Self-contained low power fluid bearing and bearing seal

A bearing using magnetic fluid for lubrication is provided with a magnetic seal circuit to retain the magnetic fluid in the baring. The magnetic seal circuit comprises two annular ring magnets surrounding a shaft and placed on opposite sides of the bearing. The two ring magnets are magnetically joined by a shunt. The shunt controls stray flux and permits the disposition of the magnet at a small distance from the shaft, thus concentrating the flux in the magnetic seal gap.

FIELD OF THE INVENTION 
This invention relates to seals and bearings, and more particularly, to 
sealed bearings of the fluid bearing type. 
BACKGROUND OF THE INVENTION 
As spindles and bearings get smaller, requirements for high precision 
performance increase and the need for the sealing of the annulus around a 
shaft arises, it becomes increasingly difficult to utilize conventional 
design in the field of bearings and seals. 
Accordingly, efforts have been made to use magnetic fluid seals to contain 
or prevent the magnetic fluid from migrating outside the region of the 
bearing and potentially contaminate the exterior region, which may contain 
such devices as magnetic data storage disks. With the use of relatively 
large size shafts, ball bearings and fluid bearings have been preferred to 
reduce friction between the hub and the shaft. With ball bearings, there 
is relatively low friction; however, ball bearings are lubricated with 
oils or greases. The magnetic seal may be a separate element of the 
assembly, merely preventing the lubricants of the ball bearing and the air 
contained within that chamber from passing coaxially to the shaft. 
As shaft diameters are reduced along with the size of the devices within 
which they are contained, the practicality of miniature ball bearings 
becomes an issue. In some cases, the shaft diameters are reduced below two 
millimeters and, accordingly, the normal tolerances for the miniature ball 
bearings become so great in proportion to the component sizes that 
accuracy of positioning of the rotating element is degraded beyond 
acceptable limits 
Efforts have been made to utilize fluid bearings in place of the ball 
bearings with varying degrees of success. The stability of the revolving 
hub requires that the bearings be spaced apart from each other along the 
axis of the shaft to the greatest possible extent The separation of the 
bearings requires that either each individual bearing be designed for 
individual containment, thereby typically requiring an end plugging scheme 
to contain the fluid in the bearing; or alternatively, the use of magnetic 
seals to confine the fluid in the bearing cavity. Thus, in the past, the 
use of magnetic seals for each bearing has embodied two magnetic seals for 
each bearing or, alternatively, the inclusions of two or more bearings 
within a common fluid cavity sealed by two seals, one at each end of the 
fluid cavity. The inclusion of more than one bearing within the single 
cavity dictates that the cavity extend over substantial lengths to 
accommodate the multiple bearings. To fill this cavity with magnetic fluid 
becomes cost significant in view of the exceedingly expensive cost of the 
magnetic fluid used as a lubricant in the bearings and as a sealing fluid 
in the seals. 
Any compromise in the length of the chamber in order to reduce the fluid 
capacity requires that the bearings be displaced closer together, thus 
degrading the stability of the rotating hub surrounding the shaft. 
Degraded hub stability directly correlates to degraded or failed 
operability of the disks attached to the hub. 
Inasmuch as the hub, at least in the preferred embodiment, supports 
magnetic storage data disks which are rotated at high speed and these 
disks are radial flanges mounted on the exterior of the hub, the stability 
of the rotating hub is exceedingly important to prevent the flanges' 
fluctuation relative to the position of the read/write heads associated 
with the disks. Fluctuation of the disk surfaces during rotation can cause 
collisions between the disks and the read/write heads mounted in 
exceedingly close proximity thereto, thereby damaging the disks and/or the 
heads with resulting loss of stored data. 
With the bearings displaced from each other as far as the disk drive 
assembly design permits, the stability of the hub can be maximized and 
undesired displacement of the read/write point on a disk minimized by the 
use of close tolerances and a fluid bearing. 
Another consideration which is key to the operation and reliable recording 
of data on the disks is the control of magnetic flux. The disks are 
magnetic material coated for receiving electromagnetic signals and storing 
those electromagnetic patterns; it is essential that the stray magnetic 
flux not be permitted to influence the magnetic coating of any of the 
magnetic disks carried on the revolving hub. 
