A sheet feeding apparatus

Abstract: A sheet feeding apparatus includes a feeding roller (4) for feeding a sheet material from a stack of sheet materials, a separation device (5,6) for cooperating with the feeding roller to allow only one sheet material to be fed, a transportation roller (7) for cooperating with the feeding roller to transport the sheet material separated by the separating device.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to a sheet feeding apparatus usable with an image forming apparatus such as a copying machine and a printer. Referring to Figure 1, a conventional sheet feeding apparatus includes a sheet tray 2, a sheet feeding spring 3 and a feeding roller 4, wherein a transfer material 1 on the sheet tray 2 is urged to the feeding roller 4 by the feeding spring 3. When the feeding roller 4 rotates, the leading edge of the transfer material (sheet) 1 is advanced into the space formed between a sheet guide 2' and the feeding roller 4, by friction. The coefficients of friction are set such that, when a plurality of transfer materials is advanced, the coefficient of friction µ₁ between the transfer materials 1, the coefficient µ₃ between the feeding roller 4 and the transfer material 1, and the coefficient µ₂ between the transfer material 1 and a separation pad 6 which is urged towards the feeding roller 4 by a separation spring 5, satisfy µ₁ < µ₂ < µ₃. Because µ₁ < µ₂, the plurality of sheets is stopped from the sheet being closest to the separation pad 6, and, finally, because µ₂ < µ₃, only one transfer material 1a is conveyed between the feeding roller 4 and the separation pad 6, as shown in Figures 2 and 3. Immediately after the transfer material 1a has been passed between the feeding roller 4 and the separation pad 6, the speed Va of the transfer material (Figure 1) may vary depending on the relationship among the frictional coefficients µ₁, µ₂ and µ₃, and, therefore, the speed is unstable. After the leading edge of the transfer material 1a is caught by a couple of registration rollers 13 and 14, the drive of the feeding roller 4 is stopped. The feeding roller 4 is provided with an unshown one-way clutch, so that the feeding roller 4 follows to rotate by the friction between the transfer material 1a and itself. When the trailing edge of the transfer material 1a departs from the feeding roller 4, the rotation of the feeding roller 4 stops, and therefore, the next transfer material 1b is not fed. Since the pressure between the registration rollers 13 and 14 is large enough as compared to the frictional load between the transfer material 1a and the separation pad 6, the conveying speed Vb of the transfer material 1a is stabilized. On the other hand, a light beam 9 produced in an optical scanning means 8 is imaged on a photosensitive member 10 to form a latent image thereon, which, in turn, is developed with toner by a developing device. The toner image on the photosensitive member 10 is transferred onto the transfer material 1a at a point P. The toner image transferred onto the transfer material 1a is fixed by a fixing device 12. The conventional example employs a couple of registration rollers 13 and 14 in order to stabilize the speed of the transfer material at the point P where the image transfer is effected, and to provide an accurate image. However, the employment of the registration-roller couple 13, 14 makes the apparatus bulky and expensive. More particularly, the conventional example relies on the relationship between the frictional forces, wherein the frictional coefficient (and therefore force) µ₃ between the feeding roller 4 and the transfer material 1 is larger than the frictional coefficient (and force) µ₂ between the separation pad 6 and the transfer material 1 when the transfer material or materials 1 are located between the feeding roller 4 and the separation pad 6, functioning as a separating means. For this reason, there always exists a slipping between the separation pad 6 and the transfer material 1 to be conveyed. This results in a braking force always applied to the feeding action of the feeding roller 4, and the braking force varies in accordance with the material, thickness and surface properties of the transfer sheets and also with the number of sheets involved. These are the reasons why the feeding speed Va of the feeding roller 4 is not stabilized. In order to obviate these problems, the conventional example uses the couple of registration rollers 13 and 14 between the feeding roller 4 and the transfer drum 10 so as to stabilize the feeding speed Vb at a position where the stabilized speed is required, for example, the transfer station P where the image is transferred from the photosensitive drum 10 to the transfer material 1. However, this makes the apparatus bulky and requires additional means, that is, a driving means for the registration roller couple 13, 14, with the result that the costs of the apparatus are