the direction from the right side of the figures to the left side of the same). The rotating member  76  can rotate in the direction of retracting from a sheet of paper P moving in the sheet feeding direction (the direction from the left side of the figures to the right side of the same).  
         [0103]     The rotating member  76  is in the form of a flap piece disposed in an opening  71  formed on the cassette cover  71 , and it is rotatably supported by a support shaft  77  at a top end thereof. The support shaft  77  is rotatably positioned directly above the opening  71 . A support member  78  is integrally supported by the support shaft  77  to be rotatable along with the rotating member  76 , and it is normally disposed above an edge of the opening  71  and supported such that it is always positioned higher than the opening  71 . The rotating member  76  is supported by the support member  78  so as to protrude a predetermined distance below the opening  71  at a predetermined angle. The end of the rotating member  76  is positioned at a height H that is the specified amount of sheets of paper P, and the end abuts on an excess sheet P 1  which is about to be loaded in the excess of the specified amount.  
         [0104]     In order to reliably prevent the excess sheet P 1  from moving in the sheet loading direction when the rotating member  76  abuts on the excess sheet P 1 , an abutting portion  76   a  is provided in a position where it is opposite to the sheet loading direction in order to abut on the excess sheet P 1  substantially at a right angle. The rotating member  76  is also provided with a slide-contact portion  76   b  formed with a surface having a great curvature in a position where it is opposite to the sheet feeding direction.  
         [0105]     In the stopper mechanism  75 , when there is an excess sheet P 1  which is about to be loaded in the excess of the height H or the specified amount, the excess sheet P 1  abuts on the abutting portion  76   a  of the rotating member  76 , and it is prevented from moving in the sheet loading direction further.  
         [0106]     Let us assume that sheets of paper P within the specified amount are contained in the sheet cassette  5 , and a sheet of paper P is about to move in the sheet feeding direction to be fed. Then, although the rotating member  76  contacts the sheet, no resistive force is generated against the movement of the sheet P in the sheet feeding direction because the member is rotatable in the direction of retracting from the sheet P moving in the sheer feeding direction as shown in  FIG. 11  (upward). As thus described, the stopper mechanism  75  reliably prevents the loading of an excess sheet P 1  and allows the sheets of paper P contained to or below the height H or the specified amount to be smoothly fed, and it therefore constitutes a one-way stopper mechanism which works effectively in a quite simple configuration.  
         [0107]     A description will be made on a configuration of a stopper mechanism to be used when a sheet loading direction and a sheet feeding direction are the same.  
         [0108]     A sheet loading direction and a sheet feeding direction may be the same when the sheet cassette  5  has a configuration in which the front wall  54  extends further forward (to the left in  FIG. 5 ) or a configuration in which the front wall  54  opens and closes. A stopper mechanism to be used in such a case is required to be switchable between an operating condition for preventing a sheet from moving regardless of the moving direction of the sheet and a non-operating condition for allowing a sheet to move. That is, a stopper mechanism switchable between an operating condition for preventing a sheet from moving and a non-operating condition for allowing a sheet to move may be employed and switched to the operating condition when a sheet is loaded and to the non-operating condition when the sheet cassette is mounted or a sheet is fed from the same. Thus, sheets of paper contained within a specified amount can be smoothly fed while preventing the loading of an excess sheet.  
         [0109]      FIGS. 12 and 13  show an example of a stopper mechanism switchable between an operating condition and a non-operating condition.  
         [0110]     First,  FIG. 12  shows a stopper mechanism  85  in an operating condition. The stopper mechanism  85  includes a roller  86  which contacts an excess sheet P 1  being loaded in the direction of the arrow B at an outer circumferential surface thereof to generate a frictional force, a support shaft  87  which constitutes a rotating shaft of the roller  86  and for which a predetermined movable range is set, a lock portion  89  which engages the support shaft  87  to stop the rotation, and an urging portion  90  for urging the support shaft  87  toward the lock portion  89 . The roller is switched from a rotation disabled condition to a rotatable condition as shown in  FIG. 13  by moving the support shaft  87  against the urging force of the urging portion  90  to disengage it from the lock portion  89 .  
