Hydrodynamic bearing

Abstract: A hydrodynamic bearing apparatus for supporting a rotating member on a support shaft (10) provided on a stationary member through a radial hydrodynamic bearing (12) and a thrust hydrodynamic bearing (13), the apparatus comprising a magnetic member (26) provided on the rotating member; and a magnet provided on the rotating member opposing the magnetic member (26) provided on the stationary member; whereby the magnetic member (26) is attracted by the magnetic force from the magnet so that a force acting on the thrust hydrodynamic bearing (12) are cancelled or reduced.

Description: The present invention relates to a hydrodynamic bearing apparatus used to support a rotating member on a stationary support shaft through a radial hydrodynamic bearing and a thrust hydrodynamic bearing. More particularly, the present invention relates to a hydrodynamic bearing apparatus which enables minimization of the torque required at the time of the starting of rotation. A hydrodynamic bearing apparatus for supporting a rotating member on a stationary support shaft through a radial hydrodynamic bearing and a thrust hydrodynamic bearing is known. A hydrodynamic bearing apparatus as defined in the preamble of claim 1 is for example disclosed in figure 5 of EP-A-0 410 293. Further, for example, Fig. 4 is a sectional view showing the structure of a rotating mirror optical deflector that employs a conventional hydrodynamic bearing apparatus. The rotating mirror optical deflector has a structure in which a mirror having a polyhedral outer periphery, i.e., a polygon mirror 11, rotates around a support shaft 10 through hydrodynamic bearings, as illustrated. The above-described hydrodynamic bearing apparatus comprises a radial bearing 12 having a sleeve like radial bearing member 12a mounted on stationary support shaft 10, an upper thrust bearing 13 having an upper thrust plate 13a, and a lower thrust bearing 14 having a lower thrust plate 14a. The lower thrust plate 14a, the radial bearing member 12a and the upper thrust plate 13a are secured to the support shaft 10 by fastening a nut 23 with a washer 22 interposed therebetween. It should be noted that reference numeral 15 denotes a sleeve like hydrodynamic bearing movable member that constitutes a movable part of the radial bearing 12 and the upper and lower thrust bearings 13 and 14. The support shaft 10 is secured to a motor base 16, and a motor coil 17 is disposed on the top surface of the motor base 16 and on the outer periphery of the lower end of the support shaft 10. A motor rotor 18 is secured to the outer periphery of the hydrodynamic bearing movable member 15, for example, by shrink-fitting, and a motor magnet 19 is secured to the lower surface of the motor rotor 18 so that the motor magnet 19 faces the motor coil 17. The polygon mirror 11 is disposed on the top of the motor rotor 18, and a support member 20 made of an aluminum material is disposed on the polygon mirror 11. The polygon mirror 11 is clamped between the motor rotor 18 and the support member 20 by fastening screws 21. It should be noted that reference numeral 24 denotes a motor cover that is secured to the motor base 16, and 25 a cap that is attached to the top of the motor cover 24. In the rotating mirror optical deflector employing the above-described hydrodynamic bearing apparatus, when the motor coil 17 is supplied with an alternating current, a revolving magnetic field is generated, so that attraction or repulsion occurs between the revolving magnetic field and the motor magnet 19, causing the motor rotor 18, the hydrodynamic bearing movable member 15, the support member 20 and the polygon mirror 11 to rotate around the support shaft 10 as a unit. In the meantime, the outer peripheral surface of the radial bearing member 12a that constitutes the radial bearing 12 is formed with dynamic pressure generating grooves, e.g., herringbone grooves, so that dynamic pressure is generated in between the outer peripheral surface of the radial bearing member 12a and the inner peripheral surface of the hydrodynamic bearing movable member 15. In addition, that surface of the upper thrust plate 13a which faces the hydrodynamic bearing movable member 15 and that surface of the lower thrust plate 14a which faces hydrodynamic bearing movable member 15 are formed with respective dynamic pressure generating grooves, e.g., spiral grooves, so that dynamic pressure is generated in between each of these surfaces and the corresponding surface of the bearing movable member 15. Thus, the bearing movable member 15 rotates around the radial bearing 12 in non-contact manner. In the hydrodynamic bearing apparatus having the above-described arrangement, when the motor rotor 18 reaches a predetermined rotational speed after the starting of the rotation, dynamic pressure is generated at each of the sliding surfaces of the radial bearing 12 and the upper and lower thrust bearings 13 and 14, so that the bearing movable member 15 rotates in a non-contact manner. However, when the motor rotor 18 starts rotation, no dynamic pressure is available, so that a large torque is needed for a large friction due to the weight of the rotating member, the attraction force acting on the motor magnet and the like, and the motor rotor 18 may fail to start rotation under certain circumstances. In view of the above-described circumstances, it is an object of the present invention to provide a hydrodynamic bearing apparatus which enables minimization of the torque required at the time of the starting of the rotation. To attain the above-described object, the present invention provides a hydrodynamic bearing apparatus comprising a rotating member rotatably supported on a support shaft provided on a stationary member through a radial hydrodynamic bearing and a thrust hydrodynamic bearing, characterised in that the hydrodynamic bearing apparatus further comprises: a magnetic member provided on said rotating member; and a magnet provided on said stationary member opposing said magnetic member provided on said rotating