Outer-rotor type spindle motor

Abstract: In an outer-rotor type spindle motor: 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 (11) is loaded) of a rotor yoke (6) forming a rotating member; and, the front-end portion having been passed through the central portion of the hub plate (7a) of the rotor yoke (6) forms a head portion (4c) which is formed into a semispherical shape having been obliquely cut.

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 is firmly fitted in a center of a rotor yoke from which it projects and the spindle is adapted to rotate together with the rotor yoke. The problem of reducing 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. A prior art spindle motor for driving the disk of a magnetic recording unit is shown in Figure 6 and 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. 7, 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 by stabilizing the motor in operation. This 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. A prior art outer-rotor spindle motor including a pair of unshielded bearings each of which includes a ring of ball bearings is described in JP-A-63255869. A further example is described in JP-A-02162566. In order to attempt to solve the above problem, there has been proposed the construction shown in Figure 6, 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 Figure 8, 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. An outer-rotor spindle motor with a bearing such as that shown in Figure 8 is described in JP-A-02071461. In any of these proposed constructions, since the metal ring 16 of a sliding type producing a relatively large frictional resistance in comparison with a ball bearing, the motor using such a metal ring tends to: increase its electric consumption; reduce its service life due to wearing of the ring metal; and be inferior in reliability compared with a motor using a ball bearing. These are disadvantages inherent in the conventional motor using a sliding metal ring bearing. A further prior art outer-rotor type spindle motor is described in JP-A-01076454 in which a coil spring is arranged to bias an inner bearing ring of an upper ball bearing towards that of a lower ball bearing. 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. 7 ) as possible. However, a 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 the thickness of which is remarkably reduced. It is a further object of the present invention to provide an outer-rotor type spindle motor in which each ball bearing has inner and outer rings having a small width and has balls with small diameters, which enables the motor to be of a very thin type and be provided with : (1) a sufficient rated load; and, (2) a small frictional resistance, which prevents a large loss of torque and reduces the electric consumption. From one aspect, the present invention consists in an outer-rotor type spindle motor comprising: a rotor yoke which has its central portion fitted to a spindle and has a permanent magnet mounted on an outer peripheral flange portion; a ball bearing which has balls interposed between an inner ring and an outer ring, each of the balls having a diameter of "D", the inner ring being fitted to the spindle, the ball bearing having a thickness of "H" defined by the equation 0.8 H ≦ D; and a stator which is firmly fitted to the outer ring of the ball bearing directly or through a stator holder mounted on an outer peripheral surface of the outer ring, the stator being provided with a pole plate on which a solenoid is mounted. From a further aspect, the present invention consists in an outer-rotor type spindle motor comprising: a rotor yoke which has its central portion fitted to a spindle and has a permanent magnet mounted on an outer peripheral flange portion; a pair of ball bearings used in series and each of which bearings has balls interposed between an inner ring and an outer ring, each of the balls having a diameter of "D", the inner rings of the pair of ball bearings being fitted to the spindle in a condition in which the inner rings are urged toward each other, each said ball bearing having a thickness of "H" defined by the equation 0.8 H ≦ D, the outer rings of the pair of ball bearings being firmly fitted to a corresponding inner surface of a stator holder so as to be axially spaced apart from each other, whereby a gap is formed between the outer rings; and a stator which is firmly fitted to the outer rings of the ball bearings directly or through the stator holder being mounted on an outer peripheral surface of the outer rings, the stator being provided with a pole plate on which a solenoid is mounted. In the accompanying drawings: Fig. 1 is a longitudinal sectional view of an embodiment of the motor of the present invention; Fig. 2 is a longitudinal sectional view of a ball bearing used in the motor of the present invention shown in Fig. 1 ; Fig. 3 is a longitudinal sectional view of an essential part of a further embodiment of the motor of the present invention; Fig. 4 is a partially enlarged longitudinal sectional view of ball bearings used in the motor of the present invention shown in Fig. 3; Fig. 5 (a) is a plan view of the shielding plate; Fig. 5 (b) is a longitudinal sectional view of the shielding plate shown in Fig. 5 (a); Fig. 6 is a longitudinal sectional view of a left half of an example of a conventional motor; Fig. 7 is a longitudinal sectional view of the ball bearing used in the conventional motor shown in Fig. 6; and Fig. 8 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. Fig. 1 shows an embodiment of the motor of the present invention, in which a single ball bearing is used. The same parts as those of the conventional motor which have been described are denoted by the same reference numerals as those used in the above description of the conventional motor and are not described here so as to avoid redundance in description. As is clear from Fig. 2, the embodiment of the present invention shown in Fig. 1 is characterized by the following equation: 0.8H ≦ D where: H is a thickness of a ball bearing 21; and D is a diameter of each ball 22 of the ball bearing 21. Fig. 3 shows a further embodiment of the present invention, in which a pair of ball bearings 23, 24 (each of which is characterized by the above equation) are mounted on the spindle in series, and have their inner rings 23a, 24a pressed towards each other. Both the outer rings 23b, 24b are fixed to the stator holder 2 so as to be spaced apart from each other to produce a gap 25 between corresponding axial inner-end surfaces thereof. The inner rings 23a, 24a are sandwiched between a lower-end surface of a central boss portion 26 of a rotor yoke 6 and a spring 27 so as to have their axial inner-end surfaces abut on each other in a pressed condition. As for the ball bearings 23 and 24, the inner rings 23a and 24a are axially misaligned with their corresponding outer rings 23b and 24b by an amount β. As a result, the outer rings 23b and 24b are axially spaced apart from each other by a gap of 2β. In the embodiment shown in Fig. 3, the spring 27 urging the inner ring 24a upward is constructed of a plurality of raised segments formed in the vicinity of a central portion of the shielding plate 28 which prevents dust from entering the interior of the motor and is fixed to the spindle 4 by means of a screw 29. Incidentally, in Fig. 2, the reference numeral 30 denotes a retainer. Since the motor of the embodiment of the present invention shown in Fig. 3 uses ball bearings 23, 24 (each of which is defined by the above equation) for supporting the spindle 4, the motor of the present invention can be further reduced in thickness compared with a motor using the conventional ball bearing defined by an equation of H>D, which enables the motor of the present invention to be of a thin type. Further, since the pair of the ball bearings described above are used in the motor of the present invention, it is possible for the motor of the present invention to have reduced vibration in operation, to be excellent in safety and to be reliable. This is possible without reducing its ability to withstand temperature variations as a result of providing the axial gap between the outer rings of the ball bearings and applying a pressure to the inner rings thereof. The motor of the present invention can also be formed into a very thin type of motor.

