Patent Publication Number: US-8118298-B2

Title: Sheet feeding device and image forming apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application NO. 2009-130437, which was filed on May 29, 2009, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The present invention relates to a sheet feeding device capable of feeding a sheet such as paper, and an image forming apparatus using the sheet feeding device. 
     A general sheet feeding device used in an image forming apparatus or the like includes a pickup roller which comes in contact with a paper sheet, which is positioned on an upper end, in a stack direction, of a stack of a plurality of paper sheets held in a holding unit, such as a paper supplying cassette. The pickup roller rotates to transport the paper sheet to an image forming unit. 
     The pickup roller is configured to apply a transport force to a paper sheet positioned on the upper end in the stack direction while rotating in contact with the paper sheet. For this reason, as the position of the paper sheet positioned on the upper end in the stack direction is separated from the pickup roller due to the decrease in the number of the paper sheets in the holding unit, the contact surface pressure between the pickup roller and the paper sheet decreases. As a result, sufficient transport force is not applied to the paper sheet causing transport performance to be deteriorated. 
     Consequently, in order to reliably transport a sheet such as paper, it is preferable that the contact surface pressure between the pickup roller and the paper sheet positioned on the upper end in the stack direction is constantly maintained without large fluctuations. 
     In this regard, for example, a related apparatus maintains a substantially constant contact surface pressure by installing a paper pressing plate, which displaces all the paper upwardly so that all the loaded paper sheets remain close to the pickup roller as the number of the loaded paper sheets decreases. 
     Further, for example, another related apparatus maintains the contact surface pressure to be substantially constant by displacing a stationary end (an end fixed to a body side of an apparatus) of a tension spring which generates contact surface pressure in cooperation with an operation of a solenoid, wherein the solenoid displaces the pickup roller to bring the pickup roller in contact with the paper sheet or to separate the pickup roller from the paper sheet. 
     SUMMARY 
     However, since the related apparatus requires a mechanism for upwardly displacing the paper pressing plate as the number of the paper sheets decreases, there creates a problem in that it is difficult to reduce a manufacture cost of the sheet feeding device. 
     In addition, the other related apparatus displaces the stationary end of the tension spring in cooperation with the operation of the solenoid, but this is unrelated to displacing the stationary end of the tension spring as the number of the loaded paper sheets decreases. Therefore, it is difficult to maintain substantially constant contact surface pressure. 
     An object of an exemplary embodiment of the present invention is to reduce manufacturing costs of a sheet feeding device by eliminating a mechanism for upwardly displacing a paper pressing plate and to reliably transport a paper sheet by suppressing significant fluctuations in the contact surface pressure between the paper sheet and a pickup roller. 
     In order to achieve the object, the exemplary embodiment of the present invention provides a sheet feeding device comprising: 
     a holding unit configured to hold a stack of sheets; 
     a pickup roller disposed in contact with a sheet placed on top of the stack held in the holding unit, the pickup roller being configured to rotated and feed the sheet from the stack; 
     a holder arm having a distal end to which the pickup roller is attached, wherein the holder arm has a swing center set at a position closer, in a stack direction of the stack, to the pickup roller side than the holding unit, and the distal end of the holder arm extends from the swing center to the holding unit side; and 
     a resiliently-deformable unit, which generates a swing moment to swing the holder arm in a direction which increases contact surface pressure between the sheet held in the holding unit and the pickup roller, 
     wherein the swing moment increases as a contact angle increases, 
     wherein the contact angle is an angle between an extension direction of the holder arm and the sheet held in the holding unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a central cross-sectional view of an image forming apparatus according to an embodiment of the invention. 
         FIG. 2  is an enlarged perspective view of a paper feeding device according to an embodiment of the invention. 
         FIG. 3  is a plan view of a paper feeding device according to an embodiment of the invention. 
         FIG. 4  is a view of a paper feeding device, which is viewed at a horizontal direction, according to an embodiment of the invention, when a holding unit holds a full stack of sheets. 
