Disk centering method and apparatus for centering disks for disk drives

A method of centering a disk on the rotational axis of a motor, the motor having a hub protruding into a center hole in the disk, and the disk having an outer diameter and an inner diameter, is provided. The method includes the steps of (a) placing a contact element in contact with the outer diameter of the disk; (b) applying a damping force and a spring force to said contact element, said damping force and said spring force, urging said contact element towards said rotational axis, wherein the magnitude of said damping force is greater than the magnitude of said spring force, and said spring force is of insufficient amplitude to move said disk with respect to said hub; and (c) rotating said contact element to rotate said disk. Also provided is an apparatus for centering a disk on the rotational axis of a motor having a hub protruding into a center hole of the disk, the disk being loosely fixed to the hub and having an outer diameter and an inner diameter defining the center hole. The apparatus includes means for contacting the outer diameter of the disk, and for rotating the disk. The apparatus further includes shock absorber means, coupled to the means for contacting, for applying a first force and a second force toward the rotational axis of the motor, where the first force being constantly applied and is of insufficient amplitude to move the disk with respect to the hub, the second force urges the means for contacting in the direction of the rotational axis when the disk is rotated to generate a third force in the direction away from the rotational axis, the second force being greater than the first force and of sufficient amplitude to move the disk with respect to the hub.

CROSS REFERENCE TO RELATED APPLICATION/PATENT 
"DISK CENTERING METHOD AND APATUS FOR CENTERING DISKS FOR DISK DRIVES", 
application Ser. No. 07/315,148, filed Feb. 24, 1990, inventor John P. 
Ross, now U.S. Pat. No. 4,933,927. 
The above cited Patent is hereby incorporated by reference. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of centering a single disk or 
multiple disks for a disk drive on the hub of a disk drive motor and an 
apparatus for performing disk centering. 
2. Description of the Related Art 
Motors in disk drives usually spin the disk mounted on the motor at 
approximately 3600 rpm. The sensitivity of disk drives to vibration 
requires that the disk and motor assembly be balanced so that the motor 
assembly does not vibrate beyond a specified degree during operation of 
the disk drive. The disk and motor are usually balanced with weighted 
screws threaded into the motor or by using other weighing methods. 
FIG. 1A illustrates a disk drive motor 10 having a first type of hub 12. 
Hub 12 has a first hub portion 13.sub.1 which protrudes into the center 
hole of disk 14 and a second hub portion 13.sub.2 which protrudes through 
the center hole of disk 14 and through disk clamp 16. Disk 14 is secured 
to motor 10 by disk clamp 16 and screws 18.sub.1-2. Disk 14 has a 
ring-like shape with an outer diameter 20 and an inner diameter 22; inner 
diameter 22 defines the center hole of disk 14. 
If weighted screws are to be added for balancing, more holes than are 
necessary to hold disk 14 in place are provided in the hub 12 of motor 10 
and the weighted screws are threaded into the extra holes to balance disk 
14. In addition, screws 18, which are usually all of the same weight, may 
be replaced with screws of varying weights during the balancing process. 
As used herein, "hub" means the rotating portion of a disk drive motor, or 
a rotating spindle or shaft attached to a motor. 
The balancing procedure is conventionally performed by spinning the disk, 
detecting an out-of-balance condition, providing screw(s) of the 
appropriate weight in the appropriate hole(s) in the hub, and repeating 
the procedure until the disk is balanced. Alternatively, weight can be 
added in different manners (e.g., by placing lead tape on the hub 12 or 
disk 14). 
Several problems are associated with the weighted balancing procedure. 
First, the balancing procedure is time and labor intensive, and extremely 
difficult to automate. Second, the efforts associated with the balancing 
procedure are often wasted because of poor disk placement relative to the 
hub, particularly first hub portion 13.sub.1 which protrudes into the 
center hole in disk 14. If the disk 14 is not centered on the hub 12, a 
portion of inner diameter 22 of disk 14 is closer to first hub portion 
13.sub.1 than the remaining portions of inner diameter 22; in some cases a 
portion of inner diameter 22 may even contact first hub portion 13.sub.1. 
