William M. Needelman

Filtration Science Solutions, Huntington, New York

Erwin V. Zaretsky

Glenn Research Center, Cleveland, Ohio

Recalibrated Equations for Determining Effect of

Oil Filtration on Rolling Bearing Life

NASA/TM—2014-218071

December 2014
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William M. Needelman

Filtration Science Solutions, Huntington, New York

Erwin V. Zaretsky

Glenn Research Center, Cleveland, Ohio

Recalibrated Equations for Determining Effect of

Oil Filtration on Rolling Bearing Life

NASA/TM—2014-218071

December 2014

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The authors would like to thank the Donaldson Company for supplying the multipass test data used to create Figures 6 and 8 and 

Equations (5) and (6).

Available from

NASA STI Information Desk

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Hampton, VA 23681-2199

National Technical Information Service

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Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identi¿ cation 

only. Their usage does not constitute an of¿ cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

This work was sponsored by the Fundamental Aeronautics Program 

at the NASA Glenn Research Center.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2014-218071 1

Recalibrated Equations for Determining Effect of 

Oil Filtration on Rolling Bearing Life 

William M. Needelman 

Filtration Science Solutions 

Huntington, New York 11743 

Erwin V. Zaretsky*

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Summary 

In 1991, Needelman and Zaretsky presented a set of empirically derived equations for bearing fatigue 

life (adjustment) factors (LFs) as a function of oil filter ratings. These equations for life factors were 

incorporated into the reference book, “STLE Life Factors for Rolling Bearings.” These equations were 

normalized (LF = 1) to a 10-Pm filter rating at ȕx = 200 (normal cleanliness) as it was then defined. Over 

the past 20 years, these life factors based on oil filtration have been used in conjunction with 

ANSI/ABMA standards and bearing computer codes to predict rolling bearing life. Also, additional 

experimental studies have been made by other investigators into the relationship between rolling bearing 

life and the size, number, and type of particle contamination. During this time period filter ratings have 

also been revised and improved, and they now use particle counting calibrated to a new National Institute 

of Standards and Technology (NIST) reference material, NIST SRM 2806, 1997. This paper reviews the 

relevant bearing life studies and describes the new filter ratings. New filter ratings, Ex(c) = 200 and 

Ex(c) = 1000, are benchmarked to old filter ratings, Ex = 200, and vice versa. Two separate sets of filter LF

values were derived based on the new filter ratings for roller bearings and ball bearings, respectively. 

Filter LFs can be calculated for the new filter ratings. 

Introduction 

It has long been recognized that lubricant contamination can affect bearing life, reliability, and 

performance. In 1976, Tallian (Refs. 1 and 2) was the first to publish a systematic study of the effect of 

contaminated lubrication on rolling-element fatigue life. He presented a probabilistic model (Ref. 1) to 

predict rolling-element fatigue life under conditions where the elemental surfaces in contact (raceways 

and balls or rollers) incur progressive damage during stress cycling from contamination in the lubricant. 

He subsequently correlated his analysis to surface density damage from experimental fatigue life data for 

ball bearing inner raceways (Ref. 2). 

Hirano and Yamamoto (Ref. 3) reported that contaminants added to various lubricants could initiate 

scuffing in rubbing contacts. Dalal et al. (Refs. 4 and 5) reported that ball bearing lives in excess of 

50 times ANSI/ABMA†

 (American National Standards Institute/American Bearing Manufacturers 

Association) bearing manufacturers’ catalogue calculations (Refs. 6 and 7) were achieved by operating 

with prefiltered ultraclean lubricant, in which the only source of metallic contamination was the test 

bearing itself. However, Dalal et al. 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 in-service contaminant-caused 

 

*

Distinguished Research Associate.

†Formerly AFBMA, Anti-Friction Bearing Manufacturers Association.
NASA/TM—2014-218071 2

indentations; although reduced from the ultraclean values, the fatigue lives were still longer than the 

ANSI/ABMA calculations. 

Fitzsimmons and Clevenger (Ref. 8) 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 that 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 example, between the roller ends and the cone large-end flange. Fitzsimmons and 

Clevenger found that wear of these surfaces increased linearly with the concentration of hard contaminant 

particles: In a discussion to the Fitzsimmons and Clevenger paper (Ref. 8) Kirnbauer and Ferris reported 

that 3-Pm filtration prevents circulation of the hard particles that cause abrasive wear. 

There were two independent investigations to determine the effect of oil filtration on rolling-element 

bearing life. These were those of Loewenthal et al. (Refs. 9 to 11) from 1978 to 1982 for ball bearings at 

NASA Lewis (now Glenn) Research Center in Cleveland, Ohio, and Bhachu et al. (Refs. 12 and 13) for 

roller bearings in 1981 at the Imperial College in London, England. In general, the results reported by 

both Loewenthal et al. and Bhachu et al. verified the trends of Tallian’s analysis (Refs. 1 and 2). 

Subsequent to the research reported above, additional experimental and analytical studies have been 

made by other investigators into the relationship between rolling bearing life and the size, number, and 

types of particle contamination (Refs. 14 to 20). Gabelli, Morales-Espejel, and Ioannides (Ref. 20) 

provide a comprehensive review of the literature, analysis, and a limited database related to particle 

damage in Hertzian contacts and rolling bearing life ratings. The reported research results from these 

references were similar to that reported by Tallian (Refs. 1 and 2), Fitzsimmons and Clevenger (Ref. 8), 

Loewenthal et al. (Refs. 9 to 11), and Bhachu et al. (Refs. 12 and 13). For a defined operating condition 

and bearing size and type, bearing life was a function of lubricant cleanliness and the number, size, and 

material properties of particles entering the Hertzian contact of the rolling element and raceway. 

However, there is conflicting opinion as to whether the elastohydrodynamic (EHD) film thickness to 

surface composite roughness, or the ȁ ratio, mitigates the negative effect of lubricant cleanliness on 

rolling-element fatigue life. That is, is the effect of contamination on bearing life less severe with 

increasing film thickness? 

There are numerous published papers that have studied the relation of debris dents on the EHD film 

thickness and rolling-element fatigue. Most of these papers are summarized and discussed by Gabelli, 

Morales-Espejel, and Ioannides (Ref. 20). Wedeven and Cusano (Ref. 21) and Kaneta, Kanada, and 

Nishikawa (Ref. 22) experimentally studied the effects of moving dents and grooves on the EHD film 

thickness. Numerical solutions to the effect of simple debris dents on the EHD film thickness and surface 

and subsurface stresses are given by Venner (Ref. 23); Ai et al. (Refs. 24 to 26); Nelias and Ville 

(Ref. 27); Ville et al. (Refs. 28 and 29); and Chapkov Colin, and Lubrecht (Ref. 30). The results of these 

analyses and experiments suggest that the debris dents reduce the EHD film thickness at the dent site, 

increasing the contact and subsurface stresses, thereby reducing rolling-element bearing fatigue life.

