IMAGE FORMING APPARATUS

Abstract: An image forming apparatus comprising an image bearing member for bearing an image, a rotating member for urging a recording sheet against the image bearing member, a driving means for rotationally driving the image bearing member, and a driving force transmission means for transmitting a driving force from the driving means to the rotating member through the image bearing member, thereby rotating the rotating member so that peripheral speed of a portion of the rotating member which portion urges the sheet against the image bearing member is higher than peripheral speed of the image bearing member.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an image forming apparatus used with copying machines, laser beam printers and the like, and, more particularly, it relates to an image forming apparatus including an image bearing member (referred to as "photosensitive drum" hereinafter) and a transfer roller. Related Background Art In a conventional image forming apparatus including a photosensitive drum or exposure drum and a transfer roller, for example, as shown in Figs. 10 to 15, a transfer roller 26 made of elastic material is pressed against an photosensitive drum 22 by a spring 6, and a toner image formed on the photosensitive drum 22 is transferred to a recording sheet carried by the transfer roller 26 by applying bias voltage to the transfer roller. In this case, a contacting portion 26b of the transfer roller 26 (referred to as "nip portion" hereinafter) which is being pressed against the photosensitive drum 22 will be gradually deformed and be decreased in its diameter, with the result that the peripheral speed of the nip portion 26b of the transfer roller will be slower than that of the other portion of the transfer roller 26. In order to avoid such inconvenience, as disclosed in the Japanese Patent Laid-Open No. 56-126872, the transfer roller 26 was driven independently of the photosensitive drum 22 so that the peripheral speed of the nip portion 26b was equal to that of the photosensitive drum 22. More particularly, the transfer roller was driven by a discrete driving means different from a driving means for driving the photosensitive drum, in such a manner that the peripheral speed of a point situated on the nip portion 26b of the transfer roller 26, which has the smallest diameter, was the same as the peripheral speed of the exposure drum 22 and the peripheral speed of the other portion of the transfer roller was higher than that of the photosensitive drum. By the way, so long as either the photosensitive drum 22 or the transfer roller 26 (for example, photosensitive drum 22) is subjected to a driving force, the other (transfer roller 26) can be driven by such driving (photosensitive drum 22). In this case, as shown in Fig. 14, even if a gear 17 of the photosensitive drum 22 is not actually engaged by a driving gear 16 (of the driving means for driving the photosensitive drum), the photosensitive drum 22 is rotated by a rotational force F T (Fig. 13) from the contacting transfer roller 26. On the other hand, when the photosensitive drum 22 and transfer roller 26 are drivingly rotated by driving both of these elements 22, 26 by means of the respective driving means, so that the peripheral speed of the portion, having the smallest diameter, of the nip portion 26b of the transfer roller 26 is higher than the peripheral speed of the photosensitive drum 22, as shown in Fig. 15, the transfer roller 26 is subjected to a driving force f₂ from the driving means through a driving gear 14 (of the driving means) and a gear 15 (of the transfer roller) and to a resistance force F D for resisting the rotation of the transfer roller from the photosensitive drum 22; on the other hand, the photosensitive drum 22 is subjected to the rotational force F T tending to accelerate the rotation of the drum from the transfer roller 26, and at the same time, is subjected to a rotational force f₃ tending to decelerate the rotation of the drum from the associated driving means through the gears 16 and 17. In this case, in a condition that the driving force is not transmitted to the photosensitive drum 22 from the driving gear 16 of the driving means (i.e., a tooth of the gear 16 is not engaged by a tooth of the gear 17 or the photosensitive drum is subjected to the force tending to resist the rotation thereof), if a leading edge of a recording sheet S is struck against a fixing device (Fig 2(11)), shock or impact force will be transmitted to the photosensitive drum 22 and transfer roller 26 through the recording sheet S, thereby returning the photosensitive drum to a position shown by a dotted line (Fig. 15), where the tooth of the gear 17 of the photosensitive drum 22 is contacted to the tooth of the driving gear 16, thus stopping or decelerating the photosensitive drum temporarily. Thereafter, the photosensitive drum 22 is rotated at a normal speed by the driving force f₁ (Fig. 12) from the driving gear 16. Accordingly, in such a case, there arises a problem that an image transferred to the transfer roller is distorted or sheared due to the uneven rotation of the photosensitive drum. Such problem will also arise in the case where a sheet conveying path is curved in the downstream of the image transferred position, since the sheet is subjected to variable friction forces from such a curved path. SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming apparatus which can obtain a clear or distinct image without distortion and/or shear. In order to achieve the above object, an image forming apparatus according to the present invention comprises an image bearing member for carrying an image thereon a rotating member for urging a sheet against the image bearing member, said rotating member transferring at a nip between the image bearing member and the rotating member the image on the image bearing member onto the sheet, a driving means for rotationally driving the image bearing member, and a driving force transmission means for transmitting a driving force from the driving means to the rotating member through the image bearing member, thereby, upon image transferring, rotating the rotating member so that the peripheral speed of the nip portion of the rotating member is higher than the peripheral speed of the nip portion of the image bearing member. