GB2266009A

GB2266009A - Air bearing in brushless electric motor.

Info

Publication number: GB2266009A
Application number: GB9207735A
Authority: GB
Inventors: Ronald William Woolley
Original assignee: FLUID FILM DEVICES Ltd
Current assignee: FLUID FILM DEVICES Ltd
Priority date: 1992-04-08
Filing date: 1992-04-08
Publication date: 1993-10-13
Anticipated expiration: 2012-04-08
Also published as: GB9207735D0; GB2266009B

Abstract

A data storage disc drive unit 1 comprises a brushless d.c. electric motor 2 having a central stator structure 3 surrounded, in a concentric manner, by a co-operating rotor structure 4. The stator 3 and surrounding rotor 4 together define a small annular gap 5, which is used to establish a plain, sell-acting, air bearing film between the stator 2 and rotor 4. The drive unit 1 is also provided with rotor thrust bearing means 6 operable to restrain axial movement of the rotor 4 relative to the stator 2 and along the rotational axis 7 of the rotor. The magnets 29 and 18 are aligned with each other and are of opposite polarity, whereby they mutually attract to each other. This attraction urges the rotor 4 in the direction of arrow 30, pre-loading the ball bearing 20 and restraining axial movement of the stator 4 in a direction opposite to that indicated by arrow 30, so as to hold the rotor 4 in place, regardless of the attitude of the drive unit 1. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO ELECTRIC MOTORS This invention relates to electric motors and is primarily, but not exclusively, concerned with direct current brushless electric motors.

The invention is more particularly concerned with data storage disc drive units which employ electric motors.

According to one aspect of the present invention, an electric motor comprises co-operating stationary and rotatable structures, disposed concentrically, the two structures together defining a fluid bearing gap.

The fluid bearing may comprise a self-acting or hydrodynamic fluid bearing.

Alternatively, the fluid bearing may comprise a pressurised or hydrostatic fluid bearing.

The bearing fluid preferably comprises air.

The two structures of the electric motor may comprise a stator and a rotor.

As the above-defined aspects of the invention apply to electrical generators as well as to electric motors, as used herein the term "motor" includes "generator".

According to another aspect of the present invention, a data storage disc unit comprises an electric motor having co-operating stationary and rotatable structures disposed concentrically and means for establishing a fluid bearing film between the two structures and means operable to restrain axial movement of the structures relative to each other.

The fluid bearing preferably comprises an air bearing, which may be a self-acting or hydrodynamic air bearing.

The last-mentioned means preferably make use of magnetic forces to restrain axial movement of the rotor.

In addition, use may also be made of a ball bearing.

The various aspects of the invention will now be described by way of example only, with reference to the accompanying drawings, wherein: Figures 1 to 8 are fragmentary side views, in medial section, of various forms of data storage disc drive units.

In the figures, like reference numerals refer to like components and features.

With reference to Figure 1, a data storage disc drive unit 1 comprises a brushless direct current electric motor 2 having a central stator structure 3 surrounded, in a concentric manner, by a co-operating rotor structure 4. The stator 3 and surrounding rotor 4 together define a small annular gap 5, which is used to establish a plain, i.e. non-grooved, air bearing film between the stator 2 and rotor 4. The drive unit 1 is also provided with rotor thrust bearing means 6 operable to restrain axial movement of the rotor 4 relative to the stator 2 and along the rotational axis 7 of the rotor.

The stator 3 comprises an upright pillar 15 which forms part of the casing 16 of the unit 1. The pillar 15 carries the annular windings 17 of the motor 2 which are disposed around the pillar. The upper end of the pillar 15 carries a permanent magnet 18 of annular form, disposed around a central, frusto-conical, recess 19, which forms a bearing seat for a ball bearing 20.

The rotor 4 comprises a cap-shaped structure 25, the inner surface of which carries a ring of permanent magnets 26, distributed over said surface. The capshaped structure 25 is formed with an external flange 27 upon which the data storage discs (not shown) are stacked, to be retained in place by a releasable locking component, (also not shown), in a conventional manner.

The upper interior surface of the structure 25 carries a central disc 28 of carbide material which is in contact with the ball bearing 20 and serves as a thrust bearing pad. The upper interior surface of the structure 25 also carries a permanent magnet 29 of annular form disposed co-axially relative to the disc 28 and axis 7.

The magnets 29 and 18 are aligned with each other and are of opposite polarity, whereby they mutually attract to each other. This mutual, axially-acting attraction urges the rotor 4 in the direction of arrow 30, pre-loading the ball bearing 20 and restraining axial movement of the stator 4 in a direction opposite to that indicated by arrow 30.

This restraint holds the rotor 4 in place, regardless of the attitude of the drive unit 1. Thus, should the unit 1 be turned upside down, for example, the rotor 4 will not fall away from the stator 3.