member; whereby said magnetic member is attracted by the magnetic force from said magnet so that the weight of the rotating member and/or other forces acting on said thrust hydrodynamic bearing are cancelled or reduced. The force acting on the thrust hydrodynamic bearing may be a weight of the rotating member. However, when said rotating member includes a motor rotor having a motor magnet, and a motor coil cooperating with said motor magnet is provided on said stationary member, said force may be the weight of said rotating member and/or the attracting force acting on said motor magnet. The rotating member may include a polygon mirror and said apparatus could be constituted as a rotating mirror optical deflector. The polygon mirror may be supported on said rotating member by a support member formed by a magnetic material and constituting said magnetic member. Alternately, a part of said support member may be formed by a magnetic material or a magnet may be provided on said supporting member. The rotating member may be covered with a cover member provided on said stationary member and said cover member has a cap provided on the top thereof. The magnet may be provided on the inner surface of said cap, and the magnet force of said magnet is adjustable by shifting said magnet to and away from said magnetic member by moving said cap relative to said cover member. According to another aspect of the invention, said magnet is provided on a magnet securing member secured to said support shaft at a position which faces said support member. The magnet securing member may be provided on said support shaft so that the height thereof is adjustable in order to adjust the attraction force of said magnet. According to a further aspect of the invention, the rotating member can be rotated by rotating said cap relative to said cover member. In this case a spacer may be removably interposed between said cap and said motor cover and said magnet can be adhered to said magnetic member by removing said spacer from between said cap and said motor cover. According to the present invention, since a magnet is provided on the stationary member to attract a magnetic member provided on the rotating member, even if the weight of the rotating member and other force are applied to the thrust hydrodynamic bearing, the attraction force from the magnet acts in a direction in which the applied force is cancelled or reduced. Accordingly, the friction that occurs at the rotating member of the hydrodynamic bearing apparatus is reduced and, the torque required at the time of starting is minimized. When the rotating member is rotated by moving the cap relative to the cover member so as to allow the magnet to adhere to the magnetic member and then rotating the cap relative to the cover member, the rotating member can be rotated externally, and it is therefore possible to prevent dust or the like from entering the inside of the cover and hence possible to keep the interior clean. Accordingly, when the invention is applied to a rotating mirror optical deflector having a polygon mirror provided on a rotating member, optical adjustment can be made with the clean state maintained. The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative examples. Fig. 1 is a sectional view showing the structure of a rotating mirror optical deflector employing one embodiment of the hydrodynamic bearing apparatus according to the present invention, Fig. 2 is a sectional view showing the structure of a rotating mirror optical deflector employing another embodiment of the hydrodynamic bearing apparatus according to the present invention, Fig. 3 is a sectional view showing the structure of a rotating mirror optical deflector employing still another embodiment of the hydrodynamic bearing apparatus according to the present invention, Fig. 4 is a sectional view showing the structure of a rotating mirror optical deflector employing a conventional hydrodynamic bearing apparatus, Fig. 5 is a graph showing experimental results of a hydrodynamic bearing apparatus according to the prior art, and Fig. 6 is a graph showing experimental results of a hydrodynamic bearing apparatus according to the present invention. Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Fig. 1 is a sectional view showing the structure of a rotating mirror optical deflector that employs one embodiment of the hydrodynamic bearing apparatus according to the present invention. In this figure, portions or members which are denoted by the same reference numerals as those in Fig. 4 are the same as those shown in Fig. 4; therefore, the repetitive description thereof is omitted (the same is the case with the other drawings). As illustrated, a single-pole magnet or magnets 26 is or are provided at the lower end of the inner surface of the cap 25 at a proper distance from the support member 20, and a suitable magnetic material is employed for the support member 20. In case a plural magnets 26 are provided, they are circumferentially spaced from each other. Thus, the magnetic flux from the magnet 26 causes an attractive force to act on the support member 20 so as to cancel or reduce the force applied to the thrust bearing 14 owing to the weight of a rotating member which comprises the bearing movable member 15, the motor rotor 18 having a motor magnet 19, the polygon mirror 11 and the support member 20 and the attraction force acting on the motor magnet 19. In other words, the attraction force of the magnetic flux from the magnet 26 is adjusted so as to balance with the weight of the rotating member and the attraction force acting on the motor magnet 19. The attraction force of the magnet 26 can be adjusted by moving the cap 25 vertically. When the rotating mirror optical deflector is used in a horizontal position, that is, when the motor is placed in a horizontal position, since the weight of the rotating member does not apply to the thrust bearing 14, the attraction force of the magnet 26 is adjusted so as to balance with the attraction force acting on the motor magnet 19. When the motor is