Outer-rotor type spindle motor

Abstract: In an outer-rotor type spindle motor an upper portion of a stationary spindle (4) extends through a central portion of a hub plate (7c) of a rotatable rotor yoke (6) onto which a disk (11) is loaded. An upper head portion (4c) of the spindle (4) has the form of a hemisphere with a flat face (4d) disposed obliquely to a central longitudinal axis of the spindle (4).

Description: The present invention relates to an outer-rotor type spindle motor used to drive a disk such as magnetic disks and optical disks of recording units wherein data is stored on the disks. In general, an outer-rotor type spindle motor is used as a drive motor of a recording unit wherein data is stored on the disk. In this type of spindle motor: a head portion (i.e., a front-end portion) of a spindle firmly fitted in a center of a rotor yoke is projected; and the spindle is adapted to rotate together with the rotor yoke. The problem of reducing the office automation instruments (such as personal computers and word processors) in size and thickness has been a matter of increasing importance, which also makes it necessary to reduce in size and thickness a drive mechanism of the recording unit used in the office automation instrument. In such drive mechanism of the recording unit in the office automation instrument, a spindle of a drive motor for the recording unit has its head portion projected from a rotor yoke which is supported by the spindle. Consequently, it is possible to reduce the drive motor in thickness when a projected portion of the spindle is reduced in length. This is, however, practically impossible for the following reasons. That is, in the recording unit, a suitable media, for example such as a 3.5-inch floppy disk, is fitted to the spindle of the motor by means of an ejector of a loading mechanism so as to be loaded onto a disk chucking mount. In this loading operation, the media or disk is horizontally moved to the vicinity of the spindle, and then moved downward in a direction inclining a very fine angle when the disk reaches a position immediately in front of the spindle, which enables the disk to have its mounting hole fitted to the head portion of the spindle. As for the shape of the spindle, since the disk enters the loading mechanism of the recording unit in a fixed direction, it is necessary to form the head portion of the spindle into a hemispherical shape which makes it possible to load the disk onto the spindle in any angular position in which the spindle stops to stay. On the other hand, as shown in Fig. 5, a spindle motor for driving the disk of a magnetic recording unit has a construction in which: a flange 6a extending downward is formed in an outer peripheral portion of a rotor yoke 6; a plurality of permanent magnets 5, i.e., N's and S's are so arranged as to be spaced alternately with each other on an inner surface of the flange 6a; the rotor yoke 6 has its central portion firmly fitted to a spindle 4 of the motor; a ball bearing 21 has a plurality of balls interposed between an outer ring 21b and an inner ring 21a, and has its inner ring 21b fitted to the spindle 4; a stator holder 2, which also acts as a bearing housing, is mounted on the outer ring 21b or outer peripheral surface of the outer ring 21b; and, a stator 3 is firmly fitted to an outer peripheral surface of the stator holder 2, and has a plurality of soft pole plates 3a which radially extend, on each of which plates 3a a solenoid 3b is mounted. The stator holder 2 is fixedly mounted on a board 1. Upon energizing of the motor, the spindle 4 is rotatably driven together with the rotor yoke 6. As described above, in the motor having the above construction, the bearing 21 is interposed between the spindle 4 and the stator 3. In case of the ball bearing which includes the inner and the outer ring, as shown in Fig. 6, since shielding plates 15 are provided between the inner and the outer ring so as to shield opposite sides of the balls from dust, the bearing 21 is large in thickness, i.e, large in a longitudinal direction of the spindle 4. In the motor, it is required to prevent the spindle from vibrating in stabilizing the motor in operation, which is accomplished by providing a pair of ball bearings in series on the spindle. However, in case of the conventional shielded ball bearings, since even one of these conventional bearings is considerably large in thickness, it is impossible to provide these conventional bearings in series on the spindle, because they become quite large in thickness and make it impossible to reduce the motor in thickness. In order to solve the above problem, heretofore proposed are: a construction shown in Fig. 5, in which a ball bearing 21 and an oil-impregnated metal ring 16 are arranged in series on the spindle; and another construction shown in Fig. 7, in which the ball bearing is not used and only the metal ring 16 is arranged on the spindle to reduce the motor in thickness. In any of these proposed constructions, since the metal ring 16 is of a sliding type producing a relatively large frictional resistance in comparison with the ball bearing, the motor using such metal ring tends to: increase its electric consumption; reduce its service life due to wearing of the ring metal; and be inferior in reliability to a motor using the ball