Hydrodynamic seal for disc drive spindle motor

A disc drive includes a disc drive motor and a chassis for providing a substantially contaminant-free cavity. The disc drive motor includes a fixed member coupled to the chassis and a rotor rotatable about the fixed member for rotating a magnetic storage disc in the contaminant-free cavity. A bearing interconnects the fixed member and the rotor. A viscous pump seal is located between the contaminant-free cavity and the bearing. The viscous pump seal includes a first pump surface carried by one of the fixed member and the rotor and a second, grooved pump surface carried by the other of the fixed member and the rotor. The first and second pump surfaces are separated by a gap. The viscous pump seal inhibits transfer of contaminants from the bearing to the contaminant-free cavity.

BACKGROUND OF THE INVENTION 
The present invention relates generally to the field of disc drive data 
storage devices. More specifically, the invention relates to a 
hydrodynamic (viscous pump) seal for a disc drive spindle motor. 
Disc drive data storage devices are well know in the industry. Such devices 
use rigid discs coated with a magnetizable medium with for storage of 
digital information in a plurality of circular concentric data tracks. The 
information is written to and read from the discs using a transducing head 
mounted on an actuator mechanism which moves the head from track to track 
across a surface of the disc under control of electronic circuitry. The 
discs are mounted for rotation on a spindle motor which causes the discs 
to spin and the surfaces of the discs to pass under the heads. 
As magnetic storage densities have increased, magnetic disc drives have 
been required to operate with increasingly greater precision. This 
requirement has meant that magnetic recording heads have been placed 
increasingly close to the surface of the magnetic disc. The interaction 
between the magnetic head and the recording surface has also become 
increasingly precise. This has required the environment of the magnetic 
disc to be free from particulate and liquid contaminants. Typically, the 
disc environment is sealed during manufacture so that contaminants cannot 
enter the housing and contact the storage disc or the magnetic recording 
head. Additionally, it is important that the disc environment within the 
chassis remain contaminant free following manufacturing and during 
operation of the disc drive system. Even minute contaminants can have 
catastrophic results on disc operation. For example, particulate build up 
between the transducing head and the disc can cause degradation in the 
read back signal, head crashes and damage to the disc surface. 
One source of particulate and liquid contaminants in the sealed chassis is 
the disc drive spindle motor which rotates the storage disc. The disc 
rotates at speeds in excess of several thousands of RPM, and the 
rotational speed in present day disc drives continues to increase. 
Although the motor is sealed, the seal in not perfect and contaminants 
tend to escape from the motor into the compartment containing the disc. 
A number of attempts have been made to reduce the tendency of contaminants 
to travel from the motor to the disc compartment. For example, U.S. Pat. 
No. 5,011,165, issued Apr. 30, 1994, to Cap, entitled "SEALING DEVICE 
ESPECIALLY FOR HARD DISK DRIVES," describes a ferrofluid seal which is 
used to isolate the environment of the drive motor from the sealed disc 
environment. The ferrofluid seal is a fluidic seal made of ferrofluid 
which is held in place by a magnet. The fluid extends across a gap between 
a fixed portion of the disc drive motor and the rotor, and thereby 
prevents contaminants from the motor from entering the sealed disc 
environment. However, one problem with the ferrofluid seal is that the 
ferrofluid may leak from the seal and enter the disc environment which 
leads to the problems discussed above. Further, the ferrofluid seal may 
leak into the motor which may cause damage. Additionally, leakage of the 
ferrofluid reduces the effective quantity of the ferrofluid in the seal, 
thereby reducing the effectiveness of the seal. 
Another type of seal is "labyrinth" seal. Typically, a labyrinth seal is a 
small gap at a small diameter of the motor which extends over a long path. 
This arrangement tends to inhibit contaminants from the motor from 
escaping through the labyrinth into the sealed disc compartment. The 
labyrinth seal can be made more effectively be reducing the gap and 
lengthening the path. However, this requires precision machining which is 
both difficult, time consuming and expensive. Although labyrinth seals 
tend to be less expensive than ferrofluid seals, labyrinth seals are 
typically not as effective in isolating the motor from the disc 
environment. 
There is a continual need for improving the isolation between the spindle 
motor in a disc drive and the contaminant-free disc environment. 
