Some of the advances in tribology that have been associated with aerospace mechanisms are discussed. The needs of aerospace have been the dominant forces leading to improvements in understanding and applying tribology technology. In the past two decades improvements in understanding bearing torque, elastohydrodynamic lubrication, lubricant distribution, cage stability, and transfer film lubricants have been made. It is anticipated that further developments will be made in response to future aerospace requirements

Dufrane, K. F.

Kannel, J. W.

English

NASA Technical Reports Server

N87- 16330
_- _tAN_ NOT FIL_-_
ROLLING ELEMENT BEARINGS IN SPACE
J. W. Kannel* and K. F. Dufrane*
The paper discusses some of the advances in tribology that have been
associated with aerospace mechanisms. The needs of aerospace have been
the dominant forces leading to improvements in understanding and apply-
ing tribology technology. In the past two decades improvements in under-
standing bearing torque, elastohydrodynamic lubrication, lubricant distri-
bution, cage stability, and transfer film lubricants have been made. It
is anticipated that further developments will be made in response to
future aerospace requirements.
INTRODUCTION
The development of aerospace mechanisms has required considerable
advances in the science of friction, wear, and lubrication (tribology).
The aerospace industry has been the dominant driving force in tribology
and has given tribology the popularity and respectability it now enjoys.
Further, many of the tribology techniques developed for aerospace have
been applied to mechanical components for non-aerospace systems and have
been a benefit to all technology. Figure i shows a qualitative chart
which illustrates the growth of tribology problems With aerospace and the
advances that have been made. Despite significant advances in tribology,
the insatiable demands of aerospace systems seem to grow faster than the
solutions.
Major tribology advances have been made in several areas, with rolling
element bearings being a particularly important example. Without the
advance made in rolling-bearings or alternative concepts, many of the space
systems would not be possible. The paper discusses a small part of their
advances and how these advances can be applied to assist design. The paper
discusses bearing torque for small and medium sized bearings, cage sta-
bility, lubricant distribution, and transfer film lubrication. The paper
does not deal with the emerging area of high temperature tribology, which
is an area needing major advances to permit the practical use of engines
and mechanisms athigh temperatures.
TRIBOLOGY DEVELOPMENT
Gyroscope Bearing Lubrication
Early in the space program the need for special lubricants for preci-
sion gyroscope systems became apparent. The most popular lubricant soon
became KG-80, which was a replacement for the original fluid Teresso V-78.
Interest in fluids at various viscosity levels prompted the development of
*Battelle Columbus Division, Columbus, Ohio.
]2]
https://ntrs.nasa.gov/search.jsp?R=19870006897 2020-03-20T12:14:51+00:00Z
fthe Super Refined Gyroscope (SRG) lubricants. These SRG fluids represent
a homologous group that allows the users to select a specific viscosit_
and the researchers to vary viscosit_ in controlled experiments with a
reasonable degree of confidence. Selecting the proper viscosity involves
several important considerations depending on the application.
Operating torque is one important consideration in selecting the vis-
cosity. Typical viscosity-temperature data are given in Figure 2 for the
SRG fluids. Several experiments have been conducted with an R-6 bearing
to define the torque and lubrication capabilities of the SRG lubricants.
Figure 3 shows torque as a function of viscosity. This bearing was lubri-
cated with a few milligrams of the selected lubricant by using a Freon
carrier. The Freon evaporates and leaves a residual film of lubricant.
The torque is related to the shearing of the viscous layer between the
balls and races. Figure 3 clearly shows the torque penalty associated with
increasing the viscosity of the bearing lubricant.
One advantage of increasing viscosity is illustrated in Figure 4.
Lift-off speed is defined as the speed at which a lubricant film is formed
that completely separates the balls and races in the bearing. The lubri-
cant film condition is monitored by an electrical continuity approach.