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a plan view of an essential part of an image forming apparatus according to the present invention; Fig. 1B is an end view of the apparatus of Fig. 1A; Fig. 2 is a sectional elevational view of the image forming apparatus according to the present invention; Fig. 3 is a sectional plan view of the apparatus of Fig. 2, a right section of an apparatus frame being disengaged from the left section and rotated into a horizontal position. Fig. 4 is an end view showing an essential part of the image forming apparatus according to the present invention; Figs. 5A, 5B and 6 are explanatory views showing an operation of the image forming apparatus according to the present invention; Figs. 7, 8 and 9 show other embodiments of the present invention; and Figs. 10, 11, 12, 13, 14 and 15 show an example of a conventional image forming apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be explained with reference to the accompanying drawings, as an example that the present invention is applied to a laser beam printer. As shown in Figs. 2 to 4, a laser beam printer 1 includes a sheet supply tray 3 which protrudes from a frame 2 of the printer and on which a plurality of sheets S is stacked, a sheet separating pad 4 for separating a single sheet S from the sheet stack and a sheet feed roller 5 for feeding the separated sheet one by one to the printer, which elements 4, 5 are arranged near an inner end of the supply tray 3. Auxiliary rollers 20 freely rotatably supported on a shaft 5a of the feed roller 5 are arranged to abut against the separating pad 4 when the sheet is not fed by the feed roller 5. Further, a sheet conveying roller 21 pressed against the auxiliary rollers 20 is arranged downstream of the separating pad 4 and is connected to a motor M through an appropriate gear train (not shown). An image forming portion or station 9 arranged downstream of the sheet conveying roller 21 can form an image by a laser signal from a laser scanner portion 7 for modulating a laser beam emitted by a laser generator 8. The image forming station 9 includes a charging roller 23 arranged around a photosensitive drum 22, for charging the photosensitive drum 22, a developer unit 25, and a transfer roller 26 pressed against the photosensitive drum 22 by means of a spring 6. The transfer roller 26 is constituted by a cylindrical metallic body, and a sponge layer covering the peripheral surface of the metallic body and made of silicone oxide mixed with electric conductive material therein; and, thus, a portion of the transfer roller which is pressed against the photosensitive drum 22 can be elastically deformed. Downstream of the image forming station 9 there is a fixing device 11, downstream of which a sheet ejector guide 27 and a sheet ejector sponge roller 29 for cooperating with the ejector guide to pinch the sheet therebetween and eject the sheet are arranged. Further, downstream of the ejector roller 29, there is arranged a sheet receiving tray 30 for collecting the sheets having the image thereon with the imaged surface upside. The above-mentioned laser scanner portion 7 is constituted by a polygonal mirror 10, a spherical lens 12 and an Fϑ lens 13. As shown in Fig. 1A, driving power from the above-mentioned motor M is transmitted to a driving gear 22a through an appropriate gear train (not shown). The photosensitive drum 22 is rotated integrally with the driving gear 22a. The rotational force of the photosensitive drum 22 is transmitted to the transfer roller 26 through a gear 22b fixed to the photosensitive drum 22 and a driving gear 26a for the transfer roller 26, thereby rotating the transfer roller 26. As shown in Fig. 1B, if a minimum radius of the portion of the transfer roller 26 which is pressed against the photosensitive drum 22 (i.e., the depressed portion deformed by the contacting exposure drum) is R', a radius of the remaining portion of the transfer roller is R, a radius of the photosensitive drum 22 is R D ' angular velocity of the photosensitive drum is ω D , and angular velocity of the transfer roller 26 is ω T , a relation between the gear 22b and the driving gear 22a (Fig. 1A) is so selected as to meet the following requirements: R D ω D < R'ω T In this case, a difference between the value R D ω D and the value R'ω T is selected to be included in a range that such difference does not have bad influence upon the transferring of the image. For example, it may be designed that a diameter of the transfer roller 26 is 16.2 mm, a diameter of the photosensitive drum 22 is 30 mm, the contacting force between the drum and roller is that of 600 grams, the number of teeth of the gear 22b is 50, the number of the teeth of the driving gear 22a is 26, modules of these gears are 1, respectively, a profile shift of the driving gear 22a is -0.1, hardness of the driving gear 22a is lower than 40° in Masker hardness, preferably 25° - 30°, and transfer roller 26 is made of urethane foam material. In addition, an outer peripheral surface of the transfer roller 26 is normally polished to obtain a desired outer diameter thereof. Further, a direction to which the transfer roller 26 is biased by means of the spring 6 is perpendicular to a plane inclined by the pressure angle α with respect to the common tangential plane of the rotating member 26 and