Sheet feeding apparatus and image forming apparatus

Abstract

A sheet feeding apparatus including: a separating roller which abuts against a feed roller and is rotated in association with the feed roller; and a torque-limiter for allowing the separating roller to rotate in association with the feed roller when a rotation torque larger than a predetermined torque is acted on the separating roller and for preventing the separating roller from rotating in association with the feed roller when the rotation torque equal to or smaller than the predetermined torque is acted on the separating roller, in which sheets fed from a sheet feed cassette are separated one by one by the feed roller and the separating roller, the separating roller urged against the feed roller is supported by a guide member in a slidable manner, and the separating roller is guided with a predetermined angle in a direction different from a direction in which the feed roller is opposed to the separating roller.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a sheet feeding apparatus and an image forming apparatus, and more particularly, to a structure of a sheet separating means for separating sheets fed by a sheet feeding means.

[0003] 2. Related Background Art

[0004] Conventionally, an image forming apparatus such as a printer, a copying machine, and a facsimile is provided with sheet feeding apparatus for separating the sheets one by one to feed the sheets from a sheet stacking portion, on which plural sheets of recording paper or originals (hereinafter referred to as "sheets") are stacked, to an image forming portion.

[0005] As a sheet separating means of the sheet feeding apparatus described above, one involving a separation method using a separating pad is known. In such the separation method using the separating pad, in a case where plural sheets are fed between a feeding roller and the separating pad, sheets other than an uppermost sheet are dammed up by an action of friction separation caused between the feeding roller and the separating pad. It should be noted that a feature of the sheet separating means using such the separating pad resides in that a sheet separating portion can be constructed to be small in area.

[0006] Next, a detailed structure of such the sheet separating means using such the separating pad with reference to the drawings. FIG. 19 is a schematic diagram showing a structure of a printer adopting a separating pad as the sheet separating means.

[0007] In FIG. 19, reference numeral 101 denotes a printer main body (hereinafter referred to as "apparatus main body"), reference symbol 101A denotes an image forming portion. Below the image forming portion 101A, 2-stages of sheet feeding portions 102 and 106 constructed of an upper sheet feeding portion 102 and a lower sheet feeding portion 106, are arranged so as to overlap with each other. The image forming portion 101A is provided with a laser scanner 114, an image forming process unit 113 including a photosensitive drum 113a serving as an image bearing member, a transfer roller 113b constituting a transferring portion for transferring a toner image formed on the photosensitive drum 113a onto a sheet S, and so on.

[0008] Reference numeral 115 denotes a fixing device for fixing the toner image transferred by the transferring portion onto the sheet. After the toner image is fixed by the fixing device 115, the sheet S is sequentially discharged to and stacked on a discharge tray 119 provided on an uppermost portion of the apparatus main body. Further, reference numeral 131 denotes an option sheet feeding apparatus provided with a sheet feed cassette 131A, and the option sheet feeding apparatus 131 is selectively mounted on a bottom surface of the apparatus main body 101.

[0009] As shown in FIG. 20, the upper sheet feeding portion 102 includes an upper sheet feed cassette 105A, an upper feed roller 103 for feeding out sheets S stored in the upper sheet feed cassette 105A, and an upper sheet separating portion 104 provided with an upper separating pad 104a for separating the sheets S fed from the upper feed roller 103 one by one.

[0010] It should be noted that reference symbol 104b denotes an upper separating pad supporting portion for bearing the upper separating pad 104a, and is provided to the upper sheet separating portion 104 in a rotatable manner. The upper separating pad supporting portion 104b is urged against the upper feed roller side by an upper separating pad spring 104c so as to pressure-contact with the upper feed roller 103.

[0011] In addition, reference symbol 105a denotes an upper sheet stacking plate for stacking the sheets S, and is provided to the upper sheet feed cassette 105A in a rotatable manner. The upper sheet stacking plate 105a is pressed upwardly by an upper sheet stacking plate pressure spring 105b from a back surface side (downside in FIG. 20) of the upper sheet stacking plate 105a. A leading end of the uppermost sheet S1 stacked on the upper sheet stacking plate 105a is pressed against the upper feed roller 104.

[0012] In the upper sheet feeding portion 102 with this structure, the upper feed roller 103 rotates counterclockwise by a drive of a driving motor (not shown) and conveys the uppermost sheet S1, which is stacked on the upper sheet stacking plate 105a, between the upper feed roller 103 and the upper separating pad 104a. At this time, in a case where plural sheets S are conveyed between the upper feed roller 103 and the upper separating pad 104a, sheets other than the uppermost sheet S1 are dammed up by the friction separating effect caused by the upper separating pad 104a, and only the uppermost sheet S1 is conveyed to a downstream side. It should be noted that the lower sheet feeding portion 106 and an option sheet feeding portion 132 of the option sheet feeding apparatus 131 are constituted in a similar manner as the upper sheet feeding portion 102.

[0013] In FIG. 20, reference numeral 120 denotes a conveying path for an option sheet feeding apparatus for conveying the sheet S fed from the option sheet feeding apparatus 131 upwardly (downstream side), the conveying path 120 for the option sheet feeding apparatus merges into a conveying path for a lower sheet feeding portion 121 for conveying the sheet S fed from the lower sheet feeding portion 106 upwardly (downstream side).

[0014] Further, a conveying path 122 for the upper sheet feeding portion for conveying the sheet S fed from the upper sheet feeding portion 102 upwardly (downstream side) merges into the conveying path 121 for the lower sheet feeding portion. Thus, the sheet fed from the option sheet feeding apparatus 131, the lower sheet feeding portion 106, and the upper sheet feeding portion 102 passes through a sheet feeding portion merging conveying path 123 to be conveyed to the downstream side.

[0015] Then, the sheet S which has passed through the sheet feeding portion merging conveying path 123 to be conveyed to the downstream side is, in synchronization with an image formed on the photosensitive drum 113a by a registration roller 112 shown in FIG. 19, conveyed to the transferring portion constituted of the photosensitive drum 113a and a transfer roller 113b. In the transferring portion, a toner image is transferred onto the sheet S.

[0016] It should be noted that the sheet S onto which the toner image has been transferred is then conveyed to the fixing device 115, and the toner image is fixed on the sheet S by applying a pressure and heat at the fixing device 115. After that, the sheet S is conveyed through a pair of conveying rollers 116 and 117 and a pair of discharge rollers 118 to be discharged and stacked sequentially on the discharge tray 119.

[0017] By the way, according to the sheet separating means adopting such the separation method using the separating pad, the following structure can be employed by taking an advantage of being capable of constructing the sheet separating portion within a small space.

[0018] In other words, in a conventional sheet feeding apparatus, as shown in FIG. 20, since a bottom surface of the upper sheet feed cassette 105A of the upper sheet feeding portion 102 is overlapped with the lower sheet feeding portion 106, the height of the apparatus main body 101 can be reduced by the overlapped portion. In addition, since the sheet separating portion requires only a small space, a lower feed roller 107 of the lower sheet feeding portion 106 can be provided in adjacent to the upper sheet separating portion 104 side. As a result, a compact product can be realized in which the height of the apparatus is made low, while suppressing the width.

[0019] Further, in such a type of printer that the option sheet feeding apparatus 131 is to be added, when the conveying path 120 for the option sheet feeding apparatus is arranged so as to be adjacent to a lower sheet separating portion 108, the lower sheet separating portion 108 requires only a small space, thereby making it possible to obtain a compact product capable of suppressing the apparatus width. Therefore, with this structure, a compact product, which allows a printer for A3-size paper to be arranged on a desktop, is known.

[0020] On the other hand, as disclosed in JP 62-105834 A, a sheet separating means adopting a separating roller with a torque-limiter function is known as another example of the sheet separating means to be mounted to the sheet feeding apparatus. It should be noted that in such the sheet separating means, the separating roller, which is positioned in coaxial with the torque-limiter or is built in the torque-limiter, is pressure-contacted with the feed roller, to thereby separate the sheets by using a braking torque of the torque-limiter.

[0021] For example, when only one sheet is placed between the feed roller and the separating roller, a large rotation torque is acted on the torque-limiter, thereby the torque-limiter allows the separating roller to rotate in association with the feed roller. On the other hand, when plural sheets are placed between the feed roller and the separating roller, a relatively small rotation torque is acted on the torque-limiter, thereby the torque-limiter prevents the separating roller from rotating in association with the feed roller. As a result, the feed roller conveys one sheet and the separating roller rotates in an opposite direction from the feed roller, to thereby prevent another sheet from being conveyed.

[0022] Thus, the braking torque of the torque-limiter controls the braking torque within a range from a lower limit, at which the torque-limiter prevents the separating roller from rotating in association with the feed roller when plural sheets locate between the feed roller and the separating roller, to an upper limit, at which the torque-limiter allows the separating roller to rotate in association with the feed roller when only one sheet is placed between the feed roller and the separating roller, to thereby obtain a sheet separating function and a sheet feeding performance. Then, with this structure, it is known that, as compared with the sheet separating means using the separating pad, the sheet separating means with high durability, without strange sound (noise) cause by chattering marks of a pad and a sheet, and capable of performing stable sheet feeding can be obtained.

[0023] Next, a detailed structure of the sheet separating means provided with such the separating roller will be described.

