U.S. patent application number 12/195646 was filed with the patent office on 2009-02-26 for methods of hydraulic compensation for magnetically biased fluid dynamic bearing motor.
This patent application is currently assigned to Seagate Technology LLC. Invention is credited to Troy Michael Herndon, Jeffrey Arnold LEBLANC.
Application Number | 20090052817 12/195646 |
Document ID | / |
Family ID | 31498712 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090052817 |
Kind Code |
A1 |
LEBLANC; Jeffrey Arnold ; et
al. |
February 26, 2009 |
Methods of Hydraulic Compensation for Magnetically Biased Fluid
Dynamic Bearing Motor
Abstract
A fluid dynamic bearing motor including a stationary sleeve, a
rotating shaft axially disposed through the sleeve, a journal gap
between the shaft and the sleeve, the gap defined by first and
second interfacial surfaces of the shaft and sleeve, at least one
set of fluid dynamic grooves formed on the first interfacial
surface of the journal gap, and at least one step defined on the
second interfacial surface of the journal gap.
Inventors: |
LEBLANC; Jeffrey Arnold;
(Aptos, CA) ; Herndon; Troy Michael; (San Jose,
CA) |
Correspondence
Address: |
SEAGATE TECHNOLOGY LLC;C/O NOVAK DRUCE & QUIGG LLP
1000 LOUISIANA, SUITE 5350
HOUSTON
TX
77002
US
|
Assignee: |
Seagate Technology LLC
Scotts Valley
CA
|
Family ID: |
31498712 |
Appl. No.: |
12/195646 |
Filed: |
August 21, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10602471 |
Jun 23, 2003 |
7422370 |
|
|
12195646 |
|
|
|
|
60401797 |
Aug 6, 2002 |
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Current U.S.
Class: |
384/107 ;
29/898 |
Current CPC
Class: |
H02K 7/085 20130101;
H02K 5/1675 20130101; F16C 17/026 20130101; F16C 2370/12 20130101;
H02K 7/09 20130101; Y10T 29/49636 20150115; G11B 19/2018 20130101;
F16C 33/107 20130101 |
Class at
Publication: |
384/107 ;
29/898 |
International
Class: |
F16C 32/06 20060101
F16C032/06; B21D 53/10 20060101 B21D053/10 |
Claims
1. A method of operating a motor having a fluid dynamic bearing
formed between a fixed sleeve and a rotatable shaft disposed in the
sleeve, and the bearing having a fluid with a variable viscosity
disposed therein, the method comprising: generating a thrusting
force for lifting the rotatable shaft from fluid pressure created,
at least in part, by interaction between a surface of one of the
shaft and sleeve and pumping grooves disposed on the other of the
shaft and the sleeve, whereby the thrusting force produces an
amount of lift determined at least in part by the fluid viscosity;
and dynamically adjusting the thrusting force to regulate the
amount of lift by varying a size of a gap formed between the
surface and the pumping grooves.
2. The method of claim 1, wherein the size of the gap is varied by
relative axial movement of the shaft and the sleeve, wherein the
surface includes one or more steps proud from a remainder of the
surface, such that as the pumping grooves become more aligned with
the steps, the gap decreases, causing the fluid pressure to
increase, and produce more thrusting force.
3. The method of claim 2, wherein the pumping grooves are disposed
on an outer diameter of the shaft, and the surface comprises an
inner diameter of the sleeve.
4. The method of claim 2, wherein the pumping grooves are disposed
on an inner diameter of the sleeve, and the surface comprises an
outer diameter of the shaft.
5. The method of claim 1, wherein the pumping grooves are
asymmetric to establish pumping pressure toward an end of the
shaft.
6. The method of claim 1, wherein generating the thrusting force
for lifting the rotatable shaft further comprises a thrusting force
generated by pumping grooves on at least one of a counterplate and
a facing surface of the shaft.