bearing. These are disadvantages inherent in the conventional motor using the metal ring. In order to reduce the motor in thickness, it is necessary to use as thin a ball bearing (a thickness of which is denoted by the reference character "H" shown in Fig. 6) as possible. However, the ball bearing having a small thickness H naturally requires small-diameter balls. As a result, such ball bearing suffers from: (1) a small rate load; and (2) a large frictional resistance, which causes a large loss of torque and increases the electric consumption. It is an object of the present invention to solve the above problems by providing an outer-rotor type spindle motor a thickness of which is remarkably reduced. It is another object of the present invention to provide an outer-rotor type spindle motor in which: a head portion of a spindle of the motor is formed into a semispherical shape which is then obliquely cut so as to further reduce a projected portion of the spindle in length in comparison with that of complete semispherical shape, the thus obliquely cut head portion of the spindle enabling the disk to be loaded in a recording unit in a stable manner. The present invention consists in an outer-rotor type spindle motor in which: a front-end portion of a spindle forming a stationary member passes through a central portion of a hub plate (onto which a disk is loaded) of a rotor yoke forming a rotating member; and, the front-end portion having been passed through the central portion of the hub plate of the rotor yoke forms a head portion which is formed into a semispherical shape having been obliquely cut. In the accompanying drawings:- Fig. 1 is a partially broken plan view of an embodiment of the outer-rotor type spindle motor of the present invention; Fig. 2 is a longitudinal sectional view of the motor of the present invention shown in Fig. 1; Fig. 3 is an enlarged longitudinal sectional view of an essential part of the motor of the present invention shown in Fig. 1; Fig. 4 is an enlarged longitudinal sectional view of an essential part of another embodiment of the motor of the present invention; Fig. 5 is a longitudinal sectional view of a left half of an example of a conventional motor; Fig. 6 is a longitudinal sectional view of the ball bearing used in the conventional motor shown in Fig. 5; and Fig. 7 is a longitudinal sectional view of a left half of another example of a conventional motor. Now, the present invention will be described in detail with reference to the drawings. Figs. 1 to 3 show an embodiment of an outer-rotor type spindle motor of the present invention. As shown in Figs. 1 to 3, a board 1 is provided with a hole to which a lower portion of a stator holder 2 is firmly fitted. The stator holder 2 is provided with a bottom plate 2a an outer peripheral portion of which is formed into a cylindrical rib portion 2b so that the stator holder 2 assumes a cylindrical shape having a bottom plate. Firmly fitted to the rib portion 2b of the stator holder 2 is a central portion of a stator 3 which is provided with a plurality of pole plates 3a which radially extend from the central portion of the stator 3 and are made of soft magnetic material, on each of which pole plates 3a a solenoid 3b is mounted. A spindle 4 is fixedly mounted in a center of the bottom plate of the stator holder 2 so as to extend upward in a direction perpendicular to the bottom plate. A rotor yoke 6 made of soft magnetic material is provided with: an outer peripheral flange on which a permanent magnet 5 is mounted; and a central hub base 7 which is rotatably mounted on the spindle 4 through ball bearings 8 and also acts as a bearing housing. That is, the hub base 7 is provided with a cylindrical housing portion 7a which has an outer peripheral part of its upper portion formed into a chucking magnet portion 7b onto which a disk is loaded. The housing portion 7a of the hub base 7 is firmly fitted to a central hole of the rotor yoke 6. Firmly fitted to an inner surface of the housing portion 7a of the hub base 7 are outer rings 8b of the ball bearings 8 inner rings 8a of which are firmly fitted to the spindle 4. On the other hand, one of the inner rings 8a has its lower surface supported by a boss portion 9 of the stator holder 2, the boss portion 9 being formed in a central portion of the bottom plate of the stator holder 2. The other one of the inner rings 8a abuts on a stop means such as a snap ring 10 fitted to a groove of the spindle 4. A hub plate 7c is mounted on an upper-end surface of the housing portion 7a of the hub base 7 to act as a shielding plate closing the upper portion of the hub base 7. Consequently, in operation, the rotor yoke 6 is rotatable, while the spindle 4 is stationary. In the motor of the present invention: as shown in Fig. 3, the spindle 4 has its upper portion 4a passed through the hub plate 7c upward, so that the thus passed portion or head portion 4c extending upward from a round neck portion 4b of the spindle 4 is formed into a semispherical shape which is obliquely cut to form an oblique surface 4d. This oblique surface 4c has the substantially same tilt angle as that of a direction in which disk 11 is obliquely moved downward when loaded, whereby the disk 11 can be easily engaged with the head portion 4c of the spindle 4 in loading thereof. As is clear from Fig. 