Knock-up plate lifting device and image forming apparatus having the same

Abstract

A knock-up plate lifting device for lowering a knock-up plate while inserting and withdrawing a paper feeding cassette, and lifting the knock-up plate when the paper feeding cassette is completely mounted. The knock-up plate lifting device includes a mounting board. A first link is pivotably mounted by a first hinge to the mounting board and has arms extended away from the first hinge. A resilient member is connected to one of the arms of the first link. A second link is pivotably mounted by a second hinge to the mounting board and extends away from the second hinge by a predetermined angle. One arm of the second link is connected with the resilient member and the other arm moves up and down in accordance with the operation of the first hinge.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit under 35 U.S.C. .sctn. 119(a) of Korean Patent Application No. 2004-81005, filed Oct. 11, 2004, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a paper feeding cassette for an image forming apparatus. More particularly, the present invention relates to a knock-up plate lifting device for raising and lowering a knock-up plate of a paper feeding cassette.

[0004] 2. Description of the Related Art

[0005] Generally, an image forming apparatus includes a paper feeding cassette for receiving a stack of a printing medium to automatically supply the printing medium, and a pickup roller for separating and conveying the stacked printing medium sheet by sheet. The paper feeding cassette includes a knock-up plate for biasing the stacked printing medium toward the pickup roller. One end of the knock-up plate is hinged to the paper feeding cassette while the other end is resiliently supported by a resilient member such as a spring to bias the stacked printing medium toward the pickup roller. However, since the knock-up plate is resiliently supported, it may collide with the pickup roller while inserting and withdrawing the paper feeding cassette with respect to a main body of the image forming apparatus and thereby disturb the insertion and withdrawal of the paper feeding cassette. Therefore, a knock-up plate lifting device is preferably configured to lower the knock-up plate during the insertion and withdrawal of the paper feeding cassette and lift the knock-up plate toward the pickup roller after completing the insertion.

[0006] Referring to FIGS. 1 and 2, a conventional knock-up plate lifting device 10 comprises a link 40, a resilient member 30, and a rectilinear-moving member 20. The link 40 has a bent form and is fixed by a hinge 43 within a main body 70 (FIG. 3) of the image forming apparatus that the knock-up plate lifting device 10 is mounted in. The hinge 43 is formed at a joint portion of the link 40. The link 40 comprises a short arm 41 and a long arm 42 with respect to the hinge 43. An end of the short arm 41 is connected to one end of the resilient member 30 whereas an end of the long arm 42 has a guiding projection 51 for guiding vertical motion of the link 40. A guiding groove 52 is formed in front of the guiding projection 51 to correspond to a moving path of the guiding projection 51.

[0007] The resilient member 30 connects the link 40 and the rectilinear-moving member 20. The long arm 42 of the link 40 moves up and down by a predetermined angle in accordance with the motion of the rectilinear-moving member 20. A tension coil spring may be used for the resilient member 30.

[0008] The rectilinear-moving member 20 comprises a moving part 21 and a guiding part 22 for guiding the linear motion of the moving part 21. The guiding part 22 is disposed within the main body 70 and the moving part 21 is connected to the other end of the resilient member 30. On both sidewalls 81 of the paper feeding cassette 80, operation projections 85 are formed to push the moving part 21 of the rectilinear-moving member 20 in a direction for mounting the paper feeding cassette 80, as shown in FIG. 3.

[0009] As the paper feeding cassette 80 is inserted into the main body 70 of the image forming apparatus, the operation projections 85 formed on the sidewalls 81 of the paper feeding cassette 80 push the moving part 21 of the rectilinear-moving part 20 in the mounting direction of the paper feeding cassette 80, that is, the direction illustrated by the arrow A in FIG. 2. Accordingly, tension is applied to the resilient member 30, thereby applying a force onto the short arm 41 of the link 40. The link 40 is pivoted with respect to the hinge 43 by a predetermined angle by the force, such that the end of the long arm 42 is lifted, as illustrated by the solid lines in FIG. 2. Therefore, if the end of the long arm is connected to the knock-up plate 83 of the paper feeding cassette 80, the knock-up plate 83 of the paper feeding cassette 80 is also lifted by the link 40. The motion of the long arm 42 is guided by the guiding projection 51 and the guiding groove 52.

