Photoreceptor drum driving mechanism

Abstract: A photoreceptor drum driving mechanism includes a drum drive gear provided at a leading end of a drive shaft, which is in mesh with an internal gear mounted to a photoreceptor drum and a roller coaxially formed with the drive shaft. The mechanism further includes a circular arc surface in contact with the roller in a deflecting direction of the drive shaft. As a result, the drive shat of the drum drive gear can be prevented from being deformed in a pressure angle direction of the drum drive gear. As this eliminates irregularities in rotations of the photoreceptor drum, a distortion of an image in an image forming process or in a transferring process can be prevented.

Description: FIELD OF THE INVENTION The present invention relates to a photoreceptor drum driving mechanism for actuating a rotation movement of a photoreceptor drum by an internal gear mounted to an inner circumference of the photoreceptor drum for use in an image forming apparatus provided with a photoreceptor drum such as a copying machine, a printer, etc. BACKGROUND OF THE INVENTION As shown in Fig. 18, there are known image forming apparatuses such as copying machines, etc., provided with a cylindrical photoreceptor drum 271. In such image forming apparatus, the surface of the photoreceptor drum 271 is charged by a main charger 272, and is exposed with light emitted from an exposure unit 273. Then, the resulting electrostatic latent image is developed by a developer unit 274, and the developer image is transferred onto a sheet by a transfer charger 275. After the transfer, a developer remaining on the surface of the photoreceptor is removed by a cleaning blade (not shown) of a cleaning unit 276, and the developer image on the sheet is permanently affixed thereto by a fixing unit 277. In the described image forming process, the photoreceptor drum 271 is rotatably driven in one direction by a drive unit. For the described driving mechanism for the photoreceptor drum, the photoreceptor drum is typically provided with a gear at one end. For simplification of the structure, the external gear system is adopted in most cases. However, with an increasing demand for miniaturization of the apparatuses, the internal gear system is more and more adopted, as this permits members of the apparatus to be positioned in a simple manner. For example, Japanese Unexamined Utility Model Application No. 155863/1986 (Jitsukaisho 61-155863) discloses a cylindrical photoreceptor holding device. As shown in Fig. 7 and Fig. 8, the cylindrical photoreceptor holding device includes a plurality of rollers 241 for supporting a photoreceptor drum 240 in a vicinity of both ends of the photoreceptor drum 240, wherein the photoreceptor drum 240 is driven by a gear 244 in mesh with a drive force transmitting member 243 mounted on an inner circumference of the photoreceptor drum 240. Japanese Unexamined Patent Application No. 120265/1983 (Tokukaisho 58-120265) discloses a drum driving mechanism for a recording device having the arrangement shown in Fig. 9, which permits a shorter drive force transmission path in the drum driving mechanism. In the drum driving mechanism of this citation, an internal gear 252 mounted on an inner circumference of a drum 250 in a vicinity of a center in the lengthwise direction is in mesh with a gear 254 of a motor 253 provided inside the drum 250. However, the described photoreceptor devices have such a drawback that as one side face perpendicular to the shaft of the photoreceptor drum is an opening, a sufficient strength of the photoreceptor drum cannot be obtained. In order to counteract the described problem, a photoreceptor drum provided with a flange formed on the closed side face perpendicular to the shaft of the photoreceptor drum is disclosed. Such driving mechanism for the photoreceptor drum, for example, has the arrangement shown in Fig. 10. That is, a flange 262 including an internal gear section 261 is provided at an end portion of a photoreceptor drum 260 so as to be fitted thereto, and the photoreceptor drum 260 is rotatably driven by a driving system including a driving-use small gear 263. The flange 262 includes an internal gear support section 264 formed on the surface perpendicular to the shaft of the photoreceptor drum 260 so as to support the internal gear section 261 and a bearing member 265 mounted at the center of the internal gear support section 264. The described arrangement provides a solution to the aforementioned problem by maintaining a sufficient strength of the photoreceptor drum 260 and preventing deviation of shaft center by the bearing member 265. Here, the positioning precision of the small gear for driving the photoreceptor drum has a great effect on the rotation movement of the photoreceptor drum, i.e., the image quality. Therefore, it is especially important to ensure such positioning precision for driving the photoreceptor drum. However, in the described driving mechanism with the internal gear, there arises another problem related to the stabilization of a backlash. Here, it is disadvantageous to have a large backlash as abrasion and noise are generated, and the transmission efficiency is lowered, etc. Therefore, increase in backlash is highly undesirable. Actually, the direction in which errors are generated from the regular position of the driving-use external gear was measured respectively with the combination of the external gear and the external gear and the combination of the external gear and the internal gear. The results are summarized in Fig. 11. Fig. 11 shows that when the center of the external gear A or the internal gear B deviates up or down, the center of the