Pivot bearing assembly providing damping for unit-to-unit consistency

A pivot bearing assembly for mounting to an enclosure of a head disk assembly to support a head stack assembly of a disk drive is disclosed. The pivot bearing assembly comprises a shaft for being affixed to the enclosure and defining a longitudinal axis, a first and second set of bearings, the second set of bearings longitudinally spaced from the first set of bearings, and each set of bearings surrounding a respective portion of the shaft. The pivot bearing assembly also comprises an inner sleeve member and an outer sleeve member, both sleeve members surrounding the shaft. A pair of channels is located between the outer and inner sleeve members. Alternatively, the pair of channels is located between the shaft and the sets of bearings. The channels contain a cured cast-in-place material, preferably, an ultra violet cured polyurethane compound having a hardness between approximately 24 Shore A to 79 Shore A. The material provides damping of any vibratory wave propagating between an outer surface of the outer sleeve member and the shaft. Furthermore, a portion of the cured cast-in-place material defines an exterior surface of the pivot bearing assembly.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to hard disk drives. More specifically, it relates 
to a pivot bearing assembly for incorporation into a head disk assembly of 
such a disk drive. 
2. Description of the Prior Art and Related Information 
In a contemporary hard disk drive, a pivot bearing assembly provides for 
rotatably supporting a head stack assembly within a head disk assembly so 
that each transducer head incorporated within the head stack assembly can 
be swung into a desired position relative to a respective recording 
surface of a disk. The head stack assembly typically comprises an actuator 
body portion surrounding a bore opening, a voice coil motor portion, and a 
set of head gimbal assemblies each carrying at least one transducer head 
for reading and writing on such a recording surface. The actuator body 
portion of the head stack assembly is typically attached to the pivot 
bearing assembly by inserting the pivot bearing assembly into the bore of 
the actuator body portion. 
A conventional pivot bearing assembly comprises a shaft that is fixed to 
the base of the enclosure for the head disk assembly. The conventional 
pivot bearing assembly also includes two sets of bearings, and an outer 
sleeve. Each set of bearings typically has an inner race, an outer race, 
and ball bearings between the inner and outer races. Typically, an inner 
surface of each inner race abuts a portion of an outer cylindrical surface 
of the shaft. Typically, an outer surface of each outer race abuts a 
portion of an inner cylindrical surface of the outer sleeve. As installed 
within the head disk assembly, the outer surface of the cylindrical sleeve 
abuts the interior cylindrical surface of the bore of the actuator body. 
As a result, the actuator body portion is able to rotate about the fixed 
shaft of the pivot bearing assembly. 
A contemporary disk drive needs to meet exacting standards with respect to 
the speed with which data can be accessed. These exacting standards 
involve high speed seek operations during which the head stack assembly 
receives high torque for angular acceleration to depart from a starting 
track and ramp up to a high angular velocity, then receives high torque 
for angular deceleration to ramp down the angular velocity and bring the 
active transducer head to a stable position at a target track for track 
following. 
During such high-speed seek operations, various structures involved in 
supporting the transducer heads can vibrate in a manner characterized by 
at least one resonant frequency. So long as the active transducer head is 
vibrating by a sufficient amount after the deceleration torque is removed, 
the drive cannot begin writing to or reading from the target track. Also, 
the vibrations propagate from the head stack assembly through the pivot 
bearing assembly and into a base plate and cover causing increased 
acoustical noise. 
One source of such vibrations involves the ball bearings within the pivot 
bearing assembly. The ball bearings are characterized by a finite radial 
stiffness. The existence of such finite radial stiffness, and particularly 
the tolerances associated with it (both initial tolerances and variations 
resulting from wear of the bearings), pose problems in disk drive design. 
