Magnetic thrust bearing for polygon mirror scanner

Abstract: A scanning optical apparatus has a stationary shaft and a rotary sleeve rotatably fitting to each other. The apparatus is arranged to deflection-scan a light beam by rotating a rotary polygon mirror (13) attached to the rotary sleeve (23). The apparatus comprises a magnetic thrust bearing in the second permanent magnet (31), which is repulsive to a first permanent magnet (30) mounted on the stationary shaft (22), is disposed on a lower end side of the rotary sleeve (23). Further, a third permanent magnet (32) is placed on a base (21a) to which the stationary shaft (22) is fixed so that the third magnet can provide a repulsive force in a direction to urge the second permanent magnet toward the first permanent magnet.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a scanning optical apparatus having a rotary polygon mirror for scanning a light beam such as a laser beam on a photosensitive member. Related Background Art In these years, the scanning optical apparatus of this type is required to rotate the polygon mirror at high speed or at high accuracy, and particularly, laser beam printer apparatus use a bearing rotating apparatus rotating in non-contact in order to achieve a high-accuracy deflection-scanning apparatus. In the bearing rotating apparatus for rotating the rotary polygon mirror used in the conventional scanning optical apparatus, as shown in Fig. 1, a stationary shaft 2 of a ceramic material is standing from a housing 1 mounted on a frame or the like, a rotary sleeve 4 made of a ceramic material with a drive magnet 3 attached thereto is rotatably fit over the stationary shaft 2, and a rotary polygon mirror 6 is fixed by a spring 5 to the rotary sleeve 4. On the housing 1 there is a base 8 to which a stator 7 is mounted at a position opposite to the drive magnet 3, thus constituting a drive motor for rotating the rotary sleeve 4. Further, a permanent magnet 9 is attached to the lower end of the rotary sleeve 4, and a second permanent magnet 10 is fixed to a lower portion of the stationary shaft 2 so as to be vertically opposed to the permanent magnet 9 and to be repulsive thereto, thus supporting a load in the thrust direction by repulsion between the permanent magnets 9, 10. Once the driving motor is driven, an air membrane is formed between the rotary sleeve 4 and the stationary shaft 2. This air membrane supports the rotary sleeve 4 in the radial direction, and the rotary sleeve 4 and rotary polygon mirror 6 can rotate in non-contact. Since the thrust bearing was kept floating only by a repulsive force between the pair of permanent magnets in the scanning optical apparatus as in the above-described conventional example, the apparatus, however, had such defects that thrust bearing stiffness was low, that accuracy of height position of the rotary sleeve could not be enhanced so much, and that control of a motion in the thrust direction due to disturbance etc. was difficult. US-A-4 726 640 discloses an optical deflector for deflecting a laser beam from a laser by rotating a polygon mirror using a dynamic pressure pneumatic bearing as a radial bearing and a magnetic bearing as a thrust bearing. The magnetic bearing uses three different magnets arranged in the order as given in the introductory part of claim 1. US-A-50 19738 discloses a beaving using three magnets. US-A-50 19738 does not disclose the fact that a third magnet can provide a repulsive force toward, a second magnet, such as that said second magnet moves. An object of the present invention is to provide a bearing apparatus and a scanning optical apparatus, solving the above problems, which is improved in thrust bearing stiffness of the driving motor and which is enhanced in height position accuracy of the rotary sleeve. This object is solved by a bearing apparatus and a scanning optical apparatus according to the independent claim. Preferred embodiments are subject of the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a major part structural drawing of a conventional bearing rotating apparatus for rotating a rotary polygon mirror; Fig. 2 is an overall structural drawing of a laser beam printer apparatus using the scanning optical apparatus of the present invention; Fig. 3 is a major part structural drawing of a first embodiment of the bearing rotating apparatus for rotating the rotary polygon mirror according to the present invention; Fig. 4 is an explanatory drawing of arrangement of permanent magnets; Fig. 5 is a major part structural drawing of a second embodiment of the bearing rotating apparatus for rotating the rotary polygon mirror according to the present invention; Fig. 6 is a major part structural drawing of a third embodiment of the bearing rotating apparatus for rotating the rotary polygon mirror according to the present invention; and Fig. 7 is a major part structural drawing of a fourth embodiment in which the bearing rotating apparatus of the present invention is applied to a magnetic disk driving apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail based on the embodiments as shown in Fig. 2 to Fig. 7. Fig. 2 is a structural drawing of the whole of a laser beam printer apparatus using the scanning optical apparatus of the present invention, in which laser light emitted from a laser unit 12 provided on a frame 11 is deflection-scanned by a rotary polygon mirror 13 so as to be focused through image-forming lenses 14, 15 on a photosensitive member 16 as a recording medium. Fig. 3 is a major part structural drawing of a first embodiment of the bearing rotating apparatus of the present invention, for rotating the above-described rotary polygon mirror 13. A motor housing (base) 21 mounted on the frame 11 of the scanning optical apparatus has a shaft supporting portion 21a projecting downward, and the lower end of a vertical, stationary shaft 22 made of a ceramic is fixed in a tubular