external gear C reaches the hatched region where the backlash becomes worse. When comparing Fig. 11(a) with Fig. 11(b), it can be seen that when adopting the internal gear B for use with a given external gear C (drive gear), a hatched region becomes larger than the case of adopting the external gear A. Namely, the combination of the internal gear B with the external gear C results in a greater reduction in the positioning precision than the case of adopting the combination of the external gear A and the external gear C, and thus it is required to have a still higher mounting precision of the two gears to compensate for this deficiency. However, the described arrangement has the following drawback. That is, as shown in Fig. 10, as the driving-use small gear 263 drives the internal gear section 261 by an overhang type shaft, it is difficult to ensure the precision. In order to drive the rotation movement of the photoreceptor drum by making the drive gear mesh directly or indirectly with the gear mounted to the photoreceptor drum, a play such as backlash, etc., between gears, is always produced. Further, in the developing process, the developer roller is rotated at an increased peripheral velocity in the rotation direction of the photoreceptor drum, and the developer roller is pressed onto the photoreceptor drum, and causes a frictional force, thereby presenting the problem of irregularities and deviations in rotation within the play. Japanese Unexamined Utility Model Application No. 055043/1992 (Jistukaihei 4-055043) discloses a drum brake shown in Fig. 23. That is, a flange 282 with an inner circumference 282a is integrally formed at one end of the photoreceptor drum 281, and a brake pad 284 held by an elastic member 283 is arranged in contact with the inner circumference 282a so as to generate a brake force. Japanese Unexamined Patent Application No. 345173/1992 (Totukaihei 4-345173) discloses a photoreceptor driving device having the arrangement shown in Fig. 24. That is, a brake gear 293 supported via a torque limiter (not shown) by a fixing shaft 294 is arranged in mesh with a drum gear 291a mounted to the end of the photoreceptor drum 291 independently of the drive gear 292 for driving the drum gear 291a so as to generate a brake force. In any of the described conventional arrangements, irregularities and deviations in rotation are prevented by suppressing the looseness due to play by applying the brake force to the photoreceptor drum. However, in the described drum brake of Japanese Unexamined Utility model Application No. 055043/1992, although the irregularities in rotation for the backlash of the photoreceptor drum 281 can be prevented by the brake force, with an abrasion of the brake pad 284, a deformation of the elastic member 283 becomes small, resulting in a smaller brake force. To compensate for the small brake force, if the contact pressure applied by the elastic member 283 is increased, the initial brake force would become too strong, which causes the deformation on the side of the elastic member 283. On the other hand, according to the photoreceptor driving device of Japanese Unexamined Patent Application No. 345173/1992, the rotation shaft for the drive gear 292 and the fixing shaft 294 for the brake gear 293 are provided on a peripheral circumference of the photoreceptor drum 291 independently of the rotation shaft for the photoreceptor drum 291. However, as the brake gear 293 is mounted to che fixing shaft 294, the difference between the number of rotations internally applied to the torque limiter and the number of rotations externally applied to the torque limiter is limited, which restricts the available mechanisms for the torque limiter. Furthermore, two shafts are provided on the peripheral circumference of the photoreceptor drum 291 independently of the photoreceptor drum 291. However, as there are provided many members constituting the image forming apparatus on the peripheral circumference of the photoreceptor drum 291 such as the charger, the developer unit, the cleaning unit, etc., it is difficult to ensure a space required for the additional structure. Therefore, it is difficult to meet the recent demand for miniaturization with the described mechanism. JP-A-2039971 discloses a drive arrangement for a paper feed drum having an internal driven gear at an end thereof. The drive gear of a motor unit mounted on a pivotal support arm is urged into engagement with the driven gear by a tension spring attached to the arm. JP-A-1088561 discloses a drive arrangement for a photosensitive drum having an external driven gear. A frictional drive means including a torque limiter is additionally provided at the end of the drum. JP-A-6118851 discloses a drive arrangement for a photosensitive drum having external driven gears at its opposite ends and also an internal gear midway along the drum. Vibrations at the end external gears is cancelled by vibration at the central internal gear. SUMMARY OF THE INVENTION An object of the present invention is to provide a photoreceptor drum driving mechanism which eliminates the looseness of the photoreceptor drum due to a backlash to prevent irregularities in rotations. According to the present invention there is provided a photoreceptor drum driving mechanism, comprising a rotatable photoreceptor drum including an internal driven drum gear; a drive shaft rotatably mounted by bearings so that an end portion of the drive shaft overhangs from the bearing closest to that end of said drive shaft; a drum drive gear mounted on the overhanging end portion of the drive shaft and arranged to drive said internal driven drum gear; and a driving unit for rotatably driving said drum drive