increased. Additionally, even in the case that the couple of the registration rollers 13 and 14 is used, the feeding speed Va between the feeding roller 4 and the registration roller couple 13, 14 varies depending on the frictional coefficient of the transfer sheets 1 or on the ambient conditions, even to such an extent that the sheet 1 does not reach the registration roller couple 13, 14 in time. A further sheet feeding apparatus is known from the document JP-A-59 31 225. According to this prior art, the sheet feeding apparatus comprises a feeding roller having a feeding portion for contacting and applying a feeding force to a sheet material of a stack of sheet materials, and a non-feeding portion having the same diameter as the feeding portion, and not applying a feeding force to the sheet material. Further, a separating means cooperates with the feeding roller so as to allow only one sheet to be fed, and additional rollers are disposed co-axially with the feeding roller, wherein conveying rollers cooperate with the additional rollers in order to transport the sheet material separated by the separating means. Although this sheet feeding apparatus is less bulky as compared to the aforementioned prior art, it suffers from the fact that, during the feeding action of the feeding roller, the load for rotation of the feeding roller is large due to the frictional contact with the separating means even when the feeding apparatus does not apply the feeding force to the sheet. This promotes the wearing of the separating means, which, in turn, may cause an unstable feeding speed. SUMMARY OF THE INVENTION Therefore, the object of the invention is to provide a sheet feeding apparatus, the functionality of which is increased as compared to that of the known feeding apparatus, and, in particular, by which the feeding of the sheet material can be performed stably and reliably. This object is achieved by the features indicated in the independent claim 1. According to the invention, a second rotatable member which is disposed co-axially with a first rotatable member having a feeding portion and a non-feeding portion, has a diameter smaller than that of the feeding portion and larger than that of the non-feeding portion so that the latter is prevented from contacting a separating means. Accordingly, the load of the first rotatable member is small when the sheet feeding mechanism does not apply the feeding force to the sheet. In addition, in a stand-by state prior to the feeding operation, the second rotatable member contacts the separating means as well as a third rotatable member which cooperates with the second rotatable member so as to transport the sheet material separated by the separating means. As a result, the sheet material can be transported by the second and third rotatable members irrespective of the rotation of the first rotatable member, wherein the first rotatable member does not obstruct the sheet transportation and the next sheet material is prevented from being double-fed. As a result, the sheet feeding operation is stabilized. Advantageously developed embodiments of the invention are subject matter of the dependent claims 2 to 7. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a sectional view of a conventional sheet feeding apparatus. Figure 2 is an enlarged sectional view around a feeding roller of the conventional sheet feeding apparatus. Figure 3 is an enlarged sectional view around the feeding roller of Fig. 2, wherein a sheet is fed. Figure 4 is a sectional view of an image forming apparatus incorporating a sheet feeding apparatus according to an embodiment of the present invention. Figure 5A illustrates a driving system for the sheet feeding apparatus of Figure 4. Figure 5B is a sectional view taken along B-B of Figure 5A. Figure 5C is a sectional view taken along C-C of Figure 5A. Figures 6A,B are sectional views taken along D-D of Figure 5A. Figure 7A shows a driving system for a sheet feeding apparatus according to a further embodiment of the present invention. Figure 7B shows a driving system for a sheet feeding apparatus according to a further embodiment of the present invention. Figure 8A is a cross-sectional view of a feeding roller of a sheet feeding apparatus according to a further embodiment of the present invention. Figure 8B is a cross-sectional view of a feeding roller of a sheet feeding apparatus according to a further embodiment of the present invention. Figure 8C is a cross-sectional view of a feeding roller of a sheet feeding apparatus according to a further embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figures 4 - 6B an embodiment of the present invention will be described. A plurality of transfer materials (cut sheets) 201 are stacked on a feeding tray 202, and the leading edge portions of the cut sheets 201 are urged towards feeding rollers 214, 214' and idler rollers 215 and 215' by a supporting plate 213 rotatably supported and biased by