Rolling-element bearing life is influenced by the number, size, and material properties of particles entering the Hertzian contact of the rolling element and raceway. In general, rolling-element bearing life increases with increasing level of oil filtration. Based upon test results, two equations are presented which allow for the adjustment of bearing L(sub 10) or catalog life based upon oil filter rating. It is recommended that where no oil filtration is used catalog life be reduced by 50 percent

Needelman, William M.

Zaretsky, Erwin V.

English

NASA Technical Reports Server

NASA Technical Memorandum 104350
Quantifying Oil Filtration Effects on Bearing Life
WilliamM. Needelman
Pall Corporation
Glen Cove, New York
and
Erwin V. Zaretsky
Cleveland, Ohio
June 1991
_- (NASA-TM-I04350)- QUANTIFYING OIL FILTRATION
EFFECTS ON BEARING LIFE (NASA) 14 p
- CSCL 131
Uncl as
G3]37 0014413
https://ntrs.nasa.gov/search.jsp?R=19910014199 2020-03-19T17:47:34+00:00Z

QUANTIFYING OIL FILTRATION EFFECTS ON BEARING LIFE
William M. Needelman
Pall Corporation
Glen Cove, New York 11542
and
Erwin V. Zaretsky
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
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SUMMARY
Rolling-element bearing life is influenced by the number, size, and
material properties of particles entering the Hertzian contact of the rolling
element and raceway. In general, rolling-element bearing life increases with
increasing level of oil filtration. Based upon test results, two equations
are presented which allow for the adjustment of bearing LIO or catalog life
based upon oil filter rating. It is recommended that where no oil filtration
is used catalog life be reduced by 50 percent.
INTRODUCTION
It has long been recognized that lubricant contamination can affect
bearing life, reliability, and performance. It is the number, size, and
material properties of particles entering the Hertzian contact of the rolling
element and raceway that impact bearing life. The nature of the particles in
the oil is a function of several processes:
I. Manufacturing processes (swarf, chips, grit)
2. Internal generation, including wear debris and chemical attack of
surfaces
3. Ingression from the environment (sand, dust)
4. Maintenance activities (making/breaking fittings, new oil)
5. Lubricant breakdown products (sludges, precipitates, coke)
Along with selected test contaminants, typical particle size distributions
from a variety of mechanical systems are shown in figure 1. The greater number
of smaller particles in each of the lubrication systems is due in part to wear
mechanisms that generate smaller particles, and, in part, to removal processes
that tend to remove larger particles. For comparison purposes two AC Test
Dusts are shown which represent mineral derived contaminants such as silica
sand. In addition to wear generated particles, Steele (1983) reported a wide
range of particulate levels in unused turbine oils.
Hirano and Yamamoto (1958) reported that contaminants added to various
lubricants could initiate scuffing in rubbing contacts. Dalai et al. (1974,
1975) reported that ball bearing lives in excess of 50 times AFBMA
calculations (catalog value) were achieved by operating with prefiltered
ultraclean lubricant in which the only source of metallic contamination was
the test bearing itself. However, Dalai et al. (1974, 1975) did not run a
control test lot of bearings without filtration to determine the exact life
improvement attributable to filtration. They later induced raceway damage in
their test bearings with a hardness indenter in an attempt to simulate
contaminant caused indentations in service; although reduced from the
ultraclean values, the fatigue lives were still longer than the AFBMA
calculations.
Fitzsimmons and Clevenger (1976) carried out an extensive test program
with tapered roller bearings using a variety of lubricants and contaminants
with controlled particle sizes, types, and concentrations. They noted that
two-body wear occurs when a hard, rough surface plows a series of grooves in
an opposing softer surface. They stated that solid contaminants in lubricants
are conducive to three-body abrasive wear, which occurs when particles are
introduced between sliding surfaces and abrade material off both surfaces.
They added the caveat that a certain amount of abrasive wear may be tolerable,
depending on the application. As an example, noise in gears which results
from looseness in the bearing system rather than from surface failure may
constitute cause for "failure" of the application.
In tapered roller bearings wear normally occurs on the surfaces where
there is combined rolling and sliding contact, for instance between the roller
ends and the cone large-end flange. Fitzsimmons and Clevenger (1976) found