SUMMARY OF THE INVENTION 
The disc drive of the present invention includes a disc drive spindle motor 
and a chassis for providing a substantially contaminant-free cavity. The 
disc drive spindle motor includes a fixed member coupled to the chassis 
and a rotor rotatable about the fixed member for rotating a magnetic 
storage disc in the contaminant-free cavity. A bearing interconnects the 
fixed member and the rotor. A viscous pump seal is located between the 
contaminant-free cavity and the bearing. The viscous pump seal includes a 
first pump surface carried by one of the fixed member and the rotor and a 
second, grooved pump surface carried by the other of the fixed member and 
the rotor. The first and second pump surfaces are separated by a gap. The 
viscous pump seal inhibits transfer of contaminants from the bearing to 
the contaminant-free cavity. 
In one embodiment, the viscous pump seal includes first and second annular 
plates, which are coaxial with the central axis. The first annular plate 
defines the first pump surface, and the second annular plate defines the 
second, grooved pump surface. However, grooves can be placed on both the 
first and second pump surfaces, in which case both pump surfaces act as an 
individual pump. The first and second annular plates can be press-fit or 
otherwise attached to the fixed member and the rotor, or can be integral 
with the fixed member and the rotor. The grooved pump surface or surfaces 
can have a spiral groove pattern or a herring bone groove pattern, for 
example. The spiral and herring bone groove patterns are preferably 
arranged to pump air from the inner radius and the outer radius of the 
plates toward the middle radius of the plates. This creates a pressure 
build-up between the plates which restricts the transfer of contaminants 
from the bearing to the contaminant-free cavity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a plan view of a disc drive 2 for use with the present 
invention. Disc drive 2 includes a base member 4 to which internal 
components of the unit are mounted. Base member 4 couples to top cover 6 
which forms a sealed environment (cavity) for critical parts of disc drive 
2. 
Disc drive 2 includes a plurality of discs 8 which are mounted for rotation 
on a spindle motor, shown generally at 10. Motor 10 is described below in 
greater detail. A plurality of magnetic read/write heads 12, usually on 
per disc surface, are mounted to an actuator 14. In the example shown at 
drive 2, actuator 14 is a rotatory actuator which is mounted for pivoting 
about a pivot axis 16. Actuator 14 includes a number of head mounting arms 
18 which couple heads 12 to the actuator body via a plurality of load 
beam/gimbal assemblies 20. Actuator motor 22 is also coupled to actuator 
body 14 to provide a force to move heads 12 to a desired position on the 
surface of disc 8. 
FIG. 2 shows a spindle drive motor 10 of a fixed shaft design in cross 
section in accordance with one aspect of the invention. Drive motor 10 is 
mounted to base 4 and includes fixed shaft 30 which is screwed into base 
4. Rotor hub 32 includes hub 34 and rotor 36 which rotate about fixed 
shaft 30 on upper bearing 38 and lower bearing 40. Hub 34 supports disc 8. 
Upper bearing 38 and lower bearing 40 are positioned within bearing cavity 
41, between fixed shaft 30 and rotor 36 for rotatably coupling rotor 36 to 
fixed shaft 30. Stator assembly 42 is mounted to base 4 by screws 44. 
Permanent magnets 46 are attached to rotor 36 proximate stator assembly 
42. Electrical signals supplied to windings 48 of stator assembly 42 
create a magnetic field which interacts with permanent magnets 46 to cause 
rotor hub 32 to rotate. 
Although disc 8 is contained in a sealed, contaminant-free cavity 50 formed 
by base 4 and cover 6, various contaminants from motor 10, such as metal 
particles or lubrication used with bearings 38 and 40, tend to leak from 
bearing cavity 41 and enter contaminant-free cavity 50, along airflow path 
52. The present invention provides a hydrodynamic or "viscous pump" seal 
54 which reduces the tendency of contaminants from entering 
contaminant-free cavity 50. Viscous pump seal 54 is positioned within air 
flow path 52, between rotor 36 and fixed shaft 30. Viscous pump seal 54 
includes annular plates 56 and 58. Annular plate 56 is carried by rotor 36 
and extends from rotor 36 toward fixed shaft 30. Annular plate 58 is 
carried by fixed shaft 30 and extends from fixed shaft 30 toward rotor 36. 
