Anti-friction bearing and a motor including such a bearing

Details Anti-friction bearing and a motor including such a bearing Anti-friction bearing and a motor including such a bearing

2001-11-28

2002-11-05

Joyce, William C (Department: 3682)

Bearings

Rotary bearing

Antifriction bearing

C384S450000, C384S513000, C384S516000, C384S504000

Reexamination Certificate

active

06474871

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an anti-frictional bearing constituting a rotationally supporting portion of a spindle motor of a hard disk drive device, VTR, and so on. The present invention further relates to a motor into which such a bearing is incorporated.
2. Description of the Prior Art
The compact hard disk device to be incorporated into a personal computer includes magnetic disk or disks driven by means of spindle motor in high rotational speed. The rotational member of such motor is adapted to be journalled through an anti-friction bearing having an inner diameter of 4-6 mm and an outer diameter of 8-15 mm.
Recently, a remarkable development or improvement is achieved in the hard disk device regarding the miniaturization and high packaging density. Especially for the hard disk device the size of which is equal to or less than 3.5 inch, the packaging density is increased rapidly. More recently, the hard disk device of the size of 2.5 inch to be incorporated into the hand held personal computer of the note book type is also demanded to have substantially the same memory capacity as that of the hard disk device of 3.5 inch in spite of its small size.
In order to enlarge the memory capacity of the hard disk device of the size of 2.5 inch, it is necessary to increase both of the track recording density and the track density. The presently demanded track density from 10 KTPI to 14 KTPI (TPI: Track Per Inch) can be satisfied by the track pitch less than 2.54 &mgr;m. This value of the track pitch corresponds with the track density of 10 KTPI.
In either hard disk device tge size if which is 3.5 inch or 2.5 inch, it is necessary to increase the number of revolutions of the magnetic disk or disk to increase the data transfer rate of the hard disk device. For example, the hard disk device of 3.5 inch requires the number of revolution from 5400 rpm to 7200 rpm, and the hard disk device of 2.5 inch requires the number of revlution from 4000 rpm to 5000 rpm.
When it is intended to read or write datum accurately into the magnetic disk of increased track density, it is necessary to reduce the rotational run out of the magnetic disk. The rotational run out is apt to increase in proportion to the number of revolutions of the magnetic disk. It is therefore important in the high packaging density of the hard disk device to improve the precision of the rotational run out of the magnetic disk. It is therefore important in the high packaging density of the hard disk device to improve the precision of the rotational run out of the magnetic disk.
In order to reduce the rotational run out of the magnetic disk, it is necessary to reduce the run out attributable to the anti-frictional bearing itself of the spindle motor for driving the magnetic disk. The counter measures having been taken for the problem of the rotational run out of the magnetic disk are to improve the sphericity of the rotational bodies of the anti-frictional bearing and to effect the high precision working on the raceway surface of the inner and/or outer races to reduce the working tolerance to the minimum.
However, a microscopic undulation formed inevitably during working or processing on the raceway surface of the inner and/or outer race will change the relative position of the inner and outer races during the operation of the bearing. This changing of the relative position will cause the rotational run out. This rotational run out can be observed as an irregular run out in which the positional relationship between the inner race and the outer race is asynchronous with the rotation of the bearing. This run out is known as an asynchronous rotational run out referred to as NRRO (non-repeatable run out).
When the asynchronous rotational run out is increased beyond the allowable maximum extent, the magnetic head for reading and/or writing date can not be moved accurately relative to the magnetic disk of high tracking density. This will cause an error in effecting the reading and/or writing datum into disk. In conclusion, the asynchronous rotational run out will fail the reliability of the hard disk device.
In other words, the asynchronous rotational run out of the anti-frictional bearing to be incorporated into the spindle motor will interfere the high packaging density and the high speed of the hard disk device, i.e. the reduction of the asynchronous rotational run out of the anti-frictional bearing to be incorporated into the spindle motor is extraordinarily important in achieving the high packaging density and the high speed of the hard disk device.
The asynchronous rotational run out is influenced by the shape of the undulation on the raceway surface of the inner and/or outer races and the number of rotational bodies interposed therebetween. In order to reduce the asynchronous rotational run out, it is necessary to measure the out of roundness of the raceway surface accurately, to wake a harmonic analysis on thus obtained value of measurement as described below, and to select the inner and/or outer races which are suitable for the number of rotating bodies to be interposed therebetween. The harmonic analysis will now be described as follows.
Each of the inner and/or outer races has a raceway surface representing a complex undulation. This undulation can be seized as a function, the frequency of which is one revolution, i.e. the function is a composite of a plurality of harmonic vibrations.
In concretely, the shape of raceway surface as shown in FIG.
19
(
a
) can be seized as a composite of a harmonic vibration of the frequency of ⅓ revolution (tertiary vibration) as shown in FIG.
19
(
b
), a harmonic vibration of the frequency of {fraction (1/7)} revolution (seventh vibration) as shown in FIG.
19
(
c
), and a harmonic vibration of the frequency of {fraction (1/50)} revolution (fifty vibration) as shown in FIG.
19
(
d
).
In this connection, the undulation presented on the raceway surface can be expressed as a following function f(t) including a plurality of frequencies &ohgr;, 2&ohgr;, 3&ohgr; . . .
f
(
t
)=
C
0
+C
1
cos (&ohgr;
t+&phgr;
1
)+
C
2
cos (2&ohgr;
t+&phgr;
2
)+
C
3
cos (3&ohgr;
t+&phgr;
3
)+ . . .
In the harmonic analysis, the constant C
0
, C
1
, C
2
, . . . , &phgr;
1
, &phgr;
2
, . . . are determined.
In the harmonic analysis effected on the shape of the raceway surface of the inner and/or outer races, the harmonic vibration of 1
revolution forms a shape of an undulation including crests the number of which is n. In this connection, the harmonic vibration of 1
revolution is referred to as the undulation including crests the number of which is n. The medial magnitude (C
1
, C
2
, . . . ) of the displacement amplitude of the shape varying in a sinusoidal manner is referred to as unilateral amplitude of each number of crests.
The shape of the raceway surface of each inner raceway, the out of roundness of which are 0.13 &mgr;m, 0.096 &mgr;m, 0.084 &mgr;m, and 0.055 &mgr;m is shown in
FIGS. 20-23
respectively in the magnification of 100,000. The results of the harmonic analysis made on each shape of the raceway surface are shown in
FIGS. 24-27
respectively.
The numbers put on the left column of each table are basic number N, the numbers to be added to the basic number N are put on the upper row of each table, and the values listed on the table are the values of unilateral amplitude.
For example, in the table of
FIG. 24
, the value put on the field of N+0 of the second row (N=7) leans that the component of vibration of {fraction (1/7)} revolution of the shape of the raceway surface as shown in
FIG. 20
is 0.002 &mgr;m, that is the unilateral amplitude in the case that the number of crests are seven is also 0.002 &mgr;m. The value put on the field of N+1 of the second row means that the unilateral amplitude in the case that the number of crests are eight is 0.006 &mgr;m, and the value put on the field of N+2 of the second row means that the unilateral amplitude in the case that the n

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