With this in mind, it can be clearly seen that the use of large or very 
strong magnets in the magnetic circuit is undesirable inasmuch as these 
strong magnets may propagate stray flux at substantial distances within 
the housing. Inasmuch as the dimension of the shaft diameter is two 
millimeters and typically smaller, it can be seen that the magnet placed 
in a sealing circuit very easily could propagate stray flux lines passing 
a substantial distance away and inadvertently affect the magnetic material 
coating of the magnetic storage disks. 
Several approaches to sealing and capturing magnetic fluid in the region of 
a fluid bearing are known. One approach utilizes a single magnetic seal 
and a physical barrier to contain the magnetic fluid within the bearing. 
Examples wherein a magnetic seal is used to contain magnetic fluid within a 
cavity where the other end of the cavity is a physical barrier to the 
migration or loss of the magnetic fluid are U.S. Pat. Nos. 4,526,484 to 
Stahl et al.; 4,734,606 to Hajec, and 4,938,611 to Nii et al. Stahl et al. 
utilizes a magnetic seal to contain the magnetic fluid within a 
cylindrical opening formed into a block where the cylindrical opening 
terminates within the block and there is no second opening thereto. 
Hajec utilizes a screw-threaded plug member which is inserted into the main 
housing, which acts as a thrust bearing surface. 
The Nii et al. reference shows a closed well formed by the bearing housing 
elements which contains the magnetic fluid in addition to the containment 
of the magnetic seal structure. The magnetic seal structure only encircles 
the shaft at one location along its axis point. 
An effort has been made to seal fluid bearings utilizing magnetic fluid on 
both ends of the bearing by the utilization of a single magnet and two 
pole pieces, as illustrated in FIG. 2 of U.S. Pat. No. 4,598,914. This 
figure is labeled as prior art to the patent in which it appears and its 
origin is unknown. 
FIG. 2 of U.S. Pat. No. 4,598,914 illustrates a seal arrangement forming a 
magnetic circuit around and containing the bearing surfaces of a fluid 
bearing. The arrangement disclosed and described utilizes a single magnet 
and two annularly shaped pole pieces. The single magnet is a hollow 
tubular magnet surrounding the bearing at some substantial distance from 
the bearing surface. The magnet's interior cylindrical surface supports a 
non-magnetic bearing material likewise formed in a hollow cylindrical 
shape surrounding the bearing with the inner surface of the bearing 
material in close proximity to the exterior surface of the bearing on the 
shaft. 
The washer shaped or annular pole pieces act to focus the magnetic flux 
from the ends of the magnet cylinder into close proximity with and into 
the shaft creating a high flux density in the gap between the pole pieces 
and the shaft. In this regard, the magnetic fluid is trapped in the two 
gaps between the pole pieces and the shaft and in the gap between the 
bearing surface on the shaft and the bearing surface of the non-magnetic 
bearing material carried by the magnet. In order to insure that an 
effective flux density will be present between the pole pieces and the 
shaft, a strong and relatively large magnet is required due to its 
displacement from the sealing gaps. 
With such a strong and relatively large magnet, the stray flux, which is 
inherent with a magnet, will tend to branch outside the bounds of the 
magnet and the pole pieces and possibly to adversely effect any magnetic 
disks which may be attached to the housing. 
A second problem is encountered when utilizing the arrangement described 
immediately above because FIG. 2 of U.S. Pat. No. 4,598,914 illustrates a 
housing, a magnet and a non-magnetic bearing material assembled together 
with the magnet sandwiched between the non-magnetic bearing material and 
the housing. In the typical environment requiring fluid bearings, such as 
in small confined areas and those areas requiring extreme precision, the 
parallelism of the interior and exterior cylindrical surfaces of the 
magnet and the non-magnetic bearing material and the interior cylindrical 
surface of the frame are all extremely critical. The introduction of an 
additional member and any variables associated with it, over and above 
that which is absolutely essential, is highly undesirable. The 
undesirability of that arrangement lies in tolerance build-up and 
non-parallelism of any of the surfaces discussed above which will then 
potentially result in bearing failure, due to the fact that the gap 
between the inner and outer bearing surfaces is varied in an axial 
direction thus resulting in inadequate lubrication and hydrostatic 
pressure at one end or the other of the bearing, potentially resulting in 
early bearing failure. 