recess 21b provided in the shaft supporting portion 21a. A ceramic rotary sleeve 23 in a length for the lower end thereof to reach inside the tubular recess 21b is fit over the stationary shaft 22 so as to be rotatable relative to the stationary shaft 22, and a flange 24 made of a non-magnetic metal material such as aluminum or brass is attached to the rotary sleeve 23, for example, by shrinkage fit. Also, a drive magnet 25 is attached to the periphery of flange 24 by means of adhesion. A motor base 26 on which electric components are mounted is attached to the top surface of motor housing 21, and a stator 27 opposed to the drive magnet 25 is set on the base 26 so as to constitute a drive motor. A rotary polygon mirror 13 is fit on the rotary sleeve 23, and is fixed by a plate spring 28 on the top surface of flange 24. A first permanent magnet 30 formed in a ring shape is attached to a lower part of stationary shaft 22 at a position below a second permanent magnet 31 attached to the rotary sleeve 23 and at a position in contact with an inner bottom surface of the tubular recess 21b in the shaft supporting portion 21a. The second permanent magnet 31 of a same cross section as the rotary sleeve 23 is attached to the lower end of the rotary sleeve 23 so as to be repulsive to the first permanent magnet. Further, a third permanent magnet 32 is attached to the housing 21 at a position above the second permanent magnet 31 and at the upper edge of the tubular recess 21b in the shaft supporting portion 21a above the first permanent magnet 30 in such an orientation that a repulsive force thereof urges the second permanent magnet 31 toward the first permanent magnet 30. Thus, the third permanent magnet 32 is repulsive to the second permanent magnet 31. Here, because the third permanent magnet 32 is located at the upper edge of the tubular recess 21b formed in the housing 21, the inner diameter d thereof is larger than the outer diameter D of the rotary sleeve 23 and second permanent magnet 31. Because of this, in order to make the repulsive force acting in the direction to urge the second permanent magnet 31 toward the first permanent magnet 30, such a magnetic pole arrangement needs to be set, for example as shown in Fig. 4, that the first and second permanent magnets 30, 31 are set with the N poles thereof opposed to each other and that the third permanent magnet 32 has the inner diameter d larger than the outer diameter D of the second permanent magnet 31 and is arranged with the N pole thereof facing down. The same effect can be achieved by the completely inverted pole arrangement of the N and S poles of the three permanent magnets. In the scanning optical apparatus constructed in the above arrangement, when the rotary sleeve rotates under drive of the driving motor, laser light emitted from the laser unit 12 is deflected by the rotary polygon mirror 13 and is focused by the image-forming lenses 14, 15 at a predetermined position on the photosensitive member 16 in a laser beam printer attached to this apparatus. An air membrane is formed between the rotary sleeve 23 and the stationary shaft 22 to support the rotary sleeve 23 in non-contact in the radial direction. On the other hand, the repulsive force between the first and second permanent magnets 30, 31 keeps the rotary body floating in a completely non-contact state with respect to the stationary side in the thrust direction. Further, because the third permanent magnet 32 urges the second permanent magnet 31 toward the first permanent magnet 30, the repulsive force in the thrust direction is reinforced so as to increase the thrust bearing stiffness, to enhance the position accuracy of the rotary sleeve 23 in the thrust direction, and to restrict a motion in the thrust direction due to thrust load variations or disturbance by the bi-directional repulsive forces to stabilize the motion. Since the stationary shaft 22 and rotary sleeve 23 each are made of a ceramic, a possibility of scuffing or the like is low even if they come into contact with each other because of mixing of dust or vibration during high-speed rotation. If high-strength silicon nitride (Si 3 N 4 ) is used among ceramics, the possibility of wear or scuffing can be further reduced. Fig. 5 shows a second embodiment of the bearing rotating apparatus of the present invention. Members having the same functions as those in the first embodiment are denoted by the same reference numerals and the description thereof will be omitted herein. A cover member 40 covering the top of the stationary shaft 22 is disposed at the upper end of the rotary sleeve 23 in the first embodiment, and an air reservoir 41 is formed among the cover member 40, the rotary sleeve 23, and the stationary shaft 22. An air vent hole 42 is formed through the cover member 40, and a sealing plug 43 is coupled to the hole 42. When the rotary sleeve 23 is brought into fit over the stationary shaft 22, the sealing plug 43 is kept in a taken-off state after fit, the hole 42 is sealed by the sealing plug 43, thus forming the air reservoir 41 among the upper end of the stationary shaft 22, the rotary sleeve 23, and the cover member 40. Since the air reservoir 41 works as an air damper even with application of an external force, which would vertically vibrate the rotary sleeve 23, the motion of the rotary sleeve 23 in the thrust direction can be further suppressed. The second embodiment can also be applied to the laser beam printer apparatus as described with Fig. 2 like the first embodiment. Fig. 6 shows a third embodiment of the bearing rotating apparatus of the present invention. Members having the same functions as those in the first embodiment are denoted by the same reference numerals and the description thereof is omitted herein. In the present embodiment, the first permanent magnet 30 and third permanent magnet 32 in the first embodiment are fit through a cylindrical