[0010] When the paper feeding cassette 80 is withdrawn from the main body 70, the operation projections 85 are separated from the moving part 21, and, accordingly, the force applied to the moving part 21 of the rectilinear-moving member 20 is removed. The recovery force of the resilient member and the weight of the link 40 apply a force in the opposite direction, and, as a result, the moving part 21 is restored. After recovery, the long arm 42 of the link 40 is parallel with the bottom of the paper feeding cassette 80 (as shown in dashed lines), and, therefore, the knock-up plate 83 supported by the link 40 also becomes parallel with the bottom.

[0011] Therefore, when the paper feeding cassette 80 is withdrawn with respect to the main body 70, the knock-up plate 83 is lowered by the knock-up plate lifting device 10, and when the paper feeding cassette 80 is completely inserted into the main body 70, the knock-up plate 83 is lifted so as to bring the printing medium stacked on the knock-up plate 83 into tight contact with the pickup roller (not shown), thereby enabling the pickup of the printing medium.

[0012] However, in the conventional knock-up plate lifting device 10 described above, the lifting and lowering of the knock-up plate 83 depends on the rectilinear motion of the rectilinear-moving part 20. Thus, if the guiding part 22 is jammed with foreign substances or if the sliding surfaces of the guiding part 22 and the moving part 21 become rough due to long-term use, the knock-up plate lifting device 10 may not operate smoothly. This may cause difficulty in inserting and withdrawing the paper feeding cassette 80 with respect to the main body 70, thereby deteriorating reliability of the product.

[0013] In addition, since the knock-up plate lifting device 10 is disposed at a lower corner portion of the main body 70, assembly is complex. Therefore, productivity deteriorates and manufacturing costs increase.

[0014] Furthermore, the knock-up plate lifting device 10 is not easily seen from the outside of the main body 70. Therefore, maintenance and repair of the knock-up plate lifting device 10 is not easy, and this may increase maintenance costs.

[0015] Accordingly, there is a need for an improved knock-up plate lifting device that is more reliable and easier to manufacture and maintain.

SUMMARY OF THE INVENTION

[0016] An aspect of the present invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide an improved knock-up plate lifting device which is not influenced much by foreign substances located in the lifting device or by roughness of sliding surfaces in the lifting device.

[0017] Another aspect of the present invention is to provide a knock-up plate lifting device which can be mounted on the sidewalls of the paper feeding cassette to save assembly time and reduce manufacturing and maintenance costs.

[0018] Yet another aspect of the present invention is to provide an image forming apparatus with a knock-up plate that is not influenced much by foreign substances or by roughness of sliding surfaces and that reduces manufacturing and maintenance costs.

[0019] In order to achieve the above-described aspects of the present invention, a knock-up plate lifting device is provided. The knock-up plate lifting device includes a mounting board. A first link is pivotably mounted by a first hinge to the mounting board. The first link has first and second arms extending in opposite directions away from the first hinge. A resilient member is connected to the first arm of the first link. A second link is pivotably mounted by a second hinge to the mounting board. The second link has first and second arms extending away from the second hinge. The first arm of the second link is connected with the resilient member so that the second arm moves up and down when the first link is pivoted.

[0020] The second arm of the second link is disposed at a lower position when the first link is vertical to the ground.

[0021] The arms of the second link have different lengths.

[0022] The knock-up plate lifting device further comprises a guiding projection formed at the second arm of the second link, and a guiding groove formed at the mounting board to guide the vertical motion of the guiding projection.

[0023] The knock-up plate lifting device further comprises an extension member formed at an end of the first arm of the first link which is connected to an end of the resilient member.

[0024] According to another aspect of the present invention, an image forming apparatus comprises a pickup roller mounted to a main body and a paper feeding cassette removably mounted to the main body under the pickup roller. The cassette includes a knock-up plate for pushing a printing medium toward the pickup roller. A first link is mounted to a sidewall of the paper feeding cassette by a first hinge and has first and second arms extending in opposite directions from the first hinge. A resilient member is connected to the first arm of the first link. A second link is mounted to the sidewall of the paper feeding cassette by a second hinge and has first and second arms. The first arm of the second link is connected with the resilient member and the second arm of the second link lifts and lowers the knock-up plate in accordance with the operation of the first link. An operation projection is formed on the main body to rotate the first link when the paper feeding cassette is mounted into the main body.