Further, the clearance between inner diameter 22 of disk 14 and first hub 
portion 13.sub.1 is on the order of 0.0004 to 0.008 inches, and therefore 
manually locating disk 14 to prevent contact with first hub portion 
13.sub.1 is difficult if not impossible. The problems associated with 
centering the outer diameter 20 of the disk 14 relative to the axis 11 of 
motor 10 are compounded by runout of motor 10 and non-concentricity of 
outer diameter 20 and inner diameter 22 of disk 14. 
Thermal expansion of hub 12 and/or thermal contraction of disk 14 will 
cause a portion of hub 12 and disk 14 to contact one another, if they are 
not already in contact. Contact between disk 14 and first hub portion 
13.sub.1 causes disk 14 to move relative to hub 12, placing disk 14 in an 
out-of-balance condition. This thermal expansion/contraction problem is 
enhanced by the different coefficients of thermal expansion of the disk 
material and the hub material, e.g., the aluminum-based disk and 
steel-based motor hub, and the difficulty in placing disk 14 on hub 12 
without contact between first hub portion 13.sub.1 and disk 14. 
One solution to this problem, conceived by one of the inventors of the 
present invention, is disclosed in U.S. Pat. No. 4,933,927, ("the '927 
patent") inventor John P. Ross. The method and apparatus disclosed in the 
'927 patent include placing an element in contact with outer diameter 20 
of the disk 14, moving the element towards the rotational axis of the 
motor until a portion of the inner diameter contacts hub 12, and 
simultaneously rotating disk 14 and hub 12 while moving the contact 
element away from the rotational axis of the motor until the contact 
element no longer contacts the disk. 
In the method and apparatus of the '927 patent, a drive head having a 
number of pins is lowered onto disk clamp 16, which includes a number of 
holes for receiving the drive head pins. The method and apparatus 
disclosed in the '927 patent thus cannot easily be adapted to a single 
screw disk clamp such as shown in FIG. 1B, wherein single screw disk clamp 
17 is attached to a second type of hub 15 by a single center screw 19. 
Single screw disk clamp 17 substantially reduces time and labor involved 
in attaching disk 14 to drive motor 10. 
With the method disclosed in the '927 application, there is some difficulty 
in centering disks on drives utilizing more than one disk. Generally, 
multiple disks are arranged in a stacked configuration about the same 
rotational axis of the drive motor. In such a configuration, the disks are 
spaced apart by spacing elements that are themselves in contact with the 
disk. In such drives, difficulty has arisen in centering a number of disks 
at the same time because movement of any one of the disks causes the 
spacer element to move, thereby shifting the position of one or more of 
the other disks. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention, to balance a disk for 
a disk drive by centering the disk with respect to the rotational axis of 
a motor. 
A further object of the present invention is to automate the disk centering 
and/or balancing processes. 
Another object of the present invention is to provide a method of centering 
a disk with respect to the rotational axis of a motor so that the outer 
diameter of the disk has a runout which is less than a specified value. 
Another object of the present invention is to provide a disk balancing 
process which does not require the addition of weight to the motor and 
disk assembly. 
A further object of the present invention is to provide a disk centering 
process that overcomes the deficiencies of previous disk centering methods 
with respect to centering multiple disks about a single axis. 
A further object of the present invention is to provide a disk centering 
method that operates independent of the pre-centering position of the disk 
in relation to the rotational axis of the motor. 
A further object of the present invention is to provide a disk centering 
method which provides greater accuracy than previous disk centering 
methods. 
A method, in accordance with the present invention, of centering a disk on 
the rotational axis of a motor, the motor having a hub protruding into a 
center hole in the disk, the disk having an outer diameter and an inner 
diameter, includes the steps of (a) placing a contact element in contact 
with the outer diameter of the disk; (b) applying a damping force and a 
spring force to said contact element, said damping force and said spring 
force, urging said contact element towards said rotational axis, wherein 
the magnitude of said damping force is greater than the magnitude of said 
spring force, and said spring force is of insufficient amplitude to move 
said disk with respect to said hub; and (c) rotating said contact element 
to rotate said disk. 