a compression spring 203. The feeding rollers 214 and 214' are provided with portions having smaller diameters, respectively, so that at least one circumferential portion of each of the feeding rollers 214 and 214' has a shape to provide an initial position in which it does not contact the cut sheet 201 or conveying rollers 216 or 216'. The feeding rollers 214 and 214' may have a length smaller than the minimum width of the sheet usable with this apparatus, and they are spaced apart in a longitudinal direction as shown in Figure 5B. The feeding rollers 214 and 214' are fixed on a driving shaft 217 which is controlled by a spring clutch 218 and a solenoid 219 adjacent a longitudinal end thereof which is effective to provide a one turn rotation control. Outside the maximum width of the cut sheet usable with this apparatus, cam plates 220 and 220' are mounted to the driving shaft 217 so as to swing the supporting plate 213. The supporting plate 213 is provided with cam followers 221 and 221' corresponding to the cam plates 220 and 220'. Therefore, by the control of the rotation of the driving shaft 217, the supporting plate 213 moves substantially vertically so as to selectively urge the stacked cut sheets 201 towards the feeding rollers 214 and 214' and the idler rollers 215 and 215'. Because of this structure, when the operator loads the cut sheets 201 into this apparatus, the supporting plate 213 takes a lower position as shown in Figure 4 and Figure 6A, thus allowing smooth loading of the cut sheets 201. Additionally, the operator is not required to perform an action of lowering the supporting plate 213. Adjacent outside longitudinal end of the feeding rollers 214 and 214', the idler rollers 215 and 215' are mounted to the driving shaft 217 for smooth rotation relative to the driving shaft 217. The idler rollers 215 and 215' have a diameter which is slightly smaller than that of the feeding rollers 214 and 214', for example, by 0.5 - 1 mm. The conveying rollers 216 and 216' are swingable about a drive input shaft 224 by a swinging arm 222, and are press-contacted to the idler rollers 215 and 215' by a spring 223 stretched between a frame of the apparatus and the conveying roller assembly. The driving force is applied to the conveying rollers 216 and 216' through a driving gear 224' fixedly mounted to the drive input shaft 224 and a conveying gear 225 fixedly mounted to the conveying rollers 216 and 216'. The driving and conveying gears 224', 225 are preferably disposed between the conveying rollers 216 and 216' as in this embodiment, since, then, the application of the driving force does not result in an unbalanced pressing force of the conveying rollers 216, 216', and, therefore, the pressing force is stabilized. To the feeding rollers 214 and 214' and the idler rollers 215 and 215', a friction member is press-contacted to separate the cut sheets 201. The friction member is supported at its longitudinal center with a spring 226 for equalizing and pressing it. The separation member according to this embodiment consists of separation pads 227 and 227' which are formed from rubber material containing cork. The separation pads may be separated into two pads 227 and 227', as shown in Figure 5B, corresponding to the feeding rollers 214 and 214' and the idler rollers 215 and 215'. In this case, the portion not provided with the separation pad functions as a sheet guide 228 for guiding the cut sheets 201 to the conveying rollers 216 and 216'. The sheet feeding apparatus according to this embodiment is provided with an additional or second cut sheet inlet 229 which is effective to introduce a cut sheet into the nips formed between the conveying rollers 216 and 216' and the idler rollers 215 and 215' from other than the feeding tray 202. Therefore, it is possible that an optional feeding means such as a sheet deck capable of accommodating a great number of cut sheets may be disposed below the apparatus so as to feed the sheets from the feeding means to the second cut sheet inlet, and, therefore, the apparatus is ready for expansion. Downstream of the nips formed between the conveying rollers 216 and 216' and the idler rollers 215 and 215', a sensor lever 230 is disposed, which is supported on the swingable arm 222 to detect the leading edge of the cut sheets 201 with the aid of a photointerruptor 231. The sensor may otherwise be constructed by a transparent type or reflection type sensor other than the combination of the sensor lever 230 and the photointerruptor 231. After the cut sheet 201 has actuated the sensor lever 230 by its leading edge, it is guided by a guiding portion 232 having a radius of curvature which is slightly larger than that of the feeding rollers 214 and 214', and is introduced into a nip formed between a photosensitive drum 301 and a transfer roller 233. The transfer roller 233 is of a semiconductor rubber to which a bias voltage of about 1 KV DC is applied, and