[0024] FIG. 21 shows a schematic diagram for explaining a structure of a printer mounted with the sheet separating means provided with a separating roller using a torque-limiter. It should be noted that in FIG. 21, the same reference symbols as those of FIG. 19 indicate the identical or corresponding portions.

[0025] In FIG. 21, reference numeral 153 denotes a sheet feeding portion, the sheet feeding portion 153 is constituted of a sheet feed cassette 152, a pickup roller 154 for feeding out the sheet S stored in the sheet feed cassette 152, a feed roller 155, and a separating roller 156a being pressure-contacted with the feed roller 155. The sheet feeding portion 153 includes a separating portion 156 for separating the sheets S fed by the pickup roller 154 one by one.

[0026] It should be noted that the separating roller 156a, as shown in FIG. 22, is rotatably held at a rotating end of a supporting member 156c which is supported rotatably about a rotation axis 156d, and is pressed against the feed roller 155 by a spring 156e. In addition, the separating roller 156a is coupled onto a rotation axis 156b through a torque-limiter (not shown).

[0027] Further, reference symbol 152a denotes a sheet stacking plate which is rotatably provided to the sheet feed cassette 152. The sheet stacking plate 152a is pressed upwardly by a sheet stacking plate pressure spring 152b from a back surface side (downside in FIG. 22) of the sheet stacking plate 152a, and a leading end of the uppermost sheet S1 stacked on the sheet stacking plate 152a is pressed against the pickup roller 154.

[0028] Then, in the sheet feeding portion 153 with this structure, the pickup roller 154 and the feed roller 155 rotate counterclockwise by a drive of a driving motor (not shown) and convey the uppermost sheet S1, which is stacked on the sheet stacking plate 152a, to a nip portion between the feed roller 155 and the separating roller 156a. Then, in the nip portion between the feed roller 155 and the separating roller 156a, the sheets are separated by a braking torque of the torque-limiter.

[0029] By the way, as the sheet separating means with this structure, as disclosed in JP 2003-02634 A, there is known a sheet separating means for feeding and separating sheets by one roller having a large diameter which serves as a pickup roller and a feed roller in common and for conveying the sheets to an image forming portion or an image reading portion. With this structure, since the number of parts can be reduced, thereby making it possible to reduce the cost, minimize a space for the sheet separating portion, and to downsize the apparatus.

[0030] FIG. 23 is a diagram for explaining an example of the sheet separating means which can be used commonly as a pickup roller and a feed roller. In FIG. 23, reference numeral 175 denotes a feed roller having a large diameter which can be used as a pickup roller. In addition, reference symbol 176a denotes a separating roller, the separating roller 176a being rotatably held at a rotating end portion of a supporting member 176c which is supported rotatably about a rotation axis 176d, and is pressure-contacted with the feed roller 175 by a spring 176e. The separating roller 176a is, similarly to the above conventional art, coupled onto a separating roller rotation axis 176b through a torque-limiter (not shown).

[0031] The feed roller 175 is pressed against a leading end of the uppermost sheet S1 stacked on the sheet stacking plate 152a, and is pressed against the separating roller 176a at a downstream side in a circumferential direction. At the nip portion between the feed roller 175 and the separating roller 176a, the sheets are separated by the braking torque of the torque-limiter.

[0032] In the sheet separating means adopting such the separating roller, the sheet separating performance is based on the function of the braking torque of the torque-limiter coupled onto the separating roller rotation axis. Next, such the function of the torque-limiter will be described by using a conventional example in which both of a pickup roller and a feed roller are used as shown in FIG. 23.

[0033] First, when a set torque of a torque-limiter is given as T and a radius of the separating roller 176a is given as Rr, a tangential force Ta on a circumferential force necessary for rotating the separating roller 176a through the torque-limiter (hereinafter referred to as "rotation allowable tangential force") can be expressed by the following expression (1). Ta = T Rr ( 1 ) ##EQU1##

[0034] Then, in a case where there is no sheet S between the separating roller 176a and the feed roller 175, and the feed roller 175 which is rotating is directly brought into contact with the separating roller 176a, a force larger than the rotation allowable tangential force Ta is acted on the separating roller 176a. In other words, a large rotation torque is acted on the torque-limiter of the separating roller 176a, thereby the torque-limiter allows the separating roller 176a to rotate in association with the feed roller 175.

[0035] In this case, when an urging force of the separating roller 176a against the feed roller 175 is given as N, .mu.g.times.N which expresses a rotating force in a clockwise direction shown in FIG. 23 is added to the separating roller 176a by a coefficient of friction pg between the separating roller 176a and the feed roller 175. At this time, the following expression (2) is established, thereby the separating roller 176a is driven by the feed roller 175 to rotate in a clockwise direction. Ta<.mu.g.times.N (2)

[0036] Further, in a case where only one sheet S is conveyed between the feed roller 175 and the separating roller 176a, a force larger than the rotation allowable tangential force Ta is acted on the separating roller 176a. In other words, a large rotation torque is acted on the torque-limiter of the separating roller 176a, thereby the torque-limiter allows the separating roller 176a to rotate in association with the feed roller 175.

[0037] In this case, when a coefficient of friction between the separating roller 176a and the sheet S is given as .mu.r, the following expression (3) is established, thereby the separating roller 176a rotates in a clockwise direction to deliver the sheet S as shown in FIG. 23. Ta<.mu.r.times.N (3)

[0038] Further, in a case where the pickup roller 174 delivers two or more sheets S between the feed roller 175 and the separating roller 176a, a force smaller than the rotation allowable tangential force Ta is acted on the separating roller 176a. In other words, a rotation torque smaller than the set value is acted on the torque-limiter of the separating roller 176a, thereby the torque-limiter prevents the separating roller 176a from rotating in association with the feed roller 175.

[0039] In this case, when a coefficient of friction between the sheets S is given as .mu.S, the following expression (4) is established, thereby the separating roller 176a stops rotating. In this case, the upper sheet S in contact with the feed roller 175 is conveyed, and the lower sheet S in contact with the separating roller 176a is prevented from being conveyed by the separating roller 176a which is a halt state.

[0040] This is because surfaces of the feed roller 175 and the separating roller 176a are formed of a rubber material or the like having a relatively larger coefficient of friction and are usually constituted so as to satisfy .mu.r>.mu.S. As a result, a slippage is caused between the sheet S in contact with the feed roller 175 and the sheet S in contact with the separating roller 176a, to thereby separate the two sheets S from each other. Ta>.mu.s.times.N (4)

[0041] By the above operation of the torque-limiter, the sheets S are separated and conveyed to a downstream side.

[0042] In FIG. 24, the rotation allowable tangential force Ta of the separating roller 176a is set to an abscissa and an urging force N with respect to the feed roller of the separating roller 176a is plotted to an ordinate (hereinafter, this graph is referred to as "relationship diagram between the rotation allowable tangential force and the urging force"). Hereinafter, a coordinate on the relationship diagram between the rotation allowable tangential force and the urging force is expressed by (Ta, N).

[0043] When the above-mentioned expressions (3) and (4) are solved with respect to N, the following inequalities (5) and (6) are obtained. N > Ta .mu. .times. .times. r ( 5 ) N < Ta .mu. .times. .times. s ( 6 ) ##EQU2##

[0044] Here, when [straight line L1: N=Ta/.mu.r] obtained by the above inequality (5) is plotted onto the relationship diagram between the rotation allowable tangential force and the urging force, in a case where the coordinate (Ta, N) is present at a right side with respect to the straight line L1, a normal paper feeding operation cannot be performed due to a slip between the feed roller 175 and the sheet S even when only one sheet S is placed between the feed roller 175 and the separating roller 176a.

[0045] Further, when [straight line L2: N=Ta/.mu.s] obtained by the above inequality (6) is plotted onto the relationship diagram between the rotation allowable tangential force and the urging force, in a case where the coordinate (Ta, N) is present at a left side with respect to the straight line L2, a normal separating/feeding operation also cannot be performed since the separating roller 176a is rotated in association with the feed roller 175 and the two sheets S are not separated to be conveyed together.

[0046] As described above, in FIG. 24, a normal separating/feeding function can be obtained by collaborating the feed roller 175 with the separating roller 176a within the range indicated by a cross-hatching portion which is from the left side with respect to the straight line L1 to the right side with respect to the straight line L2. It should be noted that the cross-hatching portion is referred to as a feeding area.

[0047] Accordingly, the normal separating/conveying operation can be performed by collaborating the feed roller 175 with the separating roller 176a in the following situations. That is, the rotation allowable tangential force Ta of the separating roller 176a is set such that in a case where the radius Rr of the separating roller 176a is fixed in the above expression (1), for example, the set torque T of the torque-limiter is varied, and the urging force N of the separating roller is set within a range from N1 to N2 in a case where the torque-limiter varying within a range from Ta1 to Ta2 shown in FIG. 24 is used.

[0048] Next, an operation of the urging force of the separating roller 176a will be described.

[0049] As described above, with the structure in which the separating roller 176a is rotatably held at a rotating end portion of a supporting member 176c which is supported rotatably about a rotation axis 176d, a force increasing in accordance with the torque of the torque-limiter is generated in addition to a spring force of the spring 176e added by an urging force of the separating roller 176a.