Other attempts to solve this problem include U.S. Pat. Nos. 4,630,943 and 
4,673,997. Both of these patents illustrate bearings and seals displaced 
from each other axially along a shaft with a complete magnetic seal and 
magnetic circuit at each end of the cavity. Such an arrangement requires 
filling of the bearing cavity over its entire length and the use of 
relatively large quantities of very expensive magnetic fluid. Each 
magnetic seal at each end of the cavity is a complete magnetic circuit in 
and of itself and independent of the other seal at the other end of the 
cavity. 
SUMMARY OF THE INVENTION 
A magnetic seal for a fluid bearing which utilizes magnetic fluid both as 
the sealing media and as the lubricating fluid for the bearing is 
described having two separate magnets formed in annular shape to surround 
a rotating shaft or to rotate about a fixed shaft. The two magnets are 
magnetically coupled by a single magnetically permeable pole piece or 
shunt engaged with one end of each of the magnets; each of the magnets has 
a reversed pole relation from the other such that the pole piece or shunt 
will engage the North and the South pole on each of the magnets. The 
interior surface of the annularly shaped magnets surround and are closely 
proximate to the exterior cylindrical surface of the shaft. The shaft is 
formed of magnetically permeable material such as steel. The interior 
diameter of the pole piece or shunt is sufficiently large to form a cavity 
between the two magnets, the shunt and the shaft. This cavity may be large 
enough to accommodate a fluid bearing or bearings. The bearing or bearings 
may be contained within the cavity. A non-magnetic material may be used to 
provide the bearing surface for the shunt or pole piece which is a part of 
a hub surrounding the shaft. The bearing surface of the shaft is formed 
onto the exterior surface of the shaft itself. 
The cavity will typically be slightly longer than the length of the bearing 
surfaces and may also include one or two thrust bearing surfaces extending 
radially to the shaft. The two annularly shaped magnets utilized in the 
seal assembly may be relatively weak magnets, due to their close proximity 
to the shaft at the point where the gap between the magnet and the shaft 
exists. The small gap thus formed will then be loadable with magnetic 
fluid to form the seal as well as with the magnetic fluid introduced into 
the cavity to lubricate the bearings. 
It is, therefore, an object of the invention to seal a magnetic fluid 
bearing with two relatively weak magnets in close proximity to the shaft 
forming the magnetic gap of the seal. 
It is another object of the invention to control undesired stray flux and 
to restrict it to regions where it will have no deleterious effect on 
magnetic recording media in close proximity thereto. 
As is apparent from the foregoing, the disadvantages and shortcomings of 
the prior art are overcome while the objects of the invention are 
accomplished by the utilization of two relatively weak magnets in close 
proximity to the sealing gap formed thereby; thus producing a reduction of 
stray flux and control of whatever inherent stray flux there may be with 
engagement of the non-gap poles of the magnet with a pole piece or shunt 
of high magnetic permeability.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the drawings and particularly to FIG. 1, a drive assembly for 
a magnetic storage disk unit is illustrated. The drive motor in the 
preferred embodiment of the best mode for carrying out the invention 
comprises a fixed shaft 10 with the field coils 12 mounted thereto and 
which remain stationary. The motor magnets 14 are mounted on the interior 
surface of the hub 16. Shaft 10 is fixedly mounted to a base 18. The shaft 
10 may be mounted to base 18 after the assembly of the motor elements and 
bearings on the shaft 10 or prior to assembly, as desired. 
Top bearing and seal assembly 20 includes a journal bearing 22 and thrust 
bearing surface 24. Bottom bearing 26 is provided with only a journal 
bearing 22. 
The electric motor comprising shaft 10, windings 12 and magnets 14, acts to 
rotate the hub 16 around shaft 10. 
Hub 16 may carry a disk pack or multiple magnetic storage disks thereon and 
the disks are arranged as flanges extending radially outwardly from the 
axis of shaft 10. 
Refer now to FIG. 2, which illustrates in greater detail and clarity, the 
arrangement of the upper bearing 20 as illustrated in FIG. 1. 