In 1991, based on the research of Loewenthal et al. (Refs. 9 to 11) and Bhachu et al. (Refs. 12 and 

13), Needelman and Zaretsky (Ref. 31) presented a set of empirically derived equations for bearing 

fatigue life adjustment factors (LFs) as a function of the oil filter ratings (FR), where ȕx = 200. These 

equations for LF were incorporated into the reference book, “STLE Life Factors for Rolling Bearings” 

(Ref. 32). These equations (Ref. 31) were normalized (LF = 1) to a FR of 10 Pm at ȕx = 200 (normal 

cleanliness) as it was then defined by ISO Standard 4572 (Ref. 33) and a ratio of the EHD film thickness 

to surface composite roughness, ȁ, of 1.1. 
NASA/TM—2014-218071 3

For over 2 decades these life factors based on oil filtration (Refs. 31 and 32) have been used in 

conjunction with ANSI/ABMA standards (Refs. 6 and 7) and bearing computer codes (Ref. 34) to predict 

rolling-element bearing life. However, in 1999 filter ratings underwent a revision and improvement 

(Ref. 35). They are now based on ISO 16889 (Ref. 36), replacing the older and now disavowed ISO 

4572:1981 (Ref. 33). 

A primary difference between these filter ratings is how particle size is specified and measured. The 

older filter test method used particle counters calibrated per ISO 4402:1991 (Ref. 37), with a variable 

material comprising irregularly shaped particles, AC Fine Test Dust (AC FTD). The new filter test 

method employs particle counters calibration per ISO 11171:1999 (Ref. 38), based on spherical particles 

traceable to a National Institute of Standards and Technology (NIST) reference material (SRM 2806,

1997) (Ref. 39), providing more accurate and verifiable results (Refs. 35 and 40). In addition, the new 

filter rating method uses a somewhat different test dust (ISO Medium Test Dust, ISO MTD) (Ref. 41), 

replacing the no-longer-available dust previously used, AC FTD. 

The Needelman-Zaretsky life factors published in the early 1990s (Refs. 31 and 32) do not 

correspond to this revised and improved system of filter ratings. This paper focuses on the improved 

methods for measuring filter performance (improved filter ratings), and recalibrated bearing life factors 

incorporating these new ratings. The improved system of filter rating provides more accurate 

measurement of industrial filter performance, including performance improvements achieved by 

advancements in the design, materials, and manufacture of filters over the past 20 years. 

In 2007, the ISO Standard 281, “Rolling Bearings—Dynamic Load Ratings and Rating Life” 

(Ref. 42), was radically changed. It is based on the inclusion of a “fatigue limit” in the bearing life 

calculations and the addition of a “contamination factor for circulating oil lubrication with on-line filters.” 

This “contamination factor” is based on the analytical work of Ioannides et al. (Ref. 17) and on a 

“cleanliness code according to ISO 4406” (Refs. 43 and 44). ISO 4406:1987 (Ref. 43) is the cleanliness 

code based on particle counters calibrated to the now obsolete ISO 4402 calibration (Ref. 37) that used 

irregularly shaped AC FTD. ISO 4406:1999 (Ref. 44) uses the approved ISO 11171 calibration method 

(Ref. 38) based on the NIST spherical-particle reference material. ISO 281:2007 (Ref. 42) incorporates 

the new ISO 4406:1999 cleanliness code (Ref. 44) but does not specify filter ratings to the levels of 

contamination. 

Based on the above discussion it became the objective of the work reported herein to (1) review 

methods and data for determining the effects of lubrication oil particle quantity and size for calculating 

the bearing L10, (2) experimentally calibrate older filter ratings that used AC FTD to new filter ratings 

using the NIST traceable particle counting calibration and ISO MTD, (3) recalibrate the NeedelmanZaretsky equations for determining effect of oil filtration on rolling-element bearing life to new filter 

ratings per ISO 16889 (Ref. 36), (4) determine filter ratings and related life factors based on new 

cleanliness codes per ISO 4406:1999 (Ref. 44) for ISO 281:2007 (Ref. 42), and (5) compare recalibrated 

filter life adjustment factors to cleanliness ratings presented in ISO 281:2007. 

Nomenclature 

C1 empirically determined constant 

C2 empirically determined exponent 

Cu fatigue load or stress endurance limit of the bearing material

Dp bearing pitch diameter, mm 

Ex particle removal efficiency, (1 – 1/Ex) u 100, percent 

eC lubricant contamination or oil cleanliness factor 

FR filter rating, micron 

h elastohydrodynamic (EHD) film thickness, Pm 

hc central or average elastohydrodynamic (EHD) film thickness, Pm 
NASA/TM—2014-218071 4

L10 bearing 10-percent or catalog life or the operating time that is exceeded by 90 percent of 

a group of bearings of a given type, hours or millions of inner-ring revolutions 

L50 bearing 50-percent or median life or the time that is exceeded by 50 percent of a group of 

bearings of a given type, hours or millions of inner-ring revolutions 

LF life adjustment factor for effect of filtration on bearing L10 life 

m Weibull slope or modulus 

NDx average number of particles downstream of the filter whose particle sizes are greater than 

x Pm 

NUx average number of particles upstream of the filter whose particle sizes are greater than 

x Pm 

P applied dynamic equivalent (applied) load to the bearing, N 

x particle size, Pm 

aISO life adjustment factors LF in ISO Standard 281:2007 

E filtration factor or filter rating, NUx/NDx

ȕx old filter rating, micron 

Ex(c) new filter rating, micron 

ț ratio of the actual viscosity of the lubricant in the bearing to a reference viscosity where 

ț § ȁ1.3

/ lubricant film parameter, hc/V

V composite surface roughness,  1 2 2

2 2 V1  V , Pm 

V1, V2 root mean square (rms) surface roughness of contacting bodies, Pm 

Z1,2 rotational speed, rpm 

1 body 1 or ball or roller 

2 body 2 or bearing raceway 

Failure Morphology 

It is generally accepted that if a rolling-element bearing is properly designed, manufactured, installed, 

lubricated, and maintained, “classical” rolling-element fatigue is the failure phenomenon that limits 

bearing life (Ref. 45). Rolling-element fatigue is extremely variable but is statistically predictable 

depending on the steel type, steel processing, heat treatment, bearing manufacturing and type, and 

operating conditions. This type of fatigue is a cycle-dependent phenomenon resulting from repeated stress 

under rolling-contact conditions and is considered high-cycle fatigue. Sadeghi, et al. (Ref. 46) provide an 

excellent review of this failure mode. 

Rolling-element fatigue can be simply categorized as either surface or subsurface initiated (Fig. 1). 

The subsurface-initiated fatigue failure is that referred to as ‘classical’ rolling-element fatigue. The 

fatigue failure manifests initially as a pit or spall that is generally limited in depth to the zone of the 

resolved maximum shearing stresses and in diameter to the width of the contact area (Fig. 1) (Ref. 45). 