U.S. Pat. No. 5,491,598 to Stricklin et al. (the '598 patent) discloses one 
prior art approach to addressing this problem. The '598 patent teaches a 
pivot bearing assembly that incorporates a so-called "tuned mass damper" 
designed to provide damping at a particular frequency determined by the 
amount of mass of a ring portion of the tuned mass damper and by the 
radial stiffness of a supporting material portion of the tuned mass 
damper. The '598 patent does not address the issue of tolerance on the 
frequency at which an adverse resonance occurs, either initially or after 
bearing wear occurs. 
The assignee of this invention has developed disk drives that incorporate 
an invention disclosed and claimed in application Ser. No. 08/561,344, 
filed Nov. 21, 1995, and titled "Disk Drive Having Elastomeric Interface 
In Pivot Bearing Assembly" Docket No. K35A0205!, which is directed to the 
same problem. 
FIG. 1 illustrates the prior art structure involved in the above-identified 
invention of the assignee. As shown in FIG. 1, O-rings 100 and 102 are 
compressed between portions of a pivot bearing assembly 110. A fixed shaft 
104 has a cap portion 106 and an inner sleeve 108 disposed thereon. O-ring 
100 is compressed between cap portion 106 and inner sleeve 108. Similarly, 
O-ring 102 is compressed between inner sleeve 102 and fixed shaft 104. 
However, the damping provided by the O rings is tolerance dependent and 
hence, is not consistent among a common lot of O rings. Therefore, when 
slight manufacturing differences exist among a common lot of O rings, the 
damping provided by the O rings changes since the O rings compress or 
stiffen as a function of the manufacturing differences. For example, the 
compression is greater when the opening where the O rings fit is smaller. 
Thus, a need exists to provide unit-to-unit consistency to dampen resonant 
vibrations induced in operation of a hard disk drive. 
SUMMARY OF THE INVENTION 
This invention can be regarded as a pivot bearing assembly for mounting to 
an enclosure of a head disk assembly to support a head stack assembly of a 
disk drive. The pivot bearing assembly comprises a shaft for being affixed 
to the enclosure and defining a longitudinal axis, a first and second set 
of bearings, the second set of bearings longitudinally spaced from the 
first set of bearings, and each set of bearings surrounding a respective 
portion of the shaft. The pivot bearing assembly also comprises a sleeve 
means defining an inner sleeve means surface and an outer sleeve means 
surface. The inner sleeve means surface has longitudinally spaced-apart 
cylindrical portions abutting the first and second sets of bearings. The 
outer sleeve means surface is cylindrical to mate with a portion of the 
head stack assembly. The pivot bearing assembly also comprises means 
defining a pair of generally ring-shaped, longitudinally spaced-apart 
channels where cured cast-in-place material occupies each of the channels 
to provide damping of any vibratory wave propagating between the outer 
sleeve means surface and the shaft. Furthermore, a portion of the cured 
cast-in-place material defines an exterior surface of the pivot bearing 
assembly. 
In an embodiment, each channel is located between the inner sleeve member 
and the outer sleeve member. In another embodiment, each set of bearings 
has an inner race surface abutting a respective portion of each channel 
and each inner surface of each channel abuts a respective portion of the 
shaft such that each channel is located between each respective set of 
bearings and the shaft.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 2, a pivot bearing assembly 200 includes a shaft 202 having a 
longitudinal axis 214, a first set of bearings 203, a second set of 
bearings 205, an inner sleeve member 204 and an outer sleeve member 206. 
It also includes a pair of generally ringed-shaped channels 210a and 210b 
located between inner sleeve member 204 and outer sleeve member 206. The 
term "ringed-shaped" refers to the shape of the channel when viewing the 
top of generally cylindrical shaped pivot bearing assembly 200. Channels 
210a and 210b are formed by cutting into outer sleeve member 206. 
Alternatively, the channels can be formed by cutting into inner sleeve 
member 204. Channels 210a and 210b contain cured cast-in-place material 
208, preferably an ultraviolet cured polyurethane compound which has a 
hardness approximately between 24 Shore A to 79 Shore A. 