Annular plates 56 and 58 have pump surfaces 56a and 58a which oppose one 
another and are separated by a gap 60. One of the pump surfaces 56a or 58a 
is grooved. As plate 56 rotates with rotor 36 relative to plate 58, the 
grooved pump surface creates a pumping action on the air within gap 60 
according to viscous flow principles. The pumping action creates a 
pressure buildup between plates 56 and 58 which is a function of the 
surface area of the plates, the physical properties of the medium (e.g. 
air) in an ambient condition, the rotational speed rotor 36 and the width 
of gap 60, for example. The groove pattern determines the direction of 
pumping. Preferably, the direction of pumping is selected to create an 
airflow restriction within airflow path 52 which inhibits transfer of 
contaminants from bearing cavity 41 to contaminant-free cavity 50. In one 
embodiment, the groove pattern is selected to pump air from the outer 
radius and inner radius of plates 56 and 58 toward a middle radius of 
plates 56 and 58 such that the net flow in either direction is zero. This 
creates a pressure build-up between the plates that inhibits transfer of 
contaminants in either direction through viscous pump seal 54. 
In another embodiment, the groove pattern is selected to pump air between 
plates 56 and 58 radially inward toward shaft 30 and thus toward bearing 
cavity 41, which inhibits contaminants from escaping the bearing cavity. 
In yet another embodiment, the axial positions of plates 56 and 58 are 
reversed and the groove pattern is selected to pump air radially outward 
toward rotor 36 and thus toward bearing cavity 41. 
Plates 56 and 58 can be easily placed in any of the traditional ferrofluid 
seal locations by press-fitting the plates onto the inner diameter of 
rotor 36 and the outer diameter of shaft 30, respectively. Plates 56 and 
58 can be formed of any suitable material, such as stainless steel or a 
cooper alloy. Alternatively, pump surfaces 56a and 58a can be formed by 
surface features integrated within the material of rotor 36 or shaft 30. 
Viscous pump seal 54 can thus replace a ferrofluid seal or be an addition 
in any spindle motor where contamination is a concern. 
FIG. 3 is a plan view of pump surface 56a of plate 56, as viewed from gap 
60. Pump surface 56a has a plurality of grooves 70a-70h which have a 
generally V-shape, herring bone pattern. Each groove 70a-70h extends from 
an inner radius 72 to an outer radius 74 of plate 56 and has an apex 76 at 
approximately the middle radius of plate 56. Each groove 70a-70h can have 
a variety of cross sections, such as rectangular, semicircular or 
triangular. Grooves 70a-70h can also have a variety of depths and widths, 
depending upon the particular application. In one embodiment, grooves 
70a-70h have a depth of 4-15 micrometers, with gap 60 (shown in FIG. 2) 
having a width of 2-10 micrometers, for example. 
During operation, plate 56 rotates with rotor 36 about central axis 62 in 
the direction indicated by arrow 78. Grooves 70a-70h pump air within gap 
60 radially outward from inner radius 72 toward the middle radius of plate 
56, at apex 76, and radially inward from outer radius 74 toward the middle 
radius of plate 56. Pressure builds between plates 56 and 58 creating an 
air flow restriction within air flow path 52 (shown in FIG. 2). This air 
flow restriction tends to limit passage of contaminants from bearing 
cavity 41 to contaminant-free cavity SO. 
Incompressible flow principles were applied to estimate the load and 
stiffness created by viscous pump seal 54 based on a narrow groove 
application. Although air is a compressible fluid, incompressible fluid 
flow principles were applied since air behaves almost like an 
incompressible fluid under the operating conditions of the viscous pump 
seal. Table 1 shows the estimated load and stiffness. 