contacts the photosensitive drum 301 to rotate therewith. The guiding portion 232 may preferably be constructed using a casing for a developing device 302 disposed below the photosensitive drum 301, since then the accuracy relative to the photosensitive drum 301 can easily be achieved, and since the number of parts is decreased. Around the photosensitive drum 301, there are located an exposure station A for exposing the photosensitive drum 301 to a laser beam 402 emitted from a laser scanning optical system 401 disposed at a left hand side of the apparatus in Figure 4, a developing station B provided with a developing sleeve 303 for developing a latent image formed on said photosensitive drum 301 by the image exposure at the exposure station A, a transfer station C provided with the transfer roller 233, a cleaning station D provided with a means 304 for cleaning the photosensitive drum 301 after a developed image has been transferred from the photosensitive drum 301 to the transfer sheet 201, and a primary charging station E provided with charging means such as a corona charger 306 for uniformly charging the photosensitive drum 301 before exposure. By cooperation of these stations, an image is formed on the transfer sheet 201. The cut sheet 201 having received the toner image transferred thereonto at the transfer station C is introduced along a transportation guide which also functions as an inlet guide for an image fixing device. The image fixing device is provided with a heating roller 235 containing therein a heater, and a pressing roller 236 made of a heat-resistive elastic member such as silicone rubber, which form a nip into which the sheet 201 is inserted. When the sheet 201 passes through the nip, the image is fixed on the sheet 201. Thereafter, the sheet 201 is discharged to the outside of the image forming apparatus by a roller 237 disposed downstream of the fixing device, made of an elastic material such as rubber and NORSOREX (tradename, available from CDF CHIMIE, France) and having fins. In operation, an unshown motor which is a driving source starts to rotate prior to the start of the sheet feeding operation. Then, a conveying roller driving gear 238 fixed to the drive input shaft 224 for the conveying rollers 216 and 216' starts to rotate, by which the rotation is transmitted to the conveying or transporting rollers 216 and 216' via the drive input shaft 224, the driving gear 224' and the conveying gear 225. Since the conveying rollers 216 and 216' are press-contacted to the idler rollers 215 and 215', respectively, the conveying rollers 216 and 216' and the idler rollers 215 and 215' are rotated together, respectively. At this time, even if the idler rollers 215 and 215' are in contact with the cut sheet 201, the cut sheet 201 is not fed since the frictional force between the cut sheet 201 and the separation pads 227 and 227' is larger than the frictional force between the idler rollers 215 and 215' and the cut sheet 201. However, in the stand-by state, as described hereinbefore, the supporting plate 213 takes a lower position by the cam plates 220 and 220' and the cam followers 221 and 221' so that the cut sheet 201 does not contact the idler rollers 215 and 215'. By the rotation of the conveying roller driving gear 238, a sheet feed driving gear 239 rotates which is connected via the spring clutch 218 to the driving shaft 217 for the feeding rollers 214 and 214'. When the solenoid 219 is not energized, the rotation of the sheet feed driving gear 239 is not transmitted to the driving shaft 217. The spring clutch 218 is provided with a one turn controlling means (control ring) 240, so that a one turn drive transmission is performed in response to the on/off pulse of the solenoid 219. When the solenoid 219 is energized in response to a feed start signal, the pawl of the control ring 240 is moved away, with the result that the driving force is transmitted from the sheet feed driving gear 239 to the driving shaft 217 via the spring clutch 218. Then, the driving shaft 217 starts to rotate, and the cam plate 220 rotates so that the supporting plate 213 is urged upwardly by the spring 203, as shown in Figure 6B. By this, the cut sheets 201 stacked on the tray 202 are urged towards the feeding rollers 214 and 214' and towards the idler rollers 215 and 215'. Even if the sheet 201 is contacted to the rotating idler rollers 215, 215', the sheet 201 is not fed out because the frictional force between the sheets 201 is larger than the frictional force between the idler roller 215, 215' and the sheet 201. However, simultaneously with, slightly before or after the urging, those portions of the feeding rollers 214 and 214' which have a diameter larger than that of the idler rollers 215 and 215' come into contact with the cut sheet 201, by which the feeding rollers 214 and 214' feed the cut sheet 201 out. Then, the cut sheet 201 reaches the separation pad 227 at which the