Figure 1(a) illustrates the sequential progression of a subsurface-initiated crack from a nonmetallic 

inclusion such as a hard oxide inclusion that acts as a stress raiser. With repeated stress cycles the crack 

propagates to form a crack network that reaches the surface, resulting in a spall shown sequentially in the 

bearing race track. At this point the bearing is no longer fit for its intended purpose and should be 

removed from service. Bearing rating life L10 as defined by the standards ANSI 9–1990 (Ref. 6), 

ANSI 11–1990 (Ref. 7), and ISO 281:2007 (Ref. 42) is based on subsurface origin (classical) 

rolling-element fatigue (Ref. 45).
NASA/TM—2014-218071 5

When the bearing is operated under conditions that deviate from the rating or reference condition, the 

fatigue origin can be of “surface” origin as illustrated in Figure 1(b). In this instance the spall can initiate 

from a defect or stress raiser on and/or near the surface of the bearing raceways and/or rolling elements 

(Refs. 45, 47, and 48). In the instance of surface-initiated fatigue spall from hard-particle contamination, 

the load zone of the race and/or rolling element is indented by the contaminant (Fig. 2). The indent acts as 

a stress raiser from which the crack initiates and then propagates to form a crack network into the 

subsurface region of resolved maximum shearing stresses (Fig. 1(b)). A spall similar in appearance to that 

of the surface-initiated spall is formed. The characteristic difference between the surface- and subsurfaceinitiated spalls is that surface-initiated spalls result in a “arrowhead-type” geometry at the leading edge or 

point of origin on the rolling-element or raceway surface (Fig. 1(b)). 
NASA/TM—2014-218071 6

The number, size, and material properties of particles entering the Hertzian contact of the rolling 

element and raceway impact bearing life. The nature of the particles in the oil is a function of several 

processes: 

1. Manufacturing processes (swarf, chips, and grit) 

2. Internal generation, including wear debris and chemical attack of surfaces 

3. Ingression from the environment (sand and dust) 

4. Maintenance activities (making/breaking fittings and new oil) 

5. Lubricant breakdown products (sludges, precipitates, and coke) 

Typical particle size distributions from a variety of mechanical systems are shown in Figure 3 

(Refs. 31 and 32). 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 

more large particles than small ones (Ref. 31). Steele (Ref. 49) reported a wide range of particulate levels 

in unused turbine oils. 

Work reported by Tonicello et al. (Ref. 47) showed that for pairs of disks in rolling contact where one 

pair comprises silicon nitride (Si3N4) against AISI M–50 bearing steel and the second pair comprises AISI 

M–50 against AISI M–50, and the oil particle contamination is AISI M–50 wear debris, the dent 

indentation on the raceway of the respective AISI M–50 disk was three times deeper with the Si3N4 disk 

than with the AISI M–50 steel disk on a AISI M–50 steel disk (Fig. 4). Greater stress concentration in the 

Hertzian contact results from deeper dents. It can be reasonably concluded that ceramic balls or rollers 

can lead to deeper dents in a mating steel raceway (Ref. 47). 
NASA/TM—2014-218071 7

Morales-Espejel and Gabelli (Ref. 50) discuss “the different hypotheses available to explain the 

interaction of sliding (and rolling) with the indentation marks in both gears and rolling bearings” under 

EHD lubrication. They present theory and analysis that are qualitatively verified by experiment. The 

micropitting phenomenon occurring around indentation marks are described by them with the same 

physical model of Morales-Espejel and Brizmer (Ref. 51) that takes into account the progression of 

surface fatigue induced by locally reduced lubrication conditions. 

Filter Rating Procedure 

The basic procedure for rating filters is shown in the flow diagram of Figure 5. During the multipass 

test, slurry of silica particles is continuously fed into a recirculating system. 

Particles flow into the filter, where some are captured, and others return to the reservoir where they 

continue to recirculate. Throughout the test, particles upstream and downstream of the filter are quantified 

with electronic automatic particle counters. The filter factor, or filter rating, for particle size x is defined 

as the ratio of upstream to downstream counts recorded during the test, denoted Ex and termed the “beta 

ratio”: 

Ex = (Upstream counts t x Pm) y (Downstream counts t x Pm) = 

x

x

ND

NU

 (1) 

where NUx and NDx are the average number of particles upstream and downstream of the filter, 

respectively, whose particle size is greater than x Pm. 

Originally, automatic particle counters were calibrated using AC FTD, per ISO 4402:1991 (Ref. 37). 

This material comprises small irregularly shaped particles of silica sand. Although widely used, AC FTD 

lacked traceability, had batch-to-batch variations, and reported size distributions of dubious accuracy 

especially below 10 Pm. In order to make testing more reproducible, calibration methods were developed 

traceable to an NIST standard reference material, NIST SRM 2806 (Ref. 39). However, the new 

calibration method (per ISO 11171:1999) (Ref. 38) reports the size of particles (in microns) as the 

diameter of an equivalent sphere, rather than the longest dimension as in the original method. These 

alterations, in effect, changed the “micron ruler.” As shown in Table I (Ref. 38), the particle sizes 

originally below 10 Pm are reported larger, and sizes above 10 μm are reported smaller. For example, a 

silica particle reported to be 3 Pm in size by a particle counter calibrated to the old standard is now 

reported as 5.1 Pm in size using the new calibration standard. 
NASA/TM—2014-218071 8

TABLE I.—EXPERIMENTAL COMPARISON OF CONTAMINANT 

PARTICLE SIZE CLASSIFICATION BASED ON 

ISO STANDARDS 

[Adapted from Ref. 38.] 

Old size, 

ISO 4572:1981 

method (Ref. 33), 

AC FTD calibration 

ȕx = 200, 

Pm 

New size, 

ISO 16889:2008 

method (Ref. 36), 

ISO MTD calibration 

ȕx(c) = 200, 

Pm 

1 4.2 

2 4.6 

3 5.1 

5 6.4 

7 7.7 

10 9.8 

15 13.6 

20 17.5 

25 21.2 

30 29.4 

40 31.7 

Along with calibrating automatic particle counters, AC FTD was also used as the test contaminant in 

the previous version of the multipass test. When it became unavailable in the 1990s, an alternative test 

material with similar chemical composition and size distributions was adopted, ISO MTD. Although 

using a slightly different test contaminant influences test results, the changes are less significant than 

those produced by the new particle size calibration described above. The Ex values obtained via the 

revised multipass test (ISO 16889:2008) (Ref. 36), using ISO MTD and particle counters calibrated to the 

NIST standard, are now reported as Ex(c) values, where x is the micron size per Equation (1), and c

emphasizes the new calibration method. 
NASA/TM—2014-218071 9

Twenty years ago leading filter manufacturers rated filters at the particle size x where Ex = 200, as 

calculated by Equation (1). For example, a filter rated at E5 = 200 has 1 out of every 200 particles equal to 

or greater than 5 Pm pass through the filter during testing. However, most engineers and end-users prefer 

to think of filter ratings as the size where essentially no particles pass through the filter. In an attempt to 

reach this ideal, many manufacturers now also rate filters at the particle size x where Ex = 1000. At this 

higher rating, only 1 particle in 1000 passes through the filter during testing. Using the new ISO standard, 

modern filter ratings are Ex(c) = 200 and Ex(c) = 1000. Typical removal efficiency results are shown in 

Figure 6. 

The removal efficiency Ex of any particle size x can be related to the E factor as follows: 

Ex = (1 – 1/Ex) u 100 (2) 

For any filter there is a large particle size above which essentially nothing passes, Ex t 104

 and 

Ex ĺ 100 percent. In contrast, there is also a small particle size for which Ex ĺ 1 and Ex ĺ 0 percent, so 

that nearly all particles this size and smaller freely pass through the filter and accumulate to copious 

amounts in recirculating systems. For intermediate sizes, a fraction of the particles are captured and the 

rest pass downstream. As an example, a filter with E10(c) = 1000 removes 99.9 percent of all particles 

t10 Pm in size during a multipass test. 