the image bearing member 22. (refer to Fig. 6). In the embodiment mentioned above, when an image signal is applied to the laser beam printer 1 from an appropriate external apparatus (not shown), an image based on the image signal from the external apparatus is formed in the image forming station 9 through the laser scanner portion 7. In this case, only the uppermost recording sheet S of the sheet stack in the supply tray 3 is separated from the sheet stack and picked up by the separating pad 4 and the feed roller 5, and this recording sheet S is conveyed by the auxiliary rollers 20 and the conveying roller 21 in synchronism with the image formed in the image forming station 9. After the image is printed on the recording sheet S in the image forming station 9, the recording sheet is then conveyed to the fixing device 11 by the conveying action of the photosensitive drum 22 and transfer roller 26. In the illustrated embodiment, as shown in Figs. 5A and 5B, since the driving force for driving the photosensitive drum 22 and transfer roller 26 is obtained by a force f transmitted from a gear 40 to the driving gear 22a, the gear 40 and the driving gear 22a must be always engaged by each other (if these gears are disengaged from each other the driving force cannot be transmitted.) Further, the transfer roller 26 is rotated while slipping on the photosensitive drum 22, and, accordingly, the transfer roller is subject to, from the photosensitive drum 22, a resistance force F D for resisting the rotation of the roller. On the other hand, to counteract this resistance force F D , the driving gear 26a is subject to a force f D by continuously engaging with the gear 22b which is rotated integrally with the photosensitive drum 22. That is to say, if the gears 26a and 22b are not engaged by each other, the roller 26 cannot be rotated faster than the photosensitive drum 22. Incidentally, since the outer surface of the transfer roller 26 is constituted by the sponge layer made of silicone oxide to have high elasticity, as stated above, even when the transfer roller 26 is rotated while slipping on the photosensitive drum 22, the outer surface of the photosensitive drum 22 is not affected adversely by such slipping movement. Accordingly, during the printing operation, even if the leading edge of the recording sheet S is struck against the fixing device 11 to generate the shock or impact force, the photosensitive drum 22 and the transfer roller 26 can be rotated at the constant speed, thus preventing the distortion and/or shear of the image. In the fixing device 11, the image is fixed on the recording sheet S; thereafter, the sheet S is ejected on the sheet receiving tray 16 with the imaged surface thereof upside. Further, when the sheet conveying path is curved, even if the sheet is subjected to the impact force from the curved path, the distortion and/or shear of the image can be effectively prevented for the reasons mentioned above. Incidentally, in the illustrated embodiment, while an example that the transfer roller 26 is pressed against the photosensitive drum 22 by means of the spring 6 was explained, the present invention is not limited to such example; the transfer roller may, for example, be biased toward the photosensitive drum by means of a leaf spring 31, as shown in Fig. 7. Further, in the illustrated embodiment, while an example that the driving force is transmitted from the photosensitive drum 22 to the transfer roller 26 through the gear 22b and the driving gear 26a was explained, the present invention is not limited to such example; such driving force may, for example, be transmitted through a wrapping connector 32 such as a belt, chain and the like, as shown in Fig. 8. In addition, in the illustrated embodiment, while an example that the rotational force from the motor M is transmitted to the driving gear 22a through the gear train (not shown) was explained, it should be noted that the present invention is not limited to such example; the photosensitive drum 22 may be driven by a gear 41 directly coupled to an output shaft of the motor M and meshed with the gear 22b of the photosensitive drum, as shown in Fig. 9. Further, by orienting the direction to which the transfer roller 26 is pressed against the photosensitive drum 22 to a direction substantially perpendicular to a plane inclined by the pressure angle with respect to a common tangential plane of said rotating member and said image bearing member, any force other than the driving force transmitted between the gears 22b and 26a can be effectively prevented from acting on the gears. In addition, since the force acting between the gear 22b and the driving gear 26a does almost not affect the influence upon the bias force by which the transfer roller 26 is pressed against the photosensitive drum 22, the transfer roller 26 is biased toward the photosensitive drum substantially by the force of the spring 6 alone. Consequently, if the transfer roller 26 is drivingly rotated, the bias force is kept constant, thus permitting the stable transferring of the image and preventing the deviation of the transfer roller 26 from its original position due to the force acting on the driving gear 26a of the transfer roller 26 from the gear 22b of the photosensitive drum 22. Lastly, in the illustrated embodiments, while the photosensitive drum was used as the image bearing member, it should be noted that the image bearing member is not limited to the photosensitive drum, but may be constituted by a photosensitive belt and the like. Further, in place of the transfer roller, another rotating member such as a belt pressed against the image bearing member may be used. Also, in this case, a sponge layer made of silicone oxide mixed with electric conductive material therein is disposed on an outer surface of the belt-shaped rotating member.