[0025] First links, resilient members, and second links are formed on both sidewalls of the paper feeding cassette, and operation projections are formed on both sides of the main body to correspond to the first links.

[0026] The second arm of the second link is disposed at a lower position when the first link is vertical to the bottom of the paper feeding cassette.

[0027] The arms of the second link are different lengths.

[0028] The image forming apparatus further comprises a guiding projection formed at the second arm of the second link to lift and lower the knock-up plate, and a guiding groove formed at the sidewall of the paper feeding cassette to guide the vertical motion of the guiding projection.

[0029] The image forming apparatus further comprises an extension member formed at an end of the first arm of the first link which is connected to an end of the resilient member.

[0030] According to the embodiments of the present invention described above, the knock-up plate is operated by rotation of the first link, without using a sliding device. Thus, the operation of the knock-up plate is not influenced much by foreign substances or by roughness of sliding surfaces.

[0031] Also, since the knock-up plate lifting device can be mounted on the sidewalls of the paper feeding cassette, assembly time for the knock-up plate lifting device can be reduced as compared with the assembly time when the knock-up plate is mounted within the main body of the image forming apparatus. Therefore, maintenance and manufacturing costs can be saved.

[0032] Additionally, the image forming apparatus applying the above knock-up plate lifting device has reduced maintenance and manufacturing costs.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0033] The above and other objects, features, and advantages of certain embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:

[0034] FIG. 1 is a schematic view of a conventional knock-up plate lifting device;

[0035] FIG. 2 is a schematic view illustrating the operation of the knock-up plate lifting device of FIG. 1;

[0036] FIG. 3 is a perspective view of an image forming apparatus including the knock-up plate lifting device of FIG. 1;

[0037] FIG. 4 is a schematic view of a knock-up plate according to an embodiment of the present invention;

[0038] FIG. 5 is a schematic view illustrating the operation of the knock-up plate of FIG. 4;

[0039] FIG. 6 is a perspective view showing an image forming apparatus including the knock-up plate lifting device according to an embodiment of the present invention, before the paper feeding cassette has been inserted; and

[0040] FIG. 7 is a partial, sectional view illustrating the knock-up plate lifting device when the paper feeding cassette is mounted in the image forming apparatus of FIG. 6.

[0041] Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features, and structures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0042] The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of the embodiments of the invention. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness.

[0043] Referring to FIGS. 4 and 5, a knock-up plate lifting device 110 according to an embodiment of the present invention comprises a mounting board 100, a first link 120, a second link 140, and a resilient member 130.

[0044] The first and the second links 120 and 140 are mounted on the mounting board 110 by first and second hinges 123 and 143, respectively, so that the links may rotate. By using a mounting board 110, the knock-up plate lifting device 100 can be attached as a separate unit. The mounting board 110 is attached to a place employing the knock-up plate lifting device 100, such as main body of a paper feeding cassette or an image forming apparatus, by a fastening means (not shown) such as a screw. When a knock-up plate lifting device 100' (FIG. 6) does not utilize a dedicated mounting board 110, the first and the second links 120 and 140 are directly hinged to a sidewall 181 (FIG. 6) of a paper feeding cassette 180 (FIG. 6) by the hinges 123 and 143.

[0045] The first link 120 is mounted by the first hinge 123 so that it can rotate with respect to the mounting board 110. An upper (or first) arm 121 and a lower (or second) arm 122, extend in opposite directions away from the first hinge 123. The first link 120 is vertical to the ground when no force is applied, and rotated when applied with force due to interference between the upper arm 121 and an operation projection 171 (FIG. 7). The lower arm 122 of the first link 120 is connected to an end of the resilient member 130. The lower arm 122 of the first link 120 may further comprise an extension member 124 for connection with the end of the resilient member 130. The connection part 125 of the resilient member 130, for connection with the extension member 124, is preferably disposed so that the distance between the first hinge 123 and the connection part 125 is almost constant whether the first link 120 is in a vertical posture and in a horizontal posture.