7. A motor having a fluid dynamic bearing formed in a gap disposed
between a fixed sleeve and a rotatable shaft disposed in the
sleeve, lubricating fluid disposed in the gap, the motor formed by
a method comprising: providing a base under the shaft; defining one
or more steps proud from a surface of one of the fixed sleeve and
the rotatable shaft; and defining pumping grooves on a surface of
the other of the fixed sleeve and the rotatable shaft, the pumping
grooves positioned relative to the one or more steps to establish,
at least in part, during operation, an asymmetric fluid pressure
profile that supports the shaft for rotation over the base, and
responds to effects from fluid viscosity changes that affect the
asymmetric fluid pressure profile by an relative axial movement
between the grooves and the one or more steps, thereby countering
the effects from fluid viscosity changes to the asymmetric fluid
pressure profile.
8. The method of claim 7, wherein the pumping grooves are
asymmetric to establish pumping pressure toward an end of the
shaft.
9. The method of claim 7, wherein defining the one or more steps
comprises adding material to the surface of one of the fixed sleeve
and the rotatable shaft by at least one of the following methods:
plating, coating, and sputtering.
10. The method of claim 9, wherein the material is added by
sputtering a Diamond Like Coating upon the surface of one of the
fixed sleeve and the rotatable shaft.
11. The method of claim 7, wherein defining the one or more steps
comprises removing material from the surface of one of the fixed
sleeve and the rotatable shaft by at least one of the following
methods: turning, grinding, electrochemical machining, and
electrical discharge machining.
12. The method of claim 7, wherein the pumping grooves are defined
on an inner diameter of the sleeve, and the surface opposite the
pumping grooves comprises an outer diameter of the shaft.
13. The method of claim 7, wherein the pumping grooves are defined
on an outer diameter of the shaft, and the surface opposite the
pumping grooves comprises an inner diameter of the sleeve.
14. The method of claim 7, wherein the pumping grooves are
asymmetric to establish pumping pressure toward an end of the shaft
proximate the base.
15. The method of claim 7 further comprising defining pumping
grooves on at least one of the base under the shaft and a surface
of the shaft opposite the base to aid in establishing the pressure
profile that supports the shaft for rotation over the base.
16. A method for countering effects from fluctuating fluid
viscosity of fluid disposed in a gap of a fluid dynamic bearing
comprising: moving a first surface axially relative to a second
surface during operation at least in part by the effects from
fluctuating fluid viscosity; generating hydraulic force by
interaction between at least one set of fluid dynamic grooves on
the first surface and at least one step extending from the second
surface; and countering the axial movement of the first surface
relative to the second surface by, at least in part, the hydraulic
force changing to compensate for the effects from fluctuating fluid
viscosity that would otherwise cause increased axial movement of
the first surface relative to the second surface.
17. The method of claim 16, wherein the fluid dynamic grooves are
defined on an inner diameter of a sleeve, and the second surface
comprises an outer diameter of the shaft.
18. The method of claim 16, wherein the fluid dynamic grooves are
defined on an outer diameter of a shaft, and the second surface
comprises an inner diameter of the sleeve.
19. The method of claim 16, wherein the pumping grooves are
asymmetric to establish pumping pressure toward an end of the shaft
adjacent the base.
20. The method of claim 16, wherein the moving comprises operating
a motor, in which the fluid dynamic bearing is a part, the
operating resulting in changing fluid viscosity
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 10/602,471, filed Jun. 23, 2003, now U.S. Pat. No. 7,422,370,
which claims the priority of U.S. Provisional Application No.
60/401,797, filed Aug. 6, 2002 by LeBlanc et al. (entitled
"Hydraulic Compensation For Magnetic Bias FDB Motor"), which is
herein incorporated by reference.
FIELD OF THE INVENTION
[0002] The invention generally relates to fluid dynamic bearing
motors and, more particularly, to magnetically biased fluid dynamic
bearing motors.
