Bearing system

A bearing system includes backup bearings for supporting a rotating shaft upon failure of primary bearings. In the preferred embodiment, the backup bearings are rolling element bearings having their rolling elements disposed out of contact with their associated respective inner races during normal functioning of the primary bearings. Displacement detection sensors are provided for detecting displacement of the shaft upon failure of the primary bearings. Upon detection of the failure of the primary bearings, the rolling elements and inner races of the backup bearings are brought into mutual contact by axial displacement of the shaft.

BACKGROUND OF THE INVENTION 
The U.S. Government has rights in this invention pursuant to Contract No. 
DE-ATO3-84SF11963. 
The invention relates generally to bearing systems and more particularly to 
a bearing system wherein a shaft is supported by primary bearings under 
normal conditions and is supported by backup or secondary bearings upon 
failure of the primary bearings. 
Although not limited to any particular application, the invention will be 
described in connection with a motor driven turbine. In such applications, 
the detection of failure of the primary bearings and the automatic 
shifting of the loads to the backup bearings is justified from cost and 
safety standpoints. 
Bearing assemblies wherein backup or secondary bearings are provided to 
support loads upon failure of primary bearings are shown and described in 
U.S. Pat. No. 3,708,215; U.S. Pat. No. 3,454,309; and U.S. Pat. No. 
4,425,010. Means for detecting shaft displacement due to bearing wear are 
disclosed in U.S. Pat. No. 4,434,448. 
SUMMARY OF THE INVENTION 
It is a general object of the present invention to provide an improved 
bearing system wherein backup or secondary bearings support a rotating 
shaft upon failure of one or more primary bearings. 
It is a more particular object of the present invention to provide a 
bearing system wherein forced axial displacement of a rotating shaft is 
effected in response to failure of primary bearings to enable support of 
the shaft by secondary or backup bearings. 
It is a further object of the present invention to provide a bearing system 
including rolling element bearings movable between a first position in 
which the rolling elements in each bearing contact only one of the 
associated races and a second position wherein the rolling elements 
contact both races. 
Further objects and features of the invention are set forth below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawing, the invention is preferably embodied in a bearing 
system 10 for supporting a rotating shaft 12 on a housing 13. The 
illustrated bearing system 10 comprises a pair of primary bearings 14, 16 
for supporting the shaft 12 under normal conditions and a pair of backup 
bearings 18 20 for supporting the shaft 12 in the event of failure of one 
or more of the primary bearings 14, 16. The system 10 is configured so 
that when the primary bearings 14, 16 are functioning properly, the shaft 
12 is maintained in a position such that the backup bearings 18, 20 are 
not loaded. In the event of failure of the primary bearings 14 and 16, the 
shaft 12 shifts axially to a position wherein the backup bearings 18, 20 
support the shaft 12 axially as well as radially. 
Each backup bearing 18, 20 comprises an outer race 22,24; an inner race 
26,28; a plurality of rolling elements 30, 32 disposed between its 
respective races; and a retaining ring 34, 36 for constraining the rolling 
elements 30, 32. 
In accordance with one feature of the present invention, the system 10 is 
configured so that during normal operation, the rolling elements 30, 32 in 
each bearing contact only one of its races, and in the event of failure of 
the primary bearings 14, 16, axial displacement of the shaft 12 brings the 
other race of each bearing into contact with the associated rolling 
elements 30, 32. 
In accordance with a second feature of the present invention, means are 
provided to apply axial force to the shaft 12 to displace it to the 
position wherein it is supported by the backup bearings 18, 20 in response 
to failure of the primary bearings 14, 16. 
Turning to a more detailed description of the illustrated embodiment, the 
housing 13 comprises a generally cylindrical peripheral wall 38 for 
supporting the bearings, and has first and second end walls 40, 42 at its 
opposite ends. The primary bearings 14, 16 are supported on the interior 
of the peripheral wall 38 of the housing 13. The illustrated primary 
bearings 14, 16 are non-contacting bearings which employ magnetic fields 
to support the shaft 12. Such bearings are available from Magnetic 
Bearings, Inc., Radford, Va. Primary bearings of the type employing a 
fluid film rather than a magnetic field could also be used. 
