GB2303498A - Bearing bias in a spindle motor - Google Patents

Description

SPINDLE MOTOR The invention relates to a spindle motor which rotates at high speed a magnetic disk in a personal computer, a word processor, or the like, a rotary drum of a video tape recorder (VTR), and particularly to a spindle motor in which, in a starting or stopping process of the motor, it is possible to prevent a flaw or a contact scar from being formed on a dynamic pressure bearing surface.

In an office automation (OA) business machine such as a personal computer or a word processor, or an audio visual (AV) machine such as a video tape recorder, a rotation driving apparatus for rotating at high speed a magnetic disk or a rotary drum is incorporated. In such a rotation driving apparatus, used is a spindle motor having a dynamic pressure bearing structure which utilizes a lubricant fluid in order to ensure the accuracy during rotation at high speed and prevent noises and vibrations from occurring.

Fig. 7 is a longitudinal section view showing the structure of a prior art spindle motor. A stator 76 is disposed on the outer periphery of a cylindrical portion 74 which is disposed on a base 70a of a stationary member 70 so as to elongate in a direction upstanding from the base 70a (hereinafter, referred to as axial direction). A sleeve 73 is fixed to the inner periphery of the cylindrical portion 74. A shaft 72 fixed to the center portion of a rotary member 71 is fitted into the sleeve 73 with forming a small clearance therebetween. A cylindrical portion 71a which elongates toward the base 70a of the stationary member 70 and in parallel with the cylindrical portion 74 is formed in the outer periphery of the rotary member 71. A rotor 75 configured by magnets is disposed on the inner periphery of the cylindrical portion 71a and at a position opposing the stator 76.As shown in the figure, on the outer periphery of the shaft 72 or the inner periphery of the sleeve 73, a plurality of dynamic pressure grooves 72a and 72b which elongate in the circumferential direction and have a herringbone-like shape or a spiral shape are formed in two portions separated from each other in the axial direction.

In other words, the shaft 72 and the sleeve 73 form a dynamic pressure bearing. The lower face of the shaft 72 is supported by means of a pivot support formed by the shaft and a plate 77 fittingly attached to the cylindrical portion 74 of the stationary member 70, or by forming a thrust bearing.

As shown in Fig. 7, in the spindle motor wherein the sleeve 73 is fittingly fixed to the cylindrical portion 74 of the stationary member 70 on which the stator 76 is disposed and the shaft 72 fixed to the rotary member 71 is fitted into the sleeve so as to attain the rotating operation, the shaft 72 of the rotary member 71 is held to the stationary member 70 by a magnetic force exerted between the rotor 75 and the stator 76. However, the holding due to the magnetic force is not sufficient for preventing the shaft 72 from slipping off from the sleeve 73 in the axial direction.

In order to solve the problem of axial movement the spindle motor shown in Fig. 4 has been developed. The spindle motor is the subject of our co-pending U.K. application No. G ff SC i 49 and has the following configuration.

A shaft 42 is fixed to a base 40a of a stationary member 40. A stator 48 is disposed on the outer periphery of a cylindrical portion 46 which is disposed so as to elongate in a direction upstanding from the base 40a (hereinafter, referred to as axial direction) with forming a constant gap between the inner periphery of the cylindrical portion and the shaft 42. The shaft 42 is fitted into a sleeve 44 which is fittingly attached to the center portion of a rotary member 41, with forming a small clearance between the shaft and the sleeve. A rotor 47 configured by magnets is disposed on the inner face of an outer cylindrical portion 41a of the rotary member 41 and at a position opposing the stator 48.On the surface of the shaft 42, a plurality of dynamic pressure grooves 42a and 42b which are used for generating a dynamic pressure, elongate in the circumferential direction, and have a herringbone-like shape or a spiral shape are formed in two portions separated from each other in the axial direction.