BACKGROUND OF THE INVENTION
[0003] Disk drives are capable of storing large amounts of digital
data in a relatively small area. Disk drives store information on
one or more recording media, which conventionally take the form of
circular storage disks (e.g. media) having a plurality of
concentric circular recording tracks. A typical disk drive has one
or more disks for storing information. This information is written
to and read from the disks using read/write heads mounted on
actuator arms that are moved from track to track across the
surfaces of the disks by an actuator mechanism.
[0004] Generally, the disks are mounted on a spindle that is turned
by a spindle motor to pass the surfaces of the disks under the
read/write heads. The spindle motor generally includes a shaft
mounted on a base plate and a hub, to which the spindle is
attached, having a sleeve into which the shaft is inserted.
Permanent magnets attached to the hub interact with a stator
winding on the base plate to rotate the hub relative to the shaft.
In order to facilitate rotation, one or more bearings are usually
disposed between the hub and the shaft.
[0005] Over the years, storage density has tended to increase, and
the size of the storage system has tended to decrease. This trend
has lead to greater precision and lower tolerance in the
manufacturing and operating of magnetic storage disks.
[0006] From the foregoing discussion, it can be seen that the
bearing assembly that supports the storage disk is of critical
importance. One bearing design is a fluid dynamic bearing. In a
fluid dynamic bearing, a lubricating fluid such as air or liquid
provides a bearing surface between a fixed member of the housing
and a rotating member of the disk hub. In addition to air, typical
lubricants include gas, oil, or other fluids. The relatively
rotating members may comprise bearing surfaces such as cones or
spheres, or may alternately comprise fluid dynamic grooves formed
on the members themselves. Fluid dynamic bearings spread the
bearing surface over a large surface area, as opposed to a ball
bearing assembly, which comprises a series of point interfaces.
This bearing surface distribution is desirable because the
increased bearing surface reduces wobble or run-out between the
rotating and fixed members. Further, the use of fluid in the
interface area imparts damping effects to the bearing, which helps
to reduce non-repeat run-out. Thus, fluid dynamic bearings are an
advantageous bearing system.
[0007] Many current fluid dynamic bearing designs employ a
combination of journal and thrust bearings. Frequently, these
designs include a shaft journal bearing design having a thrust
plate at an end thereof. The journal bearings typically include two
grooved surfaces facing the journal (either on the shaft or on the
sleeve), the thrust plate bearings typically include two grooved
surfaces, one facing each of the gaps defined by the thrust plate
and sleeve, and by the thrust plate and counter plate. Net
hydraulic pressure created by the journal bearings establishes a
thrust force on the end of the shaft (i.e., toward the thrust plate
bearings) that displaces the shaft axially; an opposing force,
generated, for example, by a magnetic bias force, is needed to
stabilize the motor.
[0008] However, as the temperature fluctuates in the motor, the
viscosity of the fluid in the bearings changes as well. While the
magnetic bias force remains constant regardless of temperature, the
hydraulic pressure (thrust force) generated by the journal bearings
varies with the changing fluid viscosity. Thus, the opposing forces
(thrust force vs. magnetic bias force) may not be of sufficient
magnitudes to offset each other, allowing the rotor to move axially
as temperature changes.
[0009] Therefore, a need exists for a magnetically biased fluid
dynamic bearing design that can compensate for changing temperature
and fluid viscosity in the motor.
SUMMARY OF THE INVENTION
[0010] A fluid dynamic bearing motor comprising a stationary
sleeve, a rotating shaft axially disposed through the sleeve, a
journal gap between the shaft and the sleeve, said gap defined by
first and second interfacial surfaces of the shaft and sleeve, at
least one set of fluid dynamic grooves formed on the first
interfacial surface of the journal gap, and at least one step
defined on the second interfacial surface of the journal gap.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] So that the manner in which the above recited embodiments of
the invention are attained and can be understood in detail, a more
particular description of the invention, briefly summarized above,
may be had by reference to the embodiments thereof which are
illustrated in the appended drawings. It is to be noted, however,
that the appended drawings illustrate only typical embodiments of
this invention and are therefore not to be considered limiting of
its scope, for the invention may admit to other equally effective
embodiments.