The shaft 12 has a pair of oppositely-facing transverse surfaces 44, 46 
thereon for transmitting thrust loads to the primary bearings 14, 16 
through magnetic fields. Radial loads on the shaft are supported by 
annular longitudinal surfaces 48, 50 adjacent the transverse surfaces 44, 
46. The illustrated shaft 12 has its rearward end 51 disposed within the 
housing and a forward end 52 extending through an opening 56 in the 
forward end wall 42 of the housing 13. 
The bearing system of the present invention might be used in any of various 
types of systems. The illustrated system 10 is configured for use with a 
motor-driven turbine. Thus, a rotor 53a and stator 53b for an electric 
motor 53 may be disposed between the primary bearings 14 and 16, and a 
turbine 54 may be mounted on the forward end 52 of the shaft 12. 
To support the outer races 58, 60 of the backup bearings 18, 20, the 
peripheral wall 38 of the housing 13 has a pair of annular channels 62 and 
64 formed on its interior, one located near the rearward end of the 
housing 13, and one located near the forward end thereof. It will be 
appreciated that the distance between the inner races 26 and 28 is subject 
to dimensional tolerances and thermal strain. Accordingly, it is desirable 
that the distance between the outer races 22 and 24 be variable. To this 
end one of the backup bearings (herein, the forward backup bearing 20) 
preferably has its outer race 24 mounted so as to be axially movable, and 
the other backup bearing (herein, the rear backup bearing 18) has its 
outer race 22 locked in position so as to be constrained against axial 
movement. The rearward channel 62 is dimensioned to fit tightly against 
the forward and rearward surfaces of the outer race 22 of the rear 
bearings, and the forward channel 64 has a width or axial dimension 
greater than the width or axial dimension of the outer race 60 of the 
bearing 20. The outer race 60 of the forward bearing is biased rearwardly 
by a spring 66 so as to rest against a rear transverse surface 68 of the 
forward channel 64 under normal conditions. 
In the illustrated embodiment, the backup bearings 18, 20 are angular 
contact bearings with spherical rolling elements or balls 30, 32. The 
inner races 26, 28 of the backup bearings 18,20 are disposed in spaced 
relation to the balls 30, 32 during normal operation. The inner races 26, 
28 are configured so that excessive displacement of the shaft 12 in any 
direction brings the inner races 26, 28 into contact with their associated 
balls 30, 32. To this end, each of the annular bearing surfaces 26a, 28a 
of the inner races 26, 28, as viewed in cross-section as in the FIGURE, 
defines an annular radius of curvature slightly larger than the radius of 
the associated balls 30, 32. The inner races 26, 28 are configured so that 
when the shaft is in the position shown in the FIGURE, the clearances 
between the inner races 26, 28 and their associated rolling elements 30, 
32 are less than the clearances between each of the primary bearings 14, 
16 and the corresponding support surfaces 44, 46, 48 and 50 on the shaft 
12. This ensures that failure of one or more of the primary bearings 14, 
16 will not result in contact between the primary bearings 14, 16 and the 
shaft 12. 
The rear backup bearing 18 preferably bears the majority of the thrust load 
on the shaft 12. To this end, the spring 66 applies force to the outer 
race 24 of the forward backup bearing 20 of magnitude less than the thrust 
load carried by the rear backup bearing 18, and the contact angle "a" of 
the rear bearing 18 is accordingly greater than the contact angle "b" of 
the forward bearing 20. The magnitude of the spring force is selected to 
provide sufficient axial load on the forward bearing 20 to ensure 
concentricity of the inner races 26, 28. 
In the FIGURE, the distances between the inner races 26, 28 and their 
associated rolling elements 30, 32 have been exaggerated for illustrative 
purposes. However, the FIGURE is intended to illustrate an embodiment 
wherein radial clearances are provided between the balls 30, 32 and the 
inner races 26, 28 and wherein the transverse radius of curvature of the 
bearing surfaces 26a, 28a--i.e., the radius of curvature of the sections 
shown in the FIGURE, which are taken along a plane transverse to the 
direction of movement of the balls 30, 32--is slightly larger than 
conventional relative to the associated balls 30, 32. 