3, the head portion 4c of the spindle 4 extends upward from a central hole of the hub plate or shielding plate 7c. The reference numeral 12 denotes a drive pin which has one of its ends fixed to a free end of a leaf spring 13 (which is fixedly mounted on a lower surface of the rotor yoke 6) and permits the other of its ends to pass through a hole 14 (which is formed in both the rotor yoke 6 and the hub base 7) upward under the influence of a resilient force exerted by the leaf spring 13, whereby the drive pin 12 engages with a reference hole of the disk 11 loaded onto the hub base 7 and rotatably drive the disk 11 in operation. Incidentally, the drive pin 12 is depressed so as to be retracted from the upper surface of the hub base 7 under the influence of the weight of the disk 11 when the disk 11 has its reference hole misaligned with the drive pin 12 in loading operation thereof. When the reference hole of the disk 11 is aligned with the drive pin 12, the drive pin 12 is pushed into the reference hole of the disk 11 by the leaf spring 13. In this embodiment of the present invention, as shown in Fig. 3, since the head portion 4c of the spindle 4 passing through the hub plate 7c upward is formed into the obliquely-cut semispherical shape, this shape is lower in height than a complete semispherical shape shown in dotted line by an amount of Δ α , which reduces a required disk-loading space over the hub base 7 by the amount of Δ α , and thereby a disk-loading unit can be reduced in thickness. In addition, since an upper surface of the head portion 4c of the spindle 4 is not formed into a plane perpendicular to the longitudinal axis of the spindle 4 but the oblique surface 4d, the head portion 4c has its one side be lower than the other side thereof. Consequently, the disk 11 can be loaded onto the hub plate 7c in a stable manner even when the disk 11 is loaded in a condition in which the disk 11 has a center line of its reference hole be slightly misaligned with the longitudinal axis of the spindle 4, because the obliquely-cut semispherical head portion 4c of the spindle 4 can smoothly guide the reference hole of the disk 11 so as to be aligned with the neck portion 4b of the spindle 4, whereby the disk 11 is steadily loaded onto the hub plate 7c. Fig. 4 shows another embodiment of the present invention. In this embodiment, the head portion 4c of the spindle 4 is formed into a large-diameter portion having a lower shoulder which abuts on an upper surface of the inner ring of the ball bearing so as to urge the inner ring downward. The remaining construction of this embodiment shown in Fig. 4 is the same as that of the embodiment shown in Fig. 3. In the embodiment shown in Fig. 4, the head portion 4c of the spindle 4 extends upward from the neck portion 4b of the spindle 4 to pass through the hub plate 7c upward, on which plate 7c the disk is loaded. Such head portion 4c of the spindle 4 is formed into an obliquely-cut semispherical shape. Consequently, such head portion 4c of the spindle 4 is lower in height than the head portion 4c assuming the complete semispherical shape, which enables the required disk-loading space over the hub plate 7c to be reduced. Since the head portion 4c of the spindle 4 is not formed into a cylindrical shape but into an obliquely-cut semispherical shape, the disk 11 can be loaded onto the hub plate 7c in a stable manner even when the disk 11 is loaded in a condition in which the disk 11 has a center line of its reference hole be slightly misaligned with the longitudinal axis of the spindle 4, because the obliquely-cut semispherical head portion 4c of the spindle 4 can smoothly guide the reference hole of the disk 11 so as to be aligned with the neck portion 4d of the spindle 4, whereby the disk 11 is steadily loaded onto the hub plate 7c. In operation, since the spindle 4 stays stationarily in operation to have its oblique surface 4d oriented in a fixed direction in which the disk is loaded onto the hub plate 7c, there is no fear that the head portion 4c of the spindle 4 (which portion 4c assumes the obliquely-cut semispherical shape) impairs loading and unloading operations of the disk. Further, in a disk recording unit provided with a disk drive mechanism using the motor of the present invention, it is possible to reduce in thickness a disk loading mechanism which loads the disk onto the hub plate of the rotor yoke in a stable manner and ensures the disk the writing and retrieving operations of data without any error.

Claims: An outer-rotor type spindle motor in which: a front-end portion of a spindle (4) forming a stationary member passes through a central portion of a hub plate (7c) (onto which a disk is loaded) of a rotor yoke (6) forming a rotating member; and, said front-end port ion having been passed through said central portion of said hub plate (7c) of said rotor yoke (6) forms a head portion (4c) which is formed into a semispherical shape having been obliquely cut.

Inventors: FUJITANI SAKAE, INAGAKI MASAAKI, SUZUKI YUZURU, UCHIDA HITOSHI, FUJITANI, SAKAE, INAGAKI, MASAAKI, SUZUKI, YUZURU, UCHIDA, HITOSHI, Fujitani, Sakae c/o Minebea Co. Ltd. R&amp;D Center, Uchida, Hitoshi c/oMinebea Co.Ltd. Karuizawa Mf.U.
Applicants: MINEBEA KK, MINEBEA KABUSHIKI-KAISHA
Application year: 1992
Main classification: G11B19
IPC codes: H02K5, G11B19