[0046] At a predetermined distance from the first link 120, the second link 140 is rotatably mounted on the mounting board 110 by the second hinge 143. The second link 140 includes a right (or first) arm 141 and a left (or second) arm 142 extending away from the second hinge 143. The right arm 141 (refer to FIG. 4) is connected to the other end of the resilient member 130. The left arm 142 (refer to FIG. 4) is lifted and lowered by a predetermined angle according to the motion of the right arm 141. As shown in FIG. 4, when the first link 120 is vertical to the ground, the left arm 142 of the second link 140 is at a lower position, that is, horizontal with respect to the ground, and the left arm 142 is longer than the right arm 141. The second link 140 is formed as a bent member wherein the left arm 142 and the right arm 142 are bent at a predetermined angle with respect to each other, preferably in the range of approximately 90.about.180.degree.. A reinforcing piece 145 may be provided between the left and right arms 142 and 141 of the second link 140 to strengthen the arms 141 and 142. As long as the left arm 142 of the second link 140 is lifted and lowered by the predetermined angle, the second link 140 may have various configurations according to the length of the first link 120, the positional relationship with the first link 120, and the lifted height of the knock-up plate 183 (FIG. 6).

[0047] The left arm 142 of the second link 140 has a guiding projection 151 while the mounting board 110 has a guiding groove 152 for guiding the vertical motion of the guiding projection 151. The guiding groove 152 is shaped to correspond to the arc of the movement of the end of the left arm 142 of the second link 140. The lower end of the guiding groove 152 is located where the left arm 142 of the second link 140 becomes horizontal, and the upper end of the guiding groove 152 is located where the knock-up plate 183 is pressed to its maximum lift height. The guiding groove 152 and the guiding projection 151 define the angle of lifting and lowering the second link 140 and stably guide the vertical motion of the second link 140.

[0048] The resilient member 130 is interposed between the first and the second links 120 and 140 to allow the left arm 142 of the second link 140 to move up and down in accordance with rotation of the first link 120. One end of the resilient member 130 is connected to the lower arm 122 of the first link 120 or the extension member 124, while the other end of the resilient member 130 is connected to the right arm 141 of the second link 140. When the first link 120 is vertical, the resilient member 130 applies no force to the second link 140. On the other hand, when the second link 140 is horizontal, the resilient member 130 lifts up the left arm 142 of the second link 140 and applies a force for pushing a printing medium stacked on the knock-up plate 183 into tight contact with the pickup roller 175 (FIG. 7). Any resilient material capable of transmitting a force, preferably a coil spring, can be used for the resilient member 130.

[0049] The operation of the knock-up plate lifting device 100 will now be described with reference to FIG. 5. When a member for rotating the first link 120, such as the operation projection 171 (FIG. 6), interferes with the upper arm 121 of the first link 120, the first link 120 rotates by approximately 90.degree. to be horizontal.

[0050] When the first link 120 rotates by approximately 90.degree., the resilient member 130 in connection with the lower arm 122 of the first link 120 pulls the right arm 141 of the second link 140 to the right with respect to FIG. 5. Therefore, the second link 140 rotates clockwise about the second hinge 143, thereby moving the left arm 142 of the second link 140 upward with the assistance of the guiding projection 151 and the guiding groove 152. If the guide projection 151 formed at the end of the left arm 142 of the second link 140 is connected with the knock-up plate 183, the knock-up plate 183 is lifted up.

[0051] As the interference is removed from the first link 120, the guiding projection 151 returns to the lower end of the guiding groove 152 due to the weight of the second link 140. Accordingly, the left arm 142 of the second link 140 becomes horizontal and the first link 120 returns to vertical by its own weight and the recovery force of the resilient member 130.

[0052] Thus, by the knock-up plate lifting device 100 according to an embodiment of the present invention, the left arm 142 of the second link 140 can be moved up and down in accordance with the rotation of the first link 120.

[0053] FIG. 6 is a perspective view showing the image forming apparatus comprising a knock-up plate lifting device 100' according to another embodiment of the present invention. FIG. 7 is a partial, sectional view for illustrating the operation of the knock-up plate lifting device 100' shown in FIG. 6 when the paper feeding cassette is mounted in the image forming apparatus.

[0054] The paper feeding cassette 180 comprises a knock-up plate lifting device 100' formed on a sidewall 181 of the cassette 180, and the knock-up plate 183.