In summary, the changes to the ISO filter rating standard were as follows: 

1. Particle counter calibration 

 a. Changed from AC FTD to NIST calibration 

 b. Increased accuracy and reproducibility 

 c. Changed the “micron ruler” 

2. Test contaminant 

 a. Changed from AC FTD to ISO MTD 

 b. Increased accuracy and reproducibility 

3. Highest filter rating changed from Ex = 200 to Ex(c) = 1000 

a. Closer to concept of “absolute rating” 

Examples: 

1. For E5(c) = 200, 1 out of every 200 particles or 5 out of every 1000 particles greater than 5 Pm 

passes through filter during test. 

2. For E5(c) = 1000, 1 out of every 1000 particles greater than 5 Pm passes through filter during test. 
NASA/TM—2014-218071 10

Results and Discussion 

In 1991, based on the experimental research of Bhachu et al. (Refs. 12 and 13) and Loewenthal et al. 

(Refs. 9 to 11), Needelman and Zaretsky (Ref. 31) presented a set of empirically derived equations for 

bearing fatigue life (adjustment) factors (LF) as a function of oil filter ratings (FR). These equations were 

normalized (LF = 1) to a 10 Pm filter rating at normal cleanliness (as it was then defined, where ȕx = 200 

per ISO 4572:1981 (Ref. 33). The life factor equations were incorporated into the reference book, “STLE 

Life Factors for Rolling Bearings” (Ref. 32). 

The Needelman and Zaretsky oil filtration life factors (Ref. 31) have been used, in conjunction with 

ANSI/ABMA standards (Refs. 6 and 7) and with bearing computer codes (Ref. 34). Experimental studies 

made by other investigators verify the relationship between rolling-element bearing life and the size, 

number, and types of particle contamination (Refs. 1, 2, 4, 5, 8, 14 to 20, 47, and 48). The ISO 281:2007 

(Ref. 42) incorporates a rolling-element bearing life factor based on lubricant cleanliness and EHD film 

thickness based on the work of Ioannides et al. (Ref. 17) but does not relate the lubricant cleanliness to 

the filter ratings. 

Filter ratings have been revised and improved (Ref. 35). They are now based on an upgraded filter 

rating method per ISO 16889:2008 (Ref. 36), employing particle counts calibrated to an NIST standard 

(Refs. 36 and 39). The work reported here was undertaken to calibrate the “old” and obsolete filter ratings 

to the “new” filter ratings, and to recalibrate the previously published bearing life factors based on the old 

filter ratings to life factors based on the new filter ratings. The revised rolling-element bearing life factors 

were then compared to the life factors in the ISO 281:2007 Standard (Ref. 42) that are based solely on 

lubricant cleanliness levels. 

Recalibration of Filter Ratings 

Old Ex = 200 filter ratings (AC FTD calibration) were converted to new Ex(c) = 200 filter ratings using 

Table I and plotted in Figure 7. Although this transformation does not take into account the change in test 

contaminant to ISO MTD, alterations in counter calibration dominate over the change in test contaminant. 

The equation relating old Ex = 200 filter ratings (FROLD200) with new Ex(c) = 200 filter ratings (FRNEW200) is 

FRNEW200 = 0.722(FROLD200) + 2.97 (3a) 

or 

FRȕx(c)200 = 0.722(FRȕx200) + 2.97 (3b) 

and 

FROLD200 = 1.39(FRNEW200) – 4.11 (4a) 

or 

FRȕx200 = 1.39(FRȕx(c)200) – 4.11 (4b) 
NASA/TM—2014-218071 11

Multipass filter tests were then performed according to the new ISO 16889:2008 (Ref. 36) method 

using 25 different filters over a wide range of filter efficiencies from five different manufacturers. This 

allowed plotting Ex(c) = 1000 filter ratings with Ex(c) = 200 ratings. The relationship is plotted in Figure 8, 

and approximated by the equations 

FRNEW1000 = 1.17 (FRNEW200) + 0.650 (5a) 

 or 

FRȕx(c)1000 = 1.17 (FRȕx(c)200) + 0.650 (5b) 

and 

FRNEW200 = 0.855 (FRNEW1000) – 0.556 (6a) 

 or 

FRȕx(c)200 = 0.855 (FRȕx(c)1000) – 0.556 (6b) 

By cross-plotting the data of Figure 7 with those of Figure 8, a relation between old and obsolete 

Ex = 200 filter rating and new filter rating Ex(c) = 1000 was obtained (Fig. 9) described by the equations 

FRNEW1000 = 0.848 (FROLD200) + 4.14 (7a) 

 or 

FRȕx(c)1000 = 0.848 (FRȕx200) + 4.14 (7b) 

and 

FROLD200 = 1.18 (FRNEW1000) – 4.88 (8a) 

 or 

FRȕx200 = 1.18 (FRȕx(c)1000) – 4.88 (8b) 
NASA/TM—2014-218071 12
NASA/TM—2014-218071 13

TABLE II.—EFFECT OF OIL FILTRATION ON THE ROLLING-ELEMENT FATIGUE LIFE OF 

25-Pm-BORE ROLLER BEARINGS 

[Radial load, 2975 N; original surface composite roughness (rms) 0.33 Pm. From Bhachu et al. (Refs. 12 and 13).] 

Test filter 

rating 

(Ex t

200), 

Pm 

Composite 

surface 

roughness, 

V after 

testing, 

Pm 

Calculated 

film 

thickness, 

Pm 

Film 

parameter 

after 

testing, 

/

Experimental life, millions of inner-race 

revolutions 

Weibull 

slope, 

m 

Failure 

indexa

Predicted L10

life from 

Eq. (9), 

106

 innerrace 

revolutions 

10-

percent 

life, 

L10

90-percent 

confidence

limits 

50-percent 

life, 

L50

90-percent 

confidence

limits 

40 

25 

6 

3 

2.5 

Magnetic 

0.41 

.36 

.32 

.26 

.22 

.20 

0.58 1.4 

1.6 

1.8 

2.2 

2.6 

2.9 

1.5 

2.5 

4.5 

8.0 

6.5 

5.0 

1.0 to 2.0 

2.0 to 3.0 

3.4 to 5.8 

5.5 to 11.6 

3.6 to 11.8 

2.9 to 8.5 

2.1 

3.0 

5.9 

12.0 

12.4 

10.0 

1.9 to 2.3 

2.9 to 3.1 

5.4 to 6.3 

10.7 to 13.5

9.9 to 15.5 

8.3 to 12.0 

5.8 

9.4 

7.0 

4.7 

2.9 

3.2 

10 out of 10 1.9 

2.5 

5.5 

8.0b

8.8 

--- 

40 

3 

40/3 

0.41 

.26 

.41 

0.99 

.99 

.99 

2.4 

3.8 

2.4 

1.8 

9.3 

1.7 

------------- 

------------- 

-------------

3.3 

16.2 

3.1 

------------- 

------------- 

------------- 

3.1 

3.4 

3.3 

10 out of 10 2.2 

9.3b

2.2 a

Number of fatigue failures out of number of bearings tested. 

b

Life prediction normalized to 3-Pm filter, Ex = 200. 

Rolling Bearing Fatigue Life Factors 

There were two independent investigations to specifically determine the effect of oil filtration on 

rolling-element bearing life. These were the studies of Loewenthal et al. (Refs. 9 to 11) and Bhachu et al. 

(Refs. 12 and 13). 