TABLE 1 
______________________________________ 
Axial P max 
GAP Groove Load Stiffnes 
(Pascal 
(.mu.m) 
RPM Depth (N) s (N/m) 
gauge) 
______________________________________ 
2 7000 4 1.2297 1.337e6 
23,704 
2 7000 11 0.5617 2.20e5 
10,813 
5 7000 11 0.194 0.79e5 
3,728 
10 7000 15 0.0474 0.121e5 
927 
10 7000 11 0.0404 0.115e5 
832 
2 10,000 4 1.756 1.91e6 
33,863 
2 7000 4 1.2297 1.337e6 
23,704 
2 10,000 11 0.802 3.15e5 
15,448 
5 10,000 11 0.277 1.13e5 
5,326 
10 10,000 15 0.0677 0.172e5 
1,325 
10 10,000 11 0.0577 0.165e5 
1,189 
Example 1,236 
of a 
ferro- 
seal 
______________________________________ 
FIGS. 4a and 4b are plan views of viscous pump plates having alternative, 
spiral groove patterns. Plate 80 has a pump surface 80a with a plurality 
of spiral-shaped grooves 82a-82h which extend from outer radius 84 toward 
a middle radius 86 of plate 80. Plate 88 has a pump surface 88a with a 
plurality of spiralshaped grooves 90a-90h which extend from inner radius 
92 toward a middle radius 94 of plate 88. When plate 80 is attached to the 
inner diameter of rotor 36 (shown in FIG. 2), similar to plate 56, and 
plate 88 is attached to the outer diameter of shaft 30, similar to plate 
58, pump surfaces 80a and 88a oppose one another across gap 60. As plate 
80 rotates in the direction of arrow 96 relative to plate 88, grooves 
82a-82h of plate 80 pump air radially outward from inner radius 82 toward 
middle radius 86 of plate 80 and grooves 90a-90h pump air radially inward 
from outer radius 92 toward middle radius 94 of plate 88. This causes a 
pressure build-up at the middle radii 86 and 94 of plates 80 and 88 which 
restricts flow in either direction and thereby inhibits the transfer of 
contaminants from bearing cavity 41 to contaminant-free cavity 50. 
In the embodiment shown in FIGS. 4a and 4b, half of the spiral groove 
pattern is one pump surface and the other half is on the other pump 
surface. The number of grooves and the length of the grooves in each pump 
surface is preferably selected to maintain a balance in the opposing flow 
rates generated by each pump surface. 
FIG. 5 is a sectional view of a disc drive spindle motor 100 having a 
rotating shaft 102 in accordance with an alternative embodiment of the 
present invention. Spindle motor 100 further includes a central axis 104, 
a hub 106, a stationary member 108, a stator 110 and a rotor magnet 112. 
Hub 106 is coupled to shaft 102 and carries a magnetic disc 114 for 
rotation about central axis 104. Stationary member 108 is secured to 
housing 116. Stator 110 is attached to the outer diameter of stationary 
member 108 and includes a stator winding 118 and a stator lamination 120. 
Rotor magnet 110 is attached to the inner diameter of hub 106. Ball 
bearings 122 and 124 interconnect shaft 102 with stationary member 108. 
Ball bearings 122 and 124 are contained within a bearing cavity 126. 
As in the embodiment shown in FIG. 2, magnetic disc 114 is sealed within a 
substantially contamination-free cavity 128 by housing 116. A viscous pump 
seal 130 is positioned between shaft 102 and stationary member 108 to 
inhibit transfer of contaminants from bearing cavity 126 to 
contaminant-free cavity 128. Viscous pump seal 130 is similar to viscous 
pump seal 54 and can have a variety of configurations and positions in 
alternative embodiments. For example, viscous pump seal 130 can be placed 
between wall 140 of hub 106 and wall 142 of stationary member 108. The 
cylindrical surfaces of walls 140 and 142 would form the pump surfaces, 
and one or both of the pump surfaces would be grooved depending upon the 
type of grooves used. The grooves can have a spiral pattern or a herring 
bone pattern that are arranged to pump air both inward and outward toward 
a predetermined axial position along walls 140 and 142, for example. 
The viscous pump seal of the present invention has several advantages over 
seals of the prior art, such as ferrofluidic seals. The viscous pump seal 
is contamination-free in that the seal, itself, is not a source of 
contamination as in ferrofluidic seals. The viscous pump seal of the 
present invention has an efficiency which increases with disc rotational 
velocity and with temperature. This allows the seal to be particularly 
effective in high speed spindle motor applications and in rugged 
environments. Further, the viscous pump seal of the present invention can 
be manufactured in mass production much cheaper than ferrofluidic seals 
and machining-intensive labyrinth seals. 
Although the present invention has been described with reference to 
preferred embodiments, workers skilled in the art will recognize that 
changes may be made in form and detail without departing from the spirit 
and scope of the invention. For example, the viscous pump seal of the 
present invention can be used in both ball bearing and hydrodynamic 
bearing applications. The seal can be formed between a pair annular plates 
or between opposed pump surfaces formed on a stationary member and a 
rotating member in the disc drive. Either pump surface or both pump 
surfaces can be grooved, and the grooved pump surface or surfaces can be 
stationary, rotating or both. Also, the pump surfaces can have a radial or 
axial orientation with respect to one another.