Upper bearing 20 comprises two bearings, a journal bearing 22 and a thrust 
bearing 24, and a seal for containing the magnetic lubricating fluid of 
the bearings 22 and 24. 
Shaft 10 is provided with shallow grooves 26 formed into its periphery. 
Grooves 26 are preferably chevron or herringbone shaped. The grooves 26 
provide a pumping action with respect to the magnetic lubricating fluid 
and thus creates sufficient pressure build up to separate the external 
surface of shaft 10 from the internal surface of the non-magnetic bearing 
insert 28. The non-magnetic bearing insert 28 is preferably a ceramic 
material which has a smooth interior surface. The ceramic material is 
chosen both for its wear characteristics and for the characteristic of 
non-permeability to magnetic flux. 
Ceramic bearing insert 28 is also provided with a bearing surface 24 which 
may engage a shoulder of shaft 10 or an insert 35 which is disposed in the 
bearing and seal structure. The insert 35 is preferably formed of a steel 
identical to that of the shaft 10 or at least chemically compatible 
therewith when in contact with the magnetic fluid contained within the 
seal assembly of FIGS. 1 and 2. 
With sufficient magnetic fluid between the surfaces of the journal bearing 
22, shaft 10 and the ceramic non-magnetic bearing insert 28 will rotate 
freely relative to each other. The chevron/herringbone shaped grooves act 
to build pressure between the shaft 10 and ceramic bearing insert 28, to 
insure lubrication of the bearing surfaces. 
In order to contain the magnetic fluid 60 within the bearing 20 and to seal 
the bearing fluid 60 from the motor chamber and from the disk environment, 
two magnets 30 and 32 are positioned to surround shaft 10 and ceramic 
bearing insert 28. Magnet 30 and magnet 32 are both annularly shaped 
magnets, typically of barium ferrite or similar material, and fabricated 
such that the outer diameter of the magnets 30, 32 may be positioned 
within an annular ring 34, typically fabricated of steel. The annular ring 
34 will act as a magnetic shunt 34 as will be described later. Alternative 
materials for the magnets include alnico, neodymium-iron-boron or samarium 
cobalt. 
Since the annular magnet structure is typically a sintered powder and 
particles of the magnet material may dislodge and cause failure of the 
bearings 20, it is highly desirable to coat the magnets 30, 32 with a 
containment coating such as an epoxy paint. Coatings on precision 
toleranced parts destroys tolerance control. Destruction of the fine 
tolerances required militates against the adoption of the arrangement and 
design of FIG. 2 of U.S. Pat. No. 4,598,914, discussed above. 
Magnets 30, 32 may be further provided with a cylindrical piece of 
material, typically magnetically non-permeable, such as a plastic sleeve 
36 which is engaged with the interior cylindrical surface of magnets 30, 
32. This sleeve 36 acts to protect the surfaces of magnets 30, 32. Magnets 
30, 32 may be inserted into shunt 34 and attached to shunt 34 by means of 
an adhesive material or, alternatively, by press fitting. Press fitting is 
preferable over the adhesive attachment because the concentricity of the 
magnets 30, 32 and the shunt 34 are more difficult to maintain when 
utilizing an adhesive. 
Journal bearing member 28 is provided with a smooth bearing surface 24 on 
its lower face which may then be engaged, when assembled, with surface 31 
of steel insert 35 to form a thrust bearing. Surface 31 may be configured 
with spiral grooves, as is conventionally known and therefore not shown, 
to provide the necessary pressurization and pumping action for a thrust 
fluid bearing. 
The shouldered configuration with shoulder surface 40 on shunt 34 provides 
a load carrying surface for engagement with the ceramic bearing insert 28 
so that the thrust force may be exerted from steel insert 35 to ceramic 
bearing insert 28 and shoulder 40 to support shunt 34 and hub 16. 
Referring now to FIG. 3, which illustrates the lower bearing as shown in 
FIG. 1, shaft 10 is likewise provided with a chevron/herringbone type 
bearing groove pattern 26 as described with respect to FIG. 2. Ceramic 
bearing insert 28, although slightly different in shape, provides the same 
function with respect to bearing properties as bearing insert 28 in FIG. 
2. 