gear; characterised by a support member located in a fixed predetermined position relative to the drum and acting on said drum drive gear or on said overhanging end portion of said drive shaft in the region of said drum drive gear so as to inhibit lateral deflection of said overhanging end portion of said drive shaft and thereby hold said drum drive gear, while the latter is active in driving said internal driven drum gear, in a fixed positional relationship with said driven drum gear. With such an arrangement, when the drum drive gear is in mesh with the internal drum driven gear to transmit the drive force, the fixedly located support member inhibits lateral deflection of the shaft and thus also of the drum drive gear. As this limits a backlash between the drum drive gear and the internal drum gear, irregularities in rotation of the photoreceptor drum hardly occur. As a result, the photoreceptor drum can be driven with high precision, and the distorted image due to irregular rotations of the photoreceptor drum can be prevented. In a described embodiment the supporting member includes a roller coaxial with said drive shaft and a rolling surface formed in a circular arcuate shape at a predetermined position relative to a rotation shaft of said photoreceptor drum, a curved surface of said roller being in contact with said rolling surface. In another described embodiment the drum drive gear is coaxial with a rotation axis of said photoreceptor drum, and said support member comprises a plurality of intermediate gears provided at equal intervals around said driven drum drive gear, said intermediate gears being in mesh with said internal driven drum gear and said drum drive gear. In this arrangement, the forces in the pressure angle direction generated from the plurality of intermediate gears provided at equal intervals are balanced with each other, and the drive force is dispersed and transmitted to the internal drum gear from the drum drive gear. As a result, the load of each intermediate gear is reduced, thereby suppressing the distortion and abrasion of the teeth. In this embodiment, the photoreceptor drum driving mechanism further comprises a respective roller coaxially formed with each of said plurality of intermediate gears, each said roller being externally in contact with a rolling surface formed on an inner circumference of said photoreceptor drum. In this arrangement, as the roller permits the position of the photoreceptor drum to be maintained constant, a proper backlash between each intermediate gear and the drum drive gear can be maintained. Further, as this prevents the deviation of the axis of the photoreceptor drum, distorted image due to irregularities in rotation of the photoreceptor drum can be prevented. Preferred embodiments of the invention will now be described by way of example with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view showing an example of a photoreceptor drum driving mechanism in accordance with the present invention. Fig. 2 is a side view schematically showing an arrangement in a vicinity of a roller of the photoreceptor drum driving mechanism. Fig. 3 is a perspective view showing a driving force transmission system of the photoreceptor drum driving mechanism. Fig. 4 is a cross-sectional view showing another example of the photoreceptor drum driving mechanism in accordance with the present invention. Fig. 5(a) is a cross-sectional view of another example of the photoreceptor drum driving mechanism in accordance with the present invention. Fig. 5(b) is an arrangement drawing showing a relative position of the gears in the photoreceptor drum driving mechanism of Fig. 5(a). Fig. 6 is a perspective view showing a structure of supporting the photoreceptor drum in a conventional photoreceptor drum in a conventional photoreceptor drum driving mechanism. Fig. 7 is a partial cutaway perspective view of a driving system of the conventional photoreceptor drum driving mechanism of Fig. 7. Fig. 8 is a partial cutaway perspective view of another driving system of the conventional photoreceptor drum driving mechanism. Fig. 9 which shows still another driving system in the conventional photoreceptor drum driving mechanism is a cross-sectional view of essential parts of the driving system. Fig. 10(a) which shows a region of increasing a backlash when gears on the driving side and on the driven side are in mesh with each other is an explanatory view showing an engagement in the case where the driven side is also the external gear. Fig. 10(b) is an explanatory view showing the engagement in the case where the driven side is the internal gear. Fig. 11 is a cross-sectional view schematically showing a structure of a copying machine to which the photoreceptor drum driving mechanism of the present invention is applicable. Fig. 12 is a cross-sectional view schematically showing an arrangement of the image forming apparatus in the copying machine of Fig. 11. Fig. 13 is a cross-sectional view showing another photoreceptor drum driving mechanism in accordance with the present invention. Fig. 14 is an arrangement drawing showing a relative position of the gears in the photoreceptor drum driving mechanism of Fig. 13. Fig. 15 is a cross-sectional view showing still another photoreceptor drum driving mechanism of the present invention. Fig. 16 is a cross-sectional view showing still another photoreceptor drum driving mechanism in accordance with the present invention. Fig. 17 is a cross-sectional view schematically showing a typical arrangement of the conventional copying machine having a cylindrical photoreceptor drum. Fig. 18 (a) is a cross-sectional view showing an arrangement of a brake mechanism of the