[0055] In the previously discussed embodiment, a sub-assembly comprising the mounting board 110 (FIG. 4), the first and the second links 120 and 140, and the hinges 123 and 143 is formed and the sub-assembly is fixed onto the sidewall 181 of the paper feeding cassette 180 to thereby mount the knock-up plate lifting device 100. In this embodiment, the knock-up plate lifting device 100' includes first and second links 120 and 140 that are mounted directly on the sidewall 181 of the paper feeding cassette by hinges 123 and 143. The resilient member 130 connects the first and the second links 120 and 140. The first link 120 is mounted directly to the paper feeding cassette 180, but the structure of the first link 120 is the same as in the previous embodiment. Therefore, a detailed description of the first link 120 is not repeated. The guiding groove 152 is formed on the sidewall 181 of the paper feeding cassette 180 to guide the guiding projection 151 of the left arm 142 of the second link 140, and the guiding projection 151 of the second link 140 is mounted to support the bottom of the knock-up plate 183. Since the second link 140 has the same structure as in the previous embodiment, a detailed description is not repeated. The knock-up plate lifting device 100' may be formed on only one sidewall 181 of the paper feeding cassette 180. Preferably, however, knock-up plate lifting devices 100' are provided on both sidewalls 181 to more stably support the vertical motion of the knock-up plate 183.

[0056] The main body 170 of the image forming apparatus that mounts the paper feeding cassette 180 has an operation projection 171 to operate the knock-up plate lifting device 100'. As the paper feeding cassette 180 is inserted in the main body 170, the operation projection 171 rotates the first link 120. When the paper feeding cassette 180 is completely inserted, the operation projection 171 maintains the horizontal state of the first link 120, as shown in FIG. 7. When knock-up plate lifting devices 100' are formed on both sidewalls 181 of the paper feeding cassette 180, a second operation projection 171 is formed to corresponding to the second knock-up plate lifting device.

[0057] With reference to FIGS. 6 and 7, the vertical motion of the knock-up plate 183 of the paper feeding cassette 180 in the above-structured image forming apparatus will now be described.

[0058] As shown in FIG. 6, when the paper feeding cassette 180 is drawn from the main body 170, the knock-up plate 183 is placed on the bottom of the paper feeding cassette 180. In this state, the second link 140 of the knock-up plate lifting device 100' is horizontal and the first link 120 is vertical to the bottom of the paper feeding cassette 180.

[0059] When the paper feeding cassette 180 is inserted into the main body 170 and advanced by a predetermined distance, the upper arm 121 of the first link 120 of the knock-up plate lifting device 100' contacts the operation projection 171 formed on the main body 170. As the paper feeding cassette 180 is completely inserted, the first link 120 of the knock-up plate lifting device 100' is kept horizontal by the operation projection 171.

[0060] When the first link 120 rotates, tension is applied to the resilient member 130. The resilient member is connected to the lower arm 122 of the first link 120 and the right arm 141 of the second link 140. Thus, the right arm 141 is pulled to the right with respect to FIG. 7 by the resilient member 130. Accordingly, the second link 140 rotates clockwise about the second hinge 143 and the guiding projection 151 formed at the end of the left arm 142 of the second link 140 moves upward along the guiding groove 152 of the paper feeding cassette 180. Consequently, the knock-up plate 183, which is supported by the guiding projection 151, is lifted. Since the knock-up plate 183 is supported by the guiding projection 151, the pickup roller 175 disposed above the knock-up plate 183 is pushed with a certain force by the knock-up plate 183, as shown in FIG. 7.

[0061] When the paper feeding cassette 180 is withdrawn from the main body 170, the upper arm 121 of the first link 120 escapes from the interference with the operation projection 171, thereby rotating the first link 120 clockwise. As the paper feeding cassette 180 is further withdrawn, the first link 120 completely escapes from the interference of the operation projection 171 and becomes vertical to the bottom of the paper feeding cassette 180. Also, the left arm 142 of the second link 140 is moved to the lower end of the guiding groove 152 by the weight of the second link 140 and the knock-up plate 183 so that it is horizontal. The knock-up plate 183 is located at the bottom of the paper feeding cassette 180 at this time and does not interfere with the pickup roller 175 mounted in the main body 170 above the paper feeding cassette 180. Therefore, the paper feeding cassette 180 can be withdrawn smoothly from the main body 170 of the image forming apparatus.

[0062] After the paper feeding cassette 180 has been refilled with the printing medium P and is remounted to the main body 170, the knock-up plate 183 is lifted by the knock-up plate lifting device 100' to bias the printing medium P toward the pickup roller 175, thereby enabling the pickup roller 175 to pick up and convey the printing medium P sheet by sheet.

