Bearings – Rotary bearing – Antifriction bearing
Reexamination Certificate
2000-04-13
2002-03-19
Hannon, Thomas R. (Department: 3682)
Bearings
Rotary bearing
Antifriction bearing
C384S907100
Reexamination Certificate
active
06357923
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ball bearing comprising a surface-coated metal ball. More particularly, the present invention relates to a bearing suitable for hard disk drive (hereinafter referred to as “HDD”) spindle motor device or the like comprising rolling elements which have a hard ceramic coating film or diamond-like carbon coating film formed on the surface thereof to exhibit an enhanced fretting resistance and prevent the unintentional release of preload due to temperature change.
2. Description of the Related Art
Despite its shorter history than other industries, the computer-related industries are making an extremely rapid technical innovation. In particular, HDD industry has introduced new techniques to make successive development of new compact models having a smaller power loss requirement, a high response and a high precision. Under these circumstances, bearing performance corresponding to these properties has been required.
Referring to bearing for HDD device, for example, a small-sized deep groove ball bearing is often used for spindle motor shown in FIG.
6
and swing arm motor shown in FIG.
7
. The ball bearing
1
for spindle motor is used to allow a cup-shaped flange
2
on which a magnetic disc (not shown) is mounted to be smoothly driven rotatively around a shaft
4
provided standing on a base
3
by a motor M. Thus, the ball bearing
1
is required to have remarkably excellent running performance and acoustic properties. The ball bearing
1
for swing arm is used to allow a swing arm
7
to be swung smoothly around a shaft
9
provided on a base
8
. The swing arm
7
allows a head
6
to be accessed and positioned on the effective area on a magnetic disc D. A preload is applied to these ball bearings
1
at room temperature to enhance the shaft supporting rigidity. However, since the motor for HDD device is required to have a reduced size, constant-pressure preload process, which requires some space, cannot be employed. Therefore, constant-position preload process is employed in which an inner race
1
n
and an outer race
1
g
of two ball bearings are fixed to shafts
4
,
9
and the inner wall of a flange
2
or sleeve
10
as a rotary body, respectively, with an adhesive while being pressured by applying a load downward.
As the material of ball bearing to be used in the foregoing HDD device, there is often used SUJ2 (JIS), which is a high carbon chromium bearing steel, SUS440C (JIS) which is a martensitic stainless steel, 0.7C—13Cr stainless steel, or the like. These steel materials are hardened and tempered to obtain desired hardness or wear resistance. Thus, steel materials, the hardness of which has HRC of 58 to 64 are used.
However, the ball bearing
1
for HDD device is subject to adverse effect on acoustic properties or vibration properties due to fretting wear developed by the microvibration of the rotary portions (flange
2
, sleeve
10
) in the rotating direction during the transportation of the device. Fretting wear takes place on the balls B in the bearing
1
. As a countermeasure against fretting abrasion, the use of ceramics as bearing ball B has begun. This is because the surface properties, hardness, mechanical strength, chemical stability and wear resistance of ball made of ceramics are better than that of ball made of steel such as bearing steel.
The ball made of ceramics has excellent surface properties but has a linear thermal expansion coefficient which is 70% smaller than that made of steel and a modulus of a longitudinal elasticity which is 50% greater than that made of steel. Thus, when the temperature rises during the use of the device, ball bearings employing constant-position preload process such as one for motor for HDD device is subject to great. change in the maximum contact stress between ball and its rolling surface such as to cause a great drop in the bearing rigidity (preload). Thus, in an extreme case; so-called release of preload, i.e., zeroing of preload during use can take place.
Referring to the possibility of release of preload, its mechanism will be studied hereinafter.