conventional photoreceptor drum driving mechanism. Fig. 18 (b) is a cross-sectional view taken on line A-A of Fig. 18 (a). Fig. 19 (a) is an arrangement drawing showing the arrangement of the gears in the brake mechanism of the conventional photoreceptor drum driving mechanism. Fig. 19 (b) is a cross-sectional view schematically showing the brake mechanism. DESCRIPTION OF THE EMBODIMENTS In order to explain embodiments of a photoreceptor drum driving mechanism in accordance with the present invention, an example of a copying machine adopting the photoreceptor drum driving mechanism will be explained in reference to Fig. 12. The copying machine has within it a cylindrical photoreceptor drum 1. Along the circumference of the photoreceptor drum 1, there are provided a main charger 2, a blank lamp 3, a developer unit 4, a transfer charger 5, a separation charger 6, a cleaning unit 7 and a removing lamp 8. Further, above the photoreceptor drum 1 is provided an exposure unit 9. These members constitute the process elements for use in the image forming process. The exposure unit 9 includes an exposure lamp 9a, a plurality of mirrors 9b, a lens 9c and an automatic exposure sensor 9d. On the exposure unit 9, is provided a document platen 10 made of a transparent hard glass. The copying machine also includes a transportation belt 11, a fixing unit 12 and a control unit 13. In the described copying machine, an image is formed in the following manner. A sheet placed on the document platen 10 is exposed by the exposure lamp 9a of the exposure unit 9, and the light reflected therefrom is projected onto the photoreceptor drum 1 through the plurality of mirrors 9b and the lens 9c. In this state, the photoreceptor drum 1 is charged to a predetermined potential by a corona discharge from the main charger 2, and is rotated at a constant velocity in the direction shown by an arrow. As a result, in the photoreceptor drum 1, the potential of the region irradiated with the reflected light drops, i.e., the photoreceptor drum 1 is exposed, thereby forming an electrostatic latent image on the surface of the photoreceptor drum 1. Then, charges in the non-image forming region of the photoreceptor drum 1 are removed by projecting thereon light from the blank lamp 3 in accordance with the sheet size. The electrostatic latent image is developed by the developer (toner) supplied from a developer roller 4a of the developer unit 4, i.e., the toner to form a toner image. In this state, the toner is supplied to the developer unit 4 from a toner hopper 14. This supplying of toner is performed based on the result of detection by a toner concentration sensor 15 mounted to the developer unit 4. The toner is stirred with a stirring roller 4b to be charged to an opposite polarity to the charged electric potential of the photoreceptor drum 1. The toner image is transferred onto a sheet (not shown) supplied between the photoreceptor drum 1 and the transfer charger 5 by the transfer charger 5 to be a visible image. Here, an attractive force is exerted between the sheet after the transfer and the photoreceptor drum 1. Thus, by applying an AC corona onto the sheet by the separation charger 6 so as to reduce the potential of the sheet to the same potential as the surface of the photoreceptor drum 1, the attractive force is eliminated, thereby separating the sheet from the surface of the photoreceptor drum 1 by a separation piece 18 (see Fig. 13) using the rigidity of the sheet. Then, the sheet is carried on the transportation belt 11 to the fixing unit 12, where the toner image is made permanent on the sheet. After the toner image has been transferred, the residual toner remaining on the surface of the photoreceptor drum 1 is collected by the cleaning unit 7, and the residual electric potential of the photoreceptor drum 1 is removed by reducing the electrical resistance of a photoconductive layer by projecting thereto a light emitted from the remover lamp 8. The structure of the circumferential portion of the photoreceptor drum 1 will be explained in further detail with reference to Fig. 13. The cleaning unit 7 wipes off the residual toner remaining on the surface of the photoreceptor drum 1 after the transfer by a cleaning blade 7a which is arranged in tight contact with the surface of the photoreceptor drum 1, and the toner thus wiped off is transported to a prescribed waste toner container by a transportation screw 7b. The remover lamp 8 is provided for projecting light onto the photoreceptor drum 1 via an anti-toner adhesion filter 16 onto the remover lamp 8. The main charger 2, the transfer charger 5 and the separation charger 6 are corona chargers which do not come in contact with the photoreceptor drum 1. On the sheet feeding side between the photoreceptor drum 1 and the transfer charger 5, is provided a paper stop roller 17 for adjusting a transportation timing of the sheet. Here, a pre-transfer charger may be adopted under the developer roller 4a which aids the operation of the transfer charger 5. Concrete examples of the driving mechanism of the photoreceptor drum 1 to be adopted in the copying machine will be explained below. With reference to Fig. 1, the photoreceptor drum driving mechanism is constituted by installing a photoreceptor unit 30 and a driving system unit 40 in a main frame 36. The photoreceptor unit 30 includes the photoreceptor drum 1, etc. The driving system unit 40 includes a drum drive gear 41, a drive shaft 42, etc. The described structure permits easy dismantling and assembling, and an excellent maintenance efficiency. As shown in Fig. 3, at the driven end side of the photoreceptor drum 1, a drum flange 31 (hereinafter simply referred to as