[0050] Next, the mechanism of the force will be described with reference to FIG. 25. In FIG. 25, when reference symbol F denotes a tangential force which is received by the separating roller 176a from the sheet S to be conveyed, reference symbol L denotes a length between a center of the rotation axis 176d of the supporting member 176c and a center of the separating roller 176a (hereinafter referred to as "supporting member length"), reference symbol Sp denotes a spring force received by the spring 176e, and reference symbol a denotes an angle made by the supporting member 176c with respect to a tangential direction between the feed roller 175 and the separating roller 176a (hereinafter referred to as a "supporting member angle").

[0051] In this case, by taking into consideration a balance of moment with the rotation axis 176d of the separating roller 176a being as a center, the urging force N is solved, thereby the following expression (7) can be obtained. Sp .times. L + F .times. ( Rr + L .times. .times. sin .times. .times. .alpha. ) = N .times. L .times. .times. cos .times. .times. .alpha. .times. .times. N = Sp .times. L L .times. .times. cos .times. .times. .alpha. + F .times. ( Rr + L .times. .times. sin .times. .times. .alpha. ) L .times. .times. cos .times. .times. .alpha. ( 7 ) ##EQU3## Here, in a case where F is smaller than Ta, the separating roller 176a does not rotate. At the time point when F is equal to Ta, the separating roller 176a starts rotating, and after the time point, F is kept constant. Therefore, in a case where F is equal to Ta, a relationship between the urging force N and the rotation allowable tangential force Ta when the separating roller 176a is rotating can be expressed by the following expression (8). N = Sp .times. L L .times. .times. cos .times. .times. .alpha. + Ta .times. ( Rr + L .times. .times. sin .times. .times. .alpha. ) L .times. .times. cos .times. .times. .alpha. .times. .times. N = ( Rr + L .times. .times. sin .times. .times. .alpha. L .times. .times. cos .times. .times. .alpha. ) .times. Ta + Sp cos .times. .times. .alpha. ( 8 ) ##EQU4##

[0052] At this time, when Rr, L, Sp, and .theta. are kept constant, the expression (8) can be expressed as the following expression (9). N = kTa + Y .times. ( Where , k = Rr + L .times. .times. sin .times. .times. .alpha. L .times. .times. cos .times. .times. .alpha. = const . , Y = Sp cos .times. .times. .alpha. = const . ) ( 9 ) ##EQU5##

[0053] From the expression (9), it is apparent that when the separating roller 176a is rotating, the urging force N is linearly increased in accordance with the rotation allowable tangential force Ta of the separating roller 176a, that is, the set torque value of the torque-limiter of the separating roller 176a.

[0054] FIG. 26 shows that the expression (9) is plotted onto the relationship diagram between the rotation allowable tangential force and the urging force shown in FIG. 24. It is apparent that the urging force N is linearly increased in accordance with the rotation allowable tangential force Ta of the separating roller 176a, that is, the set torque value of the torque-limiter of the separating roller 176a.

[0055] It should be noted that a straight line L4 shown in FIG. 26, as shown in FIG. 27 for example, a separating roller 186a is constituted so as to be slidable in a direction toward a feed roller 185 and be urged by a spring 186e. The urging force N in the structure in which the urging force N is determined only by the force of the spring 186e of the separating roller 186a is plotted. It is known that, as compared with this structure, in the structure in which the separating roller is supported by the supporting member which is rotatably supported, the rotation allowable tangential force Ta having a range larger than the feeding area indicated by a cross-hatching portion, that is, the torque value of the torque-limiter having a wider range can be set. For example, the technique is disclosed in JP 03272572 B.

[0056] The above-described operations of the torque-limiter and the urging force are similarly acted on the sheet feeding portion in which the pickup roller and the feed roller are provided separately.

[0057] However, the conventional sheet feeding apparatus, the image forming apparatus, and the image reading apparatus having a rotation axis with a structure in which a separating roller is supported by a supporting member which is rotatably supported have a limitation to minimize the size of the sheet separating portion.

[0058] Next, the reason for the above will be described. FIG. 28 is a diagram showing a state in which, in the structure described in FIG. 23, the sheet S is conveyed by the feed roller 175 and the conveyed sheet S collides against the separating roller 176a. A direction of a conveying force of the conveyed sheet S colliding against the separating roller 176a is acted on a direction in which the separating roller 176a is apart from the feed roller 175 in proportion to a relationship between the supporting member 176c and the rotation axis 176d.

[0059] At this time, when the separating roller 176a is made apart being overwhelmed by the conveying force of the sheet S, the separating roller 176a which is rotating driven by the feed roller 175 stops rotating. When the separating roller 176a is thus stopped, the collided leading end of the sheet S may be damaged and folded. Accordingly, with the structure in which the separating roller 176a is supported by the supporting member 176c which is rotatably supported, a spring force enough to bear the conveying force of the sheet S must be required.

[0060] Here, in FIG. 28, reference symbol .beta. denotes an angle (hereinafter referred to as a "sheet feeding angle") made by a line connecting a contact point between the feed roller 175 and the sheet S and a center of the feed roller 175 and a normal line between the feed roller 175 and the separating roller 176a, reference symbol .gamma. denotes an angle (hereinafter referred to as an "entry point angle") made by a line connecting an entry point of the sheet S into the separating roller 176a and a center of the separating roller 176a and a tangential line of the feed roller 175 and the separating roller 176a, and reference symbol P denotes a conveying force of the sheet S applied by the feed roller 175.

[0061] Here, the spring force for bearing the conveying force P of the sheet S must be satisfied even when the tangential force received by the separating roller 176a from a rotation of the feed roller 175 is excluded. Next, the conditional expression obtained from the balance of rotation moment at the rotation axis 176d of the supporting member 176c, is expressed as follows. Sp.times.L+P cos .beta..times.(L sin .alpha.+Rr sin .gamma.)-N.times.L cos .alpha.+P sin .beta..times.(L cos .alpha.+Rr cos .gamma.) In this expression, for the spring pressure of the separating roller enough to bear the conveying force of the sheet S, the spring pressure of the separating roller which satisfies N>0 is an essential condition. Then, the condition of N>0 is added to the above expression, a condition for the spring force for bearing the conveying force of the sheet is given by the following expression (10). Sp > P L .times. { L .times. sin .function. ( .beta. - .alpha. ) + Rr .times. sin .function. ( .beta. - .gamma. ) } ( 10 ) ##EQU6## For example, FIG. 29 shows arrangements by four combinations (A to D) of the length and the angle between the supporting member 176c and the rotation axis 176d. With respect to the four combinations (A to D), the spring pressure of the separating roller for bearing the conveying force of the sheet is calculated, and the result is plotted on the relationship diagram between the rotation allowable tangential force and the urging force shown in FIG. 24.

[0062] Here, when the radius Rr of the separating roller 176a is set to 20 mm, the sheet feeding angle .beta. is set to 38.degree., and the entry point angle .gamma. is set to 57.degree.. In obtaining the conveying force P of the sheet to be entered, when the pressure of the sheet stacking plate pressurizing spring 152b is set to 0.4 N and the coefficient of friction between the feed roller 175 and the sheet S is set to 2 (in this case, the pressure of the sheet stacking plate pressurizing spring 152b and the coefficient of friction are need to be calculated on the maximum value), the conveying force P of the sheet is 0.8 N, which is used to calculate the conveying force P of the sheet to be entered.

[0063] In addition, reference symbols A to D indicate various combinations of the separating roller supporting angle .alpha. and the separating roller supporting member length L. The following Table 1 shows a relationship between a and L, the spring pressure Sp of the separating roller for bearing the sheet conveying force P obtained from the above expression (10), and the proportional multiplier k with respect to the rotation allowable tangential force Ta obtained from the above expression (8). TABLE-US-00001 TABLE 1 .alpha.(Deg) L(mm) Sp(N) k A 0 20 0.35 0.5 B 15 20 0.18 0.79 C 15 30 0.22 0.61 D 30 30 0 0.96

[0064] It is apparent that when the separating roller supporting angle .alpha. is increased in the relationship between A and B shown in Table 1, the spring pressure Sp of the separating roller for bearing the sheet conveying force P is decreased and the proportional multiplier k is increased. It is also apparent that when the separating roller supporting member length L is increased in the relationship between B and C shown in Table 1, the spring pressure Sp of the separating roller for bearing the sheet conveying force P is increased and the proportional multiplier k is decreased. In the position D, since the rotation axis 176d of the supporting member 176c is positioned at the lower part of FIG. 29 with respect to the direction of the conveying force P of the sheet S, the conveying force P of the sheet S is not worked in the direction in which the separating roller 176a is made apart from the feed roller 175, and the conveying force P of the sheet S is worked in the direction in which the separating roller 176a is jammed into the feed roller 175. Therefore, in the position D, the spring pressure Sp of the separating roller can be arbitrarily set without taking into consideration the effect of the sheet conveying force P.