Roller Bearings 

Bhachu et al. (Refs. 12 and 13) 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 with 25-mm bore roller bearings having a 

2957 N radial load. For each test series, gear oil flow was passed through one of four possible filters of 

different ratings from 2.5 to 40 Pm or through an electromagnetic separator and continuously supplied to 

a parallel roller-bearing fatigue tester. EHD film thickness and ȁ values during testing are shown in 

Table II. 

Significantly, tests run with 40-Pm filtration for only 30 min before switching to 3-Pm filtration 

showed substantially the same lives as if all running had been with a 40-Pm filter. Apparently the early 

damage could not be healed, at least in these small roller bearings. The test results are also shown in 

Table II. These results show that life increased with improved filtration. 

The original filter ratings were based on the old filter rating method, ISO 4572:1981 (Ref. 33), with 

ȕx = 200. From these roller bearing life data (Table II), it was assumed in (Ref. 31) that the filter life 

factor takes the following form: 

  2 1

C LF C FR  | (9a) 

C1 is an empirically determined constant and C2 is an empirically determined exponent. The 

experimentally determined L10 lives from Table II for filter ratings Ex200 = 3 and 25 Pm are 8 and 

2.5 million inner-race revolutions. Solving for C1 and C2 and normalizing LF = 1 when FR = 10 Pm at 

ȕx = 200 (normal cleanliness), the following empirical relation was obtained 

 LF § 3.5(FROLD200)

–0.55 (9b) 
NASA/TM—2014-218071 14

 or 

LF § 3.5(FRȕx200)

–0.55 (9c) 

Substituting the new filter ratings into Equation (9) from Equations (4) and (8), respectively, allows 

calculating bearing life factors for new filter ratings: 

For Ex(c) = 200: 

LF § 3.5 [1.39(FRNEW200) – 4.11]–0.55 (10a) 

 or 

LF § 3.5 [1.39(FRȕx(c)200) – 4.11]–0.55 (10b) 

For Equation (10) above, where FR ȕx(c)200 d 4, LF § 2.8. 

For Ex(c) = 1000: 

LF § 3.5[1.18(FRNEW1000) – 4.88]–0.55 (11a) 

 or 

LF § 3.5[1.18(FRȕx(c)1000) – 4.88]–0.55 (11b) 

For Equation (11) above, where FRȕx(c)1000 d 5, LF § 3.5. 

Using Equation (9), the L10 lives for roller bearings were predicted by normalizing Equation (9) to the 

experimentally obtained L10 life using the 3-Pm-rated (ȕx = 200) filter. These results are presented in 

Table II for comparison purposes. 

The posttest inner-raceway measurements for 40-Pm (ȕx = 200) filtration showed greater out-ofroundness than in the untested bearing. Less out-of-roundness was observed with finer filtration down to 

the 8-Pm (ȕx = 200) rating. Virtually no out-of-roundness was observed when the 3-Pm (ȕx = 200) filter 

was used. Below the 3-Pm (ȕx = 200) level the measurement was similar to that of the unrun bearing. 

Bhachu et al. (Refs. 12 and 13) suggested that particles smaller than 3 Pm (ȕx = 200) 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 the data of Table II that the lack of contamination contributes to 

improvement in bearing raceway surface finish during operation. There appears to be a correlation between 

the lubricant film parameter / after testing and rolling-element fatigue life as evidenced by these data. 

Ball Bearings 

Loewenthal et al. (Refs. 9 to 11) performed a series of tests to measure the quantitative effects of 

filtration on rolling-element fatigue life. Four levels of filtration were investigated using full-flow 

(ȕx = 200) filters rated at 3, 30, 49, and 105 Pm. The 3-Pm (ȕx = 200) filter used for these tests had been 

developed to replace the original 40-Pm (ȕx = 200) filter for a helicopter gas turbine lubrication system. 

During service these new filter elements were not only found to provide a much cleaner lubricant with 

less component wear but contrary to prior belief, greatly extend the time between filter and oil changes, as 

discussed by Loewenthal et al. 

The test bearings were 65-mm deep-groove ball bearings run at 15 000 rpm under a radial load of 

4580 N, which produced a maximum Hertz stress of 2410 MPa. The lubricant contaminant rate was 

0.125 g/hr 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. The test lubricant was a MIL–L–23699 type, which produced a / value of 

3.3 based on race and ball pretest surface finish measurements. The test contaminant was similar to the 

particulate matter found in the lubricant filters of 50 IT8D commercial engines (Jones and Loewenthal 
NASA/TM—2014-218071 15

TABLE III.—COMPARISON OF BALL BEARING FATIGUE LIFE RESULTS WITH AN ULTRACLEAN LUBRICANT 

AND WITH DIFFERENT LEVELS OF FILTRATION IN A CONTAMINATED LUBRICANT 

[Radial load, 4580 N; speed, 15 000 rpm; temperature, 347 K; test lubricant, 

MIL–L–23699 type; film parameter /, 3.3. From Loewenthal et al. (Refs. 9 to 11).] 

Test series 

(lubricant 

condition) 

Test filer rating 

(ȕx t 200), 

ȝm 

Experimental life, 

hr 

Weibull 

slope, 

m 

Failure indexa Confidence 

number,b

percent 

Predicted 

L10 life 

from 

Eq. (12),

hr 10-percent 

life, 

L10

50-percent 

life, 

L50

L10 L50

Ultraclean 

Clean 

(baseline) 

3 

49 

1099 

672 

1741 

2276 

4.1 

1.5 

5 out of 9 

9 out of 32 

-- 

76 

-- 

-- 

1099d

547 

Contaminated 

Contaminated 

Contaminated 

Contaminated 

3 

30 

49 

105c

505 

594 

367 

------ 

993 

857 

533 

----- 

2.8 

5.1 

5.1 

--- 

10 out of 16 

11 out of 16 

20 out of 32 

--------------- 

93 

96 

99 

-- 

99 

99 

99 

-- 

505e

284 

251 

208 a

Number of fatigue failures out of number of bearings tested. 

b

Probability (expressed as a percentage) that bearing fatigue life in a given test series will be inferior to the life obtained with ultraclean 

lubrication. A 90-percent or greater confidence number is considered statistically significant. c

Test series was suspended after 448 test hours on each of the test bearings due to excessive bearing wear. No fatigue failures were encountered. d

Life prediction normalized to 3-ȝm filter, ȕx = 200 and L10 = 1099 hr. e

Life prediction normalized to 3-ȝm filter, ȕx = 200 and L10 = 505 hr. 

(Ref. 52)). Because this engine has a number of carbon-graphite bearing sump seals, replication of oil 

contaminants in engines with “windback-type” labyrinth seals demanded the use of a contaminant made 

of 88 percent carbon-graphite dust, 11 percent Arizona test dust, and 1 percent stainless steel particles. 

The results of these tests are summarized in Table III. As with the work of Bhachu et al. (Refs. 12 and 

13), 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-Pm filters. Because of the severe wear 

obtained, the contaminated 105-Pm 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 for 

ȕx = 200 filter rating for ball bearings: 

LF § 1.8(FROLD200)

–0.25 (12a) 

 or 

LF § 1.8(FRȕx200)

–0.25 (12b) 

Using Equation (12), the L10 lives of ball bearings were predicted by normalizing Equation (12) to the 

experimentally obtained L10 life using the 3-Pm (ȕx = 200) rated filter. These results are presented in 

Table III for comparison purposes. 