Claims: An image forming apparatus (1), comprising: an image bearing member (22) for carrying an image thereon; a rotating member (26) for urging a sheet (S) against said image bearing member, said rotating member (26) transferring at a nip between said image bearing member (22) and said rotating member (26) said image on said image bearing member onto the sheet (S); and a driving means (M) for rotationally driving said image bearing member (22), characterized by: a driving force transmission means (22a, 22b, 26a) for transmitting a driving force from said driving means (M) to said rotating member (26) through said image bearing member (22), thereby, upon image transferring, rotating said rotating member (26) so that the peripheral speed of the nip portion (26b) of the rotating member (26) is higher than the peripheral speed of the nip portion of said image bearing member. An image forming apparatus (1) according to claim 1, wherein, when said rotating member (26) does not press said sheet (S) against said image bearing member (22), said image bearing member and said rotating member are both rotated with a slipping movement therebetween. An image forming apparatus (1) according to claim 1, wherein said driving force transmission means includes a first gear (22b) provided on said image bearing member (22), and a second gear (26a) provided on said rotating member (26) and meshed with said first gear (22b), said driving force being transmitted from said first gear (22b) to said second gear (26a). An image forming apparatus (1) according to claim 3, further including another force transmission means (22a, 40) for transmitting the driving force from said driving means (M) to said image bearing member (22). An image forming apparatus (1) according to claim 3, further including a bias means (6) for biasing said rotating member (26) toward a direction substantially perpendicular to a plane inclined by a pressure angle with respect to the common tangential plane of said rotating member (26) and said image bearing member (22). An image forming apparatus (1) according to claim 1, wherein said image bearing member (22) comprises a photosensitive drum. An image forming apparatus (1) according to claim 1, wherein said image bearing member (22) comprises a photosensitive belt. An image forming apparatus (1) according to claim 1, wherein said rotating member (26) comprises a transfer roller. An image forming apparatus (1) according to claim 1, wherein said rotating member (26) comprises a belt member. An image forming apparatus (1) according to any of claims 1 to 9 comprising: a latent image forming means (9) for forming a latent image on said image bearing member (22); a developing means (25) for developing said latent image formed on said image bearing member; said rotating member (26) being used for transferring the developed image on said image bearing member to said sheet (S). An image forming apparatus (1) according to any of claims 1 to 9 comprising: a conveying means (5, 21) for conveying a sheet (S) to said image bearing member (22); a fixing means (11) provided downstream of said image bearing member in a sheet conveying direction, for pinching and conveying said sheet and for fixing the image transferred to said sheet. An image forming apparatus (1) according to claim 11, wherein said fixing means (11) comprises a pair of rollers. An image forming apparatus (1) according to claim 3 or 4, wherein, upon image transferring, said image bearing member (22) receives a first force from said rotating member (26) in a rotational direction of said rotating member, said first gear (22b) receives a second force from said second gear (26a) in a direction opposite to said first force. An image forming apparatus (1) according to claim 4, wherein said another force transmission means (22a, 40) has a driving gear (40) and a third gear (22a) provided on said image bearing member (22) to engage with said driving gear (40), and the drive force from said driving means (M) is transmitted from said driving gear to third gear. An image forming apparatus (1) according to claim 14, where said third gear (22a) serves also as said first gear. An image forming apparatus (1) according to one of claims 1 to 15, wherein said rotating member (26) is elastically deformable. An image forming apparatus (1) according to claim 16, wherein said rotating member (26) has a sponge layer. An image forming apparatus (1) according to claim 13, wherein said second force maintains the engaged condition between said first gear (22b) and said second gear (26a). An image forming apparatus (1) according to one of claims 1 to 18, wherein a voltage for image transferring is applied between said image bearing member (22) and said rotating member (26). An image forming apparatus (1) according to claim 14, wherein, upon image transferring, said image bearing member (22) receives a first force from said rotating member (26) in a rotational direction of said rotating member, and said first gear (22b) receives a second force from said second gear (26a) in a direction opposite to said first force. An image forming apparatus (1) according to claim 20, wherein said first gear (22b) receives the second force from said second gear (26a) so as to maintain the engagement between said third gear (22a) and said driving gear (40). An image forming apparatus (1) according to claim 13 or 20, wherein said second force is a force in a direction opposite to a rotational direction of said image bearing member (22).

Inventors: KAWAGUCHI, HIDESHI
Applicants: CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: G03G15
IPC codes: G03G15