[0065] FIG. 30 shows a result that the result of Table 1 is substituted for the above expression (8) and the result is plotted on the relationship diagram between the rotation allowable tangential force and the urging force. In this case, a coefficient of friction .mu.S between the sheets S and a coefficient of friction .mu.r between the sheet S and the separating roller 176a are set to the value approximate to the value described in JP 03272572 B, a coefficient of friction .mu.S between the sheets S is set to 0.8, and a coefficient of friction .mu.r between the sheet S and the separating roller 176a is set to 1.0. The rotation allowable tangential force Ta is set within a range from 0.2 N to 0.4 N.

[0066] Then, as shown in FIG. 30, in the case of A, the value of the spring pressure Sp of the separating roller is too high to intersect with the feeding area. In the case of B, though the value of the spring pressure Sp of the separating roller is small, the proportional multiplier k is so large that an overlapping area sufficient for the feeding area cannot be obtained. In the case of C, though it is better as compared with the cases of A and B, an overlapping area sufficient for the feeding area cannot be obtained.

[0067] On the other hand, in the case of D, since the sheet conveying force P does not affect the spring pressure Sp of the separating roller, the spring pressure Sp of the separating roller can be arbitrarily set. With such the advantage, the spring pressure of the separating roller can be set in accordance with the desired rotation allowable tangential force Ta as shown in FIG. 30.

[0068] However, even in the case D, it is necessary that the separating roller supporting angle .alpha. is increased and the supporting member length L is increased in order to secure the overlapping area sufficient for the feeding area. In other words, it is necessary that the supporting member 176c is inclined in a direction in which the sheet width is increased and the length of the supporting member 176c is increased in a direction of the sheet width.

[0069] As described above, in the conventional rotation supporting method of supporting the separating roller by the supporting member which is rotatably supported, the supporting member having at least a width about twice or more of the width of the separating roller is required. As a result, there is a limitation to minimize the size of the sheet separating portion. Further, since the separating roller and the sheet separating portion constituted of the separating pad described above have substantially the same width, it is difficult to compactly arrange the sheet separating portion as in the sheet feeding portion mounted on the printer described with reference to FIGS. 19 and 20 by adopting the rotation supporting method of supporting the separating roller by the supporting member which is rotatably supported.

SUMMARY OF THE INVENTION

[0070] The present invention has been made in view of the above problems, and therefore has an object to provide a sheet feeding portion (sheet feeding apparatus), an image forming apparatus, and an image reading apparatus which are capable of securing the sheet feeding performance and minimizing the space for the sheet feeding portion.

[0071] According to one aspect of the present invention,

[0072] a sheet stacking member configure to support sheets;

[0073] a sheet feeding member configure to feed the sheet from said sheet stacking member;

[0074] a sheet separating portion including a separating roller provided so as to abut against said sheet feeding member and a torque-limiter for allowing said separating roller to rotate in association with said sheet feeding member when a rotation torque larger than a predetermined torque is acted on said separating roller and for preventing said separating roller from rotating in association with said sheet feeding member when the rotation torque equal to or smaller than the predetermined torque is acted on said separating roller;

[0075] a guide member configure to support said sheet separating portion in a slidable manner and guiding said sheet separating portion with a predetermined angle in a direction different from a direction along which said sheet feeding member is opposed to said separating roller; and

[0076] an urging member configure to urge said sheet separating portion so that said separating roller is in pressure contact with said sheet feeding member.

BRIEF DESCRIPTION OF THE DRAWINGS

[0077] FIG. 1 is a schematic diagram of a structure of an electrophotographic printer as an example of an image forming apparatus including a sheet feeding apparatus according to a first embodiment of the present invention;

[0078] FIG. 2 is a diagram for explaining a structure of the sheet feeding apparatus;

[0079] FIG. 3 is a diagram for explaining a method of connecting a separating roller to a torque-limiter in a sheet separating portion provided to the sheet feeding apparatus;

[0080] FIG. 4A is a diagram for explaining another method of connecting a separating roller to a torque-limiter in a sheet separating portion, and FIG. 4B is a diagram for explaining another pressurizing method for a separating roller spring in the sheet separating portion;

[0081] FIG. 5 is a diagram for explaining a function of an urging force with respect to a feed roller of the separating roller in the sheet separating portion;

[0082] FIG. 6 is a diagram for explaining a state of a force working when the separating roller in the sheet separating portion collides with a sheet;

[0083] FIG. 7 is a relationship diagram between the rotation allowable tangential force and the urging force in the sheet separating portion;

[0084] FIGS. 8A and 8B are diagrams for explaining a method of changing an angle made by a guide surface of a guide member in the sheet separating portion.

[0085] FIG. 9 is a schematic diagram of a structure of a printer according to an application example of the first embodiment;

[0086] FIG. 10 is a diagram for explaining a structure of a sheet feeding apparatus provided to the printer;

[0087] FIG. 11 is a diagram for explaining a structure of a drawer of a sheet feed cassette of the printer;

[0088] FIG. 12 is a perspective view of the sheet separating portion of the sheet feeding apparatus provided to the printer;

[0089] FIG. 13 is a diagram for explaining a structure of a sheet feeding apparatus according to a second embodiment of the present invention;

[0090] FIG. 14 is a schematic diagram for showing a structure of an electrophotographic printer as an example of an image forming apparatus including a sheet feeding apparatus according to a third embodiment of the present invention;

[0091] FIG. 15 is a diagram for explaining a structure of a multiple sheet feeding portion provided to the printer;

[0092] FIGS. 16A and 16B are diagrams for explaining a structure of a sheet feeding apparatus according to a fourth embodiment of the present invention;

[0093] FIGS. 17A and 17B are diagrams for explaining another structure of the sheet separating portion provided to the sheet feeding apparatus;

[0094] FIGS. 18A and 18B are diagrams for explaining another structure of the sheet separating portion provided to the sheet feeding apparatus;

[0095] FIG. 19 is a schematic diagram for showing a structure of a conventional printer;

[0096] FIG. 20 is a diagram for explaining a structure of the sheet feeding portion provided to the conventional printer;

[0097] FIG. 21 is a schematic diagram for showing a structure of another conventional printer;

[0098] FIG. 22 is a diagram for explaining a structure of the sheet feeding portion provided to the conventional printer;

[0099] FIG. 23 is a diagram for explaining another structure of a conventional sheet feeding portion;

[0100] FIG. 24 is a relationship diagram between the rotation allowable tangential force and the urging force in a conventional sheet separating portion;

[0101] FIG. 25 is a diagram for explaining a function of a conventional urging force N;

[0102] FIG. 26 is a graph in which the urging force N is plotted on the relationship diagram between the rotation allowable tangential force and the urging force in the conventional sheet separating portion;

[0103] FIG. 27 is a diagram for explaining an urging method for a conventional separating roller;

[0104] FIG. 28 is a diagram for explaining a state in which the sheet collides with the separating roller;

[0105] FIG. 29 is a diagram for explaining a state in which the angle and the length of a conventional supporting member are modified; and

[0106] FIG. 30 is a graph in which the structure shown in FIG. 29 is plotted on the relationship diagram between the rotation allowable tangential force and the urging force in the conventional sheet separating portion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0107] Hereinafter, the best modes for carrying out the present invention will be described with reference to the drawings.

[0108] FIG. 1 is a schematic diagram of a structure of an electrophotographic printer as an example of an image forming apparatus including a sheet feeding apparatus according to a first embodiment of the present invention. However, dimensions, materials, configurations, relative arrangement, and the like of the structural elements described in this embodiment do not limit the present invention thereto as long as there is no particular specific description.

[0109] In FIG. 1, reference numeral 1 denotes a printer main body (hereinafter referred to as an "apparatus main body") and reference numeral 1A denotes an image forming portion. Below the image forming portion 1A, a sheet feeding apparatus 3 is arranged. The image forming portion 1A includes a laser scanner 10, an image forming process unit 9 including an image forming drum 9a serving as an image bearing member, and a transfer roller 9b for transferring a toner image formed on the photosensitive drum 9a onto a sheet S.

[0110] Reference numeral 11 denotes a fixing device for fixing the toner image on the sheet after the toner image formed by the image forming portion 1A is transferred onto the sheet. After the toner image is fixed on the sheet by the fixing device 11, the sheet S is sequentially discharged to and stacked on a discharge tray 14 provided on the uppermost part of the main body of an apparatus.

[0111] Further, as shown in FIG. 2, a sheet feeding apparatus 3 includes a sheet feed cassette 2 serving as a sheet stacking member, a feed roller 5 serving as a sheet feeding member for feeding the sheets stored in the sheet feed cassette 2, a separating roller 6a brought into pressure contact with the feed roller 5, and a sheet separating portion 6 for separating the sheets S fed by the feed roller 5 one by one.

[0112] Here, the feed roller 5 also serves as a pickup roller and is rotatably provided to the sheet feed cassette 2. The feed roller 5 is in contact with the uppermost sheet S1 stacked on a sheet stacking plate 2a for stacking the sheets S, and at the same time, the feed roller 5 is in contact with the separating roller 6a at a downstream side in a sheet conveying direction. It should be noted that the sheet stacking plate 2a is pressed upwardly by a sheet stacking plate pressure spring 2b from the back surface side (lower side in FIG. 1) of the sheet stacking plate 2a, and a leading end of the uppermost sheet S1 stacked on the sheet stacking plate 2a is pressed against the feed roller 5.