a flange) is press-fitted with the internal gear 31a (internal drum gear) formed along the inner circumference thereof. The drive force produced by a drive motor 45 is transmitted to the internal gear 31a via the drum drive gear 41 in mesh with the internal gear 31a, an intermediate gear 43 mounted to the other end of the drive shaft 42 of the drum drive gear 41 and a pinion 44 in mesh with the intermediate gear 43, thereby driving the photoreceptor drum 1. Further explanations are given referring back to Fig. 1. In the described arrangement, the flange 31 includes the internal gear 31a, an internal gear support member 32 which serves as a disk face perpendicularly formed with respect to the shaft of the photoreceptor drum 1 so as to support the internal gear 31a, and a rotation bearing member 33 formed at the center of the internal gear support member 32. Fitted to the rotation bearing member 33 via a bearing 34 is a rotation shaft 35 that is fixed to the main frame 36 of the copying machine main body. The photoreceptor drum 1 rotates about the rotation shaft 35. On the other hand, to the drive shaft 42 supported by bearings 47a and 47b with respect to the driving system unit 40, is mounted a roller 46 via a bearing 46a so as to be adjacent to the drum drive gear 41. The roller 46 is provided for preventing the drive shaft 42 from deflecting in the backlash increasing direction when the internal gear 31a is driven by the drum drive gear 41. The peripheral surface of the roller 46 is in contact with a circular arc surface (rolling surface) 38a formed on a photoreceptor unit frame 38 around the rotation shaft 35. As shown in Fig. 2, the circular arc surface 38a is formed along a part of the circumferential surface (circular arc) of the roller 46 on the side of the rotation shaft 35 of the photoreceptor drum 1. Therefore, by arranging the roller 46 in contact with the circular arc surface 38a, the roller 46 can be positioned with reference to the position of the rotation shaft 35. Thus, as the photoreceptor drum 1 is also positioned based on the rotation shaft 35, respective positions of the internal gear 31a and the drum drive gear 41 are ultimately determined. By thus positioning the roller 46, the drive shaft 42 (see Fig. 1) is supported at a position closer to the drive gear 41, and a backlash between the drum drive gear 41 and the internal gear 31a in the active state is not likely to deviate from a predetermined value. Namely, the problem of looseness is less likely to occur, and the internal gear 31a can be driven smoothly by the drum drive gear 41 without generating irregularities in rotation movement. Moreover, in the main frame 36, an opening 36a is formed with a size which allows the roller 46 to pass therethrough, and with the drum drive gear 41 attached to the driving system unit 40, the installation and removal of the driving system unit 40 can be performed. Instead of the described arrangement of the photoreceptor drum driving mechanism shown in Fig. 1, the arrangement shown in Fig. 4 may be adopted. In this case, a roller 56 is mounted to the leading end of the drive shaft 42 with respect to the drum drive gear 41, and is in contact with a rolling surface 32a provided in the internal gear support member 32. Here, the deflection of the drive shaft 42 can be reduced as it is held by the effect of the roller 56 and the rolling face 32a, at the position where the generated deflection is greatest, thereby improving a positioning precision of the drum drive gear 41. However, when, as in the arrangements shown in Fig. 1 and Fig. 4, three or more bearings are formed with respect to the drive shaft 42, the force exerted in the bending direction to be loaded to the drive shaft 42 is in the statically indeterminate state, which hinders the smooth rotation of the drive shaft 42. Therefore, it is required to set the mating clearance between the bearing 47b and a drive frame 47 to fall in a range of from 0.1 to 0.2 mm, that is larger than a normal range of from 0.01 to 0.05 mm. As a result, the stress imposed on the drive shaft 42 and the bearings 47a, 47b, 46a and 56a can be reduced, and the rotation movement of the drive shaft 42 is not disturbed. Furthermore, the photoreceptor drum driving mechanism may be arranged as shown in Fig. 5(a). At the driven end of the photoreceptor drum 1, the flange 31 with the internal gear 31a is press-fitted. The flange 31 includes the internal gear 31a and the internal gear support member 32 which serves as a disk surface formed on the face perpendicular to the axis of the photoreceptor drum 1 so as to support the internal gear 31a. Intermediate gears 68 (see Fig. 5(b)) are provided respectively on the three rotation shafts 69 which are fixed in such a manner that respective central angles are equal to each other around the rotation shaft (drive shaft 42) of the photoreceptor drum 1. In this arrangement, when driving the drive motor (not shown), the three intermediate gears 68 are in mesh with the internal gear 31a, and the drive force of the drum drive gear 41 is transmitted to the internal gear 31a, and this, in turn, actuates the rotation movement of the photoreceptor drum 1 about the drive shaft 42. In the described arrangement, the number of the intermediate gears 68 is selected to be three as the most stable balance of the forces produced by the intermediate gears 68 can be achieved although it is possible to disperse the force in the direction of pressure angle with at least two intermediate gears 68. In the described arrangement, as the three intermediate gears 68 for driving the internal gear 31a are provided at equal intervals with respect to the drum drive gear 41, the force