Substituting the filter rating for FR (ȕx = 200) from Equations (4) and (8), respectively, into 

Equation (12), filter life factors can be calculated for the new filer ratings where 

1. For Ex(c) = 200 filter rating: 

LF § 1.8[1.39(FRNEW200) – 4.11]–0.25 (13a) 

 or 

LF § 1.8[1.39(FRȕx(c)200) – 4.11]–0.25 (13b) 
NASA/TM—2014-218071 16

for Equation (13), where FRȕx(c)200 d 4, LF § 1.6. 

2. For Ex(c) = 1000 filter rating: 

LF § 1.8[1.18(FRNEW1000) – 4.88]–0.25 (14a) 

or 

LF § 1.8[1.18(FRȕx(c)1000) – 4.88]–0.25 (14b) 

For Equation (14), where FRȕx(c)1000 d 5, LF § 1.8. 

Table IV provides results from Equations (6), (9), and (12) for various filter ratings. The resultant life 

adjustment factors can be used to adjust the calculated bearing L10 or catalog life to account for filtration 

level in the lubricant system. These LF values are normalized to filter ratings (FR) of 10 Pm at Ex(c) = 200 

and 13 Pm at Ex(c) = 1000 (normal cleanliness) and are independent of the ȁ and/or ț values, loading 

conditions, and bearing size. Based upon the data of Loewenthal et al. (Refs. 9 to 11), it is not 

recommended to use a life adjustment factor less than 0.5 even when no filter is used. Further, 

Equations (9) and (12) may reflect the differences in the effect of particle damage between roller and ball 

bearings. 

Technology for improved oil filtration is commercially 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 and ambient mineral dusts, all-too-frequent high 

contamination levels in new oil also requires good filtration. 

No reported testing has been performed comparing grease lubrication, which entraps wear debris, 

with oil lubrication for the same bearings, with or without oil filtration. It is suggested that for long-term 

application a LF = 0.5 for grease lubrication be considered the same as oil lubrication for bearings 

without filtration where no periodic regreasing of the bearing occurs. Where the bearing is continuously 

or periodically regreased, a LF = 1 should be considered. 

TABLE IV.—NEEDELMAN-ZARETSKY OIL CLEANLINESS (FILTER) 

LIFE FACTORS (LFs) BASED ON OIL FILTER RATING (FR) 

Filter rating, FR, 

ȝm 

Life factor, 

LF

(Old) 

ȕx = 200 

(New) 

ȕx(c) = 200 

Eq. (3) 

(New) 

ȕx(c) = 1000 

Eq. (7) 

Roller bearing,

Eqs. (9) to (11) 

Ball bearing, 

Eqs. (12) to (14) 

3 5 7 2.0 1.4 

6 7 9 1.4 1.2 

8 9 11 1.1 1.1 

10a 10 13 1.0 1.0 

12 12 14 0.9 1.0 

25 21 25 0.6 0.8 

40 32 38 0.5 0.7 

49b 38 46 0.4b 0.7 

60b 46 55 0.4b 0.6 

105b 79 93 0.3b 0.6 a

Normalized to FR = 10 ȝm at (ȕx = 200) b

For filter ratings at ȕx = 200 exceeding 40 ȝm; with no filtration it is not recommended to use LF  0.5. 
NASA/TM—2014-218071 17

Comparison of Filter Life Factors (LFs) to ISO Standard 281:2007 

In 2000 the International Organization for Standardization (ISO) modified the standard ISO 

281:1990, “Rolling bearings—Dynamic Load Ratings and Rating Life,” to include a fatigue limit 

(Ref. 53). The endurance or fatigue limit as applied to rolling-element bearings is based on the theoretical 

work presented in 1985 by Ioannides and Harris (Ref. 54). It is a theoretical load or shearing stress (based 

on a Hertzian contact stress) below which no fatigue failure is assumed to occur, and therefore where 

fatigue life is infinite. ISO 281:2007 replaced ISO 281:1990 as modified in 2000. The 2007 standard 

adopted the 1999 approach to bearing life calculations presented by Ioannides, Bergling, and Gabelli 

(Ref. 55) and includes the effects of lubricant contamination on bearing life. 

The ISO 281:2007 Standard (Ref. 42) incorporates a new service life formula that integrates all life 

adjustment factors LF in what is now called aISO. The life factor aISO includes four interdependent factors: 

(1) lubrication regime, ț; (2) lubricant contamination or oil cleanliness, eC; (3) applied dynamic 

equivalent load (applied load) to the bearing, P; and (4) fatigue load or stress endurance limit of the 

bearing material, Cu. 

(15) 

From the ISO 281:2007 Standard (Ref. 42), “...when the lubricant is contaminated with solid 

particles, permanent indentations in the raceway(s) (and rolling elements) can be generated when these 

particles are over rolled. At these indentations, local stress risers are generated, which will lead to a 

reduced life of the rolling bearing. This life reduction due to contamination in the lubricant film is taken 

into account by the contamination (life) factor eC.” In the ISO 281:2007 Standard (Ref. 42), the 

contamination factor eC is given in a table based upon levels of contamination that are not tied to specific 

filter ratings. 

The standard states that the contamination life factor is dependent on the following: 

1. Type, size hardness and quantity of the (contaminant) particles. 

2. EHD lubricant film thickness (viscosity ratio, ț). 

3. Bearing size (bearing pitch diameter, Dp). 

The lubrication regime is defined by EHD theory. In the standard it is defined by the parameter ț, the 

ratio of the actual viscosity of the lubricant in the bearing at operating temperature to a reference 

viscosity. The reference viscosity is that which would produce a lubricant film thickness equal to the 

composite surface roughness of the rolling element and the raceway, or / = 1. If ț  1, the contact is in a 

boundary lubrication regime where the surface asperities of the rolling element and the raceway are in 

contact. It is preferable to operate the bearing in a lubrication regime where ț t 1. As ț increases, bearing 

life increases. The ț value in the standard is based on what is termed in EHD theory as the lubricant 

factor, or ȁ. The ȁ is equal to the EHD film thickness h divided by the composite surface finish ı of the 

rolling elements in contact with the raceway: 

V

/ h (16a)

where 

  2

2

2 V V1  V (16b) 

»

1⁄4

o «

¬

a ,N

P

e C

a f C u ISO
NASA/TM—2014-218071 18

and V1 and V2 are the root mean square (rms) surface roughness of contacting bodies. Unfortunately, in 

the ISO 281:2007 Standard (Ref. 42), ț is based on an undefined lubricant and lubricant properties and an 

undisclosed composite surface finish, ı. However, from the standard where ȁ can be calculated, 

ț § ȁ1.3 (17) 

an approximate correlation can be established between filter ratings and the contamination levels. For 

example, BFPA/P5:1999 (Ref. 56) correlates ISO contamination levels with Ex = 75 and Ex = 200 filter 

ratings. Using a similar approach, we correlated filter ratings at Ex(c) = 1000 per ISO 16889:2008 

(Ref. 36) to the contamination levels listed in ISO 281:2007 (Ref. 42). These contamination levels and 

filter ratings, together with the life factor eC, are shown in Table V. Also, listed are the NeedelmanZaretsky filter life factors at Ex(c) = 1000 from Equations (11) and (14) for comparison purposes. 

The contamination life factors, eC, from ISO 281:2007 (Ref. 42) are differentiated by both filter rating 

and bearing pitch diameter. The Needelman-Zaretsky life factor equations do not differentiate based on 

bearing size but do distinguish between ball and roller bearing types (Ref. 31). 