[0113] Then, in the printer provided with the sheet feeding apparatus 3 having the above structure, when the feed roller 5 rotates counterclockwise in FIGS. 1 and 2 owing to a drive of a driving motor (not shown), the uppermost sheet S1 stacked on the sheet stacking plate 2a is received, and then the sheet S1 is separated from other sheets by the sheet separating portion 6 to be conveyed in a downstream direction.

[0114] Thus, the separated sheet S1 is then conveyed to the transfer portion constituted of the electrophotographic photosensitive drum 9a and the transfer roller 9b through a pair of conveying rollers 7 and a pair of registration rollers 8. At this time, on a surface of the electrophotographic photosensitive drum 9a, a toner image is formed by a laser beam outputted from the laser scanner 10 positioned above the electrophotographic photosensitive drum 9a. The toner image is then transferred onto the conveyed sheet S1 at the transfer portion.

[0115] Then, the sheet S1 onto which the toner image has been transferred is conveyed in a downstream direction, and the toner image is fused and fixed on the sheet S1 after being heated and pressurized by the fixing device 11. After that, thus processed sheet S is sequentially stacked on the discharge tray 14 through a pair of conveying rollers 12 and a pair of discharge rollers 13 which serve as sheet discharge means.

[0116] On the other hand, the sheet separating portion 6 of the sheet feeding apparatus 3 includes, as shown in FIG. 2, the separating roller 6a, a supporting member 6c, a separating roller spring 6d serving as an urging member, and a guide member 6e. The separating roller 6a is positioned at a downstream side of a contact point between the feed roller 5 and the uppermost sheet S1. In addition, the guide member 6e is fixed to the apparatus main body 1 and for slidably supporting the separating roller 6a, the supporting member 6c, and the separating roller spring 6d. The separating roller 6a is slidable in a vertical direction while being guided to a flat guide surface of the guide member 6e.

[0117] Here, since the separating roller 6a is rotatably supported at a top end of the supporting member 6c and is urged on the upper side by the separating roller spring 6d through the separating member 6c, the separating roller 6a is contacted with the feed roller 5 slidably in a vertical direction.

[0118] In other words, in this embodiment, the separating roller 6a is not slidably contacted by pressure in a direction in which the feed roller 5 is opposed to the separating roller 6a, that is, in a direction in which the separating roller 6a surfaces the center of the feed roller 5, but is slidably constituted by the guide member 6e in a vertical direction. Thus, the separating roller 6a is pressure-contacted with the feed roller 5 by a predetermined angle with respect to a direction in which the separating roller 6a surfaces the center of the feed roller 5. Then, by constituting the separating roller 6a which is slidable in a vertical direction as described above, the sheet separating portion 6 can be constituted with an area substantially equal to the width (diameter) of the separating roller 6a.

[0119] On the other hand, the separating roller 6a is not driven by a motor or the like and is connected to a torque-limiter 6b on the rotation axis. FIG. 3 is a diagram for explaining a method of connecting the separating roller 6a to the torque-limiter 6b which is viewed from a sheet conveying direction of the sheet separating portion 6.

[0120] Here, the separating roller 6a is rotatably connected to a rotation axis 6f through the torque-limiter 6b. The separating roller 6a is rotatable when rotation torque equal to or larger than the set torque of the torque-limiter 6b is generated. The method of connecting the torque-limiter 6b is not limited to this, and it is possible to connect the torque-limiter 6b so as to be substantially embedded in the separating roller 6a as shown in FIG. 4A.

[0121] Further, a pressurizing method for the separating roller spring 6d is not also limited to the method shown in FIG. 3, it is possible to adopt a pressurizing method for two separating roller springs 6d, for example, as shown in FIG. 4B, as long as elasticity working in a direction in which the feed roller 5 is pressurized along the guide member 6e is obtained.

[0122] The sheet separating performance of the sheet separating portion 6 is based on the function of the braking torque of the torque-limiter 6b in connection with the separating roller rotation axis.

[0123] Here, in relation to the function and sheet separating performance of the torque-limiter 6b, the above-described relationships (1) to (6) are established. In the relationship diagram between the rotation allowable tangential force and the urging force shown in FIG. 24, the feeding area is established. Accordingly, in a case where one sheet is placed between the feed roller 5 and the separating roller 6a, the rotation allowable tangential force Ta and the urging force N are required to be controlled to be within the feeding area of FIG. 24 so that the torque-limiter 6b allows the separating roller 6a to rotate in association with the feed roller 5 without a slip. When two sheets are placed between the feed roller 5 and the separating roller 6a, the torque-limiter 6b prevents the separating roller 6a from rotating in association with the feed roller 5 to separate the sheets.

[0124] Next, a function of the urging force N with respect to the feed roller 5 of the separating roller 6a according to this embodiment will be described. Also in this embodiment, as described above, the urging force N is increased in accordance with the rotation allowable tangential force Ta of the separating roller 6a, that is, the set torque value of the torque-limiter 6b of the separating roller 6a when the separating roller 6a is rotating.

[0125] Such the mechanism will be described with reference to FIG. 5. FIG. 5 is a diagram for explaining a state in which one sheet S is conveyed while being sandwiched between the feed roller 5 and the separating roller 6a. It should be noted that in FIG. 5, the forces working similarly to those of FIG. 25 are indicated by the identical reference symbols and the explanation thereof is omitted. In addition, reference symbol "f" denotes a reaction force received from the guide surface of the guide member 6e, reference symbol "n" denotes a friction force between the guide surface of the guide member 6e and the supporting member 6c, and reference symbol .theta. denotes an angle made by the guide surface of the guide member 6e which is a predetermined angle with respect to a direction in which the feed roller 5 is opposed to the separating roller 6a (hereinafter referred to as a "guide surface angle").

[0126] In FIG. 5, when a horizontal direction is set to an X direction and a vertical direction is set to a Y direction, the following expression can be obtained with respect to a balance of two directions with the separating roller 6a as a center.

[0127] Expression representing a force balance in the X direction: N.times.sin .theta.+F.times.cos .theta.=f (11)

[0128] Expression representing a force balance in the Y direction: N.times.cos .theta.+n=F.times.sin .theta.+Sp (12) Here, in a case where F is smaller than Ta, the separating roller 6a does not rotate. At the time when F is equal to Ta, the separating roller 6a starts rotating, and after that, F is kept constant and N is also made constant. In addition, when a coefficient of friction between the guide member 5e and the supporting member 6c is .mu.n, n=.mu.n.times.f is established.

[0129] The above conditional expression is substituted for the expressions (11) and (12), and such the expression is solved with respect to N, a relational expression between the urging force N when the separating roller 6a is rotating and the rotation allowable tangential force Ta of the separating roller 6a is expressed as the following expression (13). N = sin .times. .times. .theta. - .mu. .times. .times. n .times. cos .times. .times. .theta. cos .times. .times. .theta. + .mu. .times. .times. n .times. sin .times. .times. .theta. .times. Ta + 1 cos .times. .times. .theta. + .mu. .times. .times. n .times. sin .times. .times. .theta. .times. Sp ( 13 ) ##EQU7## Then, as apparent from the expression (13), when a position of the separating roller 6a is fixed, in other words, when the value .theta. is fixed, N = kTa + Y .times. ( Where , k = sin .times. .times. .theta. + .mu. .times. .times. n .times. cos .times. .times. .theta. cos .times. .times. .theta. - .mu. .times. .times. n .times. sin .times. .times. .theta. = const , Y = 1 cos .times. .times. .theta. - .mu. .times. .times. n .times. sin .times. .times. .theta. .times. P = const . ) ( 14 ) ##EQU8## is obtained. When the separating roller 6a is rotating, the urging force N is linearly increased in accordance with the rotation allowable tangential force Ta of the separating roller 6a, that is, the set torque value of the torque-limiter of the separating roller 6a.

[0130] Further, assuming that the value .mu.n is sufficiently small, the expression (13) can be expressed by the expression (15). N = sin .times. .times. .theta. cos .times. .times. .theta. .times. Ta + Sp cos .times. .times. .theta. .times. .times. N = ( tan .times. .times. .theta. ) .times. Ta + Sp cos .times. .times. .theta. ( 15 ) ##EQU9##

[0131] Next, a function in a case where the sheet S fed from the feed roller 5 collides with the separating roller 6a with reference to FIG. 6. FIG. 6 is a diagram for explaining a state of a force working when the separating roller 6a according to this embodiment collides with the sheet S.

[0132] Here, when the sheet S collides with the separating roller 6a, the separating roller 6a is required to be contact with the feed roller 5 without being apart from the feed roller 5 by being overwhelmed by the conveying force of the sheet S even when the tangential force received from the feed roller 5 is excluded.

[0133] Then, as a condition for the separating roller 6a not to be apart from the feed roller 5 but to be contact with the feed roller 5, the following expression can be obtained with respect to a balance of two directions with the separating roller 6a as a center in a case where an angle between a conveying direction of the sheet S and the guide member 6e is set to .delta. and a horizontal direction and a vertical direction are set to X and Y directions, respectively, as shown in FIG. 6.