         [0111]     The roller  86  is disposed in an opening formed on the cassette cover  58 , and it can be put in contact with sheets of paper P inserted below the cassette cover  58 . A lower end of the roller  86  is in a position which is substantially the same the height H that is the specified amount of sheets of paper P, and the end abuts on an excess sheet P 1  which is about to be loaded in the excess of the specified amount. An outer circumferential surface of the roller  86  is lined with rubber, and the outer circumferential surface generates a frictional force when put in contact with the sheet.  
         [0112]     The support shaft  87  constituting the rotating shaft of the roller  86  extends in the axial direction of the roller  86  to be supported in a recess  93  formed on the cassette cover  58 . In the recess  93 , the support shaft  87  is supported so as to be movable within a predetermined range in the sheet loading direction and the sheet feeding direction. The support shaft  87  is always urged in the sheet feeding direction by an urging portion  90  which is energized by repulsion of a spring  91  supported on a based member  92  positioned on the cassette cover  58 . The lock portion  89  which is formed with an engaging groove  89   a  is provided opposite to the urging portion  90 , and a protrusion  88  formed on the support shaft  87  is urged by the urging force of the urging portion  90  into engagement with the engaging groove  89   a  to prevent the support shaft  87  from rotating.  
         [0113]     As a result, the stopper mechanism  85  is in a rotation disabled state in which the rotation of the roller  86  is stopped. When the roller  86  is in the rotation disabled state, an excess sheet P 1  inserted in the excess of the specified amount, the roller contacts the outer circumferential surface of the roller  86 . Since the sheet is disallowed to move in the sheet loading direction further by a frictional force between the roller  86  and itself, the loading of the sheet in the sheet cassette  5  is prevented.  
         [0114]     In a printer  1  having a cassette containing portion  17  in and from which a sheet cassette  5  having such a stopper mechanism  85  can be mounted and removed, switching means is preferably provided to put the stopper mechanism  85  in the non-operating condition at least when a sheet is to be fed.  
         [0115]     For example, as shown in  FIG. 13 , a lever member  95  may be provided in the cassette containing portion as switching means for moving the support shaft  87  against the urging force of the urging portion  90  to disengage the support shaft  87  from the lock portion  89  with the sheet cassette  5  mounted in the cassette containing portion  17 . In this case, since the stopper mechanism  85  is automatically switched to the non-operating condition in conjunction with an operation of mounting the sheet cassette  5 , a sheet feeding operation can be smoothly performed without being hindered by the stopper mechanism. Referring to  FIG. 13 , since the roller  86  can rotate in both directions, the sheet loading direction indicated by the arrow B and the sheet feeding direction indicated by the arrow C for a sheet in the sheet cassette can be made the same.  
         [0116]     In a printer  1  having a sheet cassette  5  having such a stopper mechanism  85  and the lever member  95 , the stopper mechanism  85  is in the operating condition to prevent the loading of an excess sheet P 1  when the sheet cassette  5  is removed from the cassette containing portion  17 . When the sheet cassette  5  is mounted in the cassette containing portion  17 , the stopper mechanism  85  is in the non-operating condition to allow a sheet to be smoothly fed without preventing the sheet feeding. Therefore, the stopper mechanism  85  can be always kept in a proper condition by the operation of mounting and removing the sheet cassette  5 .  
         [0117]     The lever  95  may switch the stopper mechanism  85  to the non-operating condition in an occasion other the mounting of the sheet cassette  5  in the cassette containing portion  17 . For example, the lever  95  may operate in conjunction with a sheet feeding operation of the sheet feeding unit  22  to switch the stopper mechanism  85  to the non-operating condition.  
         [0118]     The sheet cassette  5  having the stopper mechanism  85  can be used also in a printer which has no lever member  95 . In this case, a member to substitute the lever member  95  may be provided on the sheet cassette  5  to switch the stopper mechanism  85  to the non-operating condition before the cassette is mounted in the cassette containing portion  17 .