         FIG. 5  is a view of a paper feeding device, which is viewed at a horizontal direction, according to an embodiment of the invention, when a holding unit is empty. 
         FIG. 6  is an enlarged perspective view of a paper feeding device including a paper supplying tray according to an embodiment of the invention. 
         FIG. 7  is a graph illustrating a relationship between a swing moment and a contact angle. 
         FIG. 8  is a graph illustrating a relationship between a normal force and the contact angle. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In this embodiment, a sheet feeding device is applied to a paper feeding device of an image forming apparatus, and an embodiment of the invention will be described with reference to the accompanying drawings. 
     1. Broad Configuration of an Image Forming Apparatus 
     An image forming apparatus  1  includes, as shown in  FIG. 1 , an image forming unit  2  and a paper feeding device  10 . The image forming unit  2  forms (prints) an image on a paper sheet, an OHP sheet or the like (hereinafter referred to as a paper sheet), and the paper feeding device  10  feeds the paper sheet to the image forming unit  2 . 
     The image forming unit  2  according to this embodiment is an electrophotograph-type image forming unit comprising a process cartridge  3 , an exposure unit  4 , and a fixing unit  5 . The process cartridge  3  is equipped with a photosensitive drum  3 A carrying a developer image, and a charging unit  3 B to electrostatically charge the photosensitive drum  3 A. 
     The paper sheet fed from the paper feeding device  10  towards the image forming unit  2  passes between a paper powder collecting roller  6 A and a transport roller  6 B where paper powder is removed from the paper sheet, and is then transported to a pair of registration rollers  7 , so that a skew is corrected by the pair of registration rollers  7 . Then, the paper sheet is transported to the photosensitive drum  3 A. In this instance, the transport roller  6 B presses the paper sheet against the paper powder collecting roller GA. 
     The charged photosensitive drum  3 A is exposed to light by the exposure unit  4 , so that an electrostatic latent image is formed on an outer circumference of the photosensitive drum. A developer (i.e., powder type toner in this embodiment) is supplied onto the photosensitive drum  3 A and a developer image is carried or formed on the outer circumference of the photosensitive drum  3 A. 
     In this instance, the transported paper sheet is interposed between the photosensitive drum  3 A and a transfer roller  8 , which is placed opposite to the photosensitive drum  3 A, and a charge, which has a polarity opposite to the polarity of the charge applied to the developer, is applied to the transfer roller  8  so that the developer image carried on the photosensitive drum  3 A is transferred onto the paper sheet. 
     The developer transferred on the paper sheet is then heated and fixed to the paper sheet in the fixing unit  5 . A feeding direction of the paper sheet with the image formed thereon is converted in an upward direction, and the paper sheet is discharged to a paper discharge tray  9  disposed at an upper end side of the image forming apparatus  1 . 
     2. Detailed Structure of the Paper Feeding Device 
     The paper feeding device  10  is a device capable of separating one at a time the paper sheet positioned on an upper end (i.e., the uppermost end in a vertical direction in this embodiment) in a stack direction of a plurality of paper sheets held in the holding unit  11 A of the paper supplying tray  11 , as shown in  FIG. 1 , and transporting and supplying the paper sheet to the image forming unit  2  side. In this embodiment, the paper supplying tray  11  is detachably mounted on an apparatus body (i.e., body frame or housing) to which the image forming unit  2  or the like are assembled. 
     The pickup roller  12  is configured to feed the paper sheet by rotating in a state in which the pickup roller  12  comes in contact with the paper sheet positioned on the uppermost end held in the holding unit  11 A. The pickup roller  12  is assembled to a holder body  13 A in a rotatable manner, as shown in  FIG. 2 . 
     The holder body  13 A is formed as a rectangular frame of resin, which extends substantially parallel with a feeding direction of the paper sheet when seen from an upper surface side, as shown in  FIG. 3 . One end side (a downstream side of the feeding direction in this embodiment) of the holder body  13 A in the extending direction is swingably attached to the body frame  1 A through a swing shaft  14 , as shown in  FIG. 2 . 