An apparatus for centering a disk on the rotational axis of a motor having 
a hub protruding into a center hole of the disk, the disk being loosely 
fixed to the hub and having an outer diameter and ar inner diameter 
defining the center hole, comprises means for contacting the outer 
diameter of the disk, including means for rotating said disk, and shook 
absorber means, coupled to said means for contacting, for applying a first 
force and a second force toward the rotational axis of the motor, said 
first force being constantly applied and being of insufficient amplitude 
to move said disk with respect to said hub, said second force urging said 
means for contacting in the direction of said rotational axis when said 
disk is rotated to generate a third force in the direction away from said 
rotational axis, said second force being greater than said first force and 
of sufficient amplitude to move said disk with respect to said hub.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
The disk centering method and apparatus for centering disks for disk drives 
in accordance with the present invention will be described with reference 
to FIGS. 1-8. 
The purposes of the method of the present invention include centering the 
outer diameter 20 of disk 14 on axis 11 of motor 10, allowing for space 
between the hole in disk 14 and hub 12, (particularly first hub portion 
13.sub.1) and/or balancing the combined structure of motor 10 and disk 14. 
The centering operation of the present invention is dependent on the 
trueness of outer diameter 22 of disk 14, and the balancing operation of 
the present invention is dependent on the trueness of outer diameter 22 
and the balance of motor 10. The disk centering method, as shown in FIGS. 
2A-B, is performed in the following manner. The disk centering method will 
be described with reference to hub 12 and disk clamp 16, as shown in FIG. 
1A, however, it should be understood that the method works equally well 
with hub 15 and disk clamp 17, as shown in FIG. 1B. Disk 14 is mounted on 
hub 12 with a disk clamp 16, as shown in FIG. 1A. Then, disk 14 is loosely 
fixed to motor 10. As used herein, "loosely fixed" means that disk 14 may 
be moved relative to hub 12 by the application of a small force, but is 
sufficiently secured so that the disk 14 will not move under normal 
circumstances, for example, during rotation of the disk 14 and hub 12. A 
contact element 30 is brought into contact with the outer diameter 20 of 
disk 14. In FIG. 2A, the outer diameter 20 of disk 14 is closest to first 
hub portion 13.sub.1 at the point where outer diameter 22 and contact 
element 30 are in contact, e.g., the point which has the shortest 
effective radius between rotational axis 11 and outer diameter 20. 
However, contact element 30 may be brought into contact with outer 
diameter 20 at any point, having any radius relative to first hub portion 
13.sub.1. As will be understood from the following discussion, the nature 
of forces F.sub.1 and F.sub.2 will ensure that contact element 30 begins 
the centering process when disk 14 is in the position shown in FIG. 2A. 
Contact element 30 is driven to rotate, for example, in a counterclockwise 
direction when in contact with disk 14, thereby driving disk 14 into 
rotation about rotational axis 11 in a clockwise direction. Force F.sub.1 
is a spring force having insufficient magnitude to move the disk with 
respect to the hub portion 13.sub.1, but of sufficient magnitude to 
maintain contact element 30 in engagement with disk 14 Force F.sub.2 is a 
damping force which acts on contact element 30 only when disk 14 provides 
force F.sub.3 against contact element 30. Force F.sub.3 is provided when 
those portions of outer diameter 20 with a larger effective radius with 
respect to rotational axis 11 than the radius defined by the point where 
contact element 30 contacts disk 14 and rotational axis 11 are rotated 
towards contact element 30. Force F.sub.3 is related to the rotational 
velocity of disk 14 and the eccentricity of the disk with respect to 
rotational axis 11. 