in the pressure angle direction generated between the internal gear 31a and the intermediate gear 68 would be reduced to one third. Further, as forces in respective directions are balanced each other, the possible deflection of the rotation shaft 69 of the intermediate gear 68 which causes the irregularities of rotations can be eliminated. Moreover, rollers 68a are coaxially formed with the intermediate gears 68 respectively, and a rolling surface 31b is formed with respect to the rollers 68a on the side of the flange 31. Accordingly, the position of the photoreceptor drum axis is stabilized by the rollers 68a and the rolling surface 31b, and the deviation in the rotation center can be prevented. Conventionally, in order to ensure the precision, a ball bearing which is inferior in its durability is adopted as the bearing. In contrast, the preferred embodiment of the present invention is provided with the structure of improving a positioning precision for the drum drive gear 41, and this permits the adoption of a radial roller bearing such as represented by a needle bearing, etc., which shows fairly high durability. By adopting such radial roller bearing, an improved precision can be ensured for a long period of time. As described, by preventing the looseness and irregularities in rotations of the photoreceptor drum 1, a possible distorted image can be reduced. The following descriptions will discuss photoreceptor drum driving mechanisms for driving the photoreceptor drum 1 in accordance with further embodiments of the present invention with reference to Figs. 13 to 16. The photoreceptor drum driving mechanism in each of these embodiments is arranged such that a drum drive gear and a brake gear are in mesh with a drum gear mounted to the photoreceptor drum 1, and the photoreceptor drum 1 is driven with an application of a brake force, thereby preventing the looseness due to a backlash, etc., thereby preventing irregularities or deviation in rotations, etc. As shown in Fig. 13, a drum gear 121a (a drum gear unit, an internal drum gear) and a flange 121 with a rotation bearing member 121b are press-fitted to one end of the photoreceptor drum 1, and the rotation bearing member 121b is rotatably supported by a drum support shaft 123 (corresponding to the rotation shaft in the previous embodiments) that is caulked to a frame 122. A similar flange, though without the drum gear, is rotatably provided in the described manner at the other end (not shown) of the photoreceptor drum 1. The described arrangement permits rotation of the photoreceptor drum 1. A drive motor 125 for driving the photoreceptor drum 1 is fixed to a drive frame 124, and at the leading end of a motor shaft 126 projected from the drive motor 125, a pinion 126a is formed. The drive fcrce produced by the drive motor 125 is transmitted to a drive shaft 128 through an intermediate gear 127 in mesh with the pinion 126a, and is further transmitted by the gears between a drum drive gear 129 mounted to the leading end in the direction of the photoreceptor drum 1 of the drive shaft 128 and the drum gear 121a, thereby rotating the photoreceptor drum 1. On the other hand, a brake gear 130 is also in mesh with the drum gear 121a. The brake gear 130 is mounted inside the drum gear 121a, and the axis of the drive shaft 128, the axis of the drum support shaft 123 and the axis of the rotation shaft 131 which is the center of rotation of the brake gear 130 are in one plane (see Fig. 14). The brake gear 130 is supported via a torcue limiter 133 on a hollow shaft section 132a extending from the intermediate gear 132 mounted on the rotation shaft 131 so as to be rotatable. Further, the rotation force of the drive shaft 128 is transmitted to the brake gear 130 by an intermediate gear 134 that is rotatably supported by the drum support shaft 123 so as to be in mesh with the drum drive gear 129. Here, if there exists a relative difference between the speed of rotation of the drum gear 121a produced by driving the drum drive gear 129 and the speed of rotation of the drum gear 121a produced by driving the brake gear 130, the torque limiter 133 is actuated, and the brake force is applied to the drum gear 121a. With the described combination of gears, the brake gear 130 is rotated in the rotating direction of the drum gear 121a. Here, the speed reducing ratio is set such that the speed of rotation of the drum gear 121a produced by the brake gear 130 does not exceed the speed of the rotation of the drum gear 121a produced by the drum drive gear 129. Therefore, as the number of rotations to be transmitted to the brake gear 130 via the drum gear 121a from the drum drive gear 129 becomes larger than the number of rotations to be transmitted from the drum drive gear 129 to the intermediate gear 134, the intermediate gear 132 and the brake gear 130, the brake force is exerted from the brake gear 130 to the photoreceptor drum 1. For example, assume the respective number of teeth of the drum gear 121a, the drum drive gear 129, the brake gear 130, the intermediate gear 132, the pinion 126a, and the intermediate gear 134 are 72, 22, 18, 26, 7, and 40, and the number of rotations of the drive motor 125 is 1,500 rpm, then the number of rotations of the photoreceptor drum 1 would be 80.2 rpm, while the number of rotations to be applied to the photoreceptor drum 1 by the brake gear 130 would be 55.5 rpm. Therefore, under the described condition, the difference between a number of rotations internally applied to the torque limiter 133 and a number of rotations externally applied to the torque limiter 133 would be 24.7 rpm, and this difference would cause the brake force to be generated with respect to the