     The swing shaft  14  is installed at a position closer to the pickup roller  12  than the holding unit  11 A (i.e., the upward side of the paper supplying tray  11  in this embodiment), as shown in  FIG. 4 . Meanwhile, the pickup roller  12  is attached to a distal end side of the holder body  13 A, which extends from the swing shaft  14  toward the holding unit  11 A side. 
     Further, a spring  15  is a resiliently-deformable unit, which is configured to generate a moment (hereinafter, this moment is referred to as a swing moment M 1 ) to swing the holder body  13 A in a direction which increases the contact surface pressure between the paper sheet held in the holding unit  11 A and the pickup roller  12 . 
     The spring  15  is a coil-type tension spring, of which one axial end is connected to a distal end side of a hook arm  13 B, which extends in a diameter direction of the swing shaft  14  from a swing center O 1  of the holder body  13 A, and the other axial end is fixed to the body frame  1 A. 
     As shown in  FIG. 2 , the hook arm  13 B is attached to the swing shaft  14 , in a swingable manner, at a position which deviates from the holder body  13 A in an axial direction of the swing shaft  14 . As shown in  FIG. 3 , the hook arm  13 B is integrally formed with a pressing lever  13 C which extends from the swing shaft  14  towards the pickup roller  12  side and comes in contact with the holder body  13 A, as shown in  FIG. 3 . 
     The pressing lever  13 C is formed of a substantially L shape, and comprises an arm portion  13 E extending from the swing shaft  14  toward the pickup roller  12  side, and a pressing portion  13 F extending in parallel with the axial direction of the pickup roller  12  from the distal end of the arm portion  13 E. The pressing portion  13 F comes in contact with a portion of the holder body  13 A closer to the pickup roller  12  side. Meanwhile, the holder body  13 A is provided with a pressure receiving portion  13 D, which comes in contact with the pressing portion  13 F to receive a swing moment M 1  from the pressing lever  13 C. 
     For this reason, in this embodiment, the swing moment M 1  of the spring  15  acts on the holder body  13 A via the pressing lever  13 C, and the holder body  13 A receives the swing moment M 1  via the pressure receiving portion  13 D, so that the paper sheet is pressed by the pickup roller  12 . In this embodiment, the holder body  13 A, the hook arm  13 B, and the pressing lever  13 C constitute a holder arm  13 . 
     In this instance, the expression ‘a portion of the holder body  13 A closer to the pickup roller  12  side’ means a portion of the holder body  13 A closer to the pickup roller  12  side than an intermediate (center) portion from the swing shaft  14  to the pickup roller  12 . 
     As shown in  FIG. 4 , a connection portion  15 A of the spring  15  and the hook arm  13 B, and a stationary end  15 B of the spring  15  and the body frame  1 A are all set at a side of a contact portion P 1  opposite to the holding unit  11 A of the paper supplying tray  11 . In the contact portion P 1 , the pickup roller  12  contacts the uppermost sheet held in the holding portion  11 A, That is, the connection portion  15 A and the stationary end  15 B of the spring  15  are set above the holding unit  11 A. 
     Further, a separation mechanism  16  is provided at a downstream side of the pickup roller  12  in the feeding direction of the paper sheet. The separation mechanism  16  separates plural paper sheets fed by the pickup roller  12  and supplies the paper sheet to the pair of registration rollers  7 . The separation mechanism  16  includes a separation pad  16 A which comes in contact with the paper sheet fed by the pickup roller  12  to apply a predetermined transport resistance to the paper sheet, and a separation roller  16 B, which rotates while pressing the paper sheet against the separation pad  16 A. 
     The separation roller  16 B receives a rotation force from the swing shaft  14  and is thus rotated in a state in which the separation roller  16 B is supported by the swing shaft  14 . Meanwhile, the separation pad  16 A is attached to a wall portion of the paper supplying tray  11  in a swingable manner via the pad holder  16 C, as shown in  FIG. 4 . 