[0134] Expression representing a force balance in the X direction: P .times. .times. cos .function. ( .pi. 2 - .delta. ) + N .times. .times. sin .times. .times. .theta. = f ( 16 ) ##EQU10##

[0135] Expression representing a force balance in the Y direction: P .times. .times. sin .function. ( .pi. 2 - .delta. ) + Sp = N .times. .times. cos .times. .times. .theta. + n ( 17 ) ##EQU11##

[0136] When the conditional expression n=.mu.n.times.f is substituted for the expressions (16) and (17), and when the substituted expression is solved with respect to N, the following expression is obtained. N = P .times. .times. cos .times. .times. .delta. - .mu. .times. .times. n .times. .times. P .times. .times. sin .times. .times. .delta. + Sp cos .times. .times. .theta. + .mu. .times. .times. n .times. .times. sin .times. .times. 0 ( 18 ) ##EQU12## In the expression (18), with respect to the spring pressure of the separating roller enough to bear the conveying force of the sheet S, the spring pressure of the separating roller of N>0 is a necessary condition, so when the above expression (18) is added with the condition of N>0, a spring force enough to bear the conveying force of the sheet is obtained by the following expression (19). Sp>P(.mu.n P sin .delta.-cos .delta.) (19) At this time, assuming that the value .mu.n is sufficiently small, the expression (19) is expressed as the following expression (20). Sp>P(-cos .delta.) (20) Then, when .delta. is smaller than 90.degree. in this expression (20), P (-cos .delta.)<0 is established, and it is apparent from the expression that the spring pressure of the separating roller can be arbitrarily set regardless of the conveying force of the sheet S. To the contrary, when .delta. is larger than 90.degree. in this expression (20), P (-cos .delta.)>0 is established, and it is apparent from the expression that the spring force is affected by the conveying force of the sheet S and the spring force for bearing the conveying force of the sheet S must be set.

[0137] This is because when .delta. is smaller than 90.degree., a component of force of the sheet conveying force P is received by the reaction force "f" from the guide member 6e and the reaction force of the feed roller 5 (urging force N), so the force is acted on the separating roller 6a in an upward direction in FIG. 6 and is not worked in a downward direction in FIG.6. In other words, since the separating roller 5a is not made apart from the feed roller 4 by the collision of the sheet S, the leading end of the sheet S is prevented from being damaged by stopping rotation of the separating roller 5a as described above.

[0138] However, when 5 is larger than 90.degree., a component of force of the sheet conveying force P is received by the reaction force "f" from the guide member 6e and the force Sp of the separating roller spring 6d, so the force is acted on the separating roller 6a in a downward direction. Accordingly, a spring for not being overwhelmed by the conveying force of the sheet S must be set on the basis of the above expression.

[0139] As described above, when the angle 6 between the conveying direction of the sheet S and the guide member 6e is set to be equal to or smaller than 90.degree., the sheet conveying force P can be prevented from working in a direction in which the separating roller 6a is made apart from the feed roller 5. As a result, since the spring pressure of the separating roller spring 6d of the separating roller is not limited by the collision of the sheet S, the spring pressure of the separating roller can be arbitrarily set in order to stabilize the separating/conveying operation to be described later.

[0140] Next, an influence on the sheet feeding performance will be verified when the structure according to this embodiment is plotted on the relationship diagram between the rotation allowable tangential force and the urging force.

[0141] FIG. 7 is the relationship diagram between the rotation allowable tangential force and the urging force in the sheet separating portion 6 according to this embodiment. As described above, when the separating roller 6a and the guide member 6e are arranged so that the angle .delta. between the conveying direction of the sheet S and the guide member 6e is set to be equal to or smaller than 90.degree., the urging force N (Y-intercept of FIG. 7) when Ta is equal to 0 can be arbitrarily set since the sheet S is not limited by the collision with the separating roller 6a. In addition, a slope can be changed in accordance with the angle made by the guide member 6e, and the guide surface angle .theta. can be selected in accordance with the desired rotation allowable tangential force Ta, that is, the torque value of the torque-limiter.

[0142] For example, it is assumed that a straight line A passing through the points (200, 250) and (400, 400) is set with the separating roller 6a, the guide member 6e, and the separating roller spring Sp. From the above expression (15), the slope of the straight line A is obtained by tan .theta.. Thus, the guide surface angle .theta. is expressed as the following expression. tan - 1 .times. .theta. = 400 - 250 400 - 200 = 150 200 = 0.75 ##EQU13## .theta. = 36.8 .times. .times. ( Deg ) ##EQU13.2##

[0143] Further, from the expression (15), the value of Y-intercept of FIG. 7 is expressed as Sp/cos .theta.. Thus, Sp=0.8N can be set regardless of the influence of the sheet conveying force P.

[0144] A straight line B shown in FIG. 7 passes through the point (200, 250) and the slope of the strait line B is 1. In a case where the straight line B is set with the separating roller 6a, the guide member 6e, and the spring pressure Sp of the separating roller, the guide surface angle .theta. and the spring pressure Sp of the separating roller may be set as follows. tan.sup.-1.theta.=1 .theta.=45(deg.) Further, from the above expression (15), since the value of Y-intercept of FIG. 7 is expressed as Sp/cos .theta., Sp=0.34 N is established.

[0145] As described above, the guide surface angle .theta. and the spring pressure Sp of the separating roller can be arbitrarily set as described above. It should be noted that the upper limit of the slope of FIG. 7 is expressed as the following expression (21) so as to generate an intersection with the straight line of N=1/.mu.S.times.Ta. tan .times. .times. .theta. < 1 .mu. .times. .times. s ( 21 ) ##EQU14##

[0146] Accordingly, the condition for .theta. is expressed as the following expression (22). .theta. < tan - 1 .function. ( 1 .mu. .times. .times. s ) ( 22 ) ##EQU15##

[0147] For example, when .mu.S is equal to 0.8, the upper limit of .theta. is 51.3.degree..

[0148] As described above, when the guide member 6e is arranged so that the angle .delta. between the conveying direction of the sheet S and the guide member 6e is set to be equal to or smaller than 90.degree., the sheet conveying force P can be prevented from working in a direction in which the separating roller 6a is made apart from the feed roller 5. As a result, since the spring pressure Sp of the separating roller is not limited by the collision of the sheet S, the spring pressure of the separating roller can be arbitrarily set.

[0149] Further, the guide surface angle .theta. can be set with the upper limit as the above expression (22) in accordance with the desired rotation allowable tangential force Ta, that is, the desired torque-limiter value of the separating roller 6a.

[0150] As a result, in the sheet separating portion 6 constituted within a small space having a width substantially equal to that of the separating roller as described above, the sheet feeding performance of preventing double feeding and slip can be secured, and thus the space for the sheet separating portion 6 and the sheet feeding apparatus 3 can be minimized.

[0151] Therefore, the sheet separating portion 6, which can give the sheet feeding performance more stable than the sheet separating means using the separating pad, can be constituted by a width equal to that of the sheet separating means using the separating pad. In addition, the sheet separating portion 6 can be constituted by a half width as compared with the rotation supporting method of supporting the separating roller by a supporting member which is supported by a conventional rotation axis as shown in FIG. 21 and the like as described above.

[0152] As a method of changing the guide surface angle .theta., there are two methods: a method of changing the guide surface angle .theta. by rotating the separating roller 6a and the guide member 6e with the feed roller 5 as the center as shown in FIG. 8A; and a method of changing the guide surface angle .theta. by rotating the guide member 6e with the separating roller 6a as the center as shown in FIG. 8B. In both of the methods, when .delta. is smaller than 90.degree. and the condition of the expression (22) is satisfied, the above effect can-be obtained.

[0153] As described above, when the sheet separating roller 6a is slidably supported by the guide member 6e and is guided with a predetermined angle in a direction different from a direction in which the feed roller 5 is opposed to the separating roller 6a, the sheet separating portion 6 can be constituted by a width substantially equal to that of the separating roller. With this structure, the space for the sheet separating portion 6 and the sheet feeding apparatus 3 can be minimized while securing the feed performance.

[0154] Next, as an application example of this embodiment, a printer including a multiple-stage sheet feeding apparatus and an option sheet feeding apparatus will be described.

[0155] FIG. 9 is a schematic diagram of a structure of a printer according to an application example of this embodiment. In FIG. 9, a reference numeral 51 is a printer main body (hereinafter referred to as an "apparatus main body"), reference numeral 51A is an image forming portion. Below the image forming portion 51A, an upper sheet feeding apparatus 52 and a lower sheet feeding apparatus 56 are arranged by overlapping with each other. The image forming portion 51A is provided with a laser scanner 64, an image forming process unit 63 including a photosensitive drum 63a as an image bearing member, a transfer roller 63b for transferring a toner image formed on the photosensitive drum 63a onto a sheet S, and the like.

[0156] Reference numeral 65 denotes a fixing device for fixing a toner image on a sheet after the toner image formed by the image forming portion 51A is transferred onto the sheet. After the toner image is fixed by the fixing device 65, the sheet S is sequentially discharged to and stacked on a discharge tray 69 provided on an uppermost portion of the apparatus main body. In addition, reference numeral 81 denotes an option sheet feeding apparatus provided with a sheet feeding apparatus 82, and the option sheet feeding apparatus 81 is selectively mounted on the bottom surface of the apparatus main body 51.