A thrust plate 50 is fixed to the shaft 42. The thrust plate 50 is placed in an annular recess 51 which is defined by a step portion 44a disposed on the inner side of the sleeve 44 of the rotary member 41, and the lower face 49a of a ring member 49 fitted into the inner upper portion of the sleeve 44. The upper and lower faces 50b and 50c of the thrust plate 50 extend so as to be perpendicular to the axis of the shaft 42, and in parallel with the surface of the step portion 44a of the sleeve 44 and the lower face 49a of the ring member 49. In the motor, as shown in Fig. 5, dynamic pressure grooves 50a which are used for generating a dynamic pressure and have a herringbone-like shape or a spiral shape are formed in the upper and lower faces of the thrust plate 50.

In the thus-configured spindle motor, not only a horizontal dynamic pressure support due to the shaft 42 and the sleeve 44, but also a vertical dynamic pressure support due to the thrust plate 50 are exerted, and the vertical movement is limited. Therefore, the spindle motor can constitute a rotation driving apparatus which generates reduced noises and vibrations and has a high accuracy.

In the spindle motor shown in Fig. 4 where the thrust plate 50 is vertically sandwiched so as to configure a dynamic pressure bearing, the structure in which, when the spindle motor is stopped, only the weight of the rotary member is applied to one of the thrust faces has been considered acceptable. In the spindle motor, the stator 48 is coincident in the axial center line X-X with the rotor 47, and hence the horizontal magnetic forces are in equilibrium with each other. In the case where the rotary member 41 is light, when the rotational speed is low, for example, in a starting or stopping process of the motor, swings and vibrations of the rotary member 41 increase a moment load due to the magnetic forces so that, as shown in Fig. 6, the shaft 42 and the sleeve 44 are relatively inclined.It has been found that such a relative inclination causes the upper and lower end faces of the thrust plate 50 attached to the shaft 42 to eccentrically abut against the surface of the step portion 44a formed in the sleeve 44 and the lower face 49a of the ring member 49, with the result that a flaw or a contact scar is formed in the outer peripheries of the thrust faces of these components.

The invention has been conducted in view of the abovediscussed problems. It is an object of the invention to provide a spindle motor in which, even at a low speed, a thrust plate is prevented from eccentrically abutting against a thrust face of a rotary member so that the motor can be smoothly started or stopped.

In order to solve the above-discussed problems, it is an object of the invention to provide a spindle motor in which a stator is disposed on a stationary member with a constant radius, a rotor configured by magnets is disposed on a rotary member and at a position opposing the stator, a shaft is fixed to one of the stationary and rotary members, a sleeve is fixed to the other one of the stationary and rotary members, the shaft is fitted into the sleeve, and a dynamic pressure bearing is formed between the shaft and the sleeve, wherein a thrust plate is fixed to the shaft, the thrust plate is placed in an annular recess space formed in an inner face of the sleeve, a thrust dynamic pressure bearing is formed between each of upper and lower faces of the thrust plate and the annular recess, and an axial center line of the stator disposed on the stationary member is shifted from an axial center line of the rotor disposed on the rotary member in a vertical direction of the shaft.

Furthermore, it is another object of the invention to provide a spindle motor wherein the thrust dynamic pressure bearings are formed by forming dynamic pressure grooves in the upper and lower faces of the thrust plate. It is a further object of the invention to provide a spindle motor wherein the thrust dynamic pressure bearings are formed by forming dynamic pressure grooves in upper and lower faces of the annular recess of the sleeve.

When a spindle motor is configured as described above, the center line of the stator is shifted from that of the rotor and hence vertical components of magnetic forces always act on the rotary member. In other words, even in the case where the rotary member is light, the rotary member is acted on by magnetic forces in a direction along which the thrust plate uniformly makes in contact with a step portion of the sleeve or a ring member constituting a part of the sleeve. Even in a state such as a starting or stopping process where the rotary member is rotated at a low speed, therefore, the thrust plate as a whole is not in a surface contact state but in a floating state as shown in Fig. 2. When the motor is stopped, the thrust plate is stopped in a surface contact state.Therefore, the upper and lower end faces of the thrust plate are prevented from eccentrically abutting against the surface of the step portion of the sleeve or the lower face of the ring member constituting a part of the sleeve, and hence a flaw, a contact scar. or the like is not formed.