wear of these surfaces increased linearly with the concentration of hard
contaminant particles. In their discussion of this work, Kirnbauer and Ferris
(Fitzsimmons and Clevenger, 1976) report that 3 pm filtration prevents
circulation of the hard particles that cause abrasive wear.
The objective of this paper is to develop a method for calculating
bearing LIO or catalog life to account for the effects of particle size on
bearing fatigue failure.
NOMENCLATURE
I_.X
FR
hc
LIO
L50
LF
NDx
particle removal efficiency, 1 - 1/Bx
filter rating, pm
central or average elastohydrodynamic (EHD) film thickness, pm
bearing 10 percent or catalog life or the operating time which is
exceeded by 90 percent of a group of bearings of a given type
bearing 50 percent or median life or the time which is exceeded by
50 percent of a group of bearings of a given type
life adjustment factor for effect of filtration in bearing LIO life
average number of particles downstream of the filter whose particle
sizes are greater than x _m
l_l.J X
X
A
tY
from the lubrication system. Figure 2 shows the spectra of efficiencies
average number of particles upstream of the filter whose particle sizes
are greater than x t_m
particle size pm
filtration factor, NUx/ND x
lubricant film parameter, hc/a
2_1/2composite surface roughness, a_ + _2J , Win
root mean square (rms) surface roughness of contacting bodies, wm
body I or ball or roller
body 2 or bearing raceway
OIL FILTRATION
Oil filtration is a means to control and eliminate particle contamination
(B)
for various filter ratings as a function of particle size, where
NU
X
X
NUx and NDx
the filter, respectively, whose particle size is greater than x pm. The
data of figure 2 were generated using AC Fine Test Dust. A 3 pm filter is
defined as B3 _ 200. Similarly, a 25 _m filter is defined as B25 _ 200.
Ex of any particle size x can be related to
(1)
are the average number of particles upstream and downstream of
E - t- !- (2)
x Bx
The removal efficiency
the B factor as follows:
From this relationship it becomes apparent that there is a large particle size
above which essentially nothing passes. There is an intermediate particle
size for which a reasonable fraction of the particles are captured and the
rest pass downstream. Finally, there Is a smaller slze particle below which
part|cles freely pass and accumulate to coplous quantities, in rec|rculating
lubrication systems. As an example, the 3 pm and 25 Nm filters will filter
99.5% of all partlcle sizes over 3 pm and 25 pm, respectively.
The distribution of filtration factor B measured on a test stand Is not
necessarily the same as that obtained from operating machinery In the field.
However, experience shows (flg. l) that thls rating (flg. 2) qualitatively
predicts differences in contamination levels In actual machine operation. As
an example, the 3 pm fllter used for the T-56 gas turbine engine will keep the
lubrication system approximately two orders of magnitude cleaner than a 40 pm
fllter. Addltlonally, the T-700 gas turbine engine used in the Blackhawk and
Seahawk helicopters wlth 3 Nm lubrication filters has no scheduled oll changes
for its 2000 hr tlme between removal. By inhibiting catalytic Induced oll
oxidation by flne metallic wear particles, finer filters inhibit the blockage
of oii passages and coating of heat exchanger surfaces by deposits of varnish,
resins, and coke (oll oxidation products), and minimize chemlcal attack of
bearing surfaces by organic acids found through oxidation.
LIFE EFFECTS
There were two independent investigations to determine the effect of o11
filtration on ro111ng-element bearing llfe. These were those of Loewenthal
et al. (1978, 1979, and 1982) at NASA Lewis Research Center, Cleveland, Ohio
and Bhachu et al. (1981) at the Imperial College, London, England.
Bhachu et al. (1981) used a gear test machine to generate wear debris.
The gear wear debris, verified by ferrography to be representative of that
found In helicopter gearboxes, was used as the contaminant Rolling-element
fatigue tests were conducted wlth 25-mm bore roller bearings having a 2957 N
(665 Ib) radial load. For each test serles, gear oll flow was passed through
one of four posslb_e filters of different ratings from 2.5 to 40 Nm or through
an electromagnetic separator, and continuously supplied to a parallel roller
bearing fatigue tester. Elastohydrodynamic film thickness and A ratios
durlng test are shown In table 1. Significantly, tests run with 40 _m
filtration for only 30 m|n before switching to 3 Nm filtration showed
substant|ally the same lives as if all running had been with a 40 _m filter.
Apparently the early damage could not be healed, at least In these small
roller bearlngs. The test results are also shown in table I. These results
show that llfe Increased wlth Improved flltratlon where it Is suggested from