photoreceptor drum 1. As described, when the number of rotations to be applied to the torque limiter 133 is small, for example, a spring clutch type torque limiter represented by a torque limiter unit NTS series (NTN Co., Ltd.) may be used. By the described brake force, the teeth of the drum gear 121a would always receive a force exerted in an opposite direction to the rotating direction of the drum drive gear 129 with respect to the teeth of the drum drive gear 129. As a result, looseness in backlash is eliminated, thereby preventing fluctuation between gears. In the above described embodiment, the drum drive gear 129 and the brake gear 130 are formed inside the photoreceptor drum 1, and this permits the arrangement of preventing the fluctuation between the gears to be added to the image forming apparatus without adversely affecting other arrangements disposed outside of the photoreceptor drum 1, such as the layout of the members including the charger, the developer unit, the cleaning unit, etc. The photoreceptor drum driving mechanism may be arranged as shown in Fig. 15. As shown in Fig. 15, a flange 151 which is press-fitted to one end of the photoreceptor drum 1 includes an internal drum gear 151a formed along the circumference thereof and an external brake drum gear 151b mounted to a rotation bearing member 151c of the flange 151. The photoreceptor drum 1 is rotatably supported by a drum support shaft 123 which is caulked to the frame 122 and a drum support shaft (not shown) formed at the other end. To the leading end of the drive shaft 128 which receives the drive force from the drive motor 125, the drum drive gear 129 in mesh with the drum gear 151a is mounted as well as a brake gear 152 in mesh with the brake drum gear 151b via a torque limiter 153. In this case, the direction of rotations to be applied to the brake gear 152 by the brake drum gear 151b is opposite to the direction of rotations to be applied to the drum gear 151a by the drum drive gear 129. As a result, in the torque limiter 153, slipping occurs between the drive shaft 128 and the brake gear 152, and the rotating force to be applied to the brake gear 152 by the brake drum gear 151b is exerted as the brake force, and a possible looseness due to a backlash can be avoided. Here, as rotations to be applied to the brake gear 152 has an opposite direction to the rotating direction of the drive gear 128, the relative number of rotations of the brake gear 152 with respect to the drive shaft 128, i.e., the difference in number of rotations to be applied to the torque limiter 153 becomes larger. In the described situations, for example, a powder clutch system torque limiter represented by the powder clutch OPL series (Ogura Clutch Co., Ltd.) which offers a stable load at high-speed rotation range (around 50 to 300 rpm) may be used. In each of the described further embodiments, as the restrictions on the torque limiter to be adopted can be reduced by altering the setting of the gear ratio, an optimal selection of the torque limiter can be made with ease in consideration of the brake force, the durability and the cost, etc. The photoreceptor drum driving mechanism may be arranged as shown in Fig. 16. That is, a flange 161 which is press-fitted to one end of the photoreceptor drum 1 is provided with a drum gear 161a with the internal teeth formed along the circumference. Further the photoreceptor drum 1 is rotatably supported by the drum support shaft 123 which is caulked to the frame 122 and a drum shaft at the other end (not shown). On the other hand, to the drive shaft 128 which receives the drive force from the drive motor 125, are mounted a drum drive gear 129 in mesh with the drum gear 161a and a brake collar 162 (brake force application member) through the torque limiter 163. The peripheral portion of the brake collar 162 is made in tight contact with the circumference of a center shaft portion 161b (braking surface) of the flange 161 by a reaction force generated by the engagement between the drum drive gear 129 and the drum gear 161a. Further, the friction force generated between the center shaft portion 161b and the brake collar 162 by the contact force with pressure is selected to be larger than the torque which differentiates the torque limiter 163. Therefore, when the drum drive gear 161a is driven by the drum drive gear 129, the brake collar 162 is pressed against the center shaft portion 161b of the flange 161. As a result, by a friction force generated between the center shaft portion 161b and the brake collar 162, the torque limiter 163 slides. With respect to the brake collar 162, the rotation applied by the center shaft portion 161b has an opposite direction to the driving direction applied to the brake collar 162 from the drive shaft 128. This increases the difference in numbers of rotations of the torque limiter 163. In the described arrangement, for example, a torque limiter of a powder clutch type represented by the powder clutch OPL series (Ogura clutch Co., Ltd.) which permits a stable load in the high speed range (around 50 to 300 rpm) may be used. As a result, a brake force is exerted onto the photoreceptor drum 1, and the teeth of the drum gear 161a always receive a force in an opposite direction to the rotating direction of the drum drive gear 129 by the brake force. As this permits the looseness due to the backlash to be eliminated, the fluctuation between the gears can be prevented. In the above so-called further embodiments the constructional features of the earlier embodiments for inhibiting deflection of the device shaft are combined with constructional features for inhibiting play between the drum gear and the drum drive gear.