The annular, air bearing gap 5 has a width typically of 0.0025mm or less.

In operation of the drive unit 1, a hydrodynamic or self-acting air bearing film is established in the annular gap 5 by virtue of relative motion between the stator 3 and rotor 4, whereby air present in the gap becomes pressurised.

Figure 2 illustrates a modified disc drive unit 101 wherein the cap-shaped structure 125 thereof is mounted on the upper, flanged end 131, of a central shaft 132, located by a bearing bush 133. The shaft 132 and bush 133 together define an annular gap 105 wherein a self-acting air bearing film is established.

The presence of the bearing bush 133, shaft 132 and air bearing film therebetween tend to stiffen the stator 103 against side-acting loads W.

In this modification, the ball bearing 120 is located at a substantially lower position than corresponding ball bearing 20 of Figure 1. A pad corresponding to pad 28 can be provided if deemed desirable. The magnet 129 is mounted on the lower end of the shaft 132, and the magnet 118 on a circular block 134 which defines the bearing seat 119.

In the modification illustrated by Figure 3, the disc drive unit 201 employs a central shaft 232 upon which is mounted a close-fitting magnet 235 of tubular form. The magnet 235 co-operates with another magnet of tubular form, but of opposite polarity, namely coaxially disposed magnet 236. The magnets 235, 236 together comprise radially-acting rotor thrust bearing means 206. A self-acting air bearing gap 205 is defined by the stator 203 and surrounding rotor 204.

In the modification illustrated by Figure 4, three, axially acting permanent magnets, namely magnets 329, 318 and 337 are provided, all of annular form. The upper and lower magnets 329, 327 are of the same polarity; whereas the central magnet 318 is of opposite polarity, so that there is mutual attraction. The polarities could be reversed if deemed desirable.

Stationary magnets 329 and 327 are mounted on the inner surface of bearing bush 333; movable magnet 318 is mounted on the lower end of shaft 332.

Figure 5 illustrates a modification using radially-acting co-operating magnets 418 and 429 of annular form, disposed in co-axial relationship.

Rotatable magnet 429 is mounted on the lower end of the shaft 432. The stationary magnet 418 is mounted on the inner surface of bearing bush 433. A self-acting air bearing gap 405 is defined by the shaft 432 and the bush 433.

Figure 6 illustrates a modification which employs another axially-acting permanent magnet arrangement. In this case using annular magnets 529, 518, 527. A selfacting air bearing gap 505 is defined between the central shaft 532 and the surrounding bush 533.

Figure 7 illustrates a modified disc drive unit 601 which is similar to unit 1 of Figure 1 and has a self-acting air bearing gap 605. The figure illustrates a form of air bearing surface.

In Figure 7, the outer surface of the stator 603 is formed with upper and lower, axially-spaced, selfacting air bearing surfaces 640, 641 of herringbone groe form. The predetermined herringbone groove pattern may be formed by etching or grinding. The groove depth may vary according to operational requirements, but is typically of the order 0.0127mm or less.

The oppositely disposed, inward face of the rotor 602 presents a smooth, non-grooved surface.

In the modified disc drive unit 702 of Figure 8, the central shaft 732 and stationary bearing bush 733 thereof together define an annular gap 745 within which an air bearing film is formed. The air bearing is of the self-acting type and is provided by forming upper and lower, axially-spaced air bearing surfaces 746, 747 of herringbone form. The inner face of the bearing bush 733 presents a smooth co-operating bearing surface.

Air bearing patterns other than those of herringbone form may be used, if deemed desirable.

In Figures 7 and 8, thrust bearing means have not been shown for the sake of clarity.

Inter alia, the present invention provides an inexpensive data storage disc drive unit, particularly applicable to 1.8 inch (45.72mm) discs.

The invention provides the drive unit with journal (e.g. 105), and thrust (e.g. 106), bearings fulfilling different roles.

One of the limitations to increasing the storage of concentric lines of data in disc drives is concerned with random variations in the spin axis of the drive shaft supporting the disc, mainly caused by minor inaccuracies in the ball bearings supporting the shaft.

One of the beneificial features of a fluid film bearing of the type described herein is that Non Repeatable Run Out (NRRO) is almost totally eliminated, due to the averaging effect of the fluid pressure acting on the oppositely-facing bearing surfaces.

An instability known as "half speed whirl" can occur in fluid film bearings. Known methods of suppressing "half speed whirl" include the use of herringbone grooves, lobes or pockets formed in the bore or shaft as the situation demands. However a very simple technique for suppressing half speed whirl which obviates the need for forming grooves etc, is to allow a small level of unbalance to be present. A small level of rotor unbalance, which can occur as a result of small manufacturing errors, will suppress half speed whirl should plain air bearings having exceptionally fine radial clearances be used, as is the case in the present application.