The invention will be further described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a longitudinal section view showing the structure of a spindle motor forming an embodiment of the invention; Fig. 2 is a partial longitudinal section view showing the structure of the spindle motor of Fig. 1 showing a state where a thrust plate floats from a thrust face in a starting process; Fig. 3 is a longitudinal section view showing the structure of a modification of the spindle motor of Fig. 1; Fig. 4 is a longitudinal section view showing the structure of a spindle motor having a thrust dynamic bearing of the type described in our co-pending application No. q C 9 6 1 4 S49 4; Fig. 5 is a plan view of a thrust plate of the motor of Fig. 4;; Fig. 6 is a partial longitudinal section view of the spindle motor of Fig 4 and showing a state where a thrust plate eccentrically abuts against a thrust face in a starting or stopping process; and Fig. 7 is a longitudinal section view showing the structure of a prior art spindle motor.

Hereinafter, modes of embodying the invention will be described with reference to the accompanying drawings.

AS shown in Fig. 1, a shaft 3 is fixed to a base la of a stationary member 1 in a direction upstanding from the base la (hereinafter, referred to as axial direction). A plurality of dynamic pressure grooves 3a and 3b which extend in the circumferential direction and have a herringbone-like shape or a spiral shape are formed in two portions of the shaft 3 separated from each other in the axial direction. A stator 7 is disposed on the outer face of a cylindrical portion 5 which elongates in the axial' direct ion and is placed at a position radially separated from the shaft 3 of the stationary member 1 by a constant distance. The shaft 3 is fitted into a sleeve 4 which is fittingly attached to the center portion of a rotary member 2, with forming a small clearance between the shaft and the sleeve.The sleeve 4 is fitted into the cylindrical portion 5 with forming a constant gap between the outer face of the sleeve 4 and the cylindrical portion. The dynamic pressure grooves 3a and 3b may be formed in the inner face of the sleeve 4, instead of on the shaft 3.

On the inner face of an outer cylindrical portion 2a of the rotary member 2, a rotor 6 configured by magnets is disposed at a position opposing the stator 7 which is disposed on the outer periphery of the cylindrical portion 5. In the embodiment, one of the axial center line 7a of the stator 7 and the axial center line 6a of the rotor 6 is shifted upward in the axial direction so as to form a gap d (any one of the axial center lines may be positioned in the upper side). In the case of Fig. 1, the axial center line 6a of the rotor 6 is positioned higher than the axial center line 7a of the stator 7.

A thrust plate 9 is fixed to the shaft 3 which is fixed to the center portion of the stationary member 1. The thrust plate 9 is placed so as to face an annular recess 10 which is defined by a step portion 4a formed in the upper portion of the sleeve 4 fixed to the center portion of the rotary member 2, and the lower face 8a of a ring member 8 fixed to the upper end of the sleeve 4. In the embodiment, as shown in Fig. 5, dynamic pressure grooves 9a which are used for generating a dynamic pressure and have a herringbone-like shape or a spiral shape are formed on the upper and lower faces 9b and 9c of the thrust plate 9. The upper and lower faces 9b and 9c of the thrust plate 9 extend so as to be perpendicular to the axis of the shaft 3, and in parallel with the surface of the step portion 4a of the sleeve 4 and the lower face 8a of the ring member 8.

Dynamic pressure grooves may be formed in the surface of the step portion 4a of the sleeve 4 and the lower face 8a of the ring member 8 which oppose the thrust plate 9.

In the spindle motor, the center of gravity of the rotary member 2 is positioned in the upper portion of the shaft 3, and hence the degree of deflection caused by the moment load can be reduced by disposing a thrust dynamic pressure bearing at or in the vicinity of the center of gravity.

In the thus configured spindle motor, since the axial center line 7a of the stator 7 and the axial center line 6a of the rotor 6 are shifted from each other by the gap d, vertical magnetic forces always act on the rotary member 2.