The illustrated backup bearings 18, 20 have their inner races 26, 28 formed 
integrally on the shaft. It will be appreciated that the inner races in 
other embodiments may be separate pieces mounted on the shaft 12. As noted 
above, means are provided to shift the shaft 12 forwardly upon failure of 
the primary bearings 14 and 16. This is accomplished in the illustrated 
embodiment by fluid pressure acting against the rear end 51 of the shaft 
12. 
To sense failure of one or both of the primary bearings 14, 16, a pair of 
displacement sensors 70, 72 are mounted on the housing 13 adjacent the 
primary bearings 14, 16. Failure of a primary bearing may be caused by a 
malfunction of the bearing itself, or by overloading thereof as due to the 
turbine 54 becoming unbalanced. Either type of failure increases shaft 
vibration. By way of example only, commercially available displacement 
sensors 70, 72 may be obtained from Bentley Nevada Corp., Minden, Nev. In 
response to increased shaft vibration, the displacement sensors 70, 72 
send an electric signal to open a normally closed solenoid valve 74 on a 
line 75 between a source 76 of high pressure fluid and an inlet nozzle 78 
at the rear of the housing 13. The inlet nozzle 78 communicates with a 
chamber 86 defined between a forwardly facing surface 88 on the rear end 
wall 40 of the housing 13 and the rearwardly facing surface 80 at the rear 
of the shaft 12. Once the solenoid valve 74 is opened, the high pressure 
fluid flows from the source 76 through the solenoid valve 74 and through 
the nozzle 85 into an axial passage 84 in the shaft 12, and rearwardly 
therefrom into the chamber 86 behind the shaft 12 where it exerts forward 
pressure on the shaft 12 to shift it into its forward position wherein the 
shaft is supported by the backup bearings 18, 20. 
It may be noted that the fluid also exerts pressure on the surface 82 at 
the forward end of the passage 84, which contributes additional force to 
displace the shaft 12 forwardly. The pressure on the shaft 12 is 
sufficiently high to move it into the forward position even if the forward 
primary bearing 16 is still functioning properly and opposing such 
movement. 
In the illustrated embodiment, the high pressure fluid functions as a 
lubricant for the backup bearings 18, 20 in addition to providing pressure 
for axial displacement of the shaft 12. To this end, the high pressure 
fluid preferably comprises a mixture of a liquid lubricant such as oil and 
a gas such as helium, and means are provided to enable the high pressure 
fluid to flow to the backup bearings 18, 20. Herein, flow to the inner 
races 26, 28 of the respective backup bearings 18, 20 is provided by a 
plurality of radially extending passages 90, 92 communicating with the 
axial bore 84 of the shaft 12. Each of these passages 90, 92 has its inner 
end at the bore 84, and has its radially outer end at the annular curved 
surface of the inner race 26, 28 which engages the rolling elements 30, 32 
of the respective bearings 18, 20. 
To provide additional lubrication for the bearings 18, 20, and particularly 
for the outer races 22, 24 thereof, fluid flows along the exterior of the 
shaft 12 near its rear end 51 from the chamber 86 at the rear of the shaft 
12 into the space between the shaft and the interior of the housing 13. 
It will be appreciated that it is desirable to maintain relatively high 
pressure in the chamber 86 at the rear of the shaft 12 and in the interior 
bore 84 of the shaft, while permitting flow of high pressure fluid from 
the nozzle 78 to the outer races 22, 24 of the backup bearings 18, 20. To 
this end, labyrinth seals 94, 96 and 98 are employed on the exterior of 
the nozzle 78 and at the interfaces near the rear end of the housing 13 
and the front end of the housing 13 between the housing and the shaft 12. 
The labyrinth seals 94, 96 and 98 impede flow of high pressure fluid to a 
sufficient degree that the fluid pressure maintains the shaft 12 in the 
desired axial position, while permitting sufficient flow of high pressure 
fluid to enable lubrication of the outer races 22, 24 of the backup 
bearings 18, 20. Each of the labyrinth seals comprises a series of 
circumferential grooves machined into an annular surface to create 
turbulence in axial flow over the surface. 