The dynamic electrical resistance of the bearing is interpreted in terms of
percentage of lubrication, i.e., the percentage of the time no electrical
shorting occurs. At a temperature of 60 C the R-4 bearing would generate
a good elastohydrodynamic (ball race) lubricant film at a speed of 80 rpm
if a KG-80 fluid were used. If an SRG-40 fluid were used, the speed
required for good lubrication would be 160 rpm. The higher viscosity fluid
would be preferred if the primary bearing operating speed was to be in this
range. However, if the operating speed is much higher, the lower viscosity
lubricant may be preferable because of the associated lower operating
torque.
The lubricant viscosity is also a factor in evaporation rates, which
can be important for vacuum applications where extended life is desired.
In general, evaporation rates for a given type of fluid decrease as the
viscosity is increased. For example, Table 1 presents measured evaporation
rate data for two different lubricants at different viscosities. For long-
life applications the lower loss rates associated with higher viscosities
are desirable, but the high viscosity lubricants have the accompanying dis-
advantage of hi_her operating torques.
Case Stability
Gross bearing torque can be controlled by proper selection of the
lubricant and lubricant quantity in the bearing. However, bearing torque
variations are much more difficult to control. Small torque variations in
a gyroscope of a despun antenna support in space can result in intolerable
errors in positioning and control. One major source of torque variation is
erratic motions of the cage separating the balls. Under ideal conditions
the cage simply rotates with the balls. However, under some conditions the
122
cage incurs rapid secondary motion, which is known as cage instabilit_
Cage instability generates erratic friction in a bearing, which is observed
as erratic torque. Figure 5 is an example of torque fluctuation measured
in a bearing operating in an unstable mode.
The key to cage stability lies in the tangential stiffness of the
ball-race interfaces and the rate at which energy can be absorbed at these
interfaces. When the cage strikes a ball, the ball will slip and generate
a reactive force against the cage. If the slip occurs easily, such as
when the balls and races are completely separated by a film of low viscos-
ity fluid, the reaction force is small. Under such conditions the cage
energy is absorbed by shear losses in the lubricant film. Conversely, if
the ball and races are separated by a highly viscous fluid, the ball slip-
page will be small, the shear losses much less, and the cage motion will
be undamped. Depending on the parameters of the particular bearing, cage
instability can be a serious problem.
A cage stability criterion is given by:
232C
Dp ..... L
M Csl
(i)
where
C_
Csl
M
is a ball-race traction parameter inversely related to
film thickness
is the ball-cage spring rate (linearized)
is the cage mass
C_ _avgA/h (2)
where
gavg
A
h
is the average contact zone viscosity --(_i06 Cp for a mineral oil)
is contact area
is film thickness
As h gets small or # gets large, CB gets large.
avg
The larger the value of Dp the higher the likelihood the cage will be
unstable. An approximate criterion for cage stability is given in Fig_ire 6.
To check stability, values of the ball-cage friction coefficient, f, and a
factor ef must be known, where:
ef = exp (-_/mp-l) (3)
123
An accurate assessment of cage stability requires analyses of the cage
motions using comprehensive computer models.
Lubricant Distribution
Large spacecraft starting with Skylab have brought a new set of tribo-
logical problems. Smaller spacecraft could be stabilized by spinning the
spacecraft or by the use of control jets. The large extended mission
spacecraft require large control moment gyroscopes (CMG's), which are
capable of handling large slew loads resulting from astronauts moving
around the crafts and from changing the orientation. The large CMG's have
considerable weight and operate at high speeds. Small bearings using
lubricant-impregnated retainers no longer suffice for these units; positive
lubrication of large bearings is required. The lubrication has to be
highly reliable, but extremely compact to meet weight and space requirements.
One major tribological challenge is insuring that the lubricant gets
into the bearing to lubricate the ball-race interfaces. Devices such as
centrifugal oilers attached to the rotating shaft were successfully used in