these data that
LF = 3.5(FR) -0.55 (3)
Using equation (3), the predicted LIO lives for the roller bearings are
presented In table I for comparison purposes.
The post-test inner-raceway measurements for 40 Nm filtration showed
greater out-of-roundness than In the unrun bearing. Less out-of-roundness was
observed with finer filtratlon down to the 8 pm rating. Virtually no out-of-
roundness was observed when the 3 Nm filter was used. Below the 3 Nm level
the measurement was slmllar to that of the unrun bearlng. Bhachu et al. (1981)
suggested that particles smaller than 3 pm were too small to have any effect on
roundness and merely passed through the contacts of the rollers and raceways.
There Is a strong suggestion from these data, that the lack of
contamination contributes to improvement in bearing raceway surface finish
during operation. There appears to be a correlation between the lubricant
f11m parameter A after testing and rolllng-element fatigue llfe as evidenced
by the data of table I.
Loewenthal et al. (1978, 1979, and 1982) performed a series of tests to
measure the quantltatlve effects of filtration on rolllng-element fatigue
6
life. Four levels of filtration were investigated using full-flow filters
rated at 3, 30, 49, and 105 _m. The 3 _m filter used for these tests had been
developed to replace the original 40 _m filter for a helicopter gas turbine
lubrication system. The new filter elements were not only found to provide a
much cleaner lubricant with less component wear, but contrary to prior belief,
to greatly extend the time between filter and oil changes as discussed by
Loewenthal et al. (1978, 1979, and 1982).
The test bearings were 65-mm deep groove ball bearings run at 15 000 rpm
under a radial load of 4580 N (1030 lb) which produced a maximum Hertz stress
of 2410 MPa (350 000 psi). The lubricant contaminant rate was 0.125 gram per
hour per bearing. The test environment was designed to simulate an aircraft
lubrication system containing multiple bearings, pumps, and other components
commonly found in such systems. Test temperature was 347 K {165 °F). The
test lubricant was a MIL-L-23699 type which produced a A ratio of 3.3. The
test contaminant was similar to particulate matter found in the lubricant
filters of 50 IT8D commercial engines (Jones and Loewenthal, 1980). Because
this engine has a number of carbon-graphite bearing sump seals, replication of
oil contaminants in engines with "windback" type labyrinth seals may demand a
slightly different composition.
The results of these tests are summarized in table 2. As with the work
of Bhachu et al. {1981), improved filtration increased bearing life. However,
for the contaminated tests, there appears to be no statistical difference in
life obtained between the 3 and 30 _Jm filters. Due to the severe wear
obtained, the contaminated 105 _m filter test series was suspended after
448 hr on each bearing. No fatigue failures were encountered, because of the
gross wear of the bearing races. Based upon the test results between the
3 and 49 pm filters the following life relation is suggested from these data:
LF _ 1.8(FR) -0"25 (4)
Using equation (4), the predicted LIO lives of ball bearings are presented in
table 2 for comparison purposes.
Table 3 compares equation (3) with equation (4) for various filter
ratings. The resultant life adjustment factors can be used to adjust the
calculated bearing LIO or catalog life to account for filtration level in
the lubricant system. Based upon the data of Loewenthal et al. (1978, 1979,
and 1982), it is not recommended to use a life adjustment factor less than 0.5
even when no filter is used. Further, equation (4) appears to give more
conservative results than equation (3). These two equations may reflect the
differences in the effect of particle damage between roller and ball bearings.
The technology for improved oil filtration is commerically available. By
minimizing the number of harmful particles entering a rolling-element bearing,
oil filtration can substantially extend bearing life. In addition to machine
generated wear debris, the comparatively high contamination of new oil also
demands good filtration.
_o reported testing has been performed comparing grease lubrication which
entraps wear debris with the same bearings being oil lubricated with or without
oil filtration. It is suggested that for long term application, grease
lubrication be Considered the same as oil lubrication for bearings without
filtration where no periodic regreasing of the bearing occurs.
REFERENCES
Bhachu, R., Sayles, R., and MacPherson, P.B., "The Influence of Filtration on
Rolling Element Bearing Life," Innovation for Maintenance Technology
Improvements, T.R. Shives and W.A. Willard, eds., NBS-SP-640, NASA CR-172821,
1981, pp. 326-347.
8