     In this instance, the pad holder  16 C is pressed against the separation roller  16 B by a spring  16 D, and the contact surface pressure between the separation pad  16 A and the paper sheet is automatically adjusted by the spring  16 D so as to set the pressure in a predetermined range. 
     In this structure, the separation roller  16 B is integrally formed with the swing shaft  14 , so that the separation roller  16 B directly receives rotation force from the swing shaft  14 . However, the pickup roller  12  is rotated by a rotation force received from the swing shaft  14  via an intermediate roller  12 A, which is rotatably attached to the holder body  13 A, as shown in  FIG. 3 . 
     3. Operation of the Paper Feeding Device 
     As described above, the swing moment M 1  always acts on the holder arm  13 , so that the pickup roller  12  is always pressed toward the paper sheet (i.e., a bottom portion of the holding unit  11 A). As shown in  FIGS. 4 and 5 , the holder arm  13  swings so that the pickup roller  12  continuously comes in contact with the paper sheet positioned on the uppermost end of the stack, irrespective of the number of paper sheets (a height H 1  of the stack direction) held in the holding unit  11 A. 
     In this embodiment, an angle θ 2  of the hook arm  13 B, a position of the stationary end  15 B of the spring  15 , and the like are set in order to increase the swing moment M 1  as a contact angle θ 1  between an extending direction L 1  of the holder arm  13  and the paper sheet held in the holding unit  11 A increases. 
     The extending direction L 1  of the holder arm  13  represents a direction of a straight line connecting the swing center O 1  and a center of rotation of the pickup roller  12 . The angle θ 2  of the hook arm  13 B represents an angle between the extending direction L 1  extending from the holder arm  13  and a straight line L 2  extending from the swing center O 1  to an operating point P 2 , on which the resilient force of the spring  15  acts. In this connection, since the paper sheet is substantially horizontally held in the holding unit  11 A, an angle θ 3  between a horizontal straight line L 3  passing through the swing center O 1  and the extending direction L 1  extending from the holder arm  13  is identical to the contact angle θ 1 . 
     That is, in this embodiment, the stationary end  15 B of the spring  15  is set at a position closer to the pickup roller  12  side than the swing center O 1 . The angle θ 2  of the hook arm  13 B is a fixed value which is set as an angle larger than an angle of 90 degrees. In the case in which the number of the paper sheets held in the holding unit  11 A is large (i.e., the holding unit  11 A holds a full stack of paper sheets), as shown in  FIG. 4 , the contact angle θ 1  decreases, and an angle between the extending direction L 1  extending from the holder arm  13  and an axis L 4  of the spring  15  decreases so as to be substantially in parallel with the extending direction L 1 . In this instance, the axis L 4  of the spring  15  is an imaginary line passing the connection portion  15 A (i.e., the operating point P 2 ) and the stationary end  15 B. 
     For this reason, as the contact angle θ 1  increases, as shown in  FIG. 5 , a moment angle θ 4  approaches 90 degrees, but as the contact angle θ 1  decreases, as shown in  FIG. 4 , the moment angle θ 4  decreases. Herein, the moment angle θ 4  is defined as an angle between a direction of a resilient force F 1  provided by the spring  15  and the straight line L 2  extending from the swing center O 1  to the operating point P 2 . 
     In this embodiment, the distance between the swing center O 1  and the operating point P 2  is constant, and as the contact angle θ 1  increases, the moment angle θ 4  increases. As a result, as the contact angle θ 1  increases, the distance L O  (a value obtained by multiplying sin θ 4  and the distance between the swing center O 1  and the operating point P 2 ) between the swing center O 1  and the axis L 4  of the spring  15  extends. 
     Accordingly, in this embodiment, the swing moment M 1  determined by the multiplication of the distance L O  and the resilient force F 1  increases as the contact angle θ 1  increases. 