some applications. However, positive lubricators in space require a hard
look at how lubricant migrates around the bearing cavity in a vacuum envi-
ronment. Typical results of creep tests are shown in Figure 7.
A drop of lubricant approximately 0±5 mm in diameter was positioned on
a polished steel plate in a ]0 mPa (I0 torr) vacuum chamber. The
spreading of the drop was monitored over a period up to 400 hours. For the
two lubricants shown in Figure 7, the rate of creep would decrease over an
extended time period. The higher viscosity fluid, SRG 160, tended to creep
much more slowly than the low viscosity fluid, SRG 60. The lubricants
would not creep around sharp edges or over a debris track of Synthane.
Lubricant tended to creep in the direction of a thermal gradient of 4C, as
shown in Figure 8. Lubricant would not creep against the .thermal gradient.
These results demonstrated that the lubricant must be resupplied directly
to the ball tracks on the races or to the balls themselves. Since the
quantity of make-up lubricant is very small, the delivery system must be
arranged for the centrifugal force field to permit direct impingement on
the races, as opposed to controlling the direction through a jet. The tri-
bology technology developed for large CMG's permitted the design of auto-
matic lubricators that functioned for extended missions.
Transfer Film Lubrication
One major key to the success of the NASA space efforts has been the
use of highly efficient systems and the minimization of weight. Large tri-
bology devices cannot be tolerated and every effort must be given to opti-
mizing system performance. In some applications, such as the liquid
oxidizer and fuel pumps in the Space Shuttle main engine, the bearings must
be cooled and lubricated with the cryogenic fluid. Cryogenic fluids tend
to be good for bearing cooling, but are poor lubricants. One method to
lubricate bearings in this environment is by transfer film lubrication.
124
Transfer film lubrication implies that the lubricating cage material
is transferred to the ball and, in turn, to the races, Transfer film tech-
nology is in its infancy as a tribological science and little is known
about materials or operating conditions required to optimize this type of
lubrication. In most applications, transfer films are difficult to achieve
and even more difficult to substantiate. Enough is known about transfer
films to realize both their importance and their limitations.
Figure 9 shows the results of transfer film studies with Rulon-A and
5 percent MoS_. A 440C ball (Figure i0) was loaded against an oscillating
440C cylindrical ring. A Rulon-A wiper was loaded against the ball to
create a transfer film for the ball-race interface. As the load (contact
stress) between the ball and ring was increased, the roughness of the ring
wear track decreased slightly. At a mean stress level of 1.38 GPa
(200,000 psi) the transfer film was insufficient to prevent roughening of
the surfaces. The experiments imply that there is a limiting stress for
effective transfer film lubrication.
The traction associated with solid surface films can be computed using
elasticity theory for the traction of surface layers, as shown in Figure ii.
The smaller the value of v in the Figure, the lower is the surface film
elasticity. Values of y of 0.01 might be associated with MoS2, and values
of 7 of 0.002 would be indicative of a PTFE coating. The traction-slip
curves are, in many respects, similar to those for liquid lubricants. An
effective viscosity for a transfer film can be expressed:
2Poh CTV
_eff - 3V AV (4)
where
Po is the maximum contact pressure
CT is the traction coefficient
h is film thickness
V is rolliug velocity
_V is slip velocity
For a solid-film-lubricated bearing, Equation 4 in conjunction with
Equations 1-3 show the factors affecting cage stability. Large values of
the traction coefficient, for a given value of slip, can product an unstable
cage. The lower the resilience of the surface film, the greater the proba-
bility for cage unstability.
125
CONCLUSIONS
Numeroustribology problems have been encountered in the development
of aerospace mechanisms. These problems have led to research activities
which have not only yielded solutions to the problems but have also aided
in overall technology development. Examples of aerospace tribology
advances include better understanding of bearing torque, elastohydrodynamic
lubrication, lubricant distribution, cage stability, and transfer film