Force F.sub.2 is of variable magnitude and responsive to force F.sub.3. The 
magnitude of force F.sub.2 is sufficient to provide a damping force in the 
direction of the rotational axis, such that when outer diameter 20 is 
rotated past the contact element 30, force F.sub.2 causes disk 14 to move 
toward rotational axis 11 to center the disk. 
As shown in FIG. 3, the magnitude of force F.sub.2 is proportional to the 
square of the velocity of contact element 30 in the direction away from 
rotational axis 11. As those regions of outer diameter 20 which are the 
greatest distance from rotational axis 11 rotate towards contact element 
30, the velocity of the movement of contact element 30 away from the 
rotational axis 11 will increase, thereby providing greater damping force 
F.sub.2 to move disk 14 towards rotational axis 11. The greater the 
magnitude of F.sub.2, the more the pre-load force (disk slip force) 
provided by disk clamp 16,17 is overcome, and disk 14 is shifted into a 
centered position about rotational axis 11. Rotation of disk 14 and 
interaction of forces F.sub.2 and F.sub.3 continues until such time as an 
equilibrium between damping force F.sub.2 and outward force F.sub.3 is 
reached as shown in FIG. 2B. It can be seen that force F.sub.1 is minimal 
with respect to force F.sub.2. Eventually, the magnitude of damping force 
F.sub.2 and outward force F.sub.3 will equal zero. At this point, disk 14 
will be centered about rotational axis 11. Utilizing this method, the disk 
14 can be rotated for a relatively infinite amount of time to achieve the 
centering of disk 14 with respect to rotational axis 11. 
If inner diameter 22 is substantially concentric with outer diameter 20, 
the entire circumference of inner diameter 22 of disk 14 will be equally 
spaced from first hub portion 13.sub.1. Providing a space between inner 
diameter 22 and hub 12 avoids the thermal expansion/contraction problem. 
The disk 14 is rotated at rotational speeds varying from approximately 
1,500 to approximately 2,500 rpm; however, smaller and larger numbers of 
revolutions per minute may be utilized, provided that the relationships of 
disk clamp force, and the forces acting on contact element 30, are 
adjusted accordingly. The inventors of the present invention have 
determined, through trial and error, that the optimal velocities utilized 
in the method are as follows: approximately 2,400 rpm for a 31/2" form 
factor disk drive, and approximately 1600 rpm for a 22" form factor disk 
drive. 
The inventors have determined that a disk for a 31/2 inch form factor disk 
drive can be out-of-balance if the center of gravity is approximately 
0.00025 inches or greater off center with respect to axis 11. The present 
method can relieve centering of the disk about the rotational axis 11 
within 0.00025 inches. FIG. 4 is a top level diagram of a first apparatus 
of performing the method of the present invention. Motor 10 is mounted on 
disk drive base 28 prior to performing the centering/balancing operation 
so as to allow base 28 to be used to support motor 10. 
FIG. 4 shows the contact element 30 rotatably mounted on a support arm 32 
which includes a means for driving contact element 30 to spin contact 
element 30 when in contact with disk 14, thereby rotating disk 14. Forces 
F.sub.1 and F.sub.2 are provided by shook absorber 35, which includes 
piston 36, spring 37, one-way check valve 38, and hydraulic valve 39. 
Shook absorber 35 acts as a biased hydraulic dampener to provide the 
spring force F.sub.1 and the hydraulic force F.sub.2 having a minimum 
ratio of approximately 0.5:99.5, based on a maximum damping force F.sub.2. 
This ratio will vary with piston speed as set out in FIG. 3. 
Shock absorber 35 includes a cylindrical housing 31 divided into two 
hydraulic chambers 33 and 34, containing a hydraulic fluid such as oil. 
During movement of piston rod 36 under force F.sub.3, one-way check valve 
38 inhibits fluid flow and, as fluid is forced thorough hydraulic valve 
39, heat is generated and a pressure drop ensues, thereby providing 
damping force F.sub.2. When force F.sub.3 is removed, spring 37 provides 
force F.sub.1 as one way check valve 38 unseats and allows fluid flow 
between chambers 33 and 34. 