Although we consider the effect of ȁ on rolling-element fatigue life independent of and separate from 

the filter life factors, the Needelman-Zaretsky life factors (Ref. 31) are normalized at ȁ § 1.1 and LF = 1. 

The contamination life factor eC from Table V can be used to adjust the calculated bearing L10 or 

catalogue life to account for filtration level in the lubricant system. These results are shown in Table VI. 

The values of eC for Dp  100 mm were used. For the data of Bhachu et al. (Refs. 12 and 13), ț § 2.8. For 

the data Loewenthal et al. (Refs. 9 to 11), ț § 4.7. For each respective set of data the EHD film thickness 

was assumed by us to remain unchanged. Hence, the effect of ȁ and/or ț was not factored into the 

predicted lives shown in Table VI. 

The results from the Bhachu et al. (Refs. 12 and 13), and Loewenthal et al. (Refs. 9 and 11), suggest 

the following: (1) For filtration levels between 4 and 34 Pm at Ex(c) = 1000, representing “extreme 

cleanliness” to “typical contamination,” the ISO 281:2007 Standard (Ref. 42) provides a reasonable 

qualitative estimate of the effect of particle damage on rolling bearing life; and (2) At conditions of severe 

contamination and above where the filter ratings are t35 Pm at Ex(c) = 1000, the ISO 281:2007 Standard 

(Ref. 32) correlates with the Bhachu et al. results. In contrast, the ISO 281 is conservative compared to 

the Loewenthal et al. test results. This may be attributed to the use in the Loewenthal et al. tests of a 

contaminant based on carbon-graphite particles that may act as a solid lubricant while the ISO 281 life 

ratings are primarily concerned with common hard steel contamination that can be found in industrial 

gearboxes and used by Bhachu et al. in their tests. 

Gabelli, Morales-Espejel, and Ioannides (Ref. 20) provide a discussion of the theoretical basis for the 

calculation of the contamination factor eC that correlates with the curves of contaminant life factors

versus ț values presented in the ISO 281:2007 Standard (Ref. 42) and the contaminant life factors 

presented in Table V that are from the standard. According to Gabelli, Morales-Espejel, and Ioannides 

(Ref. 20), the following variables should apply in determining a contamination factor eC: (1) mean 

bearing (pitch) diameter, (2) level of contamination (filter size), and (3) lubrication rating of the bearing 

(ț value). Gabelli, Morales-Espejel, and Ioannides reduce the variables by the elimination of the fatigue 

limit. Their theoretical results were similar to those in Annex A of the ISO 281:2007 Standard (Ref. 42). 

There is an issue as to whether the EHD film thickness or ȁ (ț value) mitigates the negative effect of 

lubricant contamination on rolling-element fatigue life. That is, is the effect of contamination on bearing 

life less severe with increasing film thickness? In order to benchmark their analysis, Gabelli, MoralesEspejel, and Ioannides (Ref. 20) presented endurance data of 172 bearing population samples obtained 

over several years comprising 14 types and sizes of rolling-element bearings. It was reported by them that 

“each bearing sample is normally formed of a group of 30 bearings; several thousand bearings were 

endurance tested for this set of experimental results.” 
NASA/TM—2014-218071 19

TABLE V.—COMPARISON OF EFFECT OF OIL FILTER RATING ON ROLLING-ELEMENT BEARING FATIGUE LIFE 

FACTOR (LF) BETWEEN ISO 281–2007 (REF. 42) AND NEEDELMAN-ZARETSKY, EQS. (11) AND (14) 

Level of contaminationa Filter rating, 

Pm, 

(ȕx(c) = 1000) 

Contamination life factor,a

eC 

Needelman-Zaretsky

filter life factor, 

LF

Dp  100 mm Dp t 100 mm Roller 

bearing 

Ball 

bearing 

Extreme cleanliness 

(Particle size of the order of lubricant film thickness; 

laboratory conditions) 

d4 1 1 3.5 1.8 

High cleanliness 

(Oil filtered through extremely fine filter; conditions 

typical of bearing greased for life and sealed) 

5 to 9 0.8 to 0.6 0.9 to 0.8 3.5 to 1.3 1.8 to 1.2 

Normal cleanliness 

(Oil filtered through fine filter; conditions typical of 

bearings greased for life and shielded) 

10 to 14 0.6 to 0.5 0.8 to 0.6 1.2 to 0.9 1.1 to 1.0 

Slight contamination 

(Slight contamination in lubricant) 15 to 24 0.5 to 0.3 0.6 to 0.4 0.9 to 0.6 1.0 to 0.8 

Typical contamination 

(Conditions typical of bearings without integral 

seals; course filtering; wear particles and ingress 

from surroundings) 

25 to 34 0.3 to 0.1 0.4 to 0.2 0.6 to 0.5 0.8 to 0.7 

Severe contamination 

(Bearing environment heavily contaminated and 

bearing arrangement with inadequate sealing) 

t35 0.1 to 0 0.1 to 0 0.5 0.7 to 0.5 

Very severe contamination None 0 0 0.5 0.5 

a

From Reference 42. 

TABLE VI.—COMPARISON OF PREDICTED ROLLING BEARING FATIGUE LIVES 

BASED ON CONTAMINANT LIFE FACTOR, eC, FROM TABLE V 

Level of contamination 

(see Table V) 

Test filter rating 

ȕx(c) = 1000 (ȕx = 200), 

Pm 

L10 life, 

inner-race 

revolutions 

Predicted L10 life based 

on contaminant life 

factor eC, 

106 inner-race 

revolutions 

Roller bearing data from Bhachu et al. (Refs. 12 and 13), Table II 

High cleanliness 

High cleanliness 

Normal cleanliness 

Typical contamination 

Severe contamination 

6.5 (2.5) 

7a

 (3) 

10 (6) 

32 (25) 

46 (40) 

6.5u106

8.0a

4.5 

2.5 

1.5 

~9u106

8a

~6 

~3 

~1 

Ball bearing data from Loewenthal et al. (Refs. 9 to 11), Table III 

High cleanliness 

Severe contamination 

Severe contamination 

Severe contamination 

7a (3) 

36 (30) 

56 (49) 

116 (105) 

455u106a 

535 

330 

403b

455u106a

~56 

~56 

~56 a

Normalized to 7-ȝm filter rating where ȕx(c) = 1000 for high cleanliness level of contamination. b

See Table 3. Test series was suspended after 448 test hours on each of the test bearings because of excessive 

wear. No fatigue failures were encountered. 
NASA/TM—2014-218071 20

The Gabelli, Morales-Espejel, and Ioannides data for ȁ values varying from 0.4 to 2.9 (ț = 0.3 to 4) 

comprises three contamination levels. They classified their contamination conditions as follows: 

1. The first contamination condition was classified as their “standard cleanliness tests.” The filtration 

level at Ex(c) = 1000 was d7 Pm and their range for eC varied from 0.8 to 1. This would be equivalent to 

“high cleanliness” in Table V. 

2. The second contamination condition was classified as “slight contamination.” This is equivalent to 

Ex(c) = 1000 filter range of 15 to 24 Pm in Table V. They reported that under the given test conditions the 

expected contamination (life) factors eC can range from 0.3 to 0.5. Their actual life data showed eC values 

that ranged from 0.1 to 0.5. 