Dalai, H., Cotellesse, G., Morrison, F., and Ninos, N., "Progression of
Surface Damage in Rolling Contact Fatigue," SKF Report AL74TO02, SKF
Industries, 1974 (Avail. NTIS, AD-780453).
Dalai, H., Hanh, D., and Ninos, N., "Progression of Surface Damage and Oil
Wear Debris Accumulation in Roiling Contact Fatigue," SKF Report AL75TO07,
SKF Industries, 1975.
Fitzsimmons, B., and Clevenger, H.D., "Contaminated Lubricants and Tapered
Roller Bearing Wear," ASLE Transactions, Vol. 20, No. 2, 1977, pp. 97-107.
Hirano, F., and Yamamoto, S., "Four-Ball Test on Lubricating Oils Containing
Solid Particles," Wear, Vol. 2, 1958-9, pp. 349-363.
Jones, W.R., Jr., and Loewenthal, S.H., "Analysis of Wear Debris from
Full-Scale Bearing Fatigue Tests Using the Ferrograph," ASLE Transactions,
Vol. 24, No.3, 1980, pp. 323-330.
Loewenthal, S.H., Moyer, D.W., and Sherlock, J.J., "Effect of Filtration on
Rolling-Element-Bearing Life in a Contaminated Lubricant Environment," NASA
TP-1272, 1978.
Loewenthal, S.H., and Moyer, D.W., "Filtration Effects on Ball Bearing Life
and Condition in a Contaminated Lubricant," Journal of Lubrication
Technolo_sr, Vol. 101, No. 2, 1979, pp. 171-179.
Loewenthal, S.H., Moyer, D.W., and Needelman, W.M., "Effects of Ultra-Clean
and Centrifugal Filtration on Rolling-Element Bearing Life." Journal of
Lubrication Technology/, Yol. 104, No.3, 1982, pp. 283-292.
Steele, F.M., "Contamination Control in Turbomachinery Oil Systems,"
Lubrication Engineering, Vol. 40, No. 8, 1984, pp. 487-497.
9
ILl.-
° #" L._
1.
Q.
0 • • • ° • • • • °
Z
W
"!'
._ O0 O0 O0 !
_ IC_ __ Ii_0 • , ....
U
- _g_ .........
-g_ N
L _ U =
_ N
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o
(_;
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,_ i. f-
_"_ 0"_ _ "_,
• =, ) • • ,
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o .
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.=
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10
0
0 u
,--_®
g_
>_
_._ .
_._ _
= .-! "o
TABLE 3. - LIFE ADJUSTME_Yr FACTOR
BASED ON FILTER P_.TING
Filter rating
(where _ Z 2OO),
FR
8
10
12
25
40
49 a
60 a
105 a
Life factor, LF
Roller bearing
equation (3)
1.9
1.3
1.1
1
0.9
.6
.5
.4
.4
.3
Bail bearing
equation (4)
1.4
1.2
1.1
1
1
0.8
.7
.7
.6
.6
aFor filter ratings exceeding 40 _ or
with no filtration it is not recov,'nended
to use a life adjustment factor less
than 0.5.
(_ 10 4
103
,_'_ 10 2
1o
"6 1
E
3 .1
Z
105 -
__'_rn fi'-Paper _archine
,- Transit bus
\_.._ _ r transmission
_PaperPa_.eh__v,_-"_, _"___ "'--.1
  Oc°arse
• \ transmission "_
_ _ _ \ 61.tmfilter
_ Tf6gas \ .....
-- turbine. .A AC fine ,
3prn filter--" \ test dust J
I I I \1 L__A_LJ_J
.01 5 10 20 30 40 50 60 708090100
Particle size, pm
Figure 1 .--Effect of oil filtration on particle contamination
for mechanical systems.
11
iT.
5OOO Filter rating
(where 13z 200), pm
2OOO
12
IOO0
500
2OO
100
5O
20
10
5
2
I I I I I I ,
0 4 8 12 16 20 24 28
Particle size, pm
Figure 2._Particle size as a function of filtration ratio _ for
different filter ratings.
I
34
12
N/ ISA Report Documentation Page
NaUonal Aeronautics and
Space Adminlstr a_on
1. Report No. I 2. Government Accession No. 3. Recipient's Catalog No.
INASA TM - 104350
4. Title and Subtitle
Quantifying Oil Filtration Effects on Bearing Life
7. Author(s)
William M. Needelman and Erwin V. Zaretsky
i
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135 - 3191
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546 - 0001
5. Report Date
June 1991
6. Performing Organization Code
8. Performing Organization Report No.
E - 5862
10. Work Unit No.
505 - 63 - 1B
t 1. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
William M. Needelman, Pall Corporation, Glen Cove, New York 11542; Erwin V. Zaretsky, NASA Lewis Research
Center. Responsible person, Erwin V. Zaretsky, (216) 433 - 3241.
16. Abstract
Rolling-element bearing life is influenced by the number, size, and material properties of particles entering the Hertzian
contact of the rolling element and raceway. In general, rolling-element bearing life increases with increasing level of
oil filtration. Based upon test results, two equations are presented which allow for the adjustment of bearing LI0 or
catalog life based upon oil filter rating. It is recommended that where no oil filtration is used catalog life be reduced by
50 percent.
17. Key Words (Suggested by Author(s_,)
Lubrication; Filtration; Bearings; Life durability;
Durability; Antifriction bearings; Roller bearings;
Ball bearings
19. Security Classif. (of the report)
Unclassified
18. Distribution Statement
Unclassified - Unlimited
Subject Category 37
20. Security Classif. (of this page)
Unclassified
21. No. of pages 22. Price*
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Quantifying oil filtration effects on bearing life

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