Operation of the bearing system 10 of the illustrated embodiment may be 
summarized as follows. During normal operation, the shaft 12 rotates on 
the primary bearings 14, 16, and is maintained concentric with respect 
thereto. The shaft 12 is loaded primarily by thrust loads urging the shaft 
12 forwardly. The inner races 26, 28 of the backup bearings 18, 20 are 
spaced from the associated rolling elements 30, 32, which herein are 
balls. The balls 30, 32 are held in place by retaining rings 34, 36 and 
are stationary during normal functioning of the primary bearings 14, 16. 
The balls thus do not add to the mass of the shaft 12 and do not create 
frictional losses to impede rotation of the shaft 12. Neither the balls 
30, 32 nor the respective races of the backup bearings 18, 20 are subject 
to substantial wear while the primary bearings 14, 16 are functioning, and 
no lubricant is needed for the backup bearings under these circumstances. 
Upon failure of one or more of the primary bearings 18, 20, shaft vibration 
increases are detected by one or both of the displacement sensors 70, 72, 
resulting in an electric signal which opens the solenoid valve 74. The 
electric signal may also be operative to interrupt power to the motor 53 
if it is desired to limit the time period during which the shaft 12 
rotates while supported by the backup bearings 18 and 20. 
The shaft vibration may be of sufficient magnitude to cause intermittent 
contact between the inner races 26, 28 and the rolling elements 30, 32. 
Due to the clearances described above, the shaft 12 is prevented by the 
backup bearings 18, 20 from contacting the primary bearings 14, 16 even 
prior to shifting of the shaft 12 forwardly. 
When the solenoid valve 74 opens, high pressure fluid passes from the 
source 76 through the line 75 and through the nozzle 78 into the axial 
bore 84 of the shaft 12. From there, a portion of the fluid flows radially 
outwardly through the passages 90, 92 to the inner races 26, 28 of the 
respective backup bearings 18, 20. Another portion of the fluid flows 
rearwardly past the labyrinth seal 94 on the exterior of the nozzle 78 
into the chamber 86 at the rear of the shaft 12 to pressurize the chamber 
86 and drive the shaft 12 forward, then flows forwardly through the 
labyrinth seal 96 at the rear of the exterior of the shaft 12, flows 
through the backup bearings 18, 20 and finally exits the housing 13 
through the labyrinth seal 98 at the forward end of the housing 13. 
The pressure in the chamber 86 drives the shaft 12 forward so that the 
inner race 28 of the forward backup bearing 20 moves into contact with its 
associated rolling elements 32 and begins rotation thereof. Contact occurs 
along lines which intersect a plane perpendicular to the shaft axis at 
angle "b". The shaft 12 continues to travel forward, displacing the outer 
race 24 forwardly and increasing compression of the spring 66. The inner 
race 26 of the rear backup bearing 18 then moves into contact with its 
associated rolling elements 30, and the rear backup bearing 18 assumes the 
major portion of the thrust load on the shaft 12. Contact between the 
inner race 26 and the elements 30 of the rear backup bearings occurs along 
lines defining contact angle "a" with a transverse plane. It will be 
appreciated that the thrust load on the shaft 12 includes loads resulting 
from turbine 54 as well as the pneumatic force provided by the high 
pressure fluid. The amount of force which the forward backup bearing 20 
exerts in response to forwardly-directed thrust loads on the shaft 12 is 
determined by the spring 66. The spring force need only be great enough to 
insure that loading on the forward bearing 20 is sufficient to maintain 
shaft concentricity. 
As the inner races 26, 28 move into contact with their respective sets of 
rolling elements 30, 32, the rolling elements begin to rotate, and travel 
in circular paths between their associated inner and outer races. Some 
friction occurs between the inner races 26, 28 and their respective 
rolling elements 30, 32 as the rolling elements are accelerated. To 
minimize such friction, it is desirable that the rolling elements 30, 32 
be of relatively low mass. 
From the foregoing it will be appreciated that the present invention 
provides a novel and improved bearing system. While a preferred embodiment 
of the invention is described and illustrated herein, there is no intent 
to limit the invention to this or any particular embodiment.