3. The third contamination condition was classified as “typical to severe contamination.” From 

Table V this would comprise Ex(c) = 1000 filter range of 25 Pm or greater, although no filters appear to 

have been used in this test series. Their actual life data showed eC values that ranged from 0.01 to 0.3. 

The Gabelli, Morales-Espejel, and Ioannides experimental data, if statistically significant, shows a 

relation between the contaminant life factors eC and ț as presented in Annex A of the ISO 281:2007 

Standard (Ref. 42). 

In 1985 Lorosch (Ref. 14) reported that “The influence of contaminants is great with small bearings 

and decreases with increasing bearing size. Consequently, large bearings have a larger capacity than 

calculated.” In other words, for a particular contamination level or oil cleanliness, the effect of lubricant 

contamination is less severe for larger pitch diameter bearings than for smaller pitch diameter bearings. 

The effect of bearing size on the contamination factor eC is incorporated in Annex A and Table 13 

(Table V of this paper) of ISO 281:2007 Standard (Ref. 42). For pitch diameters ranging from 25 to 

2000 mm, the contamination (life) factors eC increased with increasing pitch diameter or bearing size. 

That is, the larger the bearing the less effect of contamination on the life of the bearing. 

The pitch diameters for the bearing tests reported by Gabelli, Morales-Espejel, and Ioannides 

(Ref. 20) were between 25 and 200 mm. Their data did not show a statistical relation between the 

contamination levels and bearing size. They explained, “the range of bearing sizes limited the range for 

comparison with bearing size.” However, the sizes and types of bearings were reasonably representative 

of those used in most rotating machinery applications. 

A comparison of the data in Table II, for the roller bearings from Bhachu et al. (Refs. 12 and 13), and 

those data for ball bearings in Table III from Loewenthal et al. (Refs. 9 to 11), suggest that the roller 

bearing lives are more sensitive to changes in contamination level than those of the ball bearing. This is 

reflected in the Needelman-Zaretsky contamination life factors in Table V. 

Summary of Results 

In 1991, Needelman and Zaretsky (Ref. 31) presented a set of empirically derived equations for 

bearing fatigue life (adjustment) factors (LF) as a function of oil filter ratings (FR). These equations for 

life factors were incorporated into the reference book, STLE Life Factors for Rolling Bearings (Ref. 32). 

These equations were normalized (LF = 1) to a 10 Pm filter rating at ȕx = 200 (normal cleanliness) as it 

was then defined and ȁ of 1.1. Over the past 20 years, these life factors based on oil filtration have been 

used in conjunction with ANSI/ABMA standards and bearing computer codes to predict rolling bearing 

life. Also, additional experimental studies have been made by other investigators into the relationship 

between rolling bearing life and the size, number, and types of particle contamination. During this time 

period, filter ratings have also been revised and improved and are now based on particle counts calibrated 

to a NIST standard reference material in the ISO 11171:1999 Standard (Ref. 38). It was the objective of 

the work reported herein to (1) review methods and data for determining the effects of lubrication oil 

particle size for calculating the bearing L10 or catalog life, (2) experimentally correlate older and obsolete 

filter ratings and the new ISO filter ratings, (3) recalibrate the Needelman-Zaretsky equations for 
NASA/TM—2014-218071 21

determining effect of oil filtration on rolling-element bearing life to the new filter ratings, (4) relate the 

new filter ratings to contamination levels listed in the ISO 281:2007 Standard, and (5) compare 

recalibrated filter life adjustment factors to those cleanliness ratings presented in ISO 281:2007 (Ref. 42). 

The following results were obtained: 

1. Using two transformations, old obsolete filter ratings can be converted to new ISO filter ratings and 

vice versa. Approximate equations relating the old Ex = 200 FR values with the new Ex(c) = 200 and 

Ex(c) = 1000 FR values are 

For new Ex(c) = 200 filter rating; FRȕx(c)200 = 0.722 (FRȕx200) + 2.97 

For new Ex(c) = 1000 filter rating; FRȕx(c)1000 = 0.848 (FRȕx200) + 4.14 

2. Two separate sets of life factors (LF) based on lubricant cleanliness for roller bearings and ball 

bearings, respectively, were derived based on the new Ex(c) = 200 and Ex(c) = 1000 ISO filter ratings. These 

LF values are normalized to FR values of 10 Pm at Ex(c) = 200 and 13 Pm at Ex(c) = 1000 and are 

independent of ȁ and/or ț, loading conditions, and bearing size. These are: 

For roller bearings and new Ex(c) = 200 filter rating; LF § 3.5 [1.39(FRȕx(c)200) – 4.11]–0.55 

For roller bearings and new Ex(c) = 1000 filter rating; LF § 3.5 [1.18(FRȕx(c)1000) – 4.88]–0.55

For ball bearings and new Ex(c) = 200 filter rating; LF § 1.8 [1.39(FRȕx(c)200) – 4.11]–0.25

For ball bearings and new Ex(c) = 1000 filter rating; LF § 1.8 [1.18(FR ȕx(c)1000) – 4.88]–0.25

3. ISO 281:2007 Standard provides a reasonable qualitative estimate of the effect of particle damage 

on rolling-element bearing fatigue life for filtration ratings ranging from d4 Pm at Ex(c) = 1000 (extreme 

cleanliness) up to E34(c) = 1000 (typical contamination). At conditions of severe contamination and above, 

where the filter ratings are t35 Pm at Ex(c) = 1000, the ISO 281:2007 Standard correlated with test results 

obtained with common hard steel contamination that can be found in industrial gearboxes.

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Fatigue Life of EHL Contacts. J. Engr. Tribology, Proc. Inst. Mech. Engrs., part J, vol. 220, no. 3, 

2006, pp. 287௅294. 
NASA/TM—2014-218071 23

31. Needelman, W.M.; and Zaretsky, E.V.: New Equations Show Oil Filtration Effect on Bearing Life. 

Power Trans. Des., vol. 33, no. 8, 1991, pp. 65௅68. 

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14. ABSTRACT

In 1991, Needelman and Zaretsky presented a set of empirically derived equations for bearing fatigue life (adjustment) factors (LFs) as a

function of oil filter ratings. These equations for life factors were incorporated into the reference book, “STLE Life Factors for Rolling

Bearings.” These equations were normalized (LF = 1) to a 10-μm filter rating at ȕx

 = 200 (normal cleanliness) as it was then defined. Over

the past 20 years, these life factors based on oil filtration have been used in conjunction with ANSI/ABMA standards and bearing computer

codes to predict rolling bearing life. Also, additional experimental studies have been made by other investigators into the relationship

between rolling bearing life and the size, number, and type of particle contamination. During this time period filter ratings have also been

revised and improved, and they now use particle counting calibrated to a new National Institute of Standards and Technology (NIST)

reference material, NIST SRM 2806, 1997. This paper reviews the relevant bearing life studies and describes the new filter ratings. New

filter ratings, ȕx(c)

 = 200 and ȕx(c) = 1000, are benchmarked to old filter ratings, ȕx = 200, and vice versa. Two separate sets of filter LF

values were derived based on the new filter ratings for roller bearings and ball bearings, respectively. Filter LFs can be calculated for the

new filter ratings.

15. SUBJECT TERMS

Rolling bearing; Life; Bearing; Fatigue; Life; Lubricant; Reliability

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