     In this instance, if the contact angle θ 1  (the moment angle θ 4 ) increases, since the connecting portion  15 A (the operating point P 2 ) approaches the stationary end  15 B, the resilient force F 1  decreases, and thus the swing moment M 1  determined by the multiplication of the distance L O  and the resilient force F 1  may also decrease. Accordingly, in this embodiment, a resilient coefficient of the spring  15  is set to a low value to prevent the swing moment M 1  from decreasing with the increase in the contact angle θ 1 . 
     In connection with this, when the paper supplying tray  11  is detached from the apparatus body, the pickup roller  12  may interfere with a lateral wall of a front surface side of the paper supplying tray  11 . Accordingly in this embodiment, a retraction mechanism (not shown) retracts the pickup roller  12  and the holder arm  13  towards the apparatus body (i.e., the upper side of the paper supplying tray  11  in this embodiment) in cooperation with a detaching operation of the paper supplying tray  11 . 
     4. Characteristic of the Image Forming Apparatus (in Particular, the Paper Feeding Device) According to this Embodiment 
     In this embodiment, if the number of the paper sheets held in the holding unit  11 A varies, the contact angle θ 1  also varies. Since a normal force acting on the paper sheet by the pickup roller  12  is equal to (swing moment M 1 ×cos θ 1 )/(a distance between the swing center O 1  and the swing center of the pickup roller  12 ), if the contact angle θ 1  increases while the swing moment M 1  is constant, the normal force exerted on the paper sheet by the pickup roller  12 , i.e. the contact surface pressure, starts to decrease. 
     However, this embodiment is configured in such a way that as the contact angle θ 1  increases, the swing moment M 1  increases, as shown in  FIG. 7 . Therefore, as the contact angle θ 1  increases, it is possible to suppress the normal force from being decreased, as shown in  FIG. 8 , and to thus suppress the contact surface pressure from being decreased. 
     Consequently, it is possible to reduce the manufacture cost of the paper feeding device  10  (the image forming apparatus  1 ) by eliminating a mechanism for upwardly displacing a paper pressing plate or the like. In addition, it is possible to transport the paper sheet reliably by suppressing the contact surface pressure between the paper sheet and the pickup roller  12  from remarkably varying. 
     In this embodiment, by setting the normal force as a reference value when the contact angle θ 1  is minimized (i.e., the holding unit  11 A holds a full stack of paper sheets), the angle θ 2  of the hook arm  13 B, the position of the stationary end  15 B of the spring  15  and the like are set in such a way that a variance of the normal force is within a range of ±20% of the reference value. 
     If the connection portion  15 A (the operating point P 2 ) for connecting the spring  15  with the hook arm  1313  and the stationary end  15 B of the spring  15  are set on the same side as the holding unit  11 A, with respect to the contact portion P 1  where the pickup roller  12  contacts the paper sheet, the spring  15  has to be installed at a position which deviates from the holding unit  11 A in a direction parallel with the paper sheet (e.g., a vertical direction of a paper surface in  FIG. 4 ). As a result, since a space for installing the spring  15  is newly required, the size of the paper feeding device may be increased. 
     In this embodiment, since the connection portion  15 A and the stationary end  15 B are installed at a side of the swing center O 1  opposite to the holding unit  11 A, that is, at the upper side of the holding unit  11 A, the spring  15  can be installed by using the existing space, thereby preventing the paper feeding device from increasing in size. 
     Additionally, in this embodiment, since the holder arm  13  swings around the rotation center of the separation roller  16 B as the swing center, for example, it is possible to decrease the size of the paper feeding device  10 , compared with a case where the swing center O 1  of the holder arm  13  is set at a portion other than the rotation center of the separation roller  16 B. 
     Further, in this embodiment, since the swing moment M 1  of the spring  15  acts on the holder body  13 A via the pressing lever  13 C, it is possible to reliably press the pickup roller  12  against the paper sheet, in a case where bending stiffness of the holder body  13 A is not large. 
