Bearings – Rotary bearing – Antifriction bearing
Reexamination Certificate
2000-10-31
2002-06-25
Hannon, Thomas R. (Department: 3682)
Bearings
Rotary bearing
Antifriction bearing
C384S537000
Reexamination Certificate
active
06409390
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to compact duplex bearing mounts, particularly for those bearings which must operate with high precision in high vibration environments.
Duplex bearings are well known in the art and may be generally described as having concentric annular inner and outer races which rotate relative to each other, and an annular arrangement of load-supporting rotating elements, such as balls or rollers, located between the races. High precision applications for duplex bearings are often subjected to vibratory conditions which are severe. For example, duplex bearings must operate with high precision during, and after, the launching of a spacecraft, the vibration caused by an airplane propeller, or the operation of a vehicle on a rough road. The vibration loading on the bearing or its mounts can also adversely affect the bearing running characteristics, resulting in jitter. Further, these loads can also affect the individual components comprising the system supported by the bearing, resulting in deformations that change the modal properties or natural frequencies of the system. These changes in natural frequencies, which are indicative of a change in the loading on the bearing, can result in the generation of objectionable, audible sounds by the bearing-supported system, or vibrations that can adversely affect the life of the bearing, or other systems near the bearing.
For high precision rotation applications, such as those requiring very low rotational jitter (on the order of 10 microseconds of rotational period variation per revolution), the preload force applied to the bearing is critical. Variation in preload may result from yielding of the bearing retaining system and will affect the running torque characteristics of the bearing, which can increase jitter and adversely affect the natural frequencies of the system supported by the bearing.
A previous duplex ball bearing mount for a rotary device is shown in the rotary device depicted in
FIG. 1A
, which is an electric motor assembly. This mount is very susceptible to variations in the tolerances of the individual parts and tolerances in the preload applied through torquing of the bolts. These variations resulted in variations in jitter and natural frequency as the motor was subjected to vibration and changes in its thermal environment.
Motor assembly
30
includes first and second housing portions
32
and
34
, respectively, located on and abutting the opposite axial ends of stator
36
. Bolts
37
secure the housing portions and the stator together. Rotor
38
is disposed within stator
36
, and has an axis of rotation which is colinear with bore
40
extending through the rotor. Shaft
42
is interference fitted within bore
40
, rotatably fixing it to the rotor. End
44
of the shaft extends through first housing portion
32
, and is operatively coupled to a device (not shown) driven by the motor. Referring to
FIG. 1B
, shaft
40
is provided with shoulder
46
between portion
48
, which is fitted into the rotor bore, and portion
50
, which is supported by duplex bearing
52
. Referring again to
FIG. 1A
, at the end of shaft
40
which is opposite end
44
, portion
54
is supported by duplex bearing
56
, which is identical to duplex bearing
52
. Shoulder
58
is located between shaft portions
48
and
54
. Shoulders
46
and
58
respectively abut inner race
60
of bearings
52
and
56
, through which shaft portions
50
and
54
extend. Outer race
62
of bearing
56
is received in counterbore
64
of second housing portion
34
, and abuts annular face
66
thereof, which forms a bearing retainer.
Outer race
62
. of bearing
52
is received in counterbore
68
of first housing portion
32
. Disposed between annular face
70
of counterbore
68
and outer race
62
of bearing
52
is wave spring
72
, shown in greater detail in FIG.
1
C. Spring
72
has free height H
1
and variable, loaded height H
2
. As bolts
37
are tightened, spring
72
is compressed to its loaded height H
2
and urges shaft
42
leftward as viewed, through bearing
52
and shoulder
46
, and clamps the bearings axially. This bearing mount device is rather sensitive to the amount of torque exerted on bolts
37
, therefore bolts
37
must be precisely torqued. The bearing mount device, through deflection of spring
72
, preloads the duplex bearing axially, in the direction of the shaft axis of rotation. As the bearing is clamped, spring
72
and, to a lesser extent, the other components of the retaining system, will deflect.
Those skilled in the art will recognize that in the type of duplex ball bearing depicted, an axially directed force is transferred between inner and outer races
60
,
62
though interfacing, radially offset shoulders located between the inner and outer races. Balls
74
, which roll on these shoulders, support both the axial and radial loads exerted on the bearings. Each race of a bearing may be provided with only one annular shoulder, as shown in
FIGS. 1A and 1B
. Alternatively, one of the inner or outer bearing races may be formed with a circumferential groove forming a pair of shoulders; during assembly of the bearing, the balls are first disposed in the circumferential groove provided in the inner or outer race, and the other race, having a single shoulder, is then positioned so as to capture the balls between the two races. Notably, each duplex bearing must be oriented and assembled into its mount such that axial forces exerted on one of its inner and outer races will be directed through its shoulder and the balls, to the shoulder of the other bearing race; otherwise the bearing may come apart.
As the bearing is clamped, spring
72
will deflect, applying a load to outer race
62
of bearing
52
that is proportional to the deflection. Thus, changes in the deflection of the spring will result in changes in the load on the bearing.
FIG. 2
shows the substantially linear relationship between spring deflection and bearing load. The clamping torque, which can be measured as the torque to rotate the inner bearing races relative to their associated outer bearing races, is shown on the abscissa of FIG.
2
and is analogous to the deflection of the spring. Thus, increased torque results in an increased load on the bearing.
The Smalley Steel Ring Company produces wave springs of the type used in motor assembly
30
; disc springs and finger disc springs may also used. These spring designs, however, have a load variation on the order of +/−25%. This is not acceptable for high precision mechanisms, which require stable performance, especially over a long period of time under severe environment conditions, as required of space flight mechanisms such as, for example, an Advanced Very High Resolution Radiometer (AVHRR) (not shown), the primary imaging instrument on certain polar orbiting meteorological satellites. In such applications, an open loop scanner is positioned by a motor having a shaft supported by duplex bearings.
FIGS. 4-7
show a number of other previous spring-based concepts by which various configurations of shaft
42
′ is supported by a plurality of duplex ball bearings
52
,
52
′,
56
, and/or
56
′. Each of these concepts employ coil springs
76
, which are more linear in their force versus deflection characteristics vis-a-vis wave spring or disc springs. Coil springs, however, require much more axial space. If coil springs
76
were eliminated from the designs of
FIGS. 3-7
, however, it would be impossible to clamp the bearings without changing the effective preload. Further, if coil springs
76
were not present, the load applied when inner and outer bearing retainers
78
,
80
are clamped would result in deflection of other parts of the system, such as bearing races
60
,
62
, any bearing spacers
82
, or the bearing retainers themselves.
Notably, too, if the resulting stress from the applied clamping load were greater in outer race
62
than in inner race
60
, or vice versa, the resulting strain would cause a change in the effe
Bouzakis George Elias
Bowman James Edward
Devine Edward J.
Joffe Benjamin
Segal Kenneth Neal
Baker & Daniels
Hannon Thomas R.
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