Material 208 damps any vibratory wave propagating between surface 218 of 
outer sleeve member 206 and shaft 202. An annular clearance region or air 
gap 212 extends longitudinally between channels 210a and 210bd and serves 
to separate inner 204 and outer 206 sleeve members. An inner race surface 
222 of first 203 and second 205 sets of bearings abuts shaft 202. Each 
bearing set abuts a surface 216 of inner sleeve member 204, and a surface 
218 of outer sleeve member 206 mates with a portion of a head stack 
assembly, see FIG. 5. A portion 220 of cured cast-in-place material 208 
defines an exterior surface of pivot bearing assembly 200. 
Still referring to FIG. 2, material 208 is a viscous gel-like material 
until it is exposed to ultraviolet light. It is cured by exposing material 
208 to a high intensity ultraviolet light. After curing, material 208 
becomes an elastic solid having a predetermined hardness, preferably 
approximately between 24 Shore A and 79 Shore A. Material 208 is available 
from Nippon Zeon Co., Ltd., of Tokyo, Japan. When material 208 cures, it 
adheres to both outer sleeve member 206 and inner sleeve member 204. Due 
to the elastic nature of cured material 208, outer sleeve member 206 is 
elastically bonded to inner sleeve member 204. Also, material 208 is 
generally too viscous to leak into clearance region 212, the dimensions of 
which are better shown in FIG. 3. 
Pivot bearing assembly 200 can be tuned to damp vibrations that occur at 
specific resonant frequencies within a disk drive. The tuning is 
controlled by varying the size of channels 210a and 210b cut into outer 
sleeve member 206. Hence, the amount of material 208 which occupies 
channels 210a and 210b also varies. Alternatively, the tuning is 
controlled by varying the hardness of material 208 by choosing different 
polyurethane compounds having a different hardness. 
In FIG. 3, an enlarged portion 226 of FIG. 2 is shown. Clearance region 212 
is very small (approximately 10 mils wide) compared to inner sleeve member 
204, outer sleeve member 206, and channel 210a. Hence, material 208 
generally does not leak into clearance region 212 due to its viscosity. 
In FIG. 4, pivot bearing assembly 504 includes a shaft 506 having a 
longitudinal axis 518, a pair of generally ring-shaped channels 510a and 
510b, a first set of bearings 508, a second set of bearings 509, and a 
sleeve means including a sleeve member 512. Channels 510a and 510b are 
located between shaft 506 and a respective bearing set such that an inner 
race surface 500 of each bearing set abuts a respective portion of each 
channel, and an inner surface 502 of each channel abuts a respective 
portion of shaft 506. The same cast-in-place material 208 as shown in FIG. 
2 is deposited into the channels and cured such that a portion 522 of the 
material defines an exterior surface of pivot bearing assembly 504. The 
cured material is then post processed, e.g., trimmed of excess material, 
to provide a desired shape. Pivot bearing assembly 504 is tuned in the 
same manner as pivot bearing assembly 200. 
In FIG. 5, a disk drive 10 includes a head disk assembly 400 and a printed 
circuit board assembly 14. Head disk assembly 400 includes an enclosure 
having a base 401 and a cover 402. It also includes disks 404 mounted on a 
spindle motor 406, and a head stack assembly 408. Head stack assembly 408 
includes a bore 412 and a plurality of actuator arms 410. A pivot bearing 
assembly 414 of this invention is affixed to base 401 and is fitted into 
bore 412 of head stack assembly 408. Cover 402 is affixed to pivot bearing 
assembly 414 to form the enclosure. 
An advantage of this invention is that the damping of the resonant 
frequencies is not tolerance dependent. Hence, unit to unit consistency to 
dampen the resonant frequencies induced in a disk drive is achieved. 
Moreover, the pivot bearing assembly is tuned to damp specific resonant 
frequencies that characterize a particular line of disk drives.