Claims: An outer-rotor type spindle motor comprising: a rotor yoke (6) which has its central portion fitted to a spindle (4) and has a permanent magnet (5) mounted on an outer peripheral flange portion (6a); a ball bearing (21) which has balls (22) interposed between an inner ring (21a) and an outer ring (21b), each of said balls having a diameter of"D", said inner ring being fitted to said spindle, and said ball bearing having a thickness of "H" defined by the equation 0.8H ≦ D; and a stator (3) which is firmly fitted to said outer ring of said ball bearing directly or through a stator holder (2) mounted on an outer peripheral surface of said outer ring, said stator being provided with a pole plate (3a) on which a solenoid (3b) is mounted. An outer-rotor type spindle motor comprising: a rotor yoke (6) which has its central portion fitted to s spindle (4) and has a permanent magnet (5) mounted on an outer peripheral flange portion (6a); a pair of ball bearings (23, 24) used in series and each of which bearings has balls (22) interposed between an inner ring (23a, 24a) and an outer ring (23b, 24b), each of said balls having a diameter of "D", said inner rings of said pair of ball bearings being fitted to said spindle in a condition in which said inner rings are urged toward each other, each said ball bearing having a thickness of "H" defined by the equation 0.8H ≦ D, said outer rings of said pair of ball bearings being firmly fitted to a corresponding inner surface of a stator holder (2) so as to be axially spaced apart form each other, whereby a gap (25) if formed between said outer rings; and a stator (3) which is firmly fitted to said outer rings of said ball bearings directly or through the stator holder (2) being mounted on an outer peripheral surface of said outer rings, said stator being provided with a pole plate (3a) on which a solenoid (3b) is mounted. The outer-rotor type spindle motor as set forth in claim 1 or 2, wherein: a central portion of a shielding plate (28) is firmly fitted to said spindle (4) to prevent dust from entering the interior of said motor. The outer-rotor type spindle motor as set forth in claim 2, wherein: a central portion of shielding plate (28) is firmly fitted to said spindle (4) to prevent dust from entering the interior of said motor, said central portion of said shielding plate being provided with a spring portion (27) applying a pressure to an end surface of one said inner ring (24a) of said ball bearings thereby urging said inner rings (23a, 24a) towards each other.

Inventors: FUJITANI SAKAE, INAGAKI MASAAKI, SUZUKI YUZURU, UCHIDA HITOSHI, FUJITANI, SAKAE, INAGAKI, MASAAKI, SUZUKI, YUZURU, UCHIDA, HITOSHI
Applicants: MINEBEA KK, MINEBEA KABUSHIKI-KAISHA
Application year: 1992
Main classification: G11B19
IPC codes: H02K7, H02K5, H02K21, G11B17, G11B19, F16C19