[0063] While the invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A knock-up plate lifting device comprising: a mounting board; a first link pivotably mounted by a first hinge to the mounting board and having first and second arms extending away from the first hinge; a resilient member connected to the first arm of the first link; and a second link pivotably mounted by a second hinge to the mounting board and having first and second arms extending away from the second hinge, wherein the first arm of the second link is connected with the resilient member and the second arm moves up and down in accordance with the operation of the first hinge.

2. The knock-up plate lifting device of claim 1, wherein the second arm of the second link is disposed at a lower position when the first link is vertical to the ground.

3. The knock-up plate lifting device of claim 2, wherein the first and second arms of the second link have different lengths.

4. The knock-up plate lifting device of claim 3, wherein the knock-up plate lifting device further comprises: a guiding projection formed at the second arm of the second link; and a guiding groove formed at the mounting board to guide the motion of the guiding projection.

5. The knock-up plate lifting device of claim 3, further comprising: an extension member formed at the end of the first arm of the first link and connected to an end of the resilient member.

6. The knock-up plate lifting device of claim 1, wherein the first and second arms of the second link are at a predetermined angle with respect to each other.

7. An image forming apparatus comprising: a pickup roller mounted to a main body; a paper feeding cassette removably mounted to the main body under the pickup roller and having a knock-up plate for pushing a printing medium toward the pickup roller; a first link mounted to a sidewall of the paper feeding cassette by a first hinge and having first and second arms extended in opposite directions from the first hinge; a resilient member connected to the first arm of the first link; a second link mounted to the sidewall of the paper feeding cassette by a second hinge and having a first arm connected with the resilient member and a second arm lifting and lowering the knock-up plate in accordance with the operation of the first hinge; and an operation projection formed on the main body to rotate the first link when the paper feeding cassette is mounted into the main body.

8. The image forming apparatus of claim 7, wherein both sidewalls of the paper feeding cassette include a first link, a resilient member, and a second link, and both sides of the main body includes an operation projection corresponding with the first links.

9. The image forming apparatus of claim 7, wherein the second arm of the second link is disposed at a lower position when the first link is vertical to the bottom of the paper feeding cassette.

10. The image forming apparatus of claim 9, wherein the first and second arms of the second link have different lengths.

11. The image forming apparatus of claim 10, further comprising: a guiding projection formed at the second arm of the second link to lift and lower the knock-up plate; and a guiding groove formed at the sidewall of the paper feeding cassette to guide motion of the guiding projection.

12. The image forming apparatus of claim 7, further comprising: an extension member formed on the end of the first arm of the first link and connected to an end of the resilient member.

13. The image forming apparatus of claim 7, wherein the first and second arms of the second link are at a predetermined angle with respect to each other.

14. An image forming apparatus comprising: a pickup roller mounted to a main body; a paper feeding cassette removably mounted to the main body, a knock-up plate mounted to the paper feeding cassette for pushing a printing medium toward the pickup roller; and a first knock-up plate lifting device, comprising: a first link pivotably mounted to a sidewall of the paper feeding cassette, the first link having first and second arms extending away from the first hinge; a resilient member connected to the first arm of the first link; a second link pivotably mounted to the sidewall of the paper feeding cassette, the second link having a first arm connected with the resilient member and a second arm for supporting the knock-up plate; and an operation projection formed on the main body to rotate the first link when the paper feeding cassette is mounted into the main body, wherein when the first link is rotated by the operation projection, the second arm raises the knock-up plate.

15. An image forming apparatus according to claim 14, further comprising: a second knock-up plate lifting device, comprising: a third link pivotably mounted to a second sidewall of the paper feeding cassette, the third link having first and second arms extending away from the first hinge; a second resilient member connected to the first arm of the third link; a fourth link pivotably mounted to the second sidewall of the paper feeding cassette, the fourth link having a first arm connected with the second resilient member and a second arm for supporting the knock-up plate; and a second operation projection formed on the main body to rotate the third link when the paper feeding cassette is inserted into the main body.

16. An image forming apparatus according to claim 14, further comprising: a guiding projection formed at the second arm of the second link to support the knock-up plate; and a guiding groove formed on the sidewall of the paper feeding cassette to guide the motion of the guiding projection.

16. An image forming apparatus according to claim 14, wherein the first and second arms of the second link are at a predetermined angle with respect to each other.