lubrication.
There are several tribology related problems currently facing industry
and specifically the aerospace industry. For example, lack of tribology
materials and lubricants are major limitations to high temperature engine
development. Lack of low temperature lubricant technology is a limitation
to cryogenic lubrication. Bearing precision is a limitation to very accur-
ate mechanical control devices. The research required to address these and
other problems promises to insure a productive future in tribology.
TABLE i. EFFECT OF VISCOSITY ON EVAPORATION.RATE OF
TWO LUBRICANTS IN A 0.133 MPa (10 -b TORR)
VACUUM
Fluid
Published Viscosity
at 40C, i0 -v m /s
Weight Loss
Rate a_ 40C
mg/cm -hr
Super-refi ned
Paraffinic Mineral
Oil (SRG 30)
Super-refi ned
Paraffinic Mineral
Oil (SRG 40)
Perfluoro Ether
Perfluoro Ether
Perfluoro Ether
14
27
8
28
357
18
13
2.2
0.19
0.0002
126
1950
--. _ Spacecraft
Space Z complexity
• Stay _ Tribology
Voyager/ /-- problems
SkylabJ /High
Apollo / j / Temperature
/ / Transfer
/" / Lub;k:_bn --.,-T "----
/ /'Distribution _......--"---
1960 1970 1980 J990 2000 20|0
Years
Figure i. - Tribology challenge.
>
_3
O
E
I0,000
i000
I00
I0
Temperature, C
0 50 IO0 150
= I I I I
I
0 4O0
I I I
_oo 200 300
Temperature, F
Figure 2. - Viscosity versus tem-
perature for a homologous series
of super-refined mineral oils.
127
2oI
E 1.5
,Q
1.0
0
0
- 0
_ _ 0
30 60 90 120 150
Kinematic Viscosity at 38 C, IO-6mZ/sec
Figure 3. - Measured bearing torque as a func-
tion of lubricant viscosity for various lub-
ricants at 25C for an R-6 bearing at 480 rpm.
18o
L)
o
==
E
120
tOO
80
60
40
20
-2o t
0 180
°
20 40 60 80 I00 120 140 160
Lift-Off Speed (90% Film), rpm
2O0
Figure 4. - Lift-off speed as a function of
temperature for an R-6 bearing loaded to
980 MN/m 2 maximum Hertzian contact stress
(inner race) and lubricated with two super-
refined mineral oils.
128
Torque
Measurement
I I FilmMeasurement
0 film
a. Dry bearing.
Chart speed: t div=5 sec (left to right)
_Jiili_!_I[i_,,ilii__J]iiil._iJil h,JiU]_!kl!!l!i_I ,u uLl'_-'.-,
Film
b. Two drops of oil in bearing.
Chart speed: Idiv= 5 sec (left to right)
Measurement
Very thin film
Figure 5. - Bearing torque and film thickness meas-
urement with no lubricant and meager lubricants.
Torque spikes imply instability.
1.0
__ 0.8_
:;o,l _0nst0.
o SO.
0`2 --
I I I I I I I I0
0 0.1 0.2 0`3 0.4 0`5 0.6 0.7 0`8 0.9
Restitution Factor, ec
LO
Figure 6. - Quick check of cage stability.
129
_°/ _ s_-6o
,.sI- __ _
1.2
1.0
O.S
06 SRG-160
_- II _..,o-- --'o- o o ,-, o ,., o- T
o.4_i_If-- _ . Top edge of dropO.Z
i,. _
1.2 ....========================::::::::::::::::::::::: ::: i
0 I0 20 30 40 50 60 70 80 90 I00 340
Elopsed Time, hours
Figure 7. - Results of creep experiments in presence
of a synthane transfer film SRG-60 and SRG-160 oils.
75F ,,
Top-Bottom
75 F -"
Heater off
87 F b ,. .
I
9IF ---
Heoter on
Figure 8. - Sketch showing migration patterns of KG-80 in the
presence of a 4 F thermal gradient.
]30
zoo1- e I
L75' T
125 _ 5
OC
,o0 ,._ 4
E
075 _ 3
0 50 o 2
0
025 --
Heavy debris /
No film in mox.._lr
Stress region_"
- y
/ Heavy debris
(p Heovy film
Slkjht debris Debris at ends /// Lay
wear
Medium film Light film
No lay wear Slight _y wear / Debris
De '•'''---'-_ •arm _'1---.-_ Oj41 Spotfy film
w / Lay wear
Heavy film Debris •
No lay wear Heavy film No debris Slight debris
I- Lay wear MedDum film Medium film
No Ioy weor _1._ .oy weor
o ' I , I , I , I , l , I , 1 , I , I = I I
40 60 80 I00 120 140 160 180 200 2;>0 240 260
Meon Hertzion Co, tort Stress, ksi
.... I , , , , I .... I ,, , , I , , , , I .... I
0.50 0.75 I.oo *.25 I._ 1.7.5
Meon Hertzion Contort Stress, GPo
Figure 9. - Relationship between race wear track
surface finish and ball-race contact stress for
a race lubricated with a rulon-A + 5 percent
MoS 2 transfer film.
Spherical
(boll)
Retainer
W I
Cylindrical
(race)
+
Figure I0. - Schematic drawing of apparatus for transfer analysis.
131
.A
0.30
0.25
i.- o.20
¢_)
0,15
u
8
oJo
0.05
0
"f = E surfoce/F'substrote
h = I00/,.in.
f=O.I
----- f=O.2
- ._-- --- ..--....-_--.._--.._,--- ........-.--_.-......:-....-.-.,-
- / / 1 "_ i _ \ \ \ =0004/ ./ 11 I "1 \ \,,=oo_'
./ // /I 11 _-=i "
J __ / I / •
_ / // --
_ /_-,_j \ \ -_,:o.oo4
/j,._ \ -..--o.o,
y--I
0 Q_2 0._4 0.006 0.008 0.010 0.012 QOI4 _0_
Slip, _v/v
Figure ii. - Theoretical traction slip curves for various
coatings.
0.018
132


Rolling element bearings in space

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