Magnets 30, 32 and sleeve 36 in FIG. 3 are all analogous to their earlier 
described counterparts of FIG. 2. 
The bearing illustrated in FIG. 3 is a journal bearing and has no thrust 
bearing capability. Accordingly, there is no thrust bearing surface on the 
ceramic bearing insert 28. 
Shunt 34 provides the same function and properties as the earlier described 
shunt 34 in FIG. 2. 
Referring now to both FIGS. 2 and 3, passages 50 are formed through the 
ceramic bearing member extending from the region of magnet 32 to the 
region of magnet 30. Passage 50 provides for circulation of the magnetic 
lubricating fluid 60 throughout the cavity formed by shaft 10, magnets 30, 
32 and shunt 34. The circulation of the fluid 60 is essential to prevent 
overheating and undue degradation of the lubricating qualities of the 
magnetic fluid 60. 
In both FIGS. 2 and 3, magnets 30 and 32 are oriented opposite to each 
other with regard to polarity. It can be seen from the arrangement of 
magnet 30 with its North pole in contact with shunt 34 and the arrangement 
of magnet 32 with its South pole in contact with shunt 34 and the 
remaining poles in close proximity to shaft 10, a magnetic circuit is 
formed wherein the shaft 10 completes the circuit formed by magnet 30, 
shunt 34 and magnet 32. This circuit will not only seal in the regions 
between shaft 10 and magnet 30 and the region between bearing insert 28 
and thrust bearing insert 35 and magnet 32; but the circuit will contain 
the magnetic fluid 60 enclosed within the interior of the flux path thus 
maintaining adequate magnetic fluid 60 for lubrication and proper 
operation of the bearing assembly 20. 
An alternative embodiment of the invention is shown in FIG. 4. The journal 
bearing ceramic insert 28 is provided with an upper thrust bearing surface 
74 and lower thrust bearing surface 70. These two surfaces 70, 74 are 
parallel and respectively face against thrust bearing rings 80, 82. The 
two opposed pairs of thrust bearing surfaces will constrain the ceramic 
insert 28 from axial movement along the axis of shaft 10. 
The magnets 30, 32 are functionally identical to those in FIGS. 1 and 2. 
As with the thrust bearing surfaces 24, 31 of FIG. 2, the surfaces 70, 72 
and 74, 76 may be provided with spiral grooves to provide the pumping 
action on the magnetic fluid. 
Positioning ring 84 is provided to position ring 82 and journal bearing 
insert 28 relative to ring 80 and control spacing between the thrust 
bearing surfaces 70, 72 and 74, 76. 
Shunts 86, 88 magnetically connect the magnets 32, 30 respectively to hub 
90 to complete the flux path through the hub 90. 
From the description of FIG. 4, it is appreciated that the seal circuit 
comprising two relatively weak magnets 30, 32, a shunt path 86, 90, 88 and 
the shaft 10 may enclose a journal bearing as well as two thrust bearings. 
By utilizing the magnets 30, 32 themselves at the gaps 37 and disposing the 
magnets 30, 32 such that the interior surface of the annular magnet ring 
30 or 32 is disposed proximate to the exterior of shaft 10, a very high 
flux density at the gap 37 is accomplished without the use of an unduly 
strong or large magnet. Interconnecting the two magnets 30, 32 with the 
shunt 34 acts to complete the flux path and, at the same time, constrains 
and controls the flux generated by magnets 30, 32 and prevents undue stray 
flux from escaping from the flux circuit path. By the use of steel in 
shunt 34, the shunt 34 controls and constrains the flux to the interior of 
the shunt 34 to a very high degree; stray flux is prevented from 
propagating into the disk environment, inadvertently affecting the 
magnetic storage disks, and potentially destroying valuable data stored on 
the disks. 
The need for a large, strong single magnet is overcome and the possibility 
of the large, strong single magnet propagating stray flux into the 
magnetic storage disk environment is eliminated by the use of the weaker 
magnets 30, 32 positioned to focus their flux in a very concentrated area 
of the gaps 37 between the magnets 30, 32 and the shaft 10. 
The foregoing preferred embodiment of the best mode for carrying out the 
invention has been described, but it should be understood that variations 
and changes may be made without departing from the scope and spirit of the 
invention as set forth by the claims.