relationship between the friction coefficients is as in the conventional case, so that only the topmost sheet is conveyed to the downstream of the separation pad 227. Sooner or later, the cut sheet 201 reaches the rotating conveying rollers 216 and 216', and thereafter, the conveying speed of the cut sheet 201 is stabilized by the conveying rollers 216 and 216'. Then, the cut sheet 201 reaches a detecting station H at which the leading edge of the sheet 201 is detected, in response to which the image light is emited onto the photosensitive members 301. By doing so, the cut sheet 201 is continuously conveyed by the conveying rollers 216 and 216' without interruption. Therefore, it can be avoided that a shock resulting from actuation or deactuation of the registration roller is imposed on an image. In order to perform the laser image writing controls in response to the detection of the leading edge of the cut sheet 201, the length L1 of the sheet passage from the leading edge detecting position H to the transfer station C is longer than the peripheral length L2 of the photosensitive drum 301 from the image exposure station A to the transfer station C (L2 < L1). When the leading edge of the sheet 201 reaches the roller 233, it is detected by a detecting sensor or by a timer starting with the output of the sensor 230. Then, the solenoid 219 is operated to stop the roller 214. However, the roller 216 continues to rotate, whereas the roller 215 rotates idly, and therefore, the roller 215 does not interfere with the sheet feeding by the roller 214. If the sheet sensing position by the sensor lever 230 of the sheet leading edge sensor H is located adjacent an inside end of the minimum sheet width which is opposite to the reference position, a warning or prohibition can be performed when a user erroneously uses a sheet having a width smaller than the minimum usable width. As a result, a sheet having a width smaller than the minimum width cannot reach the photosensitive drum 301. Also, the image writing on the photosensitive drum 301 can be prevented, so that damage and contamination to the drum 301 can be minimized. Referring to Figures 7A and 7B, a further embodiment of the present invention will be described. In the foregoing embodiment, the sheet reference position is disposed at a lateral end, and on the basis thereof the minimum width range and the maximum width range are determined. However, the present invention is not limited to this, and applicable to a central reference position feeding system. As shown in Figures 7A and 7B, the feeding rollers 514 and 514', idler rollers 515 and 515' and conveying rollers 516 and 516' may not be divided but may be continuous rollers. In Figure 7A, an idler roller 515 is disposed in the middle, whereas the feeding rollers 514 and 514' are disposed adjacent opposite ends of the idler roller 515, and an integral separation pad 527 is opposed to the feeding rollers 514 and 514'. Also, the conveying roller 516 is an integral single roller. The conveying roller 516 is not swingable to be pressed against the idler roller 515, but it is urged via bearings 542 and 542' which are spring-biased by compression springs 541 and 541'. The conveying roller 516 receives a driving force from a driving gear 538. The separation pad 527 may be urged via a swingable arm 543. Or, as in this embodiment, the structure is such that the conveying roller 516 contacts only the idler roller 515. With such structures, the sheet is prevented from being conveyed while being gripped between the feeding rollers 514 and 514' and the conveying roller 516 which provide a strong conveying force, and, therefore, the production of paper dust can be prevented, and the conveying load can be decreased. The rotation of the gear 538 is transmitted from the shaft 524 to the shaft 617. Figure 7B is a modification of the structure shown in Figure 7A. In this modification, the feeding roller 614 is disposed in the middle, whereas the idler rollers 615 and 615' are disposed adjacent the opposite ends of the feeding roller 614. An integral separation pad 627 is contacted to the idler rollers 615 and 615'. The conveying rollers 616 and 616' are disposed opposed to the idler rollers 615 and 615'. In this structure, the driving force and the pressure are applied in the middle position. Referring to Figures 8A, 8B and 8C, there are shown further embodiments. In Figure 8A, a circular rubber member 714 is eccentrically fixed on a driving shaft 717. With this structure, the manufacturing is easy, and the contact between the cut sheet and the pad or the like is continuous and smooth. In Figure 8B, a cam member 814 made of rubber material is employed which has a continuously increasing diameter. Also, with this structure, the contact between the cut sheet and the pad or the like is smooth. In Figure 8C, the control of the feeding roller 914 is not an one-rotation control, but it is a half-turn control.