placed obliquely or upside down, the attraction force of the magnet 26 is adjusted so as to balance with the rotating member according to its position. As stated above, since the magnet 26 is provided and a magnetic material is employed for the support member 20 as described above, the attraction force from the magnet 26 cancels or reduces the weight of the rotating member and the attraction force acting on the motor magnet 19, so that it is possible to minimize the torque required at the time of starting the motor. Fig. 2 is a sectional view showing the structure of a rotating mirror optical deflector that employs another embodiment of the hydrodynamic bearing apparatus according to the present invention. As shown in Fig. 2, a disc-shaped magnet securing member 27 is provided between the upper thrust plate 13a on the upper portion of the support shaft 10 and the washer 22, and the magnet 26 is secured to the lower surface of the magnet securing member 27 and at a position which faces the support member 20 at a proper distance. Since the magnet 26 is secured to the magnet securing member 27 secured to the support shaft 10 at a proper distance from the support member 20, the attraction force from the magnet 26 cancels or reduces the weight of the rotating member and the attraction force acting on the motor magnet 19, so that it is possible to minimize the torque required at the time of starting the motor. It is also possible to adjust the attraction force of the magnet 26 by adjusting the height of the magnet securing member 27, by, for example, adjusting the thickness of a spacer interposed between the magnet securing member 27 and the upper thrust plate 13a. Fig. 3 is a sectional view showing the structure of a rotating mirror optical deflector that employs still another embodiment of the hydrodynamic bearing apparatus according to the present invention. The arrangement shown in Fig. 3 is the same as that shown in Fig. 1 in that the single-pole magnet 26 is provided at the lower end of the inner surface of the cap 25 at a proper distance from the support member 20 and a magnetic material is employed for the support member 20. However, in this embodiment, a spacer 28 is removably interposed between the cap 25 and the motor cover 24. Since the spacer 28 is removably interposed between the cap 25 and the motor cover 24 as described above, if the spacer 28 is removed, the cap 25 is lowered and the support member 20 is attracted to adhere to the magnet 26. Thus, by rotating the cap 25 in this state, the polygon mirror 11 is rotated, and the optical adjustment can be made. It should be noted that, if the attraction force of the magnet 26 is strong, the polygon mirror 11 can be rotated for the optical adjustment simply by rotating the cap 25 without vertically shifting the magnet 26 to attract the support member 20. Thus, it is unnecessary to open the cap 25 during the optical adjustment, and it is possible to prevent dust or the like from entering the inside of the motor cover. Accordingly, the optical adjustment can be conducted with a high degree of cleanness maintained. It is also possible to adjust the attraction force of the magnet 26 by adjusting the heightwise dimension of the spacer 28. Fig. 5 is a graph showing experimental results of a hydrodynamic bearing apparatus according to the prior art shown in Fig. 4, and Fig. 6 is a graph showing experimental results of the hydrodynamic bearing apparatus according to the present invention, shown in Fig. 1. As will be clear from Fig. 5, in the prior art the frictional torque is large due to the attracting force acting on the motor magnet and the weight of the rotating member, so that no adequate starting torque can be obtained. In contrast, in the present invention, since the rotating member is attracted by the magnet 26, the frictional torque decreases, and a sufficiently high motor starting torque can be obtained, as shown in Fig. 6. Although in the foregoing embodiments the hydrodynamic bearing apparatus of the present invention is used in a rotating mirror optical deflector having the polygon mirror 11, it should be noted that the hydrodynamic bearing apparatus of the present invention is not necessarily limited thereto and that the present invention is widely applicable to hydrodynamic bearing apparatuses designed to support a rotating member on a support shaft provided on a stationary member. Also, in the foregoing embodiments, although the support member 20 constituting the rotating member is formed from a magnetic material it should be noted that the present invention is not necessarily limitative thereto and that a part of the support member 20 may be formed using a magnetic material. It is also possible to provide a magnetic member, e.g., a magnet, on the top of the support member 20. The magnet 26 may be either a permanent magnet or an electromagnet. In particular, when an electromagnet is employed, it is possible to change the magnetic force for balancing the rotating member in accordance with the position in which the motor is used. As has been described above, the present invention provides the following advantageous effects: (1) Since a magnet is provided on the stationary member to generate an attraction force with respect to a magnetic member provided on the rotating member, even if the weight of the rotating member and other force are applied to a thrust hydrodynamic bearing that constitutes the hydrodynamic bearing apparatus, the attraction force from the magnet acts in a direction in which the applied force is cancelled or reduced. Accordingly, the torque required at the time of starting the rotating member is minimized. (2) When the rotating member is rotated by moving or without moving the cap relative to the cover member so as to allow the magnet to adhere to the magnetic member and then rotating the cap relative to the cover member, the rotating member can be rotated externally, and it is therefore possible to prevent dust or the like from entering the inside of the cover and hence possible to keep the interior clean.