Claims: A photoreceptor drum driving mechanism, comprising: a rotatable photoreceptor drum (1) including an internal driven drum gear (31a; 121a; 161a); a drive shaft (42; 128) rotatably mounted by bearings (47a, 47b) so that an end portion of the drive shaft overhangs from the bearing (47b) closest to that end of said drive shaft; a drum drive gear (41; 129) mounted on the overhanging end portion of the drive shaft and arranged to drive said internal driven drum gear; and a driving unit (45; 125) for rotatably driving said drum drive gear; characterised by a support member (38, 46; 32a, 56; 68; 134; 162) located in a fixed predetermined position relative to the drum and acting on said drum drive gear (41; 129) or on said overhanging end portion of said drive shaft (42; 128) in the region of said drum drive gear so as to inhibit lateral deflection of said overhanging end portion of said drive shaft and thereby hold said drum drive gear, while the latter is active in driving said internal driven drum gear, in a fixed positional relationship with said driven drum gear. A photoreceptor drum driving mechanism according to claim 1, wherein said support member includes a roller (46; 56; 162) coaxial with said drive shaft and a rolling surface (38a; 32a; 161b) formed in a circular arcuate shape at a predetermined position relative to a rotation shaft of said photoreceptor drum, a curved surface of said roller being in contact with said rolling surface. A photoreceptor drum driving mechanism according to claim 2, further comprising: a main frame (36) for supporting said photoreceptor drum, said main frame having an opening (36a) with a size that allows said roller (46) and said drum drive gear (41) to pass therethrough. A photoreceptor drum driving mechanism according to claim 2, wherein: said roller (56) is formed at a leading end of the overhanging end portion of the drive shaft (42). A photoreceptor drum driving mechanism according to any of claims 2 to 4, further comprising: a drive frame (47) for supporting the drive shaft (42) via said bearings (47a, 47b), wherein when three or more bearings are provided for the drive shaft of said drum drive gear, a clearance for engagement between said drive frame (47) and said closest bearing (47b) of said drive frame is selected to be substantially in a range of from 0.1 mm to 0.2 mm. A photoreceptor drum driving mechanism according to any preceding claim, wherein said bearings are radial roller bearings. A photoreceptor drum driving mechanism according to claim 1, wherein said drive shaft (42) is coaxial with a rotation axis of said photoreceptor drum (1), and said support member comprises a plurality of intermediate gears (68) provided at equal intervals around said driven drum drive gear, said intermediate gears being in mesh with said internal driven drum gear (31a) and said drum drive gear (41). A photoreceptor drum driving mechanism according to claim 7, wherein the number of said intermediate gears (68) is three. A photoreceptor drum driving mechanism according to claim 7 or claim 8, further comprising: a respective roller (68a) coaxially formed with each of said plurality of intermediate gears, each said roller (68a) being externally in contact with a rolling surface (31b) formed on an inner circumference of said photoreceptor drum. A photoreceptor drum driving mechanism according to any of claims 7 to 9, wherein the bearings are radial roller bearings.

Inventors: HARADA YOSHIKAZU, NAKA HIROSHI, NAKAHAMA YASUJI, YAMANAKA TOSHIO, HARADA, YOSHIKAZU, NAKA, HIROSHI, NAKAHAMA, YASUJI, YAMANAKA, TOSHIO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1996
Main classification: G03G15
IPC codes: G03G15