Referring to elastic deformation and stress on the contact area at which the ball comes in rolling contact with the rolling surface, Herz's theory of elastic contact can be applied. In general, as shown in
FIG. 8A
, when two objects I and II which are elastic materials having a smooth surface come in contact with each other, main planes of curvature
1
and
2
crossing each other at right angle in symmetrical planes exist in the vicinity of the contact point. As shown in
FIG. 8B
, the object I has radii r
I1
and r
I2
of main curvature in the section of main planes of curvature, respectively. The object II has radii r
II1
and r
II2
of main curvature in the section of main planes of curvature, respectively. The reciprocal of these radii r
I1
, r
I2
, r
II1
and r
II2
of main curvature (distinguished by signs + and −, which means that the curvature is convex or concave, respectively) are defined to be &rgr;
I1
, &rgr;
I2
, &rgr;
II1
and &rgr;
II2
, respectively.
Formed at the contact area is a contact ellipsoid A having two radii (major radius a and minor radius b) crossing each. other.
Supposing that when a vertical load Q is applied to the contact ellipsoid A, the maximum contact stress acting on the. center of the contact ellipsoid A is &sgr;
max
and the amount by which the elastic objects I and II displacement each other is &dgr;, &sgr;
max
and &dgr; can be given by the following equations, respectively.
&sgr;
max
=3/2&pgr;·1/&mgr;&ngr;·
3
{square root over ( )}[1/(3/2)
2
{(1−1
/m
2
I
)/
E
I
+(1−1
/m
2
II
)/
E
II
}
2
·(&Sgr;&rgr;)
2
·Q]
&dgr;=3/4·2
k/&pgr;&mgr;·
3
{square root over ( )}[2/3·{(1−1
/m
2
I
)/
E
I
+(1−1
/m
2
II
)/
E
II
}
2
&Sgr;&rgr;·Q
2
]
A further study will be made by applying the foregoing equations to the contact of ball B with the rolling surface in the outer race
1
g
and the rolling surface in the inner race
1
n in a deep groove ball bearing
1
as shown in FIG.
9
. It is supposed that the ball B is made of ceramics and the outer race
1
g
and inner race
1
n
are made of steel.
The ceramics has a modulus of longitudinal elasticity E
1
of 313.6 GPa, a Poisson's ratio m
1
of 10/2.7, a linear thermal expansion coefficient A
1
of 3.2×10
−6
/° C. (same as that of silicon nitride Si
3
N
4
) and a thermal conductivity B
1
of 10.8 W/(m·k).
The steel has a modulus of longitudinal elasticity E
II
of 207.8 GPa, a Poisson's ratio m
II
of 10/3, a linear thermal expansion coefficient A
II
of 11.8×10
−6
/° C. and a thermal conductivity B
II
of 76 W/(m·k). In the linear thermal expansion coefficient, an average of minimum and maximum are used among that of representative steel materials, i.e., martensite stainless steel (10.1×10
−6
), bearing steel SUJ2 (12.5×10
−6
), middle, low carbon steel (13.5×10
−6
), to which the invention is applied.
The maximum contact stress &sgr;
max
and the displacemnet &dgr; are represented by the following equations:
&sgr;max=210×(1/&mgr;&ngr;)
3
{&Sgr;&rgr;)
2
Q}
(1)
&dgr;=(1.13/10
3
)(2
K
/&pgr;&mgr;)
3
{&Sgr;&rgr;Q
2
} (2)
wherein &mgr;&ngr; and 2K/&pgr;&mgr; are a function of &rgr;.
Supposing that in
FIG. 9
the diameter of ball B is d, the radius of curvature of the curved surface of groove in the section of the race groove on the outer race
1
g
including the bearing axis is r
0
, the radius of curvature of the curved surface of groove in the section crossing the bearing axis at right angle is R
0
, the radius of curvature of the curved surface of groove in the section of the race groove on the inner race
1
n
including the bearing axis is r
i
, and the radius of curvature of the curved surface of groove in the section crossing the axis 1 at right angle is R
i
, the sum of main curv
Horike Shoji
Kinno Dai
Saito Tsuyoshi
Sato Chuichi
Sumita Yuichi
Hannon Thomas R.
NSK Ltd.
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