Claims: A hydrodynamic bearing apparatus comprising a rotating member rotatably supported on a support shaft (10) provided on a stationary member (16) through a radial hydrodynamic bearing (12) and a thrust hydrodynamic bearing (14), characterised in that said hydrodynamic bearing apparatus further comprises: a magnetic member (20) provided on said rotating member; and a magnet (26) provided on said stationary member (16) opposing said magnetic member (20) provided on said rotating member; whereby said magnetic member (20) is attracted by the magnetic force from said magnet (26) so that the weight of the rotating member and/or other forces acting on said thrust hydrodynamic bearing (14) are cancelled or reduced. A hydrodynamic bearing apparatus according to Claim 1, wherein said rotating member includes a motor rotor (18) having a motor magnet (19), and a motor coil (17) for cooperating with said motor magnet (19) is provided on said stationary member (16). A hydrodynamic bearing apparatus according to Claim 2, wherein said rotating member includes a polygon mirror (11) and said apparatus constitutes a rotating mirror optical deflector. A hydrodynamic bearing apparatus according to Claim 3, wherein said rotating member includes a bearing movable member (15) having a radial and thrust bearing surfaces, and a support member (20) for supporting said polygon mirror. A hydrodynamic bearing apparatus according to Claim 4, wherein said support member (20) is formed by a magnetic material and constituting said magnetic member (20). A hydrodynamic bearing apparatus according to Claim 4, wherein a part of said support member is formed by a magnetic material and constituting said magnetic member (20). A hydrodynamic bearing apparatus according to Claim 4, wherein said magnetic member is a magnet provided on said supporting member (20). A hydrodynamic bearing apparatus according to any one of Claims 5, 6 and 7, wherein said rotating member is covered with a cover member (24) provided on said stationary member (16), said cover member (24) having a cap (25) provided on the top thereof, said magnet (26) is provided on the inner surface of said cap (25), and the magnetic force of said magnet (26) is adjustable by shifting said magnet (26) to and away from said magnetic member (20) by moving said cap (25) relative to said cover member (24). A hydrodynamic bearing apparatus according to Claim 1, wherein said magnet (26) is a permanent magnet or an electromagnet. A hydrodynamic bearing apparatus according to Claim 4, wherein said magnet (26) is provided on a magnet securing member (27) secured to said support shaft (10) at a position which faces said support member (20). A hydrodynamic bearing apparatus according to Claim 10, wherein said magnet securing member (27) is provided on said support shaft (10) so that the height thereof is adjustable and thereby adjust the attraction force of said magnet (26). A hydrodynamic bearing apparatus according to Claim 8, wherein said rotating member can be rotated by rotating said cap (25) relative to said cover member (24). A hydrodynamic bearing apparatus according to Claim 12, wherein a spacer (28) is removably interposed between said cap (25) and said cover member (24), said magnet (26) can be adhered to said magnetic member (20) by removing said spacer (28) from between said cap (25) and said cover member (24).

Inventors: CHIBA KATSUTOSHI, HISABE YASUSHI, KOMATA KIMIO, MACHIDA AKIHIRO, OTANI TOSHIHIRO, CHIBA, KATSUTOSHI, HISABE, YASUSHI, KOMATA, KIMIO, MACHIDA, AKIHIRO, OTANI, TOSHIHIRO
Applicants: EBARA CORP, EBARA CORPORATION
Application year: 1992
Main classification: F16C32
IPC codes: G02B26, F16C17, F16C32, F16C39, F16C33