     That is, if the hook arm  13 B is integrally formed with the holder body  13 A, the swing moment M 1  is transmitted to the pickup roller  12  via the holder body  13 A. As a result, in the case where the bending stiffness of the holder body  13 A is not large, the holder body  13 A is deformed, so that the swing moment M 1  may not be reliably transmitted to the pickup roller  12 . 
     In this embodiment, since the swing moment M 1  acts on the holder body  13 A via the pressing lever  13 C, the swing moment M 1  can be transmitted to the pickup roller  12 , without being greatly influenced by the bending stiffness of the holder body  13 A. Therefore, it is possible to reliably press the pickup roller  12  against the paper sheet. 
     Further, in this embodiment, since the pressing lever  13 C comes in contact with a portion of the holder body  13 A closer to the pickup roller  12  side and the swing moment M 1  acts on the holder body  13 A, when the bending stiffness of the holder body  13 A is not large, it is possible to suppress the holder body  13 A from being largely deformed, thereby pressing reliably the pickup roller  12  against the paper sheet. 
     In addition, since the pressing lever  13 C is provided with the pressing portion  13 F extending in parallel with an axial direction of the pickup roller  12 , it is possible to press the whole pickup roller  12  against the paper sheet uniformly and thus feed the paper sheet reliably. 
     In this embodiment, since the hook arm  13 B is attached to a position which deviates from the holder body  13 A in the axial direction of the swing shaft  14 , the hook arm  13  does not interfere with the spring  15  and the holder body  13 A, when the holder arm  13  swings. 
     Consequently, for example, when the spring  15  is placed in such a way that the spring  15  overlaps the holder body  13 A when seen from a vertical direction (i.e., a stack direction of the paper sheet), a degree of flexibility in design increases, and thus a development term can be shortened. 
     5. Corresponding Relationship Between Subject Matters of the Invention and the Embodiment 
     In this embodiment, the paper feeding device  10  corresponds to a sheet feeding device set forth in the claims, and the spring  15  corresponds to a resiliently deformable unit set forth in the claims. 
     Other Embodiments 
     In the above embodiment, the holder arm  13  comprises the holder body  13 A, the hook arm  13 B and the pressing lever  13 C, and the swing moment M 1  of the spring  15  is transmitted to the holder body  13 A via the pressing lever  13 C. However, the invention is not limited thereto, and the hook arm  13 B may be integrally formed with the holder body  13 A to transmit the swing moment M 1  to the holder body  13 A without passing through the pressing lever  13 C. 
     Further, although the above embodiment relates to the sheet feeding device in which the paper sheet is loaded in a substantially horizontal direction and is stacked in a vertical direction, the invention is not limited thereto. For example, the stack direction of the paper sheet may be inclined with respect to the vertical direction. 
     In addition, although the spring  15  serving as the resiliently deformable unit is a coil-type torsion spring in the above embodiment, the invention is not limited thereto. For example, other resiliently deformable units such as a compression spring or a torsion coil spring may alternatively be employed. 
     Moreover, although the connection portion  15 A (the operating point P 2 ) is placed at a fixed position with respect to the hook arm  13 B in the above embodiment, the invention is not limited thereto. For example, the swing moment M 1  may be varied by providing a cam groove with the connection portion  15 A to vary the position of the connection portion  15 A, that is, the distance between the swing center O 1  and the operating point P 2 , according to the variation of the contact angle θ 1 . 
     Further, although the swing center O 1  of the holder arm  13  and the rotation center of the separation roller  16 B are coincident with each other, the invention is not limited thereto. 
     Although an illustrative embodiment and examples of modifications of the present invention have been described in detail herein, the scope of the invention is not limited thereto. It will be appreciated by those skilled in the art that various modifications may be made without departing from the scope of the invention. Accordingly, the embodiment and examples of modifications disclosed herein are merely illustrative. It is to be understood that the scope of the invention is not to be so limited thereby, but is to be determined by the claims which follow.