[0012] FIG. 1 depicts a plan view of one embodiment of a disk drive
that comprises a motor in which the invention is used;
[0013] FIG. 2 depicts a side sectional view of a magnetically
biased fluid dynamic bearing motor according to a first embodiment
of the invention;
[0014] FIG. 2B depicts a groove pattern in accordance with the
present invention.
[0015] FIG. 3 depicts a side sectional view of a magnetically
biased fluid dynamic bearing motor according to a second embodiment
of the invention;
[0016] FIG. 4 depicts a side sectional view of a magnetically
biased fluid dynamic bearing motor according to a third embodiment
of the invention; and
[0017] FIG. 5 depicts a side sectional view of a magnetically
biased fluid dynamic bearing motor according to a fourth embodiment
of the invention.
[0018] To facilitate understanding, identical reference numerals
have been used, where possible, to designate identical elements
that are common to the figures.
DETAILED DESCRIPTION
[0019] FIG. 1 depicts a plan view of one embodiment of a disk drive
10 for use with embodiments of the invention. Referring to FIG. 1,
the disk drive 10 includes a housing base 12 and a top cover plate
14. The housing base 12 is combined with cover plate 14 to form a
sealed environment to protect the internal components from
contamination by elements outside the sealed environment. The base
and cover plate arrangement shown in FIG. 1 is well known in the
industry; however, other arrangements of the housing components
have frequently been used, and aspects of the invention are not
limited by the particular configuration of the disk drive
housing.
[0020] Disk drive 10 further includes a disk pack 16 that is
mounted on a hub 202 (see FIG. 2) for rotation on a spindle motor
(not shown) by a disk clamp 18. Disk pack 16 includes one or more
of individual disks that are mounted for co-rotation about a
central axis. Each disk surface has an associated read/write head
20 that is mounted to the disk drive 10 for communicating with the
disk surface. In the example shown in FIG. 1, read/write heads 20
are supported by flexures 22 that are in turn attached to head
mounting arms 24 of an actuator 26. The actuator shown in FIG. 1 is
of the type known as a rotary moving coil actuator and includes a
voice coil motor (VCM), shown generally at 28. Voice coil motor 28
rotates actuator 26 with its attached read/write heads 20 about a
pivot shaft 30 to position read/write heads 20 over a desired data
track along a path 32.
[0021] FIG. 2 is a sectional side view of a fluid dynamic bearing
motor 200 according to one embodiment of the present invention. The
motor 200 comprises a rotating assembly 205, a stationary assembly
203, and a bearing assembly 207.
[0022] The rotating assembly 205 comprises a shaft 202 affixed at a
first end 221 to a hub 204 that supports at least one disk (not
shown) for rotation. A second end 223 of the shaft 202 is distal
from the first end 221. The hub 204 additionally supports a magnet
assembly 252 comprising a back iron 211 with a magnet 209 affixed
thereon. In one embodiment of the invention, the magnet assembly
252 is positioned on the inside circumferential surface 254 of the
hub 204.
[0023] The stationary assembly 203 comprises a sleeve 208 mounted
on the base 12. The sleeve 208 further comprises a bore 231 through
which the shaft 202 is disposed axially. A stator 210 mounted on
the base 12 cooperates with the magnet 209 in the hub 204 to induce
rotation of the shaft 202 and hub 204 relative to the sleeve 208.
The stator 210 comprises a plurality of "teeth" 235 formed of a
magnetic material such as steel, where each of the teeth 235 is
wound with a winding or wire 237.
[0024] The bearing assembly 207 is formed in a journal (or gap) 217
defined between the facing surfaces of the inner diameter 215 of
the sleeve 208 and the outer diameter 219 of the shaft 202. A fluid
214 such as air, oil or gas is disposed between the shaft 202 and
the sleeve 208. The journal 217 further comprises fluid dynamic
grooves 300; an example is formed on one or both of the interfacial
surfaces 215, 219 (in FIG. 2, the fluid dynamic grooves 300 are
formed on the outer surface 219 of the shaft 202).