The thrust bearing provides load carrying capacity at a low noise level, without the precision required of the journal bearing.

Where possible, any of the features of the arrangements described and/or illustrated, may be substituted for, or may be added to, other features.

Thus, for example, a disc drive unit could be provided with the air bearing arrangements of Figures 7 and 8.

Furthermore, the ball bearing 20 or equivalent surface to surface bearing means may be dispensed with, if deemed desirable.

Claims

1. An electric motor (as defined herein), comprising co-operating stationary and rotatable structures, disposed concentrically, the two structures together defining a fluid bearing gap.

2. An electric motor as claimed in claim 1, wherein the fluid bearing comprises a self-acting fluid bearing.

3. An electric motor as claimed in claim 1, wherein the fluid bearing comprises a pressurised fluid bearing.

4. An electric motor as claimed in claim 1 or 2, wherein the bearing fluid comprises air.

5. A data storage disc drive unit comprising an electric motor as claimed in any one of claims 1 to 4, provided with means operable to restrain axial movement of the two structures relative to each other.

6. A disc drive unit as claimed in claim 5, wherein the last-mentioned means make use of magnetic forces to restrain axial movement of the structures relative to each other.

7. A disc drive unit as claimed in claim 6, provided with permanent magnets of opposite polarity, disposed opposite to each other.

8. A disc drive unit as claimed in claim 7, provided with a pair of permanent magnets of annular form, the magnets being disposed co-axially, on the axis of rotation of the rotatable structure.

9. A disc drive unit as claimed in claim 7, provided with a pair of permanent magnets of tubular form, disposed co-axially, one within the other, on the axis of rotation of the rotatable structure.

10. A disc drive unit as claimed in claim 7, provided with three permanent magnets of annular form disposed co-axially on the axis of rotation of the rotatable structure, the central magnet being of opposite polarity to the outer magnets.

11. A disc drive unit as claimed in any one of claims 5 to 10, wherein said axial restraint means also comprise ball bearing means.

12. A disc drive unit as claimed in any of claims 5 to 11, provided with bush means for stiffening the stationary structure against side-acting loads.

13. A disc drive unit as claimed in any one of claims 5 to 12, wherein the fluid bearing employs a predetermined groove pattern.

14. An electric motor, substantially as hereinbefore described, with reference to any one of Figures 1 to 8 of the accompanying drawings.

15. A data storage disc drive unit, substantially as hereinbefore described, with reference to any one of Figures 1 to 8 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. An electric motor (as defined herein), comprising co-operating stationary and rotatable structures, disposed concentrically, the two structures together defining a fluid bearing gap.

2. s electric motor as claimed in Claim 1, wherein the fluid bearing comprises a plain fluid bearing and the bearing gap is not more than 0.0025 mm in width.

3. An electric motor as claimed in Claim 2, wherein the rotatable structure is an unbalanced rotatable structure.

4. An electric motor as claimed in Claim 1, 2 or 3, wherein the fluid bearing comprises a self-acting fluid bearing.

5. An electric motor as claimed in Claim 1, 2 or 3, wherein the fluid bearing comprises a pressurised fluid bearing.

6. An electric motor as claimed in any one of Claims 1 to 5, wherein the bearing fluid comprises air.

7. A data storage disc drive unit comprising an electric motor as claimed in any one of Claims 1 to 6, provided with means operable to restrain axial movement of the two structures relative to each other.

8. A disc drive unit as claimed in Claim 7, wherein the last-mentioned means make use of magnetic forces to restrain axial movement of the structures relative to each other.

9. A disc drive unit as claimed in Claim 8, provided with permanent magnets of opposite polarity, disposed opposite to each other.

10. A disc drive unit as claimed in Claim 9, provided with a pair of permanent magnets of annular form, the magnets being disposed co-axially, on the axis of rotate / the rotatable structure.

11. A disc drive unit as claimed in Claim 9, provided with a pair of permanent magnets of tubular form, disposed co-axially, one within the other, on the axis of rotation of the rotatable structure.

12. A disc drive unit as claimed in Claim 9, provided with three permanent magnets of annular form disposed co-axially on the axis of rotation of the rotatable structure, the central magnet being of opposite polarity to the outer magnets.

13. A disc drive unit as claimed in any one of Claims 7 to 12, wherein said axial restraint means also comprise ball bearing means.

14. A disc drive unit as claimed in any of Claims 7 to 13, provided with bush means for stiffening the stationary structure against side-acting loads.

15. A disc drive unit as claimed in any one of Claims 7 to 14, wherein the fluid bearing employs a predetermined groove pattern.

16. An electric motor, substantially as hereinbefore described, with reference to any one of Figures 1 to 8 of the accompanying drawings.

17. A data storage disc drive unit, substantially as hereinbefore described, with reference to any one of Figures 1 to 8 of the accompanying drawings.