[0157] In FIG. 10, reference numeral 70 denotes a conveying path for the option sheet feeding apparatus for conveying the sheet S fed from the option sheet feeding apparatus 81 upwardly (to a downstream side), the conveying path 70 for the option sheet feeding apparatus merges into a conveying path 71 for the lower sheet feeding portion for conveying the sheet S fed from the lower sheet feeding apparatus 56 upwardly (to the downstream side).

[0158] Further, a conveying path 72 for the upper sheet feeding portion for conveying the sheet S fed from the upper sheet feeding apparatus 52 upwardly (to the downstream side) merges into the conveying path 71 for the lower sheet feeding portion. Thus, the sheets fed from the option sheet feeding apparatus 81, the lower sheet feeding apparatus 56, and the upper sheet feeding apparatus 52 pass through a sheet feeding portion merging conveying path 73 to be conveyed to the downstream side.

[0159] Then, the sheet S which has passed through the sheet feeding portion merging conveying path 73 to be conveyed to the downstream side is conveyed to the transferring portion constituted of the photosensitive drum 63a and a transfer roller 63b, in synchronism with an image formed on the photosensitive drum 63a by a registration roller 62 shown in FIG. 9. In the transferring portion, a toner image is transferred onto the sheet S.

[0160] It should be noted that the sheet S onto which the toner image has been transferred is then conveyed to the fixing device 65, and the toner image is fixed on the sheet S by applying pressure and heat at the fixing device 65. After that, the sheet S is conveyed through conveying roller pairs 66 and 67 and a discharge roller pair 68, and is sequentially discharged to and stacked on the discharge tray 69.

[0161] Next, the upper sheet feeding apparatus 52 and an upper sheet feed cassette 55 provided to the upper sheet feeding apparatus 52 will be described. As shown in FIG. 10, similar structures apply to the sheet feeding apparatus 82 provided with a feed roller 83 and the sheet separating portion 84 which are provided to the option sheet feeding apparatus 81, the lower sheet feeding apparatus 56 provided with the feed roller 57 and the sheet separating portion 58, and sheet feed cassettes 59 and 85 provided to the option sheet feeding apparatus 81 and the lower sheet feeding apparatus 56, respectively.

[0162] The upper feeding cassette 55 is provided with a sheet stacking plate 55a for stacking the sheets S in a rotatable manner as shown in FIG. 10. The sheet stacking plate 55a is pressed upwardly by a sheet stacking plate pressure spring 55b serving as the urging member, from a back surface side (lower side in FIG. 10) of the sheet stacking plate 55a. A leading end of the uppermost sheet S1 stacked on the sheet stacking plate 55a is pressed against the upper feed roller 53.

[0163] Then, the upper sheet feed roller 53 is contacted with the upper sheet separating portion 54 at a downstream side of the sheet conveying direction. It should be noted that functions of a separating roller 54a, a torque-limiter 54b, a supporting member 54c, a separating roller spring 54d, and a guide member 54e which constitute the upper sheet separating portion 54, a positional relationship between each of those components and the upper feed roller 53, and an angle made when the sheet S collides with the separating roller 54a are similar to those of the above embodiment. Accordingly, since the effects and the function thereof are similar to those of the embodiment, an explanation thereof will be omitted.

[0164] Further, the upper sheet feed cassette 55 is structured so as to be drawn out in a width direction orthogonal to the sheet conveying direction as shown in FIG. 11. The upper sheet feed cassette 55 is drawn out along rails 75a and 75b which are positioned at both ends of the upper sheet feed cassette 55, as shown in FIGS. 9, 10, and 11.

[0165] In FIG. 11, reference symbol 55c denotes an upper sheet-side regulating plate for regulating a position in a width direction of a sheet bundle prepared on the sheet stacking plate 55a, and reference symbol 55d denotes an upper sheet-back-end regulating plate for regulating a back-end position of a sheet bundle prepared on the sheet stacking plate 55a. The upper sheet separating portion 54 is mounted at a side of the sheet conveying direction of the upper sheet feed cassette 55. In other words, in this application example, the upper feed roller 53 remains in the apparatus main body 51, the upper sheet separating portion 54 is detachably attached to the apparatus main body 51 together with upper sheet feed cassette 55 as shown in FIG. 12.

[0166] In this application example, as shown in FIG. 10, a bottom surface of the upper sheet feed cassette 55 is overlapped with the lower feed roller 57 of the lower sheet feeding apparatus 56, to thereby reduce the height of the apparatus main body by the overlapped portion. Since the upper sheet separating portion 54 requires only a small space, the upper sheet separating portion 54 can be close to a position in a lateral direction of the lower feed roller 57. As a result, a compact product can be realized in which the width and height of the apparatus main body can be suppressed.

[0167] In addition, when the option sheet feeding apparatus 81 is added and the conveying path 70 for the option sheet feeding apparatus is arranged to be adjacent to the lower sheet separating portion 58, the lower sheet separating portion 58 requires only a small space. Thus, a compact product can be realized in which the width of the apparatus main body can be suppressed.

[0168] In the above description, though the feed roller which can be used as a pickup roller is used, the present invention is not limited to this. It is possible to adopt a sheet feeding apparatus using a feed roller and a pickup roller separately.

[0169] Next, a second embodiment of the present invention will be described in which a feed roller and a pickup roller are separately used.

[0170] FIG. 13 is a diagram for explaining a sheet feeding apparatus according to the second embodiment of the present invention. It should be noted that in FIG. 13, the same reference symbols as those of FIG. 2 indicate the identical or corresponding portions, and explanations thereof are omitted.

[0171] In FIG. 13, reference numeral 4 denotes the pickup roller and reference numeral 5A denotes a feed roller. The feed roller 5A is positioned at a downstream side of the pickup roller 4. The separating roller 6a provided to the sheet separating portion 6 opposite to the feed roller 5A is contacted with the feed roller 5A. The separating roller 6a is supported by the guide member 6e with respect to the feed roller 5A with a predetermined angle in a slidable manner. In addition, the sheet stacking plate 2a is urged by the sheet stacking plate pressure spring 2b such that a leading end of the uppermost sheet S1 stacked on the sheet stacking plate 2a is pressed against the pickup roller 4

[0172] Then, when the pickup roller 4 is rotated counterclockwise as shown in FIG. 13 by a drive of a driving motor (not shown), a conveying force is applied to the uppermost sheet S1 stacked on the sheet stacking plate 2a, and then the sheet S1 is conveyed between the feed roller 5A and the separating roller 6a. After that, the sheets S are separated one by one to be conveyed to the image forming portion or the image reading portion (not shown). It should be noted that a function of the torque-limiter between the feed roller 5A and the separating roller 6a, a change of the urging force N, and securing of the paper feed performance are similar to those of the first embodiment, so explanations thereof are omitted.

[0173] Here, also in this embodiment, the following two conditions are satisfied.

[0174] 1. The guide member 6e is disposed so that the angle .delta. formed by the conveying direction of the sheet S and the guide member 6e is equal to or smaller than 90.degree..

[0175] 2. The guide surface angle .theta. is obtained as follows. .theta. < tan - 1 .function. ( 1 .mu. .times. .times. s ) ##EQU16##

[0176] When the guide surface angle .theta. and the spring pressure Sp of the separating roller are set to satisfy the above two conditions and aim to obtain the feed area in the relationship diagram between the rotation allowable tangential force and the urging force as shown in FIG. 7, the sheet feeding performance can be secured.

[0177] As described above, similarly to the first embodiment, the sheet feeding apparatus 3 in which the pickup roller 4 and the feed roller 5A are separately arranged can be constituted with a width of the sheet separating portion 6 substantially equal to that of the separating roller while maintaining the sheet feeding performance, thereby making it possible to minimize the space for the sheet separating portion 6 and the space for the sheet feeding apparatus 3.

[0178] Next, a third embodiment of the present invention will be described.

[0179] FIG. 14 is a cross-sectional view for explaining a structure of an electrophotographic printer as an example of an image forming apparatus including a sheet feeding apparatus according to the third embodiment of the present invention. It should be noted that in FIG. 14, the same reference symbols as those of FIG. 1 indicate the identical or corresponding portions.

[0180] In FIG. 14, reference numeral 23 denotes a multiple sheet feeding portion serving as a sheet feeding apparatus provided at a right surface side of the apparatus main body 1. As shown in FIG. 15, the multiple sheet feeding portion 23 includes a multiple cover 22c serving as the sheet stacking member which can be opened and closed with respect to the apparatus main body 1 and is arranged with a certain angle, a multiple sheet stacking plate 22a for stacking the sheets S, the multiple sheet stacking plate 22a being supported by the multiple cover 22c in a swingable manner, and a multiple feed roller 25 serving as a sheet feeding member.

[0181] The multiple sheet stacking plate 22a is pressed upwardly by a multiple sheet stacking plate pressure spring 22b serving as the urging member from the back surface side (lower side in FIG. 15) of the multiple sheet stacking plate 22a. A leading end of the uppermost sheet S1 stacked on the multiple sheet stacking plate 22a is pressed against the multiple feed roller 25. The multiple cover 22c fixes an angle made by the multiple sheet stacking plate 22a with respect to the apparatus main body 1 and serves as a pedestal for the multiple sheet stacking plate pressure spring 22b.