[0025] The fluid dynamic grooves 300 form a circumferential ring
around an interfacial journal surface 215, 219 and may comprise a
V-shaped pattern or a chevron, spiral or sinusoidal pattern or
other pattern (not shown). The pattern, generates a pressure
distribution across the bearing surface that provides improved
bearing rocking stiffness.
[0026] The fluid dynamic grooves 300 may be formed asymmetrically,
where the length of one leg of the pattern leading to the pattern's
pressure apex is greater than the length of the leg on the other
side of the pattern's apex. When asymmetry of the pattern is
created by legs with different lengths, a net flow of fluid 214 is
pumped toward the leg with the shorter length. As the hub 204 and
shaft 202 rotate, a net hydraulic pressure is generated by the
journal bearing grooves 300 toward the second end 223 of the shaft
202. Pressure is also generated as a function of the size of the
gap between the shaft 202 and sleeve 208 in the areas of the
grooves 300 (and depending on the size of the gap, symmetric
grooves 300 may also be used, and the same effect achieved). This
pressure exerts a positive thrust force on the second end 223 of
the shaft 202 that displaces the shaft 202 axially.
[0027] One way to balance the asymmetry pressure acting on the
shaft 202 is to offset the magnet 209 and stator 210 relative to
each other to create a magnetic bias force that biases the hub 204
downward and stabilizes the motor 200. As illustrated in FIG. 2,
the center lines of the magnet 209 and stator 210 are separate by a
vertical distance of d. This method has generally proven to be
effective; however, temperature changes in the motor may limit or
hinder the ability of the magnetic force to bias the hub 204. This
is because the viscosity of the fluid 214 varies with changes in
temperature, which means that the journal asymmetry pressure is not
constant, but rather may be a function of temperature. Therefore,
because the magnetic force can not be varied accordingly to address
and counter the changes in journal asymmetry pressure, temperature
variations will cause the shaft 202 and hub 204 to move
axially.
[0028] One solution to this problem would be to use the axial shaft
displacement to change the length of the asymmetry created by the
journal bearings 300. However, the axial displacement required to
effectively counter the pressure changes would likely be too great
to be practically incorporated. In the embodiment illustrated in
FIG. 2, journal asymmetry pressure fluctuations are countered by
changing the gap width between the shaft 202 and sleeve 208 in the
asymmetric portions of the journal bearings 300A. This is
accomplished by creating a step 260 on the journal surface 215, 219
that is opposite the asymmetric grooves 300. In FIG. 2, the step
260 is located on the inner diameter 215 of the sleeve 208,
opposite the journal bearing grooves 300 on the shaft 202. The step
260 is also offset axially from the grooves 300, so that when the
motor 200 is at rest, the gap separating the grooved portion of the
shaft 202 from the sleeve 208 is a standard width w.sub.1. Although
there is a small axial overlap of the step and grooves, the apex
304 of the grooves 300 is generally adjacent a gap of standard
width w.sub.1. Thus, as the shaft 202 moves downward, the grooves
300 move closer axially to the step 260, and the width of the gap
separating the upper portion of the grooved area (i.e., mostly the
upper leg of the groove pattern) of the shaft 202 from the sleeve
208 shrinks to a gap w.sub.2. As the gap in this region tightens,
more pressure is built up at the bottom of the shaft 202, and the
pressure pushes the shaft back up. Furthermore, this design
provides additional stiffness (pressure change vs. axial movement
of shaft) to the motor, reducing or eliminating the need for either
a thrust plate with grooves or for a tight thrust gap, which draws
constant power.
[0029] Typical fluid dynamic bearing motors have journal bearing
gaps on the order of five microns or less, and changes to the gap
must be controlled to a fraction of that number. Therefore,
processes used to create the steps 260 and must be very precise.
Steps 260 may be created either by removing material from the shaft
202 or sleeve 208 (e.g., by processes including, but not limited
to, turning, grinding, electrochemical machining, or electrical
discharge machining), or by adding material to the surfaces 202,
208 (e.g., by processes including, but not limited to, plating,
coating, or sputtering). For example, Diamond Like Coating (DLC)
may be sputtered onto the appropriate area.