Claims: A sheet feeding apparatus, comprising a first rotatable member (214, 214'; 514, 514'; 614; 714; 814; 914) having a feeding portion for contacting and applying a feeding force to a sheet material (201) of a stack of sheet materials, and a non-feeding portion not applying the feeding force thereto, a second freely rotatable member (215, 215'; 515; 615, 615') disposed co-axially with said first rotatable member, separating means (227, 227'; 527; 627) for cooperating with said first rotatable member to allow one sheet material to be fed, and a third rotatable member (216, 216'; 516; 616, 616') for cooperating with said second rotatable member to transport the sheet material separated by said separating means, characterized in that said second rotatable member (215, 215'; 515; 615, 615') has a diameter smaller than that of said feeding portion and larger than that of said non-feeding portion of said first rotatable member (214, 214'; 514, 514'; 614; 714; 814; 914), and, in a stand-by state prior to the feeding operation, said second rotatable member contacts said separating means (227, 227'; 527; 627) and said third rotatable member (216, 216'; 516; 616, 616'). An apparatus according to claim 1, wherein in said stand-by state prior to the feeding operation, said second rotatable member (215, 215'; 515; 615, 615') resiliently contacts said separating means (227, 227'; 527; 627) and said third rotatable member (216, 216'; 516; 616, 616'). An apparatus according to claim 1, further comprising a driving source for driving said third rotatable member (216, 216') and clutching means (218) provided between said driving source and said first rotatable member (214, 214'), wherein, after said first rotatable member has stopped, the sheet material (201) is transported by cooperation between said third rotatable member (216, 216') and said second rotatable member (215, 215'). An apparatus according to claim 1, further comprising means (202) for stacking the sheet materials (201), wherein said first rotatable member (214, 214') is actuable on the topmost sheet material of the stack. An apparatus according to claim 1, wherein said second rotatable member is divided into two rollers (215, 215'; 615, 615') with said first rotatable member (214, 214'; 614) being interposed therebetween. An apparatus according to claim 1, wherein said first rotatable member is divided into two rollers (514, 514') with said second rotatable member (515) being interposed therebetween. An image forming apparatus, comprising a sheet feeding apparatus according to one of the preceding claims, and an image forming means (233, 235, 236, 301 - 304, 306, 401, 402) for forming an image onto said sheet material (201) fed by said sheet feeding apparatus.

Inventors: ENDO YOSHIFUMI, KANOTO MASANOBU, KAWAGUCHI HIDESHI, KIKUCHI YUTAKA, OHSAWA ICHIRO, SATO HIDEKI, ENDO, YOSHIFUMI, KANOTO, MASANOBU, KAWAGUCHI, HIDESHI, KIKUCHI, YUTAKA, OHSAWA, ICHIRO, SATO, HIDEKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: B65H3
IPC codes: B65H3