In one embodiment of the invention, force F.sub.2 provided by shock 
absorber 35 is adjustable to provide varying degrees of magnitude for 
resistive force F.sub.2 with respect to movement of disk 14 and outward 
force F.sub.3. (See FIGS. 6-9). 
Also shown in FIG. 4 is retraction cylinder 40 which may be utilized to 
force support arm 20 to a position where contact element 30 is free of 
disk 14. 
FIG. 5 shows a second apparatus for performing the method of the present 
invention. FIG. 5 shows an apparatus which utilizes eccentric element 40 
in place of shock absorber means 35 of the apparatus shown in FIG. 3. 
Spring 44 is utilized to provide force F.sub.1 and eccentric element 40, 
rotating about axis 43, provides force F.sub.2 to maintain contact element 
30 abutting disk 14. Again, force F.sub.1 provided by spring 44 provides 
contact between the contact element 30 and the disk 14; however, the 
magnitude of force F.sub.1 is not great enough to shift disk 14 in 
relation to rotational axis 11. Force F.sub.2, provided by eccentric 
element 40, may be implemented by rotation of eccentric element 40 by a 
synchronized motor. 
The preferred embodiment of the apparatus for centering disks in accordance 
with the above-described method is illustrated in FIGS. 6-9. The preferred 
apparatus includes a base table 50 having mounted thereon the disk 
mounting assembly 60, pivot arm assembly 70 including contact element 30, 
shock absorber assembly 80, and clamp hold-down assembly 90. 
Disk mounting assembly 60 is mounted to base table 50. Disk drive base 28 
for mounting disk 14 slides into the apparatus on rails 42.sub.1-2. Guide 
blocks 91.sub.1 and 91.sub.2 ensure positioning of drive base 28 along the 
x-axis, and dowel pin 97 acts as a ensures position stop along the y-axis. 
Blocks 91.sub.1 and 91.sub.2 are mounted to air cylinders 62.sub.1 and 
62.sub.2. Block 91.sub.1 includes a lip 44, to secure disk 14 and base 28 
securely in place. Clearly, many different structures may be utilized to 
secure base 28 to the disk centering apparatus. 
Clamp hold-down assembly 90 is mounted to vertical slider rods 97.sub.1 and 
97.sub.2. Two shock absorbers 92.sub.1 and 92.sub.2, such as Model TK 21-3 
manufactured by Endyne, Inc., 7 Center Drive, Orchard Park, N.Y., 14127 
are secured in place by jam nuts 93 and act to cushion engagement of 
O-ring 95 to disk clamp 16,17. Clamp hold-down assembly 90 includes a 
clamp head 94 linearly mounted on arm 93 by O-ring 95, mounts to head 94 
and engages disk clamp 16,17. This allows the clamp hold-down assembly 90 
to engage disk 14, and specifically, the disk clamp (16,17), without 
damaging the disk. Arm 93 includes hole 96 which, when the apparatus is 
used with single screw disk clamp 17, allows tightening of mounting screw 
19 before disk 14 is removed from the apparatus. Clamp hold-down assembly 
90, mounted on posts 97.sub.1 and 97.sub.2, may be manually raised and 
lowered along the z-axis when mounting and unmounting drive base 28, or 
may be mounted on pneumatic slides 97.sub.1 and 97.sub.2, which may be 
selected to raise or lower assembly 90 to seat on disk 14. Disk 14 is 
loosely fixed to hub 12 by the downward force of lightly tightening the 
clamp hold-down screws (18.sub.1, 18.sub.2, 19) on the disk clamp (16,17). 
A pre-load force of approximately 3-4 lbs. (in the case of a 3.5" disk) is 
provided by the lightly tightened screws and this force is sufficient to 
create enough friction so that disk 14 will rotate with hub 12 while 
allowing disk 14 to move laterally during the centering/balancing process. 