In other words, even in the case where the rotary member 2 is light, the rotary member 2 is acted on by magnetic forces in a vertical direction so that the thrust plate 9 uniformly makes contact with the step portion 4a functioning as a thrust face of the sleeve 4 or the lower face 8a of the ring member 8 constituting a part of the sleeve 4. Even in a state such as a starting or stopping process where the rotary member 2 is rotated at a low speed, therefore, the thrust plate 9 as a whole is not in a surface contact state but in a floating state as shown in Fig. 2.

When the motor is stopped, the thrust plate is stopped in a surface contact state. Therefore, the upper and lower end faces of the thrust plate 9 are prevented from eccentrically abutting against the surface of the step portion 4a of the sleeve 4 or the lower face 8a of the ring member 8, and hence a flaw, a contact scar, or the like is not formed.

Fig. 3 shows a modified embodiment of the spindle motor of the invention. In the embodiment, a sleeve 4 is fixed to a stationary member 1 by means of press fitting or the like.

A stator 7 is disposed on the outer periphery of the sleeve 4. A shaft 3 on which dynamic pressure grooves 3a and 3b are formed is fixed to a rotary member 2 and fitted into the sleeve 4. A rotor 6 configured by magnets is disposed on the inner face of an outer cylindrical portion 2a of the rotary member 2 and at a position opposing the stator 7.

Also in the embodiment, one of the axial center line 7a of the stator 7 and the axial center line 6a of the rotor 6 is vertically shifted so as to form a gap d.

A thrust plate 9 which is fixed to the shaft 3 and on which dynamic pressure grooves 9a are formed is disposed in an annular recess 10 which is defined by a step portion 4a of the sleeve 4 disposed on the stationary member 1, and the lower face 8a of a ring member 8 pressingly inserted into the upper end of the sleeve 4 (the step portion 4a and the lower face 8a will function as thrust faces).

Also in the modified embodiment, vertical magnetic force components always act on the rotary member 2 including the thrust plate 9 and the shaft 3. In other words, even in the case where the rotary member 2 is light, the rotary member 2 is acted on by magnetic forces in a direction along which the thrust plate 9 uniformly makes in contact with the step portion 4a of the sleeve 4 or the lower face 8a of the ring member 8.

As described above in detail, according to the spindle motor of the invention, the rotary member is acted on not only by horizontal magnetic forces but also by magnetic forces along a vertical direction of the shaft. In a starting process, therefore, the thrust plate disposed on the shaft is always in a floating state transferred from a surface contact state. When the motor is stopped, the thrust plate is stopped in a surface contact state.

Therefore, the upper and lower edges of the end portion of the thrust plate do not make a flaw or a contact scar on the thrust faces of the sleeve and the ring.

Claims (5)

CLAIMS:

1. A spindle motor in which a stator is disposed on a stationary member with a constant radius, a rotor configured by magnets is disposed on a rotary member and at a position opposing said stator, a shaft is fixed to one of said stationary and rotary members, a sleeve is fixed to the other one of said stationary and rotary members, said shaft is fitted into said sleeve, and a dynamic pressure bearing is formed between said shaft and said sleeve, wherein a thrust plate is fixed to said shaft, said thrust plate is placed in an annular recess space formed in an inner face of said sleeve, a thrust dynamic pressure bearing is formed between each of upper and lower faces of said thrust plate and said annular recess, and an axial center line of said stator disposed on said stationary member is shifted from an axial center line of said rotor disposed on said rotary member in a vertical direction of said shaft.

2. A spindle motor according to claim 1, wherein said thrust dynamic pressure bearings are formed by forming dynamic pressure grooves in said upper and lower faces of said thrust plate.

3. A spindle motor according to claim 1, wherein said thrust dynamic pressure bearings are formed by forming dynamic pressure grooves in upper and lower faces of said annular recess of said sleeve.

4. A spindle motor comprising a stator and a rotor having an axial direction, a dynamic pressure bearing being provided between the stator and rotor and acting radially of the axial direction, and a thrust dynamic pressure bearing being provided between the stator and rotor to licit movement in the axial direction, wherein magnetic planes of the stator and rotor are offset in the axial direction to bias the stator in an axial direction relative to the rotor.

5. A spindle motor substantially as hereinbefore described with reference to Figs. 1 and 2 or Fig. 3 of the accompanying drawings.