[0182] The multiple feed roller 25 is contacted with the uppermost sheet S1 stacked on the multiple sheet stacking plate 22a, and at the same time, is contacted with a multiple separating roller 26a provided to a multiple sheet separating portion 26 at a downstream side of the sheet conveying direction.

[0183] The multiple sheet separating portion 26 includes, in addition to the multiple separating roller 26a, a torque-limiter 26b coaxially connected to the multiple sheet separating portion 26, a supporting member 26c for rotatably supporting the multiple separating roller 26a through the torque-limiter 26b, a multiple separating roller spring 26d for pressing the supporting member 26c from the back surface side of the supporting member 26c (lower side in FIG. 15), and a guide member 26e including a guide surface having a predetermined angle with respect to a direction in which the multiple feed roller 25 is opposed to the multiple separating roller 26a.

[0184] The multiple separating roller 26a is rotatably supported by the supporting member 26c through the toque-limiter 26b, and multiple separating roller 26a is slidably held by the guide member 26e. It should be noted that a method of connecting the multiple separating roller 26a to the torque-limiter 26b is similar to that of FIG. 3 described above.

[0185] Further, an angle formed by the guide member 26e and a direction along which the multiple feed roller 25 is opposed to the multiple separating roller 26a and an angle formed by the guide member 26e and a direction along which the sheet S stacked on the multiple sheet stacking plate 22a collides with the multiple separating roller 26a are similar to the relation among the feed roller 5, the separating roller 6a, and the conveying sheet S, described in the first embodiment. Accordingly, a function of the torque-limiter between the multiple feed roller 25 and the multiple separating roller 26a, a change in the urging force N, and securing of the paper feed performance are similar to those of the first embodiment.

[0186] Therefore, also in this embodiment, the following two conditions are satisfied.

[0187] 1. The guide member 26e is disposed so that the angle .delta. formed by the conveying direction of the sheet S and a guide surface of the guide member 26e is equal to or smaller than 90.degree..

[0188] 2. The guide surface angle .theta. is obtained as follows. .theta. < tan - 1 .function. ( 1 .mu. .times. s ) ##EQU17##

[0189] When the guide surface angle .theta. and the spring pressure Sp of the separating roller are set to satisfy the above two conditions and aim to obtain the feed area in the relationship diagram between the rotation allowable tangential force and the urging force as shown in FIG. 7, the sheet feeding performance can be secured.

[0190] Therefore, even in the multiple sheet feeding portion 23 (sheet feeding apparatus) including the multiple cover 22c (sheet stacking member) in which an angle for stacking sheets S is not substantially horizontal with respect to the apparatus main body 1, and in the printer (image forming apparatus) including such the multiple sheet feeding portion 23, when the guide member 26e is arranged with a predetermined angle with respect to a direction in which the multiple feed roller 25 is opposed to the multiple separating roller 26a, and the multiple separating roller 26a is slidably supported by the guide member 26e, it is possible to constitute the multiple sheet separating portion 26 with a width substantially equal to that of the separating roller while maintaining the sheet feeding performance, as described in the second embodiment.

[0191] As described above, as long as the condition for the guide surface angle .theta. and the angle .delta. formed by the sheet conveying direction and the guide member 26e is satisfied in the multiple sheet feeding portion 23 (sheet feeding apparatus), the angle with respect to the apparatus main body 1 can be arbitrarily set. In other words, as long as the condition for the guide surface angle .theta. and the angle .delta. formed by the sheet conveying direction and the guide member 26e is satisfied, even if the multiple cover 22c is arranged perpendicularly to the apparatus main body 1, or even if the feed roller is arranged below the sheet stacking portion (bottom separating method), it is possible to constitute the sheet separating portion with a width substantially equal to that of the separating roller while maintaining the sheet feeding performance.

[0192] Next, a fourth embodiment of the present invention will be described.

[0193] FIGS. 16A and 16B are diagrams for explaining a structure of a sheet feeding apparatus according to the fourth embodiment of the present invention. It should be noted that in FIGS. 16A and 16B, the same reference symbols as those of FIG. 2 indicate the identical or corresponding portions.

[0194] In FIG. 16A, reference numeral 86 denotes a supporting member for rotatably supporting the separating roller 6a through the torque-limiter 6b, and reference numeral 87 denotes a guide member. As shown in FIG. 16A, the supporting member 86 is provided with ribs 86a and is shifted in a vertical direction while sliding the ribs 86a along the guide member 87.

[0195] With such a structure in which the supporting member 86 is shifted in a vertical direction while sliding the ribs 86a along the guide member 87, a contact area with the guide member 87 when the supporting member 86 is shifted can be made small, thereby making it possible to suppress the effect of a frictional resistance caused when the supporting member 86 is shifted. As shown in FIG. 16B, even when ribs 89a are provided to a guide member 89, it is possible to suppress the effect of the frictional resistance caused when the supporting member 88 is shifted.

[0196] Further, as shown in FIG. 17A, it is possible to make a shape of each of ribs 90a of a supporting member 90 to be arcuate. In this case, each of the ribs 90a is in a point contact with a guide member 91, thereby making it possible to suppress the sliding resistance caused when the supporting member 90 is shifted. Alternatively, as shown in FIG. 17B, it is also possible to make a shape of each of ribs 93a of a guide member 93 to be arcuate. In this case, each of the ribs 93a is in a point contact with the supporting member 92, thereby making it possible to suppress the sliding resistance caused when the supporting member 92 is shifted.

[0197] Further, alternatively, as shown in FIG. 18A, supporting member 94 is provided with rollers 94a in a rotatable manner in place of the ribs, and the supporting member 94 is shifted in a vertical direction while rotating the rollers 94a along a guide member 95.

[0198] With such a structure in which the supporting member 94 is shifted in a vertical direction while rotating the rollers 94a along the guide member 95, a contact area with the guide member 95 when the supporting member 94 is shifted can be made small, thereby making it possible to suppress the effect of a frictional resistance caused when the supporting member 94 is shifted. As shown in FIG. 18B, even when rollers 97a are provided to a guide member 97 in a rotatable manner, it is possible to suppress the effect of the frictional resistance caused when the supporting member 96 is shifted.

[0199] With any structure described with reference to FIGS. 16A and 16B, FIGS. 17A and 17B, and FIGS. 18A and 18B, the effect of the present invention can be obtained. In addition, the structure is not limited to those described above, it is possible to adopt any structure as long as the feed roller and the separating roller are slidably arranged with a predetermined angle with respect to a direction in which the feed roller and the separating roller are opposed to each other.

[0200] In this embodiment, the description is made by taking as an example the case where the present invention is applied to an image forming apparatus for forming an image on a sheet, but the present invention is not limited thereto. It is also possible to apply the present invention to an auto original feeder, which is an example of the sheet feeding apparatus used for the image reading apparatus or the like.

[0201] This application claims priority from Japanese Patent Application No. 2005-128757 filed Apr. 26, 2005, which is hereby incorporated by reference herein.

Claims

1. A sheet feeding apparatus comprising: a sheet stacking member configure to support sheets; a sheet feeding member configure to feed the sheet from said sheet stacking member; a sheet separating portion including a separating roller provided so as to abut against said sheet feeding member and a torque-limiter for allowing said separating roller to rotate in association with said sheet feeding member when a rotation torque larger than a predetermined torque is acted on said separating roller and for preventing said separating roller from rotating in association with said sheet feeding member when the rotation torque equal to or smaller than the predetermined torque is acted on said separating roller; a guide member configure to support said sheet separating portion in a slidable manner and guiding said sheet separating portion with a predetermined angle in a direction different from a direction along which said sheet feeding member is opposed to said separating roller; and an urging member configure to urge said sheet separating portion so that said separating roller is in pressure contact with said sheet feeding member.

2. A sheet feeding apparatus according to claim 1, wherein said sheet separating portion further comprising a supporting member for integrally supporting said separating roller and said torque-limiter, and said supporting member is supported by said guiding member in a slidable manner and is urged against said sheet feeding member by said urging member.

3. A sheet feeding apparatus according to claim 1, wherein an angle formed by the direction along which said sheet feeding member is opposed to said separating roller and a direction along which said sheet separating portion slides is equal to or lower than tan - 1 .function. ( 1 .mu. .times. s ) ##EQU18## where a coefficient of friction between sheets is given as .mu.s.

4. A sheet feeding apparatus according to claim 1, wherein said guide member comprises a flat guide surface, and said supporting member is supported so that said supporting member is slide-able along said guide surface.

5. A sheet feeding apparatus according to claim 4, wherein said supporting member comprises a rib, and said supporting member slides while bringing said rib into contact with said guide surface.

6. A sheet feeding apparatus according to claim 4, wherein said supporting member comprises a rotatable roller, and said supporting member slides along said guide surface while rotating said roller.

7. A sheet feeding apparatus according to claim 1, wherein said guide member comprises a rib, and said supporting member slides while being in contact with said rib.

8. A sheet feeding apparatus according to claim 1, wherein said guide member comprises a rotatable roller, and said supporting member slides while rotating said roller.

9. An image forming apparatus, comprising: a sheet feeding apparatus as recited in claims 1; and an image forming portion for forming an image on a sheet fed from said sheet feeding apparatus.