This force pre-load also holds disk 14 in place while center screw 19 is 
tightened to securely fix disk 14 to hub 12, as discussed above. 
Also included on base 56 is a dial indicator assembly 55 for monitoring 
alignment of disk 14. 
Pivot arm assembly 70 is rotatably mounted about pivot point 71 to allow 
contact element 30 to be brought into contact with disk 14. Pivot arm 
assembly 70 is coupled to shock absorber assembly 80 as, for example, by 
bench head screws coupling flange bellow 82, which secures the shock 
absorber bellow 84 to the casing of the pivot arm assembly 70 and bracket 
83. Bracket 83 is further coupled to air cylinder 89, secured to mounting 
88, to retract pivot arm assembly 70 and contact element 30 during drive 
installation and removal. A shook absorber 85 is secured to mounting 88 by 
jam nut 86. Shock absorber 85 comprises, for example, Model OEM 0.25 
manufactured by Endyne, Inc. Shock absorber 85 provides a coil spring 
force of approximately 0.8 pounds extended and 1.7 pounds compressed 
(F.sub.1) In addition, shock absorber 85 provides a maximum energy of 30 
in-lbs. per cycle, and a maximum of 125 lbs. of shock force. 
As shown in FIG. 8, pivot arm assembly 70 includes means for propelling 
contact element 30 to rotate disk 14 when contact element 30 is in contact 
with disk 14. Motor assembly 72 provides rotational drive to shaft 73, 
which is coupled to first pully 74. First pully 73 is interconnected to 
second pully 75 by belt 76. Second pully 75 is coupled to shaft 77, which 
is surrounded by bearings 81.sub.1, and 81.sub.2 and secured to mounting 
structure 78 by jam nut 79. 
Pivot arm assembly 70 is affixed to base 50 by assembly mounting 65, which 
includes bearings 66.sub.1 and 66.sub.2, surrounding pivot arm assembly 
shaft 68, seated in assembly mounting 65. A spacer 67 is provided between 
bearings 66.sub.1, and 66.sub.2. 
Contact element 30 is precisely machined from a material which is hard 
enough to be machined and to retain its shape, but soft enough so that it 
does not damage disk 14. The material for contact element 30 may be, for 
example, Delrin. A precision bearing 94 is used to mount contact element 
30 to shaft portion 90 of eccentric 52. Other low-friction, non-abrading 
elements may be used to contact outer diameter 20. In operation, disk 14 
is loosely secured to base 28 and is mounted on the centering apparatus. 
As noted above, the preload force on disk 14 is provided by the lightly 
tightened disk clamp screw(s) (18.sub.1, 18.sub.2, 19). Contact element 30 
is brought into contact with disk 14 and rotated to spin disk 14 to begin 
the centering process. The process proceeds, as discussed above, for 
approximately 3-5 seconds. Pivot arm assembly 70 is then retracted to move 
contact element 30 away from disk 14 to allow disk 14 to stop. Clamp hold 
down assembly 90 is then lowered onto disk 14 to secure disk 14 in 
position while clamp hold down screws 18.sub.1, 18.sub.2 , 19 are 
tightened to secure disk 14 on base 28. 
One major advantage of using the present method is that there is no need to 
accurately locate the axis of the disk drive and the contact element 
because disk 14 is propelled by providing a rotational drive to contact 
element 30. This feature has the advantage over the prior art in that the 
disk drive need not be precisely mounted below a drive element as was the 
case in the prior art. Further, there is no need to precisely align the 
drive head of the centering apparatus with the disk clamp. Still further, 
the accuracy of the method and apparatus is not dependent on the aoouracy 
of the drive motor. In addition, the use of a number of contact elements 
and a number of shock absorbers may be utilized to center a drive having a 
plurality of disks. 
These and other features and advantages of the centering/balancing method 
and the apparatus for performing same in accordance with the present 
invention will be apparent to those of ordinary skill in the art from the 
foregoing description and the drawings. Further, the following claims are 
intended to cover all modifications and equivalents falling within the 
scope of the invention.