magnet mounting member 50 in the tubular recess 21b in the shaft supporting portion 21a, whereby the first, second, and third permanent magnets 30, 31, 32 are set in the same positional relation as in the first embodiment. The first permanent magnet 30 and third permanent magnet 32 are attached to a same member, i.e., the magnet mounting member 50. By this arrangement, the distance relation between the first permanent magnet 30 and the second permanent magnet 31 can be set at high accuracy, because it is determined only by machining accuracy of the magnet mounting member 50. Thus, the position accuracy in the thrust direction can also be enhanced. The third embodiment can also be applied to the laser beam printer apparatus as described with Fig. 2 like the first embodiment. Although the first to third embodiments were described with the examples of inner rotor type scanning optical apparatus advantageous for high-speed rotation, the present invention can be applied to apparatus of the outer rotor type or the face-to-face type as well. Fig. 7 is a major part structural drawing of a fourth embodiment in which the bearing rotating apparatus of the present invention is applied to a magnetic disk driving apparatus (HDD). A stationary shaft 102 is attached to a base 101, and a cylindrical rotary sleeve 103 is provided to fit over the stationary shaft 102. Magnetic disks 104 are mounted on the upper part of rotary sleeve 103. A rotor 105 is attached to the lower part of the rotary sleeve 103 so as to be opposed to a stator coil 106 attached to the base 101. Then these members are covered by a case 107. A second permanent magnet 202 is attached to the lower end of the rotary sleeve 103 with the magnetic disks 104 attached thereto, and a first permanent magnet 201 is attached to a lower part of the stationary shaft 102 so as to be repulsive to the permanent magnet 202. Further, a third permanent magnet 203 acting as to urge the second permanent magnet 202 toward the first permanent magnet 201 by its repulsive force is attached to the base 101. Since the third permanent magnet 203 urges the second permanent magnet 202 toward the first permanent magnet 201, the repulsive force in the thrust direction is reinforced so as to increase the thrust bearing stiffness. As described above, because the bearing rotating apparatus according to the present invention is so arranged that the third permanent magnet provided on the base gives the repulsive force in the direction to urge the second permanent magnet provided on the rotary sleeve in the driving motor toward the first permanent magnet provided on the stationary shaft, the repulsive force is reinforced so as to increase the thrust bearing stiffness and the motion of the rotary sleeve due to disturbance can be restricted in the thrust direction, thus enhancing the height position accuracy.

Claims: A bearing apparatus comprising: a stationary shaft (22); q rotary sleeve (23) rotatably fitting with said stationary shaft (22); a second permanent magnet (31) provided on said rotary sleeve (23); a first permanent magnet (30) provided below said second permanent magnet (31), said first permanent magnet (30) being repulsive to said second permanent magnet (31); a base (21) to which said stationary shaft (22) is fixed; wherein said second permanent magnet (31) is provided on a lower end side of said rotary sleeve (23). characterized in that a third permanent magnet (32) is provided above said second permanent magnet (31); and in that said third permanent magnet (32) can provide a repulsive force toward said second permanent magnet (31), such that said second permanent magnet (31) move wherein. The apparatus according to claim 1, wherein said third permanent magnet (32) provides said repulsive force in a direction to urge said second permanent magnet (31) towards said first permanent magnet (30). The apparatus according to claim 1 or 2, wherein said first permanent magnet (30) and said third permanent magnet (32) are attached to a same member (50). The apparatus according to any one of the preceding claims, wherein said first permanent magnet (30) is provided on said stationary shaft (22). The apparatus according to claim 4, wherein said third permanent magnet (32) is provided on said base (21). The apparatus according to any one of the preceding claims, wherein the inner diameter of said third permanent magnet (32) is greater than the outer diameter of the second permanent magnet (31). The apparatus according to any one of the preceding claims, wherein said stationary shaft and said rotary sleeve are made of a ceramic material. The apparatus according to any one of the preceding claims, wherein said stationary shaft and said rotary sleeve are made of a silicon nitride. The apparatus according to any one of the preceding claims, further comprising a cover member (40) provided at an upper end of said rotary sleeve (23), for covering said stationary shaft (22) so as to form an air reservoir. The apparatus according to any one of the preceding claims, further comprising driving means (25, 26, 27) for rotating said rotary sleeve (23). A scanning optical apparatus for deflection-scanning a light beam, comprising a bearing apparatus according to claim 10, and further comprising a light source (12); and a deflector (13) attached to said rotary sleeve (23), for deflection-scanning the light beam from said light source (12). The apparatus according to claim 11, wherein said deflector (13) is a rotary polygon mirror. A laser beam printer apparatus comprising a scanning apparatus according to claim 11 or 12, and further comprising a photosensitive member (16) for receiving the light beam deflection-scanned by said deflector.

Inventors: NAKASUGI MIKIO, NAKASUGI, MIKIO, NAKASUGI, MIKIO, C/O CANON K.K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1995
Main classification: G02B26
IPC codes: F16C39, G02B26, H02K7, H02K5