[0030] FIG. 3 illustrates a second embodiment of the present
invention. The motor 400 is configured similarly to the motor 200
in FIG. 2. However, in FIG. 3, the step 460 is formed on the outer
diameter 419 of the shaft 402, rather than on the inner diameter
415 of the sleeve 408. The step 460 operates in the same manner as
the step 260 in FIG. 2, to narrow the bearing gap and thus counter
hydraulic pressure variations.
[0031] A third embodiment of the invention is illustrated in FIG.
4. The motor 500 is similar to the motors 200 and 400 illustrated
in FIGS. 2 and 3. However, the journal 517 comprises two steps
560A, 560B located across the journal from each set of asymmetry
grooves 300. Although the steps 560A, 560B are depicted as formed
on the outer diameter 519 of the shaft 502, it will be appreciated
that the steps 560A, 560B may also be formed on the inner diameter
515 of the sleeve 508, as the step 260 is located in FIG. 2.
[0032] FIG. 5 illustrates a fourth embodiment of the invention. The
motor 600 is similar to the motor 500 in FIG. 4 and uses the same
principle of a double step. However, the journal steps 660A, 660B
are made larger than in the previous embodiments so that they
interact with larger areas of the bearing grooves 300. The grooves
300 comprise two legs: L.sub.1 that pumps upward toward the apex,
and L.sub.2 that pumps downward toward the apex. When the motor 600
is at rest, a majority of each set of grooves 300 is adjacent a gap
of standard width w.sub.1, while the edges of the grooves are just
bordered by a narrower gap w.sub.2 created by the steps 660A, 660B.
However, as the shaft 602 moves downward axially, the gap narrows
over a larger portion of the grooves 300, and narrows completely
over the upper legs L.sub.2, to a width of w.sub.2. The narrower
gap over the upper legs L.sub.2 takes away the downward pumping of
these legs, and also diminishes the upward pumping of the lower
legs L.sub.1. Thus each individual set of bearing grooves 300 is
affected to a larger degree over both legs L.sub.1, L.sub.2, unlike
the previous embodiments that affected the gaps adjacent smaller
portions of the bearing grooves, and more particularly narrowed the
gaps mostly adjacent the upper legs of the grooves.
[0033] Thus the present invention represents a significant
advancement in the filed of fluid dynamic bearing motor design. A
magnetically biased fluid dynamic bearing motor is provided in
which axial movement of the shaft and hub is limited despite
temperature-induced pressure fluctuations in the journal. The
design also provides improved stiffness to the motor, reducing or
eliminating the need for thrust plate bearing grooves or tight
thrust gaps. In addition to the thermal compensation effects, the
motor doesn't need a thrust bearing as the thrust is created by the
journal asymmetry. This asymmetry is created by asymmetric bearing
grooves in the journal bearing (as described above) and/or by the
proper location of the step or steps 260 relative to the groove
pattern in the journal bearing. Positioning of the step or steps
260 alters the pressure profile in the journal bearing and thus the
pressure on the bottom of the shaft to support the shaft for
rotation over the base. In other words, when the reduced gap width
provided by the step 260 is over the grooves, the effect is
asymmetry whether or not the groove pattern itself being
asymmetric. Also, either the end surface of the shaft (e.g. 280,
FIG. 2) or the facing surface of the base 12 or counterplate 282
may be grooved to provide a quicker take-off when the motor shaft
202 spins up. The groove pattern would typically be designed to
pump toward the center of the shaft. However, even in this instance
the thrust stiffness is primarily created by journal bearing
asymmetry, established in whole or in part by the step facing the
journal groove pattern.
[0034] While the foregoing is directed to embodiments of the
invention, other and further embodiments of the invention may be
devised without departing from the basic scope thereof, and the
scope thereof is determined by the claims that follow.
* * * * *